Author: Michael H. Darin and Barbaros Demircan

Year: 2023

Series: Open-File Report 2023-03

Version: first edition, March 2023; supersedes Open-File Report 2022-01

Format: map sheet: 30.5 x 41 inches, color; text: 10 pages, b/w

Scale: 1:24,000

The Boulder Canyon 7.5-minute quadrangle is located east of Las Vegas, Nevada and almost entirely within the boundary of the Lake Mead National Recreation Area. The south half of the quadrangle overlaps the Nevada-Arizona state line along the north shore of Lake Mead and rugged topography at the northern end of the Black Mountains. The only access to this area is via Northshore Road (Nevada State Road 167), which intersects the north half of the quadrangle, or by boat. New detailed geologic mapping for this report was conducted by the authors between April 2021 and June 2022, and builds on various M.S. theses completed at Northern Arizona University (San Filippo, 2008; Winston, 2016; Demircan, 2019) and the University of Nevada, Las Vegas (Naumann, 1987; Eschner, 1989), as well as detailed geologic mapping in adjacent quadrangles (Anderson, 2003; Duebendorfer, 2003). This report and the accompanying geologic map provide useful insights into the geologic framework of the western Lake Mead structural domain, its complex deformation and magmatic evolution, and the potential for recent Quaternary faulting and seismic hazards proximal to the rapidly expanding Las Vegas urban area and within the Lake Mead National Recreation Area.

The quadrangle is situated precisely at a major structural and stratigraphic boundary within the central Basin and Range that separates thick Paleozoic, Mesozoic and Cenozoic sedimentary rocks to the north from Proterozoic crystalline rocks and Cenozoic igneous rocks to the south. This boundary corresponds with the northeast-striking, left-lateral Lake Mead fault system, which accommodated at least 20 to 65 km of cumulative displacement mostly between ~13 and 8 Ma in the western Lake Mead area (Anderson, 1973; Bohannon, 1979; Weber and Smith, 1987; Duebendorfer and Wallin, 1991). In the north half of the quadrangle, the oldest exposed rocks are early Permian and occur in the Echo Hills, a structurally complex transpressional pop-up block within a restraining step-over along the Bitter Spring Valley fault zone (Campagna and Aydin, 1991; San Filippo, 2008). Permian, Triassic, and Jurassic strata are unconformably overlain by Oligocene-Miocene strata of the Rainbow Gardens and Horse Spring Formations, all of which are intensely faulted, tilted, and locally gently folded as a result of Miocene transtension. In the south half of the quadrangle, map units consist of middle to upper Miocene (ca. 16–10 Ma) volcanic and sedimentary rocks and hypabyssal plutonic rocks that represent the northernmost and structurally shallowest part of the ca. 13.5–12.5 Ma Wilson Ridge pluton, which was emplaced during rapid extension across the Lake Mead region. Along the north shore of Lake Mead, two relatively distinct kilometer-scale screens of sedimentary and volcanic rock at Bearing Peak and Guardian Peak show pervasive hornfels-facies metamorphism and appear to be intruded by, and suspended within, the Wilson Ridge pluton. The Bearing Peak screen (~1.6 km2) consists of a >1-km-thick, gently to steeply dipping sequence of bedded conglomerate, sandstone, tuff, and angular megabreccia composed entirely of clasts of diverse Precambrian metamorphic rocks that are not exposed in or near the quadrangle; these strata are tentatively correlated with the Thumb Member of the Horse Spring Formation. The Guardian Peak screen (~2.0 km2) is composed of a >500-m-thick sequence of similar Thumb Member strata underlain by dolomitic breccia and Proterozoic orthogneiss and overlain by mafic Miocene(?) lavas. East of the Boulder Wash fault is a >600-m-thick sequence of dacitic lavas, breccias, and tuffs referred to as the volcanic rocks of Boulder Wash (ca. 14.2–12.5 Ma); geochemical correlations indicate that the volcanic rocks of Boulder Wash are the extrusive equivalents of the Wilson Ridge pluton (Naumann, 1987). These volcanic rocks are unconformably overlain by mixed gypsum, volcaniclastic sandstone and conglomerate, and tuffs of the red sandstone unit (ca. 12.5–10.0 Ma). Both of these units are interbedded with distal alkaline mafic to intermediate lava flows derived from the Hamblin-Cleopatra stratovolcano (ca. 13.0–10.7 Ma).

The dominant structures within the quadrangle include the ENE-striking, left-lateral Hamblin Bay fault and the subsidiary Bitter Spring Valley fault, which together represent the main strands of the Lake Mead fault system. Left-lateral slip is estimated to be at least ~15–20 km on the Hamblin Bay fault (Anderson, 1973) and at least ~5–11 km on the Bitter Spring Valley fault (San Filippo, 2008). While these major faults accommodate the majority of strain in the quadrangle, the structural architecture is expressed as an integrated network of ~NE-striking left-lateral, ~NW-striking right-lateral, and ~N-striking normal faults, and several E- to NNE-trending open folds. North of the Hamblin Bay fault in Bitter Spring Valley, the onset of regional extension was coeval with deposition of the Horse Spring Formation ca. 16–13 Ma (Beard, 1996; Umhoefer et al., 2010; Demircan, 2019; Lamb et al., 2022). South of this, however, a contemporaneous stratigraphic record is not present, making it difficult to evaluate the onset of extension there. Significant ~E-W extension certainly occurred during and after emplacement of the Wilson Ridge pluton and Boulder Wash volcano between 13.5 and ~10 Ma. Extension was therefore at least partly synchronous with ca. 13–8 Ma left-lateral slip on the Hamblin Bay fault. Post-13-Ma volcanic and sedimentary strata are generally west-tilted in the immediate hanging wall of the major NNW-striking, east-side-down Boulder Wash fault, but stratal dips roll over to become east-tilted toward the eastern map boundary. Total normal displacement on the Boulder Wash fault is at least 1,600 m, and normal slip on the west-dipping Ransome fault is at least 500 m. More regionally, sinistral transtension along the Lake Mead fault system was coeval and kinematically linked with the NW-striking dextral faults of Las Vegas Valley shear zone to the northwest (Anderson, 1973; Longwell, 1974; Duebendorfer and Wallin, 1991; Fryxell and Duebendorfer, 2005), and with E-W extension in the Lower Colorado River Extensional Corridor to the southwest (Anderson, 1971; Weber and Smith, 1987; Faulds et al., 2001).

Quaternary surficial deposits are extensive in the north half of the quadrangle in Pinto Valley and southern Bitter Spring Valley, and comparatively sparse in the south half of the quadrangle in the incised, high-relief terrain of the northern Black Mountains. In Pinto Valley, remnants of older alluvial terraces occur at elevations of no more than ~50 m above modern drainages, suggesting relatively slow post-tectonic erosional denudation and dissection since early(?) Pleistocene time. Although this study did not reveal evidence of Quaternary fault activity within the quadrangle, further investigation with the aid of high-resolution light detection and ranging (LiDAR) elevation data would be an essential step toward a more robust evaluation of seismic hazards for the area.

This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G20AC00390, 2020, and G21AC10973, 2021.

Suggested Citation:

Darin, M.H., and Demircan, B., 2023, Geologic map of the Nevada part of the Boulder Canyon quadrangle, Clark County, Nevada: Nevada Bureau of Mines and Geology Open-File Report 2023-03, scale 1:24,000, 10 p.

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