Thrust Belt Structures and Paleozoic Stratigraphy of the Scott Butte and Snaky Canyon Quadrangles, Southern Beaverhead Mountains, Idaho
Clayton, Robert W. and William W. Little
Brigham Young Universtiy- Idaho, Rexburg, ID
Geologic mapping by faculty and students at Brigham Young University - Idaho has revealed a structurally-complex area involving Mississippian through Permian strata that have been subjected to folding (some overturned), thrust faulting, high-angle reverse faulting, and normal faulting at the southern end of the Beaverhead Mountains between the Idaho-Montana border and the Snake River Plain. This location is believed to be the north-westward extension of thrust faulting previously mapped in the Big Hole Mountains and Snake River Range to the south of the Snake River Plain; although, specific fault correlations have yet to be made. Structure of the southern Beaverhead Mountains is dominated by a northwest-striking normal fault that drops thrusted and complexly folded Pennsylvanian- Permian strata down to the west. The normal fault is thought to be Miocene in age because of its orientation parallel to the current mountain front. A second set of smaller normal faults with E- to ENE-strikes truncate or merge with the NW-striking normal fault and may be related to passage of the Yellowstone hot spot beneath the adjacent Snake River Plain during late Miocene time. In the Snaky Canyon Quadrangle, a thrust fault places the shallowly-dipping Lower Pennsylvanian to Upper Mississippian Bluebird Mountain Formation over the steeply-dipping Upper Mississippian to Lower Permian Snaky Canyon Formation. This thrust fault is in turn cut by a subvertical, NW-striking fault that forms an abrupt stratal boundary. The Mississippian South Creek Formation pinches out abruptly southward in the Scott Butte Quadrangle, and the shaly Mississippian Big Snowy Formation pinches and swells near faults and in folds.
AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah