Internal structure of shoreface ranks revealed by high-resolution seismic reflection in a macrotidal environment (Dunkerque Area, Northern France)

The large-scale internal structure of shoreface banks located off Dunkerque (N France) in the Southern Eight of the North Sea is imaged in a very-high resolution seismic reflection study. An interpretation is proposed of the processes controlling the construction and migration of these banks located in a macrotidal coastal environment. The Dunkerque coastal system is characterized by strong, coast-parallel, tidal currents, and northerly moderate storms. The banks are coast-parallel and almost emergent during low-water spring tides. Their length, width and maximum height reach 8 km, 1.5 km and 12 m respectively. They have an asymmetrical profile with a steeper flank dipping either landward or seaward. Bathymetric investigations show that some parts of the banks migrate actively landward, while others tend to elongate or to migrate seaward. The steeper flank indicates the direction of the dominant migration. Seismic data reveal one main feature: the reflectors observed beneath the landward flank dip landward, and the reflectors observed beneath the seaward flank dip seaward. This large-scale internal structure conforms with the bank morphology and reflects the present-day migration mechanisms. According to bathymetric, hydrodynamic and seismic data, the landward migration component is induced by northern storm wave action and the longshore/seaward component is tide-controlled. Seismic data show that this migration pattern was occasionally reversed, some parts of the banks that presently migrate longshore having migrated landward at an earlier time (and vice versa). The predominance of storm-induced landward migration over tide-induced longshore migration (and the reverse) is related to water depth and surrounding seabed morphology evolution. The banks are thought to be recent sedimentary features of a few centuries. They represent very active sand bodies the migration of which induces significant morphodynamic modifications of the coastal system, especially of. the beach domain to which they tend to attach.