Integrated chronostratigraphy of Proterozoic-Cambrian boundaryudbeds in the western Anabar region, northern Siberia

摘要

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