Holocene coastal changes and infilling of the La Perroche marsh (French Atlantic coast)

The La Perroche marsh is a small Holocene marsh on the south-western coast of the Oleron island (Bay of Biscay, France) presently occupied by a freshw ater swamp and separated from the ocean by a continuous sand dune ridge. Th e study of its infill shows first basal conglomeratic deposits with calcare ous pebbles on a Cenomanian rough substrate. The Holocene infill since 6000 years BP begins with a very fast sedimentation of sands. Then, the sedimen tation becomes finer, with lagoonal to freshwater influences recorded by os tracods. The distribution of these deposits inside the marsh is asymmetric, with a thick sandy-gravely layer in the eastern part and a succession of s ands, clays and organic layers in the western part. Communication with the sea is episodic, and the sedimentation is controlled by an active channel, as shown by the presence of displaced coastal-marine faunas at some sites. Around 5000 years BP, a coastal marsh began to develop with high organic ma tter contents. These organic deposits presently outcrop on the beach, sugge sting a more seaward location of the shoreline at that time. Generally, the se sediments are azoic. When fauna is present, it indicates brackish to fre shwater environments showing a progressive isolation of the system. A secon d phase of terrigenous sedimentation occurred after a transgressive event a round 2800-2500 years BP, but the communication with the sea was then very reduced, as indicated by mainly freshwater faunas. This transgressive event appears to be synchronous with a positive sea-level tendency recorded, thr oughout north-western European coasts, between 3000 and 2000 years BP. The definitive closing of the marsh occurred around 2100 years BP, after a seco nd phase of freshwater marsh. (C) 2001 Ifremer/CNRS/IRD/Editions scientifiq ues et medicales Elsevier SAS.