Title: Paleozoic and Mesozoic stratigraphy of the northern part of the Ruby Range, southwestern Montana
Abstract: The northern third of the Ruby Range, southwestern Montana, contains about 5,000 feet (1,525 m) of Paleozoic marine sedimentary rocks overlying a basement of Precambrian metamorphic rocks.Mesozoic rocks are represented by a nonrnarine conglomerate, the Beaverhead Formation.The Cambrian sequence, composed largely of carbonate rocks, comprises the Flathead Sandstone at the base, Wolsey Shale, Meagher Limestone, Park Shale, Pilgrim Dolomite, and the Red Lion Formation.The Meagher Limestone is mainly dolomite in the southern part of the area, limestone in the northern part.The Pilgrim Dolomite is divisible into two main parts: cyclically interbedded dolomitic strata 2-to 10-inch (5-25 cm) thick in the lower part, mainly dolomitic quartz sandstone and sandy dolomite in the upper part.The Red Lion Formation is composed mainly of dolomite; the upper third is dominated by bedded, dome-shaped, and columnar stromatolites.The Devonian is represented by the Jefferson Formation, a dolomite unit containing several interstratified units of silty shale, and the lower part of the Three Forks Formation.The Three Forks is composed largely of clay shale but contains a prominent medial limestone unit.The upper (Mississippian) part of the Three Forks is overlain by three Mississippian formations.The Lodgepole Limestone is constituted of limy shale in the lower half and fossiliferous limestone in the upper half.It is overlain by the Mission Canyon Limestone, a formation that locally contains solution breccias and minor amounts of interbedded dolomite.The overlying red beds, made up of dolomite and sandstone, are questionably assigned to the Kibbey Formation.Pennsylvania rocks are represented by the Quadrant Sandstone, made up of interbedded limestone and sandstone in nearly equal amounts.The Mesozoic is represented by a nonmarine conglomerate that is correlated with the Beaverhead Formation.It is composed of clasts derived from Paleozoic carbonate units and Precambrian (Belt Supergroup) quartzite.