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

Abstract

Soil-geomorphic studies indicate that six major allostratigraphic units occur in Crater Flat, Nevada, adjacent to Yucca Mountain. These units are, from youngest to oldest, Crater Flat, Little Cones, Late Black Cone, Early Black Cone, Yucca, and Solitario. Presence and degree of differentiation of Av, Ak, Bw, Bt, Btk, Btkq, and Bqkm genetic soil horizons characterize units, confirm relative ages, and aid in estimating numerical ages. Stratigraphic order and soils allow correlation with similar alluvial sequences in adjacent Basin and Range areas. Minimum-limiting ages-by 14 C accelerator mass spectrometry (AMS) and cation-ratio dating of rock varnish and by 230 Th/ 234 U dating of pedogenic carbonate-support allostratigraphic order and are in reasonable agreement with numerical ages estimated for correlative regional units. Consistent, clustered, 14 C AMS varnish ages from widely separated, same-age surfaces suggest that the ages, although minima, do not significantly underestimate true ages. Rock-varnish 14 C AMS ages on Late Black Cone and younger units, and K-Ar ages from volcanic lava cones, provide calibration points for a Crater Flat cation-leaching curve. This curve differs somewhat from a previous Yucca Mountain curve and yields calculated cation-ratio ages younger by factors of two to three for the younger units. If the 14 C AMS varnish ages provide reasonably close minimum ages, as we believe they do, the Little Cones and Late Black Cone units collectively form an extensive late Wisconsin-early Holocene deposit not previously described in Crater Flat. The Late Black Cone unit (>17 to >30 ka) correlates with units in the Lower Colorado River, Death Valley, Mojave Desert, and Las Vegas areas-all likely products of climatically induced, late Wisconsin pluvial alluviation. Similarly, the Little Cones unit (>6 to >11 ka) correlates with regional units thought related to alluviation during climatic transition from the late Wisconsin maximum pluvial to the arid Holocene. The areal distribution of late Pleistocene units demonstrates that the Crater Flat piedmont and valley floor were extensively alluviated during the last glacial episode. Ages of three older, mid-Quaternary units are uncertain, but they are largely younger than Bishop ash (730 ka). The Early Black Cone and Yucca units are estimated from rock-varnish cation-ratio dating to be from >159 to >201 ka and >375 ka, 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 ''surficial deposits'' stratigraphy used in the Yucca Mountain area. Although the earlier stratigraphy has some units numerically equivalent in age to our allostratigraphic units, we found soil features in deposits of these ages different from those previously described.

Understanding the Spatial Variability of Environmental Change in Drylands with Rock Varnish Microlaminations

“…The key to sample survival in the final thinning is in binding epoxy to epoxy, rather than binding dissimilar materials (epoxy on glass)-an approach used in prior studies (Dorn 1992;Peterson et al 1995). Unlike prior sections, which frequently were too thick to see the subdivi- segment sions within the Mn-rich bands, the new method produced sections that were thin enough to see all layers-and preparation rates were faster and failures rates (6 percent) were lower.…”

Low‐frequency radar sounding investigations of the North Amargosa Desert, Nevada: A potential analog of conductive subsurface environments on Mars

“…In what follows we provide a brief description of the two investigated sites; a more detailed geological description is given by Swadley and Carr [1987] for the Big Dune site and Peterson et al [1995] and Faulds et al [1994] for the Little Cones at the Crater Flat area.…”

“…Our particular test area was selected because its geologic and hydrologic properties have been studied in detail as part of a regional technical evaluation of Yucca Mountain, Nevada, related to its potential for serving as a geologic repository for high‐level radioactive waste and spent nuclear fuel. The area is well characterized both geologically [ Peterson et al , 1995; Faulds et al , 1994; Swadley and Carr , 1987; Carr and Parrish , 1985; Carr , 1982] and geophysically [ Farrell and Sandberg , 2003; La Femina et al , 2002; Langenheim , 2000; Brocher et al , 1998] and hence offers a unique opportunity to understand loss mechanisms that are due to subsurface geological and geophysical complexity.…”

Microchemistry of small desert varnish samples, western New South Wales, Australia

“…Varnish is composed mainly of silicate clays (Potter and Rossman, 1977) and iron and manganese oxides and hydroxides. Some of the minor and trace elements present within varnishincluding potassium, calcium, titanium, phosphorus, sulphur, magnesium, sodium, copper and nickel (Lakin et al, 1963;Duerden et al, 1986) -have been examined for their potential to provide relative and absolute dating in both archaeological (Loendorf, 1991) and geomorphological (Dorn et al, 1986;Dorn, 1988;Peterson et al, 1995) contexts. By using a combination of chemical and morphological characteristics of the material, varnish has also been investigated as a palaeoclimatic indicator (Dorn, 1994).…”

“…Misleading cation ratio results can arise from diverse sources, many of which have been identified (Krinsley et al, 1990). The absolute ages of varnishes have also been investigated by radiocarbon dating of organics trapped within or beneath the varnish (Dorn et al, 1986;Peterson et al, 1995) and these ages have been linked to cation ratios. The interpretation of varnish radiocarbon ages raises different questions which will not be discussed here.…”