ORIGINAL GEOLOGIC MAPPING IN FIELD CAMP: FINDINGS, PRODUCTS, AND LESSONS LEARNED

For ten consecutive Field Camps, we mapped the southern Beaverhead Mountains in the Sevier thrust belt of central Idaho. We chose this location because we recognized unresolved structural and stratigraphic problems. Our mapping revealed a distinctive structural style of east- to northeast-vergent fault propagation folding, with fold amplitudes from a few meters to over a kilometer, and thrust fault offsets increasing with structural depth. The largest fault propagation fold is along the Copper Mountain thrust, which has stratigraphic throw of over 450 meters at the south end of the range, gradually decreasing to zero at a higher structural level northward near Bluebird Mountain. The mountain block at Skull Canyon is an uplift along the Blue Dome normal fault, which is separate from the range-front fault to the north and south. The Long Canyon normal fault is the connector between northern and southern range-front faults. Minor normal faults trending NNW or NE are present in the southern and northern areas. The Big Snowy formation consists of interfingered siltstone, shale, and carbonate units, and is marked by landslides along its entire length. Neogene basalts and rhyolites are present at the south end of the range, and two small remnants of the Eocene Challis volcanics are present in Willow Creek Canyon in the north. This Field Camp experience taught us lessons including the value of doing original work (not canned projects), minimizing travel time, managing logistics, teaching on the outcrops, preparation in previous field trips and courses, and progressing from small- to large-scale field methods.