The Frasnian-Famennian events in a deep-shelf succession, Subpolar Urals: biotic, depositional, and geochemical records

The Frasnian–Famennian (F–F) boundary is well biostratigraphically documented in the Palmatolepis−rich deposits exposed along the Syv’yu River in the lower slopes of the Subpolar Urals. The thin−bedded calcareous−clayey−siliceous deep−slope succession of the Vorota Formation appears to represent continuous Domanic−type deposition throughout the world−wide carbonate crisis time, without evidence for the basal Famennian hiatus or a large−scale sedimentary perturbation within a regressive setting. The northernmost Laurussian sequence exhibits many well known signatures throughout the broad F–F timespan: the appearance of organicand clay−rich deposits, icriodontid and radiolarian blooms, and a correlative shift of several geochemical proxies towards hypoxic and high−productivity regimes, perfectly recorded by positive 13Ccarb excursions of +3.5‰. Integrative biotic, microfacies and geochemical data substantiate a longer−term oceanographic destabilization, attributable to multiple Earth−bound triggering factors in (episodically enhanced?) greenhouse climate and punctuated eustatic sea−level highstands, superimposed on the elevated deposition of organic carbon−rich sediments during the Upper Kellwasser Event. Unsteady eutrophicated, and oxygen−depleted ecosystems during the F–F biotic crisis interval could be assumed, especially when intensified by various spasmodic tectono−volcanic phenomena in the incipiently closing Ural Ocean.