Reconnaissance Geology Between 

Lake Mead and Davis Dam 

Arizona-Nevada

GEOLOGICAL SURVEY PROFESSIONAL PAPER 374-E
Reconnaissance Geology Between 

Lake Mead and Davis Dam 

Arizona-Nevada

By CHESTER R. LONGWELL

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 374-E

Bedrock and structural features in a belt along 

the Colorado River

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEW ART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D.C.
CONTENTS

Abstract---_---_---------_--_--____________________

Introduction -_----__-----_--_-_____________________

Purpose and method of present study_____________

Acknowledgments. _ ______________________________

Climate and vegetation________________._________

Surface features-__-------___-__________________

Earlier work in the area_________________________

Geology of the area_________________________________

General statement______________________________

Precambrian rocks______________________________

Sedimentary rocks______________________________

Formations northwest of Lake Mead__________

Pliocene(?) basin deposits.-__________________

Muddy Creek formation.________________

Landslide breccia.__________________

Older alluvium _____________________________

Chemehuevi formation ______________________

Younger alluvium.__________________________

Cretaceous(?) and Cenozoic igneous rocks__________

Intrusive bodies. ___________________________

Large plutons.-_--_-___________.______.

Dikes, sills, and plugs___________________

Volcanic rocks._____________________________

Patsy Mine volcanics_-__________________

Golden Door volcanics__________________

Mount Davis volcanics--__-_____________

Volcanic rocks of Muddy Creek formation.

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	Page 

Geology of the area Continued

Cretaceous (?) and Cenozoic igneous rocks Continued 

Volcanic rocks Continued

Comparison with adjacent areas._________ E31

Pleistocene(?) igneous rocks._____________ 33

Structure ______________---_---__-___-__-------- 34

Structure of Black Mountains.-______________ 35

Section along Boulder Canyon__-_---_-__- 35

Structure near Fortification Hill __________ 36

Faulted western border._________________ 37

Structure north and west of Boulder Basin_____ 39

Structure west of Black Mountains ___________ 39

Boulder Basin to Eldorado Wash _________ 39

Northern Eldorado Mountains block. _ 39

Nelson block._._____________--_--_- 40

Eldorado Wash to Newberry Mountains. __ 41

Structure east of Newberry Mountains _ _ _ _ 42

Dating of faults.____________-_-_------- 43

Mineral deposits-____-_--__------__------------- 43

Geologic history of the area__________________________ 43

Paleozoic era_______________-_____--_---_-__---- 43

Mesozoic era.____________________--_----------- 44

Cenozoic era_____________________-__--__------_ 45

References._ ____________________________---_----__- 46

Index._____-_____-_-__-___-__-__----------_-----_- 49

ILLUSTRATIONS

	[Plate 1 in Pocket]

PLATE 1. Geologic map and sections of area along the Colorado River between Lake Mead and Davis Dam, Arizona and

	Nevada. Page 

FIGURE 1. Index map showing location and relations of principal geographic features..____________-_-____---_------_--- E2

2. Columnar sections north and south of Lake Mead. __-_____________.__________-__--_-___-____-_------_--- 7

3. View westward on north side of Fortification Hill.__--______-__--_____________--__---_-____--__----_-_--- 8

4. Cliff of cemented gravels on south side of Eldorado Wash__. -______-___-_________________-__-_------------ 11

5. General relations among sedimentary units designated alluvium._______-__--_-_____-_-_____-_-_---_--_--- 12

6. View generally eastward, north of Davis Dam site__________________________________-________-__-_____--_ 13

7. Newberry's section through Elephant Hill______________________________________________________________ 15

8. Unconformity between Patsy Mine and Golden Door volcanics-________-____-_-_______-_________-----__--- 20

9. Airview southeastward across Hoover Dam_____________________________________________________________ 23

10. Three units of volcanic rocks north of Nelson.____-_______________-__-____-_---__-_--__-----__-__--__--- 25

11. Ratios in nine porphyritic volcanic glasses___-___--_---_-____-_-__-___-_____---_--_---_--_---_-_-------- 30

12. Scarp of Fortification fault-- ____-__-____-__--_--_-___________-__-_--__-__--__-----__--_____-__------- 36

13. Geologic section along line H-H' ______________________________________________________________________ 39

14. East-west fault north of Malpais Mesa_________________________________________________________________ 40

15. Fault zone in Precambrian rocks at Davis Dam_ ________________________________________________________ 42
TABLES

Page 

TABLE 1. Sequence of Tertiary rocks near Lake Mead_--____--_-___-------------------»--------------------------- E10

2. Chemical analyses of 13 volcanic glasses-_________________--_____-___----------------------------------- 28

3. Bulk composition versus groundmass composition in 9 volcanic glasses___________________-__-___-__-_----_- 30

4. Volcanic rocks of Oatman district, Arizona________--___-_-_---_-_--------------------------------------- 32

5. Comparison of chemical compositions of two volcanic rocks_______________-______-____--_------_-----_---- 32

IV
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

RECONNAISSANCE GEOLOGY BETWEEN LAKE MEAD AND DAVIS DAM, ARIZONA-NEVADA

By CHESTER R. LONGWELL

ABSTRACT

Between Hoover and Davis dams the Colorado River valley, 

trending south-southeast, separates the Black Mountains on the 

east from the Eldorado and Newberry Mountains on the west. 

For more than 20 miles directly south of Lake Mead the river 

is deeply incised in Black Canyon; farther south the stream 

course is in large part between wide alluviated slopes. Maximum relief within the area is about 4,500 feet.

Thick Paleozoic and Mesozoic formations found in ranges 

north and west of Lake Mead are not present south of the 

lake except as fragmentary xenoliths within igneous bodies. 

The principal bedrock units in a wide area bordering the 

Coloi-ado River consist of Precambrian gneiss, schist, and granitic rocks; volcanic rocks in four distinctive series, with intercalated clastic sediments; and numerous plutonic bodies, 

ranging in size from stocks to small dikes, diverse in composition and in texture. Some plutons, through which the oldest 

lavas in the area were erupted, may be contemporaneous with 

tuff beds in Cretaceous formations north of the lake. The 

Patsy Mine volcanics, oldest of the eruptive series, consist 

mainly of brown andesitic lavas and agglomerates with maximum thicknesses of several thousand feet. The second episode produced the more siliceous Golden Door volcanics, equally 

thick and widespread, containing abundant glass and lightcolored tuff. A third series, the Mount Davis volcanics, consists largely of basalt and dark andesite but at several horizons includes conspicuous units of high-silica glass and purniceous tuff. Thick beds of extremely coarse sedimentary debris, 

much of it derived from Precambrian bedrock, are interspersed 

through the upper part of this volcanic formation.

The three older volcanic assemblages have been strongly 

tilted, and commonly they are discordant one on another. All 

have been cut by intrusive bodies. In the northern part of 

the area the Muddy Creek formation, made up of clastic sediments, saline beds, and thick basaltic lavas, is unconformable 

on all older rock units. Still later igneous activity is recorded 

in local basaltic flows and dikes associated with weakly cemented slope gravels.

The area holds a network of faults, some of large displacement and at least two with important reverse movement. The Black Mountains block is a compound horst, west 

of which repetitive faulting has tilted some blocks eastward, 

others westward. Considerable movement was in progress 

during the Mount Davis volcanism, as indicated by abundant coarse debris mingled with lavas near large faults. 

In Muddy Creek time great landslide masses moved from 

the rising west margin of the Black Mountains. Colorado

River gravel beds of Pleistocene age were strongly deformed 

near some faults.

Evidence now in hand establishes an order of events 

within the area but not an exact chronology. All the volcanic assemblages appear to be younger than the Horse 

Spring formation, which is tentatively dated as early Tertiary. A lead-alpha analysis on a sample of quartz monzonite from a pluton intrusive into Golden Door volcanics 

gave an age of about 50 million years, suggesting that these 

volcanics and the older Patsy Mine volcanics are at least 

as old as Eocene. Mount Davis volcanics grade upward 

into sediments mapped as part of the Muddy Creek formation, now dated tentatively as Pliocene. The Colorado River, 

as a through-flowing stream in its present course west of 

the Plateau, originated later than the interior-basin conditions of Muddy Creek time. The river was temporarily 

impounded late in the Pleistocene epoch, as indicated by 

numerous remnants of lake deposits that lie on and are 

capped by river gravels.

INTRODUCTION 

PURPOSE AND METHOD OF PRESENT STUDY

The main body of Lake Mead, from Boulder Basin 

eastward, marks a general boundary between wide 

areas characterized by very different bedrock. Ranges 

directly north of this latitude reveal sedimentary formations that total many thousand feet in thickness; 

south of the lake, volcanic, plutonic, and metamorphic 

rocks are predominant at the surface, as has been 

known since Ives (1861) made his exploratory trip 

up the Colorado Eiver in 1857-58. Published maps 

and reports dealing with the geology near the river 

south of the lake are based on rapid reconnaissance, 

and therefore this wide region offers prospects of new 

information that may be critical for deciphering the 

history of an important fraction of the Basin and 

Eange province. The present study was undertaken 

to extend and supplement a mapping program covering several ranges north and northwest of Lake Mead

(% 1).

El
E2 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

FIGURE 1. Index map showing location and relations of principal geographic features mentioned in text.
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E3

Fieldwork in the spring of 1949 was timed to anticipate completion of Davis Dam, then under construction by the United States Bureau of Reclamation. Although this dam has only moderate height, 

and the lake it impounds (Lake Mohave) is both 

narrow and shallow in comparison with Lake Mead, 

some features of local importance that were visible 

in 1949 are now under water. The area studied is 

more than 60 miles long and averages about 18 miles 

wide. So large an area, much of it with rugged 

surface, could not be covered in detail within the 4 

months available for fieldwork, and the resulting map 

is necessarily of reconnaissance nature. Considerably 

more time and effort could not have produced a strictly 

accurate result on the base maps then available, especially on the Arizona side. The Boulder Canyon, Nelson, and Camp Mohave preliminary sheets of the U.S. 

Geological Survey, on the scale 1:96,000 and with contour interval 100 feet, cover the Nevada side and a 

limited area in Arizona northeast of Hoover Dam. 

These maps, based on surveys by engineers of the Los 

Angeles Metropolitan Water District, are fairly accurate. For the belt in Arizona between Hoover and 

Davis Dams, the topographic base of plate 1 was improvised from the old Camp Mohave sheet (scale 

1:250,000) of the U.S. Geological Survey and from 

special maps issued by the National Park Service. In 

that part of plate 1, representation even of major 

topographic features is inaccurate, and a contour interval less than 200 feet would not be justified.

In preparation for the Lake Mohave project, the 

U.S. Bureau of Reclamation completed in 1939 an 

accurate map, on a scale of 1 inch to 1,000 feet and 

with a 10-foot contour interval, of a narrow belt along 

the Colorado River between Hoover Dam and a point 

several miles south of the Davis Dam site. The only 

aerial photographs available in 1948 for the area of 

the present report were made by the U.S. Forest Service and covered a strip along the river somewhat wider 

than the Reclamation map. After the field study was 

completed, the U.S. Geological Survey issued three 

15-minute topographic maps: the Henderson and 

Hoover Dam quadrangles bordering the lower part of 

Lake Mead and the Davis Dam quadrangle. Parts of 

these three maps and the corresponding aerial photographs have been useful in correcting the northern 

and southern parts of plate 1 in the present report.1

Until recent years access to much of the area was 

difficult, but with developments along the Colorado 

River, new roads have been constructed and old roads 

improved. Boats on the artificial lakes Mead and 

Mohave now facilitate work in the main valley, particularly in Black Canyon where the river had several

1 Fifteen-minute quadrangles now available include the entire area 

of this report.

stretches of swift rapids. The jeep is indispensable 

for covering much of the rough country still remote 

from good roads.

ACKNOWLEDGMENTS

Several organizations cooperated with the U.S. Geological Survey in support of this study. The Geological Society of America aided with a generous grant 

from the Penrose Bequest. Yale University gave the 

writer extended leave of absence in the season most 

favorable for fieldwork along the Colorado River. 

Later stages of the study were helped by results of a 

lengthy field season northwest of Lake Mead, made 

possible by a grant in 1954 from the National Science 

Foundation. Officers of the Bureau of Reclamation 

and of the National Park Service supplied maps and 

photographs, and extended other courtesies.

Thin sections for petrographic study, made by the 

U.S. Geological Survey, were studied by Gerald V. 

Carroll and Matt Walton. A grant from the Higgins 

Fund of Yale University paid for 13 chemical analyses of volcanic glasses. Philip Orville made the study 

of these glasses summarized in figure 11. Howard R. 

Gould, at that time a member of the U.S. Geological 

Survey, made measurements of fine clastic grains in 

the beds of the Chemehuevi formation (p. E14).

Grateful acknowledgment is hereby extended to the 

above organizations and individuals for their aid. 

Special thanks go to Henry C. Rainey, who as field 

assistant gave invaluable help and pleasant companionship.

CLIMATE AND VEGETATION

Because the climate of the region is arid, soil cover 

is poorly developed, and on the steeper slopes excellent examples of bedrock are exposed. The desert 

vegetation is sparse, and most of the individual plants 

are small, though locally along the river, mesquite, 

willow, cottonwood, and a few other large shrubs and 

trees once grew in clumps and thickets on some of the 

low ground now covered by the lake. Yuccas of several kinds are the largest plants on upland surfaces. 

"Joshuas" (tree yuccas), found as scattered individuals on alluvial ground at altitudes near 3,000 feet, 

grow in small groves on many higher slopes. Spanish 

bayonet and smaller kinds of yucca are common 

through a considerable range of altitudes. Creosote 

brush and the thorny ocotillo thrive on many of the 

gravelly slopes, and many kinds of cactus are widely 

dispersed. Sage thrives locally at altitudes above 3,000 

feet.

Annual precipitation averages little more than 5 

inches. During the summer months, temperatures 

generally are too high for effective fieldwork in any
E4 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

part of the area. From October to the end of May, 

conditions for field study are as a rule comfortable, 

though in exceptional years midwinter brings short 

periods with moderately low temperatures and occasional snow, even at low altitudes.

SURFACE FEATURES

Between Lake Mead and Davis Dam, the Colorado 

River valley, with average trend somewhat east of 

south, is flanked on the east by the Black Mountains 

and on the west by the Eldorado and Newberry Mountains (fig. 1; pi. 1). Each of these ranges has moderately high and rugged stretches separated by much 

lower divides. The highest points in each range have 

altitudes of about 5,000 feet, and maximum relief of 

the valley sides is about 4,500 feet in a horizontal distance of 6 to 9 miles from the river.

Topography of the valley slopes varies widely. 

From the wide Boulder Basin at the lower end of 

Lake Mead, the old course of the river enters the 

narrow confines of Black Canyon, the walls of which, 

either as sheer cliffs or in successive steps, reach several hundred feet above the former river bed (pi. 1). 

Along the 25-mile length of the canyon most of the 

tributary washes enter with grades that are steep, 

many of them precipitous. Below the mouth of Black 

Canyon, the valley widens and its slopes for the 

most part are covered with gravels derived from 

adjacent highlands. Considerable thicknesses of the 

gravel are unconsolidated, but at the lower exposed 

levels much of it is cemented and has been incised 

by the river and tributary washes to depths of some 

tens of feet (fig. 4). Locally hills of bedrock, some 

of them steep and of large size, protrude through the 

widespread gravel cover. The most thorough grading of slopes, on both sides of the river, has been 

accomplished in a stretch east of Searchlight, where 

the gentle, alluvium-covered valley sides are about 

20 miles wide. Farther south the steep eastern front 

of the Newberry Mountains extends to the river, and 

near Davis Dam the valley is again incised to form 

a gorge with rather low discontinuous walls.

The several kinds of bedrock represent a wide 

range in resistance and in structure, reflected in the 

highly varied topography. Gneiss, schist, and granite, together with younger plutonic rocks, underlie 

high parts of the Black, Eldorado, and Newberry Mountains. Although these crystalline rocks are extremely 

heterogeneous in composition and in detailed structure, in many large masses they have responded to 

erosion generally as homogeneous bodies. Exceptionally large granitic plutons are more resistant than 

the bordering metamorphic complex, as is conspicuous in the highest part of the Newberry Mountains (pi. 

1). In parts of the Black Mountains, however, the 

gneiss and schist underlie the highest topographic 

features. The steepest walls of Black Canyon are supported by thick volcanic flows and breccias. Tuffs, on 

the other hand, have yielded to vigorous attack and are 

commonly found in lowlands or valleys. Weak sedimentary deposits, some of considerable thickness, form 

badlands. Even the weakest rocks are protected locally in buttes and mesas capped by gently dipping 

flows of basalt, as in Fortification Hill and Malpais 

Mesa (fig. 3).

Surface forms of large extent within the area have 

important bearing on the history of drainage in a 

wide region. The western rim of the Colorado River 

valley borders active basins of interior drainage, only 

a few miles from the river and at much higher altitude. The eastern rim of one of these basins, containing the large playa southwest of Boulder City, 

is only 3 miles from Black Canyon and 1,600 feet or 

more above the river. Short tributaries to the river 

are cutting headward into some of the basin deposits. About 1,800 feet above the river, on the west 

rim of Black Canyon, remnants of caliche form eastfacing cliffs as high as 30 feet. These remnants wedge 

out westward and apparently represent the capping 

of a deposit that occupied the adjacent part of the 

Colorado River valley. Remnants of a once extensive 

fill, ranging in coarseness and composition from large 

angular rock fragments to gypsiferous clay, appear beneath high basalt caps, as in Fortification 

Hill, and at much lower altitude in the area of badlands east of Willow Beach. These deposits are like 

those found in basins with interior drainage; therefore, they raise a question of the date at which the 

Colorado River had its beginning as a through-flowing stream.

Terraces and other surface features that record definite events in the history of the river are discussed 

on subsequent pages.

EARLIER WORK IN THE AREA

The earliest published map representing the general geology in the area here considered was offered 

by Jules Marcou (1858), who in 1853-54 was geologist in the expeditionary force charged with finding 

a route suitable for a railroad to the Pacific coast. 

Marcou's crude map had scant basis in field observation. A more informative report was given by J. S. 

Newberry (1861), who as geologist with the Ives expedition of 1857-58 ascended the Colorado River 

from the Gulf of California to the mouth of the 

Virgin River and made numerous traverses into the
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E5

adjacent country. G. K. Gilbert (1875), geologist 

with the Wheeler Survey in 1871-73, followed the 

river from the southern tip of Nevada into the Grand 

Canyon; he made notes on the principal kinds of 

rocks along the route and on locations of major 

faults.

A more detailed geologic reconnaissance in the area, 

by Willis T. Lee (1908), of the U.S. Geological Survey, resulted in a small-scale (about 1:580,000) geologic map showing the distribution of principal rock 

types on the Arizona side of the river, accompanied 

by explanatory text and a series of structure sections. A supplemental study by F. C. Schrader (1909) 

emphasized mineral deposits of a large area in western Arizona and produced a geologic map of larger 

scale (1:250,000) covering a belt along the Colorado 

River extending south of lat 36° and westward into 

Nevada to long 114° 45'. At about the same time, 

F. L. Ransome (1907) studied several mining properties in the Searchlight and Eldorado districts of 

southern Nevada and published brief notes and a 

reconnaissance map. N. H. Barton's geologic map 

of Arizona (1924, scale 1:500,000) covers the part 

of the area of this report that lies east of the river.

Carl Lausen (1931) contributed some details for 

the Katherine mining district, near the present site 

of Davis Dam.

During early stages of construction work on 

Hoover Dam, Ransome made a careful geologic study 

of an area along Black Canyon for the Bureau of 

Reclamation. His map, with parts of his descriptive text, was published nearly 20 years later (U.S. 

Bur. Reclamation, 1950). In 1941 a group of U.S. 

Geological Survey geologists examined the region 

from Lake Mead southward for promising ore deposits, especially those containing managanese. Two 

of the resulting publications (Hunt, McKelvey, and 

Wiese, 1942; McKelvey, Wiese, and Johnson, 1949) 

have information useful for the present report. Written communications from several workers in the area 

were also made available to the present writer.

GEOLOGY OF THE AREA

GENERAL STATEMENT

Precambrian metamorphic and igneous rocks, and 

much younger igneous rocks, both extrusive and intrusive, are overwhelmingly dominant in the exposed 

bedrock. Sedimentary deposits that merit formation status occupy only very limited areas within a 

radius of 15 miles from Hoover Dam. Large quantities of sedimentary debris, chiefly coarse and locally derived, are interlayered with and therefore 

mapped with sequences of volcanic rocks. Alluvium

that covers wide surfaces embraces deposits of several kinds: unconsolidated slope gravels, playa silt, 

stream alluvium of varied coarseness, remnants of 

lake beds, and still older gravel and silt that are 

well cemented.

PRECAMBRIAN ROCKS

Metamorphic and coarse-grained igneous rocks 

make up a large part of the bedrock exposed in the 

Black, Eldorado, and Newberry Mountains, and underlie other areas near the Colorado River. Several 

kinds of gneiss, schist, and granitic rocks are included. Although this assemblage cannot be dated 

directly in relation to known Cambrian strata within 

the area of this report, comparison with rocks not 

far distant to the north and northeast makes the correlation reasonably certain. At the west base of 

Frenchman Mountain, about 20 miles northwest of 

Hoover Dam, beds containing Lower Cambrian fossils 

lie unconformably above a crystalline complex strikingly similar to that in the Black, Eldorado, and 

Newberry Mountains. The Precambrian rocks of the 

southern Virgin Mountains and at the base of the 

Grand Wash Cliffs also closely resemble those so 

classified south of Lake Mead. Suggestions that 

some of the metamorphic rocks in the river belt may 

belong in Paleozoic or even later systems do not pass 

the test of regional comparative study.

The complex here classed as Precambrian is highly 

varied lithologically. Most conspicuous are strongly 

banded gneisses, many with granitic composition, 

others consisting largely of quartz, hornblende, and 

biotite. Schists, both micaceous and chloritic, are 

an important ingredient, and local masses of considerable size consist of hornblende schist. Garnets, 

some of large size, are conspicuous at many localities 

in gneiss, schist, and granite. Some prominent 

quartzose bands may be strongly metamorphosed beds 

of a sedimentary sequence. Coarse granitic rock, both 

pink and gray, is intrusive into the gneisses and 

schists as layers, irregular dikelets, and scattered 

bodies of larger size. Some irregular masses of pink 

granite have pegmatitic texture, but well-defined 

veins of pegmatite are uncommon. One of the most 

conspicuous rock types is a crudely banded gneiss 

rich in biotite and thickly studded with large porphyroblasts of feldspar. This rock forms large 

masses in Newberry Mountains and on both sides of 

the river near Davis Dam.

Banding of the Precambrian rocks generally has 

steep dips and northerly strikes prevail (pi. 1) ; but 

adjacent to large faults and younger intrusive igneous 

bodies, the attitude has been considerably modified. 

In the Black Mountains east of Willow Beach, a

661-325 0-63-2
E6 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

general westward dip at about 50° steepens to nearly 

vertical near the reverse fault along the west base 

of the range. On Saddle Island, at the west end of 

Lake Mead, the dips are westward at angles as low 

as 15° to 20°. The dominant rock in this isolated 

outcrop is a distinctive greenish schist (Longwell, 

1936, p. 1406).

Many bodies of younger granitic rocks cut the Precambrian rocks on both sides of the Colorado River 

valley. Only a few of these are indicated on the 

map, and these with approximate boundaries; many 

others are not differentiated in areas mapped generally as Precambrian. Moreover, the map does not 

show numerous dikes and irregular intrusive bodies, 

ranging in lithology from basalt to rhyolite, which 

cut the Precambrian rocks in many areas. Some of 

these younger bodies are isolated; but many are 

grouped and at several localities form swarms. All 

these intrusive bodies, large and small, have escaped 

the intensive shearing and granulation that are characteristic of granitic rocks in the Precambrian complex. On evidence explained in a subsequent section, 

the igneous bodies younger than Precambrian are 

supposed to be no older than Mesozoic, and probably 

they are in large part Cenozoic. Many of these intrusive masses are directly related to the volcanic 

rocks described elsewhere.

SEDIMENTARY BOCKS 

FORMATIONS NORTHWEST OF LAKE MEAD

One of the most notable facts about the geology 

south of Lake Mead is the absence of the thick sedimentary section that characterizes all the high ranges 

directly north and northwest of the lake. Only a 

few miles northwest of Las Vegas Wash, in Frenchman Mountain and in the hilly country east of it, a 

thick section of Paleozoic and Mesozoic strata is 

tilted steeply eastward. The formations are mostly 

those recognized in the belt of country north of Lake 

Mead from the Grand Wash Cliffs westward through 

the Virgin and Muddy Mountains. As shown in the 

columnar section (fig. 2), all Paleozoic systems except the Silurian (and questionably the Ordovician) 

are represented, a total thickness of about 8,000 feet. 

In normal development and with combined thickness 

of about 3,000 feet, the Triassic Moenkopi and Chinle 

formations are overlain by typical Aztec (Jurassic?) 

sandstone, which is here much reduced beneath an 

angular unconformity from its normal thickness of 

more than 2,000 feet.

Above the. imconformable contact lies 2,000-3,000 

feet of red beds, largely siltstone and sandstone, interbedded with fresh-water limestone, considerable

gypsum, and great lenses of coarse conglomerate and 

sedimentary breccia. This thick section constitutes 

the Thumb formation the new formation whose 

name is suggested by Thumb Valley, a small valley 

tributary to Las Vegas Wash about 4 miles southeast 

of Frenchman Mountain. A large part of the section is well exposed in this valley, though the base, 

resting on Triassic and Jurassic formations, is a short 

distance to the west. At this type section the beds 

are in homoclinal attitude, with eastward dips ranging from 45° to more than 60°. Several members 

are distinguished a's follows:

Horse Spring formation.

Erosional unconformity.

Thumb formation: Feet

Breccia, coarse; probably of landslide origin: 

made up of Precambrian metamorphic and granitic rock fragments ________________ 50-300

Sandstone, brown and flaggy; grades laterally 

into gypsiferous siltstone and claystone; locally includes thick section of basaltic lavas_ 900 ±

Sandstone and siltstone, reddish-brown, and thick 

lenses of coarse conglomerate. Locally include 

large bodies of bieccia made of Precambrian 

rock fragments ________________ _ 750 ±

Coarse conglomerate, thinning and growing less 

coarse northward ____________-_-_ 150 ±

Sandstone, yellow and fine-grained; above and 

below thick-bedded gypsum and gypsiferous 

clay __________________________ 200-250

Fresh-water limestone, gray and yellowish, thinbedded ; lower and upper parts interlayered 

with siltstone ____________________ 200-300

Sandstone and siltstone, reddish-brown; lenses 

of conglomerate and fresh-water limestone___ 300-400

Basal conglomerate, well-indurated; cobbles and 

boulders derived from all older formations exposed in surrounding area _____________ 30-150

Angular unconformity.

Jurassic and Triassic formations.

No fossil evidence has been found to date the 

Thumb formation. From its stratigraphic relations 

and some general similarities to the Willow Tank 

formation and the Baseline sandstone exposed 35 

miles to the northeast near Muddy Valley, the Thumb 

formation is tentatively classified as Cretaceous(?). 

Above it lies the Horse Spring formation, more than 

2,000 feet thick, consisting largely of fresh-water 

limestone but including basaltic lavas, some siltstone, 

and lenses of coarse fan deposit. This formation 

also has yielded no diagnostic fossils, but from its 

stratigraphic position in the Muddy Mountains, it 

may be either Late Cretaceous or Tertiary in age. 

It is tentatively assigned to the Eocene (?).2 Strong 

angular unconformity separates the formation from

2 Two recent analyses of biotite in volcanic ash near the base of the 

formation indicate a date of 22-23 million years. This suggests Miocene age.
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA 

A

E7

RECENT Alluvium

PLEISTOCENE Lake deP°slts KLLISIULtlNt Cemented grave| /

Muddy Creek

PLIOCENE(?) 

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Horse spring 

EOCENE(?) 

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JURASSIC(?) Aztec sandstone

Chinle formation

Shinarump member 

TRIASSIC

Moenkopi formation

Kaibab limestone

Toroweap formation 

PERMIAN

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Callville

PENNSYLVANIAN

limestone

MISSISSIPPIAN Monte Crist°

MISSISSIK^IAIN formation

DEVONIAN Sultan formation 

ORDOVICIAN(?) \ ________

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Taoeats sandstone

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Chemehuevi formation

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Mount Davis volcanics

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rhyolite glass and tuff; 

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PRECAMBRIAN

Gneiss, schist, granite, inclosing younger igneous 

bodies

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flow

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flows and tuff

Sills and dike

FIGURE 2. Columnar sections north and south of Lake Mead: A, Section in Frenchman Mountain block, 13-20 miles northwest 

of Hoover Dam; B, Composite section between Lake Mead and Davis Dam.
E8 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

the overlying sands, silts, and clays of the Pliocene (?) Muddy Creek formation, which extend beneath the western basin of Lake Mead.

Parts of the Thumb and Horse Spring formations 

reach a short distance south of Las Vegas Wash, in 

the northwestern part of the map area, where they end 

against younger volcanic and intrusive rocks. In general, however, the entire Frenchman Mountain section which, from Cambrian to Tertiary (?) units inclusive, has a total thickness of about 17,000 feet, is 

absent from most of the area of this report and from 

a much larger contiguous area on the east, south, and 

west. There is some evidence that at least part of 

the Paleozoic section once extended farther south. 

In the Black Mountains directly south of Boulder 

Canyon, a large fault block made up of Cambrian 

shale and limestone is intruded and virtually engulfed by igneous rock (Longwell, 1936, pi. 2). Numerous blocks of limestone and dolomite, lithologically like that in several Paleozoic formations, occur 

as xenoliths in plutonic bodies in the southeastern 

part of the River Mountains, near the head of Hememvay Wash. Barite in and around some of these 

xenoliths has been prospected extensively.

A distinctive section of sedimentary rocks exposed 

north of Boulder Canyon, between the Ransome fault 

and Boulder Wash, consists of yellowish dolomite, 

extremely coarse breccia, and thin beds of sandstone, 

shale, and limestone. The higher part of the section 

encloses sheets of andesitic lava, which become dominant upward (Longwell, 1936, p. 1407-1409). This 

isolated unit of unknown age is included on plate 1 

with the group of igneous rocks labelled Tiv.

Indirect but strongly suggestive evidence bearing 

on the former extent of some formations is supplied 

by the Cretaceous (?) redbeds section east of Frenchman Mountain, in which northward-tapering wedges 

of coarse debris indicate a highland source directly 

to the south. The basal conglomerate, which bevels 

across the Triassic and Jurassic (?) formations from 

south to north, is made up largely of limestone fragments, many of boulder and cobble size; lithologic 

peculiarities and included fossils indicate their derivation from several Paleozoic and Mesozoic formations. Scattered pebbles from Precambrian bedrock 

also are included. Higher in the section great lenses 

of breccia, probably of landslide origin, contain fragments of Precambrian rocks only (Longwell, 1951, 

p. 352). It is inferred, therefore, that a broad area 

south of Las Vegas Wash and Lake Mead was strongly 

uplifted in late Mesozoic time and denuded of its 

Mesozoic and Paleozoic sedimentary formations. The 

area of uplift was later a theater of prolonged igneous activity, and resulting lava accumulations conceal the older bedrock which supplied the debris of 

the breccia lenses. The ridge that forms Saddle 

Island, which possibly marks a high part of the old 

land surface, has been exhumed by erosion following 

localized deformation (Longwell, 1936, pi. 2).

Because the Horse Spring formation and older 

sedimentary units occupy only a small fraction of 

the map area considered here, they are represented 

in somewhat generalized fashion on plate 1.

PMOCENE(?) BASIN DEPOSITS

MUDDY CREEK FORMATION

Along the river south of Lake Mead, the oldest 

sedimentary beds, other than local accumulations of 

coarse detritus interbedded with volcanic rocks, were 

deposited in small enclosed basins. These deposits 

are strikingly like those of the Muddy Creek formation, which are widely exposed east of the Frenchman Mountain block and were well displayed in the 

wide basin directly north of Black Canyon before 

the formation of Lake Mead (Longwell, 1936). A 

thick section of these characteristic beds underlying 

the basal cap of Fortification Hill (fig. 3) doubtless 

was once continuous with similar beds that emerge 

locally from the alluvial cover southeast of Hoover 

Dam. East of Willow Beach the alluvium has been 

effectively eroded, exposing the Muddy Creek formation continuously over an area of several square miles. 

There the section near a fault contact with Precambrian rocks of the Black Mountains is made up of

FIGURE 3. View westward on north side of Fortification Hill. Visible 

capping of superposed basaltic lavas is more than 400 feet thick. 

Deposits of the Muddy Creek formation, bouldery and poorly bedded in foreground (near Fortification fault) are finer grained and 

better stratified near middle of view.
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E9

coarse gravels containing angular fragments of gneiss, 

schist, and granite. Within a few hundreds of feet 

to the west, these gravels intertongue with and grade 

into layers of sand, silt, and clay that locally contain 

considerable gypsum. On the west side of the outcrop area, the section again coarsens and includes 

angular pieces of gneissic and volcanic rocks. Small 

remnants of basalt on layers of fine-grained sediments 

near the Willow Beach road apparently correlate with 

the basalt cap of Malpais Mesa, about 4 miles to the 

southwest.

All known evidence suggests deposition within a 

basin that contained a lake during at least a part of 

the sedimentary history; standing water seems a requirement to rationalize the rapid lateral gradation 

from coarse marginal to fine-grained interior deposits, 

the regular thin bedding of the fine sediments, and 

the recurrence of bedded gypsum, gypsiferous clay, 

and sporadic limestone. These characteristics are 

shared with the Muddy Creek formation farther north 

(Longwell, 1928, p. 90; 1936, p. 1419). Three of the 

limestone beds, 8 inches to 2 feet thick, carry a low 

percentage of manganese in exposures half a mile in 

extent (McKelvey and others, 1949, p. 99).

On the east side of Malpais Mesa, the cover of 

basalt lies on very coarse debris, which in part grades 

or intertongues westward into regular layers of gray 

siltstone and yellowish clay like that in the section 

east of Willow Beach. These fine-grained layers are 

well displayed in walls of a small valley that heads 

near the northwest corner of Malpais Mesa. The 

arrangement of coarse and fine sediments in this section suggests that a basin of deposition extended 

westward from the position of the mesa, and presumably the greater part of the deposit was eroded after 

strong uplift near and west of the position now occupied by the river. The basal part of the section 

around Malpais Mesa has particular interest. It lies 

on a rugged surface fashioned on Precambrian rocks 

east of the mesa, on volcanic rocks to the west. This 

basal deposit is a very coarse conglomerate and breccia in which the laregst blocks range in length from 

1 to 4 feet. East of the mesa, fragments that make 

up the conglomerate in its lower part are almost exclusively of Precambrian rock types; west of the mesa, 

fragments of volcanic rock predominate in the deposit, but Precambrian types are important. The 

coarse well-cemented debris, of a pronounced brownish color, fills an old steep-walled valley hundreds of 

feet deep, reaching below the present level of the 

river and heading south-southwest. Overlapping of 

angular cobbles and small slabs indicates that a swift

stream flowed southward in this valley. The Muddy 

Creek deposition clearly began on very rugged topography, which became much modified as sediments 

accumulated.

A third area underlain by typical deposits of the 

Muddy Creek formation lies from 2 to 3 miles east 

of Boulder City. The best exposures are in the upper part of Eifle Range Wash where, as noted by 

C. B. Hunt (written communication), the formation 

"is represented by about 75 feet of gypsiferous and 

manganiferous tuff, clay, silt, and sand." These 

beds, dissected into subdued badland topography, 

cover a considerable area and disappear toward the 

south and west beneath unconsolidated gravel. A 

shaft and two drill holes put down by the U.S. Bureau of Mines revealed that a silty gypsum bed, 60 

to 65 feet thick, contains 3 to 5 percent manganese 

(McKelvey and others, 1949, p. 99). The manganiferous beds are dark gray to nearly black; other beds 

have the tan color that is dominant in fine-grained 

deposits of the Muddy Creek formation. North of 

Rifle Range Wash, the sediments coarsen, by gradation and intertonguing, and are in sedimentary contact with older bedrock. Along the east side of the 

outcrop area, the section includes at least one flow of 

basalt and is faulted down against older rocks.

Directly south of Boulder City, beds of sedimentary 

breccia that include tuff are faulted down against 

Precambrian rocks and quartz monzonite in Boulder 

Hill. These beds are probably in the Muddy Creek 

sequence, but alluvial cover conceals their relation to 

outcrops farther east.

The Muddy Creek formation in the vicinity of Las 

Vegas Wash has been described and mapped by Hunt 

and others (1942) and McKelvey and others (1949), 

who report that the section contains three distinct 

members, separated at least locally by angular unconformities. The lowest member, conglomeratic and 

of irregular thickness, locally contains some flows of 

basalt and andesite. A thick section of similar volcanic rocks underlies the lowest of the sedimentary 

units, generally with strong angular unconformity. 

These older volcanic rocks, which Hunt and McKelvey 

included with the basal member of the Muddy Creek 

formation, are now recognized as part of the Mount 

Davis volcanics and are so represented on plate 1. 

Apparently the deposition of some sediments in the 

lower part of the Muddy Creek formation began during late stages of the Mount Davis volcanism. Movements on faults, which initiated basins of deposition, 

are reflected widely in angular unconformity. As the
E10 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

movements and the volcanic activity doubtless were 

irregular in space and time, the base of the Muddy 

Creek formation is not everywhere sharply defined. 

Widespread eruption of basaltic lavas was renewed 

later in Muddy Creek time. The Fortification basalt 

member, locally found in the upper part of the formation, is mapped as a distinct unit because of its

wide distribution in contact with rocks older than the 

Muddy Creek formation.

The following stratigraphic table, modified from 

Hunt and others (1942, p. 301) and McKelvey and 

others (1949, p. 87), represents the formation at the 

Three Kids mine and near Boulder Basin of Lake 

Mead.

TABLE 1. Sequence of Tertiary rocks near Lake Mead

Age

Pliocene (?)

Miocene(?)

Oligocene or

Eocene(?)

Formation and Member

Muddy Creek 

formation

Mount Davis 

volcanics

Golden Door 

volcanics

Member 3

Unconformity 

Member 2 

Unconformity

Member 1 

Unconformity 

Unconformity

Description

Marginal conglomerate and sandstone, grading basinward into 

siltstone, clay, tuff, and gypsum. Includes thick sheets of 

basalt; these form cap of Fortification Hill, and their 

probable equivalents to east and south are mapped 

separately as the Fortification basalt member of the 

Muddy Creek.

Conglomerate, sandstone, shale, and gypsum

(c) Gypsum (locally manganif erous) , limestone, clay, siltstone, and sandstone, 

(b) Tuffaceous sandstone and siltstone, generally manganiferous, some beds with high content of manganese, 

(a) Bedded conglomerate, with some locally interlayered 

basalt flows.

Basalt and andesite flows and agglomerate. Coarse sedimentary debris included locally.

Complex of volcanic rocks, latites to rhyolites, with much 

glass and tuff. Cut by dikes, stacks, and irregular intrusive 

bodies.

Thickness (feet)

0-2,000

0-300

0-1,500 

0-150 

0-500

0-2,000 +

Thousands 

of feet

Cumulative evidence in a wide region indicates that 

sediments of the Muddy Creek formation were deposited in several adjacent basins under conditions 

of interior drainage (Longwell, 1928, p. 95; 1936, 

p. 1422). Coarse sediments near upland sources 

graded into and intertongued with fine-grained elastics and chemical precipitates in basin interiors. Evidently the Muddy Creek sedimentation began on a 

surface made rugged by faulting, and there were recurrent movements and volcanic activity while the 

deposition was in progress. Although the several 

areas of outcrop south of the lake may represent separate basins, possibly all these coalesced as the sections 

built upward and spread laterally. As the formation 

east of the Black Mountains includes evaporites, 

chiefly halite and gypsum, hundreds of feet thick 

(Longwell, 1936, p. 1423), and at a maximum the 

total section measures thousands of feet, perhaps the 

Muddy Creek interval was of long duration. During 

that time the Colorado Kiver, as a through-flowing 

stream, could not have existed in its present location 

(Longwell, 1946, p. 831).

High in the formation, Stock (1921, p. 260) found 

some fragmentary vertebrate fossils which he considered suggestive of Miocene age. Later study indicates that these remains have no diagnostic value

(M. C. McKenna, written communication, 1956). No 

additional fossils have been reported, and no evidence 

for definite correlation is available for sections south 

of Hoover Dam, which are referred to the Muddy 

Creek formation tentatively on the basis of physical 

characteristics and relations. The formation as it is 

widely exposed in areas bordering the Muddy Mountains is strikingly like the Panaca formation, about 

100 miles farther north, which is well dated as Pliocene (Stock, 1921, p. 257).

LANDSLIDE BRECCIA

Coarse breccia that consists typically of broken 

fresh rock, with little or no matrix of weathered material, covers an area of several square miles southeast of Hoover Dam. This material, discussed in detail elsewhere (Longwell, 1951), had its source in the 

Precambrian bedrock of the Black Mountains and is 

most logically explained as a landslide mass that 

moved westward over basin deposits. The mass is an 

erosion remnant with a maximum thickness of several hundred feet. Much of it rests on the Muddy 

Creek formation; and since locally, near its southern 

limit, some of the breccia also lies below beds of this 

formation, the mass seems to be an exceptional unit 

of the Muddy Creek formation.
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA Ell

The base of the breccia is well exposed in deep 

washes. In its lower part the breccia is finely comminuted; and the basal surface of the mass, polished 

by abrasion in movement, has a low westward dip. 

Striae marking this surface, in the direction of inclination, increase its similarity to a fault surface. 

Upward in the section the rock fragments increase 

in size to maximum lengths of several hundred feet. 

The larger fragments retain the banding characteristic of Precambrian bedrock, though in detail most 

of the rock is minutely shattered. Blocks of basalt, 

locally mixed with the broken gneiss and granite, 

probably came from a flow like the one still partly 

preserved on the flank of the range 3 miles east of 

Willow Beach (pi. 1), or from local dikes.

Near the southern limit of outcrop, the breccia consists of two distinct units; a lower part, brown and 

softened by chemical weathering, is separated by 

stream-laid gravels from fresh breccia above. The 

gravels, 25 to 40 feet thick, have in the lower beds 

some reworked material from the weathered breccia 

beneath. Since the two units of breccia are similar 

fractions of the same formation, the contrast in degree of weathering suggests a radical change in local 

environment. Presumably the older mass, after its 

emplacement, was exposed for a considerable time to 

attack by the weather under a climate more humid 

than that of the present. Some change brought on 

rapid aggradation, and the younger landslide mass, 

deeply buried under basin deposits, was sealed from 

weathering until it was exposed by erosion in the 

Colorado River drainage area. Chemical decay under the present aridity is extremely slow.

Similar breccia, locally 200 to 300 feet thick, occurs at several places between flows in a volcanic 

assemblage older than the Muddy Creek formation. 

The most striking of these breccias were seen northwest of Malpais Mesa, and in sections of volcanic 

rocks exposed on the slope east of the Eldorado 

Mountains and northwest of Mount Davis. Probably 

landslide masses of this kind came from steep fronts 

of upthrown blocks along active faults (Longwell, 

1951, p. 349).

The oldest breccia of this type recognized within 

the report area is within the Thumb formation, discussed above (p. E8) as probably of Cretaceous age.

OLDER ALLUVIUM

The older part of the deposits mapped as alluvium 

is complex in origin, and probably various parts of 

it differ widely in age. At several localities beds of 

well-indurated gravel are tilted as much as 20° to 25°, 

and possibly some of these beds are older than Quaternary. A cover of later alluvium conceals older deposits over wide areas, and there is no practical way 

of correlating isolated outcrops of similar clastic beds 

that yield no fossils.

In the wide valley of the Colorado River southward 

from Black Canyon, widely distributed well-cemented 

gravels are exposed in steep cliffs facing the river and 

along many large washes (fig. 4). These gravels 

closely resemble the unconsolidated gravels now on 

the floors of active washes and on slopes between the 

intermittent stream channels. The rock fragments, 

most of which were derived from lavas or from Precambrian bedrock, range from sand size to large boulders and are either angular or subangular.

Sections of the cemented gravels exhibit differences 

in arrangement, coarseness, and lithology of the rock 

fragments. In the lower part of Eldorado Wash (fig. 

4), the basal part of each cliff contains many angular 

boulders as much as 2 feet long, derived from lavas 

of several lithologic types. Beds extend below the 

river level. An erosional unconformity separates this 

unit from overlying beds that are more regular, less 

coarse in average texture, and made up chiefly of 

fragments derived from Precambrian rocks. At the 

top of the section is a massive layer, about 20 feet 

thick, in which fragments are of moderate size, with 

isolated blocks as much as a foot long. In large part 

the arrangement is disordered; suggestion of layering 

and size sorting is faint and localized. Crude bedding indicates that the two lower units were deposited by swiftly running water, whereas disorder 

in the upper layer suggests that it accumulated on a 

slope removed from active stream channels, under 

influence of slope wash at times of exceptional runoff, 

supplemented by slow creep and perhaps by mudflow.

FIGURE 4. Cliff of cemented gravels on south side of Eldorado Wash. 

Channel of Colorado River is short distance to the left of view. 

Gravels in three distinct units: a, fragments derived from volcanic rocks; 6, fragments chiefly of Precambrian rocks ; c, unstratifled unit. Cliff is about 40 feet high.
E12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Events in the history of the Colorado River are 

recorded in these deposits, and possibly a thorough 

study would reveal important details of the story. 

Does the unconformity above the lower unit in Eldorado Wash record lowering of the river bed, with 

consequent erosion of widespread alluvium? Abrupt 

passage from the middle to the upper member in the 

section also suggests an important change in regimen, 

for which more than one hypothesis may be offered. 

Assuredly large shifts in the river level occurred, as 

evidenced by the highest remnants of cemented river 

gravel. In the wide basin east of the river and northeast of the Newberry Mountains, and particularly on 

slopes a few miles southwest of the Portland Mine, such 

remnants outline a surface which, 3 miles from the 

river, has an altitude near 1,300 feet and a westward 

slope of approximately 100 feet per mile (fig. 5). 

Since the surface presumably leveled off in the middle of the basin, alluvium must have filled the valley 

to a height more than 500 feet above the modern 

riverbed. Perhaps fluctuations, both upward and 

downward through hundreds of feet, occurred during 

development of the river valley. Evidence of cutting 

to a level more than a hundred feet below the modern bed was found at the Hoover Dam site (Longwell, 1936, pi. 17) and during earlier investigations 

by test boring (La Rue, 1925, pi. X).

Cemented Colorado River gravels are exposed at 

several localities, most of them along washes where 

concealing slope gravels have been removed. One of 

the best exposures is beside the road about midway 

between the Portland and Katherine mines, on the 

east side of the valley opposite the Xewberry Mountains, 

where the sediments consist of crossbedded sand and 

gravel, the latter containing well-rounded pebbles of 

lava, gneiss, granite, quartz, chert, and limestone. 

Only the river could have brought together such an 

assemblage and developed the forms of resistant pebbles that are evidence of long transport. This outcrop, which is nearly 3 miles from the modern channel and more than 600 feet above it, records one 

location of the stream during its long and varied history of lateral migration and vertical fluctuation. 

The cemented slope gravels, which support remnants 

of a widespread high-level surface (fig. 5), were deposited across the beds of river gravel after these 

beds were tilted locally, probably by faulting, to dips 

as high as 30°.

It is to the credit of Newberry that he saw these 

cemented gravels about 100 years ago, and differentiated those laid down by the river from the local 

slope deposits.

Relations among the several kinds of alluvium 

along the Colorado in the wide valley above Davis 

Dam are shown in figure 5. The river trenched coarse 

deposits of local origin and laid down thick beds 

made of far-traveled gravel and sand. After the 

channel shifted, the abandoned stream deposits were 

cemented, locally deformed, and in many places buried 

under local gravels which in turn became consolidated. Development of the wide and thick cap of 

caliche indicates an episode of stability. We have 

no evidence to suggest the exact geologic date represented by the widespread caliche; but it was clearly 

formed while the Colorado River was in operation, 

and therefore long after the Muddy Creek sediments 

were laid down in interior basins. The caliche cap 

is older than the lake beds of the Chemehuevi formation of Pleistocene age.

CHEMEHUEVI FORMATION

Evidence for a deep lake that occupied a vast area 

in the lower part of the Colorado River valley has 

been discussed elsewhere (Loiigwell, 1946, p. 827). 

Remnants of sediments that now are recognized as 

lacustrine were reported by Lee (1908, p. 41), who 

called the assemblage the Chemehuevis gravel and 

interpreted the entire sedimentary unit as a deposit 

made by the Colorado River. Lee, in his reconnaissance over a wide region, was impressed by the abundant loose river gravel that covers many remnants 

of the fine-grained sediment to a depth of several 

feet. Actually the gravel was deposited at various 

levels as the river cut down toward its old course at

s .1 .

River gravel Beds of Chemehuevi E

Mixed slope and river 

gravels, unconsolidated

Local gravels weakly

cemented 

Precambrian bedrock

VERTICAL EXAGGERATION 2X 

0

Caliche capping

Local gravels Beds locally 

firmly cemented deformed

FIGURE 5. General relations among sedimentary units designated alluvium on the geologic map, as seen on long slope

east of Newberry Mountains.
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA

the end of the lake episode. Gravels in basal beds 

of the few Chemehuevi remnants preserved near the 

present river level were no doubt deposited in the incipient stage of filling, while the lake was growing 

headward from the obstruction that blocked the earlier river.

Many dozens of the remnants of lake deposits, some 

of large size, are distributed in a belt on either side 

of the river; they are best preserved in areas free 

from large washes, particularly where hills of bedrock shield the weak material from erosion. Few 

remnants remain in the wide section of the valley 

between Black Canyon and the Newberry Mountains. 

Large quantities of the lake sediment have survived 

in pockets among the hills northeast of Davis Dam 

(fig. 6), and clay from that vicinity was used to 

make the impervious core of this earthfill dam. 

Characteristic lake deposits, consisting chiefly of clay 

in thin laminae, lie at various altitudes because the 

lake was deep and its floor was on rugged topography.

FIGURE 6. View generally eastward, north of Davis Dam site, to Black Mountains. Precambrian rocks exposed in lower and middle parts 

of view, volcanic rocks in distant range. Light-colored clay and silt of the Chemehuevi formation fill former channel of river and lie 

in rpinnants at higher levels. Tailings from old Katherine mill in wash, upper left. Skyline at right is about 10 miles from foreground. 

(U.S. Bureau of Reclamation photograph.)

661325 O 63 3
E14 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Near Davis Dam the laminated clays extend to a 

height about 300 feet above the river, where there is 

an abrupt change to crossbedded fine sand, remarkably uniform in grain size except in local wedges 

containing coarser, angular fragments that were 

brought in by tributary washes. Howard R. Gould, 

of the U.S. Geological Survey, kindly analyzed samples of the Chemehuevi sediments from the Davis 

Dam vicinity, and others from a large remnant of 

the sediments west of Boulder Canyon. At each locality the median particle diameter of the clay is 

less than 3 microns. Gould found the samples markedly similar to each other and also to clay in the 

bottomset beds of the present delta of Lake Mead, 

where the average median is about 2 microns. Sand 

in the upper member of the Chemehuevi has a median diameter of about 300 microns, and Gould found 

that the sand has about the same texture as the sand 

of the topset and foreset beds in the present delta 

(written communication, 1949).

The sum of evidence from the area of the present study supports the conclusion regarding the former 

lake on the basis of the sediments upstream from 

Hoover Dam (Longwell, 1946, p. 827). The finegrained sedimentary unit represents conditions on the 

bottom of a deep lake far from the mouth of any 

large permanent stream. Local wedges of angular 

fragments are interpreted as delta fans contributed 

by intermittent streams from bordering highlands 

(Longwell, 1936, pi. 15). The abrupt change to the 

overlying sand marks arrival of the foreset beds as 

filling proceeded and the delta front advanced southward. No doubt the extreme reach of these foreset 

sands was many miles in advance of topset beds, as IP 

true of the Lake Mead delta: therefore, the basal 

foreset sands of the Chemehuevi lake may well have 

been deposited in deep water, in accord with evidence 

that the upper deltaic member, predominantly of sand, 

was hundreds of feet thick in the deeper parts of the 

lake. As the area of total fill advanced down the lake, 

the river played laterally over the area, cutting and 

redepositing; as a result, over much of the delta the 

topmost member was a coarse stream deposit. In 

contrast with the Lake Mead delta, to which the 

Colorado River in flood contributes nothing coarser 

than sand, the stream in Chemehuevi time spread vast 

quantities of coarse gravel. Most of it was fartraveled, as is indicated by the advanced rounding of 

pebbles, cobbles, and boulders of peculiar lithologic 

composition, many of which were moved tens or even 

hundreds of miles from parent bedrock.

The contrast in stream loads noted above suggests 

that the river of Chemeheuvi time was more powerful

than the present Colorado. As the terraces floored 

with the coarse gravel have downstream slopes closely 

comparable to the modern stream gradient, presumably a decline in the volume of flow since the terraces 

were formed is responsible for the decrease in carrying 

power. Probably, then, the regional climate during 

Chemehuevi time was more humid than now, at least 

in the headwaters areas of the river.

Many surface features are related to the lake history. Remnants of terraces capped by characteristic 

river gravels are at many altitudes, and wherever the 

gravels lie directly on the lake deposits, they record 

some stage in downcutting from the highest lake level. 

The lake sediments, especially the topset sands, are 

inherently weak, and it is not likely any of these have 

survived at the highest level. Coarse river gravels, 

on the other hand, are highly persistent if they are 

on a firm base. The shifting stream on the advancing 

topset beds must have impinged against bedrock at 

many points on the valley sides and at the margins 

of high island masses. At several places large remnants of gravel are on bedrock at altitudes somewhat 

above 1,500 feet; one of these is on Delmar Butte, on 

the south slope of the South Virgin Mountains (Longwell, 1936, p. 1454). Another is at Sugarloaf Mountain, a rounded peak of volcanic rock and a conspicuous landmark on the Arizona side south of Hoover 

Dam. A broad shoulder along the east and south 

base of the steep knob carries an extensive deposit of 

typical Colorado River gravels. This conspicuous 

bench, fashioned on bedrock and veneered with gravel, 

may actually be a fragment of shoreline of the former 

lake. Its inner margin is at about 1,530 feet altitude, 

or nearly 900 feet above the stream bed in Black 

Canyon before the dam was built.

About a mile northeast of and a little lower than 

the Sugarloaf bench is a group of large potholes containing river gravel (Ransome, 19236). Presumably 

these features mark a temporary channel of the river 

at some stage of adjustment on the Chemehuevi sediments.

If fhe bench at Sugarloaf represents the highest 

level the river reached on the Chemehuevi fill, the lake 

at maximum extent was nearly 1,000 feet deep in the 

vicinity of Davis Dam, and 9 to 10 miles wide in 

the broad part of the valley south of Mount Davis. 

Farther south, in the basin north of the Mohave 

Mountains, the extreme width of the lake must have 

exceeded 15 miles. The basin still farther south, 

between the Mohave and Whipple Mountains, is known 

as Chemehuevi Valley and properly gives its name to 

the deposits, which are exceptionally displayed there. 

South of this basin, part of which is now occupied
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E15

by Havasu Lake behind Parker Dam, many remnants 

of Chemehuevi sediments lie in protected recesses 

along the canyon cut through the Whipple Mountains. 

The lowland south of these mountains is extensive 

both east and west of the river, and although slopes 

are well graded and generally veneered with recent 

alluvium, scattered exposures indicate that the lake 

covered an immense area within which some mountain 

masses stood as islands.

J. S. Newberry (1861, p. 38), apparently the first 

to study these sediments, found near the river, in 

gravels that lie beneath clay of the Chemehuevi formation "a very large and perfect tooth of Elephas primigenius" The gravel bed accords with the beds 

above (fig. Y) and probably is a stream deposit near 

the temporary head of the growing lake. Newberry's 

account mentioned the clay overlying the gravel, but 

he supposed all the sediments were laid down by the 

river.

Mammal bones and numerous mollusk shells have 

been found more recently, not directly in the Colorado Eiver valley but in tributary valleys and in 

deposits that seem to be closely related to the Chemehuevi formation (Longwell, 1946, p. 828). A Pleistocene age was assigned to the bones by Simpson (1933), 

and to the shells by F. C. Baker (written communication, 1932).

A map of the Chemehuevi formation would have 

value if it represented nearly all the remnants on a 

highly accurate topographic base, and preferably with 

differentiation of the significant kinds of sediment. 

Since most of the remnants are small and scattered 

over an enormous area, such a map would be very 

expensive in time and labor. In the present report 

the deposits are included among those mapped together as alluvium. They are highly distinctive deposits, and fortunately many remnants lie well above 

the level of Lake Mohave, though only a few escaped submergence or wave erosion by Lake Mead.

YOUNGER ALLUVIUM

Although the larger washes are only intermittent, 

many are fairly well graded, particularly in the wide 

segment of the valley between the Xewberry Mountains 

and Black Canyon. The washes are floored with rock 

debris that ranges in size from silt to boulders. 

Coarse fragments are predominantly angular or subrounded, but some well-rounded pebbles are interspersed, probably contributed by old river bars left 

at high levels. As suggested by the topographic map, 

large interwash areas in the wide part of the valley 

have fairly uniform and gentle slopes. East of Searchlight, the slopes on either side of the valley range 

between 200 and 250 feet per mile through distances 

of 8 to 10 miles. These slopes are floored with gravelly 

debris, made up chiefly of coarse fragments but containing also much silt and sand that probably was 

contributed, at least in part, by the Chemehuevi fill. 

Locally, recent gullying has exposed remnants of the 

old lake clay veneered with slope gravel. The proportion of coarse gravel increases generally upslope and 

toward hills of bedrock.

Before the closing of Davis Dam, much of the river 

south of Black Canyon was margined by alluvial flats 

that varied in width and in height above low-water 

stage. The most impressive of these flats, now covered by the widest part of Lake Mohave, lay on both 

sides of the stream, from the latitude of Searchlight 

southward beyond Cottonwood Island (pi. 1). In 

this area the nearly level surfaces, underlain chiefly 

by stream-laid silt and sand, extended a maximum 

of more than a mile back from the river. The surface

w

'' ^ - ' ^ _^

 -£;_' vc" -_, -- ^5^^'^^^^^^*

 £ I c d * f

a Trap hills rising eastward to the Black mountains

b Bluffs of stratified Quaternary gravel and sand

c Elephant Hill

d Alluvial bottom land

e Bed of river

f Tertiary conglomerates, inclined and covered with horizontal beds of gravel

FIGURE 7. Newberry's section through Elephant Hill, near mouth of Eldorado Wash (copied from report of 1861). Elephant 

tooth came from basal unit of Chemehuevi formation, above c. His Tertiary conglomerates are the cemented gravels 

shown in figure 4 of present report.
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

of this old flood plain was on the average about 20 

feet above low-water mark, and in many places steep 

banks adjacent to the stream indicated active lateral 

cutting. The larger tributary washes kept their channels incised below the level of the plain and formed 

small delta fans that forced the river toward the opposite, bank. These terminal deposits of tributaries contained debris much coarser than that beneath the 

alluvial flat.

Large remnants of terraces on old river gravels 

were numerous on both sides of the stream course, at 

altitudes of 60 to 100 feet above low water stage. One 

of the most extensive of these lay northeast of the 

Old Ferry at the outer edge of the wide flood plain 

east of the river. The terrace front sloped up from 

the flat at the angle of repose of the coarse gravel, 

made up largely of cobbles with diameters as much as 

8 inches, nearly all well rounded and many smoothly 

polished. The top of the terrace, 60 to 70 feet above 

low river stage, sloped gradually upward through a 

width of several hundreds of feet to its outer margin. 

A similar terrace west of the river and south of 

Aztec Wash was nearly 100 feet above stream grade. 

The coarseness of these gravels and the large original 

volume indicated by the remnants are evidence of 

great power of the river down to a late stage of its 

return to the pre-Chemehuevi grade. Pebbles and 

cobbles in the gravel represent many lithic origins, 

some of which are common in the Precambrian bedrock and in younger igneous rocks of the immediate 

region, whereas others, particularly quartzites, fossiliferous limestones, and cherts, are not known in any 

outcrops nearer than the Virgin Mountains.

In the broad part of the valley, remnants of river 

terraces at high levels are exceptional and small. On 

the resistant bedrock along Black Canyon numerous 

gravel bars, some of large size, record changing levels 

of the stream. Above the cliffs a mile upstream from 

Willow Beach, on each side of the present lake, extensive terrace remnants lie as much as 140 feet above 

the river bed of 1949. A mile directly west of Willow 

Beach, a wide saddle in the high ridge north of the 

river is floored with the river gravels at altitude 

1,225 feet; and directly east of the saddle,.pockets of 

the gravel extend to 1,320 feet, or about 700 feet 

above the channel of 1949. More than a mile farther 

downstream and on the south wall of the valley, remnants of terrace gravels lie near 1,300 feet altitude. 

Of many other occurrences in or near Black Canyon, 

the exceptional remnant at Sugarloaf is the only one 

thus far recorded as high as the 1,500-foot contour.

West of the river valley the alluvial cover is in two 

basins of interior drainage, separated by a low divide 

just north of Searchlight. Each basin is margined

with an irregular assemblage of fans, which pass outward into a well-graded bajada slope furrowed by 

shallow washes. On many areas between washes, the 

slope gravels have settled into the smooth mosaic pattern of desert pavements. A long arm of the northern 

basin drains northward to the flat floor, where gravelly 

slope debris merges with clay and silt in the floor of an 

extensive play a.

East of the Black Mountains the long flat-floored 

Detrital Valley, draining northward to Lake Mead, 

is veneered with alluvial deposits that in kind and 

general arrangement resemble those of interior basins, 

although no playa is present in Detrital Valley. Detrital Wash has a clearly defined channel, with a low 

gradient to the north, although a stream carries 

through only in times of exceptional runoff, and during dry periods windblown deposits locally obstruct 

the streambecl. Fine sediments in considerable quantity lie on the valley floor, not in continuous accumulation but undergoing slow, halting transportation 

down the valley. The advent of Lake Mead has of 

course created a new control, and doubtless there will 

be a growing deposit of sediment, dominantly fine 

grained, near the mouth of the wash, and steady decrease in a carrying power that was feeble before the 

lake was formed.

CRETACEOUS(P) AND CENOZOIC IGNEOUS BOCKS

Igneous rocks, both extrusive and intrusive, are of 

major importance in the bedrock of the area. The 

larger intrusive bodies are holocrystalline, and at least 

some of them are appreciably older than any of the 

volcanic rocks. On the other hand, many of the smaller 

injected bodies cut thick sections of lavas and tuffs, 

and some of these bodies occupy positions of vents 

through which volcanic products issued. Of these 

products lavas are predominant, but tuffs and explosion breccias are widespread, and some extensive sheets 

of perlitic glass may represent fusion of ash distributed in nuees ardentes.

The diverse igneous units are considered in a group 

separate from the sedimentary formations, because 

the position of these igneous rocks in the geologic 

column is uncertain. But all are geologically young 

in comparison with the Precambrian bedrock, and 

probably most of them originated in association with 

uplift and orogeny that affected the region in a relatively late stage of its history.

INTRUSIVE BODIES

LARGE PLUTONS

The geologic map (pi. 1) shows only part of the 

intrusive masses that have been recognized in the 

field; doubtless many others cut the Precambrian
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E17

rocks in parts of the area not examined closely. For 

example, a large body of coarse-grained granite and 

another of quartz diorite are mapped as a single pluton near the head of Boulder Canyon, and a definite 

southern boundary of these plutonic rocks is not indicated because it is not known. Most of the boundaries shown on the map are inexact. The outline of 

a large mass of granite forming the highest part of 

the Newberry Mountains is estimated from hasty examination. Representation of a large granitic pluton 

in the upper part of Aztec Wash is much generalized. 

A special pattern is used where plutonic rocks make 

irregular contacts with gneiss in the northern part of 

the Black Mountains. Many bodies have highly complex borders that ramify the Precambrian basement. 

The present discussion, based on a preliminary sampling only, may have some value as a guide for later 

workers.

The larger bodies range in composition from diorite 

through quartz monzonite to granite. Outcrop areas 

of these bodies range from about a mile to more than 

4 miles in largest dimension. Contacts with older 

adjacent rocks are for the most part steeply transgressive; but the mass exposed at Boulder City and 

extending more than 4 miles eastward is an unroofed 

sill or laccolith, whose base is generally conformable 

on lavas. Rocks that formed the roof have been 

wholly eroded from the pluton as exposed. About a 

mile east of Boulder City erosion has cut through the 

pluton along an anticlinal axis, from which the underlying lavas dip to the northeast and southwest. Farther east these lavas are essentially horizontal, and 

the contact with the pluton recedes and advances along 

walls of crosscutting canyons. Through 20 to -10 feet 

near the contact the lavas are conspicuously stained 

with iron oxide; this staining extends irregularly into 

the pluton also. Locally, the volcanic rocks have 

been recrystallized and have some resemblance to the 

adjoining plutonic rock, which near the contact is 

fine grained because of chilling.

In much of its exposed area, the Boulder City pluton .stands above its surroundings, and in its eastern 

part the border of the mass forms steep slopes and 

precipitous cliffs. The ragged outline of its border 

along transgressive valleys suggests that the original 

lateral extent of the mass has been considerably reduced by erosion. In and south of Boulder City, 

alluvium conceals the mass except for scattered exposures, and the southern limit of outcrop is uncertain. At its western limit the outcrop is narrow, and 

apparently the body is covered by volcanic rocks of 

the River Mountains, though exposures in the critical 

area do not reveal an actual contact.

In specimens collected from the eastern part of the 

Boulder City pluton, the rock is granodiorite, with 

quartz highly poikilitic. Samples from the vicinity 

of Boulder City are quartz monzonite of medium to 

coarse grain size, with plagioclase zoned from An50 

to An20 . Some of this rock, collected in 1956 by Earl 

Pampeyan and analyzed in the U.S. Geological Survey Laboratory by the zircon (lead-alpha) method 

(Larsen and others, 1952), gave an age of about 50 

million years. This suggests that the pluton was emplaced in Eocene time.3 Implications regarding date 

of the associated lavas are discussed on later pages.

The isolated Boulder Hill south of the city contains 

quartz monzonite similar to that exposed in Boulder 

City but coarser grained. In the southern part of the 

hill, the plutonic rock encloses masses of gneiss, presumably Precambrian, some of which merge imperceptibly with the quartz monzonite. In this area the 

igneous rock is less uniform in appearance and in size 

of grain than the rock in the northern part of the hill. 

Possibly the quartz monzonite in Boulder Hill represents part of a boss that served as feeder for the 

sheetlike body exposed in and east of Boulder City.

Granodiorite similar to that east of Boulder City 

underlies a large area east and northeast of Fortification Hill. Alteration along joints, perhaps in connection with later igneous activify, has developed 

bluish amphiboles, probably crossite and crocidolite 

(R. G. Coleman, written communication), that make 

conspicuous patches and streaks in the gray rock. 

South and west of Fortification Hill the lower slopes 

are on altered rock, much of it soft and claylike, characterized by bright colors which have earned for this 

area the name "Paint Pots." Local masses that show 

"little or no alteration consist chiefly of gray and 

brownish monzonite porphyry. Ransome (U.S. Bur. 

Reclamation, 1950, p. 88) concluded that the widespread alteration was caused by introduction of finely 

disseminated pyrite, and subsequent oxidation with 

formation of sulfuric acid. The original minerals in 

the rock were changed to clay minerals, iron oxide, 

and other secondary products.

An assemblage of diverse intrusive bodies occupies 

much of the area "extending southeast from Fortification Hill to the junction of the Horsethief and Indian 

Canyon faults. In the upper part of Kingman Wash, 

the bedrock is a complex of Precambrian gneiss and 

younger intrusive rocks of several kinds. South of 

Kingman Wash the intrusive rock, much of it rich in 

biotite, is dispersed irregularly through Precambrian 

gneiss east of the Fortification fault; in the block 

between this fault and the Horsethief fault, the intrusive rock surrounds large masses of siliceous lava.

3 A potassium-argon analysis made in 1962 gave an age of about 23 

million years. This indicates a Miocene age.
E18 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

A similar network of intrusive bodies that engulf 

masses of volcanic rock lies west of the river and is 

transected by Kifle Range Wash. Some of the intrusive rock in the latter area is dark diorite.

Along the upper part of Eldorado Wash intrusive 

masses, shown on plate 1 as a continuous body, form 

discontinuous outcrops that make highly irregular 

contacts with Precambrian and volcanic rocks. East 

and west of Nelson the northern boundary of this 

intrusive complex transects a thick, tilted section of 

lavas, with contacts partly intrusive, partly faulted. 

Apparently this boundary is a complicated example 

of what Ransome (1904, p. 11) called an "intrusion 

fault/' whereby the intruding body displaced upward 

great masses of the older volcanic rock. Ore deposits 

once extensively mined along and near Eldorado Canyon occurred in veins along crush zones in the intrusive rocks (Ransome, 1907, p. 76). Directly south 

and southwest of Nelson, large masses of volcanic 

rock are surrounded by and partly engulfed in monzonite porphyry that has conspicuous phenocrysts of 

feldspar in a dark-gray aphanitic groundmass.

North of the old Techatticup mine, the thick section of lavas dips steeply eastward and strikes almost 

at right angles to the nearly straight contact with a 

pluton of quartz monzonite. Dioritic rock along the 

north border may be part of a separate intrusive body. 

South of the mine, near an irregular boundary with 

Precambrian rocks, masses of gneiss and schist are 

engulfed in the pluton.

South and southwest of Nelson the contacts between 

intrusive and Precambrian rocks are highly irregular. 

Farther south, in the upper part of Aztec Wash, a 

large intrusive body has a composition at the borderline between quartz monzonite and granodiorite. 

Orthoclase in what appear to be phenocrysts actually 

is late porphyroblastic, replacing plagioclase. Biotite 

and hornblende make up to 15 percent of the groundmass. Analysis, in the U.S. Geological Survey Laboratory, of samples from this body by the zircon (leadalpha) method gave an age of 35 million years. 4 Many 

prospects of metallic minerals have been explored in 

a wide area around this pluton.

The rock of lightest color seen in any of the large 

intrusive bodies is a granite that forms a prominent 

outcrop in the highest part of the Xewberry Mountains. This rock has coarse texture, is rich in quartz, 

contains some oligoclase with the predominant orthoclase, small amounts of muscovite and biotite, and 

scattered grains of magnetite and zircon.

Study of all large intrusive bodies in the area 

would show a wide range in composition. Of the

4 A potassium-argon analysis made in 1962 reduces this age to 27 

million years.

rocks that have medium to coarse grain the dominant 

kinds are quartz monzonite and granodiorite.

DIKES, SILLS, AND PLUGS

Small intrusive bodies are numerous and widely 

distributed; no attempt was made to map them. Dikes 

and sills range in thickness from a few inches to 100 

feet or more and range in composition from basalt 

to rhyolite. In large part they are associated with 

volcanic rocks, and they occur in largest number near 

recognized centers of eruption. Many sills were injected between lava flows in thick volcanic sequences; 

some of the injected sheets are distinguished from 

flows only by generally coarser grain, by chilled upper 

margins, and by local crosscutting relations. Swarms 

of dikes that cut Precambrian rocks in wide exposures are assigned to a late geologic age because the 

dike rocks generally have not been sheared or crushed, 

and are similar, both in composition and in texture, 

to rocks in the volcanic sequences.

Several agglomerate plugs mark vents, and other 

irregular bodies perhaps had this origin. Two of the 

best examples are in the midst of a glass-and-pumice 

complex directly south of Square Butte, a prominent 

landmark 6 miles northeast of Nelson. Each plug 

has a nearly circular plan and nearly vertical walls; 

one is 300 feet in diameter, the other 150 feet. These 

and other similar bodies are more fully described in 

connection with the associated volcanic rocks.

VOLCANIC ROCKS

Five distinct episodes of volcanism are recognized 

along the river south of Lake Mead. Four of the 

resulting assemblages of rocks are widely distributed, 

and three of these, Eocene(?) to Miocene(?) in age, 

are here designated by formation names, from oldest 

upward, as follows: Patsy Mine volcanics, Golden 

Door volcanics, and Mount Davis volcanics. Volcanic 

rocks of the Muddy Creek formation make up the 

fourth assemblage. The fifth and latest episode is 

represented by local flows of basalt included in slope, 

gravels that probably are of Pleistocene age.

Confident correlation of volcanic sequences over 

distances exceeding 60 miles may be greeted with 

some skepticism; but each of the named assemblages 

has distinctive lithologic characteristics, and the stratigraphic order of occurrence, wherever two or more 

of the units are recognized, is wholly consistent. Good 

evidence indicates more than one eruptive center for 

each assemblage, and there is of course no assurance 

that eruptions from several centers supplying similar products were exactly synchronous. Nevertheless, the striking consistency in order according to
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E19

average lithologic composition, together with evidence for considerable crustal movement and erosion 

between successive units, clearly marks five chapters 

in the history of eruption. As noted in the several 

descriptions below, each of the three older assemblages has varied lithology, but the overall composition of each major unit is characteristic. A common 

kind of lava that recurs in two or more of the units 

presents problems of correlation wherever it forms 

isolated outcrops.

Locally, in several parts of the map area, complications caused by intrusive bodies and by widespread 

chemical alteration make identification of the volcanic 

rocks difficult or impossible.

PATSY MIKE VOLCANICS

The oldest of the volcanic assemblages is widely 

exposed in a belt north of Nelson, in two areas along 

Black Canyon, and on both sides of Pope Wash. 

Smaller exposures lie along the road between Eppersons corral and the Portland mine, and remnants too 

small to map lie on the Precambrian rocks west of 

Union Pass. A thick and representative section that 

extends west to east through the vicinity of the Patsy 

mine, about 3 miles northwest of Nelson, is chosen 

as the type section, although the base is not there 

exposed.

The section consists dominantly of lavas, with some 

thick units of explosion breccia. Tuffaceous layers 

are local and of minor importance. The characteristic brown to gray brown of the rock, in varied shades 

but dominantly dark, is an important aid in recognizing the unit. Flows vary in thickness from a few 

feet to more than a hundred feet, but are common 20 

to 40 feet thick. Many are vesicular or amygdaloidal 

in their upper parts, and some that contain large 

angular fragments in an aphanitic matrix probably 

were brecciated during flow. A few thick layers made 

up largely of angular fragments, both small and moderate in size, seem to be products of explosive eruption. The rock is predominantly aphanitic or has 

only scattered and rather obscure phenocrysts, chiefly 

of plagioclase; however, some of the layers are conspicuously porphyritic. Some flows of basalt in the 

upper part of the section are studded with reddish 

grains of iddingsite.

Under the microscope many specimens show both 

augite and pigeonite, with plagioclase zoned from 

An00 to An30 . Some are olivine-labraclorite basalts, 

but more are pyroxene andesites. The latter predominate in the lower part of the section, basalts in the 

upper.

In the area north of Nelson, the thick succession 

of dark-colored lavas is sharply interrupted by a

light-colored glassy unit, as much as 100 feet thick, 

in which yellowish welded tuff at the base is succeeded by gray to pinkish-brown aphanitic rhyolite 

containing lenses and small spheroidal aggregates of 

brown and greenish glass. At several horizons are 

numerous whitish spherulites, 2 to 18 inches in diameter, which weather free; many, especially the 

largest, are covered with smooth, warty excrescences. 

Some spherulites have concentric zoning and also conspicuous meridional markings. Chemical analysis of 

brown glass from this unit gave the result shown in 

table 2, p. E28 (sample 307). The glassy unit forms 

a ridge that parallels the road north of Nelson.

Offsetting and duplication of this highly distinctive horizon marker indicate the locations of several 

faults with large displacement. Above the glassy unit 

the flows are largely dark-brown basalt having a total 

thickness of perhaps as much as 1,000 feet, though 

some duplication by faulting may be present. Below the glass many strike faults break the section; 

nevertheless, the estimated thickness of this lower 

part exceeds 2,000 feet and may be more than 3,000 

feet; thus, the total thickness of the Patsy Mine section appears to be at least 3,000 or 4,000 feet east of 

the large fault on which the basal part is dropped 

against Precambrian rocks and concealed. On the 

south, both west and east of Nelson, the continuous 

outcrop of lavas ends rather abruptly against bodies 

of intrusive rock that have surrounded and apparently engulfed large masses of the lavas.

Two major eruptive centers for the Patsy Mine 

volcanics are recognized. One of these, about 5 miles 

northeast of Nelson, is marked by a maze of intrusive bodies over an area covering more than a square 

mile. Near the center of the area great numbers of 

dikes radiate from large irregular plugs; many of 

these dikes cut across sections of lava and merge into 

either flows or sills. All the rock in this complex has 

the lithologic character and the brown color predominant in the Patsy Mine volcanics. To the south 

and northeast the complex passes into thick sections 

of the lavas resting on Precambrian basement. The 

second conspicuous eruptive center is along Pope 

Wash, about 3 miles east of the river, where lavas of 

the Patsy Mine volcanics dip outward from a large 

plug of gray-brown andesite porphyry. A complex 

of dikes, small plugs, and irregular bodies that cut 

the lavas forms a network outside the central body 

of porphyry. The total thickness of lavas in the 

section along Pope Wash appears to be well over 

5,000 feet.

Another possible locus of eruption for the Patsy 

Mine volcanics is near Hoover Dam. About half a 

mile downstream from the dam, the characteristic
E20 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

brown lavas, inclined steeply eastward, emerge at 

stream level from a cover of younger rocks (fig. 8). 

The base of the volcanic mass is concealed west of 

the river, where blocks of the next younger series are 

lowered by faulting. The Patsy Mine lavas at the 

river, however, are crowded with fragments, large 

and small, of coarse-grained quartz monzonite; therefore, a plutonic mass was part of the bedrock through 

which the old volcanic material issued. Farther 

downstream the section is much confused by intrusive bodies of brown porphyry. The exact boundaries of these bodies were not mapped, and they are 

included as part of the Patsy Mine volcanics, with 

which they seem closely related lithologically. About 

4 miles downstream from the dam, the lavas dip 

southward; and the strike swings in a wide arc northeastward, marginal to a massive body of the brown 

porphyry. Large dikes extend from this body into 

the lavas. Younger rocks at the eastern edge of 

Glass Plateau are faulted down against a mass of 

the brown porphyry, which there contains numerous 

large xenoliths of Precambrian gneiss, schist, and 

pegmatite. Thin sections of the porphyry reveal 

zoned andesine and oligoclase, with biotite, pyroxene, 

and much sanidine.

The base of the Patsy Mine volcanics, resting on 

Precambrian rocks, is well exposed at several localities in and near Black Canyon; also at many points 

farther southwest, and east of the river near the head 

of Gold Bug Wash. Although at some places the 

volcanic rocks are in direct contact with the old basement rocks, commonly a layer of coarse conglomerate 

or sedimentary breccia, a few feet to 40 or 50 feet 

thick, separates the two units. Cobbles and boulders 

in this deposit are made of the several types of Precambrian bedrock, of volcanic rocks presumably older 

than the Patsy Mine volcanics, or exceptionally of 

quartz monzonite probably derived from bodies intrusive into the Precambrian. At many places, particularly along Black Canyon, the basal unit of the 

volcanic series is a layer, 100 or more feet thick, of 

resistant explosion breccia. Many sheer cliffs adjacent to the river are on this resistant unit.

Eelations at the exposed upper limit of the Patsy 

Mine assemblage are highly varied. In the wide 

belt of exposure northeast of kelson, including the 

type section, there is an abrupt change in lithology 

to the next higher volcanic unit, with no apparent 

unconformity. In much of the map area, however, 

considerable disturbance and erosion occurred between 

eruption of the Patsy Mine and later volcanic rocks. 

Downstream from Hoover Dam a thick sedimentary 

breccia is essentially concordant on the Patsy Mine 

volcanics, which dip to the east much more steeply

FIGURE 8. Unconformity between Patsy Mine and Golden Door volcanics. Older lavas dip 40° E., younger series nearly horizontal 

(note remnant at upper right). View southward, about 1 mile 

south of Hoover Dam.

than the overlying Golden Door volcanics (fig. 8). 

The breccia is conspicuously exposed at the base of 

the canyon walls at and below the dam, and Eansome 

(U.S. Bur. Reclamation, 1950, p. 91), in his study 

of the damsite, reported the thickness of the coarse 

detrital deposit as 350 to 400 feet. Logically, and in 

keeping with his sardonic humor, Ransome called this 

unit the Dam breccia. Since it thins and disappears 

within half a mile downstream, evidently the breccia 

represents a local accumulation on a surface with 

considerable relief. The fragments, as long as 1 foot, 

consist of porphyry and brown volcanic rock.

West of Union Pass the brown Patsy Mine volcanics are exposed only in small isolated remnants, 

and the younger Golden Door volcanics in general 

rest directly on the Precambrian basement. The 

Patsy Mine volcanics may be present northwest of 

Eppersons Corral, but if so they have been dropped 

below the present erosion surface by the Epperson 

fault. South of Square Butte and also in the upper 

part of Pope Wash, the Mount Davis volcanics rest 

directly on the Patsy Mine volcanics, with angular 

unconformity in varying degree.

GOLDEN DOOR VOLCANICS

The second major unit in the succession of volcanic 

rocks in the map area consists of lavas, breccias, tuffs, 

and glasses that average considerably higher in silica 

content than the next older and younger units. This 

distinctive lithology is expressed in color terms: the 

Golden Door volcanics are generally light colored, in 

contrast to the somber brown tones of the Patsy Mine 

volcanics and the dark gray, brown, and black of the 

Mount Davis volcanics. A thick section of the second volcanic assemblage is particularly well displayed
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E21

in a large area on the flank of the Black Mountains 

Avest and southwest of Mount Perkins. In a limited 

part of this area near the road to Old Searchlight 

Ferry, the contact between the Golden Door and the 

younger Mount Davis volcanics is clearly exposed. 

The volcanic rocks are in fault contact with Precambrian rocks for several miles north of the road, and 

this relation continues southward to a point about a 

mile southwest of Eppersons Corral, where typical 

lavas of the Patsy Mine volcanics in the footwall 

block dip southward in normal contact with Golden 

Door volcanics.

The type section near the Golden Door mine, about 

3 miles southwest of Mount Perkins, is essentially 

complete for the general locality, though the top and 

base are not exposed.

Thickness of the Golden Door volcanics at the type 

locality appears to be at least 5,000 feet, though there 

may be some duplication by obscure faults. Absence 

of these rocks from a large area near the head of 

Pope Wash may be the result of uplift and erosion 

before the next younger assemblage was erupted. 

Near Hoover Dam the thickness of Golden Door volcanics is somewhat more than 1,500 feet; but outcrops 

are absent in a large area south of the dam, where 

strong uplift attended by extensive erosion is indicated by prominent exposures of the Precambrian 

basement in the river belt. Locally in this area the 

Mount Davis volcanics lie directly either on the Patsy 

Mine volcanics or on Precambrian rocks; probably 

considerable movement occurred after the Golden 

Door volcanism and before or during eruption of the 

next younger assemblage. This history is indicated 

particularly by relations north and northeast of Eldorado Wash. In a belt northeast of Nelson the Golden 

Door volcanics, stepped down by a succession of faults 

west of the Eldorado fault, have a thickness as great 

as 1,200 feet, thinning eastward in each footwall block. 

East of the Eldorado fault these rocks are absent, 

and a thick section of Mount Davis volcanics lies 

with strong unconformity on Patsy Mine volcanics 

(pi. 1, sec. D-D'}.

Rhyolites form an important part of the Golden 

Door volcanics; but there is a wide range in composition between rhyolite and andesite, and the average 

composition of the sequence varies from one locality 

to another. Generally the abundance of lavas high 

in silica appears to increase southward in the belt 

along the river. Ehyolite and its close relatives make 

up half or more of the section east of Davis Dam, and 

at least a third of the type section near the Golden 

Door mine. By contrast, no rhyolite has been recognized near Hoover Dam, where latites predominate; 

but rhyolite is an important constituent higher in the

section, near Boulder City. Andesites and latites are 

the abundant types in the lower part of the type section, whereas rhyolite becomes more abundant upward. 

In view of the evidence for extensive erosion of these 

rocks in the northern part of the area, the Hoover Dam 

and Nelson sections may represent only a lower fraction that escaped destruction before the Mount Davis 

eruptions began. This would not imply, of course, 

that the basal parts of sections at any two localities 

are essentially identical. Aside from the probability 

that products erupted at several centers differed among 

themselves, crustal movements attended by vigorous 

local erosion may have been active during the volcanism, and the part of the assemblage preserved at 

one locality may be the time equivalent of any fraction, high or low, in the thick type section.

Detailed measurement and analysis of the thick type 

section have not been attempted. The lower half consists of flows and breccias, light brown to gray, aphanitic with subordinate porphyritic texture. Megascopically the predominant lithologic type is andesite, 

but a considerable part of the rock may be latite. 

Dikes and sills, some of rhyolitic composition, intrude 

the volcanic rocks locally, and sills may form an appreciable part of the total thickness. Above the 

middle of the section, dacite and rhyolite are increasingly abundant, and the upper 1,000 feet consists 

chiefly of rhyolite, rhyolite tuff, and rhyolite glass. 

Some units that appear to be flows are on microscopic 

examination revealed as welded tuffs. Much of the 

glass is perlitic, and discontinuous layers of light-gray 

flow-banded aphanitic rock included in thick yellowish 

tuff is found by thin-section study to be perlitic rhyolite.

Thin-section study of two flows in the lower part of 

the type section shows one to be pyroxene andesite, 

the other quartz latite. A third specimen, probably 

from a sill, is rhyolite porphyry. Flows in the upper 

part of the section are identified as perlitic rhyolite, 

dacite porphyry, rhyolite porphyry, and welded tuff; 

the latter is full of glassy and spherulitic rock fragments and contorted shards. Layers of dark-gray perlitic glass, also near the top of the type section, have 

the compositions shown in table 2, specimens 111 and 

185. Glass (specimen 148) from a similar section in 

Gold Bug Wash is remarkably like specimen 185 in 

chemical composition but has no phenocrysts.

An exceptional section of the Golden Door volcanics, 

in the area east of Davis Dam, has considerable rhyolite in the lower part. One flow of spherulitic rhyolite 

vitrophyre contains sanidine, embayed quartz, and 

frondlike groups of crystallites, with some sphene and 

biotite. Several mines and prospects are in rhyolite 

at horizons not far above the Precambrian basement

661-325 O - 63 - 4
E22 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

rocks, which in the large outcrop area west of Union 

Pass are cut by numerous rhyolite dikes. One of 

these, a striking rhyolite porphyry, includes phenocrysts of oligoclase jacketed by albite, sanidine, and 

quartz. Near the summit of Union Pass, a thick whitish tuff contains masses of yellow translucent opal.

A block of volcanic rocks faulted down against 

Precambrian gneiss several miles west of Davis Dam 

is indicated on the map as part of the Golden Door 

sequence, though some Mount Davis volcanics may be 

included. The lower part of the exposed section contains rhyolite breccia and tuff, which are cut by 

doleritic sills and irregular bodies that at a higher 

level seem to merge with basaltic lavas. Similar intrusive bodies of dolerite are common also in a wide 

belt east of Davis Dam. Directly west of Union Pass 

along the highway, an extensive mass of this dark 

rock appears to be intrusive into Precambrian rocks 

below the Golden Door volcanics. Between this location and the river, numerous smaller masses of dolerite 

cut both the Precambrian and the volcanic rocks. 

Probably some of these intrusive bodies represent 

channelways through which the Mount Davis volcanics 

were erupted.

The rocks exposed in the vicinity of Hoover Dam 

(fig. 9) were studied by F. L. Ransome for the Bureau 

of Reclamation, before the dam was built; and his 

report (U.S. Bur. Reclamation, 1950), of which only 

a part,was printed in modified form has been available 

to the present writer. Ransome recognized an "older 

volcanic series" the gray-brown assemblage downstream from the damsite, here called the Patsy Mine 

volcanics and a "younger volcanic series," including 

all higher units in the sequence of volcanic rocks in 

the vicinity of the damsite. As Ransome noted, the 

coarse-grained and locally thick sedimentary unit that 

he called the Dam breccia shows an eastward tilt comparable to that of the underlying brown lavas, whereas the younger units have been less disturbed. Ransome's "younger series" is here correlated in part with 

the Golden Door volcanics, in part with the younger 

Mount Davis volcanics. This separation is based 

chiefly on lithology; the lower units of the series are 

definitely more siliceous than higher units, which consist largely of basalt or trachydolerite. Structural 

definition between the two sequences is not as distinct 

as in other parts of the area; evidence of some gradational overlap is discussed in later paragraphs.

The lowest and the thickest lithologic unit in the 

Golden Door section near the damsite is a latite flow 

breccia, light brown to light red brown on fresh 

surfaces, littered with fragments, both angular and 

somewhat rounded, of rock identical with that forming 

the matrix. This mass, making a large part of the

steep canyon walls at and near the dam, ranges in 

thickness from 300 to more than 600 feet. Apparently 

its top was much eroded, and depressions so formed 

were filled with coarse sedimentary rubble, containing 

fragments as much as 6 feet long. This rubble, firmly 

cemented to form what Ransome called the Spillway 

breccia (U.S. Bur. of Reclamation, 1950, fig. 34), was 

in part contemporaneous with a thick flow which contributed many of the fragments. This flow, resting 

on the basal volcanic unit or locally on the Spillway 

breccia, consists of brownish aphanitic rock, with small 

phenocrysts sparsely distributed. Ransome commented 

that this rock has the appearance of andesite, but he 

called it "basic latite" because it has a high content 

of potassium. Near the head of Black Canyon, this 

rock interfingers with the Spillway breccia in a way 

that suggests a local intrusive relation.

Although the latite, with thickness locally exceeding 300 feet, is extensively distributed on both sides 

of Black Canyon, it is conspicuously absent over considerable areas. These discontinuities may have resulted from extrusion on a highly irregular hill-andvalley topography, with continued vigorous erosion 

during and after emplacement of the latite sheet. The 

Spillway breccia of Ransome supports this suggested 

history by its highly irregular thickness and also by 

local variations in its content from fragments representing wholly bedrock units older than the latite to 

a dominance of latite fragments.

A discontinuous deposit of tuff, locally as much as 

200 feet thick, rests on the basic latite along the north 

flank of Sugarloaf Mountain. On the southwest slope, 

where the tuff is best displayed, the latite flow is 

absent, and the tuff lies on the older flow breccia. 

East of the dam the tuff is broken by faults, and a 

segment exposed beside the highway includes a layer 

of dark-gray perlitic glass (table 2, specimen 414).

The next unit in the section is a widespread lava 

sheet, 200 to 400 feet thick, diagnosed by Ransome 

(U.S. Bur. Reclamation, 1950, fig. 34) as "biotite 

latite." The fresh rock is light red to gray brown 

and contains conspicuous phenocrysts of biotite set in 

a groundmass in part aphanitic, in part glassy. The 

feldspar is andesine. Some oxyhornblende accompanies the more abundant biotite, and calcite pseudomorphs after the ferromagnesian minerals are present 

locally. At a few localities two or more flows can 

be distinguished, but elsewhere the sheet appears to 

be undivided.

Partial chemical analyses of five specimens show a 

range in silica content of 66 to TO percent; yet these 

specimens have no visible free quartz. The analyses 

resemble strongly that of the underlying glass, and 

probably this glass and accompanying tuff
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E23

sent the initial eruption leading to formation of the 

lava sheet.

The entire Golden Door section in the vicinity of 

Hoover Dam is complicated by irregular intrusive 

bodies, large and small, of a dark rock with the megascopic appearance of basalt but with chemical composition like that of trachydolerite. Some of the intrusive bodies, well exposed in the canyon walls, are 

ordinary dikes; others have nondescript form, with 

overall dimensions as much as half a mile long and

several hundred feet wide. This rock contains scattered reddish specks described by Eansome as "ferruginous pseudomorphs after olivine." The trachydolerite in this vicinity resembles, in general appearance and in mode of occurrence, irregular bodies intrusive into Golden Door volcanics near Davis Dam 

(p. E27) and in many other parts of the map area.

The boundary between the Golden Door and Mount 

Davis assemblages east of Hoover Dam is not sharply 

defined. A coarse sedimentary deposit, named by

FIGURE 9. Airview southeastward across Hoover Dam and eastward-dipping volcanic rocks. The Dry Camp breccia of Ransome (U.S. 

Bureau of Reclamation, 1950) is locally the basal member of Mount Davis volcanics. Dissected jrravel deposits (top of view) accumulated in jrreat thickness on subsiding block southwest of Horsothief fault, Golden Door voloauics exposed in canyon wall. Bench at 

left of Sugnrloaf Mountain is mantled with Colorado River gravels, at altitude above 1,500 feet. (U.S. Bureau of Reclamation 

photograph.)
E24 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Ransome (U.S. Bur. Reclamation, 1950, fig. 34) the 

Dry Camp breccia (fig. 9), lies on the biotite latite 

and on local lenses of waterlaid tuff. The breccia, 

with thickness of as much as 200 feet, contains fragments chiefly of monzonite porphyry locally admixed 

with numerous fragments of basaltic rock. Thus, although some of the intrusive bodies of trachydolerite 

cut the Dry Camp breccia, and the section directly 

above is made up dominantly of basaltic flows, clearly 

a considerable quantity of these dark rocks was exposed at the surface while deposition of the Dry 

Camp breccia was in progress. In fact, a flow of the 

basalt lies on the biotite latite and beneath the sedimentary breccia less than a mile southeast from the 

east portal to the Hoover Dam area. Confused contacts among several lithologic units in a belt east of 

the dam probably reflect simultaneous volcanism and 

crustal disturbance, attended by locally rapid erosion 

and sedimentation. Later faulting and erosion further complicated the pattern. It seems best to include 

in the Mount Davis sequence Ransome's Dry Camp 

breccia, and also basaltic flows that locally underlie 

this breccia.

West and southwest of Hoover Dam, beyond the 

limited area studied by Ransome, the highest lavas 

exposed in the Golden Door section include several 

thick flows of rhyolite. Apparently the thickness of 

the section increases westward into the River Mountains, where a complex of volcanic rocks and intrusive 

bodies presumably represents an important eruptive, 

center. The Boulder City pluton (p. ElT), which is 

similar in chemical composition to the underlying 

lavas, probably had its origin at this general center 

during an advanced stage of the Golden Door igneous 

activity. Most of the rocks in the River Mountains 

block are similar to the Golden Door volcanics. Two 

miles north of Boulder City, a small stock of quartz 

monzonite has a nearly circular outcrop within a 

wide frame made of steeply upturned lavas that are 

cut by a network of dikes and irregular intrusive 

bodies.

North and east of Nelson the Golden Door volcanics 

consist chiefly of explosion breccias and tuffs in thick 

layers. No petrographic or chemical analyses of these 

pyroclastic units have been made. In that part of 

the area, the appearance of the light-colored pyroclastics marks an abrupt lithologic change from the 

underlying dark lavas in the Patsy Mine volcanics, 

but no structural discordance is apparent.

MOUNT DAVIS VOLCANICS

A third thick assemblage of volcanic rocks, which 

in many places lies with angular unconformity on the 

Golden Door volcanics or on older rocks, is well exposed at its type locality on and near Mount Davis, 

in western Mohave County, Arizona, and is here called 

the Mount Davis volcanics. This assemblage consists 

largely of lavas, commonly intercalated with coarse 

gravels. Although basalt and dark andesite are the 

most abundant kinds of volcanic rock in the formation, 

at some horizons sheets of glass and pumiceous tuff 

with compositions corresponding to latite and rhyolite 

are widespread. White tuffs in particular make a 

striking contrast with the large thicknesses of basaltic 

flows below and above (fig. 10). These interruptions 

in the section are similar to the occurrence of rhyolitic 

glass in the Patsy Mine sequence north -of Nelson.

The following features and relations of the Mount 

Davis volcanics suggest crustal unrest before, during 

and after the period of eruptions: (1) In some places 

the sequence rests on Golden Door volcanics conformably, elsewhere with strong unconformity. (2) Over 

large areas the Mount Davis rocks lie on Patsy Mine 

volcanics, usually with strong unconformity. Commonly, a fault or fault zone with large throw separates blocks, in one of which the Mount Davis sequence 

rests on Golden Door volcanics, in the other on Patsy 

Mine volcanics. (3) At. several localities the Mount 

Davis volcanics lie on Precambrian rocks, only a short 

distance from normal contacts of these lavas on Patsy 

Mine or on Golden Door volcanics. (4) In many 

places great quantities of extremely coarse rubble, 

derived largely from Precambrian bedrock, are interspersed with the Mount Davis volcanics. Several 

of the larger sedimentary units, exposed along important faults, have a chaotic arrangement suggesting 

landslides from steep scarps. (5) The exposed sections of Mount Davis rocks are tilted, commonly to 

dips of 30° or more; and many faults with large displacement transect these sections together with older 

rocks.

Where the Mount Davis rests on Golden Door volcanics, the change in lithology above the contact is 

abrupt, and the dark lavas in the younger unit generally continue without interruption through a large 

thickness. In this respect the outcrops directly north 

of Searchlight Ferry road and about 7 miles east of 

the old ferry location are somewhat exceptional. There 

a thick flow of rhyolite with abundant quartz phenocrysts is enclosed in dark basaltic lavas about 300 

feet above the top of the Golden Door section. Above 

the solitary flow of rhyolite the exposed section consists wholly of dark basalt.

Marked differences among sections, both in thickness 

and in general composition, doubtless reflect differences in topography at times of eruption and also 

varying distance from eruptive centers. Thus at
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E25

Mount Davis the lower part of the section, overlying 

Golden Door volcanics with slight angular discordance, consists of dark basalt flows hundreds of feet 

thick. Both units dip steeply westward (pi. 1). This 

general relation continues along the strike through 

several miles northward, though in Gold Bug Wash 

the angular discordance is more pronounced and a 

layer of coarse conglomerate 10 to 12 feet thick underlies the basal flow of the Mount Davis. Five miles 

farther northeast, along Pope Wash, the Golden Door 

section is absent; and the Mount Davis volcanics lie 

directly above Patsy Mine volcanics, both major units 

dipping steeply eastward though with noticeable angular discordance. The following section is exposed:

Mount Davis volcanics and Brown Patsy Mine volcanics in

upthroirn block east of large north-south fault 

Mount Davis volcanics (partial thickness) in downthrown 

block; lavas and beds dip 30° E. Xine units recognized as 

follows:

Feet

Basalt flows, brownish ____________ _ 100 

Gravel, bedded, very coarse, weakly cemented.

Pebbles of Precambrian and volcanic rocks____ 60 

Andesite and basalt, dark-gray, in flows 15 to 40 ft

thick ____________________________ 250 

Gravel, bedded, coarse, with pebbles and angular

blocks of Precambrian and volcanic rocks_____ 75 

Basalt flow, brownish __________:________ 25 

Coarse sedimentary debris in crude beds, weakly 

cemented. Blocks, as much as 5 ft long, of Precambrian and volcanic rocks ____________ 300 

Andesite flows, gray-brown, alternating with beds

of coarse gravel _____________________ 100 

Andesite and basalt flows, gray-brown, many weathering to rubble of blocks and sand-size particles 650 

Basalt and andesite flows, 15 to 40 ft thick, alternating with beds of gravel and gray silt_____ 250 

Unconformable contact.

Brown Patsy Mine volcanics, dipping 25° to 40° E. Large 

thickness.

The total of the above measurements, 1,810 feet, 

probably is much too small for the tilted section, which 

is iy2 miles in width of outcrop. Correction was 

made for duplication by strike faults; but there are 

no confident key horizons for use in tracing faults 

and matching accurately between blocks. At any rate, 

this section is incomplete; its base probably is above 

the base of the sequence at Mount Davis as explained 

below; and the upper part, with unknown thickness, 

has been eroded from the hanging wall of the large 

fault on the east. Other belts of outcrop in the general area present similar difficulties; nearly all are 

broken by faults, and the top of the Mount Davis 

assemblage in its typical development was nowhere 

certainly identified. But several partial sections appear to be fully 3,000 feet thick, and maximum thicknesses may well exceed 4,000 feet.

FIGURE 10. Three units of volcanic rocks north of Nelson. View 

looking northeast. Light-colored breccia and tuff, left foreground, 

is in upper member of Golden Door rolcanics. Thick section of 

overlying Mount Davis volcanics is largely basaltic but includes 

layers of whitish tuff. Horizontal Fortification basalt member of 

Muddy Creek formation forms mesa cap in middle background about 

2 miles from camera.

Some of the best exposures of the Mount Davis volcanics are in the rugged hill country about 5 miles 

north of Nelson, where valleys with steep walls run 

nearly at right angles to the strike (fig. 10). There 

the basal flows of dark basalt are nearly conformable 

to the yellow-white tuff and breccia of the Golden 

Door volcanics. As at Mount Davis the lower part 

of the younger unit consists chiefly of basalt and dark 

andesite flows. Dikes that cut the section of flows 

represent feeding channels that supplied magma for 

numerous sills of dark porphyry, and probably also 

for flows at higher levels. Above the middle of the 

section, layers of white pumiceous tuff contrast sharply 

with the dark lavas. One of these old ash beds is the 

basal member of a tripartite unit, highly distinctive, 

which recurs in widely separated localities; the most 

extensive outcrop is under Glass Plateau, a few miles 

south of Hoover Dam.

The section north of Nelson (fig. 10) is very thick, 

though large strike faults and the lack of key horizons 

make an exact measurement impossible. An estimate 

of the thickness, from the base to the large fault cutting off the outcrop on the east, is 3,200 feet, of which 

the lower 2,200 feet consists almost wholly of volcanic 

rocks and related sills, whereas in the upper 1,000 feet 

large quantities of coarse sedimentary debris are interspersed with lavas. This higher part of the section 

is generally like the complete sequence measured near 

Pope Wash; the lower part, made almost strictly of 

volcanic rocks, is like the sequence from Mount Davis 

northward. But the higher part of the section west 

of Mount Davis also includes many thick beds of
E26 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

coarse-grained sediment. The isolated ridges west of 

the river at that latitude are tilted fault blocks in 

which the bedrock consists of typical basalt and andesite flows of the Mount Davis volcanics, interspersed 

with beds of coarse conglomerate and breccia. Some 

of the sedimentary debris exhibits the disordered arrangement characteristic of landslide masses, and the 

fragments were derived almost entirely from Precambrian bedrock. Presumably the source was the Eldorado Mountain block, against which the Mount Davis 

volcanics are downfaulted. The abundant rubble intercalated with the lavas suggests that the fault was 

active in Mount Davis time.

Along Pope Wash and at several other localities, 

the coarse-grained sediments in the Mount Davis sequence extend from the base upward; such sections 

perhaps correspond mainly to the higher member in 

the general assemblage. In Pope Wash the layers of 

sedimentary breccia thicken and coarsen northward 

toward an area of Precambrian bedrock north of the 

Pope fault. Rock types represented in the conglomerate are largely Precambrian, in part volcanic rocks 

including basaltic lavas like those in the Mount Davis 

sequence. This evidence regarding provenance, found 

in several similar sections, suggests that the Mount 

Davis volcanism occurred while faulting was active. 

The thickest sections of lavas accumulated on blocks 

that were relatively sinking. Rising blocks were subject to rapid erosion, and the resulting debris, derived 

partly from lavas, partly from older bedrock, was 

deposited in maximum thicknesses near the borders 

of the positive blocks. During recurrent volcanism 

some of the lavas buried slope gravels, and in favored 

locations the alternating sedimentary and volcanic 

record has been preserved. Abrupt appearance of 

coarse sediments in a distinct upper member, as in 

the section north of Nelson, may signify the resumption of active faulting near the border of a subsiding 

block on which a large thickness of volcanic rocks 

previously had accumulated.

The widest belt covered with Mount Davis volcanics 

is west of the river and extends several miles southward from Glass Plateau. Within this belt the prevailing dips are eastward, in many places at a steep 

angle, through a distance of 5 or 6 miles. The impression of great thickness is strong, though strike 

faults with large throw have caused much duplication. Lavas make up the greater part of the section, 

and lack of key horizons defeats efforts to map most 

of the faults continuously or to reckon accurate thicknesses and displacements. Another outcrop area, as 

much as 4 miles across the average strike, lies west

of the river and north of Eldorado Wash. There the 

dip is generally eastward, above a base that along 

its outcrop bevels strongly across the Patsy Mine volcanics onto the Precambrian basement. This relation 

suggests accumulation on or bordering a rising upland; and this suggestion is reinforced by the quantities of coarse sedimentary breccia interspersed 

through the section of lavas. Probably this area is 

crossed by strike faults not shown on the map, and no 

estimate of the thickness is now possible. Directly 

northeast of the area, several faults of larger throw 

are conspicuous because of the contrast between the 

dark Mount Davis volcanics and underlying whitish 

tuffs and breccias in the Golden Door section.

A short distance farther north, directly west of 

Malpais Mesa, the Mount Davis volcanics again rest 

directly on the brown Patsy Mine volcanics, both 

units dipping steeply to the east. In that vicinity 

the Mount Davis section has particular interest for 

two reasons: from a composition dominantly of basaltic lavas near the southwest corner of the mesa 

there is gradation northward into a section with 

quantities of very coarse sedimentary debris; and the 

section grades upward, without detectable break, into 

tan siltstones that appear to be part of the Muddy 

Creek formation. A steep-walled valley leading to 

the river from the western slope of the mesa shows 

the following succession:

Thickness 

(feet)

Siltstone, cream-yellow, with some thin-bedded clay, 

apparently part of Muddy Creek formation; grades 

downward into layers that have many pebbles and 

scattered angular cobbles; dip 12° to 15° E________ 100+

Conglomerate beds, coarse; many of the pebbles angular ; Precambrian and volcanic rock types represented; 

dip increases downward in section to about 30° E 110

Limestone, gray, single layer, aphanitic __ 1

Conglomerate beds, coarse; dip increases downward to

about 45° E _____________________ 75

Breccia, extremely coarse sedimentary; bedding crude 

or in part lacking; contains many blocks of gneiss 

100 to 200 ft long, shattered but intact; probably of 

landslide origin_________ _ _ 480

Basalt flows with iddingsite, dark; abrupt contact with

sedimentary layers above and below; dip 50° E 300

Breccia, very coarse sedimentary, in part crudely bedded. Much of it, with no perceptible bedding, contains blocks of gneiss 10, 20, and 30 ft long______ 800 ±

Breccia, volcanic, lavender-gray ____ 45

Breccia, sedimentary; fragments largely of volcanic 

rocks but some of gneiss __ 300

Breccia, massive, sedimentary; blocks chiefly of Precambrian gneiss ____________ _ 100

Breccia, bedded, sedimentary; fragments largely of volcanic rocks _____________ _ 225

Breccia, massive, sedimentary; blocks chiefly of Precambrian gneiss ___ _ _ _ 150
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E27

Breccia, bedded, sedimentary; fragments of volcanic

rock with several thin layers of tuff interspersed___ 100

Basalt flows with iddingsite, brown; interlayered with 

coarse sedimentary breccia containing fragments of 

gneiss and of volcanic rocks; dip 55° E ________ 200

Slight unconformity.

Patsy Mine volcanics, dipping steeply eastward.

The northward increase in coarse sediment, much 

of it unassorted breccia containing blocks of Precambrian rock tens of feet long, suggests that the faults 

bounding the Precambrian block west and south of 

Willow Beach were active while Mount Davis volcanism was in progress. Flows that accumulated in 

large thickness in the area southwest of Malpais Mesa 

thinned and wedged out northward against an aggrading slope adjacent to the rising block of basement 

rocks. The abundant coarse rubble that recurs 

through a thick section testifies to a south-facing 

scarp that was renewed time after time by faulting.

Southeast of Hoover Dam a peculiar glassy member that elsewhere is recognized high in the Mount 

Davis volcanics lies only a few hundred feet above 

the base; therefore, the total thickness is comparatively small in that vicinity. In the southern part of 

the map area, the section of dark lavas is very thick, 

though it was not studied in detail. A few miles 

north of Union Pass these lavas, resting on Golden 

Door volcanics, dip steeply northeastward, and the 

belt of outcrop extends east of the area mapped. 

Northeast of the Portland Mine and west of a large 

fault, glassy rocks that belong high in the Mount 

Davis assemblage lie on brown Patsy Mine volcanics. 

In view of the thick Golden Door section a short distance northeast of and dipping toward this fault, 

presumably there was important displacement during 

as well as after the Mount Davis volcanism.

Two miles northwest of Davis Dam, a block of dark 

lavas interlayered with coarse conglomerate, apparently part of the Mount Davis, is faulted against 

Precambrian and Golden Door bedrock. Dikes, sills, 

and irregular intrusive bodies of basalt and andesite 

in that vicinity probably are related to the Mount 

Davis volcanics.

Although the lavas of the Mount Davis volcanics 

are predominantly dark and are properly described 

as basaltic, flows of dark-gray andesite make up considerable thicknesses, and an exhaustive study would 

be required to determine the relative proportions of 

basalt and andesite. A random specimen from the 

lower part of the type section near Mount Davis 

shows in thin section the following features:

No. 146. Glassy basalt. Dark groundmass, in large 

part glassy. Many minute grains of pyroxene. Abundant olivine, altered marginally to iddingsite.

Similar rock, dark brown in many flows, is abundant 

in the wide area of outcrop 8 to 9 miles south-southeast from Boulder City. A sample thin section is 

the following:

No. 168. Brown amygdaloidal basalt. Dark, glassy

groundmass. Labradorite has outer zones

crowded with glass inclusions. Much

olivine, in part altered to iddingsite and

serpentine. Some grains of pigeonite.

Amygdales consist largely of zeolites.

Many of the dark flows are glassy in such degree

that little or no feldspar can be identified in thin

section. Two specimens from the vicinity of Square

Butte, one of them a flow directly overlying a layer

of pumiceous tuff, can be diagnosed simply as glassy

olivine basalts, with iddingsite and minor pigeonite

and biotite. These aphanitic dark-gray to nearly

black specimens are common in large thicknesses of

the Mount Davis assemblage.

On the other hand, some gray flows are identified 

as andesites, even on megascopic examination. A 

specimen collected northwest of Davis Dam has the 

following thin-section description: 

No. 374. Groundmass of fine grain, rich in andesine. 

Some grains of pigeonite, augite, and 

magnetite masking biotite.

Gray rock similar to this makes up many of the flows 

that are interlayered with conglomerate, along Pope 

Wash and also in the area of outcrop 2 to 3 miles 

south of Square Butte. Flows with this lithology are 

not resistant to arid-climate weathering; many outcrops are partly masked by heaps of disintegrated 

rock, much of it in sand-size particles.

Glassy rocks with high content of silica make a 

subordinate but conspicuous part of the Mount Davis 

sequence. Two eruptive centers for this glassy material are conspicuous: one nearly 5 miles north of 

Nelson and the other directly south of Square Butte. 

At each of these localities, plugs made of glassy 

agglomerate cut across the underlying lavas, which 

are steeply upturned and disrupted through a radius 

of a quarter of a mile or more. Layers of pumiceous 

tuff and dark glass are particularly thick around the 

centers of eruption. At both localities much of the 

glassy section was strongly upturned and broken before it was covered discordantly by flows of dark 

basalt. These relations may suggest successive eruptions from a source of highly siliceous magma, followed by resumption of effusions low in silica. But
E28 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY*

more probably the glassy materials were erupted incidentally at a few isolated centers, while the more 

mafic lavas continued to be poured out in the general 

area.

The plugs of glassy agglomerate were moved appreciably as solid units. Their outer surfaces are strongly 

slickensided and show vertical striae and flutings, and 

the agglomerate is much fractured. No doubt the 

siliceous magma stiffened rapidly on nearing the surface, at least when activity was waning, and each 

congealing plug was driven upward by pressure from 

the magma chamber. The two plugs at the Square 

Butte center were punched up through whitish porcelaneous glass. Three or four hundred feet south of 

these plugs, a mass of brecciated glass 250 feet long 

and more than 100 feet wide was driven up through 

thick layers of glass which locally are vertical or dip 

steeply westward, in overturned attitude.

At a distance from eruption centers, where a glassy 

section may be seen in normal order, generally the 

basal part consists of nearly white pumiceous breccia and tuff, including some fragments of basalt, the 

layer totaling 10 to 100 feet in thickness. Above this 

is compact glass, in part brecciated, much of it dark 

gray or nearly black in bulk, though transparent and 

almost white on thin edges. Some sections have successive irregular layers with colors ranging through 

brown, red, orange, yellow, and other colors. Many 

of the glasses have phenocrysts, but some are glassy 

throughout.

A high content of silica appears to be characteristic 

of the glasses in the Mount Davis volcanics. Of seven 

specimens analyzed (table 2) the lowest in silica came 

from the section 5 miles north of Nelson, high in the 

thick member of lavas but below the member containing coarse sediments.

One glassy unit high in the sequence of Mount 

Davis volcanics is remarkably widespread and is 

characterized by a persistent combination of features. 

The upper member of the unit forms the cap of Glass 

Plateau, and cliffs at the south rim of that plateau 

show the full makeup of the unit, as follows:

Feet

Felsite or "stony glass," chocolate-brown_ _ 60-100 

Perlitic glass, black _________________ 20-40 

Coarse breccia, fragments of black glass and

gray pumice ____________________ 10-15

Tuff and breccia, white, pumiceous _____ _ 20-60

Flows of basalt and andesite, thick section.

Exposures along a wash 3 miles southeast of Hoover 

Dam and east of the highway show:

Cemented gravels of Muddy Creek(?) formation. 

Unconformity.

Feet 

25 

50

Basalt with iddingsite, vesicular _____________

Felsite, chocolate-brown _______ _______

Perlitic glass, dark-gray to black, locally replete with brownish spherical bodies 2 to 3 

in. in diameter _____________ __ 10-40

Tuff and breccia, gray, containing large angular blocks of pumice ______________ 50-200 

Irregular contact.

Basalt flows, brown to blackish.

TABLE 2. Chemical analyses of 18 volcanic glasses 

[Rock Analysis Laboratory, University of Minnesota. Eileen Oslund, Analyst]

Lab. specimen _ ... __ .......

Original No................. _ ..

SiOz... ...........................

AlaOs 

FezOa.--. ..... .............

FeO.... ........................ .

MgO ..........................

CaO ...... .....................

NajO . --....._

K2 0._ ---- ...._....__ ...

H2 0+... .........................

HzO-.. ...............

TiOj.. ...........................

PtOt.. ...........................

MnO._ ........ ..

Total.-...--........ . .

R1542

307

76.19

12.62

.68

19

.79

4.23

3.32

1.04

.11

.01

03

99.83

R1543

311

72.21

12.34

.59

18

.12

Q7

3.33

4.16

4.75

1.00

.12

.01

.05

99.83

R1536

111

69.40

13.62

QQ

.63

.46

1.31

9 Q4

5.33

.21

30

.07

04

QQ an

R1537

148

70 QQ

12 09

RO

.26

1ft

fti

3 OQ

4.46

3.77

OO

.12

.01

04

99 74

R1538

1QR

73.93

12.46

.55

.30

.16

.75

3 O1

4.74

3.34

.16

.15

.01

04

QQ SO

R1546

65.86

15.18

1.99

an

1.12

2.86

3.52

3.83

3.19

4Q

.42

18

.06

99.50

R1539

207

66.85

15.25

1.26

.57

.46

1.38

3.90

5.74

3.53

.18

.41

.07

.06

99.66

R1540

297

71.55

13.50

.80

.28

.24

1.02

3.58

5.02

3.06

.26

.18

.03

.06

99.58

R1541

298

70.94

13.41

1.12

.03

.34

1.42

3.44

5.00

3.08

.31

.18

.03

.06

99.36

R1544

345

72.91

12.53

.43

.30

.13

.69

3.67

4.52

3.99

.24

.13

.01

.04

99.59

R1545

350

73.65

12.78

.78

.09

.19

.79

3.49

4.58

2.96

.20

.11

.01

.05

99.68

R1547

421

72.40

12.94

.73

.16

.34

.79

3.27

4.99

3.35

.40

.13

.01

.05

99.56

R1548

424

79.46

10.57

.64

.09

.17

.68

2.92

4.16

.45

.20

.12

.02

.04

99.52

Specimen Description and locality

307. Uniform glass with no phenocrysts, from glassy member in Patsy Mine volcanics, in ridge east of Welcome fault and 1 mile northwest of Techatticup 

mine, Eldorado district, Nevada. Specimen is one of many brownish spheroids, with diameters 1-3 in., more resistant than enclosing glassy matrix.

311. Uniform glass, no phenocrysts, from glassy matrix enclosing specimen 307.

111. Glass, dark-green to black, forming layer 25 ft thick in Golden Door volcanics, 

about 500 ft below base of Mount Davis volcanics, directly north of Searchlight Ferry road, Arizona, about 7 miles east of Colorado River. Small 

phenocrysts in sample.

148. Nonporphyritic glass, light-gray, from thick glassy unit in Golden Door volcanics at Gold Bug Wash, about 2 miles east of Colorado River.

185. Glass, dark-gray, with whitish phenocrysts, from Golden Door volcanics in 

wash !} £ miles west of Golden Door mine, Arizona.

414. Perlitic glass in Golden Door volcanics, half a mile northeast of Sugarloaf Mountain, at east side of large loop to south in road leading east from Hoover Dam.

207. Glass, dark-gray to black, from layer high in Mount Davis volcanics, on west 

side of Welcome fault near north end of the fault as exposed.

Specimen Description and locality

297. Porphyritic glass, dark-gray, in upper part of Mount Davis volcanics, east of 

highway and 2 miles (straight map distance) southeast of Hoover Daii. 

Glass layer is below thick section of brownish felsite and above thick pumiceous 

tuff-breccia.

298. Glass, reddish-brown, from block in tuff breccia below glass layer of specimen 

297.

345. Glass, dark-gray, from eruptive center in Mount Davis volcanics south of Square 

Butte, from layer steeply tilted away from plug of agglomerate, has no phenocrysts.

350. Glass, light-brown, from layer directly below basaltic lavas of the Mount Davis 

volcanics, at east side of eruptive center south of Square Butte.

421. Glass, dark-sray, from layer directly below brown felsite, half a mile southeast 

of Three Kids mine. Glass overlies pumiceous tuff.

424. Brown felsite above glass layer, locality of specimen 421. The assemblage 

resembles that in upper part of Mount Davis volcanics.
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E29

South of Las Vegas Wash, near the Three Kids 

manganese mine, the following sequence is at the top 

of a thick section of volcanic rocks:

Muddy Creek formation. 

Angular unconformity.

Felsite, chocolate-brown, porphyritic; in thick 

flows _________________________ 200±

Perlitic glass, black; in part thin banded___ 50

Tuff and breccia, with large blocks of pumice_ 100+

Thick lavas and volcanic breccias.

Many other exposures of these distinctive glassy 

rocks range at least as far south as the latitude of 

the Portland Mine. Hunt (written communication, 

1941) reported a similar glassy unit east of the Black 

Mountains and north of the Colorado Kiver, at a location more than 60 miles from the Portland Mine. 

Presumably, therefore, this unit of the Mount Davis 

volcanic records a volcanic episode that affected a very 

large area. Although the unit is in general conformable within the Mount Davis, there are several 

local departures from this relation. Three miles southeast of Hoover Dam, the irregular contact between 

the glassy unit and underlying basalt appears to be 

an erosional unconformity. Along the north edge of 

Glass Plateau, the glassy unit lies on intrusive rocks 

apparently older than any part of the Mount Davis 

assemblage. Near the Three Kids Mine, the lavas 

directly beneath the glassy unit consist of latite and 

more siliceous rock types and apparently belong to 

the Golden Door sequence. This varied relation merely 

strengthens the conclusion, based on other evidence 

cited above, that crustal movements and local erosion 

were pronounced while the Mount Davis volcanism 

was in progress.

Comparison of three specimens (297, 298, 421) by 

chemical analyses given in table 2, shows the high content of silica in these similar units of glass. Specimen 

424, the felsite directly above specimen 421, is abnormally high in silica, some of which may be secondary.

A study of nine glass specimens containing phenocrysts for thin-section examination gave the results 

summarized in figure 11. Six of the glasses (207, 

297, 298, 350, 421, 424) are from the Mount Davis 

volcanics and are closely comparable to the Golden 

Door glass (185) that is highest in silica. A perlitic 

glass (414) from latite in the Golden Door volcanics 

near Hoover Dam has a rather distinctive composition.

VOLCANIC ROCKS OF MUDDY CREEK FORMATION

Although the volcanic rocks in this assemblage are 

part of the Muddy Creek formation, they merit separate treatment as a fourth major division in the record of volcanism within the map area. Basalt flows

locally interlayered with clastic sediments in the basal 

member of the formation, as at the Three Kids mine 

(p. E10), probably represent closing stages of the volcanism that produced the Mount Davis volcanics. 

Widespread sheets of basaltic lavas in the upper part 

of the Muddy Creek formation indicate renewal of 

volcanic activity after a considerable interval of time. 

A remnant of these lavas forms the cap of Fortification Hill, which has an area of nearly 2 square miles. 

In the thickest part of this prominent residual, about 

50 superposed flows of olivine basalt have a combined 

thickness of 500 feet.

Other large remnants of similar basalt in the map 

area appear to represent a widespread cover that once 

was continuous over hundreds of square miles. Some 

remnants are conformable on sedimentary beds that 

probably belong in the Muddy Creek formation; other 

remnants lie unconformably on older rocks. The largest known remnant, covering more than 10 square 

miles, lies east of the Black mountain divide.

Because of its prominence and distinctive lithology, 

this upper unit in the Muddy Creek formation is 

mapped separately as the Fortification basalt member 

(table 1).

Vigorous local erosion in early Muddy Creek time, 

as indicated by coarse sediments of highly varied 

thickness, suggests active faulting in the closing phases 

of Mount Davis volcanism. Basalt flows in these 

basal deposits of the formation are succeeded by 

layers of andesite tuff and tuffaceous sand, which indicate that the Mount Davis volcanism closed with 

explosive eruptions from reservoirs in which the magmas had become less mafic. There followed a lull in 

volcanic activity, as indicated by nearly 2,000 feet 

of sandstone, siltstone, clay, and saline materials in 

which no volcanic rocks have been observed.

The thick and uninterrupted succession of basalt 

flows resting on an equally thick sedimentary sequence, both in Fortification Hill and in Malpais 

Mesa, suggests that the flood of lavas in late Muddy 

Creek time began abruptly and continued rapidly. 

Although no Muddy Creek deposits now cover the 

cap of Fortification Hill, the base of the lava sequence 

is conformable on the beds below. Northwest of the 

hill the lavas have been downfaulted and tilted northward; and north of Boul-der Basin a closely similar 

but thinner section of basaltic flows, tilted gently 

southward, is included in the Muddy Creek sedimentary sequence. Study of this basin before Lake Mead 

was formed led to correlation of the basalts of the 

Fortification Hill area with the uppermost of those 

exposed north of the lake (Longwwell, 1936, p. 1421).

In the vicinity of Lake Mead, therefore, the Fortification basalt member, as here defined, is part of the
E30 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

MgO+FeO

luO percent 

MgO + FeO

EXPLANATION

Bulk composition

Values for specimens 350 and

424 are identical

Groundmass composition

 

Superposition of above

For specimen 421

50 percent

K20 +Na20,

CaO

FIGURE 11. (K2O + Na2O), (FeO+MgO), CaO ratios in nine porphyritic volcanic glasses, plotted from data in table 3.

Muddy Creek formation. At Malpais Mesa the thick 

cap of basaltic flows is conformable on beds that 

resemble Muddy Creek deposits; but no sedimentary 

layers now cover any part of the cap. Remnants of 

basalt forming1 tops of small buttes in the area mapped 

as Muddy Creek formation east of Willow Beach are

logically correlated with the lavas on Malpais Mesa 

and Fortification Hill. Other residuals south of Lake 

Mead rest on varied types of older rock and in themselves give no hint of any relationship to the Muddy 

Creek. Several of the larger remnants, lying with 

strong angular unconformity on tilted Mount Davis

TABLE 3. Bulk composition versus groundmass composition l in nine volcanic glasses, in percent.

[Philip Orville, analyst]

Specimen

111.............................

185..___ . __ ....

414- . -____ ______-_ .

207-.... -.

297_ -.-.. . ... . .-

998

350. . . - _ 

421-... ... .. ... .

424_ .._. _. ___ ... ...

SiO2

Bulk

fi9.40 

73.93 

65. 86 

66.85 

71.55 

70.94 

73. 65 

72.40 

79.46

Groundmass

70.57 

74.63 

69.19 

67.70 

72.85 

71.69 

74.46 

73.12 

80.07

A1 2 0 3

Bulk

13.62 

12.46 

15.18 

15.25 

13.50 

13.41 

12.78 

12.94 

11.57

Groundmass

12.61 

11.85 

12.99 

13.87 

12.40 

12. 69 

12.05 

12.32 

11.29

FeO

Bulk

1.09 

.46 

1.92 

1.03

.52 

.37 

.28 

.50 

.26

MgO

Groundmass

0.75 

.47 

.46 

9° 

.44 

.27 

.24 

.49 

.21

CaO

Bulk

1.31

.75 

2.86 

1.38 

1.02 

1.42 

.79 

.79 

.68

Groundmass

0.96 

.75 

2.13 

.92 

.63 

1.37 

.55 

.75 

.62

Na 2 O

Bulk

2.94 

3.21 

3.52 

3.90 

3.58 

3.44 

3.49 

3.27 

2.92

Groundmass

2.40 

3.14 

2.85 

3.51 

2.98 

3.14 

3.28 

3.12 

2.82

K2 0

Bulk

5.33 

4.74 

3.83 

5.74 

5.02 

5.00 

4.58 

4.99 

4.16

Groundmass

5.69 

4.26 

4.50 

5.97 

5.58 

4.96 

4.87 

4.67 

4.14

Volcanic 

unit 2

G.D. 

G.D. 

G.D. 

M.D. 

M.D. 

M.D. 

M.D. 

M.D. 

M.D.

1 Groundmass composition is the difference between the bulk chemical composition 

and the chemical composition calculated for the phenocrysts.

2 G. D., Golden Door volcanics, M. D., Mount Davis volcanics.
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E31

volcanics, form prominent mesas in a wide area between Xelson and Glass Plateau (fig. 10). A smaller 

but equally conspicuous residual makes the cap of 

Square Butte. East of Willow Beach an elongate 

remnant of basaltic lava rests on Precambrian rocks 

and extends from the summit of the Black Mountain 

block to U.S. Highway 93 (pi. 1). The largest residual, several miles south of Boulder Canyon, is 

closely associated with large basaltic dikes.

There is no positive proof that all these remnants 

represent one and the same sequence of lavas, but 

inference of this relation is very strong. All consist 

of vesicular or amygdaloidal olivine basalt, in individual flows that are thin or of moderate thickness. 

In all remnants the flows are nearly horizontal or dip 

gently; the steepest dip, about 10°, is in the large 

residual mass east of Willow Beach and is readily 

explained by some tilting of the Black Mountains 

block in post-Pliocene faulting. Altitudes at the bases 

of some remnants differ as much as a few hundreds 

of feet; but topographic relief in Muddy Creek time 

must have been considerable, and there has been later 

deformation by faulting and warping (p. E10). In a 

logical reconstruction the flows of the Fortification 

basalt member, as they appeared west of the Black 

Mountains before their dissection by the Colorado 

River drainage, would form a thick and continuous 

sheet extending at least 30 miles from north to south, 

10 to 15 miles from east to west, overlapping both 

rims of the present river valley and with the top of 

the sheet at an average height at least 2,500 feet above 

the present river channel. Wide extension of the 

flows across older rocks bordering the basins of sedimentary deposition indicates that upland sources of 

sediments had been reduced to moderate relief.

The lavas of the Fortification member range in color 

from nearly black through gray brown to dark brown. 

Some flows have numerous small vesicles, irregular in 

form; others have rounded vesicles with diameters 

as much as three-quarters of an inch, many filled with 

white amygdales made of zeolites. Most of the rock 

is glassy olivine basalt, in which the olivine is considerably altered to serpentine and iddingsite. Some 

hand specimens show conspicuous aggregates, as much 

as half an inch long, made up wholly of olivine; 

others have large grains of plagioclase crowded with 

glass inclusions. In Fortification Hill a few layers of 

dark-brown slaggy tuff are included between flows. 

The only layers of sedimentary material seen in any 

section of the lavas are local lenses of conglomerate 

near the base of the section at the southeast corner of 

Fortification Hill.

Probably the lavas of the Fortification member were 

erupted in large part from fissures. Many dikes of

dark basalt transect the Mount Davis and older rocks, 

and such dikes are conspicuous in a considerable area 

adjacent to Kingman Wash. But one central vent is 

marked by a plug, about 300 feet in diameter, etched 

into relief by erosion near the highest part of Fortification Hill. Near this plug are numerous brown 

slaggy bombs, which, with enclosing scoriaceous tuff, 

indicate some localized explosive activity in addition 

to widespread eruption of lavas.

COMPARISON WITH ADJACENT AREAS

Kansome (1907), Lee (1908), and Schrader (1909) 

gave brief and generalized descriptions of volcanic 

rocks in parts of the area here considered. All were 

limited to hasty and scattered field observations in 

rapid reconnaissance. In an area covering more than 

6,000 square miles, Schrader recognized three general 

assembages which he designated from oldest to youngest: (1) chloritic andesite; (2) undifferentiated andesites trachytes, rhyolites, and latites; (3) basalt. Ransome (1923a) later studied in some detail the geology 

of the Oatman mining district, Arizona, in the Black 

Mountains about 10 miles south of Union Pass. His 

classification of volcanic rocks exposed within an area 

of about 75 square miles is shown in table 4. Lausen 

(1931), in a later study of the district, adopted Ransome's volcanic sequence with little change.

The geologic map of the limited area centering at 

Oatman is more detailed than any part of the map 

in the present report! Ransome (1923a) and Lausen 

(1931) recognized more lithologic units than does the 

present report, and they applied to each unit a precise petrologic designation. But Ransome noted that 

attempts to distinguish among andesite, trachyte, and 

latite present difficulties because these kinds of rock 

grade almost insensibly one into another. Moreover, 

widespread alteration, perhaps caused by ore-bearing 

solutions, has obscured the original nature of many 

flows in the Oatman section, making exact classificacation impossible. Finally, Ransome's succession at 

Oatman is incomplete; the upper part of the eastward-dipping section lies beyond the eastern boundary 

of his map. For several reasons, then, correlation of 

the volcanic rocks near Oatman with the sequences 

only a moderate distance to the north is more difficult 

than might be expected.

Probably the Oatman andesite and the two underlying units of trachyte correspond in a general way 

to the Patsy Mine volcanics. Largely on the basis 

of megascopic examination, brown andesite seems 

predominant in the latter unit, but detailed petrographic study may show that much of the rock is 

trachyte. Ransome's descriptions (1923a) indicate a 

wider variety of colors in his three lower units than
E32 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

TABLE 4. Volcanic rocks of Oatman district, Ariz. 

[Summarized from F. L. Ransome (1923a)]

Unit

Sitgreaves tuff _

Meadow Creek and 

Flag Creek trachyte. 

Gold Road latite

Oatman andesite

Alcyone trachyte

Bedded breccia

Precambrian complex _

General character

Pumiceous tuff, cream _ _____ _ ___ __

Complex assemblage of lavas and tuffs. Lavas have abundant 

grains of biotite and feldspar. Considerable rhyolite included.

Lavas, greenish-gray, in part porphyritic; considerably altered. ___

Flows of gray lava, in part porphyritic. Locally altered, greenish; 

some gray shale in upper part. 

Fragments derived from Precambrian and volcanic rocks. Has 

of shale and sandstone.

Gneiss, schist, and granite.

Thickness (feet)

0-1, 000

500-600 

200-300 

250 ±

3, 000-4, 000

800-2, 700 

0-1, 000 

1, 000-2, 800

100-200

Probable equivalence

Fortification basalt 

member of the 

Muddy Creek 

formation.

Golden Door 

volcanics.

Patsy Mine volcanics.

is evident in the Patsy Mine volcanics, but this difference may reflect more extensive chemical alteration 

in the Oatman district than farther north. In favor 

of the suggested correlation are (1) similar average 

composition of the two assemblages, near or identical 

with that of andesite; (2) location in the lower part 

of each section; (3) similar maximum thickness 

5,000 to 6,500 feet.

Neither Ransome nor Lausen mentioned, as occurring below the top of the Oatman andesite, any lavas 

with high content of silica such as the spherulitebearing rhyolite within the Patsy Mine section near 

Nelson (p. 19). A distinctive unit of this kind, if it 

were widely distributed, would be useful in attempts 

at correlation.

Eansome's (1923a) Gold Road latite, marking an 

abrupt and enduring change in composition of lavas, 

is similar in many ways to the latite section in the 

Golden Door sequence at Hoover Dam. At the latter locality the latite is strongly unconformable on 

the older andesite; but this is not the invariable relation between the Golden Door and Patsy Mine units 

(p. E20). Lausen (1931) described the colors of the 

latite near Oatman as dark to light gray, lavender, 

and light brown; he stated that the typical rock has 

well-developed phenocrysts of feldspar, sparkling 

flakes of biotite, and some pyroxene, and that in thin 

section the predominant feldspar is recognized as 

andesine, and orthoclase is conspicuous. These descriptions apply well to the Black Canyon latites. 

Although no complete chemical analyses of the typical 

latite near Hoover Dam are available, perlitic glass 

in the section east of the dam is strikingly like some

of the latite near Oatman, as shown in the following 

table:

TABLE 5. Comparison of chemical compositions of two volcano

rocks

Si O2 -_-- -----------------------

A1>O3

Fe2 03 ----------------------

FeO --__---_- -_ 

MgO_ __ - __

CaO

Xa2O . --__- - _- -_--_--_

K2O _-_ ----- -- ~

H2 0 + _____-___-_-___---_---___-

H2 0- _______ ___________________

TiO9

P2 0S __-.-- _ - -

MnO... __--_-_ _- __-___ - _-__

Total _ __________________

(1)

65.86

15. 18

1. 99

. 80

1. 12

2. 86

3. 52

3. 83

3. 19

. 49

. 42

. 18

. 06

99. 50

(2)

62. 9t>

15. 36

2. 57

2. 09

2. 50

4. 26

3. 84

3. 96

1. 37

. 23

. 72

. 28

. 04

100. 18

1. Perlitic glass beside highway, a quarter of a mile (in 

straight line) southeast of Hoover Dam. Analysis by Univ. of 

Minnesota Rock Analysis L boratory.

2. Latite 1 mile southwest of Sunnyside mine, Oatman 

District, Arizona. Analysis by R. C. Wells, U.S. Geol. Survey.

Because of these similarities, Ransome's Gold Road 

latite may be correlated with the lower part of the 

Golden Door volcanics, which in the type section 

has a considerable thickness of flows resembling the 

latite of Black Canyon. The trachyte, tuff, and rhyolite above the Gold Road unit also are correlated 

with the Golden Door volcanics, and perhaps considerably more rhyolite higher in the section lies 

east of Ransome's (1923a) map area. Therefore the 

Oatman section as described by Ransome (1923a) 

and Lausen (1931) may omit much of a sequence
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E33

equivalent to the Golden Door volcanics, and all the 

Mount Davis volcanics.

The highest volcanic unit recognized by Schrader, 

Eansome, and Lausen consists of olivine basalt in 

flows that are nearly horizontal and unconformable to 

older volcanic rocks. Schrader (1909), in his generalized map covering much more than a square degree, included in one category flows probably equivalent to the Fortification basalt member of the Muddy 

Creek formation and others, much younger, enclosed 

by Quaternary (?) gravels. The basalt mapped near 

Oatman occurs in remnants at a high altitude, and 

both in composition and in mode of occurrence it is 

strongly suggestive of the Fortification basalt member.

PLEISTOCENE(?) IGNEOUS ROCKS

Flows of basalt and several mafic dikes are associated with weakly cemented gravels in a limited 

area 6 to 8 miles southeast of Hoover Dam. The outcrops of basalt are erosion remnants of sheets, originally of small or moderate extent, that were erupted 

on the alluvial slope west of the Black Mountains 

and at least in part buried as the coarse fan debris 

continued to build up. In more recent dissection by 

washes tributary to the Colorado River, the sheets 

have been more or less laid bare and cut away. Steep 

cliffs at the edges of some remnants show well-developed columnar structure. The sheets, about 20 feet 

in greatest thickness, consist of slaggy rock at the 

top, vesicular rock extending downward through 3 or 

4 feet, and a varied thickness of compact basalt grading into vesicular rock at the base. On fresh surfaces 

the rock is nearly black. Thin sections show a groundmass aphanitic to glassy, small laths of labradorite, 

and scattered grains of olivine, pyroxene, and magnetite. Composition is virtually uniform in all the 

flows.

Near the flows of basalt several camptonite dikes 

cut the deposits of weakly cemented slope gravels. 

Campbell and Schenk (1950, p. 672) mapped three 

general lines of the dike outcrops, all discontinuous 

and with some abrupt offsets in strike. The area including these outcrops extends about 2 miles from 

north to south and has maximum width of about 

1,500 feet. The dikes have a northward trend, with 

a range in strike from N. 20° W. to N. 25° E.; the 

easternmost of the three lines has an average direction slightly east of north. Thickness of the dikes 

ranges from 2 to 6 feet; in dip the average is near 

vertical, with extreme variations from 65° E. to 75° 

W. Probably the best single exposure is in the east 

wall of a deep artificial cut along U.S. Highway 93, 

8 miles southeast of the dam in a straight line or 

nearly 9 miles as measured along the road. The dike

is essentially vertical, has a nearly uniform thickness 

of about 4 feet, and cuts rather evenly across the 

coarse fan deposit in which there is little suggestion of bedding. Numerous phenocrysts of black 

amphibole in the middle of the dike are 2 to 4 inches 

long, but the size decreases outward to small fractions 

of an inch, and no phenocrysts are present in the 

chilled margins. Many vesicles in the marginal parts 

of the dike, elongate parallel to the walls, suggest 

light overburden at the time of emplacement.

The groundmass of the dike is largely microaphanitic and encloses crystals of somewhat altered 

olivine, as much as half an inch in diameter, together 

with scattered small grains of calcic andesine, magnetite, and apatite. Sporadic grains of quartz are 

probably xenocrysts derived from the fan deposits 

or other rocks cut by the dike. According to Campbell and Schenk (1950, p. 683), the crystals of amphibole, which on fractured surfaces resemble obsidian, 

appear to be kaersutite or a near relative. One chemical analysis of the dike rock shows 42.26 percent 

SiO2 , slightly higher than the average of 40.70 percent for camptonites listed by Hatch, Wells, and 

Wells (1949, p. 353).

Near the southern limit of these dike exposures an 

old vent, irregularly circular in plan and about 400 

feet in diameter, is filled with brown tuff-breccia and 

some extrusive camptonite. The breccia encloses 

breadcrust bombs 1 to 2 feet long and angular fragments of basalt as large as 2 feet in diameter. This 

material, indicating eruptive activity, is closely adjacent to and probably connected with one of the 

camptonite dikes and supplies additional strong evidence that the dikes as now exposed were emplaced 

at very shallow depth.

Campbell and Schenk (1950, p. 688) were impressed by the decrease in size of phenocrysts from 

the middle of the dike outward, and they concluded 

that the crystals formed between emplacement and 

solidification of the magma. They recognized the 

evidence for very shallow emplacement and estimated 

that the crystals of amphibole must have formed in 

a period no longer than 25 days. S. Warren Carey 

(oral communication, 1959), in an alternative hypothesis, supposed that a magma chamber at considerable depth was stationary long enough for 

growth of the large phenocrysts. Crystals that 

started forming in the upper part of the chamber 

slowly sank, and eventually there was gradation in 

size from incipient crystals near the upper boundary 

to large crystals at some depth. When magma was 

forced upward to form the dikes, the upper part of 

the fluid, wedging the walls aside, became the marginal parts of a dike; fluid containing crystals of small
E34 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

and medium size followed; and material from a 

deeper part of the chamber carried the large crystals to the middle of the growing dike. Under this 

concept the large phenocrysts present no special 

problem.

The location and orientation of the dikes suggest 

strongly that intrusion occurred along fractures related to the north-south fault at the west base of the 

Black Mountains. All exposures of the main fracture in the vicinity of the dikes indicate that it is a 

reverse fault dipping steeply eastward. The three 

nearly parallel lines of dikes in a belt more than a 

quarter of a mile wide suggest a fault zone of considerable width, in large part hidden under fan debris. Perhaps repeated movements along several major fractures in this belt, in part after the slope 

gravels were weakly cemented, extended the fractures 

upward across the fan debris and so prepared the way 

for emplacement of the dikes.

STRUCTURE

A principal objective of the present study is the 

deciphering and mapping of major structural features south of Lake Mead, for comparison with those 

directly north and west of the lake. Ranges within 

the latter belt are characterized by thick sections of 

sedimentary formations that have large areal extent; 

such bedrock is ideal for effective display and accurate mapping of faults and folds. South of the lake 

much of the bedrock is far less favorable for structural studies. The complex of Precambrian metamorphic rocks and later plutons has no effective horizon markers that outline structural forms or give any 

satisfactory measure of displacements. Fortunately 

those needs are in some degree served by the volcanic 

rocks that cover large areas on both sides of the 

Colorado River.

Recognition of four distinctive assemblages within 

the thick volcanic accumulation provides, over considerable parts of the river belt, a crude substitute 

for extensive sedimentary formations. Displacement 

of major volcanic units makes many faults clearly 

evident; and at exceptional localities it is possible to 

measure displacements precisely through use of distinctive markers such as layers of peculiar glass or 

tuff. Generally, however, quantitative measurements 

are impossible, because the volcanic formations are 

lithologically monotonous over wide areas and along 

the strike are subject to abrupt changes in character 

and in thickness. Such variations, inherent in deposits built up around scattered eruptive centers, are 

intensified in the present case because faulting movements during the volcanism provided local basins for

thick accumulations, with bordering ridges from which 

all volcanic products were quickly eroded.

Great thrust faults, conspicuous in many ranges of 

southern Nevada, are not known in the area of the 

present report. Failure to find such faults in the belt 

south of Lake Mead is not surprising, because within 

that area we must rely chiefly on volcanic rocks for 

evidence of deformation, and probably all such rocks 

that have been preserved were formed after the regional thrusting ended. Moreover the large thrusts 

of southern Nevada, so far as they have been reported, 

are confined to the section of Paleozoic and Mesozoic 

sedimentary formations. Perhaps some of these 

faults "root" in the Precambrian complex, and this 

possibility was kept in mind while the field study was 

in progress. But any thrusts cutting the metamorphic and plutonic rocks south of Lake Mead probably 

would be obscure at best and might be wholly concealed by the widespread cover of volcanic rocks and 

later alluvium. One fault zone revealed by excavations during construction of Davis Dam has low dip 

and possibly represents local thrusting; this exceptional fault (fig. 15) is described elsewhere in the 

report.

The principal structural features between Lake 

Mead and Davis Dam are steep faults, many with 

large displacement. Most of these faults are normal, 

but at least two of the largest have reverse displacement along much of their exposed lengths. Blocks 

between faults generally are tilted, and in the tilting 

movements several large normal faults were rotated 

to low angles of dip. Strikes of faults range widely; 

but northerly components prevail, and in general the 

displacements seem to be related to relative uplift of 

the Black Mountains block east of the river, and of 

the Eldorado-Newberry Mountains block west of the 

river. Through most of its length in this map area, the 

river valley follows a structural depression between the 

two major units of uplift. Although much of the depressed area is masked by alluvium, excavation by 

the river has exposed large masses of bedrock and 

many structural features. Therefore the exhibit of 

basin-range structure offered by this area is in some 

respects more satisfactory than any to be found in 

the Great Basin, where interior drainage is covering 

most of the downthrown blocks with increasing thicknesses of debris.

In an overall view the structural pattern southward 

from the Boulder Basin of Lake Mead falls into three 

well-defined units. (1) In a northern division, extending south to the latitude of Eldorado Wash, blocks 

are tilted consistently eastward toward a major fault 

zone along the west base of the Black Mountains.
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E35

(2) In a middle division, between Eldorado Wash 

and the Dead Mountains, the tilt of fault blocks is 

conspicuously westward, away from the Black Mountains block. (3) In the southern part of the area, 

tilting generally to the east is pronounced on the 

Arizona side, and apparently this general attitude 

continues to the south through- the Oatman mining 

district (Ransome, 1923a; Lausen, 1931).

Faulting has been intensive in nearly all parts of 

the area, and numerous faults visible in the field are 

not shown on the map. The aim has been to represent faults and fault zones that seem most significant 

in the structural picture and to omit many related 

fractures that are grouped too closely to be shown 

clearly on a map of small scale. Of the faults represented on plate 1, some are very clear field exhibits 

that can be traced mile after mile without question; 

others are well exposed in some stretches but elsewhere are obscure because of monotonous bedrock, 

igneous intrusions, or alluvial cover. Some fault zones 

that are complicated by multiple fractures or en 

echelon displacements are represented as single faults, 

usually with broken-line symbol. No doubt many 

faults that merit places on the map were not seen in 

the time available for study of so large an area.

The following detailed discussion considers first the 

dominant Black Mountains block, and proceeds to 

the several structural divisions west of this range, 

taking them in order from north to south.

STRUCTURE OF BLACK MOUNTAINS

SECTION ALONG BOULDER CANYON

Faults mark the western edge of the Black Mountains structural unit through much of its extent within 

the area of this report. Near Boulder Canyon, where 

stream erosion has provided exceptional exposures, 

the mountain mass is revealed as a horst, bounded by 

large faults along both eastern and western margins. 

Other important faults, with northerly trends and 

varied displacements, subdivide the mass into several 

distinct blocks. The overall width of the compound 

horst is about 8 miles.

The Boulder Wash and Ransome faults are paired in 

bounding a high horst, which is transected by the 

narrowest and deepest part of Boulder Canyon. Directly south of the canyon and west of the horst, beds 

of Cambrian limestone and shale dip toward the 

Ransome fault. Before Lake Mead was filled, the 

lower part of the Cambrian exposure was at river 

level, at an altitude near 700 feet. In the ridge east 

of the fault, Precambrian rocks are at an altitude of 

over 3,700 feet. Hence, the minimum throw on the 

fault is about 3,000 feet, and perhaps the full measure is much more, as prolonged erosion may have 

reduced the top of the ridge far below the basal 

Cambrian horizon. Probably the total throw on the 

Boulder Wash fault is still greater, as the footwall 

block contains the thick Muddy Creek section, whereas 

all rocks in the downthrown block of the Ransome 

fault are older than Muddy Creek.

The block west of the Ransome fault is a graben 

about 3 miles wide, bounded 011 the west by the Emery 

fault. This downthrown block and the bounding 

faults have clear topographic expression because of 

differential erosion (pi. 1).

Complex faulting marks the west border of the 

Black Mountains. Directly north of the lake the Precambrian rocks are in fault contact with cemented 

Colorado River gravels and older deposits. Probably 

the fault dips steeply eastward,5 and compression attending the reverse displacement deformed the gravel 

beds in a wide belt (Longwell, 1946, p. 827; pi. 5). 

South of the lake two east-dipping faults that strike 

nearly north cut the complex of Precambrian gneiss 

and younger plutonic rocks. Along one of these 

breaks, here called the Indian Canyon fault, deformed 

bands of gneiss indicate reverse movement but tell 

nothing about the measure of displacement. This 

fault shows clearly on aerial photographs and can be 

traced southward nearly 10 miles to its intersection 

with the Kingman Road fault. The fault a short distance west of Indian Canyon joins the larger fault 

north of present exposures (Longwell, 1936, pi. 2).

The reverse fault north of the lake that displaces 

Colorado River gravels appears to be continued southward as the Fortification 6 fault, on which, however, 

there is large normal displacement. Directly south of 

the river the fault surface was well exposed before 

Lake Mead was formed; the dip is 65° to 75°W., and. 

dragged up on the side of downthrow are fan deposits of the Muddy Creek formation and also old 

Colorado River gravels (Longwell, 1936, fig. 10). As 

these gravels are either Pleistocene or late Pliocene 

(Longwell, 1946, p. 828), movement on this part of 

the Fortification fault continued until late geologic 

time. The scarp is bold (fig. 12), but probably it was 

in large part buried and later exposed by erosion of 

weak deposits in the hanging wall. In Fortification 

Hill, 4 miles south of the lake, coarse Muddy Creek 

deposits near the fault suggest derivation from the 

steep front of the footwall block (fig. 3). As the

6 This fault has been represented as normal (Longwell, 1936, pi. 2, 

section E-E'). but reconsideration of the field evidence has changed 

this earlier verdict (Longwell. 1946, fig. 2).

e Earlier named the Callville fault (Longwell, 1936, fig. 10), but 

it seems best to use the name of the adjacent ridge as printed on 

the Hoover Dam topographic map.
E36 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

FIGURE 12.^-Scarp of Fortification fault, in view southeastward across Colorado River and ruins of old Fort Callville before Lake Mead was 

formed. Mountain mass is composed of granitic rocks intrusive into Precambrian gneiss. Muddy Creek formation and overlying 

cemented river gravels, here shown along the former stream, are in fault contact with rocks in the scarp.

cap of the Fortification basalt member continues across 

the fault trace without perceptible dislocation, probably most of the displacement on this fault occurred 

before the end of Muddy Creek time; and the later 

movement involving Colorado River gravels was confined to the part of the fault north of Fortification 

Hill.

A considerable part of the Mead Slope fault is concealed by Lake Mead; along this part, once well exposed (Longwell, 1936, pi. 2), beds of coarse breccia 

in the Muddy Creek formation were brought up by 

reverse movement against old Colorado River gravels. 

At present the best exposures of this fault are about a 

mile northwest of Fortification Hill, where Muddy 

Creek deposits and included basalt are downthrown 

against altered igneous rocks. Farther northeast, 

much of the trace is obscured by gravel cover on the 

dissected slope.

STRUCTURE NEAR FORTIFICATION HILL

A group of faults with diverse trends outlines several blocks in a roughly triangular area around Fortification Hill. In this group the Indian Canyon and 

Fortification faults strike nearly north, the Mead

Slope fault strikes northeast, and the Horsethief 

fault strikes northwest. At the north end of the triangular area, the Fortification fault is the logical 

boundary of the Black Mountains. In the southern 

part of the triangle, relations are less definite. The 

Fortification fault ends abruptly against the Horsethief fault, and neither of these has the strong topographic expression that marks the Fortification fault 

near Lake Mead (fig. 12). Directly south and west 

of Fortification Hill, all bedrock older than the Muddy 

Creek formation has been chemically altered, locally 

with such intensity that the rock has become a weak, 

claylike mass retaining little evidence of its origin 

(p. E17). Because of this change the rock varies in 

resistance to erosion, and positions of faults are in 

part obscured.

South of the basalt-capped mesa, the Fortification 

fault divides to form a complicated zone of fractures 

in varied bedrock. A complex of plutonic bodies intruded into Precambrian gneiss forms the foot-wall, 

and porphyritic intrusive rocks enclosing large xenoliths of Golden Door volcanics make up the hanging 

wall, in which several prospects of metallic minerals 

have been opened.
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E37

The Horsethief fault is best exposed along White 

Kock Wash, where a clearly defined fracture dipping 

70° NE. separates plutonic rocks and gneiss in the 

hanging wall from brown andesitic lavas in the footwall. The lavas, dragged up to dip steeply southwest, are mapped on the basis of color and lithologic 

character as part of the Patsy Mine volcanics. Beds 

of the Muddy Creek formation that lie on the lavas 

with strong angular unconformity also are dragged up 

to moderate southwest dips. Presumably much of the 

displacement occurred before these beds were laid 

down, though possibly in the earlier part of Muddy 

Creek time. No evidence has been found for dating 

the latest movements on this fault.

In large part the footwall block of the Horsethief 

fault is hidden by alluvium, but the nearly straight 

edge of the hanging-wall block clearly suggests a continuous fault trace (pi. 1), along a line that leads to 

confirmatory outcrops near the lake. These outcrops 

show the altered igneous rocks -south of Fortification 

Hill faulted upward against Golden Door and Mount 

Davis volcanics, and beds of the Muddy Creek formation. Although nothing in the visible evidence gives a 

trustworthy measure of throw on the fault, comparison of bedrock on opposite sides indicates large vertical displacement. Volcanic rocks identified locally 

in the belt of altered bedrock south of Fortification 

Hill are very like the Golden Door volcanics. If this 

correlation is correct, a short distance south of the 

lake the fault displacement at a minimum must approximate the combined thicknesses of the Golden 

Door and Mount Davis sequences, which near Hoover 

Dam is 2,000 feet or more. A much smaller throw 

is suggested by beds of the Muddy Creek; but these 

beds were deposited on an uneven surface, probably 

through various stages of the faulting. Beds on one 

side of the fault may differ considerably in date of 

deposition from those on the opposite side. Moreover, as noted above, the beds of the Muddy Creek 

exposed in White Rock Wash must have been laid 

down after a large part of the displacement on the 

fault had occurred.

Convergence of the traces of the Horesthief and 

Mead Slope faults, and reverse movement on both, 

may suggest that a wedge-shaped block including 

Fortification Hill was forced to move westward and 

upward under compressive stress, but it is not known 

that these faults were in operation at the same time. 

The Horsethief fault was active after some part of 

Muddy Creek deposition but is not known to have 

involved Colorado River gravel. So far as the evidence is known, the Mead Slope fault may have originated after the older river gravels were laid down.

FAULTED WESTERN BORDER

South of the Horsethief fault the border of the 

range has four nearly straight segments that alternate 

in direction between southeast and south through a 

distance of 14 miles. Faulting along this part of the 

range border is strongly implied, though actual exposures of the fault are few. Conceivably this fault, 

here called the Kingman Road fault, is continuous 

with either the Horsethief or the Indian Canyon 

fault. But the three features are distinctive in themselves, and discussion is simplified if each has a distinguishing name.

In its northern segment the Kingman Road fault 

is closely parallel to conspicuous layering in the adjoining Precambrian rocks, but the succeeding northsouth segment cuts across the layering at a small angle. 

Through the two segments, evidence for faulting is of 

two kinds: (1) general straightness of the contact 

between bedrock and alluvium and (2) numerous 

fractures in the bedrock that are nearly parallel to 

the border. These fractures are seen to best advantage in and near workings of a mineral prospect at 

the north side of West Petroglyph Wash. Presumably these fractures are genetically related to the border fault. Most of the observed fractures dip steeply 

eastward and suggest reverse displacement on the 

border fault. This suggestion is supported by observations south of U.S. Highway 93, where the Muddy 

Creek formation is in contact with the fault. Although few actual exposures of a fault surface were 

seen, the beds of the Muddy Creek are turned up to 

or past the vertical near the fault and are much 

deformed in a belt more than 1,000 feet wide. In 

the adjacent Precambrian rocks the gneissic layers, 

which near the range crest dip westward at a moderate angle, steepen progressively down the slope and 

near the fault are vertical or even overturned to a 

steep dip eastward. The sum of evidence strongly 

indicates reverse movement on the fault.

South of the Willow Beach road, the Muddy Creek 

overlaps fault blocks that consist largely of dark 

basaltic lavas, apparently part of the Mount Davis 

sequence. In some of the tilted blocks, the basalt lies 

directly on Precambrian gneiss. Presumably the 

whole assemblage of blocks is faulted against the high 

block forming the range, though the actual trace of 

the principal fault is concealed by waste. In this 

segment of the Kingman Road fault, therefore, the 

dip of the fault surface is not known.

Near the Pope Mine the western border of the 

Black Mountains Precambrian block is offset by an 

important northwesterly fault. If the Kingman Road 

fault extends to this locality, its position is effectively
E38 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

hidden, but a fault with about the same strike is 

conspicuous from the Pope fault southward. A fault 

extending eastward along the upper course of Pope 

Wash is logically the extension of the Pope fault offset by the north-trending fault. Possibly the latter 

fault continues northward, and this possibility is 

suggested on plate 1; but as no evidence of the continuation was found, the border fault from Pope 

Wash southward is considered as a separate structural unit. It is represented on the map as running 

continuously southward about 22 miles to the vicinity of Eppersons Corral, but it has not been fully 

walked out; the map representation is the result of 

extrapolating fault segments studied in a number of 

separate localities, and the line as mapped may represent a zone of related fractures. The name Epperson fault is chosen because evidence of large displacement is well displayed near Eppersons Corral.

In the 3-mile stretch between Pope and Gold Bug 

Washes, the fault is clearly marked and has large 

throw. Kesistant andesite flows of the Patsy Mine 

volcanics maintain a prominent scarp on the side of 

upthrow; weaker basalts and gravel beds in the Mount 

Davis sequence underlie a lower surface on the downthrown block. The fault surface was not seen, but it 

is assumed to dip steeply westward, as do three parallel faults that cut the downthrown block. These faults 

cause repetition in the Mount Davis section, and much 

coarse alluvium further hides relations; but thickness of the younger volcanic sequence exposed here is 

computed as more than 1,800 feet (p. E25) ; in estimating throw on the Epperson fault, to this figure must 

be added an unknown but probably large thickness of 

Patsy Mine volcanics eroded from the foot wall.

Near and for some distance south of Gold Bug Wash 

the structure is obscure, especially at the supposed 

location of the northwestward trending Gold Bug 

fault. Because of poor exposures this location is 

uncertain; but no indication was seen that this fault 

continues east of the Epperson fault. Farther south 

the Golden Door volcanics, dipping consistently westward, are normally downfaulted against Precambrian 

gneiss in the range. The fault is revealed in particularly clear exposures directly east of the Golden 

Door mine and on both sides of the Searchlight Ferry 

road. The westerly dip of the fault, at angles ranging from 45° to 65°, is seen in outcrops and is expressed in retreat of the Precambrian-Golden Door 

contact in the valley followed by the road. Near the 

road there is a marked change in topographic expression of the main bedrock units. Northward for about 

17 miles the volcanic rocks in the hanging-wall block

form a hilly belt, locally rugged but distinctly lower 

than the mountain ridge on Precambrian gneiss. 

South of the road the fault crosses diagonally to the 

east side of the ridge, which through several miles is 

on Golden Door volcanics, whereas the Precambrian 

footwall block is low on the eastern slope.

South of Eppersons Corral the structure is complex and at some points not entirely clear. Along 

the road the Precambrian rocks disappear southward 

under brown andesite identified as part of the Patsy 

Mine volcanics. These andesites in turn dip southward beneath Golden Door volcanics and are cut off 

to the west by the Epperson fault, which apparently 

dies out southward in a zone marked by sharp change 

of strike from southeast to east. A normal fault east 

of the road drops Golden Door volcanics with steep 

westward dip against the foot wall block of Patsy 

Mine volcanics and Precambrian gneiss; this fault has 

a sinuous trace because it dips to the east no more 

than 20°. A later fault, inclined more steeply eastward and with large throw, transects the highly deformed Golden Door rocks and drops against them, 

on the east, Mount Davis volcanics that dip westward. 

Probably these faults are related to sharp downwarping along an axis trending north of west, which is 

responsible for the broad outcrop of Mount Davis 

rocks athwart the range between Eppersons Corral 

and Union Pass. This downwarp is in line with the 

low, alluviated pass west of the river, between the 

Eldorado and Newberry Mountains. Probably the 

structural depression is reflected near Searchlight in 

wide outcrops of volcanic rocks (not mapped), which locally extend eastward into the broad pass.

Several miles southwest of Eppersons Corral, the 

Mount Davis volcanics, dipping southwestward, are 

faulted down against brown andesites of the Patsy 

Mine sequence. In the upthrown block, glass and tuff 

that normally are found high in the Mount Davis 

section rest directly on the andesite of the Patsy Mine 

volcanics. Absence here of the Golden Door rocks, 

which form a thick section less than 2 miles to the 

northeast, and representation of Mount Davis time 

by only the upper part of the normal section suggest 

local uplift and much erosion in the vicinity of th« 

present wash, during and perhaps before the Mount 

Davis episode of volcanism. Many faults and small 

intrusive bodies, not shown on plate 1, cut the Mount 

Davis rocks, especially near the old Portland Mine.

The downwarp south of Eppersons Corral breaks 

the continuity of the Black Mountains topographically, lithologically, and structurally. Precambrian 

basement rocl^s are again dominant farther south, near
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E39

Union Pass; but the structure of that area, closely 

related to the section near Davis Dam, is discussed 

with that section on later pages.

STRUCTURE NORTH AND WEST OF BOUIJDER BASIN

Directly north of Boulder Basin, where the bedrock consists chiefly of weak deposits in the Muddy 

Creek formation with included and older basaltic 

lavas, the structure is not of critical value in the present discussion. The extreme northwestern part of the 

map area presents a sample of the Frenchman Mountain block, where the geology contrasts sharply with 

that southward from Lake Mead; the structure section (fig. 13) illustrates some of the differences. On 

the other hand, the Frenchman block lacks the largescale thrust faults common in neighboring ranges directly to the north and west; its structure of fault 

blocks tilted steeply eastward is generally similar to 

the structure near Hoover Dam.

South of Las Vegas Wash, limestone of the Horse 

Spring formation lies unconformably below volcanic 

rocks that probably are part of the Golden Door sequence; but the relation of these limestone beds to 

older volcanic rocks is not visible. In and around the 

Three Kids mining district, the Muddy Creek formation is strongly faulted and tilted. Nothing in the 

pattern of this deformation suggests a close relation 

to the structural pattern south of the lake. Deformation in the Three Kids neighborhood is marginal to 

the River Mountains mass, which has a complex pattern of faults and tilted blocks that can be deciphered 

only after a systematic study of the volcanic and plutonic bedrock.

The isolated outcrop of Precambrian schist in Saddle Island may seem anomalous, but sediments in the 

Thumb formation serve as evidence that in late Mesozoic time the old basement rocks formed an upland 

extending westward beyond the present River Mountains (p. E8). No doubt the old rocks now exposed 

in Saddle Island and in areas south of the lake were 

mantled with volcanic and sedimentary deposits until they were laid bare by erosion in late Cenozoic 

time.

STRUCTURE WEST OP THE BLACK MOUNTAINS

BOULDER BASIN TO ELDORADO WASH

In a large area south of Boulder Basin, many rock 

units are distributed in irregular fashion and broken 

by faults that have diverse trends. There is, however, considerable order in the apparent chaos. The 

large outcrops of Precambrian rocks along Black 

Canyon, with overlying Patsy Mine volcanics, represent a northeastward trend of the Eldorado Mountain block; near Willow Beach the interval between 

that block and the Black Mountains is reduced to a 

minimum. Exposures of Precambrian rocks father 

west, including Saddle Island, part of the hill south 

of Boulder City, and the large area 5 miles west of 

Nelson, seem to mark high parts of a complex structural block west of and tilted toward the main Eldorado block. This western structural unit, here called 

the Nelson block, is bounded on the east and subdivided by a set of large normal faults, nearly all 

with the downthrow on the west, which strike more 

northerly than the Eldorado axis, toward which they 

die out southward to form a crude en echelon pattern.

The two divisions of the area north of Eldorado 

Wash, as defined above, are now described and discussed as distinct structural units.

NORTHERN ELDORADO MOUNTAINS BLOCK

This unit, from Eldorado Wash northward, is tilted 

generally to the east and faulted down against the 

Black Mountains. The abrupt eastern boundary of 

this block consists of the Kingman Road and Horsethief faults which, in contrast to all the large faults

2000'

1000*

Moenkopi and 

Chinle formations

0

DATUM IS SEA LEVEL 

FIGURE 13. Geologic section along the line H-H' on geologic map (pi. 1).

2 MILES

J
E40 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

west of them, have reverse displacement. Measure of 

the throw along this fault boundary can only be conjectured, as no stratigraphic markers are available for 

comparison on hanging-wall and footwall sides; but 

the total vertical displacement must be thousands of 

feet. East of Hoover Dam a minimum figure is given 

by the combined thicknesses of the volcanic sequences, 

Patsy Mine through Mount Davis, which in the downthrown block dip toward the fault boundary.

A point of interest is the appearance in White Rock 

Wash of brown andesite of the Patsy Mine volcanics, 

turned up steeply against the Horsethief fault. Absence of younger volcanic rocks in the outcrop there 

may be attributed to concealment beneath the Muddy 

Creek formation. Deformation in the footwall perhaps affects a zone of considerable width; and volcanic rocks younger than the brown andesites may lie 

under cover at some distance from the fault. This 

possibility is in accord with the strong disturbance of 

the Muddy Creek formation along the Kingman Road 

fault east of Willow Beach (pi. 1, section C-C'}. 

Presumably the displacement on the two large reverse faults was cumulative through a long time interval. The Muddy Creek, unconformable on lavas 

that dip more steeply, contains coarse rubble probably shed from a growing scarp to the east; and 

these beds in their turn have been deformed in later 

stages of the movement.

Deformation within the major structural blocks is 

expressed in groups of faults with varied patterns, 

and in warping effects. Near Hoover Dam, where 

exposures in the walls of Black Canyon are almost 

continuous, a set of faults nearly parallel to the edge 

of the main block occupies a belt about 2 miles wide. 

Nearly all these faults are normal, with downthrow 

on the west; some surfaces with horizontal striae suggest important strike-slip movements, and a few reverse faults have local development. With mapping 

in detail, no doubt the larger faults in this group can 

be traced farther southeast than they are shown on 

plate 1. General parallelism of the set to the Horsethief fault suggests a genetic relation. Another set, 

cutting the volcanic rocks south of the Willow Beach 

road junction, closely parallels the Kingman Road 

fault. Two faults in this set are exceptional, so far 

as movements have been deciphered, in having downthrow toward the Black Mountains block. This type 

of displacement may reflect adjustment to the weight 

of the overriding hanging-wall block of the main 

fault.

Warping is especially pronounced around the two 

large outcrops of Precambrian rocks west and southwest of Willow Beach. Structure in these areas sugFIGURE 14. View northeastward across the east-west fault north of 

Malpais Mesa. Dark lavas (a) of the Patsy Mine Tolcanics, dipping away from Precambrian rocks in foreground, are faulted down 

against Precambrian rocks (b) forming high ridge. Mount Davis 

volcanics (c), beneath low ground in right middle distance, overlie 

Patsy Mine volcanics in tilted section.

gests effects of strong local doming complicated by 

faulting. Around the Precambrian outcrop north of 

Square Butte the lavas dip strongly outward in nearly 

all directions. The set of faults striking northwesterly into this dome, all with downthrow on the southwest, may reflect torsional strain; a strong vertical 

couple is suggested by the east-west portion of the 

Jeep Pass fault, with upthrow on the south, and the 

large east-west fault with opposite throw directly 

north of Malpais Mesa (fig. 14). Complex local 

strains are indicated also by the faults northeast and 

«ast of the mesa, which in combination have large displacement. Two of these, largely in Precambrian 

rocks, are clearly recognized only where remnants of 

volcanic rocks are preserved on the downthrown side.

NELSON BIX)CK

The Nelson structural unit is broken by many faults, 

some with large displacement. In the northern part 

of the block, the eastern margin is marked by the 

Jeep Pass fault, in whose hanging wall a thick section of Mount Davis volcanics dips steeply toward the 

footwall of Precambrian gneiss and Patsy Mine volcanics. Northward the fault divides both directly and 

en echelon, and displacement appears to die out south 

of Hoover Dam. The Rifle Range fault borders the 

west side of a horst which, with the fault at its eastern border, disappears southward under glassy volcanic rocks that floor Glass Plateau. South of this 

plateau the section of Mount Davis volcanics is duplicated by many local faults and by at least four 

normal faults of considerable magnitude, the easternmost of which extends northward from the east-west 

extension of the Jeep Pass fault. More than a mile
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E41

farther west in the Hidden Valley fault, also with 

northerly strike, clearly visible where Precambrian 

gneiss is in the footwall but obscure farther north in 

the section of basaltic lavas. At the south this fault 

is cut off by another, striking northwest, which hinges 

out to the southeast in Patsy Mine volcanics. The 

Fuller Road fault has caused large displacement of 

the Mount Davis volcanics, but the trace has not been 

followed to an assured limit, either north or south. 

West of it is the Eldorado fault, which has exceptionally clear expression through several miles because of contrasting rocks on opposite sides. This 

fault, on which the throw is among the largest in the 

map area, may extend considerably father than shown 

on the map; it is obscured at the south in plutonic 

rocks and Precambrian gneiss, at the north in basaltic 

lavas. Through 5 miles or more from Eldorado Wash 

northward, the vertical displacement presumably is 

measured in thousands of feet, even if allowance is 

made for possible eastward thinning of the 3 volcanic assemblages west of the fault. Farther north 

the throw may decrease, though the Mount Davis section is very thick and the measure of duplication by 

faulting seems indeterminate.

Faults in a set west of the Eldorado fault are 

clearly expressed because of the distinctive pattern 

made by offsetting of the three volcanic units, Patsy 

Mine through Mount Davis. The tuffs and breccias 

of the Golden Door volcanics, lighter colored and 

more resistant to weathering than the lavas above 

and below, mark locations of faults unmistakably. 

All faults in the set dip to the west and have normal 

displacement. The easternmost member of this set 

makes oblique junction northward with the Eldorado 

fault; at the south this member, and the one next 

west of it, end abruptly near the old Techatticup mine. 

Although waste hides the critical relations, clearly 

the thick volcanic sequences also end along a nearly 

straight line, trending somewhat north of west, at 

the boundary of a coarse-grained intrusive body. This 

boundary is interpreted as an intrusion fault (Ransome, 1904, p. 11), south of which the volcanic rocks 

were lifted by the rising pluton and eroded. The 

Welcome fault offsets the pluton; possibly part of the 

displacement on this fault also occurred while the intrusion was in progress.

West of the Welcome fault, relations between volcanic and intrusive rocks are obscure. South and 

west of Nelson, masses of the old lavas are surrounded 

by plutonic rock and boundaries are indistinct. The 

thick section of Patsy Mine volcanics on the north, 

repeated by faults and tilted to the east, strikes southward directly into a wide zone occupied in large part

by irregular intrusive bodies. The northern boundary of this zone is vaguely marked. Possibly many 

intrusion faults, trending generally east, traverse the 

belt of intrusive bodies, and masses of the volcanic 

rocks that were lifted by intrusive action have been 

eroded, though many engulfed blocks remain. North 

of the old Wall Street mine a large mass of clay developed by hydrothermal action has been used in 

brickmaking.

Several miles northwest of Nelson, a large fault 

with downthrow on the east separates the Patsy Mine 

volcanics from Precambrian gneiss. The north and 

south limits of this fault are not determined. Broad 

anticlinal structure involves the Patsy Mine section 

at the north end of the Precambrian outcrop. Along 

the structure section D-D', plate 1, the outcrop of 

Patsy Mine volcanics is nearly 4 miles across the 

strike, and prevailing dips to the east are particularly steep in the eastern half of this distance. A 

number of large strike faults repeat the section; but 

it seems impossible to follow some of the traces 

through the brown lavas, and even where a distinctive horizon of glass provides a guide, only generalized traces have been mapped, as shown on plate 1. 

Dips of the faults thus represented are low, locally 

no more than 20°. Vagueness of the structural details defeats attempts to make a reliable estimate of 

thickness in this section.

West of the Colorado River the structure characterized by east-dipping fault blocks ends abruptly on the 

south near the latitude of Eldorado Wash. East of 

the river the structural boundary is more irregular. 

In the hanging-wall block of the Ives fault, westward 

dips prevail as far north as the lower end of Black 

Canyon; but east of this fault the belt of eastward 

dips extends to and irregularly south of Pope Wash. 

In the wide outcrop area of Patsy Mine volcanics traversed by this wash, dips are roughly quaquaversal 

around an igneous plug. The large northwesterly 

faults bordering this large block are of considerable 

interest. Three of these the Pope fault, the fault 

paired with it to bound a northwest-trending graben, 

and the Gold Bug fault are closely parallel, and they 

are similar in strike to the set of northwesterly faults 

between Square Butte and the river. The Ives fault, 

on the west border of the Pope Wash block, becomes 

an important member of that set in its extension west 

of the river.

ELDORADO WASH TO NEWBERRY MOUNTAINS

South of the Gold Bug fault, dips to the west are 

general; abruptness of the change in structure along 

this fault is shown in the structure section E~E', 

plate 1. Southward from the lower end of Black
E42 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Canyon, widening sheets of alluvium hide much of 

the bedrock; but through a distance of 15 miles, outcrops in scattered ridges reveal that between the Black 

and Eldorado Mountains the Golden Door and Mount 

Davis volcanics make up several fault blocks that are 

tilted consistently to the west. Section F-F', plate 1, 

illustrates this structure, some elements of which must 

be inferred. The Epperson fault dips west; the Big 

Blackstop fault dips east; and close similarity of exposed structure from Mount Davis westward suggests 

that the other faults also dip to the east. Outcrops 

of Golden Door volcanics along the river indicate an 

important fault east of them. Evidence supporting 

the two faults next in order to the west is obscured 

by alluvium; but they are suggested by changes in 

dip, probably caused by drag. The fault nearest the 

west end of the section is established by the dip of 

volcanic rocks toward the nearly straight border of 

the range.

The plug of andesite porphyry at the main border 

fault along the east side of the Eldorado Mountains, 

and several bodies of similar rocks alined to the south, 

indicate that intrusion was controlled in some degree 

by the fault zone. Northward branching of the fault 

and irregularity of the range front farther north imply that faulting at this border is complex, perhaps 

on a set of en echelon fractures. Large thicknesses of 

the exposed volcanics indicate very large displacement 

in the vicinity of the section F-F'.

In the large alluviated area south of Mount Davis, 

there are no outcrops of the older bedrock; but in the 

Black Mountains belt the westward dips persist, except in the zone of sharp flexure south of Eppersons 

Corral. The axis of downwarp south of the Portland 

mine, in line with the north end of the Newberry Mountains, is the logical southern limit of the structural 

unit with prevailing dips westward. Though outcrops

are limited in this unit as a whole, apparently the 

structure is much simpler than in the wide belt north 

of Eldorado Wash.

STRUCTURE EAST OF NEWBERRY MOUNTAINS

The Newberry Mountains afford the widest continuous outcrop of Precambrian rocks within the map 

area. Generally the banding in these rocks trends 

west of north, and west of the river the average dip 

of the banding is steep toward the east. Persistence 

of Precambrian outcrops across the broad valley and 

eastward to Union Pass marks a broad belt of uplift 

along an axis trending somewhat north of west, parallel to the Portland mine downwarp.

Most of the faults in this part, of the area are revealed by their effects on volcanic rocks, but excavations during the building of Davis Dam gave information on an exceptional fault zone that involves 

Precambrian bedrock exclusively (Lundgren, written 

communication, 1949). The only surface indication 

of this feature was a saddle floored with crushed rock 

and clay. Exploration by trenching and core drilling 

showed a continuous though irregular zone with average strike N. 60° to 65° E., dipping 15° to 25° SE.; 

it is continuous at least 1,000 feet along the strike, 

and extends underground at least 800 feet southeast 

of the power plant, which is east of the river channel 

(fig. 15). The full zone includes in its upper part 

an irregular layer of crushed gneiss, as thick as 30 

feet, crossed by seams of clay 1 inch to 2 feet thick. 

Ten to 100 feet vertically below this layer is another 

with similar content. In many parts of the fault zone, 

the power shovels could excavate without blasting.

Nothing in available descriptions of this zone suggests the nature and measure of displacement. The 

low average dip and the extent of thorough crushing 

suggest thrust faulting. Because of the limited obService Bay

200 FEET

J

FIGURE 15. Fault zone in Precambrian rocks at Davis Dam, exposed by excavation east of river. Zone strikes N. 60°-65° E., dips 15°-25 e 

SE. (modified from W. K. Lundgren, U.S. Bureau of Reclamation written communication, 1949).
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E43

serrations the fault is not shown on the map, plate 1, 

but its position is indicated in section G Crf .

About 2 miles west and northwest of Davis Dam, 

volcanic rocks are downthrown on faults that make an 

irregular pattern. In one block with steep eastward 

tilt, basaltic lavas interspersed with beds of gravel 

appear to be part of the Mount Davis volcanics. An 

adjoining block tilted to the west has typical Golden 

Door volcanics, including basaltic sills and dikes. Several miles east of the dam, a set of normal faults with 

northwest strike repeats a section of Golden Door 

volcanics. The large outcrop of Precambrian rocks 

west of Union Pass appears to represent a muchfaulted arch whose axis trends northwest. Some of 

the faults are clearly marked by remnants of lava and 

tuff, as at the old Frisco mine. The large fault at 

the Arabian mine, exceptional in direction of strike, 

has exposed in the hanging wall south of the mine a 

very coarse breccia that includes quantities of gray 

limestone. Conceivably the source was in Paleozoic 

beds that have been eroded from the footwall block.

DATING OF FAULTS

In the area south of Boulder Basin, none of the 

many faults can be assigned definitely to geologic 

epochs, and few can be bracketed closely in relation 

to lithologic units. For example, two,faults about a 

mile west of Willow Beach cut only Precambrian 

rocks and Patsy Mine volcanics, but as no younger 

bedrock has survived erosion there, an upper limit for 

the date of faulting is indeterminate. Northeast of 

Nelson, the Welcome fault cuts a thick Mount Davis 

section that is strongly tilted; but flows of the Fortification basalt member of the Muddy Creek formation, 

covering a nearly even surface of erosion, cross the 

fault unbroken. The Rifle Range fault, and another 

2 miles west of it, have lowered beds of the Muddy 

Creek with strong drag effects. Movements on many 

faults may have continued to a date as late as on the 

Mead Slope fault, which is favorably located to show 

involvement of the early Colorado River gravels.

No faulting of recent alluvium was detected with 

certainty anywhere in the area, although topographic 

features southwest of Railroad Pass suggest possible 

breaks in the fan slope that are not yet obliterated by 

erosion and deposition.

MINERAL DEPOSITS

The region south of Lake Mead has long been a 

prospectors' paradise, as evidenced by claim notices 

bearing a wide range in dates and by widely distributed prospecting pits. Some old mines whose records 

show large production lie within the area represented

on plate 1. The Eldorado Canyon district, with its 

center near Nelson, boasted the Techatticup and Wall 

Street mines, credited with total yield of gold and 

silver valued at several millions of dollars (Ransome, 

1907). Production was begun at the Techatticup in 

1863 and continued with interruptions until 1942. 

Development of some properties within or near this 

district has been started or renewed recently, as at 

Knob Hill. The Searchlight district, once very active, 

has been the scene of only small operations in recent 

years. On the Arizona side of the river, the Katherine mine ended a varied record about 1930. Among 

other properties that prospered for a time were the 

Gold Bug and Mocking Bird (Schrader, 1909), and 

the Arabian and Frisco (Lausen, 1931). Gold and 

silver were the chief objectives in the older operations, 

though more recently some other metals, including 

lead, zinc, and copper, have received attention.

In the present study no attempt was made to locate 

and catalog the active prospects and mines within the 

area; but a few mines, most of them now inactive, are 

shown on plate 1 to serve as reference points.

Nearly all mineral deposits exploited within the 

area have been in igneous rocks, either plutonic or volcanic, or in closely associated metamorphic rocks. Exceptions are the manganese ore mined in the Three 

Kids district from sedimentary beds of the Muddy 

Creek formation, and gypsum quarried from several 

bedded formations northwest of Lake Mead.

GEOLOGIC HISTORY OF THE AREA

Evidence now in hand from a large area south of 

Lake Mead outlines a sequence of events but gives 

little basis for fixing definite dates. A large part of 

the history must be deduced indirectly from evidence 

in the thick stratigraphic sequence directly north and 

northwest of Lake Mead. Unfortunately the formations of most critical interest, representing the Cretaceous period and much of Tertiary time, have thus 

far yielded few diagnostic fossils. But at least the 

physical evidence given by the sediments has large 

value, and eventually we may have a more satisfactory basis for dating features and events. Conceivably some local deposits, particularly waterlaid ashbeds, that are interlayered with volcanic rocks south 

of the lake may hold fossils of value.

PALEOZOIC ERA

The section of Paleozoic formations totals about 

8,000 feet in Frenchman Mountain, and somewhat 

less in the Muddy Mountains, only a few miles from 

Lake Mead. Ranges farther north and northwest
E44 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

show a much larger total thickness, and regional stratigraphic evidence indicates clearly a thinning of the 

Paleozoic toward the southeast. This evidence has 

been summarized and shown graphically by McKee 

(1951). His isopach maps indicate for each Paleozoic 

system a marked decrease in thickness southward 

through the area of the present report, though according to his logical extrapolations the full Paleozoic 

section may have measured more than 4,000 feet in 

the vicinity of the Davis Dam site.

At Frenchman Mountain the continuous section 

from Middle Cambrian through Mississippian is made 

up almost wholly of marine carbonate rocks. Only 

the Pennsylvanian and lowermost Permian beds show 

a considerable increase in content of sand and silt as 

compared with the same part of the section in the 

Spring Mountains to the northwest. In general, therefore, the evidence suggests gradual overlap through a 

considerable distance toward the southeast, as represented by McKee (1951, pi. 1, 2).

MESOZOIC ERA

Marine limestone in the Lower Triassic Moenkopi 

formation is much thinner near Frenchman Mountain 

than in the Spring Mountains, whereas a much greater 

abundance of gypsum suggests lagunal conditions near 

shore. Probably the shoreline of that time was not 

far south of the present position of Lake Mead, and 

through all later Triassic and Jurassic time continental conditions extended much farther north. Finegrained clastic deposits of the Moenkopi and Chinle 

formations, and the Jurassic crossbedded sandstones, 

probably covered the area of this report; McKee's 

(1951, pi. 2) estimate of about 1,500 feet for each system seems reasonable. We may suppose, then, that 

at the close of Jurassic sedimentation the thickness of 

sedimentary rocks in the area was several thousand 

feet.

Widespread deformation occurred before deposition 

of the Thumb formation, which lies with angular unconformity on the Triassic and Jurassic rocks. The 

old erosion surface cuts across successively older formations from north to south; this cross-cutting indicates that uplift increased toward the south. A widespread conglomerate at the base of the Thumb 

contains pebbles and cobbles that represent several 

Paleozoic formations and the Precambrian complex. 

Higher in the Thumb formation are numerous great 

lenses of shattered Precambrian bedrock; probably 

these breccias moved northward by landsliding from 

a fault scarp at the border of a rising land mass from

which nearly all rocks younger than Precambrian had 

been eroded (Longwell, 1951, p. 352).

The Thumb formation is provisionally correlated 

with the Willow Tank and Baseline formations of the 

Muddy Mountain area, which are orogenic deposits 

of Cretaceous age (Longwell, 1949, p. 931). If this 

correlation is correct, the deformation in the area of 

the present report probably began as early as midCretaceous time, in connection with the development 

of thrust sheets in southern Nevada and related to the 

orogeny farther north (Spieker, 1946, p. 150). Presumably the emplacement of plutons in the Black and 

Eldorado Mountains accompanied this orogeny, and 

some of the bodies invaded the cover of sedimentary 

formations before these were stripped away, as evidenced by xenoliths or roof pendants consisting of 

Cambrian beds. Extensive layers of tuff in the Willow Tank formation of the northern Muddy Mountains (Longwell, 1949, p. 931), and in the Thumb 

formation near Lake Mead, mark eruptive activity 

probably related to the rise of plutons now exposed in 

the Black and Eldorado Mountain blocks. Provisional dating of the older plutons there as Cretaceous 

is based on this supposed relationship. The generally 

acidic composition of the Cretaceous tuffs and of the 

older intrusive bodies is consistent with this correlation.

The Patsy Mine volcanics were erupted through 

plutonic bodies, and basal layers of the sequence contain fragments of older volcanic rocks. Moreover the 

base of the Patsy Mine volcanics, wherever exposed, 

rests on a surface that transects Precambrian rocks 

and younger intrusive bodies that presumably are 

Cretaceous in age. A considerable interval marked 

by wide uplift and erosion must have preceded the 

Patsy Mine episode, and the Thumb rocks bear witness to at least a part of this interval. The basaltic 

flows in the upper part of the Thumb formation (fig. 

2) can hardly represent any part of the Patsy Mine 

volcanism, which from some of its nearby vents would 

have flooded with distinctive andesitic lavas the 

Thumb basin if in active subsidence. Probably the 

Patsy Mine volcanics were erupted after the Cretaceous beds were laid down and deformed. The land 

surface that received the lavas may have extended 

northward across Cretaceous and younger bedrock; 

any Patsy Mine volcanics deposited there must have 

been removed by later erosion. Presumably, therefore, 

the only volcanic rocks of Cretaceous age preserved 

within the area of this report are the basaltic lavas 

and tuffaceous beds included in the sedimentary section north of Las Vegas Wash.
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E45

CENOZOIC ERA

The Horse Spring formation is separated from the 

Thumb formation by an erosional surface only; there 

is no perceptible angular divergence between the two 

units, which were deformed together in equal degree. 

Possibly the Horse Spring was deposited in latest 

Cretaceous or in Paleocene time; the Eocene date now 

assigned is tentative and uncertain. Many features 

common to the two formations indicate deposition in 

subsiding basins, with frequent interruptions. Freshwater limestone, which forms an important fraction 

of the Horse Spring section, occurs also, with similar 

lithology but smaller thickness, in a unit of the Thumb 

formation. Abrupt wedges of very coarse debris are 

present in the Horse Spring, though less conspicuous 

than in the Thumb. Similar basaltic and tuffaceous 

rocks occur in the two formations, in comparatively 

small amounts. No lavas resembling the brown andesites of the Patsy Mine volcanics are present in Horse 

Spring sections; surely these great effusions would 

have filled any subsiding basins near the present location of Lake Mead. Therefore the reasoning on 

which the Patsy Mine eruptive history is dated as 

post-Thumb is equally valid in assigning that history 

to a date later than the Horse Spring sedimentation.

Volcanic activity, intermittent and moderate in this 

immediate area until the end of Horse Spring sedimentation, later became more pronounced. Evidence 

that this activity was accompanied by large-scale 

movements on fault blocks has been cited on earlier 

pages, in descriptions of the volcanic formations. 

These formations establish an order of events, and 

the absolute-age value of 50 million years for the 

Boulder City pluton (p. E17) gives a clue in the search 

for geologic dates. This value indicates a middle 

Eocene date for the pluton, and the relation of this 

body to adjoining lavas suggests a closely similar date 

for the Golden Door volcanics. The Patsy Mine volcanics are older; but the early part of the Eocene 

epoch was long, and this volcanic episode may have 

occupied only a small fraction of that time. As all 

available evidence indicates that the Horse Spring 

formation was laid down before the Patsy Mine episode, that formation is dated logically as Eocene or 

older. Additional radiogenic age values are needed 

to check the few now available.7

The Mount Davis volcanics can now be dated only 

within wide limits. South of Rifle Range Wash lavas 

and tuffs in the upper part of the Mount Davis sequence overlie a surface of erosion that transects part 

of the Golden Door volcanics. At the northwest corner of Malpais Mesa, sedimentary deposits in the upper part of the Mount Davis sequence grade upward,

7 See footnote, p. E17.

without perceptible break, into a section mapped as 

part of the Muddy Creek formation. If this correlation and the tentative date for the Muddy Creek are 

accepted, the Mount Davis history is bracketed between middle Eocene and Pliocene dates.

Sediments of the Muddy Creek formation reflect 

relief maintained by continued movements on faults 

(figs. 3, 12). Coarse deposits at the margins of basins 

grade into or intertongue with fine sediments laid 

down on playas or in open lakes. Several large basins 

of this kind, with sediments reflecting centripetal 

drainage and containing great quantities of saline deposits, lie athwart the present course of the Colorado 

River. Therefore the river, as a through-flowing 

stream, was not in its present location in Muddy Creek 

time (Longwell, 1946, p. 821). Great thicknesses of 

gypsum and anhydrite in the lower part of the Muddy 

Creek formation east of Boulder Canyon pose a problem of origin. Estuarine beds in southwestern Arizona record a northward encroachment of the sea in 

Pliocene or Miocene time (Wilson, 1933, p. 31). Conceivably the arm of the sea extended 200 miles north 

of these known deposits to the present site of Lake 

Mead; but this speculation has no. support in known 

evidence. Perhaps the saline deposits of the Muddy 

Creek were derived from older formations and concentrated in interior basins.

Widespread lavas in the lower and upper parts of 

the Muddy Creek are almost entirely basaltic. Progressive changes in average composition of volcanic 

rocks in the general region are of considerable interest. The Patsy Mine volcanics, which were erupted 

through more silicic plutons that probably gave off 

volcanic products, are predominantly andesitic; but 

the upper part of the Patsy Mine volcanics includes 

a unit of rhyolite glass, overlain by considerable 

basalt. The Golden Door sequence starts with latites 

and in general becomes more siliceous upward to a 

predominance of rhyolites. Mount Davis rocks are 

predominantly basalts and dark andesites, interrupted 

sharply at several horizons by tuffs and glasses that 

range in composition from latite to rhyolite. The 

rule of basalt begun in Muddy Creek time continued 

into the Pleistocene. Thus there has been more than 

one pronounced swing in predominance from high to 

low silica content in the volcanic products, and several brief but pronounced changes in composition during each major episode.

Any outline of structural development within the 

area can be in general terms only until more specific 

information may become available. Large-scale thrust 

faulting, widespread in the region on the north and 

west, did not directly involve the Frenchman Moun-
E46 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

tain block which was tilted strongly and broken by 

normal faults (fig. 13). But probably this tilting 

occurred in connection with movement on large strikeslip faults that were active in late stages of the regional thrusting. Since the Horse Spring formation 

is tilted as steeply as older formations, deformation 

under regional compression may have continued from 

Cretaceous into Cenozoic time.

Vertical displacements on steep faults have been 

recurrent through several long epochs. Landslide breccias in the Thumb formation suggest that an eastwest fault or fault zone, with upthrow on the south, 

was active near the latitude of Boulder Basin in late 

Cretaceous time. Presumably the large normal faults 

that break the Frenchman Mountain block were developed and rotated to their present attitudes during 

the tilting movement that began early in the Cenozoic era. Important faulting in the several episodes 

of volcanism is attested by abrupt terminations of 

thick volcanic sequences, and by localized occurrences 

within these sequences of very coarse sedimentary debris, presumably shed from growing scarps. As this 

kind of evidence is especially pronounced in the 

Mount Davis sections, perhaps the faulting activity 

was at a maximum during that volcanic episode, which 

on the basis of the present tentative chronology continued into the Pliocene epoch. Some faults, occurring singly or in sets, are localized near plutonic 

bodies and probably reflect stresses during intrusion. 

But the larger faults that were active at the time of 

Mount Davis volcanism and later have the northerly 

orientation that is characteristic in neighboring parts 

of the Basin and Range province. This regional pattern was well established through Muddy Creek time.

The earliest known Colorado River deposits in the 

area are unconf ormable on beds of the Muddy Creek, 

and are of Pliocene or Pleistocene age (Longwell, 

1946, p. 828). Presumably the course of the stream 

in deep gorges such as Boulder Canyon and Black 

Canyon, a course that appears illogical in relation to 

present topography, has resulted from superposition 

as the river cut down from a higher horizon where 

it had its initial course on the weak Muddy Creek 

formation. Local remnants of these weak beds, capped 

by the Fortification basalt member (fig. 3), testify 

that the formation once obliterated much of the rugged relief now supported by resistant bedrock. Possibly the important movements on faults that have 

occurred at least locally since the stream course was 

established have increased relief in the vicinity of 

Boulder Canyon.

Apparently no major changes in the course of the 

river have occurred between Boulder Canyon and

Davis Dam. One may suspect that a former course 

extended southward from the lower part of Kingman 

Wash; a depression filled with old slope debris from 

the Black Mountains has some resemblance to a large 

abandoned stream valley (fig. 9). But no Colorado 

River gravels are found there, and the depression 

probably owes its origin to progressive eastward tilting of the block southwest of the Horsethief fault. 

Minor changes in the course occurred as the river cut 

down through the lake deposits of the Chemehuevi 

formation. At Davis Dam the modern valley runs 

obliquely across the earlier trench which is filled with 

the lake sediments (fig. 6). Many temporary locations of the stream channel at various levels on the 

lake beds were abandoned in later adjustments; the 

high-level potholes and terrace remnants near Hoover 

Dam mark one such location.

Origin of the Pleistocene Lake Chemehuevi presents a problem of much interest. Through hundreds 

of miles downstream from the Grand Canyon, many 

remnants of the basal lake deposits lie close to the 

river channel (fig. 6); therefore, the stream returned 

almost exactly to its earlier profile, once the obstruction that caused ponding was removed. A natural 

dam in a narrow canyon, formed by lavas or a landslide, seems the most likely cause. South of Davis 

Dam the only restricted part of the valley with walls 

high enough to meet requirements is Aubrey Canyon 

in the Whipple Mountains, where Parker Dam is located. No evidence of a breached natural dam has 

been found there; moreover, many remnants of lake 

beds lie downstream from Aubrey Canyon. Solution 

of the problem will require careful study in a large 

part of the lower Colorado River valley.

REFERENCES CITED

Campbell, lan, and Schenk, E. T., 1950, Camptonite dikes near 

Boulder Dam, Arizona: Am. Mineralogist, v. 35, nos. 9-10, 

p. 671-692.

Barton, N. H., and others, 1924, Geologic map of the State of 

Arizona: Arizona Bur. Mines, in cooperation with U.S. 

Geol. Survey.

Gilbert, G. K., 1875, Report on the geology of portions of 

Nevada, Utah, California, and Arizona: U.S. Geog. and 

Geol. Surveys W. 100th Meridian Kept. (Wheeler), v. 3, 

p. 17-187.

Hatch, F. H., Wells, A. K., and Wells, M. K., 1949, The petrology of the igneous rocks: 10th ed., London, Thomas 

Murby and Co.

Hunt, C. B., McKelvey, V. E., and Wiese, J. H., 1942, The Three 

Kids manganese district, Clark County, Nev.: U.S. Geol. 

Survey Bull. 936-L, p. 297-319.

Ives, J. C., 1861, Report upon the Colorado River of the 

West: Washington, U.S. Govt. Printing Office.

Larsen, E. S., Jr., and others, 1952, Method for determining 

the age of igneous rocks' using the accessory minerals: 

Geol. Soc. America Bull., v. 63, no. 10, p. 1045-1052.
RECONNAISSANCE GEOLOGY, LAKE MEAD-DAVIS DAM, ARIZONA-NEVADA E47

LaRue, E. C., 1925, Water power and flood control of Colorado River below Green River, Utah: U.S. Geol. Survey 

Water Supply Paper 556, 176 p.

Lausen, Carl, 1931, Geology and ore deposits of the Oatman 

and Katherine districts, Arizona: Arizona Bur. Mines Bull. 

131, Geol. ser. 6, (Univ. Bull., v. 2, no. 2), 126 p., 30 figs.

Lee, W. T., 1908, Geologic reconnaissance of a part of western 

Arizona: U.S. Geol. Survey Bull. 352, 96 p.

Longwell, C. R., 1928, Geology of the Muddy Mountains, Nev.: 

U.S. Geol. Survey Bull. 798, 152 p.

 1936, Geology of the Boulder Reservoir floor, ArizonaNevada: Geol. Soc. America Bull., v. 47, no. 9, p. 1393- 

1476.

 1946, How old is the Colorado River?: Am. Jour. Sci., 

v. 244, no. 12, p. 817-835.

 1949, Structure of the northern Muddy Mountain area, 

Nevada: Geol. Soc. America Bull., v. 60, no. 5, p. 923- 

968.

 1951, Megabreccia developed downslope from large 

faults: Am. Jour. Sci., v. 249, no. 5, p. 343-355.

McKee, B. D., 1951, Sedimentary basins of Arizona and adjoining states: Geol. Soc. America Bull., v. 62, no. 5, p. 

481-506.

McKelvey, V. B., Wiese, J. H., and Johnson, V. H., 1949, Preliminary report on the bedded manganese of the Lake 

Mead region, Nevada and Arizona: U.S. Geol. Survey 

Bull. 948-D, p. 83-101 [1952].

Marcou, Jules, 1858, Geology of North America, etc., 144 p., 

map: Zurich, Switzerland. Review in Am. Jour. Sci., 

v. 26, p. 323-333.

Newberry, J. S., 1861, Geological report, in Ives, J. C., Report 

upon the Colorado River of the West: U.S. 36th Cong., 1st 

sess., House Executive Doc. 90, pt. 3,154 p.

Ransome, F. L., 1904, Description of the Globe quadrangle 

[Ariz.]: U.S. Geol. Survey Geol. Atlas, Folio 111.

 1907, Preliminary account of Goldfleld, Bullfrog, and 

other mining districts in southern Nevada: U.S. Geol. 

Survey Bull. 303, 98 p.

 1923a, Geology of the Oatman gold district, Ariz., a preliminary report: U.S. Geol. Survey Bull. 743, 58 p.

 1923b, Ancient high-level potholes near the Colorado

River [abs.]: Science, New ser., v. 57, p. 593. 

Schrader, F. C., 1909, Mineral deposits of the Cerbat Range,

Black Mountains, and Grand Wash Cliffs, Mohave County,

Ariz.: U.S. Geol. Survey Bull. 397, 226 p. 

Simpson, G. G., 1933, A Nevada fauna of Pleistocene type and

its probable association with man: Am. Mus. Novitates,

v. 667, p. 1-10.

Spieker, B. M., 1946, Late Mesozoic and Early Cenozoic history of central Utah: U.S. Geol. Survey Prof. Paper

205-D, p. 117-161. 

Stock, Chester, 1921, Later Cenozoic mammalian remains from

the Meadow Valley region, southeastern Nevada: Am.

Jour. Sci., 5th ser., v. 2, p. 250-264. 

U.S. Bureau of Reclamation, 1950, Geological investigations

in U.S. Bur. Reclamation, Boulder Canyon Project final

reports: pt. 3, Bull. 1, 231 p.

Wilson, B. D., 1933, Geology and mineral deposits of southern Yuma County, Arizona: Arizona Bur. Mines Bull. 134,

Geol. ser. 7, 236 p.

INDEX

Page 

Absolute age determinations, potassium-argon

and zircon methods.--___ _ E17,18

Access. ...______________.____ 3

Acknowledgments..-_-_.-__.____ 3

Alluvium, older. ________________ 11

relations in Chemihuevi formation..__. 12-14

younger.---.-......____....___... 15-16

Alluvial flats, below Black Canyon,...._... 15-16

Andesite, Golden Door volcanics- __.... 21

Mount Davis volcanics_--.-._-_.... 24,27

Anhydrite, origin...___._..__. 

Arabian mine, fault- ______.__ 

Aubrey Canyon, Pleistocene damsite. 

Aztec sandstone.....................

Aztec Wash area, high-level terraces 

B

Basalt, in Mount Davis volcanics...__.. 24,25,27 

in Patsy Mine volcanics _____..... 19

Basalt flows_________.________ isBasalt-capped buttes, stratigraphic relations.. 29-31 

Base maps...__..____________.. 3 

Basin deposits, Pliocene__......___._ 8-11

Basin-range structure, exhibit of.,........._ 34

Biotite latite of Ransome__......._____ 22-23

Black Canyon area, high-level terraces._.__ 16 

history.......___..______.__.. 46

latite.---..-.-----._.................... a.

Patsy Mine volcanics---_________ 19 

Black Canyon, physiography....______ 4

structure...__________.__.._ 40,41 

Black Mountains area, Cambrian rocks...... 8

emplacement of plutons.................. 44

Epperson fault...._..........__.... 38

fault zone...._________._..._ 34-35

physiography....___.__.....___ 4

Plutonic rocks..___.___.____... 17 

Precambrian rocks..___..._____ 4,5,37 

structural relations.____......___. 35-37

structure west of....._._...._...... 39-43

Black Mountains block, uplift .._____ 34 

faults......__.._ _---_ _-_ -- 40,41

Boulder Basin, structure northward and westward_____.___________ 39 

structure westward..____.._..... 43

Boulder Basin area, Muddy Creek basaltic

flows.________........___ 29

physiographic description...... __...... 4

Boulder Basin-Eldorado Wash area, structure. 39-41 

Boulder Canyon, section along...._...._.._ 35-36

Boulder Canyon area, basalt dikes...._. .

history..._________...__........

Plutonic rocks..____.._.___..... 

Boulder City area, Mount Davis volcanics....

Muddy Creek formation.._______________

rhyolite of Golden Door volcanics..__....

structural relations...___.____..... 

Boulder City pluton, absolute age ..........

composition and age determination....--.

relation to Golden Door activity .....

31 

46 

17 

27 

9 

21 

39 

45 

17 

24 

Breccia, explosion.....__...__.......... 19, 24

Golden Door...-.___._____......_ 21,25

sedimentary............_._..__._...__. 23-24,26

Camptonite dikes..__......_.________._..__ 33,34

Cenozoic history of area.-,--..-__..--._-... 45

Page

Chemihuevi formation, conditions of deposition. __ _. E14 

fossils__ _- _.- -- --___ 15

lake deposits.-...-____________ 12-14 

particle size._ . .... . . 14

Chemihuevi lake, history...__________ 14,46

Chinle formation. _____._._.___ 6,44

Climate._____________________ 3-4

Colorado River history___________ 10,35,46

Chemihuevi time._____________ 14

Colorado River valley, older alluvium_..._ 11-12

physiographic description,............... 4

Precambrian rocks...--_____._- 5 

Columnar sections....__._._._____- 7

D

Dacite, Golden Door..___.___._._.. 21 

Dam breccia, of Ransome...___... __ 22 

Davis Dam....__________________ 3

Davis Dam area, Chemihuevi lake deposits.- 14-15 

Colorado River history..------_-.-_-.._ 46

fault zone.....----.....-__-_---._.....- 34,42-43

Golden Door volcanics.----_______ 21-22

Mount Davis andesite_.._-,..____- 27 

older alluvium......._____.__..... 12

Paleozoic history..-__.__._........... 44

rhyolite........... .................... 21

Deformation....___._.________.. 44

Detrital Valley, alluvial deposits..____.. 16 

Dikes and sills, Golden Door volcanics------- 21

Dry Camp breccia, of Ransome.---------_. 23-24

E 

Earlier work In area... 4

Eldorado Canyon, ore deposits.-------------- 18

Eldorado Canyon mining district_______ 43

Eldorado Mountains, emplacement of plutons- 44

landslide breccia___......_-..._.. 11

northern block..--.----.-...---.----.-... 39-40

physiography..._ ._.._-----._--.... 4,38

Precambrian rocks.-...---.- - 5,40

Eldorado-Newberry Mountains block, uplift. 34,35

Eldorado Wash area, gravel in._..-...-.._ 11

Intrusive masses..--...--.--.-.. .- 18

Mount Davis volcanics_--------- --.. 26

older alluvium__...._--------__-... 11

structural features - ---... 41 

Eldorado Wash to Newberry Mountains,

structural features__...__...-. 41-42 

Eruptive centers.__._-----___---_---_.-___.__ 34

F 

Faulting, age...--_---__..._-___..__...--___.- 46

displacement...--.___---___--_-,-------.- 46

historyof --- -- 35,43,45-46 

Muddy Creek time..__...._-.-.----- 10,29,31

recent..-------.------------.------------- 43

regional pattern..._....____---_-.--._-.-- 46

Faults, Boulder Wash -------- -.-.--.- 35

Callville. See Fortification fault._.__--__ 35

Faults, dating of... __-..--..--- -- ---- 43

Eldorado -------------------------- 21,41

Emery-------------------..-------------- 35

Epperson... . .-. .. -- 20,38 

Fortification---..-.--..-..-.,--.----- 35,36.37

Fuller Road...---.-.-_-.-_-.--__--------- 41

Gold Bug..-------------.------------- 41

Hidden Valley...... 41

TTorsethief -*- 17,23,36,37,39,40

Page 

Faults Continued

in Patsy Mine volcanics---- ------_. E19

Indian Canyon_____._____ 17,35,36,37 

Ives._-__ __-_-_ --_-__ 41 

Jeep Pass_ -------------- ---------- 40

Kingman Road ____.. __ 35,37,39,40 

mapping of. - -.. 34 

Mead Slope fault . 36,37,43

Pope.................. . 38,41

Ransome. - ---------- 35

Rifle Range.._____._______-__ 40

rotation and displacements..._ ._- - 34

Welcome___________________ 41,43

Fleldwork..... 3

Flows, Golden Door volcanics.___...__. 21 

Folds, mapping of...__------- ------- 34

Fortification basalt member of Muddy Creek

formation, basal basalt unit. _..-. 29 

composition of. - .- 31

correlation in Lake Mead area..---------- 29-30

equivalence in Oatman district... -. 32,33 

erosion remnants------------------ . 30-31

eruption of_. - -- - - 31

Muddy Creek formation- __.__ __ 10,36 

original extent_....._ . 31

Fortification fault, Intrusive bodies near 17 

Fortification Hill area, Muddy Creek basaltic

flows ------------------------- 29,30

Muddy Creek formation.... .- 8 

olivine basalt ... 29,31

"Paint pots"._ ___.. 17

physiography... ---------- 4

Structure ------------------ - - 36-37

tuff.

Fossils, Chemihuevi formation...- __.__ 15

Frenchman Mountain area, Cretaceous redbeds. - - 8

Lower Cambrian rocks ... ------- 5

Mesozoic history--.--.-. -------- 44

Muddy Creek formation- . 8 

Paleozoic history..-. ... ------- 43-44

Precambrian rocks__.. -- - ------ 5

sedimentary rocks.-_-.. __-. 6

Frenchman Mountain block, structural history---------------------- 39,45-46

Frisco mine, faults... . .____.. 43

G 

Garnet--.. --------- ----------------- 5

Geologic history of the area . - 43-46 

Geological Society of America, Penrose grant- - 3 

Geology of area, general----------- ------- 5

Golden Door volcanics. -- 21

Mount Davis volcanics ----- ------- 24

chemical and thin-section analysis.... 28-29

Patsy Mine volcanics, chemical analysis 28 

faulting.. - 20

horizon marker. - -------- 19

Glass Plateau area, basalt-capped buttes 31 

faulting in Patsy Mine volcanics. ------ -'0

Mount Davis volcanics --- - 26,28 

Gold Bug mine_ --------------- - -- 43

Gold Bug Wash, Golden Door volcanics.- . 21 

fault near ------------------------------

Golden Door mine, structural relations near.. 

Golden Door volcanics---- -- -----------

38 

38 

18

age.--...--_ ---- 45 

bulk and groundmass compositions - 30 

compositional change..------------------- 45

E49
E50 INDEX

Page 

Golden Door volcanics Continued

character and distribution____-..___ E20 

equivalence in Oatman district__.__. 32 

intrusive bodies in___________... 23,27

rocks of-..--..---.--..-----..---------.--. 21-24

stratigraphic relations.----------_-.-_-. 20,21

structural relations-_________ 37,41,42,43 

xenoliths..--_______------_.-._ 36

Oneiss___---__.__.___-----__ 5

Orand Wash Cliffs, Precambrian rocks..__. 5 

Granite_______._--__...-___... 5 

Granitic rocks, younger--. --.---.---.. 6 

Gypsum__----------__..-__.__...... 43,44

in Muddy Creek formation_.....__ 9,10

origin.._- .- .- ...---. 45

H

Hooker Dam area, eruptive complex--.-.._ 19-20

glassy unit-.. . .-- . 27

Golden Door section_________ 23

Golden Door volcanics...________.. 21-24

landslide breccia_-__--._______- 10-11

latite. ............ .-. --.. -- 21

Mount Davis volcanics_..-._-.--....- 28

Muddy Creek formation..._____.- 8

older alluvium....._....____ _. 12

Patsy Mine volcanics__________.___.-_ 20

Pleistocene basalt sheets and dikes_ _-..-. 33 

sedimentary breccia.-.__...---.--. 20,23-24

volcanic rocks._____-__-.---..----.- 22

Horse Spring formation, age.._ ___ _.- 6,45

Igneous plug, structure__.....__------- 41

Igneous rocks, Cretaceous and Cenozoic. ----- 16

Pleistocene-________-____--._ 33-34

sediments in__________._____.. 29.31

Intrusion, structural control of__..--.__.. 42

Intrusive bodies......__....._._........-. 16-18

Kingman Wash area, igneous complex. 

Knob Hill, mining activity..__.....

Lake Mead, access for field work._......._ 3

Lake deposits, Chemihuevi formation-.__.. 12-14

Lake Mohave, alluvial flats beneath_ .. _.. 15

fieldwork_____....._._.....___. 3

Lake Mohave project_.____..__... 3 

Landslide breccia, Muddy Creek formation _ 10 

Las Vegas Wash area, Cretaceous volcanic

rocks....--..-...-...-..-...-..--- 44

Mesozoic uplift___..................... 8

Mount Davis volcanics, glassy rocks ..... 29

Muddy Creek formation._____...... 9

sedimentary rocks..__..__.----.----.- 6,8

Latite, Golden Door volcanics..... __. ___ 21, 22

Mount Davis volcanics_................ 24

Lava sheet, in Golden Door volcanics........ 22

M

Malpais Mesa, basalt-capped buttes_....... 30

landslide breccia._.___-------------- 11

Mount Davis volcanics, measured section. 26-27 

Muddy Creek basaltic flows.............. 29,30

Muddy Creek formation ..---.--.------. 8,9

physiographic description ............... 4

structural relations.............._...... 40

Manganese_............___..----..---- 9

Marine carbonate rocks, Frenchman Mountain.-......-........--..-..-.- 44

Page

Marker beds...-_._ . E34 

Mesozoic history of area....-_---------_-. 44

Mineral deposits..--._____ --_-.-..--. 43

Mocking Bird mine.-- ..._ . 43 

Moenkopi formation_________._--.... 6

marine limestone-gypsum proportions.... 44

Mount Davis, type locality of Mount Davis

volcanics- _. - ----- 24

Mount Davis area, landslide breccia..-.-...-. 11

Mount Davis volcanics____.___._ 18,24-29

age .. - -. 46

associated unrest.. 24,26

bulk and groundmass composition of

volcanic glasses__._._...... 30

displacement.___._-.-.-_-----.-. . 41

eruptive centers. 27,28 

faulting during volcanism....... _.. 46

glassy units, high-silica__._._.. 27,28 

petrographic description....._.. 27

lithologic description-... _. _.-....-.-- 27-28

measured section.. ..-- 26-27 

sedimentary breccia.-.--.---.---. .- 26 

sedimentary debris.._._.. . 25,26 

sediments in______._------ .. 29,31

stratigraphic relations.... ..... 20,21

structural relations..... ___ 25,27,37,38,42,42

type locality______ 24 

Mount Davis volcanism, associated faulting., 29,38 

Mount Davis-Qolden Door contacts,... . 24-25 

Mount Perkins area, Qolden Door volcanics.. 21 

Muddy Creek formation.... .... .. 8-10

age-.......--_-.....--..-..---._------ 10,45

at Boulder Wash fault...- ...... . 35

basin deposition...... 9,10

compositional change..... . 45

Horse Spring formation_... 6 

manganese ore__.... -- 43

Paleozoic history.______ . 43-44

structural relations.- -. . 35,37,40

Muddy Creek time, displacement during 37

topographic relief..... 31

N

National Park Service, cooperation__. ... 3 

National Science Foundation, grant.. 3 

Nelson area, basalt-capped buttes.... 30-31

eruptive complex__-_----- -... 19

Qolden Door volcanics-------. . 24

hlgh-slllca glass... - -- 27

Intrusive contacts.... - 18

monzonlte porphyry. . 18

Mount Davis volcanics 25,26

Patsy Mine volcanics--..- 19

structural relations. 39,41,43

Nelson block, structural features - 40-41

Newberry Mountains area, granite.. -- 18

older alluvium. - . 12

physiography.-. ...... ..... 4,38

plutonic rocks.. .- - 17 

Precambrian rocks. - --- 5 

structure east of.-.-.- 42-43 

Newberry Mountains-Eldorado Wash, structure.---------------------------- 41-42

O

Oatman district, Arizona, volcanic rock classification... - 31-33 

Opal. - 22

Paleozoic history of area - 43-44

Patsy Mine volcanics.- 18,19-20,37,40

age 44,45

Page

Patsy mine volcanics, comparison with Oatman rocks____________ E31-32 

compositional change - _._ 45 

distribution and description_._____ 19 

Patsy Mine volcanics, eruption of. _.___ 19,44 

stratigraphic relations--.----- -----. 20,21,25

structural relations... ..-.--- ?8,41

type section. 19

Pleistocene Igneous rocks 33,45

Pleistocene Lake Chemihuevt, origin.. _.... 46

Plutonic rocks, relations to volcanic rocks.... 17

Plutons,. ..-- ...- . -- - 16-18 

composition. 17

dating of. _ 44

erosion of- 17

Pope mine, fault near...... 37

Pope Wash area, eruptive complex.. .. 19,41 

Mount Davis volcanics_-. ___ 26,27 

Mount Davis-Patsy Mine contact, measured section. 25,41 

Patsy mine volcanics - - 19 

structural relations... 38

Portland mine, faulting and folding near.. 38,42 

Precambrian rocks... - 5-6,36,38,39,40,41,42-43

R

Railroad Pass, recent faulting.. 43 

Ransome, F. L., classification of rocks,

Hoover Dam-. 22-23

classification of rocks, Oatman mining

district -- 31-33

References cited . ... 46-47 

Relief.. 4

Rhyollte, In Qolden Door volcanics- -- 21,22,24

In Mount Davis volcanics.- 24 

Rifle Range Wash area, igneous complex.. 18 

River Mountains, rhyolite . 24

zenoliths..... . 8

structural relations. 39

S

Saddle Island, schist.... - 6,8,39 

Schist.. - 5,6

Searchlight Ferry area, basin deposits 16

Mount Davis Qolden Door contact.. 24

structural relations. 38

Searchlight mining district 43

Sedimentary deposits - 5

Sedimentary formations, northwest of Lake

Mead... 6-8

Spherulites, Patsy Mine volcanics 19

Spillway breccia, of Ransome 22

Square Butte, basalt cap.. 31

doming_...._._.....------------- 40

eruptive centers. 27-28 

high-silica glass 27 

Mount Davis andesite.- 27 

Structural blocks, deformation within. 40 

tilting.. - 34-35

Structural development.. 45-46 

south of Lake Mead 34-39

Structural pattern. 34-35 

Sugarloaf Mountain area, Qolden Door

volcanics 22,23

high-level terraces..-.-- 14,16 

Surface features ... --- 4

Techatticup mine, gold and silver. 43

intrusive rocks... 18

intrusion fault 41

Terraces, high-level.. 16

Tertiary rocks of region, sequence of 10
INDEX E51

Page

Three Kids Mine area, glassy rock______ E29 

structural relations..___________ 39

Tertiary sequence...______......__ 10

Thumb formation, age of basaltic flows and

tuffs..__....__..__........ 44

basal conglomerate... ________ __ 44 

correlations.._......-_______..._ 44

landslide breccia._______. ____ 8,10,46 

stratigraphic section__ __. ___ _. 6 

Topography._.._______________. 4 

Trachydolerite bodies, in Golden Door volcanics_________________ 23,27

Tuffs, in Golden Door volcanics________ 24,25

Mount Davis volcanics________.... 24

U

Union Pass area, Golden Door volcanics__ 22,43 

Mount Davis volcanics__.._......_. 27

Precambrian rocks- ___________ 39,42

structure..___ ... ? -..-...._ 42

U.S. Bureau of Reclamation, Lake Mohave

project,...________....__ 3

Page

U.S. Forest Service, aerial photography___ E3 

Uplift....................................... 44

V

Vegetation. _______-___-___...... 3

Virgin Mountains, Precambrian Mountains.. 5

Volcanic glasses, bulk and groundmass composition.....__._.___..... 30 

chemical analysis and petrographic description..__._.-.-.-...-.--.. 27-28

Volcanic rocks__________________ 18-34 

age______________________ 18,44 

assemblages_____________.... 34 

correlation___________.______ 18 

progressive compositional changes........ 45

Volcanic rocks of Muddy Creek formation__ 29-31 

comparison with adjacent areas...____ 31-33 

comparison with Ransome's classification. 31-32

Volcanism, faulting movements during___ 34 

of Muddy Creek time....... ---------- 29

W Page

Wall Street mine, gold and silver ....___ E43

Wall Street mine area, hydrothermal clay._ 41 

Washes, younger alluvium in...__._...... 15

West Petroglyph Wash, Kingman Road fault. 37 

Whipple Mountains area, Chemihuevi lake

deposits..__________..._ 15

White Rock Wash, Horsethief fault- __._ 37,40

Willow Beach area, basalt-capped buttes__ 30,31

higr-level terraces.____________.. 16

Muddy Creek formation__... 8

physiography__....... 4

structural relations..........._...... 39,40,43

unassorted breccia.... . 27

Yale University, cooperation.. 3

Z 

Zircon method, age determinations.. .. 17,18

o