Geologic Map of the Washoe City Quadrangle, Washoe County, Nevada

Authors: Chad W. Carlson, Richard D. Koehler, and Christopher D. Henry
Year: 2019
Series: Open-File Report 2019-04
Version: supersedes Urban Maps UM5Ag and UM5Ak
Format: plate: 34.5 x 37 inches, color; text: 7 pages, b/w
Scale: 1:24,000

View, download, or purchase the Washoe City Quadrangle map.

This quadrangle encompasses Washoe Valley, an internally drained basin located between the Reno/Sparks and Carson City urban areas in northern Nevada. The seismically active eastern range front of the Sierra Nevada (Carson Range) extends along the western side of the quadrangle. Washoe Lake, a popular recreational area, extends from the south into the central part of the quadrangle. The eastern side of the quadrangle contains the rural communities of New Washoe City and Pleasant Valley, located along the western foothills of the Virginia Range. Major infrastructure within the quadrangle includes Interstate 580 concurrent with U.S. Highway 395, which extends north-south through the quadrangle west of Washoe Lake, and the ~73 megawatt Steamboat Hills geothermal power plants, with expansion plans for an additional 20 megawatts, occupying the northeasternmost part of the quadrangle.

Bedrock exposures in the quadrangle consist of Jurassic to Triassic metasedimentary and metavolcanic rocks of the Gardnerville Formation, Cretaceous granite and granodiorite, and Tertiary volcanic and sedimentary rocks. The Tertiary section includes Oligocene ash-flow tuffs and a complex section of Miocene volcanic rocks, intrusions, and volcaniclastic sedimentary rocks. Miocene volcanic rocks are basaltic to dacitic lavas and breccias interfingering across the northern parts of the quadrangle. The volcanic rocks were part of an ancestral Cascades arc that consisted of two recognized pulses in the Washoe City quadrangle: ~5.5–7.1 Ma lavas and breccias that extend east from the Mount Rose quadrangle into the Steamboat Hills and ~15 Ma lavas and breccias that extend west from the Virginia City quadrangle. Quaternary, 1.2 Ma rhyolite lava and tuff and 2.2 Ma basaltic andesite lava in the Steamboat Hills are some of the youngest volcanic rocks in western Nevada. Holocene sinter is being deposited by the active Steamboat geothermal system.

Principle Quaternary surficial deposits include middle Pleistocene to modern alluvial fan, landslide, and debris-flow deposits, middle to late Pleistocene glacial outwash and moraine deposits, late Pleistocene to modern lacustrine and eolian deposits, as well as active alluvial and colluvial deposits. A major debris flow complex sourced from the flank of Slide Mountain (Mount Rose) occupies the Ophir Creek canyon and is associated with at least five distinct flows including the 1983 debris flow, which caused significant damage to residential structures and infrastructure. Numerous other debris-flow deposits occur within smaller drainages of the eastern Carson Range. A massive landslide deposit along the northeastern side of Pleasant Valley is associated with large intact blocks of bedrock. The Mount Rose fan complex sourced from Jones, Whites, and Galena creeks records a long history of fan deposition (early to late Pleistocene) that includes fan deposits eroded from the Cascades arc volcanic rocks and multiple pulses of glacial outwash.

The east-dipping Carson Range fault bounds the eastern flank of the Carson Range and displaces Quaternary alluvial-fan, debris-flow, and glacial deposits across east-facing scarps that range in height from 2 to over 60 m. North of Washoe Valley, the Carson Range fault becomes distributed and is characterized by a broad zone of west- and east-facing scarps and grabens. The east-dipping Little Valley fault within the Carson Range displaces glacial outwash and moraines. A component of right-lateral displacement along the Little Valley fault is suggested by offset drainages along the eastern flank of Slide Mountain. West-dipping faults mapped and interpreted from gravity data along the eastern boundary of Washoe Valley similarly diffuse and anastamose with east-dipping faults in the northern part of the quadrangle to develop a structural accommodation zone occupied by the Steamboat Hills geothermal power plants.

This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G18AC00198, 2019.