Paleogeography and Depositional History of the Middle and Upper Devonian in Central Idaho

Abstract

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 circulation as early as Givetian time. 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 erosionmore » 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.« less