Submerged gravel shorelines of the Inner to mid-shelf offshore Hottentot’s Bay, Namibia.

Abstract

The western margin of southern Africa hosts extensive diamond placer deposits spanning from the lower Orange River to the southern Namibian coastline. These diamonds are associated with gravels deposited by surfzone processes during sea level fluctuations. During the postglacial transgression (23,000 to 6,000 BP), these diamondiferous gravels were concentrated and submerged. This study focuses on Area 4 of ML103A offshore Hottentots Bay, employing pseudo-3D seismic profiles, multibeam bathymetry, backscatter data, and over 6,400 drill cores to analyse the geomorphology and seismic stratigraphy of the inner to mid-shelf region. With the use of a pseudo-3D grid of ultra-high-resolution sub-bottom profiles and extensive borehole data, this study reconstructs the morphology of the paleo-land surface (H1) on which the current shelf sediments lie. H1 comprises a weathered bedrock surface overlain by saprolite or clay and mantled by Eocene-age sandstones. The overlying unconsolidated sediment consists of four seismic and lithostratigraphic units (C-F), including lowermost gravel Units C and D deposited in multiple phases from late Oligocene to late Pliocene/Pleistocene. These deposits formed lowstand lags in small channels on the inner shelf edge and developed into a thickly embayed beach on the innermost shelf. The bedrock-hosted embayment provided preferential accommodation for multi-stacked gravel beaches, contrasting with barrier spit and back barrier sequences to the south, closer to the Orange River. Once accommodation in the embayment was filled, material spilled into the strait, forming linear beaches anchored by a landward bedrock high, resistant to wave reworking and preserved through overstepping during significant sea level rises, notably MWP-1B of the Holocene. The dissertation emphasizes the need for further dating of these gravel beaches, challenging due to the polycyclic nature of their materials.