40

80

35

70

55

70

75

60

60

65

75

70

80

60

60

35

5

45

80

45 35

80

60

50

80

50

55

30

75

45

15

15

20

15

25

10

60

35

40

10

10

10 10

10

20

65

40

70

70

40

45 60

80

65

50

85 80

60

80

50

45

40

45

35

30

15

70

60

20

25

10

20

30

40

55

65

70

80

70

30

65

50

40

20

8

8

85

80

80

65

70

40

60

70

60

80

80 85

55

80

75

80

30

25

25

55

70

30

15

15

10

20

25

5

5

35

10

20

10

10

20

5

?

?

?

FAULT

CANYON

MEADOW

?

GRABEN

CANYON

CORCORAN

Little Table

Mountain

(west spur)

Tlt4?

Tlt4

Otq

Tspl

Tlt4?

Tlt4

Ti

Tlt3

Tlt1

Trc

Tmc

Qf

Qa2a

Tac

Qa3

Qa1

Tmcp

Ozal?

Ozal

Qt

Ozl

zs

Otlj

Otd

Otl

Trp

Otl

(Ols)

Ozal

Tmj

Qa2b

Qt

Ozl

Ozl

Otq

Ota

Qa1

Qf

Qa2a

(Ol) Tmcp

Tac

Qt

Tmc

Qc

Tpm Tmc Qt

Qa2b

Ozl

Ozl

Ozl

Ozl

Ozal

Qc

Ozl

Otq

Ota

Otl

Qt

Otd

Otl

Otl Ozd

Ozl

Qa2a

Tac

?

?

Tmc

Ozl

Ozal

Qa2b

Otl

Kba

Ozlc

Kba Qa2b

Otl

(Trc?)

Ti

Qc

Tpu1

Three

Summits

Hill

t4

a1b

a1a

Ti

j?

b

?

Tlt4

Tbt

Tspl

b

c

Tmj?

Qa2b

Tlt1

Tlt1

Qt

Tspu

Tspl

Qa1b

Tspl

Qc

Tlt2

Qc

Ti

Tspl

Tmj

Ql

Qc

Tlt4

Qa1

Tlt2?

Tspl

Tlt1

Qc

Ti

Qt

Qa2b

Tlt4

Qf

Tbt?

Ti

Tbt

Tspu

Tspl

Qf

Qf

Tmj

Tmj?

Tlt1?

Qt

Tpl

Tlt3

Tspl

Tpl

Ql

Tpu2

Qt

Qa2b

Ti

Tlt4

Qa2a

Ql

Qt

Tlt3

Qa2b

Tlt1

Qt

Qa2a

Tmj

Qa2b

Tmj

Tlt4

Qa2a

Qa3

Tmjv

Qc

Qa2a

Ti

Ti

Qt

(Tspl)

Tmcs

Qa3

Qa3

Qa1b

Thb

Qt

Trc

Trc

Tbt

Tmc

?

Tmc

Tac

Trp

Qf

Qf

Qf

Qa3

Tmc

Qt

Qt

Qt

Tmc

Trc

Qf

Ota

Qc

Ota

Ozl

Qa2a

Trc

Ozl

Qt

Trc

Qt

Qc

Trp

Trc

Tmc

Qa2b

Qa2b

Tmc

Qf

Ozd Qa2a

Otlj Otl Ota

(Trc?)

(Trc?)

Tlt1?

Tlt4

Tlt1?

Tlt4?

Ti

Tmj

Qt

Qa1 Qa2b Tmj

Qa1 Qa2b

Tmj

Tlt1

Tlt4

Tlt3?

Ql

Ql

Ti

Tlt3

Qc

Tlt4

Tlt4

Tlt1?

Ql

Ql

Tlt3

Tlt3?

Qf

Tspl

Tlt1

Tspl

Tbt

Tlt1

Qa2a Tpl

Qc

?

Qt

Qc

Qa2b

?

Qt

Tpl Tpl

Tbt

Qa2a

Tpu2

Tspu

Tspl

Tbt

Tspu

Ti

Tspl? Ql

Tlt4?

Ql1

Ql2

Ql

Ql

Tlt4

Ql Ql Tlt3Qf

Qc

Qa2a

Tlt1

Tlt1

Qc

Tpu1

Tspl

Tlt4

Tum

(Tlt1)

(Tlt2)

Tmj?

Tlt3?

Tspu

Tpb

Tbt

Tpu1

Tpu1

(gq)

Trc

Trc

Tmcs

Tmc

ms

Ozal

Ozl

ms

gqs

mf

Tmc

ms

Otq

Otl

Qa3

p

mf

mf

Qa2b

Ota

Otd

Otq

Otlj

mf

ms

mf

Qa1

Trc

2

3

A

4

3 1

5

4

3

2

R2 R1

3

G'

P

E'

D'

C'

O'

K'

F'

A'

N'

M'

B'

P'

22.8–22.7 Ma

23.7 Ma26.00 Ma

26.4 Ma

26.68 Ma26.65 Ma26.9–26.6 Ma27.17(?) Ma32.56(?) Ma36 Ma36 Ma84.5 Ma25.43–25.37 Ma

26.82 Ma

89.6 Ma

Ql1

Holocene

Pleistocene(?)

Pleistocene

early

Miocene(?)

early

Miocene

late

Oligocene

early

Oligocene

Late

Cretaceous

Middle

Ordovician

lower Middle

Ordovician

Lower

Cambrian

QUATERNARYTERTIARYCRETACEOUSORDOVICIANCAMBRIAN(?)CAMBRIAN

CORRELATION OF MAP UNITS

mf

Qu

Qa2Qa3Qa1

Qa2b

Qa2a

Qa1b

Qa1a

Qf

Qc Qt

Ql Ql2

? ? ?

?

?

Tbi

Tpu2Tpu1

Tbt

Tpl

Trus

Thb Trp Ts

?

?

Tspu

Tspl

Ti

Tlt4Tlt3

Tlt2Tlt1

Tum

Tpm

Tjsi Tjsg Tjss

TpbTpbTpb

Tcg

Tjsl

Tmju

Tmj Tmjv Tmjb Tmja Tmjs Tac Tmc Tmcp Tmcs Trc

Tjc

Tgd

Tr

Kbq

Kba Kbgf

Kbgp

Kbgs

Ota

Kra Krgf

Krg Krgi Krgt

Otl Otlj Otd Otq

Ozal Ozl Ozlc Ozlj Ozd Ozs

mf

ms

gs

gq

gsq

gqs

gl

glc

Unconformity

Unconformity

Thrust faultThrust faultThrust faultThrust fault

Tbf

st

LIST OF MAP UNITS

Manmade fill (modern)Surficial alluvial deposits, undivided (Quaternary)Alluvium (Holocene, Pleistocene?, and Pleistocene)

Youngest alluvium (Holocene)Older alluvium, undivided (Holocene and Pleistocene?)

Older alluvium b (Holocene)Older alluvium a (Holocene and Pleistocene?)

Oldest alluvium, undivided (Pleistocene)

Oldest alluvium b (Pleistocene)Oldest alluvium a (Pleistocene)

Alluvial-fan deposits (Holocene)Colluvium and slope wash (Holocene)Talus (Holocene)Landslide deposits, undivided (Holocene and Pleistocene?)

Younger landslide deposit (Holocene)Older landslide deposit (Pleistocene?)

Biotite-bearing ash-flow tuff (lower Miocene?)Tuff of Pipe Organ Spring (lower Miocene, upper Oligocene)

Upper member (lower Miocene)

Unit 2Unit 1

Lower member (upper Oligocene)Mesobreccia lenses (lower Miocene)

Tuff of Clipper Gap (lower Miocene)Bates Mountain Tuff, unit D (lower Miocene and upper Oligocene)Round Rock Formation (upper Oligocene)

Eruptive megabreccia of Silver Creek

Heterolithic breccia (upper Oligocene)Rhyolite plugs (upper Oligocene)Unexposed stocks (upper Oligocene?)Shingle Pass Tuff (upper Oligocene)

Upper memberLower member

Isom-type welded ash-flow tuff formation (upper Oligocene)Megabreccia of Bull Frame Canyon (upper Oligocene)Volcaniclastic rocks of Little Table Mountain (upper Oligocene)

Tuffaceous siltstone, sandstone, and conglomerate, and tuffZeolitic tuffAsh-flow tuffTuffaceous siltstone, sandstone, and conglomerate, and tuff

Mesobreccia with abundant fragments of Paleozoic rocks (upper 

Oligocene)

Unnamed megabreccia unit (upper Oligocene)Megabreccia of Jefferson Summit (upper Oligocene)

Intrusive phase of vent megabrecciaEruptive phase of megabreccia containing mostly granitic clasts

Siltstone, sandstone, and conglomerate (lacustrine)

Eruptive phase of megabreccia with dominant schist and quartzite clasts

Tuff of Mount Jefferson (upper Oligocene)

Upper memberPrincipal member

VitrophyreTuff brecciaAltered ash-flow tuffIntensely sheared ash-flow tuff

Tuff of Antone Canyon (upper Oligocene)Megabreccia of Meadow Canyon (upper Oligocene)

Megabreccia with abundant Paleozoic rock clastsTuffaceous sandstone

Tuff of Ryecroft Canyon (upper Oligocene)Megabreccia of Jefferson Canyon (lower? Oligocene)Granodiorite of Dry Canyon (lower Oligocene)Rhyolite dikes and plug (early Oligocene)Granite of Shoshone Mountain (Late Cretaceous)

Belmont pluton

Quartz-feldspar pipesApliteFine-grained granitePorphyritic graniteSparsely porphyritic granite

Round Mountain pluton

ApliteFine-grained graniteCoarse-grained granite

Altered and iron-mineralized graniteTourmaline-mineralized granite

Toquima Formation (Middle Ordovician)

ArgilliteLimestone

Jasperized limestone (jasperoid)

DolostoneQuartzite

Zanzibar Formation (lower Middle Ordovician)

Argillite and subordinate limestoneLimestone

Calc-silicate-mineralized limestoneJasperized limestone

DolostoneSchist

Siltstone formation (Cambrian?)Mayflower Formation (Cambrian?)

Phyllitic argilliteKnotted schist

Gold Hill Formation (Lower Cambrian)

Phyllitic schistQuartziteSchist and subordinate quartziteQuartzite and subordinate schistLimestone

Calc-silicate-mineralized limestone

mf

Qu

Qa3Qa2

Qa2b

Qa2a

Qa1

Qa1b

Qa1a

Qf

Qc

Qt

Ql

Ql2Ql1

Tbi

Tpu2Tpu1

Tpl

Tpb

Tcg

Tbt

Trus

Thb

Trp

Ts

Tspu

Tspl

Ti

Tbf

Tlt4Tlt3Tlt2Tlt1

Tpm

Tum

Tjsi

Tjsg

Tjsl

Tjss

Tmj

Tmjv

Tmjb

Tmja

Tmjs

Tac

Tmc

Tmcp

Tmcs

Trc

Tjc

Tgd

Tr

Kba

Kbgf

Kbgp

Kbgs

Kra

Krgf

Krg

Krgi

Krgt

Ota

Otl

Otlj

OtdOtq

Ozal

Ozl

Ozlc

Ozlj

Ozd

Ozs

st

mf

ms

gs

gq

gsqgqs

gl

glc

Kbq

Tmju

GEOLOGIC INVESTIGATIONS SERIES

I–2670

Pamphlet accompanies map

GEOLOGIC MAP OF THE JEFFERSON QUADRANGLE, NYE COUNTY, NEVADA

By

Daniel R. Shawe

1999

Any use of trade names in this publication is fordescriptive purposes only and does not imply endorsement by the U.S. Geological SurveyThis map was produced on request, directly fromdigital files, on an electronic plotterFor sale by U.S. Geological Survey Information ServicesBox 25286, Federal Center, Denver, CO 802251-888-ASK-USGSThis map is also available as a PDF at http://greenwood.cr.usgs.gov

Geology mapped in August 1981, July and August 1982,June–August 1984, June–August 1985, and September 1995Editing and digital cartography by Alessandro J. DonatichGeology digitized by Geologic Data Systems, Inc.Cross sections drafted by Springfield and SpringfieldManuscript approved for publication November 3, 1998

11,00010,000

9,0008,0007,0006,0005,000

SECTION

I–I'

SECTION

L–L'

SECTION

M–M'

SECTION

P–P'

Slaughterhouse

Canyon

Sawmill

Canyon

SOLDIER SPRING

FAULT

Trail

Canyon

11,00010,0009,0008,0007,0006,0005,000

1,000 ft=305 m

FEET

A

FEET

A'

MOUNT JEFFERSON CALDERA

Qu Qu

Qu

Qu

Qu

Qu

Qu Qu

Qu Qu Qu

Tpl?

Tbt?

Ts?

Ts?

Ti Ti Ti Ti

Ti

Tlt4

Tlt4

Tlt4

Tlt3?

Tlt3 Tlt3

Tlt1

Tlt1

Tlt1

Tlt1?

Tlt1

Tcg?

Tcg?

Tmj

Tmj

Tmj

Tmj

Tmj

Tmj

Tmj

Tmj Tmj

Tmj

Tmj

Tmj

Tmj

Tmjv

Tmjv Tmjv

Tmjv

Tmjs

Tmjs

Tspl Tspu

Tspl?

4,000

10,000

9,0008,0007,0006,0005,000

4,00010,0009,0008,0007,0006,0005,000

SECTION

I–I'

BEND IN

SECTION

SECTION

J–J'

SECTION

L–L'

SECTION

M–M'

SECTION

P–P'

SECTION

H–H'

Wild

Horse

Canyon

Sawmill

Canyon

Horsethief

Canyon

SOLDIER SPRING

FAULT

Meadow

Creek

JEFFERSON CANYON

FAULT

1,000 ft=305 m

FEET

B

FEET MOUNT JEFFERSON CALDERA B'

?

?

Qu

Tpu1?

Tlt3

Tmj

Tmj

Tmj

Tmj

Tmj Tmj

Tmj Tmjv Tmjv Tmjv

Tmja

Tmja Trp Tmja

Tmja

Ozl

ms

Qu Qu

Qu

Qu

Qu Tspl Qu Tspl

Qu Qu

Tmjv

Tlt1?

Tlt1

gs

gs

gqgl

Tlt4

Tlt4

Tlt4

Tlt4

Tbt Tbt?

Tbt

Ti

Tspu

Tspl

Tspl

ms?

Structural margin

of Mount Jefferson

caldera

Tmj

Tlt1?

Tlt3?

Tspl?

Tbt?

Tpu2 Tpu1

Tbt Tpl Ti

Tpb Tpb Tpb

Tcg Tpu2

Tpl Tpl

Tpb?

Tlt4?

Tlt4?

Tlt4?

Tjsg

Tmj Tmj Tmj

Tmj

Tpu1

Tpu1?

Tbt Tbt

Tlt4

Tlt3

Tlt3? Tlt1 Tlt1?

Tlt1?

Tlt1? Tlt1?

Tpl

Tpu2

Tpb

Tpu1

Tspl

Tspl

Tspl

Qu

Qu

Qu

Qu

Qu Qu

Qu Qu Qu

Qu

10,000

9,0008,0007,0006,0005,000

10,0009,0008,0007,0006,0005,000

SECTION

L–L'

SECTION

M–M'

SECTION N–N'

SECTION P–P'

SOLDIER

SPRING

FAULT Meadow

Canyon

Three Summits

Hill

?

?

?

?

?

? ? ?

?

? ?

?

? ?

?

1,000 ft=305 m

MOUNT JEFFERSON CALDERA

FEET

C

FEET

C'

mf?

mf?

mf?

Tpu2

Tbt Tbt Ti Ti Tcg Tlt4 Tlt4 Tlt4 Tlt4 Tlt2

Tlt2

Tlt1?

Tlt1 Tlt1

Tum

Tjsg

Tmj

Tmj

Tmj

Tmj

Tmj

Tmj

Tlt3 Tlt1

Tlt3 Tlt3

Tspl

Tspl

Tspl Tspl

Qu Qu Qu

Qu

Qu Qu

Qu Qu Qu

Qu Qu Qu

FEET

D D'

DU

DU

DU

4,000

10,000

9,0008,0007,0006,0005,000

4,00010,0009,0008,0007,0006,0005,000

SECTION

L–L'

SECTION

M–M'

SECTION

N–N'

SECTION P–P' SOLDIER SPRING FAULT Meadow Canyon Round Meadow

Canyon

Inferred structural marginof Mount Jefferson caldera

?

?

? ? ? ?

1,000 ft=305 m

MOUNT JEFFERSON CALDERA

? ? ?

mf?

mf mf?

mf? Thb?

Trp

Ti Tlt3

Tlt2

Tlt1

Tmj

Tmj

Tac

Tmc

Tmc

Trc?

Trc?

Tspl

Trc?

Tac

Trc?

Tmc

Qu Qu Qu Qu Qu Qu Qu

4,0009,0008,0007,0006,0005,000

4,0009,0008,0007,0006,0005,000

SECTION

F–F'

SECTION

M–M'

SECTION

N–N'

SECTION P–P'

Meadow

Canyon

Round

Meadow

Canyon

Tmc source vent

1,000 ft=305 m

FEET FEET

E E'

ms

ms

ms?

mf?

mf?

mf?

gq?

gq ms

Kra

Krg

Krg

Krg

KrgfKra

KrgfKra?

Tbf Tbf Tpm

Tpm

Tlt2 Tlt2 Tlt1

Tmj

Tac

Tac

Tac

Tac Tmc

Tmc

Trc

(Trc?)

Trc?

Tac Tac

Trc?

Tlt1

Trc

Trc?

Qu Qu Qu Qu Qu Qu Qu

Qu Qu

SECTION

E–E'

SECTION

L–L'

BEND IN SECTION

SECTION M–M'

SECTION

N–N'

SECTION P –P'

Meadow

Canyon

Round

Meadow

Canyon

11,00010,0009,0008,0007,0006,0005,000

11,00010,000

9,0008,0007,0006,0005,000

?

?

?

?

1,000 ft=305 m

q

ROUND MOUNTAIN

PLUTON

FEET

F

FEET

F'

A T

?

?

Kba?

Kbgf

KbgfKba?

Kbgf-Kba Kbgf-Kba

Kbgp?

Kbgp

Kbgp

Kbgp

Kbgp

Kbgs?

Kra

Kra

Krgf Krgf

Krgf?

Krg Krg

Krg

Krg

Ozal

Ozs

Ozs

Ozl Ozl Ozlc Ozlc

Ozlc

Ozlc

st

ms

ms?

Qu

Qu Qu

Qu

Qu Qu

Qu Qu Qu Qu

Qu

gs

gq

?

?

?

?

Krgf?

Krgf

Krgf

Krgf?

Kra

gq?

gq?

?

KbgfKba

KbgfKba

Kbgp?

c

11,00010,000

9,0008,0007,0006,0005,000

11,00010,0009,0008,0007,0006,0005,000

SECTION

O–O'

SECTION

N–N'

SECTION

K–K'

SECTION

M–M'

SECTION

L–L'

SECTION

J–J'

Shoshone

Canyon

Shoshone

Mountain

Antone

Canyon

1,000 ft=305 m

FEET FEET

G G'

ROUND MOUNTAIN PLUTON

BELMONT PLUTON

ms

ms

gs?

gq gs Ozl Ozlj

Krgi

Krgt? Krgt?

Krgt

Kra?

Krgi

Krgt

Trp

Tmj

Tmjb Tmjv

Tmja

Tmja

Tmja?

Tmja?

Tgd?

Tmja

Tmj?

Qu Qu Qu

FEET

H

FEET

H'

SECTION

B–B'

4,0009,0008,0007,0006,0005,000

4,0009,0008,0007,0006,0005,000

Davenport

Canyon

Berlin

Canyon Jefferson

Canyon

Structural margin

of Mount Jefferson

caldera

Wild Horse

Canyon

1,000 ft=305 m

f

ROUND MOUNTAIN PLUTON MOUNT JEFFERSON CALDERA

Tmj

Tmj

?

FEET I'

11,00010,0009,0008,0007,0006,0005,000

Jefferson

Canyon

JEFFERSON CANYON

FAULT

Mogul

Canyon

Grassy

Canyon

Structural margin ofMount Jefferson caldera

Horsethief

Canyon

Shoshone

Canyon

Spanish

Peak

(east spur)

Shoshone

Mountain

(northwest

spur)

SECTION

B–B'

SECTION

BEND IN

SECTION

G–G' 11,000

10,000

9,0008,0007,0006,0005,000

11,00010,0009,0008,0007,0006,0005,000

1,000 ft=305 m

FEET

J

FEET

J'

ROUND MOUNTAIN PLUTON

MOUNT JEFFERSON CALDERA

BELMONT PLUTON

Qu

Qu

Qu

Qu

Qu

Qu

Qu

Qu Qu Qu

Qu

Qu

Qu

Qu Qu

Kbq

Kbq Kbgf- Kba

Kbgp

Ozl ms

Kba

Kba

Kbgs

Kbgs

Krgf

Krgf

Krgf?

Krgf

Krg

Krg

Krg

Krg

Krgi Krgi?

Krgt?

Krgt?

Tjsi?

Tgd

Tgd

Tjsi

Tjsg Tmjv

Tmja

Tmja?

Tmjv

Tmj

Tmj

Tmj

Tmj

ms?

Krg

Krgf?

Krgi

? ?

Krgi

Tjsi?

Krg

Tmj

SECTION

N–N'

SECTION

P –P' SECTION

E–E'

SECTION

G–G'

SECTION

L–L'

SECTION

M–M' Snowbird Canyon

4,000

10,000

9,0008,0007,0006,0005,000

4,00010,0009,0008,0007,0006,0005,000

Meadow

Canyon

Ryecroft

Canyon

1,000 ft=305 m

FEET

K

FEET

K'

A T

BELMONT PLUTON

MEADOW CANYON

FAULT

Qu

Qu

Qu

Qu Qu Qu

Qu

KbgfKba

Kbgp

Kbgp

Kbgs

Kbgs

Ozl

Ozl

Ozl

Ozd

Ozlc

Ozs

Ozal?

mf Ozal

ms ms

ms

ms

mf?

Thb?

Thb

Trp

Tmc

Tmc

gqs

Ozl

Ozal Ozl

Ozl Ozl Ozl Ozal

KbgfKba

Ozal?

Ozal?

Ozl?

Tmc

mf

gs?

Tjsg Tjsg Tjsg Tjsg 

Tjsg Tjss Tjsl 

Tmju?

Tmj

Tmj

Tmj

Tmj Tmj

Tmju

Tmjv

Tmjv

Tmj?

Qu

Qu

FEET

L'

SECTION

D –D'

SECTION

C –C'

BEND IN

SECTION

SECTION

BEND IN

SECTION

B –B'

SECTION

A –A'

10,0009,00012,00011,0008,0007,0006,0005,000

Jefferson

Summit

Mount

Jefferson

?

Inferred structural margin

of Mount Jefferson caldera

MOUNT JEFFERSON CALDERA

st

ms

ms

ms?

ms?

mf?

mf?

gs

gs

gs

gq gl

mf?

Ozal

Ozl

Ozl

Ozl Ozl Ozl

Ozl

Ozlc

Ozlc

Ozl

Kbgf?

Kbgf

Kbgf

Kbgp

Kbgp

Kbgs

Tbi Tpu2 Tpu2

Tpl

Ti

Ti

Ti

Ti

Ti

Ti

Ti

Tpb

Tpb Tcg?

Tpm Tbf

Tac?

Tac?

Tbt

Tbt

Tlt4

Tlt2 Tlt2 Tlt1

Tlt1 Tlt1?

Tlt1?

Tlt1? Tlt1?

Tmj?

Tmj Tmj

Tmj

Tmj

Tmj Tmj

Tlt4

Tlt4 Tlt4

Tlt4

Tlt1? Tlt3?

Tlt3?

Tlt3?

Tlt3?

Tlt4?

Tpu1 Tpu1 Tspu

Tspu

Tspu Tspu

Ti

Tspl

Tspl Tspl

Tspl Tspl

Tmj

Qu

Qu Qu Qu Tlt3

Qu

Qu Qu Qu

Qu

Qu

M

FEET

M'

BEND IN SECTION

SECTION

K –K'

SECTION

G –G'

SECTION

E –E'

SECTION

D –D'

SECTION

C –C'

SECTION

B –B'

SECTION

P –P'

SECTION

A –A'

BEND IN

SECTION 10,000

9,0004,0008,0007,0006,0005,000

10,0009,0004,0008,0007,0006,0005,000

Antone

Canyon

Bull

Frame

Canyon Meadow

Canyon

Three Summits Hill

Pipe Organ

Spring

Corcoran

Canyon

graben

1,000 ft=305 m

SECTION F –F'

Inferred structural marginof Mount Jefferson caldera

BELMONT PLUTON

MOUNT JEFFERSON CALDERA

Tpu1

Tbt

Tpl

Ti

Tbt

Tlt4

Tlt4

Tlt3

Tlt3

Tlt1

Tlt1

Tlt1

Tmj Tmj

Tmj

Ti Tspl

Tspl Tspl Tspl

Tspl

Tspu Tspu 

Qu

Qu

Qu

SECTION

C –C'

SECTION

P –P'

10,0009,0008,0007,0006,0005,000

Little Table

Mountain

(west spur)

?

?

?

?

?

?

? ? 

ed structural margin

unt Jefferson caldera

MOUNT JEFFERSON CALDERA

FEET

N'

SECTION

P –P'

SECTION

G –G' 10,000

9,0008,0007,0006,0005,000

10,0009,0008,0007,0006,0005,000

Hillen

Canyon Snowbird

Canyon

Toll

Canyon

1,000 ft=305 m

FEET

O

FEET

O'

BELMONT

PLUTON

Qu Trc

Qu

Qu Tmc

Qu

KbgfKba

Kbgp

Ozl

Ozlc

ms

ms

ms

Ota

Ota

Ota

Otl

Otl

Otl Otl

Otlj

Ozal

Ozl

Ozl

Ozlc Ozl Ozd

mf

Otq

Otq

Otq

Ozal

KbgfKba

Ozs

Ozs

Ozl?

ms?

mf?

mf

mf mf

?

mf

mf

mf?

? ? ?

?

?

?

?

Tbt

Tbt

Tbt

Tbt Tbt

Thb

Thb

Thb? Trp

Trp?

Ti

Tlt4

Tlt4

Tlt4

Tlt3

Tlt3

Tlt3

Tlt1

Tlt1

Tlt1

Tmc

Tmc

Tmc

Tmc

Tmc?

Tum Tum

Tmj

Tmj

Tmj

Tmj

Tmj

Tmj

Tmj

Trc

Trc

Trc?

Tlt1 Tlt1

Tpu1 Tpu1 Tpl

Thb?

Tmcs

Tmc

Tbt

Tmc

Tmc?

Tspu Tspl

Tspl Tspl Tspl

Tlt1? Tmj

Qu

Qu

Qu

Qu

Qu

Qu

Qu

Qu

Qu

Qu

Qu Qu Qu Qu Qu

Qu

SECTION

O –O'

SECTION K –K'

SECTION E –E'

SECTION

F –F'

SECTION

D –D'

SECTION

C –C'

SECTION

N –N'

BEND IN

SECTION

SECTION B –B'

BEND IN

SECTION

SECTION

A –A'

SECTION

M –M'

A T

10,000

9,0008,0007,0006,0005,000

10,0009,0008,0007,0006,0005,000

Toll

Canyon

Ryecroft

Canyon Meadow

Canyon

Assumed source

area of Tmc before

emplacement of Trp

Inferred structural margin of

Mount Jefferson caldera

Little Table

Mountain

(west spur)

Corcoran Canyon graben Andrews

Creek

FEET

P

FEET

P'

1,000 ft=305 m

MOUNT JEFFERSON CALDERA

Contact—Dashed where approximately located or gradational; dotted where concealed; 

queried where uncertain. Arrow shows dip

Internal contact within map unit—Generally shows bedding plane on map; shows form 

line in cross section. Dashed and (or) queried where uncertain

High-angle fault—Dashed where approximately located; dotted where concealed. Bar and 

ball on downthrown side where relative movement is known. Opposed arrows show strike-slip movement; opposed arrows on fault symbol show horizontal grooves. Arrow shows dip of fault plane; where number is missing, dip could not be measured; cross indicates vertical dip. On cross section only, 

A indicates displacement away, 

T indicates 

displacement toward; 

U indicates displacement up; 

D indicates displacement down

Thrust fault—Dashed where approximately located; dotted where concealed; queried where 

uncertain. Arrow shows dip. Teeth on upper plate

Overturned thrust fault—Dotted where concealed. Teeth point to overturned lower plateCaldera structural margin—Approximately located. Coincides with Jefferson Canyon fault 

in northwest part of quadrangle

Margin of megabreccia pipe—Dotted where concealed. Hachures inside pipe. Arrow 

shows dip of pipe margin

Anticline—Showing trace of axial planeSyncline—Showing trace of axial planeDrag-fold axis—Showing trend and plungeStrike and dip of layered rocks—Structural complexity prevents determination of original 

orientation of most layers

InclinedInclined—Showing horizontal lineationsVerticalOverturned—Original orientation knownOverturned—Original orientation inferred

Strike and dip of foliation—Schistosity in metasedimentary rocks; foliation in granite; flow 

layering in rhyolite plug; compaction foliation in ash-flow tuff. Arrow shows trend and plunge of lineation or minor fold axis

Inclined—Showing horizontal lineations, or plunge of lineations. 

s indicates attitude of 

schlieren

VerticalShear foliation in granite and ash-flow tuff (Tmjs)Vertical shear foliation in graniteHorizontal compaction foliation in ash-flow tuff

Foliation in granite—In cross sections only. Density of lines suggests relative intensity of 

foliation

Cleavage

Inclined—Showing trend and plunge of lineationsVertical—Showing plunge of lineations

Vein—Arrow shows dip; cross indicates vertical vein. q, quartz vein (h indicates heubnerite); 

c, calcite vein; f, fissure vein

Brecciated rockMylonite layer—Uncertain projection based on geometry of mylonite layer exposed to the 

south in adjacent Belmont West quadrangle. Open teeth on upper side of mylonite layer

Large block in megabreccia—Colored as megabreccia; label in parentheses indicates 

specific map unit where recognized. In unit Tac, parentheses around Trc?, Ol, Ols, 

Os, gq, and 

mf indicate large isolated blocks in ash-flow tuff

Amazonite localityQuartz-feldspar pipe localityRadiometrically dated rock sample localityLocation of rock sample with modal analysis—Number indicates category assignment. 

Position may coincide with horizontal foliation symbol to yield 

70

60

10

70

25

10

2070

60

40

35 55

35

s

75

80

70

25

q

?

A

Kbq

R1

4

(Trc?)

117°07'30" 117°00' 116°52'30" 116°45'

38°30'

38°37'30"

38°45'

INDEX MAP

ROUND

MOUNTAIN

GQ–1756

BELMONT

WEST

GQ–1801

BELMONT

EAST

I–2675

CORCORAN

CANYON

I–2680

JEFFERSON

I–2670

MANHATTAN

GQ–1775
Geologic map of the Jefferson quadrangle, 

Nye County, Nevada

By Daniel R. Shawe

Geologic Investigations Series I–2670

1999

Ruin at Jefferson site: a miner's cabin, relic of early mining in Jefferson Canyon. Walls of the structure are of 

blocks and slabs of tuff of Mount Jefferson. Photograph by the author, 1985.

U.S. Department of the Interior

U.S. Geological Survey
1

DESCRIPTION OF MAP UNITS

[Phenocryst contents of most of the ash-flow tuffs and a few other volcanic rocks described here were

determined by modal analyses of thin sections by F.M. Byers, Jr. (U.S. Geological Survey). Between

2,000 and 4,000 points per thin section of crystal-rich rocks (20 percent or more phenocrysts)

generally were counted; points per thin section counted ranged to more than 10,000 points for some

crystal-poor rocks. Phenocryst contents are given as volume percent of total rock, and phenocryst

minerals are given as volume percent of total phenocrysts; Q, quartz; K, alkali feldspar (generally sanidine); P, plagioclase; B, biotite; H, hornblende; C, clinopyroxene; O, opaque-oxide minerals; M, unspecified mafic minerals. Other specific mineral names are spelled. Accessory minerals occur in trace

amounts; their relative amounts are described as common or moderate, sparse, and rare. In some exposures of the volcanic rocks described here, hydrothermal alteration resulted in partial or complete replacement of feldspars and mafic minerals by secondary minerals. Chemical analyses (major oxides determined by X-ray fluorescence; FeO by potentiometric titration; CO2 by coulometric titration) are recalculated on a volatile-free basis; analyses by D.F. Siems, J.S. Mee, and J.E. Taggart, Jr. (U.S.

Geological Survey), and by Actlabs, Inc., Wheat Ridge, Colo. Volcanic rock names are based on the

modal data showing phenocryst content; they are mostly the same as the field terms used during mapping. Rock names based on the IUGS chemical classification (Le Bas and others, 1986) are shown in

parentheses following the listed chemical data. Cited K-Ar dates that were determined before 1977

have been converted using new constants given in table 2 of Dalrymple (1979). Queries on map-unit

symbols indicate uncertain assignment]

mf Manmade fill (modern)—Mostly mine dumps and mill tailings

Qu Surficial alluvial deposits, undivided (Quaternary)—Shown only on cross sections

Alluvium (Holocene, Pleistocene?, and Pleistocene)—Sand, silt, and gravel.

Subdivisions of alluvium described below are based on relative age in local areas

mostly determined by relative height above current stream levels. I was unable to

correlate specific units by age everywhere in the quadrangle; thus sediment mapped

as one unit (for example, Qa3) may be the same age as sediment mapped elsewhere

as a different unit (for example, Qa2b). Studies of soil characteristics that are a

function of deposit age were not attempted here. Qa3, Qa2, and Qa1 are not subdivided on cross sections (shown as Qu).

Colors of all alluvium units described here are varied, ranging from yellowish

brown, light yellowish brown and light brown, to light brownish gray and light

gray. All alluvium units are unconsolidated unless otherwise noted. Poorly to moderately sorted and comprised of poorly rounded to well-rounded clasts

Qa3 Youngest alluvium (Holocene)—Active alluvium in drainage channels. Characterized

by braided strands of recently deposited gravel, sand, and silt. Locally grades into

alluvial fan deposits (Qf). Maximum thickness a few meters

Qa2 Older alluvium, undivided (Holocene and Pleistocene?)—Stabilized alluvium in

stream channels, stream terraces, and flat valley-fill alluvial fans topographically

higher (a few meters) than youngest alluvium (Qa3); map unit commonly is being

eroded. Clasts generally are only slightly weathered and commonly are moderately

sorted. Some layers contain pebble-size or smaller clasts whereas other layers

contain clasts of cobble or boulder size. Maximum thickness about 10 m

Qa2b Older alluvium b (Holocene)—Forms narrow to broad stream courses of mostly

inactive alluvium, or remnant patches of inactive alluvium marginal to and standing 1

m to a few meters higher than Qa3

Qa2a Older alluvium a (Holocene and Pleistocene?)—Extensively eroded deposits laced

with strands of active alluvium too small to map; older parts may be as old as

Pleistocene

Qa1 Oldest alluvium, undivided (Pleistocene)—Stabilized alluvium that underlies stream

terraces higher than older alluvium (Qa2) and commonly 20–40 m above adjacent

drainage bottoms; locally contains boulders. Large clasts (cobbles and small boulders), especially of volcanic rocks, may be significantly weathered. Presently being

eroded. Maximum thickness about 35 m underlying flats northwest of Pipe Organ

Spring; oldest alluvium possibly in part resting on a pedimented surface
2

Qa1b Oldest alluvium b (Pleistocene)—Forms small erosional remnants within a few

hundred meters of drainage courses

Qa1a Oldest alluvium a (Pleistocene)—Alluvial deposits standing several meters higher

than Qa1b; most extensive northwest of Pipe Organ Spring

Qf Alluvial-fan deposits (Holocene)—Active fans commonly formed at foot of short

drainages that enter canyon bottoms. Locally grade into youngest alluvium (Qa3).

Maximum thickness a few meters

Qc Colluvium and slope wash (Holocene)—Gently sloping accumulations of disintegrated

rock nearly in place, or deposited by slope wash. Locally grades upslope into talus

(Qt) or downslope into alluvium (Qa3 or Qa2b). Maximum thickness a few meters

Qt Talus (Holocene)—Steeply sloping accumulations of angular rock, deposited below cliffs

and steep slopes; mapped where accumulations are well defined or where they obscure geologic contacts. Contains blocks several tens of meters long beneath a

cliff of rhyolite (Trp) on north side of Meadow Canyon at east margin of the quadrangle. Locally grades downslope into colluvium and slope wash (Qc). Maximum

thickness a few tens of meters

Ql Landslide deposits, undivided (Holocene and Pleistocene?)—Generally heterogeneous

mixture of rock fragments and soil derived by slope failure from nearby higher

bedrock and surficial materials. Formed mostly of nonwelded and poorly consolidated tuffs and tuffaceous sedimentary rocks in northeast part of the quadrangle.

In places derived almost wholly from single bedrock units (such units indicated in

parentheses). Maximum thickness several meters

Ql2 Younger landslide deposit (Holocene)—Rejuvenated landslide on west slope of hill

about 3 km east of Meadow Canyon Guard Station; upper margin marked by a

scarp in higher and older landslide deposit (Ql1)

Ql1 Older landslide deposit (Pleistocene?)—Remnant of older landslide deposit near top

of hill above younger landslide deposit (Ql2)

Tbi Biotite-bearing ash-flow tuff (lower Miocene?)—Light-gray, crystal-poor, partially to

moderately welded rhyodacitic ash-flow tuff. Contains flattened pumice lapilli as

long as about 2 cm. Phenocrysts (1–2 mm in diameter) comprise 10 percent each

of two samples of tuff, and consist of: Q, less than 1 each; K, 14 and 17; P, 72

and 69; B, 10 and 9; O, 2 and 3; and H plus C, 1–2. Zircon and apatite are accessory minerals. Chemical analyses of the same two samples show the following

percentages of components: SiO2 , 73.0 and 73.2; Al2O3, 14.4 and 14.2; K2 O,

4.56 and 4.62; Na2O, 4.41 each; CaO, 1.03 and 1.00; Fe2O3, 1.45 and 1.26; FeO,

0.36 and 0.49; MgO, 0.37 and 0.33; Ti02, 0.30 and 0.31; P2O5, 0.10 and 0.09;

and MnO, 0.08 and 0.09 (rhyolite). Map unit caps summit of Three Summits Hill;

slightly unconformable on (appears to channel into) underlying tuff of Pipe Organ

Spring (Tpu2), as an erosional remnant no more than about 15 m thick

Tuff of Pipe Organ Spring (lower Miocene, upper Oligocene)—Nonwelded to densely

welded, crystal-poor rhyolitic ash-flow tuffs. An upper member of two mappable

units (Tpu2 and Tpu1) overlies a lower member (Tpl), each considered to be a separate ash flow or cooling unit. Units of upper member as well as lower member each

pinch out locally within the quadrangle. A few thin lenses of mesobreccia occur

within or between the various units. Mesobreccia is here considered to contain

clasts no more than about 1–2 mm in diameter. Source of tuff of Pipe Organ

Spring is unknown. Lower member (Tpl) appears to be equivalent to Boden’s

(1986; 1992) member 2 of tuffs and sedimentary rocks of Road Canyon; Boden referred to his member 2 as “tuff of Pipe Organ Spring.” The name is derived from

the vertically fluted cliff, which consists of lower member (Tpl), just south of Pipe

Organ Spring. Interlayered in tuff of Pipe Organ Spring are two units that are correlated to widely exposed dated units in central and east-central Nevada: tuff of

Clipper Gap (Tcg) and unit D of Bates Mountain Tuff (Tbt)

Upper member (lower Miocene)

Tpu2 Unit 2—Upper part is light-gray to light-brownish-gray, moderately to densely

welded, crystal-poor rhyolitic ash-flow tuff; weathers light brown. Contains locally

abundant dark-gray flattened pumice lapilli as much as 1 cm long. Weathered rock

has a mottled appearance. Modal analyses of six thin sections of upper part of unit
3

2 indicate 4–9 percent phenocrysts (1–3 mm in diameter) that consist of the following phases (in percent): Q, 15–33; K, 43–76; P, 3–30; B, 0–3.5; O, 0.2–2; and H

and C, 0–3. Zircon and rare apatite are accessory minerals. A chemical analysis of

a sample of upper part of unit 2 indicates (in percent): SiO2 , 76.0; Al2 O3, 13.1;

K2O, 4.75; Na2O, 3.95; CaO, 0.61; Fe2O3, 1.04; FeO, 0.16; MgO, 0.13; TiO2 ,

0.12; P2O5, 0.08; and MnO, 0.05 (rhyolite). Upper part of unit 2 is conformable

upon lower part of unit 2. Occurs on or near ridge and hill tops east, north, and

west of Meadow Canyon Guard Station, where its maximum thickness is about 35

m.

Lower part is light-grayish-brown to light-yellowish-brown, poorly to moderately welded, crystal-poor rhyolitic ash-flow tuff. Weathers to rubbly rock debris.

Lower part of unit 2 is characterized by light-gray, flattened pumice lapilli (2–4 cm

long); locally contains abundant lithic fragments 1 cm or less long. Three modal

analyses indicate 6–8 percent phenocrysts (1–2.5 mm in diameter) composed of:

Q, 16–22; K, 48 each; P, 25–30; B, 3–5; O, 0.3–2; and M, 0–2. Zircon and rare

allanite and apatite are accessory minerals. Appears conformable on underlying unit

1 (Tpu1) and on tuff of Clipper Gap (Tcg). Lower part of unit 2 occurs near ridge

and hill tops east, north, and west of Meadow Canyon Guard Station; 25–75 m

thick

Tpu1 Unit 1—Light-yellowish-brown to light-yellowish-gray, poorly to moderately

welded, crystal-poor rhyolitic ash-flow tuff. Tuff contains abundant black glass

shards and small (about 1 cm long) dark-gray glassy fiamme, and small (about 1–2

cm long) lithic and light-colored pumice fragments in places. Modal analyses of six

samples of unit 1 indicate 3–11 percent phenocrysts (1–3 mm in diameter) that

consist of: Q, 18–30; K, 49–55; P, 16–28; B, trace–3.5; O, trace–3.5; and M, 0–2.

Minor anorthoclase is evident in some thin sections. Zircon and rare allanite and

apatite are accessory minerals. Two chemical analyses show: SiO2 , 74.6 and

75.5; Al2O3, 14.0 and 13.8; K2O, 5.67 and 4.88; Na2O, 2.66 and 3.56; CaO, 0.91

and 0.60; Fe2O3, 1.23 and 1.06; FeO, 0.32 and 0.23; MgO, 0.31 and 0.12; TiO2,

0.15 and 0.13; P2O5, 0.08 and 0.07; and MnO, 0.05 and 0.06 (rhyolite). Appears

conformable on underlying lower member (Tpl) and on Bates Mountain Tuff (Tbt).

Widely exposed on lower slopes of Three Summits Hill. Thickness 0–100 m

Tpl Lower member (upper Oligocene)—Light-yellowish-brown, nonwelded to poorly

welded, moderately crystal-rich rhyolitic ash-flow tuff. Contains abundant slightly

flattened pale-yellowish-brown pumice fragments mostly less than 1 cm in size in a

groundmass of uncompacted glass shards. Modal analyses of five samples of lower

member show 10–21 percent phenocrysts (1–3 mm in diameter) that consist of:

Q, 5–25; K, 17–40; P, 30–46; B, 5–13; O, 1–3; and M, 0–4. Zircon and minor allanite and apatite are accessory minerals. A chemical analysis indicates: SiO2 ,

74.3; Al2O3, 14.3; K2O, 5.40; Na2O, 2.34; CaO, 1.42; Fe2O3, 1.17; FeO, 0.23;

MgO, 0.46; TiO2, 0.21; P2O5, 0.10; and MnO, 0.06 (rhyolite). L.W. Snee (written

commun., 1997) reported sanidine 40Ar/39Ar dates of 25.37±0.05 Ma (rock

sample locality R1) and 25.43±0.05 Ma (locality R2) for samples of lower member

collected south and southeast of Pipe Organ Spring. Tuff forms impressive fluted

cliffs near Pipe Organ Spring on both north and east slopes of Three Summits Hill.

Map unit pinches out southwestward on Three Summits Hill, but reappears farther

west on lower slopes just west of Meadow Creek. Tuff is disconformable on

underlying upper member of Shingle Pass Tuff (Tspu), as indicated by local absence

of Tspu below lower member. Thickness 0–95 m

Tpb Mesobreccia lenses (lower Miocene)—Light-yellowish-brown ash-flow tuff containing

abundant rounded fragments of welded tuff that consist mostly of Bates Mountain

Tuff (Tbt) and possibly also other older welded tuffs, as large as large-boulder size (1

m). Map unit occurs as lenses at various horizons within unit 2 (Tpu2) and unit 1

(Tpu1) of upper member of tuff of Pipe Organ Spring. Mesobreccia lenses in Tpu2

contain fragments that may be from tuff of Clipper Gap (Tcg). Thickness of unit

ranges from 0 m to a few meters. Smaller lenses were not mapped

Tcg Tuff of Clipper Gap (lower Miocene)—Pale-lavender-gray, partially to moderately welded,

crystal-poor rhyolitic ash-flow tuff; weathers light pinkish gray. In places contains
4

abundant greatly flattened pale-gray pumice lapilli as long as 5 cm. Modal analyses

of three thin sections indicate 4–6 percent phenocrysts (1–2.5 mm in diameter)

consisting of the following phases: Q, 31–49; K, 37–56; P, 12–25; B, 0–0.4; O,

0.4–1; and M, 0–1. Zircon is an accessory mineral. A chemical analysis of a sample

of tuff of Clipper Gap indicates: SiO2, 76.3; Al2O3, 12.8; K2O, 4.92; Na2O, 3.57;

CaO, 0.64; Fe2O3, 1.20; FeO, 0.09; MgO, 0.12; TiO2 , 0.11; P2 O5, 0.09; and

MnO, 0.10 (rhyolite). A sanidine K-Ar date for the Clipper Gap in the northern

Toquima Range is 22.8±2–3 percent Ma (McKee, 1976); another sanidine K-Ar

date for the unit near Eureka 100 km northeast of the quadrangle is 22.7±0.6 Ma

(Grommé and others, 1972). Unit forms relatively thin isolated lenses (0–30 m

thick) interlayered in tuff of Pipe Organ Spring on south slope of Three Summits

Hill, and farther west on slope west of Meadow Creek; small exposures west of

Windy Pass near north margin of map area are possibly of tuff of Clipper Gap.

Tuff of Clipper Gap in the Jefferson quadrangle is correlated with the unit

elsewhere on the basis of stratigraphic position, phenocryst content, and lithologic

character (Page and Dixon, 1994). It occurs in the quadrangle as erosional remnants, sandwiched between overlying unit 2 of upper member of tuff of Pipe Organ

Spring (Tpu2) and underlying rocks. These underlying rocks are unit 1 of upper

member (Tpu1) and lower member (Tpl) of tuff of Pipe Organ Spring, and unit D of

Bates Mountain Tuff (Tbt). The tuff was deposited during a hiatus in emplacement

of tuff of Pipe Organ Spring

Tbt Bates Mountain Tuff, unit D (lower Miocene and upper Oligocene)—Pinkish-gray to

lavender-gray to gray, moderately to densely welded, crystal-poor, somewhat peralkaline (high potassium-feldspar content) rhyolitic ash-flow tuff; weathers light

orangish brown to reddish brown. Characterized by greatly flattened gray to

brown pumice lapilli (1–3 cm) that impart a strong foliation to the rock. Weathered

surfaces commonly contain numerous holes 1–5 cm long, giving the rock a “swiss

cheese” appearance (for example, McKee, 1976, fig. 34). Modal analyses of six thin

sections of samples of unit D indicate 2–4 percent phenocrysts (0.5–2 mm in diameter) composed of the following phases: Q, 0–12; K (both anorthoclase and sanidine), 72–89; P, 0–11; B, trace; O, 1–5; C, 1–9; and H, trace. Zircon and rare

allanite and apatite are accessory minerals. Chemical analyses of two samples of

unit D indicate the following compositions: SiO2, 76.2 and 74.2; Al2O3, 12.0 and

13.5; K2O, 4.63 and 5.23; Na2O, 3.65 and 4.24; CaO, 0.78 and 0.33; Fe2 O3,

1.99 and 2.02; FeO, 0.01(?) and 0.22; MgO, 0.29 and 0.05(?); TiO2, 0.22 each;

P2O5, 0.10 and 0.08; and MnO, 0.08 and 0.04 (rhyolite). Unit D of Bates

Mountain Tuff extends around west and south slopes of Three Summits Hill, and

occurs as scattered patches east, north, and west of hill. According to Grommé

and others (1972), unit D has a sanidine K-Ar age of 23.7±0.6 Ma (MioceneOligocene boundary). Unit forms relatively thin lenses 0–30 m thick. The tuff was

deposited during an erosional hiatus that followed deposition of lower and upper

members of Shingle Pass Tuff (Tspl and Tspu) and lower member of tuff of Pipe

Organ Spring (Tpl), and before depositon of tuff of Clipper Gap (Tcg)

Round Rock Formation (upper Oligocene)—The Round Rock Formation consists chiefly

of caldera fill of the Manhattan caldera and it is widely exposed in adjacent Round

Mountain quadrangle (Shawe, 1995), Manhattan quadrangle (Shawe, 1999), and

Belmont West quadrangle (Shawe, 1998). The formation is largely absent from the

Jefferson quadrangle. Only eruptive megabreccia of Silver Creek (Trus), part of

upper member of Round Rock Formation, is present

Trus Eruptive megabreccia of Silver Creek—See Shawe (1995) for a more complete description. In the Jefferson quadrangle megabreccia consists of granite boulders (as

much as 1 m in size) in tuff matrix. Unit occurs on a low ridge in southwest corner

of the quadrangle where it is only a few meters thick

Thb Heterolithic breccia (upper Oligocene)—Breccia of mixed rock types including rhyolite

(Trp), tuff of Ryecroft Canyon (Trc), pumice, and several varieties of Paleozoic

rocks, in a pulverized matrix of the several rock types and tuff matrix of megabreccia

of Meadow Canyon (Tmc). Breccia fragments are as much as about 1 m in size.

Unit forms a shell that surrounds, and extends outward as much as 200 m from,
5

two closely spaced plugs (Trp) in Meadow Canyon at east margin of the quadrangle. Breccia is related to emplacement of the plugs: within a few meters of plugs,

slablike breccia fragments are crudely oriented parallel to nearly vertical walls of

plugs, and in places blocks form vertical trains, suggesting laminar flow of material

resulting from drag that accompanied emplacement of plugs. A rind of comminuted

rock a few centimeters thick is plastered against plug walls. As seen in thin section, rind consists of crudely aligned, mostly sharply angular fragments of volcanic

and Paleozoic rocks mostly less than 2 mm long in a fine-grained glassy(?) matrix

Trp Rhyolite plugs (upper Oligocene)—Unit consists of four rhyolite plugs. One plug occurs

about 0.5 km from mouth of Wild Horse Canyon. Plug is light-gray, flow-layered,

nearly aphanitic rhyolite that intrudes hydrothermally altered tuff of Mount

Jefferson (Tmja); weathers light yellowish brown. Surface dimensions are about

0.35 km × 0.7 km. Locally near margin of plug, rhyolite is strongly brecciated, and

rock is extensively hydrothermally altered (silicified and iron mineralized).

Phenocrysts (1 mm and smaller) constitute about 1 percent of rock and consist of

mostly quartz and lesser amounts of plagioclase and sanidine. Groundmass is devitrified and in places mostly replaced by silica; vugs and fractures in brecciated

rhyolite are lined with quartz and chalcedony, and commonly contain oxidized pyrite

crystals.

Three plugs intrude megabreccia of Meadow Canyon (Tmc) in Meadow

Canyon. These plugs are light-yellowish-brown to light-orangish-brown, flow-layered, moderately crystal-rich rhyolite; weather brown. Two plugs (each somewhat

more than 100 m in diameter) are at east margin of the quadrangle, and a third plug

(300–400 m in diameter) is about 1 km farther up canyon. A modal analysis of a

sample of rhyolite indicates 16 percent phenocrysts (3–5 mm in diameter) that

consist of: Q, 43; K, 34; P, 19; B, 3.5; and O, 0.4. Sparse zircon and allanite constitute accessory minerals. A chemical analysis of the same sample shows the following percentages of components: SiO2 , 73.9; Al2O3, 14.2; K2O, 4.85; Na2O,

3.59; CaO, 1.45; Fe2O3, 0.88; FeO, 0.27; MgO, 0.37; TiO2, 0.28; P2 O5, 0.13;

and MnO, 0.03 (rhyolite). Biotite K-Ar date of plug exposed 1 km west of east

margin of the quadrangle is 26.4±0.8 Ma (McKee and John, 1987)

Ts Unexposed stocks (upper Oligocene?)—Subsurface stocks inferred to underlie intensely

sheared tuff of Mount Jefferson (Tmjs). Inferred stocks may reflect emplacement of

resurgent magmas within Mount Jefferson caldera that caused a pistonlike upward

movement of overlying tuff, and partial arching of the core of the caldera. Shown

only on cross section A–A'

Shingle Pass Tuff (upper Oligocene)—Consists of two distinctive members, an upper

member (Tspu) and a lower member (Tspl), that underlie Bates Mountain Tuff (Tbt)

and lower member of tuff of Pipe Organ Spring (Tpl). Members appear to be conformable below younger ash-flow tuffs; however, the two members are themselves

separated by an erosional unconformity. Shingle Pass members in the Jefferson

quadrangle are correlated with their equivalents 60–200 km farther east in Nevada

(Page and Dixon, 1994) on the basis of stratigraphic position, phenocryst contents, and lithologic characteristics

Tspu Upper member—Light-gray to light-brownish-gray, nonwelded to moderately welded,

moderately crystal-rich latitic ash-flow tuff; weathers light brown to light reddish

brown. Contains sparse moderately compacted light-yellowish-brown pumice lapilli

(as much as 2 cm long). Groundmass is characterized by minute (about 1 mm in diameter) devitrification(?) crenulations. Modal analyses of thin sections of five samples of upper member indicate 8–14 percent phenocrysts (1–4 mm) that consist of:

Q, 0–1; K, 33–42; P, 52–57; B, 3–9; O, 1.5–3, and H and C, 0.5–2. A modal analysis of one thin section indicates: Q, 2; K (sanidine), 42; anorthoclase, 9; P, 38; B,

8;, O, 0.4; and H, 0.4. Zircon and rare apatite and allanite are accessory minerals.

Chemical analyses of two samples of upper member indicate the following percentages of components: SiO2 , 76.0 and 78.1; Al2O3, 12.9 and 11.4; K2O, 5.02 and

4.76; Na2O, 3.18 and 2.48; CaO, 0.96 and 1.13; Fe2O3, 1.14 and 1.28; FeO, 0.22

and 0.09; MgO, 0.23 and 0.31; TiO2, 0.21 and 0.19; P2O5, 0.08 and 0.10; and

MnO, 0.04 and 0.06 (rhyolite). A 40Ar/39Ar sanidine date for upper member is
6

26.00±0.03 Ma (Best and others, 1989). Upper member of Shingle Pass Tuff is

exposed in relatively small patches east of Three Summits Hill and north of Pipe

Organ Spring. It lies unconformably on underlying lower member of Shingle Pass

(Tspl). North of Pipe Organ Spring, upper member crops out in a narrow, sinuous

band that extends for about 2.5 km northeastward upon underlying lower member

and fills an old stream channel cut as much as 15 m into lower member. Elsewhere

erosional remnants of upper member are as much as 15 m thick

Tspl Lower member—Light-gray, moderately crystal-rich, partially to densely welded

quartz latitic ash-flow tuff; weathers light yellowish brown to light yellowish gray.

Characterized throughout by black glass shards. Contains numerous nearly white

moderately compacted pumice lapilli and some lithic fragments (about 1 cm long)

near base; a horizon of larger less compacted whitish pumice fragments (as long as

10 cm) occurs higher in member on southeast flank of Three Summits Hill. Modal

analyses of thin sections of seven samples of lower member indicate 7–28 percent

phenocrysts (1–3 mm) that consist of: Q, 0–17; K, 18–35; P, 49–64; B, 5–10; O,

1–2; H, 0–4; and C, 0.2–2. Accessory minerals are common zircon and rare apatite and allanite. Chemical analyses of two samples of lower member indicate:

SiO2, 72.9 and 73.3; Al2 O3, 14.5 each; K2 O, 5.51 and 5.40; Na2 O, 3.24 and

3.30; CaO, 1.34 and 1.36; Fe2O3, 1.43 and 1.35; FeO, 0.32 and 0.37; MgO, 0.41

and 0.34; TiO2, 0.24 each; P2O5, 0.10 each; and MnO, 0.06 and 0.07 (rhyolite).

Best and others (1989) reported a 40Ar/39Ar date of 28.68±0.03 Ma for the unit.

Lower member is widely exposed in northeast part of map area, where it is about

20–100 m thick

Ti Isom-type welded ash-flow tuff formation (upper Oligocene)—Crystal-poor rhyodacitic

ash-flow tuff: dark-gray, moderately to densely welded lower part that weathers

brownish gray, and light-brownish-gray to light-gray, moderately to densely

welded upper part that weathers orangish-brown. The term “Isom compositional

type” or “Isom type” was used by Page and Dixon (1994) to include a number of

ash-flow tuff units in eastern Nevada and western Utah that are similar in phenocryst composition to the distinctive Isom Formation of eastern Nevada and

western Utah, which is about 27 Ma (Best and others, 1989). Upper part is missing in places. Lower part is characterized by abundant black glass shards and flattened black glass pumice lapilli as long as 15 cm. Lower part is altered (chloritized

and (or) zeolitized?) locally where dark grayish green. Upper part characterized by

flattened light-yellowish-brown to brown pumice lapilli as long as 4 cm. Small lithic

fragments are common throughout. Modal analyses of four samples of Isom-type

tuff indicate 8–11 percent phenocrysts (1–2 mm in diameter) made up of the following phases: Q, 4–7; K, 4–19; P, 63–72; B, trace–4; O, 2–5; C, 6–25; and H, tr–1.

Apatite and rare allanite are accessory minerals. Chemical analyses of three samples of Isom-type tuff indicate the following ranges of composition: SiO2 , 65.5–

66.9; Al2O3, 15.6–15.7; K2O, 3.91–5.04; Na2 O, 2.84–3.29; CaO, 2.72–3.97;

Fe2O3, 2.85–4.56; FeO, 0.46–2.49; MgO, 1.23–1.67; TiO2 , 0.65–0.81; P2 O5,

0.23–0.30; and MnO, 0.06–0.09 (trachydacite). Unit is exposed in northeast part

of the quadrangle where it crops out locally, mostly as narrow bands. It lies nearly

conformably on underlying upper unit of volcaniclastic rocks of Little Table

Mountain (Tlt4), commonly on a thin chert-rich conglomerate layer. Similar in

modal composition and age (sanidine 40Ar/39Ar date of 27.17±0.04 Ma for a sample collected in adjacent Corcoran Canyon quadrangle; L.W. Snee, written commun., 1996) to Isom-type welded ash-flow tuffs in southeastern Nevada and

southwestern Utah (Page and Dixon, 1994; Best and others, 1989). Thickness 0–

35 m

Tbf Megabreccia of Bull Frame Canyon (upper Oligocene)—Ash-flow tuff containing small

to large fragments (1–30 m in diameter) of a variety of rock types. Apparent matrix

of unit at east end of exposures consists of light-yellowish-brown, light-greenishbrown, and light-orangish-brown, nonwelded, crystal-rich rhyolitic ash-flow tuff

similar in composition to tuff of Ryecroft Canyon (Trc). A modal analysis of one

sample of tuff matrix shows about 38 percent phenocrysts (3–4 mm) that consist

of the following components: Q, 38; K, 31; P, 26; B, 1; and M, 1. Common
7

accessory minerals are zircon and allanite. In west part of outcrop area, fragments

in tuff consist of blocks of dense rhyolitic welded ash-flow tuff and minor amounts

of granite, aplite, and Paleozoic silicic and schistose rocks as much as 2 m in

diameter. In east part of outcrop area, large rounded blocks as much as 30 m in

diameter of light-gray dense rhyolitic welded ash-flow tuff characterize unit.

Rhyolitic tuff blocks appear similar in composition to rhyolitic ash-flow tuff matrix.

Lenses of fluvial gravel are interlayered locally in megabreccia; this gravel may

indicate that unit consists of reworked megabreccia of Meadow Canyon (Tmc)

Volcaniclastic rocks of Little Table Mountain (upper Oligocene)—A thick sequence of

volcaniclastic rocks underlies, beneath a low-angle unconformity, Isom-type and

younger welded ash-flow tuffs in northeast part of the Jefferson quadrangle,

northwest and southwest of Little Table Mountain which lies just outside the quadrangle in adjacent Corcoran Canyon quadrangle to the east. Squence appears to

be unconformable on underlying rocks. The volcaniclastic rocks are divided into

four units, from base upward: tuffaceous sedimentary rocks (siltstone, sandstone,

and conglomerate) and tuff (Tlt1); ash-flow tuff (Tlt2); zeolitic tuff (Tlt3); and a second unit of tuffaceous sedimentary rocks (siltstone, sandstone, conglomerate) and

tuff (Tlt4). Some units are locally absent. Units locally contain minor lenses or

layers of different lithology, and in places units intertongue (not mapped)

Tlt4 Tuffaceous siltstone, sandstone, and conglomerate, and tuff—Pale-greenish- to

yellowish-brown and light-greenish- to yellowish-brown, in part nearly white, interlayered tuffaceous siltstone, sandstone, granule sandstone, pebble sandstone, and

conglomerate. Granules and pebbles (angular to subrounded and as large as 2 cm)

are mostly chert (black, varied shades of green, reddish brown, and yellowish

brown); in one place dark-gray fragments (as long as 5 cm) of graptolitic shale from

Ordovician Toquima Formation (Ota) were observed in platy siltstone. Siltstone

and sandstone are evenly bedded lacustrine strata; coarser beds are probably both

lacustrine and fluvial. Subordinate layers a few centimeters to about 1 m thick of

thin-bedded pumice- and biotite-bearing tuff are interbedded locally. Some beds near

top of unit contain abundant small (less than 1 mm in diameter) grains of black pyroxene. Most finer grained rocks of unit consist largely of devitrified volcanic glass,

lesser amounts of granule-size lithic fragments (mostly devitrified rhyolite), and

crystals of plagioclase, alkali feldspar, quartz, and mafic minerals. Uncommon thin

(as thick as 1 m) beds of light-greenish- to yellowish-brown lacustrine limestone, in

part algal limestone, occur near top of unit. Unit crops out around south base of

Three Summits Hill, and more widely northeast of hill. Thickness 0–100 m

Tlt3 Zeolitic tuff—Pale-greenish- to yellowish-brown and light-greenish- to yellowishbrown tuff, probably deposited in a lake. In places light yellowish brown to pale

brown. Tuff weathers to flaky to platy fragments or to rubble; beds about 10 cm

thick display conchoidal fractures. An X-ray-diffraction analysis of a sample of zeolitic tuff by R.A. Sheppard (U.S. Geological Survey) indicated the presence of

about 60 percent clinoptilolite (possibly with minor mordenite), 20 percent smectite,

20 percent glass, and traces of quartz and biotite. A modal analysis of this sample

indicates a zeolitic-smectitic groundmass that contains about 6 percent crystals

(0.5 mm in diameter) made up of the following components: Q, 5; K, 2; P, 79; B,

12; O, 0.6; and M, 2. Zircon and rare sphene and apatite are accessory minerals.

Locally zeolitic tuff contains silty and sandy layers. Unit is exposed in an easttrending strip south of Three Summits Hill and mostly northeast of Meadow Canyon, and in scattered strips northeast of Three Summits Hill. Thickness 0–65 m

Tlt2 Ash-flow tuff—Light-greenish-brown to light-yellowish-brown, pumiceous, crystalpoor, nonwelded to partially welded quartz latitic ash-flow tuff. Locally light lavender gray in upper part. Characterized by abundant nearly white uncompacted

pumice fragments and sparse but conspicuous devitrified rhyolite lithic fragments

less than 1 cm in size. Abundant dark-gray flattened pumice fragments as long as 3

cm occur just south of Meadow Canyon Guard Station. A modal analysis of a

sample of ash-flow tuff indicates 10 percent phenocrysts (1.5 mm in diameter) made

up of the following phases: Q, 23; K, 21; P, 48; B, 7; H, 2; and M (O and C), 0.4.

Apatite is an accessory mineral. Minor rhyodacitic tuff layers in unit contain
8

numerous conspicuous black pyroxene crystals. A modal analysis of a sample of

this rock shows about 20 percent phenocrysts (0.5–2 mm in diameter) that consist

of: Q, 3; K, 9; P, 59; B, trace; O, 4; C (and minor orthopyroxene), 23; and H, 1.

Apatite is an accessory mineral. Chemical analyses of two samples of quartz latitic

tuff show the presence of: SiO2 , 71.0 and 72.6; Al2O3, 15.3 and 15.0; K2O, 4.44

and 4.68; Na2O, 2.45 and 2.06; CaO, 2.89 and 2.24; Fe2O3, 1.75 and 1.55; FeO,

0.53 and 0.36; MgO, 1.16 and 1.04; TiO2, 0.31 and 0.30; P2O5, 0.16 and 0.09;

and MnO, 0.05 and 0.07 (rhyolite). Unit is not everywhere well defined, particularly where poorly exposed; in places it contains thin-platy light-yellowish-brown

tuffaceous siltstone layers, and in west part of its exposure it contains tuff layers

charged with granite boulders as large as 1.5 m. The boulders typically are stained

green, a common characteristic of large granite fragments in these Tertiary tuffs.

Unit is exposed south and southwest of Three Summits Hill, but it is present only

locally north of hill. Thickness 0–35 m

Tlt1 Tuffaceous siltstone, sandstone, and conglomerate, and tuff—Light-yellowishbrown, pale-greenish- to pale-yellowish-brown, and nearly white tuffaceous siltstone, sandstone, and conglomerate, and tuff. In a few places rocks weather with

an orangish-brown to brown surface. Unit is dominantly lacustrine thin-bedded and

evenly bedded, interlayered siltstone, sandstone, and tuff. Locally, thin-bedded siltstone shows small-scale crumpled folds suggestive of soft-sediment slumps. Rare

mud cracks and “worm tracks” appear in places. Siltstone breaks into flaky to platy

fragments; tuff layers and ash-rich siltstone and sandstone tend to show conchoidal fractures and to weather to rubbly to hackly fragments. Tuffaceous rocks

are locally zeolitized. Some layers of granule conglomerate and small-pebble conglomerate appear in upper part of unit in northeast corner of the quadrangle. A few

siltstone beds are limy. Unit lies unconformably on older rocks. Unconformity is

most evident on north slope of hill 8262 just east of Round Meadow where it

slopes north about 25°, cutting down through nearly flat layers of interlayered

megabreccia of Meadow Canyon (Tmc) and tuff of Antone Canyon (Tac). Unit

Tlt1 is exposed in an east-trending belt south of Three Summits Hill, and in patches

in northeast corner of the quadrangle. Thickness 0–120 m

Tpm Mesobreccia with abundant fragments of Paleozoic rocks (upper Oligocene)—

Fragments of Paleozoic rocks commonly 1–20 cm long, less commonly as large as

1.5 m, and rarely as large as 10 m are mixed together; matrix is generally not evident. Some larger blocks are brecciated. Rock types include quartzite and schist of

Cambrian Gold Hill Formation (gq, gs); knotted schist and phyllitic argillite of

Cambrian Mayflower Formation (ms, mf); shale, limestone, and silicified shale

and limestone of Ordovician Zanzibar (Oz units) and Toquima (Ot units) Formations,

and minor Cretaceous granite and aplite, and volcanic rocks. Gold Hill quartzite

clasts dominate in southwest part of exposed mesobreccia, Ordovician limestone

clasts dominate in south-central part, and shale, silicified shale and limestone, and

schist clasts dominate elsewhere. Sparse granite and aplite fragments occur in

west part of unit, and minor volcanic fragments are found in east part of unit.

Angular fragments occur in a pale-greenish- to yellowish-brown tuff matrix on steep

south slope of Antone Canyon 1.4 km southwest of Meadow Canyon. Almost everywhere else matrix is not evident, apparently because its soft character allows

ready erosion. Unit is exposed in a strip as much as 700 m wide south of Meadow

Canyon, extending east-southeastward from Bull Frame Canyon to beyond

Antone Canyon. Because unit appears to be interlayered locally with older rocks

toward the east, it may in fact consist in part of rocks that are older than most of

the rest of unit. In western two-thirds of outcrop area, mesobreccia unit lies unconformably upon units of Gold Hill Formation (g units), tuff of Antone Canyon

(Tac), and units of volcaniclastic rocks of Little Table Mountain (Tlt units), and it is

overlain unconformably by megabreccia of Bull Frame Canyon (Tbf). It lies unconformably upon limestone of Zanzibar Formation (Ozl) and tuff of Antone Canyon

(Tac) in eastern one-third of the outcrop area. It also may tongue into megabreccia

of Meadow Canyon (Tmc) at east end of its exposure. In exposures near confluence of Meadow and Bull Frame Canyons it appears to form layers within or at
9

base of volcaniclastic rocks of Little Table Mountain; however, these exposures

may be incorrectly mapped as interlayers in volcaniclastic rocks, and they may instead represent unconformable remnants on underlying rocks. Thickness of unit is

0–60 m

Tum Unnamed megabreccia unit (upper Oligocene)—Megabreccia characterized by common

and distinctive well-rounded and polished boulders of light-gray densely welded

rhyolitic ash-flow tuff 1–3 m in size, as well as larger blocks as long as 5 m of other

volcanic rock types. Unit is generally a light-brown aggregate of poorly sorted materials ranging in size to as little as about 1 cm; fine-grained ash matrix is not evident. The one small exposure lies along east-central margin of the quadrangle.

Unit is more widely distributed farther east in the Corcoran Canyon quadrangle

Megabreccia of Jefferson Summit (upper Oligocene)—Unit consists of two eruptive

megabreccia phases (Tjsg and Tjss) and one intrusive phase (Tjsi). Intrusive phase

is exposed in three pipelike bodies and a dike, presumed vents from which other

phases were erupted. The proposed vents are filled with varied rock types. The

most widespread eruptive phase (Tjsg), forming a tilted to nearly flat-lying outflow

apron, consists mostly of clasts of granite of the Round Mountain pluton (Krg and

related rocks) and minor amounts of other rocks. Within unit Tjsg locally are thin

layers of sedimentary rocks (Tjsl). The other extrusive phase (Tjss), also a tilted

layer, consists mostly of clasts of schist and quartzite of Gold Hill Formation (gs

and gq)

Tjsi Intrusive phase of vent megabreccia—Three inferred pipes and a dike are filled with

vent megabreccia. The largest pipe, crudely oval shaped and about 600 m long

east-west and 300 m wide north-south, occupies an amphitheater-shaped enlargement of Grassy Canyon just west of Jefferson Summit. An intermediate pipe, 500

m long and 60–120 m wide, is exposed farther south on a spur on north slope of

Bull Frame Mountain. The smallest pipe (about 220 m long and 60 m wide) crops

out on another spur about 700 m to the northwest. The dike, still farther northwest, is 20–30 m wide through most of its length of about 1.5 km, is as much as

300 m wide at its northwest end, and tapers to a pinchout at its southeast end.

The largest pipe is filled with a mixture of extremely brecciated quartzite of

Gold Hill Formation (gq), in part consisting of blocks and slabs as long as 500 m

along periphery of pipe, and blocks of strongly brecciated granite (Krg) as large as

about 100 m elsewhere. A thin section of matrix of brecciated quartzite shows

only pulverized quartzite. A thin section of matrix of brecciated granite shows

about 50 percent granite fragments (less than 1 cm in size) in a groundmass of angular crystal fragments (1–2 mm long) that consist of: Q, 72; perthite, 4; P, 22; B,

0.6; O, 0.6; and muscovite, 0.6. Virtually all crystals are derived from granite;

however, rare pumice-like fragments are present. Where exposed in Grassy

Canyon, pipe appears to have a nearly vertical north wall against tuff of Mount

Jefferson (Tmj) and altered tuff (Tmja). This pipe wall is about 100 m in vertical dimension, extending downward below overlying outflow apron of megabreccia

(Tjsg).

The second pipe, on north spur of Bull Frame Mountain, is exposed on a relatively smooth slope unbroken by outcrops of unbrecciated granite that conspicuously surround pipe. Northeast end of pipe extends into schist of Gold Hill

Formation (gs) that forms wall of Round Mountain granite pluton. Strongly

sheared and brecciated schist a meter or two wide lies adjacent and parallel to margin of pipe at its northeast end. Clusters of rounded to subrounded blocks 1–10 m

in size of mostly coarse-grained granite, lesser amounts of aplite and fine-grained

granite, and minor pegmatite occur locally across surface of pipe. Only rare individual blocks are themselves brecciated. In places mixed in with granite blocks are

fragments of schist and quartzite of Gold Hill Formation (gs and gq), some of

them brecciated. Matrix of megabreccia is nowhere evident. Two zones of chalcedony breccia (brecciated chalcedony in chalcedony matrix) 30–50 m in diameter

within megabreccia pipe are interpreted as mineralized pipes. One zone occupies

northeast end of megabreccia pipe and the other lies about 150 m south, near east

margin of pipe. Both zones show multiple generations of chalcedony, and both
10

zones contain thin (as wide as 30 cm) irregular veins of fluorite. Both zones also

contain local anomalous amounts of gold, antimony, and mercury (Shawe, unpub.

data, 1988).

The third and smallest pipe, exposed on a spur northwest of second pipe, is

filled mostly with granite blocks 1–2 m in size. Some blocks are brecciated and iron

mineralized. Rare fragments of schist of Gold Hill Formation (gs) as large as 2 m

are present.

The dike of megabreccia intrudes granite of Round Mountain pluton through

most of its length; at its northwest end limestone and schist of Gold Hill Formation

(gl and gs) form its wall rocks. The dike contains mostly rounded blocks 3–4 m

in size that consist of various phases of granite (coarse and fine grained, and aplite)

through intervals of 200–300 m along dike. Matrix of granite megabreccia, as seen

in thin section, consists of pulverized granite minerals, angular crystal fragments,

and granite fragments about a centimeter in size. In places, clasts of schist of Gold

Hill Formation (gs), of schist intruded by granite, and of greenish-gray andesite

porphyry as long as 10 m, are common. Near northwest end of megabreccia dike,

just west of Mogul Canyon where dike is about 200 m wide, a 50-m-long block of

granite is surrounded by fragments of schist and andesite porphyry. A modal analysis of a sample of andesite porphyry shows 51 percent phenocrysts (1–7 mm in

diameter) made up of the following phases: Q, 7; K, 2; P, 69; B, 10; O, 1.4; and C,

11. A chemical analysis of a clast of andesite porphyry from the dike shows the

following percentages of components (recalculated without CaO attributable to calcite—based on CO2 content of 3.84 percent—and other volatiles): SiO2 , 62.9;

Al2O3, 15.4; K2O, 2.49; Na2O, 2.41; CaO, 2.42; Fe2O3, 3.20; FeO, 3.63; MgO,

6.59; TiO2, 0.61; P2O5, 0.23; and MnO, 0.11 (andesite). Composition is quite

similar to 36-Ma andesite (Ta) and granodiorite (Tgd) mapped in the Round

Mountain quadrangle about 4 km farther west (Shawe, 1995). L.W. Snee (written

commun., 1997) reported a biotite 40Ar/39Ar date of 33.18±0.06 Ma for an

andesite fragment collected from dike (rock sample locality R6). Possibly a thermal

event associated with emplacement of megabreccia dike (at approximately 26 Ma?)

reset the date somewhat. I infer that an intrusion or intrusions of 36-Ma andesite

porphyry underlie the megabreccia dike. About 400 m northwest of 50-m-long

block of granite, an exposure of dike shows rounded fragments of andesite

porphyry, granite, and aplite in a matrix of biotite-rich tuff. A rare fragment of lightyellowish-brown flow-layered rhyolite found near northwest end of megabreccia

dike is quite similar to 36-Ma rhyolite (Tr) found in dikes and a plug about 3 km and

farther west in the Round Mountain quadrangle (Shawe, 1995). As seen in thin

section, rhyolite is nearly aphyric, containing rare crystals of quartz and biotite less

than 0.5 mm long. Again, the megabreccia dike likely tapped a subsurface rhyolite

dike during eruption

Tjsg Eruptive phase of megabreccia containing mostly granitic clasts—Blocks of granite

in outflow megabreccia are mostly rounded to subrounded, 1–10 m in diameter; a

25-m-long brecciated granite block occurs 0.8 km north-northeast of Jefferson

Summit. Blocks of schist and quartzite (gs and gq, respectively) are locally present, and they are abundant about 1.7 km north of Jefferson Summit. Only rare

fragments of andesite porphyry were observed. Most of matrix of granite

megabreccia as seen in thin section appears to be pulverized granite. A modal analysis of one sample shows about 57 percent granite fragments (to 1.5 cm in size) in

a groundmass of broken crystals made up of: Q, 62; orthoclase, 1; P, 32; B, 5; and

muscovite, 0.6. In cliffs exposed 200 m south of Jefferson Summit, matrix of

megabreccia consists of layers of material having grain size about 0.1 mm, to layers

having grain size of several millimeters, disposed as graded layers molded around

adjacent clasts. As seen in thin section, most of the material is pulverized granite;

however, sparse biotite microlites and sanidine, and rare shard forms, suggest a minor volcanic component. A chemical analysis of a portion of this matrix indicates

the following percentages of components: SiO2 , 89.2; Al2O3, 7.19; K2 O, 1.75;

Na2O, O.43; CaO, 0.45; Fe2O3, 0.55; FeO, 0.04; MgO, 0.23; TiO2, 0.15; P2O5,

0.06; and MnO, less than 0.01. High silica content suggests that mineral
11

components other than quartz were winnowed from matrix prior to deposition.

Two kilometers north of Jefferson Summit a layer of tuff that contains abundant

granite fragments as large as about 15 cm long is exposed at base of megabreccia

layer. A modal analysis of matrix of the tuff indicates lithic fragments as large as 8

mm composed of granite (21 percent of matrix), metasedimentary rocks (3 percent),

and minor pumice. Phenocrysts 1–2 mm in diameter constitute about 10 percent of

rock and consist of: Q, 47 (including abundant broken crystals likely derived from

granite); K, 12 (one-fourth of which is sanidine); P, 36; B, 2.5; O, 1; and muscovite,

1. Monazite, apatite, and zircon are accessory minerals. Near southeast end of

granite megabreccia outflow apron, matrix is light-greenish-brown to lightyellowish-brown tuff.

The granite megabreccia outflow apron, which spreads northwestward 4 km

and northward 2 km from Jefferson Summit, forms a relatively flat unit, as indicated

on cross sections C–C', D–D', I–I', J–J', and L–L'. It is warped upward at about 5°,

north of Jefferson Summit, possibly as a result of resurgence of the Mount

Jefferson caldera following emplacement of megabreccia. Unit dips 15° northward

south of Jefferson Summit (section L–L'), and about the same amount eastward

southeast of Jefferson Summit, perhaps in part reflecting ground slope at time of

emplacement, or uplift of granite of Round Mountain pluton following emplacement.

Unit appears conformable on underlying tuff of Mount Jefferson, and unconformable on underlying granite. Thickness 0–50 m

Tjsl Siltstone, sandstone, and conglomerate (lacustrine)—Thin evenly bedded layers

of platy siltstone, slabby sandstone, and conglomerate are interlayered in granite

megabreccia (Tjsg) near Jefferson Summit. Lacustrine siltstone also is exposed in

one place at base of granite megabreccia 2 km north of Jefferson Summit.

Individual beds of unit mostly are no more than about 10 cm thick. Buff siltstone in

places displays oscillatory ripple marks characteristic of lacustrine sediments. Lightbrown sandstone and conglomerate appear to consist chiefly of granite fragments;

some conglomerate layers contain angular aplite fragments about 25 cm long.

Aggregate thickness of layers, less than 10 m

Tjss Eruptive phase of megabreccia with dominant schist and quartzite clasts—Forms

a layer conformably underlying eruptive phase of megabreccia containing mostly

granite clasts (Tjsg), and unconformably overlying schist of Gold Hill Formation

(gs), southeast of Jefferson Summit. Most fragments are schist and quartzite of

Gold Hill Formation (gs and gq), are less than about 2–3 m in length, and commonly are brecciated. Fragments are rarely as long as about 10 m. Matrix was not

observed. Granite fragments in small amounts occur in places, and a fragment of

greenish-gray and gray laminated calc-silicate-mineralized limestone of Gold Hill

Formation (glc) was observed in one place. Thickness about 25 m

Tuff of Mount Jefferson (upper Oligocene)—The tuff of Mount Jefferson constitutes a

great thickness of intracaldera welded ash-flow tuff units, individual ash flows, and

simple and compound cooling units, within the Mount Jefferson caldera, the south

part of which occupies the north one-third of the Jefferson quadrangle. The intracaldera tuff of Mount Jefferson is at least about 2,000 m thick, judged from relief on

exposed tuffs between Big Smoky Valley northwest of the quadrangle, and summit

of Mount Jefferson just north of the quadrangle. In the Jefferson quadrangle I

made sporadic unsuccesful attempts to map individual flows or cooling units, but

these efforts were overcome by time limitations and rugged terrain of Mount Jefferson. I nevertheless show some mappable units, as detailed below, as well as

some contacts between internal units, to better depict attitudes and configurations

of units.

That part of the tuff of Mount Jefferson in the northeast corner of the map

area, east of Soldier Spring fault, was previously designated tuff of Trail Canyon by

Boden (1986; 1992), and considered by him to represent infill of a caldera (Trail

Canyon) younger than the Mount Jefferson caldera. Data (petrographic, chemical,

and radiometric) presented here show that the tuff of Trail Canyon is essentially

identical to the tuff of Mount Jefferson; this conclusion is confirmed also by

microprobe analyses of phenocrysts of the two units (Boden, 1994; F.M. Byers,
12

Jr., written commun., 1995). A small amount of outflow facies of the tuff of Mount

Jefferson, considered part of the principal member (Tmj), lies just south of the

caldera margin.

Tuff of Mount Jefferson, as defined here, is composed of a thin upper member

(Tmju), and a thick lower, principal, member (Tmj). The upper member is equivalent

to the lower unit of a capping member of Boden (1992); the principal member is

equivalent to Boden’s (1992) upper member. Boden (1986, 1992) attributed an age

of 25.8±0.5 Ma to 26.6±0.6 Ma to his upper member based on sanidine and biotite

K/Ar dates; Boden’s (1992) lower member, exposed in north part of the Mount

Jefferson caldera, does not crop out but presumably underlies the principal member

in the Jefferson quadrangle. Within the principal member, the following units are

defined: vitrophyre (Tmjv), tuff breccia (Tmjb), altered welded ash-flow tuff (Tmja),

and intensely sheared welded ash-flow tuff (Tmjs).

Modal analyses of a large number of samples of tuff of Mount Jefferson allowed assigning samples into five petrographic categories on the basis of phenocryst percentages. Assignments are based on generally increasing plagioclase

and mafic mineral contents, and decreasing quartz and alkali feldspar contents, from

category 1 to category 5. A “phenocryst index” (PI), determined by the sum of

quartz plus alkali feldspar contents divided by the sum of plagioclase plus mafic minerals (biotite, hornblende, pyroxene, and opaque minerals) contents, allows assignment of modal data into five categories as follows: category 1, PI 0.81–1.00; category 2, PI 0.61–0.80; category 3, PI 0.41–0.60; category 4, PI 0.21–0.40; and

category 5, PI 0.00–0.20. Tuffs of category 1 are generally low in the stratigraphic section, and tuffs with higher PI numbers are progressively higher in the

section, although in places samples that do not fit this pattern may indicate local

reversals in phenocryst compositions. Locations and category assignments of

modally analysed samples are shown on the map

Tmju Upper member—Gray, moderately welded, crystal-rich rhyodacitic ash-flow tuff;

weathers brown. Modal analyses of two samples of member indicate 29 and 38

percent phenocrysts (2–3 mm in diameter) made up of the following phases: Q, 3

and 5; K, 5 and 9; P, 72 and 67; B, 14 each; O, 3 and 2; H, 3 and 1; and C, 0.2 and

1. Zircon and apatite are common accessory minerals. Samples are included in petrographic category 5 of analyzed rocks of tuff of Mount Jefferson based on this

petrography. Chemical analyses of the same two samples indicate the following

percentages of components: SiO2 , 67.9 and 66.3; Al2O3, 16.1 and 16.6; K2 O,

4.27 and 4.09; Na2O, 3.95 and 3.89; CaO, 2.78 and 3.20; Fe2O3, 2.73 and 3.08;

FeO, 0.72 and 1.00; MgO, 0.73 and 1.02; TiO2, 0.45 and 0.51; P2O5, 0.21 and

0.25; and MnO, 0.09 and 0.08 (trachydacite). Upper member is exposed as two

small erosional remnants at north margin of the quadrangle, near summit of Mount

Jefferson (which is just north of the quadrangle). A small patch of outflow facies of

tuff of Mount Jefferson belonging to petrographic category 5 overlies megabreccia

of Meadow Canyon (Tmc) south of Meadow Canyon. A modal analysis of a sample

of this rock shows 34 percent phenocrysts (1.5–3 mm) that consist of the following phases: Q, 2; K, 9; P, 65; B, 16; O, 4; H, trace; and C, 2. Zircon and apatite

are common accessory minerals. Remnant thickness 0–40 m

Tmj Principal member—Quartz latitic to rhyodacitic welded ash-flow tuff (petrographic

categories 1–5). At north-central edge of the quadrangle, principal member underlies upper member (Tmju) conformably. However, in west-central part of the quadrangle, principal member underlies megabreccia of Jefferson Summit (Tjsg) conformably, and in northeast corner of the quadrangle, principal member unconformably underlies lowest unit of volcaniclastic rocks of Little Table Mountain (Tlt1).

Apparently considerable erosion took place before later units (Tjsg and Tlt1) were

deposited. L.W. Snee (written commun., 1997) reported sanidine 40Ar/39Ar dates

of 26.61±0.05 (1σ) Ma (rock sample locality R3), 26.64±0.04 (1σ) Ma (locality R4),

and 26.72±0.04 (1σ) Ma (locality R5), and a biotite 40Ar/39Ar date of 26.77±0.05

(1σ) Ma (locality R5) for samples of principal member. Rock samples R3 and R5, inferred to be from lower part of exposed principal member, were collected from zone

of altered rock (Tmja) associated with plug of rhyolite (Trp) that intrudes principal
13

member 0.7 km northeast of Jefferson Canyon in northwest part of the quadrangle. Rock sample R4 was collected from a stratigraphically higher part of the principal member. Possibly the determined younger date of sample R3, from lower

(older) part of member, may have resulted from thermal resetting related to emplacement of a rhyolite plug (Trp). Several biotite and sanidine 40Ar/39Ar dates of

samples collected in adjacent Corcoran Canyon quadrangle from principal member

are about 26.6–26.9 Ma (L.W. Snee, written commun, 1996). About 500 m of

principal member is exposed in the Jefferson quadrangle; base is not exposed.

Category 5 rocks—Light-lavender-gray, moderately welded, crystal-rich rhyodacitic ash-flow tuff; weathers brownish gray. Modal analyses of nine samples of

this group indicate 24–36 percent phenocrysts (1–3 mm in diameter) consisting of

the following phases: Q, 2–11; K, 4–11; P, 61–74; B, 12–16; O, 1–4; H, trace–3;

and C, 1.5–5. Accessory minerals are common zircon, moderate apatite, and rare

allanite. A chemical analysis of a sample of category 5 tuff indicates the following:

SiO2, 69.7; Al2O3, 15.4; K2O, 4.66; Na2O, 3.75; CaO, 2.20; Fe2O3, 2.24; FeO,

0.63; MgO, 0.77; TiO2, 0.39; P2O5, 0.20; and MnO, 0.05 (rhyolite). Samples in

category 5 are from relatively high stratigraphically in principal member.

Category 4 rocks—Very light gray to light-brownish-gray, partially to

densely welded, crystal-rich rhyodacitic ash-flow tuff; weathers light brown to light

orangish brown. Category 4 tuffs are massive to flaggy; spheroidally weathered in

places. Modal analyses of 23 samples of this category indicate 15–44 percent phenocrysts (1.5–3 mm in diameter) consisting of the following phases: Q, 3–16; K,

7–23; P, 53–68; B, 5–20; O, 0.2–5; H, 0–5; and C, trace–7. Accessory minerals

are common zircon, moderate apatite, and rare allanite. Tuff is strongly magnetic

in a few places. Samples of category 4 occur generally in middle to upper parts of

exposed principal member.

Category 3 rocks—Light-gray, light-lavender-gray, and light-olive-gray, partially to densely welded, crystal-poor to crystal-rich rhyodacitic ash-flow tuff;

weathers brownish gray, olive gray, and reddish brown. Massive to platy;

spheroidally weathered in places. Modal analyses of 13 samples of this category

indicate 9–41 percent phenocrysts (2–4 mm in diameter) of the following phases:

Q, 7–18; K, 15–25; P, 46–55; B, 6–15; O, 0.5–3; H, 0–6; and C, trace–2.

Accessory minerals are common zircon and apatite, and rare allanite. Tuff is

strongly magnetic in a few places. Samples in category 3 occur generally in middle

to upper parts of exposed principal member. Outflow facies of tuff of Mount

Jefferson along Meadow Creek, similar to category 3 samples, is light-yellowishbrown, pumice-rich, nonwelded to partially welded, crystal-rich rhyodacitic ash-flow

tuff; weathers light brown. Modal analysis of one sample from upper part of outflow facies indicates 29 percent phenocrysts (2–3 mm in diameter) that consist of

the following phases: Q, 10; K, 20; P, 53; B, 13; O, 2; and M, 1.5. Common zircon and sparse apatite are accessory minerals. Chemical analysis of one sample of

category 3 tuff indicates the following composition: SiO2 , 70.0; Al2 O3, 15.2;

K2O, 4.65; Na2O, 3.87; CaO, 2.14; Fe2O3, 2.02; FeO, 0.72; MgO, 0.79; TiO2 ,

0.37; P2O5, 0.18; and MnO, 0.07 (rhyolite).

Category 2 rocks—Light-gray, light-lavender-gray, and to light-pinkish-gray,

partially to densely welded, crystal-rich rhyodacitic to quartz-latitic ash-flow tuff;

weathers light brown, light orangish brown, and reddish brown. Modal analyses of

five samples of category 2 tuffs indicate 22–39 percent phenocrysts (2–5 mm in diameter) made up of the following phases: Q, 15–24; K, 16–27; P, 46–48; B, 6–10;

O, 1–3; H, 0–2; and C, trace–2. Common zircon, sparse to common apatite, and

rare allanite are accessory minerals. Modal analysis of a sample from lower part of

outflow facies indicates 30 percent phenocrysts (3–5 mm) that consist of the following phases: Q, 11; K, 29; P, 49; B, 8; and O, 2; abundant zircon and sparse

sphene are accessory minerals. Chemical analyses of two samples of category 2

tuff show the following compositions: SiO2, 70.9 and 71.5; Al2O3, 15.0 and 15.1;

K2O, 4.96 and 4.36; Na2O, 3.39 and 3.32; CaO, 1.87 and 1.93; Fe2O3, 2.14 and

2.38; FeO, 0.36 and 0.25; MgO, 0.77 and 0.62; TiO2, 0.33 and 0.36; P2O5, 0.17
14

each; and MnO, 0.06 and 0.02 (rhyolite). Category 2 tuffs occur mostly in middle

and lower parts of exposed principal member of tuff of Mount Jefferson.

Category 1 rocks—Light-gray, light-lavender-gray, and light-pinkish-gray,

moderately to densely welded, crystal-rich rhyolitic to quartz-latitic ash-flow tuff;

weathers light brown, light orangish brown, and reddish brown. Tuff contains

abundant flattened pumice lapilli as long as 3 cm in some layers. In places contains

abundant lithic fragments of metasedimentary rocks, rhyolite, and intermediate volcanic rocks as much as 2 cm long, and small noncompacted pale-yellowish-brown

pumice fragments. Mostly massive and locally spheroidally weathered. Modal analysis of one sample of category 1 tuff shows 34 percent phenocrysts (2.5–4 mm in

diameter) that consist of the following phases: Q, 22; K, 28; P, 38; B, 7; O, 2; H,

2.5; and C, trace. Accessory minerals are zircon and apatite. A modal analysis of a

crystal-poor sample of a thin (a few meters) ash-fall tuff layer stratigraphically onethird of the way above exposed base of principal member in northwest part of the

quadrangle indicates 8 percent phenocrysts (1–1.5 mm) that consist of: Q, 19; K,

27; P, 47; B, 4; O, 1; H, 2; and C(?), 0.5. Chemical analysis of the same sample of

category 1 tuff shows the following composition: SiO2 , 74.2; Al2O3, 13.7; K2O,

4.86; Na2O, 2.66; CaO, 1.47; Fe2O3, 2.00; FeO, 0.13; MgO, 0.56; TiO2 , 0.24;

P2O5, 0.10; and MnO, 0.01 (rhyolite). The two samples identified as category 1

tuff appear to be in lower part of exposed principal member

Tmjv Vitrophyre—Dark-gray to black, glassy, partially to densely welded rhyodacitic

ash-flow tuff. Lenticular layers of vitrophyre 0–30 m thick occur sporadically

throughout principal member (Tmj). They were mapped in zones identified by modal

analyses as belonging to categories 1, 3, 4, and 5.

Category 5 rocks—Modal analysis of one sample of crystal-rich vitrophyre

indicates 30 percent phenocrysts (1.5–2 mm) that consist of: Q, 4; K, 8; P, 66; B,

14; O, 2; H, 4; and C, 2. Zircon and apatite are accessory minerals. A chemical

analysis of the same sample indicates the following composition: SiO2 , 67.1;

Al2O3, 16.3; K2O, 3.60; Na2O, 4.17; CaO, 3.09; Fe2O3, 2.78; FeO, 1.02; MgO,

1.15; TiO2, 0.48; P2O5, 0.24; and MnO, 0.09 (trachydacite-dacite).

Category 4 rocks—Modal analyses of four samples of crystal-rich vitrophyre

show 25–34 percent phenocrysts (2–3.5 mm in diameter) that consist of the following phases: Q, 6–9; K, 12–16; P, 55–60; B, 12–17; O, 2–3; H, 2–3; and C, 1–

3. Zircon and apatite are common accessory minerals, and allanite is a sparse accessory mineral. Chemical analysis of one sample of category 4 vitrophyre indicates the following percentages of components: SiO2 , 68.6; Al2O3, 15.8; K2 O,

4.49; Na2O, 3.76; CaO, 2.63; Fe2O3, 2.24; FeO, 0.83; MgO, 0.97; TiO2 , 0.41;

P2O5, 0.21; and MnO, 0.09 (trachydacite).

Category 3 rocks—A modal analysis of a sample of crystal-poor vitrophyre

indicates 9 percent phenocrysts (1–1.5 mm in diameter) made up of the following

phases: Q, 18; K, 18; P, 56; B, 6; O, 0.5; H, 0.5; and C, 1. Zircon is a common

accessory mineral, and apatite and allanite are sparse. Unusually, a sparse amount

of white mica is present. A chemical analysis of the same sample indicates: SiO2 ,

77.4; Al203, 11.7; K2O, 8.09; Na2O, 0.75; CaO, 0.46; Fe2O3, 0.69; FeO, 0.23;

MgO, 0.35; TiO2, 0.23; P2O5, 0.12; and MnO, 0.02 (rhyolite). Presence of white

mica and the unusually high silica content of this sample suggest possible contamination of the vitrophyre by granitic material.

Category 1 rocks—A modal analysis of a sample of crystal-rich vitrophyre

indicates 35 percent phenocrysts (2–4.5 mm in diameter) that consist of the following phases: Q, 30; K, 18; P, 45; B, 6; and M, 1.5. Zircon and apatite are accessory minerals. A chemical analysis of the same sample indicates: SiO2 , 73.4;

Al2O3, 14.6; K2O, 4.45; Na2O, 3.62; CaO, 1.55; Fe2O3, 0.83; FeO, 0.64; MgO,

0.43; TiO2, 0.28; P2O5, 0.12; and MnO, 0.05 (rhyolite)

Tmjb Tuff breccia—Welded ash-flow tuff containing abundant fragments chiefly of

Paleozoic rocks and granite; fragments are mostly 20 cm and less in size. Occurs

as a layer within sequence of tuffs that constitute the Mount Jefferson caldera fill.

Exposed for about 700 m along the margin of the caldera south of Jefferson
15

Canyon and about 2 km from the west boundary of the quadrangle. Thickness is a

few meters

Tmja Altered ash-flow tuff—Light-yellowish-brown to light-brown, locally iron-stained,

strongly altered, welded ash-flow tuff of principal member (close stipple on map), and

light-yellowish- to light-greenish-brown and light-greenish-gray, moderately altered,

welded ash-flow tuff of principal member (open stipple on map). Strongly altered tuff

as seen in thin section is characterized by ramifying veinlets of quartz, chalcedony,

and calcite, by sericitized and calcite-replaced feldspars, and by strongly devitrified

groundmass, commonly partially silicified and pyritized. Moderately altered tuff as

seen in thin section is characterized by sericitized and chloritized biotite, by sericitized and calcite-replaced feldspars, and by partially silicified and chloritized groundmass. Strongly altered tuff forms a halo surrounding rhyolite plug (Trp) in Wild

Horse Canyon, a zone about 100 m wide along north margin of rhyolite plug and

spreading south a kilometer and more to fault that bounds southwest margin of the

Mount Jefferson caldera. Most of the mineralized rock in the Jefferson mining district that lies within the caldera is within strongly altered tuff. Moderately altered

tuff forms an irregular zone extending northwest and southeast beyond strongly

altered tuff, and lying adjacent to structural margin of caldera

Tmjs Intensely sheared ash-flow tuff—Olive-gray, welded ash-flow tuff. Forms two

crudely oval-shaped zones marked by intensely sheared tuff in north-central part of

the quadrangle. The larger zone, about 1.5 km × 2.5 km within the quadrangle, is

bisected by Slaughterhouse Canyon; the smaller zone, about 0.5 km × 1.5 km, is

bisected by Sawmill Canyon. Probably the canyons were localized within sheared

zones because of greater susceptibility of weakened rock to erosion. Sheared tuff

is characterized by numerous steeply dipping anastomosing shear planes, spaced 1–

3 cm apart. Sheared zones are interpreted to have resulted from a pistonlike, differential uplift of the zones by emplacement of igneous plugs at depth (see cross section A–A'). Shears generally are oriented at a high angle to originally subhorizontal compaction foliation. Therefore, local divergence of shear planes from nearly

vertical may indicate tilting of tuff layers because of intrusion of plugs. Unit contacts mapped elsewhere in tuff of Mount Jefferson appear to have been obliterated

within sheared zones

Tac Tuff of Antone Canyon (upper Oligocene)—Light-greenish-gray to whitish-gray, lithic-,

pumice-, and crystal-rich, nonwelded rhyolitic ash-flow tuff; weathers pale yellowish

brown to pale yellowish gray. Abundant (about 15 percent) poorly compacted, paleyellowish-brown pumice lapilli (as much as 1 cm long) are present. Unit contains

abundant lithic fragments, mostly of Paleozoic formations, as much as 5 cm long,

locally larger; and in places it contains huge isolated blocks and slabs as much as

200 m long of Ordovician limestone (Ol), silicified limestone (Ols), and schist (Os),

quartzite of Cambrian Gold Hill Formation (gq), and Cambrian Mayflower Formation (mf). A few large fragments of rhyolitic welded ash-flow tuff (Trc?) and rhyolite lava also are present. The large blocks, each monolithologic, commonly are

strongly brecciated. Modal analyses of three samples of tuff indicate 19–20 percent

phenocrysts (2–3 mm in diameter) composed of the following phases: Q, 28–31; K,

28–32; P, 31–36; B, 6 each; and M, 0.5–1.5. Zircon and allanite are common accessory minerals, and apatite is sparse. The tuff of Antone Canyon is exposed in

an area about 1 km × 2 km along Antone and Round Meadow Canyons where they

enter Meadow Canyon. Unit unconformably underlies outflow facies of tuff of

Mount Jefferson (Tmj), intertongues with megabreccia of Meadow Canyon (Tmc)

and tuff of Ryecroft Canyon (Trc) (see cross sections E–E' and F–F'), and unconformably overlies Paleozoic formations. Tuff that forms tongues into megabreccia

is partially welded in places. Maximum thickness of unit about 150 m.

Although tuff of Antone Canyon has a phenocryst content similar to that of

tuff of Ryecroft Canyon, and as described below the two units are of comparable

age, the units are petrologically dissimilar, inasmuch as the Antone Canyon is generally nonwelded and contains large blocks of foreign materials whereas the

Ryecroft Canyon is variously welded and is devoid of large blocks of foreign
16

materials. Possibly tuff of Antone Canyon is an aberrant part of tuff of Ryecroft

Canyon

Tmc Megabreccia of Meadow Canyon (upper Oligocene)—Eruptive megabreccia consisting of

small to huge, either slablike or rounded clasts (1 m to more than 100 m long,

commonly brecciated) chiefly of welded ash-flow tuff and other volcanic rocks, in a

nonwelded to partially welded ash-flow tuff matrix. [Much of the megabreccia of

Meadow Canyon is within the Corcoran Canyon quadrangle east of the Jefferson

quadrangle.]

Matrix of megabreccia is pale-greenish- to pale-yellowish-brown and lightgreenish- to light-yellowish-brown, nonwelded to partially welded, mostly structureless, crystal-rich rhyolitic ash-flow tuff. Modal analyses of two samples of matrix

show 19–28 percent phenocrysts (2–5 mm in diameter) that consist of: Q, 26–33;

K, 28–37; P, 31–32; B, 3–6; and M, 1–3. Accessory minerals are common zircon

and sparse allanite and apatite. These samples are similar in phenocryst composition to tuff of Ryecroft Canyon (Trc) described below. A third sample of matrix

contains 24 percent phenocrysts (2–3 mm in diameter) that consist of: Q, 13; K,

30; P, 49; B, 8; and M, 1. Accessory minerals are common zircon and sparse apatite and allanite. This sample is similar in phenocryst composition to category 2

samples of tuff of Mount Jefferson (Tmj). Inasmuch as ages of tuff of Mount

Jefferson and tuff of Ryecroft Canyon are comparable in part, this sample may represent intermixing of tuff of Mount Jefferson during deposition of megabreccia of

Meadow Canyon.

Large blocks in central part of megabreccia, in an area about 1 km2 on northeast side of Meadow Canyon, are mostly slablike, and stand vertically in surrounding ash-flow tuff matrix. Their upright attitudes suggest a significant component of

vertical movement in their emplacement, perhaps resulting from eruption through a

conduit underlying the area. Presence of rhyolite plugs (Trp) in Meadow Canyon

on margins of this area indicates conduits penetrating deep into the crust, and suggests a zone of weakness where eruption of megabreccia was facilitated. Many

large blocks elswhere in megabreccia are strongly brecciated and rounded, and

some are noticeably smoothed or polished as though abraded during transport in

ash flow during emplacement. In places unbrecciated blocks sit side by side with extremely brecciated but indurated blocks, suggesting that the mechanism of brecciation was not by transport in ash flow. Evidently brecciated blocks were brecciated

and indurated prior to incorporation into ash flow.

Modal analyses of three samples of clasts of moderately welded rhyolitic ashflow tuff within megabreccia indicate 21–34 percent phenocrysts (1.5–7 mm in diameter) that consist of the following phases: Q, 23–31; K, 22–34; P, 38–41; B, 4–

5; and M (O and H), 1–3. Common zircon and minor apatite are accessory minerals. These modes are quite similar to those of tuff of Ryecroft Canyon (Trc). A

chemical analysis of a sample of one of these clasts indicates the following composition: SiO2 , 75.7; Al2O3, 13.4; K2O, 5.69; Na2O, 2.91; CaO, 0.75; Fe2O3, 0.66;

FeO, 0.36; MgO, 0.30; TiO2, 0.13; P2O5, 0.08; and MnO, 0.04 (rhyolite). Modal

analyses of two samples of clasts of autobrecciated rhyolite show 19 and 25 percent phenocrysts (4–7 mm in diameter) made up of: Q, 29 and 22; K, 30 each; P,

36 and 41; B, 5 each; and 0, 1 each. Zircon is an accessory mineral. These modes

also are similar to those of tuff of Ryecroft Canyon. A clast of rhyolitic ash-fall(?)

tuff contains about 12 percent phenocrysts (1.5–3 mm in diameter) and has the

following modal composition: Q, 39; K, 38;, P, 13; B, 9, and M, 1. Zircon and apatite are accessory minerals. A quartz latite lava clast contains 25 percent phenocrysts (3–10 mm in diameter) that consist of: Q, 11; K, 44; P, 39; B, 5; O, 1;

and M, 0.5. Accessory minerals are zircon and apatite. A modal analysis of a sample of a clast of nonwelded rhyodacitic ash-flow(?) tuff indicates 12 percent phenocrysts (2 mm in diameter) consisting of: Q, 9; K, 15; P, 59; B, 11; O, 1.5; and

H, 5. Zircon, apatite, and monazite(?) are accessory minerals. A rhyodacitic lava

clast contains 19 percent phenocrysts (2–4 mm in diameter) that consist of: Q, 2;

K, 7; P, 78; B, 9; O, 1.5; and M, 2. Zircon and sparse apatite are accessory
17

minerals. These last two samples are similar in modal composition to upper member

of tuff of Mount Jefferson (Tmju).

Complex relations of the megabreccia formation to adjacent and internal units

are illustrated in cross sections E–E', F–F', N –N ', and P–P'. Main part of

megabreccia formation within the quadrangle along Meadow Canyon is structureless. Stratigraphically higher in formation and to the west, megabreccia is

distinctly layered, and to the northwest and southwest it intertongues with tuff of

Antone Canyon (Tac). A unit (Tmcp) in west part of megabreccia formation is

dominated by large fragments of Paleozoic rocks, and it is described separately below. Thin layers of sedimentary material (Tmcs) are associated with megabreccia

formation at east margin of the quadrangle. A unit of bedded tuffaceous siltstone

and sandstone overlies heterolithic breccia (Thb) and in turn is overlain by a layer of

megabreccia of Meadow Canyon just north of a rhyolite plug (Trp) in Meadow

Canyon. A layer of gravel consisting of mostly rounded fragments as large as

small boulders (made up of Paleozoic quartzite, chert, limestone, and silicified shale

and limestone, as well as Tertiary welded ash-flow tuff and vitrophyre) lies at base

of megabreccia of Meadow Canyon 600 m south of Meadow Canyon. Thin lenses

of welded ash-flow tuff of Ryecroft Canyon (Trc) lie at or near base of megabreccia

formation near mouth of Antone Canyon, and an erosional remnant of tuff of

Ryecroft Canyon (Trc) lies on megabreccia formation about 0.8 km northeast of

mouth of Antone Canyon. At the margin of the Jefferson quadrangle about 3 km

farther south, megabreccia of Meadow Canyon overlies a thick section of tuff of

Ryecroft Canyon (Trc), and it is in turn overlain by tuff of Ryecroft Canyon.

About 1.3 km south of mouth of Antone Canyon, a remnant of megabreccia overlies mesobreccia with abundant fragments of Paleozoic rocks (Tpm). Where eruptive megabreccia is interpreted to be outflow, it is as much as 100 m thick; in the

vicinity of interpreted vent area, formation has a relief of as much as 200 m.

Affinity of megabreccia of Meadow Canyon to tuff of Ryecroft Canyon (Trc)

is clear. Because the megabreccia is significantly different petrologically, however,

it is described separately. Tuff of Antone Canyon (Tac), tuff of Ryecroft Canyon

(Trc), and megabreccia are of comparable age and appear to be related genetically

Tmcp Megabreccia with abundant Paleozoic rock clasts—Clasts of a variety of lithologies

derived from Paleozoic formations, including shale or slate, phyllitic shale, silicified

argillite, limestone, schist, and quartzite, mostly strongly brecciated and indurated.

Clasts are as much as 2–3 m in size, as much as tens of meters long locally, and

occur in a matrix of pale-greenish- to pale-yellowish-brown, pumiceous, nonwelded

or only slightly welded ash-flow tuff. Contains a few well-rounded, densely welded

rhyolitic tuff and lava fragments as large as a few meters in diameter. One large

phyllitic shale clast about 50 m long, located just below the point of a spur about

400 m south of the confluence of Antone and Meadow Creeks, exhibits thin fingers

of tuff penetrating its margins along irregular fractures; block is thoroughly brecciated, but bedding layers have crude continuity through block. These features, not

observed in megabreccia blocks elsewhere in the quadrangle, are suggestive of collapse of block into ash-flow tuff during eruption of ash flow, and commensurate partial disaggregation of block. Lack of such features in other megabreccia blocks has

led me to doubt a collapse origin of most megabreccias mapped in the Jefferson

quadrangle

Tmcs Tuffaceous sandstone—Light-yellowish-brown tuffaceous sandstone forms a layer 3–

10 m thick that lies on top of heterolithic breccia (Thb) that surrounds rhyolite plug

(Trp) in Meadow Canyon at east margin of the quadrangle. Where tuffaceous

sandstone is thickest (in adjacent Corcoran Canyon quadrangle) if fills a deep channel in underlying heterolithic breccia. Sandstone displays bedding locally that is

suggestive of fluvial deposition. Unit is capped by a thin layer of megabreccia of

Meadow Canyon (Tmc)

Trc Tuff of Ryecroft Canyon (upper Oligocene)—Light-gray, light-yellowish-brown to lightyellowish-gray, and light-lavender-gray, moderately to densely welded, crystal-rich

rhyolitic ash-flow tuff. Weathers light pinkish brown, light grayish brown, light

orangish brown, and light brown. Unit contains common flattened pale-yellowish-
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brown pumice lapilli as long as 5 cm, and in places small lithic fragments of Paleozoic

and volcanic rocks. Where tuff is vitrophryic, pumice fragments are black glass.

Tuff is strongly magnetic in some places. Modal analyses of 10 samples of tuff of

Ryecroft Canyon indicate 22–35 percent phenocrysts (2–4 mm in diameter) made

up of the following phases: Q, 21–30; K, 23–35; P, 32–42; B, 4–8; O, 0.5–2; and

H, trace–3. Zircon is a common accessory mineral, and allanite, monazite, and

apatite are less common. Chemical analyses of three samples of tuff indicate the

following ranges in compositions: SiO2 , 73.2–73.4; Al2 O3, 14.1–14.2; K2 O,

4.68–4.75; Na2O, 3.46–3.60; CaO, 1.44–1.60; Fe2O3, 1.51–1.64; FeO, 0.29–

0.36; MgO, 0.37–0.41; TiO2, 0.28–0.30; P2O5, 0.13–0.14; and MnO, 0.04–0.07

(rhyolite). Lower part of tuff of Ryecroft Canyon crops out in roughly 120 m of

exposure in Ryecroft Canyon in southeast corner of the quadrangle. There the

tuff is overlain nearly conformably by a 30-m-thick layer of megabreccia of Meadow

Canyon (Tmc), which in turn is overlain by a nearly conformable 60-m-thick layer

of Ryecroft Canyon. Also, a thin lens of Ryecroft Canyon lies at base of

megabreccia of Meadow Canyon at confluence of Antone and Meadow Canyons,

and a small erosional remnant of Ryecroft Canyon lies on top of megabreccia about

600 m to the northeast.

Tuff of Ryecroft Canyon was named by Boden (1986, 1992) for exposures in

the canyon of that name; he considered it to be substantially younger than tuff of

Mount Jefferson (Boden, 1986, 1992). A 40Ar/39Ar date on sanidine from a sample I collected from tuff of Ryecroft Canyon in adjacent Corcoran Canyon quadrangle, about 2 km from quadrangle boundary, is 26.82±0.04 (1σ) Ma (L.W. Snee,

written commun., 1996). This date is within the range of dates determined for tuff

of Mount Jefferson by Boden (1992) and as given in this report; the two formations in part are equivalent in age

Tjc Megabreccia of Jefferson Canyon (lower? Oligocene)—A small area of megabreccia of

Jefferson Canyon extends into the quadrangle from adjacent Round Mountain

quadrangle to the west, 600 m south of Jefferson Canyon. Unit was described by

Shawe (1995). Boden (1986) assigned a K-Ar date of 32.3±0.7 Ma (biotite) to

unit, and L.W. Snee (written commun., 1997) determined a 40Ar/39Ar date of

32.56±0.07 Ma (biotite) for a sample of vitrophyre I collected in the Round

Mountain quadrangle. However, I have pointed out considerable contamination by

granitic material in vitrophyric rocks analyzed, and I have considered unit to be related to tuff of Mount Jefferson (Tmj), and age of unit to be about 27 Ma (Shawe,

1995).

A small (50 × 80 m) pluglike body of breccia that intrudes tuff of Mount

Jefferson (Tmj) 300 m southwest of Jefferson Canyon and 1.7 km from west

boundary of the quadrangle contains granite fragments as large as 2 m in diameter,

and minor small (a few centimeters in diameter) fragments of Paleozoic rocks in tuff

matrix. Tuff of Mount Jefferson near the pluglike body is brecciated and quartz

veined

Tgd Granodiorite of Dry Canyon (lower Oligocene)—Presence of a subsurface stock inferred

from abundant andesite porphyry clasts of similar composition in a megabreccia dike

(Tjsi), as shown only on cross section J–J'. Shawe (1995) described the 36-Ma

granodiorite of Dry Canyon as exposed in Round Mountain quadrangle west of

Jefferson quadrangle

Tr Rhyolite dikes and plug (early Oligocene)—Light-gray, nearly aphyric to sparsely porphyritic, flow-layered rhyolite; weathers light yellowish brown. Two thin dikes a

few meters thick occur at west margin of the quadrangle, one near southwest corner of the quadrangle, and a second extending into northwest part of the quadrangle 1 km south of Jefferson Canyon. A modal analysis of a sample of first dike indicates about 1 percent phenocrysts (1 mm in diameter) composed of the following

phases: Q, 13; K, 54; P, 24; B, 6, and O, 3. A small plug about 100 m in diameter

lies near the quadrangle boundary about 700 m south of Jefferson Canyon. A

modal analysis of a sample of the plug, which is hydrothermally altered, shows

about 16 percent phenocrysts (2–5 mm in diameter) consisting of 13 percent

quartz, 84 percent altered feldspars, 2.5 percent mafic psuedomorphs, and 0.6
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percent opaque oxide minerals. Accessory minerals are sparse zircon and rare

apatite. Rhyolite dikes and plug are part of a group of rhyolite intrusions in adjacent

Round Mountain quadrangle that are about 36 Ma (Shawe, 1995)

Granite of Shoshone Mountain (Late Cretaceous)—Mostly coarse-grained two-mica

granite that forms two large oval-shaped plutons, Round Mountain pluton which is

half in east part of adjacent Round Mountain quadrangle (Shawe, 1995) and half in

west part of the Jefferson quadrangle, and Belmont pluton which is mostly in north

part of adjacent Belmont West quadrangle and partly in south part of the Jefferson

quadrangle.

The Round Mountain and Belmont plutons appear to have been emplaced as

flat-topped intrusions that, several million years following solidification, were domed

by rise of granitic magma from a deeper level. The Belmont pluton differentiated

into layered phases while its roof was still nearly horizontal. Possibly the plutons

were emplaced initially as thick sill-like bodies at a crustal level where magma pressure was sufficient to raise overlying rocks. As mentioned in the description of the

megabreccia of Jefferson Summit as well as in a later section, the Round Mountain

pluton was intruded by Tertiary megabreccia bodies that brought schist from a

deeper level, suggesting that the pluton was bottomed in part on schist

Belmont pluton—The Belmont pluton consists of several intrusive bodies and rock

types, including a core of coarse-grained two-mica granite that is not exposed in

this quadrangle but is exposed in the Belmont West quadrangle (see Kgb of Shawe,

1998). Coarse-grained granite grades outward into sparsely porphyritic granite

(Kbgs) that in turn grades outward into porphyritic granite (Kbgp). These differentiated phases form an oval-shaped, domed pluton whose long axis strikes northwest; pluton is exposed in southwest and south-central parts of the quadrangle.

Porphyritic outer shell of pluton (Kbgp) is widely intruded by irregular masses to

dikelike bodies of fine-grained granite (Kbgf), and aplite dikes (Kba), some of which

penetrate into adjoining wall rocks. Some bodies of intermixed aplite and finegrained granite are labeled Kbgf-Kba. Thin-section examination shows that plagioclase and potassium feldspars have sharply defined optical properties in places in

some rock types, but elsewhere feldspar crystals intergrade in complex ways, as for

example where individual crystals are composed of patches of different feldspars, and

where feldspars appear indistinct and ragged with only faint evidence of twinning.

A few small pipes of quartz and feldspar (Kbq) were recognized in sparsely porphyritic granite near southwest corner of map area. Thin quartz veins are locally

abundant in pluton. Emplacement age of Belmont pluton, based on Rb-Sr wholerock data (John and Robinson, 1989), is 84.5±3.4 Ma (average of two isochrons).

As described by Shawe and others (1986, 1987) and by John and Robinson

(1989), the seven biotite and muscovite K-Ar ages of 80–82 Ma for the Belmont

pluton (see John and Robinson, 1989, for references) reflect an episode of later

doming, metamorphism, and mineralization of pluton.

The Belmont pluton is foliated parallel to its contact in the outer shell of porphyritic granite (Kbgp), and otherwise shows only sparse steep, northweststriking, postmetamorphic shears probably related to strike-slip deformation at the

northeast margin of the northwest-striking Walker Lane tectonic zone, a structural

“lane” that has been dominated by right-lateral deformation since late Mesozoic

time. A thin zone of northeast-dipping mylonite in porphyritic granite is projected

into the quadrangle from the northeast corner of the Belmont West quadrangle

south of the Jefferson quadrangle (Shawe, 1998)

Kbq Quartz-feldspar pipes—Several quartz and quartz-feldspar pipes occur in granite

(Kbgs) in southwest corner of map area. A quartz pipe 3 m in diameter is about 200

m from west boundary and 350 m from south boundary of map area. Another pipe

about 400 m north of south boundary of the quadrangle consists of a barren, massive, white quartz core about 5 m in diameter surrounded by a shell 1–2 m wide of

pale-yellowish-brown myrmekitic feldspar containing local patches of coarse-grained

muscovite. Nearby are two smaller similar pipes (not shown). Two other pipes

(shown as one location on map), about 800 m northeast of these pipes and about 7

m from each other, each consists of a core of barren white quartz 5 m in diameter
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surrounded by a shell of pale-yellowish-brown, coarse-grained myrmekitic feldspar.

This myrmekite, as observed under the microscope, consists of perthitic microcline

that contains abundant subparallel, anastomosing lenses and layers, 1–3 mm wide,

of quartz, and scattered small flakes of muscovite. A late 1-mm-wide veinlet of biotite, muscovite, and fluorite cuts quartz lenses and layers. I think the pipes formed

as a result of late-stage crystallization of magma at a deeper level, which resulted in

upward streaming of volatile components that precipitated quartz, feldspar, muscovite, and minor biotite and fluorite in zoned cylindrical bodies

Kba Aplite—Pale-gray to pale-yellowish-brown, leucocratic aplite. Forms dikes intruding

granite, and sills and dikes that intrude or form apophyses into Paleozoic metasedimentary wall rocks. Pods of quartz and pegmatite occur locally in aplite. Aplite is

varied in composition. A modal analysis of a sample from a zone of intermixed finegrained granite and aplite (Kbgf-Kba) in southeast corner of the quadrangle indicates about 48 percent quartz, 27 percent potassium feldspar (80 percent orthoclase and 20 percent microcline), 23 percent plagioclase, 2 percent hornblende, and

about 0.5 percent opaque mineral and 0.5 percent monazite. Traces of zircon and

apatite are present. Aplite has a granophyric texture; grain size is 1.5 mm and less.

Monazite appears as tiny pinkish grains in hand specimen. A modal analysis of a

second sample of aplite from a similar setting in southwest part of the quadrangle

indicates about 34 percent quartz, 34 percent orthoclase (virtually no microcline),

30 percent plagioclase, 1 percent biotite, 1 percent muscovite, and trace amounts

of opaque mineral, monazite, and zircon. This rock also has a granophyric texture,

grain size 0.1–1 mm.

One pegmatitic zone in aplite in southeast corner of map area contains amazonite and several unidentified minor pegmatite minerals (location labeled “A”)

Kbgf Fine-grained granite—Pale-gray to pale-yellowish-brown, fine- to medium-grained

granite. Commonly leucocratic or alaskitic (nearly white). Rock has a granophyric

texture; crystals are as large as 2–4 mm. One modal analysis indicates: Q, 34; K

(virtually all orthoclase and very little microcline), 30; P, 33 (in part myrmekite); B,

2.5; O, 0.2; and muscovite, 0.2. Apatite, zircon, and monazite are trace accessory

minerals. Forms irregular masses, dikes, and sills a few meters to a kilometer or

more across intruded into porphyritic granite (Kbgp) and Paleozoic wall rocks near

pluton margin in south part of the quadrangle. In some areas, unit was mapped as

fine-grained granite where remnants of pophyritic granite have been intricately invaded by fine-grained granite. In other places, masses of fine- and medium-grained

granite and aplite (Kbgf-Kba) were mapped together. Near mineralized zones at

margin of Belmont pluton, these rocks are altered to clay, sericite, and iron minerals, but were not mapped separately

Kbgp Porphyritic granite—Light-gray, coarse-grained granite. Contains about 15 percent or more large phenocrysts of orthoclase and (or) microcline 2–10 cm long.

Locally phenocrysts form closely spaced masses. In one locality near quartzfeldspar pipes (Kbq) I observed nearly vertical trains of phenocrysts in coarsegrained granite. Rock has a hypidiomorphic-granular texture; locally it shows mortar structure. Orthoclase poikilitically encloses quartz and other feldspars in places.

Muscovite commonly partially replaces biotite and plagioclase. Zircon and apatite

are common accessory minerals. In several places in north part of Belmont pluton,

porphyritic granite is altered to clay, sericite, and iron minerals, but was not mapped

separately

Kbgs Sparsely porphyritic granite—Pale-gray to pale-yellowish-brown, coarse-grained,

two-mica granite. Contains 0 to about 15 percent large phenocrysts 1–8 cm long

of orthoclase or microcline. Phenocrysts 1–2 cm long are more abundant than in

porphyritic granite (Kbgp), and phenocrysts 8 cm long are less abundant. Rock

has a hypidiomorphic-granular texture. Mineral composition of rock in the Belmont

West quadrangle just south of the Jefferson quadrangle was described by Shawe

(1998). A sample of hydrothermally altered rock collected south of mineralized area

near Barcelona Summit, as seen in thin section, is silicified and pyritized.

Plagioclase crystals are extensively silicified and partially replaced by sericite, biotite

margins are replaced by muscovite, and sparse fluorite blebs are scattered
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throughout. Sparsely porphyritic granite forms an annular shell 1–2 km wide in

outcrop pattern within peripheral porphyritic granite shell (Kbgp) that surrounds

pluton

Round Mountain pluton—Pluton consists of a principal mass of coarse-grained granite (Krg), the east half of which occupies west-central part of the quadrangle. It is

foliated most prominently near its margins where foliation, defined in part by biotite

and muscovite aligned in folia, parallels the intrusive contact of the pluton against

contact-metamorphosed Paleozoic wall rocks. Core of pluton west of Shoshone

Mountain consists of a large body of fine-grained granite, and southeast margin of

pluton is intruded by intermixed masses of fine-grained granite (Krgf) and aplite

(Kra). Small poorly defined bodies of fine-grained granite are present elsewhere in

pluton. Parts of pluton have been iron mineralized (Krgi) and tourmaline mineralized

(Krgt). Emplacement age of Round Mountain pluton, on the basis of Rb-Sr wholerock and mineral data (John and Robinson, 1989), is 89.6±3.3 Ma. Biotite and

muscovite K-Ar ages of about 80–81 Ma are thought to indicate time of doming,

metamorphism and mineralization of pluton (Shawe and others, 1986; Shawe,

1995). Foliation, which is defined partly by aligned muscovite and biotite flakes, reflects shearing that accompanied metamorphism rather than laminar flow associated with granite emplacement

Kra Aplite—Light-gray, pale-gray, and nearly white aplite bodies containing small zones

of pegmatite locally; locally weathers pale yellowish brown. Aplite forms small

masses and dikes in granite, and thin (a few meters wide) dikes and sills in Paleozoic

metasedimentary wall rocks. Aplite has xenomorphic-granular or granophyric to

hypidiomorphic-granular texture; locally aplite shows mortar structure. Typical examples were described by Shawe (1995). An atypical sample of aplite from south

part of pluton has the following mode: Q, 23; K (mostly orthoclase and only minor

microcline), 63; P, 9; B, 3; and O and muscovite, about 1 each. Zircon and apatite

are accessory minerals. This rock is strongly foliated, and its grain size is 0.1–2

mm. A thin quartz veinlet (4–5 mm wide) transgresses foliation; veinlet is lined with

a thin selvage of muscovite. As in the Round Mountain quadrangle (Shawe, 1995),

some aplite dikes are foliated and some are not, indicating that foliation, likely

formed at time of doming and metamorphism of pluton, took place during period of

aplite emplacement

Krgf Fine-grained granite—Pale-gray, fine- to medium-grained granite; locally weathers

pale yellowish brown. Commonly leucocratic or alaskitic (nearly white). Rock has

a granophyric texture and crystals are as large as 2–4 mm. Fine-grained granite

contains subequal amounts of quartz, potassium feldspar, and plagioclase, and minor

biotite and muscovite; a more detailed description of composition was given by

Shawe (1998). Rock forms irregular masses several hundred meters to a kilometer

or more across, which intruded into coarse-grained granite (Krg); also it forms a

large northwest-trending body almost 3 km long and 1 km wide that intruded core

of pluton. Core body is flanked by inward-dipping, arcuate faults, north and south,

that may have broken as a result of upward thrust of emplacement of fine-grained

granite (see cross sections G–G' and J–J'). Remnant patches of coarse-grained

granite are common within areas mapped as fine-grained granite. In places, masses

of fine- and medium-grained granite and aplite (Krgf-Kra) were mapped together

Krg Coarse-grained granite—Light-gray to pale-gray granite; locally weathers pale

yellowish brown. Foliation is pronounced near margin of pluton; it is conformable

with the contact and diminishes inward. Texture of granite is hypidiomorphicgranular to xenomorphic-granular. A modal analysis of a sample of coarse-grained

granite (1–10 mm in diameter) shows: Q, 16; K (25 orthoclase and 18 microcline),

43; P, including 2.5 myrmekite, 37; B, 3; and muscovite, 1.5. Opaque mineral, zircon, and apatite are accessory minerals. Muscovite replaces biotite and plagioclase.

(See chemical data for coarse-grained granite in the Round Mountain quadrangle in

Shawe, 1995)

Krgi Altered and iron-mineralized granite—Light-yellowish-brown to lightbrown, coarse-grained granite. Rock has been argillized and (or) sericitized and

pyritized; small pyrite cubes are altered to iron oxide that stains rock and lines
22

numerous fractures. Numerous thin, irregular quartz veins that are locally vuggy

occur throughout altered granite; a few veins are mineralized with base-metal

sulfides, and some veins carry the manganese tungstate huebnerite, and fluorite.

Chalcedonic-silica veins occur in places. Extensive, irregular zones of altered and

iron-mineralized granite occupy much of north and northeast parts of pluton and a

smaller zone is found near south margin of pluton

Krgt Tourmaline-mineralized granite—Light-gray, fine- to coarse-grained granite; weathers light yellowish brown to light yellowish gray. Tourmaline-mineralized

zone is a continuation of a halo surrounding the stock of 36-Ma granodiorite of

Dry Canyon (Tgd) in adjacent Round Mountain quadrangle (Shawe, 1995). A subsurface extension of stock into the Jefferson quadrangle is inferred on the basis of

tourmaline-mineralized granite at the surface (cross section H–H'). Rock contains

sparse to abundant crystals and clusters of black tourmaline, commonly forming

sunbursts or aggregates as much as 1 cm in size. As seen in thin section, feldspars

are partially to almost wholly replaced by sericite and muscovite; biotite is replaced

by muscovite; and tiny cubes or blebs of oxidized pyrite are scattered throughout.

Accessory minerals are zircon, apatite, and a high-relief mineral (monazite?) that

includes abundant opaque dust. A chemical analysis of rock indicates the following

percentages of components: SiO2 , 75.2; Al2 O3, 16.6; K2 O, 5.43; Na2 O, less

than 0.15; CaO, 0.19; Fe2O3, 1.28; FeO, 0.28; MgO, 0.45; TiO2 , 0.30; P2 O5,

0.14; and MnO, 0.02. Spectrographic analysis (by R.T. Hopkins of the U.S.

Geological Survey) indicates the presence of about 200 ppm boron. Unit forms a

zone about 1 km long at north margin of pluton, at west edge of the quadrangle

Toquima Formation (Middle Ordovician)—Typically thin-bedded, interlayered, marine

sedimentary rocks and their jasperized and (or) contact-metamorphosed equivalents.

Divided into argillite (Ota), limestone (Otl), dolostone (Otd), and quartzite (Otq);

limestone is jasperized in places (Otlj). Lithologies may be transitional from one

unit to an adjacent unit; minor amounts of other rock types may be present in

some units. Rocks are strongly deformed by folding and thrust faulting, probably in

several episodes from late Paleozoic to Late Cretaceous time. Units in the Toquima

have undergone extreme deformation and disruption such that some lithologic units

are out of stratigraphic sequence and the original sequence cannot be determined.

Thus, units are shown in the “Correlation of Map Units” as though they are of

correlative age. The Toquima correlates with part of Ordovician Palmetto

Formation in southern Nevada (for example, Ferguson, 1924; McKee, 1968) and

Middle and Upper Ordovician part of Vinini Formation in northern Toquima Range

(McKee, 1976). Formation constitutes a thrust plate overlying Zanzibar

Formation; the Toquima locally is internally cut by thrusts. Main part of plate is exposed in southeast corner of the quadrangle, and a small outlier crops out in southcentral part of the quadrangle south of Antone Creek. Original thickness is uncertain because of deformation but probably the formation is several hundred meters to

more than 1,000 m thick

Ota Argillite—Gray argillite; in places weathers light brownish gray, light pinkish gray, or

light purplish gray; somewhat phyllitic or fissile; locally silty, limy, or silicified. Platy,

laminated, and (or) thin bedded. In thin section, a sample of laminated silty argillite

consists of alternating layers that contain either sparse or abundant detrital grains

of quartz (0.02–0.1 mm in diameter). Matrix of layers is made up of minute intergrown mosaic quartz crystals, clay(?), and subparallel flakes of chloritoid(?). Quartz

grains in some laminae are graded. Cubes and blebs of oxidized pyrite about 0.05

mm in size are scattered throughout rock. Where gray argillite is in contact with

quartzite (Otq) graptolites commonly are preserved; graptolites appear to be absent

where argillite is light pinkish gray to light purplish gray and in contact with

quartzite. Graptolites collected in adjacent Round Mountain quadrangle were identified by R.J. Ross, Jr. (then of the U.S. Geological Survey) as of Mohawkian (Middle

Ordovician) age (Shawe, 1995). Argillite units about 30–60 m thick

Otl Limestone—Light-gray to dark-gray, thin-bedded and (or) thin-platy to laminated limestone; light brown to light orangish brown from iron stain where weathered.

Laminae are due mostly to tectonic shearing related to deformation that resulted in
23

folding and thrust faulting. Laminae generally parallel bedding but locally laminae

transgress bedding sharply. Some limestone beds are silty. In part, light-gray

limestone and dark-gray limestone form alternating layers a few meters to several

tens of meters thick. Color generally reflects abundance of grains and flakes of

graphite streaked along laminae; darker-gray limestone contains more abundant

graphite. Some dark-gray silty layers are silicified (“case hardened”) such that they

stand out on weathered surfaces, giving appearance of thin chert layers. Other

thin silicified limestone layers consist of mosaic quartz grains ranging from less

than 0.01 mm to 0.1 mm, containing wispy patches of graphite grains streaked

along layers. Sparse to abundant calcite crystals (as much as 0.1 mm long) are locally present. Larger units of silicified limestone (jasperoid, Otlj) are mapped seperately. A few beds of fetid, phosphatic limestone exhibit tiny fragments of phosphatic brachiopods and other shelly fossils (cell structures visible in thin section).

Thin-section examination also shows scattered isotropic, oval blebs (as long as 1

mm) of solid organic matter (“dead oil”?) whose walls are lined with an unidentified

low-birefringent mineral that is present also as tiny crystals suspended within the

organic matter. A few organic blebs show crystals of apatite growing out from

walls of blebs. Much limestone is mildly contact-metamorphosed and contains

sparse tiny crystals of tremolite. In a few places, especially close to thrust faults,

limestone is strongly brecciated or contorted. Much of thrust plate of Toquima

Formation in southeast corner of the quadrangle consists of limestone, which is

probably about 200 m in maximum thickness

Otlj Jasperized limestone (jasperoid)—Dark-gray rock; commonly stained reddish or

yellowish brown as a result of oxidation of pyrite during weathering; a siliceous

(cherty) rock that replaced limestone. Rock commonly is brecciated. Silicified darkgray layers, as seen in thin section, consist of mosaic quartz in interlocking grains

as much as 0.2 mm in diameter that form streaks a few millimeters wide; in some

streaks terminated quartz crystals indicate open-space filling. Some streaks are

crowded with wispy, anastomosing trains of graphite grains and flakes that parallel

layering in rock. Scattered oxidized pyrite cubes and blebs are common. Jasperoid

forms one layer a few tens of meters wide and more than a kilometer in length and

a patch several hundred meters across within thrust plate containing Toquima

Formation. Scattered patches of jasperoid in limestone (Otl) are too small to map

Otd Dolostone—Light-gray, laminated to thin-bedded dolostone; weathers light yellowish

brown. Nearly white locally. A few laminated beds contain streaks of graphite, and

give off a fetid odor when rock is broken. A thin section of one such sample consists of mosaic grains of interlocking dolomite crystals as much as 0.5 mm in diameter; minor mosaic quartz is present. Thin 1-mm-wide veinlets of calcite and quartz

traverse the dolostone. Two layers of dolostone 40–50 m thick are interlayered in

limestone (Otl) in southeast part of the quadrangle. One bed appears to grade laterally into limestone locally, suggesting that dolostone is a replacement of limestone rather than of sedimentary origin

Otq Quartzite—Gray to light-gray, fine-grained, massive to laminated quartzite; locally

weathers brown to light yellowish brown; locally bleached nearly white to very pale

yellowish brown. Some beds are siltstone. Quartz grains are closely packed and

mosaic textured, mostly less than 0.5 mm in diameter. In places mortar-structured

quartz grains (0.0l–0.05 mm) fill interstices among larger rounded quartz grains.

Minor sericite-muscovite clusters are scattered through rock, or form interstitial

screens or patches among quartz grains. Some slightly foliated quartzite contains

scattered sheaves (as much as 1.5 mm long) of crudely aligned tremolite crystals.

Sparse detrital black opaque oxide grains and a trace of zircon grains are present in

some quartzite. Euhedral crystals (mostly less than 0.05 mm, rarely as much as

0.1 mm in size) of metamorphic apatite are common in some quartzite. Cubes and

pyritohedrons of pyrite (mostly smaller than 1 mm in size) as well as black specks

and streaks of graphite are interstitial to quartz grains. Quartzite commonly is

veined with white quartz, and in a few places it contains masses of white quartz as

much as 5 m long. Quartzite constitutes only a small part of Toquima Formation.

It forms beds one to several meters thick that are locally brecciated, contorted, and
24

discontinuous as though boudinaged during extreme deformation. Probably at least

two quartzite beds are present in klippen in southeast part of the quadrangle, as

some quartzite is associated with graptolitic gray shale and some is associated with

nongraptolitic reddish- to pinkish-gray siliceous shale. A small klippe of quartzite of

the Toquima about 20 m in diameter lies on schist of Gold Hill Formation (gs) 300

m south of Jefferson Canyon and about 600 m from west boundary of the

quadrangle. Map unit is correlative with Middle Ordovician Eureka Quartzite of

eastern and southern Nevada on the basis of comparable graptolite faunas (for

example, Nolan and others, 1956)

Zanzibar Formation (lower Middle Ordovician)—Generally thin- to medium-bedded interlayered argillite and subordinate limestone (Ozal), limestone (Ozl), and dolostone

(Ozd). Limestone (Ozl) is further subdivided into calc-silicate-mineralized limestone

(Ozlc) and jasperized limestone (Ozlj). Some contact-metamorphosed argillite is

mapped as schist (Ozs). Because deformation has obscured original stratigraphic

sequence of units of formation, they are described as lithologic units rather than as

stratigraphic units. Formation forms a thrust plate sandwiched between an upper

thrust plate of Toquima Formation and lower thrust plates of Cambrian(?) siltstone

formation (st), Cambrian Gold Hill Formation (g units), and Cambrian Mayflower

Formation (m units). It is intruded by both Belmont and Round Mountain granite

plutons, against which limestone and argillite have been contact metamorphosed

(to Ozlj and Ozlc, and to Ozs). The Zanzibar is widely exposed in south part of the

quadrangle, and in a small area in northwest part of the quadrangle. Age of formation is Whiterockian (early Middle Ordovician, based on fossil identifications by

W.B.N. Berry, R.J. Ross, Jr., and J.W. Huddle of samples from the Manhattan

quadrangle; Shawe, 1999). Formation is correlative with part of Palmetto

Formation in southern Nevada (for example, Ferguson and Cathcart, 1954). It also

is in part the same age as Antelope Valley Limestone in eastern Nevada (Ross,

1970), on the basis of comparable faunas

Ozal Argillite and subordinate limestone—Medium- to dark-gray, platy, noncalcareous to

limy, locally laminated or phyllitic argillite; and subordinate interlayered light- to

dark-gray, thin-platy, laminated limestone. Argillite and limestone are in part silicified or silty. Close to sole thrust fault of Zanzibar thrust plate, argillite is reddish

gray to light reddish brown or brown. Rock has been mildly metamorphosed regionally. Argillite consists of oriented minute flakes and grains of clay, sericite or

chloritoid, and quartz, all of which are commonly intergrown with lenses of calcite;

argillite has moderately developed phyllitic cleavage. Graphite dust may be strung

out along cleavage planes. Faint traces of graphite that transgress cleavage appear

to reflect original bedding. Limy argillite adjacent to or as lenses within granite may

be contact metamorphosed to quartz-calcite-chloritoid schist, commonly containing

small amounts of sericite, tremolite, and graphite. In places argillite and limestone

are contorted and isoclinally folded. Argillite and subordinate limestone form units

as much as about 100 m thick interlayered in limestone (Ozl) in south part of the

quadrangle

Ozl Limestone—Light-gray to dark-gray, thin-platy to thin- to medium-bedded, laminated

limestone (laminae due mostly to shearing related to thrust faulting probably at time

of mild regional metamorphism); in places weathers light brown, light orangish

brown, or light brownish gray. Locally, limestone is argillaceous, silty, and (or)

silicified. A few layers contain thin zones of nodular chert, and elsewhere thin,

dark-gray layers of silicified (“case-hardened”) silty limestone. Most beds are a few

centimeters to 10–20 cm thick; uncommonly beds are as thick as 70 cm and massive. In places beds are strongly contorted and (or) brecciated. Beds may be isoclinally folded, broken, or boudinaged. Thin-section examination shows laminated

limestone to consist chiefly of crude, interlocking lensoids of calcite ranging from

less than 0.1 mm to 0.5 mm long, commonly lined with dusty streaks of graphite.

Graphite varies from almost absent to very abundant, resulting in limestone coloration that ranges from light gray to almost black. Scattered crystals of tremolite

(as long as 2 mm) and clear to seived crystals of diopside (as long as 4 mm) are

found in laminated limestone near granite plutons. Tiny cubes and blebs of oxidized
25

pyrite are common; where they are abundant, iron oxide derived from pyrite accounts for brownish stains on weathered rocks. Limestone consititutes bulk of

Zanzibar Formation; it is widespread in south part of the quadrangle and is present

in lesser amount in northwest part of the quadrangle near Jefferson Canyon.

Aggregate thickness of unit is uncertain, but it is at least several hundred meters

Ozlc Calc-silicate-mineralized limestone—Light-gray, light-greenish-gray, light-yellowish-gray and light-yellowish-brown, dense calc-silicate-mineralized limestone; weathers light brown. Rock is the result of contact metamorphism near or in contact

with both Belmont and Round Mountain plutons in south part of the quadrangle.

Thin layers commonly are interlayered with lesser amounts of dark-gray silicified

limestone and light-gray to medium-gray limestone. Several types of calc-silicatemineralized limestone are recognized in the quadrangle. A common type contains

abundant ragged and seived, in part interlocking, crystals of diopside (0.5–3 mm

long) in a matrix of mosaic quartz (grain size about 0.5 mm); abundant small (as

large as 0.5 mm) sphene crystals are scattered throughout. Some samples contain

tremolite also, and sparse to abundant graphite may be present. One sample, as

seen under the microscope, consists of a fine-grained (0.01–0.1mm) schistose aggregate of mosaic quartz, semi-aligned subhedral andalusite crystals, and abundant

tiny blebs of pyrite(?). Ragged crystals and aggregates of andalusite, diopside, and

uvarovite garnet, 1–5 mm long, are present in schistose aggregate. In vicinity of

Barcelona Summit, coarse-grained (to 1 cm) varieties of calc-silicate-mineralized

limestone contain brown grossularite(?) garnet and epidote, as well as sulfide minerals, including molybdenite. Thickness a few meters to several tens of meters

Ozlj Jasperized limestone—Dark-gray silicified limestone; commonly stained yellowish

brown to reddish brown as a result of weathering and oxidation of pyrite.

Commonly brecciated; in places, folded bedding layers are evident. Common veinlets of white to light-yellowish-brown quartz; uncommon thin layers of pale-green

fluorite along light-gray bedding layers 0.8 km southeast of Barcelona Summit.

Some dark-gray, brown-weathering jasperoid has the fine-grained, dense appearance of hornfels. In a few places gossan is associated with intensely iron-mineralized jasperoid. Forms beds laterally continuous for tens of meters or more, or irregular masses that grade along bedding from unaltered limestone. Jasperized limestone is localized along faults or bedding. Masses of jasperized limestone range

from thin layers to as much as hundreds of meters across. Not all thin layers were

mapped

Ozd Dolostone—Light-gray, laminated dolostone; weathers light yellowish gray to light

yellowish brown. In places contains numerous laminated limy chert layers, and

nodules (as much as 30 cm long); chert is boundinaged. Chert commonly displays a

selvage of white tremolite a few centimeters thick. Dolostone as seen in thin section consists of intergrown oblong grains of dolomite (0.02–0.2 mm long) aligned

with laminae, which contain numerous scattered needles, sheaves, and sunbursts of

tremolite as long as 3 mm. Dolostone grades laterally into light-gray limestone.

Forms part of a single bed about 30 m thick and 3 km long in limestone in southeast corner of the quadrangle. Segments of bed are limestone, suggesting that

dolomite is secondary rather than of sedimentary origin

Ozs Schist—Dark-gray, foliated and lineated schist; weathers brown. Locally contains

varied amounts of tremolite, muscovite, and graphite. A thin section of a dark-gray

schist sample indicates abundant films of graphite along folia that are defined by

strings of grains of mostly(?) quartz (grain size about 0.01 mm). Scattered sheaves

(as much as 2 mm long) of tremolite, and cubes and blebs of oxidized pyrite (0.01–

0.05 mm in diameter), appear to postdate foliation. A few andalusite crystals (as

much as 0.1 mm long), with hour-glass-shaped concentrations of graphite, are present. Elongate concentrations of graphite oblique to foliation form compositional

layering in schist. Schist occurs where thin apophyses of aplite and (or) finegrained granite of Belmont and Round Mountain plutons invaded argillite, or where

thin leaves of argillite were incorporated into margins of plutons, in south part of

the quadrangle
26

st Siltstone formation (Cambrian?)—Medium-gray, laminated, limy arkosic(?) siltstone;

weathers medium brownish gray. Thin and evenly bedded. Thin-section examination shows a fine-grained (0.01–0.1 mm) mosaic aggregate of quartz, orthoclase(?),

and calcite grains that contains abundant subparallel chloritoid flakes that are commonly ragged and less than 0.1 mm long. Orientation and varied abundance of

chloritoid flakes define laminations that may or may not be parallel to bedding.

Abundant rounded and corroded detrital grains of zircon are surrounded by

pleochroic halos in adjacent chloritoid flakes. Formation forms a thrust plate that

underlies plate of Zanzibar Formation (Oz units) and overlies plate of Gold Hill

Formation (g units), in south-central part of the quadrangle. Maximum thickness

of thrust plate probably less than 100 m. Siltstone formation correlates with a

thrust plate of Cambrian(?) arkosic siltstone in the Manhattan quadrangle that is

lithologically similar and that occupies a similar structural position (Shawe, 1999)

Mayflower Formation (Cambrian?)—Phyllitic argillite (mf) and knotted schist (ms).

Within about 2–3 km of the Cretaceous granite plutons, formation is metamorphosed to knotted schist; farther from plutons, formation consists of phyllitic

argillite that generally exhibits well-developed axial-plane(?) cleavage. The transition

from argillite to schist is gradational and occurs through an interval of perhaps a

kilometer. Schist is considered to be a result of contact metamorphism whereas

phyllitic argillite is considered to be a result of regional metamorphism. These

strata are extremely deformed by shearing and isoclinal folding. They are widely

exposed in south-central and southeast parts of the quadrangle, and in small

patches south of Jefferson Canyon in northwest part of the quadrangle. Map unit

underlies thrust plates of Gold Hill Formation (g units), Cambrian siltstone formation (st), Ordovician Zanzibar Formation (Oz units), and Ordovician Toquima

Formation (Ot units). Formation was considered by Ferguson (1921; 1924) to be

Ordovician(?); unfossiliferous phyllite referred to Mayflower in Toiyabe Range to

the west was considered by Finney and Perry (l991) to be Lower Ordovician; formation is here believed to be Cambrian(?) on the basis of similarity to known

Cambrian rocks elsewhere in region. As the entire formation is nowhere exposed,

its original thickness is unknown, but it likely was several hundred meters to a kilometer or more; considerably more than this may remain in the subsurface.

Ferguson (1924, p. 20–25), as well as Ervine (1972, p. 44), Stewart (1980),

and Kleinhampl and Ziony (1984, p. 46–47), following Ferguson, considered the

Mayflower and Zanzibar Formations in the southern Toquima Range to be parts of

a stratigraphic sequence. Ferguson (1924) and Stewart (1980) included the

Toquima Formation as well in the sequence. However, mapping in the Round

Mountain (Shawe, 1995) and Manhattan (Shawe, 1999) quadrangles suggests that

the Mayflower is separated from the overlying Zanzibar and Toquima by a thrust

plate of Gold Hill Formation. The Mayflower appears unrelated to the Zanzibar and

Toquima

mf Phyllitic argillite—Medium-gray, mostly phyllitic argillite or shale; weathers olive gray

to brownish gray. Grades from platy shale a few kilometers from Cretaceous

granite plutons, to phyllitic schist closer to plutons. Thin-section examination of a

laminated phyllitic schist sample indicates a mass of strongly oriented clay and

sericite flakes parallel to foliation, and quartz grains, mostly less than 0.01 mm in

size. Scattered throughout are specks and streaks of iron oxide (probably from oxidized pyrite), and sparse streaks of graphite dust. In places bedding laminae are

tightly folded and intersected by generally steep axial-plane(?) cleavage. Rocks are

locally brecciated, and are mineralized by quartz and iron. Several hundred meters

of the section of the phyllitic argillite is exposed in southeast corner of the

quadrangle

ms Knotted schist—Dark-gray, well-foliated knotted schist; weathers dark olive gray to

brown. Extremely deformed by shearing and probable isoclinal folding. Rock is locally fractured, brecciated, and iron mineralized. In places it contains irregular veinlets of iron-mineralized quartz. Thin-section examination shows a well-developed foliation manifested by abundant chloritoid and (or) biotite, phlogopite, or sericite

flakes. Flakes are commonly less than 0.02 mm long, locally as long as 0.5 mm,
27

and are embedded in a granular mass of quartz grains of similar size. Sparse to

moderaterly abundant tiny grains of oxidized pyrite are scattered throughout, and

graphite dust and films are streaked along the foliation. Rock is characterized by

abundant oval knots, mostly about 5 mm long; knots consist of quartz and chloritoid, generally oriented parallel to the foliation. One thin section shows knots to

consist of lensoid forms of andalusite(?), 4 mm–1 cm long, aligned parallel to

schistosity. Knots contain abundant flakes of chloritoid and sericite mostly less

than 0.01 mm long oriented parallel to schistosity, and contain scattered specks of

graphite. Widely exposed near Cretaceous granite plutons in south-central part of

the quadrangle, and in smaller patches south of Jefferson Canyon in northwest

corner of the quadrangle

Gold Hill Formation (Lower Cambrian)—Phyllitic schist (gs), quartzite (gq), and

limestone (gl) deposited as marine shale, sandstone, and limestone, respectively.

Units that are schist with subordinate interlayered quartzite beds are mapped as

gsq, and units that are quartzite with subordinate interlayered schist are mapped

as gqs. Some limestone is calc-silicate mineralized (glc). The Gold Hill

Formation is exposed in central part of the quadrangle, and in northwest corner of

the quadrangle in and near Jefferson Canyon. Parts of formation are at least several hundred meters thick, but because of thrust faulting and folding, original thickness of formation is not known; it probably was at least a kilometer thick. Forms a

thrust plate that underlies plates of Zanzibar Formation (Oz units) and Cambrian(?)

siltstone formation (st), and that overlies a plate of Mayflower Formation (m

units). Age in part Early Cambrian, based on the presence of Olenellus gilberti

Meek in similar rocks in Toiyabe Range about 25 km to the west (Ferguson and

Cathcart, 1954). Correlative with upper part of Prospect Mountain Quartzite in

eastern Nevada and Zabriskie Quartzite in southern Nevada

gs Phyllitic schist—Olive-gray, light-olive-brown, and olive-brown phyllitic argillite to

mica schist; weathers light yellowish brown to dark brown. Silty in part. Minor

thin layers of quartzite and limestone are present in places. Schist is more strongly

metamorphosed near granite contacts. A thin section of schist collected near

granite shows a mosaic groundmass of quartz grains (0.05–0.1 mm) through which

are scattered abundant, crudely aligned, ragged to euhedral flakes (0.1–0.3 mm) of

choritoid and muscovite that define foliation of rock. Sericite-flake aggregates (as

large as 2 mm) and sparse, ragged oxidized-pyrite patches (as large as 0.5 mm) are

scattered through rock. A nearby similar sample shows larger aggregates (as large

as 5 mm) of sericite flakes and muscovite crystals (as large as 1 mm). Euhedral

crystals of metamorphic apatite, and sparse detrital zircon crystals, are present.

L.W. Snee (written commun., 1997) reported a white mica (sericite) 40Ar/39Ar

date of 79.74±0.12 Ma for sulfide-mineralized rock collected at a prospect in unit

within a few meters of granite contact of Round Mountain pluton (rock sample

locality R7). The date indicates an episode of mineralization related to the inferred

80- to 82-Ma episode of doming and metamorphism of the 89.6-Ma pluton.

Individual units of schist are locally as thick as 200 m

gq Quartzite—Light-yellowish-brown, mostly fine-grained, thin-bedded to massive

quartzite; weathers light brown to dark brown. Beds generally are 0.1–1.5 m thick;

in part, unit has low-angle crossbeds, and in part it is massive. Beds commonly

separated by schistose-shaly partings. Some quartzite contains extensive worm

borings, probably Skolithos, as much as 1 cm in diameter and 15 cm long; softsediment slump structures are uncommon. As seen in thin section, rock consists

chiefly of closely packed, well-rounded, somewhat oblong grains of quartz (0.1–0.3

mm in diameter), partly surrounded by interstitial sericite and very fine grained mosaic quartz. Sparse metamorphic tourmaline, oxidized pyrite blebs and cubes, and

detrital zircon are scattered through rock. One deformed worm boring(?) is filled

with sericite, iron oxide, and tourmaline. Another quartzite sample contains moderately abundant detrital zircon, oxidized pyrite, and metamorphic tourmaline. Quartz

grains form a mosaic texture; metamorphic muscovite occurs as scattered flakes as

long as 1 mm. Individual quartzite units are locally as thick as 150 m
28

gsq Schist and subordinate quartzite—Light-olive-brown to light-yellowish-brown phyllitic schist; sparse to moderate interbeds, a few centimeters to a meter or more

thick, of medium-gray to light-yellowish-brown, brown-weathering quartzite. Map

unit forms layers a few tens of meters thick in central part of the quadrangle

gqs Quartzite and subordinate schist—Light-yellowish-gray, olive-gray, light-gray, and

nearly white quartzite (as beds about 0.2–1.5 m thick) interlayered in places with

shaly-silty phyllitic schist intervals (10–30 cm thick); weathers light to dark brown.

Quartzite locally contains abundant worm borings, especially in an isolated exposure

of Gold Hill Formation (gqs) about 1 km southwest of Meadow Canyon near east

edge of the quadrangle. Worm borings, as much as 1 cm in diameter and disposed

normal to bedding, are probably Skolithos, a characteristic form in quartzite in

Wood Canyon Formation, Zabriskie Quartzite, and Harkless Formation, age equivalents of Gold Hill Formation is southeastern California and southern Nevada (J.H.

Stewart, written commun., 1997; see also Link and others, 1993, p. 552). This

isolated exposure may be incorrectly mapped as Gold Hill Formation. Ervine (1972)

described Skolithos in quartzite that he mapped as parts of the Zabriskie and Wood

Canyon Formations in an area from south of Bull Frame Canyon southwestward

to near Barcelona Summit. I have included these rocks also as part of Gold Hill

Formation. Low-angle crossbeds and scours that are convex upward indicate that

quartzite beds are overturned in places. Map unit is exposed in northwest and

southeast parts of the quadrangle and locally is as much as 150 m thick in southeast area

gl Limestone—Light-gray, light-pinkish-gray, and light-yellowish-brown, evenly to irregularly layered, in part laminated limestone; weathers light brownish gray, light

orangish brown, and brown. In places, beds about 1 cm thick are lenticular to

nodular. Locally, 2- to 30-cm-thick beds of chert and argillite are interlayered.

Rock commonly is sheared, brecciated, and iron mineralized. A thin section of fractured limestone shows disoriented fragments of limestone of varied grain size

(0.02–0.3 mm), veined with calcite; abundant oxidized pyrite cubes (0.05 mm in size)

are scattered throughout. Strongly brecciated limestone shows much pulverized

matrix that contains significant iron oxide. Near granite contacts, thin layers in

limestone commonly are mineralized by calc-silicate phases. L.W. Snee (written

commun., 1997) reported an actinolite 40Ar/39Ar date of 84.1±0.3 Ma for material

from the metamorphosed base of a limestone layer about 100 m from granite contact of Round Mountain pluton (rock sample locality R8). The date suggests metamorphism associated with emplacement of the 89.6-Ma pluton. The date may have

been reset somewhat by a thermal event that followed intrusion, probably the

younger (80–82 Ma) episode of doming, metamorphism, and mineralization of the

pluton. Limestone units, chiefly interlayered in phyllitic schist (gs) in southcentral and northwest parts of the quadrangle, are as much as about 20 m thick

glc Calc-silicate-mineralized limestone—Gray, light-greenish-gray, and green, dense

calc-silicate-mineralized limestone; weathers light brown, brownish gray, and

brownish green. Locally, bedding characteristics of original limestone are present;

in other places rock is structureless. A coarse-grained hand specimen of unit glc

contains abundant sheaves of actinolite needles (as much as 2 cm long) intergrown

with calcite and phlogopite crystals (about 0.5 cm in size), set in a dense, finegrained matrix of granular diopside. Unit glc was mapped separately only along

Berlin Canyon in northwest part of the quadrangle, where unit is about 20 m thick
29

SUMMARY OF GEOLOGIC EVENTS

I infer that the Cambrian marine clastic

and carbonate rocks of the Mayflower

Formation (m units), Gold Hill Formation (g

units), and Cambrian(?) siltstone formation (st)

in the Jefferson quadrangle were deposited as

part of a westward-thickening sedimentary

wedge on the continental shelf at what was

then the western edge of the North American

continent (Stewart, 1980). Ordovician marine

rocks of the Zanzibar (Oz units) and Toquima

(Ot units) Formations were probably deposited

as transitional facies rocks between predominantly carbonate-facies sediments on the continental shelf and siliceous-facies sediments in a

deeper western ocean basin. Rocks of the

Toquima Formation are more siliceous than

those of the Zanzibar Formation, and they may

have been deposited west of the Zanzibar.

Although opinions differ as to whether or

not the southern Toquima Range lies within the

region of late Paleozoic–early Mesozoic compression and thrust faulting (for example,

Stewart, 1980, p. 39, 55–59), the structural and

lithologic characteristics of the rocks in the

southern Toquima Range indicate that it is

within that region. No sedimentary rocks of

known late Paleozoic and Mesozoic age are present in the Jefferson quadrangle. However, the

presence of ophiolite (interlayered serpentinite,

greenstone, and chert) 8 km south of the quadrangle (Shawe, 1998, 1999) proves that oceanic

rocks were thrust into the region of the

quadrangle. The oceanic rocks are interleaved

with strongly deformed allochthonous rocks of

Ordovician age: the Toquima Formation lies in

thrust contact upon ophiolite south of the quadrangle and the Toquima there is part of the same

plate of Toquima exposed in the Jefferson quadrangle. Elsewhere in the region such rocks have

been interpreted to have been emplaced during

the Late Devonian–Early Mississippian Antler

and (or) the Late Permian–Early Triassic

Sonoma orogenies (Poole and Desborough,

1973). Also, fragments of Permian Diablo

Formation in eruptive volcanic megabreccia in

the Manhattan quadrangle (Shawe, 1999) 10 km

southwest of the Jefferson quadrangle indicate

that the Golconda allochthon (emplaced during

the Sonoma orogony) extends into the region.

These occurrences thus suggest that periodic

compressional deformation in the region started

in the late Paleozoic and continued into the

Mesozoic.

The present configuration of plates of

Cambrian and Ordovician rocks reflects such deformation: a lower plate of the Mayflower

Formation is overlain on a strongly folded thrust

fault by a plate of the Gold Hill Formation, which

in turn is overlain successively by plates of the

Cambrian(?) siltstone formation, the Zanzibar

Formation, and the Toquima Formation. The

higher thrust faults generally are less folded

than are the lower thrust faults, and in places

the higher faults truncate the lower ones, indicating a sequence of thrusting from lower to

higher. Some thrust faults appear more tightly

folded near granite plutons (Kr and Kb units),

and part of the folding may have resulted from

intrusion of the plutons.

Because orientation and configuration of

structures in the Paleozoic rocks may have resulted in part from the shouldering effect of intrusion of the Cretaceous granite plutons (K r

and Kb units), the structures probably cannot be

used to infer direction of thrust faulting. For example, in the south part of the quadrangle, tight

folds in the thrust fault that separates the

Zanzibar Formation from underlying Mayflower

Formation, as well as foliation attitudes in the

metamorphosed sedimentary rocks, appear to

wrap around the north margin of the Belmont

pluton. Thrust faulting probably continued into

the Late Cretaceous, as evidenced by sheared

granite sills emplaced along thrust faults in the

Manhattan quadrangle to the southwest

(Shawe, 1999), and by the foliated character of

the Cretaceous plutons, which formed millions

of years following emplacement of the plutons,

and at the time of pluton doming. The foliation

thus is suggestive of compressional deformation.

The Late Cretaceous granite plutons were

emplaced during the waning stages of compressional deformation. Initially, they may have

formed thick lensoid sills intruded along major

deep-seated thrust breaks. The Round Mountain

pluton (Krg) was emplaced at about 90 Ma and

the Belmont pluton (Kbg) was emplaced at about

85 Ma. The Belmont pluton before solidification

differentiated into an upper porphyritic granite

layer (Kbgp), an underlying sparsely porphyritic

granite layer (Kbgs), and a lower coarse-grained

granite layer (Kbg of Shawe, 1998). Evidence

of graded layering and low-angle crossbeds in

the sparsely porphyritic layer in the Belmont

West quadrangle (Shawe, 1998) indicates sedimentation from convection currents prior to

complete consolidation of the granite magma.

Both the Round Mountain and the Belmont plutons were invaded by aplites (Kra and Kba) and

fine-grained granite (Krgf and Kbgf) during an

episode of doming that saw development of foliation localized near and subparallel to their margins, at about 82–80 Ma. The plutons also were

partly metamorphosed and mineralized during
30

this stage of deformation (see Shawe, 1995,

1998, 1999). Some of the details of mineralization were described by Shawe (1988).

Emplacement of the Pipe Spring granite pluton

(exposed in the Manhattan and Belmont West

quadrangles, Shawe, 1998, 1999) took place at

about 80 Ma, coincident with the late episode of

deformation, intrusion, metamorphism, and

mineralization of the Round Mountain and

Belmont plutons.

At about 36 Ma, when the presently exposed rocks were still beneath a cover probably

more than 1 km thick, a swarm of northeasttrending rhyolite dikes and a rhyolite plug (Tr)

were emplaced into and near the Round

Mountain pluton west of the quadrangle; immediately thereafter, a granodiorite stock (Tgd) and

associated andesite dikes (Ta of Shawe, 1995)

were intruded into the northeastern part of the

rhyolite dike swarm. The rhyolite intrusion

event barely reached into the Jefferson quadrangle, but evidence of the granodiorite event

extends about 1 km into the quadrangle in the

form of a tourmalinized granite halo adjacent to

subsurface intrusion of granodiorite (see cross

section H–H'). The mineralizing event that introduced tourmaline in the granite also accounted for extensive iron mineralization of

granite outward from the tourmaline halo, and

localized along the north margin of the pluton.

Possibly an early stage of precious-metal

mineralization occurred in this area at that time,

in what is now the Jefferson mining district.

The tuff of Ryecroft Canyon (Trc) was

emplaced in the southeast part of the quadrangle

at about 27 Ma. The source of the Ryecroft

Canyon may be a caldera in Monitor Valley on

the east side of the Toquima Range (Shawe and

Byers, 1999). The great thickness of the

lower, main part of the formation (at least 300

m; estimated to be as much as 1,000 m by

Boden, 1992) just east of the quadrangle, suggests a close source. Possibly the Ryecroft

Canyon within the Jefferson quadrangle was

erupted from a northwest-trending structure extending 3 km from the west margin of the proposed caldera underlying Monitor Valley. Such a

vent might be analogous to vents proposed by

Ekren and others (1980) along strike-slip zones

elsewhere within the Walker Lane. Cross section P–P' presents a possible subsurface configuration of the formation related to the zone of

northwesterly strike-slip faulting that coincides

with the lower reach of Meadow Canyon. The

widely divergent attitudes within the tuff of

Ryecroft Canyon, some exceptionally steep,

suggest that the formation collapsed somewhat

chaotically into its present form following

eruption.

Before the final stage of emplacement of

an upper relatively thin part of the Ryecroft

Canyon (Trc), a layer of the megabreccia of

Meadow Canyon (Tmc) 50 m or so thick was

deposited on the lower part of the Ryecroft

Canyon. Also, thin layers of the tuff were emplaced locally in the lower part of the megabreccia formation. The contemporaniety of the two

formations introduces a puzzling aspect of their

emplacement, which is discussed further below.

I consider that the megabreccia of

Meadow Canyon (Tmc) was erupted through a

conduit, evidence for which is the presence of

rhyolite plugs (Trp) that attest to a conduit penetrating into the deep crust. Within the inferred

vent zone of megabreccia eruption, large slabs of

welded ash-flow tuff stand vertically, implying

that they were oriented thus by some vertical

movement of the enclosing tuff matrix during

emplacement. Many large blocks within the

megabreccia were thoroughly brecciated before

emplacement; inasmuch as such brecciated rock

is nowhere in evidence at the surface within ashflow tuff formations, I conclude that the blocks

were derived from depth (see Shawe and

Snyder, 1988, for a more complete discussion of

eruptive megabreccia). Most of the blocks are of

the tuff of Ryecroft Canyon (Trc); a few consist

of welded ash-flow tuff of Mount Jefferson

(Tmj), autobrecciated rhyolite, rhyolite lava, and

rhyodacite lava. The latter three rock types are

not known at the surface, suggesting possible

derivation from depth. The matrix of the

megabreccia has a phenocryst composition similar to that of the Ryecroft Canyon, attesting to

the affinity of the two units.

The tuff of Antone Canyon (Tac) intertongues with the megabreccia of Meadow

Canyon, and it is similar in phenocryst content

to the tuff of Ryecroft Canyon. The three

units, of comparable age, appear to be genetically related. Their puzzling intertongueing relation poses a question as to manner of emplacement. Perhaps the megabreccia is akin to a pyroclastic-flow lag deposit, and the Antone

Canyon is a distal facies from which much of the

clast fraction has been winnowed. The

Ryecroft Canyon may have been emplaced intermittently through conduits that tapped the

same source magma, but not with the explosive

force that entrained the large blocks that characterize the megabreccia.

Although outflow facies of the tuff of

Mount Jefferson lies upon the tuff of Antone

Canyon, the determined age of the Mount
31

Jefferson is similar to that of the Ryecroft

Canyon (about 27 Ma), and hence also close to

the age of the Antone Canyon and of the

megabreccia of Meadow Canyon. The surface

disposition of the megabreccia of Meadow

Canyon relative to the Mount Jefferson caldera

(for example, cross section N–N'), as well as the

intermixing of the tuff of Mount Jefferson as

both matrix and clasts in the megabreccia, suggest that the megabreccia may have formed by

collapse into the Mount Jefferson caldera. I

think, however, that the evidence for eruption of

the megabreccia countervails this supposition;

intermixing of the formations probably was a result of the more or less contemporaneous activity of two separate sources.

Chemical and phenocryst compositions of

tuffs in the tuff of Mount Jefferson within the

Jefferson quadrangle suggest episodic eruption

from a differentiated magma chamber, such that

earlier eruptions tapped higher portions of a

chamber that contained rhyolite magma, and

later eruptions tapped lower portions that had

differentiated to trachydacite composition.

However, there are local inconsistencies in this

pattern, and faulting has disrupted the volcanic

section, such that stratigraphic correlations

within the tuff of Mount Jefferson are uncertain.

Reversals of compositional trends resulting from

long pauses in eruptive activity, or from magma

mixing, may have occurred.

Following ash-flow eruption and collapse

of the Mount Jefferson caldera, erosion produced an irregular topography that was covered

in rather rapid succession by a sequence of unrelated rock units. The megabreccia of Jefferson

Summit (Tjs units) was erupted from pipes and a

dike, up through the Round Mountain pluton and

wallrocks, and was spread as an outflow apron

across the middle part of the tuff of Mount

Jefferson within the quadrangle. (Presence of

schist fragments in the dike and plugs within the

Round Mountain pluton indicates that schist exists below the granite, and supports my suggestion that the granite plutons were originally

thick sills, and hence bottomed on schist wallrocks.) The megabreccia vents are aligned with

the northwest-striking southwest margin of the

caldera; the aligned vents and caldera margin

probably reflect control by northwest-striking

strike-slip structures at the margin of the Walker

Lane. A broad zone of altered granite (Krgi)

bounds the trend of dike and pipes, suggesting

an episode of hydrothermal alteration related to

emplacement of the dike and pipes.

A thick sequence of volcaniclastic rocks of

Little Table Mountain (Tlt units) was deposited

next, in the northeast part of the quadrangle

and east of the Soldier Spring fault. Clastic materials may have been derived mostly from erosion of the Mount Jefferson center; the source

of the interlayered tuffs may have been latestage volcanism of the Mount Jefferson caldera.

Mesobreccia with abundant fragments of

Paleozoic rocks (Tpm) and the megabreccia of

Bull Frame Canyon (Tbf) that overlie the volcaniclastic rocks may be related to the cycle of

emplacement of the megabreccia of Meadow

Canyon. The mesobreccia, where exposed on

the steep slope of Antone Canyon, is seen to

have a tuff matrix; the unit may have been emplaced as an eruptive blanket, although no

source vent is recognized. Alternatively, it may

have formed by dumping of erosional debris,

from topographically higher areas to the west,

into volcanic ash derived either from the Mount

Jefferson caldera or from the source of the tuff

of Antone Canyon (Tac).

The Isom-type welded ash-flow tuff formation (Ti) was spread across much of the volcaniclastic rocks of Little Table Mountain (Tlt units)

about 27 Ma, and derived from a source probably

to the east. It was quickly followed by emplacement of the lower and upper members of the

Shingle Pass Tuff (Tspl and Tspu), between

about 27 and 26 Ma, from an eastern source.

Despite the rather rapid accumulation of these

units, their deposition was punctuated by periods

of moderate erosion.

A northwest-trending array of rhyolite

plugs (Trp) was emplaced into the tuff of Mount

Jefferson (Tmj) and the megabreccia of Meadow

Canyon (Tmc). The trend of the plugs follows

closely the alignment of the southwest margin

of the Mount Jefferson caldera and a strike-slip

fault in the lower reach of Meadow Canyon.

The one dated plug has an age of about 26.4

Ma.

The inferred strike-slip structure that localized the plugs may have been responsible for the

formation of a local basin lying east of the Soldier Spring fault. Perhaps sporadic right-lateral

movement on the structure caused sagging on

its northeast side, and east of the Soldier Spring

fault. The volcaniclastic rocks of Little Table

Mountain (Tlt units), the Isom-type welded ashflow tuff formation (Ti), and the lower and upper

members of the Shingle Pass Tuff (Tspl and

Tspu) were deposited and preserved in the basin

east of the Soldier Spring fault.

Following a fairly long interval of erosion,

the tuff of Pipe Organ Spring (Tp units) was deposited in the persistent shallow basin just east

of the Soldier Spring fault. The source of the

Pipe Organ Spring is unknown, but presumably it

lies east of the Jefferson quadrangle. Minor
32

offset of the northwest-trending strike-slip fault

that intersects the 26.4-Ma rhyolite plug (Trp)

indicates that the basin may have continued

gradual subsidence subsequent to the plug’s emplacement. During the interval of Pipe Organ

Spring deposition, thin layers of unit D of the

Bates Mountain Tuff (Tbt), at about 24 Ma, and

the tuff of Clipper Gap (Tcg), at about 23 Ma,

were emplaced. These units also were derived

from an eastern source.

A biotite-rich rhyodacitic ash-flow tuff

(Tbi) from an unknown source marks the last

event of volcanic activity manifested in the

Jefferson quadrangle.

The oldest Quaternary deposit in the area

is Pleistocene alluvium (Qa1a). It indicates that

20–40 m of erosion along stream courses has

occurred since its deposition. Varied younger

alluvial deposits have since been laid down, the

youngest being active alluvium in present stream

courses (Qa3).

ACKNOWLEDGMENTS

The numerous modal analyses of thin sections by F.M. Byers, Jr., have been invaluable in

identification and correlation of the volcanic

units. R.F. Hardyman, then of the U.S.

Geological Survey, gave valuable suggestions in

the field regarding volcanic rock petrology and

stratigraphy. The U.S. Geological Survey’s

F.G. Poole, during many field excursions in past

years, provided significant information on the

character of Paleozoic sedimentary rocks in and

near the Jefferson quadrangle. Valuable reviews

of the map, cross sections, and descriptions

were provided by Robert B. Scott and John H.

Stewart.

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