Report and survey of reconnaissance geology between Lake Mead and Davis Dam Arizona-Nevada, 1963


Report and survey of reconnaissance geology between Lake Mead and Davis Dam Arizona-Nevada, 1963
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Geological survey of the area between Lake Mead and Davis Dam.
Additional Information
Geological Survey Professional Paper 374-E
Between Hoover and Davis dams the Colorado River valley, trending south-southeast, separates the Black Mountains on the east from the Eldorado and Newberry Mountains on the west. For more than 20 miles directly south of Lake Mead the river is deeply incised in Black Canyon : farther south the stream course is in large part between wide alluviated slopes. Maximum relief within the area is about 4,500 feet. Thick Paleozoic and Mesozoic formations found in ranges north and west of Lake Mead are not present south of the lake except as fragmentary xenoliths within igneous bodies. The principal bedrock units in a wide area bordering the Colorado River consist of Precambrian gneiss, schist, and granitic rocks; volcanic rocks in four distinctive series, with intercalated clastic sediments; and numerous plutonic bodies, ranging in size from stocks to small dikes, diverse in composition and in texture. Some plutons, through which the oldest lavas in the area were erupted, may be contemporaneous with tuff beds in Cretaceous formations north of the lake. The Patsy Mine volcanics, oldest of the eruptive series, consist mainly of brown andesitic lavas and agglomerates with maximum thicknesses of several thousand feet. The second episode produced the more siliceous Golden Door volcanics, equally thick and widespread, containing abundant glass and lightcolored tuff. A third series, the Mount Davis volcanics, consists largely of basalt and dark andesite but at several horizons includes conspicuous units of high-silica glass and pumiceous tuff. Thick beds of extremely coarse sedimentary debris, much of it derived from Precambrian bedrock, are interspersed through the upper part of this volcanic formation. The three older volcanic assemblages have been strongly tilted, and commonly they are discordant one on another. All have been cut by intrusive bodies. In the northern part of the area the Muddy Creek formation, made up of clastic sediments, saline beds, and thick basaltic lavas, is unconformable on all older rock units. Still later igneous activity is recorded in local basaltic flows and dikes associated with weakly cemented slope gravels. The area holds a network of faults, some of large displacement and at least two with important reverse movement. The Black Mountains block is a compound horst, westof which repetitive faulting has tilted some blocks eastward, others westward. Considerable movement was in progress during the Mount Davis volcanism, as indicated by abundant coarse debris mingled with lavas near large faults.In Muddy Creek time great landslide masses moved from the rising west margin of the Black Mountains. Colorado River gravel beds of Pleistocene age were strongly deformed near some faults. Evidence now in hand establishes an order of events within the area but not an exact chronology. All the volcanic assemblages appear to be younger than the Horse Spring formation, which is tentatively dated as early Tertiary. A lead-alpha analysis on a sample of quartz monzonite from a pluton intrusive into Golden Door volcanics gave an age of about 50 million years, suggesting that these volcanics and the older Patsy Mine volcanics are at least as old as Eocene. Mount Davis volcanics grade upward into sediments mapped as part of the Muddy Creek formation, now dated tentatively as Pliocene. The Colorado River, as a through-flowing stream in its present course west of the Plateau, originated later than the interior-basin conditions of Muddy Creek time. The river was temporarily impounded late in the Pleistocene epoch, as indicated by numerous remnants of lake deposits that He on and are capped by river gravels.
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United States Government Printing Office, Washington D. C.
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G 4354 N4 C5 1963 L65
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Relations; hln001271
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