ИСТИНА

The importance of buried soils as pedostartigraphic units of loess-paleosol sequences for paleogeographic interpretations could be hardly overestimated. On the other hand, the influence of pedogenesis on lithostratigraphic units of loess strata attracts much less attention. Though, since the very beginning of its scientific study loess was regarded as a result of deep transformation of primary sediment by pedogenesis (Richthofen, Berg, Obruchev). I.P Gerasimov regarded loess units, comprised between buried soils in temperate regions as primitive synlithogenic cryoaridic soils. Studying of the upper loess strata within the profile of surface soils in the center of Russian Plain allow to clarify the role of pedogenetic processes in the formation of characteristic features of loess. Under the influence of periglacial environment a network of thermokarst depressions was formed, occupying 20-30% of terrain. Constitution of loess strata depends on its position in a paleocryogenic complex. A clear evidence of sequential loess sedimentation, accompanied by slope processes and pedogenesis is presented in soil profiles within depressions. Sequential sedimentation is confirmed by cryomorphic features (ice wedges, frost cracks, pseudomorphs) at different levels of loess strata. Slope processes resulted in differentiation of the loess material into brownish and whitish microstripes. Periods of soil formation produced buried humus horizons at different depth (black second humus horizons at a depth of 30-40 cm, dirty brown horizons of a primitive cryohydromorphic soils at a depth of 140-150 cm from the surface, etc). Interrelation of cryomorphic features, solifluction bands and buried soil horizons confirm that sedimentation of loess material, slope processes and pedogenesis was mobile equilibrium, changing within the upper 3 m of loess strata. So that periods of low rates of sedimentation resulted in developed humus horizons of cryohydromorphic soils. Comparison of soil profiles within depressions and in the inter-depression areas confirm, that final stages of loess accumulation occured when thermokarst depressions already existed. Loess in inter-depression areas looks like uniform strata, though its correlation with corresponding layers within depressions confirm sequential accumulation, combined with primitive pedogenesis that leaved no developed soil profiles. Pedogenesis resulted in formation of a complex of secondary carbonates (impregnation of plasma, pseudomicellia, white powder, hard and soft nodules, etc.), formation of loess structure and pore space of both biogenic and cryogenic nature, as well as other features, characteristic for primitive soils of cold arid environment. Uniform intensity of these features within the upper loess strata confirms that synlithogenic pedogenesis accompanied loess accumulation.