Ordovician reefs of the Ural Mountains, Russia: A review

The Upper Ordovician reefs of the Urals were formed at a subsiding shelf-margin during an early Late Ashgillian (Sur'ya time interval) regressive phase. Reefs of this age were studied in detail from the western slope of the Northern, the Subpolar and the Polar Urals with respect to lithofacies, biotic composition and paleogeographical patterns. The thickness of the reefs varies between 100 and 500 m. The backreef areas are characterized by lagoons with increased salinity and sabkha development. Microbial associations and a diverse algal flora (Cyanophyta, green and red algae and alga incertae sedis) are the main constituents of reefal boundstones. Tabulate and rugose corals, heliolitids, calcareous sponge-like fossils, bryozoans and problematic hydroids were also part of the reef communities. Each reef exhibits a characteristic framework-building association. Reef development was terminated by a rapid and abrupt sea-level rise at the end of the middle Upper Ashgillian connected with the global Late Ordovician glaciation.