Water-Resources Investigations Report 01-4220
Hydrogeologic Framework of Antelope Valley and Bedell Flat, Washoe County, West-Central Nevada
The study area lies in a transitional zone between major tectonic structures of the Sierra Frontal Fault Zone and the Walker Lane System (Bell, 1981, p. 35). Structural features associated with these provinces, in part, define the hydrogeologic framework and control the regional movement of ground water in the study area. The Sierra Frontal Fault Zone, which separates the Sierra Nevada from the Basin and Range physiographic province, consists of a series of north-trending faults extending from Reno south to Owens Valley, California. Fault displacement in the study area is dominantly normal (Bell, 1981, p. 17) with some component of strike-slip movement (VanWormer and Ryall, 1980). The Walker Lane System is a right-lateral shear zone trending northwest-southeast and extending to southern Nevada (Bell 1981, p. 17). At oblique angles to the main zone of right-lateral movement is a series of conjugate fault sets.
Bonham (1969, p. 42) suggests that southern Washoe County has undergone two main periods of structural deformation. Because of the absence of pre-Mesozoic rocks, the earliest deformation identified took place in the late Mesozoic Era, resulting in pre-Tertiary sedimentary and volcanic rocks being folded, faulted, and regionally metamorphosed prior to the intrusion of granitic plutons during the Cretaceous period. The second period of deformation began in the middle to late Tertiary and has continued to the present. Structural features associated with this deformation include normal faulting that has formed the structural depression beneath Bedell Flat and the existing topographic features in the study area.
Rock types and deposits in the study area were grouped into five hydrogeologic units (modified from Bonham, 1969). The five units are (1) metamorphic rocks of Triassic and Jurassic age; (2) plutonic rocks, mostly granodiorite in composition, of Jurassic to early Tertiary (?) age; (3) volcanic rocks of Tertiary age; (4) older basin-fill deposits of Quaternary and Tertiary age; and (5) younger basin-fill deposits of Quaternary age. Pre-Cenozoic basement consists of units 1 and 2, and the Cenozoic deposits consist of units 3, 4, and 5.
The oldest rocks exposed in the study area are metamorphic rocks, probably of the Peavine sequence (Bonham, 1969, p. 7). These rocks occur as an isolated roof pendant in the plutonic rocks that make up Freds Mountain (Gimlett, 1967, p. 13). The most widespread and abundant consolidated rock type in the study area is plutonic rock that is thought to be indistinguishable from the most common Sierran types (Gimlett, 1967, p. 6). This rock covers about 35 percent of the study area, makes up most of the bounding mountains, and is assumed to underlie the valleys. The lines of combined thickness of the Cenozoic deposits shown in plate 1 define the depth to the top of this hydrogeologic unit. Although this unit has little if any primary permeability, ground water may occur in openings developed through fracturing, faulting, and associated weathered zones. Reported transmissivities of granitic-type rocks range from 30 to 1,000 ft2/d (Harrill, 1973, p. 30). Because of the abundance of plutonic rocks in the study area, their effect on local ground-water movement probably is significant, particularly in terms of inter-basin flow where they generally impede ground-water movement except along faults and fractures (Harrill, 1973, p. 44).
Exposures of Tertiary volcanic rocks make up a small percentage (about 6 percent) of the total area of Bedell Flat and are not exposed in Antelope Valley. In Bedell Flat, the volcanic rocks are part of the Hartford Assemblage and occur as erosional remnants that unconformably overlie Mesozoic granitic rocks (Bonham, 1969, p. 22, 52). The assemblage consists predominantly of volcanic rocks and lesser amounts of intercalated sedimentary rocks. Transmissivities of fractured rhyolite thought to be part of the Hartford Assemblage in adjacent Lemmon Valley range from about 150 to nearly 400 ft2/d (Harrill, 1973, p. 20).
Basin-fill deposits make up the principal aquifers in the study area, although ground water probably is present in all five hydrogeologic units. Exposures of older basin-fill deposits cover about 30 percent of the drainage area in Bedell Flat and are not exposed in Antelope Valley (pl. 1). Where they are saturated, the older basin-fill deposits may form important aquifers for low- and possibly medium-yielding wells that would be suitable for domestic uses, livestock, and small public supply requirements. Younger basin-fill deposits cover about half of the valley floor in Bedell Flat and the entire floor of Antelope Valley. The younger basin-fill deposits form the most important water-bearing formations in the study area and are capable of yielding large supplies of water to wells where the hydrogeologic unit is thick and saturated.
Cenozoic deposits beneath Antelope Valley are relatively thin and probably do not exceed more than about 300 ft in depth (pl. 1). The shallow depth to basement suggests a limited volume of ground water in storage in the basin-fill aquifer. Several structural depressions exist in the pre-Cenozoic basement in Antelope Valley, including the largest beneath the area occupied by the playa.
Bedell Flat is underlain by an elongated structural depression in the pre-Cenozoic basement that generally trends to the northeast. The maximum thickness of Cenozoic deposits is about 2,500 ft beneath the south-central part of the valley. Minimum depth estimates to pre-Cenozoic basement beneath seismic-profile C-C′, computed using 12,000 ft/s and 17,000 ft/s as representing basement velocities, range from 1,900 to 2,500 ft. The maximum thickness of deposits in Bedell Flat is similar to that estimated by Schaefer and Maurer (1981, p. 8) in adjacent Lemmon Valley. Other isolated basement depressions are indicated beneath the north-central and southern parts of the valley. The abrupt decreases in the thickness of Cenozoic deposits along both the northwest and southeast sides of the largest structural depression suggest that the depression is fault-controlled. The geometry of the basement also suggests that the range-bounding faults of the Sand Hills are about 2 mi southeast of the mountain front. These faults generally have no surface expression except for one mapped fault near the northwest part of the depression. A structural high in the extreme southern part of Bedell Flat nearly coincides with the hydrographic-area boundary and probably acts as a ground-water divide. The thickness of Cenozoic deposits decreases toward the northwest corner of Bedell Flat; this decrease may help explain the presence of shallow ground water beneath the northwest part of the valley. Ground water appears to exit Bedell Flat beneath the narrow draw to Red Rock Valley (Rush and Glancy, 1967, p. 43). As Cenozoic deposits thin toward the northwest, ground-water flow becomes constricted and is forced toward land surface.
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