Unraveling the Geologic History of the Blacktail Mountains, SW Montana

Document Type

Grant Proposal

Date Accepted

Summer 2004

Project Description/Abstract

This proposal is a continuation of my collaborative research with Dr. Peter Muller in understanding the geologic evolution of the Blacktail Mountains in southwestern Montana. Our recent field mapping has identified a more extensive sequence of volcanic and sedimentary rocks that occur at the southeastern end of the range then has previously been reported. The lowermost part of this sequence consists of a maroon colored, coarse-grained volcanic breccia that sits directly atop 2.7 billion year old metamorphic gneisses. The breccia unit is subsequently overlain by a series of finer-grained maroon rhyolite lava flows that locally display flow banding textures. Overlying these volcanic units is a previously unrecognized sequence of alternating layers of sedimentary rocks interlayered with volcanic ash deposits. This study will document the map pattern, composition, texture, structure, and origin of these rocks. Our results will be incorporated into our developing model for the formation of the Blacktail Mountains.

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