New Concept of the Structure of Upper Neopleistocene Deposits in the Main River Valleys of Western Siberia

The problem of differentiation and correlation of the Upper Quaternary sediments comprising the valleys of the main rivers in Western Siberia (primarily the Ob and Yenisei) was actively discussed in the second half of the 20th century and has not been solved conclusively at the end of the first quarter of the 21st century. Traditionally, the regional Quaternary history of the river valleys was considered as the “terrace staircase” paradigm, under which, in the Neogene–Quaternary, alluvial terraces formed sequentially from the older upper to the younger lower ones. Neotectonics, paleoclimate dynamics that changed the water abundance of the rivers, sea transgressions and regressions that affected the erosion base level, and frequent blocking of river valleys by northern glaciers leading to the formation of dammed lakes and the periglacial alluvium in the extraglacial zone were considered to be the factors that controlled their formation. However, the heights, age, and even the number of terraces differ significantly not only in the published works, but also on the official maps of Quaternary deposits. Significant data have been accumulated that contradict the concepts about a vertical staircase of successively downcut cycloclimatic terraces and periglacial alluvium facies that replaced the glacier-dammed northern lakes in the southern part of Western Siberia. A refined concept of the sediment structure in the Upper Neopleistocene in the main valleys of this region that takes into account the data on three glacier outburst floods from intermountain basins to the plain has been formulated and substantiated. The intense erosional and accumulative activity has continued to be underestimated.