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. MontaƱez, 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.