EVOLUTION AND ALLUVIAL ARCHITECTURE OF THE ELDORADO MOUNTAINS, SE NEVADA, USA: OBSERVATIONS FROM REMOTELY-SENSED DATA AND FIELD RECONNAISSANCE

Reconnaissance of soil and alluvial fan deposits along the Eldorado Mountains of Clark County, SE Nevada was conducted through geomorphic survey, field sampling, and analysis of a 7, 4, 2 false-colour Landsat Thematic Mapper image. Located along the eastern edge of the Basin and Range physiographic province, the study area is at the perimeter of the Mogollon Transition Zone with respect to the Colorado Plateau. The Eldorado Valley is bounded by the W-NW-striking transverse of the Lake Mead-Las Vegas Valley Shear Zone, and the Mojave-Sonoran Desert Breakaway Fault. The Eldorado Mountains (~2300 masl high) are a gold-bearing volcanic caldera of Tertiary rhyolites, andesites, ignimbrites, and basalts, cored with older metasediments, gneisses, and various Precambrian granites. Clast lithology counts link the composition of debris flows and the fan surface alluvium to weathering of the bedrock headlands, which are expressed as distinctive tonal variations on the Landsat. Comparative differences in ages of surface units are inferred from mountain-front sinuosity (S), stratigraphic relations, degree of desert varnish on surface reg, soil complexity and carbonate development stage. Younger fans are more active, with lower S and incipient-to-moderately developed soils, the northernmost of which support Artemesia (sage) and other Great Basin flora in addition to Larrea tridentata (creosote bush). The oldest, most stable fan is located along the southeastern end of the mountain-front, and is characterized by a well-developed soil with stage 2-3 carbonates. The older fans (high S) support a relict stand of Mojave (winter-precipitation dominant) floral elements including an undocumented stand of Yucca brevifolia (Joshua tree), in addition to Larrea tridentata, which is a typical Sonoran (summer-monsoonal precipitation dominant) plant. The modern plant distributions within the Eldorado Mountains reflect local controls, as well as palaeoclimatic conditions that have influenced the boundaries of the xeric floralistic provinces across the Great Basin, Sonoran, and Mojave desert regions. Because the Eldorado Mountains are at the edges of the modern phytogeographical range of both Yucca brevifolia and Larrea tridentata, it is an ideal locality for further investigating the recent development of mosaic communities, polyploidism and speciation since the Last Glacial Maximum.