This report was produced in cooperation with the Department of Energy 

through the Interagency Agreement DE-AI52-01NV13944 

Inversion of Gravity Data to Define the 

Pre-Cenozoic Surface and Regional 

Structures Possibly Influencing 

Groundwater Flow in the Rainier Mesa 

Region, Nye County, Nevada 

By Thomas G. Hildenbrand, Geoffrey A. Phelps, and 

Edward A. Mankinen 

2006 

Open File Report 2006–1299 

U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark Myers, Director 

U.S. Geological Survey, Reston, Virginia 2006 

Revised and reprinted: 2006 

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Suggested citation: 

Hildenbrand, T.G., Phelps, G.A., and Mankinen, E.A., 2006, Inversion of gravity data to 

define the pre-Cenozoic surface and regional structures possibly influencing 

groundwater flow in the Rainier Mesa Region, Nye County, Nevada: U.S. Geological 

Survey Open-File Report 2006-1299, 29 p. http://pubs.usgs.gov/of/2006/1299/

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2 
Contents 

Abstract.............................................................................................................................. 3 

Introduction ...................................................................................................................... 3 

Acknowledgments ........................................................................................................... 5 

Geologic Setting................................................................................................................ 5 

Gravity Inversion ............................................................................................................. 6 

Methodology................................................................................................................. 6 

Generalized density-depth Functions....................................................................... 9 

Regional area ............................................................................................................ 9 

Yucca Flat, Timber Mountain, and Pahute Mesa areas .................................... 10 

Rainier Mesa ........................................................................................................... 11 

Composite Thickness Map and Resulting Accuracy ............................................ 12 

Discussion ....................................................................................................................... 13 

References ....................................................................................................................... 15 

Figures ............................................................................................................................. 18 

2 
Abstract 

A three-dimensional inversion of gravity data from the Rainier Mesa area and 

surrounding regions reveals a topographically complex pre-Cenozoic basement surface. 

This model of the depth to pre-Cenozoic basement rocks is intended for use in a 3D 

hydrogeologic model being constructed for the Rainier Mesa area. Prior to this study, our 

knowledge of the depth to pre-Cenozoic basement rocks was based on a regional model, 

applicable to general studies of the greater Nevada Test Site area but inappropriate for 

higher resolution modeling of ground-water flow across the Rainier Mesa area. The new 

model incorporates several changes that lead to significant improvements over the 

previous regional view. First, the addition of constraining wells, encountering old 

volcanic rocks lying above but near pre-Cenozoic basement, prevents modeled basement 

from being too shallow. Second, an extensive literature and well data search has led to an 

increased understanding of the change of rock density with depth in the vicinity of 

Rainier Mesa. The third, and most important change, relates to the application of several 

depth-density relationships in the study area instead of a single generalized relationship, 

thereby improving the overall model fit. In general, the pre-Cenozoic basement surface 

deepens in the western part of the study area, delineating collapses within the Silent 

Canyon and Timber Mountain caldera complexes, and shallows in the east in the Eleana 

Range and Yucca Flat regions, where basement crops out. In the Rainier Mesa study 

area, basement is generally shallow (< 1 km). The new model identifies previously 

unrecognized structures within the pre-Cenozoic basement that may influence groundwater flow, such as a shallow basement ridge related to an inferred fault extending 

northward from Rainier Mesa into Kawich Valley. 

Introduction 

Underground nuclear tests at the Nevada Test Site from 1951 to 1992 introduced highly 

toxic contaminants into the ground-water system, which are presently migrating south 

from Pahute Mesa (Laczniak and others, 1996; Russell and Locke, 1996; Kersting and 

others, 1999). The Pahute Mesa and Rainier Mesa underground test areas in the 

northwestern part of the Nevada Test Site (NTS, fig. 1) were the sites of 85 and 61 tests, 

respectively (U.S. Department of Energy, 1994). 

3 
Rainier Mesa, a southeast extension of Pahute Mesa (fig. 1), is a highland of volcanic 

rocks overlying denser, pre-Cenozoic sedimentary carbonate and siliceous rocks and 

felsic and intermediate plutonic basement rocks. It lies between Yucca Flat to the east 

and the Silent Canyon and Timber Mountain caldera complexes to the west. The selected 

Rainier Mesa study area (red line, fig. 1) encompasses the Rainier Mesa and Shoshone 

Mountain Corrective Action Units, regions designated for detailed study by the 

Department of Energy. Gravity anomalies within the study area (fig. 2) suggest a 

complex basement surface, the result of transitioning from Basin and Range faulting of 

Yucca Flat to the collapse structures of Silent Canyon caldera and Timber Mountain 

caldera complexes. 

In the present study, we build on previous geophysical studies by Hildenbrand and others 

(1999), McKee and others (1999), and Phelps and others (1999) that inverted gravity data 

to generate a three-dimensional view of the basement surface, defined as the contact 

between pre-Cenozoic and younger rocks. The goal of the previous studies, as in this 

study, was to define the interface between the dense pre-Tertiary rocks (e.g., Mesozoic 

granitoids, Paleozoic and Precambrian carbonate and siliceous sedimentary rocks) and 

the less dense Tertiary and Quaternary rocks and sediments. This interface is important 

because it defines the unconformity between the basin rocks and sediments with either 

carbonate rocks belonging to regional carbonate aquifers, or silicic rocks belonging to 

regional confining units. These regional rock packages significantly influence the 

groundwater flow within and between basins of the region. Additionally, in the western 

Rainier Mesa region many of the overlying Tertiary volcanic rocks are aquifers (Laczniak 

and others, 1996), and their overall thickness provides important insights on structures 

possibly influencing ground-water flow. 

To a first order, gravity anomalies in our study area (fig. 2) reflect variations in the 

thickness of low-density sedimentary and volcanic rocks above denser pre-Tertiary rocks. 

The gravity data can therefore be used to model the depth to the pre-Tertiary rocks by 

estimating the density-depth relationship of the rocks within and around the basin. The 

previous geophysical studies mentioned above each developed a model of the depth to 

the pre-Tertiary rocks using density-depth relationships customized according to the rock 

stratigraphy in their respective study areas. We adopt these earlier results, whose study 

4 
areas partially lie within the current study area, but improve on the modeling method by 

incorporating and merging these results and by utilizing a more representative depthdensity relationship for the Rainier Mesa area than the ones previously used. 

Acknowledgments 

Rick Blakely and Victoria Langenheim, USGS colleagues, and Alan Gogbill and Gayle 

Pawloski, Members of the DOE Geologic Subcommittee of the Technical Working 

Group, as reviewers and consulting colleagues, provided valuable suggestions related to 

both the technical and applied aspects of this study, leading to a greatly improved paper. 

We are also grateful to Sig Drellack, Lance Prothro, Margaret Townsend, and Leigh 

Justet for providing critical borehole density and geologic data. The Department of 

Energy through the Interagency Agreement DE-AI52-01NV13944 funded this project. 

Geologic Setting 

The study area (fig. 1) lies within the northern part of the Nevada Test Site, where 

Precambrian crystalline rocks underlie a thick sequence (as much as 8 km) of latePrecambrian and Paleozoic sedimentary rocks. These rocks are intruded by Jurassic and 

Cretaceous granitic rocks, especially to the west near the California-Nevada border. 

Mesozoic compressional deformation produced folds and faults of all types, including 

regional thrust faults (Armstrong, 1968). In the Miocene (specifically 16-7 Ma) 

southwestern Nevada experienced pronounced volcanism over an area of >10,000 km2

, 

known as the SW Nevada volcanic field (SNVF, Christiansen and others, 1977), where 

ash-flow tuffs and lava flows reach thicknesses locally of several kilometers. The 

Miocene ash-flow sheets and lava flows of the southwest Nevada volcanic field were 

deposited on the complexly deformed basement of Mesozoic granitic rocks and Paleozoic 

and Precambrian sedimentary and metamorphic rocks (Slate and others, 1999). 

During Miocene magmatic events, a series of overlapping calderas developed in the 

SNVF that includes the Silent Canyon and Timber Mountain caldera complexes (Nobel 

and others, 1991; Sawyer and others, 1994). A relatively dense, shallow resurgent dome 

is associated with the Timber Mountain caldera complex (Kane and others, 1981). 

Voluminous sheets of welded and non-welded ash-flow tuff and numerous lava flows 

erupted from volcanic centers associated with caldera structures, and thus the calderas 

serve as major aquifers and confining units within the Rainier Mesa area (Laczniak and 

5 
others, 1996). Thus knowledge of the thickness of the caldera fill and identification of 

caldera structures are important in our understanding of the ground-water flow system. 

For example, a step or an abrupt shallowing of pre-Cenozoic basement may represent a 

structural barrier or conduit channeling groundwater and generating a steep hydrologic 

gradient. 

Gravity Inversion 

Methodology 

The isostatic residual gravity field in figure 2 is based on 4930 existing gravity 

measurements (black circles, fig. 3) and 39 new gravity stations (magenta circles; 

unpublished data, Robert Morin, U.S. Geological Survey, written communication, 2005). 

The overall distribution of gravity stations is about 4.6 stations within 1 km2 on the 

average. Although dense coverage exists within Yucca Flat, gravity stations are more 

widely spaced in remaining parts of the study area. Gravity data were reduced using the 

Geodetic Reference System of 1967 and referenced to the International Gravity 

Standardization Net 1971 gravity datum (Morelli and others, 1974; see also Mankinen 

and others (1998) for details on reducing the data to complete Bouguer anomaly values). 

The isostatic residual anomaly was calculated using a reduction density of 2,670 kg/m3, 

crustal thickness of 30 km, and a mantle-crust density contrast of 350 kg/m3 (see Simpson 

and others, 1986). All data were gridded at a spacing of 1 km using a minimum 

curvature algorithm of Webring (1982). 

To first order, the isostatic residual gravity field reflects the pronounced contrast between 

dense, pre-Cenozoic rocks (about 2,670 kg/m3) and significantly 

lower-density volcanic caldera fill (Healey, 1968; Grauch and others, 1999). The gravity 

low over the Silent Canyon caldera region is one of the most prominent lows in the 

western U.S., suggesting an unusually thick accumulation of low-density volcanic rocks. 

Higher residual isostatic gravity anomalies occur over outcrops of relatively dense 

Paleozoic rocks, such as in the Eleana Range (Slate and others, 1999). 

Using the gravity inversion method derived by Jachens and Moring (1990; modified to 

include drill hole data; Bruce Chuchel, U.S. Geological Survey, written communication, 

1997), we separated the isostatic residual gravity anomaly into two components: a basin 

gravity field and a basement gravity field. The basin gravity field reflects variations in 

6 
the thickness of low-density Cenozoic volcanic and sedimentary rocks. The basement 

gravity field reflects changes in density related to lithologic variations within the denser 

pre-Cenozoic rocks. To reduce edge effects, the area of the inversion extended well 

beyond the area shown in figure 2 (from lat. 36° 30’ to 37° 30’ and from long. 115° 52.2’ 

to 117°). 

The inversion allows the density of pre-Cenozoic basement rocks to vary horizontally, 

but the densities of basin-filling deposits are fixed using a representative density-versusdepth relationship. Wells that extended to basement rocks constrain the calculations 

(yellow circles, fig. 4; Bechtel, Nevada, written communication, 2005). Additionally, 

fourteen drill holes (green circles, fig. 4; Bechtel, Nevada, written communication, 2005) 

bottomed in older volcanic rocks (i.e., lying above but very near pre-Cenozoic basement 

such as Oak Springs Butte confining unit) and provided a means to constrain depths of 

basement by adding ~ 300 m (based on the depth relationship of the stratigraphic layer at 

the bottom of the drill hole with pre-Cenozoic basement rocks; Ted McKee, U.S. 

Geological Survey, oral communication, 2006) to the total depth of the well as an 

estimate of basement depth (discussed below). Two locations in the model were 

artificially fixed using fictitious drill hole basement intercepts. In these areas instabilities 

in the gravity inversion generated geologically unreasonable results. One fictitious well 

near the Silent Canyon caldera complex (green circle with a white X, fig. 4) was adopted 

from Hildenbrand and others (1999), who constrained basement depth in the SE region of 

the caldera where control on basement depth is absent. They selected a depth estimate (2 

km) at this location, based on projection of the geology at the bottom of several nearby 

drill holes to a pre-Cenozoic depth (Ted McKee, U.S. Geological Survey, oral 

communication, 1999). In a similar fashion, we also fixed basement depth at about lat. 

37° 10’ and long. 116° 15’ to avoid unrealistic results. The inversion estimated 

thicknesses greater than 5 km over a 36-km2 area, although nearby control wells 

encountered basement at a depth of only about 2 km. A rapid change in basement density 

may be the cause of this inversion instability. To rectify the problem, the geology in the 

nearby constraining drill holes was projected to the site of the fictitious well, leading to 

an estimated 2.5 km to basement. 

7 
In this iterative approach, a first approximation of the basement gravity field is derived 

from those gravity measurements made on exposed pre-Cenozoic rocks (blue circles, fig. 

4), augmented by basement gravity values calculated at sites where depth to basement is 

known (or estimated) and where variation of density with depth for the Cenozoic deposits 

is assumed. This field, an approximation because the gravity effects of nearby basins are 

ignored, is subtracted from the observed gravity, which provides the first approximation 

of the basin gravity field. Using an assigned density-depth relationship, the thickness of 

the basin-filling deposits is calculated. The gravitational effect of this first approximation 

of the basin-filling layer is computed at each known basement station. This effect is 

subtracted from the first approximation of the basement gravity field and the process is 

repeated until the change in basin thickness and thus basement gravity is negligible. 

Because the study area has a complex tectonic history, leading to complex geology and 

thus to a wide variation in subsurface densities, we were forced to use more than one 

depth-density relationship (fig. 5) to adequately represent the lateral change in geology 

(thus density) that is input to the modeling process. These changes in geology include 

thick low-density volcanic rocks in the Silent Canyon caldera complex (such as beneath 

Pahute Mesa), dense volcanic rocks related both to a resurgent dome in the Timber 

Mountain caldera complex and to cap rocks of welded tuffs in the Rainier Mesa area, and 

locally low-density sedimentary and volcanic rocks in the Yucca Flat area. These four 

local depth-density relationships are shown in figure 5 and discussed in the next section. 

This is an improvement over previous work because it incorporates lateral changes in the 

density-depth relationship, a factor not taken into account by Hildenbrand and others 

(1999), McKee and others (1999), and Phelps and others (1999). Furthermore, the 

density-depth relationships incorporated into this model are based on an extensive 

literature search that resulted in greater detail being incorporated into the vertical changes 

in the density-depth relationships. 

In the calculation of the isostatic residual gravity anomaly, rock mass above sea level was 

assumed to have a density of 2,670 kg/m3. This estimated reduction density is probably 

appropriate for areas where the near-surface layer consists chiefly of pre-Cenozoic rocks 

(e.g., along the Eleana Range). However, in areas with volcanic and sedimentary rocks 

in the near-surface layer (average density generally between 2,100 and 2,400 kg/m3

), the 

8 
basin analysis method described above will overestimate basin thicknesses. For example, 

the change in elevation between Pahute Mesa (where the average density of the rocks 

above sea level is roughly 2,200 kg/m3 as discussed below) and the Eleana Range (where 

Paleozoic rocks with a represented density of 2,670 kg/m3 are exposed) is about 300 m 

(fig. 6). The anticipated overestimate in relative basement depth or basin thickness, 

related to an erroneously high reduction density at Pahute Mesa, is roughly 700 m (2,300 

ft) in our final results. 

Generalized density-depth Functions 

The accuracy of the basin thickness calculations is highly dependent on the assumed 

density-depth relationship of Cenozoic rocks and the initial density assigned to basement 

rocks. Cenozoic rocks are almost entirely of volcanic origin in this area, although thin 

alluvial deposits are found locally. Several factors complicate the task of selecting 

representative densities at a particular depth: the degree of welding and alteration of the 

ash-flow tuffs; structural complexities due to, for example, landslides and shallow, dense 

volcanic domes, and volcanic ponding; and water saturation. Because density generally 

increases with depth in drill hole data due to compaction, a horizontally layered density 

model is assumed for the Cenozoic deposits, although the lenticular nature of the volcanic 

rocks is acknowledged. 

The density-depth relationship for the entire Nevada Test Site and surrounding regions 

(partially shown as the regional area in fig. 5; Hildenbrand and others, 1999) was 

modified with more local density-depth relationships based on previous studies of Pahute 

Mesa (McKee and others, 1999) and Yucca Flat (Phelps and others, 1999). For our 

study, new density-depth relationships were developed for the Timber Mountain and 

Rainier Mesa areas utilizing density and drill hole data provided by Kane and others 

(1981) and Carroll (1989), respectively. 

Regional area 

The regional density-depth relationship for Cenozoic basin fill in table 1 (fig. 5) adopts 

the same depth intervals used by Jachens and Moring (1990) in compiling a Nevada 

Cenozoic basin thickness map. However, densities within the various zones (1,900, 

2,100, 2,300 and 2,450 kg/m3) used by Hildenbrand and others (1999) are somewhat 

lower than the ones used by Jachens and Moring (1990) for exposed volcanic rock 

9 
(2,220, 2,270, 2,320 and 2,420 kg/m3

) and sedimentary rock (2,020, 2,120, 2,320 and 

2,420 kg/m3). Due to their limited study area compared to the entire state of Nevada, 

Hildenbrand and others (1999) argued that the density data in their study area leads to a 

single regional density-depth function adequately representing terranes with both 

volcanic and sedimentary rocks at the surface. Although this generalized relationship 

may be a reasonable approximation for the large region used by Hildenbrand and others 

(1999) (from lat. 36° 30’ to 37° 30’ and from long. 115° 52.2’ to 117°), it may lead to 

significant errors in more local areas. 

Yucca Flat, Timber Mountain, and Pahute Mesa areas 

More detailed basin studies in local areas like Yucca Flat show that revisions to the 

regional density-depth relationship yield better results when existing drill hole 

information is available to revise the density-depth relationship and provide constraints 

on the inversion analyses. Published borehole gravity data for nine wells within Yucca 

Flat provided information on the change of density with depth within Yucca Flat basin 

(see Phelps and others, 1999). A graph of the depth versus density of the basin sediments 

and volcanic rocks within Yucca Flat shows a linear trend that increases with depth. 

Phelps and others (1999) fitted a linear regression to the data, leading to a depth-density 

relationship with five depth intervals: 0-200, 200-400, 400-600, 600-800, and > 800 m 

(fig. 5). 

Based on density data from Kane and others (1981), another generalized depth-density 

relationship is warranted for the resurgent dome of the Timber Mountain caldera 

complex. Although only a small portion of the dense dome defined by a pronounce 

gravity high lies within the study area, a different density-depth relationship is included 

here for completeness. Also, it should be noted that all calculations were made over an 

area much larger than that shown in figure 5, and thus a more complete model using a 

Timber Mountain depth-density relationship is useful in more regional studies of the 

northern NTS. Kane and others (1981) concluded that a density of 2,400 kg/m3 is 

generally applicable in the immediate vicinity of Timber Mountain or the resurgent dome 

where densely welded tuffs and intrusive rhyolites are found. Thus a 2,400 kg/m3 density 

was used for the layers above pre-Cenozoic rocks (fig. 5) in an area defined by a gravity 

high over the Timber Mountain resurgent dome. Kane and others (1981) also mentioned 

that a lower density of 2,200 kg/m3

 represents volcanic rocks in the moat of the Timber 

10 
Mountain caldera complex and beyond, which agrees with the density proposed by 

Hildenbrand and others (1999) for shallow volcanic units (fig. 5). 

Low-density tuff layers are observed in the Silent Canyon caldera complex such as 

beneath Pahute Mesa. Borehole gravimeter studies (Ferguson and others, 1994; Warren 

and other, 1999) reveal densities of 2,100 kg/m3

 extending in many areas to depths of 

more than 1,000 m. In such situations, calculated basement depths using the regional 

depth-density relationship will be overestimated. Thus in generating a Cenozoic 

thickness map, McKee and others (1999) used a different density-depth relationship 

based on a careful analysis of the density data from Warren and others (1999). They 

showed that below 600 m the basin fill within the Silent Canyon caldera complex is less 

dense than the rocks outside the caldera, resulting in the density model shown in table 1 

(fig. 5) for Pahute Mesa. 

Rainier Mesa 

Carroll (1989) provided an excellent study and review of known density measurements in 

the Rainier Mesa area, which leads to a wealth of data justifying variation of the regional 

density-depth relationship. The primary data sets are density logs from 10 drill holes and 

core density measurements from 10 other wells. For this area, the volcanic rocks 

overlying pre-Cenozoic basement range from densely-welded tuffs to friable and vitric 

ash-flow tuffs. Carroll (1989) noted a density boundary (~ 200 kg/m3 density change) at 

the top of the zeolitization zone, separating unsaturated tuffs from underlying denser 

saturated zeolitized ash-fall and ash-flow tuffs. An overall recurring depth-density 

relationship of the volcanic section is a thin dense cap rock of either welded Rainier Mesa 

or Belted Range tuff (Grouse Canyon Member), low-density unsaturated nonwelded 

volcanic rocks, moderately-dense zeolitized bedded tuffs, denser welded tuffs, and preCenozoic basement. In Carroll (1989), generalized density boundaries emerge when 

carefully analyzing the data in his table 2 listing representative densities for several 

lithologic units, in his figures 9-11 showing lithologic and density logs, and in his figures 

8 and 18-22 showing histograms of density logs for individual lithologic units. 

Representative densities based on his data are 2,200 kg/m3 for the cap rock in a 75 m 

surface interval, 1,700 kg/m3 for the underlying nonwelded volcanic rocks that are 

roughly 125 m thick, and 1,900 kg/m3 for the zeolitized tuffs extending from about 200 to 

600 m. Beneath a depth of about 600 m, density for the older welded tuffs significantly 

11 
increases to roughly 2,300 kg/m3

. Because of the limited information on density for 

deeper units, we assumed compaction led to an increased density of 2,450 kg/m3 at 1,200 

m, which are values that match most of the density-depth relationships (table 1 in fig. 5) 

based on the work of others. The boundary shown in figure 5 for the Rainier Mesa 

density-depth relationship was first drawn around an area encompassing outcrops of the 

Rainier Mesa and Belted Range tuffs. This boundary was necessarily modified so that 

the results of the modeled thickness of Cenozoic deposits in the Rainier Mesa area 

matched the results of neighboring areas (discussed below). 

Composite Thickness Map and Resulting Accuracy 

Using the gravity inversion method derived by Jachens and Moring (1990; modified to 

include drill hole data), we separated the isostatic residual gravity anomaly into a basin 

gravity field and a basement gravity field using each of the depth-density relationships in 

table 1 (fig. 5). The basin gravity fields for the five depth-density relationships reflect 

variations in the thickness of low-density Cenozoic volcanic and sedimentary rocks and 

were used to generate a basin thickness map shown in figures 7 and 8. The gravity 

inversion model is thus the result of several modeling stages in which the different 

density-depth relationships were used to optimize the thickness model to local changes in 

general geology. In the inversion process, the density of basement was allowed to vary 

horizontally but the density of basin-filling deposits is fixed using the five representative 

density versus depth relationships (table 1 in fig. 5). This resulted in five Cenozoic 

thickness data sets based on the five density-depth relationships. To generate a 

composite thickness map shown in figures 7 and 8, the results within each of the areas 

shown in figure 5 were merged to adjacent thickness results. For the Yucca Flat and 

Rainier Mesa areas, their boundaries were determined by selecting a line where their 

thickness agreed with adjoining modeling results. For the Pahute Mesa and Timber 

Mountain areas, a 2-km-wide buffer zone was generated where thicknesses were masked 

and then filled in using a minimum curvature algorithm so that a smooth transition in 

thickness occurred across adjoining density-depth models. Thus the final composite 

thickness model obeys the density-depth relationships for the five areas, and also the 

thickness results across boundaries between the various area models are smooth. 

12 
The accuracy of these results varies due primarily to the necessary step of 

oversimplifying the density-depth relationships. In regions where good basement control 

exists (either from basement outcrops or well data, fig. 4) the results are probably within 

roughly 300 m as discussed below (e.g., the Eleana Range and Yucca Flat areas). The 

expected variability in local density-depth functions and the lack of basement control in 

some areas (e.g., the Silent Canyon caldera complex and Timber Mountain caldera 

complex) strongly argue that the relative change in basin thickness rather than its 

magnitude should be stressed in studying the basement surface. 

The data from the 14 constraining drill holes bottoming in old volcanic rocks (green 

circles, figs. 4 and 8), however, offer an opportunity to study the accuracy of the 

thickness results by carrying out the inversion analysis with and without the use of these 

data. Comparison of the results at these drill holes reveals that at 5 locations (labeled in 

fig. 4), pre-Cenozoic basement was determined to be shallower than the Micocene 

volcanic rocks at the bottom of the drill holes. The gravity inversion model is 

underpredicting the basement depth at these locations. The differences between depth to 

the bottom of these 5 holes and calculated depth to basement ranged from 18 to 127 m 

and averaged 50 m. The actual basement probably lies roughly between 200 and 400 m 

below the bottom of these drill holes (Ted McKee, U.S. Geological Survey, oral 

communication, 2006). The gravity inversion model therefore underpredicts basement 

depth at these 5 locations by between about 250 and 450 m. The calculated depths to preCenozoic basement for the remaining 9 constraining drill holes were between 34 m and 

623 m deeper than the depths to the bottom of the drill holes, averaging 232 m. Since 

these wells encountered volcanic units of similar age, we can conclude that the gravity 

inversion model is more accurate at these 9 drill-hole locations. In general we conclude 

from this analysis that in areas of good basement depth control from outcrops and drills 

holes that encountered pre-Cenozoic rocks (e.g., the general area of the 14 constraining 

drill holes; fig. 4), the inversion accuracy is approximately 300 m. 

Discussion 

An improved basement depth map of the Rainier Mesa area was produced and is intended 

for use in 3D hydrogeologic modeling. In areas of good well control the accuracy of the 

depth results is roughly 300 m, based on the analysis discussed in the previous section. 

13 
On the other hand, accuracies deteriorate away from outcrop and well control, and in 

areas where basement deepens, the accuracy of results become more suspect. With these 

caveats and the previously described accuracy limitations in mind, we discuss the 

complex nature of the basement surface, clearly evident in figures 7 and 8. In general, 

the pre-Cenozoic basement surface deepens over 4 km to the west in the collapse calderas 

and shallows to the east in the Eleana Range and toward Yucca Flat, where basement 

crops out. In the Rainier Mesa region, basement is generally shallow (< 1 km). This 

corresponds, to some extent, with the intrusion of the Gold Meadows stock, which forms 

a broad dome roughly 5 km in diameter intruding Rainier Mesa (fig. 8). 

An interesting aspect of the northern portion of the study area is the presence of a buried 

ridge that extends from the Gold Meadows stock northward into Kawich Valley. Its 

linearity and proximity to the Gold Meadows stock may suggest a dike origin during 

Tertiary volcanism. On the other hand, this previously unrecognized structure roughly 

parallels the Belted Range thrust to the east (fig. 8) and may be associated with older 

faulting. It extends northward along the eastern flank of Kawich Valley (fig. 6). Just east 

of this ridge in Kawich Valley, the gravity inversion indicates the presence of a deep 

basin (> 5 km). The subsurface ridge-and-basin structure could be indicative of a zone 

of previous thrust faulting during Mesozoic time associated with the Belted Range thrust. 

On trend with the interpreted ridge, and about 6 km north of the study area (highlighted 

in fig. 1), Cambrian rocks crop out and support the northward extension of the basement 

ridge and a relationship to faulting. Conceivably, erosion of Paleozoic and Mesozoic 

rocks and stresses related to flexure resulted in a basin forming in front of the thrust fault, 

which is now covered with a thick volcanic pile. However, Mesozoic thrust faulting 

generally juxtaposed pre-Cenozoic rocks of similar density, and a near-surface, local 

gravity anomaly associated with these thrusts is thus not anticipated but is observed along 

the interpreted ridge. For example, the Belted Range thrust fault (fig. 2) produces a large 

regional gradient (fig. 2) but not a local effect like the anomaly associated with the ridge 

(fig. 2). An alternative fault-related hypothesis for the buried ridge and associated basin 

is that these features formed as part of the Basin and Range extension, although the ridge 

appears to extend southward past the limit of the southern end of Kawich Valley. 

14 
Whatever the origin of the ridge, it may have implications for ground-water flow. 

Southward inflow to the NTS from Kawich Valley and Gold Flat is the largest source of 

ground water into the Pahute Mesa and Rainier test areas (Blankennagel and Weir, 1973). 

The interpreted ridge striking SSW from Kawich Valley to Rainier Mesa (and possibly 

beyond) may represent a major barrier and/or conduit (e.g., Rowley, 1998; Mankinen and 

others, 2003) resulting in a southward channeling of groundwater into the NTS. 

Prominent depressions (> 4 km) in basement occur between the calderas and Rainier 

Mesa and are highlighted with question marks in Figures 7 and 8. We have no 

explanation for their origins other than they may be related to caldera formation and thus 

could delineate areas of significant accumulations of low-density volcanic rocks. 

References 

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American Bulletin, v. 79, p. 429-459. 

Blankennagel, R.K., and Weir, J.E., Jr., 1973, Geohydrology of the eastern part of Pahute 

Mesa, Nevada Test Site, Nye County, Nevada: U.S. Geological Survey 

Professional Paper 712-B, 35 p. 

 Christiansen, R.L., Lipman, P.W., Carr, W.J., Byers, F.M., Jr., Orkild, P.P., and Sargent, 

K.A., 1977, The Timber Mountain-Oasis Valley caldera complex of southern 

Nevada: Geological Society of America Bulletin, v. 88, p. 943-959. 

Carroll, R.D., 1989, Density logging and density of rocks in the Rainier Mesa area, 

Nevada Test Site: U.S. Geological Survey Open-File Report 89-329, 72 pp. 

Ferguson, J.F., Cogbill, A.H., and Warren, R.G., 1994, A geophysical-geological transect 

of the Silent Canyon caldera complex, Pahute Mesa, Nevada: Journal of 

Geophysical Research, v. 99, p. 4323-4339. 

Grauch, V.J.S., Sawyer, D.A., Fridrich, C.J., and Hudson, M.R., 1999, Geophysical 

framework of the southwestern Nevada volcanic field and hydrogeologic 

implications: U.S. Geological Survey Professional Paper 1608, 39p. 

Healey, D.L., 1968, Application of gravity data to geologic problems at Nevada Test Site, 

in Eckel, E.B., ed., Nevada Test Site: Geological Society of America Memoir 

110, p. 147-156. 

Hildenbrand, T. G., Langenheim, V. E., Mankinen, E. A., and McKee, E. H., 1999, 

Inversion of gravity data to define the pre-Tertiary surface of the Pahute MesaOasis Valley region, Nye County, Nevada, Open-File Report, OF 99-0049, 29p. 

Jachens, R.C., and Moring, B.C., 1990, Maps of the thickness of Cenozoic deposits and 

the isostatic residual gravity over basement for Nevada: U.S. Geological Survey 

Open-File Report 90-404, 15 p. 

Kane, M.F., Webring, M.W., and Bhattacharyya, B.K., 1981, A preliminary analysis of 

gravity and aeromagnetic surveys of the Timber Mountain area, southern Nevada: 

U.S. Geological Survey, Open-File Report 81-189, 40p. 

15 
Kersting, A.B., Efurd, D.W., Finnegan, D.L., Rokop, D.J., Smith, D.K., and Thompson, 

J.L., 1999, Migration of plutonium in ground water at the Nevada Test Site: 

Nature, v. 397, p. 56-59. 

Laczniak, R.J., Cole, J.C., Sawyer, D.A., and Trudeau, D.A., 1996, Summary of 

hydrogeologic controls on ground-water flow at the Nevada Test Site, Nye 

County, Nevada: U.S. Geological Survey Water-Resources Investigations Report 

96-4109, 59p. 

Mankinen, E.A., Hildenbrand, T.G., Fridrich, C.J., McKee, E.H., and Schenkel, C.J., 

2003, Geophysical setting of the Pahute Mesa–Oasis Valley region, southern 

Nevada: Nevada Bureau Mines & Geology Report 50, CD-ROM, 45 p. 

Mankinen, E.A., Hildenbrand, T.G., Roberts, C.W., and Davidson, J.G., 1998, Principal 

facts for new gravity stations in the Pahute Mesa and Oasis Valley areas, Nye 

County, Nevada: U.S. Geological Survey Open-File Report 98-498, 14p. 

McKee, Edwin H., Hildenbrand, Thomas G., Anderson, Megan L., Rowley, Peter D., 

Sawyer, David A., 1999, The Silent Canyon caldera complex; a three-dimensional 

model based on drill-hole stratigraphy and gravity inversion, Open-File Report, 

OF 99-0555, 79p. 

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Bela, Uotila, U.A., and Whalen, G.T., 1974, The International Gravity 

Standardization Gravity Net 1971 (I.G.S.N. 71): Paris Bureau Centrale de 

l’Association Internationale de Geodesie Special Publication 4, 193p. 

Nobel, D.C., Weiss, S.I., and McKee, E.H., 1991, Magmatic and hydrothermal activity, 

caldera geology, and regional extension in the western part of the southwestern 

Nevada volcanics field: in, Raines, G.L., Lisle, R.E., Schafer, R.W., and 

Wilkinson, W.H., eds., Geology and Ore Deposits of the Great Basin, Geological 

Society of Nevada, Reno, Nevada, P. 913-934. 

Phelps, G.A., Langenheim, V.E., and Jachens, R.C., 1999, Thickness of Cenozoic 

deposits of the Yucca Flat, Nevada Test Site, Nevada, inferred from gravity data: 

U.S. Geological Survey Open-File Report 99-310, 29p. 

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western U.S. -- Their tectonic and economic implications, in Faulds, G.E., and 

Stewart, J.H., eds., Accommodation Zones and Transfer Zones - The Regional 

Segmentation of the Basin and Range Province: Geological Society of America 

Special Paper 323, p. 195-228. 

Russell, G.M., and Locke, G.L., 1996, Summary of data concerning radiological 

contamination at well PM-2, Nevada Test Site, Nye County, Nevada: U.S. 

Geological Survey Open-File Report 96-599, 84 p. 

Sawyer, D.A., Fleck, R.J., Lanphere, M.A., Warren, R.G., Broxton, D.E., and Hudson, 

M.R., 1994, Episodic caldera volcanism in the Miocene southwestern Nevada 

volcanic field: Revised stratigraphic framework, 40Ar/39Ar geochronology, and 

implications for magmatism and extension: Geological Society of America 

Bulletin, v. 106, p. 1304-1318. 

Slate, Janet L., Berry, Margaret E., Rowley, Peter D., Fridrich, Christopher J., Morgan, 

Karen S., Workman, Jeremiah B., Young, Owen D., Dixon, Gary L., Williams, 

Van S., McKee, Edwin H., Ponce, David A., Hildenbrand, Thomas G., Swadley, 

W. C., Lundstrom, Scott C., Ekren, E. Bartlett, Warren, Richard G., Cole, James 

C., Fleck, Robert J., Lanphere, Marvin A., Sawyer, David A., Grunwald, Daniel 

J., Laczniak, Randell J., Menges, Christopher M., Yount, James C., Jayko, Angela 

16 
S., 1999, Digital geologic map of the Nevada Test Site and vicinity, Nye, Lincoln, 

and Clark counties, Nevada, and Inyo County, California, revision 4: Open-File 

Report, OF 99-0554-A, p. (1 sheet). 

Simpson, R.W., Jachens, R.C., Blakely, R.J., and Saltus, R.W., 1986, A new isostatic 

residual gravity of the conterminous United States, with a discussion of the 

significance of the isostatic residual anomalies: Journal of Geophysical Research, 

v. 91, p. 8348-8372. 

U.S. Department of Energy, 1994, United States nuclear tests July 1945 through 

September 1992: U.S. Department of Energy, Nevada Operation Office, 

DOE/NV-209 (rev. 14), 105 p. 

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petrographic/geochemical database and structural framework for the 

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Geological Survey Open-File Report 81-1224, 43 p. 

17 
Figures 

Figure 1. Topographic map of the northern Nevada Test Site (NTS). Black lines 

represent caldera structural boundaries as drawn by Slate and others (1999). Red line 

shows study area used in the remaining figures. Green areas denote pre-Cenozoic 

outcrops. Small black circle in the southeastern Kawich Valley highlights an outcrop 

discussed in the text. 

Figure 2. Isostatic residual gravity anomaly field of the Rainier Mesa region. 

Anomalies express to first order the average density of the middle and upper crust. PreCenozoic outcrops highlighted with heavy white lines. Light dashed white lines show 

roads. 

Figure 3. Location of gravity stations. Black circles denote existing stations 

and magenta circles represent stations newly acquired for this study. Red lines show 

boundaries of pre-Cenozoic outcrop. Purple thin lines show roads. 

Figure 4. Drill-hole and outcrop constraints used in the inversion process. Yellow circles 

denote wells that encountered pre-Cenozoic basement. Green circles locate constraining 

drill holes that bottomed in older volcanic rocks above basement but provide estimates of 

basement depth using expected stratigraphy and unit thicknesses. Selected drill holes 

numbered (1-5) are discussed in text. Two green circles with a white X are locations 

where depth to basement was pinned using expected unit thicknesses from known 

geology. Red lines show boundaries of pre-Cenozoic outcrop. Blue circles locate gravity 

stations on basement outcrop. 

Figure 5. Assumed density-depth relationships in separate areas within the Rainier Mesa 

study area. Labeled blue areas define local density-depth relationships that deviate from 

the regional density model (white areas). Depth-density relationships for each area 

shown in the table 1. 

Figure 6. Topography of the Rainier Mesa region. Roads shown in blue. 

Figure 7. Three-dimensional shaded view of the pre-Cenozoic surface beneath the 

Rainier Mesa region. Question marks highlight deep basins outside caldera boundaries of 

unknown origin. Interpreted ridge in northern part of study area is shown. 

18 
Figure 8. Thickness of Cenozoic deposits based on the inversion of gravity data. Yellow 

lines highlight the mapped Belted Range thrust fault and an interpreted ridge extending 

NNE from Rainier Mesa into Kawich Valley (dashed where inferred). Question marks 

locate a few deep basins outside caldera boundaries of unknown origin. White and green 

circles identify pre-Cenozoic basement wells and constraining wells, respectively. White 

lines show roads. 

19 
116°30 ́ 116°

37°

0 10 20 KM

 950

1200

1450

1700

1950

2200

meters

YUCCA

FLAT

RAINIER

MESA

KAWICH

VALLEY

GOLD

FLAT

STUDY AREA

Nevada

Test

Site

Shoshone

Mountain

Frenchmen

Flat

Fig. 1 Topography of the 

Northern NTS

Highlighted outcrop

Eleana Range

118° 116°

36°

38°

0 100 200 KM

NTS

FIG. 1 

AREA

NEVADA

CALIFORNIA

SILENT CANYON

CALDERA COMPLEX

TIMBER MOUNTAIN

CALDERA COMPLEX

Pre-Cenozoic

Outcrop

Pahute Mesa

Area 20

Area 18

Area 19
Figure 2. Isostatic Gravity Anomaly

-56

-48

-40

-32

-24

-16

 -8

 0

mGal

116°20' 116°10'

37°

37°10'

Nevada Test Site

Study Area

Nevada

California

Yucca Flat

Kawich

 Valley

Timber Mtn. 

 Caldera

Shoshone Mountain

0 5 10KM

Gold Meadows

Stock

Eleana Range

Silent 

Canyon

 Caldera

Complex

Belted

Thrust Fault

Range

RAINIER

MESA
116°20 ́ 116°10 ́

37°

37°10 ́

0 5 10 KM

Fig. 3 Gravity Stations

Timber Mtn. 

 Caldera

Complex

Silent 

Canyon

 Caldera

Complex

Kawich

 Valley

RAINIER

MESA

Shoshone Mountain

Eleana Range
0 5 10 KM

116°20 ́ 116°10 ́

37°

37°10 ́

Silent

Canyon

Caldera

Complex

Yucca Flat

Kawich

 Valley

Timber Mtn. 

 Caldera

Complex

Gold Meadows

Stock

RAINIER

MESA

Fig. 4 Model Constraints

Shoshone Mountain

Eleana Range

x

x

1

2 3

4

5
116°20 ́ 116°10 ́

37°

37°10 ́

Yucca

Flat

Area

Rainier

Mesa

Area

0 5 10KM

Scale 1:125,000

Table 1. Generalized density-depth functions used to 

determine the thickness of Cenozoic deposits.

Depth Range Density (kg/m ) for 5 models

 (meters)Regional PahuteYucca RainierTimber 

 0 - 75 1,9001,900 1,820 2,2002,400

 75 - 200  1,9001,900 1,820 1,7002,400

 200 - 400  2,100 2,000 1,940 1,9002,400

 400 - 600  2,100 2,000 2,060 1,9002,400

 600 - 800  2,300 2,100 2,180 2,3002,400

 800 - 1,200  2,300 2,100 2,300 2,3002,400

 >1,200  2,450 2,450 2,300 2,4502,400 

3

Timber

Mtn.

Area

Regional

Area

Fig. 5 Density-depth relationships

Pahute

 Mesa
1350

1500

1650

1800

1950

2100

Meters

0 5 10 KM

116°20 ́ 116°10 ́

37°

37°10 ́

Silent 

Canyon

 Caldera

Yucca Flat

Timber Mtn.

Caldera

Complex

Gold Meadows

Stock

RAINIER

MESA

Eleana Range

Shoshone Mountain

Fig. 6 Topography

Kawich

 Valley
Basin

Thickness

0 km

10

20

30

40

10

20

30

40

E

S Silent 

Canyon

CalderaInterpreted 

Timber Mtn.

Caldera

Complex Eleana Range

Shoshone Mountain

Yucca Flat

Figure 7. Three-dimensional view of the 

 estimated basement thickness

Gold

Meadows

Ridge

Stock ?

? ?

1.01

5.01

3.01

0.01 km
4.21

3.91

3.61

3.31

3.01

2.71

2.41

2.11

1.81

1.51

1.21

0.91

0.61

0.31

0.01

KM

116°20 ́ 116°10 ́

37°

37°10 ́

0.0

Silent 

Canyon

 Caldera

Yucca Flat

Timber Mtn. 

 Caldera

Complex

Eleana Range

Shoshone Mountain Interpreted 

?

?

?

Ridge

0 5 10 KM

Fig. 8 Basin Thickness

Gold Meadows

Stock

Belted

Thrust Fault

Kawich

 Valley

X

X

Range