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 14C accelerator mass spectrometry (AMS) and cation-ratio dating of rock varnish and by 230Th/234U dating of pedogenic carbonate—support allostratigraphic order and are in reasonable agreement with numerical ages estimated for correlative regional units. Consistent, clustered, 14C AMS varnish ages from widely separated, same-age surfaces suggest that the ages, although minima, do not significantly underestimate true ages. Rock-varnish 14C 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 14C 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.

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