St. George 30’x60’ Quadrangle: A New Regional-Scale Geologic Map of Southwest Utah

By Robert F. Biek

The Utah Geological Survey will soon release a preliminary geologic map of just over 2000 square miles in southwest Utah, comprising the St. George 30′ x 60′ quadrangle and the easternmost (Utah) part of the Clover Mountain 30′ x 60′ quadrangle. The geologic map is one of many in our 1:100,000-scale series, and is part of a larger effort to map the entire state at that scale (one inch on the map equals 1.6 miles on the ground). The map covers the area from the Beaver Dam and Bull Valley Mountains on the west, eastward through the St. George area to Zion National Park, and from the Arizona border north through the Pine Valley Mountains.

The map was compiled from existing geologic maps of 36 individual 7.5′ quadrangles (most mapped by UGS geologists over the past 10 years) and significant new unpublished field mapping in the Bull Valley and Pine Valley Mountains. This regional-scale map is the culmination of a decade-long effort to geologically map areas of rapid population growth and highuse recreational and natural areas in the greater St. George area. Following technical review over the coming year, which will include extending the map to cover all of Washington County, the map will be published in full color as a poster-size map. Funding for this new map, and many of the previously published 7.5′ quadrangles on which it is based, was through a cooperative agreement between the UGS and the U.S. Geological Survey under the National Cooperative Geologic Mapping Program. The Washington County Water Conservancy District also contributed funding.

The area covered by the new St. George map is one of amazing geologic diversity and importance. With its dry climate and colorful rock layers, southwest Utah is justly famous for its exceptional exposures of structures such as the Virgin anticline (an upwarp of the Earth’s crust that marks the eastern limit of folding associated with an ancient mountain-building episode) and Hurricane fault (an active fault capable of generating large earthquakes). The Pine Valley Mountains that tower over the St. George basin may be the world’s largest laccolith. This laccolith, a shallow intrusion of magma, was emplaced so rapidly just below the Earth’s surface that it domed up the rocks above, causing catastrophic gravity slides many square miles in size that literally uncorked the magma, which then erupted lava flows and tuffs (see Survey Notes, September 2002). The St. George area is known worldwide for its examples of inverted topography – lava flows that originally covered valley floors but that now form elevated, sinuous ridges (see Survey Notes, September 2002). In the past, petroleum and mineral occurrences in the area played important roles in the development of Utah’s natural resources. The first oil produced in Utah came from the Virgin field, where oil seeps can still be found, and the Silver Reef mining district, near Leeds, is centered on an unusual silver chloride ore deposit that occasionally still interests prospective mining companies. Today, ground water is the hot commodity, and this map shows the location of the Navajo Sandstone, the region’s principal aquifer. Residents of the area, and hundreds of thousands of visitors each year, are drawn to Zion National Park, at the east edge of the St. George map; to Snow Canyon, Quail Creek, Sand Hollow, and Gunlock State Parks, each with their unique geologic story; and to the St. George Dinosaur Discovery Site at Johnson Farm (see Survey Notes, October 2000) and a multitude of dinosaur track sites across the region.

There is no better way to get a sense of how the landscape has evolved than through the use of a regional-scale geologic map, and the new St. George-area map shows the geology of southwest Utah in unprecedented detail.

Soon to be available: The interim map will be released as a print-on-demand color version: Interim Geologic Map of the St. George 30′ x 60′ quadrangle, and the east part of the Clover Mountain 30′ x 60′ quadrangle, Washington and Iron Counties, Utah, by Robert F. Biek, Peter D. Rowley, David B. Hacker, Janice M. Hayden, Grant C. Willis, Lehi F. Hintze, R. Ernest Anderson, and Kent D. Brown, Utah Geological Survey Open-File Report 478, available at the Department of Natural Resources Map & Bookstore. The final version, including GIS files, should be available in about a year.

Survey Notes, v. 38 no. 3, September 2006