Stratigraphy and geological situation of the upper Paleozoic organogenic structure on the Kosyu river (western slope of the Subpolar Urals)

Organogenic buildups are reliable indicators of the tectonic regime and paleogeographic conditions, important hydrocarbon reservoirs. Middle Carbon- iferous-Lower Permian buildups are traced as sep- arate massifs along the entire western slope of the Urals. They are distributed within the Pre Ural foredeep and the West Ural megazone. Their structural tectonic position and relationships with replacing, overlapping and underlying facies in the section remain controversial. The paleogeographic position of the Upper Paleozoic buildups is inter preted ambiguously. Previously, based on lithological and paleoecological studies, the Middle Carboniferous Lower Permian organogenic buildups of the Northern and Subpolar Urals were assigned to skeletal mounds.The age, structure, and material composition of these buildups have been studied unevenly. One of the littlestudied is an organogenic buildup on the Kosyu river, which is located directly on the boundary of two large tectonic structures.Based on structural constructions, the structure of the geological section and the sequence of the Upper Paleozoic deposits on the Kosyu river were revealed. The deposits are divided into five packs upward the section: 1) the Mychkovsky horizon of the Moscovian stage with thickness of about 30 m,2) the Protriticites pseudomontiparus Obsoletes obsoletus zone of the Kasimovian stage with thickness of 75 m, 3) the Triticites acutus Rauserites quasiarcticus zone with thickness of 20 m,4) Upper Carboniferous Gzhelian stage and Asselian Sakmarian stages of the Lower Permian with thickness of 100 m and 5) overlapping claysiliceous calcareous deposits of the Artinskian stage with thickness of 18 m. The first four packs make an organogenic buildup.As a result of the research, the authors came to the following conclusions: 1) the types of rocks and structural and texture features of the organogenic structure are similar to the previously known skeletal mounds. 2) the tendency of changes in the composition of rock forming organisms upward the section was established in the lower middle upper Carboniferous part algal limestones were widespread, and the upper one was composed mainly of biocementolites, tubifites bryozoans, microclots and micritic limestones, 3) the struc ture of the Upper Paleozoic section on the Kosyuriver shows that the change of shallow water carbonate (organogenic buildup) sedimentation to deep water terrigenous one does not occur immediately, but through the transitional slope setting.