New Geologic Maps in Clark County: Boulder City and Apex Quadrangles


Preliminary Geologic Map of the Boulder City Quadrangle, Clark County, Nevada

Authors: Seth Dee, Nicholas H. Hinz, R. Ernie Anderson, and Racheal Johnsen
Year: 2016
Series: Open-File Report 16-4
Format: plate: 39 x 29 inches, color; text: 6 pages, some color
Scale: 1:24,000

 A 1:24,000 scale preliminary geologic map of the Boulder City 7.5-minute quadrangle in Clark County, Nevada. This quadrangle covers portions of the southeastern River Mountains, the northern Eldorado Mountains and straddles a segment of the drainage divide between the hydrologically closed Eldorado Valley and the through-flowing Colorado River Basin. Boulder City is located in the northern part of the quadrangle, as is the in-progress Interstate 11, Boulder City Bypass. This publication includes a combination of new mapping and integration with existing mapping by Ernie Anderson.

The bedrock exposures in the quadrangle are dominated by Tertiary plutonic, volcanic, and sedimentary rocks with lesser Proterozoic metamorphic rocks and Paleozoic sedimentary rocks. In the northern Eldorado Mountains the basal volcanic section includes the early to middle Miocene, intermediate to felsic composition Patsy Mine volcanics. The Patsy Mine volcanics are locally intruded by the composite, middle Miocene Boulder City pluton. This pluton is variably hydrothermally altered with actinolite present in altered groundmass and fracture-fill veins. Both the Boulder City pluton and the Patsy Mine volcanics are locally overlain by the middle Miocene Mount Davis volcanics which consist of a syn-extensional, bimodal sequence of lavas and tuffaceous sediments. In the southeastern River Mountains, the Tertiary strata are dominated by the middle Miocene, intermediate to felsic composition Red Mountain volcanics. These rocks are intruded by a middle Miocene granitic stock, which is probably related to widespread hydrothermal alteration of the Red Mountain volcanics. Locally overlying the Red Mountain volcanics are the less altered, intermediate to silicic, volcanics of Bootleg Wash, and unaltered Mount Davis volcanics.

The bedrock in the northern Eldorado Mountains is faulted by numerous north-striking, down-to-east and down-to-west normal faults. The northeast-striking, sinistral Hemenway Wash fault transects the northwest quarter of the quadrangle, separating the Eldorado Mountains from the River Mountains. The Hemenway Wash fault is one of the faults that makes up the >100 km-long, Lake Mead Fault System. In the northernmost Eldorado Mountains, fault strikes curve from N-S to NW and to E-W as they approach the Hemenway Wash fault zone, possibly due to oroclinal flexure. Provisional analysis of new geochronologic and geochemical data acquired in this study indicate that the plutonic and volcanic strata exposed in the southeastern River Mountains may correlate directly to strata in the northern Eldorado Mountain, providing a means to evaluate a range of possible displacements across the Hemenway Wash fault.

Surficial sediments in the quadrangle are largely alluvial fan and pediment deposits ranging from historic to Pliocene in age. Early Pleistocene to late Pliocene surficial deposits are characterized by 2+ m thick pedogenic carbonate horizons, which form resistant geomorphic surfaces on the east side of the Eldorado basin and cap many of the bedrock highlands in the Eldorado range. Fan deposits eroded from altered Boulder City plutonic rocks or from Miocene basin sediments may contain redistributed actinolite-bearing clasts. One possible fault scarp was observed in a late to middle Pleistocene aged fan. No other evidence for Quaternary faulting was recognized in the quadrangle.

This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G15AC00240, 2016.

apexPreliminary Geologic Map of the Apex Quadrangle, Clark County, Nevada

Author: Robert G. Bohannon
Year: 2016
Series: Open-File Report 16-3
Format: plate: 31 x 32 inches, color; text: 5 pages, color
Scale: 1:24,000

The Apex quadrangle (1:24,000 scale) is centered approximately 22.5 km northeast of downtown Las Vegas in Clark County, Nevada. Rocks in the quadrangle are exclusively sedimentary and include a mostly conformable sequence of Paleozoic continental shelf and platform rocks in its north half. The south half of the quadrangle is mostly Miocene interior-continental-basin deposits. Dissected alluvial deposits of Quaternary and latest Tertiary age, only the oldest of which are consolidated, discontinuously cover large lowland parts of the quadrangle. The Paleozoic rocks are deformed and in some places overturned by Mesozoic thrust faults associated with the Late Cretaceous Sevier Orogenic disturbance. A large east-vergent thrust fault with a northeast orientation, the Dry Lake Thrust, cuts the eastern half of the quadrangle, where it is mostly concealed by post-Cretaceous deposits. The thrust juxtaposes lower Paleozoic shelf rocks in the hanging wall above upper Paleozoic continental platform rocks in the footwall. Tertiary normal faults are also common and are mostly oriented northeast where they cut Paleozoic rocks. The Miocene beds, most of which post-date the northeast normal faults, are deformed by a few east-west or east-northeast-oriented faults and numerous small folds that are associated with them. The younger faults might be associated with very late-stage movement on the Las Vegas Valley shear zone which projects into the quadrangle from beneath Las Vegas Valley to the east, but the shear zone is otherwise concealed by the Miocene deposits.

Preliminary Geologic Map of the Sloan Quadrangle, Clark County, Nevada

Nicholas H. Hinz, Alan R. Ramelli, and Seth Dee
Year: 2015
Series: Open-File Report 15-7
Format: plate: 37 x 26 inches, color, includes 1 cross section; text: 4 pages, b/w
Scale: 1:24,000

A 1:24,000 scale, preliminary geologic map of the Sloan 7.5-minute quadrangle in Clark County, Nevada. This quadrangle straddles Interstate Highway 15 along the south side of Las Vegas Valley and abuts the drainage divide with Ivanpah Valley. The actively managed Sloan Quarry is located in the NW quarter of the quadrangle. The north half of the quadrangle includes new housing developments in the City of Henderson, the Henderson Executive Airport, and new developments in the unincorporated communities of Paradise, Enterprise, and Sloan. The southeastern margin of this map area covers the north end of Hidden Valley and contains parts of each of the following land use features: the Sloan Canyon National Conservation Area, the Sloan Canyon Rock Art Site, and the Sloan Rock Art Area of Critical Environmental Concern.

The bedrock exposures in the quadrangle consist of Paleozoic carbonate basement and Tertiary volcanic and sedimentary rocks. The Paleozoic stratigraphy includes the Early to Middle Devonian Sultan Limestone, the Late Devonian to Early Mississippian Crystal Pass Limestone, the Mississippian Monte Cristo Group, and the Late Mississippian to Early Permian Bird Spring Formation. Four sub-units in the commonly undivided lower part the Bird Spring Formation were distinguished in the Sloan quadrangle. The Tertiary section includes a complex section of fluvial gravels and volcanic rocks. The fluvial gravels reach 350 m thick, fill a previously unrecognized paleovalley segment with as much as 500 m of preserved paleotopography. The volcanic rocks include middle Miocene basalt, basaltic andesite, andesite, and dacite that were locally erupted. The lower-most volcanic rocks are intercalated with the uppermost fluvial gravels. The Paleozoic section was initially deformed during the Sevier and/or Laramide orogenies and were displaced along reverse faults, thrust faults, and strike-slip faults. Normal faults associated with Basin and Range extension cut the Paleozoic strata and the lower half of the Tertiary section. Faults were not observed cutting the upper half of the Tertiary strata or the Quaternary surficial deposits.

This map was prepared as a part of the STATEMAP component of the National Cooperative Geologic Mapping Program in cooperation with the U.S. Geological Survey.

View or purchase here:

Desert National Wildlife Range—new report

Open-File Report 14-3
An updated mineral and energy resource assessment of the Desert National Wildlife Range, Clark and Lincoln counties, Nevada
by John Muntean, Nick Hinz, Tim Cramer, David Davis, Greg Dering, and Chris Sladek


In March 2013, the U.S. Fish and Wildlife Service (USFWS) contracted the Nevada Bureau of Mines and Geology (NBMG) to update the mineral and energy resource assessment of the Desert National Wildlife Range (DNWR). The first such assessment of the DNWR was completed by NBMG in January of 1993, and was published as NBMG Open-File Report 93-2, Energy and Mineral Resource Assessment of the Desert National Wildlife Range, Eastern Section, Clark and Lincoln Counties, Nevada. That assessment covered approximately 740,000 acres managed by USFWS east of the Nellis South Range military complex and was required to evaluate lands within the DNWR for withdrawal from mineral entry. In order to extend the period of withdrawal from entry for another 20 years, an updated assessment and report is required. This report provides the needed assessment.

This new report (Open-File Report 14-3) updates Open-File Report 93-2:
text (126.8 Mb), plate 2 (10.4 Mb), plate 3 (1.2 Mb)

Open-File Report 14-3:
162 pages (plus cover and title pages), color, $36.00, or available free on the Web:

Earthquake Faults in Las Vegas—video on News 3

Late Quaternary faults in Las Vegas are in need of a modern state-of-the-art analysis of their earthquake potential. This news clip, prepared by Channel 3 in Las Vegas, reviews some of the Las Vegas Valley faults with a local reporter and how we can wisely develop around faults with ground rupture potential. A resident is featured on the clip that was unaware of the fault and earthquake hazard potential in Las Vegas. Unfortunately, this may be the norm and not the exception. It underscores the need for all Nevadans to be aware of their earthquake risks and do what they can to reduce them (please see for mitigation information). Practicing the safest response to the next Nevada earthquakes can help prevent injuries from those events. Thus, we encourage everyone to sign up for the annual Nevada earthquake drill, the Great Nevada ShakeOut in October, and to practice Drop, Cover, and Hold during that exercise (see

…message from Craig dePolo

Link to the video:

New OFR on Eglington Fault—Clark County

Open-File Report 13-12
Evidence for High Contemporary Slip Rates along the Eglington Fault, Clark County, Nevada
by Craig M. dePolo, Wanda J. Taylor, and James E. Faulds


8 pages, color, $2.40:

or free online:

The Eglington fault in northern Las Vegas Valley is an unusual fault in that it is expressed as a faulted warp at the surface and accommodated a large vertical surface offset (10-14 m) in latest Pleistocene sediments relative to its short length of 11 km. Coupled with a competing hydro-compaction hypothesis for faults within Las Vegas Valley, the earthquake hazard of the Eglington fault has been poorly understood and likely underrepresented. Radiocarbon dates from faulted sediments in the area indicate that the vertical displacement across the fault has occurred in the last ~22 kyr. A preferred vertical fault slip rate of 0.6 m/kyr, and range of 0.25 to 0.9 m/kyr, are estimated using available data. How single event displacements are manifested along the Eglington fault and what the size of those displacements might be is not known, generating uncertainty in estimating the potential earthquake recurrence interval for the fault.

Ute quadrangle – two new maps

Map 177–Geologic map of the Ute quadrangle, Clark County, Nevada, by Craig M. dePolo and Wanda J. Taylor; 2012


44 x 27.5 inches, color; 17-page text, b/w; folded or rolled, $23.00 ($18.00 for map only) or free on Web:

A 1:24,000-scale, color geologic map of the Ute 7.5-minute quadrangle in Clark County, Nevada with descriptions of 59 geologic units and 2 cross sections. Accompanying text includes full unit descriptions and references. GIS zip file also available online.

The Ute quadrangle covers the central part of the California Wash basin and a small section of the westernmost North Muddy Mountains, where Paleozoic and Mesozoic rocks were highly deformed by the Sevier orogeny. Tertiary and Quaternary sedimentary rocks record the history of basin filling and dissection. Noteworthy on the quadrangle are several resistant Quaternary petrocalcic surfaces, with as much as Stage VI carbonate development, that form pediment caps on the Tertiary basin deposits. The map includes the northern part of the Holocene-active California Wash fault and the southernmost part of the Quaternary-active Hogan Spring fault.


Open-File Report 12-4–Preliminary erosional impact potential map of the Ute quadrangle, Clark County, Nevada, by Craig M. dePolo and Irene M. Seelye, 2012


20.5 x 32.5 inches, color, scale 1:24,000; folded or rolled, $14.00 or free on Web:

There is a wide range of erosional susceptibility associated with different geologic units on the Ute quadrangle that might be considered when planning cross-country motor vehicle events, which are common in this valley. Tracks from motor races that can be seen in 1976 photography were hard to find in 2011 if they were on the harder petrocalcic surfaces, but caused long-lasting disruption of softer geologic units and surfaces with desert pavements on them. When more easily erodible units are disrupted, there is enhanced erosion from water runoff and significantly more dust is generated until the surface is stabilized with a new pavement, vegetation, or erodes down to a resistant layer. Using the distribution of the geologic units and their general erosional susceptibility, this Preliminary Erosional Impact Potential Map of the Ute Quadrangle was produced. This map can be used to plan race routes with the least erosional impact.

There are three general categories on the map:

1) more resistant units with lower erosional impact,

2) softer units that can be disrupted and eroded relatively easily, and

3) stream channels that have their surfaces refreshed when the channel flows with water.

The more resistant units include petrocalcic surfaces and pre-Tertiary limestones. Softer units include Tertiary basin sediments and Quaternary surfaces with pavements and an underlying loose silty layer. Stream channels are areas that have water flowing annually to every decade or so and have the ability to refresh themselves and recover from the effects of vehicular traffic. The map is experimental and is a simple derivative map from the Geologic Map of the Ute Quadrangle (NBMG Map 177).

New geologic map-Jean quadrangle

Map 176—Geologic map of the Jean quadrangle, Clark County, Nevada by Larry J. Garside, P. Kyle House, B. Clark Burchfiel, Stephen M. Rowland, and Brenda J. Buck2012 one 39×27.5-inch color platescale 1:24,00011-page text, b/wfolded or rolled$19.00 …

Map 176—Geologic map of the Jean quadrangle, Clark County, Nevada
by Larry J. Garside, P. Kyle House, B. Clark Burchfiel, Stephen M. Rowland, and Brenda J. Buck


one 39×27.5-inch color plate
scale 1:24,000
1-page text, b/w
folded or rolled
$19.00 or free on the Web:

A 1:24,000-scale, color geologic map of the Jean 7.5-minute quadrangle in Clark County, Nevada with descriptions of 46 geologic units and 2 cross sections. Accompanying text includes full unit descriptions and references.