GEOLOGIC MAP OF THE EAST HALF OF THE BONNERS FERRY 30 X 60 MINUTE QUADRANGLE, BOUNDARY COUNTY, IDAHO, AND LINCOLN COUNTY, MONTANA

Geologic mapping of the east half of the Bonners Ferry 30 x 60’ quadrangle by the Idaho Geological Survey combines bedrock geology with subdivision of Quaternary units to provide a comprehensive view of regional geologic history. Surficial deposits were mapped over several years by R.M. Breckenridge through 2012; bedrock geology was refined from unpublished 7.5’ maps by F.K. Miller and R.F. Burmester, and other local maps, by our fieldwork 2008-2012. This is the latest in a series of regional I.G.S. maps of northern Idaho. Lithologic unit definitions and contact placements of Mesoproterozoic Belt Supergroup rocks are the same as used on the adjoining Sandpoint 30 x 60’ map and as close as possible to others farther south.

Low metamorphic grade metasedimentary rocks of the Mesoproterozoic Belt-Purcell Supergroup underlie most of the map. Subdivision of the Prichard Formation was partly based upon regional work by Cominco geologists. Most Ravalli Group units differ little from previously studied areas to the south, although thick quartzite cosets of the Revett Formation are scarce. The middle Belt carbonate (Wallace Formation) of earlier mapping was revised in order to use recently resurrected Piegan Group terminology. Strata of the Missoula Group are similar to those to the south, but upper units are cut out by the Moyie fault. Higher metamorphic grade rocks of the Priest River complex, probably derived from the Prichard Formation, and Cretaceous intrusives dominate the western edge of the map, separated from lower grade rocks by the Purcell Trench fault; contacts and unit definitions there differ little from those of Miller’s unpublished maps. The Moyie fault in the east juxtaposes east-facing middle and lower Prichard Formation against west-facing strata of Ravalli, Piegan and Missoula groups on the west limb of the Sylvanite anticlinorium. Deformation that began with syndepositional extension was episodic thereafter, primarily during Cretaceous contraction and Eocene extension, but probably at other times as well. A late result was the Purcell Trench, the major structural and physiographic feature of the area, which held the lobe of the Cordilleran ice sheet that dammed Lake Missoula and now hosts a rich Quaternary history.