Abstract

The Lima region which includes the southwest corner of Beaverhead County, Montana, and an adjacent portion of Clark County, Idaho, covers most of the Tendoy, Blacktail, and Snowcrest ranges, parts of the Beaverhead and Ruby ranges, and the intermontane basins.

Metamorphic rocks include the Pony and Cherry Creek groups and the Dillon granite gneiss, all pre-Beltian. The sedimentary rocks represent all systems from Beltian through Quaternary, except the Silurian. The pre-Cretaceous rocks were deposited in a tectonic shelf environment and in adjacent parts of the miogeosyncline to the west. This tectonic differentiation was especially marked during the Mississippian. The latest Jurassic and post-Jurassic sediments are terrestrial deposits related to the Nevadan and Laramide orogenies. A Laramide granite stock crops out at the western margin of the region. Extrusive rocks include basaltic and rhyolitic lavas and pyroclastics, ranging in age from Eocene through Pleistocene.

Pre-Beltian diastrophism metamorphosed the Pony and Cherry Creek rocks and the partly syntectonic Dillon granite complex. The Laramide orogeny began with northeast folding, probably Paleocene, which resulted in deposition of the thick and coarse Beaverhead conglomerate. This was followed in early Eocene time by northwest folding and overthrusting to the northeast. The low-angle Medicine Lodge thrust, which may have had a displacement of 10 miles or more, carried the geosynclinal sedimentary facies close to the shelf facies. The Johnson and Limekiln thrusts probably are branches of this fault. These low-angle thrusts were subsequently broken, dislocated, and tilted by the high-angle Nicholia, Cabin, and Tendoy thrusts. Post-Laramide diastrophism consiste' mostly of block faulting, which may have started in mid-Tertiary time and, in part, continues. It disrupted a late Tertiary peneplain and in part formed the present ranges. Northeast drainage on this surface maintained itself across the Tendoy Range and cut several deep gorges. Fresh-water sediments and volcanic material accumulated in the basins and were tilted.

Glaciers of the Wisconsin stage shaped some of the valleys, depositing drift in two sub-stages. The present cycle of erosion is in the mature stage.

Mineral resources in the region are largely undeveloped. One small lead-zinc mine is in operation, but in general possibilities for metal mining are poor. Coal and gypsum are, or have been, mined, and phosphate and associated elements may become important in the future. The region is not believed to have major oil and gas potentialities.

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