Title: Geology of the Whitehall area, Montana
First published: 1955
Description: The Whitehall area is located in southwestern Montana north of the Jefferson River and northward from the Tobacco Root Mountains. The area lies near the southeastern border of the Boulder batholith and near the junction of three Rocky Mountain tectonic provinces, each of which has unusual structural and stratigraphic features having important bearing on the geologic history of the Whitehall district. The area contains sedimentary rocks of all systems; volcanic rocks of late Cretaceous - Paleocene age; and intrusive rocks of Eocene age. During the late pre-Cambrian time a major fault-scarp crossed the southern part of the area, following the line of high-angle fault which separated the Tobacco Root Highland on the south from the down-dropped Beltian geosyncline-of-sedimentation on the north. Following Beltian sedimentation, both in the main trough and in the Lombard-Harlowton embayment, gentle epeirogenic movements affected the area during Paleozoic to Middle Cretaceous time.

Strong Laramide mountain-building movements were felt in the Whitehall area, including several puses of deformation and intrusion during Upper Cretaceous, Paleocene and Eocene time. Late Creaceous doming within a part of the area definately caused local truncation of all strata down through a portion of the Mississippian Madison Limestone, and possibly down to the upper part of the pre-Cambrian Greyson shale. Following this erosion 9000 or more feet of volcanic rocks were deposited. Then (Paleocene?) compression (from the northwest) formed several folds, which are oriented northeast-southwest, probably because of local drag along a southwestward extension of the Lombard thrust-fault. This folding involved pre-Cambrian, Paleozoic, and Mesozoic formation and was an integral part of the folding and thrusting which affected the whole Beltian trough of northwestern Montana.

During the later phases of this period of compression, basic magma, rising from depth, exerted a vertical force-component which broke the local crustal rocks into a number of differentially-tilted blocks or plates separated from each other by high-angle reverse faults. Shortly afterward (probably in the early Eocene) the area was intruded at depth by marginal portions of the Boulder batholith, and as a concluding phase of this igneous activity small masses of the intrusive extended upward toward the surface, with mesothermal mineralization occurring in the surficial rocks of the area. Eocene streams developed a pre-Oligocene mature topographic surface showing a local differential reflief of at least 4,100 feet, and durin ghte Oligocene and Miocene epochs more than 1,000 feet of Bozeman beds accumulated in these Eocene valleys.

By early Pliocene time the area had a gently rolling surface developed in part by pedimentary cutting of the pre-Oligocene upland rocks, and in part by truncation of the Tertiary valley fills. Subsequent (Pliocene -Recent) erosion has partially re-excavated the Eocene valleys; has cut deep canyons (where the Jefferson and North Boulder were superposed); and has developed several sets of benches or minor pedimentary surfaces along the valley-sides.
Subjects: Geology
Places: Montana, Whitehall Area