FRENCH THRUST IN SUN RIVER CANYON
FIELD TRIP STOPS – THIS STOP SHOWS THE MISSISSIPPIAN THRUST OVER CRETACEOUS SHALES
LOCATION: West of Augusta, Montana on Sun Canyon Rd. (Route 208) shortly after entering Sun River Canyon.
GEOLOGIC FEATURES: Thrust faulting of older over younger strata.
DESCRIPTION: At this locality we see an exposure of the French Thrust, one of several imbricate thrusts whose fault planes dip to the west. The Mississippian Castle Reef Dolomite (equivalent to the Mission Canyon Fm.) is thrust over the Cretaceous Blackleaf Shale. Due to their relative softness, the shales act as a slippery plane over which the brittle Mississippian carbonates have been shoved.
(1) Draw a cross-section showing the emplacement of the French Thrust through strata which were originally laid down horizontally.
(2) CHALLENGE: Using words or drawings, illustrate the meaning of imbricate thrust faults as found in Sun River Canyon.
-Bentley, Callan (Jan. 3, 2013). Inside the French Thrust. AGU Blogosphere. Accessed on 4/29/2020: https://blogs.agu.org/mountainbeltway/2013/01/03/inside-the-french-thrust/
-Bentley, Callan (March 6, 2011). French Thrust. AGU Blogosphere. Accessed on 4/29/2020: https://blogs.agu.org/mountainbeltway/2011/03/06/french-thrust/
-Devaney, Sarah Anne. “Structural Geology of the Rock Mountain Front.” The Montana Heritage Project. Accessed on 4/19/20: https://serc.carleton.edu/research_education/mt_geoheritage/sites/augusta_choteau/structural_geol.html
-Mudge, Melville. 1972. Structural Geology of the Sun River Canyon and Adjacent Areas, Northwestern Montana. Geological Survey Professional Paper 663-B, 52 pp.
Figure 1 - The Mississippian (M) strata are thrust from the west over the Cretaceous (K) shales whose relative softness allow the brittle carbonates to be thrust over the younger rocks. (Photo 1983)
Figure 2 - A closer view of Figure 1. Though not seen in this photo, pieces of the Mississippian rock have been sheared off in the thrusting process and are incorporated within the Cretaceous shales. (Photo 1983)