Geology of the Hiller Mountains Quadrangle: Clark County, Nevada

Howard, Keith; Hook, Simon; Phelps, Geoffrey; Block, Debra

Text and references to accompany Nevada Bureau of Mines and Geology Map 137. INTRODUCTION The Hiller Mountains Quadrangle lies within the Lake Mead National Recreation Area. The quadrangle was mapped as part of a team effort studying the Lake Mead 30' x 60' Quadrangle. Geologic mapping was supported by the National Cooperative Geologic Mapping Program of the U.S. Geological Survey, in cooperation with the National Aeronautics and Space Administration (NASA) and the National Park Service. The Nevada part of the quadrangle was previously mapped in reconnaissance by Volborth (1962) and Longwell and others (1965). The quadrangle includes part of Lake Mead, which is impounded by Hoover Dam 50 km to the west. The quadrangle lies 20 km downstream from the mouth of the Grand Canyon, where the west-flowing Colorado River course, now drowned beneath Lake Mead, begins its path through the Basin and Range province. In the southern part of the quadrangle, the lake narrows through Virgin Canyon, which connects two broad basins that are partly within the quadrangle, the Temple Basin on the west and Gregg Basin on the east. Steep-walled Virgin Canyon is 300 m deep to the lake surface and another 100 m to the drowned Colorado River bed, and it marks the incised path of the Colorado River across the first major range of mountains downstream from the Grand Canyon. This north-trending range comprises the White Hills south of Virgin Canyon, the Hiller Mountains north of Virgin Canyon, and in the northern part of the quadrangle north of Jumbo Pass it forms part of the Gold Butte area of the South Virgin Mountains. Field work and map interpretation benefited from NASA airborne surveys in 1997 and 1998 using the Thermal Infrared Multispectral Scanner (TIMS) and MODIS/ASTER Airborne Simulator (MASTER) (Hook and others, 2001a). These systems measure the energy emitted from the surface, which is a function of the temperature and emissivity of the surface. Differences in emissivity can be related to the composition of surface materials. In the case of silicate rocks, the emissivity differences relate to the type of silicate bonds in the rock and can be used to infer the bulk composition of the rock. In sparsely vegetated arid areas like the Hiller Mountains Quadrangle, the TIMS and MASTER data have been used to measure compositional variation (Hook and others, 1994). It proved possible in the Hiller Mountains Quadrangle to use the MASTER data to map quantitatively a close approximation of silica content of the surface materials (Hook and others, 2001b). Processed MASTER images of the Hiller Mountains area show distinct patterns due to differing-composition silicate rock bodies that are not detectable in color air photographs or Landsat multispectral scanner images. Because of ubiquitous rock varnish, some of the rock distinctions that are imaged so well by MASTER are obscure even in the field until the rocks are carefully examined in hand specimen. The MASTER images demonstrate an extraordinary potential for using thermal-infrared multispectral imaging for identifying and mapping complex patterns of silicate rocks in remote arid areas. For this map, we combined geologic fieldwork with interpretation of the NASA imagery, calibrated in spots by geochemical and mineralogic studies of rock units (Hook and others, 2001b). One map unit (quartz-poor gneiss) was mapped mostly from the imagery.