Geology of north end of the Pioneer Mountains, Beaverhead County, Montana
The north end of the Pioneer Mountains is underlain by the following rocks: lower Proterozoic gneiss and amphibolite; middle proterozoic clastic rocks of the Missoula Group (Mount Shields Formation.); basal Cambrian clastic unit; Cambrian-Devonian-Carboniferous shelf sequence; Permian Phosphoria Formation; and Lower Triassic Dinwoody Formation. Jurassic rocks are missing. The Lower Cretaceous Kootenai Formation is of lagoonal to fluviatile facies, overlain by a thick (approx. = 2 km, 1 mi) section of fluviatile Colorado Group. The sedimentary rocks are cut by calc-alkalic plutonic rocks (80 to 65 m.y.B.P.), the oldest of which are synchronous with upper Colorado sedimentation. The youngest pre-quaternary rocks are Eocene and Oligocene calc-alkalic lavas and Oligocene pumiceous tuff. The Missoula Group is entirely in thrust sheets that postdate the Colorado, so the thrusting is no older than Campanian, but the thrusts are cut by 72 to 74 m.y.B.P. plutons. In addition to the thrust sheets two families of high-angle faults dominate. The eastward projection of the mountain front at Maiden Rock, just south of Divide, resulted from block displacement along two strands of this fault. The second family of high-angle faults trends north-northeast. Wise River valley is interpreted to be a graben in this system. The west-northwest fault system began at least before the Eocene lava flows, but complex field relations between the two high-angle fault systems indicate their growth must have overlapped in age, possibly through the late Tertiary.
- Research Organization:
- Geological Survey, Reston, VA
- OSTI ID:
- 6497257
- Report Number(s):
- CONF-8309274-
- Journal Information:
- Am. Assoc. Pet. Geol., Bull.; (United States), Vol. 67:8; Conference: AAPG Rocky Mountain Section meeting, Billings, MT, USA, 18 Sep 1983
- Country of Publication:
- United States
- Language:
- English
Similar Records
Early tectonic history of the Greater Antilles
Structural geology and tectonics of the Orville Coast region, southern Antarctic Peninsula, Antarctica