BEAVERHEAD IMPACT STRUCTURE
by: Charles O’Dale
- Type: Peak ring
- Age(ma): ~600a – PROTEROZOIC
- Diameter: ~100km
- Location: SW Montana and Idaho, USA. N 44° 15’ W 114° 12’
- Shock Metamorphism: Shatter cones
a Dating Method: pre-1977 K-Ar, Ar-Ar and Rb-Sr ages recalculated using the decay constants of Steiger and Jager (1977).
The following three images are courtesy of Daniel P. Connelly.
* allochthonous – found in a place other than where they and their constituents were formed.
Petrographic and transmission electron microscopy (TEM) study reveals that planar fractures, pseudotachylite (PST) veinlets, and planar deformation features (PDF) in quartz and K-feldspar grains are developed only within ~2 mm of the surface of shatter cones in Proterozoic arkosic sandstones from the Beaverhead (Montana) impact structure. This unique association reinforces the impact-shock origin attibuted to all these shock-metamorphic features. No high-pressure SiO2 polymorphs were found in the PST, but possibly this is because these cones formed far from the center of the impact structure, in a shock-pressure zone below the stability field of the high-pressure polymorphs. (Hargraves and White, 1996)
Compare the location of the points labeled Grouse Peak and Beaverhead Site on the two diagrams. Geologists believe that, since the impact happened, the top part of the crust moved several miles to the east causing these two points to be offset from their original position on opposite sides of the structure. This type of “thrust faulting” was common in the area during the formation of the Rocky Mountains. The yellow area indicates location of bedrock that provided the unusual magnetic and gravity readings. (McCafferty 1995)
Flying over the Beaverhead structure we note that there is no visible crater form. The structure, which extends across at least 100 km (but may be as wide as 150 km) has no circular outline. The impact predates the Rocky Mountain orogeny, tectonic activity has destroyed its morphology.
Millions of years after the Beaverhead impact event, during the 25,000 to 10,000 years ago glaciation, most of Canada and parts of the northern United States were covered by massive ice sheets. These glaciers dammed the Clark Fork River creating Glacial Lake Missoula just north of where I was standing on the Beaverhead impact structure. The ice dam eventually collapsed several times causing the Glacial Lake Missoula to empty catastrophically at 130 km/hr (80 miles per hour).
[see – METEORITE]
Brent Dalrymple, Radiometric Dating Does Work! Reports of the National Center for Science Education
Fiske, P. S.; Hougen, S. B.; Hargraves, R. B. (1992), Breccia Dikes from the Beaverhead Impact Structure, Southwest Montana., International Conference on Large Meteorite Impacts and Planetary Evolution p 26, 09/1992
Hargraves R.B., Cullicott C.E., Deffeyes K.S., Hougen S., Christiansen P.P., and Fiske P.S. (1990), Shatter cones and shocked rocks in southwestern Montana: The Beaverhead impact structure Geology, September, 1990, v. 18, p. 832-834
Hargraves R.B., and White J.C. (1996) Micro-Shock Deformation adjacent to the Surface of Shatter Cones from the Beaverhead Impact Structure, Montana, The Journal of Geology Vol. 104, No. 2 (Mar., 1996), pp. 233-238
McCafferty. A.E. , (1995), Assessing the presence of a buried meteor impact crater using geophysical data, south-central Idaho: Masters Thesis, Colorado School of Mines, 88p.
Steiger R.H. and Jager E. (1977), Subcommission of geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth Planet Sci. Ltee., 36 359