Composition, structure, formation environment of the Yaptikshor deposits of the Shezhim facies complex (Upper Katian, Ilych River, Northern Urals)

Research subject . The section of the Upper Katian deposits of the Shezhim facies complex, exposed on the Bol’shaya Kos’yu River (a right tributary of the Ilych River, Northern Urals), exhibits a total thickness of 42.3 metres. The factual material is represented by a collection of more than 60 rock samples, 100 thin sections, and 10 slabs. Aim. Reconstruction of the sedimentation conditions of the Upper Katian deposits of the Shezhym facies complex. Methods. Standard lithological and paleontological methods of studying the section were used to dissect the section. Sampling was carried out after no more than 1 m. The sections were studied by the classical optical microscopic method. Results. The Upper Katian deposits are characterized by numerous remnants of conodont and shell fauna, including the zonal brachiopod species Proconchidium cf. muensteri (St. Joseph) , exhibiting a highly heterogeneous lithological composition. At the base, there lies a pack of bioclastic algal-crinoid limestones, which are replaced with tectonic contact by limestone mixed-grained sandstones and limestone block breccias upward the section. Above, they overlap with layered bioclastic limestones with numerous fauna typical of this age range and widespread within the Timan-North Ural region. Conclusions. According to the stratigraphic position and faunal characteristics, the Upper Katian deposits are reliably correlated with the stratotype of the Yaptikshor formation of the Subpolar Urals. Established on the basis of studying a variety of sections of the Northern, Subpolar, Polar Urals, and Chernyshev Ridge, the transformation of the outer margin of the Pechora Plate from rimmed platform in the Middle Katian to an open platform and to a ramp in the Late Katian (Yaptikshor Time) can also be traced in the section on the Bol’shaya Kos’yu River. The deposits characterize the shift of sedimentation conditions from shallow to deeper ones, which corresponds to the transgressive stage of the sedimentary basin development. The section on the Kosyu River differs from the stratotype of the Yaptikshor Formation by the significant presence of lithoclastic limestone differences, which were most likely formed as a result of the manifestation of tectonic activity of the Pechora Plate. This fact reflects the regional features of the North Ural margin of the carbonate platform, due to its block structure and different intensity of tectonic processes.