Structural Geology of the Rocky Mountain Front

Author: Sarah Anne Devaney, Department of Earth Sciences, Montana State University


The area of the Rocky Mountain Front is famous for its rugged mountain ranges. The Sawtooth Range rises dramatically from the plains west of Choteau and Bynum, and trending NW-SE across Pondera, Teton and Lewis and Clark Counties. The Sawtooth Range formed at the frontal part of the Cordilleran thrust system on the massive Lewis thrust system (see: DeCelles, P.G., 2004, Late Jurassic to Eocene evolution of the Cordilleran thrust belt and foreland basin system, western USA: American Journal of Science, v. 304, p. 105–168, doi:10.2475/ajs.304.2.105). To the north of the Sawtooth Range, the Lewis Thrust is very visible at the surface of the Earth in Glacier National Park . This thrust then crosses the Continental Divide close to Marias Pass and then again south of the Sun River forming the south border of the Sawtooth Range. An early study of the Structural Geology of the Sun River Canyon and Adjacent Areas, Northwestern Montana was published by Mudge (1972), U.S. Geological Survey Professional Paper 663-B (Acrobat (PDF) 23.5MB May16 18), and Mudge and Earhart (1980) The Lewis Thrust Fault and Related Structures in the Disturbed Belt, Northwestern Montana (Acrobat (PDF) 8.4MB May16 18), U.S. Geological Survey Professional Paper 1174.. A recent detailed study of the structural geology of this area was published by: Facundo Fuentes, Peter G. DeCelles, Kurt N. Constenius (2012) Regional structure and kinematic history of the Cordilleran fold-thrust belt in northwestern Montana, USA (Acrobat (PDF) 2.9MB May15 18). Geosphere ; 8 (5): 1104–1128. Master's thesis completed on this area by graduate students in the Department of Earth Sciences, Montana State University, supervised by Dr. David Lageson include: D. Scott Singdahlsen (1986) Structural Geology of the Swift Reservoir Culmination, Sawtooth Range, Montana, Bethany Ihle (1988) Internal Deformation in the Backbone Thrust Sheet, Sawtooth Range, Montana and Paul Thom (1996) Characterization of Spaced Cleavage, Sawtooth Range, Montana. A really nice overview of the Thin-Skinned Thrusts Along the Rocky Mountain Front was developed by Rob Benson, Helena High School, on his Montana Earth Science Picture of the Week website.

Tectonic Setting

Plate tectonics is the great unifying theory of geology that explains the distribution of the great mountain belts of the world and the processes that formed them. The "ring of fire" that surrounds the Pacific Basin is the result of subduction of oceanic (Pacific) plate underneath the adjacent continental (North American) plate. Subduction produces the active volcanic chains of the Andes, Cascades and Aleutians, and the belts of deep-seated seismic activity (e.g., the Cascadia Seismic Zone megathrust system. One of the consequences of convergent tectonics is whole-scale crustal shortening that can produce fault systems hundreds of miles inboard of a subduction zone near the coast. Subduction of the now extinct Farallon Plate resulted in the onset of the thin-skinned Sevier Orogeny at about 125 million years before present, and the Laramide Orogeny (involving deep-seated faults mobilizing crystalline basement rocks) about 85 million years before present. Continued subduction of the Juan de Fuca, Cocos, and Kula (also completely subducted ~40 million years ago) Plates continue to impact the seismicity, magma production, and deformation of the North American continent through to the present. The western North American continent is characterized by the Cordilleran fold-and-thrust belt which runs from Alaska all the way south past Tierra del Fuego to the Scotia Arc and the mountains of the Antarctic Peninsula.

The American Cordillera forms the backbone for mountain ranges (orogenic belts) like the Rocky Mountains, the Sierra Nevada and the Andes, and with that the volcanic arc along the eastern part of the Pacific Ring of Fire. In Montana, the Laramide Orogeny (mountain building process) occurred in several episodes throughout the Late Cretaceous (~75 million years ago until ~45 million years ago) and played a major role in the creation of the landscape we see today. Often, the Laramide Orogeny used pre-existing faults of the regions for its uplift of Archean basement rocks on high-angle reverse faults. Laramide-style mountain ranges are seen primarily in SW Montana in the Beartooth, Madison, Tobacco Root, and Ruby Ranges. In addiiton, the "thin-skinned" Sevier Orogeny overlapped in space and time with Laramide-style structures. The Sevier Orogeny was a result of the subduction of the Farallon Plate underneath the North American Plate which began approximately 125 million years ago (creating the Canadian Rocky Mountains and the Sevier Mountains in western Utah and eastern Nevada) and probably ended about 50 million years ago. The Rocky Mountain Front in the Augusta-Choteau area is a classic example of thin-skinned, fold-and-thrust belt, Sevier-style deformation. The following figure places the Sun River Canyon area in a regional geologic context (from Fuentes et al., 2012).

Thrust Faults of the Rocky Mountain Front

The Rocky Mountain Front is characterized by the fold-and-thrust belt system (see Geologic Map of Choteau Quadrangle; thrust faults are mapped with the 'sawteeth' on the upper block (hanging wall) of the thrust fault. Generally, the thrust faults run northwest-southeast and dip (point) to the southwest (propagating the formations towards the east-northeast).

Thrust faults often have a deep-seated sole thrust (a decollement), and as the upper plate is transported, a series of subsidiary thrust faults "ramp up" through the stratigraphic section. This creates an imbricate thrust zone. The thrust faults propogate through the system repeating the stratigraphic units as shortening continues. The youngest movement will occur along the front edge of the thrust system. This figure illustrates the sequential stages of thrusting in a model system.

The following images show general geologic maps and a cross section of the Disturbed Belt in the Northern Rocky Mountains of Montana, from Mudge and Earhart (1980). Key tectonic features include the Lewis Thrust, Eldorado Thrust, and Continental Divide Syncline.

The following map and cross section are the more recent interpretations of the Sun River Valley area by Facundo Fuentes, Peter G. DeCelles, Kurt N. Constenius (2012) Regional structure and kinematic history of the Cordilleran fold-thrust belt in northwestern Montana, USA (Acrobat (PDF) 2.9MB May15 18). Geosphere ; 8 (5): 1104–1128. Key findings in this report are:

  • At the surface, the foothills show deformed Mesozoic and Paleocene rocks; at depth, reflection seismic data indicate numerous thrust faults carrying Paleozoic strata.
  • The Sawtooth Range, south from the Lewis thrust salient, is defined by steeply dipping imbricate thrusts that detach at the basal Cambrian stratigraphic level.
  • A new ∼145-km-long balanced cross section indicates ∼135 km of shortening,
  • Most shortening in the Lewis thrust system, Sawtooth Range, and foothills occurred roughly between mid-Campanian and Early Eocene time (ca. 75–52 Ma), yielding a shortening rate of ∼5.9 mm/yr.

Here is a helpful visualization of the fault system 3D Block Diagram of the geology of the Castle Reef Quadrangle, Montana

Exploring the Faults and the Stratigraphy of the Rocky Mountain Front

You can see many of the thrust faults by taking one of the recommended roads from Dupuyer, Choteau or Augusta until they end at a natural barrier, the largest being the Bob Marshall Wilderness which itself offers many backpacking possibilities (even with pack mules).

For more information on tour possibilities and the Bob Marshall Wilderness check out these links:

  • State of Montana Tourism Site
  • Here is a link to a road log taking you from St. Mary, MT over the Going-to-the-Sun Road in Glacier National Park for more exposure of the Belt Supergroup.

The different faults of the region can be explored at various spots. Here, are three suggestions along with some literature for more information. Be aware that some of them are gravel roads and that weather conditions can change quickly! For more information on the rock formations of this area and their ages see Stratigraphy where you can find a list with formations and units to each of the following road logs.

South - Sun Canyon Road and trail along Gibson Reservoir

There is a possibility of driving a loop back to Augusta on the Barr Creek Road passing Black Butte. Also, when hiking the trail along Gibson Reservoir which leads into the famous Bob Marshall Wilderness please be mindful when encountering pack mule trains. They can sometimes be easily startled which puts their cargo and fellow mules in danger! Please move uphill, sit down and be still. If there is no possibility to move uphill, turn around and find a big open space where you can let the mules pass. The paved road dead-ends at the reservoir passing a campground. The trail along the reservoir leads past the Palmer and Allen Thrust (both described below) and leads you to the Scattering Springs.

From Augusta to Gibson Reservoir passing Teton Pass Ski Resort, along Teton River, ending at West Fork Teton Recreation Site:

The descriptions below are an adaptation of David Lageson's and his GSA publication from 1987, along with personal field camp experience, notes and images.
Click to enlarge route map

The distances are given from the intersection of Manix St (turns into Sun Canyon Rd) and Main St/U.S. 287, so reset your odometer for this trip.

About 17 mi (27 km) into the trip (now on Sun Canyon Rd, south of the road, at about 9:00) the first possible stop is a well-exposed thrust-faulted anticline which is overturned. This asymmetric anticline is in the Cretaceous Blackleaf Formation (gray marine mudstone) and its fore- and back-limbs have both been thrust-faulted making this an excellent example of the style of deformation the Cretaceous rocks in the foothills of the thrust belt have experienced.

The Diversion Thrust crosses the road 2.6 mi (4.2 km) later forming the entrance to Sun River Canyon and the east boundary of the Sawtooth Range. This thrust places Mississippian Allen Mountain limestone (medium- to dark-gray) and Mississippian Castle Reef dolomite (light-gray with fine- to medium crystals) over the Cretaceous Blackleaf Formation (gray marine mudstone). Further up the road just to the right is Diversion Lake Dam. Almost halfway up the lake is the Home/Sawtooth Thrust system but near the road it is covered by Quaternary alluvium of the Sun River and its flood plains. This thrust places Devonian sedimentary rocks on Cretaceous Blackleaf Formation. 2.2 mi (3.5 km) after the previous stop, the roadcut on the south side of the road (with a ery distinct pull out) exposes the French Thrust, which has placed Castle Reef dolomite over Cretaceous Blackleaf Formation (here, black shale, look for slickensides). Here, you can observe the features that are associated with a thrust fault contact. Usually these are covered up or hard to find. The shales got sheared to where slickensides were produces, yet the Mississippian carbonates are relatively undeformed. Just 0.3 mi (0.5 km) from here to the west the trace of the Norwegian Thrust Fault becomes visible, placing Mississippian Allen Mountain limestone (medium- to dark-gray) on top of Cretaceous Kootenai Formation (gray-green and maroon mudstone). This is one of three closely placed thrust faults cropping out in this area. If you follow the short trail along the river you can see some petroglyphs in the hanging wall. After another 0.3 mi (0.5 km) turn right and cross the bridge over the Sun River continuing on the main road. Gibson Dam is to your left. The Beaver Thrust can be seen at the next stop, 23.7 mi (38 km) into the trip (about 1.6 mi or 2.6 km after the last stop), on the steep hillside to the right (3:00). It has thrusted Mississippian Allen Mountain Limestone (medium- to dark-gray) over the red beds of the Cretaceous Kootenai Formations. After a bit of a climb stop at the Gibson Dam overlook (0.4 mi or 0.6 km from the previous stop), which is on the crest of the ridge formed by the resistant Mississippian Castle Reef Dolomite (light-gray with fine- to medium crystals). To the east you can see the lower Sun River Canyon as well as the major imbricate thrust faults that include the eastern Front Ranges of the Disturbed Belt. The view to the south shows you how the Beaver and Norwegian thrust sheets form enormous dip-slopes on the Mississippian Castle Reef dolomite. At the end of the road, 1.9 mi (3 km) later, is the boat access to Gibson Reservoir as well as the trailhead for the U.S. Forest Service Trail 201 which leads into the Bob Marshall Wilderness area. If you take a short hike along the reservoir you will pass the Palmer Thrust. To go further into the wilderness area, you need to be well prepared. It is advisable to contact the local forest service office in Augusta.

Middle - Little Teton and Teton Anticline on Teton Canyon Road

Somewhere along that road you pass the dirt road to Egg Mountain surrounded by the Pine Butte Swamp Preserve which lets grizzlies roam freely (until they wander too far into town). For more on Egg Mountain check out our page on paleontology.

From Choteau passing Eureka Reservoir and Teton Pass Ski Resort along the Teton River, ending at West Fork Teton Recreation Site:

Following highway 89 north turn left onto Teton Canyon road after about 5 miles north of Choteau. Along Teton Canyon road you can see many structural features and sedimentary rocks, starting out with Quaternary alluvium of the youngest terrace, which lies about 10 ft. above the modern flood plain. The Eureka Reservoir lies within the Cretaceous Telegraph Creek Formation (inter-bedded sandstone and mudstone, where weathered the sandstone appears olive gray) and the Cretaceous Virgelle Formation (dark yellowish brown to yellowish gray on weathered surfaces). The prominent rim rocks south, west and northwest of Choteau are formed by the Virgelle sandstone. The top of the formation is erosion resistant due to its titanium containing magnetite beds which protect the underlying sandstone which is easily eroded. After the reservoir, the road passes through Quaternary glacial outwash deposits sourced from the Quaternary older gravel which are higher in elevation (about 250 m above modern stream alluvium). Twin Lakes lie in Quaternary glacial till, which was deposited by mountain glaciers. For more on glacial deposits and other features visit our page on Landscapes of the area. The Quaternary deposits cover up two faults. This road turns into gravel once it enters the Lewis and Clark National Forest. Remember that conditions can change quickly! Be prepared and be safe! The road follows the North Fork Teton River and ends at the Mountain Wright trail and the West Fork Teton Recreation Site.

North - Swift Reservoir/Swift Dam Road and its Swift Reservoir Culmination (SRC)

From Dupyuer (east to west), past Swift Reservoir, along the North Fork Birch Creek:

Mitten Lake Thrust can't always be seen at the surface in the northern SRC. But where it is visible it places Devonian dolomite (greenish-gray to light-grayish-brown, younger ones are breccia), Mississippian limestone (dark to medium gray) and Dolomite (light to medium-gray) and Jurassic dolomitic and calcareous mudstone over Cretaceous Marias River shale (dark-gray marine mudstone).

Fish Lake Thrust can be seen in the northern and southern part of the SRC, where it places mudstone of the Cretaceous Kootenai Formation (gray-green and maroon mudstone) over Cretaceous Marias River shale (dark-gray marine mudstone). It is named after the nearby Fish Lake by the former graduate student from Montana State University (MSU), D. S. Singdahlsen (1986), who worked on his master's thesis in this area. This thrust fault continues 9.8 km N-NW from Birch Creek merging into the Major Steele Backbone Thrust (see below). It is a significant thrust because it is the easternmost major thrust of the SRC which uses the structurally incompetent shale unit for the displacement that creates a stratigraphic separation of almost 3,600 ft. (1200 m).

Major Steele Backbone Thrust gets its name from the prominent ridge north of the Swift Reservoir and south of Feather Woman mountain and can be distinguished into two halves, north and south. In the north, it represents the boundary of the SRC placing Cambrian quartz sandstone (light-gray), shale (gray), limestones (medium- to dark-gray, some with grayish-green shale), dolomitic limestone (gray-brown) and massive dolomite (very light gray) over Cretaceous mudstones (maroon and gray-green, greenish-gray, gray), shales (dark-gray),calcareous sandstones (dark-gray and light-gray) and sandstone (light-gray). In the south it often places Cambrian over Mississippian limestone (dark to medium gray) and dolomite (light to medium-gray), and locally Cambrian Steamboat limestone (gray-brown) over Cretaceous Kootenai (maroon and gray-green mudstone) and Blackleaf Formations (dark-gray, gray shale and gray, greenish-gray mudstone).

Lookout Ridge Thrust is named for the prominent ridge of Cambrian strata and is the easternmost thrust that transportsCambrian rocks to the NE. But further to the NW the fault becomes more distinguishable, placing Steamboat limestone (gray-brown) onto Switchback shale (greenish-gray, locally maroonish-gray), then Steamboat onto Devils Glen Dolomite, and eventually Switchback onto Devils Glen dolomite (light gray).

Haywood Creek Thrust gets its name from the creek that starts to the south-southeast of the Major Steele Backbone at a confluence draining into Birch Creek where the north fork of Birch Creek empties into the Swift Reservoir. From here the thrust runs north placing Cambrian rocks on Cambrian (mainly Steamboat limestone on Steamboat limestone, gray-brown) which produces a stratigraphic separation of 219 ft. and more, but it also splits up into to faults.

Saddle Ridge Thrust doesn't get its name from any specific nearby mountain, it's just a random name but this thrust is one of the more continuous ones in the SRC. Starting at Birch Creek it runs NW and repeats the Cambrian Steamboat limestone (gray-brown). It creates a stratigraphic separation of about 225 ft. (75 m) and in the westernmost imbricate zone it is the only fault.

Hungry Man Creek Thrust is named after the creek that runs NW-SE following the thrust. This one starts at Birch Creek as well and runs to the west placing Cambrian on Cambrian creating a stratigraphic separation of about 450 ft. (150 m). N-NW of Birch Creek is a slight lateral displacement (or step-over) producing a small overlap of Steamboat limestone (gray-brown) on the Devonian Maywood Formation (greenish-gray, dark-gray to gray-brown) which results in a stratigraphic separation of more than 1,095 ft. (365 m). This occurs where the three streams flow together making up the headwaters of Hungry Man Creek. Further to the NW, this thrust places Cambrian on Cambrian rocks. There might be a localized discontinuity or a structural tear which this pattern of anastomosing and overlapping thrust faults may indicate.

Jefferson Ridge Thrust is nearby a prominent ridge which has an exposure of Devonian Jefferson dolomite (light-gray-brown) and is another fault which begins at the Birch Creek. Initially, it places Cambrian Steamboat limestone (gray-brown) on Devonian Jefferson Formation (grayish-brown to light-gray-brown) but further to the NW the thrust splits into an eastern and western branch. The eastern splay places Devonian on Devonian (greenish-gray, dark-gray to gray-brown, grayish-brown, light-gray-brown, some solution breccia) and the western splay places Steamboat limestone on Devonian Jefferson (light-gray-brown dolomite). The western splay, though, runs into the eastern limb of a major anticline and dies-out.

Killem Horse Creek Thrust gets its name from the creek nearby which begins at Mt. Poia draining into Birch Creek. From here going NW the fault first places Cambrian on Cambrian and then starts cutting up-section through Steamboat limestone to the Mississippian Castle Reef dolomite (medium- to light-gray) before going back down into Devonian Jefferson dolomite (light-gray-brown). Here, an unusual (in the SRC) overprinting of major structures can be found: within the process of cutting up-section, the fault starts out truncating the western limb of a major syncline (with Devonian and Mississippian rocks), then moves across the center of this syncline (Mississippian) and then passes its eastern limb. To the south runs a splay fault (possibly a small backthrust) which created an isolated outcrop of Cambrian Steamboat limestone, a so-called "pop-up" structure. This thrust is also one of the major ones in this area as it is the most posterior thrust bearing Cambrian units in the SRC.

Heart Butte Thrust is special since it runs NE-SW (perpendicular to all the others), dipping to the NW, and it connects the Mitten Lake thrust and Major Steele Backbone thrust. It moves Mississippian (dark-, medium- to light gray) and Devonian (greenish-gray, dark-gray to gray-brown, grayish-brown, light-gray-brown, some solution breccia) units to be in contact with the Cretaceous Marias River shale (dark-gray, gray). Overall the transport direction in the SRC is E-NE but this thrust implies that the fault has an oblique-slip to the right. As the Heart Butte Thrust connects with the two others it displaces the movement from the Major Steele Backbone Thrust to the Mitten Lake Thrust.