The Role of Late Pleistocene Permafrost Processes in the Formation of Thufurs in the Southern Cis-Urals Steppe

Abstract

In the moist meadow areas of Southern Cis-Urals, on lake banks and low river terraces, one can find small, bumpy relief, represented by low oblong hummocks separated by hollow depressions. In the secondary scholarly literature, it was named “thufurs.” The microrelief forms a polygonal grid and resembles the migration of permafrost mounds. This paper is the first to study the distribution, morphological structure, and formation of thufurs in the Southern Cis-Urals steppe. Our research revealed a complex of relict (polygonal terrain with a polygon side of about 2.5 m, ground wedges, cryogenic crushing in ground wedges) and modern (fresh microreliefs of hummocks, traces of frost sorting, seasonal ice formation, and rock heaving in conditions of high pre-winter humidity) cryogenic processes. The report indicates that the development of the thufur microrelief is caused by paleo-cryogenic processes: the dissection of the surface by frost-breaking cracks, the growth of polygonal-vein ice in them, and the formation of a complex soil cover after their thawing in the Holocene. Currently, the microrelief is maintained through contemporary processes of differential frost heave. Thufur soils combine paleo- and modern cryogenic features.