Large Meteorite Impacts and Planetary Evolution
Allochthonous impact-shocked rocks and superimposed deformations at the Beaverhead site in southwest Montana
Published:January 01, 1992
R. B. Hargraves, K. S. Kellogg, P. S. Fiske, S. B. Hougen, 1992. "Allochthonous impact-shocked rocks and superimposed deformations at the Beaverhead site in southwest Montana", Large Meteorite Impacts and Planetary Evolution, B. O. Dressier, R.A.F. Grieve, V. L. Sharpton
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Upward-pointing shatter cones in sandstones of uncertain age (Middle Proterozoic? to Lower Cambrian?) and older crystalline basement rocks are exposed over an area of approximately 25 × 8 km in southwestern Montana. These shatter cones, together with pseudotachylites and breccias of various types (particularly in basement gneisses), are inferred to be products of a meteorite or cometary impact. However, Late Cretaceous contraction and Tertiary extension have contributed to the structural complexity of the area, and distinguishing unequivocally the shock brecciation from that due to younger tectonism is difficult. Stratigraphic constraints suggest the structure is Late Proterozoic or Cambrian in age....