The importance of geological conditions for the formation of past thermokarst closed depressions in the loess areas of eastern Poland

Closed depressions (CDs) are common forms occurring in the European loess belt. So far, investigations of CDs in Europe have suggested various natural or/and anthropogenic processes leading to their formation. The origins of CDs occurring in the loess areas of Poland have been the subject of few investigations, and their results have not clarified the problem. Most frequently, the age of CDs is linked with the post-glacial period, or the final stages of the formation of the loess cover. The investigations of CDs carried out in eastern Poland (Nałęczów Plateau) have so far revealed some patterns with regard to the morphometric characteristics and distribution of CDs on the regional scale. They also suggest the impact of thermokarst processes on the formation of the CDs. Five main lithogenetic types of sediment underlying the loess cover have been documented: glacial tills, clay and clayey loams, patches of glacial tills and sandy deposits, sands with gravels, and the bedrock. The relief under the loess cover has also been documented. The types of sediment as well as the relief under the loess cover have an impact on local differences of water content in the loess sediments. It was found that the variatiability of the geological conditions in the Nałęczów Plateau in the Pleistocene had an impact on the local variation of the ice content in the upper part of the former permafrost. This led to local predispositions for the development of thermokarst CDs during the Last Glacial. Areas with a high density of CDs have less permeable sediments (glacial tills, clay and clayey loams) and small relative heights under the loess cover. In the Pleistocene, these areas had higher ice content in the upper part of the permafrost. A model of thermokarst CDs development in the loess areas in eastern Poland is proposed. The model shows that the distribution and size of thermokarst CDs depends on the thickness of the loess cover as well as the types and relief of sediments underlying the loess. The development of thermokarst in the region studied may have consisted of multiple stages resulting in superimposed CDs. Between two and four stages of thermokarst CDs development can be distinguished on the Nałęczów Plateau. The two main stages occurred in MIS 4/3 (~58 ka or 55-50 ka Oerel and Glinde interstadials) and MIS 2/1 (~12 ka). These landforms may have developed also during the Denekamp Interstadial (32-28 ka) and ~15 ka. The present investigations indicate significant morphogenetic effects of permafrost melting on the contemporary relief of the loess areas.