Abstract

The Pahranagat area volcanic sequence consists of over 3000 feet of extremely widespread distinctive rhyodacitic ignimbrites interbedded with Miocene sediments and locally capped by basalts. The ignimbrites are deposits of fissure-type nuées ardentes (glowing avalanches) and range from completely welded to nonwelded. Field work in over 700 square miles of the Pahranagat, Alamo, and Hiko ranges has revealed that more-or-less continuous emission of nuées ardentes was punctuated by periods of fluvio-lacustrine deposition of reworked volcanic ejecta and fresh-water limestones.

The volcanic sequence is divided into four formations. These are in ascending order: (1) Hells Bells Canyon Formation (0–1000 feet thick), basal non-volcanic conglomerate, basal welded rhyodacitic crystal-vitric tuff, interbedded fresh-water limestones, welded and nonwelded vitric tuffs, upper water-laid tuffs, includes several cooling units; (2) Hiko Tuff (450–1000 feet thick), dacitic crystal-vitric welded tuff, basal ash, pseudogranitic texture, bouldery outcrop pattern, largely one cooling unit; (3) Alamo Range Formation (400–1000 feet thick), semiconsolidated pumiceous vitric tuff overlain by welded vitric tuff, locally underlain by water-laid tuff, in part one cooling unit; (4) Badger Valley Basalts (200 feet thick), basalt flows.

Except for the basalts, these formations are essentially conformable.

Utilization of the exceptional value of the ignimbrites as stratigraphic-structural datum planes and the presence of the sparingly fossiliferous freshwater limestones demonstrate that the nuées ardentes were extruded over faulted and tilted Paleozoic rocks during Miocene and possibly late Oligocene time, on a subdued topography and under climatic conditions more humid than those existing today. Furthermore, the volcanic sequence was extensively faulted as part of the later formation of Basin and Range structures.

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