Buried sandbodies within present-day estuaries (Atlantic coast of France) revealed by very high resolution seismic surveys

Studies of sandbanks within estuaries and macrotidal bays are critical for management of those coastal environments where human activity concentrates. They are also excellent analogues of reservoirs. New very high resolution (VHR) seismic profilers are now available for studying, at the outcrop resolution scale, sandbodies in very shallow water. We present here a VHR seismic study, constrained by vibrocores, repetitive bathymetric surveys and radiocarbon dating, which provide evidence of buried sandbodies in mixed tide and wave influenced estuaries of the French Atlantic coast. The three selected buried sandbodies are chosen because they are located at various water depths and have different internal architecture and outer shape. They include (1) a very shallow water (around −2 m under low tide) elongated nearshore sandbody, emplaced during the last centuries; (2) a −6 to −7 m deep, elongated nearshore sandbody dated to 2965±30 BP and (3) sandwaves lying around −17 m of water depth over the top of a large rounded shape sandbody dated to 5260±30 BP. Deepest (−6 and −17 m) sandbodies are interpreted as remnants of lower sea levels and thus transgressive sandbodies. All those sandbodies are buried under estuarine mud, which have been mainly emplaced during the last millenium. Such major sedimentary transition from basal sand to upper mud is assumed to record a major environmental change in relation to both natural infilling of estuaries, climate change and increasing human activities. Anthropogenic perturbations are however believed to be the most important factor. They have mainly consisted of land reclamation and are responsible for (1) rapid infilling of tidal bays, (2) tidal prism and tidal current energy decreases and (3) deposition of muddy sediments upon sandbodies.