Carbon and oxygen isotope variation curves for Vendian-Lower Cambrian carbo
nates of the Bokson Group are correlative with those characterizing coeval
sections in Siberia and western Mongolia. The curves demonstrate the follow
ing 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 thousan
d 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 b
oth values (down to -5 parts per thousand and 25-26 parts per thousand, res
pectively) in the lower part of the Tabinzurta Formation like in Siberian a
nd West Mongolian sections close to the base of the Manykaian (Nemakit-Dald
ynian) 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 Tommoti
an-Atdabanian boundary is assumed to be at the level dividing the Tabinzurt
a 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 even
ts are discussed.