CHEMOSTRATIGRAPHY OF THE AVZYAN FORMATION (SOUTHERN URALS, RUSSIA) AND ITS RELATIONSHIP TO THE MID-MESOPROTEROZOIC CARBON ISOTOPIC SHIFT

Mesoproterozoic seawater d¹³C values underwent a significant change from ~ 0‰ before 1300 Ma to about +3.5‰ by 1200 Ma. However, geochemical data have not yet been obtained from a single succession that both records the shift and constrains its age, thus necessitating the biological, geochemical, and stratigraphic correlation of sequences that capture parts of this transition. Several successions have been examined that record portions of this shift. Carbonates from the Allamoore Formation of Texas (1250 Ma) have d¹³C values between 0‰ and +4 ‰. In contrast, older carbonates of the Dismal Lakes Group, Arctic Canada (>1270 Ma), are characterized by a series of d¹³C shifts from 0‰ to +2 ‰. The Avzyan Formation of the southern Ural Mountains, Russia, is younger than 1348 Ma and older than 1080 Ma, and thus may preserve part of the isotopic shift. By comparing Avzyan d¹³C values with those of other successions, we can better constrain the age of the Uralian succession and the nature of the d¹³C shift. The upper Avzyan Revet Member yields a pronounced positive d¹³C shift from 0‰ to +2‰. Chemostratigraphic comparison suggests possible correlation with either the Allamoore Formation or the Dismal Lakes Group. Each of these late Mesoproterozoic successions contains segments with positive d¹³C shifts from 0‰ to +2‰. Analyses of the two lower carbonate members of the Avzyan Formation are expected to distinguish between two hypotheses: (1) if the lower Avzyan contains d¹³C values £ +2‰, then chemostratigraphic correlation with some part of the Dismal Lakes Group is likely; alternatively, (2) if d¹³C values from the lower Avzyan are predominantly > +2‰, the succession is best correlated with the Allamoore Formation. Strengthening geochemical correlations further refines the global isotopic curve for the late Mesoproterozoic and constrains possible mechanisms for change.