Bearpaw Mountains, Montana
Route
Start point
Havre, MT
End point
Havre, MT
Roads
U.S. 2, County Route 23, U.S. 87, tribal roads on Rocky Boy Reservation
Total distance
73.3 miles
Geology
Summary
This trip examines volcanic and shallow intrusive rocks in the Bearpaw Mountains which were emplaced during the period from about 54 to 50 Ma, in the late early Eocene and early middle Eocene. Within this area, wwo volcanic fields lie north and south of a central anticlinal arch in which uppermost Paleozoic and Upper Cretaceous sedimentary rocks and many intrusions are now exposed. The trip follows a route across the northern volcanic field into the core of the central arch to examine the variety of rock types in the Rocky Boy stock and the youngest volcanic rocks on Centennial Mountain (Chippewa Cree Tribe). Land traversed is on Hill County park land and private land; permission is required for access and collection of samples.
Key Lithologic Features
- mafic phonolite flows
- Beaver Creek stock and monzonite dike with xenoliths
- dike of pseudoleucite-sodalite tinguaite
- Rocky Boy stock: shonkinite, biotite pyroxenite, nepheline syenite, nepheline syenite pegmatites, monzonite, pseudoleucite tinguaite
- sericitized potassic syenite plug
- carbonatite dikes
- mafic analcime phonolite flows and dikes
- Black Butte
Structures
- graben and tilted volcanic units at Wild Horse Ridge
Landforms
- Bearpaw Mountains
- Centennial Mountain
- Boxelder Butte
- Black Butte
Other Features
- rare mineral collecting localities
Reference
Hearn, B.C., Jr., 1989, Bearpaw Mountains, Montana,
in Hearn, B.C., Jr., Dudas, F.O., Eggler, D.H., Hyndman, D.W., O'Brien, H.E., McCallum, I.S., Irving, A.J., and Berg, R.B.,
Montana High-Potassium Igneous Province : 28th International Geological Congress, Field Trip Guidebook T346: Washington, D.C., American Geophysical Union, p. 51-61.
Availability
Order from the American Geophysical Union Bookstore. Go to the
AGU On Line Book Catalog for ordering information.
