, I

Structural Control of

Ground-Water Movement in

Miogeosynclinal Rocks of South-Central Nevada-J

ISAAC J. WINOGRADU.S. Geological Survey

1401 East Adelaide Drive, Apt. 4, Tucson, Arizona, 85719

WILLIAM THORDARSON

U.S. Geological Survey, Federal Center, Denver, Colorado

ABSTRACT

Regional interbasin movement of ground water in highly deformed miogeo-synclinal carbonate rocks of the eastern Great Basin has received considerableattention in the literature since 1960. That these regional carbonate aquifersystems - some of which may integrate as many as 13 intermontane basins -are actually compartmentalized by major structural features, has not receivedadequate emphasis.

In south-central Nevada, major wrench, thrust, and normal faults and foldsexert marked control on ground-water movement. Deformation of the car-bonate rocks results in regions of high transmissibility, but juxtaposition byfaulting or folding of thick clastic strata against carbonate aquifiers results inprominent ground-water barriers, some of which are more than ten mileslong. The apparent hydraulic gradients across the thick clastic aquitards varyfrom 150 to 1300 ft per mile; by contrast gradients in the adjacent carbonateaquifers vary from 0.5 to 10 ft per mile. Barriers may also result from gougedeveloped along the major fault zones.

Recognition of the structural barriers as well as the conduits is essentialfor construction of an initial working flow net of a region. In upland areas

35

I

36 NEVADA TEST SITE WINOGRAD AND

the clastic aquitards may exert control on distribution of recharge. Wherepresent in central parts of the flow system, the aquitards act as prominentground-water dams, and localize minor spring discharge. In discharge areasthey localize major spring lines.

Major structures with hydrologic reflections include the Las Vegas Valleyshear zone and the Tippinip thrust fault.

CONTENTS

Introduction .............................................................Emigrant Valley .........................................................

Yucca Flat ..............................................................Indian Springs Valley .....................................................Conclusions .............................................................References cited .........................................................

363742444647

depend upon the construpaper seeks to emphasi2syncline are actually coiform prominent hydrauliand second, that the idmapping, is a logical fi:complex aquifer systems

At the Nevada Test Sof Paleozoic age have f1,000,000 gpd (gallonsthe transmissibility neargpd per ft, according toof thick clastic rocks of Isippian ages, on the otl1000 gpd per ft, accordilow transmissibility of tlgeneral absence of high-,the carbonate rocks hasto exceed 200,000 acre-mational history as thetured -the clastic rocklow susceptibility to soltheir tendency to deforr

Because of their marlpect that structural juxtresult in prominent hycontinuities are present(

FIGURE

I. Index map showing areas discussed .............. ........................ 382. Areas of Precambrian and Lower Cambrian outcrops

and subsurface control points, Emigrant Valley ..... .................. 393. Geologic and hydrologic cross section, Yucca

Flat to Groom Lake playa .......................................... 404. Distribution of pre-Tertiary rocks at 2400-ft altitude,

and their potentiometric surface, Yucca Flat ........................... 425. Geologic and hydrologic cross section. west-central Yucca Flat .... .......... 436. Potentiometric contours and positions of two major hydraulic

barriers, southern Indian Springs Valley ....... ...................... 447. Geologic and hydrologic cross section, southern Indian Springs Valley ... ..... 45

INTRODUCTION

Regional interbasin movement of ground water in the highly deformedmiogeosynclinal carbonate rocks of the eastern Great Basin has received con-siderable attention since 1960, principally by the U.S. Geological Survey.Test drilling at the Nevada Test Site in south-central Nevada has proven thatat least five, and probably seven, intermontane basins are hydraulically con-nected by movement of ground water through the Paleozoic carbonate strata(Winograd, 1962, 1963; Winograd and Eakin, 1964). In east-central NevadaEakin (1966) has inferred-on the basis of spring discharge and water-level data - that 13 valleys, bordering or near the White River, may consti-tute a single ground-water basin. In northeastern Nevada, Maxey (1964) hassuggested that interbasin movement best explains the uniform discharge ofseveral groups of large springs. The qualitative studies made to date thusindicate that interbasin movement of ground water probably occurs through-out much of the eastern Great Basin, Semiquantitative studies of these com-plex aquifer systems - either for economic or academic purposes - will

Figure 2 shows partThe western half of thiPlaya -is bordered onLower Cambrian clastickie and Carrara Format:clastic and younger Palvalley probably is part ofaulting, brought Precanface over a wide region.western Emigrant Valle,Posits (D. L. Healey anm

Figure 3 is a geologitfrom northeastern Yucc,mately at right angles t

__ _�I_

W'INOGRAD A\D T!IORDARSON-GROUND-WATER MOVEMENT 37

depend upon the construction of "working" flow nets of each reservoir. Thispaper seeks to emphasize, first, that the carbonate aquifers of the miogeo-syncline are actually compartmentalized by major structural features whichform prominent hydraulic barriers between or within the intermontane basins;and second, that the identification of such structural features, by geologicmapping, is a logical first step for the construction of flow nets for thesecomplex aquifer systems.

At the Nevada Test Site and vicirity, the miogeosynclinal carbonate rocksof Paleozoic age have fracture transmissibilities which range from. 1000 to1,000,000 gpd (gallons per day) per ft, according to pumping test analyses;the transmissibility near the principal discharge area may exceed 5,000,000gpd per ft, according to regional-flow analysis. The fracture transmissibilityof thick clastic rocks of Precambrian, Early Cambrian, Devonian, and Missis-sippian ages, on the other hand, is extremely low by comparison, less than1000 gpd per ft, according to hydraulic tests. The best evidence for uniformlylow transmissibility of the clastic sequences within the miogeosyncline is thegeneral absence of high-yield springs in these rocks; the spring discharge fromthe carbonate rocks has been estimated by Eakin (1966) and Maxey (1964)to exceed 200,000 acre-feet annually. Although subjected to the same defor-mational history as the carbonate strata - and consequently as highly frac-tured -the clastic rocks are poorly permeable, probably because of theirlow susceptibility to solution and, in the case of the argillaceous rocks, totheir tendency to deform plastically.

Because of their marked difference in transmissibility, it is possible to ex-pect that structural juxtaposition of the clastic and carbonate rocks shouldresult in prominent hydraulic discontinuities. Three examples of such dis-continuities are presented (Fig. 1).

EMIGRANT VALLEY

Figure 2 shows part of Emigrant Valley adjacent to the Nevada Test Site.The western half of this valley-the alluvium apron west of Groom Lakeplaya-is bordered on the east, south, and southwest by Precambrian andLower Cambrian clastic rocks of the Johnnie, Stirling, Wood Canyon, Zabris-kie and Carrara Formations. This outcrop pattern and the regional dip of theclastic and younger Paleozoic carbonate rocks suggest that this part of thevalley probably is part of a highly faulted mega-anticline which, prior to blockfaulting, brought Precambrian and Lower Cambrian clastic rocks to the sur-face over a wide region. A gravity survey suggests that the bedrock beneathwestern Emigrant Valley is overlain by as much as 4000 ft of Cenozoic de-Posits (D. L. Healey and C. H. Miller, written commun., 1965).

Figure 3 is a geologic and hydrologic section through six wells extendingfrom northeastern Yucca Flat to Groom Lake playa. The section is approxi-mately at right angles to the major structural trends. Water levels in three

_1L. - .. 1.&xF^;V

38 NEVADA TEST SITE WINOGRAD.

IIr, 1160 als0

0Ouicrop of C'

and PrecorrCIsOtic ro

370 15

3241:

0 10 20 30 MILESI I I I F L AT

Figure 1. Index map of south-central Nevada, showing areas discussed in the text.(A) Emigrant Valley, area of Figure 2; (B) Yucca Flat, area of Figure 4; (C) southernIndian Springs Valley, area of Figure 6.

widely spaced wells tapping the valley-fill aquifer in western Emigrant Valleyrange from 4340 to 4371 ft above sea level, and indicate a very gentle hy-draulic gradient, which slopes toward the playa at about 4 ft per mile. Waterlevels in wells tapping older rocks immediately east and west of westernEmigrant Valley, however, are considerably lower than the levels within thevalley-fill aquifer. The water levels in two wells tapping Tertiary tuff beneaththe south border of Groom Lake playa are about 400 to 800 ft lower thanthe water level in well 4340, 2 miles away along the western border of theplaya. (Well numbers on illustrations indicate water-level altitude.) Thesethree wells indicate an apparent eastward hydraulic gradient of 220 to 820 ftper mile. The water level in well 3918 along the northeast border of YuccaFlat is about 450 ft lower than the water level in the valley-fill aquifer ofEmigrant Valley; this well taps Precambrian clastic and carbonate rocks.Finally, the water level in well 2413 is about 2000 feet lower than levels inwest Emigrant Valley; this well taps carbonate aquifiers, and its potentiometriclevel (2413 ft altitude) is representative of the carbonate aquifers of Yucca

Figure 2. Areas ofcontrol Points, Emigraradjacent to wells repres

Flat. The two wells il-from Emigrant V.

The high and relatWithin western Emigdeveloped in the oldtcauses: (1) marked],Or' the flanking area<the presence of reserxEmigrant Valley; (3pressure differentialsa higher discharge ou

k

WINOGRAD AND THORDARSON-GROUND-WATER MOVEMENT. .n In~~~~~~~~1-a-

39

i

37Th15

t.n

0 2 4 6 MILESI I I I

rnehneeItaIf

nC

a

Figure 2. Areas of Precambrian and Lower Cambrian outcrops and subsurfaceControl points, Emigrant Valley. A-B, line of section shown on Figure 3; numbersadjacent to wells represent altitude, in feet above sea level, of water level in well.

Flat. The two wells in Yucca Flat suggest an apparent southwestward gradient-from Emigrant Valley to Yucca Flat -of about 160 to 1300 ft per mile.

The high and relatively flat potentiometric surface in the valley-fill aquiferWithin western Emigrant Valley and the apparent steep hydraulic gradientsdeveloped in the older aquifers in the flanking areas could be due to severalcauses: (1) markedly greater precipitation on western Emigrant Valley thanon the flanking areas; (2) uniform precipitation throughout the region, butthe presence of reservoir rocks of considerably lower porosity beneath westernEmigrant Valley; (3) the perching of water in the valley-fill aquifer; (4)pressure differentials caused by semipermeable membrane phenomena; (5)a higher discharge outlet for the valley-fill aquifers than for the older aquifers;

40 NEVADA TEST SITE WINOGRAD AND T

WEST EA ST

241 3918 4 3 14340 3951 3541

4000' 16 OFT/MI 44000

3000, FT/MI LEVEL 82020 FT/MI -3. l

2000 -l10 FTJMI L2000'_________ VERTICAL EXAGGERATION IOX

YUCCA s EMIGRANT VALLEY -oFLAT NOk. 20 2uW

GROOA4 LAKE PL4YA2413 O3918 Tt TI 43T1 4340 395 354140OTY' OTv OTY OTv

4000 .... .T 2000'

SEA LEVEL \~\XC~E EEA~~~~~~~~~~~~~~~~~~

I

IIi

i

i

pEd VERTICAL EXAGGERATION 2X

0 1 2 3 4 5 MILESI I I I I

Figure 3. Geologic and hydrologic cross section, Yucca Flat to Groom Lake playa.Line of section shown on Figure 2; WHI numbers represent altitude, in feet above sealevel, of water level in well. QTv, Quaternary and Tertiary valley fill; Tt, Tertiary tuff;D-Cr, Devonian to Cambrian carbonate rocks; Cper, Lower Cambrian to Precambrianclastic rocks; pCd, Precambrian dolomite.)

or (6) a lower gross transmissibility of, and lower hydraulic outlet for theclastic rocks which surround and underlie western Emigrant Valley. Of these,only the last two satisfactorily explain the hydraulic gradients, for the follow-ing reasons. First, significant differences in precipitation and porosity clearlydo not exist. Second, the uniform yield of well 4340 and data from well 3918appear to negate the possibility of perched water within the valley-fill aquifer;moreover, perched water has not been found in the valley fill beneath thebajadas of adjacent areas. Third, the chemistry of the ground water precludespressure differentials caused by membrane phenomena (such as those de-scribed by Back and Hanshaw, 1965).

The apparent steep hydraulic gradients on both ends of the cross section(Fig. 3) are believed to reflect the movement of water through the thickclastic aquitards toward points of lower head in Yucca Flat and in easternEmigrant Valley. The potentiometric surface in both Cenozoic and Paleozoicaquifers in northern and eastern Yucca Flat is well documented and is 2000 ftlower than levels in western Emigrant Valley. The presence of lower levelsin eastern Emigrant Valley is not documented by well data, but that part ofthe valley is underlain chiefly by carbonate rocks, and therefore, the steepeastward gradient probably reflects drainage to lower levels in the carbonateaquifers. Data from well 3918 supports the above interpretation of the steepgradients in three respects. First, its water level gives a control point on thegradient within the clastic rocks. Second, a pumping test of this well showsthat the transmissibility of about 4200 ft of Precambrian quartzite, siltstone,and dolomite is less than 600 gpd per ft, and half of this interval reflects a

fault zone between the Jothe absence of a change itests, indicates that thesemiperched.

The relatively high wadue also to the relativelyGroom Lake playa, but;Precambrian quartzite risioutlier, as well as a gravithe Groom and Papoose ]at very shallow depth aleThe gravity survey indi(steeply in both direction:Miller, written commun.,elastic strata, the buried ireservoir, which, incidenThe very low hydraulic E

high transmissibility of ihydraulic barrier was fiicommun., 1959), whofuture site of well 4340

In summary, both thethe cross section are prnpermeable PrecambrianEmigrant Valley from awest.

Two qualifications oftion may be raised thatshould not be considerecambrian and Paleozoiccedure is valid in this arnSprings Valley, and datein the Cenozoic aquifenlying Paleozoic acquiferexist. Because the watt100 ft, our hypothesisleast one control pointabove generalization.

The second qualificaand also values presenwithin the clastic stratadiscohtinuous levels witfaults. For this reasongradients.- Whether thterially change the inte

-

WINOGRAD AND THORDARSON-GROUND-WATER MOVEMENT 41

fault zone between the Johnnie Formation and the Noonday Dolomite. Third,the absence of a change in head with depth, as observed in several drill-stem

,0' ; tests, indicates that the water level in the clastic rocks is not perched orsemiperched.

o01 The relatively high water level within the valley-fill reservoir (Fig. 3) isdue also to the relatively impermeable clastic rocks. Along the west side of

'°, Groom Lake playa, but about 1000 ft northeast of well 4340, an outlier ofPrecambrian quartzite rises a few feet above the playa surface (Fig. 2). Thisoutlier, as well as a gravity survey, indicates that the clastic rocks composingthe Groom and Papoose Ranges form a continuous buried ridge which occursat very shallow depth along the west side of the playa and under well 4340.

)0' The gravity survey indicates further that the depth to bedrock increasesto 0, steeply in both directions from this buried ridge (D. L. Healey and C. H., LEVEL Miller, written commun., 1965). Because of the low transmissibility of the

00'clastic strata, the buried ridge probably acts as the spill point of the valley-fillreservoir, which, incidentally, is saturated to within 100 ft of the surface.The very low hydraulic gradient within the valley fill reflects the moderately

e playa. high transmissibility of thatyaquifer. That the quartzite probably acts as a0ove seary tuff; hydraulic barrier was first hypothesized in 1957 by Omar Loeltz (written

ambrian commun., 1959), who correctly predicted that the depth to water at thefuture site of well 4340 would be less than 150 ft.

for the In summary, both the steep and the gentle hydraulic gradients shown onfor thes, the cross section are probably caused by a thick sequence of relatively im-f these,o permeable Precambrian and Lower Cambrian clastic rocks separating westernfollow- Emigrant Valley from areas of lower ground-water potential to the east andclearly west.:11 3918 Two qualifications of the above argument are necessary. First, an objec-

aquifer; tion may be raised that water levels in the Tertiary and Quaternary aquifers!ath the should not be considered a reflection of hydraulic gradients within the Pre-recludes cambrian and Paleozoic clastic strata. Nevertheless, we believe such a pro-iose de- cedure is valid in this area. Drilling in Yucca and Frenchman Flats and Indian

Springs Valley, and data in the Amargosa Desert indicate that the water levelsection in the Cenozoic aquifers is usually within 100 ft or less of that in the under-

he thick lying Paleozoic acquifers or aquitards, except where true perched conditionsl eastern exist. Because the water-level changes cited are considerably greater than;aleozoic 100 ft, our hypothesis seems valid. In addition, in each area discussed ati 2000 ft least one control point is available in the older rocks for confirmation of theer levels above generalization.thpart of The second qualification pertains to the values of hydraulic gradient citedthe steep and also values presented in succeeding examples. The hydraulic gradientsthe steep Within the clastic strata may be reasonably continuous, or they may represent

fit on the discontinuous levels within blocks of rock separated by relatively impermeableell shows faults. For this reason these slopes are referred to as "apparent hydraulicsiltstone, gradients." Whether the gradients are continuous or steplike does not ma-reflects a terially change the interpretations presented.

a-

I42 NEVADA TEST SITE WINOGRAD A

YUCCA FLAT Yucca Flat generallpotentiometric levelfrom about 2900 tgradient of 240 to 9

A second example of the role of the clastic rocks in controlling regionalground-water movement within the miogeosynclinal strata is available fromtest drilling in Yucca Flat. Figure 4 shows the approximate distribution ofcarbonate and clastic rocks at 2400 ft altitude, and the potentiometric levelfor each rock type. The central and east-central part of Yucca Flat is under-lain principally by carbonate aquifers, Cambrian through Devonian in age,whereas the western half of the valley contains principally Devonian andMississipian clastic rocks of the Eleana Formation; the northeastern part ofthe valley contains Precambrian and Lower Cambrian clastic strata. TheDevonian and Mississippian strata are part of the upper plate of the Tippinipthrust fault. In effect, Figure 4 is a crude geologic map of pre-Tertiary rocksat the 2400-ft level, which is about 1500 ft below the floor of Yucca Flat.This level was chosen because the potentiometric surface within the carbonateaquifer ranges from 2380 to only 2415 ft throughout this valley.

The hydraulic gradient in the carbonate aquifer in northern and eastern

WEST

2000 I VETI(CAIL a

cSYNCLINE

RIDGE-Pr Pr

400002 0 0 0r -I -*

SEA LEVEL 1A2000- Dffi

EXPLANATION

Stipple indicates outcrop area;outcrop of carbonate rocksnot shown

Figure 5. Geologisection shown on Fig'lPr, Pennsylvanian c(Eleana Formation);

Mississippian and Devonianclastic rocks and minorPennsylvanion carbonate rocks

Devonian to Cambriancarbonate rocks

Lower Cambrian and Precambriarclostic rocks

- 2900-Potentiometric surface, in

feet above sea level

section through cechanges in hydraulgradients also existin northeastern Ytwell 3918 (Fig. 3northern Yucca Flby the bordering clbasins into the carconsequently be coAn important distirclastic aquitard whiand Precambrian clYucca Flat the Dethousands of feet cthe clastic strataonly by the crystalnecessarily retardwest, because such.aquifers, at depthsthe Eleana Formatfault; if the thrust

1 I | 36052'30"0 5 MILES 11552' 30'

" ' . i '

Stratigraphic and (or)hydrologic control

Figure 4. Approximate distribution of pre-Tertiary rocks at 2400-ft altitude and theirpotentiometric surface, Yucca Flat. Tertiary and Quaternary rocks, which extend below2400 ft altitude in the central part of the valley, are not shown. The Cambrian throughDevonian carbonate aquifers are saturated only below 2420 ft altitude, whereas thePrecambrian and Lower Cambrian and Devonian and Mississipian clastic rocks aresaturated to altitudes up to and above 3900 ft. The centrally located carbonate wedgethus serves as an hydraulic sink for ground water in the clastic strata. C-D, line of sectionof Figure 5. 40,000-ft grid based on Nevada State Co-ordinate System (Central Zone).

!!;it

1IWINOGRAD AND THORDARSON-GROUND-WATER MOVEMENT 43

)lling regionalavailable fromfistribution ofJiometric levelFlat is under-

,onian in age,Devonian andastern part ofic strata. TheIf the TippinipTertiary rocksf Yucca Flat.the carbonate

_y.

Yucca Flat generally ranges from one to ten ft per mile. By contrast, thepotentiometric level within the clastic rocks of the Eleana Formation rangesfrom about 2900 to 3800 ft above sea level, with an apparent hydraulicgradient of 240 to 940 ft per mile. Figure 5 is a geologic and hydrologic cross

WEST EAST

DLIE I*

UEIJ lProed.cidl YUCCA FLAT(Pro;.cted I'

ofv 371 ?"Z~ ot *!1 0,lv 2402

Xn and easternSEA

.ANATION

ates outcrop orea;carbonate rocks

and Devoniancks and minoron carbonate rocks

to Cambrian)fote rocks

rion and Precomb iar3tic rocks

-2900-etric surface, ineve sea level

Iphic and (or)ogic control

0 Z 2MILES

Figure 5. Geologic and hydrologic cross section, west-central Yucca Flat. Line ofsection shown on Figure 4. QTv, Quaternary and Tertiary valley fill; Tt, Tertiary tuff;FPr, Pennsylvanian carbonate rocks; MDr, Mississippian and Devonian elastic rocks(Eleana Formation); DEr, Devonian to Cambrian carbonate rocks.

4

section through central Yucca Flat which better illustrates the significantchanges in hydraulic gradient within the Eleana Formation. Steep hydraulicgradients also exist within the Precambrian and Lower Cambrian clastic rocksin northeastern Yucca Flat, which was suggested previously by data fromwell 3918 (Fig. 3). In effect, the carbonate aquifer beneath central andnorthern Yucca Flat is isolated from carbonate aquifers in adjacent valleysby the bordering clastic aquitards; that is, any ground water moving betweenbasins into the carbonate aquifer would have to pass through-and wouldconsequently be controlled by the transmissibility of - the clastic aquitards.An important distinction exists, however, between the Devonian-Mississippianclastic aquitard which bounds the valley on the west and the Lower Cambrianand Precambrian clastic rocks bordering it on the northeast; namely, in westernYucca Flat the Devonian and Mississippian strata probably are underlain bythousands of feet of lower and middle Paleozoic carbonate aquifers, whereasthe clastic strata beneath northeastern Yucca Flat probably are underlainonly by the crystalline basement rocks. Thus, the Eleana Formation need notnecessarily retard the movement of ground water into Yucca Flat from thewest, because such movement might occur through the underlying carbonateaquifers, at depths of several thousand feet. How effective a hydraulic barrierthe Eleana Formation is depends on the nature at depth of the Tippinip thrustfault; if the thrust dips steeply for several thousand feet, it could isolate the

altitude and theirlich extend belowambrian through

ude, whereas theelastic rocks arecarbonate wedge

-D, line of sectionCentral Zone).

I44 NEVADA TEST SITE WINOK

carbonate strata beneath the central part of the valley from equivalent stratato the west. If it does not, then the ground water in the Eleana Formationmay be semiperched above the underlying carbonate rocks.

INDIAN SPRINGS VALLEY

Figure 6 shows the potentiometric contours for both the Quaternary valleyfill and Cambrian and Ordovician carbonate aquifers in southern IndianSprings Valley. These contours suggest that two prominent east-trendinghydraulic barriers occur between the Nye-Clark County line and IndianSprings, a distance of about 12 miles. The water-level altitude in the valley-fill aquifers south of U.S. Highway 95 is about 3300 ft above sea level, andthe level of Indian Springs, which emerges from Pennsylvanian carbonaterocks south of U.S. Highway 95, is about 3 175 ft. The shut-in-pressure ofthis spring, if one could be obtained, would of course, be higher than 3175 ft.

The altitude (U.S. Highwayrocks, is abotlevels in twoare 2731 andthe same aquinferred hydri

Figure 7 slsouth cross seroughly withwell, 1960; 1dence to sugzone of saturibarrier may tbarrier, on tfbrian and ol2745 and 27

That the pto steep gracHale of the Ilevels in the

I

I

INORTz

5000',0

4500'-

4000 -s

3500 -

3000 -

WEL.2506 _~ OFF

1240(2000'

5300Potentiometric surface,

volley-fill aquifer

0 2 4 6 8 10 MILESI , I

_70a0= -____-__Potentiometric surface, Approximate position

carbonate aquifer of hydraulic barrier

4000'7-z20 0 0

' -

SEA LEVEL.- -

2000 -03312

Well topping Quaternary andTertiary valley-fill

02415Well tapping pre-Tertiary

aquifer or aquitfard

Numbers ore altitudes, in feet above sea level, of water levels

Figure 6. Potentiometric contours for Quaternary and Tertiary valley fill andCambrian and Ordovician carbonate aquifers, and approximate positions of two majorhydraulic barriers, southern Indian Springs Valley. E-F, line of section shown on Figure 7.

Figure 7.Line of sectioituff; O& r, Or.

I WINOGRAD AND THORDARSON-GROUND-WATER MOVEMENT 45

from equivalent stratathe Eleana Formation

rocks.

l the Quaternary valley-rs in southern Indian'rominent east-trending3unty line and Indiani altitude in the valley-ft above sea level, andnnsylvanian carbonateFhe shut-in-pressure ofbe higher than 3175 ft.

The altitude of Cactus Springs, which emerges from valley fill adjacent toU.S. Highway 95 but probably is fed by the shallow underlying carbonaterocks, is about 3240 ft. However, one mile north of U.S. Highway 95 thelevels in two test holes tapping Cambrian and Ordovician carbonate rocksare 2731 and 2745 ft altitude. Still farther northward the regional level withinthe same aquifers is about 2400 ft. The approximate position of the twoinferred hydraulic barriers is shown by the broken lines on the illustration.

Figure 7 shows the relation of the hydrology and the geology in a north-south cross section. The position of the southern hydraulic barrier coincidesroughly with the inferred position of the Las Vegas Valley shear zone (Long-well, 1960; Longwell and others, 1965; Burchfiel, 1965). There is no evi-dence to suggest the presence of clastic rocks within the upper part of thezone of saturation adjacent to U.S. Highway 95, and accordingly the hydraulicbarrier may be created by gouge developed along the shear zone. The northernbarrier, on the other hand, may result from the presence of the Lower Cam-brian and older clastic rocks within the zone of saturation north of wells2745 and 2731.

That the-prominent difference in water-level altitude in this area is not dueto steep gradients within the carbonate aquifer has been suggested by W. E.Hale of the U.S. Geological Survey on the basis of two facts. First, the waterlevels in the two wells between the barriers indicate a gentle gradient of 5 ft

115030'

NORTH SOUTH

5000', -5000'

4500'-

4000'-

INFERRED AHYDRAULIC/ \

BARRIER/ \Jl/ \ ~~~~~~~INFERRED

\ / \ ~~~~~HYDRAULICA< \ 2745 BARE

| ,\/3200 3300FOOIT27~ ~ OLEE / LEVEL

WELL CONTROL F?-J ?__ __OFF SECTION N POTENTIOMETRIC |SURFACE

-4500'

-4000'

3500 -

3000 -

-3500,

-3000'

2500'-

-nn'

-2500'2400-FOOT LEVEL

VERTICAL SCALE X 6.7

LESE F

SPOTTED RANGE INDIAN SPRINGS VALLEY

______

Approximate positionof hydraulic barrier

4000�_ O�cr 0 Ocr C, 2745 3330OTv 4000

2000 .-- 2000'11

SEA LEVEL " 11 .�-SEA LEVEL

- 1�I2000'_ 1 � 2000'*2415

apping pre-Tertiaryuifer or aquitard N .,. , _ _ _ _ I

-- LAS VEGAS VALLEYSHEAR ZONE

Of woter levels 0 I � MILESL

Tertiary valley fill andte positions of two majoriection shown on Figure 7.

Figure 7. Geologic and hydrologic cross section, southern Indian Springs Valley.Line of section shown on Figure 6. QTv, Quaternary and Tertiary valley fill; Tt, Tertiarytuff; 016r, Ordovician and Cambrian carbonate rocks; -Gr, Lower Cambrian clastic rocks.

i

I46 NEVADA TEST SITE

per mile to the west. Second, pumping tests of these wells indicate trans-missibilities of 10,000 to 20,000 gpd/ft, respectively. It is unlikely that steephydraulic gradients could develop in such rocks. The carbonate rocks tappedby these wells are, then, part of a relatively permeable block bounded onboth north and south by hydraulic barriers, which separate it from otheraquifer blocks. It is interesting to note that although the potentiometric con-tours on Figure 6 suggest a regional northerly movement of water, the prin-cipal movement within the carbonate aquifers between the barriers may actu-ally be to the west, as is the regional movement of ground water north of thetwo barriers.

4

CONCLUSION

The preceding examples demonstrate how fault blocks containing the thicksequences of clastic rocks of Precambrian, Lower Cambrian, Devonian, andMississippian age, and perhaps also major shear zones, may compartmentalizeor isolate the fault block&containing the highly permeable carbonate aquifers.The compartmentalization of the aquifer may occur within a single valley orbetween valleys. From these and several other examples we believe that thePrecambrian and Lower Cambrian clastic rocks constitute the hydraulic base-ment for ground-water movement within the miogeosynclinal strata, exceptin those places where the clastic strata are thrust over carbonate rocks in flatthrust sheets. The Devonian and Mississippian clastic rocks, on the otherhand, by virtue of their stratigraphic position need not necessarily retardinterbasin movement of ground water at great depth. The relatively thin clasticunits of the miogeosyncline, such as the Cambrian Dunderberg Shale, theOrdovician Ninemile Formation, and the Ordovician Eureka Quartzite, arenot likely to form extensive hydraulic barriers, because normal faulting fre-quently offsets them; however, locally, knowledge of their subsurface positionmay be important.

The three areas discussed were chosen specifically because adequate welland geologic control was available to demonstrate clearly the intuitively sus-pected role of the clastic rocks in regional ground-water movement. In prac-tice, however, the hydraulic barriers cited could have been detected from thewater-level data alone, without the aid of geologic mapping. But, the ex-amples cited were chosen from areas close to the margins of the flow systemin the carbonate aquifers at and near the Nevada Test Site. In the centralpart, and near the discharge end of this system in the Amargosa Desert, thehydraulic gradients are usually less than 20 ft per mile, and detection of theclastic barriers there probably will not be possible without intensive wellcontrol. Particularly in such areas, but also throughout the flow system,geologic mapping of the clastic sequences seems to offer an economical methodof detecting major hydraulic barriers in lieu of costly test drilling. Presentgeophysical methods offer little or no promise of distinguishing the clasticaquitards from the carbonate aquifers at depth.

WIN

Recognitiowater in theFirst, and rrboundaries ran interbasindemically orchemistry ofthe flow systselection an(and extent ovolume of anent springexplorationpotential forby the regioFinally, in Imonitoring cthe site ofof the positi(study of theshear zonesstudy of theof the mioge

Back, Williarin hydro

Burchfiel, B.its regior

Eakin, T. E.,area, sot

Longwell, C.GeologyBull. 62.

Longwell, C.Nevada

Maxey, G. B.the Grea

Winograd, I.Site, in S450-C, p.

- 1963, ALas Vegito the efiJoint Co:

-

iells indicate trans-i unlikely that steep3onate rocks tapped

block bounded on'arate it from otherpotentiometric con-t of water, the prin-.e barriers may actu-d water north of the

containing the thick)rian, Devonian, andlay compartmentalizeie carbonate aquifers.hin a single valley ors we believe that thete the hydraulic base-nclinal strata, exceptarbonate rocks in flatrocks, on the other

tot necessarily retardrelatively thin clastic

)underberg Shale, theeureka Quartzite, are

normal faulting fre--ir subsurface position

I WINOGRAD AND THORDARSON-GROUND-WATER MOVEMENT 47

Recognition of the influence of clastic strata upon movement of groundwater in the miogeosynclinal carbonate rocks is important for many purposes.First, and most critical, it may permit the partial assignment of hydraulicboundaries needed for the construction of an initial "working" flow net ofan interbasin aquifer system. A flow net, however crude, is essential for aca-demically oriented studies, such as those involving the dating or the geo-chemistry of ground water, because the major heterogeneities introduced intothe flow system by the hydraulic barriers must be carefully considered duringselection and evaluation of sampling points. Knowledge of the distributionand extent of the clastic rocks is a prerequisite in a hydrologic analysis of thevolume of a reservoir or of the interference between a well field and a promi-nent spring discharge area. In prospecting for oil within the miogeosyncline,exploration of basins surrounded by clastic strata appears to offer greaterpotential for entrapment than those groups of basins connected hydraulicallyby the regional movement of ground water through the carbonate aquifers.Finally, in problems involving radiological safety, drilling of wells for themonitoring of the movement of radionuclides from a reactor site, or fromthe site of an underground nuclear detonation, clearly requires knowledgeof the position of all clastic aquitards, even the relatively thin ones. Geologicstudy of the areal distribution of the thick clastic rock sequences and majorshear zones thus appears to be an important first step for semiquantitativestudy of the movement of ground water within the complex carbonate aquifersof the miogeosyncline.

REFERENCES CITED

Back, William, and Hanshaw, B. B., 1965. Chemical geohydrology, in Advancesin hydroscience, v. 2: New York, Academic Press, p. 95-98.

Burchfiel, B. C., 1965, Structural geology of the Specter Range quadrangle, andits regional significance: Geol. Soc. America Bull., v. 76, p. 175-192.

Eakin, T. E., 1966, A regional interbasin ground-water system in the White Riverarea, southeastern Nevada: Water Resources Research, v. 2, no. 2, 251-271.

LOngwell, C. R., Pampeyan, E. H., Bowyer, Ben, and Roberts, R. J., 1965,Geology and mineral deposits of Clark County, Nevada: Nevada Bur. MinesBull. 62.

Longwell, C. R., 1965, Geology and mineral deposits of Clark County, Nevada:Nevada Bur. Mines Bull. 62.

Maxey, G. B., 1964, Occurrence and motion of ground water in the limestones ofthe Great Basin (abs.): Geol. Soc. America Spec. Paper 82, p. 128-129.

Winograd, 1. J., 1962, Interbasin movement of ground water at the Nevada TestSite, in Short papers in geology and hydrology: U.S. Geol. Survey Prof. Paper450-C, p. C108-C111.1963, A summary of the ground water hydrology of the area between the

Las Vegas Valley and the Amargosa Desert, Nevada, with special referenceto the effects of possible new withdrawals of ground water, in U.S. Congress,Joint Committee on Atomic Energy, Sept. and Oct. 1963, Nevada Test Site

Because adequate wellrly the intuitively sus-,r movement. In prac-*een detected from thenapping. But, the ex-ins of the flow system

,st Site. In the centralAmargosa Desert, the

,, and detection of thewithout intensive welliout the flow system,an economical method

y test drilling. Presentstinguishing the clastic

48 NEVADA TEST SITE ICommunity, Hearings: U.S. 88th Cong., 1st sess., p. 197-226; also U.S. Geol.Survey TEI-840, open-file report, 79 p.

Winograd, I. J., and Eakin, T. E., 1964, Interbasin movement of ground waterin south-central Nevada -The evidence (abs.): Geol. Soc. America Spec.Paper 82, p. 227.

Suli

I A

U.S

Yucca Flacipally by adeveloped wlfans formedrubble mixecand in placecoarse gravethey grade ithe fans arelate Pleistoctsist of thickplaces; colle(sand Local]Paleozoic ro(are now deelonce bordert

Yucca Fl,montane ba,

- - -

13 7&q(cC'I

I The Geological Society of America, 1mu-

Me"MiriIto

Nevada Test Site /

EDWIN B. ECKEL, Editor

.awrence Radiation Laboratory,Jniversity of California

alluvium by a buriedater is 1200 feet across

p 'IIVell

AN '; - ' - ..:--- C.-- - -C).-

NOV - 6 1975Li b rary

'Washingtofl, D. C. 2O555

1968