Gravity Survey of the Western Mojave Desert California

By DON R. MABEY

GEOPHYSICAL FIELD INVESTIGATIONS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 316-D

A study of the relation of the gravity anomalies to the geology with special reference to the distribution and thickness of the Cenozoic rocks

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1960 UNITED STATES DEPARTMENT OF THE INTERIOR FRED A. SEATON, Secretary

GEOLOGICAL SURVEY Thomas B. Nolan, Director

For sale by^the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. CONTENTS

Page Abstract.______51 Gravity anomalies.______5*? Introduction ______51 Garlock fault zone.______Location and cultural features.______51 Randsburg-Harper Lake area__-. Purpose of geophysical investigations. 52 Boron-Kramer Junction area___. 64 Acknowledgments______64: 53 Barstow-Cajon Pass area. ______6" Physical and geologic features.______53 Antelope Valley area______Physiography and drainage.______53 Significance of the gravity anomalies. 6° Geology ______53 Geologic significance______6^ Gravity survey.______55 Economic significance____._-__. 7? Fieldwork and reduction of data-____ 55 Conclusions. ______71 Bouguer anomaly map.______56 Selected references.______71 Problems of interpretation.______56 Index______7?

ILLUSTRATIONS

[Plates are in pocket] PLATE 10. Bouguer anomaly and generalized geologic map of the western Mojave Desert. 11. Bouguer anomaly and generalized geologic map of the area in the vicinity of the Kramer borate district. 12. Map showing major gravity trends, faults, and inferred faults in the western Mojave Desert. Pag-? FIQUKE 22. Index map of southern California showing area of this report in relation to the Mojave Desert and other physiographic features______-______-___-_-_---_------_-_-__-____ 52 23. Density range of the major rock types in the western Mojave Desert______58 24. Graph showing relation between the density contrast and the thickness of a layer of infinite horizontal extent re­ quired to produce a 1-milligal anomaly__-______-___-_-______-_-_--_--__- 59 25. Profile A-A' across gravity anomaly southeast of the Garlock fault northeast of Tehachapi Pass------60 26. Profile B-B' across Cantil Valley.-..______.-__ 62 27. Profile C-C" across gravity anomaly south of the Rand Mountains____--_-______-----_--____-----_-___ 63 28. Profile D-D' across gravity anomaly west of Iron Mountain______-_-______-__-_---__-__----_-__ 65 29. Profile E-E' across gravity anomaly south of the Kramer Hills_-----_--__----_--_---_-_------_-_- 66 3Q. Profile F-F' across gravity anomaly south of Rosamond Lake___------_---_---_------_------_------67 31. Profile G-G' across gravity anomaly in western Antelope Valley.______._____._.______-__-____-_--__-_--_- 68 in

GEOPHYSICAL FIELD INVESTIGATIONS

GRAVITY SURVEY OF THE WESTERN MOJAVE DESERT, CALIFORNIA

By DON R. MABEY

ABSTRACT Information inferred from the gravity data relative to ib«. The western Mojave Desert lies in the southwest corner of distribution of the Cenozoic deposits and the structural fer- the Basin and Range province between the San Andreas and tures effecting these deposits is an aid to the development cf Garlock fault zones. The region is characterized by low hills the ground-water resources of the region and in exploration exhibiting no pronounced trends and by closed alluvium-cov­ for saline deposits in the Cenozoic section. This same infor­ ered basins. It is underlain by a pre-Tertiary basement com­ mation will be helpful in evaluating the possibility of whether plex composed of plutonic igneous rocks enclosing pendants of important petroleum deposits may occur in the western Mojave metamorphic rocks. The basement complex is overlain by Desert. folded, faulted, and eroded remnants of sedimentary, pyro- INTRODUCTION clastic, and volcanic rocks of Tertiary age which were depos­ LOCATION AND CULTURAL FEATURES ited in local basins. Alluvial sediments of Quaternary age cover nearly all the valley areas. The western Mojave Desert is in southern California The density contrast that exists in the western Mojave Des­ (fig. 22) in parts of Los Angeles, Kern, and San ert between the Cenozoic rocks and pre-Cenozoic rocks pro­ Bernardino Counties. It lies in a westward-pointing duces gravity anomalies that can be readily defined by gravity surveys. Gravity anomalies resulting from density variations wedge formed by the junction of the San Andreas an-i within the pre-Cenozoic basement complex are also evident. Garlock fault zones. The region is bounded on the To define the major gravity anomalies, a gravity survey con­ northwest and north by the Tehachapi Mountains, the sisting of 1,900 gravity stations was conducted over the west­ Sierra Nevada, and the El Paso Mountains and on ern Mojave Desert. The data are reduced to the complete the south and southwest by the Transverse Kange^. Bouguer anomaly and presented on a contour map along with the generalized geology. The eastern boundary is not clearly defined but for The gravity anomaly map indicates the areas in which the this study is approximately longitude 117° west, which basement complex is overlain by large thicknesses of Cenozoic lies just east of Barstow. East of this line the mour- deposits and the order of magnitude of the depth to the base­ tain ranges become more prominent. ment complex. Gravity anomalies are associated with several The principal towns are Lancaster, Palmdale, Mo­ major faults. Along profiles across seven major gravity lows, two-dimensional theoretical analyses are presented to show the jave, Barstow, and Victorville. Several major higK distribution of Cenozoic deposits that could produce the meas­ ways cross the region, including U.S. Highways 66, ured anomaly. Lateral density variations within the Cenozoic 466, 91, 395, and 6. The Atchison, Topeka and Santa deposits complicate the interpretation of the anomalies. Fe Kailway, the Union Pacific Kailroad, and tl?«i Just southeast of the Garlock fault zone the gravity anom­ Southern Pacific Lines serve the region. The indus­ alies trend generally parallel to the fault zone. Two major areas of subsidences are indicated along the Garlock fault zone tries include agriculture, manufacturing, mining, an<3 northeast of Tehachapi Pass. One of the subsidences, in Can- activities associated with the several defense depar*- til Valley, is estimated to contain about 2 miles of Cenozoic ment installations. Most of the agricultural develop­ rocks. In the western part of Antelope Valley, near the junc­ ments are dependent upon irrigation. As in most arH tion of the Garlock and San Andreas fault zones, there is a regions, water is a critical item. The limits to which major east-trending gravity low. The gravity contours in the region of the San Andreas fault zone trend generally parallel the region can be developed are dependent upon tH to the fault zone, but no important local anomalies associated availability of water for industrial, agricultural, and with the fault zone were recognized. Southeast of Rosamond household uses. Lake there is a major gravity low trending east-northeast. An Annual precipitation is less than 6 inches over mont extensive irregularly shaped gravity low occurs north of Cajon Pass. In the central part of the surveyed area there are sev­ of the Mojave Desert. Near the bordering mountair^ eral anomalies of smaller areal extent and varying trends. the precipitation increases, and areas in the San Ber- 51 52 GEOPHYSICAL FIELD INVESTIGATIONS 118C

INDEX MAP OF CALIFORNIA

INDIAN WELLS VALLEY

'//////s Region covered by

34°- O LOS ANGELES

COLORADO \

FIGBKB 22. Index map of southern California showing area of this report in relation to the Mojave Desert and other phys'ographic features.

nardino Mountains receive more than 40 inches an­ however, the current production is small. The tung­ nually. The summers are hot and daytime tempera­ sten deposits at Atolia, now largely inactive, have been tures of more than 100° are common. Minimum winter a major source of tungsten, temperatures are as low as 10°. Important deposits of borates, tungsten, limestone, PURPOSE OF GEOPHYSICAL INVESTIGATIONS gold and silver have been developed in the western The need for some method of obtaining subsurface Mojave Desert. At present the borate deposit at Boron information on the Cenozoic geology is very important is the leading source of borate minerals in the world. in the western Mojave Desert because an extensive Limestone produced from pre-Tertiary deposits is used cover of Quaternary sediments obscures much of the in the manufacture of cement, as an agricultural min­ rather complex Tertiary geology. In 1953 the U.S. eral, and as a metallurgical flux. Gold and silver have Geological Survey tested several methods of geo­ been mined extensively in the past in the R-osamond- physical exploration in the desert areas of southern Mojave area and in the Randsburg-Red Mountain area; California in an effort to determine which of the GRAVITY SURVEY OF THE WESTERN MOJAVE DESERT, CALIF. various methods were useful in studying the geologic trast to the areas to the north, west, and south. The^e problems of the region. Seismic reflection and refrac­ is no apparent pattern to the trend of the isolated hills tion, ground and airborne magnetic, and gravity meth­ that rise above the gently eastward-sloping plain. In ods were tested under a variety of conditions. The contrast, the Sierra Nevada to the northwest, the data collected during these tests indicate that the Transverse Eanges to the south, and the part of the seismic methods are generally useful in the areas of Basin and Kange province north of the Garlock fault the present playas and that both reflection and refrac­ show strong relief and well-defined trends. tion techniques yield information on the thickness, The western Mojave Desert is for the most part an structure, and stratigraphy of the basin fill. Fault undrained alluvium-covered plain that is separated into traces were located in areas removed from the playas valleys or closed basins by isolated hills, groups of by seismic-refraction techniques, but the general hills, and some low mountains. The bottoms of the heterogeneous nature of the Cenozoic fill and adverse closed basins have playas which contain water only near-surface conditions severely limit the usefulness of after occasional heavy rains. The only major surface seismic exploration. The magnetic anomalies asso­ drainage channel is the Mojave Kiver, which starts in ciated with Tertiary volcanic rocks occasionally yielded the San Bernardino Mountains and flows northward, information relative to the Cenozoic fill, and some mostly underground, thence eastward through the cen­ anomalies can be interpreted in terms of the depth of tral Mojave Desert to Soda Lake. the basement surface and intrabasement compositional The lowest point in the surveyed region is abont variations. The density contrast between the Cenozoic 1,900 feet above sea level at Koehn Lake in Cantil sediments and the pre-Cenozoic basement rocks pro­ Valley. The relief is low to moderate in most of the duces gravity anomalies that are useful in studying the region, and altitudes range between 1,900 to 4,000 feet. Cenozoic geology. Some regional structural trends There are several hills within the desert area higher are also indicated by the gravity anomalies. than 4,000 feet; the highest is Ked Mountain at an As a result of these tests the regional gravity survey, altitude of 5,220 feet. Mount San Antonio in the San which is the subject of the present report, was con­ Gabriel Mountains to the south rises to an altitude of ducted over the western Mojave Desert during two 10,080 feet, and Double Mountain in the Tehachapi 4-month winter field seasons in 1953, 1954, and 1955. Mountains to the northwest rises to an altitude of The purpose of the survey was to apply the technique 7,998 feet. of gravity exploration to obtain subsurface informa­ GEOLOGY tion in a study of the geology of the western Mojave The western Mojave Desert is underlain by a Desert. The gravity study was coordinated with other Tertiary crystalline basement complex composed largely geologic investigations being conducted by the U.S. of plutonic igneous rocks enclosing pendants of meta- Geological Survey in the Mojave Desert. sedimentary and metavolcanic rocks. The basement complex is in places overlain unconformably by non- ACKNOWLEDGMENTS marine sedimentary, pyroclastic, and volcanic rocks of L. C. Pakiser, Jr., U.S. Geological Survey, was Tertiary age, mostly deposited in local basins. T" rain corrections for the remaining stations were inter­ (Oliver, 1956, p. 1724) and its significance will not be polated. The terrain corrections thus determined discussed here. In the San Bernardino Mountains the through zone O (about 100 miles radius) are believed highest anomaly value observed was about -104 milli­ to be correct to within 10 percent of the total terrain gals; however, not enough data are available in this effect or 0.2 milligal, whichever is larger. area to confirm the existence of an aromalous gravity variation. BOUGUEB ANOMALY MAP The Sierra Pelona and Liebre Mountains constitute To illustrate the gravity anomalies, 1,000 milligals a structurally high mass of basement rock. The Sierra was added to the complete Bouguer anomaly values to Pelona is made up almost entirely of Pelona schist make all the values positive. Such a presentation fa­ (Precambrian?) (Bailey and Jahns, 1954, p. 99). The cilitates the study of the local anomalies while the gravity high that the available data indicate is develop­ absolute complete Bouguer anomaly values are still ing over the schist dominates a considerable area on apparent. The Bouguer anomaly map (pi. 10) shows the gravity-anomaly map of the Palmdale-Fairmont the anomaly values contoured at a 2-milligal interval region of the Mojave block. An explanation of this superimposed upon a generalized geologic map. The gravity anomaly will not be attempted in this report geology on the map is intended to show only the because the anomaly lies largely outiide the Mojave approximate location and extent of major outcrops and Desert and is for the most part a bro^d feature prob­ structural features. ably associated with thinning of the earth's crust to­ In the western Mojave Desert gravity lows relative ward the Pacific Ocean. to adjoining areas were observed in all places where important thicknesses of Cenozoic fill are known to PROBLEMS OE INTEBPRETATION exist. The Bouguer anomaly map shows the location Areas of great geologic interest exist where informa­ and relative magnitude of the thickness of the major tion inferred from the gravity data is lacking or is Cenozoic deposits. Several important structural fea­ inconclusive. Using gravity data alore, it is generally tures are indicated on the anomaly map. Several not possible to differentiate between different types of gravity anomalies of large areal extent reflect intra- Cenozoic rocks. If a near-ideal situation existed it basement features. would be possible to determine the approximate density Along the San Andreas fault zone the gravity con­ of the Cenozoic rocks and thus infer something about tours trend generally parallel to the fault zone. The the rock type; however, it is not usually possible to do trends appear to be in part related to extensive intra- this. basement anomalies not defined by the data collected Structures which do not affect the relative vertical in the western Mojave Desert. The Garlock fault zone position of materials of appreciable different densities northeast of Tehachapi Pass has pronounced gravity will not produce gravity anomalies. Thus, structures expression resulting from depressions that have de­ affecting only intrabasement rocks may not be indicated veloped along and within the fault zone. For several by the gravity data. Likewise, features within the miles to the south of the Garlock fault the major Cenozoic section such as saline deposits and water- gravity features trend generally parallel to the Garlock producing strata will not generally be directly detected fault. In areas removed from the San Andreas and by gravity observation. GRAVITY SURVEY OF THE WESTERN MOJAVE DESERT, CALIF. Although the extent of important accumulations of that the intrabasement anomaly is much smaller in Cenozoic rocks can be inferred from the gravity data, amplitude than the anomaly produced by the density it is not possible to define the limits of the basins in contrast between the basement complex and the Ceno­ which the rocks were originally deposited. Postdep- zoic rocks, and for interpretation the total anomaly is ositional deformation may disrupt the continuity of attributed to the Cenozoic rocks. the deposit so that what was originally a continuous The densities of samples of the major rock typ-js basin may now appear as two or more separate occur­ found in the region were determined. For sever0! rences of Cenozoic rocks. reasons these densities are significant only as an in­ Using gravity data alone, it is not possible to de­ dication of the general range in density of the different termine a unique solution for the geologic feature rock types. Over most of the western Mojave Desert producing a gravity anomaly. However, when as­ the basement rock is probably weathered to consider­ sumptions can be made concerning the density contrast able depths, and, while care was taken to collect and (or) one or more boundaries between the dis­ samples from the less weathered outcrops, it is not tributing body and the surrounding material, it is known how closely the density determinations made possible to infer from the gravity data reasonable on samples collected from these outcrops indicate tH values for the unknown dimensions. Before a gravity density of the rocks at depths of more than a few anomaly associated with a single geologic feature can hundred feet. Because of the nature of the sampling be satisfactorily analyzed, it is necessary to isolate the technique, it is not possible to obtain accurate data on anomaly from other variations in the earth's gravity the density of the Tertiary rock. Only the more re­ field. In reducing the data an effort is made to elim­ sistant, and presumably the more dense, Tertiary units inate the effects of variations in the latitude, and occur as unweathered outcrops. The densities deter­ elevation between observation points and for the topog­ mined for samples of these outcrops indicate a density raphy surrounding the points. It is also necessary to higher than the aggregate density of the Tertiary isolate the anomaly produced by the geologic feature section. A few density determinations were made on being considered from other anomalies resulting from drill cores of the less resistant Tertiary units, but it other density variations in the materials of the earth. is not known how closely the observed densities in­ Anomalies of markedly different areal extent can be dicate the density of the undisturbed rock at depth. easily separated, but, if anomalies of similar or un­ Even if a good approximate determination of the ag­ known areal extent overlap, it becomes difficult or gregate density of the Tertiary section in one area of impossible to separate them. Tertiary deposition could be determined, this density Most of the gravity anomalies considered in this determination could not be applied with any degree report can be classified into two types. One type is of of certainty to an unknown area. generally broader areal extent and is produced by Figure 23 illustrates the density range of four major lateral density variations within the pre-Cenozoic base­ rock types indicated by samples collected in the Mo­ ment rock. The second type is produced by the density jave Desert region. The estimated average density contrast between the Cenozoic deposits and the pre- indicated is based on an inspection of the density data Cenozoic rocks. Quantitative calculations will be made and is not the result of any statistical analysis. Tl Q, on only the latter type. These anomalies, because they density determinations were made on samples selected are smaller in areal extent, can usually be separated to indicate the range of the density variations and from the broader major intrabasement anomalies. were not random samples; therefore, the results of an y There is an indication that minor intrabasement statistical analysis would be misleading. anomalies are associated with the structurally low areas The densities of the pre-Tertiary rocks range from in which the Cenozoic rocks occur. The gravity 2.57 to 3.02 g per cm3 ; however, a majority of the pre- gradients over outcrops of the basement complex ad­ Tertiary rocks in the western Mojave Desert are in trs jacent to some of the important accumulations of density range of 2.65 to 2.70 g per cm3. The density Cenozoic rocks are increasing away from the areas of variations within the pre-Tertiary basement are im­ Cenozoic rocks slightly more steeply than can be ac­ portant, but they would not prohibit making approxi­ counted for by the presence of the adjacent Cenozoic mate quantitative interpretations of the gravity anoma­ rocks. Thus, in some areas there seems to be an intra­ lies produced by the density contrast between the pre- basement anomaly with approximately the same areal Tertiary and the Cenozoic rocks. The Tertiary rocks, extent as the anomaly produced by the accumulation and probably the Quaternary deposits to a lesser extent, of Cenozoic rocks. As it is impossible to separate are much more evenly distributed throughout the rang*? the gravity effects of the two influences, it is assumed indicated in figure 23. The densities of the Cenozoic 58 GEOPHYSICAL FIELD INVESTIGATIONS

cr Sedimentary rocks

Volcanic rocks

a: Plutonic rocks ocJ K-UJ UJ Metamorphic rocks o: a.

1.5 2.0 2.5 Density, in grams per cubic centimeter FIGURE 23. Density range of the major rock types In the western Mojave Desert. sedimentary rocks are determined by the density of the and vertical density distribution within the basin fill. mineral matter making up the rock and to a very im­ The problems that are imposed by density variations portant extent by the amount of induration of the similar to those described can make it impossible to rock. The amount of sorting is also important, at make an accurate quantitative interpretation of the least for the poorly indurated near-surface rocks. gravity data. The density variations can be particu­ Large density variations occur between basins of larly troublesome in interpreting the gravity data in Cenozoic deposits and even within an individual basin. the vicinity of a frontal fault that w$s active during The data indicate that, in the area of this survey, if the deposition of the basin fill. If the faulting fol­ no direct evidence is available an assumption that a lowed earlier deposition over a more extensive area, density contrast of 0.4 g per cm3 exists between the the lateral density variations may not be an important Cenozoic and pre-Cenozoic rocks is reasonable. consideration. Each individual gravity anomaly must The limited evidence available indicates that in most be considered separately. All availaHe geologic in­ places the density of the fill within a single basin of formation must be considered in determining how much deposition may vary over wide limits. For example, significance should be placed on the various character­ the coarse poorly sorted material that would be con­ istics of the gravity anomaly and how much and what centrated along the margins of a basin as fanglomerates type of quantitative interpretation is justified. is more dense than the finer lacustrine sediments de­ Once an anomaly has been isolated, the interpreta­ posited in the center or low part of the basin. Also tion procedure involves determining a geometric form an increase in density resulting from compaction at for the disturbing body that is geologically reasonable, depth occurs. Thus, a gravity anomaly can be pro­ and then, by varying the dimensions an^I density of the duced by density variations entirely within the basin body, obtaining a theoretical anomaly that matches fill. Although in some places a simple increase in the form and amplitude of the observed anomaly. In density with depth can be handled quantitatively, with considering the negative anomalies associated with the amount of subsurface geologic information avail­ Cenozoic fill in the Mojave Desert, it can be assumed able in the western Mojave Desert the gravity anoma­ tha,t the material producing the anoirraly extends to lies produced by the lateral density variations are the surface. Thus, one boundary of the disturbing difficult, if not impossible, to separate from the anoma­ body is always known. The density contrast may vary lies produced by the density contrast between the Ceno­ within a limited range, but any solution that requires zoic fill and the basement rock. The interpretation of a density contrast outside this range can be rejected the gravity anomalies in the western Mojave Desert has as being geologically unreasonable. Calculation of the been further complicated by the migration and de­ gravity effect of three-dimensional forms is often formation of the areas of deposition throughout much difficult; however, if one horizontal dimension of the of Cenozoic time, producing a complicated horizontal form is several times longer than the other, the cal- GRAVITY SURVEY OF THE WESTERN MOJAVE DESERT, CALIF. 59 dilations can be greatly simplified by assuming that 8001- this long dimension is infinite. Then a cross section normal to the long dimension and not near the ends 700 - can be treated as a two-dimensional form. Such an analysis is particularly useful in the interpretation of gravity anomalies associated with many fault struc­ tures. In a simple-step or sloping-step structure such as a fault, the magnitude of the total gravity effect is dependent upon the density contrast between the two materials in contact at the step and the height of the step but is independent of the form of the step or depth of burial. The form of the anomaly is dependent upon the form of the step and depth of burial but 100 - independent of the density contrast. If an observed anomaly is produced by a single step fault and there 0.0 04 0.5 0.7 1.0 1.5 are no other features influencing the anomaly, it would DENSITY CONTRAST, IN GRAMS PER CUBIC CENTIMETER be possible to determine the configuration of the fault FIGURE 24. Graph showing relation between the density contract and the thickness of a layer of infinite horizontal extent required from the gravity data. In practice, however, this is to produce a 1-ruilligal anomaly. often not possible. The general procedure followed in interpreting these in which the thickness of an infinite slab necessary to data was to collect all available control information produce a 1-milligal anomaly is dependent upon the and, within the limits imposed by the available control, density contrast. It is apparent that, if a density construct a geologically reasonable subsurface config­ contrast of 0.4 g per cm3 is assumed in the depth erti- uration that would produce the amplitude and general mate and the actual contrast is 0.7 g per cm3, the actual form of the observed anomaly. Profiles have been depth will be about 55 percent of the estimated depth. taken across the major anomalies that seem to be However, as the density contrast approaches 0.0 the amenable to two-dimensional quantitative interpreta­ percent error increases rapidly. For the western tion. An attempt was made to select the profiles Mojave Desert the actual depth is probably not l°-ss normal to the long dimension of the anomaly and at than 55 percent of the depth computed, using a density positions where the gravity relief is greatest and where contrast of 0.4 g per cm3 ; however, the actual depth a maximum of gravity and geologic control is available. may be several hundred percent greater than the com­ The assumption was then made that the gravity puted depth. anomaly and geologic features have an infinite extent GRAVITY ANOMALIES normal to the profile. The error introduced by making GARLOCK FAULT ZONE this assumption is not prohibitive in view of the other uncertainties involved. Any regional gravity gradient From its intersection with the San Andreas fault that was apparent was removed and a two-dimensional near Gorman, the Garlock fault zone crosses the south­ graticule (Dobrin, 1952 p. 96-99) was used to deter­ ern end of the Tehachapi Mountains to Tehachapi Pass mine a geologic configuration that would produce a northwest of the town of Mojave, and then follows the gravity anomaly that would approximate the observed southeastern base of the Sierra Nevada to Can til anomaly. Unless otherwise stated, a density contrast Valley. At the southwest end of Cantil Valley the between the Cenozoic and pre-Cenozoic rocks of 0.4 g principal fault branch diverges from the mountrin per cm3 has been used. front and continues into Cantil Valley where the sur­ It is unlikely that the aggregate density of any of face expression disappears. Northwest of Cantil Val­ ley two roughly parallel major faults are recognized the large masses of Cenozoic rocks in the western as part of the Garlock fault zone. The El Paso fault Mojave Desert is less than about 2.0 g per cm3. There­ is at the base of the El Paso Mountains and the fore, the maximum Cenozoic-pre-Cenozoic density con­ Garlock fault is located about 1 mile to the south in trast is about 0.7 g per cm3. Large masses of Cenozoic Cantil Valley. The two faults converge into a single rock with densities approaching that of the pre- fault to the northeast, which continues to the north­ Cenozoic rocks are known to exist in the region. Thus, east into the southern end of Searles Lake basin. the minimum density contrast that might be encoun­ Gravity coverage was not obtained over the western tered approaches 0.0. Figure 24 illustrates the manner end of the Garlock fault where it is within the Tel a- 628174 60 3 60 GEOPHYSICAL FIELD INVESTIGATIONS chapi Mountains. On the segment of the Garlock of pre-Tertiary rock extending from the Sierra Nevada fault zone along the southeastern base of the Sierra to the southwest end of the Rand Mountains. The Nevada it was practical to obtain gravity observations existence of this ridge is confirmed by a drill hole along only one line across the fault zone, and this line (pi. 10) bottomed at 390 feet in crystalline rock was at a relatively low angle to the fault zone. The (Dibblee, written communication, 1957). data show a gravity low southeast of, and trending In the area where the buried ridge of pre-Tertiary parallel to, the fault zone. A profile across the gravity rock joins onto the Sierra Nevada, the, Sierra Nevada low is illustrated in figure 25. The profile shows a front swings northward from the trend of the Garlock gravity relief of about 20 milligals and 2 areas of low fault zone. The surface trace of a major branch of closure. There is a moderate regional gradient ap­ the Garlock fault extends into Cantil Valley for a parent across the anomaly, along which anomaly values distance of about 5 miles. The gravity data indicate decrease northwestward toward the Sierra Nevada. that this fault continues on through Cantil Valley with An analysis of the gravity profile, as shown in fig­ the northwest side displaced relatively downward. This ure 25, indicates that about 4,000 feet of Cenozoic major branch of the Garlock fault zor Q- is referred to rocks is in contact with the plutonic rock along the in this report as the Cantil Valley fault. Garlock fault. The meager gravity data near the Along the northwest side of Cantil Valley, two major contact indicate a near-vertical feature. Southeastward parallel faults, the El Paso and the Garlock, have from the Garlock fault the surface of the basement been mapped from surface evidence. Both faults have complex is nearly level or rises gradually over a dis­ gravity expression indicating movement down on the tance of about 7 miles toward the southeast. About southeast side. 7 miles from the Garlock fault is a local depression, Cantil Valley is a closed basin between the Rand about 2% miles wide and 6 miles long, elongated ap­ Mountains on the southeast and the El Paso Mountains proximately parallel to the Garlock fault zone. The on the northwest. The basin is about 25 miles long floor of depression is estimated to be about 1,000 to and about 6 miles wide. Toward the northeast the 2,000 feet below the projected level of the basement basin floor rises gradually to a low pass north of surface. The spacing of the gravity stations does not Randsburg. Toward the southwest the basin opens permit any positive conclusions of the configuration into the Mojave Desert. Near the center of the basin of the depression. However, the gravity gradient along is Koehn Lake. Commercial salt, gypsum, and ulexite the southeast margin seems to indicate the presence have been produced from the deposits in or around of a fault parallel to the Garlock fault. The depres­ this lake. sion is probably a graben-type structure developed The El Paso Mountains rise abruptly to altitudes between faults parallel to the Garlock fault zone. The as much as 3,000 feet above the bssin floor. The local depression seems to be related to a locality that is southeast side of the range is characterized by steep a relatively good producer of ground water. V-shaped canyons and slightly rounded peaks and The major gravity low northeast of Tehachapi Pass ridges representing mature stages of erosion. The core is bounded on the north by an east-trending high of the range is made up of metamorphic and granitic gravity area. The gravity data indicate a buried ridge rocks. On the northeastern part of the range 35,000

EXPLANATION SE.

Bouguet anomatr Residual anomaly a Computed anomaly

A'

r^i-EI^-EI-Ejrlir-iilr-::!^ £5r_£uM?ff H^^fLr^I^^^r^^-EZ-EE-^Er--

0 3 MILES i I J FIGURE 25. Profile A-A', shown on plate 10, across gravity anomaly southeast of the Garlock fault northeast of Telachapi Pass. GEAVITY SURVEY OF THE WESTERN MOJAVE DESERT, CALIF. 61 feet of the Garlock series (Dibblee, 1952, p. 15) is The gravity low in Cantil Valley has the greatest exposed. The Garlock series is slightly metamorphosed relief of any local anomaly in the western Mojave sedimentary and volcanics rocks in part, if not all, Desert. The low is about 27 miles long, 6 miles wide, of Permian age. Cenozoic rocks occur along the north­ and its minimum lies about 1 mile southeast of Salt- western flanks of the range (Dibblee, 1952, p. 25). dale. The long axis parallels the Garlock fault zone. The Rand Mountains rise rather abruptly from A profile across Cantil Valley is illustrated in fig­ Cantil Valley to a maximum altitude of about 3,000 ure 26. Along this profile there is a gravity relief of feet above the basin floor. To the southeast the range about 43 milligals. The gravity gradient (15 mgal slopes gently into the Mojave Desert. The mountains per mile) in the southeastern part of the profile is the are composed of granitic and Precambrian (?) meta- steepest mapped in the western Mojave Desert. A morphic rocks; the granitic rocks predominate in the regional gradient across the valley is evident on the southwest and the metamorphic rocks in the northeast. profile and is partly responsible for the steep gradient. The El Paso fault lies along the southeast base of The regional gradient is probably part of a mp.jor the El Paso Mountains. Dibblee (1952, p. 37) states crustal gravity anomaly associated with the Sierra that there is no evidence of recent lateral movement Nevada. To facilitate the interpretation along the along the fault, and that the movement has been for gravity profile, an assumed regional gradient of about the most part, if not entirely, vertical. He estimates 2 milligals per mile was removed. the maximum vertical displacement is about 10,000 feet. The character and position of the gravity minimum Near the northeast end of Cantil Valley the El Paso within 1 mile of the Garlock fault indicate a mass fault joins the Garlock fault. The El Paso fault deficiency just south of the Garlock fault. The gravity apparently dies out to the southwest. minimum could result from either a dip of the base­ The Garlock fault was originally studied near the ment-rock surface toward the Garlock fault from the town of Garlock in Cantil Valley where there is evi­ southeast or an accumulation of unusually light sedi­ dence of recent left-lateral movement (Hess, 1910, ments near the surface just southeast of the fault. p. 25-26). Northeast of the junction of the El Paso The general position of the low can be accounted for and Garlock faults, the Garlock fault lies along the by a slope of the basement-rock surface; however, the base of the El Paso Mountains. Southeast of this sharpness of the gravity minimum indicates a near- junction the Garlock diverges from the mountain front. surface effect, such as an accumulation of lighter sedi­ A graben about 1 mile long, a quarter of a mile wide, ments under the present playa. Both a sloping base­ and more than 50 feet deep lies between the El Paso ment-rock surface and an accumulation of less dense and Garlock faults just southwest of their junction. sediments were assumed to construct the profile shown Southwest of this graben, except in the vicinity of in figure 26. Garlock, the trace of the Garlock fault is concealed An analysis of the gravity profile indicates that the by a cover of alluvium; however, the location of the vertical displacement along the northwest side of the fault can be inferred from surface evidence. South gravity minimum did not occur entirely at the El Faso of Jawbone Canyon the Garlock fault appears to swing fault. Much of the vertical displacement occurred to the south along the front of the Sierra Nevada and southeast of the El Paso fault, probably at the Garlock join the branch of fault zone that lines along the fault but perhaps at other parallel faults. In figure 26 front of the Sierra Nevada northeast of Tehachapi the displacement has been attributed to vertical move­ Pass. ment at only the El Paso and Garlock faults. Dibblee suggests that a buried fault may exist at the Along the southeast side of Cantil Valley the steepest base of the Rand Mountains. There is, however, only gravity gradient occurs out in the valley over a rule indirect surface evidence to back up this possibility. removed from the surface contact between baseirent A series of piedmont alluvial fans extend into rock and the alluvium. The conclusion is drawn from Cantil Valley from both sides. The alluvial deposits this part of the gravity profile that there is no mr.jor are underlain by continental sediments of the Ricardo fault along the contact between the surface baseirent formation (Pliocene to Pleistocene) (Dibblee, 1952, rock and the alluvium, but that the concealed Cantil p. 29-30). Northwest of Cantil and near Garlock the Valley fault with considerable vertical displacement Ricardo formation is exposed between the Garlock and is within the valley. El Paso faults. Several oil test wells have been drilled If the indicated position for the Cantil Valley fp.ult near Cantil to depths ranging from 1,859 to 5,065 feet on the profile is extended to the southwest along the (Dibblee, written communication, 1958). All the wells steep part of the gravity gradient, it will join the were bottomed in continental Tertiary sedimentary segiment mapped from surface evidence in the south­ rocks. west corner of the valley. Extended northeast from 62 GEOPHYSICAL FIELD INVESTIGATIONS

NW. SE. O-i EXPLANATION

Bouguer anomaly 10- Residual anomaly

X Computed anomaly \

N 30- \ N- / / \ \\ -.- « --- /' \\ x* o-^^' S/ 40- \ / \ / \ / \ /

50- B' 5000'--B

:: Density = 2.0 grams per cubic centimeter^^ SEA LEVEL-

Density = 2.3 grams per cubic centimeter

5000'-

'/ Density = 2.7 grams per cubic centimeter 10,000' f * JJff***J*ffff*rJfftfl

3 Miles

FIGDEB 26. Profile B-B', sbown on plate 10, across Cantll Valley. the profile the indicated position of the fault will less complex anomalies in the region. However, the coincide with the position of the two springs indicated major features of Cantil Valley can be inferred from on the map (pi. 10) and will lie southeast of a fault the gravity data, though the details may vary con­ in the northeast end of Cantil Valley located by siderably from the cross section of figure 26. Dibblee on the basis of surface evidence (Dibblee, Cantil Valley seems to be a deep graHn within the 1957). Garlock fault zone. On the northwest the graben is The major features contributing to the gravity anom­ bounded by the El Paso and Garlock faults, and the aly in Cantil Valley are large and occur in a small vertical displacement is distributed between them and area; therefore, their gravity effects overlap to an perhaps other parallel faults lying between them. The important degree, and it is impossible to isolate the floor of the graben probably slopes downward toward effects of the different features. Also, the regional the Garlock fault and the greatest thiclrness of Ceno- gradient is large and is not well understood. Thus, zoic deposits occurring just south of the Garlock fault. the ambiguities involved in the interpretation of this The Cantil Valley fault forms the soutHastern boun­ anomaly are greater than those involved in the other dary of the graben. The thickness of the Cenozoic GRAVITY SURVEY OF THE WESTERN MOJAVE DESERT, CALIF. 63 deposits in the graben is about 2 miles. Large lateral there is suggestive gravity evidence indicating that the variations in the density of the Cenozoic deposits prob­ right-lateral movement along this fault zone may have ably occur so that fine, less dense lake sediments are offset the axis of the syncline. In the vicinity of in the structurally low part of the graben and coarser, Harper Lake the gravity data indicate that a thin rc- more dense materials toward the edges. cumulation of Cenozoic deposits covers a basement- The gravity gradient indicates that the fill thickens complex surface of moderate relief. Drilling tas rapidly to the east away from the Sierra Nevada at revealed that the surface of the basement complex is the southwest end of the graben. At the southwest generally within 1,200 feet of the surface. At the end of Cantil Valley near the mouth of Jawbone northwest end of Harper Lake there is a gravity low Canyon, the Garlock fault zone is intersected at nearly indicating a local depression. a right angle by the fault along the east front of the North of Hinkley the east-striking gravity feature Sierra Nevada. There is no gravity expression of this with a strong gradient decreasing northward is difficult fault apparent, though exposures of the fault on both to explain. A major part of the feature occurs over sides of the canyon show Tertiary rocks on the east an area where the basement complex crops out. The in fault contact with plutonic rock on the west. The line of gravity stations crossing the feature north of absence of any gravity expression of this fault in Jaw­ Hinkley passes between two outcrops of quartz mon- bone Canyon indicates that there is not a thick con­ zonite. Perhaps there is a considerable depth of fill cealed Tertiary section just east of the fault. between these outcrops with the result that the con­ tours based on these stations do not reflect the trie RANDSBURG-HARPER LAKE AREA anomaly pattern in the area. More detailed grav'ty Because of the lack of access roads over the Rand observations in the Harper Lake area would probaMy Mountains, it was only possible to complete one traverse serve to define gravity anomalies associated with sev­ over the range near the northeast end. However, the eral important geologic features that are not indicated data collected clearly indicate the existence of a gravity by the data now available. high, though the magnitude may be considerably Southwest of the gravity ridge between the Fremont greater than is indicated on the contour map. Peak range and the Rand Mountains and trending The area south of the Band Mountains is typical of parallel to it is a major gravity low with a closure of the western Mojave Desert. Isolated hills rise above about 15 milligals. The low is bounded on the sou*h- the gently sloping alluvial-covered plain. To the east west by the Lockhart fault. Along the northeast edge the northwest-trending Fremont Peak range is one of of the gravity low, the gravity contour lines trend the most prominent topographic features in the area parallel to a branch of the Lockhart fault. One deep surveyed. Although the gravity data over the Fremont hole has been drilled in the area of the low. The Peak range are not complete, a gravity high appears depth is reported to be 1,750 feet; it did not penetrate to coincide with the range. A gravity high extends basement rock (Gale, 1946, p. 373-374). from the northern end of the Fremont Peak range to A profile across the anomaly is illustrated in fig­ the Rand Mountains. This high corresponds approxi­ ure 27. The gravity minimum lies within about 1 rrile mately to a topographic high, and probably represents a partly buried basement ridge. sw. EXPLANATION NE. O-i Northeast of Fremont Peak and south of Cuddeback Bouguer anomaly Lake there is a strong northwest-striking gravity Residual anomaly gradient which seems to be associated with the Harper 0 fault zone. The gravity gradient indicates that the Computed anomaly fault zone extends to the northwest for several miles from the location where the recognized surface expres­ sion terminates. Although recent movement along this 20- fault zone has been right lateral and relatively down sow.; on the southwest, the gravity gradient indicates that south of Cuddeback Lake there has been an overall SEA LEVEL - relative downward displacement of the northeast block of about 7,000 feet. 3000'

Northwest of Barstow a gravity low is associated '3 Mile with the Barstow syncline. The Harper fault zone _J FIGURE 27. Profile 0'-G't shown on plate 10, across gravity anom­ transects the western end of the Barstow syncline and aly south of the Rand Mountains. 64 GEOPHYSICAL FIELD INVESTIGATIONS of the Lockhart fault and the northeast edge of a base­ in which the Kramer deposit is located. The extent ment outcrop. It seems probable that the greater and thickness of the several basalt flows that occur in thickening of the Cenozoic deposits along the south­ the Tertiary section are not known; it is not possible, west edge of the gravity low occurs along the Lockhart therefore, to evaluate the effect the dense basalt has on fault, and that this fault is nearly vertical. There is the gravity anomaly. There is a possibility that the a slight steepening of the gravity gradient in the basalt has an important gravity effect and is displac­ general area where a projection of the north branch of ing the position of the gravity minimum. However, the Lockhart fault crosses the profile. The steepening as there is no definite evidence to support this possi­ may indicate that the branch extends to the northwest bility, it is assumed that the position of the gravity and has a vertical component of displacement. minimum indicates the location of the thickest Ceno­ zoic section. If this is the actual condition, then the BORON-KRAMER JUNCTION AEEA borate deposit does not occur in th°- area of the Just north of the town of Boron in the central part thickest Cenozoic section. of the surveyed area is the Kramer borate district. The gravity low centered about 6 miles west of For about 30 years this district has been the world's Boron is connected with the low associated with the leading source of borate minerals (Gale, 1946, p. 329 ; Kramer deposit. The low west of Borm has a north­ Mumford, 1954, p. 20-22). west trend and is bounded on the southwest by a grav­ Borate minerals occur in a mineralized area roughly ity gradient that probably marks the trace of a north­ 4 miles long and 1 mile wide. The deposit occurs in west-trending fault. No surface indication of this a fine-grained colloidal-type shale of Tertiary age fault has been recognized. The results of recent drill­ apparently laid down in standing water. The lake ing for the U.S. Geological Survey in the southern beds are underlain by a basalt flow which in turn is part of the area of the low indicate that there is underlain by several hundred feet of sedimentary and present a Cenozoic section containing over 2,328 feet pyroclastic rocks. The sodium borate body, chiefly tin- of alluvial sands and silts (Benda anc1 others, 1960). cal (Na2B4O7*10H2O) and kernite (Na2B4(V4H2O), A high-gravity trend extends from the pre-Tertiary lies at depths of from 300 to 1,000 feet below the outcrops north of Boron to those east of Kramer surface. The deposit contains impurities of clay and Junction. This high-gravity trend suggests that the other insoluble materials, but the massive borate deposit alluviated valley area between these exposures of pre- does not contain appreciable amounts of other soluble material. Colemanite (C2B6O11*5H2O) and ulexite Tertiary rock is underlain by pre-Tertiary rock at (NaCaB5(V8H2O) occur in the clay and shale that shallow depths. Another gravity low north of Kramer surround the sodium borate body. Quaternary al­ Junction is connected to the low associated with the luvium completely covers the surface over the borate Kramer deposit. The gravity trend along the south­ deposit, and no surface indication of the rich deposit west margin of this low probably is produced by a has been recognized. northwest-trending fault. Drilling for the U.S. Geo­ The Kramer ore body lies along the south edge of logical Survey has confirmed the existence of at least a gravity low (pi. 11). The gravity minimum is about 3,500 feet of Cenozoic fill containing some layers of 2 miles north of the borate body. South of the borate Colemanite (Benda and others, 1960). An isolated body is a steep gravity gradient, striking parallel to the gravity low lies south of Kramer Junction. A single long axis of the borate body, which probably marks hole drilled for the U.S. Geological Survey near the the position of the fault near the Western Borax center of this anomaly penetrated through 3,500 feet mine postulated by Gale (1946, p. 341). From the of conglomerate and sandstone (Benda and others, gravity data it is estimated that the fault is located 1960). in the approximate position indicated on the geologic BABSTOW-CAJON PASS ABEA map and that at present the surface of the basement The Kramer Hills, Iron Mountain, and the Shadow is displaced along this fault relatively down to the Mountains have only moderate topcnraphic relief. north from 1,500 to 2,000 feet. The borate-bearing Each of the ranges has a core of pre-Tertiary basement lake beds were probably deposited in a small local rocks. Several thousand feet of Tertiary strata crop basin bounded on the south by this east-trending fault out in the Kramer Hills, but no Tertiary rocks have that was active during the period of deposition. been recognized in the vicinities of the Shadow Moun­ No reliable drill-hole report has indicated the depth tains nor Iron Mountain. Gravity higli s are associated of the basement rock in the area of the gravity low with each of the three topographic features; however, GRAVITY SURVEY OF THE WESTERN MOJAVE DESERT, CALIF. e-5 the gravity highs generally are more extensive than around the basement highland associated with tH the topographic highs. Shadow Mountains. Between the Kramer Hills and Iron Mountain is North of the axis of this low an oil test was drilled a north-trending gravity trough having a relief of to 4,130 feet without reaching basement rock (Dibbles, more than 20 milligals. The anomaly is bounded on written communication, 1957). The hole reportedly the east by a steep gravity gradient. This steep bottomed in a granite cobble conglomerate and sand­ gradient may be interpreted to indicate a fault which stone, which may be near the base of the fill. However, is the eastern boundary of a trough containing about this well did not penetrate, and possibly did not reach, 5,000 feet of Cenozoic deposits (fig. 28). An oil-test the 1,000-foot section of Tertiary volcanic flows ard hole drilled in the northwestern part of this low lake beds that overlies the basement complex in tiN struck basement rock at 2,640 feet (Bowen, 1954, Kramer Hills 4 miles north, and therefore may have p. 182). The depth to basement reported in this hole been bottomed more than 1,000 feet above the bas> is consistent with the depths shown on the profile in ment complex. If a density contrast of 0.4 g per cm3 figure 28. The gravity gradient continues northward is assumed in interpreting this anomaly (fig 29), tl e to the Lockhart fault and southward to the area where bottom of this hole would seem to be near the basement the Helendale fault enters the surveyed area. Indirect complex and the depth of the basement complex near surface evidence indicates that the Lockhart fault or the axis of the gravity low would be about 4,500 feet. a branch of it extends southeastward toward Hinkley The surface of the basement complex rises toward Valley (Dibblee, oral communication, 1957), such an the Kramer Hills and the Shadow Mountains, but there extension is not inconsistent with the gravity data is no definite indication in the gravity data of a fault adjacent to either of these topographic features. Th°- though no great vertical displacement is indicated. west end of the gravity low is a steep gravity gradient The fault inferred from the steep gravity gradient striking a little east of north. The gradient approxi­ along the east side of the gravity low is probably mately coincides with the western limit of the outcrops continuous with the Lockhart fault and may also be in this area, and may be produced by a fault of con­ related to the Helendale fault. siderable vertical displacement. Northeast of the Shadow Mountains and south of the The gravity low south and southwest of Barstow Kramer Hills is an east-trending gravity low that may indicate a westward extension of the Dagge^t joins the low east of the Kramer Hills. At the west­ basin, which has been mapped to the east of Barstow. ern end of the low is a moderate gravity gradient Between Victorville and Cajon Pass the north- trending a little east of north. South of the low is sloping surface is underlain by fanglomerate and al­ a rather steep gravity gradient that forms an arc luvium of Quaternary age deposited as part of a large

SW. NE. On EXPLANATION

Bouguer anomaly

Id- Residual anomaly e Computed anomaly

20-

30-

Density = 2.3 grams per cubic centimeter

tensity = 2.7 grams per cubic centimeter f''f'fSffSSJJf*'JJJfJJ'J

3 Miles j FIGURE 28. Profile D-D', shown on plate 10, across gravity anomaly west of Iron Mountain. 66 GEOPHYSICAL FIELD INVESTIGATIONS

EXPLANATION NE.

Bouguer anomaly

2 Residual anomaly © Computed anomaly

3000' j n Density=2.3 grams per cubic centimeter

SEA LEVEL-

% Density = 2.7 grams per cubic centimeter 3000

1 3 Miles FIGURE 29. Profile E-E', shown on plate 10, across gravity anomaly south of the Kramer Hills. alluvial fan that once extended northward from the have been reported in the area of tHs low, but it San Bernardino and San Gabriel Mountains. Head- probably marks a deep local depression. A well south­ ward erosion of Cajon Creek has captured most of the west of Adelanto is reported to have been drilled in drainage that deposited the fan. In the Cajon Pass crystalline limestone, schist, and granite from 1,350 area more than 6,000 feet of Cenozoic stream-laid feet to 3,216 feet (Bowen, 1954, p. 182). This drill deposits is exposed; these deposits dip generally north­ hole is on a gravity high that extends southward from ward under this large alluvial fan. Shadow Mountains. As the intrabasenent anomalies North of Cajon Pass is an extensive gravity low in the region north of Cajon Pass are so poorly under­ area with four low closures. The main part of the stood, no quantitative interpretation of the gravity anomaly, which contains three of the four recognized anomaly has been attempted. Unless there are large low closures, trends easterly. The western end of the lateral density variations within the Cenazoic section, anomaly swings to the north and continues with dim­ the greatest depth to the basement complex occurs inished amplitude into the area of Mirage Lake. In near the gravity minimum and is about 10,000 feet. Cajon Valley along the southwest margin of the Along the San Andreas fault zone tl ^ gravity con­ anomaly there is a steep northwest-striking gravity tour lines trend generally parallel to the fault zone; gradient. The strike of the gradient is parallel to the however, no significant local gravity anomalies asso­ Cajon Valley fault (Noble, 1954b, pi. 5), but the ciated with the fault zone were recognized from the steepest part of the gradient is about 1 mile to the gravity data collected. More detailed coverage would northeast of the indicated surface position of the indicate numerous gravity anomalies rssociated with fault. The gradient indicates that the depth to the local features along the fault zone. top of the basement complex increased rapidly to the The gravity data indicate that a basement high northeast from the indicated position of the Cajon extends across the east end of Antelope Valley from Valley fault, perhaps along steep faults parallel to the Cajon Valley fault. An oil test about 1 mile Pearland to Lovejoy Buttes. The irrejrularity of the northeast of the indicated position of the fault was gravity contour lines in the Lovejoy Buttes-Black drilled to 5,800 feet without reaching the basement Butte area probably indicate intrabasement effects plus complex (Dibblee, written communication, 1956). buried basement topography. Between Adelanto and Victorville there is a north- northeast-trending gravity minimum, in part separated ANTELOPE VALLEY AREA from the main body of the low by a gravity high Antelope Valley is a broad nearly level alluvium- extending southwest from Victorville, No deep holes floored valley in which a few outcrops of Tertiary GRAVITY SURVEY OF THE WESTERN MOJAVE DESERT, CALIF. C7 and pre-Tertiary rocks rise above the alluviated sur­ gravity saddle there is a minor gravity low which face. The gravity high trend which extends south­ probably indicates a local subsidence. In the western westerly from Love joy Buttes to the San Andreas part of this low there is an outcrop of tuff of Tertiary fault is flanked on the northwest by a major closed age dipping eastward about 30°. gravity low which extends northeastward from Lan­ In the western end of Antelope Valley there is r,n caster to the south end of Eogers Lake. An oil-test extensive gravity low. The main body of the anomaly hole on the northwest flank of this gravity low was is bounded on the south by a steep east trending drilled through Cenozoic continental deposits to bot­ gradient. Along the northwestern part of the anomaly tom at 5,560 feet (Dibblee, written communication, the gradient is more gentle and trends parallel to tH 1955). A profile across the anomaly through the drill Garlock fault and the front of the Tehacliapi Moun­ hole is illustrated in figure 30. The existence of 5,560 tains. A branch of the anomaly extends along tH feet of Cenozoic fill at the drill-hole location on Rosa­ Tehachapi Mountains to the vicinity of Soledad Moun­ mond Lake would not have been inferred from the tain. gravity anomaly, using a density contrast of 0.4 g per Figure 31 illustrates a profile across the anomaly cm8. If the assumption is made that the basement through Antelope Buttes. The gravity anomaly is not surface is at least 5,560 feet deep at the point indicated well suited to a two-dimensional analysis; however, on the profile and that there is a single density contrast such an analysis will indicate some of the important of 0.3 g per cm3 between the Cenozoic deposits and the features of the subsurface configuration producing tl <* basement rock, the observed anomaly can be accredited anomaly. The Tertiary rocks northwest of Antelope to the basement-surface profile illustrated in figure 30. Buttes are dipping toward the gravity minimum at The maximum depth to basement rock would then be angles of 30°-45° (Dibblee, written communication, 10,000 feet in the vicinity of the gravity minimum. 1956). An analysis of the gravity gradient indicates The interpretation along this profile may be compli­ cated by intrabasement anomalies that cannot be iden­ that the interface between the basement rock and tH tified and possibly by lateral density variations within Tertiary rocks has a considerably greater dip toward the Cenozoic section. the gravity minimum. It may be that the Tertiary- The gravity high centered over the pre-Tertiary ex­ basement interface dips more steeply than the Tertiary posures north of Rosamond Lake extends southwest­ rocks, or perhaps the steep gravity gradient may H erly across Antelope Valley to the San Andreas fault produced by a fault. In either case the basement sur­ with a shallow gravity saddle midway between Rosa­ face rises abruptly along the south and southeast mar­ mond and the San Andreas fault. Northwest of this gins of the gravity low.

NW. SE, on EXPLANATION

Bouguer anomaly

10- Computed anomaly

20-

30-

SEA LEVEL

~_zr_r- Density = 2.4 grams per cubic centimeter 4000'

y Density = 2.7 grams per cubic centimeter />^ 8000'

3 Miles J FIGURE 30. Profile F-f" shown on plate 10, across gravity anomaly south of Rosamond Lake. 68 GEOPHYSICAL FIELD INVESTIGATIONS

NW. SE. On EXPLANATION

Bouguer anomaly o C/> Computed anomaly

FIGURE 31. Profile G-G', shown on plate 10, across gravity anomaly in western Antelope Valley.

Northwest of the gravity minimum the anomaly not made a significant contribution to the study of values rise more gently toward the Tehachapi Moun­ the local pre-Tertiary geology. tains. There is no evidence in the gravity data of During much of early Tertiary time the western major faulting in this area. An oil-test hole at the Mojave region was rising and the products of erosion west end of this low passed through 4,150 feet of rocks were removed from the region. Begirning in middle and sediments of Cenozoic age without reaching base­ Miocene and continuing intermittently to present time, ment (Wiese, 1950, p. 58). The log of a drill hole local depressions have developed and have been filled 2.7 miles northeast of the profile reported basement with volcanic material and debris derived from ad­ rock at 3,300 feet (Oakshott and others, 1950, p. 18). joining highlands. A middle Pliocene orogeny de­ If this hole is projected along a line parallel to the formed the earlier Tertiary sedimentary rocks, and gravity contours, it would cross the profile in figure 31 local thrust faults developed (Hewett, 1954a). at a point where a depth to basement of 4,000 feet is Hewett (1954a, p. 17), in reporting on the Tertiary indicated. The maximum depth to basement rock in­ basins in the Mojave block states: dicated along the theoretical profile is 7,000 feet. (1) none of Eocene or Oligocene age are yet known; (2) some Local gravity highs are apparent over Antelope basins contain thick sections of middle and upper Miocene sedi­ Buttes and Quartz Hill, and there is an indication of ments, but none seem to contain superimpowd Pliocene sedi­ possible faulting (down to the south) along the south­ ments; (3) several basins containing lower Pliocene sediments are known, but none show underlying Miocene sediments or ern margins of both. The overall gravity trend is overlying middle Pliocene sediments; and (4) the only middle parallel to the San Andreas fault, but local east-west (lower) Pliocene section is not underlain by older Pliocene or gravity trends are evident along the southern margin Miocene sediments. of Antelope VaHey^ This situation seems to indicate some shifting of areas of downwarpr with accompanying shifts in basins of deposition. SIGNIFICANCE OF THE GRAVITY ANOMALIES In the western part of the Mojave block excluding GEOLOGIC SIGNIFICANCE Cantil Valley, the present centers of deposition, as indicated by the location of the major playas, do not Although sedimentary and volcanic rocks of Paleo­ occur over the areas of thickest Cenozoic deposits, as zoic and possibly Precambriaii and Mesozoic age are revealed by the gravity data. Rogers Lake is centered present, the pre-Tertiary history of the western Mojave in an area where the basement comp^x is near the Desert is not well understood. In middle or late surface and there is a small basement outcrop in the Mesozoic time enormous volumes of plutonic igneous lake. Rosamond, Harper, Mirage, snd Cuddeback rock were emplaced. Although one or more late Lakes coincide with Cenozoic deposits of considerable Mesozoic orogenies had considerable effect on the tjiicknesses, but, as indicated by the gravity data, they western Mojave region, very little information is avail­ are all offset from the thickest section of Cenozoic able from studies within the region on the nature or sediments. Debris derived from recently uplifted date of the orogenic events. The gravity studies have masses adjoining the Mojave block is altering the GRAVITY SURVEY OF THE WESTERN MOJAVE DESERT, CAI^IF. drainage pattern along the margins of the Mojave the gravity anomalies indicate trends of Cenozoic de­ block. However, both the geologic and gravity ob­ formation as it is reflected in the relief of the basement- servations indicate that, since the beginning of Tertiary rock surface. It should be kept in mind, however, deposition in the Mojave block, the basins of deposi­ that major structural features need not have any tion have been shifting because of deformation within gravity expression if they do not result in differentir.l the Mojave block. movement of rocks with different densities. Areal mapping within the western Mojave block has The well-defined trends in the topography of the revealed several major and numerous minor northwest- pre-Tertiary rock surface as interpreted from the trending faults parallel to the San Andreas fault. On gravity data are shown on plate 12 along with the some of these faults there is evidence of reversal of faults located on the basis of the areal mapping. For vertical displacement, resulting in an apparent scissors- the gravity features in an area north and west of type displacement. This is readily apparent on the Rogers Lake a trend parallel to the Garlock fault zone Lockhart fault and also on the Harper fault zone. is dominant. East and northeast of Rogers Lake the On these faults and on several others there is geologic dominant trend for both the features having surface evidence of right-lateral displacement on a small scale. expression and those inferred from the gravity anoma - It seems probable that the relative vertical displace­ lies is northwest. In the Mirage Lake-Victorville ment along these faults and other wrench faults in area north of Cajon Pass a north-south trend of the the region is not only reversed at different places gravity features is evident. In a strip northeast cf along the lateral extent of the fault, but that the di­ the San Andreas fault several eastward-trending fea­ rection of vertical movement may not be persistent tures are indicated by the gravity anomalies. The^e at one place. Thus, the vertical displacement apparent features make an angle of about 30° with the San at the surface may not indicate the direction of net Andreas fault zone. displacement. Similarly, the direction of net vertical In the region just southeast of the Garlock fault displacement as inferred from the gravity data may zone the trend of all the major gravity anomalies is not correspond to the direction of the most recent parallel to the Garlock fault zone. This is in markei vertical movement. The lack of persistence of the contrast to the fault pattern developed from the arerl vertical movement along individual faults of this mapping. As shown on plate 10, only in a part cf northwest-trending system and other related wrench Rand Mountains and in the hills north of Rosamond faults is probably a contributing factor to the migra­ are the faults alined parallel to the Garlock fault zone tion of areas of deposition throughout late Tertiary apparent from areal mapping. The gravity anomaly and Quaternary time. map, however, reveals that nearly all the major fer.- The western Mojave Desert lies in a region of pro­ tures having gravity expression are elongated parallel found tectonic activity. To the north and northeast to the Garlock fault zone. This trend extends as far are elongated basins and ranges with striking evidence south as Rosamond Lake and is evident not only in tli e of recent deformation. To the northwest and southwest negative anomalies associated with accumulations cf are the recently uplifted Sierra Nevada and Transverse Cenozoic rock but also in the positive anomalies over Kanges. Surrounded on three sides by regions of major basement-rock mass in the Rand Mountains and great relief and pronounced structural trends, the west­ north of Rosamond Lake. ern Mojave Desert exhibits none of the surface fea­ Most of the important faults located in this region tures that distinguish the neighboring regions. The in the areal mapping trend northwest approximately surface of low relief that now characterizes the west­ parallel to the San Andreas fault zone. Several cf ern Mojave Desert is in marked contrast with the these faults are apparent in alluvium-covered area?; gravity anomaly map that has pronounced local and some form the contact between the bedrock and al­ regional variations and several well-defined local luvium, indicating recent movement. Nearly all the trends. northeast-trending faults which have been located in The gravity lows are no doubt largely an expres­ the areal mapping occur within the Tertiary or older sion of the present dispositions of the major ac­ rocks. This can be interpreted as indicating that an cumulations of Cenozoic rocks with densities appre­ older system of northeast-trending faults along which ciably less than the pre-Cenozoic crystalline rocks. considerable vertical movement has occurred in Ceno­ The Cenozoic rocks have either been deposited or zoic time is now relatively quiescent, and the north­ preserved in areas that have been downwarped or down- west-trending system which is now dominate has not faulted during Cenozoic time. Thus, the trends of produced major vertical displacement in this region. GEOPHYSICAL FIELD INVESTIGATIONS In the northeastern part of the surveyed region the disregard foliation, joint patterns, and bediing planes fax the trend of the gravity anomalies agrees with the struc­ adjacent rocks * * *. . Within the area that lies northeast of the assumed N. 45° W. tural trends apparent from the areal mapping. Sev­ line, there are only a few proven faults as far north as the eral of the northwest-trending faults have expression Garlock fault. The trend of these faults ! » diverse, and evi­ on the gravity anomaly map with considerable vertical dences of Recent movement are rarely observed. displacement indicated for some. The deep subsidence, the offset of tl xft. Garlock fault The northerly trend of the gravity anomalies in the zone, and the change in trend of the fault zone are Helendale-Victorville area is not well defined and three anomalous conditions which indicate that this perhaps not particularly important. It is, however, area is worthy of further study. Additional geo­ evident that several major gravity anomaly features physical and geologic investigations in adjoining areas, are not in alinement with the northwest trend which particularly to the north and east, combined with more dominates the gravity anomaly map in the region to detailed studies in the Cantil Valley area may yield the north and is also the dominate trend of the surface considerable information on the tectonics of this region. faults in this region. The only evidence of a northerly The break between the Sierra Nevada block and the trend apparent on the geologic map is a vague trend Basin and Eange province may have had a profound of the contact between the bedrock and alluvium north effect on the Garlock fault zone east of Sierra Nevada and south of Helendale. Until more is learned con­ and upon the Mojave block. Other tectonic events out­ cerning the nature of the apparently quiescent de­ side the western Mojave Desert also probably exerted formation which resulted in the present gravity pat­ considerable influence on the region. TTien the gravity tern, it is difficult to speculate on the causes. It is, data obtained in the western Mojave Desert are con­ however, interesting to note that this anomalous region sidered along with geologic and geophysical data from occurs directly north of Cajon Pass where the San adjoining regions, they will probaWy make a con­ Andreas fault zone crosses the Transverse Ranges be­ siderable contribution to the understanding of the tween the San Gabriel and San Bernardino Mountains. regional tectonics. The easterly trend of several minor gravity anomalies north of the San Andreas fault zone probably is an ECONOMIC SIGNIFICANCE expression of structure closely related to the fault The information on the Cenozoic geology inferred zone. On the geologic map several faults diverge from the gravity data has important significance rela­ from the trend of the fault zone with approximately tive to several economic problems. The gravity data the same trend as the minor gravity anomalies. How­ reveal the concealed areal extent of several known ever, none of the gravity anomalies can be directly basins of Cenozoic sedimentary fill ard several basins correlated with known surface faults. heretofore unknown, that are comp1etely concealed One area where the gravity data may make an im­ by Quaternary alluvium. The deposit of borates at the portant contribution to the understanding of regional Kramer borate district is in one of the basins of tectonics is in the vicinity of Cantil Valley where the Cenozoic fill. Several of the other basins have been east front of the Sierra Nevada intersects the Garlock tested or partly tested by deep drilling; others are fault zone. Here the fault zone consists of 3 major branched between which a subsidence of perhaps 2 miles unexplored. Noble (1926a, p. 4:6) has described the discovery of has occurred. In this area the southwest segment of the borate deposit at Boron as follows: the fault zone is offset to the south. From Cantil Valley west to the San Andreas fault the trend of the The discovery of the deposits was purely rccidental, for they lie beneath a broad alluvial plain whose featureless surface of Garlock fault zone is about N. 60° E. From Cantil sand and gravel affords no clue to what ir beneath. In 1918 Valley eastward the strike of the fault becomes more a well was being drilled for water in this plain on the desert easterly until it becomes approximately east in the homestead ranch of Dr. John K. Suckow, a physician of Los vicinity of Leach Lake northeast of the surveyed area. Angeles. The site of the well was at least a mile from the Also, the line Hewett (1954b, p. 17) proposes to draw nearest outcrop of rock. After the drill ha-t gone through the alluvial deposits of sand and gravel that form the surface of separating the Mojave block into two parts extends the plain and had penetrated the bedrock beneath the allu­ S. 45° E. from the northeast end of Cantil Valley. Of vium, it struck a hard, crystalline materirl which proved to the significance of this line Hewett states (p. 17): be colemanite, one of the hydrous calcium borates. The area southwest of this line contains many closely spaced Since the discovery of the Kramer deposit, wildcat faults, most of which trend northwest or roughly parallel to the San Andreas fault * * *. in the few places where the drilling has been conducted in alluvium-covered areas dips of these faults are observable, they are steep * * *. in the surrounding region. However, the absence of In the places where these faults have been studied, they seem to any surface indications of buried deposits, the large GRAVITY SURVEY OF THE WESTERN MOJAVE DESERT, CALIF. 71 areas involved, and the great depths at which a similar to the ground water are available. Most of the grour d deposit might occur present serious problems to any water being utilized in the region is produced from wildcat-drilling program. aquifers within the Cenozoic section. Some of the^e Logically the first step in the search for a borate aquifers receive recharge water from the mountain deposit of the type being exploited at Boron is to locate regions adjoining the desert where precipitation is the areas in the general region where important heavier than on the desert. Other areas are dependent amounts of Tertiary sediments occur. The second upon local rainfall to sustain the reserves. step is to select the geologic settings from these areas In general, ground water moves away from source that would favor the accumulation and preservation of areas through bedrock channels and structurally a borate deposit. A regional gravity survey of the areas. Some of the major bedrock channels and type described in this report is believed by the author areas are revealed by the gravity survey even with tH to be the best method yet tested for performing the spacing of stations used in this survey. However, to first step. It is also a valuable tool in performing the define many of the channels adequately^ much more second step, but it should be used in conjunction with detailed coverage would be required. Although xo other techniques. comprehensive study of the hydrology in the western In the vicinity of Boron and Kramer Junction sev­ Mojave Desert was made in conjunction with th«> eral test holes in basins inferred from the gravity gravity survey, several of the best water producing results have been drilled for the U.S. Geological Sur­ areas seem to occur near the margins of the gravity vey. The basin 3 miles south of Kramer Junction was low, probably because coarser materials occur away tested to a depth of 3,500 feet, but only sandstone from the center of the basins. Several faults are conglomerate were found to that depth. The basin known to act as ground-water barriers. If these faults 5 miles west of Boron was tested to a depth of 2,328 have sufficient vertical displacement, they could H feet, but the core showed only alluvial sands and silts. located by gravity measurements. In the basin just north of Kramer Junction 3 test holes were put down, the deepest to 3,500 feet; the core CONCLUSIONS of all 3 holes contained lake-bed clays and some cole- In the western Mojave Desert gravity exploration is manite. Further drilling is needed to determine if an effective and economical method of obtaining sut - this basin contains borate deposits of economic value. surface information in areas where the cover cf In the western Mojave Desert considerable evidence Quaternary sediments obscures most of the underlying indicates that the low point in a basin of Cenozoic fill geology. When considered on a regional scale, the may migrate during deposition over a considerable gravity data reveal significant regional trends. area. Therefore, the fine sediments and salines gen­ Because the location and extent of the major ac­ erally deposited in the low part of a basin are not cumulations of Cenozoic fill can be inferred from ths necessarily confined to the present topographic low gravity data, these data can make an important cor- area in a basin or the area of thickest fill as indicated tribution to the exploration for, and the developmert by gravity data. Thus, the search for saline deposits of, the mineral resources occurring within the Cenozoic should not be confined to areas below the modern fill. By indicating the approximate depth of burkl playa surfaces or to the thickest Cenozoic section. of the pre-Cenozoic rock, the gravity data indicate Many unsuccessful oil tests have been drilled in the covered areas where it might be feasible to explore for western Mojave Desert. Some of the tests have been mineral deposits within the pre-Cenozoic rocks. drilled at sites selected on the basis of very meager The geologic information inferred from the gravity geologic evidence. The western Mojave Desert is not data is an important step toward a better under­ considered by most authorities to be favorable for the standing of the geology of the western Mojave Desert. origin and accumulation of petroleum deposits. The This information can serve as a guide to direct more information inferred from the gravity data relative intensive geophysical and geologic studies of the many to the Tertiary deposits, which are most likely source problems that exist in the region. of petroleum, should be helpful in evaluating the possibilities and in locating areas for additional tests. SELECTED REFERENCES The relationship between the gravity anomalies and Bailey, T. L., and Jahns, R. H., 1954, Geology of the Transverse the occurrence and movement of ground water in the Range province, southern California: California Div. Mine* Bull. 170, chap. 2, p. 83-106. western Mojave Desert is not completely understood Benda, W. K, Erd, B. C., Smith, W. C., 1960, Core logs fror at present. The relationship is not consistent over all five holes near Kramer, in the Mojave Desert, Calif.: U.P the region, and in large areas little or no data relating Geol. Survey Bull. 1045-F [in press]. 72 GEOPHYSICAL FIELD INVESTIGATIONS Bowen, O. E., Jr., 1954, Geology and mineral resources of the Mumford, R. W., 1954, Deposits of saline minerals in southern Barstow quadrangle, California: California Div. Mines California: California Div. Mines Bull. 17'), chap. 8, p. 15-22. Bull. 165. Nettleton, L. L., 1940, Geophysical prospect'ng for oil: New Buwalda, J. P., 1954, Geology of the Tehachapi Mountains, Cali­ York, McGraw Hill, p. 444. fornia : California Div. Mines Bull. 170, chap. 2, p. 131-142. Noble, L. F., 1926a, Borate deposits in the Kramer district, Crowell, J. C., 1952, Geology of the Lebec quadrangle, California: Kern County, California: U.S. Geol. Survey Bull. 785-C, California Div. Mines Spec. Kept. 24. p. 46-61. Dibblee, T. W., Jr., 1952, Geology of the Saltdale quadrangle, ______1926b, The San Andreas rift and some other active California: California Div. Mines Bull. 160. faults in the desert region of southeastern California: _____ 1957, Simplified geologic map of the western Mojave Carnegie Inst. Washington Yearbook 25, p. 415-428. Desert, California: U.S. Geol. Survey open-file report. ______1953, Geology of the Pearland quadrangle, California: Dobrin, M. B., 1952, Introduction to geophysical prospecting: U.S. Geol. Survey Geol. Quad. Map GG-24. New York, McGraw Hill, p. 435. ___ 1954a, Geology of the Valyermo quadrangle and vicin­ Gale, H. S., 1946, Geology of the Kramer bo rate district, Kern ity, California; U.S. Geol. Survey Geol. Quad. Map GG-50. County, California: California Jour. Mines and Geology, ___ 1954b, The San Andreas fault zone from Soledad Pass v. 42, p. 325-378. to Cajon Pass, California: California DH. Mines Bull. 170, Hess, F. L., 1910, Gold mining in the Randsburg quadrangle, chap. 4, p. 37-38, pi. 5. California: U.S. Geol. Survey Bull. 430, p. 23-47. Oakeshott, G. B., Braun, L. T., Jennings, C. W., and Wells, Ruth, Hewett, D. F., 1954a, General geology of the Mojave Desert re­ 1950, Exploratory wells drilled outside of oil and gas fields gion, California: California Div. Mines Bull. 170, chap. 2, in California to December 31, 1950: California Div. Mines p. 5-20. Spec. Rept. 23, 75 p. _____ 1954b, A fault map of the Mojave Desert region: Cali­ Oliver, H. W., 1956, Isostatic compensation for the Sierra Ne­ fornia Div. Mines Bull. 170, chap. 4, p. 15-18. Hill, M. L., 1954, Tectonics of faulting in southern California: vada, California [abs.]: Geol. Soc. America Bull. v. 67, no. 12, p. 1724. California Div. Mines Bull. 170, chap. 4, p. 5-13. Hill, M. L., and Dibblee, T. W., Jr., 1953, San Andreas, Garlock, Simpson, E. C., 1934, Geology and mineral deposits of the Eliza­ and Big Pine faults: Geol. Soc. America Bull., v. 64, p. 443- beth Lake quadrangle, California: California Jour. Mines 458. and Geology, v. 30, p. 371-415. Hulin, C. D., 1925, Geology and ore deposits of the Randsburg Swick, C. H., 1942, Pendulum gravity measurements and isostatic quadrangle of California: California Bur. Mines Bull. 95, reductions: U.S. Coast and Geod. Survey Spec. Pub. 232, p. 11-148. 82 p. Jahns, R. H., 1954, Investigations and problems of southern Thompson, D. G., 1929, The Mojave Desert region, California: California geology: California Div. Mines Bull. 170, chap. 1, U.S. Geol. Survey Water-Supply Paper 578. p. 5-29. Troxell, H. C., and Hofmann, W., 1954, Hydrology of the Mojave Mabey, D. R., 1956, Geophysical investigations in the intermon- Desert: California Div. Mines Bull. 170 chap. 7, p. 13-17. tane basins in southern California: Geophysics, v. 21, p. 839- Wiese, J. H., 1950, Geology and mineral resources of the Neenach 853. quadrangle, California: California Div. Mines Bull. 153. McCulloh, T. H., 1954, Problems of the metamorphic and igneous Wright, L. A., and Troxel, B. W., 1954, Western Mojave Desert rocks of the Mojave Desert: California Div. Mines Bull. and Death Valley region: California Div. Mines Bull. 170, 170, chap. 7, p. 13-24. Geol. Guidebook 1, 50 p. INDEX

Page Pare Acknowledgments__-__ - . _____.______53 Jawbone Canyon. 61,63 Alluvial fan.._.--______...... _.____._.. 61,66 Alluvial sediments, Quaternary______. ______52,53,54,64,65,69,70 KoehnLake______-______. ______-__ 53,60 Antelope Buttes_-______54,67,68 Kramer borate district___.______.______- __ 64,70 Antelope Valley.... _. 54,66-67 KramerHills 54,64,65 Barstow syncline-_____...______63 Lake beds, borate-bearing.._ _ 64 Basalt, lava flow______--______54,64 Lava Mountains...___..._ - -- 54 Basement complex, pre-Tertiary_._____ 53-54,57, 58, 59,61,63,64,65,66,67,68,69 Leach Lake______.__-- - - 70 Basin and Range province._.__ _ .______53,70 Liebre Mountains....__..-...-- . ... - 56 Basins...- . . 53,54,58,60,64,70,71 Location of area.. _____ - _ - .- 51 Black Butte area...______.______66 Lockhart fault...._... .__ ___------_....._...... _ . 63,64,65,69 Borate-bearing lake beds.. ... - - . ___....__-..__ 64 Lovejoy Buttes. -----_ .. _...--_..___._ - 66,67 Borate minerals-..______.______..______60,64,71 Boron, discovery of borate deposit- . _. ______70 McCulloh, T. H., quoted 53-54 Bouguer anomaly map....____.. _,. -______56 Magnetic methods ... 53 Mineral deposits 52,60,64,70,71 Cajon Creek___...__ _. - ....-______66 Mirage Lake-.. _ _- 66,68,69 Cajon Pass..- .-..__.....____._.___._._.______54,65,66,69,70 Mojave block __ _ 54, 55, 56,68-69,70 Oajon Valley-______.___-----_-___-____-__.____ 66 Mojave River. ______.___ 53 Cajon Valley fault __.___.._____ 66 Mount San Antonio___...... - 53 Canta Valley __ 53,54,59,60,61,62,63,70 Cantil Valley fault...... _.__.. 60,61 Noble, L. F,, quoted._. - ... ----- 70 Cenozoic rocks.. . 53, 56,57-58, 59,60,61,62-63,64,65,67,68,69,70,71 Conclusions . 71 Oro Grande series-._ ... M Cnddeback Lake______63,68 Cultural features.._____.____ _. _ -.-______51 Pelona schist. . Pennisular Ranges. 54 Daggett basin______.__...____...__.-.______65 Playas _ - 53, f« Density contrast .__ . ____.______59 Plutonic igneous rocks ______. _ __ _ -- 54 Density range of major rock types.. _ ..______57-58 Precipitation, annual __ . . 51-52 Double Mountain_.__ .. ______53 Purpose of investigations _ ...... 52-J3 Drainage. . . __-...... 53,54,69 Drill holes._ 60,61,63,64,65,66,67,68,71 Quartz Hill..... _ ...... ^------^ Quaternary deposits __ ... . __ .. 54,55,57,65,71 El Paso fault...... _... . 59,60,61,62 El Paso Mountains .. ______51,59,60,61 Rand Mountains _____ ...... 60,61,63,69 Elevation-correction factor_._ ___ .______56 Red Mountain ______.. . . . £