Submerged barrier evolution along the Mittag inner continental shelf, southern Namibia.

Abstract

The inner continental shelf of southern Namibian coast hosts the world's largest diamond mega-placer deposit, which is intricately linked to the evolution of the Orange River and the formation of its submerged delta. The entrapment of diamonds is enhanced by the unique geological topography of the coast. Once such area where diamond prospecting is intended, the inner continental shelf offshore Mittag of southern Namibia, provides an opportunity to examine a series of submerged, and potentially diamond-rich, gravel shorelines. The inner shelf of Mittag comprises a Cretaceous siltstone basement seaward and Precambrian schist in the inner regions, overlain by enigmatic, and continuous coast parallel ridges observed in seismic reflection. These features span depths between -100 m and -55 m, showing composite, progradational, aggradational multistacked and retrogradational patterns. These features are characterised by an external peaked morphology and a channelled-high amplitude chaotic reflector internal geometry. Occasional landward directed moderate amplitude reflectors overlie the peaks and infill the channels. These are in turn overlain by low amplitude/acoustically transparent muds, with a prominent transgressive sandsheet along the inner shelf. Cores reveal the unusual subcropping features to comprise gravels, interpreted as gravel-barriers and spits that formed during lower periods of sea level. The proposed evolutionary model demonstrates a complex interaction of sediment supply, sea-level changes, and bedrock control. Initial development created a broad sheltering lee in the bedrock, followed by gravel deposition during repeated regressions and transgressions, forming prograded barrier spits, downstepping barriers, and mixed sand-gravel shorefaces. Subsequent transgressions led to barrier rollover and back-barrier depression, forming extensive barrier spit-lagoon systems. The stratigraphy records stepped sea-level rises and emphasizes the significant role of geological control in barrier system development and preservation. Given the erosional nature of this shelf, their preservation suggests links to rapid rates of sea-level rise (melt water pulses-MWPs). The rhythmic fluctuations of sea level during the Pleistocene have acted to create and preserve a series of barrier spit and back barrier environments on the shelf via normal regression and later overstepping of the barrier and back barrier complex. These barriers thus provide useful archives for sea level change from a far field site.