Willis H. Nelson

Crandall Conglomerate, An Unusual Stream Deposit, and Its Relation to Heart Mountain Faulting

The Crandall Conglomerate (Eocene) is a channel deposit, more than 350 ft (100 m) thick, believed to have formed as a result of preliminary movement of the Heart Mountain detachment fault in northwestern Wyoming. Initial movement of the Heart Mountain fault opened a deep rift in which the conglomerate was deposited. The rift was less than a mile (1.6 km) wide and was bordered by 2,000-ft (600 m) cliffs, mostly of Paleozoic limestone. Before the gravel was deposited, unconfined Cambrian shale below the rift was deformed into the Blacktail fold, a sharp anticline without apparent roots, while streams carried away the upwelling shale and cut a channel several hundred feet deep. The debris that accumulated in this channel is the Crandall Conglomerate. Deposition of the conglomerate was followed by Cathedral Cliffs volcanism, by movement on the Reef Creek detachment fault, and by the main movement on the Heart Mountain detachment fault. The main movement on this fault left the lower part of the conglomerate in place but carried the upper part with deposits of the upper plate roughly 15 mi (24 km) southeastward. Most of the deposits of the lower plate rest directly on the Blacktail fold. Of the 15 known deposits of Crandall Conglomerate, five are in place but have been overridden by the upper plate of the Heart Mountain fault, and ten have been transported as part of the upper plate. After this movement, volcanic rocks of the Wapiti Formation blanketed the region.

The Heart Mountain Detachment Fault: A Volcanic Phenomenon? A Discussion

Neither the dispersed nature of the Heart Mountain fault blocks nor the fault breccia are compatible with the hovercraft mechanism proposed by C. J. Hughes. The upper plate was not emplaced as a coherent sheet; it broke up into numerous blocks soon after movement began, and the blocks became widely separated before movement ceased. If they had been supported initially by high gas pressure, the pressure would have been lost long before the blocks came to rest. The fault breccia was examined at thirty localities; volcanic rock fragments were found only at one, and there they apparently were derived from rocks that are older than the faulting.