Subsurface geology of the Silent Canyon caldera, Nevada Test Site, Nevada

Abstract

Deep drilling in the vicinity of Silent Canyon on eastern Pahute Mesa, Nevada, has revealed a Tertiary volcanic section locally thicker than 14,000 feet. The area drilled covers most of the Silent Canyon caldera and some of the surrounding area. The caldera is rudely elliptical in plan and measures 10 by 14 miles. Except on its east edge, the caldera is completely obscured by younger volcanic rocks, including ash-flow sheets from several other centers. The structure, originally inferred from surface mapping and a 20-mgal gravity low, has been confirmed by drilling at 21 sites. Petrographic, chemical, and magnetic studies of more than 4,000 feet of drill core have revealed a complex sequence of volcanic rocks. Precaldera rocks include thick talc-alkalic lavas and tuffs. Peralkaline lavas and tuffs of the Silent Canyon center include the Belted Range Tuff (13-14 m.y.), eruption of which resulted in caldera collapse. The Belted Range Tuff has been downdropped 5,000-7,000 feet in the caldera. Subsequently, the depression was partially filled in the eastern part by genetically related peralkaline lavas and tuffs and in the western part by later calc -alkaline lavas and tuffs apparently genetically unrelated to the peralkaline rocks of the Silent Canyon center. Renewed subsidence probably occurred during their eruption. Isopach maps of younger tuffs show that the caldera was topographically low during deposition of later (12.5-7 m.y.) ash-flow sheets from other volcanic centers.