The nature of the stratigraphical record

2

of Eacies 3 ThePersistence

TheNatureof theStratigraphical Record

Similarly, at the other end of the belt, Chalk was later discovered in south-west Ireland (where it must have been noticed by the early surveyors,but they had evidently been too scared of their autocratic director to record such an unlikely phenomenon). Later still it was found covering extensiveareas of the sea-floorsouth of Ireland. Now this spread of a uniform faciesis remarkableenough, but it must alsobe rememberedthat chalk is a very unusual sediment: an extremely pure coccolithlimestone which is almost unique in the stratigraphicalcolumn. Nevertheless,there is even worse to come, for on the other side of the Atlantic in Texas,we find the Austin Chalk of the same age and character, and later Cretaceouschalks (still contemporaneouswith the European development) are found in Arkansas, Mississippi and Alabama. And most surprising of all, much farther away still in Western Australia, we have the Gingin Chalk of Late Cretaceousage, with the sameblack flints and the same familiar fossils, resting-as in north-west Europe-on glauconitic sands. Unfortunately I have not yet visited Australia, but Dr Andy Gale tells me (personalcommunication, 1,989)that the succession of the Upper Cretaceousin WesternAustralia is really remarkably like that in Britain. It is virtually identical, for example, with that in Antrim, Northern lreland. Thus glauconiticsandsor gaizeare followed by glauconiticmarls or mulattoand then by chalk marl and chalk with flints, containing the same invertebrategenera. He suggeststhat the fantasticabundanceof coccolithsforming the chalk (ten times what it is at present) would have modified the climate dramatically. Coccolith blooms release dimethyl sulphide which causesnatural acid rain, increasesthe albedo reflecting solar radiation and reversesthe 'greenhouseeffect'. This fits in with my ideas of Nature as being the biggest polluter, since the new-fangled plants first poured that dreadful element oxygen into our planet's pure atmosphereof methane and carbon dioxide. Indeed the recent eruption of Pinatubo in the Philippines has spread dust around the world and has outdone SadamHussain's efforts in setting fire to the oil-wells of Kuwait, which it was feared would have disastrous effects on the atmosphere. Some generalexplanationis surely neededfor such a wide distributionof such a uniquefaciesas the chalkc'luring,r short

period of geologicaltime. What is more, there has been no other deposit quite like it either before or since,exceptperhaps some Miocenechalks which themselvesare remarkably widespread: in the western approachesto the English Channel, in Malta, Cyprus and the Middle East and all the way to New Zealand. But now let us climb slowly down the stratigraphical column to see what other widespread facies we can find. ,URGONIAN'

LIMESTONES

'l'owards

the end of early Cretaceoustimes, at the Barremian/ Aptian level, a massivelimestonewas developedthat is usually t'alledthe 'Urgonian facies'.This can be seenin Portugal where it forms the karstic, touristic coastwest of Estoril, with strange

I igurr l.l

Sltrtitr(l)VA) lll'rrrrirlrt *tilrlnniltl ill ('u(nn, srrttl/r-r'rts/

4

Record TheNatureof the Stratigraphical

of Facies 5 The Persistence

erosionalshapessuch as the Boca do Inferno (Mouth of Hell). In Spain it weathers into the mushrooms, ships and other weird pinnacles of the Ciudad Encantada(EnchantedCity) near where it shadesthe back streetsof Cuenca (Figure 1.1). It then forms the magnificent cliffs at Cassis,east of Marseilles.It dominates the sceneryin the outer ranges of the Alps (for example, the towering escarpment of Glandasse above Hannibal's route through the sub-Alps). It caPsmany of the textbook ridges in the easternpart of the Jura Mountains (Figure 1'.2).It is well developed all round the Carpathians,in Czechoslovakia,Poland and Romaniaand then through the BalkanMountains of Bulgaria (Figure 1.3) and acrossthe Black Sea to the Crimea and the Caucasus(Figure 1.4). Typical$, the Urgonian limestonesare thought of as rudist reef deposits, and locally they show tremendous in-situ growths of these aberrant bivalves (for example, near the tragic city of Guernicain northern Spain). Elsewhere-as in the Jura-corals are more important than rudists and usually it is only a reef limestone in the broadest Gallic senseof that term; that is to say, a massive limestone without bedding planes and commonly recrystallised.It also tends to develop in huge lensesthat pass

Bulgaria nearGabroao, aboaeDrvanoaamonastery, t igura1.3 l)rgonianlimestone

cappinganticlinalridgeof Montagnedc la Bnlnrctrcor FigureL.2 L)rgonianlimestone in the Frenchlura (DVA) Bellegarde

l.rtcrally into other facies(well seen, for example, in Bulgaria). Sirrcethe publication of the first edition of this book, I have seen ,r rcmarkably similar rock of the same age-the Cupido l,inrestone-dominating the Mesozoic scenery in north-east Mexico. A comparablefaciesalso extends down into northern Af r i c a . 'l'lre term 'Urgonian' has become almost a dirty word in 'rt.t.rcct)us ( stratigraphf , for it is not one of the internationallystagt'namesand it is said ttl be a diachronous,southern .tt'r'cp'rl1'd but hardly by more than l,rt'ies,Adnrittedlyit is diachron()us,

6

The Nature of the StratigraphicalRecord

ThePersistence of Facies 7 aptychi, nautiloid jaws, specialisedbelemnitesand the aberrant brachiopodwith the hole in the middle Pygope.Inthe Jura the 'fithonian can be seen in placesto be a coral reef limestone, but usually the corals have been obliterated by dolomitisation and cledolomitisationas is commonly the fate of reefs. It might also be said that the Portland Limestone of southern Irrrgland is the Tithonian limestone in yet another form, with what is mainly a molluscan fauna of limited diversity. But the tcrm 'Tithonian', though not quite so lacking in respectability .rs the 'Urgonian', is usually reserved for the carbonatefacies of alpine Europe, and for years competed with the 'Volgian' the Ironour of being the acceptedinternationalterm for the topmost st;rgeof the Jurassic. In its reef or reef-like facies, the Tithonian continues round thc Alps, the Carpathians, the Balkan Mountains to the ('.rucasus.Where best developed, for exampleat Stramberk in ('zcchoslovakia,it contains a rich and varied fauna including rrrassivecompound corals(though the fossilsare more obvious

Figure 7.4 Urgonianlimestone aalleyof RiaerHeory, Georgia, formingescarpment, USSR(DVA)

half a stageor so, and it is still a valid generalisationto say that a massivelimestoneis developedover a very wide areatowards the end of Early Cretaceoustimes.

TITHONIAN

LIMESTONES

Below the towering Urgohian cliffs of southern Europe, there is commonly a second great escarpmentformed by a massive limestoneat the top of the Jurassicsuccession.Again it runs from northern Africa through Spain and up into the Alps. It forms, for example,the cliffs of the Porte de Franceat Grenoble. It also capsmany of the Jurabox-folds,where the Cretaceoussuccession has been eroded away. Here it illustrates the point that it is the deposition of carbonatethat is important and not the precise nature of the sediment. Thus in the outer Alps, the Tithonian is commonlyin the form of aCalpionella limestone,with abundant tintinnids and such otherwise-unusualforms as anrlnonitr-

'lrttn I igurr L5 I'ithorriutrlinn'slonrcnpyittglurnssit'srrrlr'ssirur nt tltt' Gates', uthere tltr l)uttulu'llotos lltrou'tlt llrc ('nrynlltittrs ltcltt,t,tuRonwria fun the right) and Yljrrs/rtt,irt(otr lltt hlt) (l)VA)

8

TheNatureof the Stratigraphical Record

in the collectionsfrom many years of quarrying than in the quarries themselves).Whether or not these were in-situ reefs is still a matter of dispute, but the dominance of the limestone is everywhere obvious. It dominates, for example, the rapids of the 'Iron Gates', where the Danube roars through the Carpathiansbetween Romania and Yugoslavia (Figure 1.5). The point has therefore now been made with three examples that it is usually carbonatefaciesthat are so remarkablypersistent in a lateralsense.It might be said that these representno more than quiet, low-energy conditions on an extensivecontinental shelf (or shelves). This does not fit, however, with the reef limestonesand it certainly doesnot fit with the other faciesthat show the same persistence. As we continue down the stratigraphical column we find examples in other kinds of sediment and in other kinds of quite high energy facies. THE 'GERMANIC'TRIAS Every student knows, or should know, the classictrinity of the Germanic Triassic: Keuper Muschelkalk Bunter It is not generallyemphasisedin textbooks,however, how very widespread thesesedimentsare. Thus a Briton can drive through the Betic Cordillera in southern Spain and instantly recognise the gypsiferous, red and green marls of the Keuper. In the Celtiberic Mountains of easternSpain, the three-fold division of the Trias is as clear as in Germany and the cross-bedded,red 'Bundsandstein' (for example, along the Rio Cabriel, south-west Teruel) of is exactly like the road-cuttings near Bridgnorth in the English Midlands. \tVhat is more, it is also exactly like the cliff sections in the Isker gorge, north of Sofia and elsewhere in Bulgaria, at the other end of Europe (Figure 1.6). The basal conglomeratein England is full of boulders of a distinctive purple, 'liver-coloured'and white quartzitesthat have been matched with the GrdsdeMay and the Gris Armoricainright acrossthe other side of the EnglishChannelin Brittany(thotrgh

of Facies 9 The Persistence

Bulgaria(DVA) north-zuest at Belogradchik, sandstone I igure1.6 LowerTriassic

I rcgard with some scepticism the notion that the boulders here travelled so far). Along the Rio Cabriel in Spain, it is the same/ lrrrt there the source quartzite outcrops immediately below. Near Ilt'logradchik, in north-west Bulgaria, again the basal conglomerate rs largely composed of exactly similar purple quartzite pebbles (r t'sting on Permian breccias also like those of Midland England). I vt'rr if one postulates continent-wide uplift to produce the r onglomerate in such widely separated places, it is very difficult to t'xplain why the source rock is also so remarkably similar from .rlt' cfld of Europe to the other. Agirin, we can go even farther afield. It is well known that, .rP,lrt from its basalts, the Newark Supergroup of the eastern rr',rboerrdof the United States is exactly like the Trias of northrvt,st lJurope, and both are now known to have been largely .lr,P1rsiledin fault-controlled basins. The similarities are almost 'Building Stones' of the basal l.rrrghal'rle,even to the extent of the like the remarkably being KctrPcr near Birmingham, England, 'brownstone' houses of much of s,rrrtlstonewhich provided the ,'ltl New York C-'ity.If we go to the High Atlas of Morocco, we Irrrrlt'ven ckrser similirrities,with basic intrusions and extrusions n itlritt tht fanriliar rccl sirlrcistone. acrossthe Atlantic, I lon't'r't r, n'lrt'n I tnakc tltt'st' colttp-r.tris()lts 'platc tt'cttlnics'.Otrvitlusly I r.rr),rlnr()stht.ar tttV rt'.trlt'rssaying:

of Facies 11 ThePersistence

10 TheNatureof the Stratigraphical Record if we closeup the ocean again,the resemblanceswould not be so startling. V"ry well, but then let us go right down to the southwest corner of the United Statesand look at the Moenkopi and associatedformations of Arizona. The glorious colours of the Painted Desert are produced by the same sort of red and green 'marls' as we Europeanshave in our Keuper. The road cuttings along Highway 40 in Arizona show red and green marls and thin sandstones with layers of gypsum, all of which would be perfectly at home along the banks of the River Severn (Figure 1.7). Similar sedimentswith evaporitesare seenin the coresof salt-domesbelow the Cupido Limestone mentioned above in north-eastMexico. \Atrhatis more, a recent stamp from Argentina showed a red sandstone pinnacle in the depression of Ischigualasto(the Valley of the Moon) which a visiting Chinese immediately identified as Triassic.I have sinceseen red Upper Triassicmudstones near Daye in southern China.

THE COAL MEASURES Again continental drift may be held to account for the remarkable similarity of the Upper Carboniferous (Pennsylvanian) Coal

Measureson both sides of what one of the airlines now likes to call the 'Atlantic River'. As a non-specialist,I found it quite easy, with my scanty knowledge of the plants of the British Coal Measures,to identify most of the diverse flora of the famous Mazon Creek locality in Illinois. Perhapsif I had been more of an expert the differenceswould have been more aPParent,but experts always tend to obscure the obvious. Certainly there are differences,especiallyin the better develop' ment of marine sedimentsin the American Pennsylvanian,but these in a way have obscured the resemblances;for work in America has concentrated on the marine fossils, whereas in Europe we have usually been forced to fall back on the nonrnarinefaunas and floras. It is now known, however, that as with the plants, the non-marine bivalves of the American Mid-West .rre very like those that extend from Ireland to Russia. \A/hateverthe vertical and lateral changesin the Coal Measures, we still have to accountfor a generalfaciesdevelopmentin Late ('arboniferous times that extends in essentiallythe same form .rll the way from Texasto the Donetz coal basin, north of the ('aspian Sea in the USSR. This amounts to some 170" of Iongitude, and closing up the Atlantic by a mere 40" does not rt'ally help all that much in explaining this remarkable Phenomenon. One looks in vain for a similar geographical situation at the present day. The nearest approach I can think ol' is around the deltas of south-eastAsia.

LOWER CARBONIFEROUS LIMESTONE

Figure7'7 Road-cutting in o'!,;uTr;;:"{'#;';riiKation'

Highznav 40 near

ln llritain, the greatlimestonedevelopmentof Early Carboniferous 'Mountain Limestone' lMississippian)age used to be calledthe lrt'causeit formed so much of our upland scenery.In the early tlays of mapping in the United States,geologists(no doubt with ,r liurope-oriented education)had no difficulties in tracing the t,rrniliar'CoalMeasures'and the 'Mountain Limestone'ofwestern lrtrropefrom the Appalachiansright acrossthe Mid-West. 'fhe 'Carribre Napol6on'in the Boulonnaisregion of northern 'Empire Statequarry' l;r.lnce(Figure1.8) looks exactlylike the irr lncliana(Figure1.9).Bothusecirculatingwiresto cut smooth limt'stone,and whereasthe first l.rt'r'sin the IrarlyCarbtlniferous

12

The Nature of the StratigraphicalRecord

of Facies L3 The Persistence

ErnP"f;f;;Quarry, limestone, I igure1.9 Mississippian

Figure 1.8 LowerCarbbniferous limestone,CaruiireNapol1on,nearMarquise(Pas de Calais),France(DVA)

was used to build the high monument to the Grande Arm6e that overlooks Boulogne (With Napoleon at the top firmly turning his back on England), the Indiana quarry produced the stone facings for the Empire State Building in New York. AII the physiographicalfeatures of the Mid-Western Mississippian are familiar to the man from the English Pennines or the Mendips. The Mammoth Cave of Kentucky is nothing more than a rather larger Americanisedversion of Wookey Hole in Somerset or the Dan-yr-Ogof cavesin South Wales. However, this is a casewhere the stratigraphical wood cannot be seenfor the nomenclatural trees. Whereasthe British, in their old-fashioned way, have usually stuck to the general term 'CarboniferousLimestone' to cover all the varied carbonatefacies of this age, the Americans-for very good local reasons-have

Indinna, nearBloomington,

,rllowedthe proliferation of formation names to obscurethe unity ol the whole. So the Early Carboniferouswas again a time of very widespread r.rrbonate deposition. Not only were limestones deposited in l rrropeas far south as Cantabriaand right acrossthe Mid-West, tlrt'y also went a lot further. Thus in Arizona, the Redwall I irrrcstoneof this age forms the steepest cliff in the Grand ( ,rrlyon (Figure 1.10), and the name refers to the red staining .l tlrc rock from the overlying Permo-Triassicred beds, just as rrr the Avon Gorge at Bristol, the topmost Carboniferous deposits. I rrnt'stoneis reddenedby the overlying Permo-Triassic right up in the Canadian Rockies,the Mississippian "rnrilirrly, l(rrrrcllcLimestoneforms an impressiveescarpment,for example ,rl',o\,(' the town of Banff in Alberta (Figure1.11).In Alaska, it r.. llrr' l,isburneLimestone,with very similar characters. \Vt. crrn also trace the early Carboniferouslimestones in the 'rlll)()sitedirectioninto Asia. Thus in Kashmir, there is a thick Irrrrt,stone of this age, very like its British counterpartand with ,r sirnilarfaunallist. The persistenceof fossilswill be the subject rrl lht.ncxt chaptt'r,but a cautionaryntlte should perhapsbe ..orrrrtk'ti ht'rt.,for thc firmiliaritytlf tht'ftlssilnamesmay merely

1.4 The Nature of the StratigraphicalRecord

The Persistence of Facies 15

(Mississippian), I tturc '1.11 RundleLimestone of Mount forming the escarpment Rundle, aboaeBanff, Alberta, Canada(DVA)

in the 7.10 RedwailLimestone(Mississippian) Figure formi.ngthe.mostobztious.diff Grand Canyon,as seenfrorn the Powell Memoial, Arizona, USA (DVA)

reflect the fact that they were studied by British palaeontologists. I am told that it also occurs in Western Australia. FRASNIAN REEFS Still climbing down the column, the next great limestone developmentwe meet is in the lower part of the Upper Devonian. This is the Frasnian Stageand presentsus with what is, perhaps, the most remarkable exampleof all. This was the heyday of reefs built by rugose coralsand stromatoporoids'In some areasthey started earlier (in the Givetian); elsewherethey lasted on into the Famennian, but in the Frasnian Stage reefs and reef limestones(in their broadestsense)were exPeriencingtheir finest

lr,'rrr. This is true, in a humble way, in the so-called type area , 'l the Devonian in south-west England. It is true in the classic r r,r,lsof Belgium, northern France and south-west Germany. It r,, true in the beautiful karst country of Moravia in central t zr,t'hoslovakia. It is also true in southern Morocco, in the \rrrt'rican Mid-West and in the Canadian Rockies. where the , ,r\'('r'r1ousrocks of these reefs form the most important oil r,,st'rvoirs and the chief source of wealth in the province of \ llrt'rta (Figure 1,.12). lrr Western Australia too, magnificent reefs of this age are ,l,'r t'krped, perhaps the best in the world, notably in the splendid ,,r,rtions of the Windjana Gorge (Figure 1.13).

THE OLD RED SANDSTONE llrr,other great facies of the Devonian is the continental red '.,rntlstonc development which extends across the north of Irrrol'rt.fronr lreland to the Russian Platform. It has also been ,llst ribt'clfronr t'astern Canada with fish remains very like those ol lht'cl,rssic Scottisl'rst'ctions. It crrn also be seen below the

76

The Nature of the StratigraphicalRecord

1n

oa l-\

TG'

ri=

"\s Eq *-n'

*s! o-

$E x'=

i5I

>.,

InEs \..q J,\ q-

oa

F* st!

9.s

.S=

R< i3

.R ^: 6bo

Figure1.12 UpperDeaonian limestone escarpment with reet' deoelopments in thelower (Frasnian) (DVA) part, Chinaryan's Leap,nearCanmore, Alberta,Canada ;.- s-

s2 Carboniferous Limestone mentioned earlier in Kashmir. There. not only does the fish fauna closely resemble that of the Middle Old Red Sandstone in Scotland, but the sediments themselves are said to be exactly like the Thurso Flagstone Group of Caithness. So it is not merely marine deposits that are so incredibly persistent about the earth's surface. It is also found, with similar fish fossils such as Bothriolepis, in Australia.

si gc $v 'n.R

^a> -\

*j

s'S oa E*

MID.SILURIAN LIMESTONES

HO sr m"\

The great time of carbonate deposition during the Silurian Period was what we would call Wenlock time in Britain. The escarpment of Wenlock Edge in Shropshire is formed by a massive limestone

tsS

\x

*,*

;i--:

of Facies 19 The Persistence

Record 18 TheNatureof theStratigraphical (more thinly bedded below) of Mid Silurian age. Minireefs, or 'ballstones', are developed in most outcrops of this Wenlock Limestone, though they nowhere approach the size of the reefs discussedearlier. Similar reef limestones extend up to Scandinavia,where they reach their finest development in the island of Gotland out in the Baltic. On a much grander scaleare the Niagaranlimestonesaround the Great Lakes in North America, where the reefs reach tremendous proportions, such as the splendid Thornton Reef on the outskirts of Chicago.But in time terms, theselimestones are very much of the sameage as those in Europe and, as I have said elsewhere, 'the Niagara Falls are nothing more than the NiagaraRiver falling over an escarpmentof Wenlock Limestone'.

ARENIG QUARTZITES In the Ordovician of my corner of the world, the most remarkable exampleof persistenceof faciesis that of the purple and white

...:

:lji;il , t i::.ll

(luartzites in the lower part of the System. Every British geology student knows about the 'liver-coloured' quartzite pebbles which ,rrt' found in our Triassic conglomerates (referred to earlier) and rvhich are said to have come all the way from the Ordovician t lris Armoricsin and Grds de May of Brittany (Figure L.L4), even tlrough this implies the transportation of pebbles up to 20 or 30cm .li.rmeter for several hundred kilometres up to the English \lirllands. Perhaps it is the distinctiveness of this particular ,rrr.rtomicalcolour that makes us forget the pure white quartzites tlr,rt also occur at this level, even in our own Midlands (as the of west Shropshire). But whether our "tiperstones Quartzite l',.1'lrlescome all the way from Brittany (as still seems possible) { ,r lrom hidden or eroded local sources is almost irrelevant, for tlrl outstanding fact about these quartzites is their persistence. lrorn England they go across to Brittany, where they are seen rr s(,a cliffs and along into Normandy where they form, for ''\,unple, the great escarpment at Falaiseon which stands William tlr,, (-onqueror's castle. From there they go right.down to rrrrtherfl Spain. In Cantabria, the barrier they formed (as the ll,rr rios Quartzite) played a major role for centuries in the military lrr.,toryof the Iberian Peninsula. West from there they are seen r rrr tlrc north coast in the gentler province of Galicia. They appear ,r1',,rin on the south and east sides of the Spanish Meseta and tlrr.rron way down into Africa. There are massive quartzites of ',rrrril.rrtype in the Ordovician of other parts of the world, from Itrrll',,rriato the Canadian Rockies, but in my ignorance I will not r r.,ks.rying that they are of exactly the same age. What is more, tlr. rt'assernblageof continents/ as now envisaged, does not '.rrrrPlifythe picture at all; it makes it far more complex.

THE BASAL CAMBRIAN QUARTZITE

Figure 1.14 Lower Ordoaician Grds armoricain t'ttrnringheadlandnear Carrrnrct CrozottPttrinsuln.Brittnrrv, Itratrcc(DVA)

| \ r'n nr()r(rremarkable than the basal Ordovician quartzlte is the ,'rrr,tlr.rt is found, almost all over the world, at the bottom of tlrr' ( .tnrbrian. Here dating becomes more and more trsthe time spans become longer and longer. One I'r, 'l,lt'rrraticirl r'. tt'rrrPtt'clto get mixed up with arguments about the origins ,,t lilt' ,rrrcl thc l'rcginning of tht' main fossil record, of the r r r v s t r , r ' i o u s ' 1 , i p . 1 1I in, t1t 1' r1v a l ' t l r a t w . t s t l t c c f t t v o u r e r da, n d o f

22 TheNatureof the Stratigraphical Record fauna and flora of great diversity, including ammonites which date it quite precisely as early Kimmeridgian in age. This is wonderful enough; but in the western part of the southern FrenchJura, the Cerin Lithographic Stoneis of similarly limited extent behind reefs, of similar lithology, of similar fauna and flora and of similar age (Figure 1.15). Unfortunately no Archaeopteryxhas yet been found there, but otherwise the resemblance is startling. Curiouserand curiouser, on the other side of the Pyreneesin Spain, in the high escarpmentnear the attractivevillage of Ager, north of Lerida, is yet a third lithographic limestone, again restricted to a very small area and exactly like the other two in lithology, fauna, flora and age (and even yielding a feather). Other similar deposits of about the same age are said to occur

ThePersistence of Facies 23 at Nusplingen in the Swabian Jura, at Talbrager in New South Wales and in the central Congo. The above are selected examples from among many, of discontinuous distributions of similar synchronous deposits. 'fhere are even more examplesof very thin units that persist over fantastically large areas in particular sedimentary basins. l.ithological units of 30m or less in the Permian of western ('anada, have been shown to persist over areas up to 470000km2. The thin basal member of the Trias, about a metre thick, can be found all round the Alpine chain. The occurrenceof banded ironstones around the world in late I'recambrianrocks is well known. Particularly noteworthy is the t,conomicallyimportant Animikie Basin, with the fabulous Mesabi, Marquette and other ranges, at the west end of Lake Superior in North America. Others of about the same age (i.e. about 2000 rrrillion years BP) are the TransvaalBasin in South Africa, the llamersley Basin in WesternAustralia and the Dharwars Series ol' India. All have the banded or varved iron formations that are r'haracteristicof this episodein earth history. Even more remark,rlrlc, however, is the fact that individual bands can be traced ovt r vast areas.Thus in the valuable Brockman Iron Formation rrl the HamersleyBasin, bands2-3 centimetresthick are said to be t orrelatableover an areaof some52000km2and even microscopic r'.rrveswithin those bands can be traced over nearly 300km. l.ike wartime bomb stories,every geologistseemsto have his orvn favourite exampleto cap all others. Like a politician, I may Ir,rve overstated my case to make my point. Those who are l,rst'inatedby the minutiae of stratigraphicalcorrelationmay be lrorrified at my generalisations.But I find myself left with what rrr,rybe called the first main proposition of this book: Al cartaintimesin earth history, particulartypesof sedimentary rttlironment werepreaalentoaer aast areasof the earth's surface. I lris may be calledthe 'Phenomenonof the Persistenceof Facies'. REFERENCES

Figure 7.15 Cerin LithographicStone,Upperlurassic (Kimmeidgian) near Cerin (Ain), Frenchlura (DVA)

'On someTurkishSediments', ,\pit.r,l). V. (1958). Geol. Mag.,Vol.95, u3-u4. I'l'). A trriefnote recording(amongother things)north-westEuropean tvlrt C'halkon the southsideof the BlackSea.

|..J @

HALORELLA ond

HALORELLOIDEA

A i/0 4n \S / t')

(le

^

*9,.\-

-/<\

t) o Rccorded occurrcnce in Uppcr. Triossic O Confirmed occurrencc in Upper Triossic

Y^b v

Figure 2.1 (a) Distribution of the distinctiaebrachiopods Halorella and its closerelationHalorelloidea in the late Triassic

P E R E G RI N E L L A

Figure 2.7 (b) Distribution o/ Peregrinella in the early Cretaceous

N)

M

The Nature of the StratigraphicalRecord

More Gaps than Record 45

andArchaean marinesediments Figure3.7 llnconformitybetwemUpperCretaceous (DVA) gneissof the BohemianMassif, nmr Kolin, Czechoslaaakia

In shallow-water areas, of course, the rates are higher. Carbonatedeposition on the Great BahamaBank is said to have averagedhalf a metre per thousand years, even though most of the sediment is continuously swept into deeper water. The average rate of accumulation of the 6000m of limestone under the Bahamasis only 4 or 5cm Per thousand years. From this disparity in rates, ProfessorNorman Newell deduced that these Cretaceousand Tertiary limestonesrePresentno more than a tenth of Cretaceous and Caenozoic times. These sediments are only slightly compactible, but with others (particularly muds) it must be realised that the thickness of new sediment is not the same thing as the thickness of the rock that results from it. Nevertheless we are always faced with a contradiction between rates of deposition and the known thickness of rock for a particular period of geologicaltime. \Atrhenchallenged with this sort of argument, most practitioners of the doctrine of continuous sedimentation then change their 'Oh no, we just mean without significant ground and say: breaks'. But what is significant? Obviously there are plenty of unconformities where the break is obvious, such as the splendid unconformity between the Upper Cretaceous and the Precambrian of the Bohemian Massif shown in Figure 3.1..

PLIENSBACHIAN KIMMERIDGIAN OXFORDIAN CALLOVIAN BAJOCIAN

O'5m

AALE NIAN T O A R CI A N

: ' I N E M U Rl A l . l

I'igure 3.2 ldcaliscd diagron ol lht cunlenscdsequences presentedin solution hollows '1965 irt lurnssic/irrrr'siulr'sof tutst Sicilv (from infonnatittn in Wendt, antl'1971)

46

The Nature of the StratigraphicalRecord

(IsDre) d'oisans Figure3.3 rhick,weIIr,oo,lflT1l7f;'Xr;:t'67l;aboaeBourg However, as our studies continue, more and more concealed breaks becomeapparent, such as the remarkable situation in the |urassic limestonesof western Sicily, where severalstagesare packed away into thin solution pipes in what otherwise look like unbroken limesto.nesequences(Figure 3.2). 'continuous' Supposewe look at some of these areasof thick sedimentation.Look at the spectacularcliffs of the Lower Jurassic sediment of the Dauphinois trough above Bourg d'Oisans in the French Alps (Figure 3.3). Hundreds of metres of shales and mudstonesrepresentone small part (and I suspectone small part of that small part) of the Jurassic.Here, if anlrwhere,one would think we must have had continuous sedimentation. But what are all those bedding planes?What is any bedding plane if it is not a mini-unconformity? If we really had continuous sedimentation then there would surely be no bedding planes at all. In fact the only time we seeunbedded sediments, apart from comparatively small thicknessesof in-sifu reef development, we can almost always find evidence of the destruction of the bedding planes by recrystallisation or by the burrowing activity of organisms. ProfessorGilbert Kelling has pointed out to me that, in certain circumstances,bedding planes can be produced 'continuous by textural and diagenetic differences within

MoreGapsthanRecord 47 sedimentation'. But I still maintain that most bedding planes show evidenceof a pausein sedimentation, if not actual erosion. Ideally, I suppose, we should look around for the thickest available development of a particular unit if we are to find anything approaching continuous sedimentation.Even then it has been calculated, taking systems as a whole, that the maximum rate of sedimentationwould have been something like 300cm in a thousand years, In fact we have a paradoxhere in that the areasmost commonly cited as those of continuous sedimentation without breaks, such as the late Ordovician-early Silurian of the Southern Uplands of Scotland and the Jurassicto early Oligocene of the Italian Apennines, are also those of thinnest sedimentation. Clearly, in such developmentsthere may be few, if any, erosionalbreaks, but there must be immensenon-depositionalbreaks. And even in deposits such as the flysch of northern Spain or the Polish Carpathians,there is a great deal of evidenceof erosion by the turbidity currents that laid down the sediment. Having a sentimental attachment for them, I cannot resist mentioning the Cotswold Hills of western England where the formation still quaintly known by William Smith's original name of the 'Inferior Oolite', reacheswhat is for us the tremendous thickness of about 30m. These are the Aalenian and Bajocian Stagesof the Middle Jurassicand compare favourably with the equivalent on the Dorset coastof southern England, where the limestonesof this age are condensedinto a mere 3.4m with two obvious breaks (Figure 3.4). But it has long been known from careful palaeontological studies that even in the thick development in the Cotswolds, there is evidenceof two major breaks and a period of folding in what otherwise was a very peacefulperiod in British geologicalhistory. What is more, if we krok farther afield, our magnificent 30 m dwindles into insignificance.In Alaska we read that the Middle Bajocianalone .tmounts to more than 1200m. Again, the childlike wonder appears when we read for t'xampleof more than 2000m of Kimmeridgian (Upper Jurassic) irr New Zealand or more than 3000m of Frasnian (Lower part of the Upper Devonian)in Arctic Canada,or more than 5000m of Arenigian(Lower Ordovician)in westernIreland.\A/hatI think of as a few steps akrng thc beachin the Isle of Wight suffices

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50

The Nature of the StratigraphicalRecord

More Gapsthan Record 51

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I igure 3.7 Stratigraphicalbreakutith boredphosphaticnodulesin the 'stratotype' rtl the Volgian(uppermostlurassic)at Gorodishchinear Ulyanoask,USSR (DVA)

basins of sedimentation in between (Figure 3.5a). If we put in actual thicknesses, the 'axes' are not so obvious, but they are still there (Figure 3.5b). However, if we look at just one part of that story in detail, we find complications. Thus if we trace the zones of the Lower Lias (as it is called) from the Dorset 'basin' to the Mendip 'axis' , we find that 11 of the 12 zonesthin as they ought (Figure3.6) with no signsof any breaksin the succession, but the twelfth actually thickens markedly! Such misbehaviour of stratain their most classicsectionsleads me to have serious doubts (in fact, positive hatred) of the concept of the 'stratotype' so much favoured by many workers on the European continent. This idea of type section for a particular stratigraphicaldivision will be discussedin a later chapter; all I must sayhere is that no type sectionknown to me can possibly

pretend to be representativeof a whole unit of the stratigraphical r'olumn, however small. Keeping, naturally enough, to the Irrrassic(sinceit was the birthplace of stratigraphy), let us take .rsan examplethe Volgian Stage.Previouslythis competedwith the Tithonian (discussedin Chapter 1) for a place at the top of the furassic.A lesserrival was the Portlandian(cum'Purbeckian') ,rl'England,to say nothing of the quaintly named Bononian and llolonian of France. 'lhe 'stratotype' of the Volgian Stageis at Gorodishchi, along thc river from the birthplace of Lenin (Ulyanovsk, formerly \imbirsk). It is an excellentsection,packedwith fossils,but with vt.ry obviousbreaksmarkedby prominent bandsof phosphatic nodules and borings (Figure 3.7). Clearly any break is a tlisadvantagein a sectionthat setsout to typify a whole division

56

CatastrophicStratigraphy 57

TheNatureof the Stratigraphical Record

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(DVA) Eigure4.L LrnterCretaceous flyschnearBielskoBialain thePolishCarpathians

the rest of the Oligocene alone reaches as much as 3000 m. This is the Macigno Sandstone. The story is that slow sedimentation in deep water produced first the Diaspri (Upper furassic radiolarian cherts) and then the Scaglia or Scisti policromi (Cretaceous to Oligocene vari-coloured shales). Then, at the top of the Scaglia,there was a sudden change in sedimentation to a brecciated limestone full of large foraminifera (the Brecciole nummulitich e). This heralde d the incomp arably f aster deposition 'turbidites'. of the Macigno 'catastrophic' Even more deposits were to follow in the Apennines, for with the earth movements indicated by the synorogenic sediments mentioned above, there came into Italy from the west an allochthonous series of nappes. This was 'scaly formerly known as the ArgiIIe scaglioseor clays'. It was thought to be nothing more than a shattered argillaceous deposit, produced by repeated submarine landslipping and carrying with it exotic blocks ranging in size from small fragments (Figure 4.2) up to whole mountains. Some of these are unknown in the autochthonous succession of the Italian mainland, for example the great masses of serpentinite and associatedrocks scen near the west coast. Their size diminishes as tlrt'y t{r't lttrlltt'r (l\/ay

I t.qltrt 1.,i

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Record 58 TheNatureof the Stratigraphical from their sourceareain the Tyrrhenian Sea(Figure4.3). More recent work has demonstratedthat the allochthon is not quite as confusedaswas formerly thought, and the term Argille scagliose has been dropped in Italy (though it is still used in many other parts of the world). The poeticname has been replacedby terms 'chaotic complex' and the pseudo-scientific iuch as 'olistostromes'without really adding to our knowledge, but the essentialpoint is that submarine landslipping is now thought to be only part of the processrather than the whole. Modern thought has reverted to earliernappe theoriesin which sizeable slices of recognisablestratigraphy have been pushed forward considerabledistancesfrom the south-west. At the same time there has been extensive sliding, especiallyat the front of the nappes. But rather than reducing the amount of such sliding, modern ideas seem to have increased7t, fot olistostromesare now recognisedin all parts of the succession,autochthonousas well as allochthonous. This examplewas worth discussingin more detail than most type of 'catastrophic' deposit is now becausetJneArgille scagliose recognisedin many parts of the world, from California to Taiwan. In Turkey, near the Chalk cliff mentioned at the beginning of Chapter 1, there is a great jumble of Cretaceousblocks resting, with a very irregular junction, on Eocenenummulitic marls. This had been described as a thrust, but it is almost certainly a submarinelandslip deposit like those in Italy. In fact, many years ago I was rash enough to suggestthat the great Ankara m1lange itself (humorously called by the locals tilrlil gilaec-a Turkish type version of Lancashirehotpot) might be an Argille scagliose of deposit. I was duly slapped downby -y more knowledgeable tectonic seniors, and tried to forget the brief publication in question, but later work has now led met to suspectthat I might have been right after all. Certainly there are many smashed-up-lookingdeposits around the world carrying huge exotic blocks far from their place of origin. One of the most remarkableI have heard of is in the island of Timor, where there is a deposit, known as Bobonaro Scaly Clay, which extends for nearly 1000km of outcrop, is nearly 100km wide and is 2.5km thick. In fact, if its allegedoccurrence on other islandsis correct,it extendsfor at least1600km. Figure tt'itlr some 4.4 shows a rounded exoticpebble in this c-lt'1'xr5i1,

CatastrophicStratigraphy 59

I r,",

I tgrtrr1.1 l.tttlic rottttdnll,lotl' itt lltt lloltotnroScnlqClay, trcarBobonaro in eastern .ntttt: ttilr !ltt rtttttl,roi,irlitt,\tt srtlr tl lltt lnllottt of llrc ltlock(photogra\thkindly r r. t ) . A r t r l l t t l - t ' l t n r l r s ) I t t t t i , i r l t l' rt rl yl ' 1 1 t l r : , t r[ 1

62

The Nature of the StratigraphicalRecord

CatastrophicStratigraphy 63 that certain sedimentsin the stratigraphicalrecord can be best interpreted in terms of violent storms. By analogy with 'turbidites' he has coined the term 'tempestites' for graded beds of shallow water sedimentsthat may have been churned up by storms and allowed to settle again. They differ from turbidites in their general environmental setting and in the relative paucity of the basal sole marks that record the erosional passageof a turbid current. Professor Kelling used the term first for Carboniferousdepositson the Moroccan Meseta,but I have seen the same sort of thing in Jurassic carbonateson the Polish foreland and they have been describedfrom many other horizons and areas,though not previously interpreted in this way. In fact the literature is now full of storm deposits and my students are finding them everywhere(without any pressurefrom me). One

Figure 4.5 Supposedstorm depositsin Upper lurassic near Imouzzer-des-lda-oua ilistinct storm Tinane, westernHigh Atlas, Morocco;eachot' the bedsreptesents episode,with blackcarbonatebelowand paler carbonateabooe(DVA)

(ltclow)algal of oneof theaboueunits; blacklaminateddcposits Figure4.6 Close-up 'cleaner'sedinuttt (nbopc); mits, possiblyrippedup andgradedin the frontoll''511pr, (l)VA) thc coinis a dirham,2.4cm across

'tcnry,stitc'irr Louer I igure 4.7 Sultpttsed Carboniferous limestonenearCanfranc :lnciur I irrtlrc Slturishltyrun'cs;nnglilnrlilnckfragnrcntsareseenin a palegreymatix, tlrr ulritr cnlcitrcrnckitrg is rtslrtlr.lJlo llrt dcripedltlocks,zphichprobablyrepresent olltr slnllow-u,nltrnlyl rlr'|osils-llrclrnnnnrris 29cm |oug(DVA)

66 TheNatureof the Stratigraphical Record Catskill delta, a flash-flood (itself an example of a modern catastrophicevent) uncovered a whole forest of in-situ Devonian trees up to 12m high. By such means it is possible, within the Lancashire Coal Measures for example, to demonstrate that very rapid sedimentation alternated with very slow sedimentation and that the former was responsiblefor the bulk of at least some parts of the record. If we turn to volcanic deposits,which can hardly be regarded as exceptionalin earth history, we can find many examplesof great thicknessesaccumulatingvery rapidly indeed. At Builth in Central Wales, a complicatedhistory has been worked out for one part of the Ordovician. First, spilitic lavas were extruded, layer upon layer, and weathered to produce a staircase-like scenery (or 'trap topography'). Their weathering gave rise to sandy and pebbly deposits.Then a seriesof keratophyreswere extruded on top of the spilites and weathered into rounded hills with detrital pyritous sand banked up against them. Finally the seaencroachedon this topography producing steepcliffs, inlets, seastacksand sandy or shingly beachdeposits. A reconstruction of the supposed scenery of this time is given in Figure 4.8 (it will be noted that the view is looking westwards and that the shadows are therefore coming from the north, evidently proving that the British Isles were then in the southern hemisphere!). But the most surprising fact about this is that all these events took place during the deposition of a single graptolite zone. Admittedly Ordovician graptolite zones must have lasted much longer than the ammonite zones of the Mesozoic, but nevertheless the time scale is still surprising. The great problem of uniformitarianism was always the amount of time needed to explain what was known to have happened in the history of the earth (including the evolution of all its species)if one could only postulate present processes.James Hutton recognised this right from the start with his famous aphorism 'no vestige of a beginning, no prospect of an end'. CharlesLyell was a student of William Buckland who taught that 'Geology is the efficient auxiliaryand handmaid of religion' and who saw evidenceeverywhere of 'direct intervention by a divine creator', of a 'creative power transcendingthe operirtion of known lawsof nature'.Lyell insteadwantedan t'nrPirir',rl tlrt'ory

Stratigraphy 67 Catastrophic

times,near in earlyOrdoaician scenery Figure4.8 Reconstruction of thesupposed of Sir WilliamPugh Builth,Wales(fromlonesandPugh,1949,by kindpermission Societyof London) and the Geological founded on observation and practical experience that did not depend on any preconceived ideas from the Bible or elsewhere and which provided a methodology-a means of explaining geological phenomena. 'Methodological uniformitarianism', as it is sometimes called, makes the simple assumption (as in all other sciences) of the in'Substantive uniformitarianism' on the variance of natural laws. other hand (towards which Lyell also inclined)presumes uniform rates or conditions. It is this second concept which has caused all the trouble, certainly (for example) in view of what we now know about the very different world of early Precambrian times. But what chiefly concerns me here is simply the amount of time needed for what we know to have happened in the geological past 'normal if we can only postulate processes'. Thus the other great tun iformitarian of the last century-Charles Darwin-estimated verstlylonger periods than are accepted today. For example, he postulated 300 million years since the Cretaceous to allow for thc scooping out of thc Wealden anticline in south-eastEngland. t ritrnrphec'lbecauseit provided a general U rrifornrit.rri.rrrisnr tht.ory tltat rr'.ls.rt ottt't' logit'al atrcl set'rnirrgly'scicntific'.

74

The Nature of the StratigraphicalRecord

Figure 5.1 Laminatedsedimentbeingerodedat the side of a creekin the Wash, easternEngland(DVA)

recently: 'The shelf off easternUnited Statesis covered almost entirely with relict nearshore sands of the (Pleistocene) transgression.' Even where sediment is recorded, it is frequently in the form of sand wavesthat move from placeto place and do not accumulate. Most of the sediment in fact seems to be accumulating close inshore and very little getsto the outer shelf or the deeps. It has been calculatedthat there has been an averageof about 9m of deposition closeinshore during the last 5000years. Coupled with this, however, we have to rememberthe huge concentrationof such sediment in deltas such as that of the Mississippi, where it has been accumulatingat a fantasticrate (perhaps3000m in the sameperiod of time). Similarly, around the mouth of the Orinoco in Venezuela, the area of rapid sediment accumulation is remarkablylimited. Beyond the shelf, accumulationseemsto be concentratedin a comparatively few deep-water basins. All this, of course, is during a time of exaggeratedrelief following the Pleistoceneglaciation.It is for thesereasonsthat sedimentologists have been forced to work to death the few modern examples they have (such as the poor old BahamasBanks)for analogies with ancient sedimentation.

Catastrophic Uniformitarianism75 Years ago, Arthur Holmes made an interesting calculation dividing the present area of sea-floorby the total amount of sediment being brought down annually by all the rivers of the world. He estimated 8xl-0etonnes transported annually to the sea,which works out at 0.025kg per squaremetre of the sea-floor.If the averagedensity of the sediment is 2000kg/m3, the averagerate works out at about 1cm per thousand years. This is even lessthan someof the rates mentioned in Chapter 3. It seemsto me, from a number of recent papers (and from common sense)that the rare event is becomingmore and more recognised as an important agent of recent sedimentation. Papershave been written on 'the significanceof the rare event in geology' and one must never forget the significance of the old truism that given time, the rare event becomesa probability and given enough time, it becomesa certainty. We certainly have enough time in geology. A study of the 1961hurricane 'CarLa'and the 1963hurricane 'Cindy' in the southern United States showed that they had considerably modified both the form of the affectedcoastlineand the distribution of sediments there. The suggestionwas that just as the energy of electrons is discharged in discrete amounts, or quanta, so energy is expended in near-shoresedimentary environments within short time intervals that are separatedby long periods of relative calm. In other words, the changesdo not take place gradually but as sporadic bursts, as a series of minor catastrophes. It has been calculatedthat, in the Gulf of Mexico, there is a 95o/oprobability that a hurricane will pass over a particular point on the coast at least once in 3000years. The maximum amount of sediment likely to be deposited over that period along the coast generally is about 30cm and we know that hurricanes will certainly rearrangethat amount of material. In other words, the rare hurricane is likely to be the main agent recorded in the stratigraphicalcolumn of certain parts of the world, even in our present climatic set-up. Similarly, it has been shown that tsunamis, or 'tidal waves' trsthey were for a long time mis-named,have an immense effect on shorelines, both in erosion and in the shifting of great quantitiesof sediment.To quote a recentauthor on the subject: '. . . the actirlnof tsunamisis short and extremelyviolent . . .'. It has bct'n suggt.stt'cltlr.rt setr-floorsediments as deep as

Record 78 TheNatureof the Stratigraphical However, I would not wish it to be thought that this was necessarilya unique example. Even in the same region of the western United States there was the catastrophicbreaking of the morainic dam of 'Lake Bonneville' (ancestorof the Great Salt Lake) about 30 000years ago. This flooded the wide Snake River Plateau in Idaho with similar effects. Around the world generally one finds similar examples, albeit on less than an American scale, and there are probably many more not yet known in the world's literature. One reads of 52m of debris being depositedin an hour as the result of a cloud-burst. One sees huge deposits, such as the high cliffs of gravel around Embrun in the French Alps, and not far away, the heap of great boulders at Claps de Luc, where half a mountain fell into the valley of the Dr6me one wet afternoon inL442 (Figure5.2). The largest landslide of all in the Alps, that of Flims in Switzerland, is calculatedto have brought down a mass of more than 1,2km3 of material. We know, therefore, that the frequency of landslides is quite enough to account for a major part of the wearing down of new mountain chains. Particularly disastrous, but not uncommon, have been the effectsof landslips and rock-fallsinto bodies of water. In L958,

t landslipat Claps de Luc (Driurrr) itr lltt I'rt:ttcl Figure 5.2 Fifteenth-century Altts(DVA)

Catastrophic Uniformitarianism79 40 million cubic meters of rock fell into Lituya Bay on the coast of Alaska, producing a great surge which destroyed a forest and reached more than 500m up the mountainside on the far side of the bay. In1792, a similar rock-fallinto ShimbaraBay, on the |apaneseisland of Kyushu, causedthree surges which drowned 15000 people. The most spectacularknown to me in Europe were the great rock-falls that occurred from the sheer face of Ramnefjell (Raven Mountain) into Loenfjord in central Norway in 1905and1936, producing waves that wiped out local communities and carried a steamer a considerable distanceinland. Most countries of the world have their records of great natural catastropheswhich changed the local face of the earth. One thinks of the change in the course of the Hwang-ho River in China, which in less than 80 years moved its mouth some 400km from way to the south of Shantung on the Yellow Sea up nearly to Tientsin on the Gulf of Pohai. The Brazos River in Texas is said to change its course abruptly once every 10 years or so. Alec Smith has told me of his studies on the bottom sediment of Lake Windermere in north-west England. The rush of visitors to the area since the popular romanticism of the 'Lakeland poets' has led to major changes in the micro-organismsand the deposition of their remains, due largely to the effects of human effluent. He has therefore aptly calledthe higher layers the 'post-Wordsworthian'.This shows how rapidly a new fauna and/or flora can migrate into an area and change the record albeit on a very small scale. Volcaniceffectshave been even more sudden and disastrous, ranging from the explosion of the island of Krakatoa, between java and Sumatra, in 1883to the even more catastrophiceruption of Santorini (or Thira) in the Aegaean about 1470 BC. This eruption, or series of eruptions, which resulted in the huge collapsed caldera in the sea beside the present island, must have been the greatestcatastropheever witnessedby man and may well have been heard as far away as Britain. The whole cruption probably lastedonly about 100days, but was probably indirectlyresponsiblefor the destructionof the Minoan civilisation on Crete,nearly l(X)kmaway. This seemedto fit in well with I)l.rto's.'rccour'rl of tht' t.ntl of Atlirntis.The correlation

Record 80 TheNatureof the Stratigraphical of the eruption and the destructionwas long disputed, but now seemsto be confirmed. Neverthelessvolcanicbombs were hurled this distance comPared with a mere 40km from the more publicised Krakatoa eruption. It is, however, still very easy io pick up tufa from the latter on the coast of eastern Africa, to which-it took 6 months to float acrossthe Indian Ocean' one of the most spectacularsightsever seenby man must have been the mile-higi (1.6km) fiery cascadewhen a lava flow poured into the Grand Canyon in Arizona. Earlier lava flows, Lefore the coming of man, date back a million years, but since that time the Colorado River has only cut down about 15m' The canyon itself, more than \lzkm deep, cannot have started more than 1,0million years ago, so here too there must have been some very rapid erosion at some time. So we come bacl again and again to the notion of the rare catastrophichappenings playing a major role in the working out of the stratigraphic record as we find it today. Examples of this sort are in direct contrast to what has been, in effect, the subconsciousattitude of most geologistsfor the last 100 or more years.The opposing attitude was perhapsbe-stexpressed by the great Frenih naturalist, the Comte de Buffon, back in the eighteenthcentury.In 1781,he wrote: We ought not to be affected by causes which seldom act, and whose action i6 always sudden and vioient. Thesehave no placein the ordinary course of naiure. But operations uniformly repeated, motions- which succeedone another without interruption, are the causeswhich alone ought to be the foundation of our reasoning.

This may be a reasonable phitosophical argument when we are thinking in brief human terms, but the stratigraphical record and I boih seemto prefer the doctrine of Thomas Osbert 'One crowded hour of glorious life is worth an age Mordaunt: without a name.' The hurricane, the flood or the tsunami may do more in an hour or a day than the ordinary Processesof nature has achievedin a thousand years. Given all the millennia we have to play with in the stratigraphicalrecord, we can expect our perlodic catastrophesto do all the work we want of them' PeterGretenerhas emphasisedthe need to distinguishbetween and I wottltl likt'to adapt anddiscontinuous continuous Processes

Catastrophic Uniformitaianism 81 his list of examples as follows, ranging from biological to astronomicalprocesses: Continuous Phyletic gradualism Erosion by a meandering river Compaction of sediments Reef growth Subsidence Diapirism Uplift Pelagicdeposition Heat flow Sea-floorspreading Magnetic field Cosmic rays

Discontinuous Punctuated equilibria Flash flood Internal collapse of sediments Storm beach Landslip Faulting Earthquake Turbidity currents Intrusions Continental collision Magnetic reversal Meteoritic impact

So the contrasting processesrange through every aspectof what we call geology. Gretenerwent on to suggesta scale,basedon a 95o/oprobability of a particular event happening, as follows:

Regular events Common events Ilecurrent events ()ccasionalevents I{are events

Peiodicity in years 102,i.e. once in a human lifetime L03,i.e. once in human history 1,06.i.e. once in the smallest stratigraphical unit L08,i.b. once in an era 10e.i.e. once in the historv of the earth

Of course it does not matter what we call them, but this rnerely illustrates how the rare event merges into the regular. We must not dismiss the occasionaland rare events as wild speculation;statisticallythey must occur sooneror later, just as sooner or later that chimpartzeewill type Hamlet.All the time rvc must ask ourselves: 'Is the Present a long enough key to lx.netratethe deep lock of the Past?' It is particularly instructive to look at the stratigraphical rt't'ord of our kindred scienceof archaeology.This is close I'lr()ughto us in tinrt' to rltralifyfrlr the 'present'end of the /()p(,rations urrifornrit,rri.rrr lJtrflirrr's c'ltrtrirrt.. uniformlyrepeated',

I

82 TheNatureof the Stratigraphical Record the operations, 'which succeedone another without intemrption', these are the ploughing, sowing and reaping, the building and decay of habitations, the births, marriages and deaths of human history. But they make very little showing in the archaeologicalrecord. It is the floods and the fires, the battles and the bombardments, the eruptions and the earthquakes which have preserved so much of the human story. When the palaceis allowed to decay and the stones are taken away to build the shanty town, then the frescoesand the tesselated pavements, the statues and the beautiful pottery are all lost. It is only when the barbarian reduces the palace to a heap of stones in the desert and slaughters all the inhabitants, that the record of art and thought and everyday life is preserved for us. Think of that most perfect of all archaeological records-the city of Pompeii-where we have everything preserved from the statue in the elegant garden to the beans in the cooking pot, from the political graffiti on the walls to the obscenepaintings in the brothel. These were all preserved by a single, brief catastrophe-the eruption of Vesuvius on the 24 August AD 79. One interesting geological effect of the Vesuvius eruption nineteen hundred years ago was on the famous pillars o] the so-called 'Temple of Serapis' atPozzuoli on the opposite side of the Bay of Naples (Figure5.3). Thesehave long been associated with the archpriest of uniformitarianism, Charles Lyell, who used them as the frontispiece of his great proselytising work, Principlesof Geology.The pillars show borings made by marine organisms several metres above the present highest sea-level and so provide undeniable evidence of marine invasions and retreatswithin historic times. Thesechangeshave been related to volcanic activity in this eruptive neighbourhood. They rose sharply, for example, during the eruption of Monte Nuevo in 1538.In fact what strikes me most forcibly about Lyell's pillars is not their evidence of placid uniformitarianism but rather of episodic'catastrophism'.The borings are concentratedhigh on the pillars but are remarkably lacking on the masonry lower down. A gradual advanceor retreat of the seafrom its highest point would surely have produced pock marks all the way down to present sea-level.Instead they show to my prejudiced eyes that the sea changeswere very rapid indeed.

CatastrophicUniformitarianism 83

I igure 5.3 Lyell's pillars at PozzuolinearNaplesshozaingboringsmadeby maine organismsduring a marineinaasionin post-Romantimes (DVA)

lkrwever, I would not for one moment deny the continuity ,rrrcithe gradualnessof the processeswhich are changing the t,arth. But we must always distinguish between the nature oi the processand the nature of the record. I do not deny rrrriformitarianism in its true sense,that is to say,of interpreting tlrt'p:rst by means of the pr()cesses that we see going on at

88

The Processof Sedimentation 89

The Nature of the StratigraphicalRecord

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an offshore sandbank facies; a lagoonal'facies; an intertidal or deltaic facies; a fluviatile facies; and an estuarine facies cutting acrossthe other four.

The lagoonal facieshas been disputed and the estuarine facies is complicatedin various ways and confusesthe issue, so let us just considerthree of these facies:the offshore, the deltaic and the fluviatile. All three are very well represented as sediments, shelly fossilsand trace-fossils.There is no dispute about this side of the interpretation, nor with the conclusion that the offshore deposits are mainly developed to the north and the fluviatile (continental) deposits to the south. There is disagreement,however, whether or not they can be interpreted as lateral time equivalents of one another. This is expresseddiagrammaticallyin Figure 6.2. In Interpretation 1 (top) we have the 'gentle rain from heaven' approach,with all three

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Figure 6.2 Two possibleinterpretationsof the relationshipsbetueenthe dit'ferent faciesof the Marada Formation, in the Miocene of the Sirte Basin of eastern Libya

basic environments being preserved simultaneously. In Interpretation 2 (bottom) we have the 'moving finger writes' approach, which was basicallythe interpretation put forward by the palaeontologist who followed on in this particular research project. He found a certain amount of fossil evidence that the time planes were not parallel with the lithological boundaries and he cameto the generalconclusion that the faciesto the north are in the main younger than those to the south. Clearly it is difficult to be dogmatic about faunas of this age, when such short time-spans are involved, and it is also difficult to be sure that particular species are more time-controlled than facies-controlled. What is more, it is probably utterly unfair to this particular study to oversimplifyit in the way shown in Figure 5.2, which

100 TheNatureof the Stratigraphical Record Locality B. In such a case(which must be, theoreticallyat least, very common), 'event stratigraphy, is more accurate, in a chronologicalsense,than either lithostratigraphy (in the usual sense)or biostratigraphy. over a period of many years I had a successionof research students working on the Eocenestrata of the Isle of wight, off the south coast of England. No doubt if their theses eve-ntually

Marxist Stratigraphyand the GoldenSpike 101 appear they will contradict my oversimplified ideas on the matter, 'event but this seems to me an ideal place to demonstrate stratigraphy'. Cyclic sedimentation has long been recognised here, with the succession at the east end of the island largely marine, and that at the west end largely continental. Correlation by fossils is, for the most part, impossible. Correlation by lithology always leads one into an impossible tangle. There are basically three kinds of sediment involved (Figure7.2): (i) cross-bedded yellow and buff sands; (ii) laminated beds of alternating layers of clay (or lignite) and sand; and (iii) glauconitic or sandy clays with marine fossils.

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If one tries to correlate one of these (say the laminated beds) from one end of the island to the other, one finds that there are just too many units at the east end. The paradoxical solution, it seems to me, is to correlate the unlike lithologies as shown in Figure 7.2. The glauconitic clays at the east end should therefore be equated with the laminated beds at the west end. 'marineIt may be expressed as a correlation of the degrees of ness'. We are then correlating equivalent points on the cycle. Anyway, it seems to work! This has, of course, been done before elsewhere, but the principle is not, I believe, sufficiently appreciated. Undoubtedly, comparatively sudden and very widespread events, such as major marine transgressions, did occur at various times during the earth's history. The most famous of these is the Cenomanian transgression, exemplified by Figure 3.L, where Late Cretaceous sediments rest, with marked unconformity, on the Precambrian rocks of the Bohemian Massif. To call it the 'Cenomanian transgression' is something of an oversimplification, for it is often Albian or Turonian in age, but a major transgression at about the beginning of Late Cretaceous times seems to have occurred almost all over the world. It presumably represents an epoch of geomorphological maturity and plate stability. Litholtlgical continuity or marine transgressionsor orogenic prhtrsesor irny other physical phenomennn, must always be nreirsurccl(in tlrc irtrsenceof arrything better) against the scale

136 TheNatureof the Stratigraphical Record down beneath the rising Alpine mountains. This would seem to contradict, however, the notion of major transgressionsbeing the much-delayedafter-effectsof an orogeny. The remarkable stability of northern Europe and eastern North America after this mid Cretaceousspasm is surely clear evidence of the episodic nature of some of the plate movements. The persistence of the white chalk facies outside the new mountain belts must clearly indicate a long period when the Eurasian and American plates were not being affected by any major continental collisions. It still remains difficult to explain the Gingin Chalk of farawayAustralia, though one might guess that this is part of the same phenomenon that produced the almost world-wide 'Cenomaniantransgression'.But we must not forget the great flysch troughs which were developed in Alpine Europe through much of Cretaceoustime and which in many casescontinued on into the Tertiary. By this time, the southern part of the north Atlantic was presumably wide open, and I strongly suspect a crack going up as far as east Greenland. I have earlier suggested that the presenceof Tethyan elements (of Late Jurassicand Early Cretaceousage) in Greenland might be explained in terms of an early Gulf Stream sweeping its way into an incipient North Atlantic. So.might also the presence of other Mediterranean genera and species in the more westerly outcrops of Britain. The main opening of the North Atlantic, however, was evidently a Tertiary affair. The volcanic history of that event does not need restatement here, but the complexity of the stratal history makes this part of the column both the most confusing and the most controversial of all. The widespread regression at the end of Cretaceoustimes may be related to three major plate phenomena: (i) the Laramide orogeny along the western edge of the American plate, (ii) the opening of the northern part of the North Atlantic, and (iii) the further grinding together of the African and Eurasian plates to produce the early Tertiary Pyreneeanfolding of Cantabria, the Pyrenees and Provence in southwest Europe.

TheNatureof the Control 137 New east-west fold mountains immediately began to be destroyed again, producing mountains of conglomeratesuch as the fantastic shapesof Montserrat, near Barcelona(Figure 8.3) which is remarkable even in a country of conglomerateslike Spain. The Atlas folding in north-west Africa conforms remarkably with that of the Pyrenees,which provide probably the best evidence we have in Europe of the coincidence of Variscan and Alpine fold belts. The Variscanmassifsof Europe, such as the Spanish Meseta, the Massif Central of France,the Eifel of Germany, the Bohemian Massif of Czechoslovakiaand the Rhodope Massif of Bulgaria are characterised by roughly north-south tensional grabens filled with Tertia! non-marine sediments. They also display all the featuresof a volcanicity that lasted late enough to terrify Palaeolithicman and perhaps to provide him with his fire. Some of the craters look so fresh that one almost expects the rocks still to be warm. But this volcanicity also testifies to the great tensional stresses that were still affecting Europe, producing vast fissure eruptions and long straight lines of volcanic cones, such as the Chaine de Puys near Clermont Ferrand (Figure8.4) and the puy-like volcanoesin the 'CzechAuvergne' on the west side of the Bohemian Massif (Figure 8.5).

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Mtnrtserrat, near Barcelona, Spain; a strangehl shapcd mttuntain of Mioctttc cotrglonreratc(DVA)

138

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The Nature of the StratigraphicalRecord

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The Nature of the Control 139

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Figure 8,4 k ChatnedesPuys, a line of late Caenozoicaolcanoesalong a fracture line in the Auaergne,seenfrom the top of the Puy de D6me,near ClermontFerrand,France(DVA)

The main spasm of the Alpine orogeny came in Oligocene and early Miocene times, with the African plate grinding against Europe and the alpine chains spreading out to the north and south like an opening fan. So much is now known of the Alpine fold belts, the times and forms of their movements, and so much is now being deduced about the relationship of all this to the theories of plate tectonics, that I marvel at my audacity in saying anything at all at this stage. Clearly it is more than just a picture of two continent-carryingplates coming together. As with every theory that seems at first to have an obvious answer to every problem, the beautifully simple picture is

becoming smudged; the two obvious large plates of Africa and Europe are being confused by the 'microplates' of the eastern Mediterranean. The most apparent anomaly is that the Mediterranean, although it has in Cyprus something comparableto a mid-oceanicridge, does not have the magnetic 'striping', with reversals,of other oceans.The easternMediterranean is still evidently an active tectonic area of colliding plates, with Crete and adjacentridges as part of a small island arc parallel to the Hellenic trench. The western Mediterranean, however, is quite different, having alpine folds to the south as well as to the north. Here, perhaps, all the ocean-floormaterial has been carried up into the mountains. Flysch-bearingtroughs and ophiolites are major constituents of the allochthonous elementsin the Alpine chains, including those in the Apennines discussedearlier. A similar belt of Mesozoic ophiolites has been traced from the Taurus Mountains of southern Turkey, just into northern Syria and Iraq and then on through Iran to Oman. These appear to relate to an earlier phase of ocean spreading during Late Triassic times, and were carried to their present positions in nappes formed during the Cretaceous, as in easternEunlpe.