The Moscovian–Kasimovian boundary problem in the Northern Urals: A case study of the Moleben-Iz Section, Ilych River

The Kasimovian lower boundary is usually determined by the first appearance of Obsoletes and advanced Protriticites fusulinids; however, this criterion is not always met. In the northern part of the Urals and Pai-Khoi, the Moscovian–Kasimovian boundary is determined by the abrupt appearance of fusulinids of the Subfamily Pulchrellinae. In this paper we present results of our study of the Moleben-Iz section, in an attempt to define the Moscovian–Kasimovian boundary. In this section, the upper Moscovian and lower Kasimovian deposits are represented by shallow-marine carbonates, including a diverse fusulinid complex. The most important stratigraphic level here is the lower part of bed 6, characterized by the abrupt appearance of the fusulinids Kanmeraia, Eowaeringella and Dagmarella. This level is also associated with the diversification of Usvaella (species U. plicata (Shamov and Shcherbovich) and U. porifera Remizova appear) and Pulchrella (Pul. whitensis (Ross and Sabins), Pul. hayasakai (Watanabe), and Pul. pokojamensis (Lebedeva) appear). The later appearance of Obsoletes obsoletus may be related to the later migration of this species from the Russian platform to the Urals. Placement of the Kasimovian lower boundary in the Northern Urals and Pai-Khoi at the base of the Kanmeraia ozawai-Usvaella usvae Biozone allows us to correlate this boundary with the Missourian base of the North American continent.