Geologic Map of the Bodie Hills, California and Nevada

By David A. John, Edward A. du Bray, Stephen E. Box, Peter G. Vikre, James J. Rytuba, Robert J. Fleck, and Barry 

C. Moring

Pamphlet to accompany

Scientific Investigations Map 3318

2015

U.S. Department of the Interior

U.S. Geological Survey
Cover: Photograph showing approximately 6 Ma, flow-banded rhyolite of Big Alkali dome intruding lower Paleozoic metasedimentary rocks 

at the mouth of Cinnabar Canyon.
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

John, D.A., du Bray, E.A., Box, S.E., Vikre, P.G., Rytuba, J.J., Fleck, R.J., and Moring, B.C., 2015, Geologic map of 

the Bodie Hills, California and Nevada: U.S. Geological Survey Scientific Investigations Map 3318, 64 p., 2 sheets, 

scale 1:50,000, http://dx.doi.org/10.3133/sim3318.

ISSN 2329-132X (online)

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U.S. Government.

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iii

Contents

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

Previous and Present Geologic Mapping.........................................................................................................................................1

Regional Geologic Setting of the Bodie Hills....................................................................................................................................4

Cenozoic Magmatism in the Bodie Hills ...........................................................................................................................................4

Miocene Magmatism of the Bodie Hills Volcanic Field.................................................................................................................5

Masonic Center........................................................................................................................................................................5

Mud Spring Canyon Center....................................................................................................................................................27

East Canyon Center...............................................................................................................................................................27

Aurora Center.........................................................................................................................................................................27

Rough Creek Center ..............................................................................................................................................................27

Rancheria Center...................................................................................................................................................................27

West Brawley Peak Center ....................................................................................................................................................27

Trachyandesite of Clark Canyon............................................................................................................................................28

Bridgeport Canyon Center......................................................................................................................................................28

Del Monte Center...................................................................................................................................................................28

Aurora Canyon Center ...........................................................................................................................................................28

Mount Biedeman Center ........................................................................................................................................................28

High-Silica Rhyolite Centers...................................................................................................................................................28

Rhyolite of the Bodie Hills ......................................................................................................................................................29

Silver Hill, Potato Peak, and Willow Springs Centers.............................................................................................................29

Big Alkali Center.....................................................................................................................................................................29

Pliocene to Late Pleistocene Magmatism of the Aurora Volcanic Field .....................................................................................29

Miocene Sedimentary Rocks and Deposits....................................................................................................................................30

Surficial Deposits ...........................................................................................................................................................................30

Structural Geology of the Bodie Hills..............................................................................................................................................31

Faults .........................................................................................................................................................................................31

Veins and Mineralized Faults .....................................................................................................................................................32

Paleomagnetic Studies and Vertical Axis Rotation of the Bodie Hills.........................................................................................32

Tilting..........................................................................................................................................................................................32

Volcanic Landforms....................................................................................................................................................................33

Magmatic Doming(?) of the Central Bodie Hills .........................................................................................................................33

Model for Evolution of the Late Cenozoic Stress Field in the Bodie Hills ...................................................................................34

Mineral Deposits and Alteration Zones in the Bodie Hills...............................................................................................................34

Additional Descriptive Notes ..........................................................................................................................................................35

Description of Map Units ................................................................................................................................................................39

Surficial Deposits .......................................................................................................................................................................39

Rocks of the Aurora Volcanic Field ............................................................................................................................................40

Sedimentary Deposits ................................................................................................................................................................44

Distally Sourced Ash-Flow Tuffs ................................................................................................................................................45

Rocks of the Bodie Hills Volcanic Field......................................................................................................................................46

Pre-Bodie Hills Volcanic Field Rocks .........................................................................................................................................57
iv

Basement Rocks ........................................................................................................................................................................57

References Cited ...........................................................................................................................................................................60

Figures

Figure 1. Location map showing regional geologic setting of the Bodie Hills and major physiographic features ........................2

Figure 2. Index map showing principal sources of previous geologic mapping ...........................................................................3

Figure 3. Maps showing major volcanoes and eruptive centers and types of eruptive products in the Bodie Hills .....................6

Figure 4. Total alkali-silica variation diagram for rocks of the Bodie Hills and Aurora volcanic fields ..........................................8

Figure 5. Map showing mining districts and alteration zones in the Bodie Hills...........................................................................9

Figure 6. Modes of Mesozoic granitoid rocks in the Bodie Hills plotted on a quartz-alkali feldspar-plagioclase diagram..........37

Tables

Table 1. Characteristics of eruptive centers of the Bodie Hills and Aurora volcanic fields. ........................................................10

Table 2. K-Ar and 40Ar/39Ar ages of rocks in the Bodie Hills and Aurora volcanic fields.............................................................17

Table 3. Summary of petrographic characteristics for rocks of the Bodie Hills volcanic field.........................................................

........................................................................................available for download at http://dx.doi.org/10.3133/sim3318

Table 4. Modal data for Mesozoic granitoid rocks in the Bodie Hills, California and Nevada.....................................................37
Geologic Map of the Bodie Hills, California and Nevada 

By David A. John, Edward A. du Bray, Stephen E. Box, Peter G. Vikre, James J. Rytuba, Robert J. Fleck, and 

Barry C. Moring 

Introduction

The Bodie Hills covers about 1,200 km2 straddling the California-Nevada state boundary 

just north of Mono Lake in the western part of the Basin and Range Province, about 20 km east of 

the central Sierra Nevada (fig. 1). The area is mostly underlain by the partly overlapping, middle to 

late Miocene Bodie Hills volcanic field and Pliocene to late Pleistocene Aurora volcanic field 

(John and others, 2012). Upper Miocene to Pliocene sedimentary deposits, mostly basin-filling 

sediments, gravel deposits, and fanglomerates, lap onto the west, north, and east sides of the Bodie 

Hills, where they cover older Miocene volcanic rocks. Quaternary surficial deposits, including 

extensive colluvial, fluvial, glacial, and lacustrine deposits, locally cover all older rocks. Miocene 

and younger rocks are tilted ≤30° in variable directions. These rocks are cut by several sets of highangle faults that exhibit a temporal change from conjugate northeast-striking left-lateral and northstriking right-lateral oblique-slip faults in rocks older than about 9 Ma to north- and northweststriking dip-slip faults in late Miocene rocks. The youngest faults are north-striking normal and 

northeast-striking left-lateral oblique-slip faults that cut Pliocene-Pleistocene rocks. Numerous 

hydrothermal systems were active during Miocene magmatism and formed extensive zones of 

hydrothermally altered rocks and several large mineral deposits, including gold- and silver-rich 

veins in the Bodie and Aurora mining districts (Vikre and others, in press).

Previous and Present Geologic Mapping

This new 1:50,000-scale digital geologic map of the Bodie Hills (sheet 1) covers about 

1,400 km2 and combines extensive new geologic mapping with significant modification of 

previously published geologic maps (fig. 2). These maps include those of Johnson (1951), Stanford 

Geological Survey (1961), Al-Rawi (1969), Chesterman and Gray (1975), Kleinhampl and others 

(1975), Dohrenwend (1981, 1982), Dohrenwend and Brem (1982), Stewart and others (1982), 

Chesterman and others (1986), Lange and Carmichael (1996), Silberman and Kleinhampl (unpub. 

mapping; M.L. Silberman, written commun., 2000), Reheis and others (2002), and Smailbegovic 

(2002).

Field studies for this new geologic map began in 2000–2001 by D.A. John, although most 

fieldwork and map compilation were undertaken from 2006 through 2013. Field studies focused on 

rocks of the Miocene Bodie Hills volcanic field, which host most of the mineral deposits in the 

Bodie Hills. Pre-Cenozoic basement rocks, Miocene sedimentary rocks fringing the Bodie Hills, 

Pliocene to late Pleistocene rocks of the Aurora volcanic field, and surficial deposits were studied 

in less detail. Many contacts (sheet 1) of basement rocks and Aurora volcanic field contacts are 

from published maps (Chesterman and Gray, 1975; Lange and Carmichael, 1996; Silberman and 

Kleinhampl, [unpub. mapping; M.L. Silberman, written commun., 2000]; and Smailbegovic, 

2002). John and others (2012) presented a preliminary version of the new geologic map and 

discussed the geologic history of the Bodie Hills.
2

Figure 1. Map showing regional geologic setting of the Bodie Hills (BH) and major physiographic features. Mina deflection (vertical red line pattern) and 

outline of the Walker Lane (red shaded area) modified from Faulds and Henry (2008), Busby (2013), and Carlson and others (2013). Red dashed 

line is initial 87Sr/86Sr=0.706 isopleth for Mesozoic plutonic rocks in California (Kistler, 1990) and Nevada (Tosdal and others, 2000). Blue dashed line 

is 208Pb/204Pb=38.8 isopleth for Mesozoic plutonic rocks in Nevada (Tosdal and others, 2000). Yellow box outlines area of figure 2.
3

Figure 2. Index map showing principal sources of previous geologic mapping used as a basis for the new 

geologic map, demarked by purple line.
4

Regional Geologic Setting of the Bodie Hills

The Bodie Hills covers a roughly 40 by 30 km area along the California-Nevada border and 

rises to an elevation of 10,210 ft (3,112 m) at Potato Peak, about 1,100 to 1,400 m above 

Bridgeport and Fletcher Valleys, Mono Basin, and the East Walker River (fig. 2). The central 

Sierra Nevada rises nearly 2,000 m above Bridgeport Valley and Mono Basin about 8 km west of 

the Bodie Hills. The Bodie Hills is in a complex tectonic domain near the west edge of the Walker 

Lane and the Basin and Range physiographic province and at the northwest limit of the Mina 

deflection (fig. 1; Stewart, 1988; Faulds and Henry, 2008; Busby, 2013). The Walker Lane is a 

broad, northwest-striking zone of right-lateral shear that accommodates about 20 percent of the 

right-lateral motion between the Pacific and North America plates (Oldow, 2003; Faulds and 

Henry, 2008; Busby, 2013). The Mina deflection is a 60-km right step in the Walker Lane (Stewart, 

1988; Oldow, 1992; Faulds and Henry, 2008), in which slip is transferred across an approximately 

80-km-wide, complex array of northwest-striking right-lateral faults, northeast-striking normal 

faults, and east- to east-northeast-striking left-lateral faults (Oldow, 1992; Wesnousky, 2005). The 

Bodie Hills is at the northwest corner of the Mina deflection, where east-striking left-lateral faults 

rotate into northeast-striking normal faults. Slip transfer in the Mina deflection apparently began 

after approximately 11 Ma (Faulds and Henry, 2008) and may have been concentrated along the 

east-northeast-trending Neoproterozoic continental margin generally considered to correspond to 

the initial 87Sr/86Sr (Sri)=0.706 and (or) 208Pb/204Pb=38.8 isopleths in Mesozoic granitic plutons 

(fig. 1; Kistler and Peterman, 1978; Stewart, 1988; Tosdal and others, 2000).

Late Cenozoic igneous rocks of the Bodie Hills were erupted onto now discontinuously 

exposed pre-Tertiary basement rocks. Pre-Tertiary rocks consist of (1) Lower Paleozoic hornfelsed 

argillite, sandstone, chert, and pebble conglomerate; (2) Mesozoic (Triassic?) metamorphosed 

sandstone, siltstone, chert, tuff, and pillow basalt; (3) Mesozoic, probably Jurassic, meta-andesite, 

metatuff, and metavolcaniclastic rocks; and (4) Mesozoic (probably mostly Late Cretaceous) 

granitic rocks of the Sierra Nevada batholith (Chesterman and Gray, 1975; Stewart and others, 

1982; Robinson and Kistler, 1986). Granitic rocks crop out mostly around the margins of the Bodie 

Hills, whereas metavolcanic rocks are exposed in the northern and eastern parts, and 

metasedimentary rocks occur in the southwestern and northern parts.

The Bodie Hills area likely was a topographic high during the early and mid Tertiary. 

Tertiary sedimentary and volcanic rocks generally are not present beneath the Miocene volcanic 

rocks. Paleosols that developed on pre-Tertiary basement rocks are exposed locally, notably south 

of the Homestead Mine in the northeastern part of the Bodie Hills volcanic field and at the north 

end of Bridgeport Canyon. Unlike many other areas in the western Great Basin and Sierra Nevada, 

early to mid Tertiary, westward-draining paleochannels apparently did not cross the 40 km northsouth extent of the Bodie Hills (Henry, 2008; Busby and Putirka, 2009; Henry and Faulds, 2010).

Cenozoic Magmatism in the Bodie Hills

Cenozoic volcanic rocks in the Bodie Hills are products of two long-lived volcanic fields, 

the Miocene Bodie Hills and Pliocene to late Pleistocene Aurora fields. The Bodie Hills volcanic 

field is a continental margin arc sequence (part of the ancestral Cascades arc), and formed during 

and shortly after subduction of the Farallon Plate beneath southwestern North America (Atwater 

and Stock, 1998; John and others, 2012; du Bray and others, 2014). In contrast, the Aurora 

volcanic field postdates subduction and formed in an extensional setting near the western edge of 

the Walker Lane and the Basin and Range Province (fig. 1; Lange and Carmichael, 1996; John and 

others, 2012).
5

Miocene Magmatism of the Bodie Hills Volcanic Field

Magmatic activity associated with the Bodie Hills volcanic field spanned 9.5 m.y., from 

about 15.0 to 5.5 Ma (tables 1 and 2). Volcanic rocks erupted during this period are exposed over 

about 600 km2 and may underlie a much larger area now covered by younger rocks and surficial

deposits. The Bodie Hills volcanic field consists of about 25 eruptive centers, which include 

several large, intermediate to mafic composition stratovolcanoes and more abundant intermediate 

to silicic composition lava dome complexes and exogenous rhyolite intrusions (fig. 3A and table 1). 

Eruptive activity across the field was nearly continuous between about 15 to 8 Ma, although 

eruption rates were notably higher between about 15 to 14 Ma and 10 to 8 Ma (John and others, 

2012). A final, volumetrically minor pulse of rhyolitic volcanism between about 6.2 to 5.5 Ma 

followed an approximately 1.8 m.y. hiatus in volcanic activity. Clastic sedimentary rocks were 

deposited around eruptive center margins during much of the Miocene volcanic activity, and the 

approximately 9.4 Ma Eureka Valley Tuff of the Stanislaus Group (hereafter referred to as the 

Eureka Valley Tuff), which is inferred to have erupted from the Little Walker caldera

approximately 20 km west of the Bodie Hills (fig. 1; Pluhar and others, 2009), flowed in eastwarddraining paleochannels carved around and across the eruptive centers. Small outcrops of the 12.0 

Ma tuff of Jacks Spring, which may have erupted from the Adobe Hills-Huntoon Valley area 50 

km southeast of the Bodie Hills (Stewart, 1981), are exposed locally at the base of the Miocene 

section in the southwest part of the Bodie Hills.

Compositions of rocks in the Bodie Hills volcanic field range nearly continuously from 

trachybasalt to high silica rhyolite (about 50 to 78 weight percent SiO2), but most rocks have >55 

percent SiO2 (fig. 4). Most intermediate and mafic composition Bodie Hills volcanic field rocks are 

porphyritic, commonly containing 15–35 volume percent phenocrysts of plagioclase, pyroxene, 

and hornblende±biotite and olivine; rhyolites commonly contain minor quartz, sanidine, 

plagioclase, biotite, and hornblende (table 3, available for download online at 

http://dx.doi.org/10.3133/sim3318).

Notable features of the most significant eruptive centers of the Bodie Hills volcanic field 

are briefly described in the following paragraphs and summarized in table 1. Major volcanic 

features are shown in figure 3A. The 40Ar/39Ar dates for most units are tabulated in table 2.

Masonic Center

The Masonic center, the largest and oldest eruptive center in the Bodie Hills, constitutes a 

dissected trachyandesite stratovolcano about 16 to 18 km in diameter near Masonic Mountain. It is 

composed of interbedded sequences of lava flows and debris-flow beds intruded by small plugs. 

Thick lava flows dominate exposures near the center of the volcano, including prominent southtrending ridges just east of Logan Spring. Debris-flow deposits are dominant in more distal locales, 

notably on the south side of Aurora Canyon and between Cow Camp Creek and Paramount and in 

Masonic Gulch and Red Wash on its north side. Small intrusions on the flanks of the volcano may 

define vents for some lava flows; these include a prominent columnar jointed plug and associated 

lava flows exposed near the Homestead Mine on the northeast side of the volcano. A plug on the 

southwest flank of Masonic Mountain may be one of the central vents of the volcano. 40Ar/39Ar 

ages indicate that the Masonic center formed between about 15.0 to 14.1 Ma. The volcano is a 

highly dissected horst, cut by several sets of northeast-striking faults; basement rocks are exposed 

in the center of the volcano on Masonic Mountain. Voluminous debris flows emplaced on the north 

side of the volcano in the Masonic mining district and in the Red Wash alteration zone were 

hydrothermally altered during emplacement of the Masonic Gulch and Lakeview Spring domes at 

about 13.5 and 12.9 Ma, respectively (fig. 5).
6

Figure 3. Maps showing major volcanoes and eruptive centers and types of eruptive products in the Bodie Hills. A, Bodie Hills volcanic field. B, Aurora 

volcanic field. Black lines on B outline volcanic features mentioned in the text.
7

Figure 3. Maps showing major volcanoes and eruptive centers and types of eruptive products in the Bodie Hills. A, Bodie Hills volcanic field. B, Aurora 

volcanic field. Black lines on B outline volcanic features mentioned in text.—Continued
8

Figure 4. Total alkali-silica variation diagram for rocks of the Bodie Hills and Aurora volcanic fields. Field boundaries are from Le Maitre (2002). 

Alkaline-subalkaline boundary is from Irvine and Baragar (1971).
9

Figure 5. Map showing mining districts and alteration zones in the Bodie Hills.
10

Table 1. Characteristics of eruptive centers of the Bodie Hills and Aurora volcanic fields.

Eruptive center 

or unit

Map 

units 

Volcanic 

landform

Eruptive products Composition Area; dimensions Age (Ma)1 Notable features

Bodie Hills volcanic field

Masonic Tma, 

Tmai

Stratovolcano Lava flows, debris 

flows, volcaniclastic 

sedimentary rocks, 

plugs and domes, 

minor block and ash 

flows

Trachyandesite 

and basaltic 

trachyandesite; 

with minor 

basaltic andesite 

and dacite

145 km2 exposed area,

inferred total area 

>200 km2

; circular, 

~16–18 km diameter

15.0 to 

14.1

E-W-trending horst of 

Mesozoic rocks bifurcates unit; 

debris flows and volcaniclastic 

rocks more abundant near 

margins of unit

Mud Spring

Canyon

Tams Domes Lava domes Trachyandesite 8.5 km2; 1 by 8 km 

elongated east-west

13.9 Series of domes and thick lava 

flows

East Canyon Tec Domes Lava domes and 

small intrusions

Trachydacite 2.7 km2; 2 domes 0.5

by 2–2.5 km and 3 

very small intrusions

13.8 Abundantly porphyritic lava 

domes, northwest elongated

Sinnamon Cut Tasc Intrusions Hypabyssal 

intrusions and lava 

flows(?)

Trachyandesite 0.5 km2; 2 bodies each 

0.3 by 0.8 km

13.7 Hornblende-rich, platy jointed

Masonic Gulch Tamg Domes Lava domes Trachyandesite ~2.2 km2; largest 

dome 0.6–1.2 by 2 km

13.5 to 

13.4

Three domes

Aurora Taa, 

Taai, 

Tadi

Stratovoclano(?) Lava flows, debris 

flows, block and ash 

flows, small shallow 

intrusions, minor 

volcaniclastic 

sedimentary rocks

Trachyandesite 

with minor 

andesite

~20 km2; 2–4 by 8 km 

but largely covered by 

younger rocks

13.1 to 

12.6

Hydrothermally altered in most 

places

Lakeview Spring Tals Domes Lava domes and 

flows, small 

intrusions

Andesite 0.35 km2; largest dome 

0.3 by 1 km

12.9 Domes postdate adjacent 

hydrothermal alteration in the 

Masonic mining district

Rough Creek Trcd Domes Lava domes, 

carapace breccias, 

and minor block and 

ash flows

Trachydacite and 

trachyandesite

>20 km2, largest mass 

~4 by 4 km; extensive 

cover by younger 

deposits

12.9 to 

11.7

Abundantly and coarsely 

porphyritic, northwestelongated flow domes; 

unconformably overlie 

hydrothermally altered 

Masonic center rocks

1

K-Ar and 40Ar/39Ar ages; see table 2
11

Table 1. Characteristics of eruptive centers of the Bodie Hills and Aurora volcanic fields.—Continued

Eruptive center 

or unit

Map 

units 

Volcanic 

landform

Eruptive products Composition Area; dimensions Age (Ma)1 Notable features

Bodie Hills volcanic field

Hornblende 

trachyandesite 

intrusions

Thai Intrusions Small plugs Trachyandesite 0.2 km2; largest plug 

0.3 by 0.3 km

11.8 Hornblende-rich

Rancheria Trwa, 

Trwai

Unknown Lava flows, small 

plug

Basaltic 

trachyandesite

~5 km2; largest 

exposure about 1.5 by 

2.3 km; extensive 

cover by younger 

deposits

11.7 Olivine-rich

West Brawley 

Peak

Twba, 

Twbi

Stratovolcano Central plug, lava 

flows and breccias

Trachyandesite, 

andesite, and 

trachydacite

~16 km2 exposed area; 

semicircle ~3–4 km in 

diameter with flow 

lobe extending 2 km 

farther south; 

southwest part covered 

by younger rocks 

11.5 to 

11.3

Outward-dipping lava flows 

and flow breccias on south, 

west, and north sides of plug 

forming West Brawley Peak; 

buttress unconformity with 

hydrothermally altered Aurora 

center rocks forming East 

Brawley Peak 

Clark Canyon Tcc Intrusions Small plugs, lava 

flow

Trachyandesite 0.4 km2 11.3 Series of 7 east-northeast 

aligned plugs and small lava 

flow

Trachydacite 

intrusions

Tdi Domes Small plugs and 

domes

Trachydacite 0.4 km2; 5 exposures; 

0.5 km in diameter 

Dome Hill largest 

body

11.3 to 

11.2

Biotite rich

Aurora Creek 

rhyolite

Tra Domes Flow domes, 

carapace breccias, 

lava flows, and 

lithic-rich tuffs

Rhyolite 10 km2; L-shaped, 4 

km, north-south, 5 km 

east-west, ~1 km wide 

11.2 Nearly aphyric, sparse 

phenocrysts

Del Monte 

Canyon rhyolite

Tdm Domes Lava flows and 

pyroclastic rocks

Rhyolite 3 km2; ~1 by 3 km 11.2 to 

11.1

North-dipping sequence of 

coarsely porphyritic lava flows 

and underlying lithic tuffs that 

flowed down Del Monte 

Canyon

1

K-Ar and 40Ar/39Ar ages; see table 2
12

Table 1. Characteristics of eruptive centers of the Bodie Hills and Aurora volcanic fields.—Continued

Eruptive center 

or unit

Map 

units 

Volcanic 

landform

Eruptive products Composition Area; dimensions Age (Ma)1 Notable features

Bodie Hills volcanic field

East Brawley 

Peak

Treb Domes Small domes and 

lava flows

Rhyolite 0.5 km2; three masses, 

largest 0.2–0.5 by 1 

km 

11.2 Small domes intrusive into 

Aurora and West Brawley 

Peak centers

Pyroxene 

rhyolite

Tr Domes Lava domes Rhyolite ~0.3 km2; 4 exposures, 

largest 0.1 by 0.8 km

11.2 Small flow domes north of 

Bald Peak

Trachydacite of 

Bridgeport 

Canyon

Tdbc Domes Lava flows and 

domes

Trachydacite 0.9 km2; several flow 

domes largely covered 

by younger deposits 

11.1 Small domes and associated 

lava flows locally erupted onto 

Paleozoic basement rocks

Del Monte Tdm Unknown;

stratovolcano?

Lava flows, flow 

breccias, debris 

flows, volcaniclastic 

sedimentary rocks

Basaltic 

trachyandesite, 

trachyandesite, 

and trachydacite

~26 km2 exposed in 

four main masses 

separated by younger 

rocks

11.0 Northern and central masses: 

lava flows, flow breccias, 

debris flows, and possible vent 

in Rough Creek; southern 

masses: interbedded lava 

flows, debris flows, 

sedimentary rocks; unit filled 

paleotopography (ancestral 

Bodie Creek)

Aurora Canyon Tac Domes Lava domes, 

carapace breccias, 

and minor block and 

ash flows

Trachyandesite 

and trachydacite

7.5 km2, largest body 

~1 by 5 km 

10.5 to 

10.3

Abundantly porphyritic, 

northeast-elongated lava 

domes

Bodie Creek Trbc Domes Domes and lava 

flows

Rhyolite 2.1 km2; 4 small 

domes, largest ~1 by 

1.5 km

10.2 Erosional domes largely 

covered by PliocenePleistocene lava flows

1

K-Ar and 40Ar/39Ar ages; see table 2
13

Table 1. Characteristics of eruptive centers of the Bodie Hills and Aurora volcanic fields.—Continued

Eruptive center 

or unit

Map 

units 

Volcanic 

landform

Eruptive products Composition Area; dimensions Age (Ma)1 Notable features

Bodie Hills volcanic field

Mount Biedeman Tamb, 

Tsmb, 

Timb

Stratovolcano Lava flows, debris 

flows, intrusions, 

and minor block and 

ash flows

Trachyandesite 

and basaltic 

trachyandesite 

with minor basalt 

and dacite

~100 km2 exposed 

area with extensive 

younger cover; 

circular volcano ~5–6 

km in diameter with 

debris aprons 

extending 3 to 15 km 

farther outward

9.9 to 8.9 Central intrusion forming 

Mount Biedeman surrounded 

by outward dipping lava flows 

and more distal debris flows 

and volcaniclastic sedimentary 

rocks

Bodie Hills Trbh Domes Lava domes and 

minor pyroclastic 

rocks

Rhyolite and 

minor dacite

~8 km2; largest body 

~2 by 2 km; extensive 

cover by younger 

rocks

9.9 to 9.6 Porphyritic rhyolite domes 

partly surround Mount

Biedeman stratovolcano and 

extend 15 km to northwest; 

includes several outcrops of 

rhyolite with mingled 

hornblende andesite blobs

Bald PeakParamountTrb, 

Tpst

Domes Lava domes and 

carapace breccia 

(Bald Peak), lithicrich tuff and 

volcaniclastic 

sedimentary rocks 

(Paramount basin)

Rhyolite Bald Peak: 6.5 km2; 

dome ~2.5 by 3 km; 

Paramount basin: ~20 

km2

, 2–3 by 8 km; 

extensive younger 

cover

9.7 North-northeast elongated, 

nearly aphyric rhyolite domes 

surrounded by cogenetic lithic 

tuffs and volcaniclastic 

sedimentary rocks mostly 

deposited in shallow northeastelongated basin to southwest of 

domes

Silver Hill Tsd, 

Tdsd, 

Tsdi

Domes Lava domes, 

carapace breccias, 

block and ash flows, 

debris flows,

volcaniclastic 

sedimentary rocks

Dacite, 

trachydacite, 

andesite, and 

trachyandesite

~46 km2 exposed area 

with extensive 

younger cover; 

circular field ~7–8 km 

in diameter

9.1 to 8.9 Host rock for Bodie Au-Ag 

deposits

1

K-Ar and 40Ar/39Ar ages; see table 2
14

Table 1. Characteristics of eruptive centers of the Bodie Hills and Aurora volcanic fields.—Continued

Eruptive center 

or unit

Map 

units 

Volcanic 

landform

Eruptive products Composition Area; dimensions Age (Ma)1 Notable features

Bodie Hills volcanic field

Potato Peak Tpp, 

Tppd

Domes Lava domes, lava 

flows and flow 

breccias, block and 

ash flows, debris 

flows

Trachydacite and 

trachyandesite

~80 km2; 3–8 by 14 

km with ~3 by 6 km 

debris flow apron on 

south side

9.0 to 8.8 Thick sequences of lava flows 

with rubbly flow tops and 

fronts forming prominent flow 

lobes on west, north, and east 

sides; flow-banded intrusions 

exposed on south side

Cinnabar 

Canyon

Tdcc Domes Lava flows and flow 

breccias

Trachydacite and 

rhyolite

0.3 km2 8.8 Three small exposures north of 

Clearwater Creek intruding 

debris flows associated with 

the Mount Biedeman 

stratovolcano

Willow Springs Twa Domes Lava domes, lava 

flows, minor flow 

breccias

Trachydacite and 

trachyandesite

~60 km2 exposed area; 

~9 by 12 km with 

extensive younger 

cover

8.6 to 8.0 Coarsely porphyritic lava 

flows and flow domes

Hot Springs 

Canyon

Ths Domes Lava domes Dacite 0.5 km2; largest 

exposure ~0.5 by 1 km 

8.1 Small intrusions and associated 

flows emplaced into Willow 

Springs lavas

Big Alkali Tbar Domes Lava domes, minor 

air fall tuff

Rhyolite, dacite, 

and trachydacite

~6 km2; largest body 

circular ~1 km in 

diameter

6.2 to 5.4 North-south-trending series of 

glassy, spongy, locally flowbanded domes

Rock Springs 

Canyon

Trs Domes Lava domes Rhyolite 7 km2; two lobes, 1.5 

by 2.5 km and 2 by 2 

km

<14 and 

>10.5

Two east-west elongated 

elliptical lobes of nearly 

aphryic rhyolite; local zones of 

black obsidian

1

K-Ar and 40Ar/39Ar ages; see table 2
15

Table 1. Characteristics of eruptive centers of the Bodie Hills and Aurora volcanic fields.—Continued

Eruptive center 

or unit

Map 

units 

Volcanic 

landform

Eruptive products Composition Area; dimensions Age (Ma)1 Notable features

Aurora volcanic field

Locomotive

Point

Tmlp Lava flows Lava flows and 

small cone

Basaltic andesite 4.8 km2; flows extend 

2.5 to 4 km from vent 

3.9 Series of west- to southwesttrending lava flows emanating 

from plug-filled vent 

Trachyandesite 

of Cedar Hill

Tach Lava flows and 

lava cones

Lava flows Trachyandesite ~24 km2; numerous 

exposures

3.7 Numerous ridges composed of 

lava flows and composite lava 

cones near Cedar Hill and 

extending east of map area

Spring Peak Trsp Domes Lava domes, plugs, 

and dikes

Rhyolite 22 km2; largest 

exposure 3 by 4 km; 

extensively covered by 

younger rocks

3.6 Series of flow domes

Hornblende 

trachydacite

Thd Lava flows Lava flows Trachydacite ~7 km2; 2 by 5 km 

north elongated

3.5 Series of lava flows northeast 

of Beauty Peak

Hornblende 

trachydacite of 

Cedar Hill

Thdch Lava flows and 

lava cones

Lava flows Trachydacite 46 km2; largest 

exposure ~4.5 by 6 km

3.5 to 2.4 Several series of lava flows 

and composite lava cones on 

and near Cedar Hill

Beauty Peak Tba Shield volcano Lava flows, central 

plug

Basaltic 

trachyandesite, 

trachyandesite

25 km2; 2–4 by 8 km 2.9 Series of lava flows centered 

on Beauty Peak

Pyroxene 

trachyandesite of 

Cedar Hill

Tpach Lava flows and 

lava cones

Lava flows Trachyandesite 14.5 km2; largest 

exposure ~1.7 by 6.2 

km

2.8 Composite lava cones and lava 

flows on southwest side of 

West Brawley Peak and on 

Cedar Hill

Trachybasalt of 

Cedar Hill

Tbch Cinder cone Cinder cone, lava 

flows

Trachybasalt 2.4 km2; 0.5–1.5 by 

2.5 km

2.6 Cinder cone and interbedded 

lava flows

Aurora Peak Tdap Domes Domes and lava 

flows

Trachydacite 3.9 km2; 2 by 2.5 km 

elliptical dome

2.8 to 2.6 Flow dome with two vents 

centered on Aurora Peak

Trachyandesite 

south of Aurora 

Peak

Taap Lava flows Lava flows Trachyandesite 1.8 km2; lobes 

2–2.5 km long

2.6 to 2.3 Erosional remnants of two 

lobes of lava flows 

1

K-Ar and 40Ar/39Ar ages; see table 2
16

Table 1. Characteristics of eruptive centers of the Bodie Hills and Aurora volcanic fields.—Continued

Eruptive center 

or unit

Map 

units 

Volcanic 

landform

Eruptive products Composition Area; dimensions Age (Ma)1 Notable features

Aurora volcanic field

Martinez Hill Tmr Dome Lava domes Rhyolite 0.7 km2; triangular 

0.6 by 1.2 km

2.5 Three coalescing domes

Cow Camp 

Creek

Tacc Lava flows Lava flows and 

scoria deposits

Trachyandesite 7.4 km2 with extensive 

talus; largest exposure 

~3 by 4.5 km

2.0 Lava flow sequence possibly 

related to shield volcano; 

minor scoria deposits

Mount Hicks Qmh Shield volcano Lava flows Trachyandesite 31 km2; 3.5–6 

by 10 km NWelongate, draped over 

older rocks

1.6 Shield volcano centered on 

Mount Hicks

Andesite Qa Lava flows Lava flows Andesite(?) 2.9 km2; 6 km long, 

northeast- to northtrending

1.6 to 1.4 Lava flow filling paleochannel 

draining Pleistocene Lake 

Russell

Trachyandesite 1 Qta1 Shield volcano Lava flows Trachyandesite 15 km2; 4 by 4.5 km undated; 

1.6 to 1.3

Numerous lava flows forming 

shield volcano

Trachyandesite 2 Qta2 Lava flow and 

small cone

Lava flow and plug Trachyandesite 10.5 km2; 1.5–2 by 7 

km west-trending

1.3 Fills west-trending 

paleovalley; prominent vent at 

east end

Aurora Crater Qaca Cone Lava flows and 

cinder deposits

Trachyandesite ~23 km2; 4.5 by 7 km 0.3 to 0.5 Elliptical cone with prominent 

central crater

Mud Spring Qmsa Lava flow Lava flow Trachyandesite 10 km2; 1–2.5 by 6 km 0.04 to 

0.1

West-northwest-trending lava 

flow emanating from 

prominent vent and filling 

paleodrainage

1

K-Ar and 40Ar/39Ar ages; see table 2
17

Table 2. K-Ar and 40Ar/39Ar ages of rocks in the Bodie Hills and Aurora volcanic fields.

Number Sample number Longitude 

(°W)1

Latitude 

(°N)1

Unit, mine, or alteration zone (AZ) Age, 

Ma2

Uncertainty3 Material 

dated

Method Data 

source4

Aurora volcanic field

1 EL-KA-6 −118.8169 38.3635 Trachyandesite of Mud Spring 0.04 0.04 Groundmass Ar-Ar 12

2 158 −118.8120 38.3680 Trachyandesite of Mud Spring 0.11 0.08 Groundmass K-Ar 6

3 USGS(M)-672-2 −118.8825 38.3128 Trachyandesite of Aurora Crater 0.26 0.05 Whole rock K-Ar 2

4 160 −118.8960 38.3570 Trachyandesite of Aurora Crater 0.47 0.19 Groundmass K-Ar 6

5 EL-KA-9 −118.8461 38.3630 Trachyandesite 2 1.32 0.08 Groundmass Ar-Ar 12

6 EL-KA-2 −118.7799 38.3047 Andesite 1.44 0.06 Whole rock K-Ar 12

7 EL-KA-7 −118.7901 38.3306 Andesite 1.6 0.02 Groundmass Ar-Ar 12

8 120 −118.8290 38.2400 Trachyandesite of Mount Hicks 1.6 0.1 Whole rock K-Ar 1

9 09-BA-1 −119.0441 38.3197 Trachyandesite of Cow Camp Creek 1.99 0.04 Whole rock Ar-Ar 13

10 USGS(M)-670G4 −118.8842 38.2811 Rhyolite of Martinez Hill 2.6 0.1 Biotite K-Ar 2

11 USGS(M)-611-5 −118.8558 38.2578 Trachyandesite south of Aurora Peak 2.3 0.1 Whole rock K-Ar 2

12 USGS(M)-611-5 −118.8558 38.2578 Trachyandesite south of Aurora Peak 2.6 0.2 Hornblende K-Ar 2

13 23 −118.8600 38.1560 Trachybasalt of Cedar Hill 2.60 0.06 Groundmass K-Ar 6

14 USGS(M)-610-1B −118.8586 38.2778 Trachydacite of Aurora Peak 2.6 0.2 Biotite K-Ar 2

15 USGS(M)-610-1B −118.8586 38.2778 Trachydacite of Aurora Peak 2.8 0.2 Hornblende K-Ar 2

16 466 −118.8205 38.1785 Pyroxene trachyandesite of Cedar Hill 2.8 0.2 Biotite K-Ar 1

17 469 −118.8260 38.1790 Pyroxene trachyandesite of Cedar Hill 2.8 0.8 Plagioclase K-Ar 1

18 25 −118.9730 38.2610 Basaltic trachyandesite of Beauty Peak 2.9 0.1 Whole rock K-Ar 1

19 468 −118.8185 38.1775 Hornblende trachydacite of Cedar Hill 2.9 0.4 Plagioclase K-Ar 1

20 USGS(M)-7346-3 −118.9810 38.2160 Hornblende trachydacite of Cedar Hill 2.8 0.1 Whole rock K-Ar 4

21 1069 −118.8500 38.1880 Hornblende trachydacite of Cedar Hill 2.6 0.3 Plagioclase K-Ar 1

22 1069 −118.8500 38.1880 Hornblende trachydacite of Cedar Hill 2.6 0.1 Biotite K-Ar 1

23 139 −118.8800 38.2090 Hornblende trachydacite of Cedar Hill 3.5 0.3 Biotite K-Ar 1

24 139 −118.8800 38.2090 Hornblende trachydacite of Cedar Hill 3.2 0.3 Biotite K-Ar 1

25 139 −118.8800 38.2090 Hornblende trachydacite of Cedar Hill 2.4 1.6 Plagioclase K-Ar 1 1

NAD27 coordinates

2

All 40Ar/39Ar ages relative to Fish Canyon Tuff sanidine=28.02 Ma; pre-1976 K-Ar ages recalculated using 1977 decay constants (Steiger and Jäger, 1977)

3

1 sigma

4

Data sources: 1, Gilbert and others, 1968; 2, Silberman and McKee, 1972; 3, Kleinhampl and others, 1975; 4, Silberman and Chesterman, 1972; 

5, Fleck and others, in press; 6, Lange and others, 1993; 7, L.W. Snee and B.R. Berger, written commun., 2012; 8, L.W. Snee, written commun., 2002; 

9, L.W. Snee and F. Breit, written commun., 2012; 10, L.W. Snee, B.R. Berger, and E.A. Anderson, written commun., 2012; 11, Marvin and Dobson, 1979; 12, 

Reheis and others, 2002; 13, M.A. Cosca, written commun., 2013
18

Table 2. K-Ar and 40Ar/39Ar ages of rocks in the Bodie Hills and Aurora volcanic fields—Continued

Number Sample number Longitude 

(°W)1

Latitude 

(°N)1

Unit, mine, or alteration zone (AZ) Age, 

Ma2

Uncertainty3 Material 

dated

Method Data 

source4

Aurora volcanic field

26 199 −119.0080 38.1530 Hornblende trachydacite of Cedar Hill 3.3 0.7 Plagioclase K-Ar 1

27 5-152-9 −118.9261 38.3548 Hornblende trachydacite 3.02 0.03 Whole rock Ar-Ar 10

28 5-152-9 −118.9261 38.3548 Hornblende trachydacite 3.5 0.2 Plagioclase Ar-Ar 10

29 144 −118.8940 38.1840 Trachyandesite of Cedar Hill 3.7 0.4 Plagioclase K-Ar 1

30 1092A −118.8100 38.2550 Rhyolite of Spring Peak 3.7 0.1 Sanidine K-Ar 1

31 203195 −119.1479 38.2823 Basaltic trachyandesite of Locomotive 

Point

3.9 0.06 Groundmass Ar-Ar 13

Bodie Hills volcanic field

32 USGS(M)-BH6 −119.1512 38.1655 Rhyolite of Big Alkali 5.9 0.2 Biotite K-Ar 4

33 USGS(M)-BH20 −119.1490 38.2200 Rhyolite of Big Alkali 5.3 0.3 Biotite K-Ar 4

34 USGS(M)-BH20 −119.1490 38.2200 Rhyolite of Big Alkali 5.5 0.6 Hornblende K-Ar 4

35 077-7C −119.1583 38.1968 Rhyolite of Big Alkali 5.48 0.02 Plagioclase Ar-Ar 5

36 088-24A −119.1395 38.2528 Rhyolite of Big Alkali 6.201 0.026 Plagioclase Ar-Ar 5

37 098-13A −119.1279 38.2575 Rhyolite of Big Alkali 6.173 0.028 Plagioclase Ar-Ar 5

38 09-BA-42 −119.1489 38.2700 Rhyolite of Big Alkali 5.455 0.026 Biotite Ar-Ar 5

39 39509-10 −119.2276 38.2153 Dacite of Hot Springs Canyon 8.070 0.017 Plagioclase Ar-Ar 5

40 414 −119.1930 38.1770 Trachyandesite of Willow Springs 8.0 0.2 Plagioclase K-Ar 1

41 USGS(M)-BH19A −119.1720 38.2220 Trachyandesite of Willow Springs 8.2 0.2 Biotite K-Ar 4

42 077-7A −119.1948 38.1786 Trachyandesite of Willow Springs 8.00 0.04 Biotite Ar-Ar 5

43 077-7A −119.1948 38.1786 Trachyandesite of Willow Springs 8.05 0.02 Plagioclase Ar-Ar 5

44 077-7B −119.1524 38.1918 Trachyandesite of Willow Springs 8.12 0.03 Biotite Ar-Ar 5

45 077-7B −119.1524 38.1918 Trachyandesite of Willow Springs 8.07 0.02 Hornblende Ar-Ar 5

46 077-7B −119.1524 38.1918 Trachyandesite of Willow Springs 8.09 0.03 Plagioclase Ar-Ar 5

47 088-24B −119.1412 38.2550 Trachyandesite of Willow Springs 8.575 0.022 Plagioclase Ar-Ar 5

48 088-24F −119.0898 38.2018 Trachyandesite of Willow Springs 8.15 0.02 Plagioclase Ar-Ar 5

49 08-BA-51 −119.1544 38.1745 Trachyandesite of Willow Springs 8.305 0.0169 Plagioclase Ar-Ar 5 1

NAD27 coordinates

2

All 40Ar/39Ar ages relative to Fish Canyon Tuff sanidine=28.02 Ma; pre-1976 K-Ar ages recalculated using 1977 decay constants (Steiger and Jäger, 1977)

3

1 sigma

4

Data sources: 1, Gilbert and others, 1968; 2, Silberman and McKee, 1972; 3, Kleinhampl and others, 1975; 4, Silberman and Chesterman, 1972; 

5, Fleck and others, in press; 6, Lange and others, 1993; 7, L.W. Snee and B.R. Berger, written commun., 2012; 8, L.W. Snee, written commun., 2002; 

9, L.W. Snee and F. Breit, written commun., 2012; 10, L.W. Snee, B.R. Berger, and E.A. Anderson, written commun., 2012; 11, Marvin and Dobson, 1979; 12, 

Reheis and others, 2002; 13, M.A. Cosca, written commun., 2013
19

Table 2. K-Ar and 40Ar/39Ar ages of rocks in the Bodie Hills and Aurora volcanic fields—Continued

Number Sample number Longitude 

(°W)1

Latitude 

(°N)1

Unit, mine, or alteration zone (AZ) Age, 

Ma2

Uncertainty3 Material 

dated

Method Data 

source4

Bodie Hills volcanic field

50 12-BA-1 −119.1651 38.1900 Trachydacite of Cinnabar Canyon 8.548 0.028 Plagioclase Ar-Ar 5

51 406 −119.0850 38.2340 Trachydacite of Potato Peak 8.6 0.2 Plagioclase K-Ar 1

52 USGS(M)-856-10 −119.0280 38.2050 Trachydacite of Potato Peak 9.3 0.2 Biotite K-Ar 4

53 USGS(M)-856-10 −119.0280 38.2050 Trachydacite of Potato Peak 9.2 0.2 Hornblende K-Ar 4

54 USGS(M)-BM2 −119.0570 38.2210 Trachydacite of Potato Peak 9.3 0.1 Biotite K-Ar 4

55 088-21B −119.0750 38.2642 Trachydacite of Potato Peak 8.927 0.010 Plagioclase Ar-Ar 5

56 088-21D −119.0921 38.2433 Trachydacite of Potato Peak 8.81 0.07 Plagioclase Ar-Ar 5

57 088-21E −119.0858 38.2346 Trachydacite of Potato Peak 8.86 0.05 Plagioclase Ar-Ar 5

58 088-24C −119.1121 38.2479 Trachydacite of Potato Peak 8.996 0.025 Plagioclase Ar-Ar 5

59 088-24D −119.1038 38.2268 Trachydacite of Potato Peak 9.09 0.04 Plagioclase Ar-Ar 5

60 088-24E −119.1019 38.2181 Trachydacite of Potato Peak 8.93 0.03 Plagioclase Ar-Ar 5

61 08-BA-61 −119.1017 38.2077 Trachydacite of Potato Peak 8.93 0.71 Plagioclase Ar-Ar 5

62 08-BA-68 −119.0684 38.1932 Trachydacite of Potato Peak 8.99 0.02 Plagioclase Ar-Ar 5

63 098-10C −119.0920 38.2492 Trachydacite of Potato Peak 8.982 0.023 Plagioclase Ar-Ar 5

64 098-13C −119.0922 38.2507 Trachydacite of Potato Peak 8.998 0.021 Plagioclase Ar-Ar 5

65 108-12A −119.0483 38.2425 Trachydacite of Potato Peak 8.963 0.016 Plagioclase Ar-Ar 5

66 108-12B −119.0078 38.2383 Trachydacite of Potato Peak 9.032 0.022 Biotite Ar-Ar 5

67 10-BA-66 −119.1013 38.1725 Trachydacite of Potato Peak 8.749 0.019 Plagioclase Ar-Ar 5

68 10-BA-66 −119.1013 38.1725 Trachydacite of Potato Peak 8.734 0.015 Sanidine Ar-Ar 5

69 10-BA-46 −119.1151 38.1738 Trachydacite of Potato Peak 8.779 0.013 Plagioclase Ar-Ar 5

70 15 −118.9610 38.1940 Dacite of Silver Hill 9.1 0.2 Plagioclase K-Ar 1

71 USGS(M)-BH15 −119.0030 38.2010 Dacite of Silver Hill 8.8 0.4 Hornblende K-Ar 4

72 USGS(M)-BH15 −119.0030 38.2010 Dacite of Silver Hill 8.9 0.2 Biotite K-Ar 4

73 USGS(M)-S1 −119.0080 38.1880 Dacite of Silver Hill 8.7 0.4 Whole rock K-Ar 4

74 USGS(M)-S1 −119.0080 38.1880 Dacite of Silver Hill 8.5 0.4 Plagioclase K-Ar 4

75 USGS(M)-S1 −119.0080 38.1880 Dacite of Silver Hill 9.1 0.4 Plagioclase K-Ar 4 1

NAD27 coordinates

2

All 40Ar/39Ar ages relative to Fish Canyon Tuff sanidine=28.02 Ma; pre-1976 K-Ar ages recalculated using 1977 decay constants (Steiger and Jäger, 1977)

3

1 sigma

4

Data sources: 1, Gilbert and others, 1968; 2, Silberman and McKee, 1972; 3, Kleinhampl and others, 1975; 4, Silberman and Chesterman, 1972; 

5, Fleck and others, in press; 6, Lange and others, 1993; 7, L.W. Snee and B.R. Berger, written commun., 2012; 8, L.W. Snee, written commun., 2002; 

9, L.W. Snee and F. Breit, written commun., 2012; 10, L.W. Snee, B.R. Berger, and E.A. Anderson, written commun., 2012; 11, Marvin and Dobson, 1979; 12, 

Reheis and others, 2002; 13, M.A. Cosca, written commun., 2013
20

Table 2. K-Ar and 40Ar/39Ar ages of rocks in the Bodie Hills and Aurora volcanic fields—Continued

Number Sample number Longitude 

(°W)1

Latitude 

(°N)1

Unit, mine, or alteration zone (AZ) Age, 

Ma2

Uncertainty3 Material 

dated

Method Data 

source4

Bodie Hills volcanic field

76 USGS(M)-B271 −119.0030 38.2150 Dacite of Silver Hill 8.6 0.2 Hornblende K-Ar 4

77 USGS(M)-B271 −119.0030 38.2150 Dacite of Silver Hill 8.7 0.3 Plagioclase K-Ar 4

78 USGS(M)-856-8 −119.0040 38.2140 Dacite of Silver Hill 9.4 0.5 Hornblende K-Ar 4

79 USGS(M)-BH17 −119.0120 38.1930 Dacite of Silver Hill 9.4 0.5 Hornblende K-Ar 4

80 USGS(M)-BH32 −118.9950 38.2000 Dacite of Silver Hill 9.0 0.2 Biotite K-Ar 4

81 USGS(M)-856-32 −118.9970 38.2090 Dacite of Silver Hill 9.1 0.3 Biotite K-Ar 4

82 USGS(M)-854-1 −119.0030 38.1820 Dacite of Silver Hill 8.9 0.2 Biotite K-Ar 4

83 USGS(M)-BH29 −118.9580 38.1960 Dacite of Silver Hill 8.9 0.2 Biotite K-Ar 4

84 077-7F −119.0263 38.2025 Dacite of Silver Hill 8.93 0.03 Plagioclase Ar-Ar 5

85 09-BA-26 −119.0002 38.2036 Dacite of Silver Hill 9.132 0.020 Biotite Ar-Ar 5

86 108-12C −118.9834 38.1893 Dacite of Silver Hill 9.089 0.014 Plagioclase Ar-Ar 5

87 108-12D −118.9780 38.1732 Dacite of Silver Hill 9.018 0.014 Plagioclase Ar-Ar 5

88 108-12E −118.9649 38.2076 Dacite of Silver Hill 9.140 0.020 Plagioclase Ar-Ar 5

89 077-6F −119.0085 38.1881 Dacite of Silver Hill-Sugarloaf plug 9.09 0.03 Plagioclase Ar-Ar 5

90 08-BA-66 −119.0568 38.1863 Dacite of Silver Hill debris flow clast 9.07 0.024 Plagioclase Ar-Ar 5

91 USGS(M)-BH9 −119.1160 38.1570 Trachyandesite of Mount Biedeman 9.5 0.3 Whole rock K-Ar 4

92 USGS(M)-MTB1 −119.0730 38.1460 Trachyandesite of Mount Biedeman 9.8 0.2 Hornblende K-Ar 4

93 098-11E −119.1584 38.1110 Trachyandesite of Mount Biedeman 9.019 0.028 Plagioclase Ar-Ar 5

94 09-BA-22 −119.1797 38.1430 Trachyandesite of Mount Biedeman 8.996 0.101 Plagioclase Ar-Ar 5

95 09SB015A −119.1484 38.1231 Trachyandesite of Mount Biedeman 8.895 0.061 Plagioclase Ar-Ar 5

96 108-12F −119.0992 38.1634 Trachyandesite of Mount Biedeman 9.939 0.024 Plagioclase Ar-Ar 5

97 108-9A −119.0982 38.1654 Trachyandesite of Mount Biedeman 9.947 0.019 Plagioclase Ar-Ar 5

98 10-BA-61 −119.0155 38.1401 Trachyandesite of Mount Biedeman 9.676 0.061 Plagioclase Ar-Ar 5

99 119-20B −119.0761 38.1345 Trachyandesite of Mount Biedeman 9.073 0.033 Plagioclase Ar-Ar 5

100 12-BA-22 −119.1240 38.1016 Trachyandesite of Mount Biedeman 8.931 0.041 Plagioclase Ar-Ar 5 1

NAD27 coordinates

2

All 40Ar/39Ar ages relative to Fish Canyon Tuff sanidine=28.02 Ma; pre-1976 K-Ar ages recalculated using 1977 decay constants (Steiger and Jäger, 1977)

3

1 sigma

4

Data sources: 1, Gilbert and others, 1968; 2, Silberman and McKee, 1972; 3, Kleinhampl and others, 1975; 4, Silberman and Chesterman, 1972; 

5, Fleck and others, in press; 6, Lange and others, 1993; 7, L.W. Snee and B.R. Berger, written commun., 2012; 8, L.W. Snee, written commun., 2002; 

9, L.W. Snee and F. Breit, written commun., 2012; 10, L.W. Snee, B.R. Berger, and E.A. Anderson, written commun., 2012; 11, Marvin and Dobson, 1979; 12, 

Reheis and others, 2002; 13, M.A. Cosca, written commun., 2013
21

Table 2. K-Ar and 40Ar/39Ar ages of rocks in the Bodie Hills and Aurora volcanic fields—Continued

Number Sample number Longitude 

(°W)1

Latitude 

(°N)1

Unit, mine, or alteration zone (AZ) Age, 

Ma2

Uncertainty3 Material 

dated

Method Data 

source4

Bodie Hills volcanic field

101 6-215-1 −119.0825 38.1344 Trachyandesite of Mount Biedeman 9.24 0.08 Hornblende Ar-Ar 10

102 MC-12-18 −119.0783 38.1558 Trachyandesite of Mount Biedeman 9.18 0.09 Hornblende Ar-Ar 13

103 08-BA-65 −118.9781 38.3107 Rhyolite of Bald Peak 9.690 0.01 Sanidine Ar-Ar 5

104 10-BA-67 −118.9830 38.2620 Rhyolite of Bald Peak 9.652 0.016 Sanidine Ar-Ar 5

105 507 −119.0890 38.1540 Rhyolite of Bodie Hills 9.8 0.3 Plagioclase K-Ar 1

106 507 −119.0890 38.1540 Rhyolite of Bodie Hills 9.5 0.3 Biotite K-Ar 1

107 USGS(M)-BH27 −119.0560 38.1400 Rhyolite of Bodie Hills 9.3 0.2 Biotite K-Ar 4

108 077-7D −119.0998 38.1672 Rhyolite of Bodie Hills 9.86 0.01 Sanidine Ar-Ar 5

109 077-7D −119.0998 38.1672 Rhyolite of Bodie Hills 9.86 0.01 Plagioclase Ar-Ar 5

110 088-21C −119.0838 38.2513 Rhyolite of Bodie Hills 9.776 0.017 Biotite Ar-Ar 5

111 088-21C −119.0838 38.2513 Rhyolite of Bodie Hills 9.806 0.019 Plagioclase Ar-Ar 5

112 088-21C −119.0838 38.2513 Rhyolite of Bodie Hills 9.740 0.023 Plagioclase Ar-Ar 5

113 088-21C −119.0838 38.2513 Rhyolite of Bodie Hills 9.824 0.046 Biotite Ar-Ar 5

114 08-BA-50 −119.0966 38.2482 Rhyolite of Bodie Hills 9.757 0.013 Sanidine Ar-Ar 5

115 098-10B −119.0838 38.2514 Rhyolite of Bodie Hills 9.771 0.003 Sanidine Ar-Ar 5

116 098-13B −119.1322 38.2680 Rhyolite of Bodie Hills 9.813 0.027 Sanidine Ar-Ar 5

117 09-BA-20 −119.0474 38.1566 Rhyolite of Bodie Hills 9.609 0.013 Sanidine Ar-Ar 5

118 09-BA-47 −119.1382 38.2841 Rhyolite of Bodie Hills 9.877 0.013 Sanidine Ar-Ar 5

119 USGS(M)-Baghdad −118.9467 38.2786 Rhyolite of Bodie Creek 10.2 0.3 Sanidine K-Ar 2

120 USGS(M)-Baghdad −118.9467 38.2786 Rhyolite of Bodie Creek 10.5 0.4 Plagioclase K-Ar 2

121 077-9B −118.9506 38.2697 Rhyolite of Bodie Creek 10.16 0.03 Sanidine Ar-Ar 5

122 108-11A −118.9481 38.2783 Rhyolite of Bodie Creek 9.890 0.008 Sanidine Ar-Ar 5

123 11-BA-32 −118.9780 38.3364 Rhyolite of Bodie Creek 10.127 0.013 Sanidine Ar-Ar 5

124 077-8C −119.0778 38.2715 Trachyandesite of Aurora Canyon 10.45 0.01 Sanidine Ar-Ar 5

125 077-8C −119.0778 38.2715 Trachyandesite of Aurora Canyon 10.43 0.03 Plagioclase Ar-Ar 5 1

NAD27 coordinates

2

All 40Ar/39Ar ages relative to Fish Canyon Tuff sanidine=28.02 Ma; pre-1976 K-Ar ages recalculated using 1977 decay constants (Steiger and Jäger, 1977)

3

1 sigma

4

Data sources: 1, Gilbert and others, 1968; 2, Silberman and McKee, 1972; 3, Kleinhampl and others, 1975; 4, Silberman and Chesterman, 1972; 

5, Fleck and others, in press; 6, Lange and others, 1993; 7, L.W. Snee and B.R. Berger, written commun., 2012; 8, L.W. Snee, written commun., 2002; 

9, L.W. Snee and F. Breit, written commun., 2012; 10, L.W. Snee, B.R. Berger, and E.A. Anderson, written commun., 2012; 11, Marvin and Dobson, 1979; 12, 

Reheis and others, 2002; 13, M.A. Cosca, written commun., 2013
22

Table 2. K-Ar and 40Ar/39Ar ages of rocks in the Bodie Hills and Aurora volcanic fields—Continued

Number Sample number Longitude 

(°W)1

Latitude 

(°N)1

Unit, mine, or alteration zone (AZ) Age, 

Ma2

Uncertainty3 Material 

dated

Method Data 

source4

Bodie Hills volcanic field

126 077-8C −119.0778 38.2715 Trachyandesite of Aurora Canyon 10.47 0.04 Biotite Ar-Ar 5

127 088-21A −119.1396 38.2736 Trachyandesite of Aurora Canyon 10.358 0.025 Plagioclase Ar-Ar 5

128 088-23A −119.1002 38.2785 Trachyandesite of Aurora Canyon 10.58 0.03 Biotite Ar-Ar 5

129 088-23A −119.1002 38.2785 Trachyandesite of Aurora Canyon 10.27 0.11 Plagioclase Ar-Ar 5

130 088-23B −119.0991 38.2783 Trachyandesite of Aurora Canyon 10.54 0.05 Biotite Ar-Ar 5

131 088-23B −119.0991 38.2783 Trachyandesite of Aurora Canyon 10.31 0.02 Plagioclase Ar-Ar 5

132 088-23D −119.1021 38.2811 Trachyandesite of Aurora Canyon 10.46 0.032 Biotite Ar-Ar 5

133 088-23D −119.1021 38.2811 Trachyandesite of Aurora Canyon 10.265 0.052 Plagioclase Ar-Ar 5

134 088-25A −119.0908 38.2732 Trachyandesite of Aurora Canyon 10.43 0.03 Plagioclase Ar-Ar 5

135 10-BA-62 −119.0064 38.3633 Unwelded tuff, Fletcher Basin 

sediments

10.464 0.042 Biotite Ar-Ar 5

136 10-BA-62 −119.0064 38.3633 Unwelded tuff, Fletcher Basin 

sediments

10.582 0.023 Plagioclase Ar-Ar 5

137 08SB032 −118.9558 38.3456 Unwelded tuff, Fletcher Basin 

sediments

11.075 0.041 Plagioclase Ar-Ar 5

138 077-9E −118.9100 38.3191 Trachyandesite of Del Monte 10.95 0.03 Plagioclase Ar-Ar 5

139 08SB038 −118.9738 38.4083 Trachyandesite of Del Monte 10.98 0.05 Biotite Ar-Ar 5

140 11-BA-41 −118.9809 38.3506 Trachyandesite of Del Monte 10.942 0.024 Plagioclase Ar-Ar 5

141 09SB020A −119.1328 38.1323 Trachydacite of Bridgeport Canyon 11.080 0.019 Plagioclase Ar-Ar 5

142 098-11B −119.1713 38.1245 Trachydacite of Bridgeport Canyon 11.067 0.022 Plagioclase Ar-Ar 5

143 10-BA-5 −118.9970 38.3394 Pyroxene rhyolite 11.153 0.011 Plagioclase Ar-Ar 5

144 10-BA-27 −118.9050 38.2750 Rhyolite of East Brawley Peak 11.178 0.016 Sanidine Ar-Ar 5

145 USGS(M)-NTS10B −118.8967 38.3021 Rhyolite of Aurora Creek 11.2 0.2 Biotite K-Ar 2

146 USGS(M)-NTS10B −118.8967 38.3021 Rhyolite of Aurora Creek 11.3 0.2 Sanidine K-Ar 2

147 10-BA-20 −118.9113 38.3168 Rhyolite of Aurora Creek 11.177 0.013 Plagioclase Ar-Ar 5

148 10-BA-21 −118.8974 38.3031 Rhyolite of Aurora Creek 11.181 0.022 Sanidine Ar-Ar 5

149 077-9C −118.9304 38.2766 Rhyolite of Del Monte Canyon 11.19 0.02 Sanidine Ar-Ar 5 1

NAD27 coordinates

2

All 40Ar/39Ar ages relative to Fish Canyon Tuff sanidine=28.02 Ma; pre-1976 K-Ar ages recalculated using 1977 decay constants (Steiger and Jäger, 1977)

3

1 sigma

4

Data sources: 1, Gilbert and others, 1968; 2, Silberman and McKee, 1972; 3, Kleinhampl and others, 1975; 4, Silberman and Chesterman, 1972; 

5, Fleck and others, in press; 6, Lange and others, 1993; 7, L.W. Snee and B.R. Berger, written commun., 2012; 8, L.W. Snee, written commun., 2002; 

9, L.W. Snee and F. Breit, written commun., 2012; 10, L.W. Snee, B.R. Berger, and E.A. Anderson, written commun., 2012; 11, Marvin and Dobson, 1979; 12, 

Reheis and others, 2002; 13, M.A. Cosca, written commun., 2013
23

Table 2. K-Ar and 40Ar/39Ar ages of rocks in the Bodie Hills and Aurora volcanic fields—Continued

Number Sample number Longitude 

(°W)1

Latitude 

(°N)1

Unit, mine, or alteration zone (AZ) Age, 

Ma2

Uncertainty3 Material 

dated

Method Data 

source4

Bodie Hills volcanic field

150 077-9C −118.9304 38.2766 Rhyolite of Del Monte Canyon 11.26 0.14 Plagioclase Ar-Ar 5

151 077-9D −118.9280 38.2856 Rhyolite of Del Monte Canyon 11.14 0.02 Sanidine Ar-Ar 5

152 077-9D −118.9280 38.2856 Rhyolite of Del Monte Canyon 11.20 0.03 Plagioclase Ar-Ar 5

153 AU-6/60/DD61 −118.9280 38.2850 Rhyolite of Del Monte Canyon 11.60 0.03 Biotite Ar-Ar 7

154 USGS(M)-710-7 −118.9281 38.2822 Rhyolite of Del Monte Canyon? 11.5 0.2 Biotite K-Ar 2

155 077-8B −119.0562 38.2833 Trachydacite intrusion 11.16 0.03 Plagioclase Ar-Ar 5

156 09-BA-35 −119.0464 38.2905 Trachydacite intrusion 11.269 0.034 Biotite Ar-Ar 5

157 098-13D −119.1606 38.2766 Trachyandesite of Clark Canyon 11.341 0.017 Plagioclase Ar-Ar 5

158 09-BA-43 −119.1683 38.2636 Trachyandesite of Clark Canyon 11.27 0.11 Plagioclase Ar-Ar 5

159 077-7G −118.9667 38.2344 Trachyandesite of West Brawley Peak 11.32 0.03 Plagioclase Ar-Ar 5

160 108-10C −118.9062 38.2805 Trachyandesite of West Brawley Peak 11.514 0.019 Plagioclase Ar-Ar 5

161 547 −118.9480 38.2550 Trachyandesite of West Brawley Peak 11.5 0.2 Plagioclase K-Ar 1

162 USGS(M)-712-1 −118.9175 38.2522 Trachyandesite of West Brawley Peak 11.5 0.2 Biotite K-Ar 2

163 USGS(M)-712-1 −118.9175 38.2522 Trachyandesite of West Brawley Peak 11.2 0.3 Plagioclase K-Ar 2

164 USGS(M)-712-1 −118.9175 38.2522 Trachyandesite of West Brawley Peak 11.9 0.3 Hornblende K-Ar 2

165 6 −119.1697 38.1256 Basaltic trachyandesite of Rancheria 11.6 0.9 Whole rock K-Ar 3

166 742-45 −119.1431 38.0869 Basaltic trachyandesite of Rancheria 

plug

11.7 0.5 Whole rock K-Ar 11

167 MC-12-20 −119.1261 38.2701 Hornblende trachyandesite intrusion 11.82 0.2 Hornblende Ar-Ar 13

168 10-BA-6 −119.0247 38.3338 Trachydacite of Rough Creek 12.945 0.020 Plagioclase Ar-Ar 5

169 10-BA-29 −119.0145 38.3647 Trachydacite of Rough Creek 11.675 0.016 Plagioclase Ar-Ar 5

170 11-BA-40 −119.0191 38.4116 Trachydacite of Rough Creek 12.804 0.023 Plagioclase Ar-Ar 5

171 108-10A −118.9127 38.2729 Trachyandesite of Aurora 12.580 0.024 Plagioclase Ar-Ar 5

172 00-BA-16 −118.8908 38.2708 Trachyandesite of Aurora 13.13 0.44 Hornblende Ar-Ar 8

173 00-BA-26 −118.8841 38.2542 Trachyandesite of Aurora 13.13 0.08 Hornblende Ar-Ar 8

174 07-BA-38 −119.1450 38.3605 Andesite of Lakeview Spring 12.93 0.03 Plagioclase Ar-Ar 5

175 5 −119.1475 38.0978 Trachyandesite of Sinnamon Cut 13.8 0.4 Hornblende K-Ar 3 1

NAD27 coordinates

2

All 40Ar/39Ar ages relative to Fish Canyon Tuff sanidine=28.02 Ma; pre-1976 K-Ar ages recalculated using 1977 decay constants (Steiger and Jäger, 1977)

3

1 sigma

4

Data sources: 1, Gilbert and others, 1968; 2, Silberman and McKee, 1972; 3, Kleinhampl and others, 1975; 4, Silberman and Chesterman, 1972; 

5, Fleck and others, in press; 6, Lange and others, 1993; 7, L.W. Snee and B.R. Berger, written commun., 2012; 8, L.W. Snee, written commun., 2002; 

9, L.W. Snee and F. Breit, written commun., 2012; 10, L.W. Snee, B.R. Berger, and E.A. Anderson, written commun., 2012; 11, Marvin and Dobson, 1979; 12, 

Reheis and others, 2002; 13, M.A. Cosca, written commun., 2013
24

Table 2. K-Ar and 40Ar/39Ar ages of rocks in the Bodie Hills and Aurora volcanic fields—Continued

Number Sample number Longitude 

(°W)1

Latitude 

(°N)1

Unit, mine, or alteration zone (AZ) Age, 

Ma2

Uncertainty3 Material 

dated

Method Data 

source4

Bodie Hills volcanic field

176 12-BA-9 −119.0738 38.3368 Trachydacite of East Canyon 13.782 0.025 Plagioclase Ar-Ar 5

177 098-12E −119.1240 38.3914 Trachyandesite of Masonic Gulch 13.441 0.026 Plagioclase Ar-Ar 5

178 MAS10-72 −119.1372 38.4019 Trachyandesite of Masonic Gulch 13.485 0.017 Plagioclase Ar-Ar 5

179 MAS10-73 −119.1347 38.3964 Trachyandesite of Masonic Gulch 13.467 0.016 Plagioclase Ar-Ar 5

180 MAS10-75 −119.1155 38.3917 Trachyandesite of Masonic Gulch 13.467 0.023 Plagioclase Ar-Ar 5

181 12-BA-3 −118.8473 38.3497 Trachyandesite of Mud Spring Canyon 13.87 0.11 Plagioclase Ar-Ar 13

182 088-22B −119.0780 38.3794 Trachyandesite of Masonic 14.07 0.08 Plagioclase Ar-Ar 5

183 098-12F −119.1142 38.3754 Trachyandesite of Masonic 14.094 0.017 Plagioclase Ar-Ar 5

184 10-BA-69 −119.1365 38.3054 Trachyandesite of Masonic 14.107 0.014 Plagioclase Ar-Ar 5

185 08-BA-62 −119.0494 38.3201 Trachyandesite of Masonic 14.13 0.03 Plagioclase Ar-Ar 5

186 10-BA-15 −119.1157 38.3413 Trachyandesite of Masonic 14.141 0.017 Plagioclase Ar-Ar 5

187 088-23F −119.1170 38.3107 Trachyandesite of Masonic 14.193 0.039 Plagioclase Ar-Ar 5

188 10-BA-13 −119.1042 38.3515 Trachyandesite of Masonic 14.410 0.016 Plagioclase Ar-Ar 5

189 09-BA-7 −119.0873 38.3379 Trachyandesite of Masonic 14.674 0.025 Plagioclase Ar-Ar 5

190 098-12B −119.1814 38.3876 Trachyandesite of Masonic 14.715 0.025 Plagioclase Ar-Ar 5

191 098-12D −119.1194 38.3995 Trachyandesite of Masonic <14.80 0.11 Plagioclase Ar-Ar 5

192 12-BA-20 −119.1596 38.3340 Trachyandesite of Masonic 14.136 0.02 Plagioclase Ar-Ar 5

193 MAS10-76 −119.1153 38.3817 Trachyandesite intrusions of Masonic 14.190 0.021 Hornblende Ar-Ar 5

194 10-BA-9 −119.0207 38.3528 Trachyandesite intrusions of Masonic 14.998 0.019 Plagioclase Ar-Ar 5

195 MC-12-19 −118.7815 38.3195 Trachyandesite of Borealis 15.96 0.10 Hornblende Ar-Ar 13

Distally sourced ash-flow tuffs

196 31 −118.9900 38.2470 Eureka Valley Tuff 9.8 0.3 Biotite K-Ar 1

197 27 −118.9900 38.2470 Eureka Valley Tuff 9.9 0.3 Biotite K-Ar 1

198 W24 −119.0660 38.1020 Eureka Valley Tuff 9.4 0.28 Biotite K-Ar 1

199 W25 −119.1810 38.1310 Eureka Valley Tuff 9.6 0.37 Biotite K-Ar 1

200 W25 −119.1810 38.1310 Eureka Valley Tuff 9.2 0.27 Biotite K-Ar 1 1

NAD27 coordinates

2

All 40Ar/39Ar ages relative to Fish Canyon Tuff sanidine=28.02 Ma; pre-1976 K-Ar ages recalculated using 1977 decay constants (Steiger and Jäger, 1977)

3

1 sigma

4

Data sources: 1, Gilbert and others, 1968; 2, Silberman and McKee, 1972; 3, Kleinhampl and others, 1975; 4, Silberman and Chesterman, 1972; 

5, Fleck and others, in press; 6, Lange and others, 1993; 7, L.W. Snee and B.R. Berger, written commun., 2012; 8, L.W. Snee, written commun., 2002; 

9, L.W. Snee and F. Breit, written commun., 2012; 10, L.W. Snee, B.R. Berger, and E.A. Anderson, written commun., 2012; 11, Marvin and Dobson, 1979; 12, 

Reheis and others, 2002; 13, M.A. Cosca, written commun., 2013
25

Table 2. K-Ar and 40Ar/39Ar ages of rocks in the Bodie Hills and Aurora volcanic fields—Continued

Number Sample number Longitude 

(°W)1

Latitude 

(°N)1

Unit, mine, or alteration zone (AZ) Age, 

Ma2

Uncertainty3 Material 

dated

Method Data 

source4

Distally sourced ash-flow tuffs

201 077-9A −118.9744 38.2209 Eureka Valley Tuff 9.40 0.02 Biotite Ar-Ar 5

202 077-9A −118.9744 38.2209 Eureka Valley Tuff 9.38 0.03 Plagioclase Ar-Ar 5

203 098-12A −119.1992 38.3592 Eureka Valley Tuff 9.453 0.030 Plagioclase Ar-Ar 5

204 09-BA-49 −119.0475 38.2639 Eureka Valley Tuff 9.267 0.019 Biotite Ar-Ar 5

205 AU-10/61/DD61 −118.9810 38.2600 Eureka Valley Tuff 9.26 0.04 Biotite Ar-Ar 7

206 6-289-2 −118.9993 38.3729 Eureka Valley Tuff 9.26 0.06 Biotite Ar-Ar 10

207 098-11A −119.1695 38.1224 Tuff of Jacks Spring 12.069 0.015 Sanidine Ar-Ar 5

208 09-BA-23 −119.1749 38.1285 Tuff of Jacks Spring 12.044 0.016 Sanidine Ar-Ar 5

209 6-181-12 −119.1039 38.1193 Tuff of Jacks Spring 11.95 0.16 Sanidine Ar-Ar 10

210 6-181-12 −119.1039 38.1193 Tuff of Jacks Spring 11.98 0.14 Sanidine Ar-Ar 10

Hydrothermal alteration minerals

211 39509-3K −119.2275 38.2079 Highway 395 AZ 8.169 0.013 Alunite Ar-Ar 5

212 11-BA-21 −119.0092 38.2037 Noonday mine dump, Bodie 8.124 0.006 Adularia Ar-Ar 5

213 BOD11-3A −119.0035 38.2196 Upper Hobart Tunnel dump, Bodie 8.194 0.005 Adularia Ar-Ar 5

214 USGS(M)-B270 −119.0040 38.2160 McClinton shaft dump, Bodie 8.2 0.2 Adularia K-Ar 6

215 11-BA-7G −119.0065 38.2042 Oro shaft dump, Bodie 8.217 0.012 Adularia Ar-Ar 5

216 AU-17/64/DD61 −118.9970 38.2150 Tioga shaft dump, Bodie 8.28 0.04 Adularia Ar-Ar 7

217 077-6A −119.0037 38.2149 Roseclip glory hole, Bodie 8.29 0.02 Adularia Ar-Ar 5

218 12-BA-17 −119.0002 38.2296 West side of Bodie Bluff, Bodie 8.358 0.029 Adularia Ar-Ar 5

219 AU-16/63/DD61 −119.0040 38.2150 Roseclip glory hole, Bodie 8.38 0.02 Adularia Ar-Ar 7

220 BOD11-4E −119.0040 38.2126 Standard Hill mine dump, Bodie 8.426 0.003 Adularia Ar-Ar 5

221 AU-18/65/DD61 −119.0040 38.2010 Red Cloud shaft dump, Bodie 8.46 0.02 Adularia Ar-Ar 7

222 11-BA-22 −119.0040 38.2126 Dump at base of Standard Hill, Bodie 8.498 0.005 Adularia Ar-Ar 5

223 BOD09-5 −119.0066 38.2043 Oro shaft dump, Bodie 8.63 0.007 Adularia Ar-Ar 5

224 BOD9-3 −119.0048 38.2015 Red Cloud shaft dump, Bodie 8.63 0.014 Adularia Ar-Ar 5

225 09-BA-28I −119.0036 38.2015 Red Cloud shaft dump, Bodie 8.85 0.013 Adularia Ar-Ar 5 1

NAD27 coordinates

2

All 40Ar/39Ar ages relative to Fish Canyon Tuff sanidine=28.02 Ma; pre-1976 K-Ar ages recalculated using 1977 decay constants (Steiger and Jäger, 1977)

3

1 sigma

4

Data sources: 1, Gilbert and others, 1968; 2, Silberman and McKee, 1972; 3, Kleinhampl and others, 1975; 4, Silberman and Chesterman, 1972; 

5, Fleck and others, in press; 6, Lange and others, 1993; 7, L.W. Snee and B.R. Berger, written commun., 2012; 8, L.W. Snee, written commun., 2002; 

9, L.W. Snee and F. Breit, written commun., 2012; 10, L.W. Snee, B.R. Berger, and E.A. Anderson, written commun., 2012; 11, Marvin and Dobson, 1979; 12, 

Reheis and others, 2002; 13, M.A. Cosca, written commun., 2013
26

Table 2. K-Ar and 40Ar/39Ar ages of rocks in the Bodie Hills and Aurora volcanic fields—Continued

Number Sample number Longitude 

(°W)1

Latitude 

(°N)1

Unit, mine, or alteration zone (AZ) Age, 

Ma2

Uncertainty3 Material 

dated

Method Data 

source4

Hydrothermal alteration minerals

226 CC09-9D2 −119.1705 38.1987 Cinnabar Canyon AZ 8.68 0.02 Alunite Ar-Ar 5

227 CC09-9D1 −119.1705 38.1987 Cinnabar Canyon AZ 8.82 0.29 Alunite Ar-Ar 5

228 FA1 −118.8914 38.2931 Aurora 10.11 0.03 Adularia Ar-Ar 9

229 AU-1/58/DD61 −118.9000 38.2660 Esmeralda vein, Aurora 10.47 0.1 K-feldspar Ar-Ar 7

230 37 −118.8870 38.2870 Aurora 10.6 0.2 Adularia K-Ar 3

231 AU-2/59/DD61 −118.8960 38.2820 Rio Del Monte mine dump, Aurora 10.35 0.05 K-feldspar Ar-Ar 7

232 08-BA-46 −119.1039 38.2443 Potato Peak AZ 10.83 0.06 Alunite Ar-Ar 5

233 PP09-10A1 −119.1384 38.2531 Aurora Canyon AZ 10.866 0.065 Alunite Ar-Ar 5

234 SAW11-9 −118.9166 39.2904 Sawtooth AZ 11.105 0.018 Alunite Ar-Ar 5

235 AUR10-3 −118.8992 38.2375 East Brawley Peak AZ 11.954 0.016 Alunite Ar-Ar 5

236 FA66 −118.9008 38.2375 East Brawley Peak AZ 12.42 0.04 Alunite Ar-Ar 9

237 MAS11-1 −119.0837 38.3604 Perini mine, lower dump, Masonic 12.956 0.020 Alunite Ar-Ar 5

238 MAS11-2B −119.0868 38.3606 Perini mine, upper dump, Masonic 12.960 0.014 Alunite Ar-Ar 5

239 088-22A −119.1073 38.3673 Maybelle mine, Masonic 13.018 0.061 Alunite Ar-Ar 5

240 07-BA-40 −119.1161 38.3655 Pittsburg-Liberty mine, Masonic 13.02 0.05 Alunite Ar-Ar 5

241 MAS07-3 −119.1268 38.3597 Sarita mine, Masonic 13.26 0.05 Alunite Ar-Ar 5

242 MAS10-55 −119.1026 38.4142 Redwash AZ 13.27 0.02 Alunite Ar-Ar 5

243 MAS09-1 −119.1327 38.3578 Red Rock mine area, Masonic 13.32 0.14 Alunite Ar-Ar 5

244 RW08-1 −119.1241 38.4112 Red Wash AZ 13.338 0.035 Alunite Ar-Ar 5

245 MAS09-1A −119.1382 38.3585 Red Rock mine area, Masonic 13.37 0.11 Alunite Ar-Ar 5

246 MAS07-1A −119.1483 38.3499 Chemung mine, Masonic 13.39 0.07 Alunite Ar-Ar 5 1

NAD27 coordinates 2

All 40Ar/39Ar ages relative to Fish Canyon Tuff sanidine=28.02 Ma; pre-1976 K-Ar ages recalculated using 1977 decay constants (Steiger and Jäger, 1977)

3

1 sigma

4

Data sources: 1, Gilbert and others, 1968; 2, Silberman and McKee, 1972; 3, Kleinhampl and others, 1975; 4, Silberman and Chesterman, 1972; 

5, Fleck and others, in press; 6, Lange and others, 1993; 7, L.W. Snee and B.R. Berger, written commun., 2012; 8, L.W. Snee, written commun., 2002; 

9, L.W. Snee and F. Breit, written commun., 2012; 10, L.W. Snee, B.R. Berger, and E.A. Anderson, written commun., 2012; 11, Marvin and Dobson, 1979; 12, 

Reheis and others, 2002; 13, M.A. Cosca, written commun., 2013
27

Mud Spring Canyon Center

The 14.0 Ma Mud Spring Canyon center forms a series of coarsely porphyritic 

trachyandesite lava domes that are exposed beneath mafic lava flows of the Aurora volcanic field 

on the east side of the Bodie Hills. This unit is the oldest of numerous coarsely and abundantly 

porphyritic, intermediate composition (silicic trachyandesite to dacite) dome fields in the Bodie 

Hills.

East Canyon Center

The 13.8 Ma East Canyon center forms two small intrusions and two northwest-elongated 

trachydacite lava domes emplaced into the Masonic center on the east side of Masonic Mountain.

Aurora Center

The 13.1 to 12.6 Ma Aurora center consists of trachyandesite lava flows, debris-flow 

deposits, and small intrusions that likely formed a now highly dissected stratovolcano centered near 

the townsite of Aurora. Younger rocks of the Aurora volcanic field overlie and conceal a 

significant part of this center. Nearly all rocks in the Aurora center are hydrothermally altered; the 

center hosts both important epithermal quartz-adularia gold-silver vein deposits in the Aurora 

mining district and older epithermal quartz-alunite gold prospects in the East Brawley Peak 

alteration zone (fig. 5).

Rough Creek Center

The Rough Creek center forms a series of prominent, coarsely porphyritic trachydacite lava 

domes in the north-central part of the Bodie Hills. 40Ar/39Ar ages suggest that it may consist of two 

sets of nearly identical, coarsely porphyritic domes emplaced at about 12.9 to 12.8 Ma and at 11.7 

Ma; rocks of the two ages are indistinguishable and are not portrayed separately on the map. The 

domes are unaltered and postdate hydrothermal activity related to the Red Wash-East Walker River 

alteration zone; they are, in turn, unconformably overlain by the sedimentary rocks of Fletcher 

Valley.

Rancheria Center

The 11.7 Ma Rancheria center consists of a series of basaltic trachyandesite lava flows 

discontinuously exposed beneath the west side of the Mount Biedeman stratovolcano in the 

southwest corner of the Bodie Hills. The olivine-rich Rancheria flows generally have the most 

mafic compositions of any unit in the Bodie Hills volcanic field.

West Brawley Peak Center

The 11.6 to 11.3 Ma West Brawley Peak center is a conspicuous composite cone on the 

southwest margin of the Aurora center. A plug of vertically flow-banded trachyandesite, probably a 

feeder to lava flows, forms West Brawley Peak. This plug is surrounded on its south, west, and 

north sides by thick sequences of outward-dipping lava flows. The east side of the volcano is a 

buttress unconformity against resistant ledges of quartz-alunite altered andesite from the Aurora 

center that form East Brawley Peak.
28

Trachyandesite of Clark Canyon

The trachyandesite of Clark Canyon forms a northeast-aligned array of small plugs that 

intrude debris-flow deposits of the Masonic center in Aurora Canyon. The pronounced alignment 

of these 11.3 Ma intrusions suggest emplacement along a northeast-trending structure.

Bridgeport Canyon Center

The 11.1 Ma Bridgeport Canyon center is composed of trachydacite domes and associated 

lava flows in the southwest corner of the Bodie Hills. These coarsely porphyritic rocks are notable 

for their high-K contents and are petrographically and geochemically similar to, but slightly older 

than, the 10.5 Ma Table Mountain Latite, which emanated from vents in the Little Walker caldera

and in the Sonora Pass area 30 to 50 km to the northwest (Putirka and Busby, 2007; Busby and 

others, 2013).

Del Monte Center

The 11.0 Ma Del Monte center consists of four areas of trachyandesite lava and debris-flow 

deposits on the northeast and east sides of the Bodie Hills volcanic field. Lavas in all four areas are 

geochemically and petrographically similar. Lava flows in the northern three areas have 

indistinguishable 40Ar/39Ar ages (table 2); rocks in the southernmost area are undated but are in a 

similar stratigraphic position.

Aurora Canyon Center

The 10.5 to 10.3 Ma Aurora Canyon center forms a discontinuous series of trachydacite 

lava domes emplaced into the southern part of the Masonic center. The largest exposures form a 

prominent, northeast-elongated series of coalescing domes suggesting emplacement along a 

northeast-trending structure.

Mount Biedeman Center

The 9.9 to 8.9 Ma Mount Biedeman center is an eroded trachyandesite stratovolcano that 

forms the second largest volcanic edifice in the Bodie Hills volcanic field. It consists of a central 

intrusion at Mount Biedeman surrounded by outward dipping lava flows and more distal debrisflow deposits and volcaniclastic sedimentary rocks. The intrusion under Mount Biedeman likely 

defines a lava-flow vent. The volcano is about 10 to 14 km in diameter with debris flow deposits 

extending 3 to 15 km farther away from Mount Biedeman. The generally hornblende- and 

pyroxene-rich Mount Biedeman rocks are presently exposed over approximately 100 km2.

High-Silica Rhyolite Centers

High-silica rhyolite centers composed of exogenous domes, lava flows, and minor 

pyroclastic rocks are common in the center of the Bodie Hills volcanic field and include the Rock 

Springs Canyon (undated but >10.5 Ma), Aurora Creek, Del Monte Canyon, East Brawley Peak 

(all about 11.2 Ma), Bodie Creek (10.2), and Bald Peak (9.7 Ma) units. The rhyolite of Rock 

Springs Canyon contains several large volumes of black obsidian that were used by Native 

Americans for tool manufacture. The rhyolite of Aurora Creek hosts some of the gold-silver veins 

in the northern part of the Aurora mining district (fig. 5). The rhyolite of Bald Peak forms a 

prominent lava dome centered on Bald Peak that is largely covered by lava flows from the Pliocene 

Beauty Peak shield volcano. The Bald Peak dome is flanked on its southwest, west, and north sides 
29

by sedimentary rocks and tuff of Paramount, a sequence of rhyolite tuffs and volcaniclastic 

deposits that likely erupted from the Bald Peak dome and filled the shallow, northeast-elongated 

Paramount basin to the southwest.

Rhyolite of the Bodie Hills

The 9.9 to 9.6 Ma rhyolite of the Bodie Hills is a more widely distributed rhyolite unit that 

forms both a semicircular ring of outcrops around the north, east, and south flanks of the Mount 

Biedeman stratovolcano and a discontinuous series of lava domes that extend about 25 km north 

from the west side of Mount Biedeman to the north side of Masonic Mountain. Several exposures 

on the flanks of Mount Biedeman consist of quartz-rich rhyolite containing abundant hornblenderich andesite blobs. Mingled magmas and overlapping 40Ar/39Ar ages suggest the presence of two 

discrete magmas beneath Mount Biedeman that locally mixed during emplacement of the rhyolite 

of the Bodie Hills at about 9.8 to 9.6 Ma.

Silver Hill, Potato Peak, and Willow Springs Centers

Magmatic activity in the Bodie Hills volcanic field culminated between about 9.1 to 8.0 Ma 

with emplacement of three large silicic trachyandesite to dacite lava dome complexes: Silver Hill 

(9.1 to 8.9 Ma), Potato Peak (9.0 to 8.8 Ma), and Willow Springs (8.6 to 8.0 Ma). Each complex is 

composed of numerous domes, lava flows, and related pyroclastic rocks and debris flows that 

together form the core and highest parts of the Bodie Hills; these rocks occupy an area of about 200 

km2. Each of the dome complexes forms a discrete elliptical body with little overlap; lava flows of 

the Potato Peak center locally lap over rocks of the Silver Hill center, and lava flows of the Willow 

Springs center locally abut and ramp over Potato Peak lava and debris flows. Rocks from each of 

the complexes have distinct petrographic and geochemical characteristics suggesting their 

derivation from discrete magma reservoirs; the focus of magmatism migrated 20–25 km westward 

as the complexes were successively emplaced.

Big Alkali Center

Miocene magmatic activity resumed following an approximately 2 m.y. hiatus with 

emplacement of the small silicic trachydacite to rhyolite domes of the Big Alkali center. Most 

outcrops form a narrow north-trending alignment of domes approximately 20 km long and centered 

on Big Alkali; an isolated dome crops out 6 km farther north on the north side of Masonic 

Mountain.

Pliocene to Late Pleistocene Magmatism of the Aurora Volcanic Field

The eastern and southeastern parts of the Bodie Hills volcanic field and Miocene 

sedimentary rocks on the southern edge of Fletcher Valley are unconformably overlain by Pliocene 

to late Pleistocene (approximately 3.9 to 0.1 Ma) rocks of the Aurora volcanic field (Fig. 3B; tables 

1 and 2; Gilbert and others, 1968; Al-Rawi, 1969; Chesterman and Gray, 1975; Kleinhampl and 

others, 1975; Lange and others, 1993; Lange and Carmichael, 1996; Kingdon and others, 2013). 

The Aurora volcanic field covers approximately 325 km2 and contains many well-preserved 

volcanic features, including Aurora Crater, Beauty Peak, and the trachyandesite of Mud Spring 

lava flow. These postsubduction rocks erupted in an extensional tectonic setting and are composed 

of high-K and shoshonitic lavas with a continuum of compositions from trachybasalt through 

trachydacite and high-silica rhyolite (about 48 to 76 weight percent SiO2). However, unlike the 
30

Miocene Bodie Hills volcanic field rocks, the Aurora rocks have broadly bimodal compositions, 

and rocks with SiO2 contents between 66 and 75 weight percent are conspicuously absent (fig. 4).

The Aurora volcanic field consists primarily of monogenetic lava and cinder cones (for 

example, Aurora Crater), valley-filling lava flows (for example, the trachyandesite of Mud Spring), 

shield volcanoes such as Mount Hicks and Beauty Peak, and trachydacitic (Cedar Hill and Aurora 

Peak) to rhyolitic (Spring Peak) domes. Several lava flows or series of lava flows (for example, 

Qmsa, Qta2, and Qa) filled paleocanyons that had (prior to eruption of these lavas) allowed 

northward spillage of Lake Russell (Pleistocene Mono Lake) into the Lahonton drainage basin 

(Reheis and others, 2002).

The 3.9 Ma Locomotive Point center, the oldest unit in the Aurora volcanic field, consists 

of small vent-filling plug and basaltic andesite lava flows that extend west and southwest from the 

vent. This small eruptive center forms a prominent topographic high at Locomotive Point about 5 

km south of Masonic Mountain and about 10 km west of other exposures of Pliocene-Pleistocene 

volcanic rocks in the Bodie Hills. The plug lies along the trend of Big Alkali domes suggesting that 

the associated magma ascended along the same north-trending structure as that responsible for the 

domes.

Miocene Sedimentary Rocks and Deposits

Miocene volcanic rocks in the Bodie Hills are locally interbedded with and overlain by 

poorly consolidated late Miocene and Pliocene(?) fluvial, alluvial-fan, and lacustrine deposits 

consisting of fanglomerate, conglomerate, sandstone, and siltstone. These sedimentary deposits 

form extensive pediment surfaces along the margins of the Bodie Hills and extend under 

Quaternary surficial deposits into Bridgeport Valley to the west and Fletcher Valley to the 

northeast. Sedimentary rocks along the northern and eastern edges of the Bodie Hills are mostly 

coarse- to fine-grained fluvial and lacustrine deposits (Tswr and Tfv), whereas deposits on the east 

side of Bridgeport Reservoir and in the upper parts of Masonic Gulch, Red Wash, and Rough 

Creek, are coarse gravels and fanglomerates (Tog and Tyg).

The oldest sediments are unconsolidated gravel deposits (unit Tog) that contain exotically 

derived granite clasts and locally underlie the distally sourced 9.4 Ma Eureka Valley Tuff and 11 

Ma trachyandesite of Del Monte lava flows. The gravels and the Eureka Valley Tuff fill 

paleochannels carved into the trachyandesite of Masonic and other earlier erupted products of the 

Bodie Hills volcanic field. On the northeast side of the Bodie Hills, these older gravel deposits 

transition into finer-grained fluvial and lacustrine sedimentary rocks of Fletcher Valley (Tfv) that 

constitute the southern part of the Coal Valley Formation. The Coal Valley Formation was 

deposited in a late Miocene extensional basin that extends approximately 50 km north of the Bodie 

Hills (Gilbert and Reynolds, 1973).

Younger sedimentary units, consisting both of coarse gravels and conglomerates (Tyg) and 

finer-grained, locally conglomeratic, alluvial fan, fluvial and lacustrine sediments (Tswr) overlie 

the Eureka Valley Tuff and form extensive terraces above the East Walker River and Bridgeport 

Reservoir and along Masonic Gulch.

Surficial Deposits

Pleistocene to Holocene surficial deposits, including fluvial, colluvial, glacial, and 

lacustrine deposits, locally blanket all older rocks. Prominent older gravel banks containing wellrounded cobbles of metamorphic and granitic rocks are present in Bridgeport Canyon and 

Clearwater Creek. The south end of the Bodie Hills is cut by numerous wave-cut terraces and is 
31

locally lapped by beach gravels related to Lake Russell, the Pleistocene predecessor to Mono Lake 

(Reheis and others, 2002). Several large landslides are present in the Bodie Hills, notably on both 

sides of Bodie Creek between Beauty Peak and West Brawley Peak, in Rough Creek northwest of 

Beauty Peak, east of Rancheria Gulch, and north of Mount Hicks.

Structural Geology of the Bodie Hills

Many primary volcanic features in the Bodie Hills are well preserved and the rocks 

generally are only gently tilted. However, the structural geology of the Bodie Hills is locally 

complex and multiple generations and types of faults transect rocks in the Bodie Hills volcanic 

field. In addition, there has been significant vertical axis rotation of the Bodie Hills in the late 

Cenozoic (Carlson and others, 2013). Mineralized structures in the Masonic, Aurora, and Bodie 

mining districts, which represent time slices at approximately 13, 10.5 and 8.9 Ma, respectively, 

provide evidence of the Miocene strain field (John and others, 2012; Carlson and others, 2013). 

Carlson and others (2013) present a kinematic model for the middle Miocene to Holocene for the 

Bodie Hills, which includes a change in principal stress orientation and vertical axis rotation that is 

consistent with their paleomagnetic data for the 9.4 Ma Eureka Valley Tuff and with the kinematic 

fault data of John and others (2012).

Faults

Relatively few faults have been identified in the Bodie Hills (sheet 1). Most mapped faults 

have demonstrable stratigraphic offset. Others are the interpreted structural controls of linear zones 

of hydrothermally altered rock. Many features previously mapped as faults, especially those of 

Chesterman and Gray (1975) in the Potato Peak area, are reinterpreted as depositional contacts 

between lava flows or their lobes. Many linear features on air photos are not faults, because 

demonstrable offset of map units is absent. Units that contain bedded sedimentary rocks, such as 

the sedimentary rocks and tuff of Paramount and the sedimentary rocks of Fletcher Valley, 

commonly have erratic bedding orientations suggesting the presence of unrecognized faults that 

might extend into other rock units. However, these inferred faults do not appear to have large 

displacements or to significantly offset contacts between major units.

Mapped faults (sheet 1) have three dominant orientations—northeast to east-northeast, 

north to north-northeast, and northwest to west-northwest. Kinematic data on small mineralized 

faults in the Masonic and Aurora mining districts (John and others, 2012) that parallel these 

structures suggest that the oldest faults form a conjugate set of north-striking right-lateral and 

northeast-striking left-lateral oblique-slip faults, which affected older units, including rocks in the 

Masonic (15 to 14 Ma), Aurora (13.1 to 12.6 Ma), and Aurora Creek (11.2 Ma) centers and the 

approximately 9.7 Ma sedimentary rocks and tuff of Paramount. These faults likely guided 

emplacement of the 11.3 Ma trachyandesite of Clark Canyon intrusions and the 10.5 to 10.3 Ma 

trachydacite of Aurora Canyon lava domes, and channelized hydrothermal fluids forming 

approximately 10.3 Ma epithermal veins in the Aurora mining district. They likely were active 

from about 13.4 to ≤9.7 Ma (John and others, 2012).

Younger north- to north-northeast-striking normal faults offset both the 9.4 Ma Eureka 

Valley Tuff and 9.1 to 8.9 Ma rocks of the Silver Hill center. These faults localized formation of 

approximately 8.9 to 8.1 Ma epithermal veins in the Bodie mining district (John and others, 2012). 

On the east side of Bridgeport Reservoir, one of these faults is covered by fanglomerate deposits 

(Tyg) and might extend farther south.
32

Northwest- to west-northwest-striking faults are mostly limited to the west part of the Bodie 

Hills and cut rocks of the 8.6 to 8.0 Ma Willow Springs center. The sense of displacement on these 

faults is uncertain.

The youngest faults in the Bodie Hills are north-striking normal and northeast-striking leftlateral oblique-slip faults that offset rocks of the Aurora volcanic field and late Pleistocene eolian 

deposits and lake beds (Gilbert and others, 1968; Dohrenwend, 1981; S.E. Box, unpub. data, 2012); 

some of these faults remain active (Wesnousky, 2005; Jennings and others, 2010). The northstriking faults formed a series of stepover grabens in a system of northeast-striking left-lateral 

faults at the northwest edge of the Mina deflection (fig. 1; John and others, 2012). On the west side 

of the Bodie Hills, a north-striking fault also may have controlled emplacement of the Big Alkali 

domes between about 6.2 and 5.5 Ma and localized the vent for the 3.9 Ma Locomotive Point lava 

flows.

Veins and Mineralized Faults

The orientation of the regional strain field during Miocene volcanism in the Bodie Hills can 

be estimated from orientations of mineralized structures. Because mineralization is well dated for

the Masonic (approximately 13.4 to 12.9 Ma), Aurora (approximately 10.3 Ma), and Bodie 

(approximately 8.9 to 8.1 Ma) mining districts, mineralized structures in these districts record the 

temporal variation of the principal stress orientations (John and others, 2012). 

In the Masonic mining district, faults and fractures dominantly have either N-S or N60°E 

trends. The N-S fractures have right-lateral offset and NE fractures have left-lateral offset, and they 

are interpreted as a conjugate strike-slip fault set consistent with N60°W extension and N30°E 

compression (John and others, 2012). Veins in the Aurora mining district constitute a conjugate 

fracture pair, consisting of a right-lateral north-striking set and a left-lateral N60°E set, also 

consistent with N60°W extension and N30°E compression (John and others, 2012). Some quartz 

veins in the Bodie mining district consist of mm- to cm- layers of quartz with inward-facing 

crystals. Symmetrically layered veins represent repeated tensional dilation of the host fractures 

(John and others, 2012). Other veins are fault breccias replaced by quartz and sulfide minerals. 

Veins at Bodie strike about N10°E to N50°E with an average of N28°E in fractures dominated by 

normal (extensional) faulting. These data indicate that the directions of maximum extension in all 

three mining districts are subparallel to each other; however, the intermediate and maximum 

principal stresses are reversed, which indicates that regional deformation changed from a regime of 

strike-slip (transtension) to one of normal (extensional) faulting between about 10 to 9 Ma.

Paleomagnetic Studies and Vertical Axis Rotation of the Bodie Hills

Paleomagnetic studies of the 9.4 Ma Eureka Valley Tuff indicate that pre-9-Ma rocks in the 

Bodie Hills have been rotated 10–30° clockwise (Carlson and others, 2013, C.J. Pluhar, oral 

commun., 2013). The northern part of the Bodie Hills, including the Masonic mining district, was 

rotated about 10° clockwise, whereas other parts of the Bodie Hills were rotated about 30° 

clockwise. Consequently, estimates of original fault and principal stresses orientations in the Bodie 

Hills must be corrected for this clockwise rotation. Early faults in the Masonic and Aurora mining 

districts restore to right-lateral north-northwest-striking and left-lateral northeast-striking, whereas 

faults in the Bodie mining district restore to north-striking and are dip-slip.

Tilting

In general, Cenozoic rocks in the Bodie Hills are only slightly tilted. Miocene sedimentary 

rocks on the margins of the Bodie Hills generally dip ≤30°. These rocks are not systematically 
33

tilted, although beds in the sedimentary rocks of Fletcher Valley most commonly dip away from 

the Bodie Hills into Fletcher Valley and consolidated beds on the east side of Bridgeport Valley 

(Tswr) typically dip into the Bridgeport Valley. Volcaniclastic sedimentary rocks interbedded in 

debris flows in the Masonic and Mount Biedeman centers also dip ≤30°, usually away from the 

inferred volcanic center; consequently, some of this inclination may reflect primary dip. Pliocene 

and younger rocks of the Aurora volcanic field are nearly flat lying. For example, the top of 3.9 Ma 

lava flows of the basaltic trachyandesite of Locomotive Point dip 3–4° west to southwest and the 

top surface of the 1.3 Ma trachyandesite flow 2 (Qta2) dips <1° west. In contrast, compaction 

foliation in densely welded Eureka Valley Tuff is highly irregular, and although the base of the tuff 

is generally conformable with underlying, shallowly dipping Miocene units, compaction foliation 

within these isolated exposures is commonly steep (as much as 90°). The irregular, steep foliation 

in the tuff may be the result of (1) primary dip due to banking and welding against steep 

topography, (2) structural or magmatic doming of the Bodie Hills, and (or) (3) tilting due to 

unrecognized faults (John and others, 2012).

Volcanic Landforms

Bodie Hills volcanic field eruptive centers are dominated by subcircular composite 

volcanoes and large dome fields that directly overlie deeply rooted intrusions, inferred from 

geophysical data (John and others, 2012). In aggregate, the 9.1 to 8.0 Ma trachyandesite to dacite 

dome complexes are elongate in an east-west direction, and the complexes decrease in age 

progressively to the west. In contrast, late rhyolite of the Big Alkali domes are localized in a 

narrow north-trending array mostly within the 8.6 to 8.0 Ma trachyandesite of Willow Springs 

dome field along the west flank of the Bodie Hills.

Dikes, fissure-fed eruptions, monogenetic volcanoes, and elongate grabens or rectilinear 

volcanotectonic depressions are uncommon to absent in the Miocene Bodie Hills volcanic field, 

with the notable exception of the northeast-elongate Paramount basin. The paucity of these features 

and the previously described faulting orientations are consistent with minimal variation in 

differential horizontal stress during formation of the Bodie Hills volcanic field (for example, 

Nakamura, 1977; Hildreth, 1981; Takada, 1994; Tosdal and Richards, 2001). In contrast, the 

Pliocene-Pleistocene (3.9–0.1 Ma) Aurora volcanic field in the Bodie Hills is characterized by 

north-northeast-elongated monogenetic volcanoes that are locally offset by north-northeastelongate stepover grabens linked to east-northeast-striking left-lateral faults along the north Mono 

Basin escarpment. This fault pattern suggests somewhat greater variation in differential horizontal 

stress during formation of the Aurora volcanic field than during earlier times.

Magmatic Doming(?) of the Central Bodie Hills

In the central part of the Bodie Hills, the present-day elevation of the base of the Eureka 

Valley Tuff progressively increases from about 6,500 ft (1,980 m) at Bridgeport Reservoir to 9,190 

ft (2,800 m) on the north side of Potato Peak and then decreases eastward to <8,200 ft (<2,500 m) 

in Rough and Bodie Creeks. Because the tuff flowed down paleodrainages from the Little Walker 

caldera about 25 km to the northwest to a basin in Fletcher Valley and to Mono Basin, this 

asymmetric domal pattern centered on Potato Peak suggests significant deformation of the tuff 

following its deposition. Similarly, the distribution of sinter terraces in the Paramount basin on the 

east side of Potato Peak, which vary in elevation from about 9,020 ft (2,750 m) near Paramount to 

7,050 ft (2,150 m) north of Bald Peak, and complex faulting inferred within the Paramount basin 

indicate significant deformation not explained fully by mapped faults (Vikre and others, in press). 

In addition, the approximately 500 m elevation difference between the nearly simultaneously 
34

formed Aurora Canyon and Potato Peak alteration zones (fig. 5) on the west side of Potato Peak at 

10.8 to 10.7 Ma is not consistent with mapped faults (Vikre and others, in press). These 

observations suggest structural doming of the Bodie Hills following eruption of the 9.4 Ma Eureka 

Valley Tuff, possibly due to emplacement of a shallow magma reservoir that sourced the 

trachydacite of Potato Peak dome complex at about 9 Ma. Gravity and aeromagnetic data suggest 

that relatively silicic but strongly magnetic pluton(s) may underlie this complex in the center of the 

Bodie Hills (John and others, 2012). Alternatively, this pattern might indicate rollover in the upper 

plate of the arcuate set of normal faults dipping beneath the northwest side of the Bodie Hills or 

other unrecognized faults.

Model for Evolution of the Late Cenozoic Stress Field in the Bodie Hills

As described above, rocks of the Bodie Hills volcanic field record a progressive change in 

the orientation of the regional stress field since middle Miocene time. Prior to about 10 Ma, low 

differential horizontal stresses with northerly compression and westerly extension (corrected for 

tectonic rotations) resulted in subcircular, polygenetic volcanoes with episodes of associated 

hydrothermal mineralization controlled by conjugate sets of strike-slip faults. After about 10–9 Ma, 

the maximum compressive stress axis became vertical, and extension-related veins accompanied a 

progressive westerly migration of dacitic to trachyandesitic magmatism. The stress-field change is 

approximately coincident with arrival of the Mendocino triple junction and cessation of subduction 

beneath the Bodie Hills (Atwater and Stock, 1998). Eruption of the rhyolite of Big Alkali in the 

western part of the Bodie Hills at approximately 6 Ma, following a 2 m.y. hiatus, may indicate 

magma localization along normal faults reflective of more westerly-directed extension. PliocenePleistocene magmatism was even more strongly localized by north-striking extensional faults, 

which formed stepover grabens in a system of northeast-striking left-lateral faults related to the 

Mina deflection.

Mineral Deposits and Alteration Zones in the Bodie Hills

The Bodie Hills volcanic field contains several significant epithermal mineral deposits, and 

large areas of hydrothermally altered rock are intimately associated with development of the field 

(fig. 5; John and others, 2012; Vikre and others, in press). The most important occurrences are 

gold-silver deposits in the Bodie and Aurora mining districts and a sulfur resource in Cinnabar

Canyon. In contrast, rocks in the Aurora volcanic field are unaltered and do not contain known 

mineral deposits. 

In the Bodie Hills, the largest reported production is from vein deposits in the Aurora and 

Bodie mining districts. In the Bodie mining district, reported production is 1.46 million troy ounces 

(Moz) gold and 7.3 Moz silver that were recovered from approximately 1.5 Mt of ore (Long and 

others, 1998). Veins at Bodie formed at approximately 8.9 to 8.1 Ma (table 2) and are hosted by 

flows, domes, and volcaniclastic deposits of the approximately 9 Ma dacite of Silver Hill dome 

complex. Reported production from veins in the Aurora mining district is 1.91 Moz gold and 20.6 

Moz silver that were recovered from approximately 3.9 Mt of ore (Long and others, 1998). Veins 

in the Aurora mining district formed at approximately 10.5 to 10.1 Ma (Vikre and others, in press). 

Most veins are hosted in the 13.1 to 12.6 Ma lava flows and breccias of the trachyandesite of 

Aurora, although a few veins occur in the 11.2 Ma rhyolite of Aurora Creek. Veins in both districts 

strike north to northeast, dip east and west at high to moderate angles, and extend for tens to 

hundreds of meters. Veins are tabular and mostly < 1 m thick. The most productive veins consist of 

tens to hundreds of symmetrical layers of quartz, lesser adularia, sericite and calcite, and small 
35

amounts of metallic minerals. Sections of veins that were mined contain up to several percent silver 

sulfide minerals, electrum, and other sulfide minerals in some quartz layers. Ore was largely mined 

within several hundred meters of the surface. Significant amounts of tetrahedrite, chalcopyrite, 

galena, sphalerite, and other sulfide and sulfosalt minerals are present in some veins in the Bodie 

mining district but relatively little ore was recovered from them (Vikre and others, in press).

Reported production from tabular fault breccias and replacement deposits at Masonic is 

0.06 Moz gold and 0.04 Moz silver that were recovered from 0.08 Mt of ore (Long and others, 

1998). Ore was produced from 1–2 m-wide breccia zones and centimeter-wide veins in a series of 

en echelon north- and northeast-striking curvilinear faults cutting Mesozoic granitic rocks, 

metamorphic rocks, and trachyandesite flows and volcaniclastic rocks of the Masonic eruptive 

center. Ore also was mined from stratiform zones in volcaniclastic strata. Fault breccia matrices, 

veins, and stratiform deposits consist of quartz, alunite, kaolinite-dickite, enargite, electrum, 

numerous Cu-As-Sb-Ag-Bi-Te-Se-S minerals, and barite (Vikre and Henry, 2011; Vikre and 

others, in press).

The unexposed sulfur resource in Cinnabar Canyon (Cinnabar Canyon sulfur deposit, 

approximately 2.9 million tons sulfur; Ward, 1992) is hosted by a sequence of volcaniclastic rocks 

and trachyandesite lavas correlated with the approximately 10 to 9 Ma Mount Biedeman 

stratovolcano. The resource occurs within an approximately 30 km2 area of lavas and permeable 

volcaniclastic strata that were pervasively leached and replaced at approximately 8.7 Ma by quartzalunite-kaolinite-pyrite-sulfur assemblages (Vikre and Henry, 2011; Vikre and others, in press). 

Other large (approximately 25–30 km2) alteration zones extend from the north side of Bald 

Peak to the Paramount Hg Mine (Paramount-Bald Peak alteration zone, fig. 5), and from the East 

Walker River to Fletcher Valley (Red Wash-East Walker River alteration zone, fig. 5). The 

stratiform Paramount-Bald Peak alteration zone is mostly hosted in approximately 9.7 Ma 

volcaniclastic deposits (sedimentary rocks and tuff of Paramount) deposited in the northeastelongate Paramount basin. This alteration zone includes sinter at the Paramount Hg mine, sinter 

and silicified, clay-altered sandstone and conglomerate south and north of the Paramount mine, and 

sinter and silicified or clay-altered tuff and tuff breccia with chalcedonic silica veins between 

Atastra Creek and Bald Peak (Vikre and others, in press). The stratiform and largely stratabound, 

east-west-oriented Red Wash-East Walker River alteration zone formed in coarse volcaniclastic 

strata associated with the approximately 15 to 14 Ma Masonic stratovolcano. The permeable 

volcaniclastic rocks were pervasively replaced by quartz, alunite, kaolinite, pyrite, and other clay 

minerals at about 13.4 Ma, but contain no identified mineral resources. Smaller quartz-alunitedominant alteration zones are present on the northwest side of Potato Peak (Potato Peak alteration 

zone, fig. 5), in Aurora Canyon (Aurora Canyon alteration zone, fig. 5), on East Brawley Peak 

(East Brawley Peak alteration zone, fig. 5), and 1 km west of Aurora (Sawtooth Ridge alteration 

zone, fig. 5). A small area of silicified hydrothermal eruption breccia (Four Corners alteration zone, 

fig. 5) is in distal volcaniclastic deposits associated with the Masonic center. Sinter (unit QTsn) 

overlying Mesozoic rocks (Spring Peak sinter, fig. 5) contains small Hg prospects.

Additional Descriptive Notes

Volcanic and plutonic rock unit names used in the following Description of Map Units are 

in accord with the IUGS nomenclature system (LeMaitre, 2002; Streckeisen, 1976). The 

compositional aspect of each rock unit name is its average composition computed from data 

presented by du Bray and others (2013) but compositions of all analyzed samples of each unit are 

also listed. Volcanic-rock phenocryst abundances (table 3, available for download online at 
36

http://dx.doi.org/10.3133/sim3318) and grain sizes are based on petrographic microscope 

observations. Volcanic rock unit ages (table 2) are based mostly on 40Ar/39Ar dates (John and 

others, 2012; Fleck and others, in press). Compositions of Mesozoic plutons are based on modal 

analyses of stained slabs (table 4) and are plotted on a ternary quartz-alkali feldspar-plagioclase 

diagram (fig. 6). K-Ar ages of granitoid rocks are calculated using the decay constants of Steiger 

and Jäger (1977).

Nearly all Bodie Hills volcanic rocks are porphyritic. Most phenocrysts are fine grained (<1 

mm) to medium grained (1–5 mm); however, the trachyandesite of Willow Springs, trachydacite of 

Cinnabar Canyon, trachyandesite of Mud Spring Canyon, trachydacite of Bridgeport Canyon,

trachyandesite of West Brawley Peak, and andesite of Lakeview Spring contain distinctive, coarse 

(≥5 mm) plagioclase phenocrysts. Average phenocryst sizes (by mineral) as well as the average 

size of the largest phenocryst (by mineral) in all samples of a particular unit are enumerated in 

table 1. Extent of porphyritic texture development is quantified as aphyric (rare to no phenocrysts), 

sparsely (<10 percent phenocrysts), moderately (10 to 24 percent phenocrysts), strongly (≥25 

percent phenocrysts). Euhedral, albite-twinned plagioclase laths are a nearly ubiquitous component 

of Bodie Hills volcanic rocks. Almost all plagioclase phenocrysts are oscillatory zoned and some, 

especially in intermediate-composition lava flows, are variably sieve textured. Many units contain 

multiple plagioclase populations defined by size and (or) distinctive reaction rims, zones that 

contain mineral and (or) glass inclusions, and resorption textures. Brown to green pleochroic 

hornblende forms euhedral to subhedral acicular crystals that have distinctive, variably developed, 

black opacite reaction rims (Rutherford and Hill, 1993); in weathered or altered samples 

hornblende is completely replaced by variable assemblages that include alunite, quartz, mica, clay 

minerals, pyrite, and black amorphous material. Clinopyroxene, common in many Bodie Hills 

volcanic field rocks, forms pale tan to pale green subhedral to euhedral crystals. Less-common 

orthopyroxene is colorless to rosy tan and forms euhedral to subhedral crystals. Biotite is 

subhedral, tan to deep reddish-brown, and like hornblende, is completely altered in many samples. 

Relatively uncommon olivine forms variably altered subhedral phenocrysts in some mafic- to 

intermediate-composition Bodie Hills volcanic field rocks. Small amounts of magnetite and less 

abundant ilmenite are ubiquitous. Quartz is absent in all but the most silicic units; where present, it 

forms variably resorbed and embayed, rounded, anhedral to subhedral phenocrysts. Sanidine is less 

common than quartz but forms variably and weakly perthitic Carlsbad-twinned, euhedral to 

subhedral phenocrysts in several of the rhyolite to silicic trachydacite units.

The groundmass of most Bodie Hills volcanic field rocks contains variable

microphenocryst (typically 0.05 to 0.2 mm long) assemblages dominated by plagioclase, but also 

includes combinations of Fe-Ti oxide minerals, clinopyroxene and, less commonly, hornblende and 

(or) biotite. Metastable volcanic glass is another major groundmass component. As discussed in the 

unit descriptions below, groundmass is variably devitrified (crystallized) and the extent of 

devitrification characteristic of each unit is qualitatively identified, as follows: (1) undevitrified 

fresh glass is isotropic and contains essentially no microlites; (2) weakly, devitrified glass includes 

scarce microlites; (3) strongly devitrified glass is almost completely crystallized; (4) moderately 

devitrified glass has crystallized to an extent intermediate to that of either weakly or strongly 

crystallized groundmass glass.

Exotic lithic fragments or inclusions are rare but glomerocrysts as much as several 

millimeters in diameter are present in many samples.
37

Figure 6. Modes of Mesozoic granitoid rocks in the Bodie Hills plotted on a quartz-alkali feldspar-plagioclase 

diagram (Streckeisen, 1976). Data listed in table 4.
38

Table 4. Modal data for Mesozoic granitoid rocks in the Bodie Hills, California and Nevada.

[Qtz, quartz; Alk fspr, alkali feldspar; Plag, plagioclase; mafics, sum of hornblende plus biotite plus opaque Fe-Ti oxides; Q, quartz; K, alkali feldspar; P, plagioclase]

Points counted Relative abundances, percent Ternary proportions, 

percent

Sample Unit Qtz Alk fspr Plag Mafics Total Qtz Alk 

fspr Plag Mafics Q K P

203224 Granodiorite of Masonic 367 246 601 46 1,260 29.1 19.5 47.7 3.7 30.2 20.3 49.5

203239 Granodiorite of Masonic 319 294 623 39 1,275 25 23.1 48.9 3.1 25.8 23.8 50.4

203242 Granodiorite of Masonic 323 238 528 38 1,127 28.7 21.1 46.9 3.4 29.7 21.9 48.5

203319 Granodiorite of Masonic 274 330 485 46 1,135 24.1 29.1 42.7 4.1 25.2 30.3 44.5

203322 Granodiorite of Masonic 362 267 551 39 1,219 29.7 21.9 45.2 3.2 30.7 22.6 46.7

203377 Granodiorite of Masonic 261 287 486 37 1,071 24.4 26.8 45.4 3.5 25.2 27.8 47

203293 Quartz monzodiorite of 

Masonic Mountain 30 200 807 124 1,161 2.6 17.2 69.5 10.7 2.9 19.3 77.8

203363 Quartz monzodiorite of 

Masonic Mountain 133 190 775 90 1,188 11.2 16 65.2 7.6 12.1 17.3 70.6

203364 Quartz monzodiorite of 

Masonic Mountain 134 160 771 64 1,129 11.9 14.2 68.3 5.7 12.6 15 72.4

203350 Monzogranite of 

Murphy Creek 322 403 455 17 1,197 26.9 33.7 38 1.4 27.3 34.2 38.6

203351 Monzogranite of 

Murphy Creek 322 427 514 17 1,280 25.2 33.4 40.2 1.3 25.5 33.8 40.7

203383 Quartz monzodiorite of 

Aurora 209 278 626 40 1,153 18.1 24.1 54.3 3.5 18.8 25 56.2

203384 Monzogranite of 

Rattlesnake Gulch 236 398 400 50 1,084 21.8 36.7 36.9 4.6 22.8 38.5 38.7

203385 Monzogranite of 

Rattlesnake Gulch 330 395 428 41 1,194 27.6 33.1 35.8 3.4 28.6 34.3 37.1

203387 Monzogranite of 

Rattlesnake Gulch 258 277 517 45 1,097 23.5 25.3 47.1 4.1 24.5 26.3 49.1

12-BA-29 Quartz monzodiorite of 

Mormon Meadow 60 229 552 308 1,149 5.2 19.9 48 26.8 7.1 27.2 65.6
39

DESCRIPTION OF MAP UNITS

[Absolute age determinations of volcanic rock units listed in table 2]

SURFICIAL DEPOSITS

md Tailings and mine dumps (Holocene)—Tailings consist of fine-grained 

sediment (produced during ore processing) slurried into holding ponds 

near mills associated with mining operations. Mine dumps are unsorted 

piles of subeconomic and unmineralized waste rock produced during 

mining of mineral deposits

Qtt Travertine hot springs deposits (Holocene and Pleistocene)—Very-paleorange to yellow deposits of calcium carbonate and other minerals 

precipitated from geothermal water associated with hot springs activity. 

Deposits are limited to two small areas southeast of Bridgeport near active 

hot springs

Qtf Tufa deposits (Holocene and Pleistocene)—Several 3–5-m tall spires about 2 

km southeast and 4 to 9 km northeast of the mouth of Bridgeport Canyon 

on the north side of Mono Lake are composed of highly porous, 

yellowish-gray thinolitic tufa. The tufa deposits are composed of calcium 

carbonate minerals precipitated where freshwater springs discharged 

beneath Mono Lake and interacted with its alkaline water (Shearman and 

others, 1989)

Qal Alluvium (Holocene and Pleistocene)—Silt, sand, and gravel deposits that occur 

principally in drainage bottoms. Includes slope wash deposits and minor 

talus at valley margins

Qes Eolian sand deposits (Holocene and Pleistocene)—Well-sorted, medium to fine 

sand and silt. Derived from lacustrine deposits of Mono Lake and 

Pleistocene Lake Russell and reworked pyroclastic fall deposits associated 

with the Mono-Inyo Craters volcanic chain (Bailey, 1989). Form partly 

stabilized dunes on the north side of Mono Lake (Dohrenwend, 1981), and 

wind-deposited accumulations elsewhere, most notably in the central part 

of the Aurora volcanic field

Qf Fan deposits (Holocene and Pleistocene)—Massive, poorly sorted, 

unconsolidated deposits with silt- to boulder-size clasts in a silty to sandy 

matrix; form aprons at the break in slope where drainages emerge from 

steeper upland areas

Qls Landslide deposits (Holocene and Pleistocene)—Lobate accumulations of 

poorly sorted soil and rock debris forming hummocky slopes downslope 

from bedrock scarps. Contain unsorted, chaotic accumulations of intact 

bedrock blocks; dominant rock type within each landslide deposit is 

identified using rock-unit labels

Qpd Pediment deposits (Holocene and Pleistocene)—Fan-shaped deposits of poorly 

sorted sand and coarse gravel; thinly mantles relatively smooth pediment 

surfaces cut on underlying Miocene volcanic and sedimentary rocks on 
40

north and northeast sides of the Bodie Hills. Clast compositions reflect the 

wide array of rock types exposed upslope of each particular pediment

Qpl Playa deposits (Holocene and Pleistocene)—Light-colored, reworked silt, clay, 

and evaporate minerals deposited in lakes within broad, flat, restricted 

basins of the Aurora volcanic field

Qt Talus (Holocene and Pleistocene)—Unconsolidated, poorly sorted, angular rock 

fragments shed from cliffs and steep slopes; forms cones, aprons, and 

scree slopes

Ql Lacustrine deposits (late Pleistocene)—Shallow-water lacustrine deposits of 

late Pleistocene Lake Russell (Reheis and others, 2002); commonly 

includes fine-grained laminated beds, eolian sand, and beach-gravel 

deposits

Qgd Glacial deposits (Pleistocene)—Glacial deposits exposed in the southwest part of 

the map area. Deposits are part of the 820 ka (Moore and Moring, 2013) 

Sherwin Till (Chesterman and Gray, 1975), and lack distinct depositional 

form. The glacial deposits form soil-covered slopes that include rounded 

and weathered Mesozoic granitoid boulders and less weathered, angular 

clasts of fine-grained Paleozoic and Mesozoic metamorphic rock; clasts of 

Miocene volcanic rock are subordinate

Qog Older gravel deposits (Pleistocene)—Massive, poorly sorted, unconsolidated 

deposits with silt- to boulder-size clasts in a silty to sandy matrix; form 

aprons at the base of topographic highs and distinct gravel banks above 

younger alluvium and gravel deposits, and mantle some valley walls. 

Clasts, including petrified wood and hydrothermally altered rocks, reflect 

derivation from a variety of bedrock sources

ROCKS OF AURORA VOLCANIC FIELD

Qmsa Trachyandesite of Mud Spring (Pleistocene)—Medium-dark-gray, moderately 

porphyritic trachyandesite lava flow; vesicular in most places. Contains 

about 10 percent phenocrysts of hornblende, plagioclase, and 

clinopyroxene in a weakly devitrified groundmass; accessory opaque 

oxides form scarce, small crystals. Forms a distinctive, rubbly lava flow, 

with the geomorphology of a recently erupted, essentially unweathered 

flow, which overlies unconsolidated gravel deposits in the northeast part 

of the Aurora volcanic field, east of Mud Spring. Determined ages 0.04 

±0.04 Ma and 0.11±0.08 Ma

Qaca Trachyandesite of Aurora Crater (Pleistocene)—Dark-gray, sparsely 

porphyritic trachyandesite and minor andesite lava flows and cinder 

deposits; vesicular in most places. Contains about 3 percent phenocrysts of 

clinopyroxene and orthopyroxene and minor olivine in an intergranular 

intergrowth of weakly devitrified glass, plagioclase, clinopyroxene, and 

opaque oxides. Erupted from Aurora Crater, an elliptical 4.4 by 5.6 km 

composite cone in the north-central part of the Aurora volcanic field. 

Determined ages 0.26±0.05 Ma and 0.47±0.19 Ma
41

Qta2 Trachyandesite 2 (Pleistocene)—Pale brown, almost aphyric trachyandesite lava 

flow and plug; distinctly flow laminated in some places. Contains about 1 

percent small phenocrysts of olivine in an aphanitic, intersertal 

intergrowth of plagioclase, clinopyroxene, and opaque oxides. The lava 

flow, in the northeast part of the Aurora volcanic field, south of Mud 

Spring Canyon, preserves primary, flow-morphology features. Determined 

age 1.32±0.08 Ma

Qta1 Trachyandesite 1 (Pleistocene)—Medium-light-gray, sparsely porphyritic 

trachyandesite and minor basaltic trachyandesite lava flows; vesicular in 

most places. Contains about 5 to 10 percent (average, about 7 percent) 

phenocrysts of plagioclase, hornblende, and olivine, in some places, in a 

pilotaxitic intergrowth of plagioclase, olivine, clinopyroxene, and opaque 

oxides; accessory apatite. Numerous lava flows form a shield volcano, 

centered on peak 8172T about 3.5 km north of Mount Hicks, in the 

northeast part of the Aurora volcanic field. Absolute age unknown but 

constrained to between 1.3 to 1.6 Ma by geologic relations

Qa Andesite (Reheis and others, 2002) (Pleistocene)—Andesite lava flow fills

channel that drained Pleistocene Lake Russell. Determined ages 1.44 

±0.06 and 1.6±0.02 Ma

Qmh Trachyandesite of Mount Hicks (Pleistocene)—Dark-gray, almost aphyric 

trachyandesite lava flows. Contains about 3 percent phenocrysts of olivine 

that are enclosed in a felty intergrowth of plagioclase, clinopyroxene, and 

opaque oxides. Numerous lava flows form a shield volcano, centered on 

Mount Hicks, which dominates the east-central part of the Aurora volcanic 

field. Determined age 1.6±0.1 Ma

Qacc Trachyandesite of Cow Camp Creek (Pleistocene)—Medium-gray, moderately 

porphyritic trachyandesite; includes basaltic trachyandesite and 

trachydacite. Contains about 5 to 30 percent (average, about 22 percent) 

phenocrysts, principally plagioclase, clinopyroxene, and olivine (but 

includes hornblende and orthopyroxene in some samples) in a weakly 

devitrified groundmass; accessory opaque oxides. The trachyandesite 

forms a ≤170-m-thick series of flows associated with an inferred shield 

volcano probably centered north of Cow Camp Creek; may also include 

minor moderate-reddish-orange scoria deposits north of Beauty Peak. 

Determined age 1.99±0.04 Ma

QTai Trachyandesite (Pleistocene or Pliocene)—Medium-light-gray, sparsely 

porphyritic trachyandesite. Contains about 15 percent olivine phenocrysts 

in an aphanitic, felty intergrowth of plagioclase, clinopyroxene, and 

opaque oxides. Forms an isolated plug about 15 km northeast of 

Bridgeport, significantly northwest of all other Aurora volcanic field 

rocks. Absolute age unknown

QTopb Olivine pyroxene basaltic trachyandesite (Pleistocene or Pliocene)—Mediumdark-gray, moderately porphyritic basaltic trachyandesite. Contains about 

20 percent phenocrysts of plagioclase, olivine, and clinopyroxene in a 

groundmass that consists of intergrown plagioclase, clinopyroxene, and 

opaque oxides; accessory mineral assemblage limited to opaque oxides. 
42

Forms an isolated outcrop about 6 km west southwest of the mouth of 

Trench Canyon. Absolute age unknown

QTob Olivine basaltic trachyandesite (Pleistocene or Pliocene)—Medium-light-gray, 

strongly porphyritic, vesicular basaltic trachyandesite. Contains about 45 

percent phenocrysts of plagioclase and olivine in a groundmass that 

consists of intergrown plagioclase, opaque oxides, and clinopyroxene. 

Forms two small isolated outcrops about 1 km north of Mount Biedeman. 

Absolute age unknown

QTaap Trachyandesite south of Aurora Peak (Pleistocene or Pliocene)—Mediumdark-gray, moderately porphyritic vesicular trachyandesite. Contains about 

15 percent phenocrysts of hornblende and plagioclase in a moderately 

devitrified groundmass; accessory opaque oxides. Forms several isolated 

outcrops southwest, south, and southeast of Aurora Peak. Determined ages 

2.3±0.1 and 2.6±0.2 Ma (Silberman and McKee, 1972)

QTbbc Basaltic debris-flow deposits of Cedar Hill (Pleistocene or Pliocene)—

Grayish-black debris-flow deposits that contain angular 5–50 cm clasts of 

olivine basalt; forms an isolated exposure about 1 km north of Kirkwood 

Spring; may be correlative with the trachybasalt of Cedar Hill

QThch Hornblende trachydacite of Cedar Hill (Pleistocene or Pliocene)—Mediumgray, moderately porphyritic trachydacite and trachyandesite (hornblende 

andesite of Lange and Carmichael, 1996). Contains about 15 to 30 percent 

(average, about 19 percent) phenocrysts of plagioclase and hornblende and 

trace amounts of biotite and clinopyroxene in a weakly devitrified 

groundmass; accessory opaque oxides and apatite. Forms composite lava 

cones and flows in the southern part of the Aurora volcanic field. Eight 

age determinations range from 2.4 to 3.5 Ma

Tmr Rhyolite of Martinez Hill (Pliocene)—Light-gray, almost aphyric, high-silica 

rhyolite. Contains about 4 percent phenocrysts of quartz, sanidine, 

plagioclase, and biotite in a groundmass composed of fresh glass; 

accessory opaque oxides, titanite, zircon, and allanite. Lava forms three 

small-volume, coalescing domes at Martinez Hill, south of Gregory Flats 

near the center of the Aurora volcanic field. Determined age 2.6±0.1 Ma

Tdap Trachydacite of Aurora Peak (Pliocene)—Light-gray to pinkish-gray, 

moderately porphyritic trachydacite. Contains 15 to 20 percent 

phenocrysts of plagioclase, hornblende, and biotite; accessory apatite and 

opaque oxides. Forms a lava dome with two vents near the top of Aurora 

Peak. Determined ages 2.6 ±0.2 Ma and 2.8±0.2 Ma

Tbch Trachybasalt of Cedar Hill (Pliocene)—Medium-gray, moderately porphyritic 

trachybasalt (basalt of Lange and Carmichael, 1996). Contains about 5 to 

20 percent (average, about 15 percent) phenocrysts of plagioclase, olivine, 

and clinopyroxene in an intergranular groundmass; accessory opaque 

oxides. Forms a composite cone composed of bedded cinder deposits and 

interbedded lava flows in the southern part of the Aurora volcanic field. 

Determined age 2.60±0.06 Ma
43

Tpch Pyroxene trachyandesite of Cedar Hill (Pliocene)—Light-gray to brownishgray, moderately porphyritic trachyandesite (two-pyroxene andesite of 

Lange and Carmichael, 1996). Contains 15 to 30 percent (average, about 

22 percent) phenocrysts of plagioclase, clinopyroxene, orthopyroxene, and 

olivine in a moderately devitrified groundmass that includes plagioclase 

and opaque oxides; accessory apatite. Forms composite lava cones and 

flows in the southern part of the Aurora volcanic field. Two small 

outcrops about 3 km west of Trench Canyon are atypically biotite rich. 

Determined ages 2.8±0.2 and 2.8±0.8 Ma

Tba Basaltic trachyandesite of Beauty Peak (Pliocene)—Light-gray, sparsely 

porphyritic basaltic trachyandesite. Contains about 2 to 5 percent olivine 

phenocrysts in a groundmass that is an intersertal intergrowth of 

plagioclase, clinopyroxene, and opaque oxides; accessory apatite. The 

basaltic trachyandesite forms a ≤250-m thick series of flows that emanate 

from a shield volcano centered on Beauty Peak. Determined age 2.9±0.1 

Ma

Thd Hornblende trachydacite (Pliocene)—Dark-gray, sparsely porphyritic 

trachydacite lava flows. Contains about 6 percent phenocrysts of 

plagioclase, hornblende, and a trace of clinopyroxene in a weakly 

devitrified groundmass; accessory scarce, small crystals of opaque oxides. 

Forms a set of flows that crop out in a relatively small area in the 

northwest part of the Aurora volcanic field. Determined age 3.5±0.2 Ma

Trsp Rhyolite of Spring Peak (Pliocene)—Very-pale-orange, sparsely porphyritic, 

high-silica rhyolite. Contains trace to about 10 percent (average, about 6 

percent) phenocrysts of principally quartz, plagioclase, and biotite (but 

includes sanidine and hornblende in some places) in a fresh glass 

groundmass; accessory opaque oxides, titanite, allanite, and apatite. Forms 

a moderately voluminous series of lava flows and domes and scarce dikes 

in the central part of the Aurora volcanic field. Determined age 3.7±0.1 

Ma

Tach Trachyandesite of Cedar Hill (Pliocene)—Dark-gray, strongly porphyritic 

trachyandesite (basaltic andesite of Lange and Carmichael, 1996); 

includes minor basaltic trachyandesite. Contains about 15 to 45 percent 

(average, about 27 percent) phenocrysts of plagioclase, clinopyroxene, and 

olivine in a moderately devitrified groundmass; accessory assemblage 

limited to opaque oxides. Forms composite lava cones and flows in the 

southern part of the Aurora volcanic field. Determined age 3.7±0.4 Ma

Tmlp Basaltic andesite of Locomotive Point (Pliocene)—Medium-light-gray, sparsely 

porphyritic basaltic andesite. Contains about 5 to 10 percent (average, 

about 7 percent) phenocrysts of clinopyroxene and olivine in an aphanitic, 

felty intergrowth of plagioclase, clinopyroxene, and opaque oxides; 

accessory mineral assemblage limited to opaque oxides. Forms a 

prominent series of lava flows emanating from a vent about 8 km 

northeast of Bridgeport, significantly west of all other Aurora volcanic 

field rocks. Determined age 3.9±0.06 Ma
44

SEDIMENTARY DEPOSITS

QTsn Sinter deposits (Pleistocene to Miocene)—White to light-gray, opaline to 

chalcedonic sinter that forms several small exposures about 1 km 

southwest of Spring Peak in the eastern part of the map area. The largest 

sinter terrace has a maximum thickness of ~5 m on its northeastern margin 

and is <1 m thick elsewhere. Much of the northeastern terrace margin 

consists of several-meter-long to smaller blocks that have been 

dismembered and slightly rotated by foundering. Thin, 0.5 to severalmeters-thick altered gray- to brown-weathering, pervasively silicified 

volcaniclastic strata, predominantly composed of well-sorted and locally 

bedded sand to silt, lie beneath sinter and unconformably on pre-Tertiary 

rocks

Tsm Sedimentary rocks of Martinez Hill (Pliocene to Miocene)—Bedded 

conglomerate filling a paleochannel (Smailbegovic, 2002). About 60 m 

thick; contains subrounded 6–40 cm clasts of metavolcanic rock and 1–3 

cm andesite and rhyolite clasts. Underlain by trachyandesite of Aurora and 

overlain by the rhyolite of Martinez Hill, which indicate an age between 

2.6 to 12.6 Ma

Tyg Younger gravels (Pliocene and (or) Miocene)—Massive, poorly sorted, 

unconsolidated gravels that contain silt- to boulder-size clasts in a silty to 

sandy matrix; mantles Miocene volcanic rocks along the range front 

between Sonoma Gulch and the East Walker River, and east of Bridgeport 

Reservoir. Clasts, including Eureka Valley Tuff, reflect derivation from 

proximal bedrock sources

Tswr Sedimentary rocks of East Walker River (Pliocene and (or) Miocene)—In the 

vicinity of lower Sonoma and Masonic Creeks, near their junctions with 

East Walker River, unit consists of gray, alternating fine lacustrine and 

coarse fluvial strata dominated by volcanic clasts and, in some places, 

interbedded with angular sedimentary breccias composed of Mesozoic 

metasedimentary rocks; includes rare interbedded tuffs. Some deposits, 

south of the East Walker River, contain small bone fragments of unknown 

vertebrate taxa and age. On the east side of Bridgeport Valley, east of 

Bridgeport (where these deposits are >70 m thick), unit consists of braided 

stream deposits that contain rounded granitic and lesser volcanic clasts; 

presumed to underlie all or most Bridgeport Valley. Finer-grained facies 

are clast-supported, moderately well-sorted sandstone that contains 0.1–2 

mm quartz, perthitic alkali feldspar, plagioclase, fine-grained sedimentary 

rock clasts, and minor epidote-bearing metavolcanic hornfels grains; 

cement is isotropic and possibly opaline. Mafic silicate minerals 

(biotite>green hornblende>clinopyroxene) compose about 1 percent of the 

detrital grains. Trough crossbedded volcanic-clast conglomerate and 

interbedded white lacustrine(?) shale are exposed along the Masonic Road 

and locally along the east shore of Bridgeport Reservoir. Underlain by the 

trachyandesite of Willow Springs and the Eureka Valley Tuff and 

therefore younger than 8.0 Ma; minimum age uncertain
45

Tog Older gravels (Miocene)—Massive, poorly sorted, unconsolidated deposits that 

contain silt- to boulder-size clasts in a silty to sandy matrix. Clasts include 

cobbles of exotic Mesozoic granitoid rock, petrified wood, and Bodie Hills 

volcanic field rocks. Some deposits mantle prominent topographic 

benches deep within the Bodie Hills. Other deposits conspicuously 

underlie channelized exposures of Eureka Valley Tuff and may represent 

material deposited within channels that locally controlled emplacement of 

Eureka Valley Tuff. In Fletcher Valley, unit consists of massive, poorly 

sorted, unconsolidated deposits that contain silt- to boulder-size clasts in a 

silty to sandy matrix. Most clast compositions reflect derivation from 

proximal bedrock sources. Deposits mantle ridges and the valley bottom 

about 1.5 km southeast of the Homestead Mine; a much smaller exposure 

is immediately west to northwest of the Homestead Mine. Age bracketed 

between 9.4 and 11.0 Ma by ages of underlying and overlying units

Tfv Sedimentary rocks of Fletcher Valley (Miocene)—Very light-gray braided 

stream deposits (at least 200 m thick); contain well-rounded volcanic 

pebbles and, in places, interbedded felsic tephra. Reasonably well-sorted, 

clast-supported sandstone and siltstone contain 0.1–2 mm subangular 

clasts of plagioclase and lesser quartz. Mafic silicate minerals 

(clinopyroxene>olive green hornblende>biotite) compose about 1 percent 

of detrital grains. Also contains a significant amount of lithologically 

diverse volcanic lithic fragments. Overlies the trachydacite of Rough 

Creek and interbedded with trachyandesite of Del Monte. Interbedded 

tephras yielded ages of 10.5 to 11.7 Ma (Fleck and others, in press; Gilbert 

and Reynolds, 1973; and John and others, 2012)

DISTALLY SOURCED ASH-FLOW TUFFS

Tev Eureka Valley Tuff (Miocene)—Grayish-black to light-brown, moderately 

porphyritic trachydacite ash-flow tuff. Contains about 5 to 20 percent 

(average, about 14 percent) phenocrysts of plagioclase, biotite, 

clinopyroxene, and trace amounts of hornblende in a grayish-black to ironstained moderate-reddish-brown glassy groundmass; accessory minerals 

include opaque oxides and apatite. In the Bodie Hills, where the tuff is 

moderately to densely welded, most exposures appear to fill 

paleochannels. Correlated with Eureka Valley Tuff erupted from the Little 

Walker caldera (Noble and others, 1974; Priest, 1979; Pluhar and others, 

2009). Presence of multiple cooling units, abundance of biotite 

phenocrysts, and variable magnetic polarity (Carlson and others, 2013) 

indicate that most Eureka Valley Tuff exposures in the Bodie Hills 

correlate with the Tollhouse Flat and Upper Members. Determined age 

about 9.4 Ma (table 2; Pluhar and others, 2009; Chris Pluhar, oral 

commun., 2013)

Tjs Tuff of Jacks Spring (Miocene)—Very-light-gray to pinkish-gray, porphyritic 

rhyolite ash-flow tuff. Contains about 10 to 20 percent (average, about 15 

percent) phenocrysts of plagioclase, sanidine, quartz, and biotite in a 

groundmass composed of fresh to weakly devitrified glass shards; 
46

accessory minerals include opaque oxides, apatite, zircon, titanite, and 

allanite. Tuff is lithic-rich and poorly welded. In the Bodie Hills, 

volumetrically insignificant exposures (perhaps atop basement highs) 

underlie volcanic rocks of the Bodie Hills volcanic field. Correlated with 

tuff of Jacks Spring (Stewart, 1981), southeast of Huntoon Valley, on the 

basis of age and lithologic characteristics. Determined age about 12.0 to 

12.1 Ma

ROCKS OF BODIE HILLS VOLCANIC FIELD

Rocks of Big Alkali center (Miocene)

Tbar Rhyolite of Big Alkali (Miocene)—Light-gray to pinkish-gray, moderately 

porphyritic rhyolite; includes trachydacite and dacite. Contains about 5 to 

30 percent (average, about 20 percent) phenocrysts of plagioclase, 

hornblende, biotite, and trace amounts of quartz and (or) sanidine in a 

weakly devitrified, locally glassy groundmass; accessory minerals include 

opaque oxides, apatite, and combinations of titanite, allanite and zircon in 

some places. Forms a prominent, north-trending array of small-volume 

domes from south of Mormon Meadow to north of the Sarita Mine near 

the west edge of the map area. Determined age 5.5–6.2 Ma

Rocks of Willow Springs center (Miocene)

Twa Trachyandesite of Willow Springs (Miocene)—Medium-light-gray, 

moderately porphyritic, massive but locally flow-laminated, 

trachyandesite; includes andesite, trachydacite, and dacite. Contains about 

10 to 40 percent (average about 24 percent) phenocrysts of plagioclase, 

hornblende, biotite, clinopyroxene, and rarely olivine in a moderately 

devitrified, locally glassy groundmass; accessory minerals include opaque 

oxides, apatite and minor zircon. Plagioclase phenocrysts are 

characteristically larger (average size of the largest phenocryst in samples 

of this unit is 4.8 mm) than those in most Bodie Hills volcanic field rocks. 

Forms a large-volume, north-northwest elongate set of coalesced lava 

domes and flows in the low hills southeast of Bridgeport. Includes several 

prominent flow breccias in cliffs west of Willow Springs. Determined age 

8.0–8.6 Ma

Ths Dacite of Hot Springs Canyon (Miocene)—Very light-gray, moderately 

porphyritic dacite; includes trachyandesite and trachydacite. Contains 

about 15 to 25 percent (average, about 18 percent) phenocrysts of 

plagioclase, hornblende, biotite, and trace amounts of clinopyroxene in a 

moderately devitrified groundmass; accessory minerals include opaque 

oxides and apatite. Forms a series of small-volume, isolated, weakly to 

strongly altered lava domes about 5 km south of Bridgeport, near west 

edge of the map area. Determined age 8.07±0.02 Ma

Tdcc Trachydacite of Cinnabar Canyon (Miocene)—Light-gray, strongly porphyritic 

trachydacite. Contains about 25 to 30 percent phenocrysts of plagioclase, 

biotite, hornblende, and trace amounts of quartz in a glassy groundmass; 

accessory minerals include opaque oxides, apatite, and zircon. Forms two 

very-small-volume, recessive-weathering lava dome exposures about 2 km 
47

south of Big Alkali, in the west part of the map area. Plagioclase 

phenocrysts are characteristically larger (5.0 mm, average size of the 

largest phenocryst in each sample of this unit) than those in most Bodie 

Hills volcanic field rocks. Determined age 8.55±0.03 Ma

Rocks of Potato Peak center (Miocene)—Coalesced lava flows and exogenous 

lava domes and lesser block-and-ash flow deposits and debris-flow 

deposits define a large dome field encompassing the crest of the Bodie 

Hills in central part of the map area. Age determinations indicate 

emplacement at 8.7 to 9.1 Ma. Consists of:

Tppd Debris-flow deposits associated with trachydacite of Potato Peak 

(Miocene)—Very-light-gray to pinkish-gray, recessive-weathering, poorly 

sorted, matrix-supported debris-flow deposits shed during the lateral 

collapse of trachydacite of Potato Peak lava domes; clast population 

dominated by trachydacite of Potato Peak blocks up to 3 meters in 

diameter but includes clasts of older Bodie Hills volcanic field rocks, 

especially trachyandesite of Mount Biedeman and dacite of Silver Hill. 

Weakly consolidated ashy and clay-rich matrix present in isolated 

outcrops but absent in most places, which yields a residual surface strewn 

with a large-boulder lag deposit. Age, approximately coeval with 

trachydacite of Potato Peak

Tpp Trachydacite of Potato Peak (Miocene)—Medium-gray to grayish-orangepink, massive but locally flow-laminated, moderately porphyritic 

trachydacite; includes trachyandesite, andesite, and dacite. Contains about 

5 to 30 percent (average, about 18 percent) phenocrysts of plagioclase, 

hornblende, and biotite in a moderately devitrified groundmass; accessory 

minerals include opaque oxides, apatite, and minor zircon. In the westcentral part of the map area, forms a large-volume elliptical eruptive 

center consisting of coalesced lava flows and exogenous lava domes, 

especially in the rugged terrane south of Bodie Mountain. Rubbly breccias 

form the tops and toes of many flows. Massive, steeply jointed outcrops at 

Potato Peak may constitute a shallow, subvolcanic plug. Recessive 

weathering areas may reflect the locus of soft, cogenetic block-and-ash

and debris-flow deposits. Well-indurated block-and-ash deposits crop out 

prominently in exposures on the southeast side of Rough Creek, in the 

upper part of its watershed. Determined 40Ar/39Ar age 8.7–9.1 Ma

Rocks of Silver Hill center (Miocene)—Lava domes, related intrusions, and an 

apron of sedimentary rocks and debris-flow deposits define a moderatesize dome field in south-central part of the map area. Age determinations 

indicate emplacement at 8.9 to 9.1 Ma. Consists of:

Tdsd Debris-flow deposits associated with dacite of Silver Hill (Miocene)—

Light-gray to very-pale-orange, recessive-weathering, poorly sorted, 

matrix-supported debris-flow deposits shed during the lateral collapse of 

dacite of Silver Hill lava domes; clast population dominated by dacite of 

Silver Hill blocks as large as several meters in diameter but includes clasts 

of older Bodie Hills volcanic rocks, especially trachyandesite of Mount 

Biedeman. Glassy, prismatically jointed blocks of dacite of Silver Hill are 
48

common and indicate debris-flow deposition during associated lava dome 

development. Weakly consolidated ashy and clay-rich matrix present in 

isolated outcrops but absent in most places, which yields a residual surface 

strewn with a large-boulder lag deposit. Age, approximately coeval with 

dacite of Silver Hill

Tisd Intrusive phase of dacite of Silver Hill (Miocene)—Petrographically and 

chemically identical to the dacite of Silver Hill. Forms intrusions at 

Sugarloaf and three surrounding, topographically prominent features. 

Determined age 8.8–9.1 Ma

Tsd Dacite of Silver Hill (Miocene)—Medium-light-gray to light-brownish-gray, 

strongly porphyritic, massive but locally flow-laminated dacite; includes 

trachyandesite, trachydacite, and andesite. Contains about 15 to 35 percent 

(average, about 23 percent) phenocrysts of plagioclase, hornblende, 

biotite, and trace amounts of quartz and (or) sanidine in a moderately 

devitrified (rarely glassy) groundmass; accessory minerals include opaque 

oxides, apatite, and minor zircon and titanite. Forms a series of lava 

domes, coalesced to a roughly circular area, in the south-central part of the 

map area. Determined age 8.9–9.1 Ma

Rocks of Mount Biedeman center (Miocene)—Lava flows, interbedded laharic 

debris-flow beds, an apron of sedimentary rocks and debris-flow deposits, 

and related plugs define a moderate-size stratovolcano approximately 

centered on Mount Biedeman in the south-central part of the map area. 

Age determinations indicate emplacement at 8.9 to 9.9 Ma. Consists of:

Timb Trachydacite intrusions of Mount Biedeman (Miocene)—Grayish-orangepink to medium-gray, moderately porphyritic trachydacite. Contains about 

10 percent phenocrysts of plagioclase, hornblende, and clinopyroxene in a 

felty intergrowth of plagioclase and opaque oxides; accessory minerals 

include apatite and opaque oxides. Forms three plugs east of Bridgeport 

Canyon and three very small-volume plugs west of Bridgeport Canyon; all 

have prominent, near-vertical jointing and intrude debris-flow deposits of 

Mount Biedeman or lava flows of the trachyandesite of Mount Biedeman. 

Age, approximately coeval with the trachyandesite of Mount Biedeman

Tsmb Sedimentary rocks and debris-flow deposits of Mount Biedeman 

(Miocene)—Poorly sorted lahar, block-and-ash-flow, and associated 

sedimentary deposits shed from the flanks of the Mount Biedeman 

stratovolcano; includes some trachyandesite of Mount Biedeman lava 

flows. Clast population dominated by trachyandesite of Mount Biedeman

but locally includes blocks of the Eureka Valley Tuff (Tev) and rhyolite of 

the Bodie Hills (Trbh). Most block-and-ash-flow deposits are brownishblack and clast-supported. Light-gray, recessive-weathering lahar deposits 

are massive, poorly sorted, and contain silt- to boulder-size clasts in an 

ash- to clay-rich matrix. Very-light-gray sedimentary facies deposits are 

remarkably well exposed in road cuts 1.5 km east of Mormon Meadows 

and near the mouth of Cottonwood Canyon. These matrix-supported 

deposits contain rounded, compositionally diverse clasts several 

centimeters to 0.5 meters in diameter. Overlies and locally underlies the 
49

Eureka Valley Tuff (Tev) in southwest part of map area. Age, likely 

approximately coeval with trachyandesite of Mount Biedeman. A glassy 

block contained in a debris flow of this unit in Cottonwood Canyon has an 

age of 9.68±0.06 Ma and interbedded lava flows in the west part of this 

unit range in age from 8.9 to 9.0 Ma

Tamb Trachyandesite of Mount Biedeman (Miocene)—Light- to medium-gray, 

strongly porphyritic trachyandesite; includes basaltic trachyandesite, 

trachydacite, andesite, and dacite. Contains about 15 to 50 percent 

(average, about 33 percent) phenocrysts of plagioclase, hornblende, 

clinopyroxene, and trace amounts of biotite or olivine in a moderately 

devitrified groundmass; accessory minerals include opaque oxides and 

apatite. Phenocrysts are trachytically aligned in some places; prominent 

hornblende phenocrysts (≥3 mm) are a characteristic feature in some 

places. Determined ages 8.9 to 9.9 Ma

Tpst Sedimentary rocks and tuff of Paramount (Miocene)—Pinkish-gray to palered-purple, and dark-tan, sparsely porphyritic rhyolite tuff and associated 

volcaniclastic rocks; weakly to strongly hydrothermally altered and ironstained in some places. Pyroclastic rocks contain trace amounts of 

plagioclase and quartz phenocrysts in a recrystallized ashy groundmass 

(replaced by secondary silica in some places); accessory minerals include 

opaque oxides and rare zircon. Contains abundant 0.5- to 10-cm 

unflattened pumice blocks and exotic lithic fragments of porphyritic 

intermediate-composition lava as well as almost aphyric rhyolite lava. 

Interbedded sedimentary rocks include poorly sorted, lithic-and pumicerich sandstone, conglomerate, and several prominent silica sinter terraces. 

Tuff likely associated with eruption of the rhyolite of Bald Peak; with 

cogenetic volcaniclastic deposits, fills northeast-elongated Paramount 

Basin near Bald Peak in the center of the map area. Approximately coeval 

with rhyolite of Bald Peak (9.7 Ma) and older than the Eureka Valley Tuff 

(9.4 Ma)

Trbh Rhyolite of Bodie Hills (Miocene)—White to medium-light-gray and pinkishgray, moderately porphyritic, locally flow-laminated and flow-folded 

rhyolite. Contains 1 to about 25 percent (average, about 15 percent) 

phenocrysts of plagioclase, quartz, sanidine, biotite, hornblende, and 

rarely clinopyroxene in a weakly devitrified, locally glassy groundmass; 

accessory minerals include opaque oxides, apatite, titanite, zircon, and 

allanite. Forms a series of small-volume lava domes, most centered on 

Mount Biedeman in the southwest part of the map area, but also forms a 

discontinuous north-trending array that extends just north of the Sarita 

Mine on the north side of Masonic Mountain. White, sparsely porphyritic 

ash-fall tuff locally preserved on margins of lava domes. Many lava flows 

include carapace and (or) basal flow breccias. Several distinctive outcrops 

surrounding Mount Biedeman consist of mingled magma in which 2–20 

cm blocks of trachyandesite of Mount Biedeman are contained in a matrix 

of seemingly partially hybridized, quartz-rich rhyolite of Bodie Hills. 
50

Trachyandesite inclusions are locally flattened and have near vertical 

elongation. Determined age 9.6–9.9 Ma

Trb Rhyolite of Bald Peak (Miocene)—White, pinkish-gray to light-greenish-gray, 

nearly aphyric flow-laminated rhyolite. Contains trace to about 10 percent 

(average, about 3 percent) phenocrysts of quartz, sanidine, and trace 

amounts of plagioclase and biotite in fresh to recrystallized glass; 

accessory minerals include trace amounts of opaque oxides, zircon, and 

titanite. Forms a prominent, small-volume lava dome and associated flows 

at Bald Peak near the center of the map area. Determined age 9.7 Ma

Trbc Rhyolite of Bodie Creek (Miocene)—Light-gray to pale-pinkish-gray, 

moderately porphyritic rhyolite, flow-laminated in some places. Contains 

about 5 to 20 percent (average, about 15 percent) phenocrysts of 

plagioclase, quartz, biotite, sanidine, and rare hornblende in devitrified, 

locally glassy groundmass; accessory minerals include opaque oxides, 

titanite, apatite, and rare allanite and zircon. Forms a series of smallvolume lava domes, mostly covered by lava flows of the basaltic 

trachyandesite of Beauty Peak, in the central part of the map area; includes 

a ~150-m thick, prominently jointed flow in Bodie Creek. Determined age 

9.9–10.2 Ma

Tac Trachyandesite of Aurora Canyon (Miocene)—Light- to medium-gray, locally 

flow-laminated, moderately porphyritic trachyandesite; includes 

trachydacite, andesite, and dacite. Contains about 15 to 35 percent

(average, about 21 percent) phenocrysts, trachytically aligned in some 

places, of plagioclase, hornblende, biotite, and clinopyroxene in a weakly 

devitrified, locally glassy groundmass; accessory minerals include opaque 

oxides, apatite, and rare titanite and zircon. Forms a discontinuous, 

northeast-trending series of small-volume lava domes in the west-central

part of the map area. Coarse block-and-ash-flow deposits and (or) 

carapace breccias are preserved along the margins of some domes. 

Determined age 10.3–10.6 Ma

Tsbr Silicified breccia (Miocene)—Dark-red, orange, and brown, densely silicified 

breccia, finely bedded cavern-filling pebbly sandstone, and dripstone. 

Consists of two low, crudely bedded outcrops forming an outward dipping 

semicircle tentatively interpreted as silicified eruption breccias and 

overlying hydrothermal sediments. Overlies dark-red, clay-rich paleosol 

formed on trachyandesite of Masonic and lies between domes of the 

rhyolite of Rock Springs Canyon and trachydacite of Aurora Canyon. 

Absolute age unknown

Trs Rhyolite of Rock Springs Canyon (Miocene)—Light-gray to pinkish-gray, 

almost aphyric rhyolite. Contains trace to 3 percent (average, about 1 

percent) phenocrysts of plagioclase, sanidine, hornblende, and biotite in a 

weakly devitrified, locally glassy groundmass; accessory minerals include 

opaque oxides and rare zircon. Forms two prominent, small-volume lava 

domes in the west-central part of the map area that intrude the 

trachyandesite of Masonic and are overlain by the Eureka Valley Tuff. 
51

Includes flow lobes entirely composed of gray to black obsidian. Absolute

age unknown

Tdm Trachyandesite of Del Monte (Miocene)—Medium-gray, strongly porphyritic, 

commonly strongly vesicular trachyandesite; includes basaltic 

trachyandesite and trachydacite. Contains about 10 to 40 percent (average, 

about 25 percent) phenocrysts of plagioclase, biotite, hornblende, and 

clinopyroxene in a weakly devitrified groundmass; accessory minerals 

include opaque oxides and apatite. Forms a discontinuous, north-trending 

series of exposures that consist of lava flows and interbedded, 

heterogeneous, laharic debris-flow and volcaniclastic deposits in the 

north-central and northeast parts of the map area. Debris-flow deposits are 

massively bedded, poorly sorted, clast- to matrix-supported mass wasting 

deposits with a clay- and ash-rich matrix. Clasts are 1 cm to about 1 m in 

diameter, angular to subangular, and principally volcanic rock fragments 

similar to interbedded lava flows but in some places include quartz-rich 

rhyolite, possibly the rhyolite of Del Monte Canyon. Determined age 

10.9–11.0 Ma

Tdbc Trachydacite of Bridgeport Canyon (Miocene)—Medium- to light-gray, 

moderately porphyritic trachydacite. Contains about 10 to 25 percent 

(average, about 20 percent) phenocrysts of plagioclase, clinopyroxene, and 

trace amounts of hornblende in a moderately devitrified groundmass; 

accessory minerals include opaque oxides and apatite. Forms several 

isolated, small-volume, topographically prominent lava domes west of 

Bridgeport Canyon in the southwest part of the map area. Plagioclase 

phenocrysts are characteristically larger (6.0 mm, average size of the 

largest phenocryst in each sample of this unit) than those in most Bodie 

Hills volcanic field rocks. Composition is distinctly more alkaline than 

those of Bodie Hills volcanic field rocks but similar to those associated 

with the Little Walker caldera, 25 km to the northwest. Determined ages 

11.07±0.02 and 11.08±0.02 Ma

Trdc Rhyolite of Del Monte Canyon (Miocene)—Light-gray to pale-grayish-redpurple, moderately porphyritic rhyolite. Contains about 5 to 25 percent 

(average, about 18 percent) phenocrysts of plagioclase, quartz, 

hornblende, biotite, sanidine, and rare clinopyroxene in glassy (perlitic in 

places) to moderately devitrified groundmass; accessory minerals include 

opaque oxides, apatite, titanite, and rare zircon and allanite. Forms a set of 

low-volume lava domes and associated pyroclastic deposits in the eastcentral part of the map area. Pyroclastic deposits are well bedded, lithic 

rich, and underlie the lava flows. Determined age 11.1–11.6 Ma

Tr Pyroxene rhyolite (Miocene)—Light- to medium-gray, moderately porphyritic, 

flow-laminated and locally vesicular rhyolite. Contains trace to about 25 

percent (average, about 15 percent) phenocrysts of plagioclase, 

orthopyroxene, and clinopyroxene in a glassy groundmass (perlitic in 

many places); accessory minerals include opaque oxides, apatite, and rare 

zircon. Forms a series of very-small-volume, discontinuously exposed 
52

intrusive lava domes in Rough Creek and west of Sugarloaf Mountain in 

the central part of the map area. Determined age 11.15±0.01 Ma

Treb Rhyolite of East Brawley Peak (Miocene)—Very-light-gray to pinkish-gray, 

sparsely porphyritic rhyolite. Contains about 5 to 10 percent (average, 

about 8 percent) phenocrysts of quartz, plagioclase, sanidine, and biotite, 

in a moderately devitrified, altered, and recrystallized groundmass; 

accessory minerals include opaque oxides and rare zircon. Forms a 

discontinuously exposed series of very-small-volume lava domes in the 

east-central part of the map area. Most exposures are strongly 

hydrothermally altered. Determined age 11.18±0.02 Ma

Tra Rhyolite of Aurora Creek (Miocene)—Light- to medium-gray, sparsely 

porphyritic rhyolite. Contains trace to about 10 percent (average, about 4 

percent) phenocrysts of plagioclase and trace amounts of combinations of 

quartz, sanidine, hornblende, clinopyroxene, and biotite in a glassy 

groundmass; accessory minerals include opaque oxides, apatite, and, in 

some places, titanite and zircon. Forms a group of small-volume lava 

domes (locally composed of obsidian, perlitic in some places) in the eastcentral part of the map area. Includes minor cogenetic, lithic-rich 

pyroclastic flow deposits, especially in the lower reaches of Bodie Creek. 

Most outcrops are altered to clay minerals, alunite, and opaline silica 

owing to hydrothermal processes that affected rocks in the Aurora mining 

district and Sawtooth alteration zone (Vikre and others, in press).

Determined age 11.2–11.3 Ma

Tdi Trachydacite intrusions (Miocene)—Medium-gray to light-brownish-gray, 

strongly porphyritic, locally flow-laminated trachydacite; includes minor 

dacite. Contains about 15 to 25 percent (average, about 21 percent) 

phenocrysts of plagioclase, biotite, hornblende, and trace amounts of 

quartz, and clinopyroxene, in a locally glassy, variably devitrified 

groundmass; accessory minerals include opaque oxides, apatite, and rare 

zircon and titanite. Forms several widely dispersed, very-low-volume 

intrusions into the trachyandesite of Masonic in the west-central part of 

the map area. Determined age 11.2–11.3 Ma

Tcc Trachyandesite of Clark Canyon (Miocene)—Light- to medium-gray, strongly 

porphyritic trachyandesite; includes andesite and trachydacite. Contains 

about 15 to 35 percent (average, about 22 percent) phenocrysts of 

plagioclase, clinopyroxene, and trace amounts of hornblende and biotite, 

in a moderately devitrified groundmass; accessory minerals include 

opaque oxides and apatite. Forms a series of very-small-volume plugs 

intruding debris-flow deposits in the trachyandesite of Masonic and 

arrayed along an east-northeast trend in Aurora Canyon in the west-central 

part of the map area; includes a very-small-volume lava flow about 1.5 km 

south of Aurora Canyon. Determined ages 11.27±0.11 and 11.34±0.02 Ma

Rocks of West Brawley Peak center (Miocene)—Lava flows and a plug that 

forms West Brawley Peak define a moderate-volume composite volcano 

in the east-central part of the map area. Age determinations indicate 

emplacement at 11.3 to 11.5 Ma. Consists of:
53

Twbi Trachyandesite intrusion of West Brawley Peak (Miocene)—Mediumgray, strongly porphyritic, vertically flow-laminated trachyandesite. 

Contains about 30 percent phenocrysts of plagioclase, hornblende, and a 

trace of biotite and clinopyroxene in a devitrified groundmass; accessory 

minerals include opaque oxides and apatite. Plagioclase forms tabular to 

blocky crystals as much as 2 cm long. Forms a prominent, small-volume 

plug that intrudes the trachyandesite of West Brawley Peak at West 

Brawley Peak. Likely intrudes vent for lava flows of trachyandesite of 

West Brawley Peak that radiate away from West Brawley Peak on its 

south, west, and north sides. Likely approximately coeval with 

trachyandesite of West Brawley Peak

Twba Trachyandesite of West Brawley Peak (Miocene)—Medium-light-gray to 

pinkish-gray moderately porphyritic trachyandesite; includes andesite and 

trachydacite. Contains about 10 to 35 percent (average, about 21 percent) 

phenocrysts of plagioclase, hornblende, biotite, clinopyroxene, and trace 

amounts of olivine in a moderately devitrified groundmass; accessory 

minerals include opaque oxides, apatite, and rare zircon. Forms a series of 

lava flows erupted from the moderate-volume composite volcano centered 

on West Brawly Peak, in the east-central part of the map area. Toes of 

some flows are strongly flow-banded and have prominent flow ramps. 

Plagioclase phenocrysts are characteristically larger (6 mm, average size 

of the largest phenocryst in each sample of this unit) than those in most 

Bodie Hills volcanic field rocks. In some places, contains abundant, 

rounded, fine-grained hornblende-rich mafic (cognate?) inclusions up to 

15–20 cm in diameter. Determined 40Ar/39Ar age 11.3–11.5 Ma

Rocks of Rancheria center (Miocene)—Lava flows and a very small volume 

plug that appear to represent the products of a mafic-composition volcano 

now largely overlain by products of the Mount Biedeman center in the 

southwest part of the map area. Age determinations indicate emplacement 

at 11.6 to 11.7 Ma. Consists of:

Trwi Basalt of Rancheria plug (Miocene)—Medium-gray, moderately porphyritic 

basalt. Contains about 10 percent phenocrysts of olivine and 

clinopyroxene in an intergranular intergrowth of plagioclase, 

clinopyroxene, and opaque oxides; accessory assemblage limited to 

opaque oxides. Forms a very-small-volume composite plug that intrudes 

the monzogranite of Rattlesnake Gulch about 1.5 km east of Conway 

Summit in the southwest corner of the map area. Plug contains abundant 

0.1–1 m rounded cognate fragments, abundant 3–50 cm clasts of the 

Cretaceous monzogranite of Rattlesnake Gulch, and xenocrysts of 

plagioclase and quartz, likely derived from the host monzogranite. Likely 

coeval and cogenetic with compositionally similar basaltic trachyandesite 

of Rancheria. Determined age 11.7±0.5 Ma

Trwa Basaltic trachyandesite of Rancheria (Miocene)—Medium-light-gray to 

medium-dark-gray, moderately porphyritic basaltic trachyandesite; 

includes basalt, trachyandesite, basaltic andesite, and andesite. Contains 

about 5 to 20 percent (average, about 13 percent) phenocrysts, 
54

trachytically aligned in some places, of olivine, clinopyroxene, and 

locally, minor plagioclase and hornblende, in a moderately devitrified 

groundmass or, in some places, in an intergranular intergrowth of 

plagioclase, clinopyroxene, and opaque oxides; accessory minerals include 

opaque oxides and apatite. Forms a set of small-volume, discontinuous 

exposures of lava flows in the southwest part of the map area beneath the 

Eureka Valley Tuff (Tev) and sedimentary rocks and debris-flow deposits 

of Mount Biedeman (Tsmb); appears to represent the products of a maficcomposition volcano that predates the Mount Biedeman stratovolcano. 

Determined age 11.6±0.9 Ma

Rocks of Trachydacite of Rough Creek (Miocene)

Trcd Trachydacite of Rough Creek (Miocene)—Very-light-gray to light-gray, 

moderately porphyritic trachydacite; includes trachyandesite and minor 

andesite and dacite. Contains about 15 to 30 percent (average, about 19 

percent) phenocrysts of plagioclase, hornblende, biotite, and trace amounts 

of combinations of clinopyroxene, olivine, and quartz in a locally glassy, 

moderately devitrified groundmass; accessory minerals include opaque 

oxides, apatite, and zircon. Forms a moderate-volume group of coalesced 

lava domes and flows in the northeast part of the map area; includes minor 

block-and-ash-flow deposits. Plagioclase phenocrysts are characteristically 

larger (5.8 mm, average size of the largest phenocryst in each sample of 

this unit) than those in most Bodie Hills volcanic field rocks. In some 

places, contains rounded, fine-grained mafic (cognate?) inclusions. 

Determined age 11.7–12.9 Ma

Thai Hornblende trachyandesite intrusions (Miocene)—Light- to medium-lightgray, moderately porphyritic trachyandesite. Contains about 10 to 20 

percent (average, about 14 percent) phenocrysts of plagioclase, 

hornblende, and clinopyroxene in a moderately devitrified groundmass; 

accessory minerals include opaque oxides and apatite. Forms three verysmall-volume plugs hosted in trachyandesite of Masonic in Aurora 

Canyon in the west-central part of the map area; plugs are distinguished by 

noteworthy 1–5 mm hornblende phenocrysts. Determined age 11.82±0.2

Ma

Rocks of Aurora center (Miocene)—Series of lava flows and interbedded 

laharic debris-flow beds and volcaniclastic deposits and small-volume 

intrusions define a moderate-volume stratovolcano centered near the 

Aurora mining district in the eastern part of the map area. Age 

determinations indicate emplacement at 12.6 to 13.1 Ma. Consists of:

Taai Trachyandesite intrusion of Aurora (Miocene)—Medium-light-gray 

strongly porphyritic trachyandesite. Contains about 20 to 30 percent 

phenocrysts of plagioclase, clinopyroxene, and chloritized hornblende in a 

devitrified intergrowth of plagioclase and opaque oxides; accessory 

minerals include opaque oxides and apatite. Forms a small plug in the 

Aurora mining district. Absolute age unknown

Tadi Dacite intrusions of Aurora (Miocene)—Light-olive, porphyritic dacite 

intrusions emplaced into trachyandesite of Aurora. Contains phenocrysts 
55

of plagioclase, hornblende, and biotite and in a gray to dark-green 

groundmass; compositionally similar to trachyandesite of Aurora 

(Smailbegovic, 2002). Pervasively hydrothermally altered to abundant 

chlorite, illite, calcite, and epidote. Absolute age unknown

Taa Trachyandesite of Aurora (Miocene)—Medium-light-gray or light- to darkgreen moderately porphyritic trachyandesite; includes andesite and 

trachydacite. Contains about 5 to 40 percent (average, about 19 percent) 

phenocrysts, trachytically aligned in some places, of plagioclase, 

hornblende, and trace amounts of clinopyroxene and biotite in a 

moderately devitrified groundmass; accessory minerals include opaque 

oxides and rare apatite. Hornblende phenocrysts 1–5 mm long (locally as 

long as 2.5 cm) are distinctive. In the east-central part of the map area, 

near the Aurora mining district, forms a moderate-volume series of lava 

flows and associated laharic debris-flow beds and volcaniclastic deposits 

similar to those included in the trachyandesite of Del Monte. The 

abundance of debris-flow deposits interbedded with compositionally 

equivalent lava flows, both intruded by numerous small, intermediatecomposition intrusions, suggests that these rocks may also have been 

erupted from a stratovolcano formerly centered near the Aurora mining 

district. Pervasive hydrothermal alteration in the Aurora mining district 

resulted in abundant secondary chlorite, calcite, epidote, clay minerals, 

and quartz. Determined age 12.6–13.1 Ma

Tals Andesite of Lakeview Spring (Miocene)—Medium-light-gray, moderately 

porphyritic andesite; includes trachyandesite. Contains about 15 to 20 

percent phenocrysts of plagioclase, hornblende, clinopyroxene, and trace 

amounts of olivine in a moderately devitrified groundmass; accessory 

minerals include opaque oxides and apatite. Forms a small-volume lava 

dome at Lakeview Spring and two small intrusions into the trachyandesite 

of Masonic (Tma) about 1 km farther north in the northwest part of the 

map area. Plagioclase phenocrysts are characteristically larger (5.6 mm, 

average size of the largest phenocryst in each sample of this unit) than 

those in most Bodie Hills volcanic field rocks. Determined age 12.93±0.03 

Ma

Tamg Trachyandesite of Masonic Gulch (Miocene)—Medium-light-gray, strongly 

porphyritic trachyandesite; includes minor andesite. Contains about 20 to 

35 percent (average, about 30 percent) phenocrysts, trachytically aligned 

in some places, of plagioclase, hornblende, clinopyroxene, and biotite in a 

weakly devitrified groundmass; accessory minerals include opaque oxides 

and apatite. Forms a discontinuous series of small-volume lava domes 

intrusive into trachyandesite of Masonic in the northwest part of the map 

area. Determined age 13.4–13.5 Ma

Tasc Trachyandesite of Sinnamon Cut (Miocene)—Medium-light-gray, sparsely 

porphyritic trachyandesite. Contains about 5 to 15 percent, trachytically 

aligned, hornblende phenocrysts in a weakly devitrified groundmass; 

accessory assemblage limited to opaque oxides. Forms volumetrically 
56

minor, platy jointed, shallow intrusions in the southwest part of the map 

area. Determined age 13.8±0.4 Ma

Tec Trachydacite of East Canyon (Miocene)—Light- to medium-gray, locally flowlaminated, moderately porphyritic trachydacite; includes trachyandesite.

Contains about 15 to 25 percent (average, about 19 percent) phenocrysts, 

trachytically aligned in some places, of plagioclase, hornblende, biotite, 

and clinopyroxene in a weakly devitrified, locally glassy groundmass; 

accessory minerals include opaque oxides, apatite, and rare zircon. Forms 

small-volume, northwest-elongated lava domes in the west-central part of 

the map area. Coarse block-and-ash-flow deposits and (or) carapace 

breccias are preserved along the margins of some domes. Determined age 

13.78 ±0.03 Ma

Rocks of Mud Spring Canyon Center (Miocene)

Tams Trachyandesite of Mud Spring Canyon (Miocene)—Medium-light gray, 

strongly porphyritic trachyandesite. Contains about 25 to 30 percent 

phenocrysts, trachytically aligned in some places, of plagioclase, 

hornblende, clinopyroxene and biotite in a moderately devitrified 

groundmass; accessory minerals include opaque oxides and apatite. In the 

northeast part of the map area, forms an east-west elongate series of low 

hills composed of several thick lava flows. Plagioclase phenocrysts are 

characteristically larger (average size of the largest phenocryst in samples 

of this unit is about 5 mm) than those in most Bodie Hills volcanic field 

rocks. Determined age 13.96±0.11 Ma

Rocks of Masonic center (Miocene)—Thick sequence of interbedded lava flows, 

laharic debris-flow deposits, scarce block-and-ash-flow deposits, and 

associated volcaniclastic deposits intruded by cogenetic lava domes and 

small-volume plugs define the large-volume Masonic stratovolcano in the 

northwestern part of the map area. Age determinations indicate 

emplacement at 14.1 to 15.0 Ma. Consists of:

Tma Trachyandesite of Masonic (Miocene)—Light- to medium-dark-gray, 

strongly porphyritic trachyandesite; includes basaltic trachyandesite, 

trachydacite, basaltic andesite, and andesite. Contains about 10 to 40 

percent (average, about 28 percent) phenocrysts, trachytically layered in 

some places, of plagioclase, clinopyroxene, hornblende, and trace amounts 

of biotite or olivine in a variably devitrified groundmass or, less 

commonly, in a felty intergrowth of plagioclase, opaque oxides, and 

clinopyroxene; accessory minerals include opaque oxides and apatite. 

Euhedral acicular hornblende phenocrysts (2 to 5 mm) are a distinctive 

constituent in many places. Includes a thick sequence of interbedded lava 

flows, heterogeneous laharic debris-flow deposits, scarce block-and-ashflow deposits, and associated volcaniclastic deposits intruded by cogenetic 

lava domes and small-volume plugs; combined, these rocks define the 

Masonic stratovolcano. This nearly circular composite volcano, 

approximately centered on Masonic Mountain in the northwest part of the 

map area, has the largest eruptive volume of any volcanic center in the 

Bodie Hills. Debris-flow deposits are massively bedded, poorly sorted, 
57

clast- to matrix-supported mass-wasting deposits with a clay- and ash-rich 

matrix. Clasts are 1 cm to several meters in diameter, angular to 

subangular, and principally volcanic rock fragments similar to the 

associated trachyandesite of Masonic. Debris-flow deposits are 

interbedded locally with poorly sorted volcanic sandstones and pebble 

conglomerates. Lava flows are most abundant near the inferred core of the 

eruptive center, whereas volcaniclastic deposits are most abundant in 

progressively more distal exposures. Most volcaniclastic rocks north of 

Masonic Mountain dip north, whereas those to the south dip south, 

consistent with a location for central volcanic vents near Masonic 

Mountain. Determined age 14.1–14.7 Ma

Tmai Trachyandesite intrusions of Masonic (Miocene)—Medium- to dark-gray 

to black, strongly porphyritic trachyandesite; includes minor basaltic 

trachyandesite. Contains about 10 to 35 percent (average, about 25 

percent) phenocrysts of plagioclase, hornblende, clinopyroxene, and trace 

amounts of biotite or olivine in an intersertal intergrowth of plagioclase, 

clinopyroxene, and opaque oxides or in a weakly devitrified groundmass; 

accessory minerals include opaque oxides and sparse apatite. Form a 

series of very-small-volume, locally columnar jointed plugs intruded into 

the trachyandesite of Masonic in the northwest part of the map area. 

Determined ages 14.19±0.02 and 15.00±0.02 Ma

PRE-BODIE HILLS VOLCANIC FIELD ROCKS

Tabo Trachyandesite of Borealis (Miocene)—Medium-gray, trachyandesite debrisflow deposits. Monolithologic clasts are strongly porphyritic and contain 

about 25 percent phenocrysts of plagioclase, hornblende, clinopyroxene, 

and trace amounts of biotite in a devitrified groundmass; accessory 

minerals include opaque oxides and apatite. Euhedral acicular hornblende 

phenocrysts (1 to 3 mm) are a distinctive constituent. In the northeast part 

of the map area, forms a thick sequence of laharic debris-flow deposits 

that crop out in a single mapped polygon; tentatively correlated with 

volcanic rocks in the Borealis mining district about 8 km to the north. 

Debris flows are massively bedded, poorly sorted, clast- to matrixsupported mass wasting deposits with a clay- and ash-rich matrix. Clasts 

are 1 cm to several meters in diameter, angular to subangular, and of 

remarkably uniform composition. Determined age 15.96±0.10 Ma

BASEMENT ROCKS

[K-Ar ages of granitoid rocks calculated using the decay constants of Steiger and Jäger 

(1977)]

Kgdr Monzogranite of Rattlesnake Gulch (Cretaceous)—Very-light-gray to pinkishgray, medium-grained, hypidiomorphic inequigranular monzogranite. 

Contains about 3 percent reddish-brown biotite and 1 percent tan to olive 

green hornblende; accessory minerals include opaque oxides, zircon, 

allanite, apatite, and titanite. Weathers to rounded, grus-covered slopes in 

the extreme southwest part of the map area, east of Conway Summit. 
58

Emplaced into metamorphosed and hornfelsed Mesozoic volcanic and 

sedimentary rocks; northeast part of the pluton is overlain by rocks of the 

Bodie Hills volcanic field. Determined age 95.2 Ma (Robinson and 

Kistler, 1986)

Kgm Granodiorite of Masonic Mountain (Cretaceous)—Very-light-gray to pinkishgray, medium-grained, hypidiomorphic granular, porphyritic granodiorite 

(includes minor monzogranite). Contains prominent, euhedral, perthitic, 

pink potassium feldspar megacrysts 2–5 cm long and about 3 percent redbrown biotite; accessory minerals include opaque oxides, titanite, apatite, 

and zircon. Exposed in the core of the Masonic stratovolcano, where it 

forms two northeast-elongate masses and a much smaller mass about 6 km 

to the east. Emplaced into metamorphosed and hornfelsed Mesozoic 

volcanic rocks; forms a plutonic mass covered in most places by the 

trachyandesite of Masonic. Determined age 98.0 Ma (Robinson and 

Kistler, 1986)

Kgmc Monzogranite of Murphy Creek (Cretaceous)—Pinkish-gray, medium-grained, 

xenomorphic granular seriate monzogranite. Contains 2 percent pale-tan to 

tan, weakly altered biotite; accessory minerals include opaque oxides, 

zircon, and apatite. Potassium feldspar crystals are strongly perthitic. 

Forms prominent cliffs east of the East Walker River; overlain to the east 

by the trachyandesite of Masonic

KJpm Granite of Powell Mountain (Cretaceous or Jurassic)—Medium- to coarsegrained hypidiomorphic granular, leucocratic monzogranite (John, 1983). 

Contains 2 percent biotite. Limited exposures are restricted to the east 

edge of the map area

KJqmd Quartz monzodiorite of Aurora (Cretaceous or Jurassic)—Very-light-gray to 

yellowish-gray, medium-grained, hypidiomorphic granular porphyritic 

quartz monzodiorite. Contains prominent, euhedral, perthitic, pink 

potassium feldspar megacrysts 2–3 cm long and about 2 percent tan to 

greenish-brown biotite, mostly replaced by chlorite; accessory minerals 

include opaque oxides, apatite, titanite, allanite, and zircon. Emplaced into 

metamorphosed and hornfelsed Mesozoic volcanic rocks about 5 km south 

of Mount Hicks; forms a plutonic mass largely covered by rocks of the 

Aurora and Bodie Hills volcanic fields. Intruded by a dense nexus of aplite 

and pegmatite bodies

KJsm Quartz monzonite of Sinnamon Meadow (Cretaceous or Jurassic)—Grayish, 

medium- to coarse-grained biotite-hornblende quartz monzonite 

(Chesterman and Gray, 1975). Forms single outcrop west of Virginia 

Creek near southwest edge of map area

KJmm Quartz monzodiorite of Mormon Meadow (Cretaceous or Jurassic)—

Medium-light-gray, medium-grained, hypidiomorphic granular 

inequigranular quartz monzodiorite. Contains about 25 percent 

hornblende; accessory minerals include titanite, apatite, and zircon. 

Exposure is limited to a very small area about 1.5 km southeast of 

Mormon Meadow
59

KJmgd Quartz monzodiorite of Masonic Mountain (Cretaceous or Jurassic)—Verylight-gray to pinkish-gray, medium-grained hypidiomorphic 

inequigranular quartz monzodiorite (to monzodiorite). Contains about 6 

percent tan to brown biotite; accessory minerals include opaque oxides, 

zircon, and apatite, some of which is altered to chlorite. Rock is mildly 

schistose; quartz is polycrystalline. Forms a small intrusion along the west 

side of the granodiorite of Masonic. Much of the pluton is altered to a 

quartz-sericite-pyrite assemblage

mv Metavolcanic rocks (Mesozoic)—Medium-dark-gray, greenish-gray, yellowishgray, and dark-green metavolcanic hornfels and schist. Relict 0.2- to 3-

mm igneous feldspar phenocrysts are contained in a very fine-grained 

intergrowth of feldspar and quartz that includes biotite and (or) muscovite 

in some places. Protoliths for these rocks probably include intermediatecomposition lava flows and breccias and felsic tuffs. Many of these rocks 

contain abundant metamorphic epidote and chlorite as replacements of 

primary minerals. Form roof pendants and septa adjacent to Mesozoic 

granitoid rocks throughout the Bodie Hills and are likely correlative with 

Mesozoic rocks in roof pendants throughout the Sierra Nevada

sv Metasedimentary and metavolcanic rocks (Mesozoic)—Light- to dark-gray 

quartzofeldspathic hornfels, quartzite, metachert, tuff, and pillow lava 

(Chesterman and Gray, 1975). These schistose rocks, restricted to the 

southwest part of the map area east of Conway Summit, are moderately 

metamorphosed siltstone, sandstone, and chert, with prominent cleavage 

in many places. Some of these rocks are likely Triassic and may be 

correlative with the Excelsior Formation in Nevada (Chesterman and 

Gray, 1975)

ms Metasedimentary rocks (lower Paleozoic)—Dark-gray to black hornfels, 

metargillite, metachert, and quartzite; may contain graphite and andalusite 

in some places (Chesterman and Gray, 1975). These rocks, restricted to 

the southwest part of the map area, south and west of Mormon Meadow, 

are likely correlative with Ordovician to Devonian(?) rocks of the Roberts 

Mountains allochthon in the Log Cabin Mine and Saddlebag Lake roof 

pendants in the Sierra Nevada (Kistler, 1966; Chesterman and Gray, 1975; 

Schweickert and Lahern, 1993; Stevens and Greene, 1999)
60

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