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At least 829 records · Page 46Linked to original sources
USGS1982

Volcano-tectonic history of Crater Flat, southwestern Nevada, as suggested by new evidence from drill hole USW-VH-1 and vicinity

New evidence for a possible resurgent dome in the caldera related to eruption of the Bullfrog Member of the Crater Flat Tuff has been provided by recent drilling of a 762-meter (2,501-foot) hole in central Crater Flat. Although no new volcanic units were penetrated by the drill hole (USW-VH-1), the positive aeromagnetic anomaly in the vicinity of the drill hole appears to result in part from the unusually thick, densely welded tuff of the Bullfrog. Major units penetrated include alluvium, basalt of Crater Flat, Tiva Canyon and Topopah Spring Members of the Paintbrush Tuff, and Prow Pass and Bullfrog Members of the Crater Flat Tuff. In addition, the drill hole provided the first subsurface hydrologic information for the area. The water table in the hole is at about 180 meters (600 feet), and the temperature gradient appears slightly higher than normal for the region.

Nevada↗
USGS1983

Stratigraphic evidence for Devonian tectonism on lineaments at Allegheny Front, West Virginia; supporting material

At the Allegheny Front in northeastern West Virginia, five large structural lineaments trend northwesterly to westerly across the structural grain. This paper concludes that two and perhaps three of them separated structures that were active in Devonian time. Along strike over about 200 km of the Front, J. M. Dennison and coworkers recognized and mapped numerous, mostly thin, shale, siltstone, sandstone, and limestone units in 18 measured sections between the Early Devonian Oriskany Sandstone and the Late Devonian Hampshire Formation. Many of these units terminate in intervals between measured sections. Nonparametric statistical analysis reveals that significant numbers of stratigraphic terminations occur in and between intervals in which three of the five lineaments intersect the Front. The Petersburg and Fairmont-Rowlesburg and perhaps the Bartow lineaments were loci of structural control over topography, bathymetry, or both, and of structural influence on patterns of influx and dispersal of clastic sediment. It is not clear whether this pre-Alleghany activity reflects intermittent motion (1) on a strike-transverse basement fault, as occurred under similar lineaments in Pennsylvania and Alabama, or (2) between Paleozoic thrust masses advancing somewhat independently of each other. However, if the Petersburg lineament formed over a basement fault, it cannot still overlie it because the rocks containing the lineament have been detached and transported too far to the northwest. Of the five lineaments considered, the one most likely to have formed over and to still overlie a basement fault is the Parsons lineament, especially in its northwestern portion in the eastern Plateau province.

Open-File Report↗
USGS1983

Geochronology, geochemistry, and tectonic environment of porphyry mineralization in the central Alaska Peninsula

Porphyry type sulfide systems on the central Alaska Peninsula occupy a transition zone between the Aleutian island magmatic arc and the continental magmatic arc of southern Alaska. Mineralization occurs associated with early and late Tertiary magmatic centers emplaced through a thick section of Mesozoic continental margin clastic sedimentary rocks. The systems are of the molybdenum-rich as opposed to gold-rich type and have anomalous tungsten, bismuth, and tin, attributes of continental-margin deposits, yet gravity data suggest that at least part of the study area is underlain by oceanic or transitional crust. Potassium-argon age determinations indicate a variable time span of up to 2 million years between emplacement and mineralization in a sulfide system with mineralization usually followed by postmineral intrusive events. Finally, mineralization in the study area occurred at many times during the time span of igneous activity and should be an expected stage in the history of a subduction related magmatic center.

Open-File Report↗
USGS1984

Stratigraphic and volcano-tectonic relations of Crater Flat Tuff and some older volcanic units, Nye County, Nevada

The Crater Flat Tuff is herein revised to include a newly recognized lowest unit, the Tram Member, exposed at scattered localities in the southwest Nevada Test Site region, and in several drill holes in the Yucca Mountain area. The overlying Bullfrog and Prow Pass Members are well exposed at the type locality of the formation near the southeast edge of Crater Flat, just north of U.S. Highway 95. In previous work, the Tram Member was thought to be the Bullfrog Member, and therefore was shown as Bullfrog or as undifferentiated Crater Flat Tuff on published maps. The revised Crater Flat Tuff is stratigraphically below the Topopah Spring Member of the Paintbrush Tuff and above the Grouse Canyon Member of the Belted Range Tuff, and is approximately 13.6 m.y. old. Drill holes on Yucca Mountain and near Fortymile Wash penetrate all three members of the Crater Flat as well as an underlying quartz-poor unit, which is herein defined as the Lithic Ridge Tuff from exposures on Lithic Ridge near the head of Topopah Wash. In outcrops between Calico Hills and Yucca Flat, the Lithic Ridge Tuff overlies a Bullfrog-like unit of reverse magnetic polarity that probably correlates with a widespread unit around and under Yucca Flat, referred to previously as Crater Flat Tuff. This unit is here informally designated as the tuff of Yucca Flat. Although older, it may be genetically related to the Crater Flat Tuff. Although the rocks are poorly exposed, geophysical and geologic evidence to date suggests that (1) the source of the Crater Flat Tuff is a caldera complex in the Crater Flat area between Yucca Mountain and Bare Mountain, and (2) there are at least two cauldrons within this complex--one probably associated with eruption of the Tram, the other with the Bullfrog and Prow Pass Members. The complex is named the Crater Flat-Prospector Pass caldera complex. The northern part of the Yucca Mountain area is suggested as the general location of the source of pre-Crater Flat tuffs, but a caldera related to the Lithic Ridge Tuff has not been specifically identified.

Nevada↗