Rhyolitic rocks were erupted from vents in and adjacent to the Owyhee Mountains and Owyhee Plateau of southwestern Idaho from 16 m.y. ago to about 10 m.y. ago. They were deposited on a highly irregular surface developed on a variety of basement rocks that include granitic rocks of Cretaceous age, quartz latite and rhyodacite tuffs and lava flows of Eocene age, andesitic and basaltic lava flows of Oligocene age, and latitic and basaltic lava flows of early Miocene age.
The rhyolitic rocks are principally welded tuffs that, regardless of their source, have one feature in common-namely internal characteristics indicating en-masse, viscous lavalike flowage. The flowage features commonly include considerable thicknesses of flow breccia at the bases of various cooling units. On the basis of the tabular nature of the rhyolitic deposits, their broad areal extents, and the local preservation of pyroclastic textures at the bases, tops, and distal ends of some of the deposits, we have concluded that the rocks were emplaced as ash flows at extremely high temperatures and that they coalesced to liquids before final emplacement and cooling. Temperatures of l090?C and higher are indicated by iron-titanium oxide compositions.
Rhyolites that are about 16 m.y. old are preserved mostly in the downdropped eastern and western flanks of the Silver City Range and they are inferred to have been erupted from the Silver City Range. They rarely contain more than about 2 percent phenocrysts that consist of quartz and subequal amounts of plagioclase and alkali feldspar; commonly, they contain biotite, and they are the only rhyolitic rocks in the area to do so. The several rhyolitic units that are 14 m.y. to about 10 m.y. old contain only pyroxene-principally ferriferous and intermediate pigeonites-as mafic constituents. The rhyolites of the Silver City Range comprise many cooling units, none of which can be traced for great distances.
Rocks erupted from the Owyhee Plateau include two sequences that were traced over areas having diameters of about 100 km. These two sheets are the herein-named Swisher Mountain Tuff, which is about 13.8 m.y. old, and the Little Jacks Tuff, which is about 10 m.y. old. The Swisher Mountain Tuff was erupted from the Juniper Mountain volcanic center, a gentle dome that is not bounded by arcuate faults indicative of cauldron subsidence. The tuff is 200 m thick over a considerable area in and adjacent to its source. It apparently thins gradually toward its distal edges, and it is inferred to be uniformly distributed around its source at Juniper Mountain. The unit contains vitrophyres at various intervals from base to top, and, although the vitrophyres are, in general, flow layered and commonly flow brecciated, they occasionally contain well-defined pumice clasts. The vitrophyres indicate compound cooling, and, near the distal edges of the sheet, some of them probably represent complete cooling breaks.
The Little Jacks Tuff onlaps the Swisher Mountain Tuff in expo sures east of Juniper Mountain, and it is inferred to have been erupted from a source on the part of the Owyhee Plateau that lies just east of the area studied. This inferred source area, like that at Juniper Mountain, is also expressed today as a gentle dome without structural features indicative of cauldron subsidence. The Little Jacks Tuff, in most exposures in the deep canyons of the Plateau, consists of at least four cooling units, and, in places in the eastern part of the studied area near the source area, it possibly comprises as many as six. Although there is no obvious evidence of erosion between the various cooling units, magnetic polarity measurements indicate that there were at least two magnetic reversals during the eruption interval of the Little Jacks Tuff. Like the Swisher Mountain Tuff, the Little Jacks has flattened pumice clasts in a few outcrops-principally at the bases of the various cooling units.
The two tuff sequences are calc-a
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USGS Numbered Series
High-temperature, large-volume, lavalike ash-flow tuffs without calderas in southwestern Idaho