Late Quaternary geomorphology and soils in Crater Flat,Yucca Mountain area, southern Nevada

Frederick F. Peterson Emeritus Professor of Soil Science, University of Nevada, Reno, Nevada 89557John W. Bell Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557Ronald I. Dorn Department of Geography, Arizona State University, Tempe, Arizona 85287Alan R. Ramelli Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557Teh-Lung Ku Department of Geological Sciences, University of Southern California, Los Angeles,

California 90089

ABSTRACT

Soil-geomorphic studies indicate that sixmajor allostratigraphic units occur in Cra-ter Flat, Nevada, adjacent to Yucca Moun-tain. These units are, from youngest to old-est, Crater Flat, Little Cones, Late BlackCone, Early Black Cone, Yucca, and Soli-tario. Presence and degree of differentiationof Av, Ak, Bw, Bt, Btk, Btkq, and Bqkm ge-netic soil horizons characterize units, con-firm relative ages, and aid in estimating nu-merical ages. Stratigraphic order and soilsallow correlation with similar alluvial se-quences in adjacent Basin and Range areas.Minimum-limiting ages—by 14C acceleratormass spectrometry (AMS) and cation-ratiodating of rock varnish and by 230Th/234U dat-ing of pedogenic carbonate—support allo-stratigraphic order and are in reasonableagreement with numerical ages estimated forcorrelative regional units. Consistent, clus-tered, 14C AMS varnish ages from widely sep-arated, same-age surfaces suggest that theages, although minima, do not significantlyunderestimate true ages. Rock-varnish 14CAMS ages on Late Black Cone and youngerunits, andK-Ar ages from volcanic lava cones,provide calibration points for a Crater Flatcation-leaching curve. This curve differssomewhat from a previous Yucca Mountaincurve and yields calculated cation-ratio agesyounger by factors of two to three for theyounger units.If the 14C AMS varnish ages provide rea-

sonably close minimum ages, as we believethey do, the Little Cones and Late BlackCone units collectively form an extensivelate Wisconsin–early Holocene deposit not

previously described in Crater Flat. TheLate Black Cone unit (>17 to >30 ka) cor-relates with units in the Lower ColoradoRiver, Death Valley, Mojave Desert, andLas Vegas areas—all likely products of cli-matically induced, late Wisconsin pluvialalluviation. Similarly, the Little Cones unit(>6 to >11 ka) correlates with regionalunits thought related to alluviation duringclimatic transition from the late Wisconsinmaximum pluvial to the arid Holocene. Theareal distribution of late Pleistocene unitsdemonstrates that the Crater Flat piedmontand valley floor were extensively alluviatedduring the last glacial episode.Ages of three older, mid-Quaternary

units are uncertain, but they are largelyyounger than Bishop ash (730 ka). TheEarly Black Cone and Yucca units are esti-mated from rock-varnish cation-ratio dat-ing to be from >159 to >201 ka and >375ka, respectively, and the Solitario unit,which contains the Bishop ash, is from>433 to >659, but <730 ka.Our allostratigraphic units differ in age

by factors of 2–10 from a previous ‘‘surficialdeposits’’ stratigraphy used in the YuccaMountain area. Although the earlier stra-tigraphy has some units numericallyequivalent in age to our allostratigraphicunits, we found soil features in deposits ofthese ages different from those previouslydescribed.

INTRODUCTION

Crater Flat is an alluvium-filled structuralbasin on the west side of Yucca Mountain,Nevada—the site under consideration for a

high-level nuclear waste repository. TheQuaternary alluvium of the Nevada TestSite region was first described by Hoover etal. (1981). Their ‘‘surficial deposits’’ chro-nology was the principal stratigraphicframework for most surficial geologic stud-ies in the Yucca Mountain area, and geo-logic quadrangle maps of the Yucca Moun-tain and Crater Flat region used theirchronology (Swadley, 1983; Swadley andCarr, 1987; Swadley and Parrish, 1988;Swadley and Hoover, 1989a, 1989b). Nu-merical ages for the units were estimated bySwadley et al. (1984) and Rosholt et al.(1985). Taylor (1985) and Harden et al.(1991) described soils associated with surfi-cial deposits on the east side of YuccaMountain.North-trending, late Quaternary faults

offset alluvium in Crater Flat both along thecanyons of the western flanks of YuccaMountain and out on the piedmont slope(Fig. 1). The Hoover et al. (1981) strati-graphic framework was used in some previ-ous paleoseismic studies near Yucca Moun-tain. Swadley and Hoover (1983) found noevidence of young faulting in exploratorytrenches in Crater Flat. Swadley et al. (1984)concluded that none of 32 identified faults inthe Yucca Mountain area have offsetsyounger than 40 ka and noted that no Hol-ocene faulting had been recognized. How-ever, subsequent fault studies (Whitney etal., 1986; Ramelli et al., 1988, 1989) identi-fied late Pleistocene and Holocene faults inCrater Flat. Reheis (1986) found that lateQuaternary soils had experienced faulting indeposits along the Bare Mountain faultbounding the west side of Crater Flat, im-

GSA Bulletin; April 1995; v. 107; no. 4; p. 379–395; 7 figures; 6 tables.

379

Data Repository item 9502 contains additional material related to this article.

plying these deposits to be generallyyounger than suggested by the ‘‘surficial de-posits’’ stratigraphy.We believe the initial lack of recognition

of young fault offsets at Yucca Mountainwas in part due to unrecognized late Qua-ternary stratigraphy. Regional Quaternarychronologies in other, similar alluvial-geo-morphic settings of the arid Basin andRange Province (cf. Wells et al., 1990a; Bull,1991) suggest that many basins were exten-sively alluviated during the late Quaternary,particularly during the Wisconsin glacial-pluvial episode (ca. 12–75 ka). We hypoth-esize that alluviation in the Yucca Mountainregion was more active during the late Qua-ternary than previously thought and thatearlier stratigraphic studies may not be de-tailed enough to allow identification of lateQuaternary deposits.Because detailed Quaternary stratigraph-

ic relationships are required for character-izing seismic hazards at the proposed repos-itory site (U.S. Department of Energy,1988), we tested our hypothesis in severalways. We collected geomorphic and soildata that allowed us to compare and test theprevious work while developing a refinedlate Quaternary chronology for Crater Flat.In contrast to previous work, we used allo-stratigraphic relationships and experimentalrock-varnish and uranium-series datingtechniques to define and scale stratigraphicrelationships. We compare our results withprevious soils and surface-exposure datingstudies, correlate our stratigraphy withother late Quaternary units in the southernNevada, Death Valley, and Mojave Desertareas, and provide new stratigraphic datarelevant to understanding climatic-alluvialprocesses in the Basin and Range Provinceduring the late Quaternary.

DEFINITION OF STRATIGRAPHICUNITS

We mapped geomorphic surfaces usingtopographic relations and soils to establishboundaries and characterize the units (cf.Peterson, 1981). These Quaternary units aretermed allostratigraphic units, defined by theNorth American Commission on Strati-graphic Nomenclature (1983) as mappablestratiform bodies of sedimentary rock de-fined and identified by bounding disconti-nuities—geomorphic surfaces and soils inthis case. Allostratigraphic units are analo-gous to the alluvial-geomorphic units of Kuet al. (1979) and Bull (1991). We contrastour units with the ‘‘surficial deposits’’ unitsof previous workers (Table 1), which are hy-brids of time-stratigraphic and rock-strati-graphic units, not allostratigraphic units. Us-ing geomorphic surfaces and related soils, weproduced a map delineating allostratigraphicunits for interpreting the Quaternary historyof the Crater Flat piedmont (Fig. 1).

ALLOSTRATIGRAPHIC UNITS INCRATER FLAT

Six major units comprise the piedmontslopes of Crater Flat; from youngest to old-est, they are the Crater Flat, Little Cones,Late Black Cone, Early Black Cone, Yucca,and Solitario units. At one or more loca-tions, a surface remnant of each unit is top-ographically clearly above or below a phys-ically adjacent, next-younger or next-olderfan remnant—an inset relationship that is aclear demonstration of relative age. Also,surface remnants of each unit have charac-teristic and distinctively different soils fromthose of the physically adjacent remnant ofa younger or older unit. At some locations,

units that are fan aprons are thin (,1 mthick) and veneer over, and feather into,older units. Soil examination is required tomap such areas. Table 2 shows the correla-tion of units with the ‘‘surficial deposits’’units used by Swadley and Parrish (1988) tomap Crater Flat. Table 3 describes illustra-tive pedons for each of our allostratigraphicunits. Additional representative pedon de-scriptions from Crater Flat are available inAppendix 1.In Crater Flat, presence or absence and

degree of differentiation of the following sixgenetic soil horizons help identify geomor-phic surfaces: the vesicular A horizon (Av),a recarbonated lower-A subhorizon (Ak), acambic or cambic-like horizon (Bw), variousargillic horizons (Bt, Btk, Btkq), a stronglyopalized argillic subhorizon (Btkq), andduripans (Bqkm). The first two soil horizonsare not defined in Soil Taxonomy (Soil Sur-vey Staff, 1975) but are characteristic ofmany desert soils (cf. Eckert et al., 1978;Nettleton and Peterson, 1983).

Crater Flat Unit

The Crater Flat unit consists of active andrecently active washes, inset fans, fan skirts,and fan aprons (landform terms are fromPeterson, 1981). The unit is erosionally insetbelow all older surfaces in Crater Flat andcontains one or more geomorphic subunitsnot differentiated here because of the scaleof the study.The soils of the Crater Flat unit are coarse

textured, thermic, Typic Torriorthents (pe-don CFP-41; Table 3) without any organizeddesert pavement. They have neither signifi-cant genetic soil horizons (i.e., no Av, cam-bic, or argillic horizons) nor any differ-entiated horizon of carbonate or opal accu-

TABLE 1. SURFICIAL-DEPOSITS STRATIGRAPHY

Unit Age (ka)* Distinguishing soil characteristics

Q1a 0–0.15 Soils developed on oldest subunit (Q1c) are incipiently developed with only minimal A andQ1b 0.15–4.0 stage I carbonate (Bk) horizons.

Q1c 7.0–9.0

Q2a ;40 Soils have Av, Bw, and stage I Bk horizons. Unit is only recognized in 3 locations on Nevada TestSite where it appears to be a single debris flow.

Q2b 160–250 Soils have Av, Bw, and stage I Bk horizons; 1–3 mm carbonate coatings on clast bottoms.Recognized only as strath terraces in Q2c and QTa.

Q2c 270–800 Contains 3 different soils: Oldest soil contains 30- to 70-cm-thick Bt and 1–2 m thick stage IV Bkhorizons; intermediate-age soil contains 50-cm-thick Bt and 1-m-thick stage III–IV Bk horizons;youngest soil contains 50-cm-thick Bw and 0.5- to 1-m-thick stage II–III Bk horizons. Bishop ash(;730 ka) occurs in lower part of unit.

QTa 110–2000 Deeply dissected; soils are stripped with 3-m-thick stage IV1 Bk horizon remaining.

Note: From Hoover et al. (1981) and other subsequent papers.*From Swadley et al. (1984) and Rosholt et al. (1985).

TABLE 2. COMPARISON OF CRATER FLATALLOSTRATIGRAPHIC UNITS WITH SURFICIAL-

DEPOSITS UNITS

Allostratigraphicunit*

Age (ka) Surficial-depositsunit†

Age (ka)

Modern 0 Q1a 0–0.15Crater Flat .0.3 to .1.3 Q1a 0–0.15

Q1b 0.15–4Q1c 7–9

Little Cones .6 to .11 Not mapped n.i.§Late Black Cone .17 to .30 Q2b (as part

of Q2bc)160–250

Early Black Cone .159 to .201 Q2c 270–800Yucca .375 Not mapped n.i.Solitario .433 to .659

but ,730QTa 1100–2000

*This study.†Hoover et al. (1981) unit as mapped by Swadley and Parrish

(1988).§n.i. 5 no information.

PETERSON ET AL.

380 Geological Society of America Bulletin, April 1995

TABLE3.ILLUSTRATIVEPEDONSFORCRATERFLATALLOSTRATIGRAPHICUNITS

Horizon

Depth

(cm)

Color

Texture

Size(%vol)

Size(%wt)

Argillans

Structure

Consistency

Cementation

CaCO,

effervescence

&morphology

pHPores

Roots

Lower

boundary

Dry

Moist

GravelCobbleStoneSand

Silt

Clay

Dry

Moist

Wet

CRATERFLATUNIT(SITECFP-41)

AC

0–19

10YR6/2

10YR4/2

EGS

650

095

32

none

SGLO

LO

SO/PO

none

EO&E

8.6

noV’s

1VF

GW

Ck

19–100

10YR6/2

10YR4/2

EGS

653

095

32

none

SGLO

LO

SO/PO

none

E8.6

N.D.

1VF-F,v1M

N

LITTLECONESUNIT(SITECFP-26)

Av

0–5

10YR7/3

10YR4/3

GVFSL

200

058

357

none

2VCOPR

SHVFR

SO/PS

none

ES

8.6

3VF-MV

none

AS

Ak

5–19

10YR7/3

10YR5/3

GSL

300

068

247

none

1FSBK

VSH

VFR

SO/PO

none

ES-CISF&SPB

8.8

N.D.

v1VF

CS

Bwk

19–29

10YR7/3

10YR5/3

GSL

300

074

206

none

MSO

VFR

SO/PO

none

EV-CISF&SPB

9.0

N.D.

1VF

GS

Bk

29–54

10YR7/3

10YR5/3

VGSL

500

076

195

none

MSO

VFR

SO/PO

none

EV-CSPB

9.0

N.D.

1VFv1F

CW

2Bk1

54–78

10YR7/3

10YR5/3

EGSL

650

080

155

none

MSO

VFR

SO/PO

none

EV-CSPB

8.4

N.D.

1VFv1F

CW

2Bk2

78–100

2.5Y7/1

2.5Y5/2

VGCOS

505

095

32

none

SGLO

LO

SO/LO

none

ES-CSPB

8.2

N.D.

none

N

LATEBLACKCONEUNIT(SITECFP-32)

Av

0–6

10YR7/2

10YR4/3

L13

00

4840

12none

2VCOPR

SHVFR

SS/PS

none

EO

8.6

3VF-FV,2MVnone

AS

Ak

6–15

10YR6.5/2

10YR4.5/3

GSL

200

064

288

none

1FPL&1VFSBK

VSH-SOVFR

SO/PO

none

ES&EV-D,SPB,SF

9.4

N.D.

1VF-F

CW-I

Bt

15–33

10YR6/3

8YR4.5/3

VGSL

500

060

2515

INPBC&PO

1VF-FSBK

SHFR

VSS/VPS

none

EO&E

8.4

N.D.

1VF

CW

Btk

33–51

7.5YR6/3

7.5YR4/4

EGSL

680

070

2010

NPBCC

1VFSBK

VSH

VFR

VSSPO

none

ES&EV&EO-D,SP

8.4

N.D.

none

AW-I

Bqkm

51–711

10YR8/1&

10YR6/210YR7/2&

10YR4/3

EGLS

650

0N.D.N.D.N.D.none

2VCOPL

EH

EFI

n.a.

CS

EV-D,SP

N.D.N.D.

none

N

EARLYBLACKCONEUNIT(SITECFP-37)

Av

0–5

10YR7/2

10YR4/2

L10

00

4540

15none

3COPR

SHVFR

SS/PS

none

EO&EV

8.4

3FV

none

AS

AB

5–11

10YR6/2

10YR4.5/3

GCL

150

036

3529

none

3COPR

&IMPL

SHVFR

S/P

none

EO&EV

8.4

1VFV

v1VF

AW

Btk1

11–35

7.5YR6/4

7.5YR4/4

VGCL

500

037

2538

2NCO

3VFABK

SHFR

VS/P

none

EO&ES

8.6

N.D.

1VF,v1F

CW

Btk2

35–48

7.5YR6/4

7.5YR5/4

EGLCOS

652

080

155

none

1VFSBK

LO

VFR

SO/PO

none

EO&EV

8.8

N.D.

1VF

VAW

Bqkm

48–130

10YR8/110YR7/3

N.D.

705

0N.D.N.D.N.D.none

MSH-VH

LO-VFIn.a.

none-CS

EV

N.D.N.D.

v1VF-M

N

EARLYBLACKCONEUNIT(SITECFP-29)

Av

0–5

10YR7/2

10YR4.5/2

GL

200

048

4012

none

2COPR

SHVFR

SS/SP

none

EO&E

8.8

3VF-MV

none

AS

Ak

5–12

10YR6.5/3

10YR4.5/3

SL10

00

7418

8none

1VFSBK

SOVFR

SO/PO

none

ES

8.4

N.D.

1VF

AW

Btqy

12–24

7.5YR6/3

7.5YR4/4

GCL

202

027

3538

2MKPF&PO

2VFGR-ABK

SO-SH

VFR

S/P

none

EO&ES

8.2

N.D.

none?

AS

Btq

24–47

7.5YR6/3

7.5YR4/4

‘‘SL’’

505

0n.a.

n.a.

n.a.

INPO

&PBC

1COPL&3FABK

H-VH

VFI

SO/PO

CW

EO

8.2

N.D.

none

CW

Bqkm

47–551

10YR8/1&

10YR7/210YR7/3&

10YR4.5/3n.a.

6010

0n.a.

n.a.

n.a.

MVH-EH

VFI-EFIbrittle

CS

ES-D,SP,SF

8.2

N.D.

none

N

YUCCAUNIT(SITECFP-39)

Av

0–8

10YR7/2

10YR4/2

L5

00

5335

18none

3COPR

&1MPL

SHVFR

SS/P

none

EO&E

8.8

3F-MV

none

AW

Ak

8–17

10YR6.5/3

10YR5/3

L10

00

5035

15none

1MPL&2VFSBK

SHVFR

SS/PS

none

ES-SPB

9.0

N.D.

1VF-F

CW

Bt

17–38

7.5YR5/4

7.5YR4/6

VGC

3010

028

3042

4KPF

3VF-FABK

SHFR

VS/VP

none

E8.8

N.D.

v1VF

CS

Btk

38–57

10YR6/6

10YR4.5/6

VGC

452

022

3840

4KPF&CO

2FSBK

HFI

VS/VP

none

EO&E-SPB

8.6

N.D.

none

VAW

Bqkm

57–611

10YR8/1&

10YR7/410YR7/4&

9YR5/4.5

n.a.

50N.D.

0N.D.N.D.N.D.N.D.

MEH

EFI

brittle

CS

ES

N.D.N.D.

none

N

YUCCAUNIT(SITECFP-38)

Av

0–7

10YR6.5/1

10YR4.5/2

GL

200

050

3515

none

3COPR

H-SH

VFR

S/P

none

EO&E

8.6

3F-MV

v1VF

AW

Ak

7–17

10YR6/2

10YR4.5/2

GL

200

042

4018

none

1MPF&2VFSBK

SHVFR

SS/PS

none

ES-SPB

9.0

N.D.

1VF,v1M

AW

Bt

17–28

7.5YR6/4

7.5YR4/6

FG‘‘L’’

200

0n.a.

n.a.

n.a.

4KPF,BR,CO

1MPL&3VFABKH

FI

SS/P

VCWpeds

EO

8.4

N.D.

2VF,1M

VAW

Btqkm

28–72

7.5YR6/4

7.5YR4/4

G‘‘LS’’

150

0n.a.

n.a.

n.a.

3KPF

3MPL

HFI

SO/PO

CW

EO&EV

8.4

N.D.

none

VAW

Bqkm

72–771

10YR8/1&

10YR7/210YR7/3&

10YR7/3

n.a.

300

0n.a.

n.a.

n.a.

N.D.

MVH

EFI

n.a.

CS

EV

N.D.N.D.

none

N

SOLITARIOUNIT(SITECFP-40)

Av

0–5

10YR7/2

10YR4.5/3

VFSL

100

067

258

none

3COPR

SHVFR

SO/PS

none

E&EO

8.8

3F-MV

none

AS

Ak

5–9

10YR7/3

10YR5/3

L8

00

4235

23none

3COPR

SHFR

S/P

none

EV

8.8

N.D.

none

AW-B

Btk

9–19

7.5YR6.5/47.5YR5/4

VGCL

500

030

3535

?1VFSBK

SHFR

S/P

none

EO&EV

8.8

N.D.

1VF-F

VAW

Bqkm

19–211

10YR8/2-310YR7/3

n.a.

500

0n.a.

n.a.

n.a.

N.D.

MEH

EFI

brittle

CI

EV

N.D.N.D.

3VF-F

N

Note:AbbreviationsrefertoterminologyofSoilSurveyStaff(1975).N.D.5

notdetermined;n.a.5

notapplicable.

381Geological Society of America Bulletin, April 1995

mulation. The alluvium forming these soilsis ubiquitously a very gravelly to extremelygravelly sand or loamy sand, so the soils arein sandy-skeletal soil families. At most, mi-crotopographic muting of the surface is ac-companied by very slight accumulations ofpedogenic carbonate as filaments on pebblebottoms at 20–50 cm depths—that is, StageI carbonate accumulation (Gile et al., 1966).

Little Cones Unit

The Little Cones unit consists primarily ofa large fan-piedmont remnant surroundingthe Little Cones basalt cones in southwest-ern Crater Flat and several other large rem-nants around Black and Red Cones. Thesesurface remnants are freshly eroded by theyounger, expanding Crater Flat surfaces andare themselves erosionally inset below BlackCone remnants. Compared with the raw,rough, bar-and-swale microtopography ofthe Crater Flat surface, the Little Conessurface typically is fully smoothed, with ab-stracted drainageways and a slightly var-nished desert pavement on a clearly differ-entiated soil.Little Cones soils have minimal but dis-

tinct desert pavements and sandy loamAv horizons. Calcareous cambic horizons(Bwk), with some pedogenic carbonate onpebble bottoms, and late Stage I Bk hori-zons occur under the Av horizons; the Bkdoes not contain enough pedogenic carbon-ate to be a calcic horizon, in the sense theterm calcic horizon is used in Soil Taxonomy(Soil Survey Staff, 1975). In some pedons,the bottom of the Bwk horizon is too shallowto meet the Soil Survey Staff’s (1975) arbi-trary 50 cm depth requirement for a cambichorizon; hence, the soils may be identifiedas either Camborthids (with a cambic hori-zon) or Torriorthents (without a cambichorizon), depending on where they areexamined.Pedon CFP-26 (Table 3) illustrates soil

features in a pavette1 near the Little Cones

basalt cones, where the parent material ismixed volcanic and limestone alluvium.The Av horizon is only 5 cm thick and

barely exhibits the dilatancy2 of common Avhorizons, but is coherent, is cracked intopolygons, and is coarsely vesicular.There is a prominent but noncemented,

late Stage I, 2Bk horizon that has relativelythick pebble-bottom carbonate coatings. Afew, very thin carbonate laminae occur onthe bottoms of a few of the Av horizon’scoarse prisms; these laminae and the Ak ho-rizon are interpreted as evidence of shallowcarbonate accumulation during late Holo-cene time. High pH values (pH . 8.5) justbelow the Ak horizon are evidence of eolianaddition of soluble sodium salts and theiraccumulation just below the current depthof carbonate accumulation. The relativelythick carbonate coatings on the pebbles inthe 2Bk horizon are considered earlier Hol-ocene additions.

Black Cone Unit

The Black Cone unit consists of small-to medium-sized, fan-piedmont remnantsranging in size from tens of square meters totens of square kilometers, and in aggregateabout as extensive as the Crater Flat surface.This unit has been more closely and moredeeply dissected than the Little Cones sur-face, but less so than the Yucca or Solitariosurfaces. Its ubiquitous remnants are broad,flat, microtopographically muted interfluvescovered by darkly varnished, closely spaceddesert pavement. Boundaries with youngerunits are evident because they commonlyare at erosional scarps. However, bound-aries between the Black Cone unit and theolder Yucca or Solitario unit remnants aregenerally less evident, because the darknessof the rock varnish and the closeness, exten-siveness, and sorting of the desert pave-ments are not observably different.Soil differences are demonstrable where

Black Cone remnants are inset below Yuccaor Solitario surfaces (Fig. 2), but there arealso noticeable differences in soil develop-ment between various remnants of the BlackCone unit that are related to surface-age dif-

ferences confirmable by inset relations inthe field. Differences in soil developmentbetween certain Black Cone remnants aredistinct enough that, together with the nu-merical age relationships discussed later,they allow subdivision and mapping of theBlack Cone unit as Late and Early BlackCone subunits.Late Black Cone Subunit. Soils of the

Late Black Cone subunit have prominent,loam-textured Av horizons under moder-ately well-varnished desert pavement, haveeither argillic or cambic horizons, and havehaplic (weak or discontinuous) duripans orcalcic horizons. The argillic horizons are lessclayey than those of Early Black Cone agesoils. Pedon CFP-32 (Table 3) is a HaplicDurargid formed in volcanic alluvium fromYucca Mountain, and is located betweenRed and Black Cones on an elongate, low,fan-piedmont remnant (Fig. 1). It has a min-imal, very gravelly, sandy-loam argillic hori-zon and a haplic duripan. Its desert pave-ment is well sorted, closely spaced, andmoderately darkly varnished. The underly-ing loam-textured Av horizon is moderatelythick, dilatant, leached of carbonates, andhas a prominent crust when dry. This soil isone of the younger, less-differentiated Dur-argids of the later part of the Black Coneunit. Its argillic horizon has clay skins but arelatively low clay content (;15%). Noncal-careous parts in the Av and Bt horizons areevidence that these horizons were previ-ously leached free of carbonates and havebeen recarbonated. The soil’s haplic duri-pan has only a single thin lamina on top, andalthough quite coherent in situ, it is onlyweakly cemented.Near the Bare Mountain piedmont

(Fig. 1), alluvium of the Late Black Conesubunit contains limestone resulting in theformation of Typic Camborthids rather thanDurargids. An illustrative Calciorthid has a10-cm-thick, coarsely vesicular Av horizonand a calcareous cambic (Bwk) horizon. Its2Bk horizon contains ;20% by volumeStage II–III, weakly carbonate-cementedgravel lenses in a Stage I matrix. Because theLate Black Cone Durargids and Calcior-thids occur on same-age geomorphic sur-faces, they confirm previous work (Nettle-ton and Peterson, 1983) suggesting thatmoderate amounts of limestone parent ma-terial will prevent the formation of an argil-lic horizon.Early Black Cone Subunit. Soils of the

Early Black Cone subunit, where formed involcanic alluvium, have prominent, loam-textured Av horizons under darkly var-

1Desert pavement commonly is most closelyspaced and prominent between coppice dunesand biocoppices on barren, flat to slightly de-pressed areas or along the barren crests of old,rounded, fan-piedmont remnants. These most-prominent patches of desert pavement are calledpavettes here, and are where Av horizons and un-derlying B horizons are best examined. Coppicedunes are low, sandy, eolian accumulationsaround the base of desert shrubs. Biocoppices aremicrotopographic aggregates of several coppicedunes that are joined together by a thin, sandy,eolian sheet. Desert pavement is absent from cop-

pice dunes and biocoppices where burrowing ro-dents concentrate and bioturbate (i.e., mix) thesoils.2Dilatancy is the tendency of a crushed and

worked mass of wet soil material to flow when itis jarred. The greater the flow and the lower thewater content, the higher the content of silt andvery fine sand and the lower the content of clayand humus.

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382 Geological Society of America Bulletin, April 1995

nished desert pavement, argillic horizonsthat generally are clay loams, and typic orhaplic duripans. The argillic horizons may ormay not be opalized in their lower part. Pe-don CFP-37 (Table 3), on the summit ofan elongate fan-piedmont remnant promi-

nently inset below large Yucca-age fan-pied-mont remnants in northern Crater Flat, is anexample of a Typic Durargid with an argillichorizon only minimally opalized. The verygravelly clay loam texture of the argillic ho-rizon suggests that it is older than the very

gravelly sandy loam Bt horizons of the LateBlack Cone Durargids. Noncalcareous partsin the Av and Bt horizons are evidence thatthe A and Bt were previously leached freeof carbonate and have been recarbonated.Other than a few thin opal coatings on peb-

Figure 1. Allostratigraphic map of the central portion of Crater Flat.

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383Geological Society of America Bulletin, April 1995

ble bottoms, there is no morphological sug-gestion that the Bt horizon has been opal-ized. The duripan has only a single laminaon top, but strongly to weakly cementedlenses are imbricated closely enough belowit to allow the pan to be called typic.Pedon CFP-29 (Table 3) is an example of

an Early Black Cone Typic Durargid thathas a prominently opalized argillic horizonabove its duripan. The distinctly reddish-hued (7.5YR), sticky, plastic, argillic hori-zon has at least a clay-loam texture. Itslower, platy part (Btq) reflects an interme-diate stage of the progressive accumulationof pedogenic opal that can convert an argil-lic horizon into a duripan (cf. Flach et al.,1969).

Yucca Unit

The Yucca unit consists of several fairlyextensive fan-piedmont remnants that aredeeply dissected and narrow but flat-topped.It can be distinguished from the Black Cone,Little Cones, and Crater Flat units by aerialphotograph patterns, topographic separa-tion, and soil differences. On both color andblack-and-white aerial photographs, trans-verse bands of darkly varnished desert pave-ment on low, Av-horizon slump-steps con-trast with bands of light-colored biocoppiceareas on step-risers to give a distinctivelybanded photographic pattern for many rem-

nants. These contrast with the photographicpattern for both Late and Early Black Conefan-piedmont remnants, where dark patchesof desert pavement are irregularly shaped,or longitudinally elongated, and light-col-ored biocoppices are semicircular spots, cre-ating a mottled photographic appearance.Yucca-age soils are Typic Durargids that

have darkly varnished desert pavements andloam-textured, strongly crusting Av hori-zons; Ak horizons are somewhat thickerthan those of younger soils. Their argillichorizons are thicker and more clayey thanthose of the younger soils, and some havebeen so strongly opalized (microcemented)that the apparent field texture is a loam orloamy sand. Where exposed in wash cuts,the duripans are strongly cemented andthick.Pedon CFP-39 (Table 3) illustrates a Yuc-

ca-age Typic Durargid that has minimalopalization of a 40-cm-thick argillic horizon.This soil is on a Yucca fan remnant insetbelow a Solitario-age remnant near themouth of Solitario Canyon. Pedon CFP-38(Table 3) illustrates a Yucca-age Typic Du-rargid that has a 55-cm-thick argillic horizonso strongly opalized that the material, orig-inally clay textured, now has the texture of aloam in its upper part and a loamy sand in itsplaty lower part. Both soils have as much as0.5 m of strongly cemented duripan. Al-though duripans on well-preserved Yucca

surface remnants could not be adequatelyexamined in soil pits because of theirstrongly cemented nature, wash exposuressuggest that they are in general thicker andmore strongly cemented than duripans onBlack Cone remnants. Pedon CFP-38 hasseveral opaline surficial laminae on its duri-pan, whereas pedon CFP-39 has only a sin-gle surficial opal lamina on its equally ce-mented duripan.

Solitario Unit

A few ballenas (fully rounded ridge-linefan remnants; cf. Peterson, 1981) comprisethe Solitario unit. One at the mouth of Soli-tario Canyon (Fig. 1) is 1.5 m higher than anadjacent, flat-topped, Yucca-age fan rem-nant that is clearly inset below the Solitarioremnant, demonstrating their relative ages.Bedrock spurs in Solitario Canyon—and inother canyons northeast of Crater Flat—ap-pear to be bedrock-pediment remnants of aSolitario-age surface that was once muchmore extensive. A Solitario remnant in thenortheast corner of Crater Flat (Fig. 1) con-tains reworked Bishop ash (ca. 730 ka) at5–7 m depth (Marith Reheis, 1991, writtencommun.).No well-preserved Solitario-age soils were

found. Rather, there is a complex of TypicDurorthids with small areas of Typic Dur-argids on the rounded crests of the ballenasstudied. Ballenas form by erosional round-ing of formerly flat-topped fan remnants,with stripping of A and Bt horizons from thesoil of the original fan-remnant summit. Ap-parently the original soil was a Durargid;subsequent stripping shattered the upperpart of the duripan and mixed the detrituswith later dust accumulations, leaving aTypic Durorthid.Pedon CFP-40 (Table 3) is a Typic Dur-

argid on a ballena summit that representsthe truncated remnants of the original soil.The ballena is littered with chips of duripanlaminae from the Bqkm horizon, indicatingthat the shattered top of the duripan hasbeen extensively bioturbated. The ballena isdistinctively light colored on aerial photo-graphs, apparently due to the chips of duri-pan laminae strewn across its surface. In asecond, adjacent pavette location, there is aTypic Durorthid that has only an Av (0–5cm) and an Ak (5–19 cm) over the duripan.In a third pavette location, the Durorthidhas only an Av (0–7 cm) over the duripan.The duripans commonly have 1–3 cm of

thick, strongly opalized laminae on theirtops. Fault trenches dug into the sides of

Figure 2. Late Black Cone surface offset by the Windy Wash fault at U.S. GeologicalSurvey trench CF-3. The surface is inset below a Solitario-age ballena, the site of trenchCF-2, seen in the middle ground; Yucca Mountain lies in the background. The trenchexposes thick A and Av horizons and polygenetic Bk and Bqkm horizons. A rock-varnish14C AMS age of 28 920 6 400 yr B.P. was obtained for the surface at this location.

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384 Geological Society of America Bulletin, April 1995

ballenas show that the duripans extend fromthe crest on over the sides of the ballenas.The strongly cemented, laminar part of theduripan, however, is thickest (;0.5 m) at theballena crest and thinner on the youngersideslopes. The shallow sola (Av, Ak, Btk)likely do not represent the original Solitario-age soil; rather, they probably represent dustfall and soil formation on top of a strippedduripan since about Late Black Cone time.The strongly cemented, thickly laminae-cov-ered duripans at the ballena crests are somemeasure of the original Solitario soil.

AGES OF ALLOSTRATIGRAPHICUNITS

The lack of radiometrically datable strat-igraphic horizons in the YuccaMountain re-gion, apart from the Bishop ash, requires theapplication of less-conventional dating tech-niques. Previous numerical age estimates(Table 1) are almost exclusively from ura-nium-trend dating of soils and sedimentsfrom the Nevada Test Site area (Swadley etal., 1984; Rosholt et al., 1985). The open-system uranium-trend technique estimatestime of initial sediment deposition (Rosholt,1980; Muhs et al., 1989) rather than merelythe timeof secondary depositionof thepedo-genic carbonates in the sediment, which ismeasured by the closed-system uranium-series (230Th/234U disequilibrium) method(Ku, 1976). Harrington and Whitney (1987)used some uranium-trend dates for theHoover et al. (1981) Q2b and Q2c units tocalibrate a rock-varnish cation-leachingcurve for the Yucca Mountain area. Whit-ney and Harrington (1993) dated colluvialslope deposits at Yucca Mountain using therock-varnish cation-ratio curve and 36Clanalyses. Harden et al. (1991) used urani-um-trend ages from Rosholt et al. (1985) tocalibrate soils in Fortymile Wash on the eastflank of Yucca Mountain. For this study, weused four different rock-varnish and urani-um-series methods to test our soil-geomor-phic relations and to scale and correlate ourunits.

Rock-Varnish Dating Methods

Rock varnish is a dark Fe- and Mn-richlayering that accretes on subaerial rock sur-faces in arid and semiarid environments.Three separate rock-varnish methods—ac-celerator mass spectrometry radiocarbon(14C AMS) dating, cation-ratio (CR) anal-ysis, and varnish microlamination layering—were used to estimate ages for clasts lying on

unit surfaces. As a surface-exposure datingtechnique, rock-varnish age estimation pro-vides only a minimum age for the geomor-phic surface and soil that characterize a unit,because varnish forms only after the clastsare exposed by erosion or the surface isdepositionally stabilized. In this study, var-nish fragments were collected from multipleclasts at each sampling site to assess the po-tential for disparate ages.We believe our most accurate estimates

of minimum surface-exposure age are fromthe 14C AMS varnish-dating technique.Sampling of rock varnish for this procedurerequires a layered, microstratigraphic con-text, where varnish encapsulates organicmaterial (Fig. 3). The first step is mechanicalremoval of the outer layers of varnish under10–453 magnification. The remaining or-ganic material includes that at the varnish/rock interface and that in the weatheringrind under the varnish. The second step isremoval of the basal layer of varnish and therock immediately underneath. The thirdstep is pretreatment of the subvarnish or-ganic material with HF, hydroxylamine hy-drochloride, dithionite, and HCl.

The two most important aspects in the14C AMS varnish-dating method are the useof layered varnishes and the pretreatmentwith HF. If the varnish is not layered, ityields 14C ages far younger than correspond-ing independent 14C age control (Dorn etal., 1989, 1992; Nobbs and Dorn, 1993). Ifthe HF pretreatment is not used, youngerorganic material adsorbed onto clays is notremoved, and the 14C AMS ages are againfar younger than independent age controls(Dorn et al., 1989).Dorn et al. (1989) tested 14C AMS dating

of varnish by comparing varnish ages in avariety of settings with independent 14C agecontrols. Harden et al. (1991) used rock-varnish 14C AMS and shell 14C dates to cal-ibrate soils at Silver Lake, California. In cen-tral Nevada, varnish 14C AMS dates onalluvial-fan surfaces are consistent with theages of underlying late Quaternary tephraand with conventional 14C dates (Yount etal., 1993; J. W. Bell, unpubl. data). A discus-sion of uncertainties associated with rock-varnish14C AMS dating is presented below.CR dating is based on the observation

that concentrations of Ca and K in rock var-

Figure 3. Examplesof pockets of organicmatter beneath rockvarnish. (A) Second-ary electron micro-scope view of organicmatter beneath rockvarnish at CFP-26(Table 4). The upperline separates var-nish from subvarnishorganic matter. Thelower line shows thevarnish/rock bound-ary. White arrowsidentify broken edgeswhere spot energy-dispersive analysis ofX-rays (EDAX) wastaken. Bar scale 510 mm.

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385Geological Society of America Bulletin, April 1995

nish decrease over time with respect to Ti(Dorn, 1983). Cation ratios for all sampleswere measured by wavelength-dispersiveelectron microprobe. Current CR practice issummarized in Dorn (in press); Dorn(1988a, 1989) and Dorn et al. (1990) de-scribed the CR procedure used for thisstudy. A smaller duplicate sample set wasanalyzed in 1988 by proton-induced X-rayemission (PIXE) at Crocker Nuclear Labo-ratory, University of California, Davis.These samples were also analyzed by wave-length-dispersive electron microprobe, andno significant disparities were found in thecation ratios between the two data sets.We use here only the wavelength-disper-

sive microprobe data. We note that thisanalysis discriminates all elements used inthe CR dating, and it is thus not affected bythe Ba and Ti problem encountered in othermethods (Dorn et al., 1990; Whitney andHarrington, 1993). The microprobe andPIXE data sets are available in Appendix 2.CR dating is still experimental, and its

greatest uncertainties are the environmentalvariables other than time. More than twodozen non-time-dependent variables havebeen isolated (Dorn, 1989, 1992; Krinsley etal., 1990; Dorn and Krinsley, 1991). Reneauand Raymond (1991) argued that there is nosystematic cation-leaching process and thatobserved CR change is an artifact of sam-pling rock substrate. However, Dorn andKrinsley (1991) provided textural, chemical,and laboratory-simulation evidence for cat-ion leaching in rock varnishes. A more com-plete discussion of the controversies associ-ated with cation-ratio dating and reasonswhy certain samples were rejected is avail-able in Appendix 2.

Figure 3. (Continued). (B) Comparison ofbackscatter view (upper) and secondaryview (lower) of a polished thin section ofvarnish at CFP-32 (Table 4). The backscat-ter image reflects the average atomic num-ber, so the Mn-rich layer is brightest, theMn-poor layer is darker, and the organicmatter is black. In contrast, the secondaryelectrons (lower) image shows topography;the mapped organic matter is also indicatedby a strong carbon signal in wavelength dis-persive electron microprobe measurements.Note also the presence of areas of cationleaching and the layering of Mn-poor onMn-rich varnish.

ä

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386 Geological Society of America Bulletin, April 1995

Manganese:iron ratios in rock-varnish mi-crolaminations reflect the alkalinity of thevarnish-forming environment. High ratiosdevelop in less alkaline (more humid) envi-ronments, whereas low ratios develop inmore alkaline (arid) environments (Dorn,1988b, 1990; Jones, 1991). By calculatingMn:Fe ratios for successive varnish layersand by correlating sequences of varnish lay-ers, it is possible to construct a relative-agesequence.

Rock-Varnish Dating Results

Rock-varnish samples were collectedfrom 22 widely separated sites on CraterFlat allostratigraphic units (Fig. 1, Table 4).Samples from 12 sites were analyzed in 1988(Dorn, 1988a), and an additional 10 wereanalyzed in 1989 (Bell et al., 1989). Twelveof the 22 varnish sites were dated by 14CAMS. Eleven finite 14C AMS ages were ob-tained on the Crater Flat, Little Cones, andLate Black Cone units. One infinite age of.40,120 yr B.P. was obtained on an EarlyBlack Cone surface, providing an internalcheck of the method.Rock-varnish 14C AMS ages and K-Ar

ages on basalt cones in Crater Flat are used

to calibrate the Crater Flat cation-leachingcurve (Fig. 4). Since first presented by Dorn(1988a), the Crater Flat cation-leachingcurve has evolved as new 14C AMS calibra-tion points became available. Earlier CRages (Dorn, 1988a) were recalculated usingindividual CR measurements, as describedin Dorn et al. (1990). In addition, the La-throp Wells basalt flow calibration point in-cluded on the original curve was dropped,because recent studies (Wells et al., 1990b,1992; Turrin et al., 1991; Zreda et al., 1993)indicate that the history of the LathropWells center is still unresolved. The CR cal-ibration values listed in Table 4 are averagesof four or more individual measurements,and are used to construct the followingleast-squares semilog regression in Figure 4:

CR 5 13.52 2 1.83 log10 age. (1)

The 2s error envelope shown in Figure 4was constructed to account for errors in cat-ion ratios and calibrated ages. The upperline is a least-squares semilog regression ofthe 12s cation-ratio and age errors:

CR 5 13.93 2 1.87 log10 age. (2)

The lower line is a least-squares semilogregression of the 22s cation-ratio and ageerrors:

CR 5 13.49 2 1.89 log10 age. (3)

Calibrated CR ages are assigned for eachseparate cation ratio using equation (1). By

Figure 4. Rock-varnish cation-leaching curve for Crater Flat. Calibration based on 14Crock-varnish dates for geomorphic surfaces (this study) and on K/Ar dates for basaltic lavaflows (Smith et al., 1990). Error bars indicate uncertainty for each data point used toconstruct the least-squares semilog regression. The error envelope accounts for62s errorsin both cation ratios and calibration ages. The triangular area below the regression en-closes a possible range of curves extending from CFP-32 if the sampled lava flow surfacesare actually younger than the K-Ar ages.

TABLE 4. RESULTS OF ROCK VARNISH ANALYSES

Allostratigraphic unit(this study)

Sample 14C AMS date(yr B.P.; AMS lab #)

(K1Ca)/Ti(ave. 6 1s)

Cation-ratio age (yr)22s (ave.) 12s

Crater Flat CFP-41 9.17 6 0.25 150 (300) 600*JWB-36 1 320 6 70 (ETH 5264) 7.95 6 0.17 800 (1100) 1600†

Little Cones CFP-2 6 645 6 245 (ETH 3197) 6.47 6 0.13 §

JWB-38 8 425 6 70 (ETH 5268) 6.37 6 0.13 §

CFP-26 10 180 6 270 (ETH 3187) 6.13 6 0.13 §

JWB-41 11 135 6 105 (ETH 5270) 5.99 6 0.13 §

Late Black Cone CFP-33 17 280 6 370 (ETH 3191) 5.75 6 0.05 §

CFP-31 5.67 6 0.15 14 000 (19 000) 27 000JWB-39 19 660 6 240 (ETH 4483) 5.68 6 0.15 §

CFP-27 25 700 6 360 (ETH 3188) 5.42 6 0.12 §

CFP-35 26 970 6 375 (ETH 3192) 5.34 6 0.15 §

CFP-36 28 920 6 400 (ETH 3190) 5.32 6 0.07 §

CFP-32 30 320 6 460 (ETH 3189) 5.14 6 0.07 §

Early Black Cone CFP-37 3.98 6 0.19 110 000 (159 000) 218 000CFP-29 .40 120 (ETH 5259) 3.98 6 0.32 116 000 (167 000) 229 000JWB-42 3.90 6 0.11 122,000 (176 000) 241 000JWB-20 3.79 6 0.12 139 000 (201 000) 275 000

Yucca CFP-39 3.29 6 0.11 254 000 (375 000) 508 000CFP-38 3.29 6 0.11 253 000 (373 000) 505 000

Solitario JWB-43 3.17 6 0.10 293 000 (433 000) 585 000CFP-40 2.95 6 0.10 383 000 (572 000) 769 000JWB-40 2.84 6 0.09 440 000 (659 000) 885 000

Allostratigraphic unit(this study)

Sample K-Ar age(yr B.P.)

(K1Ca)/Ti(ave. 6 1 s)

Cation-ratio age (yr)22 s (ave.) 12 s

Little Cones basalt # 770 000 6 40 000 2.63 6 0.08 §

Red Cone basalt # 950 000 6 80 000 2.63 6 0.12 §

Black Cone basalt # 1 090 000 6 120 000 2.53 6 0.06 §

*Error are asymmetric due to the semilog nature of the 2s error curves.†Samples were collected from two localities at JWB-36. One was used for calibration; the other was assigned the CR

age.§Varnish cation ratio used in calibration.#K-Ar ages from Smith et al. (1990).

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387Geological Society of America Bulletin, April 1995

treating each CR as a separate time indica-tor, intersample variability will thus be in-dicative of the time-transgressive growth ofthe varnish. The mean of each calibratedage is presented in Table 4 as the averageage; the 2s errors are derived from equa-tions (2) and (3) and the microprobe meas-urements contained in Appendix 2. It shouldbe noted that although CR averages maybe identical (e.g., samples CFP-29 and CFP-37), the average of the separate age calcu-lations can differ due to the semilog natureof the regressions.To define the older end of the curve, we

used recent K-Ar dates for basalt flows inCrater Flat (Smith et al., 1990; Table 4). Forthe younger end of the curve, a 14CAMS ageof ca. 1300 yr B.P. is available (sample JWB-36; Table 4) for calibrating all but the young-est site. Sample CFP-41, which has the high-est CR, is ,,1300 yr B.P. and has aprovisional age of ca. 300 6 200 yr B.P.based on an extension of the curve. We notethat CR values for the Early Black Cone,Yucca, and Solitario units fall between the14C AMS and K-Ar calibration points;hence, these ages are ‘‘hybrids’’ between thetwo time scales.The CR values for the Black Cone, Red

Cone, and Little Cones basalts are impor-tant calibration points on Figure 4, becausethey establish the older end of the curve. Weassume that the samples collected from thebroad constructional lava surfaces are closeto the original flow surfaces. Detailed map-ping of flow patterns indicates that althoughthe lava flows have been locally eroded, theoriginal flow surfaces are still well preservedwith ,1–2 m of erosion (Faulds et al., 1994;E. I. Smith, 1993, personal commun.). Al-though we attempted to sample varnishesfrom areas on each flow that were felt to beclosest to an original texture, we cannotcompletely exclude the possibility that someof the sampled surfaces have been eroded. Ifso, a range of curves could exist; these wouldfall within the area indicated on Figure 4.Nonetheless, our basic conclusions wouldremain unchanged, while details may varyslightly; that is: (1) the cation ratios couldonly be used to define relative clusters ofstatistically distinct populations for theEarly Black Cone, Yucca, and Solitariounits, and (2) the true ages of the samplesdependent on the K-Ar ages would have tobe even younger than those calculated here(Table 4).Selected varnish alkalinity ratios were

measured by wavelength-dispersive micro-probe, and several trends are evident (Figs. 5

and 6) when evenly layered subaerial var-nishes are used (cf. Dorn, 1990; Krinsley etal., 1990). Younger varnishes have fewerMn:Fe layers than older varnishes. CraterFlat and Little Cones varnishes are only Mnpoor, reflecting a period of enhanced alka-linity during the Holocene (Dorn, 1990).Late Black Cone varnishes have a basallayer of reduced alkalinity, possibly corre-sponding with a more moist late Pleistoceneperiod in the Nevada Test Site area (Spaul-ding, 1985; Claassen, 1986; Szabo et al.,1994). Early Black Cone, Yucca, and Soli-tario varnishes have progressively more com-plex sequences.Based on the rock-varnish results, we es-

timate approximate minimum ages for theallostratigraphic units. Rock-varnish 14CAMS ages suggest the Crater Flat unit is lateHolocene (.0.3 to .1.3 ka), the LittleCones unit is latest Pleistocene to earliestHolocene (.6 to .11 ka), and the LateBlack Cone unit is late Wisconsin (.17 to.30 ka). Because the CR dating method hassignificant analytical uncertainties, we usebroad CR age ranges for grouping the olderunits. The Early Black Cone unit is esti-mated to be from .159 to .201 ka. TheYucca and Solitario units are taken to bemid-Pleistocene, with approximate mini-mum ages of .375 ka and .433 to .659(but ,730 ka), respectively.Reliability of Rock-Varnish 14C Ages.

Rock-varnish 14C AMS ages date the onsetof varnish formation and thus are inter-preted as minima for the subaerial exposureof clasts on the sampled units. Although wedo not know precisely how much the rock-varnish 14C AMS ages underestimate thetrue numerical ages, we infer that the resultsprovide reasonably close minimum ages forthe following reasons: (1) Dorn et al. (1989)found that 14C AMS varnish ages underes-timate true exposure ages of control samplesby ;10% due to the lag between exposureand varnish accretion. Other studies basedon independent controls and stratigraphicassociations have produced similar results(Moore and Clague, 1991; Benson et al.,1992). (2) We sampled widely separated lo-calities from each unit to stratigraphicallytest the consistency of the method. Relativeconsistency and clustering of both 14C AMSages and cation ratios from the same allo-stratigraphic units suggest that either a sys-tematic bias influences the ages, that theages closely approximate true ages, or both.(3) All eleven 14C AMS varnish ages fromLate Black Cone and younger units are fi-nite, suggesting these units are young

enough to be within the range of 14C dating(,35–40 ka). This is consistent with the mi-crochemical signals (Figs. 5 and 6) suggest-ing that the Late Black Cone and LittleCones varnishes were deposited in the lateWisconsin and Holocene. (4) Uranium-series ages are consistent with the varnishresults. Three 230Th/234U ages on pedogeniccarbonate from a Late Black Cone soilrange between 17.1 and 39.4 ka (discussedbelow); at the same locality, surface clastsyielded a 14C AMS varnish age of 28.9 ka(Table 5; Fig. 7).Uncertainties associated with the 14C

AMS varnish ages include the nature of theorganic material and replication. Organicmatter encapsulated beneath rock varnishincludes lichen, charcoal, fungal mats, bac-teria, microcolonial fungi, algae, pollen,plant remains, and unidentified organic ma-terial (Dorn and DeNiro, 1985; Dorn et al.,1989; Nagy et al., 1991; Watchman, 1992;Nobbs and Dorn, 1993; Dragovich, 1993).The samples dated in this study included en-dolithic algae, lichen remains, and uniden-tified organic material. Future tests areneeded to resolve what types of materialyield the most reliable ages.Replicate 14C AMS varnish measure-

ments overlap within 2s errors about half ofthe time (Dorn et al., 1989, 1992; Nobbs andDorn,1993;WhitleyandDorn,1993).Watch-man (1992) was unable to obtain overlap-ping measurements on two replicate varnishsamples from different locations. The lack ofoverlap in ages is not surprising, however,because the organic material was likely en-capsulated at different times as the varnishaccreted. This further argues that the agesare minima.Cation-Leaching Curve. The Crater Flat

cation-leaching curve (Fig. 4) differs some-what from a previous cation-leaching curvefor Yucca Mountain (Harrington and Whit-ney, 1987; Whitney and Harrington, 1993).Although both curves have similar cation ra-tios for the older Red and Black Cones ba-salt calibration points, they deviate substan-tially at their younger ends. For Early BlackCone and younger units, the Crater Flatcurve yields calculated CR ages younger byfactors of two to three than those of Har-rington and Whitney (1987).We sampled varnish from the 40 ka Cra-

ter Flat locality used in the Harrington andWhitney (1987) calibration. Our varnishanalysis yielded a 14C AMS age of 28.96 0.4ka and a mean cation ratio of 5.32 6 0.07(sample CFP-36, Table 4). The cation ratioof 6.0 6 1.0 reported by Harrington and

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388 Geological Society of America Bulletin, April 1995

Figure 5. Visual microlaminations revealing black (Mn-rich, less alkaline) and orange (Mn-poor, more alkaline) layers from fivegeomorphic surfaces. (Top left) CFP-2, varnish thickness is ;10 mm. (Middle left) CFP-33, thickness is ;25 mm. (Bottom left) CFP-32,thickness is 35 mm. (Top right) CFP-29, thickness is ;60 mm. (Middle right) CFP-29, second microbasin, thickness is ;60 mm. (Bottomright) CFP-40, thickness is ;100 mm. Although colors do not reveal precise chemical values, the images show trends with more complexlaminae in progressively older morphostratigraphic positions. A section fromCFP-40 (bottom right) reveals two problems found in varnishsedimentology: an unconformity (at the top of the varnish, perhaps from aeolian abrasion) and interruption of layers (at the right, perhapsfrom microcolonial fungi secreting acids and ‘‘boring a hole’’ into layers).

389Geological Society of America Bulletin, April 1995

Whitney (1987) for varnish at this localitywould correspond to a calculated CR age ofca. 12–13 ka based on the Crater Flat curve,and is a ratio we found more closely relatedwith the Little Cones unit.

We do not know the reasons for this dis-parity, but speculate that it may be related toone or more of the following factors: (1)curve calibration methods (14C versus ura-nium trend); (2) cation-ratio measurement

methods (scraping electron microprobe ver-sus in situ scanning electron microscope);(3) field sampling and laboratory preparationtechniques; or (4) stratigraphic uncertainties.

Uranium-Series Dating of PedogenicCarbonate

Six samples of pedogenic carbonate werecollected from trenches CF-1 and CF-3 inCrater Flat (Figs. 1 and 7) and analyzed bythe uranium-series dating method of Luoand Ku (1991). Trench CF-1 crosses a faultdisplacing a Solitario-age ballena veneeredby a Late Black Cone–age slope deposit;trench CF-3 crosses a fault displacing a LateBlack Cone alluvial surface (Fig. 2). Sam-ples U1, U3, and U6 are from the upper-most Bqkm laminae and are assumed to bethe most recent layers added to the petro-calcic horizons at these locations. SamplesU2, U4, and U5 were collected from car-bonate rinds on clasts located 20–75 cm be-low the laminar horizons and were analyzedfor a stratigraphic test of the dating method.The isochron method of Luo and Ku

(1991) is based on the concept that 230Th inthe samples has two components. One com-ponent is that part of the 230Th initiallypresent when the deposit formed; this com-ponent is associated with all phases bearing232Th. The other component is that part ofthe 230Th produced by in situ decay of 234Usince the sample formed. It is this lattercomponent, called the authigenic compo-nent, that is used in the age estimation. Thisauthigenic 230Th will grow with time inphases such as carbonate and opal, whichcoprecipitate some uranium as they form. Inessence, the ages derived estimate the timeof uranium coprecipitation. It should benoted that the sampled pedogenic carbonatematerial was as much as 5 mm in thickness.The ages reported here, therefore, repre-sent means for the periods over which thecarbonate accumulated. They are minimum

Figure 6. Averaged elec-tron-microprobe analyses(wavelength dispersivemode) of Mn:Fe microlami-nations from different allo-stratigraphic units. As inDorn (1990), the alkalinityindex represents normal-ized Mn:Fe values. Zero isthe lowest ratio (higher al-kalinity), and 1 is the high-est ratio (less alkalinity).Years before present (in103) were derived by nor-malizing depth to the 14C orcation-ratio age. Alkalinityindex values were then re-gressed for every 5000 yr.The central line indicatesthe average alkalinity-indexvalues for 20 transectsacross layered varnishesfrom five different rocks,and bars represent 2s.

TABLE 5A. RADIOCHEMICAL AND UNCORRECTED AGE DATA ON PEDOGENIC CARBONATESAMPLES FROM CRATER FLAT

Sampleno.*

238U(ppm)

234U/238U(ppm)

230Th/234U(ppm)

230Th/232Th(ppm)

Uncorrected age(ka)

U1(a) 7.86 6 0.31 1.61 6 0.06 0.307 6 0.014 1.99 6 0.10 38.9 6 3.6(b) 6.78 6 0.17 1.55 6 0.04 0.331 6 0.012 2.13 6 0.13 42.6 6 2.7(c) 5.67 6 0.12 1.51 6 0.03 0.291 6 0.009 1.81 6 0.09 36.6 6 2.1

U2(a) 5.24 6 0.17 1.60 6 0.05 0.588 6 0.022 6.19 6 0.54 89.9 6 6.0(b) 4.76 6 0.13 1.68 6 0.04 0.644 6 0.022 4.41 6 0.31 102.4 6 6.2(c) 3.52 6 0.11 1.41 6 0.04 0.716 6 0.032 1.61 6 0.10 125.0 6 11.0

U3(a) 7.75 6 0.25 1.57 6 0.07 0.164 6 0.029 2.98 6 0.18 19.2 6 4.8(b) 8.13 6 0.19 1.48 6 0.03 0.186 6 0.007 2.23 6 0.13 22.1 6 1.6(c) 6.98 6 0.21 1.59 6 0.04 0.193 6 0.009 1.27 6 0.08 22.9 6 2.1

U4(a) 10.15 6 0.20 1.48 6 0.02 0.357 6 0.012 3.17 6 0.20 46.7 6 2.4(b) 5.45 6 0.17 1.44 6 0.05 0.419 6 0.018 1.50 6 0.09 57.2 6 4.5(c) 4.95 6 0.19 1.44 6 0.05 0.479 6 0.022 1.41 6 0.08 68.1 6 5.9

U5(a) 18.98 6 0.48 1.46 6 0.02 0.321 6 0.006 19.5 6 1.3 41.1 6 1.6(b) 6.82 6 0.16 1.29 6 0.03 0.568 6 0.015 4.10 6 0.20 87.5 6 4.7(c) 6.82 6 0.18 1.28 6 0.02 0.522 6 0.012 4.52 6 0.19 77.5 6 3.6

U6(a) 14.17 6 0.36 1.48 6 0.03 0.448 6 0.009 17.8 6 1.2 62.1 6 2.5(b) 13.77 6 0.25 1.42 6 0.02 0.454 6 0.006 12.0 6 0.4 63.5 6 1.8(c) 12.10 6 0.31 1.42 6 0.02 0.462 6 0.006 8.76 6 0.24 64.9 6 2.2

*(a), (b), and (c) denote subsamples.

TABLE 5B. ISOCHRON-DERIVED AUTHIGENIC RATIOSOF 234U/238U AND 230Th/234U, AND AGES CALCULATED

THEREFROM

Sampleno.

(234U/238U)a (230Th/234U)a Age (ka)

U1 N.D. N.D. 39.4 6 3.0*U2 1.66 6 0.03 0.555 6 0.044 82.4 6 9.4U3 1.56 6 0.03 0.147 6 0.017 17.1 6 2.5U4 1.50 6 0.02 0.302 6 0.018 38.2 6 2.9U5 1.49 6 0.02 0.288 6 0.009 36.2 6 1.7U6 1.53 6 0.01 0.434 6 0.004 59.5 6 0.9

*From the uncorrected ages of three subsamples (see Ta-ble 5A). It represents a maximum age for the sample. N.D. 5not determined.

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390 Geological Society of America Bulletin, April 1995

ages with respect to the initiation of the ped-ogenic process at the sampling sites.Each of the six samples was divided into

three subsamples with different U/Th ratios.After total dissolution of the subsamples,measurements of their 238U, 234U, 232Th,and 230Th were made using isotope-dilutionalpha spectrometry. Besides the assumptionof a closed system, the age calculations assumethat, for a given sample, the subsamples arethe same age and had a common 230Th/232Th ratio when the sample formed.The analytical results with uncertainties

(1s) based on the counting statistics arelisted in Table 5A, together with the uncor-rected ages calculated without consideringthe 230Th initially present in the deposit.The results were used to construct isochronplots from which the authigenic 234U/238Uand 230Th/234U activity ratios were derivedfor each sample from the slopes of the plot.These ratios and calculated ages are listed inTable 5B; isochron plots are available in Ap-pendix 3. The uncertainties for the isochron-derived ages include both the counting errorand errors associated with the linearity ofthe isochron plots (Luo and Ku, 1991).With the exception of sample U1, the

samples all gave isochron plots with good

linearity. For sample U1, separation intosubsamples with sufficiently different 238U/232Th ratios was unsuccessful, and no mean-ingful isochron was obtained. The age ofsample U1 (Table 5B) is an average of theuncorrected ages for its three subsamples,and it represents the maximum age for thesample.The calculated ages for the six samples

range from 17.1 6 2.5 to 82.4 6 9.4 ka (Ta-ble 5B). At trench CF-1, the Bqkm horizonis polygenetic, with a Late Black Cone soildeveloped on a truncated Solitario soil. Theuranium-series ages of 36.26 1.7 and 59.560.9 ka are interpreted to date the carbonateadded during Late Black Cone time, al-though we note that sample ages are re-versed with respect to stratigraphic position.The younger age of sample U-5 is, however,close to a 230Th/234U date of 33 6 4 ka pre-viously obtained from this trench by Szaboand O’Malley (1985) (Sample TSV-387,Fig. 7).The best results are from trench CF-3,

where the carbonate horizon is polygenetic,with a Late Black Cone Bk horizon super-imposed on an Early Black Cone Bqkm ho-rizon. Uranium-series ages of 17.1 6 2.5,38.2 6 2.9, and 39.4 6 3.0 ka from the up-

permost laminae and the underlying rindcarbonate are consistent with a Late BlackCone age for the geomorphic surface. Afourth older age of 82.4 6 9.4 ka on rindcarbonate may in part be dating the Bqkmhorizon of the older Early Black Cone soil.Rosholt et al. (1985) reported three urani-um-trend ages from sediments in this trench(Fig. 7); their date of 40 6 10 ka (SampleCF-1) is in reasonable agreement with ourresults. We interpret their older ages of 1906 50 ka (Sample CF-6) and 270 6 20 ka(Sample CF-2) to be dating the Early BlackCone horizons.A rock-varnish 14C AMS age of 28.96 0.4

ka was obtained for clasts on the upthrownside of the Late Black Cone surface abovesamples U-3 and U-4 at trench CF-3 (Sam-ple CFP-36; Table 4).

DISCUSSION

Age-Related Soil Features

Based on consistent field relations and pe-don characteristics (Table 3), the followingage-related soil features are good criteriafor identifying and mapping surficial strati-graphic units in Crater Flat: (1) Vesicular

Figure 7. Reinterpretation of Swadley et al. (1984) logs from trenches CF-1 (upper) and CF-3 (lower). Uranium-series ages are (Ta-ble 5B) U-1 (39.4 6 3.0 ka), U-2 (82.4 6 9.4 ka), U-3 (17.1 6 2.5 ka), U-4 (38.2 6 2.9 ka), U-5 (36.2 6 1.7 ka), and U-6 (59.5 6 0.9 ka).Sample TSV-387 is 33 6 4 ka (Szabo and O’Malley, 1985). Uranium-trend ages are CF-1 (40 6 10 ka), CF-2 (270 6 20 ka), and CF-6(1906 50 ka) (Rosholt et al., 1985). A rock-varnish 14C AMS age (CFP-36) of 28.96 0.4 ka was obtained for clasts on the surface overlyingsamples U-3 and U-4. Qflb 5 Late Black Cone, Qfeb 5 Early Black Cone, and Qfs 5 Solitario.

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391Geological Society of America Bulletin, April 1995

surface horizons (Av) form in coarse, sand-textured alluvium by dust fall on the surfaceand infiltration of very fine sand, silt, andclay which lends coherence to the original,loose sand and creates a material in whichbubble-like, vesicular pores can form by wet-ting and drying. Minimal, sandy, loam-tex-tured Av horizons have formed in coarse-textured alluvium only since early Holoceneor latest Pleistocene time. Prominent, loam-textured Av horizons occur in soils of atleast late Pleistocene age. In soils older thanlate Pleistocene, Av horizon morphology isnot an adequate criterion for establishingyet greater age, because the morphologydoes not change and erosion might preventprogressive thickening of the Av horizon onmany sites. (2) There has been shallow, lim-ited accumulation of eolian calcium carbon-ate and introduction of significant amountsof eolian sodium salts in all soils since earlyHolocene. Thus, older soil horizons that hadbeen leached under moister pluvial condi-tions have been recarbonated and alkalized.(3) Presence of an organized desert pave-ment is indicative of at least early Holoceneage. If darkly varnished, the pavement is noyounger than late Pleistocene age and pos-sibly much older. (4) Calcic horizons andduripans form in mixed limestone-volcanicand purely volcanic alluvium, respectively.Neither horizon occurs in soils younger thanlate Pleistocene age. (5) In volcanic allu-vium, minimally typic duripans are foundonly in the younger of the mid-Pleistocene-age soils, whereas fully typic duripans arefound in the older of the mid-Pleistocenesoils. (6) Obvious opalization of an argillichorizon occurs only in mid-Pleistocene-agesoils, but it may occur in soils of widely dif-fering ages, and it does not occur consist-ently in soils of equivalent age. Opalizationis an indicator of considerable age, butwhether or not it occurs probably dependson the local soil-forming environment.

Comparison of Allostratigraphic Unitswith Previous Work

When comparing the results of our allo-stratigraphic study with previous work, wenoted several important differences. OurCrater Flat unit generally is included withinthe Q1 delineations mapped by Swadley andParrish (1988). Three subdivisions of theCrater Flat unit that might be made basedon microtopography are (1) raw, activechannels; (2) raw, rough, bar-and-swaleswith little smoothing; and (3) inset fans withrounded or muted bar-and-swales and

proto-desert pavement. The last subdivisionwould correspond to unit Q1c of Hoover etal. (1981), estimated to be 7–9 ka (Ta-ble 2)—that is, about the age of the LittleCones unit.No unit correlative to the Little Cones

unit was included in the Hoover et al. (1981)chronology or mapped by Swadley and Par-rish (1988). Apparently, it is containedwithin their unit Q1c, but that was describedas having a substantially less-developed soil(Table 1). Little Cones soils have minimalbut distinct desert pavements and sandyloam Av horizons—characteristics not pre-viously described.In comparison with the Swadley and Par-

rish (1988) map, the Black Cone unit mostclosely corresponds to their unit Q2bc (un-differentiated Q2b and Q2c), but the Q2band Q2c soil characteristics and numericalages are different from those for the BlackCone unit (Tables 1 and 2). Unit Q2b wasmapped only as minor strath terraces, withsoils having Bw and Stage I Bk horizons.Unit Q2c soils have morphologies compa-rable to the Early Black Cone subunit (Ta-bles 1 and 3), but unit Q2c was inferred byHoover et al. (1981) to contain Bishop ash(ca. 730 ka). We found Bishop ash in theolder Solitario unit. The Late Black Conesubunit probably corresponds most closelyto unit Q2a of Hoover et al. (1981) (Ta-ble 2), which was not mapped by Swadleyand Parrish (1988) in Crater Flat. The Q2aunit, however, lacks the well-developed Bthorizon and haplic duripan of the LateBlack Cone subunit.Both the Yucca and Solitario units are in-

cluded, for the most part, in the Swadley andParrish (1988) QTa delineations (ca. 1.1–2.0Ma; Table 1). Our mapping and the occur-rence of Bishop ash in the Solitario unit in-dicate, however, that QTa is younger thanthis.

Regional Correlation and Climatic-Alluvial Implications

Our Quaternary stratigraphy for CraterFlat correlates well with similar sequencesdescribed for the southern Nevada, lowerColorado River, Death Valley, and MojaveDesert regions (Table 6). Though more lim-ited in areal extent, our units are additionalevidence for regional climatic control of al-luvial deposition in these dry landscapes.Bull (1991) proposed that regional fan allu-viation occurred during the mid- to lateWis-consin (10–75 ka) and during the transitionfrom the late Wisconsin maximum (ca. 18ka) to the arid Holocene.The Late Black Cone subunit is similar in

stratigraphic position and soil morphologyto unit Q2c (12–70 ka) of Bull (1991) in thelower Colorado River region; both unitshave the youngest Bt (argillic) and Bk (stageII–II) horizons, generally indicative of soilformation under more moist conditions ofthe late Wisconsin period (Nettleton et al.,1975). Estimated minimum ages for theLate Black Cone subunit are also consistentwith estimated ages of other regional units,many of which are similarly dated by mini-mum-limiting techniques. The Late BlackCone unit also is time correlative with the13–50 ka unit Q3 of Dorn (1988b) in south-ern Death Valley and with the ,34 to .45ka unit Qf4 of Wells et al. (1990a) in theeast-central Mojave Desert; it may also becorrelative with the 75 ka unit of Sowers etal. (1989) in the Kyle Canyon, Nevada, area.In nearby Las Vegas and Indian SpringsValleys, Haynes (1967), Quade (1983,1986), and Quade and Pratt (1989) de-scribed a well-dated late Wisconsin unit Dthat was deposited under paludal-alluvialconditions between 15 and 30 ka, an ageclose to that estimated for the Late BlackCone subunit.

TABLE 6. COMPARISON OF CRATER FLAT ALLUVIAL CHRONOLOGY WITH OTHER CHRONOLOGIESIN THE REGION

Crater Flat (this study) LowerColorado River(Bull, 1991)

Las Vegas and IndianSprings Valleys

(Quade, 1986; Quadeand Pratt, 1989)

S. Death Valley(Dorn, 1988b; Hookeand Dorn, 1992)

East-central Mojave(Wells et al., 1990a)

Modern (0) Q4b (0) Modern (0) Modern (0) Modern (Qf9) (0)Crater Flat (.0.3 to .1.3) Q4a (0.1–2) Unit G (0–4.0) Q4c (0.5–2.5) Qf8 (,0.3 to .0.7)

Q3c (2–4) Unit F (4.0–8.0) Q4b (2.0–4.5) Qf6,7 (2–8)Q3b (4–8)

Little Cones (.6 to .11) Q3a (8–12) Unit E (8.6–14.0) Q4a(6–11) Qf5 (8–15)Late Black Cone (.17 to .30) Q2c (12–70) Unit D (15–30) Q3 (13–50) Qf4 (,34 to .45)Early Black Cone (.159 to .201) Q2b (70–200) Unit C (.30) Q2a, Q2b (110–190) Qf3 (.47 to .130)

Unit B (.40 to .60) Qf2 (,160 to .320)Yucca (.375) Q2a (400–730) Unit A Qla, Qlb (.500 to .800)Solitario (.433 to .659 but ,730) Q1 (,1200) Unit A Q1a, Q1b (.500 to .800) Qf1 (,3800)

Note: Listed ages (in ka) are in parentheses.

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392 Geological Society of America Bulletin, April 1995

If the rock-varnish 14C AMS ages reason-ably approximate true numerical ages, as webelieve they do, the Late Black Cone sub-unit was largely deposited during the lateWisconsin (.10 and,35 ka). This late Wis-consin alluviation may have been related topluvial conditions, or it may have been re-lated to a pulse of warming during otherwiseaverage glacial-pluvial conditions (Bull,1991). Paleoclimatic evidence from packratmiddens in the Nevada Test Site region(Spaulding, 1985) indicates that average an-nual precipitation during the 10–38 ka pe-riod was between 10% and 40% greater thanat present. Full-glacial conditions existed inthe southern Nevada region at ca. 18 ka,gradually declining during the next fewthousand years (Spaulding, 1985; Claassen,1986). Although the estimated age of theLate Black Cone unit best fits a late Wis-consin age, the radiometric ages are never-theless minima, and we cannot completelyexclude the possibility that this unit is some-what older. Some other regionally correla-tive units may also be of mid-Wisconsin age,and Bull (1991) suggested that alluviationalso occurred during the mid-Wisconsin in-terstadial (ca. 30–60 ka).The Little Cones unit apparently corre-

lates with unit Q3a (8–12 ka) of the lowerColorado River region (Bull, 1991). Its Bwhorizon (and lack of a Bt) and stage I Bksuggest an early to mid-Holocene age (Gile,1975; Nettleton et al., 1975). The LittleCones unit is time correlative with unit E ofHaynes (1967), Quade (1983, 1986), andQuade and Pratt (1989) in Las Vegas andIndian Springs Valleys. Numerous 14C agesranging between ca. 8 and 14 ka were re-ported by Quade (1986) from the fine-grained, valley-bottom facies of unit E,which grades upslope into extensive alluvialfan complexes (Quade, 1983). Although unitE is not tied clearly to climatic control(Quade and Pratt, 1989), its deposition isclose to the Pleistocene-Holocene temper-ature-precipitation transition (Spaulding,1985; Claassen, 1986). Early Holocene allu-viation may have been associated with mon-soonal conditions during the transition fromfull-pluvial to interpluvial climate (Bull, 1991).Whitney and Harrington (1993) used var-

nish cation ratios to estimate a mid-Pleisto-cene glacial age (170 ka) for some colluvialboulder deposits on the eastern flank ofYucca Mountain and to infer paleoclimaticconditions. This age is close to the minimumwe estimate for the Early Black Cone unit,but the cation ratios are different. Theircation ratio of 4.52 6 0.55 is greater (i.e.,

younger) than those we measured on EarlyBlack Cone varnishes (Table 4) and wouldcorrespond to a calculated CR age of ca. 80ka based on the Crater Flat cation-leachingcurve (Fig. 4). If correct, this younger agemay indicate that the colluvial boulder de-posits formed during the early Wisconsin.Whitney and Harrington (1993) also con-

cluded that the late Wisconsin was a time ofsediment storage on Yucca Mountain hill-slopes, with relatively little sediment beingdeposited on the valley floors. Our results,however, suggest that the late Wisconsinwas a time of widespread piedmont and val-ley-floor fan alluviation. Based on the arealdistribution of the Late Black Cone and Lit-tle Cones remnants (Fig. 1), we infer thatmost of Crater Flat was alluviated duringthe late Wisconsin. The principal drainagesoriginating on the western flank of YuccaMountain (e.g., Solitario Canyon) weresources of debris- and flood-flow depositsthat covered most of the preexisting alluviallandscape. Although these deposits may bethin, they are areally extensive and thereforecomprise a significant volume. This alluvialdebris was derived from deposits storedwithin the canyons and on the slopes ofYucca Mountain, which may account for theabsence of slope deposits of this age notedby Whitney and Harrington (1993).Harden et al. (1991) compared four late

Quaternary soil chronosequences from thesouthern Great Basin, including a set of soilsfrom Fortymile Wash near Yucca Moun-tain. They calibrated soil ages and develop-ment rates in Fortymile Wash based on theHoover et al. (1981) stratigraphy and on theuranium-trend ages of Rosholt et al. (1985).Comparing the soil-development rate inFortymile Wash with that at Silver Lake,California, they concluded that Pleistocenesoils in Fortymile Wash developed at a ratelower by as much as a factor of 10 than theHolocene soils at Silver Lake. Our resultssuggest that soil ages in Crater Flat areyounger by factors of 2–10 than the calibra-tion ages used by Harden et al. (1991) atFortymile Wash, and that soil-developmentrates are in closer agreement with the SilverLake rates.

CONCLUSIONS AND IMPLICATIONSFOR FUTURE STUDIES

Six major allostratigraphic units were iden-tified and mapped in Crater Flat using geo-morphic surfaces and soils. Rock-varnish14C AMS and cation-ratio dating togetherwith uranium-series dating provide minimum

age estimates in good agreement with relativestratigraphic relations and soil morphology.Although 14C AMS varnish ages are minima,the relative consistency in ages and cation ra-tios from equivalent allostratigraphic surfacestogether with varnish alkalinity ratios, urani-um-series dates on pedogenic carbonate, andregional correlations suggest that these mini-mum ages do not significantly underestimatetrue ages.Our three youngest units—the Crater

Flat, Little Cones, and Late Black Coneunits—correlate well by stratigraphic order,soils, and estimated numerical age withother regionally recognized, alluvial-strati-graphic sequences, but the latter two werenot included in earlier studies of the NevadaTest Site region. The Little Cones and theLate Black Cone units collectively form anextensive late Quaternary stratigraphic se-quence, which was not recognized in previ-ous mapping of Crater Flat. Although the‘‘surficial deposits’’ stratigraphy has unitsnumerically equivalent in age (Q1c, 7–9 ka,and Q2a, ca. 40 ka; Table 1), soil featuresdescribed in this previous work do not fitthose we found in deposits of these ages.Q1c soils were described as only minimallydeveloped, whereas the Av, Bw, and Bk ho-rizons of the Little Cones unit are readilyvisible and differentiated. Q2a soils weresaid to have only Bw and stage I Bk hori-zons, morphologies characteristic of a Hol-ocene age (Gile, 1975), whereas the LateBlack Cone soils display distinct Bt andstage II–II Bk horizons typical of pre-Hol-ocene soils (Nettleton et al., 1975).We conclude that the Late Black Cone

unit best fits a late Wisconsin age and is con-tained within map units Q2b and Q2bc ofSwadley and Parrish (1988) based on simi-larities in map delineations and surficialcharacteristics such as desert pavement.However, previously reported pre-Wiscon-sin uranium-trend ages of 160–250 ka andpost-Wisconsin–type soil (Bw) morpholo-gies for Q2b deposits (Table 1) are incon-sistent with soils and ages found for the LateBlack Cone subunit.Stratigraphic position, soil development,

and estimated rock-varnish cation-ratio age(.159 to .201 ka) indicate that the EarlyBlack Cone unit is at least oxygen-isotopestage 5 age ($125 ka), but the true age isuncertain. It is likely correlative with Bull’s(1991) unit Q2b (70–200 ka) for which aclimatic origin was inferred. The correlative‘‘surficial deposits’’ unit is Q2c (Table 2),which was estimated to range in age from270 to 800 ka. From map delineations and

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393Geological Society of America Bulletin, April 1995

soil descriptions, we infer that most parts ofQ2c mapped by Swadley and Parrish (1988)include the Early Black Cone unit. We con-clude that their correlation of the lower partof unit Q2c with Bishop-age and older de-posits is in error. Rather, the Bishop ash oc-curs in the Solitario unit, which is strati-graphically older and likely includes theQTa unit of Hoover et al. (1981).We believethat the uranium-trend ages of 300–500 kareported by Rosholt et al. (1985) for Q2cdeposits are too old for the stage III–V Bkand haplic duripan horizons we found inEarly Black Cone soils, morphologies thatare more indicative of a younger Pleistoceneage (Gile et al., 1966; Bull, 1991).As elsewhere in the now arid Basin and

Range Province, Crater Flat was likely allu-viated during the late Wisconsin and duringthe transition from a full glacial to the aridHolocene climate. In contrast to previouswork indicating that alluvial inactivity char-acterized the late Wisconsin at Yucca Moun-tain (Whitney and Harrington, 1993), weconclude that much of the preexisting Cra-ter Flat alluvial landscape was covered bydeposits originating in the drainages on thewest flank of Yucca Mountain, suggestingthat debris- and flood-flow processes werequite active during full and late glacial time.The extent of late Pleistocene and early

Holocene stratigraphy found in this studymay be significant for assessment of seismichazards at the proposed Yucca Mountainhigh-level nuclear waste facility. Based onour allostratigraphy, several faults in CraterFlat displace Late Black Cone and youngerdeposits, indicating that tectonic activity hasbeen more recent than previously estimated.In comparison with previous work, our al-lostratigraphic units are younger by factorsof 2–10. Such age differences can result inseismic slip-rate calculations that signifi-cantly underestimate seismic hazard. Iffaulted deposits inferred to be of Q2c age(270–800 ka) are offset 1 m, an average sliprate of 0.001–0.004 mm/yr would be calcu-lated, a rate that is indicative of relativelylow tectonic activity (cf. Slemmons and de-Polo, 1986). In contrast, if the offset unitwere actually of Late Black Cone or LittleCones age (ca. 10–30 ka) an average sliprate of ;0.03–0.1 mm/yr would be calcu-lated, a rate indicative of a more active tec-tonic setting.

ACKNOWLEDGMENTS

The authors thank J. Clark, C. dePolo,C. Purcell, and T. Sawyer for their assistance

in field investigations. We are grateful toShangde Luo for assistance in the U-seriesdating.We also thank J. Knox, D.Muhs, andan anonymous reviewer for helpful reviewcomments. S. Wells reviewed an early ver-sion of the manuscript. This study was sup-ported by funding from the Nevada NuclearWaste Project Office to the Center for Neo-tectonic Studies, University of Nevada,Reno.

APPENDIX 1. PEDON DESCRIPTIONS3

APPENDIX 2. CHEMICAL ANALYSES OFROCK VARNISH3

APPENDIX 3. U-SERIES ISOCHRON PLOTS3

REFERENCES CITED

Bell, J. W., Ramelli, A. R., dePolo, C. M., Bonham, H. F., 1989,Task 1, Quaternary tectonics, progress report for the period1 July 1988 to 30 September 1989, in Evaluation of the ge-ologic relations and seismotectonic stability of the YuccaMountain area, Nevada, Nuclear Waste Site Investigations(NWSI): Reno, University of Nevada, Center for Neotec-tonic Studies, 1989 Progress Report, 38 p.

Benson, L. V., Currey, D. R., Lao, Y., and Hostetler, S., 1992,Lake-size variations in the Lahontan and Bonneville basinsbetween 13,000 and 9,000 14C yr B.P.: Palaeogeography,Palaeoclimatology, Palaeoecology, v. 94, p. 19–32.

Bull, W. B., 1991, Geomorphic responses to climatic change: NewYork, Oxford University Press, 326 p.

Claassen, H., 1986, Late-Wisconsin paleohydrology of the west-central Amargosa Desert, Nevada, U.S.A.: Chemical Geol-ogy (Isotope Geosciences Section), v. 58, p. 311–323.

Dorn, R. I., 1983, Cation-ratio dating: A new rock varnish age-determination technique: Quaternary Research, v. 20,p. 49–73.

Dorn, R. I., 1988a, A critical evaluation of cation-ratio dating ofrock varnish, and an evaluation of its application to theYucca Mountain repository by the Department of Energyand its subcontractors, in Evaluation of the geologic rela-tions and seismotectonic stability of the Yucca Mountainarea, Nevada Nuclear Waste Site Investigation (NNWSI),Appendix A: Reno, University of Nevada, Center for Neo-tectonic Studies, 73 p.

Dorn, R. I., 1988b, A rock varnish interpretation of alluvial-fandevelopment in Death Valley, California: National Geo-graphic Research, v. 4, p. 56–73.

Dorn, R. I., 1989, Cation-ratio dating of rock varnish: A geograph-ical perspective: Progress in Physical Geography, v. 13,p. 559–596.

Dorn, R. I., 1990, Quaternary alkalinity fluctuations recorded inrock varnish microlaminations on western U.S.A. volcanics:Palaeogeography, Palaeoclimatology, Palaeoecology, v. 76,p. 291–310.

Dorn, R. I., 1992, Comment on accuracy of rock-varnish chemi-cal analyses: Implications for cation-ratio dating: Geology,v. 20, p. 470–471.

Dorn, R. I., in press, Dating rock varnish, in Beck, C., ed., Datingin surface context: Albuquerque, University of New MexicoPress.

Dorn, R. I., and DeNiro, M. J., 1985, Stable carbon isotope ratiosof rock varnish organic matter: A new paleoenvironmentalindicator: Science, v. 227, p. 1472–1474.

Dorn, R. I., and Krinsley, D. H., 1991, Cation-leaching sites in rockvarnish: Geology, v. 19, p. 1077–1080.

Dorn, R. I., Jull, A. J. T., Donahue, D. J., Linick, T. W., andToolin, L. J., 1989, Accelerator mass spectrometry radio-carbon dating of rock varnish: Geological Society of Amer-ica Bulletin, v. 101, p. 1363–1372.

Dorn, R. I., Cahill, T. A., Eldred, R. A., Gill, T. E., Kusko, B. H.,Bach, A. J., and Elliot-Fisk, D. L., 1990, Dating rock var-nishes by the cation-ratio method with PIXE, ICP, and theelectron microprobe: International Journal of PIXE, v. 1,p. 157–195.

Dorn, R. I., and eight others, 1992, Rock varnish on Hualalai andMauna Kea volcanoes, Hawaii: Pacific Science, v. 46,p. 11–34.

Dragovich, D., 1993, Distribution and chemical composition of

microcolonial fungi and rock coatings from arid Australia:Physical Geography, v. 14, p. 323–341.

Eckert, R. E., Jr., Wood, M. K., Blackburn, W. H., Peterson, F. F.,Stephens, J. L., and Meurisse, M. L., 1978, Effects of surfacesoil morphology on improvement and management of somearid and semi-arid rangelands, in Proceedings, InternationalRangeland Congress, 1st, Denver: Denver, Colorado,American Society of Range Management, p. 299–302.

Faulds, J., Bell, J. W., Feuerbach, D., and Ramelli, A. R., 1994,Geologic map of the Crater Flat area, Nevada: Nevada Bu-reau of Mines and Geology Map 101, 1:24 000.

Flach, K. W., Nettleton, W. D., Gile, L. H., and Cady, J. G., 1969,Pedocementation: Induration by silica, carbonates, andsesquioxides in the Quaternary: Soil Science, v. 107,p. 442–453.

Gile, L. H., 1975, Holocene soils and soil-geomorphic relations inan arid region of southern New Mexico: Quaternary Re-search, v. 5, p. 321–360.

Gile, L. H., Peterson, F. F., and Grossman, R. B., 1966, Morpho-logical and genetic sequences of carbonate accumulation indesert soils: Soil Science, v. 101, p. 347–360.

Harden, J. W., Taylor, E. M., Hill, C., Mark, R. K., McFadden,L. D., Reheis, M. C., Sowers, J. M., and Wells, S. G., 1991,Rates of soil development from four soil chronosequencesin the southern Great Basin: Quaternary Research, v. 35,p. 383–399.

Harrington, C. D., and Whitney, J. W., 1987, Scanning electronmicroscope method for rock-varnish dating: Geology, v. 15,p. 967–970.

Haynes, C. V., 1967, Quaternary geology of the Tule Springs area,Clark County, Nevada, inWormington, H. M., and Ellis, D.,eds., Pleistocene studies in southern Nevada: CarsonCity, Nevada State Museum of Anthropology Paper 13,p. 1–104.

Hooke, R. LeB., and Dorn, R. I., 1992, Segmentation of alluvialfans in Death Valley, California: New insight from surfaceexposure dating and laboratory modelling: Earth SurfaceProcesses and Landforms, v. 17, p. 557–574.

Hoover, D. L., Swadley, W. C., and Gordon, A. J., 1981, Corre-lation characteristics of surficial deposits with a descriptionof surficial stratigraphy in the Nevada Test Site region: U.S.Geological Survey Open-File Report 81-512, 27 p.

Jones, C. E., 1991, Characteristics and origin of rock varnish fromthe hyperarid coastal deserts of northern Peru: QuaternaryResearch, v. 35, p. 116–129.

Krinsley, D. H., Dorn, R. I., and Anderson, S., 1990, Factors thatmay interfere with the dating of rock varnish: Physical Ge-ography, v. 11, p. 97–119.

Ku, T. L., 1976, The uranium-series methods of age determination:Annual Reviews of Earth and Planetary Science, v. 4,p. 347–379.

Ku, T. L., Bull, W. B., Freeman, S. T., and Knauss, K. G., 1979,Th230-U234 dating of pedogenic carbonates in gravelly desertsoils of Vidal Valley, southeastern California: GeologicalSociety of America Bulletin, v. 90, p. 1063–1073.

Luo, S., and Ku, T. L., 1991, U-series isochron dating: A gen-eralized method employing total-sample dissolution:Geochimica et Cosmochimica Acta, v. 55, p. 555–564.

Moore, R. B., and Clague, D. A., 1991, Geologic map of HualalaiVolcano, Hawaii: U.S. Geological Survey Miscellaneous In-vestigation Series Map I-2213, scale 1:50 000.

Muhs, D. R., Rosholt, J. N., and Bush, C. A., 1989, The urani-um-trend dating method: Principles and application forsouthern California marine terrace deposits: QuaternaryInternational, v. 1, p. 19–34.

Nagy, B., Nagy, L. A., Rigali, M. J., Jones, W. D., Krinsley, D. H.,and Sinclair, N., 1991, Rock varnish in the Sonoran Desert:Microbiologically mediated accumulation of manganiferoussediments: Sedimentology, v. 38, p. 1153–1171.

Nettleton, W. D., and Peterson, F. F., 1983, Aridisols, inWilding,L. P., Smeck, N. E., and Hall, G. F., eds., Pedogenesis andsoil taxonomy Volume II: The soil orders: Amsterdam,Netherlands, Elsevier, p. 165–216.

Nettleton, W. D., Witty, J. E., Nelson, R. E., and Hawley, J. W.,1975, Genesis of argillic horizons in soils of desert areas ofthe southwestern United States: Soil Science Society ofAmerica Proceedings, v. 39, p. 919–926.

Nobbs, M. F., and Dorn, R. I., 1993, New surface exposure ages forpetroglyphs from the Olary province, South Australia: Ar-chaeology in Oceania, v. 28, p. 18–39.

North American Commission on Stratigraphic Nomenclature,1983, North American stratigraphic code: AmericanAssociation of Petroleum Geologists Bulletin, v. 67,p. 841–875.

Peterson, F. F., 1981, Landforms of the Basin and Range Provincedefined for soil survey: Reno, University of Nevada, NevadaAgricultural Experiment Station Technical Bulletin 28, 52 p.

Quade, J., 1983, Quaternary geology of the Corn Creek Springsarea, Clark County, Nevada [Master’s thesis]: Tucson, Uni-versity of Arizona, 135 p.

Quade, J., 1986, Late Quaternary environmental changes in theupper Las Vegas Valley, Nevada: Quaternary Research,v. 26, p. 340–357.

Quade, J., and Pratt, W. L., 1989, Late Wisconsin groundwaterdischarge environments of the southwestern Indian SpringsValley, southern Nevada: Quaternary Research, v. 31,p. 351–370.

Ramelli, A. R., Bell, J. W., and dePolo, C. M., 1988, Evidence fordistributive faulting at Yucca Mountain, Nevada: Geologi-cal Society of America Abstracts with Programs, v. 20,p. 383.

3GSA Data Repository item 9502, Appendixes1, 2, and 3, is available on request from Docu-ments Secretary, GSA, P.O. Box 9140, Boulder,CO 80301.

PETERSON ET AL.

394 Geological Society of America Bulletin, April 1995

Ramelli, A. R., Sawyer, T. L., Bell, J. W., Peterson, F. F., Dorn,R. I., and dePolo, C. M., 1989, Preliminary analysis of faultand fracture patterns at Yucca Mountain, southern Nevada,in Proceedings, FOCUS ’89—Nuclear Waste Isolation inthe Unsaturated Zone: Las Vegas, Nevada, American Nu-clear Society, p. 336–343.

Reheis, M. C., 1986, Preliminary study of Quaternary faulting onthe east side of Bare Mountain, Nye County, Nevada: U.S.Geological Survey Open-File Report 86-576, 14 p.

Reneau, S. L., and Raymond, R., Jr., 1991, Cation-ratio dating ofrock varnish: Why does it work?: Geology, v. 19, p. 937–940.

Rosholt, J. N., 1980, Uranium-trend dating of Quaternary sedi-ments: U.S. Geological Survey Open-File Report 80-1087,65 p.

Rosholt, J. N., Bush, C. A., Carr, W. J., Hoover, D. L., Swadley,W. C., and Dooley, J. R., Jr., 1985, Uranium-trend dating ofQuaternary deposits in the Nevada Test Site area, Nevadaand California: U.S. Geological Survey Open-File Re-port 85-540, 72 p.

Slemmons, D. B., and dePolo, C. M., 1986, Evaluation of activefaulting and associated hazards, in Active tectonics: Wash-ington, D.C., National Academy Press, National ResearchCouncil, Panel on Active Tectonics, p. 45–62.

Smith, E. I., Feuerbach, D., Naumann, T., Faulds, J. E., and Cas-cadden, T., 1990, Annual report for the period 10/1/89 to9/30/90: Las Vegas, University of Nevada, Department ofGeoscience, Center for Volcanic and Tectonic Studies,State of Nevada Nuclear Waste Project Office Report 41,94 p.

Soil Survey Staff, 1975, Soil taxonomy: Washington, D.C, U.S.Government Printing Office, U.S. Department of Agricul-ture Handbook 436, 746 p.

Sowers, J. M., and nine others, 1989, Geomorphology and pedol-ogy on the Kyle Canyon alluvial fan, southern Nevada, inRice, T. J., ed., Soils geomorphology relationships in theMojave Desert, California, Nevada: Soil Science Society ofAmerica Field Tour Guidebook, p. 93–112.

Spaulding, W. G., 1985, Vegetation and climates of the last 45,000years in the vicinity of the Nevada Test Site, south-centralNevada: U.S. Geological Survey Professional Paper 1329,83 p.

Swadley, W. C., 1983, Map showing surficial geology of theLathrop Wells quadrangle, Nye County, Nevada: U.S.Geological Survey Miscellaneous Investigations SeriesMap I-1361, scale 1:48 000.

Swadley, W. C., and Carr, W. J., 1987, Geologic map of the Qua-

ternary and Tertiary deposits of the Big Dune quadrangle,Nye County, Nevada, and Inyo County, California: U.S. Ge-ological Survey Map I-1767, scale 1:48 000.

Swadley, W. C., and Hoover, D. L., 1983, Geology of faults ex-posed in trenches in Crater Flat, Nye County, Nevada: U.S.Geological Survey Open-File Report 83-608, 15 p.

Swadley, W. C., and Hoover, D. L., 1989a, Geologic map of thesurficial deposits of the Topopah Spring quadrangle, NyeCounty, Nevada: U.S. Geological Survey Miscellaneous In-vestigations Map I-2018, scale 1:24 000.

Swadley, W. C., and Hoover, D. L., 1989b, Geologic map of thesurficial deposits of the Jackass Flats quadrangle, NyeCounty, Nevada: U.S. Geological Survey Miscellaneous In-vestigations Map I-1994, scale 1:24 000.

Swadley, W. C., and Parrish, L. D., 1988, Surficial geologic map ofthe Bare Mountain quadrangle, Nye County, Nevada: U.S.Geological Survey Map I-1826, scale 1:48 000.

Swadley, W. C., Hoover, D. L., and Rosholt, J. N., 1984, Prelim-inary report on late Cenozoic faulting and stratigraphy inthe vicinity of Yucca Mountain, Nye County, Nevada: U.S.Geological Survey Open-File Report 84-788, 42 p.

Szabo, B. J., and O’Malley, P. A., 1985, Uranium-series dating ofsecondary carbonate and silica precipitates relating to faultmovements in the Nevada Test Site region and of calicheand travertine samples from the Amargosa Desert: U.S. Ge-ological Survey Open-File Report 85-47, 12 p.

Szabo, B. J., Kolesar, P. T., Riggs, A. C., Winograd, I. J., andLudwig, K. R., 1994, Paleoclimatic inferences from a120,000-yr calcite record in Browns Room of Devil’s Hole,Nevada: Quaternary Research, v. 41, p. 59–69.

Taylor, E. M., 1985, Impact of time and climate on Quaternarysoils in the Yucca Mountain area of the Nevada Test Site[Master’s thesis]: Boulder, University of Colorado, 217 p.

Turrin, B. D., Champion, D., and Fleck, R. J., 1991,40Ar/39Ar ageof the Lathrop Wells volcanic center, Yucca Mountain, Ne-vada: Science, v. 253, p. 654–657.

U.S. Department of Energy, 1988, Site characterization plan,Yucca Mountain Site, Nevada Research and DevelopmentArea, Nevada: Washington, D.C., U.S. Department of En-ergy Office of Civilian Radioactive Waste Management,Volumes I–VIII, no consecutive pagination.

Watchman, A., 1992, Investigating the cation-ratio calibrationcurve: Evidence from South Australia: Rock Art Research,v. 9, p. 106–110.

Wells, S. G., McFadden, L. D., and Harden, J., 1990a, Preliminaryresults of age estimations and regional correlations of Qua-

ternary alluvial fans within the Mojave Desert of southernCalifornia, inReynolds, R. E., Wells, S. G., and Brady, R. J.,eds., At the end of the Mojave: Quaternary studies in theeastern Mojave Desert: Redlands, California, San Bernar-dino County Museum Association, p. 45–53.

Wells, S. G., McFadden, L. D., Renault, C. E., and Crowe, B. M.,1990b, Geomorphic assessment of late Quaternary volcan-ism in the Yucca Mountain area, southern Nevada: Impli-cations for the proposed high-level radioactive waste repos-itory: Geology, v. 18, p. 549–553.

Wells, S. G., Crowe, B. M., andMcFadden, L. D., 1992, Measuringthe age of the Lathrop Wells volcanic center at YuccaMountain: Technical comment: Science, v. 257, p. 555–556.

Whitley, D. S., and Dorn, R. I., 1993, New perspectives on theClovis versus pre-Clovis controversy: American Antiquity,v. 58, p. 626–647.

Whitney, J. W., and Harrington, C. D., 1993, Relict colluvial boul-der deposits as paleoclimatic indicators in the Yucca Moun-tain region, southern Nevada: Geological Society of Amer-ica Bulletin, v. 105, p. 1008–1018.

Whitney, J. W., Shroba, R. R., Simonds, F. W., and Harding, S. T.,1986, Recurrent Quaternary movement on the Windy Washfault, Nye County, Nevada: Geological Society of AmericaAbstracts with Programs, v. 18, p. 787.

Yount, J. C., Bell, J. W., dePolo, C. M., Ramelli, A. R., Cashman,P. H., and Glancy, P. A., 1993, Neotectonics of the WalkerLane, Pyramid Lake to Tonopah—Part II, road log, inLahren, M. M., Trexler, J. H., Jr., and Spinosa, C., eds.,Crustal evolution of the Great Basin and the Sierra Nevada:Reno, Nevada, Cordilleran/Rocky Mountain Sections of theGeological Society of America, 1993 Field Trip Guidebook,p. 391–408.

Zreda, M. G., Phillips, F. M., Kubik, P. W., Sharma, P., and El-more, D., 1993, Cosmogenic 36Cl dating of a young basalticeruption complex, Lathrop Wells, Nevada: Geology, v. 21,p. 57–60.

MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 19, 1993REVISED MANUSCRIPT RECEIVED APRIL 29, 1994MANUSCRIPT ACCEPTED MAY 6, 1994

LATE QUATERNARY GEOMORPHOLOGY AND SOILS, SOUTHERN NEVADA

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