EVIDENCE FOR SHORT-TERM RETREAT OF THE BARRIER SHORELINES

in many places on the present shoreface, offshore bar or beach sands w ith a sharp-basal contact overlie thin-bedded fine sands and muds. Anc ient analogues are commonly interpreted as lowstand shoreface deposits lying on an erosional surface cut into the shelf during a relative se alevel fall. New observations may challenge this interpretation by pro viding evidence that such sequences develop during a recent shoreline retreat under highstand conditions without any significant change in s ea level. Two examples of such a superimposition are provided by wave- dominated elastic shorefaces in the microtidal environment from the Se negal coast (Saloum delta, Sangomar spit) and in a non-tidal environme nt on the French Mediterranean coast in the Gulf of Lions. In the Salo um, 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 anc ient beach barrier whose remnants are dated 3150 BP. The retreat of th is ancient beach barrier is demonstrated by mapping the former offshor e extension of a littoral sand unit which is defined clearly in the re gion by a grain-size signature. Other observations in the region of th e Saloum delta give evidence of the widespread occurrence of coastal r ecession along the whole Sangomar sedimentary spit. In the Gulf of Lio ns, vibrocores were used to sample the beach and the shoreface of the Thau lagoon lido. The geological record of recent and present sediment ation exhibits varied littoral sands overlying ancient deposits of fin e muddy sand and, at the very bottom of the sections, shoreface sands including fragments of beachrock. Radiocarbon datings give ages of 205 0-6700 BP for the underlying muddy fine sand which has been interprete d 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 rec ent (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 m igrated successively landwards as the littoral sedimentary prism has r eceded. Copyright (C) 1996 Elsevier Science Ltd