Paléoécologie holocène des Marais du Rocher en Maillezais (Vendée), dans le Marais Poitevin (Littoral Atlantique - France)

This study allows the succession of palaeoenvironments in this marshland to be followed from approximately 7000 BP. The process of progressive emersion is particularly clearly documented. The succession of palaeoenvironments is as follows : — a coastal, sea-edge environment, with the shoreline following the limestone relief (first half of the armorican atlantic period), — a marine marshland, at first relatively landlocked, but later in closer contact with the sea (second half of the armorican atlantic period), — a lagoon environment, extending over more of the limestone as a result of the infilling of the marshland basin (5 700 to 3600 BP), — continued infilling, linked perhaps with a general marine regression, leading to a marshland environment of inland type. In situ peat deposits allow the level reached by the highest seas at this period to be determined (2 630 to 370 BP), — finally, an environment once more of coastal type, but disturbed by human intervention (building of dams) (after 370 BP). The deposits encountered here correspond to those described in the notice of the 1 : 50,000 geological map for Marans : marine alluvium attributed to the Flandrian (MFya : clay with Scrobicularia, green or blue, labelled "bri ancien"), resting on a Jurassic bedrock (J3c2 : clayey limestone of upper Callovian age), and covered by marshland sediments (FTz : peaty alluvium). This sequence is valid throughout the Maillezais area for land under 3 m above sea level.