C and Sr isotope chemostratigraphy of Vendian-Lower Cambrian carbonate sequences in the central Siberian Platform

摘要

We propose a detailed delta C-13 curve for the Vendian and Lower Cambrian (Tommotian) strata of the central Siberian Platform. Two positive carbon isotope excursions identified near the base of the Yuryakh Formation (up to 5.5 parts per thousand) and in the lower Bilir Formation (up to 5 parts per thousand) are assigned to the lowermost and middle Tommotian, respectively. This correlation is supported by paleontological data, specific Sr-87/Sr-86 values (0.70845-0.70856), and similar C isotope record in coeval Early Cambrian basins. The documented minor vertical oscillations (a few meters) of these isotope excursions relative to the formation boundaries in remote boreholes is presumably caused by the spatiotemporal migration of facies. A high-amplitude negative delta C-13 excursion (-8 to -11 parts per thousand) in the upper Nepa Regional Stage putatively corresponds to the global Shuram-Wonoka negative carbon isotope excursion (Middle Ediacaran). Carbonates of the lower Nepa Regional Stage (Besyuryakh Formation) demonstrate positive delta C-13 values (up to 57 parts per thousand) and minimum Sr-87/Sr-86 ratios of 0.70796-0.70832. The C and Sr isotope record of the Nepa Regional Stage provides its robust correlation with the Dal'nyaya Taiga and Zhuya Groups of the Patom Foredeep. Micropaleontological data herein reported and glacial diamictites documented at the base of the Vendian sedimentary cover both in the central Patom Foredeep and on its periphery suggest a full stratigraphic volume of the Ediacaran System in the most stratigraphically complete sections of the central Siberian Platform. (C) 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.