Long term evolution of deep banner banks offshore Brittany (France): paleo-coast line markers and interactions between hydrodynamics and seafloor morphology

This study focusses on the development and evolution of a banner banks set located offshore the Brittany coast (Iroise Sea, France) using multibeam bathymetric data, high-resolution seismic data, and grab samples. It aims to provide a comprehensive understanding of environmental parameters underpinning the long-term evolution and preservation of these major morpho-sedimentary bedforms. These banks have developed on a highly dispersive tide-and-storm-dominated shelf. Despite their very contrasting morphologies, the base of each of them lies at a similar depth range (100 to 90 m). The presence of dunes superimposed on the banks suggests that they remained active until today excepting for the Armen bank. Five seismic units have been also distinguished within each bank, separated by pronounced erosional unconformities. The surfaces are interpreted as the product of the gradual flooding of the bedrock outcropping across the shelf that modified the prevailing tidal conditions and directions of sediment transport. It thus appears that correlation between sea-level rise, the opening of straits across the shelf and the intensification of tidal currents are the key parameters of the long-term sand bank evolution offshore Brittany. The stepped morphology of the bedrock also appears to be a crucial factor in the initiation of the bank and its lasting anchoring between external and internal shelf. The preservation of their original nucleus indeed attests to their low lateral migration. With the exception of the silico-clastic basal unit, the banks are biogenic and fed by a process of self-recycling the sand within the sedimentary cell controlling the preservation of the bank. This study sheds light on the mechanisms of the formation and the deep anchorage of these deep banner banks, as well as their inherent link. It also highlights the dynamic nature of these large deep bedforms that are sustained by a fossil sedimentary stock.