ABOUT THE GENESIS OF THE RIDGE-HOLLOW RELIEF IN THE SOUTH OF WESTERN SIBERIA

Abstract

In the south of Western Siberia, predominantly in the territories of the Ishim Plain and Baraba Lowland, significant areas areoccupied by complexes of ridges and inter-ridge depressions elongated in a north-easterly direction. In the literature such forms of relief are called‘griva’ (ridges), and the type of relief – ridge-hollow.The key question in the study of these geomorphological complexes is related to their genesis. Among the numerous views on the originof these forms, there are two most popular groups of hypotheses, opposite in terms of the prevailing forming process they indicate, — aeolian andaqueous. Proponents of the first one insist on the decisive role of wind accumulation; the second one, omitting the variability of paleogeographicalconditions, considers all possible factors associated with the impact of water: accumulation, erosion, their combined effect, etc. It is worth notingthat many hypotheses either lack palaeogeographic references or are outdated. By analyzing the accumulated materials about this problem, as well as by comparing the ridge complexes with morphologically similarforms in other regions, we revealed discrepancies in the lithological description and morphology of ridge-hollow complexes within different hypotheses. The differences in a number of characteristics of individual ridge complexes have also been pointed out.Basing on the modern ideas about the paleogeographical conditions of the region, the identified differentiation of the ridge-hollow complexes, and taking into account the revealed discrepancies, we propose the polygenetic hypothesis to solve the problem of the genesis of the ridges,according to which the formation of the loessoid deposits, which make up the ridges, begins in the Middle Sartanian time (about 19 thousand yearsago) and occurs in the cryo-arid conditions. An arid climate leads to the development of active aeolian activity within drained lakes and shallowriver valleys; in addition, active cryogenic weathering leads to the grinding of material to the silty fraction, characteristic of the loessoids in thesouth of Western Siberia. The formation of dune relief is a consequence of these processes. The created ridge forms continue to be influenced bypermafrost processes. A further tendency toward climate mitigation, consisting primarily in increased wetness, causes the development of thermokarst processes and waterlogging of ridges located in lake hollows and flood plains of large river valleys. These transformations have a modelingeffect, smoothing out the shapes of the ridges and gradually bringing them closer to their modern outlines. The result is ridge-hollow complexes.