Abstract

Evidence presented by Armstrong and Oriel (ibid., v. 49, p. 1847-1866, 1965) for dating the Absaroka and Paris thrusts is not convincing, and perhaps they cannot be dated exactly. The age of the Prospect thrust is not doubted. The Evanston Formation, used in dating the Absaroka thrust as latest Cretaceous, has not been found directly overlying the fault; therefore, the Absaroka might be as young as early Eocene. Conglomerates used in dating the Paris thrust as latest Jurassic and earliest Cretaceous could have been derived from other source areas now beneath the Meade or Paris thrust sheet instead of from the Paris sheet. The sequence of thrusting from west to east cannot be positively demonstrated. Interestingly, the most accurate dating of thrusting in both the Alberta and Idaho-Wyoming thrust belts is approximately the same--Eocene.

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