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Late Pleistocene–early Holocene fluvial facies and depositional processes in the Fehmarn Belt, between Germany and Denmark, revealed by high-resolution seismic and lithofacies analysis



In the narrow offshore border zone between Germany and Denmark, 550 km of high-density 3·5-kHz subbottom seismic reflection profiles were recorded within a 70-km2 area in order to reconstruct the seismic stratigraphy of late Pleistocene to early Holocene lacustrine and fluvial environments. Using detailed line drawings, seismic facies analyses and a hierarchy of bounding surfaces, a depositional unit was recognized and subdivided into subunits 4a (oblique-parallel), 4b (mound, oblique-tangential), 4c (sigmoid, oblique), 4d and 4e (shingled and parallel). The base of this seismic facies association defines a wide U-shaped valley with well-defined scours and, in the valley sides, ‘steps’ are located above deep steep-dipping reflections. Stratigraphic control was available from 32 coring sites (5- to 12-m-deep vibrocores). Subunit 4b represents coarsening-up silt and sand, and samples from subunit 4d show fining-up fine sand, silt and clay. The seismic facies association is proposed to have formed by a fluvial event of short duration some time in the period between 10·3 14C ka BP and 9·0 14C ka BP. Subunits 4a to 4e represent gradually decreasing flow power. A peak flow initiated the fluvial event, after which water discharge and level fell rapidly. Subsequently, the normal background discharge from the Baltic Sea area dominated the flow style. Reflections beneath the ‘step’-like valley side with high dip angles are interpreted as faults. This tectonic activity resulted in subsidence in the analysed area and could possibly have influenced the fluvio-dynamic development. The seismic stratigraphic succession reveals a high-resolution record of sediments in this area. In particular, the stepwise uncovering of the morphology of the subunits, preserved in high-resolution seismic facies associations, is proposed as a useful tool in modelling the dynamic development of the near sea-floor environment.