The late Quaternary terrigenous sediments of the continental margin and abyssal plains of the western equatorial Atlantic consist predominantly of hemipelagic silty clay rich in organic detritus with interbeds of redeposited silts and sands; deposition was cyclic and was controlled by glacial-interglacial climatic fluctuations. The redeposited beds are graded, as much as several metres in thickness, and were episodically deposited by turbidity currents and related mass flows. Although the terrigenous component of the hemipelagic sediments was also transported to abyssal depths via gravity-controlled bottom flows, the pelagic component (foraminifera) of these sediments indicates that accumulation occurred relatively slowly (5 to >30 cm/103 yr) and continuously for periods of as much as 100,000 yr. Thus some near-bottom process (other than simple downslope flow), such as redistribution and deposition by contour-following bottom currents, apparently was responsible for deposition of the hemipelagic sediment; however, evidence for such contour-current deposition is sparse.
The hemipelagic and redeposited sediments were continuously deposited during relatively long intervals (∼90,000 yr) of low sea level that accompanied the Wisconsin and previous glacial phases. During these intervals the continental shelf was emergent, and river sediments were discharged directly into submarine canyons from where they could easily be transported downslope to the continental margin and abyssal plains. In contrast, during the relatively short intervals (5,000 to 20,000 yr) of high sea level that accompanied the Holocene, beginning of the Last Interglaciation (125,000 to 115,000 yr B.P.), and beginning of previous interglacial phases, the locus of river sedimentation migrated landward across the continental shelf. The low gradient and great width of the shelf plus strong longshore currents prevented seaward movement of sediment beyond the innermost shelf. Thus terrigenous sedimentation was briefly, but completely, halted throughout the entire basin and only pelagic sediments were deposited.