U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY



in cooperation with



MOJAVE WATER AGENCY



A product of the Southern California Areal Mapping Project:

A geologic mapping project sponsored jointly by 

the U.S. Geological Survey and the 

California Division of Mines and Geology





Geologic map database of the El Mirage Lake Area,

San Bernardino and Los Angeles Counties, California



By David M. Miller1 and David R. Bedford1

Digital preparation by D.R. Bedford1



Open-File Report OF 00-222

version 1.0

http://wrgis.wr.usgs.gov/open-file/of00-222



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

(USGS) editorial standards or with the North American Stratigraphic Code.  Any use of trade, product, or firm 

names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

	This database, identified as "Geologic map database of the El Mirage Lake Area, San Bernardino and Los 

Angeles Counties, California" has been approved for release and publication by the Director of the USGS.  

Although this database has been reviewed and is substantially complete, the USGS reserves the right to 

revise the data pursuant to further analysis and review.  This database is released on condition that neither the 

USGS nor the U.S. Government may be held liable for any damages resulting from its use.





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









 TABLE OF CONTENTS



INTRODUCTION	2

INFORMATION ON THE DATABASE	3

SUMMARY OF GEOLOGY	3

	Geologic setting	3

	Geomorphology 	4

	Surficial deposits	4

	Bedrock deposits	6

YOUNG FAULTS	8

GROUNDWATER	10

DESCRIPTION OF MAP UNITS	12 

ACKNOWLEDGMENTS	15

REFERENCES CITED	15

Appendix 1 (Description of database)	17

Appendix 2 (Quality measures) 	26





INTRODUCTION



	This geologic map database for the El Mirage Lake area describes geologic materials for the dry lake, parts 

of the adjacent Shadow Mountains and Adobe Mountain, and much of the piedmont exten-ding south from the 

lake upward toward the San Gabriel Mountains.  This area lies within the western Mojave Desert of San 

Bernardino and Los Angeles Counties, southeastern California (Fig. 1).  The area is traversed by a few paved 

highways that service the community of El Mirage, and by numerous dirt roads that lead to outlying properties.  

An off-highway vehicle area established by the Bureau of Land Management encompasses the dry lake and 

much of the land north and east of the lake.  The physiography of the area consists of the dry lake, flanking 

mud and sand flats and alluvial piedmonts, and a few sharp craggy mountains.	

	This digital geologic map database, intended for use at 1:24,000-scale, describes and portrays the rock units 

and surficial deposits of the El Mirage Lake area.  The map database was prepared to aid in a water-resource 

assessment of the area by providing surface geologic information with which deeper groundwater-bearing units 

may be understood.  The area mapped covers the Shadow Mountains SE and parts of the Shadow Mountains, 

Adobe Mountain, and El Mirage 7.5-minute quadrangles (Fig. 2).   The map includes detailed geology of surface 

and bedrock deposits, which represent a significant update from previous bedrock geologic maps by Dibblee 

(1960) and Troxel and Gunderson (1970), and the surficial geologic map of Ponti and Burke (1980); it 

incorporates a fringe of the detailed bedrock mapping in the Shadow Mountains by Martin (1992).  The map data 

were assembled as a digital database using ARC/INFO to enable wider applications than traditional paper-

product geologic maps and to provide for efficient meshing with other digital data bases prepared by the U.S. 

Geological Survey's Southern California Areal Mapping Project (http://geology.wr.usgs.gov/wgmt/scamp).





INFORMATION ON THE DATABASE



The Geologic Map Database of the El Mirage Lake Area consists of three parts: 1) the geologic map and 

database explanation/discussion (this document), 2) the GIS dataset of the geology of the area, and 3) printable 

plotfiles of the database map.  The GIS datasets consists of seven layers compiled in ARC/INFO (Environmental 

Systems Research Institute, Redlands, CA) version 7.2.1, and version 8.0.1, and distributed in ARC/INFO export 

format.  Detailed descriptions of the parts of this publications can be found in appendix 1 of this report.   The 

entire set of files is available on the internet at URL:  http://wrgis.wr.usgs.gov/open-file/of00-222/.





SUMMARY OF GEOLOGY



Geologic Setting



	The geologic history of the western Mojave Desert was most recently summarized by Glazner and others 

(1994).  The El Mirage area lies near the San Andreas fault, and on the North American plate.  The crust in this 

area formed during the early Proterozoic, and late Proterozoic and Paleozoic strata were laid on this crust as it 

gradually subsided (Stewart, 1970).  Late Paleozoic and early Mesozoic continental-margin tectonics resulted in 

more com-plex sedimentation patterns of shallow marine and continental sediments (Stevens and Stone, 1988; 

Stone and Stevens, 1988; Martin and Walker, 1992).  The area later lay within multiple Mesozoic magmatic 

arcs, which created a composite batholith of granitoids, most of which range in age from mid-Jurassic to latest 

Cretaceous.  Pre-Jurassic rocks are now present as metamorphic rocks in pendants and screens between 

plutons in this batholith.

	During the mid-Cenozoic, the area was fragmen-ted along normal and strike-slip faults into sedimen-tary 

basins bounded by mountains, as the crust was sliced by the San Andreas fault system and related 

contractional and extensional basins (Glazner and Loomis, 1984).  Several hundreds of kilometers of strike-slip 

separation occurred along the San Andre-as fault and its precursors, ultimately placing the rocks of the San 

Gabriel Mountains against the bed-rock of the El Mirage area (Powell, 1992).  During the late Pliocene, uplift 

formed the San Gabriel Mountains (Meisling and Weldon, 1989), creating a rain shadow to the east and the 

beginnings of the topography and climate of today.

	Rocks of the San Gabriel Mountains and the adjacent Mojave Desert differ significantly.  Although both 

areas expose many granitoids from the Meso-zoic batholith, the pre-batholithic rocks are distinct.  The San 

Gabriel Mountains contain Proterozoic gneisses and the distinctive Pelona Schist, whereas the nearby desert 

contains Paleozoic continent-margin strata along with slightly older and younger strata.



Geomorphology



	The El Mirage Lake map area lies in the Mojave Desert north of the San Gabriel Mountains, one of several 

massive, high-altitude mountain ranges in the Transverse Ranges.  The interplay of these two geomorphic 

realms - the Transverse Ranges and the desert - controls the geomorphology of El Mirage Lake.  The San 

Gabriel Mountains contain dozens of peaks higher than 2500 m, and some higher than 3000 m; steep slopes 

commonly are 25 to 65 per-cent.  In contrast, the mountains to the north in the desert are much more subdued 

topographically; they rise 100 to 200 m above flatter lands that are about 850 to 1000 m in altitude, and steep 

slopes in moun-tains commonly are 12 to 30 percent.  As a result, the San Gabriel Mountains have steeper 

slopes and capture more precipitation, and both of these factors cause greater production of alluvium from the 

Trans-verse Ranges than from the desert ranges.

	El Mirage Lake lies at the junction of a desert semi-bolson to the north and a huge alluvial pied-mont to the 

south (Fig. 2).  The semi-bolson forms El Mirage Valley between the Adobe Mountain area and the Shadow 

Mountains; both mountains are fringed by broad pediments and gentle piedmonts.  Valley-axis alluvial deposits 

are carried by intermittent streams southward toward El Mirage Lake.  To the south of El Mirage Lake, an 

alluvial piedmont fring-ing the San Gabriel Mountains is rapidly aggrading over its ~25 km distance from head 

to toe.  The greater depositional rates in this piedmont, and par-ticularly in Sheep Creek fan, compared to pied-

monts of the semi-bolson indicate that desert mountains and piedmonts are being buried, and desert valleys 

being dammed.  One of these dammed valleys forms El Mirage Lake.  El Mirage Lake receives sediments from 

the two lithologically distinct provinces - San Gabriel Mountains and Mojave Desert - and its sedimentary 

record there-fore documents the interplay of sedimentation from these provinces.



Surficial deposits



	Much of the map area was mapped in recon-naissance by Ponti and Burke (1980), who distin-guished 

pediments, alluvium of Holocene and Pleistocene age, and eolian and playa deposits.    Our more detailed 

mapping has subdivided these units and distinguished several ages of alluvial cover on the pediments.  

	El Mirage (dry) Lake lies in the center of the map area.  The dry lake is covered by unvegetated sandy 

playa muds, as well as scattered eolian sand and sand-rich muds that support spotty vegetation.  Playa muds 

are poorly sorted, ranging from minor fine sand to common clay and silt.  Texture is punky with common voids.  

The playa muds grade out-ward to other map units by the increase of eolian and alluvial materials.  The playa 

bed of El Mirage Lake is at 2833 feet altitude.  A smaller playa lies to the east and slightly higher, at 2855 feet 

altitude.  The outlet of the eastern playa leads eastward to Fremont Wash, which leads to the Mojave River.  

Both playas were dammed by Holocene alluvium from the Sheep Creek fan, as indicated by distinc-tive clasts 

in sediments crossing the east ends of the playas.  Smaller playas east and west of Mt. Elmo and east of Nash 

Hill (Fig. 2) represent local-ized ponding of alluvial fines among sand sheets and dunes.

	The lowest topographic threshold for El Mirage Lake lies at its east end, about 10 m (40 feet) above the 

playa bed, raising the possibility that one or more ancient lakes as deep as 10 m once existed.  However, few 

signs of pluvial lakes, such as shore-line deposits or lacustrine deposits ringing the playa or at depth, are 

present.  The alluvial deposits underlying the eastern threshold are loose and fine grained and likely would 

rapidly erode if overtopped by a lake outlet.  These lines of evidence indicate a large lake probably never 

formed and that before the last progradational event of the Sheep Creek fan, external drainage to Fremont 

Wash probably existed.  However, there are a few hints that a lake existed.  A 2-m deep pit in the playa at a 

landfill site (~2845 ft altitude, border sec. 8 & 5, T. 6 N., R. 7 W.) exhibits a sequence of playa and alluvial 

sediments, including two thin laminated green clay beds.  The well-sorted clay in the beds was probably 

deposited in a quiet-water lake or alluvial setting.  However, the clay beds do not contain ostracodes or other 

lake fauna.  In addition, along the northeast edge of Gray Mountain possible remnant shorelines and lake 

deposits may be present about 6 m (27 ft) above the playa bed (~2860 ft).  Well sorted sand and dark mud lie 

below a subtle bench, possibly a shoreline.  The dark brown mud contains abundant plant remains, apparently 

representing marshy deposits.  Neither the sediment in the pit nor the possible lake deposits near Gray 

Mountain show significant soil development; they are therefore probably Holocene in age.  Further support for a 

past lake lies in the linear pattern of active sand dunes south of the lake, at 2865 to 2875 feet altitude, 7 to 10 

m above the playa.  The dunes may represent eolian reworking of sands accumulated along a lake shoreline, a 

com-mon feature in Great Basin pluvial lakes.  However, no direct evidence for a shoreline exists.  We con-

clude that only sparse evidence for a lake exists, and at most the playa may have undergone one or two brief 

lacustrine cycles during the Holocene.

	Dockter (1980) reported the findings of a 81 m (265 ft) hole drilled near the center of the playa.  The 

uppermost 7 m (22 ft) are described as greenish gray calcareous clay, below which are sands, gravels, and 

muds.  The color and composition of the uppermost part are suitable for a parentage from Pelona Schist 

materials, whereas the remainder of the hole is described as yellow-brown in color and the few descriptions of 

pebble compositions indicate a granitic source with little mica.  We consider that the sediments below 7 m 

represent alluvial fan and valley axis materials derived from El Mirage Valley, north of the playa, and probably 

do not represent any sediment from Sheep Creek fan.  This inference is consistent with a model of northward 

progradation of the Sheep Creek fan causing playa sedimentation only recently.

	Materials surrounding the lake vary greatly by position.  North and northwest of the lake, alluvium grades to 

playa deposits at the edge of the lake by increase of mud content and decrease of grain size to medium and 

fine sand, largely in detritus derived from granitoids.  Scattered eolian deposits generally complicate the 

transition, and areas that have signifi-cant eolian materials mixed with alluvium are mapped as hybrid units 

(Qyef and Qyfe).  East of the lake, alluvial and eolian deposits flank the lake.  In many places, the eolian 

component overwhelms the alluvial component, but eolian sands are generally reworked by alluvial processes, 

creating hybrid deposits.  South of the lake, adjacent sands are green and gray color (in contrast with the tan or 

brown sand of the alluvial sections north of the lake) and composed of debris from the Pelona Schist in the San 

Gabriel Mountains.  Most deposits here are alluvial, but a complex cover of wind-blown deposits, also green in 

color, exists. 

	The Sheep Creek fan, which lies south of El Mirage Lake, is distinctive because it is almost entirely 

composed of debris from Pelona Schist.  Troxel and Gunderson (1970) noted the presence of Pelona Schist 

clasts in alluvial sediments south of El Mirage Lake and stated in their map descrip-tion that the sediments 

were deposited by debris flows deriving from the San Gabriel Mountains, some 33 km distant near the vicinity 

of Wrightwood.  Wrightwood area debris flows have been studied by Sharp and Nobles (1969) and by Morton 

and others (1979), who demonstrated that these sources of Pelona Schist material are periodically very active 

and send debris considerable distances down the alluvial fan.  However, these workers did not docu-ment 

debris flow mechanisms operating to the toe of the fan, at El Mirage Lake.  The debris fan (Sheep Creek fan) 

leading from Wrightwood to El Mirage Lake stretches 25 km horizontally and 1200 m vertically and currently is 

distinctive because of its green, nearly monolithologic sediment (Fig. 2).  It is bordered to the east and west by 

alluvium with granitic parentage, typically a light brown or tan color.

	Eroded alluvial deposits of the Sheep Creek fan are widespread beneath recently active eolian and alluvial 

deposits.  These older alluvial materials characteristically form a sequence of gravelly sand with cobbles as 

large as 40 cm that lies on low-relief exposures of greenish-brown muddy fine sands.  The gravelly sand forms 

lenticular channel deposits that are commonly poorly size-sorted; cobbles and pebbles lie in medium to coarse 

sand.  Most peb-bles and cobbles are well-rounded, but many are distinctively subangular to angular, 

supporting a debris-flow hypothesis.  Clasts include Pelona Schist, gneisses, granitoids (some mylonitic), and 

marble; less common are granitoid dike rocks and thin-bedded quartzite.  The clast assemblage is appropriate 

for derivation from the San Gabriel Mountains.  Consistent lack of mud in the gravelly sand argues for a fluvial 

origin and against a debris-flow origin.  

	The muddy fine sand in the lower interval of the sequence represents deposits similar to modern deposits 

higher on the fan nearer Wrightwood.  The overwhelmingly fine grain sizes probably are due to the unusual 

source of altered schist, which rapidly breaks down to mica, feldspar, quartz, and clay, forming a deposit of 

clay, silt, and fine sand.  Bed-ding in the muddy sand deposit is laterally contin-uous over tens of meters, 

suggesting that these deposits formed on a very regular surface, probably during one or more periods of rapid 

aggradation.  Individual fining-upward beds, less than one meter thick, are common.  Parallel lamination and 

ripple lamination occur less commonly.  Buried weakly developed soils, buried plants, and other evidence of 

subaerial exposure occur in some cases within the muddy section but generally are not conspicuous.  Grain 

size increases up the fan in the muddy sand but appears to decrease up the fan in the overlying gravel and 

sand.  

	The interpretation and significance of a consis-tently muddy sand deposit, greater than 4 m thick and 

probably greater than 10 m thick, that covers the entire fan, but is lacking in gravel, is unclear at this time.  This 

deposit has a volume of roughly 2.5 km3, and is so large that it is a conspicuous feature from space.  Likewise, 

the observation that the muddy sand is overlain in many places by gravel channel deposits is of uncertain 

interpretation.  It appears that one or more voluminous depositional events took place over a short duration, 

perhaps a few years to a few hundred years.  Either only the last event carried alluvial gravel across the fan, or 

each pulse of sediment efficiently removed lags of prev-iously deposited gravel.  A lack of voluminous gravel 

deposits near El Mirage Lake, at the surface or at depth, argues for the former interpretation.  The event or 

events of deposition near El Mirage Lake may have relevance for understanding Holocene climate history and 

perhaps even earthquake-triggered landsliding and debris flows, since land-slides trigger most debris flows 

(Morton and others, 1979).  A weakly to moderately developed Av hori-zon, weak B horizon, and incipient calcic 

horizon characterize soil development near the surface of the alluvial deposits.  By analogy with soil develop-

ment elsewhere in the Mojave Desert, and acknow-ledging a possible fast rate of development of Av and B 

horizons due to the unusual clay and silt composi-tion of the deposit, we infer a middle to early Holo-cene age 

for the deposit.  Given the 140- to 200-year recurrence interval for San Andreas seismic events (Sieh, 1984; 

Weldon and Sieh, 1985), the poor reso-lution on the age of these deposits does not permit evaluation of the 

hypothesis that they are earthquake-generated deposits at this time.

  	Although most sediment south of the lake is alluvium of the Sheep Creek fan, eolian sand is widespread on 

the alluvium.  Green-colored musco-vite is ubiquitous in these deposits.  Most active sediment deposits in this 

area are of eolian origin because many washes farther up the fan have been dammed during the last few 

decades, but a few act-ive washes remain on the west side.  These active washes carry muds of Pelona 

parentage to El Mirage Road, near El Mirage Lake, many times each winter.  

	Piedmonts north of the lake exhibit well-devel-oped soils at shallow depths, including thick, bright-red, clay-

rich B horizons and strongly developed calcic zones, both of which contrast with the imma-ture soil 

development in piedmont deposits south of the lake.  The soil development north of the lake closely matches 

changes in micro-topography of alluvial surfaces, both of which correspond to soils and micro-topography 

nearby (McFadden and Wel-don, 1987), in the central Mojave Desert (Yount and others, 1994), and in the San 

Joaquin Valley (Ponti and Burke, 1980).  This correlation indicates that much of the piedmont north of El Mirage 

Lake is composed of deposits tens of thousands to hun-dreds of thousands of years old.  Young alluvial fan 

deposits flanking the Shadow Mountains are similar in age to the Sheep Creek fan deposits, but are generally 

10 to 30 cm thick and at a maximum reach about 1 m thickness near the lake.  

	The piedmont deposits in general are shed from pediments that are covered by a thin veneer of alluvium.  

We mapped deposits as veneer on pedi-ment where shallow bedrock was evident, such as (1) outcrops of dike 

rock protruding from an other-wise flat surface, (2) gruss and granite along walls of incised channels, and (3) 

presence of abundant sand derived from underlying granite that could not have eroded from farther upslope.  In 

general, areas of pediment veneer have much flatter topo-graphy than do alluvial fans.





Bedrock deposits



	All rock exposed around El Mirage Lake is granitoid or metamorphic.  Dibblee (1960) first mapped the 

rocks and noted the Mirage fault.  Troxel and Gunderson (1970) later mapped marble, schist, calc-silicate rock, 

and quartzite in the Sha-dow Mountains in much greater detail and distin-guished felsic and mafic igneous 

rocks.  Bowen (1954) reported brachiopods suggestive of a Pen-nsylvanian age for some strata in the Shadow 

Hills that are similar to strata in the map area and Brown (1983) made a strong case that some of the meta-

morphic rocks in the area are Paleozoic in age.  Martin (1992) mapped the Shadow Mountains metamorphic 

rocks in detail and correlated them with Late Proterozoic and Paleozoic strata that were earlier extrapolated 

across the Mojave Desert (Stewart, 1970; Stewart and Poole, 1975), as well as early Mesozoic strata (Miller 

and Carr, 1978; Miller, 1981).  We have largely adopted Martin's criteria for stratigraphic units and age 

assignments.

	Granitoid rocks make up most of the bedrock.  Two composite pluton complexes are identified, one in the 

Shadow Mountains and the other in the El Mirage Valley and Adobe Mountain area.

	Granitoids in the Shadow Mountains form a complex ranging from (1) felsic granite, to (2) a wide range of 

intermediate-composition rock such as mafic granodiorite and biotite-hornblende quartz monzodiorite, and (3) 

mafic rocks such as gabbro, quartz diorite, and diorite.  Martin (1992) described intrusive relations indicating 

that the several igneous rock types are closely related, but that in general the mafic rocks are oldest and felsic 

rocks youngest.  The youngest rock he described is leucocratic tourmaline-garnet-muscovite pegmatite.    

	Much of the pediment northeast of El Mirage Lake is underlain by compositionally uniform med-ium- to 

coarse-grained biotite-hornblende granodi-orite.  This pluton probably intruded the complex of intermediate and 

mafic rocks, which displays consi-derable compositional and textural variability.  In this latter complex, cone-

shaped sheets of gabbro 70 to 200 m thick are prominent across an area 3 km wide, forming a ring-shaped 

outcrop pattern (Troxel and Gunderson, 1970).  Pegmatite and aplite dikes cut all rocks.

	Martin (1992) used U-Pb methods to date zircon and monazite from several samples from the com-plex.  

Quartz diorite from the mafic rocks group was dated at about 148 Ma.  Felsic hornblende granite and biotite 

granite yielded ages of 143 to 148 Ma.  The leucocratic pegmatite yielded a monazite age of 141.5  1 Ma.  

Rocks of the complex other than the youngest dikes are therefore of latest Jurassic age, although as the age of 

the Jurassic-Cretaceous boundary becomes better defined, some rocks may be reclassified as earliest 

Cretaceous.

	Felsic biotite granite along the west edge of the Shadow Mountains is widely exposed in the pedi-ment in 

the northern part of the map area.  It closely matches granite in plutons to the west in the Adobe Mountain area 

that appear to belong to a single intrusive suite on the basis of overlaps in composi-tion (varying degrees of 

biotite and hornblende as the mafic minerals) and texture (distinctive shapes of biotite crystals).  In addition, all 

plutons in this suite are riddled (up to 25% of the rock) with aplite dikes, some with pegmatitic segregations.  

Most dikes strike east or east-northeast.  

	The main intrusive phases in the Adobe Mount-ains suite are:  Hornblende granodiorite, monzogran-ite of 

Adobe Mountain, and monzogranite of Nash Hill; microgranite is a minor phase.  Hornblende granodiorite is 

medium- to coarse-grained with abun-dant hornblende, some as phenocrysts, and con-spicuous sphene.  

Monzogranite of Adobe Mountain is medium-grained to sparsely porphyritic, containing mostly biotite with 

sparse hornblende and sphene.  Monzogranite of Nash Hill is fine to medium grained but carries all grains in 

sizes characteristic of the Adobe Mountain unit distributed through the rock; as a result, texture is seriate.  A 

notable characteristic of the Adobe Mountain and Nash Hill phases is pres-ence of barrel-shaped biotite 

crystals.  Although aplite is common as dikes in the Adobe Mountain intrusive suite, other very fine grained 

rocks occur, generally in accumulations too small to map.  An exception south of Adobe Mountain is mapped 

as microgranite.  It crops out over less than one km2, forming brown-weathering outcrops with close-spaced 

jointing.  The rock is pinkish and very fine- to fine-grained, and has gradational contacts to the surrounding 

Nash Hill granite.

	Other than the observation that all of these phases predate the aplite dike swarm, intrusive relations among 

plutons in the Adobe Mountains suite are ambiguous.  The Adobe Mountains, Nash Hill, and microgranite 

phases grade into one ano-ther over distances of a few meters and share many textures, so we consider them 

to be closely related.  Hornblende granodiorite shows less similarity to the other plutons, and may possibly 

represent unrelated intrusions.  All granitoids are highly jointed and commonly fractured.  The hornblende 

granodiorite commonly is altered to silica, epidote, clay, and sericite, and hosts mineral deposits in a few 

places.  The Nash Hill monzogranite is chloritized and silici-fied at Black Mountain but otherwise only rarely is 

altered; minor sericite was noted at a few locations at Adobe Mountain.

	The Adobe Mountains granitoid suite is Creta-ceous in age.  K-Ar dates by Miller and Morton (1980) for 

samples collected at Gray Mountain and the west side of Adobe Mountain, of the Nash Hill and Adobe 

Mountain phases, respectively, indicate that the intrusive suite is approximately 76 to 84 Ma.  Hornblende that 

was analyzed by Miller and Morton (1980) from the Nash Hill phase at Gray Mountain appears to conflict with 

the prevalence of biotite as its mafic phase, but the rock contains as much as 2% hornblende in places.  The 

hornblende grano-diorite unit has not been dated.

	The Jurassic and Cretaceous granitoids intrude masses of metamorphosed rocks in several places.  At Mt. 

Elmo, thin screens and wispy inclusion trails of quartzite, schist, and hornblende gneiss appar-ently represent a 

siliciclastic stratigraphic section with minor carbonate rocks; we correlate these rocks with the Late Proterozoic 

and Cambrian strata of the Mojave Desert.  Similar metamorphic rocks lie on the east side of Black Mountain.  

The crest of Black Mountain is underlain by unusual metamor-phic rocks that must represent considerable 

skarn replacement (metasomatism):  hornblendite, Ca-amphibolite, and garnet-rich rock that are virtually 

monomineralic.  These rocks probably are derived from a carbonate and siliciclastic source.

	The southern Shadow Mountains exhibit relic stratigraphy of marble, quartzite, and calcsilicate rock in 

places, which Martin (1992) interpreted as a metamorphosed sandstone and carbonate sequence that most 

likely is late Paleozoic in age.  The rocks range widely in composition, metamor-phism, and structural overprint 

and can only be assigned ages based on general lithologic assoc-iations.  The presence of carbonate breccia 

and siliciclastic rocks in the impure carbonate sequence are the best indicators that the sequence, if intact, is 

likely late Paleozoic in age.  This sequence was metamorphosed at amphibolite facies and then folded on all 

scales.  Metamorphism produced minerals such as tremolite, diopside, and garnet, and was interpreted by 

Martin (1992) as having occurred during the Middle to Late Jurassic.  Retro-grade metamorphism associated 

with Late Jurassic pluton emplacement produced hornfels with charac-teristic minerals such as chlorite, 

muscovite, wollas-tonite, tremolite, and diopside.  In the southern Shadow Mountains, approximately northward 

plun-ging open folds deform the strata.  Folds represent a syncline and two adjacent anticlines; vergence is not 

clearly defined.  Cleavage associated with the major folds and associated minor folds is locally deformed by 

nearly coaxial folds that are also open and upright.

	Farther north in the Shadow Mountains fine-grained quartz-rich mica schist and calcsilicate rock were 

interpreted by Martin (1992) as metamor-phosed latest Paleozoic and Early Mesozoic strata.  Schist is 

composed of biotite, quartz, and feldspar and is interlayered with minor quartzite and calcsil-icate rock.  The 

nearby calc-silicate map unit con-sists of fine-grained dark hornfels and lesser calcite marble.  It is similar to 

some rocks in the southern Shadow Mountains, but was considered by Martin (1992) to lie higher in the 

structural and stratigraphic sequence.

	Unlike the deformed rocks of the Shadow Moun-tains, which were intruded and metamorphosed by 

Jurassic plutons and therefore were deformed during the Jurassic or earlier, the mylonitic rocks at Mt. Elmo 

and Black Mountain affect plutons to which we assign Cretaceous ages.  The mylonitic rocks at Mt. Elmo are 

the hornblende granodiorite phase of the Adobe Mountain intrusive suite.  There, foliations dip steeply west and 

lineations plunge moderately south.  At Black Mountain, mylonitic rocks are greenschist grade as indicated by 

extensive chlorite development in the zones, and are restricted to metamorphic rocks and the hornblende 

granodiorite.  The Nash Hill phase of the intrusive suite cuts the mylonite zones.  Foliation dips steeply west 

and lineation plunges steeply south.





YOUNG FAULTS



	There is little information about the early Ceno-zoic evolution of this part of the Mojave Desert, but 

Mesozoic granites were deeply eroded before wide-spread early to middle Miocene volcanic and sedi-mentary 

rocks were deposited northeast of El Mirage Lake (Glazner and others, 1994) and younger faults developed.  

Strike-slip faults that cut the early to mid-dle Miocene volcanic and sedimentary sequence are inferred by many 

workers to be late Miocene or Pliocene to Holocene in age.  Dokka and Travis (1990) termed part of this strike-

slip realm the East-ern California shear zone, and followed previous syntheses and models such as Dibblee 

(1961), Garfunkel (1974), and Dokka (1983) in describing several kilometers of dextral separation on most 

faults and proposing that some blocks bounded by the faults rotated as fault separation accumulated.  The 

shear zone as defined by Dokka and Travis (1990) does not extend west to the El Mirage Lake area although 

the several faults in this area parallel those in the shear zone (Fig. 1). 

	Because the Eastern California shear zone accommodates part of the ongoing movement between North 

American and Pacific plates, under-standing whether faults in the El Mirage Lake area belong to the shear zone 

has implications for seis-mic hazards.  Much of the central part of the East-ern California shear zone is 

seismically active, and scattered seismicity extends to the western part of the zone, making careful assessment 

of recency of movement on faults necessary to assess seismic shaking and related hazards.  For example, the 

Landers earthquake on June 28, 1993 ruptured faults across a large area east of El Mirage Lake, including 

several areas in which active faults were not known (Hauksson and others, 1993). 

 

	Quaternary faults were shown by Dibblee (1960) and Troxel and Gunderson (1970) along the southwest 

flank of the Shadow Mountains; the prin-cipal fault was named the Mirage fault by Dibblee (1960).  The Mirage 

fault is a system composed of three segments about 6 to 10 km long that together are about 23 kilometers long 

(Ponti and Burke, 1980).  Ward and others (1993) mapped more faults as the extension of the Mirage fault 

farther to the northwest where it crosses the Rogers Lake playa and continues northwest along the Bissell 

Hills.  During field studies, we have mapped many fault scarps in addition to the Mirage fault (Fig. 3) and 

determined that many cut Quaternary deposits.  Table 1 presents data for the faults, including sur-face deposits 

cut by faults that have relative ages determined by qualitative soil and microtopography studies.  Faults were 

identified by physical trunca-tions of sediment, breccia and gouge in rock, and topographic, vegetation, and 

spring-mound linea-ments.  Many Quaternary faults strike northwest, but strikes vary as much as 30 degrees 

from this main orientation.  Dips are steep.  West- and west-northwest-striking faults in the southern Shadow 

Mountains dip steeply north (Martin, 1992) and do not provably cut Quaternary deposits in exposures examined 

by us; they may be early Quaternary or much older.  Some of these faults displace steeply- to moderately-

dipping markers in a sinistral and (or) normal sense.  One northwest-striking fault southwest of Nash Hill cuts 

Quaternary materials and offsets east-striking, moderately south-dipping dikes in a sinistral sense.  The fault 

may be sinistral, unlike most northwest-striking faults of the Eastern California Shear Zone, or it may be dextral 

with a large down-to-the-northeast component of dip slip (contrary to its scarp, which suggests down-to-the-

west), or it may be polygenetic.



Table 1. Quaternary and possible Quaternary faults near El Mirage Lake

[map numbers are plotted on Figure 3; field stations identfy notebook entries.  Map numbers with * indicate rupture 

at this location is known to be Quaternary]



Map No.	Field Station	Orientation	Offset			Geomorphic and			Age Constraints

								Identifying Features

1*	M98EM-70	305		> 1 m down to W		rounded scarp, spring mounds	cuts Qof2; Qyf uncut

2*	M98EM-82			2 m down to W 		rounded scarp, spring mounds	cuts Qof1; Qyf uncut

3*	M98EM-80	305		1.5 m down to W		rounded scarp			cuts Qof3; Qyf uncut

4*	M98EM-81	305		1.5 m down to W		rounded scarp			cuts Qof1; Qyf uncut

5	M98EM-105	315					probable spring mounds,		Qof2 uncut 

								linear depression

6	M98EM-104	315					line of spring mound remnants	Qof2 uncut

7*	M97EM-17	320		down to W		bedrock limited to upslope	cuts Qvof; Qof2 uncut 

								side; 2.5 m rounded scarp

8	M98EM-102	309, 64 NE				1-m-wide breccia zone		Qof1 uncut

9	M98EM-101	290					possible spring mound deposits

10	M98EM-100	290					large spring mound		Qof2 uncut

11	M98EM-99	325		down to SW		line of degraded spring mounds

12*	M97EM-16	317		> 1 m down to W 	bedrock limited to upslope 	cuts Qof2; Qyf2 uncut

								side; 1.0 -1.5 m rounded scarp

13	M98EM-98	290					degraded spring mound

14	M98EM-97	290					line of spring mounds, linear 

								termination of dikes

15	M98EM-96	309, 68 NE				breccia zone

16	M98EM-95	290, 72 S				breccia zone

17	M98EM-94	290					spring mound, little eroded

18	M98EM-93	290					degraded spring mound		Qof2 uncut

19	M98EM-91	290					linear truncation of rocks	Qof2 uncut

20*	M97EM-15	330, 70 SW	down to W		bedrock only on upslope side;	cuts Qvof; Qyf2 uncut

								gouge zones

21	M98EM-88	315

22*	M98EM-64	330, 74 NE	down to W		breccia zone			cuts stage III calcic 

												zone; Qof2 uncut

23*	M97EM-14	317		>1 m down to W		fault exposure in wash cut bank	cuts Qvof; Qof2 uncut

24	M97EM-13	360					vegetation lineament, breccia	possibly cuts stage IV 

												calcic horizon

25*	M97EM-12	350 - 340	down to W		1 m high linear rounded scarp,	cuts Qvof; cuts or

								spring mounds			bends Qof; Qyf2 uncut

26*	M98EM-65	320		1 m down to W		subtle scarp, spring mounds	cuts Qof2

27	M98EM-63	285, 80 N	~100 m sinistral

28	M98EM-62	280, 83-55 N	~75 m sinistral		breccia zone			Qof2 uncut

29	M98EM-59	300, 70N	50 m sinistral		breccia zone			Qof2 uncut

30	M97EM-25	300		down to E		vegetation linear, vague scarp	pre-Holocene

31	M98EM-53	315		1-2 m down to E		prominent vegetation linear 	Qyfe uncut

								10 m wide, rounded scarp

32	M98EM-54	310					pale linear depression 10 m 	uncertain

								wide, 1 to 1.5 m deep 

33	M98EM-27	320		2-3 m sinistral; 	spring mound remnants,		Qyf2 uncut

					>1 m down to W		fractured rock, vegetation 

								lineament

34	M98EM-33	320					eroded spring mounds

35	M98EM-29	325		3-4 m sinistral		spring mound remnants, 		Qyf2 uncut

								fractured rock, vegetation 

								lineament

36	M98EM-30	325					eroded spring mounds

37	M98EM-36	342, 42 NW	down to W?		caliche in fault zone in 	Qyf2 uncut

								granite

38*	M98EM-35	315		>3-4 m down to E	3-4 m rounded scarp, spring 	Qyf2 uncut; Holocene fans

								mound remnants,  vegetation 	not completely in

								lineament			equilibrium

39	M98EM-34	45		down to W		eroded spring mounds, vague 

								scarp

40	M98EM-37	315		down to E		spring mound remnants, rounded 	Qyf2 uncut

								scarp  0.5 m

41	M97EM-21	006, 70W	3-15 m dextral		fractured granite		Qyf2 or Qyf3 uncut; 

												Qof2 prob. uncut

42			138		500 m dextral		fractured granite, linear 	Qof uncut

								depression

43			135					linear depression, sericitic 	Qyf uncut

								breccia









	The Mirage fault appears to splay into many faults southeastward.  North of the map area, one splay 

breaks from the main trace and passes immediately northeast of the area we mapped.  Within the area we 

mapped, the main trace in the north part, as marked by scarps and spring mounds, ends in an area where the 

pediment is strongly dis-sected, exposing outcrop of bedrock farther into the valley floor than elsewhere.  In this 

zone, the main trace appears to divide into two and possibly three splays, the southernmost of which is the 

most recently active and is defined by scarps.  The latter splay appears to terminate next to the southern 

Shadow Mountains, because all faults crossing the mountains do not provably cut young Quaternary materials.  

We could find no evidence for a fault bordering the western front of the southern Shadow Mountains, as shown 

by Troxel and Gunderson (1970).  The large-scale splaying pattern of the Mirage fault is consistent with an 

interpretation that the fault is terminating in a horse-tail splay.  If this interpretation is correct, it provides 

partial explanation for the existence of the southern Shadow Mountains:  the splay is a compressive feature, and fault 

movement may contribute to the topographic relief. 

	Dissected Pleistocene deposits present 10 km southwest of El Mirage depart from the typical less 

dissected Holocene deposits on the piedmont north of the San Gabriel Mountains.  It is possible that an 

unexposed fault borders the northeast margin of the dissected deposits, much as the Llano fault mapped 10 

km farther southwest (Dibblee, 1961) borders older uplifted deposits.

	The approach used for determining ages of rupture on faults is that used by Yount and others (1994) and 

Miller and others (1994) at Fort Irwin.  We define surface deposits cut by and overlapping the faults and assign 

ages to those units by correla-ting soils and microtopography as defined by Yount and others (1994), 

McFadden (1987), and Reheis and others (1989).  Ages of these surface deposits are given in Table 2. 



Table 2.  Ages for chief alluvial fan deposits.



Map unit	Approximate age of deposit

Qf		<~200 yr

Qyf1		~100 to 1,000 yr

Qyf2		~1,000 to 15,000 yr

Qof1		~15,000 to 50,000 yr

Qof2		~20,000 to 180,000 yr

Qof3		~180,000 to 500,000 yr

Qvof		>500,000 yr



	By these age measures, many faults probably last ruptured during the middle to late Pleistocene, but a few 

deform latest Pleistocene deposits and may have experienced latest Pleistocene or early Holocene rupture.  No 

unambiguous examples of faulted Holocene sediment were observed.  Trench-ing is required for full 

investigation of the seismic potential for the faults.

	The newly mapped faults that strike northwest in the Adobe Mountain area are similar to those of the 

Eastern California shear zone in orientation and age.  If included in the shear zone, they would extend the zone 

westward to about 25 km from the San Andreas fault, raising questions about whether it is a discrete zone or 

simply a broad area of faulting northeast of the San Andreas, much as faults exist in a broad zone southwest of 

the San Andreas fault.  The locations of seismically active faults within the broad zone over the last several 

million years and patterns of strain through time are more important for understanding the zone and its seismic 

implications than are arbitrary boun-daries defined for the zone.





GROUNDWATER



	Groundwater in sufficient quantities for utiliz-ation is commonly found in unconsolidated sedi-ment, and less 

commonly in fractured bedrock.  In both cases the chief source of recharge for the El Mirage area probably has 

been from surface runoff from the San Gabriel Mountains.  Young faults may influence the movement of 

groundwater by acting as impediments or conduits.  Geologic evaluation of groundwater therefore involves 

extrapolating var-ious kinds of bedrock and sediment to depth and inferring fracture distributions and locations 

of young faults (California Department of Water Resources, 1967).

	Near-surface water near El Mirage Lake is reported by residents to be alkaline, and to lie at about 5 m (16 

ft) depth.   Deeper water at the south edge of the lake is potable; in one case at the north end of Gray 

Mountain, it is found at 92 m (280 ft) in fractured bedrock.  Residents near the dry lake apparently have trouble 

predicting whether they will find potable water and at what depth.  Near the cen-ter of the playa, Dockter (1980) 

reported finding the first aquifer (TDS=818 mg/l) at a depth of 53 m (175 ft).  Groundwater in the Sheep Creek 

fan is relatively shallow near the toe of the fan and deepens south-ward, higher on the fan (Stamos and 

Predmore, 1995).  Depth to groundwater 4 to 6 km south of the dry lake, near the village of El Mirage, is 12 to 

31 m (38 to 102 ft), whereas it is 94 to 133 m (308 to 436 ft) up the fan near the south edge of the map area.  

Thus, the water table appears to progressively decline about 60 m in altitude northward in the Sheep Creek fan 

toward El Mirage, over a distance of about 10 km.  It may connect with near-surface saline waters under El 

Mirage Lake, in which case it is partitioned from a deeper (50-90m) freshwater aquifer.  Although uncertainties 

in interpretation exist, it appears that changes in depth to fresh water from El Mirage village to the dry lake (a 

distance of about 4 km over which surface altitude declines ~8 m) varies by 50 to 100 m and therefore lateral 

and/or vertical partitioning of groundwater is likely in this valley-bottom setting.

	Geomorphology of desert mountains and pres-ence of broad flanking pediments suggests that bedrock is 

shallow (2 to 20 m) under alluvial pied-monts flanking the Shadow Mountains and Adobe Mountain.  In contrast, 

hills southwest of El Mirage Lake lack pediments and extensive alluvial aprons, suggesting that bedrock may 

be deeper under the flanking sediments, which are derived from the dis-tant San Gabriel Mountains.  Sediment 

shed from the San Gabriel Mountains apparently is burying any bedrock highs and their flanking piedmonts that 

once lay between the San Gabriel Mountains and El Mir-age Lake.  As a result, Quaternary groundwater-

bearing sediment a few hundred meters thick prob-ably is present in much of the area south of the lake, where 

it forms a generally southward-thickening wedge of alluvium.  However, buried desert topography probably 

exists beneath this alluvial wedge, and detailed gravity surveys are necessary for completely describing its 

geometry.

	Deeper low-density sediment may exist beneath this alluvial wedge as a result of small basins formed by 

faults near the San Andreas fault.  Biehler and others (1983) showed a northwest-trending shallow gravity 

trough in the southern part of our map area.  Total gravity relief from the trough to bedrock of the southern 

Shadow Mountains is 15 mgal.  A prelim-inary calculation of depth-to-bedrock using these gravity data predicts 

low-density material as thick as 1350 m (4500 ft) near the south edge of the map area.  This deeper low-

density material may be old sediment and characterized by lower water yield (California Department of Water 

Resources, 1967) than modern alluvium.  A gentle gravity low in El Mirage Valley probably is not caused by 

significant thicknesses of alluvial materials, since it exists in areas where granite is exposed continuously 

across the valley.  Gravity is essentially flat across El Mir-age Lake between exposures of granite, suggesting 

that significant accumulation of sediment does not exist beneath the playa.

	We could find no faults cutting sediment in the Sheep Creek fan, but buried older faults probably exist.  To 

the west, a few northwest-striking faults splay northward from the San Andreas fault into the Mojave Desert, 

where they are marked by uplifts of early Quaternary and older materials (Dibblee, 1961).  It is possible that the 

dissected terrane of Pleistocene gravels we mapped 10 km southwest of El Mirage has such an origin, 

although we did not find direct evidence for a fault there.  In addition, the Mirage fault and faults in the Adobe 

Mountain area probably extend southeast from mapped exposures to the alluvial piedmont south of the lake.  

We con-sider it likely that buried Quaternary faults exist between the dry lake and the San Gabriel Moun-tains, 

and that they influence groundwater flow.

	A more complete analysis of the geohydrology requires improved knowledge of the locations of buried 

faults, depths to basement (including sub-basins partitioned by faults), and information on the water yield of 

buried materials.  This information can be acquired by conducting detailed gravity studies and integrating drill-

hole lithologic logs with surface geology. 

	Holocene muddy sands form a continuous sheet approximately 10 m thick across the entire distal part of 

the Sheep Creek fan extending from the San Gabriel Mountains.  If similar deposits lie deeper in the buried fan, 

as they probably do, they could serve as aquitards that partition groundwater flow into gently north-dipping 

sheets of high trans-missivity gravel deposits.  As a result, recharging groundwater basins under the Sheep 

Creek fan may be best achieved by injection wells rather than by percolation ponds.







DESCRIPTION OF MAP UNITS



Mapping Conventions



	Surficial geologic units in places exist as thin (<1 m) veneers over older units.  In areas where this 

relationship is common the unit designators are shown on the map as being separated by a slash (/).  The 

younger, or overlying, unit is indicated first.  Thus, Qyf/Qof indicates an area where a veneer of young alluvial fan 

deposits overlies old alluvial deposits and Qyf/Kah indicates an area where a veneer of young alluvial fan deposits 

overlies Cretaceous hornblende granodiorite.  The lateral extent of individual deposits is commonly so small that 

each deposit cannot be shown individually at the 1:24,000 map scale.  Where areas are made up of deposits too 

small to show individually, the designators of deposits present are separated by a plus sign (+), with the most 

common deposit listed first.  Thus, Qyf + Qof indicates an area with both Qyf and Qof deposits and associated 

surfaces, and that Qyf is more common than Qof.  For the combined units, the color of the youngest unit is 

displayed on the map.  For thin deposits over granite, a faint pattern is added that is keyed to the kind of granite 

beneath the deposit.  The speckled pattern indicates deposits in which most clasts had a source of Pelona Schist 

(principally derived from the Sheep Creek fan in the San Gabriel Mountains).



Qp	Active playa deposits (Holocene)--Dark brown and green micaceous clay and silt with minor fine to 

	medium sand.  Underlies El Mirage playa and smaller playas to east and north.  White, reflective surface 

	is characterized by mud-cracks.  Little or no vegetation

Qps	Active playa deposits overlain by sand (Holocene)--Micaceous muddy playa deposits interbedded 

	with and/or overlain by complex of eolian and alluvial sands, generally less than 50 cm thick.  White and 

	pale brown on aerial photographs.  More vegetated than playa deposits (unit Qp) but does not support 

	dense cover 

Qe	Active eolian sand deposits (Holocene)--Active barchan dunes and sand ramps of fine- to coarse-grained sand;

	dunes as tall as 4 m.  Unvegetated or sparsely vegetated.  Most eolian sand south of El Mirage Lake is green

	in color due to large component of mica derived from Pelona Schist; transport is to the east

Qa	Active valley axis deposits (Holocene)--Active sand and muds with subordinate gravel in braided washes within

	semi-bolson axes

Qf	Active alluvial fan deposits (Holocene)--Poorly sorted gravel, sand, and silt in active alluvial systems.  

	Deposited along braided channels on alluvial fans, as alluvial floodplains bordering streams, and as 

	braided-stream sediment in incised stream channels

Qyc	Young colluvial deposits (Holocene)--Active and inactive gravel and sand on steep slopes of Black Mountain

Qvp	Pediment veneer deposits (Holocene and Pleistocene)--Poorly sorted gravel, sand, and silt in thin active and 

	inactive alluvial systems.  Overlie bedrock; less than 2 meters thick

Qyess	Young eolian sand sheets (Holocene)--Largely inactive sheets of fine- and medium-grained green sand and minor

	silt; laminated to lenticular bedded with minor evidence of ripples.  Surface is mounded, irregular.  

	Thickness 2 to 8 m.  Most eolian sand is derived from Pelona Schist

Qyef	Young eolian and alluvial fan deposits (Holocene)--Eolian sand sheets and mounds with subordinate young 

	alluvium.  In southeast part of map area, largely represents granitic sediments carried north on Sheep Creek

	fan in gullies and then blown eastward into sand sheets.  Thickness 1 to 4 m

Qyfe	Young alluvial fan and eolian deposits (Holocene)--Young alluvium consisting of gravel, sand, and silt, along

	with subordinate eolian sand in mounds and small sheets.  East of El Mirage Lake, consists of alluvium that 

	reworks green eolian sand blown from playa.  About 1 to 2 m thick

Qyf	Young alluvial fan deposits (Holocene)--Poorly sorted gravel, sand, and silt.  Primarily deposited as alluvial

	fans, which in many places are coalesced to form piedmonts flanking mountain ranges.  Alluvial fan deposits

	emanating from Shadow and Adobe Mountains mostly consist of coarse sand and gravel derived from granitic rocks.

	In Shadow Mountains, generally 10-30 cm thick, resting on old alluvium.  In Adobe Mountains, generally more than

	1 m thick; base not exposed.  Commonly covered by extensive biotic crusts.  In places, subdivided into two units

	based on inset relationships and soil development:

Qyf1		Young alluvial fan deposits--Inactive alluvial surfaces with bar and swale topography, no soil 

		development 

Qyf2		Young alluvial fan deposits--Inactive alluvial surfaces with subdued bar and swale topography, incipient

		Av horizon of sandy silt, incipient desert pavement

Qyfg	Young fan gravel (Holocene)--Poorly sorted cobbles and pebbles in a matrix of medium- to coarse-grained sand.

	Lenses of cobbles.  Many cobbles derived from Pelona Schist, granite, and gneiss.  Generally less than 1 m thick;

	partly covered by thin eolian deposits in many places

Qyfs	Young fan sand (Holocene)--Moderately to poorly sorted micaceous medium and fine sand, silt, and clay in a poorly

	bedded sequence greater than 4 m thick underlying young fan gravel (Qyfg).  Unit colored medium to dark green 

	due to abundant mica derived from Pelona Schist.  Beds laterally continuous for greater than 50 m distance.  

	Clay-rich beds typically parallel- and ripple-laminated.  Lenticular bedding in sand and silt beds.  Partly 

	covered by thin eolian deposits in many places

Qof	Old fan deposits (Pleistocene)--Moderately compact gravel, sand, and mud deposits; typically underlies inactive,

	partly dissected surfaces characterized by well-developed pavements and varnished clasts.   Soil development 

	generally includes moderate to strong Av horizon, moderate to strong B horizon with white calcite stringers, 

	and Stage II to III calcic horizon 10 to 150 cm thick.  Deposits largely consist of sand near El Mirage Lake.

	In places, divided into subunits where inset relations exist and soils differ:

Qof1		Old fan deposits--Alluvial fan deposits 1 to 2 m above active washes.  Characterized by nearly flat 

		surfaces with relic bar and swale topography visible, ~8 cm thick Av horizon that contains a little sand 

		with the silt; slightly to moderately developed argillic horizon, Stage I+ calcic horizon, desert 

		pavement, and moderate varnish

Qof2		Old fan deposits--Alluvial fan deposits 2 to 3 m above active washes.  Characterized by flat surfaces, 

		~10 cm thick silt Av horizon; moderately developed argillic horizon ~1.5 m thick, Stage II+ and III 

		calcic horizon greater than 1 m thick, strong desert pavement and strong varnish

Qof3		Old fan deposits--Alluvial fan deposits 3 to 5 m above active washes.  Characterized by dissected 

		surfaces, ~10 cm thick silt Av horizon; strongly developed argillic horizon ~1.5 m thick, Stage III+ 

		calcic horizon 2.5 m thick, strong desert pavement and strong varnish

Qosm	Old spring mound deposits (Pleistocene)--White, punky, calcium carbonate deposits with pelletal structure and

	abundant plant impressions.  Form mounds and degraded remnants of mounds along piedmonts bounding Adobe Mountain

	and Shadow Mountains; commonly associated with young faults

Qvof	Very old fan deposits (Pleistocene)-- Moderately compact gravel, sand, and mud deposits; typically underlie 

	inactive, well-dissected ballenas with only fragments of original surfaces exposed.  Soil development includes

	relic strong red B horizon and Stage IV calcic horizon greater than 2 m thick

Tg	Gravel (Tertiary)--Moderately consolidated, crudely bedded, alluvial boulder- to pebble-gravel and sand 

	underlying two hills north of El Mirage Lake.  Sand is arkosic, derived from granitoids.  Boulders consist 

	of granitoid and metamorphic rock derived from Shadow Mountains.  No obvious geomorphic 

	connection to present topography remains



GRANITOID ROCKS OF ADOBE MOUNTAIN AND SOUTH AND EAST



	Suite of granitoids underlying Adobe Mountain area.  All are cut by numerous aplite and pegmatite 

	dikes.  Divided into:

Kag 		Monzogranite of Adobe Mountain (Cretaceous)--Medium-grained porphyritic biotite monzogranite 

		containing minor hornblende.  Ranges in a few places to coarse grained.  Biotite 8-15%, hornblende 

		1-3%, quartz 30-35%.  Potassium feldspar phenocrysts commonly 12 mm but as large as 17 mm.  Quartz 

		light gray to milky.  Feldspars white to creamy.  Minor sphene.  Matrix to phenocrysts is subequigranular

		to seriate.  Biotite partly as distinctive barrel-shaped crystals 4 mm x 10 mm.  White to light gray, 

		weathering to pale brown; commonly forms disintegrated granite on pediments.  Dated by K-Ar at Adobe

		Mountain at 76.2  2.3 Ma (biotite) by Miller and Morton (1980).  Dike in granite near Shadow Mountains

		dated at 74.1  2.2 Ma (biotite) by Miller and Morton (1980).  Underlies much of low terrain south of 

		Adobe Mountain and in Mirage Valley 

Kan		Monzogranite of Nash Hill (Cretaceous)--Fine- to medium-grained, seriate, leucocratic monzogranite 

		containing minor biotite (CI=4 to 8).  Biotite 2 to 8 mm diameter; partly as distinctive barrel-shaped 

		crystals as large as 4 mm x 10 mm that commonly include plagioclase.  Quartz light gray to milky, about

		35%.  Feldspars white to creamy; sparse potassium feldspar phenocrysts as large as 1.5 cm.  Trace sphene.

		Forms craggy resistant hills with closely spaced jointing.  Pale gray, weathering light tan.  Crops out

		at Nash Hill, Gray Mountain, and nearby areas.  Appears to grade to monzogranite of Adobe Mountain.  

		Highly silicified at Black Mountain.  Quartz veins and muscovite selvages along joints in Nash Hill.  

		Dated by K-Ar at Gray Mountain at 82.2  2.5 Ma (biotite) and 84.3  2.5 Ma (hornblende) by Miller and 

		Morton (1980)

Kam		Microgranite (Cretaceous)--Very fine- to fine-grained, hypidiomorphic, leucocratic biotite monzogranite.

		Biotite 2-5 mm diameter, 2-5%.  Quartz light gray to milky, about 35%.  Feldspars white to creamy to 

		pink.  Light gray and pink on fresh surfaces, weathering to light brown to rust-brown. Forms low hills

		south of Adobe Mountain

Kah 		Hornblende granodiorite (Cretaceous)--Medium- and-coarse grained biotite-hornblende granodiorite, 

		generally subequigranular to porphyritic with sparse hornblende phenocrysts.  Hornblende predominates 

		over biotite; together they compose 12 to 30 % of rock, and range in size from 6 to 10 mm.  Quartz about

		25% of rock.  Sphene prominent:  it is medium grained and 4-6% of rock.  White to light tan or gray, 

		weathering dark to medium gray.  Locally mylonitic at Mt. Elmo.  At Mt. Elmo and Black Mountain, contains

		common inclusions, screens, and wispy trails of metamorphic rocks:  hornblendite, hornblende gneiss and

		schist, tremolite schist, and chlorite schist.  Altered in many locations to combinations of epidote,

		chlorite, clay, sericite, and silica



GRANITOID ROCKS IN THE SHADOW MOUNTAINS



Jgr		Granodiorite, granite and related rocks (Jurassic)--Biotite granite, hornblende granite, biotite-

		hornblende granodiorite and biotite-hornblende quartz monzodiorite.  Medium to coarse grained, but 

		compositionally and texturally variable.  Granodiorite is most common and more mafic rocks such as quartz

		monzodiorite are relatively uncommon.  Aplite and pegmatite dikes are common; one unusual composition 

		contains tourmaline, muscovite, and garnet.   Dated by Martin (1992) at 143 to 148 Ma

Jgb		Gabbro, diorite, and quartz diorite (Jurassic)--Dark brown to black hornblende gabbro, hornblende 

		diorite, hornblende-biotite quartz diorite, and mafic porphyritic hornblende-biotite quartz monzonite.

		Medium- and coarse-grained.  Mafic minerals are biotite, hornblende, and augite.  Age of quartz diorite

		is 148 Ma (Martin, 1992)



PRE-BATHOLITHIC METAMORPHIC ROCKS OF THE SHADOW MOUNTAINS - Assigned generic unit names because correlations uncertain



m7	Schist (Triassic? and Permian?)--Quartz-feldspar-biotite schist with minor quartzite and calc-silicate rock, 

	generally fine grained.  Correlated by Martin (1992; map unit PM4) to late Paleozoic and early Mesozoic strata 

	of the central and western Mojave Desert

m6	Calc-silicate rocks (Triassic? and Permian?)--Calc-silicate rock and calcite marble, generally fine-grained and

	dark in color.  Correlated by Martin (1992; map unit PMcl) to late Paleozoic and early Mesozoic strata of the 

	central and western Mojave Desert

	Marble (Permian? and Pennsylvanian?)--Calcite marble, siliceous marble, calcsilicate rock and minor quartzite.

	Divided by Martin (1992) into four stratigraphic units and units of uncertain position:

m5		Metamorphosed siliceous rocks, including layered calcsilicate, feldspathic quartzite, and interlayers 

		of marble.  About 750 m thick (unit P4 of Martin, 1992)

m4		Calcite and dolomite marble.  White to tan, massive.  About 30 m thick (unit P3 of Martin, 1992)

m3		Schistose feldspathic quartzite.  Dark gray to brown, fine- to medium-grained, and thin to medium 

		layered; less commonly massive.  About 30 to 100 m thick (unit P2 of Martin, 1992)

m2		Calcite marble, dark gray and thinly layered.  Includes distinctive layers of metamorphosed carbonate

		pebbles and less common siliceous marble and white quartzite.  About  500 m thick (unit P1 of Martin,

		1992)

m1		Siliceous marble, thinly layered.  Greater than 100 m thick (underlies unit P1 of Martin, 1992)



PRE-BATHOLITHIC METAMORPHIC ROCKS OF BLACK MOUNTAIN AND MOUNT ELMO



CZs	Siliciclastic rocks (Cambrian and Late Proterozoic)--Interlayered biotite schist, biotite-bearing quartzite, 

	and quartzite, with minor tremolite schist and mafic gneiss.  May represent the Zabriskie Quartzite, Wood 

	Canyon Quartzite, Stirling Quartzite and Johnnie Formation.  Present at Mt. Elmo and at Black Mountain

cs	Calc-silicate rocks (age uncertain)--Undivided green and brown calc-silicate rocks at Black Mountain consisting

	of variably foliated rock with varying proportions of Ca-amphibole, pyroxene, quartz, chlorite, calcite, and

	plagioclase.  Much rock is fine-grained green amphibole skarn, but some is coarse actinolite schist and gneiss

	with minerals as long as 6 cm.  Fine-grained chlorite schist is distinctive component; it carries actinolite 

	phenocrysts, as much as 1.5 cm long, in some cases.  Locally mylonitic

hg	Hornblende gneiss (age uncertain)--Black and dark brown, hornblende-rich gneiss and schist, hornblendite, and

	minor calc-silicate rock at Black Mountain. Hornblendite is finely laminated, fine-grained, and contains 75 to

	95% hornblende.  Locally mylonitic and retrograded to chlorite schist

gcs	Garnet calc-silicate rocks (age uncertain)--Calc-silicate rocks at Black Mountain similar to the calc-silicate 

	rock unit described above, but containing more muscovite and two distinctive subunits a few meters thick of 

	brown rock composed nearly entirely of zoned euhedral garnet.  Most rock is medium- to coarse-grained gneiss 

	composed of Ca-amphibole and muscovite; hornblende schist, chlorite  actinolite schist, and 

	calc-silicate-bearing marble are distinctive minor components.  Locally mylonitic





ACKNOWLEDGEMENTS

		We are grateful for assistance and advice from Brett Cox, Doug Morton, Russ Campbell, and Les McFadden.  

Reviews by Fred Miller and Kevin Schmidt improved this report, as did a review of the database and metadata by 

Scott Graham.



REFERENCES CITED



Biehler, S., Tang, R.W., Ponce, D.A., and Oliver, H.W., 1983, Preliminary Bouguer gravity map of California, 

	San Bernardino sheet:  U.S. Geological Survey Open-File Report 83-96, scale 1:250,000.

Bowen, O.E., Jr., 1954, Geology and mineral deposits of the Barstow quadrangle, San Bernardino County, 

	California:  California Division of Mines and Geology Bulletin 165, 208 p.

Brown, H.J., 1983, Possible Cambrian miogeoclinal strata in the Shadow Mountains, western Mojave Desert, 

	California: Geological Society of America Abstracts with Programs, v. 15, p. 413.  

California Department of Water Resources, 1967, Mojave River ground water basin investigation: California 

	Department of Water Resources Bulletin 84, 151 p.

Dibblee, T.W., Jr., 1960, Preliminary geologic map of the Shadow Mountains quadrangle, Los Angeles and San 

	Bernardino Counties, California:  U.S. Geological Survey Mineral Investigations Field Studies Map MF-227, 

	scale 1:62,500.

Dibblee, T.W., Jr., 1961, Evidence of strike-slip movement on northwest-trending faults in the western Mojave 

	Desert, California:  U.S. Geological Survey Professional Paper 424-B, p. B197-199.   

Dockter, R.D., 1980, Geophysical, lithologic, and water quality data from El Mirage Dry lake, San Bernardino 

	County, California:  U.S. Geological Survey Open-File Report OF 80-1200.

Dokka, R.K., 1983, Displacements on late Cenozoic strike-slip faults of the central Mojave Desert, California:  

	Geology, v. 11, p. 305-308.   

Dokka, R.K., and Travis, C.J., 1990, Role of the eastern California shear zone in accommo-dating Pacific

	North American plate motion:  Geophysical Research Letters, v. 17, p. 1323-1326. 

Fitzgibbon, T.T., 1991, ALACARTE installation and system manual (version 1.0): U.S. Geological Survey, Open-

	File Report 91-587B.

Fitzgibbon, T.T., and Wentworth, C.M., 1991, ALACARTE user interface - AML code and demonstration maps (version 1.0):

	U.S. Geological Survey, Open-File Report  91-587A.

Garfunkel, Z., 1974, Model for the late Cenozoic tectonic history of the Mojave Desert, California, and its 

	relation to adjacent areas:  Geological Society of America Bulletin, v. 85, p. 1931-1944.

Glazner, A.F., and Loomis, D. P., 1984, Effect of subduction of the Mendocino fracture zone on Tertiary 

	sedimentation in southern California:  Sedimentary Geology, v. 38, p. 287-303.

Glazner, A.F., Walker, J.D., Bartley, J.M., Fletcher, J.M., Martin, M.W., Schermer, E.R., Boettcher, S.S., Miller, 

	J.S., Fillmore, R.P., and Linn, J.K., 1994, Reconstruction of the Mojave block, in McGill, S.F., and Ross, 

	T.M., eds., Geological Investigations of an Ancient Margin:  San Bernardino County Museum Association, 

	Redlands, CA, p. 3-30. 

Hart, E.W., 1988, Fault-rupture hazard zones in California; Alquist-Priolo Special Studies Zones Act of 1972 

	with index to special studies zones maps: California Division of Mines and Geology Special Publication 42.

Hauksson, E., Jones, L.M., Hutton, K., and Eberhart-Phillips, D., 1993, The 1992 Landers earthquake 

	sequence:  Seismological obser-vations:  Journal of Geophysical Research, v. 98, p. 19835-19858. 

Martin, M.W., 1992, Stratigraphic and structural evolution of the Shadow Mountains, western Mojave Desert, 

	California:  Implications for the tectonic development of the central and western Mojave Desert:  Unpubl. Ph.D. 

	dissertation, University of Kansas, 196 p. 

Martin, M.W., and Walker, J.D., 1992, Extending the western North American Proterozoic and Paleo-zoic 

	continental crust through the Mojave Desert:  Geology, v. 20, p. 753-756.

Martin, M.W., and Walker, J.D., 1995, Stratigraphy and paleogeographic significance of metamorphic rocks in the 

	Shadow Mountains, western Mojave Desert, California:  Geological Society of America Bulletin, v. 107, p. 354-

	366.

McFadden, L.D., and Weldon, R.J. II, 1987, Rates and processes of soil development on Qua-ternary terraces 

	in Cajon Pass, California:  Geological Society of America Bulletin, v. 98, p. 280-293.

Meisling, K.E., and Weldon, R.J. II, 1989, Late Cenozoic tectonics of the northwestern San Bernardino 

	Mountains, southern California:  Geological Society of America Bulletin, v. 101, p. 106-128.

Miller, D.M., Yount, J.C., Schermer, E.R., and Felger, T.J., 1994, Preliminary assessment of the recency of faulting 

	at southwestern Fort Irwin, north-central Mojave Desert, California:  San Bernardino County Museum 

	Association Special Publication 94-1, p. 41-52. 

Miller, E.L., 1981, Geology of the Victorville region, California, Part I and II:  Geological Society of America Bulletin, 

	v. 92, p. 160-163; 554-608.

Miller, E.L., and Carr, M.D., 1978, Recognition of pos-sible Aztec-equivalent sandstones and associated Mesozoic 

	meta-sedimentary deposits within the Mesozoic magmatic arc in south-western Mojave Desert, California, in 

	Howell, D.G., and McDoug-all, K.A., eds., Mesozoic paleogeography of the western Untied States:  Pacific 

	Section, Society of Economic Paleon-tologists and Mineralogists, Pacific Coast Paleogeography Symposium 

	v. 2, p 283-289.

Miller, F.K., and Morton D.M., 1980,  Potassium-argon geochronology of the eastern Transverse Ranges and 

	southern Mojave Desert, southern California:  U.S. Geological Survey Professional Paper 1152, 30 p.

Morton, D.M., Campbell, R.H., Barrows, A.G., Jr., Kahle, J.E., and Yerkes, R.F., 1979, Wright Mountain mudflows:  

	Spring 1969, in Landsliding and mudflows at Wrightwood, San Bernardino County, California:  California 

	Division of Mines and Geology Special Report 136, p. 7-21.

Ponti, D.J., and Burke, D.B., 1980, Map showing Quaternary geology of the eastern Antelope Valley and vicinity, 

	California:  U.S. Geological Survey Open File Report 80-1064, scale 1:62,500.

Powell, R.E., 1992, Balanced palinspastic reconstruc-tion of pre-late Cenozoic paleogeology, southern California:  

	Geologic and kinematic constraints on evolution of the San Andreas fault system, in Powell, R.E., Weldon, 

R.J. II, and Matti, J.C., The San Andreas fault system:  Geological Society of America Memoir 178, p. 1-106.

Reheis, M.C., Harden, J.W., McFadden, L.D., and Shroba, R.R., 1989, Development rates of late Quaternary 

	soils, Silver Lake Playa, California:  Soil Science Society of America Journal, v. 53, p. 1127-1140.   

Sharp, R.P. and Nobles, L.H., 1965, Mudflow of 1941 at Wrightwood, southern California:  Geological Society of 

	America Bulletin, v. 64, p. 547-560.

Stamos, C.L., and Predmore, S.K., 1995, Data and water-table map of the Mojave River ground-water basin, 

	California, November 1992:  U.S. Geological Survey Water-Resources Investigations Report 95-4148.

Sieh, K.E., 1984, Lateral offsets and revised dates of large pre-historic earthquakes at Pallet Creek, southern 

	California:  Journal of Geophysical Research, v. 9, p. 7641-7670.

Stevens, C.H., and Stone, P., 1988, Early Permian thrust faults in east-central California:  Geological Society of 

	America Bulletin, v. 100, p. 552-562.

Stewart, J.H., 1970, Upper Precambrian and Lower Cambrian strata in the southern Great Basin, California and 

	Nevada:  U.S. Geological Survey Professional Paper 620, 205 p.

Stewart, J.H., and Poole, F.G., 1975, Extension of the Cordilleran miogeosynclinal belt to the San Andreas fault, 

	southern California:  Geological Society of America Bulletin, v. 86, p. 205-212.

Stone, P., and Stevens, C.H., 1988, Pennsylvanian and Early Permian paleogeography of east-central California:  

	Implications for the shape of the continental margin and the timing of conti-nental truncation:  Geology, v. 16, 

	p. 330-333.

Troxel, B.W., and Gunderson, J.N., 1970, Geology of the Shadow Mountains and northern part of the Shadow 

	Mountains SE quadrangles, western San Bernardino County, California: California Division of Mines and 

	Geology Preliminary Report 12, scale 1:24,000.

Ward, A.W., Dixon, G.L., and Jachens, R.C., 1993, Geologic setting of the East Antelope Basin, with emphasis on 

	fissuring on Rogers Lake, Edwards AFB, Mojave Desert, California:  U.S. Geological Survey Open File Report 

	93-263, 9 p.

Weldon, R.J. II, and Sieh, K.E., 1985, Holocene rate of slip and tentative recurrence interval for large earthquakes 

	on the San Andreas fault, Cajon Pass, southern California:  Geological Society of America Bulletin, v. 96, p. 

	793-812. 

Wentworth, C. M., and Fitzgibbon, T. T., 1991, ALACARTE user manual (version 1.0):  U. S. Geological Survey 

	Open-File Report 91-587C.

Yount, J.C., Schermer, E.R., Felger, T.J., Miller, D.M., and Stephens, K.A., 1994, Preliminary geologic map of Fort 

	Irwin Basin, north-central Mojave Desert, California:  U.S. Geological Survey Open File Report 94-173, 27 p., 

	scale 1:24,000.









Appendix 1.   Description of database 



	The geologic map database delineates map units that are identified by general age, lithology, and clast 

size following the stratigraphic nomenclature of the U.S. Geological Survey.  For descriptions of the units, their 

stratigraphic relations, and sources of geologic mapping, consult the Description of Map Units section of this 

report, the combined geologic map and explanation (of00-222_5a.eps or of00-222_5a.pdf), or the explanation 

sheet (of00-222_5c.eps or of00-222_5c.pdf).  The scale of the source map limits the spatial resolution (scale) of 

the database to 1:24,000 or smaller. 



For those interested in the geology of El Mirage Lake Area and vicinity who do not use an ARC/INFO compatible 

Geographic Information System (GIS), but would like to obtain a paper map and explanation, PDF and PostScript 

plot files containing map images of the data in the digital database, as well as PostScript and PDF plot files of the 

explanation sheet and explanatory text, have been included in the database package (please see the section 

"Digital Plot Files", page 18).  The PostScript plot files require a gzip utility to access them.



For those without computer capability, we can provide users with the PostScript or PDF files on tape that can be 

taken to a vendor for plotting. Paper plots can also be ordered directly from the USGS (please see the section 

"Obtaining Plots from USGS Open-File Services", page 18)

  

The content and character of the database, methods of obtaining it, and processes of extracting the map database 

from the tar (tape archive) file are described herein.  The map database itself, consisting of six ARC/INFO 

coverages, can be obtained over the Internet or by magnetic tape copy as described below. 



The database was compiled using ARC/INFO versions 7.2.1 and 8.0.1, a commercial Geographic Information 

System (Environmental Systems Research Institute, Redlands, California), with version 3.0 of the menu interface 

ALACARTE (Fitzgibbon and Wentworth, 1991, Fitzgibbon, 1991, Wentworth and Fitzgibbon, 1991).  The 

ARC/INFO coverages are stored in uncompressed ARC export format (ARC/INFO version 7.x).  All data files have 

been compressed, and may be uncompressed with gzip, which is available free of charge over the Internet via 

links from the USGS Public Domain Software page (http://edcwww.cr.usgs.gov/doc/edchome/ndcdb/public.html).  

ARC/INFO export files (files with the .e00 extension) can be converted into ARC/INFO coverages in ARC/INFO 

(see below) and can be read by some other Geographic Information Systems, such as MapInfo via ArcLink, and 

ESRI's ArcView.





DATABASE CONTENTS



The digital map database consists of digital files representing the 5 (a revisions list, the Open File Text, spatial 

data, metadata, and plot files) parts of the database.  The names of the files are unique designators based on the 

report identifier, of00-222, followed by part numbers and an extension indicating file type.  The larger files, which 

have been compressed with gzip, have .gz extensions as well.  



1.	Revision List

A list of the parts of the report and at what version number of the report each was last revised (if at all) followed by 

a chronologic list that describes any revisions:

	a. of00-222revs1.txt			ASCII file



2.	Open File Text

The text of the Open-Rile Report (this document), which discusses the geology of the area, and describes the 

database and how to obtain it:

	a. of00-222_2a.txt			unformatted ASCII text

	b. of00-222_2b.eps			PostScript file

      	c. of00-222_2c.pdf			PDF file



3.  Geologic Map Database

The geologic map coverages, which are stored as uncompressed ARC/INFO export files compressed with gzip, 

are described below.



------------------	---------------------	------------	-------------------------------------------

ARC/INFO	      	Size of gzip	       	Resultant	Description of

export file	      	compressed	       	Coverage	Coverage	       	

      		  	export file	      	(using	

		      	(uncompressed)		import.aml)

------------------    	---------------------	------------	-------------------------------------------

of00-222_3a.e00		615 Kb (2.8 MB)		elm-geol	Depositional contacts, faults and unit labels

of00-222_3b.e00      	11 kB (124 kB)		elm-str 	Fold axes, strike and dip information

of00-222_3c.e00      	2 kB (14 kB)		elm-spr         Locations of spring mound deposits

of00-222_3d.e00     	3 kB (11 kB)		elm-kar     	Locations of dated samples	

of00-222_3e.e00    	8 kB (44 kB)		elm-srcs     	Areas of previous mapping

of00-222_3f.e00    	3 kB (15 kB) 		elm-strat	Areas depicting mapping strategies

of00-222_3g.e00      	12.8 MB (68.6 MB)	elm-topo 	Topographic base map

of00-222_3h.e00		14kB (122kB)		elm-obs		Locations of field observations

of00-222_3i.tar		13.3 MB (13.4 MB)			Tar file containing all 7 of the above

								files, and import.aml



ARC export files promote ease of data handling, and are usable by some other Geographic Information Systems 

in addition to ARC/INFO (see below for a discussion of working with export files).  



4.	Metadata

FGDC compliant metadata for the spatial data in this report was compiled using ARC/INFO version 8  

ArcCatalog's built-in metadata editor.  Metadata was checked for FGDC compliance using mp version 2.4.30 

(http://geology.usgs.gov/tools/metadata/).  Metadata is available in the following formats:



--------------			---------------------

Filename			Description

--------------			---------------------

of00-222_4a.txt			FGDC metadata in formatted text

of00-222_4b.html		FGDC metadata in MP formatted HTML

of00-222_4c.html		FGDC metadata in MP formatted FAQ HTML

of00-222_4d.sgml		FGDC metadata in MP formatted SGML 

of00-222_4e.xml		FGDC metadata in MP formatted XML 



5.	 PostScript and PDF plot files

The geologic map with explanation of the El Mirage Lake Area and smaller sized separate sheets of geologic map 

and accompanying explanation sheet (the PDF files are not compressed):

---------------		-----------------	--------------------------------------------------

Plot file		Size of gzip	

			compressed		Description of plot file

			plot file	

			(uncompressed)

---------------		-----------------	--------------------------------------------------

of00-222_5a.eps       	3.2 MB (24 MB)    	El Mirage geologic map  with explanation 

						(40 X 50 inch, PostScript) 

of00-222_5b.eps		3 MB (23 MB)		El Mirage geologic map sheet 

						(36 X 50 inch, PostScript)

of00-222_5c.eps		216 kB (3.4 MB)		El Mirage explanation sheet 

						(20 X 32 inch, PostScript)

of00-222_5a.pdf		3.3 MB			El Mirage geologic map with explanation

						(40 X 50 inch, PDF)

of00-222_5b.pdf		3.2 MB			El Mirage geologic map sheet (36 X 50 inch, PDF)

of00-222_5c.pdf 	300 kB			El Mirage explanation sheet (20 X 32 inch, PDF)





OBTAINING PAPER MAPS FROM THE USGS 



The U.S. Geological Survey will make plots on demand from map files such as those described in this report. 



Be sure to include with your request the Open-File Report number and the exact names, as listed in the Database 

Contents section above, of the plot file(s) you require.  An Open-File Report number and its letter alone are not 

sufficient, unless you are requesting plots of all the plot files in this report.  You may wish to determine the price 

before placing an order.



Order plots from:



USGS Information Services

Box 25286

Denver Federal Center

Denver, CO 80225-0046

(303) 202-4200

1-800-USA-MAPS

FAX:  (303) 202-4695



e-mail:  infoservices@usgs.gov





DIGITAL PLOT FILES



For those interested only in the map or explanation images, who don't use an ARC/INFO compatible GIS system, 

but would like to obtain paper maps, we have included separate PostScript and PDF plot files.



The plot files are available in any of the three ways described below, including the World Wide Web pages.  The 

PostScript plot files have been compressed with gzip and they must be uncompressed before use.  The gzip utility is 

available free of charge over the Internet via links from the USGS Public Domain Software page:



http://edcwww.cr.usgs.gov/doc/edchome/ndcdb/public.html



The geologic map with explanation image is 40 inches wide by 50 inches long, and therefore will require a plotter 

capable of plotting files of this size.  To provide for smaller format plotters, the map is also available as separate 

sheets of a 36 X 50 inch geologic map sheet, and a 20 X 32 inch explanation sheet.  All of the map sheets require a 

large format plotter to produce paper copies at the intended scale. 





OBTAINING THE GEOLOGIC MAP DATABASE OR DIGITAL PLOT FILES



The PostScript and PDF plot files can be obtained in any of three ways:



1.	Send a tape with request



Geologic map data file(s) and/or plot file (s) can be obtained by sending a tape with request to: 



 	El Mirage Lake Area Map Database and/or Plot File

	c/o Database Coordinator

	U.S. Geological Survey

	345 Middlefield Road, MS 975

	Menlo Park, CA 94025



The file (s) will be returned on the tape.  The acceptable tape types are:



4.3	or 5.0 GB, 8 mm Exabyte tape.



In the request be sure to include with your request the Open-File Report number and the exact names, as listed in 

the Database Contents section above, of the file(s) you require.  An Open-File Report number and its letter alone 

are not sufficient, unless you are requesting all of the files for this report.



If you are obtaining a plot file on tape to give to a vendor to plot, make sure your vendor is capable of reading 

these tape types and PostScript and/or PDF plot files. 



2.  Anonymous ftp over the Internet



To obtain files by ftp:



The files in these reports are stored on the U.S. Geological Survey Western Region FTP server.  The Internet ftp 

address of this server is:



	wrgis.wr.usgs.gov



Connect to this address directly using ftp or through a browser, log in with the user name `anonymous', and enter 

your e-mail address as the password.  This will give you access to all the publications available from the server 

wrgis.



The files in this report are stored in the subdirectory:



	pub/open-file/of00-222



3.  From the Western Region Geologic Information Web Page



The U.S. Geological Survey supports a set of graphical pages on the World Wide Web from which digital 

publications such as this one can be obtained.  The web server for digital publications from the Western Region is 

"http://wrgis.wr.usgs.gov".  Once at the main page, click on 'Geologic Map Databases' under the heading 'Data 

Online'; next click on 'California.'  Scroll down to the link 'Open-File Report 00-222', which will take you to the 

web page for this report.  Set your web browser to save to a local disk and click on the link for each desired file to 

download it.





CONVERTING ARC EXPORT FILES



ARC export files are converted to ARC coverages using the ARC command IMPORT with the option COVER.   

Change directories to the database directory.  An import routine written in ARC MACRO LANGUAGE (AML) has 

been included to aid in the extraction of the coverages from the ARC export files.  From the ARC command line 

type:



Arc:  &r import.aml



ARC export files can also be read by some other Geographic Information Systems.  Please consult your GIS 

documentation to see if you can use ARC export files and the procedure to import them. 





DIGITAL COMPILATION



Initial mapping by the authors of this report was conducted by tracing lines originally drawn on areal photographs 

onto USGS Orthophoto quadrangles, and then inked onto mylar.  The mylar sheets were then scanned on an 

Altek monochrome scanner at a resolution of 600 dots per inch. The scanned images were vectorized and 

transformed from scanner coordinates to projected coordinates with digital tics placed by hand at quadrangle 

corners using ARC/INFO.  The maps were then merged and attributed to create a single spatial database of 

geologic lines and polygons.



Further field mapping was conducted using Rockwell PLGR GPS units and 'palm-top' data gathering devices.  

Daily mapping using the GPS and data gatherers was incorporated into the original spatial database using 

ArcView GIS (Environmental Systems Research Institute, Redlands, California) version 3.1 by heads up digitizing 

of areal photograph linework on Digital Orthophoto Quadrangles (DOQ's).  The revised geologic map was then 

brought into ARC/INFO for further topologic corrections and for map plotting.



The authors have compiled a spatial database of the general strategies used while collecting field data and 

preparing the map.  This data is intended to inform the user of the level of detail of the studies.  For instance, this 

information shows whether we were using aerial photograph reconnaissance or ground observations as the main 

interpretive tool.  These general strategies provide information on how confidently we expect the geology as 

mapped (both location and content of information) to be correct.  The information is summarized in Table 3 of the 

Geologic Description portion of this report, and is available as a spatial dataset in (of00-222_3f.e00, or elm-strat).





BASE MAP



The base map was created by scanning the four paper U.S. Geological Survey 7.5 minute quadrangle maps in the 

map area on an Altek monochrome scanner at a resolution of 600 dots per inch. The scanned images were 

vectorized and transformed from scanner coordinates to projection coordinates with digital tics placed by hand at 

quadrangle corners using ARC/INFO.  Because the base map is for reference only, scanning artifacts have not 

been removed from this map.





SPATIAL RESOLUTION



Uses of this digital geologic map should not violate the spatial resolution of the data.  Although the digital form of 

the data removes the constraint imposed by the scale of a paper map, the detail and accuracy inherent in map 

scale are also present in the digital data.  The fact that this database was edited at a scale of 1:24,000 means that 

higher resolution information is not present in the dataset.  Plotting at scales larger than 1:24,000 will not yield 

greater real detail, although it may reveal fine-scale irregularities below the intended resolution of the database.  

Similarly, where this database is used in combination with other data of higher resolution, the resolution of the 

combined output will be limited by the lower resolution of these data.





FAULTS



This database is not sufficiently detailed or comprehensive to identify or characterize site-specific hazards 

represented by the faults shown; the faults shown do not distinguish active faults, nor do they take the place of 

fault-rupture hazard zones designated by the California Division of Mines and Geology (see, for example, Hart, 

1988).  





DATABASE SPECIFICS



The map databases consist of ARC coverages stored in UTM projection using the North American Datum of 1983 

(Table 1). 



Table 1 - Map Projection

The map is stored in State Plane projection



PROJECTION UTM	

UNITS METERS		-on the ground

ZONE 11			 

DATUM NAD83

PARAMETERS	 	-none



The content of the geologic database can be described in terms of the lines and areas that compose it.  

Descriptions of the database fields use the terms explained in Table 2.





Table 2 - Field Definition Terms



ITEM NAME		name of the database field (item)

WIDTH			maximum number of digits or characters stored

OUTPUT			output width

TYPE			B-binary integer, F-binary floating point number, I-ASCII integer, C-ASCII character string

N. DEC.         	number of decimal places maintained for floating point numbers 





LINES



The lines (arcs) are recorded as strings of vectors and are described in the arc attribute table 

(Table 3).  They define the boundaries of the map units, the faults, and the map boundaries.  These distinctions, 

including the geologic identities of the unit boundaries, are recorded in the LTYPE field according to the line types 

listed in Tables 4 and 5.



Table 3 - Content of the Arc Attribute Tables  (ELM-GEOL.AAT, ELM-STR.AAT)



ITEM NAME	WIDTH        OUTPUT  TYPE  N. DEC	



FNODE#		4		5	B		starting node of arc (from node)   

TNODE#		4		5	B		ending node of arc (to node)

LPOLY#		4		5	B		polygon to the left of the arc

RPOLY#		4		5	B		polygon to the right of the arc

LENGTH		4		12	F	3	length of arc in meters 

<coverage>#	4		5	B		unique internal control number

<coverage>-ID	4		5	B		unique identification number

LTYPE			35		35	C	line type (see Tables 4 and 5) 

		

Table 4 - Line Types Recorded in the LTYPE Field in ELM-GEOL



contact, approx. located

contact, certain

contact, gradational

fault, approx. located

fault, approx. located, queried

fault, certain

fault, concealed

fault, concealed, queried

map boundary, certain

scratch boundary





		

Table 5 - Line Types Recorded in the LTYPE Field in ELM-STR



f.a., anticline, certain, n. plunge

f.a., syncline, certain, n. plunge

photo lineament



			

AREAS



Map units (polygons) and other areal features are described in the polygon attribute table (Table 6).  The 

identities of the map units are recorded in the PTYPE field by map label  (Tables 7 and 8).  For a 

description of the map units, consult the "Description of Map Units" portion of this report, or the PostScript 

or PDF plot files with an explanation (of00-222_5a.eps, of00-222_5a.pdf, of00-222_5c.eps, of00-

222_5c.pdf).  Areal features in the geology layer (ELM-GEOL) that require attributes other than a map 

unit, are given unique identification numbers in the ITEMID field.  ITEMID provides a common field to 

which other tabular data (ELM-GEOL.IDTAB) can be used in a relational database.  ELM-GEOL.IDTAB is 

described in Table 8, and a listing of unique values in ELM-GEOL.IDTAB is given in Table 9.  Mapping 

strategies of the authors of this report are represented as polygons depicting the strategies used in 

mapped area.  The Polygon Attribute Table (ELM-STRAT.PAT) for this coverage is described in Table 10.  

Table 11 lists the unique records in ELM-STRAT.PAT



Table 6 - Content of the Polygon Attribute Tables  (ELM-GEOL.PAT)



ITEM NAME	     WIDTH           OUTPUT    TYPE    N. DEC	

	

AREA			4		12	F	3	area of polygon in square meters

PERIMETER		4		12	F	3	length of perimeter in meters

<coverage>#		4		5	B		unique internal control number

<coverage>-ID		4		5	B		unique identification number 

PTYPE			35		35	C		unit label (see Tables 7 and 8)

ITEMID			10		10	I		attribute for individual features

								(relates to ELM-GEOL.PID)





Table 7 - Map Units in ELM-GEOL

(listed alphabetically)



CZs		Qf+Qyf			Qof1+Qyf/Kag	Qvof		Qyf1+Qyf2		hg 

Jgb		Qf+Qyf+Qof		Qof2		Qvp		Qyf1+Qyf2+Qof		m1

Jgr		Qof			Qof2+Qof3	Qyc		Qyf2			m2

Kag		Qof+Qyf			Qof2/Jgr	Qyef		Qyf2+Qyf1		m2?

Kag?		Qof+Qyf+Qf		Qof3		Qyess		Qyfe			m3

Kah		Qof+Qyf/Kag		Qof3/Kag	Qyf		Qyfe+Qa			m4

Kam		Qof/Jgr			Qof3/m2		Qyf+Qf		Qyfg			m5

Kan		Qof/Kag			Qof3?		Qyf+Qof		Qyfs			m5?

Qa		Qof/m3+m5		Qosm		Qyf+Qof+Qf	Tg			m6

Qe		Qof1			Qp		Qyf+Qof/Kag	cs			m

Qf		Qof1+Qof2		Qps		Qyf/Kag		gcs			





Table 8 - Description of INFO Table ELM-GEOL.IDTAB

NOTE: only features with added attributes are given a non-blank SRC_ROCK attribute



ITEM NAME	         WIDTH           OUTPUT   TYPE   N. DEC	

ITEMID			10		10	  I		used to link to ITEMID field in ELM-GEOL.PAT

SRC_ROCK		35		35 	  C		Description of the source material for 

								certain alluvial deposits





Table 9 - Unique values in INFO Table ELM-GEOL.IDTAB

Pelona Schist





Table 10 - Content of the Polygon Attribute Table  (ELM-STRAT.PAT)



ITEM NAME	     WIDTH            OUTPUT   TYPE   N. DEC	

	

AREA			4		12	 F	3	area of polygon in square meters

PERIMETER		4		12	 F	3	length of perimeter in meters

<coverage>#		4		5	 B		unique internal control number

<coverage>-ID		4		5	 B		unique identification number

FIELDID			10		10	 N	0	Identification number of area

GEOLOGIST		35		35	 C      	Last name(s) of geologist mapping inthat area

METHODS			200   		200      C     		Description of the methods and tools 

								used in mapping that area

					

Table 11 - Unique combinations of Mapping Strategies (ELM-STRAT.PAT)



FIELDID		1

GEOLOGIST	Miller

METHODS		Detailed reconnaissance of surficial materials by aerial photography and

		examination of key outcrops, supplemented by observations along roads.





FIELDID		2

GEOLOGIST	Miller

METHODS		Detailed study of surficial materials by aerial photography and field

		investigations supplemented by field verification of bedrock geology by Troxel and 

		Gunderson(1970) and Martin(1992).



FIELDID		3

GEOLOGIST	Miller

METHODS		Detailed field studies of bedrock; reconnaissance of surficial materials by aerial 

		photography and field studies



FIELDID		4

GEOLOGIST	Miller and Bedford

METHODS		Detailed field studies of bedrock and surficial materials



FIELDID		5

GEOLOGIST	Bedford

METHODS		Detailed field studies of bedrock supplemented with aerial photography





POINTS



Point information (strikes and dips, sample localities, etc) is recorded as coordinate pairs and related 

information is described in the Point Attribute Table (Table 12).  The identities of point types recorded in 

the PTTYPE field of the ELM-STR.PAT table are listed in Table 13.  Datasets not representing structural 

features (elm-spr, elm-kar, elm-obs) do not contain the DIP and STRIKE fields.  Identities recorded in the 

PTTYPE of the ELM-SPR.PAT table are listed in Table 14, identities recorded in the PTTYPE field of the 

ELM-KAR.PAT table are listed in Table 15, and identities recorded in the PTTYPE field of the ELM-

OBS.PAT table are listed in Table 16.





Table 12 - Content of the Point Attribute Tables in ELM-STRC.PAT



ITEM NAME	        WIDTH	      OUTPUT  TYPE   N. DEC	

	

AREA			4		12	F	3	not used 

PERIMETER		4		12	F	3	not used

<coverage>#		4		5	B		unique internal control number

<coverage>-ID	 	4		5	B		unique identification number

PTTYPE			35		35	C		point type (see Table 10)

DIP			3		3	I		dip angle in degrees

STRIKE			3		3	I		strike angle in degrees

 

								

Table 13 - Point Types  (ELM-STR.PAT)



bedding

foliation

vert bedding



Table 14 - Point Types (ELM-SPR.PAT)



Spring Mound



Table 15 - Point Types (ELM-KAR.PAT)



K-Ar Sample



Table 16 - Point Types (ELM-OBS.PAT)



Field Observation



REGIONS



Region information allows polygonal information to be stored in a way that areas, or regions, can be 

overlapping, or non-contiguous in nature.  Attribute information for regions is stored in tables that have the 

name of the coverage Polygon Attribute Table (PAT), followed by the name of a region.  Regions may 

have different table definitions (items) than the base Polygon Attribute Table, and regions within the same 

coverage may have different table definitions (items).  The index of previous mapping (ELM-SRCS), 

consists of four regions: prevmap1, prevmap2, prevmap3, and prevmap4.  Each region has identical table 

definitions, described in Table 17.  The Polygon Attribute Table (ELM-SRCS.PAT) has no attributes with 

the exception of the default ARC/INFO attributes described in Table 6.   A listing of the attributes in each 

region of ELM-SRCS is provided in Table 18.  For complete bibliographic references, see the 

"REFERENCES CITED" section of this report.



Table 17 - Content of the Region Tables for ELM-SRCS.PATPREVMAPx)



ITEM NAME	        WIDTH           OUTPUT   TYPE   N. DEC	

	

AREA			4		12	F	3	area of polygon in square meters

PERIMETER		4		12	F	3	length of perimeter in meters

<coverage>#		4		5	B		unique internal control number

<coverage>-ID		4		5	B		unique identification number 

AUTHORS			200		200	C		Authors of previous mapping

YEAR			10		10	N	0	Year of publication or report

TITLE			200		200	C		Title of publication or report

PUBLICATION		200		200	C		Publication name and number, 

								including agency of publication

SCALE			15		15	C		Scale of mapping





Table 18 - Listing of Attributes for ELM-SRCS



Table: ELM.SRC.PATPREVMAP1

AUTHORS: Dibblee, T.W., Jr.

YEAR: 1960

TITLE: Preliminary geologic map of the Shadow Mountains quadrangle, Los Angeles and San Bernardino Counties, California

PUBLICATION: U.S. Geological Survey Mineral Investigations Field Studies Map MF-227

SCALE:



Table: ELM.SRC.PATPREVMAP2

AUTHORS: Ponti, D.J, and Burke, D.B.

YEAR: 1980

TITLE: Map showing Quaternary geology of the eastern Antelope Valley and vicinity, California

PUBLICATION: U.S. Geological Survey Open File Report 80-1064

SCALE: 1:62,500



Table: ELM.SRC.PATPREVMAP3

AUTHORS: Troxel, B.W., and Gunderson, J.N.

YEAR: 1970

TITLE: Geology of the Shadow Mountains and northern part of the Shadow Mountains SE quadrangles, western San 

	Bernardino County, California

PUBLICATION: California Division of Mines and Geology Preliminary Report 12

SCALE: 1:24,000



Table: ELM.SRC.PATPREVMAP4

AUTHORS: Martin, M.W.

YEAR: 1992

TITLE: Stratigraphic and structural evolution of the Shadow Mountains, western Mojave Desert, California:  Implications 

	for the tectonic development of the central and western Mojave Desert

PUBLICATION: Unpubl. Ph.D. dissertation, University of Kansas

SCALE: 1:12,000









Appendix 2.   Quality Measures



	Location Accuracy.  Accuracy of line locations is carried in the database and displayed by different line 

symbols (solid vs. dashed lines) in the cartographic display.  For the purpose of this database, solid lines 

represent contacts and faults located with an accuracy greater than 10 m and commonly greater than 5 m.  

Dashed lines represent two kinds of less certainly located features, each distinguished in the database:  1) 

contacts and faults located with an accuracy of 10 to 15 m, 2) contacts that represent gradational 

boundaries between adjacent units.  Gradational contacts represent interleaving of sediments and smooth 

gradations in characteristics such as percentage of eolian sand.  In general, these gradations take place 

over distances of 50 to 150 m.   Lineaments and fold axes are lines whose positions do not affect location 

of polygons; they are located with an accuracy of 25 m.



	Attribute accuracy and interpretation.  Accuracy of polygon attributes depends on many factors.  We 

provide three measures of quality and interpretation, each of which provides different kinds of information.  

	(1)  Observation sites. [ELM-OBS]  This data set shows locations that we visited while conducting 

field work, at many of which we made specific recorded observations.  We observed many more points en 

route to these sites, so the data set is not a complete catalog of observations.  Rather, it is an indication of 

the areas that we most comprehensively studied.  



	(2)  Areas mapped by previous authors. [ELM-SRCS]  These areas are shown in index map form 

on the map, and the information on previous mapping is provided in the database.  The original sources 

can be consulted to evaluate agreements and disagreements about interpreting the geology in these 

areas.  Previous mapping was not intended to be as comprehensive as the present product.  Dibblee 

(1960) and Ponti and Burke (1980) conducted reconnaissance studies to identify the essential geologic 

features of the region.  Bedrock mapping by Troxel and Gunderson (1970) and Martin (1992) focused 

primarily on exposures in mountains and provided less information on pediment and piedmont geology.  

This information is expanded in the following annotated bibliography, which is referenced in fields 

AUTHORS, YEAR, TITLE, PUBLICATION, and SCALE in the database and identified on the "index map 

for previous geologic mapping". 

Dibblee (1960).  [Dibblee, T.W., Jr., 1960, Preliminary geologic map of the Shadow Mountains 

	quadrangle, Los Angeles and San Bernardino Counties, California:  U.S. Geological Survey Mineral 

	Investigations Field Studies Map MF-227, scale 1:62,500.]  Distinguished a few surficial geologic units, 

	four granitoid units, and several metamorphic rock units.  Mapped the most evident faults, as well as folds 

	in metamorphic rocks.

Troxel and Gunderson (1970).  [Troxel, B.W., and Gunderson, J.N., 1970, Geology of the Shadow 

	Mountains and northern part of the Shadow Mountains SE quadrangles, western San Bernardino County, 

	California: California Division of Mines and Geology Preliminary Report 12, scale 1:24,000.]  Distinguished 

	a few surficial geologic units, a few granitoid units, and several metamorphic rock units.  Mapped granitoid 

	and metamorphic rock units in detail, as well as faults and folds in metamorphic rocks.  Mapped 

	lineaments and faults in pediments.

Ponti and Burke (1980).  [Ponti, D.J., and Burke, D.B., 1980, Map showing Quaternary geology of the 

	eastern Antelope Valley and vicinity, California:  U.S. Geological Survey Open File Report 80-1064, scale 

	1:62,500.]  Mapped a wide range of surficial geology with a focus on areas with active faulting.  Mapped 

	several previously unrecognized faults in the El Mirage Lake area but did not break out many surficial 

	geology units in this area.

Martin (1992).  [Martin, M.W., 1992, Stratigraphic and structural evolution of the Shadow Mountains, 

	western Mojave Desert, California:  Implications for the tectonic development of the central and western 

	Mojave Desert:  Unpubl. Ph.D. dissertation, University of Kansas, 196 p.]   Conducted very detailed 

	(1:12,000) mapping of bedrock in Shadow Mountains but none in pediments.  Distinguished several map 

	units in metamorphic rocks and deduced sedimentary, metamorphic, and structural history.



  (3)  General strategies. [ELM-STRAT]  The general strategies we used while collecting field data and 

preparing the map are intended to inform the user of the level of detail of our studies.  For instance, this 

information shows whether we were using aerial photograph reconnaissance or ground observations as 

the main interpretive tool.  These general strategies provide information on how confidently we expect the 

geology as mapped (both location and content of information) to be correct.  The information is 

summarized in Table 3.



Table 1.  Field mapping strategies



Field ID	Geologist		Methods

1.		Miller			Detailed reconnaissance of surficial materials by aerial photography and 

					examination of key outcrops, supplemented by observations along roads.

2.		Miller			Detailed study of surficial materials by aerial photography and field 

					investigations supplemented by field reconnaissance verification of bedrock 

					geology by Troxel and Gunderson (1970) and Martin (1992).

3.		Miller			Detailed field studies of bedrock; reconnaissance of surficial materials by 

					aerial photography and field studies.

4.		Miller and Bedford	Detailed field studies of bedrock and surficial materials.

5.		Bedford			Detailed field studies of bedrock supplemented with aerial photography.