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James B. Paces

Publications and source records attributed to James B. Paces.

At least 55 records · Page 3Linked to original sources
USGS2013

Loess origin, transport, and deposition over the past 10,000 years, Wrangell-St. Elias National Park, Alaska

Contemporary glaciogenic dust has not received much attention, because most research has been on glaciogenic dust of the last glacial period or non-glaciogenic dust of the present interglacial period. Nevertheless, dust from modern glaciogenic sources may be important for Fe inputs to primary producers in the ocean. Adjacent to the subarctic Pacific Ocean, we studied a loess section near Chitina, Alaska along the Copper River in Wrangell-St. Elias National Park, where dust has been accumulating over the past ∼10,000 years. Mass accumulation rates for the fine-grained (<20 μm) fraction of this loess section are among the highest reported for the Holocene of high-latitude regions of the Northern Hemisphere. Based on mineralogy and geochemistry, loess at Chitina is derived from glacial sources in the Wrangell Mountains, the Chugach Mountains, and probably the Alaska Range. Concentrations of Fe in the silt-plus-clay fraction of the loess at Chitina are much higher than in all other loess bodies in North America and higher than most loess bodies on other continents. The very fine-grained (<2 μm) portion of this sediment, capable of long-range transport, is dominated by Fe-rich chlorite, which can yield Fe readily to primary producers in the ocean. Examination of satellite imagery shows that dust from the Copper River is transported by wind on a regular basis to the North Pacific Ocean. This Alaskan example shows that high-latitude glaciogenic dust needs to be considered as a significant Fe source to primary producers in the open ocean.

Alaska↗
USGS2013

234U/238U and δ87Sr in peat as tracers of paleosalinity in the Sacramento-San Joaquin Delta of California, USA

The purpose of this study was to determine the history of paleosalinity over the past 6000+ years in the Sacramento-San Joaquin Delta (the Delta), which is the innermost part of the San Francisco Estuary. We used a combination of Sr and U concentrations, d87Sr values, and 234U/238U activity ratios (AR) in peat as proxies for tracking paleosalinity. Peat cores were collected in marshes on Browns Island, Franks Wetland, and Bacon Channel Island in the Delta. Cores were dated using 137Cs, the onset of Pb and Hg contamination from hydraulic gold mining, and 14C. A proof of concept study showed that the dominant emergent macrophyte and major component of peat in the Delta, Schoenoplectus spp., incorporates Sr and U and that the isotopic composition of these elements tracks the ambient water salinity across the Estuary. Concentrations and isotopic compositions of Sr and U in the three main water sources contributing to the Delta (seawater, Sacramento River water, and San Joaquin River water) were used to construct a three-end-member mixing model. Delta paleosalinity was determined by examining variations in the distribution of peat samples through time within the area delineated by the mixing model. The Delta has long been considered a tidal freshwater marsh region, but only peat samples from Franks Wetland and Bacon Channel Island have shown a consistently fresh signal (<0.5 ppt) through time. Therefore, the eastern Delta, which occurs upstream from Bacon Channel Island along the San Joaquin River and its tributaries, has also been fresh for this time period. Over the past 6000+ years, the salinity regime at the western boundary of the Delta (Browns Island) has alternated between fresh and oligohaline (0.5-5 ppt).

California↗
USGS2013

Evaluation of Pleistocene groundwater flow through fractured tuffs using a U-series disequilibrium approach, Pahute Mesa, Nevada, USA

Groundwater flow through fractured felsic tuffs and lavas at the Nevada National Security Site represents the most likely mechanism for transport of radionuclides away from underground nuclear tests at Pahute Mesa. To help evaluate fracture flow and matrix–water exchange, we have determined U-series isotopic compositions on more than 40 drill core samples from 5 boreholes that represent discrete fracture surfaces, breccia zones, and interiors of unfractured core. The U-series approach relies on the disruption of radioactive secular equilibrium between isotopes in the uranium-series decay chain due to preferential mobilization of 234 U relative to 238 U, and U relative to Th. Samples from discrete fractures were obtained by milling fracture surfaces containing thin secondary mineral coatings of clays, silica, Fe–Mn oxyhydroxides, and zeolite. Intact core interiors and breccia fragments were sampled in bulk. In addition, profiles of rock matrix extending 15 to 44 mm away from several fractures that show evidence of recent flow were analyzed to investigate the extent of fracture/matrix water exchange. Samples of rock matrix have 234 U/ 238 U and 230 Th/ 238 U activity ratios (AR) closest to radioactive secular equilibrium indicating only small amounts of groundwater penetrated unfractured matrix. Greater U mobility was observed in welded-tuff matrix with elevated porosity and in zeolitized bedded tuff. Samples of brecciated core were also in secular equilibrium implying a lack of long-range hydraulic connectivity in these cases. Samples of discrete fracture surfaces typically, but not always, were in radioactive disequilibrium. Many fractures had isotopic compositions plotting near the 230 Th- 234 U 1:1 line indicating a steady-state balance between U input and removal along with radioactive decay. Numerical simulations of U-series isotope evolution indicate that 0.5 to 1 million years are required to reach steady-state compositions. Once attained, disequilibrium 234 U/ 238 U and 230 Th/ 238 U AR values can be maintained indefinitely as long as hydrological and geochemical processes remain stable. Therefore, many Pahute Mesa fractures represent stable hydrologic pathways over million-year timescales. A smaller number of samples have non-steady-state compositions indicating transient conditions in the last several hundred thousand years. In these cases, U mobility is dominated by overall gains rather than losses of U.

Nevada↗
USGS2013

Ion-probe U–Pb dating of authigenic and detrital opal from Neogene-Quaternary alluvium

Knowing depositional ages of alluvial fans is essential for many tectonic, paleoclimatic, and geomorphic studies in arid environments. The use of U–Pb dating on secondary silica to establish the age of Neogene-Quaternary clastic sediments was tested on samples of authigenic and detrital opal and chalcedony from depths of ∼25 to 53 m in boreholes at Midway Valley, Nevada. Dating of authigenic opal present as rinds on rock clasts and in calcite/silica cements establishes minimum ages of alluvium deposition; dating of detrital opal or chalcedony derived from the source volcanic rocks gives the maximum age of sediment deposition. Materials analyzed included 12 samples of authigenic opal, one sample of fracture-coating opal from bedrock, one sample of detrital opal, and two samples of detrital chalcedony. Uranium–lead isotope data were obtained by both thermal ionization mass spectrometry and ion-microprobe. Uranium concentrations ranged from tens to hundreds of μg/g. Relatively large U/Pb allowed calculation of 206 Pb/ 238 U ages that ranged from 1.64±0.36 (2 σ ) to 6.16±0.50 Ma for authigenic opal and from 8.34±0.28 to 11.2±1.3 Ma for detrital opal/chalcedony. Three samples with the most radiogenic Pb isotope compositions also allowed calculation of 207 Pb/ 235 U ages, which were concordant with 206 Pb/ 238 U ages from the same samples. These results indicate that basin development at Midway Valley was initiated between about 8 and 6 Ma, and that the basin was filled at long-term average deposition rates of less than 1 cm/ka. Because alluvium in Midway Valley was derived from adjacent highlands at Yucca Mountain, the low rates of deposition determined in this study may imply a slow rate of erosion of Yucca Mountain. Volcanic strata underlying the basin are offset by a number of buried faults to a greater degree than the relatively smooth-sloping bedrock/alluvium contact. These geologic relations indicate that movement on most faults ceased prior to erosional planation and burial. Therefore, ages of the authigenic opal from basal alluvium indicate that the last movement on buried faults was older than about 6 Ma.

Earth and Planetary Science Letters↗
USGS2012

Lower-crustal xenoliths from Jurassic kimberlite diatremes, upper Michigan (USA): Evidence for Proterozoic orogenesis and plume magmatism in the lower crust of the southern Superior Province

Jurassic kimberlites in the southern Superior Province in northern Michigan contain a variety of possible lower-crustal xenoliths, including mafic garnet granulites, rare garnet-free granulites, amphibolites and eclogites. Whole-rock major-element data for the granulites suggest affinities with tholeiitic basalts. P–T estimates for granulites indicate peak temperatures of 690–730°C and pressures of 9–12 kbar, consistent with seismic estimates of crustal thickness in the region. The granulites can be divided into two groups based on trace-element characteristics. Group 1 granulites have trace-element signatures similar to average Archean lower crust; they are light rare earth element (LREE)-enriched, with high La/Nb ratios and positive Pb anomalies. Most plot to the left of the geochron on a 206 Pb/ €204 Pb vs 207 Pb/ €204 Pb diagram, and there was probably widespread incorporation of Proterozoic to Archean components into the magmatic protoliths of these rocks. Although the age of the Group 1 granulites is not well constrained, their protoliths appear to be have been emplaced during the Mesoproterozoic and to be older than those for Group 2 granulites. Group 2 granulites are also LREE-enriched, but have strong positive Nb and Ta anomalies and low La/Nb ratios, suggesting intraplate magmatic affinities. They have trace-element characteristics similar to those of some Mid-Continent Rift (Keweenawan) basalts. They yield a Sm–Nd whole-rock errorchron age of 1046 ± 140 Ma, similar to that of Mid-Continent Rift plume magmatism. These granulites have unusually radiogenic Pb isotope compositions that plot above the 207 Pb/ €204 Pb vs 206 Pb/ €204 Pb growth curve and to the right of the 4·55 Ga geochron, and closely resemble the Pb isotope array defined by Mid-Continent Rift basalts. These Pb isotope data indicate that ancient continental lower crust is not uniformly depleted in U (and Th) relative to Pb. One granulite xenolith, S69-5, contains quartz, and has a unique peraluminous composition. It has the lowest εNd and εHf values of the suite. Its isotopic compositions indicate that it is significantly older than the other granulites. Broken zircon cores encased by younger overgrowths suggest that this granulite includes a large component of pre-existing sedimentary rocks. Two distinct populations of zircons from S69-5 were dated by sensitive high-resolution ion microprobe. Abundant rounded zircons yield ages of 1104 ± 42 (2σ) Ma, which coincide with the Mid-Continent Rift flood basalt eruptions. Their morphology is similar to those found in lower-crustal rocks that have undergone granulite-facies metamorphism and thus they are considered to represent the age of Group 2 granulites. Also present are less abundant elongate zircon grains that yield a mean age of 1387 ± 32 (2σ) Ma. Their elongate shapes indicate growth from a melt or fluid, possibly associated with 1·3–1·5 Ga anorogenic granite magmatism exposed in the shallow crust to the south in Wisconsin, or related to an initial encroachment of the Keweenawan plume upon the lower crust. Older ages recognized in zircon cores are less well constrained but may be related to tectono-magmatic events in the southern Superior craton. Within the studied suite only S69-5 was recognized as a remnant of the Late Archean lower crust into which the Group 1 and 2 mafic granulite precursor basalts were intruded. Collectively, the data show that the lower crust beneath northern Michigan formed in Archean times and underwent a variety of tectono-magmatic processes throughout the Proterozoic, including orogenesis, partial melting and mafic magmatic underplating in response to upwelling mantle plumes.

Michigan↗
USGS2012

Precise U‐Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent Rift System

Precise resolution of the timing of igneous activity is crucial to understanding the dynamic processes associated with continental rifting. Mafic intrusive rocks constitute a major portion of the exposed 1100 Ma (Keweenawan) Midcontinent Rift system in northeastern Minnesota; however, prior to this study, geochronological data were insufficient to allow rigorous interpretation of intrusive histories and their relationships to extrusive suites. Eight anorthositic and gabbroic intrusives were chosen to represent both the temporal and spatial ranges of plutonic activity that formed the Duluth Complex and related mafic intrusions. U‐Pb isotopic analyses from zircons and baddeleyites result in U‐Pb concordant ages with little or no ambiguity introduced by inherited components, Pb loss or common Pb. The earliest Keweenawan plutonism exposed in Minnesota occurs along the northeastern flank of the Duluth Complex as a series of layered gabbros (Nathan's layered series) emplaced at 1106.9 ± 0.6 Ma. This sequence of gabbro sheets shares temporal, spatial, and compositional similarities with the nearby Logan sills in Ontario. Four Duluth Complex anorthositic and troctolitic series samples from widely separated areas have unresolvable ages between 1099.3 ± 0.3 and 1098.6 ± 0.5 Ma, indicating a very short duration for peak intrusive activity (0.5–1 m.y.). The unresolvable ages between anorthositic and troctolitic plutons suggest that these two magma series are more closely related than previously modeled and argue strongly for the need to reexamine their fundamental petrogenetic relationships. These dates also imply that the major reverse‐to‐normal magnetic polarity switch, used throughout the rift system as an important correlation tool, occurred prior to 1099 Ma. This date is several million years earlier than previously suspected and emphasizes the need for further paleomagnetic and geochronological data from the overlying volcanics. Much of the hypabyssal intrusive suite within the volcanic pile overlying Duluth Complex plutons may be significantly younger than the main pulse of plutonic activity. Two hypabyssal bodies, the Sonju Lake intrusion and gabbro at Silver Bay, were emplaced at 1096.1 ± 0.8 Ma and 1095.8 ± 1.2 Ma, respectively. Dates reported here and in previous studies support the concept of episodic tectonomagmatic rift development where magmatism was apparently concentrated in episodes of short duration (<1–3 m.y.) interspersed with longer hiatuses (2–8 m.y.).

Michigan, Minnesota, Ontario, Wisconsin↗
USGS2012

The paleohydrology of unsaturated and saturated zones at Yucca Mountain, Nevada, and vicinity

Surface, unsaturated-zone, and saturated-zone hydrologic conditions at Yucca Mountain responded to past climate variations and are at least partly preserved by sediment, fossil, and mineral records. Characterizing past hydrologic conditions in surface and subsurface environments helps to constrain hydrologic responses expected under future climate conditions and improve predictions of repository performance. Furthermore, these records provide a better understanding of hydrologic processes that operate at time scales not readily measured by other means. Pleistocene climates in southern Nevada were predominantly wetter and colder than the current interglacial period. Cyclic episodes of aggradation and incision in Fortymile Wash, which drains the eastern slope of Yucca Mountain, are closely linked to Pleistocene climate cycles. Formation of pedogenic cement is favored under wetter Pleistocene climates, consistent with increased soil moisture and vegetation, higher chemical solubility, and greater evapotranspiration relative to Holocene soil conditions. The distribution and geochemistry of secondary minerals in subsurface fractures and cavities reflect unsaturated-zone hydrologic conditions and the response of the hydrogeologic system to changes in temperature and percolation flux over the last 12.8 m.y. Physical and fluid-inclusion evidence indicates that secondary calcite and opal formed in air-filled cavities from fluids percolating downward through connected fracture pathways in the unsaturated zone. Oxygen, strontium, and carbon isotope data from calcite are consistent with a descending meteoric water source but also indicate that water compositions and temperatures evolved through time. Geochronological data indicate that secondary mineral growth rates are less than 1&ndash;5 mm/m.y., and have remained approximately uniform over the last 10 m.y. or longer. These data are interpreted as evidence for hydrological stability despite large differences in surface moisture caused by climate shifts between the Miocene and Pleistocene and between Pleistocene glacial-interglacial cycles. Secondary mineral distribution and &delta;18O profiles indicate that evaporation in the shallower welded tuffs reduces infiltration fluxes. Several near-surface and subsurface processes likely are responsible for diverting or dampening infiltration and percolation, resulting in buffering of percolation fluxes to the deeper unsaturated zone. Cooler and wetter Pleistocene climates resulted in increased recharge in upland areas and higher water tables at Yucca Mountain and throughout the region. Discharge deposits in the Amargosa Desert were active during glacial periods, but only in areas where the modern water table is within 7&ndash;30 m of the surface. Published groundwater models simulate water-table rises beneath Yucca Mountain of as much as 150 m during glacial climates. However, most evidence from Fortymile Canyon up gradient from Yucca Mountain limits water-table rises to 30 m or less, which is consistent with evidence from discharge sites in the Amargosa Desert. The isotopic compositions of uranium in tuffs spanning the water table in two Yucca Mountain boreholes indicate that Pleistocene water-table rises likely were restricted to 25&ndash;50 m above modern positions and are in approximate agreement with water-table rises estimated from zeolitic-to-vitric transitions in the Yucca Mountain tuffs (less than 60 m in the last 11.6 m.y.).

Nevada↗
USGS2012

Transient effects on groundwater chemical compositions from pumping of supply wells at the Nevada National Security Site, 1951-2008

Nuclear testing and support activities at the Nevada National Security Site have required large amounts of water for construction, public consumption, drilling, fire protection, hydraulic and nuclear testing, and dust control. To supply this demand, approximately 20,000 million gallons of water have been pumped from 23 wells completed in 19 boreholes located across the Nevada National Security Site starting as early as the 1950s. As a consequence of more or less continuous pumping from many of these wells for periods as long as 58 years, transient groundwater flow conditions have been created in the aquifers that supplied the water. To evaluate whether long-term pumping caused changes in water compositions over time, available chemical analyses of water samples from these 19 boreholes were compiled, screened, and evaluated for variability including statistically significant temporal trends that can be compared to records of groundwater pumping. Data used in this report have been extracted from a large database (Geochem08, revision 3.0, released in September 2008) containing geochemical and isotopic information created and maintained by primary contractors to the U.S. Department of Energy, National Nuclear Security Administration, Nevada Site Office. Data extracted from this source were compiled for the entire period of record, converted to uniform reporting units, and screened to eliminate analyses of poor or unknown quality, as well as clearly spurious values. The resulting data are included in accompanying spreadsheets that give values for (1) pH and specific conductance, (2) major ion concentrations, (3) trace element concentrations and environmental isotope ratios, and (4) mean, median, and variance estimates for major ion concentrations. The resulting data vary widely in quality and time-series density. An effort has been made to establish reasonable ranges of analytical uncertainty expected for each analyte and eliminate analyses that are obvious outliers. Analysis of chemical trends in this report primarily rely on specific conductance measurements and major ion concentrations, data considered to be the most accurate and reliable over the entire time span of investigation. The analysis uses parametric and non-parametric evaluations to provide a statistical basis for trend identification. Trace element and isotope data are examined for consistency, but typically are too inaccurate or infrequent to provide a reliable long term basis for trend evaluation. Groundwater withdrawal records compiled in a companion report are included graphically in this report to allow qualitative comparisons between water quality and pumping history. Data for each supply well include (1) a borehole description and summary of pumping history, (2) a description of water-quality parameters, (3) an evaluation of temporal variations of specific conductance and major ion concentrations, and (4) an examination of supporting information from trace element and isotope data. A range of responses are observed for individual supply wells that likely include the effects of both aquifer dynamics and changing borehole conditions. Data from most wells show little or no evidence for temporal variation in water-quality parameters indicating that aquifers at the Nevada National Security Site are capable of producing large volumes of compositionally uniform water over many years of pumping. A smaller number of wells show evidence of transient changes. Wells that have ceased pumping commonly show compositions that shift toward lower concentrations in subsequent bailed samples, which indicates that more dilute water entered the well over time due to either leakage of meteoric water into the well casing or more wide-spread recharge into the shallow phreatic zone. Wells that show systematic changes in water compositions during episodes of pumping commonly have multiple open intervals whose contributions to water in the well may change over time due to hydraulic conditions or well dynamics.

Nevada↗
USGS2011

Chronology, sedimentology, and microfauna of groundwater discharge deposits in the central Mojave Desert, Valley Wells, California

During the late Pleistocene, emergent groundwater supported persistent and long-lived desert wetlands in many broad valleys and basins in the American Southwest. When active, these systems provided important food and water sources for local fauna, supported hydrophilic and phreatophytic vegetation, and acted as catchments for eolian and alluvial sediments. Desert wetlands are represented in the geologic record by groundwater discharge deposits, which are also called spring or wetland deposits. Groundwater discharge deposits contain information on the timing and magnitude of past changes in water-table levels and, thus, are a source of paleohydrologic and paleoclimatic information. Here, we present the results of an investigation of extensive groundwater discharge deposits in the central Mojave Desert at Valley Wells, California. We used geologic mapping and stratigraphic relations to identify two distinct wetland sequences at Valley Wells, which we dated using radiocarbon, luminescence, and uranium-series techniques. We also analyzed the sediments and microfauna (ostracodes and gastropods) to reconstruct the specific environments in which they formed. Our results suggest that the earliest episode of high water-table conditions at Valley Wells began ca. 60 ka (thousands of calendar yr B.P.), and culminated in peak discharge between ca. 40 and 35 ka. During this time, cold (4–12 °C) emergent groundwater supported extensive wetlands that likely were composed of a wet, sedge-rush-tussock meadow mixed with mesic riparian forest. After ca. 35 ka, the water table dropped below the ground surface but was still shallow enough to support dense stands of phreatophytes through the Last Glacial Maximum (LGM). The water table dropped further after the LGM, and xeric conditions prevailed until modest wetlands returned briefly during the Younger Dryas cold event (13.0–11.6 ka). We did not observe any evidence of wet conditions during the Holocene at Valley Wells. The timing of these fluctuations is consistent with changes in other paleowetland systems in the Mojave Desert, the nearby Great Basin Desert, and in southeastern Arizona, near the border of the Sonoran and Chihuahuan Deserts. The similarities in hydrologic conditions between these disparate locations suggest that changes in groundwater levels during the late Pleistocene in desert wetlands scattered throughout the American Southwest were likely driven by synoptic-scale climate processes.

California↗
USGS2010

Cosmogenic nuclide and uranium-series dating of old, high shorelines in the western Great Basin, USA

Closed-basin pluvial lakes are sensitive recorders of effective moisture, and they provide a terrestrial signal of climate change that can be compared to marine and ice records of glacial-interglacial cycles. Although the most recent deep-lake cycle in the western Great Basin (at ca. 16 ka) has been studied intensively, comparatively little is known about the longer-term Quaternary lacustrine history of the region. Lacustrine features higher than those of the most recent highstand have been discovered in many locations throughout the western Great Basin. Qualitative geomorphic and soil studies of shoreline sequences above the latest Pleistocene level suggest that their ages increase as a function of increasing altitude. The results of cosmogenic nuclide dating using chlorine-36 depth profiles from three sites in Nevada (Walker Lake, Columbus Salt Marsh, and Newark Valley), combined with uranium-series and radiocarbon ages, corroborate the geomorphic and soil evidence. The 36 Cl results are consistent with available 14 C ages and together indicate that the most recent highstands of all three lakes occurred ca. 20–15 ka, late in marine isotope stage (MIS) 2, as shown by previous ages. The 36 Cl ages indicate that older lakes in all three basins reached highstands between 100 and 50 ka, and most likely during MIS 4. Shorelines of this age are at about the same or higher altitudes as the younger, MIS 2 shorelines in those basins. The 36 Cl results combined with uranium-series ages and one tephra correlation obtained on shorelines higher in altitude than those of MIS 4 and 2 lakes suggest that there were also major lake highstands in the western Great Basin at ca. 100–200 ka, likely corresponding with MIS 6, and during at least two older periods. From these results, we conclude that the preserved shorelines show an apparent decrease in maximum levels with time, suggesting long-term drying of the region since the early middle Pleistocene.

California, Oregon, and Nevada↗
USGS2007

Strontium Isotopic Composition of Paleozoic Carbonate Rocks in the Nevada Test Site Vicinity, Clark, Lincoln, and Nye Counties, Nevada, and Inyo County, California

Ground water moving through permeable Paleozoic carbonate rocks represents the most likely pathway for migration of radioactive contaminants from nuclear weapons testing at the Nevada Test Site, Nye County, Nevada. The strontium isotopic composition (87Sr/86Sr) of ground water offers a useful means of testing hydrochemical models of regional flow involving advection and reaction. However, reaction models require knowledge of 87Sr/86Sr data for carbonate rock in the Nevada Test Site vicinity, which is scarce. To fill this data gap, samples of core or cuttings were selected from 22 boreholes at depth intervals from which water samples had been obtained previously around the Nevada Test Site at Yucca Flat, Frenchman Flat, Rainier Mesa, and Mercury Valley. Dilute acid leachates of these samples were analyzed for a suite of major- and trace-element concentrations (MgO, CaO, SiO2, Al2O3, MnO, Rb, Sr, Th, and U) as well as for 87Sr/86Sr. Also presented are unpublished analyses of 114 Paleozoic carbonate samples from outcrops, road cuts, or underground sites in the Funeral Mountains, Bare Mountain, Striped Hills, Specter Range, Spring Mountains, and ranges east of the Nevada Test Site measured in the early 1990's. These data originally were collected to evaluate the potential for economic mineral deposition at the potential high-level radioactive waste repository site at Yucca Mountain and adjacent areas (Peterman and others, 1994). Samples were analyzed for a suite of trace elements (Rb, Sr, Zr, Ba, La, and Ce) in bulk-rock powders, and 87Sr/86Sr in partial digestions of carbonate rock using dilute acid or total digestions of silicate-rich rocks. Pre-Tertiary core samples from two boreholes in the central or western part of the Nevada Test Site also were analyzed. Data are presented in tables and summarized in graphs; however, no attempt is made to interpret results with respect to ground-water flow paths in this report. Present-day 87Sr/86Sr values are compared to values for Paleozoic seawater present at the time of deposition. Many of the samples have 87Sr/86Sr compositions that remain relatively unmodified from expected seawater values. However, rocks underlying the northern Nevada Test Site as well as rocks exposed at Bare Mountain commonly have elevated 87Sr/86Sr values derived from post-depositional addition of radiogenic Sr most likely from fluids circulating through rubidium-rich Paleozoic strata or Precambrian basement rocks.

Data Series↗
USGS2006

Geochemical and isotopic evaluation of groundwater movement in corrective action Unit 97: Yucca Flat/Climax Mine, Nevada Test Site, Nevada, rev. no.: 0

This report describes the results of a comprehensive geochemical evaluation of the groundwater flow system in the Yucca Flat/Climax Mine Corrective Action Unit (CAU). The main objectives of this study are to identify probable pathways for groundwater flow within the study area and to develop constraints on groundwater transit times between selected data collection sites. This work provides an independent means of testing and verifying predictive flow models being developed for this CAU using finite element methods. The Yucca Flat/Climax Mine CAU constitutes the largest of six underground test areas on the Nevada Test Site (NTS) specified for remedial action in the ''Federal Facility Agreement and Consent Order''. A total of 747 underground nuclear detonations were conducted in this CAU. Approximately 23 percent of these detonations were conducted below or near the water table, resulting in groundwater contamination in the vicinity and possibly downgradient of these underground test locations. Therefore, a rigorous evaluation of the groundwater flow system in this CAU is necessary to assess potential long-term risks to the public water supply at downgradient locations.

Nevada↗
USGS2005

Evaluation of groundwater movement in the Frenchman Flat CAU using geochemical and isotopic analysis

The principal pathway for radionuclide migration from underground tests in Frenchman Flat, on the Nevada Test Site, to the accessible environment is groundwater flow. Two potential pathways for radionuclide transport via groundwater have been identified from hydrologic data: (1) radionuclide transport downward from the alluvial and volcanic aquifers into the underlying carbonate aquifer; and (2) radionuclide transport laterally to the carbonate aquifer surrounding Frenchman Flat. This report presents an evaluation of geochemical and environmental isotopic data to test these potential pathways and to identify other groundwater flowpaths in, and out of, Frenchman Flat.

Nevada↗
USGS2005

Geologic and geophysical maps of the Las Vegas 30' x 60' quadrangle, Clark and Nye counties, Nevada, and Inyo County, California

Las Vegas and Pahrump are two of the fastest growing cities in the US, and the shortage of water looms as among the greatest future problems for these cities. These new maps of the Las Vegas 30 x 60-minute quadrangle provide a geologic and geophysical framework and fundamental earth science database needed to address societal issues such as ground water supply and contamination, surface flood, landslide, and seismic hazards, and soil properties and their changing impact by and on urbanization. The mountain ranges surrounding Las Vegas and Pahrump consist of Mesozoic, Paleozoic and Proterozoic rocks. A majority of these rocks are Paleozoic carbonate rocks that are part of Nevada's carbonate rock aquifer province. The Spring Mountains represent a major recharge site in the province, where maximum altitude is 3,632 m (Charleston Peak) above sea level. Rocks in the Sheep and Las Vegas Ranges and Spring Mountains contain correlative, northeast-striking, southeast-verging thrust faults that are part of the Cretaceous, Sevier orogenic belt. These thrusts were offset during the Miocene by the Las Vegas Valley shear system (LVVSZ). We conducted new mapping in the Blue Diamond area, highlighting refined work on the Bird Spring thrust, newly studied ancient landslides, and gravity-slide blocks. We conducted new mapping in the Las Vegas Range and mapped previously unrecognized structures such as the Valley thrust and fold belt; recognition of these structures has led to a refined correlation of Mesozoic thrust faults across the LVVSZ. New contributions in the quadrangle also include a greatly refined stratigraphy of Paleozoic bedrock units based on conodont biostragraphy. We collected over 200 conodont samples in the quadrangle and established stratigraphic reference sections used to correlate units across the major Mesozoic thrust faults. Quaternary deposits cover about half of the map area and underlie most of the present urbanized area. Deposits consist of large coalescing alluvial fans that grade downslope to extensive areas of fine-grained sediment indicative of groundwater-discharge during the Pleistocene. In the central areas of Las Vegas and Pahrump valleys, Quaternary fault scarps associated with past ground-water discharge deposits suggest a genetic relationship. In collaboration with NBMG and University of Nevada, a variety of ages of gravelly alluvium are newly mapped using surficial characteristics and soil development, along with reassessment of previously published mapping during compilation. Reconnaissance geochronology (thermoluminescence and U-series) of eolian and authigenic components of surficial and buried soils and spring deposits is applied to test hypotheses of geomorphic and hydrologic response to climate change over the past 100 k.y.). The major structure in the Las Vegas quadrangle is the LVVSZ. Because the LVVSZ is concealed by thick basin-fill deposits of Quaternary and Tertiary age, it was characterized primarily based on geophysics. Likewise, the newly described State line fault system in Pahrump Valley has also been characterized by geophysics, where geophysically inferred structures correlate remarkably with surface structures defined by our new geologic mapping in the Mound Spring and Hidden Hills Ranch 7.5-minute quadrangles.

Scientific Investigations Map↗
USGS2003

Geologic map of the Mound Spring quadrangle, Nye and Clark Counties, Nevada, and Inyo County, California

The Mound Spring quadrangle, the southwestern-most 7.5' quadrangle of the area of the Las Vegas 1:100,000-scale quadrangle, is entirely within the Pahrump Valley, spanning the Nevada/California State line. New geologic mapping of the predominantly Quaternary materials is combined with new studies of gravity and geochronology in this quadrangle. Eleven predominantly fine-grained units are delineated, including playa sediment, dune sand, and deposits associated with several cycles of past groundwater discharge and distal fan sedimentation. These units are intercalated with 5 predominantly coarse-grained alluvial-fan and wash gravel units mainly derived from the Spring Mountains. The gravel units are distinguished on the basis of soil development and associated surficial characteristics. Thermoluminescence and U-series geochronology constrain most of the units to the Holocene and late and middle Pleistocene. Deposits of late Pleistocene groundwater discharge in the northeast part of the quadrangle are associated with a down-to-the-southwest fault zone that is expressed by surface fault scarps and a steep gravity gradient. The gravity field also defines a northwest-trending uplift along the State line, in which the oldest sediments are poorly exposed. About 2 km to the northeast a prominent southwest-facing erosional escarpment is formed by resistant beds in middle Pleistocene fine-grained sediments that dip northeast away from the uplift. These sediments include cycles of groundwater discharge that were probably caused by upwelling of southwesterly groundwater flow that encountered the horst.

Miscellaneous Field Studies Map↗