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DEHLER, CAROL M., and GEORGE W. GRADER

*, University of New Mexico, Albuquerque, and University of Idaho, Moscow

Abstract: Devonian Stratigraphy in East-Central Idaho

Shelf-to-basin stratigraphic studies of Devonian rocks in east-central Idaho reveal new paleogeographic resolution of depositional environments across E-W and SE-NW transects. Lower and Middle Devonian mixed carbonate and siliciclastic strata (less than or equal to 600 m thick) include restricted shallow-marine deposits of the Carey Dolomite (and associated strata) and discontinuous paleovalley deposits of the Beartooth Butte Formation. Fluviokarst, estuarine and inner shelf lithofacies with micro- and macro-vertebrate faunal assemblages allow basinward correlation with conodont-bearing deposits. Overlying these rocks are the Jefferson Formation carbonate ramp deposits of Middle and Late Devonian age.

The Jefferson Formation varies in thickness from ~ 900 m in east-central Idaho to ~100 m in the Beaverhead Mountains of SE Montana. Distinctly bedded dolostone, limestone, and quartz arenites suggest peritidal and shallow subtidal environments. Shallowing-upward successions feature bioturbated and fossiliferous carbonate overlain by algal and laminated lithofacies with breccia caps. Back-stepping Givetian and Frasnian shelf environments were followed by a regional unconformity and restricted Famennian environments. These upper Jefferson rocks feature stratiform carbonate-evaporite solution breccias, shallow water sandstone units and cryptalgal dolostones.

Devonian stratigraphy reflects differential regional subsidence, pronounced environmental changes and long-term shallow-water lithofacies thinning over the Tendoy Arch (sensu Scholten, 1957). Extrabasinal Famennian glacio-eustatic drawdown and brecciation of the Jefferson shelf is interpreted to overprint Antler foreland controls.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana