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Freek Busschers, Deltares/TNO Geological Survey of the Netherlands / Vrije Universiteit Amsterdam (Netherlands)
Kim Cohen, Utrecht University (Netherlands)
Ronald Van Balen, Vrije Universiteit Amsterdam (Netherlands)
Cees Kasse, Vrije Universiteit Amsterdam (Netherlands)
Jakob Wallinga, Delft University of Technology (Netherlands)
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We present new data from the late Middle and Late Pleistocene record (Marine Isotope Stages 6-2) of the Rhine-Meuse fluvial system in the Dutch sector of the southern North Sea Basin. Quartz OSL-dating and pollen-data allowed time-correlation of fluvial degradation-aggradation cycles and channel belt migration to phases of proglacial lake formation and glacio-isostatic crustal movements that are known be related to Fennoscandian glaciations.
Proglacial lake formation
Enigmatically elevated proglacial Rhine-Meuse sediments suggest that during the late Middle Pleistocene Drenthe glaciation (MIS6), the Rhine-Meuse fluvial system entered a vast proglacial lake that existed in the southern North Sea area south of the coalescing Scandinavian and British ice-sheets. The lake covered the entire southern North Sea area and had shoreline positions close to the current sea-level. We postulate the lake level to indicate the elevation of a saddle along a former drainage divide to the southwest of the North Sea Basin (Strait of Dover and Southern Bight) consisting of Cretaceous-Jurassic and/or Tertiary strata. We identified the development of a deeply incised valley during the terminal phase of the Drenthe glaciation, to represent the first Pleistocene Rhine-Meuse palaeovalley that continued through the Dover Strait. The valley, of which initial formation started due to spillage over the saddle, reflects the final down cutting event of the Strait of Dover to depths below interglacial sea level, allowing exchange of marine waters between the English Channel and North Sea during the following sea-level high stands (MIS5e, MIS1) postdating the proglacial-lake event.
Preliminary results from the northern Netherlands suggest that more proglacial lake phases existed in the MIS12-7 timeframe although better chronological control is needed to place these in a correct chronostratigraphic framework.
Glacio-isostatic uplift
Geophysical models that use ice limit data for input and sea-level field data for calibration, predict glacio-isostatic uplift of the foreland area around the late MIS3/MIS2 ice margins. We found indications that the Rhine-Meuse system was strongly influenced by this glacio-isostatic controlled forebulge upwarping. Upwarping-controlled lateral valley tilting deflected the Rhine-Meuse channel belts after 35 ka. We observed a phase of strong incision shortly after (30-24 ka) which is explained as ongoing response to the glacio-isostatic upwarping and adaptation of the Rhine's longitudinal profile in order to maintain equilibrium profile as it traversed towards the Dover Strait. During a later stage (24-16 ka) glacio-isostatic controlled incision was overruled by high climate controlled sediment input and initial glacio-isostatic subsidence of the area. Re-migration of channel belts towards the direction of the former uplift indicates that glacio-isostatic forebulge collapse influenced Rhine-Meuse palaeogeography until far into the MIS2/1 timeframe.
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