Elsterian ice sheet dynamics in a topographically varied area (southern part of the Racibórz–Oświęcim Basin and its vicinity, southern Poland)

Abstract

During the Elsterian Glaciation, the Scandinavian Ice Sheet, before reaching its maximum extent, advanced over the mountainous foreland of Southern Poland, which was characterized by a substrate of increasingly varied relief. One such area was the hilly Rybnik Plateau of the southern part of the Raciborz–Oświecim Basin, located directly north of the Ostrava Basin and Moravian Gate, where the ice sheet advanced far south and formed a large lobe. In this study, till from the Łaziska site, located in southern part of the Rybnik Plateau, was analysed and interpreted. The till was deposited at the top of an W–E oriented ridge composed of Neogene clays. The study indicates that a high basal water pressure occurred at the base of the ice sheet, favouring a basal mechanism of ice sheet movement. Till fabric and kinematic structures indicate that ice flowed easily from the west over the ridge, parallel to its axis. Reconstructed patterns of ice flow direction reflect an ice sheet that was characterized by a strong spatially varied dynamics. The Odra Valley was the main corridor of fast flowing ice to the Ostrava Basin, from where ice was distributed radially in different directions. The second corridor of actively flowing ice was probably located in the Ruda palaeovalley to the north of the Rybnik Plateau. In the central part of the Rybnik Plateau, ice flow was restricted. This less dynamic ice sheet behaviour resulted mostly from the much higher location of the area. This study indicates that the large morphological and lithological variations of the ice sheet substrate induced varied frictional resistance, producing a spatially varied stress field within the ice sheet. This generated active zones of streaming ice adjacent to more passive zones. Potential switching between neighbouring active zones transporting ice towards the ice sheet margin is postulated.