The Beaverhead Impact Structure: Discovery and Investigation of an Allocthonous Impact Structure in SW Montana

In 1989, Rob Hargraves identified shatter-coned sandstone cobbles in a glacial till in the Tendoy Mts., SW Montana. Subsequent investigations by Hargraves and co-workers discovered shatter cones in Precambrian sandstones and gneisses over a 25x8 km region and pseudotachylites over a more restricted region - thus defining the remains of the 50-100 km diameter, 600M to 1B year old, Beaverhead Impact Structure. Though one of 170+ impact structures identified on Earth, Beaverhead remains distinctive and important for several reasons. First, it is the only example to date of an allocthonous fragment of an impact structure, tectonically dissected and transported eastward by tens of kilometers. Second, it preserves pseudotachylies in several lithologies with varying morphologies and features (such as large vesicles) that suggest they were exhumed very quickly after formation. Finally, the orientation of shatter cones relative to sandstone bedding and cross-bedding suggests that a previously unidentified episode of tectonic deformation predated the impact (the first documented use of shatter cones as an indicator of paleo-orientation of strata). To date, nearly all the impact craters recognized on Earth retain some vestige of their original circular geometry. The discovery of Beaverhead suggests that many more meteorite impact structures may be partially preserved in orogenic belts, and that careful observation of petrologic and outcrop-scale features is the key to their discovery. Hargraves' discovery of Beaverhead (the largest impact structure yet identified in the United States) in a region already extensively mapped and studied by geologists, is only one of many examples of the insight, careful observation, and undogmatic thinking that characterized his outstanding scientific career. By bringing together regional structural geologists, planetary scientists, geophysicists and sedimentary geologists, Hargraves and his co-workers illuminated an unusual and important event in Earth history.