Paleogeography of the Horse Spring Formation in relation to the Lake Mead fault system, Virgin Mountains, Nevada and Arizona

Abstract

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

Importance of groundwater in propagating downward integration of the 6–5 Ma Colorado River system: Geochemistry of springs, travertines, and lacustrine carbonates of the Grand Canyon region over the past 12 Ma

“…9) has peaks at 1.65, 1.42, 1.05 Ga, and 610 and 415 Ma peaks. An absence of young grains is compatible with models for local derivation in alluvial fans derived from adjacent fault blocks (Beard, 1996).…”

A proposed Laramide proto-Grand Canyon

“…This precursor canyon would have begun cutting into the Paleozoic section of the Colorado Plateau as the Basin and Range down-dropped. While Mississippian to Cambrian rock was being carved on the Hualapai Plateau side of the scarp (Young, 1985;Beard, 1996;Young, 2008), the Kaibab, Toroweap, Coconino, Hermit, and Supai Formations were still in existence and being eroded along the north side of the scarp, thus supplying red sand, silt, and evaporites to the sandstone-siltstone-gypsum facies of the Muddy Creek Formation at the mouth of the forming canyon.…”

“…3). Beard (1996) placed the unroofing of the Gold Butte granite at N16 Ma and thought that the entire Paleozoic section was most likely eroded from part of the Gold Butte block prior to extension. Beard (1996) also showed the Shivwits scarp of Young (1985) in a position south of the northern Gold Butte block.…”

“…Beard (1996) placed the unroofing of the Gold Butte granite at N16 Ma and thought that the entire Paleozoic section was most likely eroded from part of the Gold Butte block prior to extension. Beard (1996) also showed the Shivwits scarp of Young (1985) in a position south of the northern Gold Butte block. Therefore, it is not unreasonable that the unroofing of the Cambrian Tapeats Sandstone and Precambrian granite/biotite schist exposed in the northern part of the Gold Butte area could have supplied these clast types eastward to Grassy Mountain (Fig.…”

“…As early as 1978, Young and McKee proposed that a Laramide-age drainage system had flowed northward across successively higher stratigraphic units to exit in the Claron basin of southern Utah, and Young (1982Young ( , 1987 and Beard (1996) showed this drainage as having followed an approximate Hurricane monocline trend northward to Utah. Hill and Ranney (2007) proposed a more specific route for a "Laramide proto-Grand Canyon," basing their model mainly on the tracing of gravels north of the canyon.…”