Beaverhead - Hypervelocity Impact Crater

Alternate Names
Local Language
Coordinates 44° 39' 3" N; 114° 6' 59" W
Notes
  1. Beaverhead County, SW Montana.
Country United States of America
Region Montana
Date Confirmed 1990
Notes
  1. Originally confirmed based on presence of well-developed shatter cones up to 40 cm in length, occurring in abundance over an area of 25 km by 8 km, in massive feldspathic arenite/sandstone and banded gneisses (Hargraves et al., 1990).
Buried? No
Drilled? No
Target Type Mixed
Notes
  1. Archean gneisses, Proterozoic-Cambrian quartzites, Palaeozoic carbonate and clastic sedimentary rocks.
Sub-Type Carbonate, Clasticsediments, Gneiss, Quarztite
Apparent Crater Diameter (km) 60 km
Age (Ma) 470 - 900
Notes :
  1. 470 to 900 Ma was estimated based on 40Ar/39Ar of impact melt rocks (Hargraves et al., 1994). Absence of shatter cones in strata of younger age suggest an age of approximately 600 Ma (Hargraves et al., 1990).

Method :
  1. 40Ar/39Ar
Impactor Type Unknown

Advanced Data Fields

Notes

Erosion
7
  1. Only a small part of the impact structure is preserved in the complexly faulted terrain.
Final Rim Diameter
Unknown
Apparent Rim Diameter
60 km
  1. Diameter of 15 km is the extent of the shatter cones (Hargraves et al., 1990). May be as large as 60 km based on distribution of shatter cones to reconstructed dimensions at other craters.
Rim Reliability Index
4
  1. No crater morphology visible.
Crater Morphology
Complex
Central Uplift Diameter
km
Central Uplift Height
Unknown
Uplift Reliability Index
Structural Uplift
Unknown
Thickness of Seds
Target Age
Precambrian Palaeozoic
Marine
No
Impactor Type
Other Shock Metamorphism
Shatter Cones
Yes
  1. Well-developed shatter cones (\"pointing steeply upward\") are present in abundance over an area ~25 km x 8 km, in massive feldspathic arenites/sandstone and also in banded gneisses (but poorly developed) (Hargraves et al., 1990) (Hargraves et al., 1994). Shatter cones up to ~40 cm in length are visible on a photograph presented in (Hargraves et al., 1990). Apical angle of ~90°; the diameter of the structure is estimated on the basis of shatter cone distribution. Complete individual cones are rarely found, nested cone segments being the typical form of occurence (Hargraves and White, 1996). \"Distinctive deformation features (pseudotachylite, PFs, and PDFs [only 1 set]) were noted only within ~2 mm of the shatter cone surface\" (Hargraves and White, 1996). [\"Conchoidal fracture with a grooved surface\" were described by Lucchitta (1966, p18), but not recognized as shatter cones; (Lucchitta, 1966).
Planar Fractures
No
Planar Deformation Features
Yes
  1. PDF in quartz, rare grains in breccia dykes (Koeberl and Fiske, 1991) (Fiske et al., 1992) (Hargraves, 1992).
Diaplectic Glass
No
Coesite
No
Stisovite
No
Crater Fill
  1. Pseudotachylite dykes and pods (ultracataclasite) in and around shatter coned areas (Shand et al., 1916) (Fiske et al., 1992) (Hargraves et al., 1994) (Reimold, 1995).
Proximal Ejecta
Distal Ejecta
Dykes
P, LB
Volume of Melt
Depth of Melting

References

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