Dynamique de construction d'un prisme sédimentaire littoral en régime mégatidal (la Baie du Mont-Saint-Michel)
The Bay of Mt-St-Michel is a megatidal environment, the Holocene sedimentary infill of which began some 8000 years ago. The present-day Bay consists in various sub-environments: an embayment in the West, an estuary in the East, and a sandy barrier subject to wave action in the NE. Thanks to VHR seismic data and cores, different architectures are highlighted: progradational to aggradational in the embayment, aggradational to chanelized in the estuary and along the NE shoreline. The infill model established with all the data is characterised as follows: a transgressive systems tract of low volume; a well-developed highstand systems tract; a tidal ravinement surface of large extent, reaching locally the substrate; a wave ravinement surface locally present along the estuary margins; an accommodation for sequence preservation of tidal origin in the estuary, and of eustatic origin along the margins. At the basin scale, discrepancies in time-and-space of the preservation potential of sedimentary sequences observed into the infill are very important. As soon as the transgression rate slow down from 6500 yB.P., the key-control of local factors (hydrodynamics, sediment supply, substrate morphology) is evidenced. Sequences preserved along the estuary margins record environmental changes interpreted as the result of climate crisis of 1500-year period. Although the origin of these climate cycles is uncertain (Bond cycles, long term tidal cycles, combination of both?), their control on coastal system evolution should be considered as major.