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The AAPG/Datapages Combined Publications Database

Wyoming Geological Association

Abstract


Resources of the Bighorn Basin; 47th Annual Field Conference Guidebook, 1996
Pages 109-136

Revised Stratigraphy of Eocene Volcanic Rocks in the Lower North and South Fork Shoshone River Valleys, Wyoming

David H. Malone

Abstract

This paper presents a revised interpretation of Eocene volcanic rocks of the Absaroka Volcanic Supergroup in the North and South Fork Shoshone River valleys. The rocks studied during this investigation are part of the Wapiti Formation, a 5000 ft succession of dark-colored lava flows and epiclastic sedimentary rocks exposed near Wapiti, Wyoming. Stratigraphic resolution of the volcanic rocks was improved by the definition and correlation from other areas of several formal and informal members of the Wapiti Formation. The basal (tuff-breccia and lower stratified) members of the Wapiti Formation consist of distal and medial facies epiclastic volcanic rocks and represent relatively quiet sedimentation from a northerly source, with fluvial, lacustrine, and paludal environments dominant. Sedimentation of distal-facies units was interrupted by the collapse of a volcanic edifice to the northwest, and the region was buried by a debris-avalanche deposit (Deer Creek Member). The Deer Creek Member consists of blocks (individually as large as several km2 in area) of vent-medial-facies lava flows, breccias, and sandstones within a thin, heterogeneous matrix of boulder- to sand-sized volcaniclastic material. It is interpreted as the deposit of a debris avalanche, formed by the collapse of a large stratovolcano within the Absaroka Range during the early middle Eocene. The currently known areal extent and volume of the Deer Creek Member are ~175 mi2 (450 km2) and ~24 mi3 (100 km3), respectively. It is possible that much of the material mobilized as parts of the debris-avalanche deposit are time-equivalents of the distal-facies units that were overridden. After the emplacement of the Deer Creek Member, sedimentation of distal-facies rocks resumed. The first dark-colored medial-facies rocks overlying these distal-facies (lower breccias member) units signal the initiation of volcanism at the Sunlight Peak vent-complex. These rocks are overlain by the massive columnar flows of the Jim Mountain Lavas which represent a shield building stage at the Sunlight Peak vent-complex. The Jim Mountain Lavas are overlain by the crudely stratified proximal- and vent-facies breccias of the upper breccias member; these units reflect the systematic growth and maturation of this vent-complex.


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