Kasimovian stage in the Moleben-Iz section (Ilych River, Northern Urals)

Abstract

Research subject. This study is devoted to the Moleben-Iz section on the Ilych River (Northern Urals) composed of Upper Moscovian (Myachkovian horizon) and Kasimovian rocks. Material and methods . The composition of rock and fossil inclusions in 49 samples was investigated. Results . The upper part of the Moscovian is represented by an alteration of bioclastic grainstones and packstones with rare interlayers of dolostones and cherts. Microcodium structures were found in three layers, which is indicative of three episodes of sub-aerial exposure. The Kasimovian lower boundary, which coincides with that of the Upper Carboniferous, is outlined by the appearance of diverse fusulinids of the usvaella genus, as well as by the first appearance of Eowariengella, Dagmarella, and Kanmeraia genera, since Pulchrellinae representatives dominate across this stratigraphic interval in northern-eastern sections of the European part of Russia, as well as in Canada and USA. At the base of the Kasimovian stage, the authors distinguished the Kanmeraia ozawai – Usvaella usvae biozone as being most consistent with the fauna composition. The Kasimovian is represented by the deposits of the Orlovskian and Kerzhakovian horizons. The deposits of the Orlovskian horizon are composed, in its lower part, by alternating grainstones and packstones, and, in its upper part, by boundstones and lime mudstones. Two levels of the development of biohermal rocks were established, representing two phases of organic buildup formation. The middle part of the Kasimovian (Montiparus montiparus zone) in the studied section was not established, which is likely to have resulted from a break in sedimentation. Upper Kasimovian deposits (Kerzhakovian horizon) are represented by highly silicified bryozoans packstones. These rocks serve as an excellent marking horizon, which is typical of the Northern Urals and partly of the Chernyshev Ridge. Conclusion . The Moleben-Iz section represents a range of biologic and geologic events, out of which the appearance of new Pulchrellinae genera and a marking horizon at the base of Kerzhakovian horizon are the most notable events.