Ammonite faunal exchanges betweenSouth Tethyan platforms and South Atlantic during the Uppermost Cenomanian-Lower most/Middle Turonian in the Benue trough (Nigeria)

摘要: Abstract The Cenomanian and Turonian transgressions (Late Cretaceous) formed seaways through the Benue Trough (Nigeria) connecting “Saharan Platforms” to north (Niger) “Atlantic Margin” slope edge (“South Atlantic”) south. A biostratigraphical framework precise correlations at ammonite subzone horizon level have been established providing a sound basis on which compare palaeontological samples interpret successive stages of colonization by ammonites or exchanges among various faunal groups. Quantitative study finds shows: several biogeographical entities are permanent features; “Benue” assemblages consist principally Vascoceratinae in uppermost superseded bytthe Pseudotissotiinae Lower Turonian; “Atlantic” composed mainly Acanthoceratinae and/or other Vascoceratine Mammitinae, Desmocerataceae, Phylloceratina Lytoceratina;and Lytoceratina; base series (Dumbli Horizon) Middle (Ingens more uniform throughout Trough; whichever horizons considered, diversity regularity increase from characterized taxa displaying little variation (overall shell shape), with very simple suture lines short body chambers; they generally well adapted shallow environments. By contrast, morphologically highly diverse, but long chamber complicated line. These forms appear better suited deeper open marine Fossil reflect biological associations; no evidence has ever produced for substantial post mortem transportation shells. Precise knowledge geological phenomena occurring during Cenomanian-Turonian indicate there were five major steps colonization. coincide N-S S-N transgressive flows; result interaction three factors: mode life, morphological adaptation changeable environments competition. 1) Dumbli Horizon, period widespread uniformity: ubiquitous species Metengonoceras dumbli appears early south is dominant this area. It emphasizes uniformity resulting double advance. 2) Latest colonizations (uppermost Juddii Zone) trend characterizes end late highstand: partial Niger (N-S) groups related Saharan (Nigericeras gadeni N. cauvini); initially restricted then large-scale globose Vascoceras evolved Atlantic (S-N). 3) Faunal regression onset (Coloradoense concides clear regressive stage: confined margin seaway between platforms margin. 4) Recolonization (Nodosoides Zone): “Pseudotissotiinae”, derived Vascoceras, reach Upper (Thomasites gongilensis), (Pseudotissotia nigeriensis), indicating new flow. During influences thought be indicated if numerically small presence Choffaticeras. This step reflects subsequent relative highstand 5) mid-Turonian (Hoplitoides ingens marks period: index found (Maghreb) unknown (continental deposits?); connection earliest mid-Turonian. should emphasized that: episodes 1 5 correspond times uniformization oxycone shells predominant both instances (Metengonoceras Hoplitoides); strictly (Mammitinae, Acanthoceratinae, Fagesia, Neoptychites, etc.) rare particularly periods when few dispersed units mixed faunas.