Abstract

More than 5000 ft (1550 m) of Eocene volcanic rocks of the Wapiti Formation, Trout Peak Trachyandesite, and Wiggins Formation are exposed in the upper South Fork Shoshone River valley. In this area, a distal facies of the Deer Creek Member of the Wapiti Formation, a large debris-avalanche deposit, has been recognized within the volcanic succession. The unit occurs above a series of light-colored, well-stratified sandstones, mudstones, and conglomerates of the lower Wapiti Formation, and beneath darker, massive breccias and lava flows of the upper Wapiti Formation and Trout Peak Trachyandesite.

The debris-avalanche deposit is very poorly sorted and consists of particles ranging from silt- to boulder-size. The unit is matrix-supported, and most clasts consist of well- to subrounded, dark gray trachyandesite, red pyroxene andesite porphyry, and light gray hornblende andesite. Several large, irregular inclusions of trachyandesite lava and epiclastic breccias occur throughout the deposit. These inclusions range in size from about 10ft (3 m) to more than 500 ft (160 m) in diameter. Stratification within these inclusions is often apparent from a distance, but upon closer inspection it is difficult to discern. Where present, this layering appears to be highly contorted, and it is truncated along the margins of the inclusions.

The unit is sheet-like in geometry, but it thins gradually to the south and west. The unit averages 500 ft (160 m) in thickness, and ranges from a low of 300 ft (90 m) to a high of more than 1600 ft (500 m). The top of the unit is mostly flat, and the variation in thickness is mainly a function of relief on the lower surface. The relief on the lower surface can be spectacular; at one locality more than 1000 ft (320 m) of relief occurs in less than 500 ft (160 m) lateral distance. For the most part, the relief on the basal surface is less dramatic, and varies less than 150m / km. The total transport distance from the inferred source area at Sunlight Peak to the extreme southern exposure of the unit in this area is at least 35 mi (56 km). This increases the previously known transport distance by some 16 km. The existence of the Deer Creek Member in the upper South Fork Shoshone River valley increases the known areal extent and volume of the unit to more than 1500 Km² and 250 Km³, respectively.