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Los Alamos National Laboratory is operated by the University of California for the United States Department of Energy under contract W-7405-ENG-36.

TITLE: LATE CENOZOIC EVOLUTION OF FORTYMILE WASH: MAJOR CHANGE INDRAINAGE PATTERN IN THE YUCCA MOUNTAIN, NEVADA REGION DURING LATEMIOCENE VOLCANISM

AUTHOR(S): Scott C. Lunstrom, U.S. Geological Survey, 101 Convention Center Drive, Las Vegas, NV89109

Richard G. Warren, Los Alamos National Laboratory, EES-1, Geology/Geochemistry, MSD462 Los Alamoa, NM 87545

SUBMI'FIED TO: 1994 International High-Level Radioactive Waste Management ConferenceMay 22-26, 1994Las Vegas, Nevada

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By acceptance of this article, the publisher recognizes that the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce thepublished form of this contribution, or to allow others to do so, for U.S. Government purposes.

The Los Alamos National Laboratory requests that the publisher identify this article as work performed under the auspices of the U.S. Department of Energy.

_z:_(_ _ /_ _ _ _;:_ (____ Los Alamos National LaboratoryLos Alamos, New Mexico 87545

FORM NO. 836 R4ST. NO. 2629 561

_...J

DIgTRIB1JTtON OF THIS D©CUMENT IS UNLIMITED

LATE CENOZOIC EVOLUTION OF FORTYMILE WASH: MAJOR CHANGI_ IN DRAINAGE PATTERN INTHE YUCCA MOUNTAIN, NEVADA REGION DURING 1.ATE MIOCENE VOLCANISM

Scott C. Lundstrom Richard G. Warren

U.S. Geological Survey Los Alamos National Laboratory101 Convention Center Drive EES-1, Geology/Geochemistry, MS D462Las Vegas, NV 89109 Los Alamos, NM 87545(702) 794-7450 (505) 667-7063

ABSTRACT INTRODUCTION

Analysis of sedimentary provenance and altitude The site characterization of Yucca Mountain, NV as

distribution of volcanic strata along Fortymile Wash, the a potential high level nuclear waste repository includesprimary desert wash east of Yucca Mountain, NV, indicates study of the surficial deposits as a record of thea major change in surface drainage basins related to late paleoenvironmental history of the Yudca MountainMiocene volcanic disruption. This event resulted in the region la. An important aspect of this history is an

establishment of the modern Fortymile Wash basin before understanding of the evolution of paleogeography leading3 Ma, and probably by latest Miocene time. An to establishment of the present drainage pattern.understanding of this event is useful for evaluation of Establishment of drainage basin evolution is needed beforeextensive alluvium east of Yucca Mountain and its relation geomorphic response to paleoclimate and tectonics can beto paleoclimate, hydrology and tectonics. To the northeast assessed, became a major change in drainage basin geometryof Yucca Mountain, Fortymile Wash provides southward can predominantly affect the sedimentary record. Becausesurface drainage from 60% of the area of the 11 Ma Timber alluvial aquifers are significant to regional hydrology 3, aMountain calderaviaFortymile Canyon, a majorbreach in major change in surface drainage resulting in buriedthe caldera wall. In the southeast caldera moat, the alluvium could have hy.drogeologic significance. In thisdistribution of volcanic units that predate and include the paper, we report on geologic evidence for a major

9.4 Ma Thirsty Canyon Group and the characteristics of modification in surface drainage pattern in the Yuccaintercalated sediments indicate a northward paleoslope and Mountain region, resulting in the probable establishment ofsediment transport from a major drainage divide near Dome the Fortymile Wash drainage basin by latest Miocene time.Mountain, a shield volcano now deeply incised byFortymile Canyon. Eruption of the Thirsty Canyon PhysiographyGroup from the Black Mountain area, 10 km northwest of

the Timber Mountain caldera, is likely to have dammed a Yucca Mountain, a ridge in the southwestern part ofcounterclockwise drainage system of the east moat. the Great Basin, is flanked by alluviated basins, and is

Following drainage disruption, the east moat filled with located to the south of the upland of the southwesternsediment up to the level of a new southward outlet at the Nevada volcanic field. Most of this upland comprises thesaddle between Dome Mountain and the onlapping rhyolite northeastern and uppermost portion of the Amargosa Riverof Shoshone Mountain. An older canyon south of this Basin, an ephemeral surface drainage system which gradessaddle received the overflow from the east moat and became southward for 90 km before entering Death Valley at itsthe throughgoing Fortymile Canyon, integrating the east southern end. Fortymile Wash provides the surface

moat basin with a lower base level in Jackass Flats. Well- drainage to a major part of the upper Amargosa basin (Fig.integrated southward drainage existed by the time the 1), including the highest uplands of eastern Pahute Mesa andtrachybasalt flows of Buckboard Mesa (2.8 Ma) were Timber Mountain. The modern watershed of Fortymileemplaced, because basal elevations of these flows slope Wash is characterized by a lower southern basin, includingsouthward about 100 m above modern Fortymile Wash. the east slope of Yucca Mountain and the west side ofAn older gravel exposed just south of Fortymile Canyon Jackass Flats, separated from an upper northern basin by thehas a clast population distinctive from that for Quaternary 25 km long Fortymile Canyon (Fig. 1), with a maximumgravels of Fortymile Wash; this older gravel is a deposit of depth of about 500 meters. The upper basin includes thethe older lower canyon, preceding the establishment of a east moat of the Timber Mountain caldera, formed duringthroughgoing Fortymile Canyon. its latest cycle of collapse and resurgence about 11.4 Ma4'5'6.

The timing of the establishment of Fortymile

,i ;

] I outh rnBasinandi¢angep,'ovince.Thevolc.' nicunits'_"" v. o_": -_ exposed along Fortymile Wash (Fig. 2) range in age from/ I _, [ ' -- ¢

,,,,,_ i ..- _=_ .... the 12.9 Ma rhyolite of Calico Hills to the 2.8 Mai "_ _ trachybasalt of Buckboard Mesa6'12; we focus on theI distribution of units that postdate the Timber Mountain

"'-, ,_/ _ / i ,llllT_]_., bq_ .v/_ GroL,p. These units are mostly rhyolitic and basaltic flows

!_ // \ \_[_][ _2 54__ extruded near ring fractures of the Timber Mountaincaldera, though ash flows of the 9.4 Ma Thirsty Canyon

"i .-x _"_*ll[!_..a. _,_i 3r'N Group originated from the Black Mountain center 6''2,

_.,_ J 5_ ."" '__ [_om,,,,,k northwest of the Timber Mountain caldera. Volcanic.- -...... - ..... s - .... _¢"./2"//./.1_.__: "R°"y°' stratigraphy predating and including the Thirsty Canyon

.\ /_ v,_, j[_S_t0"_..._!.._) \ Group defines a paleogeography that contrasts with that of-. I\ .o,,u_._ [ ..... ) the modern Fortymile Wash basin. We infer major changes

"- ',\ "t.:-'_.' I a,c_, ._ in drainage direction and paleogeography from an analysisC*\NV ',,N N t_[_x']'_I)_'" of the altitude distribution of volcanic units along

\ ""., "_'_q_"'\_-. _//'7 Fortymile Wash from previous high-quality geologic0 10 20 ., ,% ""- ...... /I/ quadrangle mapping 13J4'lsJ6. Units will be discussed inK,_._,,, . ',-_ _'/ .......... stratigraphic sequence, as well as northward from central

-,. \'., ie'l "-......

"- -. I .¢_.,_,,_. _,_ ...... Fortymile Canyon into the east moat of the caldera.

\\ I Fortymile Canyon• ,x116"30"W

The former rhyolite of Fortymile Canyon consists of

Figure 1. Geography of the upper Amargosa River surface multiple rhyolite flows and pyroclastics exposed withindrainage basin, showing major channels (dash-dot lines), the Fortymile Canyon _s.Recent work 9demonstrated that muchsub-basin of Fortymile Wash (light shading), Fortymile of the rhyolite of Fortymile Canyon, formerly defined asCanyon (dark shading), Yucca Mountain (horizontal lines), postdating the Timber Mountain Group *s,erupted duringthe boundary of the Timber Mountain Caldera (dashed, earlier magmatic cycles, Only the rhyolites of Chukarhachured line with hachures towards moat), and central Canyon and Max Mountain (10.7 Ma by K-.&r dateg; Figs.

resurgent dome (vertical lines). Other major uplands of the 2 and 3), emplaced within the southeast moat of the Timberbasin of Fortymile Wash are diagonally cross-hatched, Mountain caldera, postdate the caldera. These relations

including Dome Mountain (D), Pinnacles Ridge (P), obviate any need to explain the distribution of rhyoliteShoshone Mountain (S) and the Calico Hills (C). Other flows as evidence for ancestral canyons preceding the flows _.locations mentioned in paper are Rocket Wash (RW), Within this well-exposed volcanic sequence of rhyolite

Stockade Wash (SW), and well U18-J1 (X). flows, there are no recognized intercalated fluvial gravelsrequiring a former canyon.

Canyon is significant in resulting in a four-fold expansion ofthe Fortymile Wash drainage basin. Previous hypotheses Dome Mountain is a mafic shield volcano deeplyfor the age of establishment For_ymile Canyon range from incised along its east flank by Fortymile Canyon (Figs. 2immediately following caldera resurgence _ to middle and 3). It overlies the 10.7 Ma rhyolite of Max Mountain _

Quaternary capture from Beatty Wash ". In this paper, we and is onlapped by the 10.3 Ma rhyolite of Shoshoneanalyze the spatial distribution of volcanic stratigraphy, in Mountain .6'_. The main outcrops of the mafic lavas ofcombination with provenance considerations of intercalated Dome Mountain _sa6'_sin central Fortymile canyon (Figs. 2,sediments along Fortymile Wash, to infer the history of 3) are..gently dipping flows that occur high on both sides ofdrainage-basin evolution leading to the establishment of the canyon. However, mafic lavas of Dome Mountain haveFortymile Canyon. also been mapped near the central canyon bottom .5J6.

Correct recognition of the origin of these exposures isALTITUDE DISTRIBUTION OF VOLCANIC STRATA critical to understanding the evolution of Fortymile

Canyon; we consider three explanations. First, exposures

The volcanic stratigraphy of the southwestern Nevada near the floor of Fortymile Canyon might represent veryvolcanic field _'s'_'9'_°'_'_ is one of the more thoroughly late flows of mafic lava of Dome Mountain that occurred

studied sequences of caldera-related magmatism, and after the canyon was cut. This scenario would require aprovides a geologic framework (Fig. 2) for understanding pause in eruption long enough to cut Fortymile Canyon tothe Neogene drainage evolution in this critical area of the its present depth. Marsh and ResminP have estimated that

phenocrysts in mafic lavas at Dome Mountain had residence

times of about 10-20 years, indicating rapid magmatic B :i,:, i:':evolution, consistent with rapid evolution of the earlier i: .

Paintbrush Group 6. Thus the first explanation would ....J ::[;,'V" '"require that Fortymile Canyon be cut to its present depth - • ,..'=

during the apparently rapid magmatic evolution of theshield volcano at Dome Mountain, then undergo no further

deepening during the next 10 Ma. Such a scenario isimplausible. Second, the exposures near the floor of "Fortymile Canyon might have been misidentified. Ourpreliminary petrographic analysis of a thin section of maficlava of Dome Mountain from an outcrop at the floor ofFortymile Canyon (location A on Fig. 3) shows significant

olivine, orthopyroxene, and ilmenite, a combination ofphenocryst phases absent in nearly all marie units of theregion, but characteristic of the andesitic, upper part of themarie lavas of Dome Mountain. This confirms an earlier

lithologic assignment 15, but the geomorphic position ofthese exposures is suggestive of landslides, a thirdexplanation. Landslides of basalt are common within : ::canyons cut deeply through basalt into underlying 9

sediments, and can extensively preserve minimally deformed Jackassbasalt in slump blocks 19. Therefore, the lack of apparent _ Flats

deformation that we observe in some exposures, as at _ _ <_location A (Fig. 3) does not preclude their origin aslandslides. We conclude that the outcrops of mafic lava ofDome Mountain near the floor of central Fortymile ",,Canyon must be landslides. _.

A northeastward paleogeographic slope opposite the x._,_trend of Fortymile Canyon is defined by the altitudinal ,_ ."

profile of the mafic laws of Dome Mountain (Fig. 4), _" Known vent 0 10 20excluding the landslides described above. Altitudes of basal "1""rCaldera boundary I I Iand upper surfaces of this lava sequence generally decrease Kilometers

northeastward along Fortymile Wash from the summit area Undivided alluvium (mostly Quaternary)of Dome Mountain, the inferred vent. There is no evidence

for a pre-existing Fortymile Canyon at the time the lavas of Trachybasalt of Buckboard Mesa (2.8 Ma)Dome Mountain were erupted. Instead, the decrease inbasal altitude and the gradual northward decrease in Younger (post-Tt) sediments of east moat

thickness of lavas of Dome Mountain along Fortymile Wash Thirsty Canyon Group (9.4 Ma),(Fig. 4) are evidence for a paleogeography in which most of from the Black Mountain caldera (B)

the marie lavass of Dome Mountain flowed northward from Older (pre-Tt) gravels of east moata topographically high area of the southeastern TimberMountain moat that had been built up through extrusion of Gravel of Line EW-25the only slightly older rhyolite of Max Mountain.

Rhyolite of Shoshone Mountain (10.3 Ma)

Figure 2. Geology of the region of Fortymile Wash 1m2' Mafic lauas of Dome Mountain

with age control6'9'n'C Geographic locations indicated areAmargosa River (AR), Black Mountain (13),Bare Mountain Rhyolite of Max Mountain (10.7 Ma) and(Ba), Beatty Wash (BW), the east moat of the Timber Chukar Canyon (10.6 Ma)

Undivided older rhyolites of Fortymile CanyonMountain caldera (EM), Fortymile Wash (FW), Pinnacles that predateTimber Mountain calderaRidge (P), Scrugham Peak (sp), Shoshone Mountain (S),Rocket Wash (RW), and Yucca Mountain ('YM). Undivided Post-11 Mabasalts

Approximate scale is about 1:600,000. Undivided Pre-11 Ma rocks

• , [ •

The rhyolite of Shoshone Mountain" (Figs. 2 and 3) Figure 3. Geology of central l:ortymile Canyon modifiedoverlaps the east flank of Dome Mountain and is only from previous work 9'1_'14Js'16. Approx. scale is 1:66,000.slightly younger 17 (10.3 Ma). Its surface also defines anorthward constructional slope from a vent area marked byan intrusive plug at the summit of Shoshone Mountain'.Again, there is no evidence that Fortymile Canyon hadbeen established prior to extrusion of Shoshone Mountain,as the basal altitude of its dissected western margin occurs ataccordant elevations along the eastern rim of FortymileCanyon. Instead, the northwest margin of the rhyolite of

Shoshone Mountain appears to have localized the laterestablishment of a southwestward trending reach ofFortymile Canyon over the east flank of the DomeMountain shield volcano.

The east moat of the Timber Mountain caldera

The Thirsty Canyon Group forms a critical datum inthe east moat and in the evolution of Fortymile Wash,because it is the youngest stratigraphic unit to define anortherly paleoslope opposite to the modern slope ofFortymile Wash. The Pahute Mesa and Trail Ridge Tufts ofthe 9.4 Ma Thirsty Canyon Group 6'.2are exposed in theeast moat along Fortymile Wash beneath Buckboard Mesa

and as a northeast-dipping cuesta within sediments of theeast moat *u2'13to the southeast of Bucki_oard Mesa (Fig. 2).These ash flow tufts thicken northwestward from the eastmoat toward their source in the Black Mountain caldera 2°.

A northward component of dip is defined by the basal andupper contacts of the Thirsty Canyon Group in following I I ! IStockade Wash northeast from its confluence with 1 0.s 0 1km

Fortymile Wash at the south end of Buckboard Mesa to ...... Fortymi!e Washdrill hole U18-J121 (Figs. 1 and 4). This northward dip is

likely to reflect a northward paleoslope, because the Thirsty I O I Quaternary colluvium and alluviumCanyon Group also thickens northward along this transect

from 30 m at the south end of Buckboard Mesa to 55 m in _ Landslides of TfdIithology, formerly mapped Tfddrill hole U18-J12'

A - Landslideof TfdsampledforgeochemistryIn contrast to the older volcanics of the post-Timber ' ITTrrTIITI1TI_

Mountain Group along Fortymile Wash discussed above, _ Rhyoliteof ShoshoneMountain

the 2.8 Ma trachybasalt of Buckboard Mesa 12'13'22'23'24(Figs.2 and 4) defines a southward slope in the east moat. The Maficlavasof DomeMountain

base of the flows of Buckboard Mesa, where unfaulted, Rhyolitesof MaxMountain(10.7 Ma)slope southward, parallel to and about 100 m above the and ChukarCanyon (10.6 Ma)longitudinal profile of modern Fortymile Wash. Buckboard

Mesa clearly defines a southward-trending paleovalley ..J- Paleo-Topographicwall of TimberMountalnCalderabottom at an altitude about 300 m lower than the highestexposed basal contacts of mafic lavas of Dome Mountain in I Tfro I OlderRhyolitesof FortymileCanyon

!

Fortymile Canyon, requiring the existence of a IpTmc;! UndividedPaintbrushGroupand

I

throughgoing Fortymile Canyon at a probable grade about _ Calico Hills formation100 m higher than present by about 3 Ma.

We consider the slopes of volcanic units in Figure 4 to SEDIMENTARY (;ttAF, A(;TERISTI(;S ANI)

not have overiding components of structural rotation that PROVENANCEwould yield erroneous paleogeographic slope direaio,.Though significant tectonism in the past 10 Ma is known to The sedimentary record, including the sedimenthave occurred outside the caldera, only minor faulting has characteristics of clast lithology, rounding, and size, as well

cut the exposed units postdating caldera resurgence in the as sedimentary structures, reflect the changingeast moat. Furthermore, the general southerly trend of paleogeography inferred from the volcanic stratigraphy. ToFortymile Wash is sub-parallel to the direction of most the north of Fortymile Canyon, sediments intercalated withfaults in the area and to likely associated axes of tilting, the volcanic sequence in the east moat provide additionalMinor faults crossed by Fortymile Wash along more evidence for the formerly northward-sloping

easterlyjogsin its course are expressed as local reversals in paleogeography of the east moat, opposite the modernthe basal slopes of Dome Mountain and Buckboard Mesa drainage direction of Fortymile Wash. To the south offlows, but inferred vent locations and thickness trends of Fortymile Canyon, Quaternary gravels that postdatethese mafic flows provide further support that their original canyon establishment lithologically contrast with an olderbasal slope directions have not been tectonically reversed, gravel interpreted to predate a throughgoing canyon.

Sediments of the east moat

,, _, The oldest sediments recognized within the east moat

) 20 _,_l_ _' underlie the Thirsty Canyon Group and are intercalatedwith the mafic lavas of Dome Mountain 15a6'18(Fig. 2); these

0 Tgp • (<< 1%) _' sandy gravels to gravelly sands will be referred to as theolder gravels of the east moat. They are moderately

_----FortymileCanyon_----}-east moat- diagenetically cemented. In a northeast direction from

a. IA Dome Mountain, opposite the gradient of Fortymile Wash,- - - Bsseof Tr,cm,bas,Uof i_ these older moat gravels thicken and become more fluvial in

Buckboard Meas (2.S Ma) _ I -" " -'

1500 -- --Ashnowso, ThlrstyCen,o."_ "_ character; rounded cobbles and cross beds are common at"" "group (9.4 Ms) __=1p.-,,4.2._].= ,. the north end of Fortymile Canyon.

"_ _ l_IMafic flows of Dome _ UIS-JI

- ,o2...f<C) Mountain-1 To the north of the mapped limit of mafic lavas of--- Dome Mountain _3,older gravels of the east moat are more

1000 _ _ _-, Modern channel of extensively exposed just south of the northeastward-dippingTgp FortymileWash -- cuesta of the Thirsty Canyon Group (Fig. 2). Like the

South North stratigraphically lower sediments beneath the northernI...... I I exposed extent of Dome Mountain lavas, these older moat

20 40 60 sediments are predominantly fluvial sandy gravels, withDistancein kmalongmodernchannel common subrounded gravel clasts and cross bedding. The

upwash(north)of US95 presence of mafic clasts in these sediments provides evidencefor a northeastward direction of fluvial transport. Most

Figure 4. Longitudinal profiles of parameters along mafic clasts are oxidized scoria which is not conducive toFortymile Wash: geochemical or petrographic identification. We have founda. Altitude profiles of geologic units along Fortymile Wash. some clasts that appear to be Dome Mountain lithology,Altitudes of basal (dashed) and upper (solid) contacts of although this hand-sample identification is provisional,volcanic units are from published geologic pending geochemical and petrographic analysis. However,quadrangles_3'u'_sa% and surface altitudes of gravel units are given their stratigraphic position, there are no other known

from published U.S. Geological Survey topographic maps. potential sources of mafic clasts in areas north or east of theb. Percent of mafic lithologies in Late Quaternary alluvium East Moat basin. The northeastward fluvial transport awayand older gravel of Jackass Flats along Fortymile Wash. from a Dome Mountain source is opposite the presentEach data point for late Quaternary gravels represents a drainage direction of Fortymile Wash.natural exposure where at least 85 clasts greater than 8 mmintermediate diameter, within a fixed distance from In contrast, sediments which overly the Thirsty

regularly spaced grid points, were classified as mafic or Canyon Group in the east moat (Fig. 2) show evidence forfelsic . The data point for Tgp, the gravel of Line EW-25 a major change in &positional environment from that of(dot), represents 310 clasts counted at 3 sites, older gravels of the east moat. These younger sediments of

Lv,J=*,-.,. ,,,,d 1t,',,.,-_., 5"

the east moat are poorly exposed, but ,oadcut exposures Clast counts of gross clast lithology (mafic or felsictypically show thinly bedded tuffaceous sands, volcanics) were made of late Quaternary alluvium and theunconformably overlain by gravels. The sands represent a gravel of Line EW-25 along Fortymile Wash to quantifydistinct textural fining relative to the pre-Thirsty Canyon differences that would reflect whether Fortymile Canyonfluvial gravels; the fine bedding may represent lacustrine existed when the gravel of Line EW-25 was deposited (Fig.conditions. We interpret the fine, thinly bedded sediments 4b). The proportion of mafic lithologies in late Quaternaryas closed- basin deposits resulting from the disruption of the gravels of Fortymile Wash incre_ses from 6% to 24% withinprevious drainage system by the eruption of the Thirsty upper Fortymile Canyon, then decreases more graduallyCanyon Group. down wash to about 12% south of the exposed gravel of

Line EW-25 (Figs. 2 and 4). This pattern clearly shows thatGravels along lower Fortymile Wash mafic lavas of Dome Mountain, incised by Fortymile

Canyon, are the main source of the mafic clasts in theTo the south of Fortymile Canyon, extensive Quaternary. alluvium. Downwash of the southern limit Of

Quaternary alluvium along Fortymile Wash 2s'26 has mafic lavas of Dome Moun',:ain, the proportion of mafic

generally buried most older alluvium that might correlate to clasts in Quaternary alluvium decreases southward,sediments in the east moat. However, a distinctive older probably as a result of addition of exclusively non-maficgravel of limited exposure, termed the gravel of Line EW-25, sources in southern Fortymile Canyon, but remains higher

(Tgp, Figs. 2 and 4) has been recognized during recent than 12 percent south of the exposure of the gravel of Linemapping of surficial deposits near Yucca Mountain 26.This EW-25 . In contrast, mafic clasts in this older gravel are

older gravel is exposed as a rounded hill veneered by very sparse relative to any of the Quaternary gravels ofQuaternary alluvium along the east side of modern Fortymile Wash, comprising much less than 1% (Fig. 4b).Fortymile Wash, 7 km east of the crest of Yucca Mountain, These maf.c clasts, with typically abundant vesicles, a_e also

beneath a powerline (EW-25). The gravel of Line EW-25 likely to be derived from mafic lavas of Dome Mountain,is similar to the older gravels of the east moat in the degree based on preliminary hand sample identification.of diagenetic cementation and sparse presence of mafic

clasts, but is quite distinctive from adioining Quaternary The characteristics of the gravel of Line EW-25 provide

terrace gravels of Fortymile Wash (Table 1). The a basis for estimation of its age. The presence of any DomeQuaternary gravels deposited by Fortymile Wash are not Mountain and Shoshone Mountain clasts requires Tgp to bediagenetically cemented, and include a common and no older than 10.3 Ma. We interpret the sparsity of maficobvious component of mafic clasts. Thoughdistinctive in its clasts in the gravel of Line EW-25 to preclude the existence

content of mafic clasts and cementation, the gravel of Line of a throughgoing Fortymile Canyon when this gravel wasEW-25 is similar to the Quaternary gravels of Fortymile deposited. Instead, the provenance of this older gravelWash (Table 1)as a cross-bedded sandy gravel with common included a drainage in the position of lower Fortymilerounded cobbles and boulders, in the abundance of Canyon which headed near the southern limit of Domespherulitic rhyolite and lithologies of the Calico Hills Mountain lavas at that time. Because the gravel of Line EW-

formation, and in the presence of aphyric obsidian from the 25 is a coarse fluvial deposit similar to the Quaternaryrhyolite of Shoshone Mountain. These lithologies require a gravels, the major difference in their proportions of maficprovenance that includes the area around lower Fortylnile clasts is unlikely to be a climatic effect. If the gravel of Line

Canyon. EW-25 predates the establishment of a throughgoingFortymile Canyon, the timing of the canyon establishment

Table 1. Comparison of gravels of lower Fortymile Wash after 9.4 Ma and well before 2.8 Ma provides a lower limit

to the age of the gravel of Line EW-25 ; it was probablyQuaternarygravel gravelof LineEW-25differCnqes deposited in latest Miocene time.

commonbasalticclasts(12-25%) verysparsebasalticclastsDISCUSSION

non-cementedto locallycemented pervasivediageneticcementin isolatedcalichehorizons

We propose a new hypothesis for the evolution ofextensive along Fortymile Washvery limited exposure Fortymile Wash (Fig. 5), based on the above volcanic and

_imilarities sedimentary stratigraphy along Fortymile Wash. Insandygravelcommonroundedcobblesandboulders contrast to a previous hypothesis 7, we see no evidence for a

commoncross-bedding throughgoing Fortymile Canyon prior to extrusion of anyabundantspheruliticrhyoliteclasts of the volcanic units along Fortymile Wash other than the

commonoxidizedrhyoliteclasts,aswell as 2.8 Ma trachybasalt of Buckboard Mesa. Within about oneaphyricobsidianof rhyoliteof ShoshoneMountain

million years of the final collapse of the Timber Mountain From > 11 Ma to 9.4 Ma ,.r- -r-caldera 6 (11.4 Ma), its resurgence was closely followed by a .,,. ,

volcanic sequence in the southeastern moat that formed a Post- resurgent f,'j_ _?,--, ,,_"_-.v- "r.. imajor drainage divide (Fig. 5a). A persistent northward volcanism forms _ <'-e'_t,e. _, I" - _ '_o_..' Ipaleoslope from this divide into the east moat is defined by major drainage _" ( Timb_ _°e_'-_ )

the upper and lower surfaces of volcanic units transected by divide in SE moat _ _n )_41

upper Fortymile Wash (Fig. 4a), for units as old as the mafic ' _f/lavas of Dome Mountain, and as young as the ash flows ofthe Thirsty Canyon group 6'z2(9.4 Ma). This northward I_T_sT! Pre-Fortymile N,paleoslope is also defined by northward fluvial transport in i 9 ! Canyon sediments '_

the east moat prior to eruption of the Thirsty Canyon [_ 10.3 Ma ll.l\V/pix,_.. _L./ Major %Group. This fluvial transport must have either terminated Yucca I__ I drain.age \..... Mountain

in a closed basin (Fig. 5a) or an outlet existed. An outlet to _ 10.9 Ma _1 _s

northward drainage from the east moat could only havebeen in the northern moat, which is presently a topographic _ 10.7 Masaddle between Rocket Wash and Fortymile Wash.

We suggest that the eruption of the Thirsty Canyon b. "_> • . ' "Group disrupted the surface drainage pattern of the Timber .,.4 Ma to i # :[_;_ _Mountain caldera, and led to a reversal of drainage direction >> 2.8 Ma i _._>P_"., .,<, "r'i --r-,/.',. _i

in the east moat (Fig. 5b). A former counterclockwise (by 8 Ma'?.)l .....iii)_ .._?":_>[_ ,,. ]

drainage in the east moat would have been in alikely ----> i:[_'))

_"/_ ;,;;" .5;:_ :.:5' . :'" ; ;" f5

position to be deeply buried and disrupted by eruption of "_:: +:7;:;"5:_"

ash flows and silicic lavas of the Thirsty Canyon Group 1. Black Mountain _ r 'V "_: " _ _ __ i

from the Black Mountain caldera,, located near the Caldera(B) "\_::__._ I

northwest margin of the Timber Mountain caldera. Magmatism: k ""--._...!]1__ |Following drainage disruption, closed basin sedimentation, I A _< DDt__/with possibly lacustrine conditions represented by finely I,<Ca_y0n_Gro_ _"

moato e,eve'new ",,southward outlet (Fig. 5b). The new outlet was at a passbetween the constructional forms of the Dome Mountain 2. Post 9.4 Ma" _/ _ _I _/3. Older lower

shield volcano and the overlying rhyolite of Shoshone closed basir \nl _ I _' FortymileMountain. With recently erupted tufts of the Thirsty li_di_tati0nl ult,! II canyonsouthwarduntil L _i. | | erodesCanyon Group mantling the landscape, it is likely that spill into head "1[_ I ] northwardbasin filling was rapid until the level of the new outlet was of older canyon v V tointegratenew

drainage ofeast moat

5. Paleogeographic history of Fortymile Wash: c.a. Eruption of rhyolites of Max Mountain 9 (Tfmc, 10.7 Ma) 2.8 Ma to present _-and Shoshone Mountain 12J7(Tfs, 10.3 Ma), and intercalated (_

shield volcano of mafic lavas of Dome Mountain (Tfd) intosoutheast part of newly formed Timber Mountain calderamoat forms a major drainage divide, with northward and Extrusionof trachybasalt I / _ "P_'_(southward fluvial transport and aggradation from the Dome of Buckboard Mesa[_ _ \ (,_ I_i. "{ I

(2.8Ma) intobasin similar- . _ _ / llt_ v IMountain area prior to 9.4 Ma;to present, followed by X _ _._-". I.

b. The 9.4 Ma Thirsty Canyon Group 6J2is erupted from lOOmIncisionineast mo 'v,,at ',- I P'. • " FortVt_ile

the Black Mountain Caldera to block outlet for drainage of ]_ '_oneast moat. This causes closed basin aggradation until new (/',,southward drainage is established for '!he east moat at new _ joutlet between Dome Mountain (D) and Shoshone lMountain (S); I Quaternaryc. By 2.8 Ma, when the trachybasah of Buckboard Mesa t3_4 J aggradation

• I in Jackassis erupted, a well integrated southward drainage similar to _| Flatsthe modern Fortymile Wash basin has been established.

reached; tt_is initial establishment of southward drainage Hydrogeologic considerationsfrom the east moat probably occurred closer to 9 Ma thanto 3 Ma. The uppermost portion of an older southward A hydrologic implication of this proposed evolution ofdrainage system in the present position of lower Fortymile Fortymile Wash is that a former drainage in the northernCanyon must have been located near the proposed outlet to moat of the Timber Mountain caldera may be buried by thereceive this new overflow from the east moat. Subsequent Thirsty Canyon Group. A buried drainage system thatintegration of the east moat to the lower base level of included a coarse alluvialaquifer would probably be a zoneJackass Flats accompanied the cutting of upper Fortymile of locally high groundwater transmissivity. It is possibleCanyon, probably as northward migration and diffusion of that a volcanically buried alluvial aquifer could contributea nickpoint from the former saddle in the drainage divide, to the anomalously low southwesterly gradient ofnow located above the deepest pan of the canyon. The groundwater potential from Pahute Mesa beneath thealtitude of the outlet at the inception of the southward surface drainage divide of the north caldera moat towardsdrainage from the east moat is presently about 1525 m. Oasis ValleyCBelow 1525 m in the east moat, drainage courses weresuperimposed across and independent of former CONCLUSIONSconstructional volcanic slopes of Dome Mountain andShoshone Mountain, providing evidence for sedimentary fill Fortymile Canyon must have been established after 9.4during the transition from northward to southward Ma and well before 2.8 Ma, probably in the late Miocene.drainage. The spatial distribution and characteristics of volcanic and

sedimentary stratigraphy in the upper Fortymile WashThe incision of Fortymile Canyon and integration of basin indicate a northward slope and direction of fluvial

drainage within the east moat must have been well transport from Dome Mountain, opposite the presentestablished before eruption of the 2.8 Ma flows of drainage direction, prior to eruption of the Thirsty CanyonBuckboard Mesa(Fig. 5c). Basalcontactsofflowsextruded Group. These relations suggest that a formerfrom the vent area at the north end of Buckboard Mesa counterclockwise drainage system in the east moat of thedefine a southward sloping paleovalley, now a Timber Mountain caldera was disrupted and forcedtopographically inverted mesa parallel to but about 100m southward by eruption of the Thirsty Canyon Group. Aabove the present grade of Fortymile Wash (Fig. 4a). gravelexposed near the mouth of Fortymile Canyon, near

Yucca Mountain, appears to predate the establishment ofTectonic implications Fortymile Canyon.

Though we consider volcanism to be a probable factor ACKNOWLEDGEMENTS

in the drainage reversal in the east moat, the extensional This study is supported by the Department of Energytectonism resulting in the opening of the Jackass Flats basin Yucca Mountain Site Characterization Project undermay also have been significant in lowering base level. A Interagency Agreement No. DE-A108-78ET44802. Wegeomorphic response to lowering of base level in Jackass greatly appreciate the constructive reviews of C.J. Fridrich,Flats would be the headward erosion to the north of a C.M. Menges, and D.T. Vaniman, and the graphicalcanyon in the position of lower Fortymile Canyon. The expertise of R.J. Danat.presence of a southward drainage in this position isnecessary to our hypothesis, and is supported by the REFERENCEScharacteristicsof the gravel of Line EW-25 at the mouth of

Fortymile Canyon, especially its clast composition. The 1. U.S. Department of Energy, Site CharacterizationPlan:exposure of this gravel of probable late Miocene ageat the YuccaMountain Site, Nevada Researchand Developmenesurface is potentially significant as evidence that perhaps Area, Nevada (1988).much of the sedimentary fill in Jackass Flats took place inthe late Miocene. However, until the nature of the 2. U.S. Department of Energy, Studyplan for analysisof tbedistribution of this unit in the subsurface is better known, paleoenvironmentalhistoryof the Yucca Mountain Regionits very limited exposure warrants caution in this (1991).interpretation, especially considering the large volume of

material eroded during the cutting of upper Fortymile 3. I.J. Winograd and W. Thordarson, "Hydrogeologic andCanyon and integration of the east moat. hydrochemical framework, south-central Great Basin,

Nevada-California, with special reference to the NevadaTest Site", U.S. GeologicalSurvey ProfessionalPaper712.C, (1975).

4. F.M. Byers, Jr., w.j. Carr, P.P. Orkild, W.D. 13. F.M. Byers, Jr., C.L. Rogers, W.J. Carr, and S.J.Quinlivan, and K.A. Sargent, "Volcanic suites and Luft,"Geologic map of the Buckboard Mesa quadrangle,related cauldrons of Timber Mountain-Oasis Valley Nye County, Nevada", U.S. Geological Survey GeologicCaldera Complex, Southern Nevada", U.S. Geological Quadrangle Map GQ.552 (1966).Survey Professional Paper 919 (1976).

14. W.J. Carr and W.D. Quinlivan, "Geologic map of the5. R.L. Christiansen, P.W. Lipman, W.J. Cart, F.M. Timber Mountain Quadrangle, Nye County, Nevada",

Byers, Jr., p.p. Orkild, and K.A. Sargent, "Timber U.S. GeologicalSurvey Geologic Quadrangle Map. GQ-503Mountain-Oasis Valley caldera complex of Southern (1966).Nevada", Geological Society of America Bulletin, v.

88, p. 943-959 (1977). 15. R.L. Christiansen and P.W. Lipman, "Geologic map ofthe Topopah Spring NW Quadrangle, Nye County,

6. D.A. Sawyer, R.J. Fleck, M.A. Lanphere, R.G. Nevada", U.S. Geological Survey Geologic QuadrangleWarren, and D.E. Broxton, "Episodic volcanism in Map GQ.444 (1965).the Miocene southwest Nevada volcanic field:

stratigraphic revisions, Ar/Ar geochronologic 16. P.P. Orkild, andJ.T. O'Connor, "Geologic map of theframework, and magmatic evolution", Geological Topopah Spring Quadrangle, Nye County, Nevada",Society of America Bulletin, (in press). U.S. Geological Survey Geologic Quadrangle Map GQ-849

(1970).7. N.K. Huber, "Late Cenozoic evolution of the upper

Amargosa River drainage system, southwestern Great 17. D.C. Noble, S.I. Weiss, and E.H. McKee, "MagmaticBasin, Nevada andCalifornia", U.S. Geological Survey and hydrothermal activity, caldera geology, andOpen-File Report 87.617, (1988). regional extension in the western part of thw

southwestern Nevada volcanic field", Geology and ore8. U.S. Geological Survey, "A summary, of geologic studies deposits of the Great Basin, Raines, G.L., Lisle, R.E.,

through January 1, 1983, of a potential high-level Sharer, R.W. and Wilkinson, W.W., eds., Symposiumradioactive waste repository site at Yucca Mountain, Proceedings, Geological Society of Nevada, p. 913-934southern Nye County, Nevada", U.S. GeologicalSu_ey (1991).Open-FileReport 84-792, p. 16, (1984).

18. S.J. Luft, "Mafic lavas of Dome Mountain, Timber

9. R.G. Warren, F.W. McDowell, F.W., Byers, F.W., Jr., Mountain Caldera, southern Nevada", U.S. GeologicalBroxton, D.E., Carr, W.J., and Orkild, P.P., "Episodic survey Professional Paper 501-D, p. D14-D21 (1964).leaks from Timber Mountain Caldera. new evidence

from rhyolite lavas of Fortymile Canyon, SW Nevada 19. M.A. Dungan, R.A. Thompson, J.S. Stormer, and J.M.volcanic field", Geological Society of America Abstracts O'Neill, "Rio Grande rift volcanism: Northeastern

with Programs, v. 20, n. 3, p. 241, (1988). Jemez zone, New Mexico", Field excursions to volcanicterranes in the western United States, Volume h Southern

10. D.E. Broxton, R.G. Warren, and F.M. Byers, Jr., Rocky. Mountain region, eds. C.E. Chapin and J. Zidek,"Chemical and mineralogic trends with the Timber , New Mexico Bureau of Mines and Mineral Resources

Mountain-Oasis Valley Caldera Complex, Nevada: Memoir46, p. 435-486 (1989).evidence for multiple cycles of chemical evolution in a

long-lived silicic magma system", Journal of Geophysical 20. D.C. Noble, T.A. Vogel, S.I. Weiss, J.W. Erwin, E.H.Research, v. 94, n. B5, p. 5961-5985, (1989). McKee, and L.W. Younker, "Stratigraphic relations and

source areas of ash-flow sheets of the Black Mountain

11. V.M. Frizzell, Jr., and J. Shulters, "Geologic Map of the and Stonewall Mountain volcanic centers, Nevada",

Nevada Test Site, southern Nevada", U.S. Geological Journal of Geophysical Research, v. 89, n. B10, p. 8593-Survey Map 1.2046, (1990). 8602 (1984).

12. S.A. Minor, D.A. Sawyer, R.R. Wahl, V.A. Frizzell, Jr., 21. W.L. Emerick, "Summary of the lithology in the u18j-s.p. Schilling, R.G.Warren, P.P. Orkild, J.A. Coe, M.R. 1 drill hole, Area 18, Nevada Test Site", U.S. GeologicalHudson, R.J. Fleck, M.A. La_phere,WC Swadley, J.C. Survey Technical Letter NTS.22 (1962).Cole, "Preliminary Geologic map of the Pahute Mesa

30' x 60' Quadrangle, Nevada", U.S. Geological Survey 22. B.M. Crowe, D.T. Vaniman,and W.J. Carr, "Status ofOpen-File Report 93-299 (1993) volcanic hazard studies for the Nevada Nuclear Waste

Storage Investigations", Los Alamos NationalLaboratory Report LA-9325-MS(19_3).

23. B.M. Crowe, K.H. Wohletz, D.T. Vaniman, E.Gladney, and N. Bower, "Status of volcanic hazardstudies for the Nevada Nuclear Waste StorageInvestigations", Los Alamos National LaboratoryReport LA-9325-MS,v. II (1986).

24. B.M. Crowe, "Basaltic volcanic episodes of the YuccaMountain Region", In High Level Radioactive WasteManagement, Amer. Soc. Civil Eng. and Amer. Nuc.Soc., Proc. First Internat. Conf., Las Vegas, NV, p. 65-73 (1990).

25. WC Swadley, D.L. Hoover, and J.N. Rosholt,"Preliminary report on late Cenozoic faulting andstratigraphy in the vicinity of Yucca Mountain, NyeCounty, Nevada", U.S. Geological Survey Open.FileReport 84-788(1984).

26. S.C. Lundstrom, J.R. Wesling, F.H. Swan, E.M. Taylor,and J.W. Whitney, "Quaternary allostratigraphy ofsurficial deposit map units at Yucca Mountain, Nevada:a progress report", Geological Society of AmericaAbstracts with Programs, v. 25, n. 5, p. 112, (1993).

27. R.K. Blankkennagel, and J.E. Weir, Jr., "Geohydrologyof the Eastern Part of Pahute Mesa, Nevada Test Site,Nye County, Nevada", U.S. Geological SurveyProfessional Paper 712-B, (1973).

28. B.D. Marsh and R.G. Resmini, "Longevity of magma inthe near subsurface: a study using crystal sizes inlavas"In High Level Radioactive Waste Management,Amer. Soc. Civil Eng. and Amer. Nuc. Soc., Proc. 3rdInternat. Conf., Las Vegas, NV, p. 2025-2032 (1992).

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