The Geology of the NevadaTest Site and Surrounding Area

Clark and Nye Counties, NevadaJuly 5-7, 1989

Field Trip Guidebook T 186

Leaders:Paul P OrkildH. Lawrence McKague Steven R Mattson

Contributions By:F M. Byers Bruce M. Crowe E. D. Davidson

Holly A. Dockery Terry A. Grant E. L HardinRobert A. Levich A. C Matthusen Robert C Murray

H. A. Perry Donna Sinks

American Geophysical Union, Washington, D.C.

Copyright 1989 American Geophysical Union

2000 Florida Ave., N.W., Washington, D.C. 20009

ISBN: 0-87590-636-2

Printed in the United States of America

THe Geology of the NevadaTest Site and Surrounding Area

COVER A view southeast across Mercury, NV towards the snowcovered Spring Mountains. Mt. Charleston is the high peak in theSpring Mountains near the left edge of the photograph. The bare ridgeJust beyond Mercury is the southwestern extension of the SpottedRange. Mercury Valley is the northwest extension of the Las Vegasshear zone.

Leaders:

Lawrence McKagueLawrence Livermore National Laboratory

P.O. Box 279, L-279Livermore, CA 94550

Paul OrkildU.S. Geological Survey

Steven MattsonS.A.I.C.Suite 407

Valley Bank Building101 Convention Center Dr.Las Vegas, NV 89109

IGC FIELD TRIP T186THE GEOLOGY OF THE NEVADA TEST SITE AND SURROUNDING AREA:A FIELD TRIP FOR THE 28th INTERNATIONAL GEOLOGICAL CONGRESS

H. Lawrence McKague(l), Paul P. Orkild(2), Steven R. Mattson(3)

With Contributions by

F. M,. Byers(4), Bruce M. Crowe(4), E. D. Davidson(3)Holly A. Dockery (1), Terry A. Grant(3), E. L. Hardin(3)Robert A. Levich(5), A. C. Matthusen(3), Robert C. Murray(3),

H. A. Perry(3), and Donna Sinks(3)

INTRODUCTIONThe Nevada Test Site (NIS) was

established on December 18, 1950, toprovide an area for continental testing ofnuclear devices. In January of 1951,testing began with an airdrop intoFrenchman Flat in conjunction withOperation Ranger. In addition to airdrops,above ground testing included surfacedetonations, tower shots, and balloonsuspensions. Underground testing began in1957, and since 1963, all events have beenburied in large-diameter drill holes ortunnels. Geologists from the U.S.Geological Survey (USGS) mapped much of theNTS region between 1960 and 1965. Thesemaps formed the basis for subsequentstudies by Lawrence Livermore NationalLaboratory, Los Alamos National Laboratory,Sandia National Laboratories, and theUSGS. A good understanding of thestratigraphy, structure, geochemistry, andphysical properties of the rocks isessential for containment of undergroundnuclear tests. Many of the recent geologicstudies at NTS, particularly in Yucca Flat,Pahute Mesa, and Mid Valley, are aimed atunderstanding subsurface geology to helpensure complete containment. The potentialnuclear waste site at Yucca Mountain islocated approximately 100 miles (160 km) byroad northwest of Las Vegas, Nevada, andsituated on land controlled by threeFederal agencies; the Bureau of LandManagement, the Department of Energy(Nevada Test Site), and the U.S. Air Force(Nellis Air Force Range).

(l)Lawrence Livermore National Laboratory,Livermore, CA(2)U.S. Geological Survey, Denver, CO(3)Science Applications InternationalCorp., Las Vegas, NV(4)Los Alamos National Laboratory,Los Alamos, NM(5)Department of Energy, Las Vegas, NV

By 1978 approximately thirty sites hadbeen identified as potential localities fora mined underground nuclear wasterepository. Work on the Yucca Mountainsite began in this year. In 1982 theNuclear Waste Policy Act was passed byCongress and in February 1983 the U.S.Department of Energy narrowed the field ofpossible sites to be characterized down tonine sites. In 1984 through considerationof the Draft Environmental Assessment foreach site the field was reduced to fivesites. In May of 1986 the environmentalassessments were published for each ofthese sites and based upon this informationthree sites were chosen for continuedinvestigation. These three sites includedYucca Mountain, Nevada (silicic ash flowtuff); Hanford, Washington (basaltic lavaflows); and Deaf Smith County, Texas(bedded salt). By an amendment to theNuclear Waste Policy Act, Congress decided,in late 1987, to characterize only onesite: Yucca Mountain, Nevada. The projectwas formerly known as Nevada Nuclear WasteStorage Investigations (NNWSI) Project andis now known as the Yucca Mountain Project(YMP). Since 1978, the massive ash-flowtuff beds under Yucca Mountain have beenintensively studied to determine theirsuitability as a radioactive wasterepository.

FIGURE 1 is a regional map that showsthe field trip route and many of thegeologic features of southwestern Nevada.The older rock sequence at NTS is composedof upper Precambrian and Paleozoic rockswhich were complexly deformed by Mesozoiccompressional tectonism. Table I is ageneralized pre-Cenozoic stratigraphiccolumn for the area covered by this fieldtrip. The stratigraphy can most easily bethought of as an alternating sequence ofcarbonate and clastic rocks (Table I, righthand column). The carbonate sequences actas aquifers. The clastic sequences usually

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FIGURE l. Map of southwestern Nevada showing major geologic structures,physiography and field trip stops.

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TABLE I

Pre-Cenozoic Rocks Exposed in Southwestern Nevada(modified from Orkild, 1982)

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act as aquitards. Tertiary volcanic rocksand Tertiary and Quaternary alluvium(Table II) overlie the older rocks andwere deposited concurrent with Cenozoicextensional faulting. The upper Mioceneash-flow tuffs and lavas found in thisarea issued primarily from the TimberMountain caldera complex located in thewestern part of NTS (FIGURE 1). Studiesperformed in conjunction with nucleartesting and radioactive waste isolationhave addressed many aspects of thegeologic history of NTS, which have inturn greatly enhanced our understanding ofthe geology of the southern Great Basin.

This guidebook has had a number ofpredessors. In the 1960s and 1970s theU.S. Geological Survey prepared severalinformal guidebooks. Some of the materialin this guidebook can be traced back tothese informal reports. The guidebookprepared by Dockery et al. (1985) was usedextensively for the geologic descriptionsalong U.S. 95, and at stops 1-5, 2-1, 3-2and 3-4. This guidebook is easier to usewhen traveling west from Las Vegas on US95.ROAD LOGFirst Day Las Vegas, NV to Beatty, NVMileage (cumulative)0.0 Mileage starts on I-15 and at the

Tropicana Avenue on ramp. Proceedtoward south on I-15. From1:00-4:00 the Spring Mountains are anorthwest-trending range ofpredominantly Paleozoic rocks. Thetransition from Paleozoic craton tomiogeosynclinal facies occurs fromeast to west across the mountains.To the east and south of here theuppermost Precambrian and Paleozoicmiogeosynclinal deposits aredrastically thinner, while to thewest and north they thicken. TheMcCullough Range is to the southeast(10:00-11:00). The northern part ofthis range is dominated by a 12 to15 Ma andesitic stratovolcano. Tothe south the volcanic section liesunconformably on the Precambrianbasement (Smith et al, 1988).

3.6 Exit 33 to Pahrump, on NV 160.At 2:00 are the Wilson Cliffs,massive, light colored cliffscomposed of Triassic (?) andJurassic Aztec Sandstone.

6.4 Railroad crossing (mileagecheck). Hills at 9:00 are composedprincipally of Pennsylvanian BirdSpring Formation overlying theMississippian Monte Cristo

Limestone. Blue Diamond Hill is at2:00.

10.7 High peak at 11:00 is PotosiPeak. Potosi Mountain is underlainby folded and faulted Ordovician toMississippian carbonate rocks.Lead, silver, and some zinc havebeen mined in this area.

13.6 Road junction to Blue Diamond andRed Rock Canyon. At 11:00 is blueDiamond Hill where gypsum is minedfrom four gypsum units in the LowerPermian Kaibab Limestone (Papke,1986). Based on lithology, presenceof redbeds and minor dolomite, andthe occurrence of nodular gypsum,the Blue Diamond gypsum deposit isbelieved to have formed in a sabkhaenvironment. The gypsum is mined inopen pits and used for buildingmaterials.

15.0 A large outcrop of Tertiary(?)nonmarine limestone occurs at 2:00(Bohannon and Morris, 1983).Resting on Triassic MoenkopiFormation, Burchfiel and Davis(1988) show this contact as the BirdSpring thrust (see stop 1-1).

19.2 Stop 1-1. Spring Mountains GeologyThree Mesozoic thrust sheets

occur along the east front of theSpring Mountains (Burchfiel andDavis, 1988). These thrusts arepart of a continuous foreland foldand thrust belt that extends throughthe United States from Canada toMexico. In much of the Basin andRange, Cenozoic extensionaldeformation has obscured therelationships between the thrustsystems. However, the SpringMountains have been little affectedby the Cenozoic deformation andprovide an opportunity to work outMesozoic structural relationships.

Three thrust fault systems havebeen recognized in this area. Theyhave resulted in an estimated 37 to45 km of shortening (Burchfiel etal., 1974; Burchfiel and Davis,1988). They are, from east to west,the: 1) Bird Spring, 2) RedSpring-Wilson Cliff-Contact, andKeystone thrust systems. Thethrusts systems are higher andyounger from east to west (Burchfieland Davis, 1988).

The Bird Spring fault occurs tothe east of the hills northeast ofthis stop. It appears to be theoldest of the thrust faults and

TABLE II

PRINCIPAL GENOZOIC VOLCANIC AND SEDIMENTARY UNITS(modified from Orkild, 1982 et al 1986

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marks the western limit of theautochthon of the North Americancraton (Burchfiel and Davis,1988). The next thrust systemconsists of the Red Spring thrust tothe north at the base of the LaMadre Range; the Wilson Cliffsthrust which occurs at the top ofWilson Cliffs to the northwest; andthe Contact thrust to the south ofthe Cottonwood fault. These threefaults are considered by Burchfieland Davis (1988) to be part of thesame system. Mapping by Burchfieland Royden (1984) has shown that theKeystone thrust occurs above and tothe west of the Wilson Cliffs thrust.

The Cottonwood fault is ahigh-angle northwest-strikingfault. While it offsets the WilsonCliffs-Contact thrusts, it causeswarping in the Keystone plate and isconsidered to be post Keystone byBurchfiel and Davis (1988). At thisstop the fault is to the west andpasses through the mountain in thecanyon the road follows.

To the southwest, dark-grayCambrian to Pennsylvanian carbonaterocks underlie Potosi Mountain.These rocks form a wedge between theContact and the Keystone faults.Along the skyline to the south, thePaleozoic rocks are folded into ananticline overturned to the east.Further to the south several otherfolds rest directly above theContact thrust.

20.6 In the canyon at 2:00 theCottonwood fault drops thrustedPaleozoic rocks against the AztecSandstone. A small younger fault onthe south side of the road placesalluvium against Paleozoic limestone.

22.9 Mountain Spring Pass Summit(elev. 5493 ft). Historic markerfor Old Spanish Trail. To reachthis pass from Las Vegas was a 2 daytrip by wagon before theautomobile. This trail ran fromSanta Fe, NM to Mission San Gabrielnear Los Angeles, and was later usedby many who went in search ofCalifornia gold in 1849 and later.

25.7 Spring Mountains at 2:00. Hillsat 12:00-1:00 are Mesozoic rocks innormal stratigraphic sequence aboveKeystone thrust. Ridge of KaibabLimestone at 1:00. Steeply dippingBird Spring Formation at 10:00.

27.8 Across Pahrump Valley the Nopah

Mountains in California are visibleat 10:30-1:00.

36.6 Folded Triassic rocks at 3:00.Red Triassic continental rocks andgray Kaibab Limestone surround acore of Bird Spring Formation in asouthward plunging syncline.

38.9 Junction to left with road toTecopa, CA. At 2:00 CharlestonPeak, elevation 11,918 feet, ishighest peak in Spring Mountains.

41.1 View of Pahrump Playa at 12:00.Trout Canyon at 3:00. The LeeCanyon thrust (Stop 3-6) emergesfrom Spring Mountains along thesouth side of Trout Canyon.Cambrian limestones and dolomitesare thrust over Pennsylvanian BirdSpring formation.

42.9 Pahrump, NV at 12:00. At1:00-2:00 the hill is a doublyplunging anticline of Devonianthrough Pennsylvanian and Permiancarbonate rocks.

47.7 Nye County line. 1:00-3:00Devonian Sultan Limestone underlyingMississippian Monte Cristo Limestone.

50.4 Dissected lake beds at 3:00.52.2 Town limits of Pahrump, NV.

(Mileage check.)53.6 Left on Route 372 to west. Nopah

Range at 11:00-12:00, and theResting Spring Range at 12:00-1:00.

60.4 The low hills on both sides ofthe road are underlain bymegabreccia of Bonanza KingFormation.

60.6 Stop 1-2 Stewart Valley, NopahRange, and Resting Spring RangeGeology

Bonanza King megabreccia to thenortheast, California and theResting Spring Range across thevalley to the west. Northern end ofNopah Range is to the southwest.Chicago Valley is between theranges. Chicago Pass thrust is bestseen on the northern Nopah Range.The jagged topography is underlainby various overturned units ofCambrian through Mississippiancarbonate rocks in the Shaw thrustplate described by Burchfiel et al.(1983). The jagged morphology ofthese gray-weathering carbonaterocks gives way abruptly to theright to smooth, brown-weatheringslopes formed by Precambrian andCambrian clastic rocks of theChicago Pass thrust plate (Wernickeet al., 1988). The thrust dips

shallowly off to the north betweenthese two units. Wernicke et al.(1988) correlate this thrust withthe Wheeler Pass thrust in theSpring Mountains. Both thrustsplace Precambrian clastic rocks onBird Spring strata, a stratigraphicthrow of about 5000 m.. Continuenorthwest on Stewart Valley road.

62.4 At 3:00 Late Proterozoic StirlingQuartzite (reddish brown) in faultcontact with small hill of grayBonanza King megabreccia.

65.2 Wood Canyon Formation (darkreddish brown) at 3:00. Behind andto the north, with vertical,light-colored fissure fillings, andforming the high jagged hills is theBonanza King Formation

67.4 Wood Canyon Formation-StirlingQuartzite contact at 10:00. BothWood Canyon and Stirling Quartziteform smooth rounded hills. The highpeak is Shadow Mountain, thenorthernmost part of Resting SpringRange.

70.7 Road junction, left at fork.Hill at 2:00 is Bonanza KingFormation dipping 30 to west.Amargosa Valley at 12:00.

73.5 Tertiary tuffaceous sediments inhill at 3:00. Smooth hills tosouthwest (9:00) are underlain byWood Canyon Formation and StirlingQuartzite.

75.0 On both sides of the roadlight-green altered tuffaceoussediments, capped by Quaternaryalluvium. The Tertiary tuffaceoussediments are commonly composed ofash-fall tuffs and tuffaceoussedimentary rocks. Low temperatureground-water alteration iswidespread; clinoptilolite (zeolite)the most common alteration product.

77.9 Junction, turn right.78.6 Keep to the left at the clay pit

with altered green tuffaceoussediments.

79.1 Sign to East/West Minerals,proceed straight ahead. Southern endof Ash Meadows spring system at3:00. Ash Meadows Ranch on theright.

79.7 Tuff mounds on both sides ofroad. Funeral Mountains acrossvalley (9:00).

82.1 Junction, turn right at T. Pointof Rocks Ridge, at 3:00, isunderlain by the Banded MountainMember of the Bonanza King Formation.

83.4 Turnoff to Devils Hole to left(west).

83.5 Stop 1-3. Devils Hole Geologyand Hydrology

The Devils Hole area is locatedwithin a series of northwest-striking steep ridges of BonanzaKing Formation that are controlledby northwest-striking folds andfaults in the Paleozoic rocks (Carr,1988). Although northwesterlytrending structural grain is themost prominent trend in the area,small faults and fractures ofnortheast strike are the mostimportant hydrologically. Theycontrol the location of Devils Hole,other collapse depressions and theorientation of calcite veins in thebedrock.

From general relationships in thearea, it is concluded that most ofthe structural disturbances occurredwell before about 4 Ma, butestablishing the age of undatedolder Tertiary rocks in and near AshMeadows is critical to dating theperiods of important structuralactivity. The openings at DevilsHole considered together withorientation of the sinkholes,fractures, and faults in thesurrounding area, are, in accordwith a general stress field model(Carr, 1974, 1984) for this part ofthe Great Basin. In the last 5million years or so the minimumprincipal stress direction has beenoriented northwest-southeastaccording to Carr's model. Faultsand fractures of northwest strikewould tend to be closed, whereasthose of northeast strike would tendto open. The youthful appearance ofsome of these features suggests thatthe proposed stresses are currentlyactive.

The depressions in carbonate rockrange from holes or shallowdepressions approximately 0.3 m indiameter, to Devils Hole, which isan opening approximately 22.7 m by7.6 m in plan view and 15.2 m deep.Devils Hole is the only such featurein the Ash Meadows area whosesubsurface extent is even partiallyknown. According to Alan C. Riggs(U.S. Geological Survey, writtencommun., 1985; 1986), Devils Holeextends to more than 300 feet belowthe water table, and a network ofpassages extends at least 91.4 m to

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the northeast, including anuncollapsed subterranean openingabove the water table called"Brown's room". The average widthof the passageways is about 1.8 m.

The passageways and room wallsseem to be largely controlled byfault planes. Riggs (U.S.Geological Survey, written commun.,1985) reports that, near thenorth-eastern end of "Brown's room",a chimney extends about 15.2 m abovethe water table, which is at analtitude of about 719 m. Surfaceelevation above this point isestimated to be about 739 m, so thatthe room probably comes to withinabout 3.0 or 4.6 m of the groundsurface, yet no sign of surfacecollapse is evident. It is believedthat most of the structuraldisplacements in the Devils Holearea occurred well before about 4Ma.

The Devils Hole area is highlytransmissive hydrologically asevidenced by the large discharge ofwater from many springs in the AshMeadows area (Winograd andThordarson, 1975). Winograd andPearson (1976) discussed animportant carbon-14 anomaly in theage of water being discharged fromthe larger springs. Briefly, theyfound that water from all but one ofthe springs had a similar carbon-14content (about 2.4-percent modern),whereas one of the largest springs,Crystal Pool had a carbon-14 contentalmost 5 times larger (about 11.1percent modern) than all the otherspring waters analyzed. Theyconclude that water discharging fromCrystal Pool is following apreferred pathway from rechargeareas many miles to the northeast.This pathway enables the water tomove much more rapidly than waterreaching other springs. BecauseCrystal Pool is centrally locatedwithin the discharge zone, it islikely that a natural pipeline or"megachannel" is located relativelynear the discharge area.Furthermore, the flow direction isprobably from the northeast(Winograd and Thordarson, 1975), adirection normal to the alignment oflarger springs at the spring line,so that the ground water thatdischarges at Crystal Pool must passthrough the Devils Hole area. The

top of the regional lower clasticaquitard (Winograd and Thordarson,1975) is about 1,067 m below thewater table in the Devils Hole area.

83.7 Return to road and turn left(north) on Ash Meadows-CrystalRoad. The hills on both sides ofthe road are Bonanza King Formation.

85.7 Summit-Amargosa Flat and SpecterRange at 12:00, Spring Mountains at2:00.

87.6 Stop 1-4 Amargosa Desert claydeposits

The Amargosa Desert is anintermontane basin that drainssouthward. It is underlain andnearly surrounded by highly foldedand faulted Paleozoic rocks such aswere seen at and between the earlierstops. To the northwest the basinis bordered by Miocene volcanicrocks. These basement rocks areoverlain by an assemblage ofmoderately to highly deformedfanglomerates, siltstones,limestones, and tuffs that have beencorrelated with the Upper MioceneFurnace Creek Formation of DeathValley (Naff, 1973). Unconformablyoverlying these rocks are slightlydeformed Pliocene and Pleistocenesediments that fill the presentgeographic basin (Hay et al.,1986). These deposits consistlargely of spring related carbonaterocks, Mg clays, and detritalmontmorillonite-rich claystones.Poorly indurated clastic andcalcareous sediments occur in thewest central and northern parts ofthe basin.

The Pliocene Mg clay deposits ofthe Amargosa Desert are the largestknown deposits in the western UnitedStates (Hay et al., 1986). These2.3-3.2 Ma sediments were depositedin flood plains, marshlands, ponds,and playas. Three laterallyequivalent lithofacies wererecognized by Pexton (1984). Onelithofacies occurs within 1 to 2 kmof the Paleozoic hills to the southof this stop. This facies ischaracterized by a continuous beltof limestones with interbeddedclay. The limestones wereprecipitated in marshlands and pondsfed by springs in Paleozoic hills tothe south. Hay et al. (1986),interpret the distribution oflimestone to indicate perenniallywet conditions near the Paleozoic

carbonate hills grading outward intoan area in which wet and dryconditions alternated.The second facies consists largely

of claystones and limestones. Themajor clay mineral is detritalmontmorillonite that was derivedfrom the volcanic rocks to the north(Hay et al., 1986). These sedimentscccur to the west of the firstfacies, and are its basinwardextension.

The third facies occurs north ofthe first facies and represent playaand marshland environments. Thisfacies is characterized by Mg clay,limestones, and dolomites. Mgsmectite is the dominant claymineral. Deposits of relativelypure sepiolite occur within thisfacies, such as at this stop.Authigenic illite and K-feldspar andoxygen isotope data (Hay et al.,1986) indicate the water in theplaya was saline and alkaline andresulted from evaporation.Sepiolite deposited from lowsalinity water is found in or nearareas of spring discharge.

The distribution, mineralogy, andgeochemistry of these clay depositssuggest that in the Piiocene springswere more widespread and had greaterdischarge. This wetter period endedabout 2.5 Ma. Return to AshMeadows-Crystal road and turn north.

88.6 Turn left (west), onto privateroad, drive with caution, watch fortrucks.

88.2 Specter Range to north. Darkercolored, lower Paleozoic formationsto west; to east Silurian carbonaterocks form light colored, higherhills. Funeral Mountains 12:00,Death Valley is on west side ofmountains.

95.6 Fairbanks Spring at 9:00 is thenorthernmost spring of the AshMeadows spring system.

100.8 Junction with State Route 29 turnright (north). Highway to left goesto Death Valley Junction, CA, and toDeath Valley via Furnace Creek Wash.

113.7 Stop 1-5 Geology southern portion ofNTS

Stop is at a roadside park inVillage of Amargosa Valley, NV,formerly Lathrop Wells, NV. To thenorthwest is Yucca Mountain, sitefor the proposed high level nuclearwaste repository. Skyline behindYucca Mountain is Pinnacles Ridge,

which forms the south rim of theTimber Mountain caldera. Alsovisible are Fortymile Wash, east ofYucca Mountain, and the varicoloredvolcanic rocks of the Calico Hillsfurther to the east.

The southwestern Nevada volcanicfield is a complex assemblage ofrocks covering an area of several-thousand square miles, mainly insouthern Nye County. Most of theserocks are silicic ash-flow tuffs.The central area of the field is anormally faulted and dissectedvolcanic plateau of about 2,500mil2 (6,475 km2) that extendsnorth from the village of AmargosaValley. Several volcanic centershave been located in thesouthwestern Nevada volcanic field;and those associated with thelarge-volume ash-flow unitsgenerally are collapse calderas.Calderas in the immediate vicinityof the NTS include Silent Canyoncaldera, Crater Flat caldera, ClaimCanyon caldera, the Timber Mountaincaldera, and the Black Mountaincaldera (FIGURE 2). Collectively,greater than 3000 km3 of tufforiginated from these calderas.Several other calderas in thesurrounding region have beendescribed by various Investigators.

The Tertiary volcanic section ofthe southwestern Nevada volcanicfield includes many units. Oneneeds to be familiar with only a fewof these units for the purposes ofthis trip, but Table II lists allthe major Tertiary units of the NTSarea for general reference. Majorunits related to the same volcaniccenters and having mappablelithologic and petrographicsimilarities are named asformations; individual ash-flowcooling units (ignimbrites) aredesignated as members.

117.0 Fortymile Wash crosses U.S. 95.To left at 11:00 is Big Dunecomposed of eolian sand.

119.3 Southernmost end of YuccaMountain just north of U.S. 95 onright. Outcrops are MiocenePaintbrush and Crater Flat Tuffsrepeated by northeast-strikingfaults.

120.2 Lathrop Wells cinder cone at 3:00.(Stop 2-1.)

121.2 Outcrop of vitrophyre in MioceneRainier Mesa Member of Timber

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FIGURE 2. Map showing outline of calderas in southwestern Nevadavolcanic field. Stonewall Mountain northwest of Black Mountain calderais not shown. Modified from Dockery et al., 1985, Figure 2.

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Mountain Tuff at 3:00.Crater Flat is to the north,

Yucca Mountain is at 3:30, the southwall of Timber Mountain caldera ison skyline at 3:00.

Hills are composed ofmonolithologic megabreccia of theCambrian Bonanza King Formation ontop of Miocene tuffs.

Junction to right with road toCrater Flat through Steves Pass. At2:00 is south end of Bare Mountain.Low rounded foothills, are underlainby Stirling Quartzite, Wood CanyonFormation, and Zabriskie Quartzite.The latter underlies Wildcat Peak.Mountain at 4:00 is a slide block,from Bare Mountain, of CambrianBonanza King Formation overlyingMiocene tuffs.

131.4 At 9:00 - U.S. Ecology "Low LevelRadioactive Waste Repository". TheBeatty, NV disposal site was thefirst licensed commercial disposalsite in the United States (Clancy,et al., 1981). The site, licensedby the U.S. Atomic Energy Commissionin 1962, is owned by the State ofNevada and operated by U.S. Ecology,Inc. The site is 80 acres andincludes a non-radioactive chemicalwaste site adjacent to the low-levelradioactive waste site.

The valley floor is alluviumcomposed of detritus from thesurrounding mountain ranges. Basedon a drill hole and surface gravitymeasurements, the alluvium isestimated to be 175 m thick(Clebsch, 1962, Nichols, 1987). Thebedrock is probably Paleozoicformations, similar to those at BareMountain to the north.

The principal drainage is theAmargosa River, which is dry most ofthe year. Average annual rainfallis from 63.5 to 127 mm, but canrange from 22 mm to 250 mm (Clancyet al., 1981).

The low-level waste has generallybeen disposed at the site by meansof a cut-and-cover trenchoperation. The dimensions oftrenches are variable with lengthsfrom 91.4 to 198.1 m, widths from1.2 to 27.4 m and depths from 1.8 to9.1 m (Clancy et al., 1981). Aminimum of 0.3 m of cover isrequired above the waste. Inaddition, the trench cover must bemounded to provide drainage awayfrom the trenches.

135.1 Road to Carrara townsite at3:00. In the early 1900s marblequarries were established atCarrara. The marble was toostrongly fractured and weathered tobe marketable. A complete, northdipping Proterozoic and Paleozoicmiogeosynclinal section is exposedalong the range.

138.8 Beatty Mountain at 12:30.139.5 Beatty scarp at 3:00. Trenching

across the 7 km long lineament hasnot revealed a fault, and has beenattributed to erosional processes,based on mapping and geophysicaldata (Swadley et al., 1987).

140.2 Left turn on road to Beattyairport.

143.4 Intersection with Nevada 58between Beatty and Death Valley.Turn left.

145.2 Road to Rhyolite - right turnfrom Nevada 58.

149.0 Road to left. Remnants ofbuildings and dwellings to north arethe historic town sites of Bullfrogand Rhyolite.

150.5 Road to right, go west.152.6 Y. go to west.153.1 Junction, go left, mine and

exploration workings for gold arepreserved on hills and slopes north.of road.

155.0 Junction, go to left on oldrailroad right of way.

155.6 Heap leach to left. Piles of 1-5cm size crushed ore are treated withcyanide solution to leach out gold.

155.7 Entering Death Valley Monument(mileage check).

155.9 Stop 1-6 Original Bullfrog mineThis mine was discovered in 1904

and described by Ransome et al.(1910). At this stop a metamorphiccore complex and associatedlow-angle detachment faults can beseen. Along the south side of theBullfrog Hills the extended terraincontains at least two proposeddetachment faults. At this stop theupper detachment fault separates amiddle plate composed of a highlyattenuated incomplete sequence oflower and middle Paleozoic clasticand carbonate rocks from anoverlying sequence of block-faultedMiocene volcanic, volcaniclastic,and sedimentary rocks (Maldonado,U.S.G.S. 1988, private comm.). Aproposed lower detachment fault, notexposed at this stop, separates the

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middle plate from a lower plate(?)of amphibolite grade metamorphicrocks. These Proterozoic rocks areexposed in the hills approximately 1mile southwest of this stop. Theserocks consist of quartzofeldspathicgneiss, biotite schist, marble, andamphibolite, all cut by graniticpegmatites. These rocks havetentatively been correlated with theJohnnie Formation (Table I). K-Arage on minerals from these rockssuggest the uplift and cooling ofthe metamorphic complex occurredduring the late Miocene (16.3-10.5Ma).

The Miocene rooks of the upperplate dip moderately to steeply tothe east into the upper detachment,or where the middle plate is notpresent into the lower detachment.The Miocene rocks are repeatedlyfaulted and tilted resulting in aterrain extended generally more than100 percent and locally more than200 percent. These detachmentsurfaces have been mapped 22.5 km tothe east along the north side ofBare Mountain. East of Beatty, NVthis fault is called the FluorsparCanyon fault. The faulting of a 6.5Ma tuff in the upper plate suggeststhe extensional faulting occurred asrecently as 6.5 Ma. As notedearlier, the uplift and cooling ofthe metamorphic core complexoccurred between 16.3 and 10.5 Ma.Thus the period of extension appearsto have extended at least from 16.5to 6.5 Ma. The recent discovery ofeconomic amounts of finelydisseminated gold in the Miocenetuffs and Paleozoic carbonate rocksabove and below the detachment hasrevived the mining interest in thisarea. Return to Nevada 58.

162.4 Junction with Nevada 58, turnleft.

156.1 Third and Main, Beatty, NV, turnright, proceed south on U.S. 95.

167.4 Amargosa Narrows, turn left onFluorspar Canyon road, proceedacross the Amargosa River on gradeddirt road.

168.0 Outcrop of Cretaceous graniticsill. Turn to left.

168.4 Stop 1-7 Fluorspar Canyon fault.The Fluorspar Canyon fault passes

through the saddle on the southflank of Beatty Mountain,approximately 100 m west of the mineroad (Carr and Monsen, 1988).

A gently dipping fault (25 N)is proposed separating middleMiocene volcanic and sedimentaryrocks above from the underlying lateProterozoic and Cambrianmetasedimentary rocks. An exhumedsurface on top of the carbonaterocks south of the saddle reflectsthe approximate attitude of thefault. In the hanging wall plate ofthe fault, Miocene volcanic rocksare tilted eastward in blocksbounded by moderately to steeplynorthwest-dipping faults. Thefaults in the hanging wall plate arepostulated to terminate at or mergewith the low-angle Fluorspar Canyonfault at depth.

Rocks in the footwall are part ofa structurally attenuated section oflate Proterozoic and Cambrianmetasedimentary rocks, betterexposed in Conejo Canyon to the east(Carr and Monsen, 1988). Theserocks dip northward in normalstratigraphic order, but largeintervals of the stratigraphicsection have been cut out alonglow-angle faults that nearlyparallel bedding. The uppermost ofthese units (Middle Cambriancarbonate rocks) has been intrudedby a granitic sill, which is datedas Cretaceous on the basis ofU-Th-Pb age data from includedzircon (Carr and Monsen, 1988).

According to Carr and Monsen(1988) the Fluorspar Canyon fault ispart of a regional, low-angle normalfault system that propagated to theland surface in the Bare Mountainarea during the late Miocene. Thefault projects westward across theAmargosa Narrows, where it continuesas the low-angle Original Bullfrogfault (Ransome et al. 1910) at thebase of the extended Tertiaryvolcanic terrane of the BullfrogHills (Stop 1-6). The fault isproposed to extend eastward to thehead of Fluorspar Canyon, Return toroad.

168.7 Turn left up canyon.171.3 Left turn. At 12:00 is Daisy

Fluorspar Mine. The fluorspardeposit occurs in dolomite of theNopah Formation (Cornwall andKleinhampl, 1961).

171.6 Bullfrog Member of Crater FlatTuff to left - across fault.

172.3 Road to left goes to Crater Flat.172.6 Stop 1-8 Extended Terrane North

of Fluorspar Canyon Fault (Optional)At this stop we can view the

extensional terrane above theFluorspar Canyon fault to the north(Carr and Monsen, 1988). The smallridge of volcanic rocks to the northcontains a conformable orparaconformable sequence of middleMiocene tuff and tuffaceoussedimentary units, dippingmoderately eastward. To thenortheast, this section flattensbeneath the volcanic mesas that leadto the Timber Mountain caldera; therugged terrain on the northeasternhorizon. The middle Miocenegeomorphology seems only slightlymodified in that area. To thenorthwest, the same middle Miocenesection forms the jumble offault-bounded blocks that are tiltedeastward along northwestward-dippingfaults in the upper plate of theFluorspar Canyon fault. Theyoungest rocks affected by thisfaulting are basalt inferred to be10.5 Ma by Carr (1984). The basaltis apparently conformable with the,sequence of middle Miocene ash-flowtuffs.

Nearly flat-lying alluvial fandeposits, which are inferred to belate Miocene materials on the basisof their similarity to isotopicallydated deposits east of Bare Mountain(7.7-8.7 Ma, J. K. Nakata, writtencommun., 1986, in Carr and Monsen,1988), lap unconformably over thestrongly faulted and tilted middleMiocene rocks north of BareMountain. These relationshipssuggest that much, if not all, ofthe extension above the FluorsparCanyon fault occurred between about10.5 and 7.5 Ma in the Bare Mountainarea. The boundary between thestrongly deformed extensionalterrane to the northwest and theless-deformed plateau to thenortheast is the fault on the westside of the small ridge north ofthis stop. This fault isinterpreted as part of the breakawayfor the Grapevine-Bullfrog-northernBare Mountain extensional allochthon.Retrace route back to Beatty.

END DAY 1Second Day Beatty, NV to Yucca Mountainmileage (cumulative)

0.0 Leave Burro Inn travel east fromBeatty on Highway 95

0.9 Amargosa Narrows, Fluorspar

Canyon turn of f2.0 Original Bullfrog/Fluorspar

Canyon detachment fault is near baseof hills at 3:00

2.4 Airport Road (mileage check)

10.2 See mile 131.4 first day.12.8 Road to Crater Flat. At 10:00

gravity slide of Bonanza KingFormation from Bare Mountain, sittingon Tertiary sediments.

16.6 Lake beds containing camel andmastadon fossils at 10:00.

18.6 View of Yucca Mt at 9:3019.6 Type locality of Crater Flat Tuff

(slow drive by). Miocene volcanicunits and type locality of theCrater Flat Tuff (Dockery et al.,1985). This section from baseupward consists of: (1) vitricash-fall tuffs overlain by (2) aboulder debris flow (yellowish-greenlayers near base of hill), (3)Bullfrog Member (dark vitrophyrenear base) and Prow Pass Member ofthe Crater Flat Tuff, and (4)Topopah Spring and Tiva CanyonMembers of the Paintbrush Tuff (onskyline). This is the only knownsection where the Crater Flat Tuffis vitric and unaltered; the TramMember, the oldest unit, is missinghere.

20.3 Southernmost end of YuccaMountain just north of U.S. 95 onleft. Outcrops are MiocenePaintbrush and Crater Flat Tuffs(see Table II) repeated bynortheast-striking faults.

21.3 Stop 2-1 Lathrop Wells cindercone (Road side stop) The CraterFlat area (also see Stop 2-4)contains over 15 small basalticvolcanic centers composed of cindercones and associated lava flows.Only the youngest center is visibleat this stop. The distribution,petrology, and tectonic setting ofthe basalts have been described byCrowe and Carr (1980), Vaniman andCrowe (1981), Vaniman et al. (1982),Crowe et al. (1982), and Crowe etal. (1983a and 1983b). The rocksare divided into three eruptivecycles based on geologic fieldrelations, potassium-argon ages, andmagnetic polarity determinations.The K-Ar ages listed below were doneby R. J. Fleck, USGS (writtencommun., 1979, in Dockery et al.,1985) and R. F. Marvin, USGS(written commun., 1980, in Dockery

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et al. 1985).3.7-m.y. cycle Rocks of the

oldest cycle consist of deeplydissected cones and flows withlocally exposed feeder dikes. Theyoccur in the central andsoutheastern part of Crater Flat(Fig. 1).

1.2-m.y. cycle (Ob) Basalticrocks of this cycle consist ofcinder cones and lava flows locatedalong a northeast, slightly arcuatetrend near the center of Crater Flat(Fig. 1). From northeast tosouthwest, the major centers in thiscycle include unnamed cone, Black,Red, and Little Cones.

100.000 yr cycle (Ob) Theyoungest cycle is marked byessentially undissected cones andflows of the Lathrop Wells center.

The Lathrop Wells volcanic centeris the youngest basalt center in theYucca Mountain region and isbelieved to be 20,000 yrs or less inage (Wells et al., 1988; Crowe andTurrin, in prep). It consists ofblocky aa lava flows that wereerupted from multiple sources alongthree fissures. These include twoen echelon northwest-trendingfissures located northeast of thescoria cone and a thirdeast-northeast trending fissurelocated north of the scoria cone.The main scoria cone overlies thefissure- and lava-flow units.Tephra from the youngest eruptionsof the main scoria cone overliesmall satellite scoria cones thathave mostly been removed byquarrying activity. Pyroclasticsurge deposits are presentthroughout the main scoria conesequence and along the northwest andsoutheast flanks of the scoriacone. They also are locally presentamong the older fissure depositsindicating episodes of hydrovolcanicactivity throughout the eruptivestages of the center.

The age of the volcanic centerhas proven to be problematic. Wholerock, K-Ar age determinations ofsamples of the lava flows haveyielded ages that range from 730,000to 8,000 yrs B.P. (Sinnock andEasterling, 1983). The inconsistentresults are thought to be from avariable component of excess Ar andaccordingly the youngest ages mayrepresent the true age of the lavas

(40,000 to 70,000 yrs B.P.).Several lines of evidence indicatethe main scoria cone issignificantly younger than the lavasand may be as young as or youngerthan 20,000 yrs B.P. This evidenceincludes: 1) smooth, unrilled coneslopes, 2) lack of development of acone-slope apron, 3) poordevelopment of soils on the coneslopes, 4) presence of local soilzones between deposits of the scoriacone and older lavas, and 5)differing measured paleomagneticpole positions for the scoria coneand the lavas. These data require apolycyclic origin (multipleeruptions separated by significanttime intervals) for the LathropWells volcanic center.

The ages of the Quaternaryalluvial deposits are consistentwith ages for the basalt. Beforethe eruptions, alluvium of middlePleistocene age locally developed adense K-horizon that gave a uraniumseries age of about 345,000 yr B.P.The pyroclastic material becameincorporated locally in upperPleistocene alluvium, and a loessialsilt deposit accumulated on thecinder cone and regionally on the Q2alluvium before about 25,000 yr ago.

The structural control for thelocation of the center is notobvious. The cone, summit crater,satellite cones and fissure systemsare aligned northwesterly, probablydue to northwest-trending structuralcontrol. Faults strikingnorth-northeast are also presentthough poorly exposed. The centeris located on a regionalnortheast-trending structurallineament marking the western edgeof the Spotted Range-Mine Mountainnortheast-trending structural zone(Fig. 1); faults west of thislineament have a more northerlytrend. It is suggested that thestrike of the faults influenced thelocation of the Lathrop Wellscenter; that is, the eruptions werefed from dikes whose trends werecontrolled by the regional stressfield, with a NE least compressivestress direction.

The basalts of the Lathrop Wellscenter are sparsely porphyritic witholivine as the major phenocrystphase (3 modal percent). They differfrom the 1.2-m.y. cycle basalts by

having a slightly greater olivinecontent and a greater amount of

unaltered basalt glass. Also theccres of olivine phenocrysts areslightly more forsteritic(F8 0 7 7 ) than olivines of the

1.2-m.y. cycle (F077 -76), asdetermined by electron microprobeanalysis. Ground mass phases alsoinclude plagioclase (zoned fromAm68 to more alkalinecompositions) and minor amounts ofolivine, pyroxene, and iron-titaniumoxides plus interstitial glass.Textures of the basalts of theLathrop Wells center arehyalopilitic to pilotaxitic. Adetailed discussion of themineralogy and geochemistry of theLathrop Wells center is found inVaniman and Crowe (1981) and Vanimanet al. (1982).

22.2 South end of Yucca Mt.Specter Range 12:00Spring Mountains 1:00Rock Valley 11:00Funeral Mountains 3:00-4:10Calico Hills 9:00-9:30Amargosa Valley 2:00-3:00Yucca Mountain 8:00-8:30

24.7 Fortymile Wash.27.2 Road to NTS, turn left.28.2 Striped Hills 3:00, Little Skull

Mt 2:00.29.1 Entrance to NTS through Lathrop

Wells guard gate. Badge check.33.2 At approximately 3:00 view Little

Skull Mountain, capped by Miocene(approximately 10 Ma., R. F. Marvin,USGS, written commun., 1980 inDockery et al., 1985) basalt ofSkull Mountain, underlain by faultedMiocene Topopah Spring Member of thePaintbrush Tuff and tuffs of theWahmonie Formation. Low hills atfoot of mountain contain outcrops ofthe Tram and Bullfrog Members of theCrater Flat Tuff. Busted Butte at10:00 is a complete section of theTopopah Spring Member overlain byTiva Canyon Member of the PaintbrushTuff. Low, white water tank is atWell J-12 at the edge of FortymileWash. Long ridge on skyline tonorthwest 11:00 is Yucca Mountain.

37.1 Low hills at 10:00 are TopopahSpring Member capped by 9.6 Ma.basalt of EMAD (R. F. Marvin, USGS,written commun., 1980, in Dockery etal., 1985).

38.5 North side of Skull Mountain at1:30. From top to bottom is basalt

of Skull Mountain, Rainier MesaMember of Timber Mountain Tuff,Topopah Spring Member of PaintbrushTuff, and Wahmonie lavas.

40.7 Turn left onto road next to theNevada Research and Development Area(NRDA) facility.

40.8 Stop 2-2 DOE Sample ManagementFacility (SMF)

The U.S. Department of Energy(DOE) operates a state-of-the-artSMF which processes, documents, andpreserves Yucca Mountain Projectgeologic samples to satisfy qualityassurance requirements for licensinga geologic repository. The SMFincludes the physical facilitydesigned to process and preservethose samples, as well asmanagement, quality assurance, andoperations staff. One of the SMF'sprimary responsibilities is todocument the life cycle of a samplefrom the time it is collected in the.field, through transport,processing, analysis, and storage.

The SMF staff includes a manager,a curator, a facilities geologist, afield operations manager, severalgeotechnicians, and other supportstaff which are experienced andtrained in sample management andquality assurance (QA) code andstandards. The SMF is operated byScience Applications InternationalCorporation, the current Technicaland Management Support Servicescontractor.

42.3 On skyline at 12:00 ShoshoneMountain is capped by 8.9 Ma.rhyolite lavas (R. F. Marvin, USGS,written commun., 1980, in Dockery etal, 1985).

42.8 Turn left and proceed west towardYucca Mountain.

43.4 At 9:00 is the EngineMaintenance, Assembly andDisassembly (EMAD) facility toleft. Originally used for nuclearrocket engine maintenance, nowoperated by Westinghouse forhandling and temporary storage ofnuclear waste.

43.9 Rocket assembly facility at 3:00,one of several built in conjunctionwith Nuclear Rocket DevelopmentStation in 1960s.

46.9 Busted Butte at 11:00.48.4 Road to water Well J-13. Fran

Ridge at 12:00 is composed ofTopopah Spring Member, overlain bylight-colored bedded tuff and Tiva

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Canyon Member.48.6 Crossing Fortymile Wash.49.1 Turn left at sign for drill hole

USW G-3.51.3 Round southern end of Fran

Ridge. To south at 9:00 is BustedButte composed of Paintbrush Tuffcut by a narrow structural slicecontaining parts of the entire Tivaanyon Member. Dips range fromsteeply westward to overturnedwithin a 100-m-wide zone. On theright, along the wash, are exposuresof Topopah Spring Member withlithophysal cavities andnorth-northwest-striking fractures.

51.6 On the skyline at 10:00 is YuccaCrest, at 12:30 Boundary Ridge, at2:00 Bow Ridge, at 2:45 P-1 Hill,and at 3:30 Fran Ridge, all exposingthe Paintbrush tuff. Ridges arecreated by west-dipping major normalfaults on west side of each ridge.Strata underlying ridges dipeastward; major normal faults areaccompanied by highly brecciated andsteep west-dipping strata.

53.2 To right along Boundary Ridge, a20 angular unconformity existsbetween 11.3 Ma. Rainier Mesa Memberand the underlying 12.6 Ma. TivaCanyon Member (Marvin and others,1970). Rainier Mesa Member lapsacross major faults with only minordisplacement.

53.8 Road to Well WT 1.54.0 The generalized map and

accompanying detailed geologicsection (FIGURE 3) show this part ofYucca Mountain to consist of aseries of north-trending,eastward-tilted structural blocks,repeated by west-dipping normalfaults. West-dipping strata alongthese normal faults are interpretedas the result of shear along thefault planes. On Yucca Crest stratadip eastward at 5° to 7°; however,to the east strata also dipeastward, but commonly from 20° tovertical. Coincident with the dipsgreater than 20 are abundantwest-southwest-dipping faults with 1m to 5 m of vertical displacement.These faults and related fracturesare nearly perpendicular tofoliations in the tuff, suggestingrotation. In addition to requiredrotation of the fault planes andintervening blocks, graben-likefeatures suggest a geometric controlby the shape of major normal

faults. The dip of major faultsdecreases from the average of 70 atthe surface to 60 at depth assuggested by data from some drillholes. On Busted Butte, rotatedfault slices extend to depthsgreater than 200 m; if this geometryis typical, then any decrease in dipon the major normal faults mustoccur at greater depths.

55.2 Crest of Yucca Mountain, turnleft.

55.6 Stop 2-3 Overview Regional GeologyMost of the areas and locations

discussed can be found on FIGURE 1.To the north is the rim of theTimber Mountain and Oasis Valleynested caldera complex (FIGURE 2).This caldera complex was active fromapproximately 9-17 m.y. with silicicvolcanism active to approximately 7Ma. in the general region. Thetuffs at Yucca Mountain were eruptedfrom this nested volcanic center.Yucca Mountain consists of thenorth-south ridge upon which you arestanding and the east-westcontinuation of this ridge to thenorth.

To the northeast are the CalicoHills, a domal structure withTertiary tuff units overlyingPaleozoic strata of the EleanaFormation. From this location thetuff units can be observed dippingto the west and south. The rocks atthe Calico Hills can be highlyaltered. They lie along a lineareast-west magnetic high trendingacross the northern top of YuccaMountain for 10-25 km. The magnetichigh may represent a pluton at depth(Carr, 1984) or may represent theformation of conductors (magnetite)in the metamorphosed MississippianEleana Formation (Bath and Jahren,1984).

Jackass Flats is the broadalluvial valley to the east whichleads into the larger east-westtrending Amargosa Valley to thesouth. To the east-northeast in thebackground, the Wahmonie area(Lookout Peak) and volcanic centerconsists of basaltic, andesitic, andsilicic volcanics erupted more than13 Ma. To the southeast in thedistance are Skull Mountain andLittle Skull Mountain which arecomposed of Miocene tuffs capped by8 Ma. basalts. To the east in theforeground, the rocks comprising the

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GENERALIZED GEOLOGIC STRIP MAP ACROSS YUCCA MOUNTAIN[COULD NOT BE CONVERTED TO SEARCHABLE TEXT]

north-south trending Fran Ridge andBusted Butte to the southeast are ofcomparable age and stratigraphy tothe tuffs at Yucca Mountain. Justbeyond Busted Butte and Fran Ridge,Fortymile Wash can be seen. Thesurface facilities for the site areplanned to be located in the smallvalley in the foreground between thenorth end of Fran Ridge and the baseof Yucca Mountain.

To the southwest, in thedistance, the Funeral Mountains canbe observed. These mountainsconsist of Paleozoic rocks withcomplex structural features. On aclear day one can see the crest ofthe Sierra Nevada Range (over 100miles to the west). The alluvialvalley to the west is Crater Flatand in the distance Bare Mountain.Crater Flat has been proposed to bean ancient caldera (Carr et al.,1986), a valley bounded by classicalGreat Basin normal faults, orunderlain by a detachment fault(Scott, 1986). Basaltic cindercones of about 1.1 Ma. occur in thecentral part of the Crater Flat.Eroded remnants of 3.7 m.y. oldvolcanos occur in the southeasternpart of Crater Flat. Bare Mountain,due west, is a complexly faulted(normal and thrust faults) andfolded terrane composed of Paleozoiccarbonate rocks, dolomites, andshales. There is approximately 600ft of relief from where you arestanding to the valley in SolitarioCanyon immediately to the east. Thewest side of Yucca Mountain isbounded by the North-trendingSolitario Canyon fault.

As described at Stop 2-1 thereare several low-volume basalticcinder cones and lava flows of"weak" alkalic series affinity(Vaniman et al., 1982; Crowe et al.,1983a) located in Crater Flat and atthe south end of Yucca Mountain.These volcanic features range in agefrom 3.7 Ma. to as young as 20,000years B.P. These low-volumebasaltic eruptions are part of alarger several hundred kilometerlong linear belt of volcanismtrending north-northeast fromsouth-central Nevada (Lunar Craterfield) to Death Valley, CA; theDeath Valley-Pancake Range zone(e.g., Crowe et al., 1986; Vanimanet al., 1982). To the west and

southwest of where you are standing,the cinder cones are Black Cone, RedCone, and Little Cone and these forma northeast-trending arc inconjunction with the lava flows andother cinder cones in the area. Thetop of the Lathrop Wells basalticcinder cone (Stop 2-1) should bevisible to the south. The basalticvolcanism near Yucca Mountain hasexhibited a decrease in the eruptivevolume with time and a progressiveshift to the southwest with time(Crowe et al., 1983b, 1986).Multiple studies have been performedto assess the nature andsignificance these basalticeruptions could have on wasteisolation (Crowe and Carr, 1980:Crowe et al., 1982, 1983a, 1983b,1986). Three additional studies areplanned to be completed during sitecharacterization (U.S. Department ofEnergy, 1988).

56.7 Well USW G-3 and GU-3, turnaround and go north.

58.1 Road to crest of Yucca Mountainto right continue north.

59.1 Stop 2-4 Overview Yucca MountainGeology and High Level Nuclear WasteRepository

The rocks at Yucca Mountainconsist of a gently eastward dippingsequence (1,800 m thick) of Mioceneash-flow tuffs, lava flows, andvolcanic breccias intercalated withrelatively thin volcaniclastic rocksand air-fall tuffs (Table II). Thissequence is flanked by youngeralluvial deposits. The stratigraphybelow 1,800 m is inferred fromgeophysical data and theinterpolation and extrapolation ofinformation from the surroundingregion. Estimates of the depth tosubvolcanic surface (theunconformity between the Tertiaryvolcanic rocks and the Paleozoicrocks) ranges from 1,200 m beneaththe southeastern portion of YuccaMountain (drill hole UE-25p#l) tomore than 3,500 m beneath thenorthern portion of Yucca Mountain(U.S. Geological Survey, 1984).

Table II lists the major Cenozoicstrata found at Yucca Mountain. Theformations contain several membersthat are differentiated based uponlithological, physical, or chemicalproperties. The volcanic membersare laterally continuous and rangein thickness from approximately 70

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to 370 m (U.S. Department of Energy,

The densely welded section of theTopopah Spring Member of the middleMiocene Paintbrush Tuff is beingconsidered for the repositoryHorizon. The Topopah Spring Memberconsists of 1) lower nonwelded tomoderately welded zone, 13-42 mthick, 2) a basal vitrophyre zone,10-25 m thick, 3) a lowernonlithophysal zone, 27-56 m thick(the probable host rock for theproposed repository), 4) a lowerlithophysal zone, 43-117 m thick, 5)a middle nonlithophysal zone, 20-50m thick, 6) an upper lithophysalzone, 54-96 m thick, and 7) acaprock zone, 39-62 m thick (U.S.Department of Energy 1988). TheTopopah Spring Member iscompositionally zoned, ranging fromcrystal-poor rhyolite at the base toa crystal-rich latite at the top(Lipman et al., 1966; Schuraytz etal., 1986).

The faults that are known tooccur in the repository block haverelatively minor amounts ofdisplacement. The largest of thesefaults, the Ghost Dance fault, islocated about 1.2 km east of ourlocation. The Ghost Dance fault isabout 3 km long and is an element ofthe north-trending, anastomosing setof faults found in the YuccaMountain area. The fault dips verysteeply (80 to 90) westward andthe western side is down-dropped.Displacement in the Miocene tuffs is38 m at the south end of therepository block and decreases to anunmeasurable amount toward the northend of the block. Recognized offseton all other faults in therepository block is less than 5 m(U.S. Department of Energy, 1988).

Return to bus. Turn around andretrace route to turn off for NNWSIdrill hole G-3, i.e. mileage 49.1.

69.1 Turn left on main paved road atsign "Underground Storage, Waste(USW) drill-hole USW G-4".

70.1 North end of Fran Ridge to left.Alice Ridge to north. EnteringMidway Valley. Surface facility forrepository will be sited in MidwayValley.

74.8 Stop 2-5 Exploratory ShaftThe Exploratory Shaft Facility

(ESF) is a statutory requirement ofthe Nuclear Waste Policy Act (NWPA)

78.078.578.6

of 1982, as amended 12/87. It wasconceived by the architects of theNWPA as a necessary part of sitecharacterization, because ofintrinsic limitations ofsurface-based testing from drillholes. As presently planned theYucca Mountain ESF will consist oftwo shafts connected by workings atthe proposed repository level, about320 m (1050) ft deep. Each shaftwill be mined using drill-blast-muckmethods, and lined withapproximately 0.13 m (1 ft) ofnonreinforced concrete. The shaftcollars will be locatedapproximately 218 m (715 ft) N50E(ES-1) of USW G-4 and ES-2 91 m (300ft) 175E of ES-1, both on the northside of Coyote Wash near the mouthof the canyon. In this way theshaft collars will be above thecalculated crest of probable maximumflood for the location. Thetwo-shaft plan allows for extensivescientific work in ES-1, concurrentwith expedited penetration anddevelopment of the main test levelusing ES-2.

Turn around and retrace route toTrench 14 turnoff.

Trench 14 turn left.Turn right on jeep trail.Stop 2-7 Trench 14 and Surface

FacilityDeposits of calcite, opaline

silica, and with or without clayand/or sepiolite occur along severalfaults in the vicinity of YuccaMountain. Trench 14, where you arepresently standing, is locatedbetween Exile Hill and the easternflank of Yucca Mountain,perpendicular to the Bow Ridgefault. The trench crosses thelargest and most extensive of thesedeposits discovered in the area todate. These deposits werediscovered as a result of trenchexcavations across known faultsduring tectonic studies to detectand assess Quaternary fault movementin the vicinity of Yucca Mountain(Vaniman et al., 1988). Mapping ofthe trench walls began in the early1980s. The initial interpretationof these deposits suggestednear-surface accumulations in soiland rock from infiltrating meteoricwater.

Current hypotheses for the originof these deposits include upward,

T186: 19

downward, and lateral migration ofeither hot (>30° C) or cold (<30 C)water under a variety of drivingforces, including pedogenesis,springs, hydrothermal emplacement,and seismic pumping.

In Trench 14 (main trench) theupthrown portion of the faultconsists of the Tiva Canyon Memberand the downthrown side alluvialdeposits. In Trench 14b theupthrown side (east) of the fault isthe Tiva Canyon Member and thedownthrown side a tuff unit whichmay be either the Tiva Canyon Memberor Rainier Mesa Member (Vaniman andLevy, pers. comm.) Along the faultzone both trenches contain angularblocks of Tiva Canyon tuff, derivedfrom a higher stratigraphic positionin the Tiva Canyon Member. Trenches14c and 14d have alluvium on eitherside of the fault zone.

Abundant calcite, opaline silica,and sepiolite are found along thefault in veins and also in the soilsat Trench 14. Calcite and opal withor without sepiolite is a commonmineral assemblage in pedogenicaccumulations. These vein depositsconsist of many small depositionallayers or bands, some of which arecross cutting (Vaniman et al.,1988). The banded nature of thedeposits indicates that thedepositional mechanism wasrepetitive and persisted over a longtime span.

Relatively fresh basaltic ashfills some fractures at the centersof vertical calcite andopaline-silica veins, crosscuttingother laminae in these veins(Voegele, S.A.I.C. private comm.).The ash itself is undated butbelieved correlative with similarashes in Crater Flat with ages of1.2 and 0.27 Ma. (Taylor andHuckins, 1986).

From Trench 14 walk up the smallrise to the east to the top of ExileHill. The small valley in front ofyou is Midway Valley. The centralsurface facilities will includeareas for receiving and repackagingwaste shipments, site operations,and general logistical support.These facilities will occupyapproximately 75 acres directly tothe east of Exile Hill on an area ofgently sloping alluvial fans. Thesurface facilities will likely be

operational for 50 yr or longer.Geologic siting criteria which ledto the selection of this siteinclude the surface slope,protection from flash flooding,location of structural features suchas major faults, and proximity torock outcrops which might be usedfor the waste emplacement rampportal. Additional siting criteriaincluded the availability of asufficiently large contiguous area,the length and inclination of thewaste emplacement ramp to thecentroid of the subsurfacefacilities, and the protection offaunal and botanical species as wellas preservation of archaeologicalremains.

Return via Jackass Flats road toMercury, NV.

157.3 Turn right at NRDA campfacilities.

158.7 Proceed through intersection;Bare Reactor Experiment-Nevada(BREN) Tower (height 480 m) at 10:00.

162.7 At the divide, Skull Mountain isseparated from Little Skull Mountainto southwest by northeast-strikinghigh-angle fault system, down to thenorthwest. There is also probably astrong left-lateral strike-slipcomponent.

163.6 Light-colored massive tuff at3:00 is nonwelded Bullfrog Member ofthe Crater Flat Tuff.

164.7 Specter Range at 12:00 iscomposed of lower Paleozoiccarbonate rocks.

166.6 Road crosses southeast-facingeroded fault scarp in alluvium;fault is part of Rock Valley systemof northeast-striking Quaternaryfaults. About 5 km to the northeasttwo trenches were dug across one ofthe most prominent Quaternarystrands of the Rock Valley faultsystem. This fault system is themajor northeast-striking,seismically active, structural zonein the southeastern NTS area.Trench RV-2 is cut mainly in Q2alluvium, whose age is generallybetween about 35,000 and 750,000 yrB.P. (Hoover and others, 1981, Yountet al., 1987). At least twofaulting events appear to berecorded on the fault zone exposedin trench RV-2. It is suggestedthat the older event occurred afterabout 300,000 yr ago. The youngerevent can only be dated as less than

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4 7. Town of Amargosa Valley, junctionwith Nevada State Route 29 (mileagecheck).

50.8 Lathrop Wells Paleozoic section(Sargent, et al., 1970), in StripedHills, at 9:00 to 10:00, begins inWood Canyon Formation just above thesand fan. Essentially completeCambrian section is vertical toslightly overturned. In ascendingorder to north - Wood CanyonFormation, Zabriskie Quartzite,Carrara, Bonanza King, and NopahFormations. The Bishop ash bedoccurs in sandy alluvium forminglarge fan on south slope of hills at9:00. The materials in the Bishopash bed erupted from Long Valleycaldera, approximately 235 km to thenorthwest about 730,000 yr ago.Hills at 3:00 are underlain byBonanza King Formation.

51.3 Fresh water limestone beds in lowhills on right and left. Equivalentbeds have been dated at 29.3 + 0.9Ma. in Frenchman Flat area.

52.3 Rock Valley Wash.53.5 Exposure of Bonanza King

Formation on right.54.8 Outcrops of Bonanza King

Formation to left.55.0 Low hills of Stirling Quartzite

to right and left of highway. At11:00 low hills of Carrara and WoodCanyon Formations.

59.0 Exposure of Ely Springs Dolomiteat 8:30-9:30 (dark band), underlainby Eureka Quartzite just abovevalley fill and overlain byundifferentiated Silurian dolomite.Amargosa Desert is to the right.

59.5 Low pass with Silurian LoneMountain Dolomite on left and NopahFormation on right.

63.5 Light-gray hills at 9:00 arehighly faulted Antelope ValleyLimestone.

64.5 Intersection of U.S. 95 and roadto Pahrump. On skyline at 1:00 arethe Spring Mountains.

65.2 On right, Miocene and Pliocenegravels contain at their baseash-fall tuff layers correlativewith those at the base of the middleMiocene Paintbrush Tuff (Table II),whose source is the Timber MountainCaldera, 56 km to the northwest.

65.8 On left, contact between LateProterozoic Stirling Quartzite and

Wood Canyon Formation.66.8 On left, large fault brings

Cambrian Bonanza King Formation downagainst late Proterozoic and LowerCambrian Wood Canyon Formation.

67.7 Telephone relay station on left.Bonanza King Formation at 9:00,Nopah Formation at 3:00.

68.0 Left side of road is southeastend of the Specter Range; right sideis northwest end of SpringMountains. Rocks in canyonalongside highway are largelyBonanza King Formation of Cambrianage.

71.2 Army #2 well site on left.73.5 Mercury interchange. (Mercury

camp 4:00). Turn right off dividedhighway and go to Mercury.

77.5 Gate to Mercury, NV. BadgeOffice to right.

78.5 Stop 3-1 U.S. Geological SurveyCore Library

The Geologic Data Center and CoreLibrary, maintained by the USGS atthe NTS, is a depository forsystematic processing, cataloguing,and storage of drill hole and otherrock samples from the NTS and othertest areas. The facility maintainsreference files of reports, maps,aerial photographs, downhole videotapes of selected drill holes,geophysical logs for NTS and othertest areas. Handling of watersamples for both chemical andradiological analyses is expeditedin a hydrologic-chemicallaboratory. The facility serves asfield headquarters for USGSgeologists, hydrologists, andgeophysicists and serves as a workarea for earth scientists in supportof weapons testing andwaste-management projects of theDepartment of Energy (DOE).

The Data Center complex comprisesthe three cojoined buildings at Stop3-1 and three other buildings. Todate, storage has been provided forabout 760,000 m of drill-holesamples stored in about 50,000boxes. Drill-hole samples include

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drill-bit cuttings, nominallycollected each 3 m of drilledinterval, borehole sidewall samples,percussion-gun borehole sidewallsamples, and conventionaldiamond-bit core samples. Detailedrecords, comprising thousands ofdata cards, are maintained on allsamples, including date received atthe library, source, and finalstorage or disposition.

Leave Mercury heading north onMercury Highway from housing area.View of Red Mountain-Mercury Ridgegeology (Barnes and others, 1982).Red Mountain, between 9:00 andi2:30, is composed of gray and brownOrdovician Antelope Valley Limestonethrough Eureka Quartzite on left,Ely Springs Dolomite and Siluriandolomite on right. Strata on RedMountain generally dip eastward.Mercury Ridge, between 1:00 and2:00, is composed mainly of DevonianNevada Formation and Devils GateLimestone. North Ridge, between2:00 and 3:00, is composed of Middleand Upper Cambrian carbonate rocksthrust over Devonian andMississippian rocks (Spotted Rangethrust) in the axial portion of theSpotted Range syncline. SouthRidge, between 2:30 and 4:00,consists of Ordovician throughMississippian rocks that form thesoutheast limb of the Spotted Rangesyncline. Tower Hills, at 4:00, areDevils Gate Limestone. SpecterRange in distance, between 7:00 and9:00, contains Cambrian throughDevonian rocks, and a major thrustfault (Specter Range thrust) thatbrings Upper Cambrian and Ordovicianrocks over middle and upperPaleozoic rocks (Sargent andStewart, 1971). The Spotted Rangethrust to southeast and the SpecterRange thrust may be parts of asingle major thrust system (CPthrust) in the NTS area.Northeast-trending topography iscontrolled by N 450-600 E trendingTertiary left-lateral strike-slipfaults of the Spotted Range-MineMountain structural zone.

80.3 Checkpoint Pass, Gate 200.81.2 Old Mercury Highway Junction to

left (mileage check).Stop 3-2: Over view of Frenchman

Flat Geology. At Pump Station No. 4

at 2:00 are composed ofsoutheast-dipping Paleozoic rocksfrom carbonate rocks of theOrdovician Pogonip Group throughDevonian Nevada Formation. OlderTertiary gravels form low hills inforeground. To north-northeast isFrenchman Lake playa and beyond isNye Canyon, containing severalbasalt centers dated between 6.0 and7.0 Ma. (R. F. Marvin, USGS, writtencommun., 1980, in Dockery et al.,1985). High peak on skyline is BaldMountain in the Groom Range 80 km tonorth-northeast. French Peak andMassachusetts Mountain at 12:00 onthe northwest side of Frenchman Flatconsist primarily of faultedPaintbrush and Timber MountainTuffs. Flat-topped mountain ondistant skyline at 11:30 is OakSpring Butte at north end of YuccaFlat.

At northwest corner of FrenchmanFlat are CP Pass and CP Hogback(named after Control PointHeadquarters). To left of CP Passare the CP Hills composed ofCambrian rocks and Mississippianrocks overlain by Tertiary volcanicrocks. High skyline in far distanceat 11:00 is Rainier Mesa. Directlyto the left of Rainier Mesa on theskyline is Tippipah Point. At 10:00on skyline is Shoshone Mountain,which forms part of the southeastrim of Timber Mountain caldera. Inthe intermediate foreground at 10:00are the intermediate lavas of theWahmonie-Salyer volcanic center onthe northeast end of SkullMountain. Hample Hill at 9:30 inintermediate distance is capped bythe Ammonia Tanks Member of theTimber Mountain Tuff, which isunderlain by eolian sandstone.

At 10:00 and 2:00 in the neardistance (1.6 to 3 km) are hills ofTertiary gravels and the tuffaceoussedimentary rocks of Pavits Spring.Light-colored lacustrine limestonesof the underlying Horse SpringFormation are seen at 7:00 to 8:00where they onlap or are faultedagainst the Paleozoic rocks. TheHorse Spring Formation contains atuff bed dated at 29.3 Ma. (Marvinand others, 1970), which is probablyair-fall tuff of the Needles RangeFormation of eastern Nevada (Barneset al., 1982).

The valley is underlain by playaFacing north and looking

counterclockwise: Range Mountains

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deposits and alluvium, probably525 m (5,000 feet) deep in the

center of the basin. Tertiaryvolcanic deposits are thick in thenorthern part of the valley, whichmarked the edge of the Tertiaryvolcanic basin, and are virtuallyabsent on the south side.

Water supply for Mercury isobtained from Army Well 1 located onU.S. Highway 95 east of the junctionto Jackass Flats, and from wells 5A,5E, and 5C at Frenchman Flat. Waterfrom Army Well 1 is pumped fromPaleozoic carbonate aquifers; waterfrom the wells in Frenchman Flat isobtained from valley fill andvolcanic rocks. Water from wells inFrenchman Flat is of the sodiumpotassium bicarbonate type; calciummagnesium bicarbonate water isobtained from the Paleozoiccarbonate aquifers in Army Well 1.Several air-burst tests wereconducted in Frenchman Flat prior tothe moratorium on nuclear testing in1958. The north end of the valleyhas been developed as a test areafor underground explosions sincethat time, but is not currentlyused. Return south along MercuryHighway 0.5 mile.

82.5 Junction Old Mercury Highway,hard right turn onto old road.

83.3 White hills at 10:30 areunderlain by Eureka Quartzite.Along sky line at 10:00 EurekaQuartzite underlain by AntelopeValley Limestone and overlain by ElySprings Dolomite can be seen.

84.2 Low hill at 9:00 is composed ofHorse Spring Formation of Oligoceneage; and is the oldest Tertiaryformation on the NTS. At 3:00 hillsof younger Oligocene and Miocenerocks of Pavit Springs.

84.8 Stop 3-3 Southwest Frenchman FlatDetachment

Dirt road to left, on curve. Buswill remain on road, with smallervehicles drive off road and maketurn back to south. Follow jeeptrail. Walk or drive southapproximately 1/2-3/4 mi. Crosswash to west.

"Structural relationshipsinterpreted from detailed USGSgeologic maps of the Nevada TestSite area suggest that Tertiarystrata are tectonically detachedfrom their Paleozoic substrate.Mapping of about 5 km2 of the hill

country along the southwest marginof Frenchman Flat revealed exposuresof a low-angle tectonic contactbetween a massive, undulatingpavement of Ordovician limestone andoverlying strongly fracturedTertiary strata. The Tertiary rocksare conformable to moderatelyinclined to the smooth, unstriatedfloor. The resistant Ordoviciansection dips gently eastward, is notfolded, but is broken by easterlyand northeasterly trending highangle faults. The overlyingTertiary section, originally mappedas the Horse Spring Formation ofOligocene age, is composed ofincompetent siltstone and claystone,minor lacustrine limestone beds, anda distinctive conglomerate bed.These marker beds demonstrate thatthe Tertiary strata are stronglyfolded and locally overturned. Thefold geometry largely reflects theshape of the Paleozoic floor, whichis partly paleotopographic andpartly due to high-angle faulting,and implies that the Tertiaryblanket was folded as it wasdetached and transported over theirregular surface. These eventspost-date regional volcanic activityas young as 11.5 MA. The high-anglefaults were probably active at thesame time that the Tertiary stratawere moving across the Paleozoicfloor because the Tertiary blanketgenerally is not cut by thesefaults. The extent of thedetachment, its movement direction,and the magnitude of lateraltransport have yet to be defined.Preliminary mapping at the north endof Yucca Flat, 60 km to the north,suggests similar relationshipsbetween a floor of Paleozoicsedimentary rocks intruded byCretaceous granite and the overlying16 Ma tuffs. Detachment of theTertiary strata apparently is not alocal phenomenon related tooroclinal bending at the northwestend of the Las Vegas shear zone."(From Myers, 1986). Return to bus.Lunch. Proceed north. From Stop3-3 road passes through low hillscomposed of rocks of Pavit Spring.

87.5 Junction with new MercuryHighway, turn left.

92.8 Turn right onto 5-07 road.93.5 At 9:00 dark re-entrant is

vitrophyre lava of the Wahmonie

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Formation onlapped by Topopah SpringMember of the Paintbrush Tuff.

95.0 Turn right onto gravel road.95.3 Stop 3-4 Radionuclide Migration

(RNM) project siteThe RNM project was initiated in

1974 to study rates of theunderground migration of radio-nuclides from explosion-modifiedzones at NTS. The Cambric event,detonated in Frenchman Flat in 1965,was chosen for the study for severalreasons. The Cambric explosioncavity is within the NTS Area 5water-supply aquifer, where leakagecould have contaminated the watersupply. Hydrologic modelingindicated that sufficient time hadelapsed for ground water to fill thecavity and chimney to the preshotstatic water level, which is 73 mabove the detonation point. TheCambric detonation point is only 294m below ground surface, and thus there-entry drilling and samplingoperations were less difficult andexpensive than for more deeplyburied tests. The site is also farenough from the areas of activenuclear testing so that damage orinterruption of the re-entry andsampling operations from thoseactivities would be unlikely.Sufficient tritium (3H or T) waspresent to provide an easilymeasurable tracer for water from thecavity region. The postshot debrisand ground water in the cavity andchimney also contained enoughplutonium, uranium, and fissionproducts so that they could bemeasured and compared. The smallnuclear yield from the Cambric eventwas expected to have little effecton the local hydrology. Further,the alluvium also constituted a goodmedium for hydrologic studiesbecause it is more permeable thantuff and does not have largefissures or cracks through which thewater might selectively flow.

The Cambric field studies beganwhen the Cambric cavity region wasre-entered in 1974, and samples weretaken to determine the radionuclidedistribution between the solidmaterial and water. Beginning inOctober 1975, water was pumped froma satellite well located 91 m fromthe Cambric cavity; this induced asufficient artificial gradient todraw water from the Cambric cavity

and provide an opportunity to studyradionuclide transport under fieldconditions.

The RNM-25 satellite well hasbeen pumped nearly continuouslysince 1975 at the rate of about 600gal/min. Samples are analyzedweekly for tritium. In the summerof 1978, tritium was first detectedand reached a peak of 700 pCi/ml bylate summer of 1980, when theconcentration of tritium began todecrease. By September 30, 1982,over 42% of the tritium from Cambrichad been removed by the satellitewell. These tests significantlyenhance our understanding of theground-water transport ofradionuclides from nuclear explosioncavities in general (Daniels, 1983).

At 8:00-10:00east-northeast-trending Quaternaryfault scarps may be visible in fansat the base of the RangerMountains. Return to paved road,turn right (east).

95.5 Turn right onto 5-01 road.98.0 Y in road. At 3:00 man made

structures on playa were tested bynuclear blasts.

101.0 Gravel pits to left providematerial used in the backfilling ofdrill holes used for nuclear tests.Thickest alluvium (1220 m) inFrenchman Flat, as determined bygravity, is approximately 3 kmnorthwest of Frenchman Lake, nearStop 3-3.

102.0 To right, look along Rock Valleywhere Quaternary fault scarps havebeen recognized. Fault zone crossesroad at approximately this point andcontinues northeast to foot ofRanger Mountains.

103.5 At junction of Mercury Highwayproceed south toward Mercury.

109.0 Gate 100. Leaving NTS. Northend of Spring Mountains is at10:00-12:00. Low rounded hillsacross U.S. 95 arePrecambrian-Cambrian quartzites.Specter Range, containing Paleozoiccarbonate rocks is at 12:00-3:30.Complexly deformed Spotted Range isat 9:00.

Mercury Valley to southeast isthe northernmost topographicexpression of the northwest-trendingLas Vegas Valley or La Madre shearzone. Northeast-strikingstructures, including thrusts in theSpecter Range (Sargent and Stewart,

T186: 24

1971) and Spotted Range, can becorrelated across Mercury Valleywith little or no offset. Nosignificant northwest-strikingfaulting is present in Pliocene andPleistocene deposits of MercuryValley.

Junction with U.S. 95, turn lefttoward Las Vegas.

Massive gray cliffs at 9:00 arethe Palliseria -bearing limestone inthe middle part of the OrdovicianAntelope Valley Limestone (lowerpart of the Aysees Member of theAntelope Valley Limestone in theRanger Mountains). Underneath arebrown slopes of theOrthidiella-bearing silty limestone(Ranger Mountains Member of theAntelope Valley Limestone in theRanger Mountains).

113.3 Low ridges between 8:30 and 10:30are Ordovician Antelope ValleyLimestone. Ridge on skyline between11:00 and 1:00 consists of EurekaQuartzite through Devils GateLimestone.

116.9 Stop 3-5 Spotted Range Geology(optional)

View of Paleozoic units in theSpotted Range between 6:00 and10:00. Park off highway on rightside near sign designating Nye-ClarkCounty line. Rocks seen to thenorth in the Spotted Range aretypical thick miogeoclinal stratasimilar to those in the LathropWells section. Visible unitsinclude limestone of the OrdovicianPogonip Group, Eureka Quartzite, andEly Springs Dolomite (see Table I);Silurian and Lower Devoniandolomite; Lower and Middle Devoniandolomite and quartzite of the NevadaFormation of former usage, andMiddle and Upper Devonian DevilsGate Limestone (includes somedolomite and quartzite). UppermostDevonian and Lower and UpperMississippian rocks cannot be seenfrom here, but are present in anoverturned syncline on the far sideof the ridge on the skyline. Strataseen generally dip 30 to 40northwestward and form the southeastlimb of the Spotted Range syncline.The rocks are displaced by aprominent system ofnortheast-trending faults. Whitequartzite member of the Eureka justabove valley fill at 7:30 isoverlain by black dolomite of the

lower member of the Ely Springs.Ridge on skyline between 6:30 and8:30 is South Ridge capped by DevilsGate Limestone. Nevada-Devils Gatecontact is on skyline at 7:30.Prominent black band with brownishslope-former below is the lower partof the Nevada Formation and can bestbe seen in middle part of rangebetween 8:00 and 8:30.

119.3 Brown and gray outcropsimmediately north of highway arePogonip Group.

122.5 Road to right leads to test well4 - continue straight ahead. Lakebeds of the Las Vegas Formation formthe yellowish-gray badlandtopography along highway. Thesebeds, marking a significantshoreline of a large lake, continuewestward only a few more miles wherethey reach a maximum altitude ofabout 1100 m. They are continuousfrom that point back to an altitudeof about 800 m in the Las Vegasarea, suggesting a southeasterlytilting during the last one millionyears of approximately 5 m/km.

124.3 Prominent ridge on skylinebetween 3:00 and 6:00 is northwestend of Spring Mountains; WheelerPeak at 3:30, Mount Stirling at 4:30.

126.6 Village of Cactus Springs.Prominent black and white bandeddolomite on ridge between 7:00 and9:00 is upper part of NopahFormation.

129.9 Village of Indian Springs.Indian Springs Valley is at 9:00.White and brown outcrops in distanceat 7:00 are Eureka Quartzite. Grayand brown outcrops forming prominentridge south of town, 3:00 to 5:00,are Bird Spring Formation.

131.0 Light-gray outcrop, at 9:00, ismostly Devonian carbonate rocks.Near this point, the trend of LasVegas Valley changes fromeast-southeast to southeast pastIndian Springs, reflecting either abend in the Las Vegas Valley shearzone or the presence of a conjugatenortheast-trending fault.

133.0 Southwest end of Pintwater Rangebetween 7:00 and 9:00 is composed ofOrdovician, Silurian, and Devonianrocks. Ridge at 4:00 consists ofgray cliffs of Monte CristoLimestone and alternating brownsilty-sandy limestone and graylimestone of the Bird SpringFormation.

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137.1 State Correctional Facility, CampBonanza (Boy Scouts of America), andCold Creek Road on left. Continuestraight ahead.

139.0 One of the Playa of Three LakesValley at 8:00. Indian Ridge at4:30 is composed of Cambrian andOrdovician rocks. Ridge between2:00 and 4:00 is composed of BirdSpring Formation. The Wheeler Passthrust probably separates these tworidges.

142.5 Lee Canyon turnoff. Pull ontoNevada 52 to right.

142.6 Stop 3-6 Las Vegas Shear Zone andSheep Range Geology

The Las Vegas Valley iscoincident with the Las Vegas shearzone. A major northwest-trendingfeature, the shear zone separatesthe relatively unextended SpringMountain block from the extendedterrain to the northeast thatincludes from southeast to northwestthe Las Vegas Range, Sheep Range,Desert Range and Spotted Range. Theoffset of Gass Peak thrust, at thebase of the Las Vegas Range and theWheeler Pass thrust to the northwestof this stop is cited as evidencefor right lateral displacement alongthe zone. The contrast insedimentary facies and stratigraphicthickness on opposite sides ofvalley offers corroborative evidenceof lateral movement.

To the northeast the rocks of theSheep Range are the typical thickmiogeoclinal section of easternNevada. The two prominent blackbands at 3:00 are the lower memberof the Ely Springs Dolomite repeatedby faulting. Beneath the upper ofthe two black bands is thelight-colored Eureka Quartzite. TheEureka is underlain by brownish-graycarbonate rocks of the PogonipGroup, which in turn are underlainby the Nopah Formation, theuppermost part of which hasprominent black and white stripes.Above the black lower member of theEly Springs is a unit of light-graydolomite representing the uppermember of the Ely Springs and lowerpart of the Silurian section. Thethin black band is a dark dolomiteunit within the Silurian section.The Devonian rocks above are similarto the Nevada Formation of formerusage and the Devils Gate Limestoneof the NTS.

The Sheep Range detachment(Wernicke et al., 1984) has resultedin the eastward rotation of theSheep Range. Along the west side ofthe Sheep Range is the Hoodoo Hillshavoc (Guth et al., 1988). Thesehills consist of complexly deformedPaleozoic rocks. Black Basin alongthe west side of the Hoodoo Hillswas formed by extensional deforma-tion in the Miocene (Guth et al.,1988). Guth et al. (1988)interpreted the Hoodoo Hills havocas a series of fault slivers along amajor extensional fault and a seriesof gravity slides into the BlackHills Basin as it was developing.

The Lee Canyon thrust, a smallerthrust, between the major Keystoneand Wheeler Pass thrusts, goes upLee Canyon to the west.

146.0 Badland topography at 9:00developed on Las Vegas Formation.Near Las Vegas, similaryellowish-gray fine-grained bedshave yielded fossil mollusks andmammals of Pleistocene age.

147.7 Lucky Strike Canyon Road toright. Road to left leads to CornCreek Springs Field Station of U.S.Fish and Wildlife Service, whichmanages the Desert Wildlife GameRange. The ridge east of LuckyStrike Canyon consists of a muchthinner section than in the SheepRange and contains several differentlithofacies. At 4:00 a white streakrepresenting the distal end of theEureka Quartzite may be seen justbelow a prominent black unit, whichis probably equivalent to theIronside Member of the SultanLimestone. A thin light-graydolomite separates the Eureka andIronside. This dolomite intervalconsists of the Ordovician ElySprings Dolomite and possibly a thinsequence of Silurian rocks. TheDevils Gate Limestone forms theremainder of the ridge above theblack Ironside. The MississippianMonte Cristo Limestone forms thenorth-dipping slope of the mainridge and cannot be seen from here.The well-bedded outcrops at 5:00,north of Lucky Strike Canyon, arethe Pennsylvanian and Permian BirdSpring Formation. Below the Eureka,at Lucky Strike Canyon, the sectionis gray and brown silty and clayeycarbonate rocks of the PogonipGroup. The black dolomite just

T186: 26

above the valley fill at 3:30 is theupper part of the Nopah Formation.

Fossil Ridge at 10:00 is composedof Cambrian and Ordovician rocks.Gass Peak thrust (Fig. 1) at 8:30separates upper plate of Cambrianrocks on left from lower plate ofPennsylvanian and Permian rocks ofthe Las Vegas Range on the right.

Charleston Park Road - KyleCanyon turnoff (Nevada State Highway39). The Kyle Canyon alluvial fanis one of the largest fans on theeast side of the Spring Mountains.It was built by ephemeral streams,principally Kyle Canyon Wash. Fourgeomorphic surfaces, with soildevelopment features indicate thedevelopment of the fan was episodic(Sowers et al., 1988).

View of La Madre Mountainstratigraphy between 3:00 and 4:00.Lower thin black band is dolomite ofDevonian age (probably IronsideMember of Sultan Limestone). Itrests with apparent unconformity ona very thin gray dolomite ofDevonian or possibly Silurian age,which in turn rests unconformably ongray and brown silty and clayeycarbonate rocks of the Pogonip Groupof Ordovician age. Above theIronside is limestone and dolomiteof the Devonian Sultan Limestone.The main ridge is capped by MonteCristo Limestone of Mississippianage. Small outlier just north ofthe end of the main ridge iscomposed of the Bird SpringFormation of Pennsylvanian andPermian age.

162.9 Craig Road turnoff. PotosiMountain at 2:30 is capped by MonteCristo Limestone of Mississippianage. Prominent ridge between 2:00and 3:00 is capped by Permian KaibabLimestone. Wilson Cliffs between2:30 and 3:30, composed of buff andred Aztec Sandstone of Triassic(?)and Jurassic age, form the lowerplate overridden by Wilson Cliffsthrust. Narrow ridge at 3:30 is anerosional remnant of Keystonethrust; the ridge is capped by grayGoodsprings Dolomite of Cambrian andOrdovician age overlying red AztecSandstone. On La Madre Mountainbetween 3:30 and 5:00 are exposedcarbonate rocks of Cambrian,Ordovician, Silurian(?), Devonian,Mississippian, Pennsylvanian, andPermian age. On Las Vegas Range

between 7:00 and 9:00 most outcropsare the Bird Spring Formation ofPennsylvanian and Permian age.Muddy Mountains at 8:00. Sunriseand Frenchman Mountains between10:30 and 11:30.

169.3 Decatur Blvd. overpass.172.0 Las Vegas, intersection of

Interstate 15 and U.S. 95. Proceedsouth on U.S. I-15.

176.9 Junction I-15 and TropicanaAvenue.

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