Global Upper Kellwasser Event and its paleontological and litological reflection in the Bolshaya Barma, Akkyr, Ryauzyak and Kuk-Karauk carbonate sections of the western slope of the South Ural

The Frasnian and Famennian (F/F) boundary is one of the most discussed problems in the stratigraphy and paleontology. This interval is characterized worldwide by a global geological crisis and the associated the largest extinction event of the Phanerozoic, known as the Upper Kellwasser Event (UK). In the studied carbonate sections of the western slope of the South Urals (Bolshaya Barma, Akkyr, Ryauzyak and Kuk-Karauk) the UK event is reflected as a global bio event. A sharp change of the taxonomic composition of conodont complexes is clearly recorded in the brachiopod shell stones at the Famennian Barma Horizon bottom. In the uppermost Askyn Horizon, close to the F/F boundary, Frasnian species of the genera Ancyrognathus and Palmatolepis die out, and the genera Ancyrodella and Belodella become totally extinct. At the end of the Frasnian time in all studied sections the palmatolepid (pelagic) biofacies gives way to the icriodid (shallow water) one.