Unusual faunal associations during Upper Cenomanian-Lower Turonian floodings on the Niger ramp (central West Africa)

The Upper Cenomanian-Lower Turonian marine deposits of the Iullemmeden Basin and of the northern Chad Basin (Niger) consist of interbedded mudshales and fossil-rich limestones that formed on a ramp as a result of a Tethyan transgression. The faunal content of the limestone beds is characterized by (1) the absence of forms typically found on the Tethyan platforms: rudistids, corals and commonly associated forms (large benthic foraminifers, bryozoans, calcareous sponges, red and green algae); (2) the absence of pycnodonts and of brachiopods, and (3) the abundance of Gryphaeidae and Ostreidae, as well as the relative abundance of ammonites. This unusual faunal composition is ascribed to several factors. Rudistids, corals and associated forms may have failed to occur because of the general decrease in the diversity of these organisms in the Upper Cenomanian-Lower Turonian and because of the lack of suitable shoals on the Niger ramp for reefs to develop. Pycnodonts may have failed to gain a foothold in the face of severe competition from pioneer and opportunistic bivalves (Gryphaeidae, Ostreidae) that quickly colonized the ramp. The absence of brachiopods has not yet been explained satisfactorily; it could be due to several factors, including severe competition with pioneer bivalves and/or excessive turbidity of bottom waters.