LATE QUATERNARY GEOMORPHOLOGY AND SOILS IN CRATER-FLAT, YUCCA MOUNTAIN AREA, SOUTHERN NEVADA

Soil-geomorphic studies indicate that six major allostratigraphic unit s occur in Crater Flat, Nevada, adjacent to Yucca Mountain. These unit s are, from youngest to oldest, Crater Flat, Little Cones, Late Black Cone, Early Black Cone, Yucca, and Solitario. Presence and degree of d ifferentiation of Av, Ak, Bw, Bt, Btk, Btkq, and Bqkm genetic soil hor izons 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-14 accelerator mass spectrometry (AMS) and cation-r atio dating of rock varnish and by Th-230/U-234 dating of pedogenic ca rbonate-support allostratigraphic order and are in reasonable agreemen t with numerical ages estimated for correlative regional units. Consis tent, clustered, C-14 AMS varnish ages from widely separated, same-age surfaces suggest that the ages, although minima, do not significantly underestimate true ages. Rock-varnish C-14 AMS ages on Late Black Con e and younger units, and K-Ar ages from volcanic lava cones, provide c alibration points for a Crater Flat cation-leaching curve. This curve differs somewhat from a previous Yucca Mountain curve and yields calcu lated cation-ratio ages younger by factors of two to three for the you nger units. If the C-14 AMS varnish ages provide reasonably close mini mum ages, as we believe they do, the Little Cones and Late Black Cone units collectively form an extensive late Wisconsin-early Holocene dep osit not previously described in Crater Flat. The Late Black Cone unit (>17 to >30 ka) correlates with units in the Lower Colorado River, De ath Valley, Mojave Desert, and Las Vegas areas-all likely products of climatically induced, late Wisconsin pluvial alluviation. Similarly, t he Little Cones unit (>6 to >11 ka) correlates with regional units tho ught related to alluviation during climatic transition from the late W isconsin maximum pluvial to the arid Holocene. The areal distribution of late Pleistocene units demonstrates that the Crater Flat piedmont a nd valley floor were extensively alluviated during the last glacial ep isode. Ages of three older, mid-Quaternary units are uncertain, but th ey 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 Mountai n area. Although the earlier stratigraphy has some units numerically e quivalent in age to our allostratigraphic units, we found soil feature s in deposits of these ages different from those previously described.