U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY 

GROUND-MAGNETIC STUDIES OF THE AMARGOSA DESERT 

REGION, CALIFORNIA AND NEVADA 

Richard J. Blakely, John W. Hillhouse, and Robert L. Morin 

U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 

Open-File Report 2005-1132 

2005 

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or 

with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only 

and does not imply endorsement by the U.S. Government. 
GROUND-MAGNETIC STUDIES OF THE AMARGOSA DESERT 

REGION, CALIFORNIA AND NEVADA 

Richard J. Blakely, John W. Hillhouse, and Robert L. Morin 

ABSTRACT 

High-resolution aeromagnetic surveys of the Amargosa Desert region, California and Nevada, 

exhibit a diverse array of magnetic anomalies reflecting a wide range of mid- and upper-crustal 

lithologies. In most cases, these anomalies can be interpreted in terms of exposed rocks and 

sedimentary deposits. More difficult to explain are linear magnetic anomalies situated over 

lithologies that typically have very low magnetizations. Aeromagnetic anomalies are observed, 

for example, over thick sections of Quaternary alluvial deposits and spring deposits associated 

with past or modern ground-water discharge in Ash Meadows, Pahrump Valley, and Furnace 

Creek Wash. Such deposits are typically considered nonmagnetic. To help determine the source 

of these aeromagnetic anomalies, we conducted ground-magnetic studies at five areas: near 

Death Valley Junction, at Point of Rocks Spring, at Devils Hole, at Fairbanks Spring, and near 

Travertine Springs. Depth-to-source calculations show that the sources of these anomalies lie 

within the Tertiary and Quaternary sedimentary section. We conclude that they are caused by 

discrete volcanic units lying above the pre-Tertiary basement. At Death Valley Junction and 

Travertine Springs, these concealed volcanic units are probably part of the Miocene Death 

Valley volcanic field exposed in the nearby Greenwater Range and Black Mountains. The linear 

nature of the aeromagnetic anomalies suggests that these concealed volcanic rocks are bounded 

and offset by near-surface faults. 

Cover photo: Greenwater Range, Death Valley National Park. View is to west from Ash Meadows. Note

gently dipping lava flows and interbedded tuffaceous beds, part of the Death Valley volcanic field. 

2 
INTRODUCTION 

High-resolution aeromagnetic surveys of the Amargosa Desert region (Figs. 1 and 2) exhibit a 

diverse array of magnetic anomalies reflecting the wide range of mid- and upper-crustal 

lithologies in this area. In most cases, the anomalies are easily understood in terms of exposed 

rocks and deposits. Linear, high-amplitude anomalies in the northern part of Figure 2, for 

example, are caused largely by exposed, sub-horizontal, strongly magnetic layers of Miocene 

ash-flow tuff and interbedded air-fall tuffs, offset by north-striking normal faults (Workman and 

others, 2002; Ponce and others, 1995). 

More difficult to explain are linear, low-amplitude magnetic anomalies situated over lithologies 

that typically have very low magnetizations. In the Funeral Mountains, for example, a series of 

north- to north-northeast-striking anomalies cross exposures of pre-Tertiary quartzite and 

carbonate rocks (Fig. 2, longitude 116°45’W, latitude 36°39’N). Early interpretations of these 

aeromagnetic anomalies (Blakely and others, 2000a, 2000b) suggested that they are caused by 

secondary mineralization along near-surface faults, and, because the anomalies are on strike with 

important springs near Furnace Creek, it was further speculated by the same authors that these 

faults may influence ground-water flow into Death Valley National Park. More recent groundmagnetic studies (Hillhouse and Morin, 2003) have shown, however, that these low-amplitude 

bedrock anomalies are actually caused by steeply dipping, weakly magnetic beds within the late 

Proterozoic Stirling Quartzite, and thus are not related directly to ground-water flow. 

Subtle aeromagnetic anomalies are also observed over thick alluvial deposits in Ash Meadows, 

Pahrump Valley, and Furnace Creek Wash. The sources of these anomalies are also problematic 

because Quaternary alluvium is typically considered nonmagnetic. In Pahrump Valley, magnetic 

anomalies over alluvium are spatially associated with the State Line fault (Blakely and others, 

2000a), suggesting that faulted contacts in or beneath the alluvium may be responsible for the 

anomalies. Displacement along the fault may have juxtaposed rocks with contrasting magnetic 

properties, or secondary mineralization may have deposited magnetic minerals along the faulted 

contacts. Other explanations are possible, of course. The magnetic anomalies in Pahrump 

Valley, for example, also lie over the deepest parts of the modern-day basin, presumably 

marking its long-term depocenter. Thus, near-surface magnetic anomalies may originate from 

magnetic minerals and detritus concentrated by fluvial processes along this axis of deposition. 

It is important to understand the origin of these alluvial magnetic anomalies. If they are caused 

by faults, for example, existing high-resolution aeromagnetic anomalies will facilitate mapping 

the spatial distribution of Quaternary faults otherwise concealed in this region, thus providing an 

important constraint in understanding ground-water flow into Death Valley. This paper 

describes ground-magnetic studies in Ash Meadows and Furnace Creek Wash with the purpose 

of determining the cause of these low-amplitude aeromagnetic anomalies over Quaternary 

alluvial deposits. 

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5 
GEOLOGIC SETTING 

The study area (Fig. 3) lies within the Death Valley extended terrane, a region of diverse geology 

and dramatic tectonic evolution. Wright and Troxel (1973) viewed the opening of Death Valley 

and surrounding basins as occurring along normal faults that terminate at relatively shallow 

depth (1.5 to 9 km) against gently dipping detachment surfaces. Whereas Wright and Troxel 

(1973) envisioned relatively small horizontal translations of mountain ranges, others (Stewart, 

1983; Hamilton, 1988; Wernicke and others, 1988; Holm and Wernicke, 1990) have proposed 

more dramatic crustal translations along one or more regionally continuous detachment surfaces. 

In any case, there is no disagreement that northwest-striking, right-lateral strike-slip faults have 

played an important role in opening the largest basins of the Death Valley extended terrane. 

Burchfiel and Stewart (1966) first proposed that central Death Valley developed as a pull-apart 

basin accommodating strain between the Furnace Creek and Death Valley fault zones, and 

Wright (1988) proposed that basins beneath Pahrump Valley and Ash Meadows may have 

opened along right-stepping offsets of the State Line fault zone (Wright, 1988). 

Most of the mountain ranges of the study area (Fig. 3) are underlain by Paleozoic and older 

carbonate (limestone and dolomite) and crystalline rocks (e.g., Workman and others, 2002). 

Carbonate rocks are believed to be an important aquifer in this part of the Basin and Range 

(Winograd and Thordarson, 1975; Dettinger, 1989; Laczniak and others, 1996; McKee, 1997). 

Intervening basins, like Death Valley, Amargosa Desert, and Pahrump Valley, are filled with 

relatively young sedimentary deposits (Fig. 3), consisting of non-marine sedimentary and 

volcanic rocks, generally Oligocene and younger in age. They include fluvial conglomerate, 

sandstone, siltstone, lacustrine claystone and limestone, volcanic ash deposits of various kinds, 

and local lava flows. The upper parts of the basins typically are covered by Quaternary 

sediments, mostly alluvial, playa, and eolian deposits (Workman and others, 2002). 

The density of basin-fill deposits is sharply lower than the pre-Tertiary rocks that underlie the 

basins. These contrasting densities are reflected in gravity data as low anomalies over basins 

and positive anomalies over mountain ranges. By assuming a simple basin-and-range model, 

gravity anomalies can be mathematically inverted to estimate the three-dimensional shape of the 

basins (e.g., Jachens and Moring, 1990). An inversion of gravity anomalies from the entire 

Amargosa Desert region (Blakely and others, 1998) found a structural trough extending 

northward from the State Line fault, through Ash Meadows, to the Yucca Mountain region. The 

eastern margin of the Amargosa trough corresponds to a fault (sometimes referred to as the 

“gravity fault”) identified in various studies (Schweickert and Lahren, 1997; Winograd and 

Thordarson, 1975; Connor and others, 2000). In Ash Meadows, the estimated thickness of 

Cenozoic deposits generally exceeds 1 km (Fig 3A) and sometimes exceeds 2.5 km. 

Tertiary volcanism also played an important role in the extensional history of the Amargosa 

Desert region. Miocene ash-flow and air-fall tuffs of the southwestern Nevada volcanic field 

dominate much of the scenery north of our study area (Fig. 2). West and south of our study area, 

Miocene lava flows and tuffs of the Death Valley volcanic field are distributed throughout the 

Greenwater Range and Black Mountains (Workman and others 2002). The Amargosa trough 

spatially connects these two volcanic complexes. 

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GROUND-MAGNETIC STUDIES 

Aeromagnetic data collected in 1999 and 2001 reflect this diverse geologic setting. Highamplitude, short-wavelength magnetic anomalies are evident over Tertiary tuffs and basaltic 

intrusions of the Yucca Mountain area (Fig. 2) and over Tertiary volcanic rocks of the Death 

Valley volcanic field west of Shoshone (Figs. 2, 3, and cover photo). Aeromagnetic anomalies 

are relatively subdued in the intervening region, reflecting the low magnetization of pre-Tertiary 

bedrock and Cenozoic basin-filling deposits. Although subdued, a number of low-amplitude 

anomalies are evident throughout Ash Meadows and Furnace Creek Wash (Fig. 3). Various 

explanations are possible. (1) The aeromagnetic anomalies may reflect a non-uniform 

distribution of magnetic detritus within the Quaternary alluvium, perhaps concentrated by fluvial 

processes. (2) They may reflect discrete magnetic units now buried by alluvium, probably 

volcanic rocks. The linear nature of the anomalies may indicate dikes or lava flows truncated by 

near-surface faults. (3) The anomalies may reflect secondary mineralization along near-surface 

faults. 

To help determine the source of these magnetic anomalies, we conducted ground-magnetic 

studies at several locations within Ash Meadows and Furnace Creek Wash. Seven groundmagnetic profiles from five areas are described here: across Carson Slough near Death Valley 

Junction, at Point of Rocks Spring, at Devils Hole, at Fairbanks Spring, and near Travertine 

Springs (Fig. 3). 

Ground-magnetic profiles were conducted on foot with a Geometrics 858 magnetometer system, 

which includes a cesium-vapor magnetic sensor and real-time GPS navigational equipment. 

Measurements were made at 1-second intervals while walking at a normal pace. GPS navigation 

was of high quality due to the lack of tree cover. A stationary base station was established near 

each ground-magnetic traverse and operated simultaneously to correct for diurnal variations of 

the earth’s magnetic field and external field fluctuations. The base-station sensor was a 

Geometrics 856 proton-precession magnetometer. Desert conditions are ideal for these types of 

studies. Some “noise” was observed in all profiles, due to culverts and other metallic objects 

and due to slightly magnetic road materials. However, the geologic “signal” was many times 

stronger and generally longer in wavelength, making it relatively easy to distinguish from noise. 

The following figures that describe individual ground-magnetic profile show both the actual 

measurements (after correcting for time-varying fields) and a running average. The running 

average helps to distinguish the geologic signal from the near-surface, short-wavelength noise. 

Magnetic susceptibility measurements were also made at numerous locations (Figs. 3 and 4; 

Table 1) in order to characterize magnetic properties of exposed lithologies. 

Depth-to-source calculations 

Where appropriate, we used a graphical approach to estimate the depth to causative magnetic 

sources. The technique, known as “Peters’ method” (Peters, 1949), is based on an important 

property of magnetic anomalies: The horizontal gradient at the margins of a magnetic anomaly 

depends on the depth to source; i.e., a shallow magnetic body produces a sharper magnetic 

anomaly relative to an anomaly caused by the same body at deeper depth. Peters’ method is 

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Table 1. Magnetic susceptibility measurements in the Amargosa Desert region. Mean and SD 

are average and standard deviation, respectively, of N readings, with 1 unit = 0.001 SI. See 

Figures 3 and 4 for site locations. 

Site Latitude Longitude Mean SD N Comment

02FC001 36 28.759 116 51.758 0.122 0.001 10 Siltstone, Furnace Creek Formation

02FC002 36 28.813 116 51.609 0.123 0.001 8 Siltstone

02FC003a 36 28.949 116 51.431 0.161 0.029 8 Fanglomerate cobbles; calcium-carbonate

 cemented

02FC003b 36 28.949 116 51.431 0.194 0.001 9 Pebbly matrix

02FC004a 36 29.164 116 51.117 0.591 $0.154 12 Clayey siltstone

02FC004b 36 29.352 116 50.795 0.044 0.001 8 Siltstone, laminated

02FC005 36 29.370 116 50.760 0.069 0.000 10 Fine sandstone, tuffaceous(?), white

02FC006 36 29.417 116 50.679 0.069 0.000 8 Siltstone; laminated, green-white

02FC007 36 29.402 116 50.732 0.035 0.001 10 Siltstone, laminated, tuffaceous

02FC008a 36 26.335 116 49.199 0.249 0.070 9 Siltstone, white, resistant

02FC008b 36 26.335 116 49.199 0.693 0.739 11 Fanglomerate; above tuffaceous bed

02CS001 36 18.829 116 23.476 1.730 0.044 4 Pebbly surface

02CS002 36 19.191 116 22.721 0.514 0.008 5 Dried mud

02CS003 36 19.399 116 22.280 0.578 0.003 4 Pebbly deposit on playa

02CS004 36 19.683 116 21.727 0.478 0.004 4 Pebbly deposit on playa

02CS005 36 19.923 116 21.156 0.896 0.010 5 Pebbles on sand

02CS006 36 20.274 116 20.406 0.833 0.002 4 Pebbles on sand

02AM001 36 23.604 116 17.413 0.238 0.009 6 Scraped surface; calcium carbonate deposit

02AM002 36 23.586 116 16.722 0.444 0.016 7 Scraped surface; calcium carbonate deposit

02AM003 36 23.557 116 16.322 0.038 0.001 6 Caliche rubble

02CV001a 36 22.155 116 39.503 0.100 0.005 10 Calcite crack fillings; iron stained (see Fig. 11)

02CV001b 36 22.155 116 39.503 0.066 0.002 5 Fanglomerate next to calcite-filled cracks

02FS001 36 28.655 116 21.664 0.055 0.001 8 Siltstone; white and gray

02FS002 36 29.118 116 20.962 0.163 0.044 9 Siltstone with calcite nodules

02FS003 36 29.413 116 20.602 0.269 0.037 9 Siltstone; white and tan

02FS004 36 29.496 116 20.328 0.005 0.000 6 Calcareous spring deposit

02FS005 36 29.730 116 19.570 1.447 0.319 9 Calcite cemented conglomerate; volcanic

 and metasedimentary clasts (see Fig. 8)

02FS006 36 30.172 116 18.463 0.860 0.105 12 Cobbles on desert pavement

02FS007 36 25.886 116 17.110 0.045 0.001 8 Dolomite at Devils Hole

02FS008 36 25.511 116 17.452 0.032 0.001 5 Dolomite at Devils Hole

02TS001 36 26.648 116 49.798 0.127 0.006 6 W hite siltstone in road bed

02TS002 36 26.707 116 49.727 0.446 0.008 9 Fanglomerate; Furnace Creek Formation

02TS003 36 26.874 116 49.727 0.263 0.017 10 Fanglomerate; Furnace Creek Formation

graphical in nature and illustrated in each of the following profile figures: Draw two parallel 

lines (dotted lines in the figures) with slope equal to one half of the maximum gradient (dashed 

lines in the figures) of the anomaly, one line tangent to the peak roll-off of the anomaly and the 

other tangent to the minimum part of the anomaly. The horizontal separation of the two lines is 

proportional to the depth to the top of the body. The proportionality constant is 1.2 for very thin 

bodies and 2.0 for very thick bodies. A value of 1.6 is often used, and we have done likewise. 

Peters’ method assumes that the magnetic anomaly is caused by a two-dimensional body with 

vertical sides and uniform and nearly vertical magnetization. Although the magnetic anomalies 

under investigation here are two-dimensional in nature, the other underlying assumptions are less 

likely to be realized. Several factors in particular may tend to increase our depth estimates. 

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First, the direction of magnetization at these latitudes will generally not be vertical, and 

anomalies caused by bodies with non-vertical magnetization have somewhat gentler gradients 

than anomalies caused by the same bodies with vertical magnetization. Second, some of the 

profiles were forced to cross the anomalies at an angle oblique to the strike of the anomaly, 

which reduces profile gradients. In any case, our depth estimates should be treated as 

approximate. 

Death Valley Junction (profile DVJ) 

A ground-magnetic profile was conducted along the road extending northeast of Death Valley 

Junction to the California-Nevada state line (Fig. 3, profile DVJ). The profile crosses a flat 

expanse of Ash Meadows, underlain mostly by fine-grained alluvium of latest Pleistocene to 

Holocene age. The topographically lowest point of the traverse corresponds with Carson 

Slough, which drains parts of Ash Meadows southward. The aeromagnetic data show a 

pronounced linear, north-striking anomaly along Carson Slough (Figs. 2 and 3), but the source of 

the anomaly must lie at depth because magnetic susceptibility measurements of these finegrained alluvial deposits are very small (Table 1, Fig. 4, sites 02CS001-006). 

The ground-magnetic profile (Fig. 5) shows a single anomaly with peak-to-trough amplitude of 

90 nT, a steep western gradient, and a broad eastern gradient. The location of the groundmagnetic anomaly agrees precisely with the aeromagnetic anomaly. On the basis of the western 

gradient, we estimate the maximum depth to the magnetic source to be 980 m. As described 

above, this estimate is probably somewhat overestimated because the profile crossed the 

anomaly obliquely (Fig. 5). The depth to pre-Tertiary basin exceeds 1 km beneath the entire 

profile (Fig. 3), so the source of the anomaly lies deep within the overlying Tertiary and 

Quaternary section. The amplitude of the ground-magnetic anomaly requires a lithology with 

significant magnetization, and a volcanic body, perhaps a basaltic lava flow, is our favored 

interpretation. The linear nature of the anomaly suggests that the horizontal extent of the 

volcanic unit is fault controlled. 

Gravity inversions (Blakely and others, 1998) show that Ash Meadows is underlain by several 

small sub-basins. The southwestern end of profile DVJ lies over one sub-basin 2.5 km deep; the 

northeastern end lies over a second sub-basin about 2.0 km deep. The intervening saddle rises to 

about 1.5 km deep, has a northward strike, and corresponds closely with the position of the 

aeromagnetic and ground-magnetic anomalies. The gravity inversion assumes, however, that the 

density of basin-filling deposits remains uniform in all horizontal directions. If sediment 

densities do in fact vary, the gravity inversion will find erroneous undulations in basin depth. 

We suggest that the saddle seen in gravity inversions beneath profile DVJ is an artifact caused 

by the slightly higher density of the faulted volcanic unit at 1 km depth. 

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Point of Rocks Spring (profiles POR) 

Two ground-magnetic traverses were conducted along east-west dirt roads near Point of Rocks 

Spring (Fig. 3, profiles POR). The profiles are underlain by alluvium and spring deposits 

associated with modern and past ground-water discharge. The ground-magnetic profiles cross a 

small north-striking aeromagnetic anomaly (Fig. 3B) evident in both profiles (Fig. 6). The 

northern profile displays the anomaly with well-defined gradients and a peak-to-trough 

amplitude of about 20 nT. In the southern profile, the anomaly has an amplitude of about 15 nT 

and poorly defined margins. 

Alluvial and spring deposits at the surface have very low magnetic susceptibilities (Table 1, Fig. 

4, sites 02AM001-003) and do not contribute significantly to the magnetic anomaly. On the 

basis of horizontal gradients of the anomaly, we estimate that the magnetic source lies 390 m 

beneath the northern profile and 240 m beneath the southern profile. Based on gravity inversion, 

pre-Tertiary basement lies about 250 m beneath the anomaly on the northern profile and 400 m 

beneath the anomaly on the southern profile (Blakely and others, 1998). Taken at face value, 

therefore, the southern part of the magnetic unit lies above pre-Tertiary basement rocks, but the 

northern part is within the pre-Tertiary section. It is unlikely that pre-Tertiary basement rocks 

cause the anomaly, however. No aeromagnetic anomalies are observed over exposed preTertiary rocks in the immediate vicinity, and later we will show that a sharp rise in pre-Tertiary 

basement at Devils Hole produces no magnetic anomaly at ground level. It is more likely that 

the modest disagreement in depth to magnetic source, based on ground-magnetic profiles, and 

depth 

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to pre-Tertiary basement, based on gravity inversion, reflects the approximate nature of these 

calculations. We conclude that the source of the anomaly near Point of Rocks Spring is caused 

by a magnetic unit lying near the bottom of the Cenozoic section, possibly a small sliver of 

Tertiary volcanic rocks. The anomaly in the northern profile has higher amplitude and a deeper 

source than the southern profile, suggesting that the causative body is volumetrically larger 

beneath the northern profile. 

Fairbanks Spring (profile FBS) 

A ground-magnetic profile (Fig. 3, profile FBS) was conducted along a dirt road extending from 

Fairbanks Spring northeastward toward the intersection of State Route 16 and U.S. Route 95. 

The profile is underlain by older alluvium (middle Pleistocene to late Tertiary) and spring 

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deposits associated with modern or past ground-water discharge. The profile also crosses 

several linear, north-northwest-striking aeromagnetic anomalies (Fig. 3A). The southwestern 

end of the profile corresponds with a broad positive aeromagnetic anomaly with sharp 

northeastern margin, and the northeastern end crosses at least three low-amplitude, linear 

anomalies with similar strike. All of these aeromagnetic anomalies have a similar northnorthwest strike. 

Ground-magnetic profile FBS (Fig. 7) reflects the same anomalies: A broad magnetic high at 

the southwestern end and a series of short-wavelength anomalies at the northeastern end. 

Individual anomalies at the northeast end are low in amplitude, approximately 10 to 30 nT peakto-trough, but they coalesce to form a broad anomaly with significant amplitude, 85 nT peak-totrough. We estimate the depth to the shallowest part of the magnetic source at the northeastern 

end to be about 592 m. 

Some alluvial deposits along the northeastern part of profile FBS have significant amounts of 

scoria and other volcanic rocks (Fig. 8). Collectively, these deposits have magnetic 

susceptibilities (Table 1, Fig. 4, site 02FS005) higher than most alluvial deposits encountered in 

our study, and we should consider the possibility that concentrations of similar deposits at depth 

cause the magnetic anomalies in profile FBS. We think this is unlikely, however. The 

anomalies at the northeastern end of profile FBS are within a broader region of generally highamplitude, short-wavelength aeromagnetic anomalies (Fig. 3B). This pattern is very similar to 

the pattern between Furnace Creek and Shoshone (Fig. 3B), where the aeromagnetic anomalies 

are known to be caused by exposed Tertiary volcanic rocks of the Death Valley volcanic field. It 

is likely that aeromagnetic anomalies northeast of Fairbanks Spring are caused by similar 

volcanic rocks concealed by younger alluvium and spring deposits. 

Gravity inversions (Blakely and others, 1998) indicate that profile FBS crosses a saddle in the 

pre-Tertiary basement. The southwestern end of the profile lies over a sub-basin about 2 km 

deep, and the northeastern end lies over a second sub-basin about 0.8 km deep. The intervening 

saddle rises to 0.2 m depth and is located about midway along profile FBS. It is possible that 

some of this calculated basement topography is an artifact of non-uniform densities of overlying 

basin-filling sediments. In any case, the sources of the short-wavelength anomalies at the 

northeastern end lie above the pre-Tertiary basement. 

Travertine Springs (profiles TVS) 

Several short ground-magnetic profiles were conducted cross-country across a small 

aeromagnetic anomaly near Travertine Springs in Furnace Creek Wash (Fig. 3, profile TVS). 

The aeromagnetic anomaly (Fig. 3B) strikes northwest and consists of a single peak. The 

ground-magnetic profile (Fig. 9) reveals a more complex structure: three narrow anomalies with 

amplitudes of 20 to 70 nT peak-to-trough. The sources of these anomalies lie very near the 

topographic surface: We estimate the depth to the most prominent anomaly at only 61 m below 

the surface. 

Profile TVS is underlain by Miocene to Pliocene sedimentary rocks and modern spring deposits. 

Based on gravity inversions, Pre-Tertiary basement lies at depths of 600 to 700 m beneath 

profiles TVS. Thus, the source of the aeromagnetic and ground-magnetic anomalies lies within 

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the overlying Tertiary and Quaternary section. Fanglomerate and other alluvial deposits are 

exposed along the length of profile TVS, but these deposits are only weakly magnetic (Table 1, 

Fig. 4, sites 02TS001-003). The aeromagnetic anomaly is located near and has a character 

similar to the pattern of anomalies to the south, known to be caused by Tertiary volcanic rocks of 

the Death Valley volcanic field (Fig. 3). We conclude that the aeromagnetic anomaly at 

Travertine Springs is caused by a sliver of Miocene volcanic rocks now concealed by younger 

sediments. The narrow, linear nature of the aeromagnetic anomaly suggests that the source is 

bounded by northwest-striking faults. 

17 
Devils Hole (profile DVH) 

Finally, we include here a ground-magnetic profile with no pronounced magnetic anomaly in 

order to make an important point: pre-Tertiary rocks do not contribute appreciably to 

aeromagnetic anomalies in this area. The traverse was conducted along the dirt road passing by 

Devils Hole (Fig. 3, profile DVH), where no aeromagnetic anomalies are observed. From south 

to north, the route passes from relatively flat Quaternary alluvium into a narrow gap between 

topographically elevated areas. The surrounding hills are underlain by faulted and folded 

Bonanza King Formation, dolomite and limestone of Upper and Middle Cambrian age. If these 

rocks were significantly magnetic, the sharp rise in pre-Tertiary basement from south to north 

would produce a distinctive aeromagnetic anomaly at about Devils Hole and strong groundmagnetic anomalies within the topographic gap. Profile DVH (Fig. 10), however, shows only a 

gradual rise in amplitude, indicating very low magnetizations. This is confirmed by low 

magnetic susceptibilities measured at sites 02FS007 and 02FS008 (Table 1, Fig. 4). Thus, preTertiary rocks in this area do not contribute significantly to aeromagnetic anomalies. 

CONCLUSIONS 

A number of explanations should be considered for aeromagnetic and ground-magnetic 

anomalies observed over alluvium in the Amargosa Desert region. First, we can rule out preTertiary carbonate and crystalline rocks as potential magnetic sources. Although exceptions do 

exist (e.g., Hillhouse and Morin, 2003), such rocks typically have very low magnetizations. We 

have confirmed this with susceptibility measurements of and a ground-magnetic transect through 

exposed dolomites near Devils Hole. Moreover, the depth to the sources of the aeromagnetic 

and ground-magnetic anomalies place the causative magnetic bodies above the pre-Tertiary 

basement. 

Second, it is possible that the alluvium itself is magnetic. Near Fairbanks Spring, fanglomerate 

exposures have significant fractions of volcanic detritus (Fig. 8) and an average magnetic 

susceptibility of 1.447X10-3 SIU (Table 1, Fig. 4, site 02FS005). Although somewhat more 

magnetic than alluvium elsewhere in our study, unrealistically large volumes of these deposits 

would be required in order to account for the amplitudes of observed ground-magnetic 

anomalies. 

Third, secondary mineralization, perhaps along faulted contacts, may introduce magnetic 

minerals into the alluvial section. Calcite veins along the highway between Furnace Creek and 

Death Valley Junction (Fig. 11) provide an example of what we might expect. These veins 

display banded iron-oxide staining, presumably secondary hematite. Hematite is only weakly 

magnetic (Telford and others, 1990, p. 74), as demonstrated by our measured magnetic 

susceptibilities (Table 1, Fig. 4, site 02CV001a). Indeed, the iron-stained veins are only slightly 

more magnetic than the surrounding fanglomerate deposits (Table 1, Fig. 4, site 02CV001b). 

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Fourth, the ground-magnetic and aeromagnetic anomalies over alluvium may be caused by 

discrete magnetic units at depth. Considering the abundance of lava flows, ash-fall tuff, and 

other volcanic deposits in the nearby Greenwater Range (Fig. 3), we believe Tertiary volcanic 

rocks of the Death Valley volcanic field are likely candidates. The linear nature of these 

anomalies suggests that the concealed volcanic rocks may be bounded and offset by faults. 

Our study shows the utility of ground-magnetic investigations to facilitate interpretation of 

aeromagnetic anomalies. The 1999 and 2000 aeromagnetic surveys over Amargosa Desert and 

Death Valley were flown along flight lines spaced 400 m apart and at an altitude of 150 m above 

terrain, or as low as safely permitted. Because of steep terrain, actual flight altitudes were often 

significantly higher that 150 m above terrain. Nevertheless, our ground-magnetic transects 

confirm that these aeromagnetic data can resolve weakly magnetic lithologic units in the alluvial 

section. 

ACKNOWLEDGMENTS 

We are grateful to Dan Scheirer and Darcy McPhee for helpful comments and for reviews of our 

initial manuscript. This work was partially funded by the National Park Service under 

Interagency Agreement IA2380990002, Task Order 02-12. 

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