Ordovician deposits of the western slope of the Southern Urals and their correlation based on conodonts and chitinozoans

Biostratigraphic subdivision and correlation of Ordovician different-facies sections on the western slope of the Southern Urals are carried out based on conodonts and chitinozoans. Upper Ordovician conodonts are defined in the West Zilair zone, in the carbonate-terrigenous section near Nabiullino Village. They belong to important cosmopolitan taxa which allow distinguishing biostratigraphic units corresponding to the North Atlantic standard conodont scale. Successions of Middle and Upper Ordovician conodont and chitinozoan assemblages are identified in the terrigenous-siliceous sections of the East Zilair zone. Middle Ordovician chitinozoans are found in the Uraltau zone, in the metamorphic rocks of the Belekei Formation of the Suvanyak complex, previously assigned to the Precambrian. Most of the taxa in the conodont and chitinozoan assemblages have a good correlation potential due to their wide geographic range. A positive 513C excursion of 3.3%c was first recorded in the Nabiullino section (West Zilair zone), suggesting the initial phase of the HICE isotopic events in the Southern Urals. The development of shallow-water conodont biofacies at the top of the section suggests a sea level fall in the Late Ordovician.