The Sheep Hole Mountains quadrangle covers an area of the Mojave Desert characterized by basins and ranges. Alluviated valleys and playas (dry lakes) are as low as 165 m elevation at Cadiz Lake playa. The mountain ranges they separate are as high as 1490 m elevation, for example in the Old Woman Mountains. Rock units are well exposed owing to low rainfall and sparse vegetation.
The Sheep Hole Mountains 30' x 60' Quadrangle covers an area of the Mojave Desert characterized by desert ranges separated by broad basins. Ranges include parts of the Old Woman, Ship, Iron, Coxcomb, Pinto, Bullion, and Calumet mountains as well as Lead Mountain and the Kilbeck Hills. Basins include part of Ward Valley, part of Cadiz Valley including Cadiz Lake playa, and broad valleys occupied by the Bristol Lake and Dale Lake playas. Bedrock geologic units in the ranges range in age from Proterozoic to Quaternary. The valleys expose Neogene and Quaternary deposits.
Proterozoic granitoids in the quadrangle include the Early Proterozoic Fenner Gneiss, Kilbeck Gneiss, Dog Wash Gneiss, Granite of Joshua Tree, the Gneiss of Dry Lakes valley, and a Middle Proterozoic granite. Other Proterozoic rocks include the Pinto Gneiss and the Quartzite of Pinto Mountain.
A Cambrian to Triassic sedimentary rock 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.
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. Mafic and felsic dikes that probably are part of the Late Jurassic Independence dike swarm intrude the Jurassic batholithic rocks.
A Mesozoic ductile fault, 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 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. 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.
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. Brine and salt have been commercially exploited from the playas.
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Updated: October 19, 2007 (bwr, mfd)
