Tatiana Vladimirovna Kropotova

The work provides a detailed description of four stacked soil profiles that make up the “crimson clay” pedocomplex in two Urzhumian reference sections of the Middle Volga region, Cheremushka ravine and Grebeni. Paleosol profiles have a similar structure with eluvial-gley, illuvial-carbonate and structural differentiation of the profile. Pedogenic processes were reconstructed: gleyization from the surface and along the root network, redistribution and segregation of iron oxides, slitogenesis, redistribution of carbonates. Four stages of pedogenesis have been identified, with a total duration of at least 20–25 thousand years, which occurred on flat fluviolacustrine plains in conditions of a warm semi-arid climate with pronounced seasonality of precipitation. The difference in the structure of the pedocomplex in the studied sections is determined by differences in the facies conditions of pedosedimentogenesis: an alluvial-floodplain regime for the Cheremushka ravine section and a lake-playa regime for the Grebeni section. The pedocomplex of “crimson clays” reflects a long stage of geomorphic stability of the landscape, it has been traced in sections of the right bank of the Volga River over a distance of about 100 km and is a pedostratigraphic marker, or Geosol.
To discuss the actual results obtained, the article provides a review of the author's data on paleosols on Permian red beds of the Middle Volga region, including Ufimian, Kazanian, Urzhumian, Severodvinian and Vyatkian deposits. For each stratigraphic interval, characteristic pedogenic and sedimentary features of paleosols and host rocks are given, on this basis, soil-forming processes and landscape-climatic factors were reconstructed. In general, the Permian red paleosols have a chemical and mineralogical composition inherited from the host rocks and a high degree of weathering of the siliciclastic material associated with ancient soils and weathering crusts of the provenance areas. The autochthonous nature of the gley features of paleosols formed during the process of pedosedimentogenesis are shown. Most mature soils are classified as chromic stagnicvertic calcic Paleocambisols or chromic stagnicverticPaleocalcisols. Mean annual precipitation calculated from geochemical indicators of illuvial horizons of paleosols shows cyclical changes. The minimum amount of precipitation — 400 mmyear–1 — was typical for the Urzhumian (Wordian) time, the maximum values — 777 mmyear–1 — were revealed for the Severodvinian (Capitanian) time.
Evaluation of the pedostratigraphic potential of Permian red-bed deposits and identification of the role of microorganisms and organic matter in the formation of paleosols are tasks for further study.