Geologic and Geophysical Maps of the 

Las Vegas 30′ × 60′ Quadrangle, Clark and 

Nye Counties, Nevada, and Inyo County, 

California 

By William R. Page,1 Scott C. Lundstrom,1 Anita G. Harris,1 Victoria E. Langenheim,2 

Jeremiah B. Workman,1

 Shannon A. Mahan,1 James B. Paces,1 Gary L. Dixon,3 

Peter D. Rowley,4

 B.C. Burchfiel,5 John W. Bell,6 and Eugene I. Smith7 

Prepared in cooperation with the Southern Nevada Water Authority 

Pamphlet to accompany Scientific Investigations Map 2814 

1

U.S. Geological Survey, Denver, CO 

2

U.S. Geological Survey, Menlo Park, CA 

3

Southwest Geology, Inc., Blackfoot, ID 

4

Geologic Mapping, Inc., New Harmony, UT 

5

Massachussetts Institute of Technology, Cambridge, MA 

6

Nevada Bureau of Mines and Geology, Reno, NV 

7

University of Nevada, Las Vegas, NV 

U.S. Department of the Interior

U.S. Geological Survey
Contents 

CHAPTER A 

GEOLOGY OF THE LAS VEGAS 30′ × 60′ QUADRANGLE 

Introduction ................................................................................................................................................... 1


Methods ......................................................................................................................................................... 2


Description of Map Units............................................................................................................................. 2


Quaternary and Tertiary Rocks.......................................................................................................... 2


Gravelly Alluvium, Predominantly of Alluvial Fans ................................................................ 2


Footslope Colluvium and Alluvium ........................................................................................... 7


Fine-grained Deposits ................................................................................................................ 7


Basin-fill Deposits..................................................................................................................... 10


Volcanic Rocks .......................................................................................................................... 11


Mesozoic, Paleozoic, and Proterozoic Rocks ............................................................................... 12


Stratigraphy of the Las Vegas 30′ × 60′ Quadrangle............................................................................. 21


Proterozoic Rocks.............................................................................................................................. 21


Paleozoic Rocks ................................................................................................................................. 21


Mesozoic Rocks ................................................................................................................................. 25


Tertiary Rocks..................................................................................................................................... 26


Quaternary .......................................................................................................................................... 26


Structure of the Las Vegas 30′ × 60′ Quadrangle .................................................................................. 27


Introduction ........................................................................................................................................ 27


Mesozoic Thrust Faults..................................................................................................................... 27


Spring Mountains ..................................................................................................................... 27


Wheeler Pass Thrust....................................................................................................... 27


Lee Canyon and Macks Canyon Thrusts ..................................................................... 28


Deer Creek Thrust............................................................................................................ 28


Keystone and Red Spring Thrusts................................................................................. 28


Bird Spring Thrust............................................................................................................ 30


Sheep Range and Las Vegas Range...................................................................................... 30


Gass Peak and Valley Thrusts ....................................................................................... 30


Correlation of Thrusts .............................................................................................................. 31


Age of Thrusts ........................................................................................................................... 31


High-angle Normal and Strike-Slip Faults ..................................................................................... 32


Spring Mountains ..................................................................................................................... 32


Trough Spring Fault.......................................................................................................... 32


La Madre Fault ................................................................................................................. 32


Griffith Fault ...................................................................................................................... 32


Wheeler Graben............................................................................................................... 32


Sheep and Las Vegas Ranges ................................................................................................ 33


Las Vegas Valley Shear Zone.................................................................................................. 33


State Line and West Spring Mountains Fault Zones........................................................... 34


Cenozoic Landslide Deposits ........................................................................................................... 34


Quaternary Faults .............................................................................................................................. 35

iii 

Introduction ............................................................................................................................... 35


Las Vegas Valley ....................................................................................................................... 36


Pahrump Valley ......................................................................................................................... 37


References Cited ........................................................................................................................................ 37


CHAPTER B 

GEOPHYSICAL FRAMEWORK OF THE LAS VEGAS 30′ × 60′ QUADRANGLE 

Introduction ................................................................................................................................................. 45


Aeromagnetic Data .................................................................................................................................... 45


Isostatic Gravity Data................................................................................................................................. 45


Basins .......................................................................................................................................................... 47


Las Vegas Valley ................................................................................................................................ 47


Corn Creek Springs............................................................................................................................ 48


Pahrump Valley .................................................................................................................................. 48


Pre-Cenozoic Basement ............................................................................................................................ 49


Spring Mountains .............................................................................................................................. 49


Frenchman Mountain ........................................................................................................................ 49


Las Vegas Range................................................................................................................................ 49


Conclusions ................................................................................................................................................. 54


References Cited ........................................................................................................................................ 54


Figures 

1. Map Sheet 1 (Index of Geologic Mapping)


2. Map Sheet 1 (Paleozoic Correlations in major thrust sheets)


3A. Map Sheet 1 (Major structures)


3B. Map Sheet 1 (Eastern Spring Mountains Structural Model 1)


3C. Map Sheet 1 (Eastern Spring Mountains Structural Model 2)


4. Pamphlet page 46, Chapter B (Index Map)


5. Map Sheet 2 (Residual Aeromagnetics)


6A. Map Sheet 2 (Basin Thickness of Las Vegas Valley)


6B. Map Sheet 2 (Basin Thickness of Pahrump Valley)


7. Map Sheet 2 (Basement Gravity with Aeromagnetic Contours)


8. Pamphlet Page 50, Chapter B (Magnetic Model)


9A. Pamphlet Page 51, Chapter B (Cross Section D–D’)


9B. Pamphlet Page 52, Chapter B (Cross Section E–E’)


9C. Pamphlet Page 53, Chapter B (Cross Section F–F’)


Tables 

1. Pamphlet Page 3, Chapter A (Thermoluminescence Data) 

2. Pamphlet Page 3, Chapter A (Uranium Series Data) 

3. Pamphlet Page 47, Chapter B (Density-Depth Functions) 

4. Pamphlet Page 47, Chapter B (Densities and Magnetic Susceptibilities) 
Chapter A 

Geology of the Las Vegas 30′ × 60′ Quadrangle 

By William R. Page,1 Scott C. Lundstrom,1 Gary L. Dixon,2 Peter D. Rowley,3 Anita G. Harris,1 

Victoria E. Langenheim,4

 Jeremiah B. Workman,1 Shannon A. Mahan,1 James B. Paces,1 and 

John W. Bell5 

1

U.S. Geological Survey, Denver, CO 

2

Southwest Geology, Inc., Blackfoot, ID 

3

Geologic Mapping, Inc., New Harmony, UT 

4

U.S. Geological Survey, Menlo Park, CA 

5

Nevada Bureau of Mines and Geology, Reno, NV 

Introduction 

This report of the Las Vegas 30′ × 60′ quadrangle was 

completed by the U.S. Geological Survey’s Las Vegas Urban 

Corridor Project, National Cooperative Geologic Mapping 

Program. As one of the fastest growing cities in the country, Las Vegas, Nevada, is challenged by major issues such 

as water supply and contamination, land subsidence due to 

ground-water withdrawal, and seismic and flood hazards. The 

report was designed to respond to the rapid urbanization experienced by Las Vegas by providing earth science information 

that can be used to investigate these land-use issues. 

This report contains two parts: the first, chapter A, 

describes the geology of the map area and includes the geologic map of the Las Vegas quadrangle, cross sections, and 

text with description of map units and discussion of the stratigraphic and structural framework. The second part, chapter B, 

describes the geophysics of the map area and includes isostatic 

gravity, aeromagnetic, and gravity inversion maps, geophysical cross sections of Las Vegas and Pahrump Valleys, and 

text describing the geophysical framework. The integration of 

geologic and geophysical information contained in the report 

offers a unique opportunity to apply basic geoscience 

techniques to help solve urban growth issues in this arid 

population center. 

The Las Vegas quadrangle is in the Basin and Range 

Province, in the southern part of the North American Cordillera. The major valleys in the quadrangle are Las Vegas and 

Pahrump Valleys; both contain thick Quaternary and Tertiary 

basin-fill deposits derived from the Spring Mountains and Las 

Vegas and Sheep Ranges. The Spring Mountains are in much 

of the western and central parts of the quadrangle, and are the 

highest mountains in southern Nevada. The southern Sheep 

and Las Vegas Ranges define the northern margin of Las 

Vegas Valley. 

Quaternary deposits cover about half the quadrangle, 

underlying most of the urbanized areas. Deposits consist of 

large coalescing fans that grade downslope to extensive areas 

of fine-grained sediment indicative of Pleistocene groundwater paleodischarge (Haynes, 1967; Quade, 1986). In the 

central parts of Las Vegas and Pahrump Valleys, the association of Quaternary fault scarps with these sediments suggests 

a genetic relationship. The synthesis of new mapping of 

Quaternary geology for this report illustrates the distribution 

and recurrence of ground-water discharge in Las Vegas and 

Pahrump Valleys and provides clues for understanding past 

and present-day ground-water flow paths and water resources. 

Rocks in the Spring Mountains and Las Vegas and Sheep 

Ranges are mostly Paleozoic marine strata that formed in the 

transition between the Cordilleran miogeocline and the craton. 

The distribution of Paleozoic rocks is important because they 

are part of the Nevada carbonate rock province (Dettinger and 

others, 1995), and are major ground-water aquifers in Nevada. 

These rocks are deformed by east-directed Mesozoic thrust 

faults. The Keystone fault in the eastern Spring Mountains is 

the easternmost thrust of the Cordillera foreland thrust belt in 

the region, and is one of the best exposed thrust faults in the 

entire Cordillera. 

The Las Vegas Valley shear zone, which offsets Paleozoic 

strata and Mesozoic thrust faults in the Spring Mountains and 

Las Vegas Range, has about 50 km of right-lateral horizontal 

(strike-slip) displacement. This west-northwest-striking shear 

zone is concealed beneath thick basin-fill deposits of Las 

Vegas Valley, and it played a significant role in the tectonic 

development of Las Vegas Valley. Similarly, the development 

of northwest-striking Pahrump Valley is closely associated 

with movement along the State Line fault zone. Both fault 

zones are right-lateral strike-slip fault zones that formed 

during episodes of extensional faulting in Cenozoic time. In 

addition, the Las Vegas Valley shear zone has been termed a 

transverse zone, oriented nearly parallel to the extension 
2 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

direction, and accommodating different rates, amounts, and 

types of extensional strain north and south of it (Rowley, 

1998). 

Along the eastern Spring Mountains is Spring Mountain 

State Park, managed by the State of Nevada, and Red Rock 

Canyon National Conservation Area, managed by the U.S. 

Bureau of Land Management (BLM). These are two of the 

region’s most popular parks. The quadrangle also includes part 

of the Desert National Wildlife Refuge in the southern Sheep 

and Las Vegas Ranges, managed by the U.S. Fish and Wildlife Service. The National Park Service Lake Mead National 

Recreation Area is in the adjacent Lake Mead 1:100,000-scale 

quadrangle. This report provides information for Las Vegas 

visitors about how the rocks and landscape of the area’s scenic 

parks and lands formed. 

This report provides an integrated geologic and geophysical framework for ground-water investigations in the Las 

Vegas region. Geologic and geophysical mapping conducted 

for this report helped to identify faults, fractures, and other 

permeable zones that are pathways for ground-water flow, and 

thus it helps to determine the distribution of potential aquifers 

and ground-water flow paths and barriers. Several important 

ground-water studies were completed in conjunction with 

our mapping, including evaluation of the Las Vegas Valley 

shear zone for ground-water resource potential (Langenheim 

and others, 1997, 1998) and assessment of western Las Vegas 

Valley as a potential artificial recharge site (Langenheim and 

Jachens, 1996). 

Methods 

Bedrock map units were compiled from existing published geologic maps in the quadrangle (fig. 1) (Axen, 1985; 

Bohannon and Morris, 1983; Bingler, 1977; Burchfiel and 

others, 1974; Carr, 1992; Carr and McDonnell-Canan, 1992; 

Carr and others, 2000; Ebanks, 1965; Lundstrom and others, 

1998; Maldonado and Schmidt, 1991; McDonnell-Canan and 

others, 2000; Matti and others, 1993; Page and others, 1998), 

with some reinterpretations and modifications following 

field checking. The geologic map of Clark County, Nevada 

(Longwell and others, 1965), was an important source of 

information used in our compilation. New geologic mapping 

of bedrock units was completed in the Las Vegas Range, Blue 

Diamond area, and the Cold Creek area in the northwestern 

Spring Mountains (fig. 1; W.R. Page, new mapping, this 

report). Several sets of vertical aerial photographs considerably aided our new mapping, including 1977, 1978, and 1980 

U.S. Bureau of Land Management natural color photographs 

at 1:30,000 scale, and 1973 and 1980 U.S. Geological Survey 

black and white photographs at 1:30,000 scale. 

The age of many Paleozoic units in the Las Vegas quadrangle is based on conodont biostratigraphy. More than 100 

new conodont age determinations were added to the many 

determinations previously published for the study area (Miller 

and Zilinsky, 1981; Stevens and others, 1996, among others). 

Systematic collections for conodonts were taken in measured 

sections through lower and middle Paleozoic successions 

in the major thrust plates in order to characterize changes 

in stratigraphy and to correlate Paleozoic units across thrust 

plates (fig. 2; Harris and others, in press). These collections 

improved correlations locally and regionally by allowing more 

precise dating of stratigraphic units and unconformities as well 

as permitting the recognition of units that were misidentified 

or overlooked in previous studies. 

Quaternary map units were defined and delineated on the 

basis of lithostratigraphy, soil development (U.S. Soil Conservation Service, 1985; Sowers and others, 1988; Reheis and 

others, 1992), geochronology, surface characteristics, and photogeologic characteristics, calibrated by iterative field checking during 1996-99. Several sets of vertical aerial photographs 

were used, including 1977, 1978, and 1980 U.S. Bureau of 

Land Management natural color photographs (1:30,000 scale), 

which were the most useful and main set used; 1973 and 

1980 U.S. Geological Survey black and white photographs 

(1:30,000 scale); and U.S. Geological Survey (1:40,000 and 

1:80,000 scale) black and white and false-color IR photographs. Stereoscopic delineation of features was registered to 

1:24,000-scale base maps using a PG-2 plotter and by use of 

orthophotos. Previous Quaternary geologic mapping and fault 

studies were adapted for this map (fig. 1: Haynes, 1967; 

Bingler, 1977; Matti and Bachuber, 1985; Matti and others, 1987; Matti and others, 1993; Quade, 1983; Sowers, 

1986; Bell, 1981; Bell and others, 1998, 1999; Weide, 1982; 

McDonnell-Canan, 1989; Dohrenwend and others, 1991; 

Hoffard, 1991; Maldonado and Schmidt, 1991; Anderson and 

O’Connell, 1993; Anderson and others, 1995a, b; dePolo, 

1998; dePolo and others, 1999; Amelung and others, 1999). 

These studies were adapted through additional field checking, photointerpretation, and map registration where needed. 

Samples for thermoluminescence (methods of Millard and 

Maat, 1994) and U-series dating (methods of Ludwig and 

Paces, 2002) were collected and analyzed from sites within the 

map area (tables 1 and 2). 

Description of Map Units 

Quaternary and Tertiary Rocks 

[Pedogenic carbonate stages from Gile and others, 1966] 

Gravelly Alluvium, Predominantly of Alluvial Fans 

Coarse gravelly alluvium is subdivided below on the 

basis of age-related characteristics. Provenance accounts for 

some variability in alluvial fan characteristics that is independent of age. Within the map area, limestone and dolostone 

dominate the lithology of gravel clasts in alluvial fans derived 

from much of the Spring Mountains, Las Vegas Range, and 

Sheep Range. However, different fan compositional characteristics exist where other rock types comprise significant 

areas of exposed bedrock source area. In the areas flanking 
Table 1. Thermoluminescence data. Sample Latitude Longitude Depth Stratigraphic Modal grain size % moisture Dose rate (grays/ky) ED (1

degrees°, minutes', seconds" (cm) position field saturated field saturated (grays) 

LV-1 36°16'02" 115°03'44" 300 in Qso silt 4 74 2.3 1.3 519 LV-2 36°18'22" 115°06'52" 120 basal Qay fine sand 0.2 27 2.7 2.1 44.2 LV-3 36°17'07.7" 115°07'46.5" 200 in Qsab fine sand 3 21 3.85 3.18 382 LV-6 36°04'53.7" 115°03'48.3" 800 in Qfw fine sand 0.4 42 3.6 2.2 834 LV-13 36°22'50" 115°05'44" 200 in Qai fine sand 3 21 3.85 3.18 382 LV-14 36°20'54.3" 115°17'15.1" 270 in Qse silt 0.9 49 4.6 2.9 49 LV-15 36°20'54.3" 115°17'15.1" 260 in Qse silt 0.2 42 4.6 3 44 LV-16 36°20'58.8" 115°17'08.7" 400 in Qsc silt 1 40 3.6 2.4 86 LV-17 36°19'06.7" 115°11'46.3" 280 in Qsd silt 1.4 57 6.7 4 162 LV-18 36°19'09.6" 115°11'54.5" 350 in Qsc silt 3.7 50 4.3 2.7 150 LV-19 36°19'08.9" 115°11'27.9" 600 in Qsa silt 2 64 4.8 2.8 630 LV-20 36°19'08.9" 115°11'27.9" 350 in Qsb silt 2 51 4.5 2.8 402 LV-21 36°07'10.6" 115°53'15.5" 240 basal Qsc silt 13 42 5.3 4.1 204 LV-22 36°07'10.6" 115°53'15.5" 150 in Qsc silt 6 37 6.4 4.7 132.4 LV-23 36°07'00.8" 115°53'04.7" 120 in Qse silt 2.2 27 3.7 2.9 74.5 LV-24 36°04'28" 115°56'23" 220 in Qd fine sand 2.6 24 4.3 3.43 8.53 LV-25 36°03'03" 115°54'53" 800 in Qbo silt 5.4 38 2.53 1.87 750.2 LV-26 36°03'03" 115°54'53" 500 in Qbo silt 5.6 33 2.78 2.13 584.3LV-27 36°02'00" 115°53'18" 200 in Qby silt 5.2 37 2.92 2.17 616.5 LV-28A 36°08'05.9" 115°23'58.5" 500 in Qao fine sand 0.2 44 1.2 0.79 562 LV-28B 36°08'05.9" 115°23'58.5" 500 in Qao fine sand 0.2 44 1.2 0.79 468 LV-29 36°08'06" 115°23'58.5" 500 in Qayy fine sand 2.6 24 4.3 3.43 8.53 Note: ED (1) is equivalent dose at peak temperature T of glow curves 

Table 2. Uranium series dates. Sample Latitude Longitude Unit Depth (m) U ppm Th ppm 234U/238U 230Th/238U 230Th/232Th A

degrees°, minutes', seconds" 

TSQ2C-A 36°20'57" 115°17'18" Qsd surface 1.88 2.05 2.144+10.9 1.0477+9.8 2.87 

TSQ2C-B 36°20'57" 115°17'18" Qsd surface 3.12 1.82 2.5242+5.45 0.52816+11.7 3.11 2WM1A-A 36°05'03" 115°04'00" Qao vein at 2m 1.1 0.211 1.7138+1.33 1.89+1.69 30.4 

WM1A-B 36°04'58" 115°03'52" Qao vein at 2m 1.6 0.3571 1.657+1.49 1.82+1.74 24.1 MSQ1A-A 36°07'13" 115°53'13" Qsc 2 3.13 3.12 2.5421+10.7 1.055+8.44 3.17 

MSQ1C-A 36°07'13" 115°53'13" Qsd 0.3 1.33 2.66 2.3031+30.6 0.74068+40.8 1.33 

MSQ2B-A 36°07'13" 115°53'13" Qsd surface 3.33 2.89 2.315+8.3 0.61701+14.9 2.46 3

MSQ3A-D 36°07'05" 115°53'02" Qai 2 1.1 0.94 1.6712+6.41 0.96469+7.71 3.45 MSQ4B 36°05'04" 115°59'16" Qbw 1 0.86 0.06 1.4249+0.773 1.523+1.37 65.47 
4 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

the southern part of the Spring Mountains, the fans of Lovell relict depositional microrelief is evident on 

Wash, Red Rock Wash, and Tropicana Wash include a greater aerial photographs. Desert pavement ranges 

proportion of sandstone clasts, matrix sand, and interbedded from absent on deposits in modern chansand derived from Lower Permian redbeds and the Jurassic nels to loosely packed and weakly develAztec Sandstone. In contrast, fans derived from the northwest oped (especially in areas of relatively low 

Spring Mountains are dominated by resistant angular quartzite dust-flux, as on the upper part of a fan) to 

clasts. These fans are generally more bouldery because the moderately well packed in areas of higher 

quartzite does not weather into pebble-size clasts as readily dust flux. Rock varnish, which does not form 

as do the Paleozoic and Mesozoic carbonates and sandstones. on most limestone clasts, is generally weakly 

Desert varnish also more readily develops on quartzite clasts. developed to absent on more siliceous rock 

In the southeast corner of the map area, fans derived from the types (including siliceous carbonates) except 

Tertiary volcanics of the McCullough Range also have abun­ for relict rock varnish not abraded during 

dant boulders and have well-developed rock varnish on older transport. Typical weak soil development 

fans. The relatively high degree of pedogenic carbonate devel­ is characterized by a cambic Bw horizon, 

opment in the older fan units in this area may relate, at least by the presence of non-cemented stage I-II 

partly, to the more resistant nature of these bouldery gravels. secondary carbonate morphology (mostly 

The coarse alluvial gravel units described below were thin coats on clast undersides), and by a 

deposited predominantly but not exclusively as alluvial fans. gradual increase of sand toward the surface 

Gravelly alluvium within alluvial fans grades valleyward into through the top 0.5 m of the deposit. The 

pediment veneers, terraces, and washes that overlie or are inset surficial sand component is considered to be 

within older fine-grained deposits. Fan gravel at the distal toes pedogenically mixed and infiltrated eolian 

of alluvial fans is also commonly interbedded with, and a gra­ sand deposited after fluvial deposition of 

dational facies equivalent to, correlative fine-grained deposits. fan gravel. Age control by radiocarbon and 

For any given fan or wash, there generally is a decrease in thermoluminescence indicates that Qay 

maximum and average clast size away from upland source is predominantly Holocene, and locally 

areas. This affects the size of the original depositional bar- as old as 14 ka. Near basin centers, Qay 

and-swale morphology and how rapidly such morphology is either overlies or is inset within fine-grained 

modified by surficial processes and pedogenesis. deposits with abundant radiocarbon dates 

For all alluvial map units, gravel and sand are interbed- ranging from about 8 to 12 ky B.P. (Haynes, 

ded and intermixed, poorly to moderately well sorted, mas­ 1967; Quade, 1986; Quade and others, 1995; 

sive to well bedded, and clast supported to matrix supported. Bell and others, 1998, 1999), so it is likely 

Gravel is angular to subrounded, ranging in size from granules that most alluvium at the surface of Qay is 

to boulders; maximum clast size decreases with increasing Holocene. Qay includes deposits correladistance from upland source areas. Individual unit descriptions tive to youngest alluvium (Qayy) and older 

below emphasize surface characteristics, associated soil devel­ deposits (Qayo). Minimum thickness of Qay 

opment, and geochronology that distinguish each unit. ranges from less than 1 m to at least 3 m, as 

exposed in borrow pits; base of unit is generQay Young fan alluvium (Holocene and latest Pleis­ ally not exposed 

tocene)—Noncemented alluvial-fan gravel Qayy Youngest alluvium (Holocene)—Noncemented 

and sand with weakly developed soil. In- alluvial-fan gravel and sand of intermitcludes Holocene and locally latest Pleisto­ tently active wash complexes. Qayy is 

cene (younger than about 15 ka) deposits delineated separately from Qay only where 

between modern channels, and deposits there is a markedly greater proportion of 

in numerous modern channels that are too modern channels with minimal development 

narrow (less than 30 m) to map separately. of surface etching, varnish, pavement, and 

Etching on surficial limestone clasts ranges soil. Includes deposits of modern channels 

from absent on deposits of modern chan­ that are too narrow (less than 30 m) to map 

nels to incipient and sparse to moderately separately, as well as Holocene deposits 

developed and common on deposits between between modern channels. Etching on 

modern channels. Bar-and-swale deposi­ surficial limestone clasts ranges from absent 

tional morphology ranges from prominent on deposits of modern channels to incipiin modern channels to variably modified ent and sparse on deposits between modern 

and muted by addition of eolian sediment channels. Bar-and-swale depositional morin areas between modern channels. Even in phology ranges from prominent in modern 

the most muted cases, cobbles and boul­ channels to variably modified and muted by 

ders protrude from eolian sand cover, and addition of eolian sediment in areas between 
Geology of the Las Vegas 30′ × 60′ Quadrangle 5 

modern channels. Desert pavement ranges units of Bell and others (1998, 1999) and 

from absent on deposits of modern channels 

to loosely packed and weakly developed on 

Liu and others (2000). Minimum thickness 

of Qayo ranges from less than 1 m to at least 

deposits between active channels. Rock varnish, which does not form on most limestone 

clasts, is generally very weakly developed to 

Qaiy 

3 m, as exposed in borrow pits 

Younger intermediate fan alluvium (late Pleistocene)—Cemented alluvial-fan gravel, with 

absent on more siliceous rock types (including siliceous carbonates) except for relict 

interbedded sand; poorly to moderately 

well sorted; massive to well bedded; clastrock varnish not abraded during transport. supported to matrix-supported. Gravel is 

Typical non-cemented, weak soil development is characterized by the presence of 

angular to sub-rounded, ranging in size from 

granules to boulders. Surface is characterstage I–II secondary carbonate morphology ized by a moderately to tightly packed 

(mostly thin coats on clast undersides), and 

by a gradual increase of sand toward the surdesert pavement. Though nonsiliceous 

limestone clasts do not have rock varnish, 

face in the upper 10–30 cm of the unit. The siliceous clasts possess dark varnish, which 

surficial sand component is considered to be 

a pedogenically mixed and infiltrated eolian 

impart a darker tone to this unit on aerial 

photographs. This surface morphology is 

sediment deposited after fluvial deposition associated with a soil that typically includes 

of fan gravel. Spaulding and Quade (1996) 

reported four radiocarbon dates ranging from 

a weakly to moderately developed reddishbrown argillic (Bt) horizon and cemented 

1070 to 3310 years B.P. (analytical error stage II–IV carbonate morphology. Within 

40–60 years) on bristlecone pine logs in and the soil profile, the upward decrease in 

on debris flow deposits of Qayy deposits in 

the Yucca Forest area, northern Gass Peak 

quadrangle, (included on this map as Qay). 

proportion of gravel is due to the addition of 

eolian material concurrent with pedogenesis. 

Within and beneath the zone of maximum 

This unit grades into partly correlative finegrained facies mapped as Qfy. Minimum 

pedogenic carbonate development, limestone gravel generally is distinctly more cethickness is 1–2 m; base of unit is generally mented than in younger Holocene alluvium. 

Qayo 

not exposed 

Older young alluvium (Holocene and latest 

Pleistocene)—Noncemented gravel and sand 

Depositional microrelief is minimal relative 

to other alluvial units; bar-and-swale morphology is generally absent, and surfaces are 

with weakly developed soil of alluvial-fan 

remnants. Associated with surfaces that are 

2 m to greater than 10 m above modern 

channels. Etching on surficial limestone 

clasts is moderately developed and common. 

smooth between limited areas of erosional 

dissection. Where distinguished from unit 

Qai, which has similar soil development and 

surface characteristics, Qaiy consists of surfaces that are inset relative to Qai, especially 

Surface is generally smooth, with gen- within the incised fanhead areas of major 

eral absence of depositional bar and swale 

morphology. Desert pavement is moderately 

washes that provide drainage from relatively 

high areas of the Spring Mountains, includpacked. Rock varnish, which does not form 

on limestone clasts (Liu and others, 2000), is 

moderately developed on more siliceous rock 

types (including siliceous carbonates). Typiing Lovell Wash, Harris Wash, and Willow 

Wash. Unit Qaiy is not dated, but is hypothesized to be correlative to Lundstrom and 

others’ (1999) unit Qfiy on Fortymile Wash 

cal non-cemented and weak soil development 

is characterized by a cambic Bw or incipient 

Btj horizon, by the presence of stage I–II 

near Yucca Mountain, which they dated 

between about 25 and 50 ka, and to unit 

Qscd on this map which we dated between 

secondary carbonate morphology (mostly 

thin coats on clast undersides), and by a 

gradual increase of sand toward the surface 

about 25 to 40 ka. Where mapped separately, unit Qaiy is a strath terrace generally 

less than 2 m thick but grades basinward to 

through the top 0.5 m of the deposit. The 

surficial sand component is considered to be 

pedogenically mixed and infiltrated eolian 

sediment deposited after fluvial deposition of 

fan gravel. Unit locally overlies Qse, dated 

at about 10–16 ka (described below). Qayo 

Qai 

thicker alluvial fan gravels included in unit 

Qai 

Intermediate fan alluvium (late and middle? 

Pleistocene)—Cemented alluvial-fan gravel, 

with interbedded sand; poorly to moderately well sorted; massive to well bedded; 

includes unit E2

 of Haynes (1967) and Q3 fan clast-supported to matrix-supported. Gravel 
6 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

is angular to sub-rounded, ranging in size 

from granules to boulders. Includes modern 

channels too narrow to map separately. Surface between modern channels is somewhat 

erosionally rounded where it is more than 

about a meter above grade of adjoining 

channels. Surface between modern channels 

is characterized by a moderately to tightly 

packed desert pavement and smooth surface, 

which generally lacks bar and swale depositional morphology. Though nonsiliceous 

limestone clasts do not have rock varnish, 

siliceous clasts possess dark varnish, which 

impart a darker tone to unit Qai relative to 

younger units. This surface morphology is 

associated with a soil that typically includes 

a reddish-brown argillic (Bt) horizon and 

cemented stage II–IV carbonate morphology. Within the soil profile, the upward 

decrease in proportion of gravel is due to the 

addition of eolian material concurrent with 

pedogenesis. Within and beneath the zone 

of maximum pedogenic carbonate development, limestone gravel generally is distinctly 

more cemented than in younger Holocene 

alluvium. Depositional microrelief is minimal relative to other alluvial units; bar-andswale morphology is generally absent, and 

surfaces are smooth between limited areas 

of erosional dissection. At site LV-13 (SW 

1⁄4 sec. 19, T. 18 S., R. 62 E.), the eolian part 

of a buried soil at about 2 m depth within 

unit Qai yielded a thermoluminescence date 

of about 100-120 ka (table 1). We estimate 

Qai to have been deposited between about 

50 ka and 130 ka (the early part of the late 

Pleistocene, including the last interglacial). 

Exposed minimum thickness of Qai ranges 

from less than 1 m to at least 5 m; base of 

unit not exposed 

Qau Undivided young and intermediate alluvium 

(Holocene and late Pleistocene)— 

Cemented and noncemented alluvial-fan 

gravel, with interbedded sand; poorly to 

moderately well sorted; massive to well 

bedded; clast-supported to matrix-supported. 

Gravel is angular to subrounded, ranging 

in size from granules to boulders. Qau 

represents areas in which young fan alluvium (Qay), including common intermittent 

active channels, exists as discontinuous 

but common (30–50 percent of Qau), thin 

(<1m) veneers over intermediate fan alluvium (Qai) in patches that are too narrow 

to map separately. In the area of the State 

Line fault zone to the northeast of Pahrump 

Valley playa in the southwest part of the 

map area, Qau refers to undivided Quaternary alluvium reworked from QTa and Qbw 

(described below) 

Qao Old alluvium (Pleistocene)—Cemented gravel and 

sand of partly eroded alluvial-fan remnants 

and terraces with well-developed, but degradational soil. The erosional upper surface 

consists of broadly rounded, accordant 

ridges that are gradational basinward to 

less deeply dissected surfaces. Depositional 

microrelief is absent. The generally wellpacked surface pavement that developed on 

this erosional surface includes variable but 

often darkly varnished clasts and abundant 

clasts of pedogenic calcite and calcite-cemented gravel; these clasts impart a lighter 

surface tone to this unit than to adjoining 

surfaces of younger map units. Soil typically 

includes a laminar (stage IV, Gile and others, 

1966) but partially eroded K horizon at least 

0.5 m thick; an overlying argillic horizon of 

variable thickness and expression is commonly present. The surface characteristics 

indicate that the original upper depositional 

surface of Qao has been partially eroded. 

The pavement, and at least some of the 

soil, clearly were formed after the latest 

erosional episode. Accordancy of ridge tops 

of Qao indicates a common single grade 

in an area that the ridge tops define; this 

grade is inferred to approximate the original 

depositional top of Qao before erosion. This 

morphology contrasts with the non-accordant ridge tops of QTa (described below), 

indicating that no original depositional top 

of QTa remains. The accordant surface defined by Qao ridge tops is inset within QTa 

where these units are juxtaposed. Where 

Qao adjoins Qai, the upper part of Qao is 

clearly at a higher grade and more eroded 

than Qai. Qao is not dated. Because the 

accordant ridge tops of Qao can be traced 

to areas within a few meters of the grade 

of Qai, which has yielded late Pleistocene 

dates, we infer that Qao includes middle and 

late Pleistocene deposits. Minimum exposed 

thickness of Qao is at least 4 m; base of unit 

is generally not exposed 

Qaoi 	 Undivided Pleistocene alluvium—Cemented sandy 

gravel and interbedded sand of alluvial fans; 

consists of numerous inset to superposed 

veneers of intermediate fan alluvium (Qai) 

on old alluvium (Qao), such that surfaces 
7 

of both units are interspersed at a scale too 

small to map separately 

QTa Gravelly basin-fill alluvium (Pleistocene to late 

Miocene?)—Generally well-cemented, 

sandy gravel and interbedded finer deposits 

of undivided basin fill. Poorly to moderately 

well sorted; massive to well bedded; clastsupported to matrix-supported. Gravel 

is angular to subrounded, ranging in size 

from granules to boulders. The erosional 

upper surface consists of rounded, nonaccordant ridges between intervening deep 

gullies. Depositional microrelief is absent. 

Variably well-packed surface pavement 

includes abundant clasts of pedogenic 

calcite and calcite-cemented gravel; these 

clasts impart a lighter surface tone to this 

unit than is present on adjoining surfaces 

of younger map units. Soil typically 

includes a laminar (stage IV, Gile and 

others, 1966) but partially eroded K 

horizon at least 0.5 m thick; an overlying 

argillic horizon of variable thickness and 

expression is commonly present. The 

surface characteristics indicate that the 

original upper depositional surface of 

QTa has been substantially eroded, and its 

original depositional thickness is unknown. 

Pavement and soil development are younger 

than the latest erosional episode. Where 

QTa adjoins Qao, QTa is clearly at a higher 

grade and more eroded than Qao. QTa 

locally includes interbedded megabreccia 

deposits in the Blue Diamond area (Page and 

others, 1998) and in the west-central Willow 

Peak quadrangle. Megabreccia layers that 

have not been mapped separately crop out 

in the Mount Stirling quadrangle (N1/2 sec. 

19, T. 18 S., R. 54 E.; sec. 1 and sec. 2, T. 18 

S., R. 53 E.) and Horse Spring quadrangle 

(sec. 24, T. 18 S., R. 53 E.). Minimum 

exposed thickness of QTa is at least 50 m; 

base of unit is generally not exposed. In the 

Pahrump Valley, QTa includes gravel that 

contains sparse granitic clasts that were 

probably derived from the Kingston Range. 

Unit caps a chain of rounded hills that have 

a northwest alignment parallel to the trend 

of escarpments to the east. To the southeast 

along the general alignment of QTa within 

the Stump Spring quadrangle (McMackin, 

1999), better exposures of this unit include 

abundant and lithologically varied volcanic 

clasts in beds striking northwest and 

dipping northeast beneath Qb. Malmberg 

(1967) reported a white to light-green tuff 

Geology of the Las Vegas 30′ × 60′ Quadrangle 

that underlies these gravel deposits, but 

good exposures of the gravel or underlying 

stratigraphy were not noted in the map area. 

The flanks of the gravel-covered hills are 

draped with thick colluvium containing a 

younger eolian sand component. QTa in 

the subsurface is considered here to include 

basin-fill deposits that may be equivalent to 

parts of the Muddy Creek and Horse Spring 

Formations (Bohannon, 1984; Maldonado 

and Schmidt, 1991) 

Footslope Colluvium and Alluvium 

Qcf Hillslope colluvium and alluvium (Holocene 

and Pleistocene)—Interbedded colluvial 

and debris-flow diamicton beds that grade 

into and are interbedded with alluvium of 

a wide age range. Unit generally occurs 

on lower, concave-upward footslopes 

(Peterson, 1981), which are typically 

partially dissected. Consists of angular 

and subangular gravel ranging in size from 

granules to boulders, generally supported 

by a matrix with variable proportions of 

sand, silt, and clay; matrix material inferred 

to be at least partly of eolian origin. Qcf 

is massive to finely bedded and locally 

includes multiple buried soils indicating 

Pleistocene episodes of pedogenesis and 

eolian deposition. Includes boulder levees 

that adjoin and are parallel to modern 

gullies and that were formed by debris 

flows. Includes deposits of possible glacial 

or periglacial origin in the southeast part of 

the Charleston Peak quadrangle (sec. 27, T. 

19 S., R. 56 E.), just south and southwest 

and upslope of Big Falls. The contact with 

adjoining alluvium is well defined only 

where unit Qcf is truncated by younger 

alluvium. Exposed thickness ranges from 1 

to 4 m 

Fine-grained Deposits 

Qd Dune sand (late Holocene)—Noncemented fine 

sand in intermittently active to inactive 

dunes partially vegetated by mesquite. 

Most dunes in the Pahrump Valley and 

Corn Creek Springs area occur on and near 

northwest-trending escarpments. The dunes 

mapped by Bingler (1977), Matti and Bachuber (1985), and Matti and others (1987, 

1993) in Las Vegas Valley have been mostly 
8 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

obliterated by urbanization. Most dunes that 

still exist can be considered intermittently 

active, with evidence for recurrent activity 

during the late Holocene. In the Corn Creek 

Springs area, radiocarbon dates on archaeological hearths buried by dunes range from 

about 4,000 to 5,200 years B.P. (Williams 

and Orlins, 1963). In the Pahrump Valley, a 

TL date (LV-24, table 1) on dune material 

from about 3 m depth in a mesquitecovered dune that mantles a scarp is about 

2 ka. Thickness ranges from 1 to about 5 m. 

Eolian sand is also mapped in this unit as 

sand ramps along fluvial scarps of Red Rock 

Wash. However, eolian sand is not mapped 

separately where it is a common component 

of surface and buried soils in alluvial gravels 

and in fine-grained deposits associated with 

past ground-water discharge 

Qpy Modern playa sediment (late Holocene)—Fine 

mud, silt, and clay of modern playa in 

Pahrump Valley, southwest corner of map 

area. Smooth, flat, high albedo surface has 

several parallel, straight lineaments apparent 

on aerial photographs that are provisionally 

interpreted as buried, probable right-lateral 

strike-slip faults of the State Line fault zone. 

Margins of playa are transitional to unit 

Qfy with greater amount of fluvial sand and 

fluvial morphology. Thickness unknown; 

only upper surface of unit is exposed 

Qsyy Youngest spring deposits (late Holocene)— 

Historic and prehistoric ground-water 

discharge areas, including spring mounds, 

composed of fine-grained calcareous to 

organic-rich silt, clay, and mud. Unit 

includes the area of historically active 

artesian springs that now includes the north 

(main) well field for the Las Vegas Valley 

Water District. Unit also includes some 

spring mound areas, including some that 

were historically active and some which 

yielded late Holocene dates younger than 

2,000 years B.P. (Haynes, 1967; Bell and 

others, 1998, 1999) 

Qfy Intermittently active fluvial fine-grained alluvium 

(late Holocene)—Brown to gray sand, silt, 

mud, and interbedded gravel. Includes units 

F and G of Haynes (1967) and Quade (1986) 

and distal facies of alluvial fans. Erosional 

and depositional fluvial bar and channel 

morphology ranges from fresh to muted. 

Qfy includes a thin pediment veneer across 

older fine-grained deposits. This thin veneer 

of intermittently active fluvial sediment 

grades up-wash to coarser gravelly youngest 

alluvium (Qayy), with which Qfy is partly 

correlative. Thickness ranges from less than 

50 cm to greater than 150 cm; base is not exposed, especially down wash to southwest in 

the Pahrump Valley, where this unit grades 

into modern playa sediment (Qpy) 

Qfo Older fine-grained deposits (Holocene)—Noncemented fluvial sand and mud. Erosional 

and depositional fluvial bar and channel 

morphology ranges from fresh to muted, 

especially relative to adjoining unit Qfy. 

Map unit in up-wash position may represent 

a thin pediment veneer across older finegrained units. Thickness ranges from less 

than 50 cm to greater than 150 cm; base is 

not exposed 

Qse Unit E—Young fine-grained deposits associated 

with past ground-water discharge (early 

Holocene to latest Pleistocene)—Lightgray to light-brown unconsolidated silt, 

sandy silt, silty sand, and mud; locally 

light-greenish-gray mud. Locally contains 

abundant dark-gray peat, charcoal, and 

organic-rich horizons (black mats) (Quade 

and others, 1998), aquatic and terrestrial 

mollusc shells (Taylor, 1967; Quade, 1986; 

Quade and Pratt, 1989; Brennan and Quade, 

1997; Bell and others, 1998, 1999), and 

fossils of late Pleistocene megafauna, including mammoth, bison, horse, and camel 

(Mawby, 1967). Numerous radiocarbon 

dates (Haynes, 1967; Quade, 1986; Quade 

and others, 1995, 1998; Brennan and Quade, 

1997; Bell and others, 1998) range from 

about 8,000 to about 15,000 yrs BP, with 

a significant number of black mats yielding dates of about 10,000 to 11,000 yrs BP 

(Quade and others, 1998). We obtained 

thermoluminescence dates in this age range 

from a site (LV-14, LV-15, table 1) in which 

5 radiocarbon dates in this age range were 

obtained on black mats by Bell and others 

(1998). Commonly exists as channel fills 

inset within unit D (as in Quade and others, 

1998). Bedding defined by changes in color, 

organic content, grain size, and sedimentary 

structures. Ranges from massive to well 

bedded with fine-scale crossbeds. Unit E 

was originally defined and described by 

Haynes (1967). Includes non-fossiliferous 

eolian/phreatophyte flat facies of Quade and 

others (1995). Thickness is greater than 4 m; 

base commonly not exposed 

Qsy Undivided young spring deposits (Holocene and 

late Pleistocene)—Includes units Qse and 

Qsyy. Thickness is 1–4 m; base is locally 

not exposed 
Geology of the Las Vegas 30′ × 60′ Quadrangle 9 

Qscd Unit C and D—Intermediate fine-grained deposits associated with past ground-water 

discharge (late Pleistocene)—Top 1–2 m is 

Unit B is underlain by unit A, which consists 

of a red-brown buried soil at least 1 m thick 

in silt and mud similar to unit B. The buried 

characteristically resistant light-gray calcare­ soil in unit A is clay-rich with a wellous mud that is partially cemented with developed angular blocky structure and 

calcite which weathers to curving and common manganese oxide stains on ped 

branching to platy nodules; some of these faces. We obtained a TL date (LV-19, table 

may be trace fossils of cicada burrows 1) of 131–225 ka on the type unit A at the 

(Quade, 1986). Grades downward to underlying unit C(?), which is tan-brown (more 

oxidized), about 2–3 m thick, fine sandy 

Tule Springs site of Haynes (1967). Unit A 

may correlate in part with unit Qso. Unit 

Qsab is mapped along the Eglington fault 

silt and mud. This middle brown unit may 

include one buried soil, marked by a slight 

increase in prismatic structure and darker 

Qso 

scarp (from Haynes, 1967) 

Old fine-grained spring deposits (middle Pleistocene)—Light-toned, variably cemented fine 

tone. Includes occasional fluvial cross- sand, mud, and marl, generally associated 

bedding and common carbonate nodules. with areas of past ground-water discharge. 

These units were first defined by Haynes Generally calcareous, with different degree 

(1967) in the northern Las Vegas Valley. of cementation between beds, ranging from 

In the Pahrump Valley, the intermediate weakly to strongly cemented. Subvertical 

brown unit overlies a sharp contact with tubules within densely cemented layers may 

a strongly developed buried soil that may be plant casts. In the south-central part of the 

correlate with unit A of Haynes (1967) in the 

Las Vegas Valley. Numerous radiocarbon 

Valley quadrangle (Lundstrom and others, 

1998), where Qso is crossed by Interstate 

dates on unit D range from about 16 to 30 I-15, white calcareous layers are interbedky B.P., with most around 25 ky B.P. At the ded with reddish-brown, silty, gypsiferous 

type section of unit D defined by Haynes beds about 30–60 cm thick with medium to 

(1967) in trench K at the Tule Springs site, strong blocky structure. These may represent 

we obtained a TL date (LV-17, table 1) of buried soils. Two or three of these beds are 

24–41 ka. Unit C is generally lacking in exposed in eroded buttes south of Interfossils, but Bell and others (1998) obtained state I-15, where they are interbedded with 

a radiocarbon date on snails of 29 ky B.P.; strongly cemented carbonate-rich layers. At 

at this site we obtained a TL date (LV-16, site LV-1 (SW 1⁄4 of sec. 29, T. 19 S., R. 62 

table 1) of 24–36 ka. At the type section of E.), a thermoluminescence date of 226 to 

unit C defined by Haynes (1967) in trench 399 ka (table 1) was obtained on the upperK at the Tule Springs site, we obtained a TL 

date (LV-18, table 1) of 35–56 ka. TL dates 

most reddish-brown bed at about 3 m below 

the carbonate-cemented top of the unit. Qso 

on this unit in Pahrump Valley (LV-21, LV- is geomorphically expressed as rounded, 

22, table 1) are generally in this same time poorly exposed badlands and less commonly 

range. Observed thickness ranges from 2 to as resistant buttes mantled by thin colluQsu 

Qsab 

6 m; base commonly not exposed 

Undivided young and intermediate fine-grained 

deposits associated with past groundwater discharge (early Holocene and late 

Pleistocene)—Includes units Qse and Qscd 

Older fine-grained deposits units A and B (late? 

to middle Pleistocene)—Unit B, locally 3–4 

vium, sheetwash, and eolian sand. Unit may 

partly correlate to unit Qx of Haynes (1967). 

Minimum exposed thickness is 5 m; base is 

generally not exposed 

Basin fill of Browns Spring (Pleistocene and 

Pliocene?)—Fine-grained bedded mud, fine 

sand, marl, and limestone with minor 

m thick, consists of fine-grained greenish- interbedded gravel; mapped only in the 

gray to brown muds and marls as defined by Pahrump Valley. Upper, middle, and lower 

Haynes (1967) where well exposed at the parts of unit were mapped based on position 

Tule Springs archaeological site. Where ex- relative to a resistant limestone bed in the 

posed in nearly vertical exposures, this unit 

is not mappable at 1:100,000-scale. Locally Qby 

middle unit 

Upper part (middle Pleistocene)—Fine-grained, 

fossiliferous, including molluscs and mega- brown to whitish-tan mud, marl, and fine 

fauna. We obtained a TL date (LV-20, table sand interbedded with minor 10–80-cm1) of 89–144 ka on the type section of unit thick beds of pebble gravel. Gravel is similar 

B at the Tule Springs site of Haynes (1967). to surficial gravel in that it is subangular to 
10 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

subrounded and composed of limestone rock 

types probably derived from the Mesozoic 

and Paleozoic rocks of the Spring Mountains. Gravel probably represents the distal 

ends of alluvial fans interbedded with the 

fine-grained deposits. Fine-grained beds 

are variably cemented with calcite and 

include beds interpreted as pre-Wisconsinan 

ground-water discharge cycles (Spaulding 

and Quade, 1996). This part of the section 

is generally poorly exposed and covered by 

colluvium. At Browns Spring, in muds about 

3 m above the top of the resistant Qbw bed, 

we obtained a TL date (LV-27, table 1) of 

211–284 ka. Minimum exposed thickness is 

about 50 m, but this unit is disconformable 

with overlying units 

Qbw Middle white limestone (middle Pleistocene)— 

Massive, 2-4-m-thick densely cemented 

limestone that forms a resistant escarpment 

extending to northwest and southeast of 

Browns Spring in the southwest part of the 

map area. Included in this unit is calcitecemented limestone pebble gravel 30–60 cm 

thick, which overlies the massive limestone 

bed. At Browns Spring the gravel appears 

to be enclosed in the top of the massive 

limestone. U-series analysis from this unit 

yielded a date of about 370 ka (MSQ4B, 

table 2). Thickness is about 5 m 

QTbo Lower part (middle and early? Pleistocene to 

Pliocene?)—Fine-grained brown to whitishtan muds and marls interbedded with minor 

pebble gravel. Marls form resistant ledges 

in escarpment below Qbw and include plant 

casts (stems of unknown taxa), and probably represent intervals of ground-water 

discharge. QTbo also contains interbedded lenses and channels of pebble gravel 

composed of limestone rock types. We 

obtained TL dates of about 210–400 ka (LV25, LV-26, table 1) from this unit. Exposed 

thickness at least 40 m; base of unit is not 

exposed 

Qfw Fine-grained sediments of Whitney Mesa (middle Pleistocene)—Light-toned, variably 

cemented, reddish-brown, fine sand, mud, 

and marl, with local interstratified gravel. 

Exposed along the prominent escarpment 

along the eastern margin of Whitney Mesa 

in the southeast part of the map area. Generally calcareous, but with highly variable 

degree of cementation of beds, ranging from 

weakly to strongly cemented, and locally 

very nodular. Much of the carbonate may 

be secondary and formed by evaporation of 

ground water where it reaches the escarpment free face, as locally occurs in present 

conditions. Luminescence dating of a silty, 

fine sand near the top of the unit, about 2 m 

beneath the highly cemented caliche which 

overlies the unit, yielded a TL date of about 

232–379 ka (LV-6, table 1), similar to Useries dates on a travertine vein in the caliche 

cap (WM1A-A, WM1A-B, table 2) 

QTs Undivided fine-grained sediments of the Las 

Vegas Valley (Quaternary and Tertiary?)—Light-toned, variably cemented 

and interstratified fine sand, mud, marl, 

and minor gravel, probably associated with 

areas of past ground-water discharge and 

with distal facies of alluvial fans. Generally 

whitish to light gray but includes interstratified light-greenish-gray to yellowish-gray to 

pinkish-gray fine sand and silt, reddish-orange fine sand, and red-brown mud. Proportion of interbedded gravel in subsurface 

increases toward the west (Donovan, 1996). 

Generally calcareous, with highly variable 

degree of cementation of beds, interbeds, and 

locally capping caliche, ranging from weakly 

to strongly cemented; locally gypsiferous. 

Subvertical tubules within densely cemented 

layers may be plant casts. Generally corresponds to QTs as mapped by Matti and 

Bachuber (1985) and Matti and others (1987, 

1993), to QTss as mapped by Bingler (1977), 

and to Qx as mapped by Haynes (1967). In 

most of the mapped area, this unit is poorly 

exposed because of urbanization. Because of 

the poor exposure, lack of age control, local 

similarity to each of the previously described 

fine-grained units and to the Muddy Creek 

Formation (Tm), and existence near and 

potential juxtapositon by faults, this unit 

could represent a wide range of ages and 

correlate locally to any of the previously 

described fine-grained units or Tm. Thickness estimated (from geophysics; Langenheim and others, 1997) to be as much as 4–6 

km in the Las Vegas and Pahrump Valleys 

QTu Undifferentiated fan alluvium and basin-fill 

deposits (Quaternary to Miocene?)— 

Shown only in cross sections. Includes 

alluvial fan gravels and interbedded sands of 

units Qay and Qai at surface, and probably 

units Qao and QTa at unknown depth 

Basin-fill Deposits 

Tm Muddy Creek Formation (Pliocene and upper 

Miocene)—Reddish-brown to greenish-gray 
11 

gypsiferous mudstone and fine sandstone; 

found only in a few small exposures in the 

southeast part of the map area beneath pediment gravels flanking the southwest margin 

of Frenchman Mountain, but is more extensive and better exposed in the Lake Mead 

area (Bell and Smith, 1980; Bohannon, 

1984). Castor and others (2000) estimated 

the Muddy Creek Formation to be as great 

as 300 m thick in the Frenchman Mountain 

quadrangle 

Th Horse Spring Formation, undivided (Miocene 

and Oligocene)—Adapted from Maldonado 

and Schmidt (1991) in the Gass Peak quadrangle. Exposed in Sheep and Las Vegas 

ranges where conglomerate and lacustrine 

members have been recognized. 

Conglomerate member—Gray to brownish-gray, poorly sorted, massive to vaguely 

thick bedded, coarse conglomerate; moderately consolidated. Detritus derived from 

local Paleozoic and Late Proterozoic sedimentary rocks. Includes landslide megabreccia. Depositional top not exposed; much of 

upper part probably removed by erosion. 

Minimum exposed thickness is about 100 m, 

but member is probably thicker than 200 m. 

Lacustrine member—White and subordinate variegated red, yellow, and brown, 

laminated to thick-bedded, tuffaceous claystone and siltstone representing shallow lake 

deposits; locally interbedded with sandstone 

and pebble-cobble conglomerate. Some Tpm 

lake beds are calcareous and gypsiferous; 

subordinate thin limestone beds have algal 

mat structure. Includes light-gray air-fall tuff 

beds less than 1 m thick. K-Ar age dates of 

interbedded volcanic units in the member 

in the Gass Peak basin range from 12 to 16 

Ma (Guth and others, 1988). Base is not 

exposed: exposed thickness about 200 m 

Thb Basalt unit of Horse Spring Formation (Miocene)—Several thin flows of alkali basalt 

interbedded with the conglomerate member 

of the Horse Spring Formation; found only 

in the Las Vegas Range in the northeast 

corner of the map area (Gass Peak NE quadrangle). Consists of 25–30 percent olivine 

phenocrysts. Dated at 16.4±0.6 Ma (K-Ar 

whole-rock date; Feuerbach and others, 

1993). Basalt flows are 10–20 m thick 

Volcanic Rocks 

Tmd Mount Davis Volcanics (Miocene)—Mostly 

resistant, light- to dark-gray and tan, 

vesicular, flow-foliated, fresh, crystal-poor 

Geology of the Las Vegas 30′ × 60′ Quadrangle 

to crystal-rich dacite, andesite, and basaltic 

andesite lavas and flow breccia, and minor 

interbedded gray and reddish-brown, volcanic mudflow breccia, air-fall tuff, and fluvial 

pebbly sandstone and sandy conglomerate. 

Exposed only in the McCullough Range in 

southeastern part of the quadrangle, especially along the road to, and near, the radio 

towers in sec. 25, T. 22 S., R. 62 E. Upper 

parts of unit, downfaulted and exposed 

northeast of radio towers, include moderately resistant, light-gray and tan, dacitic 

ash-flow tuff containing 10–20 percent 

phenocrysts of plagioclase, sanidine, biotite, 

and hornblende; the tuff here is overlain 

by an aphanitic gray basaltic andesite flow. 

Tmd occupies broad stream channels that 

were cut in gently tilted and eroded rocks of 

the Patsy Mine Volcanics (Tpm). Unit was 

correlated by Bingler (1977) with the Mount 

Davis Volcanics of the Eldorado Mountains 

(Anderson, 1977), about 15 km southeast of 

the map area. In nearby areas to the south 

and east, mapped as informal andesite unit 

within the volcanic rocks of the McCullough 

Range (Anderson, 1977; Bell and Smith, 

1980; and Boland, 1996). 40Ar/39Ar dates of 

the unit, 40 km south of the Las Vegas quadrangle in the McCullough Pass area, range 

from 13.1 to 14.0 Ma (Faulds and others, 

1999). Thickness is about 300 m 

Patsy Mine Volcanics (Miocene)—Moderately 

resistant, tan, red, gray, and yellow, amygdaloidal, mostly crystal-poor andesite and 

basaltic andesite lavas and flow breccia and 

interbedded tan and light-green volcanic 

mudflow breccia, fluvial conglomerate, and 

fluvial sandstone. Generally propylitically 

altered. Flows contain 5–20 percent phenocrysts of plagioclase, hornblende, pyroxene, 

and olivine. Exposed only in McCullough 

Range in southeastern part of the quadrangle. Correlated by Bingler (1977) with 

the Patsy Mine Volcanics of the Eldorado 

Mountains (Anderson, 1977), which are 

about 15 km southeast of the quadrangle. In 

nearby areas to the south and east, mapped 

as informal altered andesite unit within the 

volcanic rocks of the McCullough Range 

(Anderson, 1977; Bell and Smith, 1980; 

Boland, 1996). An 40Ar/39Ar date of about 

15.6 Ma was obtained from the unit 40 km 

south of the Las Vegas quadrangle in the 

McCullough Pass area (Faulds and others, 

1999). Thickness is about 200 m 
J�kc 

�m 

�mu 

�ml 

12 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

Mesozoic, Paleozoic, and Proterozoic Rocks �mu 

[Abbreviations for major Mesozoic thrust plates in the quadrangle: Wheeler Pass, WPT; Gass Peak, GPT; Valley, VT; Dry 

Lake, DLT; Lee Canyon/Macks Canyon, LMCT; Deer Creek, 

DCT; Keystone, KT; and Bird Spring, BST. For example, the 

Wheeler Pass thrust plate (WPT) occupies the hanging wall of 

the Wheeler Pass thrust fault.] 

Ja Aztec Sandstone (Jurassic)—Exposed along the 

east flank of the Spring Mountains in the �ml BST. Unit is tan, pale-red, and pink quartzose sandstone containing well-sorted, subrounded quartz grains, hematite and calcite 

cement, and tabular-planar crossbeds as 

great as 10 m thick. Top of unit is truncated 

by the Keystone thrust fault. The Aztec 

Sandstone is conformable with the underlying Jurassic and Triassic units. Maximum 

exposed thickness is about 650 m 

J�kc Kayenta Formation (Lower Jurassic), Moenave 

Formation (Lower Jurassic), and Chinle 

Formation (Upper Triassic), undivided— 

Exposed along the east flank of the Spring 

Mountains (BST). Upper part includes 

about 180 m of crossbedded sandstone and 

siltstone equivalent to the Lower Jurassic 

Kayenta and Moenave Formations of the 

Colorado Plateau region. Chinle Formation 

(Upper Triassic) is about 280 m thick and 

consists of sandstone, siltstone, shale, and 

local conglomeratic lenses. Sandstone and 

siltstone is red and yellowish brown and is 

generally thin bedded. Shale is red, olive 

green, and brown. At base, includes 10–30-

m-thick Shinarump Member, which consists 

of conglomerate, pebbly carbonate, and 

calcareous pebbly sandstone; conglomerate 

clasts are composed mainly of distinctive 

reddish-brown, light-brown, and yellowishbrown chert. Unit forms slope beneath the 

massive cliffs of the overlying Aztec Sandstone. Thickness of map unit is about 460 m 

Moenkopi Formation (Middle? and Lower 

Triassic)—Exposed in the Blue Diamond 

area in the low foothills of the easternmost 

Spring Mountains (BST) and in the eastern Pkt 

Spring Mountains (KT). Consists of upper 

red member and undivided lower unit. Combined thickness is about 585 m 

�m Moenkopi Formation, undivided (Middle? and 

Lower Triassic)—Includes upper red member and lower unit combined, and is shown 

in cross section only (cross section C–C’, 

map sheet 1) 

Upper red member (Middle? and Lower 

Triassic)—This informal member consists 

of two parts. The uppermost part weathers light gray to grayish white and contains 

interbedded claystone, dolostone, limestone, 

siltstone, and shale. Lower part weathers 

mostly reddish brown and contains siltstone, 

sandstone, claystone, mudstone, and some 

thin beds of limestone and gypsum. Some 

siltstone beds contain ripple marks on bedding planes. About 335 m thick 

Lower unit (Lower Triassic)—Includes from top 

to base, Virgin Limestone Member, lower 

red member, and Timpoweap Conglomerate Member. The Virgin Limestone Member 

contains thin-bedded, medium-gray to yellowish-brown fossiliferous limestone, sandy 

limestone, dolostone, calcareous sandstone, 

and siltstone. Limestone is commonly 

nodular due to burrowing, and contains 

some thin discontinuous chert layers. Fossils 

include brachiopods, gastropods, pelecypods, and pelmatozoan ossicles identified 

as Pentacrinus (Hewett, 1931). Conodonts 

in the Virgin Limestone Member are Early 

Triassic in age (Harris and others, in press). 

Lower red member consists of reddishbrown siltstone, sandstone, and shale, and 

thin beds of light-gray, yellowish-gray, and 

moderate-orange-pink dolomitic limestone, 

sandy limestone, and gypsiferous siltstone. 

Timpoweap Conglomerate Member is 

dominantly conglomerate with minor pebbly 

carbonate and calcareous sandstone beds. 

Conglomerate clasts are composed mostly 

of subrounded to subangular gray, pink, and 

pale-red limestone and dark-brown chert 

in a carbonate matrix, and beds are tabular 

and graded. Upper contact of Timpoweap is 

gradational with lower red member. At base, 

Timpoweap generally rests unconformably 

on Kaibab Formation (Lower Permian). In 

parts of the Mountain Springs, Lost Cabin 

Spring, and Griffith Peak quadrangles, however, the Timpoweap lies disconformably on 

Lower Permian redbed unit. Thickness of 

lower member is about 250 m 

Kaibab and Toroweap Formations, undivided 

(Lower Permian)—Best exposures are in 

the Blue Diamond area (BST); also exposed 

in the eastern Spring Mountains (KT) and at 

Frenchman Mountain. The Kaibab Formation consists of the Harrisburg and Fossil 

Mountain Members, recognized in Blue 

Diamond area (Carr, 1992; Carr and 

McDonnell-Canan, 1992). Harrisburg 
13 

Member is about 75 m thick and consists 

of light-gray micritic limestone and dolostone and bedded gypsum (individual beds 

as great as 1 m thick); includes yellowishgray-weathering limestone and dolomitic 

limestone and gypsiferous red shale and 

claystone; pectinid valves and pelmatozoan PMc 

ossicles observed; chert common in nodules and layers. Fossil Mountain Member 

is about 75 m thick and consists of mostly 

medium-gray, thick-bedded fossiliferous 

limestone and dolomitic limestone with 

brown-weathering chert layers and nodules; 

includes brachiopods, bryozoans, and pelmatozoan columnals. Conodonts collected 

in the Blue Diamond area indicate the age of 

the Kaibab Formation is late Early Permian, as old as late Kungurian and as young 

as Roadian (Harris and others, in press). 

The Toroweap Formation consists of the 

Woods Ranch, Brady Canyon, and Seligman 

Members (Carr, 1992; Carr and McDonnell-Canan, 1992). Woods Ranch Member 

is about 30 m thick and consists of lightreddish-gray-weathering interbedded shale, 

dolostone, siltstone, and some gypsum. 

Brady Canyon Member is about 67 m thick 

and consists of medium-gray to pinkish-gray 

limestone and dolomitic limestone with 

common chert nodules and layers. Seligman Member is 60 m thick and consists of 

tan to red interbedded siltstone, sandstone, 

and gypsum. Some dolostone and dolomitic 

sandstone in upper part. Unit forms ledgy to 

massive cliffs. Combined thickness of the 

two formations is about 250–300 m in the 

Blue Diamond area, 0–170 m in the eastern Spring Mountains, and about 300 m at 

Frenchman Mountain 

Pr Lower Permian redbeds—Exposed in the Blue 

Diamond area along low hills that make up 

the easternmost Spring Mountains (BST), 

the eastern Spring Mountains (KT), and 

at Frenchman Mountain. Consists of red, 

white, and yellowish-orange crossbedded 

sandstone and some interbedded siltstone 

and shale; locally gypsiferous. Contact with 

overlying Toroweap Formation is sharp 

and disconformable. In Lovell Canyon area 

(KT), unit is reported to be gradational with Pbu 

underlying Bird Spring Formation (Ho, 

1990). In the Blue Diamond area (Carr 

and others, 2000; McDonnell-Canan and 

others, 2000), unit was correlated with the 

Hermit Formation of the Grand Canyon and 

Colorado Plateau regions. Unit forms ledgy 

Geology of the Las Vegas 30′ × 60′ Quadrangle 

slope. Unit about 300 m (Carr and others, 

2000; McDonnell-Canan and others, 2000) 

to 580 m (Bohannon and Morris, 1983) thick 

in Blue Diamond area and about 400 m thick 

at Frenchman Mountain (Matti and others, 

1993) 

Callville Limestone (Permian, Pennsylvanian, 

and Mississippian)—Unit mapped only at 

Frenchman Mountain in the eastern part of 

the map area. Includes stratigraphic equivalents of the Pakoon Formation (Lower Permian) of McNair (1951) and the Indian Springs 

Formation (Upper Mississippian) of Webster 

and Lane (1967). Upper 270 m (Pakoon Formation equivalent) is generally medium- to 

light-gray, thin-bedded limestone, dolostone, 

and gypsum (Matti and others, 1993). Middle 

220–297 m (Callville Limestone, Pennsylvanian) is thin- to thick-bedded, medium- to 

dark-gray, fossiliferous, finely to mediumcrystalline limestone and dolostone (Langenheim and Webster, 1979; Matti and others, 

1993). Also includes layers and nodules of 

dusky-yellowish-brown-weathering chert; 

brown-weathering sandstone beds common in upper half. The Upper Mississippian 

Indian Springs Formation is about 25–30 m 

thick at Frenchman Mountain; consists of 

thin-bedded sandstone, siltstone, limestone, 

dolostone, and intraclastic limestone conglomerate (Langenheim and Webster, 1979). 

Contact between Indian Springs Formation 

and underlying rocks of Yellowpine Limestone of Monte Cristo Group is sharp and 

disconformable. Maximum thickness of unit 

about 600 m (Matti and others, 1993) 

Bird Spring Formation (Lower Permian to 

Upper Mississippian)—Bird Spring 

Formation subdivided into upper and lower 

informal members (see map units Pbu and 

PMbl below) in Las Vegas Range (VT and 

DLT) and in Lucky Strike and Lee Canyon 

areas in the northern Spring Mountains (DCT 

and LMCT). Elsewhere in the quadrangle, 

the Bird Spring Formation was mapped undivided (PMb). Bird Spring Formation map 

unit includes Indian Springs Formation of 

Webster and Lane (1967) at base 

Upper member (Leonardian and uppermost 

Wolfcampian)—Upper part of member 

mostly Leonardian in age and about 900 

m thick; consists of cyclic thin- to thickbedded limestone, silty limestone, and 

dolostone that contain layers and nodules 

of dusky-yellowish-brown chert as 
14 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

well as beds of calcareous siltstone and discontinuous layers and nodules of darkbrown-weathering sandstone. Fossils gray, dusky-yellowish-brown-weathering 

include brachiopods, bryozoans, corals, chert; locally, some beds in member contain 

fusulinids, gastropods, and pelmatozoan more than 50 percent chert. Member also 

fragments. Very latest Wolfcampian contains medium-gray to yellowish-gray 

conodonts, including Sweetognathus dolostone. Abundant fossils include brawhitei, are in base of upper part (Harris chiopods, bryozoans, corals (Syringopora, 

and others, in press), but are succeeded by Chaetetes, and solitary rugosans), and 

Leonardian fusulinids such as Schwagerina pelmatozoan columnals; fusulinids in upper 

crassitextoria in Lee Canyon area (Rich, part. Member about 1,045 m thick in south1961). Ho (1990) reported even younger ern Las Vegas Range (Lundstrom and others, 

Leonardian fusulinids, such as Parafusulina 1998) and 1,010 m in Lee Canyon area in 

apiculata, in the Lee Canyon area. Lower 

part of member forms regional marker 

within the thick Bird Spring Formation, 

PMb 

the Spring Mountains (Rich, 1963) 

Bird Spring Formation, undivided (Lower Permian to Upper Mississippian)—Consists of 

and it was described by Rich (1961, 1963) yellowish-gray, light-brown to gray, thin- to 

and Page (1992, 1993, 1998). Consists thick-bedded, bioclastic, and arenaceous 

mainly of alternating thinly laminated beds limestone and dolostone. Also contains 

of limestone, silty limestone, mudstone, brown sandstone, gray to yellowish-gray 

calcareous siltstone, and chert. Limestone calcareous siltstone, pale-red to light-gray 

and mudstone are dark gray to olive gray; shale, and layers and nodules of dusky-yelrocks are organic rich and locally pyritic. lowish-brown chert; some beds have greater 

Silty limestone and calcareous siltstone than 50 percent chert. Unit contains abunlaminae weather yellowish gray to moderate dant macrofossils; fusulinids are found in 

brown. Some beds are turbiditic and much of the upper part of the Bird Spring 

convolutedly bedded. Macrofossils are Formation. Macrofossils include brachiogenerally sparse and include brachiopods, pods, bryozoans, colonial and solitary 

echinoderms, ichthyoliths, and trilobites; corals, gastropods, pelecypods, pelmatozoan 

microfossils are more common including columnals, sponges, and trilobites. Unit 

locally abundant sponge spicules, less forms ledgy slopes and, locally, massive 

abundant conodonts, and poorly preserved cliffs. Maximum exposed thickness is 2,355 

radiolarians. Marker unit contains submarine m in Lee Canyon area (LMCT) in the Spring 

debris-flow conglomerate in channels and Mountains (Ho, 1990; Rich, 1963); maxisheets. Clasts in conglomerate are bioclastic mum exposed thickness in southern Las 

and contain shallow-water forms such Vegas Range (VT) is about 2,500 m; total 

as corals, fusulinids, and pelmatozoan thickness in Lovell Canyon area (KT) is 

fragments. Phosphatic concretions 0.5–4 cm 

in diameter locally present at base of marker 

1,300 m (Ho, 1990) 

Indian Springs Formation (Chesterian)—Conunit in Lucky Strike Canyon area and sists of interbedded bioclastic limestone, 

southernmost Las Vegas Range; concretions shale, and quartzite. Limestone is medium 

commonly cored by fish fragments. Marker gray, grayish red, and moderate yellowish 

unit thickens west-northwestward in brown, mostly thin bedded, bioclastic, finely 

southern Las Vegas Range, from 350 m to to coarsely crystalline. Shale is mostly in 

600 m (Lundstrom and others, 1998). Rich lower part of formation and is dusky red to 

(1963) reported the marker unit to be 666 m grayish black. Quartzite is olive gray to light 

thick in Lee Canyon area. Total maximum gray and composed of fine, subrounded, 

PMbl 

thickness of upper member is about 1,500 m 

Lower member (upper Wolfcampian to 

Chesterian)—Limestone, dolostone, chert, 

and moderately sorted quartz grains. Fossils 

include the biostratigraphically diagnostic 

Chesterian brachiopod Rhipidomella nesiltstone, silty limestone, sandstone, and vadensis, other spiriferid and productid brashale. Limestone is medium gray, light gray, chiopods, bryozoans, solitary rugose corals, 

and yellowish gray. Commonly arenaceous and pelmatozoan fragments. Stigmaria is 

and bioclastic; some beds oolitic. Limestone in sandy and silty beds at base of formation 

finely to coarsely crystalline, thin to thick in Gass Peak area (VT). Contact between 

bedded; some beds contain planar lamina- Indian Springs and overlying Bird Spring 

tions and trough crossbeds. Abundant Formations apparently gradational in 
15 

quadrangle; unit disconformably overlies 

Battleship Wash Formation or units of the 

Monte Cristo Group. Indian Springs Formation forms slope and is 52 m thick at type 

section at Indian Springs (WPT) (Webster, 

1969), just north of quadrangle; about 60 

m thick in southern Las Vegas Range (VT) 

(Lundstrom and others, 1998); 8-15 m thick 

in La Madre Mountain area (KT) (Axen, 

1985) 

Mm Monte Cristo Group (Upper and Lower Mississippian)—Includes following formations, 

from top to bottom: Yellowpine Limestone, 

Bullion Limestone, Anchor Limestone, and 

Dawn Limestone; series and stage assignments based chiefly on conodont data (fig. 2; 

Harris and others, in press; Stevens and others, 1996). Map unit also includes Battleship 

Wash Formation of Webster (1969) overlying the Monte Cristo Group in southern 

Las Vegas Range (Lundstrom and others, 

1998) and Lee Canyon area (LMCT) in 

the central Spring Mountains (Langenheim 

and Webster, 1979). Monte Cristo Group 

(exclusive of Battleship Wash Formation) 

about 304 m thick in northwestern Spring 

Mountains (WPT) (Vincelette, 1964), 300 

m in La Madre Mountain area in the eastern 

Spring Mountains (KT) (Axen, 1985), about 

253 m at Mountain Springs Pass (KT) (Langenheim and Webster, 1979), 400–480 m in 

southern Las Vegas Range (VT) (Lundstrom 

and others, 1998), and 242 m at Frenchman 

Mountain (Matti and others, 1993) 

Battleship Wash Formation (lower Chesterian 

and upper Meramecian)—Exposed in 

southern Las Vegas Range (VT); a thin 

continuation of this unit was identified by 

Webster (1969) in Lee Canyon area in the 

Spring Mountains. Formation composed of 

limestone and minor quartzite. Limestone is 

medium dark gray to light olive gray, mostly 

coarsely crystalline, and thin to massive 

bedded. Basal part of formation consists of 

thin beds of quartzite and sandy limestone. 

Fossils in formation include solitary corals 

and spiriferid brachiopods. In southern Las 

Vegas Range, conodonts indicate that the 

formation is no older than late Meramecian and no younger than early Chesterian 

(Harris and others, in press). Upper contact 

usually covered, but where exposed, shale or 

quartzite of Indian Springs Formation disconformably(?) overlies bioclastic limestone 

of Battleship Wash Formation. Forms ledgy 

Geology of the Las Vegas 30′ × 60′ Quadrangle 

cliffs. Formation 42 m thick in southern Las 

Vegas Range 

Yellowpine Limestone (Meramecian and upper 

Osagean)—Medium-dark-gray to light-olive-gray, medium-crystalline, and thin- to 

thick-bedded limestone; contains sparse 

nodules of gray to brown chert. Formation 

distinct from other units of Monte Cristo 

Group in quadrangle because it contains unusually large solitary rugose corals (as much 

as 25 cm long), especially near its top. Also 

contains Syringopora and Lithostrotionella 

colonial corals, pelmatozoan columnals, and 

middle to late Osagean conodont Eotaphrus 

burlingtonensis at its base in southern Las 

Vegas Range (Harris and others, in press) 

and late Meramecian conodont Cavusgnathus unicornis near its top in Lee Canyon 

area in the Spring Mountains (Stevens and 

others, 1996). Only 12 m of unit exposed 

just north of quadrangle in Indian Springs 

area (WPT) (Stevens and others, 1996); unit 

45 m thick at Mountain Springs Pass (KT) 

(Langenheim and Webster, 1979) and 77–80 

m thick in southern Las Vegas Range (VT) 

(Lundstrom and others, 1998) 

Bullion Limestone (lower Meramecian to middle 

Osagean)—Medium-light-gray encrinitic 

wackestone to grainstone typifies Bullion 

Limestone. Contains some beds of duskyyellowish-brown-weathering chert. Fossils 

include abundant pelmatozoan ossicles 

and some brachiopods and solitary rugose 

corals. Conodonts indicate much of the 

formation is middle Osagean (anchoralislatus Zone); Stevens and others (1996) 

reported Eotaphrus burlingtonensis and 

bactrognathids near base in Lee Canyon in 

the Spring Mountains, and Harris and others 

(in press) found E. burlingtonensis and 

Hindeodella segaformis s.f. (redeposited?) 

at top in southern Las Vegas Range. Contact 

between Bullion Limestone and Yellowpine 

Limestone gradational, and it is placed at 

base of limestone sequence that is darker 

gray, thinner bedded, and richer in solitary 

rugose corals than underlying beds of 

Bullion Limestone. Forms massive cliffs. 

Bullion Limestone is about 60 m thick in 

Indian Springs area (WPT) (Stevens and 

others, 1996), 118 m at Mountain Springs 

Pass (KT) (Langenheim and Webster, 1979), 

and 100–180 m in southern Las Vegas 

Range (VT) (Lundstrom and others, 1998) 

Anchor Limestone (Osagean)—Alternating thinbedded limestone and chert. Limestone is 
16 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

medium gray to light olive gray, partly bur- limestone and quartzite. Sandy limestone is 

rowed, and fine to medium crystalline. Chert laminated, medium gray to yellowish brown, 

is dark gray and weathers moderate orange and contains scattered planispiral gastroto light brown. Anchor Limestone contains pods, pelmatozoan ossicles, and subrounded 

brachiopods, solitary and syringoporid cor­ intraclasts. Some poorly exposed laminated 

als, and pelmatozoan fragments. A 5–6-m- beds of pinkish-gray argillaceous shaly limethick bluish-gray, bedded spiculitic chert stone are in upper part. Unit also includes 

marks base of Anchor in northwest Spring brown quartzite beds that contain SkoliMountains (WPT). Anchor Limestone forms thus burrows and laminated beds of sandy 

ledgy cliffs and is 246 m thick in Indian carbonate and dolomitic sandstone. Unit 

Springs area (WPT), just north of Las Vegas produces Kinderhookian conodonts as well 

quadrangle (Stevens and others, 1996), 67 as reworked middle and early Famennian 

m at Mountain Springs Pass, 80 m at Kyle conodonts (Harris and others, in press; F.G. 

Canyon (KT) (Langenheim and Webster, Poole, written commun., 1997). Forms slope 

1979), and 140-180 m in southern Las Vegas beneath cliff-forming Dawn Limestone; conRange (VT) (Lundstrom and others, 1998) 

Dawn Limestone (lower Osagean and Kinderhookian)—Dark-gray, brownish-gray, and 

tact between these two units is sharp. Lower 

contact with Guilmette Formation is also 

sharp and disconformable. Unit 30–40 m 

light-olive-gray, thin-bedded to massive, 

bioclastic, oolitic limestone. Commonly 

contains elongate nodules and discontinuous 

thick in northwest Spring Mountains (WPT) 

Crystal Pass Limestone (lower Kinderhookian 

and upper Famennian)—Light-gray, thinlayers of brown chert. Abundant macrofos­ to thick- and massive-bedded, planar- and 

sils include brachiopods, bryozoans, solitary locally cross-laminated micrite to mudstone 

rugose corals, Lithostrotionella sp. corals, containing sparse gastropods, intraclasts, 

gastropods, and pelmatozoan ossicles. Con- and burrow mottling. At most locations in 

tact with underlying Crystal Pass Limestone Las Vegas quadrangle, upper part of Crystal 

gradational through 2 m in Kyle Canyon Pass contains the conodonts Siphonodella 

area (KT). A disconformity separates the praesulcata and (or) Si. sulcata (fig. 2) 

Dawn Limestone and the underlying Guil­ indicating a very earliest or early Kindermette(?) Formation in the Las Vegas Range hookian and (or) very latest Famennian 

(VT), and a disconformity also separates the age. Crystal Pass Limestone forms massive 

Dawn Limestone and the underlying cliffs; it is about 70 m thick in Lucky Strike 

Narrow Canyon Formation in northwest Canyon area (DCT) (Bereskin, 1982), 75 m 

Spring Mountains (WPT) (fig. 2). Dawn in Kyle Canyon area (KT), 64 m at MounLimestone is 75 m thick in Indian Springs tain Springs Pass (KT) (Langenheim and 

area, just north of Las Vegas quadrangle Webster, 1979), and 60 m at Frenchman 

(WPT) (Stevens and others, 1996), 24 to 

50 m in Mountain Springs Pass and Kyle 

Canyon areas (KT) (Langenheim and WebMountain (Rowland and others, 1990) 

Guilmette Formation (Upper and Middle 

 Devonian)—Equivalent to Devils Gate 

ster, 1979), and 70 m in southern Las Vegas Limestone mapped in northwestern Spring 

MDu 

Range (VT) (Lundstrom and others, 1998) 

Mississippian and Devonian rocks, undivided 

(Lower Mississippian and Upper and 

Middle Devonian)—In WPT and GPT, 

Mountains (WPT) and in part to Sultan Limestone mapped in central Spring 

Mountains (LMCT and DCT) (Burchfiel 

and others, 1974). Medium-dark-gray to 

unit includes Lower Mississippian Narrow light-brownish-gray dolostone and limestone 

Canyon Formation and underlying Upper and minor dolomitic quartzite. Limestone 

and Middle Devonian Guilmette Forma- and dolostone beds are cyclic and commonly 

tion. As exposed in DCT and LMCT, unit laminated and burrow-mottled. Fossils 

includes Mississippian and Devonian Crys­ include brachiopods, solitary and colonial 

tal Pass Limestone and underlying Upper corals, gastropods, and Amphipora sp. as 

and Middle Devonian Guilmette Forma- well as other stromatoporoids. Upper part of 

tion. In eastern Spring Mountains (KT) and formation contains several brown dolomitic 

at Frenchman Mountain, includes Sultan quartzite beds. Middle part of formation 

Limestone 

Narrow Canyon Formation (middle Kinderhookian)—Mostly burrowed, sandy 

mostly cliff-forming, dark-gray dolostone 

with abundant stromatoporoids. Lower part 

of formation forms ledgy slope beneath 
Geology of the Las Vegas 30′ × 60′ Quadrangle 17 

cliff-forming dolostone and contains interbeds of yellowish-gray and light-gray doloStringocephalus sp. is near the top of formation in northwest Spring Mountains. Base 

micrite. Lower part is “yellow bed” of the of the formation consists of a 10-m-thick, 

Guilmette (Tschanz and Pampeyan, 1970) 

and marks the base of the Guilmette Formabrown dolomitic quartzite unit; quartz 

grains are fine to very fine, subrounded, 

tion in many parts of Clark and Lincoln and moderately well sorted. Forms ledgy 

Counties and elsewhere in Nevada. Guilmette Formation is 490 m thick in southcliffs. About 190 m thick in southern Sheep 

Range (Maldonado and Schmidt, 1991) and 

ern Sheep Range (GPT) (Maldonado and 120–130 m thick in northwest Spring MounSchmidt, 1991), 365-450 m in northwestern 

Spring Mountains (WPT), and 380-420 m in 

central Spring Mountains (LMCT and DCT) 

Sultan Limestone (Lower Mississippian to 

Middle Devonian)—In eastern Spring 

Du 

tains (Vincelette, 1964) 

Devonian rocks, undivided (Middle and Lower 

Devonian)—Unit mapped in central Spring 

Mountains (LMCT and DCT); includes 

strata below the basal yellow-bed sequence 

Mountains (KT) and at Frenchman Moun­ of Guilmette Formation (Upper and Middle 

tain, unit follows definition of Hewett 

(1931). Sultan Limestone mostly equivalent 

Devonian) and above the Ely Springs 

Dolomite (Upper Ordovician). In LMCT, 

to but thinner than Guilmette Formation; upper part is about 60 m thick and consists 

includes, from top to base, Crystal Pass, 

Valentine, and Ironside Members. These 

members form a dark band across west face 

of alternating light- to dark-gray burrowed 

dolostone containing Amphipora sp., other 

stromatoporoids, and some brachiopods; 

of Frenchman Mountain composed chiefly probably equivalent with part of Simonson 

of dark-gray to brown dolostone with 

numerous stromatoporoids (Matti and 

Dolomite. Lower part in LMCT is about 20 

m thick and consists of light-gray-weatherothers, 1993); these rocks lie unconformably ing, cryptalgal laminated and thin-bedded, 

on Nopah Formation (Upper Cambrian). 

Thickness of all three members at Frenchmicritic to finely saccharoidal dolostone 

containing discontinuous layers and nodules 

man Mountain is 190 m (Matti and others, of chert; no macrofossils noted. These lower 

1993). In eastern Spring Mountains (KT), 

Crystal Pass Member consists of light-gray 

rocks are stratigraphically and lithologically 

similar to rocks that lie above the Silurian 

micritic limestone. Valentine and Ironside Laketown Dolomite and which we correlate 

Members consist of cycles of thin-bedded, dark-gray limestone and thick-bedded, 

with the Lower Devonian Sevy Dolomite 

in the northwest Spring Mountains (WPT) 

light-gray dolostone. Valentine Member, like and southern Sheep Range (GPT). In Lucky 

upper part of Guilmette Formation, contains 

several brown dolomitic quartzite beds 

Strike Canyon area (DCT), upper and 

lower parts are about 50 and 10 m thick, 

generally less than 1 m thick. Sultan is about respectively. Map unit is 60 (DCT) to 80 m 

Dsi 

175-200 m thick in eastern Spring Mountains (Axen, 1985) 

Simonson Dolomite (Middle Devonian)—Mapped 

DSu 

(LMCT) thick 

Devonian and Silurian rocks undivided (Lower 

Devonian and Lower Silurian)—Exposed 

only in northwest Spring Mountains (WPT) 

and southern Sheep Range (GPT). Upper 

part of Mountain Springs Formation (DOm) 

in southern Sheep Range (GPT) and 

northwestern Spring Mountains (WPT). Unit 

includes Silurian Laketown Dolomite and 

in eastern Spring Mountains (KT) and much 

of the Devonian rocks, undivided (Du), in 

central Spring Mountains (DCT and LMCT) 

overlying Lower Devonian strata. In Spring 

Mountains, upper 30 m probably include 

Lower Devonian strata likely equivalent 

include partial equivalents of the Simon- to part of Sevy Dolomite; uppermost 17 

son. Simonson also equivalent to Nevada 

Formation of Burchfiel and others (1974). 

m of these strata are light-brownish-grayweathering, medium- to medium-dark-gray, 

Unit consists mostly of alternating beds of laminated sandy dolostone containing 

medium- to dark-gray and light-olive-gray, 

slightly argillaceous, finely crystalline, 

anastomosing, medium-gray chert nodules 

and lenses. These rocks are equivalent to 

thin- to thick-bedded dolostone. Fossils Lower Devonian cherty argillaceous unit, 

mainly Amphipora sp. as well as bulbous 

and tabular stromatoporoids; the Givetian 

originally defined as an informal member 

of the Sevy Dolomite by Osmond (1962), 

(late Middle Devonian) brachiopod but later redefined by Johnson and others 
18 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

(1989) as a separate unit. Contact with member, about 60 m thick, is restricted 

overlying basal quartzite of Simonson to Middle Devonian as only early Middle 

Dolomite is sharp; as much as 3 m of relief Devonian (Eifelian) conodonts were found 

observed locally along unconformity. (conodonts identified by Miller and ZilLower 13 m of Sevy Dolomite equivalent insky, 1981, as Early Devonian in age are 

strata is light-gray, thin- to thick-bedded, Middle Devonian forms), and it is equivafinely saccharoidal dolostone containing lent to part of Simonson Dolomite. Upper 

cryptalgal laminations, fenestral fabric, member characterized by cycles of dark- to 

and rare pelmatozoan ossicles. Dolostone medium-light-gray and brownish-gray, buris partly vuggy and burrow mottled, and row-mottled, finely to coarsely saccharoidal 

contains scattered intraclasts. Beneath dolostone grading up into light- to very light 

Lower Devonian strata, Laketown Dolomite gray and pinkish-gray, thin- to thick-bedded, 

displays its typical tri-part character that is laminated, finely saccharoidal dolostone to 

widely recognized in eastern Great Basin: dolomudstone with scattered rip-up clasts. 

upper dark, middle light, and lower dark Lower part of cycle locally contains disardolostone parts. Upper dark part is medium­ ticulated, thick-shelled pentamerid brachiodark-gray, mostly finely saccharoidal pods and stromatoporoids and, more rarely, 

dolostone containing brachiopods, halysitid bryozoan, coral, and pelmatozoan fragments. 

corals, and pelmatozoan ossicles. Middle Middle member of Late Ordovician age, 

part is light-gray to light-orange-gray, 6–8 m thick, disconformably overlies and 

massive-bedded dolostone that contains underlies lower and upper members, and is 

silicified pentamerid brachiopods. Lower equivalent to part of Ely Springs Dolomite. 

part is gray, vuggy and burrowed, finely Beds are poorly exposed, light-pinkish-graysaccharoidal dolostone with brachiopods, weathering, brownish-gray, medium-bedded, 

corals, pelmatozoan ossicles, and burrow-mottled to even-bedded, bioclastic 

stromatoporoids; also contains some beds (mainly pelmatozoans), moderately sacand nodules of dark-brown-weathering charoidal dolopackstone. Lower member, 

chert. The Laketown Dolomite is Early about 225 m thick, contains abundant and 

Silurian in age (at least late Llandoverian diverse Early Ordovician conodont assemand early Wenlockian) based on conodonts blages in upper 175 m (this report, fig. 2, 

collected in Corn Creek Springs quadrangle, section 6; Harris and others, in press) but 

southern Sheep Range (fig. 2, section 1). lower 50 m paleontologically unproductive 

Forms ledge-and-slope topography. Map and may include rocks of Late Cambrian 

unit about 280 m thick in southern Sheep age; thus lower member equivalent to part 

Range and 275 m in northwest Spring of Pogonip Group. Although considerably 

DOm 

Mountains 

Mountain Springs Formation (Middle Devonian, 

Upper Ordovician, and Lower Ordovician)—Exposed only in eastern Spring 

thinner than the Lower Ordovician part of 

Pogonip Group in southern Sheep Range 

(GPT), many of the low-latitude, shallowwater Laurentian province (North American 

Mountains (KT). Defined by Gans (1974) Midcontinent conodont province of older usand subsequently modified by Miller and age) Lower Ordovician conodont zones are 

Zilinsky (1981) based on conodont recognized in the La Madre Spring section 

biostratigraphy. Hintzman (1983) and (fig. 2, section 6). Lower member consists of 

Cooper (1987), using Miller and Zilinsky’s light-gray to pinkish-gray, platy to massiverevisions, subdivided the Mountain Springs bedded, mostly burrowed, finely to coarsely 

Formation into three informal members: (1) saccharoidal, dolomitized wackestone, 

upper member of Middle and Early Devoni­ packstone, and mudstone with common 

an age; (2) middle member of Late Ordovi- orange-weathering argillaceous partings and 

cian age; and (3) lower member of Middle some chert. Rocks contain stromatolites, 

and Early Ordovician age. New closely cryptalgal lamination, intraclasts, and fenesspaced conodont samples from Miller and tral fabric common in middle and lower part 

Zilinsky’s (1981) measured section, of member; locally contains oncoids and 

west-southwest of La Madre Spring, further intraclasts. Notable karstification features 

refines age assignments. Near La Madre including solution surfaces, collapse breccia 

Spring (fig. 2, section 6), Mountain Springs beds (prominent collapse breccia containing 

Formation about 295 m thick. Upper associated sinkhole-fill deposits that include 
O�p 

�n 

Geology of the Las Vegas 30′ × 60′ Quadrangle 19 

rounded chert clasts averaging 2 cm at about 700 m in northwestern Spring Moun168–174 m below top of formation). Macro­ tains (WPT) (Vincelette, 1964), and about 

fossils include large gastropods (averaging 2 360–400 m in central Spring Mountains 

cm in diameter) and sponges. Thickness and (LMCT and DCT) 

age limits of informal members will likely Antelope Valley Limestone (Middle and Lower 

vary from place to place in KT because Ordovician)—Dark-yellowish-orange- to 

older and (or) younger beds may have been dark-yellowish-brown, medium-dark- to 

differentially removed or preserved along medium-gray, massive- to medium-bedded, 

the major unconformities that separate these coarse- to fine-grained, bioclastic limestone 

members with locally abundant ooids, oncoids, and 

Oes Ely Springs Dolomite (Upper Ordovician)—Up- intraclasts; beds are commonly burrow 

per 20–25 m is generally light-olive-gray, mottled to churn burrowed. Also contains 

thin- to thick-bedded, burrow-mottled, vari- dark-yellow-orange silty to argillaceous 

ably bioclastic, finely saccharoidal dolostone laminae and nodules and layers of brown 

containing ooids, oncoids, and pelmatozoan chert. Fossils include brachiopods, orthoossicles. Most of Ely Springs Dolomite is cone cephalopods, gastropods (Palliseria sp. 

medium-dark-gray, burrow-mottled, irregu­ and Maclurites sp.), pelecypods, pelmatozolarly thin- to thick-bedded with common an columnals, sponges (Receptaculites sp.), 

planar laminations, finely saccharoidal do- and trilobites. Rocks in the upper part of 

lostone; fossils include brachiopods, colonial formation contain shallowing-upward cycles 

(Halysites sp. and favositids) and solitary of bioclastic-oolitic packstone/grainstone 

corals, gastropods, pelmatozoan ossicles, grading up into bioturbated lime mudstone, 

and stromatoporoids (including amphipo­ laminated lime mudstone, and, finally, mudrids). Lower contact with Eureka Quartzite cracked, cryptalgal laminated, argillaceous 

is disconformity; lower Middle Ordovician dolomitic lime mudstone. Ooids, oncoids, 

quartzite of Eureka is overlain by Upper and intraclast beds (shoal facies) typify 

Ordovician dolostone of Ely Springs Dolo- lower 370 m of Antelope Valley in southern 

mite. Formation is 150 m thick in southern Sheep Range. Forms cliffs 

Sheep Range (GPT), 120 m in northwest Goodwin Limestone (Lower Ordovician and 

Spring Mountains (WPT) (Vincellete, 1964), Upper Cambrian)—Medium-gray to gray76–107 m in LMCT (Vincellete, 1964), and ish-orange-weathering, thick-bedded, bur90 m thick in DCT row-mottled to laminated, bioclastic (chiefly 

Oe Eureka Quartzite (Middle Ordovician)—Light- trilobites and brachiopods), mostly medium 

to moderate-brown-weathering, white to grained grainstone and packstone that 

light-brown, thin- to thick-bedded, fine- to characteristically contain brown-weathering 

medium-grained, rounded to subrounded, chert layers and nodules, moderate-reddishmoderately well sorted quartzite and friable orange silty partings, and intraclastic beds; 

sandstone. Contains tabular planar commonly oolitic. Samples of the Goodwin 

crossbeds and, less commonly, trough Limestone from several localities in the Las 

crossbeds; crossbed sets average about 0.4 Vegas quadrangle have produced very late, 

m thick; also contains Skolithus burrows. but not latest Cambrian conodonts from its 

Minor sandy carbonate beds. Local col- lower beds (fig. 2, sections 1 and 2; Harris 

lophane nodules in Lee Canyon and Lucky and others, in press). Contact with underlyStrike Canyon areas (Ross, 1964). Forms ing Nopah Formation is sharp 

rounded cliffs. Unit 6 m thick in Lucky �n Nopah Formation (Upper Cambrian)— 

Strike Canyon area (DCT) and as much as Dolostone, dark- to light-gray, mottled, 

120 m thick in both northwestern Spring medium-crystalline and thin- to thickMountains (WPT) (Vincellete, 1964) and bedded with discontinuous layers and 

southern Sheep Range (GPT) (Maldonado nodules of dusky-yellowish-brownand Schmidt, 1991). Unit is absent in the weathering chert. Alternating light- and 

eastern Spring Mountains (KT) and French- dark-gray beds form distinctive color bands, 

man Mountain similar to those in Bonanza King Formation, 

O�p Pogonip Group (Middle Ordovician to Upper but bands are thicker and more prominent. 

Cambrian)—Includes Antelope Valley and Stromatolites, oncoids, and brachiopod 

Goodwin Limestones. Pogonip Group is 710 fragments are common, pelmatozoan 

m thick in southern Sheep Range (GPT), ossicles less common. Dunderberg Shale 
�bk 

�c 

�b 

�t 

�Zw 

20 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

Member makes up the base of unit and thick in northwest Spring Mountains (WPT) 

consists of silty limestone and olive- (Vincelette, 1964), and about 200 m of unit 

green shale; common trilobite fragments is exposed in southern Las Vegas Range 

(Dunderbergia sp.) and grayish-orange (GPT) (Maldonado and Schmidt, 1991) 

intraclasts. Dunderberg Shale Member is �b Bright Angel Shale (Middle and Lower Cambriconformable with underlying Bonanza King an)—Exposed only at Frenchman Mountain. 

Formation. Most of the Nopah Formation Includes rocks equivalent to parts of the 

forms massive cliffs, but Dunderberg Shale Chisholm Shale, Lyndon Limestone, and 

Member forms ledgy slopes. Map unit is Pioche Shale. Chisholm Shale equivalent 

380 m thick in southern Sheep Range (GPT) consists of 25 m of greenish-gray shale and 

(Maldonado and Schmidt, 1991), about 360 minor limestone and limy shale; Glossopleum in the northern Spring Mountains (WPT) ra sp. trilobites reported by Palmer (1981). 

(Vincelette, 1964), 345 m in eastern Spring Lyndon Limestone equivalent consists of 

Mountains (KT) (Axen, 1985), and 33 m 30 m of dark-gray, thin-bedded limestone 

at Frenchman Mountain (Matti and others, containing light-gray to brownish-gray bur1993) row mottling. Pioche Shale equivalent about 

�bk Bonanza King Formation (Upper and Middle 128 m thick and composed mostly of oliveCambrian)—Consists of the Banded green phyllitic shale with local thin beds of 

Mountain and Papoose Lake Members. The dark-reddish-brown to purple sandstone; 

Banded Mountain Member generally is contains olenellid trilobite fragments (Pack 

light- to dark-gray, fine- to medium-crystal- and Gayle, 1971) 

line dolostone and limestone; alternating �t Tapeats Sandstone (Lower Cambrian)—Exposed 

light- to dark-gray colors give the member only at Frenchman Mountain. White to lightits distinctive banded appearance. Dark- brown sandstone and quartzite with conbrown to orange burrow mottling is preva­ glomeratic beds at base; thin- to thick-bedlent in the member, and several intervals ded, with trough cross bedding. Thickness is 

have chert nodules and discontinuous about 48 m (Hardy, 1986) 

layers. Base of the member is marked by �Zw Wood Canyon Formation (Lower Cambrian and 

the regionally extensive “silty unit” (Barnes Late Proterozoic)—Exposed in northwest 

and Palmer, 1961; Guth, 1980; Gans, 1974), Spring Mountains (WPT) and southern Las 

which is red-, orange-, and tan-weather- Vegas Range (GPT). Quartzite, sandstone, 

ing silty dolostone about 15 m thick, and siltstone, and shale. The quartzite is similar 

contains Middle Cambrian trilobites (Guth, to that in the underlying Stirling Quartz1980). The Papoose Lake Member is mostly ite, but it forms slopes in contrast to the 

dark-gray dolostone with distinctive brown- cliff-forming Stirling Quartzite. Top of unit 

ish-gray to grayish-orange burrow mottles; consists of a 7–8-m-thick ledge of white vitalso includes sparse limestone and silty do­ reous quartzite correlative with the Zabriske 

lostone beds. Map unit is 900 m thick in the Quartzite (Vincellette, 1964). Shaly and 

southern Sheep Range (GPT) (Maldonado silty units increase in abundance in upper 

and Schmidt, 1991), 853 m in the northwest- part of formation. Basal part of formation is 

ern Spring Mountains (WPT) (Vincellette, tan-weathering silty sandstone that forms a 

1964), about 445 m in the eastern Spring slope above the resistant Stirling Quartzite. 

Mountains (KT) (Axen, 1985), and 484 m Olenellid trilobites reported from near top 

at Frenchman Mountain (Matti and others, of formation in Sheep Range, north of the 

1993) quadrangle (Guth, 1980). Thickness from 

�c Carrara Formation (Middle and Lower Cam­ 640 to 820 m in Spring Mountains (Vincebrian)—Interbedded limestone, siltstone, lette, 1964); maximum exposed thickness in 

sandstone, and shale. Upper part is gray, Las Vegas Range is 450 m (Maldonado and 

platy limestone containing yellow- and Schmidt, 1991) 

red-weathering silt or clay partings. Contact Zs Stirling Quartzite (Late Proterozoic)—Exposed 

with overlying Bonanza King Formation in northwest Spring Mountains (WPT) and 

is marked by a change from slope-form- in southern Las Vegas Range (GPT). Unit 

ing, yellowish-weathering silty limestone is mostly purple, pink, maroon, gray, and 

of the Carrara Formation to more massive, white conglomeratic quartzite, quartzite, 

cliff-forming, gray carbonate rocks of the and sandstone, with minor beds of light- to 

Bonanza King Formation. Unit is 457 m dark-brown micaceous siltstone. Lowermost 
21 

20 m mostly white quartzite. Unit locally 

metamorphosed to sub-greenschist and 

lower greenschist facies in parts of 

northwestern Spring Mountains. Unit 

forms cliff. Thickness in Spring Mountains 

about 975 m (Nolan, 1924). Only the 

uppermost 100 m is exposed in Las Vegas 

Range (Maldonado and Schmidt, 1991) 

Zj Johnnie Formation (Late Proterozoic)— 

Exposed only in northwest Spring 

Mountains (WPT). Only the uppermost 

600 m of unit is exposed in quadrangle. 

Quartzite, sandstone, siltstone, shaly 

siltstone, and shale. Uppermost 60 m is 

micaceous siltstone with interbeds of darkbrown- to pale-red-weathering calcareous 

sandstone. Sandstone beds increase in 

number upward to become transitional with 

the overlying Stirling Quartzite. Next lower 

30 m is green-weathering phyllitic shale and 

siltstone with interbeds of brown sandstone. 

Below, the next lower 90–120 m is darkto light-brown-weathering dolomitic 

sandstone, siltstone, and quartzite. 

Underlying this unit is a marker horizon that 

consists of a 6-m-thick ledge of light-tanweathering oolitic dolostone underlain by a 

15-m-thick slope-forming blue-green shaly 

siltstone. Lowest exposed beds are reddishbrown, tan, gray, and green quartzite, 

sandstone, and siltstone. Formation is 

reported to be transitional with overlying 

Stirling Quartzite (Vincellette, 1964); base 

is not exposed 

Xg Gneiss (Early Proterozoic)—Exposed only at 

Frenchman Mountain. Gray microclinequartz-biotite garnet gneiss and banded 

pink to white leucocratic gneiss. Gneiss 

is cut by coarse-grained biotite granite. 

Rowland (1987) correlated unit with Vishnu 

Schist in Grand Canyon area, with an age 

of 1.7 Ga in the Grand Canyon area (Elston, 

1989) 

Stratigraphy of the Las Vegas 30′ × 60′

Quadrangle 

[Abbreviations for major Mesozoic thrust plates in the 

quadrangle: Wheeler Pass, WPT; Gass Peak, GPT; Valley, 

VT; Dry Lake, DLT; Lee Canyon/Macks Canyon, LMCT; 

Deer Creek, DCT; Keystone, KT; and Bird Spring, BST. For 

example, the Wheeler Pass thrust plate (WPT) occupies the 

hanging wall of the Wheeler Pass thrust fault.] 

Geology of the Las Vegas 30′ × 60′ Quadrangle 

Proterozoic Rocks 

Proterozoic rocks in the quadrangle are exposed at 

Frenchman Mountain and in the northwest Spring Mountains 

(WPT) and the southern Las Vegas Range (GPT). The oldest 

rocks in the quadrangle are gneissic crystalline rocks of Early 

Proterozoic age exposed at the base of Frenchman Mountain. 

Although these rocks have not been dated, similar rocks in 

the Grand Canyon area are 1.7 Ga (Elston, 1989). At Frenchman Mountain, the Early Proterozoic crystalline rocks are 

unconformably overlain by the Cambrian Tapeats Sandstone; 

this impressive unconformity represents a geologic time gap 

of about 1 b.y. Geophysical modeling suggests similar Early 

Proterozoic rocks may underlie the Spring Mountains. Such 

modeling is based on isostatic gravity and magnetic anomalies 

centered over the range (Langenheim and others, chapter B of 

this report). 

Late Proterozoic rocks are exposed only in the northwest 

Spring Mountains (WPT) and southern Sheep Range (GPT) 

and include the Johnnie Formation, Stirling Quartzite, and the 

lower part of the Wood Canyon Formation. These are mostly 

clastic rocks deposited in shallow marine waters along a passive continental margin in what is now western North America 

(Stewart, 1976; Stewart and Poole, 1972). Late Proterozoic 

rocks are interpreted to represent initial deposits of the Cordilleran miogeocline (Stewart, 1972, 1976; Stewart and Poole, 

1972), a broad paleogeographic feature that extends from 

Canada to northern Mexico. The miogeocline is characterized 

by a westward-thickening sequence of Late Proterozoic clastic 

and lower Paleozoic carbonate shelf rocks deposited along the 

North American continental margin as a result of Late Proterozoic rifting (Stewart, 1976). 

Paleozoic Rocks 

Paleozoic rocks make up the greatest proportion of 

bedrock units in the quadrangle. Their distribution is important 

because they are part of the Nevada carbonate rock province 

(Dettinger and others, 1995) and are the major ground-water 

aquifers in eastern Nevada. Cambrian through Devonian rocks 

form the upper part of the miogeoclinal sequence. Mesozoic thrust faults in the quadrangle juxtapose miogeoclinal 

sequences above progressively shallower shelf sequences, 

from west to east. For example, rocks of the WPT and GPT 

are miogeoclinal sequences that have been thrust above inner 

miogeoclinal sequences in the LMCT, VT, and DCT. Inner 

miogeoclinal rocks were thrust above cratonic margin rocks 

in the KT. At Frenchman Mountain, these rocks represent a 

cratonic sequence (Rowland, 1987). 

Burchfiel and others (1974), Cooper (1987), and Cooper 

and Edwards (1991) discussed major unconformities within 

the Cambrian through Devonian sequence in the Spring Mountains; these unconformities account for dramatic stratigraphic 

thinning of these units across the Las Vegas quadrangle. We 

further refined the stratigraphic relations along these unconformities based on conodont biostratigraphy; these results 
22 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

are shown in figure 2. In the WPT and GPT (fig. 2, sections 

1 and 2), unconformities exist between Middle Devonian and 

underlying Lower Devonian units, Silurian and underlying 

Upper Ordovician units, and between Upper Ordovician and 

Middle Ordovician rocks. At least two of these unconformities 

are traced eastward to the cratonic margin sequence in the KT 

(fig. 2, sections 5 and 6), where they exist within the Ordovician to Devonian Mountain Springs Formation. Here, Middle 

Devonian Simonson Dolomite equivalent rocks overlie a very 

thin Upper Ordovician section correlative with the Ely Springs 

Dolomite, and these rocks, in turn, overlie Lower Ordovician rocks equivalent to the lower part of the Pogonip Group. 

Cooper and Edwards (1991) described the lower unconformity 

between Pogonip Group and Ely Springs Dolomite equivalent 

rocks as a type-1 sequence boundary marking the top of the 

Sauk sequence of Sloss (1963), and they described the upper 

unconformity between Upper Ordovician and Middle Devonian rocks as a type-1 sequence boundary marking the top of 

the Tippecanoe sequence of Sloss (1963). 

Cambrian rocks in the quadrangle include part of the 

Wood Canyon, Carrara, Bonanza King, and Nopah Formations. Also included are the Tapeats Sandstone and Bright 

Angel Shale, which represent the cratonic sequence at 

Frenchman Mountain. Stewart (1970) correlated the Tapeats 

Sandstone with part of the Wood Canyon Formation, and the 

Bright Angel Shale with part of the Carrara Formation. These 

rocks are mostly clastic units consisting of sandstone, quartzite, conglomerate, siltstone, and shale, but they also include 

subordinate carbonate rocks. During Middle Cambrian time, 

a gradual change occurred from clastic to predominantly carbonate deposition along the miogeocline. Middle and Upper 

Cambrian rocks of the Bonanza King and Nopah Formations 

consist of dolostone, limestone, silty limestone and dolostone, 

shale, and siltstone, and these formations formed mostly in 

inner-shelf depositional environments that fluctuated from 

near-shore intertidal, shoal, and lagoonal environments to offshore inner shelf settings. These rocks typically contain stromatolites and trilobites. The Dunderberg Shale Member of the 

Nopah Formation is a distinctive marker unit in the sequence 

and consists of green to brown shale and siltstone and limestone, with abundant Dunderbergia sp. trilobite fragments. 

Ordovician rocks in the Las Vegas quadrangle include the 

Pogonip Group, Eureka Quartzite, and Ely Springs Dolomite. Miogeoclinal sections of the Pogonip Group are in the 

northwest Spring Mountains (WPT) and in the southern Sheep 

Range (GPT), where the group is about 700 m thick. Pogonip 

Group equivalent rocks thin to about 225 m in the eastern 

Spring Mountains (KT) in the cratonic margin sequence, represented by the lower member of the Mountain Springs Formation; Ordovician rocks are absent at Frenchman Mountain. 

Although traditionally assigned an Ordovician age, 

samples of the Goodwin Limestone of the Pogonip Group in 

the quadrangle have produced very late, but not latest Cambrian conodonts from its lower beds (fig. 2, sections 1 and 2). 

The Antelope Valley Limestone of the Pogonip Group contains 

abundant, generally diverse conodont assemblages except in 

rocks deposited in restricted very shallow water facies (such 

as cryptalgal laminated, mudcracked, lime mudstones). Nearly 

all of the low-latitude, shallow-water Laurentian province 

conodont zones of the Lower to lower Middle Ordovician 

(R. manitouensis Zone to at least H. holodentata Zone) have 

been identified, thus allowing precise correlation across thrust 

plates that represent significant variations in depositional environment (fig. 2). Conodont samples from the Pogonip Group 

typically represent wide-ranging open marine depositional 

environments ranging from deep-water open ocean to shallow 

inner-shelf settings. Distinctive fossils in the Antelope Valley 

Limestone are the spongiomorph Receptaculites sp. and the 

gastropods Palliseria sp. and Maclurities sp. Palliseria sp. are 

abundant, large (as much as 8 cm across), and magnificently 

exposed on the south side of Yucca Gap in the southern Sheep 

Range (map A). 

Rocks of the Eureka Quartzite conformably overlie 

Pogonip Group strata in the quadrangle. The Eureka Quartzite is a superb Paleozoic marker unit in the Great Basin. 

The light-tan quartzite contrasts sharply with the dark-gray 

dolostone of the overlying Ely Springs Dolomite. In most of 

the quadrangle, the unit consists of fine- to medium-grained 

orthoquartzite, but it also includes some sandstone and local 

sandy carbonate beds. In the western part of the quadrangle 

(WPT and GPT), the Eureka attains a maximum thickness of 

about 120 m. The Eureka Quartzite is 80 m thick in the LMCT 

but thins to only 6 m in the Lucky Strike Canyon area, where 

it is in the upper plate of the Deer Creek thrust fault. In the 

Apex area of the Dry Lake Range (in the adjacent Lake Mead 

1:100,000-scale quadrangle), the Eureka Quartzite is also thin, 

only 11 m, where it is in the upper plate of the Dry Lake thrust 

fault. In addition to similar thickness, Ross (1964) identified 

collophane nodules in the Eureka Quartzite in both the Lucky 

Strike Canyon and Apex sections. Based on these similarities, Ross (1964) suggested these two sections palinspastically 

restore along the Las Vegas Valley shear zone. Ross’ interpretation supports correlation of the Deer Creek thrust with the 

Dry Lake thrust as proposed in this report and in Lundstrom 

and others (1998). The Eureka Quartzite is absent in the eastern Spring Mountains (KT) and at Frenchman Mountain. 

The Eureka Quartzite is disconformably overlain by the 

Ely Springs Dolomite of Late Ordovician age. These rocks are 

mostly dark-gray, bioclastic dolostone, and yield conodonts 

no older than middle Edenian and no younger than middle 

Maysvillian at the base and could be as young as Richmondian at the top of the formation (Harris and others, in press). 

Because of the regional extent and relatively rapid transgression represented by the unit, the base of the Ely Springs Dolomite was chosen as the primary datum, or tie line, for correlating Paleozoic units across major Mesozoic thrust plates in the 

quadrangle (fig. 2). 

Silurian rocks in the quadrangle appear to be limited to 

the northern Spring Mountains (WPT) and southern Sheep 

Range (GPT). The Laketown Dolomite displays its typical 

tri-part character that is widely recognized throughout the 

Basin and Range Province; upper dark, middle light, and 
23 

lower dark dolostone. Here, as elsewhere in the region, 

the Laketown Dolomite has halysitid corals and silicified 

pentemarid brachiopods. The formation is about 250 m thick 

in the quadrangle and is Early Silurian in age, at least late 

Llandoverian and early Wentlockian on the basis of conodonts 

collected in the southern Sheep Range (fig. 2, section 1; Harris 

and others, in press). Conodonts indicate that rocks of the 

Laketown Dolomite formed in a shallow-water marine-shelf 

depositional setting. The Laketown Dolomite disconformably 

underlies probable Lower Devonian strata equivalent to 

the Sevy Dolomite, and disconformably overlies Upper 

Ordovician strata of the Ely Springs Dolomite. The Laketown 

Dolomite is presumably absent in thrust plates east of the 

WPT and GPT, where it was removed along a regional preDevonian unconformity. 

Devonian rocks are widespread in the quadrangle and are 

mostly limestone and dolostone deposited on a shallow-water 

carbonate shelf that fluctuated between normal- and restrictedmarine depositional settings. Dolomitic quartzite units are 

present at several horizons in the Devonian sequence, at the 

base of the Middle Devonian Simonson Dolomite and in the 

upper part of the Guilmette Formation and Sultan Limestone. 

Stromatoporoids are notable in dolostone units of the Simonson Dolomite and Guilmette Formation and Sultan Limestone. 

Devonian units in the quadrangle have a maximum thickness 

of 650 m in the southern Sheep Range and a minimum thickness of about 200 m at Frenchman Mountain. 

Lower Devonian rocks are exposed in the northwest 

Spring Mountains (WPT) and southern Sheep Range (GPT) 

and in the central Spring Mountains (LMCT and DCT). Based 

on the presence of algal laminations and fenestral fabric and 

on the general absence of conodonts, these rocks were deposited in a shallow-water, partly restricted carbonate shelf environment. In the WPT and GPT, the sequence includes rocks 

equivalent to the Sevy Dolomite and cherty argillaceous unit 

of Johnson and others (1989). These rocks have a maximum 

thickness of 50 m, and the top of the sequence is truncated 

along an erosional surface with local relief of about 3-5 m. 

Above this unconformity is a distinctive quartzite unit that 

marks the base of the Middle Devonian Simonson Dolomite 

in the quadrangle, as well as in much of eastern Nevada. In 

Lee and Lucky Strike Canyons (LCMT and DCT), the cherty 

argillaceous unit is absent, and Lower Devonian rocks are only 

10–20 m thick, respectively. In the eastern Spring Mountains 

(KT) and at Frenchman Mountain, Lower Devonian rocks are 

absent. 

Middle and Upper Devonian units in the quadrangle 

include the Simonson Dolomite, the Guilmette Formation, and 

the lower part of the Sultan Limestone. The Middle Devonian 

Simonson Dolomite is exposed in the WPT and GPT; equivalent rocks are present in thrust sheets to the east (LMCT, 

DCT), where the basal quartzite unit is absent and correlative 

strata are much thinner and more typical of an inner miogeoclinal facies. Simonson Dolomite equivalent rocks are only 60 

m thick in the KT and are represented by the upper member 

of the Mountain Springs Formation that formed in a cratonic 

Geology of the Las Vegas 30′ × 60′ Quadrangle 

margin setting. The Simonson Dolomite is Eifelian and early 

Givetian in age in most parts of the quadrangle (Harris and 

others, in press). Stringocephalus brachiopods (Givetian) 

are found at the top of the formation in the northwest Spring 

Mountains (WPT). 

The Middle and Upper Devonian Guilmette Formation 

and the lower part of the Sultan Limestone are, for the most 

part, correlative lithologically. In this report, the Guilmette 

Formation consists of rocks of miogeoclinal to inner miogeoclinal facies in the WPT-GPT and LMCT-DCT, respectively, 

and the Sultan Limestone consists of rocks of cratonic margin 

facies in the KT and cratonic facies at Frenchman Mountain. 

The lower part of the Sultan Limestone is thinner than the 

Guilmette Formation and lacks the yellow bed unit (Tschanz 

and Pampeyan, 1970) that defines the lower part of the 

Guilmette Formation throughout the eastern Basin and Range 

province. 

In most parts of the quadrangle, the Crystal Pass Limestone overlies the Guilmette Formation, but in the KT and at 

Frenchman Mountain, it is the upper member of the Sultan 

Limestone as defined by Hewett (1931). The Crystal Pass 

Limestone forms a distinct mappable unit, and its light-gray 

appearance and overall micritic texture make it a good marker 

bed; for these reasons, the unit deserves formation rank. 

Although originally defined as Devonian in age (Hewett, 

1931), conodonts establish that it is also partly early Kinderhookian in age in southeastern Nevada (Harris and others, in 

press; Stevens and others, 1996). A disconformity separates 

the upper Famennian lower part of the Crystal Pass from the 

lower Kinderhookian upper part of the formation. In the southern Las Vegas Range (VT), rocks that are below the Dawn 

Limestone of the Monte Cristo Group and above the Guilmette 

Formation are atypical of the Crystal Pass Limestone and contain quartzite beds, sandy ooid limestone, and burrowed dolostone. These rocks were deposited in a very high energy depositional environment in contrast to the laminated micrite that 

typifies the Crystal Pass Limestone elsewhere in the Las Vegas 

quadrangle. These stratigraphic relations suggest that the Crystal Pass is absent and the Dawn Limestone rests unconformably on the Guilmette Formation, or that Crystal Pass equivalent rocks are present and are represented by a different facies. 

We tentatively assign these Crystal Pass equivalent rocks to 

the Guilmette Formation (fig. 2). Poole and Sandberg (1991) 

interpreted a near-shore, lagoonal depositional setting for the 

Crystal Pass Limestone in the region. 

In the northwest Spring Mountains (WPT), rocks of the 

Lower Mississippian Narrow Canyon Formation are below the 

Monte Cristo Group and above the Guilmette Formation, and 

the Crystal Pass Limestone is presumably absent. Rocks of the 

Narrow Canyon Formation are part of the Mississippian 

central facies belt of Stevens and others (1996), a more western facies compared with the southeastern belt represented by 

the Monte Cristo Group in most of the quadrangle. The type 

area of the Narrow Canyon Formation and associated 

Mississippian central facies rocks is in the Spotted Range area, 

northwest of the Las Vegas quadrangle (Stevens and others, 
24 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

1996). Although the Narrow Canyon Formation is middle 

Kinderhookian in age, it typically contains reworked early to 

middle Famennian conodonts (Harris and others, in press). 

Rocks of the Lower and Upper Mississippian Monte 

Cristo Group are widely distributed throughout the quadrangle. These rocks are composed mostly of gray limestone 

deposited on an extensive carbonate platform across southern 

Nevada. Stevens and others (1996) defined these rocks to 

represent the southeastern facies belt of the southern Nevada 

and east-central California region. The rocks are typically 

bioclastic wackestone, packstone, and grainstone with abundant crinoids, solitary and colonial corals, and brachiopods. 

Most beds of the Monte Cristo Group are massive, but parts of 

the Anchor Limestone form slopes. The Monte Cristo Group 

maintains a thickness of about 300 m across the quadrangle, 

although the maximum thickness of the group is 480 m in the 

southern Las Vegas Range (Lundstrom and others, 1998). 

The Dawn Limestone is the lowermost formation of the 

Monte Cristo Group. It is a dark-gray, abundantly fossiliferous limestone that overlies and markedly contrasts with the 

light-gray micrite of the Crystal Pass Limestone. Conodonts 

indicate the age of the Dawn is Kinderhookian and early

 Osagean and suggest that the contact with the underlying 

Crystal Pass Limestone ranges from conformable to disconformable (Harris and others, in press). The Anchor Limestone 

of early to late Osagean age (Harris and others, in press; 

Stevens and others, 1996) overlies the Dawn Limestone. 

Unlike other formations in the group, the Anchor contains 

10–40 percent chert beds, some of which are spiculitic. The 

formation most likely represents slope deposition off a carbonate shelf margin during continued deepening (Poole and 

Sandberg, 1991). The succeeding Bullion Limestone is mostly 

massive encrinitic grainstone indicating progradation of a 

carbonate shelf westward and above the slope deposits of the 

Anchor. Conodonts indicate that much of the Anchor and Bullion Limestones in the Las Vegas Range (VT) formed during 

the middle Osagean, but near Indian Springs (WPT), just north 

of the Las Vegas quadrangle, conodonts indicate boundaries of 

Bullion Limestone are slightly younger, from late Osagean to 

early Meramecian (Stevens and others, 1996). The base of the 

Bullion in the WPT is also late Osagean (fig. 2). The Yellowpine Limestone, the uppermost formation of the Monte Cristo 

Group, yields late Osagean and Meramecian conodonts (Harris 

and others, in press). The Yellowpine contains conspicuously 

large solitary rugose corals in the Las Vegas Range (VT) and 

eastern Spring Mountains (KT). 

The Mississippian Battleship Wash Formation locally 

overlies the Yellowpine Limestone of the Monte Cristo Group 

in the southern Las Vegas Range. Webster (1969) noted a 

thin Battleship Wash Formation in the Lee Canyon area of 

the Spring Mountains. The Battleship Wash Formation is late 

Meramecian to early Chesterian based on conodonts collected 

in the southern Las Vegas Range (Harris and others, in press). 

The unit is a shallow marine deposit that commonly has spirifer brachiopods and corals. 

The Mississippian Indian Springs Formation of Webster 

and Lane (1967) was combined with the Bird Spring Formation and Callville Limestone, and it represents the basal part 

of these map units in the Las Vegas quadrangle. The Indian 

Springs Formation is Chesterian in age (Webster, 1969), and 

it is a good marker unit containing the biostratigraphically 

diagnostic Rhipodomella nevadensis and Stigmaria, as well as 

distinctive red and black shale beds. The formation represents 

deposition in a near-shore, marginal marine environment. 

Throughout the quadrangle, the Indian Springs Formation disconformably overlies the Monte Cristo Group or the Battleship 

Wash Formation and conformaby underlies the Bird Spring 

Formation. 

Pennsylvanian and Lower Permian rocks in the quadrangle include part of the Bird Spring Formation and equivalent 

Callville Limestone, succeeding Lower Permian redbeds, and 

the Lower Permian Kaibab and Toroweap Formations. The 

Bird Spring Formation is widely exposed in the Las Vegas 

quadrangle and reaches a maximum thickness of 2,500 m in 

the Las Vegas Range (Lundstrom and others, 1998). The Bird 

Spring Formation includes carbonate shelf, slope, and basinal 

deposits. 

We subdivided the Bird Spring Formation into two 

separate map units in the Las Vegas Range (VT) and in the 

northern Spring Mountains (LMCT and DCT). This subdivision allowed us to map important structures previously 

overlooked in the Las Vegas quadrangle. One of these structures is the Valley thrust fault and associated folds in the Las 

Vegas Range (Lundstrom and others, 1998). The lower unit 

of the Bird Spring Formation includes carbonate-shelf rocks 

of mostly Pennsylvanian age, and the upper member includes 

Lower Permian, uppermost Wolfcampian to Leonardian beds. 

The lower unit of the Bird Spring Formation is about 1,000 

m thick in the quadrangle and consists of silty limestone with 

subordinate dolostone, siltstone, shale, and chert; these rocks 

are abundantly fossiliferous. 

The upper unit of the Bird Spring Formation marks a 

significant change in depositional regime from a shelf depositional setting that typifies most of the Bird Spring in the region 

to a deeper water basinal and carbonate slope depositional 

regime. These rocks were deposited in the Bird Spring basin 

(Bissell, 1974), the dominant basin in the southern Nevada 

region that resulted from a major late Wolfcampian marine 

transgression and associated tectonism (Page, 1993, 1998). 

During this time, equivalent basin and slope facies rocks 

accumulated in the Dry Mountain Trough, a similar sedimentary basin in east-central Nevada (Gallegos and others, 1991; 

Stevens, 1991). During Leonardian time, carbonate-shelf 

deposits gradually prograded over the upper Wolfcampian 

slope to basin deposits of the Bird Spring Formation, reflecting a regional marine regression and filling of the Bird Spring 

basin. 

Deeper water carbonate slope and basinal facies characteristic of the upper member of the Bird Spring Formation are 

absent in the eastern Spring Mountains (KT) and northern Bird 

Spring Range (BST) and are cratonic platform and inner shelf 
25 

deposits resembling the Callville Limestone in Frenchman 

Mountain area. For example, evaporites are in the upper part 

of the Bird Spring Formation 4 km south of Blue Diamond 

(BST) near the southern edge of quadrangle. 

Strata overlying the Bird Spring Formation and Callville 

Limestone include the Lower Permian redbeds that reflect a 

significant shift from dominantly marine to mostly continental 

deposition. The Lower Permian redbeds are the basal sediments of the upper Paleozoic and Mesozoic continental platform assemblage (Dickinson, 1992) in the southern Nevada 

region. These rocks are mostly cross-bedded sandstone and 

subordinate siltstone, shale, conglomerate, and gypsum. Carr 

and others (2000) and McDonnell-Canan and others (2000) 

used the name Hermit Formation for these rocks in the Blue 

Diamond area from correlation with the Hermit Shale in the 

Grand Canyon region. At Frenchman Mountain, Longwell 

and others (1965) suggested correlation of the Lower Permian 

redbeds with parts of the Queantoweap Sandstone, the Supai 

Group, and the Coconino Sandstone—units defined in the 

Colorado Plateau region. The absence of fossils in these rocks, 

however, prevents a more definitive correlation. 

The Permian Kaibab and Toroweap Formations were 

mapped here as one unit and are similar to units exposed in 

the Colorado Plateau region. Formation members defined in 

the plateau region were recognized in the Blue Diamond area 

by Carr (1992), Carr and others, (2000), Carr and McDonnellCanan (1992), and McDonnell-Canan and others (2000). The 

Kaibab and Toroweap Formations form two prominent limestone ridges above the less resistant Lower Permian redbeds, 

and they contain abundant chert layers as well as subordinate 

dolostone, siltstone, shale, and gypsum. Rocks of the Kaibab 

and Toroweap Formations formed in shallow marine shelf 

to marginal marine environments. The Kaibab Formation 

contains nearly pure bedded gypsum in the upper Harrisburg 

Member, and it is locally mined and processed at the James 

Hardee Mine, at the top of Blue Diamond Hill in the southern 

part of the quadrangle. Exposures of the Kaibab and Toroweap 

Formations in the KT represent their westernmost limit. 

Mesozoic Rocks 

Mesozoic rocks in the quadrangle include the Triassic 

Moenkopi and Chinle Formations and the overlying Jurassic 

Aztec Sandstone. Most of these rocks are terrigenous clastic 

beds with subordinate marine rocks. Mesozoic formations in 

the quadrangle have been correlated with those in the Colorado Plateau region, but some members are absent because of 

stratigraphic thinning along major unconformities (Marzolf, 

1990). The Moenkopi Formation and Aztec Sandstone have 

been correlated with rocks in the Jurassic arc terrane in the 

Mojave Desert of southeastern California (Marzolf, 1990). 

The Timpoweap Conglomerate Member at the base of 

the Moenkopi Formation unconformably overlies rocks of the 

underlying Kaibab and Toroweap Formations, but in parts of 

the eastern Spring Mountains, it directly overlies the Lower 

Geology of the Las Vegas 30′ × 60′ Quadrangle 

Permian redbeds. Rocks in the lower part of the Moenkopi 

Formation above the Timpoweap Conglomerate Member are 

mostly terrigenous sandstone, siltstone, shale, and gypsum, 

with minor marine carbonate rocks. These rocks are overlain 

by the Virgin Limestone Member, a shallow marine deposit 

containing pelecypods, brachiopods, and pelmatozoans. 

Conodonts from the Virgin Limestone Member in the Blue 

Diamond area are Early Triassic in age (Harris and others, in 

press). Marine limestone in the Virgin Limestone Member 

represents the final stages of marine incursion across southern 

Nevada (Longwell and others, 1965). The Virgin Limestone 

Member of the Moenkopi Formation is overlain by an upper 

member composed of reddish-brown claystone, siltstone, 

sandstone, and subordinate limestone and gypsum. Hewett 

(1931) reported volcanic clasts and ashy beds in the member 

south of the quadrangle, just west of Goodsprings, Nevada. 

The Upper Triassic Chinle Formation rests unconfomably on the Moenkopi Formation in the quadrangle. The 

Chinle consists of terrigenous sandstone, siltstone, shale, and 

conglomerate; no marine beds have been recognized in the 

formation in the southern Nevada area (Longwell and others, 1965). The erosional base of the formation is defined by 

the Shinarump Member, which contains distinctive clasts of 

reddish- to yellowish-brown chert. Volcanic clasts have been 

reported in the Shinarump Member in the Spring Mountains, 

and volcanic-derived bentonitic clays are in other parts of the 

unit in the region (Longwell and others, 1965). Above the Shinarump Member, the Petrified Forest Member of the Chinle 

Formation is recognized in the southern Nevada region; these 

rocks are unconformably overlain by strata equivalent with 

parts of the Moenave and Kayenta Formations (Wilson and 

Stewart, 1967; Marzolf, 1990). In this report, the Moenave and 

Kayenta Formation equivalent rocks are included in the upper 

part of the Chinle Formation. 

The Jurassic Aztec Sandstone conformably overlies the 

Chinle Formation in the quadrangle. The Aztec is equivalent to 

the Navajo Sandstone in southwest Utah. The formation consists predominantly of cross-bedded sandstone; quartz grains 

are fine to medium grained. The Aztec Sandstone is magnificently exposed in the Wilson Cliffs in the eastern Spring 

Mountains, where it is truncated by the Keystone thrust fault. 

The conglomerate of Brownstone Basin (Axen, 1985) 

may represent the only Cretaceous unit in the quadrangle; 

the absence of datable material within the unit has prevented 

determination of its age. The conglomerate of Brownstone 

Basin is too thin to show on map A, but its distribution is 

shown by Axen (1985) in the La Madre Mountain area. The 

conglomerate of Brownstone Basin unconformably overlies 

the Jurassic Aztec Sandstone locally in the La Madre Mountain area north of the La Madre fault, and in the Wilson Cliffs 

area south of the La Madre fault; like the Aztec Sandstone, 

the unit is truncated by both the Red Spring and Keystone 

thrusts (Davis, 1973). Axen (1985) interpreted the conglomerate of Brownstone Basin as a synorogenic deposit that formed 

in frontal parts of the Red Spring thrust fault. Burchfiel and 

Davis (1988) also reported small outcrops of the unit below 
26 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

the Keystone thrust in the Wilson Cliffs area. The unit is 

from 2 to 5 m thick locally and consists of mixed Paleozoic 

and Mesozoic clasts in a matrix of red sandstone derived 

mostly from the Aztec Sandstone. Axen (1985) correlated the 

conglomerate of Brownstone Basin with the Lavinia Wash 

sequence, a similar synorogenic deposit exposed south of the 

quadrangle in the Goodsprings area (Carr, 1980; Fleck and 

Carr, 1990). The conglomerate of Brownstone Basin may also 

correlate with part of the Baseline Sandstone in the Muddy 

Mountains, a synorogenic deposit in the lower plate of the 

Muddy Mountain thrust, an equivalent to the Keystone thrust. 

Tertiary Rocks 

Tertiary rocks in the quadrangle include the Horse Spring 

Formation (Longwell and others, 1965; Bohannon, 1984), 

volcanic rocks in the McCullough Mountains, and the Muddy 

Creek Formation. The Horse Spring Formation is Miocene to 

Oligocene in age, volcanic rocks in the quadrangle are middle 

Miocene, and the Muddy Creek Formation is Pliocene to late 

Miocene. Tertiary rocks are significant because they provide 

relative age constraints for Tertiary extensional structures in 

the quadrangle. 

Rocks equivalent to the Horse Spring Formation are 

exposed in several shallow basins in the southern Las Vegas 

Range. These rocks are deformed and are considered “syntectonic” in terms of regional Tertiary extension. Correlation 

of these basin-fill deposits in the quadrangle with the Horse 

Spring Formation is controversial. Some workers (Guth 

and others, 1988) argue that these rocks were deposited into 

uniquely separate basins from those of the Horse Spring 

Formation in the Lake Mead area (Bohannon, 1984). We follow Maldonado and Schmidt (1991) and Longwell and others 

(1965) and use the name Horse Spring Formation for these 

deposits as well because of similar depositional age and the 

fact that these deposits collectively help to constrain the age of 

Cenozoic extension in the region. 

The Gass Peak basin (Guth and others, 1988) lies 

between Fossil Ridge and Gass Peak, two east- to northeasttrending ranges oroflexurally bent during movement along the 

Las Vegas Valley shear zone. Guth and others (1988) informally named the deposits filling Gass Peak basin the “Gass 

Peak formation.” Maldonado and Schmidt (1991) correlated 

these deposits with the Horse Spring Formation and subdivided the unit into several members. The deposits consist of 

alluvial gravel, megabreccia, lacustrine sediments, and some 

volcanic rocks. K-Ar ages from interbedded volcanic rocks of 

the Horse Spring Formation in the Gass Peak basin range from 

12 to 16 Ma (Guth and others, 1988; Maldonado and Schmidt, 

1991). The deposits are tilted and faulted against Proterozoic 

to lower Paleozoic rocks on the flanks of Fossil Ridge and 

Gass Peak. The Horse Spring Formation is also exposed in a 

small basin in the northeastern corner of the quadrangle. Here, 

conglomerates and some lacustrine beds and several beds of 

alkali basalt are tilted about 15–20° to the east along the 

Hidden Valley fault. 

Andesitic lava flows were deposited in the southeastern 

part of the map and were correlated with the Mount Davis and 

the Patsy Mine Volcanics (Bingler, 1977; Anderson, 1977). 

These rocks have their sources in the McCullough Range 

(Smith and others, 1988) and Eldorado Mountains (Anderson, 

1971) which are in the northern part of the Lower Colorado 

River extensional corridor (Faulds and others, 1994). The 

rocks were erupted during periods of maximum extension in 

the region, and at some locations in the Eldorado Mountains, 

they are tilted as much as 90° (Feurbach and others, 1993). 

Deposits of the Muddy Creek Formation are preserved at 

several localities on the southwest flank of Frenchman Mountain. These sediments are part of more extensive exposures 

of the formation near Frenchman Mountain in the adjacent 

Lake Mead 1:100,000-scale quadrangle. These rocks are partly 

gypsiferous, red and gray sandstone and mudstone. Most of 

the sediments of the Muddy Creek in the region are lacustrine 

and are interbedded with alluvial gravels deposited mostly 

along Cenozoic basin margins. The Muddy Creek Formation is 

generally undeformed compared with underlying sediments of 

the Horse Spring Formation, and Longwell (1974) concluded 

that movement on the Las Vegas Valley shear zone must have 

ended by the onset of Muddy Creek deposition because the 

shear zone apparently does not cut these rocks. 

Quaternary 

To evaluate the antiquity of early man in the area, Haynes 

(1967) described and established the Quaternary stratigraphy (units A through G) in the Tule Springs area of the 

northern Las Vegas Valley. This work has remained a sound 

basis for subsequent Quaternary studies and mapping in the 

area, including this map. Though Haynes (1967) recognized 

paleospring point discharge associated with stratigraphic units, 

he interpreted some of the deposits as lacustrine, as Longwell 

and others (1965) had done earlier. In a Statewide evaluation of pluvial lakes and climate, Mifflin and Wheat (1979) 

showed that characteristics of valleys with pluvial shorelines 

contrasted with those of valleys with evidence of past groundwater discharge without pluvial lakes, and inferred that most 

Southern Nevada valleys were in the latter category. Through 

studies of facies relationships and various paleoenvironmental 

data, Quade (1986) established the association of the finegrained units with expanded regional ground-water discharge 

in the Las Vegas Valley during late Quaternary climates that 

were significantly cooler and moister than present. Later 

studies (Quade and Pratt, 1989; Quade and others, 1995, 

1998) have expanded the paleoenvironmental, geochemical, 

and geochronologic databases to show that late Pleistocene 

ground-water discharge recurred episodically in many valleys 

of the region in response to climate change and its effects on 

ground-water systems. 
27 

This 1:100,000-scale geologic map, which synthesizes 

recent geologic maps of 7.5’ quadrangles (Bell and others, 

1998, 1999; dePolo and others, 1999; Lundstrom and others, 

1998, 2003), shows the extent of fine-grained deposits and the 

relationships of these deposits to faults, alluvial fan gravel, 

bedrock geology, and physiography. There is a common 

spatial association of the fine-grained deposits with Quaternary faults that offset these deposits. Faults that juxtapose 

strata of differing transmissivities can form hydrologic barriers 

that increase surficial discharge of shallow ground water. 

Extensive areas of fine-grained deposits dominate the 

surface of lower parts of the Las Vegas and Pahrump Valleys. Most of these deposits were formed in various types 

of wetlands during episodes of past ground-water discharge 

that were much larger than historically observed. The most 

areally extensive period of discharge during the past 50,000 

years occurred between about 40 ka and 25 ka when map unit 

Qscd was deposited, preceding and overlapping the last global 

glacial maximum. Comparison to proxy climate records (for 

example, Spaulding, 1990) indicates that this was a period of 

relatively high effective moisture and recharge. A less extensive period of past discharge occurred during 13–8 ka, when 

unit Qse was deposited. 

Extensive coalescing alluvial fans exist between the finegrained areas of past discharge and the upland sources of the 

fans. Like past discharge, fan sedimentation was also episodic 

and hydroclimatically driven. Extensive Holocene alluvial-fan 

sedimentation (Qay, especially Qayo) overlapped the younger 

period of prehistoric ground-water discharge at about 13–8 ka 

(Liu and others, 2000). Somewhat wetter than historic climates 

with surface flood events of larger than historic magnitude 

produced these fans. In contrast, the most extensive period of 

past discharge (40–25 ka) did not coincide with fan-building, 

but overlapped and preceded major fanhead incision (Qaiy) in 

watersheds that included the highest parts of the Spring Mountains. The fanhead incision required concentrated runoff and 

low sediment yield, and was probably dominated by 

snowmelt. The latter part of the last major episode of 

carbonate cementation in soils in the area occurred during this 

period of enhanced late Pleistocene effective moisture. The 

carbonate cementation from this period helps to distinguish 

well-cemented Pleistocene alluvial soils from non-cemented 

Holocene soils. Prior to fanhead incision, the penultimate 

period of extensive fan deposition (Qai) and aggradation 

similar to that in the Holocene occurred during 120–50 ka, 

probably in response to high-intensity rainfall, runoff, and 

erosion of uplands. Similar cycles of fan building and groundwater discharge probably occurred during the early and middle 

Pleistocene and perhaps earlier, but deposits including Qao, 

QTa, and QTs are not dated nor exposed well enough to test 

such speculation. 

Geology of the Las Vegas 30′ × 60′ Quadrangle 

Structure of the Las Vegas 30′ × 60′

Quadrangle 

Introduction 

The first major structural episode that affected the rocks 

in the quadrangle was the development of thrust faults and 

folds related to the Sevier orogeny in Cretaceous time. Major, 

southeast-directed, Mesozoic thrust faults are exposed in the 

Spring Mountains and Sheep and Las Vegas Ranges; they 

are, from west to east, the Wheeler Pass and equivalent Gass 

Peak thrusts, the Lee Canyon and Macks Canyon thrusts and 

equivalent Valley thrust, the Deer Creek thrust, the Keystone 

thrust, and the Bird Spring thrust (fig. 3A, map A). Burchfiel 

and others (1974) reported 36–75 km of shortening across the 

Spring Mountains as a result of Mesozoic thrusting. 

Late Tertiary extension in the Las Vegas quadrangle 

resulted in regional strike-slip and normal faulting, landsliding, and volcanism. The major extensional structures in the 

quadrangle include the dextral Las Vegas Valley shear zone 

and the State Line fault zone (fig. 3A). Other structures that 

formed during extension include high-angle normal and 

strike-slip faults and landslide deposits shed off the Spring 

Mountains. 

Quaternary faults are exposed in both Pahrump and Las 

Vegas Valleys, where they are generally associated with paleospring discharge deposits of Pleistocene age. These faults are 

discontinuous along strike, and have low slip rates. The origin 

of these faults is controversial, and recent studies suggest that 

many of these faults are indeed tectonic features. 

Mesozoic Thrust Faults 

Spring Mountains 

Wheeler Pass Thrust 

The Wheeler Pass thrust (fig. 3A) juxtaposes Late 

Proterozoic and Lower Cambrian clastic and carbonate units in 

the upper plate against mostly overturned Permian beds of the 

Bird Spring Formation in the lower plate. Stratigraphic separation on the thrust is approximately 4.8 km (Vincelette, 1964), 

and Fleck (1970) estimated about 7 km of horizontal shortening. This southeast-vergent thrust strikes northeast like other 

major thrusts in the quadrangle but dips more steeply (as much 

as 50°) (Nolan, 1929; Vincelette, 1964; Burchfiel and others, 

1974). The Wheeler Pass thrust and parts of the equivalent 

Gass Peak thrust, as well as the Valley thrust, are technically 

reverse faults, but we refer to these as “thrusts” to maintain 

consistency with past publications that discuss these 
28 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

structures. Along the southern segment of the Wheeler Pass 

thrust (southwest of the Trough Spring fault), upper-plate 

rocks of the Wood Canyon, Carrara, and Bonanza King 

Formations are folded into a broad north-northeast-trending 

syncline (Wheeler syncline of Nolan, 1924). The thrust here is 

cut by several northwest-striking high-angle faults, the most 

prominent being the Trough Spring fault (map A). The central 

fault segment (northeast of the Trough Spring fault) has upperplate rocks composed of Stirling Quartzite and Wood Canyon 

Formation (cross section C–C′, map sheet 1) deformed by 

folds and associated small-offset imbricate thrusts. The northern fault segment is concealed by Tertiary and Quaternary 

basin-fill deposits south of Indian Ridge (map A). 

About 15 km northwest of the Wheeler Pass thrust in the 

northwest part of the quadrangle, the Stirling Mine fault (map 

A) strikes northeast and, although its trace is concealed by 

Quaternary alluvium, the fault separates Lower Cambrian and 

Late Proterozoic rocks on the northwest side from Cambrian 

and Ordovician rocks on the southeast side. Stratigraphic 

throw on the fault was estimated at about 1.5 km (Burchfiel 

and others, 1974). Snow (1992) proposed that the Stirling 

Mine fault is a segment of a hypothetical thrust he called the 

Kwichup thrust, which is at a higher structural level than the 

Wheeler Pass thrust. Detailed mapping of contractile structures in the block northwest of the fault by Abolins (1999) 

supports the existence of the Kwichup thrust. Snow (1992) 

interpreted that other parts of the Kwichup thrust plate were 

excised by normal faults and concealed in the subsurface. 

Lee Canyon and Macks Canyon Thrusts 

The Macks Canyon and Lee Canyon thrusts (fig. 3A) 

are discussed together because the Macks Canyon thrust is 

an upper plate splay of the Lee Canyon thrust (Burchfiel 

and others, 1974). Both faults strike northeast and lose 

displacement in that direction. Southwest of the La Madre 

fault (fig. 3A), the Macks Canyon thrust has a small amount 

of displacement compared to the Lee Canyon thrust, offsetting 

chiefly Cambrian and Ordovician rocks, and it terminates to 

the south against the Trough Spring fault where it most likely 

is concealed in the subsurface. Northeast of the La Madre 

fault, upper-plate Mississippian rocks are in fault contact 

with mostly Pennsylvanian rocks in the lower plate. Here, the 

thrust has utilized the shaly Indian Springs Formation (Upper 

Mississippian) as a glide plane. 

Fleck (1970) estimated maximum stratigraphic separation 

on the Lee Canyon thrust at 4.8 km and the amount of horizontal shortening at about 7 km. Along the southern segment 

of the Lee Canyon thrust, carbonates of the Bonanza King 

Formation in the upper plate are juxtaposed against mostly 

west-dipping rocks of the Bird Spring Formation in the lower 

plate. The central fault segment in upper Lee Canyon has 

upper plate rocks consisting mostly of Upper Cambrian Nopah 

Formation above lower plate rocks ranging from Ordovician 

to Pennsylvanian. These rocks are highly faulted by local 

imbricate thrusts that are in turn cut by northwest-striking 

high-angle faults. Northeast of the La Madre fault, Ordovician 

rocks in the upper plate are juxtaposed against overturned beds 

of the Bird Spring Formation in the lower plate. At the 

northernmost exposures of the thrust in lower Lee Canyon, 

like the Macks Canyon thrust, Mississippian rocks in the upper 

plate are juxtaposed against the Bird Spring Formation in the 

lower plate. We interpret the two thrusts to merge in this area 

(fig. 3A). 

Deer Creek Thrust 

Fleck (1970) estimated a maximum stratigraphic separation on the Deer Creek thrust of about 3 km and lateral 

shortening of about 5.3 km. The southern segment of the Deer 

Creek thrust is bounded on the north by the La Madre fault, 

and on the south by the Griffith fault (fig. 3A, map A). Along 

this segment, the thrust juxtaposes Nopah Formation in the 

upper plate against thrust-imbricated Mississippian and Pennsylvanian rocks in the lower plate. The central segment of the 

fault is exposed in upper Lucky Strike Canyon, where rocks of 

the Nopah Formation in the upper plate are juxtaposed against 

rocks of the Bird Spring Formation in the lower plate (map A). 

The northern segment of the fault is exposed in lower Lucky 

Strike Canyon, where the fault juxtaposes Nopah Formation in 

the upper plate against Mississippian rocks in the lower plate. 

The Kyle Canyon thrust is exposed about 3.5 km east of 

the southern Deer Creek thrust segment where it offsets only 

rocks of the Bird Spring Formation (cross section C–C’, map 

sheet 1). This thrust is probably a frontal splay of the Deer 

Creek thrust because both faults are in the same structural 

block and terminate at the Griffith fault. Fleck (1970) estimated maximum stratigraphic separation on the thrust to be 

about 0.9 km, and lateral shortening about 0.8 km. 

Keystone and Red Spring Thrusts 

The Keystone thrust is exposed for nearly 70 km along 

the length of the eastern Spring Mountains. It is one of the 

best exposed thrust faults in the entire Cordillera, and forms 

the eastern limit of the Cordilleran fold and thrust belt in the 

region. We follow Longwell and others (1965) and use the 

name Keystone thrust for the spectacularly exposed thrust 

above the Wilson Cliffs, between the La Madre and Cottonwood faults (fig. 3A; map A), although Burchfiel and others 

(1997) alternatively refer to this thrust as the Wilson Cliffs 

thrust. The Keystone thrust juxtaposes gray carbonate rocks 

of the Cambrian Bonanza King Formation in the upper plate 

against red- and tan-weathering beds of the Jurassic Aztec 

Sandstone in the lower plate. The thrust is well exposed in 

Red Rock Canyon, just south of the La Madre fault. Here, the 

thrust plane is in a ramping configuration (dipping 30° to the 

northwest) above an overturned footwall syncline in Triassic 

and Jurassic rocks (cross section C–C′, map sheet 1). Burchfiel 

and others (1974) estimated values ranging from 11.6 to 23 km 

horizontal displacement along the Keystone thrust based on 

several respective models they considered for the thrust. 
29 

Near the southern boundary of the quadrangle, the 

Keystone thrust is offset by several high-angle faults. The 

most prominent is the Cottonwood fault, a northwest-striking 

fault that forms the southern boundary of the Wilson Cliffs 

(fig. 3A). The Cottonwood fault accommodated down-to-thesouthwest displacement, and juxtaposed Paleozoic rocks in the 

downthrown block against Aztec Sandstone in the upthrown 

block. In contrast to this large stratigraphic throw, the fault 

only offsets the Keystone thrust about 60 m (Davis, 1973). 

The La Madre fault cuts the Keystone thrust at the north 

end of Wilson Cliffs. Northeast of the La Madre fault in the 

La Madre Mountains area, the Keystone thrust is in fault 

contact with the Red Spring blocks (fig. 3A). The Red Spring 

blocks are three northeast tilted structural blocks bound by 

the Keystone thrust on the northwest, and by the La Madre, 

Turtlehead Mountain, Brownstone Basin, and Box Canyon 

faults, from southeast to northeast. Between the La Madre 

and Brownstone Basin faults, the Keystone thrust is at the 

boundary between the northeast-striking beds of La Madre 

Mountain (upper plate) and the northwest-striking beds of the 

southern two Red Spring blocks (lower plate) (Axen, 1985). 

Axen (1985) interpreted the Keystone thrust northeast of the 

Brownstone Basin fault to cut up-section and juxtapose rocks 

as young as the Upper Cambrian Nopah Formation (upper 

plate) against a discordant Cambrian through Mississippian 

sequence (lower plate) in the northernmost Red Spring block 

(map A). The upper plate rocks are highly deformed by small 

imbricate thrusts, tear faults, and folds. Several smaller highangle faults offset the Keystone thrust above the northern Red 

Spring blocks. 

The northwest-striking Red Spring thrust is exposed 

repeatedly in all three Red Spring blocks (fig. 3A). The thrust 

dips from 35° to 40° to the northeast, and like the Keystone 

thrust, juxtaposes upper-plate rocks of Cambrian Bonanza 

King Formation against Jurassic Aztec Sandstone in the 

lower plate (map A). The structural relationship between the 

Keystone and Red Spring thrusts is controversial because of 

complex structural relations. The controversy is long-lived, 

dating to the 1920s when Longwell (1926) first interpreted the 

Red Spring thrust as a separate thrust that pre-dated the Keystone thrust and was subsequently overridden by the younger 

Keystone thrust. Longwell (1960) later reinterpreted the 

Red Spring thrust to be equivalent with the Keystone thrust, 

and that the Red Spring blocks were fragments of the Keystone plate broken off the frontal edge and overridden by the 

advancing Keystone thrust. Several structural models for these 

thrusts evolved from Longwell’s interpretations. 

Model 1, shown in figure 3B, is similar to Longwell’s 

1926 interpretation and assumes that the Red Spring thrust 

pre-dates and was overridden by the younger Keystone 

thrust (Axen, 1985; Davis, 1973; Burchfiel and Davis, 1988; 

Burchfiel and Royden, 1984; Burchfiel and others, 1997). 

Proponents of the model further assume that the Wilson Cliffs 

thrust and the Cottonwood thrust in the Goodsprings area correlate with the Red Spring thrust, and were all overidden by 

the younger Keystone thrust. The fault traditionally shown on 

Geology of the Las Vegas 30′ × 60′ Quadrangle 

other published maps as the Keystone thrust (Longwell, 1926; 

Secor, 1963; Burchfiel and others, 1974) above the Wilson 

Cliffs (fig. 3A) was named the Wilson Cliffs thrust by 

Burchfiel and Royden (1984) and Burchfiel and others (1997) 

(fig. 3B). They mapped the Keystone thrust at yet a higher 

structural level to the west, where in the Mountain Springs 

area they demonstrated that the fault was continuous and 

similar in style to the type Keystone thrust of the Goodsprings area. In the Mountain Springs area, Burchfiel and 

others (1997) distinguished the Keystone and Wilson Cliffs 

thrusts based on internal structure and detailed mapping of the 

Bonanza King Formation. They determined that the rocks in 

the Keystone plate are less deformed than in the Wilson Cliffs 

plate. The Keystone thrust juxtaposes relatively undeformed 

older units against younger units of the Bonanza King Formation that are highly deformed by imbricate thrusts and isoclinal 

folds. 

Model 1 is based mostly on (1) the presence of highangle faults (La Madre and Cottonwood faults) that cut and 

accommodate significant offset of the Red Spring, Wilson 

Cliffs, and Contact thrust plates but fail to appreciably offset 

the Keystone thrust and (2) the fact that the Red Spring and 

Contact thrusts lie topographically below the Keystone plate. 

Recognition of pre-Keystone high-angle faults led Davis 

(1973) and Burchfiel and Davis (1988) to propose that the Red 

Spring, Wilson Cliffs, and Contact thrusts were emplaced, 

faulted by northwest-striking high-angle faults, such as the 

Cottonwood fault and faults that bound the three Red Spring 

blocks, and then the part of the thrust plate between the La 

Madre and Cottonwood faults was uplifted and eroded away. 

These events were followed by emplacement of the younger 

Keystone thrust. 

Model 2 (fig. 3C) is adapted from Longwell’s 1960 interpretation of the Keystone and Red Spring thrusts, and requires 

only one episode of thrusting. Longwell (1960) proposed 

that the Red Spring and Keystone thrusts are equivalent and 

that the Red Spring thrust plates were parts of the Keystone 

plate that were broken and downdropped from frontal parts of 

the thrust plate, then overridden by the advancing Keystone 

thrust. Matthews (1988, 1989) also believed that the Keystone 

and Red Spring thrusts are equivalent, but he disagreed with 

Longwell’s “overriding” concept and proposed an alternative model for the thrust faults in the Red Spring blocks (fig. 

3C). Although the Red Spring blocks appear to lie structurally 

below the Keystone thrust, Matthews (1988) argued that the 

blocks stand topographically above the Keystone thrust and 

were displaced from the Keystone plate through a combination of vertical and horizontal axis rotation. He believed that 

this deformation occurred during oroclinal bending caused by 

Miocene movement on the Las Vegas Valley shear zone. In the 

southern two Red Spring blocks, Matthews (1989) interpreted 

the Keystone thrust segments (as shown on map A) as 

south-dipping normal faults that downdropped and rotated the 

blocks from La Madre Mountain to conceal the trace of the 

true Keystone thrust (fig. 3C). He interpreted the mapped 
30 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

Keystone thrust in the northernmost Red Spring block (map 

A) as a Cenozoic thrust that formed as a result of extreme 

shortening accommodating oroclinal bending of the block by 

the Las Vegas Valley shear zone. 

Matthews (1989) considered the Wilson Cliffs thrust of 

Burchfiel and Royden (1984) and Burchfiel and others (1997) 

to be the traditionally mapped Keystone thrust, and the thrust 

Burchfiel and Royden (1984) and Burchfiel and others (1997) 

identified as the Keystone thrust, as part of a deformed zone 

in the upper plate. Although Davis (1973), Axen (1985), and 

Burchfiel and Davis (1988) correlated the Contact thrust in 

the Goodsprings area with the proposed pre-Keystone Red 

Spring and Wilson Cliffs thrusts, Fleck and Carr (1990) 

and Matthews (1988) interpreted the Contact and Keystone 

thrusts as contemporaneous structures. The Cottonwood fault 

was interpreted by Matthews (1988) as connecting with the 

Contact thrust to form a lateral ramp of a lower plate duplex 

zone of the Keystone thrust, rather than a pre-Keystone age 

high-angle fault. 

Longwell’s (1960) interpretation and model 2 are appealing because of their simple view of the thrusts along the 

eastern Spring Mountains as one integrated system of the same 

age. Even Longwell (1960) commented that identical rocks in 

the Red Spring and Keystone thrust plates implied their equivalence, and Fleck and Carr (1990) concluded coeval emplacement of the Contact and Keystone thrusts in the Goodsprings 

area. Nevertheless, thrust models in the La Madre Mountain 

area and Red Spring blocks remain equivocal and need further 

evaluation. 

Bird Spring Thrust 

The Bird Spring thrust is the structurally lowest thrust 

in the quadrangle (fig. 3A, cross section C–C’, map sheet 1). 

Its trace is subparallel to the Keystone thrust, and it extends 

northward more than 30 km from the southern Bird Spring 

Range to the Blue Diamond area in the southern part of the 

quadrangle; its throw decreases northward along strike. At 

its southernmost exposures, south of the quadrangle in the 

Goodsprings area, rocks of the Bonanza King Formation in 

the upper plate are juxtaposed against lower-plate rocks of the 

Aztec Sandstone (Burchfiel and Davis, 1988). In the quadrangle, along the east side of Blue Diamond Hill, upper-plate 

Permian redbeds are juxtaposed against lower-plate rocks 

of the Virgin Limestone Member and upper red member of 

the Triassic Moenkopi Formation (cross section C–C’, map 

sheet 1). Beds of the Virgin Limestone Member are locally 

highly folded at the base of the thrust, especially just north of 

State Highway 160. The thrust appears to terminate at the La 

Madre fault. However, new mapping for this report indicates 

that it continues northward across the fault and is concealed 

by a shallow landslide block composed of the Kaibab and 

Toroweap Formations (see landslide section below). 

Sheep Range and Las Vegas Range 

Gass Peak and Valley Thrusts 

Major Mesozoic structures in the Sheep and Las Vegas 

Ranges are the Gass Peak thrust and the Valley thrust and 

structurally related overturned limbs of an anticline-syncline 

pair (fig. 3A, map A, cross sections A–A′ and B–B′, map sheet 

1). Strike of these structures changes from east-west near the 

margin of Las Vegas Valley to northeast in the northern part of 

the quadrangle. This pattern resulted from oroclinal bending 

along the Miocene right-lateral Las Vegas Valley shear zone. 

Nelson and Jones (1987) documented more than 50° of clockwise, vertical-axis rotation of rocks (Cambrian Bonanza King 

Formation) in the Las Vegas Range, near the shear zone. They 

concluded that surface deformation resulting from oroclinal 

bending is characterized by small fault-bounded blocks (less 

than 5 km in length) that rotated independently due to ductile 

deformation related to the shear zone at depth. Lundstrom and 

others (1998) detailed this style of deformation in their mapping of Paleozoic rocks adjacent to the shear zone. 

The Gass Peak thrust has Proterozoic and Cambrian 

rocks in the upper plate and Lower Permian (Leonardian) 

rocks in the lower plate (Guth, 1990; Maldonado and Schmidt, 

1991; Ebanks, 1965). Guth (1981) reported the thrust to have 

as much as 5.9 km of stratigraphic separation and greater than 

30 km of horizontal displacement. Dips of the thrust range 

from 40° to 47° to the northwest, and Guth (1990) interpreted 

a ramp-flat-ramp geometry with a decollement in the Proterozoic Johnnie Formation. Ebanks (1965) described a complex 

imbrication zone along the thrust near the northern edge of the 

quadrangle, involving rocks of the Stirling Quartzite and Carrara Formation (map A). 

Structure in the southernmost Las Vegas Range is 

dominated by the Valley thrust and overturned limbs of an 

anticline-syncline pair (fig. 3A, map A, cross sections A–A’ 

and B–B’, map sheet 1). These structures are subparallel to and 

in the lower plate of the Gass Peak thrust. The Valley thrust 

(Lundstrom and others, 1998) strikes northeast and is traced 8 

km north from the margin of Las Vegas Valley, where it is at 

the base of Mississippian rocks that form the core of an asymmmetric, southeast-verging anticline (Valley anticline) (fig. 

3A, map A). The thrust placed Mississippian rocks in the upper 

plate above an overturned syncline (Valley syncline) in lower 

plate Pennsylvanian and Permian rocks. 

Two segments of the Valley thrust are separated by 

a broad, northwest-trending reentrant in the southernmost 

Las Vegas Range, referred to as East Pass (fig. 3A; map A) 

(Ebanks, 1965). The thrust is best exposed southwest of East 

Pass, where it dips from 40° to 45° to the northwest and 

ramps along shaly beds of the Mississippian Indian Springs 

Formation (map A). Northeast of the pass, Mississippian 

rocks die out and we interpret that the thrust terminates 

into the Valley anticline. In the East Pass area, the Valley 

thrust and folds are offset subhorizontally about 1 km by 
31 

a northeast-dipping detachment fault (map A). This fault 

is concealed beneath Quaternary deposits in East Pass, but 

exposed northeast of the pass in the Bird Spring Formation, 

where it dips 25° to the northeast (map A). Because the fault 

offsets Mesozoic structures, we conclude that it formed during 

oroflexural bending and structural adjustment associated with 

movement on the Las Vegas Valley shear zone during the 

Miocene. Recognition of the Valley thrust and folds has led 

to refinement of correlation of major Mesozoic thrust faults 

across the shear zone (Lundstrom and others, 1998). 

Correlation of Thrusts 

Mesozoic thrust faults in the Sheep and Las Vegas ranges 

correlate with thrusts in the Spring Mountains. The thrusts 

were offset right-laterally along the northwest-striking Las 

Vegas Valley shear zone during Miocene time and indicate 

50 km of horizontal displacement (Duebendorfer and Black, 

1992; Longwell, 1960; Wernicke and others, 1988). Of these 

thrusts, the Gass Peak thrust in the southern Sheep Range 

correlates with the Wheeler Pass thrust in the northern Spring 

Mountains (Guth, 1981, 1990; Wernicke and others, 1988; 

Burchfiel and Davis, 1988). The Wheeler Pass thrust equivalent, to the south and west of the quadrangle, however, is 

controversial. Snow (1992) correlated the Wheeler Pass thrust 

with the Montgomery thrust, exposed in the Montgomery 

Mountains west of the quadrangle. Snow (1992) proposed that 

these thrusts were offset right-laterally during the Miocene, 

and estimated 20 km of right-lateral displacement between the 

Spring and Montgomery Mountains. 

Snow (1992) correlated the Kwichup thrust (Stirling 

Mine fault in northwest Spring Mountains) with the Lemoigne thrust in the Cottonwood Mountains of the Death Valley 

region. Abolins (1999) alternatively correlated the Kwichup 

thrust with the Montgomery thrust, and suggested that the 

Wheeler Pass thrust equivalent is south of the Spring Mountains and is concealed beneath Pahrump Valley, or it may be in 

the Kingston Range, 40 km south of the quadrangle. This correlation is based on detailed mapping that revealed continuity 

of stratigraphic units between the Spring Mountains and Montgomery Mountains and similarity of structure between the 

Kwichup and Montgomery faults. Paleogeographic reconstruction of major thrusts in the Death Valley and Spring Mountains 

area indicates 85–115 km of west-northwest translation of the 

Cottonwood Mountains and Panamint Range with respect to 

the Spring Mountains (Abolins, 1999). This large-magnitude 

translation was accommodated in part by movement along 

major Tertiary extensional structures that separate the Spring 

Mountains from the Death Valley extensional terrain, including the State Line fault zone. 

Wernicke and others (1988) correlated the Lee Canyon 

thrust in the Spring Mountains with the Dry Lake thrust in the 

Dry Lake Range northeast of Las Vegas. Marked similarities 

between the newly mapped Valley thrust and the Lee Canyon 

Geology of the Las Vegas 30′ × 60′ Quadrangle 

and Macks Canyon thrusts along the margins of Las Vegas 

Valley suggest that it is more likely that these thrusts are 

correlative (Lundstrom and others, 1998). For example, both 

thrusts have ramp-anticlines composed of Mississippian rocks 

in their upper plates, both were thrust above overturned Pennsylvanian and Permian rocks in their lower plates, and both 

thrusts ramped along shaly units of the Mississippian Indian 

Springs Formation. In addition, the distance from the Gass 

Peak thrust to the Valley thrust (8–9 km) is more compatible 

with the distance measured from the Wheeler Pass thrust to 

the Lee Canyon and Macks Canyon thrusts (9–10 km) than the 

distance measured from the Gass Peak thrust to the Dry Lake 

thrust (22 km). Therefore, Lundstrom and others (1998) correlated the Dry Lake thrust with the Deer Creek thrust, 18–19 

km east of the Wheeler Pass thrust in the Spring Mountains. 

Both thrusts have lower Paleozoic rocks (Cambrian and Ordovician) in their upper plates and contain upper Paleozoic and 

Mesozoic rocks in their lower plates. In addition, similarities 

in the Eureka Quartzite in these thrust plates (Ross, 1964) 

supports this correlation (see Eureka Quartzite on p. 22). 

The Keystone thrust in the eastern Spring Mountains 

correlates with the Muddy Mountain thrust, which is east of 

Las Vegas in the Muddy Mountain area (Fleck, 1970; Fleck 

and Carr, 1990; Wernicke and others, 1988). Both thrusts form 

the eastern limit of Mesozoic thrusts in this part of the 

Cordillera, and both thrusts have Paleozoic rocks in their 

upper plates and Mesozoic rocks in their lower plates. 

Age of Thrusts 

The age of thrust faults in the quadrangle is poorly constrained, although most workers agree that the major episode 

of thrusting in this part of southern Nevada occurred during 

the Sevier orogeny in Cretaceous time (Fleck, 1970; Guth, 

1990; Longwell and others, 1965). The age of the Keystone 

thrust in the region is based mostly on isotopic dates of synorogenic deposits cut by correlative thrust faults south (Goodsprings area) and east (Muddy Mountains) of the quadrangle. 

In the Goodsprings area, the Keystone and Contact thrusts 

both cut deposits of the Lavinia Wash sequence (Carr, 1980). 

Preliminary conventional K-Ar dates for the Lavinia Wash 

sequence were Late Jurassic to Early Cretaceous (154±10 

Ma) and thus established a maximum age of emplacement for 

these thrusts. New laser-fusion 40Ar/39Ar methods yielded dates 

of 99±0.04 Ma for the sequence, suggesting a further refined 

maximum age for the thrusts as late Early Cretaceous (Fleck 

and Carr, 1990). 

In the Muddy Mountains, the Muddy Mountain thrust 

(Keystone equivalent) cuts synorogenic deposits of the 

Baseline Sandstone of late Albian to Cenomanian(?) age 

(Bohannon, 1984), thus establishing a maximum age of thrust 

emplacement similar to the age of the Keystone and Contact 

thrusts in the Goodsprings area. The Baseline Sandstone 
32 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

and Lavinia Wash sequence are similar in age, depositional 

environment, and structural position, and thus led Fleck and 

Carr (1990) to correlate these units along the frontal Sevier 

thrust belt. They further concluded that correlation of these 

units supports regional correlation of the Keystone and Muddy 

Mountain thrusts. 

High-angle Normal and Strike-Slip Faults 

Spring Mountains 

Trough Spring Fault 

The Trough Spring fault (Vincelette, 1964) is a highangle normal and (or) oblique-slip fault that offsets the 

Wheeler Pass thrust just south of Wheeler Pass, hence the 

most recent movement on the fault post-dated thrusting, most 

likely during the Cenozoic (fig. 3A, map A). Along most of its 

trace, the fault strikes northwest, and it accommodated downto-the-southwest displacement where upper Paleozoic rocks 

(mostly Bird Spring Formation) are downdropped against 

lower Paleozoic rocks. Vincelette (1964) estimated 1,828 m of 

displacement along the northwest segment of the fault. South 

of Clark Canyon, the fault abruptly changes strike to the northeast. Vincelette (1964) explained this curious bend by differential movement of upthrown and downthrown blocks, whereby 

the downthrown block rotated and tilted northeastward during 

faulting, independently of the relatively undeformed northwest-dipping beds in the upthrown block. 

La Madre Fault 

The La Madre fault (fig. 3A, map A) strikes northwest 

and can be traced for more than 50 km across the Spring 

Mountains. The fault has been proposed to have multiple histories (Axen, 1985; Davis, 1973). As suggested by Longwell 

and others (1965), it may have been active as a tear fault with 

mostly strike-slip movement during Mesozoic thrusting, similar to the Griffith fault. In the La Madre Mountain area, Axen 

(1985) estimated 1,130–1,230 m of vertical separation along 

the fault prior to emplacement of the Keystone thrust, and only 

100–200 m of post- or syn-Keystone separation. Latest movement on the fault was speculated by Longwell to be related 

to the Las Vegas Valley shear zone. Page (new mapping, this 

report) observed older basin-fill deposits (map unit QTa) that 

are tilted as much as 15° to the south along the La Madre fault 

in the Cold Creek area, and Axen (1985) reported that Quaternary(?) colluvial deposits may be deformed by the fault in the 

Angel Peak area. 

Strike-slip movement on the La Madre fault is implied by 

apparent right-lateral offset of the Keystone, Macks Canyon, 

and Lee Canyon thrusts in the Spring Mountains. Because 

the fault offsets major thrusts in the quadrangle, the latest 

significant movement on the fault probably occurred 

during Cenozoic extension. In the La Madre Mountain area, 

the La Madre fault is coincident with a distinct structural bend 

in the Keystone thrust plate (Longwell and others, 1965). It 

there forms a boundary separating a more deformed zone to 

the northeast (where the Keystone thrust is in contact with 

the northeast-tilted Red Spring blocks) from a less deformed 

zone to the southwest, where the Keystone thrust truncates 

the Aztec Sandstone. This pattern suggests that the La Madre 

fault, together with the Turtlehead Mountain, Brownstone 

Basin, and Box Canyon faults, accommodated oroflexural 

bending of the La Madre Mountain block along the Las Vegas 

Valley shear zone (fig. 3A). 

Griffith Fault 

The Griffith fault is a northwest-striking high-angle fault 

exposed for nearly 22 km across the central Spring Mountains 

(fig. 3A, map A). Along the southeastern half of the fault, 

highly folded Mesozoic rocks on the downthrown side (southwest) are juxtaposed against upper Paleozoic rocks of the Bird 

Spring Formation on the upthrown block. The upthrown block 

is a horst block bounded on the southeast by the Griffith fault 

and on the northeast by the La Madre fault. The same Mesozoic rocks are exposed northeast of the La Madre fault and 

southwest of the Griffith fault. In this horst block, the Deer 

Creek and Kyle Canyon thrusts terminate against the Griffith 

fault, although the fault does not offset the Keystone or Lee 

Canyon thrusts. This suggests that the Griffith fault was active 

during thrusting as a tear fault that accommodated differential 

movement across the fault (Burchfiel and others, 1974; 

Longwell and others, 1965). 

Wheeler Graben 

The Wheeler graben (named here) is exposed in the 

upper plate of the Wheeler Pass thrust in the northwest Spring 

Mountains (fig. 3A, map A). It is bounded by high-angle 

normal faults that juxtapose Devonian and Mississippian rocks 

in the downthrown blocks within the graben against Cambrian 

and Ordovician rocks in upthrown blocks outside the graben 

(map A; cross section C–C′). The graben has a maximum 

width of 7 km and it pinches out to the south, just west of 

Wheeler Well. It is partly asymmetric, with several major 

northwest- to southwest-dipping normal faults of greatest displacements (2–3 km) bounding the east side, and subordinate 

northeast-dipping antithetic faults with lesser displacements 

on the west side (cross section C–C′, map sheet 1). The graben 

corresponds to a broad saddle in the gravity field, with lowest 

gravity values occurring just east of the eastern graben margin 

(map B, map sheet 2). The breadth of this gravity low and 

its correspondence to a similar saddle in the magnetic field 

suggest that the graben and these geophysical anomalies are 

caused by deeper sources that reflect topography on the denser 

and more magnetic basement. 
33 

Sheep and Las Vegas Ranges 

Major high-angle faults in the Sheep and Las Vegas 

Ranges include from west to east the Yucca Forest, Mormon 

Pass, Fossil Ridge, Gass Spring, and Hidden Valley faults (fig. 

3A). The Yucca Forest and Mormon Pass faults bound Yucca 

Forest, a broad alluvial basin in the southern Sheep Range 

(map A). The basin extends northward about 8 km into the 

Indian Springs quadrangle, and southward it pinches out along 

Fossil Ridge near the margin of Las Vegas Valley. The Yucca 

Forest fault is mostly concealed along the eastern flank of the 

southern Sheep Range, but near the northern edge of the quadrangle, it juxtaposes rocks of the Mississippian Monte Cristo 

Group on the upthrown side with late Miocene(?) to Pleistocene alluvium (QTa) on the downthrown side. 

The Mormon Pass fault bounds the Fossil Ridge horst 

block on the southeastern side of Yucca Forest. Guth (1980) 

mapped the fault farther north in the Indian Springs quadrangle as the eastern breakaway of the Miocene Sheep Range 

detachment zone. Along Fossil Ridge, the fault places Cambrian and Ordovician rocks on the upthrown side against 

Tertiary and Quaternary deposits as young as latest Pleistocene 

on the downthrown side. 

The Gass Peak basin (Guth and others, 1988; Guth, 1990) 

(fig. 3A) is bound on the northwest by the Fossil Ridge fault, 

and on the southeast side by the Gass Spring fault; dips on 

the Gass Spring fault range from 55° to 70° to the northwest. 

Miocene deposits of the Horse Spring Formation are faulted 

against Proterozoic and Paleozoic units along these faults at 

the basin margins and are locally deformed within the basin. 

Gravity lows correspond with the Yucca Forest and Gass Peak 

basins. However, a three-dimensional inversion of gravity data 

indicates that these Cenozoic basins probably are no greater 

than 0.5 km deep (Langenheim and others, chapter B, this 

report; see cross section B–B′, map sheet 1). 

The Hidden Valley fault (named here) in the Las Vegas 

Range parallels and, in places, offsets the Valley thrust and the 

Valley anticline (map A). The fault strikes northeast and dips 

from 55° to 60° to the northwest, and offsets rocks of the Bird 

Spring Formation along most of its trace. In the northeastern 

corner of the quadrangle (map A), the fault bounds a small 

Cenozoic basin where it cuts (and accommodated tilting of) 

deposits of the Horse Spring Formation that contain interbedded olivine basalts (Thb) dated at 16.4 Ma (Feuerbach and 

others, 1993). At this location, the hanging wall of the fault is 

an east-tilted half-graben. 

High-angle normal faults in the Sheep and Las Vegas 

Ranges, such as the Mormon Pass and Hidden Valley faults 

(map A), appear to be oroflexurally bent along the Las Vegas 

Valley shear zone, implying they pre-dated major movement 

on the shear zone and played a role in the early development 

of Cenozoic extensional basins in the region from about 20 to 

15 Ma. Guth and others (1988) proposed that early movement 

on the Mormon Pass fault induced development of the Yucca 

Forest and Gass Peak basins. Most high-angle normal faulting, however, probably occurred during or shortly after major 

Geology of the Las Vegas 30′ × 60′ Quadrangle 

movement on the Las Vegas Valley shear zone from about 14 

to 8 Ma (Duebendorfer and Black, 1992), a time of prolific 

extensional faulting in southern Nevada. 

Las Vegas Valley Shear Zone 

The Las Vegas Valley shear zone (LVVSZ) (fig. 3A; map 

A) is one of the major structures in the Las Vegas quadrangle. 

Because it is concealed by thick Tertiary and Quaternary 

basin-fill deposits, its geometry and location are based primarily on geophysical studies (Langenheim and others, 1997, 

1998; chapter B of this report). The LVVSZ is a northweststriking, right-lateral strike-slip fault zone that is traced about 

150 km from Mercury, Nevada, southeast to the Lake Mead 

area. Fault segments of the LVVSZ that break the surface are 

exposed in the Corn Creek Springs area (map A). The effects 

of the LVVSZ include oroflexural bending and offset of major 

Mesozoic thrust faults and folds. Offset of Mesozoic thrust 

faults across Las Vegas Valley indicate 48±7 km of rightlateral separation (Wernicke and others, 1988); this estimate 

includes bending of the Las Vegas Range. Offset decreases at 

the eastern and western ends of the shear zone. For this reason, 

the LVVSZ and associated strike-slip faults are considered 

transverse structures (transverse zones) which separate regions 

of different rates, amounts, and types of extension and magmatism (Fleck, 1970; Guth, 1981; Wernicke and others, 1982; 

Duebendorfer and Black, 1992; Rowley, 1998). 

Paleomagnetic data (Sonder and others, 1994; Nelson 

and Jones, 1987) indicated a 20-km-wide zone of clockwise 

rotation as great as 100° in rocks as young as 13.5 Ma adjacent 

to the LVVSZ. The paleomagnetic data, along with other 

structural data, bracket the principal period of movement 

along the LVVSZ between 14 and 8.5 Ma (Duebendorfer and 

Black, 1992; Duebendorfer and Simpson, 1994). Wernicke and 

others (1982, 1988) interpreted that detachment faults are the 

primary structures in the area and the LVVSZ and Lake Mead 

fault system are in effect tear faults in the hanging wall of a 

detachment. Similarly, Duebendorfer and Wallin (1991) and 

Duebendorfer and Black (1992) suggested that normal faulting 

and strike-slip motion on the LVVSZ were coeval and thus 

root into the Saddle Island detachment fault, 25 km southeast 

of Frenchman Mountain in the adjacent Lake Mead quadrangle. The Saddle Island fault bisects Saddle Island and dips 30° 

to the northwest (Choukroune and Smith, 1985). More likely, 

however, the LVVSZ and the Lake Mead fault system are the 

major structures that control normal faults and detachments 

(Campagna and Aydin, 1994; Rowley, 1998). An alternative 

view is that of Anderson and others (1994) who suggested that 

the LVVSZ bounds the northern side of westward-flowing 

ductile rocks. 

Previous efforts to determine the location of the LVVSZ 

beneath the alluvial deposits of Las Vegas Valley were limited 

to gravity studies (Plume, 1989; Campagna and Aydin, 1994). 

Longwell and others (1965) indicate the LVVSZ as a N. 60° 

W.-striking fault extending northeast from Frenchman Mountain to Indian Springs, Nevada. Campagna and Aydin (1994) 
34 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

showed the LVVSZ as several parallel strands stepping across 

Las Vegas Valley to a point south of Frenchman Mountain and 

cutting across the northern tip of the McCullough Mountains. 

Our interpretation of the LVVSZ indicates two non-parallel 

strike-slip faults creating two sets of pull-apart basins beneath 

Las Vegas Valley (figs. 3A and 4; Langenheim and others, 

chapter B this report). The northern strand of the LVVSZ as 

shown by Campagna and Aydin (1994) coincides approximately with our interpreted northern strand of the LVVSZ. 

We, however, disagree with the location and orientation of 

the southernmost strand they proposed, shown cutting across 

the northern McCullough Mountains. Wallin and Duebedorfer 

(1995) reported no noteworthy fault in that area. Furthermore, 

the aeromagnetic anomalies generated by the volcanic rocks 

in the northern McCullough Mountains do not show evidence 

of strike-slip faults in this area (map C; fig. 5). Instead, we 

propose that a strand of the LVVSZ cuts across the valley and 

is aligned with the Frenchman Mountain fault segment that 

bounds the southern margin of Frenchman Mountain (fig. 

3A; map A). The two strands of the LVVSZ could be contemporaneous; alternatively, they may be of different ages. The 

left-lateral strike-slip faults of the Lake Mead fault system 

apparently exhibited similar behaviors: older strands strike 

more easterly (N. 60°–70° E.) than do the younger faults. This 

relationship may also apply to strands of the LVVSZ, where 

older strands may be characterized by more westerly trends. 

State Line and West Spring Mountains 

Fault Zones 

The State Line fault zone (Blakely and others, 1998) is 

a northwest-striking, right-lateral, strike-slip fault zone that 

extends through Pahrump Valley (fig. 3A; map A). The fault 

has also been referred to as the State Line fault (Hewett, 

1956), Pahrump Valley fault zone (Wright and others, 1981; 

Hoffard, 1991), Amargosa River fault zone (Donovan, 1991), 

and the Amargosa fault zone (Schweickert and Lahren, 1997). 

The fault zone has a topographic expression that extends about 

175 km northwest from Ivanpah Valley, through Mesquite 

Valley and Ash Meadows, and may continue northwestward to 

define the northeast flank of the Funeral Mountains. The fault 

zone is structurally complex, and based on gravity data, the 

bedrock basement concealed beneath alluvial valleys consists 

of a series of narrow, steep-sided sub-basins, similar to Las 

Vegas Valley. The sub-basins have been interpreted as transtensional or pull-apart basins, caused by en-echelon right steps 

during strike-slip faulting (Blakely and others, 1998). 

In the Pahrump Valley, the bedrock basement surface is 

relatively flat along the valley margins, but within the central 

part of the valley, two northwest-trending steep-sided basins 

are separated by a basement gravity ridge (Blakely and others, 

1998; figs. 6B and 9B). Based on gravity data, the southwestern basin is 2 km deep, and the northeastern basin is about 5 

km deep. The basement gravity ridge is exposed on the surface 

in the Pahrump area as a gravel-capped ridge that extends 

northwest through the Mound Spring quadrangle (Lundstrom 

and others, 2003) in the southwestern part of the quadrangle. 

The ridge is interpreted as a horst block, or as a compressional 

structure caught up between reverse faults along this segment 

of the fault zone. The basement ridge may be an important feature controlling ground-water flow in Pahrump Valley. Blakely 

and others (1998) suggested that faults bounding the northeast 

margin of the ridge may act as a ground-water barrier, where 

ground water flowing from the north and east is brought to 

the surface. This is based on the distribution of springs and 

paleospring discharge deposits restricted to areas along the 

northeast side of the ridge. 

The West Spring Mountains fault zone (fig. 3A; map A) 

(Anderson and others, 1995b) is the range front fault zone 

along the southwestern Spring Mountains. The overall fault 

zone has a sinuous trace, but most strands strike north to 

northwest. The southern end of the fault zone merges with 

the State Line fault zone in Pahrump Valley. The fault was 

determined to be mostly dip-slip, and it accommodated westside-down movement. The fault zone cuts Quaternary deposits 

as young as late Pleistocene and has a strong geomorphic 

expression. 

Cenozoic Landslide Deposits 

Well-consolidated Cenozoic landslide deposits are on the 

flanks of the Spring Mountains. These deposits are frequently 

referred to as gravity-slide blocks, because most formed by 

downslope movement influenced by gravity. The landslide 

deposits are bound at their base by low-angle shear-slip surfaces, denoted on map A by a single-hachured fault, with the 

hachures on the upper plate. Landslide deposits are generally 

of two textural types in the quadrangle. The first type consists 

of kilometer-size blocks of bedrock units that have maintained 

their stratigraphic order during landsliding, except for local 

breccia zones along their slip surfaces. The second type is 

composed predominantly of megabreccia. Although megabreccia texture is commonly associated with large landslide block 

deposits and large blocks are locally observed within megabreccia deposits, the landslide deposits discussed below were 

categorized based on their dominant textural type. 

An example of the first type of deposit is a large, composite landslide flanking the southwest Spring Mountains 

near the mouth of Trout Canyon (Burchfiel and others, 1974) 

(map A). In this area, large landslide blocks (as much as 4 by 

6 km in diameter), consisting mostly of the Mississippian to 

Permian Bird Spring Formation, slid over Cambrian rocks 

in the upper plate of the Lee Canyon thrust, and one of these 

landslide blocks partly conceals the southern segment of the 

thrust. Burchfiel and others (1974) suggested that the stratigraphic coherency of these large blocks indicated they moved 

downslope somewhat slowly. 

Another example of this type was recently recognized 

(W.R. Page, new mapping, this report) in the Red Rock Wash 
35 

area, at the northeast edge of Blue Diamond Hill in the southcentral part of the map (map A). This feature is characterized 

by a 5 by 6 km block of Kaibab and Toroweap Formations, 

just southeast of the Red Rock Canyon visitor’s center. We 

interpret this block to have detached and slid eastward from 

Blue Diamond Hill due to faulting and local relief along the 

La Madre fault scarp. The basal shear-slip surface of the landslide block is bowl-shaped, and rocks in the upper plate dip 

to the northeast and are highly deformed by closely spaced, 

high-angle normal faults (map A) that most likely sole into the 

basal shear plane. In contrast, rocks in the stable block of Blue 

Diamond Hill dip gently to the west and are relatively undeformed. A north-trending, east-verging overturned anticline at 

the eastern margin of the landslide block probably accomodated local compression at the toe of the landslide. 

Lone Mountain, an isolated 1-kilometer-diameter block 

of Devonian and Mississippian rocks east of La Madre 

Mountain in the central part of the map, is also interpreted as 

a landslide deposit of this type. Geophysical studies indicate 

that Lone Mountain is underlain by low-resistive, low-density rocks (Zohdy and others, 1992; Langenheim and others, 

chapter B, this report). Zohdy and others (1992) interpreted 

these rocks as the Aztec Sandstone, saturated with water, and 

possibly the Chinle or Moenkopi Formations. Alternatively, 

these rocks may be Tertiary and Quaternary basin-fill deposits. 

These data suggest that Lone Mountain is not rooted by older 

Paleozoic rocks and is a landslide block that detached from the 

La Madre Mountain area and slid over either Mesozoic units 

or Tertiary and Quaternary basin-fill deposits during Neogene 

extension. 

West of Willow Creek and east of the Nye–Clark County 

line in the northwestern part of the quadrangle, W.R. Page 

(map A) mapped several low-angle-normal faults in Ordovician through Mississippian rocks that may be part of a large 

composite landslide complex, shed off the northwest Spring 

Mountains. Along one low-angle fault, a 2-kilometer-diameter 

block composed of coherent upper-plate Devonian and Mississippian rocks overlies Ordovician rocks in the lower plate 

(map A). Along other low-angle faults in this area, Mississippian rocks in the upper plate are juxtaposed against Devonian 

rocks in the lower plate. These low-angle faults may represent 

shear-surfaces of a much larger composite landslide deposit 

that extends westward into the adjacent Death Valley Junction 

1:100,000-scale map, where similar low-angle normal faults 

are present. One of these faults is the Point of Rocks detachment fault (Abolins, 1999). K-Ar ages reported for volcanic 

ash beds interbedded with synextensional basin deposits in the 

upper plate of the fault range from 13 to 10 Ma, and these ages 

date, or slightly post-date, movement on the fault (Abolins, 

1999). Abolins (1999) suggested upper plate rocks were probably derived from the northwest Spring Mountains during 

extensional unroofing of the range. 

The second type of landslide deposit is chaotically mixed 

megabreccia consisting of clasts of bedrock units in a comminuted matrix of the same material. Internal texture of the 

megabreccia landslides suggests that they formed as rock 

Geology of the Las Vegas 30′ × 60′ Quadrangle 

avalanche deposits and were emplaced rapidly downslope. 

Page and others (1998) described megabreccia deposits along 

the eastern Spring Mountains that they called the Blue Diamond landslide. The megabreccia is predominantly derived 

from Paleozoic rocks interpreted to have slid nearly 10 km 

from their proposed source area above the Wilson Cliffs. 

Although the majority of the Blue Diamond landslide deposit 

is labeled on map A as unit MDu (Sultan Formation), other 

rock units are identifiable in the deposit including rocks of the 

Monte Cristo Group. Axen (1985) mapped similar megabreccia deposits in the La Madre Mountain area; these may be part 

of the Blue Diamond landslide. 

Other megabreccia landslide deposits are exposed on the 

northwest flank of the Spring Mountains, between Willow 

and Cold creeks. The upper plate of this landslide consists 

of megabreccia deposits derived mostly from the Cambrian 

Bonanza King Formation which slid northeastward above 

Ordovician rocks. We interpret that these deposits were shed 

off a splay of the La Madre fault (map A). 

The age of emplacement for landslides on the flanks 

of the Spring Mountains is poorly constrained, but we agree 

with Burchfiel and others (1974) and Axen (1985) that most 

of these well-consolidated landslides are Cenozoic in age and 

were emplaced during maximum episodes of extension in the 

Miocene (Abolins, 1999; Page and others, 1998). Miocene 

landslide deposits are reported in areas adjacent to the Las 

Vegas quadrangle and in other parts of the southern Basin and 

Range Province. Guth and others (1988) reported megabreccia 

landslide deposits on the west side of the Sheep Range that 

were emplaced between 17 and 12 Ma. Landslide deposits on 

the flanks of the northern Mormon Mountains, about 100 km 

northeast of the quadrangle, are 12.5–15 Ma (R.E. Anderson, 

U.S. Geological Survey, written commun., 1998). Minor and 

Fleck (1994) reported landslide deposits near Beatty, Nevada, 

100 km to the west of the quadrangle, that were emplaced 

between 11 and 12 Ma. Krieger (1977) documented landslide 

deposits in the basin and range of southeastern Arizona that 

are encased in Miocene basin-fill sediments. The occurrence 

of Miocene landslide deposits throughout the southern basin 

and range region suggests they shared a common genesis, 

and were generated in response to high relief formed by 

fundamental processes of crustal extension. These processes 

include development of basin-range topography through 

high- and low-angle normal and strike-slip faulting and 

development of positive areas related to the rise of plutons and 

core complexes. 

Quaternary Faults 

Introduction 

Quaternary fault activity in the Las Vegas quadrangle is 

indicated by numerous faults that displace Quaternary deposits 

at the surface. A wide range of studies on Quaternary faults in 

the Las Vegas region were presented at a seismic hazards 
36 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

conference in 1996 (dePolo, 1998). The overview and discussion below emphasizes mapped Quaternary faults in the Las 

Vegas and Pahrump Valleys, which are the two basin areas 

dominated by Quaternary surficial deposits. Because Quaternary faulting can only be recognized where Quaternary deposits and surfaces exist in appropriate positions, it is possible 

that unrecognized Quaternary faulting could be associated 

with some of the bedrock structures noted above, especially 

the high angle normal faults. 

Las Vegas Valley 

A prominent set of fault scarps exist in the central part 

of the Las Vegas Valley (fig. 3A). These intra-valley faults, 

including the Decatur, Valley View, and Cashman faults 

(Slemmons, 1992), were first systematically mapped by Bell 

(1981) who studied their relationships to subsidence due to 

ground-water withdrawal and to tectonics. At the latitude 

range of central Las Vegas, the faults are generally north-south 

trending and form large (ranging in height from 12–55 m) 

scarps in fine-grained sediments. The scarps and sediments are 

now mostly covered by urbanization, but the fault scarps are 

still obvious in the urban landscape. The basic geologic relations of these faults were reported in the four Las Vegas 7.5’ 

quadrangles (Bingler, 1977; Matti and Bachuber, 1985; Matti 

and others, 1987, 1993) through analysis of aerial photographs 

taken before 1950. 

The north-south trend of these intra-valley faults changes 

to southeastward in the southern part of the Las Vegas Valley 

and to northeastward in the northern part of the valley. In the 

southeast corner of the quadrangle, the Whitney Mesa fault 

zone (fig. 3A) has several strands forming a composite scarp 

55 m high just south of Whitney Mesa, but this fault system 

is apparently buried by the caliche-cemented gravels (Qao) 

which cap Whitney Mesa. However, discontinuous fractures 

aligned on this fault zone cut the caliche and are locally filled 

with travertine where sample WM1A-A and WM1A-B yielded 

U-series dates of 363 ± 30 ka and 317 ± 19 ka (table 2). 

The north end of the Decatur fault curves northeast to 

become the Eglington fault (fig. 3A). Similarly, the north ends 

of the Valley View and Cashman faults turn to northeasterly 

trends. Thus, the wrap-around pattern of the intra-valley 

faults is subparallel to both the west margin of the Frenchman 

Mountain block and its subsurface gravity signature, as well 

as to the concave eastward curvature of the Keystone thrust. 

Whether there is any genetic reason for this coincidence is 

unknown and deserves further study. 

All of the above intra-valley faults are in fine-grained 

deposits inferred to at least partly include deposits associated with past ground-water discharge that were documented 

as such north of the Eglington fault (Haynes, 1967; Quade, 

1986). The faults form discontinuities and barriers to the 

stratigraphically controlled basinward ground-water flow and 

thus may have been significant in localizing past ground-water 

discharge. A modern analogue is the area of historically active 

artesian springs of the Las Vegas Valley which are along the 

Valley View fault. 

Conversely, the faults and associated fine-grained deposits are known to control the patterns of subsidence related to 

ground-water overdrafts in the Las Vegas Valley (Bell, 1981; 

Amelung and others, 1999). Mifflin (1998) attributes most 

movement on these intra-valley faults to differential compaction due to dewatering of compressible fine-grained sediments to the east of each of the above faults. Bell and dePolo 

(1998) showed that the magnitude of apparent offset from 

scarp height cannot be explained by differential compaction 

alone; therefore a significant tectonic component is likely. It 

is noteworthy that a large area of recent historic subsidence 

measured by Amelung and others (1999) is on the northwest, 

upthrown side of the Eglington fault, which bounds most of 

the subsidence. This is opposite the sense needed to explain 

down-to-southeast fault movement by prehistoric groundwater compaction. 

The Eglington fault forms a broad low-angle scarp about 

12 m high that steepens downslope toward its base. The 

surface expression of this fault scarp coincides with a gravity 

ridge bounding two gravity lows (chapter B) and with offsets 

in the bedrock alluvial contact and in the valley-fill alluvium 

at depth (Plume, 1989). The Eglington fault may be bounded 

at its northeast end by the Las Vegas Valley shear zone 

(LVVSZ). 

The LVVSZ is delineated in the subsurface by a steep 

gravity gradient (discussed in chapter B), but it does not have 

a clear surface fault expression. However, the Corn Creek 

Springs fault (Haynes, 1967; Bell and others, 1999) is aligned 

on the LVVSZ and has surface expression as it cuts associated 

fine-grained deposits associated with past ground-water discharge (Quade and others, 1998). Parallel to this zone, and to 

the northeast, the northwest-trending frontal fault of the Sheep 

Range also shows Quaternary faulting that juxtaposes bedrock 

against Quaternary fan gravels. 

The Frenchman Mountain fault (Anderson and 

O’Connell, 1993) (fig. 3A) is at the western range front of 

Frenchman Mountain, but in detail has various non-aligned 

short segments cutting various ages of alluvial fans (Peck, 

1998) ranging in age from middle Pleistocene to early 

Holocene (map A). The steep gravity gradient west of the 

range front fault (discussed in chapter B) coincides with the 

entire Frenchman Mountain piedmont and indicates the main 

range-bounding fault is 1–2 km west of the range front. This 

is consistent with the occurrence of outcrops of the Muddy 

Creek(?) Formation which are overlain on this piedmont by 

thin Quaternary pediment deposits. 

To the west of the Las Vegas Valley, in the area where 

projections of the La Madre, Turtlehead Mountain, and 

Brownstone Basin faults and Bird Spring thrust all converge 

on Red Rock Wash, there is a large (about 30 m high) eastfacing fault scarp bounding map unit QTa. Just north of the 

projection of this scarp, a late Pleistocene terrace of Qai has 

a gentle monocline that displaces its grade about 15 m down 

to the east. At the hinges of the monocline, there are fractures 
and minor displacement probably related to the folding of the 

carbonate-cemented horizons of the surficial soil of Qai. 

Pahrump Valley 

Gravity investigations (discussed in chapter B) show an 

elongate northwest-trending horst along the Nevada-California 

boundary, flanked by northwest-trending basins on both sides. 

The horst and flanking basins, and their approximate alignment with similar subsurface features beneath the Amargosa 

Desert, were interpreted by Blakely and others (1999) as 

transpressional and pull-apart features, respectively, along 

the proposed, northwest-trending right-lateral State Line fault 

system. 

The Quaternary geology is consistent with this structural 

interpretation. The gravity high coincides with the surface 

occurrence of QTa, which forms a northwest-aligned chain of 

hills. This alignment is traversed and cut by the southwesternsloping, modern and late Quaternary drainage system tributary 

to the playa (Qpy) in the southwest corner of the quadrangle. 

A large, complex, southwest-facing, northwest-trending 

escarpment as much as 20 m high coincides with the outcrop 

of Qbu. The scarp is about 2 km northeast of and subparallel to the hills composed of unit QTa that are along 

the crest of the gravity ridge. Though Hoffard (1991) and 

Anderson and others (1995a) considered this scarp as a fault 

scarp of a down-to-the-southwest normal fault, we interpret 

an opposite, northeast-down sense of displacement based on 

subsurface geology indicated by the gravity gradient. Moreover, in gully exposures near Brown Spring and in the adjoining Hidden Hills Ranch quadrangle, we can trace continuous 

unfaulted beds of QTbo from gullies cut into the scarp to a 

few hundred meters southwest of the scarp. The beds exposed 

in the scarp area dip 5°–15° northeast into the scarp, and the 

main scarp is held up by the resistant limestone bed of Qbw. 

Northeast dips increase as much as 20° as the scarp is crossed 

to the southwest, suggesting the scarp may be localized along 

the east side of a northeastward-facing monocline. The northeast dips on middle Pleistocene strata in a direction opposite 

the southwest topographic slope are consistent with middle to 

late Quaternary uplift in the area of the horst. Because surface 

fault scarps do not cut the predominantly Holocene sediments 

in the valley between the scarp and the hills composed of unit 

QTa, we infer a buried, intermittently active fault with downto-the-northeast displacement under the intervening sediments, 

consistent with both the gravity data and surface geology. 

In the area of fine-grained sediments between the playa 

and the State Line horst in the southwest corner of the map, 

there are numerous curving lineaments that are prominent on 

aerial photographs (Lundstrom and others, 2003). We interpret these lineaments to be tension cracks accentuated by 

vegetation; they lack surface displacement and are difficult to 

identify on the ground. They may be related to some combination of dessication, evaporation, and ground-water discharge 

through the playa surface, and (or) extension related to the 

hypothesized strike-slip and pull-apart tectonics. 

Geology of the Las Vegas 30′ × 60′ Quadrangle 37 

About 9 km east of the Pahrump Valley playa and apparently splaying off the State Line horst is a north-northweststriking system of left-stepping en-echelon west-facing scarps 

which cut both older and younger fine-grained deposits, as 

well as Qai fan gravel. As in Las Vegas Valley, the faults 

probably formed barriers to valleyward ground-water flow 

that favored past ground-water discharge with which the 

fine-grained deposits are associated. Similarly, a fault bounds 

and probably controlled past discharge associated with finegrained deposits forming badlands along the highway near 

Pahrump (dePolo and others, 1999). Cracks and related piping 

features were observed and mapped along the upper parts of 

scarps in the late Pleistocene ground-water discharge deposits. These are probably a result of subsidence from historic 

ground-water withdrawal similar to that documented in the 

Las Vegas Valley (Bell, 1981). 

About 5–8 km to the east of the State Line horst is the 

south end of the West Spring Mountain fault zone (fig. 3A; 

Anderson and others, 1995a), which forms a prominent scarp 

and graben in Qai fan gravels. The west-facing scarp forms 

a sharp 90° bend about 1 km from the range front. Further to 

the northwest, more west-facing scarps form left-stepping segments several kilometers from the range front. 

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Stewart, J.H., 1972, Initial deposits in the Cordilleran geosyncline: evidence of a late Precambrian (<850 m.y.) continental separation: Geological Society of America Bulletin, v. 

83, p. 1345–1360. 

Stewart, J.H., 1976, Late Precambrian evolution of North 

America: plate tectonics implication: Geology, v. 4, 

p. 11–15.

Stewart, J.H., and Poole, F.G., 1972, Lower Paleozoic and 

uppermost Precambrian Cordilleran Miogeocline, Great 

Basin, western United States, in Dickinson, W.R., ed., Tectonics and Sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication 22, p. 28–57. 

Taylor, D.W., 1967, Late Pleistocene molluscan shells from 

the Tule Springs area, in Wormington, H.M., and Ellis, D., 

eds., Pleistocene studies of southern Nevada: Nevada State 

Museum Anthropological Paper no. 13, p. 396–399. 

Tschanz, C.M., and Pampeyan, E.H., 1970, Geology and Mineral deposits of Lincoln County, Nevada: Nevada Bureau of 

Mines and Geology Bulletin 73, 187 p., scale 1:250,000. 

U.S. Soil Conservation Service, U.S. Department of Agriculture, 1985, Soil Survey of Las Vegas Valley Area, Nevada: 

U.S. Soil Conservation Service, U.S. Department of Agriculture, 194 p. 

Vincelette, R.R., 1964, Structural geology of the Mt. Stirling 

quadrangle, Nevada, and related scale-model experiements: 

Stanford University, Stanford, California, Ph.D. thesis, 

141 p. 

Wallin, E.T., and Duebendorfer, E.M., 1995, Comment on 

“Basin genesis associated with strike-slip faulting in the 

basin and range, southeastern Nevada,” by D.J. Campagna 

and A. Aydin: Tectonics, p. 1056–1057. 

Webster, G.D., 1969, Chester through Derry conodonts 

and stratigraphy of northern Clark and southern Lincoln 

Counties, Nevada: University of California Publications in 

Geological Sciences, v. 79, 121 p. 

Webster, G.D., and Lane, N.R., 1967, The MississippianPennsylvanian boundary in southern Nevada, in Teichert, 

Curt, and Yochelson, E.L., eds., Essays in paleontology 

and stratigraphy, R. C. Moore Commemorative Volume: 

Lawrence, Kansas, Kansas University Press, Department of 

Geology Special Publication 2, p. 503–522. 

Weide, D.L., 1982, Surficial Geologic Map of the Las Vegas 

1° × 2° quadrangle, Nevada, Arizona, California, and Utah: 

U.S. Geological Survey Open-File Report 82-706, 9 p., 

scale 1:250,000. 

Wernicke, Brian, Guth, P.L., and Axen, G.J., 1988, Tertiary 

extensional tectonics in the Sevier thrust belt of southern Nevada, in Weide, D.L., and Faber, M.L., eds., This 

Extended Land; Geological journeys in the southern Basin 

and Range; Geological Society of America Cordilleran 

Section Field Trip Guidebook: University of Nevada at Las 

Vegas Geoscience Department Special Publication no. 2, p. 

473–499. 

Wernicke, Brian, Spencer, J.E., Burchfiel, B.C., and Guth, 

P.L., 1982, Magnitude of crustal extension in the southern 

Great Basin: Geology, v. 10, p. 499–502. 

Williams, P.A., and Orlins, R.I., 1963, The Corn Creek dunes 

site: Nevada State Museum Anthropological Paper 10, 65 p. 

Wilson, R.F., and Stewart, J.H., 1967, Correlation of Upper 

Triassic(?) formations between southwestern Utah and 

southern Nevada: U.S. Geological Survey Bulletin 1244-D, 

20 p. 

Wright, L.A., Troxel, B.W., Burchfiel, B.C., Chapman, R.H., 

and Lobotka, T.C., 1981, Geologic cross section from the 

Sierra Nevada to Las Vegas Valley, eastern California to 

southern Nevada: Geological Society of America Map and 

Chart Series MC-28M, p. 3–15, 1 plate. 

Zohdy, Adel, Bisdorf, R.J., and Dixon, G.L., 1992, Geoelectrical exporation for ground water in northwest Las Vegas and 

vicinity, Nevada: U.S. Geological Survey Open-File Report 

92-264, 111 p. 
Chapter B


Geophysical Framework of the Las Vegas 30′ × 60′

Quadrangle


By Victoria E. Langenheim,1 Richard J. Blakely,1 William R. Page,2 Scott C. Lundstrom,2 Gary L. Dixon,3 and 

Robert C. Jachens1 

1

U.S. Geological Survey, Menlo Park, CA 

2

U.S. Geological Survey, Denver, CO 

3

Southwest Geology, Inc., Blackfoot, ID 

Introduction 

An important objective of geologic mapping is to understand how surficial structures and stratigraphy project into the 

subsurface. Geophysical data and analyses are useful tools for 

reaching this objective. The gravity and aeromagnetic maps 

provide a three-dimensional perspective to the geologic map 

of the Las Vegas 30′ × 60′ quadrangle. The isostatic gravity 

anomaly map (map B, map sheet 2) reflects density variations in the upper and middle crust. Interpretation of these 

anomalies provides information on the depth of Cenozoic 

basins and on the nature of pre-Cenozoic basement rocks. 

The aeromagnetic map (map C, map sheet 2), in contrast, is 

more sensitive to the distribution of magnetic rocks, primarily 

those containing magnetite. In the Las Vegas quadrangle, these 

rocks are Tertiary igneous and Precambrian crystalline rocks. 

In particular, aeromagnetic anomalies mark abrupt spatial 

contrasts in magnetization that can be attributed to lithologic 

boundaries, potentially caused by faulting of these rocks. 

These two geophysical maps, in concert with information from 

geologic mapping, wells, and other geophysical data, provide 

constraints on the subsurface geology. 

Aeromagnetic Data 

The aeromagnetic map (map C) is based on measurements from east-west flight lines acquired in two separate 

surveys (U.S. Geological Survey, 1979; 1983). Both surveys 

were flown at 300 m (1,000 ft) above the ground along flight 

lines spaced 1.6 km (1 mi) apart. The data were adjusted to 

a common datum and then merged by smooth interpolation 

across a buffer zone along the survey boundaries. Saltus and 

Ponce (1988) published these data as a mosaic at a scale of 

1:250,000. 

To help delineate trends and gradients in the aeromagnetic data, we calculated magnetization boundaries in 

the following way: first, in order to emphasize the edges 

of shallow magnetic sources, we subtracted a numerically 

derived regional field from the actual data. The regional field 

was computed by analytically continuing the aeromagnetic 

data to a surface 100 m higher than actually measured, an 

operation that tends to smooth the data by attenuating shortwavelength anomalies (Blakely, 1995). The resulting residual 

aeromagnetic field accentuates those anomalies caused by 

shallow sources and is shown in figure 5. Second, the resulting 

residual aeromagnetic field was mathematically transformed 

into pseudogravity anomalies (Baranov, 1957); this procedure effectively converts the magnetic field to the equivalent 

“gravity” field that would be produced if all magnetic material 

were replaced by proportionately dense material. Third, the 

horizontal gradient of the pseudogravity field was calculated 

everywhere by numerical differentiation. Lastly, locations 

of the locally steepest horizontal gradient were determined 

numerically (Blakely and Simpson, 1986). These locations 

occur approximately over vertical or near-vertical contacts that 

separate rocks of contrasting magnetic properties. 

Isostatic Gravity Data 

The isostatic gravity map (map B) was created from more 

than 2,500 gravity stations. The details of the data acquisition, sources, and processing can be found in Langenheim and 

others (1999). In general, gravity highs occur over exposed or 

shallow pre-Cenozoic rocks, whereas gravity lows occur over 

thick sections of Cenozoic sedimentary and, in some cases, 

volcanic rocks. The major features on the isostatic gravity map 

are the gravity lows associated with the Las Vegas and Pahrump Valleys and the gravity high over the Spring Mountains. 

The isostatic gravity values range from a low of -33 mGal 

(centered over the Gass Peak SW 7.5′ quadrangle) to a high 

of +25 mGal over the highest part of the Spring Mountains 

near Charleston Peak. The gravity data provide information on 

major structural features such as the Las Vegas Valley shear 

zone (LVVSZ), the State Line fault zone, and the Frenchman 

Mountain fault (fig. 4). 
McCullough Mtns River Mtns 

Eldorado

Mtns 

Spring 

Mtns

 Las

Vegas

Range 

Muddy Mtns 

Desert Range 

NVCA 

Indian 

Springs 

HM 

LAKE 

MEAD FAULT 

FM 

Black 

Mtns 

LAS 

VEGAS 

VALLEY 

CC 

GH 

GP 

Sheep Range

Pintwater

Range 

Arrow Canyon Range 

Dry Lake

Range 

L VVSZ EG 

WF 

Henderson SI

PAHRUMP

VALLEY 

LAS VEGAS 30' 

× 60' 

QUADRANGLE 

FMF 

Thrust fau

 1965) 

Cenozoic s

Cenozoic v

Pre-Cenoz

EXP

Lake Mead 

Quaternary 

Hoffard 

Modern lea

Water well 

Geophysic

LVVSZ 

78E 

78E 

LM 

115°00' 114°30' 

0 50 KILOMETERS 

116°00' 115°30' 

STATE LINE FAULT ZONE 

LMS 

KT 

36°00' 

36°30' 

NV AZ 

10 20 30 40 

Figure 4. Index map of the Las Vegas region showing simplified geology (modified from Longwell and others, 1965) and major structural features. CC

Eglington fault; FM, Frenchman Mountain; FMF, Frenchman Mountain fault; GH, Gale Hills; GP, Gass Peak; HM, Hamblin Mountain; KT, Keystone th

LMS, La Madre Spring; SI, Saddle Island; WF, Whitney Mesa fault. Heavy black box outlines the Las Vegas 30’ × 60’ quadrangle. 
Geophysical Framework of the Las Vegas 30′ × 60′ Quadrangle 47 

The method of Jachens and Moring (1990) was used 

to separate that part of the gravity field caused by Cenozoic 

deposits from that caused by variations in the density of the 

basement rocks (defined here as pre-Cenozoic). The method 

was modified to include constraints based on well information 

and depths to basement derived from other geophysical data 

(B.A. Chuchel, written communication, 1995). Much of the 

following discussion is based on the results of the inversion 

and summarizes various U.S. Geological Survey Open-File 

reports (Blakely and others, 1998; Langenheim and Jachens, 

1996; Langenheim and others, 1997; Langenheim and others, 

1998). 

Basins 

Las Vegas Valley 

The broad alluvial valley of Las Vegas is characterized by 

a roughly rectangular, northwest-trending gravity low centered 

near the northern part of the valley. Another low of substantially less magnitude (minimum gravity value -15 mGal) lies 

to the southeast within the Las Vegas SE 7.5’ quadrangle 

(southeast corner of the map). The southwestern part of the 

valley is marked by higher isostatic gravity values (exceeding 

0 mGal), suggesting that this part of the valley is underlain by 

a shelf of relatively shallow pre-Cenozoic basement (Langenheim and Jachens, 1996). This region also corresponds to a 

broad aeromagnetic high, suggesting the presence of magnetic 

Precambrian basement rocks at depths of 3–4 km. To the east, 

aeromagnetic anomalies indicate shallower sources, probably 

buried Tertiary volcanic rocks, similar to those exposed in the 

McCullough Mountains. 

The gravity field over Las Vegas Valley was inverted for 

basin thickness using a density-depth function derived from 

borehole porosity data (table 3), available well data that constrain the thickness of Cenozoic fill, and depths to basement 

from seismic-reflection profile LV-4 (Langenheim and others, 

1998). The resulting distribution of basin sediments (fig. 6A), 

in general, mirrors the gravity field, although the deepest part 

of the basin is not over the lowest gravity values in the basin, 

but 5 km west of Frenchman Mountain. The apparent mismatch is caused by allowing for variations in basement density 

during the inversion method. Density measurements from 

hand samples indicate that significant density variations are 

possible between various basement rock types (table 4). 

Plume (1989) also produced a basin thickness map for 

Las Vegas Valley that differs from that shown in figure 6A. 

Our maximum basin thickness is more than 4 km, much 

deeper than his maximum estimated depth of 1.5 km. Our 

model is based on a more detailed gravity data set, independent depths to basement, and densities that increase with 

depth. Plume’s map is based on a single density contrast 

between basement and basin fill (-400 kg/m3

). 

The basement surface under Las Vegas Valley is complex 

and consists of elongated sub-basins with N. 70° W., N. 50° 

W., and N. 40° E. trends (fig. 6A). Many Quaternary fault 

scarps mapped within the valley fill coincide with changes 

in basin thickness (for example, the Eglington fault, EG on 

fig. 6A) and edges of residual aeromagnetic anomalies (fig. 

5). Cross section D–D′ (fig. 9A) illustrates the correlation of 

mapped faults with steps in the basement surface, especially 

Table 3. Density-depth functions used to determine thickness of basin-fill deposits. 

Depth range Density contrast above basement rocks (kg/m3)

 (meters) 

Las Vegas Valley Pahrump Valley 

sedimentary volcanic sedimentary volcanic

 0-200 -600 -450 -650 -450 

200-600 -450 -400 -550 -400 

600-1,200 -250 -250 -350 -350 

>1,200 -250 -250 -250 -250 

Table 4. Densities (kg/m3) and magnetic susceptibilities (10-3 SI units) of basement rocks.

 Average Susceptibility Average 

No. samples Density range density range susceptibility 

Precambrian crystalline rocks

 3 2,630–2,940 2,830 0.25–2.85 13.1 

Paleozoic sedimentary rocks

 33 2,240–2,850 2,680 0.00–0.00 0.00 

Mesozoic sedimentary rocks

 13 2,210–2,670 2,500 0.00–0.01 0.00 
48 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

those faults that strike perpendicular to D–D’. The curved 

geometry of the faults suggests that the LVVSZ may have 

played a role in their formation (Langenheim and others, 

2001). The origin of these faults is controversial; they may 

be tectonic or caused by differential compaction of sediments (due to ground-water withdrawal) or both (Bell, 1981). 

Interestingly, recent subsidence as measured by interferometric synthetic aperture radar (InSAR) data, has occurred 

on the upthrown side of the Eglington fault (Amelung and 

others, 1999), opposite to the main sense of offset measured 

by seismic data (Plume, 1989) and inferred from gravity data 

(this study). 

The deepest part of the basin under Las Vegas Valley 

trends northeast, parallel to the Frenchman Mountain fault 

system (Bell, 1981). This fault system is characterized by 

multiple shear planes (both along the bedrock-valley interface 

and within the alluvium) that (1) range in dip from as low as 

35° to near vertical and (2) young westward (Peck, 1998). The 

Frenchman Mountain fault scarps (west of the faults mapped 

along the bedrock contact) coincide with the strong gravity 

gradient. The inversion suggests that the main basin-bounding 

fault is steep, with an average dip of 60°–65° E, and it is 1–2 

km west of the bedrock contact (see also D–D’, fig. 9A). The 

gravity gradient also coincides with a strong aeromagnetic 

gradient, which suggests that the fault also offsets magnetic 

Precambrian crystalline rocks. 

The northern margin of Las Vegas Valley is marked by 

a strong gravity gradient. A weak magnetization boundary 

coincides with the location of the steep gravity gradient south 

of Gass Peak (fig. 7). However, caution should be exercised 

in interpreting this boundary because the east-west survey 

flightlines cross the LVVSZ at a highly oblique angle, and the 

filtering process may have produced artifacts in the residual 

data. Plume (1989) suggested that the gravity gradient south 

of Gass Peak marks the buried trace of the LVVSZ. Campagna 

and Aydin (1994) argued that the northern margin marks one 

of three strands of the shear zone, on the basis of their analysis 

of the gravity field. Langenheim and others (1998, 2001) 

concurred that the steep gradient south of Gass Peak (GP) represents the LVVSZ, and indicates at least two closely spaced 

strands of the shear zone along the northern margin of the 

basin. They infer another strand cutting across the valley to the 

south and connecting with a fault mapped along the southwest 

margin of Frenchman Mountain (figs. 4 and 6A). 

Corn Creek Springs 

In contrast to the basin morphology beneath Las Vegas 

Valley, the basin northwest of Corn Creek Springs is very narrow and elongate, no more than 5 km wide, and characterized 

by sub-parallel, N. 50° W.-trending edges (fig. 4). The basin 

is as much as 3 km deep. Corn Creek Springs itself is located 

along the northeastern gravity gradient of the Corn Creek 

Springs gravity low. The gravity gradient coincides approximately with the Corn Creek Springs fault(s) which documents 

late Quaternary movement (Haynes, 1967; Quade, 1986; 

Bell and others, 1999). An electrical resistivity cross-section 

derived from inverted audio-magnetotelluric soundings shows 

a ridge of somewhat higher resistivities under Corn Creek 

Springs relative to those associated with the fine-grained Las 

Vegas Valley deposits (Pierce and Hoover, 1988). The two 

margins of the basin may be related to the LVVSZ. Campagna 

and Aydin (1994) interpreted the Corn Creek Springs gravity 

low as reflecting a pull-apart basin. 

Pahrump Valley 

Another prominent and compound gravity low lies on the 

western side of the Spring Mountains over Pahrump Valley. 

The low is bifurcated by a northwest-trending gravity high that 

reaches values as high as -16 mGal. The low to the northeast 

of the gravity ridge has values as low as -32 mGal; the low 

to the southwest, as low as -24 mGal. These anomalies are 

elongated in a northwestward direction. The ridge defined 

by gravity coincides with outcrops of the oldest Quaternary 

deposits in this part of Pahrump Valley (QTa); the outcrops 

form a northwest-aligned chain of hills. The northeastern 

margin of the gravity ridge roughly coincides with mapped 

Quaternary spring discharge (Lundstrom and others, 2003). 

Hoffard (1991) mapped numerous lineaments, some of which 

are probably faults, along the margins of the ridge and the two 

basins (fig. 6B), although she and Anderson and others (1995) 

infer an opposite sense of motion for the northeastern margin 

of the ridge than we do. 

The basin thickness map for the Pahrump Valley (fig. 6B) 

is from Blakely and others (1998), who used inversion parameters somewhat different from those used east of the Spring 

Mountains (table 3). The resulting basin thickness map mirrors 

the gravity field. Most of the basement surface beneath Pahrump Valley is generally flat and lies within 200 m of the land 

surface. In the areas of the gravity low, however, the topography is punctuated by two prominent, laterally restricted basins 

(figs. 6B and 9B). The thickness of the Cenozoic deposits 

exceeds 2 km in the southwestern sub-basin and 5 km in the 

northeastern sub-basin. Some of the mapped faults and lineaments may result from fissuring of the sediments above the 

basement ridge (Lundstrom and others, 2003). These basins 

most likely formed as transtensional features along a right-lateral strike-slip fault system (State Line fault zone) paralleling 

the State line between California and Nevada (Wright, 1988; 

Blakely and others, 1998). The ridge between the basins may 

have formed because of slightly more westerly orientations of 

second-order faults within the State Line fault zone. 

As discussed in this section, we prefer the interpretations 

for a compound gravity low and that the Pahrump Valley is 

underlain by two locally restricted sub-basins separated by a 

narrow, northwest-striking ridge of pre-Cenozoic basement. 

This model (fig. 6B) assumes that the density of basin fill is 

laterally uniform. It is likely, however, that the State Line fault 

zone has promoted ground-water flow, and this may have 

increased the densities of near-surface sedimentary deposits 
Geophysical Framework of the Las Vegas 30′ × 60′ Quadrangle 49 

along the fault zone by cementation and alteration. Thus, an 

alternative interpretation for the narrow gravity high invokes 

high-density sedimentary deposits along the State Line fault 

zone rather than basement topography. We favor the basement 

ridge interpretation, although the alternative of higher-density 

basin fill may contribute to the magnitude of the gravity high 

of the ridge. 

Pre-Cenozoic Basement 

The inversion method produces not only a basin thickness map but also a map of the basement gravity field (fig. 7). 

The basement gravity field reflects the density in pre-Cenozoic 

rocks exposed and buried beneath basins within the quadrangle. Caution should be used in interpreting basement gravity 

anomalies over basins without well control or independent 

control from other geophysical data. For this reason, the following discussion is limited to the ranges where pre-Cenozoic 

rocks are exposed or areas where several wells or geophysically determined depths to basement exist. 

Spring Mountains 

The Spring Mountains are characterized by a broad 

isostatic gravity high centered roughly over Charleston Peak, 

the highest mountain in southern Nevada. The highest densities measured in the Las Vegas quadrangle are from Precambrian gneiss, exposed on the western edge of Frenchman 

Mountain (lat. 36° 11’N., long. 115° 01’W.), and Paleozoic 

dolostones exposed in the Spring Mountains (Langenheim and 

others, 1998). No crystalline basement rocks are exposed in 

the Spring Mountains, but dolostones are part of the Paleozoic 

carbonate sequence exposed in the overall antidomal structure 

of the range. Two lines of evidence suggest that, although they 

undoubtedly contribute to the gravity high, these rocks are not 

the main source of the gravity high. First, the outcrop pattern of the carbonates does not coincide well with the gravity 

high. For example, the Nopah and Bonanza King Formations 

exposed in the upper plate of the Lee Canyon thrust are on 

the western flank of the gravity high, not over the gravity 

maximum. Second, the aeromagnetic map shows a broad positive anomaly over the Spring Mountains, suggesting that the 

Spring Mountains are underlain by magnetic rocks. Limestone 

and dolostone are not magnetic and therefore not a likely 

source of the aeromagnetic high. A more likely candidate for 

the large gravity and magnetic anomalies present over the 

mountain range is Precambrian metamorphic and (or) igneous 

rocks (Blank, 1987). 

The basement gravity anomalies over the Spring Mountains generally do not coincide with mapped thrust faults, 

except for the area near La Madre Mountain. Here, a basement gravity low (relative to the Spring Mountains basement 

gravity high) trends northeast, approximately parallelling 

the Keystone thrust northeast of La Madre Spring. The low 

appears to coincide spatially with outcrops of Aztec Sandstone 

in the lower plate. The Mesozoic sandstones are characterized 

by densities of 2,400 to 2,500 kg/m3

, which would produce 

a density contrast of -100 to -300 kg/m3 with respect to the 

Paleozoic sedimentary sequence (table 4). Lone Mountain, 

composed of Devonian and Mississippian limestones, does 

not perturb the basement gravity low, suggesting that it is 

underlain by low-density rocks. This is supported by a profile 

of resistivity data across the southern end of Lone Mountain 

that indicates lower-resistivity rocks (< 150 ohm-m) at depths 

of 200–1,400 m (Zohdy and others, 1992). Zohdy and others 

(1992) suggested that the cause of the lower-resistivity rocks 

was Aztec Sandstone, saturated with water, and possibly silty 

Chinle and Moenkopi Formations. Another possible contributor to the gravity and resistivity low are Tertiary and Quaternary alluvial deposits. 

Somewhat lower basement gravity values occur in the 

area of Blue Diamond. This area has the highest magnetic 

values (+660 nT, nearly 1,000 nT higher than the low in Las 

Vegas Valley) on the map. The intense, oblong aeromagnetic 

high is superimposed on the broader aeromagnetic high present over much of the Spring Mountains within the quadrangle. 

Our model (fig. 8) is consistent with unpublished data of 

H.R. Blank, Jr. (in Conrad and others, 1990), that indicates 

a strongly magnetic body at about 4 km depth. Our model 

indicates that the magnetic Precambrian basement extends to 

the middle crust. Conrad and others (1990) suggested that the 

broad anomaly was caused by elevated Precambrian basement 

(“core complex”). They also suggest that the superimposed 

anomaly in the area of Blue Diamond is caused by a quartz 

monzonitic body that post-dated thrusting and that intruded 

Paleozoic rocks, producing contact-metasomatic magnetite. 

Frenchman Mountain 

Only the westernmost part of Frenchman Mountain crops 

out in the Las Vegas 30′ by 60′ quadrangle. Values reach as 

high as 20 mGal, nearly as high as the Spring Mountains gravity high. Unlike the Spring Mountains, Precambrian crystalline 

rocks are exposed and readily explain the gravity high. The 

Precambrian rocks are also the probable source of the positive aeromagnetic anomaly. The aeromagnetic low associated 

with the Frenchman Mountain high wraps around the margin 

of Frenchman Mountain, suggesting that the source of the 

anomaly does not extend to great depths. Modeling of the 

aeromagnetic high indicates that the source rocks may extend 

to depths of 5 km beneath Frenchman Mountain (Langenheim 

and others, 1997). 

Las Vegas Range 

Although the same Paleozoic stratigraphy as that exposed 

in the Spring Mountains crops out in the Las Vegas and Sheep 

Ranges north of Las Vegas Valley, the gravity values are substantially lower than those in the Spring Mountains (+4 mGal 
50 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

SOUTHWEST NORTHEAST 

G F F' G' 

DEPTH, IN KM MAGNETIC ANOMALY, IN nT600


200


-200 

0 

5


10


15


20


Las Vegas 

Valley basin 

.050 .038 

.088 

.044 

.013 

observed 

calculated 

SPRING 

MTNS 

Las Vegas Valley

shear zone 

0 20 40 60 80 100 120 140 160 180


DISTANCE, IN KM 

Figure 8. Magnetic model across Spring Mountains along extended cross section F–F’ (G–G’ marks the southern and northern 

boundaries, respectively, of the Las Vegas 30’ × 60’ quadrangle along the model). Numbers in model are magnetic susceptibilities 

in SI units. Vertical exaggeration 3×. Model assumes sources are greatly elongated in the east and west directions. Sediments in 

Las Vegas Valley basin are assumed to be non-magnetic. Maps B and C and figures 5, 6A, 6B, and 7 show locations of sections F–F’ 

and G–G’. 
Geophysical Framework of the Las Vegas 30′ × 60′ Quadrangle 51 

ISOSTATIC GRAVITY,

ELEVATION, IN METERS ELEVATION, IN METERS IN mGals 

AEROMAGNETIC ANOMALY,

IN nT 

D D' 

WEST EAST 

AEROMAGNETICS 

120 

80 

40 

0


-40


-80


-120


-160


-200


-240


ISOSTATIC GRAVITY 

16 

12 

8 

4 

0 

-4 

-8 

-12 

-16 

-20 

-24 

1600 

800 

SL 

-800 

-1600 

-2400 

-3200 

-4000 

1600 

800 

SL 

-800 

-1600 

-2400 

-3200 

-4000 

FRENCHMAN 

MOUNTAIN 

CENOZOIC 

SEDIMENTARY ROCKS 

PRE-CENOZOIC 

BASEMENT 

NO VERTICAL EXAGGERATION 

PRE-CENOZOIC 

BASEMENT 

BASEMENT SURFACE FROM GRAVITY INVERSION 

INTERPRETATION OF BASEMENT SURFACE 

Projected 

FaultFaultFaultFaultFaultFault 

inferred fault

FaultFaultFaultFaultFaultFault 

Fault 

Projected 

inferred fault 

Fault 

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 32000 

NO VERTICAL EXAGGERATION 

SPRING MTNS 

SPRING MTNS 

FRENCHMAN 

MOUNTAIN 

PRE-CENOZOIC 

BASEMENT 

PRE-CENOZOIC 

BASEMENT 

CENOZOIC 

SEDIMENTARY ROCKS 

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 32000 

DISTANCE, IN METERS 

A SCALE 1:100,000

Figure 9 (above and following two pages). A, Cross section D–D’ extending east-west across Las Vegas Valley; location of cross 

section shown in Figure 6A. B, Cross section E–E’ extending across Pahrump Valley; location of cross section line shown in figure 

6B. C, Cross section F–F’ extending northeast-southwest across Las Vegas Valley; location of cross section lines shown in figures 

6A and 7. Maps B and C and figures 5, 6A, 6B, and 7 show locations of sections D–D’, E–E’, and F–F’. 
52 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

E E' 

SOUTHWEST NORTHEAST 

AEROMAGNETIC

ISOSTATIC GRAVITY, ANOMALY,

ELEVATION, IN METERS ELEVATION, IN METERS IN mGals IN nT 

60 

40 

20 

0 

-20 

-40 

-60 

-80 

-100 

-120 

-140 

20 

16 

12 

8 

4 

0 

-4 

-8 

-12 

-16 

-20 

-24 

-28 

2400 

1600 

800 

SL 

-800 

-1600 

-2400 

2400 

1600 

800 

SL 

-800 

-1600 

-2400 

AEROMAGNETICS 

ISOSTATIC GRAVITY 

BASEMENT SURFACE FROM GRAVITY INVERSION 

PAHRUMP 

VALLEY 

Lineaments mapped by Hoffard in dark gray 

Mapped faults by Lundstrom and others in solid black 

Inferred faults by Lundstrom and others in dashed black 

SPRING MOUNTAINS 

PRE-CENOZOIC 

PRE-CENOZOIC ROCKS

ROCKS 

CENOZOIC 

DEPOSITS 

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 32000 34000 

INTERPRETATION OF BASEMENT SURFACE 

PAHRUMP 

VALLEY 

Lineaments mapped by Hoffard in dark gray 

Mapped faults by Lundstrom and others in solid black 

Inferred faults by Lundstrom and others in dashed black 

SPRING MOUNTAINS 

PRE-CENOZOIC 

ROCKS PRE-CENOZOIC 

ROCKS 

CENOZOIC 

DEPOSITS 

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 32000 34000 

DISTANCE, IN METERS 

B SCALE 1:100,000 
Geophysical Framework of the Las Vegas 30′ × 60′ Quadrangle 53 

F F' 

SOUTHWEST NORTHEAST 

AEROMAGNETICS 

-280 

-240 

-200 

-160 

-120 

-80 

-40 

0 

40 

80 

-120 

-160 

-200 

-240 

-280 

AEROMAGNETIC ANOMALY,

IN nT 

ISOSTATIC GRAVITY 

-36 

-32 

-28 

-24 

-20 

-16 

-12 

-8 

-4

 0 

4 

ISOSTATIC GRAVITY,

IN mGals 

BASEMENT SURFACE FROM GRAVITY INVERSION 

Inferred faultInferred fault 

-2100 

-1300 

-500 

300 

1100 

1900 LAS VEGAS 

VALLEY 

Projected LVVSZ

inferred fault 

PRE-CENOZOIC 

BASEMENT 

CENOZOIC 

SEDIMENTARY ROCKS LAS VEGAS 

RANGE

ELEVATION,

IN METERS 

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 32000 34000 36000 38000 40000 42000 Inferred fault Inferred fault

INTERPRETATION OF BASEMENT SURFACE 

inferred 

LVVSZ 

Projected 

inferred fault 

-2100 

-1300 

-500 

300 

1100 

1900 LAS VEGAS 

VALLEY 

PRE-CENOZOIC 

BASEMENT 

LAS VEGAS 

RANGE 

CENOZOIC 

SEDIMENTARY ROCKS

ELEVATION,

IN METERS 

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 32000 34000 36000 38000 40000 42000 

C 

DISTANCE, IN METERS 

SCALE 1:100,000 
54 Geologic and Geophysical Maps of the Las Vegas 30′ × 60′ Quadrangle, Nevada and California 

versus +25 mGal), even allowing for the effects of the deep 

basin beneath Las Vegas Valley. Unlike the Spring Mountains, 

the Las Vegas Range is not characterized by a large aeromagnetic anomaly, suggesting that rocks composing the basement 

north of the Las Vegas Valley shear zone are different from 

the basement rocks beneath the Spring Mountains. Our model 

(fig. 8) suggests that the shear zone approximately marks a 

possible change in Precambrian basement type and may be a 

deep-seated feature. 

Conclusions 

Gravity and aeromagnetic data provide constraints on the 

nature of the subsurface in the Las Vegas 30′ × 60′ quadrangle. 

Important constraints from the gravity data include the basin 

configuration beneath Las Vegas and Pahrump Valleys. For 

both valleys, the alluvial deposits conceal a complex basement 

topography. Both basin configurations suggest that strike-slip 

faulting played an important role in basin formation. The large 

gravity and aeromagnetic highs over the Spring Mountains 

support the deep-rooted model of the Spring Mountains block 

as proposed by Wernicke (1992). These data have important 

implications not only for the tectonic evolution of the area, but 

also for hydrogeologic studies. The LVVSZ appears to affect 

ground-water chemistry in the valley (Lyles and Hess, 1988) 

and thus may control ground-water flow. These data allow 

us to determine the location and geometry of these concealed 

structures beneath the valley floor. 

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