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Evaluation of geologic structure guiding ground water flow south and west of Frenchman Flat, Nevada Test Site

1997; United States Department of the Interior; Linguagem: Inglês

10.3133/ofr97734

2332-4899

E.H. McKee,

Hydraulic Fracturing and Reservoir Analysis

Ground water flow through the region south and west of Frenchman Flat, in the Ash Meadows subbasin of the Death Valley ground water flow system, is controlled mostly by the distribution of permeable and impermeable rocks.Geologic structures such as faults are instrumental in arranging the distribution of the aquifer and aquitard rock units.Most permeability is in fractures caused by faulting in carbonate rocks.Large faults are more likely to reach the potentiometric surface about 325 meters below the ground surface and are more likely to effect the flow path than small faults.Thus field work concentrated on identifying large faults, especially where they cut carbonate rocks.Small faults, however, may develop as much permeability as large faults.Faults that are penetrative and are part of an anastomosing fault zone are paricularly important.The overall pattern of faults and joints at the ground surface in the Spotted and Specter Ranges is an indication of the fracture system at the depth of the water table.Most of the faults in these ranges are westsouthwest-striking, high-angle faults, 100 to 3500 meters long, with 10 to 300 meters of displacement.Many of them, such as those in the Spotted Range and Rock Valley are leftlateral strike-slip faults that are conjugate to the NW-striking right-lateral faults of the Las Vegas Valley shear zone.These faults control the ground water flow path, which runs west-southwest beneath the Spotted Range, Mercury Valley and the Specter Range.The Specter Range thrust is a significant geologic structure with respect to ground water flow.This regional thrust fault emplaces siliceous clastic strata into the north central and western parts of the Specter Range.These rocks act as a barrier or aquitard that confines ground water flow to the southern part of the range, directing it southwestward toward springs at Ash Meadows.These siliceous clastic aquitard rocks and overlying Cenozoic deposits probably also block westward flow of ground water in Rock Valley, diverting it southward to the flow path beneath the southern part of the Specter Range.Winograd and Thordarson, 1975; Structural geology of the Specter Range quadrangle, Nevada and its regional significance by Burchfiel, 1965; Middle Devonian-Mississippian stratigraphy on and near the Nevada Test Site: implications for hydrocarbon potential by Trexler and others, 1996; Tertiary extension north of the Las Vegas Valley shear zone, Sheep and Desert Ranges, Clark County, Nevada by Guth, 1981; Preliminary analyses of minor structures and lithologic boundaries for the Frenchman Flat model area by Grauch and Hudson, 1995; Digital bedrock geologic map data base of the Beatty 30'X60' Quadrangle, Nevada by Carr and others, 1996.; Regional structural setting of Yucca Mountain, southwestern Nevada, and late Cambrian rates of tectonic activity in part of the southwestern Great Basin, Nevada and California, Carr, 1984.