Isotopic stratigraphy of the Bokson Group, Vendian-Cambrian of the Sayan Mountains

Carbon and oxygen isotope variation curves for Vendian-Lower Cambrian carbonates of the Bokson Group are correlative with those characterizing coeval sections in Siberia and western Mongolia. The curves demonstrate the following characteristics of sediments: (1) abnormally high values of delta(13)C (up to 6 parts per thousand PDB) and delta(18)O (up to 31 parts per thousand SMOW) at the base of the section, which are also typical of basal beds of the Tsganolom Formation of western Mongolia; and (2) a sharp decrease of both values (down to -5 parts per thousand and 25-26 parts per thousand, respectively) in the lower part of the Tabinzurta Formation like in Siberian and West Mongolian sections close to the base of the Manykaian (Nemakit-Daldynian) Stage. On the basis of isotopic correlation, the Zabit Formation can be attributed to the Vendian. The lower boundary of the Manykaian (Nemakit-Daldynian) Stage is distinguished at the level of 75-100 m above the base of the Tabinzurta Formation, whereas the lower Tommotian boundary is placed at the level of 400-450 m from the base of this formation, and the Tommotian-Atdabanian boundary is assumed to be at the level dividing the Tabinzurta and Khuzhirtai formations. Basal diamictites of the Zabit Formation seem to be correlative with the Laplandian glacial horizon of the Early Vendian. Origin of isotopic anomalies and their possible relations to climatic events are discussed.