New Data on the Lower Permian Kushtau Reef Massif, South Сis-Urals: Biostratigraphy and Lithofacies

The biostratigraphic study of the core from 12 boreholes drilled on Mount Kushtau (South Urals) and uncovering the most complete section of the Asselian and Sakmarian biohermal limestones revealed the structure of the reef massif. The deposits were dated using foraminifera. Regional substages and zones were identified, and the hypsometric position of the substage boundaries was established, indicating the bedded structure of the Shikhan Kushtau with a gentle monoclinal dip of the beds to the north-west. The character of the distribution of bioherm facies across the section and laterally was clarified, and the evolution of reef paleobiocenoses from Tubiphytes-bryozoan bioherms of the quietwater zone to fusulinid sands of the hydrodynamically active zone was shown. Granular fusulinid and polybioclastic limestones (grainstones and packstones) at the Asselian-Sakmarian boundary, which were formed in the high energy environment probably indicate regressive shelf shallowing. The boundary is drawn by the change of fusulinid fauna and is not lithologically distinct. Numerous neptunian dikes are found in the carbonate massif of Mount Kushtau. The nature of secondary changes, the degree of their influence on carbonates and the time of their appearance are shown. These data can be used to study Lower Permian reef deposits in other regions.