Pecularities of composition, structure and environments of Hirnantian deposits in the Timan-northern Ural sedimentary basin

The results of lithological and geochemical study of the Hirnantian deposits on the western slope of the Subpolar (Ko-BKB and Ko-108/01 sections) and Northern (BK-2 section) Urals are presented. At the beginning of the Hirnantian the regression in the Timan-northern Ural region on the outer zone of the carbonate platform margin (eastern sections of Ko-BKB - the Bad’yashor Fm in the Kozhym River, Subpolar Urals and BK-2 - pack 1 of the Verkh Ruchej Fm in the Ilych River basin, Northern Urals) was manifested in the formation of breccias, erosion surfaces with pockets, carbon and oxygen isotope excursions. In the inner zone of the platform margin (western section Ko-108/01 - pack 1 of the Yunkoshor Fm) was formed bioclastic sands, erosion processes have been significantly weaker. In the late Hirnantian existed shoals with crinoidal-sand facies (sections Ko-BKB - the Kamennaya Baba Fm and BK-2 - pack 2 of the Verkh Ruchej Fm) and more quiet water conditions of lower intertidal zone (section Ko-108/01 - pack 2 of the Yunkoshor Fm). The difference in the sedimentation was due to the existence of paleouplifts and paleodepressions (raised and lowered blocks of composite basement) on the carbonate platform. The similar environments in the sedimentary basin are revealed in the mid-Hirnantian by the negative excursion of carbon and oxide isotope curves clearly expressed in all sections. This shift reaching in the δ18О to 4.7‰ fixed in the section Ko-BKB. Such expressive isotope excursion can apply as the regional geochemical marker of the mid-Hirnantian deposits. This time interval characterizes an abrupt shallowing, intense continental runoff, and influence of fresh-water due to short-term extensive regional regression in the Timan-northern Ural marine basin. For the Hirnantian Stage in stratigraphic scheme of the Western Urals on the basis of sections completeness it is necessary to allocate the “Kozhym” Regional Stage with the stratotype sections in the Kozhym River on the Subpolar Urals, which must be located above the Kyr’ya Regional Stage corresponding to the upper Katian