CHEMOSTRATIGRAPHY OF PREDOMINANTLY SILICICLASTIC NEOPROTEROZOIC SUCCESSIONS - A CASE-STUDY OF THE POCATELLO FORMATION AND LOWER BRIGHAM GROUP, IDAHO, USA

Isotopic chemostratigraphy has proven successful in the correlation of carbonate-rich Neoproterozoic successions. In successions dominated b y siliciclastic rocks, chemostratigraphy can be problematic, but if th in carbonates punctuate siliciclastic strata, useful isotopic data may be obtained. The upper Pocatello Formation and lower Brigham Group of southeastern Idaho provide an opportunity to assess the potential and limitations of isotopic chemostratigraphy in overwhelmingly silicicla stic successions. The 5000 m thick succession consists predominantly o f siliciclastic lithologies, with only three intervals that contain th in intercalated carbonates. Its depositional age is only broadly const rained by existing biostratigraphic, sequence stratigraphic and geochr onometric data. The lowermost carbonates include a cap dolomite atop d iamictites and volcanic rocks of the Pocatello Formation. The deltaC-1 3 values of these carbonates are distinctly negative (-5 to -3), simil ar to carbonates that overlie Neoproterozoic glaciogenic rocks worldwi de. Stratigraphically higher carbonates record a major positive deltaC -13 excursion to values as high as +8.8 within the carbonate member of the Caddy Canyon Quartzite. The magnitude of this excursion is consis tent with post-Sturtian secular variation recorded elsewhere in the No rth American Cordillera, Australia, Svalbard, Brazil and Namibia, and exceeds the magnitude of any post-Varanger deltaC-13 excursion documen ted to date. In most samples, Sr-isotopic abundances have been altered by diagenesis and greenschist facies metamorphism, but a least-altere d value of approximately 0.7076 supports a post-Sturtian and pre-Marin oan/Varanger age for upper Pocatello and lower Brigham rocks that lie above the Pocatello diamictite. Thus, even though available chemostrat igraphic data are limited, they corroborate correlations of Pocatello Formation diamictites and overlying units with Sturtian glaciogenic ro cks and immediately post-Sturtian successions in western North America and elsewhere.