Abstract

The St. George basin is experiencing some of the most rapid urban growth in Utah. Because of its setting in a tectonically active area with extensive shallow and exposed bedrock, many geologic concerns have arisen. These include an unusually large variety of geologic hazards, limited industrial minerals needed for construction, water supply issues, unique geologic features that merit preservation, and conflicting land management plans that generally have a geologic component.

As a result of these concerns, in 1994 the Utah Geological Survey (UGS) Mapping Program began an extensive geologic mapping effort in the basin. Currently, we have completed ten 1:24,000-scale quadrangles that were funded with a 50:50 cost share under the National Cooperative Geologic Mapping Program. In 1996, the UGS Mapping Program expanded its efforts and began a project to map the eight quadrangles encompassing Zion National Park. This project is nearing completion and was funded in part by the National Park Service (NPS).

Thus, since 1994, the UGS has published or is nearing completion of 18 1:24,000-scale geologic maps in the greater St. George basin area. These maps provide unprecedented geologic map coverage of over 2,700 km2 across the western margin of the Colorado Plateau and adjacent transition zone with the Basin and Range Province. Already, the maps have served as the basis for a GIS-based geologic hazards map folio of the burgeoning St. George basin; as a guide to ongoing paleoseismic investigations of the Hurricane fault zone; as the foundation for general-interest geologic reports on Zion National Park, and Quail Creek and Snow Canyon State Parks; and will form an important GIS layer in the NPS’s Zion National Park Resource Management Area database. The maps have also been a critical resource in ongoing efforts to set aside habitat for the endangered desert tortoise, and to delineate scarce sand and gravel resources before they are covered by construction.

These geologic maps provide a wealth of new data, but like any good maps they raise as many questions as they solve. We know, for example, that long-term downcutting rates vary systematically along the Virgin River, yet river terraces remain poorly dated and poorly correlated across the region. We have mapped the extent of debris-flow deposits high on the Kolob Terrace, but the age and significance of these deposits are imperfectly understood. At the base of the Cretaceous section, we have identified strata of late Early Cretaceous age that have affinities with the Cedar Mountain Formation of central and eastern Utah, but regional correlations remain uncertain. The list goes on and on, but one thing is clear - these maps and reports should provide a regionally coherent base from which to launch future, more detailed geologic studies.