The origin of deformation structures in periglacial fluvial sediments of the Wisłok Valley, southeast Poland

Numerous deformation structures of various shapes and internal structures (drop‐like and pocket‐like structures, folds, streaks and diapirs) are present in the Weichselian alluvial terrace sands and loams of the Wisłok river (Sandomierz Basin, southeast Poland). A detailed study of the structures leads to the conclusion that they originated from cryostatic pressure, gravitational loading of sediments with reversed density gradient and thermokarst. Under cryostatic pressure, irregular structures formed in channel sands and gravels and wide folds occurred in clayey organic loams during the Greenland Stadial 2 when permafrost expanded. The variable forms and complex structure record the progress of downward displacement interrupted at various stages by water escape. Small sandy load casts and folds in the lower and middle parts of flood rhythmite also formed during the Greenland Stadial 2 when permafrost stabilised. Large deformation structures in the topmost rhythmite layers formed under the weight of overlying channel deposits during the Greenland Interstadial 1 as permafrost degraded. Copyright © 2010 John Wiley & Sons, Ltd.