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Water-Resources Investigations Report 01-4220

Hydrogeologic Framework of Antelope Valley and Bedell Flat, Washoe County, West-Central Nevada


This report describes the hydrogeologic framework of Antelope Valley and Bedell Flat, adding to general knowledge of geologic controls on regional ground-water flow in west-central Nevada. The hydrogeologic framework is defined by (1) the rocks and deposits that transmit ground water or impede its movement, and (2) the combined thickness of basin-fill deposits and volcanic rocks. For purposes of this study Quaternary and Tertiary basin-fill deposits and low-density Tertiary volcanic rocks are defined as Cenozoic deposits. Plutonic and metamorphic rocks are defined as pre-Cenozoic basement. Land-based geophysical methods were used to infer the form of structural features and to estimate the thickness of Cenozoic deposits in Antelope Valley and Bedell Flat.

Antelope Valley and Bedell Flat cover a combined area of 68 mi2 in southern Washoe County about 15 miles north of Reno, Nevada. Gravimetric data were compiled from 351 gravity stations throughout the study area. An iterative gravity inversion method was used to determine the thickness of Cenozoic deposits. Seismic-refraction data collected along five profiles were used as additional control points for gravity interpretation in areas of limited borehole data.

The study area lies in a transitional zone between the Sierra Frontal Fault Zone and the Walker Lane System. Structural features associated with these provinces, in part, define the hydrogeologic framework and control regional ground-water movement. Rock types and deposits in the study area were grouped into five hydrogeologic units: (1) Triassic and Jurassic metamorphic rocks, (2) Jurassic to early Tertiary (?) plutonic rocks, (3) Tertiary volcanic rocks, (4) Quaternary and Tertiary basin-fill deposits, and (5) Quaternary basin-fill deposits. The basin-fill deposits make up the principal aquifers in the study area, although ground water probably is present in all five hydrogeologic units.

The thickness of Cenozoic deposits beneath Antelope Valley probably is not greater than about 300 ft, which suggests that the volume of ground water in storage is limited. Bedell Flat is underlain by an elongated structural depression in the pre-Cenozoic basement. The maximum thickness of Cenozoic deposits in Bedell Flat is about 2,500 ft, similar to the thickness estimate obtained in an adjacent valley. The geometry of the structural depression suggests that the range-bounding faults of the Sand Hills are about 2 mi southeast of the mountain front. Shallow ground water in the northwest corner of Bedell Flat may be a result of the decreasing depth to the pre-Cenozoic basement, causing ground-water flow to become constricted and forced to land surface.

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