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The sub-Middle Jurassic unconformity exhumed at Swift Reservoir, in the Rocky Mountain thrust belt of Montana, exposes structures that call for a re-evaluation of the deformation history at this locale. The unconformity separates Late Mississippian Madison Group carbonate (∼ 340 Ma) from the transgressive basal sandstone of the Middle Jurassic (Bajocian-Bathonian) Sawtooth Formation (∼ 170 Ma). Fieldwork established that northwest-trending, karst-widened fractures (grikes) are filled with cherty, phosphatic sandstone and conglomerate of the Sawtooth Formation and penetrate the Madison Group for 4 meters below the unconformity. Clam borings, filled with Sawtooth sandstone, pierce the unconformity surface, some of the fracture walls, and also perforate rounded clasts of Mississippian limestone that lie on the unconformity surface within basal Sawtooth conglomerate. Following deposition of the overlying …