Geology and igneous petrology of the northern Elkhorn Mountains, Jefferson and Broadwater Counties, Montana

Abstract

The northern Elkhorn Mountains comprise an area of about 132 square miles in west-central Montana. The area includes parts of four major geologic units : ( 1) in the northern part, a narrow belt of folded and faulted upper Paleozoic and Mesozoic sedimentary rocks more than 5,700 feet thick; (2) in the eastern part, a folded and faulted sequence of Upper Cretaceous volcanic rocks more than 11,500 feet thick, and related intrusive rocks ; ( 3) in the western part and near the northeastern corne-r, plutonic rocks of the Boulder batholith, with small bodies Of Tertiary igneous rocks cutting and resting upon them ; and ( 4) along the northern edge, poorly consolidated and unconsolidated deposits df ash, tuff, and gravel of Tertiary and Quaternary age.

Geology of the Elliston region, Powell and Lewis and Clark counties, Montana

“…Klepper and others (1957, p. 44-45) described older intrusive rocks of the Boulder batholith as ranging in composition from quartz monzonite or monzonite to gabbro, and younger intrusive rocks of the batholith as granodiorite and monzogranite and small stocks of mafic rocks. Smedes (1966) batholith, as did Klepper and others (1957) and Knopf (1963). New isotopic data (table 1) suggest that granodiorite and monzogranite of the Marysville stock (79 Ma) and quartz monzonite, quartz monzodiorite, and granodiorite of the Blackfoot City stock (unit Kbc, pl.…”

Revision of middle Proterozoic Yellowjacket Formation, central Idaho, and revision of Cretaceous Slim Sam Formation, Elkhorn Mountains area, Montana

“…Reasons for assignment to the Cody rather than the Marias River Shale are presented in the section on the Cody Shale. others (1957, 1971), Freeman and others (1958), Smedes (1966), and Knopf (1963).…”

“…The geologic studies of Klepper and coworkers others, 1957, 1971;Freeman and others, 1958;Smedes, 1966) in the Elkhorn Mountains dates from the 1950's and remain the standard references for the area. The revision of the Slim Sam in this report builds on their careful mapping and related detailed studies and reflects subsequent areal, regional, and correlation work carried out by many people.…”

The Maudlow and Sedan Formations of the Upper Cretaceous Livingston Group on the west edge of the Crazy Mountains basin, Montana

“…Member D contains several fine-grained biotitic dacitic welded tuff sheets, the "red liparite" of Lindgren. The tuffs resemble ash-flow tuffs of the lower unit of the Elkhorn Mountains Volcanics (Smedes, 1966 The unconformity at the base of member A is characterized by an abrupt and persistent change in mineral constituents. The beds below the unconformity, the Telegraph Creek Formation (upper part) or the Virgelle Sandstone Member, are composed of abundant quartz, some feldspar, and varying amounts of biotite, sphene, and magnetite in a calcite matrix, whereas the basal sandstones and mudstones of member A appear to be slightly reworked crystal tuffs with abundant plagioclase, minor quartz, moderate hornblende, and pyroxene in a clay-rich zeolite or calcite matrix.…”