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A composite thickness of more than 2,000 m of Tertiary (22 m.y. to 27 m.y.) subalkaline, genetically related, ash-flow tuffs and intercalated lava flows of intermediate composition is exposed in the Gabbs Valley and Gillis Ranges in west-central Nevada. The two oldest ash-flow units can be traced for distances of more than 150 km. Six regionally extensive tuff sheets, locally separated by lava flows, are recognized: the Mickey Pass and Singatse Tuffs (names defined by Proffett and Proffett, 1976 and adopted in this report; included here together with the Petrified Spring Tuff in the Benton Spring Group, a new name), the herein-named Blue Sphinx Tuff, the hereinnamed Nugent and Poinsettia Tuff Members of the Hu-pwi Rhyodacite, and the tuff of Copper Mountain. In contrast to most tuff sheets that grade upward from silicic to intermediate composition, several of these units in western Nevada grade upward from bases of intermediate composition to silicic tops. One unit displays a remarkable change upward from rhyodacite at the base through quartz latite to rhyolite at the top. The tuff sheets show northwesterly elongated distribution patterns that reflect a pretuff topographic trough. This trough coincides with the present-day Walker Lane and probably reflects the Walker Lane as a distinct rift zone prior to ash-flow tuff eruptions. In early Miocene time (at least 23 m.y. ago), after many of the tuff sheets were erupted, the region was subjected to conjugate strike-slip faulting along northwestand northeast-trending faults. Most of the movement was right-lateral along a series of en echelon faults with northwest trends. This faulting continued into Quaternary time as indicated by fault scarps and shutter ridges in alluvium. The eruption of tuff and lava in volumes comparable to that in the Gabbs Valley and Gillis Ranges normally gives rise to a large cauldron complex, but no such complex appears possible within this area. The possibility exists that, instead of a faultbounded cauldron complex, the massive extrusions formed a southeasterly elongated depression or depressions whose axes were more or less coincident with the pretuff topographic trough. The fact that many tuffs contain intensely resorbed phenocrysts of quartz, plagioclase, and, in some units, alkali feldspar suggests that the magma was erupted from levels that were undersaturated with respect to water. Such undersaturation could indicate eruption from great depths. This possibility is suggested also by the unique zonation of the tuffs from bases of intermediate composition to silicic tops. Perhaps these depths were too great to allow the development of normally zoned magma chambers and well-defined cauldrons. 2 STRATIGRAPHY, PETROLOGY, STRUCTURE IN NEVADA

Stratigraphy, preliminary petrology, and some structural features of Tertiary volcanic rocks in the Gabbs Valley and Gillis ranges, Mineral County, Nevada