Paleogeography and Depositional History of the Middle and Upper Devonian in Central Idaho
WILER, S. W., and P. E. ISAACSON, University of Idaho, Moscow, ID
Middle and Upper Devonian rocks of central Idaho were deposited in a foreland basin east of the Antler orogenic "welt" and west of a positive feature that extends along the central Idaho-Montana border. Upper Devonian rocks are anomalously thick (more than 700 m) in central Idaho, relative to the carbonate platform (300 m) in southwestern Montana. West of the Antler orogenic "welt" Lower to Upper Devonian siliciclastic and carbonate rocks of the western Milligan Formation were deposited in an inner arc basin. East of the Antler orogenic "welt" intertidal to subtidal deposition of the lower black dolomite unit of the Jefferson Formation occurred from Givetian to late Frasnian time. Restricted conditions suggest the Antler orogenic "welt" was periodically an effective barrier to circula ion as early as Givetian time. Localized organic buildups did, however, develop and include a 41 m-thick coral-stromatoporoid biostrome, which may extend laterally as much as 37 km. Stratigraphic variation in the upper Jefferson Formation is associated with subaerial exposure along the Antler orogenic "welt" and increased foreland basin subsidence during the late Frasnian. Shallowing to supratidal and intertidal, siliciclastic channels and eolian sand probably are derived from early Paleozoic rocks along the Antler orogenic "welt." Conformably overlying the Jefferson Formation is the Famennian Trident Member of the Three Forks Formation, which contains a garnet-bearing argillaceous limestone, suggesting that erosion along the Antler orogenic "welt" had reached Precambrian basement rocks. Middle Famennian flysch deposition suggests increased subsidence in the foreland prior to the unconformity over which Early Mississippian rocks were deposited.
AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)