The role of the aeolian factor in the formation of the landforms of the Lower Volga region

The history of the study of Baer knolls (BK) goes back more than a century. Baer knolls are ridges orientedmostly in WNW-SW direction, widespread in the lower reaches of the Volga and more pronounced in theVolga delta. BK in the Volga delta have morphology different from other areas of their distribution due tothe impact of exogenous processes such as erosion by the Volga and its channels, abrasion (duringNovocaspian transgression) and wind-wave processes, which simulated, as a rule, latitudinal spreading ofridges. BKs are absent in some meridional parts of the central part of the Volga delta and near its easternmargin. During the Mangyshlak regression, actively incising watercourses eroded the ridges that hadalready existed by that time. It has been revealed that some isolated massifs of BK have remained, probably,due to their location on more elevated areas. Up to now the genesis of these notable landforms has not beendetermined with sufficient precision to prove one of the existing hypotheses. Currently, the best knownand most widely accepted scientific hypothesis is the aeolian hypothesis of the origin of the BK, which arelargely similar to the sandy ridges in the deserts of Turkmenistan, which allowed a number of researchersto consider them as aeolian forms. The purpose of this paper is to attempt to show the weaknesses of theaeolian hypothesis of the origin of the Northern Caspian Sea Baer knolls. The authors of the aeolian theorystate that the BKs are formed as a result of the equilibrium of winds from several directions. However, thecontemporary picture is somewhat different, and the dominant wind direction is difficult to identify: itvaries from west to east rumba, sometimes shifting to south or north rumba depending on the season ofthe year. It is likely that the pattern was similar at the end of the Pleistocene as well. Other researchers, alsosupporters of the aeolian origin of BK, assume that at the time of their formation the winds of one directionprevailed, either east or west, without giving any arguments to support this assumption. Also it should benoted that BK are absent on some local elevations, for example on salt domes. The aeolian hypothesis doesnot explain the following lithological features of the hillock strata: high cementation of sediment, a widevariety of layering types, the presence of interlayers and lenses of detritus and individual shells, and asharp erosion boundary between the upper and lower hillock strata of sediment. In addition, there isfrequent erosion of the hillocks from the sides and then, in some cases, accumulation of new sedimentstrata, where the type of stratification and slope of the strata and their dip azimuth are sharply differentfrom the rest of the hillock. As the detailed field investigations of the BK formation layering have shown,the most common are oblique cross-sectional, wavy with ripple marks, diagonal, horizontal and obliquelayering. This variety is not at all characteristic of aeolian type sediments, but belongs to the aqueoussedimentation type. The presence of oblique stratification in the strata is related to the flowing of thereservoir, which was unsteady and erratic, because along with the oblique stratification there is alsohorizontal, weakly visible stratification. We suggest that the BK was formed in a slightly saline flowingenvironment, when there was simultaneous erosion and associated accumulation of sandy sediments andchocolate clays. In addition, due to the presence of salt in the water, clay particles coagulated andprecipitated as flakes simultaneously with silty and sandy particles, as they do now [Badyukova, ۲۰۱۸], inparticular in near-salt areas, where silty and clay are present along with sands. We will present the newresults of the BK studies and describe many other evidences of the aqueous sedimentation.

کلیدواژه ها:

Baery knolls ، Lower Volga ، Caspian Sea ، Late Quaternary ، Northern Caspian lowland. References