Lower–Middle Pleistocene Stratigraphy of the Southern West Siberian Plain: New Data

—The Quaternary stratigraphy of the southern West Siberian Plain is considered in the context of the updated International Chronostratigraphic Scale, with the Neogene/Quarternary boundary at 2.588 Ma. New geological, lithological, paleontological, and paleomagnetic data from a reference Quaternary section in the Irtysh River valley near Isakovka Village provide more rigorous constraints on the Lower–Middle Pleistocene stratigraphy and the respective deposition conditions. The Isakovka outcrop exposes two stratigraphically expressed and paleontologically characterized units that were deposited during regional Pleistocene interglacial events of different ages: the Strunino and Serebryanoe alluvium beds lying over eroded surfaces. The species of the Corbicula genus coexisting with numerous species of Palearctic molluscs, small mammals Allophaiomys deucalion, Mimomys reidi, and last rooted lagurids Borsodia found in the Strunino alluvium are known also from continental equivalents of the warm upper Gelasian Stage in different parts of northern Eurasia. Judging by the presence of extant corbicules, the Strunino alluvium was deposited in a warm climate, with a mean annual air temperature above +16 °C and a mean winter temperature no colder than –8 °C, in rivers that remained free from ice all year round. On the basis of fauna constraints for the Serebryanoe alluvium, along with the first appearance of the European Pisidium clessini molluscs in West Siberia, the unit can be correlated with one of the earliest Middle Pleistocene interglacial events. The faunal assemblages and spore-pollen patterns of the two units indicate that the climate during the Serebryanoe deposition was slightly cooler and wetter than during the Strunino deposition, though steppe landscapes predominated on watersheds in both events. The two alluvial beds, which are traceable in river bluffs, at 30 m above the modern water level in a tectonically stable part of West Siberia, mark the hypsometrically higher position of the river network during the Early Pleistocene and early Middle Pleistocene interglacials.