High Resolution
Sequence Stratigraphy of
Latest Mississippian (Chesterian) Carbonate and Siliciclastic Facies in East
Central Idaho and Southwest Montana*
By
Search and Discovery Article #50021 (2005)
Posted November 10, 2005
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1Geological Sciences, University of Idaho, Moscow, ID 83844-3022, phone: 208 882 9430 ([email protected])
2Department of Geology, University of Idaho, Moscow, ID 83844-3022
3Department of Geology, Washington State University, Pullman, WA 99164
4Dept. of Geology, University of California, Davis, CA 95616.
Abstract
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.
Location map of field area: East central Idaho and southwest Montana
Depositional facies, Chesterian (latest Mississippian) of east central Idaho and southwest Montana