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Lecture Notes in Earth Sciences Editors: S. Bhattacharji, Brooklyn G. M. Friedman, Brooklyn and Troy H. J. Neugebauer, Bonn A. Seilacher, Tuebingen
- iZ',','g
46
Gianni Galli
Temporal and Spatial Patterns in Carbonate Platforms
Springer-Verlag Berlin Heidelberg NewYork London Paris Tokyo Hong Kong Barcelona Budapest
Author Dr. Gianni Galli Via Samacchini 5 1-40141 Bologna
"For all Lecture Notes in Earth Sciences published Ull now please see final pages of the book"
ISBN 3-540-56231-1 Springer-Verlag Berlin Heidelberg New York ISBN 0-387-56231-1 Springer-Verlag New York Berlin Heidelberg
Library of Congress Cataloging-in-Publication Data Galli, Gianni, 1956Temporal and spatial patterns in carbonate platforms / Gianni Galli. p. cm. - (lecture notes in earth sciences; 46) Includes bibliographical references. ISBN 3-540-56231 - 1 (Berlin: acid-free). - ISBN 0-387-56231-1 (New York: acid-free) 1. Rocks, Carbonate. 2. Sedimentation and deposition. I. Title. II. Series. QE471.15.C3G35 1993 552' .58-dc20 93-8317 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, re-use of illustrations, recitation, broadcasting, reproduction on microfilms or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under the German Copyright Law. © Springer-Verlag Berlin Heidelberg 1993 Printed in Germany Typesetting: Camera ready by author 32/3140-543210 - Printed on acid-free paper
ACKNOWLEDGMENTS
I w o u l d like to e x p r e s s my h e l p e d me d u r i n g t h i s 1 2 - y r
s i n c e r e t h a n k s to long work project,
all of t h o s e w h o s t a r t e d in 1981.
Prof.G.B.Vai ( U n i v e r s i t y of B o l o g n a ) a l l o w e d me to m a k e a f r e s h s t a r t at the b e g i n n i n g by i n t r o d u c i n g me to the f i e l d g e o l o g y of the C a r n i c A l p s a n d l a t e r a c t i n g as a s u p e r v i s o r of m y P h D thesis, He also oriented my mental attitude towards a neocatastrophist a p p r o a c h of g e o l o g i c a l p r o c e s s e s . Prof.R.N.Ginsburg allowed Florida platform in 1987 enjoying the fresh breeze Fullbright tenure,
me to s t u d y Pleistocene cores of and put me in the condition of of F i s h e r Island Station during a
Prof.C.G,St.C,Kendall g a v e a c c e s s to South Carolina University, probably s i m u l a t i o n p r o g r a m s e x i s t i n g today,
the one
S e d p a k p r o g r a m at the of the m o s t p o w e r f u l
D r , M i a V a n S t e e n w i n k e l q u i t e k i n d l y s e n t me h e r t h e s i s in 1 9 8 8 which allowed the development of the sequence stratigraphy a p p r o a c h to the c a s e h i s t o r i e s s u m m a r i z e d in this work. I am a l s o i n d e b t e d to Prof. J o n a t h a n T e n n e n b a u m (Fusion Energy Foundation) who pushed me to develop section V on the relativistic distribution of e v e n t h o r i z o n s ,
TABLE OF C O N T E N T S
1
INTRODUCTION ..................................................
PART
Krikogenetic
rejuvenation
Krikogenetic
quiescence
periods periods
........................
..........................
I ........................................................ INTRODUCTION ............................................. FACIES BELTS ............................................ Shallow
ramp .......................................
Thin-bedded
alternations
Thick-bedded
alternations
Proximality-distality Intermediate Deep
......................
trends
0nlap
geometry
0fflap
SEQUENCE Shelf
17
~I ~6
tract
tract tract
surface
Transgressive Maximum
tract
facies
facies
Ravinement
~I
....................................
facies
Lowstand
27 31
RAMPS . . . . . . . . . . . . . . . . . . . . . . . . . .
facies
margin
15
2A
STRATIGRAPHY ...................................
Highstand
13
.....................................
geometry
Transgressive
8 12
.......................
ramp ..................................
OF I N T R A S H E L F
A 7
.....................
ramp ..........................................
GEOMETRIES
3
..........................
............................. ..............................
.................................
surface
flooding
.........................
..............................
surface
...........................
51 55 57 57 57 58 58 59
M E C H A N I S M S OF F O R M A T I O N O F O N L A P R A M P S . . . . . . . . . . . . . . . . . . 60 FORT THOMPSON FORMATION,PLEISTOCENE,FLORIDA P L A T F O R M .... 6 9 Introduction
.......................................
69
Lithofacies
and
71
Marine
swamp
associations Shallow Deep
Maximum
Early Late
flooding
.........................
............................ surface
facies
highstand
facies facies
......................
tract
....................
tract tract
88 88
95
........
..................................
associations
83 83
...................
.........................................
Shallow
78 78
92
.......................................
studies
76
..................
FORMATION,JURASSIC,VENETIANALPS
Introduction area
..............................
surface
highstand
GRIGI"
Facies
75
surface
Transgressive
Study
73
................................
sequence
Ravinement
71
..............................
ramp .....................................
Transgressive
Previous
..............
ramp ..................................
Depositional
"CALCARI
setting
Bay ....................................
Freshwater Facies
environmental
97 97 99 I01
...............................
I01
ramp .................................
102
Vlll
Oncolite
grainstones
Bioclast-lithoclast Lime
mudstones
Intermediate
wackestones
....................
107 109
sequence
.............................
facies
margin
tract ...................
facies
highstand
tract ..................
121
tract ....................
122
D E V O N I A N CARBONATE P L A T F O R M , C A R N I C
...............................
ramp .................................
Pond
facies .............................
Intermediate
ramp ............................
Intraclast
shoal ........................
142 143
grain-wackestones
SHORELINE sequence
.............................
facies
tract ........................
Highstand
facies
facies
remarks
tract ...................
tract .......................
................................
PART II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C O M P U T E R SIMULATION OF CLASTIC WEDGES . . . . . . . . . . . . . . . . . . Introduction
......................................
pattern
Convergent
..................................
pattern pattern
Concludin~ Introduction
Highstand
155 155 159
179
sequence
MASSIF.181
.......................
thinning-upward and
unit .............
thickening-upward sequence
facies
Transgressive
154
169
depositional
Lowstand
153
................................
...................................... and
153
.................................
depositional Fining-
151 152
162
(CRETACEOUS-PALEOCENE),GARGANO
Coarsenin E-
149 151
................................
remarks
SLOPE CARBONATES
144
SEQUENCE,PLEISTOCENE .......... 149
......................................
Lowstand
Concludin~
............
model ................................
Transgressive
Second
142 143
Introduction
First
139 142
Brachiopod
Depositional
Diversent
130 139
ramp ....................................
Depositional
Parallel
124
ALPS,ITALY .......... 130
......................................
Shallow
117 I19
...............................
associations
i15 117
tract .................
facies facies
deposition
Introduction
...................
...................................
highstand
Shelf
"CAPO RIZZUTO"
I05 109
Early
Deep
105
.................
wackestones
Transgressive
Facies
105
Lithiotis Depositional
of
.... 1 0 4
..........................
ramp ....................................
Paleobathymetry
Model
..... 1 0 2
grain-packstones
Skeletal
Late
and
ramp ............................
Oolite Deep
packstones
grain-packstones
facies
184
unit ....... 187
......................
tract ........................
facies
181 184
tract ...................
tract .......................
19A 19d i97 207
IX
Shelf
margin
facies
Distal Proximal Third
sequence
facies
Transgressive Highstand
facies
Proximal
MIDDLE
Introduction Shallow
tract
...................
215
........................
216
clinoforms
.....................
216
.......................
216
................................
BUILDUPS,DOLOMITES
...........
......................................
ramp .................................
ramp .........................................
Deep PART
213 215
ramp ......................................
Intermediate
209
.......................
tract
CARBONATE
209 209
........................
clinoforms
remarks
TRIASSIC
tract
facies
Distal Concluding
....................
lobe ...........................
depositional Lowstand
tract
lobe .............................
218 223 223 225 226 227
III ....................................................
229
MODAL SEQUENCE ......................................... PART IV ..................................................... SHORT-TERM SEA-LEVEL FALLS : AN INDICATOR OF GEOIDAL PULSES? ................................................
230
Megabreccias
......................................
Seismoturbidites Are or
megabreccias vertical
Drowning Relief
PART
..................................
of
the
product
of
Hierarchy Geometrical Biological Event
2~9
tectonics?
............................
252
platforms
258
...................
.................................
......................................
Relativistic
Generation
237
carbonate
inversions
Neg-entropy
23A
oompressional
V ...................................................... RELATIVISTIC DISTRIBUTION OF "EVENT HORIZONS". ......... Introduction
233
concept
of
event
.....................
....................................... of of
singularities singularities
distribution evolution
horizons
261 281 282 282 286 288
.......................
289
........................
290
of
singularities
.........
..............................
....................................
REFERENCES ..................................................
291 295 297 300
I n t r o d u o t ion
"The geological history,as expressed by the stratigraphic column,is basically composed of cycles of sedimentation, stratification and magmatism which correlate with relative changes in sea level d e t e r m i n e d in turn by d i f f e r e n t types of crustal movements. The classical sequence of stages "transgression - inundation - differentiation - regression emergence" is b e l i e v e d to reflect the d e f o r m a t i o n p h a s e s of a g e o t e c t o n i c cycle" (Wezel,1988: p.37). The concept of g e o t e c t o n i c cycle is f u n d a m e n t a l in because it links t e c t o n i c s with s e d i m e n t a r y p r o c e s s e s .
geology
According to Wezel (1988) the geotectonic cycle is an e x p r e s s i o n of c y c l i c v a r i a t i o n s in the b e h a v i o r of the crust; more precisely,it is a g e o d y n a m i c response to the Earth's variations in the rate of r o t a t i o n (M~rner,19869 Whyte,1977~ Carey,1976).Based on a g l o b a l analysis of g e o t e c t o n i c data, synchronous e p i s o d e s of intense global swelling, governed by cyclically ordered diastrophic processes, were identified (Wezel,1985;1988). The p r o c e s s l e a d i n g to these swells was t e r m e d k r i k o g e n e s i s (Wezel, 1988).It b a s i c a l l y c o n s i s t s of not steady, localized, migratory vertical movements linked to m a n t l e diapirism and c o n c e n t r a t e d in single z o n e s . T h e o v e r l y i n g crust a d j u s t s itself to mantle motions induced by k r i k o g e n e s i s , w i t h the f o r m a t i o n of transient troughs and swells ('touche-de-piano' tectonics).This mechanism was individuated in several areas (Wezel,1988). The h i s t o r y of the E a r t h is d e s c r i b e d by six e p i s o d e s that repeat in the same way in the c o u r s e of g e o l o g i c a l time.Their d u r a t i o n p r o g r e s s i v e l y d e c r e a s e s : t h e first c y c l e has a d u r a t i o n of about 200 m i l l i o n years, the f o l l o w i n g , y o u n g e r c y c l e s lasted 1 5 0 , I 1 5 , 6 5 , A 5 and 20 m.y. These cycles can be t r a c e d along a time-spiral,its length representing the time (Fig.l). Each cycle of d e p o s i t i o n and uplift can be subdivided into the following phases: krikogenesis, inundation, regression and e m e r g e n c e . The last phase is p r e c e e d e d by a t e c t o g e n i c phase, linked to an i n c r e a s e in t e c t o n i c a c t i v i t y (Fig.2).
G EOSYNCLIlv4 I NUNDA.I.IOIv
uPPE. oo _
\
0 0
LU
co
uj
0
h..
o
0
0
+ "eln~. otv
"~
\
flOGEN! c
Fig.l - Time spiral showing the six ma~or sedimentary cycles of Earth history, each separated by a tectogenetic phase (Wezel,1976:p.87).Each cycle is divisible into four main phases. The geological history is marked by the rhythm of k riko~enetic rejuvenation and k~ikogenetic quiescence periods (Wezel,1988), each punctuated by a recurrent set of changes of different organic and inorganic processes.
== =_
== 13Z O
~oOoOoO O 0 )O ° O ~ O G o oDD • <=O DO °OOO~
RED BEDS
12--MOLASSE
COMPRESSION
11-
PDSTFLYSCH AND WILDFLYSCH
1 0 - A R E N I T I C FLYSCH <3
9 Z
-
GREENISH--GREY MARLY SHALES
--
=J U
8 - BIOCLASTIC
u3 O m
CALCITURBIDITES
7
--
6
-- BLACK SHALES
VARIEGATED SHALES AND OLISTOLITHS
o
'
5 -- CHERTY LIMESTONE
o
¢t -- LIMESTONE-- CHERT--MARL
z u/
3
--
SHALLOW--WATER CARBONATES
O
~ 9 ~ N ~97~3B
BASAL TERR,GENOUS
CLAST,CS
Fig.2 - G e o t e c t o n i c cycle showing the v e r t i c a l sequence of lithofacies and correlations with the diastrophic phases (adapted from Wezel,1988).The sequence, consisting of a transgressive hemicyole overlain by a r e g r e s s i v e hemicycle, occurs in d i f f e r e n t situations on various scales, also in c a r b o n a t e s e t t i n g s r a n g i n g from s h a l l o w - w a t e r to basinal.
Kriko~enetic
rejuvenation
periods
Periods characterized by k r i k o g e n e t i c rejuvenation r e c o r d the formation of a 'basin-and-swell' morphology (geoanticlinal ridges), high heat flow and volcanioity of shoshonite to t h o l e i t e affinity. T h e s e p e r i o d s are a s s o c i a t e d w i t h e p i s o d e s
of normal m a g n e t i c p o l a r i t y , i n c r e a s e s in the rate of s e a - f l o o r spreading,global temperature (leading to the deposition of evaporites) and climatic amelioration. There is also an increase in the seismicity at low latitudes (Whyte,1977). Several s t o r m d e p o s i t i o n a l s y s t e m s are r e p o r t e d from p e r i o d s of krikogenetia rejuvenation, as s h o w n by M a r s a g l i ~ and Klein (1983) who listed several storm occurrences dated to the 0rdovician, Devonian,Early Jurassic and late C r e t a c e o u s , all time intervals of global increases in ocean temperature. A n o t h e r f e a t u r e p e c u l i a r of k r i k o g e n e t i c r e j u v e n a t i o n seems to be the g e n e r a l i z e d appearance of b l a c k shales ('AOE' events) probably resulting from a low oxygen concentration in the deep-intermediate levels of the o c e a n due to a r e d u c e d w a t e r circulation possibly resulting in turn from an increase in temperature. Carbonate sedimentologists working on c a r b o n a t e mineralogy have observed systematic variations in the i s o t o p i c c o m p o s i t i o n of the s e a - w a t e r t h r o u g h the P h a n e r o z o i o . L o w e n s t a m (1963) found n o n - l i n e a r c h a n g e s of a r a g o n i t e - h i g h - M g c a l c i t e mineralogy of marine skeletons, linked to changes in the atmospheric carbon dioxide (Sandberg,1983). Fischer (1981) d i s t i n g u i s h e d g r e e n h o u s e p e r i o d s c o n t a i n i n g an e l e v a t e d a m o u n t of c a r b o n d i o x i d e and temperature, and icehouse periods of colder temperature and lesser amounts of carbon dioxide,As shown by S a n d b e r g (1983), such variations seem to be tied to worldwide sea-level fluctuations,ultimately global tectonics and v o l c a n i c a c t i v i t y .
Krikogenetic
quiescence
periods
P e r i o d s c h a r a c t e r i z e d by k r i k o g e n e t i c q u i e s c e n c e are l i n k e d to reverse or f r e q u e n t i n v e r s i o n of m a g n e t i c p o l a r i t y , a slowing down of tectonic activity, regressions and climatic deteriorations. Mountain uplifts produce a deflection at low latitudes which can be responsible for global cooling, hence,glaciations. T h e s e p e r i o d s are m a r k e d by b i o t i c crises (Whyte,1977), i n c r e a s e in s h a l l o w s e i s m i c i t y at h i g h l a t i t u d e s and
few s t o r m
occurrences.
A c c o r d i n g to W e z e l (1988) and W h y t e (1977) the d e c r e a s e in the rate of E a r t h ' s rotation which leads to a t r a n s i t i o n from k r i k o g e n e t i c r e j u v e n a t i o n to q u i e s c e n c e c o i n c i d e s w i t h a c h a n g e in the g e o i d a l configuration from an oblate to a prolate shape.The geotectonic cycle is seen as the p r o d u c t of such cyclical variations. The transgressive hemicyole comprising krikogenetic and geosynclinal phases records a period of acceleration in the rate of rotation ('spin maximum' of Whyte,1977). Conversely, the u p l i f t of the g e o s y n c l i n a l pile
and m o l a s s e d e p o s i t i o n r e c o r d s a d e c r e a s e in the Earth's r o t a t i o n ('spin minimum' of W h y t e , 1 9 7 7 ) .
speed
of
the
Four g l o b a l l y s y n c h r o n o u s e p i s o d e s of k r i k o g e n e t i c r e j u v e n a t i o n were identified: I) Late L o w e r J u r a s s i c - M i d d l e Jurassic; 2) A p t i a n - Senonian; 3) P a l e o g e n e ; and A) P l i o c e n e - Q u a t e r n a r y (Wezel,1988). The C r e t a c e o u s was a period of long, intense k r i k o g e n e t i c activity. The "sea-level curve" published by Haq, et ai.(1987) which shows relative sea-level fluctuations throughout the Phanerozoic, consists of a s e q u e n c e of irregular bumps and wiggles. Long-term sea level fluctuations "are most likely related to c h a n g e s in the v o l u m e of e c e a n basin driven by changes in the rate of sea floor s p r e a d i n g and ridge volume. The s h o r t e r term e u s t a t i c c h a n g e s ... are most p r o b a b l y due in large part to changes in p o l a r ice v o l u m e and p o s s i b l y part of some other, as yet u n k n o w n m e c h a n i s m " (Vail and Haq,1988). In several works M~rner introduced the concept of g e o i d a l eustasy (i.e. M S r n e r , 1 9 8 6 ) . T h e effects of s h o r t - t e r m geoidal e u s t a s y have been u n d e r e s t i m a t e d , as g e o i d a l pulses tend to be correlated with tectonic phases of glacial fluctuations. Conversely, geoidal eustasy is an independent mechanism, c o n t r o l l e d by p r o c e s s e s s e a t e d in the u p p e r mantle. The study of c a r b o n a t e sediments offers two advantages: I) c a r b o n a t e s store a m o l t i t u d e of p h y s i c a l , chemical, inorganic and o r g a n i c v a r i a t i o n s that have o c c u r r e d t h r o u g h o u t the E a r t h history; 2) carbonate sediments, more than siciclastic sediments, give a fine, precise record of v e r t i c a l crustal movements (relative sea-level changes by Kendall and Schlager,1981). Therefore, the study of c a r b o n a t e rocks is especially s u i t a b l e for d e f i n i n g a l t e r n a t i o n s of k r i k o g e n e t i c periods, and p o s s i b l y a h i e r a r c h y of k r i k o g e n e t i c processes. These may be linked to g e o i d a l eustasy. This preliminary work s e q u e n t i a l questions: I) Are there specific krikogenetic rejuvenation
has
been
centered
on
carbonate geometries and q u i e s c e n c e p e r i o d s ?
the
that
following
reflect
2) Is t h e r e a s e q u e n c e in the c a r b o n a t e world encompassing s h a l l o w - w a t e r and slope s e c t o r s e q u i v a l e n t or c o m p a r a b l e to the g e o t e c t o n i c cycle?
3) If e x i s t e n t , are such g e o m e t r i e s s h o r t - t e r m geoidal p u l s e s ?
and
sequences
indicative
of
A) A r e s h o r t - t e r m g e o i d a l p u l s e s d i s t r i b u t e d at r a n d o m t h r o u g h g e o l o g i c a l time, or c o n v e r s e l y o r d e r e d in time by some p h y s i c a l law? The f o l l o w i n g time i n t e r v a l s are c o n s i d e r e d to be p e r t i n e n t to this study: 1) Ordovician; 2) Upper DevonianLower C a r b o n i f e r o u s ; 3) A n i s i a n - L a d i n i a n ; ~ ) Pliensbachian- Aalenian; 5) V a l a n g i n i a n ; 6) C e n o m a n i a n - T u r o n i a n ; 7) U p p e r M a a s t r i c h t i a n ; 8) T h a n e t i a n ; 9) Y p r e s i a n ; I0) Chattian; II) S e r r a v a l l i a n ; 12) M e s s i n i a n ; and 13) P l e i s t o c e n e . These time i n t e r v a l s are t h o u g h t to c o r r e s p o n d to more intense short-term krikogenetic rejuvenation periods,They were i n d i v i d u a t e d by an i n t e g r a t e d a n a l y s i s of p a l e o n t o l o g i c a l and g e o l o g i c a l data w h o s e d e s c r i p t i o n is b e y o n d the scope of this work, In this w o r k I d e s c r i b e d some f e a t u r e s w h i c h I thought to be linked g e n e r a l l y to k r i k o g e n e t i c r e j u v e n a t i o n periods; the main key e l e m e n t s of this r e s e a r c h are the following: I) i n t r a s h e l f ramps; 2) d i v e r g e n t (onlap) p a t t e r n s ; and 3) m e g a b r e c c i a s .
and
convergent
(offlap)
sedimentary
Part
I
Intrashelf ramps are b a s i c a l l y sedimentary wedges (miniature ramps) d e v e l o p e d w i t h i n c a r b o n a t e s h a l l o w - w a t e r c o m p l e x e s . Onlap and offlap ramps are respectively divergent and c o n v e r g e n t prisms. R o t a t i o n a l s u b s i d e n c e is a m a i n c o n t r o l in the f o r m a t i o n of o n l a p ramps. D i f f e r e n t m o d a l i t i e s of f o r m a t i o n of onlap ramps are d i s c u s s e d in the s e l e c t e d case h i s t o r i e s d e s c r i b e d below. They c o m p r i s e Pleistocene, Jurassic (Pliensbachian), and Upper Devonian (Givetian-Frasnian) g e o l o g i c a l intervals.
Introduction
T h e p r o f i l e of s o m e b a s i n s is t h a t of a r a m p c o n s i s t i n g of a n irregular, undulating topographic outline with a deeper basinal trough and a shallower, overhanging b a s i n l o c a t e d l a n d w a r d of a s i l l or i n f l e x i o n of t h e t o p o g r a p h i c gradient. T h i s r a m p o u t l i n e is d e v e l o p e d at v a r i o u s s c a l e s , At a r e g i o n a l scale it c o n s i s t s of a m i o g e o s y n c l i n a l and a eugeosynclinal trough separated by a r i d g e of the b a s e m e n t . T h e ridge, or f l e x u r e , as w e l l as the l a n d w a r d a r e a of the m i o g e o s y n c l i n e may be the sites of formation of smaller scale ramps.These miniature ramps are most commonly developed in d i v e r g e n t p r i s m s of p a s s i v e m a r g i n s , in the areas comprised b e t w e e n the h i n g e
/, i s I
p
11
/
/
/%
1I
E°o1 0
/
,5:
0
24
..=:..:'"
km
- C r o s s s e c t i o n of M i d d l e O r d o v i c i a n sequence from East Tennessee (from Ruppel and Walker,1982). The miniature (intrashelf) r a m p o c c u r s in the f l e x u r e z o n e of the s e d i m e n t a r y prism, and the trough areas,where t h e t i m e l i n e s b e g i n to d i v e r g e f r o m one another,An example occurs in t h e M i d d l e Ordovician cross s e c t i o n of E a s t T e n n e s s e e (Fig.3).
Several
modern
carbonate
shallow-water
settings
record
a
complex bathymetry consisting of p e r i t i d a l banks and islands surrounded by o p e n marine sediments. A number of a n a l o g o u s fossil 'facies mosaics' record a complex interbedding of different, unrelated lithologies. Laporte (1967) a n d A n d e r s o n (1971) interpreted one of these facies mosaics within the Manlius Formation as a c o m p l e x array of h i g h and low-tidal flat a n d l a g o o n a l d e p o s i t s r e s u l t i n ~ f r o m a t r a n s g r e s s i o n in a n epeiric sea (Fig.4).
SUPRATIDAL
INTERTIDAL SHOALS
INTERTIDAL
LAGOON
~
elmicrosp~
skeletal pelmlcrite
T'DAAANN
°'i°o' alo"esoa"u i"
N
X --X'= 30KM
Fig. A - R e c o n s t r u c t e d depositional environment for the Coeymans and Manlius facies, Lower Devonian, New York (from Anderson,1971).Recent reappraisals of the Manlius and Coeyman F o r m a t i o n s b y G o o d w i n , et a i . ( 1 9 8 6 ) d i d n o t d i s a g r e e w i t h t h i s former sedimentological interpretation. In s o m e of t h e s e s i t u a t i o n s the p r e s e n t - d a y or a n c i e n t l a g o o n a l f l o o r r a n g e s in d e p t h f r o m a f e w m e t e r s (1.5 to 2 m) to as m u c h as 25 - 35 m. B e t w e e n shallow and deeper floor there is an i n c l i n e d s u r f a c e , 10 to 50 km w i d e (20 k m on t h e a v e r a g e ) that
10
is c o m p a r a b l e to a ramp (Ahr,1973), in spite of the s m a l l e r size and width. A m o d e r n e x a m p l e of this t o p o g r a p h i c feature is found w i t h i n the B a h a m a p l a t f o r m , in the area c o m p r i s e d b e t w e e n the B i m i n i B a n k and the A n d r o s Bank (Eberli and G i n s b u r g , 1 9 8 1 ; Fig.5). WSW STRAITS OF FLORIDA
SIMINt BANK
F i g . 5 - S e i s m i c cross and G i n s b u r g , 1 9 8 0 ) . An of A n d r o s .
STRAITS
OF ANDROS
£NE A N D R O S SANK
s e c t i o n of the B a h a m a p l a t f o r m i n t r a s h e l f ramp o c c u r s b e l o w the
(Eberli Straits
The object of the following chapters is to outline the characteristics of these miniature ramps located within carbonate platforms. They are t e r m e d i n t r a s h e l f ramps b e c a u s e of t h e i r l o c a t i o n w i t h i n c a r b o n a t e p l a t f o r m s . Their recoEnition, description and interpretation of the controlling factors were b a s e d on the s t u d y of a n u m b e r of fossil and m o d e r n c a r b o n a t e d e p o s i t i o n a l s y s t e m s (Fig.6). Study
areas
# 2,3,~,5,1~
and
15 are
described
in this
work.
ii
[]
Fig.6 - Locations of study areas. I): Florida Bay, Holocene, U.S.A.; 2): Fort Thompson Formation, Pleistocene, U.S.A.9 3): 'Caloari Grigi' Formation,Jurassic, Italy~ ~): D~rrenstein Formation, Carnian (Triassic), Italy; 5): Devonian p l a t f o r m , G i v e t i a n - F r a s n i a n (Devonian), Italy; 6): Cadeby Formation,Permian, England; 7): Asbian - Brigantian platform,Derbyshire, England; 8): Dinantian platform,Upper Carboniferous, Belgium; 9): Devonian platform, Givetian-Frasnian, Belgium; I0): Belize platform,Holocene, British Honduras; 11): Pleistocene carbonates,eastern Tunisia; 12): Manlius Formation, Lower Devonian, U.S.A.; 13): Exuma Cays, Holooene, Bahamas~ I~): Cretaceous carbonates, southern Gargano, Italy; 15): Capo Rizzuto carbonates, Pleistocene, Italy.
Faoies
An
intrashelf
ramp
is c o m p o s e d I) 2) 3)
belts
of
three
sectors:
S h a l l o w ramp; Intermediate ramp; D e e p ramp.
Some trends which are immediately apparent when examining ancient and modern intrashelf ramps are landward decreases in the d e t r i t u s / m a t r i x ratio and faunal diversity. Petrographic t r e n d s in m o s t c a s e s a r e n o t l i n e a r , as a r e s u l t of a c o m p l e x topography, variations in w i d t h of f a c i e s belts and a broad s p e c t r u m of v a r i a t i o n s in t h e r e l a t i o n s h i p s between the t h r e e sectors,More linear trends and a gradational transition between shallow and deep ramp reflect regular and negligible topographic variations along the sloping surface~ in these simpler situations the intermediate ramp is c o m m o n l y poorly developed.
SHALLOW
RAMP INTERMEDIATE
RAMP
DEEP
RAMP $.L.
graded limestone beds in micrites thinly- bedded limestones in micrites
Fig.7 ramp.
- Profile
sandwaves; beaches ; oolite bars
and
main
facies
T h i s c h a p t e r is a s u m m a r y of t h e s e c t o r s w h i c h m a y be r e c o g n i z e d
biostromes; banks ; ~catch-up } reefs
types
of
an
intrashelf,
onlap
m a i n c h a r a c t e r s of t h e s e t h r e e on stratigraphic profiles. A
]3
~eneral, representative profile of t h e m a i n l i t h o f a c i e s in r e l a t i o n to the b a t h y m e t r y is s h o w n in F i g . 7 ,
Shallow
types
ramp
The shallow ramp is the shallowest site of the ramp. The la~oonal floor is approximately located 2 m below the sea-level. The bathymetry is m a d e u n e v e n by a c o m p l e x f r a m e w o r k composed of p e r i t i d a l banks and islands surrounded by o p e n marine or lagoonal sediments, This environmental situation is c o m m o n in n e a r l y all r i m m e d s h e l v e s w h e r e t h e r a p i d H o l o c e n e eustatic sea-level rise (Im/1500 yrs.) determined a drowning within the euphotic zone and the formation of scattered d e p o c e n t e r s that h a v e k e p t p a c e w i t h the r i s i n g s e a - l e v e l , T h i s p r o c e s s o c c u r r e d a l s o in p r e - H o l o c e n e times (Read,1985: p.16~ Grotzinger,1986~Pratt and James,1986).
'% "
Fi~,8 - Distribution F l o r i d a B a y (Enos a n d
{.~
"j) j~
KM
of carbonate banks and islands Perkins,19799Ginsburg,1956).
within
Progradation mechanisms of depocenters and the dynamics of infilling of the lagoonal depressions are poorly understood, Laporte suggested some possible causes leading to the formation
14
I) migration of t i d a l c h a n n e l s ; of t h e s e m o s a i c s : of i s l a n d s ; a n d 3) s h o r t - t e r m sea-level changes.
2)
migration
Shoals range from calcarenite to c a l c i r u d i t e in g r a i n s i z e a n d h a v e an o n c o l i t e , bioclast or intraclast composition. Their w i d t h is d i f f i c u l t to r e c o n s t r u c t from ancient examples. Modern shoals, s u c h as t h o s e o c c u r r i n g in the F l o r i d a p l a t f o r m , are h u n d r e d of m to 2 k m w i d e , s e m i c i r c u l a r or e l o n g a t e d n o r m a l to the s t r i k e of the t h r e e s e c t o r s of the r a m p (Fig.8; Enos and Perkins,1979).Similar sizes were estimated by A n d e r s o n (1971) in the r e c o n s t r u c t i o n of t h e M a n l i u s F o r m a t i o n (Fig. A). In s o m e cases shoals are narrower in e x t e n t , s u c h as t h o s e o c c u r r i n g in the B a h a m a p l a t f o r m (Fig.9).
. i< ,
•
"~
~
'
"~
"'
~j" '
,
.. . . . . .
.,-
~ /';,
~,
..L~
~.,
..
i v
"~
i
~J
/~
i
~ i~i~!~
i
I Fig.9 Aerial view Island,Bahama Bank.
of
islands
and
banks
near
Andros
Intershoal areas are represented by t h i n l y b e d d e d , d a r k c o l o r e d mudstones and wackestones, T h e y a r e in s e v e r a l cases nodular and strongly bioturbated. They are a l s o p r o b a b l y incised by channels. The faunal diversity is low (algae, ostracods, thin-shelled bivalves, calcispheres), typical of poorly oxygenated environments.The very shallow intertidal mud and lithoclasts are draped by enorusting organisms, s u c h as w o r m s . Freshwater p o n d s m a y be p r e s e n t in t h e l a n d w a r d m o s t areas and ponds, as is shown by the occurrence of plane-spiraled gastropods and snails and remains of r o o t traces and plant d e b r i s . T h i s s i t u a t i o n o c c u r s in F l o r i d a B a y ( F i g . 1 0 ) , c l o s e to the ~ u n c t i o n to the land.
15
NW
SE
FLORIDABAY MURRAY KEY
FLAMINGO
EVERGLADES----~
" ~{-:.*"i : :'.;
1-
"~3 km
[ ~
skeletal wackestone hurricane mud mangrove peat fresh-water mudstone peat Pleistocene
Fig.lO - Schematic profile Everglades (Scholi,1963).
of
the
recent
sediments
close
to
the
T h e s e a r e a s m a y be t h e s i t e s of f o r m a t i o n of p i s o i d s a n d v a d o s e diagenetic features which may also form on top of shoals. Radial oolite horizons may occur in the more sheltered, hypersaline intershoal areas. Shoals and intershoal associations:
areas
are
organized
into
I) t h i n - b e d d e d alternations; 2) t h i c k - b e d d e d a l t e r n a t i o n 8
two
main
facies
and
Thin-bedded alternations
These alternations consist of a few cm to 20 cm thin, calcarenite calcisiltite layers interbedded with black, cm-thin argillaceous calcilutites which contain vegetal debris and plant remains. Bases and tops of calcarenite and calcisiltite beds are sharp. Physical sedimentary structures are gutter casts at the bases of coarser grained beds, pinch-outs within calcilutites and very thin mm to c m thin lenses of shell debris, or cm-thiok mud storm horizons (Fig.ll). Calcarenite and calcisiltite mud layers frequently display a vertical thickening-upward trend which is a c c o m p a n i e d by an increase in t h i c k n e s s a n d n u m b e r of s t o r m h o r i z o n s and grain size (Fig,12). In o t h e r cases thicker beds are homogeneous mudstones with rare ostracods.
]6
Fig.ll - Thin-bedded alternations. A,B: m u d s t o n e s intercalated w i t h m a r l s ( ' C a l c a r i G r i g i ' F o r m a t i o n , s e c t i o n s #5 a n d #19). C; very thin, storm-generated lamination (Giver i a n - F r a s n i a n platform, Belgium). D: H u r r i c a n e and winter-storm senerated l a y e r s ( C r a n e Key: F l o r i d a B a y ) . There
are
basically
two
end-members
I) c a l c a r e n i t e s with fragments 2) s t r u c t u r e l e s s mud.
of
of
thicker
bioclasts
and
beds: shell
debris;
There is a l a t e r a l transition, over short distances, between these two end members,accompanied by a set of s t r u c t u r e and compositional variations, as is s h o w n in F i g . 1 3 . A modern analogue of these marlstone - limestone alternations o c c u r s c l o s e to t h e l a n d w a r d m o s t termination of F l o r i d a Bay, c l o s e to t h e land ( F i g . l O ) . T h e r e , mud layers are aragonite and contain sparse detritus of m o l l u s k s , foraminifers and
]7
cross laminated mud with vegetal debris
hummocky
_."::L.-c..--" ---." ----,.
algal stromatolite
-L :.".'.'.'....-:-.-:
:'.:'1
•
.
,
.
-
poorly sorted calcarenite laminae
,~-4,#~.~.J,,4;,'~:A
-.... •.'..'.'"
: J,"'" ~ " ' ~ ' " ; '
15 cm
,
.
.
roots
.J
winter storm deposit
.
°
.
•
-- algal mats
t
~,---~ ~-,--~.. ~, :.~:~':.~:~ . ~%~t ~ " ~_~.'~-[t
"dli "..:' :~." ~ f e n est rae •
algal laminae
_
cmO
.
carbonate grains & algal mats poorly sorted c a l c a re n i t e algal laminated
~--sediment Fig.12 Florida
Details of recent Bay (Galli,1990),
calcareous angiosperms
algae. The and peat,
mud
thin-bedded
contains
alternations
mangrove
plant
from
debris,
Sharp bases, l a c k of b i o t u r b a t i o n and lags at the b a s e s of individual interbeds evidence for 'event' deposition. These thin calcarenite-calcisiltite interbeds are analogous to interpreted distal tempestites d e s c r i b e d by B r e t t ( 1 9 8 3 ) , A i g n e r (1985) and Van Steenwinkel (1988). Their location and stratigraphic relationships with other facies types suggest that they formed in a very shallow setting, They are interpreted as the finer tails of onshore storms directed towards progressively shallower zones. Interbeds are also similar to hurricane-generated thinly bedded grainstones interbedded with organic-rich layers d e s c r i b e d by W a n l e s s , et al. (1988) f r o m C a i c o s p l a t f o r m , Mud layers represent hurricane-generated deposits~ they are s i m i l a r to m u d l a y e r s d e p o s i t e d by h u r r i c a n e s Donna and Betsy in F l o r i d a B a y ( P r a y , 1 9 6 6 ; ~ G a l l i , 1 9 9 0 9 F i g , 1 2 ) .
Thick-bedded
These
are
characterized
by
alternations
fining-upward
calcarenites
and
18
19
=last grains 2
..~,~.. ~ ~ . 4.4 ?~.~,..~£~:..~~/ ".-..3 ~-~..'xC.¢4
shells
•"[-... '.'.-'..
,J
m0-
matr,×
~ ~ s" ~ t- -_i n_t_ _ _ _ ~racla _
grains
Fig.13 - Fining-leftward grain size trends and compositional c h a n g e s of t h i n - b e d d e d alternations (Devonian platform, Carnic Alps) :brachiopod coquina (A)-->bioclast and intraclast grains (B)-->peloidal detritus with sparse bioclasts interbedded with mud laminae (C)-->mud laminae with sparse detritus interbedded with mudstones (D). T h e s e t r e n d s r e s u l t f r o m an o n s h o r e storm c u r r e n t ( t o w a r d s t h e left). T h e s k e t c h s h o w s a reconstruction of the paleoenvironment and the corresponding strat igraphic log.
calcisiltites, normally between 0.70 and 2m thick, interbedded with thin calcareous black mudstones containing scarse fossils. Thick beds are characterized b y the f o l l o w i n g m o d a l t r i p a r t i t e division I)
2) 3)
(Fig.14):
a b a s a l s h e l l lag; an i n t e r m e d i a t e mudstone-wackestone unit; and a parallel-laminated, well sorted upper grainstone
The upper mudstones
u n i t m a y be is planar
missing. The lower contact to e r o s i o n a l , as is s h o w n
unit.
above black by erosional
20
Fig. IA Thick-bedded alternation ('Calcari Grigi' Formation:section #25). This composite bed consists of a s c o u r e d c o n t a c t a b o v e l i m e m u d s t o n e s , a b a s a l lag z o n e c o m p o s e d of L i t h i o t i s shells and other bioclasts (B), a n i n t e r m e d i a t e wackestone unit consisting of a m a l g a m a t e d dm-thick layers, and an upper, hummocky-cross bedded unit with well sorted grainstones (A). t r a c e s a n d g u t t e r c a s t s . T h e l o w e r s h e l l l a g is p o o r l y sorted and contains lithoclasts eroded from the underlying muddy interface, in a d d i t i o n to v a r i o u s t y p e s of u n s o r t e d bioclasts. Imbrications w i t h i n m o r e d e n s e l y p a c k e d u n i t s c o n t r i b u t e to t h e development of a better sorting. The transition to the intermediate u n i t is s h a r p 9 the m u d d y u n i t a p p e a r s h o m o g e n e o u s 9 sedimentary structures were possibly present (such as faint
21
PROXIMAL
DISTAL
INTERMEDIATE RAMP
SHALLOW RAMP
f ~
i
lw
THIN-BEDDED ALTERNATIONS
THICK-BEDDED
m0m 0
E
l m
22
undulations: Fig. IA) but were destroyed by non figurative b i o t u r b a t i o n w h i c h is of v a r y i n g i n t e n s i t y . The t r a n s i t i o n to the o v e r l y i n g unit is diffuse. The thinner upper unit is characterized by well sorted, laminated grainstones with h u m m o c k y , to w a v e r i p p l e l a m i n a t i o n , or p a r a l l e l lamination. This s e q u e n c e is s i m i l a r to o t h e r nearshore storm deposits d e s c r i b e d by K r e i s a (1982), B r e n c h l e y , et a i . ( 1 9 7 9 ) and K u m a r and S a n d e r s (1976), among others.Storms and hurricanes are m a i n l y r e s p o n s i b l e for the f o r m a t i o n of these sequences. High flow r e g i m e s can be i n f e r r e d from a n u m b e r of features, such as spar c a l c i t e cement, s h e l t e r p o r o s i t y cement, a m a l g a m a t e d beds, well s o r t e d laminae and h u m m o c k y c r o s s - l a m i n a t i o n . B a s e d on d i f f e r e n t p r o p o r t i o n s of the i n t e r m e d i a t e and lower units, different types of s e q u e n c e s can be r e c o g n i z e d . They r e p r e s e n t a s p e c t r u m of l a n d w a r d to s e a w a r d t r a n s i t i o n s , w i t h a landward tendency towards a decrease in shell percentage. Fig.15, t a k e n from the 'Calcari Grigi' Formation, illustrates c h a n g e s in b e d d i n g styles g o i n g from the i n t e r m e d i a t e ramp to marsh environments typical of the landwardmost site of the s h a l l o w ramp. This t r e n d is a n a l o g o u s to that o c c u r r i n g in F l o r i d a Bay9 the Holocene platform, 36 km wide, does not e x c e e d 12 m in depth. The floor which gently dips towards south and southeast
Fig.15 - A~B~C: c h a n g e s in b e d d i n g s t y l e s of t h i c k - b e d d e d and t h i n - b e d d e d a l t e r n a t i o n s from a r e a s c l o s e r to the s h a l l o w ramp (right) to the i n t e r m e d i a t e r a m p (left) in the 'Calcari Grigi' Formation (sections #19 : A,B-->#6 : C), Closer to the intermediate ramp (C) there are no m u d s t o n e interbeds within thin-bedded alternations which are distinguished from the underlying thick-bedded alternations by their small thicknesses. In more distal positions lime mudstone become predominating and t h i c k - b e d d e d alternations, interbedded with b i o t u r b a t e d t h i c k e r sets of lime m u d s t o n e s , s t a n d out c l e a r l y on o u t c r o p faces. P r o x i m a l and d i s t a l t h i n - b e d d e d a l t e r n a t i o n s are o r g a n i z e d into t h i c k e n i n s - u p w a r d t r e n d s (cf. Fis.12). D~E~F: c l o s e r v i e w s of c o m p o s i t i o n a l changes from the s h a l l o w ramp (D,E) to the i n t e r m e d i a t e ramp (F), w h e r e s h a l l o w - w a t e r forms of L ~ t h i o t ~ s are r e p l a c e d by t h i n - s h e l l e d m o l l u s k s . The sketches below show proximal (section #6) and distal (section #19) alternations, and facies types ('Calcari Grigi'Formation).
23
NW
SE
"~. mud banks
patch r e ~ s ' ~ ~ J ~
Florida Ray
sand shoal
outer reeP--
5
4
3 2 km
t
1
20m
0
sJ.
LAND
SEA
~"
"-"[
r," "
Crane Key 2 Safety Valve 3
mangrove 1 i mud bank 1 flat Murray K e y 1
.'.•_.
-.- ~ iiii
"', ::... "-'.
",.C :.'£' ...
!?I
:'.'2, •
(~
I
angiosperm
grainstone
gastropod
packstone
peneroplid
wacke-mudstone
Halimeda lm
-~-~-.- pelecypod
'-'--]
fresh
water swamps
.... .... ... •• ,...
.i
.... ... . . :-" :
- :5
Molasses 4 reef
red mangrove peat .... MBH4•.I
coral
Fig,16 Bathymetric profile of the Florida platform (Enos,1977) and s t r a t i g r a p h i c logs (I: C r a i g h e a d , 1 9 6 9 ; 2 : E n o s and P e r k i n s , 1 9 7 9 ; 3 : A i g n e r , 1 9 8 5 ; A : E n o s , 1 9 7 7 ) .
(Atlantic Ocean) is complicated by topographic relieves c o n s t i t u t e d by c a r b o n a t e shoals, banks and p a t c h - r e e f s . S e d i m e n t a t i o n took place d u r i n g a phase of r e l a t i v e s e a - l e v e l rise (Scholl,et ai.,1969). The flooding of the platform i n i t i a t e d 8500 years ago. A s l o w i n g in the s e a - l e v e l rise took place 5500 years ago (30 c m / 1 0 0 0 years). The cross s e c t i o n of Fig.16, oriented normal to the strike of the p l a t f o r m , from open to s e m i r e s t r i c t e d e n v i r o n m e n t s
northwestsoutheast, reflects a transition (Enos,1977).
The cycles a c t u a l l y d e v e l o p i n g in F l o r i d a Bay are a s y m m e t r i c , transgressive sequences c a p p e d by i n t e r t i d a l and s u p r a t i d a l d e p o s i t s (Enos and P e r k i n s , 1 9 7 9 ) . T h e y c o n s i s t of: i) a basal c o a r s e m o l l u s k g r a i n s t o n e and p a c k s t o n e (shallow m a r i n e bay);
24
2) a mud b a n k c o n t a i n i n g few s c a t t e r e d o r g a n i s m s 9 and 3) an upper island d e p o s i t consisting of algal l a m i n a t e d s e d i m e n t s and c m - t h i c k s t o r m layers. These banks are intercepted by onshore directed storms ( h u r r i c a n e s and w i n t e r storms) and in s e c t i o n show a w i n d w a r d side and a l e e w a r d side dipping downcurrent (Fig.17). The w i n d w a r d side is steep and c h a r a c t e r i z e d by a nip and a b e a c h ridge c o v e r e d by shells, i n t r a c l a s t s and v e g e t a l debris. T h e s e keys c o n s i s t of l o w - i n c l i n e d sets c o m p o s e d of s t a c k e d graded layers each given by a basal lag (packstones, w a c k e s t o n e s w i t h m o l l u s k s and f o r a m i n i f e r s ) o v e r l a i n by m o l l u s k foraminifer mudstones. The leeward side is covered by an i n t e r t i d a l - s u p r a t i d a l flat w h i c h is the site of d e p o s i t i o n of fine-grained peloidal and bioclast detritus. A similar s t r u c t u r e is shown in F i g . 1 7 (below).
In both m o d e r n and a n c i e n t s i t u a t i o n s l a n d w a r d c h a n g e s in c y c l e thicknesses, composition and grain-size trends reflect the a c t i o n of o n s h o r e - d i r e c t e d s t o r m flows d e c r e a s i n g in i n t e n s i t y towards s h a l l o w e r areas. T h e s e trends are a n a l o g o u s to those d e s c r i b e d by B o u r r o u l h - l e Jan, et a i . ( 1 9 8 5 ) and to the case h i s t o r y r e p o r t e d by A i g n e r (1985) w h o d e t a i l e d the s t r u c t u r e of spillover lobes consisting of s u c c e s s i v e inputs of onshore d i r e c t e d sediments.
Proximality-distality
trends
The d e c r e a s e of s t o r m e f f e c t s t o w a r d s deeper, offshore water was demonstrated by Aigner (1985) in modern and ancient s i t u a t i o n s d o m i n a t e d by a p r o g r a d a t i o n a l , r e g r e s s i v e trend. It shows that: "The p e r c e n t a g e of sand, s t o r m layer t h i c k n e s s and grain size, as well as the d e g r e e of a m a l g a m a t i o n show a continuously decreasing trend from s h a l l o w to d e e p e r water, while bioturbation generally decreases" (Aigner,1982,1985). This c o n c e p t is not a p p l i c a b l e in the c a s e s e x a m i n e d f u r t h e r b e l o w w h e r e the v a r i o u s p a r a m e t e r s d i s p l a y o p p o s i t e trends. A distal p o s i t i o n for t h i n - b e d d e d the f o l l o w i n g features:
alternations
is
indicated
by
25 Windward
C
o
Leeward
n
g
l
o
~
~
,
.
~
,
k Wacl<estone
\ Basal sheet lag
lOOm
0m
eagrass
20 m
,,,,,,,,,
INI "I
II
~i
I
II
II
I IIIIIIIIII
I
IIIIII
III
II
4
Fig.17 - Above: cross s e c t i o n of a m u d m o u n d from F l o r i d a Bay (Upper C r o s s Bank: Bosence,1988). Below: front view of an a n c i e n t a n a l o g u e from the 'Calcari GriEi' Formation (section #6) formed by a stacked lenses of skeletal wackestones i n t e r b e d d e d with thin lime m u d s t o n e s
I. lack of basal units; 2. flat bases; 3. s c a r c i t y of a m a l g a m a t i o n s ; 4. small t h i c k n e s s e s and fine g r a i n e d A proximal position by the following:
for
the
size
thick-bedded
of material. alternations
is
shown
I. o c c u r r e n c e of complete storm sequences (tripartite division); 2. e r o s i o n a l surfaces; 3. thick basal u n i t s with lags and a g r a i n s t o n e texture; ~. amalEamations of sequences; s t a c k i n E of basal units. S t o r m d e p o s i t s here are a main result of an o n s h o r e s e d i m e n t t r a n s p o r t in a shallow, n e a r s h o r e water, as a direct e f f e c t of w i n d - d r i f t currents. There are two p o s s i b l e c a u s e s for this: I) the deep ramp is not deep e n o u E h to f a v o u r o f f s h o r e s e d i m e n t
26
t r a n s p o r t d u e to g r a d i e n t c u r r e n t s a n d 2) the o v e r a l l t r e n d is t r a n s g r e s s i v e .
bottom
return
flows;
and
'Proximal' and 'distal' here are used in a relative meaning:they r e f e r to the n e a r n e s s to t h e s o u r c e a r e a of the material which is transported landward, Variations in the a m o u n t s of b i o c l a s t s v e r s u s m u d c o n t e n t , and thickness of b e d s interbedded with lagoonal micrites reflect proximality distality trends. P r o x i m a l b e d s o c c u r in m o r e d e e p e r l a g o o n a l a r e a s w h i c h a r e the s o u r c e of t h e b i o c l a s t m a t e r i a l transported landward; they are bioclast-rich, thick-bedded grainstones and packstones located s e a w a r d , c l o s e to t h e i n t e r m e d i a t e ramp. Distal beds are mud-supported thin beds containing scattered b i o c l a s t s . T h e f r e q u e n c y of t e m p e s t i t e s decreases landward, as is s h o w n in a ' i s o t e m p e s t i t e ' map obtained from the 'Calcari Grigi' Formation' (Galli,1990:Fig.2). In t h e s e 180 ° out
situations of phase
the proximality - distality criterions are w i t h r e s p e c t to t h e proximality-distality
Proximal
Distal
~ _ . . . . 9 ~ Y ~.'~~'.;-" ~ "
grainstone
-
lag Intraclasts Amalgamations
r-
Grain size
C
Bed thickness
-
-] Peat Bioclasts
Thick- bedded alternations Fig.18 - Proximality-distality shallow ramp.
Thin-bedded alternations trends
in
the
intermediate
-
27
trends d e s c r i b e d by A i g n e r (1985), and others, mainly b e c a u s e the s i t u a t i o n s e n c o u n t e r e d are c h a r a c t e r i z e d by t r a n s g r e s s i v e , r e t r o g r a d a t i o n a l trends, w h e r e a s the t r e n d s d e s c r i b e d by A i g n e r (1985) a p p l y to p r o g r a d a t i o n a l , r e g r e s s i v e systems. In o r d e r to avoid m i s l e a d i n g i n t e r p r e t a t i o n s , t h e use of the p r o x i m a l i t y d i s t a l i t y c r i t e r i o n must be c o n f r o n t e d w i t h other i n d e p e n d e n t p e t r o g r a p h i c and g e o l o g i c a l data.
Intermediate
ramp
The i n t e r m e d i a t e r a m p is located b e t w e e n the lower and the upper limit of wave action. This is a high-energy area c h a r a c t e r i z e d by a rough t o p o g r a p h y and i n t e r c e p t e d by waves and c u r r e n t s which are m a i n l y r e s p o n s i b l e for a c o m p l e x and l e n t i c u l a r geometry. The large size of bedforms, several m in wavelength, points to a d e p o s i t i o n as sand blankets. C o m m o n sedimentary structures are e r o s i o n a l features, channels and m e g a r i p p l e s m i g r a t i n g u n d e r e p i s o d i c h i g h - e n e r g y conditions. The distribution of sediment in close proximity to the i n t e r m e d i a t e ramp is c o n t r o l l e d by the u n d e r l y i n g t o p o g r a p h i c s u r f a c e s . T h e r e are d i f f e r e n t belts: n a m e l y I) skeletal sands9 and 2) c o l d - r i c h sands. T h e y form r e s p e c t i v e l y I) h i g h - e n e r g y skeletal beaches; and 2) sandwaves. T h e s e two belts may o c c u r
Beach sands Sandwaves Lagponal,calcareous
muds
Lagoon '! siliciclastic mu6s "~
/
,~ ~.... • •':.~.~.. -...~
Oolite shoal Offshore sediments FiS.19 - E x a m p l e of facies a s s e m b l a g e s and facies d i s t r i b u t i o n in the intermediate ramp (Dinantian platform: from Van Steenwinkel,1988:Fig.6.13).
28
in the same d e p o s i t i o n a l system; for e x a m p l e (1988) d e s c r i b e d a s i t u a t i o n w h e r e b e a c h e s are w i t h r e s p e c t to s a n d w a v e s (Fig.19).
Van Steenwinkel located landward
In the F l o r i d a p l a t f o r m the i n t e r m e d i a t e ramp is a c o m p l e x a r e a o c c u p i e d by a d i s c o n t i n u o u s alignment of n e a r s h o r e banks and small tidal deltas l o c a t e d c l o s e to the tidal inlets and to the t r a n s i t i o n from F l o r i d a Bay and the Gulf of M e x i c o (Fig.20). The d y n a m i c s of banks d e s c r i b e d by A i g n e r (1985) takes p l a c e by i n c r e m e n t s of s e d i m e n t v o l u m e s t r a n s p o r t e d by h u r r i c a n e s . The 'event' a c c r e t i o n d y n a m i c s leads to a l a n d w a r d t r a n s i t i o n from beaches and shell islands,to skeletal banks and mud banks (Aigner,1985: F i g . 1 8 ) . T h e w i n d w a r d side of lobes and s u b a q u e o u s dunes is e n r i c h e d in shells; b i o o l a s t b e a c h e s may form on t h e i r top; the l e e w a r d side c o n v e r s e l y is m u d d i e r in c o m p o s i t i o n . O n s h o r e s t o r m floods also p r o d u c e s p i l l o v e r lobes (Bali,1967), A s i t u a t i o n s i m i l a r to that d e s c r i b e d by A i g n e r (1985) from Florida Bay occurs in the Devonian platform (Carnic Alps, Italy) w h e r e s h o r e f a c e bars, s t o r m bars and b e a c h bars r e s u l t e d
SKELEB TA~L ~~'~a~~ ~ ~ 0
1
F i g , 2 0 - S k e l e t a l banks of F l o r i d a Bay, ramp (from G i n s b u r g , R . N . , w i t h p e r m i s s i o n ) .
in
2Krn
.~,"
LAND
the
intermediate
29
from the p i l i n g up of shells in a s t r a n d l i n e e n v i r o n m e n t . The c o m p o s i t i o n of the b e a c h was built up by s t e p w i s e a s s e m b l a g e s of sand b o d i e s of d i f f e r e n t p r o v e n a n c e , such as open lagoons and reef flats (Galii,1986; Fig.21). Open l a g o o n s are a s s o c i a t e d w i t h s a n d w a v e s and bars located in the i n t e r m e d i a t e ramp. T h e y are c o n s t i t u t e d by skeletal sands and p o p u l a t e d by thin- and t h i c k - s h e l l e d mollusks, o t h e r than g a s t r o p o d s , c r i n o i d s , f o r a m i n i f e r s and corals. The open lagoon facies are g e n e r a l l y transitional to the shallow ramp and located at both l a n d w a r d and s e a w a r d sides of s a n d w a v e bodies. More sheltered areas are b i o t u r b a t e d . Open lagoons located s e a w a r d are the sites of f o r m a t i o n of m - t h i c k bars, whereas those l o c a t e d t o w a r d s the s h a l l o w ramp, on the other side of the barrier, more c o m m o n l y d i s p l a y f i n i n g - u p w a r d s e q u e n c e s and b i o c l a s t lags t r a n s i t i o n a l to those of the s h a l l o w ramp. In the B a h a m a s s k e l e t a l sands are less than 2 km wide; they are w i d e r (iO km) a l o n g the n o r t h e r n m a r g i n and e x t e n d from the u p p e r s l o p e a c r o s s the o u t e r m a r g i n to w a t e r depths of about I0 m. T h e y t e r m i n a t e a b r u p t l y a g a i n s t cold sands and islands. In the J u r a s s i c 'Calcari Grigi' F o r m a t i o n the main l i t h o f a c i e s of the i n t e r m e d i a t e ramp are c o n s t i t u t e d by o o l i t e g r a i n s t o n e s and packstones, and skeletal wackestones, with subordinate amounts of nodular wackestones and mudstones. Here the intermediate ramp is d o m i n a t e d by o o l i t e sandwaves, such as that e x e m p l i f i e d in Fig,22: the sect.ion may be d i v i d e d into a lower and an u p p e r member. The basal m e m b e r is a p o o r l y s o r t e d l i t h o f a c i e s w i t h s u r f i c i a l o o l i t e s and lumps and an a d m i x t u r e of s k e l e t a l g r a i n s w h i c h c o n s t i t u t e the nuclei of the colds, intraclasts, p e l o i d s and c o a t e d grains. The u p p e r l i t h o f a c i e s is a well s o r t e d g r a i n s t o n e with t a n g e n t i a l colds, S e d i m e n t a r y structures are m a s s i v e bedding at the base, hummocky-cross b e d d i n g in the m i d d l e and wide, c r o s s - b e d d e d shallow channel fills r e m i n i s c e n t of the s w a l e y cross s t r a t i f i c a t i o n (Leckie and W a l k e r , 1 9 8 2 ) at the top. I n d i v i d u a l f i n i n g - u p w a r d s e q u e n c e s w i t h i n h u m m o c k y c r o s s - b e d d e d sets c o n t a i n internal s e q u e n c e s of structures comparable to the 'b-c-d' Bouma sequence: planar lamination (Walker,at ai.,1983) v e r t i c a l l y g r a d i n g to t r o u g h cross lamination with normal graded sets (of. Dott and Bourgeois,1982). The basal m a s s i v e bedded oolite grainstones and p a c k s t o n e s are i n t e r p r e t e d as shoals and s a n d w a v e s s i t u a t e d in a s t o r m - d o m i n a t e d area, at a s h a l l o w w a t e r depth. Ooid sand m i g r a t i o n took p l a c e u n d e r the influence of storms, Van Steenwinkel (1988) interpreted similar 'oolitic lump facies' f r o m a C a r b o n i f e r o u s ramp in B e l g i u m as a t r a n s g r e s s i v e
30
-,(--- Deep
Intermediate
ramp
Submerged sketetal bars
hwI
Emergent bars
ramp
Ridge
- - Sha{|ow
ramp
Pond
....................................................................................................................................................................................................................
".%
-% Well sorted grainstone
Mudstone 2Mudstone
3 , b ' 6 :'6-
1
,,~,'.-q~
Brachiopod coquina 2
8rachiopod coquina
Skeletal wackestone m 0
,~'~*.~,.o~: •
Poorly sorted skeletal ,int raclast packstone
,, ',¢e~o~,-
Mudstone 1
mO
Fig.21 B a t h y m e t tic profile intermediate ramp occurring platform, Italy ( G a l i i , 1 9 8 6 ) .
Skeleta! wackestone
and facies types in the Devonian
Mudstone
of the carbonate
lag d e p o s i t f o r m e d d u r i n g an i n c i p i e n t d r o w n i n g . B r i e f e p i s o d e s of bottom agitation alternated with longer, quiet periods c h a r a c t e r i z e d by a s l o w s e d i m e n t a t i o n rate, She c o m p a r e d s u c h grapestone lumps w i t h the g r a p e s t o n e lumps o c c u r r i n g at L i l y B a n k (little B a h a m a Bank) and at Cat Cay P l a t f o r m (Great B a h a m a Bank) where oolitically coated sands migrated landward in r e s p o n s e to an i n c r e a s e in the s e a - l e v e l rise and s u b s e q u e n t l y b e c a m e i n a c t i v e and the site of s e a - g r a s s g r o w t h and g r a p e s t o n e f o r m a t i o n . M i g r a t i o n takes p l a c e m a i n l y d u r i n g large s t o r m s and hurricanes. Oolitic sand shoals are common along the edges of s e v e r a l B a h a m a areas (Cat I s l a n d p l a t f o r m , J o u l t e r s Cay, B e r r y Islands, and so on). T h e y o c c u r as a c t i v e c o l d s h o a l s m a r g i n a l to the o p e n sea, and as s t a b i l i z e d b l a n k e t s h e e t s of o o l i t e sand flats w h i c h are g r a d a t i o n a l with other platform sediments (Multer, 1977). S k e l e t a l a d m i x t u r e s are g r e a t e s t in d e e p e r w a t e r areas. M o d e r n o o l i t e s a n d w a v e s m i g r a t e l a n d w a r d in r e s p o n s e to a s l o w s e a - l e v e l rise. S u b a q u e o u s d u n e s are e l o n g a t e d p a r a l l e l to the
31 4'.'o'~ ~ ' 6 ~D~
o oo
~ o
• ~ G 0 © u ' .© ©. / o
• o...
o '0'
J!:."?..o..o ; o." "oo
e e
.
.
.
".."
.. .o.
.Ovo •
'.
''
'o
o
~"
~ : -'.) j~;o.o°-: "
Oolitic grainstone
Mudstone Oncolite packstone
Oolitic lump facies
m
Fig.22 - Oolitic Formation,Venetian
v
sequence occurring Alps ( s e c t i o n #22).
in
the
'Calcari
direction of p r e v a i l i n g currents. Active shoals i n a c t i v e m i x e d o o l i t e facies and b e c o m e s t a b i l i z e d growth.
Grigi'
overlie the by s e a - g r a s s
Deep ramp The deep ramp is a deep lagoon; the lagoonal floor, located 5 to 15 as m u c h as 20 m b e l o w wave base, is t y p i c a l l y p o p u l a t e d by o l i g o t y p i c , h i g h - d e n s i t y and l o w - d i v e r s i t y mollusks. B i v a l v e s form t i g h t l y packed accumulations embedded in a w a c k e s t o n e
32
{0
o
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r.) ..o ~1
~
,~
~
o-- "o
.~, 0
"0
•,~, "~ ',4 ::~: (~ o P-, ..c: .~ •,'.I
4.~
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r"
,~ ~
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0
33
matrix (Fig.23A,B,C). Mudstones alternate with fossiliferous wackestones. These fossils and o r g a n i s m s are also found in other sectors of the ramp, but they do not form big v o l u m e s of sediment as in the deep ramp; here they b e c o m e a r r a n g e d as c a t c h - u p reefs, b i o s t r o m e s and banks r e a c h i n g 3 as much as 7 9 m in t h i c k n e s s (Fig.23 D,E). Walkden and Gutteridge (198~,1987) and Gutteridge (1987) described different styles of mud mounds from the late B r i g a n t i a n Eyam L i m e s t o n e in the D e r b y s h i r e c a r b o n a t e p l a t f o r m (Fig.2~). The mounding styles (tabular dome-shaped l a t e r a l l y and v e r t i c a l l y a c c r e t e d mounds) were d e t e r m i n e d by the w a t e r depth of d e p o s i t i o n . Laterally accreted mounds are u s u a l l y found in the s h a l l o w ramp, w h e r e a s v e r t i c a l l y a c o r e t e d mounds occur in the deep ramp. In c o n t r a s t to the laterally a c c r e t e d mounds, flank facies of v e r t i c a l l y a c c r e t e d m o u n d s are poorly developed;the s e d i m e n t s u r r o u n d i n g bases of v e r t i c a l l y a c c r e t e d mounds d i s p l a y some cross b e d d i n g e v i d e n c i n g for an initial shallow water of deposition. The core, composed of peloidal mud, does not show any z o n a t i o n w h i c h rules out a c h a n g e in the w a t e r d e p t h t h r o u g h o u t d e p o s i t i o n , except at the very top, where v a d o s e d i a g e n e t i c features point to p e r i o d s of subaerial exposure. A n a l o g o u s bank styles o c c u r in the J u r a s s i c s h a l l o w ramp in the V e n e t i a n Alps (Fig.2~). Small mounds, 1 m thick to 5 m wide, together with multiply truncation scours infilled with L i t h i o t i s shells, occur in the s h a l l o w ramp. Banks o c c u r r i n g as s t e e p l y inclined, s i g m o i d a l to u n d u l a t i n g beds, may c o r r e s p o n d to the flank facies of l a t e r a l l y accreted Lithiotis banks d e v e l o p e d close to the i n t e r m e d i a t e ramp. The g r o w t h of t h i c k L i t h i o t i s banks o c c u r r e d in deeper, subsiding areas of the lagoon, as is s u g g e s t e d by the lack of m e c h a n i c a l structures within banks, a h i g h faunal density, uniformity, large, 'in situ' shell sizes, and o c c u r r e n c e of cross b e d d i n g only at the base and top of the banks (Fig.23). The L i t h i o t i s banks were i n t e r p r e t e d by B o s e l l i n i (1972) to h a v e been d e p o s i t e d w i t h i n a system of m i g r a t i n g tidal p o i n t bars (Fig.25). Accord.ing to that i n t e r p r e t a t i o n , B-type mounds formed at the b o t t o m of point bars, C and D - t y p e m o u n d s w e r e i n t e r p r e t e d as point bars or as m e c h a n i c a l d e p o s i t s formed in the bends of m e a n d e r i n g c h a n n e l s or b e t w e e n larger c h a n n e l s . T h e model put forth by B o s e l l i n i (1972) is w r o n g (of. W r i g h t , 1 9 8 A ; Galli,1990).Nevertheless, his p r o f i l e of point bars may be taken as the p r o f i l e of an i n t r a s h e l f ramp: hence, B-type mounds formed in the deep ramp; C,D, E types d e v e l o p e d in the i n t e r m e d i a t e ramp, w h e r e a s c h a n n e l s and scours were p r o d u c e d in the s h a l l o w ramp.
34
TABULAR
MOUND
5m
•
Sm
\ \
LATERALLY ACCRETED
, '
VERTICALLY~ ACCRETED "
[~.m 100 m
'
~
r ~
~Om
1
SHALLOW RAMP
lateral accretion
INTERMEDIATE RAMP
vertical accretion DEEP RAMP
Fig.24 EnEland
Top : mud mounds in t h e A s b i a n - B r i g a n t i a n (Walkden and GutteridEe,1987). Bottom: Lithiotis banks.
platform, types of
35
A
B
lO -20. m
C /II 1 /
:
x
//// I
,,
I
,, / ,, ,,
D
I x_ I
20-30 m - -
\
X,
~
\,,,,,
,\
4-5~ I ,:1",l' .... "",",w,' " ',' '",~",'q~P
I
\\
20-40 rn .............................. D-.E.. . ..
A
. . . . . . . . . . . . . . .
y~ , . . . .
X \
X
/'""',
Y ~ D-E . .
C
-
-B ~
D-E Y
Fig.25 - Paleoenvironmental model of Bosellini (1972), He mistakenly interpreted Lithiotis b a n k s to h a v e f o r m e d w i t h i n a s y s t e m of m i g r a t i n g t i d a l p o i n t bars. H o w e v e r , t i d a l facies are absent in the s t u d y a r e a a n d t h e r e is n o t m u c h e v i d e n c e for migration of t i d a l c h a n n e l s in m o d e r n c a r b o n a t e environments. T h e m o d e l is s t i l l a p p l i c a b l e if the p r o f i l e of the 'point bar' is c o n s i d e r e d to be the p r o f i l e of an i n t r a s h e l f ramp.
The Jurassic intrashelf ramp occurring in the 'Calcari Grigi' F o r m a t i o n w a s s u b j e c t e d to a s t r o n g e p i s o d i c hydraulic action (Galli,1990). Channel morphologies reflect the former depth (Fig.26).Undulating bedforms and flattened,wide channel o u t l i n e s o c c u r in t h e d e e p ramp. D e e p l y i n c i s e d , t i g h t e r channel outlines cutting shoals are commonly found in t h e shallow ramp. The distribution of the t y p e s of c h a n n e l o u t l i n e s o c c u r r i n g in the study area of the 'Calcari Grigi' Formation (Fig.26) follows the variations in bathymetry of the ramp. Within shallowing-upward sequences, wide channels o c c u r at the b a s e (Fig.26:A), whereas semicircular s c o u r s a r e f o u n d t o w a r d s ithe top (Fig.26:B), T h i s is a l s o a p p a r e n t f r o m a n e x a m i n a t i o n of v e r t i c a l s e q u e n c e s of c h a n n e l m o r p h o l o s i e s (Fig.27).
36
R:H • 2 ~
"
f
SHALLOWRAMP
R:H= 3 ~
R:H=4
B R:H = n
DEEP RAMP
B
,..
//
~
. . • .....
.........
•
0
. A
..:i: {!i~i" B
B
jA
5KM.
SHALLOW
DEEP
RAMP
RAMP.
N
37
NW
SE
~am=ms~ssE:mmcml
0
lOre.
F i g . 2 7 - U p w a r d c h a n g e s in c h a n n e l m o r p h o l o g i e s from u n d u l a t e d to e l l i p t i c a l reflect a s h a l l o w i n g upward trend,confirmed by vertical changes in l i t h o f a c i e s ('Calcari Grigi' Formation: s e c t i o n #27). Banks o c c u r r i n g in the deep ramp are a n a l o g o u s to a n u m b e r of 'catch-up' reefs d e s c r i b e d in the l i t e r a t u r e (Fig.28), as is i n d i c a t e d by the v e r t i c a l d e c r e a s e in size of fossils and an increase in the p r o p o r t i o n of lithoclasts. The u p p e r m o s t p a r t s contain fossils more indicative for a shallower setting, scours, channels and w a v e - g e n e r a t e d structures reflecting a d e p o s i t i o n a b o v e w a v e b a s e . T r e n d s d i s p l a y e d by these s e q u e n c e s record transitions from deep, quiet water stages, to a shallower,agitated w a t e r stage that c o r r e s p o n d s to v a r i a t i o n s in the steepness of the curve of change in sea-level. Initially, the space a v a i l a b l e for s e d i m e n t a t i o n was i n f i l l e d by a mechanism of vertical sediment growth (vertical aggradation); later, by lateral m i g r a t i o n ( p r o g r a d a t i o n ) , once the s e d i m e n t a r y i n t e r f a c e c a u g h t - u p with the sea-level. In the F l o r i d a p l a t f o r m the deep ramp, described by P e r k i n s (1977), consists of a landward area characterized by a semirestricted water circulation, and a s e a w a r d area, where c i r c u l a t i o n is more limited. The c o m p o s i t i o n and g r a i n size of the s e a w a r d m o s t side of the deep ramp r e f l e c t s the p r o x i m i t y to the reefs. Sediments become finer and of more varied composition towards the i n t e r m e d i a t e ramp w h e r e the i n c l i n e d
F i g . 2 6 - A : V a r i a t i o n s of e l l i p t i c i t y of c h a n n e l o u t l i n e s as a f u n c t i o n of the h e i g h t : a m p l i t u d e (H:R) ratios with depth. B: schematic representation of the outlines of channels and scours, in the s t u d y area of the 'Calcari Grigi' F o r m a t i o n , r e l a t e d to the r e c o n s t r u c t e d bathymetry, Circular scours are m a i n l y c o n f i n e d to the s h a l l o w ramp; flattened channels occur p r e f e r e n t i a l l y in the o t h e r sectors.
38 BRITOMART REEF 041
.r.~.~ •
48-
..,..
10~
•3 ,,2
31 122l
E 16r~ 2o-
2 0 ~ •1
2428-
32 O5-
ALACRAN REEF
10,~~--~
t0-
Years B.P. (x 1ooo) 31
21
E
;15-
2
t•
25
~:~ :l~ Cor~ rubble 0
300
2 3 4 5 6 7 8910 Years B,P. {x ~ooa}-
GALETA POINT REEF 0mO r-~o~:~.--..~..t ==6 •
.
=15
5_
.
4l 3•
1 3
tO
I5
~
12 It
a
2• 15-
20, 0
Years B.P. (xloOO)
39 surface is covered by patch reefs and poorly sorted, bioturbated wackestone banks. These form a compound, elongated, disoontinuous w e d g e p a r a l l e l to the e d g e of t h e p l a t f o r m . T h e y are also asymmetric, w i t h the s t e e p e s t side located landward. Inner structures c l o s e to the i n t e r m e d i a t e ramp consist of acoretionary sets inclined at a low a n g l e (I ° - 5 ° ) a n d s t r i k i n g p a r a l l e l to the e d g e of the p l a t f o r m (Enos,1977).They m a y be a m o d e r n a n a l o g u e of a n c i e n t , inclined beds connecting the intermediate and deep ramp (Fig.29).
PUEBLO NUEVO REEF WABASH
REEF
40
i'i~!. .'.-.",|. ;,.e ..
Massive corals and corallinae algae
....~;-<j
Ma s save hemispherical Favositldae (packstone} 3O
),.".: ii '
"
. ~ 3
Massive coralsand robust branching coral~
~/.:1 Fragile tabulate,mainly Syringopora ( wacke tone)
20 - " . ' ~
10 . . . . .
? ? mO
"-i :.".".!
0
Fragile branching corals
Cora nae algae
?):;:~::]
Fig.28 - Examples of 'catch-up' reefs. Britomart Reef,Great B a r r i e r R e e f : J o h n s o n , et a l . ( 1 9 8 A ) 9 G a l e t a P o i n t Reef, P a n a m a : MacIntyre and Glynn (1976);Alacran Reef, Isla Perez,Mexioo: MacIntyre, et ai.(1977); Wabash Reef,Devonian, Indiana: Lowenstam (1950); Pueblo Nuevo Reef,Oligocene, Mexico: Frost (1977). T h e s t r a t i g r a p h y a n d e v o l u t i o n of the H o l o c e n e e x a m p l e s shows a strict dependance u p o n the r e l a t i v e s e a - l e v e l c u r v e . A s i m i l a r o o n t r o l is i n f e r r e d for the o l d e r e x a m p l e s .
40
--
W
-
OJ
O~
cO F-, -~ 0".
,.~ bO
,--I .--4
v
w l-.i
o
M
~.~
~g'~ .,..~
~
~ >. 5 • -
.0,4
>, •
0
"o
"0
.1~ 0
.1=
.,-I
-
/ ~
~ '"~
.,~, .i,~
0
~ 0 cO r-~ W ,-,4
cO • ~ .~ "0
.
~0
0.I
~,,~
I~
g
~.~
"
×
~
JT"" • I
.,-i
-
"'0
0
"13
Geometries
of
intrashelf
ramps
I n t r a s h e l f ramp g e o m e t r i e s fall into two f u n d a m e n t a l c a t e g o r i e s (Fig.30) which correspond respectively to divergent and c o n v e r g e n t p a t t e r n s d e s c r i b e d further below: I) onlap ramps; 2) o f f l a p ramps The r e c o g n i t i o n of these two a r c h i t e c t u r e s was a t t e n d a n t on the application of 'event' correlation between stratigraphic sections and logs (Ager,1981; Dixon,et aI.,1981~ Matthews, 198~;Aigner,1985).The use of p h y s i c a l bodies and s t r a t i g r a p h i c horizons as chronostratigraphic tools within shallow-water carbonate platforms offers a better strati~raphic resolution than b i o s t r a t i g r a p h i c criteria because the time interval of f o r m a t i o n of p h y s i c a l m a r k e r s such as storm d e p o s i t s is s h o r t e r than the rate of e v o l u t i o n of s h a l l o w - w a t e r o r g a n i s m s living in a platform (Sommerville,1979).
ONLAPRAMPI OFFLAPRAMPE Fig.30 - Geometries of intrashelf ramps.The onlap ramp is g e n e r a l l y d e v e l o p e d b e l o w the o f f l a p ramp and is b o u n d e d by flooding surfaces, therefore corresponding to a genetic stratigraphio sequence (Galloway,1989).
0nlap
~eometry
The o n l a p geometry is c o n s t i t u t e d by a 20 + 60 m thick, divergent prism. Isochronous lines c o n v e r g e towards a hinge zone. As a result, facies and cycles thin towards the hinge. Facies associations in the hinge zone c o m p r i s e shallow ramp facies a s s o c i a t i o n s (thin- and t h i c k - b e d d e d a l t e r n a t i o n s ) . The i n f l e c t i o n in the topographic profile, w h i c h is the site of
42
FLEXURE
TROUGH
HINGE
DEEP RAMP
INTERMEDIATE RAMP
SHALLOW
RAMP
Banks, biostromes
Sandwaves, beaches
T hick-, thin ' bedded alternations Y "-~ :-~'.5, "C
" f'.''~
~..-~ ":--L^ ".<.~ ~ . . " ". "..: " C'.' •~ .~. b Deep
Sandwave ,beach
lagoon -'A'~ ~ ? ."
,.~
.
.
~..~
Biostrome >-
.
. . ,
Thin-bedded alternations
°Tp l
Z >-~ . > -
Thick- bedded alternation
HINGE SEQUENCE
TROUGH SEOUENCE
Fig.31 - Sectors,facies onlap ramp.
associations
and
modal
sequences
of
an
development
of the intermediate facies association, is the the trough zone, the t h i c k e s t s e c t o r of t h e o n l a p lithofacies a r e t y p i c a l of t h e d e e p r a m p a s s o c i a t i o n .
flexure. In ramp,
Sedimentation t a k e s p l a c e m a i n l y by a g g r a d a t i o n . In an o n l a p geometry facies are developed over progressively wider areas (Krumbein and Sloss,1963), f r o m b o t t o m to t o p of s t r a t i g r a p h i c sections,and result from a transgressive trend. A consequence of the environmental shift towards the hinge is t h e f o r m a t i o n of deepening-upward sequences and megasequences (trends produced by s t a c k i n g of i n d i v i d u a l sequenaes).The deepeningu p w a r d t r e n d m a y n o t g o to c o m p l e t i o n as a r e s u l t of s h o r t - t e r m sea-level falls, which determine a shallowing-upward t r e n d at
43
the very top. U p w a r d faunal v a r i a t i o n s r e f l e c t r e s t r i c t e d to more o p e n lagoonal e n v i r o n m e n t s .
changes
from
An onlap g e o m e t r y c o r r e s p o n d s to IV and V order e u s t a t i c cycles (I x 10 years). It may r e p r e s e n t a depositional sequence (Vail, et a i . , 1 9 7 7 ) b e c a u s e is a ' s t r a t i g r a p h i c set c o m p o s e d of a c o n c o r d a n t s u c c e s s i o n of g e n e t i c a l l y linked s y s t e m tracts, d e l i m i t e d at the base and top by u n c o n f o r m i t i e s (or r e l a t e d paraconformities)'.Sequence boundaries are constituted by a t r a n s s r e s s i v e s u r f a c e at the base and the m a x i m u m flooding surface at the top: in which case the onlap ramp g e o m e t r y c o r r e s p o n d s to a 'genetic s t r a t i g r a p h i c sequence' d e f i n e d as 'a packase of s e d i m e n t s r e c o r d i n g a s i g n i f i c a n t e p i s o d e of basin margin o u t b u i l d i n g and basin f i l l i n g b o u n d e d by p e r i o d s of w i d e s p r e a d b a s i n - m a r g i n flooding' ( G a l l o w a y , 1 9 8 9 ) . A complete hinge sequence is a d e e p e n i n g - u p w a r d sequence c o n s i s t i n g f r o m b o t t o m to top of the f o l l o w i n g (Fig.31): I) 2) 3) ~)
t h i c k - b e d d e d a l t e r n a t i o n s ( s h a l l o w - r a m p ; h i n g e zone); t h i n - b e d d e d a l t e r n a t i o n s (shallow-ramp9 h i n g e zone); s a n d w a v e s and bars; beaches ( i n t e r m e d i a t e ramp; flexure)~ banks and b i o s t r o m e s (deep ramp).
The c v e r p o s i t i o n of t h i n - b e d d e d a l t e r n a t i o n s (located landward) above t h i c k - b e d d e d a l t e r n a t i o n s (located seaward) s u g g e s t s that the deepening recorded by the hinge sequence is not a c o n t i n u o u s p r o c e s s . The h i n g e s e q u e n c e t h e r e f o r e records two ma~or deepening episodes': the first at the base of the sequence, f o l l o w i n g a sea-level fall~ and the second at the base of s a n d w a v e s and beaches~ or b i o s t r o m e s and banks of the intermediate or deep ramp. In the intervening period,a depositional r e g r e s s i o n may take place: in fact, carbonate p r o g r a d a t i o n is a rapid p r o c e s s (Kendall and S k i p w i t h , 1 9 6 8 ) . In Florida p l a t f o r m the m a i n l a n d s h o r e l i n e is e x p e c t e d to m i g r a t e seaward d u r i n g the actual d e c r e a s e in the r e l a t i v e s e a - l e v e l rise (Scholl, et ai.,1969). A c c o r d i n g to Enos & P e r k i n s (1979) mangrove f l a t s will e v e n t u a l l y o v e r l i e the F l o r i d a Bay mud banks; the s i t u a t i o n at that time will r e s e m b l e that o c c u r r i n g in the l o w e r part of the d e e p e n i n g - u p w a r d s e q u e n c e (cf. F i g . 1 6 with Fig.31) where thin-bedded alternations overlie thick-bedded alternations. A well k n o w n d e e p e n i n g - u p w a r d s e q u e n c e s i m i l a r to the h i n g e sequence is the '~Lofer cyclothem" described by Fischer (196b).Another example of hinge sequence comes from the Devonian c a r b o n a t e p l a t f o r m o c c u r r i n g in the C a r n i c Alps, Italy (Fig,32). T h e c o m p l e t e n e s s of the r e c o r d of the h i n g e s e q u e n c e
44
, 000
0
m8-
Sprotbrough M.
Hampole discontinuity
~9
~ '
_ E'DL
_
~,,,M e ~-
_
~
-
Hampolediscontinuity" ~ ~
MARL S
'
~
L
~
zl "
~
CADEBY Km
o o
~
ON FORMATION
>' o r 0 7
~
_
%
] !Z
FORMATION A
T
E
~
!
-" I;Z
45
~
DISCONFORMITY ,
.| J
_
_ ~
_
(~(,~ ~,~(~
-CO :~
IL..
; ' . . . " :'." •
Fossiferous grainstones and wackestones with molluscs,algae, echinoids. oncolites
.:.-::
.9. I ~ nI,i, , ~ /
~ Structures of subaerial ~ / exposure.
C
~ ~ -
I
z- l --
Algal mats, mud-cracks fenestrae, restricted fauna.
Basal conglomerate. DISCONFORMITY
1 rn
~
~ ~k.)
Fig.32 - Deepening-upward sequences analogous to the hinge sequence.A: Example of hinge sequence (Devonian platform, Carnic Alps) consisting of thick-bedded alternations (intraelast grainstones/packstones passing to ostracodc a l c i s p h e r e - Amphipora m u d s t o n e s w h i c h are o v e r l a i n by a t h i c k Amphipora (deep lagoon). B: D e e p e n i n g bank c o n t a i n i n g a b u n d a n t - upward s e q u e n c e from the E n g l i s h Z e a h s t e i n (New M i c k l e f i e l d Quarry) c o n s i s t i n g of intertidal flat facies ("Hampole Beds") o v e r l a i n by cold s a n d w a v e s c h a r a c t e r i z e d by h o r i z o n t a l l y b e d d e d g r a i n s t o n e s at the base and large scale b e d d i n g i n c r e a s i n g in size u p w a r d s . ( T h e sketch b e l o w is from Smith, et ai.,1986). C : L o f e r c y c l o t h e m (Fischer,1966).
46
depends, a m o n g variations.
other
factors,
of
the
rapidity
of
the
sea-level
A h i n g e s e q u e n c e m a y c o r r e s p o n d to the l a n d w a r d m o s t t e r m i n a t i o n of clinoforms within carbonate platforms and marks the separation or d i s c o n t i n u i t y between wedges within carbonate prisms. An e x a m p l e of such a d i s c o n t i n u i t y resembling a hinge s e q u e n c e is the " H a m p o l e beds" d e s c r i b e d by Smith, et a i . ( 1 9 8 6 ) from the E n g l i s h Z e c h s t e i n (Fig.32). A trouEh sequence is an alternation of deep ramp and intermediate ramp facies associations (Fig.31). A long-term deepening-upward t r e n d e x p e r i e n c e d by a p l a t f o r m leads to the d e p o s i t i o n of a t r o u g h s e q u e n c e o v e r the h i n g e sequence,
Offlap
geometry
The o f f l a p g e o m e t r y is a s e d i m e n t a r y prism, of the type s h o w n in F i g . 3 0 , T h e s h a l l o w ramp zone is the t h i c k e s t s e c t o r of the o f f l a p ramp g e o m e t r y . The i n f l e c t i o n in the t o p o g r a p h i c p r o f i l e is the flexure. The deep ramp c o i n c i d e s w i t h the trough zone. Time lines c o n v e r g e t o w a r d s the h i n g e w h i c h is l o c a t e d in the deep ramp. E x a m p l e s of this type of i n t r a s h e l f ramp g e o m e t r y i n c l u d e the i n t r a s h e l f ramp o c c u r r i n g in the D e r b y s h i r e c a r b o n a t e p l a t f o r m (Walkden,1982;Fig.33) and the P l e i s t o c e n e in the Great B a h a m a B a n k s t u d i e d by B e a c h (1982) and B e a c h & G i n s b u r g (1980), s h o w n in Fig.3~. The f o r m a t i o n of an o f f l a p g e o m e t r y is d e t e r m i n e d l a r g e l y by autocyclic,progradational p r o c e s s e s and e u s t a t i c v a r i a t i o n s . In an offlap geometry deep ramp facies are developed over p r o g r e s s i v e l y n a r r o w a r e a s ,from b o t t o m to top of s t r a t i g r a p h i c sections, w i t h the p r o d u c t i o n of s h a l l o w i n g and c o a r s e n i n g upward sequences and megasequences,Upward faunal variations reflect changes from open to semirestricted environments. Tectonic subsidence is not an essential control in the f o r m a t i o n of o f f l a p s t r u c t u r e s } n e v e r t h e l e s s , the a c t i o n of a r o t a t i o n a l s u b s i d e n c e a r o u n d the h i n g e l o c a t e d in the d e e p ramp is not e x c l u d e d . Short-term sea-level changes (I0~-I05 years) tuned in the M i l a n k o v i t o h band are e s p e c i a l l y a p p a r e n t in the s h a l l o w - r a m p zone; a tidal flat may form in the s h a l l o w e s t areas of the
47
I
:1 I I ~( tll i I i t t t k. i I Jl)t I I I , J , il~lcLI
J
tj ,
NW
j~ ~
BASNS
~q~/--
ll/lI ~%k
/
,.
'l
t
t%ll
Fis,33 - O f f l a p ramp (Asbian - B r i g a n t i a n p l a t f o r m , D e r b y s h i r e , EnEland), I: massive-bedded Erainstones and packstones; 2: medium-bedded grey packstones and w a c k e s t o n e s ; 3: g r a i n s t o n e shoals; A: t h i n - b e d d e d d a r k s h a l e y w a c k e s t o n e s and m u d s t o n e s ; 5: knoll reef and marEinal reef; 6: lava horizon (from Walkden,1982: riEht~ and G u t t e r i d E e , 1 9 8 7 : left), s h a l l o w ramp; the deep ramp records the a c t i o n of s t o r m s , A study on s t o r m p r o c e s s e s in an ancient, o f f l a p ramp o c c u r r i n g in the D ~ r r e n s t e i n F o r m a t i o n , D o l o m i t e s (Galii,1989) s h o w e d that p r o x i m a l i t y trends are a n a l o g o u s to those d e s c r i b e d by A i g n e r (1985): they are o p p o s i t e to those r e c o r d e d in the o n l a p ramp g e o m e t r i e s (see above), WEST
30--
~
EAST
.........................
--~
45-/
m 60-
F i g , 3 ~ - O f f l a p ramp and G i n s b u r g , 1 9 8 0 )
(dotted
area)
in
the
Bahama
Bank
(Beach
48
The D ~ r r e n s t e i n Formation (Triassic, eastern Dolomites) was m i s t a k e n l y i n t e r p r e t e d by B o s e l l i n i (198A) to be a p r o d u c t of vertical sediment aggradation o c c u r r e d d u r i n g a s t i l l s t a n d of the s e a - l e v e l and a b s e n c e of d i f f e r e n t i a l subsidence. Rather than a v e r t i c a l sediment aggradation, a stil[stand of the sea-level is more likely to p r o d u c e a lateral progradation (Kendall & S c h l a g e r , 1 9 8 1 ) . Furthermore, there s no e v i d e n c e for a s t i l l s t a n d of the s e a - l e v e l in the C a r n i a n , a s result from an e x a m i n a t i o n of the e u s t a t i c c u r v e by Haq, et a i . ( 1 9 8 7 ) , w h i c h shows a f i r s t - o r d e r s l o w s e a - l e v e l rise. The depositional model, illustrated in Fig.35 (Galii,1989; Bonaga, et a i . , 1 9 8 9 ) , suggests that s e d i m e n t a t i o n took p l a c e m a i n l y by p r o g r a d a t i o n . The modal c y c l e (Fig. B5) d i s p l a y s o p p o s i t e t r e n d s w i t h r e s p e c t to the h i n E e s e q u e n c e of the o n l a p ramp, where shallow-ramp l i t h o f a c i e s u n d e r l i e the b i o s t r o m e s and b a n k s of the deep ramp. It c o n s i s t s of a c o m p o u n d s h a l l o w i n E - u p w a r d sequence composed of a f i n i n g - and t h i n n i n g - u p w a r d sequence, d e p o s i t e d b e l o w w a v e base, in the deep ramp, overlain by a coarsening-upward s e q u e n c e d e p o s i t e d a b o v e w a v e base, in the s h a l l o w - r a m p z o n e . A consequence of the lateral progradation, which is the m a i n process operating in the o f f l a p ramp, was the formation of shallowing-upward meEasequences,the shallow-ramp lithofacies and s e q u e n c e s o v e r l y i n g the deep ramp l i t h o f a c i e s .
49
Grainstone / packstone with intraclasts and oncolites
T SHALLOW RAMP
Packstone/grainstone with intraclasts
%/
Packstone with peloids
" "" . ' ,
".'2 ,
°
6-
•
,
,
~."
Grainstone/packstone with bioclasts and peloi ds
.~. " - " "N"
.
~.
•
""~"
:-5. - 6 •
DEEP RAMP
c ...:)
"
mi
PROGRADATION
•
~%~ .................. . ~ .........~ ......~ .....~ ..... \ \
:,,. ::~.
'
-~
~.~.
~
..~f.!
M.G/R
......
~
I,GIR
V
,~.
M.GIR
50
O O
40-
•
30
/
® ~IN~
~,,uoo
3 0 - -
J
Km 0
Pelmo
a
30
®
20
20
lO
lO I 10~ il 10
2O
~
.
.
.
.
.
.
.
o. o . o o o
~2 m o I
I
mo
M G/P
Fig.35 - Stratigraphic sections,modal c y c l e and d e p o s i t i o n a l model of the D ~ r r e n s t e i n F o r m a t i o n , D o l o m i t e s . I: S h a l l o w ramp facies a s s o c i a t i o n s ; 2: d e e p ramp facies (from G a l i i , 1 9 8 9 and Bonaga, et a l . , 1 9 8 9 ) . I n the study area the deposition took place by lateral p r o g r a d a t i o n .
Sequence stratigraphy
The sequence stratigraphy depositional model by Vail,et ai.(1977;1984) s h o w i n g d e p o s i t i o n a l s e q u e n c e s and facies tracts is shown in Fi8.36. maximum flooding surface type-2 sequence boundary
transgressive surface type-1 sequence boundary
Condensed section ~ ~
Sequence boundary
HST
--.
/ ~
TST
Transgressivesurface / ~
* - Seaward
~
LST Sequence boundary
Landward --,.
*-- Low
Coastal onlap Fig.36 - Depositional sequence (from Swift,et p a s s i v e m a r g i n model (from V a i i , 1 9 8 ~ 1 9 8 7 ) .
High --~ Sealevel ai.,1987)
and
52
A depositional sequence is d e f i n e d as a s u c c e s s i o n of facies tracts f o r m e d in r e s p o n s e to a r e l a t i v e s e a l e v e l cycle. It is a succession of'conformable, genetically-related strata, b o u n d e d below and above by unconformities and their correlative conformities ( V a i l , e t a i . , 1 9 7 7 ) . A facies tract is "a l i n k a g e of c o n t e m p o r a n e o u s d e p o s i t i o n a l s y s t e m s (Brown & F i s c h e r , 1 9 7 7 ) , each linked to a s p e c i f i c segment of the eustatic curve".A depositional s e q u e n c e is any s t r a t i g r a p h i c unit, from a few m to 1000 m thick, with vertical boundaries constituted by physical surfaces corresponding to t e m p o r a l d i s c o n t i n u i t i e s in the s e d i m e n t a t i o n . The unit c o m p r i s e d b e t w e e n these s u r f a c e s is a g e n e t i c a l l y h o m o g e n e o u s body. There are v a r i o u s gerarchic types of d e p o s i t i o n a l sequences which are the product of distinct orders of geological phenomena. Generally, depositional sequences are sigmoidal b o d i e s f o r m e d by d e p o s i t i o n a l systems and f o r m a t i o n s passing from the sea to the land from b a s i n a l , slope, p l a t f o r m , p a r a l i c and c o n t i n e n t a l settings. Although difficult to define, owing to the d e t a i l e d a n a l y s i s and c o r r e l a t i o n s required for t h e i r identification, depositional sequences are true n a t u r a l u n i t s which record natural geological processes, such as transgressions, r e g r e s s i o n s and r e l a t i v e s e a l e v e l changes. The d e p o s i t i o n a l s e q u e n c e s d e s c r i b e d h e r e f o r m e d on a t h e r m a l l y s u b s i d i n g margin. The h i n g e is l o c a t e d l a n d w a r d of the a r e a of sedimentation. Their facies distribution and sedimentary pattern are c o n t r o l l e d by the i n t e r a c t i o n of : l ) e u s t a t i c s e a l e v e l change; 2) subsidence~ 3) sedimentation rate and A) environmental changes (Schlager,1991).The space a v a i l a b l e for s e d i m e n t a t i o n is the r e l a t i v e c h a n g e in s e a - l e v e l w h i c h is a c o m b i n e d e f f e c t of e u s t a t i c s e a - l e v e l and s u b s i d e n c e . T h e r e may be a r e l a t i v e fall, stillstand or rise of the sea-level resulting from different combinations of sea-level fall, rise, stillstand, s u b s i d e n c e and t e c t o n i c u p l i f t (Van S t e e n w i n k e l , 1 9 8 8 ) . The rate of r e l a t i v e c h a n g e of s e a l e v e l is a d e r i v a t i v e f u n c t i o n of the e u s t a t i c s e a - l e v e l c u r v e (Fig.37) : it is the rate of a d d i t i o n or subtraction of the space available for sedimentation (Posamentier,et The
most
ai.,1988).
important
changes
in s e d i m e n t a t i o n
take
place
when
the
rate of s e a l e v e l rise or fall are h i g h e s t . Subaerial unconformities are p r o d u c e d w h e n the space a v a i l a b l e for s e d i m e n t a t i o n is taken away; they o c c u r as a result of a rapid s e a - l e v e l fall. They form s e q u e n c e b o u n d a r i e s . A type-1 sequence boundary is " c h a r a c t e r i z e d by a s u b a e r i a l
53
exposure and concurrent subaerial erosion, associated with stream rejuvenation (incised valleys), a basinward shift of facies, a landward shift in c o a s t a l onlap, and onlap of overlying strata" (Vaii,1987). It is accompanied by the d e v e l o p m e n t of a l o w s t a n d facies tract (Haq,et ai.,1987). A slower rate of s e a l e v e l fall, less than or equal the rate of basin s u b s i d e n c e at the p l a t f o r m margin, produces a type-2 s e q u e n c e boundary. "It is m a r k e d by s u b a e r i a l e x p o s u r e and a d o w n w a r d shift of c o a s t a l o n l a p l a n d w a r d of the d e p o s i t i o n a l s h o r e l i n e break, but lacks both s u b a e r i a l erosion associated with stream rejuvenation and basinward shift in facies" (Vaii,!987). In o t h e r words, the whole shelf may not be exposed. At a later stage, when the r e g i o n a l subsidence outrans the slowing rate of sealevel fall, new space available to s e d i m e n t a t i o n is c r e a t e d by the r e l a t i v e rise in s e a l e v e l and a p r o g r a d i n g l o w s t a n d wedge facies tract a c c u m u l a t e s b e t w e e n the shelf edge and the l o w s t a n d fan. Later, the lowstand wedge facies tract o v e r l i e s the l o w s t a n d fan. In a c a r b o n a t e s e t t i n g a fall in the s e a l e v e l d e t e r m i n e s non d e p o s i t i o n , s u b m a r i n e erosion, c e m e n t a t i o n and the f o r m a t i o n of lithoclast beds. The carbonate 'factory' ceases to work (Droxler & Schlager,1985). Falls of relative sealevel are associated with karst, soil development on the platform (Kendall and Schlager, 1981). A c c o r d i n g to Sarg (1988) l o w s t a n d facies tracts associated w i t h a type-i u n c o n f o r m i t y lead to s i g n i f i c a n t slope front e r o s i o n and s h e d d i n g of large v o l u m e s of c o a r s e talus into the b a s i n . L o w s t a n d s h e d d i n g is c o m m o n in s i l i c i c l a s t i c systems9 it s h o u l d be an e x c e p t i o n r a t h e r than a rule in c a r b o n a t e systems. In this w o r k it is p r o p o s e d that d e p o s i t i o n of m e g a b r e c c i a s (lowstand fan facies) at s p e c i f i c time i n t e r v a l s was triggered by global episodes of g e o i d a l deformation. A rapid relative sealevel rise determines a transgression, because the sedimentation rate is no longer sufficient to fill-up the space. A transgressive surface develops above a l o w s t a n d facies tract. W h e n l o w s t a n d d e p o s i t s are lacking, the transgressive surface coincides with a facies boundary (ravinement surface: Stamp,1922). The formation of a transgressive (retrogradational) facies tract takes p l a c e a l o n g the s t e e p e s t part of a r i s i n g sealevel curve. A condensed s e c t i o n is d e p o s i t e d s e a w a r d of the d e p o s i t i o n a l area b e c a u s e there the sedimentation rate is too low.Younger units are p r o g r e s s i v e l y t h i n n e r u p w a r d and b a s i n w a r d as a result of b a s i n starvation.
54
TIME
High t EUSTACY
Low
'I t"'-.J
SUBSIDENCE
'I
i
I
I I
I
I
,
,
'
I
~
I
l
Uplift 1 ~ I ~
, " - ~
I
Susidence I
RATE OF EUSTATIC CHANGE
l
Fall ~
I
I
Rise RATE OF SUBSIDENCE
t
I i I.
lI
Fall i
=
RATE OF RELATIVE SEA-LEVEL CHANGE
I I I I
I I ! I
-I I ! I
I I 1 I
I I 1 I
RATE OF ADDITION OF N E W SPACE
I
Rise O Fall~
Fig.37 - Relative sealevel as subsidence (from Posamentier,et
a function ai.,1988).
of
eustacy
and
T h e m a x i m u m f l o o d i n g s u r f a c e , or d o w n l a p s u r f a c e , s e p a r a t e s the transgressive f a c i e s t r a c t f r o m the o v e r l y i n g highstand facies tract. It m a r k s t h e m a x i m u m l a n d w a r d s h i f t of t h e t r a n s g r e s s i v e facies tract. As a r e s u l t of a g r a d u a l slowing of the relative sealevel rise,or generally during a decrease in t h e r a t e of r e l a t i v e c h a n g e in s e a l e v e l , there is a s l o w s u b t r a c t i o n of the s p a c e available to s e d i m e n t a t i o n a n d s e d i m e n t s a r e f o r c e d to a g g r a d e and prograde above the transgressive facies tract with the formation of a highstand facies tract. It is possible to distinguish an early aggradational, and a late progradational facies tract.The upper surface of the highstandfaoies tract m a y be a t y p e - 2 o r a t y p e - I s e q u e n c e b o u n d a r y , d e p e n d i n g of t h e r a p i d i t y o f the s e a l e v e l fall.
55
When a rapid s e a - l e v e l rise changes into a slow s e a - l e v e l fall, or a slow fall c h a n g e s into a slow rise: more g e n e r a l l y d u r i n g a m a x i m u m i n c r e a s e in the rate of r e l a t i v e c h a n g e of sea-level, the n e w l y added space is i n f i l l e d with an a g g r a d a t i o n a l progradational complex (shelf m a r g i n facies tract) above the h i g h s t a n d facies tract and a t y p e - 2 u n c o n f o r m i t y (Vaii,1987). This tract is c h a r a c t e r i z e d by stacked sequences with an i n c r e a s i n g d e e p e n i n g tendency; it r e p r e s e n t s the f i l l i n g - u p of t o p o g r a p h y b e f o r e drowning. It is o v e r l a i n by the t r a n s g r e s s i v e facies tract (Haq, et a i . , 1 9 8 7 ) . The r e l a t i v e sea-level is the e n d - p r o d u c t of two v a r i a b l e s : eustasy and subsidence. It is the space available for sedimentation (Posamentier,et ai.,1988). The rate of r e l a t i v e s e a - l e v e l c h a n g e r e s u l t s from the d i f f e r e n c e b e t w e e n rate of e u s t a t i c c h a n g e and the rate of subsidence: it is the rate of change of space available to sedimentation. It is a main controlling factor in the depositional pattern as is d e m o n s t r a t e d by Van S t e e n w i n k e l (1988).As shown above, across the curve of c h a n g e in the rate of the r e l a t i v e sea-level several tracts may by distinguished that correspond to increases or slowings of the sea-level change leading to r e g r e s s i v e or t r a n s g r e s s i v e p h a s e s . A c c o r d i n g to the t e r m i n o l o g y d e r i v e d from s e i s m i c s t r a t i g r a p h y (Haq,et ai.,1987), the facies tracts that form the d e p o s i t i o n a l sequences discussed in the case h i s t o r i e s d e s c r i b e d b e l o w are the following: I 2 3 A
-
t r a n s g r e s s i v e facies tract; shelf m a r g i n facies tract; h i g h s t a n d facies tract; l o w s t a n d facies tract.
Fig.38 summarizes as a reference the facies a s s o c i a t e d d i s c o n t i n u i t y surfaces, in r e l a t i o n to c h a n g e of the r e l a t i v e s e a - l e v e l .
Transgressive.
facies
tracts and the rate of
tract
D u r i n g this tract a rapid i n c r e a s e in the rate of the r e l a t i v e sea-level determines a drowning : sedimentation does not keep pace with the sea-level rise and 'is taken by surprise' (Kendall and S c h l a g e r , 1 9 8 1 ) . Either black micrites or p a p e r shales form d u r i n g this tract. Beds c o m p o s e d of f a s t - g r o w i n g , m o n o t y p i c e p i b e n t h i o faunal a s s e m b l a g e s may a l s o form, p r o v i d e d the organisms are c a p a b l e of keeping pace with the r i s i n g
56
LOWSTAND SYSTEM TRACT
HIGHSTAND
SYSTEM TRACT
I MAXIMUM FLOODING SURFACE ~ -
TRANSGRESSIVE
SYSTEM
TRACT
---•
TRANSGRESSIVE SURFACE
SHELF
MARGIN
SYSTEM
TRACT
RAVINEMENT
LOWSTAND
SURFACE
SYSTEM
I
TRACT
57
Shelf
margin
facies
tract
This tract is c h a r a c t e r i z e d by a slow i n c r e a s e in the rate of r e l a t i v e s e a - l e v e l rise w h i c h d e t e r m i n e s an i n c i p i e n t drowning. T a n g e n t i a l o o l i t e s and s k e l e t a l bodies form d u r i n g this tract: they may also undergo a landward migration resulting in spillover lobes, washovers, or storm bars and high-energy beaches. The g e n e r a t i o n of oolite shoals in r e s p o n s e to the s e a - l e v e l rise curve is d e t a i l e d by Hine (1977) who showed that oolites form d u r i n g a ,period of gradual rise or s l o w i n g of the sea-level, P r o g r e s s i v e increase in the rate of s e a - l e v e l rise may lead to a d e e p e n i n g and to the f o r m a t i o n of lumps above oolites and e v e n t u a l l y to skeletal beds. In the case h i s t o r y studied by Van S t e e n w i n k e l (1988) the onset of the shelf m a r g i n facies tract was more or less p r o n o u n c e d and led to d i f f e r e n t types of s e d i m e n t s , d e p e n d i n g of the local s e d i m e n t s u p p l y and bathymetry: grapestone lumps and cryptocrystalline grains s i m i l a r to those o c c u r r i n g at Lily B a n k (Hine,1977), l i t h o c l a s t beds or c r i n o i d a l sands,
Highstand
facies
tract
During the highstand facies tract the rate of relative sea-level rise decreases, This leads to a r e d u c t i o n in the space a v a i l a b l e to s e d i m e n t a t i o n , hence, to a r e d u c t i o n in the sedimentation rate. S e d i m e n t s u n d e r g o a g r a d u a l p r o g r a d a t i o n . The net effect is a r e l a t i v e depositional regression which leads to the f o r m a t i o n of a s h a l l o w i n g - u p w a r d sequence. W i t h i n the h i g h s t a n d facies tract an early and a late h i g h s t a n d phase may be distinguished.The gradual decrease in the rate of a d d i t i o n of new space results in an o v e r a l l t h i n n i n g of m - s c a l e cycles in the early h i g h s t a n d facies t r a c t . C o n d e n s e d sequences c a p p e d by textures of s u b a e r i a l d i a g e n e s i s are p r o d u c e d d u r i n g the late h i g h s t a n d , i n r e s p o n s e to a s e a - l e v e l fall,
Lowstand
facies
tract
This tract is m a r k e d by a r e l a t i v e fall of the s e a - l e v e l w h i c h may stop the lateral p r o g r a d a t i o n and t r i g g e r the f o r m a t i o n of relict lithoclast beds and v a d o s e pisolite horizons, under vadose meteoric conditions and s u b a e r i a l diagenesis.Little if
58
any sediment is p r e s e r v e d , because no accomodation space is created. Lateral migration is p r e v e n t e d by the cementation which tends to link together grains, and by the reduced sedimentation rate. This tract is r e p r e s e n t e d by d i a g e n e t i e c a p s of s h a l l o w i n g - u p w a r d s e q u e n c e s f o r m e d d u r i n g the h i g h s t a n d system and shelf margin system tracts,
A peculiar f e a t u r e of t h e s t r a t i g r a p h i c columns described in t h i s w o r k is the s t a c k i n g of u n r e l a t e d , different lithofacies delimited by discontinuity s u r f a c e s . T h e s e s e p a r a t e the s y s t e m t r a c t s a n d a r e the p r o d u c t of c h a n g e s in the r a t e of r e l a t i v e sea-level rise. E a c h is a s s o c i a t e d w i t h a c h a n g e in t h e t r e n d a l o n g the s e g m e n t of the r e l a t i v e s e a - l e v e l c u r v e ( F i g . 3 8 ) .
The the
main surfaces following:
of
discontinuity
between
system
tracts
are
I - ravinement surface9 2 - transgressive surface~ 3 - s u r f a c e of m a x i m u m f l o o d i n g .
Ravinement
surface
A ravinement surface (Stamp,1922) is a f o r m e r s u b a e r i a l s u r f a c e transformed into a marine surface by erosional shoreface r e t r e a t . It is a f l o o d i n g surface above a former subaerial surface. This discontinuity surface underlies the s h e l f m a r g i n s y s t e m t r a c t a n d r e s u l t s f r o m a s l o w r a t e of s e a - l e v e l rise.
Transgressive
surface
A transgressive surface is a p l a n a r surface overlain by t h e transgressive system tract. It m a r k s a r a p i d increase in t h e rate of sea-level rise and occurs within marine lagoonal facies, separating different lithofacies. The transgressive surface may itself represent the transgressive system tract when the sea-level r i s e is g e o l o g i c a l l y instantaneous. In s u c h c a s e s t h i s s u r f a c e is d i r e c t l y o v e r l a i n by t h e h i g h s t a n d s y s t e m tract.
59
Maximum
floodins
surface
The m a x i m u m f l o o d i n g s u r f a c e marks the m a x i m u m level r e a c h e d by the sea d u r i n g a s e a - l e v e l rise and o v e r l i e s the h i g h s t a n d facies tract. It marks the t r a n s i t i o n b e t w e e n t r a n s g r e s s i v e and highstand facies tracts, often delimiting thickeningand thinning-upward trends. This may c o r r e s p o n d to a c h a n g e in sedimentation trend from v e r t i c a l sediment growth to lateral facies m i g r a t i o n . This s u r f a c e in some instances is r e p r e s e n t e d by a coquina, or s k e l e t a l lag.
These d i s c o n t i n u i t y s u r f a c e s are c o n s t i t u t e d in several cases by skeletal concentrations. Kidwell (1991) operated a classification of skeletal concentrations into some broad groups (event, c o m p o s i t e , hiatal and lag c o n c e n t r a t i o n s ) . She also s h o w e d as c o q u i n a e are d i s t r i b u t e d through depositional sequences.According to that classification, hiatal,condensed c o n c e n t r a t i o n s may form t r a n s g r e s s i v e surfaces; hiatal, s t a r v e d concentrations r e p r e s e n t m a x i m u m f l o o d i n g surfaces; ravinement surfaces are c o n s t i t u t e d by lag c o n c e n t r a t i o n s . The existence of different facies tracts separated by d i s c o n t i n u i t i e s was h i g h l i g h t e d by G o o d w i n and A n d e r s o n (1985) according to w h i c h the g r e a t e s t part of the stratigraphic column is episodic. Their punctuated aggradational cycle ('PAC'),typically I to 5 m thick and c o v e r i n g a time interval comprised between 100.000 years (with an average of about 50.000 years),is regarded as the result of geologically instantaneous sea-level rises (Im/1000 years) followed by v e r t i c a l g r o w t h of s e d i m e n t d u r i n g a r e l a t i v e s t a b i l i t y of the sea-level. Such a s i t u a t i o n may r e p r e s e n t a t r a n s i t i o n from a transgressive to a h i g h s t a n d facies tract, according to the terminology derived from the sequence stratigraphy (Haq,et ai.,1987).
Mechanisms
of
onlap
of
formation ramps
W h e r e a s the e u s t a t i c curve is g i v e n by a r e g u l a r s i n u s o i d a l function in time, the curve of subsidence may be more irregular, even over limited stratigraphic intervals. Subsidence is the summation of three distinct subsidences ( P o s a m e n t i e r . e t a i . , 1 9 8 8 ) :I) r o t a t i o n a l s u b s i d e n c e ~ 2) l o a d i n g s u b s i d e n c e ; and 3) t e c t o n i c subsidence. M o d e l s of s e d i m e n t a t i o n a s s u m i n g a c o n s t a n t rate of s u b s i d e n c e are in r e a l i t y a simplification which is negligible when considering volumes of sediments formed over large time intervals; s h o r t - t e r m s p a s m o d i c c h a n g e s in s u b s i d e n c e rate are p r o b a b l y i m p o r t a n t for m - s c a l e cycles. The model p r o p o s e d by Cisne (1987) regards the Lofer c y c l o t h e m s (Fischer,1966) as t e c t o n i c cycles; they w o u l d result from s t i c k - s l i p faulting w h i c h is typical of p a s s i v e c o n t i n e n t a l margins; a c c o r d i n g to Cisne (1987) these movements would record changes of the litospheric flexure.Dip-slip movements alone normal faults w o u l d p r o d u c e abrupt c h a n g e s in the d e p t h of the s e d i m e n t a r y interface and m - s c a l e depressions successively infilled by sediment a g g r a d a t i o n . Cisne (1987) hypothesized subsidence pulses with a r e c u r r e n c e time of about 40.000 y e a r s , c o n s i d e r e d to be c h a r a c t e r i s t i c of n o r m a l faults.
Differential subsidence had an important control on the s t r a t i E r a p h i c p a t t e r n of d e p o s i t i o n a l s e q u e n c e s of onlap ramps e x e m p l i f i e d by the case h i s t o r i e s d e s c r i b e d below. A t e c t o n i c s u b s i d e n c e control is s u g g e s t e d by the f o l l o w i n g features: I) lack of r e g u l a r i t y of the curves of r e l a t i v e s e a - l e v e l ( i.e. 'Calcari GriEi' F o r m a t i o n ) .
change
in
the
2) I n c r e a s e s in the n u m b e r of s e q u e n c e s from the deep ramp to the s h a l l o w ramp and their t h i n n i n g - o u t t o w a r d s the h i n g e w h i c h rules out an onlap r e l a t i o n (i.e. Fort T h o m p s o n F o r m a t i o n ) . 3) A p r e d e p o s i t i o n a l t i l t i n g is e x c l u d e d by the d i f f i c u l t y in r e c o n c i l i n g the t h i c k n e s s of s e d i m e n t in the t r o u g h a r e a w i t h the l i m i t e d b a t h y m e t r i c range of s u r v i v a l or a d a p t a b i l i t y of s h a l l o w - w a t e r o r g a n i s m s and fossils found in the deep ramp.
61
Two m o d e l s
are
discussed
(Fig.39).
I - The onlap ramp is p r o d u c e d by a rotational a r o u n d a hinge line located in the inner ramp;
subsidence
2 - The r o t a t i o n a l subsidence around the h i n g e takes p l a c e in c o m b i n a t i o n w i t h p r o g r e s s i v e t i l t i n g r e s p o n s i b l e for the shift of the flexure towards the hinge. In the first model a flexure can be m i s s i n g and facies trends vary gradually normal to the strike. In the s e c o n d model the shift of the flexure d e t e r m i n e s a s t e p w i s e m i g r a t i o n of the i n t e r m e d i a t e ramp facies associations towards the h i n g e . T h e first model is not new: it was s u g g e s t e d by L o w e n s t a m (1950) and later by M e i s s n e r (1972). The e f f e c t s of the rotational subsidence can p r o d u c e great c o m p l i c a t i o n s as it is not e x p e c t e d to be c h a r a c t e r i z e d by a constant rate through space or time: in both models the creation of new space induced by rotational subsidence is greatest in the t r o u g h area and so n e g l i g i b l e close to the h i n g e that s e d i m e n t a t i o n b a s i c a l l y takes p l a c e in r e s p o n s e to e u s t a t i c shifts. It follows that interpreted eustatic cycles, such as M i l a n k o v i t c h rhythms, and t e c t o n i c c y c l e s may o c c u r a l o n g the same stratigraphic interval, respectively in the hinge and t r o u g h a r e a s . T h i s may lead to some c o n t r a s t i n g i n t e r p r e t a t i o n s b e c a u s e two w o r k e r s s t u d y i n g the two r e s p e c t i v e a r e a s , w i l l be f o r c e d to a p p l y their (correct) interpretation (tectonic vs. e u s t a t i c ) to the o t h e r area. Such a case is r e p r e s e n t e d by the T r i a s s i c D o l o m i t e s , c o n s i d e r e d f u r t h e r below. A n o t h e r f a c t o r of c o m p l i c a t i o n of the e n v i r o n m e n t a l s e t t i n g and lithofacies distribution results from a differential deformation between the hinge and the flexure with the p r o d u c t i o n of a s e c o n d a r y t r o u g h in the i n t e r v e n i n g area.
The accentuation of the flexure relief, deformation, may c h a n g e the flexure into a site of f o r m a t i o n of; 1) 2) 3) ~)
c o m p l e x facies t r a n s i t i o n s ; c o n d e n s e d sequences; v e r y r a p i d facies changes; and h i g h - e n e r g y s t r u c t u r e s and d e p o s i t s and h u m m o c k y sequences.
such
consequent which
~ill,
to is
as c r i n o i d a l
the the
sands
62
trough
hinge
sea level
ROTATIONAL SUBSIDENCE
trough
flexure
hinge
sea level
ROTATIONAL SUBSIDENCE 4DEFORMATION
trough
sill
secondary trough v
hinge
)
sea level
Fig.39
- Types
A
sequence
sill
stacking
of
of
onlap
ramp
geometries.
is a s h a l l o w i n g - u p w a r d sequence formed different,unrelated shallow-water facies.
by
the
Sill areas in some cases record the abrupt (channelized) vertical transition from deep lagoonal deposits ( d e e p ramp) to very shallow deposits, s u c h as the e x a m p l e shown in F i g , 2 7 where deep ramp deposits are scoured and infilled with a v a r i e t y of s h a l l o w - w a t e r lithofacies. A n o t h e r e x a m p l e of sill is r e o o g n i s a b l e in t h e c r o s s s e c t i o n of the P l e i s t o c e n e Florida platform ( F i g . 4 0 : c f . F i g , 3 9 : b o t t o m ) by
63
Perkins (1977).Here, a tectonic control on s e d i m e n t a t i o n is mainly inferred from a lack of parallelism between time l i n e s . A n e l e v a t e d area, intermediate between the F l o r i d a Bay and the o u t e r platform, forms the p r e s e n t - d a y Florida Keys.A comparable, h o w e v e r more c o m p l i c a t e d situation occurs in the Middle Ordovician of T e n n e s s e e , where the p l a t f o r m lagoonal areas are separated by several elongated, relieved areas (Fig. A0: L o w e r O r d o v i c i a n i s l a n d s , s h e l f edge reef t r a c t , o o l i t e shoals & reefs: Walker, 197A) w h i c h may r e p r e s e n t sill areas.
In the c a r b o n a t e w o r l d most of the m o r p h o l o g y and p h y s i o g r a p h y of e l e v a t e d features such as reefs, banks, patch-reefs show v a r i o u s d e g r e e s of i n h e r i t a n c e from the p r e e x i s t i n g t o p o g r a p h y . The B e l i z e lagoonal reefs for e x a m p l e provide an e x c e l l e n t s p e c t r u m of such r e l a t i o n s h i p s w h e r e shelf atolls and linear foundations (Choi and G i n s b u r g , 1 9 8 2 ; reefs m i r r o r P l e i s t o c e n e Choi and H o l m e s , 1 9 8 2 ; P u r d y , 1 9 7 ~ ) .
A comparison between the two styles of the depositional a s s e m b l a g e s e x a m i n e d in the case h i s t o r i e s b e l o w i n d i c a t e that the modalities of formation of intrashelf ramps:(simple rotation; rotation and tilting: Fig.39) dictate the type of c o n t r o l e x e r t e d by the p r e d e p o s i t i o n a l topography in the two facies. W h e r e the i n c l i n e d s u r f a c e of the ramp is p r o d u c e d only by a stepwise, s i m p l e r o t a t i o n a s s o c i a t e d with e u s t a t i c shifts, the control p l a y e d by the a n t e c e d e n t t o p o g r a p h y is s t r o n g (Fig.41 A) and the s t r a t i g r a p h i c column c o n s i s t s of a well defined cyclicity, shown in the P l e i s t o c e n e Fort Thompson Formation (see below). C o n v e r s e l y , a d e f o r m a t i o n a c c o m p a n i e d by a r o t a t i o n d e t e r m i n e s d i s c o n t i n u o u s v a r i a t i o n s in the t o p o g r a p h y that c o m p l i c a t e the e n v i r o n m e n t a l facies d i s t r i b u t i o n (Fig. Al B). C o m p l e x facies m o s a i c s are e x p e c t e d to form e s p e c i a l l y in this last s i t u a t i o n , o t h e r than by a u t o c y c l i c shifts and v a r i a t i o n s in the coexisting syndepositional topography (Pratt and James,1986)~the predepositional t o p o g r a p h y is not as i m p o r t a n t as in the p r e v i o u s case; it may play an i m p o r t a n t role in the f l e x u r e w h e n it is t r a n s f o r m e d into a sill. The c y c l i c i t y is less evident; rather, it is a s t a t i s t i c a l c y c l i c i t y a c c o m p a n i e d by r a n d o m facies t r a n s i t i o n s w h i c h are also a f u n c t i o n of the i n t e n s i t y and m o d a l i t y of the d e f o r m a t i o n .
64
Cross-bedded oolitic. pelletal, skeletal grain,ctone & pack,~tone
2 0 - ~°"-. o-o°
:,;.~ .~,. ." ~ " ~ .~,
SILL SEQUENCE Arenaceous,oolitic, pelletal, skeletal grainstone & packstone
Oolitic, pelletal, fossiliferous sandstone
15
Sandstone 10
Coral ,skeletal,algal 5
mO
arenaceous packstone
~\",. ".. :'o:@.(
65 NNO
SSE Florida
:.....:.....:,:. :..~:::.-:-.:~.:.:.-..:.::.-.':.::-:.
-:.Ft~
Bay
...~"..Atlantic
~
Ocean
..... .::" "',: '...
............................
.....................
.-..';~
o+ -..
_ .^ ".........7]-:;;777;........................... 7;;;;7;;;i; .................~-~ ....................i ; .......... ~ 4" e ". 4" '">:.."7..:.-:--?~~i R .-iu[ ......:-iT...~........ "..................~.."t.......... " ~ ................ ". ""~.:..!y~
/ 5
...... ;;;;.......................... 0~ ...L;:::::::::: .... ~ o O1 ......... .... ;,.~"~ .... "~ ...... ~. ........................ -"Y..'"..... ................................ ;~";;;
iL 0
30
60
,r- ~ ~:'.~:~= ~', "' < '~o \ ~ o
......;',".,~ ~
90km
\ ~. ".,
......;-....
Patch- reefs, ,. ,li,/ -. B a c k - r e e f I@ " i ~ lagoon ~llt ~ ~ ~ / iii ~l / / t Idlll'
(
f w,th
k
,VII
""
.:.,. I
I-~lancl~ ,-_.~/.' ""
L.Ord.
.-" .'" ,.,". . ,. . .
,~,;.. ~ 11 I
f i .l
z
', '/',.
~
I.
,../Y
~ ~
/,,,~,/i II
t
/,.I. 'll''
.llll t~ll~l
islands/ //iflil I ~ Shelf edge ~" reef tract ,~
/~ ii-~
]Jt'~-"/ /
~
./' / /..~.,. // /
" l"errigenous
i
-o,r~
t D e e p e r - w a t e r .-/ shale basin / 9
/ ~~,o ~ / . / .~
-,
k /
/
~'I.~
/
"
Oolite shoals & reefs
clastics
!
I I
Fig,40 - C r o s s s e c t i o n of the P l e i s t o c e n e of F l o r i d a platform (from P e r k i n s , 1 9 7 7 ) , c o r e ff34 by P e r k i n s (1977) t a k e n a l o n g the Florida Keys showing a shallowing-upward sequence developed above coral deposits, and environmental reconstruction of the Middle 0rdovician of T e n n e s s e e (Walker, 1974) showing several s u p p o s e d sill a r e a s o r i e n t e d N E - S W . The Key Largo Formation occurring in Florida platform constitutes a coral-bearing Formation. Maximum thicknesses are of a b o u t 60 m (20 on the a v e r a g e ) . The F o r m a t i o n constitutes slightly arcuate, elongated i s l a n d s w h i c h e x t e n d for a b o u t 2 4 0 km a l o n g a E N E - W S W d i r e c t i o n , 8 km i n l a n d f r o m the o u t e r b o r d e r of the p l a t f o r m . The faunal composition is h o m o g e n e o u s . Coral zonations do not o c c u r (Stanley,1966; Hoffmeister & Multer,
66
1 9 6 8 ) . M a i n c o r a l s a r e M o n t a s t r e a annularis, Diploria striEosa, Diploria labirintiformis, Porites. T h e m o s t c o m m o n f o s s i l is Montastrea annularis. Previous interpretations concerninE the formation of t h e K e y Largo Limestone are c o n t r a s t i n g . It was interpreted as an elongated s e r i e s of p a t c h - r e e f s developed behind a coral bank located seaward (Hoffmeister and Multer, 1 9 6 8 ) , o r a r e m a i n of an o u t e r r e e f . T h i s last h y p o t h e s i s contrasts with the absence of Acropora palmata a n d Millepora; and the occurrence of Halimeda p l a t e s w h i c h a r e a b s e n t in t h e o u t e r reef, b u t an important constituent of t h e K e y L a r g o F o r m a t i o n . The main difficulties arising from an exclusion of a t e c t o n i c control result from the n e e d to c o m b i n e the former depth of deposition of the Key Largo Formation with its p r e s e n t - d a y elevated topographic location. The Formation is presently located 6 m above sea-level. According to S t a n l e y (1966) it formed 9 m below sea-level (cf. t h e o c c u r r e n c e of Montastrea annularis w h i c h t y p i c a l l y g r o w s in q u i e t l a g o o n a l a r e a s , b e l o w the s u r f zone, as m u c h as 25 m b e l o w s e a - l e v e l , 9-10 m on the average).Coeval, coralline reefs a l o n e t h e b o r d e r of the o u t e r platform are located 8 m below sea-level. According to E n o s (1977) t h e P l e i s t o c e n e surface of t h e o u t e r r e e f is a d e p o s i t i o n a l surface, as t h e b u t t r e s s e s maintain the patch-reef confiEuration. This rules out an erosion of the outer reef during a sea-level lowstand. T h e l o c a t i o n of t h e K e y l a r g o F o r m a t i o n , 6 m above sea-level was explained as the r e s u l t of a d e p o s i t i o n during a previous highstand. However, this does not explain the discrepancy in elevation b e t w e e n the o u t e r r e e f a n d t h e K e y L a r g o F o r m a t i o n , because corals on the outer reef should have caught-up to sea-level at t h e s a m e r a t e as t h o s e l o c a t e d o n the K e y L a r E o Formation. The Pleistocene surface, alone the outer platform, is s l i g h t l y i n c l i n e d s e a w a r d : t h e d e p t h is of a b o u t 75 f e e t (I f e e t = 33 cm) a l o n e t h e o u t e r b o r d e r , 50 f e e t in t h e m i d d l e a n d much
less
close
to
the
Keys
(Enos,1977:P.19~7).
This inclination can not be explained as the result of a g r e a t e r e r o s i o n a l o n g t h e o u t e r b o r d e r of t h e p l a t f o r m , d u e to the greater wave energy resulting in a h i g h e r s e d i m e n t t r a s p o r t of s e d i m e n t , because sedimentary trends indicate a landward sediment transport. On the other hand, remains of Acropora palmata in t h e o u t e r p l a t f o r m (here referred to as the d e e p ramp s e c t o r ) s u E E e s t s a f o r m e r d e p t h of d e p o s i t i o n c l o s e to the surf zone.A gradual eastward dipping of the K e y s is e v i d e n t from topographic data: h e i g h t values are 18 f e e t at W i n d l e y
67
K e y , 2 - 3 f e e t at M a t e c u m b e Key, 0 feet at B i g P i n e Key, 6-8 f e e t b e l o w s e a - l e v e l at B o c a C h i c a . These differences i n d i c a t e an e l e v a t i o n d i f f e r e n c e of 8 m f r o m W i n d l e y K e y to the w e s t w a r d s i d e of the Keys. H o f f m e i s t e r and causes,suggested a Multer (1968), among the possible downfaulting to e x p l a i n t h e s e h e i g h t d i f f e r e n c e s . It is s u g g e s t e d here that rotational subsidence led emergence of the k e y s a n d to a d i f f e r e n t i a l subsidence outer platform (deep ramp).
to on
the the
Some areas along the keys underwent relief inversions and emergence.The c o r e s h o w n in Fig. A0 (core # 3 ~ by P e r k i n s ) shows the d e v e l o p m e n t of a s h a l l o w i n g - u p w a r d sequence above coral limestones. The sequence is s i m i l a r to a s i l l sequence.The emergence of the K e y s is documented in s o m e cases by the development of vadose diagenetic features and caliche-like c r u s t s a b o v e the K e y L a r g o Formation and the eteropic Miami Limestone. An alternative to a s y n d e p o s i t i o n a l t i l t i n g is r e p r e s e n t e d by a f i l l i n g of a p r e v i o u s l y downfaulted a r e a by v e r t i c a l sediment aggradation during a highstand. In the c a s e of the p r i s m a t i c s t r u c t u r e of the P l e i s t o c e n e Formation,the d e p t h of d e p o s i t i o n should have been -h6 m during the deposition of the Q2 unit,then -17 ( Q 3 ) , a n d - 9 m ( Q A ) . T h e d e p t h of the s e d i m e n t a r y interface would have diminished gradually by vertical aggradation and gradual infilling. Nevertheless, the c o r a l c o m p o s i t i o n is i n c o m p a t i b l e w i t h s u c h a hypothesis, based on b a t h y m e t r i c and ecologic c o n s t r a i n t s . In fact, a d e p t h of -A6 m c o n t r a s t s w i t h the o c c u r r e n c e of P o r i t e s (which grows above -15m), Chione cancellata (which lives in shallow lagoons) and Montastrea annularis. Conversely, corals l i v i n g at g r e a t e r depths (i.e. Meandrina m u s s a ) do n o t o c c u r . A n a l o g o u s considerations c a n be m a d e for the u p p e r , Q3 a n d Q A u n i t s . A tilting antecedent to the to t h e d e v e l o p m e n t of o f f l a p
infilling would have r a m p s (cf. F i g . 7 5 ) ,
probably
led
A situation partly similar to that found in t h e Key Largo Formation occurs in the intrashelf ramp from Belgium (Preat,198A: Fig.3) where the reef is 60 m thick. This anomalous thickness was related to a d i f f e r e n t i a l subsidence (Preat,198~).
68 CONTROL
PLAYED BY THE
PREDEPOSITIONAL
TOPOGRAPHY
Stratigraphic columns
Time line
Fig.~l - Different controls played by the predepositional t o p o E r a p h y and r e l a t e d facies s t a c k i n g in o n l a p ramps g e n e r a t e d by rotational subsidence (A) and rotational subsidence + d e f o r m a t i o n (B),
Fort
Thompson
Pleistocene,
Fo r m a t i on,
Florida
platform,
Introduction
The Fort T h o m p s o n Formation p l a t e a u from Lake O k e e c h o b e e (Fig,A2). The
stratigraphy
is o u t l i n e d
crops out in the south Florida s o u t h w a r d up to the F l o r i d a Keys
below.
The u n d e r l y i n g T a m i a m i F o r m a t i o n (Miocene) represents several inner shelf environments ranging from n e a r s h o r e to deltaic (Parker,et a i , , 1 9 5 5 ) . In the study area it c o n s i s t s of s h e l l y s a n d s t o n e s , at places c o n t a i n i n g a b u n d a n t O s t r e a shells. The C a l o o s a h a t c h e e marls were deposited in a p a r a l i c swamp a b o v e the T a m i a m i F o r m a t i o n as thin m a r l y s t r i n g s e n r i c h e d in organic debris,The rapid vertical transition to the Fort T h o m p s o n F o r m a t i o n is c h a r a c t e r i z e d by the s u d d e n d i s a p p e a r a n c e of Ostrea and an increase in the carbonate content which r e f l e c t an e n v i r o n m e n t a l change from from p a r a l i c to a f u l l y m a r i n e setting. The Fort T h o m p s o n F o r m a t i o n c o n s i s t s of several a l t e r n a t i o n s of marine brackish freshwater limestones which reflect high-frequency relative sealevel fluctuations which were c o r r e l a t e d w i t h i n t e r g l a c i a l h i g h s t a n d s and g l a c i a l lowstands. The w e d g e - s h a p e of the F o r m a t i o n , v i s i b l e in Fig. A2, c o r r e s p o n d s to an o n l a p ramp g e o m e t r y whose t r o u g h area is l o c a t e d in the south and southeast, and the hinge in the north and northwest, In the east and northeast the Fort Thompson Formation interfingers w i t h the A n a s t a s i a Formation which represents a complex of barrier beach, bar, dune and eolianite bodies (Perkins,1977) which controlled water circulation within the area of deposition of the Fort Thompson Formation during p e r i o d s of r e l a t i v e s e a l e v e l lowstands. In the s o u t h the Fort T h o m p s o n F o r m a t i o n is t r a n s i t i o n a l to the patch-reef deposits of the Key L a r g o Limestone (Stanley, 1966). The o v e r l y i n g Miami Oolite Formation represents a c o m p l e x of oolite bars and sandwaves developed along a northeast s o u t h w e s t s t r i k e (Evans and G i n s b u r g , 1 9 8 7 ) .
70 Everglades~ ~
~.
~
_~ .- ~_ , : _ .. , :~. ~,
.
:
_ .-
....
~.o
,
o-o.~.~.~.~ ~-~-.~-.~-~
.
" ' "+
~' "~-~ Key Largo
~; ~.~
Miami Oolite ~ ' #
Fort Thompson
F. j
~nlllllll|lliillll)l]llllllll
E
~"
il~illliil~ .........................
[ Caloosahatchee marls
"4
Tamiami
F. ~
10 km
"5 MIAMI
•
17
26
2"2
2
"23
~4
~8
~'
~o
STUDY AREA
>-
N
• 25
2"7
/ J
~; ~
+ 0 I
I
10 :
20 .
30 Km I
½2
Y i g , 4 2 - L o c a t i o n of the Fort T h o m p s o n F o r m a t i o n , drill c o r e s (Causaras,~987), and block-diagram (Parker, et ai,,1955) s h o w i n g the s t r a t i g r a p h y and the w e d s e - s h a p e of the Formation.
71
P r e v i o u s d e s c r i p t i o n s of the Fort T h o m p s o n F o r m a t i o n are those by Parker, et ai.(1955), Cooke, et a l . ( 1 9 A 3 ) , Brooks (1968), Perking (1977),Mitterer (1975), C a u s a r a s (1987),Goldhammer, et ai.(1990) and Galli (1991). O u t c r o p s are limited to rock pits and d r a i n a g e canals; most of the actual s u r f a c e is c o v e r e d by a d e n s e v e g e t a t i o n and a thin v e n e e r of w a t e r . T h e study is based on the d e s c r i p t i o n of 3A cores (700 m of s t r a t i g r a p h i c sections) d r i l l e d in Dade C o u n t y area by air p u m p i n g t e c h n i q u e s ( C a u s a r a s , 1 9 8 7 ) .
Lithofacies
and
environmental
setting
The Fort T h o m p s o n F o r m a t i o n is c o m p o s e d of a c o m p l e t e s p e c t r u m of l i t h o f a c i e s r a n g i n g from m a r i n e bay ( m o l l u s k g r a i n s t o n e s and packstones) to t e r r e s t r i a l c a r b o n a t e s u b e n v i r o n m e n t s (Helisoma w a c k e s t o n e s , root rock, p h y t o g e n i c breccia).
Marine
bay
(Fig. AS)
Desqription This facies consists of poorly cemented, pale-orange to yellowish-gray, clean-washed calcarenite and calcirudite containing marine shells such as Chione cancellata, corals (Archais angulatus,Montastrea annularis, Porites), rare o y s t e r s (Vermicularia). Mollusks are p r e d o m i n a t i n g : and w o r m shells they are e i t h e r d i s a r t i c u l a t e d , concave upwards, or found in life p o s i t i o n . T h e y a l s o form c o q u i n a e . Q u a r t z sand c o m p o s e d of well s o r t e d , s u b a n g u l a r g r a i n s c o n s t i t u t e s a p p r o x i m a t e l y 15% of the totail g r a i n p e r c e n t a g e . Two grain size trends occur in this lithofacies: I) f i n i n g - u p w a r d trends a c c o m p a n i e d by an u p w a r d i n c r e a s e in shell fragmentation and o c c u r r e n c e of Callianassa burrows; and 2) coarsening-upward grain-size trends consisting of an u p w a r d increase in size of s h e l l s , s o m e of w h i c h occurring in life position.
72
F i g . ~ 3 - S u b f a c i e s t y p e s of m o l l u s k g r a i n s t o n e s A patch-reef.B m a r i n e bay, C m a r s h flat. D bar.
and packstones. E estuary.
73
Interpretation The
thanatocenosis
is t y p i c a l
of p r e s e n t - d a y
marine
settings
of
Florida Bay and southwest Florida coast.This lithofacies includes several m a r i n e lagoonal subenvironments. G r a d e d beds c o m p o s e d of f i n i n g - u p w a r d m o l l u s k a n d e b r i s t y p i f y lagoons close to bars, beaches, or tidal deltas. The o c c u r r e n c e of O s t r e a reflects a deposition in an e s t u a r y (Perkins,1977~ Parkinson, 1989),more typical of the underlying Tamiami Formation. Callianassa mounds are found in lagoonal areas close to emergent and marsh flat e n v i r o n m e n t s . Coarsening-upward beds c o n t a i n i n g A r o h a i s anEulatus, M o n t a s t r e a a n n u l a r i s and Porites r e p r e s e n t p a t c h - r e e f s and bars d e v e l o p e d in a m a r i n e lagoonal environment: they are c o m m o n in the s o u t h e a s t w h e r e the Fort T h o m p s o n F o r m a t i o n g r a d e s l a t e r a l l y to the Key Largo Formation.
Freshwater
swamp
(Fig.~A)
Description This facies c o m p r i s e s wackestones.
root
rock,phytogenic
breccia
and
Helisoma
Root rock, mainly developed within preexisting sediments, c o n s i s t s of an i r r e g u l a r n e t w o r k of circular, equidimensional rods w h o s e rinds are lined w i t h r e d - c o l o r e d laminations and s t a i n e d margins. P h y t o g e n i c b r e c c i a c o n s i s t s of w h i t e - c o l o r e d , dense m a s s e s of r o o t l e t s and traces of f r e s h w a t e r p l a n t s w h i c h impart to the rock a b r e c c i a t e d a s p e c t . C a s t s of tree trunks and b l a c k p e b b l e s are locally present. The m a t r i x has a g r a n u l a r texture and c o n s i s t s of a m i x t u r e of roots and r h y z o m e s of f r e s h w a t e r swamp peat types. Helisoma wackestones are constituted by dark-brown, well c e m e n t e d mud c o n t a i n i n g a b u n d a n t p l a n e - s p i r a l e d and p u l m o n a t e gastropods (Helisoma) together with Ameria and ostracods. Organic matter and wood fragments are common components. Charcoal debris is not too frequent.Present-day cores of Helisoma wackestones show a fibrous dark-brown matrix with d a r k e r bands c o n t a i n i n g a g r e a t e r a m o u n t of o r g a n i c matter.
74
Fig,~ Freshwater s w a m p as s e e n f r o m c o r e p r o f i l e and t h i n s e c t i o n c o m p o s e d of mud, H e l i s o m a s n a i l s , roots, v e g e t a l d e b r i s and authigenic quartz grains (Galli,1991),
Interpretation Root rock facies Teeter (1975) and
was described and interpreted by B a i n and Hoffmeister & M u l t e r (1965) as b l a c k m a n g r o v e
75
roots and m a n g r o v e reef rock r e s p e c t i v e l y . The root rock facies r e p r e s e n t s the i n v a s i o n and i n f i l l i n g by r o o t l e t s of A v i c e n n i a n i t i d a of p r e e x i s t i n g u n l i t h i f i e d sediment. V e r t i c a l l y o r i e n t e d root s y s t e m s r e p r e s e n t tap root systems. Phytogenic breccias occur r e f e r r e d by D a v i e s (1980)
at p r e s e n t in the E v e r g l a d e s to as f r e s h w a t e r swamps.
and
are
Helisoma wackestones represent a deposition in a f r e s h w a t e r pond p o p u l a t e d by H e l i s o m a s n a i l s . T h e actual site of d e p o s i t i o n of H e l i s o m a w a c k e s t o n e s is c o v e r e d by few cm of w a t e r , s a w grass and s c a t t e r e d red m a n g r o v e s . The saw grass p r a i r i e is the site of a c c u m u l a t i o n of lime mud and marls and f l o c c u l a t i o n of a dense mass of rootlets, fragments and freshwater grass and sedges. The prairie is locally forested by buttonwood hummocks.The former existence of buttonwood hummocks is inferred from wood fragments observable in thin sections. C h a r c o a l o r i g i n a t e d in h u m m o c k r e l i e v e s w h i c h were the sites of aeration.The saw grass p r a i r i e is r e p r e s e n t e d by l i g h t e r mud c o n t a i n i n g fewer shells and v e r t i c a l root systems (Distichlis spicata).
Sedimentation took place in a low-energy, wide lagoonal environment, behind a complex of barrier island system represented by the Anastasia Formation and the Key Largo Formation.The lagoon, occupied by skeletal bars, patch-reefs and e s t u a r i e s populated by oysters, graded landward, in the south and s o u t h w e s t , t o muddy shorelines close to m a r s h e s and swamps, covered by mangroves9 Further landward, the depositional surface was occupied by freshwater swamps.The depositional setting is analogous to the present-day e n v i r o n m e n t s b o r d e r i n g the F l o r i d a platform. A paleoenvironmental shown in Fig.45.
profile
Facies
of
the
low-energy
shoreline
is
associations
Lithofacies and alternations between freshwater and marine lithofacies are grouped into shallow ramp and deep ramp associations. The s h a l l o w ramp is located in the south9 the deep ramp in the north. The i n t e r m e d i a t e ramp is a b s e n t and the transitions between the two sectors of the ramp are gradational.
76
SALINE
MANGROVE
ZONE
FRESHWATER SWAMPS
.::::
FLORIDA BAY
SALINE MANGROVE ZONE
FRESHWATER SWAMPS
BUTTONWOOD HUMMOCK
SAWGRA~S PRAIRIE
FLORIDA KEYS
FRESHWATER pOND
BLACK MANGROVE MARSH FLAT PEAT BED MANGROVE PEAT
MARINE BAY
CALLIANASSA MOUNDS
BARS;PATCH PEEFS
|OYSTER
BED]
(not to scale)
Fig.~5
-
1969),
and
Thompson
Present-day
environmental
paleoenvironmental
shallow
brackish
(from
Craighead,
of
the
Fort
Formation.
Shallow
The
zones
reconstruction
-
ramp
is
represented
freshwater
ramp
by
an
limestones.This
alternation sector
of
marine
consists
of
a
77
O
Sandstones (Tamiami Formation) -~
Mollusk grainstone/packstone Helisoma wackestone Mangrove peat Miami Oolite Formation
Om
Fig.~6 ramp,
-
Thickening-upward
trends
occurring
in
the
shallow
m a x i m u m n u m b e r of t h r e e alternations of f a c i e s ( F i g . ~ 6 ) w h i c h are organized into symmetrical grain-size trends (coarseningupward -->fining-upward).Remains of c a r b o n i z e d r o o t s w i t h b l a c k mangrove f a c i e s (root r o c k f a c i e s ) are common, Transitions to f r e s h w a t e r s w a m p f a c i e s a r e m o r e or less g r a d a t i o n a l . T h e s t a c k i n g of s e q u e n c e s r e s u l t s in a t h i c k e n i n g - u p w a r d trend, also visible in F i g . a T , which shows a stacking of marine lithofacies separated by discontinuity surfaces typical of hinge areas.
78
0
300m
sandstone
Fig.~7 - Stacking P e r k i n s , 1977) .
of
shelly sandstone limestone
lithofacies
Deep
in
the
hinge
laminated crust
area
(from
ramp
The deep r a m p c o n s i s t s of a r e g u l a r a l t e r n a t i o n of m a r i n e and freshwater lithofaoies that increase progressively in n u m b e r t o w a r d s n o r t h (from ~+5 to 9 a l t e r n a t i o n s : Fig.48). The m a r i n e bay lithofacies is p r e p o n d e r a n t . Close to the shallow ramp (cores #18 to 25) such a p r e d o m i n a n c e is less a p p a r e n t . M a r i n e bay facies d i s p l a y c o a r s e n i n g - u p w a r d E r a i n - s i z e trends, especially in the north9 in the s o u t h , s y m m e t r i c a l g r a i n size t r e n d s b e c o m e m o r e frequent. C a r b o n i z e d p l a n t r e m a i n s and root rock facies are less c o m m o n than in the s h a l l o w ramp. The vertical thinning-upward
stacking of alternations or a s e q u e n t i a l trends.
Depositional
results
in
sequence
The Fort Thompson Formation represents a third order d e p o s i t i o n a l sequence. The d e p o s i t i o n a l theme is s i m p l e b e c a u s e the whole Formation consists of the vertical stacking of meter-scale alternations of f r e s h w a t e r and m a r i n e l i t h o f a c i e s (parasequences) which represent high-frequency, short-term p h a s e s of s e d i m e n t a t i o n .
79 26
1B
,,~
~1
19
, -'''~
o,
/,..~
,-/.I .*..
20
21
80
0
0
0
Om
0
0
81
Q
0
Q
e
GROUND ~__--LEVEL --
I
--i
-i --i
--i
'-,:.'[o:. ::.,>:
~...?. --%,:..
Fig,48 - Core profiles freshwater lithofacies,
showing
alternations
between
marine
and
82
.~
MIAMI OOLITE SEQUENCE BOUNDARY
!:~ "'"
SEQUENCE"C"
~..~: ,==4sSEOUENCE.S.@ MARSHFLATB ®
t LATE HIGHSTAND SYSTEM TRACT
I
t
[EAR~YH'OHSTANO1
[SYSTEMTRACT
j_
~-/'
m,s
®
~-?~'[l
1
I"G~"
,\
..,~,,..~.. ~-~. ":-"'"
BARS
pond
freshwater
marsh flat
ii~
==t~ PATCH-REEFS;
sawgrass prairie
~ ~
Calllanassa coquina
mounds
~=z=mrs
bar , patch-reef === ts mangrove peats & prairie
®
mollusc packstone & gra;nstone m
®
freshwater pond ts transgressive surface
,,i,uI~,1rs ravinementsurface :==::::mfs maximum flooding surface I m
JENCE BOUNDARY IAMI FORMATION
1
lm J
Fi~.A9 - S e q u e n c e s t r a t i g r a p h y Thompson Formation.
isochronous unit A
depositional
thinning-upward trend
model
of
the
Fort
The s y s t e m tracts w h i c h c o m p o s e the d e p o s i t i o n a l s e q u e n c e are the following, from b o t t o m to top: I) t r a n s g r e s s i v e system tract; 2) e a r l y h i g h s t a n d s y s t e m tract; and 3) late h i g h s t a n d s y s t e m tract (Fig.49), The d i s c o n t i n u i t y surfaces which separate parasequences are transgressive and ravinement surfaces. A maximum flooding s u r f a c e s e p a r a t e s the t r a n s g r e s s i v e from the h i g h s t a n d s y s t e m tract. Variations in the i n t e r n a l characteristics of p a r a s e q u e n c e s
83
reflect sealevel
corresponding rises.
differences
TransBressive
in
the
rates
of
relative
surface
Transgressive surfaces are r e p r e s e n t e d by a b r u p t transitions from f r e s h w a t e r swamp to m a r i n e bay l i t h o f a c i e s occurring at the bases of lowermost p a r a s e q u e n c e s (Fig.49). They reflect a r a p i d f l o o d i n g of the lagoon located b e h i n d the b a r r i e r island c o m p l e x of the A n a s t a s i a and Miami O o l i t e Formations, during sealevel highstands.Conversely, during sealevel lowstands the lagoon was a f r e s h w a t e r lake (Fig.50).
HIGHSTAND marine bay
open
sea sea level
LOWSTAND open sea
freshwater lake
sea level
Fig.50 Thompson changes.
Control of the sedimentary pattern of Formation by high-frequency relative
Ravinement
This surface pseudobreccias
is a h o r i z o n (Fig.51)
the Fort sea-level
surface
comprising
laminar
micrites
and
Laminar micrites consist of a few mm-thick, red colored laminations draping irregular subhorizontal surfaces developed m a i n l y w i t h i n the m a r i n e bay facies. T h e s e l a m i n a r m i c r i t e s are ~omp~eed ~f an a l t e r n a t i o n of darker and lighter laminae. D a r k e r l a m i n a e are c o m p o s e d of a mottled, red colored, dense mass; l i g h t e r laminae c o n s i s t of m i c r o s p a r f i l l i n g irregular, sub-horizontal,contorted voids characterized by frequent bifurcations, labyrinthic structuress and pseudofenestral
84
fabrics, fine vertical rods p r o t r u d e downward from horizontal laminae, in s o m e c a s e s cutting through shells. As s e e n from o u t c r o p s p a r a l l e l to the b e d d i n g p l a n e , the l a t e r a l c o n t i n u i t y of this thin horizon is interrupted by subcircular holes, averaging 8 c m in d i a m e t e r . In s o m e c a s e s t h e h o l e s a r e a l i g n e d along a circular perimeter. Pseudobreccias generally overlyin~ laminar micrites, are breccia-like features composed of red-colored mm-cm thick monogenic fragments surrounded by a matrix constituted by equidimensional quartz grains, a n d i n f e s t e d by the s a m e q u a r t z grains. Marine and freshwater shells occur within the m a t r i x . Clasts l o o k like p i e c e s of a j i g s a w p u z z l e ; ~oing downward, they become smaller and more numerous. P r o f i l e s s u c h as t h a t s h o w n in F i g . 5 2 , A a r e i n t e r p r e t e d as the result of r o o t p e n e t r a t i o n by m a n g r o v e root systems. Unlike c a l i c h e s or c a l c r e t e s w h i c h a l s o o c c u r in the C a r i b b e a n region (Beach and Ginsburg,1981; James, 1972), laminar micrite horizons display sharp transitions from marine carbonates to crusts,are accretionary features, do n o t t r u n c a t e bedding and lack d i a g e n e t i c textures evidencing for a s u b a e r i a l exposure. The shell truncation p r o d u c e d by t h e s e l a m i n a r h o r i z o n s is m o r e typical for roots possessing acidic properties t h a n of c a l i c h e crusts. T h e r e d c o l o r of l a m i n a e was p r o d u c e d by t h e t a n n i n e Rhyzophora manEle. T h e i n c l u s i o n s p r o d u c e d by the red m a n g r o v e present-day p e a t p r o d u c e d by the red m a n g r o v e is r e d d i s h b r o w n to dark-brown and consists of a dense mass of rootlets. Rhyzophora mangle is r e d b r o w n . Likewise, water surrounding Circular holes visible on horizontal surfaces probably represent casts of former roots.White laminae inside the laminar micrite horizon represent the i n f i l l i n g by c a l c i t e of former thin horizontal root filaments.Laminar horizons are similar to those detailed and interpreted as root mats by Wright,et ai.(1988). Pseudobreccias are interpreted as the r e s u l t of a d i s s o l u t i o n produced by the a c i d i c p e a t of the red m a n g r o v e r o o t s y s t e m . In several cases the transition b e t w e e n c l a s t s a n d the m a t r i x is gradational.The distinction between clasts and matrix is m a d e possible by a greater abundance of equidimensional quartz g r a i n s w i t h i n the m a t r i x t h a n w i t h i n the o l a s t s . I t is p o s s i b l e that quartz grains originate from the siliceous material contained in the v a s c u l a r tissues and periderm of r o o t s , once they undergo peatification (of. H o f f m e i s t e r and Multer,1965). The
development
of
red
mangrove
peat
within
marine
bay
facies
85
F I E . 5 1 - A e r i a l v i e w s of s o m e Carolina and Georgia,thought different stages of f l o o d i n g Thompson Formation.
aras alone the coasts to be representative and shallowinE up of
of s o u t h of the the Fort
86
g surtace ~ss
Freshwater pond
Tta Marsh flat ebris l~SSa
E o3 •b
s A Marine Bay
g surface
~
".............
(laminite) Freshwater
~ i ~ £ ~ j~e ~f,,--. mi~ed tauna
Marsh flat
E
u~ ": I ~ ~ , ~ J . . . - , , m a r i n e
sh¢lls
Mar)he
Bay
B
Marsh flat
Freshwater pond
and
- Parasequences formed during a quick transgression a less r a p i d sealevel r i s e d ~ s p l a y i n g laminar mi~Pites
pseudobrecc~as
(B),
CA) and
87
,
, , ,
,
C
Fi~.53 - Ravinement surface. A P r o f i l e of the l a m i n a r m i c r i t e , pseudobreccia, root casts developed above marine bay facies. B C a s t s of p r o p r o o t s a n d l a m i n a r m i c r i t e s . C~D Pseudobreccia. Root rock showing rhyzoturbated sediment.F~G D e t a i l of l a m i n a r micrites ( t h i n s e c t i o n s ) . T h e e x a m p l e v i s i b l e in G s h o w s a r o o t lamina cutting through a shell.
88
is e x p l a i n e d by the marine to b r a c k i s h coastal, intertidal m a n g l e . It f o r m s d u r i n g t h e i n i t i a l p h a s e s s e t t i n g of R h y z o p h o r a of a r e l a t i v e sealevel rise.Quick sealevel rises prevent the development of the r e d m a n g r o v e b e c a u s e t h e l i m i t of s u r v i v a l and colonization of the r e d m a n g r o v e seedling corresponds to the u p p e r s h o r e f a c e . T h e red mangrove peat transitional between f r e s h w a t e r a n d m a r i n e b a y f a c i e s is i n t e r p r e t e d as a r a v i n e m e n t s u r f a c e b e c a u s e it r e p r e s e n t s a slow reworking of a p r e v i o u s l y e x p o s e d i n t e r f a c e by an a d v a n c i n g sea. T h e p s e u d o b r e c c i a m a y be r e g a r d e d as a b i o g e n i c t r a n s g r e s s i v e conglomerate. Parasequences formed during a quick transgression are d i f f e r e n t from those developed during a slow transgression (Fig.52).The first (Fig.52A) occur at the bottom of the Fort Thompson Formation~ the s e c o n d (Fig.52B) in its u p p e r p a r t a n d in the n o r t h e r n s e c t o r , in the d e e p ramp.
Maximum
floodin~
surface
It c o n s i s t s of a c o r a l - b e a r i n g horizon (Fig.53) containing Montastrea annularis and Porites. It is interpreted as a maximum flooding surface because records the t i m e of m a x i m u m r a t e of a c c o m o d a t i o n increase: it r e c o r d s in fact a m a x i m u m deepening as s h o w n by t h e o c c u r r e n c e of fossils such as Montastrea annular~s and Porites which actually inhabit more open and deeper lagoonal areas (from -9 to -24 m below a n d the o t h e r b i o t a c o n t a i n e d sealevel) than Chione cancellata in t h e m a r i n e b a y f a c i e s .
Transgressive
facies
traqt
The transgressive facies tract, developed above the Tamiami Formation, consists of the stacking of transgressive parasequences (Fig.53,A).Their t h i c k n e s s r a n g e f r o m 3 - % m to a b o u t 1 . 2 m. The basal part is d e v e l o p e d above a transgression surface. Lithofacies are m o l l u s k g r a i n s t o n e s and packstones (marine bay Porites and Montastrea annularis. f a c i e s ) in s o m e c a s e s w i t h T h e p r o p o r t i o n a n d s i z e s o f m o l l u s k s m a y i n c r e a s e u p w a r d s u p to the h a l f of the m a r i n e lithofacies, with the production of a coarsening-upward grain size trend.
89
N 13
Hellsoma wackestone Poorly fossiliferous packstone Mollusk gralnstone and packstone
Fig,SA - Field aspect and lateral f l o o d i n g s u r f a c e ( p a r a s e q u e n c e #3).
variations
of
the
maximum
90
The lower part of the parasequence grades upwards into a strongly bioturbated packstone, as is e v i d e n c e d by sinuous galleries and shell debris-filled irregular patches. Going upwards, shell sizes d e c r e a s e . This part in turn g r a d e s to a (Helisoma wackestone).This freshwater swamp lithofacies transition is e v i d e n c e d by a m i x e d fauna containing marine m o l l u s k s and f r e s h w a t e r g a s t r o p o d s indicative for a brackish, t r a n s i t i o n a l m a r s h flat e n v i r o n m e n t , Vertical transitions from marine bay facies to freshwater lithofacies indicate an upward shallowing in a low-energy shoreline. The basal c o a r s e n i n g - u p w a r d g r a i n size t r e n d in the sequence may by interpreted as a catch-up phase of sedimentation (Kendall and Schlager,1981) during which carbonate sediment production i n c r e a s e takes p l a c e by v e r t i c a l g r o w t h as bars or p a t c h - r e e f s in o r d e r to keep pace w i t h the i n c r e a s e d rate of s e a l e v e l rise. The s u c c e s s i v e fining-upward trend may correspond to the keep-up phase (Kendall and Schlager,1981) that evidences for a lateral accretion or outbuilding of islands during a period characterized by a reduced rate of relative sealevel rise, or a sealevel stillstand. The transition to the freshwater unit may have been a consequence of an a c c e n t u a t i o n of the rate of s e a l e v e l fall during glacial periods. Barrier island faciess in the east d u r i n g these p e r i o d s b e c a m e e m e r g e n t and i n t e r r u p t e d the w a t e r e x c h a n g e w i t h the open sea, w i t h the c o n s e q u e n t t r a n s f o r m a t i o n of the bays into f r e s h w a t e r lakes s i m i l a r to those a c t u a l l y existing inh the Everglades and nearby coastal areas (cf. Fig.51). Trangressive facies tracts form a l o n g the steepest part of rising limbs of the relative eustatic curve ( Haq, et ai.,1987). This tract h e r e is e x p r e s s e d by the d e v e l o p m e n t of aggrading patch-reefs displaying coarsening-upward grain size trends which record deepenings caught-up by sedimentation, Freshwater facies capping marine lithofacies document the interference of h i g h e r order sealevel fluctuations with the lower o r d e r f r e q u e n c y s e a l e v e l rise. Correlations between cores mark a series of northward t h i c k e n i n g w e d g e s w i t h i r r e g u l a r o u t l i n e s f o r m e d by b u l g e s and d e p r e s s i o n s that r e f l e c t the c o n t r o l p l a y e d by the a n t e c e d e n t topography in the deep and s h a l l o w ramp. The Fort Thompson Formation is a northward and eastward thickening wedge (Fig. A 2 , F i g . 5 5 ) . T h e reconstructions made by Parker,at al. (1955), P e r k i n s (1977) and C a u s a r a s (1987) clearly show a
9]
A
A I
lake Okeechobee
Q5 Q4 03
lake Okeechobee Q2
Q1
S O_
M i a m i Oolite
"?.
o .O
E
--.-....__
10-
:Z I-a. I.u
Q
20-
Tamiami
30
A 33
Formation
l 29
, . I .. 23
I
]
~
r
19
13
7
3
CORES 0
8 Km
F i g . 5 5 - 0 n l a p ramp g e o m e t r i e s of the F o r t T h o m p s o n Formation (above : from P e r k i n s , 1977) showing southward and eastward thickenin~ wedges and the control played by the antecedent topography in the t r o u g h and h i n g e a r e a s . gradual thinning-out and disappearing of p a r a s e q u e n c e s towards a h i n g e l o c a t e d in the w e s t and s o u t h that i m p l i e s a r o t a t i o n a l subsidence on a w e s t e r l y s t r i k i n g h i n g e . The
rate
temporally,
of but
relative also with
sealevel change varied not only the p o s i t i o n on the p l a t f o r m , due to
92
variations in space and time of the amount and rate of rotational subsidence. T r a n s g r e s s i v e p a r a s e q u e n c e s were formed by a r o t a t i o n on the hinge: e a c h r o t a t i o n e p i s o d e led to the f o r m a t i o n of a t r a n s g r e s s i v e surface. Biological r e w o r k i n g of the s h o r e f a c e by m a n g r o v e s was p r e v e n t e d by the r a p i d i t y of the rate of s u b s i d e n c e in such a way that the shoreface was overstepped. In the north, the space a v a i l a b l e for s e d i m e n t a t i o n during a r e l a t i v e rise was f i l l e d by c a t c h - u p reefs. Sites i n t e r m e d i a t e b e t w e e n the n o r t h e r n and the s o u t h e r n area underwent a lesser amount of rotational subsidence, and s e d i m e n t a t i o n o u t p a o e d the r e l a t i v e rise and m i g r a t e d l a t e r a l l y over coeval c a t c h - u p reefs. Close to the h i n g e line, the rate of r o t a t i o n a l s u b s i d e n c e was at a m i m i m u m ; therefore, sediment c o u l d keep pace with the relative rise of the sealevel. This led to repeated a m a l g a m a t i o n s of p a r a s e q u e n c e s . The two mechanisms: aggradation in the north and lateral sediment shift in the s o u t h , l e d to d i f f e r e n t c o n t r o l s by the predepositional topography. In fact, a s t r o n g c o n t r o l can be recognized in the deep ramp, w h e r e areas of d e v e l o p m e n t of patch-reefs and bars maintain a topographic contrast with a d j a c e n t a r e a s a l o n g the e n t i r e d e p o s i t i o n a l sequence. In the south, the c o n t r o l is less e v i d e n t and d e p o c e n t e r s oscillate r a n d o m l y from one a r e a to another. These two s i t u a t i o n s are a l s o e v i d e n t from an e x a m i n a t i o n of the b l o c k - d i a g r a m s of F i g . 5 6 which show variations of the u p p e r m o s t l i t h o s o m e g e o m e t r i e s w i t h time, The r a n d o m o s c i l l a t i o n of d e p o c e n t e r areas in the s h a l l o w ramp is a n a l o g o u s to the i r r e g u l a r island shift p r e d i c t e d by the "tidal island facies m o d e l " of P r a t t and J a m e s (1986). The r e s u l t i n g p a t t e r n a l o n g an i s o c h r o n o u s stratigraphic interval is a lateral t r a n s i t i o n f r o m c o a r s e n i n g - u p w a r d to f i n i n g - u p w a r d beds that is also demonstrated by symmetrical sequences developed in i n t e r m e d i a t e positions by a migration of the fining-upward beds o v e r c o a r s e n i n g - u p w a r d beds (according to W a l t h e r ' s law).
Earlx,
highstand
facies
trac%
This tract, developed above the maximum flooding surface, consists of three early highstand parasequences (Fig.53,B)
93
o r g a n i z e d into a v e r t i c a l t h i n n i n g - u p w a r d t r e n d ( F i g . 4 9 ) . I n the basal part of these parasequences mangrove peat facies (ravinement surfaces) are interposed between underlying f r e s h w a t e r facies and m a r i n e lithofaoies. The m a r i n e bay facies contains scattered shells of Chione cancellata, fine shell d e b r i s and several root traces. This l i t h o f a c i e s g r a d e s u p w a r d s into the root rock facies and to a l i t h o f a c i e s containing a m i x e d m a r i n e and f r e s h w a t e r fauna, i n d i c a t i v e for a b r a c k i s h environment~ less commonly, the sequence is topped by s u p r a t i d a l laminites. In c o n t r a s t to the t r a n s g r e s s i v e parasequences, these record gradual flooding rates (ravinement surfaces) and gradual sealevel falls ( transitions from catch-up reefs --> m a r s h flats--> freshwater swamp). The o c c u r r e n c e of a m a r s h flat indicates that sedimentation could outpace the relative s e a l e v e l rise as a result of its s l o w i n g down. The lower parts of these p a r a s e q u e n c e s ( mangrove peat-->marsh flat-->marine bay) are analogous to the sequence recently described by Parkinson (1989) from the southwest Florida platform which consists of 2 to 6 m thick transgressive r e g r e s s i v e c o u p l e t s (paralic swamp--> r e s t r i c t e d m a r i n e - - > o p e n marine) b o u n d e d by m a n g r o v e peat facies. It is a l s o a n a l o g o u s to the cycle of F l o r i d a Bay ( Enos & P e r k i n s , 1 9 7 9 ) . As s t r e s s e d by G o l d h a m m e r , e t ai.(1990), these cycles result to be c o n t r o l l e d by 5th order sealevel fluctuations (rapid rise followed by a decelerating sealevel rise). The similar lithofacies and p a t t e r n s of c h a n g e s recorded in the early h i g h s t a n d p a r a s e q u e n c e s suggest an e v o l u t i o n s i m i l a r to that of F l o r i d a Bay and s o u t h w e s t F l o r i d a p l a t f o r m . This part of the d e p o s i t i o n a l s e q u e n c e c o n t a i n s e v i d e n c e for a eustatic control on sedimentation.Based on calculations by M i t t e r e r (1975), the u p p e r m o s t two p a r a s e q u e n c e s result to have formed in 2 5 . 0 0 0 years (Galli,1991) which approximately fit M i l a n k o v i t c h p r e c e s s i o n cycles. S u b s i d e n c e g r a d u a l l y d i m i n i s h e d d u r i n g the d e p o s i t i o n of this facies tract~ so that m a r s h flats c o u l d d e v e l o p and e u s t a t i c effects, superimposed on t e c t o n i c s u b s i d e n c e , became more evident h i g h e r up the Formation.
94
6
/
3
.,~, oo~,~
/\
3." ~ 3 0
22
3
18
19
13 7
.,"~--'-~/.~ ~
28
22 ~
6
3
Fig. 56 Block-diagrams showing variations in volume and m o r p h o l o g y of p a r a s e q u e n c e s . A lack of p r o ~ r a d a t i o n is a p p a r e n t in the hinge, in the south, whereas a southward shift of depocenters (retrogradation) c h a r a c t e r i z e s the trough.
95
Late
hi~hstand
facies
tract
T h i s f a c i e s t r a c t is c o n s t i t u t e d by a p a r a s e q u e n c e which varies in t h i c k n e s s f r o m a dm to A m. (Fig.57). The v e r t i c a l succession of l i t h o f a c i e s w i t h few c h a n g e s is the s a m e at all sites. The sequence passes from a laminar m i c r i t e h o r i z o n to a p s e u d o b r e c c i a , to the r o o t r o c k f a c i e s a n d eventually to the freshwater phytogenic breccia which is commonly the thickest lithofacies. The upper part of this parasequence may contain oolites or m u d s t o n e s typical of the overlying Miami Oolite Formation (Evans and Ginsburg,1987) which may correspond to a s h e l f m a r g i n f a c i e s t r a c t . A s i m i l a r , m o d e r n s e q u e n c e of s e d i m e n t s p a s s i n g from mud banks to f r e s h w a t e r s w a m p f a c i e s was d e s c r i b e d by C r a i g h e a d (1969), f r o m the E v e r g l a d e s National Park (Fig.16),who s h o w e d that the Everglades has p r o g r a d e d seaward more than 8 Km, d u r i n g the H o l o c e n e s e a l e v e l rise. The late highstand parasequence is interpreted as a shallowing-upward sequence f o r m e d by a d e p o s i t i o n a l regression in r e s p o n s e to a s e d i m e n t a t i o n rate in e x c e s s of t h e r e l a t i v e sealevel rise. Lateral variations in thicknesses of the parasequence d e p e n d e d on local, f o r m e r d e p t h s of the l a g o o n a l floor: in fact, thicknesses are g r e a t e r in the d e e p r a m p a n d l i m i t e d to a f e w dm in t h e s h a l l o w ramp. By a n a l o g y w i t h the m o d e r n c y c l e d e s c r i b e d by C r a i g h e a d (1969) the p a r a s e q u e n c e records a seaward shoreline progradation of a mainland produced during a declining sealevel rise, as is a l s o supported by the v e r t i c a l transitions between mangrove and freshwater lithofacies. T h e late h i g h s t a n d facies tract records the m a x i m u m rate of a c c o m o d a t i o n decrease and a continuous shallowing-up. It v e r t i c a l l y g r a d e s to the M i a m i O o l i t e w h i c h records a change from a eustatic fall to a s l o w r i s e in the sealevel. The progressive decrease in subsidence led to a progressive decrease in the control played by the predepositional topography: eventually, the d e p o s i t i o n of the parasequence formed uniquely under eustatic controls w h i c h led to a l e v e l l i n g of the t o p o g r a p h i c r e l i e f , w i t h the f o r m a t i o n of a compensation c y c l e (cf. E n o s & P e r k i n s , 1 9 7 9 ) .
96
•
.+
~ "+c
+
'~
o"-++J., ~ ' o+-~-~+.......... bird's eye ~.+~]+:.
,~.-. ++-~, •
•+/
I
' . ~ ' t ] ~ O . ~ . ~ ,%........ black
~'+'/~.'"'+
:" (~
........ wood
~> ..........
pebble
Freshwater peat
fragment
Helisoma
,.,;~.~-~o.+++......mangrove
+~>~;++.~..1 ......roo,,o+~
I
~C~-.~..'-J
Mangrove peat ......... pseudobreccia
,++~++++++~(
~--~'~t...-..
Fig,57
- Late
highstand
.......laminar mi~rite
facies
tract,
"Calcari Grigi" Formation, Jurassic, Venetian Alps
Introduction
The J u r a s s i c C a l c a r i Grigi F o r m a t i o n crops out in the V e n e t i a n Alps, Italy, at the top of the T r e n t o p l a t f o r m (Fig.58), a rimmed, isolated platform, 8000 s q u a r e Km wide, which is a p o r t i o n of an A t l a n t i c - t y p e m a r g i n s u b j e c t e d to d r o w n i n g as a result of a c o m p l e x i n t e r p l a y of e u s t a t i c p h a s e s (Bernoulli and Jenkins,197A~ Hallam,1978, 1981~ Vail and Todd,1981), s u p e r i m p o s e d on s y n s e d i m e n t a r y t e c t o n i c s (Castellarin, 1972). The e u s t a t i c c u r v e s by H a l l a m (1978) and Vail, et a i . ( 1 9 7 7 ) show a c o n t i n u o u s rise in the Jurassic. M a j o r rises took p l a c e in the H e t t a n g i a n , late Sinemurian, Pliensbachian, and at the b e g i n n i n g of the T o a r c i a n . Conversely, s e a - l e v e l falls were of less i m p o r t a n c e (Hallam,1981). It is d i f f i c u l t to a s c e r t a i n the role p l a y e d by t e c t o n i c s in the s e a - l e v e l rise. M a j o r facies c h a n g e s were p r o d u c e d by a platform collapse in the european margin (Bernoulli & J e n k i n s , 1 9 7 4 ) . The d e s t r u c t i o n of the c a r b o n a t e p l a t f o r m b e g u n in the lower Liassic (Lemoine, et ai.,1978) but continued diachronously in the southern Alps. The destruction of the T r e n t o p l a t f o r m p r o b a b l y i n i t i a t e d at a later time ( W i n t e r e r & Bosellini,1981) , at the end of the m i d d l e L i a s s i c (179 m.a.). Listric synsedimentary faults oriented NNE-SSW producing h a l f - g r a b e n s t r u c t u r e s , were m a p p e d a l o n g the w e s t e r n m a r g i n of the T r e n t o p l a t f o r m (Castellarin, 1972). A non-isostatic subsidence antecedent to the d r o w n i n g of the platform is e v i d e n c e d in the study a r e a by n u m e r o u s features (Fig.59): synsedimentary faults~slumping features,crumpled b e d s , a n d t s u n a m i t e s (Galli,1990), S y n s e d i m e n t a r y faults w h i c h were m a p p e d in the area, p r o ~ e o t e d on a S c h m i d t net (Fig.60) intersect in the SW q u a d r a n t and individuate a potential failure wedge oriented SSW. The NE q u a d r a n t c o n v e r s e l y m a y r e p r e s e n t a t e c t o n i c a l l y r e l i e v e d area. This structural frame controlled bathymetry and facies ~ distribution.
98
J
J Quaternarycover ~
~
Jurassic-Cretaceous pelagic deposits ~
CalcariGrigiFormation Triassic
O slratlgraphicsections
[]*-
F TRENTOPLATEAU-'1
~
ENTo
PLATEAU
99
W Basso
E Asiago
Sarca
Cen°m 1 Cret,
SerriasJ I1~-.
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Titon. !
Kimm
.,..°o.E
I~--
Saioc. :'t~-I Aalen. I] i
Giur. $Up
AMMONITICO Rosso A
/1'1 - ~
ca,,. I ~ s,,o. t 4 U ~
0 -220m
20m
•
0 -9
AMM. INF. j - " ° s s ° A~ MACHELLA -~Lo.,c.E
Toa rc. I
~ ALPINA .... ALr'=r~ '
P,//
A 2m / /
OLITE DI SAN / IGILIO / 0 - lOOm /
Glut. Upper Membl ( Membro di Ro 85m
[
med.
Giur. .,°'
i.f. Midlle Memb 35m
PI;ensb.I Lower Membe 40 m
DOLOMIA PRINCIPALE 700 - 1200 m
Trlas sup.
Slnemur.f
F i g , 5 8 - L o c a t i o n of Left: f r o m B e r n o u l l i (1981),
the study area, Above: from G~hner a n d J e n k i n s (197A) a n d B o s e l l i n i ,
Study
The in
(1981), et al,
area
s t u d y a r e a is 20 x 20 K m w i d e , It the center of the platform,
is s i t u a t e d a p p r o x i m a t e l y The thickness of the
100
Fig,59 - Features indicative for t e c t o n i c instability in t h e Trento platform, A~D~G~E~F Crumpled beds and deformation features, B Intraformational discordance. C Small-scale synsedimentary fault,G Panoramic view (locality section #4-14) showing a deformed stratigraphic horizon,
101
stratigraphic interval ranges from about 20 to 60 m,The i n v e s t i g a t i o n was c o n d u c t e d on the u p p e r part of the 'Calcari Grigi' F o r m a t i o n (upper part of the Rotzo M e m b e r a c c o r d i n g to the local stratigraphic t e r m i n o l o g y ) , w i t h i n the O r b d t o p s e l l a p r a e c u r s o r z o n e , b y means of facies a n a l y s i s of 32 s t r a t i g r a p h i c s e c t i o n s a m o u n t i n g to about 800 m. Host of s t r a t i g r a p h i c sections were c o r r e l a t e d by means of 'event c o r r e l a t i o n ' by u s i n g p h y s i c a l s u r f a c e s as time lines (for example, a triple d i s c o n f o r m i t y : Riding & Wright,1981, a dm-thick level containing radial oolites; tsunami-generated h o r i z o n s ) . T h e s e s u r f a c e s p r o v i d e d a few t r a n s e c t s w h i c h a l l o w e d for the subdivision of the stratigraphic columns into isochronous units, successively r e l a t e d to d i f f e r e n t system tracts.
Previous
studies
Various aspects of the 'Calcari i n v e s t i g a t e d by a n u m b e r of authors.
Grigi'
Formation
were
The s t r a t i g r a p h y and regional g e o l o g y r e c o n s t r u c t i o n s were made by Venzo (1963),Auboin,et ai.(1965), Castellarin (1972), Bosellini (1973a,b), B e r n o u l l i & J e n k i n s (197A), Winterer & B o s e l l i n i (1981) and B a r b u ~ a n i , et ai.(1986). Most of the work on the 'Calcari Grigi' F o r m a t i o n has been c o n c e r n e d with p a l e o n t o l o g y (Parona,192~9 Fabiani & T r e v i s a n , 1939; Wesley, 19569 V e n z o , 1 9 6 3 ) . S e v e r a l of these p a p e r s are c o n c e r n e d w i t h the d e s c r i p t i o n and i n t e r p r e t a t i o n of L i t h d o t i 8 shells, a huge m o l l u s k w h i c h is p a r t i c u l a r l y a b u n d a n t in the s t u d y area: Berti Cavicchi, et ai.(1971), Bosellini (1972), Benini & B r o g l i o L o r i g a (197~), B r o g l i o L o r i g a & Neri (1976), A c c o r s i Benini & B r o g l i o Loriga (1977), G e y e r (1977), A c c o r s i Benini (1979). The s e d i m e n t o l o g y was i n v e s t i g a t e d by V e n z o (1963), Fusanti (196&), Fuganti & M o s n a (1966), B o s e l l i n i & B r o g l i o Loriga (1971), C a s t e l l a r i n (1972), C a s t e l l a r i n and Sartori (1973a,b), Clari (1975), G ~ h n e r (1980,1981) and Galli (1990).
Facies
The the
associations
lithofacies distribution is far more c o m p l i c a t e d than in p r e v i o u s case h i s t o r y . A p u z z l i n g f e a t u r e of the 'Calcari
102
N N
Fig.60 Equiareal synsedimentary faults
pro~ection in the s t u d y
Grigi' Formation is lithofacies, grouped associations: shallow
ramp
intermediate wackestones, deep
ramp
(oncolite
of area.
the complex here into
packstone,
planes
alternation of the following
wackestones
ramp (oolite packstones and grainstones and packstones);
(thick
Lithiotis
of
and
dip-slip
several facies
grainstones);
grainstones; and
bioclast
banks).
Lithiotis shells are ubiquitous ramp; t h i c k b a n k s as m u c h as f o r m e d in t h e d e e p ramp.
9
and m
occur thick
in all s e c t o r s of the are t h o u g h t to h a v e
The different bedding s t y l e s of the t h r e e s e c t o r s of the r a m p (alternations of m e d i u m and thin beds in the shallow ramp; m e d i u m a n d t h i c k b e d s in t h e i n t e r m e d i a t e ramp and thick beds in the d e e p ramp) c a n be a p p r e c i a t e d on o u t c r o p and panoramic views (Fig.61),
Shallow
0ncolite
grainstones
and
ramp
packstones
Description This
lithofacies
consists
of
m-thick
grainstone
-
packstone
103
beds containing abundant coated grains floating in a m a t r i x constituted by a p o o r l y sorted admixture of bioclasts and l i t h o c l a s t s . S e d i m e n t a r y s t r u c t u r e s a r e t r a c e s of c r o s s b e d d i n g , rare dessication and subaerial features and channel-fills. C o a t e d g r a i n s o c c u r as s u r f i c i a l a n d l a r g e o n c o i d s ('macroids' according to P e r y t , 1 9 8 2 ) . Surficial oncoids average 3 m m in diameter, have an intraclast/bioclast, monomictic core, a clastic texture and elliptical to o v o i d a l shapes. 'Macroids' r e a c h 2 cm in d i a m e t e r ; often they have a polimictic core, a c l a s t i c to b o t r y o i d a l t e x t u r e a n d a c r u s t o s e to o v a l s h a p e . This lithofacies commonly f o r m s 0.5 m t h i c k , c o a r s e n i n g - u p w a r d cycles, developed above wackestones. These are characterized by the following vertical trends: a gradual increase in g r a i n size, grain abundance and percentage of surficial oncolites (10-25%), and sparry cement. Scours, infilled with coated grains and bioclasts (brachiopods, bivalves, gastropods and Lithiotis fragments) occur in the uppermost part. At few localities t h e t o p s of o n c o l i t e - r i c h beds are mud-cracked, or constituted by thin, r e d d i s h h o r i z o n s e n r i c h e d in l i t h o c l a s t s , or also by p a r a l l e l laminated, dm-thick, yellow, silt-size sands.
Interpretation In t h e s t u d y a r e a o n c o l i t e b e d s a r e o f t e n a s s o c i a t e d in s p a c e with Lithiotis banks and oolite beds. They represent a deposition in i n t e r b a n k , i n t e r b a r a r e a s or p o n d s l o c a t e d in the o u t e r m o s t p a r t of the s h a l l o w ramp. T r a c e s of s c o u r i n g r e f l e c t episodic mechanical reworking. Oncolite deposition is a n i n d i c a t o r of b r e a k s or s l o w i n g down in the r a t e of s e d i m e n t a t i o n , in a n e a r s h o r e or v e r y s h a l l o w environment ( W e i s s , 1 9 6 9 ) . T h e o c c u r r e n c e of l a r g e q u a n t i t i e s of large oncoids within a mudstone lithofacies was taken by C a t a l o v (1983) as an e v i d e n c e for low r a t e s of s u b s i d e n c e . Coarsening-upward cycles are interpreted as s h a l l o w i n g - u p w a r d sequences related to an upward, gradual decrease in sedimentation rate following a relative fall of the s e a l e v e l . Macroids may reflect t h e o n s e t of h y p e r s a l i n e conditions and local exposures, when associated with mudcracks, reddish horizons and keystone vugs.
104
Bioclast-lithoclast
~rainstones
and p a c k s t o n e s
Description This lithofacies is r e p r e s e n t e d by m a s s i v e to thin bedded, intraclast-bioclast grainstones and p a c k s t o n e s containing in a d d i t i o n to l i t h o c l a s t s v a r i a b l e p e r c e n t a g e s of c o a t e d grains, peloids and bioclasts (bivalves, foraminifers, Lithiotis, algae, crinoids, etc.), often enveloped by algal coatings. Oolites are rare. Lithoclasts are both r o u n d e d and angular, l i g h t - g r a y to r e d d i s h in color. Examples of bedding styles, thick-bedded and thin-bedded alternations are s h o w n in F i g . 1 4 , 1 5 and 17. The most typical s e d i m e n t a r y s t r u c t u r e is g i v e n by scours r a n g i n g in a m p l i t u d e from a few dm to I0 m. Some beds may r e s u l t from m e r g i n g and s t a c k i n g of c h a n n e l structures w h i c h do not e v i d e n c e for any facies v e r t i c a l t r e n d (Fig.17). A common sequence encountered in this lithofacies ranges between 0.5 and 1.5 m in t h i c k n e s s and consists of three units.The basal part has an e r o s i o n a l base, a disorganized m a s s i v e bed c o n t a i n i n g fossils w h i c h in some cases are d i s p o s e d in p a r t i n g lineations,traces of h u m m o c k y cross-bedding and undulations of uncertain origin. This basal part appears structureless when the composition and grain size are h o m o g e n e o u s . T h e basal part passes up g r a d a t i o n a l l y to a b e t t e r sorted unit characterized by intrastratal, scalloped u n d u l a t i o n s and v a r i o u s types of w a v e - g e n e r a t e d s t r u c t u r e s such as c l i m b i n g - w a v e r i p p l e l a m i n a t i o n ( K r e i s a , 1 9 8 1 ) , or very thin, slightly undulated, flattened plane lamiantion. Traces of ripples are also p r e s e n t with amplitudes of about 8 cm and heights of 4 cm. The upper unit consists of well sorted i n t r a s p a t i t e s o r g a i n z e d into laminae, i n t e r b e d d e d in some case w i t h lime m u d s t o n e s . This c y c l e records an o v e r a l l g r a i n size fining-upward
trend.
Interpretation This
lithofacies
reflects
a
deposition
in
a
shallow
lagoonal
environment. The c y c l e is a n a l o g o u s to i n t e r p r e t e d n e a r s h o r e storm d e p o s i t s d e s c r i b e d by K r e i s a ( 1 9 8 1 ) , B r e n c h l e y & N e w a l l (1982), K u m a r & S a n d e r s (1976), M o u n t (1982), and others. T h e y are i n t e r p r e t e d as t h i c k - b e d d e d alternations (see above). The tops of these cycles c h a r a c t e r i z e d by thin s t o r m - g e n e r a t e d beds a l t e r n a t i n g with fairweather muds are interpreted as thin-bedded alternations
105
Lime
mudstones
Description T h i s l i t h o f a c i e s o c c u r s as t h i n i n t e r b e d s w i t h o t h e r f a c i e s . It overlies subaerial surfaces or m a r i n e unconformities. It is also frequently sandwiched between marine lithofacies. It c o n s i s t s of b l a c k , calcareous, clayely deposits containing at p l a c e s a b u n d a n t p l a n t d e b r i s a n d m o r e r a r e l y t r a c e s of s u l p h a t e minerals. Thicknesses average 1 0 - 2 0 cm. T h e y f o r m t h i n - b e d d e d alternations with homogeneous, dm-thick mudstones containing rare ostracods. These interbeds are completely lacking of macroskeletal constituents. Similar lithofacies i n v e s t i g a t e d by Castellarin and Sartori (1973) in a n e a r b y location revealed that the mud contains traces of i l l i t e and hematite, and/or goethite ,quartz and chlorite. Interpretation This lithofacies was interpreted as a mud flat or marsh environment (Bosellini & Broglio Loriga,1971). Marl deposits form in a n u m b e r of d i s t i n c t subenvironments, which may be c o l o n i z e d by a d e n s e v e g e t a t i o n . They were described from shallow ponds ,coastal marshes, and freshwater lakes. In the ponds and lakes, such as the E v e r g l a d e s , m a r l s o c c u r at the b a s e of a t r a n s g r e s s i v e sequence as the s e a l e v e l r i s e s a n d as a r e s u l t of a p r o g r e s s i v e r i s e of the f r e s h w a t e r lens ( M o n t y a n d H a r d i e , 1 9 7 6 ) . It m a y a l s o f o r m at the top of a r e g r e s s i v e sequence w h e n the m a r s h p r o g r a d e s over a retreating shoreline, as o c c u r s on the e a s t e r n h a l f of Andros Island,Bahamas. Marls also form over exposed surfaces during prolonged periods of lowstand. Coastal marshes can represent the transitional zone between freshwater marls and marine calcareous mud. In t h e s e cases, intrusions by m a r i n e sediments during storms produce interbedded freshwater and marine sequences (thin b e d d e d a l t e r n a t i o n s ) .
Intermediate
Oolite
srainstones
and
ramp
packstones
Description On o u t c r o p
this
lithofacies
is
a
massive
bedded,
light-gray
to
106
creamy, homogeneous packstone and grainstone containing concentrical, tangential colds (amounting to a b o u t 30-40%). Wackestones a r e m u c h less c o m m o n . This lithofacies is m o s t w i d e s p r e a d in the T r e n t o p l a t f o r m a n d in its t o p m o s t p a r t .
eastern
area
of
the
Two lithofacies can be distinguished (Fig.22). The first, p o o r l y s o r t e d , is c o m p o s e d of s u r f i o i a l o o l i t e s a n d l u m p s a n d a m i x t u r e of i n t r a c l a s t s , peloids and coated grains, other than s e v e r a l t y p e s of s k e l e t a l g r a i n s s u c h as f o r a m i n i f e r s , o s t r a c o d s and algae which constitute the n u c l e i of ooids. The second lithofaoies,less common,consists of well sorted oolite grainstones. Sedimentary structures are large-scale hummocky cross-bedding, rare tabular cross bedding, symmetrical megaripples and horizontal lamination. Smaller scale structures include flute casts, scours infilled with mud and coated grains, and dubious load casts. Half-cm thick, lenticular coquinites composed of densely packed, imbricated, both articulated and disarticulated s h e l l s of b i v a l v e s a r e f o u n d f r e q u e n t l y intercalated with some of the t h i c k e r b e d s . G r a d e d b e d s a r e c o m m o n . S o m e o u t c r o p s s h o w upward transitions f r o m t h e p o o r l y s o r t e d , to the w e l l s o r t e d oolite
lithofacies.
In the s t u d y a r e a t h i s it m o s t l y o c c u r s o n top
lithofacies of s h o a l s .
has
a
patchy
distribution:
Interpretation These massive bedded, oolite grainstones and packstones are interpreted as shoals, banks and sandwaves situated in a storm-dominated area, at a shallow-water depth. Storms were mainly responsible for the cold migration. Oolitic sand shoals are found actually along the edges of several Bahama areas (i.e. Cat Cay, Joulters Cay, Berry Islands, etc.). They occur as I) n a r r o w , active cold shoals marginal to the o p e n sea; a n d 2) as s t a b i l i z e d cold-aggregate grains-pelletal sands flats forming widespread blanket sheets behind active sand shoals and grading to other platform sediments (Multer, 1977). Skeletal admixtures are greatest in the d e e p e r s i t e s . These two modern well sorted and
sediment types correspond respectively to the poorly sorted oolite lithofaoies. Transitions
107
from the p o o r l y sorted to the well s o r t e d l i t h o f a c i e s indicate a shallowing of the s e d i m e n t a r y interface consequent to a d e p o s i t i o n a l r e g r e s s i o n (of, Van S t e e n w i n k e l , 1 9 9 0 ) .
Skeletal
wackestones
Description These places
thin-medium bedded, light-gray wackestones contain at a b u n d a n t , thin shells of b i v a l v e s (Pholadomia, Gresslya, pectinidae) and m i n o r q u a n t i t i e s of g a s t r o p o d s . Other skeletal constituents are thin-shelled brachiopods, crinoids (Isochrinus), foraminifers (Paleodasycladus, Orbitopsella occasional Lithiotis and other undetermined praecursor), microfossils. Small, reddish intraclasts are found n e a r the base of some of the less f o s s i l i f e r o u s beds, Coated grains o c c u r i n f r e q u e n t l y t h r o u g h o u t this lithofacies,
A v a r i e t y of this l i t h o f a c i e s is c o n s t i t u t e d by thin beds (I0 20 cm thick) of p o o r l y fossiliferous, dark-colored mudstones w i t h i n t e r s p e r s e d s a n d - s i z e grains. This l i t h o l o g y is t y p i f i e d by n o d u l a r i t y which gives way to p s e u d o b u d i n s , and lensoid n o d u l e s w i t h l a m i n a t e d clay s e a m s . B i o t u r b a t i o n is d o m i n a t e d by Thalassinoides burrows w h i c h may have c o n t r i b u t e d t o g e t h e r w i t h p r e s s u r e s o l u t i o n to the f o r m a t i o n of the n o d u l a r i t y . Sedimentary structures c o n s i s t of irregular, erosional scours (a few cm to some dm wide), symmetrical megaripples, gutter casts and hummocky cross-bedding. Fenestrae are rare. Coquinites are composed of I) gastropod streaks forming pebble-cluster a l i g n m e n t s p a r a l l e l or d r a p i n g the s y m m e t r i c a l u n d u l a t i o n s , and, more commonly, of 2) lenses of b i v a l v e s that are e s s e n t i a l l y t h i n - s h e l l e d , of the same size, c o n v e x - s i d e up and d r a p i n g the topsets of s y m m e t r i c a l megaripples. Few of these lenses are o r g a n i z e d into 20 cm thick fining-upward cycles composed of: I) a lower grainstone-packstone unit consisting of r a n d o m l y oriented bivalves; 2) a t h i n n e r unit with c o n v e x - u p shells; and 3) an u p p e r mud r i p p l e d top or an argillaceous, yellow cm-thick horizon. Multistored, complex lenses containing coquinites are volumetrically less r e p r e s e n t e d than w a c k e s t o n e layers.
Interpretation This lithofacies oolite grainstone
was deposited at a d e e p e r depth than the and p a c k s t o n e lithofacies, as is s u g g e s t e d by
108
Fi~,61 Beddins styles of deep ramp (A:thick banks), intermediate r a m p ( B ; t h i c k a n d t h i n b e d s ) a n d s h a l l o w r a m p (C: thin beds),The panoramic v i e w of M , T e s t o (D9 l o c a l i t y s e c t i o n s #15 and 28) shows a transition from shallow to deep ramp e v i d e n c e d by an u p w a r d i n c r e a s e in b e d t h i c k n e s s and declivity (a h i ~ h e r e r o s i o n in the s h a l l o w r a m p is f a v o u r e d by f r e q u e n t intercalations of lime m u d s t o n e s ) ,
109
the t r a n s i t i o n and lateral c h a n g e s to Lithiotis w a c k e s t o n e s . Similar bathymetric relationships b e t w e e n skeletal and o o l i t e beds occur in m o d e r n areas, for e x a m p l e at Lily B a n k (see also Hine,1977: Fig.2~) w h e r e s k e l e t a l w a c k e s t o n e s o c c u r in deeper, s e a w a r d sites (-5 to -I0 m b e l o w s e a l e v e l ) . This facies formed as lime mud thickets. Intense b i o t u r b a t i o n in more p r o t e c t e d , less p o p u l a t e d by Thalassinoides o c c u r r e d Thalassinoides burrows indicate however areas. Truncated e p i s o d i c erosion, as is s u p p o r t e d by the o c c u r r e n c e of o t h e r storm-generated structures, such as hummocky cross-bedding, wave m e g a r i p p l e s , c o q u i n i t e s , etc.
Deep
ramp
Lithiotis w a c k e s t o n e s Description
Lithiotis w a c k e s t o n e s are w i d e s p r e a d in the T r e n t o p l a t f o r m . They are t y p i f i e d by thick beds (I to 9 m) of t i g h t l y p a c k e d problematica, accumulations of huge pelecypods ( Lithiotis Cochlearites loppianus, Lithopedalium, Gervilleioperna, etc.) as much as A0 cm long and e m b e d d e d in a w a c k e s t o n e matrix. Other fossil types such as crinoids, corals, brachiopods, algae, (Orbitopsella praecursor, Glomospira,Textularia) foraminifers and sponge spicules are only accessory components, as this biofacies represents a suspension feeder, olygotypic a s s o c i a t i o n (Broglio L o r i g a & N e r i , 1 9 7 6 ) . Lithiotis display various fabrics: vertical, Shells of fanning-upward clustering, imbricated, wave knitted (bidirectional shell orientations). The u p w a r d decreases in shell sizes result in a f i n i n g - u p w a r d trend. Sedimentary structures are various types of scours and undulated bedforms (Galli,1990). In this s e c t o r of the ramp Lithiotis alternate with skeletal thick beds composed of wackestones (trough sequences: F i g . 2 3 D ; F i g . 6 1 A ) .
Interpretation Thick
banks
represent
a deposition
in d e e p e r
areas
of the
ramp
II0
111
AXES OF SCOURS
Km 0
I
2
3
4
~=
f
112
f~
J
SYMMETRICAL RIPPLES"
LITHIOTIS SHELLS ~-2~"~
\
113
Fig.62 - Contour maps and paleocurrent data of the study area. The isocoquinite map results from contouring sites characterized by the same n u m b e r s of c o q u i n i t e lenses; the v a l u e s w e r e o b t a i n e d by d i v i d i n g the n u m b e r of o o q u i n i t e lenses within skeletal wackestones by the thickness of skeletal wackestones occurring in s t r a t i g r a p h i c sections.The frequency of c o q u i n i t e lenses d e c r e a s e s t o w a r d s n o r t h e a s t , w h i c h is the s h a l l o w e s t s e c t o r of the ramp (of. the p r o x i m a l i t y distality concepts shown schematically in Fig.15).The isopac map of Lithiotis banks also reflects changing water depth because t h i c k beds of t i g h t l y p a c k e d a c c u m u l a t i o n s of big shells of Lithiotis problematica GOmbel took p l a c e in the deep r a m p . A n examination of the c o n t o u ~ maps r e v e a l s the e x i s t e n c e of an elongated lagoonal depression oriented NE-SW. The lagoonal floor was uneven due to the development of an array of shoals.Directional data i n d i c a t e a d o m i n a n t rotary h i g h - e n e r g y path oriented SW-NE driven by the lagoonal corridor c o n f i g u r a t i o n and a minor mode, o r i e n t e d S E - N W w h i c h indicates c u r r e n t s f l o w i n g o b l i q u e to the lagoonal c o r r i d o r . M e a s u r e m e n t s from shell imbrications and p a r t i n g lineations suggest that c u r r e n t s m o v e d from SW to N E . A r e f r a c t e d wave p a t t e r n w h i c h d i s p l a y s a c o u n t e r c l o c k w i s e sense of r o t a t i o n p r o b a b l y r e s u l t e d from the i m p i n g e m e n t of the S W - N E o r i e n t e d c u r r e n t upon shoals l o c a t e d in the east. P r o b a b l y most of c u r r e n t s and waves were p r o d u c e d by t s u n a m i s ( of. G a l l i , 1 9 9 0 ) . A s suggested elsewhere ( G a l l i , 1 9 9 0 ) , t h e s t o r m s y s t e m , r a t h e r than a c t i v e l y t r a n s p o r t i n g sediment, determined near-bottom oscillating currents acting through strongly pulsating bursts of e n e r g y . S t r o n g pressure p u l s e s on and b e l o w the lagoonal floor and s t r o n g shear stress produced an 'in situ' reorientation od shells. Large-scale b e d f o r m s , n o t d e s c r i b e d in this work, were p r o b a b l y g e n e r a t e d by tsunamis, as s u g e g s t e d by their formation by the a c t i o n of s u r f a c e waves, a great lateral extent of e x p o s u r e s and t h e i r r e s t r i c t i o n to the same s t r a t i g r a p h i c h o r i z o n s (for d i s c u s s i o n see Galli,1990). Earthquakes within the platform may have produced sudden oscillations of w a t e r w h i c h incorporated the w h o l e w a t e r column.
Fig,63 - North-south cross s e c t i o n showing the w e d g e - s h a p e d g e o m e t r y of the s t u d i e d p a r t of the 'Calcari Grigi' F o r m a t i o n . A lack of p a r a l l e l i s m between time lines (base of the early h i g h s t a n d s y s t e m tract and radial o o l i t e h o r i z o n i n t e r p r e t e d as a type-2 unconformity) is taken as an evidence for s y n s e d i m e n t a r y tectonics.
114
tUO
01.
t
8~;
~
9
61. cj
6
t
L
91,
g
I.I,
115
as is s u g g e s t e d by the g r e a t t h i c k n e s s e s ( as m u c h as 7 - 9 m), l a c k of e r o s i o n a l s t r u c t u r e s , lack of s u p p l y of i n t r a c l a s t s a n d a n d / or s k e l e t a l d e b r i s . W a t e r c i r c u l a t i o n was l i m i t e d as t h e faunal diversity is low. T h i n L i t h i o t i s b e d s , I to 2 m t h i c k ,were p r o b a b l y d e p o s i t e d in s h a l l o w e r a r e a s , c l o s e to the i n t e r m e d i a t e ramp. T h e s e t h i n n e r Opisoma, and beds contain in f a c t t h i n - s h e l l e d brachiopods, o t h e r s k e l e t a l f r a g m e n t s t y p i c a l of the i n t e r m e d i a t e ramp. Hummocky structures and other mechanical sedimentary structures evidence for strong episodic disturbances by w a v e s r e s p o n s i b l e for r e w o r k i n g of L i t h i o t i s s h e l l s . The occurrence fluctuations fluctuations.
of t r o u g h possibly
sequences related
suggests some bathymetric to relative sealevel
Paleobathymetry
A n e s t i m a t e of b a t h y m e t r y of the s t u d y a r e a was c a r r i e d o u t by constructing the isopach map of the maximum thicknesses of Lithiotis banks. The existence of a l a g o o n a l bucket oriented NE-SW is r e v e a l e d by t h e i s o p a c h map, Deeper lagoonal areas (deep ramp) are located in the w e s t and south. Directional data, s u m m a r i z e d in F i g . 6 2 a n d 26, c o l l e c t e d from orientations of symmetrical wave ripple crests, gutter casts, axes of coquinite lenses, channels and from the longest axes of L J t h i o t i s s h e l l s , i n d i c a t e that, w h a t e v e r the o r i g i n (storms, tsunamis,etc.) currents paths were controlled by the paleotopography and bucket configuration. In c r o s s s e c t i o n , the s t u d i e d u p p e r p a r t of the ' C a l c a r i G r i g i ' Formation is wedge-shaped (Fig,639 cf. also Fig.58). The northeastern side, 20 m t h i c k , is m a i n l y c o m p o s e d of s h a l l o w ramp lithofacies. It formed at a s h a l l o w e r depth than the southern side where the sedimentary p r i s m , 60 m t h i c k , w a s t h e s i t e of a c c u m u l a t i o n of the t h i c k e s t L i t h i o t i s b a n k s , The sedimentary wedge corresponds to an i n t r a s h e l f onlap ramp w h o s e h i n g e is l o c a t e d in the n o r t h a n d n o r t h e a s t ~ the flexure area is oriented north-south. The lagoonal trough strikes northeast southwest. A graphic simulation obtained by interactive modelling shows a hypothetical representation of the p a l e o b a t h y m e t r y of the ramp surface which was inclined
116
4\
SHALLOW
'~
1-"---"'-/ BIOCLAST ~ . BANK
~
Fig.6A
~ WASHOVER
RAMP
MARSH WA,SHOVER BIOCLASTBANK
DISCONTINUITIES
MARSH
- Transgressive
DEEP RAMP
system
tract,
117
towards west and southwest and c o r r e s p o n d i n g to s a n d w a v e s and banks
Depositional
punctuated (Fig.26).
by
sequence
The stratigraphic interval represents a third depositional s e q u e n c e w h i c h is s u b d i v i d e d from b o t t o m into a transgressive facies tract, an early and h i g h s t a n d facies tract and a shelf m a r g i n facies tract.
Transgressive
relieves
facies
order to top a late
tract
Description This facies tract is c o n s t i t u t e d by the f o l l o w i n g of l i t h o f a c i e s , s u m m a r i z e d b e l o w from b o t t o m to top I) Scoured undulating,
alternation (Fig.6~).
packstone with reddish lithoclasts passing n o d u l a r b l a c k lime m u d s t o n e s (shallow ramp);
to
2) G r a d e d bioclast layers, each characterized by an u p w a r d increase in the p r o p o r t i o n of l i t h o c l a s t s and c o a t e d g r a i n s ( w a s h o v e r d e p o s i t - t h i c k - b e d d e d a l t e r n a t i o n : s h a l l o w ramp); 3)
Lime
mudstones
(shallow
ramp);
~) S t a c k e d c o a r s e n i n g - and t h i c k e n i n g u p w a r d layers r e c o r d i n g a progressive increase in the b i o c l a s t percentage. G r a i n sizes d i s p l a y some b i m o d a l i t y ; p e l o i d a l g r a i n s are well sorted and m i c r i t i z e d ( i n t e r m e d i a t e ramp).
Interpretation The sequence analogous to W r i g h t (1981) Zechstein.
is interpreted as a hinge sequence. It is the littoral barrier described by Riding and and to the H a m p o l e beds o c c u r r i n g in the E n g l i s h
Transitions r e c o r d e d by the s e q u e n c e and m u d flats to an i n t e r m e d i a t e ramp p o i n t to a d e e p e n i n g - u p w a r d trend.
from s h a l l o w ramp p o n d s s u b m a r i n e b i o c l a s t i c bar
118
~'f---*-'-~SEA LEVEL CURVE
IOTIS BANK
HINGE
mO,
~
.~ .=
~.~,"
7m
Fi~.65 system
- Early tract.
highstand
119
Changes in the relative sealevel were discontinuous as individual assemblages of lithofacies are separated by d i s c o n t i n u i t y surfaces. The l o w e r m o s t surface is scoured. It f o r m e d above a h o r i z o n of likely s u b a e r i a l o r i g i n and is i n t e r p r e t e d as a t r a n s g r e s s i v e surface. It c o r r e s p o n d s to the lower s e q u e n c e b o u n d a r y of the depositional sequence.The former pedogenetic horizon above w h i c h the s e q u e n c e was d e p o s i t e d is r a r e l y p r e s e r v e d . It is m o s t l y inferred from a b u n d a n t r e d - c o l o r e d l i t h o c l a s t s o c c u r r i n g at the very bottom. The scarce micritization of grains c o n t a i n e d w i t h i n the lowermost g r a d e d beds (washover d e p o s i t s in the s h a l l o w ramp) and the p r e s e r v a t i o n of w a s h o v e r d e p o s i t s point to an increase in the rate of the relative sealevel r i s e . A s u c c e s s i v e s l o w i n g in the speed of the r e l a t i v e sealevel rise si indicated by the occurrence of intraclasts and m i c r i t i z e d grains on top of the s h a l l o w ramp lithofacies. The t r a n s i t i o n to the b i o c l a s t bank m a r k s an i n c r e a s e in the deepening-upward trend. The s u r f a c e which separates the two sectors of the ramp c o r r e s p o n d s to a r e t r o g r a d a t i o n a l line (see c h a p t e r #3 for the d e f i n i t i o n of r e t r o g r a d a t i o n a l line). The s e q u e n c e r e p r e s e n t s a r e t r o g r a d a t i o n a l t r a n s g r e s s i v e facies tract. Sediment was transported hingeward and formed a s t r i n g - l i k e body which may c o r r e s p o n d to a t h i n - s h e e t l i t h o s o m e as d e f i n e d by Burchettej et a ! . ( 1 9 9 0 ) . The low s e d i m e n t a t i o n rate was b a r e l y s u f f i c i e n t to c o u p l e w i t h the r i s i n g sealevel.
Early
highstand
facies
tract
Description The early h i g h s t a n d facies tract is a w a c k e s t o n e bank c o m p o s e d (Fig.65). The t h i c k n e s s d e c r e a s e s of thick shells of L i t h i o t i s from the t r o u g h area in the s o u t h w e s t (7 m: s e c t i o n # 25) to the h i n g e area in the NE (2.7 m: s e c t i o n #8). Faunal d i v e r s i t y is h i g h e r in the t r o u g h area. The thickest beds in the southwest (trough area) record vertical changes in fabric and c o m p o s i t i o n of L i t h i o t i s and sedimentary structures. Fabrics vary from p a r a l l e l (in some i n s t a n c e s found in p h y s i o l o g i c a l position) to w a v e - k n i t t e d , to r a n d o m t o w a r d s the top. L i t h i o t i s c o m p r i s e L i t h i o t i s sp. ss. and Cochlearites in the lower and m i d d l e part of the bank, and GervJlleJoperna in its upper part. Shell sizes a l s o d e c r e a s e
120
from 5cm - 2 0 + 3 0 cm at the bottom, to I-3 cm t o w a r d s the top. T r a c e s of s c o u r s f i l l e d w i t h r a n d o m l y o r i e n t e d Lithiotis shells and t r a c e s of cross b e d d i n g o c c u r at the top of the bank. This Lithiotis bank grades upwards to grainstones containing Lithiotis and o t h e r b i o c l a s t s such as small-size dispersed foraminifers, algae and g a s t r o p o d s . The top is also e n r i c h e d in o n c o l i t e s and i n t r a c l a s t s . A l l o c h e m s are m i c r i t i z e d . The u p p e r grainstone unit records also coarsening-upward as well f i n i n g - u p w a r d g r a i n size trends.
Interpretatio n This that that
bank formed in the deep r a m p . T h e lithosome geometry is of a wedge. D e c r e a s e s in t h i c k n e s s from NE to SW indicate the s e d i m e n t a r y interface was s l i g h t l y inclined towards
south. The lower p a r t of the bank c o n t a i n i n g t h i c k shells of Lithiotis f o r m e d w h e n the a c c o m o d a t i o n potential was highest: the h i g h s e d i m e n t a t i o n rate f a v o u r e d the d e v e l o p m e n t of Lithiotis; this part of the b a n k c o r r e s p o n d s to a m a x i m u m f l o o d i n g surface. The b a n k is c o m p a r a b l e to o t h e r c a t c h - u p reefs d e s c r i b e d in the literature (Fig.28) which record a transition from a quiet~ deep water stage to a shallower water stage typified by d e t r i t u s and l i t h o c l a s t s a s s o c i a t e d w i t h fossils. This early highstand facies tract records an aggradational trend d u r i n g w h i c h the space c r e a t e d by the r i s i n g s e a l e v e l was i n f i l l e d by vertical sediment growth ('catch-up phase' by K e n d a l l and S c h l a g e r , 1 9 8 1 ) . The p r o g r e s s i v e relative sealevel fall of the s e d i m e n t a r y i n t e r f a c e p r o d u c e d by the p i l i n g - u p of shells led to the d e p o s i t i o n at a s h a l l o w e r w a t e r d e p t h w h i c h thickets.Then, the favoured the f o r m a t i o n of Gervilleioperna d e c r e a s i n g s p a c e a v a i l a b l e to s e d i m e n t a t i o n f a v o u r e d a lateral facies s h i f t and p r o g r a d a t i o n w h i c h is d o c u m e n t e d by the set of compositional and fabric f e a t u r e s o c c u r r i n g at the top of the bank. T h e s e c h a n g e s r e f l e c t the i n i t i a t i o n of the late h i g h s t a n d facies tract phase of s e d i m e n t a t i o n . T h e vertical transition from L i t h i o t i s to Gervilleioperna a p p e a r s to be a primary f u n c t i o n of the d e c r e a s i n g s e d i m e n t a t i o n rate, in k e e p i n g w i t h the r e s u l t s obtained by Rey et a i . ( 1 9 9 0 ) from south Spain, r a t h e r t h a n a m a i n f u n c t i o n of d i f f e r e n t d e p t h s of d e p o s i t i o n of the two bivalves,as suggested by B r o g l i o Loriga & Neri (197~); interpretations which assign different depths to f o s s i l i z e d o r g a n i s m s w i t h i n a c a r b o n a t e p l a t f o r m are f r e q u e n t l y based on some sort of c i r c u l a r r e a s o n i n g ; conversely, their i n f e r r e d d e p e n d a n c e of the r e l a t i v e rise in the sea level may be c o n f r o n t e d
with
independent
data,
121
Late
hi~hstand
facies
tract
Description T h i s p h a s e is a p p r o x i m a t e l y 7 m t h i c k a n d c o n s i s t s of a n u m b e r of grain-supported, thinly bedded oncolite bioclast intraclast- bearing lithofacies interbedded with lime mudstones SHALLOW
RAMP
s.l.
7m OOLITES
BLACK MtCRITES COATED GRAINS
- Late
highstand
facies
tract.
Individual thin beds are organized into finingand coarsening-upward grain size trends. In the northern area, close to the hinge (sections # 8,11 and 18: Fig.63), coarsening-upward cycles are predominating. Fining-upward c y c l e s a r e m o s t c o m m o n t o w a r d s the t o p of t h i s t r a c t a n d in t h e south. In t h e m o s t p a r t of the stratigraphic sections this tract records a recurring change in the composition of grain-supported beds intercalated with lime mudstones from b i o c l a s t --> o o l i t e - - - > to o n c o l i t e . T h i s t r a c t is t o p p e d by a c m - t h i n h o r i z o n of r a d i a l o o l i t e s .
Interpretation Deposition took place in a shallow ramp.The frequency of oncolites within micrites and pisolite-oncolite grainstone beds p o i n t to a r e d u c e d s e d i m e n t a t i o n rate. S o u t h w a r d c h a n g e s in t h e thickness, l i t h o f a c i e s a n d t y p e s of b e d s s h o w t h a t the s o u t h e r n z o n e w a s d e e p e r . T h e o c c u r r e n c e of 6 - 7 m t h i c k o o l i t e b a n k s in the s o u t h ( s e c t i o n # 25: Fig. 63) a n d o o l i t e storm deposits
122
( s p i l l o v e r and w a s h o v e r d e p o s i t s ) in the n o r t h a l o n g the same stratigraphic horizon suggests northward,hingeward directed storm pro~esses. Grain-supported beds are interpreted as t h i c k - b e d d e d a l t e r n a t i o n s w h i c h formed s h a l l o w i n g - u p w a r d c y c l e s as is d o c u m e n t e d by e m e r s i o n f e a t u r e s on tops r e s u l t i n g from h i g h - f r e q u e n c y s e a l e v e l changes. The overall l i t h o l o g i c t r a n s i t i o n s of g r a i n s t o n e - p a c k s t o n e beds i n t e r c a l a t e d with lime m u d s t o n e s indicate a p r o g r e s s i v e u p w a r d s h a l l o w i n g trend. M i g r a t i o n s a r o u n d s c a t t e r e d d e p o c e n t e r s took p l a c e d u r i n g this time i n t e r v a l . T h e p h y s i o g r a p h y was p r o b a b l y a n a l o g o u s to that c h a r a c t e r i z i n g a n u m b e r of p e r i t i d a l s e t t i n g s located within carbonate platforms typified by p r o s p i c i e n t s u b m e r g e d and e m e r g e n t areas, The uppermost thin radial oolite horizon documents the e s t a b l i s h m e n t of u n i f o r m e n v i r o n m e n t a l c o n d i t i o n s in the area,
Shelf mar~in
facies.tract
Description This tract o v e r l i e s the h i g h s t a n d facies tract and is o v e r l a i n by p e l a g i c lithofacies. It is c h a r a c t e r i z e d by the p r e d o m i n a n c e of l i t h o f a c i e s typical of the i n t e r m e d i a t e ramp. The u p p e r m o s t part of the 'Calcari Grigi' F o r m a t i o n c o n s i s t s in fact of several repetitions of the following type of alternation of l i t h o f a c i e s (Fig.67) : o o l i t e g r a i n s t o n e s --> L i t h ~ o t ~ s bank--> s k e l e t a l w a c k e s t o n e s (intraclast - bioclast p a o k s t o n e s and g r a i n s t o n e s ) , This sequence averages 7 m in thickness. Its time interval of formation, c a l c u l a t e d by u s i n g the m e t h o d d e s c r i b e d by G r o t z i n g e r (1986),is of about 1 9 0 , 0 0 0 years, This tract is c h a r a c t e r i z e d by a lack of thin b e d s . F a c i e s t r a n s i t i o n s are sharp. Thin, lime m u d s t o n e h o r i z o n s o c c u r at the b o t t o m and top of the o o l i t e l i t h o f a o i e s , B i o c l a s t s are scarcely
micritized,
Interpretation T h e s e 7 m thick a l t e r n a t i o n s are the p r o d u c t of a h i n g e w a r d , s t e p w i s e m i g r a t i o n of i n t e r m e d i a t e l i t h o f a o i e s , a s is s h o w n by
123
the d i s t r i b u t i o n of o o l i t e bodies (Fi~,63). The r e l a t i v e s e a l e v e l rise i n i t i a l l y led to the f o r m a t i o n of oolite sediments; successively,the space created by the sealevel rise was c o l o n i z e d by L i t h i o t i s w h i c h formed banks t h i n n e r than those o c c u r r i n g in the deep ramp. T h e s e banks are in turn o v e r l a i n by s k e l e t a l w a c k e s t o n e s and/or bioclast and i n t r a c l a s t E r a i n s t o n e s w h i c h point to a s h a l l o w i n g - u p , These s e q u e n c e s are i n t e r p r e t e d as d e e p e n i n g - u p w a r d sequences c a p p e d by s h a l l o w e r w a t e r facies w h i c h were d e p o s i t e d f o l l o w i n ~ a s l o w i n g d o w n of the rate of sealevel rise,The deepeningu p w a r d t r e n d is also d o c u m e n t e d by the s c a r c i t y of m i c r i t i z e d grains and i n t r a c l a s t lithofacies,
m 7
lntraclast, oncolite, bloclast oohte
'@.~ ' £ ~ ! t
/
,,mestones
:- ~ ' ~ - @ ~
.-
""L
/INTERMEDIATE
,,,,,o,i. wac e.tone
/
I\
RAMP I
\
.
/
" DEEP RAMP
~ " ~ " ~
~,~o6.%%J g.~__%~$'2~4
~
Discontinuity .uotl~e . . . grams~one . .
"~"~ BmliacCrkte
/ / /
"" INTERMEDIATE RAMP .......
Rate of sea-level change
'J
•
Markovian
• Shallower
/ /
Deeper
sequences
e~ oe°
Fi~,67
- Modal
cycle
of
the
shelf
margin
facies
tract.
In the s t u d y a r e a the S M S T is o v e r l a i n by d i f f e r e n t pelagic facies of various ages (Rosso Ammonitico; Oolite di San Vigilio9 T e n n o F o r m a t i o n ) . It follows that the d i a c h r o n o u s top of the C a l c a r i Grigi c a n n o t be c o n s i d e r e d as a f l o o d i n g s u r f a c e as s t a t e d by B a r b u ~ a n i , et ai,(1986),
124
Model
of d e p o s i t i o n
The TST c o r r e s p o n d s to a b a r r i e r - l a g o o n littoral facies; the e a r l y HST to a c a t c h - u p reef; the late HST to a p r o g r a d i n g mud flat; the S M S T r e p r e s e n t s the l a n d w a r d m i g r a t i o n of an o o l i t e b a r r i e r facies. The s t a c k i n g of these d i f f e r e n t depositional faciess is typical of mature, l o n g - l i v e d p l a t f o r m s . Sedimentation took place mainly retrogradational mechanisms. A limited d u r i n g p e r i o d s of s l o w i n g - d o w n of the (late H S T and top of the SMST). A general
transgressive
trend
by aggradation and progradation occurred rate of s e a l e v e l rise
is d e m o n s t r a t e d
by the
I) facies belts are d i p p i n g towards the deep Depocenters of the i n t e r m e d i a t e ramp, namely display a progressive shift t o w a r d s the h i n g e
following:
ramp (Fig.63). oolite bodies, l o c a t e d in the
north. 2) S p i l l o v e r and w a s h o v e r d e p o s i t s (thick-bedded alternations) indicate a hingeward, onshore storm transport of lagoonalb a r r i e r s e d i m e n t s . A l o n g the v e r t i c a l , t h i c k - b e d d e d a l t e r n a t i o n s overlie
thin-bedded
3) Deep
ramp
alternations
associations
overlie
(Fig.68). shallow
ramp
facies
(Fig.69).
%/ ~5
vF0 w
Fig.68 thin-bedded
Overposition alternations.
of thick-bedded alternations over R i g h t : s e c t i o n # 7 ; l e f t : s e c t i o n #20.
125
3(
2C
IC
m0
Fig,69 A~B Stratigraphic section #15 showing gradual supplantation of intermediate and shallow ramp by thick Lithiotis banks, C Transition from shallow ramp to intermediate-deep ramp (section ~5: oncolite packstones and grainstones --> s k e l e t a l wackestones --> lime mudstones --> Lithiotis bank, m
126 The transgressive t r e n d w a s m a r k e d by s t e p w i s e p h a s e s w h i c h led to 7 m t h i c k l i t h o f a c i e s assemblages which differ depending of the p o s i t i o n on the s u r f a c e of the ramp. The a
sedimentary
w e d g e a n d the of the r a m p SHST formation.
retrogradation
the
TST
and
transgressive trend resulted from t o w a r d s the h i n g e , namely during
Differential,rotational subsidence, played an w h i c h is s u g g e s t e d , o t h e r t h a n by local t e c t o n i c features which may have b e e n local in e x t e n t ,
important role synsedimentary by the lack of
< ,Hinge
Q ~~~lFlexure
~~ Q
ill
hallowing- upward trend
/~_..:.~"~Ret rogradat ion of the sill - Deepening-upward .trend
(D Fig.70 - Progressive of sills,secondary towards
the
hinge.
deformation responsible for the f o r m a t i o n trough areas and shift of the flexure
127
parallelism between time lines (Fig.63),The discontinuous migration of the o o l i t e b a r r i e r followed the direction of m i g r a t i o n of the flexure w h i c h o f f e r e d the optimal b a t h y m e t r i c c o n d i t i o n s for t a n g e n t i a l o o l i t e f o r m a t i o n , T h e t e c t o n i c t i l t i n g determined a progressive f o l d i n g and shift of the flexure towards the hinge (Fig.70), This determined a differential deformation and downwarping b e t w e e n f l e x u r e and hinge r e s p o n s i b l e for the f o r m a t i o n of I) a
®
®
® 1
10
.'5
my
my
,,10
Sill
Secondary trough
128
East
10m
30m
~E NW 7
J tithlotls wackestones
"" " ' ' *
...............
oncobiosparrudite
~. ~ ' : :
/A
5
,.~;
,../j
oospsrite
~:====c=--=,'==ml0m~ "V/~
~0
biosparite |it hioti$ wackestone
F i g , 7 1 - E x a m p l e s of s i l l s e q u e n c e s a n d b e d d i n g s t y l e s of the sill a r e a . In t h e example below the accentuation of channel traces (transitions from flattened to s e m i c i r c u l a r scours) is s e e n as t h e r e s u l t of a p r o g r e s s i v e uplift and transformation of
a former
deep
ramp
floor
into
a sill,
129
secondary trough in the north d i s t r i b u t i o n ; and 2) sill sequences.
that
complicated
facies
Some examples of sill sequences are given in Fig,71. The f o r m a t i o n of a s e c o n d a r y trough and sill s e q u e n c e s are seen in terms of p r o g r e s s i v e increments and decreases of v e r t i c a l space c o n s e q u e n t to the d i f f e r e n t i a l tectonics (Fig.gO).This mechanism is explained by the model of relief inversion d e s c r i b e d in c h a p t e r #A. The interbedding of lime mudstones with deeper ramp lithofacies, especially recorded in the shelf margin facies tract, may have been p r o d u c e d by u p l i f t s of the p l a t f o r m w h i c h c a u s e d the e m e r s i o n of some areas. Fig.72 shows for e x a m p l e a t i l t e d s u b s t r a t e s u t u r e d by lime m u d s t o n e s .
F i g . 7 2 - T i l t e d beds o v e r l a i n by d m - t h i c k lime m u d s t o n e s . The tilting produced emersion of the interface which became a swamp. This is a small-scale example of relief inversion (chapter #&).
D e v o n i a n c a r b o n a t e platform C a r n i e Alps Italy
Introduction
Devonian limestones in the C a r n i c A l p s occur along a 20 K m e a s t - w e s t t r e n d as f a u l t e d t e c t o n i c t h r u s t s ( F i g . 7 3 ) resultin~ from the complex Hercynian and A l p i n e t e c t o n i c p h a s e s (Vai,
............
C O G L I A N S - CO LLINETTA '.
CIMA
t.
O M B L A D E T ~ ~ . ~
Sappada
~ Comeg[ians
".'C.
.......
~
_
...... ~ . . ~ : : ~ ~::~ Km 0 1 0
c Tarvisi(~;
Fig,73 - Location of the study area and outlines of the hercynian tectonic sheets (Venerandi Pirri,1977).The panoramic view shows the Cima Ombladet t e c t o n i c s h e e t in the c e n t e r a n d the C o g l i a n s - C o l l i n e t t a s h e e t in the r i g h t e m b e d d e d w i t h i n the turbiditio Hochwipfel Formation.
131
1980).They a r e p a r t of an e p i o c e a n i c shallow-water carbonate complex i n c l u d e d in a c o n t i n u o u s sequence f r o m the C a r a d o c to the W e s t p h a l i a n . T h e s t r a t i g r a p h y , tectonics and paleontology of the P a l e o z o i c of the C a r n i e A l p s w e r e e x t e n s i v e l y studied. A r e f e r e n c e list is f o u n d in V a i ( 1 9 8 0 ) . A hypothetical c r o s s s e c t i o n of the C o g l i a n s Collinetta reef complex, l o w e r to M i d d l e Devonian, was constructued by Vai (1980) a n d is s h o w n in F i g . 7 A . H o w e v e r w r o n g ( b i o h e r m a r e a s a r e overrated),such a c r o s s s e c t i o n is u s e f u l as it e m p h a s i z e s the well pronounced progradat iona i trend that characterized the platform growth during that time interval.
w
E
mo- ~
1200
-
i
-
m0
1000
Fig,74 - C r o s s
s e c t i o n of the C o g l i a n s - C o l l i n e t t a reef complex (from Vai,1980).TF:tidal flat; BR: Back-reef area~IR: inter-reef area; BI: Bioherms; FR: Fore-reef area9 PR: Peri-reef area. The stratigraphic Frasnian in age, water complex.
sequence forms the
described uppermost
here,Givetian part of the
to U p p e r shallow -
The paleoenvironmental situation of adjacent areas of the Coglians-Collinetta shallow-water complex is simple, as is exemplified by the stratigraphic sections measured in the Volaia-Coglians (Fig.75).These five stratigraphio sections i n d i c a t e a d e p o s i t i o n p a s s i n g f r o m a r e e f f l a t in the e a s t to a tidal or storm flat (Wanless,et ai.,1989) in the west.The general vertical and westward trend is fining-upward and shallowing-upward. Grain sizes show a westward decrease from 0 p h i to 4 p h i . T h e f o s s i l c o m p o s i t i o n s h o w s a c h a n g e f r o m open, agitated environments (corals, algae, brachiopods,foraminifers, (Amphipora, etc.) to semirestricted, protected conditions
calcispheres,ostracods).
132
133
/
/
3 N
16
2(
• ....
•..:~.:
D'
. .. ,...
1
10-
10-.}....
).:/..; .,..:.
1
• ,'~ -.t.[
- i}/0~
'.-...
:~.~, ,: .,'.,, mO
m(
mO
~",,,' •"
Fig,75 Cross section of M.Volaia-Coglians shallow-water complex showing vertical and landward shallowingand thinning-upward trends.(Section#~ m e a s u r e d by A , A r g n a n i ) . A~6.[ : m a s s i v e b e d s c o m p o s e d of c o r a l r u b b l e ( o u t e r r e e f flat; s e c t i o n #5).B~B':Sigmoidal calcarenite beds probably developed as sandwaves ( s e c t i o n fl3),C~C': i n n e r l a g o o n and t i d a l - s t o r m flat ( s e c t i o n s #1,2),
134
J REEFFLAT
I
DEEP.INTERMEDIATERAMP I 5 2 1 • •
4
.3
30-
I SHALLOW RAMP I 100m
20-
6
]ntraclast shoal 3
Pond
!:~':::V:A
~:~...
4
Open lagoon N
10-
Brachiopod bar
N
Reef flat
g
?-: ,... -"~. mONE
SW
101
m 5
o
I
I
I
I wackestone
packstone
1
Fig.76 - Stratigraphic sections of t h e C i m a O m b l a d e t succession and (below) oros section of t h e F l o r i d a platform (Enos,1977) which shows analogous vertical and lateral sediment trends,
135
T h e s e c h a n g e s w h i c h a l s o o c c u r a l o n g the s t r a t i g r a p h i c sections are related to a predominating progradational trend. The sedimentary i n t e r f a c e w a s i n c l i n e d t o w a r d s e a s t a n d s u b j e c t to flooding by storm currents.The physiography and sediment distribution are comparable to a 'Motu-Hoa' configuration (Bourroulh-le Jan and Talandier,1985) where onshore directed storm floods loose gradually energy and competence towards inner areas, w i t h the f o r m a t i o n of a s h o r e w a r d fining-upward grain size trend. The Cima Ombladet carbonate succession displays remarkable differences in facies organization with respect to those occurring in coeval, adjacent parts of the shallow-water limestone complex. These dissimilarities were superficially explained (Galli,198A,1985) as a r e s u l t of a d e p o s i t i o n within an i s o l a t e d a t o l l w i t h i n a m a j o r c a r b o n a t e c o m l p l e x . Such differences m a y be b e t t e r explained by Cima Ombladet succession as having formed intrashelf ramp structure.
considering the as an onlap,
In t h e C i m a O m b l a d e t carbonate succession a transition from a reef flat to semirestricted, inner lagoons is recorded (Galli,198A; 1985a,b,c; 1986). The paleobathymetry (Fig.77), reconstructed by m e a n s of i n t e g r a t i o n of s t a t i s t i c a l a n d f a c i e s a n a l y s e s , c o n s i s t s of a s e r i e s of l a g o o n s s e p a r a t e d by i s l a n d s a n d b a n k s w h i c h c o m p l i c a t e the e n v i r o n m e n t a l trends. Environmental gradients w e r e s t u d i e d by m e a n s of q u a n t i t a t i v e m o d a l a n a l y s e s of p a l e o n t o l o g i c a n d l i t h o l o g i c c o m p o n e n t s of 76 thin sections. F r o m t h e l e f t to t h e r i g h t s i d e of the d i a g r a m of F i g . 7 7 A the increase in o s t r a o o d s , calcispheres a n d Amphipora p e r c e n t a g e s corresponds to a t r a n s i t i o n f r o m the o u t e r r e e f - d e e p r a m p to shallow ramp sectors. The decrease in s e d i m e n t a r y i n f l u e n c e of the outer-inner reef flat towards the shallow ramp is gradational.For each lithofacies the a v e r a g e fossil abundance was correlated with the corresponding detritus:matrix ratio (biopeloidal + intraclast detritus : matrix + cement). This r a t i o , b e i n g a m e a s u r e of the p a c k i n g , g i v e s an e s t i m a t e of the environmental energy (Fig.77C).With the e x c e p t i o n of c r i n o i d s and some stromatoporoids, inverse correlations between fossils and detritus:matrix ratio indicate that organisms lived in (StrinEocephalus burtini, muddy habitats. Brachiopods Pentamerus) l i v e d in the d e e p ramp, p r o b a b l y in a s e r i e s of
136
80-
.-.-..a
~'"
b •
CRINOIDS
•
8RACHIOPOO$ TRYPANOPORA
js
.
g |:?:", 0
-... ".':*
t.
•
THAMNOPORA
O
AMPHIPORA
*
OSTRACOO$ & CALC$$PHERES
. ,.. ~.
....
:o '~
....
1 ~"'"
** °..
7
"'"
_
"'" 0
.1
.9
DETRITUS/ MATRIX
DETRITUS
I MATRIX
REEF FLAT
BIOPELSPARITES
DEEP
BIOMICRITES
RAMP
INTERM. RAMP
INTRASPARITES SHALLOW RAMP
MICRITES i
i
i
i
!
t
i
*
i
i
I
i
*
t
i
i
i
SIMILARITY
-
STROMATOPORA THAMOPORA GASTROPODS E A ONCOLITES DETRITUS CEMENT CRINOIDS B BRYOZOANS ALGAE TABULATES AMPHIPORA C SPONGES CALCISPHERES * MUD MATRIX OSTRACODS " ~D BRACHIOPODS TRYPANOPORA
137
A ,.,
f.' I.
•
,
"
.
[ ;"~ [ ". "REE'F'" • "
'
"
'
"
B
B
'
A
SHALLOW \
~
"
"~
RAMP
RAMP
D
C
"" " ' " • • "
--: :L
REEF
Q-MODE CLUSTERS
3B
3A
::::::::::::::::::::::::::::::
BATHYMETRY
2B 2A
:
.....
5 i
RAMP
7B
...........
7A ...... :.::~
8. ,~ !0. .
i
9
~ : ~
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :::::::5:::;5i:.':; "-5: :::: :}:):: :i:!:}:::: :-::2 : ::::::::::::::::::::::::::::::::::::::
R- MODE CLUSTERS D:M RATIO
1
FLAT
B
A
D
C
11 O
FOSSIL 96
so] / O
FOSSIL DIVERSITYs 1
\
0
Fig.77 Paleoenvironmental reconstruction and facies distribution of t h e C i m a Ombladet carbonate succession (for more details see Galii,1985), paleobathymetry, petrographic trends and Q,R-mode clusters.
138
tidal c h a n n e l s and bars c u t t i n g t h r o u g h inlets, Trypanopora lived in the deep ramp. The development of Thamnopora was confined to the i n n e r reef flat and the intermediate ramp. C a l c i s p h e r e s and o s t r a c o d s lived in the s h a l l o w r a m p . T h e R - m o d e c l u s t e r i n g (Fig.77D) g i v e s four c l u s t e r s r e p r e s e n t a t i v e of the o u t e r reef flat (B), inner reef flat (A), deep ramp (D) and s h a l l o w ramp (C).No d i s t i n c t i v e faunal a s s e m b l a g e c h a r a c t e r i z e s the i n t e r m e d i a t e ramp. I m p o r t a n t t r e n d s t o w a r d s the s h a l l o w ramp are: I) a d e c r e a s e in fossil d i v e r s i t y ; and 2) a d e c r e a s e in the detritus: matrix ratio (Fig. V 7 B ) . T h e s e trends are peculiar features of i n t r a s h e l f ramps. O t h e r b e a c h profiles, not d e v e l o p e d in onlap ramps, d i s p l a y o p p o s i t e trends, such as the e x a m p l e shown in Fig.78 (Auernig Formation, Permian - Carboniferous, eastern Carnic Alps, Italy: G a l i i , 1 9 8 6 ) . As seen f r o m the m e a s u r e d sections, faunal and lithofacies variations form reef flat to inner lagoon are rather complicated and evidence for a facies mosaic, Based on microfacies and litho!ogic composition, twelve facies were recognized (Galii,1985). The h y p o t h e t i c a l map showing the g e n e r a l facies zonation is shown in F i g . 7 7 A ( G a l l i , 1 9 8 5 b ) . The r e c o n s t r u c t e d e n v i r o n m e n t a l s e t t i n g b e a r s a g e n e r a l s i m i l a r i t y to some s i t u a t i o n s o c c u r r i n g in the E x u m a Cays~ w h e r e o p e n lagoons and b e a c h e s form just close to reef flat l o c a t e d l e e w a r d and tidal inlets (Fig.79). A peculiar feature occurring in the study area is the o c c u r r e n c e of m a s s i v e beds c o m p o s e d of i n t r a c l a s t g r a i n s t o n e s and d o l o i n t r a m i c r u d i t e s . The grain s o r t i n g is m o d e r a t e to poor. The p a c k i n g is low. I n t e r n a l sedimentary structures are first order low-angle cross-bedding and second order high-angle d i p p i n g f o r e s e t s w i t h i n first o r d e r s e t s . T h e s e beds h a v e been r e g a r d e d as the lagoonward terminations of the reef flat occurring as l i n e a r sand r i d g e s . T h e s e b o d i e s h a v e some a n a l o g y with r a m p a r t d e p o s i t s o c c u r r i n g in the reefs inside the Great Barrier Reef (Scoffin,1977). B r a c h i o p o d s are the m o s t a b u n d a n t f o s s i l s in the s u c c e s s i o n as occurr in a wide range of subenvironments, As is shown schematically in F i g . 8 0 , d i f f e r e n t biostratonomio data of these fossils c h a r a c t e r i z e the three sectors of the ramp.
139
BANK
BAR,RIDGE
...............
Ostracods
I~ JI} I~ p 2 ~
~
[
Gait ropod,~
Calci.~pherae Philloid algae
I
J
Antrachoporella Tubiphytes Epimastopora Eugonophlllum Crinoids Fusulinae
Bryozoans Faunal diversity
J
Detritus : matrix Fi~.78 - Allochem distribution in Auernig Formation ('Permo-Carbonifero Italy: G a l i i , 1 9 8 6 )
Facies
ramp consists alternations.
beach profile in the Pontebbano, Carnie Alps,
associations
Shallow
The shallow thick-bedded
a
of
ramp
pond
facies
and
thin-
and
140
141
DEEP
~
INTERMEDIATE RAMP
SHALLOW RAMP
HOMOGENEOUS
HOMOGENEOUS
HETEROGENEOUS
RAMP (BAR)
FAUNAL COMPOSITION
HOMOGENEOUS
DISSOCIATION OF SKELETAL PARTS
VALVES ARTICULATED VALVES BOTH (DISARTICULATED ARTICULATED & FOR BURROWERS} DISARTICULATED
VALVES BOTH VALVES NEARLY ARTICULATED & ALL DISARTICULATE D DISARTICULATED
ORIENTATION
LIFE POSITION, PARALLEL
IMBRICATED, INCLINED, PARALLEL
RANDOM
RANDOM
PACKING
LOW
HIGH
VARIABLE
HIGH
GEOPETAL INFILLING
INTRAMICRITIC
INTRAMICRITIC
INTRACLASTIC. NONE
BIOPELOIDAL
BRACHIOPOD
ASSEMBLAGE
66
"IN SITU"
78
90
LOCALLY TRANSPORTED TRANSPORTED
29
TRANSPORTED
Fi~.80 - Biostratinomy of b r a c h i o p o d s . Brachiopods f o u n d in the deep ramp represent an 'in situ' a s s e m b l a g e , as is s h o w n by s h e l l s f o u n d in l i f e p o s i t i o n , articulated and floating in a micrite matrix with a micrite geopetal infilling. Brachiopods of sublittoral bars in the deep ramp underwent a local selective transport, by means of tidal-longshore currents (bipolar beddings). Brachiopods occurring in the i n t e r m e d i a t e and shallow ramp (within thin- and thick-bedded alternations) are smaller, display a higher degree of transport (random orientations,a greater fragmentation, and various geopetal infillings). Brachiopods piled up in t h e intermediate ramp underwent a mass transport, as is evidenced by random orientations, poor sorting and variable packing. Unlike brachiopods of the d e e p ramp, those of the intermediate and shallow ramp underwent transport and mixing with other lagoonal f o s s i l s (see F i g . 1 3 for s o m e a d d i t i o n a l d e t a i l s ) .
142
Pond
facies
Description This facies consists of poorly fossiliferous, well-bedded, thin, black micrites and barren dolomicrites, with scarce calcispheres, o s t r a c o d s and Trypanopora ( G a l i i , 1 9 8 5 ), and v e r y thin s h e l l e d b r a c h i o p o d s . Pyrite, organic matter horizons and rare algal mat w i t h m m - s i z e l a m i n a t i o n s o c c u r . 0 t h e r s e d i m e n t a r y s t r u c t u r e s include v e r t i c a l burrows, some w a v y lamination, flat p e b b l e c o n g l o m e r a t e levels and p e b b l e clusters. I n t e r c a l a t e d w i t h this facies are t h i n - b e d d e d and t h i c k - b e d d e d alternations. The last consist of bioclast interlayers of v a r i a b l e t h i c k n e s s . R a p i d p i n c h - o u t s into m i c r i t e facies can be seen in some instances.The faunal content is h e t e r o g e n e o u s . I n t r a c l a s t s of v a r i a b l e s h a p e s c o n s i s t of b l a c k m i c r i t e s . T h e s e beds are organized into a fining-upward sequence which is composed of two units: I) a lower part, consisting of d a r k calcarenitic beds, composed of b r a c h i o p o d s , c o r a l s , crinoids, calcispheres, p e l o i d s and i n t r a c l a s t s ; and 2) an u p p e r part, composed of t h i n n e r beds with disarticulated both thin- and thick-shelled brachiopods. This u p p e r part g r a d e s q u i c k l y into c a l c i l u t i t e s w i t h thin algal laminae. Interpretation A very s h a l l o w e n v i r o n m e n t for this facies is i n d i c a t e d by vertical burrows , p a u c i t y of f o s s i l s , a l g a l layers and p y r i t e horizons.This facies represents intertidal pools and ponds whose extension and depth depended on the local intraclast shoal c o n f i g u r a t i o n , and o t h e r t o p o g r a p h i c b a r r i e r s . T h i c k - and t h i n - b e d d e d a l t e r n a t i o n s represent a deposition by storms, as w a s h o v e r d e p o s i t e d , as e v i d e n c e d by the f o l l o w i n g : sharp basal c o n t a c t s , c o u p l e t s of s h e l l y layers and l a m i n a t e d mud, grading, escape structures in the underlying mud and screening fabrics.The tripartite subdivision of the thick bedded alternations is a n a l o E o u s to that o c c u r r i n g in F l o r i d a Bay. Intermediate
Intraclast
ramp
shoal
Description This f a c i e s c o n s i s t s of p o o r l y grained intraclast grainstones
sorted, disorganized, and packstones, up
coarsely to 3 m
143
thick.Lithoclasts are of v a r i a b l e s i z e , s h a p e and c o m p o s i t i o n . Fossils are all transported and of variable provenance (lagoons, reef flat, p o n d s ) . C o m m o n sedimentary structures are vadose silt,cut-and-fills, keystone vugs, gradations, crossb e d d i n g and f l a t - p e b b l e c o n g l o m e r a t e s . This lithofacies is overlain by cryptalgal laminites, represented by light-gray, well-sorted, planarbedded, laminated intrasparites, 60 cm to 1 m thick, w i t h very thin micrite laminae, w h i c h are u n d u l a t e d , slightly inclined and stylolitic, w i t h a h o r s e t a i l f i l i g r e e pattern.
Interpretation The i n t r a c l a s t shoal facies r e p r e s e n t s a d e p o s i t i o n in a b e a c h environment,as is i n d i c a t e d by the o c c u r r e n c e of flat p e b b l e conglomerates,fringing cement, keystone rugs, vadose silt, occurrence of v a r i o u s textures and a b s e n c e of a grain size s i g n a t u r e (Davis,et a i . , 1 9 7 2 ) . The o c c u r r e n c e of w i d e t e x t u r a l and g r a i n - s i z e ranges, disorganized beds and m a s s i v e bedding indicate a d e p o s i t i o n u n d e r c o m p l e x h y d r a u l i c c o n d i t i o n s , as a result of island shifting. Cryptalgal laminites are interpreted laminations. The s e d i m e n t was d e p o s i t e d by c u r r e n t s b y - p a s s i n g ridges and bars.
Deep
Brachiopod
~rainstones
as storm
beach-ridge floods and
ramp
and w a c k e s t o n e s
Description B r a c h i o p o d w a c k e s t o n e s c o n t a i n a b u n d a n t m o n o t y p i c a s e m b l a g e s of (StrinEocephalus, Pentamerus) thick-shelled brachiopods o c c u r r i n g t o g e t h e r w i t h s m a l l e r a m o u n t s of other fossils such as crinoids, and o c c a s i o n a l s o l i t a r y corals. Brachiopod grainstones consist of brachiopod shelly layers containing well-sorted shells. This unit ranges in t h i c k n e s s from 1.5 to as much as 3 m. I m b r i c a t i o n s , geopetal structures,
144
planar and bipolar cross-bedding were observed in some instance. Some of the t h i c k e r beds c o n t a i n a g r e a t e r p e r c e n t a g e of i n t r a c l a s t s , a lesser p e r c e n t a g e of b r a c h i o p o d s , a n d a lesser shell o r i e n t a t i o n .
Interpretation Brachiopod wackestones represent a deposition in a m a r g i n a l bay. The e n v i r o n m e n t is s u b t i d a l , a s s h o w n by the o c c u r r e n c e of p a t c h y and d i s t i n c t , " i n p l a c e " a c c u m u l a t i o n s of b r a c h i o p o d s and o t h e r types of fossils. Zonations of e n d e m i c populations and t e x t u r e s i n d i c a t e that this l a g o o n was r a t h e r large and deep. Brachiopod grainstones may be r e f e r r e d to 'in situ' lagoonal bars and b a n k s . S o m e r e w o r k i n g by s t o r m a g e n t s is e n v i s a g e d for the t h i c k e r beds, w h i c h may r e p r e s e n t r e w o r k e d lagoonal bars and banks close to the intermediate ramp which was h e t e r o g e n e o u s in c o m p o s i t i o n .
~positiona!
model
The measured stratigraphic section records complex and recurring facies transitions.The paleoenvironmental reconstruction is shown in Fig.21 (Galii,1986). The Markov chain analysis (Miall,1973),applied to the succession (Galii,1985) in o r d e r to d i s c r i m i n a t e d e t e r m i n i s t i c from r a n d o m facies t r a n s i t i o n s , w a s an aid in the i d e n t i f i c a t i o n of two m a i n s e q u e n c e s c o r r e s p o n d i n g to: I) a s t o r m 2) a b e a c h
bar bar
sequence; sequence.
and
These two sequences (Fig.81) are the result opposite,different d e p o s i t i o n a l m e c h a n i s m s w h i c h were in the area: I) a g g r a d a t i o n ; and 2) p r o g r a d a t i o n .
of two operating
The c o m p l e x i t y of facies t r a n s i t i o n s , s h o w n by the r h y t h m o g r a m of F i g . 8 2 , i s a c o n s e q u e n c e of the i n t e r f e r e n c e between these two d e p o s i t i o n a l m e c h a n i s m s . The p r o g r a d a t i o n a l trend produced the d e p o s i t i o n of s h a l l o w water deposits over deep ramp facies. As indicated by the detailed microfacies analyses (Galii,1985) these shallow-water s e d i m e n t s p r o g r a d e d into the lagoon, from reef flats as l i n e a r
145
OPEN LAGOON . . . . . . . . . . "> BIOCLAST BAR ° o"~
o
POND FACIE~
°.•
oO°• o.
~.~
..."
POND .." V ..... -'9 FACIES INTRACI_AST . . . . . . . . . . . "> CRYPTALGAL ......... SHOAL < . . . . . . . . . . . . LAMINITE
Pond
Pond
Cryptalgal
3 -
Cryptalgal laminite
laminite 3 -
nkJ 2'"2- ~ .'--I Bioclast
2 -
~a~
bar 2-
Intraclast
_~_r_/_-~
shoal
~.-_/-~ _ d : g ' .o-.. ~'1 1 1 -
Open
.u:. (D..~-.Ol • f,,,~\.~. c~, 81
:l ):t,"~O" I
lagoon
mO
Pond
mO Deepening-upward
sequence
Shallowing-upward
Aggradation
Deep-intermediate
sequence
Progradation
ramp
I n t e r m e d i a t e - shallow ramp
Fig,81 - Facies flow chart showing more common than random facies transitions (Galii,1985) and modal sequences (storm bar sequence:left;beach bar: r i g h t ) . sand shoals and ramparts. Topographic h i g h s w e r e t h e s i t e s of formation of beaches.This progradational trend is a local r e s p o n s e to the d o m i n a n t p r o g r a d a t i o n a l trend recorded in the a d j a c e n t a r e a s of the s h a l l o w - w a t e r carbonate complex (Fig.74 a n d 75). The
aggradational
trend
led
to
the
deposition
of
deeper,
146
beach bar ~
beach bar ~.
~ s t o r m bar
bar cryptalgal laminite intraclast shoal -- brachiopod gr. -- brachiopod wk. -- pond -
-
Fig.82 - Rhythmogram aperiodic cyclicity chain analysis.
TIME of f a c i e s t r a n s i t i o n s evidencing (Schwarzacher,1975), obtained by
for an Markov
deep ramp and intermediate ramp facies above shallow ramp f a c i e s . It r e s u l t e d f r o m a d e e p e n i n g - u p w a r d trend, a n d led to the deposition of a h i n g e s e q u e n c e (storm bar sequence). As s e e n f r o m the v e r t i c a l changes throughout the s t r a t i g r a p h i c succession (Fig.83), the transgressive trend became progressively predominating u p w a r d s , w h e r e the d e e p r a m p f a c i e s and trough sequences overlie hinge sequences. The complexities of f a c i e s t r a n s i t i o n s suggest a scarce control exerted by the predepositional topography. This may be attributed to a m o d e l of f o r m a t i o n of o n l a p ramps involving complex mechanisms of r o t a t i o n s a n d t i l t i n g (cf. Fig.39)'. The coexistence of p r o g r a d a t i o n a l and aggradational trends in the s a m e a r e a is t y p i c a l of a r e a s l o c a t e d n e a r the b o u n d a r y of the intrashelf onlap ramp,close to the hinge area, in intrashelf ramp structures f o r m e d by a c o m b i n a t i o n of t i l t i n g and rotational subsidence. In t h e s e sites the o n l a p ramp is transitional to o t h e r areas of the platform governed by a dominant progradational trend. Interferences between progradational and aggradational trends in the h i n g e area are also recorded in the 'Calcari Grigi' Formation. Thickeningand coarseningupward trends, s u c h as t h a t s h o w n in F i g . 8 A , o c c u r in the e a s t e r n b o r d e r of the ramp, w h e r e it is t r a n s i t i o n a l to a r e a s s u b j e c t e d b y p r o g r a d a t i o n . In these stratigraphic sections, the material supplied by the nearby prograding areas produced an equilibrium between subsidence and sedimentation rate. These complex situations a r e in a c c o r d a n c e w i t h the r e s u l t of the s t u d y c o n d u c t e d by Eberli & Ginsburg (1981) on the B a h a m a platform (Fig.5).Additional work is needed to confirm the possibility that some ancient carbonate platforms, rather than
147
20
HINGE SEQUENCE
TROUGH SEQUENCES
H|NGE SEOUENCE
10
°
HINGE
SEQUENCE
m 2
m0
Fig.83 - Stratigraphic sequence showing a progressive increase in p r o p o r t i o n of deep ramp facies, l:pond facies (shallow ramp); 2: i n t r a c l a s t shoal (intermediate ramp); 3: b r a c h i o p o d g r a i n s t o n e s and w a c k e s t o n e s (deep ramp). m o n o l i t h i c s t r u c t u r e s , are c o n s t i t u t e d by c o m p l e x 'offshooting' prisms, r e s u l t i n g from the ~ u x t a p o s i t i o n and lateral t r a n s i t i o n b e t w e e n d i f f e r e n t types of i n t r a s h e l f ramp g e o m e t r i e s . The same d i s t r i b u t i o n of t h i c k e n i n g - and t h i n n i n g - u p w a r d c y c l e s and m e g a s e q u e n c e s occurs in o t h e r areas and s i t u a t i o n s . In a d i f f e r e n t s i t u a t i o n this d i s t r i b u t i o n , found in the P l e i s t o c e n e
148
- Holocene fan d e l t a system close to the city of Bologna (Italy), s t u d i e d by G a l l i , e t al.(1985),seems to be c o n t r o l l e d by d i f f e r e n t i a l t e c t o n i c control: thickening-upward trends of gravel bodies are found in the uplifted areas, whereas t h i n n i n g - u p w a r d t r e n d s are r e s t r i c t e d to the d o w n f a u l t e d sites (Fig.8A).
F i g . 8 h - L o c a t i o n of t h i c k e n i n g and c o a r s e n i n g - u p w a r d trends and t h i n n i n g and f i n i n g - u p w a r d trends in the s t u d i e d a r e a of the 'Calcari grigi' Formation (cf. Fig.26 showing the paleotopography of the r a m p ) ; e x a m p l e of t h i c k e n i n g - u p w a r d and coarsening-upward sequence in the 'Calcari Grigi' Formation (section #11)~cross section showing the distribution of Holocene thickeningand thinning-upward trends of gravel bodies in the Pc p l a i n and the t e c t o n i c s y n s e d i m e n t a r y f e a t u r e s (Galli, et a i . , 1 9 8 5 ) .
'C a p o
Rizzuto' sequence,
shoreline Pleistocene
Introduction
Pleistocene marine deposits in s o u t h e r n I t a l y o c c u r a l o n g the coastline of s o u t h e r n Italy and form the lowest ones of a s e r i e s of m a r i n e t e r r a c e s (Fig.85) deposited during the last climatic oscillation ( R i s s / W ~ r m ) , 3 0 0 . 0 0 0 to 7 0 0 . 0 0 0 y e a r s ago, Although the o v e r a l l deposition of the sedimentary sequence described below records the onset of a climate change associated with glacial-interglacial fluctuations, shoreline
•
'
' "~'"I'~
,
v
17'I
Fig.85 - Location of study area and cross Selii,1962) showing marine terraces developed a b o v e o l d e r r o c k s . I V to VII r e p r e s e n t T y r r h e n i a n
section (from unconformably terraces.
150
t
® ~o ~ ~ s bioturbated calcarenite @
®
cross bedded calcarenite @ ~
~-~.~.~_-~-~.
@~
rhodolites
i,'~. :.~.;~:o ., 1
+
UPPER SHOREFACE
COASTAL CLIFF
®
@
~ ,, ~ . : . . .':~~
pebbly mudstone p e l i t e @ LOWEL SHOREFACE
mOJ'~-----"
(A) cross bedded c a l c a r e n i t e ~
~
• .,......
~..
.
.
:
....
.". ...
. . . . . : . . . . . . . , . : . . . : . . . . . . . . :. -. . ., ~ : . ~ : ~."-;--. .'-~..-:.., . . :-,-~-.. . .-w-,.. . ~ .~.:- : • : : . . . . : . : - .
....:
~.
~, . . . . . .
: . . . . ~ : : . . ~ . . . . . .
.~<... ~ .....~ . ~ . . _ . ~ ~ . ~ • : . ~ . . . : ' . :....-'.'., . : . . . ' . : ' . :
. . . :
.
,.:
.
:
:
. . . . . s,l,4 .
• ..
.
.......
/~
.
•
.
T
: ..-....~ . ~ . : ~..'..... : : : .:.: ~. : ;" : . o :..'...: :. ~ ~. ~." ...;..: -
~ ' ~ " ~ " ._J r e l a t i v e s e a ] Rate of eustatic sea level rise > r a t e of t e c t o n i c uplift : " ~ l e v e l rise | t J 0J
P~
r3
rhodolites
1
. . . . . . s,I, 3
L)
.p(
r~ ~0
_J stillstand
Rate of eustatic sea level rise = r a t e of t e c t o n i c uplift : 7
~o i
erosional u n c o n f o r m i t y
. . . . , s.I.
. . . . 'S,12 Rate of eustatic sea level rise < r a t e of t e c t o n i c uplift :
J
relative -]
regression
151
erosion, deposition, and the altitudes were c o n t r o l l e d by (Selii,1962,1967; Desio,1973).
location combined
Depositional
of cycles tectonics
at v a r i a b l e and e u s t a s y
sequence
The m e a s u r e d s t r a t i g r a p h i c s e q u e n c e , 1 0 m thick, c o n s i s t s b o t t o m to top of the f o l l o w i n g facies t r a c t s (Fig.86):
from
I) L o w s t a n d facies tract; 2) T r a n s g r e s s i v e facies tract; 3) H i g h s t a n d facies tract.
Lowstand
facies
tract
Description This facies tract overlies bioturbated siltstones with interbedded lenses of p o o r l y sorted, coarse to f i n e - g r a i n e d c a l c a r e n i t e s c o n t a i n i n g some shells i n t e r b e d s (Fig.86A). It c o n s i s t s of a 2 m t h i c k fining-upward sequence9 the v e r y base consists of a thick unit composed of unsorted, intraformational, r o u n d e d blocks and p e b b l e s e n g u l f e d w i t h i n a moderately sorted c a l c a r e n i t e to c a l c i r u d i t e m a t r i x (Fig.86B) whose a l l o c h e m s are g i v e n by a b r a d e d skeletal fragments of sponges, Strombus bubonius, Oardium, Glycymeris, Cerithium, shark teeths and bryozoans. A discontinuous tract.
horizon
of
branching
rhodolites
caps
this
Interpretation C a l c a r e n i t e i n t e r b e d s u n d e r l y i n g the d e p o s i t i o n a l s e q u e n c e are common in nearshore to lower shorefaoe environment. They r e f l e c t a f i n e - g r a i n e d shelf or lagoonal e n v i r o n m e n t subject to an e p i s o d i c input of b i o c l a s t c a l c i r u d i t e sand. T h e s e d e p o s i t s were laid down in the lower s h o r e f a c e . The
overlying
lithofaoies
(Fig.86B)
indicates
a
deposition
152
u n d e r h i g h - e n e r g y c o n d i t i o n s , in a c o a s t l i n e e n v i r o n m e n t , as is shown by the g r a i n - s u p p o r t e d texture and the type of fauna. The r o u n d i n g of c l a s t s i n d i c a t e s a very s h a l l o w e n v i r o n m e n t s u b j e c t to a c o n s t a n t w a t e r m o t i o n . T h e f o r m a t i o n of relict l i t h o c l a s t blocks may have been a consequence of bioerosion and wave-action. Indented lateral contacts with the upper lithofacies outline an articulated, rough submarine paleotopography which suggests the existence of submarine cliffs. B r a n c h i n g r h o d o l i t e s on top suggest within a very shallow environment.
a
slow
sedimentation
rate
A r a p i d r e l a t i v e s e a - l e v e l fall p r o d u c e d r e p e a t e d r e w o r k i n g of carbonate sediments with the f o r m a t i o n of relict lithoclast beds which mark the contact between lower shoreface and c l i f f . T e c t o n i c uplift, faster than e u s t a t i c s e a l e v e l rise, led to a r e l a t i v e sea level fall and f r a g m e n t a t i o n of p r e v i o u s l y exposed carbonates into b l o c k s which subsequently underwent r e w o r k i n g and r o u n d i n g in a s h o r e l i n e e n v i r o n m e n t (Fig.86).The lower erosional contact between shoreface and overlying conglomerates does not corresponds to the onset of the T y r r h e n i a n phase, but r e p r e s e n t s p r o b a b l y a m i n o r r e l a t i v e sea level
fluctuation.
The rate of t e c t o n i c uplift, i n i t i a l l y g e o l o g i c a l l y rapid, (cf. C i s n e , 1 9 8 7 ) , g r a d u a l l y d e c r e a s e d and at a c e r t a i n time r e a c h e d the same rate as the e u s t a t i c sea level rise. C o n s e q u e n t l y , the sea level remained stable for a short time interval~ these conditions, favourable to the f o r m a t i o n of o n c o l i t e s , led to the d e v e l o p m e n t of the r h o d o l i t e horizon, at a d e p t h less than 2 m e t e r s , u n d e r a low s e d i m e n t a t i o n rate.
Transgressive
facies
tract
Description This
consists
of
coarse-grained,
moderately
sorted,
cross-
bedded biocalcarenites (Fig.86D). Some a c c u m u l a t i o n s of O s t r e a o c c u r t o w a r d s the top of this lithofacies which is d m thick. Sedimentary structures are given by trough cross-bedding and planar, sigmoidal and hummocky
cross-bedding.
153
Introduction The m u l t i s t o r e d c h a n n e l i z e d g e o m e t r y is typical of the u p p e r to lower high-energy shoreface environment (Galii,1986). The trough cross bedded sets and hummocky cross bedding were interpreted as the result of a d e p o s i t i o n by storms, above f a i r - w e a t h e r base (Galii,1989). T e c t o n i c u p l i f t was u n i m p o r t a n t and s e d i m e n t a t i o n caught sea level, m a i n l y in r e s p o n s e to the e u s t a t i c s e a - l e v e l by v e r t i c a l a g g r a d a t i o n (Fig.86).
Highstand
facies
up to rise,
tract
Description This tract is represented by calcarenites (Fig.86E) whose sedimentary structures are n e a r l y completely obliterated by root penetration and bioturbation by Callianassa and Ophiomorpha, well v i s i b l e on b e d d i n g planes.
Interpretation The r e c o r d of e p i s o d i c s e d i m e n t a t i o n is less a p p a r e n t here due to b i o t u r b a t i o n and r h i z o t u r b a t i o n w h i c h point to an u p w a r d s h a l l o w i n g and d e c r e a s e in s e d i m e n t a t i o n rate, in turn f u n c t i o n of a d e c r e a s e in the rate of sea level rise.
Concludin8
remarks
A l t h o u g h not d i r e c t l y r e l a t e d to a d e p o s i t i o n in an i n t r a s h e l f ramp, the s e q u e n c e d e s c r i b e d above has been i n c l u d e d in this work because of its s i m i l a r i t i e s to a n u m b e r of s e q u e n c e s d e s c r i b e d here, for e x a m p l e the L o f e r c y c l o t h e m and the h i n g e s e q u e n c e d e s c r i b e d above. The s u c c e s s i o n of facies tracts and the relative sea-level curve is a n a l o g o u s , a p a r t from differences in s c a l e , t o the second and t h i r d d e p o s i t i o n a l s e q u e n c e of the slope c a r b o n a t e s in the G a r g a n o m a s s i f described in c h a p t e r #3. The s e q u e n c e described in this s e c t i o n is a l s o a s m a l l - s c a l e example of relief inversion.
Computer
simulation wedges
of
elastic
Introduction
Most of s i l i c i c l a s t i c depositional outer edges of p a s s i v e c o n t i n e n t a l sedimentary wedges consisting of developed above a progradational chronostratigraphic surfaces. Each of these s u r f a c e s can and a sloping portion aggradational and the sedimentation.
s e q u e n c e s d e v e l o p e d on the m a r g i n s are p r o g r a d a t i o n a l laterally stacked bodies surface and delimited by
be d i v i d e d into a n e a r l y h o r i z o n t a l which represent respectively the progradational component of the
A g g r a d a t i o n c o n s i s t s of a v e r t i c a l b u i l d - u p of s e d i m e n t w h i c h takes p l a c e u n d e r an i n c r e a s e in the r e l a t i v e sea level rise. S t a c k i n g of units p r o d u c e s sigmoid or c o m p l e x - o b l i q u e c l i n o f o r m p a t t e r n s ( M i t c h u m , e t ai.,1977). Progradation refers to s e a w a r d results in a net decrease or Progradation produces oblique patterns (Mitchum,et ai.,1977).
building-up of sediment and stillstand of the sea-level. (tangential and parallel)
The p l a n e w h i c h ties all the lines of i n f l e x i o n ( p l a t f o r m b r e a k hinge) b e t w e e n h o r i z o n t a l platform-top s e d i m e n t s and i n c l i n e d slope s e d i m e n t s is the p l a t f o r m break p l a n e (PBP) (Doglioni and Bosellini,1989).The downlap plane (DP) (Doglioni and Bosellini,1989) is the d i a c h r o n o u s plane i n t e r s e c t i n g all the lines w h e r e the toe of p r o g r a d i n g c l i n o f o r m s f l a t t e n out into the h o r i z o n t a l b a s i n facies (downlap hinge). The d o w n l a p p l a n e may be horizontal, climbing and d e s c e n d i n g (Bosellini, 198A). The i n o l i n a t i o n of the P B P may also vary from n e a r l y h o r i z o n t a l to n e a r l y v e r t i o a l , w i t h the f o r m a t i o n of o f f l a p r e l a t i o n s h i p s (Mitchum,et ai.,1977) when lateral progradation takes place contemporaneously with aggradation, and toplap relationships (Mitchum,et ai.,1977) when inclined strata terminate on top against the u p p e r h o r i z o n t a l boundary. Also variable is the
156
lenEth and t h i c k n e s s
of the c l i n o f o r m s .
T h e s e p a r a m e t e r s are shown in Fig.87. They c h a n g e in r e s p o n s e to; I) e u s t a t i c changes; 2) t e c t o n i c s u b s i d e n c e ; 3) s e d i m e n t volume; A) s e d i m e n t p e n e t r a t i o n into the basin; 5) w i d t h of the mar~ins; and 6) h y d r o d y n a m i c factors ( B u r t o n , e t a i . , 1 9 8 7 ) .
CHRONOSTRATIGRAPHIC SURFACE PLATFORM BREAK PLANE (PBP)
CLINOF DOWNLAP PLANE (DP) /
AGGRADATION PORTION
PROGRADATION PORTION INITIAL
Fi~.87
SURFACE-~
- Key e l e m e n t s
of e l a s t i c
proErading
wedges.
This c h a p t e r p r e s e n t s the r e s u l t s of a c o m p u t e r s i m u l a t i o n of stratigraphic models of a c c u m u l a t i o n of p r o g r a d i n g elastic wedges, in r e s p o n s e to s e a - l e v e l curves, t e c t o n i c m o d e l s and s e d i m e n t inputs. For each pattern, the f o r w a r d mathematical modelling Eenerated a 'family' of solutions. They are a s i m p l i f i c a t i o n of reality; n e v e r t h e l e s s , they p r o v i d e a tool in b a s i n a n a l y s i s b e c a u s e they a l l o w for a d i s t i n c t i o n b e t w e e n the c o m p o n e n t s of the r e l a t i v e s e a - l e v e l rise, w h i c h are d i f f i c u l t to s e p a r a t e in s t r a t i g r a p h i c interpretations (Burton,et al., 1988). The m a t h e m a t i c a l m o d e l l i n g of s i l i c i c l a s t i c w e d g e s used in this simulation tracks the evolving geometry produced by the i n f i l l i n g of t w o - d i m e n s i o n a l b a s i n by a s t e p - b y - s t e p a d d i t i o n of s e d i m e n t increments, for u s e r - { d e f i n e d , c h a n g i n g values of subsidence and eustasy. The s i m u l a t i o n does not r e p r o d u c e s p h y s i c a l p r o c e s s e s of d e p o s i t i o n . It s i m u l a t e s the s e d i m e n t geometries, based on the c o n c e p t that i m p o r t a n t f e a t u r e s of d e p o s i t i o n a l ~ e o m e t r i e s are c o n t r o l l e d by m a c r o p r o c e s s e s such as c h a n g e s of eustasy, s u b s i d e n c e , d e p o s i t i o n and c o m p a c t i o n of sediment. Each d e p o s i t i o n a l s u r f a c e is d e f i n e d by an a r r a y of evenly spaced points. This gives the sediment surface a
157
staircase-like a p p e a r a n c e w h i c h is not seen, due to the large number of horizontal points. The sediment deposition is s i m u l a t e d as right t r i a n g l e s of s p e c i f i c length and t h i c k n e s s . The h e i g h t of t r i a n g l e s is a m e a s u r e of the v o l u m e of s e d i m e n t deposited~ the length is the d i s t a n c e of s e d i m e n t p e n e t r a t i o n into the basin. The s e q u e n c e of p r o c e s s e s at each time step is the f o l l o w i n g . I) c a l c u l a t i o n of the sea-level; 2) t e c t o n i c s u b s i d e n c e (or uplift); 3) e r o s i o n of the s e d i m e n t above s e a - l e v e l with a slope a n g l e g r e a t e r than the s e d i m e n t repose angle (as d e f i n e d by the user); A) d e p o s i t i o n of the sediment, i n c l u d i n g that e r o d e d in the p r e v i o u s step; 5) c o m p a c t i o n of the sediment according to the w e i g h t of the overburden; 6) i s o s t a t i c s u b s i d e n c e of surfaces in r e s p o n s e to the s e d i m e n t and w a t e r loading. The s i m u l a t i o n is c a r r i e d out for each time step as a s e q u e n c e of subsidence, sea-level change, erosion, deposition or c o m p a c t i o n , etc., as shown s c h e m a t i c a l l y in the s i m p l i f i e d flow chart of Fig.88. More details concerning application are found in ai.(1989), Helland-Hansen,
the computer technique and Burton, et a i . ( 1 9 8 7 ) , Scaturo, et ai.(1988).
its et
elastic progradational w e d g e s d e v e l o p e d on p a s s i v e c o n t i n e n t a l m a r g i n s w e r e s i m u l a t e d by u s i n g the data base d e r i v e d from the Exmouth plateau, NW A u s t r a l i a (Erskine and Vail, 1988).The initial s u r f a c e s i m u l a t e s a b l o c k - f a u l t e d basement. Sea level was a s s u m e d to f l u c t u a t e by about a m a x i m u m of 200 m o v e r a p e r i o d of 6 m i l l i o n s years. The eight h y p o t h e t i c a l s e a - l e v e l curves w h i c h have been u s e d in the simulation reproduce sea-level stillstand, rapid,slow s e a - l e v e l falls, and g r a d u a l ,rapid s e a - l e v e l rises. M o d e l l e d subsidence is w i t h i n the range of s u b s i d e n c e rates of the A t l a n t i c m a r g i n . T e c t o n i c a c t i v i t y includes l a n d w a r d and s e a w a r d tilting, uplift, basculatory tectonics, and rotational subsidence along listric faults.Sedimentation rates are t a k e n as c o n s t a n t , i n c r e a s i n g and decreasing. The g r a p h i c values of penetration.
simulation generated 71 g e o m e t r i e s under defined eustasy, subsidence and sediment volume and B a s e d on d i f f e r e n t o r i e n t a t i o n of the P B P and DP,
158
V Define depositional triangles
San,4
Shale
V Set
sl
sea level
V Set tectonic subsidence
I
1
sl
V Locate shoreline
I
V Perform erosion
]
V I
Deposit sediment
I
V
I
sl
Perform compaction
V Isostatlc
subsidence
V No
V Yes
Fig.88 1989).
-
Flow
chart
of
the
simulation
(from
Scaturo,et
al.,
as seen in d i p - o r i e n t e d c r o s s sections, the g r a p h i c a l outputs w e r e g r o u p e d into three p a t t e r n s w h i c h were f o r m e r l y d e f i n e d by D o g l i o n i and B o s e l l i n i (1989): I) p a r a l l e l ; 2) c o n v e r g e n t ; 3) d i v e r g e n t . Climbing, descending and p a r a l l e l produce for each pattern three
directions of sub-patterns.
progradation The c o m p l e t e
159
•/•/PBP
j
PBP
PBP
~ l ~ D p
DP Convergent - descending
Convergent- climbing
oP Convergent- horizontal
PBP
PBP
DP
/'
DP
/DP Divergent - climbing
Divergent - descending
Divergent - horizontal
PBP PBP
/'
&p
DP Parallel - climbing
Parallel- descending
F i g . 8 9 - Types of and D o w n l a p Plane,
Parallel - horizontal
p a t t e r n s b a s e d on the P l a t f o r m and d i r e c t i o n s of p r o g r a d a t i o n .
s p e c t r u m of p a t t e r n s c o n s i d e r a t i o n but are
Break
Plane
(composite patterns were also t a k e n in not d e s c r i b e d here) is shown in Fig.89.
Parallel
pattern
In this pattern the c l i n o f o r m s maintain a constant length b e c a u s e the d o w n l a p p l a n e keeps p a r a l l e l to the p l a t f o r m break p l a n e (Doglioni and B o s e l l i n i , 1 9 8 9 ) . This kind of p a t t e r n is c o n t r o l l e d by the different interrelationships b e t w e e n factors.
following
three
160
I) 2) 3)
Sea-level fall - s e a w a r d t i l t i n g - d e c r e a s e in t h e r a t e of s e d i m e n t a t i o n (Fig.90A). S e a - l e v e l fall - s e a w a r d , landward subsidence - decrease in sedimentation rate (FiE.90B). Sea-level stillstand - seaward sedimentation rate (Fig.90C).
The main controlling in the s e d i m e n t a t i o n sea-level.
factors are rate, a n d a
tilting
decrease
in
the
a seaward tilting,a decrease f a l l i n g or s t i l l s t a n d of the
This pattern does not s e e m to f o r m when place within a depression originated from a
progradation takes landward tilting.
T h e D P in all c a s e s is c l i m b i n g ; the steepness of the PBP varies and produces offlap relationships when subsidence o u t p a c e s t h e r a t e of s e a - l e v e l fall. A t o p l a p r e l a t i o n s h i p is visible in the e x a m p l e of F i g . 9 0 A : in t h i s c a s e the PBP is nearly horizontal: this probably results from a stillstand of the sea-level, The resulting prograding geometries are either climbing or descending with respect to the sea-level. According to the classification by M i t c h u m , et a i . ( 1 9 7 7 ) the clinoform set is a complex-oblique pattern. Oblique patterns are recognisable in t h e last c l i n o f o r m s of F i g g . 9 O A , B . The aggradation p o r t i o n is g r e a t e r in the o u t p u t s of F i g . 9 0 C , D a n d 90A, w h e r e s e d i m e n t a t i o n is c o n t r o l l e d by a s e a - l e v e l fall, a n d l e s s e r in the e x a m p l e s of F i g . 9 0 A w h i c h w a s p r o d u c e d by a sea-level stillstand, The basinal sediments should record vertically a shallowingupward sequence. E x a m p l e s of p a r a l l e l p a t t e r n s f r o m t h e l i t e r a t u r e a r e f e w as a likely result of the landward tilting undergone by p a s s i v e margins s u c h as the A t l a n t i c margin, and the NW Australian m a r g i n . P a r a l l e l p a t t e r n s m a y o c c u r in a r e a s w h i c h u n d e r w e n t a n inversion in the s e n s e of t i l t i n g , for example in t h e A p t i a n and
Oligocene.
A case history displaying a parallel pattern occurs in t h e Tusoalose Delta and Turbidite (central Louisiana: Berg,1982) (Fig.91) where the deposition appears to have been controlled by a s e a w a r d t i l t i n g , a d e c r e a s e in the s e d i m e n t a t i o n rate and a s t i l l s t a n d of t h e s e a - l e v e l ; This example is c o m p a r a b l e with t h e g r a p h i c s i m u l a t i o n of F i g . 9 0 A .
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Fig.91
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This pattern is c h a r a c t e r i z e d by a progressive decrease in l e n g t h o f t h e c l i n o f o r m s as a r e s u l t of t h e u p w a r d c o n v e r g e n c e of the P B P a n d the D P ( D o g l i o n i a n d B o s e l l i n i , 1 9 8 9 ) . It w a s I) 2)
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fall - l a n d w a r d (Fig 92A). fall landward
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5)
sedimentation r a t e ( Fig. 9 2 , B , C , D ) . Rapid sea-level fall - basculatory tectonics - decreasing sedimentation rate (Fig,92A,E). Sea-level stillstand no subsidence - decreasing rate of s e d i m e n t a t i o n (Fig.92F). Slow sea-level rise landward tilting decreasing
6)
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3) ~)
sedimentation
-
decreasing
rate.
The main controlling factors are a sedimentation rate and a fall or stillstand
decrease in the of the s e a - l e v e l .
T h i s p a t t e r n is n o t p r o d u c e d d u r i n g a r a p i d s e a - l e v e l r i s e , n o r by a s e a w a r d tilting. Differential subsidence is n o t a l w a y s present. T h e D P is a l w a y s c l i m b i n g . T h e PBP displays various degrees of steepness. It is s t e e p e r w h e n a s e a - l e v e l r i s e is a c c o m p a n i e d by a l a n d w a r d tilting (Fig.92A), slightly inclined when the sea-level fall is c o u n t e r b a l a n c e d by a landward tilting,or uplift;horizontal to s l i g h t l y descending under conditions of sea-level fall or stillstand and an absence of s u b s i d e n c e , In most situations the PBP follows the sea level curve (Fig.92A,B).
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11_
210800
234000
• • •
5_ ir~'t and ~econd Urder Sea Lev~ Curves
I
187200
8_
8_ 2 ................. - VO|U~e from Left
I
16~800 20_
20.
134_ 101_ 68_ 35_ 2_
-2700
H
17
lmmm m
-Volume from Right
mm II
167
Commonly, the s e d i m e n t a r y g e o m e t r y is climbing. A descending pattern occurs only in one g r a p h i c a l simulation (Fig.92C). According to the terminology by Mitchum,et ai.(1977) the c l i n o f o r m g e o m e t r i e s are c o m p l e x - o b l i q u e and oblique, The a g g r a d a t i o n p o r t i o n is s i g n i f i c a n t only in the s i m u l a t i o n of F i g . 9 2 A , D where the r e l a t i v e s e a - l e v e l rise r e s u l t e d from the c o m b i n e d effects of s u b s i d e n c e and a rapid s e a - l e v e l rise. C l i n o f o r m s thin-out in a b a s i n w a r d direction; they t h i c k e n in the case of Fig,92A: this may be e x p l a i n e d by a b y - p a s s i n g of sediments consequent to a t r a n s i t i o n from o f f l a p to t o p l a p upper boundary relationships. An e x a m p l e of c o n v e r g e n t , d e s c e n d i n g p a t t e r n is the P l e i s t o c e n e occurring in the continental shelf of Israel (Ryan and Cita,1978~ Fig.93).The upper toplap boundary is slightly i n c l i n e d towards the basin and the DP and PBP c o n v e r g e t o w a r d s the same direction. A c o m p a r i s o n w i t h the s i m u l a t i o n of F i g . 9 2 D indicates that P l i o c e n e s t r a t a were laid down d u r i n g a rapid sea-level fall, an a b s e n c e of s u b s i d e n c e and a decreasing s e d i m e n t a t i o n rate,
.._._.... I.
2-
3-
7
/
.....
" . . . 3. . . .
~ ~ _' ~ j -
-
J
4
4-
0
6
Fig.93 - Descending-convergent (Ryan and C i t a , 1 9 7 8 ) .
19
Km
pattern
in
the
shelf
An absence of tectonic subsidence such as f a u l t i n g is in a c c o r d a n c e w i t h the i n t e r p r e t a t i o n and C i t a (1978).
of
Israel
down-to-basin g i v e n by Ryan
An example of convergent, climbing pattern occurs in the Devonian - Mississipian from Ohio (Broadhead,et ai.,1982; F i g . 9 ~ ) , I t is c o m p a r a b l e to the g r a p h i c s i m u l a t i o n of F i g , 9 2 E w h e r e p r o g r a d a t i o n took p l a c e onto two f a u l t e d blocks s u b j e c t e d to r o t a t i o n s along listric faults. Each block underwent a rotation with a progressive landward increase in the rate of
168
DELTA P L A I N SHELF
DEEP CRATONIC BASIN
A L L U V I A L PLAIN
F i g . g A - B a s i n model of the D e v o n i a n - M i s s i s s i p i a n n o r t h e r n O h i o (Broadhead, et a i . , 1 9 8 2 ) .
turbidites
of
subsidence and a relative uplift at the basinward edge (Fig.95). The s i m i l a r i t y to the s i m u l a t i o n of F i g , 9 2 E s u g g e s t s a deposition under a sea-level fall,a decreasing sedimentation rate, and a rotational subsidence of the type sketched in Fig.95. It is p o s s i b l e that the d e p o s i t i o n of t u r b i d i t e into
turbidites
delta plain- shelf
t
Fi~.95 turbidite
Tectonic behavior of the basement d e p o s i t s of N o r t h e r n O h i o of Fig.9~.
the deep c r a t o n i c the d e l t a - p l a i n
b a s i n was shelf.
triggered
by
a
underlying
relative
uplift
the
of
A n o t h e r e x a m p l e of c o n v e r g e n t - c l i m b i n g pattern occurs in the 1500 m thick, B e r r i a s i a n - V a l a n g i n i a n c l a s t i c w e d g e l o c a t e d at the E x m o u t h plateau, N W A u s t r a l i a (Fig.96). It was s t u d i e d by Exon a n d W i l l c o x (1978), Exon,et ai.(1982), Stakelberg,et ai.(1980) and Erskine and Vail (1988) who interpreted the c l a s t i c w e d g e as a t u r b i d i t e fan.The Exmouth plateau belongs to a p a s s i v e m a r g i n which underwent rifting
169
o
/
Km
Fig.96 - Clastic Vaii,1988).
wedge
of
the
Exmouth
Plateau
(from
Erskine
&
and b l o c k - f a u l t i n g , f o l l o w i n g the b r e a k - u p of Pan~aea. A c c o r d i n g to E x o n , e t ai.(1982) the w e d g e formed as a result of a gentle, northward, regional tilt from the early Neocomian to the mid-Neocomian, onto a series of f a u l t e d blocks, The p h a s e of d o w n f a u l t i n g was f o l l o w e d from 125 to 120 m.y. by a p h a s e of t h e r m a l uplift, r e l a t e d to m o v e m e n t s along transform faults. Fig.97, from Exon and W i l l c o x (1978) shows the e v o l u t i o n of the p r o g r a d i n g wedge. The s i m u l a t i o n of Fig 92H is a s c h e m a t i c r e p r o d u c t i o n of the c l a s t i c w e d g e shown in Fig,96. The wedge formed d u r i n g a slow s e a - l e v e l rise and u n d e r a d e c r e a s i n g s e d i m e n t a t i o n rate. The t e c t o n i c c o n t r o l r e s u l t s to have been r a t h e r complex: the deposition was affected by b a s c u l a t o r y tectonics acting as e p i s o d i c s u b s i d e n c e and u p l i f t at d i f f e r e n t l o c a t i o n s ( Km 3,1 and 163: Fig.92H) and times ( subsidence: 132 + 126~ 126 + 127 m.y.~ uplift from 129 to 128 m.y.). The o b l i q u e p a t t e r n of clinoforms visible in F i g . 9 2 H and 96 results to have been produced by a combination of tectonic uplift (rates: I0 m / 2 0 0 . 0 0 0 yrs.) and a slow sea-level rise.
Divergent
This
pattern
displays
an u p w a r d
pattern
incraese
in
length
due
to the
170
® o
o
®
MIOCENE -- RECENT
LATE CRETACEOUS--EOCENE
2
~
MID. CRETACEOUS
i
EARLY JURASSIC -- EARLY CRETACEOUS
0 LATE
!"'"
TRIASSIC
EARLY & MIDDLE TRIASSIC
0~
i
-?:'.'--~5:-: ":.'.:.".:.".:":.-'.-. -::~ - : . : . :
~
0
'
"......... : :
-~.:i
[::'..::°." ::4.." :
100
KM
'
I
I A
~ D
grffle gr~
~ s
~c
Fig.97 - Schematic cross section showing the evolution of t h e Exmouth Plateau (after Exon & Wilcox,1978), A: M i d - C r e t a c e o u s muds and silt; B:Early Jurassic to Early Cretaceous sands, s i l t s a n d m u d s . C: T r i a s s i c deltaic f l u v i a l m u d s a n d s a n d s . D: Paleozoic sediments. E: oceanic basement~ F: continental basement. basinward divergence of Bosellini,1989). Divergent following
combinations - level
Slow
2)
sedimentation rate (Fig.98A). Sea-level fall - seaward tilting ( Fig.98B).
fall
and PBP (Doglioni were obtained by
variables,
I)
rate
sea
of
the DP patterns
-
landward
tilting
- constant
-
constant
sedimentation
and the
171
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Second Ordu- Se~ Level C~ves
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177
3)
Sea
- level
fall
sedimentation
rate
fall
-
landward
tilting
-
decreasing
(Fig.98D).
~)
Sea-level
5)
sedimentation (Fig.98E). Sea-level stillstand - landward
- landward
tilting
- decreasing
rate
of s e d i m e n t a t i o n (Fig.98F). 6) S e a - l e v e l s t i l l s t a n d - l a n d w a r d
tilting
- constant
rate
7)
of s e d i m e n t a t i o n (Fig.98G). Sea-level stillstand - landward
tilting
- increasing
rate
8)
of s e d i m e n t a t i o n (Fig.98H). Sea-level rise - seaward tilting
9)
sedimentation (Fig.98J). Sea-level rise - landward
sedimentation (Fig.98K), 10)Sea-level rise - seaward
tilting
rate This
increasing
- decreasing
tilting tilting
sedimentation (Fig.98I). ll)Sea-level rise - landward sedimentation (Fig.98L). 12)Sea-level rise - seaward
-
-
rate
rate
increasing
- increasing
tilting tilting
- constant
- constant
of
of
rate
of
rate
of
rate
of
sedimentation
(Fig.98M).
pattern
is
always
tilting or a stillstand Differential subsidence
formed
by
a
sea-level
and a landward a p p e a r s in all
rise
and
tilting. of t h e a b o v e
seaward
models.
M o s t of the m o d e l s w e r e o b t a i n e d b y m e a n s of a c o n s t a n t or increasing sedimentation rate; m o d e l s p r o d u c e d by a d e c r e a s i n g sedimentation r a t e g i v e w a y to g r a p h i c simulations with very s m a l l a n g l e s of d i v e r g e n c e b e t w e e n t h e D P a n d the P B P . T h e f o r m a t i o n of t h i s p a t t e r n a p p e a r s to be i n d e p e n d e n t of t h e type of sea-level change. The
DP
may
be
The first increasing)
either
descending
rise
and
climbing,
case is produced by a sedimentation rate and
the sea-level. A climbing DP results mainly rate, a landward tilting and sea-level,
or
and
2)
a
a seaward
constant (less commonly a fall o r s t i l l s t a n d of
f r o m I) a d e c r e a s i n g s e d i m e n t a t i o n a s t i l l s t a n d , rise, or f a l l of t h e
decreasing
sedimentation
T h e P B P m a y be c l i m b i n g . T h e s t e e p n e s s of t h e of the relative sea-level rise,Offlap relationships are produced by a landward relationships The
aggraded
the
steepness
are
produced
portion of
rate,
a
sea-level
tilting.
the
of
the
PBP.
by
a seaward
clastic
P B P is a f u n c t i o n upper boundary tilting9 toplap
subsidence.
wedge
is a l s o
a function
of
178
The
divergent
pattern
may
be
climbing
or d e s c e n d i n g .
The first one is the most common. Climbing progradation produces sigmoidal patterns (Mitchum,et ai.,1977). A climbing progradation results f r o m I) a l a n d w a r d t i l t i n g ; 2) a seaward tilting accompanied by a s e a - l e v e l stillstand a n d an increase in s e d i m e n t a t i o n rate,and 3) a s e a - l e v e l r i s e a n d an increasing sedimentation rate, A descending progradation gives way to oblique patterns (Mitchum,et ai,,1977). A descending progradation results from two combinations of variables: I) s e a - l e v e l rise - seaward tilting - decreasing sedimentation rate (Fig.98I,J); and 2) sea-level fall landward tilting - constant sedimentation rate (Fig.98A). T h e g r e a t n u m b e r of e x a m p l e s of d i v e r g e n t p a t t e r n s f o u n d in the literature reflects the wide range of combinations between eustasy, sedimentation rate and subsidence that can produce this pattern. An example of d e s c e n d i n g divergent pattern occurs within the 2500 m thick MiocenePliocene wedge that laps onto the continental s h e l f of S u m a t r a (Fig.g9). It fits t h e s i m u l a t i o n of F i g . 9 8 B w h i c h w a s o b t a i n e d by m e a n s of a c o n s t a n t sedimentation rate, a sea-level fall and a seaward tilting.
FOREARC BASIN
Fig.99 continental
Descending shelf (from
CONTINENTAL
divergent Beaudry and
SHELF
pattern in Moore,1985).
the
Sumatra
A n e x a m p l e of c l i m b i n g ,divergent pattern occurs in t h e L o w e r Cretaceous of N E A l a s k a , s t u d i e d by M o l e n a a r (1983~ F i g . 1 0 0 ) . It is the product of a sea-level rise, a c o n s t a n t r a t e of
179
~-).
•".::...:'.'.'..:'.'-:":.,':.':.'....,.:,-;'-c;'-'..:C::".'.'-'.:." :.
,
" " '
" " "
- ~
~
\
n~-
~Ow ~~':':.:':.~: .[:...• U
~
...... \\ ~
"~ ~ ~.Turbidites
Fig.100 - Cross section showing NE Alasks (from Molenaar,1983). sedimentation
and
a
landward
Cretaceous
tilting
Concludin~
(Fig.
/
- Lower
Tertiary
of
98L),
remarks
The knowledge of the eustatic curve, tectonic history and sedimentation rate for a g i v e n area may lead to p r e d i c t i v e geometries of c l a s t i c wedges.In fact, the forward system of modelling employed here generates a n u m b e r of t w o - d i m e n s i o n a l representations of s e d i m e n t g e o m e t r i e s t h a t a r e the p r o d u c t of eustasy,sedimentation rate and type and direction of t e c t o n i c movements.Therefore, it b e c o m e s p o s s i b l e to s e p a r a t e for each graphical output the e u s t a t i c and tectonic component of the relative sea-level c u r v e a n d to c o n s i d e r the n a t u r e of s o m e features such as sequence boundaries, offlap, toplap relationships, t h i c k n e s s of c l i n o f o r m s , downward s h i f t of the coastal onlap,an so o n . I did not describe these featuures here;nonetheless, m o s t of f e a t u r e s vary approximately in the s a m e w a y as the c o r r e s p o n d i n g f e a t u r e s of p r o g r a d i n g carbonate platforms (cf. Bosellini,1984).Some differences may be attributed to the greater lateral transport occurring in siliciolastio
than
in c a r b o n a t e
settings.
180
An interesting aspect of the simulation is that the same results can r e s u l t from a w i d e v a r i e t y of c o m b i n a t i o n s b e t w e e n tectonic behavior, e u s t a t i c c h a n g e s and s e d i m e n t a t i o n rate. In particular, divergent patterns (and p a r a l l e l p a t t e r n s ) m o s t l y result from a differential subsidence (mainly a seaward tilting); c o n v e r g e n t p a t t e r n s are p r o d u c e d by a s t i l l s t a n d or a fall of the s e a l e v e l and a d e c r e a s i n g s e d i m e n t a t i o n rate. T h e s e r e s u l t s fit the o b s e r v a t i o n s and m o d e l s of p a s s i v e m a r g i n e v o l u t i o n by M o u g e n o t , et a i . ( 1 9 8 3 ) who s h o w e d a c h a n g e from a 'sigmoidal progradation shelfbreak type' to an 'oblique progradation shelfbreak type'. According to Mougenot, et a i . ( 1 9 8 3 ) the first type is most typical of m a r g i n s a f f e c t e d by a rapid t h e r m a l c o o l i n g , w h e r e a s the s e c o n d is more c o m m o n l y associated with slightly subsiding mature margins. It seems that Lower Cretaceous margins typically display divergent patterns, such as the Exmouth plateau (Fig.96),the Lower Cretaceous Mannville Group of W e s t e r n Canada (Jackson,198A), and the L o w e r C r e t a c e o u s of N o r t h e a s t e r n C a n a d a ( M o l e n a a r , 1 9 8 3 ; Fig.100).
Slope carbonates (CretaceousPaleocene) Gargano massif
Introduction
The stratigraphic interval in the study area represents a transitional zone b e t w e e n J u r a s s i c - Cretaceous shallow-water carbonates,and Cretaceous to Paleocene basinal-hemipelagic d e p o s i t s (Fig.101). The c a r b o n a t e s t r a t i g r a p h i c s u c c e s s i o n of the G a r g a n o p e n i n s u l a (S.Italy) has been the o b j e c t of c o n t r a s t i n g i n t e r p r e t a t i o n s . B a s e d on the a b u n d a n c e of s h a l l o w - w a t e r r u d i s t f r a g m e n t s , i t was i n t e r p r e t e d as a s h a l l o w - w a t e r reef by A G I P g e o l o g i s t s and the University of Bologna working in the area (Cremononi,et ai.,1971; Mattavelli and P a v a n , 1 9 6 5 ; Pavan and P i r i n i , 1 9 6 5 ; M a r t i n i s and P a v a n , 1 9 6 5 ; Pattera, 1967). Recently, m e g a b r e c c i a s w h i c h m a r k the j u n c t i o n b e t w e e n p l a t f o r m and the b a s i n in the study a r e a w e r e i n t e r p r e t e d as g e n e r a t e d by a tectonic collapse of the Cretaceous platform along synsedimentary faults (Masse and Borgomano,19879 Sinni and Masse,1986). This i n t e r p r e t a t i o n was c o n t r a s t e d by the r e s u l t s of the field m a p p i n g made by Ferioli (1988), w h i c h r e v e a l e d that the traces of s y n s e d i m e n t a r y faults are in r e a l i t y s u b m a r i n e slide scars responsible for the f o r m a t i o n of m e g a b r e c c i a s 9 Bosellini and Ferioli (1988) and Bosellini (1989) advanced a eustatic interpretation: the f o r m a t i o n of the slide scars was c o r r e l a t e d with well pronounced lowstands of the sea-level (Haq,et a l , , 1 9 8 7 ) , o n the a s s u m p t i o n that t h e i r s h e d d i n g into the b a s i n could be r e l a t e d to e u s t a t i c s e a l e v e l falls. These discordances allowed for the subdivision of the complicated tangle of stratigraphic formations into three depositional sequences (Fig.102) whose description is the object of this section. This w o r k is b a s e d on d e t a i l e d o b s e r v a t i o n s of about 1800 m of spectacular exposures (9 s t r a t i g r a p h i c sections) c r o p p i n g out m a i n l y a l o n g road cuts and n a t u r a l c l i f f e x p o s u r e s .
t82
Peschici ;arganico lago di Varano
Vieste
Sanmcandro
I
I
tltl
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ttinata
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. ._
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Peschici-Vieste
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Cm
scale 1:5o..ooo
I
Fig,101 - Location of t h e s t u d y a r e a ( M a r t i n i s and Pavan,1967) and local stratigraphy (Cremonini, et a i , , 1 9 7 1 ) evidencing for a divergent pattern with a hinge located at M , S , A n g e l o area (Cc2,CcI,Cc3),
183
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N MONTE DEGLIANGELI FM. 7~
MATTINATAFM.
13
STRATiGRAPHiC SECTION
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SEQUENCEBOUNDARY
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SEQUENCE
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Fig.f02 - Geological map ,location and subdivision into depositional 1 9 8 7 ; F e r i o l i and B o s e l l i n i , 1 9 8 8 ) ,
I?
Maiolica
I
DEPOSITIONAL SEQiENCE
of. s t r a t i g r a p h i c sections sequences (Ferioli~1986-
184
First
depositional
sequence
The first d e p o s i t i o n a l sequence, Jurassic to L o w e r C r e t a c e o u s in age, consists of shallow-water ('M.Spigno Formation': Cremonini,et ai.,1971; 'Calcari o o l i t i c i di C o p p a G u a r d i o l a ' : Martinis and Pavan,1967), slope carbonates ('Madonna degli Angeli' and 'Mattinata' Formations) and basinal miorites (Majolica Formation). The slope deposits, located north of M.S.Angelo (Fig.101: s e c t i o n #I), c o n t a i n a g r e a t p r o p o r t i o n of m e g a b r e c c i a s , slump deposits, and graded calcarenites (a-b-o Bouma intervals) intercalated with hemipelagic micrites. The age (Pattera, (Bacinella irregularis; 1966-1967) ranges from the Aptian Cuneolina laurentis;Orbitolina sp.) to the A l b i a n (Ticinella; Globigerinoides) in the u p p e r m o s t 170 - 180 m of s t r a t i g r a p h i c exposures. The slope deposits are divisible into a lower finingt h i n n i n g - u p w a r d unit (0 - 300 m) and an u p p e r t h i c k e n i n g c o a r s e n i n g - u p w a r d unit (300-500 m ) , a s is s h o w n in Fig.103.
Fining-
and
thinning-upward
and and
unit
Description The l o w e r m o s t part (0 - 85 m) c o n s i s t s of a l t e r n a t i o n s b e t w e e n s i l i c e o u s m a r l s or chert layers, and t h i n l y b e d d e d (10-15 to 50 cm thick) white colored waokestones containing dispersed bivalves and bioclasts. Going upwards, bioclast concentrations w i t h i n beds i n c r e a s e in t h i c k n e s s w i t h the p r o d u c t i o n of a m scale, t h i c k e n i n g - u p w a r d s e q u e n c e (Fig.10d) o v e r l a i n by t h i c k e r slumped beds w i t h fragments of c h e r t nodules and bioclast, l i t h o c l a s t s and s h a l l o w - w a t e r corals. The upper segment of this lower unit contains a great proportion of g r a v i t y deposits: I) t h i n - b e d d e d micrites; 2) g r a d e d c a l c a r e n i t e beds; 3) g r a d e d - u n g r a d e d m e g a b r e c c i a s ; and A) slump d e p o s i t s . These lithofacies are organized into a g e n e r a l finingand thinning-upward trend: megabreccias are preponderant at the base; gravity deposits vertically decrease in thickness, grain-size and f r e q u e n c e and e v e n t u a l l y pass to t h i n - b e d d e d
185
500 -
z <
.2
<
7 <
C~
<
Fig.f03 breccias unit
-
First are
contains
megabreccias results
in
depositional
common a and
a
in
great
the
proportion
pebbly
coarsening
sequence.Discordances lower
mudstones and
and
unit.
Conversely,
of
to
1
whose
thickening-
25
m
thick
vertical
upward
trend.
slumped
the
upper graded
increases
186
pelagic micrites organized into intraformational t r u n c a t i o n surfaces.
packages
by
separated
micrite with chert clasts and dispersed bioclasts bioclast intercalations
calcarenite lenses thin bedded micrites with intercalated chert and shales
Fig.104 - Thickening-upward sequences first d e p o s i t i o n a l s e q u e n c e .
in
the
lower
part
of
the
Interpretation Thickening-upward s e q u e n c e s o c c u r r i n g at the base of the lower unit, similar to the thickening-upward cycles described by A i g n e r (1985), are i n t e r p r e t e d as the basal t e r m i n a t i o n s of an a p r o n or b i o c l a s t w e d g e of s e d i m e n t p r o g r a d i n g d o w n s l o p e , on a g e n t l y s l o p i n g s u r f a c e , T h e c o a r s e g r a i n - s i z e of the t h i c k e n i n g upward sequences suggest a proximity to the shallow-water p l a t f o r m , Slumped, t h i c k m i c r i t e beds on top of the t h i c k e n i n g upward sequence may have oriNinated from high-frequency sea-level fluctuations, The f i n i n g - u p w a r d and t h i n n i n g - u p w a r d t r e n d s d i s p l a y e d by the lower u n i t s u g g e s t a m e c h a n i s m of v e r t i c a l a g g r a d a t i o n , rather than a p r o g r a d a t i o n of the slope, The c o m p o s i t i o n of the s l o p e suggests a change in the slope c h a r a c t e r through geological time, from a ramp-type setting, to a depositional margin (Fig.105).
187
The a b u n d a n c e of s l u m p e d breccias at the top of the unit reflects a relatively unstable sedimentary interface which contrasts with undisturbed bedding styles of the basinal m i c r i t e s o c c u r r i n g at the base of the section. Coeval b a s i n a l limestones (Majolica Formation) in the G a r g a n o p e n i n s u l a , 4 5 0 m thick, c o n s i s t of white, well b e d d e d m i c r i t e layers, I0 - ~0 cm thick, with slumped horizons sandwiched between undisturbed beds. N e p t u n i a n dykes are also f r e q u e n t . T h e M a j o l i c a F o r m a t i o n was d e p o s i t e d on a steep slope subjected to s y n s e d i m e n t a r y t e c t o n i c s and frequent s e i s m i c activity. Therefore,it is p o s s i b l e that the c h a n g e in d e c l i v i t y of the slope , r a t h e r than b e i n g a r e s p o n s e to a e u s t a t i c c h a n g e (the sea-level curve shows a stillstand in the Aptian: Haq,et ai.,1987) was a consequence of synsedimentary downfaulting which produced a depression, infilled by a mechanism of v e r t i c a l s e d i m e n t a g g r a d a t i o n (cf. E b e r l i , 1 9 8 7 ) .
Coarsening-
and
thiokening-upward
unit
Description The u p p e r unit is c o m p o s e d of the f o l l o w i n g lithofacies: !) h e m i p e l a g i c micrites9 2) g r a d e d c a l c a r e n i t e s and c a l c i r u d i t e s 9 and 3) megabreccias. The base of this unit c o n s i s t s of m - s c a l e thickening-upward b i o c l a s t c a l c a r e n i t e cycles a n a l o g o u s to those o c c u r r i n g at the base..' of the underlying fining-upward and thinning-upward unit.These cycles and the hemipelagic micrites occur as i n t e r b e d s w i t h i n ten-m t h i c k , g r a d e d m e g a b r e c c i a s w h i c h become p r o g r e s s i v e l y t h i c k e r h i g h e r - u p the section. The u p p e r m o s t part is composed of pebbly mudstones including rounded clasts containing lithofacies indicative for an inner lagoonal environment.
Interpretation The p r e d o m i n a n c e of thick m e g a b r e c c i a s t o w a r d s the top s u g g e s t s some corresponding change in slope character through the g e o l o g i c a l time. The u p p e r unit marks a p r o g r a d a t i o n a l stage of the p l a t f o r m w h i c h led to the d e v e l o p m e n t of a b y - p a s s g u l l i e d slope margin (Fig.105).It was the response to a eustatic sea-level rise which took place in the Albian (Haq,et ai.,1987).
188
189
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190
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031 O3
Fig,t05 sequence.
-
Phases
of
evolution
of
the
first
depositional
191
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d
u.
m
E) QJ
m ,-4
0 -"I ,el
0
r~ 0 D
r~
0
D
0
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192
p%%%%%~ ,L,0~scAR
OL,NOFO~ S
PARASEQUENCES (HST}
F i g , l O T - A P a n o r a m i c v i e w of l o c a l i t y s e c t i o n t r a c e of s l i d e scar, a n d m e g a b r e c c i a (lowstand Flattening-out olinoforms composed of r u d i s t rudstones,
~2 s h o w i n g the system tract), grainstones and
193
Fig.f08 Transgressive system tract overlain by parasequence, and (below) example of a parasequence of h i g h s t a n d s y s t e m t r a c t of the s e c o n d d e p o s i t i o n a l s e q u e n c e .
a the
194
Second
depositional
sequence
The second depositional sequence,Albian to S e n o n i a n (Pattera,1966-1967),is c o m p o s e d of the f o l l o w i n g facies
in age tracts:
i) l o w s t a n d f a c i e s tract~ 2) t r a n s g r e s s i v e facies tract; 3) h i g h s t a n d f a c i e s tract9 a n d A) s h e l f m a r g i n f a c i e s tract. Some panoramic views and s h o w n in F i g . 1 0 6 - 108.
details
Lowstand
of
facies
these
facies
tracts
are
tract
Descrip.tion The lowstand f a c i e s t r a c t is a g r a v i t y deposit, approximately 220 m t h i c k , which extends from the amphiteater-like slide scar~ basinward for about some hundred of meters. The stratigraphic section (section ~2: F i g . 1 0 2 ; Fig,109) consists of a b o u t 300 m thick lithoclast megabreccia laterally and vertically transitional to clinoforms composed mainly of bioclast ( r u d i s t ) d e t r i t u s . A l o n g the v e r t i c a l , a t a first sight the m e g a b r e c c i a a p p e a r s to be a u n i f o r m ,unlayered body. In a basinward direction however, it r e c o r d s r e p e a t e d v a r i a t i o n s in size,
packing
and
composition
of
the
clasts.
The b a s a l 60 m of the s u c c e s s i o n are c o m p o s e d of w e l l p a c k e d , dm - cm s i z e c l a s t s of m a i n l y o o b i o s p a r i t e s and biosparrudites. T h e r e m a i n i n g p a r t c o n s i s t s of r e p e t i t i o n s of t h r e e u n i t s w h i c h form a modal sequence .Its r e c o g n i t i o n is n o t too evident: t h e s e t h r e e u n i t s are r e c o g n i z e d by s u b t l e increases in the percentage of d i f f e r e n t t y p e s of c l a s t s . I) T h e b a s e c o n s i s t s of d e n s e l y p a c k e d dm to s m - s i z e u n f o u n d e d clasts of o o l i t e lithofacies0The megabreccia here is c l a s t supported, poorly sorted, has a chaotic fabric, angular to sub-angular clast boundaries,absence of i n t e r s t i t i a l m u d and grading. 2) T h e i n t e r m e d i a t e p a r t is a less d e n s e l y packed parabreccia with a biosparrudite matrix. T h e u p p e r l i m i t of c l a s t s i z e is on the order of I/2 m. Clasts of rudist rudstones are particularly frequent in t h i s part. It is c h a r a c t e r i z e d by a
195
poor
sorting
inverse
3)
and
a matrix
support.There
are
also
basal
zones
of
grading.
the
supratidal provenance
upper
unit
laminites containing
is
a
breccia
with
clasts
of
oncolites,
a n d of s k e l e t a l wakestones of lagoonal s t r u c t u r e s of s u b a e r i a l e x p o s u r e .
300 -
well packed rnegabreccia
~ 20C
)0
loosely packed megabreccia
•~"~"~" ,,v9, ,,Y rudist biorudite red colored,laminated matrix C~----bioclast 10C
~
oolite rudist fragment
(~
lithoclast coated grain
([~
supratidal laminite
mO
F i g . 1 0 9 - L o w s t a n d f a c i e s t r a c t ( s e c t i o n # 2 ) . T h e l o w e r m o s t 60 m represent a lowstand fan,produced by a catastrophic slope f a i l u r e . T h e r e m a i n i n g p a r t of the s e c t i o n is i n t e r p r e t e d as a lowstand wedge facies tract. G o i n g b a s i n w a r d , s e q u e n c e s r e c o r d a p r o g r e s s i v e d e c r e a s e in the proportion of oolite lithofacies and an increase in the l i t h o f a c i e s of the u p p e r u n i t . C l a s t s of r u d i s t r u d s t o n e s a l s o b e c o m e m o r e c o m m o n u p w a r d s , e s p e c i a l l y in the u p p e r m o s t 50 m. P a r a b r e c c i a f a b r i c s a r e m o r e f r e q u e n t in the u p p e r p a r t of the section.
196
Interpretation The m e g a b r e c c i a is a g r a v i t y d e p o s i t l a i d d o w n at the b a s e a n d against the platform. Bauxite clasts dispersed in the megabreccia, laminations ponded in d e p r e s s i o n s , rounding of clasts a n d the y e l l o w - c o l o r e d matrix between clasts suggest that s o m e a r e a s of the p l a t f o r m were emergent w h e n the s l o p e failure took place. According to S a r g (1988),during the formation of a type-I s e q u e n c e b o u n d a r y t h e r e is a s i g n i f i c a n t a m o u n t of s l o p e f r o n t erosion with downslope i n p u t of l a r g e q u a n t i t i e s of m a t e r i a l . In fact, the f o r m a t i o n of t h e m e g a b r e c c i a was correlated with a c o e v a l s u b a e r i a l h i a t h u s in t h e s o u t h e r n A p p e n n i n e s . The m e g a b r e o c i a was i n t e r p r e t e d by B o s e l l i n i and Ferioli(1988) as the r e s u l t of a c a t a s t r o p h i c collapse i n d u c e d by a s u d d e n eustatic lowstand in the T u r o n i a n (90 m.y. : H a q , e t ai.,1987). The slide scar described by M u l l i n s , et ai.(1986) from the Florida plateau was also taken by these authors as an actualistic e x a m p l e of the s i t u a t i o n e n c o u n t e r e d here. This interpretation is p r o b l e m a t i c . In fact, the o c c u r r e n c e of a modal sequence points to a m u l t i e v e n t deposition for the megabreccia which excludes a single catastrophic collapse.The slide scar reported by M u l l i n s , e t ai.(1986) originated at the p e a k of a s e a l e v e l h i g h s t a n d , i n the e a r l y M i d d l e Miocene, not d u r i n g a l o w s t a n d of the s e a l e v e l as c l a i m e d by B o s e l l i n i and F e r i o l i (1988). It is n o t k n o w n w h e t h e r the type-I unconformity recorded in other areas in the s o u t h e r n Appennines represents a regional uplift of the A p p e n n i n e s , or a eustatic sea-level fall.By a n a l o g y w i t h the c a s e h i s t o r y reported by M u l l i n s , e t al. (1986) the megabreccia may have originated at the peak of a progradation phase, when the relief of the platform was highest: the m e g a b r e c c i a would represent the f o r m a t i o n of an erosional margin from a by-pass gullied slope which is represented by the top of the first depositional sequence. Exposure of the platform, documented by the occurrence of b a u x i t e c l a s t s w i t h i n the m e g a b r e c c i a , may have been produced d u r i n g a r e g i o n a l p h a s e of u p l i f t w h i c h is r e c o r d e d in s e v e r a l p l a c e s in the s o u t h e r n A p p e n n i n e s 0 The clast-support of the first unit of the modal sequence reflects a rock-fall type of g r a v i t y transport, in c o n t r a s t with the matrix-support of the intermediate unit which originated by d e b r i s flows .Rockfall deposition requires a steep slope t y p i c a l of b y - p a s s or erosional margins, whereas
197
N
0
o
1
UNCONFORMITY " - - ~ ~ " " ~ ; : ; : : : ; ~ ~
1
: -.-.~-
p-
9001
SLIDE SCAR
1050~" F i g , l l O - Slide margin reported debris
flows
scars in the west Florida by Mullins., et al, (1986),
take
place
along
less
inclined
carbonate
platform
slopes.
The modal s e q u e n c e r e f l e c t s d i f f e r e n t e p i s o d e s of a p r o c e s s of landward retreat of the carbonate escarpment, typical of h i g h - r e l i e f c a r b o n a t e s l o p e s , w h i c h is c o m p a t i b l e w i t h a p e r i o d of slow s e a - l e v e l r i s e . T h e c a t a s t r o p h i c c o l l a p s e is r e p r e s e n t e d only by the l o w e r m o s t 60 m thick, c l a s t - s u p p o r t e d m e g a b r e c c i a , The geometry of the lowstand facies tract of the second depositiona! sequence, which can be perceived from an examination of natural e x p o s u r e s , is s i m i l a r to that of the d e p o s i t i o n a l s e q u e n c e d e s c r i b e d by Mullins, et a i , ( 1 9 8 6 ) ~ w h e r e s e q u e n c e s are o r g a n i z e d into a f l a t t e n i n g - o u t c l i n o f o r m p a t t e r n and a d i v e r g e n t - c l i m b i n g p a t t e r n (Fig,llO),
Transgressive
facies
tract
The transgressive facies tract overlies and onlaps the megabreccias and rudist biorudites of the lowstand facies tract, It is a finer g r a i n e d d e p o s i t that stands out c l e a r l y on o u t c r o p f a c e s . I t is e x e m p l i f i e d by two s t r a t i ~ r a p h i c sections
198
located shelfward and basinward (sections ~3 a n d A: Fig,lll) f r o m the s l i d e scar, They demonstrate a general finingand deepening-upward trend. Both sections are divisible into a l o w e r a n d an u p p e r s e g m e n t .
Description The basinward section, 9 m thick, is s a n d w f c h e d between the megabreccias of the lowstand facies tract and those of the highstand facies tract. The lower part is c h a r a c t e r i z e d by ungraded, I0 cm to 1 m thick, mostly ungraded calcarenites and predominating couplets of t h i n b e d d e d m i c r i t e s and cm-thick shales. Micrite beds form 30 + 50 c m t h i c k t h i c k e n i n g - upward sequences s i m i l a r to the sequences occurring at the base of the first depositional sequence (Fig.10~). T h e u p p e r p a r t is c o n s t i t u t e d by g r a d e d c a l c a r e n i t e s , micrite beds and thin chert layers. The shelfward section,75 m thick,is also divisible into two segments (Fig.lll). T h e l o w e r is 20 m t h i c k a n d c o n s i s t s of u n g r a d e d m e g a b r e c c i a s , calcarenites, and thin micrite layers (Fig.ll2). Megabreccia layers (Fig.ll2,113) are A m thick and about 70 m wide. The bases exhibit concave-up outlines, load casts and gutter casts (as m u c h as 60 c m w i d e ) , infilled with rudist bioclasts. The clasts composing the m e g a b r e c c i a a r e m a i n l y of rudist rudstones and to a lesser extent of biomicrites, e s p e c i a l l y at the b a s e of t h e s e c t i o n . L a r g e r c l a s t s a r e 4 x m w i d e in c r o s s s e c t i o n , b u t t h e m o s t c o m m o n d i m e n s i o n s a r e of dm-size. Ovoidal shapes are predominating. Clast imbrications are e s p e c i a l l y frequent where breccias are poor with matrix. The megabreccias are not graded. The upper surface is channalized (Fig.ll2,113). The only internal structures within megabreccias are amalgamations and channel-fills. Gutter casts indicate a 200+35 ° N current direction. Calcarenites and micrites a r e cmto 30 c m thick: they are interlayered with megabreccias. Most of t h e c a l c a r e n i t e s are graded; they are also typically hummocky cross-bedded (Fig.f13). A thickeningand coarsening-upward trend is recognisable at the b o t t o m of the s e c t i o n , where blocks and clasts are dispersed and floating within the calcarenites
(Fig.ll3). T h e u p p e r h a l f of t h e s e c t i o n c o n s i s t s of p r e d o m i n a t i n g , graded hummocky cross-bedded rudist grainstones (Fig.ll3), organized i n t o 50 c m to 2 m t h i c k s e q u e n c e s (Fig.ll3) with a tripartite division:
199
RETROGRADATIONAL UNITS
Fis,lll Transgressive system t r a c t , T h e p a n o r a m i c v i e w shows a part of the shelfward section.The sharp transition from partly channelized megabreccias to the finer grained hummocky cross-bedded rudist beds is interpreted as a retrogradation line, R e t r o g r a d a t i o n a l units: s e c t i o n #3; basinward condensed section: s e c t i o n #~,
BASINWARD CONDENSED SECTION
'~- MAXIMUM FLOODING SURFACE
5
/
•,rr.- RE!rROGRADATtON UNE
/ / / /
m0
,<-.,- TRANSGRESSIVE SURFACE
2(10 v~ L,. O 4-~
4-) II1
f,
UI
e ¢..
O
QJ
r'
4-~ ¢-
,.~ 4.a 0
:6
I_
"D
4.a X3
=,
4,a rd 4o 0 m
¢© ¢-
tn
,,-I
,,,,@ ,.~'~
o
D~
~5
,,-I D
.g
0 11
O
E-'E ¢-
I
201
I) a l o w e r l i t h o c l a s t base; 2) an i n t e r m e d i a t e unit c o n s i s t i n g of w e l l l a m i n a t e d I+2 m m r u d i s t b i o c l a s t s 9 a n d 3) an u p p e r m i c r i t e unit.
sorted
sets
of
G r a i n s i z e s of t h e s e s e q u e n c e s d e c r e a s e t o w a r d s the t o p of t h e section. Wavelengths of h u m m o c k y - c r o s s bedding are of some meter. Gutter casts and other erosional f e a t u r e s at the b a s e of these sequences indicate a 600-85 ° N current direction.
Interpretation Channeled megabreccias are interpreted as ephemeral, anastomozing g u l l i e s w h i c h i n c i s e d the u p p e r s l o p e . The hummocky-stratified, thin-bedded calcarenites and micrites into which channels were cut indicate a deposition in the u p p e r shelf located above storm wave base, Thickening-upward sequences occurring in b a s i n w a r d and shelfward sites indicate that sedimentation rate could keep pace with an initially slowly rising sea-level, The sedimentation took place
mainly
by
aggradation.
The depositional facies of the u p p e r part of the shelfward section is interpreted as a retrogradational transgressive f a c i e s t r a c t d e v e l o p e d on a s l o p i n g s u r f a c e . It r e w o r k e d r u d i s t bioclast material delivered to the s l o p e . Its d e p o s i t i o n was concomitant with a sharp vertical decrease in g r a i n s i z e w h i c h m a y be r e l a t e d to a p r o c e s s of p r o g r e s s i v e s o r t i n g a n d loss of the c o a r s e r fraction, with extraction of f i n e s e d i m e n t s from storm flows in the u p p e r slope.Sediments were reworked into sandwaves and wave- generated structures, T h e a b s e n c e of c o a r s e n i n g and thickening-upward trends rules out
a progradational
trend.
The sharp transition recorded in t h e s h e l f w a r d section from megabreccias to fine-grained, better sorted biorudites and caloirudites with hummocky cross-bedding reflects a rapid d e e p e n i n g c o n s e q u e n t to a r e l a t i v e s e a - l e v e l rise. It is c o r r e l a t e d w i t h the c h a n g e in b a t h y m e t r y and increasing upward deepening tendency recorded in t h e u p p e r h a l f of the basinward section,where chert layers replace shaley micrites. The surface delimiting the t w o s e g m e n t s of the t r a n s g r e s s i v e f a c i e s t r a c t is t e r m e d r e t r o g r a d a t i o n a l line (Fig.ll2) It is t r a c e a b l e from abrupt increase in
the s l i d e s c a r to t h e the deepening-upward
b a s i n a n d m a r k s an trend within the
202
®
- 4 " - - PROXIMAL
--
®
-
©
DISTAL
®
~~~~:~oo,toI~ii~
Fig.ll3 - Transgressive s y s t e m tract. ( s e c t i o n #3: F i E . 1 0 2 ) . A Thickening-upward trend at the b a s e of the m e a s u r e d section with boulders floating within the calcarenites.B: m-scale, graded and thinning-upward sequence with a thick basal biorudite bed overlain by thin beds of calcarenites. C:large-scale hummocky cross-bedding within rudist biorudites and calcarenites.
203
transgressive facies tract. The retrogradational line s e p a r a t e s a lower aggradational to s l o w l y r e t r o g r a d a t i o n a l facies tract, from an upper tract characterized by a more pronounced retrogradational character.This surface corresponds to an inflexion in the profile of the transgressive facies tract w h i c h c o u l d be s e e n in s e i s m i c p r o f i l e s .
ii 1
RAPIDLY RETROGRADATIONAL TRANSGRESSIVE TRACT
MAX,MUM F L O O O , N OSURFACE
.......................
RETROGRADATION LINE .......... TRANSGRESSIVE SURFACE
AGGRADATIONAL TO SLOWLY RETROGRADATIONAL TRANSGRESSIVE TRACT
_- 300 ~
:
.
.
.
-
-
:
........ .
~
. - 800
_- 900
-1200
-1500
Fig.114 Computer modelling of a t r a n s g r e s s i v e facies tract.A retrogradational line ( s u r f a c e ) r e s u l t s f r o m a s l i g h t i n c r e a s e in s u b s i d e n c e or s e a l e v e l r i s e w h e n t h e r e l a t i v e sea l e v e l r i s e is a c o m p o u n d
effect
The computer retrogradational increase either rise, when the
of
subsidence
and
eustasy.
modelling (Fig.lla) demonstrates that lines (surfaces) are produced by a slight in the r a t e of s u b s i d e n c e or e u s t a t i c s e a - l e v e l relative sea-level rise is the summation of
subsidence and eustasy. S t r o n g e r i n c r e a s e s in the r a t e of e i t h e r the two c o m p o n e n t s of the r e l a t i v e s e a - l e v e l c u r v e l e a d to a b a c k s t e p p i n g (Fig.llS) which would have been documented in the f i e l d by h a r d g r o u n d s a b o v e the t r a n s g r e s s i v e Correlations between
surface. the two
sections
indicate
that
the
204
transgressive f a c i e s t r a c t is a w e d g e t h a t t h i c k e n s shelfward and thins basinward into a condensed s e c t i o n , as a c o n s e q u e n c e of o f f s h o r e sediment starvation.The lithosome ~eometry of t h e TST, s i m u l a t e d by c o m p u t e r m o d e l l i n g (Fig.llS),originated from tectonic tilt (seaward and landward subsidence), a r i s e in the e u s t a t i c s e a - l e v e l a n d a d e c r e a s e in the s e d i m e n t a t i o n rate,
CONTROLLING
LITHOSOME GEOMETRIES OF THE TRANSGRESSIVE SYSTEM TRACT
V
"°t
Q
900
900
FACTORS
V
sea- level rise ; decreasing sedimentation rate; landward subsidence
Q
sea-level stillstand ~ sea level rise;decreasing sedimentation rate ; no subsidence
(~
sea-level rise; decreasing sedimentation rate ; seaward subsidence
\ ~
m°t ~O0
Q (t)
sea-level stillstand ;increasing sedimentation rate; no subsidence; (2) sea-level rise ; decreasing sedimentation rate; no subsidence
Q {1)
.ool
sea-level stillstand;decreasing sedimentation rate ; landward subsidence ; {2) sea-level rise:decreasing sedimentation rate; no subsidence
30 km
- Computer modelling of d i f f e r e n t geometries of transgressive facies tract obtained by various combinations subsidence, eustatic sea-level rise and sedimentation rate.
Fig,115
the of
205
6
5c 7
.." . . . .
~......~..."-... °.° °~°~
~b ¸
"5c ~o
o
5d~ X
~5c
.i 5b
5a
mO-
.~.....
206
"0
," co
o,.,.i 0 0 J
~o
i
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Z
leen
.C,l 0
r~,~
'~ OJ " 0 0 ~ ,.(:: i= .,,0 F.I hn . •
rr ¢,q
N
_J W
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o
,,~
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0
vJ
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o
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207
Hi~hstand
facies
traGt
D.escription The highstand facies tract is bounded below by the transgressive f a c i e s t r a c t a n d a b o v e by t h e s h e l f m a r g i n f a c i e s tract.It is IA0 m t h i c k a n d c o n s i s t s of s e v e r a l 1 0 - 1 4 m t h i c k parasequences separated by hemipelagic micrites or/and intraformational discordances (Fig.ll6). These parasequences have a dominant oblique sigmoidal progradational geometry. They are characterized by abrupt bases,amalgamations, basinward decreases in grain-size and distal offlapping patterns. Compositionally, they are compound sequences formed by t h r e e lithofacies: I) b i o c l a s t beds; 2) l i t h o c l a s t c o n g l o m e r a t e s ; 3) b r e c c i a s .
and
The last lithofacies is not always present.These three lithofacies m a y be s t a c k e d o n t o a n o t h e r but m o r e c o m m o n l y are s e p a r a t e d by t h i n , o f t e n s l u m p e d h e m i p e l a g i c micrite horizons. B i o c l a s t b e d s c o n s i s t of c o a r s e - t a i l graded, 1 to 0.5 m t h i c k calcirudites and c a l c a r e n i t e s . It is possible to recognize proximal (northern) and distal (southern) end-members. In t h e n o r t h , p r o x i m a l b e d s are 0 . 4 0 as m u c h as 2 , 6 0 m t h i c k a n d c o a r s e g r a i n e d . T h e y c o n s i s t of a lower, c h a n n e l i z e d b r e c c i a or trough cross-bedded b i o r u d i t e , an i n t e r m e d i a t e u n i t c o m p o s e d of dm-thick, graded calcarenites and an upper unit given by laminated micrites, In t h e s o u t h i n d i v i d u a l b e d s a r e b e t w e e n 1 and 1.30 m thick. T h e l o w e r p a r t is up to 0 . 4 m t h i c k a n d is a o a l c i r u d i t e lag including rudist bioclasts and micrite lithoclasts. Some bioclasts are imbricated.Gutter casts and channels , v i s i b l e at places, are infilled with angular lithoclasts or rudist f r a g m e n t s . In m o s t c a s e s h o w e v e r t h e b a s a l lag u n i t is o n l y f e w cm thick.The middle part of the cycle is composed of finer-grained, laminated calcarenites with undetermined shell debris which in s o m e instances is organized into sets of l a m i n a e of p r o g r e s s i v e l y decreasing thickness and grain-size. The uppermost unit consists of p l a n e p a r a l l e l to s m a l l - s c a l e undulated lamination.Laminae both thin and fine upwards.Along the strike, these beds are 20-15 m thick and have a
208
wide,concave-up lower surface or a lower planar and concave-down upper boundary. Laterally,they pinch-out finer-grained lithofacies and mudstones.
upper into
These bioclast beds are organized into a sequence constituted by a s t a c k i n g of 5 - 7 b e d s to a m i n i m u m of 3 l a y e r s with an asequential, irregular, or thinning-upward trend. Conglomerates o c c u r as l e n t i c u l a r 1 to 0.5 m t h i c k b e d s w i t h sharp erosional bases and sharp tops. They display a crude g r a d i n g in t h e u p p e r p a r t , but m o s t of the b o d i e s h o w e v e r are asequential. T h e y c o n t a i n d m - c m , as m u c h as 30 c m size, p a c k e d , rounded clasts of micrite and some fragments of rudist lithofacies. Some clast imbrications were observed. The matrix is e i t h e r c a l c a r e n i t e - c a l c i r u d i t e , or m i c r i t e . S l u m p e d b r e c c i a s o c c u r at t h e top of the p a r a s e q u e n c e s , (above lithoclast conglomerates) and are more commonly found in the u p p e r p a r t o f the h i g h s t a n d facies tract.They sharply truncate underlying hemipelagic sediments. C l a s t s in t h e s e d e p o s i t s a r e of shallow-water and hemipelagic provenance. Shallow-water c l a s t s a r e c o m p o s e d of r u d i s t r u d s t o n e s a n d f l o a t s t o n e s . Clasts a r e m a i n l y o v o i d a l a n d t a b u l a r in s h a p e , as l a r g e as 80 c m a n d with rounded clast boundaries. They have a chaotic fabric and are vertically organized into coarsening-upward trends. Their t o p m o s t p a r t is c h a n n e l i z e d and infilled with rudist fragments and clasts. The matrix is a m i x t u r e of c l a s t s a n d b i o c l a s t s . T h e r e is n o e v i d e n c e of s t r a t i f i c a t i o n , except where clasts are imbricated~ m-size olistoliths a r e f l o a t i n g w i t h i n the m a t r i x .
Interpretation There are two parasequences: I) 2)
basic
types
of
vertical
bed
organizations
within
Thinning-upward, asequential; thickening-upward, coarsening-upward.
The first, found in bioclast-rich beds and the second characterizing lithoclast lithofacies, reflect respectively aggradational and progradational trends (infilling of depressions f o l l o w e d by f r o n t a l a c c r e t i o n ) . The parasequences reflect episodes of accretion driven relative high-frequency sea-level oscillations (tectonics eustasy).A possible mechanism for the formation parasequences
is
shown
in
Fig.llT.
by and of
209
The whole highstand facies tract is a coarseningand thickening-upward megasequence, as a r e s u l t of an i n c r e a s e in proportion and thickness of lithoclast lithofacies and breccias. The occurrence of a m e g a s e q u e n c e trend suggests some control by differential subsidence,
Shelf
margin
facies
tract
DescriR, t i o n T h e s h e l f m a r g i n f a c i e s t r a c t is c o n s t i t u t e d by a compound depositional lobe (Fig.ll6).Layers are stacked,without hemipelagic micrite interbeds, It is c o m p o s e d of a d i s t a l lobe at t h e b a s e , a n d lobe at t h e top.
Distal
70 m t h i c k vertically a
proximal
lobe
T h e l o w e r c o n t a c t w i t h the u n d e r l y i n g hemipelagic micrites is gradational. The first 8 - 1 0 m of t h e succession consist of d m - t h i n l a y e r s (a-b B o u m a i n t e r v a l s : Fig.ll8A). It is f o l l o w e d by a 20 m t h i c k c h a o t i c d e p o s i t ( s l u m p b r e c c i a ) c o n s i s t i n g of a disordered structure with variable amounts of inclusions of rudist clasts, distorted intraformational calcarenite beds floating within a calcirudite calcarenite matrix. The r e m a i n i n g p a r t of the l o b e is an a g g r a d a t i o n a l deposit, Apart from few slump horizons, this part of the d i s t a l lobe is a s t a c k i n g of m - s c a l e thick, thinningand fining-upward cycles (Fig,118B). Individual cycles pass from biocalcirudites with fragments of rudists to fine-grained with plane parallel lamination. The upper part shows evidence for bioturbation (Planolites). M o s t of the c a l c a r e n i t e beds are u n g r a d e d and structureless, w i t h s h a r p b a s e s a n d tops.
Proximal
lobe
T h e u p p e r p a r t of the lobe c o n s i s t s of 15 - 20 m t h i c k d e p o s i t s bounded or developed above slumped horizons or intraformational discordances ( t r a c e s of t r a n s l a t i o n a l slides).The deposits are thickening and coarsening-upward sequences (Fig.ll8C) characterized by an upward transition from thinly bedded calcarenite - micrite alternations to coarse- grained, thick
210
FAST SUBSIDENCE
BIORUDITES /
AUTOCHTONOUS TURBIDITES
SLOWER SUBSIDENCE ; AGGRADATION UPSLOPE ONLAPPING CALCARENITES ............. I
-
"
;
~
~
.
ALLOCHTONOUS TURBIDITES CONGLOMERA~E,S
NO SUBSIDENCE; HIGH-FREQUENCY EUSTATIC CHANGES ; PROGRADATION
UNCONFORMITIES / SLUMP BRECCIA ; MUD DRAPE
DIFFERENTIAL SUBSIDENCE F~,117 - Mechanism of formation of parasequences by an interplay of h i g h - f r e q u e n c y sealevel fluctuations and v a r y i n g subsidence rates, A l l o c h t o n o u s , l i t h o c l a s t - r i c h turbidites and upslope onlap of c a l c a r e n i t e bodies formed during h i s h s t a n d periods which f o l l o w e d a stage of e m e r E e n c e of the p l a t f o r m which favoured the formation of lithoolast beds, thick skeletal calcarenite~, to 7 8 m thick sigmoidal beds (Fig. IISD) and eventually to me~abreccias containin~ lithoclasts and fragments of rudist-bearing rudstones,
211
o
-~
~
•
v
~o .r.i
~
~
"13 %
,21 .,4 (11
X
~
.,..t
'13
<~.~ ~ ~ N
bO ~ ,~ m
~ .8~~
~
'"4 ~ "~
~
0
oo
,m ~ ...o
.,..~
4~
22~
212
F i ~ . l l 9 - C r o s s s e c t i o n of a p o r t i o n of t h e lobe s h o w i n g l o w e r thin beds draping an irregular topography (distal lobe) and upper, thicker convex-upward b e d s ( p r o x i m a l lobe).
Interpretation T h e l o b e is a s i g m o i d a l body characterized by a m a j o r e p i s o d e of a g g r a d a t i o n ( d i s t a l lobe) c a p p e d b y p r o g r a d a t i o n a l deposits ( p r o x i m a l lobe). The absence of h e m i p e l a g i c interbeds in the lobe r e f l e c t s an i n c r e a s e in the a v e r a g e s e d i m e n t a t i o n rate. The r a t e of aggradation was controlled by the accomodation potential (subsidence rate). Initially,the increase in the sedimentation r a t e on t h e s h e l f d u e to a s l o w s e a - l e v e l rise led to t h e f i l l - u p of t h e d e p r e s s i o n on the s l o p e w i t h the formation of thin, concave-up irregular beds (Fig,ll9)~ successively,the decrease in t h e a c c o m o d a t i o n t o g e t h e r w i t h the increase in the r a t e of r e l a t i v e sealevel rise determined a progradation of t h e l o b e w i t h an i n c r e a s i n g upward deepening tendency,
213
Third
depositional
sequence
This sequence is l o w e r to m e d i u m Eocene in age. The lower sequence boundary has a tight amphiteaterlike outline (Fig,lO2),It truncates the u n d e r l y i n g deposits of t h e s e c o n d depositional sequence.The upper sequence boundary is not visible,
Fig.120 Panoramic view clinostratification (Punta Rossa sediments (Peschici Formation),
of M,Saraceno Formation) overlying
showing basinal
The third depositional sequence represents the progradation of a Nummulite buildup (M,Saraceno Formation) on slope sediments (Punta Rossa Formation) above basinal sediments (Peschici Formation). The progradational character of the sedimentation is well evident from an examination of the measured stratigraphic sections and panoramic views which show clinostratification developed above basinal sediments and in t u r n o v e r l a i n by a shallow-water Nummulite buildup, (Fig,120). This depositional sequence is c h a r a c t e r i z e d the f o l l o w i n g f a c i e s t r a c t s ( F i g . 1 2 1 ) :
by
I) l o w s t a n d f a c i e s t r a c t ( t h i c k n e s s : 50 m); 2) t r a n s g r e s s i v e facies tract ( t h i c k n e s s : 20 3) h i g h s t a n d f a c i e s t r a c t ( t h i c k n e s s : 30 m).
the
cm);
stacking
of
214
B 200
A
Nummulite buildup
100 Proximal clinoform f. (HST)
150
Proximal clinoforms ( HST )
(TST)
50
100 Megabreccia (LST)
m0
SEOUENCE BOUNDARY
Distal clinoform S IHST') 50
A
B
I
I
Basinal micrites m0
Fig,121
- Third
depositional
sequence,
215
Lowstand
faaies
tract
Description It is c o n s t i t u t e d by a 50 m t h i c k megabreccia. The lower contact above thin-bedded, hemipelagic l i m e s t o n e s of the s e c o n d depositional sequence is erosional. The megabreccia is a multi-event deposit consisting of s e v e r a l channalized bodies separated by amalgamation surfaces. These bodies are a parabreccia containing m-size blocks and clasts of biocalcirudites, mainly of r u d i s t material and open lagoon sediments, embedded within a micrite matrix. Smaller micrite c l a s t s are of d m - c m size. C l a s t s are p o o r l y s o r t e d a n d a n g u l a r . Upwards, the m e g a b r e c c i a displays a normal grading and clasts b e c o m e m o r e e n r i c h e d in f o s s i l s , m a i n l y in N u m m u l i t e s .
Interpretation The m e g a b r e c c i a represents a s e r i e s of e p i s o d e s resulting from an e r o s i o n of a c a r b o n a t e slope. It is a n a l o g o u s in t e x t u r e and f a b r i c to the r u d i s t p a r a b r e c c i a occurring at the b a s e of the second depositional sequence,which have been interpreted as a l o w s t a n d fan. The m o d a l i t y of d o w n s l o p e movement t o o k p a l c e by debris flow, as is shown by the clastic, chaotic texture s u p p o r t e d by a fine m a t r i x .
Transgressive
facies
tract
Description This is constituted by a thin stratigraphic thickness consisting of t h i n m i c r i t e h o r i z o n s intercalated w i t h a few, bioclasts,a thinning-upward graded beds composed of N u m m u l i t e thin hard,round horizon and large-scale symmetrical undulations (Fig.122).
Interpretation The t h i n m i c r i t e beds i n t e r b e d d e d with wackestone g r a d e d beds represent a condensed section formed during a sealevel rise which drowned the lowstand wedge.The thinning-upward trend displayed by g r a d e d beds resulted from sediment starvation. Symmetrical undulations may represent t r a c e s of s l i d e s c a r s or the r e s u l t of s m o o t h i n g of the s e a f l o o r by c u r r e n t s or w a v e s , a s of h a r d g r o u n d s on top of is also suggested by the o c c u r r e n c e
216
~
%S~ ~
Fig.122 Symmetrical undulation transgressive f a c i e s tract.
and
hardground
in
the
the u n d u l a t i o n s . Slide scars result from gravitational instability produced by sediment overloading more commonly d u r i n g p e r i o d s of i n c r e a s e d sedimentation rate, for example during a highstand facies tract, t h a n d u r i n g p e r i o d s of s e d i m e n t s t a r v a t i o n .
Highstand
facies
tract
Description The highstand facies tract consists of a s e r i e s of proximal and distal clinoforms have been observed measured stratigraphic sections.
Proximal
clinoforms: in the two
clinoforms
Proximal clinoforms are thickeningand coarsening-upward sequences formed by m-scale, lens-shaped, sigmoidal megabreccias interbedded with hemipelagic sediments (Fig.123A). Thicker megabreccias have a tripartite division:the base consists of m i c r i t e l i t h o c l a s t s (5-10 c m in size), g a s t r o p o d s , Nummulite shells. The intermediate unit is a biorudite N u m m u l i t e s . The u p p e r u n i t c o n t a i n s containing gastropods and Nummulite shells oriented parallel to the bedding plane Nummul~te shells reflects that (FiE.123B). The inclination of of c l i n o f o r m s : going upwards, stratigraphically, the a n g l e s of inclination d e c r e a s e f r o m 35 °- 20 ° to I0 ° or less. Distal In
the
measured
area
clinoforms
distal
clinoforms
display
a
downlap
217
- H i g h s t a n d facies tract.A: p r o x i m a l c l i n o f o r m s s h o w i n g a megabreccia (right) passing to sigmoidal clinoforms (left).B;detail of proximal clinoform showing a basal lag Nummulite shells composed of gastropods and disoriented Nummulites, mainly overlain by a t h i c k e r unit composed of inclined at a low angle with r e s p e c t to the b e d d i n g plane.C: N u m m u l i t e shells. d m - t h i c k distal c l i n o f o r m s c o m p o s e d of termination against the lower h e m i p e l a g i c basinal sediments (Fig,12~),Clinoforms are I+2 m t h i c k , c o n t a i n a b u n d a n t N u m m u l i t e bioclasts with a p a r a l l e l to r a n d o m o r i e n t a t i o n to b e d d i n g planes, and lesser a m o u n t s of o t h e r shell d e b r i s , I n the distal Nummulite beds occur as dm-thick intercalations positions w i t h i n h e m i p e l a g i c m i c r i t e s (Fig,123C). Interpretation The two s e c t i o n s represent shelfward sigmoidal clinoforms of an oblique (Fig.125).
and b a s i n w a r d progradational
parts of pattern
218
Fig.12~ - Distal (section #9).
clinoforms
downlapping
The s i g m o i d a l shape is s u g g e s t e d by: downlap terminations of c l i n o f o r m s , the shell i n c l i n a t i o n s .
hemipelagic
micrites
I) u p s l o p e f l a t t e n i n g and and 2) u p w a r d c h a n g e s in
R a t h e r than h o m o g e n e o u s , these clinoforms have a multistored geometry resulting from v a r i o u s types of g r a v i t y mechanisms shells and g r a i n (debris f l o w s , s l u m p , a v a l a n c h i n g of N u m m u l J t e flows in the distal areas).The progradational and frontal accretional c h a r a c t e r of the c l i n o f o r m is i n d i c a t e d by a b r u p t bases at the p r o x i m a l areas, amalgamations, and g r a d a t i o n a l t e r m i n a t i o n s n e a r distal ends.
Concludin s remarks
The g e o m e t r y of the s e c o n d d e p o s i t i o n a l s e q u e n c e c o r r e s p o n d s to the s i g m o i d p r o g r a d a t i o n c o n f i g u r a t i o n d e s c r i b e d by M i t c h u m , e t al. (1977) : "A sigmoid progradational configuration is a prograding clinoform pattern formed by superposed si~moid (S-shaped) r e f l e c t i o n s i n t e r p r e t e d as s t r a t a w i t h thin, g e n t l y
219
DISTALCLINOFORMS ~ SE
NW ~iiatform3
:~i gastropods &nummulites ~ ~- lithoclastlag
~!
Fig.125 clinoforms.
Highstand
facies
tract.
downlapsurface
Inferred
geometry
of
d i p p i n g u p p e r and lower segments, and thicker, more steeply dipping middle segments. The u p p e r (topset) segments of the strata have horizontal or very low angles of dip and are concordant with the u p p e r surface of the facies unit. The thicker middle (foresets) segments form lenses superposed to a l l o w s u c c e s s i v e l y y o u n g e r lenses to be d i s p l a c e d l a t e r a l l y in a d e p o s i t i o n a l l y d o w n d i p direction, f o r m i n g o v e r a l l o u t b u i l d i n g or p r o g r a d i n g p a t t e r n s . . . The most d i s t i n c t i v e f e a t u r e of the sigmoidal configuration is the interpreted parallelism and c o n c o r d a n c e of the u p p e r stratal (topset) segments, suggesting a degree of c o n t i n u e d upbuilding (aggradation) of the u p p e r segments coincident with p r o g r a d i n g of the m i d d l e segments. This configuration implies relatively low sediment supply, relatively rapid b a s i n s u b s i d e n c e , and/or rapid rise in sea level to allow deposition and preservation of the topset units" The
types
of
clinoforms,
the
upper
toplap
boundary
relations
220
w i t h the s h a l l o w w a t e r N u m m u l J t e b u i l d u p a n d the l o w e r d o w n l a p t e r m i n a t i o n of the t h i r d d e p o s i t i o n a l sequence are geometrical f e a t u r e s of an o b l i q u e p r o g r a d a t i o n a l p a t t e r n ( M i t c h u m , et al., 1977): "An oblique progradational configuration ... is interpreted as a prograding clinoform pattern consisting ideally of a number of relatively steep-dipping strata terminating updip by toplap at or n e a r a n e a r l y flat upper s u r f a c e , a n d a d o w n d i p by d o w n l a p a g a i n s t the l o w e r s u r f a c e of the facies unit. Successively younger foresets segments of strata build almost entirely laterally in a depositional downdip direction. They may pass laterally into thinner bottomset segments, or terminate a b r u p t l y at the l o w e r s u r f a c e at a r e l a t i v e l y high angle. T h e y b u i l d out f r o m a r e l a t i v e l y constant upper surface characterized by l a c k of t o p s e t s t r a t a and by pronounced toplap terminations of foreset strata. Depositional dips are characteristically higher than in the sigmoid configuration, and may approach I0 ° " (Mitchum, et a i . , 1 9 7 7 : p a g e 125). The third depositional sequence formed during a first-order sea-level fall.The progradation during the highstand facies tract was the r e s p o n s e to a sealevel fall w h i c h forced the s e d i m e n t to e x p a n d l a t e r a l l y , as a r e s u l t of a d e c r e a s e in the vertical space available for sedimentation. The relatively minor thicknesses of the f a c i e s t r a c t s , c o m p a r e d to t h o s e of the second depositional sequence, imply a corresponding minor rate of a c c o m o d a t i o n potential.Tectonic subsidence was negligible to absent during the deposition of the third depositional sequence. The geometries of t h e f i r s t a n d s e c o n d depositional sequence (sigmoidal or climbing-divergent pattern) and of the third depositional sequence ( o b l i q u e or c l i m b i n g c o n v e r g e n t pattern) are the product of different combinations of allocyclic factors.The second depositional sequences formed under conditions of differential subsidence, an increasing sedimentation rate and a sea-level curve characterized by a r a p i d r e l a t i v e s e a - l e v e l f a l l --> a r a p i d sealevel rise---> a s t i l l s t a n d to s l o w s e a - l e v e l rise. The third depositional sequence formed under conditions of negligible subsidence, a decreasing sedimentation rate and a stillstand
or
fall
of
the
sealevel.
G e o m e t r i e s s i m i l a r t o t h a t of t h e s e c o n d d e p o s i t i o n a l sequence (sigmoidal or divergent patterns) simulated by computer modelling (see a b o v e ) , occur in the E x m o u t h plateau elastic wedge (Erskine and Vaii,1988; Fig.96,92H) and in the post-middle Miocene sediments filling the slide scar described
221
by M u l l i n s et al. (1986; Fig.ll0).This last e x a m p l e shows a d i v e r g e n t p a t t e r n c o m p o s e d of a set o f d e p o s i t i o n a l sequences t h i n n i n g a w a y from the slide scar; the e u s t a t i c s e a - l e v e l curve (Haq, et ai.,1987) indicates that these sediments were deposited during a sealevel highstand. The results of the computer simulation indicate that a divergent pattern which is g e n e r a t e d d u r i n g a sealevel fall forms under a landward or seaward subsidence control. The computer simulation of the E x m o u t h plateau also suggests a s u b s i d e n c e control a c t i n g by means of a b a s c u l a t o r y tectonics and a s e a w a r d tilting. The recognition of an interplay of differential tectonics d u r i n g the d e p o s i t i o n of the second d e p s i t i o n a l s e q u e n c e casts a s h a d o w of doubt on the e u s t a t i o interpretation f u r n i s h e d by Bosellini & Ferioli (1988).This also by c o n s i d e r i n g that the T u r o n i a n (the time of f o r m a t i o n of the l o w s t a n d facies tract of the second depositional sequence) is a non-glacial time interval; also the s e a - f l o o r s p r e a d i n g does not show to have u n d e r g o n e s i g n i f i c a n t c h a n g e s d u r i n g this time. On the o t h e r hand, the c o r r e l a t i o n s of the two l o w s t a n d facies tracts w i t h well p r o n o u n c e d s e a - l e v e l l o w s t a n d of the curve by Haq, et a i . ( 1 9 8 7 ) is well evident. This apparent contradiction leads s i g n i f i c a n e of these sharp lowstand of v i s i b l e from the e u s t a t i c curve.
to the
reconsidering sea-level that
the are
In the Gargano peninsula the Eocene succession at some localities (Tremiti Islands; Vieste area: Cremonini, et ai.1971) records the direct deposition of shallow-water sediments (Formazione di San Domino) above the basinal succession (Formazione di Caprara). The amount by which s e a l e v e l had f a l l e n to p r o d u c e such a s e d i m e n t a r y record is unrealistic; a tectonic uplift in these cases is more likely.That t e c t o n i c s was active d u r i n g the s e d i m e n t a t i o n of the C r e t a c e o u s and E o c e n e slopes is a p p a r e n t when one e x a m i n e s the n e a r b y coeval bedding disturbances of pelagic deposits consisting of recurren£, spectacular slumped horizons sandwiched
between
undisturbed
layers.
The relative sea-level fall on the platform and the differential subsidence in the b a s i n is in a c c o r d a n c e w i t h a model of t e c t o n i c t i l t i n g and flexure, and t e c t o n i c inversion characterizing the initial stages of the evolution of a marginal geosyncline (e.g. Wezel,1988).This mechanism, quite common, has been r e c o g n i z e d for e x a m p l e a l o n g the c o n t i n e n t a l p l a t f o r m b o r d e r i n g the T y r r e n i a n basin, in the M e d i t e r r a n e a n Sea.
222
The interpreted reconstruction of the first and second d e p o s i t i o n a l s e q u e n c e c o m b i n e s the d i f f e r e n t t h i c k n e s s e s of the two s e q u e n c e s , the l o c a t i o n of the t h i r d d e p o s i t i o n a l s e q u e n c e at a lower topographic level than the second depositional sequence, and the types of patterns of the two sequences (Fig.126). It is similar to the geometry described by Mullins,et al.(1986).It differs from the i n t e r p r e t a t i v e cross section of Bosellini (1989) which assumes that the third d e p o s i t i o n a l s e q u e n c e f o r m e d on top of the s e c o n d d e p o s i t i o n a l s e q u e n c e . l n his r e c o n s t r u c t i o n , Bosellini (1989) did not take into a c c o u n t the g e n e r a l p a l e o g e o g r a p h i c data, w h i c h i n d i c a t e that the most part of the Gargano paeninsula was already e m e r g e d at the time of d e p o s i t i o n of the t h i r d d e p o s i t i o n a l sequence (Azzaroli Cita,1967).Also~the third depositional s e q u e n c e is l o c a t e d t o p o g r a p h i c a l l y at a lower level than the second depositional sequence.
W
SLIDE SCAR
u
1LSLIDE SCAR
m 100 i
0
100
200 ~
rn
W
£
E skm
Fig.126 - Geometries sequences (above) and Bosellini
(1989)
of the second and third depositional comparison with the cross section of
(below).
Middle
Triassic carbonate buildups, Dolomites
Introduction
The M i d d l e Triassic of the Dolomites records a period of extensional tectonics attributed to i n t r a c o n t i n e n t a l rifting (Brandner,198~) or diapiric uprise (De Jong,1967).In the A n i s i a n the c a r b o n a t e p l a t f o r m u n d e r w e n t a c o m p l e x d e f o r m a t i o n l e a d i n g to its b r e a k - u p and the f o r m a t i o n of a c o m p l e x b u i l d u p m o s a i c (buildups s e p a r a t e d by p e l a g i c basins). The d i f f e r e n t i a l tectonic subsidence and u p l i f t led to the f o r m a t i o n of a ridge, the " d o r s a l e b a d i o t o - g a r d e n e s e " (Pisa,et ai.1980,) which is b a s i c a l l y a ramp s t r u c t u r e elevating the basin in the west, as r e s u l t s from an e x a m i n a t i o n of the c r o s s s e c t i o n c o m p i l e d by B o s e l l i n i (1965),
S ~ L L A PLATFORM LATEMAR
CARNIAN-
%
LADINIAN
VOLCANICS
soe
PERMIAN
BADfOTO - GARD~NESE R|OG~
Fig.127 - Schematic reconstruction b e t w e e n the P e r m i a n and T r i a s s i c ( s i m p l i f i e d from B o s e l l i n i , 1 9 6 5 ) .
CARN,AN
8*S,N
of the g e o m e t r i c a l F o r m a t i o n s of the
relations Dolomites
224
Due to the late A n i s i a n uplift, Ladinian buildups g r e w from s l i g h t l y e l e v a t e d a r e a s in the w e s t . T h i c k n e s s e s of b u i l d u p s are of about 8 0 0 - I 0 0 0 m . The e s t i m a t e d s u b s i d e n c e rate was on the order of A00 m/Ma (Bosellini,198A). These buildups were s u r r o u n d e d by a starved, a n o x i c basin. B a s i n a l s e d i m e n t s of the L i v i n a l l o n g o F o r m a t i o n (0-200 m thick) w h i c h r e c o r d a v e r t i c a l deepening-upward trend (Cros,1974).Thicknesses of the F o r m a t i o n are 800 m in the East, 600 to 500 m in the West. The d e v e l o p m e n t of s h a l l o w - w a t e r carbonate buildups display a strong antecedent topography control, as they developed on u p l i f t e d a r e a s by k e e p i n g p a c e w i t h a s t r o n g s u b s i d e n c e . They are: the S c h l e r n , S c i l i a r , R o s e t t a , Latemar, Cernera, M a r m o l a d a , C a t i n a c c i o b u i l d u p s (Fig.128). In the west i n d i v i d u a l c a r b o n a t e buildups, separated by pelagic deposits, g r e w v e r t i c a l l y and
p
0
Fig.128 - Distribution of Dolomites (simplified from
Ladinian - Carnian Leonardi,1967).
"
TO XM
buildups
of
the
225
laterally. The progradation is r a t h e r c o n s i s t e n t in the w e s t (9-10 km: see C a t i n a c c i o : Bosellini, 1984), m u c h less in the E a s t (I-2 km). S e d i m e n t a t i o n rates were also variable. The U p p e r L a d i n i a n was a t i m e of i n t e n s e t e c t o n i c and volcanic activity (Viel,1979) that led to the emersion of carbonate buildups (Blendinger,1982; Wendt & Fursich,1980). Carbonate sedimentation was resumed in the C a r n i a n , The formation of a continental flexure was surmised by Blendinger (1982) and Sarntein (1967). The buildups in the W e s t in the D o l o m i t e s w e r e r e c e n t l y s t u d i e d by Bosellini (198~); those in the East by Blendinger (198A;1985,1986).Whereas the first (198A) p r o d u c e d a model of progradation, the s e c o n d w o r k i n g in the E a s t p r o p o s e d a model of stationary to retreating platform. This led to some discussion centered on the n e e e d to delineate the spatial l i m i t s of v a l i d i t y of the two r e s p e c t i v e m o d e l s . The d i f f e r e n c e in the e v o l u t i o n b e t w e e n the b u i l d u p s l o c a t e d in the w e s t e r n a n d e a s t e r n s e c t o r s m a y be i n t e g r a t e d by e x t e n d i n g the i n t r a s h e l f r a m p m o d e l to the D o l o m i t e r e g i o n , w h i c h r e s u l t s therefore subdivided into the f o l l o w i n g s e c t o r s (Fig.129): 1)
a
'shallow
ramp',
buildups; 2) an 'intermediate Limestone~ 3) a 'deep
exemplified ramp',
and ramp',exemplified
by
the
constituted by
the
Cernera
Latemar-Catinaccio by
the
Marmolada
massif.
The s e d i m e n t a r y t r e n d s in the t h r e e s e c t o r s were respectively characterized by p r o g r a d a t i o n (Latemar-Catinaccio), aggradation (Marmolada Limestone) and retrogradation (Cernera buildup). The b a s i n progressively deepened: whereas carbonate buildups became widespread in the West and merged one into another,deeper basinal conditions became dominant in the e a s t (Gaetani,et ai.,1981).
Shallow
ramp
The L a t e m a r buildup,800 m thick, Late Anisian to L a d i n i a n in age, h a s b e e n s t u d i e d in d e t a i l by G o l d h a m m e r , e t ai.(1990) who recognized the f o l l o w i n g facies in a s c e n d i n g order: I) o p e n subtidal platform (250 m thick); 2) restricted peritidal platform (90 m t h i c k ) ; 3) t e p e e h o r i z o n (120 m t h i c k ) ~ a n d &) restricted peritidal platform with thinner cycles (210 m thick).
226
The L a t e m a r r e p r e s e n t s a t h i r d - o r d e r d e p o s i t i o n a l sequence.The v e r t i c a l c h a n g e s in l i t h o l o g y reflect a p r o g r e s s i v e d e c r e a s e in the a c c o m o d a t i o n . P r o g r e s s i v e i n c r e a s e in s u b a e r i a l f e a t u r e s on top of individual m-scale cycles records a shift from submergent conditions to some more punctuated by subaerial exposure. T h e o v e r a l l t r e n d of the m e g a s e q u e n c e is s h a l l o w i n g upward. Goldhammer,et al, (1990) distinguished: I) a transgressive s y s t e m tract ( a g g r a d a t i o n a l , a m a l g a m a t e d m e g a c y c l e s ) ; and 2) an early highstand system tract constituted by thinner cycles w h i c h r e c o r d t h i r d o r d e r sea level falls of the sea level and a d e c l i n e in the t h i r d o r d e r a c c o m o d a t i o n . The Latemar platform underwent a progradational and aggradational centrifugal growth. The model of g r o w t h is a climbing divergent pattern ('Ladinian model' by Bosellini, 198~).
Apart from differences in size of the i n d i v i d u a l buildups,the situation in this sector is broadly comparable to that occurring in the s h a l l o w ramp of F l o r i d a Bay if i n d i v i d u a l b u i l d u p s are c o m p a r e d to banks of F l o r i d a Bay w h i c h form an a n a s t o m o s i n g n e t w o r k of island and b a n k s , a n d are c h a r a c t e r i z e d by aggradation followed by progradation (mainly directed t o w a r d s the b a s i n or deep r a m p ) , v a r y i n g s e d i m e n t a t i o n rates and m e r g i n g of banks,
Intermediate
ram R
The Marmolada Limestone, I00 m thick, does not record a cyclicity. Conversely, Blendinger,et al (1982) observed at several places I00 m of s u p e r i m p o s e d sandwaves (e.g, Sasso Piatto) and the stacking of homogeneous facies. Multidirectional s a n d w a v e s o c c u r in n e a r b y areas, for e x a m p l e at V i e z z e n a , A c c o r d i n g to B l e n d i n g e r (1986) the L a d i n i a n p l a t f o r m s at this area r e c o r d a c o n t i n u o u s s u b s i d e n c e ; p r o g r a d a t i o n was p r e v e n t e d by synsedimentary faults; subsidence rate here was s t r o n g e r than in the w e s t e r n area (2:1 ratio). The r e s u l t i n g p l a t f o r m was s i m i l a r to a t e c t o n i c horst. Clinoforms here display a d e s c e n d i n g p a t t e r n (Bosellini, 1984).
227
T h e M a r m o l a d a L i m e s t o n e r e p r e s e n t s an a r e a of g e n t l e c o n n e c t i o n of the w e s t e r n s i d e of the ' s h a l l o w ramp' to the ~basin in the east.It developed under prevailing subtidal conditions (Gaetani,
et
ai.,1981).
The facies organization is a n a l o g o u s to that of L i l i Bank ( H i n e , 1 9 8 7 ) c o n s i s t i n g of : I) s a n d b a r f a c i e s 9 2) s a n d cay; 3) s a n d flat; a n d &) r e e f facies. There is a l s o an analogous environmental differentiation into a leeward, low-energy sand flat a n d a w i n d w a r d m a r g i n .
Deep
ramp
The deep ramp consists of basinal sediments, thickening upwards, and isolated buildups. Both display a deepening-upward trend.The Cernera buildup,700 m t h i c k ( B l e n d i n g e r , et a i . , 1 9 8 2 ) is c h a r a c t e r i z e d by u p w a r d c o n v e r g i n g m a r g i n s , as a r e s u l t of a strong subsidence.The platform eventually was drowned and c o v e r e d by p e l a g i c a m m o n o i d f a c i e s , The general trend is s i m i l a r to deepening-upward sequences d e v e l o p e d in the d e e p ramp. In the Upper Ladinian the Dolomites were subjected to deformation. P l a t f o r m s in t h e w e s t w e r e u p l i f t e d and underwent a s u b a e r i a l e x p o s u r e , as a r e s u l t of r o t a t i o n a l subsidence. In the C a r n i a n c a r b o n a t e deposition was resumed: the D ~ r r e n s t e i n Formation records a progradational t r e n d d e v e l o p e d as an o f f l a p r a m p ( G a l i i , 1 9 8 9 ~ c h a p t e r # ). The Ladinian Carnian stratigraphic represents an onlap - offlap geometry.
interval
therefore
228
IlJ 0 I1J
W 0
",,
-
,.':
w
~_
.'~:: i~.."."
e,. 7.,
.".'2.', 7' "~' "
a
I/1 v
0
0
0. Ill e~
i
!i! 1
/
0
'~
0x
.Q
0
,
t
3
,1~ ,,'~ •0 " ~ r' g.4 ~
W e~
0
~ m
o
,o
I1: I-
~ , - o o= o ®
/l','J~..., ,,',,, """
~ ~Z! o
-am
3J
~
.r-I
0
'~
1° 0
Modal
sequence
Onlap ramps (and divergent patterns) are the product of t e c t o n i c t i l t i n g and are a s s o c i a t e d w i t h r e l i e f i n v e r s i o n s and deepening-upward sequences (hinge sequences). Rotational tilting appears to be linked by M ~ r n e r (1986) to geoidal eustasy. The a s s o c i a t i o n of an o l i g o t y p i c fauna in the deep ramp and the f r e q u e n t i n t e r c a l a t i o n s of b l a c k shales or p o o r l y fossiliferous mudstones in the ramp reflect the poorly o x y g e n a t e d c o n d i t i o n s e x i s t i n g in the b a s i n ("AOE" events). The onlap geometry is thought to be found mainly during k r i k o g e n e t i c r e j u v e n a t i o n p e r i o d s . It is a p r a e c u r s o r e p i s o d e of the d r o w n i n g of a p l a t f o r m . Offlap ramps (and convergent patterns) are progradational units; they have o p p o s i t e c h a r a c t e r s to those of the o n l a p ramp geometry; they are linked to k r i k o g e n e t i c q u i e s c e n c e p e r i o d s . The two m a i n g e o m e t r i e s of i n t r a s h e l f ramps d e s c r i b e d in the p r e v i o u s c h a p t e r s are not r e s t r i c t e d to s h a l l o w - w a t e r c a r b o n a t e settings, Along the v e r t i c a l , o f f l a p geometries normally are d e v e l o p e d a b o v e the o n l a p g e o m e t r i e s , The r e s u l t i n g g e o m e t r y (onlap ---> offlap) is recorded in several carbonate palimpsests. One e x a m p l e is r e p r e s e n t e d by the M e s o z o i c c a r b o n a t e p l a t f o r m s in the E a s t e r n N . A m e r i c a A t l a n t i c m a r g i n w h e r e a Lower J u r a s s i c carbonate aggradational stage formed during an e p i s o d e of
BANK STAGE
'
Fig.130 - Jurassic platform m a r g i n (after J a n s a , 1 9 8 1 ) ,
1OO
km
of
the
north
America
continental
231 rifting-drifting as a r e s u l t of a rapid subsidence. In the U p p e r J u r a s s i c the r e s u m e d p r o ~ r a d a t i o n led to the d e v e l o p m e n t of a p l a t f o r m a n d l a t e r to a b a n k ( J a n s a , 1 9 8 1 ; F i g . 1 3 0 ) . Another similar transition occurs in the Devonian carbonate shelf, in the C a n n i n g Basin, where a Frasnian aggradational margin is overlain by a Famennian progradational platform (Playford,1980), The G r e a t B a h a m a Bank studied by E b e r l i and Ginsburg (1987) s h o w s an a g g r a d a t i o n a l p l a t f o r m f o r m e d in the M i d d l e C r e t a c e o u s to M i o c e n e u n d e r l a i n by a p r o g r a d i n g p l a t f o r m in the P l i o c e n e , All the above aggradational stages fall into krikogenetic rejuvenation periods, Such an a r c h i t e c t u r a l organization is a l s o f o u n d in s i l i c i c l a s t i c s ,
The o n l a p - o f f l a p g e o m e t r y r e s u l t s f r o m the f o l l o w i n g s u c c e s s i o n of e p i s o d e s : 1) t e c t o n i c tilting (leading to the e m e r s i o n of the p l a t f o r m and deepening of the b a s i n and s h e d d i n g of m e g a b r e c c i a s ) ; 2) f o r m a t i o n of the o n l a p r a m p by v e r t i c a l s e d i m e n t g r o w t h . 3) f o r m a t i o n of the o f f l a p r a m p by p r o g r a d a t i o n ,
O F F L A P RAM P REGRESSIVE HEMICYCLE
--
Fig,131
TRANSGRESSIVE HEMICYCLE
- Modal
sequence.
CONVERGENT
PATTERN
ONLAP RAMP DIVERGENT PATTERN
232
The resulting modal sequence (Fig.131) is composed of a thinning-upward and deepening-upward h e m i c y c l e o v e r l a i n by a thickening-upward shallowing-upward h e m i c y c l e . It ranges in thickness f r o m t e n s to h u n d r e d of m e t e r s as it o c c u r s at a small, p a r a s e q u e n c e s c a l e a n d at a b i g g e r , m e g a s e q u e n c e scale, (Figs. 3 1 , 3 2 , 3 6 , ~ 9 , 5 6 , 8 6 7 , I 0 3 , 1 0 8 , 1 1 6 , 1 2 1 ) therefore suggesting a h i e r a r c h y of the c o n t r o l l i n g f a c t o r s at d i f f e r e n t l e v e l s .
Short-term indicator
sea-level of
geoidal
falls :
an
pulses?
The case h i s t o r y of the G a r g a n o m a s s i f (chapter #~) has a r i s e n a p r o b l e m r e g a r d i n g the i n t e r p r e t a t i o n of the nature of the s h o r t - t e r m T u r o n i a n and Y p r e s i a n s e a - l e v e l falls r e s p o n s i b l e for the s h e d d i n g of m e g a b r e c c i a s into the basin. The chart of g l o b a l s e a - l e v e l (Haq et a i . ( 1 9 8 7 ) shows abrupt falls of the s e a - l e v e l a s s o c i a t e d with type-i u n c o n f o r m i t i e s and l o w s t a n d s y s t e m tracts (lowstand f a n ; l o w s t a n d wedge), as shown by the s c h e m e of Fig.132.
S E A - LEVEL HIGH LOW
RAPID
FALL
F i g . 1 3 2 - E l e m e n t s of s e q u e n c e s t r a t i g r a p h y a s s o c i a t e d with rapid s e a - l e v e l fall (after H e l l a n d - H a n s e n , e t ai.,1986).
a
The d e r i v a t i o n of the e u s t a t i c chart by Haq, et ai.(1987) from the a n a l y s i s of p a s s i v e margin sequences is taken by some a u t h o r s as an e v i d e n c e for its s o u r c e from global s u b s i d e n c e behavior, r a t h e r than e u s t a s y ( M i a l l , 1 9 8 4 ) . I n fact~ the chart of global s u b s i d e n c e built up by Guidish, et a l . ( 1 9 8 A ; F i g . 1 3 3 ) m i r r o r s the g l o b a l e u s t a t i c curve. A c c o r d i n g to Sloss (1984) unconformities bounding sequences (type-I unconformities) result from g l o b a l l y s y n c h r o n o u s e p i s o d e s of c r a t o n i c uplift. S t i l l e ( 1 9 2 & ) c o n s i d e r e d u n c o n f o r m i t y s u r f a c e s to be r e l a t e d to peaks of m a x i m u m r e g r e s s i o n c o r r e s p o n d i n g to t e c t o n i c phases. A possibility discussed in this c h a p t e r s e a - l e v e l falls r e p r e s e n t g e o i d a l pulses.
is
that
short-term
M ~ r n e r (1986) b e l i e v e s that the s h o r t - t e r m e u s t a t i c falls, brief g e o l o g i c a l d u r a t i o n that p u n c t u a t e the s e a - l e v e l chart
of by
235
I
I
Triassic
l
I,
I
I JurassJcl Cretace - ITertiary + + ous 4,
200
. V all. et al ~*s eustatm, sea level curve ~ .;
100
E3 O~ 10 ii 0 LU
~
o
-.-I, ~
100
O
200 .
Rate of basement subsidence
. ,
ii
ii
!.:
::
300
2o
4O0
u . ~uJ Oa. P'u.r ~'>
500
30
3oo
'2so
'2o0
'1so ~ "
~oo
'so
wm
o
AGE(Ma) Fi~.133 - Relationships between and the changes of basement ai.,1984).
the e u s t a t i c s e a - l e v e l c h a n g e s subsidence rate (Guidish, et
Haq, et ai,(1987) are produced by changes in the geoidal configuration.According to M~rner (1986) geoidal eustasy consists of s e a - l e v e l changes caused by d e f o r m a t i o n of the g e o i d a l surface, c o n t r o l l e d by m a n t l e c o n v e c t i o n m e c h a n i s m s . The f o l l o w i n g a s p e c t s of c a r b o n a t e s e d i m e n t o l o g y as key e l e m e n t s to the r e c o g n i t i o n of s h o r t - t e r m p h a s e s of g e o i d a l pulses: I. 2. 3. &, 5.
can be v i e w e d (<500,000 yrs)
o n l a p r a m p s ; d i v e r g e n t patterns; megabreccias; d r o w n i n g of p l a t f o r m s ; seismoturbidites; r e l i e f inversions.
The Cretaceous system offers a good opportunity to study g e o i d a l e u s t a t i c changes: in fact, it is a n o n - g l a c i a l systems, hence, e u s t a s y was the p r o d u c t of the i n t e r p l a y of t e c t o n i c s and geoidal eustasy, M~rner (1981) recognized Cretaceous latitudinal, n o r t h - south m i g r a t i o n s of the g e o i d a l eustasy. The Upper Cretaceous was a period of intense geoidal d e f o r m a t i o n . The most spectacular e x a m p l e of g e o i d a l change
236
Fig.13A - Paleogeographic scheme of the central Mediterranean area (simplified a f t e r J . A u b o i n a n d B r o u s s e , 1977) s h o w i n g a s e r i e s of r i d g e s s e p a r a t e d by b a s i n s ,
~N
is r e p r e s e n t e d by the foundering of the Darwin Rise in the Pacific Ocean, concomitant with the rising of the Melanesian R i s e a n d the E a s t P a c i f i c Rise (Menard,196~; Wezel,1988).
,<
2u m._~ .~_ E
D
~ g
c "E
n
E ~
-o
.~_ "r n~
.--
a
During the Cenomanian - Turonian interval the r a t e of s e a - f l o o r spreading decreased; eustasy was mainly caused by geoidal changes. This interval records at s e v e r a l .E p l a c e s complex facies characterized by slumps, facies changes, megabreccias and < unconformities. The Mediterranean region consisted of e l o n g a t e d basins separated by intrageosynclinal ridges (Fig.13A) which became the sites of carbonate platform development.Slumps, megabreccias and unconformities w e r e p r o d u c e d by u p l i f t a n d deformation of the intrageosynclinal ridges. Deformation phases were concomitant with the establishment of a n o x i c f a c i e s in the basins (Jenkins,1991 ;Wezel,1985). During the Cenomanian - Turonian there may have been tectonic phases superimposed on geoidal eustasy; nevertheless, the inference of a global episode of deformation of the g e o i d a l s u r f a c e is b a s e d on the worldwide extent of the 'events'.Geoidal deformation was not synchronous everywhere: emersion was not a generalized phenomenon: some areas remained s u b m e r g e d by a t h i n v e n e e r of w a t e r , o t h e r s were drowned and transformed into guyots and seamounts; pelagic,basinal conditions < persisted in the geosynolinal troughs. Anoxio conditions ('AOE' events) were established in t h e b a s i n s . It is p o s s i b l e that uplift caused by the geoidal deformation 'event' contributed to the development of anoxic conditions: as a matter of fact, the whole proto-Atlantic ocean appears to be a semiclosed basin
237 s u r r o u n d e d by s h a l l o w - w a t e r c a r b o n a t e -Cenomanian, as s h o w n in Fig. 135.
Fig,135 Jurassic-Cretaceous distribution of oceanic crust p l a t f o r m s (from J e n k y n s , 1 9 9 1 ) .
platforms
situation (black area)
in the
Turonian
showing the and carbonate
Megabreccias
Megabreccia debris sheets are common features in ancient reworked carbonate sequences. T h e y are i n t e r p r e t e d in g e n e r a l as the p r o d u c t of l a r g e - s c a l e c o l l a p s e of c a r b o n a t e platform megabreccias o c c u r widely throughout the margins,Carbonate Phanerozoic, especially but not exclusively in the M e d i t e r r a n e a n area. The triggering mechanism of their formation is open to
238
discussion. Possible causes are: gravitational instability p r o d u c e d by o v e r l o a d i n g ( C r e v e l l o and Schlager, 1980); f a u l t i n g and s e i s m i c shocks (Mutti and Ricci L u c c h i , 1 9 8 ~ ; Mullins and Hine,1989); undercutting of the e s c a r p m e n t by seeps and/or bottom currents (Paull,et ai.,198~; Mullins and H i n e , 1 9 8 9 ) ; b i o e r o s i o n ; and c h e m i c a l d i s s o l u t i o n . Megabreccias and a s s o c i a t e d unconformities are u b i q u i t o u s in the M e d i t e r r a n e a n region. Some occurrences of m e g a b r e c c i a s , d a t e d m a i n l y to the C e n o m a n i a n - T u r o n i a n , are r e v i e w e d b e l o w (Fig.136).
III 8
A
~112 •-=_" 11 1[
12 /
"".:"1"-7if:..:
ulI
Fig.136 - Location of Cenomanian-Turonian megabreccias and related unconformities described in the text.A: emergent areas;B:basinal siliciclastics;C:carbonate platforms.
239
Lombardy
basin
(Fig.136;#l)
The C r e t a c e o u s s e q u e n c e is c h a r a c t e r i z e d by basinal sediments p a s s i n g from p e l a g i c c a r b o n a t e s to a s i l i c i c l a s t i c flysch. The Middle/Late Cenomanian sequence interval is r e p r e s e n t e d by a s u c c e s s i o n of two c h a o t i c layers (0 to A0 m thick) i n t e r b e d d e d with thin b e d d e d t u r b i d i t e s , D e b r i s flow d e p o s i t s o c c u r also in the overlying Turonian interval (pets. obs,). They are analogous to debris flows interpreted as slope deposits described in n e a r b y areas (Bichsel and H a t i n g , 1 9 8 1 ) . The two chaotic layers d e l i m i t a wedge onlapping onto the n o r t h e r n margin of the basin (Bersezio and Fornaciari,1987). Paleocurrents indicate a northern provenance. The f o r m a t i o n of these c o n g l o m e r a t e s was r e l a t e d to an e o a l p i n e compressive phase in the southern Alps. These conglomerate layers w o u l d be a s t r a t i g r a p h i o e v i d e n c e for the b e g i n n i n g of Cretaceous activity during the Cenomanian-Turonian, as is i n d i c a t e d by D o ~ l i o n i and B o s e l l i n i (1987). C a s t e l l a r i n (1982) related the formation of megabreccias to the action of incipient transcurrent faults (extensions of oceanic t r a n s f o r m s ) l o c a t e d n o r t h of the area, b e t w e e n the A f r i c a n and the E u r o p e a n margins. According to B e r s e z i o and Fornaciari (1987), the unconformity records a first closure of the L o m b a r d y b a s i n t o w a r d s the north.
Insubric
flysch
(Fig.136:
#2;Fig.137)
A 600 m t h i c k flysch s u c c e s s i o n crops out in the a r e a c o m p r i s e d b e t w e e n M a l ~ and R u m o (Castellarin, et al., 1976; C a s t e l l a r i n , 1982),along a 12 Km long belt. The 'Upper conglomerates' (Insubric flysch) studied in detail by Castellarin, et ai.(1976) was delivered from the north by high-density currents. The t e x t u r a l f e a t u r e s of these c o n g l o m e r a t e s i n d i c a t e that phases of meteoric degradation took place prior to transport. These conglomerates would represent a narrow t u r b i d i t e fan located on the direct p r o s e c u t i o n of the n o r t h e r n slope. It is g e n e r a l l y accepted that a n o r t h e r n continental shelf gave o r i g i n to these e l a s t i c d e p o s i t s by u p l i f t i n g a n d / o r rejuvenation. T e c t o n i c m o v e m e n t s p r o d u c i n g these u p l i f t s took place in the A l b i a n - T u r o n i a n time interval, in the n o r t h e r n C a l c a r e o u s Alps. The b e g i n n i n g of the f l y s c h d e p o s i t i o n in the T u r o n i a n was interpreted as the r e m o t e effect of the Pregosau tectonic phase. The source of t h e s e clastics is l o c a t e d within the Austroalpine domain (Castellarin,1982).
240
N
?INN
..N
SLOPE
400
NI"
] DROWHED PLATFORM
BASIN
Fig.137 - Hypothetical r e l a t i o n s b e t w e e n the V e n e t i a n p l a t f o r m and the F l y s c h b a s i n d u r i n g U p p e r C r e t a c e o u s (from C a s t e l l a r i n , et a i . , 1 9 7 6 ) ,
Northern
Calcareous
Alps
(Fig.136;#8).
Gaupp,et al. (1981) described a Cenomanian-Turonian sequence developed above Triassic-Jurassic c a r b o n a t e rocks, p a s s i n g from shallow-water calcarenites to a t u r b i d i t e flysoh, deposited d u r i n g two m a j o r t e c t o n i c p h a s e s of the e a r l y A l p i n e o r o g e n y (Austrian - Mediterranean phases). A clast-supported polymictic megabreccia ('Blockbreccia: G a u p p , e t ai.1981) d e v e l o p e d d u r i n g the C e n o m a n i a n - T u r o n i a n transition, a l o n g the n o r t h e r n m a r g i n of the n o r t h e r n C a l c a r e o u s Alps, for more than I00 Km. This megabreccia is t h o u g h t to h a v e o r i g i n a t e d from b l o c k f a l l s at s u b m a r i n e fault scarps, due to i n t r a b a s i n a l uplift. Gaupp,et al. (1981) suggested i m b r i c a t i o n , f o l l o w e d by e m e r g e n c e , s u b s i d e n c e of the t h r u s t sheets.
Lombardy basin Fig,138~139).
-
Venetian
platform
a combination of nappe b l o c k f a u l t i n g and i s o s t a t i o
transition
(Fig.136:
#11;
241
One of the best d o c u m e n t e d e x a m p l e s of t e c t o n i c s y n s e d i m e n t a r y evolution of a carbonate margin is that described by Castellarin (1972). The investigated area separates the Lombardy basin to the east (deep water,cherty limestones), Jurassic to C r e t a c e o u s in age, from the Venetian platform (Fig.138).Since the b e g i n n i n g of the J u r a s s i c , synsedimentary b l o c k - f a u l t i n g was a c t i v e in this t r a n s i t i o n a l a r e a . A fault was a c t i v a t e d at the b e g i n n i n g of the Liassic, contemporaneously with the deposition of the Trento platform, in the Pliensbachian, Pliensbachian-Toarcian transition, and in the Aalenian.
Fig. IS8 platform
- Cross section of the Lombardy t r a n s i t i o n (from C a s t e l l a r i n , 1 9 7 2 ) .
Basin
-
Venetian
The e a s t e r n side of the T r e n t o p l a t f o r m , b o r d e r i n g the B e l l u n o Trough, has a zig-zag outline due to the control of synsedimentary faults. The m a r g i n was cut by c a n y o n s which delivered megabreccias (Pelf Breccia:Bosellini, et ai.,1981) into a n o x i c , p o o r l y oxygenated basin. During the L i a s s i c the m a r g i n u n d e r w e n t a r e t r o g r e s s i v e m i g r a t i o n by fault activity. The r e a c t i v a t i o n of faults in the w e s t e r n b o r d e r of the T r e n t o p l a t f o r m took p l a c e in the V a l a n g i n i a n - H a u t e r i v i a n interval, as is s h o w n by the a c c u m u l a t i o n of p o l i g e n i c m e g a b r e c c i a s . In the following time intervals (Turonian-Maastrichtian) a r e t r o g r e s s i v e , p l a t f o r m w a r d m i g r a t i o n of fault zones took p l a c e
242 (cannibalistic development of poligenic breccias). In the Turonian the 'H.Gurlo' paleofault was individuated, contemporaneously with the uplift of the platform, as is demonstrated by an i n v e r s i o n of the inclination of b l o c k s (Fig.139), s!
Upper Li;ts sl
Valangini~n $1
Alb~an
Turonlan sl
Campan;an - Lower M~astrlchtian
F i g . 1 3 9 - E v o l u t i o n of the (from C a s t e l l a r i n , 1 9 7 2 ) .
margin
in
the
Jurassic-Cretaceous
The ' B e l l i n o - G a r d a ' p a l e o f a u l t s were r e a c t i v a t e d in the U p p e r O r e t a c e o u s , w i t h the f o r m a t i o n of i m p r e s s i v e , huge d e t a c h m e n t scarps and m e g a b r e c c i a a c c u m u l a t i o n s , a s a result of a g e n e r a l u p l i f t of the V e n e t i a n p l a t f o r m ( C a s t e l l a r i n , 1 9 7 2 ) 0 According to C a s t e l l a r i n (1982), the r e a c t i v a t i o n of these fault s c a r p s was a c o n s e q u e n c e of t r a n s c u r r e n t m o v e m e n t s d u r i n g the G o s a u o r o g e n i c phase,
F,,riuli p l a t f o r m
(Fi~,136:#9;Fig,140)
The s e d i m e n t a r y and t e c t o n i c e v o l u t i o n of the e a s t e r n part of the Friuli p l a t f o r m was s t u d i e d by T u n i s and V e n t u r i n i (1986) who p r o p o s e d a model c o n s i s t i n s of a c o m p l e x series of s t e p w i s e
243
r e t r o g r a d a t i o n e p i s o d e s of the p l a t f o r m m a r g i n r e s u l t i n g from transcurrent tectonics. Figure 140 shows a wedge with u n c o n f o r m i t i e s d a t e d to the Toarcian, V a l a n g i n i a n - H a u t e r i v i a n , Cenomanian-Turonian and C a m p a n i a n ,
SW L.Maast.
NE "/'o~
L.Se no n.--- -~'~
"--~ U. C
,..; .c Alblan
~....
Berrias.
h .'.'.: :";%:: ".:~
,P, ~ 1 "-,,_._ez_z_z_z~_z_z_z~
~'l"~
!
~ -,
100,
------
U.Titon. L.Titon.--- ----- _ Kimm.--~ D°gger ~
50.
Fig. IA0 C o r r e l a t i o n s of s t r a t i g r a p h i c part of the Friuli platform.
sections
of the e a s t e r n
In the T u r o n i a n - C e n o m a n i a n the p l a t f o r m u n d e r w e n t a p e r i o d non d e p o s i t i o n and e m e r s i o n f o l l o w e d by subsidence.
Cretaceous
buildups~
south of France
of
(Fig,136:~7;Fig,l~l),
The data by M a s s e and P h i l i p (1981) show a p l a t f o r m in the south of France s e p a r a t e d by the A l p i n e B a s i n by a ridge area ("Bombement durancien") trending east-west which formed as early as the A l b i a n (Fig,IAl), An overall deepening-upward tendency, i n i t i a t e d in the C e n o m a n i a n , gradually established p e l a g i c c o n d i t i o n s d u r i n g the U p p e r T u r o n i a n and d e t e r m i n e d the reduction of the extent of the shallow-water carbonate d e p o s i t i o n in the p l a t f o r m , T h i s d e e p e n i n g - u p w a r d t e n d e n c y in some areas was c o u n t e r b a l a n c e d by d e f o r m a t i o n of the " B o m b e m e n t durangien" which led to the development of shallow-water c a r b o n a t e s in the Turonian0 M o d e s t u p l i f t s are r e c o r d e d in the V a l a n g i n i a n and T u r o n i a n in other areas of the s o u t h e r n Alps of France ( S i s t e r o n area).
244
J bement urancien"
Alpine basin-----.
aErn~aer ~a~~ gent --~
_
Provence
J
~00 k'rn Fig,l~l Paleogeographic maps of (left),Cenomanian (center) and Upper Philip,1981),
Ligurian
Briangonnais
(Fig.136:
the upper Albian Turonian (Masse and
#6;Fig.132),
The L i g u r i a n B i r a n G o n n a i s represents the p r o s e c u t i o n of the B r i a n G o n n a i s domain, a p a l e o g e o g r a p h i c unit located b e t w e e n the f o r e l a n d and the a l p i n e e u g e o s y n c l i n a l d o m a i n (Ligurian ocean). It e x t e n d s from the Corsica throughout the western Alps. Probably it r e p r e s e n t s an i n t r a g e o s y n c l i n a l ridge s e p a r a t i n g the m i o g e o s y n c l i n e from the e u g e o s y n c l i n e . The d i f f e r e n t i a t i o n
245
of the Ligurian Briangonnais domain initiated since the T r i a s s i c . An u p l i f t due to an o c e a n w a r d t e c t o n i c t i l t i n g took p l a c e in the S i n e m u r i a n - A a l e n i a n time. The M i d d l e J u r a s s i c to L o w e r C r e t a c e o u s interval was a time of s h a l l o w - w a t e r c a r b o n a t e sedimentation. S u c c e s s i v e l y , a d e e p e n i n g - u p w a r d trend, from the Upper Jurassic (Malm) led to a g e n e r a l i z e d development of a hardground in the A p t i a n - C e n o m a n i a n . Unfortunately, detailed a n a l y s e s of this area are l a c k i n g (Vanossi and G o s s o , 1 9 8 5 ) , The p e r s i s t e n c e of a h a r d g r o u n d h o r i z o n in an area s u b j e c t e d to a deepening-upward trend s u g g e s t s that the L i g u r i a n - B r i a n g o n n a i s u n d e r w e n t a r e l a t i v e u p l i f t in the T u r o n i a n - N e o c o m i a n .
Priabonian
Eocene Upper -- --
Cretaceous
Neocomian - Turonian Lower C r e t a c e o u s
Maim
Dogger Lias Rhaetian Middle Triassic
l
I
Fi~.Ia2 - Stratigraphic (Vanossi and G o s s o , 1 9 8 5 ) .
Abruzz!
platform~
central
column
Italy
of
the
(Fig.136:
Ligurian
#3,A;
Briangonnais
Fig. IA3,1A~),
The U p p e r C r e t a c e o u s of c e n t r a l Italy, s t u d i e d by C a r b o n e and S i r n a (1981) is r e p r e s e n t e d by c a r b o n a t e e p i o o e a n i c p l a t f o r m s . C a r b o n a t e s e d i m e n t a t i o n was c h a r a c t e r i z e d by the d e v e l o p m e n t of r u d i s t - c o r a l c o m m u n i t i e s , b a c k - r e e f areas and f o r e s l o p e s e c t o r s up to the C e n o m a n i a n . Progradation was interrupted in the Turonian: the shelf m a r g i n was s u b j e c t e d to d i s r u p t i o n which led to the u p l i f t of the shelf m a r g i n and to the f o r m a t i o n of deep sea ways and intrashelf ramps (pets. obs.), and d e v e l o p m e n t of b e a c h e s (Fig.143). The s u b a e r i a l e x p o s u r e of the m a r g i n was a t t r i b u t e d to tectonism. The transgressive trend continued in the Senonian, when the shelf margin was t r a n s f o r m e d into a seamount.
246
ForeMopa
Buildup
Winnowed e~ge sands
Shelf lagoon
Tidal flat
s.I.
NW
SE
ForeMope
Outer high
Sea way
lnt rashelf ramp Shoal
W
Lagoon
E 60
Km
Fig.1~3 Cross s e c t i o n s h o w i n g the e v o l u t i o n of the margin in the C e n o m a n i a n (above) and S e n o n i a n time From C a r b o n e and S i r n a (1981).
platform (below).
A d e t a i l e d a n a l y s i s of the L a t i u m - A b r u z z i p l a t f o r m was c a r r i e d out by Colacicchi (1987) who distinguished three large s e d i m e n t a r y cycles, e a c h c h a r a c t e r i z e d by a fast t r a n s g r e s s i o n f o l l o w e d by a m o r e g r a d u a l p r o g r a d a t i o n a l phase (Fig.lA~). He c o n s t r u c t e d a c h a r t s h o w i n g the e v o l u t i o n in time and s p a c e of various environmental z o n e s of the p l a t f o r m (inner p l a t f o r m margin-slope) from the T r i a s s i c to the Miocene. The first c y c l e (Lower D o g g e r to V a l a n g i n i a n ) shows a strict c o r r e l a t i o n w i t h the eustatic curve by Haq, et ai.(1987). In the second (Valanginian - Cenomanian) and third cycles (Cenomanian Maastrichtian) the t h r e e sectors of the platform follow different trends. A maximum landward shift of the inner p l a t f o r m t o o k p l a c e in the A a l e n i a n , V a l a n g i n i a n and T u r o n i a n , then in the M a a s t r i c h t i a n , Ypresian, Priabonian-Rupelian and Serravallian. These facts were taken as an evidence for tectonic interference: the d r o w n i n g of p e r i p h e r a l areas was c o u n t e r b a l a n c e d by u p l i f t up to e m e r s i o n of the inner p l a t f o r m areas.
Apulian
platform~southern
Italy
(Fig.136:#12)
Iannone studied angular shallow
and Laviano (1980) and Sinni and Borgomano (1989) a shallow water Cretaceous succession, T h e y f o u n d an u n c o n f o r m i t y and a d i s c o n t i n u i t y s u r f a c e s e p a r a t i n g two water F o r m a t i o n s : the 'Calcare di Bari' from the
247
AGES ~'°
U,
OO M, .~
L
INNER
MARGIN
SLOPE TO
BASIN
TORTONo '
SERRAVALL . BURDIGAL.~ ' AOU, TAN
0
PLATFORM
.
" .
.
RUPELIAN •
" .
.
.
.
.
.
.
. . . . .
.
.
.
•
.
~
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- -
"
*
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--
,
i
•
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M " LUTETIAN W
L,
yPRESIAN'
J
u. L'""
. "
.
°
.
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.
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.
.
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.
.
.
.
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- -
ALBIAN
.%~
"
.
"
*
.
.
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.
MAAe~B
0
"
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%"
~ ---- ~
--
~
- -
--
.
.
-i -
.
"
. 7
KIMM,
~--'~
I'7~.I~.
"
%''~'%'q"."
l"
%~b.~--
I
- -
~
-4
OXFORDIAN'11%"'~~ ~ %.-- ' t,-- %." }.~%-- %/~ [ ~
~
..~
: ~ L O V , ~ . t "~ ~ , ' - ~ ~.h ~- V'," V %-..I % - \ .I'~1"
--
--
EMERGED
~
PLATFORM
~
MARGIN
[--~
Fig. IA~ - E v o l u t i o n of the L a t i u m - A b r u z z i margin (from Colacicchi,1987).
SLOPE
carbonate
/
/
I--I
I
/ /
BASIN
platform
'Calcare di Altamura'. According to these authors this discontinuity corresponds to a g e n e r a l i z e d emersion of the c a r b o n a t e p l a t f o r m d u e to a t e c t o n i c e p i s o d e in the M i d d l e a n d Upper Turonian.
Upper Cretaceous #10;Fig.145).
Rudis t Reef
complex~Central
Tunisia
(Fig.136;
A shallow water carbonate platform, described by N e g r a , et ai.(1987) was dissected in the Turonian by synsedimentary tectonics, A spectacular o u t c r o p at S i d i A b d e l k a d e r (60 k m SW of the c i t y of H a ~ e b e l A i S u n ) s h o w s a r e c i f a l c o m p l e x d e v e l o p e d
248
above u p l i f t e d p a r t s of b a s i n a l d e p o s i t s c o n t a i n i n g A m m o n i t e s , c a l c i s p h e r e s and o t h e r u n d e t e r m i n e d biota. Faults are e v i d e n t in the field. S.L,
Q
®
S.L.
®
S.L.
®
Fig.l~5 - Interpretative relationships at Sidi
s k e t c h of t e c t o n i c Abdelkader area
- sedimentation (modified from
Touir,1986) .
Arabian
carbonate
platform
mar~in~Oman
The Arabian carbonate platform,studied by Watts and Blome (1990) in the O m a n area f o r m e d a l o n g the transitional zone b e t w e e n a p a s s i v e c o n t i n e n t a l m a r g i n and a deep o c e a n i c b a s i n in the east.The platform margin (Qumayrah and Mayhah Formations) was influenced by progressively increasing tectonism, as it approached a subduction zone, In the Cenomanian time, the Oman margin underwent uplifting and erosion; consequent to the oversteepening of the slope and d e f o r m a t i o n , a m e g a b r e c c i a was d e l i v e r e d into the basin, W a t t s and B l o m e (1990) a r g u e that a 'notable s e a - l e v e l l o w s t a n d at this time c o u l d h a v e h e l p e d t r i g g e r mass m o v e m e n t s downslope' The a b o v e a u t h o r s r e l a t e d the r e s e d i m e n t a t i o n of the Q u m a y r a h and Mayhah Formations at this time to either initial deformation and thrust faulting of the slope, or to large g r a v i t y slides. H o w e v e r , they o b s e r v e d that 'the d e f o r m a t i o n of the Mayhah Formation and synorogenic sedimentation of the Q u m a y r a h F o r m a t i o n p r e c e d e d e m p l a c e m e n t of the h i g h e r nappes'
249
Seismoturbidites
Mutti and Rioci Lucchi (198&) suggested to term s e i s m o t u r b i d i t e s the a n o m a l o u s m e g a t u r b i d i t e s c o n s i s t i n g of the delivery of e x c e p t i o n a l l y great volumes of s e d i m e n t and of great lateral extent. They are the p r o d u c t of c a t a s t r o p h i c g r a v i t y flows c a u s e d by s h a l l o w e a r t h q u a k e s .
MUDSTONE
CALCARENITE CALCIRUDITE
E O O O4 O
MEGABRECCIA
O. :3
F i g . l ~ 6 - Modal s e q u e n c e s of m e g a t u r b i d i t e s northern Appennines and southern Pyrenees Lucchi,198~).
occurring in the (Mutti and Ricci
These seismoturbidites occur as scattered intercalations b e t w e e n fans or, more in g e n e r a l , i n turbidite systems. Mutti and Ricci Lucchi (198A) p r o p o s e d two i d e a l i z e d megaturbidite sequences,one reported in Fig. I A 6 . T h e s e megaturbidites,mostly calcareous in c o m p o s i t i o n , are p a r t i c u l a r l y frequent in the e l o n g a t e t u r b i d i t e basins of the n o r t h e r n A p p e n n i n e s . The 'Contessa' megaturbidite layer,dated to the S e r r a v a l l i a n (Middle Miocene) is a megaturbidite layer, 20 m thick, t r a c e a b l e for IA0 Km a l o n g the axis of the M a r n o s o A r e n a c e a foredeep, in the northern Appennines (Ricci Lucchi,1978). A c c o r d i n g to the authors w o r k i n g in the area, the 'Contessa' m e g a b e d was formed d u r i n g a p e r i o d c h a r a c t e r i z e d by a m a x i m u m rate of m i g r a t i o n of the thrust front and h i g h rate of uplift. It represents a transgressive phase in the basin,
250
counterbalanced by a t e c t o n i c t i l t i n g in the n e a r b y c a r b o n a t e p l a t f o r m s w h i c h led to a c a t a s t r o p h i c c o l l a p s e of the c a r b o n a t e platform margin. T h e F r i u l i p l a t f o r m and a d j a c e n t S l o v e n l a b a s i n c r o p o u t in the s o u t h e r n s e c t o r of the J u l i a n A l p s (NE of I t a l y ) . T h e e v o l u t i o n of t h e p l a t f o r m records a withdrawal of t h e m a r g i n (Tunis and Venturini~1987) which underwent a progressive dismantling, In the Maastrichtian the margin was affected by a strike-slip transcurrent tectonic control and listric faults. D u r i n g t h e P a l e o c e n e the n a r r o w i n g of the b a s i n w a s c o n c o m i t a n t w i t h t h e d e p o s i t i o n of m e g a b e d s (Fig,147). In t h e L o w e r E o c e n e a huge amount of m a t e r i a l was deposited i n t o the b a s i n . It consisted of redeposited shelf limestones, slope marls and fragments of m e g a b e d s . These megabeds c r o p out w i d e l y in the eastern Friuli region (Tunis and Venturini,1987).These megabeds a r e d a t e d to the T h a n e t i a n a n d to the Y p r e s i a n . In t h e V e r n a s s o q u a r r y , the E o c e n e megabeds c o n s i s t of a t e n s
Fis. I A 7 - A_) In s i t u ; a u t o c l a s t i c breccia of e a r l y lithified carbonate material ~ probably induced by seismic shocks (Vernasso quarry).B)Parabreccia f o r m e d by 'mud b a l l s ' f l o a t i n g in a f i n e - g r a i n e d calcarenite matrix ( V e r n a s s o quarry)._C) a n d D) P a n o r a m i c v i e w of t h e P a l e o c e n e megabreccia occurring at Anhovo, Slovenia basin (Skaberne, 1987) , t r a n s i t i o n a l to t h e left to bedded turbidites ;detail stressing the internal organization a n d s i z e of m e g a c l a s t s .
251
of m to h u n d r e d of m thick, fining-upward,stacked sequence. Individual beds contain features quite analogous to the intraclast parabreccias d e s c r i b e d by S p a l l e t t a and Vai (198A) from the U p p e r D e v o n i a n c a r b o n a t e p l a t f o r m m a r g i n of the C a r n i c Alps, Italy, and i n t e r p r e t e d as s e i s m i t e s (Fig. IA7). Some of these structures are also analogous to the 'mud balls' occurring in the shallow-water carbonate platform of the V e n e t i a n A l p s (Trento p l a t f o r m ; c h a p t e r WI) and i n t e r p r e t e d as t s u n a m i - g e n e r a t e d f e a t u r e s (Galli,1991). O t h e r i n t e r p r e t e d s e i s m o t u r b i d i t e beds o c c u r in the E o c e n e Echo Group, in the s o u t h P y r e n e e s , s t u d i e d by Seguret, et a l . ( 1 9 8 A ) . This turbidite basin formed as a narrow, elongate basin d o m i n a t e d by s t r i k e - s l i p t e c t o n i c s . N i n e m e g a b e d s , 200 m thick, traceable for 150 Km t h r o u g h o u t the b a s i n (Fig.l&8),show a periodicity of d e p o s i t i o n of about 5 x I0 to 1 x I0 years (Mutti and Ricoi L u c c h i , 1 9 8 A ) . The t r i g g e r i n g m e c h a n i s m for the d e p o s i t i o n of these c a l c a r e o u s m e g a t u r b i d i t e s was t h o u g h t to result from s u p e r f i c i a l s e i s m i c a c t i v i t y due to t h r u s t faults a l o n g the edge of the basin (Seguret, et a i . , 1 9 8 ~ ) . W
IE
20 km
c
i
20 km
D
Fis. IA8 - C a r b o n a t e m e g a t u r b i d i t e s in the E o c e n e E c h o Group, south-central Pyrenees (from Mutti and Ricci Lucchi,IQ8&). A;complete,proximal megaturbidite sequences (including the basal magabreccia); B: incomplete,distal megaturbidites (calcarenite-mudstone couplets).
252
Are megabreccias tectonics?
the
product
of
compressional
or
vertical
A large number of authors tend to link the genesis of megabreccias to c o m p r e s s i o n a l orogenetic phases,linked to the emplacement of nappes which would be responsible for the formation of a p e r i p h e r a l bulge and oversteepening of the platform margin. For example, Abbate, Bortolotti and Sagri (1981) c o m m e n t : " An analysis of a v a i l a b l e examples s e e m s to indicate that the tectonic melanges are connected to accretionary w e d g e s in c o n v e r g e n t zones, w h e r e large s e c t o r s of oceanic crust and trench fillings have been or are being subducted. Olistostromes can be found in a wide range of structural settings: in m o r e or less close connection with tectonic melanges, orogenic l a n d s l i d e s a n d n a p p e s in c o n v e r g e n t and c o l l i d i n g m a r g i n s a n d , a l s o , in p a s s i v e c o n t i n e n t a l margins. Modern examples of the latter would be the afore mentioned mudflows off western Africa, earlier examples being olistostromes in the S c i s t i P o l i c r o m i of the T u s c a n sequence, a n d the m e g a b r e c c i a s and olistoliths derived f r o m the f a u l t e d rims of the c a r b o n a t e platform of the C e n t r a l A p e n n i n e s . These ancient occurrences are r e l a t e d to the p a s s i v e margins of the Adria plate. O w i n g to the c o m p l e x structural pattern of the n a r r o w T e t h y s s e a w a y , t h e r e is n o d o u b t t h a t a l s o t h e s e p a s s i v e margins experienced since Cretaceous intense tectonism, induced by c l o s e c o n v e r g e n t zones". This intriguing view is based on a model of subductionaccretion which in spite of its general acceptance and application is not d o c u m e n t e d a l o n g the c i r c u m - P a c i f i c margin, as h a s b e e n d e m o n s t r a t e d by W e z e l (1988).The results of the D e e p Sea D r i l l i n g Pro~ect produced contradictory results which f a i l e d to c o n f i r m an a c c r e t i o n a r y model. An interpretation regarding the o r i g i n of m e g a b r e c c i a s recurs to the ' p e r i p h e r a l bulge' model. Beaumont (1981) e x p l a i n e d the tectonic subsidence of the foredeep as the result of a litospheric f l e x u r e d u e to the l o a d of the t h r u s t front. The peripheral bulge progressively u p l i f t s t o w a r d s the t h r u s t f r o n t d e p e n d i n g of the v i s c o e l a s t i c behavior of the crust. T h e m o d e l was based mainly on the study of seismic profiles and mathematical modelling on W y o m i n g and Appalachians,where the e s t i m a t e d t h i c k n e s s of n a p p e s is of s e v e r a l k i l o m e t e r s . Recently, Royden and Karner (198~) c a l c u l a t e d the i n f l e x i o n of the r a m p b e n e a t h the A p p e n n i n e s a n d c a m e d o w n to the c o n c l u s i o n that the load of n a p p e s is insufficient to p r o d u c e such a bending: an a d d i t i o n a l l o a d w o u l d be r e q u i r e d to m a i n t a i n the basement inflexion. A consequence of t h e r e s u l t s of the w o r k by (198~) is t h a t the m o d e l of p e r i p h e r a l bulge
Royden is not
and Karner applicable
253
to the A p p e n n i n e s , A p e r i p h e r a l b u l g e i n t e r p r e t a t i o n of m e g a b r e c c i a s r e q u i r e s that I) the n a p p e e m p l a c e m e n t is c o n c o m i t a n t w i t h the f o r m a t i o n of megabreocias; and 2) the load of the n a p p e s is s u f f i c i e n t to p r o d u c e a d e f o r m a t i o n of the foredeep. Another problem concerns the d i r e c t i o n of peripheral bulge migration which should in normal cases p r o g r e s s towards the s u b d u c t i o n zone (basinward). In the A p p e n n i n e s the m i g r a t i o n of the interpreted peripheral bulge goes in the opposite d i r e c t i o n , as s h o w n in the work by Vai and C a s t e l l a r i n (1988). The M e s s i n i a n 'Vena del Gesso' e v a p o r i t e F o r m a t i o n d e s c r i b e d by Vai and Ricci Lucchi (1976;1977) and by M a r a b i n i and Vai (1985) c o n s i s t s of a g y p s u m lithosome, 1 5 0 - 1 7 0 m thick, c r o p p i n g out as a h o m i c l i n e along a NW-SE strike,in the A p p e n n i n e s . Its o r i g i n a l e x t e n t was of about I00 Km. The b a s i n of s e d i m e n t a t i o n was a lagoon bounded by topographic highs, which became infilled w i t h layers of m i c r o c r y s t a l l i n e gypsum interlayered with bituminous black shales. Tectonic sills separated d i f f e r e n t e v a p o r i t e basins. Vai and Ricci Lucchi (1976) d e s c r i b e d a r e g u l a r r e p e t i t i o n of six m a i n facies types w h i c h c o m p o s e a d e p o s i t i o n a l modal c y c l e c o n s i s t i n g of the f o l l o w i n g facies s u c c e s s i o n : b l a c k s h a l e s - - - > calcareous g y p s u m and algal laminite--->massive selenite---> banded selenite--->chaotic gypsum (pebbly m u d s t o n e ) --->slump breccia (megabreccia), A great proportion of the banks is r e p r e s e n t e d by m e c h a n i c a l l y reworked gypsum which underwent a basinward transportation by debris flow mechanisms in a subaerial environment, The authors proposed an autocyclic cannibalistic process whereby gypsum was eroded from the margins and redeposited basinward, The m e c h a n i c a l deposition was thought to have been the result of lowering sea-level c a u s i n g a d e p o s i t i o n a l regression, The modal cycle can be split into two parts: a lower aggradational cycle (black shales to a u t o o h t o n o u s and b a n d e d selenite), overlain by an u p p e r coarseningand thickeningupward cycle represented by allochtonous gypsum, The same organization is v i s i b l e at a m e g a s e q u e n c e scale. This modal cycle is t h e r e f o r e a n a l o g o u s to the modal sequence described above, An a l t e r n a t i v e i n t e r p r e t a t i o n to a p u r e l y a u t o c y c l i c a l model of s e d i m e n t a t i o n w o u l d be a c a n n i b a l i s t i c t e n d e n c y r e s u l t i n g from the o v e r s t e e p e n i n g of the relief due to a t e c t o n i c inversion. This c a n n i b a l i s t i c trend was c o n c o m i t a n t w i t h a r e t r o g r a d a t i o n of the c a n n i b a l i z e d margin.
254
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Fig. IA9 - S e q u e n c e of s i x s i g n i f i c a n t time intervals (dotted areas) plotted on the global eustatic curve by Haq, et ai.(1987) in o r d e r to s h o w t h e i r c o r r e s p o n d a n c e with short-term sea-level falls. These time intervals m a y c o r r e s p o n d to g l o b a l periods of short-term krikogenetic rejuvenation or geoidal deformation.These time intervals represent 'event horizons', d u e to t h e c l u s t e r i n g of s e v e r a l e v e n t s w i t h i n them, G e o l o g i c a l p r o c e s s e s a r e s e e n as t h e r e s u l t of a n o n - l i n e a r punctuation of 'events' spaced by lag-times, the s p a c i n g being regulated by the omothetic proportion (see chapter 5). Other 'event horizons', n o t p l o t t e d h e r e , o c c u r in t h e I) U p p e r D e v o n i a n Lower Carboniferous, Anisian-Ladinian; and 2) SerravallianMessinian - Pleistocene, respectively representing the endt e r i s a n d f i r s t t e r m s of t w o o t h e r t e m p o r a l s e q u e n c e s .
255
The structural s e t t i n g of the n o r t h e r n Appennines consists of narrow, arcuate belts and overthrusts delimiting elastic w e d g e s . T h e s t r u c t u r e of the A p p e n n i n e s was interpreted by V a n Bemmelen (1972) as the result of mantle diapirism and gravitational spreading of fluidized sub-crustal material. According to W e z e l (198~) the A p p e n n i n e frontal arcs are an indication of a t e c t o n i c d e f o r m a t i o n d u e to v e r t i c a l l y rising diapirie domes. Krikogenesis (see page I) results in the s u r f a c e as t r a n s i e n t c r u s t a l u p l i f t s (see W e z e l , 1 9 8 ~ , 1 9 8 5 ) , The evolution of the northern Appennines implies a "paired migration and extension and compression towards east,linked to progression of t r a n s i e n t orogenic arc systems" (Wezel,198&), The possible mechanism for the e m p l a c e m e n t of t h r u s t sheets, together with the foredeep migration consists of vertical (touche-de-piano) tectonics, as d o c u m e n t e d for e x a m p l e a c r o s s the a c t u a l J a v a trench. This implies the m i g r a t i o n of m a n t l e diapirism through time and space ("megaundations" of Van Bemmelen). In k e e p i n g with the concepts of Stifle (192A), Vai (1987) showed that orogenetic deformation in the Appennines was discontinuous; he supported a punctuated migration of the deformation in the A p p e n n i n e s w i t h a g i v e n r e c u r r e n c e time In the Messinian and the Pleistocene the stages of maximum deformation and f r o n t a l advancement fall into the following ages: 18 m . y . ~ 1 0 m.y. ~5.5 m . y . ~ 5 m , y . , 3 . 5 m.y. ~ 2 - 1 . 5 m.y.~ a n d 0.5 m.y. (Vai,198 ). The d u r a t i o n of the d e f o r m a t i o n episodes is b r i e f (I-0.I m,y.)~the quiescence is of l o n g e r duration (5-0.5 m . y . ) . I t seems that the d e f o r m a t i o n and advancement of the f o r e d e e p was s y n c h r o n o u s w i t h the e x t e n s i o n a l tectonics in the T y r r e n i a n Sea. A c r u c i a l p o i n t is that m o s t of the m e g a b r e c c i a s , so c o m m o n in the M e d i t e r r a n e a n region,both in the A l p s a n d A p p e n n i n e s , were emplaced during short-term sea-level falls,These occurrences (some of which described above) are confined within the following time intervals: Anisian-Ladinian Pliensbachian-Toareian Tithonian-Valanginian Cenomanian-Turonian Upper Maastrichtian Thanetian Ypresian Serravallian Messinian Pleistocene
256
These time intervals correspond to s h o r t - t e r m sea-level falls v i s i b l e in t h e c h a r t by Haq, et a l . ( 1 9 8 7 ) , r e p o r t e d in F i g . l ~ 9 . The same ages coincide w i t h t h e a g e s of u n c o n f o r m i t i e s which delimit the elastic wedges and Formations in t h e A p p e n n i n e s , as s h o w n in F i g . 1 5 0 .
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Fig.150 - Time-space distribution of s e d i m e n t a r y Formations in a sector of the Appennines,transversal to the CorsicaTuscan-Umbro-Marchigiano Apennine belt). Stippled areas: synorogenie migrating basins; white areas: basinal turbidites, hemipelagites). T h e d i r e c t i o n of m i g r a t i o n is t o w a r d s t h e r i g h t (East). F r o m P r i n c i p i a n d T r e v e s (198~).
257
A p o s s i b i l i t y is that m e g a b r e c c i a s and u n c o n f o r m i t i e s are the p r o d u c t s of u p l i f t s r e s u l t i n g from deep p r o c e s s e s of v e r t i c a l and lateral d i a p i r i s m s e a t e d in the u p p e r mantle, This p o s s i b i l i t y allows for a r e c o n s i d e r a t i o n of p i g g y - b a c k basins and d e n u d a t i o n e v e n t s d e s c r i b e d by Ori, et a i . ( 1 9 8 6 ) , i n t e r p r e t e d as the result of lateral tectonic compressional tectonics. The cross s e c t i o n d e s c r i b e d by Ori, et a i . ( 1 9 8 6 ) shows d e n u d a t i o n c o m p l e x e s r e s u l t i n g from the e r o s i o n of the thrust sheets, o v e r l a i n by p r o g r a d i n g c o m p l e x e s ( F i g . 1 5 1 ) . T h e geometrical relationships between fault scarp, denudation complex (megabreccias?) and progradation complex bear a striking resemblance to those of the second depositional s e q u e n c e of the G a r E a n o case h i s t o r y (Fig.106).
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- Sketch from a s e i s m i c line s h o w i n g r e l a t i o n s h i p s fault scarps, denudation c o m p l e x and p r o g r a d a t i o n a l complex in the Plio-Pleistocene foredeep in the Central A d r i a t i c Sea (from Ori, et a l . , 1 9 8 6 : F i g . 6 ) .
between
The above few e x a m p l e s , t a k e n from the a v a i l a b l e literature, allow for the following working hypothesis concerning the o r i g i n of m e ~ a b r e c c i a s : I. Megabreccias occur in a wide range of settings and p a l e o g e o g r a p h i c situations, r a n g i n g from the slope to a s h a l l o w lagoons. 2. T h e y may be i n t e r p r e t e d as l o w s t a n d s y s t e m tract, m o s t l y a s s o c i a t e d with s h o r t - t e r m s e a - l e v e l falls. 3. Most uplift, causes. ~. The
of m e g a b r e c c i a s e n c o u n t e r e d rather than of tectonic
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punctuating
the
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eustatic
258
chart by Hag, global g e o i d a l
et a i . ( 1 9 8 7 ) deformation.
represent
short-term
phases
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5. In some case m e g a t u r b i d i t e s a p p e a r to have been d e p o s i t e d d u r i n g s h o r t - t e r m s e a - l e v e l falls, d u r i n g short p e r i o d s ( less than 500.000 yrs.) of g e o i d a l deformation, by catastrophic m e c h a n i s m s i n d u c e d by s e i s m i c shocks. Therefore, it is not e x c l u d e d that g e o i d a l u p l i f t s r e p r e s e n t spasmodic phenomena.
Drownin~
of c a r b o n a t e
platforms
The drowning consists of the submergence of a carbonate platform below the photic zone and its termination.This p h e n o m e n o n is d e a l t h w i t h in a t h o r o u g h s y n t h e s i s by S c h l a g e r (1981~1991,1989). Most of w o r l d - w i d e p h e n o m e n a of d r o w n i n g of c a r b o n a t e p l a t f o r m s found in the g e o l o g i c a l r e c o r d are d a t e d to the M i d d l e - L o w e r 0rdovician, Late Devonian (Frasnian Famennian),Toarcian, V a l a n g i n i a n , T u r o n i a n , M i o c e n e and P l e i s t o c e n e . As the age of worldwide drownings falls into periods of h i g h s t a n d of the s e a - l e v e l , it is logical to a c c e p t the c o n c e p t that c a r b o n a t e p l a t f o r m s are d r o w n e d w h e n the rate of sea level rise outpaces the carbonate accumulation rate (Kendall and Schlager,1981) On the o t h e r hand, estimated g r o w t h rates of reefs d u r i n g the Late H o l o c e n e rise s u g g e s t that, in spite of a very r a p i d sea level rise, carbonate sediments in the late H o l o c e n e c o u l d r e c o v e r and b u i l t to sea l e v e l . S c h l a g e r (1981) went to the c o n c l u s i o n that o n l y an e x c e p t i o n a l s e a - l e v e l rise is c a p a b l e to e x c e e d the g r o w t h rate of a c a r b o n a t e p l a t f o r m w h i c h is of about I000 B u b n o f f . Among the causes of d r o w n i n g , an increase in the rate of s e a - f l o o r s p r e a d i n g ( H e e z e n , e t a i . , 1 9 7 3 ) , a r a p i d e u s t a t i c rise produced by v o l c a n i s m (Schlanger and P r e m o l i Silva,1981), a d e s s i c a t i o n of a y o u n g o c e a n b a s i n (Hs~ and W i n t e r e r , 1 9 8 0 ) are discussed by Schlager (1981) as possibile causes of drownings.Also, an e n v i r o n m e n t a l c h a n g e c a u s e d by e n v i r o n m e n t a l stress is a likely a l t e r n a t i v e , when sea-level rise is not s u f f i c i e n t l y fast ( S c h l a g e r , 1 9 9 1 ) . A r t h u r and S c h l a n g e r (1979) observed the coincidence of a n o x i c events and drowning of carbonate platforms, and H a l l o c k and S c h l a g e r (1986) p r o p o s e d an e x c e s s of n u t r i e n t flood as a c a u s e of d e m i s e of c a r b o n a t e platforms.
259
A n o t h e r p o s s i b l e cause of d r o w n i n g is r e p r e s e n t e d by a v e r y r a p i d sea level fall f o l l o w e d by a t r a n s g r e s s i o n , as s u g g e s t e d by Mc Laren (1970), which may have led to drowning unconformities (Schlager a n d C a m b e r , ! 9 8 6 ) . Most of anoxic events coincide with periods of d r o w n i n g of carbonate platforms which in turn coincide with short-term s e a - l e v e l falls in the chart of Haq, et a l . ( 1 9 8 7 ) . S o m e of these rapid sea-level falls may not be lowstand (i.e. the Valanginian lowstand is probably a drowning unconformity m i s t a k e n l y a t t r i b u t e d to a lowstand: see S c h l a g e r , 1 9 8 9 ) A further possibile cause of d r o w n i n g w o u l d be a p e r i o d of intense g e o i d a l d e f o r m a t i o n , w h i c h may have d e t e r m i n e d in the platform some environmental change responsible for some r e d u c t i o n in its g r o w t h p o t e n t i a l : f o r example, the u p l i f t may have c a u s e d a d i s c h a r g e of great v o l u m e s of f r e s h w a t e r into the ocean, as s u g g e s t e d by T h i e r s t e i n and B e r g e r (1978), In fact, the w o r l d - w i d e episodes of drowning of carbonate platforms coincide with periods of megabreocia shedding, formation of onlap ramps, divergent patterns, and seismoturbidites. The C e n o m a n i a n - T u r o n i a n gives a good opportunity to a t t e m p t c r o s s - c o r r e l a t i o n s b e t w e e n s e e m i n g l y i n d e p e n d e n t p h e n o m e n a . The d r o w n i n g and retreat of c a r b o n a t e p l a t f o r m s during the U p p e r Cretaceous is a w o r l d w i d e event. Most of the d r o w n i n g s of carbonate platforms took place in the C e n o m a n i a n - Turonian period (Schlager,1981,1991,Schlager and Philip, 1990;Fig.152). Several s e a m o u n t s in the w e s t e r n P a c i f i c (Heezen, et ai.,1973; M a t t h e w s ,et al.,197A), the C o m a n c h e shelf (Young,19779 Bebout and Louoks,197~), the south Florida platform (Worzel, et ai.,1969) underwent abrupt drownings represented by the deposition of pelagic chalk deposits over shallow water sediments.The western Pacific seamounts dredged by the DSD Pro~ect probably underwent emergence prior to drowning: in fact, Heezen, et a i . ( 1 9 7 8 ) report that the top of s e a m o u n t s and g u y o t s in the C e n o m a n i a n - T u r o n i a n was c o v e r e d by b i o c l a s t calcarenites: the well r o u n d e d g r a i n s e v i d e n c e for a p e r m a n e n c e in the surf zone; the flat s u r f a c e of the g u y o t s f o r m e d in the Turonian. The e x t e r m i n a t i o n of the reef fauna c o n s e q u e n t to the shallowing was followed by a penep!anization of the reef surface, followed by rounding of the grains. Diagenetic f e a t u r e s i n d i c a t e a t r a n s i t i o n from m a r i n e p h r e a t i o to m e t e o r i c p h r e a t i o c o n d i t i o n s . T h e J o r d a n Knoll in F l o r i d a was a shallow
260
F i g . 1 5 2 - L o c a t i o n of p l a t f o r m s the C e n o m a n i a n (modified after map from J e n k i n s , 1 9 9 1 ) .
d r o w n e d in the A p t i a n t h r o u g h Schlager and P h i l i p , 1 9 9 0 ; b a s e
w a t e r b a n k in the A l b i a n - C e n o m a n i a n (Bryant, et a i . , 1 9 6 9 ) . It d e e p e n e d from the C e n o m a n i a n to the S a n t o n i a n . M e g a b r e c c i a s dated to this period may have formed by tilting of the knoll.According to Worzel, et a i . ( 1 9 7 3 ) the M i d d l e C r e t a c e o u s was m a r k e d by a g e n e r a l p e r i o d of e m e r g e n c e in the Gulf C o a s t B a s i n and East Texas. In the A l b i a n - C e n o m a n i a n the C a m p e c h e shelf records pebbly mudstones interbedded or overlying calcarenite-caloisiltite beds (site #97: cores 7,8 and 9), d e l i v e r e d from the a b o v e shelf. In this case, the o r i g i n of the pebbly mudstones was t h o u g h t to be linked to p h a s e s of the Laramide orogeny of C u b a , H o w e v e r , the L a r a m i d e orogeny took p l a c e in the P a l e o c e n e , not in the C r e t a c e o u s .
26]
Relief
inversions
The accentuation of the r o t a t i o n a l tilting in s o m e s i t u a t i o n s is responsible for relief inversions consisting of the e m e r g e n c e of s o m e a r e a s of t h e s h a l l o w - w a t e r carbonate platform or even the uppermost parts of the slope,close to the platformward terminations of c l i n o f o r m s . The relief inversion is a s s o c i a t e d w i t h the f o r m a t i o n of d i v e r g e n t p a t t e r n s , t e c t o n i c tilting, fault scarps, and shedding of m e g a b r e c c i a s i n t o the basin (Fig.153). It is s e e n h e r e as an e f f e c t of r o t a t i o n a l tilting associated with geoidal eustatic changes, as hypothesized by M ~ r n e r (1981), T h e e f f e c t s of r e l i e f i n v e r s i o n s in the p l a t f o r m interiors lead to the e x p o s u r e with possible karstification, or a s u d d e n transformation of a d e e p lagoon into a p a r a l i c swamp: the r e p e a t e d mudstone interbeds within the d e e p l a g o o n a l , d e e p r a m p f a c i e s o c c u r r i n g in t h e 'Calcari G r i g i ' f o r m a t i o n , a n d the sill s e q u e n c e s ( c h a p t e r #I) a r e s e e n to h a v e f o r m e d in r e s p o n s e to r e l i e f i n v e r s i o n m e c h a n i s m s ,
KARST, SABKHA,BEACHES, LAGOONS, ETC. : .":
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Fig.153 Features associated with relief inversions of p l a t f o r m i n t e r i o r s a n d the m a r g i n of a c a r b o n a t e p l a t f o r m . In a strike-slip situation the relief inversion is produced by transpression in w h i c h c a s e the f a u l t p l a n e is e i t h e r v e r t i c a l or i n c l i n e d t o w a r d s the p l a t f o r m i n t e r i o r s . In some cases the relief inversions take place by a touche-de-piano tectonics and a retrogradation of t h e m a r g i n . T h i s p r o c e s s in the s h a l l o w - w a t e r platform is d o c u m e n t e d by the
262
I LIGURIAN
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OCEAN
CANAVESE RIDGE
100
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200 km
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FRIULI ~1~ PLATFORM--
2
L. EOC.
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Fig.15~ - Meridional Alps, Cross section from Winterer and Bosellini (1973). B e l o w : I) : C a s t e l l a r i n ( 1 9 7 2 ) ; 2): T u n i s a n d Venturini (1987).Black zones in the stratigraphic columns denote megabreccias. migration of the sill a r e a s (see c h a p t e r #I).
and
flexure
zones
towards
the
hinge
In the Meridional Alps (Fig.15~) retrogradation of the carbonate platforms was discontinuous, as testified by the deposition of m e g a b r e c c i a s w h i c h a r e d a t e d to t h e P l i e n s b a c h i a n
263
Toarcian, Tithonian - Valanginian, Turonian, Maastrichtian, T h a n e t i a n and L o w e r E o c e n e . T h e megabreccia deposition in the w e s t e r n areas was a c c o m p a n i e d by e m e r s i o n of some areas in the east i.e. in the Toarcian, where deposition of m e g a b r e c c i a s a l o n g the w e s t e r n b o r d e r of the T r e n t o p l a t f o r m was c o n c o m i t a n t w i t h e m e r s i o n and k a r s t i f i c a t i o n of some p a r t s of the Friuli platform.The stratigraphic c o l u m n s r e c o r d an i n c r e a s e in the thicknesses of megabreccia bodies, probably related to a c o r r e s p o n d i n g i n c r e a s e in the rate of t e c t o n i c tilting.
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Fi~,155 - Cross s e c t i o n of the B e t i c C o r d i l l e r a , s o u t h Spain (after M a r t i n A l g a r r a , 1 9 8 8 ) . l - 5 : Hidalga Group (shallow-water Triassic carbonates and e v a p o r i t e s ) . 6 - 9 : Libar Group (6-7: s h a l l o w - w a t e r c a r b o n a t e s ; 8 - 9 : s h a l l o w - w a t e r p e l a g i c limestones. 10-16 : E s p a r t i n a Group (pelagic, b i t u m i n o u s limestones, marls and f l y s c h o i d s a n d s t o n e s and clays).l_~7: J u r a s s i c and C r e t a c e o u s karst. R e l i e f i n v e r s i o n s are c o m m o n in c o n v e r g e n t w r e n c h zones, as a result of t r a n s p r e s s i o n mechanisms.Two e x a m p l e s come from the B e t i c C o r d i l l e r a of s o u t h S p a i n and the P a l e o z o i c s e q u e n c e from the C a r n i e AIps (northern Italy). In the first e x a m p l e (Fig.155) the N W s e c t o r of a J u r a s s i c platform was raised by a tectonic tilting in the Lower Valanginian-Hauterivian, Lower Aptian and L o w e r Albian. Upon
264
uplift, the NW s e c t o r was s u b j e c t to k a r s t i f i c a t i o n , whereas the SE sector was transformed into a 'shallow pelagic p l a t f o r m ' . L a t e r , the e m e r g e d area u n d e r w e n t d r o w n i n g and b e c a m e a s u b m a r i n e plateau. De Smet (198d) in a t h o r o u g h study of the t e c t o n i c e v o l u t i o n of the B e t i c C o r d i l l e r a , d e m o n s t r a t e d that s t r i k e - s l i p m o v e m e n t s f o l l o w e d by c o n v e r g e n t m o t i o n s o c c u r r e d in the area c o m p r i s e d b e t w e e n the S p a n i s h and the M a r o c c a n Meseta. S t r i k e - s l i p m o v e m e n t s p r o d u c e d a n t i c l i n a l and s y n c l i n a l massifs; transpression led to the formation of 'flower' s t r u c t u r e s (Harding and L o w e l l , 1 9 7 9 ) . In the second example occurring in the Carnic Alps, the r e c o g n i t i o n of relief i n v e r s i o n is r a t h e r p r o b l e m a t i c , b e c a u s e of o v e r p r i n t e d a l p i n e and h e r c y n i a n t e c t o n i c phases. The P a l e o z o i c s t r a t i g r a p h i c s e q u e n c e c o n s i s t s of : I) s h a l l o w w a t e r c a r b o n a t e s t r a n s i t i o n a l to p e l a g i c g o n i a t i t e l i m e s t o n e s (Caradoc-Devonian);2) pelagic carbonates (Clymenia limestones, Famennian-Dinantian);3) deep water radiolarites (lydites: Dinantian);A) thick, siliciclastic turbidites (Hochwipfel F o r m a t i o n ) p a s s i n g v e r t i c a l l y to an a c i d i c to basic v o l c a n i c t u r b i d i t e F o r m a t i o n ( S i l e s i a n ) ; 5 ) c o n g l o m e r a t e s , s a n d s t o n e s and s h a l l o w - w a t e r c a r b o n a t e s (Upper C a r b o n i f e r o u s - Permian). The vertical sequence of lithofacies is an example of geotectonic cycle shown in Fig.2: units range from the krikogenetic and geosynclinal stage (units #1+3), to the t e c t o g e n e t i c and e p e i r o g e n e t i c stages (units #~+5). The stage a n t e c e d e n t to k r i k o g e n e t i c r e j u v e n a t i o n , r e p r e s e n t e d by a c o n t i n u o u s s e q u e n c e from the C a r a d o c to the U p p e r D e v o n i a n ( V a i , 1 9 8 0 ) , i s m a i n l y c o n s t i t u t e d by s h a l l o w - w a t e r and p e l a g i c c a r b o n a t e s w h i c h b e c o m e the n e a r l y e x c l u s i v e l i t h o f a c i e s from the E m s i a n to the Frasnian. The F r a s n i a n r e c o r d s a w o r l d w i d e e s t a b l i s h m e n t of r e s t r i c t e d m a r i n e c o n d i t i o n s ; o r g a n i c - r i c h b l a c k shales d e v e l o p e d in the USA, for e x a m p l e in the M i c h i g a n Basin. In the C a r n i c Alps d u r i n g this p e r i o d a n o x i o c o n d i t i o n s p r e v a i l e d in the basin; corresponding changes in the p l a t f o r m are documented by a d e c r e a s e in fossil d i v e r s i t y from the G i v e t i a n to the F r a s n i a n and to the appearance of shallow-water black mudstone intercalated with other lagoonal facies. Semirestricted lagoonal facies (black m u d s t o n e s and shales i n t e r c a l a t e d with other bioclast accumulations) are t h o u g h t to r e p r e s e n t the p l a t f o r m w a r d e x t e n s i o n s of a n o x i c p e l a g i c facies o c c u r r i n g in the basin, w h i c h are w i d e s p r e a d in E u r o p e (see P r e a t , 1 9 8 8 ; K r e b s , 1 9 7 A ) . T h e F r a s n i a n r e c o r d s a c h a n g e in p l a t f o r m facies o r g a n i z a t i o n and a t r a n s i t i o n from r e e f - - b a c k - r e e f - - lagoon-tidal flat to an i n t e r - r e e f p l a t f o r m c o n f i g u r a t i o n with lagoons
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populated by an o l i g o t y p i c fauna (calcispheres a n d Amphipora) and intervening patch-reefs, composed of monospecific brachiopods (Vai,unpubl.data). The development of this l a g o o n a l stress environment is m o r e or less s y n c h r o n o u s throughout the world and may be related to s o m e drastic change in water paleocirculation (Copper,1986).A f i e l d e x a m p l e of this v e r t i c a l decrease in f a u n a l d i v e r s i t y (Fig.156) s h o w s r e e f flat f a c i e s at the b a s e containing crinoids, algae, stromatoporoids and corals, overlain by open lagoon facies with brachiopods, Amphipora, Thamnopora and Trypanopora, followed in turn by AmphJpora m e a d o w s o n t o p ( G a l i i , 1 9 8 5 , 1 9 8 6 ) . T h e e v o l u t i o n of the c a r b o n a t e p l a t f o r m d u r i n g the F r a s n i a n m a y be t a k e n as an e x a m p l e of e n v i r o n m e n t a l deterioration preceding the d r o w n i n g of the p l a t f o r m , d i s c u s s e d by S c h l a g e r (1991). The
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crisis and extinctions attributed by Mclaren (1970) and Playford, et aI.(198~) to a b o l i d e impact. In the C a r n i e Alps the d r o w n i n g is r e c o r d e d by the d e p o s i t i o n of s h a l l o w - w a t e r p e l a g i c limestones. Tectonics became progressively important,as early as the Frasnian,when intrashelf onlap ramps were formed. In the Dinantian the deepening-upward trend was accentuated as a result of the fragmentation of the p l a t f o r m by e x t e n s i o n a l tectonics. Neptunian dykes, olistoliths, olistostromes, sedimentary hiathuses and unconformities evidence for the sedimentary tectonics. The cross section evidencing the g e o l o g i c a l e v o l u t i o n of the p l a t f o r m (Spalletta, et ai.,1979; Fig.157) contains several features common to o t h e r platform margin transitions subjected to extensional tectonics and successive relief inversion such as: I) coarsening-upward trends of m e g a b r e c c i a s 9 2) d i v e r g e n t w e d g e s of p e l a g i c - b a s i n a l s e d i m e n t s at the m a r g i n of the p l a t f o r m 9 and 3) s y n s e d i m e n t a r y faults. The hercynian synsedimentary tectonics is evident from an e x a m i n a t i o n of the fence d i a g r a m of Fig. 158 w h i c h reveals a horst-graben geometry. Angular unconformities amounting to 10°-20 ° s e p a r a t e some of the p e l a g i c F o r m a t i o n s . Two r e c u r r e n t types of d i s c o r d a n c e s can be o b s e r v e d in the study area. A first type consisting of fan-shaped discordances was the product of a r o t a t i o n a l , basinward tilting.The second, more complex, was p r o d u c e d by b a s c u l a t o r y m o v e m e n t s or i n v e r s i o n s in the direction of tilting which may be related to relief inversion mechanisms. The d e v e l o p m e n t of the s t r a t i g r a p h i c s e q u e n c e was c o n t r o l l e d by strike-slip tectonics from the U p p e r Devonian to the L o w e r Carboniferous,consequent to the activation of a transform s y s t e m w h i c h d e t e r m i n e d a w e s t w a r d drift of c o n t i n e n t a l blocks s e p a r a t e d by n a r r o w seaways. The c e n t r a l areas of the h e r c y n i d e s display arched structural p a t t e r n s , d o m e - l i k e features, a b s e n c e of o p h i o l i t e s , w i d e s p r e a d high-temperature metamorphism and a g r e a t number of g r a n i t e intrusions which evidence for r a i s e d isotherms and thinned continental crust. No o c e a n i c crust was present untill the Upper Devonian.The absence of o p h i o l i t e s , nappe structures, island ares and a lack of h i g h - p r e s s u r e metamorphism in the h e r c y n i d e s led Krebs and W a c h e n d o r f (1973) to infer a d i a p i r i c o r o g e n e s i s as an a l t e r n a t i v e to a p l a t e t e c t o n i c model. Vai and Cocozza (1986) sustained that the deformation, p r o g r e s s i n g from west to east, was c o n t i n u o u s and d i a c h r o n o u s
270
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271
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F i g . 1 6 0 - E x a m p l e s of fold s t r u c t u r e s in the P a l e o z o i c of the C a r n i c Alps. A: P . s o di M . C r o c e C a r n i c o (Cantelli et a i . , 1 9 6 8 ) . An a n t i c l i n a l structure,however dismembered into slices, is clearly recognizable in the p r o f i l e . T h i s cross section was p r e v i o u s l y i n t e r p r e t e d as a series of i m b r i c a t e d t h r u s t sheets. Above :interpreted flower structure. B:Panoramic view towards east of the P.di T i m a u from P i z z o C o l l i n a (from C a n t e l l i , et ai.,1982) showing the southern limb of an anticlinal s t r u c t u r e close to the h i n g e area..C: H i n g e of an a n t i c l i n a l structure, overprinted by alpine tectonism (Spalletta, et al.,1979).D~E: Cima 0mbladet and Sasso Nero (see Fig.73), interpreted as flower structures. The n o r t h e r n flank of the Cima O m b l a d e t (sketch from H e r i t s c h , 1 9 3 6 ) r e v e a l s c o m p l e x box f o l d s . T h e s t r u c t u r e is u n c o n f o r m a b l y o v e r l a i n by the t u r b i d i t i c Hochwipfel Formation, hence it formed in the Lower Carboniferous. F: A n t i c l i n a l s t r u c t u r e at C r e t a di C o l l i n e t t a (modified from C a s t e l l a r i n , 1965: in D e s i o , 1 9 7 3 ) , interpreted here as a flower structure. G: Small-scale anticlinal and s y n c l i n a l s t r u c t u r e s at F r a u e n h ~ h e , L a g o V o l a i a (Vai,1963) (cf. with A ) . S u r p r i s i n g l y , these folded s t r u c t u r e s (and o t h e r s not shown here) have been ignored or d i s m i s s e d by i t a l i a n a u t h o r s working in the area, who made up a complicated style of i m b r i c a t e d u n d e r t h r u s t sheets (cf.Fig.162) e n g u l f e d w i t h i n the C a r b o n i f e r o u s H o c h w i p f e l Formation. Symbols:H:Hochwipfel Formation; D: D e v o n i a n ; S: Silurian; 0: 0rdovician; UD: Upper Devonian; MD: Middle Devonian; MG: m e g a b r e c c i a s ; LD: L o w e r Devonian~ R: r a d i o l a r i t e s .
274
The s p l a y i n g of the d i v e r g e n t w r e n c h i n g zone in the area was responsible for the n o r t h w a r d b l o c k m i g r a t i o n , retrogradation of fault planes, formation of the fan-shaped angular u n c o n f o r m i t i e s and the d i v e r g e n t p a t t e r n w h i c h can be p e r c e i v e d from an e x a m i n a t i o n of F i g . 1 5 8 . A s u c c e s s i v e stage in the s t r i k e - s l i p e v o l u t i o n in the area led to c o n v e r g e n c e , f o l d i n g a n d r o t a t i o n of f a u l t e d b l o c k s , a n d the disintegration of the b l o c k mosaic leading to d i f f e r e n t i a l block movements and compressive structures squeezed out of a d j a c e n t a r e a s , s u c h as 'flower s t r u c t u r e s ' The second type of discordances visible in the fence diagram of Fig.158 were probably produced during this stage, by b l o c k u p l i f t i n g and relief inversion. The carbonate thrust sheets are intensively deformed and folded, as documented formerly by Heritsch (1936) who i n t e r p r e t e d the t e c t o n i c s t r u c t u r e as a f o l d e d style c o m p o s e d of b r a c h i a n t i c l i n e s and s y n c l i n a l structures. Later, italian a u t h o r s w o r k i n g in the a r e a r e c o n s i d e r e d such an i n t e r p r e t a t i o n and p r e f e r r e d a m o r e s o p h i s t i c a t e d style of m u l t i p l e , stacked thrust sheets enguifed within the flyschioid Hochwipfel Formation.This interpretation did not rule out the o c c u r r e n c e
Fig.161 - Evolution of the flower c o n v e r g e n t w r e n c h i n g ( a d a p t e d from De
structures Smet,198~).
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275
of f o l d s : t h e r e is no d o u b t in fact sheets are strongly folded (Fig.160). the C i m a 0mbladet (Fig.160), may be structures'
that most of carbonate Some structures,such as interpreted as 'flower
The evolution of the area from the Frasnian to the Lower Carboniferous p r o p o s e d h e r e is s h o w n in F i g . 1 6 1 (adapted from De S m e t , 1 9 8 A ) . W r e n c h i n g i n i t i a l l y l e a d s to f o l d i n g a n d f a u l t i n g of the c a r b o n a t e platform. Successively the anticlinal areas are uplifted and squeezed out, with formation of 'flower structures'.The flower structure concept is a key in the interpretation of s e v e r a l t e c t o n i c s t r u c t u r e s in the a r e a w h i c h display an o p p o s i t e vergence, s u c h as t h e s t r u c t u r e s h o w n in Fig,162, bracketed by arrows,which could be interpreted alternatively as a t h r u s t . A c c o r d i n g to t h e 'flower structure interpretation', compressive phases (Asturic phase) may have reoriented the d e t a c h e d slabs of the c a r b o n a t e platform, or o v e r t u r n e d s o m e b l o c k s or limbs of f l o w e r s t r u c t u r e s .
N
S
f.
DP KmoO
10
Fis.162 - Palinspastic reconstruction of the P a l e o c a r n i c chain (Upper Permian) (after Vai,1976).The s t r u c t u r e of the c h a i n is i n t e r p r e t e d as a s e r i e s of i m b r i c a t e d u n d e r t h r u s t s , O b s e r v e the absence of a n t i c l i n a l forms in the p r o f i l e (cf. with Fig. 160),the opposed vergence of the area delimited by arrows i n t e r p r e t e d h e r e as a ' f l o w e r s t r u c t u r e ' , t h e f a u l t (a) f l o a t i n g w i t h i n the f l y s c h H o c h w i p f e l Formation.According to the t h r u s t sheet view, the volume of the Hochwipfel Formation is preponderant.Conversely, a flower structure interpretation l e a d s to r e d u c i n g s u c h a v o l u m e , i n keeping with stratigraphic data indicating an o v e r a l l total thickness of t h e H o c h w i p f e l F o r m a t i o n of a b o u t 800 m, in c o n t r a s t w i t h the t h i c k n e s s of the c a r b o n a t e p l a t f o r m w h i c h a m o u n t s to a b o u t 1 2 0 0 m. H: H o c h w i p f e l Formation (NamurianLower Westfalian); DPN: neritic-pelagic carbonates (GedinnianVisean)9 S; Silurian carbonates and shales; U:Uqua Formation (Caradoc-Ashgill)~ D;Devonian platform carbonates; Di; D i m o n F o r m a t i o n ( L o w e r W e s t f a l i a n ) .
276
A t e r r e s t r i a l - karst h o r i z o n is s a n d w i c h e d b e t w e e n u n d e r l y i n g pelagic and shallow-water carbonates and o v e r l y i n g turbidite deposits. It c o n s i s t s of s i ! c r e t e deposits and v a r i o u s karst features.Recently, Sch~nlaub, et a i . ( 1 9 9 1 ) studied in great detail this horizon and conclusively demonstrated a karst origin. It was the o b j e c t of much study by i t a l i a n g e o l o g i s t s working in the area, who considered and listed several interesting, alternative possibilities (Spalletta, et a i . , 1 9 8 2 a,b,c,d),which s e r v e d as a s t i m u l u s to w o r k e r s in the area a t t e m p t i n g at m a t c h i n g the o c c u r r e n c e of the karst h o r i z o n w i t h the e x i s t i n g model of g e n e r a l i z e d d e e p e n i n g (of. F i g . 1 5 7 ) . S o m e f e a t u r e s of this h o r i z o n o c c u r r i n g in the s t u d y a r e a are shown and d e s c r i b e d in Fig.163. Interpretations concerning this t e r r e s t r i a l h o r i z o n have been c o n t r a s t i n g , b e c a u s e of the d i f f i c u l t y in r e c o n c i l i n g an u p l i f t w i t h the p r e v i o u s i n t e r p r e t a t i o n of the b a s i n w h i c h a s s u m e d a generalized, deepening-upward trend (Spalletta, et a i . , 1 9 7 9 ) and w i t h the o c c u r r e n c e of some b a s i n a l s e q u e n c e coeval with the t e r r e s t r i a l h o r i z o n and r e p r e s e n t e d by p e l a g i c c a r b o n a t e s (Famennian-Dinantian) o v e r l a i n by deep w a t e r r a d i o l a r i t e s (late Dinantian) and the H o c h w i p f e l , turbidite Formation (Silesian) (Spalletta, 1 9 8 2 ) . T h i s p r o b l e m can be r e c o n c i l e d by a m e c h a n i s m of rotational subsidence which contemporaneously sunk the basinward areas and uplifted the platformward zones which b e c a m e 'flower s t r u c t u r e s ' . The o c c u r r e n c e of a T o u r n a i s i a n to V i s e a n emersion horizon draping the platform contradicts the theoretical scheme of d i a c h r o n o u s d e f o r m a t i o n p r e d i c t e d by Vai and C o c o z z a ( 1 9 8 6 ) . T h e m a i n i m p l i c a t i o n of the o c c u r r e n c e of this t e r r e s t r i a l h o r i z o n is that c o m p r e s s i o n a l B r e t o n i c (top D e v o n i a n ) and S u d e t i c (top Dinantian) phases can not be e x c l u d e d (according to i t a l i a n a u t h o r s o n l y the A s t u r i a n p h a s e - W e s t p h a l i a n - took p l a c e in the Carnie Alps).Mineralizations and k a r s t horizons seem to be located p r e f e r e n t i a l l y upon flower structures (they are a b s e n t in the D e v o n i a n rocks c r o p p i n g out in the A u s t r i a n a r e a ) . T h i s documents a localized tectonic control upon mineralizing agents. The exact origin for these Bretonic and Sudetic phases is obscure.As a matter of fact,the DevonianCarboniferous boundary (corresponding to the B r e t o n i c phase in the C a r n i c Alps) r e c o r d s a w o r l d w i d e emersion, as can be e v a l u a t e d from an e x a m i n a t i o n of the r e s u l t s of the 1986 s y m p o s i u m of A a c h e n on 'Late D e v o n i a n e v e n t s a r o u n d the Old Red C o n t i n e n t ' (Ministry of Econ. A f f a i r s , A d m , o f M i n e s , 1 9 8 6 ) . The
two p h a s e s
correspond
to
short-term
sea
level
falls
in
the
277
PELAGIC GARBONATES HERCYNIAN FLYSCH
• EMERSION
HORIZONS
278
charts by Johnson (1985) and Stille (192A). The emersion corresponding to the Sudetic phase (Tournaisian-Visean) is less recorded in the world~ probably the uplift was less i n t e n s e . T h e u p l i f t is d o c u m e n t e d in s e v e r a l areas, for example, in Poland, A r m o r i c a n M a s s i f and the A p p a l a c h i a n s .
F i g , ! 6 3 - D i s t r i b u t i o n of the e m e r s i o n h o r i z o n and a s s o c i a t e d mineralizations (above: a f t e r S p a l l e t t a , et a i . , 1 9 8 2 ) and some aspects of the shallow-water deposits sandwiched between pelagic carbonates and the turbiditic Hochwipfel Formation (A-E). According to the above scheme the minera!isations developed above pelagic and shallow-water carbonates are confined to a n a r r o w belt trending east-west, which became individuated by r e l i e f inversion mechanisms produced by the convergent wrenching tectonics. A): S i l c r e t e d e p o s i t s c o n f o r m a b l y o v e r l y i n g p e l a g i c c a r b o n a t e s (Sasso N e r o a r e a ) . B ) : L a m i n a r and t u b u l a r bands r e m i n i s c e n t of algal tufas (photo c o u r t e s y of L.Brigo).C__~) : D o l o m i t e layers a l t e r n a t i n g w i t h s c a l e n o e d r i c calcite.D_J_) : L o w - a n g l e l a m i n a t i o n i n d i c a t i v e of a s h o r e l i n e e n v i r o n m e n t . These deposits, in a d d i t i o n to those s t u d i e d by S c h ~ n l a u b , et al.(1991),testify for an e m e r s i o n horizon and u p l i f t of the platform (relief i n v e r s i o n ) . Similar silica concentrations or s i l c r e t e d e p o s i t s o c c u r in o t h e r areas of the e a s t e r n A l p s , f o r e x a m p l e in the D o l o m i t e s , above Ladinian buildups, and in the Hauptdolomite Formation in S w i t z e r l a n d , T h e general aspect of these s i l c r e t e d e p o s i t s is a l s o s i m i l a r to s i l c r e t e s r e p o r t e d by S m a l e (1973) from S o u t h A f r i c a and A u s t r a l i a . In all such situations silicification took p l a c e close to s y n s e d i m e n t a r y faults, c l o s e to a M g - C a b o u n d a r y , in a s h a l l o w environment resulting from s p i c u l a e concentrations.In the H a u p t o d o l o m i t e F o r m a t i o n (i.e. Munt de la Bascha, E n g a d i n e D o l o m i t e ) the site of s i l i c a d e p o s i t i o n was s u b t i d a l (less than 10-20 m deep)~ probably subjected to a syndiagenetic instability (R. Trumpy,1987:pers.comm.).Barite, fluorite, scalenoedric calcite and to a l e s s e r degree, s p h a l e r i t e , are a c c e s s o r y c o m p o n e n t s of the s i l c r e t e d e p o s i t s . T h e o c c u r r e n c e of s e d i m e n t a r y s t r u c t u r e s such as r h y t h m i t e s and the a b s e n c e of c o n t e m p o r a n e o u s v o l c a n i s m rules out an e p i g e n e t i c origin, as h y p o t h e s i z e d by S p a l l e t t a , et ai.(1982), or even a diagenetic transformation from o v e r l y i n g r a d i o l a r i t e s ( S p a l l e t t a , et a i . , 1 9 8 2 ) w h i c h are n e v e r found associated with these deposits (Galli,1980).Sponge spiculae of the type found here flourish in shallow-water carbonate environments, such as the B i m i n i Lagoon (Hay, et a i . , 1 9 8 7 ) , and in o t h e r l a g o o n a l areas in the Bahamas. S p o n s e
279
s p i c u l a e are a b u n d a n t in m a r g i n a l environments (Chouns and E l k e r n s , 1 9 7 ~ ) , as well as in s a b k h a facies. C o n c e n t r a t i o n s of sponges may have taken p l a c e in a s h o r e l i n e e n v i r o n m e n t . The b i t u m e n and the c a r b o n a c e o u s m a t e r i a l p r o d u c e d in an a n o x i c lagoon by the decay of o r g a n i c m a t t e r leads to an i n c r e a s e in the c o n c e n t r a t i o n of the c a r b o n d i o x i d e w h i c h in turn lowers the pH, therefore facilitating the silica replacement of c a r b o n a t e s (Walker,1960) and the p r e c i p i t a t i o n a l o n g fissures. Silica is i n o r g a n i c a l l y p r e c i p i t a t i n g in the e p h e m e r a l lakes a s s o c i a t e d with the C o o r o n g L a g o o n in S. A u s t r a l i a (Peterson and Von Der B o r k , 1 9 6 5 ) . N e a r b y deposits to these silcretes consist of l o w - a n g l e l a m i n a t e d , d o l o m i t e layers a l t e r n a t i n g w i t h thin c m - t h i c k h o r i z o n s c o m p o s e d of s c a l e n o e d r i c c a l c i t e (B ). Some low-angle l a m i n a t e d sets are c o m p o s e d of f r a g m e n t s of s c a l e n o e d r i c c a l c i t e . O x y g e n - i s o t o p e data from c r y s t a l s of this type (Sch~nlaub,et ai.;1991) indicate a meteoric origin. D o l o m i t e is c o m m o n l y a s s o c i a t e d w i t h b e a c h d e p o s i t s b e c a u s e the area l a n d w a r d of the beach is a d i s c h a r g e zone for c o n t i n e n t a l g r o u n d w a t e r s . An actual e x a m p l e occurs in the C o o r o n g Lagoon, in south A u s t r a l i a . Algal tufas (C) c o n s i s t of an a l t e r n a t i o n of t u b u l a r and laminar bands; they are quite s i m i l a r to some components described from present-day terrestrial cyanobacterial stromatolites (Galli and S a r t i , 1 9 8 9 ) , T h e close association of bitumen, silcrete deposits, barite, fuorite, sphalerite, dolomite, b e a c h deposits, algal tufas and o r g a n i c m a t t e r p o i n t s to a d e p o s i t i o n in e p h e m e r a l , very s h a l l o w - w a t e r lagoons ranging from hypersaline to freshwater (cf. Galli,1983).Uplifts and b l o c k - f a u l t i n g may have p r o v i d e d the diastrophic background for the f o r m a t i o n of these marginal environments. m
The U p p e r D e v o n i a n - L o w e r C a r b o n i f e r o u s interval in the C a r n i c Alps is i n s t r u c t i v e b e c a u s e c o n t a i n s several features d e v e l o p e d t y p i c a l l y d u r i n g k r i k o g e n e t i c r e j u v e n a t i o n periodsl these are: l)onlap i n t r a s h e l f ramps in the p l a t f o r m ; 2 ) a n o x i o basinal facies; 3 ) d e p o s i t i o n of m e g a b r e c c i a s and s e i s m o t u r b i d i t e s ; A ) s t r i k e - s l i p t e c t o n i c s and f o r m a t i o n of d i v e r g e n t p a t t e r n s l 5 ) h i g h - s e i s m i o i t y ( S p a l l e t t a and V a i , 1 9 8 ~ ) i 6 ) p a l e o k a r s t s and relief inversions; 7 ) a c c e n t u a t i o n of s t o r m i n c i d e n c e on the p l a t f o r m ( G a l l i , 1 9 8 6 ) i 8 ) d r o w n i n g of the c a r b o n a t e p l a t f o r m at the Frasnian-Famennian boundary.
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If one c o n s i d e r s g e o i d a l d e f o r m a t i o n as an e f f e c t of a g l o b a l c h a n g e in the shape of the g e o i d due to an i n c r e a s e in the rate of r o t a t i o n (Whyte,1977), c o n v e r g e n t w r e n c h i n g results to be a p h a s e of k r i k o g e n e t i c rejuvenation activity (Wezel,1988). The record of the punctuated, patohyly distributed geoidal d e f o r m a t i o n on the g l o b e , o r s t r i k e - s l i p t e c t o n i c s , d e p e n d s u p o n whether the deformed area is located within or outside a strike-slip megashear (cf. C a r e y , 1 9 8 8 ) w h i c h b e c o m e s a c t i v a t e d d u r i n g p h a s e s of g e o i d a l d e f o r m a t i o n (change from p r o l a t e to o b l a t e shape: W h y t e , 1 9 7 7 ) . The two case h i s t o r i e s summarized above, d o c u m e n t i n g r e l i e f i n v e r s i o n s in the V a l a n g i n i a n (Betic Cordillera) and Late D e v o n i a n - C a r b o n i f e r o u s (Carnie Alps) w e r e both localized within transcurrent megashears.
Relativistic
distribution
'event
of
horizons'
Introduction
The g e o l o g i c a l c o l u m n is p u n c t u a t e d by rare 'events', such as drownings of p l a t f o r m s , faunal extinctions,global short-term episodes of r e l a t i v e se-level change, and so on. S e v e r a l of these e v e n t s are not d i s t r i b u t e d at random, but c o n v e r s e l y are g r o u p e d w i t h i n s p e c i f i c time i n t e r v a l s ('event h o r i z o n s ' ) . The g r o w i n g i n t e r e s t in rare events, catastrophes and s u d d e n changes interrupting a presumed gradually evolving system fosters the views of the f o r e r u n n e r of c a t a s t r o p h i s m , Cuvier (1769-1832) who first reported from several stratigraphic successions sudden events represented by appearances and disappearances of fossil species ("Life ... has often been d i s t u r b e d on E a r t h by t e r r i b l e e v e n t s - c a l a m i t i e s ... w h i c h at the b e g i n n i n g , have p r o b a b l y m o v e d and o v e r t u r n e d to a g r e a t e r depth the entire outer crust of the globe" : cited from Raup ,1988: p.69). Since the b e g i n n i n g of E a r t h S c i e n c e s , catastrophists together w i t h t h e i r p r a e c u r s o r were r e g a r d e d as e c c e n t r i c p e o p l e b l i n d e d by t h e o l o g i c a l c r e e d who r e s o r t e d to i r r a t i o n a l , supernatural causes to e x p l a i n the e v o l u t i o n of n a t u r a l p r o c e s s e s . T h e r e a s o n of this v i e w regarding catastrophism has been explained by Raup (1988): "... In time, the u n i f o r m i t a r i a n v i e w of E a r t h h i s t o r y w o n d e c i s i v e l y o v e r the c a t a s t r o p h i s t view. It was a reflection of the a b h o r r e n c e of rare, unpredictable events, that is c o m m o n in m a n y fields of science. In any event, uniformitarianism (or g r a d u a l i s m , a s it is s o m e t i m e s called) has dominated geology a n d the education of ~eologists for the c e n t u r y a n d a half since the o r i g i n a l d e b a t e " . The p a r a d i g m of u n i f o r m i t a r i a n p r i n c i p l e s e s t a b l i s h e d by Lyell and H u t t o n c o n v e r s e l y s t a t e that " p r o c e s s e s that a c t e d in the past are in no way d i f f e r e n t from p r o c e s s e s a c t i n g today". The transformation of natural processes is thought to be a c u m u l a t i v e e f f e c t of small, t r i v i a l c h a n g e s , since the r e m o t e s t past. This concept is c r y s t a l l i z e d in the statement: "The p r e s e n t is the key to the past". A l s o the a c t u a l d e g r e e ,rate
283
a n d i n t e n s i t y of p r o c e s s e s a r e c o n s i d e r e d to be the s a m e as in the d i s t a n t p a s t . Stratigraphic a n a l y s i s h a s not e s c a p e d the application of the uniformitarian paradigm. The principles of Facies Models in sedimentology developed by W a l t h e r (1893) states that "only those facies and facies-areas can be superimposed primarily w h i c h c a n be o b s e r v e d b e s i d e e a c h o t h e r at p r e s e n t t i m e " . M o d a l c y c l e s a r e the p r o d u c t of the a p p l i c a t i o n of W a l t h e r ' s Law; in m o s t c a s e s h o w e v e r t h e y a r e o n l y an d e a l i z a t i o n of the r e a l i t y because they show what the situation would have been in a s c e n a r i o of g r a d u a l e n v i r o n m e n t a l shifting. T h e r e a r e s o m e b a s i c p r o b l e m s w i t h the W a l t h e r ' s Law. A f i r s t problem is the r e l a t i o n between the present-day accumulation r a t e of s e d i m e n t s a n d the g e o l o g i c a l preservation potential of a given sediment thickness. Sedimentologists studying the actual sediments are more concerned with the modality of deposition than with the preservation potential. A paradox in studying the stratigraphic sections is that most of the geological t i m e is n o t r e c o r d e d , d u e to n o n - d e p o s i t i o n or/and e r o s i o n . T h i s is p a r t i c u l a r l y evident where fluctuations of the b a s e l e v e l (level b e l o w w h i c h d e p o s i t i o n takes place under any condition) are frequent. Beds usually represent a small time compared to t h a t represented by b e d d i n g planes.Viewed under this perspective,beds are 'events'. When not produced by diagenesis, bedding planes represent changes in the environment.This means that the W a l t h e r ' s law is a p p l i c a b l e correctly only to t h e sediments representing a small time i n t e r v a l s e p a r a t e d by d i s c o n t i o n u i t i e s . The Walther's Law works well only when sequences do n o t record major breaks in sedimentary processes, when facies boundaries are g r a d a t i o n a l . The application of s t r a t i g r a p h i c g r a d u a l i s m h a s r e s u l t e d in t h e definition of F o r m a t i o n s and Members -diachronous rock unitswhich result from gradual environmental shifts. In several i n s t a n c e s F o r m a t i o n s do n o t h a v e a g e n e t i c m e a n i n g : t h e y a r e an a r t i f a c t of s t r a t i g r a p h i c gradualism; they have added confusion to t h e complexity of g e o l o g i c a l processes by leading to a complicated t a n g l e of n a m e s a n d s u b d i v i s i o n s w h i c h is f a m i l i a r to e v e r y g e o l o g i s t s t a r t i n g the s t u d y of s o m e n e w area. Goodwin and Anderson (1985) c h o s e d t h e 'PAC' as a f u n d a m e n t a l u n i t of a n a l y s i s of e p i s o d i c p u n c t u a t e d sediment accumulation; their reults contrasted a n d led to d i f f e r e n t interpretations f r o m t h o s e f o u n d e d on a g r a d u a l i s t i c approach: w h e r e a s the u s e of F o r m a t i o n s a n d M e m b e r s led to a d i s o r d e r e d stratigraphy, the u s e of P A C led to a v e r y o r d e r e d , layer-cake of s t r a t i g r a p h i c units bounded by s y n c h r o n o u s surfaces. The same approach is u s e d in s e q u e n c e stratigraphy, where sequence boundaries are
284
synchronous surfaces global process.
of
basinwide
extent,produced
by
some
The statement "The Present is t h e k e y to the p a s t " n e e d s s o m e reconsideration. For example, ~ust to c i t e f e w s i t u a t i o n s , the ancient epicontinental areas were characterized by a d i f f e r e n t water circulation and morphology} actual continental platforms underwent in the Quaternary atypical tectonic and glacial morphological modifications. Shelf regions are covered by relict sediments forming migrating sandwaves (i.e. along the e a s t e r n c o a s t s of N o r t h A m e r i c a ) . S h e l f m a r g i n m o d e l s , as w e l l as the B a h a m i a n m o d e l m u s t be u s e d w i t h c a u t i o n , a s s t r e s s e d by the same carbonate sedimentologists working in the area. Paleozoic oceans had different geometries: they were undersaturated with respect to calcium carbonate. Genetic models for Mesozoic deep-water sediments advanced by W i n t e r e r and Bosellini (1975) for t h e A l p i n e - Mediterranean region can n o t be a p p l i e d for recent oceans because of d i f f e r e n t calcium carbonate concentrations. T h e i n t e n s i t y of g e o l o g i c a l p r o c e s s e s h a s a l s o v a r i e d w i t h time: in t h e d i s t a n t g e o l o g i c a l p a s t t h e r e was a prevalence of thalassocratic conditions whereas at present an epeirocratic r e g i m e is p r e d o m i n a t i n g (emergent areas are prevailing). Some classes of d e p o s i t s and processes are better developed in t h e p a s t t h a t in t h e p r e s e n t : for e x a m p l e , the l a c k of v e g e t a t i o n a l c o v e r in t h e d i s t a n t p a s t led to m u c h h i g h e r r a t e of m e c h a n i c a l deposition. Orogeneses also increased in rate and volcanic activity since the Precambrian.Plate t e c t o n i c s p r o b a b l y d i d n o t o p e r a t e in the P a l e o z o i c . In s h o r t , t h e p r e s e n t r e p r e s e n t s o n l y a v e r y s m a l l f r a c t i o n of e v o l u t i o n of g e o l o g i c a l p r o c e s s e s c o m p a r e d to t h e s p a n i n t e r v a l of t i m e r e p r e s e n t e d by the Phanerozoic. A blind application of uniformitarian principles may correspond in s o m e c a s e s to a fixist
attitude.
The increasing i n t e r e s t in r a r e e v e n t s in t h e last d e c a d e s led to t h e n e e d to a c c o m p l i s h a distinction between continuous and discontinuous processes. Recent analyses have shown that most of p r o c e s s e s considered initially as c o n t i n u o u s , s u c h as t h o s e listed in A g e r (1981):erosion of a meandering river, reef growth, subsidence, uplift,pelagic deposition, heat flow, seafloor spreading, magnetic f i e l d , c o s m i c rays, are p u n c t u a t e d by d i s c o n t i n u i t i e s . The distinction a l s o d e p e n d s u p o n the s c a l e of o b s e r v a t i o n . The solar radiation i n t e n s i t y at v a r i o u s
for example undergoes discontinuities in s c a l e s , r e l a t e d to c h a n g e s in the m a g n e t i c
285
field below the photosphere. These discontinuities have a bearing on the terrestrial climate and weather such as an intensification of h u r r i c a n e strength, changes in t h e E a r t h ' s s p i n rate, s h i f t of c l i m a t i c z o n e s , a n d so on. The deep sea drilling pro~ect results indicate synchronous variations in the s e d i m e n t a t i o n rate of p e l a g i c deposition, formerly considered as c o n t i n u o u s . V a r i a t i o n s are related at v a r i o u s t e m p o r a l s c a l e s to c o o l i n g e p i s o d e s . The rate of sea floor spreadin~ is also punctuated by discontinuities; Schwan (1980) o b s e r v e d a coincidence between such discontinuities and unconformities of o r o g e n i c phases in orogenic belts. Orogenic deformation at a f i r s t a p p r o x i m a t i o n is a c o n t i n u o u s p r o c e s s as a n o r o g e n i c b e l t r e c o r d s a m i g r a t i o n of t h e d e f o r m a t i o n towards the f o r e l a n d (Vai,1987). Detailed s t u d i e s c o n d u c t e d in s e v e r a l o r o g e n i c b e l t s by V a i (1987) h a v e s h o w n t h a t the d e f o r m a t i o n is c o n c e n t r a t e d within specific time intervals.According to Stille (192~) orogenic phases and unconformities are episodic and correspond to t e c t o n i c p h a s e s . Conversely, uniformitarian views assume that orogenic phases are continuous9 a natural consequence of this view is t h e difficulty of i n t e r p r e t i n g the c a u s e f o r m a j o r u n c o n f o r m i t i e s delimiting depositional sequences which are normally attributed to e u s t a t i c sea level changes or to an obscure,not clear reorganization p h a s e of p l a t e t e c t o n i c a c t i v i t y (Bally,1980). Unconformities are considered by uniformitarians as perturbations of an o t h e r w i s e g r a d u a l l y e v o l v i n g p r o c e s s .
There I) 2)
are
two ways
of
considering
events
local
(Raup,1988):
e v e n t s m a y o c c u r as s u d d e n h a p p e n i n g s , totally unrelated to a p r o c e s s ~ events may subtend a process operating before the onset of the e v e n t in w h i c h c a s e t h e y m a r k t h e o v e r c o m i n g of a threshold.
Based point
on t h i s c o n c e p t , R a u p p (1988) events, a n d t h r e s h o l d e v e n t s .
made
a
distinction
between
As n a t u r a l processes become better understood, more and more point events shift into the threshold event category. For example,the t h e o r y p r o p o s e d by A l v a r e z et al., (1980) a c c o r d i n g to w h i c h the d i n o s a u r extinction was caused by a collision between an asteroid and the Earth, formerly considered as a random event, is n o w being reconsidered, in the light of
286
discoveries of o t h e r i r i d i u m a n o m a l i e s associated with minor extinction periods (Lower Devonian~Middle Miocene;end of Cenomanian; Eocene-Oligocene boundary). As our k n o w l e d g e of p h e n o m e n a increases, the idea of r a n d o m n e s s tends to be replaced with cause-effect relationships, as pointed out by a mathematician of the beginning of the twentieth century: "randomness is a m e a s u r e of our i g n o r a n c e " (Poincar4,1910). Hurricanes are n o r m a l l y considered as r a n d o m events. If we c o n s i d e r some of the s e v e r a l maps of h u r r i c a n e tracks h i t t i n g F l o r i d a p e n i n s u l a , we can c o n c l u d e w i t h o u t the s l i g h t e s t doubt that the p o s s i b i l i t y of s o m e h u r r i c a n e h i t t i n g the east F l o r i d a is r e g u l a t e d by c h a n c e . C o n v e r s e l y , t h e f i n d i n g of a t h i c k e n i n g u p w a r d s e q u e n c e g e n e r a t e d by h u r r i c a n e s and w i n t e r s t o r m layers in the Holocene Florida Bay, and successively of various analogous or identical sequences of different ages and locations in Europe (Galli,1990~ Fig.ll,12) led me to r e c o n s i d e r on one s i d e the r a n d o m n e s s of h u r r i c a n e s as r a n d o m e v e n t s , a n d , o n the o t h e r side, the use of the t e r m event as a synonymous of r a n d o m n e s s . In o t h e r words, how is it p o s s i b l e that r a n d o m e v e n t s f o r m a t r e n d ? Likewise, the q u e s t i o n I a t t e m p t e d at f o r m u l a t i n g in this w o r k is the f o l l o w i n g : how is it p o s s i b l e that events that are spaced from e a c h o t h e r s e v e r a l m i l l i o n s years form a t e m p o r a l s e q u e n c e of e v e n t s r e g u l a t e d by a g e o m e t r i c a l proportion? In fact, an i m p o r t a n t point observable from an e x a m i n a t i o n of Fig. IA9 is the t r e n d of s u c c e s s i v e lag times b e t w e e n the time intervals of s h o r t - t e r m sea level falls ( i n t e r p r e t e d as p h a s e s of p e r i o d s of g e o i d a l d e f o r m a t i o n ) . Such d i s c o n t i n u i t i e s are not d i s t r i b u t e d at r a n d o m b e c a u s e e a c h lag time b e t w e e n two t e m p o r a l d i s c o n t i n u i t i e s is a p p r o x i m a t e l y the half of the p r e c e d i n g one. Is this p u r e c h a n c e or does it underline some h i d d e n p h y s i c a l or g e o m e t r i c a l law?
Relativistic
The
study
object
of
of
'events',however
investigation
concept
new
in
of even. ~
Earth
sciences,
is
an
old
in P h y s i c s .
Events w e r e i n i t i a l l y s t u d i e d by Cusano, L e o n a r d o da V i n c i and Pacioli during the R e n a i s s a n c e period in Italy, in the X I V c e n t u r y . L e o n a r d o da V i n c i in p a r t i c u l a r s t u d i e d the m o d a l i t i e s
287
of f o r m a t i o n of impulse waves. He o b s e r v e d that sinusoidal waves u n d e r c e r t a i n c o n d i t i o n s c h a n g e t h e i r c o n f i g u r a t i o n with their t r a n s f o r m a t i o n into impulse waves, The study of discontinuities and events was successively u n d e r t a k e n by R i e m a n n , Gauss and C a n t o r in the X I X century, in Germany. It forms the basis for the development of the r e l a t i v i t y t h e o r y of Einstein. R i e m a n n d e m o n s t r a t e d that u n d e r d e f i n e d c o n d i t i o n s of a m p l i t u d e and w a v e l e n g t h the wave c h a n g e s its c o n f i g u r a t i o n from a s i n u s o i d a l and forms a d i s c o n t i n u o u s front which represents a discontinuity. This transformation takes p l a c e t h r o u g h d i s c r e t e a m o u n t s of e n e r g y , o r i n f i n i t e s i m a l q u a n t a of action. The c o n c e p t of 'event' is one of the c e n t r a l c o n c e p t s of the r e l a t i v i t y theory. An 'event' is c o n s i d e r e d as the i n t e r s e c t i o n point of the four s p a c e - t e m p o r a l c o o r d i n a t e s . T h e g e n e r a l t h e o r y of the r e l a t i v i t y implies the h y p o t h e s i s that nature, s o m e t i m e s c a l l e d the 'complex domain', can be a n a l y z e d t h r o u g h events. The space, r a t h e r than an a b s o l u t e state, as a s s u m e d by the Newtonian physics, is d e t e r m i n e d by the event distribution w h i c h c o n s t i t u t e s a non r i g i d s y s t e m of r e f e r e n c e , equivalent to a g a u s s i a n , c u r v e d q u a d r i d i m e n s i o n a l s y s t e m of c o o r d i n a t e s . The s u b s t i t u t i o n of time and space w i t h the t i m e - s p a c e d o m a i n involves the r e p l a c e m e n t of the c o n c e p t of m a t t e r with the event or e v e n t - p a r t i c l e concept. The r e a l i t y of a p h e n o m e n o n t h e r e f o r e r e p r e s e n t s a series of events. A g r o u p of events may be linked to e a c h o t h e r by a d e t e r m i n e d law; t h e i r c l u s t e r i n g may r e p r e s e n t the a r r i v a l point of s e v e r a l g e o d e s i c s (the p a t h from one e v e n t point to a n o t h e r is d e t e r m i n e d by the least action principle: a body follows the p a t h that c o r r e s p o n d to the m i n i m u m action: m a t e r i a l points move a l o n g g e o d e s i c s ) . A c l u s t e r of events, linked to a g r a v i t y center, c l o s e to a c u r v a t u r e of the s p a c e - t i m e domain, may be r e l a t e d to o t h e r s i m i l a r p o i n t s in the c h o s e d s y s t e m of c o o r d i n a t e s , w h i c h are the loci of s i m i l a r events. All these e v e n t s form a process. Matter is a mathematical construction based on the event distribution. Before E i n s t e i n Math s u b s t i t u t e d the c o n c e p t of space w i t h that of s u m m a t i o n of i n s t a n t a n e o u s d i s t a n c e s b e t w e e n m a t e r i a l points. The U n i v e r s e results to h a v e a d i s c o n t i n u o u s , c o r p u s c u l a r s t r u c t u r e w h o s e g e o m e t r i c a l p r o p e r t y is d e t e r m i n e d by the discontinuous distribution of event-matter. Several events clustered in the same space-time horizon are termed event h o r i z o n s . What e m e r g e d f r o m the r e l a t i v i t y t h e o r y developing a geometrical representation discontinuities.
was the p o s s i b i l i t y of the d i s t r i b u t i o n
of of
288
Neg-entropy
G i v e n a p o i n t 'x' in a n a t u r a l s y s t e m , o r m o r e g e n e r a l l y in t h e Universe, characterized b y a s e r i e s of p r o c e s s e s Pl ,P2 ,P3 ,,, Pn, a n y p a r a m e t e r is d e f i n e d b y t h e f o l l o w i n g f u n c t i o n : x
= g
(PI,P2...Pn)
(1)
According to this equation, organic processes control the development and geometrical properties of i n o r g a n i c processes which represent discontinuities or ' e v e n t s ' Schr~dinger associated the term neB-entropy to the p r o c e s s of formation and growth of organic processes, such as those characterizing living organisms. Neg-entropy is the o p p o s i t e than entropy; it is a m e a s u r e of the i n c r e a s e in t h e l e v e l of organization of a g i v e n p r o c e s s . L i f e is in f a c t a s s o c i a t e d to m i n i m u m v a l u e s of e n t r o p y ; it is in a c o n s t a n t disequilibrium state which contradicts the second principle of t e r m o d y n a m i c s equilibrium. A m e a s u r e of t h e n e g h e n t r o p y of a s y s t e m m a y by a c c o m p l i s h e d on a geometrical basis~ a geometrical approach stresses the relationship b e t w e e n the i n c r e a s e in t h e l e v e l of o r g a n i z a t i o n of the complex domain and the resulting increase in the associated neg-entropy~ The Gala hypothesis by Lovelock (1979) which considers the Earth as a n o r g a n i c whole, is a l s o b a s e d on the n e g e n t r o p y concept9 this hypothesis assumes that all processes on the globe are regulated by Life through an omeostatic, autoregolatory principle capable of maintaining conditions suitable to Life. This hypothesis has met with the actual orientation of reductive approach which may
resistance, probably because of science towards an opposite, be s c h e m a t i z e d by the following
equation: Y where which
= f(xl,x2...xn)
Y is a p r o c e s s , f a f u n c t i o n correspond to a s e r i e s of
(2)
of a s e r i e s differential
of p a r a m e t e r s equations.
x
Inherent in the (2) is the tendency to explain organic processes as a complicated combination between inorganic processes. T h e r e d u c t i v e a p p r o a c h is d o m i n a n t to o v e r w e l m i n g in
289 the s c i e n c e , w h i c h is c o n c e n t r a t e d on of structures which are considered complex domain.
the as
study and description the reality of the
T h e (I) c o n v e r s e l y implies that structures or discontinuities are t h e p r o d u c t of a t r a n s f o r m a t i o n of a n e g - e n t r o p i c process, m a i n l y a p r o c e s s of o r g a n i c g r o w t h .
Generation
O f singularities
A c c o r d i n g to R i e m a n n (185A) t h e o b j e c t of p h y s i c a l m a t h e m a t i c s a n d g e o m e t r y is the s t u d y of the p r o c e s s of t r a n s f o r m a t i o n from n o r d e r s e r i e s to a h i g h e r n+l order series. The infinitesimal calculus developed by Leibniz is also based on the same p r i n c i p l e s of g e n e r a t i o n of n u m e r i c a l series. One
can
consider
a given
series;
1,2,3,5,5,6
...
and two higher-order series, c u b e of the f i r s t s e r i e s :
I, I, The are
differences between the following:
4, 9, 8,27, the
(3)
respectively
the
square
and
the
16, 25, 36... 6~,125... terms
of
a
series
of
a
given
order
1, 4, 9, 16, 2 5 , 3 6 , a 9 , 6 ~ , . . 3, 5, 7, 9, 11, 1 3 , 15 T h e last s e r i e s r e p r e s e n t s a p r o c e s s of former, original series. The differences c a n a l s o be o b t a i n e d :
3, 2, The derived derives from this
case
the
This
process
5, 2,
7, 2,
9, 2,
13, 2,
transformation of the d e r i v e d
of the series
15... 2...
series of d i f f e r e n c e s indicate that each term the p r e v i o u s o n e b y a d d i n g t h e s a m e q u a n t i t y (in value is
'2').
known
as
the
invariant
law
of
transformation.
290
The process of transformation can be reverted with the g e n e r a t i o n of a f o r m e r s e r i e s t h r o u g h i n t e g r a t i o n or s u c c e s s i v e summations. In k e e p i n g w i t h R i e m a n n ("The m e t r i c of a p r o c e s s implies the generation of singularities"),numbers originate from the c o u n t i n g of s i n g u l a r i t i e s . The interval b e t w e e n two s u c c e s s i v e terms of a series of differences is characterized by an infinitesimal i n c r e m e n t dx to the s y s t e m w h i c h becomes visible only t h r o u g h the f o r m a t i o n of a new s i n g u l a r i t y . A consequence of the above conceptual scheme is that a neg-entropic process of growth takes place only with the generation of singularities which correspond to q u a n t a of action, that i s , t h r o u g h d i s c o n t i n u o u s ~umps.
Hierarchy
of s i n g u l a r i t i e s
A neg-entropic p r o c e s s c o n s i s t s of a d i s c o n t i n u o u s growth of s i n g u l a r i t i e s w h i c h takes p l a c e at d i f f e r e n t levels, This can be s c h e m a t i z e d in the f o l l o w i n g way, by c o n s i d e r i n g a s y s t e m of series of first, second, t h i r d order. xl x2 x3 xA
I, 2, I, A, I, 8, 1,16,
3, A, 5, 6,... 9, 16, 25, 36... 27, 6 & , 1 2 5 , 2 1 6 . . . 81,256...
The h o r i z o n t a l rows b e l o n g to a c l a s s or o r d e r of s i n g u l a r i t y N c h a r a c t e r i z e d by a g e o m e t r i c a l e x p a n s i o n . V e r t i c a l c o l u m n s are e x p o n e n t i a l s e r i e s N+I c o r r e s p o n d i n g to an e x p o n e n t i a l growth. Each of the s e r i e s N+I grows more r a p i d l y than any o t h e r of the f o r m e r s e r i e s b e l o n g i n g to the c l a s s N. The s e r i e s N + 2 a l o n g the d i a g o n a l correspond to a f u n c t i o n w h i c h grows e v e n more r a p i d l y t h a n the s e r i e s of t r a n s f o r m a t i o n N and N+I. The s e r i e s of higher-level
transformation N,N+I,N+2... procesds M which in turn
may be d e f i n e d by a generates M+I,M+2...
processes. A c c o r d i n g to R i e m a n n the m u l t i p l e d o m a i n is s t r u c t u r e d in this way) for example, the s c a l e of length of p h y s i c a l processes f o l l o w s an e x p o n e n t i a l geometrical increase in the o r d e r of magnitude. G e o l o g i c a l p r o c e s s e s d i s p l a y such a h i e r a r c h i c a l o r g a n i z a t i o n . The s c h e m e of d y n a m i c s t r a t i s r a p h y d e v e l o p e d by A i g n e r (1985) is an e x a m p l e of h i e r a r c h i c a l organizatioD based on three levels
of
stratigraphic
sequences
(Fig.16~).
291
O,1
m
boo,. _ , o 2 Y .
1
,o2_
Fig.16A Hierarchical stratigraphical sequences
Gemetrical
The modality neghentropic process may
,o , v.
.....]
,o4
_
,o6 y .
. ........ 1
analysis of three levels of (after Aigner,1985,with permission).
distribution
of
singularities
of g r o w t h of a l i v i n g o r g a n i s m , o r more generally,a growth, is k n o w n as s i m p l e o m o t h e t i c ~rowth. The be v i s u a l i z e d by m e a n s of a simple geometrical
scheme, In a p e n t a g o n - the t y p i c a l s y m m e t r y of l i v i n g o r g a n i s m s an omothetic g r o w t h l e a d s to an i n c r e a s e in s i z e w i t h a c o n s t a n t proportion , through the development of pentagons of progressively b i g g e r size. T h e g e o m e t r i c a l construction, quite simple, is obtained by prolonging the two sides of the triangles of F i g . 1 6 5 , until a side B and a diagonal A+B are constructed. This
growth
must
satisfy
C=A+B; The
omothetic
D=B+C;
proportion
the
following
E=C+D
proportions:
... A : B : B : C : C : D . . .
is d e f i n e d
as
follows:
292
time
axis
N÷2
E
E
E
E
D
D
N
S
S
A
F i g . 1 6 5 - Right: o m o t h e t i c g r o w t h of the p e n t a g o n a c c o r d i n g to the o m o t h e t i c p r o p o r t i o n . Left: the l o g - s p i r a l winding around the cones produces circular sections whose geometrical relationships vary according to the o m o t h e t i c proportion.C) Projection of the l o g , s p i r a l on the base of the c o n e . U p o n a complete rotation the d i s t a n c e from the axis of the cone is halved.
A+B=B:(A+B) The omothetic proportion, also known as golden section by a n c i e n t G r e e k s , is a f u n d a m e n t a l c h a r a c t e r of living b e i n g s and o r g a n i c p r o c e s s e s , or even i n o r g a n i c p r o c e s s e s d i r e c t l y d e r i v e d from o r g a n i c processes. It o c c u r s in D N A cells, microscopic organisms, trees, animals, shells, and so on. The simplest scheme of the o m o t h e t i c g r o w t h of a p o p u l a t i o n is g i v e n by the Fibonacci series (I,I,2,3,5,8,11...) whose successive rates (I/192/2~2/3~3/595/8...) r a p i d l y c o n v e r g e t o w a r d s the v a l u e s of the o m o t h e t i c p r o p o r t i o n . L i v i n g b e i n g s d i f f e r from i n o r g a n i c ones by t h e i r m o d a l i t y of g r o w t h w h i c h is r e g u l a t e d by the o m o t h e t i c p r o p o r t i o n and growth. The c o n c e p t of spiral c o n i c a l a c t i o n d e v e l o p e d by Gauss at the beginning of the X I X century is u s e f u l in e x p l a i n i n g the generation of s i n g u l a r i t i e s as the result of a p r o ~ e c t i o n of the r e l a t i v i s t i c s p a c e - t i m e c o n t i n u u m onto the e u c l i d e a n s y s t e m of c o o r d i n a t e s . The geometrical construction of v i s u a l i z e d as a s e c t i o n of a spiral
Fig.165 (right) can w h e r e the w i n d i n g of
be the
293
geodesic (or log-spiral) around a conic volume determines circular sections whose changes follow the omothetic proportion. A neg-entropic process can be t h o g h t of as an o u t w a r d and s i d e w a r d e x p a n s i o n p r o d u c e d by a c i r c u l a t o r y and r o t a t i o n a l w i n d i n g of the l o g - s p i r a l a r o u n d the c o n i c a l volume. The rate of n e g - e n t r o p i c g r o w t h is e x p r e s s e d by the angle of the cone. Singularities are formed through a 180 ° r o t a t i o n a r o u n d the cone. The f o r m a t i o n of a s i n g u l a r i t y c o r r e s p o n d s to a p h a s e change, or metrics, in the s y s t e m (Fig.166),
TIME AXIS
F i ~ . 1 6 6 - Series of d i s c o n t i n u i t i e s or s i n g u l a r i t i e s h a r m o n i c a l l y in the spiral conical action.
ordered
What is p e r c e i v e d as structures on a euclidean system of reference (i,e, F i g , 1 6 6 : l e f t ) is the p r o j e c t i o n of a p r o c e s s of c o n t i n u o u s a c t i o n from a n o n - e u c l i d e a n s y s t e m of r e f e r e n c e (the s p a c e - t i m e gaussian coordinate s y s t e m by E i n s t e i n ) , T h i s process can be visualized by the Fig.167 where the discontinuities, projected on the R i e m a n n s p h e r e , i n d i v i d u a t e a great circle~ singularities are invariant points and are maintained in the s t e r e o g r a p h i c projection o n t o a p l a n e cut normal to the axis of the ellipse; o t h e r e l e m e n t s , such as the c a r t e s i a n i n f i n i t y d i s a p p e a r as they are not i n v a r i a n t p o i n t s . A similar representation of n e g - e n t r o p i c process in c o n f o r m i t y w i t h the p r i n c i p l e s of the g e n e r a l r e l a t i v i t y was p r o d u c e d by Weyl who conceptualized the expansion of the Universe as originating from a s w a r m of particles spreading out along g e o d e s i c s from a point source, It can be c o n c l u d e d that n e g - e n t r o p i c p r o c e s s e s s h a v e a typical m o r p h o l o g y of g r o w t h and f u n c t i o n s d e r i v e d from the p r o c e s s of e x p a n s i o n of an a u t o s i m i l a r spiral structure, congruent with the o m o t h e t i c p r o p o r t i o n . A n e g - e n t r o p i c a c t i o n c o r r e s p o n d s to
294
t-m-~ I I I
| ! !
I I
I
Fig.167 - Stereographic projection of h y p e r b o l i c singularities or discontinuities o n the R i e m a n n s p h e r e . O b s e r v e t h a t on the sphere the cartesian infinities disappear. Successive j u m p s to u p p e r l e v e l s in the s y s t e m s i n d i v i d u a t e in the s p h e r e g r e a t e r spheric volumes;the ratios between spheric volumes converge towards the omothetic proportion. a s p i r a l w h i c h p r o d u c e s a c o m p l e t e r o t a t i o n b e t w e e n a s e r i e s of circles progressively increasing in size. There is an exponential acceleration u p w a r d s a n d s i d e w a r d s as a f u n c t i o n of t h e a n g l e of r o t a t i o n around the c o n i c a l action. The winding p a t h is a l o g - s p i r a l which defines on e a c h r o t a t i o n circular sections whose geometrical relations are defined by the omothetic proportion. A plane cut normal to t h e axis of the cone contains the pro~ection of t h e c i r c u l a r s e c t i o n s w h i c h a p p e a r as a s e r i e s of concentrioal circles whose distances from each other are defined by the omothetic proportion. An example of s u c h g e o m e t r i c a l organization is s h o w n b y t h e Kepler's reconstruction of the S o l a r S y s t e m . In K e p l e r ' s system, the S o l a r S y s t e m is d i v i d e d into t w o m a i n r e g i o n s ( e x c e p t i n g P l u t o ) . T h e r e a r e i n n e r p l a n e t s , c o m p o s e d of heavier elements, d e v o i d s of r i n g s a n d w i t h a s m a l l n u m b e r of satellites, and the outer planets characterized by a b i g size, a gaseous composition a n d a h i g h e r n u m b e r of s a t e l l i t e s . The division b e t w e e n t h e t w o z o n e s is t h e a s t e r o i d belt, occupied
295
by I 0 0 , 0 0 0 small bodies w h i c h c o r r e s p o n d to a p h a s e change, to a d i s c o n t i n u i t y b e t w e e n the two s e r i e s of planets, The orbits of the p l a n e t s are o r d e r e d h a r m o n i c a l l y in the way i n d i c a t e d by Gauss and Riemann: it is p o s s i b l e to c o n s t r u c t two series of a o n a e n t r i c a l , s l i g h t l y o b l i q u e s e c t i o n s for the two classes of p l a n e t s w h i c h may be c o n s i d e r e d as the p r o ~ e c t i o n s from a cone of the p l a n e s c o n t a i n i n g the orbits, n o r m a l to the axis of the cones, in each case the Sun and the A s t e r o i d Belt. The p l a n e t s a p p e a r to be located a l o n e a spiral ( l o g - s p i r a l ) on the p o i n t s that correspond to successive rotations of 180 ° of the l o g - s p i r a l a l o n e the cone. The d i s t a n c e b e t w e e n the p l a n e t s on the l o g - s p i r a l is c o n g r u e n t with the o m o t h e t i c p r o p o r t i o n .
Biolo~ical
evolution
At p r e s e n t the old c o n c e p t of g r a d u a l i s t i c evolution is b e i n g r e p l a c e d by m o d e l s of p u n c t u a t e d e v o l u t i o n , A c c o r d i n g to Dutuit (1986) the e v o l u t i o n of Life is a s t e p w i s e p r o c e s s progressing through discontinuous p h a s e changes. The e v o l u t i o n of living organisms corresponds to a neg-entropio exponential growth p r o c e s s in r e s p o n s e to an i n c r e a s e in the g l o b a l e n e r g y b u d g e t of the U n i v e r s e , D u t u i t ( 1 9 8 6 ) went b a c k to the e m e r g e n c e of the first germs of Life, 3.5 to ~ b i l l i o n y e a r s ago, P h o t o s y n t h e t i c bacteria, c o m p o s e d of a p r i m i t i v e n u c l e o u s , a p p e a r e d about 2.5 to 2 b i l l i o n years ago, as is t e s t i f i e d by the o c c u r r e n c e of microbial organo-sedimentary structures (stromatolites). Afterwards, e u k a r y o t e s a p p e a r e d about 1 b i l l i o n y e a r ago, The Ediakara fauna in A u s t r a l i a records the first a p p e a r a n c e of m e t a z o a n s , a p p r o x i m a t e l y 750 m i l l i o n y e a r s ago. Dutuit (1986) a t t e m p t e d at q u a n t i f y i n g t h r o u g h phase changes the e v o l u t i o n of v e r t e b r a t e s : their e v o l u t i o n w o u l d have taken p l a c e t h r o u g h the a c q u i s i t i o n of n e w p l a n e s of o r g a n i z a t i o n . The first stage is r e p r e s e n t e d by a d v a n c e d fished a p p r o x i m a t e l y ~50 m.y. ago, B e t w e e n the first and s the s e c o n d s t a g e , 3 0 0 to 3~0 m,y. ago, there was the a c q u i s i t i o n of carrying members, with the a p p e a r a n c e of a m p h i b i a n s . The s u p p o r t and l o c o m o t i o n i n v o l v e d an increase in the e n e r g y e x p e n s e s by a factor equal to I0. The third level involved a more advance in the o r g a n i z a t i o n m a r k e d by the e v o l u t i o n of a d v a n c e d reptiles (200 m.y. ago) t o w a r d s the m a m m a l i a n d e v e l o p m e n t , and the t r a n s i t i o n to endothermy which involved a greater action upon the e n v i r o n m e n t , T h e f o l l o w i n s level c o r r e s p o n d s to the e v o l u t i o n of mammals (135 m.y, ago). Here again, the temporal p u n c t u a t i o n of the p h a s e changes by
296
P P M 0 E P K J
?
TR P
C
D S
.
/ ~
~
~
Fig.168 - T r e e of L i f e of v e r t e b r a t e s . The arrows indicating the a p p r o x i m a t e l o c a t i o n of the 'event h o r i z o n s ' of Fig. 149, suggest a relation with phase changes in the evolution of vertebrates. N e w s t e p s of t h e e v o l u t i o n take place close or ~ust in o o r r e s p o n d a n o e with the event horizons. Dutuit
(1986)
is c o n g r u e n t
with
the
omothetic
proportion.
T h e i n c r e a s e in s i z e of i n d i v i d u a l p h i l a (i.e. e q u i d s ) m a y a l s o be t a k e n as a m e a s u r e of t h e c h a n g e in m e t r i c s o f the u n i v e r s a l evolution: in this case the increase in the energy is compensated b y a n i n c r e a s e in the m a s s of l i v i n g o r g a n i s m s , as follows from the well known formula : E = M c ~ . In r e l a t i v i s t i c t e r m s , t h e i n c r e a s e in s i z e a n d / o r c o m p l e x i t y of t h e o r g a n i s m s means that new mass is c r e a t e d because new energy is m a d e available
to
the
system,
297
Event
horizons
The g e o l o g i c a l column is p u n c t u a t e d by s h o r t - t e r m phases of global geoidal deformation (some of w h i c h c o n s i d e r e d in the above chapters), as shown in Fig. I A 9 . T h e intervals between these s t r a t i g r a p h i c h o r i z o n s are c o n g r u e n t w i t h the o m o t h e t i c proportion. These time intervals may be t h o u g h t of as s i n g u l a r i t i e s or 'events' of a s p i r a l - t y p e n e g - e n t r o p i c g r o w t h (Fig.169). T h e r e are two series (Fig. I A g ) , s e p a r a t e d by a p h a s e change dated approximately 38 m.y. ago w h i c h was a p e r i o d of increase in tectonic deformation, global diastrophic events, acceleration of the E a r t h ' s flattening pulses and g l o b a l cooling (Wezel, 1988) .
time
YPRESIA~NETIAN ~:--....2:~/)/CENOMANJAN \ TURONIAN VALANGINIAN ~--~
// V UPPERLtAS
-
I Fig.169 - Log-spiral arrangement of J u r a s s i c - T e r t i a r y event h o r i z o n s of F i g . l ~ 9 . E v e n t h o r i z o n s are s p a c e d 180 ° from each o t h e r (el. F i g . l ) , a n d b e c o m e more c l o s e l y s p a c e d w i t h t i m e , d u e to the u p w a r d w i d e n i n g of the spiral s p a c e - t i m e domain. It is not s u r p r i s i n g to find that several, d i f f e r e n t types of events are c l u s t e r e d within these h o r i z o n s , such as i r i d i u m storm accentuations, earthquakes, anomalies, extinctions, d r o w n i n g s of p l a t f o r m s , g l o b a l uplifts, p h a s e s of increase in the e v o l u t i o n of organisms. T h e s e time i n t e r v a l s c o r r e s p o n d to 'event horizons'.They may correspond to krikogenetic r e j u v e n a t i o n p e r i o d s (see I n t r o d u c t i o n ) . Event horizons may be t h o u g h t of as the point sources of attraction of events which converge towards the event horizons. Each event may be c o n s i d e r e d as a p o i n t - c o m p o n e n t of a l o g - s p i r a l p r o c e s s of w i n d i n g a r o u n d the c o n e and s u b ~ e c t e d to near the 'event horizon'.In fact, the event a deflection horizons correspond to p o i n t s of c u r v a t u r e increase of the r e l a t i v i s t i c s p a c e - t i m e domain.
298
0
~ ~
3
O
0
~/ i
I
,
;
o
\1lip
~
o
~
w
lu
NO~SS~ ~ ' ~
0
0 i N.~ £1 0~" Fig.170 - Double a r o u n d the cone.
singularity
formed
by
the
log
spiral
action
The p r o ~ e c t i o n of the 'event horizons' onto the time spiral of Fig.l shows that they are c l u s t e r e d into two zones with a 180 ° s p a c i n g from e a c h other. The two zones may c o r r e s p o n d to k r i k o g e n e t i c r e j u v e n a t i o n and k r i k o g e n e t i c q u i e s c e n c e periods. The 180 ° s p a c i n g may be e x p l a i n e d by the c o n c e p t of d o u b l e discontinuity obtained by a p r o j e c t i o n of a conical spiral a c t i o n on a p l a n e n o r m a l to the axis of the cone (Fig.170). In this respect, the m o d a l sequence described in the a b o v e c h a p t e r s ( d e e p e n i n g - u p w a r d p a s s i n g to s h a l l o w i n g - u p w a r d cycles; t r a n s i t i o n s from o n l a p to o f f l a p g e o m e t r y , d i v e r g e n t - c o n v e r g e n t patterns~ more in g e n e r a l all situations ranging from the g e o t e c t o n i c cycle: F i s . 2 to 'PAC' cycles) may be thought of as double discontinuities produced by the p r o j e c t i o n s onto our euclidean s y s t e m of c o o r d i n a t e s of a c o n i c a l spiral action (Fig.171).
299
GEOLOGICAL
LOG-SPIRAL
COLUMN
A
Fig.171
-
singularity
Modal of
sequence a
spiral-type
seen
as
process,
a
double
discontinuity
or
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