Scientific Investigations Report 2007–5261
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5261
By Donald S. Sweetkind, Lari A. Knochenmus, David A. Ponce, Alan R. Wallace, Daniel S. Scheirer, Janet T. Watt, and Russell W. Plume, U.S. Geological Survey
A hydrogeologic framework defines the physical geometry and rock types in the subsurface through which water flows. A variety of geologic and geophysical approaches have been used to improve the understanding of the hydrogeologic framework of the study area. Geologic map units and structures were compiled from digital versions of the Nevada (Stewart and Carlson, 1978; Raines and others, 2003) and Utah (Hintze and others, 2000) 1:500,000-scale State geologic maps. Drilling records and accompanying geophysical logs for oil and gas wells and exploration wells also were evaluated to understand down-hole lithology and stratigraphy, to estimate relative permeabilities of different rock types, and to augment the regional hydrogeologic framework. The new geologic data were integrated with existing information to develop a generalized hydrogeologic map (pl. 1) that portrays the configuration of rock units in the study area. The geologic units were grouped into hydrogeologic units (HGUs)—rock units that have reasonably similar hydrologic properties. HGU designations were based on lithologic, stratigraphic, and structural characteristics from published descriptions and from data collected during field mapping as part of the study. A generalized stratigraphic column and corresponding hydrogeologic unit designation for the study area are shown in figure 3.
Surface geophysical techniques were applied to take advantage of characteristic density, magnetic, electrical, and acoustic properties of different rocks in a way that provides additional insight into the subsurface geology. Detailed gravity, magnetic, electromagnetic, and seismic geophysical data (fig. 4) are used to identify faults, subsurface structure, and the interconnectivity of adjacent basins. The results of most of the geophysical investigations conducted for the BARCAS study are presented in Watt and Ponce (2007).
The geologic history of the eastern part of Nevada is preserved in rocks and geologic structures that span more than a billion years, ranging from Precambrian sedimentary rocks to widespread Quaternary alluvial deposits and active faults. The geologic framework that has resulted from the geologic events during this time profoundly affects ground-water flow. Thus, any water-resource assessment of the area must take into account the complex geologic history and consider the distribution of the diverse rocks types and geologic environments.
The geologic evolution of the study area since the end of Precambrian time may be subdivided into three general phases (Levy and Christie-Blick, 1989): (1) a late Precambrian to middle Paleozoic interval when dominantly marine sediments were deposited along a passive continental margin; (2) late Devonian to Eocene crustal shortening, compressive deformation, and changes in sedimentation patterns related to the accretion of exotic terrains along the western continental margin in western Nevada; and (3) middle to late Cenozoic extension, faulting, volcanism, and continental sedimentation. Within the context of this three-phase evolution, numerous tectonic events and accompanying changes in sedimentation patterns and igneous activity have occurred throughout geologic time in the study area (fig. 5). These tectonic-induced events have been summarized by De Courten (2003).
During the first phase of geologic evolution, from late Precambrian until middle Devonian time, the rocks in east-central Nevada were deposited in shallow to deep marine water in a stable continental shelf environment similar to that of modern-day Atlantic and Gulf Coast margins of the United States (Blakely, 1997; available at http://jan.ucc.nau.edu/~rcb7/paleogeogwus.html). The stable shelf environment produced thick and laterally extensive carbonate, quartzite, and shale deposits. Most of the widespread units of the older Paleozoic limestone and dolomite rocks (hydrogeologic unit LCU, pl. 1) were deposited in shallow water on a broad, stable continental shelf, known as a “carbonate platform” (Jackson, 1997; Cook and Corboy, 2004). To the west of the study area, correlative rocks were deposited on a gently sloping submarine surface that gradually deepened seaward of the platform (fig. 6). Sedimentary rocks accumulated to thicknesses of about 30,000 ft during this time (Kellogg, 1963; Stewart and Poole, 1974) and form the vast majority of the consolidated rocks exposed in the study area. These limestone and dolomite rocks have long been recognized as an aquifer in the Great Basin (Winograd and Thordarson, 1975; Bedinger and others, 1989; Dettinger and others, 1995; Harrill and Prudic, 1998). These rocks typically consist of an upper Precambrian and Lower Cambrian section of quartzite and shale, a Middle Cambrian to Lower Ordovician limestone section, a distinctive Middle Ordovician quartzite, and an Upper Ordovician to Middle Devonian dolomite section (Kellogg, 1963; Poole and others, 1992) (fig. 3).
From late Devonian to Eocene time, during the second major geologic phase of evolution, several episodes of east-directed compressive deformation primarily affected the central and western parts of Nevada and also influenced rocks in the study area (fig. 5). A Late Devonian to Early Mississippian compressive event, known as the Antler orogeny, interrupted carbonate sedimentation and resulted in the deposition of a thick sequence of siliciclastic rocks (Poole and Sandberg, 1977). Carbonate-shelf sedimentation resumed in Pennsylvanian and Permian time, creating a thick, widespread carbonate sequence in the study area. A late Jurassic through earliest Tertiary compressive event called the Sevier orogeny (fig. 5) resulted in the formation of regional-scale folds in the study area (Armstrong, 1968).
Starting in the middle to late Eocene through the remainder of the Tertiary period, extensional uplift and faulting, volcanism, and continental sedimentation characterized the third phase of geologic evolution in the study area (fig. 5)and adjacent areas in northern and eastern Nevada. During this time, modern basin-and-range landforms were created as a result of motion along both gently dipping and relatively high-angle faults, causing the relative rising of the ranges and sinking of adjacent basins. Generally accompanying the regional extension was the eruption of relatively large volumes of volcanic rocks, particularly ash-flow tuffs that were deposited by caldera-forming eruptions during the Tertiary (Best and others, 1989). Caldera-forming eruptions from two major centers, the Indian Peak caldera complex and the Central Nevada caldera complex (pl. 1) resulted in deposition of volcanic rocks that extend over parts of Nevada and Utah. Following Tertiary volcanism, unconsolidated sediments were deposited in the intermontane basins of the study area during the late Tertiary and Quaternary. These sedimentary deposits include Pliocene to Pleistocene fine-grained lake sediments (Reheis, 1999), and Quaternary stream and alluvial-fan sediments of sand and gravel deposited along the basin margins, and changing to finer grained silt and clay sediments within playas along basin axes.
East-central Nevada features structural domains that vary in style and intensity of deformation (Gans and Miller, 1983; Smith and others, 1991; Dettinger and Schaefer, 1996). Three principal structural domains are evident in the study area—compressional, extensional, and transverse (pl. 1). Compressional and extensional domains generally alternate spatially in the study area; for example, compressional domains represented by regional thrust belts or folds alternate with extensional domains of normal-faulted, highly attenuated stratigraphic sections (Gans and Miller, 1983). Transverse zones are regional scale, east-west structural alignments that generally are perpendicular to the regional north-south alignment of mountain ranges and valleys. Prominent structural features in the study area, including compressional thrust belts, large-magnitude extensional normal and detachment faults, and transverse zones, are shown on plate 1.
The only significant manifestation of the Mesozoic Sevier orogenic belt within the study area are two broad regional synclines, or downfolds, termed the Butte and Confusion Range synclinoria (Hose, 1977). These large folds are characterized by broadly sinuous but generally north-trending fold axes that preserve Triassic rocks and the entire underlying Paleozoic carbonate-rock section (pl. 1). The Butte synclinorium is present in the Maverick Springs Range and Butte Mountains, the central part of the Egan Range and the southern part of the Schell Creek Range (section A-A', pl. 1); the Confusion Range synclinorium is present in the Needle and Confusion Ranges of western Utah (section B-B', pl. 1).
During Cenozoic time, north-south aligned mountain ranges of carbonate, siliciclastic, or metamorphic rocks were formed in the study area by episodes of structural extension. Structural extension was not uniform across the study area, but was segmented into domains of either large-magnitude or relatively minor amounts of extension. Each domain generally is represented by specific HGUs that influence regional ground-water flow. The highly extended domains often have uplifted Precambrian to Cambrian siliciclastic rocks or metamorphic rocks of low permeability at or near the surface; whereas less-extended domains tend to preserve the entire thickness of Paleozoic carbonate rocks of higher permeability (pl. 1). Dettinger and Schaefer (1996) compared the structural setting and distribution of Paleozoic carbonate rocks with the location of regional ground-water flow systems within the carbonate-rock province. The two major ground-water flow systems in the study area, the Great Salt Lake Desert and the Colorado regional flow systems (fig. 1) were shown to correspond to areas with thick sections of Paleozoic carbonate rocks in parts of the study area that had been extended only slightly. However, the low-permeability siliciclastic rocks typically found in highly extended domains appear to completely disrupt carbonate-rock aquifer continuity resulting in ground-water flow systems of limited lateral extent.
Within highly extended domains, extension was accomplished along gently to moderately dipping, large-offset extensional detachment faults. For example, in the northern Snake Range, an abrupt, gently dipping detachment fault brings low permeability granitic rocks and ductilely deformed and metamorphosed Cambrian and Precambrian quartzite, marble and pelitic schist to the surface (fig. 7; Miller and others, 1983). Based on seismic reflection data, interpretive cross sections suggest that the moderately dipping detachment fault dips beneath Snake Valley (section B-B', pl. 1) and beneath the Confusion Range to the east of the northern and southern Snake Range. Similar structures that bring low-permeability rocks to the surface exist in the southern Grant Range in northern Nye County (pl. 1) (Kleinhampl and Ziony, 1984; Lund and others, 1993) in the northern Egan and southern Cherry Creek Ranges (Armstrong, 1972; Gans and Miller, 1983) (section A-A', pl. 1), and the Schell Creek Range (Dechert, 1967; Drewes, 1967; Armstrong, 1972).
A second style of Tertiary extension is characterized by steeply dipping, range-bounding normal faults that produced elongate mountain ranges and have controlled the subsidence of intervening, down-faulted valleys (Zoback and others, 1981; Stewart, 1998). The range-bounding faults strike northeast and have displacements of several thousands of feet, typically juxtaposing the consolidated rocks within the range blocks against Cenozoic basin fill (Kleinhampl and Ziony, 1984). Basins commonly have a half-graben form in which the basin fill and basin floor are tilted toward a major fault on one side of the basin; this fault accommodates much of the extensional deformation and subsidence, producing a tilted, asymmetric basin (Stewart, 1998). Less commonly, basins have the form of a symmetric graben, with major faults bounding both sides of the basin. Symmetric grabens typically are located along the valley axis, with shallow pediments on either side. The general relation between extensional range-bounding faults and resulting asymmetric or symmetric grabens is annotated on section C-C' shown on plate 1. Geophysical data show that basins in the study area vary in their complexity of faulting and relative development (Saltus and Jachens, 1995; Dohrenwend and others, 1996). For example, in White River Valley, along the western part of seismic line ECN-01 (section C-C' pl. 1), there are three east-dipping half-grabens increasing in size from west to east. These half-grabens are largely buried and are not evident from surface topography or bedrock outcrops. In contrast, Cave Valley is a single east‑dipping half-graben, where the floor of the graben mimics the dip of the Paleozoic rocks on the west side of the basin and a steeply dipping fault zone bounds its eastern edge.
Regional gravity data were used to assess the thickness of the Cenozoic basin-fill deposits (fig. 8). Cross sections that incorporate the geophysical data portray the three-dimensional shape of pre-Cenozoic basement, the location of major basin-bounding structures, and the presence of significant intrabasin faults (fig. 9). Typical thicknesses of the basin fill range from 0.3 to 0.9 mi; maximum thicknesses of basin fill range from about 1 mi to more than 3 mi (fig. 8). With the exception of Steptoe Valley in the north, basins in the southern part of the study area contain thicker basin-fill deposits than basins in the northern part of the study area.
Gravity-derived models of pre-Cenozoic bedrock, integrated with seismic, aeromagnetic, and drilling data, indicate that many of the basins in the study area contain buried bedrock highs (sections C-C' and F-F', fig. 9). These bedrock highs represent intrabasin divides that separate most basins into two or more subbasins (fig. 8); geologically, they are referred to as accommodation zones that developed in response to differential extension or tilting in different parts of the basin. In selected cases where the intrabasin divides are particularly shallow or distinctly separate deeper basins, these locations were chosen to subdivide hydrographic areas into subbasins (fig. 2). Subbasins do not necessarily represent individual ground-water basins, but merely areas separated by intrabasin divides where pre-Cenozoic bedrock has been uplifted and overlying basin-fill deposits are relatively thin.
Transverse zones (Faulds and Varga, 1998) generally are regional scale, east-west-trending features that have been previously identified in the study area (Ekren and others, 1976; Rowley, 1998). Transverse zones segment subbasins, hydrographic areas, or larger regions into areas of different types, rates, or relative amounts of extension. Transverse zones commonly are oriented at a high angle to the long axes of current basins and ranges and, as a result, may influence the rate or direction of ground water flowing parallel to valley axes. The influence of such zones on ground-water flow patterns is largely unknown.
Hydrogeologic units (HGUs) are defined as having considerable lateral extent and similar physical characteristics that may be used to infer their capacity to transmit water. Material properties of basin fill and consolidated rock, therefore, were used as indicators of primary and secondary permeability, such as grain size and sorting, degree of compaction, rock lithology and competency, degree of fracturing, and extent of solution caverns or karstification.
The consolidated pre-Cenozoic rocks, Cenozoic sediments, and igneous rocks of the study area are subdivided into 11 HGUs (table 1; fig. 3). Pre-Cenozoic rocks and older Cenozoic rocks were classified as consolidated rocks (commonly referred to as bedrock) that may consist of limestone, dolomite, sandstone, siltstone, and shale. Consolidated pre-Cenozoic rocks are subdivided into HGUs based primarily on the degree to which the rocks fracture and, in the case of limestones and dolomites, the presence of solution openings. Proterozoic to Early Cambrian metamorphic and siliciclastic rocks, and Paleozoic siliciclastic rocks typically form the least permeable HGU within the consolidated, pre-Cenozoic rocks. Paleozoic carbonate rocks typically form the most permeable HGUs within the pre-Cenozoic consolidated rocks. These carbonate rocks extend throughout much of the subsurface in western Utah, central and southern Nevada, and eastern California (Dettinger, 1989; Harrill and Prudic, 1998), and crop out in many of the mountain ranges in the study area (pl. 1). Younger Cenozoic sediments were classified as basin-fill deposits that may consist of unconsolidated granular material such as sand, gravel, and clay. The unconsolidated Cenozoic basin fill is subdivided into HGUs based on grain size and sorting. Igneous rocks are subdivided on the degree to which the rocks fracture and, for the volcanic rocks, on the presence or absence of soft ashy material.
The pre-Cenozoic sedimentary rocks of the study area are grouped into five HGUs: the lower siliciclastic-rock unit (LSCU), the lower carbonate-rock unit (LCU), the upper siliciclastic-rock unit (USCU), the upper carbonate-rock unit (UCU), and the Mesozoic sedimentary rock unit (MSU). This usage is similar to that established by Winograd and Thordarson (1975).
The lower siliciclastic-rock unit (LSCU) includes the oldest exposed sedimentary rocks in the study area, including the upper Precambrian McCoy Creek Group, which consists of more than 9,000 ft of siliceous and argillaceous metasediments and the Lower Cambrian Prospect Mountain Quartzite, which is as much as 4,500 ft thick of predominantly quartz-rich sandstone (fig. 10; Hose and others, 1976). Rocks of the LSCU are exposed in the Cherry Creek Range, the northern part of the Egan Range, the Schell Creek Range, and the Snake Range (pl. 1 and fig. 10). Schists and marbles also are included in the LSCU, and these rocks form, in part, the lower plates of major extensional detachment faults in the Snake and Schell Creek Ranges.
The LSCU generally has low permeability throughout the eastern Great Basin (Winograd and Thordarson, 1975; Plume, 1996). Sandstones of the LSCU commonly are highly cemented, filling much of the original pore volume, and are overlain and underlain by a significant thickness of fine-grained shales, all of which contribute to the overall low permeability of this HGU. At shallow depths, rocks of the LSCU commonly are highly fractured (fig. 10) and can support small volumes of flow, such as at Strawberry Creek in the northeastern part of Great Basin National Park (Elliott and others, 2006). Schists and marbles of the LSCU that typically have schistose foliation lack a continuous fracture network. Based on the low permeability and capacity to transmit water, the top of the LSCU, for purposes of this report, represents the base of the ground-water flow.
The LCU represents a significant volume of carbonate rock that is prominently exposed in the mountain ranges in the study area (pl. 1), and is present beneath many of the valleys. The LCU includes Cambrian through Devonian limestones and dolomites with relatively minor interbedded siliciclastic rocks. A representative stratigraphic succession of the LCU in the study area typically consists of the following units, from lower (older) in the succession, to higher (younger) in the succession: a Middle Cambrian to Lower Ordovician limestone, silty limestone, siltstone, and shale section, a distinctive Middle Ordovician Eureka quartzite, an Upper Ordovician through Middle Devonian dolomite, and a limestone and minor dolomite of the Middle and Upper Devonian Guilmette Formation (fig. 11) (Kellogg, 1963; Poole and others, 1992).
The LCU, along with the carbonate-rock units of the UCU, forms a major high-permeability consolidated-rock unit in the Great Basin (Winograd and Thordarson, 1975; Bedinger and others, 1989; Dettinger and others, 1995; Harrill and Prudic, 1998). Carbonate rocks of the LCU and UCU have three distinct types of porosity that influence permeability and associated storage and movement of ground water—primary or intergranular porosity, fracture porosity, and vug or solution porosity. Lower Paleozoic carbonate rocks from southern Nevada have relatively low primary porosity (Winograd and Thordarson, 1975). Studies of ground-water flow within the carbonate-rock province (Winograd and Pearson, 1976; Dettinger and others, 1995; Harrill and Prudic, 1998) have continued to emphasize correspondence of faults and broad structural belts with zones of high transmissivity, presumably the result of the formation of fractures during deformation. Moreover, in their analyses of hydraulic property estimates for rocks equivalent to the LCU and UCU in the carbonate-rock province, Belcher and others (2001) concluded that extensive faulting and karst development significantly enhanced hydraulic conductivity. Fracture permeability may be enhanced if vertical fractures intersect horizontal fractures, creating a well-connected network of openings through which water can move. In addition, water can dissolve carbonate rocks to form solution openings that create additional pathways. For example, as a result of periodic declines in sea level during Paleozoic time, extensive areas of carbonate rock in east-central Nevada were exposed to the air and subsequent erosion. These intervals of erosion are represented in the sedimentary record as unconformities (fig. 6)—relatively long gaps in time when the carbonate platform was above sea level and conditions were favorable for erosion, dissolution, and development of solution caverns in the exposed carbonate rocks.
The Paleozoic carbonate rocks of the LCU are overlain by a sequence of Mississippian mudstone, siltstone, sandstone, and conglomerates that form the upper siliciclastic-rock unit (USCU). These rocks were formed by the muddy and sandy sediment influxes associated with the Antler orogenic event and are represented by rocks of the Mississippian Chainman Shale, Diamond Peak Formation, and Scotty Wash Quartzite. This succession of sedimentary rocks is widely distributed across the study area and, where not structurally thinned, generally ranges in thickness from 1,000 to greater than 3,000 ft (Hose and others, 1976).
The shaly siliciclastic rocks of the USCU are fine grained and of low permeability. Because of their low susceptibility to dissolution or fracturing, the USCU also lacks significant secondary permeability. The shaly rocks of the USCU yield in a ductile manner when deformed and deformation does not result in significant fracture openings through which water can flow. For example, in southern Nevada, steep hydraulic gradients at the Nevada Test Site are attributed to the low permeability of the Mississippian siliciclastic rocks (Winograd and Thordarson, 1975; D’Agnese and others, 1997); similar properties are expected for these rocks in the study area. The low porosity of the Chainman Shale in the study area has been tabulated (Plume, 1996) from data from oil and gas exploration wells. In the western part of the study area where the Chainman Shale grades laterally and upward into the coarser conglomeratic rocks of the Diamond Peak Formation, a number of exploration wells have penetrated this unit.
The upper carbonate-rock unit (UCU) consists of thick, widespread Pennsylvanian and Permian rocks that overlie the Mississippian rocks of the USCU. In the western and eastern parts of the study area that were less disturbed by subsequent structural extension, upper Paleozoic rocks dominate outcrops in ranges and at interbasin divides (pl. 1). Within these areas, the UCU includes as much as 4,000 ft of Ely Limestone and approximately 2,500 ft of Arcturus Group limestones and silty limestones (Hose and others, 1976). The UCU and LCU possess similar secondary fracture and solution permeability and, as a result, the UCU potentially is an important conduit for recharge and interbasin ground-water flow through ranges in the northwest part of White Pine County, in the central part of the Egan and Schell Creek Ranges, and in the Confusion Range in western Utah.
The Mesozoic sedimentary rock unit (MSU) is preserved in the cores of down-folded regional synclines and, therefore, is exposed only in isolated patches throughout the study area (pl. 1). Triassic rocks of the MSU consist of interbedded siltstone and limestone (Hose and others, 1976) that typically are relatively thin in exposure, about 150 ft thick in the Butte Mountains and slightly thicker in western Utah. Equivalent MSU rocks on the Colorado Plateau, southeast of the study area, are relatively permeable, but most exposures of the MSU in the study area are too small in lateral extent and shallow to be significant conduits for ground-water flow.
The Cenozoic sediments of the study area are grouped into three HGUs: the consolidated older sedimentary rock unit (OSU), and two unconsolidated units, the coarse-grained younger sedimentary rock unit (CYSU) and fine-grained younger sedimentary rock unit (FYSU) (table 1; fig. 3). The occurrence and lithologic characteristics of Cenozoic basin-fill deposits in the study area are summarized in table 2. Characteristics of the basin-fill deposits are described in terms of the abundance and type of volcanic rocks within the basin, and the presence or absence of sedimentary rocks or Pleistocene lake deposits (Reheis, 1999). Inferences regarding the character of the basin-fill deposits are made on the basis of surrounding geologic outcrops, information from oil and gas exploration wells (Hess and others, 2004), aeromagnetic data, and seismic data.
Consolidated Cenozoic basin-fill rocks of the older sedimentary rock unit (OSU) range from late Eocene to Miocene in age and generally underlie the more recent basin-fill deposits. Eocene OSU rocks include fluvial and lacustrine limestone, sandstone, siltstone, and conglomerate and have only minor volcanogenic components compared with younger basin-filling rocks (fig. 12). Unlike the older Eocene rocks, Oligocene OSU rocks contain a major volcanogenic component, including relatively thin and areally restricted fluvial and lacustrine tuffaceous limestone, sandstone, and siltstone that are interbedded with volcanic tuff and ash (Stewart, 1980). Miocene to Pliocene OSU rocks contain coarse sandstone and conglomerate, volcanic-rich sediment, lacustrine sediments, and tectonic landslide or megabreccia deposits (fig. 12). These deposits formed during synextensional faulting and uplift in the study area (fig. 5) that resulted in a characteristically tilted and highly faulted heterogeneous assemblage of rocks (fig. 13). Examples of such synextensional basins include the sedimentary rocks in the Sacramento Pass area (Gans and Miller, 1983; Miller and others, 1999) between the northern and southern parts of the Snake Range, and the Horse Camp Formation in the northern part of the Grant Range and in Railroad Valley (Moores, 1968; Moores and others, 1968).
Analysis of rocks from southern Nevada that are similar to the OSU suggests that these consolidated rocks have significantly lower permeability than the overlying unconsolidated basin-fill deposits (Belcher and others, 2001) and could function as a low-permeability barrier between the overlying younger basin-fill and the underlying higher permeability pre-Cenozoic carbonate rocks. However, outcrops of Miocene and Pliocene OSU rocks are not widespread, and probably were never thick. As a result, the lower permeability of this unit likely has minimal influence as a barrier to ground-water flow.
Holocene to Pliocene alluvium, colluvium and, in some valleys, fluvial deposits (Plume, 1996) form the unconsolidated coarse-grained younger sedimentary rock unit (CYSU). In general, these deposits predominantly consist of sandy gravel with interbedded gravelly sand, and sand. Where deposited as alluvial fans, the grain size of the CYSU gradually decreases from proximal to distal parts of the fan (Plume, 1996). Sediments of the CYSU are not commonly cemented, but are increasingly indurated with depth. These deposits, though discontinuous, are permeable aquifers, particularly alluvial fan and stream channel deposits (Belcher and others, 2001). However, in some areas, CYSU deposits may contain intercalated, less permeable finer grained sediments or volcanic ash. The fine-grained younger sedimentary rock unit (FYSU) consists of unconsolidated Holocene to Pliocene fine-grained playa and lake deposits that are widespread throughout the study area (Stewart, 1980). FYSU sediments were deposited along basin axes and, as a result, typically are mixtures of moderately to well stratified fine sand, silt, and clay of relatively low permeability and limited capacity to transmit water. Pliocene lacustrine and fluvial deposits consist of freshwater limestone, tuffaceous sandstone and siltstone, laminated clays, and water-lain tuffs and ash that include the Panaca and Muddy Creek Formations, and the White River lakebeds (Tschanz and Pampeyan, 1970). These deposits were formed by Quaternary lakes, such as Pleistocene Lake Bonneville and more local lakes in Antelope, Spring, Lake, Cave, and Jakes Valleys (Reheis, 1999).
Igneous rocks in the study area consist of plutonic rocks and volcanic deposits that may be grouped into three primary HGUs—the intrusive rock unit (IU), volcanic tuff unit (VTU), and the volcanic flow unit (VFU) (table 1; fig. 3). The IU includes all Mesozoic and Cenozoic granitic plutonic rocks in the study area. The exposed or concealed plutonic rocks, typically granitic, are widely scattered, but most occur in the east and northeast parts of the study area (pl. 1). Geologic and aeromagnetic data indicate that plutonic rocks locally intrude the carbonate-rock units (LCU and UCU). Depending on how deeply the plutons are buried, granitic rocks may influence ground-water flow direction or magnitudes. Although small quantities of water may pass through these intrusive crystalline rocks where fractures or weathered zones exist, fractures in the IU typically are poorly connected. Where studied elsewhere, these rocks often impede ground-water flow (Winograd and Thordarson, 1975).
Volcanic rocks in the study area were divided into two principal HGUs (fig. 3), the volcanic tuff unit (VTU) and the volcanic flow unit (VFU). The use of these two HGUs follows the subdivision of volcanic rocks typically used on the State geologic maps. Rocks of the VTU include welded and nonwelded tuffaceous units of rhyolite-to-andesite composition; rocks of the VFU include basalt, andesite, and rhyolite lava flows. Relatively thick exposures of ash-flow tuffs occur in the southern and western parts of the study area (fig. 14), and these deposits also may be preserved in many of the intermontane valleys of the study area. The middle Tertiary volcanic rocks of east-central Nevada also include lavas and associated deposits that are a significant, though not especially voluminous, part of the geologic framework of this area.
In the southern parts of the study area, volcanic rocks, particularly densely welded tuffs of the VTU, are relatively thick and permeable over a considerable area. The thickness of the VTU is estimated to be greatest in the intra-caldera source areas for widely distributed ash-flow tuffs, such as in the Indian Peak caldera complex and in the Central Nevada caldera complex (fig. 14). In the northern half of the study area, the thickness of VTU is estimated to be relatively minor. Estimates of VTU thickness are based on an evaluation of volcanic rocks potentially preserved in down-faulted, Cenozoic graben valleys of east-central Nevada and west-central Utah. Fractured rhyolite-lava flows and moderately to densely welded ash-flow tuffs are the principal volcanic-rock aquifers. Rhyolite-lava flows (VFU) are laterally restricted, whereas welded ash-flow tuff sheets (VTU) are more widely distributed and may constitute a laterally continuous aquifer.
The hydrogeologic units in the BARCAS study area form three distinct aquifer systems composed of alternating more permeable and less permeable units. The three general types of aquifer materials are: basin-fill alluvium (CYSU), some volcanic rocks (VTU), and carbonate bedrock (LCU and UCU). Each of these units may include one or more water-bearing zones but are stratigraphically and structurally heterogeneous resulting in a highly variable ability to store and transmit water. The intervening lower permeability units, FYSU, OSU, VFU, USCU, and LSCU, separate the three aquifer systems.
The basin-fill aquifer occurs in each hydrographic area and subbasin, extending across most intrabasin divides and some hydrographic area boundaries. The lateral extent of the HGUs that form this aquifer vary, but in most basins, the coarser grained CYSU deposits occur near the mountain front and along drainages, the finer grained FYSU occur along valley axes. The consolidated OSU deposits typically underlie these younger basin-fill deposits and, in the southern part of the study area, contain significant quantities of volcanic ash and tuff. The volcanic aquifer primarily occurs in the western and southern parts of the study area, extending laterally beneath the basin-fill aquifer and multiple hydrographic areas. Of the two HGUs that form this aquifer, the lateral extent and thickness of the VTU typically are greater than that of the VFU. The carbonate aquifer is the most laterally extensive aquifer in the study area, underlying the basin-fill or volcanic aquifers in most hydrographic areas. The upper part of this aquifer is composed of UCU rocks and the lower part is composed of LCU rocks; these HGUs are separated by rocks of the USCU. Rocks of the LSCU underlie the carbonate aquifer. MSU and IU rocks are widely scattered throughout the study area, and generally are of lower permeability and limited aerial extent. However, depending on the depth of IU plutons, these rocks may intrude into overlying carbonate, volcanic, or basin-fill aquifers and influence the direction or magnitude of ground-water flow.
Relative differences in hydraulic properties were used to delineate aquifers from confining or semi-confining HGUs in the BARCAS study area. These evaluations primarily were based on relative differences in permeability determined from HGU material properties, or on estimates of hydraulic conductivity, a quantitatively derived parameter that serves as a measure of permeability (Todd, 1980). For the BARCAS study, differences in hydraulic conductivity also were used, in part, to evaluate the potential for ground-water flow across hydrographic area boundaries and intrabasin divides. Differences in hydraulic properties along these boundaries and divides typically are the result of structural disruption that may cause, for example, the juxtaposition of aquifers and lower permeability units, or uplifted bedrock areas where the saturated thickness of overlying aquifers is thinned. Relative differences in hydraulic conductivity of HGUs and the distribution of these HGUs along boundaries and divides are, therefore, important controls on intrabasin and interbasin ground-water flow.
Hydraulic properties can be highly non-uniform in many aquifer systems. Hydraulic conductivity is scale dependent and is affected by fracturing and chemical dissolution in the case of carbonate rocks. Consolidated rocks generally have a wider range of hydraulic conductivity compared to unconsolidated sediments. Estimates of hydraulic conductivity frequently are determined from aquifer tests in wells or boreholes. In fractured rock, at small scales on the order of inches to feet, contrasts in hydraulic conductivity result from the presence or absence of fractures. At larger scales, on the order of tens to hundreds of feet, contrasts in hydraulic conductivity arise from differences between zones of numerous, open, well-connected fractures and zones of sparse, tight, poorly connected fractures. Methods used to analyze aquifer tests that rely on simplifying assumptions is an additional complication. Violations of these assumptions may result in erroneous estimates for computed hydraulic properties (Belcher and others, 2001). Few aquifer tests have been completed in the study area and thus estimates of hydraulic properties are sparse. Because of limited data for the study area, estimates of hydraulic properties were compiled from aquifer tests in the Death Valley regional ground-water flow system (DVRFS; fig. 1; Belcher and others, 2001). Hydraulic properties for the DVRFS are considered to be representative of hydraulic properties in the study area because of similar rock types and HGUs (table 1).
Horizontal hydraulic conductivity (hereinafter referred to as hydraulic conductivity) values were grouped by HGU and statistically evaluated to determine the central tendency and range of values. Descriptive statistics, including the arithmetic and geometric means, median, and range of hydraulic conductivity for each HGU are shown in table 3. The arithmetic mean is the average value within the sampled dataset. The geometric mean is the mean of the logarithms, transformed back to their original units, and commonly is used for positively skewed data (Helsel and Hirsch, 1992). The hydraulic conductivity was calculated by dividing estimates of aquifer transmissivity by the total saturated thickness of the aquifer material tested.
For the study area, the hydraulic conductivity for an HGU can span three to nine orders of magnitude. Carbonate and volcanic rocks typically are aquifers in the study area, however, where fractures and dissolution are largely non‑existent, they are confining units. Grain size and sorting are important influences on hydraulic conductivity of the unconsolidated sediments (Belcher and others, 2001). The largest hydraulic conductivity values are associated with CYSU, VTU, UCU, and LCU. The arithmetic and geometric means are greater than or equal to 40 and 1 ft/d, respectively. The mean hydraulic conductivity of the VFU is an order of magnitude less than that for the VTU; whereas the geometric means only differ by a factor of 8 (table 3). The geometric mean of the hydraulic conductivity values of the MSU overlying the carbonate-rock aquifer, the USCU separating the upper and lower carbonate-rock aquifers, and the LSCU that underlies the carbonate-rock aquifer are a minimum of three orders of magnitude smaller than their adjacent aquifers; the LSCU that underlies the carbonate-rock aquifer has the lowest value (2.0 × 10-6 ft/d). The relatively greater hydraulic conductivity values for the FYSU, OSU, and VFU (values between those for aquifers and the aforementioned confining units) indicate that these HGUs may be semi-confining units. In some areas, these semi-confining units may be fractured to a sufficient degree to transmit water, although typically these units are not fractured and tend to retard ground-water flow.
The hydraulic connection of aquifers and confining units across HA boundaries and intrabasin divides is a principal control on interbasin and intrabasin ground-water flow in the study area. The occurrence and juxtaposition of aquifers and confining units in these areas must be understood to assess the geologic controls on the relative potential for ground-water flow across these boundaries and divides. For example, ground-water flow across HA or subbasin boundaries may not be possible if one or more permeable HGUs are not present, or may not be likely if the hydraulic conductivity of juxtaposed aquifers and confining units is relatively low.
To assess the geologic controls on the potential for ground-water flow across HA boundaries and intrabasin divides, the stratigraphic and structural features described previously were integrated with subsurface geophysical data to categorize rocks into 1 of 10 general subsurface boundary conditions that are likely to result in differing ground-water flow characteristics. Each boundary condition represents the likely influence of one or more HGUs or structural conditions on ground-water flow along or across HA or intrabasin divides. The evaluation of boundary conditions primarily is based on the interpreted presence, juxtaposition, and average hydraulic properties of specific HGUs; degree of structural disruption is considered an important but secondary control. Each HA boundary and intrabasin divide was represented as a vertical, irregularly bending cross section. Relative differences in primary or secondary permeability and the mean hydraulic conductivity for HGUs were assumed to be constant along each boundary cross section. Structural disruption is considered as a boundary condition where closely spaced high-angle normal faults disrupt a relatively broad region and where carbonate-rock aquifers are highly faulted and disrupted in the upper plates of low-angle normal faults. Because few data are available, however, the categorization does not incorporate the effects of individual faults as distinct hydrologic entities. For example, the analysis omits potential effects of impermeable, clay-rich fault core zones, fractured and potentially more permeable zones that might lie outside of the fault core, or stratabound fractured intervals in volcanic or carbonate rocks. The occurrence of each subsurface boundary condition varies throughout the study area; for example, boundaries with LCU or UCU rocks occur in many HAs and subbasins; boundaries with FYSU or CYSU deposits are limited and absent in the study area, respectively. For each of the 10 subsurface boundary conditions, the potential for ground-water flow was evaluated in one of three ways (fig. 15)—(1) permeable rocks are likely to exist at depth such that ground-water flow likely is permitted by subsurface geology, (2) relatively impermeable rocks are likely to exist at depth such that ground-water flow likely is not permitted by subsurface geology, or (3) the subsurface geology beneath the boundary or divide is not well constrained or the nature of the subsurface framework is highly uncertain such that the geologic controls on ground-water flow are uncertain.
The rationale for each of the 10 subsurface boundary conditions shown in figure 15 is described in the following paragraphs:
Intrabasin divides represent locations where the basin-fill aquifer is interrupted by buried structural highs of pre-Cenozoic bedrock; however, these areas are not necessarily barriers to ground-water flow. The intrabasin divides were evaluated using the same rationale used to classify the HA boundaries. A much greater level of uncertainty exists in envisaging the subsurface geology and potential hydraulic effects across intrabasin divides (fig. 15). Except for one area, all intrabasin divides in the study area are interpreted as ground-water flow being possible across these divides, but uncertain because the subsurface geologic framework is not well constrained. Two of these intrabasin divides, in Lake Valley and in southern Snake Valley, were located at the buried northern margin of the Indian Peak caldera complex, even though the pre-Cenozoic surface does not show significant changes in topography. In these areas, relatively thick accumulations of volcanic rocks closer to the caldera likely influence ground-water flow differently than volcanic rocks interbedded with basin fill and farther away from the calderas. However, ground-water flow likely crosses an intrabasin divide near the northern part of Snake Valley (fig. 15) where carbonate rocks occur beneath the basin-fill aquifer.
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/sir/2007/5261
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