Učënye Zapiski Kazanskogo Universiteta. Seriâ Estestvennye Nauki (Dec 2016)
Paleogeography of Accumulation of the Middle-Upper Permian Red Mudstones in the Kazan Volga Region
Abstract
In this work we focus on sedimentology, mineralogy, grain size, and geochemistry of red mudstones of the Urzhumian (Wordian) and Severodvinian (Capitinian) reference section of the Monastyrskii ravine to specify their depositional settings and paleoclimatic conditions. In the section, two types of mudstones have been identified based on their structure: a) massive and b) laminated. The former ones do not contain faunal and plant remains and are often altered by pedogenic processes, the latter ones may have ostracod and bivalve shells or fish scales and sometimes bear the sings of short-term shallowing and drying. The bulk geochemical analysis of siliciclastics has revealed a high degree of weathering (chemical index of alternation, CIA ~ 72–79) of both types of mudstones, as well as their source rocks (Permian red beds of the Cis-Ural plains). Massive mudstones have non-erosional contacts, they are confined to the regressive stages of sedimentary cycles, and their composition is dominated by fine and medium silt with angular grains. In the geochemical profile of paleosols developed on massive mudstones, under almost constant CIA values, there have been found levels with the high Ti/Zr ratio, which corresponds to the bimodal distribution of grain size. This could be an evidence of an input of clastic material during the pedogenesis process, and the surface morphology of quartz grains indicates their aeolian origin. The study of the paleosol profiles widely represented in the section has allowed reconstruction of the semi-arid climate with distinct rainfall seasonality. The clastic material has been transferred into the Urzhumian sedimentary basin from the Cis-Ural plains by the fluvial way under the semi-arid climate conditions, thereby leading to the formation of laminated mudstones in shallow lakes with periodical drying and on floodplains. An increase of the aeolian silt transport occurred in dry seasons during the stages of lake regressions, when the fluvial plains prograded basinwards simultaneously with the formation of soil cover.
