LAUSR

Abstract The aim of the research conducted in an outcrop of late Saalian sediments in Siedlątkow (Central Poland) was to reconstruct extreme meltwater flows during a jokulhlaup on the basis of the geological record. A combination of field and laboratory lithological methods was used, which included: lithofacies analysis, analyses of grain-size distribution, shape and roundness of gravelly clasts. Examination of sediments made it possible to recognise palaeohydrological, sedimentation and deformation processes during and immediately after the flood. In particular, the simultaneous operation of water flows in four paths in the immediate foreland of the ice-front was reconstructed. Mechanisms of transport and sedimentation of deposits were determined. Characteristic features of the analysed jokulhlaup deposits include erosional and deformational contact with the substrate, a high content of very coarse sediments, poorly sorted with predominant massive clast-supported gravels and boulders, the presence of rip-up clasts, inverse grading in the lower part of the profile, normal grading in the upper part, and similarities in clast shape and roundness and the ratio of local rocks to Northern origin rocks when compared to the surrounding Saalian till. The documented features allow for the following phases of jokulhlaup to be distinguished: initial, rising, high stage, and waning stage. During the reconstructed event, hyperconcentrated flow was dominant, whereas fluvial flow and debris flow were of lesser importance. The release of meltwater in the marginal part of the ice sheet came from the depression of the Kolo Basin, to the north of the analysed sites. Complex deformation processes which occurred in the flood deposits and their substratum were also interpreted. The analysed jokulhlaup event resembles some minor glacial floods in the ice-contact zone of present-day Icelandic glaciers. Further research in marginal zones of Pleistocene ice sheets should account for their occurrence, as the analysed example indicates that jokulhlaups might have had considerable influence on the course of sedimentation processes in the marginal zones of ice lobes of Pleistocene ice sheets as well as in the marginal valleys and sandars.