Structural Geology of the Sawtooth Range at Sun River Canyon, Montana Disturbed Belt, Montana

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Route

Start point

Entrance to Sun River Canyon, 20 miles NW of Augusta, MT

End point

Gibson Dam

Roads (and Trails)

Montana 435 (U.S. Forest Service Road 208), U.S. Forest Service Trail 201

Total distance

9 miles

Geology

Summary

The Sawtooth Range is one of the best exposed examples of imbricate thrust faulting in the foreland fold and thrust belt of the western United States. It is an outstanding field classroom for the demonstration of stratal shortening and the various structural features associated with thrust belt terranes. This 7-stop traverse through Sun River Canyon offers excellent exposures of thrust contacts along the road and panoramic views of stacked thrust sheets and demonstrates the dependence of structural style on relative competence of units in the stratigraphic section.

Key Lithologic Features

  • Paleozoic sedimentary rocks
    • Castle Reef Dolomite (Mississippian)
    • Allen Mountain Limestone (Mississippian)
    • Cambrian shales
  • Mesozoic sedimentary rocks
    • Kootenai Formation (Cretaceous): mostly shales
    • Blackleaf Formation (Cretaceous)
    • Two Medicine Formation (Cretaceous

Structures

  • imbricate thrust faults, ramps, duplexes, slickensides, and intensely sheared rocks in thrust footwalls

Landforms

  • Front Ranges of the Disturbed Belt

Other Features

Reference

Lageson, D.R., 1987, Structural geology of the Sawtooth Range at Sun River Canyon, Montana Disturbed Belt, Montana, in Beus, S.S., ed., Centennial Field Guide Volume 2: Rocky Mountain Section of the Geological Society of America , p. 37-39.

Availability

Out of print. Check Amazon.com. Digital access to this publication is available via the links in the references section above.