Sediment textural variability and mud storage on a large accreting sand flat in a macrotidal, storm-wave setting: the North Sea coast of France.
Résumé
The North Sea coast of France is a macrotidal, storm-wave coast globally characterized by an erosional trend, except for the area of Calais which exhibits a large (approximately 12 km²), actively accreting sand flat formed following the onshore migration, over the twentieth century, of a shoreface sand bank. The sand flat and contiguous beach exhibit large variability in sedimentation, ranging from pure sand to significant fractions of mud, depending on the morphology (multiple intertidal bars and troughs, upper beach platform and foredune). In order to highlight the variability in sedimentation, in situ wave and current measurements, and sedimentary (samples and short cores) and topographic surveys were carried out along a cross-shore profile during three field experiments involving variable wave and tidal conditions and spanning different seasons. The results show an onshore decrease in current speeds and in wave heights compatible with a fining trend, from sand in the lower sand flat, to sand and interbedded mud layers towards the upper flat. Analysis of the sediment textural and spatial patterns over a one-year period show an increase in the mud fraction in the lower sand flat and the onshore migration of these fine sediments onto the upper sand flat, resulting in significant muddy sedimentation in this latter zone. This pattern shows that an accretionary context on sand-rich, storm-wave dominated macrotidal coasts offers conditions for the significant trapping of mud, with implications for both contemporary management issues and the rock record.