AMMONITE FAUNAL EXCHANGES BETWEEN SOUTH TETHYAN PLATFORMS AND SOUTH ATLANTIC DURING THE UPPERMOST CENOMANIAN LOWERMOST MIDDLE TURONIAN IN THE BENUE TROUGH (NIGERIA)

The Cenomanian and Turonian transgressions (Late Cretaceous) formed se aways through the Benue Trough (Nigeria) connecting the ''Saharan Plat forms'' to the north (Niger) and the ''Atlantic Margin'' slope edge (' 'South Atlantic'') to the south. A biostratigraphical framework and pr ecise correlations at the ammonite subzone and horizon level have been established providing a sound basis on which to compare palaeontologi cal samples and to interpret the successive stages of colonization of the trough by ammonites or exchanges among the various faunal groups. Quantitative study of palaeontological finds shows: several biogeograp hical entities are permanent features; the ''Benue'' assemblages consi st principally of Vascoceratinae in the uppermost Cenomanian and are s uperseded by the Pseudotissotiinae in the Lower Turonian; the ''Atlant ic'' assemblages are composed mainly of Acanthoceratinae and/or other Vascoceratinae and/or Mammitinae, Desmocerataceae, Phylloceratina and Lytoceratina; at the base of the Cenomanian series (Dumbli Horizon) an d at the base of the Middle Turonian (Ingens Horizon) the assemblages are more uniform throughout the trough; whichever biostratigraphical h orizons are considered, faunal diversity and regularity increase from north to south. The ''Benue'' assemblages are characterized by taxa di splaying little variation (overall shell shape), with very simple sutu re lines and short body chambers; they are generally well adapted to s hallow environments. By contrast, the ''Atlantic'' assemblages are mor phologically highly diverse, but are generally characterized by a long body chamber and a complicated suture line. These forms appear better suited to deeper and more open marine environments. Fossil finds refl ect the biological associations: no evidence has ever been produced fo r substantial postmortem transportation of shells. Precise biostratigr aphical correlations and knowledge of the geological phenomena occurri ng during the Cenomanian-Turonian transgressions indicate there were f ive major steps of colonization. These coincide with the N-S or S-N tr ansgressive marine flows; they result from the interaction of three fa ctors: ammonite mode of life, morphological adaptation to highly chang eable environments and competition. 1) The Dumbli Horizon, a period of widespread faunal uniformity: the ubiquitous species Metengonoceras d umbli appears early in the north and south and is dominant in this hor izon throughout the area. It emphasizes uniformity resulting from a do uble S-N and N-S faunal advance. 2) Latest Cenomanian colonizations (u ppermost Juddii Zone) with a double trend which characterizes the end of the transgressive period and the late Cenomanian highstand: partial colonization of the trough from the Niger (N-S) by groups related to the Saharan Vascoceratinae (Nigericeras gadeni and N-cauvini); initial ly restricted and then large-scale colonization by globose Vascoceras evolved from Atlantic Acanthoceratinae (S-N). 3) Faunal regression at the onset of the Turonian (Coloradoense Zone) which coincides with a c lear regressive trend at the onset of the stage: the globose Vascocera tinae are confined to the Atlantic margin with no palaeontological evi dence of a seaway between the Saharan platforms and the Atlantic margi n. 4) Recolonization in the early Turonian (Nodosoides Zone): the ''Ps eudotissotiinae'', derived from Vascoceras, reach the Upper Benue (Tho masites gongilensis), then the Niger (Pseudotissotia nigeriensis), ind icating a new S-N flow. During this period Saharan influences in the t rough are thought to be indicated by the permanent if numerically smal l presence of Choffaticeras. This step reflects a new transgressive pe riod and subsequent relative highstand in the early Turonian; 5) A new period of substantial uniformity in the mid-Turonian (Hoplitoides ing ens Horizon) which marks the onset of a regressive period: the index s pecies is found north of the Saharan platforms (Maghreb) and throughou t the trough but is unknown in Niger (continental deposits?); no N-S b iogeographical connection is found for the earliest mid-Turonian. It s hould be emphasized that: episodes 1 and 5 correspond to times of subs tantial morphological uniformization with oxycone shells predominant i n both instances (Metengonoceras and Hoplitoides); strictly Atlantic a mmonites (Mammitinae, Acanthoceratinae, Fagesia, Neoptychites, etc.) a re rare in the Trough; they are found particularly at the end of the t ransgressive periods and at periods of relative highstand when a few d ispersed units are mixed with Upper Benue faunas.