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R. Ernest Anderson

Publications and source records attributed to R. Ernest Anderson.

At least 19 recordsLinked to original sources
USGS2013

Analysis of Neogene deformation between Beaver, Utah and Barstow, California: Suggestions for altering the extensional paradigm

For more than two decades, the paradigm of large-magnitude (~250 km), northwest-directed (~N70°W) Neogene extensional lengthening between the Colorado Plateau and Sierra Nevada at the approximate latitude of Las Vegas has remained largely unchallenged, as has the notion that the strain integrates with coeval strains in adjacent regions and with plate-boundary strain. The paradigm depends on poorly constrained interconnectedness of extreme-case lengthening estimated at scattered localities within the region. Here we evaluate the soundness of the inferred strain interconnectedness over an area reaching 600 km southwest from Beaver, Utah, to Barstow, California, and conclude that lengthening is overestimated in most areas and, even if the estimates are valid, lengthening is not interconnected in a way that allows for published versions of province-wide summations. We summarize Neogene strike slip in 13 areas distributed from central Utah to Lake Mead. In general, left-sense shear and associated structures define a broad zone of translation approximately parallel to the eastern boundary of the Basin and Range against the Colorado Plateau, a zone we refer to as the Hingeline shear zone. Areas of steep-axis rotation (ranging to 2500 km 2 ) record N-S shortening rather than unevenly distributed lengthening. In most cases, the rotational shortening and extension-parallel folds and thrusts are coupled to, or absorb, strike slip, thus providing valuable insight into how the discontinuous strike-slip faults are simply parts of a broad zone of continuous strain. The discontinuous nature of strike slip and the complex mixture of extensional, contractional, and steep-axis rotational structures in the Hingeline shear zone are similar to those in the Walker Lane belt in the west part of the Basin and Range, and, together, the two record southward displacement of the central and northern Basin and Range relative to the adjacent Colorado Plateau. Understanding this province-scale coupling is critical to understanding major NS shortening and westerly tectonic escape in the Lake Mead area. One north-elongate uplift in the Hingeline shear zone is a positive flower structure along a strike-slip fault, and we postulate that most other large uplifts are diapiric, resulting from extension-normal inflow of ductile substrate, rather than second-order isostatic responses to tectonic unloading. We also postulate that large steep-axis rotations, and some small ones as well, result from basal tractions imparted by gradients in southerly directed subjacent ductile flow rather than by shear coupling imparted by laterally variable elongation strains. The shortening strain recorded in the rotations and related structures probably matches or exceeds the magnitude of lengthening, even for the Lake Mead area where we do not question local large (~65 km) west-directed lengthening. We assess the results of extensive recent earth-science research in the Lake Mead area and conclude that previously published models of N-S convergence, westerly tectonic rafting, and N-S occlusion are valid and record unique tectonic escape accommodation for south-directed displacement of the Great Basin sector of the Basin and Range. Genetic ties between the south-directed displacement and plate-interaction forces are elusive, and we suggest the displacement results from body forces inherent in the Basin and Range.

Arizona, California, Nevada, Utah↗
USGS2010

Geology of the Lake Mead region: An overview

The Lake Mead region contains major Miocene disruptions of structures formed during Mesozoic tectonic shortening. Erosion by the Colorado River and its tributaries has produced exceptional exposures of diverse structures and basin deposits recording the disruptions. Here we provide an overview of the results of studies of these features that started in earnest in 1934 when Chester Longwell began assessing the geology of the reservoir floor prior to impoundment of Lake Mead. The analysis was reinvigorated in the 1970s and early 1980s with geological mapping and structural and stratigraphic studies by Ernie Anderson and Bob Bohannon, as well as geochemical and volcanological studies by Gene Smith and his students, and has culminated in numerous subsequent studies.

Arizona, Nevada, Utah↗
USGS2010

Geometry and kinematics of the eastern Lake Mead fault system in the Virgin Mountains, Nevada and Arizona

The Lake Mead fault system is a northeast-striking, 130-km-long zone of left-slip in the southeast Great Basin, active from before 16 Ma to Quaternary time. The northeast end of the Lake Mead fault system in the Virgin Mountains of southeast Nevada and northwest Arizona forms a partitioned strain field comprising kinematically linked northeast-striking left-lateral faults, north-striking normal faults, and northwest-striking right-lateral faults. Major faults bound large structural blocks whose internal strain reflects their position within a left step-over of the left-lateral faults. Two north-striking large-displacement normal faults, the Lakeside Mine segment of the South Virgin–White Hills detachment fault and the Piedmont fault, intersect the left step-over from the southwest and northeast, respectively. The left step-over in the Lake Mead fault system therefore corresponds to a right-step in the regional normal fault system. Within the left step-over, displacement transfer between the left-lateral faults and linked normal faults occurs near their junctions, where the left-lateral faults become oblique and normal fault displacement decreases away from the junction. Southward from the center of the step-over in the Virgin Mountains, down-to-the-west normal faults splay northward from left-lateral faults, whereas north and east of the center, down-to-the-east normal faults splay southward from left-lateral faults. Minimum slip is thus in the central part of the left step-over, between east-directed slip to the north and west-directed slip to the south. Attenuation faults parallel or subparallel to bedding cut Lower Paleozoic rocks and are inferred to be early structures that accommodated footwall uplift during the initial stages of extension. Fault-slip data indicate oblique extensional strain within the left step-over in the South Virgin Mountains, manifested as east-west extension; shortening is partitioned between vertical for extension-dominated structural blocks and south-directed for strike-slip faults. Strike-slip faults are oblique to the extension direction due to structural inheritance from NE-striking fabrics in Proterozoic crystalline basement rocks. We hypothesize that (1) during early phases of deformation oblique extension was partitioned to form east-west–extended domains bounded by left-lateral faults of the Lake Mead fault system, from ca. 16 to 14 Ma. (2) Beginning ca. 13 Ma, increased south-directed shortening impinged on the Virgin Mountains and forced uplift, faulting, and overturning along the north and west side of the Virgin Mountains. (3) By ca. 10 Ma, initiation of the younger Hen Spring to Hamblin Bay fault segment of the Lake Mead fault system accommodated westward tectonic escape, and the focus of south-directed shortening transferred to the western Lake Mead region. The shift from early partitioned oblique extension to south-directed shortening may have resulted from initiation of right-lateral shear of the eastern Walker Lane to the west coupled with left-lateral shear along the eastern margin of the Great Basin.

Arizona, Nevada↗
USGS2010

Integration of tectonic, sedimentary, and geohydrologic processes leading to a small-scale extension model for the Mormon Mountains area north of Lake Mead, Lincoln County, Nevada

Scattered remnants of highly diverse stratigraphic sections of Tertiary lacustrine limestone, andesite flows, and 23.8–18.2 Ma regional ash-flow tuffs on the north flank of the Mormon Mountains record previously unrecognized deformation, which we interpret as pre–17 Ma uplift and possibly weak extension on the north flank of a growing dome. Directly to the north of the Mormon dome, 17–14 Ma ash-flow tuffs and rhyolite are interstratified with landslides, debris avalanches, debris flows, and alluvial-fan deposits that accumulated to a thickness of more than 2 km in an extension-parallel basin. The source for the landslides and debris avalanche deposits is unknown, but it was probably an adjacent scarp along a transverse fault bounding an early part of the Mormon dome. An average 45° of easterly tilt of the entire Tertiary basin-fill succession represents the major post–14 Ma deformation event in the region. We question the basis for the published estimate of 22 km of westerly displacement on the Mormon Peak detachment fault and, on the basis of landslides in the upper plate having a probable source in the adjacent Mormon dome, constrain the heave to ~4 km. We interpret the dome and basin as coupled strains similar to others in the region and suggest that these strains reflect a waveform pattern of extension-normal lateral midcrustal ductile flow. Previously, doming was interpreted as an isostatic response to tectonic unloading by large-displacement detachment faults or as pseudo-structural highs stranded by removal of middle crust from adjacent areas. Moreover, we argue that the strong thinning of upper-plate rock successions throughout the Mormon Mountains and Tule Springs Hills resulted from a loss of rock volume by protracted fluid flow, dissolution, and collapse, seriously limiting the usefulness of upper-plate strain in evaluating extension magnitude. We present a geohydrologic model that couples uplift driven by ductile inflow with dissolution driven by fluid infiltration, possibly augmented by mantle-derived CO 2 -rich fluids. Karsting in the uplands led to carbonate sedimentation in adjacent lowlands. Whether or not our downward revision of extension in the Mormon Mountains is valid, extension at that latitude is isolated from extension in the Lake Mead area by a low-strain corridor between the two areas. Recognition of the isolated and potentially diminished strain impacts estimates of maximum finite elongation of the Basin and Range Province because one of three vector paths used in those estimates passes through the Mormon Mountains.

Nevada↗
USGS2010

Fluid flow, solution collapse, and massive dissolution at detachment faults, Mormon Mountains, Nevada

Dissolution has removed large volumes of rock at low-angle normal faults, i.e., detachment faults, in the Mormon Mountains and the Tule Springs Hills in the eastern Basin and Range Province, southeastern Nevada. Evidence for major dissolution includes widespread solution-collapse breccias, meter-scale stylolite structures, and high-angle accommodation faults that terminate at or merge with dissolution seams. Chemically reactive fluids moving along the fault zones led to a strong depletion of 18 O in the detachment fault breccias (e.g., a δ 18 O decrease of 8‰ relative to the unaltered rocks). These strong chemical shifts, demonstrated by (1) negative oxygen isotope values and (2) steep compositional gradients marked by metal enrichment in elements such as Au, Ag, Ti, Pb, Zn, and Cu, are generally restricted to the narrow (<1 m to 8 m) microbreccia zones. Extensional faulting and fracturing, accompanying regional uplift, opened conduits for the influx of meteoric waters from above and hydrothermal fluids from below. As the largest, most permeable structures that formed during uplift, detachment faults focused the fluid flow. In this deformation and hydrogeologic model, dissolution-caused stratal thinning is a major complement to detachment faulting and is an important process that resolves void space issues in the reconstruction of cross section.

Nevada↗
USGS1999

Yucca Mountain as a Radioactive-Waste Repository

Yucca Mountain straddles the west boundary of the Nevada Test Site in an arid, remote, and thinly populated region of southwestern Nevada. It is the potential site of a monitored geologic repository for the Nation’s commercial and military spent nuclear fuel, high-level radioactive waste derived from reprocessing of uranium and plutonium, surplus plutonium, and other nuclear-weapons materials. (Collectively, these radioactive materials are known as high-level waste [HLW] and are to be distinguished from the low-level radioactive waste to be stored at the recently opened Waste Isolation Pilot Plant in southeastern New Mexico.) Tens of thousands of metric tons of HLW is presently stored at more than a hundred sites in 40 States (fig. 1). The fundamental rationale for a geologic repository for radioactive materials is to securely isolate them from the environment and its occupants to the greatest extent possible.

Nevada↗
USGS1978

Chemistry of Tertiary volcanic rocks in the Eldorado Mountains, Clark County, Nevada, and comparisons with rocks from some nearby areas

Chemical variations of 27 rocks that erupted from in and near the Eldorado Mountains, southern Nevada, show (1) fair to good correlation suggestive of a cogenetic suite, (2) a range of uncorrected silica values from about 48 to 74 weight percent with no strong tendency toward bimodality, (3) a calc-alkaline characteristic, and (4) a shift from early eruptions having an anomalously low level of iron enrichment to later eruptions having a more normal level of iron enrichment. Analyses of rocks not known to have been erupted from the Eldorado Mountains (for example, five analyses of the tuff of Bridge Spring and two analyses of the Fortification Basalt Member of the Muddy Creek Formation) are not anomalous when compared with the other rocks, except that iron concentration in the Fortification Basalt Member may be higher than normal. Extensive K-Ar age data provide a clear time framework for the chemical changes as well as for the structural history of the area. The inception of intense extensional tectonism about 15 million years ago was marked by an abrupt shift to more iron-rich and more typically calc-alkalic volcanic products (Mount Davis Volcanics) than those erupted earlier (Patsy Mine Volcanics), but no corresponding shift in K 2 O or total alkali contents or ratios is recognized nor is there any obvious shift toward bimodality in rocks younger than 15 million years. These data, together with data from the Oatman district of Arizona, 100 kilometers to the south-southeast, indicate that the northern part of the lower Colorado River area does not fit into the published Cenozoic patterns of volcanic and tectonic evolution of the western United States. A brief review of nearby areas indicates that they may not fit well either.

Nevada↗
USGS1977

Geology and ore deposits of the Kutam Mine, Kingdom of Saudi Arabia

The ancient Kutam mine in southwestern Saudi Arabia was discovered during the course of regional geologic mapping. Malachite coatings along fractures in the ancient workings and nearby large piles of slag indicate the mine was worked for copper. Quartz porphyry of Precambrian age is the principal host rock of the deposit. The quartz porphyry is associated with volcanic rocks of mafic to intermediate composition. All rocks have been deformed and metamorphosed to various types of schist, of which quartz-sericite-chlorite schist is the most common. Mineralization is thought to be controlled by the convergence of three fault patterns that strike N 60&deg;W., N 45&deg;W and N. 20&deg;W, respectively, and dip steeply west to vertical. Cross faults that extend between faults of the N 45&deg;W. pattern have influenced deposition of sulfides locally. Southeast-plunging lineation derived from two converging directions of planar schistosity also may have exerted control on sulfide deposition. Detailed geologic mapping of surface features and geochemical and geophysical studies indicated that the deposit warranted subsurface exploration. Accordingly, eight holes were drilled that delineated possible economic-grade sulfide deposits in an area about 300 m long and which is known to continue to a depth of more than 200 m. However, drilling found copper-zinc mineralization along a strike length of 850 m. Drill findings show that copper and zinc sulfides partially replace chlorite and higher concentrations of chlorite generally define the higher-grade mineralization. The more intensely chloritized zones tend to follow the N.45&deg;W. fracture pattern. South of drill hole KA-I, the sulfide mineralization may plunge 40&deg; to 60&deg; southeast. Indicated and inferred reserves outlined by drilling total 8,056,000 metric tons, the grade of which averages 0.31 g/t Au, 6.13 g/t Ag, 1.83 percent Cu, and 0.95 percent Zn.

Open-File Report↗
USGS1975

The influence of late Cenozoic stratigraphy on distribution of impoundment-related seismicity at Lake Mead, Nevada-Arizona

At Lake Mead, contrasts in permeability of upper Cenozoic sediments show a better correlation with irregularly distributed impoundment-related seismicity than do contrasts in structure. An evaluation of structures developed during the late Cenozoic fails to explain the erratic distribution of seismicity. An evaluation of the late Cenozoic stratigraphy, however, shows a concentration of relatively impermeable evaporite beds and fine-grained clastic strata in the less seismic part of the lake basin; therefore, the authors conclude that a hydraulic connection between the lake water and the deep aquifer system that includes buried faults is needed in the Lake Mead area to cause the release of seismic energy. Where hydraulic connection is prevented by continuous or quasi-continuous upper Cenozoic basin-fill strata of low permeability, as in the eastern basin area, seismicity does not occur.

Arizona, Nevada↗
USGS1973

Geologic and hydrologic summary of salt domes in Gulf Coast region of Texas, Louisiana, Mississippi, and Alabama

There are 263 known or suspected onshore salt domes in the Texas-Louisiana-Mississippi-Alabama portion of the Gulf Coast geosyncline. The top of the salt in 148 of them is probably deeper than desirable for a waste repository site, and 79 of those that are shallow enough are probably unavailable for a site because of present use by industry for gas storage or production of oil, salt, or sulfur. In this report we have compiled the available geologic and hydrologic background data pertinent to the evaluation of the remaining 36 known or suspected salt domes as potential sites for waste storage. There are three parts to this compilations: 1) summaries of the geology and hydrology of the salt-dome province as a whole; 2) summaries of the physiography, climate, geology, and hydrology of each of the five salt-dome basins that occur within the province; and 3) an appendix of background data for each of the 36 potentially acceptable domes. The distribution of salt domes in the province is genetically related to areas of relative subsidence that formed basins or depocenters within the Gulf Coast geosyncline. In some cases, as in northeast Texas and south Louisiana, the locations of individual domes or groups of domes are related to deep movement of salt along axial trends. The salt domes in the interior salt-dome subprovince are probably more structurally stable than those of the coastal subprovince because salt diapirism is inferred to have ceased around Miocene time in the interior but may still be active in parts of the coastal subprovince. Although the size and shape of many domes is unknown or can only be roughly approximated, each of the five basins in the province appears to contain potentially acceptable domes of adequate size for a repository. We recognize no pattern to the distribution of salt-dome size. Caprock thicknesses vary greatly within each salt-dome basin,and we recognize no pattern to the variations. Among the potentially acceptable domes, the depths to the top of the salt are generally greatest in the Mississippi salt-dome basin, where all tops are more than 1,500 feet deep. Intermediate depths of about 1,000 feet are common in the east Texas-south Louisiana salt-dome basin. Depths to salt tops in the north Louisiana and northeast Texas basins are variable but most are less than 1,000 feet. Available drilling records are generally adequate to determine the number of wells drilled on or in the vicinity of individual domes and also the well locations. The numbers of wells vary widely within each salt-dome basin. More salt domes are currently available for use as repository sites in the interior subprovince than in the coastal subprovince, where the pressure for industrial use of domes is high. In the interior subprovince many of the potentially acceptable domes are located beneath hilly well-drained terrain that is not subject to flooding or other surface-water problems. Although topographic depressions occupied by shallow lakes, swamps, or 'salines' occur over several of the domes, they are generally flanked by topographically high ground where surface facilities could be sited without complications.A few of the potentially acceptable domes are located beneath floodplains where surface facilities might face hazards from flooding. In the coastal subprovince several of the potentially acceptable domes are located in relatively flat poorly drained terrain where surface flooding might constitute a potential hazard. The availability of fresh to slightly saline ground water varies considerably within each salt-dome basin. We have outlined some of the factors that are responsible for the variations and have provided or referred to maps and geohydrologic cross sections that illustrate the general distribution of fresh to slightly saline water. The top of the salt in about half of the known potentially acceptable domes is below the regional base of the fresh to slightly saline ground-water system, but in a few of

Alabama, Louisiana, Mississippi, Texas↗