Loess-like siltstones from the urzhumian (middle permian) red beds of Kazan Volga Region, Russia

Abstract

© 2020 International Multidisciplinary Scientific Geoconference. All rights reserved. Here we focused on the massive, non-pedogenic, loess-like siltstones, which are common within the fluvio-lacustrine Middle Permian red sediments of Kazan Volga region, to refine their depositional settings. Sedimentological, mineralogical and geochemical methods including field description, grain size analysis, optical microscopy, bulk geochemistry and X-ray diffraction, were used. Siltstone facies from the Urzhumian red beds of this region are represented by two types: laminated and massive, the last one is commonly modified by pedogenesis. Pedogenically non-altered massive siltstones often look like loessites and are considered by some researches as an aeolian sediments. These siltstones commonly occur in tabular beds of 1-1.2 m thick and underlain by paleosols and overlain by laminated siltstones or lacustrine carbonates. The matrix-supported internal structure, the poor grain sorting, a significant proportion of sand material, gley spots along with convolute microstructures, microlamination fragments and brecciation signs suggest the fluvial character of sedimentation of these siltstones. Lack of fossils and root traces, sheet-like tabular beds with non-erosional contacts and their structural features indicate the rapid sedimentation, most likely, by sheetfloods. Gradual transitions from the massive loess-like to pedogenically modified siltstones and then to developed paleosols can be traced in the Urzhumian succession on the different stratigraphic levels. The mineral composition of studied rocks and geochemical indices including chemical index of alteration (CIA) are similar to those of the underlying rocks, which may indicate the same provenance of siliciclastic material and/or its redeposition. Thus, the massive loess-like siltstones under study are interpreted as fluvial, floodplain or mudflat deposits, formed by rapid sedimentation of sandy-silty material.