Recent morphodynamics of shell banks in the western part of Mont-Saint-Michel Bay (France)
Résumé
Tidal environments in the western part of Mont Saint-Michel Bay, reputed for its tidal range, comprise numerous shore-parallel bars and banks consisting of coarse biogenic sands rich in well-preserved shells. The shell banks can be grouped into three morphological types according to their size and their position within the tidal environments: salt marsh banks, upper tidal flat banks and mid- to upper tidal flat banks. The mid- to upper tidal flat banks migrate shoreward, at rates of several tens of m/year, from the lower tidal flat towards the Duchess Anne dyke, which forms the present shoreline, and which was built in the 11th century to protect a coastal marsh, the Dol marsh, from marine influence. The salt marsh banks are relatively stable, their migration rates not exceeding a few cm/year. The morphodynamic study of these sedimentary bodies highlights various stages of evolution from the western to the eastern part of the bay, depending on exposure to wave action which drives bank migration. The oldest, almost fossil salt marsh banks, are situated in the western part of the bay, while recent dynamic tidal flat banks develop in the more exposed eastern part. The upper tidal flat banks are gradually forming an almost continuous barrier located a few hundred metres in front of an old shell barrier on which the Duchess Anne dyke was built.