Geologic Structure of St. George District Washington County, Utah

Abstract

The structural history, geologic structure, and results of deep drilling in an area occupying a structural transition zone between the High Plateaus of Utah and the Basin and Range Province are described. Cambrian quartzite rests unconformably on pre-Cambrian igneous rocks, and is overlain by as much as 20,000 feet of strata, which record frequent oscillations during the Paleozoic and Mesozoic, a period of uplift and erosion just prior to the Cretaceous, a period of orogeny during the late Cretaceous or early Tertiary (Laramide revolution), and post-Laramide thrust and normal faulting down to rather recent time. Major north-trending normal faults, with the downthrow on the west, divide the district into five east-dipping monoclinal blocks. The normal faults are both strai ht and curved, terminate abruptly, vary in vertical displacement along any one fault surface, and have as much as 4,500 feet of observable stratigraphic displacement. Encouraged by the finding of oil in the small Virgin field 5 miles east and by the presence of anticlinal structure, six deep wells have been drilled in the district without discovering commercial quantities of oil or gas.