The Beartooth Highway is a remarkable road that goes over the plateau of the Beartooth uplift. Much of the road is over 10,000 feet (~3000 m) and above treeline. The road is not open all year, so check for road conditions and keep an eye on the weather. The exposed rocks include a wide array of Archean igneous and metamorphic rocks, and some Paleozoic sediments, and a spectacular array of Pleistocene features including glacial lakes and valleys, patterned ground, and rock glaciers.
Map, showing Beartooth Highway and field trip stop locations.
1. Granitization hypothesis outcrop.
This first stop on the Beartooth Highway was to see one of the important outcrops where the granitization concept for the origin of granite was originally developed. This concept held that large bodies of granite and related rocks were produced by infiltration of sedimentary rocks by remarkable granitizing fluids. These fluids brought in potassium and some other elements, and removed calcium and some other elements, leaving the original rock transformed into granite. This concept has been, of course, almost entirely replaced by the igneous model of granite petrogenesis in the early 20th century. In addition to the rocks described below, this stop also has amphibolite and a metamorphosed iron formation layer.
Northern part of the
Beartooth Highway, not far below the plateau region. This nice
overlook was the location of the granitization concept outcrop.
In the image, the rock on the right is a coarse-grained granite. The rock on the left is metamorphic and is made right here of interbedded schist and quartzite. The contact between the metamorphic rock and granite is along the nearly vertical shear zone in the middle of the image (eroded and in dark shadow; this shear zone is a minor fault). The concept here is that granitizing fluids moved in from the right, converting the metamorphic rock (once sedimentary) into granite. The fluids were blocked here by the impermeable quartzite layers. It is important to recognize that the shear zone is a strike against this being a convincing example of granitization caught in the act. The original contact relations between the granite and the metamorphic rocks are obscured by the fault.
This is a closeup of some of the schist. It is strongly deformed and metamorphosed. Tight isoclinal and possibly even sheath folds are visible here, as are deformed pegmatitic layers and masses. In the granitization concept this would represent the initial influence of granitizing fluids moving parallel to the layering, transforming the rock into granite. The modern alternative, of course, is that the rock was partially melted during metamorphism. The mineral assemblage in the schist is quartz - feldspar - biotite - sillimanite - garnet ± cordierite.
Closeup of one of the quartzite layers associated with the schist that is visible to the upper left and lower right. Another strike against this being a convincing example of granitization is that the quartzites, hypothetically blocking the granitizing fluids, are discontinuous and therefore would not form an effective barrier.
The granite on the other (right) side of the shear zone, near the shear zone. The rock is layered and folded, obviously deformed. This texture was interpreted in the granitization concept as being relict layering from the original sedimentary/metamorphic rock.
he same granite several meters farther away from the shear zone. Here the granite is essentially undeformed, and euhedral phenocrysts of K-feldspar are visible along with abundant large crystals of quartz and plagioclase. The granitization hypothesis envisioned the continuous petrogenetic sequence for this outcrop: metamorphic rock - partially granitized metamorphic rock - granitized metamorphic rock with relict layering - homogeneous metamorphic rock (granite). The modern igneous model envisions a generally discontinuous petrogenetic sequence for this outcrop: unmelted metamorphic rock - partially melted metamorphic rock - [intrusive contact] - intruded granite body with a deformed margin.
2. Beartooth Highway, plateau north side
Photos from the parking area overlook on the north side of the Beartooth Mountains plateau.
This looks
southward up a beautiful U-shaped glacial valley carved into the north side
of the Beartooth Mountains, with a little hanging valley to the right.
Talus slope with a little rock glacier at the foot of the slope, visible on the east side of the overlook.
Some of the geologists on the field trip, with the Beartooth Mountain tablelands in the distance. This is, or at least is close to, the Cambrian erosion surface onto which a considerable thickness of sedimentary rocks were deposited. These rocks have mostly been eroded away after the uplift of the Beartooth Range.
3. Beartooth Highway, plateau south side
This next set of images are from the southern side of the plateau on the Beartooth Highway. The location is where the highway crosses the Christmas Lake dike.
View to the south of a lovely broad U-shaped valley with a stream and swamps along its center.
View to the southeast. The peak in the center distance is Heart Mountain, made of Paleozoic sedimentary rocks which have been emplaced in their current location along the famous but controversial Heart Mountain detachment fault.
View of the Beartooth Mountains, looking to the west.
North edge of the Christmas Lake dike, one member of an extensive Proterozoic dike swarm exposed in the Beartooths and elsewhere. This is the approximate location of the northeastern (right) dike margin, better seen in the distance as the contact between the dark brown rock (left) and lighter rock (right).
Closeup of the interior of this tholeiitic basalt dike. Light-colored plagioclase, black pyroxenes, and metallic magnetite are visible.
4a. Lunch stop
Lunch stop near Long Lake on the Beartooth Highway on the south side of the Beartooth Plateau. It is a region of Archean granites and related rocks, cut by more dikes. The dominant rock seen here is the Late Archean Long Lake Granite, which cuts and includes more mafic rocks.
View approximately south. The layered, flat-topped peak in the distance is Beartooth Butte. It is made of Paleozoic sediments that unconformably overly Archean granites and gneisses below.
Our lunch stop with the granites and gneisses in the background. Click here to see the path of the dike that passes near this spot.
4b. Granite mapping project
Our main task at this site was to map a ~150 m square region in granitoid rocks. There were several different broad classes of rock at this site.
This first is a homogeneous light pink biotite granite with weak
foliation.
Here is a contact between the light-colored homogeneous rock (lower half) and a much less homogeneous, generally darker rock (upper half). The less homogeneous rock includes a matrix rock that is somewhat more biotite-rich than the light homogenous rock, and numerous darker inclusions (xenoliths or autoliths).
Another view of the darker rock with biotite granite matrix and abundant dioritic inclusions.
The darker granitic matrix rock with larger, more blocky dioritic inclusions. This is a complex outcrop with pegmatites and more than one generation of granite emplacement.
Another example of the inclusion-rich granite. The darker, irregular inclusion is amphibolite.
The darker granite matrix rock with abundant more mafic inclusions is in the near ground, with the more homogeneous, lighter colored granite near the top and far right. The impression I get from this mapping region is that this area is crossed by several sum-parallel, wide, dike-like bodies of granite, emplaced at different times and carrying different quantities of inclusions. Detailed examination of the outcrops shows multiple episodes of granite emplacement.
Discussion of the outcrop and some of its possible interpretations. Emplacement pressure for these rocks was said to have been ~6 kbar (~20 km), where the intrusive rocks might remain mushy for awhile.
Large black allanite crystal in granite pegmatite, exposed in blocks adjacent to the road, at the same stop as lunch and the granite mapping project. Notice the fractures radiating from the allanite crystal, caused by swelling as radiation damage destroyed the allanite crystal lattice.
5. Overlook above Clark's Fork Canyon
These last images are from a scenic overlook just off the Beartooth Highway as it descends southward into Clarks Fork Canyon.
The forested area
in the near ground is mostly Precambrian gneisses and granites. Across
the valley most of the topography is made of the Absaroka Volcanics. The pointy
mountain in the center is Pilot Peak, and the one to its right is Index Peak.
Looking almost due south into Clark's Creek Canyon down a nearly dip slope on the Cambrian unconformity where the Precambrian rock dives beneath sedimentary and volcanic cover rocks of Cambrian age and younger. The layered rocks on the opposite side of the canyon are Paleozoic sediments, above which are Eocene Absaroka Volcanics. The prominent layer halfway up the slope across the valley is the Ordovician Bighorn Dolomite. Immediately above the Bighorn Dolomite is the Heart Mountain detachment fault.
Looking southeast across Clarks Fork Canyon. The layered rocks in the distance are mostly Absaroka Volcanics, with Paleozoic sediments underneath. As above, the rocks in the near ground are Precambrian.
References and further reading
Mueller, P.A., Locke, W.W., and Wooden, J.L., 1987, A study in contrasts: Archean and quaternary geology of the Beartooth Highway, Montana and Wyoming. Geological Society of America Centennial Field Guide, Rocky Mountain Section, p. 75-78.
Pierce, W.G., 1987, Heart Mountain detachment fault and clastic dikes of fault breccia, and Heart Mountain break-away fault, Wyoming and Montana. Geological Society of America Centennial Field Guide, Rocky Mountain Section, p. 147-154.
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