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~,. C\J ~ - PHYSIOGRAPHY AND GEOLOGY w C Las Vegas Valley is bounded by the Spring Mountains 10 the west, the Sheep and Las Vegas Ranges to the north, the Frenchman and Sunrise Mountains to the east, and the McCullough Range to the south (fig. I). Piedmont surfaces. representing coalescing alluvial aprons, and pediments form the transition between the steep mountain blocks and the lowlands that make up Las Vegas Valley (Dinger, 1977; Bell, 1981), The valley slopes gently to the east and southeast, toward the Whitney area. and is drained by Las Vegas Wash, Flamingo Wash, Tropicana Wash, and farther south, by Duck Creek (fig. IB). Las Vegas Wash. which empties into Lake Mead, receives the flow from the other three drainages. , _. ..".. Las Vegas Valley is underlain by as much as 5,000 ft of clastic sediments of Miocene to Holocene age. These sediments fill a structural basin that may have resulted from slippage along the Las Vegas Shear Zone to the north (Bell. 1981, p, 13; Plume, 1984, p. 21) and along normal faults that bound the east and possibly the west sides of the valley (Morgan and Dettinger, in press). The Muddy Creek Formation, of Miocene age, probably includes most of the clastic deposits that fill the basin. The thickness of this formation is inferred from welllogs to range from more than 3,000 ft near the Whitney area to less than 500 ft in the northeast part of the valley (Malmberg. 1965, p.21). The Muddy Creek Formation is composed of generally coarse sediments toward the west side of the valley and becomes finer grained toward the east and southeast (Morgan and Dellinger, in press). On the west side of Las Vegas Valley, the formation is dominated by sand, with gravel and thick clay lenses. On the east and southeast sides of the valley, the formation grades into a silty sand with clay lenses and evaporite deposits (Longwell and others, 1965, p. 48; Plume, 1984, pI. 2). These sediments may have been deposited in a lacustrine environment (Bohannon, 1984. p. 56). Younger alluvium overlies the Muddy Creek Formation in mosI of the valley. These deposits, which are of Pleistocene and Holocene age, represent poorly sorted alluvial gravel and sand (Malmberg, 1965, p. 22), that may reach thicknesses of 1,000 ft in the central and western parts of the valley. In the eastern and southeastern parts of the valley, however, these deposits are thin or non-existent where the Muddy Creek Formation is exposed , along flanks of Frenchman Mountain (Domenico and others. 1964, p. 10). These younger alluvial deposits make up the 'principal aquifers within the valley (Plume, 1984, pI. 2). In the Whitney Area, the Muddy Creek Formation is mostly fine-grained and is overlain by an approximately 30-ft section of alluvial-apron and flood-plain deposits (see further discussion in subsequent section titled "Thickness"); the finer deposits are derived primarily from Las Vegas Valley to the west, whereas the coarser deposits (mainly gravel) probably originate in nearby Frenchman Mountain to the north. Drillers' logs for the Whitney area indicate thaI the Muddy Creek Formation is dominated by clay and silt and is relatively impermeable; however, the logs provide no evidence of evaporite deposits. HYDROGEOLOGY Water-level and water-quality data were collected from observation wells at 34 sites in the Whitney area (fig. 4), Several of the sites have two wells, and the cluster-well sites have as many as five wells, screened at discrete intervals vertically through the aquifer. The design, installation, and development of all observation wells in the Whitney area are discussed in detail in a companion report by Emme and Prodic (1991). Properties of the Alluvial Deposits An understanding of the hydrogeologic setting of the Whitney area and the physical and chemical processes that influence the nature and behavior of ground water flowing through the area are important to the understanding of how the dike and slurry wall mighI affect flow and transport in the shallow aquifer. Thus, key hydraulic and physical properties of both the alluvial sediments and the ground water were estimated, -8-