Late Mississippian sequence and carbon isotope stratigraphy, east-central Idaho and southwest Montana: Proxies for Gondwana glacio-eustasy

An eastwardly thinning succession of Late Mississippian (Chesterian) carbonates and siliciclastics from east-central Idaho and southwest Montana record deposition in a distal foreland basin. Seventeen outcrops measured along depositional dip and a detailed conodont biostratigraphy provide the basis for a regional cross section and sequence stratigraphic framework for Chesterian strata. One partial and seven complete sequences provide the basis for stratigraphic interpretations. Sequences are based on parasequence stacking patterns and bound by flooding surfaces. The regional cross section indicates that tectonic subsidence in the early Chesterian resulted in flooding of the western ramp as well as the development of an intrashelf basin on the middle ramp. The intrashelf basin persisted as a bathymetric low until the middle to late Chesterian. A thick succession of carbonates formed on the western ramp during the early-middle Chesterian in response to increased accommodation. A pronounced change in the character and stacking geometry of parasequences in the late Chesterian indicates a transition from moderate to high amplitude sea level changes and reflects the onset of glacio-eustatic sea levels associated with middle Carboniferous glaciation on Gondwana.; Bulk carbonate derived carbon isotopes from the study area provide additional constraints on the timing of basin subsidence as well as on the transition from a greenhouse to icehouse climate. To assess whether bulk carbonates from east-central Idaho preserve an original seawater signature, over 150 biotic calcite and marine cement components were microsampled for comparison of their carbon isotope values to contemporaneous bulk carbonate values. The results indicate that most bulk carbonate and biotic calcite precipitated in equilibrium with seawater and retained their original carbon isotopic values through time.; Three prominent shifts occur in the Chesterian carbon isotope curve. A positive shift in the early Chesterian from -1‰ to 2.5‰ (PDB) coincides with tectonic subsidence and flooding on the ramp, and may reflect enhanced mixing with the open paleo-Pacific ocean. A negative shift from 2‰ to 0‰ in the late Chesterian coincides with the transition to high amplitude sea level cycles, and may reflect enhanced mixing with meteoric water as well as a transition to a cooler, wetter climate. The timing of a positive shift from -0.5‰ to 2‰ across the middle Carboniferous boundary coincides with a similar shift in records from the US Mid continent, Nevada, Europe and Russia. This globally recognized positive excursion may coincide with increased global carbon burial and drawdown of rhoCO 2 associated with a middle Carboniferous icehouse climate.