Geologic and Hydrologic Investigations 

of a Potential Nuclear Waste Disposal Site 

at Yucca Mountain, Southern Nevada 

U.S. GEOLOGICAL SURVEY BULLETIN 1790 
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U.S. Geological Survey Bulletin 1790 

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Figure 2.10. Residual gravity-anomaly map (wavelengths shorter than 125 km) of southern Great Basin. 

The caption for the figure appearing on page 17 should read: 

Figure 2.9. Regional gravity-anomaly map (wavelengths longer than 125 km) of southern Great Basin. 

These figures were inadvertently printed on the wrong pages. 

Geologic and Hydrologic Investigations 

of a Potential Nuclear Waste Disposal Site 

at Yucca Mountain, Southern Nevada 

By MICHAEL D. CARR and JAMES C. YC>UNT, Editors 

U.S. GEOLOGICAL SURVEY BULLETIN 1790 

CONTENTS 

1. Introduction 1 

By Michael D. Carr and James C. Yount 

2. Regional geologic and geophysical maps of the southern Great Basin 3 

By Thomas G. Hildenbrand, Albert M. Rogers, Howard W. Oliver, Stephen C. 

Harmsen, John K. Nakata, Douglas S. Aitken, Robert N. Harris, and 

Michael D. Carr 

3. Preliminary interpretation of seismic-refraction and gravity studies west of 

Yucca Mountain, Nevada and California 23 

By Hans D. Ackermann, Walter D. Mooney, David B. Snyder, and Vickie D. Sutton 

4. Volcano-tectonic setting of Yucca Mountain and Crater Flat, southwestern 

Nevada 35 

By Wilfred J. Carr 

5. Detachment faulting in the Death Valley region, California and Nevada 51 

By Warren B. Hamilton 

6. Stress field at Yucca Mountain, Nevada 87 

By Joann M. Stock and John H. Healy 

7. An evaluation of the topographic modification of stresses at Yucca Mountain, 

Nevada 95 

By Henri S. Swolfs, William Z. Savage, and William L. Ellis 

8. Preliminary study of Quaternary faulting on the east side of Bare Mountain, 

Nye County, Nevada 103 

By Marith C. Reheis 

9. Reinterpretation of the Beatty scarp, Nye County, Nevada 113 

By W C Swadley, James C. Yount, and Samuel T. Harding 

10. Preliminary results of high-resolution seismic-reflection surveys conducted 

across the Beatty and Crater Flat fault scarps, Nevada 121 

By Samuel T. Harding 

11. Uranium-trend dating of fluvial and fan deposits in the Beatty area, 

Nevada 129 

By John N. Rosholt, W C Swadley, and Charles A. Bush 

12. Relation between P-wave velocity and stratigraphy of late Cenozoic deposits 

of southern Nevada 139 

By Eduardo A. Rodriguez and James C. Yount 

13. Water-table decline in the south-central Great Basin during the Quaternary: 

implications for toxic waste disposal 147 

By Isaac J. Winograd and Barney J. Szabo 

Any use of trade names is for descriptive purposes only and does not imply endorsement by the 

U.S. Geological Survey. 

v 

1. Introduction 

By Michael D. Carr and james C. Yount 

Tectonic setting has emerged as an important issue 

during site selection for many nuclear facilities. It also will 

be an important issue in the selection of the nation's first 

site for the disposal of high-level nuclear waste. The potential 

for tectonic or volcanic activity that could adversely affect 

the performance of a repository is listed specifically by the 

Nuclear Regulatory Commission as a potentially adverse 

condition under siting criteria in the Code of Federal 

Regulations. 

Many questions regarding tectonism will not be 

answered uniquely. Questions of broad scope, such as what 

region constitutes the tectonic setting of significance to the 

proposed site and what is the character of tectonism in the 

regional setting, include topics that are at the leading edge 

of current scientific debate. More specific questions, such 

as what are the setting and history of young faults near 

enough to the proposed site to produce consequential ground 

shaking, what is the state of stress throughout the rock 

column at the site, and what are the maximum earthquake 

magnitudes and the maximum ground accelerations expected 

at the site, press the limit of current theory and research 

techniques. Realistically planners will need to accept a range 

of interpretations of the site geology in keeping with current 

concepts of the regional geologic framework, evaluating safe 

waste isolation in light of every credible hypothesis. 

Tectonic setting is an especially pertinent issue for the 

Yucca Mountain site because the site lies within the southern 

Great Basin, a region of known active tectonism and young 

volcanism. Some scientists have emphasized the relationship 

between tectonism and volcanism in the region that includes 

Yucca Mountain. Could future tectonism induce volcanism 

or vice versa? The relationships between tectonism and other 

geologic factors important to site evaluation also are 

important issues. For instance, the stability of the deep 

ground-water table is critical to plans for a repository at 

Yucca Mountain: could future tectonism alter the character 

of the hydrologic system beneath the proposed site? 

Members of the U.S. Geological Survey conducting 

investigations in the southern Great Basin (including many 

of those directly involved in the evaluation of the Yucca 

Mountain site) met in Denver, Colorado, in August 1985 to 

review available geologic and geophysical data for the region. 

Much discussion centered on current concepts of regional 

crustal extension in the southern Great Basin. Considerable 

discussion centered, as well, on methods for determining the 

state of stress in and around Yucca Mountain and on the 

results of ongoing stress studies. Geologic, geophysical, and 

geochemical techniques for establishing the age of Quaternary deposits in the region also were examined in detail 

because of their importance for determining the movement 

history of young faults near Yucca Mountain. 

Crustal extension has been the principal tectonic 

process affecting the southern Great Basin during the late 

Cenozoic. Questions focused on the mechanisms of extension, the temporal and spatial distribution of extension, and 

the amounts and rates of extension. Surface geology is 

reasonably well known for much of the southern Great Basin, 

but deep crustal structure is not well known. 

Many of the tectonic models for the southern Great 

Basin emphasize detachment faulting as a fundamental 

mechanism for extension. Most of the attendees concurred 

that the detachment faults exposed in the region played a key 

role during regional Miocene extension, but there was 

considerable disagreement concerning the geometry and 

significance of such faults in the contemporary tectonic 

framework. 

Some researchers contended that contemporary extension is merely a continuation of Miocene extension and that 

the geometry of extension has not changed significantly 

during the past 15 million years. While some of these workers 

cited waning volcanism as evidence that the rate of tectonic 

activity in the region also is slowing, others cited areas of 

extreme active tectonism such as Death Valley as evidence 

of continuing regional extension in the southern Great Basin. 

Still others contended that major changes have occurred in 

the extensional framework of the southern Great Basin during 

the late Cenozoic. These researchers suggested that although 

contemporary fault movement may be in response to 

processes similar to those controlling Miocene structures, 

the geometry and rates of contemporary movement on faults 

formed during the Miocene, as well as on newly formed 

faults, are not wholly related to the Miocene framework and 

must be viewed independently. 

There was general agreement among the researchers 

that numerous interpretations are permissible under the constraints of our current knowledge of surface geology and that 

acquisition of regional subsurface data will be important in 

evaluating these hypotheses. Determining whether the extensional setting has been uniform since the middle Miocene, 

or whether there have been major changes in the regional 

structural setting since extension began, is fundamental to 

predicting the nature of future faulting and volcanic activity 

near Yucca Mountain. If the extension occurring today 

reflects the same tectonic setting as in the Miocene, then it 

may be valid to incorporate Miocene history into predictions 

of future tectonism. If, however, the tectonic setting has 

ch1anged significantly since the late Miocene, then only 

the most recent tectonic episode can be incorporated into 

Introduction 1 
predictive tectonic analyses. Establishing an age for the 

beginning of the contemporary tectonic episode thus 

becomes an especially important problem facing 

investigators. 

Just as tectonic activity may not have been uniform in 

time, so it appears not to have been uniform in space; for 

example, recent deformation in the Death Valley region, west 

of the Furnace Creek fault zone, apparently is more severe 

than in the region farther east, closer to Yucca Mountain. 

Some researchers noted this as evidence for relative tectonic 

stability of the area that includes Yucca Mountain with 

respect to other parts of the Great Basin. Documenting the 

rates of tectonic activity and defining the structural boundaries between contemporary tectonic domains in the region 

also are important to making geologic predictions of future 

tectonism for Yucca Mountain. For example, if meaningful 

strain rates for structures in the Yucca Mountain area are 

to be determined, then such rates can be calculated only on 

the basis of data demonstrably from the structural domain 

that includes Yucca Mountain. Furthermore, it is important 

to understand the extent to which tectonic domains in the 

region are mechanically independent from, or interactive 

with, one another. 

Focal mechanisms determined from current seismicity 

are consistent with they recognition of active crustal extension in the region that includes Yucca Mountain, but the correspondence between earthquake epicenters and surface 

structures is weak. There is a slight tendency for earthquakes 

to be more abundant in regions with young faults, except 

in the Death Valley area. Measurements of the active stress 

field at Yucca Mountain also are consistent with active 

extension. 

Much of the current emphasis of tectonic research sponsored by the nuclear waste program focuses not on the 

regional tectonic setting, but rather on the history of specific 

young faults in the vicinity of Yucca Mountain. Recent 

discoveries by some of the researchers at the meeting 

demonstrate recurrent movement during the Quaternary on 

several fault systems near Yucca Mountain and possible 

Holocene displacement on at least two of these. 

Precise dating of Quaternary materials offset by faults 

remains a critical problem in determining the history of 

faulting, which is the basis for predicting future fault activity. 

Most of the dates defining the chronology of the Quaternary 

structural history of the Yucca Mountain area were done 

by the uranium-trend method, an experimental technique 

whose accuracy in specific cases was strongly debated at 

the meeting. Clearly, a variety of dating techniques must be 

used to refine the Quaternary stratigraphy of Yucca Mountain. In order to do this, a regional approach is needed, 

2 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

because directly datable materials are sparse at Yucca 

Mountain. 

A major topic of the Denver meeting was additional 

research needs and directions. The need for a thorough 

regional understanding of the tectonic setting of Yucca Mountain was emphasized. Studying Yucca Mountain out of 

context likely would lead to an inadequate level of understanding for licensing a repository when the site is placed 

under the scrutiny of the scientific community. 

The significance of detachment faulting, identification 

of contemporary structural domains, and determination of 

strain rates and patterns are problems that must be appreciated 

on a regional scale. Regional tectonic framework studies will 

require additional regional map compilation and then detailed 

mapping of critical areas, along with further integration of 

geological and geophysical data. Regional seismic-refraction 

and seismic-reflection surveys have proved to be the most 

valuable tools in many cases for better constraining the 

relation between structures mapped at the surface and the 

subsurface structural framework. Regional understanding of 

strain patterns and rates will require further paleostress 

studies, in-situ stress measurements, paleomagnetism studies, 

and isotopic dating to help determine uplift rates and 

histories. Studies of Cenozoic sedimentary basins were 

recommended for comparison of Miocene basin configuration with that of present-day basins to test continuity of structural geometry and help establish age control for major 

tectonic features. 

Detailed Quaternary fault studies must continue along 

with studies to refine the stratigraphy of Quaternary deposits. 

Testing of experimental dating techniques and duplication 

of dates by various techniques are critical to establishing the 

rates of contemporary faulting in the vicinity of Yucca 

Mountain. The Furnace Creek fault was emphasized as one 

deserving additional study, because it is the longest fault in 

the vicinity of Yucca Mountain and because it appears to be 

a major structural boundary between contemporary tectonic 

domains. 

An understanding of the local geology of Yucca Mountain and credible prediction of geologic boundary conditions 

for engineering purposes are dependent on our thorough 

appreciation of the regional geologic and geophysical setting, 

as well as detailed site-specific studies. Tectonism in the 

southern Great Basin is in response to processes on a global 

scale and cannot be understood except in a regional context. 

As conveners of the Denver meeting we wish to thank all 

of the participants. We especially thank Ernest E. Anderson, Manuel G. Bonilla, Jerry P. Eaton, Warren B. 

Hamilton, and John H. Stewart, who served as a panel of 

experts to evaluate our ongoing research program. 
2. Regional Geologic and Geophysical Maps 

of the Southern Great Basin 

By Thomas G. Hildenbrand, Albert M. Rogers, Howard W. Oliver, 

Stephen C. Harmsen, John K. Nakata, Douglas S. Aitken, 

Robert N. Harris, and Michael D. Carr 

CONTENTS 

Abstract 3 

Introduction 3 

Quaternary faults 3 

Contemporary seismicity 4 

Terrain maps 5 

Terrain data 5 

Slope 5 

Gravity anomaly maps 5 

Free-air and Bouguer gravity anomalies 5 

Separating the regional and residual gravity anomaly 6 

Isostatic residual gravity map 6 

Magnetic anomaly maps 8 

Total-intensity residual magnetic field 8 

First-vertical-derivative operation 8 

Pseudo-gravity gradient 8 

References cited 9 

Abstract 

A collection of maps of the southern Great Basin presents 

an overview of the geologic and geophysical features of the 

region that contains the proposed site for disposal of high-level 

nuclear waste at Yucca Mountain in southern Nevada. The 

collection consists of 1 :2,500,000-scale maps of the geology; 

terrain and terrain slope; Quaternary faults and earthquake 

epicenters; free air, complete Bouguer, regional, residual, and 

isostatic residual gravity anomalies; residual total magnetic 

anomaly; first vertical derivative of the magnetic field; and 

magnitude of the horizontal gradient of the pseudo-gravity field. 

INTRODUCTION 

The collection of 1 :2,500,000-scale maps of the southern Great Basin presented here gives an overview of the 

general gravity, aeromagnetic, and terrain features of the 

region that contains the proposed high-level nuclear waste 

disposal site at Yucca Mountain in southern Nevada 

(fig. 2.1). The collection includes a generalized geologic map 

(fig. 2.2, in pocket) and a map of Quaternary faults and 

epicenters of contemporary seismic events (fig. 2.3, in 

pocket) as transparent overlays for comparison with the 

various geophysical and terrain maps, in order to emphasize 

spatial relations between regional geologic and geophysical 

features, terrain, and patterns of contemporary tectonic 

activity. 

No attempt is made here to interpret these maps. 

Rather, we present the maps as a data source for those 

attempting to interpret the regional geologic and geophysical 

framework of the southern Great Basin and the proposed 

waste disposal site at Yucca Mountain. It is hoped that these 

maps will assist in identifying the boundaries of the major 

contemporary tectonic domains in the region surrounding 

Yucca Mountain. All of the maps use an Albers equal-area 

conic projection (standard parallels 29.5° N. and 45.5° N., 

central meridian 96 o W.). 

QUATERNARY FAULTS 

Late Cenozoic fault traces (fig. 2.3) were derived from 

the Quaternary fault map of the Basin and Range and Rio 

Grande Rift compiled by Nakata and others (1982) and, in 

the vicinity of Yucca Mountain, from Swadley and others 

( 1984). The compilation of Quaternary faults is presented 

here principally to show the general pattern and distribution 

of contemporary faulting for comparison with regional 

patterns of epicenters of seismic events and as a geologic 

frame of reference for comparing regional geophysical 

features. 

The Quaternary fault map of Nakata and others was 

compiled at a scale of 1 :2;500,000 using data from 55 sources 

and shows the regional pattern of known Quaternary faulting. 

Because of the problems inherent in generating a map using 

data from different sources and map scales, several cautions 

need to be kept in mind: 

1. Adjacent short faults that were mapped at a fine 

scale commonly are portrayed as a single fault line with an 

average orientation. 

2. The change in fault density from one area to another 

commonly reflects the intent of the original source maps and 

may not reflect the true density of faults. For example, a 

reconnaissance geologic map may not show as many fault 

traces as a map resulting from a project specifically directed 

to recognize Quaternary faults. There was no attempt by 

Nakata and others (1982) to equilibrate the density of faults 

in areas of similar fault density but disparate mapping. 

Regional Geologic and Geophysical Maps 3 
3. Strict control over the reliability of fault information from various sources was difficult, and data were 

therefore taken at face value. Some faults only suspected of 

Quaternary movement were included for completeness. 

CONTEMPORARY SEISMICITY 

Seismic events recorded by the southern Great Basin 

seismograph network from August 1978 through December 

0 

AREA OF MAP 

1986 (fig. 2.3) range in local magnitude from about ML =0 

to 4 (using a revised magnitude scale presented by Rogers 

and others, 1986). 

The southern Great Basin seismograph network consists 

of 47 vertical-component seismographs installed during 1978 

and 1979 over a region within 160 km of the proposed 

nuclear waste repository site at Yucca Mountain (fig. 2.4). 

Data from this network provide a basis for locating and 

studying earthquakes to aid in evaluating the seismic hazard 

at the Yucca Mountain site. The network was designed to 

50 100 KILOMETERS 

Figure 2.1. Index map of southern Great Basin. Patterned areas are upland areas underlain by bedrock; unpatterned areas 

are Quaternary basins. 

4 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
cover the tectonic features suspected to be of greatest 

significance relative to seismic hazard assessment at Yucca 

Mountain (Rogers and others, 1983; Carr, 1984). 

A six -station supplemental net was deployed on Yucca 

Mountain in May 1981 in order to lower the detection 

threshold and to improve location accuracy for earthquakes 

at the proposed site. During the fmal half of 1984, horizontalcomponent instruments were deployed at stations PRN, 

GMR, EPN, GMN, YMT4, LSM, and JON (fig. 2.4). These 

serve multiple purposes, including enhanced shear-wave 

arrival time detection, magnitude estimation for larger 

earthquakes, and earthquake radiation pattern studies. 

A PDP 11/34 computer continuously digitized the 

analog data from this seismograph network and automatically 

tested for presence of earthquakes. The digital data were 

stored on magnetic tape and later analyzed on a DEC PDP 

11170 computer. Rogers and others (1987, appendix E) 

discussed the telemetry and electronics of the network and 

derived the frequency response curves for all systems used 

in the network. The combined hardware-software package, 

including high-resolution graphics display terminals, results 

in estimation of phase arrival times within 0.01 to 0.02 

second. The uniformly high station gains (84 db), combined 

with the processing tools now in use, give the network the 

capability of locally recording earthquakes having magnitudes 

as low as ML =0.0 and a region-wide sensitivity of ML = 1.0. 

Develocorder films serve as a continuous backup when the 

computer fails, due to computer problems such as tape write 

errors. The computer downtime is about 5 to 6 percent; 

however, films were scanned for these time periods, and the 

catalog is essentially complete to roughly ML = 1.0. Known 

mining blasts and nuclear tests have been removed from the 

catalog. 

Earthquake locations have a mean horizontal standard 

error of about 0.5 km and mean depth standard error of about 

1.0 km. The network spans longitude 114.6° to 117.8° W. 

and latitude 36.0° to 38.2° N. Earthquakes located outside 

this area using data from the network have larger standard 

errors than the values given above. Furthermore, the earthquake catalog is incomplete outside this area. 

TERRAIN MAPS 

Terrain Data 

The terrain map shown on figure 2. 5 was plotted from 

a U.S. Geological Survey digital data base derived from data 

obtained from the National Geodetic Survey. The National 

Geodetic Survey received these data from the Defense 

Department's Electromagnetic Compatibility Analysis 

Center, whose primary data source was the Defense Mapping Agency Topographic Center. 

The original terrain data were generated by digitizing 

contour lines, spot elevations, and stream and ridgeline elevations from 1 :250,000-scale maps, which have contour 

intervals ranging from 50 to 200ft (15 to 61 m). The Electromagnetic Compatibility Analysis Center used the Defense 

Mapping Agency Topographic Center data and data from 

other sources to obtain point elevations rounded to the nearest 

10 m for every 30 seconds of latitude and longitude, or 

approximately every 0.85 km on the ground. Godson (1981) 

transformed these 30-second point -elevation data to average 

elevations for grid cells 30 seconds on a side by taking a 

simple average of the elevations of the four comers of each 

cell. 

The 30-second geodetic coordinates of the study area 

were changed to map coordinates using an Albers equal-area 

conic projection (standard parallels 29.5° N. and 45.5° N., 

central meridian 96 o W.). Because the machine contouring 

method requires equally spaced data, elevations were determined at intervals of 2 km by using a minimum-curvature 

gridding technique. 

Slope 

The terrain-slope map in figure 2.6 was compiled using 

the following equations: 

at ti+1,j-ti-1,j 

ax 2/:u 

and 

at ti,j+ 1 - ti,j-1 

ay 2/ly 

where x is the longitudinal coordinate, y is the latitudinal 

coordinate, and ti,j is the elevation at grid point i,j. 

GRAVITY ANOMALY MAPS 

Free-Air and Bouguer Gravity Anomalies 

The following general discussion is intended to give 

the reader a sense of significance of the various types of 

gravity anomalies. It leaves out many technical details of the 

gravity reduction process. For a more complete discussion 

of these points, the reader is referred to Lambert (1930), 

Swick (1942), Dobrin (1976), and Nettleton (1976). 

Measurements of gravity are normally compared with 

the values that would theoretically be obtained if the 

measurements were made on an idealized surface (the 

reference ellipsoid) approximated by sea level. If only the 

latitude and the vertical distance of the gravity observation 

above sea level are taken into account, the difference between 

observed and theoretical gravity is known as the free-air 

anomaly; if the mass between the point of observation and 

Regional Geologic and Geophysical Maps 5 
sea level is also taken into account, then the difference is 

known as the Bouguer anomaly. To a first approximation 

the topographic mass above sea level can be regarded as a 

simple slab, or shell, but when irregularities on the upper 

surface of the slab (topography above and below the gravity 

station) are considered in the reduction, the resulting anomaly 

is designated the complete Bouguer anomaly, the standard 

anomaly for comparison with geologic features. 

Free-air anomaly maps therefore reflect the vertical 

component of attraction of topographic masses above sea 

level as well as of anomalous masses below sea level. 

Bouguer anomaly maps, on the other hand, have had the 

effects of topographic mass removed down to sea level (at 

least to a first approximation). For this reason, Bouguer 

gravity maps present a clearer picture of the distribution of 

subsurface masses, and commonly display geologic features 

more clearly than free-air anomaly maps. 

The free-air and complete Bouguer anomaly data shown 

in figures 2. 7 and 2. 8, respectively, represent subsets of 

gravity data compiled for the Basin and Range province by 

Hildenbrand and Kucks (1982), who discussed data reduction and gravity station locations in more detail. All gravity 

values are adjusted to conform to the International Gravity 

Standardization Net of 1971 (Morelli and others, 1974). 

Bouguer gravity anomaly values were computed using the 

1967 gravity formula (International Association of Geodesy, 

1967) and a Bouguer reduction density of 2.67 g/cm3. 

Gravity stations have been terrain corrected for a distance 

of0.895 km to 166.7 km from the station, using a computer 

program by Plouff (1977). 

The free-air, Bouguer, and isostatic gravity data sets 

were gridded at a spacing of 4 km using a minimum curvature 

technique. 

Separating the Regional and Residual Gravity 

Anomaly 

A Bouguer gravity anomaly map exhibits the effects 

of source bodies of various shapes, dimensions, and burial 

depths. In any region, the gravity field usually reflects the 

superposition of the overlapping gravitational effects of many 

bodies whose individual gravity anomalies may be difficult 

to separate. The terms ''residual'' and ''regional'' are 

somewhat arbitrary, depending upon the map scale and 

source bodies of interest, but are commonly used to make 

a distinction between anomalies arising from local, nearsurface masses and those arising from larger and usually 

deeper features, respectively. There are many methods for 

preparing regional and residual maps (Grant, 1972). We have 

chosen a general wavelength (or frequency) filtering method 

to obtain a separation of long-wavelength anomalies 

(regional), typically associated with deep-crustal or subcrustal 

features, from short-wavelength anomalies (residual) that are 

associated with shallow features. The separation is not complete, however, and in particular, some long-wavelength 

6 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

anomalies can overlie broad, shallow features. The shortwavelength anomalies on the residual maps emphasize the 

contributions of shallow sources, but in many cases the 

anomaly amplitudes are distorted by the removal of the long 

wavelengths (Kane and Godson, 1985). 

The gridded data were transformed to the wave number 

domain using the fast Fourier transform and then were lowpass filtered (Hildenbrand, 1983). The low-pass filter was 

a simple rectangular window (passing long wavelengths), 

modified so that the gain of the filter drops from one to zero 

along a ramp centered at the cut-off wavelength. The ramp 

was located between 100 and 150 km; the cutoff wavelength 

was 125 km. The regional (low-pass) field (fig. 2.9) was 

calculated by taking the inverse Fourier transform of the 

product of the low-pass filter and the Fourier-transformed 

Bouguer gravity field. The residual field (fig. 2.10) was 

calculated by subtracting the computed regional field from 

the unfiltered gravity field. 

The effectiveness of the wavelength filtering process 

in calculating regional and residual gravity fields is partly 

a function of the relation of the cut -off wavelength of the 

filter and the maximum depth of sources. The residual 

gravity-anomaly map (fig. 2.1 0), composed of wavelengths 

of 125 km and less, exhibits anomalies which probably are 

associated primarily with sources within the crust; the effects 

of broad, shallow sources, however, are not present on this 

map. Conversely, the complementary low-pass map composed of wavelengths of 125 km and greater (fig. 2.9) 

generally represents the effects of deeper sources, such as 

the shape of the crust-mantle boundary and anomalous masses 

in the mantle and middle and lower crust, although the contributions of any broad, shallow masses are also included. 

Isostatic Residual Gravity Map 

In areas of mountainous terrain, perhaps the most useful 

gravity map for identifying and interpreting geologic 

structures in the upper crust is the isostatic residual gravity 

map (fig. 2.11). This map is similar to the residual gravity 

anomaly map (fig. 2.10), but has the advantage of being 

based on a physical model of the crustal structure. Deviations from the model should appear as anomalies in the 

residual field, and changes in the model can be made so as 

to fit the observed field. The gravity anomalies shown on 

figure 2 .11 reflect lateral variations in subsurface density, 

primarily in the middle and upper crust. 

It is known from empirical evidence that isostatic 

equilibrium occurs to a high degree throughout most of the 

world (Heiskanen and Vening Meinesz, 1958, chapter 7) and 

particularly well in the southwestern United States (Eaton 

and others, 1978; Oliver, 1980; Saltus, 1982; Oliver and 

others, 1982). A comparison of the terrain and Bouguer 

gravity maps of the southern Great Basin (figs. 2.5 and 2.8, 

respectively) shows a marked correlation between the higher 

topography to the north and the lower Bouguer gravity 
values. Most of this regional gravity correlation with 

topography has been removed on the isostatic residual map 

(fig. 2.11) making it possible to readily see the relations of 

local gravity anomalies with major faults and seismicity. 

The crustal model used to generate the isostatic residual 

gravity map assumes perfect Airy isostasy, that is, that high 

topography is supported by deep-seated mass deficiencies in 

the form of low-density ''mountain roots'' of crustal material 

protruding into a denser upper mantle. The parameters of 

the model are the density of the high topography (P), the 

contrast in densities between the upper mantle and the lower 

crust that displaces it (WP), and the normal "sea level" 

thickness of the Earth's crust (t). The values of the parameters used to produce figure 2.11 are p =2.67 g/cm3, 

Wp =0.4 g/cm3, and t=25 km. The 2.67 g/cm3 density is 

\..._ 

representative of felsic intrusive rocks and many of the 

Paleozoic rocks that make up many of the ranges in the 

southern Great Basin. The 0.4 g/cm3 and 25 km values for 

Wp and t, respectively, are based on seismic-refraction data 

in California summarized by Jachens and Griscom (1985, 

p. 351), although current research suggests that the normal 

crustal thickness for the Basin and Range province may be 

somewhat lower (Oliver and Saltus, 1985). The effect of 

decreasing the parameter t in the isostatic model is to raise 

the centroid of the mass deficiency under areas of high 

topography, which will increase algebraically the isostatic 

residual gravity values relative to areas of low topography. 

Three-dimensional computation of the gravity effect of 

the crustal model at each of the approximately 30,000 

regional gravity stations in the southern Great Basin was 

TPU 

0 

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MTI 

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SjlRG ___./. 0 

'PK . --...._ 0 ."-

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_____ \ ____ l_ __ - CHI~~~--7\--- ,, \. 

LA!KE'1j 1

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0 50 100 KILOMETERS 

Figure 2.4. Southern Great Basin seismograph network. 

Regional Geologic and Geophysical Maps 7 
made using a FORTRAN program, "isocomp" (Jachens and 

Roberts, 1981). The 14,477 gravity stations in the Nevada 

Test Site were ''thinned'' to a minimum spacing of0.4 km 

before operating on the data set. All existing gravity stations 

in that part of the southern Great Basin within California as 

compiled by Snyder and others (1982) were used to control 

the isostatic residual map (fig. 2.11). 

MAGNETIC ANOMALY MAPS 

Total-Intensity Residual Magnetic Field 

The magnetization of crustal rocks is the vector resultant of magnetization induced in constituent minerals (chiefly 

magnetite) by the Earth's present dynamo field, plus the 

natural remanent magnetization of those minerals (Grant and 

West, 1965). Remanent or permanent magnetization acquired 

during the rock's history can have highly variable orientations. Crystalline rocks generally have sufficient magnetic 

minerals to cause variations in the Earth's magnetic field that 

can be mapped by aeromagnetic surveys. Sedimentary rocks, 

however, are generally much more weakly magnetized and 

have a small effect on the magnetic field; thus a magnetic 

anomaly map can be used to "see through" the sedimentaryrock cover and convey information on lithologic contrasts 

and structural trends related to crystalline basement (see 

Nettleton, 1971, for example). 

The magnetic anomaly data shown in figure 2.12 are 

a subset of the aeromagnetic data of the Basin and Range 

province compiled by Hildenbrand and others (1983). The 

map provides a synoptic view of major anomalies. Reference 

geomagnetic fields, that approximate the Earth's main (core) 

field, have been subtracted from the recorded magnetic data. 

The resulting residual total intensity map exhibits contour 

patterns related primarily to crustal sources shallower than 

the Curie-point isotherm (the surface within the Earth beneath 

which temperatures are so great that rocks lose their magnetic 

properties). 

The magnetic anomaly map has been compiled from 

digital data. Although these data were obtained from aeromagnetic surveys made at different times, spacings, and flight 

elevations, a consistent data set was constructed by analytical 

continuation of all data onto a common surface 3, 810 m 

(12,500 ft) above sea level. The residual magnetic anomaly 

data in figure 2.12 were projected to the Albers equal-area 

conic projection on a 2-km grid. Hildenbrand and others 

(1983) gave more details regarding data reduction. 

Accompanying the residual magnetic map are two 

filtered magnetic anomaly maps. Analysis by filtering in 

geophysical applications involves conversion of the data into 

a form that enhances particular anomaly characteristics, such 

as wavelength or trend. Two filtering operations have been 

used here: (1) a first-vertical-derivative operation to sharpen 

anomalies of shallow origin and small areal extent, and (2) 

a pseudo-gravity transformation followed by horizontal 

8 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

gradient analysis, to delimit lithologic or structural boundaries. New interpretative information may be obtained by 

studying these filtered anomaly maps (figs. 2.13 and 2.14), 

as discussed below, but they have important limitations and 

should be used with caution and only in a qualitative analysis. 

First-Vertical-Derivative Operation 

In areas of steep, broad magnetic gradients, lowamplitude and spatially restricted anomalies (related to nearsurface features) and other subtle features or trends tend to 

escape notice on the total-intensity anomaly map. This is 

especially the case for features having amplitudes less than 

25 nT, which is the color contour interval of figure 2.12. 

To resolve short-wavelength anomalies, a first-verticalderivative filter (Bhattacharyya, 1966) was applied to the 

magnetic data. The vertical derivative anomaly map shown 

in figure 2.13 thus enhances local features and reduces the 

effects of broad regional gradients. 

Pseudo-Gravity Gradient 

Cordell (1979) made use of horizontal gravity-gradient 

maxima to map graben-bounding faults. This technique, 

designed to delineate lithologic or structural boundaries, was 

later extended to the analysis of magnetic data through the 

use of the pseudo-gravity transformation (Cordell and 

Grauch, 1982). 

Gravity and magnetic anomalies that reflect a common 

source of magnetization and density contrast are related to 

each other by Poisson's equation. Baranov (1957) suggested 

using Poisson's relation to calculate a pseudo-gravity 

anomaly map from magnetic data. The transformation of the 

magnetic field to the pseudo-gravity field requires no assumption regarding a common source of magnetization and 

density. The magnetization contrast related to a source is 

simply converted to a hypothetical density contrast to take 

advantage of analysis in terms of gravity. In our calculations, 

the ratio of the rocks' magnetization contrasts to their density 

contrasts was set to a constant value (2.5 x 10-2 SI/g/cm3), 

and induced magnetization in a uniform direction (inclination 64° N., declination 16° W.) was assumed. 

Having made the pseudo-gravity transformation, the 

magnitude of the horizontal pseudo-gravity gradient g' is 

determined using the following equations: 

a g gi+ t,j- gi-t,j 

ax 2Ax 

and 

ag =gi,j+t -gi,j-t 

ay 21ly 
where x is the longitudinal coordinate, y is the latitudinal 

coordinate, and gi,J is the pseudo-gravity field defined at grid 

point i,j. 

Pseudo-gravity gradient maxima occur immediately 

over steep or vertical boundaries separating contrasting 

magnetizations. On the pseudo-gravity gradient map (fig. 

2.14), lines drawn along ridges of high horizontal gradient 

magnitudes correspond to these boundaries. If the boundaries 

have shallow dips, if remanent magnetization is strong, or 

if contributions from adjacent sources are significant, the 

maximum gradient will be shifted a certain distance from 

the uppermost part of the boundary. 

REFERENCES CITED 

Bailey, E.H., ed., 1966, Geology of northern California: California Division of Mines and Geology Bulletin 190, 508 p. 

Baranov, V., 1957, A new method for interpretation of aeromagnetic maps-Pseudo-gravimetric anomalies: Geophysics, 

v. 22, no. 2, p. 359-383. 

Bhattacharyya, B.K., 1966, A method for computing the total 

magnetization vector and the dimensions of a rectangular blockshaped body from magnetic anomalies: Geophysics, v. 31, 

no. 1, p. 74-96. 

Carr, W.J., 1984, Regional structural setting of Yucca Mountain, 

southwestern Nevada, and late Cenozoic rates of tectonic 

activity in part of the southwestern Great Basin, Nevada and 

California: U.S. Geological Survey Open-File Report 84-854, 

109 p. 

Cordell, Lindrith, 1979, Gravimetric expression of graben faulting 

in Santa Fe County and the Espanola basin, New Mexico: New 

Mexico Geological Guidebook, 30th Field Conference, Santa 

Fe County, p. 59-64. 

Cordell, Lindrith, and Grauch, V.J.S., 1982, Mapping basement 

magnetization zones from aeromagnetic data in the San Juan 

Basin, New Mexico, in Society of Exploration Geophysicists, 

Abstracts with Program, 1982, Annual Meeting: p. 246-247. 

Dobrin, M.B., 1976, Introduction to geophysical prospecting 

(3d ed.): New York, McGraw-Hill, 630 p. 

Eaton, G.P., Wahl, R.R., Prostka, H.J., Mabey, D.R., and 

Kleinkopf, M.D., 1978, Regional gravity and tectonic patterns: 

Their relation to late Cenozoic epeirogeny and lateral spreading 

in the western Cordillera, in Smith, R.B., and Eaton, G .P., 

eds., Cenozoic tectonics and regional geophysics of the Western 

Cordillera: Geological Society of America Memoir 152, 

p. 51-92. 

Godson, R.H., 1981, Digital terrain map of the United States: U.S. 

Geological Survey Miscellaneous Investigations Map 1-1318, 

scale 1:7,500,000. 

Grant, F.S., 1972, Review of data processing and interpretation 

methods in gravity and magnetics, 1964-1971: Geophysics, 

v. 37, no. 4, p. 647-661. 

Grant, F.S., and West, G.F., 1965, Interpretation theory in applied 

geophysics: New York, McGraw-Hill, 583 p. 

Heiskanen, W.A., and Vening Meinesz, F.A., 1958, The earth and 

its gravity field: New York, McGraw-Hill, 467 p. 

Hildenbrand, T. G., 1983, FFTFIL: A filtering program based on 

two-dimensional Fourier analysis: U.S. Geological Survey 

Open-File Report 83-237, 60 p. 

Hildenbrand, T .G., and Kucks, R.P., 1982, A description of colored 

gravity maps of the Basin and Range Province, southwestern 

U.S.: U.S. Geological Survey Open-File Report 82-500, 8 p. 

Hildenbrand, T.G., Kucks, R.P., and Sweeney, R.E., 1983, 

Colored digital magnetic anomaly map of the Basin and Range 

Province: U.S. Geological Survey Open-File Report 83-189. 

International Association of Geodesy, 1967, Geodetic Reference 

System 1967: International Association of Geodesy Special 

Publication no. 3, 116 p. 

Jachens, R.C., and Griscom, Andrew, 1985, An isostatic residual 

gravity map of California-A residual map for interpretation 

of anomalies from intracrustal sources, in Hinze, W .J., ed., 

The utility of regional gravity and magnetic anomaly maps: 

Tulsa, Okla., Society of Exploration Geophysicists, p. 347-

360. 

Jachens, R.C., and Roberts, C.W., 1981, Documentation of a 

FORTRAN program, 'isocomp', for computing isostatic 

residual gravity: U.S. Geological Survey Open-File Report 

81-574, 26 p. 

Kane, M.P., and Godson, R.H., 1985, Features of a pair oflong 

wavelength (>250 km) and short wavelength ( <250 km) 

Bouguer gravity maps of the United States, in Hinze, W .J., 

ed., The utility of regional gravity and magnetic anomaly maps: 

Tulsa, Okla., Society of Exploration Geophysicists, p. 46-61. 

King, P.B., and Beikman, H.M., 1974, Geologic map of the United 

States (exclusive of Alaska and Hawaii): U.S. Geological 

Survey, scale 1:2,500,000. 

Lambert, W. D. , 1930, The reduction of observed values of gravity 

to sea level: Bulletin Geodesique, no. 26, April-June, 

p. 107-181. 

Morelli, Carlo, Gantav, C., Honkasala, Tauno, McConnel, R.K., 

Tanner, J.G., Szabo, Bela, Uotila, U.A., and Walen, G.T., 

1974, The International Gravity Standardization Net 1971 

(IGSN-1971): Paris, Bureau Central de L'Association Internationale de Geodesie, Special Publication 4, 194 p. 

Nakata, J.K., Wentworth, C.M., and Machette, M.N., 1982, 

Quaternary fault map of the Basin and Range and Rio Grande 

Rift Provinces, Western United States: U.S. Geological Survey 

Open-File Report 82-579, scale 1:2,500,000. 

Nettleton, L.L., 1971, Elementary gravity and magnetics for 

geologists and seismologists: Society of Exploration Geophysicists Monograph No. 1, 121 p. 

1976, Gravity and magnetics in oil prospecting: New 

York, McGraw-Hill, 464 p. 

Oliver, H.W., ed., 1980, Interpretation of the gravity map of 

California and its continental margin: California Division of 

Mines and Geology Bulletin 205, 52 p. 

Oliver, H.W., and Saltus, R.W., 1985, Variations in new isostaticgravity maps of the Southwestern United States based in nine 

models (expanded abstract), in Kahle, H.G., and Bartholomew, J., eds., Workshop Report of the International Association of Geodesy, SSG, 5. 97, on ''Density distribution of the 

lithosphere, static and dynamic models" held May 28-31, 1985, 

at ETH University, Zurich, Switzerland: Zurich, ETH Institute 

for Geodesy and Photogrammetry Report No. 102, p. 27-28. 

Oliver, H.W., Saltus, R.W., Mabey, D.R., and Hildenbrand, T.G., 

1982, Comparison of Bouguer anomalies and isostatic residual 

gravity maps of the southwestern Cordillera (expanded 

abstract): Society of Exploration Geophysicists, 52nd Annual 

Meeting, Dallas, Technical Program Abstracts, p. 306-308. 

Regional Geologic and Geophysical Maps 9 
Plouff, Donald, 1977, Preliminary documentation for a FORTRAN 

program to compute gravity terrain corrections based on 

topography digitized on a geographic grid: U.S. Geological 

Survey Open-File Report 77-534, 45 p. 

Rogers, A.M., Harmsen, S.C., Carr, W.J., and Spence, W., 1983, 

Southern Great Basin seismological data report for 1981 and 

preliminary data analysis: U.S. Geological Survey Open-File 

Report 83-669, 240 p. 

Rogers, A.M., Harmsen, S.C., Herrmann, R.B., and Meremonte, 

M.E., 1987, A study of ground-motion attenuation in the 

southern Great Basin, California-Nevada, using several techniques for estimates of Q5 , Log A0 , and Coda Q: Journal of 

Geophysical Research, v. 92, p. 3527-3540. 

Rogers, A.M., Harmsen, S.C., and Meremonte, M., 1987, Evaluation of the seismicity of the Southern Great Basin and its 

relationship to the tectonic framework of the region: U.S. 

Geological Survey Open-File Report 87-408. 

10 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

Saltus, R. W., 1982, A description of Bouguer anomaly and isostatic 

residual colored gravity maps of the southwestern Cordillera: 

U.S. Geological Survey Open-File Report 82-839. 

Snyder, D.B., Roberts, C.W., Saltus, R.W., and Sikora, R.F., 

1982, Description of magnetic tape containing the principal facts 

of 64,026 gravity stations in the state of California: U.S. 

Geological Survey Report, 34 p.; available from the National 

Technical Information Service, Springfield, VA, NTIS-PB82-

168279. 

Swadley, W C, Hoover, D.L., and Rosholt, J.N., 1984, Preliminary 

report on late Cenozoic faulting and stratigraphy in the vicinity of Yucca Mountain, Nye County, Nevada: U.S. Geological 

Survey Open-File Report 84-788, 42 p. 

Swick, C.H., 1942, Pendulum gravity measurements and isostatic 

reductions: U.S. Coast and Geodetic Survey Special Publication 232, 82 p. 
FIGURES 2.5-2.14 
0 50 100 KILOMETERS 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 LO LO 0 LO 0 LO 0 LO 0 LO 0 LO 0 LO 0 LO 0 LO 0 LO 0 LO 0 LO 0 LO 0 

I N N (") (") o:::t o:::t LO LO <D <D I"- I"- 00 00 0> 0> 0 0 N N (") 

ELEVATION IN FEET 

Figure 2.5. Terrain map of southern Great Basin. 

12 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
0 0 0 0 0 0 N v CO CO 0 

0 50 

METERS PER KILOMETER 

Figure 2.6. Terrain-slope map of southern Great Basin. 

100 KILOMETERS 

Regional Geologic and Geophysical Maps 13 
0 50 100 KILOMETERS 

L.-J~~~~~------~ 

0 0 0 0 0 0 0 00 CO '<:t N 0 00 CD 

I I 

0 0 0 0 0 0 '<:t N N '<:t CO 

I I 

MILLIGALS 

Figure 2.7. Free-air gravity anomaly map of southern Great Basin. 

14 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

0 0 0 0 0 0 0 00 0 N '<:t CO 00 0 

N 
0 0 0 0 1..0 .q M N N N N N 

I I I I 

0 50 

0 0 0 0 0 0 0 0 ...... o en co ~"'-- co 1..0 -=t N N 

I I 

MILLIGALS 

100 KILOMETERS 

0 0 0 0 0 0 0 M N o en co ,..... 

I I I 

Figure 2.8. Complete Bouguer gravity-anomaly map of southern Great Basin. 

Regional Geologic and Geophysical Maps 15 
0 50 

N N 0 00 ~ ~ N 0 00 ~ ~ N 0 N ~ 

~ N N I I I I I I I 

MILLIGALS 

100 Kl LOM ETERS 

0 N N N N M 

Figure 2.9. Regional gravity-anomaly map (wavelengths longer than 125 km) of southern Great Basin. 

16 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
0 0 0 0 .;:t M N .,..... N N N N 

I I I I 

0 50 

0 0 0 0 0 0 0 0') 00 !"'-- c.o l.C) N 

I 

MILLIGALS 

I 

I 

I L 

I ""' \ i / 

Iii 

~~ ... ') ll l/!•0.\1 

1- ~ LAKE I '""'-. t I \J 

I 

"'· 

; ~ 

\ 

\ 

\ \ 

'-,_1 

100 KILOMETERS 

0 0 0 0 0 0 .;:t M N 0 0'> 

I 

Figure 2.10. Residual gravity-anomaly map (wavelengths shorter than 125 km) of southern Great Basin. 

Regional Geologic and Geophysical Maps 17 
0 50 100 KILOMETERS 

~ g g ~ g ~ 0 0 0 ~ g 

I I I I I I 

MILLIGALS 

Figure 2.11. Isostatic residual gravity map of southern Great Basin. 

18 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
0 50 100 KILOMETERS 

1.{) 0 1.{) 0 1.{) 0 1.{) 0 1.{) 0 1.{) 0 1.{) 0 1.{) 0 1.{) 0 1.{) 0 1.{) 0 1.{) 0 1.{) 0 1.{) 0 1.{) 0 1.{) 0 N 0 ,....... 1.{) N 0 ,....... 1.{) 

~ 0 ,....... 1.{) N 0 ,....... 1.{) N N 1.{) ,....... 0 N 1.{) ,....... 0 N 1.{) ,....... 0 N 1.{) 

'<:t '<:t (\") (\") (\") (\") N N N I I I N N N N (\") (\") (\") 

I I I I I I I I I I 

GAMMAS 

Figure 2.12. Residual total magnetic anomaly map of southern Great Basir. 

Regional Geologic and Geophysical Maps 19 
8 ~ ~ ~ ~ ~ g 0 I I I I I I 

I' 'o::t N N 

I I 

c::i oo 

I 

0 50 100 KILOMETERS 

~ N m m M o M m m N ~ oo N ~ 

I I I 

GAMMAS PER KILOMETER 

Figure 2.13. First vertical derivative of magnetic field of southern Great Basin. 

20 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
oo·"'~~~<.O 0 0 0 0 0 

0 50 100 KILOMETERS 

N M ~ 1.!') N N N N 

'PSEUDO' MILLIGALS PER KILOMETER 

Figure 2.14. Magnitude of horizontal gradient of pseudo-gravity field of southern Great Basin. 

Regional Geologic and Geophysical Maps 21 

3. Preliminary Interpretation of Seismic-Refraction 

and Gravity Studies West of Yucca Mountain, 

Nevada and California 

By Hans D. Ackermann, Walter D. Mooney, David B. Snyder, 

and Vickie D. Sutton 

CONTENTS 

Abstract 23 

Introduction 23 

Acknowledgments 24 

Geologic setting 24 

Data collection 25 

Data analysis 25 

Results 26 

Interpretation of the Crater Flat and Beatty velocity sections 27 

Velocity, density, and depth relations 30 

Possible relation of geophysical data to detachment faults 32 

Conclusions 33 

References cited 33 

Abstract 

Crustal velocity sections based on two seismic-refraction 

profiles are presented for the area west of Yucca Mountain, Nye 

County, Nevada. The north-south Crater Flat profile nearly 

parallels the west side of Yucca Mountain and extends northward 

from the Amargosa Desert valley to the northeast corner of Crater 

Flat. The Beatty profile extends westward from northern Crater 

Flat past Beatty and the Bullfrog Hills to Death Valley. 

The Crater Flat profile is interpreted in terms of six velocity 

layers ranging from 1.5 to 6.1 km/s. The interpreted prevolcanic 

surface has a relief of over 2,000 m, ranging from approximately 2.2 km below sea level (total depth below surface of 3.2 km) 

in the center of Crater Flat to approximately sea level (total depth 

of 1.3 km) beneath southern Yucca Mountain. 

Interpretation of the Beatty profile reveals an escarpment 

near the northeast edge of Bare Mountain, where Paleozoic rocks 

are probably down-faulted 2,600 m into a volcano-tectonic 

depression in Crater Flat. 

The seismic profiles and inferred density-velocity relations 

have been incorporated into an east-west gravity model from 

Death Valley to Crater Flat, corresponding to the Beatty seismic 

profile. An important feature of this model is the inferred continuity of a layer with a model density of 2.74 g/cm3 (corresponding to a seismic velocity of 6.3 km/s) from the Grapevine 

Mountains to Bare Mountain. This layer is interpreted as the 

lower plate of a regional decollement or detachment fault. 

INTRODUCTION 

This report presents the velocity structure derived from 

two crustal seismic-refraction profiles recorded west of 

Yucca Mountain in 1983. These profiles, which we have 

named the Beatty and Crater Flat profiles (fig. 3.1), represent part of a continuing program of seismic investigations 

in the region of Yucca Mountain. 

Previous seismic investigations in the region consisted 

of three reconnaissance refraction spreads east of Yucca 

Mountain (Pankratz, 1982), which had a maximum penetration depth of about 600 m; a reconnaissance ''highresolution'' reflection survey by the Colorado School of 

Mines (Barry, 1980); two industry reflection surveys along 

the east flank of Yucca Mountain (McGovern, 1983); an 

engineering refraction survey (unpublished) by H.D. Ackermann along the east edge of Yucca Mountain; and an 

unreversed reconnaissance crustal refraction proftle (Hoffman and Mooney, 1983), which crossed Yucca Mountain 

at the proposed repository site. With the exception of the 

Hoffman and Mooney (1983) profile, none of the surveys 

produced results that could be interpreted in terms of 

stratigraphic relations at Yucca Mountain. The three reflection surveys failed to record a single coherent reflection 

event, attesting to the high degree of variability and poor 

sound transmission properties of the shallower rocks in the 

area. These negative results were further complicated by the 

shallow refraction studies, which demonstrated not only the 

inherent difficulty of producing and recording coherent sound 

signals in the volcanic rocks, but also their high degree of 

lateral variablity. 

On the other hand, good upper-crustal refraction 

returns were obtained from the reconnaissance survey by 

Hoffman and Mooney (1983) and from the work leading to 

this report. These results formed the basis for additional deep 

refraction studies in the Yucca Mountain region conducted 

in 1985, which are not reported here. 

In their interpretation of the unreversed profile, Hoffman and Mooney (1983) relied upon inferred relations 

Seismic-Refraction and Gravity Studies 23 
between seismic velocities and rock densities derived from 

the gravity modeling (Snyder and Carr, 1982) to compute 

depths to the prevolcanic surface beneath Crater Flat. The 

detailed gravity models have also provided an excellent 

framework for the interpretation and discussion of the present 

refraction results. 

An objective of the present survey was to improve on 

the earlier depth estimates by using reversed refraction profiles. The results also suggest some revisions of the gravity 

models. 

Acknowledgments 

We wish to thank our U.S. G. S. colleagues for their 

valuable critical comments on this paper. The data analyzed 

in this paper were collected by the authors and G. Bloomberg, 

M. Burns, B. Colburn, T. Conway, E. Criley, B. Echols, 

L. Greene, R. Kaderabek, P. Meador, J. Murphy, and 

L. Pace. We thank all of them for their diligent field efforts. 

Death 

Valley 

EXPLANATION 

11roo· 

Grapevine 

Mountains ,~' 0q,. (;;:- , 

(;;:- -Ao 

0.::;} (;;:-

Qj ()q,. 

' 

GEOLOGIC SETTING 

A brief discussion of the geologic framework pertinent 

to this report follows. A more thorough review is provided 

by Snyder and Carr (1984) and Carr (1984). 

The study area lies within the southern part of the Great 

Basin, a large structural and physiographic section of the 

Basin and Range province. Whereas much of the Great Basin 

is characterized by linear, fault-bounded ranges separated 

by elongate deep structural basins of late Cenozoic age, most 

of the area considered here, between Death Valley and Yucca 

Mountain, does not contain well-developed typical basinrange structure. Instead the area is one of diverse structural 

style, trends, and topography, called the Walker Lane belt 

or Walker belt by some authors (Stewart, 1980; Carr, 1984). 

The oldest rocks exposed in the region are partially 

metamorphosed sedimentary rocks of Proterozoic age, 

mainly on Bare Mountain and in the Funeral Mountains 

(fig. 3.1). These rocks, which are mostly quartzite and other 

mildly metamorphosed clastic rocks, are overlain by a very 

116"30' 

Timber Mountain - Oasis 

;¢.. ' 

e 

19 Shotpoint 

N b 

location -

. d f. 

"'~ &_,.. 

<$)/ 

~ um ers referred to m text an 1gures % , 

I 

• • •••• Seismic profile line 

0

'-'-? ' """ 

A A' Gravity profile line /'<$)/hiS' """ 1:. 

0 ,~t­ Drill hole location '1(10.' -1o 

Caldera margin 

........,__.,.. Outline of upland area - hachures point downhill 

0-'9.~ 

~-1""" 10 KILOMETERS 

' 

Figure 3.1. Map of study area showing locations of seismic and gravity profiles (Snyder and Carr, 1982) and shotpoint and drill 

hole locations. Drill hole 1, USW VH-1; 2, UE-25P1; and 3, US Borax. 

24 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
thick section of Paleozoic, largely carbonate sedimentary 

rocks. In some areas, as in the northern part of Bare Mountain, these carbonate rocks are thrust over middle Paleozoic 

clastic rocks (Eleana Formation). Granite of Mesozoic age 

crops out at the northwest corner of Bare Mountain, and 

similar granites may be present in the subsurface in an eastwest belt from the Grapevine Mountains to east of Yucca 

Mountain (Carr, 1984, fig. 13). 

Locally thick Tertiary rocks lie unconformably above 

the Paleozoic rocks in most of the area. Oligocene rocks in 

the Grapevine Mountains consist mainly of conglomerate and 

other sedimentary rocks, as well as minor tuff. Most of the 

volcanic rocks of the Bullfrog Hills and north and east of 

Bare Mountain are tuff and subordinate lava flows of 

Miocene age. Yucca Mountain and Crater Flat contain thick 

(greater than 2,000 m) sections of welded tuff. The youngest 

volcanic rocks are scattered rhyolite and basalt lavas of late 

Miocene age and small basalt flows of Pliocene and Quaternary age in and near Crater Flat. Alluvium of Pliocene and 

Quaternary age is relatively thin (less than 500 m) in most 

of the area. 

Several Miocene calderas are present. Shotpoint 11 

(fig. 3.1) is at the south edge of the large Timber MountainOasis Valley caldera complex (Byers and others, 1976). 

Another slightly older group of calderas, or a volcanotectonic depression, may be present beneath Crater Flat and 

possibly the northern part of Yucca Mountain (Carr and 

others, 1986; Carr, 1982). 

The pre-Tertiary and Tertiary rocks are moderately 

faulted in most areas. North of Bare Mountain and to the 

west in the Bullfrog Hills, the structure in the volcanic rocks 

is dominated by complex northeast- and northwest-striking 

faults, many of which have had some strike-slip displacement and may be listric, that is, level off at depth. The middle 

and southern parts of Yucca Mountain are cut by westdipping, north-south-striking basin-range faults. The structure within Crater Flat may be controlled by largely buried 

caldera ring faults; the major fault on the east side of Bare 

Mountain is probably part of this system (Carr, 1982). 

Of particular interest with respect to this report is the 

presence at Boundary Canyon in the Funeral Mountains 

(fig. 3.1) of a low-angle detachment fault (Giarmita and 

others, 1983). This structure separates metamorphosed lower 

plate, mostly Proterozoic rocks, from overlying relatively 

unmetamorphosed Paleozoic rocks. This structure probably 

continues northeastward into the Bullfrog Hills and possibly 

farther to the east. 

DATA COLLECTION 

Both profiles reported here were collected in April 

1983. The Crater Flat profile (fig. 3.1) extends northward 

from the Amargosa Desert valley, obliquely across the 

southern tip of Yucca Mountain, along the east side of Crater 

Flat, and to a point near the northeast corner of Crater Flat. 

Seven shotpoints, numbered 11 through 17, were located at 

approximately 8-km intervals along this proflle. Shotpoint 11 

lay beyond the array of seismographs about 9 km north of 

shotpoint 12. Each shot consisted of approximately 900 kg 

of ammonium nitrate emplaced in 50-m drill holes. The 

46-km-long profile length was designed to provide details 

of the crustal structure to a maximum depth at least as great 

as the prevolcanic rocks, estimated to be as much as 4± 1 

km from gravity data. 

The Beatty profile (fig. 3.1) extends west from shotpoint 12 of the Crater Flat profile, past the north edge of 

Bare Mountain to the Bullfrog Hills. Three shots with an 

average spacing of 14 km were fired along the profile. They 

are numbered 12, 18, and 19, and had the same drill-hole 

depths and charge size as those on the Crater Flat profile. 

In addition, reconnaissance measurements were made by 

placing seismographs as far west as Death Valley along the 

Titus Canyon road. Thus, the portion of the seismic profile 

between the Bullfrog Hills and northern Crater Flat is 

reversed, but the part between the Bullfrog Hills and Death 

Valley is unreversed. 

Data for each profile were recorded by 120 portable 

seismographs of recent design (Healy and others, 1982). 

These instruments are equipped with vertical-component 

seismometers having a natural frequency of 2 Hz, and the 

data were recorded in frequency modulated format on cassette 

tapes. Analog tapes were digitized in the U.S. Geological 

Survey laboratory at Menlo Park, Calif., at a sampling rate 

of 200 samples per second. 

The principal facts for these profiles ( shotpoint and 

recorder locations, timing, instrumentation, and so forth) 

were reported by Sutton (1984). 

DATA ANALYSIS 

Many methods are available for processing seismicrefraction data; their theoretical bases are well described in 

a number of texts (see Grant and West, 1965; Telford and 

others, 1976). Some of these techniques involve essentially 

trial-and-error fitting of the observations by successive model 

calculations, a process that can be as tedious for the interpreter as it is arbitrary in the final result obtained. To avoid 

the inherent uncertainties of trial-and-error model fitting, 

these refraction profiles were recorded with field parameters 

satisfying the requirements of a method for the direct computer inversion of the data (Ackermann and others, 1982, 

1983). The primary requirements are that there be close 

spatial sampling of the data and multiple shotpoints along 

the profile. 

The complete seismic record sections obtained in this 

study were presented without analysis by Sutton (1984). For 

the present analysis, arrival times were picked from expanded 

record sections to improve timing accuracy. Samples of the 

data at this scale are shown in figure 3. 2. Arrival times, complete traveltime curves, smoothed elevations, and surface 

Seismic-Refraction and Gravity Studies 25 
velocities for all shots and recorders were entered into computer data files for use in final processing. 

RESULTS 

Complete analysis of refraction data consists of two 

stages, data processing and geologic interpretation. Processing is a problem of geometrical ray tracing, done with 

minimum regard to geology and resulting in velocity sections 

indicating layers having velocities that may vary laterally. 

The processed velocity sections for the Crater Flat and Beatty 

profiles, at 6.2 times vertical exaggeration and with no 

vertical exaggeration, are shown in figures 3.3 and 3.4. The 

C/) 

0 

z 

0 

() 

w 

C/) 

z 

~ 1.0 

to 

w 

() 

:::J 

0 

w 

a: 

upper boundary (or horizon) of each layer represents the 

calculated path (or portion of the path) of a critically refracted 

ray, and the varying layer velocities represent lateral changes 

in velocity along a particular ray path. Seismic horizons need 

not conform with geologic horizons. Instead, they represent 

minimum time paths, which can pass from one geologic 

regime into another, and hence can be represented on a 

processed velocity section only as a change in the velocity 

within a layer. A case in point is shown in figure 3. 3 at the 

boundary between southern Yucca Mountain and Crater Flat. 

Here the velocity of the fifth layer changes from 4. 8 to 

5.4 km/s, which, from geologic interpretation, probably 

represents a lateral change from volcanic into prevolcanic 

rocks. 

0 

20 19 18 17 16 15 

DISTANCE FROM SHOTPOINT, IN KILOMETERS 

Figure 3.2. Seismic traces from shotpoint 17 on Crater Flat profile, plotted with reduced traveltime of 6.0 km/s. Small dashes 

on traces indicate first arrival picks. See figure 3.1 for location. 

26 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
INTERPRETATION OF THE CRATER FLAT AND 

BEATTY VELOCITY SECTIONS 

The velocity sections (figs. 3.3 and 3.4) show horizons 

as discrete boundaries at which an abrupt increase in layer 

velocity occurs. However, analysis of velocity surveys in 

boreholes in the Yucca Mountain area show that, although 

abrupt changes in velocity do occur in the subsurface, the 

velocity functions are best described by smooth curves rather 

than discrete breaks. Furthermore, analysis of borehole densities in the Tertiary tuff sequence (Snyder and Carr, 1982, 

1984) show a downward increase in density mainly related 

to closing of pore spaces rather than to primary density 

variations in the tuff. Hence, we interpret, with reasonable 

confidence, that the discrete layers shown in the velocity sections do not necessarily represent true layering in a physical 

or geologic sense, but instead are our best representation of 

a velocity function gradually increasing with depth due to 

closing of fractures and pore spaces related to the depth of 

burial. 

Two holes have been drilled near the line of sections, 

USW VH-1 (762 m total depth), 2 km west of the Crater 

Flat profile (figs. 3.1 and 3.3), and the US Borax hole (323 m 

total depth) along the Beatty proftle (figs. 3.1 and 3.4). Both 

holes are relatively shallow compared to depths obtained from 

the present survey, and neither penetrated the prevolcanic 

surface. 

Drill hole USW VH-1 (fig. 3.3) penetrated principally 

welded tuff below 155m (Carr, 1982) and bottomed in the 

densely welded Bullfrog Member of the Crater Flat Tuff. 

A downhole velocity survey depicts a ragged curve, which 

we have approximated by three layers. The upper two have 

average velocities of 1.9 and 3.0 km/s, which are somewhat 

higher than the 1.5 and 2.5 km/s velocities of layers 1 and 

2 (fig. 3.3). Layer 3 of the velocity survey, which begins 

70 m deeper than layer 3 of figure 3. 3, has an average 

velocity of 3. 6 km/ s, which is the same as that shown in the 

velocity section. Clearly, a closer match between velocities 

would be preferred in the very shallow section. Part of the 

discrepancy may be attributed to the assumed eastward dip 

of the Tertiary rocks at the location of shotpoint 14 (W .J. 

Carr, USGS, written commun., 1985), and the local presence 

of 20m of dense basalt in drillhole USW VH-1. Furthermore, analysis of shallow seismic-refraction data and velocity 

logs at Yucca Mountain shows extreme variability in the 

velocity of the shallow volcanic rocks, and values obtained 

at a single drill hole may not be representative of an area. 

Hence, average values obtained over a broad area by refraction methods may be more representative than values 

obtained at a specific point location. 

On the Beatty profile (fig. 3.4),- Paleozoic rocks occur 

near the surface west of shotpoint 18. The 4.8 km/s velocity 

in the surface layer, therefore, indicates that these rocks are 

fractured, but that fractures are essentially closed or much 

less abundant at depths ranging between 400 and 700 m, 

where velocities reach 5. 7 km/ s. East of shotpoint 18, the 

seismic line runs along the north foot of Bare Mountain, 

entirely within tuff, but generally parallel to a major fault 

that dips northward and drops the tuff down along the north 

edge of Bare Mountain. The US Borax hole east of shotpoint 

18 (fig. 3.4) bottomed at 323m in rhyodacite lava under the 

thick, welded Tram Member of the Crater Flat Tuff (W .J. 

Carr, USGS, written commun., 1985), within the seismic 

layer of 4.7 km/s velocity. However, projection of the 

northward dip of the fault at the north edge of Bare Mountain suggests that the prevolcanic surface may also lie within 

this velocity layer. Our interpretation, indicated by the 

queried stippled region in figure 3.4, assumes that fractured 

prevolcanic rocks of approximately 4. 7 km/s lie beneath the 

welded tuffs, and that fractures are closed at depths below 

approximately 300 to 1,300 m, where velocities are between 

5.7 and 5.9 km/s. 

The intrinsic seismic velocity of crystalline and other 

dense rocks devoid of cracks is approximately 6 km/s; joints, 

fractures, and weathering tend to lower this value. Beneath 

Crater Flat (fig. 3.3) the approximately horizontal layers 1 

through 4 show the increase in velocity with depth in the 

thick sequence of volcanic rocks. Layer 5, of 4. 7 to 4. 8 km/s 

velocity, begins at a depth of approximately 1,600 m, and 

continues to a maximum depth of approximately 3,200 m 

beneath shotpoint 14. The prevolcanic surface may lie within 

this layer; however, we believe that confining pressures at 

these depths should be sufficient to close most cracks and 

pores and raise velocities of a hypothetical prevolcanic surface in this layer to well above 5 km/s. Thus, we interpret 

the 4.7 to 4.8 km/s layer beneath Crater Flat as comprising 

the deeper portion of the volcanic section filling the Crater 

Flat depression and the pre-volcanic rock surface (shown by 

solid squares) as being represented by the basal 5. 7 to 6.1 

km/s layer. 

At the south boundary of Crater Flat (fig. 3.3), the 

velocity of layer 5 changes from 4.8 km/s beneath Crater 

Flat to 5.4 km/s beneath southern Yucca Mountain, accompanied by a rise in elevation of approximately 600 m. This 

velocity change implies a corresponding lateral change in 

rock properties, which we suggest represents a transition 

from the volcanic rocks beneath Crater Flat to shallower, 

prevolcanic rocks beneath southern Yucca Mountain. Thus, 

the prevolcanic surface appears to transect layer 5 and emerge 

as the top of layer 5 beneath southern Yucca Mountain. Here 

its velocity (4.9 to 5.4 km/s) is less than beneath Crater Flat 

(5.7 to 6.1 km/s), perhaps because of less lithostatic load 

on the pre-Tertiary rocks at southern Yucca Mountain. This 

interpretation, of relatively high-standing prevolcanic rocks 

below southern Yucca Mountain, agrees remarkably well 

with the gravity model of Snyder and Carr (1982) along their 

section A-A' (see fig. 3.1), which is perpendicular to the 

structural trend. Their gravity model shows an abrupt rise 

of the prevolcanic surface at the intersection with the seismic 

proftle, rising from 2,000 m below sea level to 400 m below 

sea level, as compared to our interpretation (at approximately 

20° to the structural trend) of a more gradual apparent rise 

Seismic-Refraction and Gravity Studies 27 
s N 

3 

Northwestern Southern Beatty 

Amargosa Desert Crater Flat (.) Yucca Mountain Wash Yucca Mountain I 

(.) 

C\1 ,... 2 ('t) ,... ,... 

co ,... a.. 

a.. en 

(j) ""' 

co I en 

c: ,... ,... co 

w 

I- I 

w C\. 

~ I 

0 I 

...J 3.6 3.9 ® / 

~ C\. 

Total depth 762 meters I 

z 0 4.1 I 

I 

z 4.1 @) I ""4.7 0 C\. 

I- I 

<( r}-c' ~ > -1 -6· 

w -c 

...J '1" -c w .... 

-2 

Vertical exaggeration 6.2 

-3 

-28 -24 -20 -16 -12 -8 -4 0 4 8 12 16 20 24 28 

DISTANCE, IN KILOMETERS 

N 

-24 -20 -16 -12 -8 -4 0 4 8 12 16 20 24 28 

DISTANCE, IN KILOMETERS 

Figure 3.3. Crustal velocity section along Crater Flat profile at 

6.2 times vertical exaggeration (top) and at true scale (bottom). 

Velocities (decimal numbers) are in kilometers per second. 

Inferred prevolcanic surface is shown by solid squares and zones 

of uncertainty by stipple pattern, queries, and dashed lines. Also 

28 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

shown are shotpoints (SP), layer numbers (circled), intersections 

with gravity profiles A-A', B-B; and C-C' (fig. 3.1), and projection of drill hole USW VH-1. Dots show approximate location 

of lateral changes in velocity. 
w E 3~--~----~--~--~----~--~----~--~----~--~----r----r--~r---~--~ 

Death 

Valley 

Grapevine 

Mountains Bullfrog Hills 

North edge of 

Bare Mountain 

North edge 

of 

Crater Flat 

C\1 

(f) 

a: 

w 

1-

w 

~ 

0 

_J 

~ 

z 

z 

0 

1-

2 

0 

<( -1 

> 

w 

_J 

w 

-2 

6.3 

"' ~ 

5.7 

* (6.2-6.4) 

5.9 

Vertical exaggeration 6.2 

~ 

a.. 

en 

-3----~----~--~--~----~--~----~--~----~--~----~--~--~~--~----

-44 -40 -36 -32 -28 -24 -20 -16 -12 -8 -4 0 4 8 12 16 

DISTANCE, IN KILOMETERS 

(f) 

a: w E 

w 10 1- X w 8 ell ~ I... 

0 6 "' _J co~ ~ 

~ 4 a.. Q..Cf) 

en en~ 

z 2 

z 0 · ······ ····· · 

0 

1- -2 No vertical <( 

> -4 

-44 -40 -36 -32 -28 -24 -20 -16 -12 -8 -4 0 4 8 12 16 

~ DISTANCE, IN KILOMETERS 

' Figure 3.4. Crustal velocity section along Beatty profile at 6.2 

I

, times vertical exaggeration (top) and at true scale (bottom). 

Velocities (decimal numbers) are in kilometers per second. 

!Inferred prevolcanic surface shown by solid squares and zones 

of uncertainty by stipple pattern, queries, and dashed lines. Also 

shown are shotpoints (SP), layer numbers (circled), and projection of drill hole US Borax. Dots show approximate location of 

lateral changes in velocity. Portion of profile from Crater Flat 

to Bullfrog Hills has reversed seismic coverage; that from Bullfrog 

Hills to Death Valley is unreversed. Shotpoint 12 (fig. 3.1) is 

common with Crater Flat profile. Star and double arrow indicate 

possible range in depth to a 6.2 to 6.4 km/s layer (Hoffman and 

Mooney, 1983). 

Seismic-Refraction and Gravity Studies 29 
from 1,600 m to 300 m below sea level. Furthermore, 

stratigraphic and structural data (Robert B. Scott, ill, USGS, 

written commun., 1984) from the surface in southern Yucca 

Mountain also suggest a topographic high predating the 

deposition of the Paintbrush Tuff, which may reflect a high 

on the prevolcanic rock surface. 

Southward on the Crater Flat profile, from southern 

Yucca Mountain into the Amargosa Desert valley, layer 5 

decreases in velocity to 4.7 km/s and in depth to about 700 m 

at shotpoint 17. Although a 4. 7 km/s velocity appears possible for Paleozoic or crystalline rocks at such a shallow depth, 

the actual rock type represented is uncertain. Nevertheless, 

it has been labeled with queried solid squares, suggesting 

a southward rise of the prevolcanic surface beneath the 

Amargosa Desert valley. 

Geophones were not emplaced across northwestern 

Yucca Mountain beyond shotpoint 12. Therefore the shallow 

velocity structure beneath this part of the profile could not 

be determined. However, analysis of the travel times from 

horizons 5 and 6 for shotpoint 11 in Beatty Wash, recorded 

in Crater Flat, shows that velocities both beneath shotpoint 

11 and the deeper part of the section beneath northwestern 

Yucca Mountain are significantly higher than within Crater 

Flat. The corresponding decreased delay times may be 

accounted for by projecting horizon 5 from below shotpoint 

12 to near the surface at shotpoint 11 with a velocity of 

approximately 4. 7 km/s, and projecting a corresponding rise 

of the prevolcanic surface beneath northwestern Yucca 

Mountain, shown by queried dashed lines in figure 3.3. Shotpoint 11 lies just inside the Timber Mountain-Oasis Valley 

caldera complex (fig. 3.1). The results from this single shotpoint suggest a marked velocity increase from Crater Flat 

northward into the Timber Mountain area. 

Gravity modeling (Snyder and Carr, 1982) along section C-C' (fig. 3.1) indicates that volcanic rocks could extend 

as deep as 5, 000 m beneath the surface within Crater Flat. 

Snyder and Carr also discussed a three-dimensional gravity 

model with a prevolcanic rock surface at approximately 

3,500 m depth (2,500 m below sea level). We suggest that 

the latter model is the more accurate given the present seismic 

interpretations (fig. 3.3), which indicate that volcanic rocks 

do not extend beyond 3,200 m depth. 

The unreversed seismic-refraction profile by Hoffman 

and Mooney (1983) is approximately coincident with the 

gravity traverse B-B' (fig. 3.1) of Snyder and Carr (1982), 

and was interpreted with reference to their two dimensional 

gravity model. Hoffman and Mooney concluded that a layer 

with a velocity of 5.7 km/s occurs at a depth of 2,200 m 

below sea level in central Crater Flat and interpreted it to 

represent prevolcanic rocks on the basis of its high velocity. 

The present Crater Flat profile crosses their earlier profile 

between shotpoints 13 and 14, where we have independently 

interpreted the same depth to a prevolcanic rock surface with 

a velocity between 5. 7 and 6.1 km/s. 

The salient feature of the Beatty profile (fig. 3.4) is 

the large displacement of the interpreted prevolcanic surface, 

30 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

marked by stipple, at the boundary between the north edge 

of Bare Mountain and the Crater Flat depression. Approximately 6 km to the east, near shotpoint 12, interpretation 

of the seismic profiles places the prevolcanic rock surface 

at a depth of about 2,600 m. The total fault displacement 

at this location is indeterminate due to complicating factors 

introduced by the intersecting east-west-trending fault 

discussed earlier, at the north foot of Bare Mountain. 

Approximately 3 km to the south, however, Paleozoic rocks 

at Bare Mountain are exposed at the surface, and the depth 

to the interpreted prevolcanic surface remains approximately 

the same. Therefore, we infer approximately 2,600 m of 

Tertiary(?) fault displacement near the northeast edge of Bare 

Mountain. 

The portion of the Beatty profile between Death Valley 

and the Bullfrog Hills (fig. 3.4) is unreversed. Therefore, 

layer velocities and depths are poorly constrained and 

numerous solutions are possible. For example, data from the 

indicated 4.8 and 5.7 km/s layers were recorded only within 

6 km west of shotpoint 19, due in part to the large offset 

(fig. 3.1) of the seismic line there. The boundary (dashed 

line, fig. 3.4) between these two layers has simply been 

extrapolated from shotpoint 19 west to Death Valley. 

Although velocities and depths in the Grapevine Mountains area remain highly speculative, two conclusions can 

be drawn with certainty. The first is the existence of a large 

thickness of low-velocity material at the boundary between 

the Grapevine Mountains and Death Valley, shown in figure 

3.4 by the abrupt termination of the 5. 7 km/s horizon. The 

second is a layer shown beneath the Grapevine Mountains, 

whose approximate velocity is 6.3 km/s. The possible continuation of this layer northeastward beneath the Bullfrog 

Hills could not be determined without additional shotpoints. 

Choosing alternate ray paths permits this 6.3 km/s layer to 

be moved up beneath the Grapevine Mountains by as much 

as 500 to 1,000 m, indicated in figure 3.4 by the two queried 

arrows pointing upward. A shallower depth, however, would 

require a significant downward displacement or disappearance of the 6.3 km/s layer beneath the Bullfrog Hills, but 

Hoffman and Mooney (1983) did identify a seismic layer of 

6.2-6.4 km/s at a depth 1.0-2.4 km below sea level beneath 

Beatty, as shown in figure 3. 4. Several additional shotpoints 

would have helped to resolve the problem of depth to the 

6.3 km/s layer beneath the entire Beatty profile. 

VELOCITY, DENSITY, AND DEPTH RELATIONS 

The relations of density and velocity vs depth, shown 

in figure 3. 5, were obtained from borehole gravity measurements and gamma-gamma logs in drill hole USW H 1, located 

at the proposed repository site on Yucca Mountain (Snyder 

and Carr, 1982,, 1984). It shows a linear increase in density 

with depth of 0.26 g/cm3 per kilometer. A similar gradient 

and lithologic sequence is assumed in Crater Flat. The 

layered velocity distribution at shotpoint 14 of the Crater Flat 
profile, shown by the step functions, has been roughly 

approximated on the assumption of a more uniform increase 

in velocity with depth by three linear segments labeled A, 

B, and C with respective gradients of 6, 0. 9, and 0.4 km/ s 

DENSITY, 

IN GRAMS PER CUBIC CENTIMETER 

2.0 2.5 3.0 

0 

0.5 

1.0 

(/) 

a: 

w 

Iw 1.5 ~ 

0 

_J 

~ 

z 

I 2.0 Ia.. 

w 

0 

2.5 

,, 

0 

3.0 

3. 5 ......__.~...-_.~...-_....___....___...._____.....__ .......... 

0 1 2 3 4 56 7 

VELOCITY, IN KILOMETERS PER SECOND 

Figure 3.5. Plots of velocity and density against depth. Density 

plot is from Snyder and Carr (1982, 1984). Velocity step functions are from Crater Flat portion of figure 3.3. Line segments 

A, B, and C are smoothed representations of step functions. 

per kilometer. The gradient of segment C is constrained on 

the assumption of a possible maximum velocity of 5.2 km/s 

in the volcanic section, suggested by a 4, 170-m-deep borehole (UE20f, not shown) in the Silent Canyon caldera (Pahute 

Mesa) that penetrated 468 m of lava and welded rhyolitic 

tuff in the deepest part of the hole having a maximum velocity 

of 5.1 km/s and bulk density of 2.56 g/cm3 (Carroll, 1966). 

The decrease in gradient of the velocity curve at 1,750 m 

depth, which also marks the greatest depth of measured 

borehole densities, suggests a similar decrease in the gradient 

of the density curve. 

In a plot of velocity against density (fig. 3.6), which 

may be representative of the Yucca Mountain region, 

segments A and B, obtained from the parametric plots in 

figure 3.5, represent the welded tuff. The point Pl (velocity 

6.4 km/ s, density 2. 7 5 g/ cm3) was obtained from logs in 

drill hole UE-25Pl (fig. 3.1) (Muller and Kibler, 1984) and 

is representative of 661 m of Paleozoic dolomite penetrated 

at depths below about 1,200 m. We have connected point 

P 1 with segment B in figure 3. 6, and suggest that this part 

of the density-velocity plot roughly approximates both the 

prevolcanic and the volcanic rocks having velocities greater 

than 4. 7 km/s. The point PM (velocity 5.1 km/s, density 2.56 

g/ cm3) from the deep drillhole UE20f at Pahute Mesa, plots 

slightly above this line. 

No previous density determinations have been made 

for the prevolcanic rocks beneath Crater Flat here interpreted 

to have velocities between 5.7 and 6.1 km/s. Densities 

between 2. 64 and 2. 70 g/ cm3 can be extracted from the 

density-velocity plot (fig. 3.6). These values may represent 

approximate typical density values for deeply buried 

a: 2.8 w 

1-

w 

:2 

i= 

z 

~ 2.6 

() 

in 

:::> 

() 

a: 

w 

c.. 

(/) 

:2 

<( 

a: 

~ 

~ 

-;.: 

len 

z 

~ 2.0 

P 1 (velocity 6.4 km/s; density 2. 75 g/cm3) 

,)( 

rocks beneath Crater Flat 2.64 ~ 

Density range for prevolcanic { 2.70 711" 

PM(velocity 5.1 km/s; ,'' 1 

density 2.56 g/cm3 ) ,/< ~ 5.7 6.1 

Velocity range for 

pre volcanic rocks 

beneath Crater Flat 

2 3 4 5 

VELOCITY, IN KILOMETERS PER SECOND 

7 

Figure 3.6. Plot of velocity against density. Segments A and B 

derived from figure 3.5. Point P1 obtained from prevolcanic rocks 

in drill hole UE-25P1 (fig. 3.1) on east side of Yucca Mountain, 

and point PM from volcanic rocks in drill hole UE20f (not shown) 

on Pahute Mesa. 

Seismic-Refraction and Gravity Studies 31 
prevolcanic rocks in this region. On the other hand, densities 

between 2. 7 and 2. 8 g/ cm3 were required for the rocks in 

Bare Mountain (Snyder and Carr, 1982) to satisfy gravity 

data, which implies that the Paleozoic rocks composing the 

core of Bare Mountain are different from the prevolcanic 

rocks beneath Crater Flat. However, differences may be due 

to a disordered state of the prevolcanic rocks under Crater 

Flat rather than to any lithostratigraphic difference. 

On the basis of the conclusions discussed above, a new 

gravity model (fig. 3.7) has been constructed from Death 

Valley to Crater Flat that approximates the Beatty seismic 

profile (fig. 3.4). Densities were selected to match the 

velocity values suggested by figure 3.6. This gravity model 

indicates that the 6.3 km/s (2. 74 g/cm3) layer dips eastward 

from the Grapevine Mountains to Bare Mountain at depths 

from 1,000 m above to 1,500 m below sea level. Thus, the 

gravity model suggests continuity between the 6.2-6.4 km/s 

layer beneath Beatty (Hoffman and Mooney, 1983) and the 

6.3 km/s layer beneath the Grapevine Mountains described 

in this study. 

POSSIBLE RELATION OF GEOPHYSICAL DATA TO 

DETACHMENT FAULTS 

A low-angle fault or detachment surface is exposed 

about 10 km south of the Beatty profile in the Grapevine 

Mountains at an elevation of about 1,000 m above sea level 

(Monsen, 1983; Giarmita and others, 1983). This structure, 

called the Boundary Canyon fault, juxtaposes a highly fractured Paleozoic section upon a metamorphosed Precambrian 

and lower Paleozoic section. The seismic and gravity interpretations presented here suggest the possibility that this fault 

surface may correspond to the 6.3 km/s, 2. 74 g/cm3 horizon 

and continue east of the Grapevine Mountains beyond the 

Bullfrog Hills area as far as the fault separating Bare Mountain and Crater Flat. 

20~----------~----------~------------~----------~-----------, 

(f) 

__J 

<( 0 <.9 

__J 

__J 

~ 

z -20 

>-

__J 

<( 

~ -40 0 

z 

<( 

-60 

(f) 

a: 2 w 

Iw 

~ 1 

0 

__J 

~ 0 

z 

z -1 

0 

I- -2 <( 

> 

w 

__J -3 

w -20 

EXPLANATION 

• Calculated 

0 Observed 

Death Valley 

2.2 

2.67 

0 

2.67 

2.74 

Vertical exaggeration 6.2 

20 40 

DISTANCE, IN KILOMETERS 

2.30 

2.67 

60 80 

Figure 3.7. Two-dimensional gravity model approximately coincident with seismic-refraction section in figure 3.4. Values are 

in grams per cubic centimeter and indicated bodies are assumed homogeneous. Densities from figure 3.6. Observed gravity values 

from Healey and others (1980). Dashed lines show velocity section of figure 3.4; shotpoint, SP. 

32 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
CONCLUSIONS 

The interpretation of seismic-refraction velocity sections indicates that volcanic rocks have a maximum thickness 

of about 3,200 min Crater Flat (fig. 3.3), filling a depression interpreted by Snyder and Carr (1982, 1984) and Carr 

and others (1986) as a caldera complex. The depression is 

25-30 km long in a north-south direction and about 20 km 

wide east-west. The prevolcanic rock surface exhibits some 

2,000 m of vertical relief between the south end of Yucca 

Mountain and the center of Crater Flat. A fault on the east 

side of Bare Mountain (fig. 3.4) downdrops exposed 

Paleozoic rocks about 2,600 m into Crater Flat. 

The seismic velocities of the prevolcanic rocks are 

between 5. 7 and 6.1 km/ s within the deeper parts of the 

Crater Flat depression, where lithostatic loading has closed 

fractures. At shallower depth, velocities of the prevolcanic 

rocks may be less, possibly as low as 4. 7 km/s within 

approximately 1,000 m of the surface. 

The velocities in the volcanic section beneath Crater 

Flat reach 4. 7 to 4.8 km/s at a depth of approximately 

1,600 m. However, data from a single offset shotpoint (shotpoint 11) in Beatty Wash, just within the Timber MountainOasis Valley caldera, suggest that volcanic rocks beneath the 

northwestern part of Yucca Mountain reach a 4.7 km/s 

velocity at considerably shallower depth and are near the surface beneath shotpoint 11. 

A graph of density against velocity, which may be 

representative of the volcanic and prevolcanic rocks in the 

Yucca Mountain region, has been determined (fig. 3.6). 

Based on this graph, a gravity model approximately coincident with the Beatty profile suggests that the Boundary 

Canyon detachment fault, exposed in the Grapevine Mountains, may be continuous as far east as Bare Mountain. 

REFERENCES CITED 

Ackermann, H.D., Pankratz, L.W., and Dansereau, D.A., 1982, 

A comprehensive system for interpreting seismic-refraction 

arrival-time data using interactive computer methods: U.S. 

Geological Survey Open-File Report 82-1065, 265 p. 

1983, Computer program modifications of Open-File 

Report 82-1065: A comprehensive system for interpreting 

seismic-refraction arrival-time data using interactive computer 

methods: U.S. Geological Survey Open-File Report 83-604, 

2 p. 

Barry, C.T., 1980, A study of the vertical vibrator source for 

seismic profiling in the Yucca Mountain area of the Nevada 

Test Site: Colorado School of Mines, Master of Engineering 

Thesis, ER 2429, 58 p. 

Byers, F.M., Jr., Carr, W.J., Orkild, P.P., Quinlivan, W.P., and 

Sargent, K.A., 1976, Volcanic suites and related cauldrons of 

Timber Mountain-Oasis Valley caldera complex, southern 

Nevada: U.S. Geological Survey Professional Paper 919, 70 p. 

Carr, W .J., 1982, Volcano-tectonic history of Crater Flat, southwestern Nevada, as suggested by new evidence from drill hole 

USW-VH-1 and vicinity: U.S. Geological Survey Open-File 

Report 82-457, 23 p. 

1984, Regional structural setting of Yucca Mountain 

southwestern Nevada, and late Cenozoic rates of tectonic 

activity in part of the southwestern Great Basin, Nevada and 

California: U.S. Geological Survey Open-File Report 84-854, 

109 p. 

Carr, W.J., Byers, F.M., Jr., and Orkild, P.P., 1986, Stratigraphic 

and volcano-tectonic relations of Crater Flat Tuff and some 

older volcanic units, Nye County, Nevada: U.S. Geological 

Survey Professional Paper 1323, 28 p. 

Carroll, R.D., 1966, Preliminary interpretation of geophysical logs, 

UE20f, Pahute Mesa, Nevada Test Site: U.S. Geological 

Survey Technical Letter, Special Studies 1-37, Supplement 1, 

5 p. 

Giarmita, M., Day, H.W., and Troxel, B.W., 1983, Structural 

metamorphic and plutonic evolution of the northern Funeral 

Mountains, Death Valley, California [abs.]: Geological Society 

of America Abstracts with Programs, v. 15, no. 5, p. 419. 

Grant, F.S., and West, G.F., 1965, Interpretation theory in applied 

geophysics: New York, McGraw-Hill, 584 p. 

Healey, D.L., Wahl, R.R., and Oliver, H.W., 1980, Bouguer 

gravity map of Nevada: Death Valley sheet: Nevada Bureau 

of Mines and Geology Map 69, scale 1:250,000. 

Healy, J.H., Mooney, W.D., Blank, H.R., Gettings, M.E., Kohler, 

W.M., Lamson, R.J., and Leone, L.E., 1982, Saudi Arabian 

deep-seismic refraction profile: Final project report: U.S. 

Geological Survey Open-File Report 82-37, 429 p. 

Hoffman, L.R., and Mooney, W.D., 1983, A seismic study of 

Yucca Mountain and vicinity, southern Nevada: Data report 

and preliminary results: U.S. Geological Survey Open-File 

Report 83-588, 50 p. 

McGovern, T. F., 1983, An evaluation of seismic reflection studies 

in the Yucca Mountain area, Nevada Test Site: U.S. Geological 

Survey Open-File Report 83-912, 57 p. 

Monsen, S.A., 1983, Structural evolution and metamorphic 

petrology of the Precambrian strata, northwest Bare Mountain, 

Nevada: Davis, University of California, M.S. thesis, 66 p. 

Muller, D.C. , and Kibler, J. E. , 1984, Preliminary analysis of 

geophysical well logs from drill hole UE-25P#1, Yucca Mountain, Nye County, Nevada: U.S. Geological Survey Open-File 

Report 84-649, 14 p. 

Pankratz, L.W., 1982, Reconnaissance seismic refraction studies 

of Calico Hills, Wahmonie, and Yucca Mountain, southwest 

Nevada Test Site, Nye County, Nevada, U.S. Geological 

Survey Open-File Report 82-478, 25 p. 

Snyder, D.B., and Carr, W.J., 1982, Preliminary results of gravity 

investigations at Yucca Mountain and vicinity, southern Nye 

County, Nevada, U.S. Geological Survey Open-File Report 

82-701, 35 p. 

1984, Interpretation of gravity data in a complex 

volcano-tectonic setting, southwestern Nevada: Journal of 

Geophysical Research, v. 89, no. B12, p. 10,193-10,206. 

Stewart, J.H., 1980, Geology of Nevada: Nevada Bureau of Mines 

and Geology Special Publication 4, 136 p. 

Sutton, V.D., 1984, Data report for the 1983 seismic refraction 

experiment at Yucca Mountain, Beatty and vicinity, 

southwestern Nevada: U.S. Geological Survey Open-File 

Report 84-661, 66 p. 

Telford, W.M., Geldart, L.P., Sheriff, R.E., and Keys, D.A., 

1976, Applied geophysics: Cambridge, U.K., Cambridge 

University Press, 860 p. 

Seismic-Refraction and Gravity Studies 33 

4. Volcano-Tectonic Setting of Yucca Mountain and 

Crater Flat, Southwestern Nevada 

By Wi I fred J. Carr 

CONTENTS 

Abstract 35 

Introduction 35 

Regional setting and general geologic relations 37 

Geologic framework of Crater Flat and Yucca Mountain 38 

Crater Flat Tuff and associated volcanic units 40 

Structural setting of Crater Flat and Yucca Mountain 40 

Age of faulting 40 

Geologic and geophysical evidence supporting a volcano-tectonic 

depression in Crater Flat 41 

Other volcano-tectonic structure in the Yucca Mountain area 43 

Degree of late Cenozoic tectonic stability of Crater Flat area 46 

Conclusions 48 

References cited 48 

Abstract 

Information from several cored drill holes, geophysical 

data, and detailed surface geologic studies strongly suggests the 

presence of a large volcano-tectonic depression, probably a 

caldera complex, beneath Crater Flat, a basin between Bare 

Mountain and a proposed nuclear waste repository at Yucca 

Mountain. This probable Crater Flat-Prospector Pass caldera complex, middle Miocene in age, would be related to the Crater Flat 

Tuff, a group of ash-flow sheets that preceded the better known 

and well-exposed Paintbrush and Timber Mountain Tuffs. As 

defined, the complex consists of at least two collapse segments, 

one of which is contiguous with the slightly younger Timber 

Mountain caldera. The inferred calderas lie within a prominent 

regional north-trending structural trough marked by a major 

gravity low in which volcanic rocks extend several kilometers 

below sea level. 

The exposed structure of the Yucca Mountain area is 

dominated by north-northeast striking basin-range style normal 

faults whose principal displacement occurred during a relatively 

brief period about 12 m.y. ago. Many of these faults may be 

reactivated segments of ring fractures of the buried Crater FlatProspector Pass caldera complex. 

Subsurface data from Crater Flat reinforce the concept that 

the basin was formed largely by large-scale subsidence in middle 

Miocene time, and that subsequent tectonic displacements have 

been relatively minor. Spatial and temporal associations indicate 

that some of the fault movement in late Cenozoic time accompanied specific episodes of silicic and basaltic volcanism. 

Waning of that volcanism since the late Miocene, together with 

the mechanical effects of very thick volcanic rocks in the volcanotectonic depression may help explain the relatively low rate of 

structural activity in the area during Pliocene and Quaternary 

time. 

INTRODUCTION 

Understanding the style and mechanisms of tectonic 

development of the Yucca Mountain area is especially critical 

to evaluation of the proposed site for long-term storage of 

high-level nuclear waste. If, as this report attempts to substantiate, the structure of Yucca Mountain is mainly the result 

of middle Miocene volcano-tectonic processes, a relatively 

high degree of present tectonic stability is likely. In other 

words, if the structure in the Crater Flat-Yucca Mountain 

area is largely related to 15-10 m.y.-old magmatic and 

volcanic processes, including caldera formation, rather than 

to Pliocene and Quaternary basin-range deformation, a 

greater relative tectonic stability may be expected during the 

several thousand year period critical for waste storage. This 

relative stability would be a result, not only of the length 

of time since the last main structural activity, but also of 

kinematic considerations that suggest that an inactive caldera 

regime tends to be a decoupled mass within the Earth's upper 

crust. 

The volcano-tectonic history of the Yucca Mountain 

region has been studied for more than 30 years, but only 

recently has the older volcanic history of the area been well 

enough known to illuminate some aspects of the structural 

evolution. Despite the strong evidence summarized in this 

report, proof of caldera complexes beneath Crater Flat or 

northern Yucca Mountain is not at hand. Most older calderas 

whose topographic expression is poor to nonexistent, and 

whose ash-flow units are not exposed within the caldera, are 

necessarily inferred. The evidence for the Crater FlatProspector Pass caldera complex appears as good as, if not 

better than, that for most older calderas in the Basin and 

Range province. 

Two and possibly three major caldera complexes are 

probably present in the vicinity of Yucca Mountain (fig. 4.1): 

The oldest, a pre-Crater Flat Tuff caldera, may exist beneath 

northern Yucca Mountain and northern Crater Flat (Carr, 

1982; Carr and others, 1986), but its exact location and 

history cannot be determined without deep drill holes. A 

Crater Flat-Prospector Pass caldera complex related to the 

Crater Flat Tuff has recently been described (Carr, 1982; 

Volcano-Tectonic Setting 35 
Carr and others, 1986); much remains unknown about this 

episode of volcanism, mainly because it is largely buried by 

younger volcanic rocks. The youngest, best exposed, and 

best understood of the complexes, Timber Mountain-Oasis 

Valley, has been described in detail (Byers and others, 1976b; 

Christiansen and others, 1977). 

nr 116° 

38o ,-----------------,--~~~nc-.~.--------------,-~----------, 

'\ 

~ f 

~ ~ f ~ o-f 

Goldfield '\ ~ f ~'0"" '\_ f 

I ' ~ ..,.,..- -;-r- -n- --rr- ~ F ...,..,.._ " -rr- -rr- --rr- ....,...,.._ ~ f 

I ~~~~~ 

1- I ~--~....,.... '\ ~ "7'7-"7'7-~~ I I \ ~ tO c: ~ I' Stonewall J ~~ro & \ 

I

I ~ l M . j • ..\ ·r \ ountam'/ ~ -1- -\ 

'<..-.~._ ....:v- Southwestern Nevada ~ ~ 

~ volcanic field ~ } \ 

~ ~ ,-.£7· ,_~~ ~ \ ~~', ;

1 ~-,-->-- · I I \~ \ ~ :i t Black. ~ V j Silent }•l_ ~ \\;, ~:"~t\/Canyon;/~,--·~ ~\ 

\ ~~ /~--r~"--/" ~ . c\ 

~ \' I f ~ i ,' I ~ ~ \""" ( ~ Timber Mountain- J / . ..\ ~ "'\. ' - J Oasis Valley 1 ,' 

1 

'\ 

' ~~~ ~ ~· ( t ~ I ~ 1 

'\ \, f I I ,J " · 1 ~ \;, ~ /r-'if_.__+ ........ Y , 1 1 

'\ ~ "<.~ Beatty U- f 1\--..J-~Ciaim Canyon " ;/ 

0 '\, ~"\, 0 t +7 y~ cauldron ~ (' 

(9 \ ~ Crater Flat- /f f [~/ ~ \ ~ Prospector Pass L f 4 / . · ,, ~ "<"~ I / f' Nevada Test S1te 1/ ~ 

"' ~ ~ "-. '<.. -T . boundary 7 r ~ 

~ ~~ ~ t L _.J..J...._.l..l-...J...L....U..J..I-y ~ ~ ' ~ ·r·-·~ . , " '<.~...J.J..-<..~....uh...u..Y .'\ j ~<t-v, 

< 

-

t.- ~ ' \ ~ v· ~~ 

/ '\ \ """' ' \ \ ~ < ~~ 

EXPLANATION 

-L ~ -l- Caldera boundary 

~ ....J...L __w_ Volcanic field boundary 

= == Walker Lane belt boundary 

----- • • • •· Steps in Walker Lane belt 

-+- -+- -+- Axis of gravity low 

/ '~ ~~ ~ (9 '\~ t' .J- "\ ~~ ~ f """ ' ~ J ' "k -<'~~ o~~t> / / I \ («-,..,~ 

W I /I \ o~~~ ~ /: / f \ ~", / t ' ~ ~\ Greenwater- \ """ 

./~ Shoshone -\ ' 

, ; ~. \ volcanic field ~ ''-

36o L---------------------------~~--------~ll-~~~\~--------~~------------~~---"~----------~ 50 KILOMETERS 

Figure 4.1. Location of Walker Lane belt in southwestern Nevada and Greenwater-Shoshone volcanic field and associated calderas. 

YM, Yucca Mountain; FCFZ, Furnace Creek fault zone; LVVSZ, Las Vegas Valley shear zone. 

36 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
In order to support the case for volcanism as the principal controlling process for Cenozoic tectonism in the Yucca 

Mountain area, this paper briefly reviews the setting and 

evidence for volcano-tectonic activity, mainly the caldera 

complex believed present in Crater Flat adjacent to Yucca 

Mountain. Important evidence for relative stability of the area 

since the middle Miocene also is discussed. 

Evidence to be presented includes information from 

surface geology, drill holes, and geophysics. The third 

dimension provided by subsurface data is invaluable in constructing the structural model. 

Acknowledgments.-Helpful reviews of this paper by 

F .A. McKeown and R.E. Anderson are gratefully acknowledged, but the interpretations are those of the author. 

REGIONAL SETTING AND GENERAL GEOLOGIC 

RELATIONS 

Yucca Mountain and Crater Flat are within the southwestern Nevada volcanic field (fig. 4.1) (Christiansen and 

others, 1965, 1977; Byers and others, 1976b), which consists of locally thick and, in some areas, highly faulted 

sequences of Tertiary silicic volcanic rocks, mainly lavas and 

ash-flow tuffs about 8-15 m.y. old, roughly centered around 

the Timber Mountain and Silent Canyon (Noble and others, 

1968) calderas. Volcanic and other stratigraphic units 

important to discussions in this report are summarized in 

table 4.1. 

The volcanic centers lie within the Walker Lane belt 

(WLB) (Carr, 1984a, b), a northwest-southeast-trending zone 

of diverse structure and topography, 75 to 125 km (50-75 

mi) wide, lying between the tectonically more active InyoMono (Carr, 1984a, b) and Basin-Range subsections (fig. 

4.2) of the southwestern Great Basin. Except for volcanotectonic subsidence, the WLB has been dominated by lateral 

shear rather than by large vertical displacements. Several 

northeast-striking structural zones or lineaments, some of 

which are seismically active (Rogers and others, 1983), occur within the WLB (fig. 4.2). Two of these zones are present near Yucca Mountain. The major caldera complexes at 

Silent Canyon, Timber Mountain, and Crater Flat lie along 

Table 4.1. Generalized major stratigraphic units in Bare Mountain-Crater Flat-Yucca Mountain area 

Unit 

Alluvium-------------------

Basalts--------------------

Rhyolite of Shoshone 

Mountain; basalt of 

Dome Mountain; rhyolites 

of Fortymile Canyon. 

Timber Mountain Tuff-------

Paintbrush Tuff------------

Wahmonie and Salyer 

Formations. 

Rhyolite lavas and 

tuffaceous beds of 

Calico Hills. 

Crater Flat Tuff-----------

Lavas of "intermediate" 

composition. 

Lithic Ridge Tuff----------

Older tuffs and lavas------

Tuffs and sedimentary 

rocks. 

Granitic rocks-------------

Sedimentary rocks----------

Members 

Ammonia Tanks Mbr., 

Rainier Mesa 

Mbr. 

Volcanic center 

(figs . 4 . 1 , 4 • 4 ) 

Crater Flat, Big Dune basalt 

areas. 

Shoshone Mountain, Fortymile 

Canyon area. 

Timber Mountain-Oasis Valley 

caldera complex. 

Tiva Canyon Mbr.----- Claim Canyon cauldron-----------

Topopah Spring Mbr.-- Timber Mountain-Oasis Valley 

caldera complex. 

Prow Pass Mbr., 

Bullfrog Mbr., 

Tram Mbr. 

Wahmonie-Salyer center----------

Fortymile Canyon area-----------

Crater Flat-Prospector Pass 

caldera complex. 

Various sources, locations 

poorly known. 

Various sources, locations 

poorly known. 

Various sources, locations 

poorly known. 

Age 

Quaternary to Miocene. 

Pliocene and 

Pliestocene. 

Middle Miocene. 

Middle Miocene. 

Middle Miocene. 

Middle Miocene. 

Middle Miocene. 

Middle Miocene. 

Miocene. 

Middle and early 

Miocene. 

Middle and early 

Miocene. 

Middle and early 

Miocene. 

Middle Miocene to 

middle Oligocene. 

Cretaceous. 

Paleozoic and 

Proterozoic 

Volcano-Tectonic Setting 37 
a regional gravity trough that spans the WLB in a location 

that corresponds roughly with significant right-steps in the 

WLB margins (fig. 4.1) (Carr, 1984b). The gravity low extends both north and south of the southwestern Nevada 

volcanic field, and the south end is located at the GreenwaterShoshone volcanic field east of Death Valley (fig. 4.1). 

The Tertiary volcanic terrane is developed on the 

eroded surface of gently to moderately folded and thrustfaulted Proterozoic and Paleozoic sedimentary rocks that 

were intruded in Jurassic and Cretaceous time by easttrending belts of granitic plutons (Carr, 1984a). 

' ' '\ ', \ " 

\ 

' " " ' \ \ \ \ \ \ ' ' \. \ \ \ ' '\. ,~, \ ' 

GEOLOGIC FRAMEWORK OF CRATER FLAT AND 

YUCCA MOUNTAIN 

Crater Flat, believed to be the source for the Crater 

Flat Tuff (Carr and others, 1986), is a southward-sloping 

basin about 10 km wide. It is flanked on the west by the 

prominent Bare Mountain structural block, composed of preTertiary rocks, and on the east by the fault-block terrane of 

Yucca Mountain, composed almost entirely of Miocene ashflow tuffs (fig. 4.3). On the north the basin is irregularly 

bounded by a complex volcanic terrane that lies between 

EXPlANATION 

Shear zones or major fault 

zones at margins of subsections 

Northwest-trending structures 

within the Walker Lane belt 

Northeast-striking structural 

'\. \ "' \ \ ', 

'\ < \ " 

BASIN-RANGE 

SUBSECTION YM 

zones associated with the Walker 

Lane belt 

Yucca Mountain 

' 0 Tonopah 

/~ ' " / ~ " 

50 

' ' ' 

Figure 4.2. Structural-physiographic subsections of the southwestern Great Basin, and important structures of the Walker Lane belt. 

38 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
36° 

45' 

G 

0 

() 

(::' 

s 

q (::': :r 

EXPlANATION 

Quaternary and Tertiary alluvium 

Pleistocene and Pliocene basalts 

CJ Tertiary volcanic rocks 

~~~~~ Upper Paleozoic clastic rocks 

~ Middle and lower Paleozoic carbonate rocks 

Lower Paleozoic and Precambrian clastic rocks 

----Contact 

--r--Fault-Dotted where concealed; bar and ball 

on downthrown side; arrows indicate relative 

movement 

.J... ~Caldera wall-Mostly inferred and approximately located 

........____...__..L.... Low-angle fault, thrust or glide plane-Sawteeth on upper plate; dotted where concealed 

Possible resurgent dome 

X 

® 

usw 

VH-1e 

Miocene basalt dikes 

Location of faults that have trends approximately parallel to postulated caldera wall 

Drill hole 

Tis Tertiary slide composed of lower Paleozoic 

A A' rocks 

f------1 Location of section shown in figure 4.7 

1W15' 

Figure 4.3. Geologic map of the Bare Mountain-Crater Flat-Yucca Mountain area. Volcano-Tectonic Setting 39 
Crater Flat and the Timber Mountain-Oasis Valley caldera 

complex (Byers and others, 1976a); the latter includes the 

Claim Canyon cauldron segment, the probable source of the 

Tiva Canyon Member (table 4.1) of the Paintbrush Tuff 

(Byers and others, 1976b). On the south, Crater Flat is 

rimmed by a low arcuate ridge of relatively unfaulted tuffs, 

10-m.y .-old basalt lavas, and a very extensive gravity slide 

block of Paleozoic rocks (unit Tls, fig. 4.3; Swadley and 

Carr, in press). Within Crater Flat are two groups of basalts 

(fig. 4. 3); those in the southeast corner are about 3. 8 m. y. 

old, and those along a slightly curving north-northeast trend 

in central Crater Flat are about 1.2 m.y. old. A third young 

basalt, about 0.3 m.y. old, occurs 6 km southeast of Crater 

Flat (Vaniman and others, 1982). 

CRATER FLAT TUFF AND ASSOCIATED VOLCANIC 

UNITS 

The Crater Flat Tuff (table 4.1) probably was erupted 

from an elongate caldera complex underlying most of Crater 

Flat and the area between there and the Timber Mountain 

caldera complex (Carr and others, 1986). The Tram Member, oldest member of the Crater Flat Tuff, is believed (Carr 

and others, 1986) to have originated from the Prospector Pass 

caldera segment in the northern part of the area (fig. 4.3), 

perhaps partly from within the area of the subsequent Timber 

Mountain caldera. The Bullfrog Member of the Crater Flat, 

and possibly the Prow Pass Member of the Crater Flat, are 

thought to have been erupted from a caldera in southern 

Crater Flat. 

The Crater Flat Tuff is sporadically exposed in a fairly 

large region surrounding Crater Flat. The Crater Flat 

underlies the extensive, well-exposed ash-flow sheets of the 

Paintbrush and Timber Mountain Tuffs, and has been 

penetrated by numerous drill holes in the Yucca Mountain 

area (e.g., Spengler and others, 1981; Scott and Castellanos, 

1984). In a few exposures, and in drill holes on Yucca Mountain, the Crater Flat Tuff is underlain by rhyolitic to dacitic 

lavas, by another ash-flow tuff called the Lithic Ridge Tuff, 

and by several sequences of so-called "older tuffs" (Carr 

and others, 1986). 

STRUCTURAL SETTING OF CRATER FLAT AND 

YUCCA MOUNTAIN 

The volcanic rocks on Yucca Mountain are displaced 

by a system of prominent northerly-striking normal faults, 

most with several hundred meters of displacement, spaced 

1 to 3 km apart (fig. 4.3); these faults dip westward and 

repeat the gently (5°-20°) east-dipping volcanic section 

(Lipman and McKay, 1965). It is important to note that the 

western part of this fault system is distinctly en echelon; that 

is, faults step to the left or northwestward and tend to 

decrease in displacement northward. The result (fig. 4.4) is 

40 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

a subtle arcuate net displacement down to the west along the 

northeast side of Crater Flat. Several subparallel faults just 

northeast of Yucca Mountain (Paintbrush Canyon fault 

system, fig. 4.4) curve gently to the northwest and cross into 

the Timber Mountain caldera. A few faults strike northwest 

on northeastern Yucca Mountain (fig. 4.3). 

In contrast to the structure on Yucca Mountain, only 

one major fault is exposed on the west side of Crater Flata continuous, major, east-dipping fault at the foot of Bare 

Mountain (fig. 4.4). Except near the southeast tip of Bare 

Mountain, only alluvium is exposed on the downthrown side. 

The fault trace is composed of two arcuate segments concave toward northern and southern parts of Crater Flat. Other 

north-striking normal or oblique-slip faults, mainly in central 

and northwestern Bare Mountain, are also down to the east 

in the direction of Crater Flat (fig. 4.3). 

Large displacement north-trending faults like those on 

Yucca Mountain do not occur in exposures of volcanic rocks 

of the same age on the northwest and south edges of Crater 

Flat; instead, rocks exposed in the narrow, arcuate south rim 

of Crater Flat are cut by only a few north-striking faults of 

relatively small displacement. Two faults of northeasterly 

trend (F, fig. 4.3) that are concave toward Crater Flat and 

are down toward it, could be outermost collapse faults of 

the caldera structure. These faults drop down a thick section of Crater Flat Tuff exposed on the ridge immediately 

to the south (Swadley and Carr, 1987). North of Crater Flat 

is a complex area with many faults of northeast trend, but 

also much diverse, locally chaotic structure, largely 

developed in rocks older than the Timber Mountain Tuff. 

Thus, although it is tempting to extend the prominent northtrending structural style of Yucca Mountain westward 

beneath Crater Flat to Bare Mountain, the exposed structure immediately north and south of Crater Flat does not 

support such an extrapolation, nor does available subsurface 

information, as will be brought out later. 

AGE OF FAULTING 

The age of major movements on faults exposed around 

Crater Flat is well constrained. Faults on Yucca Mountain, 

and around the northeast and southeast sides of Crater Flat, 

had their principal displacement in a relatively brief period 

after eruption of the Tiva Canyon Member of the Paintbrush 

Tuff, and before emplacement of the Timber Mountain Tuff, 

approximately 11.5-12.5 m.y. ago. This age is well established by detailed mapping in this region (see Byers and 

others, 1976a, Scott and Bonk, 1984; Swadley and Carr, 

1987), which demonstrates that the Rainier Mesa Member 

of the Timber Mountain Tuff was deposited across small fault 

scarps and at the foot of large fault scarps developed in the 

Paintbrush and older tuffs (Carr, 1984a, fig. 32). In most 

of the Yucca Mountain area, faulting of the Timber Mountain Tuff and younger rocks has, by contrast, been minor 

(Carr, 1984a; Scott and Bonk, 1984). On Bare Mountain, 
to the west, north-striking dikes dated at 13.9 m.y. (Carr, 

1982, p. 9) are little disturbed by faults of the same general 

trend. Faulting adjacent to, and locally within, the Timber 

Mountain-Oasis Valley caldera complex, however, caused 

extensive disturbance of the Timber Mountain Tuff and 

younger volcanic rocks, such as the rhyolites of Fortymile 

Canyon (table 4.1) (Byers and others, 1976a). Quaternary 

deposits show small displacement at several locations on 

Yucca Mountain, in southeastern Crater Flat, and at the east 

foot of Bare Mountain (Swadley and others, 1984). These 

Quaternary displacements occur on segments of the faults 

formed mainly about 12 m.y. ago, and occur largely on portions of these faults here interpreted to be controlled, in part, 

by caldera structure. 

116°45' 30' 

GEOLOGIC AND GEOPHYSICAL EVIDENCE 

SUPPORTING A VOLCANO-TECTONIC 

DEPRESSION IN CRATER FLAT 

Surface and near-surface geologic information bearing 

upon a volcano-tectonic and caldera interpretation of the 

Crater Flat area is summarized in table 4.2. Geophysical data 

interpreted to support the caldera model are summarized in 

table 4.3. 

Although no single feature listed in tables 4.2 and 4.3 

proves the presence of a caldera complex in Crater Flat, taken 

together the evidence is good, at least for volcano-tectonic 

subsidence of a large area west and north of Yucca Mountain. Large-scale subsidence prior to emplacement of the 

116°15' 37"~----~------------------~--------------~~--------------~~~----~----~ 

DO' 

36° 

45' 

0 

10 KILOMETERS 

Crater Flat 

caldera 

EXPlANATION 

___L __L_ Caldera margin 

..1.1.. ....LL Older tuffs caldera(?) margin 

---.--- Fault-Possibly related to caldera structure; dotted where concealed. Bar and 

ball on downthrown side 

+++++-+-+- Dike-Rhyodacite on Bare Mountain, 

usw 

G-1 0 

basalt on Yucca Mountain 

Drill hole 

E Fault of en echelon group 

Pre-Pliocene rocks 

Figure 4.4. Relation between basin-range faults and calderas, Yucca Mountain area. 

Volcano-Tectonic Setting 41 
Table 4.2. Geologic evidence of caldera complex in Crater Flat 

Evidence or feature 

Circular or semicircular shape, particularly 

of southern Crater Flat. 

Two northeast-striking curved faults occur 

near the southeast margin of Crater Flat 

(F, fig. 4.3), where they drop a thick 

section of Crater Flat Tuff down toward 

Crater Flat. 

A gently arcuate system of 13.9-m.y.-old dikes 

on eastern Bare Mountain, slightly concave 

toward Crater Flat (figs. 4.4, 4.5); dikes 

are locally in small faults that show little 

or no post-dike movement. 

Lavas similar to the dikes on Bare Mountain 

underlie the Tram Member in a large area 

roughly centered about Crater Flat (fig. 4.5). 

Northwest of Crater Flat, altered Tram Member 

appears to be abruptly thinner outside a 

partly buried curving structure. Pre-Tram 

lavas occur near this boundary (fig. 4.5). 1 

Pattern of diverse fault trends in area between 

Crater Flat and Beatty Wash occurs in an area 

of thick Tram Member. 

Distribution of the welded ash-flow tuff members 

of the Crater Flat Tuff is roughly centered 

about Crater Flat (Carr and others, 1986). 

Breccia at the top of the Bullfrog Member near 

the south rim of Crater Flat (Swadley and 

Carr, in press). 

Densely welded, granophyric Bullfrog Member 

more than 141 m thick (base not reached) 

occurs in bottom of drill hole USW VH-1 

(figs. 4.4, 4.7; Carr, 1982). 

Pliocene and Pleistocene basalts in central 

and southern Crater Flat (fig. 4.3). 

Basin-range-style structure east of Crater Flat 

is not well developed within caldera or at 

north and south margins. 

Interpretation or comment 

Arcuate, caldera-like topographic boundaries are 

present on the west, south, and southeast margins 

Crater Flat. 

The faults could be segments of the outermost caldera 

fault zone. 

The dikes are about the same age and petrographically 

similar to parts of the Tram Member and underlying 

Lithic Ridge Tuff and associated lavas. Dikes may 

occupy early incipient outer ring fractures of the 

caldera complex. 

Lavas and dikes may represent precaldera leakage of the 

magma chamber that subsequently produced the ash-flow 

eruptions of the Crater Flat Tuff. 

Structures may be near the wall of the Prospector Pass 

caldera segment, postulated source of the Tram Member. 

Fault pattern resembles that of resurgent dome; Tram 

Member of same area has characteristics of intracaldera facies. 

Thickest and most welded part of the Crater Flat Tuff 

exposed or penetrated by drill holes lies adjacent 

to Crater Flat, suggesting a nearby source. 

A wedge of monolithologic breccia of the welded part of 

the Bullfrog, lithologically unlike the exposed 

Bullfrog, may have been emplaced by collapse of a 

nearby caldera and sliding from the wall. 

Granophyric part of the Bullfrog Member in drill hole 

appears to be abnormally thick, suggesting a slowly 

cooled, intracaldera section. 

Basalts occur mainly in the moat area of the caldera 

complex and, as is common in other Great Basin 

calderas, could have risen along caldera ring 

fractures. 

Pattern of extensional faulting is interrupted by 

caldera structure. Subsurface data suggest that 

faulting is not well developed within Crater Flat. 

1 A drill hole by U.S. Borax, Inc., about 6.5 km east of Beatty, and just north of Bare Mountain, 

penetrated more than 400 m of Tram Member, underlain by rhyodacitic lava. 

Paintbrush Tuff is virtually required by the very thick 

volcanic section indicated by the gravity anomaly (Snyder 

and Carr, 1984). Data from two drill holes (USW VH-1 and 

USW VH-2) in Crater Flat (Carr, 1982; Carr and Parrish, 

1985) indicate that the subsidence, at least in central Crater 

Flat, is largely pre-Paintbrush Tuff in age. The subsided area 

was modified somewhat, mostly by post-Paintbrush Tuff 

basin-range-style faults, some of which may have partially 

utilized and reactivated caldera ring fracture faults where 

these were favorably oriented. This appears especially likely 

on the east side of Bare Mountain where the trace of the 

frontal fault zone is curvilinear or gently scalloped. I suspect, 

but cannot prove, that the en echelon faults that form much 

of the west margin of Yucca Mountain facing Crater Flat, 

are reactivated portions of eastern ring fracture faults 

42 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

(fig. 4.4); that is, gently curving and scalloped north-trending 

caldera ring fracture zones may have responded to east -west 

crustal extension by collapsing in segments arranged in en 

echelon fashion. The segments of greatest vertical displacement would correspond in position with the Crater FlatProspector Pass caldera ring-fracture segments. Carr and 

others (1986, p. 25) suggested that a series of small10-m.y .-

old basalt dikes on Yucca Mountain could mark the general 

location of the buried eastern outer ring fracture zone of the 

complex (fig. 4.3). As suggested by Snyder and Carr (1984, 

fig. 5), however, the caldera may be a "trap-door" type, 

the west side next to Bare Mountain being the area of greatest 

collapse. If this is correct, the ring fracture zone on the east 

side of the collapse, next to Yucca Mountain, may have been 

spread across a wider area and may not have been as well 
Table 4.3. Geophysical evidence of a caldera complex in Crater Flat 

Evidence or feature Interpretation or comment 

Pronounced negative gravity anomaly (nearly 50 

mGal) in Crater Flat. 

More than 4,000 m of relatively low-density volcanic 

rocks probably are present in Crater Flat (Snyder 

and Carr, 1 984) . 

Seismic velocities beneath Crater Flat generally 

agree with gravity anomaly (Hoffman and 

Mooney, 1983; Snyder and Carr, 1984). 

A very thick section of relatively low velocity rocks, 

probably volcanic, underlies Crater Flat. 

Large circular aeromagnetic positive anomaly 

centered in southern Crater Flat (Kane and 

Bracken, 1983; fig. 4.6). 

The unit with magnetic characteristics and thickness 

most likely responsible for the anomaly is the 

Bullfrog Member; the circular anomaly and relative 

nearness (about 620 m) of the Bullfrog to the surface 

at drill hole USW VH-1 (Carr, 1982) suggests a gentle 

resurgent dome in the Bullfrog about 7 km in diameter 

diameter (fig. 4.6). 

A prominent (more than 1,500 nT) negative 

aeromagnetic anomaly between northwestern 

Crater Flat and Beatty Wash (Kane and 

Interruption at the west side of Yucca Mountain 

of a persistent east-west aeromagnetic 

gradient (Kane and Bracken, 1983). 

Anomaly occurs over a thick section of Tram Member that 

has intracaldera characteristics, and is faulted in a 

pattern resembling part of a resurgent structure. 

The magnetic gradient, which is likely due to presence of 

the Eleana Formation (largely clastic rocks of 

Mississippian age) magnetized by Mesozoic granitic 

intrusive rocks (Bath and Jahren, 1984), ends near the 

margin of the postulated Prospector Pass caldera 

segment. 

developed as on the west. Because of the north-south elongation of the gravity low (Snyder and Carr, 1984) and 

postulated caldera boundaries, it is possible that the complex is similar to a sector graben, a collapsed linear segment 

on the flanks of a volcanic edifice. Subsequent formation of 

the large Timber Mountain-Oasis Valley caldera complex in 

the area where such a volcanic high might have been located, 

makes very difficult any reconstruction of the terrane and 

structure at the time of Crater Flat Tuff eruption. 

The closest deep drill hole to the Crater Flat-Prospector 

Pass caldera structure is USW H-6 (Craig and others, 1983) 

on the west side of Yucca Mountain (figs. 4.4 and 4.5). This 

hole penetrated a relatively thin section of Crater Flat Tuff, 

which rests on about 250m of dacitic lavas (Craig and others, 

1983). These rocks of intermediate composition are present, 

but thinner, in several other drill holes on Yucca Mountain 

to the northeast ofUSW H-6 (fig. 4.5), and at several other 

locations around Crater Flat, where they occur beneath both 

the Tram Member of the Crater Flat Tuff and the Lithic 

Ridge Tuff. Both the Tram and the Lithic Ridge contain abundant lithic fragments of these lavas. Thus, it is likely that 

the lavas were extensive in the Crater Flat area prior to eruption of the Crater Flat Tuff. 

The above interpretations, together with much suggestive evidence, indicate that a caldera complex related to 

the Crater Flat Tuff lies directly west of Yucca Mountain, 

but its exact boundaries and dimensions are not well constrained. That a Crater Flat Tuff caldera is not present in 

the proposed repository site area on Yucca Mountain is 

shown by the absence of abrupt thickening of the members 

of the Crater Flat Tuff in the area, their generally weak to 

moderate welding, and the lack of slide blocks or debris in 

the tuffs that might indicate the proximity of an unstable 

caldera wall. 

OTHER VOLCANO-TECTONIC STRUCTURE IN 

THE YUCCA MOUNTAIN AREA 

The question of whether an older (pre-Tram Member) 

caldera is present beneath northern Yucca Mountain is 

difficult to answer. More than 744 m of lava and tuffs 

underlie the Tram Member in drill hole USW G-1 (fig. 4.4) 

(Spengler and others, 1981). Gravity data indicate that more 

than 4, 000 m of tuffs underlie Crater Flat and part of Yucca 

Mountain (Snyder and Carr, 1984), which places the base 

of the volcanic rocks at least 2. 7 km below sea level. 

Therefore, it is evident that some sort of pre-Crater Flat Tuff 

volcano-tectonic subsidence took place in the northern Yucca 

Mountain area. Carr and others (1986, p. 27 and fig. 7) suggested that the Paintbrush Canyon fault system may mark 

the east edge of a pre-Crater Flat caldera or large volcanotectonic sag beneath northern Yucca Mountain (fig. 4.4). 

These faults coincide in general location with the northeastern 

edge of a lobe of the prominent gravity low under Yucca 

Mountain and Crater Flat (Snyder and Carr, 1984). In this 

concept, the Paintbrush Canyon fault system on the east edge 

of the older possible caldera would be analogous to the faults 

on the west side of Yucca Mountain previously suggested 

as partially reactivated segments of the caldera ring fracture 

zone of the Crater Flat-Prospector Pass complex. 

Volcano-Tectonic Setting 43 
Some discussion and reassessment of earlier ideas about 

the relations between volcanic and structural movements in 

the region seems to be required by the new surface and subsurface information. Cummings (1968) ascribed the termination of most of the basin-range-style faults at the Timber 

Mountain caldera ring-fracture zone to the inability of the 

somewhat decoupled or highly fractured caldera substructure to propagate the faults. Christiansen and others ( 1965) 

believed that the termination was due to vertical displacement on the basin-range faults being taken up by the caldera 

ring fracture zone. They also proposed that the faults were 

related to broad doming and an associated hingeline that 

developed preceding Timber Mountain Tuff eruptions and 

caldera collapse. 

Later work (Byers and others, 1976b) has demonstrated 

that rocks on the rim of Timber Mountain caldera northwest 

of Yucca Wash are part of a resurgent block of the Claim 

Canyon cauldron (fig. 4.4), the probable source of the Tiva 

Canyon Member of the Paintbrush Tuff. The resurgent block 

was eccentric to the caldera, and uplift occurred along the 

southern portion of the previously formed caldera outer ring 

fracture zone. Therefore, most of the "circumferential" 

faults of Christiansen and others (1965, fig. 2) are part of 

lWOO' 

37° 

00' 

EXPLANATION 

Exposed bedrock 

- Area of exposed dacitic to rhyolitic 

lava flow or plug 

\/I Dike 

Crater Flat Tuff (100-m isopachs. 

queried where no control) 

------ Prow Pass Member 

Bullfrog Member 

Tram Member 

USW0 

G-1 

Area of dacitic to rhyolitic lava flows 

penetrated by drill holes 

Drill hole penetrating pre-Crater 

Flat lavas 

10 

the complex resurgent dome structure. The hingeline, as 

earlier proposed (Christiansen and others, 1965), is not a 

simple arcuate feature related to doming preceding eruption 

of the Timber Mountain Tuff, but a more irregular and less 

continuous boundary that trends northwest across Yucca 

Mountain. Structure contours (Carr, 1984, fig. 30) suggest 

that the easterly dips are related to the basin-range faults, 

as stated (Christiansen and others, 1965), but the southeasterly dips in the Paintbrush Tuff in the northeastern part 

of the Yucca Mountain area probably are a combination of 

initial dip and perhaps some tilting related to resurgence of 

the Claim Canyon block. This conclusion is based on the 

general slope of the Paintbrush Tuff units away from their 

resurgent source area in a direction that is about 45 o to the 

northerly striking faults in northern Yucca Mountain (Carr, 

1984, fig. 30). 

The degree of unconformity between the Rainier Mesa 

Member of the Timber Mountain Tuff and underlying units, 

and the age of latest significant faulting, tend to increase in 

a clockwise arc from east to west, from the .southern 

Shoshone Mountain area (fig. 4.5) across Yucca Mountain 

to the north end of Crater Flat. There is a good spatial relation of faults with the age of the youngest volcanic rocks in 

30' 

20 KILOMETERS 

Figure 4.5. Exposed-bedrock and isopach map showing thickness of members of Crater Flat Tuff in southwestern Nevada Test Site. 

44 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
116°3 7'30" 

c 

C1l 

c 

:::J 

0 

~ 

QJ 

Aeromagnetic co ntour interval 

100 nT; hachure s indicate low 

~J 

4 KILOMETERS 

116°30' 

c 

C1l 

c 

:::J 

0 

~ 

q, 

u 

u 

Figure 4.6. Aeromagnetic map of southern Crater Flat area, showing caldera boundary 

and the proposed resurgent dome (shaded) associated with the Bullfrog Member of the 

Crater Flat Tuff. From U.S. Geological Survey (1978, 1979). Drill holes indicated by small 

circles. 

this arc. On Shoshone Mountain, fault displacements in the 

Rainier Mesa are commonly nearly as great as those in 

underlying units (Orkild and O'Connor, 1970); on most of 

northern Yucca Mountain, the Rainier Mesa Member is 

absent, but to the south and west it is present at several locations where it was clearly deposited at the foot of pre-existing 

Volcano-Tectonic Setting 45 
large fault scarps, and later was displaced a minor amount 

by the north-striking faults. In the northwestern part of Yucca 

Mountain, the Rainier Mesa Member and precursor lava 

flows were deposited unconformably across the tilted blocks 

of the Paintbrush Tuff and older rocks (Byers and others, 

1976a), resting on units as old as the Tram Member of the 

Crater Flat Tuff in the Beatty Wash area northwest of Yucca 

Mountain (Carr and others, 1986). In the area north of Yucca 

Wash (fig. 4.4), no Rainier Mesa is present, but the rhyolite 

lavas of Fortymile Canyon, whose minimum age is about 

9.5 m.y., are locally sharply unconformable on the Paintbrush Tuff. (Recent work has demonstrated that most of the 

rhyolite lavas of Fortymile Canyon were erupted contemporaneously with the Paintbrush and Timber Mountain Tuffs. 

Thus, faulting in those lavas is a little older than previously 

thought, and generally the same age as that on Yucca Mountain itself.) The lavas are displaced a moderate amount by 

the Paintbrush Canyon fault system, which strikes N. 

15°-20° W. in the area between Shoshone Mountain and 

the northeast edge of Yucca Mountain. This faulting is 

spatially and temporally associated with the area of the 

youngest voluminous bimodal volcanic eruptions, the rhyolite 

lavas of Fortymile Canyon, the basalts of Dome Mountain, 

and the rhyolite of Shoshone Mountain (Byers and others, 

1976a). In addition, Fortymile Canyon, near the middle of 

this zone of faults and young lavas, follows the axis of a structural low that developed after emplacement of the Timber 

Mountain Tuff (Carr, 1984a, fig. 30). All members of the 

Paintbrush and Timber Mountain Tuffs, except the Pah 

Canyon Member of the Paintbrush, thin drastically or are 

absent in the Fortymile Canyon area, yet the canyon is the 

site of a pronounced post-Timber Mountain Tuff erosional 

and structural syncline (Carr, 1984a, fig. 30). 

In spite of the tendency for the Timber Mountain Tuff 

to be faulted in many areas on the rim of the Timber Mountain caldera, the Timber Mountain-Oasis Valley caldera complex in general is not broken by basin-range-style faults of 

large displacement (Byers and others, 1976a), and the faults 

that are present show little or no movement in Pliocene or 

Quaternary time. A similar large decrease in fault activity 

(Carr, 1984a, tables 4 and 5, p. 84-96) appears to typify 

most of the Crater Flat-Yucca Mountain area, although minor 

activity has continued into the Quaternary on a few faults 

(Swadley and others, 1984). 

DEGREE OF LATE CENOZOIC TECTONIC 

STABILITY OF CRATER FLAT AREA 

Drill holes USW VH-1 and VH-2, together with other 

geologic and geophysical data, provide evidence that contributes substantially to understanding of the structural history 

of central Crater Flat during the last 10 m.y. or so. This 

information was used to produce a cross section (fig. 4. 7), 

which suggests that a relatively small amount of vertical 

46 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

displacement has taken place in central Crater Flat since the 

late Miocene. Surface exposures show that the basalts a little 

over 1 m.y. old, such as the one at Black Cone (fig. 4.7) 

(Vaniman and others, 1982), and the basalts about 3.8 m.y. 

old (Vaniman and others, 1982) near USW VH-1 are both 

at or very near the present surface of Crater Flat. The 

1.2-m.y.-old basalts are unfaulted; those about 3.8 m.y. old 

are cut by a few minor faults. The next oldest closely dated 

unit penetrated in either drill hole is another basalt at a depth 

of360 min USW VH-2 (fig. 4.7). As explained elsewhere 

(Carr and Parrish, 1985) the K-Ar age of 11.2 m.y. obtained 

for this basalt may be a little too old; the age is probably 

between 10 and 11 m. y. Assuming an age for the basalt of 

11 m.y., the rate of burial at this location in west-central 

Crater Flat has been only about 0.03 m per 1,000 yr (Carr, 

1984a, p. 80). 

Two important intervals of Paleozoic rock debris were 

penetrated by USW VH-2, one directly above the Miocene 

basalt at depths of 309 to 360 m, and the other between the 

Tiva Canyon Member of the Paintbrush Tuff and the Rainier 

Mesa Member of the Timber Mountain Tuff, from 535 to 

595 m (fig. 4.7). Most of this material consists of large 

masses of Paleozoic rocks that have the continuity and texture of gravity slide blocks. The upper zone, just above the 

basalt, is in the same stratigraphic position as an extensive 

(at least 7 km across) slide mass of lower Paleozoic rocks 

exposed along the south rim of Crater Flat (fig. 4.3). This 

nearly continuous mass of Paleozoic rock rests on basalt dated 

at 10.5 m.y. (R.J. Fleck, U.S. Geological Survey, written 

commun., 1978) that is very similar to the basalt in the USW 

VH-2 drill hole. Thus, much of southern and western Crater 

Flat was covered by large masses that probably slid from 

high terrane on or near Bare Mountain, possibly as the result 

of ground motion from nearby large earthquakes. This slide 

mass is cut by only a few small faults, and at the southeast 

comer of Black Marble hill where the large frontal fault zone 

at the east foot of Bare Mountain crosses into the Amargosa 

Desert valley (fig. 4.3), very little vertical movement appears 

to have occurred after emplacement of the slide (Swadley 

and Carr, in press). The exact age of the slide is not known; 

it is younger than 10.5 m.y. and older than the 3.8-m.y.-old 

basalts in the southeastern part of Crater Flat. Thus, emplacement of the slide, or slides, may coincide with the last major 

movements on the east frontal fault of Bare Mountain. The 

older interval of Paleozoic rock debris in USW VH-2 

between the Tiva Canyon and Rainier Mesa Members corresponds in stratigraphic position with the time of major 

faulting at Yucca Mountain, and suggests that the Bare Mountain frontal fault was also active at about the same time. 

No faults of large displacement were encountered in 

either USW VH-1 or VH-2. The apparent absence of largedisplacement north-trending faults, especially in central and 

western Crater Flat, is consistent with geophysical data. 

Except for the major fault at the foot of Bare Mountain, 

gravity data (Snyder and Carr, 1984) do not indicate any 

important buried faults of northerly strike, nor do low-level 
< 0 

?i 

AI 

~ 

t'l) 

~ 

g 

~ 

;:;· 

Ill 

~ :r 

QQ 

,f:o, 

'I 

FEET 

6000 

2000 

SEA 

LEVEL 

2000 

w 

A 

Basalt (Quaternary) 

I OTac I Alluvium and colluvium 

(Quaternary and Tertiary) 

I', T;ls ~ Gravity slide debris of 

Paleozoic rocks (Tertiary) 

I Tac j Alluvium and colluvium 

(Tertiary) 

Basalt (Tertiary) 

Crater Flat 

~--------------------v---------------------/ 

Possible resurgent dome related 

to the Bullfrog Member 0 1 2 3 4 5 KILOMETERS 

EXPLANATION 

Ammonia Tanks Member } Timber Mountain 

Rainier Mesa Member Tuff (Miocene) 

~~ Tiva Canyon Member 

r}P-Yi Topopah Spring Member 

8 Prow Pass Member 

j,,re~',,j Bullfrog Member 

[ Tc~ j Tram Member 

} 

Paintbrush 

Tuff (Miocene) 

} 

Crater Flat 

Tuff (Miocene) 

-~· Lithic Ridge Tuff (Miocene) 

I' Tot I Older tuffs (Tertiary) 

I Pzec'.j Sedimentary rocks (Paleozoic and Proterozoic) 

Contact-Approximately located 

Fault-Arrow on downthrown side; dashed 

where inferred from geophysical data 

'f' Drill hole-TO, total depth 

E 

A' 

METERS 

2000 

1500 

1000 

500 

SEA 

LEVEL 

500 

1000 

Figure 4.7. Cross section of Crater Flat between Bare Mountain and Yucca Mountain. Doming at drill hole USW VH-1 is inferred from the relatively shallow position of 

the tuffs and from geophysical evidence. See figure 4.3 for section location. 
aeromagnetic data (Kane and Bracken, 1983), which show 

very well the exposed prominent faults on Yucca Mountain. 

The rate of burial or tectonic adjustment at USW VH-2 

during about the last 11 m.y. has been estimated at 0.03 

m/1,000 yr (Carr, 1984, table 2). The complete absence of 

volcanic debris in the Tertiary and Quaternary alluvium and 

rock debris below about 120m depth in USW VH-2 suggests 

that there was no large structural tilting to the west during 

deposition of that part of the stratigraphic record. Assuming that the correct age of the basalt is about 10.5 m.y., at 

USW VH-2 only about 365 m of alluvium and slide material 

has accumulated in that time, two-thirds of it (245m) before 

the influx of volcanic clasts from the east began. Interpolation, using a uniform deposition rate, suggests that the change 

in deposition from entirely Paleozoic rock debris to mainly 

volcanic fragments took place about 3.5 m.y. ago. The 

presence of basalt a little more than 1 m.y. old at the surface at nearby Red Cone and Black Cone (fig. 4.1) suggests 

a significant slowing of the alluviation process and (or) 

stabilization of tectonic adjustments in at least the last 1 m.y. 

or so. If the incorporation of volcanic clasts in the alluvium 

began roughly 3-4 m.y. ago, the cause could have been 

gentle westward tilting that accompanied renewed displacement on the Bare Mountain frontal fault zone. Conversely, 

the ending of significant uplift of Bare Mountain could have 

slowed the accumulation of fans of Paleozoic rock debris, 

and allowed encroachment of volcanic clasts from the east. 

The cross section of figure 4. 7 assumes the latter explanation, and allows only minor structural tilting after the Tertiary 

basalts were extruded. 

Although the steep, morphologically youthfulappearing east front of Bare Mountain is obviously the locus 

of persistent faulting, the lack of large-scale downdropping 

of post -tuff units (basalt and alluvium) in central and eastern 

Crater Flat is good evidence that no major structural displacement has occurred in most of Crater Flat during the last 

10 m.y. or so. As mentioned previously, the overlap and 

onlap of large fault scarps in the Paintbrush Tuff by the 

Rainier Mesa Member around the flanks of Yucca Mountain (Scott and Bonk, 1984) also demonstrate the age of the 

major faulting. There are, however, indications that each 

basalt eruptive episode in and near Crater Flat was accompanied by some faulting. The most obvious relation was 

found in two trenches dug across faults in eastern Crater Flat 

near Yucca Mountain, where basalt ash, probably related 

to the 1.2-m.y. episode of basalt eruption (Swadley and 

others, 1984), was found at several meters depth within the 

faults (Swadley and Hoover, 1983). Large-scale landslides, 

possibly triggered by faulting and earthquakes, appear to be 

closely associated in time with the 3.8-m.y.- and 10 to 

11-m.y .-old basalt episodes. Some or all of the youngest 

minor surface faulting in the Crater Flat-Yucca Mountain 

area may have accompanied the Big Dune basalt eruption 

(approximately 0.27 m.y. ago) (fig. 4.3). This faulting, of 

less than 1 m displacement, was estimated by Swadley and 

Hoover (1983) to have occurred between 260,000 and 40,000 

48 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

years ago, with the evidence favoring the older part of this 

range, which places it at the general time of the Big Dune 

basalt eruption. 

CONCLUSIONS 

Structural development of most of the Crater FlatYucca Mountain area was closely related to volcano-tectonic 

processes, which probably included magmatic insurgent and 

resurgent doming, possible sector grabens or elongate sags, 

and caldera collapse, but the mechanisms are complex and 

not completely understood. Incontrovertible proof of calderas 

beneath Crater Flat and part of Yucca Mountain would 

require deep drill holes. 

Clearly, the youngest significant tectonic adjustments 

occurred in the areas of the youngest volcanic eruptions, but 

considerable evidence indicates that Crater Flat itself is 

mainly a relatively old (middle Miocene) volcano-tectonic 

depression, and probably a caldera complex, that has not been 

greatly modified by subsequent Pliocene or Quaternary 

tectonic activity. Important faulting that displaced the Paintbrush Tuff after Crater Flat caldera formation may have been 

controlled, in part, by reactivation of buried caldera ring fractures of favorable northerly trend near the east and west 

caldera margins, specifically the curvilinear fault at the east 

foot of Bare Mountain, and several en echelon faults 

distributed across western and northwestern Yucca Mountain. Subsequent faulting, though relatively minor in most 

areas, was spatially and temporally related to the late 

Miocene bimodal and younger basaltic volcanism. The timing 

and trend of past tectonic events, however, suggests relative 

stability for the Crater Flat-northern Yucca Mountain area 

in the future. 

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anomalies at a potential repository site located in the Yucca 

Mountain area, Nevada Test Site: U.S. Geological Survey 

Open-File Report 84-120, 40 p. 

Byers, P.M., Jr., Carr, W.J., Christiansen, R.L., Lipman, P.W., 

Orkild, P.P., and Quinlivan, W.D., 1976a, Geologic map of 

the Timber Mountain caldera area, Nye County Nevada: U.S. 

Geological Survey Miscellaneous Investigations Series Map 

1-891; scale 1:48,000. 

Byers, P.M., Jr., Carr, W.J., Orkild, P.P., Quinlivan, W.D., and 

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Report 82-457, 23 p. 

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1:48,000. 

Lipman, P.W., and McKay, E.J., 1965, Geologic map of the 

Topopah Spring SW quadrangle, Nye County, Nevada: U.S. 

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Rogers, A.M., Harmsen, S.C., Carr, W.J., and Spence, W.J., 

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U.S. Geological Survey Open-File Report 84-494; scale 

1:12,000. 

Scott, R.B., and Castellanos, Mayra, 1984, Stratigraphic and structural relations of volcanic rocks in drill holes USW GU-3 and 

USW G-3, Yucca Mountain, Nye County, Nevada: U.S. 

Geological Survey Open-File Report 84-491, 121 p. 

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10,193-10,206. 

Spengler, R.W., Byers, F.M., Jr., and Warren, J.B., 1981, 

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Survey Open-File Report 81-1349, 50 p. 

Swadley, W C, and Carr, W.J., 1987, Geologic map ofthe surficial 

and Tertiary rocks of the Big Dune quadrangle, California and 

Nevada: U.S. Geological Survey Miscellaneous Investigations 

Series Map I-1767; scale 1:48,000. 

Swadley, W C, and Hoover, D.L., 1983, Geology of faults exposed 

in trenches in Crater Flat, Nye County, Nevada: U.S. 

Geological Survey Open-File Report 83-608, 15 p. 

Swadley, W C, Hoover, D.L., and Rosholt, J.N., 1984, Preliminary 

report on late Cenozoic faulting and stratigraphy in the vicinity 

of Yucca Mountain, Nye County, Nevada: U.S. Geological 

Survey Open-File Report 84-788, 42 p. 

U.S. Geological Survey, 1978, Aeromagnetic map of Lathrop Wells 

area, Nevada: U.S. Geological Survey Open-File Report 

78-1103; scale 1:48,000. 

1979, Aeromagnetic map of the Timber Mountain area, 

Nevada: U.S. Geological Survey Open-File Report 79-587; 

scale 1:48,000. 

Vaniman, D.T., Crowe, B.M., and Gladney, E.S., 1982, Petrology 

and geochemistry of hawaiite lavas from Crater Flat, Nevada: 

Contributions to Mineralogy and Petrology, v. 80, p. 341-357. 

Volcano-Tectonic Setting 49 

5. Detachment Faulting in the 

Death Valley Region, California and Nevada 

By Warren B. Hamilton 

CONTENTS 

Abstract 51 

Introduction 51 

Bare Mountain, Bullfrog Hills, Funeral Mountains, and Yucca 

Mountain 53 

Bare Mountain detachment fault 53 

Bullfrog Hills detachment fault 55 

Funeral Mountains detachment fault 57 

Continuity of faulting between Bare Mountain, Bullfrog Hills, 

and the Funeral Mountains 59 

Imbricate faulting of Yucca Mountain 62 

Black Mountains 62 

Amargosa detachment fault and Amargosa chaos 62 

Turtleback detachment faults 65 

Upper-plate normal faults 65 

Continuity of faulting in Black Mountains 67 

Relation between detachment faults of the Black and Funeral 

Mountains 67 

Panamint Mountains 68 

Hanaupah detachment fault 68 

Tucki detachment-fault system 69 

Central Panamint Mountains 71 

Opening of Death Valley 72 

Northern Death Valley 73 

Central Death Valley 73 

COCORP seismic-reflection profiles 77 

Opening of Panamint Valley 77 

Extension west of Panamint Valley 78 

Extension south of the Death Valley region 78 

Extension east of the Death Valley region 78 

Discussion 79 

References cited 82 

Abstract 

Detachment faults of Oligocene to Holocene age are 

widely exposed in the Death Valley region, where crustal 

extension is interpreted to have amounted to at least 1 00 percent 

and is continuing at a high rate. Upper-crust fault blocks of both 

preextensional and synextensional materials, rotated to consistent eastward dips, and the west-dipping normal faults that 

separate and dismember them, all end downward at undulating 

detachment faults in the Panamint, Black, and Funeral Mountains, Bare Mountain, and the Bullfrog Hills. Beneath the detachment faults are rocks that were in the middle crust before 

extension, and thick ductile shear zones were developed during early, deep slip on the structures. The detachment faults may 

represent great normal faults that initially dipped westward but 

that have been rotated and raised as tectonic denudation 

unroofed them progressively westward. The huge upper-plate 

blocks of the Grapevine and southern Funeral Mountains were 

stranded atop raised and flattened sectors of the faults, whereas 

the upper-plate blocks of the Panamint and Cottonwood Mountains continue to slip away. Synextensional basin-fill materials 

compose half the width of upper plates at the detachments; 

panels of pre-Tertiary rocks were not in contact with one another 

above detachments except during early stages of extension. 

Although very late slip on near-surface domiform detachment 

faults is down dip into widening basins, earlier slip was regionally 

westward and northwestward, independent of present directions 

of dip of the detachment surfaces. 

In the southern Great Basin, extension farthest west has 

taken place during Pliocene and Quaternary time, that farthest 

east took place mostly in the middle Miocene; and a broad 

medial tract was extended during late Oligocene and early 

Miocene time, with a westward younging of termination of major 

extension-middle Miocene in the Nevada Test Site, late 

Miocene in the Bullfrog Hills, Pliocene in the Funeral Mountains, and still active in Death Valley. The imbricate middle 

Miocene faulting of Yucca Mountain represents the headwall 

position of the Bare Mountain-Bullfrog Hills-Funeral Mountains 

detachment system when the part of the original fault to the east 

had risen so that further slip there was precluded but when the 

part to the west still dipped westward. 

Middle Tertiary extension was approximately westward, 

relative to the continental interior, and this produced northtrending fault blocks. Late Neogene extension has been northwestward, and the old blocks, with their inherited trends, are 

being carried obliquely apart. The present east limit of major 

deformation is along the east side of Death Valley and is marked 

by northwest-trending right-slip faults in the north and south and 

by oblique-slip north-trending normal faults between them. 

INTRODUCTION 

The width of the Basin and Range province has been 

about doubled by crustal extension during middle and late 

Cenozoic time. The sort of structures on which this deformation has been accomplished are exposed particularly well in 

the Death Valley region (fig. 5.1), where rapid extension 

is continuing. This chapter describes and interprets the 

extensional structures of that region and, to a lesser extent, 

surrounding regions. Many of these interpretations are 

controversial. For a regional synthesis with conclusions 

broadly incompatible with those presented here, see Carr 

(1984). 

Detachment Faulting in the Death Valley Region 51 
Yucca Mountain, in the southwest part of the Nevada 

Test Site and the northeast corner of the Death Valley region 

(see fig. 5.2), is being considered as a repository for radioactive waste. An understanding of the late Cenozoic structure of the region is needed for evaluation of the deep 

geometry of the site and of the possible ground motions and 

disruptions that might accompany earthquakes. Neogene 

structures are exposed within the test site only at upper crustal 

levels. They are largely middle Miocene in age and consist 

mostly of normal faults, with steep to moderate dips, between 

gently rotated blocks. Moderate extension of the upper crust 

is required by this geometry, but far more extension took 

place in early Miocene and late Oligocene time. Extensional 

120° 

0 

KILOMETERS 

structures of the sort that originated within the middle crust 

and that are likely to lie deep beneath Yucca Mountain are 

exposed widely in the Death Valley region. 

Exposures are excellent in Death Valley and vicinity, 

where altitudes are near sea level, but mediocre closer to 

the test site, where altitudes are typically near 1,000 m. Compare the many photographs, presented in this chapter, of 

detachment faults exposed superbly at low altitudes with the 

two photographs (figs. 5.3B, 5.4C) of detachment faults 

exposed poorly at altitudes of about 1,000 m. Exposures are 

still poorer in much of the test site itself and throughout most 

of the Great Basin. Because structures of the type displayed 

so well in Death Valley are likely to be present much more 

ROCKY 

MOUNTAINS 

Figure 5.1. Physiographic provinces of the Western United States. Basin and Range province (shaded) of middle and 

late Cenozoic extensional faulting consists of the Great Basin and the Mojave and Sonoran Deserts. Regions of similar 

deformation in the Rio Grande Rift and north of the Snake River Plain are also shaded. 

52 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
widely in the Great Basin than has been generally recognized, 

I have included many photographs of them as possible guides 

to relations that might be sought elsewhere. 

Studies in many parts of the Basin and Range province, 

including the Death Valley region, have demonstrated that 

severe extension incorporates detachment faulting. A detachment fault is a major undulating or gently dipping extensional 

fault that attenuates a crustal section. The term has been used 

in this context by G.A. Davis (Davis and others, 1980) and 

many others. The terms "denudation fault" (Armstrong, 

1972) and "decollement" (Miller and others, 1983, as used 

in an extensional context only) are synonyms of detachment 

fault. The origin of these structures is much debated. 

Early geologists in the Death Valley region were at the 

forefront of interpretations of extensional geology. Emmons 

(1907) and Emmons and Garrey (191 0) mapped a detachment fault and recognized the extensional nature of normal 

faulting half of a century ahead of most Basin and Range 

geologists. The so-called chaos and turtleback features 

described by Curry (1954), Noble (1941), and others from 

Death Valley can now be placed in the framework of great 

extensional deformation. Hunt and Mabey (1966) were first 

to recognize a complete domiform detachment fault, the 

Tucki Mountain fault in the Panamint Mountains, as being 

of extensional origin. Hamilton and Myers (1966) discussed 

the region and reached the then-outrageous conclusion that 

Cenozoic extension had doubled the width of the Great Basin. 

Stewart (1983) recognized that the great upper-plate block 

of the Panamint Mountains had moved completely over the 

Black Mountains lower plate. Much of the mapping of the 

last several decades in the Black and Funeral Mountains has 

been done by L.A. Wright and B.W. Troxel (Wright and 

Troxel, 1973, 1984; Wright and others, 1974), who have 

emphasized extensional explanations but have interpreted 

exposed structures largely in terms of imbricate normal 

faulting related to deeply hidden detachment faults. Work 

underway by B.C. Burchfiel, B.P. Wernicke, and others is 

focused on exposed detachment faults. 

My work in the Death Valley region consists of widespread reconnaissance, which provides a basis for interpretation of mapping published by other geologists. I brought to 

the region biases developed in the course of my work in 

southeastern California and my contacts with many other 

geologists working with detachment faults in that region; 

among those to whom I am indebted for comprehension of 

these structures are G.A. Davis, G.H. Davis, K.A. Howard, 

Barbara John, and S.J. Reynolds. I developed from geologicmap and seismic-reflection data the interpretation (Hamilton, 

1982, 1987) that the middle crust is extended primarily by 

the sliding apart of lenses separated by zones of ductile shear, 

that the upper crust is broken into rotated blocks in response 

to the widening of the composite top of these separating 

lenses, and that most detachment faults are the tops of these 

lenses. A contrary model was developed by B. P. Wernicke 

(1985), who argued that master normal faults cut through 

the entire crust at gentle to moderate angles, that isostatic 

rise carries these faults toward the surface as they are denuded 

tectonically, and that detachment faults are primarily 

exposures of such master normal faults. Although in my 1987 

paper I continued to argue for the broad applicability of the 

crustal-lens model, in this paper I instead accept much of 

Wernicke's model as applicable to upper-crustal deformation; I retain the crustal-lens model for much middle-crustal 

deformation. An extensive fieldtrip with Wernicke in early 

1987 convinced me of the validity of much of his interpretation. I return to the subject of crustal mechanics in this paper 

after describing the structural geology. All photographs here 

are my own, and their lengthy captions are intended to 

augment the brief generalizations in the text. 

The southwestern part of the Great Basin, which 

includes the Death Valley region, is the most actively 

extending part of the Basin and Range province. Within the 

area shown in figure 5.2, or near it to the west and northwest, 

are the highest steep scarps, the most severe deformation of 

Quaternary materials, the deepest topographic closures of 

structural depressions, and the youngest exposed detachment 

faults. Indeed, some gently dipping detachment faults 

exposed at the surface are still active. Nonresistant Neogene 

upper-plate materials are widely preserved above the 

detachments, and late Quaternary valley fill hides much less 

structure than in less active regions. Also shown in figure 

5. 2 are detachment faults that are exposed in the Panamint, 

Funeral, and Black Mountains, Bare Mountain, and the 

Bullfrog Hills. The Argus and Greenwater Ranges and the 

Cottonwood and Grapevine Mountains display only upperplate materials but must be underlain by detachment faults 

at shallow depth. 

The following discussion proceeds in a general way 

from east to west within the Death Valley region, and thus 

from areas in which there is little active faulting to areas of 

great young activity. 

BARE MOUNTAIN, BULLFROG HILLS, FUNERAL 

MOUNTAINS, AND YUCCA MOUNTAIN 

The farthest northeast exposed detachment fault of the 

Death Valley region is that of Bare Mountain and the Bullfrog 

Hills, which is now inactive. This Bare Mountain detachment fault projects eastward beneath Yucca Mountain. The 

detachment faults exposed in Bare Mountain, the Bullfrog 

Hills, and the Funeral Mountains appear to be domiform 

exposures of a single fault surface that initially dipped westnorthwestward and was denuded as the Grapevine Mountains upper-plate block slid down it. 

Bare Mountain Detachment Fault 

The lower plate of the Bare Mountain detachment fault, 

which consists of variably metamorphosed Late Proterozoic 

and Cambrian strata, has been much extended internally by 

normal faults that now mostly dip gently (Cornwall and 

Detachment Faulting in the Death Valley Region 53 
36"30' 

36"00' 

0 

MESQUITE 

FLAT 

10 20 

54 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

116"30' 

........ 

30 

KILOMETERS 
Kleinhampl, 1961, who regarded the Neogene faults as thrust 

faults; Cornwall, 1972; Monsen, 1982; M.D. Carr and S.A. 

Monsen, oral commun., 1985, 1986). Metamorphism of the 

lower-plate rocks increases gradually from lowest greenschist 

facies in the far south (slaty pelites, nonmarbleized carbonates: my observations) to lower amphibolite facies in the 

northwest (staurolite + garnet, epidote + hornblende: 

Monsen, 1982; T.D. Hoisch, oral commun., 1987). 

Upper-plate rocks have been stripped from most of the 

Bare Mountain lower plate but are preserved in the south 

and north. At the south end of Bare Mountain, nonmetamorphosed Cambrian strata are truncated downward against the 

gently plunging detachment fault (fig. 5.3A). Middle Miocene 

ashflows, about 11 m.y. old, and slide megabreccia lie upon 

the southern upper-plate Cambrian rocks and are but gently 

tilted (Swadley and Carr, 1987). Around the north end of 

Bare Mountain, upper middle Miocene volcanic rocks, older 

than about 11.2 m. y. , dip downward to truncations against 

the detachment fault (fig. 5.3B), whereas lower upper 

Miocene volcanic and sedimentary rocks about 10.5 m.y. 

D 

m 

Ill 

I I I 

EXPLANATION 

Quaternary materials 

UPPER-PLATE ROCKS 

Tertiary, sedimentary and volcanic rocks, 

Oligocene through Pliocene 

Paleozoic, includes Mesozoic granitic rocks 

Precambrian, Middle Proterozoic plutonic rocks 

and Late Proterozoic sedimentary and 

metasedimentary rocks 

LOWER-PLATE ROCKS 

Metamorphic and plutonic rocks, of diverse ages, 

below lowest exposed faults 

Faults, dotted where concealed 

Strike-slip, arrows indicate relative movement 

Normal, hachured on downthrown side 

Detachment faults and normal faults rotated to 

gentle dips, hachures on upper plate 

Strike and dip of inclined strata 

Figure 5.2. Tectonic and generalized geologic map of Death 

Valley region, California and Nevada. Most normal faults are 

omitted. Adapted mostly from published maps cited in text. See 

figure 5.1 for location. 

old and younger lie unconformably across both the fault and 

the deformed upper-plate volcanic rocks and are merely tilted 

(Florian Maldonado and P.P. Orkild, oral commun., 1985, 

1987; compare Carr, 1984). Breccia of unmetamorphosed 

Cambrian and Late Proterozoic rocks locally intervene 

between upper and lower plates (fig. 5.3B) and attest to the 

earlier passage across the lower plate of an upper plate of 

Paleozoic strata, presumably that of the present Grapevine 

Mountains. The time of inception of faulting is not defined 

by local data, but in the Grapevine Mountains (discussed 

below), which likely slid westward from above Bare Mountain, extension probably was underway in late Oligocene 

time. The faulting about 11 m.y. ago represents only the last 

slip on the detachment fault, which thus had a history that 

predates also the faulting of Yucca Mountain. 

Although Bare Mountain is deeply eroded, its crest and 

lateral ridges define a symmetrical domiform surface which 

plunges northward and southward to approximately become 

the exposed detachment fault, and hence likely has been but 

little eroded beneath the detachment fault elsewhere. This 

surface dips under upper Quaternary alluvium on both the 

east (fig. 5.3A, C) and west sides of the range, where there 

are no exposures of upper-plate rocks. A steep range-front 

fault was inferred by Carr (1984), Cornwall (1972), and 

Cornwall and Kleinhampl (1961) to bound the east side of 

Bare Mountain. M.C. Reheis (chapter 8) found evidence for 

local disruption of alluvium at the foot of the range and 

inferred that steep faulting has broken the front within 

Holocene time. 

The absence of range-front scarps in either bedrock or 

surficial materials (fig. 5.3A, C) is strong evidence against 

the presence of a young range-front fault at the east base of 

Bare Mountain. The surface defined by the crests of the 

lateral ridges projects smoothly beneath the alluvium of 

Crater Flat with a dip of about 30°. The continuation of that 

surface beneath the alluvium with about that inclination is 

demonstrated by detailed gravity traverses across the range 

front (Snyder and Carr, 1982). No gravity gradient is present 

as would be if a steep fault was at the foot of the bedrock 

exposures. Farther east under Crater Flat, a steeper subsurface contact at considerable depth can be fitted to 

ambiguous gravity models but is not required. Carr (1984), 

Carr and others (1986), and Snyder and Carr (1982, 1984) 

inferred two Miocene calderas to lie beneath Crater Flat, but 

facies and distribution of Miocene ignimbrites require no 

ignimbrite sources in this area, and the gravity analysis by 

Snyder and Carr is in my view merely a visual aid in support of permissive rationales for such calderas. 

Bullfrog Hills Detachment Fault 

The detachment fault of the Bullfrog Hills, west of 

northern Bare Mountain, has an upper plate of Miocene 

volcanic and sedimentary rocks, as young as 7.5 m.y. old, 

which dip moderately to steeply eastward and northeastward 

Detachment Faulting in the Death Valley Region 55 
A 

B 

c 

56 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
to a truncation against the poorly exposed fault, which defines 

a gentle north-plunging dome (Florian Maldonado, written 

commun., 1985; Cornwall and Kleinhampl, 1964; Emmons 

and Garrey, 1910). Faulting continued later here than in Bare 

and Yucca Mountains nearby to the east but is now inactive 

or nearly so. 

The Bullfrog lower plate consists of Late Proterozoic 

strata metamorphosed at amphibolite facies, including 

staurolite-kyanite assemblages (T.D. Hoisch, oral commun., 

1987), Jurassic(?) pegmatite, and variably mylonitized 

undated gneiss. Between the upper and lower plates is a sheet 

of slightly metamorphosed and much disrupted Paleozoic 

strata, analogous to the carapaces seen on many other detachment faults in the region and formed presumably as a friction breccia beneath the Grapevine Mountains block as it slid 

over the Bullfrog lower plate. 

Funeral Mountains Detachment Fault 

What I regard as a continuous domiform Funeral 

Mountains detachment fault with an elliptical trace was 

mapped as many separate faults of diverse types by Giaramita 

(1984, who recognized its continuity around the north end 

of the complex), Hunt and Mabey (1966), McAllister (1971), 

Swadley and Carr (1987), and L.A. Wright and B.W. Troxel 

(written commun., 1986, unpublished map; see also Labotka, 

1980, and Labotka and Albee, in press). The lower plate has 

a domiform upper surface both where the upper plate is 

preserved and where it has been stripped away (fig. 5.4). 

Lower-plate rocks are metamorphosed clastic strata, 

and subordinate carbonate rocks, that are at least mostly Late 

Figure 5.3. Bare Mountain detachment fault. The last important 

slip on this fault occurred about 11 m.y. ago. A, View southwest 

to south end of Bare Mountain. Detachrilent fault (line on left; 

the right skyline is eroded little beneath the fault) separates 

Middle or Upper Cambrian limestone and dolomite (dark hill 

on left) from low-grade-metamorphic Late Proterozoic clastic 

rocks (rest of range to right). Upper plate slipped obliquely 

toward the right distance. 8, View east along detachment fault 

(heavy dashes) at north end of Bare Mountain. Lower plate 

(foreground and right) is of metamorphosed Lower Cambrian 

and Late Proterozoic rocks. Upper plate is of east-dipping 

ashflows and tuffs of upper middle Miocene Timber Mountain 

Tuff. A tectonic lens of unmetamorphosed but severely brecciated Middle or Upper Cambrian carbonate rocks intervenes in 

the middle distance (below light dashes). Upper plate moved 

relatively west (left). C, View north-northwest along east side 

of central Bare Mountain, which consists of low-grademetamorphic Lower Cambrian and Late Proterozoic carbonate 

and clastic rocks. The lateral ridgecrests have been little eroded 

beneath the detachment fault bounding these lower-plate rocks. 

Detailed gravity surveys show the bedrock surface to continue 

with gentle inclination beneath the alluvium. The lack of rangefront scarps in either bedrock step indicate that there has been 

little if any Quaternary faulting here. (See also part A.) 

Proterozoic but may include basal Cambrian in the far 

southeast. These rocks were metamorphosed to lower 

greenschist to upper amphibolite facies, probably during 

Jurassic time (Labotka and Albee, in press). At the southeast 

end of the exposed complex, where the detachment-fault trace 

lies along the south side of upper Echo Canyon, Proterozoic 

lower-plate clastic rocks are metamorphosed at low grade 

and are now quartzite and glossy slate and phyllite; the sparse 

clastic rocks in the directly overlying upper-plate Paleozoic 

section, which consists mostly of carbonates, are nonmetamorphosed sandstone and argillite that lack secondary mica 

(my observations; also, T.D. Hoisch, oral commun., 1987). 

Metamorphic grade increases northwestward in the lower 

plate to upper amphibolite in the far northwest (Labotka, 

1980; Labotka and Albee, in press), and the high-grade rocks 

are intruded by abundant sheets of pegmatite and muscovite 

granite. Kyanite is present in middle- and high-grade rocks, 

locally with sillimanite at highest grade; these high-grade 

rocks formed at a depth of about 20 km and a temperature 

near 700 oc (Giaramita, 1984; Labotka, 1980; Labotka and 

Albee, in press). At least the high-grade metamorphism must 

have involved introduction of magmatic heat (Labotka and 

Albee, in press). Whether the low-grade rocks of the 

southeast part of the lower plate formed at lesser depth, as 

I infer, than the high-grade northwestern rocks, or at a comparable depth but much lower temperature, has not been 

proved. 

Foliation and layering are subparallel in the transposed 

middle- and high-grade rocks of the lower plate, whereas 

cleavage cuts bedding in the low-grade rocks. Orientation 

of layering is quite variable, but dominant dips are eastward 

at gentle to moderate angles (Wright and Troxel, unpublished 

map). 

The central and northwest parts of the lower plate bear 

a retrograde greenschist-facies carapace, 10 to 150m thick, 

within which foliation and transposed lithologic layers are 

parallel to the detachment fault that bounds the carapace. 

Continuity of the carapace on ridge crests shows that the 

gently domiform topography defined by those crests 

(fig. 5.4A, B) is eroded very little beneath the detachment 

fault. The carapace displays isoclinal ductile and semiductile folding that verges in the same direction, top to the west 

(fig. 5.5A, B), as the slip of upper-plate rocks and opposite 

to fold vergence in nonretrograded amphibolite-facies lowerplate rocks beneath the carapace (fig. 5.6), where I have 

studied carapace and core in Monarch Canyon and along the 

western range front. The carapace apparently was developed 

during the early, deep-seated stage of Tertiary extension. 

Giaramita (1984) also recognized this west-verging folding 

and shearing, and its significance, of retrograded high lowerplate rocks at the north end of the lower plate. The carapace 

in many places has a veneer a few meters thick of mylonitized 

carbonate rocks, smeared out as a lubricating ductile layer 

(fig. 5.5B, C). I saw no carapace at the south end of the lower 

plate, although as I did not see actual exposures of the fault 

there, a thin carapace could be present. 

Detachment Faulting in the Death Valley Region 57 
A 

B 

c 

58 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
Upper-plate rocks, rotated mostly to dip steeply to 

moderately northeastward in the north and eastward and 

southeastward elsewhere, and sharply truncated downward 

against the detachment fault, are nonmetamorphosed Late 

Proterozoic and Paleozoic sedimentary rocks and upper 

Oligocene and lower, middle, and lower upper Miocene 

sedimentary and volcanic rocks (figs. 5.4B, C, 5.5C). Upperplate Late Proterozoic and Paleozoic rocks stand mostly as 

the high, rugged Grapevine and southern Funeral Mountains, 

whereas Tertiary units, which dominate the upper plate atop 

the intervening northern and central Funeral Mountains, have 

mostly been eroded to low ground flanking the dome and 

are broadly covered by undeformed Quaternary materials. 

Upper-plate rocks moved relatively westward above all parts 

of the lower-plate dome, including its east flank and the large 

klippe on its crest. Upper-plate pre-Tertiary strata are nonmetamorphosed in both the Grapevine and southern Funeral 

Mountains and show no apparent northwest increase in grade 

parallel to that of the lower plate. A lens, crescentic in plan, 

of coherent low-grade metasedimentary rocks intervenes 

between the main lower plate of high-grade rocks and the 

upper plate of nonmetamorphosed strata at the north end of 

the Funeral Mountains. Slide megabreccia layers are intercalated in lower Miocene strata at the west base of the Funeral 

Mountains (fig. 5.5C), so upper-plate faulting probably was 

underway by early Miocene time; a late Oligocene age of 

inception is deduced below from evidence in the Grapevine 

Mountains. 

The complex upper plate in the southeastern Funeral 

Mountains consists of panels of Paleozoic strata overlain, 

with broad concordance, by uppermost Oligocene and lower 

and middle Miocene strata (Cemen and others, 1985), rotated 

to general eastward and southeastward dips between westand northwest -dipping normal faults (McAllister, 1970, 

1971) which mostly have gentle dips and are not known to 

cut the detachment beneath. This composite of domino panels 

Figure 5.4. Domiform detachment fault of the Funeral Mountains. The last slip on this fault occurred during Pliocene time. 

A, View southeastward into the northern Funeral Mountains. The 

domiform crest is essentially the stripped detachment fault. Sense 

of slip of upper plate was westward across the dome, from distant 

left to near right. B, View northwestward along the west side 

of the Funeral Mountains, from Keane Wonder Mill. Mylonitic 

carapace (layering, right) was developed concordantly below 

detachment fault by shearing and retrogression of Late Proterozoic middle-grade metasedimentary rocks. Arrow indicates 

dip of upper-plate middle Tertiary redbeds in low bluff beneath 

late Quaternary pediment. Upper plate moved relatively leftward. 

White mass in left middle distance is a Holocene travertine 

terrace, which here covers the fault trace. Upper-plate Grapevine 

Mountains in left distance. C, View west along domiform detachment fault (lines), northern Funeral Mountains. Unmetamorphosed latest Proterozoic upper-plate clastic strata are truncated 

downward against lower-plate amphibolite-facies much older 

Late Proterozoic strata. Head of Monarch Canyon. 

is in turn broken by a range-front fault, trending southeastward, southwest of which are steeply to moderately dipping early Pliocene strata which overlie the upper Miocene 

strata of the northern Black Mountains (Cemen and others, 

1985; Hunt and Mabey, 1966; McAllister, 1970, 1973). The 

Pliocene strata are tightly to openly folded about doubly 

plunging northwest-trending axes (McAllister, 1970, 1973). 

It appears that extension at the south end of the Funeral 

Mountains continued through at least early Pliocene time and 

was followed by modest northeast-southwest shortening, and 

that the area has been internally stable during at least late 

Quaternary time. I infer from the complex configuration of 

the range-front fault that it was rotated to a gentle dip, then 

broadly folded. 

The upper plate in the Grapevine Mountains consists 

of complexly deformed but nonmetamorphosed latest Proterozoic to Pennsylvanian sedimentary rocks overlain on the 

northeast by a thick northeast-dipping section of upper 

Oligocene and Miocene sedimentary and volcanic rocks 

(Reynolds, 1969, 1976). Sedimentary breccias are intercalated in the Oligocene section, and I assume that these were 

shed from nearby fault scarps and hence that extension was 

underway in late Oligocene time. Subhorizontal faults, 

topographically high in the range, mostly place younger 

Cambrian rocks on older ones and at least in part display 

top-to-the-west slip; beneath these faults are large recumbent folds with southwest vergence. Both faults and folds 

may be products of middle Tertiary extension, the faults having been normal faults that dipped moderately westward 

before northeastward tilting of the range. Reynolds (1969) 

mapped many patches of middle Tertiary strata and sedimentary breccia, which, where attitudes are shown, mostly dip 

moderately downward to subhorizontal contacts, atop high 

ridges of Cambrian rocks; I infer these contacts to be 

segments of a rotated range-front fault. 

The Grapevine Mountains face Death Valley on the 

southwest with a high but much-eroded scarp, and the range 

is cut by deep canyons. This old frontal normal fault trends 

southeastward into the Funeral Mountains detachment fault 

but does not cut it-the range-front fault is restricted to the 

upper plate. 

The domiform gravity high of the Funeral Mountains 

lower plate plunges gently under the Grapevine Mountains 

upper plate in the northwest and the southern Funeral Mountains upper plate in the southeast (Chapman and others, 1977; 

Mabey, 1963). Apparently those great upper-plate blocks are 

perched atop the dome. 

Continuity of Faulting between Bare Mountain, 

Bullfrog Hills, and the Funeral Mountains 

Temperature of metamorphism increased northwestward in the Bare Mountain lower plate, from perhaps 350 oc 

to 500 °C, and the Bullfrog Hills lower plate records a yet 

Detachment Faulting in the Death Valley Region 59 
higher temperature and a depth of metamorphism of perhaps 

20 km. There appears to be a northwestward increase of 

depth of exposure in the Bare Mountain and Bullfrog Hills 

lower plates. The Bare Mountain and Bullfrog Hills detachment faults dip gently toward one another beneath tectonically 

overlying Miocene volcanic rocks, so likely either both 

domiform detachment faults expose parts of a single fault 

surface or else the Bullfrog Hills lower plate represents a 

deeper crustal lens that emerged from beneath the Bare 

Mountain one. 

A 

Figure 5.5. Retrograde carapace produced on Funeral 

Mountains lower plate concordantly beneath detachment fault. Transposition was in same west-directed 

sense as was slip of overlying upper plate, and represents 

early deep-seated shearing on the fault system that 

evolved into the shallow, brittle detachment fault. A, 

Carapace in north wall of Monarch Canyon. Detachment 

fault projects just above skyline; entire wall, 120 m high, 

is in carapace of rocks transposed parallel to fault. Sense 

B 

of overturning of ductile folds (most conspicuous in 

sheared white Jurassic? pegmatites) is top-to-the-left C 

(west). Host rocks are Late Proterozoic mica and hornblende schists and, most abundant high on the wall, carbonates. 

8, Mylonitized marble veneer (layered rocks, upper left half of 

view) atop carapace. Brittle detachment fault is just above the 

exposure, which is 6 m high. Dark marble layer near top is 

deformed in a west-verging isocline; thick marble beneath it is 

boudinaged; gouge lies beneath boudinaged quartz vein that 

forms base of layered sequence. Rock at lower right is phyllonitic 

60 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

This northwestward increase in probable depth of 

metamorphism parallels that in the continuously exposed 

lower plate of the Funeral Mountains, where also metapelites 

increase in grade from slate and phyllite in the southeast to 

kyanite-staurolite schist in the northwest. Biotite, garnet, and 

kyanite isograds can be projected from the Funeral Mountains northward and north-northeastward across Amargosa 

Valley to Bare Mountain and the Bullfrog Hills. 

A broad, shallow gravity low lies in Amargosa Valley, 

between the exposures of the Bare Mountain and Funeral 

augen schist. View north, 1 km northwest of Keane Wonder mill. 

C, Detachment fault, 3 km northwest of Keane Wonder mill, 

looking north. Smooth rock surface (right center) sloping gently 

left is the stripped lower plate of the detachment, which has a 

veneer 1 m or so thick of mylonitized marble (light color). Lower 

Miocene redbeds, tuff, and breccia (left center) dip steeply east 

to truncation against fault, which is in part marked by dashes. 
Mountains detachment faults (Chapman and others, 1977; 

Mabey, 1963; Snyder and Carr, 1982). I infer that lowerplate rocks are present at shallow depth throughout this area, 

that upper-plate rocks consist largely or entirely of lowdensity Cenozoic materials, and that overlying postdetachment Pliocene and Quaternary deposits are thin. Indeed, 

probable lower-plate rocks make small outcrops near the 

center of the valley (Swadley and Carr, 1987). 

The detachment faults of the Funeral Mountains, 

Bullfrog Hills, and Bare Mountain may represent upwarps 

of a continuous fault surface that initially cut west-northwestward down into the crust, from a depth of 10 km or less in 

present southeastern exposures to 20 km or more in northwestern ones, over a present distance of about 30 km. A 

depth change from 10 km to 20 km corresponds to an initial 

westward dip of about 20°. Gently dipping normal faults have 

been mapped within the lower plates of Bare Mountain by 

Cornwall and Kleinhampl (1961), Monsen (1982), and 

Monsen and M.D. Carr (written commun., 1986, 1987), and 

of the Funeral Mountains by L.A. Wright and B.W. Troxel 

(written commun., 1986), indicating that these lower plates 

were themselves extended; hence, the initial dip of the master 

fault that is now seen as the detachment was probably steeper 

than this 20 o • 

Slip of Late Proterozoic and Paleozoic rocks on this 

master fault was relatively small in the southeast, for in both 

Bare Mountain and the Funeral Mountains nonmetamorphosed strata are placed on slightly metamorphosed strata, 

but large in the northwest, where the nonmetamorphosed 

rocks of the Grapevine Mountains overlie deep-seated 

crystalline rocks of the Funeral Mountains and, presumably, 

the subsurface western projection of the Bullfrog Hills lower 

plate. The intervening tract of lower-plate rocks was denuded 

tectonically as the Grapevine block slid across it, and synextensional Tertiary sedimentary and volcanic rocks now 

form the upper plate in the gap. 

Such an explanation accords with the model of B.P. 

Wernicke, with the addition to his proposed mechanism of 

extension within the lower plate as well as of the upper plate 

and with uplift of the lower plate being progressive by sectors. A great normal fault initially dipped westward, more 

steeply than 20° by an angle not yet well constrained, to a 

depth of at least 20 km. The upper-plate rocks initially above 

the shallow end of this fault were fragmented into great and 

Figure 5.6. Lower-amphibolite facies Late Proterozoic metacarbonate rocks, below carapace in canyon 

1 km east of Keane Wonder mill. S-folds (arrows to hinges) indicate shear of top-up-to-east (left) sensethis Jurassic(?) synmetamorphic shearing had sense opposite to that in Neogene carapace (figs. 5.5A, 8). 

Exposure 4 m high. 

Detachment Faulting in the Death Valley Region 61 
small blocks, from the size of ranges such as the Grapevine 

Mountains downward. The denuded part of the lower plate 

rose isostatically, eastern upper-plate blocks were stranded 

atop the flattened sector of the fault while western blocks 

continued to move, and the age of last slip decreased 

westward as the hingeline between dipping and flattened fault 

migrated in that direction. 

Biotite and muscovite K-Ar ages of pre-Tertiary lowerplate crystalline rocks of the Funeral Mountains, Bullfrog 

Hills, and northern Bare Mountain scatter from 10 to 50 m.y. 

old, but most are between 10 and 22 m.y. (M.D. Carr, written commun., 1985; Giaramita, 1984; McKee, 1983; Stern 

and others, 1966). These ages of cooling presumably record 

Miocene uplift, by tectonic denudation, from midcrustal 

depths at which ambient temperatures were above those 

permitting full retention of argon in the micas. 

Imbricate Faulting of Yucca Mountain 

Middle Miocene volcanic rocks, broken and rotated by 

subparallel west-dipping imbricate normal faults, form a 

nearly continuous ring from Yucca Mountain westward and 

southward to the north and south ends of exposure of the 

Bare Mountain detachment fault. That fault dips gently 

eastward beneath Crater Flat (fig. 5.3A, C) and hence 

projects beneath Yucca Mountain, and volcanic rocks 

correlative with those of Yucca Mountain are rotated down 

and truncated against the detachment. The extensional 

deformation of Yucca Mountain likely is a product of slip 

on the Bare Mountain detachment fault. Yucca Mountain 

faulting is much more severe in rocks 12.5 to 13.5 m.y. old 

than in those less than 11.5 m.y. old, and was largely completed by 10 m.y. ago (Carr, 1984; Maldonado, 1985; Scott 

and Bonk, 1984). Dips of Yucca Mountain faults increase, 

and dips in its strata decrease, away from the exposures of 

the Bare Mountain detachment fault (Scott, 1986). Late 

middle Miocene rocks farther east in the Nevada Test Site 

display much less deformation than do the correlative rocks 

of Yucca Mountain. There has been little normal faulting 

since the early late Miocene, and presumably any underlying detachment faults also are inactive or almost so. 

Scott and Whitney (1987) deduced from these relations 

that the west-dipping imbricate faults of Yucca Mountain 

represent a headwall complex by which slip on the Bare 

Mountain detachment fault reached the surface, and they 

argued that the fault does not continue farther east at depth. 

I agree with their analysis insofar as it applies to the last small 

bit of detachment faulting, that which postdated 12.5 m.y. 

ago. The explanation is not applicable to the earlier very 

different configuration and vastly greater slip on the fault. 

I see Yucca Mountain faulting as having occurred above 

the I'\Jl1-m.y.-old hingeline between flattened and dipping 

master fault. By that time, the tectonically denuded fault 

surface had risen in the east to an undulating surface on which 

there was little if any continuing slip, whereas in the west, 

62 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

it still dipped moderately westward so that slip was continuing. This earliest late Miocene hingeline lay far to the west 

of successively older hingelines and associated headwall complexes, which in turn lay far to the west of the initial positions of their preextension upper- and lower-plate complexes. 

The upper-plate Grapevine Mountains block had by late 

Miocene time already gone by-most of the detachment slip 

predated the Yucca Mountain episode. 

BLACK MOUNTAINS 

The central Black Mountains display detachment faults, 

or undulating exposures of a single fault, the lower plates 

of which consist entirely of plutonic rocks, and the upper 

plates of which consist variously of Middle Proterozoic and 

younger plutonic rocks, Late Proterozoic sedimentary rocks, 

and Miocene to Quaternary sedimentary and volcanic rocks 

(fig. 5.2). Early stages of extension, recorded by ductile 

structures, were presumably of middle Tertiary age; latestage extension, dated by local involvement of upper-plate 

Neogene strata, was underway by early late Miocene time. 

Faults in the present medial and eastern Black Mountains 

are now largely inactive, and may have become so progressively with time from east to west. Along the west side 

of the range, west-dipping sectors of detachment faults are 

still active, and perched, active range-front faults in the upper 

plates end downward against those active detachment faults. 

These conclusions are based partly on maps and descriptions 

by Curry (1954), Drewes (1959, 1963), Hunt and Mabey 

(1966), Noble (1941), Otton (1977), Wright and others 

(1974), and Wright and Troxel (1984), and partly on my own 

field reconnaissance. 

Amargosa Detachment Fault and Amargosa Chaos 

Noble (1941) recognized, in the low Jubilee Pass area 

between the central and southern Black Mountains, an 

undulating fault that separates plutonic rocks beneath from 

a variety of Proterozoic, Cambrian, and Neogene materials 

above. The fault surface defines a series of antiforms, dipping and plunging gently, over an area about 15 km wide 

across the strike. Noble recognized the consistent westward 

transport of rocks above the fault relative to those below. 

He emphasized that a thick crustal section had been cut out 

across the fault, which he nevertheless assumed to represent 

Tertiary compressive overthrusting and named the 

''Amargosa thrust fault.'' I here refer to the fault as the 

Amargosa detachment fault. N able described widespread 

megabreccia in the upper plate and deduced that it formed 

by disruption of a thin overriding sheet. He classified the 

breccia, on the basis of its constituent rock types, into Virgin 

Springs, Jubilee, and Calico phases of the "Amargosa 

chaos.'' 
Much of Noble's area was mapped in detail by Wright 

and Troxel ( 1984), and I have toured the area both with 

Troxel and alone. The interpretations by Wright and Troxel 

have evolved since their map went to press; my statements 

in this section represent a mixture of their published interpretations, their subsequent interpretations as I understood 

them from discussions in 1985 with Troxel and in 1986 with 

Wright, and my own interpretations. Noble applied his terms 

very inconsistently, but his ' 'Calico phase of the Amargosa 

chaos'' typically is a coherent upper Miocene formation of 

volcanic and tuffaceous rocks which include thick marblecake 

megabreccia apparently formed by intrusion and extrusion 

of magmas into wet bentonitic mud. Noble's ''Jubilee phase'' 

typically is a coherent lower upper Miocene redbed formation that contains intercalated slide megabreccia of mixed 

plutonic rocks and Late Proterozoic and Tertiary sedimentary rocks. Much of Noble's "Virgin Springs phase" is a 

discontinuous friction breccia of severely disrupted and 

attenuated Late Proterozoic sedimentary rocks, as much as 

100m thick, at the base of the upper plate (fig. 5.7A, B), 

rubbed off a transiting upper plate of Late Proterozoic 

sedimentary rocks; Noble lumped the large block of coherent 

Late Proterozoic strata north of Jubilee Pass with this 

"chaos." The detachment-fault domes are discontinuously 

outlined by carapaces of these friction breccias derived from 

overriding Late Proterozoic rocks, and the carapaces dip both 

eastward and westward beneath structurally overlying 

Neogene rocks. This widespread friction carpet of Proterozoic strata between upper-plate Neogene rocks and lowerplate plutonic rocks demonstrates that the parts of the lower 

plate now overlain tectonically by Tertiary materials have 

been denuded tectonically by the westward slip of Proterozoic 

rocks, and hence that the slip recorded by the truncations 

of Tertiary strata is but a small fraction of the total slip of 

upper-plate rocks above the faults. 

Wright and Troxel (1984) did not make this interpretation and instead inferred the west flanks of the domes to be 

relatively minor west-dipping normal faults rotated to gentle 

dips, and the east flanks to be truncated against unexposed 

west-dipping faults; the map relations to me preclude this 

interpretation, and I see, as did Noble, an undulating underlying fault. Upper-plate units dip generally eastward to their 

truncations downward against that fault. Upper-plate rocks 

are broken by numerous west-dipping normal faults, but as 

far as I am aware these have not been proven to penetrate 

the lower plate. 

As the various detachment antiforms are separated by 

upper-plate materials, it is possible that they represent the 

tops of crustal lenses that are separated by gently dipping 

shear zones, and that there has been great extension within 

the lower plate as well as within the upper plate. 

The western antiform of the detachment surface in the 

Jubilee Pass area rises and broadens northwestward to include the plutonic rocks of the high central Black Mountains 

(fig. 5.2; Desert Hound anticline of Noble, 1941; Wright 

and Troxel, 1984). On the west, Late Proterozoic sedimentary rocks, locally with the Middle Proterozoic plutonic basement rocks on which they were deposited preserved beneath 

them, form a large east-tilted block that is truncated 

downward against the detachment fault. Beneath the detachment are plutonic rocks, which include Jurassic granitic and 

gneissic rocks (I base this correlation on the lithologic 

similarity of the distinctive but undated megacrystic granodiorite and quartz monzonite here with dated Jurassic granitic 

rocks of the Mojave Desert and regions to the south), likely 

also Middle Proterozoic plutonic rocks, and possibly Tertiary 

granite. A major perched range-front fault bounds the upperplate Proterozoic rocks against east-dipping Neogene strata 

and megabreccia in the low ground at the west foot of the 

range. This fault is limited to the upper plate-the fault trends 

northward into the detachment fault, which there dips gently 

westward, and ends against it. The west front of the central 

Black Mountains nearby to the northwest is the detachment 

surface from which tectonically overlying Neogene strata 

have been largely stripped. The detachment fault is exposed 

discontinuously along the foot of the range where upper-plate 

Pliocene redbeds and basalt are preserved in the low ground. 

The lower plate here consists of Jurassic granitic rocks; the 

juxtaposed upper-plate redbeds contain no clasts of this rock 

type but instead were derived from upper-plate Late Proterozoic rocks similar to those against which the redbeds are 

faulted by the perched range-front fault to the south, so the 

redbeds which now form the hanging wall of the detachment 

fault originated as the hanging wall of an upper-plate normal 

fault and were rotated downward against the detachment as 

extension progressed. Along much of this sector, upper-plate 

rocks are wholly buried beneath upper Quaternary Death 

Valley alluvium that laps onto the stripped footwall. 

The Amargosa detachment fault along the east side of 

the Desert Hound antiform was mapped northward to lat 

36° N. by Noble (1941) and Wright and Troxel (1984), and 

was traced about 2 km farther north, as the ''Amargosa thrust 

fault,'' by Drewes (1963). It is not clear from Drewes' reconnaissance map whether the detachment fault continues 

northward as the structural top of all plutonic rocks or 

descends into the undivided crystalline rocks; I infer the 

latter. Drewes schematically mapped breccia, near the contact between plutonic rocks and overlying Tertiary volcanic 

and sedimentary rocks, of Late Proterozoic or Cambrian 

formations which he regarded as coherent Paleozoic rocks 

shuffled with Neogene strata by networks of Neogene thrust 

and normal faults. I have not seen these complexes but deduce 

from Drewes' descriptions that the breccia might be a friction carpet atop lower-plate plutonic rocks (in which case 

they mark a detachment fault structurally high above the Mormon Point turtleback detachment fault at the west foot of the 

range), or that, more likely, they are slide breccias intercalated stratigraphically in Neogene formations (in which 

case the subjacent plutonic rocks may be the basement rocks 

for these formations and be in the upper plate of the 

Amargosa detachment fault). If the first alternative is correct, then the turtleback faults, discussed next, are the tops 

Detachment Faulting in the Death Valley Region 63 
A 

B 

Figure 5.7. Amargosa detachment fault, jubilee Pass area, Black Mountains. A, Unconsolidated upper Pliocene or lower 

Pleistocene fanglomerate (light colored, left, bedding parallel to lines) dips eastward to truncation against the gentle west 

flank of a dome in the Amargosa detachment fault. Domiform, lower-plate hill of plutonic rocks (right; Rhodes anticline 

of Noble, 1941) is little eroded beneath projected detachment fault (lines, arrows). A friction breccia of Late Proterozoic 

rocks, like that in part B, is preserved between fanglomerate and lower plate around base of dome. View northwest from 

about 4 km east-northeast of Jubilee Pass. B, Disruption along Amargosa detachment fault. Gouge (between lines near center) 

separates upper and lower plates. Exposed part of lower plate consists of crushed and altered Middle Proterozoic gneiss. 

Exposed part of upper plate is a friction breccia (part of "Virgin Springs phase of Amargosa chaos" of Noble, 1941) of 

greatly attenuated Late Proterozoic dolomite (light, top), diabase (dark, center), and other sedimentary rocks (directly above 

gouge). Exposure about 30m high, 3 km east-northeast of Jubilee Pass. Upper Neogene strata (not in view) dip downward 

to a truncation against this breccia, as in part A. 

64 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
of tectonic lenses deeper than the Amargosa detachment fault. 

The second alternative appears more likely, however, as the 

turtleback faults that emerge from beneath these structurally high plutonic rocks bear carapaces of mylonitized Late 

Proterozoic strata, from which I infer that the turtlebacks 

represent the same structural level as the Amargosa detachment fault. The denudation of the structurally high plutonic 

rocks presumably was due to Tertiary extension in any case. 

The lower plate of the Amargosa detachment fault is 

continuously exposed north to the Mormon Point turtleback 

along the west base of the Black Mountains, and I have 

arbitrarily connected the east flanks of Mormon Point and 

Desert Hound anticline structures on figure 5. 2. 

Around the southern part of the central Black Mountains, middle and upper Miocene volcanic and sedimentary 

rocks generally dip more steeply than do Pliocene ones 

(Wright and Troxel, 1984), but all are truncated by the same 

undulating Amargosa detachment fault. In the western part 

of this terrane, Pleistocene strata also are rotated, although 

to gentler dips, in the same general eastward direction 

(Wright and Troxel, 1984). The abundance of slide megabreccia in late Miocene sections attests to the presence of 

upper-plate scarps at that time. In the northern Black Mountains, steep-dipping Pliocene strata lie semiconcordantly 

above upper Miocene ones (Cemen and others, 1985; Hunt 

and Mabey, 1966). Faulting is still very active at the west 

foot of the northern Black Mountains. 

The southern Black Mountains, south of the area shown 

in figure 5.2, consist mostly of plutonic rocks, and these, 

as I read the map of the north edge of them by Wright and 

Troxel (1984; I have not been in this area), lie entirely 

beneath the Amargosa detachment fault. The rest of the 

southern Black Mountains is covered by the 1:250,000 map 

compiled by Jennings and others (1962) from reconnaissance 

maps, mostly still unpublished. The compilation map shows 

the plutonic rocks to be faulted against -overlain structurally 

by, at least in part-Late Proterozoic, Cambrian(?), and 

Tertiary strata on the east and south. I infer that the plutonic 

rocks, about 15 km by 20 km in exposed dimensions, lie 

beneath the Amargosa detachment fault, whether that fault 

is a continuous single surface or the composite top of separate 

crustal lenses. 

Turtleback Detachment Faults 

Three stripped domiform detachment faults along the 

west base of the central Black Mountains were aptly termed 

"turtlebacks" by Curry (1954), who assumed them to be 

anticlinal folds in a thrust fault. They were studied further 

by Drewes (1959, 1963), Otton (1977), and Wright and 

others (1974). Drewes regarded the faults as ancient shear 

zones along which small surficial blocks are now sliding into 

Death Valley. Wright and his associates inferred the turtlebacks to be giant mullions on a normal fault which had 

undergone large late Miocene to Quaternary slip and had 

evolved from deep and ductile to shallow and brittle. 

The turtlebacks are marked on figure 5.2 by the fault 

closures at Badwater and at Copper Canyon and by the 

northwest end, at Mormon Point, of the large southern fault 

dome. The west flank of each of these turtleback detachments 

is presently active-large active normal faults in the upper 

plates end downward against them (figs. 5.8, 5.9). The hanging wall of each normal fault becomes the hanging wall also 

of the detachment fault, down which it is now sliding. Late 

Quaternary slip of the normal-fault footwall blocks on the 

detachment faults has not yet been demonstrated. 

The turtleback domes plunge 15 a to 25 a northwestward. Flank dips are asymmetric, gentle to the northeast but 

moderate to the southwest. Concordant mylonitic carapaces 

fade out downward in the footwall plutonic rocks. Mylonitized Late Proterozoic dolomite and other metasedimentary 

rocks veneer the Badwater carapace (Curry, 1954) and the 

northeast flank of the Copper Canyon one (Otton, 1977; figs. 

5. 9, 5 .1 0). This veneer represents a smear of rocks not otherwise present locally either above or below those turtlebacks, 

unless they form part of the Mormon Point lower plate 

(Otton, 1977). Mullions and mylonitic lineations trend 

dominantly northwestward along the turtlebacks. Although 

critical for interpretation of the turtlebacks, it is not clear 

from published descriptions whether the ductile-slip features 

of the steep southwest flanks of the turtlebacks correspond 

to those of the gentle northeast flanks-that is, whether or 

not the contrasted flanks had similar histories before late 

Quaternary time. Variable brittle brecciation occurs beneath 

the faults (fig. 5.10) and postdates the ductile deformation. 

Gouge zones along the faults vary greatly in thickness, as 

do friction breccias at the bases of the overriding plates. The 

slip direction of upper-plate rocks is shown by mylonitic 

lineations and other structures to have been northwestward 

during the ductile phase, oblique to the northerly trend of 

the range, relative to lower-plate rocks. 

Upper-Plate Normal Faults 

Normal faults break the rocks above the turtleback 

detachment faults but end at the detachments. 

The Artist's Drive fault of the northern Black Mountains has an imposing west-facing scarp developed from 

upper Miocene volcanic and tuffaceous rocks (figs. 5.8A). 

The steep, young part of the scarp is as high as 500 m, and 

along much of its base is a late Holocene scarp, 5 to 10m 

high, on which northwest-plunging mullions and slickensides 

indicate oblique slip. Erosion of the 500-m scarp from the 

fault dip of 50° or so to the present rugged angle-of-repose 

slope of35° has occurred primarily by landsliding, and late 

Quaternary slide megabreccias form a hilly apron along the 

fault. 

The abundance of similar megabreccias in Tertiary sections rotated to steep dips and truncated against detachment 

faults, throughout the Death Valley region as in many other 

parts of the Basin and Range province, is to me one of many 

indications that high normal-fault scarps were formed at the 

Detachment Faulting in the Death Valley Region 65 
A 

Figure 5.8. Overlapping eastward views showing large rangefront fault of northern Black Mountains truncated downward 

against Badwater turtleback detachment fault. A, Artist's Drive 

range-front fault (marked by arrows) is a major Quaternary structure with late Holocene slip. High-standing footwall is of upper 

Neogene volcanic and tuffaceous rocks. Hanging wall (low 

varicolored hills) includes both downdropped upper Neogene 

rocks and Quaternary slide megabreccias from the scarp. 8, 

Artist's Drive fault (between left and center arrows; see part A) 

surface while major detachment faulting went on at greater 

depth. 

The Artist's Drive fault, the steeply tilted rocks of its 

footwall, and the slide breccias of its hanging wall are all 

truncated sharply against the gently dipping Badwater 

turtleback fault (fig. 5.8B), as Curry (1954) recognized. The 

extensional Artist's Drive normal fault has had Holocene slip 

and affects only the upper plate of the detachment system, 

so the detachment fault beneath the hanging wall of the 

normal fault must also have had Holocene slip. The footwall Miocene rocks are rotated downward to a sharp truncation against the turtleback, so there must be much young 

slip beneath the footwall of the normal fault also, although 

that part of the detachment fault is not known to be presently 

active. The certainly active part of the gently dipping fault 

is that which dips westward into the widening Death Valley. 

Along strike to the south, another major young normal 

fault in the upper plate is truncated southward and downward 

against the Copper Canyon turtleback. Here, the footwall 

of the normal fault comprises plutonic rocks, and the hanging wall, upper Neogene sedimentary and volcanic rocks 

rotated to moderate eastward dips; footwall and hanging-wall 

rocks and the intervening fault end against the smooth detachment fault (fig. 5.9). Drewes (1959, 1963) and Otton (1977) 

both mapped the abrupt truncation of the normal fault against 

the turtleback and recognized that hanging-wall slip shifted 

66 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

is truncated against detachment at center arrow. Turtleback surface is stripped dome in center of view. Erosion has caused retreat 

of the perched scarp (right arrow). As the extensional Artist's 

Drive fault has Holocene slip, at least the hanging wall of that 

fault has slipped on the detachment fault in Holocene time. Footwall of Artist's Drive fault is of strongly rotated upper Neogene 

strata, so it too has had much late Neogene slip on the 

detachment. 

from the normal fault to the turtleback. The turtleback, with 

its mylonitic carapace and metasedimentary-mylonitic 

veneer, continues eastward beneath the footwall of the normal 

fault, and presumably that part of the detachment is now 

inactive. 

Similar relations hold at the north side of the Mormon 

Point turtleback. A gently dipping, concave-upward fault 

bounds a block of Pliocene(?) and Quaternary strata, rotated 

to eastward dips, against the turtleback surface in the south, 

but against plutonic rocks structurally above the turtleback 

in the north. 

The hanging-wall blocks north of the Mormon Point 

and Copper Canyon turtlebacks are themselves bounded 

against the alluvium of Death Valley by active normal faults 

(fig. 5.9). Latest Pleistocene and Holocene alluvial fan 

surfaces are broken by small normal faults close to the range 

front as defined by these upper-plate faults and by the 

turtlebacks themselves. These steep faults presumably are 

refracted upward through unconsolidated or poorly consolidated materials from faults that dip more gently at shallow 

depth. Analogous coseismic structures from the 1959 Hebgen 

Lake earthquake in Montana were described by Myers and 

Hamilton (1964). 

The southwest side of each of the three turtlebacks is 

a stripped fault surface that dips markedly more steeply than 

the northeast side. The alluvial fill of Death Valley is being 
B 

Figure 5.8. Continued. 

actively offset along these steeper faults, for scraps of 

alluvium cling to the faces high above the present alluvial 

surfaces. It is not yet clear whether the steep southwest flanks 

of the turtlebacks share the ductile-slip histories of the gentle 

northeast flanks or represent younger faults cutting older, 

gentler structures in some places and merging with them in 

others. 

Continuity of Faulting in Black Mountains 

The detachment faults of the central Black Mountains 

are exposed as a number of separate plunging domes, no 

contacts between which are known to be exposed, over a 

terrane about 15 km wide and having a proved extent, in 

a north-northwest direction, of 45 km; the southern Black 

Mountains terrane, if indeed consisting of subdetachment 

rocks as discussed above, adds 20 km more to the length. 

Are the detachment antiforms structural highs in a single fault 

surface that is continuous between them, or are at least some 

of the antiforms the tops of lenses which originated at various 

structural levels? 

Lower-plate rocks consist of granite, migmatite, gneiss, 

and augen gneiss, so all these rocks may have been in the 

middle crust when extension began. The meager data 

available provide an inadequate basis for evaluating the 

possibility that there is a systematic variation in depth of 

exposure of these rocks beneath the detachment fault, or 

faults, in some direction. The presence of carapaces of 

mylonitic rocks on the turtlebacks in the northwest but of 

breccias on the domes in the Jubilee Pass area perhaps 

indicates that early slip on the former was at a greater depth 

than that on the latter, but the possible contrast has been too 

little studied for confident analysis. 

The upper plate of the central Black Mountains, above 

the turtleback detachment faults, consists of plutonic rocks 

overlain directly by upper Miocene strata. Presumably the 

plutonic rocks were exposed at the surface by tectonic 

denudation after the Panamint Mountains block had passed 

across this region, and the lower plate of that structure was 

broken in turn by structurally lower faults. 

Relation between Detachment Faults of the Black 

and Funeral Mountains 

The thick section of upper Miocene and Pliocene 

sedimentary and volcanic rocks that on the west side of the 

Black Mountains is truncated downward against the turtleback detachment faults dips steeply to moderately northeastward off the northeast side of the range to a truncation 

against the inactive range-front fault of the southeastern 

Funeral Mountains. This frontal fault likely is limited to the 

upper plate of a detachment system. The axis of the gravity 

low that presumably marks the deepest part of the base of 

the Neogene strata lies only a few kilometers from the trace 

of the range-front fault (Chapman and others, 1977; Mabey, 

1963). The shallowness of this gravity low precludes the 

presence of more than 1 or 2 km of the low-density strata, 

so the Neogene section projects downdip to a shallow truncation against gently dipping faults atop dense rocks. A 

northwest-trending gravity high in an area of upper Neogene 

strata of the northeastern Black Mountains, midway between 

the Bad water turtleback and the gravity low in front of the 

Funeral Mountains, may mark a hidden turtleback in the 

underlying detachment system. 

Detachment Faulting in the Death Valley Region 67 
Possibly Black Mountains and Funeral Mountains 

detachment faults may be continuous beneath the upper-plate 

strata that intervene at the surface, and the gravity low in this 

case should define the deepest part of the flexed fault. Alternatively, the frontal fault of the Funeral Mountains may break 

the Black Mountains detachment fault beneath the basin axis. 

The structure in any case represents but a minor late stage 

in the extension in this area, for, as discussed subsequently, 

the Panamint Mountains upper-plate block likely was torn 

from the Funeral Mountains and transported across the Black 

Mountains detachment system and rifted across Death Valley. 

PANAMINT MOUNTAINS 

The Panamint Mountains mostly comprise a great easttilted block that is allochthonous above a major detachment 

fault, exposed around Hanaupah and Death Valley Canyons 

at the east foot of the range, which I here term the Hanaupah 

detachment fault. The upper-plate block is complicated by 

many normal faults rotated and deformed with the block. 

Most of these faults are relatively minor, with slips on the 

order of a kilometer or so each, but aggregate slip on the 

large faults of the structurally high Tucki detachment-fault 

system is probably several tens of kilometers. The imbricate 

Hanaupah and Tucki detachments both cut out thick crustal 

sections. 

Figure 5.9. View east at upper-plate normal fault truncated 

downward against Copper Canyon turtleback detachment fault 

(smooth face right of arrow; has mylonitic veneer of light metacarbonates). Active normal fault (along dots on left, arrow on 

right) has footwall (above) of plutonic rocks and hanging wall 

68 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

Hanaupah Detachment Fault 

The Hanaupah detachment fault is exposed at the east 

base of the range and there dips about 15° westward. It was 

mapped and described by Hunt and Mabey (1966), who 

recognized it as an extensional structure although they 

referred to it as the ''Amargosa thrust fault. '' The lower plate 

consists of augen gneiss in which mylonitization increases 

upward toward the detachment fault. The protolith is undated 

but presumably is either Middle Proterozoic or Jurassic. The 

K-Ar ages of biotite in two samples of gneiss are 11 and 14 

m.y. (Stern and others, 1966). The gneiss is intruded by very 

abundant silicic and intermediate Tertiary dikes which, 

although undeformed, display much static argillic alteration. 

Dikes dip about 70° westward where I examined them north 

ofHanaupah Canyon, and they appear to have a similar steep 

westward dip in a photograph in Hunt and Mabey (1966, 

fig. 97), although they (p. 137) referred to the dikes in one 

locality as ''nearly vertical.'' 

Upper-plate rocks, as truncated against lower-plate 

mylonitic gneiss along the exposed part of the Hanaupah 

detachment fault, are east -dipping upper Miocene volcanic 

rocks intercalated with slide breccias, and, unconformably 

beneath them and dipping more steeply eastward, nonmetamorphosed Ordovician and older strata (Cemen and 

others, 1985; Hunt and Mabey, 1966). These Phanerozoic 

(below) of upper Neogene volcanic rocks. Fault is truncated 

against detachment fault at point of arrow. Neogene rocks are 

sliding down detachment fault and are rotated back into it. 

Detachment swings into steeper range-front fault on right; upperplate Neogene rocks are broken by range-front fault on left. 
Figure 5.10. Mormon Point turtleback detachment fault as the 

range-front fault of the central Black Mountains. Fault dips 20° 

west (obliquely toward left front). Upper plate is of Quaternary 

fanglomerate, which dips gently back to truncation against fault. 

Lower-plate friction breccia of Late Proterozoic metadolomite 

is broken into phacoids whose long axes lie in strike direction. 

rocks are the structural top of the main, east-tilted allochthonous Panamint block. Upper- and lower-plate rocks are 

brittly intersheared along the fault, and a thick breccia of 

upper-plate rocks lies atop part of it (Hunt and Mabey, 1966). 

The detachment fault is now inactive. 

I infer that lower-plate mylonitization took place during early, ductile deep-seated slip on the detachment fault, 

in or before middle Miocene time, and that the lower plate 

was still at a temperature of perhaps 200 oc when the 

postmylonitization dikes were intruded into it after middle 

Miocene time. Late, brittle slip was at least largely younger 

than the dikes. Eastward tilting of the lower plate by about 

20° after emplacement of the dikes likely is indicated by the 

Pliocene sediment, Pliocene? 

present attitude of the dikes, so the lower plate is itself 

allochthonous above some deeper structure, and the detachment fault likely dipped about 35 o westward when the dikes 

were intruded into the lower plate. 

Tucki Detachment-Fault System 

Detachment faults form a domiform complex, here 

referred to as the Tucki detachment-fault system, in the 

northern Panamint Mountains, recognized as of extensional 

origin by Hunt and Mabey ( 1966). These structures lie within 

the upper plate of the Hanaupah detachment fault. The lowest 

fault of the Tucki system defines a dome, outlined by a 

mylonitic carapace beneath the late brittle fault, which has 

flank dips of 15° to 30°, a broad crest, and a plunge of about 

20° northward (figs. 5.11, 5.12; Hunt and Mabey, 1966; 

Wernicke and others, 1986; my observations). Although 

domiform, the fault cuts structurally downward westward 

in the lower plate, which consists of Late Proterozoic strata, 

metamorphosed to greenschist and amphibolite facies, and 

Mesozoic muscovite granite. The mylonitic carapace comprises rocks retrograded to lower greenschist facies and 

severely transposed parallel to the domiform fault that caps 

the carapace, is a few tens of meters to perhaps 100m thick, 

and is developed discordantly across the higher grade rocks 

beneath (fig. s: 12B-D). Above the carapace and detachment 

fault is a zone of gouge and incoherent breccia 3 to 50 m 

thick (fig. 5.12B). 

Upper-plate units mostly dip steeply to moderately 

eastward and are truncated against the detachment fault. 

Upper-plate rocks low on the west side of the dome are 

Pliocene fluvial strata, slide megabreccia, and basalt (Hall, 

1971; J.E. Conrad and E.H. McKee, written communs., 

1985; Hunt and Mabey, 1966; Larsen, 1979; Wernicke and 

others, 1986; my observations), faulted directly against the 

lower plate (fig. 5.12£). 

strata 

WEST HORIZONTAL =VERTICAL SCALE EAST 

0 5 KILOMETERS 

Figure 5.11. Section across Tucki Mountain domiform detachment fault, near north end of Panamint Mountains. Sense of slip 

of upper plate is westward on both flanks of the dome. Paleozoic strata: -CI, -Cm, and -Cu, Lower, Middle, and Upper Cambrian; 

0, Ordovician; S, Silurian; D, Devonian. Adapted from Hunt and Mabey (1966, fig. 86). 

Detachment Faulting in the Death Valley Region 69 
A 

B 

Figure 5.12. Domiform Tucki detachment fault at the 

north end of the Panamint Mountains. A, View southsouthwestward across Death Valley to Panamint Mountains, from crest of northern Funeral Mountains. Valley 

floor lies below sea level; Telescope Peak (left skyline) 

rises to almost 3400 m. Entire Panamint block is allochthonous above the Hanaupah detachment fault, exposed 

along base of range on left. Broad dome of Tucki Mountain (right) is little-eroded lower plate ofTucki detachment 

fault. Crests of foreground ridges are little eroded beneath 

Funeral Mountains detachment fault; low hills projecting 

through fans beyond are of upper-plate middle Tertiary 

strata that mostly dip east toward detachment. B, View 

south to Mosaic Canyon and east flank of dome. Smooth 

slope on right, marked by lines, is stripped detachment 

surface; mylonitic carapace 50 m or so thick produces 

layers on top of lower plate of Late Proterozoic metasedimentary rocks. Upper plate (upper left) is of unmetamorphosed Paleozoic carbonate rocks. Sense of slip of 

upper plate was westward (upward and right). A zone of 

70 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

gouge and breccia (between lines left of mouth of canyon) 

separates plates. Top of lower plate swings along rangefront on right but upper plate has slid, or been eroded, 

away. C, Boudinaged and mylonitized Late Proterozoic 

metadolomite in retrograded carapace beneath Tucki 

detachment fault. Face is about 10 m high; top is about 

20m below the detachment fault; view eastward. Mosaic 

Canyon, near mouth. 0, Metadolomite carapace at top 

of lower plate. Left skyline is the stripped detachment fault, 

from which has been eroded south-dipping Pliocene or 

Pleistocene fanglomerate. Microbrecciated metadolomite 

(above lines) has been transposed discordantly across 

variably disrupted metaclastic rocks; both are Late Proterozoic. View eastward, about 3 km west of Mosaic 

Canyon. E, West flank of dome. Smooth bedrock slope 

is stripped detachment, dipping gently west, on Late Proterozoic metasedimentary rocks and Mesozoic muscovite 

granite. Unconsolidated upper Pliocene or lower Pleistocene alluvium dips (lines) east to truncation against detachment. Emigrant Wash. 
On the crest and east flank of the dome, two large 

normal faults (Trellis Canyon and Tucki Wash faults of 

Wernicke and others, 1986; fig. 5.2) and several lesser ones, 

each with top-to-the-west slip but now rotated to gentle dips 

to either west or east, form an imbricate array and merge 

with the basal fault (Hunt and Mabey, 1966; Wernicke and 

others, 1986). In the west part of this complex, where the 

c 

D ..... 

E 

Figure 5.12. Continued. 

aggregate slip on all of the faults is represented by the basal 

fault at Mosaic Canyon, nonmetamorphosed Cambrian strata 

are placed on metamorphosed Late Proterozoic lower-plate 

rocks (fig. 5.12B), and at least 10 km of crustal section is 

missing; but in the east, beyond the inflection away from 

the fault of the structurally higher imbricate faults, the basal 

fault (Harrisburg fault of Wernicke and others) merely places 

less metamorphosed Late Proterozoic strata on slightly older 

and more metamorphosed strata. Lower Cambrian formations lie 15 km farther west at the top of the imbricate stack 

than at the base. This distance is the minimum aggregate offset across the imbricate faults. The stratigraphically highest 

rocks in the east -dipping panels above the imbricate faults 

are present at the east foot of the northern Panamints, where 

strata of probable middle Miocene age overlie Devonian 

strata, both dipping steeply eastward, and are in hidden contact with steeply dipping unconsolidated boulder deposits 

presumed to be of late Pliocene or early Quaternary age 

(Wernicke and others, 1986; B.P. Wernicke, oral cqmmun., 

1987). 

A west-dipping range-front fault, which ended downward against the detachment, must have been eroded from 

above the Tucki dome (fig. 5.11). This fault separated preTertiary upper-plate rocks, on the east, from Pliocene ones, 

on the west. The Cottonwood Mountains consist of Paleozoic 

rocks that presumably overrode the dome and were faulted 

away from east-flank rocks, which thus had a lesser slip 

relative to the dome than did the Cottonwood rocks, but a 

greater slip than did the Pliocene materials. An upper-plate 

fault at the north edge of the dome breaks relatively old 

Quaternary fanglomerate, and upper-plate materials may 

presently be sliding down the detachment fault into the deep 

Mesquite Flat basin to the north. 

The Tucki detachment-fault system thus records a long 

period of slip that began when the lower plate was deep 

enough so that slip was ductile and synmetamorphic and that 

continued at least through Pliocene time as the lower plate 

was denuded tectonically and rose toward the surface. The 

Neogene upper-plate rocks did not exist when slip began, 

and the histories of various parts of the fault system were 

very different. The total amount of slip of pre-Tertiary upperplate rocks across the lower plate increased westward, 

although the present tectonic cover low on the west flank 

is of Pliocene strata that may have slid only a few kilometers. 

Central Panamint Mountains 

The central and southern Panamint Mountains, as 

mapped and described by Albee and others (1981), Hunt and 

Mabey (1966), Labotka and Albee (in press), and Labotka 

and others ( 1980), consist of a great block, tilted to a general 

eastward dip of about 20 o and eroded obliquely through a 

crustal section 10 or 12 km thick. This block is allochthonous 

above the Hanaupah detachment fault, which shares much 

Detachment Faulting in the Death Valley Region 71 
of the eastward rotation of the block, but mostly lies structurally beneath the faults of the Tucki detachment system. 

The down-section progression of this huge block begins in 

the east with upper Miocene (Cemen and others, 1985) 

volcanic rocks and slide breccia, which dip less steeply 

eastward than do the underlying nonmetamorphosed Ordovician and Cambrian strata and hence may have been 

deposited after extension was underway. The crustal section 

continues westward and downward through increasingly 

metamorphosed Proterozoic rocks, bearing andalusite or 

tremolite at lower grades, and sillimanite or diopside in the 

deepest and highest temperature rocks. Shallow, crosscutting Miocene(?) granite is intruded above the middle of the 

crustal section, and semiconcordant Jurassic(?) muscovite 

granite occurs low in it; the amphibolite-facies metamorphism 

resulted from heat added by Jurassic granite and fluids 

(Labotka and Albee, in press). Normal faults, rotated with 

the block so that they now dip gently, break the east -dipping 

crustal section with small top-to-the-west slips that do not 

grossly disrupt the succession, and these faults presumably 

merge with the underlying detachment fault. 

Offset on the relatively minor fault that bounds the east 

flank of the Tucki lower-plate dome decreases southward, 

as mapped by Wernicke and others (1986), and the fault 

apparently dies out within the little-broken main Panamint 

block near Wildrose Canyon and the north edge of the 

Telescope Peak quadrangle as mapped by Albee and others 

(1981). 

The gently dipping west-flank fault of the Tucki complex, by contrast, continues southward as the boundary 

between the main Panamint block and east -tilted Pliocene 

or lower Pleistocene strata. As shown by Wernicke and 

others (1986, fig. 2) and as followed also by me, the trace 

of the fault swings around the Skidoo pluton of Hunt and 

Mabey ( 1966), trends southward beneath the fill of Harrisburg Flats, emerges at Emigrant Pass, and trends southwestward. I found the fault to continue southward across Wildrose 

Canyon near its mouth (fig. 5.13A); I infer that it trends 

thence generally south-southeastward in the northwest comer 

of the Telescope Peak quadrangle to become the fault mapped 

by Albee and others (1981), between Jail and Happy 

Canyons, that dips gently westward between a footwall of 

varied crystalline rocks and a hanging wall of spectacular 

eastdipping monolithologic slide breccia (fig. 5.13B, C). 

Labotka and others (1980) calculated that the source of the 

breccias lay about 3 km updip in the footwall, so the postPliocene slip on the fault has been small. The fault must have 

been a range-front structure when the breccia was deposited. 

Weakly cemented Quaternary gravels that lie unconformably 

on tilted Pliocene strata, near the fault trace around the mouth 

of Wildrose Canyon, are broken by normal faults with 

individual offsets of as much as several tens of meters, so 

slip likely is continuing on the gently dipping fault beneath, 

but probably not on the fault at its surface trace. 

Hanging-wall breccias dip 15° to 20° eastward into the 

fault, which dips 20° to 30° westward (fig. 5.13B, C), but 

72 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

likely were deposited with dips of about 10° away from the 

fault. Hanging wall, fault, and footwall have apparently been 

rotated together about 30° eastward. The main tilting of the 

Panamint upper-plate block thus has taken place within late 

Pliocene and Quaternary time, presumably above a deep 

detachment fault. As the Hanaupah detachment fault and its 

lower plate, at the exposed east base of the Panamint block, 

have also been rotated about 20° eastward, some still-deeper 

detachment fault must be involved. 

The unconsolidated slide breccias of the hanging wall 

of the main fault are being rapidly eroded away, leaving the 

stripped footwall exposed as a high scarp (fig. 5.13C). Many 

steep range fronts of large, tilted fault blocks in the Great 

Basin likely also have been stripped from rotated faults within 

the upper plates of detachment systems, rather than having 

been, as usually interpreted, eroded back from steep faults 

that directly bound adjacent alluviated basins. 

The footwall where of granite has a thick mylonitic 

carapace (fig. 5.13B), and where of metasedimentary rocks 

has a veneer of ductile-deformation rocks (fig. 5.13C). Late 

slip on the fault, including the few kilometers of offset of 

Pliocene strata, took place near the surface; but I infer from 

the ductile structures that much greater slip took place earlier 

while the footwall was at middle-crust depths. The fault 

evolved with time from a deep detachment to a shallow rangefront fault. 

Klippen of east-dipping rocks, requiring large top-tothe-west offset above faults that now dip only very gently 

westward, cap ridges in the medial part of the main Panamint block north of upper Wildrose Canyon (Wernicke and 

others, 1986). Presumably these faults are analogous to those 

atop the Tucki dome farther north and record pre-range-front 

faulting on the detachment. 

OPENING OF DEATH VALLEY 

Death Valley is now being widened obliquely as the 

ranges to the west move relatively northwestward away from 

those to the east. The general eastern boundary of rapid 

extensional deformation in these latitudes lies within or at 

the east side of Death Valley. In the central sector, between 

the Panamint and Black Mountains, the boundary consists 

of the frontal faults along the Black Mountains. These faults, 

like the Black and Panamint Mountains and intervening central Death Valley, trend about 350 o; but kinematic indicators 

along the Black Mountains front indicate the valley side to 

have moved obliquely northwestward away from the Black 

Mountains, not directly westward. In northern and southern 

Death Valley, by contrast, there are no active frontal faults, 

and the east limit of present major deformation is defined 

by active right-slip faults well out in the valleys-the Furnace Creek fault zone in the north, Death Valley fault zone 

in the south. These faults trend northwestward and end 

approximately at the north and south ends of the Black Mountains frontal-fault system. The northwest-trending strike-slip 
faults are thus transform faults to the extension recorded by 

the oblique slip on the north-trending Black Mountains frontal 

faults. Ranges to the west of Death Valley are moving 

northwestward relative to the region to the east. 

Northern Death Valley 

A gentle gravity gradient slopes southwestward from 

the trace of the detachment fault along the southwest side 

of the central Funeral Mountains (Chapman and others, 1971; 

Mabey, 1963), and I infer that the inactive detachment fault 

continues with gentle southwest dip, beneath an upper plate 

of deformed Neogene materials, for 8 to 10 km southwest 

from the fault trace. At that distance, where the likely depth 

to the detachment is 2 or 3 km, exposed late Neogene strata 

are broken by the active, northwest-trending, right-slip Furnace Creek fault (Hunt and Mabey, 1966; L.A. Wright and 

B.W. Troxel, unpublished map). Gravity gradients steepen 

slightly across that fault to define the east side of the Death 

Valley axial low east of Tucki Mountain; the gravity gradient 

that defines the west side of this low is more gentle. To the 

west of the northern Funeral Mountains and southern 

Grapevine Mountains, the gravity gradient across the strikeslip fault is steeper, and it continues into the deep Mesquite 

Flat gravity low north of Tucki Mountain. The gravity 

gradients may indicate that the bedrock floor of Death Valley 

deepens across the strike-slip fault, either by a step or steps 

or with average slopes steeper than those close to the Funeral 

Mountains; or they may indicate that the average age and 

density of Neogene materials above the basement decrease 

across the fault which marks the east limit of late Pliocene 

and Quaternary extension of Death Valley. 

The southeast end of the Furnace Creek strike-slip fault, 

at least as a late Quaternary structure, is at the north end 

of the active, north-trending, oblique-slip Artist's Drive fault 

that defines the front of the northern Black Mountains. The 

strike-slip fault is a transform to oblique slip on the normal 

fault, and the 315 o trend of the strike-slip fault is the direction of modem motion of the Cottonwood Mountains relative 

to the Funeral and Black Mountains. The Argus Range and 

Inyo Mountains are in turn moving northwestward relative 

to the Cottonwood Mountains, as discussed below. 

The Furnace Creek strike-slip fault is commonly 

assumed to continue southeastward, past the Artist's Drive 

fault, to become the frontal fault of the Funeral Mountains, 

with which it is aligned; but the Funeral Mountains frontal 

fault has long been inactive (McAllister, 1970), and its late 

Pliocene motion had a large vertical component. Perhaps the 

two sectors shared Pliocene slip, but certainly only the sector 

within Death Valley is now active. 

The Panamint and Cottonwood Mountains and Argus 

Range all trend about 350 o, whereas the direction of their 

rapid late Neogene relative slip past and away from each 

other and the Black and Funeral Mountains is about 315 o. 

The aggregate amount of extension in this direction increases 

both north-northwestward, as successive ranges move away 

from each other, and westward across it, as adjacent ranges 

are separated by strike-slip and oblique-slip faults. Such 

oblique motion cannot be explained in terms of the simple 

sliding of upper plates down the dip of normal faults inclined 

deep into the crust, a matter discussed later. 

B.P. Wernicke (oral commun., 1987) told me that the 

Paleozoic and middle Miocene structural and stratigraphic 

assemblage of the northeastemmost Panamint Mountains is 

so similar to that of the southeasternmost Funeral Mountains 

that these blocks likely were adjacent and on north-south 

strike in middle Miocene time. The indicated 50 km of 

relative motion of the Panamint Mountains away from the 

Funeral Mountains, in the net direction of 305 o, took place 

by some combination of sliding down the ramp fault 

equivalent to the west flanks of the Funeral Mountains and 

Black Mountains detachments, by motion of the Black Mountains away from the Funeral Mountains, and by the opening 

of Death Valley. 

Central Death Valley 

The active structures that at the surface bound central 

Death Valley, between the Panamint and Black Mountains, 

are the frontal faults of the Black Mountains (fig. 5.14). 

Those faults, like the valley itself and the flanking ranges, 

trend about 350°, but kinematic indicators show upper blocks 

to have moved obliquely northwestward on at least some of 

the faults. 

That central Death Valley has opened by motion of the 

Panamint Mountains obliquely northwestward away from the 

Black Mountains is indicated also by the transform relation 

to the strike-slip Death Valley fault zone in the south. This 

active strike-slip fault, which offsets late Pleistocene 

materials, trends northwestward to end approximately at the 

southward projection of the Black Mountains frontal faults 

(Wright and Troxel, 1984). 

Stewart (1983) argued, on the basis of isopachs and 

facies of miogeoclinal Late Proterozoic and lower Paleozoic 

strata, that the Panamint Mountains upper plate had moved 

80 km relatively northwestward, above the Black Mountains 

on the Amargosa and turtleback detachment faults and across 

the full width of Death Valley on unspecified structures, from 

an initial position against the Resting Spring Range. The 

mylonitic and brecciated carapaces of Late Proterozoic rocks 

left on the Black Mountains detachment faults are remnants 

from the passage of the Panamint block, and the block of 

Late Proterozoic strata near Jubilee Pass in the Black Mountains is a fragment left behind. The Amargosa, turtleback, 

and Hanaupah detachment faults presumably are parts of the 

surface or surfaces upon which this 80 km of slip occurred. 

Great additional motion of the Resting Spring rocks away 

from the continental interior is required by other extensional 

structures farther east. 

Detachment Faulting in the Death Valley Region 73 
A 

B 

74 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
The Panamint Mountains have probably moved about 

50 km west-northwestward relative to the upper plate of the 

southern Funeral Mountains, as noted previously; the upper 

plate of the Funeral Mountains had in tum moved relatively 

westward across the lower plate. 

The physiography of central Death Valley indicates 

rapid eastward tilting (Hunt and Mabey, 1966). Along the 

turtlebacks sector, playa deposits and the evaporites of a latest 

Pleistocene lake lie in the east half of the valley, their surface as much as 85 m below sea level, and in places reach 

the basal scarps of the Black Mountains. These materials 

c 

Figure 5.13. Tucki detachment fault on west side of Panamint 

Mountains. A, Detachment fault (dashed line) in lower Wildrose 

Canyon. Upper Pliocene or lower Pleistocene fanglomerate dips 

(black lines) southeastward into northwest-dipping fault. Lower 

plate consists of dark, metamorphosed Late Proterozoic Kingston 

Peak Formation, in which shearing parallel to the fault and alteration (lightening) can both be seen to increase upward toward 

the fault. Lower plate has thin carapace of mylonitized marble 

(light; best exposed across ridge right of center) about 2 m thick. 

8, Tucki detachment fault (marked by lines), a normal fault 

rotated to gentle dip. Stripped fault surface (smooth bedrock 

slope, right and foreground) concordantly tops mylonitized 

Mesozoic muscovite granite and dips 30° west. Light-colored 

were tilted eastward as much as 6 m in late Holocene time, 

probably coseismically with the development of the young 

scarp along the Artist's Drive fault (Hunt and Mabey, 1966). 

The west half of this sector of the valley is formed of alluvial 

fans rising to an altitude of 400 m or so along the unbroken 

foot of the Panamint Mountains. 

Gravity data indicate that low-density Neogene 

materials are probably nowhere deeper than 3 km beneath 

central Death Valley and that the structural axis lies about 

40 percent of the way between the Black and Panamint Mountains (Hunt and Mabey, 1966; Mabey, 1963). The average 

Pliocene or Pleistocene landslide megabreccias of granite and 

metasedimentary rocks (skyline ridge, and nearby bluff on left) 

dip 20° east into fault. Fault had an early history of deep-seated 

motion, and a late history as a range-front fault with 3 km of 

slip; fault and breccias rotated 25° down to east. jail Canyon. 

C, Rotated normal fault being exhumed as an apparent rangefront fault. Stripped fault surface forms smooth bedrock face in 

upper slopes left of canyon, and crests of steep ridges right of 

canyon. Nonresistant slide megabreccias of the hanging wall 

form frontal hills and dip moderately back to truncation against 

fault but have been eroded from higher levels. View east at Hall 

Canyon. 

Detachment Faulting in the Death Valley Region 75 
slope of the structural or stratigraphic base of all Cenozoic 

materials-including both variably deformed upper-plate 

upper Neogene strata and undeformed valley fill-into the 

deepest part of the structure is near 20 o on the west side of 

the valley and 35° on the east side. The east subsurface 

boundary cannot be a single steep range-front fault; it could 

be a series of step faults or the downdip continuation of the 

turtleback faults or a combination· of steep and gentle faults, 

such as are, indeed, exposed in the Badwater sector of the 

range front. The western subsurface boundary of the valley 

slopes more gently than the average dip, 40° or so (Hunt 

and Mabey, 1966), of the east flank of the tilted Panamint 

block, hence is unlikely to be the eastward continuation of 

the exposed contact between Paleozoic and cotilted Miocene 

strata. 

The east side of the Panamint Mountains is close to 

the stripped contact between resistant east -dipping Paleozoic 

strata and the more easily eroded Miocene rocks that overlie 

them semiconcordantly, and both are truncated downward 

Figure 5.14. View east-northeast across central Death Valley to 

Black Mountains, from central Panamint Mountains. East-dipping 

strata of (near) foreground ridges are of Late Proterozoic and 

(more distant) early Paleozoic ages and are part of Panamint 

Mountains block. Block is allochthonous above Hanaupah 

detachment fault and lower-plate gneiss, exposed in low ground 

near mouth of canyon but not obvious in picture. Late Quaternary Artist's Drive fault (between arrows, a) in Black Mountains 

ends at right against Badwater turtleback detachment fault (tb; 

detachment marked by lines). Footwall of Artist's Drive fault is 

76 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

against the Hanaupah detachment fault exposed locally at the 

foot ofthe range. Gravity data preclude the presence beneath 

Death Valley of a large panel of tilted Neogene materials, 

and likely the shallow high-density floor of the west side of 

the valley consists largely of rocks that are lower plate with 

regard to the Hanaupah fault. No late Quaternary deformation other than tilting is required by available data for the 

west side of the valley and the adjacent east bHse of the Panamint Mountains. 

The continuing Quaternary extension of this sector of 

Death Valley may be accommodated by slip on a fault that 

dips westward from the west base of the Black Mountains. 

The continuing eastward tilting of the valley (and the tilting 

of the entire Panamint Mountains, including the Hanaupah 

lower plate, as discussed previously) presumably is a product 

of flattening of this fault. The bedrock surface of the east 

floor of the valley has an overall slope of only about 35 o. 

If that slope is of the fault surface itself, then the continuation of the fault beneath the west side of the valley and the 

of upper Neogene volcanic and sedimentary rocks; hanging-wall 

Neogene rocks and Quaternary slide breccias form low hills. 

Neogene basin fill represented by footwall of Artist's Drive fault 

forms hanging wall of normal fault (arrows, f) along southwest 

front of Funeral Mountains. Large Quaternary normal faults break 

both turtleback lower plate and, above and right of it, Black 

Mountains upper plate (which consists of plutonic rocks overlain 

directly by Neogene volcanic rocks) and outline both parts of 

range front and canyons in it. Picture taken from Telescope Peak 

(fig. 5.12A). 
Panamint Mountains is at a very gentle angle; alternatively, 

the 35 o slope might mark the top of a complex of stepped 

blocks separated by faults dipping about 60°, and the subPanamint dip could then be perhaps 40°. 

This explanation cannot be complete, for it does not 

account for the obliquity of slip, as is discussed in a subsequent section. 

COCORP Seismic-Reflection Profiles 

The Consortium for Continental Reflection Profiling 

(COCORP) of Cornell University recorded five VffiROSEIS 

lines in southern Death Valley and the region to the east of 

it which were processed and interpreted primarily by Beatrice 

de Voogd and Laura Serpa (de Voogd and others, 1986; 

Serpa and others, 1985). They inferred a structural style of 

imbrication by parallel normal faults that dip about 30° 

westward and end at subhorizontal detachment faults at a 

depth near 15 km, along which in one sector is a great sheet 

of molten basalt. These conclusions are in my view suspect. 

To enhance possible dipping reflectors, Serpa and de Voogd 

applied both increased stacking velocities and severe 

frequency-wavenumber filtering, and they appear to have 

generated pseudoreflectors, to which they mistakenly 

attached geologic significance, by so doing. I have studied 

displays of the five lines as released with their processing. 

I have studied also COCORP shot gathers and less processed 

stacks of some of the data, and lines 9 and 11 as reprocessed, 

without the two suspect procedures, by Eric L. Geist. None 

of these shows most of the apparent reflectors on which de 

Voogd and others (1986) and Serpa and others (1985) based 

their interpretations. The only profile that displays much 

apparent information even with the COCORP processing, 

line 11, lies in the narrow south end of Death Valley, only 

a few kilometers from the front of the Black Mountains; 

de Voogd and others (1986) and Serpa and others (1985) 

assumed that all apparent reflections, which are as distant 

as 6.5 s, are in the vertical plane along the traverse, but complex oblique echoes must in fact be included. Further, the 

inference of molten magma is in no way required by the 

acoustic data. 

OPENING OF PANAMINT VALLEY 

Panamint Valley, between the Panamint and Cottonwood Mountains on the east and the Argus Range and other 

mountains on the west, did not begin to form until late 

Miocene time. Middle late Miocene gravels atop the Darwin 

Plateau, west of northern Panamint Valley, were derived 

from the Panamint and Cottonwood Mountains and were 

deposited on a surface of low relief (Schweig, 1985). Late 

Miocene basalts, about 6 to 8 m.y. old, predate all, and early 

Pliocene basalt most, of the tilting and faulting of the 

Cottonwood Mountains, Darwin Plateau, and Argus Range 

(Larsen, 1979; Schweig, 1985). These basalts are in high 

fault blocks in the Argus Range, are tilted and step-faulted 

down the east flank of Darwin :t:>lateau into northern Panamint Valley, and reappear in tilted fault blocks on the west 

face and crest of the Cottonwood Mountains (Hall, 1971; 

Larsen, 1979; Schweig, 1985). Faults and dikes within the 

Darwin Plateau indicate that internal extension was mostly 

west-northwestward, oblique to the range trend of about 345° 

(Schweig, 1985). Extension in and between these western 

ranges is still at a youthful stage; range-sized blocks have 

not yet been rotated steeply, nor are major gently dipping 

faults exposed. 

The direction of relative motion of the Darwin Plateau 

away from the Cottonwood Mountains is approximately 

defined as west-northwestward by the strike-slip fault that 

trends 300° between them out of the northwest end of Panamint Valley. The post early Pliocene right slip on this fault 

has been 8 or 10 km, as constrained by the offset of a piercing 

point of lower Pliocene basalt above the margin of a large 

Jurassic pluton (Burchfiel and others, 1987). Restoration of 

this slip closes northern Panamint Valley so that the preQuaternary rocks now exposed on the two sides are almost 

juxtaposed. 

The Cottonwood Mountains in turn have slipped away 

from the Panamint Mountains, above the Tucki detachment 

fault discussed previously. The Pliocene section that dips 

east -southeastward to a truncation downward against that 

exposed fault in the northwestern Panamint Mountains is 

marked by only a slight gravity low (Mabey, 1963), hence 

likely the Tucki detachment fault continues northwestward 

with gentle dip beneath the Pliocene rocks. The Pliocene 

section is the top of the large tilted and complexly faulted 

Cottonwood Mountains panel, which thus also is allochthonous above faults of the Tucki system. If the azimuth of 

relative slip of the Cottonwood panel away from the Panamint Mountains was perpendicular to the strike of the tilted 

Pliocene strata, then that slip was toward about 285°. 

Southeastern Cottonwood Paleozoic rocks could have been 

offset from the updip projection of rocks of the main Panamint block 20 km or more to the east-southeast. Restoration 

of such an offset makes the upper-crustal Hunter Mountain 

batholith in the Cottonwoods the shallow, outspread part of 

the Jurassic plutonic complex whose middle-crust part was 

the Skidoo pluton in the Panamint Mountains. 

The slip of the Argus Range and Darwin Plateau 

relatively away from the Panamint Mountains should, ideally, 

equal the sum of the 20 + km of slip of the Cottonwoods 

from the Panamints plus the 10 km of strike slip of the 

Darwin Plateau past the Cottonwoods, or 30 km toward the 

west-northwest. Although an accurate determination is 

precluded by large uncertainties, the true value likely is 

indeed greater than the 18 km width of central Panamint 

Valley as measured along the direction of slip. The east part 

of the present Argus Range block, which consists of uppercrustal granite and contact-metamorphic wallrocks, may have 

lain above the above the high-grade metamorphic rocks of 

Detachment Faulting in the Death Valley Region 77 
the west-central Panamint Mountains before Pliocene extension rather than adjacent to them; of course, the Panamint 

block itself moved a long distance, above other detachment 

faults, relative to assemblages farther east. 

The Argus Range thus is part of the upper plate that 

slid above the detachment fault now exposed along the west 

side of the Panamint Mountains. 

Panamint Valley is marked by a broad, shallow gravity 

low of such low relief and gentle flank gradients that lowdensity materials probably extend no deeper than 1 km within 

it and cannot be bounded by high, steep bedrock contacts 

(Jones and others, 1987; Mabey, 1963). A well in central 

northern Panamint Valley penetrated 115m of unconsolidated 

clastic sediments and bottomed in Paleozoic limestone, 

without intervening basalt (Smith and Pratt, 1957). Magnetic 

anomalies preclude the presence of Neogene basalt beneath 

most of the valley (Jones and others, 1987). The shallow, 

sub horizontal bedrock floor of Panamint Valley may consist largely of rocks that were beneath detachment faults in 

early Pliocene time, rather than being the trailing parts of 

plates above such detachment faults. 

Extension is continuing at least along the west base of 

the central Panamint Mountains, where Pleistocene alluvial 

fans are offset as much as 200 m vertically on normal faults 

marked by little-eroded scarps (Albee and others, 1981). The 

surface of central Panamint Valley is tilted eastward, and 

a large playa lies close to the Panamint front; a 300-m-deep 

well in the playa penetrated only evaporites and unconsolidated clastic sediments (Smith and Pratt, 1957). Slight 

eastward tilting of a Panamint Valley bedrock panel pulled 

away from relatively minor range-front faults may be 

indicated, but major steep normal faults are precluded by 

the gravity data. The active faults lie only 2 to 4 km west 

of the trace of the western Panamint detachment fault; if the 

steep faults end downward at the detachment, then the subsurface part of that gently dipping structure is still active. 

Although dating of progressive deformation within short time 

intervals is much needed, it appears likely that the opening 

of Panamint Valley took place largely during Pliocene time, 

and that Quaternary extension has been of much lesser 

amount. 

EXTENSION WEST OF PANAMINT VALLEY 

West of the area shown in figure 5.2, a mountainous 

tract 50 km or so wide has undergone considerable internal 

fragmentation by normal faults, but only local tilting, and 

is bounded in turn by through-going Owens Valley, beyond 

which is the great raised and slightly tilted block of the Sierra 

Nevada. The extension within this region almost wholly 

postdates basalt of early Pliocene age (Duffield and others, 

1980; Elliott and others, 1984; Larsen, 1979; Novak and 

Bacon, 1986; Schweig, 1985). The separation of the Sierra 

Nevada from ranges to the east and most of its rise to present 

high altitudes was of late Neogene age (references in Jones, 

78 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

1987). Structures are seen at high crustal levels, and I know 

of no detachment faults exposed within this western region. 

Oblique- and strike-slip faults and the rotation directions of 

domino-block complexes indicate the general direction of 

extension to be northwestward relative to the Death Valley 

region. 

EXTENSION SOUTH OF THE DEATH VALLEY 

REGION 

The Death Valley-Panamint Valley-Owens Valley 

region of severe late Miocene through Quaternary extension 

is bounded on the south by the active left-lateral Garlock 

fault. South of the Garlock fault is the Mojave Desert, at 

least the eastern part of which was much extended within 

Oligocene through middle Miocene time; younger activity 

has been dominated by right slip on northwest-trending faults. 

The Garlock fault serves as a transform fault by which the 

northern region, Death Valley to Owens Valley, of active 

extension is bounded against the southern Mojave region of 

little young extension (Davis and Burchfiel, 1973; Hamilton 

and Myers, 1966). From its origin south of the Black Mountains, the Garlock fault trends westward across the south end 

of the Panamint Mountains, then curves west-southwestward 

and southwestward to become the south terminus of the Sierra 

Nevada. Slip on the fault increases from zero at its origin 

through a well-constrained 60 km in its middle sector; if the 

Pelona Schist of the southernmost Sierra Nevada and the 

Rand Schist of the northwest Mojave Desert are offset parts 

of an early Paleogene regional anticline, as I infer them to 

be, then slip in the west is about 100 km. The westwardincreasing offset on the Garlock fault is equal to about half 

the width of the sectors to the north as measured from the 

fault origin, so the northern region has been doubled in width 

within late Miocene through Quaternary time. The Panamint 

Mountains, Black Mountains, and palinspastically adjacent 

Mojave Desert all underwent middle Tertiary extension also, 

and in that sector the 100 percent extension applies only to 

the subsequent deformation. 

Late Neogene extension to the north of the Garlock fault 

has been in a generally northwestward direction relative to 

the continental interior, and northwest-trending right-slip 

faults cross the Mojave Desert. The Garlock fault has rotated 

clockwise, as though about a pivot at its east end, relative 

to the continental interior, as slip on it progressed; the initial 

fault had a much more northerly trend than does the modem 

one. 

EXTENSION EAST OF THE DEATH VALLEY REGION 

The southeastern Great Basin, a terrane 240 km wide 

between the area shown in figure 5.2 and the west margin 

of the little-faulted Colorado Plateau, was extended severely 

during late Oligocene and early and middle Miocene time. 
Prior to extension, this region was part of the Sevier belt 

of eastward-imbricated thrust sheets of pre-Tertiary strata. 

Tertiary strata deposited early in the extensional history of 

a given sector tend to be broadly concordant to underlying 

pre-Tertiary strata, and the old rocks as well as the early 

syntectonic Tertiary strata mostly now dip moderately to 

steeply eastward. This generally eastward tilting of upperplate blocks, including those in the Death Valley region, 

indicates that the extensional faults by which they were 

dismembered mostly had initial westward dips. Local rotation of upper plates to eastward dips can result from slip 

above downward-flattening west-dipping faults, whereas 

regional tilting is most easily explained as due to rotation 

toward the horizontal of a more planar west -dipping 

fault together with its upper- and lower-plate complexes 

(Wernicke, 1985). 

Extension in the northeastern part of the southeastern 

Great Basin, north and east of the latitude and longitude of 

Las Vegas, took place mostly during middle Miocene time 

(Bohannon, 1983). Large imbricate normal faults have been 

rotated to gentle dips, and one in the Mormon Mountains 

has been deformed into a broad dome; only shallow crustal 

levels are exposed beneath the faults, and lower plates consist of nonmetamorphosed Paleozoic strata (Wernicke and 

others, 1984, 1985). 

In the southeastern part of the southeastern Great Basin, 

south and east of the latitude and longitude of Las Vegas, 

a tract, 100 km wide, of Middle Proterozoic plutonic rocks 

was denuded tectonically by the slipping westward away from 

the Colorado Plateau of the Spring Mountains (Wernicke and 

others, 1982, 1984). Middle Miocene volcanic rocks were 

deposited directly on denuded Proterozoic rocks and themselves subjected to severe extension above detachment faults 

now widely exposed (Anderson, 1971; Weber and Smith, 

1987)-perhaps half of the 100 km of relative transport of 

the Spring Mountains occurred during the middle Miocene, 

and half before. The western part of the lower-plate Proterozoic basement rocks southeast of the Spring Mountains 

formed at a depth of only about 10 km (Young and others, 

1986), so a single west -dipping proto fault cannot be invoked 

to explain the slip of the Spring Mountains; the detachment 

faults must represent a number of different rotated imbricate 

normal faults or a number of midcrust lenses, or both. 

The Spring Mountains block, 90 km wide along the 

southwest side of the Las Vegas Valley shear zone but only 

about 50 km wide perpendicular to that, has remained almost 

intact, neither tilted nor fragmented, since Sevier belt 

thrusting (Burchfiel and others, 197 4). The facies and 

thicknesses of upper-plate Paleozoic and Late Proterozoic 

strata, and the specific imbrications of those by Sevier belt 

thrust faults, require that the assemblages now strewn across 

the Nopah, Resting Spring, Funeral, Grapevine, Panamint, 

and Cottonwood Mountains were closed up against the 

southwestern Spring Mountains, in part even resting 

obliquely upon one another, before separation by imbricate 

extensional faults (Stewart, 1983; B.P. Wernicke, oral 

commun., 1987). Of the present cross-strike width, 130 km, 

of these upper-plate complexes, almost 100 km represents 

Cenozoic extension. As the Spring Mountains in tum moved 

about 100 km relatively away from the Colorado Plateau, 

about 200 km of the present distance of 300 km of the 

Cottonwood Mountains from the plateau must represent 

extension. 

In the Nevada Test Site in the northwestern part of the 

southeastern Great Basin, voluminous middle Miocene 

ignimbrites and caldera complexes were erupted during ongoing extension (Byers and others, 1976; Christiansen and 

others, 1977). Successively younger middle Miocene units 

are successively less deformed, but even the oldest of them 

display only modest extension; dips are mostly gentle, and 

no large detachment faults are proved to break surface rocks 

east of Bare Mountain. The middle Miocene ignimbrites bury 

severely extended older terrane on the north. Thus, just north 

of the Nevada Test Site, upper Oligocene rocks are tilted 

with underlying Paleozoic strata to steep dips above normal 

faults rotated to gentle dips; lower Miocene rocks are less 

deformed, and middle Miocene ones still less; and upper 

Miocene rocks are little deformed (Ekren and others, 1971). 

The much-extended region (Wernicke and others, 1984) 

between the Nevada Test Site and the eastern terrane of 

middle Miocene extension likely was also deformed primarily 

during late Oligocene and early Miocene time, for although 

rocks of such ages are all but missing in this area, widespread 

deformation of this age is proved both to the north in Nevada 

and to the south in California. 

The northwest -trending right -slip Las Vegas Valley 

shear zone is a transform complex of strike-slip faults and 

oroclines developed between terranes of different extensional 

patterns (Wernicke and others, 1982, 1984). To the north 

of the zone, from the Nevada Test Site to the Colorado 

Plateau, extension was distributed relatively evenly between 

many allochthonous blocks. To the south, by contrast, the 

large Spring Mountains block was little extended internally 

but moved 100 km westward away from the Colorado 

Plateau. Aggregate right-slip faulting and oroflexing across 

the shear zone is zero in the eastern Lake Mead area, 

increases to a maximum of about 60 km at the eastern Spring 

Mountains, and decreases northwestward to perhaps 30 km 

at the northwestern Spring Mountains. The latter slip fully 

affects rocks that may be as old as Oligocene (see Hinrichs, 

1968), but it largely predated 13.5 m.y. ago, for ashflows 

of that age at Yucca Mountain are oroflexed only about 5 km 

(Scott and Rosenbaum, 1986). 

DISCUSSION 

The southern Great Basin is now 350 km wide-and 

at least 200 km of this represents crustal extension during 

middle and late Cenozoic time. Oligocene and early Miocene 

extension affected a broad medial region; middle Miocene 

extension affected this same region and also an additional 

Detachment Faulting in the Death Valley Region 79 
region to the east; late Miocene extension affected primarily 

the far western part of the previously deformed terrane; 

Pliocene and Quaternary extension has affected in turn the 

western part of that sector plus a broad area of new deformation to the west of that. The average rate of extension has 

been about 7 km per million years, or 7 mm per year. 

Extension was concentrated within less than half the width 

of the widening province at any one time, and strain within 

a broad tract of active extension likely was on the order of 

10 or 20 percent per million years. 

Evidence for this amount of extension comes from 

many criteria. Matching of offset assemblages, and the constraints of transform faults, were emphasized in the preceding 

descriptions. Half the width of upper-plate rocks now lying 

against detachment faults consists of basin-flll materials that 

did not exist when extension began, and this by itself requires 

100 percent extension. Palinspastic reconstructions of 

Mesozoic components in the cordillera make sense to me only 

in terms of Cenozoic extension on the order of 100 percent 

(Hamilton, 1978; Hamilton and Myers, 1966). 

This extreme extension has been accommodated on 

structures of different types at different levels in the crust. 

The brittle structures of the upper crust-normal faults and 

rotated blocks-are seen to end downward against detachment faults that now have undulating surfaces, and many of 

these faults are seen to have originated as ductile faults in 

the middle crust. Rocks which formed the brittle upper crust 

before this extension began are now stranded as allochthonous, rotated blocks atop undulating detachment faults. 

Between these upper-plate blocks are rotated middle and 

upper Cenozoic sedimentary and volcanic rocks, within 

which tilting commonly decreases stratigraphically 

upward-deposition and extension were concurrent. Upperplate rocks have mostly been rotated to eastward dips, so 

the faults that dismembered them mostly had initial westward 

dips; those faults have been rotated with the blocks and now 

dip gently. 

The conventional assumption that basin-range structure 

is dominated by tilted bedrock panels, the high part of each 

forming a mountain range and the low part lying beneath 

a basin, in contact with one another at range-front faults, 

is largely invalid for the southern Great Basin. It is invalid 

also for the Mojave Desert, southeastern California, and 

southern Arizona, although it may be applicable to restricted 

parts of the northern Great Basin. Such deformation is uncommon at the large scale of ranges and basins. Ranges 

instead are mostly allochthonous perched blocks or exposures 

of subdetachment complexes. Also invalid are any models 

which require that major faulting related to now-domiform 

detachment faults represented sliding down the flanks of such 

domes; late, shallow deformation may indeed have such an 

origin, but major slip on the structures is independent of 

direction of present dip of the exposed surfaces. Above all 

the detachment-fault domes of the Death Valley region, 

upper-plate strata display a general regional rotation to 

eastward dips, and hence a consistent slip relatively westward 

80 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

across what are now oppositely dipping flanks of the domes. 

The domes did not extrude upward, shrugging slide masses 

off their flanks, as some geologists have postulated. 

Rocks beneath the detachment faults of the Death 

Valley region were at midcrustal depths before extension 

began and rose to the surface as tectonic denudation removed · 

overlying materials. Early deformation on the deeper parts 

of the lower plates was ductile, but late slip was at low 

temperature. Twenty kilometers of crustal section is missing across much of the Funeral Mountains-Bullfrog Hills 

detachment fault, and likely as much across the Hanaupah 

fault, where the upper plates are of Tertiary rocks, although 

those Tertiary rocks have slid far less on the faults than did 

bedrock upper plates. Favorably oriented detachments remain 

active even at the surface where upper plates are sliding into 

presently widening basins. 

Many detachment faults studied elsewhere in the Basin 

and Range province have similarly evolved from zones of 

ductile slip, initially 10 to 25 km deep, through progressively 

less ductile regimes, to final zones of shallow, lowtemperature, brittle slip (Anderson, in press; Davis and 

Lister, in press; Davis and others, 1986). Many of the faults 

show a temporal progression from early mylonite through 

coherent microbreccia to late gouge, which corresponds to 

a decrease in temperature of deformation from above 300 oc 

for quartzose mylonites (Sibson, 1983) to near-surface 

temperatures for the final gouge. Where early ductile 

deformation resulted in formation of mylonitic gneiss, 

temperatures were above 500 oc (Anderson, in press; 

Sibson, 1983), and this may have been the case beneath the 

turtleback detachment faults of the Black Mountains and the 

Hanaupah detachment fault of the Panamint Mountains. As 

extension progressed to 100 percent at strain rates such as 

the 10 or 20 percent per million years noted above for the 

southern Great Basin, the isotherms limiting ductile behavior 

would have risen to depths on the order of 60 percent of their 

initial depths (England and Jackson, 1987), so the shallow 

limit of ductile deformation rose as tectonic denudation pro--

gressed but lagged behind the uplift of the faults. Although 

the discrimination between mylonites that represent early 

detachment slip and mylonites that are old structures reused 

by young shear systems is often ambiguous, there are a 

number of cases now known in which middle Tertiary 

granite, little older than the detachment faults that cut them, 

display severe mylonitization and even mylonitic gneissification. Rises of lower plates by 10 to 20 km in times as short 

as a few million years, hence local extension rates that may 

exceed 1 cm/yr, seem indicated (Davis and Lister, in press). 

Some detachment faults elsewhere in the Basin and 

Range province have upper-crustal rocks in their footwalls 

as well as their hanging walls and can be demonstrated to 

have cut downward through the upper crust with initial 

moderate to gentle dips and to have been rotated to still 

gentler dips or to sub horizontal or domiform configurations. 

Such a fault was tracked, in excellent seismic-reflection 

records in west-central Utah, from near-headwall outcrop 
to a depth of about 12 km by Allmendinger and others (1983). 

Wernicke and others (1984, 1985) demonstrated such uppercrustal detachment faulting by geologic mapping in the 

southeastern Great Basin. 

Wernicke (1985) argued that exposed detachment faults 

are raised parts of great normal faults that originated as ramps 

cutting gently through the entire lithosphere. He reasoned 

that imbricate upper-plate normal faults sole downward into 

such master faults, which are flattened and raised toward the 

surface by isostatic response to tectonic denudation. The 

west-northwestward increase in probable depth of metamorphism in the Bullfrog Hills-Bare Mountain-Funeral Mountains lower-plate complex accords with the inference that it 

was raised from beneath a continuous inclined surface. That 

same detachment system appears to record a cessation of slip 

progressing westward with time, and the corresponding progressive stranding of allochthonous blocks at shallow levels; 

this can be explained as due to the westward progression of 

the hinge between risen, inactivated sectors of the master 

fault from still-dipping, active sectors. 

Davis and others (1986) applied the Wernicke model 

to a middle Tertiary detachment fault in southeastern California and another in southeastern Arizona and showed that slip 

on ductile, semiductile, and brittle components of the nowundulating detachment systems was all in the same downdip 

sense, in terms of the model. Howard and John (1987) 

applied the model to detachment faults exposed in the lower 

Colorado River region, where the structures cut deeply into 

middle-crust rocks and where the indicated master faults dip 

toward and beneath the Colorado Plateau, not away from 

it as in the southern Great Basin. Howard and John 

demonstrated that the detachment faults obliquely truncated 

thick crustal sections, in both upper and lower plates, in the 

sense required by the model; they also showed that major 

anastomosing or imbricate faults had originated in the middle 

crust, not just a single master fault. 

Much of the deformation observed in the Death Valley 

region and elsewherein the Basin and Range province is 

accounted for by the Wernicke model. The complications 

must be added that different master faults have operated 

sequentially or simultaneously within each region, that lower 

plates are themselves broken by new master faults, and that 

the faults anastomose at depth. Further, the dating of 

termination of slip on different sectors of a single master fault 

indicates that the uplift of a lower plate occurs progressively, 

and that a migrating hinge separates dipping sectors on which 

slip continues from flattened or undulating sectors on which 

it has ended. Allochthonous blocks thus become stranded at 

progressively later times as that hinge migrates beneath them. 

The aggregate slip of any one allochthon is a function of the 

duration of its slippage. Stranding occurred about 11 m. y. 

ago at Bare Mountain, 7 m. y. ago in the Bullfrog Hills and 

northeastern Funeral Mountains, 4 m.y. ago in the 

southwestern Funeral Mountains and eastern Black. Mountains, and 2 m.y. ago in the medial Black Mountains; it has 

yet to occur in and west of Death Valley. 

Wernicke visualized the plates above master ramp faults 

as greatly disrupted by distributed extension but the plates 

below them as unbroken. Much extension is in fact required 

within the lower plates. The many gently dipping normal 

faults within Bare Mountain, for example, indicate much 

extension there. Major ductile or semiductile faults within 

the lower plates have not yet been proved within the Death 

Valley region but have been identified beneath ductile detachment faults elsewhere in the Basin and Range province 

(Hamilton, 1987; Howard and John, 1987). 

Further, the behavior of the deep crust, although still 

poorly understood, is unlikely to be that of the Wernicke 

model. Although Wernicke proposed that the inclined master 

faults cut completely through the lithosphere, continuity 

through the deep crust and upper mantle is improbable. Heat 

flow in much of the Great Basin requires temperatures at 

the base of the crust near those of granite melts; distinct faults 

could not be maintained under such conditions. A transition 

at some depth between structures dominated by unidirectional 

slip above and by ductile flattening below is to be expected 

on rock-mechanic grounds (England and Jackson, 1987). 

Such flattening should occur in discrete zones at relatively 

shallow depths but be more pervasive at greater depths (Kirby 

and Kronenberg, 1987). The upper mantle, by contrast, 

should display much less ductile behavior because of the 

much greater strength of olivine-rich rocks than of quartzofeldspathic rocks. Extension within lower plates in the 

southern part of the Basin and Range province, when they 

were still at high temperatures and considerable depths, produced thick zones of mylonitic gneiss, not merely ductile 

faults (Davis and Lister, in press). Basal-crustal reflectors, 

as tracked continuously across much of Nevada in reflection profiles, are unbroken by steps such as predicted by 

Wernicke (Klemperer and others, 1986). That deep reflection discontinuity is a zone, some kilometers thick, of intercalated rocks of sharply contrasted ac~ustic properties; 

rebuilding of attenuated lower crust by sheets of gabbro 

melted in the mantle can be inferred. 

Many seismic-reflection profiles in southeastern 

California and southern Arizona show the crust beneath 

undulating detachment faults to display large lenses of 

acoustically transparent rock between anastomosing zones 

of gently dipping reflectors that presumably record layering 

due to transposition by ductile flow (Hamilton, 1982, 1987; 

Morris and others, 1986; Okaya and Frost, 1986a, b; Okaya 

and others, 1986). More pervasive, subhorizontal reflection 

fabrics appear to typify the deeper crust. The lenses 

characterize what is now the upper part of the subdetachment crust, but as the rocks exposed beneath such detachment faults mostly originated in the middle crust, the lenses 

are developed in what was middle crust before extension. 

Profiles in the northern Great Basin similarly show a lower 

crust characterized by subhorizontal reflectors, and a middle 

crust that appears to display discontinuous, gently dipping 

zones of reflectors (Allmendinger and others, 1987; Potter 

and others, 1987). 

Detachment Faulting in the Death Valley Region 81 
I deduced from the presence of such lenses and from 

the geometry I perceived in the midcrust rocks exposed 

beneath detachment faults in southeastern California and 

southwestern Arizona a model of extensional deformation 

dominated by the spreading apart of lenses between zones 

of ductile shear, rather like the gravitational spreading of 

a pile of wet fish (Hamilton, 1982, 1987). This model 

resembles in important aspects that of G.H. Davis (1980), 

but not of his later work (1983). I viewed most detachment 

faults as the tops of great lenses that are separated by gently 

dipping,anastomosing ductile faults and that retain prefaulting 

fabrics in their interiors. As the lenses slide apart along 

ductile faults, the area of the composite top of the lensesthe detachment faults-increases. Shallow, brittle materials 

rotate in collapsing blocks as their substrate is widened, and 

in part are torn apart and in part are carried passively along 

on the lenses. Domiform and undulating fault shapes represent primarily deformation due to interactions between moving lenses. 

I now see that (as colleagues tried to tell me) rotated 

great normal faults are much more important than accounted 

for by my model as just described. Wernicke's (1985) model 

accounts far better than mine for much of the deformation 

observed in the field-but his model, beyond its problems 

with deep-crustal mechanics, also leaves important deformation unexplained, and so cannot yet be complete. 

High among the problems is the obliquity of extension 

in the southwestern Great Basin. Although Death Valley and 

Panamint Valley and their flanking ranges trend northward 

or north-northwestward, the relative direction of slip of 

western ranges away from eastern is northwestward, not 

westward. The east boundary of major active extension consists of the northwest -trending, right -slip Furnace Creek and 

Death Valley fault zones and an intervening north-trending 

oblique-slip fault along the Black Mountains. B.P. Wernicke 

(oral commun., 1987) suggested that the northwestward 

motion might be due to a push of upper plates from the south 

by an impinging Mojave plate. Although the northwestward 

shift of the massive Sierra Nevada can be visualized in these 

terms, they would also require that, improbably, the push 

be transmitted 300 km northward along the narrow, unshortened block of the Coso, Inyo, and White Mountains, 

which is moving northwestward with regard to terranes to 

the east. 

The boundary between the North American and Pacific 

plates is the northwest-trending, right-slip San Andreas fault 

system. This plate boundary ends at the northern California 

triple junction with the north-trending Oregon trench and 

west -trending Mendocino transform fault. The triple junction is migrating northward, relative to the continental 

interior, at the slip rate of the San Andreas system, yet it 

is positioned in a zone of sharp change in the trend of the 

continental margin. Perhaps this is a coincidence, and the 

margin of North America south of the present triple junction happened to lie parallel to the direction of relative motion 

of the Pacific plate. It appears to me more likely, consider82 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

ing the patterns of crustal extension within the Western 

United States, that the shape of the continent is changing as 

needed to maintain contact with the Pacific plate south of 

the migrating triple junction. Among the elements of this 

change might be northwestward drag of components as far 

east as the Sierra Nevada, and oblique flattening of the deep 

crust in the Great Basin. Such flattening could be in a 

separating-lens mode in the middle crust and a more pervasive mode in the lower crust. The oblique extension underway at the surface in the Death Valley region may be a 

relatively passive response to the progressively more 

pervasive flattening beneath it. 

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Detachment Faulting in the Death Valley Region 85 

6. Stress Field at Yucca Mountain, Nevada 

By Joann M. Stock and John H. Healy 

CONTENTS 

Abstract 87 

Introduction 87 

Acknowledgments 87 

Stress magnitudes 87 

Stress directions 90 

Agreement with regional data 91 

Topographic effects on the horizontal stress 91 

Tectonic implications 92 

Conclusions 92 

References cited 92 

Abstract 

Hydraulic fracturing stress measurements performed in four 

holes (USW G-1, USW G-2, USW G-3, and Ue25P1) indicate 

that at Yucca Mountain, the least horizontal stress Sh is less than 

the vertical stress Sv· Values of the greatest horizontal stress SH 

are intermediate between Shand Sv, corresponding to a normal 

faulting regime with values of + = (SH-Sh)/(Sv-Sh) between 

0.25 and 0.7. Drilling-induced hydraulic fractures seen on 

borehole televiewer logs indicate an Sh direction of N. 60° W. 

to N. 65 o W. in USW G-1, USW G-2, and USW G-3. The same 

Sh direction is inferred from breakout orientations in USW G-2 

and Ue25P1. The Sh values in the upper parts of the three USW 

G holes are less than the pressure of a column of water filling 

the borehole to the surface. Thus, the long drilling-induced 

hydraulic fractures in the shallow parts of these holes could have 

been formed in attempts to maintain circulation during drilling. 

These low Sh values may be intimately related to the low water 

table and fracture-dominated hydrology of Yucca Mountain. 

INTRODUCTION 

Information on the state of stress at Yucca Mountain 

is important to the evaluation of its suitability as a site for 

a high-level radioactive waste repository. A knowledge of 

the stress field is a key parameter in our understanding of 

the tectonic setting of the site, and the evaluation of possible 

failure behavior of preexisting faults. This knowledge permits 

estimation of the additional stresses which may be induced 

by the repository, for instance, due to excavation or thermal 

effects of the waste. Because the magnitudes of the principal 

stresses influence fluid transport through fractures, the stress 

field may be intimately related to the hydrologic regime 

present at Yucca Mountain. 

The hydraulic fracturing stress measurements performed at Yucca Mountain, as part of the Nevada Nuclear 

Waste Storage Investigations program of the U.S. Department of Energy, yield information on the directions and 

magnitudes of the principal stresses at the locations of the 

individual tests, made within holes USW G-1, USW G-2, 

USW G-3, and Ue25P1 (fig. 6.1). When collectively 

evaluated and combined with other stress field indicators, 

the results provide an important basis for more detailed 

studies, with specific reference to the depths being considered 

for the repository facility. 

Acknowledgments 

We thank W.F. Brace, Art McGarr, and Mary Lou 

Zoback for helpful comments. Hans Swolfs' suggestions 

were also much appreciated, although he does not endorse 

all of our conclusions. The field measurements were performed in cooperation with the Nevada Operations Office 

of the U.S. Department of Energy under interagency agreement DE-AI08-78ET44802. J .M. Stock thanks the Fannie 

and John Hertz Foundation for supporting her graduate work 

at the Massachusetts Institute of Technology, during which 

time this paper was completed. 

STRESS MAGNITUDES 

The hydraulic fracturing method directly measures the 

magnitude of the least horizontal principal stress, Sh, and 

indirectly yields an estimate of the value of the greatest 

horizontal principal stress, SH. The method, first proposed 

by Hubbert and Willis (1957), is now fairly standard and 

is described in detail elsewhere (see Hickman and Zoback, 

1983). Our test procedures and equipment setup are described 

in more detail by Healy and others (1984) and Stock and 

others (1984, 1986). 

This method is based on an analytic solution for stress 

around a cylindrical hole in an elastic, isotropic medium 

(Hubbert and Willis, 1957). If Shand SHare perpendicular 

to the borehole axis, the minimum compressive stress tangential to the borehole occurs at the azimuth of SH. Therefore, 

when the pressure in the holes exceeds this minimum stress, 

a hydraulic fracture should be formed at this azimuth and 

propagate perpendicularly to the direction of Sh. The Sh is 

obtained by observing the normal stress across the newly 

created hydrofracture, defined as the pressure at which the 

fracture closes, or the ISIP (instantaneous shut in pressure) 

Stress Field 87 
visible on the pressure-time curves. The SH is calculated 

from the fracture reopening pressure on the second cycle, 

according to the method of Bredehoeft and others (1976). 

The pore pressure, which is needed for the SH calculation, 

is calculated as the hydrostatic pressure from the observed 

water level in the hole. If testing has altered the pore pressure, 

so that its value is uncertain, then SH cannot be well constrained. Because this was the case in many of our tests, few 

credible SH values were obtained. 

The third principal stress must be vertical, and is 

calculated by integrating the weight of the overlying rocks, 

obtained from compensated density logs and (or) borehole 

gravimetry. The compensated density logs, gravimetry 

(where available), and density measurements of core samples 

were generally in close agreement for these four holes, so 

most of the estimates of vertical stress magnitudes are likely 

to be quite accurate. Where the hole has a large component 

of drift, such as in the lower part of USW G-3, corrections 

• USW G-1 

[£2] 

D 

Nevada 

I 

EXPlANATION 

Fault-Dotted where concealed 

Drill hole location and designation 

Alluvium (Quaternary) 

Volcanic and sedimentary rocks (Miocene) 

AREA OF MAP 

0 2 

KILOMETERS 

Figure 6.1. Generalized geologic map of Yucca Mountain, Nev., showing locations and names of drill holes discussed in text. 

Geology generalized from Scott and Bonk (1984). 

88 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
must be made for absolute depth and changes in overburden 

due to rapidly varying surface topography, thus adding uncertainty to the estimate of Sv. 

If the topography is irregular, small-wavelength variations should be averaged out before calculation of Sv at 

depth. Such corrections are most important for USW G-3. 

For instance, the elevation of the surface at USW G-3 is 

1,480 m; however, the average surface elevation within a 

circle of 700 m radius centered on the top of the hole is 

approximately 1,390 m. If the circle is centered on the surface projection of the position of our lowest test, the average 

surface elevation is 1,370 m. The Sv values listed for the 

USW G-3 tests (table 6.1) include a correction obtained using 

a 700-m radius because that is the average depth of the USW 

G-3 tests below the surface. Maximum uncertainty limits 

on these values can be estimated using the uncorrected value 

of Sv as an upper bound, and the average topography in the 

absence of the excess height of the ridge as a lower bound; 

this gives an uncertainty of ±2.0-2.5 MPa for the Sv values 

in USW G-3. An estimate for the Sv correction can also be 

derived analytically if the surface topography can be approximated as a smooth function, and density and elastic constants are assumed uniform (see Swolfs and Savage, 1985). 

Stress values obtained at Yucca Mountain are summarized in table 6.1 and figure 6.2. The corresponding 

pressure-time data have been discussed elsewhere (Healy and 

others, 1984; Stock and others, 1984, 1986). We reproduce 

here some sample test curves (fig. 6.3) to illustrate the details 

of interpretation and some unusual conditions encountered 

in the Yucca Mountain holes. 

For the deeper successful tests in these holes, the 

breakdown pressures were clear and distinct, and the ISIPs 

occurred at pressures higher than the surface hydrostat 

(pressure of a column of water filling the hole to the surface). The pressure-time curves show an inflection when the 

pressure equals the surface hydrostat, at a level below the 

Sh value obtained from ISIP and steprate injection tests on 

the final cycles. Such behavior was seen during the tests at 

1,038, 1,218, and 1,288 min USW G-1; 1,026 and 1,209 m 

in USW G-2; and 1,573 m in Ue25P1 (see fig. 6.3A). 

On some other tests, good breakdown pressures were 

observed, with ISIP values less than the surface hydrostat. 

Such behavior was seen during the three successful tests in 

USW G-3, at 1,074, 1,338, and 1,356 m; and in USW G-1, 

at 646, 792, and 945 m (see fig. 6.3B). 

Several tests showed breakdown pressures that were 

equal to the fracture reopening pressure on later cycles. This 

suggested that the effective tensile strength of the formation 

was zero, probably because a preexisting fracture was present 

in the interval and was reopened during the test. As this 

occurred in regions of the hole where preexisting fractures 

are likely (above the water table in USW G-2 and in bedded 

carbonate units at 1,564 and 1,693 min Ue25P1), we prefer 

this explanation for the repeatable character of these test 

curves (see fig. 6.3C). Preexisting fractures may not be 

oriented perpendicularly to the sh direction; thus, the 

estimated normal stress across them can only be used as an 

upper bound on the value of the minimum stress (in this case, 

Sh) at depth. 

The types of pressure-time curves discussed above 

encompass those from which values of Sh or SH were 

obtained. A fourth type of curve showed a large pressure 

drop once the interval was open to the pressure in the tubing 

string. This behavior may be due to an unidentified equipTable 6.1. Summary of hydraulic fracturing stress measurements at Yucca Mountain 

[Pore pressures are based on the water levels recorded before testing: USW G-1, 575 m; USW G-2, 526 m; USW G-3, 752 m; Ue25Pl, 385m] 

Drill 

hole 

USW G-2-----

USW G-2-----

USW G-2-----

USW G-1-----

USW G-1-----

USW G-1-----

USW G-3-----

USW G-2-----

USW G-1-----

USW G-1-----

USW G-2-----

USW G-3-----

USW G-3-----

USW G-1-----

Ue25P1------

Ue25P1------

Ue25P1------

Logged 

depth 

(m) 

295 

418 

432 

646 

792 

945 

1 ,074 

1 '026 

1 '038 

1 '218 

1 '209 

1 '338 

1 '356 

1 '288 

1,564 

1,573 

1 '693 

True 

depth 

(m) 

295 

418 

432 

646 

792 

945 11,068 

1 ,026 

1 '038 

1 '218 

1 ,209 11 '321 11,338 

1 '288 

1 '564 

1 '573 

1 '693 

sv 

(MPa) 

6.1 

8.4 

8.7 

12.9 

15.9 

19.2 

20.6±2.0 

20.8 

21.4 

25.5 

25.5 

25.8±2.5 

26.3±2.5 

27.2 

35.3 

35.6 

38.8 

sh 

(MPa) 

< 5.1 ±0.1 

< 5. 4±0. 1 

< 5.5±0.1 

4.2±0.2 

7.2±0.2 

9.0±0.2 

6.8±0.2 

11 .1 ±0. 2 

1 0.6±0.2 

12.1 ±0. 2 

12. 0±0. 2 

11. 5±0. 2 

11 .4±0. 2 

14.8±0.2 

<33.7±0.2 

-20.7±0.2 

.s_36.5±1.0 

SH 

(MPa) 

1 o. 7±1.0 

16.8±0.4 

17.3±0.4 

17. 5±0. 9 

18.1 ±0.8 

17.9 

31. 0±1 .1 

Pore 

pressure 

(MPa) 

0 

0 

0 

0.7 

2.2 

3.6 

3.1 

4.9 

4.5 

6.3 

6.7 

5.6 

5.7 

7.0 

11.5 

11.6 

12.8 

1 Depths in USW G-3 were corrected to true depth prior to calculation of pore pressure and s

8 . 

Sv values in USW G-3 have been corrected for average elevation and are therefore approximate. 

-o.28 

.59 

.39 

-.45 

-.45 

. 25 

.69 

Stress Field 89 
ment problem or to some unusual property of the rock in 

the test interval. Such behavior was common in tests in the 

tuffs and lavas of the Calico Hills unit in USW G-2. 

STRESS DIRECTIONS 

The orientations of the principal stresses can be found 

from observation of directional features in drill holes, 

including hydraulic fractures (both test-induced and drillinginduced) and directional wellbore spalling, or breakouts. 

These and other borehole features are visible on acoustic 

televiewer logs of the drill holes. Operational principles of 

the televiewer have been described by Zemanek and others 

(1969), and details of interpretation of borehole features are 

described in our previous reports. 

Preexisting fractures intersected by the borehole can 

be seen as dark, sinusoidal traces in the televiewer log. Their 

amplitude and phase can be measured to determine the strike 

and dip of the fracture. Such fractures are present in all four 

holes. Their strike directions tend to vary from north to 

northeast, they dip steeply, and they tend to become less 

steep, with more scattered orientations, at depth. Their orientations provide no direct indication of stress direction, but 

they do show that the degree of fracture anisotropy, which 

may be pronounced at shallow levels, decreases or disappears at depth. 

C/) 

...J 

<( 

(.) 

~ 10 a.. 

<( 

(!) 

w 

::E 

~ 20 

CJ) 

C/) 

w 

a: 

1-

C/) 

...J 

<( 30 

(.) 

;:::: 

a: 

w 

> 

HORIZONTAL STRESS, IN MEGAPASCALS 

EXPLANATION 

Stress measurement from 

drill hole 

• 

6. USW G-1 

0 USW G-2 

'V USW G-3 

0 Ue25 Pl 

o-. 

40 

Figure 6.2. Values of least horizontal principal stress Sh (solid 

symbols) and greatest horizontal principal stress SH (open symbols) plotted as a function of vertical stress, Sv. Horizontal 

arrows indicate that values are upper bounds. Vertical bars 

indicate range of uncertainty associated with Sv values in drill 

hole USW G-3, due to topographic effects. 

90 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

In addition to preexisting fractures, very steep to vertical fractures follow the hole for many tens of meters in the 

televiewer logs ofUSW G-1 (520-760 m depth), USW G-2 

(526-678 m depth), and USW G-3 (at intervals from 882 

to 1,018 m). In some cases these do not appear to be 

throughgoing fractures, as they have no observed trace on 

the opposite azimuth in the televiewer log. They vary in 

aspect from en echelon and jagged, connected to or interrupted by throughgoing fractures, to straight and continuous. 

These fractures have an average strike of N. 25 ° E. in USW 

G-3, N. 15° E.-N. 35° E. in USW G-1, and N. 25° E.-N. 

30° E. in USW G-2. Most notably, these fractures cannot 

be identified on corresponding sections of the drill core, suggesting that they were formed after coring but prior to the 

collection of the televiewer data. We believe that these are 

hydraulic fractures induced by drilling. The low values of 

Sh measured elsewhere in the holes, if present at the depths 

of these features, could have been exceeded by raising the 

water level to the surface by filling the hole with fluid. Such 

fractures could accept fluid when the water level was still 

below the surface of the hole, providing a reasonable 

Drill hole USW G-2 at 1026 m 

0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 

A TIME, IN MINUTES 

z!Q w 4oo r-.--.--.--,-.--.---,.---.---.--r---.-----.--. 

;~ffis 200 o..Ja.z ii ~ i 0 t----'-....__. ........................ .._____.._..__ _____ -1 

~ ~ 15 

~z~ 10 cn-a. 

~ ~w 5 Drill hole USW G-3 o.. at 1338 m ~ 0 o!:--'-..........,4'f=o---'-"""":8':f:o:--'-"""":1~2o=--'"---:-:!16~o_._'="2o!=-::o:--'-'~247.o=-' 

B TIME, IN MINUTES 

~ffi LLI200~~-~-.-~~-.--~-,--.-,-~-.--~ 

~!::!55 100 o..Ja.z 

~~ ~ o~~L--~~--~~~~~~~~~---~ 

. ~ LLI <( 

a:: u 

~z~ en-a. LLI <( 

a:: (!) 

Q. ~ 

4 I fi~ __ _j\JtJlHLL~· 2 Drill hole USW G-2 at 295 m 

o~-L--2~o~-~4o-~-6~o-~-8~0-~-~o~o-..__~120 

C TIME, IN MINUTES 

Figure 6.3. Pressure-time curves from hydraulic fracturing tests 

at Yucca Mountain. Least horizontal principal stress (instantaneous shut-in pressure), Sh; surface hydrostatic pressure, P. A, 

Clear breakdown with Sh greater than P. B, Clear breakdown 

with Sh less than P. C, Breakdown pressure equal to fracture 

reopening pressure on later cycles, indicating that a preexisting 

fracture is being reopened. Arrows show time at which test 

interval was opened to pressure in tubing string. 
explanation for the huge volumes of drilling fluid that were 

lost in the three holes and the continuous loss of circulation 

during drilling. In Ue25P1, drilling-induced hydraulic 

fractures are not observed, and circulation was maintained 

during drilling. 

If these are indeed hydraulic fractures, their strike 

implies an Sh direction of N. 60° W.-N. 65° W. in these 

three holes. This direction is also suggested by the orientations of breakouts in USW G-1, USW G-2, and Ue25Pl. 

These breakouts are spalled portions of the borehole wall 

which are visible as dark bands of the televiewer log at 

azimuths 180° apart. When the returning televiewer signal 

is plotted as traveltime against azimuth, to yield a cross section of the shape of the hole, these dark patches are seen 

to correspond to sections of the hole with unusually large 

radii. 

Breakouts in many regions provide consistent, regional 

information on the horizontal principal stress directions 

because they form at the azimuth of the maximum compressive stress around the borehole, the Sn azimuth (see Bell 

and Gough, 1983). The average N. 60° W. toN. 65° W. 

orientation of breakouts in USW G-2 (between 1,053 and 

1,219 m) and N. 60° W. azimuth of breakouts in Ue25P1 

(between 1,524 and 1 ,676 m) suggest an Sh orientation of 

N. 60° W., consistent with the orientation inferred from the 

drilling-induced hydraulic fractures. Breakouts from 1,113 

to 1,202 min USW G-1 had aS. 80° W. azimuth, indicating 

an Sh direction 40° off from all other directional data. These 

occur in an inclined portion of the hole (9° from vertical); 

however, modeling shows that this breakout azimuth could 

not have resulted from the observed hole deviation and the 

measured Yucca Mountain stress field with S H at N. 25 o E., 

Sh at N. 65° W., Sv vertical, and T, or (Sn-Sh)/(Sv-Sh), 

between 0.25 and 0.7 (Stock and others, 1985). Therefore, 

a local perturbation of the stress field seems likely. 

AGREEMENT WITH REGIONAL DATA 

The observed stress directions are in good agreement 

with directions derived from regional data showing northwest 

to west-northwest orientation of Sh (Carr, 1974; Zoback and 

Zoback, 1980; Stock and others, 1985). TheN. 60° W. to 

N. 65° W. Sh direction that we find at Yucca Mountain is 

slightly more westerly than the N. 45 o W. to N. 55 o W. 

Sh orientations reported from hydraulic fracturing in Rainier 

Mesa (Haimson and others, 1974; Warren and Smith, 1985) 

and theN. 45° W. toN. 60° W. Sh orientation derived from 

breakouts in drill holes in Yucca Flat and Pahute Mesa (Carr, 

1974; Springer and Thorpe, 1981). However, they are within 

the scatter of this regional data and are considered to be consistent with it. 

The observed stress magnitudes indicate a normal 

faulting stress regime, with Sh<Sn<Sv. This is consistent 

with regional earthquake and geologic data, which show a 

combination of normal and strike-slip faulting, implying that 

Sh<Sn<Sv (see Stock and others, 1985). The ratio T, equal 

to (Sn-Sh)I(Sv-Sh) in a normal faulting regime, indicates 

how close the stress magnitudes lie to a ''pure normal faulting 

regime'' ( T = 0) or a ''combined normal and strike-slip 

faulting regime" (T= 1). The T values obtained from these 

holes are scattered, ranging from 0.69 (at 1,573 m in 

Ue25P1) to 0.25 (1,288 min USW G-1), but the highest 

T values are found at the deepest structural levels. If the 

Yucca Mountain stress field tends toward higher values with 

structural depth, extrapolation to deeper levels should be 

based on higher T values than the average of our test 

measurements. These may also be more representative of 

the regional stress field. 

TOPOGRAPHIC EFFECTS ON THE HORIZONTAL 

STRESS 

Topographic corrections to Sv are important in USW 

G-3, as previously noted. Analytical solutions (Savage and 

Swolfs, 1986) and approximations to the stress field in 

regions of moderate relief (McTigue and Mei, 1981) indicate 

that the topographic effect on the horizontal stresses is 

generally negligible at depths greater than one wavelength 

ofthe topography (see Jaeger and Cook, 1979, p. 373). The 

topographic variation at USW G-1, USW G-2, and Ue25Pl 

has a wavelength of 500 m; its amplitude is small, and the 

surface elevations of the holes are quite close to the average 

elevation over distances equal to the depths of our tests. 

Therefore, the topographic effects on Shand Sn should be 

minimal. Although USW G-3 is in an area of steeper 

topography, modeling by Swolfs and Savage (1985) shows 

that the topographic effect on the horizontal stresses will be 

larger than the uncertainty in the hydrofrac tests only within 

a few hundred meters of the surface. Since our tests here 

were at much deeper levels, we conclude that topographic 

influence on the measured Sh values is too small to be 

detected. 

Additional evidence for this conclusion comes from the 

Sh orientations in these holes. They all show a consistent Sh 

azimuth of N. 60° W. toN. 65° W., which is maintained 

in the presence of both a north-south topographic grain (near 

USW G-3 and Ue25P1) and a northwest-southeast 

topographic grain (near USW G-1 and USW G-2). Since 

a topographically controlled stress field should have Sh directions perpendicular to the strike of the topography, it appears 

that the Yucca Mountain stress field, at the depths studied 

(below the water table), is not strongly influenced by 

topography. The continuity of stress directions from the 

volcanics into the underlying carbonate rocks argues against 

a strong component of residual or cooling stresses influencing the stress field in the volcanic rocks, suggesting that the 

stress field is primarily the result of current tectonic 

processes. 

Stress Field 91 
TECTONIC IMPLICATIONS 

The most important characteristic of these stress 

measurements is the consistently low value of Sh relative to 

Sv. As discussed in previous reports (Healy and others, 

1984; Stock and others, 1984, 1985), the measured Sh values 

fall close to the limits at which frictional sliding, in a normalfaulting sense, might be expected to occur on optimally 

oriented preexisting faults. These low Sh values have an even 

more dramatic consequence: the long drilling-induced 

hydraulic fractures present in the upper parts of the three 

USW G holes. Such fractures could propagate even in the 

absence of a drill hole to concentrate the stresses, if the pore 

pressure in small cracks locally exceeded Sh. (This situation 

has been observed in other active extensional areas; see 

Zoback and Healy, 1984.) 

Our data suggest that in the presence of such a stress 

field, if the pore pressure were to increase and exceeded sh 

(due to a natural rise of the water table or to human activities, 

such as fluid injection), cracks could open perpendicular to 

Sh. These open cracks could then act as pathways for fluid 

flow and dissipate the excess pore pressure, until it dropped 

back to the Sh magnitude and the cracks could close. This 

implies that there may be a delicate balance between the 

hydrology and the stress field at Yucca Mountain, with the 

magnitude of Sh acting as a limit to possible increases in 

water level. Because Sh increases faster with depth than does 

the hydrostatic pressure, this effect becomes less important 

with depth, and is unlikely to be of concern where Sh 

exceeds the surface hydrostat. However, within the first 

kilometer of the surface, this effect is likely to be a major 

control on the hydrology of Yucca Mountain, and influence 

the tectonic response to human activities which produce 

major changes in local pore pressure. 

CONCLUSIONS 

In hydraulic fracturing stress measurements in four 

holes at Yucca Mountain, both the least horizontal principal 

stress Sh and the greatest horizontal principal stress S H are 

less than the vertical stress Sv, indicating a normal-faulting 

stress regime. Plots of Sh and SH as a function of Sv 

demonstrate that all of the Sh values are reasonably linear 

with respect to Sv. The few SH values obtained from the 

USW holes are substantially less than Sv, whereas the one 

SH value from Ue25P1 is closer to, although less than, Sv. 

The Sh direction is N. 60° W. toN. 65° W., in good agreement with regional stress indicators. 

Upward extrapolation of these results should be done 

with caution, as topographic effects are more important near 

the surface. In order to obtain an accurate picture of the stress 

field in the unsaturated zone, in the area of the proposed 

repository, a numerical model incorporating these results and 

including actual topography, observed distribution and 

physical constants of volcanic units, and reasonable bound92 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

ary conditions should be developed. Such a model should 

be checked for consistency with future stress measurements 

(hydraulic fracturing and overcoring) made closer to the position of the proposed repository, in both the saturated and 

unsaturated zones. 

REFERENCES CITED 

Bell, J.S., and Gough, D.l., 1983, The use of borehole breakouts 

in the study of crustal stress, in Zoback, M.D., and Haimson, 

B.C., eds., Hydraulic fracturing stress measurements: 

Washington, D.C., National Academy Press, p. 201-209. 

Bredehoeft, J.D., Wolff, R.G., Keys, W.S., and Shuter, E., 1976, 

Hydraulic fracturing to determine the regional in-situ stress 

field, Piceance Basin, Colorado: Geological Society of America 

Bulletin, v. 87, p. 250-258. 

Carr, W.J., 1974, Summary of tectonic and structural evidence for 

stress orientation at the Nevada Test Site: U.S. Geological 

Survey Open-File Report 74-176, 53 p. 

Haimson, B.C., Lacomb, J., Jones, A.H., and Green, S.J., 1974, 

Deep stress measurements in tuff at the Nevada Test Site, in 

Advances in rock mechanics: Washington, D.C., National 

Academy of Sciences, p. 557-561. 

Healy, J.H., Hickman, S.H., Zoback, M.D., and Ellis, W.L., 

1984, Report on televiewer log and stress measurements in core 

hole USW G-1, Nevada Test Site: U.S. Geological Survey 

Open-File Report 84-15, 47 p. 

Hickman, S.H., and Zoback, M.D., 1983, The interpretation of 

hydraulic fracturing pressure-time data for in-situ stress determination, in Zoback, M.D., and Haimson, B.C., eds., 

Hydraulic fracturing stress measurements: Washington, D.C., 

National Academy Press, p. 44-54. 

Hubbert, M.K., and Willis, D.G., 1957, Mechanics of hydraulic 

fracturing: Journal of Petroleum Technology, v. 9, p. 153-168. 

Jaeger, J.C., and Cook, N.G.W., 1979, Fundamentals of rock 

mechanics, 3d ed.: London, Chapman and Hall, 593 p. 

McTigue, D.P., and Mei, C.C., 1981, Gravity-induced stresses 

near topography of small slope: Journal of Geophysical 

Research, v. 86, p. 9268-9278. 

Savage, W.Z., and Swolfs, H.S., 1986, Tectonic and gravitational 

stress in long symmetric ridges and valleys: Journal of 

Geophysical Research, v. 91, p. 3677-3685. 

Scott, R.B., and Bonk, Jerry, 1984, Preliminary geologic map of 

Yucca Mountain with geologic sections, Nye County, Nevada: 

U.S. Geological Survey Open-File Report 84-494, 9 p., scale 

1:12,000. 

Springer, J.E., and Thorpe, R.K., 1981, Borehole elongation versus 

in situ stress orientation: Livermore, Calif., Lawrence Livermore Laboratory Report UCRL-87018, 15 p. 

Stock, J.M., Healy, J.H., and Hickman, S.H., 1984, Report on 

televiewer log and stress measurements in core hole USW G-2, 

Nevada Test Site: U.S. Geological Survey Open-File Report 

84-172, 31 p. 

Stock, J.M., Healy, J.H., Hickman, S.H., and Zoback, M.D., 

1985, Hydraulic fracturing stress measurements at Yucca 

Mountain, Nevada, and relationship to the regional stress field: 

Journal of Geophysical Research, v. 90, p. 8691-8708. 

Stock, J.M., Healy, J.H., Svitek, J., and Mastin, L., 1986, Report 

on televiewer log and stress measurements in holes USW G-3 
and Ue-25p#1, Yucca Mountain, Nevada Test Site: U.S. 

Geological Survey Open-File Report 86-369, 91 p. 

Swolfs, H.S., and Savage, W.Z., 1985, Topography, stresses, and 

stability at Yucca Mountain, Nevada: paper presented at 40th 

U.S. Rock Mechanics Symposium, Rapid City, S.D., June 

1985. 

Warren, W.E., and Smith, C.W., 1985, In situ stress estimates 

from hydraulic fracturing and direct observation of crack orientation: Journal of Geophysical Research, v. 90, p. 6829-6839. 

Zemanek, J.R., Caldwell, L., Glenn, E.E., Holcomb, S.V., 

Norton, L.J., and Straus, A.J.D., 1969, The borehole 

televiewer: A new logging concept for fracture location and 

other types of borehole inspection: Journal of Petroleum 

Technology, v. 21, p. 702-774. 

Zoback, M.D., and Healy, J.H., 1984, Friction, faulting, and insitu stress: Annales Geophysicae, v. 2, p. 689-698. 

Zoback, M.L., and Zoback, M.D., 1980, State of stress in the conterminous United States: Journal of Geophysical Research, 

v. 85, p. 6113-6156. 

Stress Field 93 

7. An Evaluation of the Topographic Modification 

of Stresses at Yucca Mountain, Nevada 

By Henri S. Swolfs, William Z. Savage, and William L. Ellis 

CONTENTS 

Abstract 95 

Introduction 95 

Observations 96 

Analysis 97 

Topography 97 

Fracture anisotropy 100 

Conclusions 101 

References cited 101 

Abstract 

Stress components measured by the hydraulic-fracturing 

technique at Yucca Mountain, Nye County, Nevada, are in good 

agreement with a plane-strain analytical solution for the gravitational state of stress beneath a long, symmetrical ridge. Both the 

analytical model and the field observations indicate that the 

horizontal stress components beneath the base of the ridge are 

reduced in magnitude relative to horizontal stress components 

beneath the flanks of the ridge, or at locations where the ridge 

topography is less pronounced. We propose that this stress reduction is due to gravitational spreading at depth beneath the ridge 

and that this localized effect on the stress field penetrates to 

depths several times the height of the ridge. A well-developed 

set of faults and fractures, subparallel to the north-south-trending 

ridge, imparts a vertical transverse isotropy to the rock and, as 

a consequence of gravitational loading, induces unequal horizontal stresses in directions parallel and perpendicular to the 

anisotropy. 

INTRODUCTION 

Yucca Mountain, located on the west boundary of the 

Nevada Test Site (fig. 7 .1), is a north-south-trending ridge 

of moderate relief that projects southward from the higher 

terrain near Timber Mountain into the lower, broad plains 

of Crater Flat, the Amargosa Desert, and Jackass Flats. 

Regional gravity and seismic-refraction surveys (Mooney and 

others, 1982; Snyder and Carr, 1984), as well as extensive 

drilling in the area, reveal that Yucca Mountain and Crater 

Flat are underlain by a thick (3-4 km) section of ash-flow 

tuffs and associated extrusive rocks that fill a steep-sided 

north-south-trending depression in the pre-volcanic rocks. 

The east edge of this depression rises to within 1. 5 km of 

the surface just east of Yucca Mountain, where Ordovician 

and Silurian carbonate rocks have been penetrated (Carr and 

others, 1986). 

Yucca Mountain is cut by major north-south to 

northeast -southwest -striking and generally westward-dipping 

normal faults (fig. 7 .2), forming blocks of Miocene ash-flow 

tuffs tilted gently eastward (Scott and others, 1983; Scott and 

Castellanos, 1984; Scott and Bonk, 1984). In general, the 

density of these faults decreases toward the north, where 

northwest -southeast -striking strike-slip faults with apparent 

right-lateral offset become common (Scott and others, 1983). 

The rock types in these blocks include densely welded 

devitrified and vitric ash-flow tuff, nonwelded victric ashflow tuff, vitric bedded tuff, nonwelded zeolitic and 

argillaceous ash-flow tuff, and zeolitic and argillaceous 

bedded tuff. 

As revealed by structural mapping of the surface of 

Yucca Mountain and analyses of oriented cores (Scott and 

others, 1983), fractures and minor faults are abundant and 

pervasively distributed throughout Yucca Mountain. The 

dominant fractures have steep to near-vertical westward dips 

and nearly constant northward to northwestward strikes. The 

frequency of these fractures is several times greater in the 

welded tuffs than in the nonwelded tuffs. The orientation and 

attitude of these fractures impart a mechanical anisotropy to 

the rocks; in fact, Scott and others (1983) attributed the 

observed deviation from vertical of several deep drill holes, 

as well as the directions of deviation, to this anisotropy. 

Because the static water table at Yucca Mountain is 500 

to 800 m below the surface, the unsaturated zone above the 

water table is being considered as a possible location for a 

nuclear waste repository (Winograd, 1981; Roseboom, 

1983). The study and characterization of the constraints 

imposed on the design and construction of the repository by 

the geological environment include the investigation of the 

regional stress. As reported in chapter 6, measurements of 

stress using the hydraulic fracturing technique (Haimson and 

Fairhurst, 1967) have been made in several drill holes by 

the U.S. Geological Survey (Healy and others, 1984; Stock 

and others, 1984; J.M. Stock, written commun., 1984). The 

majority of these measurements were made in the saturated 

zone below the water table to a depth of 1.4 km. Extrapolation of the results of stress measurement upward into the 

unsaturated zone is unwarranted without some theoretical 

guidance. 

The aim of this report is to describe two aspects that 

may exert some influence on the local state of stress in Yucca 

Mountain. First, we consider the stresses induced at 

Topographic Modification of Stresses 95 
repository depths by topography alone using solutions of 

Savage and others ( 1985), and second we consider the 

separate effect of fracture anisotropy on the horizontal-stress 

distribution following the procedures outlined by Amadei and 

others (1987). Both approaches deal with the gravitational 

component of the stress field only. Implicit in the analyses 

is the assumption of lateral restraint (Jaeger and Cook, 1979, 

p. 371). We justify these boundary conditions as follows: 

there will be a finite distance from the ridge beyond which 

displacements due to the ridge become vanishingly small. 

Because Yucca Mountain, and especially its southern part, 

is bordered by two broad flats, we are satisfied that these 

boundary conditions apply at distances of a few kilometers 

from the ridge. 

OBSERVATIONS 

To date, 11 hydraulic-fracturing tests have been conducted to estimate the stresses in the saturated zone of Yucca 

Mountain (Healy and others, 1984; Stock and others, 1984; 

J.M. Stock, written commun., 1984) at depths from 646 to 

37030'r-------'--11'-'=160=30:._' ----------~1160 

Beatty 

• 

Ba~Ccatec 'M't~Fiat 

SITE 

J~~::·· /? F~ 

• Lathrop 

Wells Specter 

Range 

Mercury 

• 

Figure 7.1. Location of Yucca Mountain and drill holes USW 

G-1, USW G-2, and USW G-3. Shaded areas of positive relief 

expose Miocene and Pliocene ash-flow tuffs; Paleozoic sedimentary rocks underlie Bare Mountain and Specter Range. 

96 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

1,338 m (fig. 7.3). Six of these tests were done in drill hole 

USW G-1 and two in USW G-2, located north of the proposed repository block and east of the ridge crest. The three 

remaining tests were done in drill hole USW G-3, located 

in the southernmost part of the repository block on the crest 

of the ridge (fig. 7.1). The results of these tests show that 

the least horizontal stress (Sh) increases from 4.2 to 14.8 

MPa in the test intervals from 646 to 1 ,288 min holes USW 

G-1 and USW G-2, and from 6.8 to 11.5 MPa in the test 

intervals from 1,068 to 1,338 min hole USW G-3 (fig 7.3). 

The depths in hole USW G-3 have been corrected for 

borehole deviation (J.M. Stock, oral commun., 1985). Due 

to the overburden weight, the vertical stress (Sv) increases 

at an average rate of 20.6 MPa/km. The few calculations 

of the greatest horizontal stress (Sn) indicate that it increases 

from 16.8 to 17.9 MPa in holes USW G-1 and USW G-2, 

and from 10.6 to 18.1 MPa in hole USW G-3 (see table 6.1 

in chapter 6). These results indicate that the stresses at Yucca 

Mountain are such that Sn!Sv ~ 3/4 and Sh!Sv ~ 112. 

The measured stresses at shallow depths beneath Yucca 

Mountain are consistent with normal faulting or an extensional stress regime where S;>Sn>Sh. This also agrees with 

the fault orientations shown in figure 7.2. However, as shown 

in figure 7 .3, the stresses obtained in hole USW G-3 are 

smaller than those obtained at equal depths in either hole 

USW G-1 or U~W G-2. The static water levels are found 

at depths of 575 m in hole USW G-1, 526 m in hole USW 

G-2, and 752 min hole USW G-3 (Scott and others, 1983). 

Horizontal-stress orientations have been inferred from 

drilling-induced hydraulic fractures and sidewall spalling 

(breakouts) observed along sections of the drill holes by 

means of televiewer surveys (Healy and others, 1984; Stock 

and others, 1984; J.M. Stock, written commun., 1984). 

Hydraulic fractures are likely to be propagated when the 

bottom-hole pressures of the drilling fluid exceed the value 

of Sh, the least horizontal stress. At Yucca Mountain, this 

criterion is readily satisfied by filling the holes to the surface 

with drilling fluid (Ellis and Swolfs, 1983). The drillinginduced fractures are parallel to the drill holes for long 

near-vertical distances and strike north-south to northeastsouthwest. In near-vertical holes, breakouts are zones of 

limited failure that are oriented in directions normal to the 

greatest horizontal stress or at right angles to the hydraulicfracture planes (Jaeger and Cook, 1979, p. 238). The 

azimuths of the observed breakouts in holes USW G-1 and 

USW G-2 are west to west-northwest; no breakouts were 

recognized in hole USW G-3. For the purposes of this paper, 

we take the orientation of the least horizontal stress (Sh) to 

be normal to the strike of Yucca crest (fig. 7 .2), and the 

orientation of the greatest horizontal stress (Sn) to be parallel 

to the strike of Yucca crest and the vertical fractureanisotropy. Even though this introduces a discrepancy of 

about 30° in stress orientation, it is not likely to invalidate 

our general conclusions with regard to a topographic effect 

on stress beneath Yucca Mountain. The vertical stress (Sv) 

is assumed equal to the weight of the overburden. 
ANALYSIS 

Topography 

The least horizontal stresses measured beneath the base 

of Yucca Mountain in hole USW G-3 are smaller than those 

measured at comparable depths under the eastern flank of 

the ridge in holes USW G-1 and USW G-2 (fig. 7.3). In 

order to explain this observation, we use the results of Savage 

and others (1985), who derived an exact solution for topographically induced stresses by the Kolosov-Muskhelishvili 

method of complex potentials for plane elasticity. A conformal mapping function transforms an isolated symmetric 

ridge (fig. 7 .4) into a half-plane, in which expressions are 

obtained for the gravity-induced stresses in the symmetric 

ridge. Stresses given by the solution satisfy all conditions 

of the problem; that is, shear stresses vanish on the ridge 

surface and on the axial plane of the ridge, stresses normal 

to the ridge surface vanish, and at great depth and distance 

from the ridge, all stresses are compressive and approach 

the state of stress given by the assumption of lateral restraint 

(Jaeger and Cook, 1979, p. 371). 

In a contour diagram of the dimensionless least horizontal stress Sh/pgb for a symmetric ridge, where Pgb is the 

characteristic stress at the base of the ridge and b is the ridge 

height (fig. 7 .5), the ridge shape conforms closely to the 

topographic cross section in the vicinity of hole USW G-3 

(fig. 7 .2). It is clear that the stress contours do not follow 

the ridge shape, but concentrate along the flanks, and at depth 

along the axial plane of the ridge, the stress contours are 

deepened relative to those under the flanks. This reduction 

of horizontal stress under the ridge to depths several times 

the ridge height is attributed to gravitational spreading of the 

ridge. 

WEST Yucca Mountain crest 

Topopah Spring Member 

0 100 200 

East-west cross sections through Yucca Mountain 

incorporating holes USW G-1, USW G-2, and USW G-3 

are approximated by the conformal mapping function. For 

example, near USW G-3 the actual cross section of the ridge 

is modeled as 250 m high and 750 m wide between the 

inflection points on the flanks (fig. 7 .2). Assuming the ridge 

materials to be isotropic, homogeneous, and elastic, the 

topographically induced stresses are calculated for a 

measured Poisson's ratio ofv=0.32 (Ellis and Swolfs, 1983, 

fig. 3), and an average density of p =2.14 g/cm3. The 

calculated variations of the least horizontal and vertical 

stresses (Sh and Sv) with depth as solid curves in the vicinity of holes USW G-1, USW G-2, and USW G-3, respectively, are shown in figure 7.6A-C. Also shown are the least 

horizontal stress gradients (dashed lines) calculated in the 

absence of topography which, at depth, are the asymptotic 

bounds to the topographically induced components sh. These 

stress gradients shown as dashed lines in figure 7.6 find their 

origin at the intersection of the base of the ridge, or horizontal 

datum (fig. 7. 7), and the vertical depth axis. 

The least horizontal stresses (shut-in pressures) 

measured by the hydraulic fracturing technique are shown 

as open symbols for each hole in figure 7 .6. Although the 

agreement between the calculated least horizontal stresses 

and the measured components appears to be good, it is clear 

that topography has little effect on either the calculated or 

measured stresses in holes USW G-1 and USW G-2lower 

on the flanks of the ridge (fig. 7 .6A, B; see also fig. 7. 7). 

However, the effect of topography on the calculated stresses 

in hole USW G-3 under the crest of the ridge is obvious and 

significant (fig. 7 .6C). Here, the calculated component Sh 

shows little tendency to increase with depth for the first 

200 m, whereas Sv increases steadily with depth, though at 

a slightly lower rate than shown in figure 7 .6A, B. Below 

300 400 METERS 

Principal 

NNE-striking 

fault 

\ I 

EAST 

Minor NNW-striking faults 

Figure 7.2. Schematic structure section across Yucca Mountain near drill hole USW G-3 showing disposition of normal faults 

and tilted blocks in ridge. The Tiva Canyon and Topopah Spring units shown are members of the Paintbrush Tuff. No vertical 

exaggeration. (From Scott and others, 1983.) 

Topographic Modification of Stresses 97 
about 800 m, both stress components increase with depth, 

and the effect of topography is apparent because the 

magnitudes of these stresses are lower than the calculated 

and measured stresses at comparable depths in holes USW 

G-1 and USW G-2 shown in figure 7.6A, B. Although not 

shown in figure 7 .6, this discrepancy finally dissolves at a 

depth of about 2 km. 

The construction shown in figure 7. 6D is obtained by 

superposing the measurements in figure 7 .6A-C such that 

the dashed lines and their origins coincide. In effect, the 

measured stresses (Sh) in the saturated zone are corrected 

for topography by adjusting the depths with a vertical shift 

to a local datum or flat surface that passes through the base 

of the ridge (fig. 7. 7). The distribution obtained in this way 

(fig. 7. 6D) reconciles the differences between measurements 

in hole USW G-3, and holes USW G-1 and USW G-2 

(fig. 7.3). 

It is worthwhile to observe that as a result of the vertical 

shift just described, the differences in the depths to the water 

table (fig. 7.6A-C) are also accounted for. The static water 

LEAST HORIZONTAL STRESS IN MEGAPASCALS 

0 5 10 15 20 

0 I I I I I I I 

- 6 USW G-1 -

- 0 USW G-2 -

0 USW G-3 1- -

t- -

(/) 0.5 1- - a: 

w 

1-

w 1- - ~ 

0 

_J 1- - sz 

~ 1- - I 

1- 1- - a.. 

w 6 

0 1.0 1- - 0 60 

- -

- ~ -

- 8 6 -

- -

1.5 _l I I I I J I 

Figure 7.3. Least horizontal stress, Sh, measured in drill holes 

USW G-1, USW G-2, and USW G-3 as a function of depth 

below surface. (From Healy and others, 1984; Stock and others, 

1984; and J.M. Stock, written commun., 1984.) 

98 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

y 

v 

4 ~ 

u .. 

-

Figure 7.4. Conformal transformation of a symmetric ridge in 

x, y coordinates into a half-plane in u, v coordinates and definition of parameters a and b, which describe the shape of the ridge. 

See text for discussion of model. 

+I 

-I 

y/b 

-2 

-3 

0 

x/b 

2 3 4 

Figure 7.5. Contour plot of dimensionless least-horizontal stress 

component, Sh!Pgb, for a symmetric ridge. Vertical and 

horizontal scales are normalized by ridge height b. 
STRESS IN MEGAPASCALS 

5 10 15 20 25 

C/) 

a: 

w 

I- 0.5 

w 

~ 

0 

_J 

~ 

z 

:r: 

I- 1.0 CL 

w 

0 

A 

1.5 

STRESS IN MEGAPASCALS 

0 5 10 15 20 25 

0 

USW G-3 

C/) 

a: 

w 

1-

w 0.5 

~ 

0 

_J 

~ • z 

:r: 

1-

CL 1.0 w 

0 

c 

1.5 

Figure 7.6. Comparison between calculated (solid lines, Shand 

Sv) and measured stresses (open symbols, Sh) in drill holes. A, 

USW G-1, B, USW G-2, and C, USW G-3. Dashed lines represent predicted distribution of Sh in the absence of topography; 

note that they intersect ordinate at different depths. Depths to 

STRESS IN MEGAPASCALS 

5 10 15 20 25 

C/) 

a: 

w 

I- 0.5 

w • ~ 

0 

_J 

~ 

z 

:r: 

Ia.. 1.0 

w 

0 

B 

1.5 

LEAST HORIZONAL STRESS 

IN MEGAPASCALS 

0 5 10 15 20 25 

0~, I I I I ' ' 

C/) ALL HOLES - a: 

w - \ 6 USW G-1 - 1-

w USW G-2 - \ 0 

~ o USWG-3 0 - - _J 

~ 0.5- \ -

z 6\ f- -

~ 

::> f- - I- 6 

<( f- o\ - 0 

~ f- 6 -

0 1.0 f- \e::.o - _J 

w r- co ~ -

:r: f- (]:). - 1-

CL 

w f- \ 6 -

0 .... D \ -

1.5 I I I I I I I 

static water table indicated by solid, inverted triangles. 0, Composite plot of all measurements of Sh in saturated zone 

corrected for topography. Ordinate now measures depth relative 

to base of ridge, which is origin of dashed stress gradients. 

Topographic Modification of Stresses 99 
Figure 7.7. Schematic representation of location of drill holes 

USW G-1, USW G-2, and USW G-3 relative to axial plane 

of idealized ridge and to local, flat, horizontal datum shown by 

dashed line. Topographic effect on stresses measured in holes 

USW G-1 and USW G-2 located on the flank near flat datum 

is negligible, but effect is pronounced on stresses measured in 

hole USW G-3 because of its proximity to axial plane atop the 

ridge. 

levels are now found at a near constant depth of about 500 m 

below the base of the ridge. This suggests a possible feedback relation between the magnitude of the least horizontal 

stress component, Sh, and the position of the water level 

(fig. 7 .6D). A rise in the water table producing a hydrostatic 

pressure on the order of Sh would result in the reopening 

of conductive fractures and a consequent lowering of the 

water table to its former level. 

The calculated topographically induced stresses are 

affected by the relative position of the measurement holes 

with respect to the axial plane of the ridge. Near the crest, 

the calculated least horizontal stress is greater than the 

vertical stress to a depth of about 100m (fig. 7.6C). Away 

from the crest, this effect is diminished (figs. 7.6A, B). 

Calculations of maximum shear stresses, induced by 

topography, show that the stress differences are minimized 

at a depth of about 100m under the crest of the ridge. This 

result is shown in figure 7 .6C, where Sh =Sv. If the planestrain assumption is valid at this depth, Sn will be close in 

value to Sh and Sv, and the greatest structural stability would 

be expected here. 

In hole USW G-2, three attempts were made to 

estimate the shut-in pressures in sections of the hole in the 

unsaturated zone above the static water table (Stock and 

others, 1984). These estimates of the shut-in pressures, at 

which the fractures close, are obtained from pumping rate 

versus pressure plots; the results are shown as open triangles 

in figure 7 .6B at depths of295 m, 418 m, and 432 m. Because 

it is not known whether these fractures are freshly formed 

or already present, and what their attitudes are, some uncertainty exists about the interpretation of the shut-in pressures 

in terms of the least horizontal stress (Sh) (Stock and others, 

1984). What is clear, however, is that the normal stress 

across the fracture tending to close it should vary with fracture attitude or dip and range between the local calculated 

values of Shand Sv, as indicated by the data distribution at 

shallow depths in figure 7. 6B. 

The calculated values of the greatest horizontal stress 

(Sn) are only slightly greater than Sh because of the planestrain assumption in the analytical model of Savage and others 

(1985). This result is not in accord with the observations sum100 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

marized earlier. To find an explanation for the observation 

that the ratio of horizontal-to-vertical stresses Sn!Sv is about 

3/4, we proceed by characterizing the fractured rock material 

at depth under Yucca Mountain as transversely isotropic in 

vertical planes subparallel to the ridge. 

Fracture Anisotropy 

The problem to be considered here is the determination of the stress distribution in anisotropic rock induced by 

gravitational loading in the absence of the effects of topography. Solutions are given by Amadei and others (1987) for 

the case where the planes of transverse isotropy are vertical. 

Horizontal stresses parallel and perpendicular to the anisotropy are Sn and Sh, respectively. The governing equations 

are 

Sh!Sv = v' (1 +v)/(1-v' 2E/E') (1) 

and 

SniSv = (v+v' 2E/E)/(1-v' 2E/E') (2) 

where Sv=Pgh, and E, E', 11, and v' are the elastic constants. 

The Poisson's ratios, defined in terms of strain ratios and 

directions with reference to the planes of transverse isotropy, 

are v= -E.nhv = -E.viE.n and v' = -E.nhh = -E.vhh. The 

Young's moduli parallel and perpendicular to the anisotropy 

are E and E', respectively. 

As mentioned above, the stresses at Yucca Mountain 

are such that Sn1Sv~3/4 and Sh!Sv = 1/2. If we assume, as 

before, that v=0.32 (Ellis and Swolfs, 1983), then equations 

1 and 2 may be solved simultaneously for the two unknowns, 

v' and El E'. Because of the uncertainty in the measured 

magnitude of Sn at Yucca Mountain, the results are given 

in table 7.1 for a range of S HI Sv ratios. The results show 

a weak dependence on v', but a strong dependence on the 

modulus ratio. For the extreme case, where Sn!Sv =3/4, the 

modulus contrast is a factor of 3, although it decreases rapidly 

as the stress ratio is diminished. In a fractured medium, such 

as that found in Yucca Mountain, this range of modulus contrast (table 7 .1) may not be altogether unrealistic (Pratt and 

others, 1977). 

Table 7.1. Properties of transversely isotropic material 

E/E' 

0.75----------0.29----------- 3.0 

.70---------- .29----------- 2.6 

.65---------- .30----------- 2.2 

.60---------- .31----------- 1.8 

.55---------- .32----------- 1.4 
CONCLUSIONS 

We have analyzed the measured stresses at Yucca 

Mountain by using two separate theoretical models. The 

stress analyses suggest that the measured stresses at shallow 

depths in the saturated zone are predominantly of gravitational origin, a conclusion that is compatible with the fact 

that Yucca Mountain is seismically quiescent. Furthermore, 

the stresses are affected by fracture-induced anisotropy, and 

selectively, by topography. Using only a single measured 

value of Poisson's ratio as a modeling parameter, we show 

that the reduced stresses measured in drill hole USW G-3 

are due to gravitational spreading beneath the ridge. The 

method of analysis also allows for the adjustment of these 

stresses modified by topography. 

The data, now corrected for the topographic effect, may 

be interpreted in terms of the Coulomb criterion for frictional 

sliding on suitably oriented faults (Jaeger and Cook, 1979, 

p. 95). The implications of such an interpretation are 

presented by Stock and Healy in chapter 6. 

Stress states above the saturated zone at repository 

depths remain enigmatic because of the paucity of data. 

However, the model for topographically induced stresses 

provides a basis for predicting the stress magnitudes, and 

the degree of structural stability, at these depths in the 

unsaturated zone. 

REFERENCES CITED 

Amadei, Bernard, Savage, W.Z., and Swolfs, H.S., 1987, Gravitational stresses in anisotropic rock masses: International Journal of Rock Mechanics, Mining Sciences, and Geomechanical 

Abstracts, v. 24, p. 5-14. 

Carr, M.D., Waddell, S.J., Vick, G.S., Stock, J.M., Manson, 

S.A., Harris, A.G., Cork, B.W., and Byers, F.M., 1986, 

Geology of drill hole UE25p#l-A test hole into pre-Tertiary 

rocks near Yucca Mountain, southern Nevada: U.S. Geological 

·survey Open-File Report 86-175, 87 p. 

Ellis, W.L., and Swolfs, H.S., 1983, Preliminary assessment of 

in-situ geomechanical characteristics in drill hole USW -G-1, 

Yucca Mountain, Nevada: U.S. Geological Survey Open-File 

Report 83-401, 18 p. 

Haimson, B.C., and Fairhurst, Charles, 1967, Initiation and 

extension of hydraulic fractures in rock: Journal of the Society 

of Petroleum Engineers, v. 7, p. 310-318. 

Healy, J.H., Hickman, S.H., Zoback, M.D., and Ellis, W.L., 

1984, Report on televiewer log and stress measurements in core 

hole USW-G1, Nevada Test Site: U.S. Geological Survey 

Open-File Report 84-15, 47 p. 

Jaeger, J.C., and Cook, N.G.W., 1979, Fundamentals of rock 

mechanics: New York, Chapman and Hall, 593 p. 

Mooney, W.D., Snyder, D.B., and Hoffman, L.R., 1982, Seismic 

refraction and gravity modeling of Yucca Mountain, Nevada 

Test Site, southern Nevada [abs.]: Eos, Transactions of the 

American Geophysical Union, v. 63, p. 1100. 

Pratt, H.R., Swolfs, H.S., Brace, W.F., Black, A.D., and Handin, 

J. W., 1977, Elastic and transport properties of an in situ jointed 

granite: International Journal of Rock Mechanics, Mining 

Sciences, and Geomechanical Abstracts, v. 14, p. 35-45. 

Roseboom, E.H., 1983, Disposal of high-level nuclear waste above 

the water table in arid regions: U.S. Geological Survey Circular 

903, 21 p. 

Savage, W.Z., Swolfs, H.S., and Powers, P.S., 1985, Gravitational stresses in long symmetric ridges and valleys: International Journal of Rock Mechanics, Mining Sciences, and 

Geomechanical Abstracts, v. 22, p. 291-302. 

Scott, R.B., and Bonk, Jerry, 1984, Preliminary geologic map of 

Yucca Mountain with geologic sections, Nye County, Nevada: 

U.S. Geological Survey Open-File Report 84-494, 9 p., scale 

1:12,000. 

Scott, R.B., and Castellanos, Mayra, 1984, Stratigraphic and structural relations of volcanic rocks in drill holes USW GU-3 and 

USW G-3, Yucca Mountain, Nye County, Nevada: U.S. 

Geological Survey Open-File Report 84-491, 121 p. 

Scott, R.B., Spengler, R.W., Diehl, S.F., Lappin, A.R., and 

Chornack, M.P. , 1983, Geologic character of tuffs in the 

unsaturated zone at Yucca Mountain, southern Nevada, in 

Mercer, J.W., Rao, P.S.C. and Marine, I.W., eds., Role of 

the unsaturated zone in radioactive and hazardous waste 

disposal: Ann Arbor, Mich., Ann Arbor Science, p. 289-335. 

Snyder, D.B., and Carr, W.J., 1984, Interpretation of gravity data 

in a complex volcano-tectonic setting, southwestern Nevada: 

Journal of Geophysical Research, v. 89, p. 10,193-10,206. 

Stock, J.M., Healy, J.H., and Hickman, S.H., 1984, Report on 

televiewer log and stress measurements in core hole USW G-2, 

Nevada Test Site: U.S. Geological Survey Open-File Report 

84-172, 31 p. 

Winograd, I.J., 1981, Radioactive waste disposal in thick 

unsaturated zones: Science, v. 212, p. 1457-1464. 

Topographic Modification of Stresses 101 

8. Preliminary Study of Quaternary Faulting 

on the East Side of Bare Mountain, 

Nye County, Nevada 

By Marith C. Reheis 

CONTENTS 

Abstract 103 

Introduction 103 

Acknowledgments 105 

Geomorphic interpretation 105 

Field reconnaissance 105 

Faults in Tertiary and older Pleistocene units 105 

Faults in younger Pleistocene and Holocene units 106 

Faults in bedrock units 109 

Summary 110 

References cited 110 

Abstract 

Active faults bound the east side of Bare Mountain. 

Geomorphic features, stratigraphy, and soil development indicate 

that two 3-km-long segments of the range-front fault probably 

last moved in late Pleistocene or Holocene time. Other segments 

of the fault have been quiescent since the late Pleistocene. Both 

upper Pleistocene and Holocene deposits bury many faults east 

of the north end of Bare Mountain. 

Two prospect pits on the range-front fault reveal evidence 

of recurrent late Quaternary movements. Both older and younger 

deposits in one pit are faulted, but fractures in the older unit 

do not extend up into the younger unit. Soil development shows 

that the older and younger fault episodes are probably late 

Pleistocene and Holocene, respectively, in age. Another pit 

shows carbonate-cemented fractures with slickensides in an 

upper Pleistocene deposit, suggesting at least two late Pleistocene 

or Holocene fault movements. 

Tertiary to middle Pleistocene deposits show evidence of 

recurrent faulting in many locations. Faults in these deposits are 

pervaded by secondary CaC03 and silica that commonly 

exhibit slickensides. 

INTRODUCTION 

The east range front of Bare Mountain trends northsouth and lies about 15 km west of the proposed nuclear waste 

disposal site at Yucca Mountain (fig. 8.1). Bare Mountain 

consists of complexly faulted Paleozoic rocks intruded by 

Tertiary rhyodacitic dikes (Cornwall and Kleinhampl, 1961). 

The range front is structurally controlled by a system of 

normal faults that have remained active during Quaternary 

time (Swadley and others, 1984; Carr, 1984). Several small 

earthquakes (less than magnitude 3) from 1978 to 1984 had 

epicenters located along the range-front fault (Rogers and 

others, 1983; Carr, 1984). However, previous workers have 

mapped few faults in the fan alluvium that fringes the mountain front, except in the area east of the north end of Bare 

Mountain. 

Preliminary interpretation of mapping, soil studies, and 

aerial photographs indicates that the Bare Mountain rangefront fault cuts deposits of Pleistocene and probable Holocene 

age in numerous places. Evidence for recurrent activity on 

this fault in middle Pleistocene and Holocene time is found 

in several localities. 

The ages of Quaternary deposits are estimated using 

soil development and surface characteristics (Hoover and 

others, 1981). The Soil Development Index of Harden (1982; 

see also Harden and Taylor, 1983) is used to quantify soil 

properties in order to compare the ages of soils in different 

areas. Soils were compared to those developed in similar 

climates on dated alluvial deposits in Fortymile Wash, 25 km 

east of Bare Mountain (Taylor, 1985) and in Kyle Canyon, 

115 km to the southwest (Sowers, 1985). Although the Fortymile Wash area is closer, the Kyle Canyon soils formed on 

carbonate-rich deposits similar to those at Bare Mountain. 

Soils on the calcareous Kyle Canyon and Bare Mountain fans 

with similar characteristics, such as thickness of the A and 

B horizons and stage of pedogenic CaC03 and silica 

accumulation, should be of comparable age. 

In general, the terms Holocene, younger Pleistocene, 

and older Pleistocene as used in this report correspond, 

respectively, to units Q1a-c, Q2b-c, and QTa of Hoover and 

others (1981). However, age assignments of these deposits 

in Swadley and others (1984) along the Bare Mountain range 

front may be too old. For example, the latter workers used 

petrocalcic horizons to identify units Q2c and QTa, believed 

to be older than 250 ka (thousand years). Units with similar 

development were originally dated with the uranium-trend 

method in relatively noncalcareous alluvium on the Nevada 

Test Site (Hoover and others, 1981). Many of the Bare 

Mountain alluvial fans are mainly composed of carbonate 

Preliminary Study of Quaternary Faulting 103 
T 125 

T 135 

T 135 

T 145 

c 

ca 

-c 

:::s 

0 

:! 

c 

as 

c 

~ 

0 

::E 

·. 

) I ... / 

(.. 

'" / 

/ 

/ 

., : 

··. I 

'\..: 

/.t:;; \ :' "' ·. / f 

·'\ .. _ I 

\ ... 

. "'- . I -~ 

'"' ): t '•, \ I 

./ 

-··-... __ J y:· 

0 

''i 

k-- ----.. _I 

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_.a::a::o* J*:~ ~ I w w 

!'--. co 

: ;:_ ;:_ Little 

\ Cones 

2 3 

Crater 

\ 

\. Flat 

KILOMETERS 

_./ 

/ 

/ 

/ 

/ 116'45' 116'30' / 37'00' """"'..,.,.,.,....,..------:-:-~,............---. 

36'45' 

w w 

~~~ R 48E 

EXPLANATION 

Fault, bar and ball on downthrown 

side, cuts deposits of: 

Holocene fans 

+++'tt+- Younger Pleistocene fans 

*'**'** Older Pleistocene fans 

.....- Late Tertiary gravel 

...- Bedrock 

Fault buried by deposits of: 

...... Holocene fans 

•• ••• Younger Pleistocene fans 

Dashed faults inferred from air photos 

... - ... - Drainage 0 Quaternary cinder cone 

Figure 8.1. Quaternary faulting on east side of Bare Mountain. Faults with no bar and ball are down to the south 

or east. Circled numbers refer to sites discussed in text and other figures. 

104 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
sediments in which pedogenic CaC03 probably forms much 

more rapidly than in areas of other provenance. Petrocalcic 

horizons are found in similar soils dated with uraniumthorium methods at 120 ka (Sowers, 1985) on calcareous fans 

in Kyle Canyon. Thus, soils formed in calcareous alluvium 

are probably younger than soils with comparable amounts 

of CaC03 formed in noncalcareous alluvium. 

Acknowledgments 

This work was performed in cooperation with the U.S. 

Department of Energy, Nevada Nuclear Waste Storage 

Investigations Project. Discussions with W C Swadley, J.W. 

Whitney and E.M. Taylor gave useful perspectives. G. Vick 

provided able field assistance, and C.C. Reheis provided 

entertainment. 

GEOMORPHIC INTERPRETATION 

Geomorphic characteristics of the Bare MountainCrater Flat area indicate recurrent Quaternary faulting 

(fig. 8.1, and map of Swadley and others, 1984). The Bare 

Mountain front is steep, with faceted spurs. Drainages are 

deeply incised within the mountains, but are aggrading on 

the surface of steeply sloping Holocene alluvial fans that bury 

older fans adjacent to the mountain front south of Tarantula 

Canyon. This relation suggests that recent uplift along the 

range front has caused drainages to deposit young alluvium 

over older alluvium (Bull, 1964). Holocene deposits north 

of Tarantula Canyon are mainly confined to arroyos cut into 

older deposits, which suggests that faulting there has not been 

as recent. This unequal distribution of Holocene fans along 

the Bare Mountain front indicates a local tectonic control of 

stream gradients rather than a regional climatic control. 

Geomorphic features on aerial photographs (scale 

1: 12,000) also indicate probable Quaternary faults. Linear 

features such as brush lines and disturbed ground (extensive 

areas of rodent burrows) commonly mark the traces of faults 

that offset Quaternary alluvium in the area of the Nevada 

Test Site. Such features are abundant in Pleistocene and 

Holocene deposits along the range-front fault, and in 

Pleistocene deposits north of the Tarantula Canyon drainage. 

Only two obvious scarps along the range front are recognized: one on the south side of Tarantula Canyon (site 1, 

fig. 8.1), and the other, now destroyed by recent mining 

activity, on the range-front fault about 1. 5 km south of the 

boundary between townships 12 and 13 (fig. 8.1). Both 

scarps offset fans which appear to be of middle or late 

Pleistocene age. Many small drainages appear to be offset 

along the range-front fault as they debouch onto the piedmont slope. Whether the drainages are following weaknesses 

caused by recent faulting or the bedrock is not known. As 

a result of these active channels at the bedrock-alluvium contact, exposures of the range-front fault are rare. 

FIELD RECONNAISSANCE 

The faults on figure 8 .1 were mapped on the basis of 

exposures in arroyos and prospect pits and from aerial 

photographs. Arroyos cutting older Pleistocene deposits commonly expose steeply dipping fault zones with secondary 

calcium carbonate (CaC03) and amorphous silica. These 

zones correspond to linear features in aerial photographs 

(sites 1 and 2, fig. 8.1). Because faults in young deposits 

are not cemented with CaC03 or silica, they are more difficult to discern. Linear zones of disturbed ground and abrupt 

changes in the gradients of fan surfaces are believed to 

indicate faults in younger deposits. Three prospect pits (sites 

3, 4, and 5, fig. 8.1) expose faults in upper Pleistocene or 

Holocene deposits. Some fault segments along the range front 

pass from alluvium into bedrock (site 5). 

Most of the range-front faults dip between 60° to 80° 

eastward. An exception is the 35 o -45 o southeastwarddipping segment of the range-front fault that strikes northeast 

from Chuckwalla Canyon (fig. 8.1). This segment projects 

into a gently dipping normal fault in bedrock that continues 

to the head of Chuckwalla Canyon (Cornwall and 

Kleinhampl, 1961; M.D. Carr, oral commun., 1985). 

Offset is difficult to determine on most of the faults 

because of poor exposures and the uniformity of the alluvial 

fan deposits. Assuming that the faults are normal, as are 

many local faults in bedrock, most of the offsets are down 

to the east or south. Antithetic faults seem to be present at 

sites 1 and 4 (fig. 8.1), but exposures are poor and excavations are needed to determine their nature and extent. 

Faults in Tertiary and Older Pleistocene Units 

Arroyo walls and prospect pits expose faults in older 

alluvial fans along linear features noted on aerial photographs. Faults are most abundant in the area north of the 

Chuckwalla Canyon drainage (fig. 8.1). The fault zones are 

typically 0.25-1 m wide (fig. 8.2A). The zones are always 

cemented with secondary CaC03 and sometimes amorphous 

silica, which apparently precipitate from ground water or 

from rainwater and surface runoff percolating through the 

fault zones. A fault zone is commonly accompanied by 

several thinner, carbonate-cemented fractures within 30 m 

of the main zone. These fractures show no offset but are 

probably fault-related, as they occur only in areas that are 

faulted. 

Clasts within the fault zones are usually oriented with 

their long axes parallel to the boundaries of the cemented 

zone. Presumably the clasts were oriented by drag along the 

fault, although near-vertical orientations can also result when 

clasts fall into open surface fractures caused by faulting (K.L. 

Pierce, oral commun., 1985; Crone and others, 1985), or 

into animal burrows. The latter two mechanisms usually 

Preliminary Study of Quaternary Faulting 105 
A 

B 

106 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

result in a wide range of clast orientations, however, rather 

than one preferential orientation. 

Multiple offsets have occurred on a number of the faults 

in older units. The CaC03 and silica cement within the fault 

zones, emplaced after one or more fault movements, commonly exhibits slickensides that indicate the fault moved after 

at least some of the cementation occurred (fig. 8.2B). The 

slickensides on exposures of the range-front fault are usually 

perpendicular to the local trend of the mountain front. 

Accurate ages of the faults in older alluvial units are 

not known. Some of the faults cut Tertiary gravel and (or) 

older Pleistocene fans but are buried by younger Pleistocene 

and Holocene alluvium (fig. 8.1); these faults are therefore 

probably middle Pleistocene in age or older. Other faults cut 

both older and younger Pleistocene units, but are buried by 

Holocene alluvium; these may be late Pleistocene in age. 

Extensive CaC03 cementation along a fault zone probably 

indicates that at least one movement of the fault occurred 

in late Pleistocene time or earlier, for the cementation process 

is slow. Faults in upper Pleistocene and Holocene deposits 

are not cemented, although secondary CaC03 does occur 

locally along young faults. 

One radiometric date was obtained by the uraniumseries method for a cemented fault zone (site 2, fig. 8.1) along 

Bare Mountain (B.J. Szabo, oral commun., 1985). The age 

of secondary silica with slickensides (fig. 8.2B) is >350 ka. 

Thus, the fault movement that permitted cementation to begin 

probably predates 350 ka, but the age of the movement that 

produced the slickensides is known only to be younger than 

the initiation of cementation. 

Faults in Younger Pleistocene and Holocene Units 

In general, subsurface exposures of faults in young 

alluvial units are poor because they are unconsolidated, so 

a fault origin for many of the linear features seen from the 

air on the surface of these units is equivocal. Brush lines in 

these units may be caused by the plants rooting through thin, 

young deposits into fault zones in underlying older deposits. 

However, two prospect pits (sites 3 and 4, fig. 8.1) reveal 

good evidence of late Pleistocene or Holocene movement on 

the Bare Mountain range-front fault, and two other pits 

(site 5) provide supporting evidence for recent faulting. 

Figure 8.2. Secondary cementation on faults. A, Secondary 

CaC03 cementing zone along fault cutting younger Pleistocene 

alluvium on south side of Tarantula Canyon at site 1; fault zone 

is marked by hammer on large white boulder in foreground. B, 

Slickensides on secondary silica in fault exposed in prospect pit 

north of Chuckwalla Canyon at site 2. The silica is dated at >350 

ka (B.J. Szabo, oral commun., 1985). See figure 8.1 for site 

locations. 
A prospect pit exposes a small alluvial fan deposited 

against faulted Paleozoic limestone (site 3, fig. 8.1). A 

probable fault cuts the alluvium parallel to and about 1 m 

east of the bedrock outcrop (fig. 8.3A), where a gravelly bed 

of alluvium adjacent to the bedrock stops at the fault and 

appears to be cut by it. Clasts in the deposit are oriented 

parallel to the fan surface slope except along the fault zone, 

where the clasts are parallel to the fault (fig. 8.3B). The area 

of the presumed fault has animal burrows, but the clasts in 

this area have fairly uniform orientation; clasts that fell into 

burrows would probably have more randomly distributed 

orientations. 

The surface morphology and the soil developed on this 

fan suggest that the fan and the subsequent faulting are 

probably Holocene in age and no older than late Pleistocene 

in age. The surface is bouldery and has little or no desert 

pavement. These characteristics are typical of late Pleistocene 

and Holocene surfaces in the area (Hoover and others, 1981). 

The soil has a weak, sandy, vesicular A horizon, 2-3 em 

thick, and a slightly oxidized, calcareous B horizon. CaC03 

development consists of coats up to 1 mm thick on clast 

bottoms. These properties are typical of soils developed on 

deposits believed to be late Pleistocene and early Holocene 

in age in Fortymile Wash (Taylor, 1985), and soils on a 5- to 

15-ka fan in Kyle Canyon. 

Evidence of two successive movements of the rangefront fault is exposed in an excavation at site 4 (fig. 8.1). 

The north wall of this pit (fig. 8.4A) exposes gravelly sediment in fault contact with Paleozoic limestone. The fault 

plane strikes N. 35° E. and dips 58° SE. Clasts as large as 

boulders are oriented parallel to the fault plane, in a zone 

about 50 em wide along the bedrock surface. Most of the 

clasts away from this zone are oriented parallel to the ground 

surface or are randomly oriented. Two Quaternary deposits 

are recognized in the excavation. The younger deposit disconformably overlies a buried soil formed in the older deposit. 

Both deposits are very poorly sorted and poorly stratified, 

and in many places are clast -supported with very little fine 

matrix. These deposits may have accumulated at the base 

of a free face formed by faulting (Wallace, 1977). 

The buried soil contains numerous vertical fractures 

(fig. 8.4B) and has two fractures that parallel the main fault 

plane (fig. 8.4A). The east fracture is lined by secondary 

CaC03 (fig. 8.4B), but shows no evidence of offset. Movement may have occurred on the west fracture, because some 

large clasts have been rotated parallel to it (fig. 8.4A). All 

of the fractures terminate upward at the top of the buried 

soil. This soil is well preserved and may have been rapidly 

buried, perhaps in part by colluvium shed from the fault 

scarp. 

The older deposit is probably middle Pleistocene in age, 

based on soil morphology. This buried soil has remnants of 

an A horizon overlying well-developed structure in the B 

horizon. Pedogenic CaC03 and silica coat clast bottoms and 

occur in the finer matrix between clasts, but do not cement 

the soil horizons. The CaC03 and silica morphology is 

similar to that of soils formed on middle Pleistocene(?) 

deposits (Q2b) in Fortymile Wash that are believed to be 

145-160 ka in age (Swadley and others, 1984). Typical 

profile-index values of the Soil Development Index for soils 

on Q2b deposits range from 28 to 44 (Taylor, 1985); the 

profile index value calculated for the buried soil is 39. These 

values are probably comparable because this particular 

deposit contains relatively large proportions of quartzite 

rather than carbonate gravel, in contrast to most of the Bare 

Mountain alluvium. Thus, the parent material for the soils 

in this prospect pit resembles the carbonate-poor alluvium 

of uranium-trend-dated soils on the Nevada Test Site more 

closely than the carbonate-rich alluvium of soils at Kyle 

Canyon. 

The episode of faulting that fractured the buried soil 

must have occurred after most of the soil properties were 

developed, but before the deposition of the overlying 

material. The buried fractures are very clear and well 

preserved. Had the fractures formed before most of the soil 

properties, the fractures would have been disrupted and 

destroyed by processes such as bioturbation during subsequent soil formation. 

The younger deposit has definitely been faulted. Clasts 

near the main fault plane parallel the bedrock (fig. 8.4A). 

These clasts must have been oriented within the younger 

alluvium, for the dip angle is too steep to be maintained as 

a free face. At the contact between the younger deposit and 

the buried soil is a laminar CaC03 layer 1-2 em thick 

(fig. 8.4B). This layer is continuous across the exposure but 

stops at the zone of oriented clasts near the fault plane. Pieces 

of laminar CaC03 oriented parallel to the fault plane occur 

within this zone about 1. 75 m above the continuous laminar 

layer (fig. 8.4A). The laminar layer resembles those found 

in petrocalcic soil horizons. However, it is unlikely to have 

formed as part of the buried soil because the layer overlies 

the B horizon of this soil; such layers normally occur within 

or below B horizons of well-drained soils. The laminar layer 

probably formed after the younger deposit buried the older 

one, because the buried soil, being compact and weakly 

cemented, acted as a permeability barrier to percolating, 

CaC03-saturated water. If the oriented pieces of laminar 

CaC03 were derived from the laminar layer, they indicate 

that the fault moved at least 1. 75 m vertically after deposition of the younger unit and formation of the laminar layer 

at its base. The exposed limestone bedrock has a degraded 

scarp of comparable height. 

The morphology of the surface soil formed on the 

younger deposit suggests that this deposit and the fault movement that affected it are Holocene in age. Pedogenic CaC03 

and silica occur only as coats on clast bottoms; the A horizon 

is sandy, and the B horizon is weakly oxidized. These 

characteristics are similar to those of Holocene soils in Kyle 

Canyon. The profile-index value of the Soil Development 

Index for the surface soil is 20, which corresponds to values 

for soils on lower Holocene deposits (unit Q1c, believed to 

be about 9 ka) in Fortymile Wash (Taylor, 1985). 

Preliminary Study of Quaternary Faulting 1 07 
A 

B 

Figure 8.3. Fault at site 3 (fig. 8.1 ). A, North-facing prospect pit exposes fault in fan 

of probable early Holocene age overlying Paleozoic limestone. Gravelly alluvium adjacent to bedrock is cut by fault. Fan surface slopes east (left) at 25 °. B, Closeup shows 

disturbed zone and shingling of clasts at angles up to 65 o. 

108 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
A -3.5 m 

B 

Figure 8.4. Prospect pit at site 4 (fig. 8.1 ). A, Faulted upper Pleistocene or lower 

Holocene deposit (younger) overlies buried soil on faulted middle Pleistocene(?) deposit 

(older). B, Closeup shows fractures in buried soil. Laminar CaC03 layer at top of 

photograph. Secondary CaC03 outlines fracture to right of hammer. 

Faults in Bedrock Units 

Bedrock may have been offset during late Pleistocene 

or Holocene time at site 5 (fig. 8.1) on a splay of the rangefront fault. In one prospect pit (fig. 8.5), a Tertiary dike 

emplaced in Paleozoic limestone is sheared and brecciated, 

and has slickensides along its contact with the limestone at 

the base of the exposure. Near the ground surface, the 

limestone and dike rocks are separated by about 1 m of colluvium with vertically oriented clasts. Some of these clasts 

are petrocalcic fragments; secondary CaC03 occurs on the 

tops and sides of other clasts reworked from older soils. This 

wedge may represent material that fell into an open fracture 

formed by fault movement along the limestone-dike contact; 

similar wedges were observed in fractures along recent faults 

caused by the Borah Peak earthquake (K.L. Pierce, oral 

Preliminary Study of Quaternary Faulting 109 
commun., 1985; Crone and others, 1985). The soil formed 

on the colluvial wedge has a thin, sandy A horizon, a weakly 

oxidized B horizon, and little pedogenic CaC03, suggesting 

that the colluvium was deposited in early Holocene time. 

A second prospect pit at site 5 exposes the dike in contact with a younger Pleistocene deposit. The soil on this 

deposit is similar in CaC03 development to the buried soil 

at site 4, but has less pedogenic silica; the soil at site 5 is 

similar to soils formed on 40-50-ka deposits in Fortymile 

Wash (unit Q2a, Taylor, 1985). The deposit in the prospect 

pit contains a near-vertical zone of secondary CaC03 that 

is separated from but parallels the alluvium-dike contact, and 

slickensides occur on the plates of CaC03. These relations 

suggest that two movements of the fault occurred here after 

deposition of the alluvium. 

SUMMARY 

The Bare Mountain range-front fault and the associated 

northern fault complex have been active in Quaternary time. 

Relations in prospect pits and linear features on aerial 

Figure 8.5. North wall of prospect pit at site 5 (fig. 8.1 ). 

Faulted limestone-dike contact with probable Holocene 

colluvial wedge filling formerly open fracture. 

110 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

photographs suggest that late Pleistocene or Holocene faulting 

has occurred along two segments (fig. 8.1): (1) from site 

4 to 1 km north of site 3, and (2) from about 2 km south 

of Chuckwalla Canyon to Tarantula Canyon. Between the 

two segments and south of site 4, the range-front fault 

probably last moved in middle or late Pleistocene time. 

Recurrent movement has occurred on the range-front 

fault and the northern fault complex. Two episodes of faulting 

have occurred since the late Pleistocene at site 4 (fig. 8.4), 

and are indicated at site 5 (fig. 8.5). Recurrent faulting is 

also demonstrated at numerous locations by slickensides 

preserved on secondary CaC03 and silica in fault zones 

(fig. 8.2). 

Tertiary and lower Pleistocene(?) deposits are complexly faulted north of the Tarantula Canyon drainage (fig. 8.1), 

but none of the faults appear to extend upward into Holocene 

deposits, and few middle and upper Pleistocene(?) fans are 

faulted. These relations suggest that this area is relatively 

inactive. The fault complex appears to stop near the Tarantula Canyon drainage, but this may be due to burial by 

younger Pleistocene fans that extend from this drainage 

south. 

The north-northeastward trend of the northern fault 

complex roughly parallels the line of Pleistocene cinder cones 

in Crater Flat (fig. 8.1). In addition, the two active segments 

of the range-front fault trend north or north-northeast, as do 

long drainage segments in Crater Flat. This local trend is 

similar to a regional trend of recent tectonic features noted 

by Carr (1984), and suggests structural control of the Crater 

Flat drainages. 

REFERENCES CITED 

Bull, W.B., 1964, Geomorphology of segmented alluvial fans in 

western Fresno County, California: U.S. Geological Survey 

Professional Paper 352-E, p. E89-E129. 

Carr, W.J., 1984, Regional structural setting of Yucca Mountain, 

southwestern Nevada, and late Cenozoic rates of tectonic 

activity in part of the southwestern Great Basin, Nevada and 

California: U.S. Geological Survey Open-File Report 84-854, 

109 p. 

Cornwall, H.R., and Kleinhampl, F.J., 1961, Geology of the Bare 

Mountain quadrangle, Nevada: U.S. Geological Survey 

Geologic Quadrangle Map GQ-157, scale 1:62,500. 

Crone, A.J., Pierce, K.L., and Hait, M.H., Jr., 1985, The Borah 

Peak earthquake of October 28, 1983: Surface faulting, mass 

movements, and Quaternary tectonic setting of the Lost River 

Range: Geological Society of America Field Trip Guide, 1985 

Rocky Mountain Section Meeting, Boise, Idaho, 44 p. 

Harden, J. W., 1982, A quantitative index of soil development from 

field descriptions: Examples from a chronosequence in central 

California: Geoderma, v. 28, p. 1-28. 

Harden, J.W., and Taylor, E.M., 1983, A quantitative comparison 

of soil development in four climatic regimes: Quaternary 

Research, v. 20, p. 342-359. 
Hoover, D.L., Swadley, W C, and Gordon, A.J., 1981, Correlation characteristics of surficial deposits with a description of 

surficial stratigraphy in the Nevada Test Site region: U.S. 

Geological Survey Open-File Report 81-512, 27 p. 

Rogers, A.M., Harmsen, S.C., Carr, W.J., and Spence, W., 1983, 

Southern Great Basin seismological data report for 1981 and 

preliminary data analysis: U.S. Geological Survey Open-File 

Report 83-669, 240 p. 

Sowers, J.M., 1985, Pedogenic calcretes of the Kyle Canyon 

alluvial fan, southern Nevada: Morphology and development: 

Berkeley, University of California, Ph.D. thesis, 159 p. 

Swadley, W C, Hoover, D.L., and Rosholt, J.N., 1984, Preliminary 

report on late Cenozoic faulting and stratigraphy in the vicinity 

of Yucca Mountain, Nye County, Nevada: U.S. Geological 

Survey Open-File Report 84-788, 42 p. 

Taylor, E.M., 1985, Paleoclimatic interpretations of the geomorphology and Quaternary soils in the Yucca Mountain area of 

the Nevada Test Site: Boulder, University of Colorado, M.S. 

thesis, 217 p. 

Wallace, R.E., 1977, Profiles and ages ofyoung fault scarps, northcentral Nevada: Geological Society of America Bulletin, v. 88, 

p. 1267-1281. 

Preliminary Study of Quaternary Faulting 111 

9. Reinterpretation of the Beatty Scarp, 

Nye County, Nevada 

By W C Swadley, James C. Yount, and Samuel T. Harding 

CONTENTS 

Abstract 113 

Introduction 113 

Previous work 113 

Geologic setting 113 

Stratigraphy 115 

Methods and observations 116 

Mapping 116 

Scarp profiles 116 

Trenching 116 

Age determinations 118 

Seismic-refraction profiles 118 

MINI-SOSIE profile 119 

Conclusions 119 

References cited 119 

Abstract 

A prominent 1 0-km-long topographic scarp near Beatty, 

Nevada, heretofore considered a Quaternary fault scarp by 

previous workers, is interpreted to be the product of fluvial 

erosion. The discontinuous west-facing scarp ranges in height 

from 30 m to less than 2 m. An investigation of the scarp that 

included surficial mapping of the area, logging of two trenches 

excavated across the scarp, and seismic-reflection and seismicrefraction profiles found no evidence of offset. The trend of the 

scarp, parallel to the bottom lands of the Amargosa River, and 

exposures of coarse fluvial deposits of the Amargosa River at 

or near the base of the scarp indicate that the scarp was formed 

by lateral erosion by the river. 

A 14C age of 10,000±300 yr on carbonized wood 

(W-5673) in fluvial deposits at the base of the scarp indicates 

that the scarp was undercut in early Holocene time. The silt bed 

containing the carbonized wood was dated at approximately onehalf million years by the empirical uranium-trend dating method. 

The reason for the difference between these two ages remains 

unresolved. 

INTRODUCTION 

The arcuate Beatty scarp forms a prominent topographic break in slope about 2 km south of Beatty, Nevada. 

The scarp, interpreted as a Quaternary fault by previous 

workers, is within 25 km of a proposed radioactive waste 

repository at Yucca Mountain (fig. 9.1). This study was 

undertaken to evaluate the age and amount of possible 

Quaternary offset along the Beatty scarp. The work is part 

of a larger effort, done in cooperation with the U.S. Department of Energy, Nevada Nuclear Waste Storage Investigations Project, that is meant to evaluate the nature and timing 

of movement on potentially active faults in the region around 

Yucca Mountain. 

The 10-km-long Beatty scarp trends generally northsouth, approximately parallel to the west edge of Bare Mountain. Most of the scarp is formed in Pleistocene gravel; 

approximately one-fifth of the length crosses pre-Quaternary 

rocks. The scarp is discontinuous; crosscutting streams have 

either eroded the scarp or buried it under alluvial deposits 

for approximately half of its length. 

Previous work 

The arcuate trace of the Beatty scarp, which crosses 

the border of the Bullfrog and Bare Mountain quadrangles 

(fig. 9.1), has been mapped and interpreted in various ways. 

On the west, the geologic map of the Bullfrog quadrangle 

(Cornwall and Kleinhampl, 1964) shows the ct:ntral part of 

the scarp as a Quaternary fault. In the adjacent Bare Mountain quadrangle (Cornwall and Kleinhampl, 1961) the 

northern part of the scarp was mapped as a fault offsetting 

rocks of Paleozoic to Quaternary age; the south one-third 

of the scarp, which is formed entirely in Quaternary deposits, 

was not mapped as a fault. In a description of the structural 

setting of the Yucca Mountain region, Carr (1984) interpreted 

the Beatty scarp as a fault of Quaternary age. 

Geologic setting 

The Beatty scarp parallels the bottom lands of the 

Amargosa River, bordering the west side of Bare Mountain 

(fig. 9.1). Bare Mountain consists of a triangular area of 

intensely deformed Late Proterozoic and Paleozoic sedimentary rocks overlain at the north end by upper Tertiary gravel. 

The Late Proterozoic and Paleozoic rocks have undergone 

episodes of folding and faulting in the Mesozoic and a period 

of faulting and intrusion during the Tertiary (Cornwall and 

Kleinhampl, 1961). The east side of Bare Mountain is 

bounded by an east-dipping normal fault that has had Quaternary offset (see chapter 8). Along its north edge, Bare Mountain Paleozoic rocks rest in fault contact against deformed 

Tertiary volcanic flows and tuffs. A complex of Pleistocene 

Reinterpretation of the Beatty Scarp 113 
and Holocene alluvial fans covers the lower slopes of the 

west flank of Bare Mountain. The Beatty scarp is developed, 

for the most part, in these alluvial slopes. West of the narrow 

segment of the bordering Amargosa River valley, a small 

0

.___ __ _____. _ __ _____.2 KILOMETERS 

group of hills expose Paleozoic rocks similar to those of Bare 

Mountain. The area beyond the south limit of the scarp is 

largely underlain by alluvial deposits of the Amargosa River 

floodplain. 

D 

~ 

-

EXPLANATION 

Holocene and upper Pleistocene alluvial 

deposits (includes units Ola, Qlb, Ole, 

02b; see table 9.1) 

Holocene terrace deposits of Amargosa 

River (unit Of; see table 9.1) 

Pleistocene and Pliocene(?) alluvial 

fans truncated by Beatty scarp (units 

02c and QTa, undivided; see table 

9.1) 

Pre-Quaternary rocks (includes Tertiary 

volcanic rocks and Paleozoic 

bedrock) 

---Contact 

---Scarp 

Approximate margin of Amargosa River 

bottom lands 

1--1 Trench location 

·-·-·- Seismic refraction profile line 

·-·-·-Seismic reflection profile line 

• Sample locality of 14C date 

Figure 9.1. Index map and simplified geologic map of Beatty scarp area, Nevada. 

114 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
Table 9.1. Surficial units exposed along the Beatty scarp and equivalent units in the Nevada Test 

Site region 

[Nevada Test Site (NTS) area modified from Hoover and others (1981, fig. 2)] 

Beatty area NTS area 

Stratigraphy 

Age 

Holocene--------------

Late and middle 

Pleistocene. 

Early Pleistocene 

and Pliocene(?). 

Alluvi~l 

deposits 

(unit symbol) 

Q1a 

Q1b 

Q1c 

Q2b 

Q2c 

QTa 

Quaternary surficial deposits exposed along the Beatty 

scarp have been divided into seven map units (table 9.1). 

Six of these units consist of alluvial deposits of local streams; 

the seventh consists of fluvial deposits of the Amargosa 

River. The lithology of the six alluvial units and the correlation characteristics used in their mapping in the area of the 

Nevada Test Site (NTS) have been described by Hoover and 

others (1981) and are described here only briefly (table 9.1). 

The fluvial unit was not described by Hoover and others 

(1981). 

Units Q1a, Q1b, and Q1c (in order of increasing age) 

form channel deposits, two levels of low terraces, and alluvial 

fans of the modern drainage system. These units consist 

chiefly of locally derived unconsolidated gravel, sand, and 

silt. Deposits are typically moderately sorted to unsorted, 

well-bedded to nonbedded, sandy pebble to boulder gravel 

with lesser amounts of silty, gravelly sand. A Holocene age 

has been inferred for units Q1a, Q1b, and Q1c on the basis 

of their weak soil development and on correlations with 

similar deposits in adjacent areas (table 9.1). Only unit Q1c 

exhibits recognizable amounts of pedogenic carbonate (stage 

I carbonate morphology of Gile and others, 1966). 

Units Q2b and Q2c form alluvial fans and terrace remnants that are commonly topographically above units Q1a, 

Q1b, and (or) Q1c in nearby locations, but are locally 

overlain by these younger units. Units Q2b and Q2c consist 

of slightly to moderately well consolidated gravel, sandy 

gravel, and gravelly sand. They are typically poorly to 

moderately well bedded, moderately well sorted to unsorted, 

Fluvial Alluvial 

deposits deposits 

(unit symbol) (unit symbol) 

Qf Q1a 

Qf Q1b 

Qf Q1c 

Q2a 

Q2b 

Q2c 

QTa 

and consist of angular to subrounded, locally derived clasts 

as much as 0.5 m across. Minimum ages of 160 ka and 

270 ka have been inferred for units Q2b and Q2c, respectively, based on uranium-trend ages determined for soils 

developed at the surface of similar units in the NTS area 

(Swadley and others, 1984). 

Unit QTa is composed of nonbedded, poorly sorted 

gravel that is slightly to moderately well cemented with 

secondary carbonate minerals and occurs as dissected alluvial 

fans and fan remnants. Fans of unit QTa in the area of the 

Beatty scarp commonly are overlain by thin gravel deposits 

of unit Q2c. An age of less than 2 Ma and greater than 

0.8 Ma is suggested for unit QTa by radiometric ages of ash 

beds in deposits that are stratigraphically below and above 

unit QTa in the NTS area (Swadley and others, 1984). 

The fluvial unit, Qf, consists of interbedded gravel, 

gravelly sand, sand, and silt deposited by the Amargosa 

River. The deposits vary from unconsolidated to slightly consolidated, poorly bedded to moderately well bedded, and 

poorly to well sorted. The gravel consists of subrounded to 

well-rounded clasts of Tertiary basalt and rhyolitic welded 

tuff and Paleozoic quartzite and carbonate rocks. Unit Qf 

forms channel deposits and three levels of terrace deposits 

along the Amargosa River valley. A sample of carbonized 

wood fragments from a cutbank exposure of the lowest 

terrace deposit of unit Qf (fig. 9.1) has been dated by the 

14c method at 9,800±300 yr (W-5676; Meyer Rubin, 

written commun., 1985). The degree of soil development 

in the highest terrace deposits of unit Qf in the area of the 

Beatty scarp is interpreted as indicating that unit Qf may also 

be late Pleistocene age in part. 

Reinterpretation of the Beatty Scarp 115 
METHODS AND OBSERVATIONS 

Study of the Beatty scarp involved mapping of surficial 

deposits in the area of the scarp, measurement of scarp 

proftles, excavation and logging of two trenches, dating by 

radiometric methods, measurement of shallow seismic 

profiles with a high-resolution refraction system, and 

measurement of one deeper seismic profile with the MINISOSIE system. 

Mapping 

The scarp consists of a 10-km-long line of isolated scarp 

segments. Most of these segments occur in lower and middle 

Pleistocene alluvial fan gravels (fig. 9.1). Approximately 

2 km of the northern part of the scarp cuts across Paleozoic 

sedimentary rocks and Tertiary volcanic rocks. The scarp 

is preserved for about 45 percent of its overall length. The 

missing segments of the scarp either have been eroded by 

intermittent crosscutting streams that drain westward from 

Bare Mountain, or have been buried by alluvial fans 

deposited by these streams. 

Quaternary deposits truncated by the scarp consist of 

alluvial fans of units QTa and Q2c. The youngest of these 

deposits, unit Q2c, has been correlated by use of mapping 

criteria with deposits at NTS for which a minimum age of 

about 270 ka has been inferred, based on several age determinations by the uranium-trend method (Rosholt and others, 

1985). The oldest deposits that extend across the trace of 

the scarp without being cut by it are alluvial fans of unit Q1c. 

This unit has not been dated but is considered to be early 

Holocene based on the weak soil development (Hoover and 

others, 1981). 

Mapping revealed no exposures of units Q2c or QTa, 

the truncated units, on what would be the downthrown side 

of the scarp. It did show, however, a discontinuous line of 

fluvial deposits of the Amargosa River, unit Qf, that parallels 

the scarp to the west for much of its length (fig. 9.1). These 

deposits include boulders as much as a meter in diameter. 

From the proximity of these deposits to the scarp and from 

the size of clasts in these deposits, we infer that at one time 

a major channel of the Amargosa River closely followed the 

scarp. 

Scarp Profiles 

Twenty-five scarp profiles were measured by the 

method described by Wallace ( 1977). All proftles were 

measured across the central and northern parts of the scarp 

in areas where it is formed in Quaternary deposits. The maximum scarp height measured was 30 m near the midpoint 

116 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

of the scarp. Scarp height decreases northward to about 9 m 

at the north end of the scarp and southward to less than 2 m 

at the southernmost exposure. Slope angles of the scarp range 

from about 35 o where the scarp is highest, to about 20 o at 

a scarp height of 4 m. Profiles were smooth and sigmoidal, 

with no knicks or perturbations indicative of multiple scarpforming events. 

Wallace (1977, p. 1273) presented a series of histograms of the principal slope angles for groups of young fault 

scarps in north-central Nevada that range in age from 

historical to considerably older than 12,000 yr. A histogram 

of the principal slope angles of the Beatty scarp profiles 

(fig. 9.2) most closely resembles fault scarps in the Humbolt 

Range less than 12,000 yr, but more than a few thousand 

years in age (Wallace, 1977, p. 1273, fig. 8). 

Trenching 

Two trenches were excavated by bulldozer to examine 

the evidence for faulting along the Beatty scarp (fig. 9.1). 

Trench BF-1 (fig. 9.3) was sited near the midpoint of the 

scarp in order to expose the maximum scarp height in Quaternary deposits. Excavation of the trench was begun on the 

lower slope of the scarp several meters east of the scarp toe 

at a location where colluvial gravel mantles bedrock and 

alluvial fan gravel (units Pzb and QTa) that form most of 

the scarp at this site. The trench was excavated normal to 

the scarp for about 35 m to the west. The bulldozer excavated 

to a depth of 3. 5 m; both trench walls were logged with the 

aid of a 1-m square rope grid. Trench BF-1 ~xposed interbedded fluvial gravel, sand, and silt (unit Qf) overlain by 

pebbly silt (unit Q1c) that interfingers with colluvial gravel 

(Q1c) at the toe of the scarp (fig. 9.3). The contact between 

units Qf and Q1c is a slight disconformity. No soil is 

developed at this contact, and no soils are preserved within 

en 

I.LI 

....J 

u.. 

0 

0:: 

a.. 

u.. 

0 

a::: 

I.LI 

co 

~ 

::::> 

z 

10 

5 ~ 

0 

10 

I 

t r. 

-

I 

20 30 

PRINCIPAL SLOPE ANGLE, IN DEGREES 

Figure 9.2. Histogram of principal slope angles of scarp profiles 

of Beatty scarp. 
unit Qf. Paleozoic bedrock was exposed at the east end of 

the trench. No evidence of offset or disturbed strata was 

found in any of the units exposed in the trench. 

At this site, the Beatty scarp shows no stratigraphic offset and is most readily explained as an erosional scarp formed 

by lateral migration and undercutting by the Amargosa River. 

Deposition of the fluvial sediments at the base of the scarp 

occurred contemporaneously with the formation of the scarp. 

Eastward planation by the river was probably stopped when 

the river encountered the bedrock that is exposed at the east 

end of the trench. 

Trench BF-2, sited 2 km south of BF-1 (fig. 9.1), 

exposes part of an alluvial fan (unit Q1c) that extends across 

the projection of the scarp without apparent offset. The trench 

is located about 15m south of an older alluvial fan (unit Q2c) 

that is truncated by the scarp. Excavation and logging of the 

trench followed procedures similar to those at BF-1. 

Carbonate-cemented gravel (unit QTa) exposed in the south 

East 

0 

METERS 

No vertical exaggeration 

wall and in the floor of the trench unconformably underlies 

alluvial gravel of unit Q1c (fig. 9.4). Unit QTa disappears 

near the point where the projection of the scarp intersects 

the trench wall. The upper contact of unit QTa, as well as 

its western termination of exposure, appear eroded. No fault 

planes can be recognized that offset stratigraphy in the trench; 

one near-vertical zone of clast disruption exposed in the north 

wall near the projection of the scarp trace, overlies a thin 

gravel bed that is not offset. An equivalent zone of disturbed 

fabric was not present in the south wall. 

The interpretation of the trench is that the truncation 

of unit QTa and the adjacent fan deposit of unit Q2c is 

erosional. However, truncation by faulting and subsequent 

erosion of the faulted edge of unit QTa prior to deposition 

of the overlying unit, unit Q1c, cannot be ruled out at this 

exposure. The zone of disturbed fabric in unit Q1c is not 

thought to be the result of faulting because the underlying 

strata are not disturbed. 

Approximate study area of Rosholt 

and others, fig;..ll.2, this volume 

BF-3 BF-2 BF-1 l 

z 

Trench floor 

West 

Qlc 

Figure 9.3. Sketch of part of south wall of trench BF-1 (fig. 9.1 ). Trench exposes fluvial gravel, sand, and silt (unit 

Qf) overlain by silt and colluvial gravel (unit Q1 c). Paleozoic bedrock (unit Pzb) is exposed at east end of trench. Lithology 

symbols: c, cobble gravel; cb, cobble-boulder gravel; p, pebble gravel; s, sand; z, silt. Location of lens of carbonaceous 

silt containing carbonized wood shown by heavy horizontal line. Labeled vertical bars indicate location of samples 

dated by uranium-trend method (see table 9.2). 

East 

0 5 

METERS 

No vertical exaggeration 

Figure 9.4. Sketch of part of south wall of trench BF-2 (fig. 9.1 ). 

Trench exposes carbonate-cemented cobble-boulder gravel (unit 

QTa) overlain by alluvial fan deposits (unit Q1 c). Lithology 

symbols: p, pebble gravel; pc, pebble-cobble gravel; cb, cobbleWest 

boulder gravel; s, sand. Dotted contact indicates lateral gradational change in lithology. Dashed line indicates approximate 

intersection of trench wall with projection of nearby scarp. 

Reinterpretation of the Beatty Scarp 117 
Age Determinations 

Radiometric dating meant to establish the age of the 

Beatty scarp has given conflicting data. Carbonized wood 

fragments from a silt bed exposed in trench BF-1 (fig. 9.3) 

gave an age of 10,000±300 yr by the 14c method (W-5673; 

Meyer Rubin, written commun., 1985). Six sets of samples 

from localities shown on figure 9. 3 and a sample collected 

near trench BF-2 were dated by the experimental uraniumtrend method (table 9.2). 

The wood fragments that gave a 14C age of about 

10 ka came from the same stratum as uranium-trend sample 

BF-3 (fig. 9.3), which yielded an age of 480 ka. The reason 

for this considerable difference between data from the two 

dating methods remains unresolved. The 14C age indicated 

for carbonized wood from unit Qf is similar to that obtained 

for charcoal from the same stratigraphic unit in a cutbank 

exposure along the Amargosa River about 1 km southwest 

of trench BF-1 (W-5676, 9,800±300 yr). The uraniumtrend age of approximately 70 ka for unit Q2c conflicts with 

those from trench BF-1 in that it infers that the fan composed of unit Q2c truncated by the scarp is younger than 

post-scarp deposits of unit Qf exposed in trench BF-1. 

Further sampling has been carried out to resolve these age 

discrepancies. 

Seismic-Refraction Profiles 

Shallow seismic-refraction investigations were carried 

out at three sites along the Beatty scarp (fig. 9.1). The refraction system, utilizing a sledge-hammer energy source, was 

capable of detecting refracting horizons within 20 m of the 

ground surface (see Rodriguez and Yount, chapter 12). First 

arrival times read directly from the refracting waveform were 

plotted against distance from geophone to source (fig. 9.5). 

The resultant traveltime plots show no steps in the traveltime 

plots indicative of faulting in the upper 20 m of alluvium 

at any of the three sites investigated. 

Line BF-29 was located approximately 50 m south of 

trench BF-2 and was oriented parallel to the trench (fig. 9.1). 

The projection of the Beatty scarp indicated on figure 9.4 

would cross line BF-29 at approximately its midpoint. Line 

BF-29 shows evidence of nearly 3 m of young alluvium (units 

Q1a?, Q1 b?, or Q1c?) overlying alluvium possibly belonging to units Q2b, Q2c, or QTa on a westward sloping contact 

(see Rodriguez and Yount, chapter 12, for discussion of 

P-wave refraction velocity and its relation to deposit age). 

No significant displacement of any refracting horizon is 

evident on the traveltime plot for line BF-29 (fig. 9.5). 

Line BF-34 was located approximately 50 m south of 

trench BF-1 with an east-west trend (fig. 9.1). The east end 

of line BF-34 corresponds with the west end of the trench. 

Because no evidence of faulting was found at the base of the 

Beatty scarp in trench BF-1, the possibility existed that the 

Amargosa River had eroded the scarp back to the east from 

118 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

Table 9.2. Uranium-trend ages of samples 

[Rosholt and others (chapter 11) describe sampling and dating method 

used for these samples] 

Laboratory No. 

BF1 

BF2 

BF3 

BF4 

BF5 

BF6 

BT2F 

w 

Trench BF-1 

Near BF-2 

Age (ka) 

75±15 

155±35 

480±50 

530±70 

540±100 

460±90 

70±12 

200 LINE BF-34 1---Trench BF-2----l 

(f) 

~ 150 

0 

u 

w 

(f) 

:::i 100 _J 

~ 

~ 

w 50 ~ 

f= 

0 

E 

o~-L--~~--~~--~--L-~--~~--~~ 

~ 150 LINE BF- 29 z 

0 

u 

w 

~ 100 _J 

~ 

~ 50 w 

~ 

f= 

(f) 

~ 150 0 

u 

w 

(f) 

::::::J 

_J 

~ 

100 

0 

LINE BF-37 

55 

DISTANCE IN METERS 

110 

Figure 9.5. Traveltime plots for shallow refraction lines across 

or near Beatty scarp. Line BF-37 from Fluorspar Canyon; Line 

B F-34 south of trench B F-2; Line B F-2 9 south of trench B F-1 . 

Refraction velocity for each segment of plot shown in meters 

per second. Circles indicate actual first arrival time at each 

geophone. See figure 9.1 for location of lines. 
some more westerly postion. Line BF-34 shows no evidence 

of offset refractors for some 110m to the west of trench BF-1 

(fig. 9.5). Instead, approximately 2m of alluvium belonging to units Q1a, Q1b, or Q1c appears to horizontally overlie 

a sequence of deposits that have refraction velocities similar 

to those of units Q2a, Q2b, Q2c, and QTa. 

Line BF-37 was located across the projection of the 

Beatty scarp at Fluorspar Canyon (fig. 9.1). The line was 

sited on an alluvial fan composed of unit Q1c where this fan 

crosses a segment of the scarp that developed in alluvium 

of unit Q2c. The refraction line trends east-west and is 

centered on the projection of the scarp across Fluorspar 

Canyon. No evidence of offset refractors can be seen from 

the traveltime plot (fig. 9.5) at this locale. Rather, 3 m of 

alluvium of unit Q1c appears to overlie alluvium of unit Q2c 

along a slightly east dipping contact. 

MINI-SOSIE Profile 

The MINI-SOSIE (MS) system is a small, versatile 

high-resolution seismic-reflection technique that uses 

vibrating earth tampers as energy sources. Reflection data 

are acquired by an array of geophones along the line of the 

profile as a series of vibration points are occupied. The 

system and the computer processing of the data are discussed 

by Wiles (1979). Using the MS technique, a seismicreflection profile was measured across the Beatty scarp 

(Harding, chapter 10) adjacent to trench BF-2 (fig. 9.1). Two 

reflection horizons were detected within, what is here interpreted to be buried Paleozoic bedrock, at depths of about 

200 and 300m. No fault corresponding to the apparent offset of the scarp was indicated. 

CONCLUSIONS 

The Beatty scarp formed as an erosional scarp produced 

by lateral migration of the Amargosa River. This interpretation is based on the trend and location of margins of the 

bottom lands occupied by the river in late Quaternary time, 

the occurrence of fluvial deposits at or near the base of the 

scarp, the lack of conclusive evidence of faulting revealed 

by surficial mapping and trenching, and the lack of offset 

of subsurface horizons as shown by seismic-refraction and 

seismic-reflection profiles. 

The age of the scarp is inferred to be early Holocene 

from the 14c age of carbonized wood in fluvial deposits at 

the base of the scarp and a very tentative age correlation based 

on scarp profile data. The difference between this age and 

the middle Pleistocene age indicated by some of the uraniumtrend age data remains unresolved. Also unresolved is the 

conflict between the 70-ka age indicated for deposits of unit 

Q2c truncated by the scarp and ages of approximately 500 ka 

indicated for fluvial sediments about the same age as the scarp 

that are exposed at the base of the scarp in trench BF-1. 

REFERENCES CITED 

Carr, W.J. 1984, Regional structural setting of Yucca Mountain, 

southwestern Nevada, and late Cenozoic rates of tectonic 

activity in part of the southwestern Great Basin, Nevada and 

California: U.S. Geological Survey Open-File Report 84-854, 

109 p. 

Cornwall, H.R., and Kleinhampl, F.J., 1961, Geology of the Bare 

Mountain quadrangle, Nevada: U.S. Geological Survey 

Geologic Quadrangle Map GQ-157, scale 1:62,500. 

1964, Geology of the Bullfrog quadrangle and ore 

deposits related to Bullfrog Hills caldera, Nye County, Nevada 

and Inyo County, California: U.S. Geological Survey Professional Paper 454-J, 25 p. 

Gile, L.H., Peterson, F.F., and Grossman, R.B., 1966, Morphological and genetic sequences of carbonate accumulations 

in desert soils: Soil Science, v. 101, no. 5, 347-360. 

Hoover, D.L., Swadley, W C, and Gordon, A.J., 1981, Correlation characteristics of surficial deposits with a description of 

surficial stratigraphy in the Nevada Test Site region: U.S. 

Geological Survey Open-File Report 81-512, 27 p. 

Rosholt, J.N., Bush, C.A., Carr, W.J., Hoover, D.L., Swadley, 

W C, and Dooley, J.R., Jr., 1985, Uranium-trend dating of 

Quaternary deposits in the Nevada Test Site area, Nevada and 

California: U.S. Geological Survey Open-File Report 85-540, 

72 p. 

Swadley, W C, Hoover, D.L., and Rosholt, J.N., 1984, Preliminary 

report on late Cenozoic faulting and stratigraphy in the vicinity 

of Yucca Mountain, Nye County, Nevada: U.S. Geological 

Survey Open-File Report 84-788, 42 p. 

Wallace, R.E., 1977, Profiles and ages of young fault scarps, northcentral Nevada: Geological Society of America Bulletin, v. 88, 

p. 1267-1281. 

Wiles, C.J., 1979, MINI-SOSIE: New concept in high resolution 

seismic surveys: Oil and Gas Journal, v. 77, p. 94-97. 

Reinterpretation of the Beatty Scarp 119 

10. Preliminary Results of High-Resolution 

Seismic-Reflection Surveys Conducted Across 

the Beatty and Crater Flat Fault Scarps, Nevada 

By Samuel T. Harding 

CONTENTS 

Abstract 121 

Introduction 121 

Acknowledgments 121 

The MINI-SOSIE system 121 

Processing 122 

Results and interpretation 122 

Beatty scarp 122 

Crater Flat fault scarp 123 

References cited 127 

Abstract 

Two high-resolution seismic-reflection surveys were 

conducted across two scarps near Beatty, Nev. Employing the 

MINI-SOSIE high-resolution seismic-reflection technique, the first 

survey was run across the a scarp mapped as the Beatty fault 

about 6 km south of the town of Beatty. Reflection profiles from 

this site indicated no evidence of faulting that could be correlated 

with the scarp. The other survey was located across a prominent down-to-the-west fault scarp on the east side of Crater Flat 

approximately 26 km southeast of Beatty. The resulting profile 

had one distinct reflector that could be carried along the length 

of the section. We interpreted this reflector to be complexly 

faulted. A major, nearly vertical fault was projected to the surface 

fault scarp. This fault appeared on a reflector at 160 m depth 

and was located east of the fault scarp. This fault, if associated 

with the scarp, represents a strike-slip fault. Evidence for normal 

faulting that could be associated with the fault scarp could not 

be seen from the collected data. 

INTRODUCTION 

Two high-resolution seismic-reflection surveys were 

conducted southeast of Beatty, Nev., in an effort to determine if the MINI -SOSIE (MS) source system could be used 

to delineate near-surface faulting in surface alluvium and 

volcanic rocks. This equipment has led to a better understanding of fault -scarp mechanics. Locations of the survey 

sites are shown on figure 10.1. 

Acknowledgments 

Robert Bucknam and Anthony Crone proposed the two 

locations as sites whose characteristics could best serve the 

purposes of our study. Reviews and suggestions for improvement of the manuscript were done by Kathleen M. Haller 

and Ruth Ann Dwyer. I thank Ted Barnhard, Anthony Crone, 

Richard Dart, Kathy Haller, Danny Miller, John Michael, 

Susan Plymell, Kirstin Rinehart, Dave Worley, and Annette 

Yuan for their many hours in the hot sun during the acquisition of the data. Ruth Ann Dwyer's many suggestions in the 

data processing aspect were invaluable. 

THE MINI-SOSIE SYSTEM 

The MINI-SOSIE (MS) system is a small, versatile, 

high-resolution reflection technique that uses earth tampers 

as energy sources. The foot of each tamper is fitted with a 

source sensor that identifies a time break for each impact 

of the foot. Time breaks for the impacts are transmitted to 

a recording truck by radio. The tampers have a peak energy 

input at about 45 Hz but they also supply higher frequency 

energy above 100Hz (Wiles, 1979) that improves the resolution of shallow reflections. 

Time breaks from the tampers are crosscorrelated in 

the field with the signals received by the geophones. The 

crosscorrelated data are electronically stored while data from 

succeeding impulses are crosscorrelated. Vibration-point 

spacing is 18.8 m. For each vibration point, a sufficient 

amount of crosscorrelated data are stacked and autocorrelated 

to yield a good signal-to-noise ratio. 

The MS technique has several distinct advantages over 

conventional reflection-profiling methods. Because of its 

small size, the system can be easily transported into remote 

areas. The system is also capable of collecting reflection data 

in seismically noisy areas, a useful feature when working 

in urban areas. Where ambient seismic noise is a problem, 

the number of impacts stacked for each vibration point can 

be increased until the reflected signal can be distinguished 

from the undesirable noise. Another advantage of the MS 

High-Resolution Seismic-Reflection Surveys 121 
system is that it can be operated in populated areas without 

the threat of inducing ground motion that could damage nearby structures. Thus, the MS can fill gaps in conventional 

seismic reflection data associated with areas of cultural 

development. By employing specific source and receiver 

arrays, destructive interference from horizontal propagating 

waves are greatly reduced. Finally, distributing 1,500-2,000 

impacts over the entire group interval yields a distinct 

advantage over single explosive sources. 

Processing 

Field data from the Beatty scarp and Crater Flat fault 

scarp were processed by specialized computer techniques at 

the U.S. Geological Survey facilities in Denver. Processing 

steps included tape reformat and gain recovery, commondepth-point (CDP) sort, constant-velocity analyses (CVA), 

normal-moveout correction, spectral whitening, deconvolution, bandpass filtering, datum and residual statics, final CDP 

stack migation, and depth conversion. The resulting migrated 

time sections consisted of 1 s (two-way traveltime) of 12-fold 

CDP data. The time sections were then converted to depth 

(km) sections. 

Figure 10.1. Location map of survey areas southeast of Beatty, 

Nevada. Circles indicate general locations of seismic-reflection 

surveys. Hachures enclose areas of relatively higher relief. 

(Modified from Swadley and Hoover, 1983.) 

122 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

RESULTS AND INTERPRETATION 

Beatty Scarp 

The location of the 1, 123-m seismic line that crosses 

the Beatty scarp is shown in figure 10.2. The scarp corresponds to CDP 226 and the highway to CDP 246. 

The seismic-reflection time section that was obtained 

from the field data and the same time section after migration are shown in figure 10.3 and 10.4, respectively. The 

poor quality of the record section probably reflects the small 

changes in the acoustic impedance contrast between geologic 

boundaries in addition to severe residual static problems and 

near-surface scattering of source impulses by boulders. The 

stacking velocities used are listed in table 10.1. The profile 

shows two coherent bands of reflections: one at at about 

0.260 s, reflector A, and the other at approximently 0.350 s, 

reflector B. Reflector A is flat or had a slight westward dip, 

and reflector B has an eastward dip at least to CDP 290, 

where it becomes somewhat disrupted and appears to turn 

over and dip west. The depth conversion of the time section 

is shown in figure 10.5. The depths are measured in 

kilometers from a datum of 1.2 km above sea level. 

36°~~~----~--~~~---~~----------~ 

55' 

36°~--------------~~----------------~ 50' 

Figure 10.2. Location of seismic-reflection survey south of 

Beatty, Nevada. General scarp location is shown in relation 

to reflection survey. Numbers along reflection survey line are 

common-depth-point (CDP), as shown on figures 1 0.3, 1 0.4, 

and 10.5. 
The coherency of reflector B (fig. 10.4) makes it impossible to project a fault from the surface scarp to depth. 

One possibility is to project it through reflector B at CDP 

280. This may represent a fault plane with a dip of approximately 30° to the west. The apparent motion, determined 

by reflector B, would be down to the east, the reverse downto-the-west motion, indicated if the scarp were a fault. 

Therefore, in my opinion the Beatty scarp is not fault related. 

Crater Flat Fault Scarp 

A seismic reflection survey was conducted in Crater 

Flat (fig. 10.6) located between and parallel to trenches 2 

and 3 (Swadley and Hoover, 1983). The line was 1,219 m 

long and extended east 468 m from the trace of the fault in 

the trenches and extended west of the fault 750 m. The time 

section is shown in figure 10.7, the migrated time section 

is shown in figure 10.8, and the depth converted section is 

shown in figure 10.9. 

The near-surface velocity structure was found to be 

complex along this profile. A discussion of the processing 

steps utilized to determine this structure is in order. The 

West East Scarp 

t 

COMMON DEPTH POINT 

320 300 280 260 240 220 200 

~ 0.0 

z 

0 

0 

~ 0.1 

z 

w 

~ 0.2 

§-

w 

> 

<( 0.3 

0: 

t-

>-

<( :?: 0.4 

I 

0 

:?: 

t- 0.5 

( 

( 

~ 

( ~~ < ~ L}\ 

ffi1 

I I I 

i 

. ~KI .. 

I 

·~ ~ 

['~ ~x 

~ 

~ '!01 

ffi1 1 rl 

I 

1: '1, 1.1 '!' 

,I I I· ill' ·~ I, 

~ : 

~ ··~ 

rr\ 

:)1 ~ ~ 

!1i ~l(( ~X\' :u 

I . I 

·11 I, I I 

irli ',1

1

111, 1. 1!1 

I : 

1\'. :I' 

,('. 

i I 

ioN' ; A 

• .. 

~ B 

{]\ 

rJ 

·,~ 

Y.· ~ ~ I 

Figure 10.3. Computer plot of unmigrated seismic-reflection 

section, Beatty scarp, Nevada. Datum is at 1,200 m above sea 

level and time section is corrected at 1 ,500 m/s. Reflections 

A and B are bands of coherent reflected energy returns. The 

formation they arise from is unknown due to lack of well control in the area. The line represents 1,123 m of subsurface 

coverage. Surface position of common-depth-point numbers 

shown on figure 1 0.2. 

original brute stacks, using a constant velocity across the line, 

showed practically no coherent energy. Constant velocity 

analyses (CVA) were then run on 20 adjacent CDP's. 

Velocities from 1,500 m/s to 3,000 m/s were run by increasing the constant velocity by 100 m/s for each panel. The next 

10 CDP's were dropped, and the same procedure was then 

repeated on the next 20 adjacent CDP's. The entire section 

was treated in this manner. The next step was to again stack 

the data and look for further trends in coherent energy. A 

number of residual static windows were run using the velocity 

analyses on coherent bands of energy found on the velocity 

stack. These usually were only short bursts of coherent 

energy over a few CDP's, but they were enough to determine the general velocity structure. It was found that the 

sloping interface at 0.100 s from CDP 320, dipping eastward, 

to 0.190 sat CDP 190 (fig. 10.7), reflector A, appeared when 

the residual static window was run across part of the section 

from 0. 50 to 0. 300 s. The residual static operator was then 

determined from this interface using this dip to define the 

window. The constant-velocity profiles were then run using 

this horizon to make the residual static correction. This procedure was repeated to improve ( 1) the stacking velocities 

and (2) the coherent reflections. When the signal-to-noise 

West 

3 

en 0.0 c 

2 

0 

0 

w 0.1 en 

~ 

w 

~ 0.2 

tJ 

w 

> 

<( 0.3 

,o: 

t-

>-

<( :?: 0.4 

I 

0 

;: 

t- 0.5 

20 

t'i 

~ 

~ ~ 

< 

East 

Scarp 

t 

COMMON DEPTH POINT 

300 280 26 0 240 220 200 

' 

: : I 

I 'It 

II 

(~ ~~~ ·~ I 

~ : ~ 

·~ 

• - ~ ~ 

~ ~ 

A 

B 

~ I 

>N't 

Figure 10.4. Computer plot of migrated seismic-reflection section, Beatty scarp, Nevada. Datum is at 1,200 m above sea 

level and time section corrected at 1,500 m/s. Reflections A 

and B are bands of coherent reflected energy returns. The 

formations they arise from are unknown due to lack of well 

control in the area. The line represents 1,123 m of subsurface 

coverage. Surface positions of common-depth-point numbers 

shown on figure 1 0.2. 

High-Resolution Seismic-Reflection Surveys 123 
Table 10.1. Stacking velocities for common depth points, 

Beatty scarp, Nevada 

Common Traveltime Velocity 

depth (s) (m/s) 

point 

210 0.130 2,344 

.280 2,500 

.600 3,125 

1.000 3,425 

220 .100 2,500 

.200 2,969 

.600 3,281 

1.000 3,438 

270 .100 2,500 

.200 2,969 

.600 3,281 

1.000 3,438 

2980 .100 2,500 

.200 2,969 

.600 3,281 

1.000 3,438 

ratio is near a value of 1, every processing stage must be 

carefully considered to prevent creating coherent reflections 

where none exist. 

The resulting near-surface velocities varied laterally to 

a considerable degree. Figure 10.10 shows the isovelocity 

contours of the root-mean-square velocities determined from 

the CVA that was used for stacking the data and for converting 

the time section to depth (fig. 10.9). The depth section 

(fig. 10.9) did not exhibit the relatively planar, dipping 

reflector A that was observed on the time section. The lowvelocity wedge from around CDP 230 to CDP 190 (fig. 

10.1 0), was necessary to stack the coherent band of seismic 

energy (labeled A in figs. 10.7, 10.8, 10.9). The dramatic 

change between the depth and time sections across this part 

of the profile was related to the insertion of the low-velocity 

wedge. 

Correlation between the isovelocity plot (fig. 10.10) 

and the surface geology showed an apparent slowing of the 

surface velocities across the units mapped as the intermediate 

and younger fluvial gravels of Quaternary age (see fig. 10.6). 

Where the line crossed units mapped as the undivided older 

fluvial gravel deposits and sedimentary rocks unit of Tertiary 

and Quaternary age, the near-surface velocities were higher. 

The velocity analysis showed a low-velocity wedge under 

the Tertiary volcanic rocks unit. 

The depth section presented the most visible display 

of the one reflector that could be traced across the section 

124 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

(reflector A, fig. 10.9). This reflector was relatively flat from 

CDP 320 to CDP 280 and then dipped eastward to CDP 237, 

with some minor faulting interpreted as offsetting the reflection. Reflector A was then interpreted to be downfaulted 

approximately 40 m at CDP 235. Reflector A then dipped 

westward for about 5 CDP's and was faulted to a depth of 

about 150 m and appeared to be slightly bowed upward to 

CDP 200. This line may or may not have intersected the 

eastern branch of a fault (mapped by Swadley and Hoover, 

1983) that was not trenched. This fault appeared to cut the 

reflector marked B (fig. 10.9) and was the reason for 

terminating reflector A. This fault also appears to have the 

same dip as the surface fault. 

The complicated faulting shown in the middle part of 

the cross section was interpreted from faulting seen in trench 

2. While the surface expression may not have been representative of the subsurface, activity in the subsurface appears 

to be related to the scarp. The data indicated the presence 

of a negative flower structure (Harding, 1985), usually 

associated with strike-slip faulting. Other examples of this 

type of seismic reflection signature have been presented by 

Emmons (1969) and Harding (1985). Our seismic-reflection 

data from this survey showed no evidence that basinbounding down-to-the-west normal faulting or calderacollapse faulting were the mechanisms for the fault scarp 

formation. 

West East 

Scarp 

t 

COMMON DEPTH POINT 

320 300 280 260 240 220 200 

(/) 

a: 

w 

1--

w 

0.0 

~ 

0 

::::! 

~ 0.5 

~ 

:c 

1--

~ 

w 

0 

1.0 

Ground 

surface 

·A 

B 

Figure 10.5. Computer plot of migrated depth section, Beatty 

scarp, Nevada. Datum is at 1 .2 km above sea level. Surface 

locations of common-depth-point numbers shown on figure 

1 0.2. Vertical exaggeration is 1.09. Reflections A and B same 

as in figures 10.3 and 1 0.4. 
EXPLANATION 

QUATERNARY 

Younger fluvial gravel 

(units *Q 1 ab and 

*Q 1 c, undivided) 

Intermediate 

fluvial gravel 

(unit *Q2bc) 

QUA TERNARY AND 

TERTIARY 

D 

Older fluvial gravel 

deposits and 

sedimentary rocks, 

undivided 

(unit *QTa) 

TERTIARY 

Volcanic rocks 

(unit • Tv) 

*Map units of Swadley 

and Hoover ( 1983) 

_.-/'"Contact 

I Fault, bar and ball 

on downthrown 

side 

~Trench 

200 Co~mon-depth­

• po1nt numbers 

Figure 10.6. Geologic map of southeast Crater Flat, Nevada, showing seismic-reflection line with common-depth-point locations, 

t··ench locations, and fault scarps. (Modified from Swadley and Hoover, 1983.) 

High-Resolution Seismic-Reflection Surveys 125 
en 

Cl 

z 

0 

(.) 

w 

en 

~ 

w 

~ 

i= --::J 

w 

> <( 

cc 

1-

>-

<( 

~ I 

0 

~ 1-

West East 

Fault scarp 

t 

COMMON DEPTH POINT 

Figure 10.7. Computer plot of unmigrated seismic-reflection 

section, Crater Flat, Nevada. Reflector A delineates a band 

of coherent energy. Length of line is 1,219 m from commondepth-point 190 to 310. Datum is set at ground surface. 

West East 

Fault scarp 

t 

A 

Figure 10.8. Computer plot of migrated seismic-reflection 

section, Crater Flat, Nevada. Surface locations of commondepth-point numbers shown on figure 1 0.6. Reflections A and 

B are bands of coherent reflected energy returns. 

126 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

West East 

Fault scarp 

t 

COMMON DEPTH POINT 

320 300 280 260 240 220 200 

en 0.0 

cc 

w 

1- 0.1 B w 

~ A 0 0.2 ::::! 

~ 

~ 0.3 

::C 0.4 1-

a.. 

w 

Cl 0.5 

Figure 10.9. Computer plot of migrated depth section, Crater 

Flat, Nevada. Reflectors A and B same as in figure 1 0.8. Reflector 

A has been greatly changed by velocity function used from figure 

1 0.1 0. Small near-vertical bars indicate minor faults. Larger faults 

shown with sense-of-motion arrows. 

Geologic map-unit symbols 

QTa ~\~b QTa ~Q1ab} Q2bc lQ1c( 

Tv 

COMMON DEPTH POINT 

320 300 280 260 240 220 200 

en o.o------~---r----~----~---.-----r-, 

Cl 

z 

0 

(.) 

w 0.1 

en 

~ 

w~ 

~ 0.2 

_j; 

__. 

w 

> 

<( 0.3 

cc 

1-

>-

<( 

~ 0.4 

I 

0 

~ 

1- 0.5~--~--~~--~--~~----~--~~ 

Figure 10.10. Computer plot of isovelocity contours in meters 

per second of root-mean-square velocities used for stacking 

velocities and for depth conversion, Crater Flat, Nevada. 

Geologic map-unit symbols (explained in figure 1 0.6) indicate 

surficial geology crossed by seismic-reflection line. 
REFERENCES CITED 

Emmons, R.C., 1969, Strike-slip rupture patterns in sand models: 

Tectonophysics, v. 7, p. 71-87. 

Harding, T.P., 1985, Seismic characteristics and identification of 

negative flower structures, positive flower structures, and 

positive structural inversion: American Association of 

Petroleum Geologists Bulletin, v. 69, no. 4, p. 582-600. 

Swadley, W .C, and Hoover, D .L., 1983, Geology of faults exposed 

in trenches in Crater Flat, Nye County, Nevada: U.S. 

Geological Survey Open-File Report 83-608, 15 p. 

Wiles, C.J., 1979, MINI-SOSIE: New concept in high resolution 

seismic surveys: Oil and Gas Journal, v. 77, p. 94-97. 

High-Resolution Seismic-Reflection Surveys 127 

11. Uranium-Trend Dating of Fluvial 

and Fan Deposits in the Beatty Area, Nevada 

By John N. Rosholt, W C Swadley, and Charles A. Bush 

CONTENTS 

Abstract 129 

Introduction 129 

Acknowledgments 130 

Sample collection, preparation, and chemical procedures 131 

Sample sites 131 

Results 131 

Discussion 132 

Conclusions 136 

References cited 137 

Abstract 

The uranium-trend dating method is used to estimate the 

ages of Quaternary deposits to help evaluate the age and origin 

of a scarp in the Beatty, Nevada, area. For dating deposits of 

5 to 800 ka (thousand years) age, the open-system technique 

consists of determining a linear trend from analyses of six to eight 

channel samples collected at different depths in a depositional 

unit. The analytical results plotted as activity ratios of (2 38U230Th)f238U versus (234U-238U)!238U are required for the empirical model. Ideally these data points yield a linear array in 

which the slope of the line of best fit changes predictably for 

increasingly older deposits. Analyses of deposits of known age 

are required to calibrate the empirical model; calibrations were 

provided by correlations with deposits dated by independent 

radiometric methods. 

A sequence of the fluvial deposits next to the scarp and 

exposed in trench BF-1 were sampled for dating. An age of 

75 ± 15 ka was obtained for the alluvial unit of sandy silt; an 

underlying gravel unit indicated an age of 155 ± 25 ka and an 

average age of 500±60 ka was obtained for three different 

lithologic units in lower silt and gravel deposits. Four suites of 

samples representing the lower silt and gravel deposits provided 

the following ages: clayey silt, 480± 50 ka; pebble gravel, 

530± 70 ka; pebble cobble gravels, 540± 100 ka and 460± 90 

ka. For the older units, the dating method does not provide sufficient resolution to distinguish differences in ages for the deposits 

in the 500 ka group. Disseminated carbonized wood fragments 

occur in the clayey silt; a radiocarbon age of 1 0,000± 300 yr 

for this wood is not consistent with the estimated 480 ka 

uranium-trend age of the clayey silt host-sediment. 

Two separate suites representing the alluvial fan deposit 

that was truncated by the Beatty scarp adjacent to trench BF-2 

also were analyzed; uranium-trend ages of 70± 10 and 80± 10 

ka were determined on these two suites of samples. Estimates 

for the time of carbonate accumulation as rinds on pebbles in 

the same alluvial fan deposit, using the conventional 230Thf234U 

method, were 41 ± 3 and 68 ± 4 ka. 

INTRODUCTION 

Uranium-series disequilibrium dating methods, 

described by Ku and others (1979), use conventional closed 

system 230Thf234U ratios for dating pedogenic carbonates 

which form rinds on alluvial gravel. These ages provide 

reasonable estimates of the minimum age of the alluvium. 

For conventional uranium-series dating (Ku, 1976), a closed 

system is assumed to exist throughout the history of a sample, 

which means that there has been no postdepositional migration of 238u or of its daughter products (234u and 230Th). 

In contrast, open-system conditions impose no restrictions 

on migration. Results of other studies of uranium-series disequilibria indicate that uranium commonly exhibits an opensystem behavior in many near-surface deposits (lvanovich 

and Harmon, 1982). Because materials suitable for closedsystem dating are commonly absent in Quaternary deposits 

in this area of the Great Basin, an open-system dating method 

is needed. 

An open-system variation of uranium-series dating 

called uranium-trend dating has been tested extensively over 

the past decade. A preliminary model for uranium-trend 

dating was described by Rosholt (1980) with samples 

collected from a variety of Quaternary deposits including 

alluvium, eolian sediments, glacial deposits, and zeolitized 

volcanic ash. A revised model for uranium-trend systematics 

was described by Rosholt (1985). The empirical model 

requires time calibration based on analyses of deposits of 

known age; results of these calibrations are included in 

Rosholt and others ( 1985b). The uranium-trend ages of 

alluvium, colluvium, and eolian deposits at the Nevada Test 

Site area were reported by Rosholt and others (1985a). 

For uranium-trend dating of sediments, the distribution of uranium-series members during and after sedimentation must have been controlled by open-system behavior. 

Sediments and soils are penetrated continuously or episodically with water that contains at least small amounts of 

transported or locally derived uranium. As this water-borne 

uranium decays, it produces a trail of radioactive daughter 

products that are readily adsorbed on solid matrix material. 

If the trail of the daughter products, 234u and 230Th, is 

distributed through the deposit in a consistent pattern, then 

uranium-trend dating is possible. The large number of 

Uranium-Trend Dating of Fluvial and Fan Deposits 129 
geochemical variables in an open system precludes the definition of a rigorous mathematical model for uranium migration. Instead, an empirical model is used to define the 

parameters that can reasonably explain the patterns of isotopic 

distribution. 

This model requires independent time calibration with 

deposits of known age. None of the known-age deposits used 

for calibration occur in Nevada; however, results on other 

deposits in the Nevada Test Site region (Rosholt and others, 

1985a) indicate that results obtained by the uranium-trend 

method are reasonable when compared to geomorphic and 

stratigraphic relations. In rare instances at NTS, ages can 

be compared by two methods (Swadley and others, 1984). 

At Crater Flat, a uranium-trend age of 270,000 years 

(270±30 ka) was obtained for gravel deposits of unit Q2 of 

Swadley and Hoover (1983) that locally overlie and contain 

reworked cinders from a small volcano northwest of Lathrop 

Wells, Nev., which has yielded K -Ar ages ranging from 230 

to 300 ka as determined by different laboratories (V animan 

and others, 1982). 

In an open-system environment, analyses of the abundances of 238U, 234U, 230Th, and 232Th in a single sample 

do not establish a meaningful time-related pattern of isotopic 

distribution. However, analyses of several samples, each of 

which has only slightly different physical properties and only 

slightly different chemical compositions within a unit, may 

provide a consistent pattern in the distribution of these 

isotopes (Rosholt, 1985). Analyses of five to eight samples 

per unit from several alluvial, colluvial, glacial, and eolian 

deposits has yielded time-related patterns (Rosholt and others, 

1985a, b). These types of deposits range from clay-silt units 

to gravel units, most of which have isotopic distributions that 

appear to fit the model. 

The purpose of this investigation is to apply the 

uranium-trend dating technique to the geologic study of 

surficial deposits along the Beatty scarp in southern Nevada 

(fig. 11.1). Conventional closed-system uranium-series 

dating was used to determine the time of carbonate accumulation as rinds on clasts in an alluvial fan truncated by the scarp. 

The surficial geology of the Beatty scarp site is described 

by Swadley and others (chapter 9). 

Acknowledgments 

We thank R.R. Shroba and D.R. Muhs for help collecting samples of the alluvial fan; D .R. Muhs for uranium-series 

ages and valuable help with interpretation of the age estimates 

and map illustrations; and N.C. Bostick and E. C. Spiker for 

the organic petrographic studies of the carbonaceous matter. 

Figure 11.1. Generalized geology of Amargosa River area, 

Nevada, in vicinity of Beatty scarp (from Swadley and others, 

chapter 9). 

130 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

0 1 KILOMETER 

EXPLANATION 

~ Holocene and upper Pleistocene 

~ alluvial deposits 

~ 

~ 

Quaternary fluvial deposits 

of Amargosa River 

~ 

~ 

Quaternary fan deposit 

truncated by Beatty scarp 

~ Pre-Quaternary sedimentary 

L~ and volcanic rocks 

---Beatty scarp 

t----i Trench location 

BF-1 
SAMPLE COLLECTION, PREPARATION, AND 

CHEMICAL PROCEDURES 

Several samples, about 1 kg each, were collected from 

a vertical section of each depositional unit. The number of 

samples required to establish a reliable linear trend in the 

data depends on the variation in ratios of uranium and 

thorium that define the trend line; our experience indicates 

that five to eight samples in a given unit are usually sufficient. It is preferable to collect samples from a channel cut 

through deposits exposed in a trench wall or a relatively 

fresh, well-exposed outcrop. Depositional units in the Nevada 

Test Site area commonly contain abundant pebbles and larger 

fragments, which are removed by sieving. The remaining 

less-than-2-mm-size fraction is pulverized to less than 

0.2-mm size, homogenized, and retained for analysis. 

Chemical procedures used for separating uranium and 

thorium for alpha spectrometry measurements are those 

described by Rosholt ( 1985). Spikes of 236u and 229Th are 

used in the radioisotope-dilution technique to determine the 

concentrations of uranium and thorium (Rosholt, 1984). For 

defining uranium-trend slopes, each uranium separate is 

counted four different times in an alpha spectrometer, and 

each thorium separate is counted three different times. For 

conventional closed-system dating using 230Thf234u ratios, 

we sampled CaC03 rinds on the undersides of clasts found 

in the alluvial fan deposit above trench BF-2. Selective 

carbonate dissolution procedures followed Ku and Liang 

(1984) and chemical procedures followed Rosholt (1984, 

1985). Ages were calculated using the isochron-plot method 

of Szabo and Sterr (1978). 

SAMPLE SITES 

Two trenches used in the evaluation of the origin and 

age of a scarp located near U.S. Highway 95 south of Beatty, 

are described in detail by Swadley and others (chapter 9); 

map-unit symbols used for these sites are discussed in there. 

Complete logs of these two trenches also are available 

(Swadley and others, 1986). 

Trench BF-1 exposed interbedded gravel, sand, sandy 

silt, and clayey silt (unit Qt) that were deposited by the 

Amargosa River at the base of the scarp (fig. 11.1). Unit 

Qfis overlain by a thin deposit of silty alluvium (unit Q1c). 

Six sample suites, containing approximately eight samples 

each, were collected from the trench (fig. 11.2). Sample 

suites BF5 and BF6 are duplicate suites, collected 2.5 m 

apart, from the pebble-cobble gravel unit of Qf near the base 

of the trench. 

An alluvial fan (units Q2a, Q2b, Q2c) truncated by the 

Beatty scarp near trench BF-2, 1.2 km south of trench BF-1, 

was sampled at two sites to estimate the age of this fan. 

Sample suite BT2F was collected from a 1.3-m-deep hole 

excavated in the exposed side of the fan, approximately 50 m 

northwest of the trench. Sample suite 2BT2F was collected 

from a backhoe pit that penetrated the top of the same alluvial 

fan surface, approximately 55 m northwest of the trench. 

Stones found in the alluvial fan deposit, at 0.5 to 1m depth, 

were sampled to estimate the time of carbonate accumulation in the fan; conventional uranium-series dating was used 

with analyses of CaC03 rinds on the undersides of the clast. 

RESULTS 

The analytical results for the eight sample suites are 

listed in table 11.1. Uranium and thorium contents are precise 

within ±2 percent (1 sigma). Standard deviation for the 

234uf238u, 230Thf238u, and 230Thf232Th ratios are 1.5, 2, 

and 1.5 percent, respectively, based on the precision of 

repeated counts of the chemical separates. The uranium-trend 

model parameters (Rosholt, 1985) and calculated ages of the 

eight suites are shown in table 11.2. 

Uranium-trend plots of the results for alluvial fan sample suites (BT2F and 2BT2F) show acceptable ranges of 

isotopic ratios and good linearity (fig. 11.3), yielding 

estimated ages of 70± 10 ka and 80± 10 ka, respectively. 

Estimated times for the carbonate accumulation in the fan, 

41 ± 3 ka and 68 ± 4 ka (table 11. 3), are consistent with the 

approximate 75-ka uranium-trend ages for the fan deposit. 

For the two upper units from trench BF-1 (fig. 11.4) 

uranium-trend ages of 75 ± 10 ka and 155 ± 35 ka, respectively, were obtained. Results for two of the three lower 

fluvial units, (suites BF3 and BF4) indicate negative slopes 

(fig. 11.5) and uranium-trend ages of 480±50 ka and 

530 ± 70 ka, respectively. Duplicate results for the deeper 

fluvial unit, (suites BF5 and BF6) are shown in figure 11.6; 

uranium-trend ages of 540± 100 ka and 460±90 ka, respectively, were obtained. For the three lower fluvial units, the 

dating method does not provide sufficient resolution to 

distinguish differences in age. An average age and standard 

deviation of the values for the three units represented by 

suites BF3, BF4, and BF5-BF6 is 500±60 ka. 

Disseminated carbonized wood fragments ( 6 mm maximum size) occur in the fluvial clayey silt unit (suite BF3; 

fig. 11.2). A radiocarbon age of 10.0±0.3 ka was obtained 

on this carbonaceous material (W-5673; Meyer Rubin, 

U.S. G. S. , written commun., 1985). There is a great difference between the 14C age (10 ka) and the uranium-trend age 

(480 ka) for this unit. After detailed petrographic study of 

the carbonaceous material, N.C. Bostick (written commun., 

1985) determined that the carbon is not charcoal but coalified 

pure wood tissue. In a further study using 13C nuclear 

magnetic resonance spectroscopy and element abundances 

(H, C, N, 0), E.C. Spiker (written commun., 1986) found 

that the carbonized wood is similar to highly oxidized humic 

material although he could not rule out charcoal as a possible source of the material. 

Uranium-Trend Dating of Fluvial and Fan Deposits 131 
DISCUSSION 

The uranium-trend ages indicate that fluvial sediments 

exposed in trench BF-1 were deposited over a time period 

extending from the middle into late Pleistocene. We consider 

these ages to be reliable because ( 1) plots of (234 U _238u 1 

238U) versus (238u_230Th)f238u show good linearity and 

extension (Rosholt, 1985), (2) the ages have stratigraphic 

consistency, and (3) duplicate analyses of the same stratigraphic unit (suites BF5 and BF6) show concordance within 

experimental error. The uranium-trend ages suggest that all 

of the lowermost units were deposited about 500,000 yr ago, 

as dates are all concordant around this age within error limits. 

Suite BF2 yielded a significantly younger age of about 155 ka 

indicating a depositional hiatus between the units represented 

by suites BF3 and BF2 of about 345 ka. If the top of the 

fluvial silts (suites BF3) was exposed for about 345 ka before 

deposition of the upper fluvial gravels (suite BF2), then a 

well-developed paleosol should be present. The absence of 

a well-developed paleosol at the contact between these two 

units suggests a major period of erosion sometime between 

about 500 ka and about 160 ka. A similar interpretation can 

be made for the contact between the upper fluvial gravel 

(suite BF2) and the silty alluvium (suite BF1) at the surface; 

the dates suggest a depositional hiatus between about 155 ka 

and about 75 ka. Again, the absence of a well-developed 

paleosol at the contact between these two units also suggests 

a major period of erosion some time between about 155 ka 

and 75 ka. 

The uranium-trend age estimate of the uppermost unit 

(Q1c, suite BF1) in trench BF-1 is difficult to evaluate 

geomorphically or pedologically, but some constraints are 

possible. There is only weak pedogenic carbonate development in unit Q1c, but a discontinuous, 2-cm-thick zone of 

CaC03 accumulation is found at depths of 40-50 em. This 

material is 15-20 percent CaC03, based on loss on ignition. 

If the carbonate is pedogenic, it indicates enough carbonate 

accumulation to suggest a pre-Holocene age for unit Q1c, 

based on rates of carbonate accumulation in alluvium in 

similar climates as summarized by Machette ( 1985). The 

surface there also has a weak stone pavement, with rock 

varnish found on the pavement clasts (fig. 11.2). Stone 

pavements can form in as short a time as a few years in arid 

regions (Sharon, 1962), but observations by many researchers indicate that rock varnish requires several thousand 

to as much as 10,000 years to form (Dorn and Oberlander, 

1982). One can conclude from the combined carbonate and 

rock-varnish data that unit Qlc is probably older than earliest 

Holocene and perhaps considerably older. 

The results from trench BF-1 suggest that a sequence 

of several alluvial deposits were laid down by the Amargosa 

River at the base of the Beatty scarp in the last 500,000 yr. 

The fan deposits actually cut by the Beatty scarp in the 

vicinity of trench BF-1 have not been dated because suitable 

sampling sites were not found. Presumably, they predate the 

oldest unit ( "-~500 ka) exposed in trench BF-1 if the scarp 

is erosional and not tectonic. However, we have dated an 

alluvial fan truncated by the Beatty scarp approximately 

Weak stone pavement with varnish 

Interbedded 

Ole Silty sand 

Bf1 75±15ka 

gravel and silt l ---------------------------------- --------~-

Boulder-cobble Qf BF2I155±35 ka 

gravel 

Clayey silt 

BF3 480± 50 ka 

BF6 I 460±90 ,, BF5 I 540± 1 00 ,, 

L-------------;FI~oor of trench 0 

Carbonaceous silt 

10±0.3 ka 

. Pebble-cobble gravel 

2 METERS 

No vertical exaggeration 

EXPlANATION 

BF2 I Location and number of sample 

suites for U-trend dating 

--Location of carbon sample 

for I4C age 

Figure 11.2. Diagram of sampled area on south face of trench BF-1 (fig. 11.1 ). Modified from trench log of Swadley and others 

(1986). Qlc, silty alluvium (Swadley and Hoover, 1983); Qf, fluvial sediments (Swadley and Hoover, 1983). Uranium-trend ages 

shown next to sample locations. 

132 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
Table 11.1. Sample collection depths, element concentrations, and uranium-series activity ratios 

D8pth Th u 23401 2380 23oTh/2380 230Th/232Th 23801 232Th 

Sample (em) (ppm) (ppm) 

Sandy gravel of fan deposit (channel in outcrop) 

BT2F-1 36-47 7.74 1.18 1 .149 1.138 0.538 0.473 

BT2F-2 47-58 11.3 1.73 1.132 1.110 .523 . 471 

BT2F-3 58-69 11.2 1. 76 1.157 1 .133 .549 .485 

BT2F-4 69-80 11.2 1. 96 1. 211 ~986 .535 .542 

BT2F-5 80-91 1 2.2 2.43 1.279 • 830 • 513 .617 

BT2F-6 91-102 11.9 2.49 1. 295 .837 .541 .647 

BT2F-7 102-113 11.4 2.29 1.262 • 904 .562 .622 

BT2F-8 113-124 11.3 2. 21 1. 224 . 959 .579 .604 

Sandy gravel of fan deposit (channel in backhoe pit) 

2BT2F-2 28-38 10.7 1. 53 1. 112 1. 241 0.547 0. 441 

2BT2F-3 38-48 11.2 1.51 1. 1 01 1.187 .494 • 416 

2BT2F-4 48-58 1L3 1. 71 1.166 1.087 .508 .468 

2BT2F-5 58-68 1 o. 7 2.16 1 ~ 31 2 • 834 .523 .627 

2BT2F-6 68-78 10.3 2.29 1. 281 .849 .585 .690 

2BT2F-7 78-88 10.9 2.20 1.260 .926 .577 .624 

2BT2F-8 88-98 1 o. 4 2.46 1. 344 .834 . 612 .734 

Silty all uvi urn and fluvial sand, trench BF-1 

BF1-2 10-20 14.9 2.27 1. 036 1. 345 0.634 o. 471 

BF1-3 20-30 16.4 2.67 1.083 1.204 .607 .504 

BF1-4 30-40 16.2 2.51 1.114 1. 247 .596 .478 

BF1-5 40-50 16.8 2.85 1.155 1.104 .583 .527 

BF1-6 50-60 17.9 3.21 1 • 1 91 1.066 .598 .561 

BF1-7 60-70 17.6 3.25 1 .193 1.054 .607 .576 

BF1-8 70-80 19.4 2. 91 1. 118 1 ~ 154 .536 .465 

Fluvial gravel, trench BF-1 

BF2-1 85-93 16.5 2.99 1.127 1.040 0.581 0.558 

BF2-2 93-101 17.9 3.33 1.158 1.008 .578 .574 

BF2-3 101-109 17.8 3.27 1.151 .995 .564 .566 

BF2-4 109-117 17.4 3.16 1.184 1.001 .569 .568 

BF2-5 117-125 16.7 3.39 1. 205 • 953 .596 ~626 

BF2-6 125-133 17.6 3.33 1 .142 1. 011 .593 .587 

BF2-7 133-141 17.7 3.30 1.120 1. 033 .593 .574 

BF2-8 141-149 18.4 3.06 1 .099 1.145 .588 .513 

Fluvial silt, trench BF-1 

BF3-1 150-157 19.7 3.36 1. 323 1.327 0.699 0.527 

BF3-2 157-164 19.4 3.35 1. 247 1.170 .626 .535 

BF3-3 164-171 17.1 2.94 1 .227 1 .139 .604 • 531 

BF3-4 171-178 14.8 2.50 1.234 1.153 .602 .522 

BF3-5 178-185 17.6 2.95 1 .290 1. 219 .632 .519 

BF3-6 185-192 20.2 3.35 1. 449 1. 366 .700 .512 

BF3-7 192-199 15.4 2.70 1. 312 1.282 .694 .542 

BF3-8 199-206 18.1 2.93 1. 293 1. 332 .668 • 501 

Fluvial gravel, trench BF-1 

BF4-1 210-214 18.4 2. 90 1. 264 1. 267 0.618 0.488 

BF4-2 214-218 18.0 3.00 1.216 1.199 • 616 .514 

BF4-3 218-222 18.3 3.00 1. 228 1. 265 • 641 .507 

BF4-4 222-228 1 9. 4 3.15 1. 314 1. 390 .697 . 501 

BF4-5 228-232 19.7 3.01 1. 272 1. 348 .637 . 472 

BF4-6 232-236 20.1 3.35 1 .189 1.200 • 616 .514 

Uranium-Trend Dating of Fluvial and Fan Deposits 133 
Table 11.1. Sample collection depths, element concentrations, and uranium-series activity ratios-Continued 

Depth Th u 234u;238u 230rh;238u 230Th/232Th 

Sample (ern) (ppm) (ppm) 

Fluvial gravel, trench BF-1 

BF5-1 244-250 18.8 2.92 1 .163 1 .204 0.577 

BF5-2 250-256 17.7 2. 71 1.188 1.168 .553 

BF5-3 256-262 19.3 2.85 1.133 1.072 .489 

BF5-4 262-268 20.0 2.90 1. 227 1. 219 .547 

BF5-5 268-274 18.7 2.74 1.187 1.149 .520 

BF5-6 274-280 19.7 2.73 1.182 1.136 .486 

BF5-7 280-286 20.2 2.88 1.145 1 .1 00 .484 

BF5-8 286-292 19.2 2.65 1. 225 1. 272 .544 

BF6-1 222-232 16.7 2.60 1.180 1 .175 .567 

BF6-2 232-242 16.4 3.00 1. 265 1.144 .575 

BF6-3 242-252 18.4 3.07 1 .131 1.070 .561 

BF6-4 252-262 18.1 3.06 1.187 1.131 .576 

BF6-5 262-272 18.5 2.84 1 .142 1.079 .494 

BF6-6 272-282 19.1 3.03 1.137 1 .098 .566 

BF6-7 282-292 18.2 2. 91 1 .133 1 .094 .534 

BF6-8 292-302 18.4 3.06 1 .238 1.106 .609 

Table 11.2. Uranium-trend model parameters and ages of depositional units in Beatty area 

[F(O), uranium flux] 

Half period 

Sample U-trend X of F(O) 

suite Description of deposit slope intercept (ka) 

Trench BF-2 area 

BT2F Alluvial fan deposit adjacent +0.487 -0.426 100 

to trench BF-2 (outcrop 

channel). 

2BT2F Alluvial fan deposit adjacent + .554 - .400 11 0 

to trench BF-2 (backhoe 

pit channel). 

Trench BF-1 

BF1 Silty alluvium at surface----- + 0.538 - 0.404 100 

BF2 Upper fluvial gravel---------- l- .588 - .275 240 

BF3 Lower fluvial silt------------ - ~753 + • 1 43 520 

BF4 Lower fluvial gravel---------- - .570 + . 155 500 

BF5 Lower fluvial gravel---------- - .490 + .204 420 

BF6 Lower fluvial gravel (same -2. 071 - .026 680 

as BF5). 

238u;232rh 

0.479 

.473 

.457 

.449 

.452 

.428 

.440 

.427 

.482 

.502 

.525 

.509 

.458 

.516 

.488 

.551 

Age 

(ka) 

70±10 

80±1 0 

75±154 

155±35 

480±50 

530±70 

540±100 

460±90 

1.2 km south of trench BF-1 (fig. 11.1); this fan is about 

55 m northwest of trench BF-2. Samples taken from a natural 

exposure along the side of this fan yielded a uranium-trend 

age estimate of 70± 10 ka (table 11.2). In order to confirm 

this age, a backhoe pit was dug on a flat, stable part of the 

fan surface, and samples collected there gave a concordant 

uranium-trend age estimate of 80± 10 ka (table 11.2). In both 

cases, the uranium-trend plots are highly linear (correlation 

coefficients are both 0.98, significant at the 99.9 percent 

confidence level). 

134 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
As a further test of the uranium-trend age estimates, 

carbonate rinds from fan clasts were collected in the pit from 

depths of about 0. 5-1.0 m; the rinds are 1-3 mm thick. Two 

samples were analyzed for conventional closed-system 

OCl 

r<) 

N 

OCl 

r<) 

N 

A 

-0.4~~--~~--~~--~~--~~--J-~--~ 

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 

( 238 u- 230 Th) I 238 u 

B 

-0.4~~--~~--~~--~~--~~--J-~--~ 

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 

Figure 11.3. Uranium-trend plot of fan alluvium adjacent to 

trench BF-2 (fig. 11.1 ). A, Suite BT2F. 8, Suite 2BT2F. The 

x-intercept is where regression line intersects zero line for 

(234U_238U)!238U. 

23~;234u dating that yielded age estimates of 41 ± 3 ka and 

68 ±4 ka (table 11. 3). These age estimates support the 

uranium-trend age estimates, because the uranium-trend ages 

theoretically date the time of deposition of the alluvial fan 

materials whereas the uranium-series dates should be time 

integrated age estimates of the period of carbonate accumulation. Because carbonate accumulation had to occur after the 

fan deposits were stabilized, uranium-series age estimates 

for the carbonate should be no older than uranium-trend dates 

of the fan deposits themselves. 

Finally, all radiometric determinations on these fan 

deposits are supported by pedologic and rock-varnish data. 

The surface of the fan has a well-developed stone pavement, 

and clasts in the pavement have been strongly varnished. The 

soil developed in the fan deposits has Stage II carbonate morphology following the scheme of Gile and others (1966). Soils 

in similar climates with a Stage II carbonate morphology have 

been estimated to be roughly 60,000 to 120,000 yr old, based 

on previous uranium-series dates (Ku and others, 1979) and 

observations summarized by Machette ( 1985). 

The significance of the age estimates of the fan deposits 

near trench BF-2 is that they provide a maximum age for 

the Beatty scarp, because they are exposed in the scarp. The 

results of our studies here are inconsistent with the results 

from trench BF-1, which suggest that the Beatty scarp is 

older than rv500 ka. Several interpretations of this apparent 

inconsistency are possible: (1) the alluvial deposits exposed 

in trench BF-1 may be downfaulted sediments, rather than 

deposits set in by fluvial processes against a preexisting erosional scarp, in which case the Beatty scarp is of tectonic 

origin (but see discussion in Swadley and others, chapter 9); 

(2) the Beatty scarp may have different ages at different 

localities; or (3) some of the uranium-trend age estimates 

in the lower units exposed in trench BF-1 may be too old, 

despite the fact that they meet all criteria for reliable age 

estimates. 

Table 11.3. Concentrations, isotopic activity ratios, and uranium-series age estimates for carbonate pebble rinds in alluvial fan 

deposits near Beatty trench 2 

Age 

Caco1 Th u *23401 2380 

estimate 

Sample (pet (ppm) (ppm) 23401 2380 230Th/232Th 23oTh/2340 *230rh;234u (ka) 

2BT2F-A 91 

leachate------------- 1.7 1.18 1. 71 ±0. 03 1. 41 ±0. 02 o. 39±0. 01 1. 91 ±0. 06 0.32±0.01 41 ±3 

residue-------------- 8.5 1. 37 1.07± .08 .42± .01 .79± .01 

2BT2F-B 78 

leachate------------- 0.70 1. 82 1. 72± .02 6.7 ± .1 .49± .01 1. 76± .04 .48± .02 68±4 

residue-------------- 5.7 2.60 1.55± .02 1.13± .02 .52± .01 

*Corrected for detrital 23°Th and U contamination using the isochron-plot method of Szabo and Sterr (1978). 

Errors reported are based on multiple alpha counts in different detectors (±1o). 

Uranium-Trend Dating of Fluvial and Fan Deposits 135 
A serious inconsistency arises from comparison of the 

uranium-trend age of 480 ka for suite BF3 and a 14c date 

on carbonaceous material collected from this same unit. The 

carbonaceous material yielded a 14C date of 10 ka. The 

results of the petrographic study that indicate the carbonaceous material is coalified wood tissue rather than charcoal 

suggests that the carbon may be decomposed remnants of 

abundant root growth that penetrated the clayey silt horizon 

about 10 ka. We were not able to determine the exact source 

of the carbon using nc nuclear magnetic resonance 

spectroscopy. 

CONCLUSIONS 

The following conclusions can be made, based on 

uranium-trend ages on fluvial deposits from trench BF-1: 

0.6 

:::> A !X) 

I") 0.4 N 

~ ~ 

:::> 0.2 

!X) 

I") 

N 

I 0 

:::> 

v 

I") -0.2 N 

'-../ 

-0.4 

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 

( 238 u- 230 Th) I 238 u 

0.6 

:::> B 

!X) 

I") 0.4 N 

~ ~ 

:::> 0.2 

!X) 

I") 

N 

I 0 

:::> 

v 

I") -0.2 N 

'-../ 

-0.4 

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 

( 238 u- 23o Th)l 238 u 

Figure 11.4. Uranium-trend plots of alluvium and fluvial deposit 

in trench BF-1 (fig. 11.1 ). A, Suite BF1. B, Suite BF2. The 

x-intercept is where regression line intersects zero line for 

(234U_238U)!238U. 

136 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

(1) deposition of silt, sand, and gravel took place at about 

500, 155 and 75 ka; and (2) erosion took place between 500 

and 155 ka, and again between 155 and 75 ka as indicated 

both by the differences in age and by the absence of welldeveloped paleosols between depositional units. 

From uranium-trend and uranium-series ages and soil 

development on fans near trench BF-2, we conclude that the 

Beatty scarp at that locality is no older than about 80 ka. 

This conclusion is not consistent with the chronology and 

stratigraphic relations at trench BF-1, which suggests that 

(1) the Beatty scarp may be of tectonic origin, (2) the Beatty 

scarp may have different ages at different localites, or 

(3) some of our age estimates in the lower units of trench 

BF-1 may be too old. No unambiguous interpretation will 

be possible without further age control and more detailed 

examination of the field relations. 

0.5 I ·,r I I 

:=J A (() 

-¥-~ 1'0 

N 0.3 r- -

~ ~ 

:=J ' (() ' 0.1 ' 1'0 r- - N 

' I 0 

:=J ' v -0.1 - ' ' - 1'0 

N ' '-../ 

-0.3 I I I I 

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 

( 238 u- 230 Th) I 238 u 

0.6 -..... .. I I I I 

' :=J B ' (() ' 1'0 0.4 N 

~ ~ 

:=J 0.2 

1-

'~*=t, -

1- -

' (() ' 1'0 1'- N 

0 I 

' ·, 

' ' :=J ' ' "'" I") -0.2 r- ·-:::, ....;: 

N 

'-../ 

-0.4 I I I I 

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 

Figure 11.5. Uranium-trend plots of fluvial deposits in trench 

BF-1 (fig. 11.1). A, Suite BF3. 8, Suite BF4. The x-intercept is 

where regression line intersects zero line for (

234U-238U)!23au. 
0.5 

' 

::::J i' ro 

f'0 

N 0.3 -

............... 

,.---.... 

::::J 

ro 

f'0 0.1 - N 

I 0 

::::J 

-.;!- -0.1 rf'0 

N 

'-...../ 

-0.3 

-0.8 

0.6 

::::J B ro 

f'0 0.4 N 

............... 

,.---.... 

::::J 0.2 

ro 

f'0 

N 

I 0 

::::J 

"<t 

f'0 -0.2 N 

'-...../ 

-0.4 

-0.8 

I 

' -, -,_ 

I 

-0.6 

·, 

I 

·,_ ·, 

I 

-0.4 

I 

,, 

I 

-0.2 0 

·, ·,_ ·, 

I 

I 

0.2 

( 238 u - 230 Th) I 238 u 

-0.6 -0.4 -0.2 0 0.2 

( 238 u - 230 Th) I 238 u 

-

-

'·._, 

0.4 

0.4 

Figure 11.6. Uranium-trend plots of fluvial deposits in trench 

BF-1 (fig. 11.1 ). A, Suite BFS. B, Suite BF6. The x-intercept is 

where regression line intersects zero line for (234U_23Bu)!23Bu. 

REFERENCES CITED 

Dorn, R.I., and Oberlander, T.M., 1982, Rock varnish: Progress 

in Physical Geography, v. 6, p. 317-367. 

Gile, L.H., Peterson, F.F., and Grossman, R.B., 1966, Morphological and genetic sequences of carbonate accumulation 

in desert soils: Soil Science, v. 101, p. 347-360. 

lvanovich, M., and Harmon, R.S., 1982, Uranium series disequilibrium: applications to environmental problems: Oxford, 

Clarendon Press, 571 p. 

Ku, T.L., 1976, The uranium-series methods of age determination: Annual Review, Earth and Planetary Science Letters, v. 4, 

p. 347-379. 

Ku, T.L., Bull, W.B., Freeman, S.T., and Knauss, K.G., 1979, 

Th230_u234 dating of pedogenic carbonates in gravely desert 

soils of Vidal Valley, southeastern California: Geological 

Society of America Bulletin, v. 90, p. 1063-1073. 

Ku, T.L., and Liang, Z.C., 1984, The dating of impure carbonates 

with decay series isotopes: Nuclear Instruments and Methods 

in Physics Research, v. 223, p. 563-571. 

Machette, M.N., 1985, Calcic soils and calcretes of the southwestern United States, in Weide, D. L., ed., Soils and Quaternary geology of the southwestern United States: Geological 

Society of America Special Paper 203, p. 1-21. 

Rosholt, J.N., 1980, Uranium-trend dating of Quaternary sediments: 

U.S. Geological Survey Open-File Report 80-1087, 65 p. 

1984, Radioisotope dilution analyses of geological 

samples using 236u and 229Th: Nuclear Instruments and 

Methods in Physics Research, v. 223, p. 572-576. 

1985, Uranium-trend systematics for dating Quaternary 

sediments: U.S. Geological Survey Open-File Report 85-298, 

34 p . 

Rosholt, J.N., Bush, C.A., Carr, W.J., Hoover, D.L., Swadley, 

W C, and Dooley, J.R., Jr., 1985a, Uranium-trend dating of 

Quaternary deposits in the Nevada Test Site area, Nevada and 

California: U.S. Geological Survey Open-File Report 85-540, 

72 p. 

Rosholt, J.N., Bush, C.A., Shroba, R.R., Pierce, K.L., and Richmond, G.M., 1985b, Uranium-trend dating and calibrations 

for Quaternary sediments: U.S. Geological Survey Open-File 

Report 85-299, 48 p. 

Sharon, D., 1962, On the nature ofhamadas in Israel: Zeitschrift 

fiir Geomorphologie, v. 6, p. 129-147. 

Swadley, W C, and Hoover, D.L., 1983, Geology of faults exposed 

in trenches in Crater Flat, Nye County, Nevada: U.S. 

Geological Survey Open-File Report 83-608, 15 p. 

Swadley, W C, Hoover, D.L., and Rosholt, J.N., 1984, Preliminary 

report on late Cenozoic faulting and stratigraphy in the vicinity 

of Yucca Mountain, Nye County, Nevada: U.S. Geological 

Survey Open-File Report 84-788, 42 p. 

Swadley, W C, Huckins, H.E. and Taylor, E.M., 1986, Logs of 

the trenches across the Beatty Scarp, Nye County, Nevada: 

U.S. Geological Survey Miscellaneous Field Studies Map 

MF-1897. 

Szabo, B.J., and Sterr, H., 1978, Dating caliches from southern 

Nevada by 230Thf232Th versus 234U/232Th and 234uf232Th 

versus 238UJ232Th isochron-plot method, in Zartman, R. E., 

ed., Short papers of the 4th International Conference on 

Geochronology, Cosmochronology and Isotope Geology: U.S. 

Geological Survey Open-File Report 78-701, p. 416-418. 

Vaniman, D.T., Crowe, B.M., and Gladney, E.S., 1982, Petrology 

and geochemistry of hawaiite lavas from Crater Flat, Nevada: 

Contributions to Mineralogy and Petrology, v. 80, p. 341-357. 

Uranium-Trend Dating of Fluvial and Fan Deposits 137 

12. Relation Between P-Wave Velocity and Stratigraphy of 

Late Cenozoic Deposits of Southern Nevada 

By Eduardo A. Rodriguez and james C. Yount 

CONTENTS 

Abstract 139 

Introduction 139 

Refraction data 139 

Discussion 140 

References cited 140 

Abstract 

Shallow seismic-refraction studies near Yucca Mountain 

reveal a correlation between the geologic age of deposits and 

their measured P-wave velocities. The P-wave velocities of nearsurface and subsurface layers are correlated with stratigraphic 

units as mapped at the ground surface and as observed in 

trenches. This correlation shows a good temporal resolution 

within the study area, with P-wave velocity increasing with 

inferred stratigraphic age. The P-wave velocity of units with 

similar inferred stratigraphic ages does not vary significantly 

between sites separated by as much as 60 km. We suggest that 

the major control of P-wave velocities within the stratigraphic 

units may be the accumulation of secondary pedogenic 

carbonate rather than differences in grain size or in clast composition; however, no quantitative analysis of lithologic controls 

has been performed. 

INTRODUCTION 

We conducted 37 shallow seismic-refraction investigations in five areas near Yucca Mountain (fig. 12.1), as part 

of an ongoing study of recent tectonism in the vicinity of 

a proposed high-level nuclear waste repository (U.S. 

Geological Survey, 1984). Four of the study areas contain 

potentially youthful faults and the fifth, the Fortymile Wash 

area, contains a well-studied sequence of stratigraphic units. 

During these investigations, a correlation between the age 

of mapped units and their measured P-wave velocities was 

observed. 

Field studies were performed using a Bison model 1580 

signal-enhancement seismograph. Methods of operation 

follow established procedures for portable seismographs 

(Mooney, 1973; Tinsley, 1984). The seismograph was 

deployed using an array of six geophones and a sledgehammer signal source (fig. 12.2A). Line lengths ranged from 

18 to 110m (60 to 360ft) with geophones aligned away from 

the signal source at intervals varying from 1.5 to 9 m (5 to 

30ft). The maximum depth resolvable with this array in the 

materials studied is approximately 18 m (60 ft). 

Previous studies (Duke and others, 1971; Lajoie and 

others, 1975; Fumal, 1978) have shown a correlation 

between geologic age and either shear-wave or P-wave 

velocity in noncalcareous Quaternary materials. The present 

study extends these results to calcareous coarse-grained 

alluvium and fluvial sediments of southern Nevada. 

REFRACTION DATA 

The seismograph enhances the induced seismic signal 

by cumulatively recording energy from multiple blows of 

the hammer until the recorded waveforms show distinct 

P-wave arrivals for each geophone on the line. Plotting the 

P-wave arrival times at each geophone against the corresponding distance from the signal source results in points 

through which best-fit line segments can be drawn (fig. 

12.2B). Ordinary least-squares linear regression was used 

to determine best-fit lines for the traveltime plots; r2 values 

of 0.97 or greater were achieved for these line segments. 

The P-wave velocity was determined for respective 

layers encountered at each survey site. The P-wave 

velocities, intercept times, depths to refracting interfaces, 

and inferred stratigraphic units for each refraction profile 

are presented in table 12.1. The P-wave velocities corresponding to subsurface layers were tabulated by stratigraphic 

units (fig. 12.3) as mapped at the surface (Hoover and others, 

1981; Fortymile Wash: Swadley and others, 1984) and as 

observed in fault trenches (Frenchman Flat: Yount and 

Rodriguez, unpublished trench log; Rock Valley: Yount and 

others, 1987; Crater Flat: Swadley and Hoover, 1983; 

Swadley and others, 1984; Beatty: Swadley and others, 

1986). 

The ranges of P-wave velocities for stratigraphic units 

among the study sites overlap to a small degree (possibly 

owing to seismic lines being arrayed across very thin veneers 

of surficial materials) and hence may reflect the limit of 

accuracy and resolution of the method. In order to test and 

isolate any regional variation in velocity, we calculated the 

mean of P-wave velocities for each stratigraphic unit 

encountered at each study site separately. Then we calculated 

the mean of the sum of P-wave velocities for each unit at 

all study sites together (table 12.2). Finally, we grouped 

stratigraphic subdivisions together within the stratigraphic 

framework, and labeled these with the suffix "u" (for 

Relation Between P-Wave Velocity and Stratigraphy 139 
example, units Q1a, Q1b, and Q1c considered together are 

labeled Q1u in this report). These composite units were 

treated the same as individual stratigraphic units in calculating 

the means of P-wave velocities at each site and also for the 

study area as a whole (table 12.2). 

One-way analysis of variance shows that the variation 

among the mean P-wave velocities of the stratigraphic units 

at all sites is significantly greater than variation due to random 

error (calculated F-ratio=39.13; F-ratio at the 1 percent 

level=2.15). 

DISCUSSION 

Tabulation of the means of P-wave velocities for the 

various stratigraphic units shows that P-wave velocity correlates well with inferred stratigraphic age (fig. 12.4). The 

good temporal resolution within any given area contrasts with 

the distribution of P-wave velocities from site to site (fig. 

12.5). One exception to the trend that velocity increases with 

age is found in the Beatty study area. At this locale, deposits 

mapped as unit Q2b on the basis of terrace altitude and pavement development have comparatively low P-wave velocities 

(table 12.2). Recent radiocarbon dating of dispersed organic 

fragments from a trench in these deposits suggests a Holocene 

age for this unit (Swadley and others, chapter 9); however, 

dating of the deposits by uranium-trend methods yields an 

age of 70 ± 15 ka (Rosholt and others, chapter 11). Clearly, 

the velocities for this unit (367 to 546 m/s) group more 

closely with deposits (Q1a, Q1b, Q1c) of Holocene age. 

We suggest that the P-wave velocity increases with 

stratigraphic age owing to the accumulation, with time, of 

secondary pedogenic carbonate. Units Q1a, Q1b, and Q1c 

lack secondary carbonate or, at best, show weak Stage I 

carbonate development. 1 Units Q2a, Q2b, and Q2c can be 

moderately to well cemented and can even contain some 

Stage IV (laminar) development. Unit QTa consistently contains a thick, dense caliche (Stage IV) in the upper meter 

or more of the deposit. At one locality (Line RV -2; Yount 

and others, 1987) a trench adjacent to a closely spaced refraction line confirms that the principal lithologic change at the 

depth of the most prominent refractor (1m) is a moderately 

developed laminated soil carbonate within an accumulation 

of gravel of unit Q2c. 

If laminated soil carbonate cementation is controlling 

seismic refraction, then depths to refracting layers may not 

correspond exactly to stratigraphic horizon boundaries. We 

do not know how removal of laminated soil carbonate and 

subsequent deposition of a younger unit will affect derived 

P-wave velocity for this unconformable interface, because 

1

Pedogenic carbonate stage designations as used by Birkeland (1984), 

table A-4, p. 358-359. 

140 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

we have not encountered this situation in the field. We suppose that this stratigraphic configuration would either yield 

a low P-wave refraction velocity for the underlying unit, or 

result in an overestimate of the thickness of the upper unit. 

REFERENCES CITED 

Birkeland, P.W., 1984, Soils and geomorphology: New York, 

Oxford University Press, 372 p. 

Dobrin, M.B., 1976, Introduction to geophysical prospecting: New 

York, McGraw-Hill, 622 p. 

Duke, C.M., Johnson, J.A., Kharraz, Y., Campbell, K.W., and 

Malpiede, N .A., 1971, Subsurface site conditions and geology 

in the San Fernando earthquake area: University of California, Los Angeles, School of Engineering and Applied Science 

Report UCLA-ENG-7206, 118 p. 

Fumal, T.E., 1978, Correlations between seismic wave velocities 

and physical properties of geologic materials, San Francisco 

Bay Region, California: U.S. Geological Survey Open-File 

Report 78-1067, 114 p. 

Hinrichs, E.N., 1968, Geologic map of the Camp Desert Rock 

quadrangle, Nye County, Nevada: U.S. Geological Survey 

Geologic Quadrangle Map GQ-726, scale 1:24,000. 

Hoover, D.L., Swadley, W C, and Gordan, A.J., 1981, Correlation characteristics of surficial deposits with a description of 

surficial stratigraphy in the Nevada Test Site region: U.S. 

Geological Survey Open-File Report 81-512, 27 p. 

Lajoie, K. R. , and Helley, E .J., 197 5, Differentiation of sedimentary deposits for purposes of seismic zonation, in Borcherdt, 

R.D., ed., Studies for seismic zonation of the San Francisco 

Bay Region: U.S. Geological Survey Professional Paper 

941-A, p. A39-A51. 

Mooney, H.M., 1973, Handbook of engineering seismology: 

Minneapolis, Bison Instruments, 104 p. 

Swadley, W C, and Hoover, D .L., 1983, Geology of faults exposed 

in trenches in Crater Flat, Nye County, Nevada: U.S. 

Geological Survey Open-File Report 83-608, 15 p. 

Swadley, W C, Hoover, D.L., and Rosholt, J .N., 1984, Preliminary 

report on late Cenozoic faulting and stratigraphy in the vicinity 

of Yucca Mountain, Nye County, Nevada: U.S. Geological 

Survey Open-File Report 84-788, 42 p. 

Swadley, W C, Huckins, H.E., and Taylor, E.M., 1986, Logs of 

trenches across the Beatty scarp, Nye County, Nevada: U.S. 

Geological Survey Miscellaneous Field Studies Map MF-1897. 

Tinsley, J. C. , 1984, Seismic refraction studies of the thickness of 

alluvium in the Wolverton ground-water basin and beneath 

Crescent Meadow, in Wahrhaftig, Clyde, Geomorphology and 

glacial geology, Wolverton and Crescent Meadow areas and 

vicinity, Sequoia National Park, California: U.S. Geological 

Survey Open-File Report 84-400, p. 40-52. 

U.S. Geological Survey, 1984, A summary of geologic studies 

through January 1, 1983, of a potential high-level radioactive 

waste repository site at Yucca Mountain, Nye County, Nevada: 

U.S. Geological Survey Open-File Report 84-792, 103 p. 

Yount, J.C., Shroba, R.R., McMasters, C.R., Huckins, H.E., and 

Rodriguez, E.A., 1987, Trench logs from a strand of the Rock 

Valley fault system, Nevada Test Site, Nye County, Nevada: 

U.S. Geological Survey Miscellaneous Field Studies Map 

MF-1824. 
FIGURES 12.1-12.5 

AND TABLES 12.1 I 12.2 
o 116°30' 116°00' 

37 r------------,--r-----------------~~~---------------------r------------------~~~~-------------, oo' 

I NEVADA TEST i/ SITE BOUNDARY 

BEATTY 

STUDYAREA 

I FORTYMILE WASH 

STUDY AREA Frenchman 

Flat 

Crater Flat I 

0 D FRENCHMAN 

! FLAT 

............--1/STUDY 

I AREA 

AREA OF MAP 

Figure 12.1. Location map showing study areas near Yucca Mountain and the Nevada Test Site. 

Hammer 

First layer 

VIA" 450m/s 

Second Ioyer 

Vu • 786 m/s 

Third Ioyer 

v3A. 1,540 m/s 

A 

<J) 

= 

West 

i 120 

3 80 

iE 

== 40 LLJ 

:::&: 

B 

Geophones 

Second i nterfoce 

East 

20 40 60 80 100 

DISTANCE IN METERS 

Figure 12.2. Schematic vertical section and three-layer model 

(A) illustrating refracted wave paths (modified from Mooney, 

1973; Dobrin, 1976). v,A, v2AI v3A' P-wave velocity of 

respective subsurface layers. Traveltime plot (8) of typical 

reversed refraction line. TI 1A, TI 2A, Tl 1 8, and Tl28, intercept 

times. Velocity in meters per second. 

142 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

>- IX 

<C 

z IX Regional unconformity UJ 

!-

<C Q2a ::> 

0 

Q2b 

>- ~ Regional unconformity IX <-· 

~ ~ ~[J QT· 

::> UJ 

0 !-

YOUNG DEPOSITS 

(grouped under Qlu) 

Qla - Coarse fluvial deposits 

Qlb - Coarse fluvial deposits 

and debris flows 

Qle - Eolian sand in dunes 

and sheets 

Qls - Fluvial sand sheets 

Qlc - Coarse fluvial deposits, 

minor debris flows 

INTERMEDIATE DEPOSITS 

(grouped under Q2u) 

Q2a - Local debris-flow deposits 

Q2b - Fluvial deposits and 

stra th terraces 

Q2s - Fluvial sand sheets 

Q2e - Eolian sand in dunes 

and sheets 

Q2c - Coarse fluvial deposits, 

fluvial sand sheets, 

and minor debris flows 

OLDER DEPOSITS 

QTa - Debris flows, minor 

fluvial deposits 

Figure 12.3. Correlation chart showing principal surficial 

geologic units in the Nevada Test Site region (from Swadley 

and Hoover, 1983, as modified from Hoover and others, 

1981). 
Qlu 

Tp 

Figure 12.4. Averaged P-wave velocities and ranges (shaded 

regions) for all stratigraphic units (unit QTa of Hoover and others, 

1981, unit Q1 u, and unit Q2u) for all study sites together. Single 

P-wave refraction velocity determined for a Tertiary bedrock unit 

(unit Tp, rocks of Pavits Spring of Hinrichs, 1968) of interbedded 

siltstone and tuff in the Rock Valley study area. Map-unit symbols: Q1 u, units Q1a, Q1 b, and Q1c (composite) of Hoover and 

~~======-------------------- others, 1981; Q2u, units Q2a, Q2b, and Q2c (composite) of 

DISTANCE IN METERS Hoover and others, 1981. 

2,000 

1,800 

0 1,600 

z 

0 

u 

LU 

~ 1,400 V) 

~ 

LU 

rLU 

~ 1,200 

z 

0 ~ 1,000 

0 

...J 

LU 

> 800 LU 

> 

~ ~ 600 

400 

200 

0 

A B 

Figure 12.5. Histograms grouped by (A) stratigraphic unit and (8) study area, showing mean P-wave velocity. 

Symbols: RV, Rock Valley; BF, Beatty area; CF, Crater Flat; FF, Frenchman Flat; and FM, Fortymile Wash. Mapunit symbols: Q1 u, units Q1 a, Q1 b, and Q1 c (composite) of Hoover and others, 1981; Q2u, units Q2a, Q2b, 

and Q2c (composite) of Hoover and others, 1981; QTa, gravel unit QTa of Hoover and others, 1981. See figure 

12.1 for location of study areas. 

Relation Between P-Wave Velocity and Stratigraphy 143 
Table 12.1. Results of seismic-refraction survey lines 

[ln, length of survey line; int, interval between geophones; az, compass azimuth direction of survey lines; V, P-wave velocity corresponding to 

subsequent layers: subscripts 1, 2, and 3, subsequent subsurface layers; subscripts A and B, survey line directions (reversed lines). Intercept times 

were produced by projecting line segment of corresponding P-wave velocity to origin (see fig. 12.2A); subscripts same as for velocity. Depth to 

interface refers to depth from surface to refracting interface; D1A, Dm is thickness of first layer; DzA• DzB is thickness of second layer. Stratigraphic 

unit designations are as mapped at the surface from Hoover and others (1981); Fortyrnile Wash, Swadley and others (1984) and as observed in 

fault trenches; Frenchman Flat, Yount and Rodriguez (unpublished trench log); Rock Valley, Yount and others (1987); Crater Flat, Swadley and 

Hoover (1983); Swadley and others (1984); Beatty area, Swadley and others (1986). LI> L2, L3, subsequent subsurface layers; und, underlying 

material present but unit designation not determined; CaC03, calcium carbonate layer at or near surface; Tv?, possible silicic volcanic unit at depth 

in Crater Flat] 

Line Velocity 

Site 1 n i nt az v1A v1B v1 v2A v2B v2 v3A v3B v3 

and 

(m s - 1 No. (m) (m) (deg) ) 

Rock Valley 

RV-01 18 1.5 360 391 1,284 

RV-02 110 9.1 083 939 572 755 2,564 1,356 1,787 

RV-03 37 6.1 005 896 896 896 2,881 2,238 2,621 

RV-04 37 6.1 358 1,793 7,620 

RV-06 110 9.1 358 927 942 935 2,102 1,611 1,869 

RV-07 18 3.1 327 327 762 

RV-08 110 9.1 327 978 1,345 1,162 1,988 1,241 1,615 2,010 

RV-12 55 9.1 168 847 1,707 315 

RV-13 110 9.1 028 1,102 1,380 2,078 

RV-16 18 3.1 011 346 1,199 

RV-17 110 9.1 011 640 914 777 1,219 1,676 1,427 2,057 

RV-18 18 3.1 011 411 1,219 

RV-19 18 3.1 058 1,372 2,286 

RV-20 18 3.1 065 975 1,219 

RV-21 55 9.1 065 975 1,554 

Beatty area 

BF-25 18 3.1 263 391 421 406 582 544 564 

BF-26 18 3.1 165 617 

BF-27 18 3.1 082 847 2,540 1,693 1,209 577 893 

BF-28 9 1.5 082 293 508 401 569 709 637 

BF-29 110 9.1 067 608 457 411 871 728 798 

BF-30 18 3.1 020 343 391 367 632 638 639 

BF-31 55 9.1 020 535 558 546 978 1,693 1,590 1,678 

BF-32 18 3.1 014 385 456 421 1,168 924 1,034 

BF-33 55 9.1 014 376 462 419 851 717 791 1,045 1,068 1,045 

BF-34 110 9.1 098 450 486 468 786 818 817 1,540 1,301 1,405 

BF-35 18 3.1 084 448 448 448 1,411 938 1,148 1,016 1,418 

BF-36 55 9.1 084 993 689 841 1,811 1,967 1,919 

BF-37 110 9.1 085 635 672 654 942 1,007 977 

Fortymile Wash 

FM-14 55 9.1 030 586 1,139 

FM-22 55 9.1 030 671 671 671 1,143 1,219 1,199 

FM-23 18 3.1 030 381 381 381 1,250 732 937 1,341 

FM-24 18 3.1 041 351 335 343 1,036 1,128 1,110 

Crater Flat 

CF-38 18 3.1 008 762 1,088 925 1,229 1,423 1,332 

CF-39 55 9.1 177 737 732 735 2,230 2,134 2,256 

CF-40 110 9.1 095 796 756 776 1,758 1,407 1,601 

Frenchman Flat 

FF-09 18 3.1 356 346 653 1,524 

FF-10 55 9.1 073 618 1,560 

FF-11 110 9.1 040 524 775 1,696 

144 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
Intercept time Depth to interface Units 

TilA TilR TI2A TI2B 01A 018 02A D?.B L1 L2 L3 

(1xlo- 3 s) (m) 

0.0051 0.9 Qlc QTa 

.0179 0.0077 7.6 3.4 Q2c? Qta 

.0048 0.0044 2.4 2.1 02c? Tp 

.0051 4.6 QT~ und 

.0338 0.0191 18.6 10.4 Q2c QTa 

.0156 2.7 Q1c Q2c 

.0306 0.0043 0.0334 18.6 6.4 Q2c QTa und 

.0053 2.4 Q2c QTa 

.0016 0.01?.8 1.5 11.6 Q2b Q2c QTa 

.0064 1.2 Qla Q2c 

.0070 0.020 0.019 3.4 9.5 11.6 Q2b Q2c QTa 

.0048 0.9 Qlc QTa 

.0010 0.9 QTa Tp 

.0017 1.5 Q2b QTa 

.0045 2.7 Q2b QTa 

.0069 0.0055 2.1 1.5 Ole Q2c 

Q2c 

1.2 CaC03 .0017 0.0005 0.3 0.1 Qla CaC03 ---

.0249 0.0067 6.1 1.5 Ole Q2c ---

.0034 0.0036 0.9 0.9 Q2b Q2c 

.0194 0.0389 0.0344 6.4 6.4 3.4 10.1 Q2b Q2c QTa 

.0163 0.0116 3.7 2.7 Qlc Q2c 

.0091 0.0070 0.0174 0.0166 2.1 1.8 1.4 5.5 Qlc Q2c QTa 

.0067 0.0078 0.0398 0.0349 1.8 2.1 4.9 13.1 02h 02c ()T~ 

.0046 0.0034 0.0011 0.0069 1.2 0.9 02c QTa 

.0182 0.0325 8.5 15.2 Q2c QTa 

.0093 0.0070 4.0 3.1 Qlc? Q2c? 

.0076 2.7 O(c OTa 

.0055 0.0090 2.1 3.7 Q2c QTa 

.0090 0.0037 0.0110 1.8 0.9 Q2a Q2c QTa? 

.0110 0.0107 2.1 1.8 Qlc Q2a 

.0024 0.0019 1.5 1.2 Q2c QTa 

.0126 0.0112 4.9 4.3 Q2c Tv? 

1.0188 0.0227 8.5 10.1 Q2b QTa 

,.0064 0.0041 0.0148 0.9 3.1 Qlc Q2c QTa 

1.0091 3.1 Q2c QTa 

1. 0083 0.0402 3.1 14.3 Qlc Q2c QTa 

Relation Between P-Wave Velocity and Stratigraphy 145 
... 

.&;;. 

0'1 

C1 

I'D 

0 

~ ;:;-

~ 

::::s 

c.. 

:::1: 

~ 

a 

0 

CJCI 

;:;· 

5" 

;:6 

:a. 

aci" 

g. Table 12.2. Relations of map units to measured velocities, in meters per second ::::s 

~ 

< c 

I"') 

I"') 

~ 

~ 0 

c 

::::s 

S' 

J:i" 

z 

~ ~ 

c.. 

~ 

Map 

units 

Q1u 

Q1a 

Q1c 

Q2u 

Q2a 

Q2b 

Q2c 

QTa 

Tp 

Rock 

Mean 

361 

346 

376 

1,006 

----

957 

1,053 

1,629 

2,454 

Valley Beatty 

Mi n/t1ax t1ean 

346- 376 408 

---- 401 

327- 411 414 

957-1,053 630 

---- ---- 777-1,102 458 

755-1,427 802 

1,219-2,078 1,362 

2,286-2,621 ----

area Crater Flat 

Mi n/t1ax Mean Mi n/t1ax 

401- 414 ---- ---- ---- ---- ---- 406- 421 ---- ----

458- 802 812 735- 925 

---- ---- ---- 367- 546 776 756- 796 

448-1,034 812 735- 925 

977-1,919 1,467 1,332-1,601 

---- ---- ----

Frenchman Flat Fortymi 1 e Wash All areas Mean Mi n/t1ax Mean Min/Max Mean Min/Max 

435 ---- 343 ---- 387 343- 435 

---- ---- ---- ---- 374 346- 401 

435 346- 524 343 ---- 392 343- 524 

682 ---- 738 731- 745 774 458-1,053 

---- ---- 745 381-1 '110 745 381-1' 110 

---- ---- ---- ---- 730 367-1,102 

682 618- 775 731 586- 937 816 448-1,427 

1,593 1,524-1,696 1,227 1,139-1,341 1,456 977-2,078 

---- ---- ---- ---- 2,454 2,2R6-2,621 
13. Water-Table Decline in the 

South-Central Great Basin During the Quaternary: 

Implications for Toxic Waste Disposal 

By Isaac J. Winograd and Barney J. Szabo 

CONTENTS 

Abstract 147 

Introduction 147 

Acknowledgments 148 

Rates of apparent water-table decline 148 

Discussion and synthesis 150 

Implications for toxic waste disposal 151 

References cited 152 

Abstract 

The distribution of vein calcite, tufa, and other features 

indicative of paleo-ground-water discharge indicates that during the early and middle Pleistocene the water table at Ash 

Meadows, in the Amargosa Desert, Nevada, and at Furnace 

Creek Wash, in east-central Death Valley, California, was tens 

to hundreds of meters above the modern water table, and that 

ground-water discharge occurred up to 18 km up the hydraulic 

gradient from modern discharge areas. Uranium-series dating 

of the calcitic veins permits calculation of rates of apparent watertable decline; rates of 0.02 to 0.08 m/ka are indicated for Ash 

Meadows and 0.2 to 0.6 m/ka for Furnace Creek Wash. The rates 

for Furnace Creek Wash closely match a published estimate of 

vertical crustal offset for this area, suggesting that tectonism is 

a major cause for the displacement observed. In general, 

displacements of the paleo water table probably reflect a 

combination of (1) tectonic uplift of vein calcite and tufa, 

unaccompanied by a change in water-table altitude, (2) decline 

in water-table altitude in response to tectonic depression of areas 

adjacent to dated veins and associated tufa, (3) decline in watertable altitude in response to increasing aridity caused by major 

uplift of the Sierra Nevada and Transverse Ranges during 

the Quaternary, and (4) decline in water-table altitude in 

response to erosion triggered by increasing aridity and (or) 

tectonism. 

A synthesis of hydrogeologic, neotectonic, and paleoclimatologic information with the vein-calcite data permits the 

inference that the water table in the south-central Great Basin 

progressively lowered throughout the Quaternary. This inference 

is pertinent to an evaluation of the utility of thick (200-600 m) 

unsaturated zones of the region for isolating solidified radioactive 

wastes from the hydrosphere for hundreds of millenia. Wastes 

buried a few tens to perhaps 100 m above the modern water 

table-that is above possible water level rises due to future pluvial 

climates-are unlikely to be inundated by a rising water table 

in the foreseeable geologic future. 

INTRODUCTION 

Regional interbasin flow of ground water through the 

thick section of Paleozoic carbonate rocks of the south-central 

Great Basin has been the subject of numerous studies in the 

past 25 years (Hunt and Robinson, 1960; Loeltz, 1960; 

Winograd, 1962, Winograd and Thordarson, 1968; 

Winograd, 1971; Winograd and Friedman, 1972; Naff, 

1973; Winograd and Thordarson, 1975; Dudley and Larson, 

1976; Winograd and Pearson, 1976; Waddell, 1982). Flow 

through the regional carbonate-rock aquifer is directed 

toward major spring discharge areas at Ash Meadows in the 

Amargosa Desert of Nevada and toward Furnace Creek Wash 

in east-central Death Valley, California (fig. 13.1). The flow 

occurs under hydraulic gradients as low as 0.06 m/km 

(Winograd and Thordarson, 1975, pl. 1), reflecting the high 

fracture transmissivity of this aquifer. Locally, major 

hydraulic barriers compartmentalize the aquifer (Winograd 

and Thordarson, 1968, 1975). A detailed hydrogeologic and 

hydrogeochemical synthesis of this vast flow system, including potentiometric maps, is available in Winograd and 

Thordarson (1975). 

A variety of geologic evidence indicates that during the 

Pleistocene the water table in the regional carbonate-rock 

aquifer at Ash Meadows and at Furnace Creek Wash (fig. 

13.1) was tens to hundreds of meters above the modem water 

table (Winograd and Doty, 1980). The evidence consists of 

tufas; ancient spring orifices; calcitic veins and cyclindrical 

calcite-lined tubes that mark the routes of paleo ground water 

flow to spring orifices; and paleo water levels inscribed on 

the walls of Devils Hole (fig. 13.1), a fault-controlled collapse feature adjacent to the Ash Meadows discharge area. 

Most of these have been briefly described elsewhere 

(Winograd and Thordarson, 1975, p. C82-C83; Winograd 

and Doty, 1980; Pexton, 1984; Winograd and others, 1985). 

In this study we focus on the calcitic veins as indicators of 

paleo water tables because they are readily datable using 

uranium-disequilibrium methods (Szabo and others, 1981; 

Winograd and others, 1985) This report is an initial step 

toward a quantification of the observations of Winograd and 

Doty (1980). 

Water-Table Decline During the Quaternary 147 
Acknowledgments 

We thank D. Stuart-Alexander, W.J. Carr, W.W. 

Dudley, Jr., W.R. Osterkamp, N.J. Trask, Jr., R.B. Scott, 

and W.E. Wilson for helpful review comments, and we 

appreciate the excellent assistance provided by G. C. Doty 

and A. C. Riggs during the field search for the calcitic veins. 

RATES OF APPARENT WATER-TABLE DECLINE 

At Ash Meadows, the calcitic veins occur in association with, and adjacent to, a structurally controlled 16-kmlong spring discharge area (Winograd and Thordarson, 

1975). The veins occur as much as 50 m higher than and 

36'45' 

LOCATION DIAGRAM 

"-, 1-: c)~'J... o Tonopah ""1 •, 

oFresno '·· ~ Vegas Las 

•n {"- 0 :., Map area '•'-) 

116'45' 

0 

Crater 

0 Flat 

116'30' 

\ \].···.······ tr 

Q ..... \\·/·(/ 

as much as 14 km up the hydraulic gradient from the highest 

water level (altitude 719 m) at Ash Meadows-namely, that 

in Devils Hole (Winograd and Doty, 1980). Veins AM-7, 

DH-1, and AM-10 from Ash Meadows and northern 

Amargosa Flat (fig. 13.1) are, respectively, 11, 19, and 26m 

higher than the water level in Devils Hole (table 13.1). (The 

hydraulic gradient in the region between these veins is 

extremely small-0.06 m/km (Winograd and Thordarson, 

1975, pl. I)-so that for practical purposes the altitude of 

the veins can be compared directly to the water level in Devils 

Hole.) Uranium-disequilibrium dating of these veins yields 

an age of 510 ± 62 ka for the youngest laminae in vein AM-7, 

660±75 ka for the youngest laminae in vein DH-1, and 

750±52 ka for the center of vein AM-10 (table 13.1). These 

data permit calculation of the average rates of apparent waterf) 

I 

I~ ·O 

\a 

•;3 J ::::.: : 'b 

116°15' 

1'-.. ~ Angeles \I ~ . . ~ 

\(3 o II / 

/{::)* !$. 

I 

0( I 19 

\ IS' 19 r \ 36'30' I' ·. 

\ 

( % 

\

: Fairbanks 

Spring Amargosa 

: .rO "\ /. \ ,.../ .. ---·· ,_../ ~~··-·· ~ . ·jJ I \ 

\~(~ ',.., \Flat 

·.)/.-AM-7~~ . .., __ , 

~ / ~ Devil~~·:. .... 

~: 01 Hole · .. 

~ j ~ (DH-1) 

')(Cil ~ 

§ j ', Big Spring 

d( "., \ 

\. : '~ 

\ ' 

0 5 10 15 20 KILOMETERS 

Figure 13.1. Index map of south-central Great Basin, Nevada-California. Uplands shaded; x's mark location of uranium-series 

dated calcitic veins discussed in text. 

148 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 
table decline; rates on the order 0.02 to 0.03 m/ka are 

indicated (table 13.1). These rates are minimum values, first, 

because we do not know how high above outcrop the sampled veins might have extended prior to erosion; and second, 

because the youngest laminae in our veins may only record 

the time of sealing of the vein, rather than the time of cessation of ground-water discharge. Nevertheless, because the 

numerator (that is the altitude difference between vein outcrop and water level in Devils Hole) in our ratio is so much 

smaller than the denominator (vein age), any reasonable combination of values yields a very slow rate of decline. For 

example, if the altitude of vein DH-1 were 20m higher prior 

to erosion (a large value considering the present relief on 

the Pliocene and Pleistocene rocks in central Ash Meadows) 

and if ground-water discharge ceased 400 ka instead of 

660 ka ago, we still calculate an apparent rate of water-table 

decline which is less than 0.1 m/ka. 

The average rates of water-table decline cited above 

(0. 02 to 0. 03 m/ka) were calculated (table 13 .1, columns 

1-5) assuming a constant rate of decline during the middle 

and late Pleistocene, that is over times 510 to 750 ka long. 

A more realistic computation of decline rate is one involving three vein pairs that differ in age by only 90 to 240 ka. 

Such a computation (table 13.1, columns 6-7) yields rates 

of decline which are 2 to 2 ¥2 times as large as the average 

rates, namely 0.05 to 0.08 m/ka. The paired veins record 

ground-water flow in the period 510 ka to 750 ka (table 13.1). 

When these data are arranged by decreasing age and are 

coupled with the average rate of decline calculated for the 

past 510 ka (0.02 m/ka, derived from the youngest and lowest 

vein, AM-7, and the water-table altitude in Devils Hole), 

we see a suggestion of a possible reduction in rate of watertable decline during the middle Pleistocene; that is, a rate 

of 0.08 m/ka is indicated for the period 750 to 660 ka, a 

rate of 0.05 m/ka for the period 660 to 510 ka and a rate 

of 0.02 m/ka for the past 510 ka. In view of the caveats 

presented in the preceding paragraph, additional work is 

clearly in order to verify the suggested change in rate of 

water-table decline in the Ash Meadows region. In summary, 

the data of table 13 .1 indicate rates of apparent water decline 

of 0.02 to 0.08 m/ka for the Ash Meadows-Amargosa Flat 

area. 

At Furnace Creek Wash (fig. 13.1), the rate of lowering of the water table during the Quaternary is an order of 

magnitude greater than the cited rates for Ash Meadows and 

vicinity. Here a calcitic vein swarm and associated tufa occur 

at an altitude of about 855 m. Uranium-disequilibrium dating 

of vein 1 OB (fig. 13 .1) from this area indicates that groundwater flow in the fracture containing this vein ceased about 

1,000±100 ka. In the absence of wells or artesian springs, 

the water-table altitude in the regional carbonate aquifer 

beneath the vein swarm is unknown, but we can bracket the 

range of possible water-table altitudes by reference to known 

water levels both up and down the hydraulic gradient from 

the vein swarm. The altitude of the water table in the valleyfill aquifer beneath the southern Amargosa Desert, 15-20 

km up the hydraulic gradient from the vein swarm, is about 

640-670 m (Winograd and Thordarson, 1975, pl. I), or about 

185-215 m lower than the vein swarm (855 m). The altitude 

of the water table in the regional carbonate aquifer at Nevares 

Spring (fig. 13 .1) in east -central Death Valley, 18 km down 

the hydraulic gradient, is 286m or about 570 m lower than 

the vein swarm. (The water level at Nevares Spring is the 

highest level known for the regional carbonate aquifer in eastcentral Death Valley.) Due to the extreme aridity of the 

region, plus the high transmissivity of the regional carbonate 

aquifer (Winograd and Thordarson, 1975), the presence of 

a ground-water mound (a potentiometric high) in the 

Table 13.1. Uranium-disequilibrium ages of calcitic veins at Ash Meadows and Amargosa Flat, Nevada, and rates of apparent watertable decline during the Quaternary 

[Altitude ofvein AM-10 estimated from USGS 1:24,000 Specter Range, SW topographic quadrangle; altitude DH-1 from laser-altimeter survey; altitude 

AM-7 from average of four aneroid-barometer surveys. Justification for dating the calcitic veins by the 234UI238U method in Winograd and others (1985). 

Average rate of water-table decline assumes constant rate of decline between time of deposition of youngest laminae in vein and the Holocene; values 

rounded to one significant figure and represent minimum rates for reasons given in text] 

Vein Altitude 234u;238u Vein altitude above Average rate of Difference in Rate of water-

(see fig. 13.1 (m) age water-level at Devils water-table altitude (m) table decline 

for location) (ka) Hole (719 m) decline and in youngest using data of 

(m) (m/ka) age (ka) for preceding column1 

indicated vein (m/ka) 

pair 

AM-10---------- 745±3 2750±52 26±3 0.03 7, 90 (AM 10-DH 1) 0.08 

DH-1----------- 738 660±75 to 19 .03 15, 240 (AM 10-AM 7) .06 

890±92 

AM-7----------- 730±1 510±62 to 11 ±1 .02 8, 150 (DH 1-AM 7) .05 

620±66 

1values rounded to one significant figure. 2sample came from center of 1-om-thick vein; given average growth rates (0.3 mm/ka) in other Ash Meadows veins, the time 

spanned during deposition of this very thin vein is unlikely to have exceeded 50 ka. (We assume the vein grew symmetrically 

from the walls.) 

Water-Table Decline During the Quaternary 149 
carbonate aquifer between the southern Amargosa Desert and 

east-central Death Valley is extremely remote; that is, we 

have confiden~e that the potentiometric surface in the 

carbonate aquifer beneath the vein swarm at Furnace Creek 

Wash is intermediate between the cited altitudes in the 

southern Amargosa Desert and in east -central Death Valley. 

The uranium-disequilibrium age, in conjuction with the cited 

vein and water-table altitudes, indicates an average watertable lowering of 0.2 to 0.6 m/ka. 

DISCUSSION AND SYNTHESIS 

Tectonics, climate change, and erosion in response to 

tectonics and (or) climate change are obvious potential causes 

for the observed water-table displacements. A comparison 

of the cited water-table displacement rates at Ash meadows 

(0.02 to 0.08 m/ka) and Furnace Creek Wash (0.2 to 0.6 

m/ka) with average rates of vertical crustal offsets in these 

regions seemingly supports tectonism as a major cause for 

the displacement we have observed at Furnace Creek Wash. 

In east -central Death Valley, a rate of vertical crustal offset 

of0.3 m/ka has been calculated by Carr (1984) for the Black 

Mountains (fig. 13.1) utilizing the data of Fleck (1970). In 

contrast, data presented by Pexton (1984, p. 49) on the 

displacement of a 3-m.y.-old tuff at Ash Meadows indicate 

relative vertical crustal offset of about 0.01 m/ka, or onehalf to one-eighth of the indicated rate of water-table decline 

at Ash Meadows and vicinity. 

Climatic change cannot be discounted as an important 

auxiliary cause for the documented water-table displacements. Major uplift of the Sierra Nevada and Transverse 

Ranges during the Pliocene and Quaternary should have 

markedly and progressively reduced the precipitation 

reaching the Great Basin during this time. Smith and others 

(1983, p.23) suggested that 3 m.y. ago, when the Sierra 

Nevada was about 950 m lower, about 50 percent more 

moisture might have crossed the Sierra and moved into the 

Great Basin. Various lines of evidence support such notions. 

Raven and Axelrod ( 1977) and Axelrod ( 1979), using 

paleobotanical evidence, argued for increasing aridity in the 

Great Basin, Mojave Desert, and Sonoran Desert during the 

late Tertiary and Quaternary. They attributed this increasing aridity to uplift of the Sierra Nevada, Transverse Ranges, 

Peninsular Ranges, and the Mexican Plateau. Winograd and 

others (1985) described a major and progressive depletion 

in the deuterium content of ground-water recharge in the 

region during the Quaternary; the most logical explanation 

for their data is a progressive decrease in Pacific moisture 

due to uplift of the Sierra Nevada and Transverse Ranges. 

And Pexton (1984, p. 43-46, 57), on the basis of studies 

of sediment depositional environments, believed that the Ash 

Meadows area became progressively more arid during the 

Quaternary. 

The role of erosion in the apparent lowering of the 

water table is not known. We assume that in east-central 

150 Geologic and Hydrologic Investigations, Yucca Mountain, Nevada 

Death Valley, where the rate of vertical crustal offset is large, 

tectonism dominated both erosion and climate as a factor in 

water-table change during the Quaternary. This may not, 

however, be correct for the Ash Meadows region, where the 

rate of vertical crustal offset is an order of magnitude smaller 

(see above); here, the erosional history of the bordering 

Amargosa Desert-a history influenced by climate change 

and possibly also by tectonism in Death Valley-may have 

played an important role in the water-table changes we see 

at and northeast of Ash Meadows. 

We should emphasize that the evidence presented by 

Winograd and Doty (1980), and its initial quantification 

herein, suggests only an apparent lowering of the water table 

during the Quaternary at Ash Meadows and Furnace Creek 

Wash. We do not know to what degree the displacement of 

veins and tufas relative to the modem water table reflects 

( 1) tectonic uplift of the veins and associated tufas, unaccompanied by a decline in water-table altitude; (2) a lowering 

of water-table altitude in response to the tectonic downdropping of a region adjacent to the veins and tufas; (3) a lowering of water-table altitude in response to increasing aridity, 

or to erosion; or (4) some combination of these. Locally, 

uplift of the veins was probably a major cause for the 

displacements we observed. For example, the occurrence of 

the veins at Furnace Creek Wash (site lOB, fig. 13.1) at 

altitudes higher than the modern potentiometric surfaces to 

the west and east (see above) strongly suggests that major 

uplift of the Funeral Mountains and Greenwater Range 

occurred relative to both Death Valley on the west and the 

Amargosa Desert on the east. However, a synthesis of 

regional hydrogeologic, tectonic, and paleoclimatologic 

information with our observations indicates that a progressive 

and absolute lowering of the regional water table (more 

correctly the potentiometric surface) is likely to have 

occurred throughout the south-central Great Basin during the 

Quaternary. This inference is based on the following three 

considerations. (1) The several-thousand-meter topographic 

relief in Death Valley developed principally during the 

Pliocene and Pleistocene (Hunt and Mabey, 1966; U.S. 

Geological Survey, 1984), and the movement of the floor 

of Death Valley has probably been downward relative both 

to sea level and to bordering areas (Hunt and Mabey, 1966, 

p. A153). (2) Gravity-driven interbasin flow of ground water 

through the carbonate-rock aquifer is widespread in the 

region today (Winograd and Thordarson, 1975) and is 

directed toward Ash Meadows and Death Valley. Such 

interbasin flow of ground water toward Death Valley in all 

likelihood also occurred during the Quaternary in response 

to the progressive lowering of ground-water discharge outlets 

there. (3) The progressive increase in aridity of the region, 

due to uplift of the Sierra Nevada and Transverse Ranges, 

would presumably have resulted in a progressive reduction 

in ground-water recharge. 

We are aware that the regional carbonate-rock aquifer 

is hydraulically compartmentalized by faulting (Winograd 

and Thordarson, 1975, p. C63-C71) and that, consequently, 
the postulated lowering of ground-water base level in Death 

Valley during the Quaternary may not have propagated 

uniformly throughout the region, specifically northeast of 

the major hydraulic barrier at Ash Meadows (Winograd and 

Thordarson, 1975, p. C78-C83). Nevertheless, we believe 

that the combination of increasing aridity and local erosion 

in the Amargosa Desert during the Quaternary should, in 

any event, have resulted in a progressive lowering of the 

water table at and northeast of Ash Meadows. 

Yet another mechanism for water-table lowering at and 

northeast of Ash Meadows that involves neither erosion nor 

climate change, but rather extensional fracturing, was outlined by Winograd and Doty (1980). They pointed out 

(p. 74-75) that the major springs at Ash Meadows oasis 

differ in altitude by as much as 35 m and are as much as 

50 m lower than the water level in Devils Hole. Thus, 

periodic initiation of discharge from new spring orifices (or 

an increase in existing discharge) in the lower portions of 

this oasis due to faulting would have resulted in new and 

lower base levels for ground-water discharge. Implicit in their 

hypothesis is the belief that the faulting would be of extensional nature, opening new (or widening old) avenues of 

discharge from the buried Paleozoic carbonate-rock aquifer 

which underlies eastern Ash Meadows and which feeds all 

the modern springs (Winograd and Thordarson, 1975). In 

support of their hypothesis, we note that most of the calcitic 

veins in Pliocene and younger rocks at Ash Meadows strike 

N. 40° ± 10° E., that is, nearly at right angles to Carr's 

(1974) estimate of the direction of active extension in region, 

namely N. 50° W. This mechanism may also have periodically lowered the water table in east -central Death Valley 

(fig. 13.1) where the difference in altitude between the 

highest (Nevares) and lowest (Texas) major springs discharging from the regional carbonate aquifer is about 170 m 

(Winograd and Thordarson, 1975, p. C95-C97). 

Our evidence for water-table decline pertains only to 

the Paleozoic carbonate-rock aquifer, the ''Lower carbonate 

aquifer" of Winograd and Thordarson (1975, table 1). As 

mentioned in the introduction, this regional aquifer serves 

as a gigantic ''tile field'' which integrates the flow of ground 

water from perhaps as many as 10 intermountain basins 

(Winograd and Thordarson, 1975). The water-table altitude 

in this regional aquifer system presently exerts a major control on the altitude of the potentiometric surface in locally 

overlying Cenozoic welded tuff and valley-fill aquifers 

(Winograd and Thordarson,1975, p. C53-C63, and pl. 1). 

Accordingly, we suggest further that the progressive watertable decline postulated for the regional carbonate aquifer 

during the Quaternary was accompanied by a decline in 

water-table altitude in the overlying welded tuff and valleyfill aquifers of the region. 

The suggested progressive lowering of the regional 

water table throughout the Quaternary does not preclude 

superimposed and relatively rapid cyclical fluctuations in 

water level in response to the glacial (that is, pluvial) and 

interglacial climates of the Pleistocene. Indeed, preliminary 

data from Devils Hole indicate that the water table in the 

carbonate aquifer may have fluctuated as much as 10 m in 

the past 30 ka (A. C. Riggs and B.J. Szabo, oral communication, December 1986). This, in turn, indicates that vein 

AM-7 (see above), which is only 11 m above the modern 

water table, would by itself be of limited utility for determination of the postulated water-table decline since the 

middle Pleistocene. Intensive studies of paleo water level 

fluctuations are underway in Devils Hole where excellent 

records of both Quaternary paleohydrology and paleoclimatology are preserved. We hope that these studies will permit 

us to distinguish between short-term (1-10 ka) and long-term 

(100-1 ,000 ka) water-table fluctuations at Ash Meadows and 

vicinity where the difference between the highest dated paleo 

water level and the highest modern water table is only 26m 

(table 13.1). 

IMPLICATIONS FOR TOXIC WASTE DISPOSAL 

The cited evidence for an apparent lowering of the 

water table at Ash Meadows and Furnace Creek Wash and 

the inference of an absolute lowering of water table in the 

south-central Great Basin during the Quaternary are pertinent to an evaluation of the utility of the thick (200-600 m) 

unsaturated zones of the region for isolating solidified 

radioactive and toxic wastes from the hydrosphere for tens 

to hundreds of millenia (Winograd, 1981). Important information which must be obtained before using such zones for 

toxic waste disposal is the magnitude of water-table rise that 

occurred during past pluvial climates of the Pleistocene; such 

information would, by extension, provide clues to the 

likelihood of buried toxic wastes being inundated by a future 

rise of the water table. Winograd and Doty (1980) and 

Czarnecki (1985), using worst-case assumptions, suggested 

possible pluvial-related water-table rises of several tens of 

meters to 130m above the modern water table in Frenchman 

Flat and beneath Yucca Mountain (fig. 13.1). As noted in 

the preceding section, we have preliminary information 

suggesting a late Wisconsin water-table rise on the order of 

10m in the carbonate aquifer at Devils Hole. Thus, it appears 

that solidified wastes emplaced in the thick (200-600 m) 

unsaturated zones of the region-at levels a few tens to a 

hundred meters or so above the water table-should not be 

inundated by a rising water table during future pluvial 

climates. (The depth of placement within the unsaturated zone 

would, or course, be chosen to preclude exhumation of the 

wastes by erosion.) Moreover, if our inference of a progressive lowering of water table during the Quaternary is 

sustained by ongoing studies of the carbonate rock and other 

aquifers, then it is likely that wastes buried in the unsaturated 

zone will in any event become increasingly displaced from 

the water table in the foreseeable geologic future. That is, 

the continuing uplift of the Sierra Nevada (Huber, 1981) and 

Transverse Ranges, and lowering of Death Valley (Hunt and 

Mabey, 1966, p. A100-Al16), relative to surrounding 

Water-Table Decline During the Quaternary 151 
regions, should result in a continued progressive decline of 

the regional water table in the next 100,000 to 1 million yr 

(and beyond?) in response to increasing aridity and to lowering of ground-water base level. 

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