Revised stratigraphy and radiometric ages of volcanic rocks and mineral deposits in the Marysvale area, west-central Utah

Abstract

The Marysvale area, Utah, is near the northeast end of a broad belt of Tertiary igneous rocks that extends east-northeastward from southern Nevada to central Utah. In late Oligocene and early Miocene time (30-21 m.y. (million years) ago), a composite volcanic center consisting of numerous local volcanoes was active in the general Marysvale area. The volcanoes consisted of near-source lava flows, volcanic breccias, and minor pyroclastic deposits of generally intermediate composition flanked by coalescing aprons of volcaniclastic debris. Locally derived ash-flow tuffs form important marker beds within the assemblage. Concurrently with local volcanism, ash flows from distant sources in the Great Basin extended into the Marysvale area to interleave marginally with locally derived volcanic rocks.Local volcanic activity in the Marysvale area probably began about 30 m.y. ago with eruption of mafic intermediate-composition lava flows and volcanic breccias. About 29 m.y. ago, a widespread sheet of crystal-rich ash-flow tuff (Oligocene Needles Range Formation) was spread across the region from distant, as yet unidentified centers to the west or southwest. The Needles Range Formation is exposed all around the Marysvale area, but may have been excluded in places within it by local volcanoes. Accumulation of intermediate-composition volcanic rocks from local centers continued after deposition of the Needles Range Formation, and by 21 m.y. ago, a major pile of volcanic rocks had been built in the Marysvale area. The name Bullion Canyon Volcanics is applied to this assemblage of Oligocene and Miocene volcanic rocks.About 27 m.y. ago a sequence of distinctive crystal-rich ash flows was erupted from a center in the northern part of the Marysvale pile, and a small trapdoor type block subsided at the source. The resulting ash flow tuff sheet, here named Three Creeks Tuff Member of the Bullion Canyon Volcanics, made up a significant part of what was formerly considered part of the Dry Hollow Formation. The Dry Hollow previously was believed to be younger than the Bullion Canyon Volcanics. However, we have found the Three Creeks Tuff Member in Bullion Canyon, in the type locality of the Bullion Canyon Volcanics, thereby demonstrating the lateral equivalence of the Dry Hollow and part of the Bullion Canyon. We abandon the name Dry Hollow Formation.Bullion Canyon rocks overlying the Three Creeks Tuff Member consist of a poorly understood assemblage that includes a basaltic andesite shield volcano, ash-flow tuffs of diverse lithologies and sources, and intermediate-composition lava flows and volcanic breccia. The Delano Peak Tuff Member (formerly the Delano Peak Latite Member) is one of the more distinctive of the ash-flow units in the upper part.Between 21 and 18 m.y. ago, repeated rhyolitic eruptions from two source areas in the Marysvale area produced a heterogeneous assemblage of ash-flow tuffs, lava flows 'Thicknesses from Callaghan (1939) are given in the units used in the original report (1 foot=0.3 m.).