Paleozoic Stratigraphy of Southwestern Montana

Abstract

Within this area all Paleozoic systems excepting the Silurian are represented. Cambrian strata are similar to those of better known areas on the north and east, but differ in the presence of several intrasystemic unconformities. The Ordovician Kinnikinic quartzite occurs along the Idaho-Montana border and a thin edge of Bighorn dolomite is present along the eastern margin of the area in Gallatin County. Elsewhere, Cambrian strata are overlain by Upper Devonian. Devonian strata of the Jefferson and Three Forks formations are typically developed and widespread, with little lateral variation. The Mississippian system, on the other hand, is characterized by extremes of facies expression, including rapid lateral variation among redbeds, evaporites, fragmental limestones, dolomites, and black shales. Mississippian rocks range from 1,000 to more than 4,000 feet in thickness. Pennsylvanian strata are dominated by the quartzose and well cemented sandstones of the Quadrant formation which ranges in thickness from approximately 100 feet to nearly 3,000 feet. The Permian system is represented by the Phosphoria formation, composed of quartzites, limestones, dolomites, black shales, phosphorites, and bedded cherts. End_Page 2135------------------------------ Fig. 1. Regional index map, showing location of area discussed and other areas with which correlations are made: (1) southeastern Idaho, (2) central Idaho, (3) southwestern Montana, (4) central Montana, (5) northwestern Montana, (6) northern Wyoming and Big Horn Basin, (7) Black Hills, (8) Williston Basin. Fig. 2. Index map of southwestern Montana, showing location of topographic features referred to in text. End_Page 2136------------------------------