Paper No. 3
Presentation Time: 8:30 AM


WHITNEY, John W., United States Geological Survey, Denver Federal Center, MS-980, Denver, CO 80225, PERRY, Frank, Earth and Environmental Sciences Division, Los Alamos National Laboratory, EES-16, MS D452, Los Alamos, NM 87545, ROOD, Dylan H., AMS Laboratory, Scottish Universities Environmental Research Centre (SUERC), East Kilbride, G75 0QF, United Kingdom, SPENGLER, Richard, U.S. Geol Survey, Denver Federal Center, Denver, CO 80225, PACES, James B., U.S. Geological Survey, MS 963, Box 25046, Denver Federal Center, Denver, CO 80225-0046, FINKEL, Robert C., Department of Earth and Planetary Science, University of California, Berkeley, 371 McCone Hall, Berkeley, CA 94720, HARRINGTON, Charles D., P.O. Box 32586, Sana Fe, NM 87594, BUCKINGHAM, Susan E., 2749 South Kearney St, Denver, CO 80222 and HANKS, Thomas C., United States Geological Survey, Menlo Park, CA 94025,

The proposed high-level nuclear waste repository described in the 2008 DOE license application is located 200-250 m below the surface of Yucca Mountain (YM), a tilted fault block composed of 12-Ma densely welded tuff. On the basis of a stream-power erosion model Stüwe and others (2009) suggested that the repository horizon would be unroofed in 0.5-5.0 million years (m.y.). The model, which does not include local geomorphic or basin history, makes untenable assumptions that weathered debris is removed from the Amargosa Valley-Fortymile Wash closed basin and that baselevels lower through time. More than 40 boreholes logged adjacent to Yucca Mountain record more than 11 m.y. of basin-fill depositional history. In 8 boreholes well-dated Pliocene and late Miocene basalts, interstratified with basin fill alluvium, indicate long-term aggradation rates in adjacent Crater Flat basin of 35 m/m.y. and 41 m/m.y. to the east and south of YM. Scores of boreholes in the southern Amargosa Valley do not contain detritus from YM volcanic rocks; therefore, nearly all debris shed off YM is located within 30 km of the mountain. The general baselevel has risen, not declined, about 400 m since erosion began in the late middle Miocene. Recent cosmogenic dating by Cl-36, Ne-21, and Be-10 indicate a vertical rock-weathering rate of about 2 m/m.y. on bedrock surfaces. Horizontal erosion rate of cliffs and uppermost hillslopes is about 10 m/m.y. Dated segments of the main YM cliff have been preserved for more than 250 k.y. Small volumes of debris mantle YM hillslopes and U-series-dated soils on older colluvium are commonly of middle Pleistocene age and as old as ~1 Ma. Because long-term erosion and aggradation rates integrate multiple climate cycles we believe that these long-term rates are good predictors of future rates of similar geomorphic processes. Based on our data, during the next 5 m.y. the highest point on Yucca Mountain crest may become about 10 m lower vertically, upper valley walls may recede 50-100 m horizontally, and continued aggradation will likely raise the baselevel on three sides of the mountain by 150-200 m. Yucca Mountain is downwasting very slowly, continues to be buried by its own waste, and will continue to do so as long as the present closed basin exists.