Geochemical Evidence for Large­Scale Drainage Reorganization in Northwest Africa During the Cretaceous

Abstract

West African drainage reorganization during Cretaceous opening of the Atlantic Ocean is deciphered here from geochemical provenance studies of Central Atlantic sediments. Changes in the geochemical signature of marine sediments are reflected in major and trace element concentrations and strontiumneodymium radiogenic isotopic compositions of Cretaceous sedimentary rocks from eight Deep Sea Drilling Project (DSDP) sites and one exploration well. Homogeneous major and trace element compositions over time indicate sources with average upper (continental) crust signatures. However, detailed information on the ages of these sources is revealed by neodymium isotopes (expressed as ENd). The ENd(0) values from the DSDP sites show a three-step decrease during the Late Cretaceous: (1) the Albian-Middle Cenomanian ENd(0) values are heterogeneous (-5.5 to -14.9) reflecting the existence of at least three subdrainage basins with distinct sedimentary sources (Hercynian/Paleozoic, Precambrian, and mixed Precambrian/Paleozoic);(2) during the Late Cenomanian-Turonian interval, ENd(0) values become homogeneous in the deepwater basin (-10.3 to -12.4), showing a negative shift of 2 epsilon units interpreted as an increasing contribution of Precambrian inputs; (3) this negative shift continues in the Campanian-Maastrichtian (ENd(0)5215), indicating that Precambrian sources became dominant. These provenance changes are hypothesized to be related to the opening of the South and Equatorial Atlantic Ocean, coincident with tectonic uplift of the continental margin triggered by Africa-Europe convergence. Finally, the difference between ENd(0)values of Cretaceous sediments from the Senegal continental shelf and from the deepwater basins suggests that ocean currents prevented detrital material from the Mauritanides reaching deepwater areas.