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The nature of the stratigraphical record PDF

82 Pages·1993·19.07 MB·English
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2 TheN atureo f theS tratigraphicRael cord TheP ersistencoef Eacies 3 Similarly, at the other end of the belt, Chalk was later period of geologicalt ime. What is more, there has been no other discovered in south-west Ireland (where it must have been deposit quite like it either before or since,e xceptp erhaps some noticed by the early surveyors,b ut they had evidently been too Miocenec halks which themselvesa re remarkably widespread: scared of their autocratic director to record such an unlikely in the western approachest o the English Channel, in Malta, phenomenon). Later still it was found coveringe xtensivea reas Cyprus and the Middle East and all the way to New Zealand. of the sea-floors outh of Ireland. But now let us climb slowly down the stratigraphical column Now this spread of a uniform faciesi s remarkablee nough, but to see what other widespread facies we can find. it must alsob e rememberedt hat chalk is a very unusual sediment: an extremely pure coccolithl imestone which is almost unique ,URGONIAN' in the stratigraphicacl olumn. Nevertheless,t here is even worse LIMESTONES to come, for on the other side of the Atlantic in Texas,w e find 'l'owards the Austin Chalk of the same age and character, and later the end of early Cretaceoust imes, at the Barremian/ Cretaceousc halks (still contemporaneousw ith the European Aptian level, a massivel imestonew as developedt hat is usually 'Urgonian development) are found in Arkansas, Mississippi and Alabama. t'alledt he facies'.T his can be seeni n Portugal where And most surprising of all, much farther away still in Western it forms the karstic, touristic coastw est of Estoril, with strange Australia, we have the Gingin Chalk of Late Cretaceousa ge, with the sameb lack 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 (personalc ommunication, 1,989t)h at the succession of the Upper Cretaceousin Western Australia is really remarkably like that in Britain. It is virtually identical, for example, with that in Antrim, Northern lreland. Thus glauconitics andso r gaizea re followed by glauconiticm arls or mulattoa nd then by chalk marl and chalk with flints, containing the samei nvertebrateg enera. He suggestst hat the fantastica bundanceo f coccolithsf orming the chalk (ten times what it is at present) would have modified the climate dramatically. Coccolith blooms release dimethyl sulphide which causesn atural acid rain, increasest he albedo 'greenhouse reflecting solar radiation and reversest he effect'. This fits in with my ideas of Nature as being the biggest polluter, since the new-fangled plants first poured that dreadful elemento xygen into our planet's pure atmosphereo f methane and carbon dioxide. Indeed the recent eruption of Pinatubo in the Philippines has spread dust around the world and has outdone SadamH ussain's efforts in setting fire to the oil-wells of Kuwait, which it was feared would have disastrous effects on the atmosphere. Someg enerale xplanationi s surely neededf or such a wide distributiono f sucha uniquef aciesa s the chalkc 'luring,r short I igurr l.l lll'rrrrirlrt *tilrlnniltl ill ('u(nn, srrttl/r-r'rtsS/l trtitr( l)VA) The Persistencoef Facies 5 4 TheN atureo f the StratigraphicRale cord erosionals hapess uch as the Boca do Inferno (Mouth of Hell). In Spain it weathers into the mushrooms, ships and other weird pinnacles of the Ciudad Encantada( EnchantedC ity) near where it shadest he back streetso f Cuenca (Figure 1.1). It then forms the magnificentc liffs at Cassis,e ast of Marseilles.I t dominates the sceneryi n the outer ranges of the Alps (for example, the towering escarpment of Glandasse above Hannibal's route through the sub-Alps). It caPsm any of the textbook ridges in the easternp art of the Jura Mountains (Figure 1'.2).I t is well developed all round the Carpathians, in Czechoslovakia,P oland and Romaniaa nd then through the BalkanM ountains of Bulgaria (Figure 1.3) and acrosst he Black Sea to the Crimea and the Caucasus( Figure 1.4). Typical$, the Urgonian limestonesa re 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 Guernicai n northern Spain).E lsewhere-as in the Jura-corals are more important than rudists and usually it is only a reef limestone in the broadest Gallic senseo f that term; that is to say, a massive limestone without bedding planes and commonly recrystallised.I t also tends to develop in huge lensest hat pass t igura1 .3 l)rgonianl imestonaeb oaeD rvanoaam onasteryn,e arG abroaoB, ulgaria l.rtcrally into other facies( well seen, for example, in Bulgaria). Sirrcet he publicationo f 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 comparablef aciesa lso extends down into northern Af rica. 'l'lre 'Urgonian' term has become almost a dirty word in 'rt.t.rcct)us ( stratigraphf , for it is not one of the internationally- .tt'r'cp'rl1s'tda gt'namesa nd it is said ttl be a diachronous,s outhern FigureL .2 L)rgonianli mestoneca ppinga nticlinalr idgeo f Montagned c la Bnlnrctr cor l,rt'iesA, dnrittedlyi t is diachron()usb,u t hardly by more than Bellegardien the Frenchl ura (DVA) 6 The Nature of the StratigraphicaRl ecord TheP ersistencoef Facies 7 aptychi, nautiloid jaws, specialisedb elemnitesa nd the aberrant brachiopodw ith the hole in the middle Pygope.Int he Jura the 'fithonian can be seeni n placest o be a coral reef limestone, but usually the coralsh ave been obliterated by dolomitisation and cledolomitisationa s is commonly the fate of reefs. It might alsob e said that the Portland Limestoneo f southern Irrrgland is the Tithonian limestone in yet another form, with what is mainly a molluscan fauna of limited diversity. But the 'Tithonian', tcrm though not quite so lacking in respectability 'Urgonian', .rs the is usually reserved for the carbonatef acies 'Volgian' of alpine Europe, and for years competed with the the Ironour of being the acceptedin ternationalt erm for the topmost st;rgeo f the Jurassic. In its reef or reef-like facies, the Tithonian continues round thc Alps, the Carpathians, the Balkan Mountains to the ('.rucasus.W here best developed,f or examplea t Stramberk in ('zcchoslovakia,i t contains a rich and varied fauna including rrrassivec ompound corals( though the fossilsa re more obvious Figure 7.4 Urgonianl imestonefo rminge scarpmenat,a lleyo f RiaerH eory, Georgia, USSR( DVA) half a stageo r so, and it is still a valid generalisationt o say that a massivel imestonei s developedo ver a very wide areat owards the end of Early Cretaceoust imes. TITHONIAN LIMESTONES Below the towering Urgohian cliffs of southern Europe, there is commonly a second great escarpmentf ormed by a massive limestonea t the top of the Jurassics uccession.A gain it runs from northern Africa through Spain and up into the Alps. It forms, for example,t he cliffs of the Porte de Francea t Grenoble. It also capsm any of the Jurab ox-folds,w here the Cretaceouss uccession has been eroded away. Here it illustratest he point that it is the deposition of carbonatet hat is important and not the precise nature of the sediment. Thus in the outer Alps, the Tithonian 'lrttn I igurr L5 I'ithorriutrl inn'slonrc npyittgl urnssits' rrrlr'ssirunr t tltt' Gates', uthere is commonlyi n the form of aCalpionelllaim estone,w ith abundant tltr l)uttulu'llotos lltrou'tlt llrc ('nrynlltittrs ltcltt,t,tuR onwria fun the right) and tintinnids and such otherwise-unusualf orms as anrlnonitr- Yljrrs/rtt,ir(to tr lltt hlt) (l)VA)

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