CURRENT UNDERSTANDING OF THE RAVEN CANYON FAULT AT YUCCA MOUNTAIN, NEVADA

Several lines of evidence, including geophysical data, borehole data, and surface geologic data, are interpreted to indicate an inactive, large, down-to-the-west, buried fault beneath Yucca Mountain, site of a proposed nuclear waste repository. This previously postulated fault, herein referred to as the Raven Canyon fault, may affect current understanding of ground-water flow in the Yucca Mountain area. Gravity data indicate a steep gradient trending from west of the Lathrop Wells Cone northeastward towards the Calico Hills, separating thicker, less dense Tertiary volcanic rocks to the west from more dense Paleozoic rocks to the east. Previous interpretations of borehole data suggest that a tectonic feature was encountered in boreholes at Yucca Mountain that may be related to the Raven Canyon fault. New borehole data in the interpreted hanging wall of the Raven Canyon fault show that the ground-water level is lower than previously expected, indicating that the buried fault may affect ground-water flow in the southern part of Yucca Mountain. Interpretations along geologic cross sections indicate that strata of the Miocene Crater Flat Group thicken in the hanging wall of the buried fault, and that a fault-controlled basin existed there at 13.25 Ma. However, because the thickness of the overlying Miocene Paintbrush Group tuffs remains uniform across the fault, faulting activity apparently ceased by about 12.8 Ma. Surface exposures show a ponded mudflow deposit preserved within Crater Flat Group rocks at Raven Canyon, coincident with the steep gravity gradient in southern Crater Flat. Beneath the mudflow deposit, a buried NE-trending fault scarp is locally observed. The mudflow deposit contains clasts as large as 1 m, indicating that a large topographic gradient (paleo-fault scarp) probably existed at the time of emplacement.