Abstract:The Silent Canyon volcanic center consists of a buried Miocene peralkaline caldera complex and outlying peralkaline lava domes. Its location has been corroborated by geophysical data and more than 50 drill holes. Two widespread ash flow sheets, the Tub Spring and overlying Grouse Canyon members of the Miocene Belted Range Tuff, were erupted from the caldera complex and have volumes of 60–100 km3 and 200 km3, respectively. Eruption of the ash flows was preceded by widespread extrusion of precaldera comendite do… Show more
“…Major oxide analyses in table 4 were obtained by X-ray fluorescence techniques in the analytical laboratories of the U.S. Geological Survey (USGS) in Denver, Colo.; analytical methods, accuracy, and precision are described by Taggart and others (1987). Trace-element content (table 4) was determined by energy-dispersive X-ray spectroscopy (Elsass and duB ray, 1982) using 109 Cd and 241 Am sources; accuracy and precision of these analyses are described by Sawyer and Sargent (1989). Major-oxide and trace-element analyses in table 2 were obtained by combined INAA (instrumental neutron activation analysis) and ICP (inductively coupled plasma mass spectrometry) methods.…”
“…Major oxide analyses in table 4 were obtained by X-ray fluorescence techniques in the analytical laboratories of the U.S. Geological Survey (USGS) in Denver, Colo.; analytical methods, accuracy, and precision are described by Taggart and others (1987). Trace-element content (table 4) was determined by energy-dispersive X-ray spectroscopy (Elsass and duB ray, 1982) using 109 Cd and 241 Am sources; accuracy and precision of these analyses are described by Sawyer and Sargent (1989). Major-oxide and trace-element analyses in table 2 were obtained by combined INAA (instrumental neutron activation analysis) and ICP (inductively coupled plasma mass spectrometry) methods.…”
“…northeast, and the younger Area 20 caldera, to extensive than elsewhere beneath Pahute Mesa, the southwest (Sawyer et al 1994, andSargent 1989). The Timber Mountain caldera complex to the south (Figure 3 (Blankennagel and Weir 1973).…”
Section: [ 10kilometersmentioning
confidence: 99%
“…The entire stratigraphic and lithologic package is offset by younger high-angle normal faults. Recent summaries of the geology and hydrogeology of this volcanic field are contained in Laczniak et al (1996);Sawyer et al (1994);Ferguson et al (1994); Noble et al (1991); Byers et al (1989);and Sawyer and Sargent (1989). Two major buried caIderas of the southwestern Nevada volcanic field, which together form the Silent Canyon caldera complex, are located in the subsurface of the Pahute Mesa underground test area (Figure 3).…”
“…Chipera, 1988 Bish andChipera, 1989 1.4 Geologic and Hydrogeologic Setting The study area is located within the buried Silent Canyon caldera complex of (Figure 1-1). The caldera complex consists of at least two partially nested calderas, the Grouse Canyon and younger Area 20 calderas (Ferguson et al, 1994;Sawyer and Sargent, 1989). These calderas were formed and then subsequently filled by voluminous eruptions of almost entirely rhyolitic tuff and lava between approximately 13 and 14 million years ago.…”
Recent radiochemical evidence from groundwater characterization and monitoring wells in the vicinity of the TYBO and BENHAM underground nuclear tests in Area 20 of the Nevada Test Site, suggests that migration of radionuclides within groundwater beneath this portion of Area 20 may be more rapid than previously thought. In order to gain a better understanding of the hydrogeologic conditions in the TYBO-BENHAM area for more accurate flow and transport modeling, a reevaluation of the subsurface geologic environment in the vicinity of the two underground tests was conducted. Eight existing drill holes provided subsurface control for the area. These holes included groundwater characterization and monitoring wells, exploratory holes, and large-diameter emplacement holes used for underground nuclear weapons tests. Detailed and consistent geologic descriptions of these holes were produced by updating existing geologic descriptions with data from petrographic, chemical, and mineralogic analyses, and current stratigraphic concepts of the region. The updated descriptions, along with surface geologic data, were used to develop a detailed geologic model of the TYBO-BENHAM area. This model is represented by diagrams that correlate stratigraphic, lithologic, and alteration intervals between holes, and by isopach and structure maps and geologic cross sections. Regional data outside the TYBO-BENHAM area were included in the isopach and structure maps to better evaluate the geology of the TYBO-BENHAM area in a regional context. The geologic model was then evaluated with regard to groundwater flow and radionuclide migration to assess the model's implications for flow and transport modeling. Implications include: 1) confirmation of the general hydrogeology of the area described in previous studies; 2) the presence of two previously unrecognized buried faults that could act as zones of enhanced permeability within aquifers; and 3) secondary alteration within tuff confining units that is much more complex than previously described.iv This page intentionally left blank.
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