Map showing depth to pre-Cenozoic basement in the Death Valley 

ground-water model area, Nevada and California 

By R.J. Blakely and D.A. Ponce 


Prepared in cooperation with the 


Nevada Operations Office 


National Nuclear Security Administration 


U.S. Department of Energy 


(Interagency Agreement DE-A108-96NV11967)


Pamphlet to accompany 


Miscellaneous Field Studies MF-2381-E 


2001 
U.S. Department of the Interior 

U.S. Geological Survey 

1

INTRODUCTION 

This map shows the depth to pre-Cenozoic basement in the Death Valley ground-water model 

area. It was prepared utilizing gravity (Ponce and others, 2001), geologic (Jennings and others, 

1977; Stewart and Carlson, 1978), and drill-hole information. Geophysical investigations of the 

Death Valley ground-water model area are part of an interagency effort by the U.S. Geological 

Survey (USGS) and the Department of Energy (Interagency Agreement DE-AI08-96NV11967) 

to help characterize the geology and hydrology of southwestern Nevada and parts of California. 

The Death Valley ground-water model is located between lat 35°00’ and 38°15’N., and long 

115° and 118°W. 

DEPTH TO BASEMENT PROCESS 

An iterative gravity inversion method (Jachens and Moring, 1990) was used to determine the 

depth of Cenozoic basin deposits in southwestern Nevada and southeastern California, an area 

that encompasses the Death Valley ground-water model (Blakely and others, 2000). Gravity data 

used in this process were reduced using standard techniques that include terrain and isostatic 

gravity corrections. Isostatic gravity anomalies (Simpson and others, 1986) were used during the 

inversion process because they enhance or reflect shallow- to mid-crustal sources within the 

Earth. 

The method separates the gravity field into two components: the field caused by pre-Cenozoic 

basement and the field caused by overlying younger basin deposits. An initial basement gravity 

field is determined by using just those stations located on pre-Cenozoic basement outcrops. The 

initial basement gravity field is only approximate because stations located on basement are 

influenced by the gravity effect of low-density deposits in nearby basins, especially for those 

stations near the edges of the basins. The difference between the isostatic gravity and basement 

gravity fields provides the first estimate of the basin gravity field, which is inverted to provide 

the first estimate of the basin shape. The gravitational effect of the basins are subtracted from 

each station located on basement, and a new and improved basement gravity field is determined. 

This process is repeated until successive iterations converge. Inversion of the final basin gravity 

field, constrained with a density-depth function (table 1), geology, and drill-hole information 

yields an estimate of the depth to pre-Cenozoic basement. The density of basement rocks is 

allowed to vary horizontally, whereas the density of basin-filling deposits increases with depth 

according to the density-depth relationships shown in table 1. The density-depth function is 

based on density information from rock samples, geophysical well logs, and borehole gravity 

data. For sedimentary deposits, we use a density-depth profile by Jachens and Moring (1990), 

and for volcanic deposits we use a density-depth profile by Hildenbrand and others (1999), 

derived from detailed geophysical studies of Pahute Mesa and vicinity. The digital data sets used 

in this study were gridded at an interval of 500 m using a computer program (Webring, 1981) 

based on a minimum curvature algorithm by Briggs (1974). The resulting grid was then 

interpolated to a 200-m grid to minimize pixel size, and then it was color contoured. 

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A number of limitations are inherent in the gravity data themselves, as well as in the inversion 

process. Some of the uncertainties are related to the gravity data coverage, especially for stations 

on basement outcrops; the density-depth function; accuracy or scale of the geologic mapping; 

simplifying assumptions regarding concealed geology; and the distribution of basement outcrops. 

Although this map is shown at a scale of 1:250,000, the process itself was regional in scope and 

caution should be exercised when using these results at a scale greater than about 1:500,000. A 

more detailed discussion of the limitations and accuracy of the method are provided by Jachens 

and Moring (1990). 

Table 1.—Density-depth function for Cenozoic sedimentary and volcanic deposits. 

Depth Density for Density for 

Range sedimentary deposits volcanic deposits 

(m) kg/m3

 kg/m3 

0-200 2,020 1,900 

200-600 2,120 2,100 

600-1,200 2,320 2,300 

>1,200 2,420 2,420 

DISCUSSION 

In general, gravity anomalies within the Death Valley ground-water model area reflect carbonate 

rocks, calderas, deep sedimentary basins, and linear geologic features such as faults. Many of 

these features play an important role as aquifers or confining units in the region and their 

distribution is important to the understanding of the hydrogeologic framework of the area. PreCenozoic carbonate and crystalline rocks underlie most of the region and their subsurface 

distribution is especially important in evaluating the hydrogeology of the area. Tertiary volcanic 

rocks also play an important role in the hydrogeologic framework of the area and in the 

formation of large collapse calderas. Thick accumulations of these volcanic rocks are present in 

the central and northern part of the Death Valley ground-water model. Quaternary alluvial 

deposits exist throughout the study area and are composed of non-marine sedimentary and 

volcanic rocks, generally Oligocene and younger in age. These alluvial deposits play an 

important role in the saturated-zone hydrology of deep alluvial basins within the southwestern 

Nevada volcanic field (Grauch and others, 1999) and probably in other deep basins as well. 

Because of the density contrast between these lithologic types, gravity data can be inverted to 

determine the depth to basement throughout the Death Valley ground-water model. 

In general, the depth-to-basement within basins in the study is similar to that of other basins in 

the state of Nevada (Jachens and Moring, 1990); most basins are less than about 2 km deep. 

Within the Death Valley ground-water model, several basins are greater than about 5 km in 

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depth, including Death Valley, Pahrump Valley, Kawich Valley, and Tickaboo Valley. One of 

the deepest basins in the state of Nevada is related to the Silent Canyon caldera complex over 

Pahute Mesa, a volcanic center that is filled with low-density volcanic rocks that may reach 

depths as great as 6 km or more (McKee and others, 1999). 

A number of other geophysical studies were undertaken to aid in characterizing the geologic and 

hydrogeologic setting of the region. A summary of geophysical investigations at Yucca Mountain 

that includes gravity, magnetics, electrical methods, seismic methods, heat-flow, and stress data 

were provided by Oliver and others (1995). Recent studies characterized the geophysical 

framework of the southwestern Nevada volcanic field (Grauch and others, 1999; Hildenbrand 

and others, 1999; Mankinen and others, 1999), and they identify a number of subsurface features 

that might control or influence ground-water flow. These features include resurgent calderas and 

prominent geophysical lineaments that probably reflect faults. Phelps and others (1999) describe 

the depth to basement in Yucca Flat, Nevada Test Site, Nevada. And finally, Blakely and others 

(1998, 1999, 2000) describe geophysical investigations in Death Valley, Amargosa Valley, and 

the Death Valley regional ground-water model. 

The diverse physical properties of lithologic types that underlie this region are well suited to 

geophysical investigations. The contrast in density between pre-Cenozoic basement and 

overlying unconsolidated alluvium, for example, produces a distinctive pattern of gravity 

anomalies that can be used to estimate the depth of pre-Cenozoic basement in three dimensions. 

In most parts of the study area, this basement surface corresponds to the top of the carbonate 

aquifer, an important element of the hydrologic framework. 

4 
REFERENCES 

Blakely, R.J., Jachens, R.C., Calzia, J.P., and Langenheim, V.E., 1999, Cenozoic basins of the 

Death Valley extended terrane as reflected in regional-scale gravity anomalies, in Wright, 

L.A., and Troxel, B.W., eds., Cenozoic basins of the Death Valley region: Boulder, 

Colorado, Geological Society of America Special Paper 333, p. 1-16. 

Blakely, R.J., Langenheim, V.E., and Ponce, D.A., 2000, Summary of geophysical investigations 

of the Death Valley regional water-flow modeling project, Nevada and California: U.S. 

Geological Survey Open-File Report 00-189, 36 p. 

Blakely, R.J., Morin, R.L., McKee, E.H., Schmidt, K.M., Langenheim, V.E., and Dixon, G.L., 

1998, Three-dimensional model of Paleozoic basement beneath Amargosa Desert and 

Pahrump Valley, California and Nevada: Implications for tectonic evolution and water 

resources: U.S. Geological Survey Open-File Report 98-496, 29 p. 

Briggs, I.C., 1974, Machine contouring using minimum curvature: Geophysics, v. 39, p. 39-48. 

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, 39 p. 

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

gravity data to define the pre-Tertiary surface and regional structures possibly influencing 

ground-water flow in the Pahute Mesa—Oasis Valley region, Nye County, Nevada: U.S. 

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

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1999, Gravity and magnetic study of the Pahute Mesa and Oasis Valley region, Nye 

County, Nevada: U.S. Geological Survey Open-File Report 99-303, 57 p. 

McKee, E.H., Hildenbrand, T.G., Anderson, M.L., Rowley, P.D., and Sawyer, D.A., 1999, The 

Silent Canyon caldera complex—A three-dimensional model based on drill-hole 

stratigraphy and gravity inversion: U.S. Geological Survey Open-File Report 99-555, 38 

p. 

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Oliver, H.W., Ponce, D.A., and Hunter, W.C., eds., 1995, Major results of geophysical 

investigations at Yucca Mountain and vicinity, southern Nevada: U.S. Geological Survey 

Open-File Report 95-74, 35 p. 

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

Yucca Flat inferred from gravity data, Nevada Test Site, Nevada: U.S. Geological Survey 

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Simpson, R.W., Jachens, R.C., and Blakely, R.J., 1986, A new isostatic residual gravity map of 

the conterminous United States with a discussion on the significance of isostatic residual 

gravity anomalies: Journal of Geophysical Research, v. 91, no. B8, p. 8,348-8,372. 

Stewart, J.H., and Carlson, J.E., 1978, Geologic map of Nevada: Nevada Bureau of Mines and 

Geology, scale 1:500,000. 

Webring, M.W., 1981, MINC—A gridding program based on minimum curvature: U.S. 

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

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