Abstract: A Revised Interpretation of Eocene Volcanic Stratigraphy in the Lower North and South Fork Shoshone River Valleys

David H. Malone

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 of several 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 i terrupted by the collapse of a volcanic edifice to the northwest, and the region was buried by a debris-avalanche deposit (Deer Creek Member). 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.

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana