Antecedent bedrock control on the sediment-starved continental shelf of south/central Namibia

The Namibian inner continental shelf immediately north of the Orange River delta is sediment-poor, dominated by outcrop of metamorphic basement with a scarcity of sandy cover. Using a combination of high and very-high resolution geophysical tools and drill cores, an examination of the evolution of the inner shelf stratigraphy and geomorphology shows persistent antecedent bedrock controls throughout the Pleistocene.The bedrock of the inner shelf comprises schists, in places overlain by Eocene sandstones, over which a subaerial unconformity (H1) formed in the late Oligocene. H1 was repeatedly re-occupied during succeeding transgressive/regressive cycles, creating a backstop for erosion/sedimentation and creating a repeating topography on which transgressions took place. Apart from localised development of Miocene phosphatic limestones, little sediment has since accumulated. Large-scale karst weathering of the limestones reflects several periods of subaerial exposure most recently during the Quaternary lowstands of sea level including MIS 2 and 1.Thin Quaternary cover, housed within depressions on the H1 surface is predominantly preserved in the lee of bedrock ridges, behind which shallow and occasionally hypersaline lagoonal conditions developed. These preserved back-barrier sedimentary sequences developed post-MIS 5 (previous phases were either eroded/weathered or not deposited). Dolomite precipitation occurred when sea-level stabilised at −75 m 37,000 cal BP and restricted lagoonal conditions prevailed.Currently, there are very limited MIS 2/1 deposits preserved above H1. The absence of thick sand cover, adjacent to and downdrift of the Orange River delta, suggests that the onshore diversion of sand that currently supplies the Namib Sand Sea also operated during lower sea levels, related to the bedrock control and lack of accommodation in surface H1. This persistent onshore diversion of sediment throughout the Quaternary has implications for sediment source as well as budget calculations for the Namib Sand Sea.