NEW GEOLOGIC MAP OF THE WESTERN HALF OF THE SALINA 30´ X 60´ QUADRANGLE, SEVIER COUNTY, UTAH

The Utah Geological Survey recently completed field mapping of a new geologic map of the western half of the Salina 30’ x 60’ quadrangle in central Utah. GIS compilation is nearly complete, after which the map will enter a review cycle before public release. Prior to this mapping this area was poorly mapped—a 1:250,000-scale map was the best available. This mapping significantly revised stratigraphic and volcanic nomenclature, and clarified major and minor structural relationships in the area. Exposed rocks include the Middle Jurassic Arapien and Twist Gulch Formations, thick Cretaceous and Paleogene sedimentary sections, and interfingering Oligocene-Miocene volcanic and volcaniclastic rocks of the northeastern flank of the Marysvale volcanic field. Oligocene diorite intrudes Arapien outcrops in several areas. Fluvial, alluvial, colluvial, and mass-movement deposits fill valley grabens and mantle high plateaus and slopes. The western part of the map area is dominated by the Sanpete-Sevier Valley antiform, cored by Cretaceous-Eocene thrust faults that deform gypsum- and halite-bearing, calcareous mudstone in the Arapien Formation. Multiple angular unconformities, unusual stratigraphic relationships, and paleotopographic relief document a protracted Jurassic to present history of uplift of the core of the antiform. The Covenant Oil field in the southern part of the map area formed in a structural trap of deeply buried back thrusts. Most of the map area is dissected by northeast- to northwest-trending normal faults, some of which have Holocene scarps and present a significant seismic risk in the area. The huge Monroe Peak caldera of latest Oligocene age extends into the southwestern part of the map area.