Exploring the Proterozoic Big Sky orogeny in southwest Montana

What: This project will characterize, compare, and/or contrast the protoliths, the grade and time of metamorphism, and the intensity and age of tectonism of three rock suites found in the southern Gravelly Range and Raynolds Pass area of southwest Montana. Our goal is to integrate these findings into our developing understanding of the Big Sky orogeny, an arc-continent collision that occurred along the northern flank of the Wyoming province at 1.8-1.7 Ga.

When: July 24-August 20

Where: The project will be headquartered in the vicinity of Island Park, Idaho. Fieldwork will be conducted in the southern Gravelly Range and adjacent Raynolds Pass area. The field area is 30 km west of Yellowstone National Park and 120 km south of Butte, Montana.

Who: Six students

Project Faculty: Tekla Harms and Jack Cheney, Professors of Geology, Amherst College; John Brady, Professor of Geology, Smith College

Project description and goals

Precambrian rocks in the Tobacco Root Mountains preserve evidence of a P-T-t path of metamorphism and deformation that are the result of an arc-continent collision, the Big Sky orogeny, which occurred along the NW boundary of the Wyoming Province during the Middle Proterozoic (1.7-1.8 Ga) (Brady et al., 2004). The pressures and temperatures determined indicate that Tobacco Root Mountains rock suites were first buried by some 30 km of tectonic overburden, then carried in a thrust sheet up over cooler rocks, and somewhat later experienced rapid tectonic unroofing to approximately 12 km depth. The tectonic overburden once covering the Tobacco Root Mountains must have been displaced laterally by normal faulting; similarly the crust the Tobacco Root Mountains rocks were thrust over must somewhere flank the range across a major thrust fault. Thus we have testable hypotheses about the likely tectonic history of Precambrian rock suites in the ranges surrounding the Tobacco Root Mountains.

The north-south trending Gravelly Range provides a geologic transect across the inboard flank of the Big Sky orogen. Rocks in the central and Gravelly Range contrast in protolith, metamorphic grade, and thermochronology versus those in the Tobacco Root Mountains, suggesting that the Gravelly Range preserves a higher structural level or different tectonic province of the Big Sky orogen (Harms, Brady and Cheney, 2006). Three main rock suites occur in the southern Gravelly Range, including the area around Raynolds Pass (O’Neill and Christiansen, 2004).

  1. Low grade metasedimentary rocks are intruded by gabbroic bodies with contact metamorphic aureoles in the area around Standard Creek in the southern Gravelly Range.
  2. Higher grade metasedimentary rocks and interlayered metamafic rocks occur farther south between the Madison River and Henry’s Lake.
  3. A broad zone of mylonitic rocks separates the two metasedimentary suites.

Field studies focused on these three rock suites will seek to relate southern Gravelly Range geology to the geology of the Tobacco Root Mountains and to the history and architecture of the Big Sky orogen.

Student projects

Individual projects will be tailored as much as possible to the background of the student and the resources of his or her home institution. Students will collect samples in specific areas or samples of a specific rock type across the field area for petrologic, structural, geochemical, and/or geochronological study over the course of their senior year. Fieldwork will include documentation of the local geologic setting of each project, gathering structural data, and sampling. Particular student projects may include:

  1. Petrology, regional thermobarometry, and PT evolution of metamorphic rocks in the two metasedimentary rock suites and of contact metamorphic rocks in aureoles.
  2. Major, trace element, and/or isotope geochemistry of metamafic units to characterize protoliths.
  3. Monazite Pb/Pb and/or amphibole, biotite, and muscovite Ar/Ar geochronology of the metamorphic rocks.
  4. Kinematics of the mylonitic zone that juxtaposes the two suites of metasedimentary rocks.

Field conditions

Daily traverses will be largely off-road and off-trail within the National Forest. Each day we will drive Forest Service roads as close to our target rocks as possible – but most days will include a hike of 2 or more kilometers and/or a change in elevation of 1000 feet. We could encounter antelope, elk, and coyotes, as well as bears, rattlesnakes, and ticks. We will typically be working at some distance from a vehicle or cell phone service, but never alone. Summer weather in Montana is usually sunny and dry. Temperatures depend on elevation, however, and can range from 95 to 45 degrees. Afternoon thunderstorms are common. Exercising sensible precautions, good judgment, and appropriate respect will minimize the danger from such experiences and maximize the rewards of working in the Rocky Mountains. Having appropriate field gear is also a critical component of a safe work environment.
Our base of operation will be a lodge-like setting where students will bunk in several bedrooms and share a couple of bathrooms. Full kitchen facilities will allow us to provide our own meals each day.

Course Preparation

Course work in metamorphic petrology and structural geology are necessary prerequisites for participation in this project. Experience at a field camp or in a field geology course is strongly recommended but not required.


  • Brady, J.B., Burger, H.R., Cheney, J.T., and Harms, T.A., editors., 2004, Precambrian Geology of the Tobacco Root Mountains, Montana: Geological Society of America Special Paper 377, 256p.
  • Cheney, J.T., and 17 others, 2004, Proterozoic metamorphism of the Tobacco Root Mountains, Montana, in Brady, J.B., Burger, H.R., Cheney, J.T., and Harms, T.A., eds., Precambrian Geology of the Tobacco Root Mountains, Montana: Geological Society of America Special Paper 377, p. 105-129.
  • Harms, T.A., Brady, J.B., Burger, H.R, and Cheney, J.T., 2004, Advances in the geology of the Tobacco Root Mountains, Montana, and their implications for the history of the northern Wyoming Province, in Brady, J.B., Burger, H.R., Cheney, J.T., and Harms, T.A., eds., Precambrian Geology of the Tobacco Root Mountains, Montana: Geological Society of America Special Paper 377, p. 227-243.
  • Harms, T.A., Brady, J.B., and Cheney, J.T., 2006, Exploring the Proterozoic Big Sky orogeny in southwest Montana: 19th Annual Keck Symposium in Geology Proceedings, p. 171-176.
  • O’Neill, J.M., and Christiansen, R.L., 2004, Geologic map of the Hebgen Lake Quadrangle, Beaverhead, Madison, and Gallatin Counties, Montana, Park and Teton Counties, Wyoming, and Clark and Fremont Counties, Idaho: U.S.Geological Survey Scientific Investigations Map 2816, 1:100,000 scale.