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New!  Geologic Map of the Bedell Flat quadrangle, Washoe County, Nevada
Geologic Map of the Bedell Flat quadrangle, Washoe County, Nevada
 
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Title: Geologic Map of the Bedell Flat quadrangle, Washoe County, Nevada

Author: Seth Dee, Ryan Goldsby, William Junkin, Richard D. Koehler, and Larry J. Garside
Year: 2025
Series: Open-file reports
Version: OF2025-04
Format: 33" x 32.5" color map; text, 19 pages
Scale: 1:24,000

The Bedell Flat 7.5ꞌ quadrangle is located immediately north of Reno in an area known as the ‘North Valleys’. The quadrangle includes two distinct hydrologic basins. In the northwest part of the quadrangle is Bedell Flat, which is drained by ephemeral streams that flow northwest into Long Valley Creek and onward to Honey Lake. Antelope Valley is in the south-central part of the quadrangle and is a closed basin with a small seasonally inundated playa. The map area includes the Hungry Range with the summits Hungry Mountain and Warm Springs Mountain, Freds Mountain, and the southeastern flank of Dogskin Mountain. The bedrock exposure in the quadrangle consists primarily of Cretaceous plutonic rocks, primarily granite and granodiorite, with lesser gabbro and diorite, related to the Sierra Nevadan batholith. The ages of these plutonic rocks are constrained by clear crosscutting relationships and supporting U-Pb zircon ages (table 2). Along the crest of Freds Mountain, the plutonic rocks intrude metavolcanic rocks that may be equivalent to the Jurassic Peavine sequence. On the western flank of Freds Mountain, west- to northwest-dipping Oligocene ash-flow tuff deposits non-conformably overlie Cretaceous granitic rocks. Equivalent ash-flow tuff deposits are found in isolated outcrops to the east in Antelope Valley. Exposures of northwest-dipping Miocene to Pliocene clastic and fluvio-lacustrine sediments overlie the Oligocene ash-flow tuff deposits northwest of Freds Mountain. West-dipping tuffaceous Miocene sediments are exposed in the southeastern part of the quadrangle, where they are in fault contact with the plutonic rocks of the Hungry Range. Quaternary sediments in the map area consist largely of alluvial fans as well as lacustrine sediments deposited in a late Pleistocene to Holocene pluvial lake in Antelope Valley.

The quadrangle includes three distinct Quaternary fault zones. Bounding the eastern range front of Freds Mountain is the north-striking, east-dipping, normal Freds Mountain fault system, which continues north through a low divide and runs along the western edge of Bedell Flat. In Antelope Valley, the fault is characterized by generally east-facing fault scarps that displace middle to late Pleistocene-aged alluvial-fan surfaces, with fault scarps up to 11 m high (Koehler 2018; Dee et al., 2018). Smaller faults scarps (<3 m high) are present along the western edge of Bedell Flat. Subparallel, concealed traces of the Freds Mountain fault system are inferred in Antelope Valley based on range morphology and displaced bedrock lithologies. A outcrop of silicified fault breccia is exposed in the northwestern corner of Antelope Valley along the Freds Mountain range front. East of the Hungry Range, another northerly striking, east-dipping Quaternary normal fault displaces late Pleistocene alluvial-fan surfaces. The northwest-striking, dextraloblique Honey Lake fault system runs along the Dogskin Mountain range front. The fault system is made up of two principal strands: a southwestern strand with southwest-facing fault scarps in middle Pleistocene alluvial-fan surfaces, and a northeastern strand that faults alluvial-fan deposits against granitic bedrock. Along most of the mapped length of the fault, both strands have a right-lateral, down-to-the-southwest, oblique sense of motion. The southwestern strand changes polarity in the eastern part of the quadrangle, where it has northeast-facing fault scarps in alluvial-fan surfaces and an exposure of a down-dropped, fault-bound sliver of Oligocene ash-flow tuff. The Honey Lake fault system is one of the principal strike-slip faults in the northern Walker Lane (Faulds et al., 2005), a region which has a geodetically measured slip rate of ~7 mm/yr (Hammond and Thatcher, 2007) . The geometry of the Freds Mountain fault suggests that it is kinematically linked with the Honey Lake fault system in a transtensional pull-apart geometry.

Suggested Citation: Dee, S., Goldsby, R., Junkin, W., Koehler, R.D., and Garside, L.J., 2025, Geologic map of the Bedell Flat quadrangle, Washoe County, Nevada: Nevada Bureau of Mines and Geology Open-File Report 2025-04, scale 1:24,000, 19 p.

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Original Product Code: OF2025-04