Abstract

A very large debris-avalanche deposit found within the Eocene volcanic succession in the northeastern Absaroka Range, Wyoming, is a result of the failure and collapse of an ancient volcanic edifice in the vicinity of Sunlight Peak, a principal early middle Eocene center of volcanic-plutonic activity. The debris-avalanche deposit is a sheetlike body that consists of blocks, individually as large as several square kilometres, of well-stratified vent medial facies lava flows, breccias, and sandstones within a thin, heterogeneous, and complexly deformed matrix of boulder- to sand-sized volcaniclastic material. The avalanche was directed to the southeast along a broad paleovalley, and remnants have been found >40 km from the source area. The deposit initially covered an area of at least 450 km2 and had a minimum volume of 100 km3, making it one of the largest subaerial avalanches ever recognized.

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