Upper Ordovician–Lower Silurian deposits in the Zakola section (Ilych River, Northern Urals): Composition, structure, formation environments

Research subject. Section of boundary deposits, exposed in the right side of the Zakola stream (the right tributary of the Ilych River, Northern Urals) with a total thickness of 98 m, presented with rock samples (more than 100 pieces), thin sections (100 pieces), slabs (10 pieces), and chemical analyses (15 pieces). Aim. Detailed description of the most complete section of the Upper Ordovician–Lower Silurian boundary sediments of the Shantym facies complex with a stratigraphic reference of layers, lithological typification of sediments, and restoration of sedimentation conditions. Methods. Conventional lithological and paleontological methods were used to dissect the section. Sampling was carried out after 1 m; however, samples were additionally taken when detailed observations were possible to identify lithological and facial differences. The thin sections were studied by the classic method of optical microscopy. The study of the fauna complex was carried out both by collecting samples of the author’s rocks and by the materials of A.I. Antoshkina. In addition, stock materials were attracted. Results. In the section, 14 packs of rocks were identified, which are represented by 6 lithological types of secondary dolomites. Their distribution along the section, along with the presence of characteristic faunal remains, made it possible to establish three members in the section, which can be correlated with the Katian, Hirnantian, and Ruddanian stages of the General Stratigraphic Scale. Conclusion. The Katian deposits are represented by the Yaptikshor Formation (Strata 1), which was formed under the conditions of the lower sublittoral of a ramp-type carbonate platform with a transgressive carbonate tract and a diverse fauna. The beginning of the Hirnantian (Strata 2) is characterized by shallow water conditions of the upper sublittoral up to the littoral, which reflects a sharp shallowing at the Katian–Hirnantian boundary and corresponds to the beginning of the global early Hirnantian regression. The rocks exposed higher up the section indicate sedimentation with a certain rise in the sea level; however, the correlation of this part of the section with the upper part of the Verkh Ruchey Formation is complicated by the fragmentary nature of the outcrops. The overlying Ruddanian deposits (Strata 3) were accumulated on the outskirts of a ramp-type carbonate platform under the conditions of the lower sublittoral, which corresponds to the beginning of the Early Silurian global transgression.