Ash-flow fissure vent in west-central Nevada

An east-trending fissure vent filled with welded tuff is well exposed for nearly 1 km on the west flank of Gabbs Valley in northeastern Mineral County, Nevada. In its deepest exposed part, the vent is about 60 m wide. At the top of an exposure, however, where the tuff in the vent merges with an ash-flow tuff cooling unit, the vent is about 460 m wide. The abrupt widening of the vent upward through a vertical distance of only about 400 m from the base of the exposure to the top was probably accomplished by a combination of explosive action and forcible shoving aside of the country rocks. The ash-flow cooling unit continuous with the tuff in the fissure is the youngest of three genetically related units that are sporadically exposed in the vicinity of Gabbs Valley. Stratigraphic and structural relationships in the vicinity of the fissure suggest that the fissure is located on the west flank of a cauldron that is nearly completely buried in Gabbs Valley. The “vent tuff” and the underlying genetically related units in the Gabbs Valley area are highly differentiated rhyolites characterized by high silica and low Ca and Mg contents. The tuffs were erupted 25 m.y. ago at virtually the same time that other chemically and minera-logically similar tuffs were erupted farther to the east in central and east-central Nevada.