J. geol. Soc. London, Vol. 138, 1981, pp. 159-166. Printed in Northern Ireland.

Major marine cycles in the

Derek V. Ager

SUMMARY: The cycles postulated byFischer (1979)are considered inthe light of the Mesozoic stratigraphical and palaeontological record. It is suggested that the most important events were:(i) the late regression;(ii) the late mid- transgression;(iii) the late late-Triassic transgression; (iu) the late early transgression; (U) the late mid-Jurassic transgression; (ui) the end-Jurassic regression; (uii) the late early transgression; (uiii) the end-Cretaceous regression. These are related to episodicityin ocean-floor spreading, climatic changes and major happenings in the history of the shallow marine benthos.

A number of authors (e.g. Terry & Tucker 1968; Vogt poor marine record of the earliest Triassic. Only in a 1972) have suggested cycles of c. 60 Ma duration in veryfew places (e.g. E Greenland,Transcaucasia, the history of the Earth, though they have differed in southern China and the Salt Range) do there seem to their postulated causes. Following a suggestion from be the very first Triassic shallow water faunas. Even Christopher Walley, I indicated (Ager 1976) how such when the highest Permian stage is immediately over- cycles might be related to majorchanges in the history lain by the lowestTriassic stage, there is often evi- of life. In his first abstract for this meeting,Fischer dence of uplift and erosion between them, as in the (1979) proposed cycles of 32 Ma and in this paper I section from the Ladakh Himalayas (Bassoullet et al. considerhow cycles of such anorder might be 1978). The earliest Triassic seas were confined to the reflected in the Mesozoic part of earth history. Pacific, eastern Tethys, an Arctic embayment and (on The mostobvious phenomena which might show the evidence of Greenland and Spitsbergen) perhaps such a pattern are marine transgressions and regres- anearly split into the northern Atlantic. Fischer sions and the succession of shallow marine faunas. So suggested that this wasa time of abnormalmarine far as the Triassic is concerned, detailed stratigraphy, salinities, which had a disastrous effect on the marine correlation and faunal successions are not sufficiently faunas of the time. well known on a world-widescale, butHallam Generallyspeaking, major regressions in the (1963,1969,1978)has argued repeatedly for major Mesozoic do not seem to have seriously affected shal- widespreadeustatic changes in sea-levelduring the low marine benthos and it may be significant that the Jurassic; Cooper (1977), Hancock & KaufTman (1979) earliest Triassic faunas at the few localities mentioned and others have argued similarly for the Cretaceous. I above are remarkably like those of the highest Per- wish to emphasize the most important eustatic rises mian. Thus they include Palaeozoic-type , and falls in sea-level for all threeperiods and how suchas productids and chonetids. This was em- these affected the distribution and evolutionof shallow phasized recently by Zhuo-ting (1980) in his descrip- water benthos, especially brachiopods. tion of the lowermostTriassic faunas in southern China. However, the world-wide ‘salinity crisis’ may The beginning of the Triassic have made this a special case and been responsible for the of the vastmajority of the existing The persistentproblem of defining the Permian- brachiopods, together with other shallow benthos such Triassic boundary around the world is the very wide- as the blastoids andrugose corals. Certainly spread regression at the endof Tatarian times. Even in stenohaline forms seem to have been most affected. classic sections such as those of the Guadelupe Moun- tains of Texas and New Mexico or those of the west- The Ladiniau transgression ern Urals, the top of the Permian is lost in red beds. Thus Mstislavskiy (1977) defined the base of the Trias I haveargued previously that the most important over a vast area of the East European Platform andE event in the Triassic history of Europe was the trans- of the Caspian as the point where theregressive phase gression of the Upper Muschelkalk (Ager 1970). This of the Tatarian is replaced by the transgressive phase may be dated in chronostratigraphical terms as Ladi- of the Induan stage. Similarly in Australia, for exam- nian and in chronometric terms as 210 Ma. This is c. ple, erosion and non-marine deposition were general 30 Ma after the late Permianregression and c. 30 Ma at this level, apart from marginal early Triassic incur- before the Toarciantransgression. As I saidin the sions, most notably in the Fitzroy Trough in the NW abovepaper: ‘We are probablybetter justified in (Ludbrook in Moullade & Nairn 1978). referring to the Ladinian transgression in the Triassic The widespread nature of the regression at the end rocks of this country than we are to the Cenomanian of thePermian is demonstratedby the remarkably transgression in the Cretaceous’.

0016-7649/81/030(M159$02.00@ 1981 The Geological Society

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/2/159/4886437/gsjgs.138.2.0159.pdf by guest on 27 September 2021 160 D. V. Ager However,this is very muchlooking at the world cal to subspeciesoriginally described from theAu- through European spectacles. The continents in gen- strian Alps (Ager 1968). eral tell us very little about Triassic seas. Arkell (1956) The widest extent of this late Triassic shelf sea, prior spoke of marine Jurassic deposits as‘relics of marginal to the next regression, was in the Norian and charac- lappings of thesea around the edges of the terizedby particularly large forms. Apartfrom the continents. . .’. This is all the more true of the Triassic. halorellid brachiopods, the most distinctive elements Thus Africa has hardlyanything marine to show of in the shallowmarine benthoswere the giant this system and the United States only has traces in megalodontidbivalves. They may be an example of the extreme W. The same is true in Australia. In Asia the principlesuggested by Cooper (1977) that we think chiefly of the new faunas described from the (through the operation of Cope’s Rule) transgressions far E of the U.S.S.R. Even Europe would look rather are likely to becharacterized by small forms and pathetic to (say) a Triassic ammonite specialist, were it regressions by large ones. not for the greatslices of Tethys thrust up from the S. Sloss & Speed (1974)included the whole of the The end of the Triassic Triassic (and the whole of the Permian) in the middle of their ‘Absaroka oscillatory episode’. They regarded The Rhaetian episode that followed was of an unusual this as a prolonged transition between submergent and nature in many parts of the world,with distinctive emergent states, characterized by cratonic rifting and facies(notably sulphurous black shales), under-sized plate convergence at island arcs. The former iswell faunas and mass . Thus the ammonites all seen in western Europe in mid to late Triassic times but disappeared at this time, as did the megalodontid and the latter in the S around the Mediterranean. bivalves; many groups of brachiopods (e.g. the spire- However,Triassic events within the Alpinebelt bearers)suffered near eclipse. Gustomesov(1978) were, onthe whole,remarkably peaceful. This was showed that it was also a critical point in thehistory of emphasized by Triimpy (1971) concerning the gentle the Belemnoidea.Since it wasalso a time of wide- downwarping in the western Alps that startedin Ladi- spread transgression it may well have been the climax nian times. In the far E of the U.S.S.R. this seems to of one of Fischer’s ‘G-State’ episodes (1979) with high havebeen the time when massifssuch as that of sea level and ‘oceans sluggishand subject to anoxia’. It Kolyma were first flooded (Beznosov et al., in Moul- was a state of affairs that was to be repeated (though lade & Nairn 1978). withfewer ill-effects on pelagicfaunas) in Toarcian Most of the Triassic aroundthe world is poorly times. It is noteworthy that rich benthonic faunas only dated but the Ladinian transgression leading to late survived for awhile very locally in Rhaetian times, Karnian to Norian carbonate platforms can be recog- mostnotably in the Kossenerschichten of Bavaria, nizedwidely, simply fromthe occurrence of typical such as the diverse faunas monographed benthonic faunas. Thus in Kashmir,richly fossiliferous byPearson (1977), none of whichoutlasted the Ladinian strata with brachiopods and bivalves domi- period. nate the story and pass up into marine Upper Triassic In parts of the world not reached by the peculiar beds.Something similar is seenin New Guinea Rhaetian sea, the Triassic-Jurassic boundary is usually (Brown et al. 1980) where Ladinian marine strata rest only recognizable by palynomorphs. Continental beds directly on the Palaeozoic, and carbonates were de- and even coal measures arevery common from south- veloped in the late Triassic. ern Scandinavia to Australia. The continental troughs The Halorella-Halorelloidea group of highly dis- of the Newark Group down the E coast of North tinctivebrachiopods is widespread in thesestrata America are now known to range into the early Juras- (Ager 1968). Their distribution, however, indicates the sic (Lindholm 1979) and radiometric dates of that age limited nature of the epicontinental seas. Dagis (1965) are known from intrusions such as the Pallisades Sill, describedtypical examples from NE Siberia.It is New York. The dominant theme of the late Triassic, noteworthythat the faunas here, veryclose tothe from Argentina to China, was regression, red bedsand North Pole of the time as postulated on palaeomagne- evaporites. tic evidence, are typically ‘Tethyan’ (and by implica- tion tropical). Presumably this was a time of climatic The Toarcian event optimum with the carbonate belt at its widest. It is probably significant that these faunas did not reach the Blackshales are the mostobvious feature of the western end of the Mediterranean, i.e. the Betic Cor- Toarcian over most of Europe, from Britain to Bul- dillera or the Maghreb. Aubouin et al. (1977) referred garia. It seems to have been the climax of another of to a‘reconquest’ of thisend of theTethys in late Fischer’s ‘G State’episodes with high sea level, the Triassictimes, though the facies are comparable to ocean currents retarded and the bottoms poorly oxy- those of the Muschelkalk of northernEurope. genated.Pelagic organisms dominate the Toarcian Halorella did,however, reach Cordilleran North story. Where brachiopods occur they are scarce, scat- America, where forms in Oregon are virtually identi- tered and under-sized, such as the tiny Nannirhynchia

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and ‘’globulina of southernEngland. In the former whichmay indicate a climatic optimum. western Morocco the sea camein briefly from the new The Bathonian regression was merely a prelude to the Atlantic to the W and with it a few stunted shells next and most important event. (Ager 1974). Only in theIberian peninsula and in adjacent partsof NW Africa (from northern Tunisia to The Callovian transgression SE Morocco) did ‘normal’ conditions persist in Toar- cian times, with shallow water limestones and abun- The most outstanding event of the period, if not the dantbenthos. This may becompared with the era, was a world-wide transgression that began in the Kossenerschichten situation in the Rhaetian. Callovian and continued through the late Jurassic. In The oldest pelagic limestones or uniform shale se- the Sloss & Speed (1974) terminology,this was the quences in the Alps frequently belong to the Upper beginning of the ‘Zuni submergent episode’ that was Pliensbachian(Triimpy 1971), butoutside the Alps to last until the end of the Mesozoic. They correlated there is no doubt that the Toarcian marked thebegin- it witha sudden increase in the rate of ocean-floor ning of a major submergence. spreading. Hallam (1969) claimed that the elevation of Thatthe seas deepened at the beginning of the the Darwin Risein the Pacific was more than sufficient Toarcian is implied by the widespread change in facies to account for this eustatic rise in sea-level which he from the shallow, ferruginous carbonates of the late had earlier described (1963) as a progressive process Pliensbachian and the arrival of new ammonite faunas, throughout the period with temporary setbacks, nota- such as the northerly spread of dactylioceratids from bly in the Aalenian and Bathonian. Tethys at this time. Elsewhere the seas extended to This was the beginning of the ‘Bathyal Lull’ in the covervast new areas suchas theDneiper-Donetz Alps (Triimpy 1971) when reduced sedimentation of Basin and the Siberian Platform (Beznosov et al., in pelagic cherts and micritic limestones took over from Moullade & Nairn 1978). They also spread down the the muchmore varied andthicker deposits of the E African Seaway for the first time from the Middle Bajocianand Bathonian. Presumably general sub- East to Malagasy (Ager 1975a). mergence cut off the supply of land-derived sediment. The general record of pelagic rather than benthonic The Callovian transgression took the sea for thefirst faunas in other parts of theworld implies that this was time over a major part of the Russian Platform, and thegeneral state of affairs. There werewidespread Jurassic seas reached their maximum extentin the late extinctions of sessile benthosat this level, as em- Oxfordian (Hallam 1969, 1971). This was also when phasizedby Hallam (1972). In particular Spiriferina, carbonatedeposition was most widespread (Ager the last of the great host of spire-bearing brachiopods, 1975~).Reefs spread over North Africa and Europe, finally disappeared,as its ancestors had so nearly evenreaching as far as central England, though succumbed in the similar black shale episode in the perhaps only for as little as 400 years (Ali 1977, pers. Rhaetian. Only in the refuge of eastern Spain did they comm. 1978). The same Callovian-Oxfordian sea in- manage to survive a little longer, outside the suffocat- vaded the area that is now the western High Atlas of ing environment of the black muds (Thomas 1978). Morocco(Ager 1974); it spreadinto Mauritania, In theMediterranean, the Pliensbachian-Toarcian Senegal and even S of the present equator into Gabon. interval marked the break-upof an extensive Triassic- Simultaneously on the opposite sideof the new Atlan- Liassicshallow-water carbonateplatform. This has tic it laid down the invading Smackover Formation of been thoroughly documented by Jenkyns (e.g. 1970). Texas and the Zuloaga Formation of northern Mexico Block-faulting was followed by submergence and bur- with many features in common (Oivanki 1974). The ial in pelagic sediments. Hallam (1972) suggested that valuable new oil resources of Yucatan and thereabouts a general eustatic rise in sea level in early Toarcian in southern Mexico come from reef developments of times was adirect consequence of the coming into the same age. This was the ‘Jurassic reconquest’ at the being of a mid-Atlantic ridge. This may have been the western end of the Tethys of Aubouin et al. (1977), realbeginning of theAtlantic, though it mustbe though there mustremain some doubt about the emphasized that oceanic crustof this age is not known. Lower Jurassic deposits of Veracruz in Mexico. There It mustalso besaid that this was nomore than a is no evidence of anearlier Jurassic transgression relatively downward shift of the land surfaces within a comingfrom theE, and I havesuggested (Ager generally oscillatory episode. It fits in the 32 Ma cyclic 1975a) that such embayments may have come from pattern. the Pacific. By mid-Jurassic times the dominant theme was re- Much critical evidence on these various seaways lies gression,especially in theearly Bathonian (Hallam in the record of shallow marine sessile benthos. Thus 1969) just about 30 Ma after the regression at the end very similarbrachiopod faunas extend for the first of the Trias. Sandstones and shallow-water limestones time from E Africa to theArabian Peninsula, to India, are by far themost characteristic rocksof the Bajocian to NW Africaand even into NW Europe.Certain and Bathonian, from the Cotswolds to Kenya, accom- elementsreached Mexico,as did the reef corals panied by significant reef developments and oolites in (Beauvais 1974).

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When the ‘Smackover Sea’ was invading the Gulf boundarywithout significant evolution or extinction Coast region of NorthAmerica, the ‘SundanceSea’ (e.g. the brachiopods, Ager 19756). was spreading down the western states from the N as A brachiopod distribution thatis very reminiscent of far as the Four Corners region. that of Haloreila inthe late Triassicwas that of Againseas were transgressing at this time as far Peregrinella in the early Cretaceous (Ager 1968). This away as Papua New Guinea (Brown et al. 1980). The spread via the eastern European Tethys, Tibet (Hu, E African seaway deepened and extended southwards 1978, pers. comm.), Siberia (Dagis 1968) to California and marine deposition reached South Africa, Antarc- without reaching the western end of the Mediterra- tica and the Falkland Islandsin the late Jurassic (Ager nean. 1975a; Dingle, in Moullade & Nairn 1978). The most impressive rocks in the Lower Cretaceous This has been suggested (Hallam 1969) as the alter- are the ‘Urgonian’limestones thatextend from one native date for the first opening of the modern Atlan- end of Europe to the other, dominating the scenery tic, andthe oldest sediments that are somewhat outside the central alpine belt from Portugal to Geor- oceanic in type, as well as position (in and around the gia. This seems to have been an acme of carbonate Cape Verde Islands) are of late Jurassic age. deposition,at the Barremian-Aptian level, andthe widest extent of the early Cretaceous sea prior to the The end-Jurassic regression major transgression that was to follow. I wasastonished in NE Mexico to finda similar Afterthe widespread carbonatesand climatic op- massivelimestone with similar rudist reefs-the timum of lateJurassic times there wasageneral CupidoFormation-at thesame level and similarly regression of the shelf seas and the Jurassic-Cretaceous dominating the landscape above the basal Cretaceous junction is nearlyeverywhere hidden in continental Taraises Formation. This was when early Cretaceous deposits,whether it be Wessex,Morocco, or Papua epicontinental seas were most widespreadin Australia New Guinea (Brown et al. 1980).Important earth and has been similarly blamed on the initiation of new movements occurred all round the Pacific as well as in oceanfloor ridges (Ludbrook in Moullade & Nairn more familiar places such as the North Sea. 1978). On the Russian and Siberian platforms there seems It wasalso thetime whenblack, bituminous and to have been a more or less direct passage from the sulphurous deposits were particularly widespread (Jen- Jurassic to the Cretaceous in marine strata(Beznosov kyns 1980), notably in the newly opening S Atlantic. et al., in Moullade & Nairn 1978), but the Russians This may have been responsible (like the earlier black have noted a marked southerly shift of the climatic shale events) for the virtual extinction of brachiopod zones at the end of Jurassic times, as deduced from groups such as the cyclothyrids and pygopids. floral distributions (Vachrameev 1978), though this is not so evident in Asia. This seems to be confirmed by The ‘Cenomanian’ transgression the distribution of red beds and gypsum, which disap- peared generally by about Albian times with a gener- There is no formal division of geological time known ally damper climate in what was previously the arid as the mid-Cretaceous, butif there were, it would be a zone. This may be related in turn to the beginning of very important episode in the Phanerozoic history of the marine transgressions that were to dominate later the continents. We tend to thinkof it as the ‘Cenoma- Cretaceous history. The widespreadbauxites of this nian transgression’, although the inverted commas are time point to a warm humid climate over large areas, needed because we know that it is usually not exactly but these largely came to an end with the late Albian, Cenomanian in age. In southern Britain, for example, when there is general evidence of cooling. it started in the late Albian and continued, somewhat Undoubtedly the seas of Cretaceous times were as episodically until the sea reached its maximum in the oscillatoryas those of the Jurassic. Cooper (1977) Campanian (Hancock 1969). This late Cretaceous sea postulated as many as 13 or 14 transgressive episodes spread out over nearly half of the United States and a between the late Valanginian and early Maastrichtian, large part of western Canada. In the Western Interior separated by episodes of eustatic regression. He as- of North America there seem to have been two com- sumed a direct connectionwith the rate ofocean-floor plete cycles of transgression and regression contrasting spreading,and implied a marked episodicity in that with only one in Europe, but the latter may be only process. Fischer’s ‘G-state’ episodes mentioned above a superficial impression, with the complexities of the are represented by the anoxic events in the Cretace- story hidden by gaps in the record (Hancock 1975). ous described by Jenkyns (1980). Thedouble cycle may berepresented in the two The seaways that were re-established after the end- furtherepisodes of anoxicconditions postulated by Jurassic regression were, on the whole, very similar to Jenkyns (1980). This transgression was less marked in those of late Jurassic times. The fauna1 changes at the the U.S.S.R. than in the world in general, probably end of theJurassic are most obvious in the Delagic due to the Cimmerian movements (Hallam 1977). faunas. The sessile benthos passesthrough the Hallam(1969) suggested that the marked eustatic

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rise during the late Cretaceousmay have resulted from 75% of the species living on the planet at the time oceanic ridge uplift. Similarly, Hays & Pitman (1973) became extinct. argued convincingly for a direct connection between The regression at the end of the Mesozoic is almost the ‘mid‘ Cretaceous world-wide transgression and an as widespread as the one at its beginning. Such is the intense phase of ocean-floor spreading withthe forma- emphasis on the classic marinesuccessions (e.g. the tion of mid-oceanridges causing a simple displace- Maastrichtian-Danianpassage in auniform chalky ment of waterover the low-lyingcontinents. This facies in Denmark) that one forgetshow very rare and apparently happened in both the Pacific and the At- local these are. Usually the junction is lost in conti- lantic between 110 and 85 Ma, i.e. 30Ma before the nental deposits, whether one is in Mexico (as in the final regression. interfingering marine and non-marine deposits of the The so-called Cenomanian transgression is the best Difunta Formation), North Africa (as in the valuable known of all transgressions and is well known from the phosphate deposits of Morocco), or Europe (as in the Gulf Embayment to Bohemia, from the Bengal Basin red beds of the Tremp Formation in the Pyrenees). to Western Australia (though Australia generally was Alternatively, there is an erosional break as at the top emergent at this time). Almost everywhere there was a of the Chalk in NW Europe. A break appears tooccur major transgression over continental or shallow water at this level almost everywhere in the Indonesian and deposits of theearly Cretaceous. Often there is a Phillippine archipelagos (Audley-Charles, in Moullade marked unconformity, as on the Bohemian Massif, or & Nairn 1978) and in southern Africa (Dingle, ibid). disconformity, as in SW England. Everywhere there is In the western United States, the earliest marine Ter- a deepening succession up to about the Campanian, tiary strata (the Cannonball Formation) lies far from with the white chalk of Europe matched by the Austin anyimmediately preceding or laterally equivalent Chalk of the USA and the Gingin Chalk of Western strata. Australia. Again regression seems to be associated with large Hancock & Kauffman (1979) concluded that there size among the marine invertebrates, as suggested by were, in fact, 4 transgressive maxima in late Cretace- Cooper(1977). One thinks particularly of the giant ous time, on a world-wide scale. These maxima were ammonites andrudist bivalves of the late Cretaceous. allegedly reached in early Turonian, early Coniacian, In the magnificent section at Stevns Klint in Den- Middle Santonian and late Campanian times, thoughit mark, the Tertiary Danian appears to follow directly is clear that these were merely positive oscillations on on the Mesozoic Maastrichtian in a remarkably similar ageneral transgressive-regressive curve ratherthan facies. Nevertheless,as at many other localities in distinct and separate episodes. Two of them coincide southern Sweden, the actual boundary is marked by a with the late Cretaceous anoxic events postulated by pronounced‘omission surface’ (Bromley 1979). The Jemkyns (1980). famous thin ‘Fish clay’ at the junction almost certainly indicates a virtual cessation of deposition for a long period.There is, in fact,clear evidence of along The end of the era preamble of shallowing fromthe usual pure white chalk tothe bryozoanlimestones that span the The ‘Cenomanian’transgression continued into later boundary. Cretaceous times. Thusthe marginalCretaceous of Wide publicity has been given to the discovery of southern Swedenhas sizeable basement boulders in high levels of iridium in athin clay seamat the the Emscherian(Gmiacian-Santonian) andthe sea boundarywithin the limestone succession of the probably reached its deepest in the Campanian. On Apennines near Gubbio (Alvarez et al. 1979), and the palaeobotanicalevidence, Vachrameev (1978) con- even higher levels in the similar seam just mentioned cluded that temperatures also reached their maximum at the same level in Denmark. The presence of simi- in Campanian times (when the transgression was at its larly high levels in chrondritic meteorites has led to maximum), after which there was progressive cooling the suggestion of an asteroidimpact at the end of that was most marked in the Danian, at the beginning Cretaceoustimes with much dust thrown into the of Caenozoic times. atmosphere, the cutting outof light for photosynthesis The mostremarkable event of the Mesozoic Era and the starvation of the dinosaurs. It is a pity that was undoubtedly that which marked its end. Russell those over-publicized creatures so much dominate the (1979) surveyed this episode and drew attentionto the public imagination at this important level. It is difficult greatprecision with which theboundary canbe to see how such an event could have simultaneously defined in places as scattered as Mexico, Italy, Tunisia, extinguished the vastly moreabundant ammonites, Pakistan,Arctic Canada, Nigeria and Brazil. Apart belemnites,planktonic foraminifera, coccoliths and from the well-known extinctions, these boundaries are benthonicforms suchas the rudists and nerineids. also clearly defined by lithological changes. Estimates What is more, the land floras which are so important of the length of time involved range from 1000 yrs to in the above arguments show remarkably little change 1.5 Ma. During this time it has been estimated that apartfrom the extinction of the cycadeoids, which

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/2/159/4886437/gsjgs.138.2.0159.pdf by guest on 27 September 2021 164 D. V. Ager were probably the main foodof the last dinosaurs such recent analyses of radiometric and other dates (nota- as Tricerutops (Ostrom 1964). It is surprising that no bly van Hinte 1976~and b) placethe chief events controlstudies seem tohave been carried out on specified herein as follows: similar sediments at other, less catastrophic levels and MU that no consideration appears to have been given to R Beginning of Triassic 230 the relationship between rate of sedimentation and the T Beginning of Ladinian 210 accumulation of theconstant shower of meteoritic C Norian 205 dust. Clearly the thin argillaceous seam with its con- R End of Triassic 192 centration of iridium at the boundarycould amount to T Beginning of Toarcian 178 nothing more than a long halt in deposition, following C Bajocian 166 the disappearance of the maincarbonate depositing R Bathonian 160 organisms. T Beginning of Callovian 156 Without wishing to deny the possible importance of C Tithonian 140 meteoritic impacts in thehistory of the earth, it seems R End of Jurassic 135 far more likely to me (see Ager 1976) and to many T Neocomian 120 other workers (e.g. Van Valen & Sloan 1977) that the C Urgonian 105 end of the Cretaceous was a climatic event brought T Beginning of Cenomanian 100 about by major regressions inmany parts of the world. C Campanian 75 The contrast with the climatic and transgressive max- R End of Cretaceous 65 imum in the Campanian is particularly striking. R = Regression. The excellent floral record from the western United T = Transgression. States and the U.S.S.R. for the end of the Cretaceous C = Climatic optimum. Period, notably in the Hell Creek Formation of Mon- tana, showsa clear record of climatic deterioration accompanied by normal community replacement. The The regularity of some aspects of this progression is dinosaurs, for example, were already vastly outnum- obvious,notably themajor regressionsat c. 30 Ma bered bysmall mammals beforethey finally disap- intervals, each following a climatic optimum, with the peared (Van Valen & Sloan 1977). one marked exception of the Cenomanian. Thismakes In the sea the regression and climatic deterioration one wonder if thatevent wasexceptional in some evidently produced a 90% drop in specific diversity of other way. the coccoliths (Bramlette & Martini 1964; Perch- Revized dates constantlyupset the pattern, which Nielsen 1969, 1972).The disappearance of these as the might leadto the conclusion thatthere is noreal basicplankton of lateCretaceous seas produced a cyclicity. However, such are the vagaries of radiomet- collapse of the whole marineecosystem, via foraminif- ric dating and such is the stratigraphical imprecisionof era and ammonites to marine saurians.is It noteworthy the samples used that I suspect that, in the future, the that much of the shallow waterbenthos, such as dates may prove a more regularcyclicity than is appar- brachiopods,bryozoans and echinoderms passed ent at present. This may seem to bean unscientific through the boundary unscathed, though not the rud- approach,but is not so unscientificas uncritically ists, which may have required temperatures that were accepting quantified results, whether they be based on no longer available. isotope ratios or palaeomagnetic directions. Therefore, though I think catastrophic happenings It is acommon over-simplification that transgres- very importanthappenings in the stratigraphicalre- sions were ‘good’ for marine organisms in that they cord, I seeno need to postulate an extra-terrestrial opened up new areas for colonization, whilst regres- one at the end of theCretaceous. A majormarine sions were ‘good’for non-marine faunas and floras. In regression accompanied by a marked deterioration in fact, in the case of the shallow water sessile benthos, climateseems tobe sufficient to explain all the the reverseseems to havebeen the case in the changesobserved in the sedimentologicaland Mesozoic. Undoubtedlythehardest times for palaeontologicalrecords. This in turn may be attri- brachiopodsseem to havebeen at the end of the buted to increased ocean-floor spreading and the rise Triassic (with the extinction of many groups of spire- of mid-oceanic ridges and thence to internal processes bearing forms and terebratulids as well as the mollus- in the earth. canmegalodontids), in the Toarcian (with the final disappearance of the spiriferinids and many groups of rhynchonellids), in the Callovian (with the end of the Conclusions vast diversity of the Bajocian-Bathonian faunas) and the ‘mid-Cretaceous’ (withthe exit of the pygopids and I have summarized what seem to be the most signific- the cyclothyrids). What is more, the great take-overby ant events during Mesozoic times. The most detailed the angiosperms on landcoincided with the world-

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wide Albian-Cenomanian transgression. outside my terms of reference) that cyclic changes in On the other hand, regressive episodes such as the sea-level are even more marked in the Tertiary, when end of the Jurassic appear to have had little effect on ocean-floorspreading rateswere evidently attheir the shallow-water benthos. Even the supposedly all- maximum, with increasing evidence of episodicity (e.g. important event at the end of the Cretaceous did not Girdler & Styles 1974). The transgressions, however, do much to benthonic organisms, as is shown by the do not appear to have been on the same scale as, say, close affinities of brachiopods,bryozoans and those of the Callovian andCenomanian. I find the echinoderms in the similar facies above and below the arguments for episodicity particularly convincing but boundaryin the classic sections of Denmark.The perhaps that is because I see episodicity as the domin- changes,such as they were, seem to havebeen ant theme in the whole geological record. gradual, as they were in the land floras at this time. The Permian-Triassic regressive episode may have beena special case with abnormal salinites perhaps Postscript. My attentionhas been drawn to the fact on a world-wide scale, though even here what littlewe thatthough the idea of anasteroid impact causing know of earlyTriassic benthos is remarkably massextinctions atthe end of theCretaceous has Palaeozoic in appearance. recently received wide publicity through the work of If transgressions and regressions are to be related to Alvarezand his colleagues (as mentioned in this rates of ocean-floor spreading, then much of the argu- paper), the essentialhypothesis was contained in an ment must centre around the episodicity or otherwise important paper by W. M. Napier and S. V. M. Clube of thatphenomenon. It is noteworthy(though it is (Nature, London, 282, No. 5738, 455-9, 1979).

References

AGER, D. V. 1968.The supposedly ubiquitous Tethyan montrant le r6le des Madrkporaires dans la thkorie de la brachiopod Halorella and its relations. J. palaeonfol.Soc. mobilitk continentale. Bull. Soc. giol. Fr. 16, 465-9. India, 5-9 (1960-64), 54-70. BRAMLE~,M. N. & MARTINI, E. 1964. The great change in -1970. The Triassic system in Britain and its stratigraphi- calcareous nannoplankton fossils between the Maestrich- cal nomenclature. Q. J. geol. Soc. London, 126, 3-17. tian and Danian. Micropaleontology, 10, 291-322. - 1974. The western High Atlas of Morocco and their BROMLEY,R. G. 1979. Chalk and bryozoan limestone: facies, significance in the history of the North Atlantic. Proc. sedimentsand depositional environments, In: BIR- Geol. Assoc. London, 85, 23-41. KELUND, T. & BROMLEY, R. G. (eds.).Cretaceous- - 1975a. The Jurassic WorldOcean. Keynote address: TertiaryBoundary Events. I. TheMaastrichtian and Jurassic NorthernNorth Sea Symposium. NorgesPet- Danian of Denmark. Uniu. Copenhagen, 16-32. roleum Soc., Stavanger, 143. BROWN,C. M., PILGRAM, C. J. & SKWARKOS. K. 1980. - 1975b. Brachiopods at the Jurassic-Cretaceous bound- Mesozoicstratigraphy and geological history of Papua ary. Mem. Bur. Rech. geol. minim, 86, 150-62. New Guinea. Palaeogeogr. Palaeoclimafol. Palaeoecol. -1976. The natureof the fossil record. Proc. Geol. Assoc. 29, 301-22. London, 87, 131-50. COOPER,M. R.1977. Eustacy during the Cretaceous; its h,0. E. 1977.Jurassic hazards to coral growth. Geol. implicationsand importance. Palaeogeogr. Palaeo- Mag. 114, 634. climatol. Palaeoecol. 22, 1-60. ARKELL, W. J. 1956. Jurassic Geology of the World. Oliver & DAGIS, A.S. 1965. Triassic brachiopods of Siberia. Izdatel. Boyd, Edinburgh, 806 pp. Nauka. Acad. Nauk. SSSR, Siberskoe Otdel., Inst. Geol. hvm,L. W., ~VAREZ,W., ASARO,F. & ~ICHEL,H. V. Geofz., 126 pp. (in Russian). 1979.Anomalous iridium levels at the Cretaceous- - 1968. Jurassic and Lower Cretaceous brachiopods from Tertiary boundary at Gubbio, Italy: negative results of northern Siberia. Izdafel, Nauka, Akad. Nauk S.S.S.R., tests for a supernova origin. Absfr. geol. Soc. Am. 11, Siberskoe Ofdel.,Trud. Inst. Geol. Geofiz,41, 167 pp. (in No. 7, 378. Russian). AUBOUIN,J., BLANCHET,R., STEPHAN,J. F. & TARDY,M. FISCHER,A. G. 1979. Rhythmic changes in the outer earth. 1977.Tkthys (Mesogke) et Atlantique:donnkes de la Newsl. geol. Soc. London, 8 (No. 6), 2-3. gkologie. C. r. Seances Acad. Sci. Pans, 285, 1025-8. GIRDLER, R. W.& Sm~s,P. 1974. Two stage Red Sea floor BASSOULLET,J.-P., COLCHEN,M,, Gm,J., LYS, M,, IMAR- spreading. Nature, London, 247, 7-11. coux, J. & G. 1978.Permien terminal GUSTOMESOV,V. A. 1978. The pre-Jurassic ancestry of the nkritique, Scythien pklagique et volcanisme sous-marin, Belemnitida and the evolutionary changes in the Belem- indices de processus tectono-skdimentaires distensifsh la noidea at theboundary between the Triassic and the limite Permien-Trias dans un bloc exotique de la suture Jurassic. Paleont. Zh. Akad. Nauk. S.S.S.R.12 (3), 3-13 de 1’Indus (Himalaya du Ladakh). C. r. Seances Acad. (in Russian,translation Paleont. J., Am. geol. Inst. 12, Sci. Paris, 287, 675-8. 283-92). BEAWAIS,L. 1974. Quelques examples pris dans le Malm HALLAM,A. 1963. Eustatic control of major cyclic changes

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/2/159/4886437/gsjgs.138.2.0159.pdf by guest on 27 September 2021 D. V. Ager

in Jurassic sedimentation. Geol. Mag. 100, 444-50. ONANKI,S. M. 1974. Paleodepositionalenvironments in -1969. Tectonism and eustasy in the Jurassic. Earth Sci. Upper Jurassic Zuloaga Formation (Smackover), north- Rev. 5, 45-68. eastern Mexico (abstract). Bull. Am. Assoc. Petrol. Geol. - 1971. Mesozoic geology and the opening of the North 58, 2211. Atlantic. J. Geol. 79, 129-57. PEARSON,D. A. B. 1977. Rhaetian brachiopods of Europe. - 1972. Diversity anddensity characteristics of Neue Denkschr. naturhist. Mus. Wien, 1, 70 pp. Pliensbachian-Toarcianmolluscan andbrachiopod PERCH-NIELSEN,K. 1969. Die Coccolithen einiger danischer faunas of the North Atlantic margins. Lethaia, 5, 389- Maastrichtien-und Danienlokalitaten. Medd.Dansk geol. 412. Foren. 19, 51-69. - 1977. Secular changes in marine inundation ofUSSR - 1972. Les nannofossiles calcaires de la limite Crhtack- andNorth America through the Phanerozoic. Nature, Tertiare. Mem. Bur. Rech. geol. minieres, 77, 181-8. London, 269, 769-72. RUSSELL,D. A. 1979. TheCretaceous-Tertiary boundary - 1978. Eustaticcycles in the Jurassic. Palaeogeogr. problem. Episodes (IUGS newsletter), No. 4, 21-4. Palaeoclirnatol. Palaeoecol. 23, 1-32. SLOSS,L. L. & SPEED,R. C. 1974. Relationships of cratonic HANCOCK,J. M. 1969. Transgression of the Cretaceous sea in andcontinental-margin tectonic episodes. In: DICK~N- south-west England. Proc. Ussher Soc. 2, 61-83. SON,W. R. (ed.) Tectonics and Sedimentation. Spec.hbl. - 1975. The sequence of facies in the Upper Cretaceous Soc. econ. Paleontol. Mineral,22, 98-119. of northern Europe compared with that in the Western TERRY, K. D. & TUCKER W. H. 1968. Biologiceffects of Interior. Spec. Pap. geol. Assoc. Can. 13, 83-118. supernovae. Science, 159, 421-3. -& KAUFFMAN,E. G. 1979. The great transgressions of THOMAS, A. R. 1978. The Ecology, Evolution and Extinction the Late Cretaceous. J. geol. Soc. London, 136, 175-86. of Spiriferina in the Lower Jurassic. Thesis, Ph.D, Univ. HAYS, J.D. & Prrmw, W. C. 1973. Lithosphericplate Coll. Swansea (unpubl.). motion,sea level changes and climatic and ecological TRUMPY,R. 1971. Stratigraphy in mountain belts. Q. J. geol. consequences. Nature, London, 246, 18-22. SOC.London, 126, 293-318. JENKYNS,H. C. 1970. Growth and disintegration of a carbo- VACHRAMEEV, V. A. 1978. Theclimates of thenorthern nateplatform. Neues Jahrb. geol. Palaeontol.Monats- hemisphere in the Cretaceous in the lightof paleobotani- hefte, 6, 325-44. cal data. Paleontol. Zh. Akad. Nauk SSR, 2, 3-17 (in - 1980. Cretaceousanoxic events: from continents to Russian; transl. Paleont.J., Am. geol. Inst. 12,143-54). oceans. J. geol. Soc. London, 137, 171-88. VANHINTE, J. E. 1976a. A Jurassic time scale. Bull. Am. LINDHOLM,R. C. 1979. Geologic history and stratigraphy of Assoc. Petrol. Geol. 60, 489-97. the Triassic-Jurassic Culpeper Basin, Virginia.Bull. geol. - 1976b. ACretaceous time scale. Bull. Am.Assoc. Soc. Am. 90, 1702-36. Petrol. Geol. 60, 498-516. MOULLADE,M. & NAIRN, A. E. M.(ed.) 1978. The VAN VALEN, 1. & SLQAN, R. E. 1977. Ecologyand the Phanerozoic Geology of the World, 11. The Mesotoic, A. extinction of the dinosaurs. Evol. Theory, 2, 37-64. Elsevier, Amsterdam. 529 pp. VOGT, P. R. 1972. Evidencefor global synchronism in MSTI%AVSKIY, M.M. 1977. Sedimentationcycles and the mantleplume convection and possible significance for Permian-Triassic boundary, western partof the Turanian geology. Nature, London, 240,338-42. plate. Sou. Geol. No. 12, 37-49 (in Russian, translation ZHUO-TING,L. 1980. Brachiopodassemblages from the 1979 Inc. geol. Rev. 21, 153-62). UpperPermian and Permian-Triassic boundary beds, OSTROM,J. H. 1964. A functional analysis of jaw mechanics south China. Can. J. Earth Sci. 17, 289-95. in the dinosaur Triceratops. Postilla, No. 88, 1-35.

Received 30 April 1980; revised typescript received 25 Nov. 1980. DEREK V. AGER,Department of Geology,University College of Swansea, Singleton Park, Swansea SA2 8PP.

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