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

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