Structural Geology Between the Putnam Thrust and the Snake River Plain, Southeastern Idaho

Abstract The Mount Putnam area of the northern Portneuf Range, Idaho-Wyoming thrust belt, contains three structural plates of Cambrian and late Proterozoic rocks above the Jurassic-Cretaceous Putnam thrust. The Putnam places Ordovician rocks on Pennsylvanian and Permian rocks. The faults between the structural plates cut previously formed folds and are interpreted to have formed sequentially upward and westward from the Putnam thrust. The Chinks Peak area of the Pocatello (northern Bannock) Range, 25 kilometers southwest of Mount Putnam, also contains several structural plates separated by faults that cut folds. Recumbent folds, present in both areas, explain the structural overturning of rocks on Chinks Peak. A speculative geologic cross section from Mount Putnam west to the Bannock Range (hinterland of the Idaho-Wyoming thrust belt) shows a basement duplex beneath the Chinks Peak area that may be located near late Proterozoic block faults associated with deposition of the Pocatello Formation. Younger-over-older faults of the area are considered thrusts. Tertiary reactivation cannot be proven although it remains a distinct possibility. The Red Hill Fault, just west of Chinks Peak, is a Pliocene low-angle normal fault that places late Proterozoic quartzite over vertical Neogene rhyolitic tuff. It is thought to have formed during Basin and Range extension and uplift of the Chinks Peak block. The late Proterozoic and Cambrian Brigham Group is formally defined in Idaho, as containing the dominantly quartzitic rocks above the Pocatello Formation or Blackrock Canyon Limestone and below mappable Cambrian carbonate.