First Results of Determining the Age of Alluvial Deposits in the Ishim River Valley Using Optically Stimulated Luminescence Dating

The river valley morphology and depositional strata of the numerous rivers of the West Siberian Plain are the primary source of information on the region’s paleogeographic history during the Pleistocene. In particular, a complex of 3–4 floodplain terraces composed of alluvial deposits of varying composition and age has been identified for the Ishim River valley; the chronology of the alluvial formations has been established based on paleontological data. In this study, the age of the alluvial deposits of the second left-bank floodplain terrace of the Ishim River was determined for the first time using the optically stimulated luminescence (OSL) dating. The studied section is located between the villages of Bystrukha and Spirina in the Abatsky district of Tyumen region, Russian Federation At the base of the section are exposed sands of the Kazantsevo thermochron (MIS 5e), based on an OSL-date of 132 ± 11 ka. These sands are overlain by sand, sandy loam, and loam with ground veins and thin buried soils of the Karginsky age (MIS 3). This is confirmed by OSL-ages in the range of 36–30 ka. A stratigraphic hiatus of approximately 95 ka is recorded between the sediments of the Kazantsevo interglacial and the Karginsky megainterstadial. The upper part of the section is represented by cover loess-like sandy loams, which were formed between 28 and 29 ka, based on dating results, under the conditions of Sartan cryochron (MIS 2). Thus, new data obtained clarify the history of the Ishim River valley development in the Late Pleistocene. The alluvial deposits of the second floodplain terrace were formed during the Kazantsevo interglacial; their upper part was eroded; sediments corresponding to the age of second half of MIS 5 and MIS 4 were not identified in the studied section. At the end of the Karginsky thermochron and during the Sartan cryochron, a thick layer of loess-like sandy loams rapidly accumulated mostly in subaerial conditions, and active stages of aeolian material accumulation were followed by short phases of stabilization, recorded as poorly developed paleosols. In the studied section of floodplain terrace of the Ishim River Valley, two cryogenesis events were identified in the Late Pleistocene, represented by small ground veins that formed during the Karginsky period.