UNCOVERING BURIED VOLCANOES: NEW DATA FOR PROBABILISTIC VOLCANIC HAZARD ASSESSMENT AT YUCCA MOUNTAIN Page: 3 of 7
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to the conclusion that only Miocene basalt is present in Jackass Flats. This conclusion,
discussed below, will be tested by additional drilling and sample dating.
The most prominent magnetic feature in Jackass Flats is a broad linear positive anomaly
(Anomaly U) that parallels the eastern edge of the survey from central Jackass Flats to .
the northern edge of Anomaly B at the southeastern edge of the survey (Figure 1).
Anomaly U is interpreted to represent buried Miocene basalt for two reasons. First, it
includes a 9.6 Ma, fault-bounded basalt outcrop near its north end. Faults evident in the
aeromagnetic data adjacent to this outcrop both laterally displace buried basalt to the
northeast and down-drop buried basalt to the west (Figure 1). Farther to the south, the
buried basalt is progressively down-faulted to the southwest as indicated by increasingly
attenuated anomaly signals bounded by northwest-trending faults. Second, drill hole 23P
near the south end of the anomaly intersected a 10-meter-thick basalt at a depth of 400 m
that was dated at 9.5 Ma (Michael Kunk, written comm.). A series of deep (>350 m) drill
holes immediately to the west of Anomaly U intersected alluvium and Miocene tuff, but
did not encounter basalt, further limiting the possibility that buried Pliocene basalt is
present in Jackass Flats.
Two of the drill holes in the current program have been completed to date. A sequence of
four thin basalt flows (1-4 m thick) was intersected at Anomaly Q in Crater Flat between
a depth of 141 and 163 m (bottom.of hole). Landslide deposits of Paleozoic strata
emplaced during the Miocene uplift of Bare Mountain were encountered immediately
above the basalt flows. These stratigraphic relationships are identical to those
encountered in drill hole VH-2, five km south of Anomaly Q, and in outcrop at the
southern margin of Crater Flat (Figure 1). The basalts in drill hole VH-2 and in outcrop
are both dated at about 11.3 Ma, leading to the. conclusion that the basalt flows
encountered at Anomaly Q are also 11.3 Ma. By correlation, Anomalies R and 4, adjacent
to Anomaly Q in northern Crater Flat, are thus also interpreted to represent buried
Miocene basalt.
At Anomaly A in Crater Flat, basalt was encountered beneath alluvium at a depth of 148
m. Lack of flow features and evidence of internal differentiation suggest this basalt may
be a sill or a conduit. Stratigraphic relationships at this site do not allow an interpretation
of the age; sample dating will provide that information.
Implications
Although confirmation awaits the completion of drilling and sample dating, interpretation
of the new aeromagnetic data and drilling results to date suggest that magnetic anomalies
adjacent to Yucca Mountain in Crater Flat and Jackass Flats are due to buried Miocene
volcanic features. Sensitivity analyses can give insight into how much probability
estimates might be impacted if a number of Pliocene volcanoes were buried in Crater Flat
and Jackass Flats. These analyses, based on models used in the 1996 expert elicitation,
considered two primary scenarios (Bechtel SAIC, 2004). The first assumed that 22
anomalies (none of which were located in Jackass Flats) identified from previous air and
ground magnetic surveys represented buried Pliocene volcanoes. This scenario resulted in3
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Perry, F.V. UNCOVERING BURIED VOLCANOES: NEW DATA FOR PROBABILISTIC VOLCANIC HAZARD ASSESSMENT AT YUCCA MOUNTAIN, report, October 13, 2005; Oak Ridge, Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc779835/m1/3/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.