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Biomarker Geochemistry of Early Cretaceous Sediments From West Africa


The Early Cretaceous rifting of West Gondwana resulted in the formation of the South Atlantic Ocean as Africa separated from South America. The associated rift-basins, now distributed along the conjugate margins of West Africa and Brazil, have attracted significant attention owing to the known and potential petroleum accumulations there and as sedimentary archives for Early Cretaceous regional climate evolution. Following the initial development of lacustrine ecosystems, seawater intrusion from the south eventually triggered deposition of evaporites during the late Aptian. This Aptian salt layer is pervasive across all rift basins and has been utilized to correlate regional stratigraphy. Organic geochemical signatures for marine biota have rarely been reported in “pre-salt” deposits and, therefore, the initial seawater intrusion and the deposition of the Aptian salt have been regarded as roughly time equivalent. However, this idea is being progressively challenged as evidence has accumulated for seawater intrusion significantly prior to the salt deposition. In this study, we analyzed sets of Neocomian to Albian sediment samples from the Kwanza and Gabon Basins with a focus on molecular fossils, comprising steranes, hopanes and carotenoids, that are sensitive to the presence of marine biota. Our results for a Gabon Basin sediment sequence show a shift from bacterially-dominated lacustrine ecosystems in the Neocomian and Barremian to a transitional environment with low but detectable levels of marine markers in the early Aptian. In the Kwanza basin, biomarkers indicate a stronger signal for marine conditions by the early Aptian, likely associated with seawater intrusion over the Walvis Ridge. Comparatively, biomarkers for fully open marine conditions are not evident until the early Albian, after the deposition of the Loeme salt. Here we discuss, in detail, the defining biomarker signatures of the lacustrine, transitional and marine sediments in these two Early Cretaceous basin settings.