Foraminiferal sequence of the middle–upper Viséan (Mississippian) of the Sikasya River Valley in the South Urals and correlation with the Belgian standard

Late Viséan foraminiferal assemblages from a section in the Sikasya (Sikaza) River Valley (South Urals) are studied to provide a reliable framework for stratigraphy and correlation of the Viséan-Serpukhovian boundary beds on the western slope of the South Urals. The middle–upper Viséan deposits on the Sikasya River are composed of shallow shelf carbonates with abundant foraminifers, corals, and brachiopods, and are represented by four regional substages: Tulian, Aleksinian, Mikhailovian, and Venevian. This section is known as the “Sikaza Section”. The upper Viséan deposits are overlain by Serpukhovian beds composed mainly of dolomites. The Tulian and Aleksinian successions consist of bioclastic and foraminiferal wackestone and packstone. In the Mikhailovian, there are packages of dolomites with interbeds of bioclastic grainstone to packstone, containing foraminifers. The Venevian is represented by bioclastic and foraminiferal grainstone and packstone. The following foraminiferal units are discussed: in the middle Viséan: (1) Paraarchaediscus koktjubensis–Endothyranopsis compressa; in the upper Viséan: (2) Ikensieformis proikensis–Archaediscus gigas , (3) beds with Vissarionovella (corresponding to the Ikensieformis ikensis Zone), and (4) Ikensieformis tenebrosa ; the Serpukhovian is represented by (5) beds with Eostaffellina decurta .