CARBON ISOTOPE CHEMOSTRATIGRAPHY OF THE REVET MEMBER, MESOPROTEROZOIC AVZYAN FORMATION, SOUTHERN URAL MOUNTAINS, RUSSIA

Recent examination of Mesoproterozoic carbonates from Canada, the US, Siberia, China, and Australia indicates that the carbon isotopic composition of seawater changed from approximately 0 permil, prior to 1300 Ma, to about +3.5 permil by 1250 Ma. However, no single succession has yet been found that records this transition. Here, we begin our examination of carbonate sediments from the Middle Riphean (ca. 1300-1000 Ma) stratotype, southern Ural Mountains, Russia, which may contain rocks of approximately the correct age to capture this geochemical transition. This study focuses on the Revet Member, the fifth of six alternating carbonate and siliciclastic members of the Middle Riphean Avzyan Formation. Radiometric constraints indicate that the Avzyan is younger than 1348 Ma (U-Pb analysis of rhyolite within an underlying unit) and older than 1080 Ma (U-Pb analysis of cross-cutting dikes). The Revet Member comprises approximately 200 m of shallow subtidal to peritidal carbonate rocks with minor siliciclastic influx, and is well exposed in two localities. The first is near the village of Kaga, where a newly dug pipeline has exposed approximately 60 m of the Revet Member. The second exposure lies along a hillside in the village of Avzyan (~120 km south of Kaga), where the entire unit is exposed. Carbonates from both localities were collected for carbon isotopic and elemental analysis. All samples were analyzed both petrographically and via cathodoluminescence, and distinct phases were drilled to establish a paragenetic sequence for Revet carbonates. Finally, least-altered phases are being used to construct a carbon-isotopic profile for the Revet Member. Within the emerging framework of a global Mesoproterozoic carbon isotopic curve, these new data should (1) allow better correlation of Revet strata between these geographically distant sections, and (2) permit more accurate placement of the Revet Member into a global geochronologic framework.