Abstract

Recent geologic mapping along the southern edge of the Escalante Desert has focused on the volcanic stratigraphy, mineral resource potential, and structural geology of southwestern Utah. Mapped quadrangles extend from the Mount Escalante quadrangle near the Utah-Nevada border to the Silver Peak quadrangle just west of the Iron Springs mining district.

Pre-Tertiary rocks crop out in the eastern quadrangles and record a brief marine invasion in Middle to Late Jurassic time, followed by continental sedimentation throughout the Cretaceous. Tertiary volcanic rocks are the dominant lithology in the area. Ash-flow tuff sheets of the Needles Range Group, Isom Formation, Quichapa Group and Racer Canyon Tuff comprise the most voluminous products of calc-alkaline volcanism. Local accumulations of dominantly andesitic volcanics were coeval with ash-flow tuff eruptions and precluded tuff deposition in some areas. Bimodal volcanism was initiated in the mid-Miocene with the eruption of large rhyolitic complexes about 13 m.y. ago. A thick mid-Miocene volcaniclastic sequence was deposited over much of the area in a broad, east-trending trough.

Structural events recorded in the area include Sevier age thrusting, deformation related to intrusion, and mid- to late Miocene extensional tectonism. Both right- and left-lateral strike-slip faulting have been documented in the Silver Peak quadrangle and are related to extensional stresses. Late Miocene to possible Holocene basin-range style faulting produced much of the present topography.

Epithermal vein-type precious metal mineralization is present at the currently producing Escalante Silver Mine and in the Antelope Range mining district. Silver mineralization is associated with quartz-rich veins containing accessory calcite, barite, and fluorite. Mineralization is speculated to be related to hydrothermal cells generated by adjacent and contemporaneous rhyolitic volcanic centers.