Evolution of the fluvial system of the Prypiat, Desna and Dnieper during the Late Middle - Late Pleistocene [ Evolution des systèmes fluviatiles des rivières Prypiat, Desna et Dniepr au cours du Pléistocène moyen récent et du Pléistocène supérieur.

Abstract

A hypothesis that envisages Late Pleistocene braided rivers with enlarged meanders, unlike the present rivers of the European plains, anses from a tentative model for the development of the Prypiat-Dnieper-Desna alluvial plain in the late Middle - Late Pleistocene. This plain (local relief - 30-60 m) consists of a staircase of six river terraces and corresponding alluvial suites that have been forming since the Dnieper (Saale) Glaciation. Each terrace occupies a definite altitudinal step in this staircase and has a unique position within the plain as well as characteristic relief features related to terrace boundary shapes, longitudinal and cross profiles, primary fluvial and superimposed aeohan, cryogenic and erosional landforms. The alluvial suites have a cut-and-fill framework and noticeable differences in faciès composition expressed by sedimentary structures and other hthological features. Data inferred from the relief and from geological structures indicate complex fluvial system development with six main stages of varying duration and significance. These comprised one accumulative phase and five erosion-accumulation cycles, associated with weak pulsating tectonic uplift, expressed as 7-20 m of incision. The direct influence of the decay of the Dnieper ice sheet, alternations of glacial -interglacial runoff and stream load, significant fluctuations of precipitation, displacement of vegetation zones as well as permafrost thaw, all caused changes in the fluvial erosion-sedimentation regime. Glacially-fed suspended load streams, sandy meandering and braided streams, as well as sandy bedload braided and low sinuosity streams succeeded each other following the Dnieper Glaciation. These changes were accompanied by remarkable transformations of the river network general westward displacement, downstream displacement during the latest stages, abandonment of tributaries and inner fan formation. Proof of the existence of discontinuous permafrost or deep seasonal frost penetration has been obtained for the first time in the Late Pleistocene fluvial deposits of the area. Relying on the evidence of cryogenic and aeohan activity, the final phases of accumulation of the 1st Terrace Suite presumably correlate with the maximum of the Last Glaciation and the formation of the Peripheral Terrace Suite with permafrost degradation at the end of the Late Pleistocene.