Gravity and Magnetic Study of the Pahute Mesa and Oasis Valley Region, Nye County, Nevada
by
Edward A. Mankinen1, Gary L. Dixon2, Edwin H. McKee1, Christopher J. Fridrich3, and Randell J. Laczniak2
U.S. Geological Survey Open-File Report 99-303
Version 1.0
Prepared in cooperation with the
U.S. DEPARTMENT OF ENERGY
NEVADA OPERATIONS OFFICE
(Interagency Agreement DE-AI08-96NV11967)
1999
U.S. DEPARTMENT OF THE INTERIOR
BRUCE BABBITT, Secretary
U.S. GEOLOGICAL SURVEY
Charles G. Groat, Director
This report is preliminary and has not been reviewed for
conformity with U.S. Geological Survey editorial standards or
with the North American Stratigraphic Code. Any use of trade,
firm, or product names is for descriptive purposes only and
does not imply endorsement by the U.S. Government.
1Menlo Park, California 94025
2Las Vegas, Nevada 89119
3Denver, Colorado 80225
This online publication consists of an Adobe Acrobat PDF file, the abstract, introduction, and Acknowledgments for which are reproduced below. The data for this publication total 2.2 MB
Abstract
Regional gravity and aeromagnetic maps reveal the existence of deep basins underlying much of the southwestern Nevada volcanic field, approximately 150 km northwest of Las Vegas. These maps also indicate the presence of prominent features (geophysical lineaments) within and beneath the basin fill. Detailed gravity surveys were conducted in order to characterize the nature of the basin boundaries, delineate additional subsurface features, and evaluate their possible influence on the movement of ground-water. Geophysical modeling of gravity and aeromagnetic data indicates that many of the features may be related to processes of caldera formation. Collapse of the various calderas within the volcanic field resulted in dense basement rocks occurring at greater depths within caldera boundaries. Modeling indicates that collapse occurred along faults that are arcuate and steeply dipping. There are indications that the basement in the western Pahute Mesa - Oasis Valley region consists predominantly of granitic and/or fine-grained siliceous sedimentary rocks that may be less permeable to groundwater flow than the predominantly fractured carbonate rock basement to the east and southeast of the study area. The northeast-trending Thirsty Canyon lineament, expressed on gravity and basin thickness maps, separates dense volcanic rocks on the northwest from less dense intracaldera accumulations in the Silent Canyon and Timber Mountain caldera complexes. The source of the lineament is an approximately 2-km wide ring fracture system with step-like differential displacements, perhaps localized on a pre-existing northeast-trending Basin and Range fault. Due to vertical offsets, the Thirsty Canyon fault zone probably juxtaposes rock types of different permeability and, thus, it may act as a barrier to ground-water flow and deflect flow from Pahute Mesa along its flanks toward Oasis Valley. Within the Thirsty Canyon fault zone, highly fractured rocks may serve also as a conduit, depending upon the degree of alteration and its effect on porosity and permeability. In the Oasis Valley region, other structures that may influence ground-water flow include the western and southern boundaries of the Oasis Valley basin, where the basement abruptly shallows.
Introduction
The Nevada Test Site (NTS) was the primary location for testing nuclear devices within the United States from the early 1950's until September, 1992. After enactment of the Limited Test Ban Treaty of 1963, all testing was performed underground to minimize the possibility of release of radioactive materials into the atmosphere, and many of these were detonated near or below the water table. As a consequence, many highly toxic and long-lived contaminants exist in the subsurface of the NTS. Loss of containment of these hazardous materials would most likely be by movement of ground-water (Laczniak and others, 1996). Elevated tritium levels were found in well PM-2 (Russell and Locke, 1996), 270 meters from the closest nuclear test (the 1968 Schooner test). Traces of plutonium also have been found in well ER-20-5 (Kersting and others, 1999), 1.3 km south of the 1968 Benham nuclear test. Although tritium is soluble in water, other radioactive products of nuclear testing partition strongly to non-aqueous-phase materials. Kersting and others (1999) demonstrate that even some non-soluble products, such as plutonium, are mobile, probably by attaching to colloids. Ground-water within this region is the western part of the Death Valley ground-water-flow system (Laczniak and others, 1996). Water from Pahute Mesa, northwestern NTS, is thought to flow south and southwest, discharging at Oasis Valley, Amargosa Desert, and ultimately, Death Valley.
Regional gravity and aeromagnetic maps of the Pahute Mesa and Oasis Valley region (McCafferty and Grauch, 1997; Ponce, 1997) reveal deep basins underlying a large part of the area. A revised gravity anomaly map based on data from Ponce (1997) and Mankinen and others (1998) is shown in figure 1, and this area will be referred to as the study area. The basin underlying eastern Pahute Mesa was shown in the interpretive cross-sections produced by Ferguson and others (1994), who used lithologic logs from boreholes along with available gravity and seismic data. The locations and dimensions of all the basins within the study area are shown on a basin thickness map by Hildenbrand and others (1999). Our investigation expands on a regional interpretative study by Grauch and others (1999) by more closely defining the nature of the basin boundaries and several other prominent regional features with differing structural grain that are apparent on the regional basin thickness map of Hildenbrand and others (1999). Some of the features and localities to be discussed in this report are shown in figure 2. Gravity (Ponce, 1997; Mankinen and others, 1998) and aeromagnetic (McCafferty and Grauch, 1997) data, results from a concurrent magnetotelluric (MT) study (Schenkel, 1998; Schenkel and others, 1999), and new geologic mapping (C.J. Fridrich, unpublished data, 1999) are used to develop new tectonic models of the subsurface. Other useful geologic information in the vicinity of our profiles are the geologic cross sections generated by Byers and Cummings (1966), O'Conner and others (1966), Christiansen and Noble (1968), Orkild and others (1969), Byers and others (1976a, b), Scott and Bonk (1984), Frizzell and Shulters (1990), and Minor and others (1997). The models of the subsurface by Ferguson and others (1994), and the geologic and hydrologic sections of Laczniak and others (1996) were also valuable. Results are intended to provide constraints in the development of conceptual and numerical models of ground-water flow and to aid in monitoring efforts.
Acknowledgments
We thank J.E. Magner and D.A. Trudeau of the USGS (Las Vegas) for arranging the logistics on the Nevada Test Site and for facilitating our work there. Officials at the Nellis Air Force Range were very accommodating by allowing the access necessary to complete this study. Thanks, also, to landowners Glenn Coffer and David Spicer for access to their properties. C.W. Roberts and J.G. Davidson provided valuable assistance in the field and with the data processing. Discussions with D.C. Buesch, V.E. Langenheim, R.L. Morin, and J.M. Thomas were very helpful. We also appreciate the helpful reviews and informal comments by S.L. Drellack, B. Hurley, G.A. Pawloski, D.A. Ponce, M.J. Townsend and W. Hawkins. This investigation was conducted as part of an interagency effort between the USGS and the U.S. Department of Energy (DOE) under Interagency Agreement DE-AI08-96NV11967.
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To contact the senior author, email Ed Mankinen (emank@mojave.wr.usgs.gov).
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Date created: 08/25/1999
Last modified: 11/30/1999
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