Geologic map of the government wash quadrangle, Clark County, Nevada

Duebendorfer, Ernest

Text and references for Nevada Bureau of Mines and Geology Map 140... INTRODUCTION The Government Wash 7.5' Quadrangle is located north of the western part of Lake Mead in southern Nevada. The Lake Mead area, between the Grand Wash Cliffs and Las Vegas, Nevada, at about the latitude of 36 N, lies in the central Basin and Range province (terminology of Wernicke, 1992) between (and south of) the Sevier orogenic belt and the Colorado Plateau. A marked change in the style of Basin and Range extension occurs in the Lake Mead area. To the south, the metamorphic core complexes of the Colorado River extensional corridor expose ductilely deformed rocks in the lower plates of regional detachment faults (Davis and others, 1980; 1982; 1986; Howard and John, 1987; many others) and major strike-slip transfer zones (classification of Faulds and Varga, 1998) appear to be rare. For 250 km to the north, exposures of ductilely deformed lower-plate rocks are rare and strike-slip transfer zones are common (Duebendorfer and Black, 1992). The Lake Mead area contains core complex-type detachment systems, the Saddle Island and Lakeside Mine (Gold Butte block) faults, and two of the largest strike-slip transfer faults in the Basin and Range province. These faults, the Lake Mead fault system and the Las Vegas Valley shear zone, mark the northern terminus of the northern Colorado River extensional corridor (Howard and John, 1987; Faulds and others, 1990, 1992). The Government Wash Quadrangle can be divided into two physiographic and structural domains. North of the right-slip Las Vegas Valley shear zone, the Gale Hills average about 900 m in elevation and consist of a thick section of Miocene sedimentary rocks that overlie Mesozoic rocks deposited in the foreland of the Sevier thrust belt (farther north, in the Muddy Mountains, these same Miocene rocks overlie Paleozoic rocks of the Muddy Mountains thrust allochthon). South of the Las Vegas Valley shear zone is a topographically low (300–500 m) and relatively featureless terrain developed on the Boulder basin (Longwell, 1936). With the exception of areally limited exposures in the upper and lower plates of the Saddle Island detachment fault (fig. 1), pre-Tertiary rocks are not exposed south of the Las Vegas Valley shear zone in Boulder basin. Because pre-Tertiary rocks are not exposed here, and because it is possible that the Saddle Island detachment may underlie much of Boulder basin (Duebendorfer and Wallin, 1991), the sub-Tertiary geology of the southern half of the Government Wash Quadrangle is not known. Possibilities include: (1) the entire Paleozoic and Mesozoic section, exposed at Frenchman Mountain about 15 km west of the quadrangle, is present, (2) highly dismembered, fault-bounded blocks of Paleozoic and Mesozoic rock may occur at depth, and (3) the Tertiary section could be juxtaposed against Proterozoic crystalline rocks along the Saddle Island detachment, which projects northward in the subsurface toward the Government Wash Quadrangle, due to complete tectonic removal of the Paleozoic and Mesozoic sections.