GEOLOGY OF THE SAN RAFAEL SWELL, EAST-CENTRAL UTAH

The San Rafael Swell is a broad, asymmetric, north-south- to southwest-northeast-trending anticlinal structure, about 120 km long and 60 km wide, which formed due to Laramide compression during the Cretaceous through early Tertiary. Uplift and erosion have made it a showcase of Colorado Plateau geology with a colorful array of sedimentary rocks over 2100 m thick, ranging in age from Permian to Cretaceous, exposed in spectacular cliffs along cuestas, mesas, and deep canyons. The San Rafael Swell serves as a natural laboratory for students, researchers, and industry geologists to study stratigraphy, facies, structure, and outcrop analogs for hydrocarbon reservoirs worldwide. Uranium, oil and gas, carbon dioxide, helium, coal, and industrial minerals are found within the Swell.

The subsurface sedimentary section consists of Cambrian, Devonian, Mississippian, and Pennsylvanian strata overlying Precambrian crystalline basement rocks consisting of schist and granite dated at 1800 Ma. The sedimentary formations and their many members exposed in the San Rafael Swell were deposited in a wide range of environments including eolian, floodplain, fluvial, braided stream, deltaic, paludal, tidal flat, and shallow and restricted marine. Several major unconformities represent significant periods of erosion or non-deposition. Pliocene-age igneous rocks are present in the form of dikes, conduits (vertical volcanic necks and plugs), and sills intruded into exposed Triassic to Cretaceous sedimentary strata.

The rocks in the San Rafael Swell have been folded, faulted, jointed, fractured, and uplifted. The major uplift and deformation of the Swell was likely controlled by a large, blind, basement-involved reverse fault (up on the west side) bounding the east flank of the structure. Small to large subsidiary anticlines and synclines are found north to south along the uplift. Three sets of high-angle normal faults are mapped on the surface: (1) northwest-southeast striking, (2) east-west striking, and (3) north-south to northeast-southwest striking. Two styles of reverse faulting are also identified in the Swell: (1) west-directed, blind reverse faults on the east flank, and (2) east-directed, ramp-style thrusting. Sandstone beds are quartz rich and brittle, and when folded or bent produce prominent joints and fractures.