The character and origin of lacustrine source rocks in the Lower Cretaceous synrift section, Congo Basin, west Africa

Lower Cretaceous synrift lacustrine shales from the Congo Basin, west Africa, have been analyzed with sedimentological and geochemical techniques to characterize source rock quality and identify triggers for deposition of intervals richest in organic carbon. The sequence includes a lower active rift section, deposited during active faulting and subsidence, overlain by an upper late rift section, deposited during reduced faulting and subsidence. Total organic carbon (TOC) averages 2-3 wt.% throughout the active rift siliciclastic shale section, 6% in marls in the lower part of the late rift section, and 1-2% in deltaic shales in the upper part of the late rift section. Organic matter consists of mixed types I and III kerogen in the active rift shales, pure type I kerogen in the late rift marls, and a type I and III mixture in the late rift deltaic shales. Redox proxies indicate that the deep lake was relatively reducing throughout deposition of the active rift and lower late rift sections. Therefore, enhanced anoxia did not trigger deposition of the richest source rocks. Decreased sedimentation rates in the late rift do not account for the full increase in TOC nor the shift on organic matter type. The richest source rocks are associated with high rates of organic productivity and chemical sedimentation, indicating that flux of dissolved components to the rift lake, including nutrients for algae growth, was critical. We propose that reduced topography associated with the late rift was necessary for efficient cycling of plant-derived carbon into soil carbonate and ultimately the rift lake, and for enhancing chemical weathering and nutrient flux.