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

Abstract

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permanent features; the “Benue” assemblages consist principally of Vascoceratinae in the uppermost Cenomanian and are superseded bytthe Pseudotissotiinae in the Lower Turonian; the “Atlantic” assemblages are composed mainly of Acanthoceratinae and/or other Vascoceratine and/or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