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At least 1,081 records · Page 60Linked to original sources
USGS1978

Comendite (peralkaline rhyolite) and basalt in the Mitu Group, Peru: Evidence for Permian-Triassic lithospheric extension in the central Andes

The Mitu Group consists of generally coarse clastic strata and volcanic rock of Permian and (or) Triassic age filling elongate basins that parallel the general structural trend of the Peruvian Andes. Volcanic rocks of the Mitu Group include peralkaline and nonperalkaline rhyolite and subalkaline basalt. To our knowledge, the peralkaline rhyolites are the first of this rock type reported from South America. The presence of appreciable volumes of peralkaline rhyolite and basalt, supports the interpretation that the Mitu Group was deposited in major graben structures that resulted from lithospheric extension produced by rifting or possible backarc extension.

central Andes Mountains↗
USGS1978

Fusion of granodiorite by basalt, central Sierra Nevada

A trachybasalt plug, 100 m in diameter, has partially fused inclusions and wallrock of porphyritic granodiorite near Tuolumne Meadows, Yosemite National Park, Calif. Granodiorite surrounding the plug was altered within distances of about 3.5 m. Within this distance, (1) trace amounts of glass occur along fractures and grain boundaries, (2) biotite has been altered from dark olive to dark reddish brown and contains bands of fine reddish iron oxide grains, and (3) optic axial angles of potassium feldspar decrease toward the contact. As much as 20 volume percent of glass occurs in the granodiorite in a reentrant and in inclusions within the plug. Detailed study of two partially fused samples shows that, relative to unfused granodiorite, SiO 2 is clearly depleted in both samples, whereas K 2 O is depleted in one but not the other. Total iron, Al 2 O 3 , MgO, Na 2 O, and H 2 O show apparent increases in both samples; other constituents show no significant changes. Chemical analyses suggest that much original material was lost from the partially fused rocks, probably by mass migration of melt rather than by chemical diffusion. The composition of glasses now present in the partially fused rocks was dominated by melting of quartz and feldspars, whereas the composition of the early-formed "lost" melt was strongly influenced by subsolidus reaction and subsequent melting of biotite.

California↗
USGS1978

Chabazite in siliceous tuffs of a Pliocene lacustrine deposit near Durkee, Baker County, Oregon

A relatively pure chabazite deposit of at least 75 X 10 6 metric tons occurs near the Durkee type locality of erionite. This erionite is in a welded tuff interbedded with lacustrine rocks- of a closed Pliocene basin. The zeolites, chabazite, erionite, clinoptilolite, and other authigenic silicate minerals are confined to a 27-square-kilometer area of altered rocks enclosed in relatively freshwater lacustrine beds. About 135 meters of the lower part of the 350-m stratigraphic section at the southeastern part of the lake basin contain zeolitic tuff. The chabazite is an early product of the reaction between glass of rhyolitic or dacitic composition and alkaline (pH>9) saline lake waters. Erionite, clinoptilolite, analcime, and potassium feldspar follow chabazite in the paragenetic sequence. At the erionite type locality, chabazite and clinoptilolite form megascopic crystal clusters within fractures and cavities in unaltered welded tuff. Elsewhere in the basin, most zeolite crystals in the altered airfall or reworked tuffs are less than 80 micrometers long. Chemical analyses, unit-cell determinations, and mean indices of refraction show that fine-grained chabazite varies in composition and physical parameters.

Oregon↗
USGS1978

Differentiation of the sulfides in the Basal Zone of the Stillwater Complex, Montana

Chemical leaching and analysis of the sulflde portion of rocks from a section of the Basal zone of the Stillwater Complex demonstrate that mole fractions of nickel, copper, cobalt, and sulfur in the sulfldes show trends and repeating patterns when plotted against stratigraphic position. These patterns are attributed to differentiation of immiscible sulflde liquids probably in response to differentiation of basaltic magma. Changes in composition of the immiscible sulflde liquids could most readily take place during the collection of the sulflde from the basaltic magma or during the crystallization of the sulflde liquid to the present assemblages of minerals.

Montana↗
USGS1978

Recognition of oxidized sulfide minerals as an exploration guide for uranium

The difference in color between tan to red oxidized sandstone and gray reduced sandstone on either side of the reduction-oxidation (redox) interface, which is- the locus of uranium concentration in roll-type deposits, has been noted and used locally as an exploration guide within known uranium districts. Reduced sandstone is characterized in many deposits by the presence of iron sulflde minerals (particularly marcasite and pyrite) that occur as replacements of and overgrowths on iron-titanium oxide minerals and plant debris and as cement of detrital grains. Oxidation of the sulfldes by oxygenated ground water forms yellow to red ferric oxide and ferric hydroxide phases ("limonite") that replace the FeS 2 minerals. Processes other than the oxidation of sulfldes, however, can cause reddening of clastic sediments without the creation of a redox boundary, and so color alone is not a sufficient condition by which to judge the potential for uranium ore in frontier areas. Ferric oxides that form by the oxidation of iron sulflde minerals can be distinguished from other forms of ferric oxide by reflection microscopic examination of polished grain mounts- and polished thin sections'. Diagnostic features of oxidized sulfldes are limonite pseudomorphs of sulfldes and limonite containing internal textures that mimic characteristic textures of previously sulfldized detrital phases. Recognition of oxidized sulfides by reflection microscopy, then, can distinguish oxidized sandstones, which might have a redox boundary downdip and which would therefore be favorable hosts for uranium concentration, from those without such potential.

Journal of Research of the U.S. Geological Survey↗
USGS1978

Discrimination of fluvial and eolian deposits by number-frequency analysis of sediments of sand through silt size from a point bar, Rio Puerco, New Mexico

Fluvial and eolian deposits are interbedded in ephemeral-stream, arid-region point bars in the channel of the Rio Puerco, west of Albuquerque, N. Mex. Number frequency distributions of grain size for both types of deposits were obtained using an electronic automatic image analyzer for a suite of 10 samples. The number-frequency results were superior to weight-frequency (sieve) results in discriminating between eolian units and fluvial units in a vertical point-bar sequence. The image-analysis technique also identified those fluvial deposits whose size distributions have been altered significantly by winds, as well as fluvial deposits that do not show a prominent eolian-process component.

New Mexico↗
USGS1975

An occurrence of disseminated uraninite in Wheeler Basin, Grand County, Colorado

Disseminated uraninite occurs in Wheeler Basin, Grand County, Colo., about 5 mi (8 km) southeast of Monarch Lake, in Precambrian metamorphic rocks consisting of migmatized gneiss and mixed gneiss and pegmatite. An intrusion of Precambrian Y Silver Plume Granite lies within 400 ft (122 m) of the occurrence. The disseminated uraninite is confined to parts of the host rock that are rich in biotite; highest grade found was 0.73 percent uranium. The disseminated uraninite occurs as cubes and grains, generally from 0.1 to 0.3 mm across. Unit cell edge of the uraninite, ≈ 5.48 A, suggests its pegmatitic origin. The origin of the uraninite disseminations is attributed by us to remobilization and concentration of elements during metamorphism caused by the intrusion of Silver Plume Granite. Uranium and lead isotopic analyses by K. R. Ludwig of uraninite and monazite from biotite concentrations confirm an apparent age of 1,450±20 m.y. for these minerals. This age is equivalent to that reported for the Silver Plume Granite. Although the Wheeler Basin occurrence is small in size, it has many similarities to the Rossing uranium deposit in South-West Africa.

Colorado↗
USGS1978

Spectral gradient of lunar radiobrightness: Heat flow or volume scattering?

Lunar heat flow cannot be derived unambiguously from the spectral gradient of the radiobrightness. Volume scattering of microwaves by rock clasts within the lunar regolith results in a spectral component in the 5- to 30-centimeter range of wavelengths that is a significant fraction of that resulting from lunar heat flow.

Journal of Research of the U.S. Geological Survey↗
USGS1978

Accuracy of flood mapping

Information taken directly from published and unpublished reports was used to appraise the accuracy and the limitations of the three general flood-mapping methods: detailed, historical, and physiographic. In the appraisal, the probable nationwide average standard error of estimate for water depth and elevation in percentage of depth and in meters was determined for 100-year flood boundaries. Bias errors, users' errors, and certain time-variant errors were not. considered. The probable standard error for the detailed method is 23 percent. Based on 3.9 m as the national average 100-yr depth, the probable standard error of estimate is 0.9 m. The detailed method is applicable to a wide range of hydraulic and topographic conditions; it is the only method tested that is directly applicable to floodways analysis. The method requires detailed data and time-consuming analyses, however, and it has only limited application for determining flood elevations for sheet flow and for flow in channels having readily movable boundaries. The probable standard error for the historical method is 23 percent. Based on the 3.9-m average 100-yr depth, the standard error is 0.9 m. The historical method is applicable to a wide range of hydraulic and topographic conditions if specific data are available. The method is not applicable to floodways analysis, and it has only limited application for flow in moveable-boundary channels. The standard error for the physiographic method is 27 percent. In meters, the error is 1.1. .The physiographic method is simpler to apply than the other methods; however, it can be used only in natural channels having rigid boundaries. It is not applicable to sheet flow and to floodways analysis.

Journal of Research of the U.S. Geological Survey↗
USGS1978

Definition of regional relationships between dissolved solids and specific conductance, Susquehanna River basin, Pennsylvania and New York

This report presents statistical tests for assessing the accuracy and validity of two regional models of the relationship between dissolved-solids concentration and specific conductance. These models are used to estimate dissolved-solids concentrations based on specific-conductance measurements. The two regional models are compared with station models for 27 stream sites in the Susquehanna River basin. The average standard errors of estimate of two regional models, when used to estimate dissolved-solids concentrations for the 27 stream sites, are 10.7 and 10.6 percent. In comparison, the station models have average standard errors of 9.0 and 9.3 percent, respectively. Results of the statistical tests indicate that in the Susquehanna River basin the regional models can be used to estimate dissolved-solids concentrations from specific-conductance data for sites for which chemical analyses are not available.

New York, Pennsylvania↗
USGS1978

Preliminary evaluation of the floating dome method of measuring reaeration rates

The floating dome method of measuring reaeration rates was evaluated using a modified tracer technique and a disturbed equilibrium technique. The lack of agreement between results of the floating dome method and the other methods was attributed to limitations in instrument accuracy amplified by physical relationships of the dome to the water column rather than conceptual errors in the dome method. In techniques where polarographic oxygen probes are used to monitor the oxygen content within the dome atmosphere, the ratio of volume of the dome to volume of the water column under the dome must be kept below 0.02 to minimize instrument error to less than 10 percent of the predicted oxygen transport. The combination of instrument limitations, coupled with environmental limitations caused by wind and the greenhouse effects, severely restrict application of the floating dome method.

Journal of Research of the U.S. Geological Survey↗
USGS1975

Widespread late glacial and postglacial tephra deposits from Mount St. Helens Volcano, Washington

Pumice layers composing four different groups of tephra beds (termed "sets"), whose stratigraphy, age, and trend away from Mount St. Helens are fairly well known, are potentially valuable stratigraphic markers in the northwestern United States and adjacent parts of Canada. All four tephra sets are less than about 18,000 yr old. The oldest set described (set S) is between about 18,000 and 12,000 yr old; the most extensive pumice layers of the set, however, probably are no more than about 13,000 yr old. Relatively voluminous layers in the next younger tephra unit (set J) probably range from slightly less than 12,000 to slightly more than 8,000 yr old; in the overlying set Y, the most extensive layers range from about 4,000 to 3,400 yr old. The largest tephra layers in the youngest tephra set described, set W, are apparently all about 450 yr old. All the extensive tephra deposits were carried chiefly east of Mount St. Helens, and the bulk of them form an arc which extends from north-northeast of the volcano clockwise around to the southeast.

Washington↗
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↗
USGS1975

Rockfall seismicity correlation with field observations, Makaopuhi Crater, Kilauea Volcano, Hawaii

During August 7-13, 1972, intense and sustained rockfall activity occurred in Makaopuhi Crater on the east-rift zone of Kilauea Volcano. In a 4-day period (August 7-10), approximately 270,000 m 3 of rockfall debris accumulated in Makaopuhi's west pit, representing a total kinetic energy release of about 101B ergs. Because the rockfalls happened within an area with an established seismic network, it was possible to correlate the seismic record of the rockfalls with onsite field observations. The seismic signatures of rockfalls are generally distinguishable from those of earthquakes and other recorded events. Approximate magnitudes determined for some of the largest rockfalls range from 0.8 to 1.2, corresponding to calculated seismic energy releases of 2X10 11 to 10X10 11 ergs, if the magnitude-energy relationship for earthquakes is applicable to rockfalls. The August 1972 swarms of rockfalls at Makaopuhi correlate in time not with moderate or large earthquakes but rather with local eruptive activity and are inferred to have been caused by eruption-induced modifications of stress patterns of the crater walls. However, the amount and nature of the stress change required to exceed the threshold stability of the crater wall and to trigger a rockfall flurry cannot be determined. The Makaopuhi activity is typical of most major rockfall episodes in other Kilauean pit craters in recent years, which also have been associated with volcanic activity, particularly during times of changes in eruptive behavior.

Hawai'i↗
USGS1975

A characteristic pattern of disequilibrium in some uranium ore deposits

A redistribution of radium-226 in uranium ore deposits produces a characteristic pattern of disequilibrium in which uranium is greater than equivalent uranium in high-grade samples and equivalent uranium is greater than uranium in low-grade samples. The redistribution is a continuous process in uranium deposits, and the resulting pattern of disequilibrium is itself a system in equilibrium.

Journal of Research of the U.S. Geological Survey↗
USGS1975

Origin of lunar light plains

Most Cayley-type Imbrian-age plains deposits and adjacent mantled slopes, including those at the Apollo 16 site, may be composed at least near the surface of ejecta from the Orientale basin, the youngest multiringed impact basin on the Moon. The distribution and apparent age of the plains deposits and preliminary data on the highly feldspathic breccias collected by the Apollo 16 crew indicate that these surficial materials are neither locally derived nor part of the Imbrium ejecta. Stratigraphic relations, crater size-frequency distributions, and dating by erosional morphology of superposed craters have established Cayley light-plains deposits as younger than the Imbrium basin and older than mare material. All such crater-dated Cayley-type plains on both the near and far sides of the Moon are contemporaneous within the limits of the technique. Furthermore, comparisons of the crater size-frequency distributions of the Hevelius Formation (Orientale ejecta) and the plains show that the Cayley and Hevelius Formations are indistinguishable in age. The surface and near-surface materials of the Apollo 16 plains, therefore, are contemporaneous with Orientale basin ejecta not with Imbrium ejecta, in which both crater densities and crater degradation are greater than in the light plains. Imbrium ejecta may, however, be present on the surface at the Apollo 16 site where excavated by craters from depth. Limited age data now available on the Apollo 16 samples are consistent with this interpretation. We conclude that rocks of Orientale provenance may be predominant on and near the surface at the Apollo 16 site. This hypothesis implies that the catastrophic Orientale impact struck a highland area underlain by highly feldspathic material and spread it over much of the Moon. Thus, much of the lunar highlands crust need not consist of anorthositic materials to any significant depth. These conclusions apply to the plains at the Apollo 16 site and most of the other Cayley-type plains, but we do not exclude the possibility that, moonwide, the plains may be polygenetic.

Journal of Research of the U.S. Geological Survey↗
USGS1975

Computer-generated shaded-relief images

Digital image-processing techniques have been developed to make shaded-relief images from digitized topographic data. The resulting images have at least two advantages over aerial photographs: (1) The images are true map projections containing no relief-induced distortion, and (2) tonal variation is unambiguously identified with relief, rather than with snow, vegetation, or other albedo variations. Parallax can be introduced into these images in the computer, and stereoscopic pairs can thus be created.

Journal of Research of the U.S. Geological Survey↗