Proterozoic granitoids in the quadrangle include the Early Proterozoic Fenner Gneiss, Kilbeck Gneiss, Dog Wash Gneiss, granite of Joshua Tree, the (highly peraluminous granite) gneiss of Dry Lakes valley, and a Middle Proterozoic granite. Proterozoic supracrustal rocks include the Pinto Gneiss of Miller (1938) and the quartzite of Pinto Mountain. Early Proterozoic orogeny left an imprint of metamorphic mineral assemblages and fabrics in the older rocks.
A Cambrian to Triassic sequence deposited on the continental shelf lies above a profound nonconformity developed on the Proterozoic rocks. Small metamorphosed remnants of this sequence in the quadrangle include rocks correlated to the Tapeats, Bright Angel, Bonanza King, Redwall, Bird Spring, Hermit, Coconino, Kaibab, and Moenkopi formations. The Dale Lake Volcanics (Jurassic), and the McCoy Mountains Formation of Miller (1944)(Cretaceous and Jurassic?) are younger Mesozoic synorogenic supracrustal rocks in the quadrangle.
Mesozoic intrusions form much of the bedrock in the quadrangle, and represent a succession of magmatic arcs. The oldest rock is the Early Triassic quartz monzonite of Twentynine Palms. Extensive Jurassic magmatism is represented by large expanses of granitoids that range in composition from gabbro to syenogranite. They include the Virginia May Quartz Monzonite and other members of the Bullion Intrusive Suite, the Chubbock Porphyry, and rocks that form the Goat Basin pluton, Music Valley pluton, and Ship Mountains pluton. The Jurassic plutons range in emplacement depths from mid-crustal to hypabysasal. Mafic and felsic dikes that probably are part of the Late Jurassic Independence dike swarm intrude the Jurassic batholithic rocks.
A Mesozoic ductile fault (tectonic slide), the Scanlon thrust, places an inverted sequence of lower Paleozoic rocks and their Proterozoic basement over a lower plate of younger Paleozoic and Triassic rocks. The lower- plate rocks are internally sliced and folded. They in turn are superposed along an attenuation tectonic slide, the Kilbeck fault, over highly strained tectonic schist. The major tectonic slides and associated fabrics are cut by Late Cretaceous batholithic rocks.
Widespread Late Cretaceous granitoids assigned to the Cadiz Valley batholith and the Old-Woman Piute Range batholith together form a contiguous super-unit of granite and granodiorite compositions. The Old- Woman Piute Range batholith includes the granite of Sweetwater Wash in the Painted Rock pluton and the Old Woman Mountains Granodiorite forming the Old Woman pluton. The large Cadiz Valley batholith is divided into the Iron Mountains Intrusive Suite and the Coxcomb Intrusive Suite. The Iron Mountains Intrusive Suite includes the Granite Pass Granite (which forms the Granite Pass pluton), the Danby Lake Granite Gneiss, and the Iron Granodiorite Gneiss. The Coxcomb Intrusive Suite consists of many units including the Clarks Pass Granodiorite, the Sheep Hole Mountains Granodiorite (forms the Sheep Hole Mountains pluton), and the Sheep Hole Pass Granite (forms the Sheep Hole Pass pluton). The Cretaceous rocks were emplaced at a range of deep to shallow depths, and their intrusion resulted in an aureole 2-3 km wide in older rocks. Mylonitic fabrics developed through a thickness of >1.3 km, together with screens of tectonic schist, record ductile deformation associated with or immediately following batholith emplacement in a plutonic roof zone in the Iron Mountains. Post-plutonic Late Cretaceous mylonitic fabrics were also produced by extensional unroofing off both the western and eastern flanks of the incipient Old Woman Mountains.
A nonconformity above the Cretaceous rocks represents a period of deep erosion and nondeposition before lower Miocene volcanic and clastic rocks were deposited. Early Miocene magmatism is recorded by basanitoid, basalt, and dacite flows as well as dacitic intrusions of a lacolith, a stock, and the Bullion Mountains dike swarm. This magmatism coincided with early Miocene tectonic extensional tilting of rocks in the Calumet Mountains. Younger Neogene deposits of conglomerate, gravel, and breccia, and the demarcation of many of the modern ranges in the quadrangle, probably relate to strike-slip faulting and block rotations in the eastern California shear zone. This zone includes northwest- striking dextral faults and east-striking sinistral faults in the western part of the quadrangle. Faults active in Quaternary time include the Calumet fault, West Calumet fault, Dry Lakes fault, Sheep Hole fault, Cleghorn Lakes fault, Cleghorn Pass fault, Ivanhoe fault, Old Dale fault, Humbug Mountain fault, Dog Wash fault, Twentynine Palms Mountain fault, Pinto Mountain fault, Mesquite Lake fault, and two faults that exhibit Holocene movement, the West Valley Mountain fault and the East Valley Mountain fault.
Late Pliocene basalt in the Deadman Lake volcanic field and Quaternary basalt in the Amboy Crater lava flow and a flow near Lead Mountain record the youngest volcanism. Quaternary surficial deposits of alluvium, playa deposits, and windblown sand underlie more than half the quadrangle. Twentynine drill holes deeper than 100 m have penetrated the surficial deposits. Mapped sand dune crests strike mostly east to southeast. Brine and salt have been commercially exploited from the playas.
The digital geologic map database has been created as a general purpose data set that is applicable to broad land-related investigation in the earth and biological sciences. For example, it can be used for mineral resource evaluation studies, animal and plant habitat studies, studies of regional fault hazards, ground-water studies, and soil and surficial geology studies in the California Desert Conservation Area and in Joshua Tree National Park. The database is not suitable for site-specific geologic evaluations.
Use of this digital geologic-map database should not violate the spatial resolution of the data. Although the digital form of the data removes the constraint imposed by the scale of a paper map, the detail and accuracy inherent in map scale are also present in the digital data. The fact that this database was compiled and edited at a scale of 1:100,000 means that higher resolution information may not have been uniformly retained in the dataset. Plotting at scales larger than 1:100,000 will not yield greater real detail, although it may reveal fine-scale irregularities below the intended resolution of the database. Similarly, although higher resolution data is incorporated in most of the map, the resolution of the combined output will be limited by the lower resolution.
Geologic mapping, compilation and digital preparation of this report were sponsored jointly by (1) the National Cooperative Geologic Mapping Program of the U.S. Geological Survey, and (2) the Bureau of Land Management Wilderness, Reactor Hazards, Deep Continental Studies.
The areal extent of the map is represented digitally by an appropriately projected (UTM projection), mathematically generated box. Consequently, polygons intersecting the lines that comprise the map boundary are closed by that boundary. Polygons internal to the map boundary are completely enclosed by line segments which are themselves a set of sequentially numbered coordinate pairs. Point data are represented by coordinate pairs.
Preparation for 1:100,000 scale entailed necessary simplification in some areas, combining of some geologic units, definition of several new geologic names, and addition of new geologic and photogeologic mapping. Most geologic detail from the 1:24,000 maps is retained on the 1:100,000- scale map.
In addition, the data set includes the following graphic and text products: (1) an Encapsulated PostScript File (.eps), sheet1, containing the geologic map and cross sections, base data, a discussion of the geolgy including a geologic summary and a section on structural evolution, a tabulation of drill holes, and a list of references; (2) a .pdf version of sheet1; (3) sheet 2(.eps), showing the Correlation of Map Units (CMU), the Description of Map Units (DMU), modal diagrams for granitoid rocks, and an explanation for point and line symbols; (4) a .pdf version of sheet2; (5) a Readme (.pdf) file, and (6) this metadata file.
The original map plot was prepared using symbol sets that are no longer commonly used by the USGS. Consequently, look-up tables would require updating in order to implement USGS symbol sets that meet Federal Geographic Data Committee (FGDC) approval.
In no event shall the USGS have any liability whatsoever for payment of any consequential, incidental, indirect, special, or tort damages of any kind, including, but not limited to, any loss of profits arising out of use of or reliance on the geographic data or arising out of the delivery, installation, operation, or support by USGS.
This digital, geologic map database of the Sheep Hole Mountains 30' x 60' quadrangle, 1:100,000 map-scale, and any derivative maps thereof, is not meant to be used or displayed at any scale larger than 1:100,000 (e.g., 1:24,000).