Gravel and sand barrier preservation at geological (Cenozoic) timescales: Insights from the southern Namibia continental shelf

ABSTRACT An apparent disconnect exists between the morphology and behaviour of modern coastal barriers and barrier‐associated sediments preserved in the geological record, reflecting modification and selective preservation over geological timescales. The southern Namibian shelf preserves multiple submerged barrier‐related sequences spanning the Cenozoic. Seismic stratigraphy and borehole data reveal gravel‐ and sand‐dominated barrier systems developed across an irregular, stepped bedrock surface at lower‐than‐present shorelines (~50 to 70 m below present sea level). The succession records contrasting barrier behaviours, including progradational–aggradational growth, landward stepping during transgression, and overwash‐dominated development. Sediment inheritance and reworking contributed to barrier construction, while antecedent topography controlled accommodation and barrier position. Subsequent transgression, ravinement and subaerial exposure selectively removed components of formerly more complete depositional systems. Preservation consequently ranges from isolated shoreface, barrier and back‐barrier remnants to comparatively complete barrier–lagoon systems. Preservation completeness is not simply related to sediment calibre, with a younger sandy barrier system more completely preserved than older gravel‐rich systems. The preserved stratigraphic expression therefore reflects the interaction between original barrier behaviour, sediment availability, antecedent accommodation and cumulative erosional history, rather than original barrier morphology alone.