Carbonate-rich sedimentary rocks of the western Anabar region, northern Siberia, preserve an
exceptional record of evolutionary and biogeochemical events near the Proterozoic/Cambrian boundary.
Sedimentologically, the boundary succession can be divided into three sequences representing successive
episodes of late transgressive to early highstand deposition; four parasequences are recognized in the
sequence corresponding lithostratigraphically to the Manykai Formation. Small shelly fossils are abundant
and include many taxa that also occur in standard sections of southeastern Siberia. Despite this coincidence
of faunal elements, biostratigraphic correlations between the two regions have been controversial because
numerous species that first appear at or immediately above the basal Tommotian boundary in southeastern
sections have first appearances scattered through more than thirty metres of section in the western Anabar.
Carbon- and Sr-isotopic data on petrographically and geochemically screened samples collected at one- to
two-metre intervals in a section along the Kotuikan River, favour correlation of the Staraya Reckha
Formation and most of the overlying Manykai Formation with sub-Tommotian carbonates in southeastern
Siberia. In contrast, isotopic data suggest that the uppermost Manykai Formation and the basal 26 m of the
unconformably overlying Medvezhya Formation may have no equivalent in the southeast; they appear to
provide a sedimentary and palaeontological record of an evolutionarily significant time interval represented
in southeastern Siberia only by the sub-Tommotian unconformity. Correlations with radiometrically dated
horizons in the Olenek and Kharaulakh regions of northern Siberia suggest that this interval lasted approximately
three to six million years, during which essentially all 'basal Tommotian' small shelly fossils
evolved
