TO LUMP OR TO SPLIT: CHALLENGES IN THE COMPILATION OF THE HAILEY 1°X2° QUADRANGLE IN THE SOUTHWEST CORNER OF THE USGS NORTHERN ROCKY MOUNTAINS TRANSECT PROJECT, SOUTH-CENTRAL IDAHO

The Hailey 1°x2° quadrangle in south-central Idaho forms the southwest corner of the Northern Rocky Mountains Transect, a project established to produce an updated, seamless geologic map of the 2° quadrangles that constitute the majority of Idaho and western Montana, from 43°N to the US-Canada border. The Hailey quadrangle contains numerous geologic provinces, including: the western and central Snake River Plain, the Atlanta lobe of the Idaho batholith, the Challis Volcanic Field, the Pioneer metamorphic core complex, and the sedimentary packages associated with the Laurentian miogeocline.

The most recent geologic map of the Hailey quadrangle (Worl et al., 1991) was produced at 1:250,000 scale by the USGS Conterminous US Mineral Assessment Program (CUSMAP). We have collected newer maps, publications, and geochronology to assess where the 1991 map needs to be updated and how to seamlessly connect to adjacent maps. Updated mapping in the area is typically at larger scales, requiring units to be lumped together to fit the smaller scale of the 1991 map. In areas where updated mapping does not exist, some units from the 1991 map need to be split for conformity. Units from neighboring maps also require lumping or splitting so they are continuous on either side of the map border.

The southern half of the 1991 map contains numerous volcanic units of the Snake River Plain that are lumped into just a few formations. Newer maps within and surrounding the Hailey quadrangle, map these units individually and in greater detail. Our approach is to strategically lump units from the newer maps by age and composition, then extend them into areas where the 1991 map needs to be updated and early generalized units should be split.

The Challis 1°x2° quadrangle to the north has more than 30 units in the Challis Volcanic Group where it adjoins the Hailey quadrangle. Our solution requires lumping some units in the Challis quadrangle while splitting others in the Hailey quadrangle.

Where new mapping does not exist, old mapping needs to be updated to match up to where new mapping does exist. In these areas, lidar and high-resolution imagery can assist with interpolating between, or extrapolating from, newer, updated maps. However, mapping created in this manner must be properly field checked and will include sample collection, identification, and geochronology.