Triassic and Jurassic Stratigraphy of Southwestern Montana

Abstract

ABSTRACT The Triassic system is represented by marine sandstones, siltstones, limestones, and shales and by non-marine redbeds, all of Early Triassic age. These beds are correlated with the Dinwoody, Woodside, and Thaynes formations of southeastern Idaho, western Wyoming, and northeastern Utah. The system is widely distributed and ranges in thickness from zero in Silver Bow County to more than 1,500 feet in southern Beaverhead County. These sediments were deposited under relatively stable to mildly unstable conditions along the margins of the geosyncline, the axis of which lay west of this region during the Triassic, as well as during the Paleozoic. The Jurassic system is represented by marine sediments that are correlated with the Sawtooth, Rierdon, and Swift formations of the Middle and Upper Jurassic Ellis group and by non-marine deposits that represent the Upper Jurassic Morrison formation. Jurassic strata are thin or missing in the northern and central parts of the region, as the result of the development of a small but prominent positive area, and they thicken eastward into Gallatin County and southward and westward in Beaverhead County. The marine Jurassic rocks were deposited under shelf conditions, and the axis of the geosyncline still lay west of this region during the Jurassic. The presence of subgraywackes in the Morrison formation presaged the eastward shift of the geosynclinal axis into this region, which occurred in the Cretaceous.