Major Marine Cycles in the Mesozoic

J. geol. Soc. London, Vol. 138, 1981, pp. 159-166. Printed in Northern Ireland. Major marine cycles in the Mesozoic 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 Permian regression;(ii) the late mid-Triassic transgression;(iii) the late late-Triassic transgression; (iu) the late early Jurassic transgression; (U) the late mid-Jurassic transgression; (ui) the end-Jurassic regression; (uii) the late early Cretaceous 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 lowest Triassic 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 brachiopods, 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 extinction 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 hardly anything marine to show of in the shallowmarine benthoswere the giant this system and the United States only has traces in megalodontid bivalves. 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 extinctions. 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, on the 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). with fewer 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 brachiopod 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.

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