Geologic Map of the Sloan NE Quadrangle, Clark County, Nevada
Series: Open-File Report 2021-02
Format: sheet: 41 x 27 inches, color; text: 9 pages, color
The Sloan NE 7.5-minute quadrangle is located just south of Las Vegas, NV and includes housing developments in the City of Henderson, as well as a large part of the Sloan Canyon National Conservation Area. The map area straddles the crest of the northern McCullough Range including Black Mountain, the highest peak in the range. This ridgeline is part of the northwestern segment of the drainage divide between the hydrologically closed Eldorado Valley and Las Vegas Valley, which is part of the Colorado River watershed.
Except for a single exposure of Proterozoic gneiss in the east-central map area, lithologic units in the Sloan NE quadrangle exclusively consist of middle Miocene to Holocene volcanic and sedimentary rocks. The bedrock is dominated by middle Miocene volcanic centers, including: the Colony rhyodacite dome complex (~15.0–14.1 Ma) along the east flank of the range; andesitic lavas and breccias of the McCullough stratovolcano including the Dutchman Pass volcanic units (~14.7–13.9 Ma) and overlying Hidden Valley unit (~14.1–13.9 Ma); the Henderson dome complex (<13.9 Ma) at the northern end of the range; and basalt, andesite and dacite extrusions of the Sloan volcanic section (~14.0–12.9 Ma) in the southwest. The entire middle Miocene volcanic section is >1,400 m thick. The magmatic plumbing system of the McCullough stratovolcano is well-expressed along the east side of the map, where the Railroad Pass pluton and related subvolcanic intrusions intrude the base of the volcanic sequence, and a system of feeder dikes and sills intrude its upper part; most dikes form a radial pattern in map view and converge toward the east-central edge of the map and exposures of the Railroad Pass pluton in the adjacent Boulder City NW 7.5-minute quadrangle.
Miocene volcanic rocks are gently tilted toward the west and northwest in the south and north, respectively. While a portion of this inclination may reflect primary stratal dip on the flank of the McCullough stratovolcano, most tilting is likely due to flexure of the footwall along a system of E- to SE-dipping normal faults that bound the eastern front of the McCullough Range; vertical throw on this fault zone is at least 1 km based on cross section estimates. This fault system links along strike to the northeast in the adjacent Boulder City NW 7.5-minute quadrangle with SE-dipping normal faults that collectively represent the southwestern end of the ENE-striking, left-lateral Lake Mead fault system. Extension may have been partly synchronous with ~15–13 Ma volcanism in the area and continued into the late Miocene (ca. 12–6 Ma). Several NW- to NNW-striking normal or dextral(?) faults with relatively minor (<150 m) normal separation cut tilted volcanic strata, and likely post-date tilting and major down-to-the-east normal faulting along the range front fault system.
Alluvial-fan deposits of probable early Pleistocene to Pliocene age are faulted against bedrock at one location along the range-bounding fault system in the map area. No additional evidence of Quaternary fault activity was identified within the quadrangle.
This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G19AC00383, 2019.