Date of Award


Document Type


Degree Name

Master of Arts (MA)



First Advisor

W.L. Moore


The Elliston area includes four townships located along the continental divide west of Helena, Montana on the northwest margin of the Boulder Batholith and within the Laramide disturbed belt. Strata ranging in age from Precambrian through recent are present, with all but the Ordovician, Silurian, and Triassic System& represented. The sedimentary rocks are mainly marine and continental carbonates, shales, and sandstones typical of a relatively stable shelf. The Precambrian Belt "Series" comprise the oldest rocks. The lower Cretaceous Blackleaf Formation is the youngest pre-Laramide unit. Post-Laramide sediments include fine-grained mid-Tertiary basin fills, terrace gravels, a moraine, and a mantle of coarse gravel in the north which may be the remnant of an early Pleistocene drift sheet.

The oldest of four generations of igneous rocks is a sequence of pre-Laramide latitic and andesitic volcanics and related sills, probably correlative with the Lake Cretaceous Elkhorn Mountains Volcanics. A number of small post-Laramide granodioritic plutons, satellitic to the Boulder Batl1olith, intrude the volcanics and older rocks, and are responsible for extensive thermal metamorphism. In the Avon basin there are extensive later post-Laramide Tertiary rhyolite flows, which pre-date the basin-fill sediments. In the Dog Creek basin area similar rhyolites occur which appear to overlie the dirft (?) gravels and may be of Quaternary age.

The major structural feature is a large southeasterly plunging syncline, which is overthrust from the west by an adjacent snticline. These structures were modified during batholitbic intrusion by some secondary folding and much faulting. The Avon and Dog Creek basins are products of post-Laramide block-faulting.

Walker (69339 kB)

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