Sedimentary balance and sand stock availability along a littoral system. The case of the western Gulf of Lions littoral prism (France) investigated by very high resolution seismic

Abstract

Very high-resolution seismic data was acquired along the microtidal wave-dominated littoral zone of the western Gulf of Lions (SE France) with a view to estimate the volume of sand present in the upper shoreface under conditions of stable high sea level. An upper sand unit is thus identified at the top of the shoreface, bound at its base by a hard floor made of beach-rock layers or geological substrate. This upper sand unit (USU) is mobile and represents the available sand stock used for internal exchanges and beach supply. Estimate of the USU volume takes place within each sedimentary compartment and cell defined along the littoral zone. Two main results are obtained from this quantification. (1) The distribution of the sand stock fits with the littoral drift pattern, updrift source zones being characterized by low volume USU, while downdrift sink areas are characterized by much larger sand reservoirs. This distribution pattern is verified at all scales, the regional, the compartment and the cell scales; (2) the volume of the upper shoreface sand reservoir seems to match the adjacent beach behaviour. Wherever the beach evolution trend is negative, the stock is depleted and, on the contrary, the beaches associated to voluminous reservoirs do not suffer erosion.