High Resolution Sequence Stratigraphy of Latest Mississippian (Chesterian) Carbonate and Siliciclastic Facies in East Central Idaho and Southwest Montana

Liselle Batt, Geological Sciences, University of Idaho, Moscow, ID 83844-3022, phone: 208 882 9430, [email protected], P. E. Isaacson, Department of Geology, University of Idaho, Moscow, ID 83844-3022, Michael C. Pope, Department of Geology, Washington State University, Department of Geology, Pullman, WA 99164, and Isabel P. Montañez, Dept. of Geology, University of California, Davis, CA 95616.

Latest Mississippian (Chesterian) successions of mixed carbonates and siliciclastics, spanning 300 kilometers across east-central Idaho and southwest Montana, show 2nd and 3rd order sequences across depositional strike into the Idaho foreland basin. Sequences can be distinguished from subsidence events. Meter scale lithofacies analysis, conodont biostratigraphy, and carbon isotope chemistry provide the basis for interpretations. Deposition occurred on a westward facing ramp positioned between the Laurentian craton and the Antler foreland basin. Considerable lateral variability in depositional facies across the regional transect affords an excellent setting for deciphering the sedimentary response to changes in relative sea level. Third order cycles (1-5my) in Idaho are stratigraphically distinct from their updip counterparts, but exhibit similar overall trends. Lower Chesterian cycles in Idaho shallow gradually upward from massive mudstones into skeletal shoals. Updip equivalent cycles shallow into intertidal peloid and oncoid packstones. Cycle amplitude and frequency increases in the Upper Chesterian. In Idaho, thin bedded skeletal packstones are capped by prograding calcite-cemented siliciclastics. Correlative cycles in Montana are siliciclastic-dominated and commonly unconformity-bound. Well developed soils, plant fossils and lag conglomerates attest to the role of terrestrial processes in constructing these sequences. Vertical changes in cycle frequency and amplitude in these successions are mimicked in the carbon isotope curve derived from these rocks. Together the chemical and stratigraphic trends suggest superposition of a 2nd order (>10my) sea level regression that persists across the mid-Carboniferous boundary. This regression may be related to global cooling associated with the onset of a Late Carboniferous icehouse climate.