Abstract

Structures studied in the Bighorn Mountains, principally along its eastern side, lead to a hypothesis of the structural history of the range. Apparantly the Laramide uplift took place during two distinct episodes, here referred to as the primary and secondary deformations. In the primary deformation uplift by compression caused the central part of the range to be arched asymmetrically with the steeper limb on the east. The northern and southern parts were asymmetrically arched to a lesser extent, with steeper limbs to the west. During the secondary episode of deformation numerous relatively small thrust structures were superimposed on the primary structure. Evidence that the two episodes were distinct is found in the stratigraphy of early Tertiary (?) gravels deposited along the eastern front of the range.

The existence of an “ancestral” Bighorn range during the pre-Laramide history of the area is indicated by the presence of basal Chugwater stream gravels containing pebbles derived from Cambrian formations.

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