This 1:24,000-scale preliminary geologic map of the Boulder City NW 7.5-minute quadrangle in Clark County, Nevada covers portions of the southwestern River Mountains, the northeastern McCullough Range and straddles a segment of the drainage divide between the hydrologically closed Eldorado Valley and Las Vegas Valley which is part of the through-flowing Colorado River Basin. The City of Henderson extends into the north-central part of this quadrangle, and the in-progress Interstate 11, Boulder City Bypass transects the northeastern part of the quadrangle.

The bedrock exposures in the quadrangle include Tertiary plutonic, volcanic, and sedimentary rocks. The northeastern McCullough Range includes the ~14.5- to 13.8-Ma Railroad Pass pluton and corresponding ~15.0- to 13.9-Ma Dutchman Pass volcanics. The southwestern River Mountains is dominated by the pervasively hydrothermally altered Red Mountain volcanics. The Red Mountain volcanics are locally intruded by the ~14.5- to 13.5-Ma Boulder City pluton and locally overlain by the Dutchman Pass, Powerline Road, and Mount Davis volcanics.

The bedrock in the southern River Mountains contains numerous southerly strands of the dextral, NW-striking Las Vegas Valley shear zone, and the sinistral, NE-striking Lake Mead fault system, both major crustal structures during Miocene regional extension. The bedrock in the northeastern McCullough Range is cut by a system of kinematically linked north-striking, down-to-east normal faults and NW-striking dextral faults, the latter of which are associated with part of the southern termination of the Las Vegas Valley shear zone. The east side of the northern McCullough Range is bound by the Black Hills fault, which is an active range-front normal fault expressed as a continuous, 1.8- to 2.8-m-high fault scarp in late Pleistocene fan deposits. Mapping at the north end of the Black Hills fault suggests that the fault is directly linked to the sinistral, NE-striking Hemenway Wash fault, which is a major strand of the Lake Mead fault system, and that locally bounds the southern flank of the River Mountains. In contrast to the Black Hills fault, Quaternary fault scarps were not observed along the Hemenway Wash fault.

Surficial sediments in the quadrangle are largely alluvial fan and pediment deposits ranging from historic to early Pleistocene. Fan deposits eroded from altered Railroad Pass pluton locally contain clasts with actinolite and magnesiohornblende, which are naturally occurring asbestos (NOA) minerals.

This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G16AC00186, 2017.

Suggested Citation:
Hinz, N.H., Dee, S., and Johnsen, R., 2017, Preliminary geologic map of the Boulder City NW quadrangle, Clark County, Nevada: Nevada Bureau of Mines and Geology Open-File Report 17-5, scale 1:24,000, 5 p.