by: Charles O’Dale

a Dating Method: pre-1977 K-Ar, Ar-Ar and Rb-Sr ages recalculated using the decay constants of Steiger and Jager (1977).

The Beaverhead impact structure, extending across Idaho and south west Montana, is circled on this topographic map.

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.
The red dot represents the approximate area of the Beaverhead impact ~600 million years ago in the Cambrian Period.
The Beaverhead impact structure shatter cone, courtesy of Mike Plautz, Science teacher at Hellgate Elementary in Missoula.

The following three images are courtesy of Daniel P. Connelly.

The Beaverhead impact structure in SW Montana and Idaho is an allochthonous*  fragmentof a ~100 km diameter impact structure that was transported some distance eastward during the Cretaceous Sevier orogeny.

* allochthonous – found in a place other than where they and their constituents were formed.

The present evidence for impact consists of shatter cones (Hargraves et al1990), pseudotachylites, and planar deformation features in quartz. The age of the impact is not well constrained but is estimated to be Neoproterozoic to Cambrian (1000-500 Ma). A large circular gravity, magnetic and topographic anomaly, which could be the root of the impact structure, has been identified near Challis, Idaho. An enigmatic lithic tuff, identified in drill cores from the Challis area and an intraformational quartzite breccia in the Leaton Gulch area may be impact-related deposits, but no definitive evidence of shock metamorphism has been observed in these materials.
The remains of the Beaverhead Impact structure is delineated by the occurrence of shatter cones, found in an area >200 km2, occurring within the Cabin thrust plate sandstone outcrops, part of the Cretaceous Sevier fold and thrust system. (Fiske et al 1992)

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)

This is a (proposed) cross-section of what the geology of the Beaverhead Impact Structure area might have been like soon after the impact.
A cross-section of Beaverhead Impact Structure today.

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)

Bouguer gravity map of region surrounding the proposed thrust detached root to the Beaverhead structure, East Idaho.
Ground zero for the impact is depicted in the center of this map of the impact structure. Enigmatic, fluidal-textured breccia dikes have been found in three localities in this area containing shatter cones. These rocks are characterized by rounded, highly deformed clasts of wall rock, suspended in cryptocrystalline, flow-banded matrix. (Fiske et al 1992)

Aerial Exploration

Beaverhead impact structure ground zero.

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.

Ground Exploration

Yours truly standing on the Beaverhead Impact Structure point of impact – 2019.

Western USA Missoula Floods

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).



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

University of New Brunswick