structural geology, propagation mechanics and - Stanford School of ...
structural geology, propagation mechanics and - Stanford School of ...
structural geology, propagation mechanics and - Stanford School of ...
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did not report permeability values for individual b<strong>and</strong>s, Taylor <strong>and</strong> Pollard (2000) used<br />
his data to estimate that average DB permeability is about 2.3 orders <strong>of</strong> magnitude less<br />
than in the host rock. Gibson (1998) found that the permeability <strong>of</strong> DBs depends on the<br />
clay content <strong>of</strong> the parent s<strong>and</strong>stone <strong>and</strong> can vary from 1 to 4 orders <strong>of</strong> magnitude, with<br />
an average value <strong>of</strong> ~2.2 orders <strong>of</strong> magnitude.<br />
4. Characteristic DB patterns in the Aztec s<strong>and</strong>stone<br />
The Valley <strong>of</strong> Fire State Park lies about 60 km northeast <strong>of</strong> Las Vegas <strong>and</strong> about 30<br />
km west <strong>of</strong> the Utah border (Figure 6.1). The predominant outcrop unit within the park is<br />
the Jurassic Aztec s<strong>and</strong>stone, commonly considered correlative with the Navajo<br />
s<strong>and</strong>stone. The Aztec is an æolian, subarkosic s<strong>and</strong>stone up to 2 km thick typified by<br />
large-scale cross-bed sets up to 10+ meters thick (Marzolf, 1983; Myers, 1999). The<br />
Aztec has a mean grain size <strong>of</strong> about 0.25 mm, an average porosity <strong>of</strong> around 20%<br />
(Taylor <strong>and</strong> Pollard, 2000) <strong>and</strong> an average matrix permeability ranging from 0.1 to 1.0<br />
darcys (Antonellini <strong>and</strong> Aydin, 1994; Myers, 1999). The Aztec exhibits a variety <strong>of</strong><br />
brittle <strong>structural</strong> fabrics that include DBs, joints <strong>and</strong> faults, with DBs consistently<br />
appearing to be the oldest features (Hill, 1989; Myers, 1999; Taylor <strong>and</strong> Pollard, 2000).<br />
Hill (1989) hypothesized that DB formation within the Aztec was related to widespread<br />
compression during emplacement <strong>of</strong> the Summit/Willow Tank <strong>and</strong> Muddy Mountain<br />
thrusts during the Early Cretaceous Sevier orogeny.<br />
Fieldwork in the Valley <strong>of</strong> Fire has revealed at least three wide spread, systematic <strong>and</strong><br />
volumetrically significant patterns <strong>of</strong> dominantly compactive DBs in the Aztec s<strong>and</strong>stone<br />
over an area <strong>of</strong> more than 10 km 2 . All <strong>of</strong> these characteristic patterns—parallel, cross-<br />
hatch <strong>and</strong> anastomosing—have previously been described at various locations, though<br />
generally using different nomenclature (Antonellini <strong>and</strong> Aydin, 1994, 1995; Aydin <strong>and</strong><br />
Reches, 1982; Burhannudinnur <strong>and</strong> Morley, 1997; Davis, 1998; Hill, 1989; Jamison <strong>and</strong><br />
Stearns, 1982; Mollema <strong>and</strong> Antonellini, 1996; Taylor <strong>and</strong> Pollard, 2000; Underhill <strong>and</strong><br />
Woodcock, 1987; Woodcock <strong>and</strong> Underhill, 1987). Within the Aztec, the 2-D density <strong>of</strong><br />
DBs exposed in any given outcrop (area DBs/outcrop area) ranges from ~1% up to ~20%.<br />
Assuming reasonable continuity in the third dimension, as is clearly suggested by field<br />
observations in outcrop areas <strong>of</strong> high relief, the volume percent density <strong>of</strong> DBs also<br />
ranges between 1% <strong>and</strong> 20%.<br />
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