Geology of part of the Townsend Valley, Broadwater and Jefferson Counties, Montana

Abstract

The Townsend Valley, a broad intermontane basin in west-central Montana, extends from Toston to Canyon Ferry. The area described in this report includes that part of the valley west of longitude 111° 30' W. and south of latitude 46° 30' N. and the low hills west and south of the valley. The Missouri River enters the area near Townsend and flows northward through the northern half of the area. Three perennial tributaries and a number of intermittent streams flow across the area and into the river from the west. The hilly parts of the area are underlain mainly by folded sedimentary rocks ranging in age from Precambrian to Cretaceous. The broad pediment in the southwestern part is underlain mainly by folded andesitic volcanic rocks of Upper Cretaceous age and a relatively thin sequence of gently deformed tuffaceous rocks of Tertiary age. The remainder of the area is underlain by a thick sequence of Tertiary tuffaceous rocks that is partly blanketed by late Tertiary and Quaternary unconsolidated deposits. Two units of Precambrian age, 13 of Paleozoic age, 7 of Mesozoic age, and several of Cenozoic tuffaceous rock and gravel were mapped. Rocks of the Belt series of Precambrian age comprise a thick sequence of siltstone, sandstone, shale, and subordinate limestone divisible into the Greyson shale, the Spokane shale, and the basal part of the Empire shale, which was mapped with the underlying Spokane shale. The Precambrian rocks are overlain with slight angular unconformity by a parallel-bedded sequence of Paleozoic and Mesozoic sedimentary rocks. The formations of Cambrian age, named from oldest to youngest, are the Flathead quartzite, Wolsey shale, Meagher limestone, and Park shale of Middle Cambrian age, and the Pilgrim dolomite of Late Cambrian age. The uppermost Cambrian unit, the Red Lion formation, and the basal Devonian unit, the Maywood formation of Late Devonian age, are thin units that are difficult to distinguish from one another, and accordingly they were mapped together even though they are separated by an erosiOnal unconformity. The Maywood formation is overlain by the Jefferson dolomite of Devonian age, and the Three Forks shale of Devonian and Mississippian age, and these are overlain by the Lodgepole and Mission Canyon limestones, subdivisions of the Madison group of Mississippian age. The Amsden formation, which is probably separated from the Mission Canyon limestone by an erosional unconformity of negligible, relief, includes rocks of both Mississippian and Pennsylvanian age. It consists of red calcareous siltstone 481 GEOLOGICAL SURVEY s-BULLETIN 1042 PLATE 43 inches A. Meagher limestone, upper part. Dark color is dense medium-gray pure limestone. Light color is somewhat coarser grained yellowish-orange slightly calcitic dolomite. Elongation of moltlcs parallels bedding. Inches B. Pilgrim dolomite. Sample of dolomite unit showing conspicuous ribbons of dolomite weathering in relief in gray dolomite. T>pical specimens of Meagher limestone and Pilgrim dolomite si GEOLOGICAL SURVEY BULLETIN 1042 PLATE ...