Outer- to inner-shelf response to stepped sea-level rise: Insights from incised valleys and submerged shorelines

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Shorelines respond to rising sea-level through processes such as erosion, landward migration and in-situ drowning (i.e. overstepping). Submerged and preserved shorelines on the continental shelf play a key role in examining coastal response to rising sea levels as they provide important information on how modern shorelines may evolve in time and space within the context of changing climate and post-glacial sea-level rise. This study identifies and assesses the response of a continental shelf to stepped rises in sea level with particular focus on the stepwise evolution of incised valleys and shorelines from the shelf-edge to the inner shelf. Multibeam bathymetry from the mid-outer allows for the analysis of seafloor morphology, including the Protea Banks Reef (a palaeo-shoreline complex), and the adjacent incised, sediment starved continental shelf. Six seismic units and intervening surfaces are identified using interpretations from sub-bottom profiles, these include the incised acoustic basement, variable incised valley fill successions, aeolianite ridges and post-transgressive shoreface and associated sediments that withstood wave ravinement processes. The incised valleys of the outer-shelf are manifested as distinctive seafloor depressions, filled at their bases by fluvial deposits overlain, in the unfilled valley, by deposits derived from cascading subaqueous dunes which comprise the upper-most post-transgressive sediments. A core intersecting the dune material yields a maximum age of deposition of 1191–1263 cal yr BP (68% range), synchronous with a period of higher than present sea-levels in the region suggesting reworking and redistribution of coastal sediment as shelf sediment post-transgression. During the stepped rises in sea level, the shoreface has disconnected from the contemporary shoreline and is preserved by means of topographic barriers formed by antecedent topography as relict shoreface deposits. We provide a new perspective of shoreline response to stepped rises in sea level by integrating the seismic architecture of incised valley fills and shorelines across the continental shelf thus allowing for the assessment of variation in rates of relative sea-level rise since the last glacial maximum.