Evidence for short term retreat of the barrier shorelines

Abstract

In many places on the present shoreface, offshore bar or beach sands with a sharp-basal contact overlie thin-bedded fine sands and muds. Ancient analogues are commonly interpreted as lowstand shoreface deposits lying on an erosional surface cut into the shelf during a relative sealevel fall. New observations may challenge this interpretation by providing evidence that such sequences develop during a recent shoreline retreat under highstand conditions without any significant change in sea level. Two examples of such a superimposition are provided by wave-dominated clastic shorefaces in the microtidal environment from the Senegal coast (Saloum delta, Sangomar spit) and in a non-tidal environment on the French Mediterranean coast in the Gulf of Lions. In the Saloum, a littoral cross-section across the Sangomar beach barrier shows a dune sand deposit overlying a man-made shell accumulation, dated 600 BP, and a layer of green muds formed in a lagoon at the back of an ancient beach barrier whose remnants are dated 3150 BP. The retreat of this ancient beach barrier is demonstrated by mapping the former offshore extension of a littoral sand unit which is defined clearly in the region by a grain-size signature. Other observations in the region of the Saloum delta give evidence of the widespread occurrence of coastal recession along the whole Sangomar sedimentary spit. In the Gulf of Lions, vibrocores were used to sample the beach and the shoreface of the Thau lagoon lido. The geological record of recent and present sedimentation exhibits varied littoral sands overlying ancient deposits of fine muddy sand and, at the very bottom of the sections, shoreface sands including fragments of beachrock. Radiocarbon datings give ages of 2050–6700 BP for the underlying muddy fine sand which has been interpreted as a lagoonal deposit enriched in organic matter, typical of lagoon environments in the region. An evolution similar to that proposed for the Sangomar region is supported by additional evidences of a very recent (some centuries) landward displacement of the beach barrier. Lines of wrecks of different ages are observed along this part of the Gulf of Lions, showing that the shoal determined by the offshore bars has migrated successively landwards as the littoral sedimentary prism has receded.