The nature of the stratigraphical record

82 Pages·1993·19.07 MB·English

by  Derek Victor Ager

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)