Carbon isotope stratigraphy and the problem of a pre-Tommotian Stage in Siberia

Carbon isotopic oscillations are useful to elucidate the stratigraphy and b iogeochemical events around the Precambrian-Cambrian transition. New isotop ic data from the Manykaj and Emyaksin formations of the eastern Anabar Upli ft (Siberia) help to correlate the Lower Cambrian and Neoproterozoic-Cambri an transitional beds across the Siberian Platform. The similarity of trends and amplitudes of the carbon isotopic curves, together with biostratigraph ic and sequence-stratigraphic markers from the Anabar Uplift, provide a pre cise correlation with the southern part of the Siberian Platform. Diagenesi s of argillaceous limestones of the Emyaksin Formation has apparently not a ffected the primary isotopic variations. The resulting curve is nearly iden tical in sections about 100 km apart in the Tommotian-Atdabanian portion of the formation. Relatively frequent and pronounced isotopic oscillations in the lower beds of the Emyaksin Formation fit between features I and II of the southern Siberian isotopic reference scale but are undetected therein o wing to the depositional hiatus at the base of the Tommotian Stage in its t ype section. This confirms the transgressive onlap from the north suggested by previous studies, and makes the appearance of the Cambrian skeletal fos sils on the Siberian Platform less abrupt. The hiatus in the south appears to embrace at least two biostratigraphic zones as recognized in the north. The case is strengthened for a pre-Tommotian Cambrian Stage in Siberia. the biostratigraphic framework for which has been elaborated earlier.