FLUVIAL RESPONSE TO PROGLACIAL EFFECTS AND CLIMATE IN THE UPPER DNIEPER VALLEY (WESTERN RUSSIA) DURING THE LATE WEICHSELIAN AND THE HOLOCENE

摘要

The Dnieper River in the westerly-aligned reach upstream from the Russian/Belarussian border exhibits deep early MIS 2 (pre-LGM) incision that is considered to have been caused by valley tilting associated with the growth of glacial forebulge. Glacial damming occurred around the LGM downstream from the city of Smolensk, locally shifting the valley southwards and stimulating substantial aggradation, although in a limited reach of few tens of kilometres. The failure of the glacial dam induced incision down to pre-LGM elevations, which was followed, before the onset of the Holocene, by further aggradation that was probably a response to a decrease in valley gradient caused by the subsidence of the glacial forebulge. This deformation continued into the Holocene and is estimated to have caused valley tilting by 4-5 m within a 100 km valley reach, which is noticeably larger than that predicted by the ICE-5G (VM2) model (Peltier, 2004) and suggests a delayed forebulge collapse in comparison with that predicted by modeling. In the Early Holocene, the river formed very wide braided or single-thread palaeochannels. Early Holocene channel morphology implies discharges estimated at three times that of the present-day river, which indicates a considerable increase of discharge from the Upper Dnieper catchment, presumed to be climatically driven. Substantial remnants of Moscovian (MIS 6) glacigenic deposits, preserved as intra-valley residual hills, are interpreted as having been dissected and isolated during valley initiation at the end of the Moscovian and then partly buried by fluvial sediments due to valley aggradation prior to the onset of the Holocene.