Urzhumian Pedocomplexes of the Kazan Volga Region (Eastern Europe): An Example of Paleoclimate and Paleolandscapes of the Subtropics of the Continental Pangea in the Middle Permian

Abstract

The paleosol profiles of the Urzhumian from the section on the right bank of the Volga River near the village of Grebeni were studied by field and laboratory methods. Two paleosol profiles on red clayey siltstones were identified. They were classified as strongly developed Vertic Calcisols based on their pedogenic features such as horizonation, carbonate nodules, slickensides, etc. The climate during their formation was found to be seasonally wet and warm. The mean annual precipitation was calculated from the geochemical indices of the paleosols and amounted to 522 mm/year for the Late Urzhumian. The reddish-brown soils in the dry subtropics of the Mediterranean or Australia can be considered as modern analogs of these paleosols.The upper paleosol profile is a pedocomplex consisting of four single soils separated by nonpedogenic carbonate layers in the middle part. The pedocomplex structure contains one lacustrinepalustrine and two lacustrine-playa sedimentary microrhytms, which indicate the setting of lacustrinealluvial plains and make it possible to estimate the duration of its formation spanning thousands to tens of thousands of years.A pedocomplex with a similar structure was identified in the upper part of the Urzhumian of the reference section in the Cheremushka Ravine. Both pedocomplexes have very similar mineralogical, chemical, and granulometric composition of their siliciclastics, as well as the isotopic composition of pedogenic nodules. The biostratigraphic position of the pedocomplex in the Cheremushka Ravine shows that it is coeval with the pedocomplex of the Grebeni section. The correlation between them is direct, making them a useful pedostratigraphic marker. It is suggested to label this marker horizon as the “Crimson Clay pedocomplex”.