BEDROCK GEOLOGIC MAP of the YUCCA MOUNTAIN AREA, NYE COUNTY, NEVADA Robert P

Home , Yucca Mountain

36 36 FEET 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 745000 755000 750000 738100 740000 786900 765000 770000 775000 780000 785000 760000 ° ° 48'45" 52'00" cpv bt5 547100 bt5 547100 mr

C B cr A For legibility, undivided units tr, tp, ac, and p are used on this cross section as follows: 1927 North American datum Polyconic projection. Nevada coordinate system, central zone. 5,000-ft grid ticks based on by EG&G for the U.S. Department of Energy. produced from orthophoto mission flown in 1990 Base from 1:6,000-scale topographic contour data U.S. GEOLOGICAL SURVEY U.S. DEPARTMENT OF THE INTERIOR Qc A cr bt5 Management land 547500 and p includes pbt. tr includes trv and trn; tp tpul, tpmn, tpll, tpln, tpv; ac act acl; cp mr bt5 cx mr Bureau of Land Force Range Nellis Air pp mr cr mr ym cr cr 5

cr 000

Northern Windy Wash 4400 5100

4500 4600 5200 mrw

Qac Windy Wash Qac bt3 cp bt5 4800 cr 4400 Qac cpv

Crater Flat 4300 cp p cr cr cpv

116 pp

ac 116 4200 cp 4900 cp tp ? bt3 bt3 ° tr Qc ° cr pp ym 30'00" pp 30'00" mr ym bt5

cp NORTHERN Fatigue Wash cp Qac cr

Qac 80 mr

3400 cr ym cp cpv

cp tx

4100

15 cp

cpv 25 16

tpul bt5 20

3500 W b

20 20 IND cr cp Y mr mr bt5 trv

WASH ym 20 550000 cp 550000

cpv trn

550000 FAU trn 80 Qac 78

LT bt5 trn cx 65

tpmn 4700 cp

72 tpul

? 28 cr

64 80

cx 15 cpv 28 Qc cp ?

cr Qac trv

cpv 10 46

55 p

ym cx bt2 55 bt2 tpmn cpv pp FAT ym trn IGU Qc

E pbt Qac Qac

cpv WASH cp pp

tpv tr 40 act

tpul NORTHERN WINDY Qc

trv 65 FAULT cpv 10

3500 pp

Qc 9 tpmn tpul tr

WASH FAULT pp tp pp cp

tpll 5 trn

cp 200 cx 15

15 Qc cp

BOOMERANG 18 bt2 18 tpv cr

cpv

ym ym 15 pre-Prow Pass bt3 4500 tpmn SOLITARIO CANYON cp tpln tpll Qc cpv ym tpll pp tpln Tuff rocks

bt3

6 act

cpv tr ym tpmn 5500 cp

tpmn cp

17 cp 16 tr bt3 tpul POINT cr

Qc 7 tr cpv tpul

cpv cr 15 cp bt3 trn cpv

bt3

mr 10

cpv 9 pp tr FAULT pp cpv

Qac 3800 cr tp bt3 tr

3600 cr 25

mrw cpv 5 pp 552500 cpv pp

mr 5200 ym West Ridge5400 5

pp cpv ym

cr 3700 mr

p T Qc 4600 bt3 he

pp cp

ac 5500 13 5300 ym trn ym cpv

ym tp cp

FA 4900 tr

bt3 cp P

tr 4700

cr ro

U cr 13

WT-10 Ridge ym West w L ?

ym T rz 4 cp ? tp cp cpv cp cr pp

cp Boomerang Point 85 5600

Qc cr 58 Qc WT-7 00 cp Qc

trn 3900 cp cpv cpv mrw cp bt3 tpmn

mrw 3900 cpv

trv

cp 5

5000 100 ym 5700 5700 act Qc ? cr 5100 ym Qac cr cp ym

FATIGUE cp bt3 cp

Qc Jet Ridge cpv

cr cpv

ym 5000

H-6 3500 12 cp tr

WASH 12 tpv cr

pp cr tp Plug

5200 Qc

cpv 5300 tpv Hill 29

bt3

8 55 ym tpmn

FAULT cpv cp cp tpul cp

555000 Qac Ridge 5800 555000 cr cr cp trv 555000 mr Jet cp

Qc 3

75 60 cp Qc Qc cp

tpul 62

48 48

tpul

13 trv tpul

bt cr

66 trv 25

APPROXIMATE MEAN trn tpul 20

trn

bt 5700 DECLINATION, 1998 ? 4900

5400

trn 53 mrw 50

acl Qc

tpul

bt 22

cp 4100 trn trv cr px 4000 d cr trn

cp 5600

trn cp 5500 Castellated trv trn trv mr cpv d cp cpv cr Qc cpv

ym TRUE NORTH Qc px cp

trn

cr 80

3600

SOLITARIO 76

cpv 65

bt 14 bt cx

CANYON ?

MAGNETIC NORTH 10 d Qac 55

cpv °

FAULT Qc

cr 20 d 80 Solitario Canyon cpv

bt trn

cpv cp

52 cp cr tpul tpmn 5600 cr Qac cr

tr 4300

bt Qc

tpv 66 tpll acl Qac ? bt bt3

ym tpul

Ridge bt 56 tpmn cpv cp Qc

cpv Qc

px cp 50 tpmn bt

tpul

cr 45 48

Qac cr

73 tpul 11

cpv tpul

60 cp

cr 65 Qc

tpul Qc

tr Qc 4800

px 4200 tp Qac

tpmn tpul

4200 30

tx 5100 Qc 52 pp

cp tpmn 5300 tpll bt3 bt2 cp trn Qc Qc tpul cr Qc tpul ym trn cpv

Qac

20 cp pp cp 62 tpmn cp cp

ym trn

Mile High Mesa 5000

ym 557500

trn

15 pre-Prow Pass 75 trv 80 trn

trn

Qc bt3 14 tpul 80 Qac Qc cx

trn cpv 35 cr Tuff rocks

cpv

pp 4000

trn 13

cp ym bt 80 cp tpv

bt2 cp

4700 8 cpv 14 pp ac

tr 70 cpv

40 cp ?

Qac

p 65

15 cr 4900

cp cr

cr 5400 tpul ?

cpv

9 20 14 7

10 cr 2 1

pp 12

trn

trv cp 70

10 75

3800 cp Pacific Western Technologies, Inc., Denver, Colorado U.S. Geological Survey, Denver, Colorado

UZ-6s

Qc 5000 cp 460012 WT-11

UZ-6 4500

trv cr

bt2 15 tpul

cp cpv cr trn

bt 4800

bt SOLITARIO trv

ABANDONED WASH FAULT trv 5

5200 cr

14 ym Qc acl cr 15 tp

d bt2

cpv cp trv

cpv 26 CANYON cpv

ym trv

18 pp 8 ym

GU-3 10 cp

15 trv cr G-3

bt5 tpmn cr FAU cpv UZ-13

H-3 4800

cp 20

cr 8 LT

cp 10

Qac Yucca Crest

mr 15 cr

WT-11 Wash cp

Broken Broken tpmn 10 cpv

bt3 cpv

45 ym 4700

trv 15

9 10 Qc

cr 8

Little

Prow tpul 4800 Ridge Highway

trn

15 H-5

65 8

14 6

Qac SOLITARIO 5 ym

cr tr trv IRON RIDGE FAU cpv

LT 4600

Purgatory

cp 20

bt cr trv Mesa High Mile 4800 ym tpul

CANYON cr

Dead GA-1 trn 4600 cr Qac cp 3900 cp Qc

SUNDANCE

trv 85 tpul

17 cpv Qc

15 cpv trn 8

FAULT 8

tpul

trn

4600 575000 570000 565000 560000 trv 13

7 FAULT

600550 700575000 570000 565000 560000

5 4

560000 Ridge Diabolus cp

cr tpul

cpv 14 cp Limb bt

cpv

bt3 cpv UZ-14 cr

Yucca 4500 Qc

cp 12 bt

cpv

cp Wash Cup Tea

9 15 cp 4700 cp cr tr

cpv 18 8

1 MILE 0 1 Whale Back UZ-1

21 Qc

Qac bt Double Double

cpv 5100

Ridge 4300

Qc 4600

8 11 vp bt Notch Ridge Notch cr

bt cpv cp acl Qc cpv cp trv

cpv

Tonsil Ridge Tonsil 4500 G-2

cr 15 cp cpv 17

cpv

tr tr

bt 4400

cp Ridge

15 G-1

Qc 2 cp ? 5000

4200 4500 cr cp cr bt cr

Qc 4900 4500

cpv 6 4400

trv

5 ym

15 cp 15 pp tu cp

cpv

SD-7

4200 3 10

cp cp SD-12 cp

cpv Wash Hole Drill

bt 11 11

cpv 15

trv 4000 450 cp cr

bt 0

cp cpv SD-9 35

6 act

ym 25

10 4400 Ridge

18 bt3 cr cp bt Qac cp cp cpv

Ghost 430

cpv 4400 32 cr 10 cpv bt

Qac cp cp 8 cr 0 tpmn cpv cr

UZ-7a

16 Qc ? cpv

trv

cpv Wash Abandoned cp

trn 11

Dance Ridge

? 78

Qc cp 78 10 pp tpmn

15 vp 12

9 12 7

Qc 12

cpv 562500 1.50

13 tr

cr 20 cpv

Qc H-1

cp 18

tpul 8

W WT-2

3900 tp tpul 15 trn

cpv

acl 20 16

ash Qac

Forlorn Wash Forlorn cr 4 4500

bt 23

Antler Ridge Antler Qac

10 cp

19 cr 13 cpv

FAULT GHOST DANCE cr

NRG-7a

cpv 4 bt3 Qac

cp tx

/2 1/

cpv Ridge Bone Bleach ym Antler Wash Antler 4100 80

UZ-7 FAULT WASH HOLE RILL

cp Pagany

11 vp 3700 bt

Yucca Ridge Wash Wren bt Ridge Azreal

5 33 8

bt cpv 35 cpv acl

3800 4200

? Qac 4300 trn WT-2 cr

G-4

tpul 8 cr Live cp

cpv 9

trv

Qc 5

bt Qac 85

PAG

cpv Wash

trn Coyote W cr acl cp 4400 tpul

cp tr cp Qc

bt cp

cp 5

Dune Wash Dune

cr Wash

cp 11 ANY

trv tpul Wash Split bt3 tr tr tx tpmn

9 cr

tr 20 Qc Qc Qac trv WASH

WT-1 4700 pr cp act tpul NATIONAL GEODETIC VERTICAL DATUM OF 1929

W cr cpv cr

trn

tu bt 10 4600 H-4 cpv bt tpmn

T tx 20

10 pp cpv cp ? WT#6

-12 W -12 20 NRG-6

4000 pp

cp ? cp cr Bleach Bone Ridge Bone Bleach

cr FA 20 4100 ash Qac ym

bt3

a#4 7 cr

Qc 5 15

trn 10

CONTOUR INTERVAL 20 FEET ash

trv bt

U 75

3900 LT pre-Prow Pass

a#6 4200 ? cp cr a#5 tp cpv

tu Qc

cr UZ#16 12

? tu

cpv cpv Tuff rocks 565000 tpul

Drill Hole Wash Hole Drill bt3

WT#18 4300 15 3800 NRG#5

cp Nevada Test Site cr bt cpv Qc vp

tr Warren C. Day, cp pp SCALE 1:24 000 SCALE 1:24

trv

5 ?

Qc 6

Sever Sever

7 cp

trn ?

bt 16

Yucca Wash 4200

tx 4100 Qac p FAULT WASH DUNE pp Qc cp Qc ac

cpv Qc 74

15 4700

pp ? ym cr cp

tpul ? Wash

cr 15

4000 Qac

Qc cp cr 4500 a#7 Qc

cpv UZ#5

7 ym

cp 40

cp Boundary

4100 cr

? ? dc ym

? pr 59

Qc ? 4000 8

East Ridge East WT#17 pp

cp cp

tpmn tu cp Ridge ? ?

6 trn 11

? cpv

UZ#4 21

20 ? 4900 b#1

bt cp a#1 ? tu cp

tpul

cp ? mr 116 Qc 4700 SEVER WASH FAULT WASH SEVER

4600 3500 16 116

trv

trn

16 bt3

cr 10 act

cr 28

trv Isolation dc

Yucca Wash Yucca 20 12 Qac

tpul ? cp WT-12 cp cr cu cp ° tr pp ° bt pp Qac bt

pp cr 26'15"

tpul 12 vp cr 26'15" Qac Qc Qc

mrw Qc 15

Qac

9 mr 3900

Qc cp

NRG#4 BOW RIDGE FAULT RIDGE BOW 11

cpv Butte Bob's

567500 11 Qc R

cp 4500 Qac

bt2 13

cu tu

cp ? 10

3700 kt Qac

18 Qc Qc

Rainier Ridge Rainier cr

dc tr 5 4400 ge ? cp

cr kt 65

kl Qc

4100 cr

EAST 10 cp

RIDGE

3600 Qc

FAULT 4200

4400 cpv 20 ONC#1 kt cp mrw WT#4 ym Qac tr ? ? ac pr

NRG#3 15 cu Qc tu

1 dc ? cu Ridge Bow dc cr

BEDROCK GEOLOGIC MAP OF THE YUCCA MOUNTAIN AREA, NYE COUNTY, NEVADA Robert P. Dickerson,

cpv

kl cp Bow Ridge 21

cx 3800 bt

1 KILOMETER mr cp Dune Wash Dune cp mr BOW RIDGE FA Qc vp Qc

bt5 5 ULT mrw

Qc 43

cp 00 ? Qac Qc kt cu cp mr Qc bg cr kl Qac kt

cp Exile Hill

bg cp 18 cr cp

cr cp cp

Qac Qc cr 23

cr Qc

570000 ?

mrw cr c#1 ? c#2

Ambush Pass c#3

EXILE HILL FAULT BOW RIDGE FAULT act pp

kt ?

kl 4100 cx 57

? Qc 55 kl

3700 kl mr cr WT#16 cr mr

3600 cr tpul tu Qac kt cx BLACK GLASS tu pp cr cr tx bt

tr 3800

CANYON FAULT dc Qc BLACK GLASS BLACK 3600

cp 3700 tx 3900 cu

pp cu

pre-Prow Pass 3500

cp 38 San Juan Hill FAULT CANYON act Qac kt kt

p#1

Tuff rocks dc 20 cr cpv

3700 4100 ? 4000 cu

tpmn

4400 4500 tu bt 13 cp tr kl

cr 70

bt 4000 3800 Busted Butte kt acl mr dc mr kl cpv p Qac 572500 rtpv tr tpul

tpul pp 4200 4500 kt

kl kl

cp PAINTBRUSH CANYON FAULT tpul 19 cp Qc cr M

ac o ig oyRdeJake Ridge Joey Ridge Ron Ridge

Midw IDWAY VALLEY FAULT kt 34

cp 4300 pntr tpln

tr ay tr

cx Valley

cr Ridge Ron bt 22 Qac tpul cu kt dc bt

bt tu 57 WT#3 bt cpv cpv tpll pp

cp act

tpmn tpln tpv cpv tr tpll 3700 tpv tr Qc

4000 dc

cu cp

act

tr 66

Fran Ridge kt 4300 12

3800 3600 4300 90 kl kt bt

cr kl tpll

30 2 64

kl cpv Qc

cp kt tpll

WT#5 71 47 tpul kl Christopher J. Potter, tpmn

tpv

tr 74

65 66 57

tpln 56 tpln

acl

3400 Qc MAP LOCATION 15 cp cr

cpv

bt 10 74

act

tr 4000 tpll NEVADA

tpv Qac 4200 acl act

acl 18 19 575000 PAIN

#14 TBRU WT

3500 SH 56 ac FA CANYON

PAINTBRUSH CANYON kt ULT

kl 3800 kt

BUSTED BUTTE FAULT 41 7 PAINTBRUSH act cr kt

tu CANYON FAULT FAULT

? 65 Qc kl kl

cp kt

4300 13 tu 4900 3700 pp pp

cp 68 cr

76 dc 4600

3800 kl pp 25 cu 5100 cp Qc kl pp cu cu

dc kl

Point Alice cu cp 3900

3500 cu 74

cr 41

cr cp

80 kt

cr 6 3400 Ridge Joey pp

4500

4000 pp kl

5000 cp 8

4700 577500 kt ? trn

cr kt pp

86 cu 19

49 65 3900 tu cu

3600 kt

76 50

62 87

3300 85

Manuscript approved for publication March 16, 1998 Editing and design by Alessandro J. Donatich R.M. Drake II, and Alessandro J. Donatich Digital cartography by C.A. San Juan, W.C. Day, See "Index to geologic mapping of study area" 70

3200

67 4400 60 cr

4200 NEVADA OPERATIONS OFFICE, U.S. DEPARTMENT OF ENERGY

81 act

24 B' pp acl

acl Qac ac 48

WT#13 5 dc

tu 27

act 90 67

1 cu

4100 tu

p Ridge Jake J#13

Donald S. Sweetkind, tu Qac 4200 acl tu acl act tu

580000 tu pp

580000

3400 3200 4100

WT#15

acl 15 Qac Qac

Qc 3600 acl pp

3500 3700 3900 tu tu

pre-Prow Pass 4200 Prepared in cooperation with the

51 78

56 Tuff rocks 79 Jackass Flats act

? Qac

36 12 pp dc ac Qac acl acl acl pp act

act 14 act cp acl Qac

acl Qac

Qac act 3800

3300 582500 3400

3900 3700 tp 3200 Qc

tp 23

1998 Fortymile Canyon 19

cp By

Qac tr 3800 3500 Qac Qc

acl tr 15 pp Qac acl acl Qac 583900 tr A' acl 583900 cp cp tp cp tp A' 36 786900 745000 755000 750000 738100 740000 760000 765000 770000 775000 780000 785000 36 cu ° ° FEET 48'45" 1500 2500 3500 4500 5500 6500 1000 2000 3000 4000 5000 6000 52'00" 1 Carma A. San Juan, FEET 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 B For legibility, undivided units bt, tr, tp, ac, and p are used on this cross section as follows: cr 450550000 547500 Qac ac includes act and acl; p pbt. bt includes bt3, pp, and bt2; tr trv trn; tp tpul, tpmn, tpll, tpln, tpv; p igneous rock unit labels was dropped inasmuch as they are all Tertiary in age] [The letter prefix “ px tp cp tpv d FEET tr mrw Qac 1000 1500 2000 2500 3000 3500 4000 4500 5000 bt5 px mr Qc vp mr pr kt kl act cpv Fatigue Wash pre-Prow Pass Tuff rocks bt C FATIGUE For legibility, undivided units bt, tr, tp, ac, and p are used on this cross section as follows: cp 547500 ac includes act and acl; p pbt. bt includes bt3 and bt2; tr trv trn; tp tpul, tpmn, tpll, tpln, tpv; WASH FAULT Basalt dike Colluvial deposits Alluvial and colluvial deposits Timber Mountain Group Rhyolite of Pinnacles Ridge Paintbrush Group Rainier Mesa Tuff, welded Paintbrush Group, undivided tectonic breccia Bedded tuff Rhyolite of Vent Pass Rhyolite of Comb Peak Rainier Mesa Tuff, nonwelded tx Qac cr of map area on eastern flank Jet Ridge. K-Ar age is 10.0 deposits, and colluvial deposits colluvial deposits block-bounding fault zones on low hills in valley floors. compound cooling history. Exposures are restricted to hanging-wall side of devitrification, and pumice size content change abruptly. Unit has a a cooling break in lower part of welded tuff which the degree welding, brown devitrified volcanic rock. On Plug Hill in eastern Crater Flat there is in diameter; and as much 2 percent lithic fragments of dark-gray to dark- pumice (locally as much 60 percent) clasts that range from 0.3 to 10 cm cent phenocrysts of quartz, sanidine, plagioclase, and biotite; 8–20 percent welded, devitrified, massive, quartz-rich ash-flow tuff. Contains 10–20 per- areas north of Yucca Wash near caldera margin nacles Ridge area at northeastern edge of map and is restricted to plagioclase, biotite, and magnetite. Unit is as much 300 m thick in Pin- Ash-flow horizons contain 15–30 percent phenocrysts of quartz, sanidine, welded, bedded, vitric to devtrified pyroclastic-flow and fallout deposits. vitrophyre is locally preserved. Ash-flow phases are light-gray to pink, non- nocrysts of quartz, sanidine, plagioclase, biotite, and magnetite; thick basal fied, flow-banded rhyolite. Flows are crystal rich with 30–35 percent phe- dome complex. Lava flows are light-gray to black, vitric locally devitri- and Parrish, 1985) 11.6 western part of map area ciated with block-bounding faults west of Solitario Canyon fault in south- yon Tuff. Exposed predominantly in hanging-wall deformation zones asso- highly brecciated welded units of the Topopah Spring Tuff and Tiva Can- (1995) Interpreted as fallout- and pyroclastic-tuff deposits by Geslin others distinguish it from overlying unit Rainier Mesa Tuff (unit clasts of pumiceous lapilli. Preserved locally beneath the nonwelded map area blende. Unit is 0–150 m thick and crops out only in northernmost part of well as 3 percent phenocrysts of sanidine, plagioclase, biotite, and horn- ments of dark-reddish-brown to brownish-gray devitrified volcanic rocks, as densely fused, and spherulitic. Ash flows contain 10–35 percent lithic frag- locally bedded. Where vitric, ash-flow tuffs are dark gray to black, partly gray to medium dark purplish brown, nonwelded, massive, and devitrified, lithic rich, and pumice rich. Ash-flow tuffs are light greenish breccia and vitrophyre are locally preserved. Ash-flow tuffs in the flow-banding plane and commonly are silicified. Basal autoclastic blende. Flows are locally vitric and flow banded. Oblate lithophysae lie contain 2–3 percent phenocrysts of sanidine, plagioclase, biotite, and horn- are medium-gray to dark-brownish-gray dark-grayish-brown rhyolite, and Thickness ranges from 0 to 38 m contact occurs beneath a fallout tephra in very coarse, lithic-rich horizon. horizons. Unit is characteristically massive and very pumiceous. Lower welded tuff lithic fragments as long 35 cm are concentrated in lithic-rich moderately flattened pumice as long 1 cm. Gray and brown rhyolite tite, and sphene; also contains white to light-gray, devitrified, equant percent phenocrysts of sanidine, plagioclase, hornblende, magnetite, bio- ately welded, devitrified, pumiceous and lithic-rich ash-flow tuff. Contains 5 Ash-flow tuff m thick clase, hornblende, magnetite, biotite, and sphene. Unit is as much 130 rhyolitic lava flow. Contains 5–6 percent phenocrysts of sanidine, plagio- Rhyolite lava flow greater than 20 m in area not known inasmuch as top of unit eroded, but thickness ranges to tral, southern, and southwestern parts of map area. Total thickness unit bounding fault zones in valley floors. Unit crops out northwestern, cen- volcanic rock. Exposures are restricted to hanging-wall side of block- as much 2 percent lithic fragments of dark-gray to dark-brown devitrified 8–20 percent pumice clasts that range from 0.3 to 10 cm in diameter; and 12–20 percent phenocrysts of quartz, sanidine, plagioclase, and biotite; zons are bedded and grade upward into massive horizons. Unit contains welded, devitrified to partially vitric, quartz-rich ash-flow tuff. Basal hori- southwestern parts of map area. Thickness ranges from 2 to 30 m T Crater Flat Qac ” commonly used for designation of the Tertiary age bedrock tr ± 2 0.03 Ma (Sawyer and others, 1994). Exposed in southern tr —Dark-gray to black, aphanitic basaltic dikes in northwestern part Ronald M. Drake II, p cp QUATERNARY SURFICIAL DEPOSITS CORRELATION OF MAP UNITS cpv tp tu bt DESCRIPTION OF MAP UNITS —White to buff, nonwelded bedded tuff containing devitrified cpv act cp act tpv MIOCENE VOLCANIC ROCKS MIOCENE INTRUSIVE ROCKS Qac —Light-gray to pink brownish-gray, nonwelded moder- bt tpv —Boulder- to cobble-size unconsolidated talus, slope-failure bt pre-Prow Pass p Qac cr tp Tuff rocks tr cp 550000 Qac cx ac bt tp lkt kl tr —Light-gray to pinkish-gray, devitrified, flow-banded cx mr —Rhyolitic lava flows and ash-flow tuffs. Lava

SOLITARIO —Exposed only in northeastern part of map area Qac Unconformity CANYON FAULT cp act —Rhyolitic lava flows and ash-flow tuff of a flow Qac —Light-gray to pinkish-gray, partly moderately acl —Boulder- to silt-size unconsolidated alluvial and mr Jet Ridge tpmn 552500 d tpln tpul pbt bt2 bt3 ym bt5 tpv tpll act trn trv pp bg vp cu pr b p cr —Light-gray to light-tan, nonwelded partly ). Devitrified pumice clasts of unit cr tpv tp p tr rz mr Qc p cr cp tp whose pumice clasts are more vitric. cp tr cpv act cpv mr cp dc cr cpv ? 550550 550500 650550 650500 750575000 572500 570000 567500 565000 562500 560000 557500 555000 552500 bt Qac Qc bt tpv cr cr p —Tectonically juxtaposed, act mrw tp tr tpv cpv cp 555000 2 Solitario Canyon cp and Christopher J. Fridrich Qac cp cr ym Qc 40 cpv ym Ar/ cp tr tr Qac ± 0.4 Ma (Carr cx tp 39

cpv SOLITARIO tr ? p ? ym Ar age is bt bt Miocene

act CANYON ?

tp FAULT cr tp bt5 tpv bt tpv ym p act cp cpv 557500 ?

cp TERTIARY QUATERNARY

uc rs Broken Limb Ridge Yucca Crest cr Qac cpv

pre-Prow Pass cr mr Tuff rocks cpv cpv bt2 ym bt3 trv pp bg dc cp cu cx p tu bt tx cr tr rz act cp

tp cr ? tr cr ym 560000 tpv Qac Topopah Spring Tuff Pre-Pah Canyon Tuff bedded tuff Pah Canyon Tuff Rhyolite of Delirium Canyon Pre-Yucca Mountain Tuff bedded tuff Rhyolite of Zig Zag Hill Tiva Canyon Tuff Nonwelded bedded tuffs, undivided Rhyolite of Black Glass Canyon Yucca Mountain Tuff cp bt dominant component of the nonwelded bedded tuffs, undivided (unit pp thick) to map separately and is combined with the Pah Canyon Tuff (unit present in northern part of map area, but is mostly too thin (less than 2 m from 5 to 15 mm in diameter. Thickness ranges 0 10 m. Unit is grayish-red, pale-red, pale-brown, and brown lithic fragments that range 2–15 percent volcanic lithic fragments of grayish-black glass and light-gray, reddish gray. Pumice clasts are 10–35 mm in diameter. Unit contains from white, light gray, pale orange, brown, moderate to magnetite; and 15–85 percent pumice clasts. Pumice clasts range in color 2–12 percent phenocrysts of feldspar and traces biotite, pyroxene, massive to crudely stratified fallout tephra and ash-flow tuff. Unit contains brown, and reddish-brown, nonwelded, vitric to devitrified altered, pinches out to the south ness ranges to over 60 m; unit is thickest in northern part of map area, and nantly nonwelded toward central and southern part of map area. Thick- northern Windy Wash and Fatigue areas. Unit becomes predomi- has a welded core that dominates the unit in its northern extent ing in the Pah Canyon Tuff varies from north to south. The Unit is characterized by abundant, poorly sorted large pumice clasts. Weld- cent light-gray, pink, and light-green pumice 2–12 cm in average diameter. lithic fragments of devitrified rhyolite and minor obsidian; 15–25 per- clase, and traces of biotite, quartz, clinopyroxene, sphene; 0–5 percent ash-flow tuff. Unit contains 5–10 percent phenocrysts of sanidine, plagio- Lower part is typically light-purple-gray to gray, partly nonwelded, vitric welded, vitric to devitrified ash-flow tuff with local vapor-phase alteration. fied ash-flow tuff. Middle part is light-gray to moderate-brown, densely pumice clasts. Upper part is pink to lavender, nonwelded, vitric devitri- flow tuff as much 100 m thick cm, locally as large 25 cm. Lava flows much 250 m thick; ash- brown volcanic rock fragments. Fragments range in diameter from 2 to 15 pumice, 0.5–3 cm in diameter; 15–25 percent dark-brown to grayish- hornblende. Tuffs contain 5–15 percent, pink to light-gray, devitrified tains 5–7 percent phenocrysts of sanidine, plagioclase, biotite, sphene, and welded, partly vitric to devitrified, and massive poorly bedded. Unit con- light-brown, tan, ash-flow tuff. Tuffaceous parts are nonwelded to partly brown, vitric to devitrified, flow-banded rhyolite lava flow with light-gray this rhyolite was deposited. Thickness as much 10 m indicating that faulting was prevolcanic to early synvolcanic during the time Thickness increases across a small fault just west of the Fatigue Wash fault, (unit (less than 3 m thick) and is included in nonwelded bedded tuffs, undivided pp to map separately and was included in the subjacent Pah Canyon Tuff (unit it is as much 40 m thick. South of Sever Wash, unit not thick enough is thick enough to map separately only in northern part of area where tuff unit 3 of Buesch and others (1996) Moyer (1996). Unit pumiceous, vitric, nonwelded pyroclastic-flow deposits of informal bedded thin to show on this map) Tuff (unit Scott and Bonk (1984). It forms a thin horizon between the Pah Canyon of The Prow just west the Fatigue Wash fault, as originally observed by rhyolite flow. Unit is restricted to a small graben exposed on northern wall dominantly indicates others (1996). This map unit is used in southern part of area and pre- Canyon Tuff bedded tuff ( of rock. Basal contact with the underlying crystal-poor member (unit clasts as large 15 cm in diameter, which make up much 80 percent thick horizon of light-gray, rounded and partially resorbed rhyolite lithic diameter. In northern part of map area, unit has a distinctive 1- to 4-m- part of unit; lithophysae typically are equant and as much 20 cm in eter. Lithophysae (as much as 30 percent) locally are present near basal light-gray to dark-reddish-gray brown lithic fragments 1–10 mm in diam- black pumice as much 10 cm in diameter; and 3 percent traces of hornblende, sphene, and pyroxene; 5–30 percent light-gray to tains 10–15 percent phenocrysts of sanidine, plagioclase, biotite, and ly to densely welded tuff of quartz latitic composition. In general, unit con- Crystal-rich member less than 15 m but both members are discernible area north of Yucca Wash, where total thickness the Tiva Canyon Tuff is cp Undivided units of the Tiva Canyon Tuff Undivided tectonic breccia 1994) latite ash-flow tuff. Contains 60–90 percent rhyolitic groundmass, 10–15 brown to dark-red, nonwelded densely welded, vitric devitrified quartz Fran and Bow Ridges, on Busted Butte is present but poorly exposed as in the Yucca and Windy Wash areas, on rich member of the Topopah Spring Tuff. Map unit is used where zone, undivided (unit Crystal-rich member, undivided Spring Tuff is less than 20 m but both members are discernible map area north of Yucca Wash, where total thickness the Topopah tp Undivided Topopah Spring Tuff Undivided tectonic breccia ers, 1994) tuff ( and fall deposits. Unit locally includes the pre-Tiva Canyon Tuff bedded ranges from 2 to 14 m phenocrysts of sanidine, hornblende, sphene, and biotite. Thickness welded, devitrified, massive ash-flow tuff. Unit contains 3–4 percent banded rhyolite lava flow and tan to brownish-gray, nonwelded partly as 60 m thick Welding is more developed in northern part of map area. Unit as much Tuff. Unit thins to the south and pinches out in central part of map area. Tuff, but paucity of phenocrysts is characteristic the Yucca Mountain Welded middle part appears similar to welded intervals of the Tiva Canyon low to brown vitric glass shards occur in upper and lower nonwelded parts. rare grayish-purple lithic clasts less than 1 cm wide. Abundant clear to yel- containing 1 percent pink and gray pumice clasts less than cm wide flow tuff; and lower part is vitric, brownish-gray, nonwelded ash-flow tuff tuff; middle part is gray to pinkish-gray, densely welded and devitrified ash- altered. Upper part is dark-gray, moderately welded to nonwelded ash-flow it. Unit is characteristically aphyric and vitric to devitrified variably 3 to 6 m from other zones within the crystal-poor member. Thickness ranges Tuff throughout map area, and as such was mapped a separate unit Unit forms a prominent marker horizon at the base of Tiva Canyon 4A and 4B of Buesch others (1996) Moyer others, 1996). of map unit includes pre-Tiva Canyon Tuff bedded tuffs (bedded tuff units canic lithic fragments that are less than 1–2 cm in diameter. Locally, base much as 25 percent) pumice clasts; and 0–5 percent dark-reddish-gray vol- sanidine, plagioclase, and traces of hornblende; 2–15 percent (locally, as partly welded, vitric rhyolitic tuff containing 3–5 percent phenocrysts of Day and others (1998). Thickness as much 140 m and Buesch others (1996) delineated in the central block area by several informal zones and subzones described by Scott Bonk (1984) a ledge-forming horizon with prominent columnar joints. Unit contains member is grayish-red to maroon nonlithophysal tuff, which commonly has to grayish-red lithophysal-bearing zone. Basal part of welded crystal-poor sound when struck with hammer. Locally, lower part of unit is a pale-red and massive, has distinctive concoidal fracturing, makes a “clink” contains lithophysae. Middle part of unit is predominantly nonlithophysal Upper part of unit generally is light purplish gray to pinkish and and markedly decreases southward (south of latitude Abandoned Wash). lithophysal content is greatest in northern and central parts of map area Buesch and others, 1996; Day 1998). In general, total delineated based on prevalence of lithophysae (see Scott and Bonk, 1984; fragments less than 1 cm in diameter. Internal stratigraphy of unit can be as 5 percent pumice; and less than 1 light-gray volcanic lithic sanidine, plagioclase, and traces of biotite altered pyroxene; as much devitrified, rhyolitic ash-flow tuff. Contains 3–4 percent phenocrysts of Crystal-poor member and others (1998). Thickness 3–15 m Buesch and others (1996) delineated in the central block area by Day Map unit contains several informal zones and subzones described by crystal-poor member. Unit forms prominent cliffs and rounded slopes. percent and commonly prominent bronze-colored biotite) relative to the transitional and is characterized by greater abundance of phenocrysts (7–10 mr ) of the Topopah Spring Tuff. Use is restricted to northeastern part ) of the Tiva Canyon Tuff. Use restricted to northeastern part map ). In southernmost extent of map area unit is present as thin horizons ). Unit’s aggregate thickness increases southward to become the

IRON RIDGE Exploratory Vitric zone and densely welded zone, undivided Vitric zone of crystal-poor member bt4 bt cr Studies FAULT Facility ) ? tp ), the pre-Yucca Mountain Tuff bedded tuff ( pre-Prow Pass —Unit includes crystal-rich and crystal-poor members (units —Unit includes crystal-rich and crystal-poor members (units pp 1 Tuff rocks tr ) and the pre-Yucca Mountain Tuff bedded tuff (unit Qac tpv cr —Pyroclastic-flow deposit characterized by abundant, large — 562500 p 40 —Nonwelded to densely welded pyroclastic-flow depos- act — GHOST bt2 ym ? cp Ar/ trv —Pale-brown to brownish-black pinkish-gray, part- —Dark-gray to black, vitric, crystal-rich, flow-banded —Gray to pinkish-gray pale-red, densely welded,

DANCE 40 ? and minor amounts of ) and nonlithophysal zone (unit tpv cpv 39 FAULT Ar/ bt2 tp Ar age is 12.7 ? 39 p ) of Buesch and others (1996) Moyer act tr —Tectonically juxtaposed, highly brecciated —Tectonically juxtaposed, highly brecciated cpv —Light-gray to light-pinkish-gray light- tr Ar age is 12.8 cp tp —Includes vitric zone and densely welded ? Qac —Medium-gray to purplish-gray, flow- —White, light-gray, pale-orange, light- cr —Nonwelded, bedded pyroclastic-flow —White to light-brown light-gray, ? bt mr 565000 —Light-orangish-brown to gray, ±

bt Qac 0.03 Ma (Sawyer and others, ± cr 0.03 Ma (Sawyer and oth- bt3 cp and Qc bt3 Antler Ridge cp bt4 ), and the pre-Pah trn bt ym —Dark-reddish- (unit ) of the crystal- p act cr tpv tp tr cp bt4 567500 Qc bt Qac is too tr cpv cp p bt3 )

bt5 tp trv ) is cr tr ). pre-Prow Pass cp Qac , , Tuff rocks mr act bt cr

cr BOW RIDGE FAULT bt ym mr

J#13

tpv 30 tpmn

tpln tpul 12 15 16 pbt tpv tpll act acl trn ac tp

570000 b p

cpv 75 cpv EXILE HILL FAULT Approximate location of the underground Exploratory Studies Strike and dip of compaction foliation Strike and dip of bedding Strike and dip of bedding Reverse fault Fault Contact Crater Flat Group Borehole location and number Unimproved road Potential perimeter boundary surrounding the repository Calico Hills Formation mr Bullfrog Tuff Prow Pass Tuff Rhyolite lava flow Ash-flow tuff Undivided A C B Qac . Day and others (1998). . Dickerson and Drake (1998). . Mapping done for this report by W.C. Day, R.P. Dickerson, Facility contact of unit queried where inferred or uncertain. Sawteeth on upthrown side approximately located; dotted where concealed beneath Quaternary cover; horizontal movement barbed arrow indicates dip of fault plane. Opposed arrows show relative tipped line indicates rake and plunge of slickenslide lineation. Double- queried where uncertain. Ball and bar on downthrown side. Diamond- located; dotted where concealed and inferred beneath Quaternary cover; where inferred or uncertain where concealed beneath Quaternary cover or poorly exposed; queried unit not exposed lished an and others (1976), Carr (1986), Sawyer (1994), who estab- er part by the Windy Wash fault. Areal distribution was discussed Byers Windy Wash area. Complete section is not exposed due to faulting of low- sure of unit on this map is limited to area west Prow Pass in northern includes quartz, alkali feldspar, plagioclase, and characteristic biotite. Expo- tuff. Phenocryst content in map area is as much 10 percent, and at base of the Prow Pass Tuff (unit Nonwelded bedded tuff Base not exposed but thickness greater than 12 m well exposed just west of Prow Pass at northern end Yucca Mountain. (Byers and others, 1976). Type locality (and sole occurrence on map) is clase; it is unique in that orthopyroxene the dominant mafic phenocryst as much 30 percent phenocrysts of quartz, alkali feldspar, and plagio- rich pyroclastic rock. Unit contains approximately 10 percent pumice and sharp and marked by autoclastic breccia. Unit is as much 215 m thick quartz, sanidine, plagioclase, biotite, and hornblende. Lower contact is monly altered and zeolitized contains 3–4 percent phenocyrsts of banded rhyolitic lava. Locally brecciated, silicified, and spherulitic. Com- lava flows. Exposed thickness approximately 115 m rhyolite clasts. Bedded tuff is typically interstratified with massive and yellow to pale-greenish-yellow pumice; and dark-gray reddish-brown sanidine, plagioclase, biotite, and hornblende; 10–40 percent grayish- devitrified to zeolitized. Contains 1–12 percent phenocrysts of quartz, nonwelded ash-flow tuff, fallout tephra, and reworked tuff; vitric to Shown only on cross sections and Busted Butte (over 12 m thick) ly 1 percent. Unit is exposed near Prow Pass (where it about 10 m thick) er amounts of pumice and lithic clasts. Phenocryst content is approximate- Unit is predominantly made up of groundmass (over 75 percent), with less- Lower part of unit is moderate-brown to grayish-orange nonwelded tuff. partly devitrified, grayish-black to grayish-brown rhyolitic ash-flow tuff. Busted Butte in southeastern part of map area (where it is 40 m thick) Pass in northern part of map area (where it is about 10 m thick) and on Unit typically forms slopes in map area. is exposed only near Prow pronounced and developed along fractures corroded pumice voids. clasts (Buesch and others, 1996). Local vapor-phase alteration is phenocrysts, as much 15 percent pumice clasts, and 1–10 lithic Unit is made up of over 75 percent groundmass with 1–2 dusky-reddish-purple, densely welded, devitrified, rhyolitic ash-flow tuff. plagioclase, biotite, magnetite, rare quartz, and altered pyroxene and about 2 percent phenocrysts of alkali feldspar, with minor amounts percent lithophysae, 1–10 pumice clasts, 1–5 lithic brown, densely welded, devitrified, rhyolitic ash-flow tuff. Contains 5–15 much as 5 percent lithophysae lithic fragments of light-gray volcanic rock, 1–10 mm in diameter; and as or biotite; as much 3 percent devitrified pumice fragments with trace percent phenocrysts of sanidine, plagioclase, and rare oxidized hornblende light-gray, densely welded, devitrified, rhyolitic ash-flow tuff. Contains 1–2 locally prominent cliffs ation rims. Unit is as much 45 m thick and forms steep slopes (unit more equidimensional relative to lithophysae in the lower lithophysal zone cent lithophysae, which are 1 cm to greater than 3 in diameter, and fragments of pale-brownish-pink volcanic rock. Also contains 5–30 per- rare altered pyroxene(?); and 0–10 percent devitrified pumice with lithic flow tuff containing 2–5 percent phenocrysts of sanidine, plagioclase, and pale-reddish-purple, moderately to densely welded, devitrified, rhyolitic ash- ber of the Topopah Spring Tuff vitric zones (units nonlithophysal, lower lithophysal, and crystal-poor Crystal-poor member, undivided cliffs as much 30 m thick phenocrysts, including diagnostic bronze biotite. Unit forms prominent unit is transitional and mapped on the basis of greater than 7 percent crystal-rich lithophysal zone of Buesch and others (1996). Basal part of unit contains as much 15 percent lithophysae and corresponds to the is transitional into the underlying crystal-poor member. Locally, lower part dant secondary vapor-phase corrosion of pumice clasts. Lower part unit light-gray to dark pumice clasts; minor lithic clasts. Characterized by abun- tent ranges from 10 to 25 percent by volume and is mixed in color, with tite, and rare pseudomorphs of pyroxene(?) hornblende. Pumice con- mass contains 10–15 percent phenocrysts of sanidine, plagioclase, and bio- light-brown, densely welded, devitrified crystal-rich quartz latite. Ground- bearing, densely welded tuff. Middle part of unit is grayish- to pale-red thick traces of lithic fragments. Unit forms slopes; ranges to as much 8 m percent phenocrysts, 2–10 partly deformed pumice fragments, and 1994) cr —Solid where exposed or well constrained; dashed approximately INDEX TO GEOLOGIC MAPPING OF STUDY AREA

tx PAINTBRUSH C.J. Fridrich, June 1996 to 1997. C.J. Potter, D.S. Sweetkind, R.M. Drake II, and —Solid where exposed; dashed approximately located; dotted tpll CANYON Crystal-poor vitric zone Lower nonlithophysal zone Lower lithophysal zone Middle nonlithophysal zone Upper lithophysal zone Nonlithophysal zone FAULT Fran Ridge Midway Valley —Shown by on cross section ). Lithophysae have 1- to 5-mm-thick, light-gray vapor-phase alter- —Includes ash-flow tuff (unit tr bt Qac 40 ? —Solid where exposed or tightly constrained; dashed act 572500 cp cpv p tp tr —Light-gray, massive, moderately to densely welded ash-flow Ar/ tr —Pale-orange, grayish-yellow, and pale-greenish-yellow, cp —Light-pinkish-gray, moderately welded, devitrified, cystal- 39 mr cpv tpv Busted Butte tp bt —Light- to dark-gray, greenish-gray, pale-purple flow- p Ar age of 13.25 tpul This map is also available as a PDF at http://greenwood.cr.usgs.gov Box 25286, Federal Center, Denver, CO 80225 For sale by U.S. Geological Survey Information Services endorsement by the U.S. Geological Survey descriptive purposes only and does not imply Any use of trade names in this publication is for ? cr — tpv cpv cp 40 GEOLOGIC INVESTIGATIONS SERIES , tpmn cr Ar/ cp —Calculated from three-point problems on basal pre-Prow Pass act —Yellowish-gray, nonwelded, bedded ash-flow tuff bt mrw Tuff rocks cpv —Dark-reddish-brown to dark-brown, pumice- 39 , tr bt mr Ar age is 12.9

PAINTBRUSH tpll —Pale-red, grayish-red to purple, and light- —Light-gray, light- to moderate-brown, and bt —Predominantly densely welded, vitric to C cpv tp CANYON 575000 , FAULT ± —Grayish-orange-pink to light-brown cpv tpln p —Pale-red, grayish-reddish purple, to 0.04 Ma for unit. Stratigraphic base of A ). Unit is as much 12 m thick —Includes upper lithophysal, middle tr act cp

BUSTED BUTTE act B , and cpv FAULT p – act ) and rhyolitic lava flow (unit bt B tp cr tr tr ' cp ± tp tpv 0.04 Ma (Sawyer and others, Pamphlet accompanies map p ) of the crystal-poor mem- B tpv Qac tpv C' 577500 cr FEET bt 2000 2500 3000 3500 4000 4500 5000 1000 1500 Qac mr cpv I–2627 B' FEET 5500 3000 4000 5000 acl 1000 1500 2000 2500 4500 3500 ). U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY

BEDROCK GEOLOGIC MAP OF THE YUCCA MOUNTAIN AREA, NYE COUNTY, NEVADA

Warren C. Day,1 Robert P. Dickerson,2 Christopher J. Potter,1 Donald S. Sweetkind,1 Carma A. San Juan,2 Ronald M. Drake II,2 and Christopher J. Fridrich1

1998

1U.S. Geological Survey, Denver, Colorado 2Pacific Western Technologies, Inc., Denver, Colorado

Prepared in cooperation with the

NEVADA OPERATIONS OFFICE, U.S. DEPARTMENT OF ENERGY

Pamphlet to accompany GEOLOGIC INVESTIGATIONS SERIES I–2627 CONTENTS Abstract ...... 1 Introduction ...... 1 Notes on stratigraphic nomenclature ...... 4 Previous mapping ...... 4 Methodology ...... 4 Borehole designations ...... 5 Data sources ...... 5 Regional setting ...... 6 Stratigraphic setting ...... 6 Structural geology ...... 8 Description of block-bounding faults ...... 8 Description of relay structures ...... 8 Description of the prominent northwest-striking strike-slip faults ...... 10 Description of intrablock structures ...... 11 Structural domains ...... 12 Central Yucca Mountain domain ...... 12 Azreal Ridge domain ...... 14 Yucca Wash domain ...... 14 Paintbrush Canyon domain ...... 14 Fran Ridge domain ...... 15 Fortymile Wash domain ...... 15 Plug Hill domain ...... 15 Southwest domain ...... 16 East Ridge domain ...... 16 Dune Wash domain ...... 16 Discussion on the variation and timing of tectonism at Yucca Mountain ...... 17 References cited ...... 19

i FIGURES

1. Index map of study area showing regional distribution of caldera structures near Yucca ...... 2 Mountain, Nye County, Nevada. 2. Map showing locations of prominent physiographic features in the map area ...... 3 3. Map showing distribution of fault types in the map area ...... 9 4. Map showing distribution of structural domains defined for the map area ...... 13

CONVERSION FACTORS

Multiply by To obtain

millimeter (mm) 0.03937 inch (in.) centimeter (cm) 0.3937 inch (in.) meter (m) 3.281 foot (ft) kilometer (km) 0.6214 mile (mi)

The following abbreviation is also used in this report: Ma, millions of years before present.

ii ABSTRACT extensional deformation as recorded in the amount of offset along the block-bounding faults Yucca Mountain, Nye County, Nevada, has as well as an increase in the intrablock faulting. been identified as a potential site for under- The rocks in the map area had a protracted ground storage of high-level radioactive nuclear history of Tertiary extension. Rocks of the waste. Detailed bedrock geologic maps form an Paintbrush Group cover much of the area and integral part of the site characterization pro- obscure evidence for older tectonism. An earlier gram by providing the fundamental framework history of Tertiary extension can be inferred, for research into the geologic hazards and hy- however, because the Timber Mountain–Oasis drologic behavior of the mountain. This bedrock Valley caldera complex lies within and cuts an geologic map provides the geologic framework older north-trending rift (the Kawich- and structural setting for the area in and adja- Greenwater rift). Evidence for deformation dur- cent to the site of the potential repository. ing eruption of the Paintbrush Group is locally The study area comprises the northern and present as growth structures. Post-Paintbrush central parts of Yucca Mountain, located on the Group, pre-Timber Mountain Group extension southern flank of the Timber Mountain–Oasis occurred along the block-bounding faults. The Valley caldera complex, which was the source basal contact of the 11.6-Ma Rainier Mesa Tuff for many of the volcanic units in the area. The of the Timber Mountain Group provides a key Timber Mountain–Oasis Valley caldera complex is time horizon throughout the area. Other work- part of the Miocene southwestern Nevada vol- ers have shown that west of the study area in canic field, which is within the Walker Lane belt. northern Crater Flat the basal angular uncon- This tectonic belt is a northwest-striking formity is as much as 20° between the Rainier megastructure lying between the more active Mesa and underlying Paintbrush Group rocks. Inyo-Mono and Basin-and-Range subsections of In the westernmost part of the study area the the southwestern Great Basin. unconformity is smaller (less than 10°), whereas Excluding Quaternary surficial deposits, the in the central and eastern parts of the map area map area is underlain by Miocene volcanic rocks, the contact is essentially conformable. In the principally ash-flow tuffs with lesser amounts of central part of the map the Rainier Mesa Tuff lava flows. These volcanic units include the laps over fault splays within the Solitario Crater Flat Group, the Calico Hills Formation, Canyon fault zone. However, displacement did the Paintbrush Group, and the Timber Mountain occur on the block-bounding faults after deposi- Group, as well as minor basaltic dikes. The tuffs tion of the Rainier Mesa Tuff inasmuch as it is and lava flows are predominantly rhyolite with locally caught up in the hanging-wall deforma- lesser amounts of latite and range in age from tion of the block-bounding faults. Therefore, the 13.4 to 11.6 Ma. The 10-Ma basaltic dikes in- regional Tertiary to Recent extension was pro- truded along a few fault traces in the north- tracted, occurring prior to and after the eruption central part of the study area. of the tuffs exposed at Yucca Mountain. Fault types in the area can be classified as block bounding, relay structures, strike slip, and INTRODUCTION intrablock. The block-bounding faults separate the 1- to 4-km-wide, east-dipping structural This 1:24,000-scale map focuses on the blocks and exhibit hundreds of meters of dis- area surrounding the potential high-level nuclear placement. The relay structures are northwest- waste repository site at Yucca Mountain (figs. 1 striking normal fault zones that kinematically link and 2). Its purpose is to define the character the block-bounding faults. The strike-slip faults and extent of the dominant structural features in are steep, northwest-striking dextral faults lo- the vicinity of and outward from the potential cated in the northern part of Yucca Mountain. repository area. As currently conceived, the po- The intrablock faults are modest faults of limited tential repository would be a permanent under- offset (tens of meters) and trace length (less ground facility with high-level nuclear waste than 7 km) that accommodated intrablock placed in drifts excavated in the densely welded deformation. units of the Miocene Topopah Spring Tuff. The The concept of structural domains provides repository would be built in the unsaturated zone a useful tool in delineating and describing varia- approximately 250 m above the regional ground- tions in structural style. Domains are defined water table. The Yucca Mountain Project cur- across the study area on the basis of the rently is evaluating the cumulative effect of natu- relative amount of internal faulting, style of ral geologic hazards in the site area, including deformation, and stratal dips. In general, there seismic and volcanic hazards. Hydrologic inves- is a systematic north to south increase in tigations and computer-aided three-dimensional

1 117°00' 116°00'

Goldfield

Boundary of southwestern Nevada volcanic field 37°30' Silent Canyon caldera Stonewall Mountain complex caldera(?) complex Grouse Canyon caldera Black Mountain Area 20 Caldera caldera 95 ? Rainier Mesa Timber Mountain– caldera Nevada Oasis Valley ? caldera complex ? Ammonia Test Tanks ? caldera ? Site 37°00'

? Beatty Timber Mountain–Oasis Valley caldera complex Approximate boundary Yucca Claim Canyon caldera of study area Mountain

Wahmonie volcano

Mercury Amargosa Valley

° NEVADA 36 30' DEATH VALLEY NEVADA Study CALIFORNIA area

Pahrump

0 1020304050 KILOMETERS

0 5 10 15 20 25 MILES

Figure 1. Index map of study area showing regional distribution of caldera structures near Yucca Mountain, Nye County, Nevada. Modified from Carr and others (1986, fig. 1) and Sawyer and others (1994, fig. 1).

2 116°30' 116°26'15"

Prow Pass Castellated The Prow Paintbrush Canyon Ridge Black Glass Canyon

Joey Ridge

Mile High Yucca Wash 780000 Mesa Bleach Bone Ridge

Isolation Ridge West Ridge Teacup Wash

Jake Ridge ile Canyon Tonsil Ridge Pagany Wash

Sever Wash Windy Wash Little Fortym Prow Azreal Ridge Diabolus Ridge 36°52' Drill Hole Wash 770000

Jet Ridge Dead Yucca Ridge Alice Purgatory Ridge Point

Live Yucca Ridge Midway

Antler Ridge Exile Hill Valley Fatigue Wash SplitWash Fortymile Whale Back Ridge Wash Broken Limb Ridge Antler Wash

Boomerang Solitario Canyon Highway Ridge 760000 Point

Ghost Dance Boundary Wash Ridge

Abandoned Wash

Bow Ridge

Plug Hill Fran Ridge

San Juan Hill Rainier Ridge

36°48'45" Yucca Crest 750000 Crater Flat Jackass Flats Dune Wash

East Ridge

Double Notch

WT-11 Wash Forlorn Wash

Ridge W T-12 W Busted 740000 ash Butte

Ambush Pass

550000 560000 570000 580000

0 1 2 MILES

0 1 2 KILOMETERS

EXPLANATION

Quaternary deposits

Miocene volcanic bedrock

Figure 2. Locations of prominent physiographic features in the map area.

3 geologic and hydrologic framework modeling ef- block-bounding faults and some of the intrablock forts are ongoing to characterize the site as faults, and outlined the zoned compositional na- fully as possible. Knowledge of the distribution ture of the tuffaceous units that underlie Yucca of geologic units as well as the stratigraphic and Mountain. After identification of Yucca Moun- structural setting is the underpinning for such tain as a possible site for high-level nuclear investigations. This map and accompanying waste disposal by the U.S. Department of En- text describe the location, geometry, and kine- ergy, during the 1980’s Scott and Bonk (1984) matic interplay between the various fault types developed a detailed reconnaissance (1:12,000 as well as variations in the structural style. The scale) geologic map of the area. This map was concept of structural domains is applied to the used to help constrain the most favorable area map area to better describe these structural vari- at Yucca Mountain and was a guide to further ations. The timing of deformation at Yucca site-characterization studies. Of their many Mountain is discussed in the context of the Ter- contributions, Scott and Bonk (1984) presented tiary to Recent tectonic evolution for the area. a detailed stratigraphy for the volcanic units, further defined the block-bounding faults, and NOTES ON STRATIGRAPHIC mapped numerous intrablock faults. NOMENCLATURE Subsequent mapping during the 1990’s re- vealed structural complexities along some of the The stratigraphic nomenclature used here is intrablock faults (for example, Spengler and oth- modified from the informal stratigraphy pre- ers, 1993) not evident at the 1:12,000 scale. sented by Buesch and others (1996), incorpo- Recently, Day and others (1998) developed a rating lessons learned by Day and others detailed (1:6,000 scale) geologic map over the (1998). The stratigraphic nomenclature pre- central block area, which includes the potential sented in Buesch and others (1996) was an repository (see fig. 4). A detailed (1:6,000 scale) outgrowth of borehole studies, replacing map geologic map of the area north of Yucca Wash units of Scott and Bonk (1984) from a vol- in the Paintbrush Canyon area has just been canological perspective to reflect stratigraphic completed (Dickerson and Drake, 1998) and was features observed both in the boreholes and incorporated herein. outcrops. Buesch and others (1996) also incorporated the work of Sawyer and others (1994) METHODOLOGY to update definitions of formations and groups for the area. Modifications to the nomenclature This map was developed using standard geo- used herein were made to simplify the map unit logic mapping techniques. Detailed 1:6,000- labels to the standard maximum of four letters scale and 1:12,000-scale orthophoto and topo- per unit, which was especially necessary in the graphic maps were used for base maps over structurally complex areas of the map. The “T” most of the area, which were developed specifi- used by Buesch and others (1996) was dropped cally for the Department of Energy Yucca from the bedrock igneous rock unit labels inas- Mountain Project. These series of maps have much as they are all Tertiary in age. In addition, 10-ft contour intervals. As such, the final prod- the “p” for most of the units in the Paintbrush uct used supporting maps at a much larger scale Group (see Buesch and others, 1996) was (1:6,000 to 1:12,000) than the final 1:24,000- dropped. For example, a unit such as the middle scale map. nonlithophysal zone of the crystal-poor member Stratigraphic units within the Topopah of the Topopah Spring Tuff, delineated as Spring and Tiva Canyon Tuffs, which make up “Tptpmn” by Buesch and others (1996), is the bulk of bedrock outcrops in the map area, simplified to “tpmn” in this report. This con- have been refined into informal members, zones, vention follows that for symbols for standard and subzones by Buesch and others (1996) (see U.S. Geological Survey geologic maps as out- “Stratigraphic setting” section). Inasmuch as lined by Hansen (1991). the Topopah Spring Tuff is proposed as the host unit for the potential repository, the pre- PREVIOUS MAPPING sent map includes detailed zonal subdivisions of the Topopah where possible. In order to sim- During the 1960’s Lipman and McKay plify the final map, the Tiva Canyon Tuff was (1965) and Christiansen and Lipman (1965) mapped at the informal member status [crystal- mapped the Yucca Mountain area at a scale of rich and crystal-poor members (units cr and cp)], 1:24,000 during a regional mapping campaign to as was its nonwelded basal vitric zone (unit cpv), provide an overall geologic framework for the which forms a prominent marker horizon Nevada Test Site. They delineated the main throughout the map area.

4 The threshold vertical offset on faults boreholes, the series identifier and sequence mapped outside of the central block area (Day number are separated by a hypen rather than and others, 1998) was approximately 10 ft (3 the number symbol, #. The first hole planned in m). Within the central block proper, the the UZ series is off-NTS and, therefore, is des- threshold was approximately 5 ft (1.5 m) of ver- ignated “USW UZ-1”. tical offset. Placement of stratigraphic contacts The letters signify the primary purpose for falls within similar vertical thresholds of accu- which the borehole was drilled. For example, racy. In addition, detection of faults is difficult boreholes starting with G were drilled for collec- within the middle part of the crystal-poor mem- tion of geologic data, those with H were drilled ber of the Tiva Canyon Tuff. This zone is one primarily for hydrologic data, with UZ for unsat- of the most widely exposed units, but it com- urated zone data, with NRG for collection of monly forms debris-covered slopes. Faults are data along the north-ramp alignment of the un- difficult to detect in this unit not only because of derground Exploratory Studies Facility (ESF), the cover, but because it is a densely welded unit with p for collection of data on rocks of pre- that lacks internal marker horizons. However, Tertiary age, with SD for geotechnical and hy- faults that offset adjacent mappable contacts are drologic data from a statistically located system- easier to detect. As such, the intrablock faults atic drilling program, and with WT for testing with a few hundred meters or more of surface the depth of the water table. trace length were readily detected. In this same vein, the massive, densely welded middle non- DATA SOURCES lithophysal zone of the crystal-poor member of the Topopah Spring Tuff (unit tpmn) may con- Two geologic map reports were incorpo- tain minor faults that were undetected. Faults rated into this map (see “Index to geologic map- with appreciable extent cut through map unit ping” on map): the bedrock geologic map of the contacts, however, which are offset. Their central block area by Day and others (1998) and presence would have been detected. the geologic map of the Paintbrush Canyon area by Dickerson and Drake (1998). The remaining BOREHOLE DESIGNATIONS area was remapped by the authors (W.C. Day, R.P. Dickerson, C.J. Potter, D.S. Sweetkind, Each borehole depicted on the map and used and R.M. Drake, II) from June 1996 through in the study of Yucca Mountain has a unique June 1997, and by C.J. Fridrich during 1992. name or number. For the purposes of this re- The locations of faults beneath the Quater- port the prefix designators UE-25 or USW (see nary surface deposits of Midway Valley and following paragraphs) are not posted on the map Yucca Wash were inferred from the ground- because of space limitations on the map. Each based gravity and magnetic surveys of Langen- bolehole designator is unique in this study area, heim and others (1993), Langenheim and Ponce and, therefore, the prefixes are not necessary (1994), Ponce and others (1993), and Ponce on the map. (1993). A recent geophysical report on the Boreholes on the Nevada Test Site (NTS— Midway Valley and Yucca Wash areas by Ponce see geologic map for boundary) use a designa- and Langenheim (1994) was also included. All of tion that differs slightly from that of boreholes these sources were used primarily to determine off the NTS. For example, the full borehole the probable location of the concealed faults in designations for boreholes on the NTS begin Midway Valley and Yucca Wash, as well as to with UE (for Underground Exploratory), fol- estimate the direction of offset. Estimates of lowed by the NTS area number (Area 25 in this offset were also corroborated by Buesch and report; for example, UE-25). This prefix desig- others (1994). nation “UE-25” commonly is followed by one or The main source of information for the more letters signifying the purpose of the hole structural control on the interpretations pre- or simply by a sequential letter, followed by a se- sented in the cross sections was the surface quence number. Thus, the designation “UE-25 geologic mapping presented herein. Strati- c#3” specifies a hole on the NTS in Area 25 and graphic thickness information from Geslin and that it is the third hole planned, but not neces- Moyer (1995), Geslin and others (1995), Moyer sarily drilled, at a site named “c”. and others (1995), and Moyer and others For boreholes off the NTS in adjacent land (1996) was used as corroborative data to com- to the west controlled by the U.S. Bureau of pare with outcrop data to construct the cross Land Management or the Nellis Air Force sections. Data from three-dimensional computer Range, the prefix USW is used (for Under- framework models by Buesch and others (1995) ground, Southern Nevada, Waste). For USW and from the Integrated Site Model ISM2.0 by

5 R.W. Clayton and W.P. Zelinski (Woodward- (pre-circa 13 Ma) extension provided the original Clyde Federal Services, written commun., 1997) tectonic framework for volcanism and subse- were also used as corroborative information to quent tectonism at Yucca Mountain. verify stratigraphic thicknesses on the cross sections. STRATIGRAPHIC SETTING Over the decades of geologic investigations at Yucca Mountain, numerous cross sections of The study area is predominantly underlain by the Midway Valley area have been produced, Miocene volcanic rocks of the Crater Flat, Paint- each reflecting the geologic understanding at brush, and Timber Mountain Groups (Buesch that time. These corroborative cross sections and others, 1996). At Yucca Mountain, these include those of Scott and Bonk (1984), Scott are metaluminous extracaldera tuffs (Sawyer and (1990), Carr (1992), Carr and others (1986), others, 1994) made up of variably welded Geldon (1993), and Neal (1986) as well as un- pyroclastic outflow sheets and tephra fallout published cross sections by W.R. Keefer and deposits, with minor intercalated lava flows, J.W. Whitney (U.S. Geological Survey, written ground surge, and reworked volcanic deposits. commun., 1996), and geophysical interpreta- The oldest units in the map area are ash- tions by Ponce (1996). These various cross flow tuffs of the quartz-rich Crater Flat Group. sections were examined in light of present inter- The Crater Flat Group is made up of the Tram pretations to assure that known geologic data Tuff, the 13.25-Ma Bullfrog Tuff, and the Prow and structural considerations were not ignored. Pass Tuff (Byers and others, 1976; Sawyer and The present interpretations are based solely others, 1994). The Tram Tuff does not crop upon the above-cited qualified data, and the ear- out in the map area. The Bullfrog and Prow lier cross sections were used only as guidelines. Pass Tuffs (units b and p) crop out just west of Subsurface borehole data from published and Prow Pass in the northwesternmost part of the unpublished sources were also used as support- map area, but only the uppermost part of the ing data. These include data for boreholes c#1, Bullfrog Tuff is exposed in the map area. Its c#2, and c#3 (Geldon, 1993), borehole p#1 in middle and lower parts are severed by the splay Carr and others (1986), those discussed by Gib- of the Windy Wash fault. Sawyer and Sargent son and others (1992), borehole WT-5 (R.W. (1989) suggested that the source for the Bull- Spengler, U.S. Geological Survey, written frog was the Area 20 caldera, which is more commun., 1994), and boreholes WT-14 and WT- than 20 km north of Prow Pass and lies along 16 (R.W. Spengler, written commun., 1994; the northern margin of the Timber Mountain– Muller and Kibler, 1985). Oasis Valley caldera complex (fig. 1). Carr and others (1986) did not identify a source caldera REGIONAL SETTING for the Prow Pass Tuff, but suggested that its distribution is sufficiently limited that it may not Yucca Mountain, Nye County, Nevada, lies have erupted from a caldera. within the Walker Lane belt, which is a north- Regionally, the 13-Ma Wahmonie Formation west-striking zone dominated by right-lateral overlies the Crater Flat Group. It crops out wrench-zone tectonism (Stewart, 1980; Carr, poorly (<3 m thick) and only on the northeast- 1984, 1990). The middle Miocene southwestern and southeastern sides of Busted Butte ern Nevada volcanic field (fig. 1) erupted from a (Broxton and others, 1993). As such, it was series of calderas within the Walker Lane belt not mapped separately from the Quaternary de- (Byers and others, 1976; Carr, 1984; Sawyer posits on this map. The Wahmonie Formation and others, 1994). The southwestern Nevada is made up of andesitic and dacitic rocks that volcanic field is predominantly a series of silicic erupted from the Wahmonie volcano (fig. 1) east tuffs and lava flows that range in age from 8 to of the map area (Poole and others, 1965; 15 Ma. One of the principal caldera complexes Broxton and others, 1989). exposed in the southwestern Nevada volcanic The 12.9-Ma Calico Hills Formation lies field is the Timber Mountain–Oasis Valley caldera stratigraphically between the Crater Flat Group complex (Sawyer and others, 1994). The mar- and the Paintbrush Group. This unit is com- gin of the Timber Mountain–Oasis Valley caldera posed of interbedded rhyolitic tuffs and lavas re- complex lies just north of the map area and was lated to Crater Flat Group post-caldera volcan- the source for many of the volcanic units ex- ism (Sawyer and others, 1994). Dickerson and posed at Yucca Mountain. This caldera complex Drake (1998) proposed a source in the lower formed within an older north-trending Tertiary Fortymile Canyon area. rift zone, which Carr (1986, 1990) defined as Rocks of the Paintbrush Group make up the the Kawich-Greenwater rift. As such, Tertiary vast majority of the bedrock exposures in the

6 Yucca Mountain area. Buesch and others Tuff is variously made up of pre-Pah Canyon Tuff (1996) described the units of the Paintbrush bt2, pre-Yucca Mountain Tuff bt3, the distal part Group and defined the members, zones, and sub- of the Pah Canyon and Yucca Mountain Tuffs, zones in both the Tiva Canyon and Topopah and the pre-Tiva Canyon Tuff bt4 of Moyer and Spring Tuffs. The main units of the Paintbrush others (1996). Locally, unit bt2 is the predomi- Group include (from oldest to youngest) the nant tuff within unit bt. Topopah Spring Tuff, the Pah Canyon Tuff, the The quartz-poor 12.7-Ma Tiva Canyon Tuff rhyolite of Zig Zag Hill, the rhyolite of Delirium (Sawyer and others, 1994) overlies the non- Canyon, the rhyolite of Black Glass Canyon, welded bedded units. The source of the Tiva the Yucca Mountain Tuff, the Tiva Canyon Tuff, Canyon Tuff was the Claim Canyon caldera (fig. the rhyolite of Vent Pass, and the rhyolite of 1) north of the map area (Byers and others, Comb Peak. The quartz-poor 12.8-Ma Topopah 1976). Like the Topopah Spring Tuff, the Tiva Spring Tuff is made up of a lower crystal-poor Canyon Tuff is made up of a lower crystal-poor rhyolitic member (Buesch and others, 1996) rhyolitic member (Buesch and others, 1996) (units tpv, tpln, tpll, tpmn, tpul, and tp) and an (units cpv and cp) and an upper crystal-rich upper crystal-rich quartz latite member (units trn, quartz latite member (unit cr). It is overlain by trv, and tr). The source caldera for the pumiceous, pyroclastic-flow and fallout deposits Topopah Spring remains undefined; however, (unit bt5) in southern Dune Wash as well as in Byers and others (1976) proposed the Claim northern Windy Wash. Canyon caldera to the north of Yucca Mountain Pyroclastic flow deposits and lava flows of (fig. 1). Facies relations within the Topopah the rhyolites of Delirium Canyon (unit dc), Black Spring Tuff around Yucca Wash and north of Glass Canyon (unit bg), Vent Pass (unit vp), the map area are consistent with the Claim and Comb Peak (units kt and kl) are exposed Canyon caldera as the most likely source. north of Yucca Wash in the northeastern part The informal pre-Pah Canyon Tuff non- of the map area. The vent for the rhyolite of welded bedded tuff (unit bt2) of Buesch and oth- Comb Peak was defined by Dickerson and Drake ers (1996) lies above the nonwelded uppermost (1995) to be at Comb Peak, just northeast of part of the Topopah Spring Tuff. Its distribution the map area. Facies and thickness relations extends from the area of The Prow south within the rhyolite of Delirium Canyon suggest throughout the entire map area. Where tuff bt2 a source in upper Fortymile Canyon, also just is less than 2 m thick and the Pah Canyon Tuff north of the map area. The rhyolite of Black is present as a mappable unit (unit pp), such as Glass Canyon is limited to Black Glass Canyon, at The Prow, unit bt2 is lumped with the and its source likely is located there as well. The overlying Pah Canyon Tuff. Where tuff bt2 is source for the rhyolite of Vent Pass was proba- over 4 m thick and the Pah Canyon Tuff is not bly near the northern boundary of the map area. present as a mappable unit, such as in southern The quartz-rich 11.6-Ma Rainier Mesa Tuff, Fatigue Wash, unit bt2 is mapped separately. which is the oldest formation of the Timber Where tuff bt2 occurs in the interval of Mountain Group, erupted from the Timber nonwelded, bedded tuff between the Topopah Mountain–Oasis Valley caldera complex (Byers Spring and Tiva Canyon Tuffs, such as in the and others, 1976). In the map area, the Rainier southern part of the map area, it is lumped with Mesa Tuff is made up of a lower nonwelded tuff the other bedded tuffs as nonwelded bedded (unit mr) overlain by a partially to moderately tuffs, undivided (unit bt). welded rhyolitic ash-flow tuff (unit mrw). North Both the Pah Canyon Tuff and Yucca of Yucca Wash, the rhyolite of Pinnacles Ridge Mountain Tuff thicken from east to west in the (unit pr) forms a tuff cone and lava dome com- northern part of the map area and pinch out plex of limited areal extent. toward the south in Fatigue Wash and Solitario Tertiary basaltic dikes (unit d) intrude Paint- Canyon. The pre-Yucca Mountain Tuff bedded brush Group rocks along the Solitario Canyon tuff (unit bt3) is a thick, nonwelded tuff between fault and along northwest-striking faults in the the Pah Canyon and Yucca Mountain Tuffs in north-central part of the map area. Isotopic dat- the northern part of the map area. Unit bt3 is ing (K-Ar) of a basaltic dike that intrudes the as thick as 40 m in Yucca Wash, but thins to Solitario Canyon fault at Little Prow yields an the south and pinches out approximately where age of 10±0.4 Ma (Carr and Parrish, 1985). the Pah Canyon and Yucca Mountain Tuffs This dike is itself brecciated along the fault, pinch out. Southward from the pinch-out of implying that movement along the Solitario these units, the nonwelded tuff (unit bt) between Canyon Fault both predated and postdated the the Topopah Spring Tuff and the Tiva Canyon emplacement of the dike (Scott, 1990).

7 STRUCTURAL GEOLOGY include (fig. 3) the Northern Windy Wash, Fatigue Wash, Solitario Canyon, and Iron Ridge The volcanic outflow sheets form a carapace faults, several unnamed faults between the Iron of pyroclastic rocks around the southern and Ridge fault and Dune Wash, and the East Ridge, southeastern margins of the Claim Canyon Dune Wash, northern part of the Bow Ridge, caldera (fig. 1). The volcanic carapace was then Midway Valley, and Paintbrush Canyon faults. disrupted by predominantly north-striking faults, These north- and northwest-trending, block- along which the volcanic units were tilted into bounding faults are often linked kinematically by east-dipping structural blocks at Yucca northwest-trending relay faults and associated Mountain. structures, which act to distribute displacement Faults at Yucca Mountain can be classified as across several of the faults. Mineral lineations block-bounding faults, relay (and associated) and mullions developed on fault scarps indicate structures, strike-slip faults, or intrablock struc- that a subordinate component of sinistral slip tures. The block-bounding faults generally was commonly associated with the block- strike north, are spaced 1–4 km apart (fig. 3), bounding faults (Scott and Bonk, 1984; O’Neill and have throws of as much as hundreds of me- and others, 1992; Simonds and others, 1995; ters. Subordinate amounts of strike-slip motion Day and others, 1996; Day and others, 1998). are common. In some places, Quaternary to The block-bounding faults generally dip west Recent offset has been documented on these with variable dips ranging from relatively shallow faults (see Simonds and others, 1995). Relay (45°–50°) to steep (75°–85°). The amount of structures link the block-bounding faults. These secondary faulting preserved in the footwall is can be either large-scale features, commonly higher in areas along traces of the fault zones forming fault zones or systems with greater that have shallower dips. The subordinate left- than 100 m of aggregate offset, or they can be lateral component of motion within some of the small-scale features made up of one or more zones set up releasing and restraining bends lo- faults that interconnect with the block-bounding cally along the trace of the main fault zones. faults and have cumulative offset on the order of Restraining bends are along segments of the tens of meters. Internally, the fault zones can block-bounding faults that strike north-north- be made up of horst and graben structures, with east. One such area is preserved in the hanging an overall net displacement of units across the wall of the Bow Ridge fault near the south portal zone. The large strike-slip faults are restricted of the ESF (Exploratory Studies Facility). In this to the north-central part of the map area. area the Bow Ridge fault strikes north-north- Intrablock structures are relatively minor faults east. Farther to the south the strike of the fault that lie entirely within the structural blocks de- changes to the northwest at the saddle be- fined by block-bounding faults (fig. 3). The in- tween Bow and Boundary Ridges (fig. 2). An trablock faults typically have mapped lengths of east-dipping reverse fault in the hanging wall, in- less than 1 km, but are as long as 7 km, as tersected in the ESF, formed due to local com- noted in the combined length of the Ghost pression in this area of the restraining bend on Dance and Abandoned Wash faults. Bow Ridge fault. Previous workers recognized west-dipping DESCRIPTION OF BLOCK-BOUNDING strata that form coherent structural panels FAULTS within some of the hanging walls of the block- bounding fault zones (Scott and Bonk, 1984; Deformation associated with the block- Scott, 1990; Potter and others, 1996). Such bounding faults at Yucca Mountain affects sev- west-dipping panels are preserved in the hanging eral major repository issues such as the volume wall of the Solitario Canyon fault (north of Plug and quality of rock available for underground Hill), the Iron Ridge fault (north of borehole WT- construction of the potential repository, the de- 11), and the Northern Windy Wash fault (south- lineation of hydrologic flow paths, and seismic west of Prow Pass). The west-dipping panels hazard assessments. The present map depicts are bounded on their western side (west of the the distribution of structures associated with the block-bounding fault) by steeply dipping faults block-bounding faults, thus providing critical data that splay off of the west-dipping block-bounding needed to resolve these issues. Because of faults at depth. erosion and partial burial of hanging-wall blocks adjacent to the main fault trace, depiction of DESCRIPTION OF RELAY STRUCTURES structures in footwall blocks is generally more detailed than in the hanging-wall counterparts. Relay structures kinematically link the block- From west to east, the block-bounding faults bounding faults and form an integral part of the

8 116°30' 116°26'15"

Pagany 780000 Wash fault Sever Wash fault

Solitario Fatigue Wash fault Canyon fault

Northern Windy Wash fault 36°52' Drill Hole Wash 770000 Ridge fault fault

Bow Sundance fault

Midway Valley fault

Boomerang Point fault Ghost Dance fault

760000

Paintbrush Canyon fault

Abandoned Wash fault

36°48'45" 750000 B o w R Dune Wash faultid ge fault

East Ridge fault

Iron Ridge fault

Busted Butte fault 740000

550000 560000 570000 580000

0 1 2 MILES

0 1 2 KILOMETERS

EXPLANATION

Quaternary deposits Fault type—Bar and ball on downthrown side. Dotted where inferred Miocene volcanic bedrock Block-bounding fault Strike-slip fault Exploratory Studies Facility Relay structure Dominant intrablock fault

Figure 3. Distribution of fault types in the map area.

9 structural framework of Yucca Mountain. Relay two faults is more than 120 m along the major structures range in size from large zones of saddle of Yucca Crest, placing the top of the complexly anastomosing normal faults that rival Tiva (on the hanging wall) against its base (on the total displacement and rock-mass damage of the footwall). Numerous normal faults crop out many block-bounding faults, to simple small faults just east of the Iron Ridge fault north of and grabens that rival intrablock faults. One borehole WT-11. These gently west-dipping such large relay structure cuts through West faults formed in the footwall of the Iron Ridge Ridge (figs. 2 and 3), connecting the Northern fault where slices of the footwall were down- Windy Wash and Fatigue Wash faults. It con- dropped into the main fault zone along spoon- sists of a system of predominantly southwest- shaped faults. dipping normal faults with down-to-the-south- The strike of the Bow Ridge fault changes west displacement coupled with a subordinate abruptly at the saddle between Bow and Bound- number of nearly vertical, northeast-side-down ary Ridges from north-northeast on the north- normal faults. Moderate-scale (50–200 m wide) ern side of the saddle to a northwestern strike horsts and grabens occur within the relay zone. south of the saddle. The southern part of the The aggregate displacement across the zone is fault, which strikes into the Paintbrush Canyon approximately 60 m, down to the southwest. fault north of Busted Butte, can be thought of Two other relay structures effectively cut as a relay connecting the northern part of the the southern end of Jet Ridge. A northwest- Bow Ridge fault with the Paintbrush Canyon striking, southwest-side-down normal fault ex- fault. posed directly above the colluvial apron along Small relay structures also connect some of the southwestern end of Jet Ridge (west of the the block-bounding faults. Examples are exposed Boomerang Point fault) drops the crystal-rich on Exile Hill, just north of the north portal of member of the Topopah Spring Tuff (unit trn) the ESF. Four northwest-striking, down-to-the- about 20 m. Although the fault projects beneath northeast normal faults terminate against the the alluvial cover, it is inferred to connect the Bow Ridge fault on their northwestern ends and Fatigue Wash and Boomerang Point faults. An- against the east-side-down Exile Hill fault at their other relay fault is exposed east of the southern southeastern ends. Although the aggregate extent of the Boomerang Point fault. At the displacement across these faults is minimal (less southern terminus of Jet Ridge, the middle and than 20 m), the intervening rock mass is highly lower parts of the crystal-poor member of the brecciated. Another series of small northwest- Tiva Canyon Tuff (unit cp) are downthrown striking relay faults is exposed on the northern along a north-northwest-striking normal fault part of Bow Ridge, linking the Bow Ridge and against the top of the Topopah Spring Tuff. Midway Valley faults. One of these faults cuts This west-dipping, down-to-the-west fault has at through the c-hole complex (boreholes c#1, c#2, least 100 m of displacement and projects to the and c#3). The northern tip of Bow Ridge also south along the foot of Boomerang Point. A hosts several small northwest-striking faults. A splay of this fault strikes to the southeast just parallel-striking fault 350 m to the south of the west of borehole WT-7 and has over 70 m of c-hole complex has only about 10–15 m of west-side-down displacement. A thin sliver of down-to-the-southwest displacement. the crystal-rich member of the Topopah Spring Tuff (unit trn) forms a horst block within the DESCRIPTION OF THE PROMINENT normal-fault zone. Therefore, this part of the NORTHWEST-STRIKING northeastern margin of Crater Flat is defined by STRIKE-SLIP FAULTS northwest-striking, southwest-side-down relay faults, both exposed and buried. Three main northwest-striking strike-slip Another important relay structure slices faults are exposed in the northern part of the across Yucca Crest southwest of borehole G-3. map area: the Sever Wash, Pagany Wash, and Two dominant northwest-striking, southwest- Drill Hole Wash faults. They have been de- dipping faults cut the Paintbrush Group rocks scribed by Scott and others (1984), Simonds and connect the Solitario Canyon fault with the and others (1995), and Day and others (1998). Iron Ridge fault and associated faults. In the Kinematic indicators preserved along the scarps footwall of the Solitario Canyon fault, the of the Sever Wash and Pagany Wash faults indi- easternmost of these two faults places the cate that the latest motion was strike slip. Pre- middle part of the Tiva Canyon Tuff (unit cp) served along the Sever Wash fault are zones of down against the middle nonlithophysal zone of silicified breccia, subhorizontal slickensides, and the Topopah Spring Tuff (unit tpmn). Total subhorizontal mullion structures. Along the southwest-side-down displacement across these trace of the Pagany Wash fault, Reidel shears

10 are well developed in the welded part of the deformation that affects the block within a few Yucca Mountain Tuff in Pagany Wash (Simonds hundred meters of the block-bounding faults. and others, 1995). The Reidel shears form an Other intrablock faults, such as the northwest- acute angle (about 30°) measured clockwise from trending narrow grabens seen on Boundary the main fault plane, indicating dextral motion on Ridge, are present in the central and southern the fault plane during formation of the shears. parts of the map area. The grabens are local ex- Another feature of both faults is the apparent pressions of regional Tertiary extension and reversal of the vertical component of offset form minor dilation zones in the structural along their traces. Along their northwestern blocks. Intrablock and block-bounding faults part the faults show down-to-the-southwest share common geometric elements. Intrablock displacement, which changes to down-to-the- faults, such as the Ghost Dance, Abandoned northeast along their southeastern part. Wash, and Busted Butte faults, are marked by The Drill Hole Wash fault is buried beneath zones that widen upward near the surface, be- Quaternary alluvial deposits in the floor of Drill coming frameworks of upward-splaying faults Hole Wash. The main trace of the fault is inter- (Day and others, 1998). This pattern is shared sected in the subsurface in borehole a#4 by the block-bounding faults. (Spengler and Rosenbaum, 1980) and in the ESF Intrablock faults in the central and southern (Steven C. Beason, U.S. Bureau of Reclamation, parts of the map area include the Boomerang oral commun., 1995). The fault is exposed at Point, Ghost Dance, Abandoned Wash, Sun- the surface on Tonsil Ridge northwest of bore- dance, and Busted Butte faults, as well as un- hole H-1. Here the fault strikes N. 30° W., dips named faults in the block west of Dune Wash on steeply (80°–85°) to the southwest, and has a East Ridge. The Boomerang Point fault bisects cumulative throw of approximately 15 m down the Jet Ridge block (figs. 3 and 4). Down-to- to the southwest. This fault projects into the the-west displacement along this fault increases subsurface into a fault zone exposed in the ESF, southward to more than 120 m over a strike which has an apparent offset of least 1.2 m length of approximately 4 km. Rocks between (down-to-the-southwest). Exposures in the ESF the Boomerang Point fault and the Fatigue reveal subhorizontal slickensides with a dextral Wash fault contain very few faults—making it sense of slip. The variability in relative vertical one of the least deformed parts in the Yucca offset along the fault trace in the 400-m dis- Mountain area. The number of faults in the tance between Tonsil Ridge and the ESF (15 m block east of the Boomerang Point fault is at the surface compared to 1.2 m in the ESF) greater due to the increase in dip associated and orientation (N. 30° W. versus N. 10° W., with hanging-wall deformation of the Solitario respectively) is similar to that observed on Canyon fault. intrablock faults throughout the map area. The Ghost Dance fault is the main intra- Scott and Bonk (1984) inferred a north- block fault within the central part of the poten- west-striking fault beneath Yucca Wash, based tial repository area. It is a north-striking normal on their interpretation of aeromagnetic data. fault zone that dips steeply west with down-to- The wash is a prominent geomorphic erosional the-west displacement. The displacement, feature beneath which it is reasonable to infer a amount of brecciation, and number of associated fault. Significant changes in volcanic facies oc- splays vary considerably along its trace (Spen- cur across and north of Yucca Wash, but those gler and others, 1993; Day and others, 1998). facies changes do not require the presence of a The Abandoned Wash fault is interpreted to fault. Dickerson (1996) observed that several connect with the Ghost Dance fault in the area north-striking block-bounding faults can be of Ghost Dance Wash. From this area south, traced continuously across Yucca Wash in the the Abandoned Wash fault extends another 3.8 subsurface using detailed ground magnetic data km. It terminates in the wash south of borehole of Ponce and Langenheim (1994). Therefore, G-3. As such, the 7-km-long Ghost Dance– there is no direct or indirect evidence for a sig- Abandoned Wash fault is entirely an intrablock nificant fault beneath Yucca Wash. fault with no surface connection to any known block-bounding fault. Any connection at depth DESCRIPTION OF INTRABLOCK with block-bounding faults is inferential. STRUCTURES The Sundance fault zone is exposed south of Drill Hole Wash between Live and Dead Yucca Intrablock faults by definition are within the Ridges. Spengler and others (1994) first intervening rock mass between the block-bound- identified the Sundance fault as a wide zone of ing faults. In some cases, intrablock faults are minor northwest-striking faults that commonly expressions of hanging-wall or footwall show down-to-the-northeast displacement.

11 Subsequent detailed mapping by Potter and patterns that characterize individual domains in- Dickerson (in Day and others, 1998) showed clude both the geometry and intensity of fault- that the fault zone is about 750 m long, extends ing as well as the magnitude and direction of from Dead Yucca Ridge southeastward to Live stratal dips. These domains reflect protracted Yucca Ridge, and has a maximum cumulative deformation and record an integrated structural down-to-the-northeast displacement of 11 m. In response through time. In general, structural the ESF, the Sundance is a series of discon- domains can be identified at any scale; those de- tinuous narrow fault zones with minor amounts fined herein are appropriate for this 1:24,000 of vertical offset. map scale. In this context, the structural do- Numerous intrablock faults are present in mains as defined below are typically an aggre- the southern part of the map area. One of the gate of distinct structural blocks that share most prominent slices through Busted Butte. common intrablock fault characteristics and This west-side-down normal fault has one of the stratal dips. They are briefly described below, as greatest vertical exposures (over 250 m) of any a summary of the overall structural geology of intrablock fault in the map area. The upward- the map area. branching nature of faults described by Day and others (1998) for faults in the central block area CENTRAL YUCCA MOUNTAIN DOMAIN is well displayed along the fault that splits Busted Butte. The lowest exposures of the fault at the The largest domain is the Central Yucca base of Busted Butte form a zone of intense Mountain domain, which comprises three east- brecciation approximately 3 m wide. The fault tilted blocks bounded by west-dipping block- branches upward into several splays forming an bounding faults. From west to east, these approximately 110-m-wide zone. The western blocks include (1) the West Ridge block, bounded splay of the fault zone is actually an east-dipping on the west by the Northern Windy Wash fault reverse fault. The gently east-dipping crystal- and on the east by the Fatigue Wash fault; (2) rich Tiva Canyon Tuff (unit cr) in the footwall the Jet Ridge block, bounded on the west by the (west of fault splay) is overlain by a fault sliver of Fatigue Wash fault and on the east by the Soli- vertically dipping older crystal-poor Tiva (unit cp) tario Canyon fault; and (3) the central block, in the hanging wall (east of fault splay). bounded on the west by the Solitario Canyon Dips of the intrablock faults are generally fault and on the east by the Bow Ridge fault. steeper than those of the block-bounding faults. The West Ridge block is about 1 km wide and 5 The intrablock dips shown on the cross sec- km long, the Jet Ridge block is about 2 km wide tions were derived from the mapped trace of the and 7 km long, and the central block is about 4 fault intersection with topography and range km wide and 7 km long. In each of these blocks, generally from 75° to vertical. Measured scarps stratal dips for the Paintbrush Group ash-flow along the block-bounding faults vary, but dips tuffs are typically 5°–10° to the east, steepening generally are between 60° and 75° to the west. along the eastern edges of the blocks in the If all of the intrablock faults formed prior to the hanging walls of block-bounding faults to values rotation of the beds into their generally east- as large as 20°. Intrablock faults, such as the ward dip, then the intrablock faults would have north-striking Ghost Dance and Abandoned formed originally as reverse faults. Kinemati- Wash faults in the central block and the Boo- cally, this would be unlikely in that deformation merang Point fault in the Jet Ridge block, are of the Tertiary tuffs throughout the map area locally prominent (see “Structural geology” sec- was extensional. Therefore, the intrablock tion). The central block and, to a much lesser faults probably formed during and after tilting of extent, the Jet Ridge block contain a complex the beds on the block-bounding faults. zone of intrablock faulting along the eastern edge (Scott and Bonk, 1984; Day and others, STRUCTURAL DOMAINS 1998). This block-margin faulting, accompanied by steepening of stratal dips, reflects the stratal The map area can be divided into ten struc- rotation and internal deformation produced in tural domains (fig. 4), each characterized by a the hanging wall of a complex block-bounding distinctive structural style. These structural fault zone (Scott, 1990; Day and others, 1998). domains all lie within the more regional Crater The northern edge of the Central Yucca Flat tectonic domain as defined by Fridrich (in Mountain domain is more highly deformed than press). The term “structural domain” is used the central and southern parts. For example, here to denote an area characterized by a par- near The Prow (figs. 2 and 4) the frequency and ticular structural style that is distinct from that intensity of intrablock faulting increase. Along of adjacent areas. The distinctive structural the northwest-trending edge of Yucca Mountain

12 116°30' 116°26'15"

Yucca Wash domain

780000

Azreal Ridge Central WEST RIDGE BLOCK domain Yucca Mountain domain

36°52' 770000

JET RIDGE BLOCK

CENTRAL BLOCK

Paintbrush Canyon domain

760000

Fran Ridge domain

36°48'45" 750000 Dune W Fortymile Wash domain

ash dom Plug Hill

domain ain

Southwest East Ridge

domain dom

ain 740000

550000 560000 570000

0 1 2 MILES

0 1 2 KILOMETERS

EXPLANATION

Figure 4. Distribution of structural domains defined for the map area. 13 between The Prow and Castellated Ridge, Central Yucca Mountain domain, the Azreal southeast-dipping Paintbrush Group strata are Ridge domain consists of gently southeast- cut by a series of north- to north-northwest- dipping Paintbrush Group strata. Because the striking normal faults that range in offset from 2 main ridges and washes at Yucca Mountain have to 70 m; east-side-down and west-side-down generally developed parallel to the dip of the displacements are about equally divided (see Paintbrush Group welded tuffs, this southeast cross section A –A ′). Through the north- dip is expressed geomorphically as a strong western corner of the map area and to the northwest-trending topographic grain. The north of the map area, the Northern Windy southeast-dipping strata in the Azreal Ridge Wash fault maintains its character as a complex, domain actually form one limb of a broad west-side-down block-bounding fault. The southeast-plunging syncline, and the other limb Fatigue Wash fault continues through the area is defined by the east-dipping strata of the of The Prow as a west-side-down block- central block. The axis of this syncline lies bounding fault. beneath Drill Hole Wash.

AZREAL RIDGE DOMAIN YUCCA WASH DOMAIN

The Azreal Ridge domain lies between Drill North of the Azreal Ridge domain, the Hole Wash and Yucca Wash, northeast of the Yucca Wash domain records a northward in- Central Yucca Mountain domain (fig. 4). The increase in the magnitude of east-west extension, tensity of faulting in this domain is relatively low accommodated by displacements along several compared with the Central Yucca Mountain, north- to northwest-striking intrablock and Dune Wash, or Southwest domains to the strike-slip faults. These include several promi- south (see below). Faulting within the domain is nent splays of the Sever Wash fault just south dominated by the three strike-slip faults previ- of Yucca Wash, along which the amount of ously discussed (Sever Wash, Pagany Wash, stratigraphic displacement increases markedly to and Drill Hole Wash faults). The intrablock the north. This structural style is similar to that faults are minor, north-trending normal faults in the adjacent part of the Central Yucca Moun- with limited brecciation. Strata dip gently tain domain near The Prow, as previously dis- southeast (4°–9°) in the main part of the cussed, and it continues on the north side of domain. Along the eastern edge of the domain Yucca Wash within the Yucca Wash domain (fig. in the hanging wall of the Bow Ridge fault, de- 4). Bedrock dips within the Yucca Wash do- formation intensifies as recorded by the increase main, as preserved in the Paintbrush Group, are of the stratal dips (as much as 13°) and relative steeper (14°–28°) and more easterly than in the intensity of faulting. adjoining Central Yucca Mountain or Azreal The transition from the northern margin of Ridge domains. the central block to the Azreal Ridge domain (fig. 4) is marked by a decrease in the magnitude of PAINTBRUSH CANYON DOMAIN east-west extension, manifested in a fundamental change in the nature of the two block- The Paintbrush Canyon domain is char- bounding faults. West-side-down displacements acterized by closely spaced faults that divide this along the Solitario Canyon and Bow Ridge domain into a series of long, narrow, north- faults diminish to zero at hinge points within trending, east-dipping blocks ranging from 0.6 northern Yucca Mountain. North of these to 1 km in width (fig. 4). The domain is bounded hinge points, east-side-down displacements are on the east and west by the Paintbrush Canyon mapped along these faults. For the Solitario and Bow Ridge faults, respectively, and it in- Canyon fault, the hinge point is in Teacup cludes prominent north-striking faults such as Wash, and for the Bow Ridge fault, the hinge the Black Glass Canyon, Midway Valley, and point lies beneath Quaternary alluvium east of Exile Hill faults. The northern part of the do- Isolated Ridge. The Azreal Ridge domain occu- main is well exposed north of Yucca Wash, pies the little-extended area bounded by the seg- whereas the bedrock in the central part of the ments of these two faults that have undergone domain is obscured by Quaternary deposits in minimal displacements near the hinge points. the Midway Valley area. The bedrock in the In addition to the decreased magnitude of southern part of the domain resurfaces on Bow extension, the Azreal Ridge domain is character- Ridge and San Juan Hill to the east (just west of ized by a change in the direction of stratal dips the Paintbrush Canyon fault; figs. 2 and 4). and a change in intrablock faulting style. In Where exposed, the Paintbrush Group generally contrast to the easterly dips predominant in the strikes northward, but dips more steeply (as

14 much as 46°) to the east than in the adjacent north to south): Comb Peak (just off the north- Central Yucca Mountain domain. eastern part of the map area) and the bedrock In the northern part of the domain, fault tract to its south, Alice Point, Fran Ridge, and blocks are juxtaposed along down-to-the-west Busted Butte. In the northern part of the do- normal faults with extensive deformation in their main the Paintbrush Group and older rocks hanging walls. This hanging-wall deformation strike northward and dip steeply 17°–27° to the includes graben development, numerous splays, east. In the central part of the domain on Fran pull-apart structures, and wide, internally Ridge, rocks of equivalent age strike northward sheared, massively brecciated, multi-plane main and dip more gently to the east (5°–17°). To fault zones. Geophysical interpretations strong- the south on Busted Butte, the bedrock dips are ly suggest that fault blocks buried beneath surfi- gentle to the east (5°–10°). The Fran Ridge cial deposits in Midway Valley define a horst-and- domain is characterized by less internal defor- graben pattern (Ponce, 1993; Ponce and others, mation than that found in the Paintbrush 1993). Exile Hill, for example, represents a horst Canyon domain. Smaller faults, which splay block in this domain. Another buried horst oc- eastward off the northern extent of the Paint- curs to the east beneath the center of Midway brush Canyon fault, indicate a greater amount of Valley; these horsts are 0.3–0.7 km wide. footwall deformation for the Paintbrush Canyon The continuity of north-trending faults from fault than is demonstrated for the exposed faults north of Yucca Wash southward through Mid- in the Paintbrush Canyon domain. Although way Valley is one of the unifying characteristics the internal deformation is minor in most of the of the Paintbrush Canyon domain. Geophysical Fran Ridge domain, a few intrablock faults have and geologic data indicate that the main faults significant amounts of offset, such as the exposed north of Yucca Wash continue south- Busted Butte fault (75 m) and the fault south of ward beneath Quaternary deposits to the Comb Peak (130 m). Additionally, the Fran southern part of Midway Valley (Dickerson and Ridge domain is structurally higher than the Drake, 1998). This observation argues strongly adjacent Paintbrush Canyon and Fortymile against the presence of the Yucca Wash fault as Wash domains. Gaps between the topographic a major structure. highs within the Fran Ridge domain are occupied Along the east edge of the Paintbrush Can- by buried down-to-the-northeast faults that lie yon domain, the magnitude and complexity of between Busted Butte and Fran Ridge, and the Paintbrush Canyon fault increase to the between Fran Ridge and Alice Point. south. Over 200 m of displacement is recorded along this fault in upper Paintbrush Canyon FORTYMILE WASH DOMAIN (Dickerson and Spengler, 1994; Dickerson and Drake, 1998), and over 300 m of displacement Geologic relations within the Fortymile near Fran Ridge. Beneath southeastern Midway Wash domain are poorly understood because the Valley, buried northeast-striking splays are in- bedrock geology of this domain largely is con- ferred to have propagated from the main trace of cealed by the surficial deposits of Jackass Flats. the Paintbrush Canyon fault (Ponce, 1993). Furthermore, the prevailing easterly dip of the The horst-and-graben pattern that characterizes Paintbrush Group strata in the Fran Ridge most of Midway Valley is cut off by (or merges domain strongly suggests that a major west- into) these splays. At the southern end of the side-down normal fault may lie east of Fortymile Paintbrush Canyon domain the Paintbrush Wash beneath the western part of Jackass Canyon fault merges with the Dune Wash and Flats. The northern part of the Fortymile Wash Bow Ridge faults (fig. 3); the Paintbrush Canyon domain consists of the extensively faulted rocks fault system collects the aggregate displacement east of Fortymile Canyon. There is no of all of these faults. Unfortunately, the area of preferred orientation for faults in this area, as intersection of these faults, surely one of the faults strike east-west, north-south, northeast- most deformed and structurally complex areas of southwest, and southeast-northwest. It is not Yucca Mountain, is concealed beneath surficial known to what extent this random fault pattern deposits northwest of Busted Butte. characterizes other parts of the Fortymile Wash domain where bedrock is not exposed. FRAN RIDGE DOMAIN PLUG HILL DOMAIN East of the Paintbrush Canyon domain, the Fran Ridge domain is a north-trending, largely The Plug Hill domain lies in the eastern intact block between the Paintbrush Canyon margin of Crater Flat on the southwest side of fault and Fortymile Wash. It includes (from the map area. It contains low-lying hills of Tiva

15 Canyon Tuff as well as exposures of the Rainier elsewhere in the southern part of Yucca Moun- Mesa Tuff on Plug Hill itself. The domain is tain. The block-bounding fault systems in this rimmed by normal faults that drop units of the domain are west-dipping, upward-widening ar- Paintbrush Group into Crater Flat. The north- rays, with westerly dips ranging from relatively ern margin of the domain is delineated by the shallow (45°–50°) to steep (75°–85°). As the relay structures on the southern tip of Jet upward-widening fault zones attain shallower Ridge, and the eastern and southern margin by dips, the amount of secondary faulting preserved the Solitario Canyon fault, which curves from a in the footwall of the block-bounding fault sys- north strike to a more northeasterly strike. Al- tem increases dramatically. In addition, fractur- though the bedrock is poorly exposed, where ing of the footwall increases near the major re- present it is made up of highly faulted, eastward- lay faults. dipping blocks. Plug Hill is made up of Tiva Canyon Tuff overlain by nonwelded and welded EAST RIDGE DOMAIN units of the Rainier Mesa Tuff. The amount of faulting within the Rainier Mesa Tuff is hard to The East Ridge domain lies to the west of detect because it lacks internal marker horizons the Dune Wash domain (fig. 4). It is bounded on and is poorly exposed. However, Day and oth- the west by the unnamed block-bounding fault ers (1998) did map faulted Paintbrush Group west of borehole WT-12 and the down-to-the- rocks caught in the hanging-wall deformation of east block-bounding faults just east of East Ridge the Solitario Canyon fault that are overlain by on the western edge of the Dune Wash graben. Rainier Mesa Tuff, suggesting that eruption of This domain differs from the adjacent Central the Rainier Mesa postdated some of the post- Yucca Mountain domain in that the amount of Paintbrush Group deformation on the hanging intrablock faulting is more intense, recording a wall of the Solitario Canyon fault. This relation greater amount of extensional deformation. The is unique to the Plug Hill area. Here, as else- strata strike north-northeast and dip generally where, the Rainier Mesa Tuff is preserved only 15°–20° to the east, more steeply than is ob- on the hanging wall of the block-bounding faults. served in the Central Yucca Mountain domain. Implications of the distribution of the Rainier This domain also has numerous east-side-down Mesa Tuff are discussed in the “Discussion on faults, which splay off the block-bounding fault the variation and timing of tectonism at Yucca along the western edge of the Dune Wash Mountain” section. graben.

SOUTHWEST DOMAIN DUNE WASH DOMAIN

In the Southwest domain, which lies be- The Dune Wash domain (fig. 4) is defined by tween Double Notch Ridge and Crater Flat (figs. the northwest-trending Dune Wash graben, 2 and 4), north-trending, block-bounding faults which is bounded on its east side by the down- are linked kinematically by northwest-trending to-the-west block-bounding Dune Wash fault and relay faults and associated structures. An excel- on its southeast margin by the Paintbrush lent example is the complex northwest-striking Canyon fault. Its western margin is bounded by relay zone between the Solitario Canyon and the down-to-the-east block-bounding East Ridge Iron Ridge faults near the north end of the fault zone (fig. 3), which has at least 120 m of Southwest domain. Several other relay faults down-to-the-east displacement. This down-to- splay off the Solitario Canyon fault in the the-east fault zone is approximately 100 m wide, southernmost part of the map area in the contains tectonically brecciated and juxtaposed Southwest domain, transferring strain from the units of the Topopah Spring Tuff (unit tx), and main trace of the Solitario Canyon fault. The in places exhibits higher degrees of oxidation, vertical component of offset on the Solitario silicification, and alteration compared with other Canyon fault (east of Plug Hill) is over 400 m. block-bounding faults. This fault zone splays to However, at its southernmost trace (at the the south-southwest into several faults with southern margin of the map area) the offset is down-to-the-east displacement. The splays each only on the order of 60 m. The aggregate ver- have over 30 m of displacement. Within the in- tical displacement taken up by the intervening terior of the graben are numerous, smaller five relay faults is over 60 m each, accounting horst-and-graben blocks whose strata dip pre- for the missing vertical component of displace- dominantly to the east. The structure of the in- ment on the Solitario Canyon fault. terior of the graben is highly complex with nu- Northwest-striking narrow grabens are merous discontinuous, steeply dipping faults. commonly associated with relay faults, here and The Dune Wash domain is one of the areas

16 depicted by figure 6 of Scott (1990) as a broad extreme south end of Yucca Mountain, south of imbricate fault zone; although it is intensely the map area (Rosenbaum and others, 1991). faulted, it is not characterized by consistent Northwest- to north-northwest-striking splays west-side-down offset along the array of faults, are a common component of the north-striking as Scott (1990) implied by his definition and use block-bounding fault systems. In some places, of the term “imbricate fault zone.” these northwest-striking splays have developed In the northern end of the Dune Wash into major relay faults that transfer displacement graben (southwest of borehole WT-1), the between block-bounding faults. In the extreme western bounding fault of the graben steps southern part of Yucca Mountain (south of the about 200 m to the west and displacement de- map area), the strikes of the southern Windy creases; this fault places the crystal-rich member Wash and Stagecoach Road block-bounding of the Tiva Canyon Tuff (unit cr) down against faults are rotated to northeasterly orientations, the pre-Tiva Canyon, post-Topopah Spring and the subsidiary splays strike north (Simonds nonwelded bedded tuffs (unit bt). The displace- and others, 1995). ment on individual faults within the graben de- Tertiary extension in the Yucca Mountain creases to the north, but the overall horst-and- area occurred primarily between 16 and 10 Ma graben pattern is maintained. At the northern (Carr, 1984, 1990; Scott, 1990; Hudson and end of the graben, just south of the mouth of others, 1994, 1996). Evidence for pre-Paint- Abandoned Wash, the faulting resembles the brush Group (pre-12.8 Ma) deformation along closely spaced faulting that characterizes block- the block-bounding faults is buried by the tuffs of margin deformation in the central block to the the Paintbrush Group themselves. Older units north. The southern end of the Dune Wash are sparsely exposed either within or adjacent to domain, which is buried beneath Quaternary de- the map area. As reviewed by Carr (1990), the posits southwest of Busted Butte, seems to Timber Mountain–Oasis Valley caldera complex, terminate against the down-to-the-west Paint- which was the source for most of the tuffs of brush Canyon fault. the Paintbrush Group, lies within an older, north-trending rift zone, which he named the DISCUSSION ON THE Kawich-Greenwater rift. This older rift basin VARIATION AND TIMING OF formed within the Walker Lane belt and con- TECTONISM AT YUCCA MOUNTAIN trolled the locus of caldera structures in the southwestern Nevada volcanic field (fig. 1 of The variation in deformation from a less- Carr, 1990). extended northern part of Yucca Mountain to a There is indirect evidence that the Solitario more-extended southern part is generally ex- Canyon fault was active during eruption of the pressed on geologic maps by an increase in Paintbrush Group. One clear pre-Tiva Canyon throw to the south along the block-bounding Tuff growth fault, a splay off the Solitario faults. In intrablock areas, the transition is ex- Canyon fault, is well exposed along the eastern pressed by the appearance of numerous closely canyon wall midway along Solitario Canyon spaced minor faults that coalesce and gain dis- southwest of borehole H-5. The nonwelded placement to the south. These patterns are ap- units appear to thicken across that fault, consis- parent in the northern part of Dune Wash, par- tent with a growth-fault setting. Throw at the ticularly near the mouth of Abandoned Wash. top of the Topopah Spring Tuff is about 10 m, Southward-increasing east-west extension is decreasing upsection to less than 4 m at the also recorded by the southward splaying of the base of the Tiva Canyon Tuff, and dying out in Solitario Canyon fault, the development of the the upper part of the Tiva Canyon Tuff. broad, complexly faulted Dune Wash graben, and Another example of extensional deformation the convergence of several block-bounding faults during deposition of the Paintbrush Group is ex- along the west side of Busted Butte. Scott posed on the west flank of Fran Ridge. A small (1990) expressed the southward increase in north-trending graben structure, presumably deformation as an increase in the area underlain kinematically related to the Paintbrush Canyon by imbricate fault zones characterized by steeper fault, is filled with pre-Tiva Canyon Tuff non- eastward dips of strata and an imbricate pattern welded bedded tuff (unit bt), which thickens in of closely spaced, steep, west-dipping faults with the center of the graben. Faults related to the minor, down-to-the-west offsets of a few meters graben, however, do not extend upward into the or less. Clockwise vertical-axis rotations of overlying Tiva Canyon Tuff. Paintbrush Group strata increase from north to The basal contact of the 11.6-Ma Rainier south, from 5° to 10° in the central block of the Mesa Tuff is one of the key horizons to unravel- Central Yucca Mountain domain, to 30° at the ing the timing of tectonism in the Yucca

17 Mountain area. There appears to be an east- conformably overlain by a bedded basal horizon ward decrease in the amount of angular uncon- in the base of the Rainier Mesa. Above this formity at the base of the Rainier Mesa Tuff. nonwelded bedded horizon within the Rainier Fridrich (in press) has shown that a significant Mesa Tuff, distinguished from unit bt5 by vitric angular unconformity exists between the Rainier pumice, lies a nonwelded massive horizon that Mesa Tuff and the underlying Tiva Canyon Tuff may have a structural discordance of 5°–8° with west of Yucca Mountain in northern Crater Flat its bedded basal horizon. This apparent change (west of the map area). He deduced that there in dip is internal to the Rainier Mesa Tuff. was a significant pulse of regional east-west ex- The distribution of the Rainier Mesa Tuff is tension recorded by the angular unconformity. key to understanding the Tertiary tectonic The basal contact of the Rainier Mesa Tuff history of Yucca Mountain. In the western part is exposed locally in northern Windy Wash in of the region, it caps the top of ridge crests. In the western part of the map area. There, the the central and eastern parts, the Rainier Mesa bedded tuff (unit bt5) dips about 8°–10° more is restricted to valley floors adjacent to the steeply than the overlying Rainier Mesa Tuff, major block-bounding faults. Moreover, except supporting the formation of a gentle angular un- near Plug Hill, the Rainier Mesa is further conformity after deposition of the Paintbrush restricted to the hanging-wall areas of the block- Group rocks (post-12.7 Ma), but prior to erup- bounding faults and does not lap onto the tion and emplacement of the Rainier Mesa. In footwall. At Plug Hill, the Rainier Mesa does lap this same area, the Rainier Mesa is caught up in across older faults associated with the Solitario the Windy Wash fault zone, faulted against Canyon fault. The Rainier Mesa appears to be Yucca Mountain Tuff and Pah Canyon Tuff. It conformable in a west-dipping hanging-wall block was present when the major displacement oc- associated with the Iron Ridge fault (north of curred along the Northern Windy Wash fault, borehole WT-11). Its absence from the footwall which has the most displacement of any block- areas cannot be explained by erosion alone bounding fault in the map area (over 450 m). inasmuch as the Rainier Mesa Tuff also forms This evidence indicates that the Northern resistant erosional remnants on ridge crests in Windy Wash fault was active both before and af- the western part of the map area. Its general ter deposition of the Rainier Mesa Tuff and, restriction to the hanging walls of the block- therefore, had a protracted history of motion. bounding faults indicates that it was erupted The basal contact of the Rainier Mesa Tuff prior to the final phases of motion on those is preserved farther to the east in the Solitario faults and was downdropped thereafter. If it Canyon area at Plug Hill, where evidence is were simply erupted into preexisting valleys and lacking to support a major angular unconformity grabens (Carr, 1986; Scott, 1990) then it would at the base of the Rainier Mesa. Compaction lap consistently onto the footwall of the block- foliations within the Tiva Canyon Tuff dip at bounding faults, which are also in the valley about 12° to the east, an alignment similar in floors. This is in keeping with the mapping of magnitude to the dip of the welding contact in the Timber Mountain–Oasis Valley caldera com- the overlying Rainier Mesa. This is contrary to plex, in which the Rainier Mesa Tuff is highly mapping by Scott and Bonk (1984), who de- faulted within and adjacent to the caldera com- picted the basal contact of the Rainier Mesa plex (Byers and others, 1976). down-cutting into the top of the Tiva Canyon In summary, there is evidence for a pro- Tuff, a geometry that implies a significant tracted history of Tertiary extensional deforma- amount of erosion and post-Tiva Canyon, pre- tion in the Yucca Mountain area. Pre-Paintbrush Rainier Mesa deformation. The “down-cutting” Group extension formed a buried north-striking depicted by Scott and Bonk (1984) is simply a rift in which the Timber Mountain–Oasis Valley down-to-the-southwest fault that cuts the Tiva caldera complex formed. Syn-Paintbrush Group Canyon and Rainier Mesa, as was mapped origi- (circa 12.7 Ma) extension occurred along minor nally by Lipman and McKay (1965). Therefore, structures related to the Solitario Canyon and there is no significant amount of erosion be- Paintbrush Canyon faults. Evidence for contin- neath this contact at Plug Hill. ued extension along the block-bounding faults On the eastern side of the map area, the and rotation of units is recorded by an angular basal contact of the 11.6-Ma Rainier Mesa Tuff unconformity beneath the 11.6-Ma Rainier Mesa is exposed on the southwestern side of Dune Tuff in the northwestern part of the map area. Wash, just west of Ambush Pass. There, the In addition, the Rainier Mesa Tuff overlaps faults contact is conformable between a bedded tuff within the Solitario Canyon fault zone at Plug (unit bt5) and the top of the Tiva Canyon Tuff. Hill, indicating post-Tiva Canyon, pre-Rainier Additionally, the bedded tuff (unit bt5) is Mesa motion on this block-bounding fault. To

18 the east, however, the basal contact of the B.W., and Byers, F.M., Jr., 1986, Geology Rainier Mesa is conformable with the underlying of drill hole UE-25p#1—A test hole into pre- 12.7-Ma Tiva Canyon Tuff. This suggests a Tertiary rocks near Yucca Mountain, south- westward increase in extension earlier than 11.6 ern Nevada: U.S. Geological Survey Open- Ma within the map area. Extension along the File Report 86–175, 87 p. block-bounding faults continued after deposition Carr, W.J., 1984, Regional structural setting of of the Rainier Mesa Tuff inasmuch as its current Yucca Mountain, southwestern Nevada, and preservation is restricted to the hanging walls Late Cenozoic rates of tectonic activity in and faulted by the block-bounding faults. part of the southwestern Great Basin, Nevada and California: U.S. Geological Sur- REFERENCES CITED vey Open-File Report 84–854, 108 p. ———1986, Volcano-tectonic setting of Yucca Broxton, D.E., Chipera, S.J., Byers, F.M., Jr., Mountain and Crater Flat, southwestern and Rautman, C.A., 1993, Geologic evalua- Nevada, in Carr, M.D., and Yount, J.C., tion of six nonwelded tuff sites in the vicinity eds., Geologic and hydrologic investigations of Yucca Mountain, Nevada, for a surface- of a potential nuclear waste disposal site at based test facility for the Yucca Mountain Yucca Mountain, southern Nevada: U.S. Project: Los Alamos National Laboratory Geological Survey Bulletin 1790, p. 35–49. Report LA–12542–MS, 83 p. ———1990, Styles of extension in the Nevada Broxton, D.E., Warren, R.G., Byers, F.M., Jr., Test Site region, southern Walker Lane and Scott, R.B., 1989, Chemical and Belt—An integration of volcano-tectonic mineralogical trends within the Timber and detachment fault models, in Wernicke, Mountain–Oasis Valley caldera complex— B.P., ed., Basin and Range extensional tec- Evidence for multiple cycles of chemical evo- tonics near the latitude of Las Vegas, lution in a long-lived silicic magma system: Nevada: Geological Society of America Journal of Geophysical Research, v. 94, p. Memoir 176, p. 283–303. 5961–5986. ———1992, Structural model for western Mid- Buesch, D.C., Dickerson, R.P., Drake, R.M., II, way Valley based on RF drillhole data and and Spengler, R.W., 1994, Integrated geol- bedrock outcrops, in Gibson, J.D., and oth- ogy and preliminary cross section along the ers, eds., Summary and evaluation of exist- north ramp of the Exploratory Studies Facil- ing geological and geophysical data near ity, Yucca Mountain: 5th International High prospective surface facilities in Midway Val- Level Radioactive Waste Management Con- ley, Yucca Mountain Project: Sandia Na- ference, v. 2, p. 1055–1065. tional Laboratories SAND 90–491, 94 p. Buesch, D.C., Nelson, J.E., Dickerson, R.P., Carr, W.J., and Parrish, L.D., 1985, Geology of Drake, R.M., II, Spengler, R.W., Geslin, drill hole USW VH-2, and structure of J.K., Moyer, T.C., San Jaun, C.A., and Crater Flat, southwestern Nevada: U.S. Felger, T., 1995, Distribution of lithostrati- Geological Survey Open-File Report 85–475, graphic units within the central block of 41 p. Yucca Mountain, Nevada—A three-dimen- Christiansen, R.L., and Lipman, P.W., 1965, sional computer-based model, version Geologic map of the Topopah Spring YMP.R2.0: U.S. Geological Survey Open- Northwest quadrangle, Nye County, File Report 95–124, 61 p. Nevada: U.S. Geological Survey Geologic Buesch, D.C., Spengler, R.W., Moyer, T.C., Quadrangle Map GQ–444, scale 1:24,000. and Geslin, J.K., 1996, Proposed strati- Day, W.C., Potter, C.J., Sweetkind, D.S., and graphic nomenclature and macroscopic iden- Dickerson, R.P., 1996, Detailed bedrock tification of lithostratigraphic units of the geologic map of the central block area, Paintbrush Group exposed at Yucca Moun- Yucca Mountain—Implications for structural tain, Nevada: U.S. Geological Survey Open- development of the potential high-level ra- File Report 94–469, 45 p. dioactive waste repository area in Nye Byers, F.M., Jr., Carr, W.J., Orkild, P.P., County, Nevada: Geological Society of Quinlivan, W.D., and Sargent, K.A., 1976, America Abstracts with Programs, v. 28, Volcanic suites and related cauldrons of Tim- no. 7, p. A–248. ber Mountain–Oasis Valley caldera complex, Day, W.C., Potter, C.J., Sweetkind, D.S., southern Nevada: U.S. Geological Survey Dickerson, R.P., and San Juan, C.A., 1998, Professional Paper 919, 69 p. Bedrock geologic map of the central block Carr, M.D., Waddell, S.J., Vick, G.S., Stock, area, Yucca Mountain, Nye County, J.M., Monsen, S.A., Harris, A.G., Cork, Nevada: U.S. Geological Survey Miscella-

19 neous Investigations Series I–2601, scale Hudson, M.R., Minor, S.A., and Fridrich, C.J., 1:6,000, 2 plates. 1996, The distribution, timing, and charac- Dickerson, Robert, 1996, Geologic and geo- ter of steep-axis rotations in a broad zone of physical evidence for normal faulting in dextral shear in southwestern Nevada: Geo- Yucca Wash, Yucca Mountain, Nevada: logical Society of America Abstracts with Geological Society of America Abstracts Programs, v. 28, no. 7, p. A–451. with Programs, v. 28, no. 7, p. A–191 to A– Hudson, M.R., Sawyer, D.A., and Warren, R.G., 192. 1994, Paleomagnetism and rotation con- Dickerson, R.P., and Drake, R.M., II, 1995, straints for the middle Miocene southwest- Source of the rhyolite at Comb Peak, ern Nevada volcanic field: Tectonics, v. 13, southwest Nevada volcanic field: Geological no. 2, p. 258–277. Society of America Abstracts with Pro- Langenheim, V.E., and Ponce, D.A., 1994, grams, v. 27, no. 4, p. 8. Gravity and magnetic investigations of ———1998, Geologic map of the Paintbrush Yucca Wash, southwest Nevada, in High Canyon area, Yucca Mountain, Nevada: level radioactive waste management, U.S. Geological Survey Open-File Report Proceedings of the Fifth Annual Interna- 97–783, 2 plates, scale 1:6,000. tional Conference, Las Vegas, Nevada, May Dickerson, R.P., and Spengler, R.W., 1994, 22–24, 1994: LaGrange Park, Ill., American Structural character of the northern seg- Nuclear Society, v. 4, p. 2272–2278. ment of the Paintbrush Canyon fault, Yucca Langenheim, V.E., Ponce, D.A., Oliver, H.W., Mountain, Nevada, in High level radioactive and Sikora, R.F., 1993, Gravity and mag- waste management, Proceedings of the netic study of Yucca Wash, southwest Fifth Annual International Conference, Las Nevada: U.S. Geological Survey Open-File Vegas, Nevada, May 22–24, 1994: Report 93–586–A, 14 p. LaGrange Park, Ill., American Nuclear Soci- Lipman, P.W., and McKay, E.J., 1965, Geologic ety, v. 4, p. 2367–2372. map of the Topopah Spring Southwest Fridrich, C.J., in press, Tectonic evolution of quadrangle, Nye County, Nevada: U.S. Crater Flat basin, Yucca Mountain, Nevada: Geological Survey Geologic Quadrangle Map Geological Society of America Special Paper. GQ–439, scale 1:24,000. Geldon, A.L., 1993, Preliminary hydrogeologic Moyer, T.C., Geslin, J.K., and Buesch, D.C., assessment of boreholes UE-25c#1, UE- 1995, Summary of lithologic logging of new 25c#2, and UE-25c#3, Yucca Mountain, and existing boreholes at Yucca Mountain, Nye County, Nevada: U.S. Geological Sur- Nevada, July 1994 to November 1994: vey Water-Resources Investigations Report U.S. Geological Survey Open-File Report 92–4016, 85 p. 95–102, 27 p. Geslin, J.K., and Moyer, T.C., 1995, Summary Moyer, T.C., Geslin, J.K., and Flint, L.E., 1996, of lithologic logging of new and existing Stratigraphic relations and hydrologic prop- boreholes at Yucca Mountain, Nevada, erties of the Paintbrush Tuff nonwelded March 1994 to June 1994: U.S. Geological (PTn) hydrologic units, Yucca Mountain, Survey Open-File Report 94–451, 16 p. Nevada: U.S. Geological Survey Open-File Geslin, J.K., Moyer T.C., and Buesch, D.C., Report 95–397, 151 p. 1995, Summary of lithologic logging of new Muller, D.C., and Kibler, J.E., 1985, Preliminary and existing drill holes at Yucca Mountain, analysis of geophysical logs from the WT Nevada, August 1993 to February 1994: series of drill holes, Yucca Mountain, Nye U.S. Geological Survey Open-File Report County, Nevada: U.S. Geological Survey 94–342, 39 p. Open-File Report 86–46, 30 p. Gibson, J.D., Swan, F.H., Wesling, J.R., Bullard, Neal, J.T., 1986, Preliminary validation of geol- T.F., Perman, R.C., Angell, M.M., and ogy at a site for repository surface facilities, DiSilvestro, L.A., 1992, Summary and eval- Yucca Mountain, Nevada: Sandia National uation of existing geological and geophysical Laboratories SAND 85–0815, 54 p. data near prospective surface facilities in O’Neill, J.M., Whitney, J.W., and Hudson, M.R., Midway Valley, Yucca Mountain Project: 1992, Photogeologic and kinematic analysis Sandia National Laboratories SAND 90– of lineaments at Yucca Mountain, Nevada— 2491, 94 p. Implications for strike-slip faulting and oro- Hansen, W.R., 1991, Suggestions to authors of clinal bending: U.S. Geological Survey the reports of the United States Geological Open-File Report 91–623, 23 p. Survey: U.S. Geological Survey, 7th Ponce, D.A., 1994, Geophysical investigations Edition, 289 p. of concealed faults near Yucca Mountain,

20 southwest Nevada, in High level radioactive Scott, R.B., 1990, Tectonic setting of Yucca waste management, Proceedings of the Mountain, southwest Nevada, in Wernicke, Fifth Annual International Conference, Las B.P., ed., Basin and range extensional tec- Vegas, Nevada, May 22–24, 1994: tonics near the latitude of Las Vegas, LaGrange Park, Ill., American Nuclear Soci- Nevada: Geological Society of America ety, v. 4, p. 168–174. Memoir 176, p. 251–282. ———1996, Interpretive geophysical fault map Scott, R.B., Bath, G.D., Flanigan, V.J., Hoover, across the central block of Yucca Mountain, D.B., Rosenbaum, J.G., and Spengler, R.W., Nevada: U.S. Geological Survey Open-File 1984, Geological and geophysical evidence Report 96–285, 15 p., 4 plates. of structures in northwest-striking washes, Ponce, D.A., and Langenheim, V.E., 1994, Pre- Yucca Mountain, southern Nevada, and their liminary gravity and magnetic models across possible significance to a nuclear waste Midway Valley and Yucca Wash, Yucca repository in the unsaturated zone: U.S. Mountain, Nevada: U.S. Geological Survey Geological Survey Open-File Report 84–567, Open-File Report 94–572, 25 p. 23 p. Ponce, D.A., Langenheim, V.E., and Sikora, Scott, R.B., and Bonk, J., 1984, Preliminary R.F., 1993, Gravity and magnetic data of geologic map of Yucca Mountain, Nye Midway Valley, southwest Nevada: U.S. County, Nevada, with geologic sections: Geological Survey Open-File Report 93– U.S. Geological Survey Open-File Report 540–A, 7 p. 84–494, scale 1:12,000. Poole, F.G., Carr, W.J., and Elston, D.P., 1965, Simonds, F.W., Whitney, J.W., Fox, K.F., Salyer and Wahmonie Formations of south- Ramelli, A.R., Yount, J.C., Carr, M.D., eastern Nye County, Nevada, in Contribu- Menges, C.M., Dickerson, R.P., and Scott, tions to stratigraphy 1965: U.S. Geological R.B., 1995, Map showing fault activity in Survey Bulletin 1224–A, p. A36–A44. the Yucca Mountain area, Nye County, Potter, C.J., Day, W.C., and Sweetkind, D.S., Nevada: U.S. Geological Survey Miscella- 1996, Structural evolution of the potential neous Investigations Series Map I–2520, high-level nuclear waste repository site at scale 1:24,000. Yucca Mountain, Nevada: Geological Soci- Spengler, R.W., Braun, C.A., Linden, R.M., ety of America Abstracts with Programs, v. Martin, L.G., Ross-Brown, D.M., and 28, no. 7, p. A–191. Blackburn, R.L., 1993, Structural character Rosenbaum, J.G., Hudson, M.R., and Scott, of the Ghost Dance fault, Yucca Mountain, R.B., 1991, Paleomagnetic constraints on Nevada, in High level radioactive waste the geometry and timing of deformation at management, Proceedings of the Fourth Yucca Mountain, Nevada: Journal of Geo- Annual International Conference, Las physical Research, v. 96, p. 1963–1979. Vegas, Nevada, April 26–30, 1993: Sawyer, D.A., Fleck, R.J., Lanphere, M.A., LaGrange Park, Ill., American Nuclear Soci- Warren, R.G., Broxton, D.E., and Hudson, ety, v. 1, p. 653–659. M.R., 1994, Episodic caldera volcanism in Spengler, R.W., Braun, C.A., Martin, L.G., and the Miocene southwestern Nevada volcanic Weisenberg, C.W., 1994, The Sundance field—Revised stratigraphic framework, fault—A newly recognized shear zone at 40 Ar/39 Ar geochronology, and implications Yucca Mountain, Nevada: U.S. Geological for magmatism and extension: Geological Survey Open-File Report 94–49, 11 p. Society of America Bulletin, v. 106, p. Spengler, R.W., and Rosenbaum, J.G., 1980, 1304–1318. Preliminary interpretations of geologic re- Sawyer, D.A., and Sargent, K.A., 1989, Petro- sults obtained from boreholes UE-25a-4, -5, logic evolution of divergent peralkaline mag- -6, and -7, Yucca Mountain, Nevada Test mas from the Silent Canyon caldera com- Site: U.S. Geological Survey Open-File Re- plex: Journal of Geophysical Research, v. port 80–929, 33 p. 94, p. 6021–6040. Stewart, J.H., 1980, Geology of Nevada: Nevada Bureau of Mines and Geology, Spe- cial Publication 4, 136 p.