SAVE THE DETAIL - CREATING A SEAMLESS GEOLOGIC MAP OF THE ST. GEORGE 30' X 60' QUADRANGLE, SOUTHWEST UTAH

Creating a 1:100,000-scale geologic map from a composite of detailed 1:24,000-scale geologic maps is an exercise in (1) frustration at the inevitable loss of geologic detail, and (2) elation at seeing the whole map take shape, revealing more than the sum of its parts. We used ArcGIS to stitch together 25 published geologic maps, we added original, unpublished field mapping of seven quadrangles, and we then simplified the whole to regionally consistent detail using manual and automated line-removal and line-smoothing techniques. The result is "Interim geologic map of the St. George 30' x 60' quadrangle and the east part of the Clover Mountains 30' x 60' quadrangle, Washington and Iron Counties, Utah." The UGS, USGS STATEMAP component of the National Cooperative Geologic Mapping Program, and the Washington County Water Conservancy District provided funding for the project.

The St. George quadrangle straddles the transition zone between the Colorado Plateau and Basin and Range Provinces. The Cambrian to Tertiary sedimentary and volcanic stratigraphic succession is more than 11 km thick, and it ultimately rests on little-studied, Early Proterozoic gneiss exposed in the Beaver Dam Mountains. Our map reveals the distribution of 38 Quaternary units, 89 bedrock units, and more than 45 individually mapped basaltic lava flows, reflecting the geologic diversity of the region and our attempt to maintain as much detail as practical while fitting the geology to a 1:100,000-scale topographic base map. Following technical review, we hope to release the map both as a printed sheet and as a fully attributed GIS product; the latter will include links to more than 90 40Ar/39Ar ages and 500 whole-rock geochemical analyses on volcanic and intrusive rocks in the quadrangle.