Preservation potential of highstand coastal sedimentary bodies in a macrotidal basin : Example from the Bay of Mont-Saint-Michel, NW France

This study investigated the spatio-temporal evolution and preservation of a highstand coastal wedge in the Holocene deposits of the macrotidal Bay of Mont-Saint-Michel (NW France). Vibracores were collected in two contrasted environments of the Bay : (1) in the estuary, which is characterized by very strong tidal currents, and (2) along the north-eastern littoral, which is made up of an elongated wave-dominated barrier. Facies analysis associated with radiocarbon dating revealed marked differences in the preservation potential of sedimentary sequences in both environments. In the general framework of a very slow rise in sea level, these two environments are subject to different hydrodynamical, sediment supply and coastal morphology conditions. In the northeastern area, the coastal wedge is made of successive tidal lagoon infill sequences. Due to their back-barrier origin, each individual sequence has been preserved; the supply of sediments combined with wave dynamics are the main factors that control the way of the system functions. In the estuary, the sediment wedge comprises a single tidal channel infill sequence reaching down to the substratum. In this area, the sediment supply is very high and tidal currents are very powerful. As the active tidal channel occupies all the available space, lateral migration of the channel is the main factor that controls the temporal and spatial development and preservation of the sequence. The potential for the sequence preservation in the estuary is therefore minimal and only the last channel infill sequence is in fact preserved. Dating revealed that the tidal lagoon sequences and the tidal channel infill sequence correspond to millennial and centennial time scales respectively. This study showed that local factors such as the sediment supply and the hydrodynamics, combined with pre-existing topography, play a significant role in the preservation of coastal wedge sequences during sea-level highstand conditions. It also revealed the time scale of the preserved sequences