New data and a morphodynamic perspective on mid- to late-Holocene palaeoenvironmental changes in the French Flanders coastal plain,southern North Sea

Eight mechanical cores, 50 auger holes, 145 borehole logs, and 19 radiocarbon ages are used to identify the sedimentary facies and patterns of mid- to late-Holocene development of the French North Sea coastal plain near the Belgian border. Stratigraphic sections show a shallow Pleistocene basement that deepens towards the North Sea, overlain by up to 11 m of deposits comprising basal peat representative of freshwater marsh development, covered inland by essentially muddy tidal sediments, and by sandy tidal deposits near the present North Sea. The peat is conspicuously absent in a well-defined circular depression, ‘Les Moëres’, lying astride the border. A widespread layer of sandy silt up to 3 m thick, absent in Les Moëres, and commonly overlying peat elsewhere, generally forms the topmost stratigraphic unit in inland areas, while near the North Sea, a well-defined inland aeolian sand unit, the Ghyvelde dune, overlies tidal flat sediments. The radiocarbon ages, ranging from 5651 cal. BP to 1070 cal. BP (c. 4908 ± 52 yr BP to 1270 ± 48 yr BP), date the final 2—3 m of infill. The very slow sedimentation corresponding to this upper part of the overall infill appears to reflect a lower sediment accommodation space and very slow regional sea-level rise. The superposition of a layer of sandy silt over peat over much of the inner part of the plain highlights a dramatic change, probably associated with the tidal prism and tidal channel dynamics, from widespread freshwater marshes to renewed tidal sedimentation, as new sediment accommodation space was created through peat drainage and compaction. Further seaward, dominantly sand-flat sedimentation persisted throughout the Holocene, such sand flats forming the foundation for the Ghyvelde aeolian dune. This dune probably accumulated largely after c. 3500 yr BP, following the welding onto this sand-flat shore of one or more large nearshore sand banks that are the primary source of dune accretion on this southern North Sea coast. In this area, the persistence of the tidal Les Moëres depression may reflect a propensity for weaker sand supply from the sea to a coastal embayment that was being increasingly sheltered by such dune development, accompanied by increasingly underfit tidal inlets. These inlets were finally sealed off completely by the accretion of the modern aeolian dune barrier which also led to the isolation inland, within the tidal plain, of the Ghyvelde dune.