No abstract

Submarine dune dynamics are controlled by tidal currents and wind forces. According to the relative influence of these forces and the nature of dune sediment, different bedform behaviors can be observed. The footprint of the different hydrodynamic agents is recorded into the internal architecture of dunes. This paper is concerned with bedforms that compose the thick sediment wedge located in the eastern English Channel, off the Bay of Somme. This sedimentary archive constitutes an interesting feature to achieve a better understanding of seabed sediment dynamics and its timeline building stages. The dynamics of large submarine dunes, which are organized in fields, are studied thanks to bathymetric and seismic data over the periods 1937-1993 and 1993-2007. Dune morphology presents low lee and stoss side slopes (on average 8°and 3°, respectively) and dune migration rate is not very high. Dune movements are in the direction of residual tidal currents, i.e. toward the east, with mean migration rates around 0·8 to 5 Ϯ 0·25 m yr -1 and up to 6·6 Ϯ 0·7 m yr -1 , respectively, at multi-decennial and decennial time scales. The dune internal architecture is complex with superimposed eastward prograding units, displaying locally opposite progradation. Second-order discontinuities (dip of 0·5°-4°perpendicular to dune crests) constitute dune master bedding. By counting the number of second-order reflectors between 1937-1993 and 1993-2007, the formation periodicity of these bounding surfaces is estimated to range from 4 to 18 years. These time intervals coincide with the long-term tidal cyclicities and also with the inter-annual to decennial variability of storm activity in northern Europe. Two theories were made to interpret the dune internal structures: the second-order surfaces are interpreted either as the depositional surfaces corresponding to the marks of weak energy periods (weak tidal and storm action), or as erosive surfaces due to an opposite direction of dune migration provoked temporarily by exceptional storms from the northeast.