Upper Cretaceous palaeoenvironments and benthonic foraminiferal assemblages of potential source rocks from the western African margin, Central Atlantic

Abstract

Abstract Upper Cretaceous benthonic foraminiferal assemblages, clay minerals, kerogen types and carbonate microfacies are examined in organic-rich sediments from western African coastal basins along a latitudinal transect extending from southern Morocco to Nigeria. Samples from onshore sections of the Tarfaya Basin, southern Morocco, from DSDP Site 369, from offshore commercial wells on the Casamance Shelf, Senegal, from DSDP Site 367, from ODP Site 959 and from onshore sections of the Benue Trough in Nigeria, are analysed. Organic-rich sediments from these sites are characterized by smectiterich clay mineral assemblages, high amounts of marine organic matter and a typical benthonic foraminiferal biofacies dominated by buliminids and bolivinids. Benthonic foraminifera provide robust proxy indicators of palaeoproductivity and organic matter flux to the seafloor. The palaeobiogeographic distribution of benthonic foraminiferal biofacies can be used to identify areas and stratigraphic intervals of hydrocarbon accumulation. Lower Turonian benthonic foraminiferal assemblages exhibit extremely low diversity and are strongly dominated by a Gabonita biofacies, in contrast to Coniacian to Maastrichtian assemblages, which display higher diversity and contain numerous species of Buliminella, Praebulimina, Afrobolivina, Bolivina and Orthokarstenia . The palaeobathymetric distribution of these high productivity benthonic foraminiferal biofacies reflects the establishment of a productivity driven oxygen minimum zone along the western African shelf, associated with an upwelling system active off coastal Africa during most of the Late Cretaceous. The extent of the high productivity equatorial belt appears to have shifted latitudinally during the Late Cretaceous as sea level and circulation patterns changed. The zone of highest productivity was broadest in the Early Turonian, when it coincided with the maximum sea-level rise and highest atmospheric temperature in the Late Cretaceous.