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