The Quaternary geological evolution of the Belgian continental shelf, southern North Sea

With respect to the Quaternary deposits, the Belgian Continental Shelf (BCS) was one of the last unmapped and unknown areas of Belgium. Because of the absence of a distinct shelf break and the virtually complete lack of subsidence, the BCS has very little accumulation space to accommodate and preserve Quaternary sediments. The Quaternary on the BCS is very patchy and discontinuous, and has a maximum thickness of only 45 m. From this fragmented record it was very difficult to produce a coherent reconstruction of the Quaternary evolution, in times when only analogue data were available. At present, > 5000 km of analogue high-resolution seismic profiles have been scanned, converted into digital ‘segy’ format, and integrated with almost 500 core descriptions, enabling us to develop a genetic model for the Quaternary evolution of the BCS. The seismic data show seven seismic-stratigraphic units, in agreement with previous studies on one of the sandbanks of the BCS. Three basal units (U1, U2, U3) infill a large valley, incised during the Saalian ice-age in the Top-Paleogene (former Top-Tertiary) surface, i.e. the Ostend Valley. The three units, separated by tidal ravinement surfaces, represent successive phases of a transgressive estuarine infilling during the Eemian sea-level rise. After the final phase, shoreface erosion was that severe, that seismic unit U3 was completely levelled with the Top-Paleogene surface, and Eemian remnants are only found in depressions. During the subsequent Weichselian lowstand, a minor sinuous river incised in the Eemian transgressive surface, where previously the Ostend Valley was present. Directly on top of this surface no Weichselian cover sands have been encountered, but early Holocene tidal flat deposits, i.e. seismic unit U4. The tidal flat environment developed behind a coastal barrier which migrated landward with the Holocene rising sea-level. In the sand layer left by the barrier migrating over former tidal flat deposits, coastal storm-dominated banks formed (U5). These coastal banks were partly eroded when the barrier stabilised and started prograding seaward again, in reaction to the slowing down in the Holocene relative sea-level rise. The tidal flat area behind the barrier, silted up and an extensive surface peat developed. With a new tidal pulse, the barrier migrated landward again, but stabilised before it reached the present-day coastline. Until in the 15th century, storm surges induced the drowning of the island Wulpen, which caused irreversible hydrographical changes in the mouth of the Western Scheldt. Due to these changes and the consequently stronger tidal currents, the original natural and storm-induced shoreface ravinement surface was deepened. This did not happen though until the middle of the 16th century. After that, the eroded, high-organic muddy sediments (of former back-barrier deposits) could settle, alternated with sandy storm layers. This represents seismic unit U6. Since around 7000 cal BP, tidal sand banks and intervening swales (U7) developed on top, and from, the former deposits, and form now the main features of the present-day bathymetry. In an area with low accommodation potential and almost no new sediment input, amalgamation and reworking of sediments are common processes, and little accumulation of sediments takes place. Most of the deposits were strongly reworked to form new deposits, and little was preserved, which is what made it challenging to reconstruct the Quaternary evolution of the Belgian shelf.