at the Cenomanian/Turonian boundary: a global control. J.M. Philip and C. ... biozone times and was coeval with the Global Oceanic Anoxic Event (OAE2). A.
Coral Reefs (1991) 10:115-125
Coral R [s (~) Springer-Verlag 1991
The demise of the rudist-bearing carbonate platforms at the Cenomanian/Turonian boundary: a global control J.M. Philip and C. Airaud-Crumiere Centre de S~dimentologie-Pal~ontologie, Universit~ de Provence, C.N.R.S. - U.R.A. 1208, F-1331 Marseille Cedex 03, France Accepted 10 February 1991
Abstract. The demise of the rudist-bearing carbonate platforms at the Cenomanian/Turonian boundary is studied in different ways through examples from the Western Mediterranean Province. During the Late Cenomanian, North and South Tethyan carbonate platforms extended widely and were subjected to different climatic and oceanographic conditions. The onset of the demise of the carbonate platforms occurred during Upper Archaeocretacea and Helvetica biozone times and was coeval with the Global Oceanic Anoxic Event (OAE2).A major biologic turnover affected the benthic organisms (i.e. rudists and large foraminifera). The rudists underwent a severe extinction event, leading to the disappearance of the dominantly aragonite secreting rudists, while the dominantly calcitic forms were less affected. The major development of the carbonate platforms occurred during the Latest Cenomanian - Earliest Turonian, involving a transgressive highstand system tract and a keep-up carbonate organization. During the Early Turonian the carbonate sedimentation was disturbed; hard-grounds, condensed beds, terrigenous inputs developed and a gap in the carbonate platform deposition occurred. A combination of several sequentially linked factors, could explain the demise of the carbonate platform and the major change on the benthic ecosystem at the Cenomanian-Turonian boundary.
Introduction Many authors have emphasized the oceanographic crisis of the Cenomanian-Turonian boundary, and its biological and sedimentological consequences. Very detailed studies have been carried out in the oceanic areas (Schlanger et al. 1987; Arthur et al. 1987; Brumsack and Thurow 1986; Kuhnt et al. 1986; de Graciansky et al. 1987; Jarvis et al. 1988b). The biological aspects of the marine macro-and microfauna at the CenomanianTuronian crisis have been studied by Kauffman (1986),
Masse and Philip (1986), Hilbrecht et al. (1986), Elder (1987) and Jarvis et al. (1988). In contrast, few papers have been devoted to the carbonate platforms at the C/T boundary and the changes in the benthic communities (Philip 1982; Floquet et al. 1987). The aim of this paper is to give new data on this topic. The area studied is that of the Western Mediterranean Province (Philip 1982), where the Upper Cenomanian carbonate platforms are well developed on the European, Apulian and African margins. In this province, the C/T boundary is marked by drastic sedimentological changes and a general eclipse of the rudist-bearing carbonate platforms in the Early Turonian (Philip 1982). The paleogeographical, chronological, palaeontological and sedimentological patterns of this event are first presented before establishing an explanatory model. Palaeogeography During the Late Cenomanian, carbonate platforms extended broadly along the northern and southern borders of the Tethyan Realm (Fig. 1) on the passive margins of the Eurasian, African and American plates. In both extremities, the Tethys was joined with the Pacific Ocean; a general East-Westward oceanic circulation governed the palaeogeographic distribution of the rudists (Philip 1982). According to the palaeoclimatic reconstruction of Barton and Washington (1982) and Lloyd (1982), the Tethys Cenomanian carbonate platforms were located in the lower latitudes of the Northern Hemisphere. The northern boundary of the Tethys domain would have coincided with the 22 ~ C January isotherm (Lloyd 1982). The North and South Tethyan carbonate platforms differed mainly through climatic and oceanographic factors. The North Tethyan platforms were influenced by wet temperate climatic conditions as proved by the large distribution of coal deposits on the Laurentia and Angara continental areas (Parrish et al. 1982) while the marine sedimentation was characterized by the input of
116 [----'~ Emergeda r e a s ~'~
Carbonateplatforms
~
Evaporiticfacies
~
Basinalfacies
~
Chalkyfacies Coals
; 9 : ", Detrftaldeposits Oceanic currents UpwelUngs NTPF
Fig. 1. General world palaeogeographicalmap and setting of the Late Cenomaniancarbonate platforms (from Lloyd 1982, modified).See explanations in the text
NorthTethyanPlatform
8 T P F so h TethyanPlaffocm AP
Ap ,a, Platform
fine terrigenous sediments, mixed with chalk deposits. North-Southward cold-temperate currents moving through gateways between Tethyan and Boreal seas prevented the enlargement of the carbonate platforms and favored stratification of the oceanic water column and a stagnation of the bottom waters (Arthur et al. 1987). In contrast, the South Tethyan Upper Cenomanian platforms developed in an arid climate as indicated by evaporitic deposits on the northern part of the Gondwana lands (Busson 1972). During this period a communication was established between the Tethys and the South Atlantic by the Trans-Saharian Gateway (Reyment 1980). The Apulian carbonate platforms were surrounded by oceanic troughs or basins and were free of terrigenous influences and strong palaeobiogeographic affinities exist with the South Tethyan Province (Philip 1982). In these cases, oceanic upwellings increased pelagic productivity on the outer edge of the carbonate platforms (Thurow et al. 1982; Macellari and de Vries 1987; Arthur et al. 1987; Philip et ~'.. 198%). In this work, our attention has been focussed on the Mediterranean carbonate platforms which developed on the passive margins of the cratonic areas of the African and European plates. Emerged continental areas supplied terrigenous runoff and narrow, oceanic troughs, separated the different carbonate platform provinces (d'Argenio et al. 1980; Philip 1982).
Chronostratigraphy In the Mediterranean province, chronostratigraphy of the Cenomanian-Turonian boundary in the carbonate
platforms areas is well established, and the rudist or large foraminifera coenozones can be integrated with ammonite or plankton biozones (Philip 1978; Berthou 1984; Polsak et al. 1982; Bilotte 1978; de Castro 1980). Two main results are inferred from these data (Fig. 2). First, during the Late Cenomanian the platforms were extensive; during the early Turonian they became drastically reduced. Second, accurate correlations are possible between carbonate platform and oceanic biozonations (Philip et al. 1989a and b). Caprinid coenozones fit with the cushmani and Lower arehaeocretacea planctonic foraminiferal biozones. The first appearance datum of the hippuritid family is coeval with the upper part of the archaeocretacea biozone. The major demise stage of the carbonate platforms and the decrease in the diversity of the benthic carbonate platform association (rudists and foraminifera) occurred during the Upper areheoeretacea and helvetica biozones. These data strongly support the fact that the onset of the demise of the carbonate platforms was related to the Global oceanic anoxic event (OAE2)dated from the archaeocretacea zone (Schlanger et al. 1987).
Biological events It is important to notice the link between the decrease in area of the carbonate platforms during the Early Turonian and a major change in the benthic ecosystem which mainly involved rudists and benthic foraminifera. After a slight decrease in diversity, at the top of the geslinianum zone, the rudists underwent an important extinction event near the C/T boundary (Fig. 3) with the disappearance of 8 genera, and the appearance of the
117
Ages Chronostrati
graphy
. a q and al. 1987
Middle 91-
Z < i Z
O rr
Biostratigraph r Ammonites (Kennedy 1984) Kamerunoeeras
Planktonic foraminifera
Carbonate platforms Rudists reduced extent
/
broad extent
Sigali
Ocean events
Oxic
turoniense
Mammites Helvetica nodosoides
Hippuritid coenozones
Early
Anoxic Watinoceras coloradoense
92Neocardioceras
r
Archaeoeretaeea juddii
I-lypoxic (?)
Unnamed Nigericeras
Z < Z
