South Atlantic organic-rich sediments: facies, processes and environments of deposition

Organic-rich sediments (black shales) occur in parts of the South Atlantic ocean within Jurassic, Cretaceous and Neogene sections. These black shales are represented by various lithologies, including mudstones, marlstones, limestones, sandstones, pebbly mudstones and diatomites. They range from thin to very thick bedded, laminated, fissile or structureless, and show varying degrees and types of bioturbation. Texturally and compositionally they are equally varied. Organic carbon may be of principally marine or terrigenous source and forms from 1% to over 20% of the sediment. In many cases, the black shales are interbedded with organic-poor lithologies in cycles with irregular periodicities that average 20,000 to 140,000 years. The range of depositional processes of these black shales includes sliding, debris flows, high- and low-concentration turbidity currents, pelagic/hemipelagic sedimentation, and winnowing by shelf bottom currents. They occur in various palaeogeographical settings from nearshore shelf to ocean basin, and have been differently influenced by variations in bottom water oxygenation, organic matter supply and rates of sedimentation.