The composition and formation conditions of the Upper Lochkovian Valgan Formation (Lower Devonian Ilych River, Northern Urals)

Abstract

Research subject. A section of the Upper Lochkovian (Lower Devonian) Valgan Formation located in the stratotypic area on the Ilych River, Northern Urals, was investigated. Materials and methods . A study of the composition and structure of rock samples gathered from the section under consideration during fieldwork in 2015 was conducted. Results. The studied section with an exposed thickness of 51 m is characterized by the thin and uneven alterations of calcareous con-glogravelites, coarse-grained and fine-grained organic sandsones, bioclatic-pelitomorphic limestones and argillo-calcareous shales. These rocks demonstrate a gradational and convolute lamination. An individual interval of the studied section contains calcareous conglobreccias layers. The Valgan Formation is characterized by a cyclic structure, with the thickness of individual cyclites ranging from 5.0 to 25 cm. The character of their structure testifies to sedimentation in slope settings. An analysis of clastic material revealed two sources of the basin area formation: near ablation (represented by the fragments of background bioclastic-pelitomorphic limestones) and remote ablation (represented by biohermal rock clasts). It is proposed to consider the Valgan Formation as the Valgan gradation, which extends the Upper Lochkovian sulfate-carbonate Subformation to the east. Conclusions. Therefore, the Valgan gradation should be treated as an intermediate link between shallow-marine Upper Lochkovian biohermal structures and bathyal clayey and siliceous shales.