Holocene Sediment Mobilization in the Inner Continental Shelf of the Bay of Biscay: Implications for Regional Sediment Budget Offshore to Onshore
Résumé
Sediment mobilization, especially from offshore bay to the coast during the Holocene, along the inner continental shelf of Bay of Biscay is less understood. Acoustic and sediment sampling and surveys were conducted offshore of the Bay of Étel to define the shallow geologic framework and the sedimentology. Results of these campaigns are used to identify and map Holocene deposits, discuss sediment transport pathways and depth of closure using Hallermeier's approach in order to define the active sedimentary prism using the depth of closure on a meso-tidal and wave dominated coast. In the Bay of Étel, Holocene sediments are concentrated between rocky shoals and emerged rocks, passing from external zone by sand lobe, 20 m thick to the medium zone, 15 m thick, to the beaches of the Gâvres-Penthièvre beach dune system (modern coastal prism). This case study found that the thickest deposits of Quaternary sediments observed along the inner shelf have a strong dependence on coastal topography and structural heritage. The presence of offshore basement shoal has been determined to have exercised control over sediment transfers from offshore to landward regions during the last marine inundation. Due to the lack of any significant modern fluvial input of sand in the region, the Holocene deposits are inferred to have been derived by reworking of relict Pleistocene and older inner-shelf deposits. Sediment textural trends and seafloor morphology from the medium zone to the beach dune system indicate an absence of relationships (linking) between the modern sedimentary prism and the potential source of sediment to the shoreline.