CARBON ISOTOPE STRATIGRAPHY AND DIAGENESIS OF PENNSYLVANIAN (DESMOINESIAN-MISSOURIAN) CARBONATES IN EAST-CENTRAL IDAHO
Carbon isotope stratigraphy of carbonate sediments is instrumental in examining major perturbations in the global carbon cycle and in correlating strata. However, the primary isotopic signal recorded in these sediments can vary with depositional environment and diagenetic alteration. This study examines the carbon isotope stratigraphy and its relation to depositional environment, lithology, and diagenetic history in a section of the 312-307 million year old Pennsylvanian (Desmoinesian-Missourian) Snaky Canyon Formation in the Beaverhead Mountains, east-central Idaho. Petrography of 90 thin sections show carbonate rocks ranging from mudstone to boundstone containing variable amounts of eolian siliciclastics with as much as 50% silt to medium sand grains in some samples. The abundance of siliciclastic influx decreases upsection to less than 1% sand and roughly corresponds to an increase in parasequence thickness. Open marine carbonate facies include abundant crinoids, bryozoans, foraminifera, brachiopods, green algae, phylloid algae, and arthropod fragments. Chert, bioturbation features (e.g. burrows), intraclasts, pellets, coarse calcite spar, calcite-filled fractures, and fossil silicification also occur in these rocks. Cathodoluminescence (CL) analysis of thin sections reveals a lack of luminescence in nearly all components, suggesting that depositional and diagenetic waters were low in Mn2+ or high in Fe2+. Approximately 130 billets were analyzed to yield 13C/12C and 18O/16O ratios. Fractures have much lower δ18O (-13.9‰)reflecting post-depositional tectonic processes. Most of the isotopic data for micritic components are within -8 to -1‰ for δ18O and -1 to +5‰ for δ13C. Previously reported oxygen isotope values for Pennsylvanian brachiopods from the U.S. Midcontinent are -2.2 ±0.7‰, indicating outlier δ18O values in this study are altered. Most of the isotope data for carbon are within previously reported Pennsylvanian fine grained carbonate ranges of +1 to +5‰. These isotopic values appear to be independent of lithology and percent carbonate; thus, the main controlling influence on the δ13C and δ18O data may be depositional environment. Three significant trends in the δ13C data appear to correspond to thickness and boundaries of stratigraphic parasequences. Isotope values in this study were compared to those of Arrow Canyon,Nevada and showed similar trends for the Desmoinesian but not for the Missourian.
Carbon isotope stratigraphy
Wood, Stephanie (2011). CARBON ISOTOPE STRATIGRAPHY AND DIAGENESIS OF PENNSYLVANIAN (DESMOINESIAN-MISSOURIAN) CARBONATES IN EAST-CENTRAL IDAHO. Honors and Undergraduate Research. Available electronically from