Abstract

The Saypo quadrangle, in northwestern Montana, lies between parallels 47°30′ and 48° and meridians 112°30′ and 113° and contains parts of the Lewis and Clark and the Sawtooth ranges and part of the northern Great Plains. Stebinger mapped the geology west to the mountains before 1918, and the writer began mapping the mountains in 1940 for the United States Geological Survey.

The stratigraphic column, approximately 14,500 feet thick, consists of 19 formations: four of Belt, nine of Cambrian, two of Devonian, and one each of Mississippian, Jurassic, and Lower and Upper Cretaceous age. Pleistocene drift and Recent alluvium lie on all older rocks. Belt and Cretaceous sediments contain gabbro and diorite sills. Unconformities occur between Cambrian and Belt, Devonian and Cambrian, Mississippian and Devonian, and Jurassic and Mississippian formations. None are strongly angular.

The region contains two structural provinces: an eastern province characterized by westward-dipping, high-angle thrust faults, which broke steeply dipping Paleozoic and Mesozoic sediments, and a western province characterized by westward-dipping, low-angle overthrusts and westward-dipping, high-angle normal faults which broke the Belt and older Paleozoic sediments. Folds are subordinate to faults, both in number and importance.

All major structures probably formed during the Laramide orogeny. Its earlier compressional phase produced open folding, then high-angle thrusting, and finally low-angle overthrusting. The later tensional phase resulted in low-angle normal and, toward its close, in high-angle normal faulting.

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