Geology of a portion of southern Madison range, southwestern Montana

The Southern Madison Range is located in southwestern Montana. Strata range in age from Precambrian to Quaternary. Paleozoic rocks are predominantly marine carbonates, with minor units of sandstones and shales. Mesozoic rocks are predominantly fine-grained clastic rocks of nonmarine origin. All periods are represented except the Silurian. The study area lies in the zone of north-northwest-trending structures, where the Madison-Gravelly arch has moved eastward onto the Madison synclinorium. Major reverse faults, the Hilgard and Scarface faults, place Precambrian and Paleozoic strata on rocks as young as Cretaceous. These faults dip moderately to steeply west. Overturned beds occur in the lower plate. In the southern part of the study area, an east-dipping reverse fault in the upper plate is associated with the northern termination of the Hilgard fault. To the north lies the Shedhorn anticline and syncline. The Shedhorn anticline is a double plunging structure with steep dips on the northeastern limb. The Shedhorn syncline is an open structure to the west of the Shedhorn anticline. The northwestern edge of the syncline is deformed by the Scarface fault, with local overturning and an east-dipping reverse fault in the lower plate. The Taylor fault is a Cenozoic normal fault that dips steeplymore » west, with at least 2000 ft of displacement.« less