A geophysical‐geological transect of the Silent Canyon Caldera Complex, Pahute Mesa, Nevada

Abstract

Revision of lithological logs for boreholes penetrating the volcanic center at Pahute Mesa, Nevada, has led to a thorough review of the volcanic stratigraphy and geologic structure. We have combined this review with a compilation of old and newly acquired gravity and seismic travel time data, producing a unified interpretation along a northwest to southeast profile. The analysis supports a new interpretation of the Silent Canyon caldera complex. The caldera is found to be more asymmetric than previously suggested, with the southeastern boundary formed by linear, high‐angle normal faults and a more gently sloping northwestern boundary. The total thickness of volcanic units within the caldera complex does not appear to exceed 5 km. The shallow structure at Pahute Mesa could have a profound effect on the seismic response for regional and teleseismic signals from this nuclear test site. The Silent Canyon caldera complex is actually a set of nested calderas first filled by thick (>1 km) postcaldera lavas and subsequently buried by outflow sheets of the Timber Mountain caldera to the south. Thick, postcaldera lavas filled a half‐graben structure formed west of the West Greeley fault, dropping the tops of the youngest caldera‐forming units to depths in excess of 2 km. Therefore the western boundary of the caldera complex is poorly defined. East of the West Greeley fault, two overlapping calderas are defined, and stratigraphic data suggest the presence of even older calderas. The youngest caldera, the calc‐alkaline Area 20 caldera, is well defined from drill hole data. The Area 20 caldera overlaps the 13.6 Ma peralkaline Grouse Canyon caldera, which is less well defined, but apparently collapsed in trap‐door style along the Almendro fault. For both these calderas, collapse continued after the main caldera‐forming eruption, concurrent with the accumulation of thick (>1 km) lavas within the peripheral collapse zones. The geophysical interpretation indicates that the major structural boundary of the caldera complex corresponds to the NNE trending Scrugham Peak and Almendro faults, which offset the pre‐Tertiary contact more than 1 km but have less than 200 m offset in rocks of 11 Ma age. Drill hole data show that offsets along these faults increase systematically within older (up to 15 Ma) units, which are commonly rotated eastward in a style similar to units at the surface. Abrupt changes in the subsurface thickness of the caldera‐forming units occur across the faults, indicating that these linear features served as caldera boundaries.

Geology in the Vicinity of the TYBO and BENHAM Underground Nuclear Tests, Pahute Mesa, Nevada Test Site

“…The ridge coincides with a dramatic southward disappearance of mafic-poor Calico Hills Formation (Figure 3-7), which fills the Area 20 caldera, formed 13.1 Ma (Ferguson et al, 1994). We consider that this abrupt thinning coincides with the southern boundary of the Area 20 caldera, as defined in Figure 3-1. …”

“…U.S. Geological Survey (USGS) Technical Letters provided additional background and geologic information on older drill holes, Sources: DOE, 1997;RSN, 1990; unpublished data, BN Geology/Hydrology files. Warren et al, 1999 Well Cluster ER-20-5 DOE, 1997 Warren et al, 1999--U20ag Howard, 1981Ferguson et al, 1994Warren et al, 1999U20ao Howard and Wagoner, 1985Warren et al, 1999--U20as Wagoner and Clark, 1986Ferguson et al, 1994Warren et al, 1999U20c Orkild, 1965Orkild and Jenkins, 1978Warren et al, 1999UE20c Santos, 1964Orkild and Jenkins, 1978Warren et al, 1999U20y Orkild and Jenkins, 1978Warren et al, 19991-8 including U20c, UE20c, and U20y. Information for Well was compiled from various UGTA reports.…”

“…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.…”

Nevada Test Site Environmental Report 2003

“…The only surface exposure of Mesozoic-age rocks in the NTS area are granitic intrusive masses, the Gold Meadows Stock north of Rainier Mesa (Snyder, 1977;Gibbons et al,1963 ), and the Climax Stock located at the extreme north end of Yucca Flat (Maldonado, 1977;Barnes et al, 1963) (Figure A-5). Wahl et al (1997) andFerguson et al (1994). (b) Letters in parentheses are stratigraphic unit map symbols.…”

Processing and geologic analysis of conventional cores from well ER-20-6 No. 1, Nevada Test Site

“…Core-recovery information is presented in Table 212. Additional information about the coring equipment and other data can be found in the BHI coring operations report (BHI, 1996 Well ER-20-6#1 is located w i t h i n the Silent Canyon caldera complex (Figure ]-I), one of several calderas and caldera complexes w i t h i n the southwestern Nevada volcanic field (Ferguson et al, 1994). The volcanic rocks associated wifh the Silent Canyon caldera complex include bedded and nonwelded tuffs, welded ash-flow tuffs, and rhyolite lava flows.…”