Stratigraphy, chronology and paleogeography of Late Quaternary cryogenesis in Northern Caspian Lowland

The article presents a generalization of the results of field and analytical studies of cryogenic phenomena in the Lower Volga region. For the first time for this territory, pseudomorphs, soil wedges and cryoturbations were described and studied in detail. Their cryogenic genesis was substantiated. In the Lower Volga region, various structures have been identified in loess-soil series, alluvial and marine deposits. The development of cryogenesis in similar environmental conditions, but in different genetic types of sediments, leads to the formation of structures of different shapes, which directly depends on the humidity and composition of the sediments. The processes of ice degradation and accompanying changes in their morphology are of decisive importance in the final appearance of soil structures. Absolute dating of the deposits containing cryogenic structures made it possible to identify the time intervals of their formation. Six stages of cryogenesis in the Late Pleistocene were identified based on the structural features, their stratigraphic position, and the results of laboratory analyzes. Stage I is characterized by the spread of deep seasonal freezing in the region, recorded in coastal marine sediments in MIS 5d. For stages II-III (MIS 5b, MIS 4, respectively), the existence of a perennial permafrost zone is reconstructed, cryogenic forms are recorded in various genetic types of sediments. Stage IV (MIS 3c – MIS 3b) corresponds to the existence of a perennial permafrost zone only for the northern part of the region (Srednyaya Akhtuba and Raygorod sections) and thin sporadic permafrost or deep seasonal freezing for the southern part of the Volga River valley (Chernyy Yar section). Stages V (MIS 3a) and VI (MIS 2) are characterized by the spread of thin sporadic permafrost or deep seasonal freezing. The identified major stages of the development of permafrost in the Caspian Lowland significantly refine the available data on the cryogenic horizons of the East European Plain.