PublicationEvolution of the sedimentary architecture of the Flemish coastal plain since Antiquity: contribution of ground-penetrating radar data

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The Flemish coastal plain is formed by the aggradation and progradation of Holocene marine and estuarine deposits, reaching a thickness of 30 meters below the current coastline. The focus is on the edge of the Denna River, one of the medieval watercourses of the coastal plain, which was located west of Gravelines and subsequently migrated westward to become the Aa River. The architecture of the last deposits before agricultural use is reconstructed using surface geophysics. Mapping of variations in soil electrical conductivity reveals the last network of tidal channels and drainage ditches. Ground-penetrating radar profiles, lithologically calibrated using vibro-carottes, show that the last channels, filled with Scrobicularia clays, are incised into a meter-thick aggradational sand sheet containing sponge spicules. These sands also contain decametric lenses of tidalites filling breccia umbilici formed by tidal overtopping along medieval dikes. The underlying heterolithic deposits exhibit meandering channel bar geometries detectable on ground-penetrating radar profiles down to 4 m below the surface. These deposits contain, in addition to ubiquitous bivalves characteristic of transition zones, foraminifera adapted to brackish environments. These findings echo archaeological knowledge acquired in the area. The transition from meander bars to a sandy sheet, although poorly dated, would coincide with the widespread construction of dikes from the 10th century onward. The overtopping of the dikes would trap sand entering the estuary during extreme events, accelerating the silting process, which was completed by the 15th century.