Environmental conditions in the South Atlantic (Angola Basin) during the Early Cretaceous

Abstract

The South Atlantic has experienced periods of intense anoxia and black shale deposition that form an important hydrocarbon source. Here we have investigated the depositional environment during the initial opening phase of the South Atlantic during the Early Cretaceous. The period is crucial as it is characterized by extensive source rock deposition and because it sets the stage for subsequent periods of anoxia within the northern sub-basin of the South Atlantic. Within an Aptian sequence of organic-rich sediments (up to 40% total organic carbon, TOC) from Deep Sea Drilling Project (DSDP) Site 364, we found a distinct biomarker distribution, including the presence of isorenieratane and an array of thiophenic (S-containing) compounds. Our results indicate that, during the time of deposition, corresponding to the initial phase of opening of the South Atlantic, most of the water column in the Angola Basin was hypersaline (>40‰) and euxinic, with euxinia episodically reaching the photic zone. The low relative abundance of marine biomarkers in the samples suggests that these extreme conditions were unfavourable for typical marine organisms. Stratigraphically up-section, the biomarker distribution changed as the TOC content gradually decreased (< 20wt.%), isorenieratane and thiophenic compounds became less abundant and marine biomarkers became more abundant. We interpret the results to reflect a shift towards more open marine conditions, with less extensive euxinia/anoxia and normal marine salinity as the opening of the South Atlantic continued and the basin became less restricted. Our results demonstrate that the opening of the South Atlantic was the dominant control on the deposition of organic rich shales in the Angola Basin during the Aptian, highlighting the control of local basin geography on the depositional environment and formation of organic rich black shales during the Early Cretaceous.