Geology topics
Research about Nevada
Source-linked reports with geographic coverage including Nevada.
Preliminary development of the LBL/USGS three-dimensional site-scale model of Yucca Mountain, Nevada
A three-dimensional model of moisture flow within the unsaturated zone at Yucca Mountain is being developed at Lawrence Berkeley Laboratory (LBL) in cooperation with the U.S. Geological Survey (USGS). This site-scale model covers and area of about 34 km 2 and is bounded by major faults to the north, east and west. The model geometry is defined (1) to represent the variations of hydrogeological units between the ground surface and the water table; (2) to be able to reproduce the effect of abrupt changes in hydrogeological parameters at the boundaries between hyrdogeological units; and (3) to include the influence of major faults. A detailed numerical grid has been developed based on the locations of boreholes, different infiltration zones, hydrogeological units and their outcrops, major faults, and water level data. Contour maps and isopatch maps are presented defining different types of infiltration zones, and the spatial distribution of Tiva Canyon, Paintbrush, and Topopah Spring hydrogeological units. The grid geometry consists of seventeen non-uniform layers which represent the lithological variations within the four main welded and non-welded hydrogeological units. Matrix flow is approximated using the van Genuchten model, and the equivalent continuum approximation is used to account for fracture flow in the welded units. The fault zones are explicitly modeled as porous medium using various assumptions regarding their permeabilities and characteristic curves. One-, two-, and three-dimensional simulations are conducted using the TOUGH2 computer program. Steady-state simulations are performed with various uniform and non-uniform infiltration rates. The results are interpreted in terms of the effect of fault characteristics on the moisture flow distribution, and on location and formation of preferential pathways.
Endangered cui-ui of Pyramid Lake, Nevada
Cui-ui ( Chasmistes cujus ) is a large plankton-feeding fish that only occurs in Pyramid Lake, Nevada. It was put on the federal endangered list in 1967 based on declining population and absence of reproduction. A lake dweller, cui-ui is a stream spawner. Most of this century, this sucker species was unable to access the Truckee River, Pyramid Lake's only perennial tributary, to reproduce. Water diversion from the Truckee River, as a result of the nation's first Bureau of Reclamation project (Newlands Project), reduced the lake elevation and, in most years, caused an impassable delta to form at the mouth of the Truckee River. Cui-ui live more than 40 years; it is this longevity that has allowed the species to persist for as many as 19 years with virtually no recruitment (see glossary) to the adult population (Scoppettone 1988). Cui-ui is one of three remaining species of the genus Chasmistes. Of the three, its habitat is most intact, and it thus has the best opportunity for recovery (Scoppettone and Vinyard 1991). Each spring, cui-ui adults, most of which mature at 8-12 years of age, migrate to the mouth of the Truckee River at the south end of Pyramid Lake, where they aggregate, awaiting environmental cues and sufficient stream flow to enter the river (Scoppettone et al. 1986). This behavior provides an excellent opportunity to capture the adults for estimating population numbers and year-class (year hatched) structure. In this article we report changes in adult cui-ui population number and year-class structure from spring 1983 to spring 1993.
Geology, alteration, and geochronology of the Como District, Lyon County, Nevada
No abstract available.
Cyanide and migratory birds at gold mines in Nevada, USA
Since the mid-1980s, cyanide in heap leach solutions and mill tailings ponds at gold mines in Nevada has killed a large but incompletely documented number of wildlife (>9,500 individuals, primarily migratory birds). This field investigation documents the availability of cyanide at a variety of ‘typical’ Nevada gold mines during 1990 and 1991, describes wildlife reactions to cyanide solutions, and discusses procedures for eliminating wildlife loss from cyanide poisoning. Substantial progress has been made to reduce wildlife loss. About half of the mill tailings ponds (some up to 150 ha) in Nevada have been chemically treated to reduce cyanide concentrations (the number needing treatment is uncertain) and many of the smaller heap leach solution ponds and channels are now covered with netting to exclude birds and most mammals. The discovery of a cyanide gradient in mill tailings ponds (concentration usually 2–3 times higher at the inflow point than at reclaim point) provides new insight into wildlife responses (mortality) observed in different portions of the ponds. Finding dead birds on the tops of ore heaps and associated with solution puddling is a new problem, but management procedures for eliminating this source of mortality are available. A safe threshold concentration of cyanide to eliminate wildlife loss could not be determined from the field data and initial laboratory studies. New analytical methods may be required to assess further the wildlife hazard of cyanide in mining solutions.
Carbonate deposition, Pyramid Lake subbasin, Nevada: 1. Sequence of formation and elevational distribution of carbonate deposits (Tufas)
During the late Quaternary, the elevation of terrace cutting and carbonate deposition in the Pyramid Lake subbasin were controlled by constancy of lake level imposed by spill to adjoining subbasins. Sill elevations are 1177-1183 m (Mud Lake Slough Sill), 1207 m (Emerson Pass Sill), and 1265 m (Darwin Pass Sill). Carbonate deposition was favored by: (1) hydrologic closure, (2) proximity to a source of calcium, (3) elevated water temperature, and (4) a solid substrate. The thickness and aspect of tufa are a function of lake-level dynamics. Relatively thin sheets and pendant sheets were deposited during a rising or falling lake. The upper parts of thick reef-form tufas have a horizontal aspect and were deposited in a lake which was stabilized by spill to the Carson Desert subbasin. The lower parts of the reef-form tufas are thinner and their outer surface has a vertical aspect, indicating that the lower part formed in a receding lake. The thickest and most complete sequences of tufa are mounds that border the Pyramid Lake shore. The tops of the tallest mounds reach the elevation of the Darwin Pass Sill and many mounds have been eroded to the elevations of the Mud Lake Slough Sill of the Emerson Pass Sill. The sequence of tufa formation (from oldest to youngest) displayed in these mounds is: (1) a beachrock containing carbonate-cemented volcanic cobbles, (2) broken and eroded old spheroids that contain thinolitic tufa and an outer rind of dense laminated tufa, (3) large cylindrical (tubular) tufas capped by (4) coatings of old dense tufas, and (5) several generations of old branching tufa commonly associated with thin, platy tufas and coatings of thinolitic tufa, (6) young spheroids that contain poorly oriented young thinolitic tufa in the center and several generations of radially oriented young thinolitic tufas near the outer edge, (7) a transitional thinolite-to-branching tufa, (8) two or more layers of young branching tufa, (9) a 0.5-cm-thick layer of fine-grained dolomite, (10) a 2-cm-thick layer of young dense laminated tufa, (11) a 0.1-cm-thick layer of encrusting tufa that was covered by a beach deposit and (12) a 1.0-cm-thick layer of porous encrusting tufa that coated the beach deposit and the sides of tufa mounds. The elevational ranges of the principal varieties of tufa are not related to terrace or spill elevations. It seems likely that the distribution of tufa varieties is related to variation in the thermal structure or chemistry of Lake Lahontan. It is hypothesized that the thinolitic tufa (ikaite) formed in the near freezing hypolimnion and the braching tufa (calcite) formed in the seasonally warmer epiliminion of the lake.
40Ar/39Ar thermochronologic constraints on the tectonothermal evolution of the northern East Humboldt Range metamorphic core complex, Nevada
The northern East Humboldt Range (NEHR) of northeastern Nevada exposes a suite of complexly deformed migmatitic, upper amphibolite-facies rocks in the footwall of the Ruby Mountains-East Humboldt Range (RM-EHR) detachment fault. New 40 Ar/ 39 Ar data on hornblende, muscovite, biotite, and potassium feldspar help constrain the kinematic and thermal evolution of this terrain during Tertiary extensional exhumation. Hornblende samples from relatively high structural levels yield discordant age spectra that suggest initial cooling during early Tertiary time (63–49 Ma). When coupled with petrological constraints indicating a strongly decompressional P-T-t path above 550°C, the hornblende data suggest that exhumation of the RM-EHR may have initiated in early Tertiary time, approximately coincident with the initial phases of unroofing in the Wood Hills immediately to the east and with the end of thrusting in the late Mesozoic to early Tertiary Sevier orogenic belt of eastern Nevada and western Utah. This temporal coincidence suggests that gravitational collapse of tectonically thickened crust in the internal zone of the Sevier belt could have driven the initial phases of unroofing. Thermal history during the final stage of exhumation of the NEHR is constrained by discordant hornblende cooling ages of 36-29 Ma from deep structural levels and biotite, muscovite, and potassium feldspar cooling ages of 27-21 Ma from a range of structural levels. Comparison of muscovite, biotite, and potassium feldspar cooling ages with previously published fission-track cooling ages implies very rapid cooling rates at temperatures below the closure temperature for muscovite (270°–350°C), but time gaps of > 7 m.y. between hornblende and mica cooling ages suggest that cooling at higher temperatures was more gradual. In addition, comparison of 40 Ar 39 Ar mica cooling ages with previously published fission-track apatite cooling ages suggests pronounced thermal gradients between the NEHR and adjacent areas during latest Oligocene to earliest Miocene time. Such thermal gradients could be readily explained if the RM-EHR detachment fault dipped > 30° between the 300°C and 100°C isotherms. Finally, 40 Ar 39 Ar biotite cooling ages increase southward through the East Humboldt Range, compatible with northward extrapolation of a previously recognized pattern of WNW-younging biotite cooling ages from the Ruby Mountains. A simple model involving the propagation of footwall uplift in the direction of tectonic transport beneath an initially listric normal fault can explain the principle features of the Oligocène to Miocene thermochronologic data set for the RM-EHR.
500,000-year stable carbon isotopic record from Devils Hole, Nevada
The record of carbon-13 (δ 13 C) variations in DH-11 vein calcite core from Devils Hole, Nevada, shows four prominent minima near glacial terminations (glacial-interglacial transitions) V to II. The δ 13 C time series is inversely correlated with the DH-11 oxygen isotope ratio time series and leads it by as much as 7000 years. The δ 13 C variations likely record fluctuations in the δ 13 C of dissolved inorganic carbon of water recharging the aquifer. How such variations are transported 80 kilometers to Devils Hole without obliteration by water-rock reaction remains an enigma. The record may reflect (i) global variations in the δ 13 C of atmospheric CO 2 and, hence, the δ 13 C of continental biomass or (ii) variations in extent and density of vegetation in the southern Great Basin. In the latter case, δ 13 C minima at 414, 334, 246, and 133 thousand years ago mark times of maximum vegetation.
Isolation, growth, and metabolism of an obligately anaerobic, selenate- respiring bacterium, strain SES-3
A gram-negative, strictly anaerobic, motile vibrio was isolated from a selenate-respiring enrichment culture. The isolate, designated strain SES-3, grew by coupling the oxidation of lactate to acetate plus CO 2 with the concomitant reduction of selenate to selenite or of nitrate to ammonium. No growth was observed on sulfate or selenite, but cell suspensions readily reduced selenite to elemental selenium (Se 0 ). Hence, SES-3 can carry out a complete reduction of selenate to Se 0 . Washed cell suspensions of selenate-grown cells did not reduce nitrate, and nitrate-grown cells did not reduce selenate, indicating that these reductions are achieved by separate inducible enzyme systems. However, both nitrate-grown and selenate-grown cells have a constitutive ability to reduce selenite or nitrite. The oxidation of [ 14 C]lactate to 14 CO 2 coupled to the reduction of selenate or nitrate by cell suspensions was inhibited by CCCP (carbonyl cyanide m -chlorophenylhydrazone), cyanide, and azide. High concentrations of selenite (5 mM) were readily reduced to Se 0 by selenate-grown cells, but selenite appeared to block the synthesis of pyruvate dehydrogenase. Tracer experiments with [ 75 Se]selenite indicated that cell suspensions could achieve a rapid and quantitative reduction of selenite to Se 0 . This reduction was totally inhibited by sulfite, partially inhibited by selenate or nitrite, but unaffected by sulfate or nitrate. Cell suspensions could reduce thiosulfate, but not sulfite, to sulfide. These results suggest that reduction of selenite to Se 0 may proceed, in part, by some of the components of a dissimilatory system for sulfur oxyanions.
Triggering of the Ms = 5.4 Little Skull Mountain, Nevada, earthquake with dynamic strains
We have developed an approach to test the viability of dynamic strains as a triggering mechanism by quantifying the dynamic strain tensor at seismogenic depths. We focus on the dynamic strains at the hypocenter of the M s = 5.4 Little Skull Mountain ( LSM ), Nevada, earthquake. This event is noteworthy because it is the largest earthquake demonstrably triggered at remote distances (∼280 km) by the M s = 7.4 Landers, California, earthquake and because of its ambiguous association with magmatic activity. Our analysis shows that, if dynamic strains initiate remote triggering, the orientation and modes of faulting most favorable for being triggered by a given strain transient change with depth. The geometry of the most probable LSM fault plane was favorably oriented with respect to the geometry of the dynamic strain tensor. We estimate that the magnitude of the peak dynamic strains at the hypocentral depth of the LSM earthquake were ∼4 μstrain (∼.2 MPa) which are ∼50% smaller than those estimated from velocity seismograms recorded at the surface. We suggest that these strains are too small to cause Mohr-Coulomb style failure unless the fault was prestrained to near failure levels, the fault was exceptionally weak, and/or the dynamic strains trigger other processes that lead to failure.
Evaluation of measurement scale using imbibition experiments in volcanic tuffs
A major issue in the site characterization at Yucca Mountain, Nevada, a potential site for a high-level nuclear waste repository, is the relevance of laboratory-scale measurements on cores to field-scale processes, particularly water flow. Calculation of Philip`s sorptivity parameter using imbibition of water into rock was selected as a simple test to describe hydrologic parameters at both laboratory and field scales and to study effects of sample size and spatial variability. Laboratory-scale imbibition experiments were conducted on two sizes of core from two boreholes drilled in layered nonwelded tuff and fractured welded tuff. Laboratory experiments were compared with field experiments in the boreholes using neutron logs and a field-scale Mariotte system. Measured sorptivity for both sizes of core were virtually identical and both could predict field-scale sorptivity if enough samples were used to account for spatial heterogeneity. Core data was less useful in predicting the neutron log data due to the nature of the neutron probe measurement and difficulties in accounting for effects of the unique system geometry. Mean neutron log values could not predict the field results from the welded borehole due to fractures transmitting but not retaining water, and could not predict laboratory results because of scale and sampling volume differences. The mean neutron log data could, however, approximate the large-scale field results in the nonwelded borehole.
Aeromagnetic and related maps of the Getchell gold trend, Osgood Mountains, north-central Nevada
No abstract available.
Geologic map of the Pahroc Spring NE quadrangle, Lincoln County, Nevada
No abstract available.
Ground-water conditions in Las Vegas Valley, Clark County, Nevada; Part II, Hydrogeology and simulation of ground-water flow
Groundwater withdrawals in Las Vegas Valley, Nevada, primarily for municipal supplies, totaled more than 2.5 million acre-ft between 1912 and 1981, with a peak annual withdrawal rate of 88,000 acre-ft in 1968. Effects of heavy pumping are evident over large areas of the valley but are more pronounced near the major well fields. Secondary recharge from lawn irrigation and other sources is estimated to have totaled more than 340,000 acre-ft during 1972-81. Resulting rises in water-level in shallow, unconfined aquifers in the central and southeastern parts of the valley have caused: widespread water-logging of soils; increased groundwater discharge to Las Vegas Wash and its tributaries; and potential for degradation of water quality in deeper aquifers by accentuating downward vertical hydraulic potential in areas where shallow groundwater has high concentrations of dissolved solids and nitrate. A 3-dimensional groundwater flow model of the valley-fill aquifer system was constructed for use in evaluating possible groundwater management alternatives aimed at alleviating problems related to overdraft and water-logging while maximizing use of the groundwater resources. Natural recharge to the valley-fill aquifers is about 33,000 acre-ft/yr; in 1979, an estimated 44,000 acre-ft of secondary recharge infiltrated to the near-surface and developed-zone aquifers. Peak water use for lawn irrigation during summer results in rates of secondary recharge that may increase threefold from winter rates. Simulated rates of seepage to washes in the valley increased correspondingly from an average of 850 acre-ft/mo in winter to about 1,300 acre-ft/mo in the summer. Groundwater withdrawals by pumping totaled 620,000 acre-ft during 1972-81, and model results indicate that about 190,000 acre-ft of that total was derived from storage. Use of the model as a predictive tool was demonstrated by simulating the effects of using most municipal wells only during the peak-demand season of June 1 through September 20. Results of the 9-year simulation indicated that: (1) long-term rates of water-level decline near the municipal well field would be less than rates for 1972-81, but the magnitude of seasonal fluctuations would increase, and (2) total volume of water released from storage as a result of subsidence would be only 42,000 acre-feet per year, or about half the volume during 1972-81.
Hydrogeology and potential effects of changes in water use, Carson Desert agricultural area, Churchill County, Nevada
Operating Criteria and Procedures established in 1988 for delivery of water for irrigation in the Newlands Project area include regulations and methods to increase Project efficiency. Public Law 101-618 of 1990 includes a target of 75-percent Project efficiency and a program of water-rights acquisition for wetlands maintenance. The directives could result in large reductions in water used for irrigation in the Carson Desert, potentially affecting ground-water supplies. Previous studies of the area have been evaluated to determine the current understanding of how aquifers are recharged, what controls the flow and quality of ground water, potential effects of changes in water use, and what additional information would be needed to quantify further changes in water use. Inflow of surface water to the basin from Lahontan Reservoir averaged about 370,000 acre-ft/yr (acre-feet per year) from 1975 to 1992, supplying water for irrigation of more than 50,000 acres. More than half of the water released from the reservoir is lost to seepage, operational spills, and evaporation before delivery of about 170,000 acre-ft/yr to farm headgates. The volume of water delivered to farms that does not contribute to crop consumptive use (on-farm loss) is poorly known but could be as much as 60,000 acre-ft/yr. Consumptive use on irrigated land may be about 180,000 acre-ft/yr, of which 50,000 acre-ft/yr may be derived from the shallow aquifer. Outflow from irrigated land is a mixture of operational spill, runoff from irrigated fields, and ground-water seepage to drains. Total outflow averages about 170,000 to 190,000 acre-ft/yr. This water flows to wetlands at Carson Lake, Stillwater Wildlife Management Area, and Carson Sink. Three sedimentary aquifers were previously defined in the basin: a shallow aquifer having highly variable lithology and water quality, an intermediate aquifer containing principally fresh water, and a deep aquifer having water of poor quality. The deep aquifer could possibly be divided into sedimentary and volcanic zones. In addition, a near-surface zone may exist near the top of the shallow aquifer where vertical flow is inhibited by underlying clay beds. A basalt aquifer near the center of the basin is the source of public supply and is recharged by the shallow, intermediate, and deep aquifers. Water levels in the basalt aquifer have declined about 10 feet from pre-pumping levels, and chloride and arsenic concentrations in the water have increased. The average depth to ground water has decreased beneath large areas of the Carson Desert since 1904 as a result of recharge of surface water used for irrigation. Ground water generally flows from west to east, and dissolvedsolids concentrations increase greatly near areas of ground-water discharge, where State of Nevada drinking-water standards commonly are exceeded. Uncertainties in the rates of recharge to and discharge from the basin cause an imbalance in the calculated water budget. Estimates for total recharge range from 400,000 to 420,000 acreft/yr, whereas estimates for discharge range from 630,000 to 680,000 acre-ft/yr. Estimates of inflow to and outflow from aquifers of the study area are as follows: shallow aquifer, more than 120,000 acre-ft/yr; intermediate aquifer, possibly more than 25,000 acre-ft/yr; deep aquifer, unknown; and basalt aquifer, about 4,000 acre-ft/yr. Estimates for flow volumes to and from the shallow and intermediate aquifers are based on assumed aquifer properties and could be in error by an order of magnitude or more. Conceptual models of the basin show that ground-water flow is downward from the shallow aquifer to the intermediate aquifer in the western part and near the center of the basin, and is upward in the eastern part of the basin. Little is known about flow in the deep aquifer. Nearsurface clay beds inhibit vertical flow near the center and eastern part of the basin except where breached by relict sand-filled channels of the Carson River. Conceptual models of the basin show that changes in water use in the western part of the basin probably would affect recharge to the sedimentary and basalt aquifers. Near the center of the basin, water-use changes could affect the shallow and basalt aquifers but might have less effect on the intermediate aquifer. In the eastern part of the basin, changes could affect the shallow aquifer, but would probably not affect the intermediate or basalt aquifers. If seepage is decreased by lining canals, and land is removed from production, water-level declines in the shallow aquifer could be greater than 10 feet as far as 2 miles from the lined canals. Depending upon the distribution of specific yield, decreasing recharge by 25,000 to 50,000 acre-ft/yr beneath 30,000 acres could cause water levels to decline from 4 to 17 feet. Where ground water supplements crop consumptive use, water levels could temporarily rise when land is removed from production. Where water is pumped from a near-surface zone of the shallow aquifer, water-level declines might not greatly affect pumped wells where the nearsurface zone is thickest, but could cause wells to go dry where the zone is thin. The understanding of surface-water and ground-water relations, recharge and discharge of ground water, ground-water movement, and the potential effects of changes in water use in the Carson Desert can be refined by studying (1) the extent of potable water in the intermediate and basalt aquifers, (2) lithology and specific yield of aquifer materials, (3) data on ground-water levels and quality, and (4) data on surface-water flow and quality, as well as monitoring the effects of changes in water use as they take place.