Influence of the composition and properties of Khvalynsk deposits on the evolution of soils and soil cover in the north-western part of the Caspian Lowland (based on the results of mineralogical and micromorphological studies)

The lithological composition of surface sediments within the Caspian Lowland is closely related to theCaspian Sea shoreline dynamics (Svitoch, ۲۰۱۴). Sediments of the Early Khvalynsk (Khvalynian)transgression have the OSL dates between ۲۵-۱۲.۶ ka BP (Yanina et al., ۲۰۱۷). They are typicallyrepresented by morphologically and chemically specific polyfacial strata chocolate clays (ChC), which areusually found on second terraces of Volga, Akhtuba and Ural rivers and only sporadically occur within theVolga-Ural Interfluve, where their influence on soil properties is insufficiently understood. Field studieson soils and microtopography were supplemented by physicochemical, mineralogical (JEOL JSM-۶۶۱۰LVSEM elemental microanalysis for particles ≥۰.۰۱ mm and an HZG-۴a X-ray diffractometer for particles<۰.۰۰۱ mm) and micromorphological (Olympus BX۵۱ microscope) analyses. The study was conducted onSolonetz complexes with different degrees of microtopography development and soil cover complexity,including Solonetz, Chestnut soils and Meadow-Chestnut soils on silty loams within the Dzhanybek Plain(between ۲۶-۲۸ m a.s.l.) and Brown arid soils on stratified sediments underlain by ChC in Lis'ya Balka nearLake Elton (۲۰ m a.s.l.). Near Lake Elton we identified mineralogical and micromorphological diagnosticfeatures of clayey and silty layers of ChC (identified from malacofauna), which had the coefficient ofcryogenic contrast (CCC) values of ≥۱ and ≤۱, respectively. Clayey layers had cryogenic cracks. Surfaceloess deposits included numerous fragments of ChC brought by local aeolian processes. Heterochronousgypsum accumulations were observed. At the Dzhanybek Research Station we firstly described Solonetzon microhighs, at the base of which there were ChC layers similar to those in Lis'ya Balka. Soils ofmicroslopes and microlows had buried horizons with paleocryogenic microfeatures (at ۵۰-۱۰۰ cm depths).Loess with fragments of shells and ChC had the following characteristics of cryoarid pedogenesis: CCC ≥۱, ooidal peds and biogenic structuring. Loess was overlain by silty-clayey material with the CCC ≤ ۱ andmicrofeatures of solonetzic process (coatings). Loess due to its tendency to subside predetermines thedynamic development of soil cover in the course of changes in recent and paleoenvironmental conditionsand the depth of groundwater table. The occurrence of ChC at a shallow depth under loess predeterminesa low complexity of soil cover and an absence of subsidence phenomena. The genesis of loess with ChCfragments is connected with aeolian processes at the Pleistocene-Holocene boundary and subsequentpedogenetic processes. As a result of this study, we can conclude that modern soils can be regarded aspedocomplexes with shallow-buried paleosols. This work was supported by the Russian ScienceFoundation, grant no. ۲۱-۷۴-۲۰۱۲۱.

Keywords:

Soil fabric , mineralogy , Caspian Lowland , Russia