Geology of the Picture Rock Hills quadrangle, Southwestern Keg Mountains, Juab County, Utah (Thesis and maps)

The area covered by this report is the Picture Rock Hills quadrangle and comprises about 60 square miles. ft includes the southwestern Keg Mountains and a small part of the Drum Mountains, Juab County, Utah. The southwestern Keg Mountains is composed chiefly of extrusive and intrusive igneous rocks of Tertiary age. Lacustrine and alluvial deposits of Quaternary age cover the older rocks and fill in the valleys. The major intrusive rock in the area is the Keg Granodiorite Porphyry which is probably Tertiary and pre-Miocene in age. The volcanic rocks, which comprise most of the. exposure in the area, are divided into two groups, separated by an unconformity. The older volcanic rocks are poorly exposed. They include andesite porphyry, Keg Spring Andesite, and Red Mountain Crystal Tuffs. The Red Mountain Crystal Tuffs unit is the most widespread of the older rocks and is predominately ash-flow tuffs. It is the youngest unit in the older group and is probably middle Miocene. The younger volcanic rocks include the Keg Mountains Tuff, Keg Mountains Rhyolite, and Picture Rock Quartz Latite Porphyry. The Drum Mountains Rhyolite is equivalent to the Keg Mountains Rhyolite. The younger volcanic rocks probably range in age from late Miocene to late Pliocene. The rhyolite, tuff, and quartz latite porphyry are interbedded one with the other and are about the same age. Topaz, pseudobrookite, specular hematite, and bixbyite occur in cavities in the Keg Mountains Rhyolite. Several different types of spherical structures occur in the younger extrusive and intrusive rocks: radially structured spherulites, lithophysae, and ""thunder eggs"". The spherulites and lithophysae were formed by the devitrification of the rocks. Previous investigators hypothesized the existence of calderas in the area. Field investigation done for this report does not substantiate the geologic evidence used for that interpretation.