The Tertiary Horse Spring Formation in southeast Nevada records the development of strike-slip and extensional tectonism that formed the Lake Mead left-lateral fault system. Stratigraphic, geochronologic, and mapping studies in the Virgin and south Virgin Mountains clarify the details of that evolution. Exposures of the Horse Spring Formation in the Virgin Mountains area consist of only its lower two members, in ascending order, the Rainbow Gardens and Thumb Members. The Horse Spring deposits are faulted and variably tilted by normal and oblique-slip faults. The Rainbow Gardens Member (ca. 26-18 Ma) was deposited in a broad shallow basin with positive areas to the northwest and southeast; facies changes are gradual and show no influence of active faulting. In contrast, complex facies relations in the Thumb Member are the result of deposition during complex mixed-mode deformation along kinematically linked strike-slip and extensional faults of the Lake Mead fault system, about 16-14 Ma. Tilting of Tertiary and older rocks about horizontal axes apparently occurred late in the extensional episode, at about 14 Ma or later.The Rainbow Gardens Member consists of three units that each represent separate pre-extensional depositional systems. The basal conglomerate is a braid plain or pediment deposited by northeast paleoflow. The middle unit is a mixture of tuffaceous fluvial and lacustrine deposits that are the result of a reversal of drainage to the southwest, into the Rainbow Gardens lake. The change in paleoflow direction is attributed to extensive volcanism to the north that blocked the earlier northeastward flow. The upper unit consists of pedogenically altered, palustrine carbonate rocks that formed during cessation of lacustrine deposition and waning of volcanic activity to the north.A previously unrecognized unconformity that records the onset of faulting separates the Rainbow Gardens Member from the overlying Thumb Member and can be traced throughout the Virgin Mountains area. The unconformity separates the underlying pedogenically altered carbonate rocks from overlying well-bedded lacustrine limestones. Locally the unconformity is marked by conglomerate and megabreccia deposits derived from the underlying Rainbow Gardens carbonate rocks across nearby fault scarps. Carbonate rocks above the unconformity grade laterally and vertically into lacustrine gypsum and fine-grained sandstone of the Thumb Member. These rocks in turn intertongue laterally and vertically with marginal lacustrine and alluvial fan facies. Abrupt influx of megabreccia and coarse conglomerate into lacustrine deposits occurred northward from both the Gold Butte and Lime Ridge faults. At approximately the same time, megabreccia and coarse conglomerate were shed
We applied multiple geochemical tracers ( 87 Sr/ 86 Sr, [Sr], d 13 C, and d 18 O) to waters and carbonates of the lower Colorado River system to evaluate its paleohydrology over the past 12 Ma. Modern springs in Grand Canyon reflect mixing of deeply derived (endogenic) fluids with meteoric (epigenic) recharge. Travertine (<1 Ma) and speleothems (2-4 Ma) yield 87 Sr/ 86 Sr and d 13 C and d 18 O values that overlap with associated water values, providing justification for use of carbonates as a proxy for the waters from which they were deposited. The Hualapai Limestone (12-6 Ma) and Bouse Formation (5.6-4.8 Ma) record paleohydrology immediately prior to and during integration of the Colorado River. The Hualapai Limestone was deposited from 12 Ma (new ash age) to 6 Ma; carbonates thicken eastward to ~210 m toward the Grand Wash fault, suggesting that deposition was synchronous with fault slip. A fanningdip geometry is suggested by correlation of ashes between subbasins using tephrochronology. New detrital-zircon ages are consistent with the "Muddy Creek constraint," which posits that Grand Wash Trough was internally drained prior to 6 Ma, with limited or no Colorado Plateau detritus, and that Grand Wash basin was sedimentologically distinct from Gregg and Temple basins until after 6 Ma. New isotopic data from Hualapai Limestone of Grand Wash basin show values and ranges of 87 Sr/ 86 Sr, d 13 C, and d 18 O that are similar to Grand Canyon springs and travertines, suggesting a long-lived springfed lake/marsh system sourced from western Colorado Plateau groundwater. Progressive up-section decrease in 87 Sr/ 86 Sr and d 13 C and increase in d 18 O in the uppermost 50 m of the Hualapai Limestone indicate an increase in meteoric water relative to endogenic inputs, which we interpret to record progressively increased input of high-elevation Colorado Plateau groundwater from ca. 8 to 6 Ma. Grand Wash, Hualapai, Gregg, and Temple basins, although potentially connected by groundwater, were hydrochemically distinct basins before ca. 6 Ma. The 87 Sr/ 86 Sr, d 13 C, and d 18 O chemostratigraphic trends are compatible with a model for downward integration of Hualapai basins by groundwater sapping and lake spillover.The Bouse Limestone (5.6-4.8 Ma) was also deposited in several hydrochemically distinct basins separated by bedrock divides. Northern Bouse basins (Cottonwood, Mojave, Havasu) have carbonate chemistry that is nonmarine. The 87 Sr/ 86 Sr data suggest that water in these basins was derived from mixing of high-87 Sr/ 86 Sr Lake Hualapai waters with lower-87 Sr/ 86 Sr, first-arriving "Colorado River" waters. Covariation trends of d 13 C and d 18 O suggest that newly integrated Grand Wash, Gregg, and Temple basin waters were integrated downward to the Cottonwood and Mojave basins at ca. 5-6 Ma. Southern, potentially younger Bouse basins are distinct hydrochemically from each other, which suggests incomplete mixing during continued downward integration of internally drained basins. Bouse carbonates display a southward trend ...
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