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 m udshales and fossil-rich limestones that formed on a ramp as a result of a Tethyan transgression. The faunal content of the limestone beds i s characterized by (1) the absence of forms typically found on the Tet hyan platforms: rudistids, corals and commonly associated forms (large benthic foraminifers, bryozoans, calcareous sponges, red and green al gae); (2) the absence of pycnodonts and of brachiopods, and (3) the ab undance of Gryphaeidae and Ostreidae, as well as the relative abundanc e of ammonites. This unusual faunal composition is ascribed to several factors. Rudistids, corals and associated forms may have failed to oc cur because of the general decrease in the diversity of these organism s in the Upper Cenomanian-Lower Turonian and because of the lack of su itable shoals on the Niger ramp for reefs to develop. Pycnodonts may h ave failed to gain a foothold in the face of severe competition from p ioneer and opportunistic bivalves (Gryphaeidae, Ostreidae) that quickl y colonized the ramp. The absence of brachiopods has not yet been expl ained satisfactorily; it could be due to several factors, including se vere competition with pioneer bivalves and/or excessive turbidity of b ottom waters.