The stratigraphy of the Kimmeridge Clay of the Dorset type area and its correlation with some other Kimmeridgian sequences

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stratigraphy of the Kimmeridm2 Clay of the Dotset type area and is%:correlation with some other Kimmeridgian sequences

INSTITUTE OF GEOLOGICAL SCIENCES Natural Environment Research Council

Report 80/4

The stratigraphy of the Kimmeridge Clay of the Dorset type area and its correlation with some other Kimmeridgian sequences

B. M. Cox and R. W. Gallois

O Crown copyright 1981

lSBN 0 1l 884182 3

London Her Majesty's Stationery Office

1981

CONTENTS Summary 1 The Kimmeridge Clay in Dorset 1 Introduction 1 Details of the sections 4 Wyke Regis 4 Black Head and Osmington Mills 4 Ringstead B v 9 Kimmeridge area 10 Lithologies 10 Fauna, ammonite distribution and zonation Ammonite distribution and zonation 13 Pictonia baylei Zone 13 Rasenia cymodoce Zone 14 Aulacostephanus mutabilis Zone 14 Aulacostephanus eudoxus Zone 14 Aulacostephanus autissiodorenris Zone 15 Pectinatites zones 16 Pavlovia and jittoni zones 16

13

Correlation with other English sections 18

I

1

l

Introduction 18 Pictonia 18 Rasenia 18 Aulacostephanus and Xenostephanus Amoeboceras 19 Aspidoceras 19 Sutneria 22 Pectinatites 22 Gravesia 23 Pavlovia and Virgatopavlovia 23 Other macrofauna 23

18

Correlation with other European sequences 24 Correlation based on ammonites, notably Gravesia 24 The Kimmeridgian Stage 25 Acknowledgements 28 References 29 Appendix 1 32 Black Head 33 Osmington Mills 34 Ringstead Bay 35 Brandy Bay 36 Hobarrow Bay to Chapman's Pool 39

6 Correlation between the basal beds of the Kimmeridge Clay exposed at Wyke Regis, Black Head, Osmington Mills and Ringstead Bay 9 7 Sketch map of the solid geology of the Kimmeridgc area 11 8 Geological sketch sections of the Kifnmeridge Clay exposed in the cliffs between Brandy Bay and Chapman's Pool 12 9 Generalised Kimmeridge Clay rhythms 13 10 Ammonite ranges and zonal schemes applied to thc beds adjacent to the Lower-Upper Kimmeridgian boundary 17 11 Sketch map of the Kimmeridge Clay outcrop and subcrop in England 19 12 Correlation of the Lower Kimmeridge Clay sequence of the Dorset coast with those of the Warlingham Borehole and the Wash area 20 13 Correlation of the Upper Kimmeridge Clay sequence of the Dorset coast with those of the Warlingham Borehole and the Wash area 21 14 Ammonite ranges and lithologies adjacent to the eudoxus-autissiodorem's zonal boundary at Kimmeridge Bay and in the Warlingham Borehole h 15 Palaeogeography in Lower Kimmeridgian times showing the positions of outcrops referred to in the text 27

TABLES

1 Zonal schemes : past and present 3 2 Correlation between Dorset and south-west Germany sequences 25 3 Stage names : past and present 26

FIGURES

1 Kimmeridge Clay outcrops in the Dorset type area 2 2 Generalised vertical section of the Lower Kimmeridge Clay in Dorset 5 3 Generalised vertical section of the Upper Kimmeridge Clay in Dorset 6 4 Sketch map of the solid geology of the Black ~ead-Ringstead Bay areas 7 5 Correlation between the Kimmeridge Clay sequences exposed at Black Head-Osmington Mills, Ringstead Bay and Kimmeridge Bay 8

Bibliographical reference Cox, B. M. and GALLOIS,R. W. 1981. The stratigraphy of the Kimmeridge Clay of the Dorset type area and its correlation with some other Kimmeridgian sequences. Rep. Znrt. Geol. Sci., No. 8014. Authors B. M. Cox, B.Sc. and R. W. Gallois, B.Sc., F.I.M.M.. Institute of Geological Sciences, Exhibition Road, London SM.7 2DE.

The stratigraphy of the Kimmeridge Clay of the Dorset type area and its correlation with some other Kimmeridgian sequences B. M. Cox and R. W. GALLOI~ SUMMARY The Kimmeridge Clay of the type area crops out in cliff sections at Kimmeridge, Osmington Mills, Ringstead Bay and Wyke Regis in south Dorset. At Kimmeridge, the sections, although continuous, expose only the upper part ofthe formation. At Osmington Mills and Ringstead Bay, a section through the lower part ofthe Kimmeridge Clay and through part of the remainder of the formation can be determined, althoLigh only in isolated exposures which are separated from one another by landslips and complicated by tectonic structures. At Wyke Regis, only the basal beds are exposed. Descriptions and graphic sections are given for the exposed sequences. Comparison of the Dorset succession with those elsewhere in England has shown that the lithological and faunal sequences are similar throughout the country. A number of distinctive thin marker-bands have been recognised in Dorset, and these are correlated with similar bands recorded from the Warlingham Borehole, Surrey, and from boreholes in East Anglia. The ammonite faunas are discussed and the zonal scheme in current use is reviewed. It is suggested that the abundance of Gravesia has been underestimated in the past and that consideration should be given to the introduction ofa new Zone of Gravesia S@. The base ofsuch a zone would mark the Lower-Upper Kimmeridgian boundary. It would also form an important link with Tithonian and Volgian sequences elsewhere in Europe. Relationships among the Kimmeridgian, Portlandian, Tithonian and Volgian stages are discussed. It is recommended that most of the Kimmeridge Clay of the type area should be included in the Kimmeridgian Stage, and that the use of the term Kimmeridgian sensu gallico (which places only the Lower Kimmeridge Clay in the Kimmeridgian Stage) should be abandoned. THE KIMMERIDGE CLAY IN DORSET INTRODUCTION The Kimmeridge Clay was first recognised as a discrete division by William Smith on his map of 1815, where it was named in the table of strata as the Oaktree Soil. Subsequently (1817), he used the name Oaktree Clay for the clays lying between the 'Coral Rag and Pisolite' and the 'Portland Rock'. Webster (1816) provided the first descriptive details of the formation and changed the name to Kimmeridge Clay after Kimmeridge Bay in Dorset. The Kimmeridge Clay has an outcrop and subcrop in England which extends from Dorset to North Yorkshire. I t is poorly exposed inland and almost all our understanding of the stratigraphy of the formation has been made through study of the coastal sections between Weymouth and Swanage in Dorset (Figure 1) and borehole sequences elsewhere. Fitton (1836) described the general faunal and lithological characters of the formation in the Dorset cliff sections and further details

were added by Waagen (1865), Blake (1875), Woodward (1895) and Strahan (1898). As with most Jurassic strata1 terms, it can be argued that the original definition of the Kimmeridge Clay was that of a chronostratigraphical unit, in that it embraced a lithological and faunal assemblage bounded by what were believed to be isochronous surfaces. D'Orbigny (1842-1851, p. 610) introduced the term 'l'etage kimmeridgien' to cover the interval of time during which the Kimmeridge Clay was deposited and in recent years, most British workers have treated the terms Kimmeridge Clay and Kimmeridgian as if synonymous. The following notes are provided to clarify the position of these terms and their derivatives (Lower Kimmeridge Clay, Lower Kimmeridgian, and others) within the current system of stratigraphical nomenclature (Hedberg 1976; Holland and others, 1978). The Kimmeridge Clay (Formation) is an argillaceous lithostratigraphical unit which, in south Dorset, ranges from about 300 m to more than 500 m in thickness. The lithology of the formation is described on p. 10. The position of its base has remained unchanged since Waagen (1865) defined it at the base of a clay with fossils including 'Rhynchonella' inconstans, resting on a clay with iron concretions, in cliff sections near Wyke Regis (Figure 1). The base of the formation is also exposed at Ringstead Bay, 9 km north-east of Wyke Regis, where clays with 'R.' inconstans overlie a thin, ironshot, muddy limestone with corals (the Ringstead Coral Bed of Arkell (1929, p. 10) ), (see also Pictonia baylei Zone, p. 13.) Here, and in the adjacent sections at Osmington Mills and Black Head, the boundary lies at a lithological change from the mixed lithologies of the Corallian Beds to the more uniform, soft mudstones of the Kimmeridge Clay. This lithological change is accompanied by a general change in the macrofaunal assemblages. The Ringstead Coral Bed and its equivalents contain the last of the typical 'Corallian' fauna, characterised by corals, echinoids, gastropods, and large, commonly coarsely ornamented, bivalves. The abruptness of the faunal and lithological change is due to a minor, but widespread, erosion surface at this level. The base of the formation is coincident with the base of the Kimmeridgian Stage (see p. 3 for details). Brookfield (1978) has recently revived a suggestion of Blake (1875) that the basal part of the Kimmeridge Clay should be grouped with the top part of the Corallian Beds to form a 'Passage Beds Formation'. Arkell (1933, p.385) dismissed Blake's suggestion as impractical and retained Waagen's (1865) definition of the base of the formation. Although Brookfield cited faunal, lithological and mineralogical evidence to support the case for the new formation, his main contention was that the Kimmeridge Clay (mutabilis Zone and above in his definition) is composed of uniform black shales with an impoverished macrofauna and microfauna, whereas the 'Passage Beds' are dominantly arenaceous and contain a typical Corallian fauna. These observations are not in

~ l

---_

Corfs Castle

Fault. crossmark on downthrow s~de

r(z General dip an degrees

- 70

PORTLAND

60

Figure 1 Kimmeridge Clay outcrops in the Dorset type area

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I

accord with those of the present authors who regard the lithologies and faunas of the Kimmeridge Clay part of the 'Passage Beds' as typical of the Kimmeridge Clay. Throughout the baylei to eudoxus zones, the Kimmeridge Clay is made up of rhythmic alternations of silty mudstone, dark grey mudstone and pale grey, very calcareous mudstone (see p.10 for details) with a fauna that is similar in character to the remainder of the Kimmeridge Clay. The position of the upper limit of the Kimmeridge Clav in Dorset is more controversial. Arkell (1933, pp.443-6) summarised the reasons given by authors, from Fitton (1836) to Cox (1929), for their particular choices of a boundary within what is clearly a lithologically transitional sequence from clay to silt and sand. Arkell (1933; 1947) placed the boundary at the base of the Massive Bed, a prominent bed of sandstone exposed in Houns-tout cliff [SY 9505 77201, and most subsequent authors have followed this. Townson (1975) lowered the boundary to the base of the Rhynchonella Marls but Cope (1978) and Wimbledon and Cope (1978) continued to use the Massive Bed. This part of the sequence is poorly exposed and has not been remeasured in the present work; for the purpose of comparison with earlier literature, the top of the Kimmeridge Clay has been taken at the base of the Massive Bed. It seems likely however that Townson's (1975) position for the boundary will become generally accepted in the future because it is based on detailed sedimentolo~icalwork. At outcrop outside Dorset, theuhighest beds of the Kimmeridge Clay have been removed and the base of the overlying Portland Beds (or their equivalents) rests on an erosion surface. Blake (1875, p.197) was the first to divide the formation into Lower Kimmeridge Clay and Upper Kimmeridge Clay. I t has proved useful for descriptive purposes to have these terms available because of the substantial thickness of the Kimmeridge Clay in its type area; they have been widely used by subsequent authors such as Arkell (1947) and Cope (1967). Both divisions consist of relatively uniform soft mudstones, but they are easily distinguishable on the basis of their ammonite

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1

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2

3 m

4

i 80

.+- Ant~cllnalaxts a

Minor faults and folds omitted

5km l

Vert~callimb of Purbeck monocl~ne

e

s 90

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7

faunas. The Lower Kimmeridge Clay is characterised br species of Pictonia, Rasenia and Aulacostephanus and &c Upper Kimmeridge Clay by species of Pectinatitri. Pavlovia and related forms. Ammonites are abundant at the level of the Lower-Upper Kimmeridge Cla? boundary and the distinction between the two assemb lages can be made even from very small shell fragments (see p.16 for discussion). Elsewhere in southern England, this boundary has proved readily identifiable in boreholes and its outcrop position can be determined even in areas of poor exposure. Arkell (1947) introduced terms for nearly all tbc subdivisions of the Kimmeridge Clay at and adjacent to Kimmeridge Bay (for example, Cattle Ledge Shales, Hen Cliff Shales, Maple Ledge Shales, Gaulters Gap Shales, Washing Ledge Shales), defining each as tbc stratum between particularly prominent, and usually named, thin limestone bands. These names have m usefulness outside the type section, and it is suggested that they should be abandoned. D'Orbigny's original definition of the Kimmeridgiam Stage was subsequently refined by Salfeld (1913) to encompass a number of fossil zones. Arkell ( 1933; 1945: 1947; 1956), Callomon (in Torrens and Callomon, 1968; in Callomon and Cope, 1971), Casey (1967), Cope (1967; 1978), Neaverson (1925), Spath (1935; 1936) and Ziegla ( 1962a; 1964) have modified Salfeld's zonation to produce the present zonal scheme, based on ammonite (Table 1). The boundaries of most of the lower zones have not been precisely specified in the sections because tbt published descriptions of the Lower Kimmeridge Clay h Dorset are mostly of generally poorly correlated short sections, which give only an approximate stratigraptpI n modern terminology, these zones are range or assemblage biozones (biostratigraphical units), because they have been defined in terms of the ammonite fauna and are unrelated to the lithological sequence. The?cannot be used alone therefore to define the Kimmeridgian Stage because 'stages' are now considered to be units of chronostratigraphy (Hedberg, 1976). However. in fossiliferous marine clays such as the Kimmeridge

Table 1 Zonal schemes for the Kimmeridgian Salfeld 1913'

Neaverson 1925

Arkell 1933

Spath 1935; 1936

Arkell 1945; 1956

Perisphinctes pallasianw

Holcosphinctes pallasioides

Zonally unclassified

Paulouia pallasioides

Pavlouia pdlasioidcs

Ziegler , Casey 1967' Callomon 1962a; 1964 Cope 19673 ' 1968; 1971

Pavlovia rotunda

Virgatopaalovit jttoni

Paulouia pallasioides

Pavlovia rotunda

Pallasiceras rotundurn

Paulouia rotunda

Paulouia rotunda

Pavlovia rotunda

Pavlovia pallasioides

Paulouia pallasioides

Pectinatites pectinatus

Pectinatites pectinatus

Pectinatites pectinatus

~cctinatites pectinatus

Pectinatites pectinatus

Pectinatites pectinatus

Virgatosphinctoides whcatleymcis

Pectinatites hudlestoni

Subplanites ~ p p .

Pectinatites wheatleymcis

Virgatosphinctoides Subplanifcs nodifcm wheatlcymris

Virgatites rniatschkouiemis

-----

Vir atosphinctoides Virgatosphinctoides Subplanites nh+ms wheatlcyensis grandis

-----

Virgatosphinctoides Subplanites ~ p p . Subplanites wheatlcymris uimincus? Grauesia irius

Gravesia irius

'Gravesiazones'

Gravesia gigas

Pcctinatites scitulus Pectinatites elegans

Grauesia gigas

Grauesia grauesiana

Grauesia grauesiana

Aulacostephanus pseudomutabilis

Aulacostcphanus pseudomutabilis

Aulacostephanus pseudomutabilis B A . undorae

Aulacostephanw yo

Aulacostephanus YO

Aulacostephanusyo B Aspidoceras longispinurn

Rasenia mutabilis

Pararasenia mutabilis

Pararasmia mutabilis

Rasmia mutabilis

Aulacostc&nus mutabilis

Aulacostephanus mutabilis

Rasenia cymodoce

Rasmia cymodoce

Rmia uralensis B Amoebites kitchini

Ramia cymodoce

Rarmia uralensis

Rasenia cymodou

Pictonia baylei

Pictonia baylei

Pictonia baylei G3 Rmia cymadoce

Pictonia baylei

Pictonia baylci

Pictonia baylei

O

X

F

O

Cope 1978

R

D

I

A

Grauesia graucsiana Aulacostephanus pscudomutabilis

Aulacostcphanus autissiodorensis

Aulacostephanus autissiodorensis Aulacostephanus cudoxus

N

'

This sequence represented the whole of Salfeld's Kimmeridgian Stage and the lower part of his Portlandian Stage (see Figure 10). Pavlouia zones only. Pectinatites zones only. Note:

Horizontal lines do not imply precise correlation between zonal boundaries.

study in a single, gently dipping section in which the zonal boundaries have been more accurately determined (Cope, 1967; 1978). Although as defined by Cope these zones are biozones, some are probably also chronozones. Until the situation is clarified and the whole of the Kimmeridgian Stage can be divided into chronozones it is suggested, in the interests of consistency of usage, that all the zones retain their italicised fossil names. In recent years, the Kimmeridgian has been divided into two substages (Lowerand Upper). The base of the Upper Kimmeridgian was taken by Cope (1967, p.4) at the base of the Grauesia zones of Arkell (1956). Arkell (1947, p. 67) believed this division to be in accord with the most natural grouping of the ammonites; he had earlier and again subsequently (Arkell, 1945; 1956) used this level as the boundary between a Lower and Middle

Clay where correlations have been achieved in the past almost entirely by the use of fossils, the practical distinction between biostratigraphy and chronostratigraphy is slight. Recent work on the Lower Kimmeridge Clay of East Anglia (Gallois and Cox, 1976) has shown that minor erosion surfaces occur at the bases of the baylei, cymodoce, mutabilis and eudorrs zones. These same erosion surfaces, and the lithological and fauna1 marker beds that enable them to be recognised, have been recorded in Dorset in the Dresent work. The authors believe that these erosion surfaces can be regarded as isochronous for all practical purposes, and therefore enable these zones to be elevated to the status of chronozones. By contrast with the Lower Kimmeridge Clay, the whole of ihe--Upper Kimmeridge Clay is available for 3

substage in a threefold division of the stage. Cope (1967, p. 68)proposed that the use of Gravesia as a zonal index should be discontinued because of its rarity in Dorset, and placed the substage boundary at the junction of the autissiodorensis and elegans zones. In the type section, this boundary was believed to be coincident with a limestone rhat forms a convenient lithological marker (Bed 42 of Blake, 1875). The present work has reaffirmed the usefulness of Gravesia and has suggested that the zonal scheme and the substage boundary might require revision (see p.25 for details). The purpose of the present work has been to bring together new information on the faunal and lithological sequences observed on the Dorset coast and to compare this information with that from cored boreholes elsewhere in England. Graphic sections have been drawn (Appendix 1) in the hope that these will provide a framework fi5r future palaeontological, sedimentological and mineralogical research on the type sections.

DETAILS OF THE SECTIONS The Kimmeridge Clay crops out in two main coastal areas of Dorset. At Wyke Regis, Osmington Mills and Ringstead Bay (Figure l), a number of small exposures, disturbed by tectonic structure and landslip, expose most of the Kimmeridge Clay. In the Kimmeridge area, between Brandy Bay and Chapman's Pool, a broad anticline exposes beds ranging from the middle part of the tudoxus Zone to the top of the Kimmeridge Clay along about 8 km of cliffs in continuous sections that are largely unaffected by structural complication. Arkell (1947, pp. 80-85) described the broad stratigraphical features of the Osmington-Ringstead sections, and suggested that the total Kimmeridge Clay thickness in that area was about 800 ft [244 m], compared to his estimate of about 1650 ft [503 m] in the Kimmeridge Bay area. The Kimmeridge Clay is poorly exposed inland in Dorset. Between Osmington and the Ridgeway Fault at Portesham (Figure l), the soft mudstones of the formation give rise to a poorly drained clay vale lying between the escarpments of the Corallian Beds and the

Portland Beds. At the western end of this outcrop, near Abbotsbury, the lowest part of the formation is represented by a sandy limonite oolite (the Abbotsbury Iron Ore). The stratigraphical sequence of the Kimmeridge Clay in Dorset is summarised in Figures 2 and 3. Wyke Regis The lowest part of the Kimmeridge Clay (baylei and cymodocG zones) is exposed at Wyke Regis in low, slipped and weathered cliffs at East Fleet [SY 661 7661 and near Sandsfoot Castle [SY 671 7701 (Figure 1). Both sections are protected from marine erosion, the former by Chesil Beach and the latter by Portland Harbour breakwater, and are consequently degraded. Nevertheless, with some digging, almost continuous sections can be measured through the baylei and lower cymodoce zones at both localities. In the 19th century, the Sandsfoot section was the best available exposure of the Corallian Beds Kimmeridge Clay junction and for this reason it was used by Waagen (1865) for the first definition of this boundary. At present, the most prominent single bed in the Wykc Regis sections is a thin siltstone crowded with myid and other bivalves. Birkelund and others (1978, pp. 35-36) have recorded the sequence of raseniid faunas ( c y w t h z Zone) of the Fleet section and confirmed the earlier observations of Arkell (1947, p.88) and Morris (1968. p.8) that the base of the zone is marked by this siltstone which they named the Wyke Siltstone. A bed of similar lithology, occurring 1-2 m above the Wyke Siltstone, has been recorded in the same sections and at Black Head and Osmington Mills: for ease of stratigraphical reference, the higher bed is here named the Black Head Siltstone. B!ack Head and Osmington Mills The most complete section of Lower Kimmeridge Clay in Dorset is exposed in slip-faces on the east side of a prominent ridge running down from the highest point of Black Head [SY 7258 82001 (Figure 4, section 1) to the beach [SY 7259 81921. Viewed from Osmington Mills, Sca1e:l to 1000

Mudstone, undifferentiated; mostly dark grey

Mudstone, very calcareous (pale grey)

Silty mudstone and siltstone

N.B. Thicknesses for the Lower Kimmeridge Clay are based on Kimmeridge Bay (down to tha Hobarrow Bay Stone Bandland the Broad Bench No.1 Borehole with faunal datril transposed from Weymouth area sections.

Oil shale

- 1

Tabular bed Dolomitic limestone (cementstone

dD 0

Concretions

W

W

Bivalve - r i c h

db

Rhynchonellid brachiopods

W

L 0

Individual oil-shale seams and limestones not to scale Corresponding Bed Number in East Anglia (Wash area boreholel

VERl RANGE OF

Figure 2 Generalised vertical section of the Lower Kimmeridge Clay in Donet

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VERTICAL RANGE OF SECTIONS

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I

Virgatopavlovia fittoni Zone(prs1 . F -

silty mdstone and muddy siltstone

Pavlovia

Rotunda Nodules

Pectinatites pallasioides Zone

A

Freshwater Steps Stone Band

Middle White Stone Band A White Stone Band

Basalt Stone Band

*

E; m

S

n

Rope Lake Head Stone Band Blackstone

j

E r

Y 5 m Grey Ledge Stone Band

Cattle Ledge Stone Bend

Yellow Ledge Stone Bend

Blrkef Bed 42

V V V Zone (pars)

Figure 3 Generalised vertical section of the Upper Kimmeridge Clay in Dorset

crossmark on downthrow side

Fault.

_L-

m

Kimmeridge Clay

@

Measured section

2 kilometres

Figure 4 Sketch map of the solid geology of the Black Head - Ringstead Bay areas (based on Arkell, 1947, pl. XIX) showing the positions of sections referred to in the text

these slips appear to form an almost continuous black cliff from which the head takes its name. There are scattered exposures in smaller slip faces to the west of Black Head and eastwards to Osmington Mills village, and these provide additional details to the main section. Few prominent marker bands are visible from the beach but, in the middle and upper part of the ridge, a line of cementstone doggers in the mutabilis Zone, the Virgula Limestone and the Nannocardioceras Cementstone form useful datum planes (Figure 5). The eudoxus Zone above the Nannocardioceras Cementstone and the autissiodorensis Zone are poorly exposed here, but much of the sequence can be revealed by digging. Dips are generally steep throughout the section, being about 80" near the base of the Kimmeridge Clay and decreasing to 50" to 60"in the upper part of the section. Camber and landslip add to the difficulties of making accurate thickness measurements. Much of the lower part of the section is obscured by a thin crust of weathered clay, but in dry weather this can be readily cleared to expose clean sections of essentially unweathered material, often with beautifully preservecl calcareous fossils. On the western side of Black Head for a distance of about 300 m [SY 7259 8192 to 7229 81981, there are small exposures, mostly in the baylei, cynrodoce and mutabilis zones, which can be correlated with the main section. These sections, although varying from year to year as new cliff falls occur, often show continuous

exposures through the baylei Zone and much of the cynodoce Zone. There is little overlap between the Lower Kimmeridge Clay exposed at Black Head and that in Kimmeridge Bay; the beds in the range of overlap are considerably thinner at Black Head and, for the most part, poorly exposed (Figure 5). To the east of Black Head, almost as far as Osmington Mills village, there is a large landslip. Along its eastern edge [SY 7336 81861, a number d adjacent exposures, although complicated by faulting and steep dips, provide an almost continuous section from the base of the Kimmeridge Clay to a level within the upper part of the eudoxus Z.one. At the present time, the baylei and cymodoce zones are well displayed here and in a fault-bounded mass of Kimmeridge Clay at beach level [SY 7342 81741 (Figure 4, section 2). Dips in this latter exposure are steep (70" to 80") and, along the northern margin of the outcrop, clays low in the mutabilis Zone are faulted against Corallian Beds. The Corallian Beds-Kimmeridge Clay junction is well exposed with Torquirhynchia inc01utans U.Sowerby) and Pictonia common in the lowest beds of the Kimmeridge Clay. This part of the sequence can be matched in detail with that at Ringstead Bay, although at Black Head and Osmington Mills, the Ringstead Coral Bed is replaced by a shelly oolitic ironstone (Figure 6).

Kimmeridge Bay end adjacent c l i f f s west of Clavell's Herd Grey Ledge S.B.

I Cattle Ledge S.B

Black Head and Osmington Mills

/ /

Ringstead Bav

i

I I

1

1

/

Blake's Bed 41 Blake's Bed 42

Freshwater Steps S. B.

I

Washing Ledge S.B.

Nannocardioceras Cementstone -----

L- -

===% Virgula Limestone

Hobarrow Bay S. B.

I I I l

I I

I

I

I I

I

I I I not exposed:l I I

3 S

]

AI-1

Supracorallina Bed

E

thickness not

I

i I I

I

precisely known

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I

""

lnconstans and Nana beds Ringstead Coral Bed

!I I

t

I

I I

KEY

r 30 metres 25

I

I

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1

I I I

I I

I

Pictcnnia ...baylei ~ ~ n eV t , , , ,

I

Beds below Hobarrow Bav Stone Band proved in Broad Bench No.1 Borehole

Lithologies as Fig.2

I

I I

!

S-S-S-s Saccocoma -rich band

1

lnconstans and Nana beds

Ringstead Waxy Clay (Corallian Beds)

Figure 5 Correlation between the Kirnrneridge Clay sequences exposed at Black Head - Osrnington Mills, Ringstead Bay and Kimmeridge Bay

WYKE REGlS (~ANO~FDOT SY67067700

KEY Siltstone

m m

Oolitic (limonitel mudstone Sandy mudstone

Oark grey , .m du

0

1,

Limestone Erosion surface w i t h phosphatic pebbles and burrowing Scale

I' \

'.

MILLS

not exposed

(Y

D C

N

..

Pale grey mudstones with thin tabular clay ironstone

<

RINGSTEAD BAY SY 74868137

?

,

0

2 \r

not exposed

I ,

I

Dark grey mudstones with abundant Deltoideum

l

Pale grey mudstones with thin tabular clay ironstones

Oark grey mudstones w i t h abundant Deltoideum delta Nana Bed

Ringstead Waxy Clay (pars)

Figure 6 Correlation between the basal beds of the Kimmeridge ,Clay exposed at Wyke Regis, Black Head, Osmington Mills and Ringstead Bay

Ringstead Bay The Kimmeridge Clay is exposed in Ringstead Bay [SY 76811 mainly in unconnected small sections separated by landslip. The lower part is poorly exposed. Arkell (1933; 1947; 1949; 1951) described most of the sections that were available between about 1930 and 1950 and suggested that they could be correlated with one another

to provide an almost continuous sequence from the baylei to lower cudoxus zones totalling about 120 ft [36.5 m] (Arkell, 1947, pp.83-84). The Upper Kimmeridge Clay up to the Freshwater Steps Stone Band can be measured in a partly overgrown section [SY 765 8131 (Figure 4, section 6). This section, in contrast with those in the Lower Kimmeridge Clay, is structurally uncomplicated

(with steady 10" to 20° dips) and thicknesses can therefore be measured with confidence. The present work suggests that the thickness of the Lower Kimmeridge Clay at Ringstead Bay is comparable to that at Black Head where about 95 m of strata have been recorded. Much of this part of the sequence is either unexposed or too damaged by landslip at Ringstead Bay to permit accurate measurement. Arkell (1933, p. 451) recorded the Ringstead Coral Bed in about 1.5 km of continuous low cliff in the central part of Ringstead Bay but it is now largely obscured by shinge and landslips. The Corallian Beds-Kimmeridge Clay junction is visible in only two small, degraded and landslipped sections below the western end of Ringstead village [SY 7478 8139 and 7486 81371 (Figure 4, section 3). It is unfortunate that this latter sec+on (figured by Arkell 1933, plate XXI), which has been proposed by George and others (1969, p. 153) as the type section for the base of the Kimmeridge Clay and the Kimmeridgian Stage, is separated from the remainder of the Kimmeridge Clay outcrop at Ringstead Bay. Eastwards from Ringstead village, slipped masses of Kimmeridge Clay form a low, grassy cliff. Although, under certain favourable conditions at low tide, parts of the Lower Kimmeridge Clay are exposed in situ on the foreshore in front of this cliff, there is essentially no section worthy of measurement between the outcrop of the basal beds and that of the eudoxus Zone. The upper part of the Lower Kimmeridge Clay, from the upper part of the eudoxus Zone to about the middle of the autissiodorensis Zone, is exposed in two small overlapping sections [SY 7619 8147 and 7606 81471 (Figure 4, sections 4 and 5), but the Lower-Upper Kimmeridge Clay boundary is obscured by slipped material. In the Upper Kimmeridge Clay sections, the equivalents of the Grey Ledge, White and Freshwater Steps stone bands form prominent features; a thick oil shale with calcareous concretions appears to be the equivalent of the Blackstone. Saccocoma has not been recorded from this bed or the adjacent oil shales, but the material is mostly deeply weathered; it was recorded from this stratigraphical level in the nearby Poxwell Borehole (Arkell, 1947, p. 81) (see Other macrofauna, p. 23). Kimmeridge area An extensive section has been measured in the Kimmeridge area between Brandy Bay [SY 889 7951 and Chapman's Pool [SY 955 7711 (Figure 7). The structural relationships in the cliffs in the Kimmeridge area are shown in Figure 8. The beds between the White Stone Band (basal bed of the pectinatus Zone) and the Hobarrow Bay Stone Band (upper eudoxus Zone) are well exposed between Brandy Bay and Hobarrow Bay: the whole of the Kimmeridge Clay above the Hobarrow Bay Stone Band is exposed between Hobarrow Bay and Chapman's Pool. Between Brandy Bay and Chapman's Pool, the strata are readily accessible for bed by bed collecting, mostly at, or just above, high water mark. Calcareous and pyritic fossils from near high water mark are, however, subject to oxidation and saline attack. Parts of the sequence in Brandy Bay, Hobarrow Bay and Kimmeridge Bay, and between Hen Cliff and Rope Lake Head are exposed in wave-cut platforms as well as in the cliff and yield better preserved specimens. When the outcrop thickness above The Flats Stone Band is added to the Kimmeridge Clay thickness proved

below this bed in the Broad Bench No. 1 Borehole (Lees and Taitt, 1945), the full thickness of the Lower Kimmeridge Clay at Kimmeridge Bay is estimated to be about 235 m. No fault was recorded in the borehole but it is likely that at least one of the numerous small faults which crop out in the cliffs in the Broad Bench area is intersected. The thickness of the upper part of the autissiodorensis Zone, which was uncertain formerly, has been determined by tracing several marker beds (including Blake's Bed 42) from Hen Cliff into Kimmeridge Bay. In addition, the doubt that existed concerning the total thickness of the zone at Kimmeridge Bay (see Arkell, 1947, pp. 74-76 for discussion) has been resolved by a recent borehole [SY 9097 78991, which proved 24.1 m of strata between the Maple Ledge and the Washing Ledge stone bands thereby indicating that there is probably no gap in the cliff sections. The base of the Upper Kimmeridge Clay has been taken for convenience at a thin limestone band, a little above the upper limit of the range of Aulacostephanus (Arkell, 1947) and at the lower limit of the range of Pectimtites (Cope, 1967) (Blake's Bed 42). The thickness of the beds between this bed and the top of the Rotunda Nodules (the highest bed measured in the present work) is about 207 m and is similar to that recorded by Blake (1875). The highest beds of the Kimmeridge Clay, between the Rotunda Nodules arid the base of the Portland Beds, are poorly exposed mudstones, silty and sandy mudstones and fine-grained muddy sands that crop out in the steep lower slopes of the Houns-tout. Their thickness has been given recently by Cope (1978, pp. 471-472) as 66 m. If this figure is added to the present measurements for the lower beds, the total is 273 m for the Upper Kimmeridge Clay between Kimmeridge Bay and Chapman's Pool. The total thickness of the Kimmeridge Clay in the Kimmeridge area is therefore about 508m.

LITHOLOCIES The Kimmeridge Clay in Dorset is made up almost entirely of soft mudstones, calcareous mudstones and kerogen-rich mudstones (bituminous mudstone and oil shales). Coarser sediments occur in the basal part of the formation around Abbotsbury (sandy, oolitic ironstones and ferruginous sandstones) and in the top part of the formation in the Kimmeridge and Weymouth areas (sandy mudstones) (see also p.2 for discussion of the stratigraphical position of the latter beds). The formation shows rhvthmic sedimentation at a number of stratigraphical levels. In the lower part of the Lower Kimmeridge Clay, each rhythm consists of siltstone overlain in turn by dark grey mudstone and pale grey calcareous mudstone (Figure 9 Type A). In the upper part of the Lower Kimmeridge Clay and in the Upper Kimmeridge Clay, the rhythms consist of brownish grey bituminous mudstone or oil shale passing up into dark grey mudstone and then into pale grey calcareous mudstone (Figure 9 Type B). Thin bands of cementstone (muddy dolomitic limestone) occur at a number of levels, usually in the pale grey calcareous mudstones. Many of the individual rhythms can be correlated over distances of tens of kilometres. Superimposed on this rhythmic sequence are broader lithological changes, from more to less calcareous and from more to less kerogen-rich, which can be regarded as larger scale rhythms and which can be correlated throughout southern England. The Kimmeridge Clay

Younger strata

-c-

Ant~clinalaxis Boundary of gunnery range

3 kilometres

Figure 7 Sketch map of the solid geology of the Kimmeridge area (based on One Inch geological sheets 342 and 343)

z

which occur in varying combinations and amounts in each lithology. The main components are listed below. Clastic components i clay minerals - mostly illite and kaolinite with minor amounts of smectite and chlorite; generally 30 65% of whole rock ii crystalline quartz -angular silt and rare clay-grade particles; generally 15 - 23%; silt grade quartz is the major component of the thin siltstone horizons iii reworked biogenic and chemogenic materials shell and plant debris; phosphatic pebbles. Bwgenic components i calcareous macro and microfauna and flora predominantly ammonites, bivalves and foraminifera, with gastropods, brachiopods, serpulids, crinoids, ostracods and coccoliths important at some levels ii phosphatized fauna - vertebrate debris, mostly fish scales, vertebrae, faecal pellets ... 111 kerogen - palynomorphs (e.g. pollen, acritarchs, dinoflagellates), plant debris, diagenetically formed kerogen. Chcmogenic components i calcium and magnesium carbonate -diagenetically formed concretions ii phosphate - early diagenetic formation iii pyrite - early diagenetic formation. The hydrated sulphates gypsum and natrojarosite are common crystalline constituents of dried samples of Kimmeridge Clay havinglbrmed from the interaction of the oxidation products of pyrite with water and calcium carbonate.

can itself be regarded as a single rhythm when compared with other Upper Jurassic clays in that it changes with time from rhythms with silt to rhythms with oil shale, and then becomes almost uniformly very calcareous (Figures 2 and 3). The sandy and silty Kellaways Beds, the organic-rich Lower Oxford Clay and the calcareous Middle and Upper Oxford Clay are similarly related; so are the silty West Walton Beds, the dark grey lower Ampthill Clay and the calcareous upper Ampthill Clay, although in this latter example no oil shale is present. There are, of course, numerous differences of detail in the beds within these three large-scale rhythms, but their overall similarity suggests the widespread repetition of a particular sequence of events. The bulk chemistry of the commoner Kimmeridge Clay lithologies, as recorded by X-ray diffraction and calcimeter analyses, is summarised below. i dark grey mudstone: clay minerals 45 - 65%; quartz 10 - 30%; calcium carbonate 5 - 20% depending upon shell content; kerogen<2% ii medium grey mudstone: clay minerals 35 - 55%; quartz 10 - 15%; calcium carbonate 20 - 35%; kerogen< l % iii pale grey mudstone: clay minerals 25 - 45%; quartz 8 - 15%; calcium carbonate 25 - 55%; kerogen< l % iv cementstone: clay minerals 10 - 20%; quartz 2- 6%; calcium/magnesium carbonate 60 - 90%; kerogen< l % v bituminous mudstone: clay minerals 30 - 50%; quartz 10 - 20%; calcium carbonate 10 - 25%; kerogen 2 - 10% vi oil shale: clay minerals 20 - 40%; quartz 10 - 15%; calcium carbonate 10 - 25% kerogen 10 - 45% These lithologies are largely made up of a limited number of clastic, biogenic and chemogenic components 11

Axis of Kimmeridge Hobarrow Bay

7

-

I

Bese of Kimmeridge Clay a 1 8 4 m below 0.0

Clavell Tower

I

Baae of Kimmeridge Clay at ZlBm below 0.0,

St A I M S Head

-

I

Vertlcal Scale 120m

l0

Chapman's Pool

90 60

30

0.n

Minor faults and some stone bands omitted for clarity

0

05 Hor~zontalscale

Figure 8 Geological sketch sections of the Kimmeridge Clay exposed in the cliffs between Brandy Bay and Chapman's Pool

I km

TYPE B

TYPE A Siltstone as below Erosion surface

t

lnterburrowed junction

F

+4

_m

Pale and very pale grey mudstones. cohmonly with limestone doggers or septaria

8 B m

- : S 0

passing Into

0

g

o

Oil, shale, as below Junction commonly planar, interburrowed

Medium and dark grey mudstones, commonly becoming more shelly and fissile with deprh

m

E E

1

passing into

S

Medium and dark grey mudstones, commonly becoming more fissile and shelly with depth

m

-

-

_a

.-

8

Z

F

c

m

Pale and very pale grey mudstones, commonly wfth limestone dams or sepraria

2

passing into

passing into Fissile, shelly mudstone

Erosion surface

I

I

Siltstone and silty mudstone, gritty mth shell debris, phosphatrsation and/or phosphaiic pebbles at base lnterburrowed junction Pale and very pale grey mudstones

passing into Oil shale, fissile, shelly; phosphatic debris common Pale and very pale grey mudstones

Figure 9 Generalised Kimmeridge Clay rhythms

FAUNA INCLUDING AMMONITE DISTRIBUTION AND ZONATION The macrofauna of the Kimmeridge Clay is dominated by bivalves and ammonites. Gastropods, belemnites, brachiopods, echinoderms and serpulids, and fish, reptile, crustacean and plant debris occur at particular levels but, together with the bivalves, are too longranging to be of zonal use. The fauna1 and floral sequences of much of the Dorset succession have yet to be, collected and described systematically. Cope (1967; 1978) described the ammonites of the Upper Kimmeridge Clay and Ziegler (1962a) included Aulacostephanus from the Lower Kimmeridge Clay of Dorset in his monograph of that genus. Downie (1957) described the acritarch, dinoflagellate and coccolith assemblages from a range of samples from the Upper Kimmeridge Clay and Gitmez (1970) described the acritarchs and dinoflagellates from the b+i Zone of the Weymouth area sections. Lloyd (1959) made a systematic study of the arenaceous foraminifera of the Dorset sections, and Kilenyi (1969) described the ostracod succession. Spore material from some of these sections has been described by Norris (1969). Ammonite distribution and zonation Ammonites are common throughout the Kimmeridge Clay. They occur in assemblages of rapidly evolving forms which are sufficiently wellpreserved to provide an excellent basis for a zonal scheme (Table 1; Figures 2 and 3). In Dorset, the Kimmeridge Clay ammonite faunas are dominated by perisphinctids. The earliest Kimmeridgian forms belong to the genus Pictonia, successor to the Oxfordian genus Ringstcadia. Pictonia characterises the baylei Zone; other ammonites are rare at this level. The base of the overlying Ras~niacymodoce Zone is marked by the appearance of t-he genus~asenia;this is joined and then succeeded by the related genera Xenostephanus and Aulacostephanus. The last named genus provides the basis for the zonation of the remainder of the Lower Kimmeridgian. Other ammonites include the boreal cardioceratid genus Amoeboceras, which persists from the Oxfordian and occurs in the Rasenia a i d Aulacostcbhanur zones. In the latter zones, Amoeboceras (Nannocardioceras) is particularly common and is joined by the tethyan genera Aspidoceras (with its associated aptychus, Lacvaptyhus) and Sutneria.

The Upper Kimmeridgian is characterised by the progressive incoming of the perisphinctid genera Gravesia, Pectinatites, Pavlovia and Virgatopavlovia. Pictonia baylei Zone: The lowest zone of the Kimmeridgian is exposed at Wyke Regis, Black Head, Osmingtoh Mills and Ringstead Bay. The type section of the base of the baylei Zone is at Ringstead Bay (Figure 4, section 3); the base is taken at a minor erosion surface at the top of the Ringstead Coral Bed (Corallian Beds). The coral bed forms the highest part of a ferruginous oolitic clay that is patchily calcite-cemented to form an impersistent rubbly, ironshot, shelly, muddy limestone. Its distinctive fauna includes the corals Protos~riswattoni (Edwards & Haime), Thammhia arachnoides (Parkinson), T. concinna (Goldfuss) and Thccosmilia annulan3 (Fleming) (Arkell 1936), many fat serpulids, Deltoidturn delta (Wm Smith) and the bivalves Camptonectes, Chlamys, Entolium, Eopecten, Lima and Ctenostreon proboscideum U. Sowerby). At Wyke Regis, Black Head and Osmington Mills, the coral bed is absent but the ferruginous oolitic clay is present (Figure 6) and, in its highest part, is patchily cemented and contains few fossils other than Ctenostreon. The basal bed of the Kimmeridge Clay is a dark grey, intensely bioturbated clay, 0.38 to 0.40 m thick at Ringstead Bay, with wisps and burrowfills of silt, fine-grained sand and scattered limonite ooliths in its lower part. Phosphatic pebble beds occur at the base and 0.3 m above the base. Phosphatised serpulids, Pleurotomaria, Goniomya, Pholadomya, Pieuromya, Torquirhynchia inconstans U. Sowerby) and Pictonia occur in both pebble beds and throughout the intervening clay. These forms also occur as crushed shells, together with Chlamys (as clay casts), nests of Lopha, and Trigonia. The rhynchonellid brachiopod T. inconstans has long been used to define a marker horizon at this level (the Inconstans Bed of Arkell (1933; Plate XXI)). The bay& Zone was proposed by Salfeld (1913) who recorded Pictonia baylei and P. nonnandiana Tornquist from the lowest part of the Kimmeridge Clay at Ringstead Bay, with the ammonites Ringsteadia occurring below, and Rasenia above (Salfeld, 1914, p. 205). Arkell (1935; 1947) also recorded abundant Pictonia, including P. densicostata Salfeld, with 'Rhynhonella' inconstans in the clay immediately above the Ringstead Coral Bed. Morris (1968) suggested that only Pictonia baylei was present in

the Inconstans Bed, but that this included macroconch and microconch (Prorasenia) forms. The small oyster Nanogyra nuna U. Sowerby) is particularly abundant from 0.85 to 1.05 m above the base of the Kimmeridge Clay in the Wymouth area sections, in association with serpulids. Arkell (1933; 1947) named this the 'Exogyra nunu Bed' (here called the Nana Bed). The large flat oyster Deltoideum delta is common in the Inconstans Bed and occurs in the overlying few beds; at Ringstead Bay, it weathers out of the clays immediately above the Nana Bed in particularly large numbers. Rasenia cymodoce Zone: Since Ziegler ( 1962a) separated the 'fine-ribbed Raseniae' (compare with Arkell, 1947, p. 66) of the rnutabilis Zone into the genus Aulacostephanus, the range of the genus Rasenia has been taken to define the extent of the cymodoce Zone. The transitional nature of the change from ~aseniato Aulacostephanus has meant that the upper stratigraphical limit of Rasenia and the cymodoce-mutabilis zonal boundary have been somewhat arbitrary, as discussed on page 2 . To a lesser degree this has also been the case with the lower zonal boundary, which is taken at the change from Pictonia to Rasenia. The lower part of the cymodoce Zone is well exposed in the cliffs at Black Head and Osmington Mills, and is also seen at Wyke Regis in the weathered cliffs at the Fleet and Sandsfoot. Parts of the zone are exposed from time to time at Ringstead, in the foreshore and in a low, degraded cliff at the western end of the Ringstead Bay landslip. The base of the zone is marked at each of these localities by the Wyke Siltstone which forms a prominent hard band and line of seepage. The overlying Black Head Siltstone is equally prominent in most of the sections. At Wyke Regis, Black Head and Osmington Mills, the Wyke Siltstone consists of intensely burrowed, finely cross-laminated, muddy quartz silt, crowded in its lower part with bivalves, mostly myids in growth position. The overlying Black Head Siltstone is readily distinguishable from the Wyke Siltstone; it is shelly throughout with small oysters, Thracia and abundant Amoeboceras (Amoebites) in a calcite ghost preservation. Both siltstones contain common phosphatic pebbles in their lowest part and rest with an irregular, interburrowed contact on the underlying clays. The upper part of the cymodoce Zone is less well exposed a t these localities because it consists largely of pyrite-rich mudstones that weather rapidly. At Black Head and Os~]ningtonMills, a shelly mudstone rich in Rasenia and Xenostefihanus forms a useful marker bed at the to^ of the zone. In their study of the raseniid faunas of the cynwdoce Zone in Britain, Birkelund and others (1978) recognised four faunas (I to IV from below). In Dorset, they used the sections at the Fleet to demonstrate the RasetaM succession and there proved faunas I, I1 and IV, although these were not related to any measured section. Their oldest Rasenia assemblage with R. cf. cynwdoce (d'orbigny) and R. (Prorasenia) cf.tr@licata U. Sowerby) was recognised only at this locality where it occurs in the Wyke Siltstone; this bed therefore defines the base of the zone. In the area around Abbotsbury, Dorset, some 15 km west of Osmington Mills, there is an unusual sandy oolitic facies of the Lower Kimmeridgian (the Abbotsbury Iron Ore). Brookfield (1973) concluded that the ironstone was deposited in an offshore (barrier) bar environment. Arkell (1947, p. 87) recorded Rasetaia

cymodoce, R. uralenris (d'orbigny), R. involuta Salfeld, Rasenioides cf. thermarum (Oppel), Prorasenia bowerbanki Spath and P.? pseudowitteana (Salfeld) from the ironstone which has generally been assigned to the cymodoce Zone (for example, Morris, 1968). Aulacostephanus mutabilis Zone: Ziegler (1962b, p. 769) concluded that the distinction between Rasenia and Aulacostephanus was rather subjective and, by implication, that the boundary between the cymodoce and mutabilis zones which is defined by these genera was arbitrary. Lack of good exposures at this level in Dorset, togethkr with the transitional character of the fauna, has meant that the boundary between the cymodoce and mutabilis zones has been uncertain. Recent work in East Anglia (Gallois and Cox, 1976) has shown that the change from raseniid to aulacostephanid faunas occurs at a minor erosion surface; the present work has shown that the same erosion surface; characterised by interburrowing and an influx of silt, and aulacostephanid faunas, is also present in Dorset although less well developed than elsewhere. The mutabilis Zone, except for some of the middle part, is well exposed at Osmington Mills and Black Head. At the former locality, however, a complete sequence can be exposed by digging. The base of the zone has been taken at the base of a pale grey, slightly silty mudstone with widely spaced pumpkin-shaped cementstone doggers and with Aulacostephanus including A. eulepidus Schneid. The underlying shelly mudstones contain a rich ammonite fauna of R m i a (without smooth ventral band) and Xenostcphanus. The upper part of the mutabilis Zone at Black Head and Osmington Mills is composed largely of very calcareous mudstones in which a line of cementstone doggers forms a useful marker band. Above this, a bed crowded with small Astarte, the Astarte supracorallina Bed of Arkell (1947, p. 85), forms another useful marker band close to the top of the zone. Parts of the zone are poorly exposed at Wyke Regis and Ringstead Bay. Species of Aulacostephanus are common throughout the mutabilis Zone in Dorset (material figured by Ziegler (1962a)). The finely ribbed species eulepidus (Schneid), linealis (Quenstedt), peregrinus Ziegler and mutabilis U. Sowerby) occur throughout the zone: beautifully preserved iridescent specimens of the first named are es~eciallvcommon at certain levels at Black Head and 0iming;on Mills. Rare tiny Laevaptychus have also been recorded from the upper part of the zone at Black Head. Aulacostephanus eudoxus Zone: The basal beds of the eudoxus Zone are well exposed only at Black Head and Osmington Mills: some of the higher beds are exposed in the landslip area at Ringstead Bay and the whole of the zone above the Hobarrow Bay Stone Band (see Figure 2) is exposed between Brandy Bay and Kimmeridge Bay. At Black Head, the base of the zone has been taken at the base of a very fossiliferous tabular, soft, calcitecemented siltstone, 0.10 m in thickness, about 38 m above the base of the Kimmeridge Clay. This bed and the overlying one metre of slightly silty mudstone contain Aulacos&@hanurof the eudoxus (dYOrbigny)group. The base of the siltstone also marks the lower recorded limit of common Aspidoeeras, hvaptychus and Sutneria (see also p. 22). Rhynchonellid brachiopods, unusual elsewhere in the Kimmeridge Clay, are common together with small oysters, including Nanogyra, and rare wood fragments. This distinctive lithological and fauna1 assemblage can

be recognised in the cores of the Warlingham Borehole, Surrey (at 2923 ft [891 m]) (Worssam and Ivimey-Cook, 1971) and in the borehole sequences in East Anglia (Gallois and Cox, 1976), Wiltshire and Lincolnshire (Gallois, 1979b), and its base provides an excellent zonal boundary. Ziegler (1962a, pp. 10, 139) recognised a change in the Aulacostephanus faunas between the mutabilis and eudoxus zones in Dorset (material figured by Ziegler (1962a)), with the finely ribbed species listed above (p. 14) being replaced by the generally more coarsely ornamented assemblage of eudoxus eudoxus (d'orbigny), eudoxus eleg~ns Ziegler, pusillus pusillus Ziegler, mammatus Ziegler, undorae (Pavlow) , pseudomutabilis anglicus (Steuer) and volgensis (Vischniakoff). Although Ziegler's observations were not related to a,measured section, it is clear from the small range of overlap between Aulacostephanus of the eulepidus group and those of the eudoxus group that his intended position for the zonal boundary in Dorset was close to the siltstone described above. At Black Head, the lower part of the eudoxus Zone is exposed in a number of small slip faces separated by steep grassy slopes. The sections are richly fossiliferous; and well preserved, commonly iridescent specimens of Amoeboceras (Amoebites), Aspidoceras, Aulacostephanus and Sutneria are abundant at many levels. Here and in the landslip at Ringstead Bay, thick calcareous aptychal plates (Laevaptychus) weather out on the exposed surfaces of the slip faces. Arkell (1947, p. 84) referred to these levels as the Aptychus Shales. The lowest beds above the basal siltstone at Black Head contain numerous thin bands of oil shale, one of which is rich in pyritised specimens of the microcrinoid Saccocoma. Above this, a thick bed of calcareous mudstone, rich in serpulids and the oyster Nanogyra virgula Defrance and containing a band of septarian nodules with common Propectinatites?, forms a useful marker. At one level in the eudoxw Zone, these oysters are particularly common and characterised by-specimens of large size (about 30 mm long compared with the more usual 15 mm). A soft muddy limestone composed almost entirely of N. virgula (the Virgula ~im&tone)occurs at this level and is well exposed at Black Head. This bed and the adjacent Propectinatites?rich band form a distinctive sequence that can be traced throughout the Kimmeridge clay of southern England. M-ell preserved, commonly iridescent specimens of SYhVriCI tumela (d'orbigny), complete with lappets, can be collected at Black Head where they are mostly only partially crushed. This species is abundant in Dorset and has a well defined stratigraphical range between the basal siltstone of the eudoxus Zone and the Virgula Limestone. The upper part of the eudoxus Zone is poorly exposed at Black Head; but one of the highest beds visible, a nodular cementstone with common Amoeboceras (Nannocurdiocc~as)preserved in uncrushed, translucent calcite and referred to as the 'Amoeboceras cementstone' by Arkell (1949), occurs at Ringstead Bay and Hobarrow Bay and can be used to link the Weymouth area sections with those of the Kimmeridge area (Figure 5). The upper part of the eudoxus Zone is dominated by numerous bands of very shelly oil shale, most of which are crowded with crushed Amoeboceras (Nannocardioceras), that weather out as hard ribs of fissile shale with sulphur-coated surfaces. Isolated parts of the sequence are exposed in the landslip at Ringstead Bay but the full sequence, repeated several times by faulting, is exposed in the clif& and wave-eut platform between Brandy Bay

and Kimmeridge Bay. At this last locality, the top of the zone is marked by The Flats Stone Band, a thin tabular dolomite that can be readily distinguished from any other limestone in the sequence by the presence of numerous low-angle thrusts that form an intersecting pattern of sinuous ruckles in its upper surface. These thrusts do not persist into the adjacent mudstones and are believed to result from diagenetic chemical changes within the stone band (Bellamy, 1977), not from tectonism as thought previously. Below this stone band, Arkell (1947, p. 74) described abundant Aspidoceras, some of which showed aptychi within the aperture of the shell. Aulacostephanus autissiodormis Zone: The whole of the autissiodorens.is Zone is available for bed by bed collecting in a gently dipping section in the cliffs at Kimmeridge Bay. The sequence consists of a large number of small-scale rhythms (Type B of Figure 9) in which brownish grey oil shales or bituminous mudstones (forming hard ribs in the cliffs) are overlain by pale grey calcareous mudstones (weathering back as slacks with a characteristic cuboidal or 'dicey' fracture). The eudoxus-autissiodorensis zonal boundary has been taken at The Flats Stone Band (Ziegler, 1964, p. 347; Callomon and Cope, 1971, p. 158) of Kimmeridge Bay, although Cope (1969, p. A51) drew it 5 m above this (following Ziegler, 1962a, p. 12 and fig. 3). The boundary theoretically marks where the species assemblage listed above is replaced by the assemblage of autissiodormis (Cotteau), volgensis (Vischniakoff), undorae (Pavlow) and jasonoides (Pavlow) (Ziegler, 1962a, p. 139). Our observations indicate that A. autissiodorensis first appears about l m above The Flats Stone Band, while forms of the A. eudoxus group persist to about 2 m above this stone band. In the present state of knowledge, the top of The Flats Stone Band provides a useful boundary for the tudoxus-autissiodorensis zonal boundary, although systematic bed-by-bed collecting at 'this level could well refine this definition. Within the zone, the Washing Ledge and Maple Ledge stone bands provide useful lithological markers. It has been mistakenly implied that The Flats Stone Band marks the upper limit of the ranges of Aspidoceras, Laevaptychus and Nannocardioceras (Arkell, 1947, p. 73; Cope, 1969, p. A51; Ziegler, 1964, p. 347). Aspidoceras and Laevaptychus are common up to the Washing Ledge Stone Band and Nannocardioceras is abundant for at least 2.6 m above this stone band. The lower part of the autissiodorensis Zone is characterised by common Aspidoceras and Aulacostephanus and abundant Nannocardioceras; the upper part by abundant large, crushed Aulacostephanus autissiodorensis, large Propectinatites? (relatively common in the pale grey calcareous mudstones at KimmeridgeBay and Brandy Bay) and rare Gravesia. A thin bed rich in Sutneria rebholzi (Berckhemer) occurs a few metres above The Flats Stone Band and provides a useful marker close to the eudoxus-autissiodorensis zonal boundary throughout southern England. The full sequence of the autissiodorensis Zone is alsd exposed in Brandy Bay, in the western limb of the Kimmeridge Bay anticline, where the sequence can be closely matched with that m Kimmeridge Bay. At both localities, the best sections for fauna1 collecting occur in the wave-cut platform, but unfortunately, although large specimens of Aulacostephanus and Propectinatites? are readily visible most of the ammonites (probably including Gravesia) and bivalves appear to have been

L

removed by solution. The complete fauna can be revealed only by excavation. Parts of the autissiodormis Zone are poorly exposed at Black Head and in the eastern part of the landslip at Ringstead Bay. Pectinatites zones: Between Hen Cliff, at the eastern end of Kimmeridge Bay, and Freshwater Steps, the whole sequence of the Pectinatites zones is exposed in a continuous, gently dipping cliff section. The lithologies are similar to those of the autissiodormis Zone. be in^ rhythmic alternations of kerogen-rich and calcium carbonate-rich mudstones in which the stone bands and a number of other distinctive lithologies act as markers. The base of the oldest Pectinatites zone (elegans) was defined by Cope (1967, p. 68) at the earliest appearance of Pectinatites (Figure 10). At Kimmeridge Bay, pectinatitid ammonites have a considerable range of overlap with Aulacostephanus and Cope (1974, p. 29) subsequently noted that the identification of true Pectinatites depends on the presence of a horn on the peristome of the microconch, a feature not commonly preserved in Dorset. Following Cope's original (1967) definition, the base of the elegans Zone is taken at the base of a minor stone band (Bed 42 of Blake, 1875), but this position may hdve to be revised once the pectinatitids of the autissiodorensis Zone have been collected and described systematically. Aulacostephanus is abundant at Kimmeridge Bay up to 7 m below the base of Blake's Bed 42, where the genus suddenly disappears. The beds above this level, assigned by Cope to the top part of the autissiodorensis Zone, contain abundant Pectinatites s.1. which may include undescribed forms attributable to Propectinatites (Cope, 1968). Elsewhere in England, borehole cores at this stratigraphical level have shown that the extinction of Aulacostephanus is everywhere sudden and provides a more practical method of placing the junction of the Aulacostephanus and Pectinatites zones than the recognition of the incoming of true Pectinatites. Gravesia was first recorded in Dorset by Salfeld (1913, p. 425) who described flattened specimens from Kimmeridge. He recognised two species which he used as indices for two zones - G. gravesiana (d'orbigny) below and G. iriw (d'orbigny) above - represented by the clays between the Maple Ledge and the Yellow Ledge stone bands (Salfeld, 1913, p. 428). Arkell (1933, p. 450) also used these zones but subsequently (1947; 1956) restricted them to the clays between Blake's Bed 42 and the Yellow Ledge Stone Band. Later authors seem to have assumed the range of Gravesia in Dorset to be defined by Arkell's (1947; 1956) Gravesia zones. Cope (1967, p. 68) emphasised the rarity of the genus and advocated that it should be abandoned as a zonal index. He introduced zones based on Pectinatites for the beds above Blake's Bed 42, and used Ziegler's (1962a) zones, based on Aulacostephanus, for the beds below. Cope's (1967) records of Gravesia ranged from 8 ft [2.4 m] above Blake's Bed 42 to 6 ft [l .8 m] above the Yellow Ledge Stone Band (a total of about 70 ft [21.4 m]), but he concluded that Gravesia was so rare in Dorset that this might not be the full range. The apparently tacit abandonment of the Maple Ledge Stone Band as the lower limit of the range of the genus by Arkell (1947, p. 72) seems explicable. It appears from Salfeld's (19 13, p. 424) original description that his stratigraphically lowest specimens were found at beach level at the western end of Hen Cliff, just above Strahan's (1898, pl. X) 'supposed position of the Maple Ledge Stone Band', and that no specimen was found in

Kimmeridge Bay where its direct relationship to the stone band could have been determined. Arkell (1947, p. 76) later recorded Aulacostephanus from the shales at the western foot of Hen Clie he seems to have assumed that there would be no overlap in the vertical ranges of Aulacostephanus and Gravesia, and that Salfeld's specimens must therefore have come from a more easterly foreshore exposure, an exposure stratigraphically higher than Blake's Bed 42. More recently specimens have been found in Kimmeridge Bay between the Maple Ledge Stone Band and Blake's Bed 42. Ziegler (1962a, p. 13) recorded a specimen 8.15 m above the Maple Ledge Stone Band, and the present authors have found further specimens 5 m and 9 m above this band. An additional specimen has been recorded 3 m below the stone band in a borehole at Kimmeridge Bay. In our present state of knowledge, therefore, the lower limit of the range of Gravcsia in Dorset should be taken at 3 m below the Maple Ledge Stone Band. The highest recorded specimen is 1.8 m above the Yellow Ledge Stone Band (Cope, 1967), giving a total known range for the genus of about 50 m (see also p. 23). Simple descriptions of the Pectinatites zones have been published by Blake (1875) and Arkell (1933; 1947) in which the more prominent hard bands (Blake's Bed 42, the Yellow Ledge, Cattle Ledge, Grey Ledge, Rope Lake Head, Basalt, White, Middle White and Freshwater Steps stone bands and the Blackstone) are used as markers. All subsequent fauna1 and lithological descriptions have been related to them. In addition to these hard bands, groupings of distinctive strata are also mesent: notabiv thick-beds of verv calcareous mudstone in the w h a t l q m i s and hudlestoni zones (the latter including the Basalt Stone Band) that weather to form steep degraded slopes, and groups of oil-shale rich beds at the whatkymis-hudlestoni zonal boundary (this group includes the Blackstone) and at the hudlestoni-pectinatus zonal boundary; these form precipitous cliffs with numerous serrated overhanging ribs of oil shale. The whole of the Pectinatites zones is again exposed at Brandy Bay in the western limb of the Kimmeridge Bay anticline. Here dips increase westwards from 10" to 20" as the steep limb of the Purbeck Monocline is approached. The thickness of the beds up to the Cattle Ledge Stone Band at Brandy Bay is comparable to that exposed in the eastern limb of the anticline but above this, the Brandy Bay sequence is about 30% thinner than the comparable beds that crop out between Cuddle and Freshwater Steps. The reason for this is not clear; it might be due to a regional variation in the original depositional thickness but it seems more likely to have been caused by tectonic attenuation (mostly low angle shears) as the steep limb of the Purbeck Monocline is approached. Parts of the Pectinatites zones are exposed also at Black Head and more completely at Ringstead Bay. The most distinctive beds in the sequence, the Blackstone and the White Stone Band, can be identified in both sections but many of the remaining beds cannot be recognised without much excavation and detailed collecting; most of the stone bands that form such obvious markers near Kimmeridge are absent or weakly developed in the Weymouth area. Pavlovia and jittoni zones: From Freshwater Steps to Chapman's Pool, the Kimmeridge Clay exposed in the cliffs consists of uniform, very calcareous mudstones in which a few lines of limestone doggers and some thin,

Blake (1875) Bed Nos

KEY X X X X X

Kimmeridgian -Portlandian boundary of Salfeld X X X X X

Lower- Middle Kimmeridgian boundary of Arkell X X X X X

Lower-Upper Kimmeridgian boundary of Cope

metres

Maple Ledge Stone Band

Figure 10 Ammonite ranges and zonal schemes applied to the beds adjacent to the Lower-Upper Kimmeridgian boundary

weakly developed oil shales form markers. In the absence of resistant stone bands or oil shales, the beds form steep degraded slopes: in places, the cliffs have been overridden by slipped masses of the overlying topmost Kimmeridge Clay and Portland Beds, but a complete sequence is still present. The general lithologies have been described by Blake (1875) and the zonal sequence by Cope (1978). At Chapman's Pool, thin oil-shale seams are again present and form prominent hard ribs in the cliff and ledges in the wave-cut platform. Two lines of small calcareous concretions (the Rotunda Nodules) occur 3 to 6 m above the highest oil shale. These provide a useful

marker and contain uncrushed and ~artiallvcrushed pavlovid ammonites, including Paulouia rotunda U. Sowerby). The beds above consist of uniform very calcareous mudstones (the Lingula Shales) that pass up into silty and sandy calcareous mudstones with rhynchonellid brachiopods and small belemnites (the Rhynchonella Marls). These beds form a steep degraded slope in which accurate measbrement and collection is diflicult. The section has been summarised by Arkell (1933; 1947) and has been redescribed recently by Cope (1978). The upper limit of the Kimmeridge Clay is poorly exposed here but there appears to be a gradational transition from the-sandy clays of the top

part of the Kimmeridge Clay to clayey sands in the basal Portland Beds. The boundary was taken by Arkell (1947, pp. 90-91) at the base of a prominent sandstone, the Massive Bed, in Houns-tout cliff above Chapman's Pool. Following detailed mapping and sedimentological studies, Townson (1975) lowered the boundary to the base of the Rhynchonella Marls. Cope (1978) and Wimbledon and Cope (1978), however, used the Massive Bed, which they also took to mark the Kimmeridgian-Portlandian boundary.

CORRELATION OF OTHER ENGLISH SECTIONS INTROD UCTZON Although the Kimmeridge Clay has an extensive outcrop in England (Figure 11) the absence of exposure, other than in small, generally weathered sections in widely separated brick-pits, has meant that detailed correlation between th'e Dorset coastal sections and the inland area was not possible until samples became available from continuously cored boreholes. Recent work based on boreholes in East Anglia (the Wash area), has shown that the Kimmeridge Clay of the baylei to bectinatw zones can be divided into48 distinctive bids onzthe basis of faunal and lithological characters (Gallois and Cox, 1976; Cox and Gallois in Gallois, 1979a). Most of these beds, toeether with a number of " individual marker bands that provide additional correlations, have also been recognised in the borehole sequence at Warlingham, Surrey and, in the present work, in the Dorset coastal sections (Figures 12 and 13). At most stratigraphical levels, the lithological and faunal sequences are remarkablv constant over laree " areas and zonal boundaries can be identified with confidence even where they are not marked by a single distinctive lithological marker (for example, the eudoxus-autissiodorensis boundary, Figure 14). The youngest Kimmeridgian strata preserved in eastern England are of pectinat~sZone age and can be correlated with a level a little above the Freshwater Steps Stone Band in Dorset (Figure 13). The ammonite faunas of the Kimmeridge Clay recorded to date in southern and eastern England are c1oseIy similar to that described for Dorset earlier in this paper. The highest part of the Kimmeridge Clay with Virgatopuvlovia has yet to be described outside Dorset. The following notes on the ammonites and other macrofauna that have proved useful for correlation summarise current knowledge of the regional and stratigraphical distribution of these forms in other English Kimmeridgian sequences. PICTONZA In the original definition of his zone based on Picbnia, Salfeld ( 1913; 1914) recorded the genus from the lowest beds of the Kimmeridge Clay of Swindon, Westbury and Wootton Bassett (material figured by Spath (1935)) in Wiltshire, Brigg and Horncastle in Lincolnshire, and Filev Bav in North Yorkshire. since ;hen it has been recorded fmm strata ascribed to the basal beds of the Kimmeridge Clay at a number of other localities. However, the difficulty of distinguishing Pictonia from large raseniids and aulacostephanids with smooth body chambers has led to a number of rnisidentifications. The absence of any good permanent section at this stratigraphical level outside Dorset and the poorly preserved nature of much of the material

obtained from temporary sections and borehole cores has added to the difficulties. True Pictonia have been recorded in boreholes in Norfolk (Pringle, 1923; Gallois and Cox, 1976) and in an exposure at South Ferriby, Humberside (Smart and Wood, 1976). RASENIA The type locality of the genus Rasenia, created by Salfeld (1913), is at Market Rasen, Lincolnshire, where small and now defunct clay pits yielded beautifully preserved specimens (material figured by Spath ( 1935), and Birkelund and others (1978)). The stratigraphical relationship between this fauna, those of Dorset and that from Ethie, Cromarty, Scotland (Ziegler, 196213) has been uncertain but recent work has helped to clarify the position and has established a four-fold sequence of Rusenia faunas for Britain (Birkelund and others, 1978); the genus is, however, still in need of systematic revision. Elsewhere in England, raseniid faunas have been recorded fmm exposures at Westbury, Wiltshire (Birkelund and others, 1978), Littleport, Cambridgeshire (Casey, 1960) and South Ferriby (Smart and Wood, 1976). Forms previously identified as Pictonia? from Bed 5 of the Lower Kimmeridge Clay of East Anglia (Gallois and Cox, 1976, p. 23) and the Littleport specimens which Casey (1960) compared with the genus Pachypictonia, are now thought to belong to the oIdest Rasenia assemblage. It is therefore recommended that the base of the cymodoce Zone in that sequence should be lowered from the base of Bed 8 (equivalent to the Black Head Siltstone) to the base of Bed 5 (equivalent to the Wyke Siltstone) (see p. 4). AULACOSTEPHANUS and XENOSTEPHANUS In the Warlingham Borehole (Callomon and Cope, 1971) and in boreholes in East Anglia (Gallois and Cox, 1976), Aulacostephanus sequences similar to those described by Ziegler (1962a) and noted in Dorset (pp. 14-16) have been recorded. Ziegler's ( 1962a) monograph included a brief description of a coarsely ribbed evolute subgenus Xetwstephanus from the Chalky Boulder Clay of Lincolnshire. This was described more fully and the taxa Xcnostephanus (macroconch) and Xmstephunoides (microconch) were formally designated by Arkell and Callomon (1963). Ziegler (1962a, p. 136) gave the range of Xmstcphnnus as upper cymodoce and lower mutabilis zones with occurrences in drift deposits in Lincolnshire and in situ occurrences in Scotland [Culgower, Sutherland according to Kent and Casey (1963, p. 61)], Greenland and northern Russia. He assigned the Lincolnshire specimens, which he figured, to the cymodoce Zone. Arkell and Callomon ( 1963, p. 241) concluded, on the basis of the accompanying forms, that Xenostephanus belonged to the mutabills Zone with the possibility that it might define a separate subdivision or subzone. At about the same time, Kent and Casey (1963) recorded Xcnostcpha~in situ, in a sandstone (which they named the Elsham Sandstone) in north Lincolnshire [now Humberside). There it occurred in association with Aulacostephanoih of the mutabilis group and they concluded a position near the base of the mutabilis Zone. They also recorded Xcnostephanur in a sandy horizon in the No.2 Shaft of the old Dover CoIliery in Kent, associated with Aulncostephanoides cf. attenuutur Ziegler, confirming a position in the mutabills Zone. Xenestep&anus thus came to be recognised as indicating the lower part of the mrrtabiIis Zone (Callomon and Cope,

Market Rasen

Kimmeridge Clay (

Kimmeridge Clay subcrop

rset type area (see Fig.1)

-

O

100 km

P

Figure 11 Sketch map of the Kimmeridge Clay outcrop and subcrop in England

1971, p. 158). It has now been found in situ in the normal clay facies of the Kimmeridge Clay in Dorset and in boreholes in East Anglia (Gallois and Cox, 1976) where it has been shown to be at its most abundant within the upper part of the cymodoce Zone. It is accompanied by fine-ribbed Rasenia (Rasenioides) without a ventral smooth band, comparable with forms figured by Arkell and Callomon (1963, pl. 32, figures 13-21) and lies below the main range of Aulacostephanus of the mutabilis group. Since the main range of Xenostephanus is stratigraphically distinct from that of Aulacostephanur, it is proposed here that Xenostephanus should be used at generic level. Casey (in Kent and Casey, 1963, p. 61) has already implied that it might be misplaced as a subgenus of Aulacoslephanus and likened it to the genus Zlovaiskiocerar Sasonov. AMOEBOCERAS The occurrence of Amoeboceras in the upper part of the Lower Kimmeridgian, particularly the tiny forms of the subgenus Nannocardioceras, has been discussed with reference to the Warlingham Borehole by Callomon and Cope (197 1, pp. 158-1 59). At Warlingham, Nannocardioceras is abundant in two groups of oil shales, the higher group corresponding to the upper limit of Aspidoeeras and Laevaptychus and containing rare Sutneria rebholzi (Berc-

khemer) (Figure 14). Callomon and Cope's (1971, p. 158) use of the term Nannocardioceras Beds is open to misinterpretation because of the incorrect implied range of these forms in Dorset (see p. 15 for discussion). The highest recorded Nannocardioceras, both in Dorset and East Anglia, is a distinctive form with fine rectiradiate ribbing assigned to A. (N.) aff. anglicum (Salfeld). Apart from Nannocardioceras, most Kimmeridgian Amoeboceras are assigned to the subgenus Amoebites. Salfeld (1916) and Spath (1935) figured material from Market Rasen, Filey Bay and localities in Scotland. In Dorset, the Warlingham Borehole and East Anglia, AmabitGs occurs sporadically throughout the Lower Kimmeridge Clay, but is particularly common in the oil shales of the nrdoxus and autissiodorensis zones (in association with Nannocardwccras) and in the Black Head Sitstone of the gwdotc Zone.

ASPZDOCERAS The occurrence of Aspidoceras in the Kimmeridgian has been discussed by Callomon and Cope (197 1, p. 159). In the Warlingham Borehole, they recorded the genus in the upper part of the mutabilis Zone, the eudoxus Zone and the lower part of the autisswdorensis Zone. In East Anglia,

DORSET COAST or Kimmeridge Bay) --W---

WARL INGHAM BOREHOLE

THE WASH AREA ----W-

autissiodorensis

Nannocardioceras Cementstone Hobarrow Bay S.B.

/

KEY Fossil marker bands

.

:.:.:.:.:.:.; :;.;.:.

A A A A '

J

1-1-1-1-

I I !-'-l-

mutabilis

t

I-T-T-7

Sutneria rebholzi

NNNN

Nannocardioceras

P P P P

Propectinatites.7

S S S S

Saccocoma

A A A A

Astarte supracorallina

Pn h Pn

Pentacrinus

X X X X

Xenostephanus

Lithologies as Fig.2

F-T-T-T ,I-I-I,T

Sr Sr Sr

NO horizontal scale I

0 metres

I

I 1-

1-'-'-C

K I- L 1.

20

4

( XXXX!

/

cymodoce

/

....,.............& . ...,.. .. .. / ,,

,

/

ba ylei

/

Corallian Beds (Ringstead W a x y Clay)

1

Figure 14 Correlation of the Lower Kimmeridge Clay sequence of the Dorset coast with those of the Warlingham Borehole and the Wash area

DORSET COAST

KEY 0 metres

20

-

Unconformity

jE -

Fine - grained sand

Lithologies as Fig. 2 No horizontal scale

Figure 13 Correlation of the Upper Kimrneridge Clay sequence of the Dorset coast with those of the Warlingham Borehole and the Wash area 21

KlMMERlDGE BAY and HOBARROW BAY I.G.S. WARLINGHAM BOREHOLE (after Callmon and Cope, 1971)

Washing L ~ d p Stone Band

-

P

e

Aulacostephanus autissiodorensis

Zpne

C.

Ths Flats Stone Band Nannocardioceras Beds of $ Callornon & Cope

?

v

0

-

8

0

8 5

Aulacostephanus eudoxus Zone

Nannocardioceras Beds of Callornon & Cope

Nannocardioceras Cementstone Hobarrow Bay Stone Band

feet 0

10

Scale 0 metres

3 5

K@v

Oil shale Stone band undifferentiatsd

Figure 14 Ammonite ranges and lithologies adjacent to the cudoxur-autissiodorensis zonal boundary at Kimmeridge Bay and in the Warlingham Borehole

Aspidoceras first appears at the base of the eudoxus Zone, although rare tiny Laevaptychus (presumably derived from Aspidoceras) occur in the upper part of the mutabilis Zone. Similar occurrences have also been noted in Dorset (p. 14) and together these suggest that Aspidoceras has a greater stratigraphical range than yet recorded. The upper limit of the genus in East Anglia is similar to that recorded at Warlingham (Figure 14). SUTNERIA The ammonite Sutnesia has been recorded in considerable numbers in the eudoxus Zone of the Warlingham Borehole (Callomon and Cope, 1971), in the Ashdown No.1 Borehole, Sussex, where it dominates the fauna at certain levels of the eudoxus Zone (Callomon in Bristow and Bazley, 1972, p. 19), in Yorkshire (Callomon and Cope, 1971, p. 161) and in East Anglia (Gallois and Cox, 1976). A manuscript record of Sutneria in the Benniworth Borehole, Lincolnshire, made by L.F. Spath in 1940 is now confirmed. Three species have been recognised: S. eumela (dYOrbigny)and S. cyclodorsata (Moesch), with rarer S. rebholzi (Berckhemer). In the Warlingham Borehole this last named species occurs at a higher stratigraphical level within the lower autissioderensis Zone. [Author's note. This occurrence has been misplotted in Callomon and Cope (1971, PI. VI) range chart.] With the occurrences now recorded in Dorset (p. 15), it is apparent that Sutneria is an important constituent of the Lower Kimmeridgian ammonite fauna in England and together with Amoeboceras, Aspidoceras and Aulacostephanus forms part of the diagnostic ammonite assemblage for the eudoxus Zone and the basal part of the autissiodormis Zone.

P E C T I N AT I T E S Species of Pectinatites have been used by Cope (1967) to define the elegans, scitulus, wheatlgensis, hudlestoni and pectinatus zones of the Upper Kimmeridgian. He placed forms which had previously been attributed to the genera Allovirgatites, Keratinites, Lithacoceras, Pectinatites, Pectiniformites, Pseudovirgatites, Subplanites, Virgatites, Virgatosphinctoia'es and Wheatleyites within the genus. With the exception of very rare Gravesia in Dorset (see p. 16) and at Warlingham, Pectinatites is the only ammonite genus recorded to date from the elegans to pectinatus zones in England. Identification of the various subgenera and species of Pectinatites is largely based on the ribbing, particularly on the number of ribs per whorl. Well preserved, adult specimens are necessary for species identification; such material is rare in borehole cores even though pectinatitids are common throughout much of the sequence. The boundaries of the various zones of the Upper Kimmeridgian in the Warlingham Borehole and in East Anglia have therefore been placed on the basis of lithological features which can be matched with the Dorset coastal succession (Gallois and Cox, 1974). The ammonites have been used to provide only the broad framework. The stratigraphical ranges of the three subgenera of Pectinatites (Arkellites, Pectinatites and Virgatosphinctoides) are shown in Figure 3. Correlation of the Wash area numbered bed sequence (Cox and Gallois in Gallois, 1979a) with the Dorset succession suggests that the position of the scituluswheatleymFis zonal boundary as currently understood in both areas (Cope, 1967; 1974) should be adjusted. Jn East Anglia, the base of the scitulus Zone is currently taken at the base of Bed 37. There, scitulus Zone ammonites occur in Bed 37, but there is no ammonite evidence between these and the first wheatleycnsis Zone ammonites in Bed 40. In Dorset, scitulus Zone ammonites

occur between the Yellow and Cattle Ledge stone bands but no determinable ammonite has been recorded between the latter and the Grey Ledge Stone Band, above which wheatleyensis Zone ammonites occur. The scitulus-wheatleyensis Zonal boundary has therefore been placed at the base of Bed 40, at an upward lithological change from mixed oil shales, brownish grey and pale grey mudstones, into predominantly pale grey mudstone. In Dorset, the revised position of the zonal boundary is 8.1 m below the base of the Grey Ledge Stone Band. At various levels in the eudoxus and autissiodorensis zones, there occurs a poorly known group of perisphinctid ammonites that have in the past been assigned to the genera Subdichotomoceras, Sphinctoceras and 'Perisphinctes'. Cope (1968) defined a new genus Propectinatites from amongst such forms in the upper part of the autissiodorensis Zone of Brandy Bay. The new genus was characterised by the presence of short lappets on the aperture of the microconch, in contrast to Pectinatites which bears a horn. It has not been possible to distinguish apertural features in most of the crushed and fragmentary specimens that occur in Dorset and in the present account all the Lower Kimmeridgian forms have therefore been tentatively assigned to Propectinatites?. Such forms are common in the autissiodorensis Zone at Brandy Bay and Kimmeridge Bay; at one level in the eudoxus Zone a band rich in Propectinatites? forms a widespread marker throughout southern England. GRA VESIA Gravesia has been recorded previously in Britain from the Kimmeridge Clay of Dorset, Wiltshire, Surrey, Norfolk, Lincolnshire and Sutherland. Re-examination of the available specimens has confirmed only the Dorset and Surrey occurrences. The specimens from Lincolnshire (Donington on Bain (1918) Borehole; Pringle, 1919, p. 51), Norfolk (West Bilney Borehole; manuscript records by Pringle), Wiltshire (Swindon; Chatwin and Pringle, 1922, p. 165) and Sutherland (Navidale; Arkell, 1933, p.474) are not confirmed. The specimens from the West Bilney Borehole, Norfolk (Pringle in Strahan, 1920, p. 37) could not be traced. Salfeld's (1913, p. 425; in the Gottinghen Institute jide Arkell (1947, p. 76)) and Ziegler's (1962a, p. 12) specimens from Dorset were not examined. The specimens collected by Spath in Dorset (Arkell, 1933, p. 450) are fragments of very large crushed perisphinctids, distinct from Pectinatites and similar to those figured by Cope (1967, pl. 1); they are probably referable to Gravesia. Cope (in Callomon and Cope, 1971, p. 152) recorded three specimens of Gravesia from the Warlingham Borehole, Surrey. Additional specimens have been noted by the present authors in recent IGS boreholes a t Donington on Bain, Lincolnshire [TF 2399 81881 and Portesham, Dorset [SY 6214 85541. Salfeld (1913) recorded the species G. gravesiana (d'orbigny) and G. irius (d'orbigny) but since then (for example, Arkell, 1933), specimens have been assigned to gravesiana and gigas (Zieten). Arkell (1956) thought that G. gravesiana occurred below G. gigas; Cope (1967), however, thought that gravesiana occurred above gigas. In our present state of knowledge, it seems advisable to assume that the three species of Gravesia recorded to date in Britain are approximately contemporaneous. The specimens found by the present authors, from Kimmeridge Bay (referable to G. cf. gravesiana), Portesham and Donington on Bain, occur at stratigraphically similar levels to one another. (See pp. 24-25 for discussion of Gravesia as a zonal index aqd its importance for correlation with other European sequences.)

_

PA V'O VIA and VZRGA TOPA VLO VIA In addition to occurrences in Dorset, Pavlovia has been described from the Swindon Clay of Swindon, Wiltshire (Chatwin and Pringle, 1922; Neaverson, 1924; Kitchin, 1926) and the Hartwell Clay of Aylesbury, Buckinghamshire (Hudleston, 1880; Neaverson, 1925; Spath, 1936) where small brickyards worked the clays at the top of an otherwise sandy Upper Kimmeridgian sequence. Farther north, these strata are absent and Pavlovia faunas occur only as derived phosphatised fragments at the base of younger Jurassic (~ortlanhian)or Cretaceous deposits. The Hartwell Clay at Aylesbury is the stratotype for the pallasioides Zone, one of two zones based on Pavlovia. The relationship of the pallasioides fauna to the fauna of the Rotgnda Nodules (rotunda Zone) in Dorset, was for a long time in doubt. Casey (1967) showed that the pallasioides fauna occurred in the shales underlying the Rotunda Nodules, and that the two zones had previously been placed in the wrong stratigraphical order. The stratigraphy at this level has been reviewed by Cope (1978) who described and figured the pavlovid faunas and separated the youngest (which only occurs in Dorset) in a new genus Virgatopavlovia and a new zone jittoni.

0THER MACROFA UNA In addition to the ammonites, the macrofauna of the English Kimmeridge Clay includes bivalves, gastropods, belemnites, brachiopods, echinoderms, and serpulids with fish, reptiles, crustaceans and plant debris. Most of these belong to long-ranging genera or species but may be used to define stratigraphical marker horizons where they make a sudden appearance or where they occur in great abundance over a small stratigraphical range. The marker horizons defined by the bivalves Nanogyra nana, N. virgula, and Astarte supracorallina are shown in Figure 2. Some of the lowest heds of the Kimmeridge Clay, notably the Inconstans Bed and Wyke Siltstone, yield a rich bivalve infauna preserved, in growth position, in uncrushed cream-coloured phosphate. Myids, including Goniomy, Phladomy and Pleuromya, and trigoniids are particularly common. I t has already been noted (p. 3 ) that amongst the most easily recognisable widespread marker horizons in the Lower Kimmeridge Clay of southern England are the silty beds and minor erosion surfaces which define the bases of the cymodoce, mutabilis and eudoxus zones. As well as coinciding with changes in the ammonite assemblages, these silty beds commonly contain rhynchonellid brachiopods, thick-shelled oysters including stmnglyribbed Lopha and echinoid spines and plates, all ofwhich are rare elsewhere in the Lower Kimmeridge Clay. The influxes of silt are also associated with a few wood fragments. Echinoderm occurrences which are stratigraphically useful in the Kimmeridge Clay in England include four levels at which brassy pyritic plates of the pelagic crinoid Saccocoma are preserved in oil shale. Saccocoma was first recorded in the Kimmeridge Clay by Bather (1911) in the Penshurst Borehole in Kent. Kitchin (1919, p.43) subsequently recorded the genus at the level of the Blackstone (hudlestoni Zone). He noted that the occurrence over a limited stratigraphical range was widespread in southern England and proposed a Saccacoma subzone. Casey (1958) later recorded a Saccocoma-rich band in the autissiodorensis Zone and the present authors have recorded two additional levels in the eudoxus Zone a

(Gallois and Cox, 1976). Each of the four levels of occurrence has been shown to be widespread in England, although in the Dorset coast sections only the hudlestoni Zone band and one of the eudoxus Zone bands have yet been recorded, due probably to the weathered nature of the sections. The persistent occurrence, at one level, of columnals of the crinoid Pentacrinus in pale calcareous mudstones in the middle part of the mutabilis Zone, has also proved to be a useful widespread echinoderm marker horizon elsewhere in southern England. This has been named the Pentacrinus Band (Gallois and CO?, 1976).

CORRELATION WITH OTHER EUROPEAN SEQUENCES CORRELATION BASED ON AMMONITES, NOTABLY GRA VESIA At the present time, the most important genera for establishing correlations between Dorset and the other faunal provinces recognised by Ziegler (1964) in the Lower Kimmeridgian of Europe are Aspidoceras, Aulacostephanus, Rasenia and Sutneria. The genera Amoeboceras and 'Perisphinctes' may become important in the future when more svstematic work has been done on them. Correlation between the Dorset and south-western German sequences is shown in Table 2. Geyer (1969) pointed out the usefulness of Sutneria for correlation and noted its distribution in south-western Germany (14 known species), the Aargau (Switzerland), the Jura mountains, the ArdL.che, the Paris Basin, Dorset, southern Spain, the d northern alpine region, Roumania, ~ u ~ o s l a v i a - a nthe Russian Platform. Callomon (in Callomon and Cope, 1971, p. 161) showed that the three species of Sutneria recordkd from the Warlingham ~orehoie,Surrey occupy stratigraphical levels similar to those in Germany (White Jura y, 6, c, ). These species have now been recorded from the Dorset sections, where they occur at stratigraphically similar levels to those of the Warlingham Borehole (see p.22). In the Upper Kimmeridgian, Gravesia provides an important link between the Dorset and other European successions. M.A. Ziegler ( 1960, p.676) considered Gravesia to be an 'epicontinental-subboreal' genus with a limited geographical distribution centred on the borders of the Paris Basin and northern. Germany. Gravesia occurs rather commonly in the eastern parts of the Paris Basin (Arkell, 1956, p.57) and from there, de Loriol, in his monographs of t6e Upper Jurassic faunas of Yonne (de Loriol and Cotteau, 1868) and Haute Marne (de Loriol and others, 1872), described and figured Ammonites gig?, A. gravesiana and A irius from the Ammonites gigas Zone. In the former work, he described gigas as not rare and Ammonites [Aulacostephanus] autissiodorensis as very rare; of the five ammonite species described in this monograph, three belong ti Gravesia. In northern Germany, the species G. gigas has given rise to the term Gigas-Schichten (for example, Salfeld, 1914), which Arkell ( 1956, p. 138) described as yielding 'various species of the tumid involute, giant ammonites of the genus Gravesia', some of which were described and figured by Seebach (1864) and Struckmann (1887). The genus is not uncommon in southern Germany (Swabia and Franconia) (Zeiss, 1964) and the French Jura (Enay, 1966) and isolated examples have been recorded in the Balearic Islands and possibly as far south

as Madagascar (Enay, 1964, pp.369-370). Hahn (1963) noted that in southern Germany, Gravesia occurred only as an accessory faunal element. In his monograph of the genus from the Upper Jurassic of central and northwestern Europe, he examined 160 specimens which included G. gigas (68 specimens), G. gravesiana (43), G. irius (12), G. hypselostoma Hahn (9) and G. polypleura Hahn (6). All five species were recorded from northeastern France and from either north-western or southern Germany. All except G. gravesiana and G. irius were recorded from the Boulonnais and possible examples of the first three species were recorded from Dorset. Most of Hahn's figured specimens came from north-eastern France or north-western Germany. The significance of the role of Gravesia in the Russian faunas is difficult to assess. None of the works previously mentioned (Arkell, 1956; Enay, 1964; Hahn, 1963; Ziegler, 1960) has noted its occurrence. ~ m o n ~ the st more recent Russian literature (Sasonov, 1961, 1964; Mik:lailov, 1964, 1966; Gerasimov and others, 1974), the genus has been recorded from the Russian Platform area in the lowest beds of the Volgian. Only the species gigas and gravesiana have been named (Mikhailov, 1964, 1966) and it seems likely that the genus is not common. In some other areks of the USSR where the stratigraphy and palaeontology of Volgian sequences have been recently described in detail'for the first time, further records of Gravesia have been noted, for example, the species polypleura in the Subarctic Urals (Zakharov and Mesezhnikov, 1974). Grauesia thus provides a link between the various faunal provinces which developed in Europe towards the end of the Jurassic. Within these provinces, local stage names - Tithonian in the Tethyan region, Volgian in the Russian Platform area and Kimmeridgian/Portlandian in the Anglo-Paris Basin - have been used. Correlations between these areas have not been readily demonstrated. In the Tithonian, Gravesia characterises the lowest zone(s) of the stage - zones of Glochiceras lithographicum and Lithacoceras ulmense (Enay, 1964); Zone of Neochetoceras steraspis (Ziegler, 1961, 1962a, 1974; Hahn, 1963); Zone of Glochiceras lithgraphicum (or Hybonoticeras hybonotum) (Enay, 1971). In the Volgian, Gravesia occurs in the lowest zone of the stage, that of Subplanites klimovi (Mesezhnikov and others, 1977). In north-western Europe (Britain, the Boulonnais and parts of the Paris Basin), the range of Gravesia lies within the zones of Aulacostephnus autissiodorensis, Pectinatites (Virgatosphinctoi&s) e1egan.s and P. (V.) scitulus. Salfeld (1913, 1914) first used species of the genus Gravesia as indices for zones which he recognised throughout north-western Germany, northern France and England. Zones based on Gravesia remained in use for this part of the Kimmeridgian in England until Cope (1967) proposed a new zonal scheme based on the genus Pectinatites. Enay ( 1971) suggested that Cope's scheme could be applied to the Boulonnais and, to a lesser degree, the Paris Basin. For southern Germany, Salfeld (1914) used a zone of Oppelia steraspis and Gravesia zieteni in the Malm but this seems to be the only reference to the use of Gravesia as a zonal index in the southern European (Tithonian) province. Grauesia is not used as a zonal index in the north-eastern European (Volgian) province although Sasonov (1961) used G. gravesiana as index and Gerasimov and others (1974) included 'Graowicr sp.' with Subplanites klimoui as indices of the lowest ammonite zone of the Volgian.

Table 2 Correlation between the Lower Kimmeridgian of south-west Germany and England (after Geyer (1961; 1969) and Ziegler (1974). WeiDjura

SOUTH-WEST GERMANY Zone

5

Neochetoceras steraspis

E:,

Hybonoticeras beckeri

Subzone

ENGLAND Zone Pectinatites elegans

Virgataxioceras setatum

UPPER KIMMERIDGIAN

Aulacostephanus autissiodorensis l?

:

Sutneria subeumla E1

m

Sutneria pedinopleura

64 Aulacostephanus eudoxur

Substage

W

Aulacostephanus eudoxus

E

C1

X

53

K Aspidoceras acanthicum

Aulacoste~hanusmutabilis

YI

Ataxioceras hypselocylum Sutneria platynota

P2

Zdo~rasplanula

W C1

61 Y3 Katroliceras divisum ~2

m

Rasenia cymodoce

U 0 M

9 Z

Pictonia baylei Sutneria galar

Amoeboceras rosenkrant~i

UPPER

OXFOR-

PI

Consideration should be given to the re-introduction of a Zone of Gravesia s p p into the standard zonal sequence of the Kimmeridgian because of its potential importance as a link with other European sequences. he position of the base of the original G. gravesiah Zone of Salfeld (1913), which was taken by Arkell (1933, p.450) at the Maple Ledge Stone Band, was close to the present known lower limit of Gravesia in Dorset. This level also approximately marks the lower limit of the range of overlap of abundant aulacostephanid and pectinatitid ammonites. A suitable range for a new Gravesia spp. Zone in Dorset would be from 3 m below the Maple Ledge Stone Band to the base of Blake's Bed 42 (the base of the elegans Zone). More detailed collecting would be necessary before the lower boundary of the new zone could be fixed. Elsewhere in southern and eastern England, Gravesia appears to be restricted to Bed 35 of Gallois and Cox (1976). Although the introduction of a new zone would considerably reduce the thickness of the autissiodorensis Zone, the occurrence of common large Aulacostephanus autissiodorensis would still be restricted to that zone. If established, the base of the Gravesia spp. Zone described above would define the base of the Upper Kimmeridgian Substage and could provide a directlkk at this level with Tithonian and Volgian sequences. T H E KZMMERZDGZAN S T A G E

D'Orbigny (1842-185 1, p. 610) introduced the term 'l'itage kimmtridgien' to include the Kimmeridge Clay of the Kimmeridge area and a number of French formations that he believed to be of similar age. He described its characteristic fauna as Ammonites [Aspih a s ] lallieri, Ostrea deltoidea [Deltoideum delta] and Ostrea [Nunogyra] virgula. In the overlying stage, '1'Ctage portlandien', he included the Portland Sand and Portland Stone of Fitton (1836) and several French and German formations of presumed equivalent age. The

DIAN

characteristic fauna of the Portlandian was said to be Ammonites [Titanites] giganteus, Ammonites [Gravesia] irius and Trigonia [Lacvitrigonia] gibbosa. It was not until the time of Salfeld (1913) that Grauesia was recorded in

England and it was then realised that dYOrbignyhad wrongly correlated Gravesia from Kimmeridgian limestones in France with ammonites from the Portland Stone of England (Arkell, 1946, pp. 5-6). It is clear that dYOrbigny'sintention was that the Kimmeridgian and Portlandian stages should be synonymous respectively with the Kimmeridge Clay and Portland Beds of Dorset. Unfortunately the mistake in the original definition led to a misunderstanding between continental and British geologists, with the result that the terms Kimmeridgian and Portlandian have each come to be used in two ways - s m anglico and sensu gallico. A summary of the stage names that have been applied to the Kimmeridge Clay of Dorset is given in Table 3. The first definition of the Kimmeridgian and Portlandian stages in terms of ammonite zones was that of Salfeld (19 13) based on sections in England and France. He defined the base of the Portlandian Stage at the base of his G. gravesiana Zone. Salfeld's zonal scheme was subsequently modified with some authors following his stage definition ( s m gallico), and others reverting to d'orbigny's original definition (sensu anglico) (Table 3). The problem of these two interpretations has been discussed extensively but not yet resolved (Arkell, 1946; Cope and others, 1964; Enay, 1964). Following the international colloquium on the Jurassic System held in Luxembourg in 1962, the British Mesozoic Committee recommended that the terms Kimmeridgian and Portlandian should be used sensu gallico (Ager, 1963, p. 1046) but the recent Geological Society report on correlations in the British Jurassic (Cope and others, in press) use Kimmeridgian and Portlandian sensu anglico.

Table 3 Stage names applied to the Kimmeridge Clay of Dorset d'orbigny 1842-51 (alternative interpretations)

Dorset Formations1

Portland Sand

t.

Upper Kimmeridge Clay

Blake, 1881

Pavlow, 1896

Portlandian (pan)

Portlandian or Bononian (pars)

Bolonian

.g C

,

Salfeld, 1913

Arkell, 1956

Ager, 1963

Cope, 1967; Wimbledon & Cope, 1978

Casey, 1971

Portlandian (pars)

Portlandian (pars)

Portlandian (pan)

Portlandian (pars)

Middle Volgian (pars)

Upper Kimmeridgian

v

9%.e

Upper Kimmeridgian

Modern zonal scheme (as used in thils paper)

Portlandian (pars) Virgatopaulovta fittonr Pavlovia rotunda

2%

--Lower Volgian (base not defined)

----

-

m

Middle Kimmeridgian

P,

E

Pavlovta pallasrotdes Pectinatites pectinatus Pectrnatites hudlestonr Pect. whcatleycnsis

m

t:

&

Pcchnatiks scitulus

n

e

Pectinatites clegans

.G

-

----

4

\U

E

Lower Kimmeridge Clay

Kimmeridgian

B

----

Lower

Kimmeridgian Kimmeridgian

Kimmeridgian

Lower

Kimmeridgian

'

Kimmeridgian (base not defined)

Aulacostcphanus autissiodorensis

Aulacostcphanus eudoxus m

Aulacostcphanus mutabilis

.&

2

\U

Rascnia cymodocc

E E

2

Pictonia baylei

Corallian Beds

l

Corallian

As in Fitton (1836) S m anglico S m gallico

Corallian

Corallian

Oxfordian (pars)

Oxfordian (pars)

Oxfordian (pars)

Oxfordian (pars)

Pigure 15 Palaeogeography in Lower Kimmeridgian times showing the positions of outcrops referred to in

tbr t a t

The present authors are strongly in favour of defining the upper limit of the Kimmeridgian Stage at or near the

ap of the Kimmeridge Clay of Dorset because they bdicve this to be the most stratigraphically useful ddinition. The cliffs at and adjacent to Kimmeridge Bay, with the sections farther west at Ringstead Bay, Osmington Mills and Wyke Regis, are the type sections d t h e Kimmeridge Clay. The sections are well exposed, readily accessible and consist of thick sequences of kiliferous marine mudstones, apparently free from major non-sequences. The Upper Kimmeridge Clay is completely exposed in tectonically uncomplicated sectioas from Kimmeridge Bay eastwards to Chapman's Pool and in Brandy Bay. Following recent work on the ammonite sequences in the top part of the Kimmeridge Clay (Cope, 1978; Wimbledon and Cope, 1978), the position of the Kimmeridgian-Portlandian boundary has been reaffirmed as the base of the ProgalbanitGs albani Zone, which is defined in Houns-tout Cliff [SY 9505 77201 at the base of the Massive Bed (Arkell, 1947, pp. 90-91 ). The main practical reason in the past for not including the Upper Kimmeridge Clay in the Kimmeridgian Stage has been the difficulty of demonstrating correlations between the faunal provinces which developed in the late Jurassic. However, recent work by Casey (1967, 1971, 1973) has shown that faunal links existed between the AngbParis Basin and the Russian Platform when the Portland Stone was being deposited, suggesting that the Kimmeridgian and Portiandian faunas of southern n7

England may not be as geographically isolated as had previously been thought. The palaeogeography of the time suggests that the Kimmeridge Clay was deposited in a shallow shelf sea with numerous relatively small islands separated by seaways (Figure 15). There was no land barrier between the various faunal ~rovincesand they appear to have been climatically co4rolled. The strongest argument in favour of using the Kimmeridgian Stage sensu gallico is that it might in the future enable the bases of the Portlandian, Tithonian and Volgian stages (see p.24) to be placed at the same stratigraphical level throughout Europe. However, the details of this correlation have yet to be described precisely. Oppel (1865) introduced the Tithonian Stage for the condensed limestone sequence developed between the Kimmeridgian and the Cretaceous in the Tethyan region. Arkell (1946, pp. 6,21) considered the Tithonian Stage to be poorly defined and suitable only as a local name; Cope and others (1964, p. 934) also concluded that the stage was not suitable a s a basis for international correlation. A type-section for the base of the Tithonian Stage has yet to be designated but Enay (1964, p. 361), who supported the use of this stage, suggested that the stratotype of the Lower Tithonian could be in Franconia (S. Germany), where some of the best Tithonian sequences are seen in a region where tectonics are relatively simple. The lowest part of the Tithonian Stage includes horizons with Gravesia (see p. 24) and has been correlated with the former Gravesia gigas Zone of north-westem Europe (Hahn, 1963, p. 107; Zeiss, 1968).

The type section of the Volgian Stage, in the banks of the River Volga at Gorodishche near Ulyanovsk in the USSR, consists of clays, marls and glauconitic sands with seams of phosphatic nodules (Casey, 1968). The individual beds are rather variable in thickness and in total, range between 15.6 m and 21.6 m. The base of the stage is taken at the base of the Zone of Subplanitesklimovi, which at Gorodishche is 2.2 to 4.5 m thick and includes the ammonites Subplanites (Zlovaiskya) klimovi (Ilovaisky and Florensky), Glochiceras spp., Gravesia cf. gigas (d'orbigny), Haploceras sp., Neochetoceras cf. steraspis (Oppel) and Sutneria cf. subeumela Schneid (Mesezhnikov and others, 1977). This zone overlies 9 to 12.5 m (visible thickness) of clays and marls with an ammonite fauna including Amoeboceras, Aspidoceras, Aulacostephanus, Glochiceras, Haploceras, Subdichotomoceras, Sutneria and Virgataxioceras. The sequence is very condensed compared with that of Dorset. Russian authors advocate the use of the Volgian as a world-wide stage. They suggest that 'Portlandian' deposits are limited to the Anglo-Paris Basin and that the Tithonian Stage, although it is more completely developed than the Volgian, has no stratotype and has yet to be satisfactorily zoned; the Volgian Stage, on the other hand, can be recognised throughout the Boreal region. Overlap of the Volgian Stage (as defined by Nikitin, 1881, 1884) and the upper part of the Kimmeridgian Stage sensu anglico has been widely recognised for a long time and most Russian authors (Sasonov, 1964; Gerasimov and others, 1974) have correlated the Volgian Stage with the Portlandian Stage semu gallico. Casey (1967, p.132) however, has shown that an unconformity occurs between the Middle and Upper Volgian substages in the type area into which the greater part of the Portland Beds of southern England can be fitted and has suggested that this condensed and incomplete Volgian succession is not suitable as a world-wide standard. He proposed that the Volgian Stage should be retained only for strata at present grouped as Upper Volgian and that the Portlandian Stage should be defined as embracing the Middle Volgian of Gerasimov and Mikhailov (1966) and the whole of the Portland Beds of Britain. This would mean that strata at present grouped as Lower Volgian would be assigned to the Kimmeridgian and the new Kimmeridgian-Portlandian boundary, which would be taken at the base of the Pavlovia zones, would effect a compromise between the Kimmeridgian Stage sensu anglico and smu gallico. In order to clarify the position concerning the boundaries and stratotypes of the Kimmeridgian Stage the following recommendations are made: i The Kimmeridgian Stage should be defined to include most of the Kimmeridge Clay in its type area (but see v below). ii The type section for the Lower Kimmeridgian should be the cliff sections at and adjacent to Black Head, near Osmington Mills, Dorset [SY 7229 8198 to 7342 81741 combined with the beach section at Ringstead Bay, Dorset [SY 7486 81373, showing the Oxfordian-Kimmeridgian boundary. iii The type section for the Upper Kimmeridgian should be the cliff sections between Kimmeridge Bay [SY 905 7911 and Chapman's Pool [SY 955 7711. An additional section at Brandy Bay [SY 889 7951 exposes most of the sequence. iv Consideration should be given to the re-introduction of a zone based on Gravesia, the new zone to cover strata currently placed within the top part of the existing

Aulacostephanus autissiodormris Zone. The base of this new zone, which would also mark the Lower-Upper Kimmeridgian Substage boundary, would be defined by the incoming of Gravesia andlor a lithological change associated with it; the junction with the overlying P. (V.) elegans Zone would be defined by the incoming of ~ectinatitesandlor an associated lithological change.v The top of the Kimmeridgian Stage is defined by the base of the overlying Portlandian Stage. At present, this albani Zone. the is taken at the bascof the Pro~albanites a type horizon-section of which is the Massive Bed at Houns-tout Cliff [SY 9505 77201 (Wimbledon and Cope, 1978). In any future discussion concerning the relationship of the Kimmeridgian, Portlandian, Volgian and Tithonian stages, serious consideration should be given to Casey's (1967) suggestion that the Kimmeridgian~ortlandianboundaiihould be taken at the base of the Pavlouia zones.

ACKNOWLEDGEMENTS The authors are indebted to a number of colleagues and friends for their help. In particular, Miss Molly Simmons who assisted with the field work and Dr J.H. Callomon for discussion and criticism of an early draft of the paper. Details of the Kimmeridge No. 1 Borehole are published by kind permisspn of British Petroleum Ltd.

- 1968. The type-sec~onof the Volgian stage (Up.&GER, D.V. 1963. Jurassic stages. Nature, London, Vol. 198, No. 4885, pp. 1045-1046. . W L L , W. J. 1929. A monograph of British Corallian Lamellibranchia. Monog. Pal. Soc., Pt 1, pp. 1-72. - 1933. The Jurassic system in Great Britain. (Oxford: Clarendon Press). - 1935. On the Lower Kimmeridgian ammonite genera Pictonia, Rasenia, Aulacostephanus and Ataxioceras. Geol Mag., Vol. 72, pp. 246-257. - 1936. The Corallian Beds of Dorset. Proc. Dorsit .Vat. Hist. Archaeol. Soc., Vol. 57, pp. 59-93. - 1945. The zones of the Upper Jurassic of Yorkshire. Proc. Yorkshire Geol. Soc., Vol. 25, pp. 339-358. - 1946. Standard of the European Jurassic. Bull. Geo1:Soc. Amer., Vol. 57, pp. 1-34. - 1947. T h e geology of the country around Weymouth, Swanage, Corfe and Lulworth. Mem. Geol. Sum. G.B., Sheets 341-343 plus parts of sheets 327-329. - 1949. The Kimmeridge Clay succession at Burning Cliff, Ringstead. Proc. Dorset Nat. Hist. Archaeol. Soc., Vol. 70, p. 124. - 1951. Dorset geology 1940-50. Proc. Dorset Nat. Hist. Archaeol. Soc., Vol. 72, pp. 176-194. - 1956. Jurassic geology of the World. (Edinburgh and London: Oliver and Boyd.). - and CALLOMON, J.H. 1963. Lower Kimmeridgian ammonites from the Drift of Lincolnshire. Palaeontology, Vol 6, pp. 219-245. BATHER.F.A. 1911. Note on crinoid plates from the Penshurst Boring. Summ. Prog. Geol. Sum. for 1910, pp. 78-79. BELLAMY, J. 1977. Subsurface expansion megapolygons in Upper Jurassic dolostone (Kimmeridge, CTK). J. Sediment Petrol., Vol. 47, pp. 973-978. B I R K E L U N DT, . , T H U S U , B. a n d V I G R A N , J. 1978. Jurassic-Cretaceous biostratigraphy of Sorway, with comments on the British Rasenia cymodoce Zone. Palaeontology, Vol. 21, pp. 31-63. BLAKE,J.F. 1875. On the Kimmeridge Clay of England. Q. J. Geol. Soc. London, Vol. 31, pp. 196-233. - 1881. On the correlation of the Upper Jurassic Rocks of England with those of the Continent - Part 1. The Paris Basin. Q. J. Geol. Soc. London, Vol. 37, pp. 497-587. BRETOW,C.R. and BAZLEY,R.A. 1972. Geology of the country around Royal Tunbridge Wells. Mm. Geol. Sum, G.B., Sheet 303. BROOKFIELD, M.E. 1973. The palaeoenvironment of the Abbotsbury Ironstone (Upper Jurassic) of Dorset. Palaeontology, Vol. 16, pp. 261-274. - 1978. The lithostratigraphy of the upper Oxfordian and lower Kimmeridgian beds of south Dorset. Proc. Geol. Assoc., Vol. 89, pp. 1-32. CALLOMON, J.H. and COPE,J.C.W. 1971. The stratigraphy and ammonite succession of the Oxford and Kimmeridge clays in the Warlingham Borehole. Bull. Geol. Sum. G.B., No. 36, pp. 147176. CASEY,R. 1958. In Summ. Prog. Geol. Sum. for 1957, p. 48.

- 1960. In Summ. Prog. Geol. Sum. for 1959, p. 48 - 1967. The position of the Middle Volgian in the English Jurassic. pp. 128-133.

Proc. Geol. Soc. London, No. 1640, 29

per Jurassic) at Gorodische, near Ulyanovsk, USSR. Proc. Geol. Soc. London, No. 1648, pp. 74-75. - 1971. Facies, faunas and tectonics in late Jurassicearly Cretaceous Britain. Pp.153-158 in Fauna1 provinces in space and time. MIDDLEMISS, F.A., RAWSON,P.F. and NEWALL,G. (Editors). Geol. J. Spec. Issw, No. 4. - 1973. The ammonite succession at the JurassicCretaceous boundary in eastern England. Pp. 193266 in The boreal Lower Cretaceous. CASEY,R. and ~WSON P.F. , (Editors). Spec. Issue Geol. J., No. 5. CHATWIN, C.P. and PRINGLE, J. 1922. The zones of the Kimmeridge and Portland rocks at Swindon. Summ. Prog. Geol. Sum. for 1921, pp. 162-168. COPE,J.C.W. 1967. The palaeontology and stratigraphy of the lower part of the Upper Kimmeridge Clay of Dorset. Bull. Br. Mus. (Nat. Hist.) (Geol.), Vol. 15, pp. 3-79. - 1968. Propectinutites, a new Lower Kimmeridgian ammonite genus. Palaeontology, Vol. 11, pp. 16-18. - 1969. The Kimmeridge Clay. Pp. A47-A53 in Guide for Dorset and South Somerset. International Field Symposium on the British Jurassic. (Keele: University of Keele.) - 1974. Upper Kimmeridgian ammonite faunas of the Wash area and a subzonal scheme for the lower part of the Upper Kimmeridgian. Bull. Geol. Sum. G.B., No.47, pp. 29-37. - 1978. The ammonite faunas and stratigraphy of the upper part of the Upper Kimmeridge Clay of Dorset. Palaeontology, Vol. 2 1, pp. 469-533. - ,SARGEANT, W.A.S., SPALDING, D.A.E., and ZEISS, A. 1964. The Kimmeridgian-Portlandian boundary. Pp. 933-936 in C. R. et M&. Colloq. Jurassique Luxembourg 1962. (Luxembourg: Inst. Gr.-Duc. Sect. Sci. Nat. Phys. Math.) - and others. [In press.] A correlation of Jurassic rocks in the British Isles. Spec. Rep. Geol. Soc. London. Cox, L.R. 1929. A synopsis of the Lamellibranchia a n d Gastropoda of the Portland Beds of England. Proc. Dorset Nat. Hist. Archaeol. Soc., Vol. 50, pp. 131-202. DOWNIE,C. 1957. Microplankton from the Kimmeridge Clay. Q. J. Geol. Soc. London, Vol. 112, pp. 413434. ENAY,R. 1964. L'ktage Tithonique. Pp. 355-379 in C.R. et Mim. Colloq. Jurassique Luxembourg 1962. (Luxembourg: Inst. Gr.-Duc. Sect. Sci Nat. Phys. Math.). - 1966. Le genre Grauesia (Ammonitina Jurassique) d a n s le J u r a F r a n ~ a i s e t les c h a i n e s subalpines. Ann. a2 Paliontol. Inuertibr., Vol. 52, pp. 95-105. - 1971. TithoniqueIPortlandien. I n Les zones du Jurassique en France. C. R. Somm. de Siances Soc. Ghl. France, Vol. 6, pp. 76-102. FITTON,W.H. 1836. Observations on some of the strata between the Chalk and the Oxford Oolite in the south-east of England. Trans. Geol. Soc. London, Vol. 2, No. 4, pp. 103-388. GALLOIS,R.W. 1979a. Geological investigations for the Wash Water Storage Scheme. Rep. Inst. Geol. Sn', No. 78/19. - 1979b. A pilot study of oil shale occurrences in the Kimmeridge Clay. Rep. Inst. Geol. Sci., No. 78/13. -and Cox, B.M. 1974. Stratigraphy of the Upper Kimmeridge Clay of the Wash area. Bull. Geol. Sum. G.B., No. 47, pp. 1-16.

- 1976.

The stratigraphy of the Lower Kimmeridge Clay of eastern England. Proc. Yorkshire Geol. Soc., Vol. 41, pp. 13-26. GEORGE,T.N. and others. 1969. Recommendations on stratigraphical usage. Proc. Geol. Soc. London, No. 1656, pp. 139-166. GERASIMOV, P.A. and MIKHAILOV, N.P. 1966. The Volgian stage and the standard scale for the upper series of the Jurassic system. I t v . Akad. Nauk. SSSR, Ser. Geol., No. 2, pp. 118-138 [In Russian]. - , KUZNETZOVA, K.I., and MIKHAILOV, N.P. 1974. Volgian stage and its zonal subdivision. Cdiloque du Jurassique 5 Luxembourg 1967. Mim Bur. Rech. Giol. Miniire Fr., No. 75, pp. 347-355. GEYER,O.F. 1961. Monographie der perisphinctidae des unteren unterkimeridgium (Weisser Jura Y, Badenerschichten) in Siiddeutschen Jura. Palaeontographica, Vol. 117A, pp. 1-157. - 1969. The ammonite genus Sutneria in the Upper Jurassic of Europe. hthaia, Vol. 2, pp. 63-72. GITMEZ,G.U. 1970. Dinoflagellate cysts and acritarchs from the basal Kimmeridgian (Upper Jurassic) of England, Scotland and France. Bull. BY. Mus. (Nut. Hist.) (Geol.), Vol. 18, No. 7, pp. 23333 1. HAHN, W. 1963. Die Gattung Gravesia Salfeld (Ammonoidea) im Oberjura Mittel - und Nordwesteuropas. Palaeontographica, Vol. 122A, pp. 90-1 10. HEDBERG, H.D. 1976. ,International Stratigraphic Guide. (New York: John Wiley & Sons.) HOLLAND, C.H. and others. 1978. A guide to stratigraphical procedure. Spec. Rep. Geol. Soc. London., No. 11. HUDLESTON, W.H. 1880. Excursion to Aylesbury. Proc. Geol. Assoc., Vol. 6, pp. 344-352. KENT, P.E. and CASEY,R. 1963. A Kimmeridgian sandstone in north Lincolnshire. Proc. Geol. Soc. London, No. 1606, pp. 57-62. KILENYI,T. 1969. The ostracoda of the Dorset Kimmeridge Clay. Palaeontology, Vol. 12, pp. 112-1 60. KITCHIN,F.L. 1919. The fauna1 characters and correlation of the concealed Mesozoic rocks in Kent. Summ. Prog. Geol. Sum. for 1918, pp. 37-45. - 1926. A new genus of lamellibranchs (Harhucllia gen. nov.) from the Upper Kimmeridge Clay of England with a note on the position of the Hartwell Clay. Ann. Mug. Nut. Hist., Ser. 9, Vol. 18, pp. 433-455. LAUGHTON, A.S. 1972. Pp. 1155-1 179 in Initial Rep. Deep Sea Drill. Proj., Vol. 12 (Washington: U.S. Government Printing Office.) LEES, G.N. and TAITT, A.H. 1945. The geological results of the search for oilfields in Great Britain. Q. J. Geol. SOC.London, Vol. 101, pp. 255317. LLOYD,A.J. 1959. Arenaceous foraminifera from the type Kimeridgian (Upper Jurassic) . Palaeontology, Vol. 1, pp. 298-320. LORIOL,P. de and COTTEAU, G. 1868. Monographie paltontologique et giologique de 1'Ctage Portlandien du dtpartement de lYYonne. Bull. Soc. Sci. Hist. Nut. Yonne, Ser. 2, Vol.1. -, ROYER,E. and TOMBECK, H. 1872. Description gtologique et paltontologique des itages jurassique supirieurs de la Haute-Marne. Mm. SOC. Linn. Normandie, Vol. 16.

MESEZHNIKOV and others. 1977. Jurassic/Cretaceous boundary beds in the Middle Volga area (A prospectus to geological excursions). (Leningrad: VNIGRI). MIKHAILOV, N.P. 1964. Boreal late Jurassic (Lower Volgian) ammonites (Virgatosphinctinae). TT. Geol. Inst., Leningrad., Vol. 107, pp. 1-88. [In Russian.] - 1966. Boreal Jurassic ammonites (Dorsoplanitinae) and zonal subdivision of the Volgian stage. Tr. Geol. Inst. Zmingrad, Vol. 151, pp. 1-1 16. [In Russian.] MORRIS,N.J. 1968 Stratigraphical and palaeontological researches in the Upper Jurassic rocks. D. Phil. thesis, University of Oxford. [Unpublished.] NEAVERSON, E. 1924. The zonal nomenclature of the e Upper ~ i m m e r i d ~Clay. Geol. Mug., Vol. 61, pp. 145-151. - 1925. Ammonites from the Upper Kimmeridge Clay. Pap. Geol. Dep. Univ. Liverpool., No. 1, pp. 1-52. NIKITIN, S. 1881. Die Jura-Ablagerungen zwischen Rybinsk, Mologa and Myschkin an der oberen Wolga. M h . Acad. Imp. Sci. St. Pitersbourg, Ser. 7, Vol. 28, No. 5. - 1884. Allgemeine geologische Karte von Russland. Blatt 56. Jaroslaw I. M h . Com. Giol. St. Pitersbourg, Ser. 8, Vol. 1, No. 2. NORRIS,G. 1969. Miospores from the Purbeck Beds a n d m a r i n e U p p e r J u r a s s i c of s o u t h e r n England. Palwontology, Vol. 12, pp. 574-620. OPPEL, A. 1865. Die tithonische Etage. Z. Dtsch. Geol. Ges., Vol. 17, pp. 535-558. ORBIGNY, A. d'. 1842-1 851. Paliontologie jangaise. Terrains jurassiques. I Ciphalopodes. (Paris: Victor Masson). PAVLOW, A. 1896. On the classification of the strata between the Kimeridgian and Aptian. Q. J. Geol. Soc. bndon, Vol. 52, pp. 542-555. PRINGLE, J. 1919. In Summ. Prog. Geol. Suw. for 1918, p. 51. - 1923. On the concealed Mesozoic rocks in southwest Norfolk. Summ. Prog. Geol. Suw. G.B., for 1922, pp. 126-139. SALFELD, H. 1913. Certain Upper Jurassic strata of England. Q. J. Geol. Soc. London, Vol. 69, pp. 423430. - 1914. Die Gliederung des oberen Jura in nordwesteuropa. News Jahrb. Miner. Geol. Palaont. Beil-Band, Vol. 37, pp. 125-246. - 1916. Monographie der gattung Cardioceras. Z. Dtsch. Geol. Ges., Vol. 67, pp. 149-204. SASONOV, N.T. 1961. A standard scheme of stratigraphy of the Jurassic deposits of the Russian Platform (experimental). Pp. 5-17 in Work of the all-Union committee for establishing a standard scheme of stratigraphy for the Mesozoic deposits of the Russian Platform. Tr. Vses. Nauchn.-Issled. Geo1.-Razven. Neji. Inst., Vol. 29, pp. 5 4 7 . [In Russian.] - 1964. Stratigraphe des dtp6ts jurassiques de la Plate-forme Russe. Pp. 787-805 in C.R. et M h . Colloq. Jurassique, Luxembourg 1962. (Luxembourg: Inst. Gr.-Duc. Sect. Sci. Nat. Phys. Math.) SEEBACH,K. von. 1864. Der Hannonersch Jura. (Berlin: Wilhelm Hertz.) SMART,J.G.O. and WOOD,C.J. 1976. South Humberside field excursion. Proc. Yorkshire Geol. Soc., Vol. 40, pp. 586-593.

b. \V. 1815. A delineation of the strata of England

- 1964. Das untere Kimmeridgien in Europa. Pp.

d Hakes, with part of Scotland. (London: W. Smith.)

345-354 in C. R . et M&. Colloq. Jurassique, Luxembourg 1962. (Luxembourg: Inst. Gr.-Duc. Sect. Sci. Nat. Phys. Math.). - 1974. Grenzen der Biostratigraphie im Jura und Grenzen zur stratigraphischen Methodik. Colloque du Jurassique B Luxembourg 1967. Mim. Bur. Rech. Giol. M i n k Fr., No. 75, pp. 35-67. ZIEGLER,M.A. 1960. Gravesienfunde aus dem 'unteren Portland' der Gegend von Morteau (Doubs). Eclogae Geol. Helvetiae, Vol. 53, pp. 670677.

-1817.

Strata identified by organized fossils. Part III. (London: W. Smith.) Sin=. L.F. 1935. The Upper Jurassic invertebrate 6unas of Cape Leslie, Milne Land. I: Oxfordian and Lower Kimmeridgian. Medd. om Grenland, Vol. 99 (2). - 1936. The Upper Jurassic invertebrate faunas of Cape Leslie, Milne Land. 11: Upper Kimmeridgian and Portlandian. Medd. o n Grenland, Vol. 99 (3). -N, A. 1898. The geology of the Isle of Purbeck and Weymouth. Mem. Geol. Sum. G . B . - 1920. Special reports on the mineral resources of Great Britain. Mineral oil, Kimmeridge oil-shale, i i p t e s , jets, cannel coals, natural gas. Mern. Geol. Sum. G.B., Vol. 7, 2nd edition. ~ T C K M A N N , C. 1887. Die Portland-Bildungen der Umgegend von Hannover. 2. Dtsch. Geol. Ges., Vol. 39. pp. 32-67. TORRENS,H.S. and CALLOMON, J.H. 1968. The Corallian Beds, the Ampthill Clay and the Kimmeridge Clay. Chapter 15, pp. 291-299 in The geology of dc East Midlands. SYLVESTER-BRADLEY, P.C. and FORD,T.D. (Editors.) (Leicester: Leicester University Press.) T'SSON, W. G. 1975. Lithostratigraphy and deposition of the type Portlandian. J. Geol. Soc. Londn. Vol. 131, pp. 619-638. WAAGEN,W. 1865 Versuch einer allgemeinen Classijca&n (ICr Schichten des oberen Jura. (Munchen: Hermann

h.) W-R,

T. 1816. Geological observations on the of Wight. Pp. 117-238 in ENGLEFIELD, H. Description of the principal picturesque beauties, anti+S, and geological phenomena of the Isle of Wight. (London: Payne and Foss.) M~BLEDON, W.A. and COPE, J.C.W. 1978. The ammonite faunas of the English Portland Beds and th zones of the Portlandian Stage. J. Geol. Soc. M ,Vol. 35, pp. 183-190. ~DWARD H.B. , 1895. The Jurassic rocks of Bri6. LTol.V. The Middle and Upper Oolitic rocks of England (Yorkshire excepted). Mem. Geol. Surv. GB. WO.SSAM,B.C. and IVIMEY-COOK, H.C. 1971. The matigraphy of the Geological Survey Borehole at Warlingham, Surrey. Bull. Geol. Sum. G.B., No. 36, ; pp. 1-146. k-ov, V.A. and MESEZHNIKOV, M.S. 1974. The V o b a n Stage of the Subarctic Ural. Tr. Inst. Geol. Gaphys. (Novosibirsk), Vol. 196, pp. 1-216. [In RusIsle

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h. A. 1964. Zur Verbreitung der Gattung Gravesia im Malm der Sudlichen Frankenalb. Geol. Bavartta. Vol. 53, pp. 96-101. - 1968. Untersuchungen zur Palaontologie der Cephalopoden des Unter-Tithon der Sudlichen Frankenalb. Abh. Bayerische Akad. Wiss. Math. ,%-&miss K l . [Neue Folge], Vol. 132, pp. 1-190. ~ L E R B. , 1961. Stratigraphische und zoogeognphische Beobachtungen an Aulacostephanus (Ammonoidea-Obejura). Palaont. Z., Vol. 35, pp. 79-89. - 1962a. Die Ammoniten-Gattung Aulacostephanus im Obejura (Taxionomie, Stratigraphie, b l o g i e ) . Palaeontographica, Vol. 119A, pp. 1-1 72. - 1962b. Some Upper Jurassic ammonites of the genus Rasenia from Scotland. Palaeontolo.g, Vol. 5, pp. 765-769.

APPENDIX l DETAILS OF SECTIONS The accessibility and quality of the Kimmeridge Clay coastal sections varies in response to a number of artif~ialand natural factors and different parts of the sequence become more or less accessible or well exposed with time. At Wyke Regis the cliff sections, showing the basal part of the Kimmeridge Clay, have become steadily more degraded since Victorian times due to the construction of harbour works, and they seem destined to become steep, grassy banks. At Black Head and Osmingtpn Mills, the lower parts of the sections are constantly being rejuvenated by marine erosion, but the higher parts (cudoxus Zone and above) are adjacent to a large, presumed Pleistocene, landslip and are unlikely to improve unless further major landslipping occurs. At Ringstead Bay, much of the Lower Kimmeridge Clay was well exposed in the 1930's and 1940's but these sections now lie behind a continuous shingle beach and have become either degraded or hidden by landslip. The Upper Kimmeridge Clay is better exposed here but the sections are deeply weathered; they are not subject to marine erosion and are likely to deteriorate. All the sections at Wyke Regis, Black Head, Osrnington Mills and Ringstead Bay are accessible at all states of the tide.

The sections in the Kimmeridge area are much better exposed than those of the Weymouth area. The cliffs between Brandy Bay and Freshwater Steps are being actively eroded and fresh sections are almost always available. Between Freshwater Steps and Chapman's Pool, marine erosion is less active and parts of the section, including that at Chapman's Pool, have become progressively more degraded in recent years. All the sections between Brandy Bay and Chapman's Pool are accessible from the beach at most states of the tide. There are however a number of small promontories, notably Long Ebb and Broad Bench in Brandy Bay, Yellow Ledge at the foot of Cuddle, and at Clavell's Hard, Rope Lake Head and Freshwater Steps, that cannot be negotiated easily at high tide even when the sea is calm. In addition, there is no safe access down the cliff between the western end of Brandy Bay and Charnel [SY 901 7911, between Charnel and Kimmeridge Bay, between Kimmeridge Bay and Clavell's Hard and between Clavell's Hard and Freshwater Steps. The limited number of access pointwand the state of the tide therefore need to be borne in mind when visiting these areas. At present, the cliffs at Brandy Bay and Hobarrow Bay, and eastwards, to the western end of Kimmeridge Bay, fall within the Lulworth Gunnery Range and permission must be obtained fiorn the Ministry of Defence before they can be visited. At Kimrneridge Bay, the local landowner has banned the use of hammers because of the danger of cliff falls and the presumed increase in the rate of cliff erosion caused by large numbers of visiting geological parties.

KEY Mudstone, undifferentiated; mostly dark grey

IzGl L]

Mudstone, very calcareous (pale grey)

................. .......... ......... .......... : : : : : : : .: Silty rmdstone and siltstone ......... .........

* .

--

Bituminous mudstone (fa~ntly bituminous in rnutabilir Zme) or oilshale interbedded with other lithologies Very shelly mudstone

€ 1 m

(=Stone Band) bed Dolomitic ~iI'rteSt0ne

mm

concretions

Coccolith-rich limestone

1

I

ccementaone) Sheared clay with ramifying network of paper-thin calcite sheets

- Bedding-plane shear**

Scale : 1 to 100 (lcm to l m)

*No satisfactoryterminology exists for the description of kerogenrich mudstones. The term 'oil shale' is widely used to describe lithologies that yield more than about 15 gal.oil/ton shale when retorted at 500°C. In the Kimmeridge Clay, mudstones that yield from about 2 to 15 gal.oil/ton are sufficiently rich in kerogen to be

lithologically distinguishable from other mudstones and have commonly beendescribedas 'bituminous'even though they do not contain bitumen (organic-solublematerial). This terminology is used here in the absence of a more precise alternative. The bituminous mudstones in the Kimmeridge Clay mostly have potential oil yields in the range of 5 to 15 gal.oil/ton; those in the mutabilis Zone yield less than 5 gal.oil/ton.

** Laterally persistent bedding-plane shears, composed of sheared clay cut by calcite veins. occur at numerous levels in the cliffs in the Kimmeridge area; only those that provlde useful markers are shown in the measured sections.

BLACK HEAD

[W7239 8195 for baylei and cymodoce zones; Shelly with widely spaced septarian concretions

7259 8 192 to

W 7258 8200 for remainder]

I not exposed

Shelly with pyritized ammonites and bivalves

m

Shelly and sparsely shelly with well preserved iridescent ammonites including Amoebites, Aspidoceras (and Laevaptychus), Aulacostephanus (eudoxus gp) and Sutneria eumela

Shelly and sparsely shelly with crushed ammonites and bivalves including Aulacostephanus preserved in calcite or rotted pyrite

5 2

5 P =I (D

Shelly with common Aulacostephanus eulepidus and A. mutabilis Very shelly with Aspidoceras, , Aulacostephanus eulepidus, A. mutabilis, rare A. eudoxus gp and common Lopha and rhynchonellids

4

Moderately shelly becoming shelly with depth; Aulacostephanus including A. eulepidus common; small oysters common

Pumpkin-shapedseptarian concretions Very shelly with common coarsely ribbed Rasenia and Xenostephanus

4 Supracorallina Bed: abundant Astarte supracorallina and Aulacostephanus eulepidus

Sparsely shelly with pyritized ammonites and bivalves including Rasenia

Black Head Siltstone; finely crosslaminated; sparsely shelly with Amoebites Wyke Siltstone: cross-laminated and bioturbated; shelly with myid bivalves Calcareous mudstone with thin tabular beds (2 or 3) of siderite mudstone

Dark grey mudstone with abundant Deltoideum delta

5

Uniform calcareous mudstone with dicey weathering; moderately shelly with common small oysters and echinoid debris

3

5B Widely spaced septarian concretions

P 3 CD

Nana Bed Inconstans Bed Ringstead Waxy C b y (OXFORDIAN)

Shelly with abundant Aulacostephanus including A. eulepidus, myids and Lopha

Smooth textured with numerous shell plasters composed largely of bivalves and iridescent Aulacostephanus eulepidus

OSMINGTON MILLS [W7336 8186 for highest 7 beds SY 7342 8 17 4 for remainder] I

. . .. .....

BLACK HEAD (continued) Intermittent exposures show equivalents of ?Make's Bed 42, Grey Ledge S.B., Blackstone with Saccocoma, Rope Lake Head S.B. and White S.B. separated by poorly exposed mudstones.

1

Shelly siltstone; cemented in part; commpn Aulacostephanus eulepidus and oysters; rare Aspidoceras

)

)

] Nannocardioceras Cementstone: cemented oil shale with Nannocardioceras and bivalves preserved in solid calcite

Supramrallina Bed: abundant Astarte supracorallina and Aulacostephanus eulepidus Widely spaced septarian concretions

Poorly exposed; interbedded oil shale and mudstone w ~ t habundant Nannocardioceras Cementstoneconcretions with crushed bivalves and Nannocardioceras

I I I

I I

Weakly cemented siltstone not exposed c.8m;

Soil fragments indicate interbedded oil shale with abundant Nannocardioceras, and shelly and sparsely shelly mudstone

Poorly exposed mudstones and faintly bituminous mudstones with abundant crushed but well preserved iridescent Aulacostephanus eulepidus and rare A. mutabilis

Propectinatites?, Nanogyra virgula and serpulids common Virgula Limestone; soft muddy limestone composed largely of Nanogyra virgula

Abundant Aulacostephanus eulepidus

l

'

Rhythmic alternations of shelly oil shale and shelly and sparsely shelly mudstones; well preserved iridescent ammonites common including Aspidoceras, Amoebites, Aulacostephanus (eudoxus gp) and Sutneria eumela

Pumpkin-shaped septarian concretions Very shelly, partially cemented mudstone with common Rasenia and Xenostephanus

Black Head Siltstone: cross-laminated, sparsely shelly; common Amoebites

Wyke Siltstone; cross-laminated and bioturbated with abundant myid bivalves in lower part

Pyritized Saccocoma and Sutneria eumela common

I

I I

not exposed c7m;

Soil fragments indicate shelly mudstones with thin interbeds of oil shale

Calcareous mudstone with 3 thin bands of siderite mudstone Dark grey mudstone with abundant Deltoideum delta Nana Bed Inconstans Bed Ringstead Waxy Clay (OXFORDIAN)

-STEAD

BAY

[W7619 8147 and SY 7606 8147 for eudoxus and autissiodorensis zones; SY 765 813 for higher zones]

3

Freshwater Steps S.B.: finely laminated coccolith-rich limestone

cb

4 ccS cb 3 ? 9. (D

Widely spaced septarian concretions up to 0.5m long

Poorly exposed rhythmic alternations of calcareous mudstone and oil shale

cn

2. c

Almost continuous tabular cementstone with dicey weathering (?=Cattle Ledge S.B.)

L g I Poorly exposed; mostly calcareous mudstone 4.5m '

Laminated coccolith-rich oil shale (?=Middle White S.B.)

S

Sparsely shelly with pectinatitid ammonites

mn

B

exposed c.2m

I

White S.B.: finely laminated coccolith. rich limestone; oil shale parting near base

C C C C C C C C C C C C C C C

I0

)

Alternations of bituminous and calcareous mudstones with septarian concretions 0.2 to 0.4m long at three levels; sparsely shelly with pectinatitid ammonites

Poorly exposed calcareous mudstone

I I exposed c.8m

Rhythmic alternations of oil shale, bituminous and calcareous mudstones; sparsely shelly with crushed ammonites including A. autissiodorensis

Poorly exposed calcareous mudstone

Shelly oil shales with abundant Nannocardioceras

a

lnterbeddedbituminous and calcareous mudstones, shelly in part With Aulacostephanus and Sutneria

Massive oil shale with calcareous concretions (7= Blackstone)

Interbeddedthin oil shales and mudstones withpectinatitid ammonites

Nannocardioceras Cementstone: oil shale with septarian concretions with Nannocardioceras in translucent calcite Weakly cemented oil qhale Oil shales with abundant Nannocardioceras seoarated bv mudstones with many large ~ a n o ~ y r a virgula

Weakly cemented oil shale

Poorly exposed mudstone with a few thin oil shale bands

v/

3

Q

Oil shale with interbeds of mudstone; shelly with common Amoebites, Nannocardioceras and abundant large Nanogyra virgula

Cementstones with dicey weathering

(?=GreyLedge S.B.)

BRAN W BAY [W893 792 to !W 889 7951

EAST

WEST

Oil shale, cemented in part Alternations of thick calcareous mudstone and thin bituminous mudstone; form poorly accessible, steep degraded cliff Rhythmic alternations of oil shale, bituminous and calcareous mudstones; shelly in part with large crushed Aulacostephanus, including A. autissiodorensis, and large pectinatitids comrnonat many levels especially in the more calcareous beds; oil shales form prominent ribs in cliff

Rare cementstone concretions

Rhythmic alternations of bituminous and calcareous mudstones with very common, crushed large Aulacostephanus, including A. autissiodorensis

Massive oil shale with laminations picked out by pale faecal debris Rhythmic alternations of oil shale, bituminous and calcareous mudstones; shelly in part with Aulacostephanus locally common

I

Laminated oil shale

Washing Ledge S.B.: persistent tabular cementstone

,

Maple Ledge S.B.; persistent tabular cementstone

WEST

BRANDY BAY (continued) Oil shale with coccolithrich laminae Finely laminated oil shale

Oil shales, form prominent ribs

Rhythmic alternations of oil shale, bituminous and calcareous mudstones

Weakly cemented oil shale (?=decalcified limestone at Kimmeridge)

Grey Ledge S.B.: persistent tabula? cementstone with dicey weathering

Uniform calcareous mudstone; weathers back to form steep degraded slope

Rhythmic alternations of bituminous and calcareous mudstones; sparsely shelly with crushed Pectinatites (Virgatosphinctoides)

Rhythmic alternations of oil shale, b~tuminous and calcareous mudstones; shelly and sparsely shelly with crushed ammonites, including Pectinatites (Arkellites) and P. (Virgatosphmctoides), present throughout

Hard fissile mudstone with many large, crushed pyritized ammonites

'oil

Cattle Ledge S.B.: persistent tabular cementston with dicey weathering

Sparsely shelly with crushed bivalves and ammonites including Pectinatites (Arkellites) and P. (Virgatosphinctoides)

shale, cemented in part

Line of pyritic concretions Laminated oil shales

Laminated oil shale, cemented in part

Laminated oil shale with impersistent calcitic cement in lower part (?= Blake's Bed 42)

BRANDY BAY (continued) EAST

Middle White S.B. and Freshwater Steps S.B. 7m and 16.4m above White S.B. respectively

n C C C C C C C C C C C C C C C C C C C c

White S.B.; finely laminated coccolithrich limestone

oil shale with coccolith-rich laminae

Oil shales above and below Blackstone form steep cliff with overhanging ledges

Rhythmic alternations of oil shale and mudstone; sparsely shelly with Pectinatites (Arkellites) and P. (Virgatosphinctoides

-Oil shale with pyritic concretions

-.

J

Oil shale with central coccolith-rich band

3 (D

Cemented, laminated, coccolith-rich oil shale (?=Rope Lake Head S.B.)

Laminated coccolith-rich oil shale Uniform calcareous mudstone with dicey weathering; forms steep degraded slope Blackstone: massive oil shale with calcareous and pyritic concretions and pyritized Saccocorna

Rhythmic alternations of oil shale and mudstone; shelly in part with crushed Pectinatites (Virgatosphinctoides)

Weak tabular cementstone; forms prominent ledge

Basalt S.B.

Uniform calcareous mudstone with dicey weathering; forms steep degraded slope; sparsely shelly with crushed Pectinatites (Virgatosphinctoides)

HOBARROW BAY

[W8 9 6 7901 TO KlMMERlDGE BAY [W9 0 8 7911 lmpersistent weak cementstone

I

1

Very shelly oil shales with abundant ~annocardioceras,form hard ribs separated by moderately shelly mudstone and calcareous mudstone

Massive oil shale, forms overhangs in cliff

I

Rhythmic alternations of bituminous and calcareous mudstones; sparsely shelly but with common, crushed Aulacostephanus

Fissile mudstone with plasters of Nannocardioceras

CN-v0Iga.e

Washing Ledge S.B.: persistent tabular cementstone with central oil shale parting

Oil shale with widely spaced, fossiliferous concretions (?NannocardiocerasCementstone)

Rhythmic alternations of dark grey, sparsely shelly mudstone with common, crushed Aspidoceras and Aulacostephanus and shelly oil shale with abundant Nannocardioceras and common Aspidoceras and Aulacostephanus

Obscured by shingle, c2m

111111

Hobarrow Bay S.B.; tabular brown cementstone Sutneria rebholzi common at several levels

Pale grey mudstone with rare concretions Interbedded oil shale and shelly mudstone with abundant ammonites including Aspidoceras, Aulacostephanus and Sutneria rebholzi

talus, 0.7m The Flats S.B.: persistent tabular cementstone cut by polygonal netwofk of low angle thrusts

KlMMERlDGE BAY

TO CUDDLE

[m 912 782)

Partially cemented, shelly oil shale Cemented bituminous mudstone forms prominent ledge

Rhythmic alternations of dark grey and brownish grey, bituminous mudstones; sparsely shelly with crushed Aulacostephanus, pectinatitids and rare Gravesia

Maple Ledge S.B.

-

-

Rhihmic alternations of &rk grey, sparsely shgly mudstone and fis$le, shelly, bituminous m@stone; crushed pectinatitids including Pectinatites lArkellitesl

Blake's Bed 41: pale grey mudstc impersistent cementstone

Bed 42: cemented bituminous mudstone

Fault

Rhythmic alternations of bituminous and calcareous mudstones; sparsely shelly with common crushed pectinatitids

F]Rhythmic alternations of bituminous and calcareous mudstones; moderately shellvwith verycommon, crushed large Aulacostephanus including A. autissiodorensis

As above but with Aulacostephanus and pectinatitids

Alternations as above; widely spaced concretions at 2 levels

Cementedbituminous mudstone; forms Prominent rib

CUDDLE

CSY

912 7821 TO CLAVELLS'S HARD

[W920 7771

Sparsely shelly with crushed bivalves and ammonites including Pectinatites (Arkellites) and P. ~Virgatosphinctoides) Finely laminated oil shales, form distinctive marker bed

Massive oil shale: forms prominent rib

Rhythmic alternations of bituminous and calcareous mudstones; sparsely shelly with bivalves and ammonites

Fissile shelly mudstone with bivalve plasters including Nanogyra virgula

I

Grey Ledge S.B.: persistent tabular cementstone with dicey weathering

Uniformly calcareous mudstone, forms steep degraded cliff; sparsely fossiliferous with fragmentary ammonites

Yellow Ledge S.B.: persistent tabular cementstone with yellow patina

Rhythmic alternations of bituminous and calcareous mudstones; moderately shelly with crushed Pectinatites (Arkellites) and P. (Virgatosphinctoides)

I

Soft, brown, earthy mudstone; presumed decalcified limestone

Sparsely shelly with rare Pectinatites (Virgatosphinctoides)

Cattle Ledge S.B.: persistent tabular cementstone with dicey weathering

m

Basal beds of group of oil shales that form precipitous serrated upper part of cliff at Rope Lake Head

Uniform calcareous mudstone

lmpersistent tabular cementstone with dicey weathering Oil shales and bituminous mudstones above and below Blackstone form steep cliff with overhanging ledges; moderately shelly with crushed Pectinatites (Virgatosphinctoides)

Coccolith-rich limestone within oil shale

p{

4

,C,C,c-c

-c

b b b 1 . 1 .

Basalt S.B.: persistent tabular cementstone with dicey weathering

Rom Lake Head S.B.: coccolith-rich limestone

Massive oil-shale; forms prominent rib

Blackstone: massive oil shale with calcareous and pyritic concretions; forms prominent overhang; formerly worked in adits at Clavell's Hard

Rhythmic alternations of oil shale and mudstone; shelly in part; Saccocoma abundant in Blackstone and underlying bed

Finely laminated, cemented oil shale with Saccocoma

v-1

Interbedded oil shales and mudstones with abundant Saccocoma preserved in pyrite

Weakly cemented calcareous mudstone; forms prominent ledge at Clavell's Hard

Uniform, calcareous mudstone; forms steep degraded slope at and adjacent to Rope Lake Head; sparsely shelly with rare Pectinatites /Virgatosphinctoides)

ROPE LAKE HEAD [SY 9 2 7 7751 TO FRESHWATER STEPS

Sparsely shelly mudstone, darker and more fissile in lower part

Oil shales with coccolith-rich laminae

B C C C C C

[W9 4 4 7 7 2 1

Middle White S.B.: finely laminated coccolith-rich limestone

Persistent tabular cementstone

Sparsely shelly mudstone with 3 lines of widely spaced septarian concretions mostly to 0.3 to 0.4m long

lmpersistent tabular cementstone

Sparsely shelly with crushed Pectinatites s.s. Fissile mudstone with pyritic concretions giving rise to sulphurcoated surfaces: forms lipof waterfall at Freshwater Steps

Oil shale, forms prominent rib

Sparsely shelly and shelly with crushed Pectinatites s s .

White S.B.: finely laminated coccolithrich limestone Oil shales with some coccolith-rich laminae Finely laminated pale and dark grey mudstones lnterlaminated bituminous and cocwlith-rich mudstones Interbedded pale and brownish grey mudstones

Massive, highly calcareous mudstone, forms prominent tabular bed

Freshwater Steps S.B.: finely laminated coccolith-rich limestone

I

LE/

.'. ... .:; ..:...:. '

Sparsely shelly with Pectinatites (Virgarosphinctoides) and P. (Arkellites)

slightly silty mudstone, forms rib

,

Finely laminated oil shale with pyritic concretions at base

b

X

Shelly, pyritic oil shale; forms very prominent rib

lJ

Cementstone concretions, mostly burrowfills 0.1 to 0.2m long, with rare Pavlovia

Rotunda Nodules

Cementstone concretions, mostly 0.05 to 0.1 m long, with abundant Pavlovia

Highest oil shales: form prominent ledge at Chapman's Pool

Uniform calcareous mudstone with dicey weathering; sparsely shelly with crushedPavlovia; forms steep degraded slopes

Abundant crushed Pavlovia and large Protocardia

Plaster of partially phosphatised Pavlovia with belemnites and oysters

Shelly and very shelly with abundant crushed Pavlovia Oil shale, forms prominent rib

Sparsely shelly with Pavlovia and rare Pectinatites; two weak ribs formed by fissile bituminous mudstones

Shelly oil shale, forms prominent rib

Sparsely shelly calcareous mudstone with crushed Pavlovia, forms steep degraded slope in which fissile, bituminous mudstones form weak ribs Sparsely shelly with Pectinatites s.s.

Silty mudstone; forms prominent line of seepages

1