. * DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United Stales Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The viows and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. CONF-8205157—2 DE87 008461 GEOLOGIC CHARACTER OF TUFFS IN THE UNSATURATED ZONE AT YUCCA MOUNTAIN, SOUTHERN NEVADA Robert B. Scott Richard W. Spengler Sharon Diehl U.S. Geological Survey, MS 954 Denver Federal Center Denver, CO 80225 A. R. Lappin Sandia National Laboratories Box 5800 Albuquerque, NM 87185 Michael P. Chornack Fenix & Scisson Box 498 Mercury, NV 89023 ABSTRACT At Yucca Mountain, a potential site for a high-level nuclear waste repository on the Nevada Test Site in southern Nevada, evaluation of the geologic setting and rock physical properties, along with previous regional hydrologic studies, has provided background that can be used for construction of a preliminary conceptual hydrologic model of the unsaturated zone. The 500-m-thick unsaturated portion of Yucca Mountain consists of alternating layers of two contrasting types of tuff. One type consists of highly fractured, densely weld- ed, relatively nonporous but highly transrnissive ash—flow tuffs. The other type consists of relatively unfractured, nonwelded, highly porous but relatively nontransmissive, argillic and zeolitic bedded tuffs and ash-flow tuffs. The contrast between these two sets of distinctive physical properties results in a stratified sequence best described as "physical-property stratigraphy" as opposed to tradi- tional petrologic stratigraphy of volcanic rocks. Superimposed on this layering are two sets of fault6 and fractures: one strikes north-northwest (N. 15° W. to N. MASTER 289 DISTRIBUTION Of THIS VX-UNZHT (S iMJMITEO
Transcript
. *
DISCLAIMER
This report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United Stales Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viowsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.
CONF-8205157—2
DE87 008461
GEOLOGIC CHARACTER OF TUFFS IN THE UNSATURATEDZONE AT YUCCA MOUNTAIN, SOUTHERN NEVADA
Robert B. ScottRichard W. SpenglerSharon Diehl
U.S. Geological Survey, MS 954Denver Federal CenterDenver, CO 80225
A. R. LappinSandia National LaboratoriesBox 5800Albuquerque, NM 87185
Michael P. ChornackFenix & ScissonBox 498M e r c u r y , NV 89023
ABSTRACT
At Yucca Mountain, a potential site for a high-levelnuclear waste repository on the Nevada Test Site in southernNevada, evaluation of the geologic setting and rock physicalproperties, along with previous regional hydrologic studies,has provided background that can be used for construction ofa preliminary conceptual hydrologic model of the unsaturatedzone.
The 500-m-thick unsaturated portion of Yucca Mountainconsists of alternating layers of two contrasting types oftuff. One type consists of highly fractured, densely weld-ed, relatively nonporous but highly transrnissive ash—flowtuffs. The other type consists of relatively unfractured,nonwelded, highly porous but relatively nontransmissive,argillic and zeolitic bedded tuffs and ash-flow tuffs. Thecontrast between these two sets of distinctive physicalproperties results in a stratified sequence best describedas "physical-property stratigraphy" as opposed to tradi-tional petrologic stratigraphy of volcanic rocks.
Superimposed on this layering are two sets of fault6and fractures: one strikes north-northwest (N. 15° W. to N.
MASTER 289 DISTRIBUTION Of THIS VX-UNZHT (S iMJMITEO
40° W.) and dips steeply (60°-90°) westward; the otherstrikes north-northeast (N. 5° E. to N. 35° E.) and alsodips steeply westward. The north-iiortheast set constitutesthe major Basin and Range style normal faults that separatethe gently eastward-tilted major structural blocks of YuccaMountain. The hanging-wall sides of these faults commonlyexhibit increases in dip similar to rollover structurescommonly related to listric faults. The north-northwestfracture set has the higher density, and the north-northwestfault set appears to have two directions of displacement.One is right-lateral slip perhaps related to the Las VegasValley and Walker Lane shear zones; the other is dip slipperhaps related to brittle drag behavior adjacent to themajor north-northeast-striking faults.
The vast majority of recharge through the unsaturatedzone is assumed to be vertical; the dominant migration mayoccur in fractures of densely welded tuffs and in the matrixof nonwelded tuff, but the mode of fluid flow in theseunsaturated systems is undetermined. Limited lateral flowof recharge may occur at horizons where local perched watertables may exist above relatively nontransmissive zeolitizednonwelded tuffs. The pervasive north-northwest-strikingfractures may control the direction of lateral flow ofrecharge, if any, in the unsaturated zone, and certainlythat direction coincides closely with the observed south-easterly flow direction in the saturated zone under YuccaMountain. Empirical evaluation of this conceptual hydro-logic model has begun.
INTRODUCTION
The Nevada Nuclear Waste Storage Investigations proj-ect, administered by the Nevada Operations Office of theU.S. Department of Energy, is examining the feasibility ofemplacing nuclear wastes in southern Nevada. As part ofthat effort, the U.S. Geological Survey (USGS) and SandiaNational Laboratories (SNL), among other research groups,have participated in investigations of the geology, hydrol-°8y» geophysics, and rock properties of the thick sequenceof silicic tuffs that occur m.thin and contiguous to theNevada Test Site (NTS) in Nye County, Nevada (Figure 1).These efforts augment more than 20 years of geologic re-search by the USGS at the NTS [1], within the southern partof the Great Basin subprovince. Site-specific investiga-tions, carried out since 1978, have concentrated on thecharacterization of the 1.5- to 4 km-thick sequence of ash-flow tuffs and related rocks at Yucca Mountain (Figure 2).Recently, emphasis has been placed on the unsaturated zone
290
II6°3O'
EXPLANATION/^p Areoj of
positive relief
J-13 Woler well
—37600"
10 20 KILOMETERS
Figure 1. Location map. Index map at lower left shows thelocation of the Nevada Test Site (NTS) within Nevada,and the center shows an expansion of the southwestportion of the NTS.
which is some 500 to 800 m thick under Yucca Mountain. Thesequence within the unsaturated zone- consists of a widerange of contrasting rock types including densely weldeddevitrified and vitric ash-flow tuffs, nonwelded vitric ash-flow tuffs, vitric bedded tuffs, nonwelded zeolitized andargillized ash-flow tuffs, and zeolitized and argillizedbedded tuffs.
291
[ lOuatarnary alluwland colluvlum
Ralnlar Maaa Mimbarof Timber Mountain 2̂̂ ]
Tuff[_I_JT.t!l«r» Palntbruah
TuffI,*• '̂ -M Undlffarantlatad
Tartlary volcanicrocha
Normal Fault, bar andball on downthrown aid*d i ih td whir t Intmtfi);
°USW- Drill HolaG? locallona
' 1 Maaaurcd7 Sactlona
> CLAIM CANYONiNCAULDRON RIM
* - 36°52<30"
CRATERFLAT
Figure 2. Geologic map of central Yucca Mountain general-ized from the work of Christiansen and Lipman [3] andLipman and McKay [4]; and modified based on results ofrecent detailed mapping. Exact boundaries of the areaof the potential repository within this map area arenot drawn because data are still being collected thatmay affect the most favorable boundaries.
292
This chapter deals principally with the geologic char-acter and rock properties that potentially affect thehydrology of the unsaturated zone. Because the static waterlevel at Yjcca Mountain is 500 to 800 m below the surface,the unsaturated zone provides a thick sequence of welded andnonwelded tuffs for possible storage cf nuclear waste.Several reasons for serious consideration of the unsaturatedzone for waste storage in the Great Basin subprovince werereviewed by Winograd [2]; although the basic logic outlinedby Winograd emphasized burial in thick alluvium, his logiccan be extended to the unsaturated zones of fractured,densely welded ash-flow tuffs. This extension of logicincludes: 1) low recharge flux in arid climates; 2) movementof recharge through highly transmissive, thermally stable,densely welded tuffs; 3) thick unsaturated zones; A) multi-ple barriers to radionuclide migration including retardationby sorptive zeolitic and argillic nonwelded tuffs; 5) favor-able thermal loading; and 6) economic factors of relativelyshallow burial.
Among the most critical parameters of nuclear wasteisolation is the transport of radionuclides by ground water.Ground-water movement in the NTS region is largely a func-tion of effective rock conductivities [5]. Factors thatgovern effective rock conductivities in an ash-flow tuff arenumerous; several factors produce matrix conductivity: Atthe time an ash flow is initially emplaced, the glass shardmatrix becomes flattened by the welding process, providedthe interior of the ash flow is at temperatures in excess of500°C. This welding process creates an inverse relationbetween glass shard porosity and the degree of welding, buta positive relation between bulk density and the degree ofwelding. In the process of squeezing trapped gases frombetween the shards, gas-rich portions of ash-flow tuffs maydevelop gas pockets called lithophysal cavities. Most ash-flow tuffs consist of intercalated tongue-like sheets re-sulting from a series of magmatic pulses; if extruded over ashort period at high temperatures, these individual sheetseffectively weld into one sheet with decreases in degree ofwelding observable only at the margins of the sheet. Suchsheets are called simple cooling units [6]. Alternatively,cooling periods of sufficient duration between individualmagmatic pulses may cause decreases in the degree of weldingwithin the interior of the cooling unit. Such ash-flowsheets are called compound cooling units. Within eithercompound or simple cooling units, small pore sizes (lessthan tens of micrometers) and assumed high tortuosities,particularly in the densely welded interiors of ash flows,create very low matrix conductivities: typical values areabout 10"^ cm/s for vitric nonwelded tuffs and about 10
293
cm/r> for densely welded tuffs [5].Superimposed upon this cooling-unit framework and
matrix conductivity are the effects of 1) devitrification inthe slowly cooled interiors and vapor-phase crystallizationin the partially welded margins, 2) vertical extensionjoints produced during cooling, 3) alteration to secondaryphases, by reaction with pore waters to form zeolites andclays, concentrated in less welded portions of the coolingunits, and 4) faults and joints related to tectonic events.The term fracture is used to refer to any crack with unde-termined offset; those without offset are joints, and thosewith known offset are faults. A wide variety of fracturedensities, attitudes, and apertures are formed as a functionof the behavior of a particular rock type to its stressenvironment. Also, a variety of minerals may fill part orall of the fractures.
The net effect of all these processes, discussed in theparagraph above, is to alter drastically the effective rockconductivities. Highly fractured, densely welded tuffs haveeffective hydraulic conductivities about 5 or 6 orders ofmagnitude higher than their matrix hydraulic conductivities[5]. Nonwelded vitric tuffs have effective hydraulic con-ductivities only about 1 order of magnitude higher thanmatrix hydraulic conductivities, but zeolitized or argil-lized nonwelded tuffs have effective hydraulic conductiv-ities 1 to 4 orders of magnitude higher than matrixhydraulic conductivities [5,7]. A note of caution should beadded here because these effective hydraulic conductivitieswere measured in saturated rocks [5]; those for the unsatu-rated rocks of interest at Yucca Mountain may be signifi-cantly lower and may be related to the degree of saturation.These differences emphasize the need to evaluate rock prop-erties in their stratigraphic and structural framework.Thus, the principal objective of this paper is to character-ize qualitatively those rock-mass properties that may affectthe hydrology of the unsaturated zone at Yucca Mountain.
PREVIOUS GEOLOGIC INVESTIGATIONS
Prior to nuclear waste storage investigations, geologicresearch and exploration within the immediate vicinity ofYucca Mountain were limited to reconnaissance geologicquadrangle mapping [3-4]. Several summaries of the volcano-tectonic history of the NTS region that specifically addressthe origin of ash-flow tuffs exposed at Yucca Mountain havealso been published [8—10]. Regional tectonic and struc-tural relationships for the NTS and vicinity have beensummarized by Carr [11].
294
Stratigraphic Framework
Although less than 2. km of Miocene (12.5 to 14 m.y.-old) ash-flow tuffs with minor lavas and bedded tuffs havebeen penetrated by drilling with continuous core recovery[12-14] (Florian Maldonado, USGS, and Sarah Koether, Fenlx &Scisson (F&S), written commun., 1982), geophysical evidencesuggests that a sequence of tuffaceous rocks between 1.5 and4 km thick overlies a pre-Cenozoic basement under centralYucca Mountain [15]. The volcanic sequence beneath YuccaMountain is shown on a north-south cross section (Figure3). Four major ash-flow tuZfs have been been penetrated by
NORTH SOUTH
Km. T*rllary YUCCA2-> •:..::• £££;} osiv-G2 USW-GI MOUNTAIN
0-
- 1 -
usw-GU3/G3
CLAIM '51
CANYON 5
CAULDRON RIM
T7if_oi_ti!5i£—2^*
Older Unnamad Tufts
? "^^ PraCanozolc rock*
— 1
M.r«al f . . l t , arrawtojlcal.a r.tatl*. M I
— » — IHaalaale aaalacl,tfaahaa • • • Q..rlafl*
SWL Sl.tl* »•!.. L.
utm-mx
o 2 km.
Figure 3. Generalized north—south geologic section throughTertiary volcanic strata under Yucca Mountain. SWLmarks the static water level. Horizontal and verticalscales are the same. The zero depth is mean sea level.
drilling. These tuffs are listed in ascending order: 1)The informal tuff of Lithic Ridge (300+m thick) (W. J. Carr,USGS, F. W. Byers, Los Alamos National Laboratory (LANL),and Paul Orkild, USGS, written commun., 1982) consists of apartially to moderately welded, rhyolitic compound coolingunit. 2) The Crater Flat Tuff includes three rhyolitic ash-flow tuff members. The basal unit is the informal Tram unit(300±m thick), which consists of a partially to moderatelywelded, compound cooling unit. The middle member is theBullfrog Member (200±m thick), which consists of a partiallyto densely welded, compound cooling unit. The upper memberis the Prow Pass Member (100±m thick), which consists of apartially welded, compound cooling unit (W. J. Carr, USGS,
295
F. W. Byers, LANL, and Paul Orkild, USGS, written commun.,1982; R. B. Scott, USGS, and Mayra Castellanos, F&S, writtencommun., 1982). All three members are considered to haveerupted from a caldera complex now buried below Crater Flatseveral kilometers to the west of Yucca Mountain [16] (W. J.Carr, USGS, F. W. Byers, LANL, and Paul Orkild, USGS, writ-ten coiranun., 1982). 3) A nonwelded sequence of rhyoliticash-flow tuffs and bedded tuffs (30 to 275 m thick) isinformally called the tuffaceous beds of Calico Hills [12](Florian Maldonado, USGS, and Sarah Koether, F&S, writtencommun., 1982; R. B. Scott, USGS, and Mayra Castellanos,F&S, written commun., 1982). 4) The uppermost formation,the Paintbrush Tuff, here consists largely of two major ash-flow tuffs, the Topopah Spring Member (300±m thick) and theTiva Canyon Member (100±m thick). Both the Topopah Springand Tiva Canyon Members were erupted from the Claim CanyonCauldron about 2 km north of Yucca Mountain [8]. Both theTopopah Spring and the Tiva Canyon Members largely consistof moderately to densely welded tuff, and they are bothcompositionally zoned from the high-silica rhyolites thatform their basal and central portions to quartz latites thatform densely welded caprocks near the top of each member[17,8]. In the stratigraphic interval .between the TopopahSpring and Tiva Canyon Members, two minor members of thePaintbrush Tuff, the Pah Canyon and Yucca Mountain Members,are present at the north end of Yucca Mountain as distaledges of nonwelded to moderately welded sheets that pinchout toward the south and east within the central portion ofYucca Mountain [12] (Florian Maldonado, USGS, and SarahKoether, F&S, written commun., 1982). Where these twomembers become too chin to map, they are lumped togetherwith unnamed bedded tuffs. Although the Paintbrush Tuff isthe youngest (12.5 to 13.1 m.y. old) ash-flow tuff to coverthe entire Yucca Mountain area, the 11.3-m.y.-old RainierMesa Member of the Timber Mountain Tuff was locally pondedin post-Paintbrush Tuff structural depressions, coveringportions of the normal faults that delineate the majorstructural blocks of the mountain (Figure 2).
The remainder of this chapter considers the character-istics of those units that occur above the static waterlevel at Yucca Mountain. The static water level (Figure 3)within Yucca Mountain is located near the base of thePaintbrush Tuff at drill hole USW-G2, near the base of thetuffaceous beds of Calico Hills at USW-GI, and within theCrater Flat Tuff at USW-GU3/G3 [12] (Florian Maldonado,USGS, and Sarah Koether, F&S, written commun., 1982; R. B.Scott, USGS, and Mayra Castellanos, F&S, written commun.,1982).
296
Structural Framework
Paleozoic strata are not exposed within 15 km of YuccaMountain, though seismic refraction (W. D. Mooney, USGS,written commun., 1982) and gravity data [15] suggest thatPaleozoic rocks may be present under 1.5 to A km of volcanicstrata. As no drilling has penetrated these rocks underYucca Mountain, their character must be extrapolated fromnearby exposures in the Specter Range, Calico Hills, andBare Mountain (see Figure 1) [18}• Paleozoic rocks in theselocalities have been deformed by complex folding and thrustfaulting during formation of the late hesozoic Sevier oro-genic belt [19-20]. Ambiguities inherent in these region-ally complex structures do not allow reliable projections ofeither structures or lithologies under Yucca Mountain.Paleozoic carbonate or clastic rocks are likely to form thesubvolcanic rocks. Regional interpretations of theseMesozoic structures vary [11,20] (M. D. Carr, USGS, oralcommun., 1982), and possible structures under Yucca MountainInclude upper or lower plates of regional overthrusts, low-angle gravity-controlled fault complexes, or broad folds.Because Paleozoic rocks are regionally important aquifersand aquitards in the southern Great Basin [5], penetrationinto subvolcanic rocks remains one of the ultimate objec-tives of the USGS investigations at Yucca Mountain.
Regionally, three types of Cenozoic structures exist:1) Basin and Range style faults, 2) major strike-slip faultzones, and 3) volcano-tectonic structures. Relative to theheterogeneous and complex Cenozoic structure common Insurrounding areas, the structure at Yucca Mountain appearsuncomplicated. As exposures of pre-Paintbrush Tuff rocksare sparse, the apparent structural simplicity of YuccaMountain may be misleading to some degree. However, struc-tural data from cores to depths of nearly 2 km do not sug-gest the presence of structural styles or complexities notapparent on the surface. Pre-Paintbrush Tuff Tertiarystructures below the 2 km depths explored by drilling prob-ably do exist beneath Yucca Mountain, as suggested by thegravity and magnetic gradients [15] (G. D. Bath, USGS,written commun., 1982).
As shown in quadrangle maps [3,4], Yucca Mountainitself is broken by several major north-northeast-strikingand generally westward dipping Basin and Range style normalfaults with tens to hundreds of meters of vertical displace-ment, forming blocks tilted gently eastward. In general,these faults decrease in displacement and density toward thenorth (Figure 2). Basin and Range style normal faulting
297
began prior to volcanism [21,11] and continued during mostof the major eruptive activity; apparently, few If anyfaults formed after volcanism ceased, although relativelyminor additional displacements have occurred locally onpreexisting faults during the last few million years.Regional evidence of Quaternary faulting in response to aminimum principal horizontal stress direction of about N.50° W. has been reviewed by Carr [11]. Direct evidence ofthis type of faulting at Yucca Mountain is not reported inprevious studies, but has been found by recent mapping (W.J. Carr, USGS, oral r.ommun., 1982). The original geologicmapping at Yucca Mountain [3,A] did not identify volcano-tectonic structures associated with the Crater Flat Tuff;however, such structures are now believed to exist in thevicinity of Crater Flat [15].
Hydrologic Framework
Extensive summaries of the NTS region and local hydrol-ogy have been made [5,7,22-24], but no details were previ-ously available concerning the Yucca Mountain area. Theregional water table is commonly 500 to 800 m deep and has asouth and southwest flow direction toward the AmargosaDesert in most of the region. However, in local areas con-siderable variation is observed such as the southeast flowdirection under Yucca Mountain. The principal aquifers Inthe NTS region are Paleozoic carbonate rocks and highlyfractured Tertiary densely welded ash-flow tuffs. The majoraquitards are Paleozoic clastic rocks and Tertiary zeoli-tized or argillized nonwelded tuffs. For example, a fewkilometers east of Yucca Mountain in Jackass Flats (Figure1), where the Topopah Spring Member is below the watertable, the densely welded tuff is an aquifer in well J-13[24,5]. Matrix and effective hydraulic conductivities ofthe major rock types found within ash-flow tuffs at the NTSare given in Table I.
GEOLOGIC CHARACTER OF THE UNSATURATED ZONE
Physical-Property Stratigraphy
The movement of fluids through rock is fundamentally aphysical process. For this reason, a stratigraphic frame-work designed to emphasize potential differences in thehydrologic character among strata should be based upon rockphysical properties. Petrologic differences, usually iden-tified by phenocryst ratios, are traditionally used to
298
Table I . Hydraulic Conductivities of Tuffaceous Aquifersand Aquitards, NTS
Hydrologic
Unit
Rock
Type
Range of Matrix
Hydraulic
Conductivities
(cm/s)
1
Range of Effective
Hydraulic
Conductivities2
(cm/s)
Aquifer
Leaky i
Aquitard
Aquitard
Aquitard
Denselyweldedtuff3
Nonweldedvitric tuff
Nonweldedzeolitized
tuff
Nonweldedargillized
tuff
3 X 10,-10,-8to 10
10-*
2 X 10~y
to 3 X 10~5
lO-1^ to2 X 10 5
10r5to 3 X 10,-2
-3
10,-6
to 10~A
10,-6
to 10,-A
Upper range for welded tuff from page 34, lower range forwelded tuff from "Figure 18, and zeolitized and argillizedtuff from Table 5 from Winograd and Thordarson [5J.
From Table 1 of Winograd and Thordarson [5] and Thordarson(USGS, written connnun., 1965), and estimated from thick-nesses of nonwelded vitric tuffs at margins of coolingunits.
Values for both devitrified and vitric densely welded tuffare included.
Term used by Winograd and Thordarson [5].
define stratigraphic breaks in sequences of ash-flow tuffs,but for the hydrologic and engineering concerns of a nuclearwaste repository, physical property differences in porosity,bulk density, grain density, permeability, and degree ofwelding, along with the density and attitudes of fracturesand faults, are the important features. To contrast thesetwo different stratigraphic classification schemes, Figure 4compares petrologic stratigraphy with a physical-property
299
stratigraphy based on the degree of welding in the sequenceof rocks cored at holes USW-GU3/G3, -Gl, and -G2 [12] (R. B.Scott, USGS, and Mayra Castellanos, F&S, written comraun.,1982; Florian Maldonado, USGS, and Sarah Koether, F&S,written commun., 1982). The physical-property subdivisionsshown in Figure 4 were produced by placing bedded tuffs andnonwelded to partially welded ash-flow tuffs into one cate-gory and moderately to densely welded ash-flow tuffs into asecond category. The first category consists of relativelylow-density rocks that tend to be more porous and lessfractured than those in the second category, which consistsof relatively high-density rocks that tend to be less porousand more fractured. In combination with structural data,physical-property stratigraphy, as expressed in Figure 4, isparticularly useful for construction of conceptual hydro-logic models. The variability in this physical-propertystratigraphy from USW-GU3/G3 in the southern part of theYucca Mountain region to USW-G2 in the northern part of theregion is considerable, over a distance of only 8 km.
Detailed investigations of the physical properties ofthe Tiva Canyon Member of the Paintbrush Tuff were possiblebecause of the excellent three-dimensional exposures of thismember at Yucca Mountain. The Tiva Canyon Member is verysimilar in overall character to the less well exposedTopopah Spring Member. This similarity is supported by theobservation that the physical properties of surface samplesof the welded Tiva Canyon Member are comparable to those ofcore samples of the welded Topopah Spring Member [26-27].In addition to studies of the more welded units, determi-nations of physical properties have been made for the beddedand nonwelded tuffs [28]. These physical property studiesare augmented by petrological and -.hemical studies made atthe Los Alamos National Laboratory [29], which describe thecharacter of clay and zeolite alteration in both welded andnonwelded tuffs.
Mappable Zonations within the Tiva Canyon Member
Zonations within ash-flow tuffs, as defined by Smithf6], primarily reflect the effects of the thermal evolutionof ash-flow tuffs; Lipman and others [17] discussed theorigin of primary chemical zonations that reflect primarymagmatic differences. Zonations used to study ash-flowtuffs at Yucca Mountain are recognized in the field bydifferences in groundmass devitrification, degree of weld-ing, shape of eroded slopes, texture of weathered surfacescreated by both surficial cracking and tectonic fracturing,lithophysal cavity abundance, lithic fragment abundance,
Figure 4. Comparison between stratigraphic units definedby petrologic criteria and those defined by physical-property criteria. The conventional stratigraphic no-menclature based upon petrologic criteria is given fordrill hole USW-GU3/G3 on the left and the physical-property stratigraphies based on degree of welding fordrill holes USW-GU3/G3, -Gl, and -G2 are drawn in order' toward the right. The conventional stratigraphic names
301
of major units are also listed on the far right forgeneral reference but exact boundaries are not shown.The shaded zones are moderately welded to densely weld-ed; unshaded are nonwelded to partially welded. BT •bedded tuff.
phenocryst ratios, presence of vapor-phase crystallization,and color. Besides forming stratigraphic horizons particu-larly valuable for recognition of small faults, these zonesprovide a stratigraphic framework for comparison with thephysical properties of the recks. A fence diagram of thi6zonation in the Tiva Canyon Member is shown in Figure 5;each station of the fence diagram is a measured section(located in Figure 2) where samples were also collected forphysical- property studies. The informal field terms forthese zones are used on the fence diagram.
DRILL HOL£ WASH
ABANDONED
WASH
THE PROW
Figure 5. Generalized zonation within the Tiva CanyonMember. The rounded step zone grades laterally intothe clinkstone zone. Measured section numbers arelocated on Figure 2.
Bulk Density, Porosity, and Grain Density
Measured dry bulk densities, matrix porosities, andgrain densities for different zones of the Tiva CanyonMembsr are shown on Table II. Similar values have beenreported for these physical properties in the core of drillhole UE25a-l [25]. The interdependence of dry bulk densityand matrix porosity is shown in Figure 6; as expected, drybulk density, increases and porosity decreases as weldingincreases. Different degrees of welding should not bedefined by specific porosity or density limits becauseconsiderable variation in both total porosity (including
302
Table II. Physical Properties of the Tlva Canyon Member of thePaintbrush Tuff, Yucca Mountain
Dry bulk density calculated using volume of sample based on Immersion;external llthophysal cavities are generally not evident In those data.
Calculated by powdered pycnoraeter method.
Calculated value, based on the formula: percent porosity - (grain density -dry bulk denslty)/graln density; therefore, llthophysal porosity generallyis not included in these daca.
lithophysal porosity) and matrix porosity, are common atconstant degree of welding. Explanations for these vari-ations include the presence of secondary mineralization,vapor-phase crystallization, and formation of lithophysalcavities.
Small shifts in the porosity-dry bulk density relation-ship shown in Figure 6 are not related to the degree ofwelding, but instead are related to small changes in graindensity. The vitric basal portion (Figures 6 and 7) hap thelowest average grain density within the Tiva Canyon Member(2.38±0.06 g/cm3). This is the same value reported for thevitric portion of the Topopah Spring Member in drill holeUSW-G1 [26]. The average grain density within the partiallyvitric columnar zone of the Tiva Canyon Member is 2.46+0.06
304
50
40-
c/)O£TOQ.
30-
20 -O
LLJ
Q_
/ O -
nonweldad
partially
weldedUpper Portion(Caprock andupper cl i f f )
LowerPortion
(Columnarand basal)
mo derate! y
welded
densely
welded
1.5 2.0DRY BULK DENSITY g/cm3
2.5
Figure 6. Relations between dry bulk density, matrix poros-ity, and degree of welding of the Tiva Canyon Member.The central portion includes the devitrified zones: thehackly, lower lithophysal, clinkstone, and upper litho-physal zones. Brackets show the general regions typi-cal of rocks of different degrees of welding.
305
DRILL HOLE WASHABANDONED
WASH
THE PROW
2.38 10OS 2 4S1OOC
Figure 7. Grain density (g/cm ) correlation with strati-graphic zones in the Tiva Canyon Member, PaintbrushTuff.
g/cm , within the completely devitrified central portion is2.48*0.04 g/cm3, and within the upoer cliff and caprockzones is 2.51±0.03 g/cm3. The gra n densities in the TivaCanyon Member reported here are significantly lower thanthose reported for the devitrified portions of the TopopahSpring Member, which average 2.55 g/cm3 [26]. Similarresults were also obtained in the physical property studiesof ash-flow tuffs in the Timber Mountain Caldera region (W.J. Carr, USGSt written commun., 1966).
Thus, a consistent trend of increasing grain densityexists from the partially devitrified columnar zone upwardthrough the devitrified central zones to the devitrifiedupper cliff and caprock zones, consistent with the offsetsof curves in Figure 6. Several explanations for this trendcan be given. First, the increase in average grain densityof the partially devitrified columnar zone relative to thevitric basal zone is caused by the greater density of devit-rification products, quartz, tridymite, cristobalite, andfeldspar, compared to volcanic, glass in the vitric zone.Second, higher densities in the central devitrified portionbetween the columnar and the upper cliff zones are alsorelated to the greater densities of devitrified phases; theincrease of grain density near the top of the central por-tion, specifically in the upper lithophysal zone, may berelated, in part, to an increase in the phenocryst contentfrom about 3 percent in the lower zones to about 6 percentin the upper lithophysal zone. Third, the greatest averagegrain densities, those found in the upper cliff and caprockzones, are probably related to two factors, an increase inphenocryst content to about 15 percent and the more maficrock composition of those zones. Still another source ofgrain density variation within the devitrified portion of
306
the unit may be the ratios of silica polymorphs and feld-spar; in particular, the grain densities of cristoballte(2.33 g/cm3) and tridymite (2.26 g/cm3) are significantlylower than that of quartz (2.65 g/cm3). However, there isalmost no quartz in the devitrification products in the TlvaCanyon Member, and the feldspar/silica polymorph ratio isessentially constant (Dave Bish, LANL, written commun.,1982). The presence of abundant quartz in the devitrifi-cation products of the Topopah Spring Member is consistentwith the higher densities in the devitrified portion of thatunit.
Devitrification
^e devitrification textures evident in thin sectionsare a u u related to zones mapped in the field, and probablyexplain many of the differences in the character of weather-ed surfaces. For example, the hackly zone (see Figure 7)weathers to centimeter-sized irregular fragments, in con-trast to the overlying clinkstone zone, which forms meter-long, smooth, conchoidally fractured blocks; field relation-ships do not suggest an explanation for this difference insurface characteristics. In thin section, however, thedevitrification texture within the hackly zone consists ofshort, finely fibrous spherulitic patches (Figure 8a), whilethat in the clinkstone zone consists of long, coarselyfibrous to granophyric textures which both obscure and crossglass shard boundaries, forming a more uniform three-dimensional intergrowth (Figure 8b). The devitrificationtextures in the upper portions of the clinkstone zone and inthe upper cliff and caprock zones are increasingly coarsergranophyric with gradational contacts. The columnar zonehas an almost unaltered flattened glass shard axiolitictexture that gradationally becomes devitrified toward thetop of the zone. The basal zone has partially flattenedshards with a hydrated(?) rim. The boundaries between thesedevitrif?zation textures approximately follow the mappedzone boundaries (Figure 9).
Because these differences should affect the means bywhich these units fracture after devitrification, a corre-lation between devitrification zones and the density offractures is expected. In drill hole USW-GU3/G3 where corewas recovered from the Tiva Canyon Member, the calculateddensity of fractures in the densely welded portion of theunit is calculated to be 14 fractures/unit m in the upperlithophysal zone, 22 fractures/unit m in the clinkstonezone, 22 fractures/unit m3 in the lower lithophysal zone, 26fractures/unit m in the hackly zone, and 21 fractures/unit
30/
(a)
(b)
Figure tt. Photographs of thin sections of devitrified tex-tures in the Tiva Canyon Member in the (a) hackly .zoneand (b) clinkstone zone. The glass shard texture ismore distinct in the hackly zone but partially destroy-ed in the clinkstone zone*
308
DRILL HOLE WASH
ABANDONEDWASH
THE PROW
FLATTINCD •KA*DSOMADtt UPMAIB TODEVITKIFICATIOH
IT PIBMOUtM TEITUBI
Figure 9. Stratigraphy of devitrification textures in theTiva Canyon Member, Paintbrush Tuff. The dashed linesmark contacts between devitrification zonations that donot coincide with zone boundaries.
m in the columnar zone. The partially welded caprock zoneof the Topopah Spring Member has a fracture density of about18 fractures/unit nr* and should be representative of theunsampled portion of the Tiva Canyon caprock zone (an expla-nation of the fracture density units and calculation followsin a discussion of fractures). It may be significant thatthe hackly zone, with fine spherulitic devitrificationtextures,,,has the highest fracture density and that theupper lithophysal zone has the lowest fracture c2nsity,though it is apparent that zones of approximately the samedegree of welding have approximately the sane density offracturing, in spite of variations in devitrification tex-ture and lithophysal content. One exception to this generalrule is that the density of fractures within the denselywelded vitrophyres is appreciably lower than the overlyingdevitrified portion; for example, the Topopah Springvitrophyre has a density of 8.8 fractures/unit m .
In contrast, the partially welded to nonwelded base ofthe Tiva Canyon Member has only 9 fractures/unit m , and theunderlying bedded tuffs and nonwelded tuffs have less than 1fracture/unit m . Thus, devitrification textures and litho-physal content appear to have much less impact on fracturefrequency than degree of welding and vitrophyres have inter-mediate fracture densities.
Lithophysal Zones
Zones containing abundant lithophysal cavities or gaspockets may significantly affect many of the physical prop-
309
erties of ash-flow tuffs. Several lithophysal zones occurin the compound cooling unite of the Tiva Canyon and theTopopah Spring Members; these have been mapped and studiedin measured sections of the Tiva Canyon Member (Figure 5)and studied in measured sections and core of the TopopahSpring Member (Figure 10). Field relationships of thelithophysal zones in the Tiva Canyon Member suggest thatthese zones are continuous sheets, and that each lithophysalzone represents a separate gas-rich eruptive pulse. Fieldand detailed petrographic evidence suggests that the TivaCanyon Member actually consists of at Iea6t 8 eruptivepulses, only two of which were gas-rich. The somewhat moreirregular pattern of lithophysal zones in the Topopah SpringMember shown in Figure 10 probably represents a series of
Figure 10. Lithophysal zonation in the Topopah SpringMember, Paintbrush Tuff. Locations of drill holesUSW-G2, -Gl, and -GU3/G3 and the measured sections 3,7, and 8 are shown on Figure 2.
somewhat discontinuous stratigraphic tongues of gas-richmagmatic pulses that created llthophysal-rich horizons. Atleast 9 separate magmatic pulses can be recognized frompetrographic evidence in the core from drill hole USW-GU3/G3alone, and evidence for other stratigraphic units are foundelsewhere in the field and in core [17] (R. B. Scott, USGSand Mayra Castellanos, F&S, written commun., 1982). Approx-imately 2 of these pulses were sufficiently gas-rich toproduce lithophysal zones.
Lithophysal cavities consist of spherical to highlyoblate voids ranging from less than 1 cm to as much as 30 cm
310
in maximum diameter. Surrounding these voids is a thinmm thick) inner rim of vapor-phase crystals; cristobalite,tridymite, and alkali feldspar are common and plates ofhematite, prisms of pseudobrookite, and needles of alkaliamphiboles rare. Outside the vapor-phase rim itself is anouter rim of pale-colored altered rock matrix, commonlyabout 1 cm thick. In general, the alteration in this rimconsists of the products of high-temperature mineraloglcaland chemical processes, but mixed layei clays are locallyreported [29]. Lithophysal porosity varies from less than 1percent to greater than 30 percent of the rock.
Obviously these cavities will influence effectivehydraulic conductivities, especially in fractured zones;hydraulic tests of these specific zones are planned. Alsothese cavities, which are air-filled above the static waterlevel, decrease rock thermal conductivities and bulk den-sities, and will alter rock-mass engineering or mechanicalproperties. Therefore, determination of the actual three-dimensional distribution of lithophysae within the TopopahSpring Member is being given high priority.
STRATIGRAPHIC CONTROL OF STRUCTURAL CHARACTER
Expression of Fracture Density
Knowledge of the character of fractures in a rock massis essential to understanding or predicting its hydrologiccharacter. Therefore, it is critical to express the atti-tude, distribution, and density of fractures properly wheninterpreting data collected either along an outcrop or indrill core. A critical problem in interpretation is createdby fractures that intersect cores or traverses at anglesless than 90°. In such a situation, the apparent fracturefrequency per unit length along core or outcrop may besignificantly less than the true fracture frequency per unitlength perpendicular to the fracture set [30]. First,consider a simplified example where a core is cut by a setof parallel fractures that intersect the core axis at someangle less than 90°. In this example, the actual frequencyof fractures per unit length in a direction perpendicular tothe fracture set can be expressed by the relation:
Fc = [sin A ] "1 • Fffl
where Fc is the corrected fracture frequency, Fn is theapparent fracture frequency measured along the core axis,and A is the acute angle between the core axis and thefracture set (note: if the dip of the fracture is measured,use [cos dip]"*). This correction, in effect, determinesthe number of fractures that would have been encountered ifthe core were cut exactly perpendicular to the fracture
311
plane* In the real world of infinite fracture attitudes ina rock mass, fracture frequency per unit distance in allpossible directions perpendicular to all fracture planesmust be expressed. The IOCUP of that unit distance in filldirections geometrically generates a sphere. The sum of allthe corrected fracture frequencies thus becomes the numberof fractures within that spherical volume, that is, fracturedensity. The diameter of that sphere is the unit distancealong which each fracture density is calculated. Becausedistance along a core or traverse increases by distance tothe first power but volume increases by distance to thethird power, the density of fractures must be expressedrelative to a standard unit volume. In this work, thenumber '•'" -racture6 per sphere with a unit volume of 1 m isarbitarily chosen as the standard of comparison. Thus theunit distance, along which the fracture density is calcu-lated, is the diameter of the sphere; for a sphere of 1 unitm , a unit distance of 1.24 meters must be used*
As an example, over an interval of 44.10 m of core inthe highly fractured clinkstone zone of the Tiva CanyonMember, 179 fractures were observed. Using the angularcorrection described above, the sum of the calculated frac-ture frequencies along the entire interval would be about800. The density of fractures within each unit volume of 1wr is calculated by dividing the sum of the calculatedfracture frequency in the entire interval by the number ofstandard volume diameters in the cored interval. The re-sulting number, 800 X 1.24/44.10 - 22, is the average numberof fractures predicted within each sphere of 1 vr within theclinkstone zone.
Implicit within this statement is the acceptance ofincreasingly poor statistics as the angles between t\tefracture sets and the core axis or traverse direction becomesmaller. Also, the assumption is explicitly made that thetotal fracture density observed is representative of theentire stratigraphic zone from which data were collected.Thus, either the observed fractures or others with the samerepresentative attitudes and densities are present through-out the sampled zone.
Recognition of Detailed Structural Features
Among the most critical factors for understandinghydrologic flow are the distribution, density, and attitudeof minor faults. At Yucca Mountain, the recognition ofminor faults presents a particularly difficult dilemmabecause the vast majority of exposures consists of one onlycooling unit, the Tiva Canyon Member. To recognize faults
312
with less than about 3 m of displacement, detailed strati-graphic control must be available. Such control i6 providedby only a few horizons, such as the sharp transition at .thetop and base of the lower lithophysal zone and minor su6-horizons within the columnar zone. In addition, the pres-ence of tectonic breccia, gouge, slickensides, and abruptchanges in foliation attitudes were used to identify faultswhere insufficient, stratigraphic control existed to revealdisplacement. The resulting fault pattern is shown inFigure 11, and a detailed schematic east-west cross sectionin the vicinity of Abandoned Wash is shown in Figure 12.
Several important structural characteristics of YuccaMountain are noted:
1) The shallow (5° to 7°) eastward dip of primaryfoliation typical of the crest of Yucca Mountain increasesgradually toward Abandoned Wash, commonly to as much as 40°,and in a few instances over 70° (Figure 12). Where dipsexceed approximately 10° to 20°, abundant small-displacement(less than 3 m), north-northwest-striking, westward-dipping,normal faults appear. These are present throughout the morehighly tilted region around Abandoned Wash. The change inelevation of a stratigraphic horizon caused by the eastwarddip is opposite in sense to the change in elevation causedby the offset along the west-dipping normal faults. Thus,in cross section, a single stratigraphic horizon and theconnecting fault planes form a zig-zag pattern (Figure 12).
2) The abundance of north-northwest-striking (about N.15° W.) faults decreases from Abandoned Wash northward wherethe steep foliation dips are absent.
3) North of Drill Hole Wash, right-lateral strike-slipfaults are essentially parallel to and lie within several ofthe northwest-trending (N. 25°-40° W.) washes. These washesare also parallel to the trend of the dip direction of ash-flow tuffs. Therefore, the trends of these washes areprobably the consequence of both faulting and runoff direc-tion.
4) Numerous fractures are present on the surface ofYucca Mountain. Fractures can originate either from exten-sional stresses generated within the rock mass during cool-ing or from tectonic stresses applied to the rock mass aftercooling. If cooling joints were exclusively present, onlyrandom fracture strikes would be observed; therefore, anydeparture from randomness is assumed be have a tectonicorigin. The fracture densities and strikes along severaltraverses (corrected for fracture and traverse attitude asdescribed above) are shown in rose diagrams in Figure 13.Six traverses were made, 5 in the Tiva Canyon Member and onein the tuffaceous beds of Calico Hills near The Prow. Where
313
out TtmiiDrI 1 C .MSVIMI aa*
aHavlaai
•nocnvrmuR MOUNTAIN
TUFFS3 •••«•> »•••
aaa^arrvrr
I 1 k»tf««« tafl
««fnal I M Ik«f ••# ball • •
-•r •Irlas-M* IHlt«>•»• la«a«Mralallva »«llaa
ABANDONED WASH
2 KM
Figure 11. Detailed fault pattern at Yucca Mountain.Faults in the Abandoned Wash area are mapped only onsouth-facing slopes where exposures are favorable.Attitudes of primary foliation and detailed zones with-in the Tiva Canyon and Topopah Spring Members are notshown on this figure because of space limitations.Examples of foliation attitudes in the Abandoned Washarea are given in Figure 12a
31A
r.cc« Crail
FBINCIPALNHE-STKIKINO
Abmmtommt FAULT
Figure 12. Structure section across Yucca Crest andAbandoned Wasb showing the nature of the increase infoliation attitudes toward a north-northeast-strikingnormal fault and apparent dip-slip movement on repre-sentative north-northwest—striking faults.
outcrop is continuous along short traverses in the TivaCanyon Member, fracture densities are about 6 to 8/unit mmeasured along the lower lithophysal—clinkstone contact indensely welded tuff. In contrast, fracture densities in therange of only 2 to 4/unit m were observed in the TivaCanyon Member along the long traverses. Two factors con-tribute to the apparently lower fracture densities on longtraverses. First, the long traverses (one just north ofAbandoned Wash and one just south of Drill Hole Wash) weremade, by necessity, across outcrop of moderately weldedcaprock, in addition to the more densely welded zone in thecentral portion of the Tiva Canyon Member; fractures areconsiderably more abundant in densely welded tuff than inmoderately welded tuff. Both of these loig traverses arerepresented by two rose diagrams, one for the east half andone for the west half; the east halves of these long trav-erses were made in regions more extensively underlain bymoderately welded tuff. Second, by necessity the longtraverses were made in part over zones extensively coveredwith talus; talus preferentially covers gullies which arezones expected to contain high fracture densities. Theobservation that north-facing slopes, which have more exten-sive cover, have only about half the fracture densities ofthe corresponding south-facing slopes, supports this con-clusion (fracture densities are calculated only for thefraction of traverse with good exposure). These factorssuggest that the most reliable fracture densities are thosemeasured along short traverses, where both maxiumum exposureand uniform stratigraphic interval can be found. The numberof variables that cannot be quantified and are inherent tothe measure of fracture densities measured along traverses
315
Figure 13. Rose diagrams of the fracture strikes encoun-tered along traverses essentially perpendicular to theaverage fracture attitude. Five traverses were made inthe Tiva Canyon Member, two longer traverses and threeshorter traverses. One of the two longer traverses (A)was made south of Drill Hole Wash and the other (8) wasmade just north of Abandoned Wash. The northernmost ofthe traverses was made in the tuffaceous beds of CalicoHills. The outline of Yucca Mountain is the Tertiaryvolcanic rock contact with Quaternary alluvium andcolluvium.
316
suggest that minor differences In fracture densities are notsignificant.
As mentioned above, the abundance of fractures is afunction of both tectonic stresses and cooling stresses.1
Because nonwelded ash-flow tuffs and bedded tuffs have notbeen subjected to the stresses related to thermal contrac-tion during cooling, the abundance of fractures in nonweldedtuff might represent exclusively the abundance of tectonicfractures, and the difference between abundance of fracturesin welded and nonwelded tuffs might represent the abundanceof cooling fractures. This logic breaks down, however, byobservation that only a very small portion of the fracturesin the rose diagrams of Figure 13 can be assigned to randomcooling fractures. The vast majority, therefore, must betectonically induced. Thus, there is a large differencebetween the number of tectonic fractures in nonwelded andwelded tuffs. Nonwelded tuffs, therefore, have rock mai;sphysical properties that respond to a tectonic stress byformation of relatively few fractures, whereas, welded tuffshave physical properties that respond the the same tectonicstress by formation of more numerous fractures. Thi6 con-clusion is confirmed by the constant dominant strike ofthese fractures throughout the mountain, indicating thatmost of the fractures are of tectonic origin.
5) Dominant fracture strikes are essentially parallelto the north-northwest-striking (about N. 15° W.) faults inthe region near Abandoned Wash. Northeast of Drill HoleWash, however, the north-northwest-striking fractures dis-tinctly Intersect the northwest—trending washes and asso-ciated strike-slip fault traces. The relationship betweenthe north-northwest-striking fractures and the northwest-striking stike-slip faults is not yet understood, but the30° angle between them suggests that the fractures may beriedel shears or tension fractures.
6) At numerous localities, slickensides on north-northwest- and northwest-striking faults are nearly hori-zontal, characteristic of strike—slip motion. In contrast,slickensides on north- to north-northeast-striking faults inthe southern and central portions of Yucca Mountain, andparticularly those faults that strike in a more north tonorth-northwest direction, locally concentrated near YuccaWash, have down-dip attitudes, characteristic of normalfault motion. On two faults with near horizontal slicken-sides, chattermarks are present. Chattermarks form hairlinecrescent—shaped fractures [31—32] that are commonly found onglaciated rock surfaces and less commonly on shallow faults.The focus points or noses of the crescents point in therelative direction of movement of the rock face that con-tains the chattermarks. In both cases on Yucca Mountain,
317
right-lateral strike-slip motion is unambiguously indicated.Although the magnitude of strike—slip displacement is com-monly too small to be measured, at one locality west ofdrill hole USW-G2, a 10- to 30-m right-lateral offset in theoutcrop pattern at the base of the Tiva Canyon Member wa6measured (Figure 11).
In addition to the structural features descibed above,there are several characteristics of fractures, such asfracture aperture, vein mineralogy, and extent of veinfilling that are also very critical for a complete under-standing of fracture control on hydrology. Unfortunately,surface weathering processes and drilling disturbancespreclude observation of undisturbed fracture apertures.Also the mineralogy and extent of veins in fractures is sohighly variable that generalizations should be avoided.Probably, the cumulative effect of both fracture apertureand mineral filling is best measured by empirical hydrologictesting.
Correlation of Structure with Physical Property Stratigraphy
Based upon fracture attitudes and densities measureddownhole from both unoriented cores, oriented cores, anddownhole televison video tapes, an impressive positivecorrelation between degree of welding and density of frac-tures in core recovered from drill hole USW-GU3/G3 is shownin Figure 14. The density of fractures has been correctedfor the attitude relative to the core axis as describedabove, and is expressed as the number of fractures/unitm^. To depths of 940 m, the more elastic (lower Young'sModulus or stiffness) zeolitized nonwelded and partiallywelded tuffs [27] have as few as 1 to 3 fractures/unit m^,whereas the more brittle (higher Young's modulus or stiff-ness) densely welded tuffs typically have 20 to 40fractures/unit m . Fracture densities in core from drillholes USW-G2 and -Gl are somewhat lower, but the overallpatterns are similar. Below 940 m, the abundance of frac-tures decreases by nearly an order of magnitude in core fromUSW-GU3/G3. Further investigations are being conducted todetermine whether this decrease in fracture density isrelated to the more ductile behavior at higher confiningpiessures, to the somewhat more altered state of these moredeeply buried tuffs, or merely to a change in tectonicstyle, independent of depth. The degree of welding of themore extensively welded tuffs below 940 m falls more intothe moderately welded range rather than the densely weldedrange, and the degree of alteration is significantlygreater. Although there is an expected decrease in fracture
Figure 14. Density of fractures compared to physical-property stratigraphy expressed by differences indegree of welding in drill hole USW-GU3/G3 shown inFigure A. These fractures include only mineralizedcracks or cracks that have evidence of shear such asbreccia, gouge, or slickensides. The density of frac-tures has been calculated per unit m . The density offractures at drill holes USW-G1 and -G2 is somewhatlower than at -GU3/G3, but the overall correlation withdegree of welding is similar.
319
density related to these factors, the decrease from 23fractures/unit m3 In the moderately welded Tram unit above940 m to 2.3 fractures/unit m , also In moderately weldedTram unit, below 940 m requires an explanation other thaii achange In degree of welding.
The densities of fractures in densely welded tuffmeasured in core (20 to 40/unit m ) do not correlate wellwith densities measured in well-exposed outcrops of denselywelded tuff (6 to 8/unit m 3 ) . In all probability, fracturesare both more readily seen in core, and the mechanical dis-turbance of coring enhances parting along abundant hairlineaperture fractures not visible on surface exposures. Thedominant northwest strike of the fractures measured Inoriented core (N. 15° W.,,5° SW., Figure 15) from drill holeUSW-GU3/G3 (R. B. Scott, USGS and Mayra Castellanos, F&S,written comimm., 1982), however, does agree well with thosemeasurements on the surface (Figure 13). Also the averagefault attitude in this core (N. 21° W., 73° SW.) is similarto the north-northwest-striking faults mapped on the surfacein the vicinity of Abandoned Wash (Figure 11).
Drill-Hole Deviation
Drill holes on Yucca Mountain tend to deviate naturallyfrom vertical, almost uniformly to the southwest and west(Figure 16). Holes USW-G1, USW-H1, and UE25a-l, all withinDrill Hole Wash, trend S. 30° W. to S. 60° W., to a depth ofover 1 km, essentially perpendicular to Drill Hole Wash.Holes USW-G2 and USW-G3, in contrast, trend slightly southof west, from S. 80° W. to S. 83° W. Because Drill HoleWash appears to be controlled, at least in part, by fractureand fault systems that strike nearly parallel to the washand dip steeply (60°-90°) toward the southwest, the uniformdrill-hole deviation direction may also be controlled if thecore bit tends to follow the path of least resistance downnear-vertical fractures and faults. However, within theYucca Mountain block, away from influence of the structuresin the washes, the trend of dip of dominant fractures andfaults is only slightly south of west; apparently the drill-hole deviation follows a similar trend and plunge. Thus,drill-hole deviation directions appear to be sensitive indi-cators of anisotropic mechanical properties of the rock, inthis case, fracture attitudes.
320
MAXIMUM AVERAGEFRACTURE PLANE
(LOW ANGLE &NE-TRENDINGMAXIMAEXCLUDED)
TOTAL OF148 FRACTURES
FRACTURE POLEMAXIMA
UNDERLINEDsTHE MAXIMUMNO UNDERLINE^ LESSER MAXIMA
% OF POLES
TOTAL OF
21 FAULTS
Figure 15. Lower hemisphere poles to attitudes of averagefractures and faults measured in oriented core fromUSW-GU3/G3 and corrected for drilling deviation (R. B.Scott, USGS, and Mayra Castellanos, F&S, writtencommun., 1982). The average fracture plane (N. 15° W.,81° SW.) was calculated in the upper diagram by averag-ing the stereonet maxima for all the poles to fractureplanes from each stratigraphic unit. Also in the upperdiagram, the averaged direction of drill hole deviationis shown by the X. The stereonet maximum for the fault*attitude (N. 21° W., 73° SW.) is shown in the lowerdiagram. The Tiva Canyon Member (Tpc) and TopopahSpring Member (Tpt) are part of the Paintbrush Tuff.The Prow Pass Member (Tcp), Bullfrog Member (Tcb), andTram unit (Tct) from the Crater Flat Tuff. Tlr sym-bolizes the tuff of Lithic Ridge and Tt symbolizes theolder unnamed tuffs.
321
w—
1500m
UE25a- \1200m
USW-G 1
1800 m
Figure 16. Drill—hole deviation directions on YuccaMountain. Note that no horizontal scale is given be-cause the amount of lateral deviation varies* Linelengths are normalized to represent percent of hori-zontal deviation.
DISCUSSION
Regional Structural Framework
Although the dominant Tertiary faults in the NTS arenorth-northeast-striking, Basin and Range style, normalfaults [18,21], Carr [11] reported a significant overprintof strike-slip fault displacement, particularly left-lateraldisplacement on the north-northeast-striking faults. Carrdid not observe right-lateral strike-slip displacement onnorthwest-striking faults elsewhere in the NTS; he concludedthat strike-slip displacement is not expected because thesefaults would be under compression under the current stressregime. However, on Yucca Mountain, evidence of right-lateral displacement exists on north-northwest-strikingfaults south of Drill Hole Wash and on northwest-strikingfaults north of Drill Hole Wash. Major movement on thesefault systems and the north—northwest—striking fracture
322
system do not appear to affect the Rainier Mesa Member atYucca Mountain; thus, the probable age of this fracturingand faulting Is between the age of the faulted Tiva CanyonMember, about 12.5 m.y., and the age of the Rainier Mesa.Member, about 11.3 m.y. From a regional perspective, thegeneral attitude, sense of displacement, and age of thesenorthwest-striking features are very similar to those fea-tures of regional strike-slip zones including the Las VegasValley and Walker Lane shear zones (Figure 17) [33-34]. A
3 CENOZOIC VOLCANIC ROCKS
P#J^§S???A& • PRE-CENOZOIC ROCKS
it§liIiL
Figure 17. Location of the Las Vegas Valley shear zone f21]relative to the Yucca Mountain region. Note the paral-lelism of the shear zone and the elongation of YuccaWash. '
physical extension of the Las Vegas Valley shear zone itselfto Yucca Mountain seems unlikely, even though most of therocks exposed in the intervening area postdate regionalshear zone activity [36-37]. A diffuse zone of right-lateral stike-slip with small displacements is probably morelikely, but very difficult to recognize in the field.
The major observable displacement on faults in theAbandoned Wash area along both the numerous north-northwest-striking faults and the major north-northeast-striking
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block-bounding fault appears to be dip-slip and, as a re-sult, that area appears to have experienced significantextension. The general increase in eastward dip or thedown-faulted blocks west of the major block-bounding faultin this area is very similar to the roll-over features or"reverse drag" observed along growth faults in the GulfCoast region and normal faults in the Colorado Plateauregion [38-39], If the features at Yucca Mountain are ofsimilar origin, then the dips of major block-bounding faultsunder Yucca Mountain probably decrease at some as yet un-observed depth in a manner similar to those cf listricgrowth faults on the Gulf Coast, as Wernicke and Burchfiel[40] have recently suggested for extensional systems of theGreat Basin. Where large vertical displacements (about 100m) occur, such as along the eastern margin of SolitarioCanyon (Figure 11), chaotic zones of breccia and extremelyirregular fault blocks including overturned sequences arepresent on the hanging-wall side of the fault* Presumablythese chaotic zones develop where movement along the listricfault creates a significant gap at the head of the fault andblocks collapse in a disorganized fashion to fill a develop-ing gap. Abandoned Wash is an example of smaller verticaldisplacement between major blocks (about 25 m), and there-fore, smaller gap formation on the hanging-wall of thelistric system. In this case, the fractured rock in theroll-over structure has retained coherent block-to-blockcontact with the major normal fault. Drag on these surfacesproduces the offset observed; in essence, this is a brittleresponse to drag along the major fault (Figure 12). Thesefeatures further suggest that the brittle behavior seen inthe Tiva Canyon and Topopah Spring Members at the surfaceprobably goes through a transition at depth to more ductilebehavior where these faults begin to flatten. The abundantfractures characteristic to depths of about 1 km depth andthe mechanical contrast that produces uniform drill-holedeviation both reinforce the concept that the exposed por-tion of Yucca Mountain is within the upper brittle behaviorregion. Data do not exist at present to predict the depthof the transition to more ductile behavior, but the presenceof "reverse drag" dips should persist to at least 1 km.
An alternative interpretation to the listric faulthypothesis given above is that the steeper foliation atti-tudes observed in Abandoned Wash are related to rotation bya component of strike-slip displacement (W. J. Carr, oralcommun., 1982). In the absence of more discriminatoryevidence, the listric fault explanation appears most likely.
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Conceptual Hydrologic Model
Direct hydrologic investigations of the unsaturatedzone at Yucca Mountain are underway but are too preliminaryto construct an an empirical model of the hydrology at thisearly stage. However, knowledge of the gross physical prop-erty stratigraphy combined with a three-dimensional model offracture and fault attitudes and densities, in conjunctionwith regional hydrologic observations and regional concep-tual models of the hydrology of ash-flow tuffs [5,7,22],does allow construction of a very simple conceptual hydro-logic model at Yucca Mountain*
Limited physical-property data [25] are available fromcore samples from the unsaturated zone at Yucca Mountain.These data include matrix hydraulic conductivities that fallwithin the ranges listed in Table I, for the most part (2 X10"*" to 3 X 10"' cra/s for the densely welded Tiva CanyonMember, 8 X 10~10 to 2 X 10~7 cm/s for the densely weldedTopopah Spring Member, and 4 X 10~9 to 1.5 X 10~6 cm/s forthe underlying zeolitized nonwelded ash-flow and beddedtuffs). Also these data include natural, saturated, and drybulk densities, from which the degree of saturation wascalculated. The range of saturation of the welded TivaCanyon Member samples is 33 to 50 percent, the underlyingnonwelded ash-flow and bedded tuffs is 61 to 90 percent, thewelded Topopah Spring Member is 17 to 91 percent, and theunderlying nonwelded ash-flow and nonwelded tuffs is 82 to100 percent. Even below the static water level, within thedeepest interval listed above, the range of saturation isbetween 82 and 100 percent. The lower densities, higherporosities, higher degree of saturation, and higher surfaceareas of the nonwelded tuffs suggest that they may act astemporary capillary barriers (W* E. Wilson, DSGS, oralcommun., 1982), but the unsaturated zone has probablyreached a steady-state flow such that each layer is assaturated as possible with a balance of capillary forces,gravity, and back pressure of trapped gases. Thus, anypulse of water migrating through the rock would be in excessof what the rock can absorb, and therefore, would eventuallypass through these nonwelded layers.
The generalized and greatly simplified cross sectiondrawn through the unsaturated zone in Yucca Mountain inFigure 18 assumes the more welded portions of the TivaCanyon and Topopah Springs Members to be highly transmissiveunits with high effective conductivities. Because the frac-ture densities in lithophysal zones in the densely weldedtuffs of the Tiva Canyon Member (14 to 22 fractures/unit nr*)do not appear to differ appreciably from those within non-lithophysal densely welded zones (22 to 26 fractures/unit
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Wttt Ectt
Figure 18. Conceptual hydrologic model of Yucca Mountain,assuming a positive net recharge for the mountain*Determination of vertical and possible horizontal ratesof transmission of recharge through densely and moder-ately welded Tiva Canyon and Topopah Spring Members andthrough less welded vitiric, argillic and zeolitizedtuffs awaits hydrologic testing specifically designedfor the unsaturated zone. If faults are locallysealed, they may create local perched water tables,but if they are open, the faults may rapidly transmitwater.
m^), it is assumed that llthophysal zones will have hydro-logic properties similar to densely welded nonlithophysalzones. The abundance of cavities in lithophysal zones mayincrease the conductivities locally where the fracturesinterconnect with cavities. Between and below these morewelded portions of the Tiva Canyon and Topopah SpringMembers are bedded tuffs and nonwelded to partially weldedash-flow tuffs that have few fractures. Where these lesswelded units are vltric, they may act as "leaky aquitards",the term used by Winograd and Thordarson [5], because theyhave relatively high matrix conductivities. However, wherethese bedded tuffs and less welded tuffs are altered toclays and zeolites [29], their matrix conductivities areconsiderably lower than those of vitric tuffs, and they havelow hydraulic conductivities because of the low fracturedensities. These less welded altered tuffs may have thecapacity to act as capillary barriers because of the lower
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hydraulic conductivities. Observations by Thordarson [22],at Rainier Mesa northeast of Yucca Mountain In a 6imilarsetting on the NTS, Indicate that perched water does occurabove locally saturated zeolltlzed tuffs within the unsatu-rated zone.
Of the small annual precipitation that occurs In thisarid region (less than 20 cm/yr in the Jackass Flats drain-age basin), less than about 3 percent is estimated 140] toenter the hydrologic system as recharge on a basin—wideaverage. Because Yucca Mountain stands roughly 400 m higherthan Jackass Flats, it is assumed here that a positive re-charge does occur at 3 percent of 20 cm/yr (actual measure-ments of precipitation or recharge on Yucca Mountain havenot been made). Thus, roughly 0.6 cm of water would enterthe system annually and would be transmitted through thehighly fractured zone of the densely welded portion of theTiva Canyon Member. Where the water encounters vitric tuff6below the welded Tiva Canyon Member, the water may tend tobe temporarily absorbed in the porous nonwelded tuffs abovethe Topopah Spring welded tuffs; however, pore sizes invitric tuff are considerably greater than those in zeoli-tized tuffs, and eventually water would penetrate these vit-ric layers. On the other hand, where these nonwelded vitriclayers are clay-rich and have low conductivities, they maybecome essentially water saturated, and may inhibit verticalflow sufficiently to locally or temporarily pond water, pro-ducing perched water tables above the Topopah Spring Member.After vertical passage through densely welded zones and thevitrophyre of the Topopah Spring Member, the water would en-counter the deeper zone of nonwelded tuffs. Although thesetuffs are extensively zeolitized, the zeolitization is dis-continuous, particularly toward the southern portion ofYucca Mountain where the water table is deeper and the tuffsare still vitric (R. B. Scott, USGS, and Mayra Castellanos,F&S, written commun., 1982). Whether zeolitization is arequirement and whether argillization can create similarconditions conducive to perched water tables in ash-flowtuffs on Yucca Mountain is not known at the present time.Direct evidence to suggest that perched water exists withinYucca Mountain has not been found, but wet areas on thesurface exist above zeolitized tuffs along the steep wallsof Yucca Wash, indirectly suggesting that some perched watermay be present along this horizon. Such hydrologic barriersmay also cause limited lateral movement of water down dip(east and southeast) of the stratigraphic boundary and alongthe major fracture attitude (south-southeast and southeast).This general southeast direction would be essentially par-allel to flow directions down the hydrologic gradient (F. £.Rush, USGS, oral commun., 1982).
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Faults may either be sealed by formation of fault 'gougeor secondary mineralization, forming relatively nontrans-missive barriers and creating local perched water, or theymay be open, highly transmissive paths through both nonrwelded tuffs and the highly transmissive Topopah SpringMember. Eventually that portion of the recharge that movesvertically must permeate the nonwelded base of the TopopahSpring Member, the nonwelded, zeolitized tuffaceous beds ofCalico Hills, various zeolitized and argillized beddedtuffs, and the nonwelded to partially welded zeolitized FrowPass Member before reaching the static water level, unless afault acts as a short-circuit conduit. Faults may be highlyimpermeable in general, especially in nonwelded zones butobservations of open faults in core to depths greater thanthe static water level suggest that unless detailed empiri-cal data exist to support the contention that a specificfault is impermeable, high permeabilities should be consid-ered as possible, at least locally.
The validity of the simplified conceptual model shownin Figure 18 will be tested as empirical data are gathered.At present the model can best be used to design futurehydrologic testing and to recognize areas where additionalstructural and hydrologic data are needed. Obviously, thereality of hydrologic flow within the unsaturated zone underYucca Mountain is far more complex than the model is capableof suggesting; a number of unknown factors will have to beinvestigated (R. K. Waddeil, USGS, oral commun., 1982):What is the annual average precipitation and recharge atTucca Mountain? What is the relation between recharge fluxand llthophysal zones? How does water flow through layersof variable degrees of saturation, degrees of welding, andvariable densities of fracturing? Do fractured denselywelded tuffs have fracture conductivity as the dominant modeand nonwelded tuffs have matrix conductivity as the dominantmode of water flow? How much smaller are matrix and effec-tive conductivities in the unsaturated zone than in the sat-urated zone? At what rates does water flow through rockswith these variable characteristics? To what degrees doporous layers act as capillary barriers? Until these andother problems are addressed, a more realistic and complexmodel is difficult to produce. The viability of the unsat—urated zone under Yucca Mountain as a nuclear waste reposi-tory will depend, in part, upon reaching an understanding ofthe complex physical process of fluid transmission in theunsaturated zone of Yucca Mountain.
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CONCLUSIONS
Determination of the viability of nuclear waste dis-posal in the unsaturated zone of ash-flow tuffs at YuccaMountain is highly dependent upon a three-dimensional eval-uation of physical, structural, and hydrologic properties ofthe rock mass.
A) Listed below are four major conclusions reachedfrom the investigation of rock mass physical properties*
Al) The physical-property Btratigraphy in tuffs atYucca Mountain is defined principally by layers of con-trasting degrees of welding, bulk density, and porosity.Physical-property stratigraphy, rather than conventionalpetrologic stratigraphy, mupt be used to characterize theinhomogeneous and layered rock body.
A2) Within densely welded portions of the Tiva Canyonand Topopah Spring Members of the Paintbrush Tuff, the den-sity of fractures measured on core 6.25 cm in diameter iscommonly 20 to 40 fractures/unit m . Detailed surface trav-erses indicate a lower fracture density for thi6 rock type,in the range of 6 to 8 fractures/unit m •
A3) Partially welded and nonwelded tuffs have consid-erably fewer fractures than welded tuffs, commonly as few as1 to 3 fractures/unit m . Also they are commonly altered toclays and zeolites.
A4) Although a number of surface characteristics thatcreate mappable zonations (such as the clinkstone, hackly,and columnar zones) appear to be related to detailed devit-rification fabrics, the fracture density appears to be rela-tively independent of this variable. Also, the lithophysalcontent and grain density do not appear to appreciablyaffect the fracture density.
B) Listed below are four major conclusions reachedfrom the investigation of the structural geology of theunsaturated zone.
Bl) The dominant strike of fractures in both weldedand nonwelded tuffs, as measured in both outcrop and core,is about N. 15° W.. This indicates that most of the ob-served fractures are tectonically induced, as a randomdistribution of fracture strikes should result fromcontraction-induced jointing.
B2) The small-displacement north-northwest strikes offaults in the Abandoned Wash vicinity in the southern por-tion of Yucca Mountain are parallel to the dominant strikeof fractures. However, north of Drill Hole Wash, faultsappear to be more nearly parallel to the washes, about N.25° W. to N. 40° W.
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B3) Several of the northwest-striking minor faultshave sllckensidee and chattermarks compatible with right-lateral, strike-slip motion. This attitude and sense ofdisplacement suggests that a genetic relation between theregional Las Vegas Valley and Walker Lane shear systems andthese local Yucca Mountain structures may exist. Thismovement appears to postdate the Tiva Canyon Member of thePaintbrush Tuff (12.5 my old) but predate the Rainier MesaMember (11.3 my old) of the Timber Mountain Tuff.
B4) On the hanging-wall side of several major north-northeast-striking normal faults in Yucca Mountain, the in-crease in foliation dip may be analogous to roll-over struc-tures associated with listric faults. Also, the closely-spaced> small-displacement normal faults in Abandoned Washmay result from drag along the major faults. The majormovement on normal faults on Yucca Mountain occurred withinthe same time Interval as the strike-slip faults mentionedabove•
C) The conclusions reached from physical-property andstructural investigations of Yucca Mountain allow the con-struction of the following preliminary conceptual hydrologicmodel based upon the fundamental assumption that a net posi-tive recharge occurs on the mountain.
Cl) The uniform high fracture density and brittlecharacter of densely welded tuffs indicate that, in spite oftheir low matrix conductivities, their effective hyraulicconductivities are uniformly high; thi6 conclusion is sup-ported by limited in situ hydraulic conductivity measure-ments within the saturated zone of the densely weldedTopopah Spring Member near Yucca Mountain [5]. Thus, in theunsaturated zone, the fractures in densely welded tuffsshould readily transmit fluids, provided a maximum orsteady-state degree of saturation of the rock matrix exists;that is, assuming the rock matrix has already absorbed allthe water it can ret?in in the unsaturated zone.
C3) The low fracture density and low matrix hydraulicconductivities of zeolitized or argillized nonwelded tuffssuggests that they should serve as potential aquitards belowthe water table [5] and as infiltration barriers within theunsaturated zone.
C4) The relatively high matrix conductivities of non-welded vitric tuffs indicates that these tuffs should serveas relatively permeable zones, even in the absence of frac-turing [5], but the rate of transmission of flow in theunsaturated zone may be inhibited to a degree by possiblecapillary action of these porous tuffs.
C5) Although the vast majority of recharge probablymoves vertically, limited lateral flow associated withperched water may occur above relatively impermeable zeoli-
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tized or argillized nonwelded zones. Even though Thordarson[22] did not find evidence to support significant lateralflow in a similar setting in the unsaturated zone underRainier Mesa northeaEt of Yucca Mountain, he did find perch-ed water above highly zeolitized tuffs there. The east tosoutheast dip and north-northwest-striking fractures onYucca Mountain, absent at Rainier Mesa, would be conduciveto lateral flow of perched water in an east or southeastdirection.
C6) Development of this hydrologic model has identi-fied several areas'where important fundamental data areneeded for an understanding of the hydrology of the unsatu-rated zone: a) What is the range of annual precipitationon Yucca Mountain and what is the range of annual recharge?b) Is the dominant mode of transmission of fluids alongfractures in densely welded tuffs, but within the rockmatrix in zeolitized or argillized nonwelded tuffs and invitric nonwelded tuffs? c) How do rates of fluid trans-mission In the unsaturated zone differ from those in thesaturated zone within similar rocks? d) Do porous vitrictuffs act as capillary barriers in a steady state system?e) Does limited lateral flow occur and does perched waterexist? f) Do specific faults within Yucca Mountain act ashydrologic barriers or as highly permeable hydrologic shortcircuits?
ACKNOWLEDGMENTS
As in any attempt to correlate several disciplines,much credit belongs to numerous colleagues: In addition toformal USCS technical reviews, numerous USGS hydrologistsincluding Ike Winograd, Bill Wilson, Rick Waddell, MerrickWhitfield, and Jim Robison, have freely given advice. MikeCarr has also provided a particularly valuable informalgeologic review. Barry Swartz of SNL transmitted the phys-ical-property measurements for this work and Larry Teufel,also of SNL, gave advice concerning the behavior of theseash-flow tuffs under stress. Judy Brandt and Byron Cork,among others of Fenix and Scisson, provided field assist-ance. Finally Jim Mercer's understanding of interminabledelays in submitting the manuscript is deeply appreciated.
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