CHEMOSTRATIGRAPHY AS A TOOL FOR CONSTRAINING THE AGE OF SUBSURFACE PROTEROZOIC STRATA, SOUTHERN URALS, RUSSIA

Correlation of Proterozoic strata is often hindered by an absence of absolute age constraints and the relatively poor resolution of biostratigraphic markers. Our ability to place chronologic constraints on these successions, however, has been greatly enhanced through the use of C-isotopes. Global chemostratigraphic studies have shown that during the Meso- to Neoproterozoic, marine C-isotopes changed from values near 0±1‰ (>1300 Ma) to values near 3.5±1‰ (from ~1350-900 Ma), to baseline values typically >5‰ in the Neoproterozoic. This study examines two Proterozoic cores from the southern Ural Mountains, Russia. These cores, each measuring ~1000 m, contain both peritidal and deeper water carbonate facies and varying proportions of siliciclastic material. Strata crop out only in the subsurface and have been previously interpreted with conflicting results. One interpretation suggests a Late Riphean (<900 Ma) age based on the presence of large, primarily unornamented, acritarchs. A second interpretation suggests an Early Riphean (>1300 Ma) age based on lithologic similarity to other Lower Riphean strata in the southern Urals.

Within these broad time intervals, C-isotope stratigraphy should be able to distinguish between these conflicting interpretations. Intervals of predominantly carbonate strata were extracted from the cores and analyzed by standard petrographic and cathodoluminescence techniques to estimate preservation of primary depositional phases. Distinct phases were microdrilled for isotopic (C, O) and elemental (Ca, Mg, Sr, Mn, Fe) analysis, which will be used to reconstruct the diagenetic history of samples and construct chemostratigraphic profiles for the individual cores. Whereas elevated C-isotopic values (>4‰) would suggest that strata are Upper Riphean in age and support biostratigraphic interpretations, C-isotopic values near 0‰ would suggest that strata are Lower Riphean and that lithostratigraphic correlations may be correct.