Genesis and preservation of composite (Miocene to Holocene) gravel deposits on the accommodation-limited continental shelf of south-central Namibia

Gravel accumulations on the Namibian shelf off Hottentots Bay have developed during successive transgressions since the Late Oligocene. Using more than 12,000 line km of seismic data coupled with >6000 boreholes, the palaeo-land surface (H1) on which the gravel deposits occur was mapped. This is a weathered bedrock surface overlain by saprolite or clay and mantled by Eocene-age sandstones. Despite high sediment input, the limited accommodation space on the uplifted shelf means that unconsolidated sediment has accumulated only in localized depressions and topographically sheltered zones on this palaeo-land surface. The overlying unconsolidated sediment consists of four seismic and lithostratigraphic units (C F), including lowermost gravel Units C and D that were deposited in multiple phases from late Oligocene to late Pliocene/Pleistocene. These accumulated locally in relict fluvial valleys and in a bedrock-hosted embayment that provided preferential accommodation for multi-stacked gravel beaches. They formed initially as lowstand lags in small tidal channels and developed into thick embayed beaches through aggradation. By the Holocene, accommodation in the embayment was filled and excess material spilled into a palaeo-strait, forming linear gravel beaches and a gravel spit anchored by a landward bedrock high. These were preserved through overstepping during rapid sea level rise during MWP-1B. The preservation of each package during multiple transgressions over 20 million years without major reworking during intervening occupations of sea level is ascribed to a combination of: (i) inertia associated with voluminous gravel deposits; (ii) rapid burial by successive gravel shorelines; (iii) a consistently high sediment supply (both transgressive and alongshore); and (iv) preferential deposition in wave-sheltered settings. The multiple recurrence of gravel shorelines in the same locations reflects the persistence of depositional conditions in geologically constrained settings; elsewhere shoreline deposits did not survive transgression. The latest (Holocene) phase of deposition coincided with a sea-level slowstand during which a spit prograded alongshore. Subsequent rapid sea-level rise prevented its breakdown and/or rollover and left it stranded on the shelf as a discrete palaeo-shoreline feature. Any Pleistocene precursors were destroyed via wave ravinement during multiple sea-level cycles. • Pseudo 3D sub-bottom and borehole data reveal several submerged gravel beaches/spits • Deposits stack vertically due to limited accommodation in a submerged embayment • Reflect Miocene to Holocene changes in gravel supply and sea level • Preservation relates to geologically constrained antecedent coastal setting