Cretaceous transgressions in Nigeria and Sergipe-Alagoas (Brazil)

摘要: The Nigerian and Sergipe-Alagoas sedimen tary basins are structurally and historically different, the former being related to the opening of the Gulf of Guinea, while the Sergipe-Alagoas Basin is of the semi-graben type created by the splitting of the African-South American continent. The sedimentary sequence of the coastal basins is basically a reflection of local tectonic events (constant sea level), whereas the trans-Saharan transgressions echo major changes in the ocean floor (constant freeboard). Important episodes of ridge growth occurred in Late Cenomanian-Early Turonian, Late Cam panian-Early Maastrichtian, and Paleocene. Comparison is made between the transgressive and regressive cycles of the two basins and new biostratigraphical data are presented. Scattered Cenomanian deposits have been found in the Ser gipe-Alagoas Basin, and Upper Turonian has been definitely proved. Up to Early Turonian, a low-lying land contact seems to have been main tained along the northern margin of the Gulf of Guinea. This was easily swamped by eustatic sea level rises, but it was not until MiddleTuronian that a modern system of surface oceanic currents appeared, allowing free inter change of faunas between the Central and South Atlantic. The statistical analysis of geomagnetic reversals has brought out the existence of a remarkable correlation in the lengths of quiet periods and phases of epicontinental transgres sions, thence changes in the ocean floor.