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

A. Shadrin, R. Ivanova, E. Ponomarenko
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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.
帕恰-拉斯塔河段(乌拉尔北部伊里奇河)上维谢安沉积物中的叠层石
研究对象是 "Pacha-Lasta "碳酸盐岩段(乌拉尔北部伊里奇河)。这项工作的基础是研究岩石的成分、结构以及其中所含有孔虫的物种多样性。岩石由各种石灰岩组成,其中含有大量有孔虫、腕足类、珊瑚、棘皮动物、常见的钙钛矿和罕见的叠层石。根据复合体中存在的某些有孔虫属 Biseriella、Globivalvulina、各种 Eostaffella(包括 E. ikensis tenebrosa)以及 Endothyranopsis sphaerica 和 Asteroarchaediscus baschkiricus,可以可靠地确定这些沉积物的金星时代。在研究区段的两个间隔处发现了叠层石层,这不是该地区同时期沉积物的特征。它们是由蓝藻的微粒化菌落组成的,其中有奥托奈拉菌(Ortonella)和吉尔瓦奈拉菌(Girvanella)。这些地层属于骨架叠层石类型,在志留纪和泥盆纪晚期具有一定的古地理位置,并固定了碳酸盐平台的边缘。对于伊里奇河盆地,在早维谢安横断之后,相对海平面持续下降,开放的近岸条件(图拉和阿列克辛基地层)转变为岛屿浅水条件(米哈伊洛夫斯基和维涅夫斯基地层)。后者有时会出现叠层石,这不是该地区上维谢安沉积的特征。
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