Volcanic centers of southwestern Nevada: Evolution of understanding, 1960–1988

Abstract

Since about 1960, geologists of the U.S. Geological Survey and, more recently, those of Los Alamos and Lawrence Livermore national laboratories, supported largely by the U.S. Department of Energy (DOE) and its predecessors, have been unraveling a complex series of ash flow sheets, lavas, and related calderas in the southwestern Nevada volcanic field in and near the Nevada Test Site (NTS). Extensive detailed geologic mapping aided in delineation of four major calderas: Silent Canyon (∼14 Ma), Timber Mountain‐Oasis Valley (∼11.5 Ma), Black Mountain (∼7.5 Ma), and Stonewall Mountain (∼6 Ma). In the 1960s, key concepts that contributed to the understanding of volcanology were the recognition of vertical compositional zonation within ash flow sheets, the significance of caldera rim and moat lavas, the relation between caldera collapse and intracaldera breccias and ash flow facies, and the correlation of intracaldera and outflow‐sheet facies. Deep drill holes within Silent Canyon and Timber Mountain calderas provided vital information on caldera geometry and intracaldera facies. Radiometric dating has produced nearly 100 dates that define the age of the field between about 16 and 6 Ma. During the middle part of that period a major ash flow eruption occurred once in about every half million years. Continuing support by the DOE for earth science at the NTS during the 1970s and 1980s has permitted a unique longevity of studies and provided opportunities to restudy mapped areas, revise some incorrect relationships, and work out important details of caldera history and structure that otherwise would not have come to light. Petrochemical and isotopic studies contributed to the understanding of the PT environment of the magma bodies that generated the major ash flow sheets. In the last decade, specialized work has continued on stratigraphic and petrologic problems, resulting in understanding of petrochemical cycles, in wider and more accurate correlation of certain units, and in understanding the time and spatial relationship between petrochemically very similar ash flow sheets from the Black Mountain and Stonewall Mountain calderas. Drilling and detailed Earth science studies in connection with preliminary characterization of a proposed nuclear waste repository at Yucca Mountain have greatly advanced knowledge of the older and younger parts of the volcanic sequence, including the basalts. A newly defined volcano‐tectonic collapse area in Crater Flat, probably a caldera, is strikingly similar in structure, location, and geophysical expression to the Silent Canyon caldera to the north. Rhyolite lavas peripheral to the Timber Mountain‐Oasis Valley caldera complex are intercalated with the major ash flow sheets from the complex, and analyses of the lavas fit well on compositional trends determined by the ash flow sheets. Renewed studies since 1981 include eruptive dynamics and magma chemistry of the major ash flow sheets and sources of mafic and intermediate volcanism. Hydrothermal alteration and mineralization occur after major...

The CHESHIRE Migration Experiment A Summary Report

“…The entire stratigraphic and lithologic package is offset by younger high-angle normal faults. Recent summaries of the geology and hydrogeology of this volcanic field are contained in Laczniak et al (1996);Sawyer et al (1994);Ferguson et al (1994); Noble et al (1991); Byers et al (1989);and Sawyer and Sargent (1989). Two major buried caIderas of the southwestern Nevada volcanic field, which together form the Silent Canyon caldera complex, are located in the subsurface of the Pahute Mesa underground test area (Figure 3).…”

Nevada National Security Site Environmental Report 2010, Attachment A: Site Description

“…Refinement of the stratigraphy of the area was a continuous process during the decades in which geoscientists associated with the Weapons Testing Program worked to understand the complex volcanic setting (documented by Byers et al, 1989). The need to develop detailed geologic models in support of the Underground Test Area (UGTA) Sub-Project (see Chapter 12 of the main body of this document) intensified this process, and the recognition of smaller and smaller distinct volcanic units permitted a greater understanding of the three-dimensional configuration of the various types of rocks, which has been incorporated into the geologic framework.…”

Nevada Test Site Environmental Report 2003

“…Refinement of the stratigraphy of the area was a continuous process during the decades in which geoscientists associated with the Weapons Testing Program (WTP) worked to understand the complex volcanic setting (documented by Byers et al, 1989). The need to develop detailed geologic models in support of the Underground Test Area (UGTA) Project (see Section 13) intensified this process, and the recognition of smaller and smaller distinct volcanic units permitted a greater understanding of the three-dimensional configuration of the various types of rocks, which has been incorporated into the geologic framework.…”

“…This summary was compiled from various sources, including BN (2001a), Gonzales and Drellack (1999), Winograd and Thordarson (1975), Laczniak et al, (1996), Byers et al, (1989), andCole (1997) where additional information can be found.…”