This map and accompanying cross sections present an updated synthesis of the geologic framework of the Oasis Valley area, a major groundwater discharge site located about 15 km west of the Nevada Test Site. Most of the data presented in this compilation is new geologic map data, as discussed below. In addition, the cross sections incorporate new geophysical data that have become available in the last three years (Grauch and others, 1997; written comm., 1999; Hildenbrand and others, 1999; Mankinen and others, 1999). Geophysical data are used to estimate the thickness of the Tertiary volcanic and sedimentary rocks on the cross sections, and to identify major concealed structures. Large contiguous parts of the map area are covered either by alluvium or by volcanic units deposited after development of the major structures present at the depth of the water table and below. Hence, geophysical data provide critical constraints on our geologic interpretations. A companion paper by Fridrich and others (1999) and the above-cited reports by Hildenbrand and others (1999) and Mankinen and others (1999) provide explanations of the interpretations that are presented graphically on this map. This map covers nine 7.5-minute quadrangles in Nye County, Nevada, centered on the Thirsty Canyon SW quadrangle, and is a compilation of one published quadrangle map (O'Connor and others, 1966) and eight new quadrangle maps, two of which have been previously released (Minor and others, 1997; 1998). The cross sections that accompany this map were drawn to a depth of about 5 km below land surface at the request of hydrologists who are modeling the Death Valley groundwater system.

This digital geologic map compilation presents new polygon (i.e., geologic map unit contacts), line (i.e., fault, fold axis, dike, and caldera wall), and point (i.e., structural attitude) vector data for the Thirsty Canyon NW 7 1/2' quadrangle in southern Nevada. The map database, which is at 1:24,000-scale resolution, provides geologic coverage of an area of current hydrogeologic and tectonic interest. The Thirsty Canyon NW quadrangle is located in southern Nye County about 20 km west of the Nevada Test Site (NTS) and 30 km north of the town of Beatty. The map area is underlain by extensive layers of Neogene (about 14 to 4.5 million years old [Ma]) mafic and silicic volcanic rocks that are temporally and spatially associated with transtensional tectonic deformation. Mapped volcanic features include part of a late Miocene (about 9.2 Ma) collapse caldera, a Pliocene (about 4.5 Ma) shield volcano, and two Pleistocene (about 0.3 Ma) cinder cones. Also documented are numerous normal, oblique-slip, and strike-slip faults that reflect regional transtensional deformation along the southern part of the Walker Lane belt. The Thirsty Canyon NW map provides new geologic information for modeling groundwater flow paths that may enter the map area from underground nuclear testing areas located in the NTS about 25 km to the east. The geologic map database comprises six component ArcINFO map coverages that can be accessed after decompressing and unbundling the data archive file (tcnw.tar.gz). These six coverages (tcnwpoly, tcnwflt, tcnwfold, tcnwdike, tcnwcald, and tcnwatt) are formatted here in ArcINFO EXPORT format. Bundled with this database are two PDF files for readily viewing and printing the map, accessory graphics, and a description of map units and compilation methods.

Bare Mountain comprises the isolated complex of mountain peaks southeast of the town of Beatty in southern Nye County, Nevada. This small mountain range lies between the alluvial basins of Crater Flat to the east and the northern Amargosa Desert to the southwest. The northern boundary of the range is less well defined, but for this report, the terrane of faulted Miocene volcanic rocks underlying Beatty Mountain and the unnamed hills to the east are considered to be the northernmost part of Bare Mountain. The southern tip of the mountain range is at Black Marble, the isolated hill at the southeast corner of the map. The main body of the range, between Fluorspar Canyon and Black Marble, is a folded and complexly faulted, but generally northward-dipping (or southward-dipping and northward-overturned), sequence of weakly to moderately metamorphosed upper Proterozoic and Paleozoic marine strata, mostly miogeoclinal (continental shelf) rocks. The geology of Bare Mountain is mapped at a scale of 1:24,000.

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 magmatic pulses. New data on the age, structure, and distribution of the volcanic rocks have resulted in revised structural models involving volcano‐tectonic and detachment faulting processes.

In late 1987 Congress issued an amendment to the Nuclear Waste Policy Act of 1982 which directed the characterization of Yucca Mountain, Nevada as the only remaining potential site for the Nation`s first underground high-level radioactive waste repository. The evaluation of a potential underground repository is guided and regulated by policy established by the Department of Energy (DOE), Nuclear Regulatory Commission (NRC), Environmental Protection Agency (EPA), Department of Transportation (DOT), and the US Congress. The Yucca Mountain Project is the responsibility of the DOE. The purpose of this field trip is to introduce the present state of geologic and hydrologic knowledge concerning this site. This report describes the field trip. 108 refs., 6 figs., 1 tab.

The Nevada Test Site (NTS) was established to provide an area for continental testing of nuclear devices. Geologists from the US Geological Survey (USGS) mapped much of the NTS region. These maps formed the basis for subsequent studies by geologic support groups from the Los Alamos National Laboratory, Lawrence Livermore National Laboratory, Sandia National Laboratories, and the USGS. A good geologic understanding of the stratigraphy, structure, geochemistry, and physical properties of the rocks is essential for adequate containment of underground nuclear tests. Many of the recent geologic studies at NTS, particularly in Yucca Flat, Pahute Mesa, and Mid Valley, are aimed at understanding subsurface geology to help ensure complete containment. Studies performed in conjunction with nuclear testing and radioactive waste isolation have addressed many aspects of the geologic history of NTS, which have in turn greatly enhanced our understanding of the geology of the southern Great Basin. 53 refs., 3 figs., 2 tabs.

At Pahute Mesa, Nevada Test Site, the Silent Canyon caldera of about 14 Ma age is almost completely concealed beneath ash-flow tuffs of the 11.5 Ma old Rainier Mesa Member of the Timber Mountain Tuff. Structures unequivocally related to the caldera are not observed in the Rainier Mesa Member. Structure contours on top of Rainier Mesa Member at Pahute Mesa define a series of elongate, fault-bounded blocks. Between the East Boxcar and Almendro Faults these blocks tilt eastward away from westward-dipping normal faults and elsewhere they also have a strong northward component of dip, away from Timber Mountain caldera. Episodic movement along these faults controlled thicknesses of members of Paintbrush Tuff (13.3 - 12.7 Ma) and tuffs and lavas of Area 20 (14 - 13.3 Ma), which have steeper eastward and northward components of dip than the overlying Rainier Mesa Member and also thicken eastward within each structural block. Fault blocks north of Timber Mountain caldera on Pahute Mesa are very similar to blocks described at Yucca mountain south of the caldera, and probably were generated by regional Basin and Range extension and four episodes of caldera-forming volcanism at Timber Mountain. Faults bounding these blocks on Pahute Mesa formed duringmore » early episodes of caldera-forming volcanism at Timber Mountain and reactivated during later episodes, so that fault displacements and bedding plane attitudes increase with age. Because these faults have episodic activity, even a relatively small post-Rainier Mesa displacement may define the location of important displacement within underlying units.« less

The Crater Flat Tuff is herein revised to include a newly recognized lowest unit, the Tram Member, exposed at scattered localities in the southwest Nevada Test Site region, and in several drill holes in the Yucca Mountain area. In previous work, the Tram Member was thought to be the Bullfrog Member, and therefore was shown as Bullfrog or as undifferentiated Crater Flat Tuff on published maps. The revised Crater Flat Tuff is stratigraphically below the Topopah Spring Member of the Paintbrush Tuff and above the Grouse Canyon Member of the Belted Range Tuff, and is approximately 13.6 m.y. old. Drill holes on Yucca Mountain and near Fortymile Wash penetrate all three members of the Crater Flat as well as an underlying quartz-poor unit, which is herein defined as the Lithic Ridge Tuff from exposures on Lithic Ridge near the head of Topopah Wash. In outcrops between Calico Hills and Yucca Flat, the Lithic Ridge Tuff overlies a Bullfrog-like unit of reverse magnetic polarity that probably correlates with a widespread unit around and under Yucca Flat, referred to previously as Crater Flat Tuff. This unit is here informally designated as the tuff of Yucca Flat. Although older, it may be geneticallymore » related to the Crater Flat Tuff. Although the rocks are poorly exposed, geophysical and geologic evidence to date suggests that (1) the source of the Crater Flat Tuff is a caldera complex in the Crater Flat area between Yucca Mountain and Bare Mountain, and (2) there are at least two cauldrons within this complex - one probably associated with eruption of the Tram, the other with the Bullfrog and Prow Pass Members. The complex is named the Crater Flat-Prospector Pass caldera complex. 24 references, 18 figures, 1 table.« less

Research Article| July 01, 1977 Timber Mountain–Oasis Valley caldera complex of southern Nevada ROBERT L. CHRISTIANSEN; ROBERT L. CHRISTIANSEN 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar PETER W. LIPMAN; PETER W. LIPMAN 2U.S. Geological Survey, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar W. J. CARR; W. J. CARR 2U.S. Geological Survey, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar F. M. BYERS, JR.; F. M. BYERS, JR. 2U.S. Geological Survey, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar PAUL P. ORKILD; PAUL P. ORKILD 2U.S. Geological Survey, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar K. A. SARGENT K. A. SARGENT 2U.S. Geological Survey, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Author and Article Information ROBERT L. CHRISTIANSEN 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 PETER W. LIPMAN 2U.S. Geological Survey, Federal Center, Denver, Colorado 80225 W. J. CARR 2U.S. Geological Survey, Federal Center, Denver, Colorado 80225 F. M. BYERS, JR. 2U.S. Geological Survey, Federal Center, Denver, Colorado 80225 PAUL P. ORKILD 2U.S. Geological Survey, Federal Center, Denver, Colorado 80225 K. A. SARGENT 2U.S. Geological Survey, Federal Center, Denver, Colorado 80225 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1977) 88 (7): 943–959. https://doi.org/10.1130/0016-7606(1977)88<943:TMVCCO>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation ROBERT L. CHRISTIANSEN, PETER W. LIPMAN, W. J. CARR, F. M. BYERS, PAUL P. ORKILD, K. A. SARGENT; Timber Mountain–Oasis Valley caldera complex of southern Nevada. GSA Bulletin 1977;; 88 (7): 943–959. doi: https://doi.org/10.1130/0016-7606(1977)88<943:TMVCCO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Timber Mountain–Oasis Valley caldera complex lies within a volcanic field in southern Nevada that once covered 11,000 km2. The caldera complex, active from 16 to 9.5 m.y. ago, was the source of nine voluminous rhyolitic ash-flow sheets and numerous smaller rhyolitic tuffs and lava flows. Several centers of basaltic and related volcanism were active before the complex formed, continued around its periphery during caldera activity, and have since overlapped the caldera complex. Extensional normal faulting and perhaps deep-seated right-lateral deformation preceded, accompanied, and followed evolution of the caldera complex and its surrounding volcanic field.The youngest major structure of the complex is the Timber Mountain resurgent caldera, 25 by 30 km across. Several stages of its development can be documented: (1) magmatic insurgence accompanied by gentle tumescence, formation of a ring-fracture zone, and minor rhyolitic volcanism; (2) eruption about 11 m.y. ago of a voluminous ash-flow sheet, caldera collapse during the eruption, and postcollapse caldera infilling by sediments and rhyolite flows; (3) renewed ash-flow eruptions and further caldera collapse; (4) resurgent doming of the cauldron block; and (5) postcollapse rhyolitic volcanism and filling of the caldera by sediments.Only parts of the older calderas are preserved, but they can be interpreted in terms of evolutionary cycles similar to that of the Timber Mountain caldera. Major tectonic intersections appear to have controlled the locations and certain structural features of each major volcanic source area.Differentiation at high crustal levels of the silicic magmas related to the caldera complex produced compositionally zoned ash-flow sheets. High-level differentiation also is represented at most of the basaltic centers of the field. Each caldera cycle probably represents a separate batch of rhyolitic magma that rose high into the crust, differentiated in place, and partly erupted to the surface. Each of these magmas probably rose independently through the crust, but all of them were related ultimately to a single magmagenetic system, as were the basaltic magmas of the field. The silicic magma bodies consolidated to form large shallow granitic plutons, and the caldera complex now overlies a small composite granitic batholith. This content is PDF only. Please click on the PDF icon to access. 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