Precambrian–Cambrian Transition in the Biryusa Cis-Sayans Uplift (Southwestern Siberian Platform)

Herein we present an integrated chemostratigraphic (δ13С and δ18O) and paleoichnological study of the Ust’-Tagul and basal Usol’e formations of the Biryusa Cis-Sayans Uplift (southwestern Siberian Platform). Positive carbon isotope excursion in the upper 34 m of the Ust’-Tagul Formation corresponds to ZHUCE anomaly in the composite δ13С curve and suggests Tommotian age of the host strata. Our data show the diachronous onset of the massive evaporitic sedimentation, marking the base of the Usol’e Regional Stage, in different paleobasins at the southwestern margin of the Siberian Platform. The isotope record of the Ust’-Tagul Formation lacks a profound negative excursion comparable to BACE anomaly. The lowermost appearance of burrows Treptichnus pedum 90 m above the base of the formation, where the shallow-marine facies first occur, suggests the host strata are of the Nemakit–Daldynian age. Although the base of the Nemakit–Daldynian Stage cannot be precisely defined in the section, it putatively corresponds to the base of the Ust’-Tagul Formation.