ABSTRACT

Badger Pass lies at the southern edge of the Pioneer Mountains, approximately 12 miles west of Dillon. The map area, a 8x16 km zone, is a part of the Rocky Mountain thrust belt. Two thrust faults, the Ermont and Kelley, dominate the structure.

The Ermont thrust, easternmost thrust in the area, dips gently westward along a 24 km NNE-trending trace. In the overlying Ermont plate, a 4-5 kilometer wide zone of Madison limestone and younger Paleozoics have been folded and thrust eastward over late Cretaceous Beaverhead conglomerate and tuffaceous sediments The plate was later intruded by Tertiary granodiorite and andesite. Orientation of small folds, tear faults, drag folds and a window of Tertiary tuff indicate a transport direction of S70° ± 15°E and at least 5 km movement.

The Kelley thrust has brought Precambrian Belt quartzite over an eastward-overturned syncline of Madison limestone. The fault dips westward at 25°-40°, with locally steeper segments, and has been offset by east-west tear faults. A transport direction of S80° ± 15°E is inferred from orientation of tear faults and surface trace of the thrust Although precise calculation of net slip is not possible, it appears that stratigraphic throw is comparable to that of major overthrusts in the Rocky Mountain thrust belt.