Late Quaternary geomorphology and soils in Crater Flat,Yucca Mountain area, southern NevadaFrederick 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 90089ABSTRACTSoil-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—by14C acceleratormass spectrometry (AMS) and cation-ratiodating of rock varnish and by230Th/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-varnish14CAMS ages on Late Black Cone and youngerunits, and K-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 the14C 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 notpreviously 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-Quaternaryunits 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 ageby 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.INTRODUCTIONCrater Flat is an alluvium-filled structuralbasin on the west side of Yucca Mountain,Nevada—the site under consideration for ahigh-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 faultsoffset 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.379Data 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 recognitionof 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