Justification of the Age of Sands with Middle and Late Quaternary Theriofaunal Complexes in the Lower Ob’ River Near the Village Khashgort (North-Western Siberia)

For more than half a century, the question remained unresolved why the closely spaced sequences of fluvial sands overlying Middle Pleistocene diamicton in the lower reaches of the Ob’ River near the village Khashgort contain micromammal faunas of different evolutionary levels. The sequences 430 and 430a at kilometer 430 from the Ob’ River mouth yielded Late Quaternary faunas and the sequence Khashgort (= Bolshaya Ob’ 440 km) at kilometer 442 from the Ob’ River mouth yielded Middle Quaternary faunas. This contradicted 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 proposes a comparison between two geological sections using both geological and paleontological methods and suggests an updated biostratrigraphic interpretation of the materials relevant to the Karginian and Tazovian horizons of Western Siberia collected in 1980s and 2016–2022. It is established that the Late Quaternary assemblage of micromammals from the sections at kilometer 430 of the Ob’ River is confined to alluvium of the 2nd terrace above the river floodplain. In the locality Khashgort (Bolshaya Ob’ 440 km) at kilometer 442 of the Ob’ River, the micromammal assemblage is associated with fluvioglacial sands showing the signs of close redeposition of paleontological materials with the outliers comprised in the Middle Quaternary Khashgort aleuropelites bearing boulders and sands. Thus, the study makes it possible to harmonize the geological and paleontological data that were in conflict for more than half a century.