Lithogeodynamic evolution of the outer margin of the North Ural paleoshelf of the Pechora Plate in the Late Ordovician (Ilych River Basin, Northern Urals)

Upper Ordovician carbonate deposits in the Northern Urals are the most common in the Ilych River basin. At the same time, the sections drilled here, which are the objects of this study, have a fragmentary character and tectonic boundaries, which led to significantly different ideas about the age of some strata and, as a result, to the ambiguity of their sedimentation conditions. The lithological-facies, paleoecological and paleontological studies revealed that the section of the Upper Ordovician carbonate deposits in the Northern Urals was represented by two formations, the Bolshaya Kosya reef and four thickness covering three tiers of the International Stratigraphic Scale of the Upper Ordovician — Sandbian, Katian and Hirnantian. The analysis of the spatiotemporal relationships of 36 lithotypes isolated in these sediments showed that they were grouped into 10 paragenetic associations of rocks reflecting the change of sedimentation conditions from a deep-water ramp with the accumulation of terrigenous calcareous silts (Sandbian) to a carbonate shelf with a clearly defined brow and the formation of a reef shoal on it (Middle Katian) and further to a shallow ramp with the widespread development of erosion processes and the formation of sandbanks (Late Katian-Hirnantian).