Evidence for Quaternary tectonism in the northern Bighorn Basin, Wyoming and Montana

Abstract

Irregularities in the reconstructed gradients of latest Pliocene and Quaternary terraces in the northern Bighorn Basin, northwest Wyoming and south-central Montana, suggest that Quaternary movements have occurred on the Frannie anticline, the Nye-Bowler–Sage Creek fault zone, and the North Pryor fault. This area has been considered tectonically inactive. The timing of the interpreted movements is derived from age estimates of fluvial deposits determined from numerical ages and altitudes of associated deposits of volcanic ash. South of the Pryor Mountains, the 1.4 Ma Polecat Bench terrace of the Shoshone River rises 6–7 m over the axis of the Frannie anticline; this displacement may be related to folding or faulting since 1.4 Ma. Near the Nye-Bowler–Sage Creek and North Pryor faults in the Pryor Mountains, bedrock is 25 and 55 m shallower than depths predicted from the projected 1.4 Ma gradient of the Shoshone River through Pryor Gap. The 2.0 Ma terrace of Rock Creek may have been offset up to 7 m by a splay of the Nye-Bowler fault zone between 2.0 and 0.6 Ma. The downstream divergence of terraces along Rock Creek suggests ongoing uplift of the northern Bighorn Basin, perhaps caused by crustal rebound due to post-Oligocene erosion.