Geophysical, Geohydrological, and 

Geochemical Reconnaissance of the 

Luke Salt Body, Central Arizona

GEOLOGICAL SURVEY PROFESSIONAL PAPER 753
Geophysical, Geohydrological, and 

Geochemical Reconnaissance of the 

Luke Salt Body, Central Arizona

By GORDON P. EATON, DONALD L. PETERSON, and 

HERBERT H. SCHUMANN

GEOLOGICAL SURVEY PROFESSIONAL PAPER 753

Preliminary .interpretation of a buried salt body in 

the Basin and Range province of Arizona., based on 

its gravity field, attendant hydrologic effects, and 

bromide geochemistry

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY

V. E. McKelvey, Director

Library of Congress catalog-card No. 72-600111

For sale by the Superintendent of Documents, U.S. Government Printing Ofnce

Washington, D.C. 20402 - Price 40 cents (paper cover)

Stock Number 2401-2161
CONTENTS

Page 

1

1

2

2

3

5

8

8

Transmissivity of the valley-fill sediments........... 10

Abstract...........................................................................

Introduction and acknowledgments............................

Physical setting and delineation of the salt bodyTopography.............................................................

Geology....................................................................

Geophysics...............................................................

Geohydrologic data bearing on the occurrence 

of the salt...................................................

Salinity of the ground water... ..................

Page

Physical setting and delineation of the salt body Continued

Deep-well data..............................

Gravity data.........................................

Calculation of residual gravity. 

Interpretation of residual gravity....

Density data.................................

Modeling method..........................

Origin of the salt.................................

Bromide geochemistry.................

The source problem.....................

References.............................................

10

13

13

15

16

18

22

22

23

27

ILLUSTRATIONS

Page

FIGURE 1. Index map of central Arizona, showing general geology of region around western Salt River Valley. ......... 4

2-9. Maps showing:

2. Topography of western Salt River Valley.................................. .................................................................. 6

3. Fifth-degree residual topography of the Luke area....-.................................... . ... . .. 7

4. Areas of predominantly fine-grained sediments.............................................................................................. 8

5. Observed Bouguer gravity field of western Salt River Valley......................... ......................................... 9

6. Ground-water salinity in western Salt River Valley. ........... .. . .................................. 11

7. Transmissivity of valley-fill sediments of western Salt River Valley........................................................ 12

8. Distribution of deep wells within 7.5-mile radius of center of crest of the Luke salt body................. 13

9. Residual Bouguer gravity associated with the Luke salt body. .................................................................. 14

10. Chart showing density and generalized lithology of upper 1,100 feet of deep well penetrating salt body.......... 16

11. Chart of alluvium density logs and idealized depth-density functions used in preparing

gravity models of the Luke salt body. ............. ........... .......- ........... ....... .............................................. 17

12. Map and cross sections showing limiting configurational model of the Luke salt body, based on the

residual gravity field of figure 9C and alluvium density model C (fig. 11B)................................................. 19

13. Cross sections illustrating alternate interpretations of the Luke salt body.............................................................. 21

14. Chart of bromine content of halite in the Luke salt body.......... .. . ........... .................................................. 23

15. Map showing areas of occurrence of known major halite deposits and sodium chloride

waters in Arizona............ ..................................................................................................... ................................ 24

TABLE

TABLE 1. Occurrence of halite and selected sodium chloride brines in Arizona.

Page

25

in

GEOPHYSICAL, GEOHYDROLOGICAL, AND GEOCHEMICAL 

RECONNAISSANCE OF THE LUKE SALT BODY, CENTRAL ARIZONA

By GORDON P. EATON, DONALD L. PETERSON, and HERBERT H. SCHUMANN

ABSTRACT

A major salt body lies hidden beneath the floor of the 

western Salt River Valley, 17 miles west-northwest of Phoenix, 

Ariz. It appears to have an arcuate crest and a broad triangular base. Although the salt body was initially interpreted 

as a structural massif, formed by extensive plastic flow 

and that is still the preferred interpretation the data in 

hand appear to allow the alternate interpretation that it is an 

exceedingly thick steep-sided sedimentary prism representing 

long-lived evaporite deposition in the center of a clastic sedimentary basin. Its crest is marked by several local domes which 

have bowed and compacted the overlying sedimentary cover, 

but this phenomenon may be localized and without structural 

significance for the entire body.

Gravity data suggest that the base of the body is at a depth 

of at least 6,900 feet, but isolation of that part of the gravity 

field due to the salt is subjective, and the density stratification 

of the sediments in the basin is unknown. Thus the interpretation of the gravity data is uncertain. The upper 3,600 feet of 

the salt has been penetrated by drilling. The body is overlain 

locally by an anhydrite layer 90 feet thick.

Geohydrologic data indicate that the upper part of the salt 

has a pronounced local effect on the salinity of the ground 

water and an indirect effect on the transmissivity of the alluvial fill. Grain-size data suggest that the area above the 

body stood relatively higher than its surroundings throughout 

the later part of late Cenozoic time. A local earth-fracture 

system which appears to owe its location, at least in part, to 

the existence of the body indicates that adjustments are still 

occurring, probably in direct response to large ground-water 

withdrawals.

The salt constituting this body is thought to be of nonmarine 

origin. Bromine analyses of drill cuttings from the deep test 

average 2 parts per million, much below the threshold value 

of 30 parts per million characteristic of marine halite. On the 

basis of the meager information available from regional geologic studies, we believe that the salt accumulated in a longstanding saline lake sometime during the middle Tertiary, or 

possibly even earlier.

INTRODUCTION AND ACKNOWLEDGMENTS

In November 1968 Arizona Salt Co. and El Paso 

Natural Gas Co. drilled an exploratory test hole in 

the western Salt River Valley, 17 miles west-northwest of Phoenix, Ariz. The hole, 1 mile east of Luke 

Air Force Base, in sec. 2, T. 2 N., R. 1 W., penetrated the top of an anhydrite section at a depth of 

790 feet and passed from anhydrite into halite at a

depth of 880 feet. It was continued in halite to a 

depth of 4,503 feet, at which point drilling- was 

stopped. The presence of a buried salt mass in this 

area had been suspected from the finding of halite 

in two water wells (Stulik and Twenter, 1964; Kam 

and others, 1966) and from the mapping of a large 

negative gravity anomaly by the U.S. Geological 

Survey (Peterson, 1965, 1968), according to G. J. 

Grott (Southwest Salt Co., written commun., 1971).

The drilling of the exploratory well triggered considerable interest in regard to the extent and origin 

of the salt body, and as a result, the original gravity 

study was enlarged, and additional geophysical and 

geochemical data were collected (Eaton and others, 

1970, 1971). The conclusion reached as a result of 

this further study is that the salt mass, here referred to as the Luke salt body, is probably of continental origin. The body includes at least 15 cubic 

miles of halite. In gross shape it is roughly an irregular, locally domed, ridge-like mass that has an 

arcuate crest and a broad triangular base. Although 

its configuration is like that of the core of a large 

salt anticline, it may represent an undeformed prismatic sedimentary accumulation of salt, only the 

uppermost part of which has flowed plastically and 

bowed the overlying alluvial sediments. Similar, or 

somewhat smaller, volumes of apparently nonmarine 

halite are known to occur in three other basins to 

the northwest, and we suggest that appreciable volumes of salt may be found in yet other basins of 

analogous location along the edge of the plateaus 

province.

We thank all the people who provided data and 

materials to us in the course of this study. We are 

especially grateful to G. J. Grott, of the Southwest 

Salt Co., who drilled the exploratory hole and who 

made available the geophysical logs, a set of cuttings, 

and samples of brines from it. R. J. Kite, of the U.S. 

Geological Survey, was most helpful to us in suggesting the method of bromine analysis and analyzing the core sample for insolubles. He also acted as 

a critical sounding board for our evolving ideas on 

the origin of the Luke salt body.
LUKE SALT BODY, CENTRAL ARIZONA

Our gratitude to M. E. Cooley is apparent from 

the abundance of references to communications or 

publications by him. He was most generous in sharing with us his knowledge of regional Tertiary geology and in calling our attention to local occurrences 

of sodium chloride brines. H. W. Peirce, of the Arizona Bureau of Mines, provided us with the drill 

cuttings from which the bromine log was prepared 

and also gave freely of his knowledge of salt as it 

occurs in Arizona. J. H. Earl, of El Paso Natural 

Gas Co., provided ideas based on his own investigations of the Luke body. The manuscript benefited 

significantly from helpful and constructive reviews 

by M. E. Cooley, E. S. Davidson, R. J. Hite, and 

W. D. Stanley.

PHYSICAL SETTING AND DELINEATION 

OF THE SALT BODY

The topography, general geology, and gravity field 

of the region around the salt body are described and 

illustrated in this section of the report. Although the 

basic purpose is to provide a background against 

which to view some of the characteristics of the body, 

certain aspects of the setting as related to the probable origin of the body will be discussed briefly.

TOPOGRAPHY

The area in which the Luke salt body occurs is 

part of the western Salt River Valley, which lies in 

the northern part of the Basin and Range province 

of Arizona (fig. 1). The Salt River, from which the 

valley takes its name, is tributary to the west-flowing Gila River at the south edge of the area. A short 

distance to the west is the confluence of the Gila and 

the south-flowing Agua Fria River. The Agua Fria 

River skirts, and may be deflected slightly by, the 

crest of the salt body.

The western Salt River Valley is irregularly 

rimmed by several bedrock ranges, and its gently 

sloping alluvial floor represents, for the most part, 

an incised surface that is topographically adjusted 

to the foot of these ranges in its higher parts and 

to the through-flowing Salt and Gila Rivers in its 

lower part (figs. 1, 2). The surface comprises a 

series of smoothly coalesced alluvial fans and plains 

through which project isolated steep-flanked bedrock hills. As figure 2 shows, except for the channel 

of the Agua Fria River and the less pronounced 

valley of the New River, which joins the Agua Fria 

from the northeast, the surface is relatively uniform, 

and the contours swing across the area from west 

to east in broad arcs open to the south. The only 

significant interruption of the topography is a pair 

of low hills west of the Agua Fria River, southeast 

of Luke Air Force Base. These hills rise above the

alluvial surface and are characterized by quasiradial drainage. Unlike the steep-flanked mountains 

nearby, the hills are underlain by unconsolidated 

alluvium. M. E. Cooley, who is widely experienced 

in the Tertiary geology of Arizona, examined the 

alluvium exposed in these hills and described it thus 

(written commun., 1971) :

* * * The deposits comprising the hills * * * are divisible into 

two units a younger gravel and an older silt to silty sand. The 

gravel is thin and is not present on the summits of all the hills. 

Where it is present, it has the appearance of a terrace deposit, 

which tends to mask the presence of the underlying unit. The 

gravel unconformably overlies the older unit and has a maximum observed thickness of 6 feet. It consists mainly of rounded 

to subrounded pebbles composed of volcanic, granite-gneiss, 

and other silicic types. In places there are also a few rounded 

cobbles and small boulders as much as 16 inches in the long 

dimension. In its overall appearance, the gravel is similar to 

that transported by the nearby Agua Fria River and dissimilar 

to the well-rounded to rounded quartzitic and hard silicic 

pebbly to cobbly gravel exposed in terraces along the Salt 

River. The imbrication or arrangement of the gravel indicates 

that it was deposited chiefly by southwestward-moving water, 

but the range of individual measurements is from southwest, 

through south, to west-northwest.

The older unit is composed principally of buff silt and some 

thin layers of silty sand and sand. It is weakly cemented by 

limy materials. Some thin beds contain more than 75 percent 

of porous limestone (or caliche). At one exposure, the silt has 

been crumpled somewhat and is interlaced with very thin calcite veins, which dip at different angles and trend in different 

directions. In general, the deposit resembles silt-sand beds exposed near the lower Hassayampa River to the west and other 

fine-grained deposits in Arizona that are considered to be of 

late Tertiary (chiefly Pliocene) age.

These hills are believed to reflect a doming of the 

alluvium by the salt because they are near the deep 

salt test well and, as will be shown later, coincide 

with a residual-gravity trough that terminates in a 

gravity minimum east of the well, as well as with 

an anomaly in ground-water salinity. If Cooley's surmise is correct and if the capping gravel is Quaternary in age but the underlying silt and sand are 

Pliocene, then stratigraphic evidence for a dome is 

at hand and supports the topographic interpretation 

made here.

In order to study the relationship of these hills to 

the rest of the surface of the western Salt River 

Valley and, more particularly, to determine their 

topographic closure in relation to the sloping alluvial 

plain and the wells which penetrated salt, the topography of the area was digitized on a 2,000-foot grid, 

and orthogonal polynomial surfaces were fitted to 

it by digital computer. Figure 3 shows the residual 

topography obtained by subtracting the fitted fifthdegree polynomial surface from the observed surface. The fifth-degree residual topography was picked
PHYSICAL SETTING AND DELINEATION OF THE SALT BODY

for illustration because it tends to minimize the 

number and amplitude of isolated residuals throughout the area and, at the same time, to enhance the 

topography of the hills under discussion.

Three features stand out on the residual topographic map: (1) the paired hills southeast of Luke 

Air Force Base, with closures of about 45 and 55 

feet; (2) the channel of the Agua Fria River, with 

a maximum depth of about 25 feet; and (3) a broad 

ridge on the east side of the channel, rising about 25 

feet above the surrounding alluvial surface.

Two of the three wells (1 and 2 in fig. 3) which 

penetrated halite in the subsurface are between the 

+10- and +20-foot contours of residual topography. 

Well 1, the deep test, penetrated halite at a depth 

of 880 feet, and well 2, a water well, at a depth of 

1,447 feet. No known wells sufficiently deep to penetrate salt have been drilled within the higher parts 

of the hills, which we regard as marking structural 

highs on the body.

The phenomenon of circular topographic mounds 

at the surface above salt domes in the Gulf Coast 

region has been recognized for many decades. In 

the section that follows, we present evidence indicating that the area above the Luke salt mass stood 

topographically high during the latter part of the 

Neogene.

GEOLOGY

The general geology of the region is illustrated in 

figure 1. On this map, all igneous and metamorphic 

rocks, regardless of age, have been lumped together 

with one pattern. One can easily see by inspection of 

the map that these rocks, which can be dismissed as 

a possible source for the salt, make up the vast bulk 

of all the exposed consolidated rock in the vicinity 

of Luke Air Force Base.

The closest sedimentary rocks older than late Tertiary in age lie about 20 miles to the east, north of 

Tempe. They are probably middle Tertiary (Oligocene-early Miocene) in age, according to M. E. 

Cooley (written commun., 1971), and consist of 

semiconsolidated arkosic red conglomerates, sandstones, and shales of probable continental origin. 

Rocks of similar age are exposed about 40 miles to 

both the northwest and the southwest. Paleozoic and 

Mesozoic sedimentary rocks (undivided) are exposed 60 miles to the west-northwest of the Luke 

salt body. About 75 miles north-northeast of the 

Luke area, in the Verde River Valley, is the Pliocene 

Verde Formation, which contains known evaporite 

deposits. These deposits probably are younger 

than the lower part of the section occupied by the 

Luke salt body; therefore the Luke salt is probably 

pre-Pliocene in age. Just northeast of the area shown

in figure 1 (130 miles northeast of the Luke body) 

are the closest known deposits of pre-Tertiary salt. 

They occur in the marine Supai Formation of Permian and Pennsylvanian age.

We tentatively conclude, on the basis of these observations, that the Luke salt body most likely was 

not derived from a halite deposit within the local 

consolidated bedrock of the region. Possibly, Mesozoic and older sedimentary rocks containing evaporites once blanketed the region and were stripped 

erosionally from the uplifted ranges, but they lie 

buried in place beneath the floor of the western Salt 

River Valley; however, this is a speculation without 

basis in observational fact.

The upper Cenozoic sedimentary deposits of the 

western Salt River Valley consist of unconsolidated 

to semiconsolidated gravel, sand, silt, and clay. Locally they contain caliche and thin evaporites, predominantly discontinuous beds of gypsum, in the 

upper 1,500 feet of the section (Stulik and Twenter, 

1964, p. 8). These materials generally occur intermixed in laterally limited beds or lenticular layers. 

The clastic components, which were derived mainly 

from the mountainous country to the north and east, 

probably are Pliocene and Pleistocene, but at depths 

exceeding 1,500 to 2,000 feet they may be Miocene 

or older. Sedimentary rocks of middle Tertiary age 

are known to underlie upper Tertiary deposits at 

Gila Bend and in areas northeast of Phoenix and 

west of Casa Grande. According to M. E. Cooley 

(written commun., 1971), the Pliocene-Pleistocene 

deposits in the deeper parts of many of the basins in 

this region (the area shown in fig. 1) are generally 

less than 2,000 feet thick, and in some basins, such 

as at Gila Bend, they are only 1,000 to 1,200 feet 

thick.

Data from the three wells penetrating the salt 

body suggest that the upper part occurs through a 

range of stratigraphic levels. The locations of the 

exploratory well (1) and the two water wells (2 and 

3) that penetrated halite are shown in figure 3. Well 

2 penetrated 5 feet of "solid rock salt" and bottomed 

therein at a depth of 1,452 feet, after passing 

through 1,447 feet of mixed clay, sand, conglomerate, 

and gypsum, with clay predominating (Kam and 

others, 1966, p. 50). Well 3 penetrated halite and 

associated brines at a depth of about 2,320 to 2,350 

feet (Stulik and Twenter, 1964, p. 11). The section 

overlying the salt in this well consists of clay, silt, 

sand, and gravel, with clay and silt predominating. 

The total depth is not on record. The data from all 

three of these wells were used in modeling the salt 

mass.

Cenozoic deposits seem at present to be the most
LUKE SALT BODY, CENTRAL ARIZONA

113° 112°

10 20 30 40 50 MILES

10 

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20 

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30 

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40

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50 KILOMETERS 

J

FIGURE 1. Index map of central Arizona, showing general 

geology of region around western Salt River Valley (generalized from Wilson and others, 1969, with modifications 

from Cooley, 1967, and M. E. Cooley, written commun., 

1971). Note the general scarcity of lower Cenozoic and older 

sedimentary rocks throughout the region. The heavy solid 

line in the northeast corner of the map marks the Mogollon

Rim, at the southern edge of the Colorado Plateaus. South 

of the heavy dashed line is the Basin and Range province. 

The region between these lines is transitional. The area of 

the present investigation is bounded by the White Tank 

Mountains on the west, the Sierra Estrella on the south, 

the city of Phoenix on the east, and the northern border of 

the Basin and Range province on the north.
PHYSICAL SETTING AND DELINEATION OF THE SALT BODY

EXPLANATION

Mainly Quaternary deposits

Mainly upper Tertiary (Pliocene) deposits

Middle Tertiary sedimentary rocks

Mesozoic and Paleozoic sedimentary rocks

Precambrian sedimentary rocks; slightly metamorphosed

Phanerozoic and Precambrian intrusive, extrusive, and 

metamorphic rocks

Contact

Location of Luke salt body

Physiographic province boundaries

Location) ARIZONA

of maplikely enclosing rocks for the Luke salt body. A possible precedence for determining such an occurrence 

is found in Hualpai and Detrital Valleys, 170 miles 

to the northwest, in extreme northwestern Arizona 

(Pierce and Rich, 1962; Peirce, 1969). There, 500 to 

more than 1,190 feet of salt, apparently within the 

Pliocene Muddy Creek Formation, of continental origin, was tapped by drilling. In Virgin Valley, Nev., 

immediately north-northwest of that, an exploratory 

drill hole passed through 1,700 feet of halite enclosed by the Muddy Creek Formation without penetrating the base of the body (Mannion, 1963, p. 

171-173).

The only published description of the alluvium 

near Luke Air Force Base is that of Stulik and 

Twenter (1964). Because the layers constituting the 

valley fill are lenticular, these investigators digitized 

available well logs (mainly drillers' logs) in terms

of weighted percentages of fine-grained materials 

and prepared two maps which they used in a qualitative interpretation of geohydrologic parameters. 

One of these maps (their pi. 2) is reproduced here 

in modified form as figure 4. 1 It illustrates, in a 

general way, the gross grain-size variations in the 

upper 200 to 500 feet of valley fill. The area southeast of Luke Air Force Base is underlain by a high 

proportion (more than 60 percent) of fine-grained 

sediments. Surrounding this area, in moatlike fashion, are areas of coarse-grained sediments (less 

than 60 percent fines) that were probably deposited 

by major streams. The axes of discontinuous areas 

with less than 40 percent fines are shown in figure 4 

by light dashed lines. Two interpretations based on 

this map are possible for the area east and south of 

Luke Air Force Base: (1) this area stood relatively 

high throughout the period of deposition (middle to 

late Quaternary?) represented by the interval sampled, or (2) the finer grained materials are older 

(Pliocene to early Pleistocene?) sediments lifted, 

during the late Quaternary, to a higher structural 

level by doming. In the first interpretation, the high 

would have deflected the major streams around it 

in a subannular fashion, thereby producing the facies 

pattern observed. Such drainage patterns are observed commonly around salt domes (Thornbury, 

1969, p. 214-216). In the second interpretation, the 

fine-grained and the surrounding coarse-grained materials are not different facies of the same stratigraphic unit, but are of different ages. Although we 

prefer the first interpretation, there is no firm basis 

for choosing between them. The relative altitude of 

this tract today is attributed to continual arching of 

the topographic surface by the rising salt.

The facies variations noted here probably have 

tended to reduce the topographic closure or relief 

over the salt, because the finest grained (and therefore most compactible) sediments occur directly 

above the salt body, and the coarser grained, less 

compactible sediments are distributed in the surrounding area. The net effect of differential compaction would be to reduce the height of the domed 

surface relative to the surrounding terrain. Thus the 

residual topography shown in figure 3 may reflect a 

minimum value for the doming of the topographic 

surface relative to the rise of the salt beneath it.

GEOPHYSICS

The geophysical setting of the Luke salt body is 

known almost entirely from gravity data, although

 'Although Stulik and Twenter digitized each log in units of 20 percent, 

we question whether the data have real significance in increments this small. 

In part, a driller's descriptive term is based on how the material drills, 

rather than on its lithology. We have therefore broken their detailed map 

down into only two units, separated by their 60-percent contour line.
LUKE SALT BODY, CENTRAL ARIZONA

22'30" 15' 112°07'30"

33°37'30"

33° 30'

FIGURE 2. Topography of western Salt River Valley (from 

the following U.S. Geological Survey 7 %-minute quadrangle 

maps: Calderwood Butte, 1957; El Mirage, 1957; Fowler, 

1952; Glendale, 1957; Hedgpeth Hills, 1957; McMicken Dam, 

1957; Perryville, 1957; Tolleson, 1957; and Waddell, 1957). 

Contour interval is 25 feet; dashed contour lines interpolated 

from maps with 10-foot contour interval. Note hilly area

a limited amount of reconnaissance aeromagnetic 

data have been gathered also, in the hope that they 

might provide an independent means of determining 

the depth to the crystalline basement. Unfortunately, 

no aeromagnetic anomalies suitable for this purpose 

were recorded in the general area of the salt body.

southeast of Luke Air Force Base. Bedrock outcrops are 

shown in solid black near margins of map. Short heavy 

lines represent open fractures in alluvium or offsets of 

paved surfaces; solid dots indicate wells with crushed or 

collapsed casings, as indicated by Kam, Schumann, Kister, 

and Arteaga (1966). Wells 1, 2, and 3 represent wells that 

penetrated halite.

Figure 5 is a Bouguer gravity map of the western 

Salt River Valley based on the earlier work of Peterson (1965) and on 118 supplementary measurements 

made in the course of the present study. The principal features shown on the map are (1) a large 

gravity low, with a northwest-trending axis between
PHYSICAL SETTING AND DELINEATION OF THE SALT BODY

33°37'30" 

33°30' 

FIGURE 3. Fifth-degree residual topography of the Luke area. Computer-generated map derived by subtracting 

a fitted fifth-degree polynomial surface from a digitized topographic map of the area. Contour interval is 10 

feet. Solid dots with ticks represent wells that penetrated massive evaporites. Dashed line is axis of that part 

of salt body above a depth of 2,500 feet, as shown in figure 12.

Luke Air Force Base and El Mirage, parallel to the 

regional structural grain; (2) steep gradients flanking the east, south, and west sides of this low; and 

(3) a sinuous gravity high trending northwest and 

north on the east side of Glendale and Peoria. Most 

of the major gravity low is due to the presence of a 

thick sedimentary fill of moderate density. The sinuous high probably reflects a broad ridge of relatively 

dense bedrock buried at shallow depth. The steep 

flanking gradients of the low are thought to reflect 

zones of high-angle normal faulting bounding the 

grabenlike basin in which the fill accumulated.

Of particular interest here are the details of the 

central part of the gravity low. Irregularities in the
LUKE SALT BODY, CENTRAL ARIZONA

33°37'30" 

I I I

5 MILES

0 5 KILOMETERS

I i i i i I

FIGURE 4. Areas of predominantly fine-grained sediments 

more than 700 feet above sea level in the stratigraphic 

section of the western Salt River Valley (modified from 

Stulik and Twenter, 1964, pi. 2). Shading indicates areas 

where the valley fill contains more than 60-percent clay 

and silt, and local thin beds or streaks of gypsum. Light 

dashed lines are the axes of areas of coarse-grained material. Note subannular pattern of these axes about an 

area centered southeast of Luke Air Force Base. Solid dots 

with ticks represent wells that penetrated massive halite.

 90-mgal (milligal) and lower contours seem to 

define a subtle, local gravity closure, the axis of 

which trends northeast to north at a moderate angle 

across the axis of the large low. Notable is the pronounced reentrant (defined by the relatively sharp 

convexity of contours toward the southwest) caused 

by this local feature, on the south side of Luke Air 

Force Base. The gradients of the local closure are 

steeper than those of the regional low in the axial 

region of the latter, allowing the possibility, but not 

proving, that the source of the local closure is at a 

relatively shallow depth.

Crude estimates of the maximum depth to bedrock 

beneath the major low, based on density contrasts 

between valley-fill sediments and bedrock appropriate for this part of the Basin and Range province, 

and ignoring the presence of the salt, place the basement2 surface at a depth between 10,000 and 15,000 

feet. This range of approximate depths derived from 

the gravity data is in approximate accord with a

2The term "basement" is used here in the geophysical sense. It refers 

to relatively dense, well-consolidated rocks of any composition or age 

underlying the weakly consolidated to unconsolidated valley fill.

depth of roughly 11,000 feet derived from a reconnaissance seismic reflection measurement (J. H. 

Earl, oral commun., 1970).

The observation of a local gravity closure with 

a relatively shallow source suggests the presence of 

an anomalous body within the valley fill. This body 

produces a gravity minimum, which indicates that 

the structure has a bulk density less than that of the 

valley fill. This evidence, together with the observation that wells 1, 2, and 3 bottomed in halite, suggests that the peculiarities in the center of the 

principal low are related to a large mass of halite. 

If a local residual anomaly could be isolated by appropriate means, the gravity data could be used to 

determine the approximate size and shape of this 

salt mass. Such an isolation is attempted with the 

aid of other data (figs. 3, 4, 6, 7, and 8), and some 

of these data are presented next to help define the 

horizontal extent of the upper several hundred feet 

of the salt body.

GEOHYDROLOGIC DATA BEARING ON THE 

OCCURRENCE OF THE SALT

Because of rapid urbanization and agricultural 

and industrial development in the Phoenix area, an 

abundance of well data is available for the area. 

Three relatively recent studies by the U.S. Geological 

Survey (Stulik and Twenter, 1964; Kam and others, 

1966; Anderson, 1968) provide adequate detail in 

the area of interest for the delineation of specific 

anomalies, each of which appears to relate to the 

salt mass.

Stulik and Twenter (1964) noted the occurrence 

of unusually high salinity in five wells on the east 

and south sides of Luke Air Force Base. Noted also 

was the local occurrence of conspicuous earth fractures and collapsed or crushed well casings, believed 

to be associated with recent land subsidence induced 

by large-scale ground-water withdrawal.

Kam, Schumann, Kister, and Arteaga (1966) presented several maps and tables that likewise indicate 

unusual geohydrologic conditions in the vicinity of 

the base. Data selected from these two reports are 

presented here, and figure 7 was taken directly from 

the study by Anderson (1968).

SALINITY OF THE GROUND WATER

In order to study geographic variations in groundwater salinity to see if they relate to the salt mass 

under study, we tabulated specific-conductance data 

from available sources (Stulik and Twenter, 1964, 

table 4; Kam and others, 1966, tables 3 and 4) and 

converted the values to total dissolved solids. Specific 

conductance was chosen as the principal indicator of 

salinity because data in this form were available for 

many more wells than were analyses of total dissolved solids.
PHYSICAL SETTING AND DELINEATION OF THE SALT BODY

112°30' 22'30"

33°37'30'

33°30'

FIGURE 5. Observed Bouguer gravity field of western Salt River Valley. Contour interval is 2 milligals. Contour lines dashed 

in areas of sparse control. Small dots are gravity stations. Large solid dots with ticks represent the three wells which penetrated massive halite. Heavy dashed line is approximate edge of top of salt body based on data in figures 3, 4, 6, 7, and 8. 

Profile A-A' is shown in figure 9B and illustrates the derivation of residual gravity values used in modeling the salt 

mass. Stippled pattern represents exposed bedrock.

As is well known, there is a linear relation between 

dissolved-solids concentration and specific conductance in simple dilute solutions of a single salt (Hem,

1959, p. 40). Approximate values of dissolved solids 

were computed by multiplying the observed specific 

conductance values by 0.60. This factor was deter-
10 LUKE SALT BODY, CENTRAL ARIZONA

mined empirically for the Luke area from pairs of 

measurements of both parameters. Water quality 

data are available from only one of the three wells 

that penetrated the salt body. This well (well 3 in 

fig. 6) was sampled at various intervals, including 

one near the top of the evaporite sequence. The value 

for this well in figure 6 represents an average of 

values for waters from a zone 175 to 250 feet above 

the halite. The composited value, 5,300 mg/1 (milligrams per liter) reflects very clearly the effect of 

the salt on ground-water composition.

The areal distribution of computed dissolved-solids 

content is shown in figure 6. Ground water in the 

northern and northwestern parts of the area contains less than 500 mg/1 of total dissolved solids. In 

the southern part of the area, near the Salt and Gila 

Rivers, the ground water contains more than 3,000 

mg/1 of dissolved solids.

A large anomaly, shown by the crosshatching, with 

salinity values ranging from 500 to more than 9,000 

mg/1 dissolved solids, exists south and east of Luke 

Air Force Base and reflects the presence of the salt 

mass. It is in the same general area as that of anomalous topography (fig. 3) and fine-grained valley-fill 

sediments (fig. 4). It also includes the three wells 

that penetrated halite.

The dissolved-solids data should be viewed only 

as a qualitative indicator of the salt mass. Water 

sampled at each well represents a composite of 

waters from different depths and stratigraphic 

zones penetrated by the wells, and the wells themselves range greatly in depth (from 100 to more 

than 1,000 ft). After being backfilled or plugged to 

shallower depths, some of the deeper wells that had 

produced water containing large amounts of dissolved solids were found to produce less saline water 

(Stulik and Twenter, 1964, p. 26).

TRANSMISSIVITY OF THE VALLEY-FILL SEDIMENTS

Anderson (1968) presented a map of the transmissivity of the sediments in this region, and the 

relevant part of this map is reproduced here as 

figure 7. It shows a local area of low transmissivity 

existing southeast of Luke Air Force Base and extending northward along the course of the Agua 

Fria River. Comparison of this figure with figure 4 

indicates a rough correlation between areas of finegrained alluvium and areas of relatively low transmissivity, even though the grain-size data represent, 

at most, the upper 500 feet of the valley fill, whereas 

the transmissivities represent the upper 1,000 to 

1,200 feet.

Stulik and Twenter (1964) commented on the relationship between the area of low permeability (or 

transmissivity) east and south of Luke Air Force

Base and the high proportion of fine-grained materials in the same area. In our opinion, however, an 

additional factor may have influenced the distribution of transmissivities namely, the local reduction 

of pore volume in the valley-fill sediments, due to 

compaction caused by the buoyant rise of salt beneath 

the sediments and (or) by differential compaction 

over the crest of the salt. Such a reduction in porosity would serve further to reduce the permeability 

of the sediments. Alternatively, if the area of finegrained sediments represents upbowed, older valleyfill sediments, the reduced permeability may be a 

function of a greater degree of cementation.

Interestingly, in conjunction with this hypothesis 

of compaction-reduced transmissivity, a concentration of wells with a history of casing collapse and 

several open earth fissures occurs in the area (fig. 7). 

Although both phenomena, generally attributed to 

dewatering of the valley fill, are also observed north 

and northwest of Luke Air Force Base, their relative concentration in the area of the base suggests 

a localized concentration of stress giving rise to 

horizontal extension and vertical compression. Such 

a stress field is to be expected from a doming of the 

alluvium by the rising salt.

DEEP-WELL DATA

All the wells within a 7.5-mile radius of the discovery well with total depths of, or exceeding, 1,000 

feet are located in figure 8. Most of the wells that 

did not penetrate appreciable evaporites (shown as 

open circles in fig. 8) probably should be regarded 

only as fixing upper limits for the top of the salt 

body, for conceivably they may terminate only a few 

tens of feet above the body. At least one of them, 

however, sharply limits the horizontal extent of the 

uppermost part of the body. The well with a total 

depth of 1,055 feet, immediately adjacent to the discovery well (1), evidently was drilled just off the 

upper flank of the salt mass, for although it did not 

encounter halite, it did produce saline water, and 

dried cuttings of clastic sediments showed light salt 

incrustation. Similarly, the well with a total depth of 

1,605 feet, 2 miles southeast of the southeast corner 

of Luke Air Force Base, probably bottoms not far 

above the southeast-sloping southern flank of the 

body. These well data aid in further delineating the 

probable area of the upper part of the salt and provide a guide that was used in examining gravity 

profiles for apparent expressions of the salt.

Subtle details of the Bouguer gravity map (fig. 5), 

which, as noted earlier, can be interpreted as reflecting the presence of a salt massif, are here used 

in an attempt to define the approximate shape of the 

massif. Additional observations on topography
PHYSICAL SETTING AND DELINEATION OF THE SALT BODY 11

22'30" 112°07'30"

33°22'30

FIGURE 6. Ground-water salinity in western Salt River Valley. Contours show total dissolved solids, in milligrams per 

liter. Small dots are wells whose salinity values were used in preparation of the map. Crosshatching indicates area 

of brackish to saline water ranging erratically in salinity from 500 to more than 9,000 mg/1. Large dots with 

ticks show location of the three wells that penetrated halite. Solid pattern represents areas of exposed bedrock.

(fig. 3), variations in salinity of the ground water 

(fig. 6), and lithofacies variations in the alluvial 

sediments of the valley (fig. 4) all serve to delineate 

the upper part of the salt mass. Pertinent features 

of figures 3, 4, 6, and 7 roughly define the eastern 

and southern edges of the salt body. Further definition of the body is provided by two of the three 

wells that penetrated salt and by other relatively 

deep wells in the vicinity that did not. The general 

extent of the upper part of the salt body, as based 

on these data, is shown in figure 5 by a heavy dashed 

line. Note that well 3, where salt was not penetrated
12

112-30'

LUKE SALT BODY, CENTRAL ARIZONA

22'30" 15' 112°07'30"

33°37'30"

FIGURE 7. Transmissivity of valley-fill sediments of western Salt River Valley. (Modified from Anderson, 1968, pi. 2.) 

Contours indicate transmissivity, in thousands of gallons per day per foot, of the upper 1,000 to 1,200 feet of the 

section. Solid pattern represents areas of exposed bedrock. Note area of reduced transmissivity «50) south and east 

of Luke Air Force Base and concentration of collapsed wells (solid dots) and earth fissures (heavy lines) on the west 

side of the area of low transmissivity. Contour interval is 25,000 or 50,000 gallons per day per foot.
GRAVITY DATA 13

112*22'30" 112° 15'

33°37'30'

33°30'

U68

a1140

°1093

1000*,

2425oV. Peoria

1012

 '°58 ^Vso f\ 992 >2°°3 

/I? Co^ooo

1120./V v<- °

1605. I/) \y/' 1600

o!340 ^2350 ^1447

2000 

17200 ° 1000

iooo

Tolleson

0 5 MILES

I i i ii I

0 5 KILOMETERS

I i i i i I

EXPLANATION

^1447

Well which penetrated upper part of evaporite body

Number is depth, in feet, at which top was encountered

0 1490

Deep well which did not encounter appreciable 

evaporites

Number is total depth, in feet, of well

.1120

Well used as constraint in gravity modeling 

Number is total depth, in feet, of well

FIGURE 8. Distribution of deep wells within 7.5-mile radius 

of center of crest of the Luke salt body.

until a depth of 2,320 to 2,350 feet was reached, falls 

outside this line. Thus the dashed line defines only 

the upper 1,000 feet or so of the salt body.

GRAVITY DATA 

CALCULATION OF RESIDUAL GRAVITY

After the area! extent of the upper part of^the 

salt body had been roughly defined, it was feasible 

to attempt a separation of the observed Bouguer 

gravity field (fig. 5) into its individual components. 

Three sources were considered: (1) Density contrasts within or below the crystalline basement, 

(2) the density contrast at the alluvium-basement 

interface, producing an anomaly associated with the 

thick prism of alluvium in the western Salt River 

Valley, and (3) the density contrast between the 

alluvium and the enclosed salt.

The problem of separation is not amenable to an 

exact solution because of the ambiguity inherent in

the interpretation of potential field data. We do 

know, however, that the three gravitational 

sources cited above occur at successively shallower depths, in the order listed. Also, experience 

gained from gravity surveys elsewhere indicates that 

gravity fields associated with simple, symmetrical 

grabens can be closely approximated by polynomial 

surfaces of low degree.

Because the approximate location and plan of the 

upper part of the salt mass had been deduced from 

geologic and hydrogeologic information, we decided 

to attempt direct derivation of a residual, the source 

of which could be easily modeled. The principal difficulty in this effort was that the salt occurs within 

the area of closure of the anomaly associated with 

the prism of valley-fill sediments. Thus, we were 

seeking to define a closure (due to the salt) superimposed on another closure (due to the alluvial valley 

fill) of like algebraic sign. Although the approximate 

location of the first closure was known, its amplitude 

was not. In order to estimate the amplitude to a first 

approximation, we subjected the gravity data to two 

calculations. In the first, which was done to remove 

regional crustal or subcrustal effects, a gravity field 

with low curvature was derived by contouring gravity data for all those stations on bedrock in an area 

of 3,300 square miles. The data for this were taken 

from Peterson (1965). The resulting field was subtracted from the observed field, producing a first-step 

residual supposedly free of effects due to variations 

in density beneath the basement surface.

In the second calculation, the first-step residual, 

assumed to contain components associated with both 

the valley-fill sediments and the salt enclosed within 

them, was fitted with a second-degree polynomial 

surface. This surface was assumed to approximate 

the gravity field associated with the fill alone, and 

on this assumption rests the accuracy of the derived 

model. The algebraic difference between the fitted 

polynomial surface and the first-step residual yields 

a second-step residual theoretically reflecting the 

effect of the salt alone. This second-step residual is 

shown in figure 9A. The closure of the negative 

anomaly due to the salt is approximately 12 mgal. 

Surrounding it are areas of discrete positive anomalies with amplitudes of 6 to 8 mgal. The gradients 

of these positive anomalies are gentler than those of 

the low associated with the salt and are believed to 

be due either to broad topographic irregularities on 

the buried bedrock surface or to lithologic variations 

within the upper part of the basement. Maximum 

depth to the source of these positive anomalies, calculated by the method of Bott and Smith (1958), 

ranges from 10,000 to 12,000 feet.
14 LUKE SALT BODY, CENTRAL ARIZONA

112*22'30"

33°30'

0 5 MILES 

I I l l i I

0 5 KILOMETERS

I l I I l I

0 5 KILOMETERS

I l I l I I

-55

-60

-65 

V)

<-70 

g

^-75 

z -80

£-85

£-90 

0

-95

-100

-105

5 MILES

5 KILOMETERS

FIGURE 9. Residual Bouguer gravity associated with the 

Luke salt body. Contour interval 2 mgal. A, Computergenerated map of second-degree polynomial residual gravity. 

B, Profile A A' (see fig. 5 for location) shows observed 

Bouguer gravity field in relation to both the polynomial 

"regional"1 field, used in deriving map A, and the handsmoothed "regional"2 field, used in deriving map C. Shaded

areas represent, in part, the positive residual anomalies in 

A. P and P', points at which hand-smoothed regional profile approaches observed profile; 0 and 0', points of zero 

value on computer-generated polynomial residual field; X 

and X', breaks in slope. C, Residual-gravity map prepared 

from orthogonal grid of hand-smoothed profiles. Wells 1, 2, 

and 3 are those which penetrated the salt body.

^se of the term "regional" here refers to a field which consists of the 

gravitational effect of the valley fill, as opposed to the more normal usage 

of the term "regional" in referring to the effects of deep crustal and subcrustal sources only.

zUse of the term "regional" here refers to a field that consists of the 

gravitational effect of both the valley-fill sediments and the deep crustal and 

subcrustal sources.
INTERPRETATION OF RESIDUAL GRAVITY 15

The amplitudes of all the residual anomalies shown 

in figure 9A are in part a function of the method of 

fitting the polynomial surface. Skeels (1967) discussed and illustrated the deleterious effect of including with the data to be fitted by such a surface 

those points specifically related to the residual 

anomaly one is attempting to isolate. In the present 

instance, we had little alternative but to include 

them, because figure 5 shows that those parts of the 

gravity field outside the 84-mgal contour are too 

irregular to be approximated as a whole by a seconddegree surface. Because of this, we decided to restrict 

the area to be fitted to that part that we judged by 

inspection to be amenable to the fitting of a surface 

of low degree namely, that part within the rectangle 

shown (fig. 5). Unfortunately, much of this restricted area was suspected of including effects of 

the residual anomaly we wished to isolate. The rationale for proceeding in the face of this difficulty 

was that the results were to be regarded only as a 

first approximation. All the data within the rectangle, including those directly over the top of the 

salt, were incorporated in computing the polynomial 

surface. The effect was to give the resulting surface 

a slightly sharper curvature than it would have had 

otherwise and, as a result, to lead to a minimum 

value for the residual anomaly associated with the 

salt and a maximum value for positive anomalies 

bordering the residual low. For a simple, two-dimensional illustration of this effect, see figure 9J5 or 

Skeels (1967, figs. 1 and 2).

The computer-generated, second-step polynomial 

residual map (fig. 9A) was used as a guide in the 

creation of another residual map prepared from a 

grid of profiles smoothed by inspection. These profiles were drawn through the field data points shown 

in figure 5, and each profile was smoothed with reference to its counterpart profile of the polynomial 

"regional" surface, which had produced a minimum 

value for the closure of the residual anomaly related 

to the salt. The points at which the hand-smoothed 

"regional" profiles approached the observed profiles

(for example, P and P' in fig. 95) were made to lie 

outside the points of zero value (0 and 0') on the 

polynomial "regional" surface. Subtle breaks in slope

(for example, X and X') were observed on both 

flanks of most, but not all, profiles at the anticipated 

locations, and the "regional" curves were bridged 

smoothly across the observed profiles between these 

points. Pairs of "regional" values from intersecting 

profiles were then adjusted to bring them into mutual 

agreement. The final values were subtracted from 

the values of observed Bouguer gravity at each field 

station, and the results were contoured to produce 

figure 9C.

As anticipated, there is a gross similarity between 

the residual maps of figures 9A and 9C. However, 

figure 9C differs in the magnitude of its closure, the 

curvature of the residual surface, the location of the 

zero contour, and the absence of satellite positive 

anomalies. We regard the residual map (fig. 9C) prepared from the grid of profiles as the more accurate 

representation of the residual anomaly associated 

with the salt. Local irregularities are undoubtedly a 

function of the rather arbitrary method of drawing 

the smooth "regional" profiles.

INTERPRETATION OF RESIDUAL GRAVITY

The residual anomaly shown in figure 9C was used 

to derive a total of 14 models of the salt mass, of 

which about five or six were refined to the point of 

matching the residual field rather closely. Depth 

control for the top of the body was provided by the 

three wells that penetrated the salt and the seven 

that did not (fig. 8). We knew nothing of the depth, 

extent, or configuration of the base of the body, and 

in the process of modeling we made the simple 

assumption that the base was both horizontal and 

planar. Any discrepancies between the configuration 

of the actual body and the configurations of the derived gravity models which stem from the wrongness 

of this assumption are probably larger than those 

due to other sources, because of the relative insensitivity of the gravity method to density variations at 

a level about as deep as that of the probable base of 

the salt.

The residual-gravity map (fig. 9C) probably is in 

error in significant ways because of the rather arbitrary assumptions made in preparing it. Because the 

closure of the gravity anomaly due to the salt coincides in part with the closure due to the valley fill, 

as noted above, the residual map may contain elements that properly belong to the gravity field due 

to the clastic sedimentary prism of the western Salt 

River Valley. In particular, the northern and southeastern apices of the crude triangles defined by the 

residual gravity contour lines might reflect crossfault-controlled wedging out of the trough of alluvial 

sediments against a basement surface rising steeply 

both to the north and to the southeast from a central 

deep east of Luke Air Force Base. The "regional" 

map used in the preparation of the residual map 

(fig. 9C) reflects a nonfaulted gentle wedging out of 

the alluvial prism, but the ability to discriminate the 

effects of the two sources fades where the basement 

surface rises to levels comparable to those occupied 

by the salt.

Another source of error in our models relates to 

the occurrence of the anhydrite penetrated in drilling 

the exploration well. (See lithologic log in fig. 10.)
16 LUKE SALT BODY, CENTRAL ARIZONA

DENSITY, IN GRAMS PER 

CUBIC CENTIMETER

2.0 2.2 2.4 2.6 2.8

100

200

300

400

500

Sj 600 

D

700

800

900

1000

 

 __ __ IAIT yy | __ __ 

Siltstone

Sandstone

Limestone

Sandstone

Fine to coarse

elastics, consisting

predominantly of

dark-red and black

mottled grains

Anhydrite

Halite

i I i I i

FIGURE 10. Density and generalized lithology of upper 1,100 feet of deep well 

penetrating salt body (well 1 in figs. 2, 3, 

5, 6, 7, 9, and 12). Irregular curve represents five-point moving average of a 

gamma-gamma density log digitized at 

10-foot intervals. Smooth curve with 

dashed lower part is idealized depth-density function fitted to the data by inspection and employed in gravity modeling. 

Light dashed line labeled "WT" on lithologic log represents position of the water 

table at time of logging.

The anhydrite presents a problem in interpreting the 

residual gravity, for it is not continuous over the 

entire top of the structure and, because it was not 

penetrated in either well 2 or 3, its lateral extent is 

unknown. Therefore the anhydrite layer, which is 

denser than any of the other rocks in the area, was 

ignored in modeling.

In addition to the uncertainty posed by the anhydrite, we have few data bearing on the density 

of the alluvium and, therefore, on the density contrast between alluvium and salt, which is of the 

utmost importance in gravity modeling.

Each of the uncertainties cited above is sufficiently 

great that the final results should be regarded as 

only one of many possible schematic approximations 

of the shape and extent of the salt body. Certainly 

our models should not be regarded as a guide to 

further drilling, and they provide only a rough estimate of the total reserves of halite.

DENSITY DATA

A gamma-gamma density log of the exploration 

well (well 1) provides the only density control on 

alluvium for the whole of the western Salt River 

Valley. A five-point moving average of the upper 

1,100 feet of this log is shown in figure 10. With it 

is a description of lithologies penetrated, based on 

an examination of drill cuttings. The smooth curve 

shown with the log was fitted to the log by inspection 

and represents an idealized density-depth function 

for the alluvial part of the section. Such a curve is 

more amenable to use in gravity interpretation than 

the actual log, which displays pronounced, discontinuous variations. From a depth of 900 feet to the 

bottom of the well (at 4,503 ft), the gamma-gamma 

log indicates that the density varies only locally, 

over very short intervals, from an average value of 

2.10 g/cc (grams per cubic centimeter). The cuttings in this interval consist mostly of coarse, clear 

to slightly cloudy grains and cleavage fragments of 

halite, plus very minor amounts of silt and clay. A 

fragment of core from a new well drilled near the 

exploratory hole and taken from a depth of 3,425 

feet was made available to us in April 1971. This 

core fragment has a crystalline texture and is very 

coarse grained. Individual halite crystals are moderately clear and have brown inclusions of clay scattered throughout. The value of 2.10 g/cc for the salt 

interval is higher than the apparent bulk density of 

2.03 g/cc noted in other gamma-gamma density 

measurements of halite (Alger and Grain, 1966) ,3

3Although the specific gravity of halite is 2.16, gamma-gamma density 

logs for pure halite yield a bulk density of only 2.03 because the electron 

density is not quite proportional to the specific gravity for this mineral.
INTERPRETATION OF RESIDUAL GRAVITY 17

DENSITY, IN GRAMS PER CUBIC CENTIMETER 

1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6

DENSITY, IN GRAMS PER CUBIC CENTIMETER 

, 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6

500 -

1000

FIGURE 11. Alluvium density logs and idealized depth-density functions used in preparing gravity models of the Luke 

salt body.

A, Details of uppermost 1,000 feet of logs shown in B, 

plus additional data provided for comparison. 1 and 2, Borehole gravity-meter density logs from alluvium in Hot Creek 

Valley and Frenchman Flat, Nev., respectively (from Healey, 

1970); 3, gamma-gamma density log from Luke area; 

4, laboratory core density log from Safford Valley, Ariz.; 

A, idealized version of 1; B, idealized version of 2; C, idealized version of 3, displaced 0.06 g/cc to the right; and D, 

constant density used in several of the Luke models.

B, Curve 1 is Healey's (1970) gravity-meter density log 

from Hot Creek Valley, Nev., from the surface to a depth 

of 4,200 feet. Smooth curve A, fitted to it and extrapolated 

to 8,000 feet, was used as a lower bound for our density 

models and served as a basis for extrapolating curve C 

(our upper bound) to depths below 790 feet (the top of the 

evaporite column). Shaded area represents range of variation of density models used in modeling Luke salt body.

We do not know the cause for this discrepancy, but 

we suspect that it may be the contamination of the 

salt by included fine-grained clastic silicate minerals. 

The data for the alluvial part of the section are 

shown again but in more exaggerated fashion in 

figure HA (curve 3). To the right of curve 3 is a 

smooth curve (C) analogous to the curve fitted to 

the data in figure 10 but displaced to the right about 

0.06 g/cc. This curve represents a maximized density-depth function, and as such it served as a limiting envelope for the various density models we 

tested. The displacement of 0.06 g/cc was based on 

a comparison by Healey (1970, fig. 3) of gammagamma density log values with borehole gravitymeter values determined in the same well in Hot 

Creek Valley, Nev. The discrepancy between the two 

sets of values ranged from 0.01 to 0.09 g/cc for the 

upper 3,800 feet of the hole and averaged near 0.06. 

Below this depth the radioactivity log showed much

500 

1000 

1500 

2000 

3500 

4000 

2500 

3000 

BREAK

8000
18 LUKE SALT BODY, CENTRAL ARIZONA

greater variability, and comparisons were more difficult to make. Because we regard the borehole 

gravity-meter data as the more accurate, the discrepancy of 0.06 g/cc serves as a crude measure of the 

expectable error of the gamma-gamma data in a 

borehole in alluvium.

In figure 11B, smooth curve C is extrapolated to 

a depth of 8,000 feet. The extrapolation was guided 

by additional observations by Healey (1970), who, 

to our knowledge, is the only investigator who has 

measured in situ densities in alluvium to great 

depths. The Nevada base, based on subsurface gravity-meter observations, are the best in existence for 

estimating the range of densities and illustrating 

the systematic variations of density with depth in 

homogeneous alluvial sediments. Smooth curve A was 

fitted by eye to Healey's data.

The density of the alluvium over the Luke salt 

body (curve 3 in fig. 11A) is appreciably higher 

than that of the alluvium in Nevada (curves 1 and 2) 

for equivalent depths. In the depth range 300 to 600 

feet, however, it is remarkably similar to values observed in Pliocene (?) sediments in a similar geologic 

setting (Safford Valley) in southeastern Arizona 

(curve 4). The contrast with the alluvium in 

Nevada could be a function of age (Quaternary versus Pliocene) or of differences in grain density or 

in porosity, which varies widely with pronounced 

variations in the sorting of alluvium (Eaton and 

Watkins, 1970). No data on actual grain densities 

are available for either area (Nevada or central Arizona), but there is good reason to believe that, 

because of the buoyant rise of the upper part of the 

Luke salt massif, the porosity of the overlying alluvium has been reduced by compaction, and the 

density has thereby been increased. If this is the 

case, the density of alluvium surrounding the flanks 

of the salt body, at all depths, is probably less than 

that represented by curve C. Below 1,500 feet, it may 

also be less than that represented by curve D, an 

arbitrary averaged constant density employed in a 

few of our models. We used functions C and D for 

much of our modeling, although we also tested various densities falling within the shaded area between 

curves A and C. In so doing we maximized the density contrast between alluvium and salt, particularly 

in the upper part of the section, and thereby obtained 

purposefully conservative estimates of the vertical 

dimension of the salt body.

Note that the crossover depth, above which the 

density of the salt is greater than that of the surrounding rocks, appears to be unusually shallow in 

this area. It is about 950 feet for model A and less 

than 200 feet for model C.

MODELING METHOD

Fourteen three-dimensional gravity models were 

prepared for comparison with the residual gravity 

map of figure 9C, using density models C, D, A, and 

others within the shaded area of figure IIB. The 

method of Cordell and Henderson (1968) was used 

in deriving the models, and the method of Talwani 

and Ewing (1960) was used in testing them. All 

three wells that penetrated salt, plus the seven that 

did not (fig. 8), were used as constraints in preparing the models. Constant density contrasts used in 

the Cordell-Henderson method were replaced, in the 

Talwani-Ewing method, by the variable-density models of figure 11. The salt model was modified by hand 

after each run until a reasonably close fit between 

the residual gravity field and the modeled gravity 

field was achieved.

Typical of the final models is the one shown in 

figure 12. It is based on density model C (fig. 115) 

and, as such, probably represents a minimum depth 

(6,900 ft) for the base of the salt. The maximum 

depth produced by the many models tested was 9,000 

feet (using density model A). With the crest of the 

body at a depth of about 500 feet, the salt body has 

a vertical dimension somewhere in the range 6,400 

to 8,500 feet.

The small solid dots in figure 12A represent points 

at which the gravity field of the model was compared 

with the residual gravity field. For the 48 stations 

shown, the average discrepancy between the two 

values, ignoring the sign of the difference, is 0.3 

mgal. The fit is poorest beyond the edges of the body, 

where the average discrepancy is 0.7 mgal and where 

the calculated model values are consistently more 

negative than the residual values. Over the salt itself 

the average discrepancy, regardless of sign, is only 

0.2 mgal. Neither the goodness of fit over the salt 

nor the poorness of fit beyond the edges of its base 

constitutes a measure of the accuracy of the model, 

of course, because the residual anomaly on which the 

model is based may be in considerable error and 

because more than one model was derived which gave 

similar results in terms of matching the residual 

gravity field. The gross configuration of all these 

models was roughly the same, however, and all displayed a better fit over the salt than in the area 

adjacent to it.

The discrepancies observed at points off the structure have several possible explanations. It is well 

known, for example, that the gravitational effect of 

a finite slab of given thickness and density varies 

with depth. As the slab is placed at successively 

deeper levels in a postulated model, its gravitational 

effect at a point on the surface over the center of the
INTERPRETATION OF RESIDUAL GRAVITY 19

112-30' 22'30" 15' 112°07'30"

33°37'30"l

Peoria

33°30'h

Glendale

0 5 MILES

I I I I I I

0 5 KILOMETERS

I I I I I I

Tolleson

A'

12 FEET X 103 

I

METERS X 103

FIGURE 12. Limiting configurational model of the Luke salt 

body, based on the residual gravity field of figure 9C and 

alluvium density model C (fig. 115). This model has a 

minimum depth to the base of the salt; all other models 

tested had deeper bases and, therefore, greater vertical 

dimensions.

A, Map of the salt body. Depth contour lines are referred 

to a surface datum. Contour interval is 500 to 1,100 feet. 

The basal 1,100 feet of the body (5,800 or 6,900) has vertical 

flanks. Small solid dots are points at which discrete values 

of the gravity field of this model were calculated. Fraction

next to each dot provides a means of comparing values of 

observed residual gravity from figure 9C (numerator) with 

those of the modeled gravity field (denominator). Large 

solid dots with ticks are wells that penetrated halite. Small 

open circles are wells with collapsed casings. Heavy curved 

lines represent open earth fractures, as shown in figure 2. 

Solid pattern represents areas of exposed bedrock.

B, Cross sections A-A' and B-B' through the salt body. 

No vertical exaggeration. Numbers on vertical scale are 

altitudes relative to sea level, in thousands of feet.
20 LUKE SALT BODY, CENTRAL ARIZONA

slab becomes smaller, but at a point not far off the 

edge its effect increases slightly with an increase in 

depth, because the gravitational effect is proportional 

to the angle subtended by the slab. The practical 

result is that, as additional slabs are added to the 

base of a model to bring the calculated values of 

gravity over the model in line with the observed 

values, the effects at points just off the edge may be 

disproportionately large and lead to a poor fit in that 

area. This was suspected as a prime cause of the 

poor fit observed off the edges of models such as 

that in figure 12, but other models prepared to eliminate this effect by placing the base of the salt at 

shallower depths required what we regarded as unrealistically high values for the density contrast 

between salt and alluvium, in order to produce correct values at points above the salt. We prefer to 

think that the residual map itself is in error toward 

the edges of the anomaly, that the body may interfinger laterally with the enclosing sediments and 

therefore lack the sharp boundaries suggested by 

figure 12B, or, more likely, that the body tapers inward toward its base. No basis exists for choosing 

between these hypotheses short of independent geophysical measurements or deep drilling. If the flanks 

are serrated and taper downward, however, the configuration would be consistent with the facies interpretation described below and illustrated in figure 

13R

The gross shape of the model in figure 12 is unlike 

that of most deformed salt structures known to us. 

In plan, the basal part is crudely triangular. It is 

surmounted by a broad ridge trending in an arc 

southwest to north, the crest of which is marked by 

several domelike bulges which reach within 500 feet 

of the surface. At the southwest end of the body 

there is an overhang in the area of wells 2 and 3.

If one visually eliminates the southeast corner of 

the model (particularly that part of the body southeast of the 2,000-ft contour), the remainder approximates the core of an arcuate salt anticline. (For 

comparison, see Byerly and Joesting, 1959, figs. 19, 

20, and 21.) Thus one interpretation of the model is 

that it represents a large plastically deformed mass 

of salt that has undergone extensive flowage in response to tectonic deformation and (or) to the 

motive force stemming from the density contrast 

between the salt and the overlying alluvium. As such, 

the body would have deformed and shouldered aside 

a very large volume of valley-fill sediments. Unfortunately, the only subsurface structural data available for testing this hypothesis come from a single 

well (fig. 8, the 2,003-ft-deep well southwest of 

Peoria). There the sediments near the bottom of the

hole dip at 30°, but in an unknown direction. The 

question of lateral deformation resulting from doming of the salt is thus unanswerable. Nevertheless, 

in our preliminary interpretation (Eaton and others, 

1970) we referred to the body, in its entirety, as a 

halokinetic dome, and this is still the preferred interpretation. We were strongly impressed by the doming 

of the alluvium immediately over the crest of the 

body, by the localized development of open fissures 

at the surface, and by the presence of 90 feet of 

capping anhydrite at well 1. We envisioned, by 

analogy, an elongate structure somewhat akin to that 

of a Gulf Coast salt pillow, aided in its initial formation, perhaps, by northwest-trending compression.

We interpreted the cap as a residue of insoluble 

anhydrite which was originally included throughout 

the mother salt body as primary evaporite grains and 

which had accumulated at the crest of the dome in 

response to solution of the rising halite by undersaturated ground waters. As is well known, marine 

halite contains 1 to 3 percent insoluble minerals, 

most of which is anhydrite. The solution of large 

volumes of such halite leads to the accumulation of 

several tens to hundreds of feet of anhydrite caprock 

atop the typical Gulf Coast salt dome. The areal distribution of such caps on some domes is very uniform 

(Murray, 1961, figs. 5.24 and 5.98), but on other 

domes, such as the Luke massif, it is very irregular 

(Murray, 1961, figs. 5.8, 5.16, and 5.99). We believed, 

therefore, that the facts all pointed toward a large 

salt dome or pillow, somewhat irregular in plan.

Now, however, in the light of additional data, we 

believe that the salt body can be justifiably interpreted as an in situ evaporite facies of the valley-fill 

section. As such, it would represent a stratigraphically extensive, but geographically restricted, prism 

of evaporites interfingering laterally with the clastic 

valley fill. The flanks of the body, except where locally faulted, would constitute the loci, through 

time, of the margins of a restricted lagoon or saline 

lake which shrank fairly steadily as the halite accumulated, rather than quaquaversally domed, conformable contacts or structural boundaries stemming 

from diapirism. In the evaporite-prism interpretation, only the uppermost part of the body would have 

been involved in actual doming, though the dome 

thus formed would have been nourished by plastic 

flow from deeper levels in the body. The doming, as 

well as the open earth fractures and collapsed wells, 

would be a local phenomenon marking a restricted 

amount of deformation. The overhang at the southwest end of the body, viewed in the salt-pillow structural interpretation as the product of lateral flow or 

mushrooming, would represent a temporary south-
INTERPRETATION OF RESIDUAL GRAVITY 21

10,000

I

20,000 FEET

__I

2000 4000 6000 METERS

EXPLANATION

Salt Valley-fill alluvium Fine-grained valley-fill sediments

FIGURE 13. Alternate interpretations of the Luke salt body. A, Salt dome or pillow. B, In situ evaporite prism, only the 

top of which has been domed. Interpretation A is preferred. Cross-section locations shown in figure 12.

westward transgression of the saline lagoon or playa. 

Schematic models representing these alternate interpretations and based on cross sections A-A' and 

B-B' of figure 125 are shown in figure 13.

Evidence that supports the evaporite-prism interpretation follows:

1. Analysis of the insoluble residue in a core from 

a depth of 3,425 feet in a new well indicates that the 

total insoluble mineral content of the Luke halite is

less than one-half of 1 percent by weight. X-ray 

diffraction analysis reveals that about 95 percent of 

these insolubles consists of clay, quartz, feldspar, 

and calcite. The anhydrite content is, at the most, 

only 0.025 percent (R. J. Kite, oral commun., 1971). 

If this single sample is at all representative and 

we acknowledge that it may not be the anhydrite 

cap atop the Luke body probably is not the result 

of leaching of a large volume of rising halite. Pos-
22 LUKE SALT BODY, CENTRAL ARIZONA

sibly the cap represents a primary anhydrite deposit 

laid down at the close of halite deposition as a result 

of changes in the composition of the source waters.

2. In well 3, the depth interval from 2,318 to 2,784 

feet has been described as "rock salt with interbedded sandstone and conglomerate" (E. E. Komie, 

written commun., 1964). This description, though 

admittedly vague, suggests an interfingering of 

halite with clastic sediments, rather than a laterally 

intruding tongue of plastically flowing halite.

3. Detailed examination of the neutron porosity 

log and the compensated gamma-gamma density log 

for well 1 shows about 20 widely separated intervals, 

1 to 4 feet thick, where both the porosity and the 

density increase sharply within the halite section. 

These can be interpreted readily as interbedded 

clastic sediments.

4. In the Dead Sea area of the Middle East, about 

4,000 meters (13,000 ft) of salt (El Lisan) is reported to interfinger toward the margins of the rift 

valley with an equally thick sequence of coarse and 

fine elastics (Bentor, 1968, p. 143). This occurrence 

provides a prototype model for the stratigraphically 

extensive facies variation proposed here for the Luke 

massif.4

ORIGIN OF THE SALT

The origin of the Luke halite is problematic. 

Neither its age nor its specific mode of accumulation 

is known, and although we here present some geochemical data bearing on the salinity of the waters 

in which it was deposited, the interpretation of ultimate origin is closely tied to the problem of age, as 

explained below. Inferences based on regional geologic observations provide some loose guidelines as 

to age, but they hardly serve as sharply limiting 

constraints. What follows, therefore, is offered by 

way of defining the various geologic constraints that 

can be arrayed before any proposed hypothesis of 

origin. It is not yet possible to offer a theory of 

origin for the Luke salt body that accommodates all 

the known facts without additional data.

The problem of origin is particularly significant 

when viewed from the perspective of the volume of 

the salt, which we estimate to be between 15 and 30 

cubic miles. Such volumes are not known among 

continental evaporite deposits in the Western United 

States. At least one apparently comparable volume 

of alleged continental salt elsewhere in the world

 'A difference of opinion concerning the nature of the El Lisan salt body 

exists in the literature. Picard (1965, fig. 2, sec. 4) depicted it as a "salt 

horst," and Bender (1968, p. 162 and fig. 151b) described and illustrated 

arching and faulting of the rocks immediately overlying it, relating these 

phenomena to ascent of the salt. In cross section B of his geologic map of 

Jordan, however, Bender (1968) showed the El Lisan salt interfingering 

laterally with its enclosing rocks over an incomplete stratigraphic interval 

of 2,000 meters, much in the manner that Bentor (1968) described.

has been reported (Bentor, 1968, p. 155), but it 

appears to be exceptional. In most instances, volumes 

of halite on the order of tens of cubic miles represent 

marine accumulations. Thus the first step in determining the origin of the Luke salt was to establish 

whether it was of marine or continental origin. This 

was done by measuring the bromine content of the 

halite.

BROMIDE GEOCHEMISTRY

Bromide minerals, as is well known, are not found 

among the crystallization products of marine waters, 

even though such products have an appreciable bromine content (Valyashko, 1956; Holser, 1966; Raup, 

1966). Instead, the bromine occurs in solid solution, 

substituting for chlorine in the precipitating chloride 

minerals. The amount of bromine occurring in the 

various chlorides is a function of the amount of 

bromine in the original solution and the coefficients 

of solid solution of the individual chloride minerals. 

The abundance of bromine in halite is dependent on 

the concentration of bromine in the parent solution 

but is not affected by the presence or absence of 

other chlorides. As a result, the salinity of the brine 

from which the halite precipitated can be determined 

from the bromine content of the halite. Holser (1966, 

p. 268-270) surveyed the bromine content of a number of basal marine halite deposits of various geologic ages from around the world and found a 

general tendency for them to fall in the range 30 to 

50 ppm.

Theory suggests, and actual sampling confirms, 

that the bromine content of a salt bed is minimal at 

the base and that it increases upward, reflecting an 

increase in salinity of the brine during the period of 

precipitation. (See, for example, Raup, 1966, figs. 

3-6.) Thus the value of 30 ppm obtained by Holser 

in his survey of basal marine salts represents a 

threshold value for marine halite. Values of less than 

30 ppm bromine represent either nonmarine deposition of the halite or the leaching or dilution of bromine in halite after marine deposition, through the 

agent of nonmarine or brackish waters. Recrystallization in the dry state will not lower the initial 

bromine content according to Wardlaw (1970), and 

this hypothesis is borne out by the values (34 to 89 

ppm) measured by Holser (1966, p. 256) in two 

intensely deformed piercement domes of marine salt 

in the Gulf Coast region.

No cores from the drilling of the Luke massif were 

available to us, but extensive samples of cuttings 

were. Because Raup, Kite, and Groves (1970) have 

shown that bromine profiles determined from cuttings closely resemble those obtained from cores, we 

decided to determine the bromine content of the Luke 

halite from cuttings. From each of 30 samples taken
ORIGIN OF THE SALT 23

from the depth interval 985 to 4,495 feet in well 1, 

groups of grains were picked under a binocular 

microscope. The bromine content of these grains, as 

determined by X-ray fluorescence, ranged from less 

than 1 to 6 ppm and averaged 2 ppm. Thus, all 

values were well below the threshold value indicative 

of marine halite.

The log of the bromine content in well 1 is shown 

in figure 14 along with observed values for basal 

marine halites in the Western Hemisphere and highly' 

deformed marine salt of the Gulf Coast. It seems 

unlikely that the processes of resolution by fresh or 

brackish water and subsequent crystallization phenomena that would be required to produce these low 

values in marine salt would have acted on the 

Luke body to depths of 4,500 feet. We therefore conclude that the Luke halite is probably of nonmarine 

origin. The analysis of the insoluble minerals in the 

halite, described earlier, tends to corroborate this 

interpretation, because the insolubles content was 

much lower than that of typical marine halites, and 

the composition was different. The salt probably was 

deposited in a saline lake.

THE SOURCE PROBLEM

If the Luke salt body is of lacustrine origin, as 

just suggested, one is led to inquire as to the source 

of the sodium chloride, which had to be available in 

great abundance. In raising this question for continental accumulations in general, Stocklin (1968, p. 

417) argued that

cumulative salt thicknesses of the order of 100 m, (and 

more) may * * * form in closed continental basins, provided 

sources other than the open sea, e.g., salt from older formations, are available. [Italics ours]

Regarding this problem for the continental Virgin 

Valley salt deposits in the Muddy Creek Formation 

of southern Nevada, Mannion (1963, p. 174) postulated older halite deposits on the Colorado Plateaus 

as the source of the saline waters. It is tempting to 

cite here an analogous source, the marine evaporites 

in the Permian upper part of the Supai Formation 

in the Holbrook basin (Peirce and Gerrard, 1966), 

as a source for the sodium chloride waters that 

deposited the Luke halite. The location and extent of 

this Permian salt is shown in figure 15 as stippled 

area C. Near it is a large area of saline ground water 

(diagonally ruled area "a") which occurs in an overlying stratigraphic unit, the Permian Coconino Sandstone. The difficulty in postulating this region as a 

source for the waters which nourished the Luke deposit is that there is no drainage from the Holbrook 

basin to the southwest today and little likelihood that 

there was in the relevant part of the geologic past. 

The basin lies structurally below the Mogollon HighBROMINE IN HALITE, IN PARTS PER MILLION 

,0 20 40 60 80 100 120 140 160 180 200

1000

Range in bromine 

content of basal 

marine halites

2000

ppm

Range in bromine content of 

recrystallized marine halite, 

Grand Saline salt dome, Texas

3000

4000

4503"

Range in bromine content of 

recrystallized marine halite, 

Weeks Island salt dome, 

Louisiana

FIGURE 14. Bromine content of halite in the Luke salt 

body. On the left, 30 samples of Luke halite, collected 

from depths of 985 to 4,495 feet, show a range in concentration from less than 1 to 6 ppm. Shown for 

comparison are the ranges in bromine content of undeformed marine halites of a wide variety of geologic 

ages from the Western Hemisphere .(upper bar) and 

two severely deformed marine halites from the Gulf 

Coast of the United States (lower bars). Latter data 

are from Holser (1966).

lands to its south and southwest and probably did so 

in mid-Tertiary and later time. The paleogeographic 

maps of Cooley and Davidson (1963) and Cooley 

(1967, figs. 7 and 8) show an ancestral Mogollon
24 LUKE SALT BODY, CENTRAL ARIZONA

r

35°

EXPLANATION

Basins and valleys containing known 

halite deposits

Triangle indicates location of drill hole 

penetrating halite

Saline springs

Area of saline ground water 

b

Isolated well producing sodium 

chloride waters

50 

I

100 MILES 

_I

50

I

100 KILOMETERS

FIGURE 15. Areas of occurrence of known major halite deposits and sodium chloride waters in Arizona. (See table 

1 for full description, including sources of information for 

drill holes penetrating halite.) Halite, apparently of nonmarine origin and supposed Pliocene age, occurs in substantial quantity in Detrital Valley (A) and Hualpai Valley (B). 

Marine halite of Permian age occurs in the Holbrook basin 

(C) on the Colorado Plateaus. Nonmarine evaporites and 

subsurface brines are found in the Verde Valley (D), although the proportion of sodium chloride is relatively low. 

Other areas discussed in the text are Phoenix basin (E; 

outline is that of Cooley and Davidson, 1963, fig. 11), Safford Valley (F), and Tonto Basin (G). Salt springs occur 

at localities 1 through 5, and ground waters with notable 

sodium chloride content occur at localities "a" through 

"f." Area of the Mogollon Highlands is shown by the 

vertically ruled pattern.

Highlands forming a regional drainage divide between the Colorado Plateaus, where the Holbrook 

basin is located, and the Basin and Range province, 

from middle Tertiary (Oligocene) time onward. Still 

earlier in the Tertiary, before the Colorado Plateaus 

and Mogollon Highlands were elevated with respect 

to the Basin and Range province, the regional drainage was apparently northeastward across the relevant part of the region (Cooley, 1967, fig. 6). Thus 

if the Luke salt body is of Cenozoic age, it apparently 

could not have been derived, either directly or indirectly, from the Permian marine salt of the Holbrook basin.

Other sources of sodium chloride brines (saline 

springs 1 through 5 in fig. 15 and table 1) are present in scattered parts of the State, and these, or 

others like them, may have existed at the time of 

Luke salt deposition. Springs of this type, as shown 

in table 1, issue from such widely divergent rocks as 

Precambrian quartzite, Cambrian sandstone, and 

Cenozoic volcanics. The ultimate origin of the salt 

in these spring waters is not known. Two or three 

of these springs (3 and 4, and possibly 5, in fig. 15) 

are furnishing sodium chloride to the western Salt 

River Valley today.

Figure 6 indicates that ground water in the south-
ORIGIN OF THE SALT 25

TABLE 1. Occurrence of halite and selected sodium chloride brines in Arizona

Map 

symbol 

(fig. 15)

Location of

stratigraphic

unit

Source of 

information Remarks

Basins and valleys that contain known halite deposits

A Detrital Valley..

B Hualpai Valley..

C Holbrook basin..

D Verde Valley.....

. Pierce and Rich

(1962, p. 65) ; 

Peirce (1969, p. 420). 

..Pierce and Rich

(1962, p. 65) ; 

(i) Gillespie, Bentley, and Kam

(1966, p. 31-34).

..Peirce and Gerrard (1966)..........

..Twenter and Metzger (1963)............

Phoenix basin........................................... (ii) Present report..

Safford Valley..

(iii) E. E. Komie, U.S. 

Bureau of Reclamation 

(written commun., 1964). 

. (iv) M. W. Edington 

(oral commun., 1971).

Relatively pure recrystallized halite. Thickness in subsurface: 500-700 

ft. Probably occurs in the Pliocene Muddy Creek Formation (continental origin). Total area of occurrence not known.

Medium to very coarse grained halite. Some clay inclusions and minor 

silt, clay, and sand partings. Thickness of halite in subsurface exceeds 1,190 ft by an unknown amount. Thought to occur in the 

Pliocene Muddy Creek Formation (continental origin). Total area of 

occurrence not known.

...485 ft (maximum thickness) of halite in the Permian upper part of 

the Supai Formation (marine origin).

Total area of occurrence of halite, 2,300 sq mi.

.. Interbedded evaporites and fine clastic sediments in the Pliocene Verde 

Formation (continental origin). Evaporites include thenardite, mirabilite, halite, and glauberite. Aggregate thickness not known, but 

some beds consisting primarily of evaporites are as much as 70-90 

ft thick. Total area of occurrence: 75 sq mi.

...Luke salt body, confirmed by drilling; relatively pure halite from 880 

ft to total depth at 4,503 ft.

USER test hole (D-7-8) 25ccc. "Colorless, transparent, crystalline 

halite" from depth of 1,936.5 ft to total depth at 1,944 ft.

Test boring (D-7-26) 26aba. "Solid rock salt" from depth of 2,329 

to 2,339 ft, underlain by "hard clay" to total depth of 2,350 ft.

Springs that contain salt water

Several unnamed 

springs in Garnet 

Canyon, Phantom 

Canyon, and Monument 

Creek, all in Grand 

Canyon National Park.

Verde Hot Springs...............

Studevant (1926)....................................Sodium chloride brine issues from springs in the Lower and Middle

Cambrian Tapeats Sandstone. No data on salinity or temperature, 

but halite incrustations are deposited around spring orifices.

..Feth and Hem

(1963, tables 4 and 5).

3 Green Seep at Salt Banks.........

Orange Spring at Salt Banks..

4 White River Salt Springs........

5 Clifton Hot Springs..................

Springs issue from Cenozoic volcanic rocks. Total dissolved solids: 

3,100 ppm. Chloride: 545 ppm. (Also abundant bicarbonate and 

sulfate ions.) Discharge: ~ 10 gpm. Temperature: 100°-106°F.

do.......................................................Springs issue from Precambrian quartzite. Total dissolved solids:

28,400 ppm. Chloride: 15,900 ppm. Discharge: no data. Temperature:

70°-78°F. 

.do...................................................... Springs issue from Precambrian quartzite. Total dissolved solids: 37,300

ppm. Chloride: 20,800 ppm. Discharge: no data. Temperature: 70°-

78 °F. 

do....................................................... Springs issue from Cambrian sandstone and Precambrian quartzite. Total

dissolved solids: 8,450 ppm. Chloride: 4,420 ppm. Discharge: ~ 10 gpm.

Temperature: 83°F. 

.do.......................................................Springs issue from Cenozoic volcanic rocks. Total dissolved solids: 9,790

ppm. Chloride: 5,800 ppm. Discharge: 1,100 gpm. Temperature: 120°F.

Stratigraphic units in which brines have been reported from water wells

Coconino Sandstone

(Permian). Stippled area

shows geographic extent of

salty ground water. 

Valley-fill alluvium

(age unknown). 

Valley-fill alluvium

(lower Pleistocene).

Valley-fill alluvium 

(Quaternary).

Valley-fill alluvium 

(probably Pliocene).

Playa sediments 

(Quaternary).

Cooley, Harshbarger, Akers, and 

Hardt (1969, fig. 17 and 

table 8).

M. E. Cooley,

(written commun., 1971). 

M. E. Cooley and

E. S. Davidson,

(written commun., 1971). 

Hem (1950, p. 41-56,

148-167).

Peirce (1969, table 37) ;

E. S. Davidson,

(oral commun., 1971). 

Brown, Schumann, Kister, and

Johnson

(1963, p. 82).

Maximum total dissolved solids : 30,000 ppm. Maximum chloride : 10,100 

ppm. Lies stratigraphically above halite occurrence in Holbrook basin. 

(See above.)

Water from well (D-l-6) 27dda "has strong salty taste and develops 

white coating on drying" ; depth interval 1,590 to 1,940 ft.

Water from shallow wells. Total dissolved solids: 5,000 ppm. Chloride: 

2,500 ppm.

River water shows increase in sodium and chloride in this area. 

Water sample from depth of 1,250 ft. Total dissolved solids ("largely 

sodium and chloride") : 120,000 ppm.

Water sample from shallow auger hole. Total dissolved solids : 106,000 

ppm. Chloride : 40,500 ppm.

ern part of the western Salt River Valley, near and 

west of the confluence of the Salt and Gila Rivers, 

contains a relatively high proportion of dissolved 

solids. According to McDonald, Wolcott, and Hem 

(1947), waters of the Salt River have a moderately high dissolved-solids concentration and contain 

mostly sodium and chloride ions. The Gila River has 

about the same average concentration as the Salt 

River but contains proportionately more calcium and 

sulfate ions and fewer sodium and chloride ions.

The Salt River takes its name from salt springs in 

its upper reaches (Granger, 1960). Feth and Hem

(1963), who sampled the springs at Salt Banks (site 

3 in fig. 15 and table 1) commented (p. 37-40) :

Salty water emerges from quartzite of Precambrian age 

along the Salt River, notably at the junction of the Black and 

White Rivers and at the Salt Banks. The water from springs 

at the Salt Banks has a concentration of dissolved solids approximating that of sea water. With the exception of chloride, 

however, the proportions in which the constituents appear 

in the spring water are very different from those of sea water. 

Sodium and chloride greatly exceed other components of the 

Salt Banks spring inflow, giving the flow practically the 

character of a solution of common salt * * *. The strata from 

which these saline springs issue are deeply buried in the area
26 LUKE SALT BODY, CENTRAL ARIZONA

north of the Salt River Canyon * * * and * * * it seems likely, 

therefore, that the saline waters * * * may derive much of 

their dissolved matter from rocks other than the ones from 

which the springs flow.

Another saline spring (site 4 in fig. 15) occurs 

approximately 25 miles upstream on the Salt River.

Similarly, the Gila and Verde Rivers, which also 

enter the western Salt River Valley, both have saline 

springs in their drainage areas (sites 5 and 2, respectively, in fig. 15), and the Gila flows through a 

valley (F) containing salty ground water (Iocs, c, d, 

and e) and at least one locality of known subsurface 

salt (iv), indicating earlier deposition of halite. 

Aggregate discharge and salinity data (McDonald 

and others, 1947) for these three rivers (Salt, Gila, 

and Verde) indicate that under present-day conditions a total volume of salt equal to that of the Luke 

salt body is carried into (and through) the western 

Salt River Valley in a period of about 20,000 years. 

If transport by the three rivers was the mode of 

origin of the Luke halite, however, it would have 

been necessary for the basin to have been closed to 

through-flow of the Salt River and its tributaries, 

and an additional, but unknown, period of time 

would have been required for evaporation. In addition, the load of clastic sediments that these rivers 

would have carried in traction and suspension would 

have to be reconciled with the total volume of clastic 

sedimentary rocks filling the balance of the western 

Salt River Valley in order to adequately test the 

hypothesis.

Attractive as this hypothesis may seem in attempting to explain the origin of the Luke salt deposit, it 

conflicts with serious obstacles in the form of the 

following regional geologic facts given by Cooley 

(1968, p. 75-77) : The main physiographic features 

and the ancestral stages of the present drainage patterns in southeastern Arizona were developed mainly 

in Pliocene and Quaternary time. In early to middle 

Pliocene time, drainage was impounded and basins 

were internally drained. The Salt and Verde Rivers 

did not enter the Phoenix basin until late Pliocene to 

early Quaternary time, and the Gila River, until early 

post-Blancan time.

These facts are depicted in the paleogeographic 

maps by Cooley (1967, figs. 7, 8), which show 

regional drainage divides that would have prevented 

inflows to the Phoenix basin of surface waters from 

springs 2 through 5 (fig. 15) from Oligocene time 

until late Pliocene or later time. Thus if the saline 

springs described in table 1 were the sources of the 

Luke salt deposit, the salt would have to be of late 

Pliocene age or younger. We believe it to be older 

than this, in spite of the fact that the apparently

analogous halite in Virgin, Detrital, and Hualpai 

Valleys is, or is reputed to be, of Pliocene age.

The Pliocene and Pleistocene deposits of the local 

region, as noted earlier, are generally less than 2,000 

feet thick in the deeper parts of the basins, and some 

are only 1,000 to 1,200 feet thick. In contrast, the 

base of the Luke salt body is believed to be at a depth 

greater than 4,500 feet, and the gravity data suggest 

it may be as deep as 6,900 to 9,000 feet. Thus the base 

of the salt would appear to be stratigraphically well 

below the base of the Pliocene section. If the salt is 

middle Tertiary in age (Oligocene or Miocene), or 

still older (for example, Mesozoic) and no data at 

present preclude this possibility the source or 

sources for the sodium chloride remain a mystery. 

One can only postulate a source in the Paleozoic or 

Mesozoic rocks of the ancestral Mogollon Highlands, 

an admittedly ad hoc hypothesis. Brine occurrences 

such as that at locality f (fig. 15, table 1) indicate 

that highly saline waters can accumulate locally in 

internally drained basins far from any source of 

abundant sodium chloride, but it is extremely doubtful that one could account for the vast volume of the 

Luke salt deposit in this way.

If the Luke halite is, after all, contemporaneous 

with the halite in Virgin, Detrital, and Hualpai Valleys, either the western Salt River Valley is regionally anomalous in having an enormously thick (6,900 

to 9,000 ft) Pliocene-Pleistocene section or the halite 

in those three valleys is not of Pliocene age, as previously thought. Interestingly, some of the observational data for the Virgin Valley deposit seem open 

to an interpretation of greater age for the salt. In his 

description of the Muddy Creek section, Longwell 

(1936, p. 1423) noted that the "salt deposits are low 

in the section, and are brought to view only where 

they have been thickened and have domed the overlying beds * * *"; again (p. 1469) "* * * As the salt 

is seen only where it has been forced up and presumably thickened, its normal horizon within the Muddy 

Creek formation is not known."

Mannion (1963), who studied these deposits in 

greater detail later, noted that the salt exposures 

along the Virgin River are associated with severe 

structural deformation in the Muddy Creek Formation. He also commented (p. 173) that "near Echo 

Bay * * * a rather impure, thin salt section rests directly on anhydrite, sandstone, and volcanic rocks 

which may not belong to the Muddy Creek formation." Thus the halite associated with the Muddy 

Creek Formation may have originated in older rocks 

and achieved its present position within the formation by diapirism and doming. If so, then it, too, 

could be of middle Tertiary or older age.
REFERENCES 27

The occurrence of Cenozoic (or older) halite in 

Virgin, Detrital, Hualpai, Verde, and western Salt 

River Valleys suggests that other basins bordering 

the Colorado Plateaus in Arizona may enclose similar large bodies of salt. Safford Valley, with its several known occurrences of highly salty ground water 

and one confirmed bed of halite (fig. 15, table 1), and 

Tonto Basin, in which the Salt River was impounded 

for a time, both appear to be likely possibilities.

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28 LUKE SALT BODY, CENTRAL ARIZONA

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U. S. GOVERNMENT PRINTING OFFICE : 1972 O - 468- 306