INTEGRATED CHRONOSTRATIGRAPHY OF PROTEROZOIC-CAMBRIAN BOUNDARY BEDS IN THE WESTERN ANABAR REGION, NORTHERN SIBERIA

Carbonate-rich sedimentary rocks of the western Anabar region, norther n Siberia, preserve an exceptional record of evolutionary and biogeoch emical events near the Proterozoic/Cambrian boundary. Sedimentological ly, the boundary succession can be divided into three sequences repres enting successive episodes of late transgressive to early highstand de position; four parasequences are recognized in the sequence correspond ing lithostratigraphically to the Manykai Formation. Small shelly foss ils are abundant and include many taxa that also occur in standard sec tions of southeastern Siberia. Despite this coincidence of faunal elem ents, biostratigraphic correlations between the two regions have been controversial because numerous species that first appear at or immedia tely 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 petrographical ly and geochemically screened samples collected at one- to two-metre i ntervals in a section along the Kotuikan River, favour correlation of the Staraya Reckha Formation and most of the overlying Manykai Formati on with sub-Tommotian carbonates in southeastern Siberia. In contrast, isotopic data suggest that the uppermost Manykai Formation and the ba sal 26 m of the unconformably overlying Medvezhya Formation may have n o equivalent in the southeast; they appear to provide a sedimentary an d palaeontological record of an evolutionarily significant time interv al represented in southeastern Siberia only by the sub-Tommotian uncon formity. Correlations with radiometrically dated horizons in the Olene k and Kharaulakh regions of northern Siberia suggest that this interva l lasted approximately three to six million years, during which essent ially all 'basal Tommotian' small shelly fossils evolved.