Stromatolites in the Upper Visean sediments of the Pacha-Lasta section (Ilych River, Northern Urals)

Abstract

The object of the study is the carbonate section «Pacha-Lasta» (Ilych river, Northern Urals). The work is based on the study of the composition, structure of rocks and species diversity of the foraminifera contained in them. The rocks are represented by various limestones with numerous remains of foraminifera, brachiopods, corals, echinoderms, frequent calcispheres and rare stromatolites. Based on the presence in the complex of certain foraminifera genera Biseriella, Globivalvulina, various Eostaffella (including E. ikensis tenebrosa) as well as the species Endothyranopsis sphaerica and Asteroarchaediscus baschkiricus, the Venusian age of the deposits is reliably established. Stromatolite formations, which are not characteristic of the same-age deposits of the area, were established at two intervals of the studied section. They are composed of micritized colonies of cyanobacteria, among which Ortonella and Girvanella are identified. These formations belong to the type of skeletal stromatolites, which in the Silurian and Late Devonian had a certain paleogeographic position and fixed the margin of the carbonate platform. For the Ilych River basin, a consistent decrease in relative sea level is assumed after the Early Visean transgression with a change in the conditions of the open sublittoral (Tula and Aleksinsky horizons) to the conditions of insular shallow water (Mikhailovsky and Venevsky horizons). In the latter, stromatolites can sometines occur, which are not characteristic of the Upper Visean deposits of the area.