GEOLOGIC MAP OF THE WESTERN PART OF THE SALMON 30' X 60' QUADRANGLE AND REGIONAL STRATIGRAPHIC IMPLICATIONS

The new geologic map of the western part of the Salmon 30' by 60' quadrangle depicts Mesoproterozoic bedrock, Eocene though Oligocene volcanic and sedimentary strata, and Quaternary deposits from fluvial, lacustrine, and glacial processes. Together they tell a story of sedimentary deposition, plutonism, metamorphism, deformation, uplift, and erosion over the past 1.4 billion years. Geologic concepts and units for this map evolved during an 8 year collaborative mapping project by the Idaho Geological Survey and the Montana Bureau of Mines and Geology. Funding was provided in part by the Statemap component of the USGS National Cooperative Geologic Mapping Program. This map is a compilation of 14 published 1:24,000-scale geologic maps that resulted from that collaboration, updated for consistency with our current structural and stratigraphic interpretations. Bedrock is mostly Mesoproterozoic metasediments of the middle and upper parts of the Lemhi Group, the Swauger and Lawson Creek Formations, and an extensive, overlying 3-5 km thick quartzite-dominated section that we assign to the Apple Creek Formation. This metasedimentary section, at least the part above the Lemhi Group, probably correlates with the Missoula Group of the Belt Supergroup. Intruded into these metasediments are 1380 Ma megacrystic granite in the west, and Cretaceous and Eocene plutonic rocks in the north. Eocene dikes, mostly along the Beaverhead divide, and Challis volcanic and minor sedimentary rocks concentrated in the south, document normal faulting during Eocene extension. Boulders of megacrystic granite found to the southeast near Lemhi Pass likely mark a river channel that flowed from a high-relief landscape northwest of Salmon southeastward into Montana. Age of this river system is uncertain, but appears to be post-Challis and may have been active early in the Oligocene. Mostly Oligocene sedimentary and minor volcanic rocks accumulated between present mountain ranges during extension on detachment and related faults. Lack of thick Quaternary alluvial fills in the Salmon and Lemhi valleys indicates that the Salmon basin detachment along the west side of the northern Beaverhead range is inactive.