The role of an ice-sheet, glacioisostatic movements and climate in the transformation of Middle Pleistocene depositional systems: a case study from the Reda site, northern Poland

ABSTRACT This study provides a new stratigraphic and palaeogeographic scheme for the time span between Marine Isotope Stages (MISs) 8 and 6. Based on detailed sedimentological and palynological studies, the roles of the Scandinavian Ice Sheet (SIS) advances and changing climate in the evolution of sedimentary environments is considered. At the Reda site, northern Poland, vertically stacked, 25 m thick glacigenic and fluvial successions were distinguished and divided into four depositional complexes. Two glacigenic units (R1 and R 2), representing two SIS advances in study area, are separated by two fluvial units (R2 and R3). The first SIS advance recorded at the Reda site took place at the end of MIS 8. Proglacial fluvial systems transported material along the ice margin towards the NW (R1 complex). Migration of the forebulge during deglaciation led to incision and opened the outflow towards the SW. During the younger part of MIS 7, the older fluvial unit (R2 complex) was deposited in a sand-bed braided fluvial system in boreal climate conditions. Local erosion and change in the sedimentation style into a meandering one under periglacial conditions took place (R3 complex). The progressive cooling of the climate towards the end of MIS 6 resulted in another SIS transgression and deposition of coarse-grained sediments (R4 complex) in the ice-marginal zone during deglaciation. SIS played a key role in the formation of R1 and R4 complexes. Glacial processes managed the reshaping the landscape through the erosion of older sediments and in the formation of accommodation space. Glacioisostatic movement controlled accommodation space, accumulation-erosion ratio, size of catchment and morphology of the valley.