Justification of the Age of Sands with Middle and Late Quaternary Theriofaunal Complexes in the Lower Ob River near the Village of Khashgort (Northwestern Siberia)

For more than half a century, the question remained unresolved why the closely located sequences of fluvial sands overlying Middle Pleistocene diamicton in the lower reaches of the Ob River near the village of Khashgort contain micromammal faunas of different evolutionary levels: Late Quaternary (430 km from the Ob River mouth) and Middle Quaternary (442 (440) km from the Ob River mouth). This was in contradiction with generally accepted ideas, according to which the absence of Middle Quaternary alluvium was assumed in this area and called into question either geological or paleontological interpretations of the Pleistocene history of north of Western Siberia. This paper presents the results of correlating two geological sections based on both geological and paleontological methods and suggests an updated biostratigraphic interpretation of the materials relevant to the Karginian and Tazovian horizons of Western Siberia collected in the 1980s and 2016–2022. It was established that the Late Quaternary assemblage of micromammals from the sections at kilometer 430 of the Ob River is confined to alluvium of the second terrace above the river floodplain, while the Middle Quaternary assemblage at kilometer 442 (the Khashgort locality, kilometer 440 of the Bolshaya Ob River) is associated with fluvioglacial sands showing signs of close redeposition of small mammal remains in the megaclasts contained in Middle Quaternary Khashgort boulder-bearing sandy aleuropelites. Thus, the study makes it possible to harmonize the geological and paleontological data that were in conflict for more than half a century.