GRIFFIN CREEK AND BAILEY MOUNTAIN 7 1/2 MINUTE QUADRANGLES, WESTERN MONTANA

The Griffin Creek and Bailey Mountain 7 1/2 Minute quadrangles are located along the Lewis and Clark shear zone in west-central Montana. They are contiguous with the Luke Mountain and Garrison quadrangles that were previously mapped as part of the EDMAP program. The four quadrangles together reveal key elements of the structure of the Lewis and Clark zone, a major transcurrent structure of the Montana Rockies. Bedrock units range from the Mesoproterozoic Belt Supergroup to Late Cretaceous marine deposits and volcanics, exposed in large, SE- plunging en echelon folds. The folded units are overlain with angular unconformity by Eocene volcanics and Oligocene-Miocene sediments. Of special interest is a major angular unconformity between the Campanian Carten Creek and Golden Spike formations. The unconformity cuts through hundreds of meters of section across fold hinges, but the same folds were reactivated after deposition of the Golden Spike and Elkhorn Mountains volcanics, so that the unconformity itself is tightly folded. Laccoliths of alkalic andesites and basalts intruded the strata after the early folding but before the later folding. An associated dike swarm cuts the angular unconformity and is deformed. The cross-cutting relationships that are revealed by the quadrangle mapping have important implications for the structural evolution of the Lewis and Clark shear zone.