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  • BEDROCK GEOLOGIC MAP OF THE YUCCA MOUNTAIN AREA, …5400 3700 5000 4900 4500 5400 5300 4900 4400 5300 4800 4700 4400 3900 4700 4600 5200 5200 3800 4600 3900 4100 4700 4500 5100 3600
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4200 5000 5100 4900 5500 4500 5000 4700 4900 5200 4700 4900 4700 5200 4800 4300 3900 4500 4500 4600 5700 5100 4500 4400 5800 5800 4900 4300 5100 5000 4600 4200 4500 5400 5600 5700 4200 4400 4800 4300 4400 4300 4300 4200 4100 5600 3800 5700 5200 5100 4700 4300 3900 4200 4100 5600 4600 5300 5000 5500 5500 4100 4000 4700 5100 4000 4400 4600 4500 5000 5400 3700 5000 4900 4500 5400 5300 4900 4400 5300 4800 4700 4400 3900 4700 4600 5200 5200 3800 4600 3900 4100 4700 4500 5100 3600 3800 4400 5000 3800 4800 4500 4300 4000 3700 3900 3800 4900 4400 4500 3700 4500 4000 3700 4400 4800 3600 3500 4300 4200 3600 4300 4200 4200 4500 4600 4100 4400 4000 4600 4100 3500 4100 4600 3800 4400 3400 4200 3500 4000 3700 4100 3900 3400 3900 3900 4500 4000 3400 4100 4800 3500 3600 3900 3300 3800 4700 3800 3800 3700 3700 3300 4600 3700 3600 3500 3600 4000 4200 4500 4100 4300 3500 4200 4200 4000 3200 3900 4000 3900 3400 4300 3800 3600 3800 3400 3800 4200 4000 3600 3200 3200 3700 4200 3500 3500 3500 4100 3700 Antler Wash Solitario Canyon Yucca Crest Crater Flat Hill Plug Boomerang Point Forlorn Wash Double WT-11 Wash East Ridge WT-12 Wash Dune Wash Abandoned Wash Wash Dance Ghost Wash Highway Ridge Broken Bow Ridge Bow Ridge Ridge Boundary Rainier Ridge Busted Butte Ambush Pass Fran Ridge San Juan Hill West Ridge Jet Ridge Ridge The Bleach Bone Ridge Coyote Wash Live Yucca Ridge Split Wash Wren Wash Drill Hole Wash Azreal Ridge Pagany Ridge Prow Little Tea Cup Wash Purgatory Dead Diabolus Ridge Tonsil Ridge Ridge Yucca Antler Ridge Drill Hole Wash Bleach Bone Ridge Sever Isolation Ridge Mile High Mesa Bob's Butte Yucca Wash Exile Hill Alice Point Ron Ridge Joey Ridge Jake Ridge Fortymile Canyon Castellated Wash Whale Back WT-2 Prow Nellis Air Force Range Nevada Test Site Wash Notch Ridge Windy Wash Fatigue Wash Ridge Ridge Limb Dune Wash Jackass Flats Midway Valley Bureau of Land Management land WT-11 WT-10 WT-7 H-6 GU-3 G-3 H-3 UZ-13 UZ-6s UZ-6 WT#3 WT-12 WT#17 SD-7 UZ#16 UZ-7a SD-12 WT-2 WT-1 H-4 UZ-7 ONC#1 p#1 c#2 c#3 c#1 WT#13 J#13 #14 WT WT#5 G-1 UZ-14 G-2 H-5 UZ-1 GA-1 NRG-6 a#1 SD-9 NRG-7a a#7 NRG#5 WT#18 G-4 b#1 H-1 UZ#5 UZ#4 a#6 a#5 a#4 NRG#4 NRG#3 WT#16 WT#4 WT#6 WT#15 21 11 5 6 8 8 7 8 5 3 9 9 5 11 9 9 6 7 6 12 9 11 12 5 6 7 7 11 5 8 13 14 9 13 12 13 6 9 5 20 5 7 9 5 16 10 7 13 12 9 10 5 3 5 4 15 20 25 11 18 15 22 34 19 20 13 38 21 18 23 13 5 8 25 20 15 5 24 17 19 20 19 15 14 12 12 7 20 10 5 25 20 14 20 50 52 15 40 45 8 15 10 14 10 20 53 14 45 10 14 18 20 13 15 14 10 7 11 50 12 20 15 22 35 11 33 10 11 10 10 8 10 23 8 5 19 16 10 16 15 13 15 15 10 15 13 17 16 21 18 15 17 40 28 15 15 10 25 15 28 16 20 65 18 18 17 16 15 15 25 15 8 9 8 10 15 10 10 15 8 26 14 65 35 8 10 15 8 20 12 8 11 5 10 15 17 20 5 20 15 11 10 10 20 20 20 16 10 12 18 18 13 4 4 8 5 12 2 15 12 8 11 8 12 15 15 6 29 15 4 8 5 10 30 47 15 20 20 40 20 10 16 15 20 10 18 9 16 6 8 12 15 28 18 6 19 23 15 41 49 27 A B' C C' B A' ? ? ? 20 80 55 80 ? ? ? 48 75 48 62 66 69 65 62 60 73 56 66 52 48 30 55 80 76 65 80 60 ? ? ? 78 78 78 65 85 64 70 80 80 65 78 72 55 55 46 ? ? ? 80 75 85 80 ? ? 32 25 70 70 75 85 ? 80 35 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 75 ? ? ? ? 70 ? ? ? ? ? ? ? ? ? ? ? 59 65 85 74 57 71 64 65 66 74 57 56 56 90 64 74 86 66 55 57 ? 76 65 74 80 41 76 ? 50 85 68 78 86 87 81 70 67 48 90 67 60 62 65 56 68 51 78 36 56 79 SUNDANCE FAULT IRON RIDGE FAULT SOLITARIO CANYON FAULT ABANDONED WASH FAULT SOLITARIO CANYON FAULT FATIGUE WASH FAULT BOOMERANG POINT FAULT NORTHERN WINDY WASH FAULT SOLITARIO CANYON FAULT DUNE WASH FAULT BOW RIDGE FAULT EAST RIDGE FAULT BUSTED BUTTE FAULT CANYON FAULT PAINTBRUSH BLACK GLASS DRILL HOLE WASH FAULT BOW RIDGE FAULT EXILE HILL FAULT PAGANY WASH FAULT SEVER WASH FAULT CANYON MIDWAY VALLEY FAULT FAULT PAINTBRUSH PAINTBRUSH CANYON FAULT CANYON FAULT GHOST DANCE FAULT BOW RIDGE FAULT tpv Qc cp cr cr bt3 bt3 cr cp cp acl tr Qc vp Qc vp pr act acl tp pp cp pp Qc tp tp cp cp cpv cpv Qc cr tpmn bt pp ym tpll bt5 bt3 bt3 cr Qc Qc cp Qc Qc tx Qc Qc Qc Qc Qc tpmn mr tpul cr bt trv bt trv mr tpul tpln tpll Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc cp Qc cpv cp cp Qc ym cpv pp ym cp ym cr pp bt2 ym pp cp ym ym Qc Qc bt3 cpv ym cx ym bt3 pp pp bt2 Qc tx trn cpv trv cp mr cp cpv cpv cr Qc cp cr cp cr tpmn Qc Qc Qc Qc cpv ym cpv cp bt5 cpv cp tpll tpul cpv Qc cp cr tpmn cr Qc Qc cr Qc cr dc Qc tr cpv cx b ym bt cp cpv cp tr cpv cpv cpv bt pp cr cp mrw pp tr Qc tr cp cpv bt cpv cpv cpv bt trv trv tr bt cpv cp cr cpv cpv bt cpv Qac cp bt tpul tr bt cpv Qc cr cp trn cr cp cr cp cr cpv cr ym cpv cpv cpv cpv cpv cpv cpv cpv ym pp ym cp Qc Qc bt5 pp Qc tr tr p bt5 act tpv tpln tpll mr Qac mr tpmn tpul tr pp bt3 cpv cp cpv ym Qc cp cpv ym bt3 pp cp cr cr cpv pp ym ym tr tp Qac Qc Qac Qc acl tpv tpv acl tpv pp ym rz pp tp act Qc tpln tpll cr ym Qc cpv ym cpv pp cp tr tpul ym pp tpmn Qc Qc Qc cp tr tpmn pp ym cpv cp cpv ym cr cpv cp cpv ym cpv bt3 cp cpv ym cpv bt3 ym cpv cp cp cr ym cpv cp trv tpul tpul bt bt cp trn bt cpv cpv cpv cr trv trn cpv bt pr ym cr mr cr mr cr cp cpv tpln tpll tpul tpmn tr cr tr bt bt cp tx tr cp tpv tpv tr bt cr Qac cx mr mr tpul tr tr tr tpul tpul bt cp cp cr Qc cr cp Qac cr cp bt bt Qc Qc cr cp mrw tr tpv act tr tpul cr cp cp cp cp cp cp cr cr cp cr trn cp cp cr cr cp trv cr Qac cr cp cr trn cp cp cr cp cpv bt Qac tpul tpmn Qc tpul trv ym cr bt3 bt3 cr cp ym tr cp ym bt3 Qac Qac pp cr Qc cr Qc Qc bt cpv cpv cp cp cr cr trn bt cpv Qc bt cp tpul trn px trn bt cr cp Qc cp Qc cr cr tpul px bt trv mr cp bt cp trn trv tpul bt cp trv tx trn cpv trn bt cpv bt bt cpv trn Qc trv trv cpv trv trn tpul Qc cp tpmn cpv bt cpv cp trn bt tpul cr cr mrw bt cr bt5 mr cp cpv trv trn Qc tpul mrw bt bt cpv trv trn tpul bt trn bt trv bt cpv mr Qc Qc Qc cp cr trn cp trv bt trv trn Qac cr cp cp cp cr cpv trn tpul cpv bt trv tpul trn tpul cr cp cpv trv tpul Qac tpul trn cp cpv cp tpul cp Qc cp tpmn cpv cp cr cr cpv tpul bt cp bt cpv Qac Qac cpv cp Qc cr bt cpv bt cp cpv bt trv cp cr cp cp cr mr mrw cp cr Qac Qc cp cr trn bt3 cp cpv cx Qc cr trn bt2 cpv cp cpv cpv cpv trv trn bt3 bt2 trv trn cp cx cp cp Qac Qac acl acl Qc cp cr Qac pp cr cr cr mr Qac Qac cx cp cr mr Qac bt5 mr mr bt5 bt5 mr pp cpv cr cp tpul Qac bt3 ym ym ym Qc pp ym Qc Qc ym ym pp bt3 cp mrw cp ym d mr cr Qc d ym d cp cr cr Qc cr cp pp cp d cp d cr ym cp cp cp cx Qac Qac Qac cp cp cr cp cp cp cp cp cr Qac cr cr cr cp cr cr cr cr cr cp cp cp cr Qac cr cp cp cr cpv cr trv trn cr tpul cp trn trn tpul cp trn cp cr cpv cr cr cp cr cp tr Qac cp cp cr cr cp tpul trn cp cr tpmn cr cr Qac cr cp ym cp cp cp cr cp cr cpv Qac cp cp cr cr cr cp cp cp cp cp cp cr cr Qac Qac Qac cp Qac cr cp cr cp cp cp cp cr cr cp cr cr Qac cp cr cp cp Qac Qac dc bg pr vp pp tu tu pp kl kl kl kl kl kt kt pp dc kl kl Qc dc kt acl acl act act kt acl tu cu pp pp Qc acl Qac Qac acl Qac Qc kl kt kl pp Qac cu Qac cu cu acl cu Qc kt kl kt tu Qac tu tu Qac acl tu tu acl Qac acl acl Qac pp tu act act kt act dc cu cu pp pp tu tu acl act vp tu Qc kl kl Qac pp cu dc kl pp cu tu kt Qc act Qc kt acl kt kt kt cu pp cu kt kl tu acl act Qc Qc Qc cu pp Qc Qc Qc tpul tpll Qc trn dc tr bt bt cpv tx tx bt cpv bt bt cr tpmn pp tr bt3 Qc cp tpv tp tpmn bt3 acl acl cu act kt tu cx cx pp ym mr trn cr cr cp tpll tp tr trn px tpmn cr tpul Qac tpul tpul trn tpul tpmn cp Qc act pp dc dc cp cu Qac tp tr acl act kt dc dc tu vp acl act acl vp cr cp ym pp tr tpmn cr tu pp pp kt kl cu kl cp cr cr Qc cr cp cp cr cp cp cr cr tpul tpv tp tr cpv bt3 tp bt3 bt3 tr tr kt tu mrw bt2 kt pp cpv cp cp cpv Qac cr act cr tpll cpv tpmn trv bt2 cp px tpul cpv tpul tpmn tx trn tpul tpul Qc cr bt cpv tpmn cr cp tpmn cr cp cr bt cr tr cr cpv bt cp cr cp cr cp Qac cr cp Qc cp cp Qc tp bt5 mr ym pbt dc dc acl acl act act act bt5 cr cp cr tpul tpmn tr cpv Qac pp pp trn cr mr mrw cp tpmn cpv tr tpul tpmn tpul cr trv tr trn cp pp tr Qc cx trn trv Qc cp bt2 cpv cp Qc ym kl cp ym trv Qc trv px bt2 mr bt cpv cpv trv cpv trv bt tr trn cpv trv mrw tx bt act tpll tpln cpv tr tpv tpln tr Qac Qac tu bg tu kl cu cr cx 547100 738100 745000 740000 760000 755000 750000 775000 770000 765000 785000 780000 555000 550000 560000 565000 570000 575000 786900 583900 580000 36°48'45" 116°30'00" 36°52'00" 116°26'15" 555000 550000 560000 565000 570000 575000 583900 580000 116°30'00" 116°26'15" 547100 786900 738100 745000 740000 760000 755000 750000 775000 770000 765000 785000 780000 36°48'45" 36°52'00" U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY GEOLOGIC INVESTIGATIONS SERIES I–2627 Pamphlet accompanies map Prepared in cooperation with the NEVADA OPERATIONS OFFICE, U.S. DEPARTMENT OF ENERGY BEDROCK GEOLOGIC MAP OF THE YUCCA MOUNTAIN AREA, NYE COUNTY, NEVADA By 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. Fridrich 1 1998 Any use of trade names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey For sale by U.S. Geological Survey Information Services Box 25286, Federal Center, Denver, CO 80225 This map is also available as a PDF at http://greenwood.cr.usgs.gov 1 U.S. Geological Survey, Denver, Colorado 2 Pacific Western Technologies, Inc., Denver, Colorado See "Index to geologic mapping of study area" Digital cartography by C.A. San Juan, W.C. Day, R.M. Drake II, and Alessandro J. Donatich Editing and design by Alessandro J. Donatich Manuscript approved for publication March 16, 1998 Base from 1:6,000-scale topographic contour data produced from orthophoto mission flown in 1990 by EG&G for the U.S. Department of Energy. 5,000-ft grid ticks based on Nevada coordinate system, central zone. Polyconic projection. 1927 North American datum SCALE 1:24 000 1/ 2 1 0 1 MILE 1 KILOMETER 1 .5 0 CONTOUR INTERVAL 20 FEET NATIONAL GEODETIC VERTICAL DATUM OF 1929 ? NORTHERN WINDY WASH FAULT FATIGUE WASH FAULT SOLITARIO CANYON FAULT BLACK GLASS CANYON FAULT PAINTBRUSH CANYON FAULT Northern Windy Wash West Ridge Jet Ridge Mile High Mesa Yucca Wash Ron Ridge Joey Ridge Jake Ridge 1000 1000 2500 1500 2000 3000 3500 2500 1500 2000 3000 3500 4000 4500 5000 5500 4000 4500 5000 5500 6000 6500 FEET 6000 6500 A A' FEET 565000 562500 560000 557500 555000 552500 550000 547500 582500 580000 577500 575000 572500 570000 567500 cp ym pp tr tp ac p Qac mr cpv bt3 bt5 cr cr tr tp cr cp ym cp Qac pp ac cr cp Qac tr tp ac p tp p cpv bt3 bt3 pp tr cpv ym cpv tu Qac tu p cu pp tu dc kl kt cu Qac kt kl cu dc tu ac kt Qac pp Qac ac p tr pp Qac cp ym tp ac cpv ac ac pp ac kt tu cu kl dc p Qac pp tu kt ym pre-Prow Pass Tuff rocks pre-Prow Pass Tuff rocks pre-Prow Pass Tuff rocks pre-Prow Pass Tuff rocks pre-Prow Pass Tuff rocks ac ym For legibility, undivided units tr, tp, ac, and p are used on this cross section as follows: tr includes trv and trn; tp includes tpul, tpmn, tpll, tpln, and tpv; ac includes act and acl; and p includes p and pbt. FATIGUE WASH FAULT Fatigue Wash Jet Ridge Solitario Canyon SOLITARIO CANYON FAULT Yucca Crest Broken Limb Ridge GHOST DANCE FAULT Antler Ridge BOW RIDGE FAULT Midway Valley Fran Ridge FEET FEET B B' EXILE HILL FAULT PAINTBRUSH CANYON FAULT 1000 1000 1500 2000 2500 1500 2000 2500 4500 4500 3000 3500 4000 3000 3500 4000 5000 5500 5000 5500 Exploratory Studies Facility 560000 557500 555000 552500 547500 550000 577500 575000 570000 562500 567500 565000 572500 cpv bt tr act tr act tpv tpv tpv act Qac cpv cr Qc ym cp cr cr ym p act p p p Qac ym cr mr bt bt bt bt5 cr cp tr act p tr p tpv act tpv mr cp cpv bt bt cpv cp cp cp tr mr cpv mrw mr cr tr cp cpv bt mr cpv Qac Qac Qac Qac Qac cp cp cr cr cp cp cp cr bt bt bt cp ym pre-Prow Pass Tuff rocks tp p tp tp tp tp tp tp tp tp tp cr cr cr p tpv p tpv tpv cpv cpv cpv cpv cpv ? ? ? ? Qac Qc tr bt bt tr act cr tr act p tr tpv act tpv act Qc tr Qac Qc cpv tr tp cx cp ym ym ym bt tr ? ? ? cp cpv cr ym pre-Prow Pass Tuff rocks pre-Prow Pass Tuff rocks For legibility, undivided units bt, tr, tp, ac, and p are used on this cross section as follows: bt includes bt3, pp, and bt2; tr includes trv and trn; tp includes tpul, tpmn, tpll, tpln, and tpv; ac includes act and acl; and p includes p and pbt. 4500 4000 3500 3000 1500 2000 2500 1000 5000 5000 1000 2500 2000 1500 3000 3500 4000 4500 547500 550000 552500 555000 557500 560000 562500 565000 567500 570000 572500 575000 SOLITARIO CANYON FAULT Crater Flat IRON RIDGE FAULT PAINTBRUSH CANYON FAULT Busted Butte C' C FEET FEET tp tpv p cpv cp mr cp p act tpv tp cp cpv tp p p tpv cp cp cp cpv cpv mr cr cr Qac Qac Qac Qac mr bt bt cx Qac Qac cr cr cr cr tr Qac tp cp cp p bt bt cpv mr mr cr Qac ? ? ? ? tx tp tp mr tpv bt tr cpv cp cp cr cp cpv bt tr cpv bt Qac tr cp cpv bt ? ? pre-Prow Pass Tuff rocks act act tr tr p act tpv cr tr cr tr tp act tr act p tpv tp act cpv Qac Qac ? Qac tr tr cr cr tpv pre-Prow Pass Tuff rocks pre-Prow Pass Tuff rocks bt cp tp For legibility, undivided units bt, tr, tp, ac, and p are used on this cross section as follows: bt includes bt3 and bt2; tr includes trv and trn; tp includes tpul, tpmn, tpll, tpln, and tpv; ac includes act and acl; and p includes p and pbt. BUSTED BUTTE FAULT Miocene TERTIARY QUATERNARY CORRELATION OF MAP UNITS Unconformity d Qac Qc pr mr mrw kl kt vp bt5 cx cu cr cp cpv px ym bg bt3 rz dc pp bt bt2 tx tu tr tp trv trn tpul tpmn tpll tpln tpv ac act acl p pbt b J#13 15 16 30 75 12 DESCRIPTION OF MAP UNITS [The letter prefix “T” commonly used for designation of the Tertiary age of the bedrock igneous rock unit labels was dropped inasmuch as they are all Tertiary in age] QUATERNARY SURFICIAL DEPOSITS Alluvial and colluvial deposits—Boulder- to silt-size unconsolidated alluvial and colluvial deposits Colluvial deposits—Boulder- to cobble-size unconsolidated talus, slope-failure deposits, and colluvial deposits MIOCENE INTRUSIVE ROCKS Basalt dike—Dark-gray to black, aphanitic basaltic dikes in northwestern part of map area on eastern flank of Jet Ridge. K-Ar age is 10.0±0.4 Ma (Carr and Parrish, 1985) MIOCENE VOLCANIC ROCKS Rhyolite of Pinnacles Ridge—Rhyolitic lava flows and ash-flow tuff of a flow dome complex. Lava flows are light-gray to black, vitric to locally devitri- fied, flow-banded rhyolite. Flows are crystal rich with 30–35 percent phe- nocrysts of quartz, sanidine, plagioclase, biotite, and magnetite; thick basal vitrophyre is locally preserved. Ash-flow phases are light-gray to pink, non- welded, bedded, vitric to devtrified pyroclastic-flow and fallout deposits. Ash-flow horizons contain 15–30 percent phenocrysts of quartz, sanidine, plagioclase, biotite, and magnetite. Unit is as much as 300 m thick in Pin- nacles Ridge area at northeastern edge of map area and is restricted to areas north of Yucca Wash near caldera margin Timber Mountain Group Rainier Mesa Tuff, welded—Light-gray to pinkish-gray, partly to moderately welded, devitrified, massive, quartz-rich ash-flow tuff. Contains 10–20 per- cent phenocrysts of quartz, sanidine, plagioclase, and biotite; 8–20 percent pumice (locally as much as 60 percent) clasts that range from 0.3 to 10 cm in diameter; and as much as 2 percent lithic fragments of dark-gray to dark- brown devitrified volcanic rock. On Plug Hill in eastern Crater Flat there is a cooling break in lower part of welded tuff in which the degree of welding, devitrification, and pumice size and content change abruptly. Unit has a compound cooling history. Exposures are restricted to hanging-wall side of block-bounding fault zones on low hills in valley floors. 40 Ar/ 39 Ar age is 11.6±0.03 Ma (Sawyer and others, 1994). Exposed in southern and southwestern parts of map area. Thickness ranges from 2 to 30 m Rainier Mesa Tuff, nonwelded—Light-gray to light-tan, nonwelded to partly welded, devitrified to partially vitric, quartz-rich ash-flow tuff. Basal hori- zons are bedded and grade upward into massive horizons. Unit contains 12–20 percent phenocrysts of quartz, sanidine, plagioclase, and biotite; 8–20 percent pumice clasts that range from 0.3 to 10 cm in diameter; and as much as 2 percent lithic fragments of dark-gray to dark-brown devitrified volcanic rock. Exposures are restricted to hanging-wall side of block- bounding fault zones in valley floors. Unit crops out in northwestern, cen- tral, southern, and southwestern parts of map area. Total thickness of unit in area not known inasmuch as top of unit eroded, but thickness ranges to greater than 20 m Paintbrush Group Rhyolite of Comb Peak—Exposed only in northeastern part of map area Rhyolite lava flow—Light-gray to pinkish-gray, devitrified, flow-banded rhyolitic lava flow. Contains 5–6 percent phenocrysts of sanidine, plagio- clase, hornblende, magnetite, biotite, and sphene. Unit is as much as 130 m thick Ash-flow tuff—Light-gray to pink to brownish-gray, nonwelded to moder- ately welded, devitrified, pumiceous and lithic-rich ash-flow tuff. Contains 5 percent phenocrysts of sanidine, plagioclase, hornblende, magnetite, bio- tite, and sphene; also contains white to light-gray, devitrified, equant to moderately flattened pumice as long as 1 cm. Gray and brown rhyolite and welded tuff lithic fragments as long as 35 cm are concentrated in lithic-rich horizons. Unit is characteristically massive and very pumiceous. Lower contact occurs beneath a fallout tephra in a very coarse, lithic-rich horizon. Thickness ranges from 0 to 38 m Rhyolite of Vent Pass—Rhyolitic lava flows and ash-flow tuffs. Lava flows are medium-gray to dark-brownish-gray to dark-grayish-brown rhyolite, and contain 2–3 percent phenocrysts of sanidine, plagioclase, biotite, and horn- blende. Flows are locally vitric and are flow banded. Oblate lithophysae lie in the flow-banding plane and commonly are silicified. Basal autoclastic breccia and vitrophyre are locally preserved. Ash-flow tuffs locally are devitrified, lithic rich, and pumice rich. Ash-flow tuffs are light greenish gray to medium gray to dark purplish brown, nonwelded, massive, and locally bedded. Where vitric, ash-flow tuffs are dark gray to black, partly to densely fused, and spherulitic. Ash flows contain 10–35 percent lithic frag- ments of dark-reddish-brown to brownish-gray devitrified volcanic rocks, as well as 3 percent phenocrysts of sanidine, plagioclase, biotite, and horn- blende. Unit is 0–150 m thick and crops out only in northernmost part of map area Bedded tuff—White to buff, nonwelded bedded tuff containing devitrified clasts of pumiceous lapilli. Preserved locally beneath the nonwelded Rainier Mesa Tuff (unit mr). Devitrified pumice clasts of unit bt5 distinguish it from overlying unit mr whose pumice clasts are more vitric. Interpreted as fallout- and pyroclastic-tuff deposits by Geslin and others (1995) Paintbrush Group, undivided tectonic breccia—Tectonically juxtaposed, highly brecciated welded units of the Topopah Spring Tuff and Tiva Can- yon Tuff. Exposed predominantly in hanging-wall deformation zones asso- ciated with block-bounding faults west of Solitario Canyon fault in south- western part of map area Tiva Canyon Tuff— 40 Ar/ 39 Ar age is 12.7±0.03 Ma (Sawyer and others, 1994) Undivided tectonic breccia—Tectonically juxtaposed, highly brecciated units of the Tiva Canyon Tuff Undivided—Unit includes crystal-rich and crystal-poor members (units cr, cp) of the Tiva Canyon Tuff. Use restricted to northeastern part of map area north of Yucca Wash, where total thickness of the Tiva Canyon Tuff is less than 15 m but both members are discernible Crystal-rich member—Pale-brown to brownish-black to pinkish-gray, part- ly to densely welded tuff of quartz latitic composition. In general, unit con- tains 10–15 percent phenocrysts of sanidine, plagioclase, biotite, and traces of hornblende, sphene, and pyroxene; 5–30 percent light-gray to black pumice as much as 10 cm in diameter; and as much as 3 percent light-gray to dark-reddish-gray to brown lithic fragments 1–10 mm in diam- eter. Lithophysae (as much as 30 percent) locally are present near basal part of unit; lithophysae typically are equant and as much as 20 cm in diameter. In northern part of map area, unit has a distinctive 1- to 4-m- thick horizon of light-gray, rounded and partially resorbed rhyolite lithic clasts as large as 15 cm in diameter, which make up as much as 80 percent of rock. Basal contact with the underlying crystal-poor member (unit cp) is transitional and is characterized by greater abundance of phenocrysts (7–10 percent and commonly prominent bronze-colored biotite) relative to the crystal-poor member. Unit forms prominent cliffs and rounded slopes. Map unit contains several informal zones and subzones described by Buesch and others (1996) and delineated in the central block area by Day and others (1998). Thickness 3–15 m Crystal-poor member—Gray to pinkish-gray to pale-red, densely welded, devitrified, rhyolitic ash-flow tuff. Contains 3–4 percent phenocrysts of sanidine, plagioclase, and traces of biotite and altered pyroxene; as much as 5 percent pumice; and less than 1 percent light-gray volcanic lithic fragments less than 1 cm in diameter. Internal stratigraphy of unit can be delineated based on prevalence of lithophysae (see Scott and Bonk, 1984; Buesch and others, 1996; Day and others, 1998). In general, total lithophysal content is greatest in northern and central parts of map area and markedly decreases southward (south of latitude of Abandoned Wash). Upper part of unit generally is light purplish gray to pinkish gray and contains lithophysae. Middle part of unit is predominantly nonlithophysal and massive, has distinctive concoidal fracturing, and makes a “clink” sound when struck with hammer. Locally, lower part of unit is a pale-red to grayish-red lithophysal-bearing zone. Basal part of welded crystal-poor member is grayish-red to maroon nonlithophysal tuff, which commonly has a ledge-forming horizon with prominent columnar joints. Unit contains several informal zones and subzones described by Scott and Bonk (1984) and Buesch and others (1996) and delineated in the central block area by Day and others (1998). Thickness as much as 140 m Vitric zone of crystal-poor member—Light-orangish-brown to gray, partly welded, vitric rhyolitic tuff containing 3–5 percent phenocrysts of sanidine, plagioclase, and traces of hornblende; 2–15 percent (locally, as much as 25 percent) pumice clasts; and 0–5 percent dark-reddish-gray vol- canic lithic fragments that are less than 1–2 cm in diameter. Locally, base of map unit includes pre-Tiva Canyon Tuff bedded tuffs (bedded tuff units 4A and 4B of Buesch and others (1996) and Moyer and others, 1996). Unit forms a prominent marker horizon at the base of the Tiva Canyon Tuff throughout map area, and as such was mapped as a separate unit from other zones within the crystal-poor member. Thickness ranges from 3 to 6 m Yucca Mountain Tuff—Nonwelded to densely welded pyroclastic-flow depos- it. Unit is characteristically aphyric and is vitric to devitrified and variably altered. Upper part is dark-gray, moderately welded to nonwelded ash-flow tuff; middle part is gray to pinkish-gray, densely welded and devitrified ash- flow tuff; and lower part is vitric, brownish-gray, nonwelded ash-flow tuff containing 1 percent pink and gray pumice clasts less than 1 cm wide and rare grayish-purple lithic clasts less than 1 cm wide. Abundant clear to yel- low to brown vitric glass shards occur in upper and lower nonwelded parts. Welded middle part appears similar to welded intervals of the Tiva Canyon Tuff, but paucity of phenocrysts is characteristic of the Yucca Mountain Tuff. Unit thins to the south and pinches out in central part of map area. Welding is more developed in northern part of map area. Unit is as much as 60 m thick Rhyolite of Black Glass Canyon—Medium-gray to purplish-gray, flow- banded rhyolite lava flow and tan to brownish-gray, nonwelded to partly welded, devitrified, massive ash-flow tuff. Unit contains 3–4 percent phenocrysts of sanidine, hornblende, sphene, and biotite. Thickness ranges from 2 to 14 m Nonwelded bedded tuffs, undivided—Nonwelded, bedded pyroclastic-flow and fall deposits. Unit locally includes the pre-Tiva Canyon Tuff bedded tuff (bt4), the pre-Yucca Mountain Tuff bedded tuff (bt3), and the pre-Pah Canyon Tuff bedded tuff (bt2) of Buesch and others (1996) and Moyer and others (1996). This map unit is used in southern part of map area and pre- dominantly indicates bt2 and minor amounts of bt3 and bt4 (unit bt4 is too thin to show on this map) Pre-Yucca Mountain Tuff bedded tuff—White to light-brown to light-gray, pumiceous, vitric, nonwelded pyroclastic-flow deposits of informal bedded tuff unit 3 of Buesch and others (1996) and Moyer and others (1996). Unit is thick enough to map separately only in northern part of map area where it is as much as 40 m thick. South of Sever Wash, unit is not thick enough to map separately and was included in the subjacent Pah Canyon Tuff (unit pp). In southernmost extent of map area unit is present as thin horizons (less than 3 m thick) and is included in nonwelded bedded tuffs, undivided (unit bt) Rhyolite of Zig Zag Hill—Dark-gray to black, vitric, crystal-rich, flow-banded rhyolite flow. Unit is restricted to a small graben exposed on northern wall of The Prow just west of the Fatigue Wash fault, as originally observed by Scott and Bonk (1984). It forms a thin horizon between the Pah Canyon Tuff (unit pp) and the pre-Yucca Mountain Tuff bedded tuff (unit bt3). Thickness increases across a small fault just west of the Fatigue Wash fault, indicating that faulting was prevolcanic to early synvolcanic during the time this rhyolite was deposited. Thickness as much as 10 m Rhyolite of Delirium Canyon—Light-gray to light-pinkish-gray to light- brown, vitric to devitrified, flow-banded rhyolite lava flow with light-gray to light-brown, tan, ash-flow tuff. Tuffaceous parts are nonwelded to partly welded, partly vitric to devitrified, and massive to poorly bedded. Unit con- tains 5–7 percent phenocrysts of sanidine, plagioclase, biotite, sphene, and hornblende. Tuffs contain 5–15 percent, pink to light-gray, devitrified pumice, 0.5–3 cm in diameter; 15–25 percent dark-brown to grayish- brown volcanic rock fragments. Fragments range in diameter from 2 to 15 cm, locally as large as 25 cm. Lava flows as much as 250 m thick; ash- flow tuff as much as 100 m thick Pah Canyon Tuff—Pyroclastic-flow deposit characterized by abundant, large pumice clasts. Upper part is pink to lavender, nonwelded, vitric to devitri- fied ash-flow tuff. Middle part is light-gray to moderate-brown, densely welded, vitric to devitrified ash-flow tuff with local vapor-phase alteration. Lower part is typically light-purple-gray to gray, partly to nonwelded, vitric ash-flow tuff. Unit contains 5–10 percent phenocrysts of sanidine, plagio- clase, and traces of biotite, quartz, clinopyroxene, and sphene; 0–5 percent lithic fragments of devitrified rhyolite and minor obsidian; and 15–25 per- cent light-gray, pink, and light-green pumice 2–12 cm in average diameter. Unit is characterized by abundant, poorly sorted large pumice clasts. Weld- ing in the Pah Canyon Tuff varies from north to south. The Pah Canyon has a welded core that dominates the unit in its northern extent in the northern Windy Wash and Fatigue Wash areas. Unit becomes predomi- nantly nonwelded toward central and southern part of map area. Thick- ness ranges to over 60 m; unit is thickest in northern part of map area, and pinches out to the south Pre-Pah Canyon Tuff bedded tuff—White, light-gray, pale-orange, light- brown, and reddish-brown, nonwelded, vitric to devitrified to altered, massive to crudely stratified fallout tephra and ash-flow tuff. Unit contains 2–12 percent phenocrysts of feldspar and traces of biotite, pyroxene, and magnetite; and 15–85 percent pumice clasts. Pumice clasts range in color from white, light gray, pale orange, light brown, moderate brown, to reddish gray. Pumice clasts are 10–35 mm in diameter. Unit contains 2–15 percent volcanic lithic fragments of grayish-black glass and light-gray, grayish-red, pale-red, pale-brown, and brown lithic fragments that range from 5 to 15 mm in diameter. Thickness ranges from 0 to 10 m. Unit is present in northern part of map area, but is mostly too thin (less than 2 m thick) to map separately and is combined with the Pah Canyon Tuff (unit pp). Unit’s aggregate thickness increases southward to become the dominant component of the nonwelded bedded tuffs, undivided (unit bt) Topopah Spring Tuff— 40 Ar/ 39 Ar age is 12.8±0.03 Ma (Sawyer and oth- ers, 1994) Undivided tectonic breccia—Tectonically juxtaposed, highly brecciated Topopah Spring Tuff Undivided—Unit includes crystal-rich and crystal-poor members (units tr, tp) of the Topopah Spring Tuff. Use is restricted to northeastern part of map area north of Yucca Wash, where total thickness of the Topopah Spring Tuff is less than 20 m but both members are discernible Crystal-rich member, undivided—Includes vitric zone and densely welded zone, undivided (unit trv) and nonlithophysal zone (unit trn) of the crystal- rich member of the Topopah Spring Tuff. Map unit is used where unit trv is present but poorly exposed as in the Yucca and Windy Wash areas, on Fran and Bow Ridges, and on Busted Butte Vitric zone and densely welded zone, undivided—Dark-reddish- brown to dark-red, nonwelded to densely welded, vitric to devitrified quartz latite ash-flow tuff. Contains 60–90 percent rhyolitic groundmass, 10–15 percent phenocrysts, 2–10 percent partly deformed pumice fragments, and traces of lithic fragments. Unit forms slopes; ranges to as much as 8 m thick Nonlithophysal zone—Dark-reddish-brown to dark-brown, pumice- bearing, densely welded tuff. Middle part of unit is grayish- to pale-red to light-brown, densely welded, devitrified crystal-rich quartz latite. Ground- mass contains 10–15 percent phenocrysts of sanidine, plagioclase, and bio- tite, and rare pseudomorphs of pyroxene(?) and hornblende. Pumice con- tent ranges from 10 to 25 percent by volume and is mixed in color, with light-gray to dark pumice clasts; minor lithic clasts. Characterized by abun- dant secondary vapor-phase corrosion of pumice clasts. Lower part of unit is transitional into the underlying crystal-poor member. Locally, lower part of unit contains as much as 15 percent lithophysae and corresponds to the crystal-rich lithophysal zone of Buesch and others (1996). Basal part of unit is transitional and is mapped on the basis of greater than 7 percent phenocrysts, including diagnostic bronze biotite. Unit forms prominent cliffs as much as 30 m thick Crystal-poor member, undivided—Includes upper lithophysal, middle nonlithophysal, lower lithophysal, lower nonlithophysal, and crystal-poor vitric zones (units tpul, tpmn, tpll, tpln, and tpv) of the crystal-poor mem- ber of the Topopah Spring Tuff Upper lithophysal zone—Light-gray, light- to moderate-brown, and pale-reddish-purple, moderately to densely welded, devitrified, rhyolitic ash- flow tuff containing 2–5 percent phenocrysts of sanidine, plagioclase, and rare altered pyroxene(?); and 0–10 percent devitrified pumice with lithic fragments of pale-brownish-pink volcanic rock. Also contains 5–30 per- cent lithophysae, which are 1 cm to greater than 3 cm in diameter, and more equidimensional relative to lithophysae in the lower lithophysal zone (unit tpll). Lithophysae have 1- to 5-mm-thick, light-gray vapor-phase alter- ation rims. Unit is as much as 45 m thick and forms steep slopes and locally prominent cliffs Middle nonlithophysal zone—Grayish-orange-pink to light-brown to light-gray, densely welded, devitrified, rhyolitic ash-flow tuff. Contains 1–2 percent phenocrysts of sanidine, plagioclase, and rare oxidized hornblende or biotite; as much as 3 percent devitrified pumice fragments with trace lithic fragments of light-gray volcanic rock, 1–10 mm in diameter; and as much as 5 percent lithophysae Lower lithophysal zone—Pale-red, grayish-red to purple, and light- brown, densely welded, devitrified, rhyolitic ash-flow tuff. Contains 5–15 percent lithophysae, 1–10 percent pumice clasts, 1–5 percent lithic clasts, and about 2 percent phenocrysts of alkali feldspar, with minor amounts of plagioclase, biotite, magnetite, rare quartz, and altered pyroxene Lower nonlithophysal zone—Pale-red, grayish-reddish purple, to dusky-reddish-purple, densely welded, devitrified, rhyolitic ash-flow tuff. Unit is made up of over 75 percent groundmass with 1–2 percent phenocrysts, as much as 15 percent pumice clasts, and 1–10 percent lithic clasts (Buesch and others, 1996). Local vapor-phase alteration is pronounced and developed along fractures and corroded pumice voids. Unit typically forms slopes in map area. Unit is exposed only near Prow Pass in northern part of map area (where it is about 10 m thick) and on Busted Butte in southeastern part of map area (where it is 40 m thick) Crystal-poor vitric zone—Predominantly densely welded, vitric to partly devitrified, grayish-black to grayish-brown rhyolitic ash-flow tuff. Lower part of unit is moderate-brown to grayish-orange nonwelded tuff. Unit is predominantly made up of groundmass (over 75 percent), with less- er amounts of pumice and lithic clasts. Phenocryst content is approximate- ly 1 percent. Unit is exposed near Prow Pass (where it is about 10 m thick) and Busted Butte (over 12 m thick) Calico Hills Formation— 40 Ar/ 39 Ar age is 12.9±0.04 Ma (Sawyer and others, 1994) Undivided—Includes ash-flow tuff (unit act) and rhyolitic lava flow (unit acl). Shown only on cross sections Ash-flow tuff—Pale-orange, grayish-yellow, and pale-greenish-yellow, nonwelded ash-flow tuff, fallout tephra, and reworked tuff; vitric to devitrified to zeolitized. Contains 1–12 percent phenocrysts of quartz, sanidine, plagioclase, biotite, and hornblende; 10–40 percent grayish- yellow to pale-greenish-yellow pumice; and dark-gray to reddish-brown rhyolite clasts. Bedded tuff is typically interstratified with massive tuff and lava flows. Exposed thickness approximately 115 m Rhyolite lava flow—Light- to dark-gray, greenish-gray, to pale-purple flow- banded rhyolitic lava. Locally brecciated, silicified, and spherulitic. Com- monly altered and zeolitized and contains 3–4 percent phenocyrsts of quartz, sanidine, plagioclase, biotite, and hornblende. Lower contact is sharp and marked by autoclastic breccia. Unit is as much as 215 m thick Crater Flat Group Prow Pass Tuff—Light-pinkish-gray, moderately welded, devitrified, cystal- rich pyroclastic rock. Unit contains approximately 10 percent pumice and as much as 30 percent phenocrysts of quartz, alkali feldspar, and plagio- clase; it is unique in that orthopyroxene is the dominant mafic phenocryst (Byers and others, 1976). Type locality (and sole occurrence on map) is well exposed just west of Prow Pass at northern end of Yucca Mountain. Base not exposed but thickness greater than 12 m Nonwelded bedded tuff—Yellowish-gray, nonwelded, bedded ash-flow tuff at base of the Prow Pass Tuff (unit p). Unit is as much as 12 m thick Bullfrog Tuff—Light-gray, massive, moderately to densely welded ash-flow tuff. Phenocryst content in map area is as much as 10 percent, and includes quartz, alkali feldspar, plagioclase, and characteristic biotite. Expo- sure of unit on this map is limited to area west of Prow Pass in northern Windy Wash area. Complete section is not exposed due to faulting of low- er part by the Windy Wash fault. Areal distribution was discussed by Byers and others (1976), Carr (1986), and Sawyer and others (1994), who estab- lished an 40 Ar/ 39 Ar age of 13.25±0.04 Ma for unit. Stratigraphic base of unit not exposed Contact—Solid where exposed; dashed where approximately located; dotted where concealed beneath Quaternary cover or poorly exposed; queried where inferred or uncertain Fault—Solid where exposed or well constrained; dashed where approximately located; dotted where concealed and inferred beneath Quaternary cover; queried where uncertain. Ball and bar on downthrown side. Diamond- tipped line indicates rake and plunge of slickenslide lineation. Double- barbed arrow indicates dip of fault plane. Opposed arrows show relative horizontal movement Reverse fault—Solid where exposed or tightly constrained; dashed where approximately located; dotted where concealed beneath Quaternary cover; queried where inferred or uncertain. Sawteeth on upthrown side Strike and dip of bedding Strike and dip of bedding—Calculated from three-point problems on basal contact of unit Strike and dip of compaction foliation Approximate location of the underground Exploratory Studies Facility—Shown by on cross section B–B' Potential perimeter boundary surrounding the repository Unimproved road Borehole location and number Qac Qc d pr mrw mr kl kt vp bt5 cx cu cr cp cpv ym bg bt bt3 rz dc pp bt2 tx tu tr trv trn tp tpul tpmn tpll tpln tpv ac act acl p pbt b px A B C INDEX TO GEOLOGIC MAPPING OF STUDY AREA A. Mapping done for this report by W.C. Day, R.P. Dickerson, C.J. Potter, D.S. Sweetkind, R.M. Drake II, and C.J. Fridrich, June 1996 to June 1997. B. Dickerson and Drake (1998). C. Day and others (1998). NEVADA MAP LOCATION TRUE NORTH MAGNETIC NORTH APPROXIMATE MEAN DECLINATION, 1998 14°

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Page 1: BEDROCK GEOLOGIC MAP OF THE YUCCA MOUNTAIN AREA, …5400 3700 5000 4900 4500 5400 5300 4900 4400 5300 4800 4700 4400 3900 4700 4600 5200 5200 3800 4600 3900 4100 4700 4500 5100 3600

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tr

Qc

trcp

cpv

bt

cpv

cpvcpvbt

trvtrv

tr

bt

cpv

cp

cr

cpv

cpv

bt

cpv

Qac

cp

bt

tpul tr

bt

cpv

Qc

cr

cp

trncr

cp cr

cpcr

cpv

cr

ym

cpv

cpv

cpv

cpv

cpv

cpv

cpv

cpv

ym

pp

ymcp

Qc

Qc

bt5

pp

Qc

tr

tr

p

bt5

act

tpv tpln

tpll

mr

Qac

mr tpmn

tpul

tr ppbt3

cpv

cp

cpv

ym

Qc

cp

cpvymbt3

pp

cp

cr

cr

cpv

pp

ym

ymtrtp

Qac

Qc

Qac

Qc

acl

tpv

tpv

acl

tpv

ppym

rz

pp

tp actQc

tplntpll

cr

ym Qc

cpv

ym

cpv

pp

cp

trtpul

ympptpmnQc

Qc

Qc

cp

tr

tpmn

pp

ym

cpv

cp

cpvym

cr

cpv

cpcpv

ym

cpv

bt3

cp

cpv ymcpv

bt3ym

cpv

cp

cp

cr

ym

cpv

cp

trvtpul

tpul

btbt

cp

trn

bt

cpv

cpv

cpv

cr

trv

trn

cpv

bt

pr

ym

cr

mrcrmr

cr

cp

cpv

tpln

tplltpul

tpmn

tr

cr

tr

bt

bt

cptx

tr

cp tpv

tpv

trbt

crQac

cx

mr

mr

tpul

tr

tr

trtpul

tpul

bt

cp

cpcr

Qccr

cp Qac

cr

cp

bt

bt

Qc

Qc

cr

cp

mrw

tr

tpvact

tr

tpul

cr

cpcp

cp

cpcp

cp

cr

cr

cp

cr

trn

cp

cp

cr

cr

cp

trv

cr

Qac

cr

cp

cr

trn

cp

cp

cr

cp

cpv

bt

Qac

tpultpmn

Qc

tpul

trv

ym

cr

bt3

bt3

cr

cp

ym

tr

cp

ym

bt3

Qac

Qac

pp

cr

Qc

cr

Qc

Qc

btcpv

cpv

cp

cp

cr

cr

trnbt cpv

Qc

bt

cp

tpul

trn

px

trn

bt

cr

cp

Qc cp

Qc

cr

cr

tpul

px

bttrv

mr

cp

bt

cptrn

trv

tpulbt

cp

trvtx

trn

cpv

trn

bt

cpv

bt bt

cpv trn

Qc

trv

trv

cpv

trv

trntpul

Qc

cp

tpmn

cpv

bt

cpvcp

trn

bttpul

cr

cr

mrw

bt

cr

bt5

mr

cpcpv

trv

trn

Qc

tpulmrw

bt

bt

cpvtrv

trntpul

bttrnbt

trv

bt

cpv

mr

Qc

Qc

Qc cp

crtrn

cp

trv

bttrvtrn

Qac

cr

cp

cp

cp

cr

cpv

trntpulcpv

bttrv

tpul

trn

tpul

cr

cp

cpv

trvtpul

Qac

tpul

trn

cp cpv

cp

tpul

cp Qc

cp

tpmn

cpv

cp

cr

cr

cpv

tpul

btcp

bt

cpv

Qac

Qac

cpv

cp

Qc

crbt

cpv bt

cp cpv

bt

trv

cp

crcp

cp

cr

mr

mrw

cp

cr

Qac

Qc

cp

cr

trn

bt3

cpcpv

cx

Qc cr

trn

bt2

cpv

cp

cpv

cpv

cpv

trv

trn

bt3

bt2

trv

trn

cp

cx

cp

cp

Qac

Qac

acl

acl

Qc

cp

cr

Qac

pp

cr

cr

cr

mr

Qac

Qac

cx

cp

cr

mr

Qac

bt5

mr

mr

bt5

bt5

mr

ppcpv

cr

cp

tpul

Qac

bt3

ym

ym

ym

Qc

pp

ym

Qc

Qc

ym

ym

pp

bt3

cp

mrw

cp

ym

d

mr

cr

Qc

dym

d

cp

cr

cr

Qc

cr

cp

pp

cp

d

cp

d

cr

ym

cp

cp

cp

cx

Qac

Qac

Qac

cp

cpcr

cp

cp

cp

cp

cp

cr

Qac

cr

cr

cr

cp

cr

cr

cr

cr

cr

cp

cp

cp

cr

Qac

cr

cp

cp cr

cpv

cr

trv

trn

cr

tpulcp

trn

trn

tpul

cp trn

cp

cr

cpv

cr

crcp

crcp

tr

Qac

cp

cpcr

cr

cp

tpul

trn

cp

cr

tpmn

cr

cr

Qac

crcp

ym

cp

cp

cpcr

cp

crcpv

Qac

cpcp

cr

cr

cr

cp

cp

cp

cp

cp

cp

cr

cr

Qac

Qac

Qac

cp

Qac

cr

cp

cr

cp

cp

cp

cp

cr

cr

cp

cr

cr

Qac

cp

cr

cp

cp

Qac

Qac

dc

bg

pr vp

pp

tu

tupp

kl

kl

klkl

kl

kt

kt

pp

dc

kl

kl

Qc

dc

kt

acl

acl

act

actkt

acl

tucu

pp

ppQc

acl

Qac

Qac

acl

Qac

Qc

kl

kt

klpp

Qac

cu

Qac

cu

cu

aclcu

Qc

kt

kl

kt

tu

Qac

tu

tu

Qac

acl

tu

tu

aclQac

acl acl

Qac

pp

tu

act

act

kt

act

dc

cu

cu

pp

pp

tu

tu

aclact

vp

tu

Qc

kl

kl

Qacpp

cu

dc

kl

pp cutu kt

Qc

act

Qcktacl

kt

kt

kt

cu

pp cu

kt

kl

tu

acl

act

Qc

Qc

Qc

cu

pp

Qc

Qc

Qc

tpul

tpll

Qc

trn

dc

tr

bt

bt

cpv

tx

tx

bt

cpv

bt

bt

cr

tpmn

pp

tr

bt3

Qc

cptpv

tptpmn

bt3

acl acl

cu

act

kt

tu

cx

cx

pp

ym

mr

trn

cr

crcp

tpll

tptr

trn

px

tpmn

cr

tpul

Qac

tpul

tpul

trn

tpul

tpmn

cp

Qcact pp dc

dc

cp

cu

Qac

tp

tr

acl

act

kt

dc dc

tu

vp

acl

act

acl

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cr

cp

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tr

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cr

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pp

kt

kl

cu

kl

cp

cr

cr

Qc

cr

cp

cp

cr

cp

cp

cr

cr

tpul

tpv

tp

tr

cpv

bt3

tp

bt3

bt3

tr

tr

kt

tu

mrw

bt2

kt

pp

cpv

cp

cp

cpv

Qac

cr

act

cr

tpll

cpvtpmn

trv

bt2cp

px

tpul

cpv

tpul

tpmntx

trn

tpul

tpul

Qc

cr

bt

cpv

tpmn

cr

cp

tpmn

cr

cp

cr

bt

cr

tr

cr

cpv

bt

cpcr

cp

cr

cp

Qac

cr

cp

Qc

cp

cp

Qc

tp

bt5mr

ym

pbt

dc

dc

acl

acl

act

actact

bt5

crcp

cr

tpul

tpmn

tr

cpv

Qac

pp

pp

trn

cr

mr

mrw

cp

tpmn

cpvtr

tpul

tpmn

tpul

cr

trv

tr

trn

cp

pp

tr

Qccx

trn

trv

Qc

cp

bt2

cpv

cp

Qc

ym

kl

cp

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trv

Qc

trv

px

bt2

mr

bt

cpv

cpv

trv

cpv

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bt

tr

trn

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trv

mrw

tx

bt

acttpll

tpln

cpv

tr tpvtpln tr

Qac

Qac

tu

bg

tu

kl

cu

cr

cx

547100738100

745000

740000

760000

755000

750000

775000

770000

765000

785000

780000

555000550000 560000 565000 570000 575000786900

583900580000

36°48'45"

116°30'00"

36°52'00"

116°26'15"

555000550000 560000 565000 570000 575000 583900580000116°30'00" 116°26'15"

547100786900

738100

745000

740000

760000

755000

750000

775000

770000

765000

785000

780000

36°48'45"

36°52'00"

U.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY

GEOLOGIC INVESTIGATIONS SERIESI–2627

Pamphlet accompanies map

Prepared in cooperation with theNEVADA OPERATIONS OFFICE, U.S. DEPARTMENT OF ENERGY

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

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

Any use of trade names in this publication is fordescriptive purposes only and does not imply endorsement by the U.S. Geological Survey

For sale by U.S. Geological Survey Information ServicesBox 25286, Federal Center, Denver, CO 80225

This map is also available as a PDF at http://greenwood.cr.usgs.gov

1U.S. Geological Survey, Denver, Colorado2Pacific Western Technologies, Inc., Denver, Colorado

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

Base from 1:6,000-scale topographic contour dataproduced from orthophoto mission flown in 1990by EG&G for the U.S. Department of Energy.5,000-ft grid ticks based on Nevada coordinate system, central zone.Polyconic projection.1927 North American datum

SCALE 1:24 0001/ 21 0 1 MILE

1 KILOMETER1 .5 0

CONTOUR INTERVAL 20 FEETNATIONAL GEODETIC VERTICAL DATUM OF 1929

?

NO

RT

HE

RN

WIN

DY

WA

SH

FA

ULT

FAT

IGU

EW

AS

HFA

ULT

SO

LITA

RIO

CA

NY

ON

FAU

LT

BLA

CK

GLA

SS

C

AN

YO

N F

AU

LT

PAIN

TB

RU

SH

CA

NY

ON

FA

ULT

Northern Windy Wash West Ridge

Jet Ridge

Mile High Mesa Yucca Wash Ron Ridge Joey Ridge Jake Ridge

1000 1000

2500

1500

2000

3000

3500

2500

1500

2000

3000

3500

4000

4500

5000

5500

4000

4500

5000

5500

6000

6500

FEET

6000

6500

A A'FEET

565000562500560000557500555000552500550000547500 582500580000577500575000572500570000567500

cpym

pptr

tpac

p

Qacmr

cpvbt3

bt5cr

cr

tr

tp

crcp

ym

cp

Qac

pp

ac

crcp Qac

trtp

ac

p

tp

p

cpv

bt3bt3

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tuQac tu

p

cupptudckl

ktcuQac

kt

kl

cudc

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ac

ktQac

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ac

p

trppQac

cpym

tpac

cpv

acac

pp

ac

kt

tu

cu

kl

dc

p

Qacpp

tu

kt

ym

pre-Prow PassTuff rocks

pre-Prow PassTuff rocks pre-Prow Pass

Tuff rockspre-Prow Pass

Tuff rocks

pre-Prow PassTuff rocks

ac

ym

For legibility, undivided units tr, tp, ac, and p are used on this cross section as follows:tr includes trv and trn; tp includes tpul, tpmn, tpll, tpln, and tpv; ac includes act and acl;and p includes p and pbt.

FAT

IGU

EW

AS

H F

AU

LT

Fatigue Wash Jet Ridge SolitarioCanyon

SO

LITA

RIO

CA

NY

ON

FAU

LT

Yucca Crest Broken Limb Ridge

GH

OS

TD

AN

CE

FAU

LT

Antler Ridge

BO

W R

IDG

EFA

ULT

Midway Valley Fran Ridge

FEETFEETB B'

EX

ILE

HIL

LFA

ULT

PAIN

TB

RU

SH

CA

NY

ON

FAU

LT

1000 1000

1500

2000

2500

1500

2000

2500

45004500

3000

3500

4000

3000

3500

4000

5000

5500

5000

5500

ExploratoryStudiesFacility

560000557500555000552500547500 550000 577500575000570000562500 567500565000 572500

cpv

bttr

act

tr

act

tpv

tpv

tpvact

Qaccpv

crQc

ymcp

cr

crym

pact

p

pp

Qacym crmrbt

btbt

bt5

crcp

tr

actp

tr

p

tpv

acttpv

mr

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bt

btcpv

cp cp

cp

tr

mr

cpv

mrw mr

cr

tr

cp

cpvbt

mr

cpv

QacQacQac

QacQac

cp

cp

cr

crcp

cp

cp

cr

bt bt

btcp

ym

pre-Prow PassTuff rocks

tp

p

tp

tp

tp

tp

tp

tp

tp

tp

tp

crcrcr

p

tpv

p

tpv

tpv

cpvcpv

cpv

cpvcpv

?

?

?

?

Qac

Qc

trbt

bt

tr

act

cr

tr

actp

tr

tpv

acttpv

act

QctrQac Qc

cpv

trtp

cxcp

ym

ym

ymbttr

?

? ?

cp

cpv

cr

ym

pre-Prow PassTuff rocks pre-Prow Pass

Tuff rocks

For legibility, undivided units bt, tr, tp, ac, and p are used on this cross section as follows:bt includes bt3, pp, and bt2; tr includes trv and trn; tp includes tpul, tpmn, tpll, tpln, and tpv;ac includes act and acl; and p includes p and pbt.

4500

4000

3500

3000

1500

2000

2500

1000

5000 5000

1000

2500

2000

1500

3000

3500

4000

4500

547500 550000 552500 555000 557500 560000 562500 565000 567500 570000 572500 575000

SO

LITA

RIO

CA

NY

ON

FA

ULT

Crater Flat

IRO

N R

IDG

EFA

ULT

PAIN

TB

RU

SH

CA

NY

ON

FAU

LT

Busted Butte

C'CFEETFEET

tp

tpvp

cpvcp

mrcp

p act

tpv

tp

cp cpv

tp

p

p

tpvcp

cpcp

cpvcpvmrcrcr QacQac Qac Qac mr

bt

bt

cxQacQac

crcr

crcr

tr Qac

tp

cp

cp

p

bt bt cpv

mrmr

crQac

? ??

?

tx

tptp

mr

tpv

bttr

cpv cpcp

cr cp cpv bttr

cpv btQactr

cp cpvbt

? ?

pre-Prow PassTuff rocks

act

acttr

tr

p

acttpv

cr

tr

crtr

tp

act

tr

act

p

tpvtp

act

cpvQac

Qac

?

Qac

trtr

cr

cr

tpv

pre-Prow PassTuff rocks

pre-Prow PassTuff rocks

bt

cptp

For legibility, undivided units bt, tr, tp, ac, and p are used on this cross section as follows:bt includes bt3 and bt2; tr includes trv and trn; tp includes tpul, tpmn, tpll, tpln, and tpv; ac includes act and acl; and p includes p and pbt.

BU

ST

ED

BU

TT

EFA

ULT

Mio

cene

TER

TIA

RY

QU

AT

ERN

AR

Y

CORRELATION OF MAP UNITS

Unconformity

d

QacQc

pr

mrmrw

kl kt

vp

bt5

cx cucr

cp

cpv

px

ym

bg

bt3

rz

dc

ppbt

bt2

tx tu

tr

tp

trv

trn

tpul

tpmn

tpll

tpln

tpv

acact

acl

p

pbt

b

J#13

15

16

30 75

12

DESCRIPTION OF MAP UNITS

[The letter prefix “T” commonly used for designation of the Tertiary age of the bedrock igneous rock unit labels was dropped inasmuch as they are all Tertiary in age]

QUATERNARY SURFICIAL DEPOSITS

Alluvial and colluvial deposits—Boulder- to silt-size unconsolidated alluvial and colluvial deposits

Colluvial deposits—Boulder- to cobble-size unconsolidated talus, slope-failure deposits, and colluvial deposits

MIOCENE INTRUSIVE ROCKS

Basalt dike—Dark-gray to black, aphanitic basaltic dikes in northwestern part of map area on eastern flank of Jet Ridge. K-Ar age is 10.0±0.4 Ma (Carr and Parrish, 1985)

MIOCENE VOLCANIC ROCKS

Rhyolite of Pinnacles Ridge—Rhyolitic lava flows and ash-flow tuff of a flow dome complex. Lava flows are light-gray to black, vitric to locally devitri-fied, flow-banded rhyolite. Flows are crystal rich with 30–35 percent phe-nocrysts of quartz, sanidine, plagioclase, biotite, and magnetite; thick basal vitrophyre is locally preserved. Ash-flow phases are light-gray to pink, non-welded, bedded, vitric to devtrified pyroclastic-flow and fallout deposits. Ash-flow horizons contain 15–30 percent phenocrysts of quartz, sanidine, plagioclase, biotite, and magnetite. Unit is as much as 300 m thick in Pin-nacles Ridge area at northeastern edge of map area and is restricted to areas north of Yucca Wash near caldera margin

Timber Mountain GroupRainier Mesa Tuff, welded—Light-gray to pinkish-gray, partly to moderately

welded, devitrified, massive, quartz-rich ash-flow tuff. Contains 10–20 per-cent phenocrysts of quartz, sanidine, plagioclase, and biotite; 8–20 percent pumice (locally as much as 60 percent) clasts that range from 0.3 to 10 cm in diameter; and as much as 2 percent lithic fragments of dark-gray to dark-brown devitrified volcanic rock. On Plug Hill in eastern Crater Flat there is a cooling break in lower part of welded tuff in which the degree of welding, devitrification, and pumice size and content change abruptly. Unit has a compound cooling history. Exposures are restricted to hanging-wall side of block-bounding fault zones on low hills in valley floors. 40Ar/39Ar age is 11.6±0.03 Ma (Sawyer and others, 1994). Exposed in southern and southwestern parts of map area. Thickness ranges from 2 to 30 m

Rainier Mesa Tuff, nonwelded—Light-gray to light-tan, nonwelded to partly welded, devitrified to partially vitric, quartz-rich ash-flow tuff. Basal hori-zons are bedded and grade upward into massive horizons. Unit contains 12–20 percent phenocrysts of quartz, sanidine, plagioclase, and biotite; 8–20 percent pumice clasts that range from 0.3 to 10 cm in diameter; and as much as 2 percent lithic fragments of dark-gray to dark-brown devitrified volcanic rock. Exposures are restricted to hanging-wall side of block-bounding fault zones in valley floors. Unit crops out in northwestern, cen-tral, southern, and southwestern parts of map area. Total thickness of unit in area not known inasmuch as top of unit eroded, but thickness ranges to greater than 20 m

Paintbrush GroupRhyolite of Comb Peak—Exposed only in northeastern part of map area

Rhyolite lava flow—Light-gray to pinkish-gray, devitrified, flow-banded rhyolitic lava flow. Contains 5–6 percent phenocrysts of sanidine, plagio-clase, hornblende, magnetite, biotite, and sphene. Unit is as much as 130 m thickAsh-flow tuff—Light-gray to pink to brownish-gray, nonwelded to moder-ately welded, devitrified, pumiceous and lithic-rich ash-flow tuff. Contains 5 percent phenocrysts of sanidine, plagioclase, hornblende, magnetite, bio-tite, and sphene; also contains white to light-gray, devitrified, equant to moderately flattened pumice as long as 1 cm. Gray and brown rhyolite and welded tuff lithic fragments as long as 35 cm are concentrated in lithic-rich horizons. Unit is characteristically massive and very pumiceous. Lower contact occurs beneath a fallout tephra in a very coarse, lithic-rich horizon. Thickness ranges from 0 to 38 m

Rhyolite of Vent Pass—Rhyolitic lava flows and ash-flow tuffs. Lava flows are medium-gray to dark-brownish-gray to dark-grayish-brown rhyolite, and contain 2–3 percent phenocrysts of sanidine, plagioclase, biotite, and horn-blende. Flows are locally vitric and are flow banded. Oblate lithophysae lie in the flow-banding plane and commonly are silicified. Basal autoclastic breccia and vitrophyre are locally preserved. Ash-flow tuffs locally are devitrified, lithic rich, and pumice rich. Ash-flow tuffs are light greenish gray to medium gray to dark purplish brown, nonwelded, massive, and locally bedded. Where vitric, ash-flow tuffs are dark gray to black, partly to densely fused, and spherulitic. Ash flows contain 10–35 percent lithic frag-ments of dark-reddish-brown to brownish-gray devitrified volcanic rocks, as well as 3 percent phenocrysts of sanidine, plagioclase, biotite, and horn-blende. Unit is 0–150 m thick and crops out only in northernmost part of map area

Bedded tuff—White to buff, nonwelded bedded tuff containing devitrified clasts of pumiceous lapilli. Preserved locally beneath the nonwelded Rainier Mesa Tuff (unit mr). Devitrified pumice clasts of unit bt5 distinguish it from overlying unit mr whose pumice clasts are more vitric. Interpreted as fallout- and pyroclastic-tuff deposits by Geslin and others (1995)

Paintbrush Group, undivided tectonic breccia—Tectonically juxtaposed, highly brecciated welded units of the Topopah Spring Tuff and Tiva Can-yon Tuff. Exposed predominantly in hanging-wall deformation zones asso-ciated with block-bounding faults west of Solitario Canyon fault in south-western part of map area

Tiva Canyon Tuff—40Ar/39Ar age is 12.7±0.03 Ma (Sawyer and others, 1994)Undivided tectonic breccia—Tectonically juxtaposed, highly brecciated units of the Tiva Canyon TuffUndivided—Unit includes crystal-rich and crystal-poor members (units cr, cp) of the Tiva Canyon Tuff. Use restricted to northeastern part of map area north of Yucca Wash, where total thickness of the Tiva Canyon Tuff is less than 15 m but both members are discernibleCrystal-rich member—Pale-brown to brownish-black to pinkish-gray, part-ly to densely welded tuff of quartz latitic composition. In general, unit con-tains 10–15 percent phenocrysts of sanidine, plagioclase, biotite, and traces of hornblende, sphene, and pyroxene; 5–30 percent light-gray to black pumice as much as 10 cm in diameter; and as much as 3 percent light-gray to dark-reddish-gray to brown lithic fragments 1–10 mm in diam-eter. Lithophysae (as much as 30 percent) locally are present near basal part of unit; lithophysae typically are equant and as much as 20 cm in diameter. In northern part of map area, unit has a distinctive 1- to 4-m-thick horizon of light-gray, rounded and partially resorbed rhyolite lithic clasts as large as 15 cm in diameter, which make up as much as 80 percent of rock. Basal contact with the underlying crystal-poor member (unit cp) is transitional and is characterized by greater abundance of phenocrysts (7–10 percent and commonly prominent bronze-colored biotite) relative to the crystal-poor member. Unit forms prominent cliffs and rounded slopes. Map unit contains several informal zones and subzones described by Buesch and others (1996) and delineated in the central block area by Day and others (1998). Thickness 3–15 mCrystal-poor member—Gray to pinkish-gray to pale-red, densely welded, devitrified, rhyolitic ash-flow tuff. Contains 3–4 percent phenocrysts of sanidine, plagioclase, and traces of biotite and altered pyroxene; as much as 5 percent pumice; and less than 1 percent light-gray volcanic lithic fragments less than 1 cm in diameter. Internal stratigraphy of unit can be delineated based on prevalence of lithophysae (see Scott and Bonk, 1984; Buesch and others, 1996; Day and others, 1998). In general, total lithophysal content is greatest in northern and central parts of map area and markedly decreases southward (south of latitude of Abandoned Wash). Upper part of unit generally is light purplish gray to pinkish gray and contains lithophysae. Middle part of unit is predominantly nonlithophysal and massive, has distinctive concoidal fracturing, and makes a “clink” sound when struck with hammer. Locally, lower part of unit is a pale-red to grayish-red lithophysal-bearing zone. Basal part of welded crystal-poor member is grayish-red to maroon nonlithophysal tuff, which commonly has a ledge-forming horizon with prominent columnar joints. Unit contains several informal zones and subzones described by Scott and Bonk (1984) and Buesch and others (1996) and delineated in the central block area by Day and others (1998). Thickness as much as 140 m

Vitric zone of crystal-poor member—Light-orangish-brown to gray, partly welded, vitric rhyolitic tuff containing 3–5 percent phenocrysts of sanidine, plagioclase, and traces of hornblende; 2–15 percent (locally, as much as 25 percent) pumice clasts; and 0–5 percent dark-reddish-gray vol-canic lithic fragments that are less than 1–2 cm in diameter. Locally, base of map unit includes pre-Tiva Canyon Tuff bedded tuffs (bedded tuff units 4A and 4B of Buesch and others (1996) and Moyer and others, 1996). Unit forms a prominent marker horizon at the base of the Tiva Canyon Tuff throughout map area, and as such was mapped as a separate unit from other zones within the crystal-poor member. Thickness ranges from 3 to 6 m

Yucca Mountain Tuff—Nonwelded to densely welded pyroclastic-flow depos-it. Unit is characteristically aphyric and is vitric to devitrified and variably altered. Upper part is dark-gray, moderately welded to nonwelded ash-flow tuff; middle part is gray to pinkish-gray, densely welded and devitrified ash-flow tuff; and lower part is vitric, brownish-gray, nonwelded ash-flow tuff containing 1 percent pink and gray pumice clasts less than 1 cm wide and rare grayish-purple lithic clasts less than 1 cm wide. Abundant clear to yel-low to brown vitric glass shards occur in upper and lower nonwelded parts. Welded middle part appears similar to welded intervals of the Tiva Canyon Tuff, but paucity of phenocrysts is characteristic of the Yucca Mountain Tuff. Unit thins to the south and pinches out in central part of map area. Welding is more developed in northern part of map area. Unit is as much as 60 m thick

Rhyolite of Black Glass Canyon—Medium-gray to purplish-gray, flow-banded rhyolite lava flow and tan to brownish-gray, nonwelded to partly welded, devitrified, massive ash-flow tuff. Unit contains 3–4 percent phenocrysts of sanidine, hornblende, sphene, and biotite. Thickness ranges from 2 to 14 m

Nonwelded bedded tuffs, undivided—Nonwelded, bedded pyroclastic-flow and fall deposits. Unit locally includes the pre-Tiva Canyon Tuff bedded tuff (bt4), the pre-Yucca Mountain Tuff bedded tuff (bt3), and the pre-Pah Canyon Tuff bedded tuff (bt2) of Buesch and others (1996) and Moyer and others (1996). This map unit is used in southern part of map area and pre-dominantly indicates bt2 and minor amounts of bt3 and bt4 (unit bt4 is too thin to show on this map)

Pre-Yucca Mountain Tuff bedded tuff—White to light-brown to light-gray, pumiceous, vitric, nonwelded pyroclastic-flow deposits of informal bedded tuff unit 3 of Buesch and others (1996) and Moyer and others (1996). Unit is thick enough to map separately only in northern part of map area where it is as much as 40 m thick. South of Sever Wash, unit is not thick enough to map separately and was included in the subjacent Pah Canyon Tuff (unit pp). In southernmost extent of map area unit is present as thin horizons (less than 3 m thick) and is included in nonwelded bedded tuffs, undivided (unit bt)

Rhyolite of Zig Zag Hill—Dark-gray to black, vitric, crystal-rich, flow-banded rhyolite flow. Unit is restricted to a small graben exposed on northern wall of The Prow just west of the Fatigue Wash fault, as originally observed by Scott and Bonk (1984). It forms a thin horizon between the Pah Canyon Tuff (unit pp) and the pre-Yucca Mountain Tuff bedded tuff (unit bt3). Thickness increases across a small fault just west of the Fatigue Wash fault, indicating that faulting was prevolcanic to early synvolcanic during the time this rhyolite was deposited. Thickness as much as 10 m

Rhyolite of Delirium Canyon—Light-gray to light-pinkish-gray to light-brown, vitric to devitrified, flow-banded rhyolite lava flow with light-gray to light-brown, tan, ash-flow tuff. Tuffaceous parts are nonwelded to partly welded, partly vitric to devitrified, and massive to poorly bedded. Unit con-tains 5–7 percent phenocrysts of sanidine, plagioclase, biotite, sphene, and hornblende. Tuffs contain 5–15 percent, pink to light-gray, devitrified pumice, 0.5–3 cm in diameter; 15–25 percent dark-brown to grayish-brown volcanic rock fragments. Fragments range in diameter from 2 to 15 cm, locally as large as 25 cm. Lava flows as much as 250 m thick; ash-flow tuff as much as 100 m thick

Pah Canyon Tuff—Pyroclastic-flow deposit characterized by abundant, large pumice clasts. Upper part is pink to lavender, nonwelded, vitric to devitri-fied ash-flow tuff. Middle part is light-gray to moderate-brown, densely welded, vitric to devitrified ash-flow tuff with local vapor-phase alteration. Lower part is typically light-purple-gray to gray, partly to nonwelded, vitric ash-flow tuff. Unit contains 5–10 percent phenocrysts of sanidine, plagio-clase, and traces of biotite, quartz, clinopyroxene, and sphene; 0–5 percent lithic fragments of devitrified rhyolite and minor obsidian; and 15–25 per-cent light-gray, pink, and light-green pumice 2–12 cm in average diameter. Unit is characterized by abundant, poorly sorted large pumice clasts. Weld-ing in the Pah Canyon Tuff varies from north to south. The Pah Canyon has a welded core that dominates the unit in its northern extent in the northern Windy Wash and Fatigue Wash areas. Unit becomes predomi-nantly nonwelded toward central and southern part of map area. Thick-ness ranges to over 60 m; unit is thickest in northern part of map area, and pinches out to the south

Pre-Pah Canyon Tuff bedded tuff—White, light-gray, pale-orange, light-brown, and reddish-brown, nonwelded, vitric to devitrified to altered, massive to crudely stratified fallout tephra and ash-flow tuff. Unit contains 2–12 percent phenocrysts of feldspar and traces of biotite, pyroxene, and magnetite; and 15–85 percent pumice clasts. Pumice clasts range in color from white, light gray, pale orange, light brown, moderate brown, to reddish gray. Pumice clasts are 10–35 mm in diameter. Unit contains 2–15 percent volcanic lithic fragments of grayish-black glass and light-gray, grayish-red, pale-red, pale-brown, and brown lithic fragments that range from 5 to 15 mm in diameter. Thickness ranges from 0 to 10 m. Unit is present in northern part of map area, but is mostly too thin (less than 2 m thick) to map separately and is combined with the Pah Canyon Tuff (unit pp). Unit’s aggregate thickness increases southward to become the dominant component of the nonwelded bedded tuffs, undivided (unit bt)

Topopah Spring Tuff—40Ar/39Ar age is 12.8±0.03 Ma (Sawyer and oth-ers, 1994)Undivided tectonic breccia—Tectonically juxtaposed, highly brecciated Topopah Spring TuffUndivided—Unit includes crystal-rich and crystal-poor members (units tr, tp) of the Topopah Spring Tuff. Use is restricted to northeastern part of map area north of Yucca Wash, where total thickness of the Topopah Spring Tuff is less than 20 m but both members are discernibleCrystal-rich member, undivided—Includes vitric zone and densely welded zone, undivided (unit trv) and nonlithophysal zone (unit trn) of the crystal-rich member of the Topopah Spring Tuff. Map unit is used where unit trv is present but poorly exposed as in the Yucca and Windy Wash areas, on Fran and Bow Ridges, and on Busted Butte

Vitric zone and densely welded zone, undivided—Dark-reddish-brown to dark-red, nonwelded to densely welded, vitric to devitrified quartz latite ash-flow tuff. Contains 60–90 percent rhyolitic groundmass, 10–15

percent phenocrysts, 2–10 percent partly deformed pumice fragments, and traces of lithic fragments. Unit forms slopes; ranges to as much as 8 m thick

Nonlithophysal zone—Dark-reddish-brown to dark-brown, pumice-bearing, densely welded tuff. Middle part of unit is grayish- to pale-red to light-brown, densely welded, devitrified crystal-rich quartz latite. Ground-mass contains 10–15 percent phenocrysts of sanidine, plagioclase, and bio-tite, and rare pseudomorphs of pyroxene(?) and hornblende. Pumice con-tent ranges from 10 to 25 percent by volume and is mixed in color, with light-gray to dark pumice clasts; minor lithic clasts. Characterized by abun-dant secondary vapor-phase corrosion of pumice clasts. Lower part of unit is transitional into the underlying crystal-poor member. Locally, lower part of unit contains as much as 15 percent lithophysae and corresponds to the crystal-rich lithophysal zone of Buesch and others (1996). Basal part of unit is transitional and is mapped on the basis of greater than 7 percent phenocrysts, including diagnostic bronze biotite. Unit forms prominent cliffs as much as 30 m thickCrystal-poor member, undivided—Includes upper lithophysal, middle nonlithophysal, lower lithophysal, lower nonlithophysal, and crystal-poor vitric zones (units tpul, tpmn, tpll, tpln, and tpv) of the crystal-poor mem-ber of the Topopah Spring Tuff

Upper lithophysal zone—Light-gray, light- to moderate-brown, and pale-reddish-purple, moderately to densely welded, devitrified, rhyolitic ash-flow tuff containing 2–5 percent phenocrysts of sanidine, plagioclase, and rare altered pyroxene(?); and 0–10 percent devitrified pumice with lithic fragments of pale-brownish-pink volcanic rock. Also contains 5–30 per-cent lithophysae, which are 1 cm to greater than 3 cm in diameter, and more equidimensional relative to lithophysae in the lower lithophysal zone (unit tpll). Lithophysae have 1- to 5-mm-thick, light-gray vapor-phase alter-ation rims. Unit is as much as 45 m thick and forms steep slopes and locally prominent cliffs

Middle nonlithophysal zone—Grayish-orange-pink to light-brown to light-gray, densely welded, devitrified, rhyolitic ash-flow tuff. Contains 1–2 percent phenocrysts of sanidine, plagioclase, and rare oxidized hornblende or biotite; as much as 3 percent devitrified pumice fragments with trace lithic fragments of light-gray volcanic rock, 1–10 mm in diameter; and as much as 5 percent lithophysae

Lower lithophysal zone—Pale-red, grayish-red to purple, and light-brown, densely welded, devitrified, rhyolitic ash-flow tuff. Contains 5–15 percent lithophysae, 1–10 percent pumice clasts, 1–5 percent lithic clasts, and about 2 percent phenocrysts of alkali feldspar, with minor amounts of plagioclase, biotite, magnetite, rare quartz, and altered pyroxene

Lower nonlithophysal zone—Pale-red, grayish-reddish purple, to dusky-reddish-purple, densely welded, devitrified, rhyolitic ash-flow tuff. Unit is made up of over 75 percent groundmass with 1–2 percent phenocrysts, as much as 15 percent pumice clasts, and 1–10 percent lithic clasts (Buesch and others, 1996). Local vapor-phase alteration is pronounced and developed along fractures and corroded pumice voids. Unit typically forms slopes in map area. Unit is exposed only near Prow Pass in northern part of map area (where it is about 10 m thick) and on Busted Butte in southeastern part of map area (where it is 40 m thick)

Crystal-poor vitric zone—Predominantly densely welded, vitric to partly devitrified, grayish-black to grayish-brown rhyolitic ash-flow tuff. Lower part of unit is moderate-brown to grayish-orange nonwelded tuff. Unit is predominantly made up of groundmass (over 75 percent), with less-er amounts of pumice and lithic clasts. Phenocryst content is approximate-ly 1 percent. Unit is exposed near Prow Pass (where it is about 10 m thick) and Busted Butte (over 12 m thick)

Calico Hills Formation—40Ar/39Ar age is 12.9±0.04 Ma (Sawyer and others, 1994)

Undivided—Includes ash-flow tuff (unit act) and rhyolitic lava flow (unit acl). Shown only on cross sections

Ash-flow tuff—Pale-orange, grayish-yellow, and pale-greenish-yellow, nonwelded ash-flow tuff, fallout tephra, and reworked tuff; vitric to devitrified to zeolitized. Contains 1–12 percent phenocrysts of quartz, sanidine, plagioclase, biotite, and hornblende; 10–40 percent grayish-yellow to pale-greenish-yellow pumice; and dark-gray to reddish-brown rhyolite clasts. Bedded tuff is typically interstratified with massive tuff and lava flows. Exposed thickness approximately 115 m

Rhyolite lava flow—Light- to dark-gray, greenish-gray, to pale-purple flow-banded rhyolitic lava. Locally brecciated, silicified, and spherulitic. Com-monly altered and zeolitized and contains 3–4 percent phenocyrsts of quartz, sanidine, plagioclase, biotite, and hornblende. Lower contact is sharp and marked by autoclastic breccia. Unit is as much as 215 m thick

Crater Flat GroupProw Pass Tuff—Light-pinkish-gray, moderately welded, devitrified, cystal-

rich pyroclastic rock. Unit contains approximately 10 percent pumice and as much as 30 percent phenocrysts of quartz, alkali feldspar, and plagio-clase; it is unique in that orthopyroxene is the dominant mafic phenocryst (Byers and others, 1976). Type locality (and sole occurrence on map) is well exposed just west of Prow Pass at northern end of Yucca Mountain. Base not exposed but thickness greater than 12 mNonwelded bedded tuff—Yellowish-gray, nonwelded, bedded ash-flow tuff at base of the Prow Pass Tuff (unit p). Unit is as much as 12 m thick

Bullfrog Tuff—Light-gray, massive, moderately to densely welded ash-flow tuff. Phenocryst content in map area is as much as 10 percent, and includes quartz, alkali feldspar, plagioclase, and characteristic biotite. Expo-sure of unit on this map is limited to area west of Prow Pass in northern Windy Wash area. Complete section is not exposed due to faulting of low-er part by the Windy Wash fault. Areal distribution was discussed by Byers and others (1976), Carr (1986), and Sawyer and others (1994), who estab-lished an 40Ar/39Ar age of 13.25±0.04 Ma for unit. Stratigraphic base of unit not exposed

Contact—Solid where exposed; dashed where approximately located; dotted where concealed beneath Quaternary cover or poorly exposed; queried where inferred or uncertain

Fault—Solid where exposed or well constrained; dashed where approximately located; dotted where concealed and inferred beneath Quaternary cover; queried where uncertain. Ball and bar on downthrown side. Diamond-tipped line indicates rake and plunge of slickenslide lineation. Double-barbed arrow indicates dip of fault plane. Opposed arrows show relative horizontal movement

Reverse fault—Solid where exposed or tightly constrained; dashed where approximately located; dotted where concealed beneath Quaternary cover; queried where inferred or uncertain. Sawteeth on upthrown side

Strike and dip of bedding

Strike and dip of bedding—Calculated from three-point problems on basal contact of unit

Strike and dip of compaction foliation

Approximate location of the underground Exploratory Studies Facility—Shown by on cross section B–B'

Potential perimeter boundary surrounding the repository

Unimproved road

Borehole location and number

Qac

Qc

d

pr

mrw

mr

kl

kt

vp

bt5

cx

cu

cr

cp

cpv

ym

bg

bt

bt3

rz

dc

pp

bt2

tx

tu

tr

trv

trn

tp

tpul

tpmn

tpll

tpln

tpv

ac

act

acl

p

pbt

b

px

A

B

C

INDEX TO GEOLOGIC MAPPING OF STUDY AREA

A. Mapping done for this report by W.C. Day, R.P. Dickerson, C.J. Potter, D.S. Sweetkind, R.M. Drake II, and C.J. Fridrich, June 1996 to June 1997.

B. Dickerson and Drake (1998).C. Day and others (1998).

NEVADA

MAP LOCATION

TRU

E N

OR

TH

MA

GN

ETIC

NO

RTH

APPROXIMATE MEANDECLINATION, 1998

14°

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U.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY

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

By

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, Colorado2Pacific Western Technologies, Inc., Denver, Colorado

Prepared in cooperation with the

NEVADA OPERATIONS OFFICE, U.S. DEPARTMENT OF ENERGY

Pamphlet to accompanyGEOLOGIC INVESTIGATIONS SERIES

I–2627

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i

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

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ii

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.

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1

ABSTRACT

Yucca Mountain, Nye County, Nevada, hasbeen identified as a potential site for under-ground storage of high-level radioactive nuclearwaste. Detailed bedrock geologic maps form anintegral part of the site characterization pro-gram by providing the fundamental frameworkfor research into the geologic hazards and hy-drologic behavior of the mountain. This bedrockgeologic map provides the geologic frameworkand structural setting for the area in and adja-cent to the site of the potential repository.

The study area comprises the northern andcentral parts of Yucca Mountain, located on thesouthern flank of the Timber Mountain–OasisValley caldera complex, which was the sourcefor many of the volcanic units in the area. TheTimber Mountain–Oasis Valley caldera complex ispart of the Miocene southwestern Nevada vol-canic field, which is within the Walker Lane belt.This tectonic belt is a northwest-strikingmegastructure lying between the more activeInyo-Mono and Basin-and-Range subsections ofthe southwestern Great Basin.

Excluding Quaternary surficial deposits, themap area is underlain by Miocene volcanic rocks,principally ash-flow tuffs with lesser amounts oflava flows. These volcanic units include theCrater Flat Group, the Calico Hills Formation,the Paintbrush Group, and the Timber MountainGroup, as well as minor basaltic dikes. The tuffsand lava flows are predominantly rhyolite withlesser amounts of latite and range in age from13.4 to 11.6 Ma. The 10-Ma basaltic dikes in-truded along a few fault traces in the north-central part of the study area.

Fault types in the area can be classified asblock bounding, relay structures, strike slip, andintrablock. The block-bounding faults separatethe 1- to 4-km-wide, east-dipping structuralblocks and exhibit hundreds of meters of dis-placement. The relay structures are northwest-striking normal fault zones that kinematically linkthe block-bounding faults. The strike-slip faultsare steep, northwest-striking dextral faults lo-cated in the northern part of Yucca Mountain.The intrablock faults are modest faults of limitedoffset (tens of meters) and trace length (lessthan 7 km) that accommodated intrablockdeformation.

The concept of structural domains providesa useful tool in delineating and describing varia-tions in structural style. Domains are definedacross the study area on the basis of therelative amount of internal faulting, style ofdeformation, and stratal dips. In general, thereis a systematic north to south increase in

extensional deformation as recorded in theamount of offset along the block-bounding faultsas well as an increase in the intrablock faulting.

The rocks in the map area had a protractedhistory of Tertiary extension. Rocks of thePaintbrush Group cover much of the area andobscure evidence for older tectonism. An earlierhistory of Tertiary extension can be inferred,however, because the Timber Mountain–OasisValley caldera complex lies within and cuts anolder north-trending rift (the Kawich-Greenwater rift). Evidence for deformation dur-ing eruption of the Paintbrush Group is locallypresent as growth structures. Post-PaintbrushGroup, pre-Timber Mountain Group extensionoccurred along the block-bounding faults. Thebasal contact of the 11.6-Ma Rainier Mesa Tuffof the Timber Mountain Group provides a keytime horizon throughout the area. Other work-ers have shown that west of the study area innorthern Crater Flat the basal angular uncon-formity is as much as 20° between the RainierMesa and underlying Paintbrush Group rocks.In the westernmost part of the study area theunconformity is smaller (less than 10°), whereasin the central and eastern parts of the map areathe contact is essentially conformable. In thecentral part of the map the Rainier Mesa Tufflaps over fault splays within the SolitarioCanyon fault zone. However, displacement didoccur on the block-bounding faults after deposi-tion of the Rainier Mesa Tuff inasmuch as it islocally caught up in the hanging-wall deforma-tion of the block-bounding faults. Therefore, theregional Tertiary to Recent extension was pro-tracted, occurring prior to and after the eruptionof the tuffs exposed at Yucca Mountain.

INTRODUCTION

This 1:24,000-scale map focuses on thearea surrounding the potential high-level nuclearwaste repository site at Yucca Mountain (figs. 1and 2). Its purpose is to define the characterand extent of the dominant structural features inthe vicinity of and outward from the potentialrepository area. As currently conceived, the po-tential repository would be a permanent under-ground facility with high-level nuclear wasteplaced in drifts excavated in the densely weldedunits of the Miocene Topopah Spring Tuff. Therepository would be built in the unsaturated zoneapproximately 250 m above the regional ground-water table. The Yucca Mountain Project cur-rently is evaluating the cumulative effect of natu-ral geologic hazards in the site area, includingseismic and volcanic hazards. Hydrologic inves-tigations and computer-aided three-dimensional

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95

Goldfield

Beatty

Amargosa Valley Mercury

Pahrump

NEVADA

CALIFORNIA

25 MILES0 5 10 15 20

50 KILOMETERS0 10 20 30 40

Stonewall Mountaincaldera(?) complex

Boundary of southwesternNevada volcanic field

BlackMountainCaldera

SilentCanyoncaldera

complex

Timber Mountain–Oasis Valley

caldera complex

YuccaMountain

Claim Canyon caldera

Rainier Mesacaldera

Timber Mountain–Oasis Valleycaldera complex

AmmoniaTanks

caldera

Wahmonievolcano

Nevada

Test

Site

DEATH VALLEY?

?

?

?

GrouseCanyoncaldera

Area 20caldera

Approximateboundary

of study area

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).

?

?

37°30'

37°00'

36°30'

117°00' 116°00'

2

NEVADA

Study area

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EXPLANATION

Miocene volcanic bedrock

Quaternary deposits

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

0

0

1

1

2

2

MILES

KILOMETERS

Yu

cca

Cre

st

Exi

le H

ill

MidwayValley

Fran

Rid

ge

BustedButte

Am

bush

Pas

s

East Ridge

Do

ub

le No

tchR

idg

e

Plug Hill

Jet Ridge

WestRidge

The Prow

Prow Pass

Yucca Wash

FortymileWash

Isolation Ridge

Sever Wash

Pagany WashA

zreal Ridge

Bleach Bone RidgeTonsil RidgeDiabolus RidgeDead Yucca Ridge

Live Yucca Ridge

Purgatory Ridge

Antler RidgeAntler W

ash

Bow Ridge

BoundaryRidge

Highway Ridge

Broken Limb Ridge

Drill H

ole Wash

DuneWash

Crater Flat

Fati

gue

Was

h

Win

dy W

ash

Sol

itari

o C

anyo

n

Ghost DanceWash

Black G

lass Canyon

CastellatedRidge

Forlorn Wash

Joey Ridge

Jake Ridge

Paintbrush Canyon

AlicePoint

Whale Back Ridge

Mile HighMesa

SplitWash

San

Ju

an H

ill

Rainier Ridge

WT-12 W

ash

WT

-11 Wash

Fort

ymile

Can

yon

Little Prow

AbandonedWash

BoomerangPoint

Teacup Wash

116°30' 116°26'15"

36°52'

550000 560000 570000 580000

740000

750000

760000

770000

780000

Jackass Flats

36°48'45"

3

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geologic and hydrologic framework modeling ef-forts are ongoing to characterize the site asfully as possible. Knowledge of the distributionof geologic units as well as the stratigraphic andstructural setting is the underpinning for suchinvestigations. This map and accompanyingtext describe the location, geometry, and kine-matic interplay between the various fault typesas well as variations in the structural style. Theconcept of structural domains is applied to themap area to better describe these structural vari-ations. The timing of deformation at YuccaMountain is discussed in the context of the Ter-tiary to Recent tectonic evolution for the area.

NOTES ON STRATIGRAPHICNOMENCLATURE

The stratigraphic nomenclature used here ismodified from the informal stratigraphy pre-sented by Buesch and others (1996), incorpo-rating lessons learned by Day and others(1998). The stratigraphic nomenclature pre-sented in Buesch and others (1996) was anoutgrowth of borehole studies, replacing mapunits of Scott and Bonk (1984) from a vol-canological perspective to reflect stratigraphicfeatures observed both in the boreholes andoutcrops. Buesch and others (1996) also incor-porated the work of Sawyer and others (1994)to update definitions of formations and groupsfor the area. Modifications to the nomenclatureused herein were made to simplify the map unitlabels to the standard maximum of four lettersper unit, which was especially necessary in thestructurally complex areas of the map. The “T”used by Buesch and others (1996) was droppedfrom the bedrock igneous rock unit labels inas-much as they are all Tertiary in age. In addition,the “p” for most of the units in the PaintbrushGroup (see Buesch and others, 1996) wasdropped. For example, a unit such as the middlenonlithophysal zone of the crystal-poor memberof the Topopah Spring Tuff, delineated as“Tptpmn” by Buesch and others (1996), issimplified to “tpmn” in this report. This con-vention follows that for symbols for standardU.S. Geological Survey geologic maps as out-lined by Hansen (1991).

PREVIOUS MAPPING

During the 1960’s Lipman and McKay(1965) and Christiansen and Lipman (1965)mapped the Yucca Mountain area at a scale of1:24,000 during a regional mapping campaign toprovide an overall geologic framework for theNevada Test Site. They delineated the main

block-bounding faults and some of the intrablockfaults, and outlined the zoned compositional na-ture of the tuffaceous units that underlie YuccaMountain. After identification of Yucca Moun-tain as a possible site for high-level nuclearwaste disposal by the U.S. Department of En-ergy, during the 1980’s Scott and Bonk (1984)developed a detailed reconnaissance (1:12,000scale) geologic map of the area. This map wasused to help constrain the most favorable areaat Yucca Mountain and was a guide to furthersite-characterization studies. Of their manycontributions, Scott and Bonk (1984) presenteda detailed stratigraphy for the volcanic units, fur-ther defined the block-bounding faults, andmapped numerous intrablock faults.

Subsequent mapping during the 1990’s re-vealed structural complexities along some of theintrablock faults (for example, Spengler and oth-ers, 1993) not evident at the 1:12,000 scale.Recently, Day and others (1998) developed adetailed (1:6,000 scale) geologic map over thecentral block area, which includes the potentialrepository (see fig. 4). A detailed (1:6,000 scale)geologic map of the area north of Yucca Washin the Paintbrush Canyon area has just beencompleted (Dickerson and Drake, 1998) and wasincorporated herein.

METHODOLOGY

This map was developed using standard geo-logic mapping techniques. Detailed 1:6,000-scale and 1:12,000-scale orthophoto and topo-graphic maps were used for base maps overmost of the area, which were developed specifi-cally for the Department of Energy YuccaMountain Project. These series of maps have10-ft contour intervals. As such, the final prod-uct used supporting maps at a much larger scale(1:6,000 to 1:12,000) than the final 1:24,000-scale map.

Stratigraphic units within the TopopahSpring and Tiva Canyon Tuffs, which make upthe bulk of bedrock outcrops in the map area,have been refined into informal members, zones,and subzones by Buesch and others (1996) (see“Stratigraphic setting” section). Inasmuch asthe Topopah Spring Tuff is proposed as thehost unit for the potential repository, the pre-sent map includes detailed zonal subdivisions ofthe Topopah where possible. In order to sim-plify the final map, the Tiva Canyon Tuff wasmapped at the informal member status [crystal-rich and crystal-poor members (units cr and cp)],as was its nonwelded basal vitric zone (unit cpv),which forms a prominent marker horizonthroughout the map area.

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The threshold vertical offset on faultsmapped outside of the central block area (Dayand others, 1998) was approximately 10 ft (3m). Within the central block proper, thethreshold was approximately 5 ft (1.5 m) of ver-tical offset. Placement of stratigraphic contactsfalls within similar vertical thresholds of accu-racy. In addition, detection of faults is difficultwithin the middle part of the crystal-poor mem-ber of the Tiva Canyon Tuff. This zone is oneof the most widely exposed units, but it com-monly forms debris-covered slopes. Faults aredifficult to detect in this unit not only because ofthe cover, but because it is a densely welded unitthat lacks internal marker horizons. However,faults that offset adjacent mappable contacts areeasier to detect. As such, the intrablock faultswith a few hundred meters or more of surfacetrace length were readily detected. In this samevein, the massive, densely welded middle non-lithophysal zone of the crystal-poor member ofthe Topopah Spring Tuff (unit tpmn) may con-tain minor faults that were undetected. Faultswith appreciable extent cut through map unitcontacts, however, which are offset. Theirpresence would have been detected.

BOREHOLE DESIGNATIONS

Each borehole depicted on the map and usedin the study of Yucca Mountain has a uniquename or number. For the purposes of this re-port the prefix designators UE-25 or USW (seefollowing paragraphs) are not posted on the mapbecause of space limitations on the map. Eachbolehole designator is unique in this study area,and, therefore, the prefixes are not necessaryon the map.

Boreholes on the Nevada Test Site (NTS—see geologic map for boundary) use a designa-tion that differs slightly from that of boreholesoff the NTS. For example, the full boreholedesignations for boreholes on the NTS beginwith UE (for Underground Exploratory), fol-lowed by the NTS area number (Area 25 in thisreport; for example, UE-25). This prefix desig-nation “UE-25” commonly is followed by one ormore letters signifying the purpose of the holeor simply by a sequential letter, followed by a se-quence number. Thus, the designation “UE-25c#3” specifies a hole on the NTS in Area 25 andthat it is the third hole planned, but not neces-sarily drilled, at a site named “c”.

For boreholes off the NTS in adjacent landto the west controlled by the U.S. Bureau ofLand Management or the Nellis Air ForceRange, the prefix USW is used (for U nder -ground, Southern Nevada, Waste). For USW

boreholes, the series identifier and sequencenumber are separated by a hypen rather thanthe number symbol, #. The first hole planned inthe UZ series is off-NTS and, therefore, is des-ignated “USW UZ-1”.

The letters signify the primary purpose forwhich the borehole was drilled. For example,boreholes starting with G were drilled for collec-tion of geologic data, those with H were drilledprimarily for hydrologic data, with UZ for unsat-urated zone data, with NRG for collection ofdata along the north-ramp alignment of the un-derground Exploratory Studies Facility (ESF),with p for collection of data on rocks of pre-Tertiary age, with SD for geotechnical and hy-drologic data from a statistically located system-atic drilling program, and with WT for testingthe depth of the water table.

DATA SOURCES

Two geologic map reports were incorpo-rated into this map (see “Index to geologic map-ping” on map): the bedrock geologic map of thecentral block area by Day and others (1998) andthe geologic map of the Paintbrush Canyon areaby Dickerson and Drake (1998). The remainingarea was remapped by the authors (W.C. Day,R.P. Dickerson, C.J. Potter, D.S. Sweetkind,and R.M. Drake, II) from June 1996 throughJune 1997, and by C.J. Fridrich during 1992.

The locations of faults beneath the Quater-nary surface deposits of Midway Valley andYucca Wash were inferred from the ground-based gravity and magnetic surveys of Langen-heim and others (1993), Langenheim and Ponce(1994), Ponce and others (1993), and Ponce(1993). A recent geophysical report on theMidway Valley and Yucca Wash areas by Ponceand Langenheim (1994) was also included. All ofthese sources were used primarily to determinethe probable location of the concealed faults inMidway Valley and Yucca Wash, as well as toestimate the direction of offset. Estimates ofoffset were also corroborated by Buesch andothers (1994).

The main source of information for thestructural control on the interpretations pre-sented in the cross sections was the surfacegeologic mapping presented herein. Strati-graphic thickness information from Geslin andMoyer (1995), Geslin and others (1995), Moyerand others (1995), and Moyer and others(1996) was used as corroborative data to com-pare with outcrop data to construct the crosssections. Data from three-dimensional computerframework models by Buesch and others (1995)and from the Integrated Site Model ISM2.0 by

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6

R.W. Clayton and W.P. Zelinski (Woodward-Clyde Federal Services, written commun., 1997)were also used as corroborative information toverify stratigraphic thicknesses on the crosssections.

Over the decades of geologic investigationsat Yucca Mountain, numerous cross sections ofthe Midway Valley area have been produced,each reflecting the geologic understanding atthat time. These corroborative cross sectionsinclude those of Scott and Bonk (1984), Scott(1990), Carr (1992), Carr and others (1986),Geldon (1993), and Neal (1986) as well as un-published cross sections by W.R. Keefer andJ.W. Whitney (U.S. Geological Survey, writtencommun., 1996), and geophysical interpreta-tions by Ponce (1996). These various crosssections were examined in light of present inter-pretations to assure that known geologic dataand structural considerations were not ignored.The present interpretations are based solelyupon the above-cited qualified data, and the ear-lier cross sections were used only as guidelines.

Subsurface borehole data from published andunpublished sources were also used as support-ing data. These include data for boreholes c#1,c#2, and c#3 (Geldon, 1993), borehole p#1 inCarr and others (1986), those discussed by Gib-son and others (1992), borehole WT-5 (R.W.Spengler, U.S. Geological Survey, writtencommun., 1994), and boreholes WT-14 and WT-16 (R.W. Spengler, written commun., 1994;Muller and Kibler, 1985).

REGIONAL SETTING

Yucca Mountain, Nye County, Nevada, lieswithin the Walker Lane belt, which is a north-west-striking zone dominated by right-lateralwrench-zone tectonism (Stewart, 1980; Carr,1984, 1990). The middle Miocene southwest-ern Nevada volcanic field (fig. 1) erupted from aseries of calderas within the Walker Lane belt(Byers and others, 1976; Carr, 1984; Sawyerand others, 1994). The southwestern Nevadavolcanic field is predominantly a series of silicictuffs and lava flows that range in age from 8 to15 Ma. One of the principal caldera complexesexposed in the southwestern Nevada volcanicfield is the Timber Mountain–Oasis Valley calderacomplex (Sawyer and others, 1994). The mar-gin of the Timber Mountain–Oasis Valley calderacomplex lies just north of the map area and wasthe source for many of the volcanic units ex-posed at Yucca Mountain. This caldera complexformed within an older north-trending Tertiaryrift zone, which Carr (1986, 1990) defined asthe Kawich-Greenwater rift. As such, Tertiary

(pre-circa 13 Ma) extension provided the originaltectonic framework for volcanism and subse-quent tectonism at Yucca Mountain.

STRATIGRAPHIC SETTING

The study area is predominantly underlain byMiocene volcanic rocks of the Crater Flat, Paint-brush, and Timber Mountain Groups (Bueschand others, 1996). At Yucca Mountain, theseare metaluminous extracaldera tuffs (Sawyer andothers, 1994) made up of variably weldedpyroclastic outflow sheets and tephra falloutdeposits, with minor intercalated lava flows,ground surge, and reworked volcanic deposits.

The oldest units in the map area are ash-flow tuffs of the quartz-rich Crater Flat Group.The Crater Flat Group is made up of the TramTuff, the 13.25-Ma Bullfrog Tuff, and the ProwPass Tuff (Byers and others, 1976; Sawyer andothers, 1994). The Tram Tuff does not cropout in the map area. The Bullfrog and ProwPass Tuffs (units b and p) crop out just west ofProw Pass in the northwesternmost part of themap area, but only the uppermost part of theBullfrog Tuff is exposed in the map area. Itsmiddle and lower parts are severed by the splayof the Windy Wash fault. Sawyer and Sargent(1989) suggested that the source for the Bull-frog was the Area 20 caldera, which is morethan 20 km north of Prow Pass and lies alongthe northern margin of the Timber Mountain–Oasis Valley caldera complex (fig. 1). Carr andothers (1986) did not identify a source calderafor the Prow Pass Tuff, but suggested that itsdistribution is sufficiently limited that it may nothave erupted from a caldera.

Regionally, the 13-Ma Wahmonie Formationoverlies the Crater Flat Group. It crops outpoorly (<3 m thick) and only on the northeast-ern and southeastern sides of Busted Butte(Broxton and others, 1993). As such, it wasnot mapped separately from the Quaternary de-posits on this map. The Wahmonie Formationis made up of andesitic and dacitic rocks thaterupted from the Wahmonie volcano (fig. 1) eastof the map area (Poole and others, 1965;Broxton and others, 1989).

The 12.9-Ma Calico Hills Formation liesstratigraphically between the Crater Flat Groupand the Paintbrush Group. This unit is com-posed of interbedded rhyolitic tuffs and lavas re-lated to Crater Flat Group post-caldera volcan-ism (Sawyer and others, 1994). Dickerson andDrake (1998) proposed a source in the lowerFortymile Canyon area.

Rocks of the Paintbrush Group make up thevast majority of the bedrock exposures in the

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7

Yucca Mountain area. Buesch and others(1996) described the units of the PaintbrushGroup and defined the members, zones, and sub-zones in both the Tiva Canyon and TopopahSpring Tuffs. The main units of the PaintbrushGroup include (from oldest to youngest) theTopopah Spring Tuff, the Pah Canyon Tuff, therhyolite of Zig Zag Hill, the rhyolite of DeliriumCanyon, the rhyolite of Black Glass Canyon,the Yucca Mountain Tuff, the Tiva Canyon Tuff,the rhyolite of Vent Pass, and the rhyolite ofComb Peak. The quartz-poor 12.8-Ma TopopahSpring Tuff is made up of a lower crystal-poorrhyolitic member (Buesch and others, 1996)(units tpv, tpln, tpll, tpmn, tpul, and tp) and anupper crystal-rich quartz latite member (units trn,t rv , and tr ). The source caldera for theTopopah Spring remains undefined; however,Byers and others (1976) proposed the ClaimCanyon caldera to the north of Yucca Mountain(fig. 1). Facies relations within the TopopahSpring Tuff around Yucca Wash and north ofthe map area are consistent with the ClaimCanyon caldera as the most likely source.

The informal pre-Pah Canyon Tuff non-welded bedded tuff (unit bt2) of Buesch and oth-ers (1996) lies above the nonwelded uppermostpart of the Topopah Spring Tuff. Its distributionextends from the area of The Prow souththroughout the entire map area. Where tuff bt2is less than 2 m thick and the Pah Canyon Tuffis present as a mappable unit (unit pp), such asat The Prow, unit bt2 is lumped with theoverlying Pah Canyon Tuff. Where tuff bt2 isover 4 m thick and the Pah Canyon Tuff is notpresent as a mappable unit, such as in southernFatigue Wash, unit bt2 is mapped separately.Where tuff b t 2 occurs in the interval ofnonwelded, bedded tuff between the TopopahSpring and Tiva Canyon Tuffs, such as in thesouthern part of the map area, it is lumped withthe other bedded tuffs as nonwelded beddedtuffs, undivided (unit bt).

Both the Pah Canyon Tuff and YuccaMountain Tuff thicken from east to west in thenorthern part of the map area and pinch outtoward the south in Fatigue Wash and SolitarioCanyon. The pre-Yucca Mountain Tuff beddedtuff (unit bt3) is a thick, nonwelded tuff betweenthe Pah Canyon and Yucca Mountain Tuffs inthe northern part of the map area. Unit bt3 isas thick as 40 m in Yucca Wash, but thins tothe south and pinches out approximately wherethe Pah Canyon and Yucca Mountain Tuffspinch out. Southward from the pinch-out ofthese units, the nonwelded tuff (unit bt) betweenthe Topopah Spring Tuff and the Tiva Canyon

Tuff is variously made up of pre-Pah Canyon Tuffbt2, pre-Yucca Mountain Tuff bt3, the distal partof the Pah Canyon and Yucca Mountain Tuffs,and the pre-Tiva Canyon Tuff bt4 of Moyer andothers (1996). Locally, unit bt2 is the predomi-nant tuff within unit bt.

The quartz-poor 12.7-Ma Tiva Canyon Tuff(Sawyer and others, 1994) overlies the non-welded bedded units. The source of the TivaCanyon Tuff was the Claim Canyon caldera (fig.1) north of the map area (Byers and others,1976). Like the Topopah Spring Tuff, the TivaCanyon Tuff is made up of a lower crystal-poorrhyolitic member (Buesch and others, 1996)(units cpv and cp ) and an upper crystal-richquartz latite member (unit cr). It is overlain bypumiceous, pyroclastic-flow and fallout deposits(unit bt5) in southern Dune Wash as well as innorthern Windy Wash.

Pyroclastic flow deposits and lava flows ofthe rhyolites of Delirium Canyon (unit dc), BlackGlass Canyon (unit bg ), Vent Pass (unit vp ) ,and Comb Peak (units kt and kl) are exposednorth of Yucca Wash in the northeastern partof the map area. The vent for the rhyolite ofComb Peak was defined by Dickerson and Drake(1995) to be at Comb Peak, just northeast ofthe map area. Facies and thickness relationswithin the rhyolite of Delirium Canyon suggesta source in upper Fortymile Canyon, also justnorth of the map area. The rhyolite of BlackGlass Canyon is limited to Black Glass Canyon,and its source likely is located there as well. Thesource for the rhyolite of Vent Pass was proba-bly near the northern boundary of the map area.

The quartz-rich 11.6-Ma Rainier Mesa Tuff,which is the oldest formation of the TimberMountain Group, erupted from the TimberMountain–Oasis Valley caldera complex (Byersand others, 1976). In the map area, the RainierMesa Tuff is made up of a lower nonwelded tuff(unit mr ) overlain by a partially to moderatelywelded rhyolitic ash-flow tuff (unit mrw). Northof Yucca Wash, the rhyolite of Pinnacles Ridge(unit pr) forms a tuff cone and lava dome com-plex of limited areal extent.

Tertiary basaltic dikes (unit d) intrude Paint-brush Group rocks along the Solitario Canyonfault and along northwest-striking faults in thenorth-central part of the map area. Isotopic dat-ing (K-Ar) of a basaltic dike that intrudes theSolitario Canyon fault at Little Prow yields anage of 10±0.4 Ma (Carr and Parrish, 1985).This dike is itself brecciated along the fault,implying that movement along the SolitarioCanyon Fault both predated and postdated theemplacement of the dike (Scott, 1990).

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8

STRUCTURAL GEOLOGY

The volcanic outflow sheets form a carapaceof pyroclastic rocks around the southern andsoutheastern margins of the Claim Canyoncaldera (fig. 1). The volcanic carapace was thendisrupted by predominantly north-striking faults,along which the volcanic units were tilted intoeast-dipping structural blocks at YuccaMountain.

Faults at Yucca Mountain can be classified asblock-bounding faults, relay (and associated)structures, strike-slip faults, or intrablock struc-tures. The block-bounding faults generallystrike north, are spaced 1–4 km apart (fig. 3),and have throws of as much as hundreds of me-ters. Subordinate amounts of strike-slip motionare common. In some places, Quaternary toRecent offset has been documented on thesefaults (see Simonds and others, 1995). Relaystructures link the block-bounding faults. Thesecan be either large-scale features, commonlyforming fault zones or systems with greaterthan 100 m of aggregate offset, or they can besmall-scale features made up of one or morefaults that interconnect with the block-boundingfaults and have cumulative offset on the order oftens of meters. Internally, the fault zones canbe made up of horst and graben structures, withan overall net displacement of units across thezone. The large strike-slip faults are restrictedto the north-central part of the map area.Intrablock structures are relatively minor faultsthat lie entirely within the structural blocks de-fined by block-bounding faults (fig. 3). The in-trablock faults typically have mapped lengths ofless than 1 km, but are as long as 7 km, asnoted in the combined length of the GhostDance and Abandoned Wash faults.

DESCRIPTION OF BLOCK-BOUNDINGFAULTS

Deformation associated with the block-bounding faults at Yucca Mountain affects sev-eral major repository issues such as the volumeand quality of rock available for undergroundconstruction of the potential repository, the de-lineation of hydrologic flow paths, and seismichazard assessments. The present map depictsthe distribution of structures associated with theblock-bounding faults, thus providing critical dataneeded to resolve these issues. Because oferosion and partial burial of hanging-wall blocksadjacent to the main fault trace, depiction ofstructures in footwall blocks is generally moredetailed than in the hanging-wall counterparts.From west to east, the block-bounding faults

include (fig. 3) the Northern Windy Wash,Fatigue Wash, Solitario Canyon, and Iron Ridgefaults, several unnamed faults between the IronRidge fault and Dune Wash, and the East Ridge,Dune Wash, northern part of the Bow Ridge,Midway Valley, and Paintbrush Canyon faults.These north- and northwest-trending, block-bounding faults are often linked kinematically bynorthwest-trending relay faults and associatedstructures, which act to distribute displacementacross several of the faults. Mineral lineationsand mullions developed on fault scarps indicatethat a subordinate component of sinistral slipwas commonly associated with the block-bounding faults (Scott and Bonk, 1984; O’Neilland others, 1992; Simonds and others, 1995;Day and others, 1996; Day and others, 1998).

The block-bounding faults generally dip westwith variable dips ranging from relatively shallow(45°–50°) to steep (75°–85°). The amount ofsecondary faulting preserved in the footwall ishigher in areas along traces of the fault zonesthat have shallower dips. The subordinate left-lateral component of motion within some of thezones set up releasing and restraining bends lo-cally along the trace of the main fault zones.Restraining bends are along segments of theblock-bounding faults that strike north-north-east. One such area is preserved in the hangingwall of the Bow Ridge fault near the south portalof the ESF (Exploratory Studies Facility). In thisarea the Bow Ridge fault strikes north-north-east. Farther to the south the strike of the faultchanges to the northwest at the saddle be-tween Bow and Boundary Ridges (fig. 2). Aneast-dipping reverse fault in the hanging wall, in-tersected in the ESF, formed due to local com-pression in this area of the restraining bend onBow Ridge fault.

Previous workers recognized west-dippingstrata that form coherent structural panelswithin some of the hanging walls of the block-bounding fault zones (Scott and Bonk, 1984;Scott, 1990; Potter and others, 1996). Suchwest-dipping panels are preserved in the hangingwall of the Solitario Canyon fault (north of PlugHill), the Iron Ridge fault (north of borehole WT-11), and the Northern Windy Wash fault (south-west of Prow Pass). The west-dipping panelsare bounded on their western side (west of theblock-bounding fault) by steeply dipping faultsthat splay off of the west-dipping block-boundingfaults at depth.

DESCRIPTION OF RELAY STRUCTURES

Relay structures kinematically link the block-bounding faults and form an integral part of the

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Quaternary deposits

Miocene volcanic bedrock

Exploratory Studies Facility

Fault type—Bar and ball on downthrown side. Dotted where inferred

Block-bounding faultStrike-slip faultRelay structureDominant intrablock fault

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

Nor

ther

n W

indy

Was

h fa

ult

Fati

gu

e W

ash

fau

lt

PaganyWashfault

Drill Hole Wash

fault

SeverWashfault

SolitarioCanyon

fault

Boo

mer

ang

Poin

t fau

lt Sundancefault

Gh

ost

Dan

ce f

ault

Bow

Rid

ge fa

ult

Mid

way

Val

ley

faul

tPa

intb

rush

Can

yon

faul

t

Bow Ridge fault

Dune Wash fault

East Ridge fault

Ab

and

on

ed W

ash fau

lt

Iron

Rid

ge fau

lt

Busted Buttefault

EXPLANATION

0

0

1

1

2

2

MILES

KILOMETERS

116°30' 116°26'15"

36°52'

550000 560000 570000

740000

750000

760000

770000

780000

36°48'45"

580000

9

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structural framework of Yucca Mountain. Relaystructures range in size from large zones ofcomplexly anastomosing normal faults that rivalthe total displacement and rock-mass damage ofmany block-bounding faults, to simple small faultsand grabens that rival intrablock faults. Onesuch large relay structure cuts through WestRidge (figs. 2 and 3), connecting the NorthernWindy Wash and Fatigue Wash faults. It con-sists of a system of predominantly southwest-dipping normal faults with down-to-the-south-west displacement coupled with a subordinatenumber of nearly vertical, northeast-side-downnormal faults. Moderate-scale (50–200 m wide)horsts and grabens occur within the relay zone.The aggregate displacement across the zone isapproximately 60 m, down to the southwest.

Two other relay structures effectively cutthe southern end of Jet Ridge. A northwest-striking, southwest-side-down normal fault ex-posed directly above the colluvial apron alongthe southwestern end of Jet Ridge (west of theBoomerang Point fault) drops the crystal-richmember of the Topopah Spring Tuff (unit trn)about 20 m. Although the fault projects beneaththe alluvial cover, it is inferred to connect theFatigue Wash and Boomerang Point faults. An-other relay fault is exposed east of the southernextent of the Boomerang Point fault. At thesouthern terminus of Jet Ridge, the middle andlower parts of the crystal-poor member of theTiva Canyon Tuff (unit cp ) are downthrownalong a north-northwest-striking normal faultagainst the top of the Topopah Spring Tuff.This west-dipping, down-to-the-west fault has atleast 100 m of displacement and projects to thesouth along the foot of Boomerang Point. Asplay of this fault strikes to the southeast justwest of borehole WT-7 and has over 70 m ofwest-side-down displacement. A thin sliver ofthe crystal-rich member of the Topopah SpringTuff (unit trn) forms a horst block within thenormal-fault zone. Therefore, this part of thenortheastern margin of Crater Flat is defined bynorthwest-striking, southwest-side-down relayfaults, both exposed and buried.

Another important relay structure slicesacross Yucca Crest southwest of borehole G-3.Two dominant northwest-striking, southwest-dipping faults cut the Paintbrush Group rocksand connect the Solitario Canyon fault with theIron Ridge fault and associated faults. In thefootwall of the Solitario Canyon fault, theeasternmost of these two faults places themiddle part of the Tiva Canyon Tuff (unit cp )down against the middle nonlithophysal zone ofthe Topopah Spring Tuff (unit tpmn ). Totalsouthwest-side-down displacement across these

two faults is more than 120 m along the majorsaddle of Yucca Crest, placing the top of theTiva (on the hanging wall) against its base (onthe footwall). Numerous normal faults crop outjust east of the Iron Ridge fault north ofborehole WT-11. These gently west-dippingfaults formed in the footwall of the Iron Ridgefault where slices of the footwall were down-dropped into the main fault zone along spoon-shaped faults.

The strike of the Bow Ridge fault changesabruptly at the saddle between Bow and Bound-ary Ridges from north-northeast on the north-ern side of the saddle to a northwestern strikesouth of the saddle. The southern part of thefault, which strikes into the Paintbrush Canyonfault north of Busted Butte, can be thought ofas a relay connecting the northern part of theBow Ridge fault with the Paintbrush Canyonfault.

Small relay structures also connect some ofthe block-bounding faults. Examples are exposedon Exile Hill, just north of the north portal ofthe ESF. Four northwest-striking, down-to-the-northeast normal faults terminate against theBow Ridge fault on their northwestern ends andagainst the east-side-down Exile Hill fault at theirsoutheastern ends. Although the aggregatedisplacement across these faults is minimal (lessthan 20 m), the intervening rock mass is highlybrecciated. Another series of small northwest-striking relay faults is exposed on the northernpart of Bow Ridge, linking the Bow Ridge andMidway Valley faults. One of these faults cutsthrough the c-hole complex (boreholes c#1, c#2,and c#3). The northern tip of Bow Ridge alsohosts several small northwest-striking faults. Aparallel-striking fault 350 m to the south of thec-hole complex has only about 10–15 m ofdown-to-the-southwest displacement.

DESCRIPTION OF THE PROMINENTNORTHWEST-STRIKING

STRIKE-SLIP FAULTS

Three main northwest-striking strike-slipfaults are exposed in the northern part of themap area: the Sever Wash, Pagany Wash, andDrill Hole Wash faults. They have been de-scribed by Scott and others (1984), Simondsand others (1995), and Day and others (1998).Kinematic indicators preserved along the scarpsof the Sever Wash and Pagany Wash faults indi-cate that the latest motion was strike slip. Pre-served along the Sever Wash fault are zones ofsilicified breccia, subhorizontal slickensides, andsubhorizontal mullion structures. Along thetrace of the Pagany Wash fault, Reidel shears

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are well developed in the welded part of theYucca Mountain Tuff in Pagany Wash (Simondsand others, 1995). The Reidel shears form anacute angle (about 30°) measured clockwise fromthe main fault plane, indicating dextral motion onthe fault plane during formation of the shears.Another feature of both faults is the apparentreversal of the vertical component of offsetalong their traces. Along their northwesternpart the faults show down-to-the-southwestdisplacement, which changes to down-to-the-northeast along their southeastern part.

The Drill Hole Wash fault is buried beneathQuaternary alluvial deposits in the floor of DrillHole Wash. The main trace of the fault is inter-sected in the subsurface in borehole a # 4(Spengler and Rosenbaum, 1980) and in the ESF(Steven C. Beason, U.S. Bureau of Reclamation,oral commun., 1995). The fault is exposed atthe surface on Tonsil Ridge northwest of bore-hole H-1. Here the fault strikes N. 30° W., dipssteeply (80°–85°) to the southwest, and has acumulative throw of approximately 15 m downto the southwest. This fault projects into thesubsurface into a fault zone exposed in the ESF,which has an apparent offset of least 1.2 m(down-to-the-southwest). Exposures in the ESFreveal subhorizontal slickensides with a dextralsense of slip. The variability in relative verticaloffset along the fault trace in the 400-m dis-tance between Tonsil Ridge and the ESF (15 mat the surface compared to 1.2 m in the ESF)and orientation (N. 30° W. versus N. 10° W.,respectively) is similar to that observed onintrablock faults throughout the map area.

Scott and Bonk (1984) inferred a north-west-striking fault beneath Yucca Wash, basedon their interpretation of aeromagnetic data.The wash is a prominent geomorphic erosionalfeature beneath which it is reasonable to infer afault. Significant changes in volcanic facies oc-cur across and north of Yucca Wash, but thosefacies changes do not require the presence of afault. Dickerson (1996) observed that severalnorth-striking block-bounding faults can betraced continuously across Yucca Wash in thesubsurface using detailed ground magnetic dataof Ponce and Langenheim (1994). Therefore,there is no direct or indirect evidence for a sig-nificant fault beneath Yucca Wash.

DESCRIPTION OF INTRABLOCKSTRUCTURES

Intrablock faults by definition are within theintervening rock mass between the block-bound-ing faults. In some cases, intrablock faults areexpressions of hanging-wall or footwall

deformation that affects the block within a fewhundred meters of the block-bounding faults.Other intrablock faults, such as the northwest-trending narrow grabens seen on BoundaryRidge, are present in the central and southernparts of the map area. The grabens are local ex-pressions of regional Tertiary extension andform minor dilation zones in the structuralblocks. Intrablock and block-bounding faultsshare common geometric elements. Intrablockfaults, such as the Ghost Dance, AbandonedWash, and Busted Butte faults, are marked byzones that widen upward near the surface, be-coming frameworks of upward-splaying faults(Day and others, 1998). This pattern is sharedby the block-bounding faults.

Intrablock faults in the central and southernparts of the map area include the BoomerangPoint, Ghost Dance, Abandoned Wash, Sun-dance, and Busted Butte faults, as well as un-named faults in the block west of Dune Wash onEast Ridge. The Boomerang Point fault bisectsthe Jet Ridge block (figs. 3 and 4). Down-to-the-west displacement along this fault increasessouthward to more than 120 m over a strikelength of approximately 4 km. Rocks betweenthe Boomerang Point fault and the FatigueWash fault contain very few faults—making itone of the least deformed parts in the YuccaMountain area. The number of faults in theblock east of the Boomerang Point fault isgreater due to the increase in dip associatedwith hanging-wall deformation of the SolitarioCanyon fault.

The Ghost Dance fault is the main intra-block fault within the central part of the poten-tial repository area. It is a north-striking normalfault zone that dips steeply west with down-to-the-west displacement. The displacement,amount of brecciation, and number of associatedsplays vary considerably along its trace (Spen-gler and others, 1993; Day and others, 1998).The Abandoned Wash fault is interpreted toconnect with the Ghost Dance fault in the areaof Ghost Dance Wash. From this area south,the Abandoned Wash fault extends another 3.8km. It terminates in the wash south of boreholeG-3. As such, the 7-km-long Ghost Dance–Abandoned Wash fault is entirely an intrablockfault with no surface connection to any knownblock-bounding fault. Any connection at depthwith block-bounding faults is inferential.

The Sundance fault zone is exposed south ofDrill Hole Wash between Live and Dead YuccaRidges. Spengler and others (1994) firstidentified the Sundance fault as a wide zone ofminor northwest-striking faults that commonlyshow down-to-the-northeast displacement.

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Subsequent detailed mapping by Potter andDickerson (in Day and others, 1998) showedthat the fault zone is about 750 m long, extendsfrom Dead Yucca Ridge southeastward to LiveYucca Ridge, and has a maximum cumulativedown-to-the-northeast displacement of 11 m. Inthe ESF, the Sundance is a series of discon-tinuous narrow fault zones with minor amountsof vertical offset.

Numerous intrablock faults are present inthe southern part of the map area. One of themost prominent slices through Busted Butte.This west-side-down normal fault has one of thegreatest vertical exposures (over 250 m) of anyintrablock fault in the map area. The upward-branching nature of faults described by Day andothers (1998) for faults in the central block areais well displayed along the fault that splits BustedButte. The lowest exposures of the fault at thebase of Busted Butte form a zone of intensebrecciation approximately 3 m wide. The faultbranches upward into several splays forming anapproximately 110-m-wide zone. The westernsplay of the fault zone is actually an east-dippingreverse fault. The gently east-dipping crystal-rich Tiva Canyon Tuff (unit cr) in the footwall(west of fault splay) is overlain by a fault sliver ofvertically dipping older crystal-poor Tiva (unit cp)in the hanging wall (east of fault splay).

Dips of the intrablock faults are generallysteeper than those of the block-bounding faults.The intrablock dips shown on the cross sec-tions were derived from the mapped trace of thefault intersection with topography and rangegenerally from 75° to vertical. Measured scarpsalong the block-bounding faults vary, but dipsgenerally are between 60° and 75° to the west.If all of the intrablock faults formed prior to therotation of the beds into their generally east-ward dip, then the intrablock faults would haveformed originally as reverse faults. Kinemati-cally, this would be unlikely in that deformationof the Tertiary tuffs throughout the map areawas extensional. Therefore, the intrablockfaults probably formed during and after tilting ofthe beds on the block-bounding faults.

STRUCTURAL DOMAINS

The map area can be divided into ten struc-tural domains (fig. 4), each characterized by adistinctive structural style. These structuraldomains all lie within the more regional CraterFlat tectonic domain as defined by Fridrich (inpress). The term “structural domain” is usedhere to denote an area characterized by a par-ticular structural style that is distinct from thatof adjacent areas. The distinctive structural

patterns that characterize individual domains in-clude both the geometry and intensity of fault-ing as well as the magnitude and direction ofstratal dips. These domains reflect protracteddeformation and record an integrated structuralresponse through time. In general, structuraldomains can be identified at any scale; those de-fined herein are appropriate for this 1:24,000map scale. In this context, the structural do-mains as defined below are typically an aggre-gate of distinct structural blocks that sharecommon intrablock fault characteristics andstratal dips. They are briefly described below, asa summary of the overall structural geology ofthe map area.

CENTRAL YUCCA MOUNTAIN DOMAIN

The largest domain is the Central YuccaMountain domain, which comprises three east-tilted blocks bounded by west-dipping block-bounding faults. From west to east, theseblocks include (1) the West Ridge block, boundedon the west by the Northern Windy Wash faultand on the east by the Fatigue Wash fault; (2)the Jet Ridge block, bounded on the west by theFatigue Wash fault and on the east by the Soli-tario Canyon fault; and (3) the central block,bounded on the west by the Solitario Canyonfault and on the east by the Bow Ridge fault.The West Ridge block is about 1 km wide and 5km long, the Jet Ridge block is about 2 km wideand 7 km long, and the central block is about 4km wide and 7 km long. In each of these blocks,stratal dips for the Paintbrush Group ash-flowtuffs are typically 5°–10° to the east, steepeningalong the eastern edges of the blocks in thehanging walls of block-bounding faults to valuesas large as 20°. Intrablock faults, such as thenorth-striking Ghost Dance and AbandonedWash faults in the central block and the Boo-merang Point fault in the Jet Ridge block, arelocally prominent (see “Structural geology” sec-tion). The central block and, to a much lesserextent, the Jet Ridge block contain a complexzone of intrablock faulting along the easternedge (Scott and Bonk, 1984; Day and others,1998). This block-margin faulting, accompaniedby steepening of stratal dips, reflects the stratalrotation and internal deformation produced inthe hanging wall of a complex block-boundingfault zone (Scott, 1990; Day and others, 1998).

The northern edge of the Central YuccaMountain domain is more highly deformed thanthe central and southern parts. For example,near The Prow (figs. 2 and 4) the frequency andintensity of intrablock faulting increase. Alongthe northwest-trending edge of Yucca Mountain

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Azreal RidgedomainCentral

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Quaternary deposits

Miocene volcanic bedrock

Exploratory Studies Facility

Structural domain boundary

Fault type—Bar and ball on downthrown side. Dotted where inferred

Block-bounding faultStrike-slip faultRelay structureDominant intrablock fault

EXPLANATION

Figure 4. Distribution of structural domains defined for the map area.

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between The Prow and Castellated Ridge,southeast-dipping Paintbrush Group strata arecut by a series of north- to north-northwest-striking normal faults that range in offset from 2to 70 m; east-side-down and west-side-downdisplacements are about equally divided (seecross section A –A ′ ). Through the north-western corner of the map area and to thenorth of the map area, the Northern WindyWash fault maintains its character as a complex,west-side-down block-bounding fault. TheFatigue Wash fault continues through the areaof The Prow as a west-side-down block-bounding fault.

AZREAL RIDGE DOMAIN

The Azreal Ridge domain lies between DrillHole Wash and Yucca Wash, northeast of theCentral Yucca Mountain domain (fig. 4). The in-tensity of faulting in this domain is relatively lowcompared with the Central Yucca Mountain,Dune Wash, or Southwest domains to thesouth (see below). Faulting within the domain isdominated by the three strike-slip faults previ-ously discussed (Sever Wash, Pagany Wash,and Drill Hole Wash faults). The intrablockfaults are minor, north-trending normal faultswith limited brecciation. Strata dip gentlysoutheast (4°–9°) in the main part of thedomain. Along the eastern edge of the domainin the hanging wall of the Bow Ridge fault, de-formation intensifies as recorded by the increaseof the stratal dips (as much as 13°) and relativeintensity of faulting.

The transition from the northern margin ofthe central block to the Azreal Ridge domain (fig.4) is marked by a decrease in the magnitude ofeast-west extension, manifested in a fundamen-tal change in the nature of the two block-bounding faults. West-side-down displacementsalong the Solitario Canyon and Bow Ridgefaults diminish to zero at hinge points withinnorthern Yucca Mountain. North of thesehinge points, east-side-down displacements aremapped along these faults. For the SolitarioCanyon fault, the hinge point is in TeacupWash, and for the Bow Ridge fault, the hingepoint lies beneath Quaternary alluvium east ofIsolated Ridge. The Azreal Ridge domain occu-pies the little-extended area bounded by the seg-ments of these two faults that have undergoneminimal displacements near the hinge points.

In addition to the decreased magnitude ofextension, the Azreal Ridge domain is character-ized by a change in the direction of stratal dipsand a change in intrablock faulting style. Incontrast to the easterly dips predominant in the

Central Yucca Mountain domain, the AzrealRidge domain consists of gently southeast-dipping Paintbrush Group strata. Because themain ridges and washes at Yucca Mountain havegenerally developed parallel to the dip of thePaintbrush Group welded tuffs, this southeastdip is expressed geomorphically as a strongnorthwest-trending topographic grain. Thesoutheast-dipping strata in the Azreal Ridgedomain actually form one limb of a broadsoutheast-plunging syncline, and the other limbis defined by the east-dipping strata of thecentral block. The axis of this syncline liesbeneath Drill Hole Wash.

YUCCA WASH DOMAIN

North of the Azreal Ridge domain, theYucca Wash domain records a northward in-crease in the magnitude of east-west extension,accommodated by displacements along severalnorth- to northwest-striking intrablock andstrike-slip faults. These include several promi-nent splays of the Sever Wash fault just southof Yucca Wash, along which the amount ofstratigraphic displacement increases markedly tothe north. This structural style is similar to thatin the adjacent part of the Central Yucca Moun-tain domain near The Prow, as previously dis-cussed, and it continues on the north side ofYucca Wash within the Yucca Wash domain (fig.4). Bedrock dips within the Yucca Wash do-main, as preserved in the Paintbrush Group, aresteeper (14°–28°) and more easterly than in theadjoining Central Yucca Mountain or AzrealRidge domains.

PAINTBRUSH CANYON DOMAIN

The Paintbrush Canyon domain is char-acterized by closely spaced faults that divide thisdomain into a series of long, narrow, north-trending, east-dipping blocks ranging from 0.6to 1 km in width (fig. 4). The domain is boundedon the east and west by the Paintbrush Canyonand Bow Ridge faults, respectively, and it in-cludes prominent north-striking faults such asthe Black Glass Canyon, Midway Valley, andExile Hill faults. The northern part of the do-main is well exposed north of Yucca Wash,whereas the bedrock in the central part of thedomain is obscured by Quaternary deposits inthe Midway Valley area. The bedrock in thesouthern part of the domain resurfaces on BowRidge and San Juan Hill to the east (just west ofthe Paintbrush Canyon fault; figs. 2 and 4).Where exposed, the Paintbrush Group generallystrikes northward, but dips more steeply (as

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much as 46°) to the east than in the adjacentCentral Yucca Mountain domain.

In the northern part of the domain, faultblocks are juxtaposed along down-to-the-westnormal faults with extensive deformation in theirhanging walls. This hanging-wall deformationincludes graben development, numerous splays,pull-apart structures, and wide, internallysheared, massively brecciated, multi-plane mainfault zones. Geophysical interpretations strong-ly suggest that fault blocks buried beneath surfi-cial deposits in Midway Valley define a horst-and-graben pattern (Ponce, 1993; Ponce and others,1993). Exile Hill, for example, represents a horstblock in this domain. Another buried horst oc-curs to the east beneath the center of MidwayValley; these horsts are 0.3–0.7 km wide.

The continuity of north-trending faults fromnorth of Yucca Wash southward through Mid-way Valley is one of the unifying characteristicsof the Paintbrush Canyon domain. Geophysicaland geologic data indicate that the main faultsexposed north of Yucca Wash continue south-ward beneath Quaternary deposits to thesouthern part of Midway Valley (Dickerson andDrake, 1998). This observation argues stronglyagainst the presence of the Yucca Wash fault asa major structure.

Along the east edge of the Paintbrush Can-yon domain, the magnitude and complexity ofthe Paintbrush Canyon fault increase to thesouth. Over 200 m of displacement is recordedalong this fault in upper Paintbrush Canyon(Dickerson and Spengler, 1994; Dickerson andDrake, 1998), and over 300 m of displacementnear Fran Ridge. Beneath southeastern MidwayValley, buried northeast-striking splays are in-ferred to have propagated from the main trace ofthe Paintbrush Canyon fault (Ponce, 1993).The horst-and-graben pattern that characterizesmost of Midway Valley is cut off by (or mergesinto) these splays. At the southern end of thePaintbrush Canyon domain the PaintbrushCanyon fault merges with the Dune Wash andBow Ridge faults (fig. 3); the Paintbrush Canyonfault system collects the aggregate displacementof all of these faults. Unfortunately, the area ofintersection of these faults, surely one of themost deformed and structurally complex areas ofYucca Mountain, is concealed beneath surficialdeposits northwest of Busted Butte.

FRAN RIDGE DOMAIN

East of the Paintbrush Canyon domain, theFran Ridge domain is a north-trending, largelyintact block between the Paintbrush Canyonfault and Fortymile Wash. It includes (from

north to south): Comb Peak (just off the north-eastern part of the map area) and the bedrocktract to its south, Alice Point, Fran Ridge, andBusted Butte. In the northern part of the do-main the Paintbrush Group and older rocksstrike northward and dip steeply 17°–27° to theeast. In the central part of the domain on FranRidge, rocks of equivalent age strike northwardand dip more gently to the east (5°–17°). Tothe south on Busted Butte, the bedrock dips aregentle to the east (5°–10°). The Fran Ridgedomain is characterized by less internal defor-mation than that found in the PaintbrushCanyon domain. Smaller faults, which splayeastward off the northern extent of the Paint-brush Canyon fault, indicate a greater amount offootwall deformation for the Paintbrush Canyonfault than is demonstrated for the exposed faultsin the Paintbrush Canyon domain. Althoughthe internal deformation is minor in most of theFran Ridge domain, a few intrablock faults havesignificant amounts of offset, such as theBusted Butte fault (75 m) and the fault south ofComb Peak (130 m). Additionally, the FranRidge domain is structurally higher than theadjacent Paintbrush Canyon and FortymileWash domains. Gaps between the topographichighs within the Fran Ridge domain are occupiedby buried down-to-the-northeast faults that liebetween Busted Butte and Fran Ridge, andbetween Fran Ridge and Alice Point.

FORTYMILE WASH DOMAIN

Geologic relations within the FortymileWash domain are poorly understood because thebedrock geology of this domain largely is con-cealed by the surficial deposits of Jackass Flats.Furthermore, the prevailing easterly dip of thePaintbrush Group strata in the Fran Ridgedomain strongly suggests that a major west-side-down normal fault may lie east of FortymileWash beneath the western part of JackassFlats. The northern part of the Fortymile Washdomain consists of the extensively faulted rockseast of Fortymile Canyon. There is nopreferred orientation for faults in this area, asfaults strike east-west, north-south, northeast-southwest, and southeast-northwest. It is notknown to what extent this random fault patterncharacterizes other parts of the Fortymile Washdomain where bedrock is not exposed.

PLUG HILL DOMAIN

The Plug Hill domain lies in the easternmargin of Crater Flat on the southwest side ofthe map area. It contains low-lying hills of Tiva

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Canyon Tuff as well as exposures of the RainierMesa Tuff on Plug Hill itself. The domain isrimmed by normal faults that drop units of thePaintbrush Group into Crater Flat. The north-ern margin of the domain is delineated by therelay structures on the southern tip of JetRidge, and the eastern and southern margin bythe Solitario Canyon fault, which curves from anorth strike to a more northeasterly strike. Al-though the bedrock is poorly exposed, wherepresent it is made up of highly faulted, eastward-dipping blocks. Plug Hill is made up of TivaCanyon Tuff overlain by nonwelded and weldedunits of the Rainier Mesa Tuff. The amount offaulting within the Rainier Mesa Tuff is hard todetect because it lacks internal marker horizonsand is poorly exposed. However, Day and oth-ers (1998) did map faulted Paintbrush Grouprocks caught in the hanging-wall deformation ofthe Solitario Canyon fault that are overlain byRainier Mesa Tuff, suggesting that eruption ofthe Rainier Mesa postdated some of the post-Paintbrush Group deformation on the hangingwall of the Solitario Canyon fault. This relationis unique to the Plug Hill area. Here, as else-where, the Rainier Mesa Tuff is preserved onlyon the hanging wall of the block-bounding faults.Implications of the distribution of the RainierMesa Tuff are discussed in the “Discussion onthe variation and timing of tectonism at YuccaMountain” section.

SOUTHWEST DOMAIN

In the Southwest domain, which lies be-tween Double Notch Ridge and Crater Flat (figs.2 and 4), north-trending, block-bounding faultsare linked kinematically by northwest-trendingrelay faults and associated structures. An excel-lent example is the complex northwest-strikingrelay zone between the Solitario Canyon andIron Ridge faults near the north end of theSouthwest domain. Several other relay faultssplay off the Solitario Canyon fault in thesouthernmost part of the map area in theSouthwest domain, transferring strain from themain trace of the Solitario Canyon fault. Thevertical component of offset on the SolitarioCanyon fault (east of Plug Hill) is over 400 m.However, at its southernmost trace (at thesouthern margin of the map area) the offset isonly on the order of 60 m. The aggregate ver-tical displacement taken up by the interveningfive relay faults is over 60 m each, accountingfor the missing vertical component of displace-ment on the Solitario Canyon fault.

Northwest-striking narrow grabens arecommonly associated with relay faults, here and

elsewhere in the southern part of Yucca Moun-tain. The block-bounding fault systems in thisdomain are west-dipping, upward-widening ar-rays, with westerly dips ranging from relativelyshallow (45°–50°) to steep (75°–85°). As theupward-widening fault zones attain shallowerdips, the amount of secondary faulting preservedin the footwall of the block-bounding fault sys-tem increases dramatically. In addition, fractur-ing of the footwall increases near the major re-lay faults.

EAST RIDGE DOMAIN

The East Ridge domain lies to the west ofthe Dune Wash domain (fig. 4). It is bounded onthe west by the unnamed block-bounding faultwest of borehole WT-12 and the down-to-the-east block-bounding faults just east of East Ridgeon the western edge of the Dune Wash graben.This domain differs from the adjacent CentralYucca Mountain domain in that the amount ofintrablock faulting is more intense, recording agreater amount of extensional deformation. Thestrata strike north-northeast and dip generally15°–20° to the east, more steeply than is ob-served in the Central Yucca Mountain domain.This domain also has numerous east-side-downfaults, which splay off the block-bounding faultalong the western edge of the Dune Washgraben.

DUNE WASH DOMAIN

The Dune Wash domain (fig. 4) is defined bythe northwest-trending Dune Wash graben,which is bounded on its east side by the down-to-the-west block-bounding Dune Wash fault andon its southeast margin by the PaintbrushCanyon fault. Its western margin is bounded bythe down-to-the-east block-bounding East Ridgefault zone (fig. 3), which has at least 120 m ofdown-to-the-east displacement. This down-to-the-east fault zone is approximately 100 m wide,contains tectonically brecciated and juxtaposedunits of the Topopah Spring Tuff (unit tx), andin places exhibits higher degrees of oxidation,silicification, and alteration compared with otherblock-bounding faults. This fault zone splays tothe south-southwest into several faults withdown-to-the-east displacement. The splays eachhave over 30 m of displacement. Within the in-terior of the graben are numerous, smallerhorst-and-graben blocks whose strata dip pre-dominantly to the east. The structure of the in-terior of the graben is highly complex with nu-merous discontinuous, steeply dipping faults.The Dune Wash domain is one of the areas

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depicted by figure 6 of Scott (1990) as a broadimbricate fault zone; although it is intenselyfaulted, it is not characterized by consistentwest-side-down offset along the array of faults,as Scott (1990) implied by his definition and useof the term “imbricate fault zone.”

In the northern end of the Dune Washgraben (southwest of borehole W T - 1 ), thewestern bounding fault of the graben stepsabout 200 m to the west and displacement de-creases; this fault places the crystal-rich memberof the Tiva Canyon Tuff (unit cr) down againstthe pre-Tiva Canyon, post-Topopah Springnonwelded bedded tuffs (unit bt). The displace-ment on individual faults within the graben de-creases to the north, but the overall horst-and-graben pattern is maintained. At the northernend of the graben, just south of the mouth ofAbandoned Wash, the faulting resembles theclosely spaced faulting that characterizes block-margin deformation in the central block to thenorth. The southern end of the Dune Washdomain, which is buried beneath Quaternary de-posits southwest of Busted Butte, seems toterminate against the down-to-the-west Paint-brush Canyon fault.

DISCUSSION ON THEVARIATION AND TIMING OF

TECTONISM AT YUCCA MOUNTAIN

The variation in deformation from a less-extended northern part of Yucca Mountain to amore-extended southern part is generally ex-pressed on geologic maps by an increase inthrow to the south along the block-boundingfaults. In intrablock areas, the transition is ex-pressed by the appearance of numerous closelyspaced minor faults that coalesce and gain dis-placement to the south. These patterns are ap-parent in the northern part of Dune Wash, par-ticularly near the mouth of Abandoned Wash.Southward-increasing east-west extension isalso recorded by the southward splaying of theSolitario Canyon fault, the development of thebroad, complexly faulted Dune Wash graben, andthe convergence of several block-bounding faultsalong the west side of Busted Butte. Scott(1990) expressed the southward increase indeformation as an increase in the area underlainby imbricate fault zones characterized by steepereastward dips of strata and an imbricate patternof closely spaced, steep, west-dipping faults withminor, down-to-the-west offsets of a few metersor less. Clockwise vertical-axis rotations ofPaintbrush Group strata increase from north tosouth, from 5° to 10° in the central block of theCentral Yucca Mountain domain, to 30° at the

extreme south end of Yucca Mountain, south ofthe map area (Rosenbaum and others, 1991).Northwest- to north-northwest-striking splaysare a common component of the north-strikingblock-bounding fault systems. In some places,these northwest-striking splays have developedinto major relay faults that transfer displacementbetween block-bounding faults. In the extremesouthern part of Yucca Mountain (south of themap area), the strikes of the southern WindyWash and Stagecoach Road block-boundingfaults are rotated to northeasterly orientations,and the subsidiary splays strike north (Simondsand others, 1995).

Tertiary extension in the Yucca Mountainarea occurred primarily between 16 and 10 Ma(Carr, 1984, 1990; Scott, 1990; Hudson andothers, 1994, 1996). Evidence for pre-Paint-brush Group (pre-12.8 Ma) deformation alongthe block-bounding faults is buried by the tuffs ofthe Paintbrush Group themselves. Older unitsare sparsely exposed either within or adjacent tothe map area. As reviewed by Carr (1990), theTimber Mountain–Oasis Valley caldera complex,which was the source for most of the tuffs ofthe Paintbrush Group, lies within an older,north-trending rift zone, which he named theKawich-Greenwater rift. This older rift basinformed within the Walker Lane belt and con-trolled the locus of caldera structures in thesouthwestern Nevada volcanic field (fig. 1 ofCarr, 1990).

There is indirect evidence that the SolitarioCanyon fault was active during eruption of thePaintbrush Group. One clear pre-Tiva CanyonTuff growth fault, a splay off the SolitarioCanyon fault, is well exposed along the easterncanyon wall midway along Solitario Canyonsouthwest of borehole H-5. The nonweldedunits appear to thicken across that fault, consis-tent with a growth-fault setting. Throw at thetop of the Topopah Spring Tuff is about 10 m,decreasing upsection to less than 4 m at thebase of the Tiva Canyon Tuff, and dying out inthe upper part of the Tiva Canyon Tuff.

Another example of extensional deformationduring deposition of the Paintbrush Group is ex-posed on the west flank of Fran Ridge. A smallnorth-trending graben structure, presumablykinematically related to the Paintbrush Canyonfault, is filled with pre-Tiva Canyon Tuff non-welded bedded tuff (unit bt), which thickens inthe center of the graben. Faults related to thegraben, however, do not extend upward into theoverlying Tiva Canyon Tuff.

The basal contact of the 11.6-Ma RainierMesa Tuff is one of the key horizons to unravel-ing the timing of tectonism in the Yucca

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Mountain area. There appears to be an east-ward decrease in the amount of angular uncon-formity at the base of the Rainier Mesa Tuff.Fridrich (in press) has shown that a significantangular unconformity exists between the RainierMesa Tuff and the underlying Tiva Canyon Tuffwest of Yucca Mountain in northern Crater Flat(west of the map area). He deduced that therewas a significant pulse of regional east-west ex-tension recorded by the angular unconformity.

The basal contact of the Rainier Mesa Tuffis exposed locally in northern Windy Wash inthe western part of the map area. There, thebedded tuff (unit bt5) dips about 8°–10° moresteeply than the overlying Rainier Mesa Tuff,supporting the formation of a gentle angular un-conformity after deposition of the PaintbrushGroup rocks (post-12.7 Ma), but prior to erup-tion and emplacement of the Rainier Mesa. Inthis same area, the Rainier Mesa is caught up inthe Windy Wash fault zone, faulted againstYucca Mountain Tuff and Pah Canyon Tuff. Itwas present when the major displacement oc-curred along the Northern Windy Wash fault,which has the most displacement of any block-bounding fault in the map area (over 450 m).This evidence indicates that the NorthernWindy Wash fault was active both before and af-ter deposition of the Rainier Mesa Tuff and,therefore, had a protracted history of motion.

The basal contact of the Rainier Mesa Tuffis preserved farther to the east in the SolitarioCanyon area at Plug Hill, where evidence islacking to support a major angular unconformityat the base of the Rainier Mesa. Compactionfoliations within the Tiva Canyon Tuff dip atabout 12° to the east, an alignment similar inmagnitude to the dip of the welding contact inthe overlying Rainier Mesa. This is contrary tomapping by Scott and Bonk (1984), who de-picted the basal contact of the Rainier Mesadown-cutting into the top of the Tiva CanyonTuff, a geometry that implies a significantamount of erosion and post-Tiva Canyon, pre-Rainier Mesa deformation. The “down-cutting”depicted by Scott and Bonk (1984) is simply adown-to-the-southwest fault that cuts the TivaCanyon and Rainier Mesa, as was mapped origi-nally by Lipman and McKay (1965). Therefore,there is no significant amount of erosion be-neath this contact at Plug Hill.

On the eastern side of the map area, thebasal contact of the 11.6-Ma Rainier Mesa Tuffis exposed on the southwestern side of DuneWash, just west of Ambush Pass. There, thecontact is conformable between a bedded tuff(unit bt5) and the top of the Tiva Canyon Tuff.Additionally, the bedded tuff (unit bt5 ) is

conformably overlain by a bedded basal horizonin the base of the Rainier Mesa. Above thisnonwelded bedded horizon within the RainierMesa Tuff, distinguished from unit bt5 by vitricpumice, lies a nonwelded massive horizon thatmay have a structural discordance of 5°–8° withits bedded basal horizon. This apparent changein dip is internal to the Rainier Mesa Tuff.

The distribution of the Rainier Mesa Tuff iskey to understanding the Tertiary tectonichistory of Yucca Mountain. In the western partof the region, it caps the top of ridge crests. Inthe central and eastern parts, the Rainier Mesais restricted to valley floors adjacent to themajor block-bounding faults. Moreover, exceptnear Plug Hill, the Rainier Mesa is furtherrestricted to the hanging-wall areas of the block-bounding faults and does not lap onto thefootwall. At Plug Hill, the Rainier Mesa does lapacross older faults associated with the SolitarioCanyon fault. The Rainier Mesa appears to beconformable in a west-dipping hanging-wall blockassociated with the Iron Ridge fault (north ofborehole WT-11). Its absence from the footwallareas cannot be explained by erosion aloneinasmuch as the Rainier Mesa Tuff also formsresistant erosional remnants on ridge crests inthe western part of the map area. Its generalrestriction to the hanging walls of the block-bounding faults indicates that it was eruptedprior to the final phases of motion on thosefaults and was downdropped thereafter. If itwere simply erupted into preexisting valleys andgrabens (Carr, 1986; Scott, 1990) then it wouldlap consistently onto the footwall of the block-bounding faults, which are also in the valleyfloors. This is in keeping with the mapping ofthe Timber Mountain–Oasis Valley caldera com-plex, in which the Rainier Mesa Tuff is highlyfaulted within and adjacent to the caldera com-plex (Byers and others, 1976).

In summary, there is evidence for a pro-tracted history of Tertiary extensional deforma-tion in the Yucca Mountain area. Pre-PaintbrushGroup extension formed a buried north-strikingrift in which the Timber Mountain–Oasis Valleycaldera complex formed. Syn-Paintbrush Group(circa 12.7 Ma) extension occurred along minorstructures related to the Solitario Canyon andPaintbrush Canyon faults. Evidence for contin-ued extension along the block-bounding faultsand rotation of units is recorded by an angularunconformity beneath the 11.6-Ma Rainier MesaTuff in the northwestern part of the map area.In addition, the Rainier Mesa Tuff overlaps faultswithin the Solitario Canyon fault zone at PlugHill, indicating post-Tiva Canyon, pre-RainierMesa motion on this block-bounding fault. To

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the east, however, the basal contact of theRainier Mesa is conformable with the underlying12.7-Ma Tiva Canyon Tuff. This suggests awestward increase in extension earlier than 11.6Ma within the map area. Extension along theblock-bounding faults continued after depositionof the Rainier Mesa Tuff inasmuch as its currentpreservation is restricted to the hanging wallsand faulted by the block-bounding faults.

REFERENCES CITED

Broxton, D.E., Chipera, S.J., Byers, F.M., Jr.,and Rautman, C.A., 1993, Geologic evalua-tion of six nonwelded tuff sites in the vicinityof Yucca Mountain, Nevada, for a surface-based test facility for the Yucca MountainProject: Los Alamos National LaboratoryReport LA–12542–MS, 83 p.

Broxton, D.E., Warren, R.G., Byers, F.M., Jr.,and Scott, R.B., 1989, Chemical andmineralogical trends within the TimberMountain–Oasis Valley caldera complex—Evidence for multiple cycles of chemical evo-lution in a long-lived silicic magma system:Journal of Geophysical Research, v. 94, p.5961–5986.

Buesch, D.C., Dickerson, R.P., Drake, R.M., II,and Spengler, R.W., 1994, Integrated geol-ogy and preliminary cross section along thenorth ramp of the Exploratory Studies Facil-ity, Yucca Mountain: 5th International HighLevel Radioactive Waste Management Con-ference, v. 2, p. 1055–1065.

Buesch, D.C., Nelson, J.E., Dickerson, R.P.,Drake, R.M., II, Spengler, R.W., Geslin,J.K., Moyer, T.C., San Jaun, C.A., andFelger, T., 1995, Distribution of lithostrati-graphic units within the central block ofYucca Mountain, Nevada—A three-dimen-sional computer-based model, versionYMP.R2.0: U.S. Geological Survey Open-File Report 95–124, 61 p.

Buesch, D.C., Spengler, R.W., Moyer, T.C.,and Geslin, J.K., 1996, Proposed strati-graphic nomenclature and macroscopic iden-tification of lithostratigraphic units of thePaintbrush Group exposed at Yucca Moun-tain, Nevada: U.S. Geological Survey Open-File Report 94–469, 45 p.

Byers, F.M., Jr., Carr, W.J., Orkild, P.P.,Quinlivan, W.D., and Sargent, K.A., 1976,Volcanic suites and related cauldrons of Tim-ber Mountain–Oasis Valley caldera complex,southern Nevada: U.S. Geological SurveyProfessional Paper 919, 69 p.

Carr, M.D., Waddell, S.J., Vick, G.S., Stock,J.M., Monsen, S.A., Harris, A.G., Cork,

B.W., and Byers, F.M., Jr., 1986, Geologyof drill hole UE-25p#1—A test hole into pre-Tertiary rocks near Yucca Mountain, south-ern Nevada: U.S. Geological Survey Open-File Report 86–175, 87 p.

Carr, W.J., 1984, Regional structural setting ofYucca Mountain, southwestern Nevada, andLate Cenozoic rates of tectonic activity inpart of the southwestern Great Basin,Nevada and California: U.S. Geological Sur-vey Open-File Report 84–854, 108 p.

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Carr, W.J., and Parrish, L.D., 1985, Geology ofdrill hole USW VH-2, and structure ofCrater Flat, southwestern Nevada: U.S.Geological Survey Open-File Report 85–475,41 p.

Christiansen, R.L., and Lipman, P.W., 1965,Geologic map of the Topopah SpringNorthwest quadrangle, Nye County,Nevada: U.S. Geological Survey GeologicQuadrangle Map GQ–444, scale 1:24,000.

Day, W.C., Potter, C.J., Sweetkind, D.S., andDickerson, R.P., 1996, Detailed bedrockgeologic map of the central block area,Yucca Mountain—Implications for structuraldevelopment of the potential high-level ra-dioactive waste repository area in NyeCounty, Nevada: Geological Society ofAmerica Abstracts with Programs, v. 28,no. 7, p. A–248.

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Dickerson, R.P., and Drake, R.M., II, 1995,Source of the rhyolite at Comb Peak,southwest Nevada volcanic field: GeologicalSociety of America Abstracts with Pro-grams, v. 27, no. 4, p. 8.

———1998, Geologic map of the PaintbrushCanyon area, Yucca Mountain, Nevada:U.S. Geological Survey Open-File Report97–783, 2 plates, scale 1:6,000.

Dickerson, R.P., and Spengler, R.W., 1994,Structural character of the northern seg-ment of the Paintbrush Canyon fault, YuccaMountain, Nevada, in High level radioactivewaste management, Proceedings of theFifth Annual International Conference, LasVegas, Nevada, May 22–24, 1994:LaGrange Park, Ill., American Nuclear Soci-ety, v. 4, p. 2367–2372.

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Geldon, A.L., 1993, Preliminary hydrogeologicassessment of boreholes UE-25c#1, UE-25c#2, and UE-25c#3, Yucca Mountain,Nye County, Nevada: U.S. Geological Sur-vey Water-Resources Investigations Report92–4016, 85 p.

Geslin, J.K., and Moyer, T.C., 1995, Summaryof lithologic logging of new and existingboreholes at Yucca Mountain, Nevada,March 1994 to June 1994: U.S. GeologicalSurvey Open-File Report 94–451, 16 p.

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southwest Nevada, in High level radioactivewaste management, Proceedings of theFifth Annual International Conference, LasVegas, Nevada, May 22–24, 1994:LaGrange Park, Ill., American Nuclear Soci-ety, v. 4, p. 168–174.

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5300 Brochure

MATH 1314 JEOPARDY $7500 $7400$7300$7200$7100$7000$6900$6800$6700$6600$6500$6400$6300$6200$6100$6000$5900$5800$5700$5600$5500$5400$5300$5200$5100$5000$4900$4800$4700$4600$4500$4400$4300$4200$4100$4000$3900$3800$3700$3600$3500$3400$3300$3200$3100$3000$2900

Уровнемер 5300

Cepra 5300

LIGHTWEIGHT OPTIONS - Western Star Trucks -- Home OPTIONS 4800 SF SB 4900 SF SB EX 4700 ... an extra 500 pounds you can use for payload. ... getting a Star is all about choices

RHAPSODY...8 COLORWAYS (A9 CABA) SAL 19 COLORWAYS (A9 4600) HERDADE 6 COLORWAYS (A9 4900) CARVALHAL 5 COLORWAYS (A9 4700) BLISS COMPORTA 5 COLORWAYS (A9 5000) RHAPSODY INDOOR/OUTDOOR