structural geology, propagation mechanics and - Stanford School of ...

out in positive relief as distinct fins (Sternlof et al., 2005; Mollema and Antonellini, 1996)
(Figure 7.2). Millimeters to centimeters thick and tens of meters in planar extent, CBs
represent a kinematic end member of the suite of structures known collectively as
deformation bands, which also includes shear and dilation bands (Aydin, 1978;
Antonellini et al., 1994; Mollema and Antonellini, 1996; Du Bernard et al., 2002). The
petrophysical changes accommodating mechanical compaction inside CBs, in conjunction
with subsequent preferential cementation, can reduce porosity within the bands to a few
percent. Decreased porosity, and corresponding reductions in pore throat diameter and
connectivity, can in turn reduce saturated permeability within the bands by 1 to 4 orders
of magnitude relative to that of the surrounding host rock (Pittman, 1981; Freeman, 1990;
Antonellini and Aydin, 1994; Crawford, 1998; Gibson, 1998; Taylor and Pollard, 2000;
Sigda and Wilson, 2003).
Permeability anisotropy due to depositional bedding in the Aztec and Navajo
sandstones is generally less than a factor of two (Freeman, 1990; Antonellini and Aydin,
1994; Flodin et al., 2005), minimal when compared to the reduced permeability
represented by the CBs. There is no data to suggest that significant anisotropy related to
bedding persists inside the bands, and no reason to expect that the consequences of any
such remnant anisotropy would be significant. In the analyses to follow, we therefore
make the simplifying assumption of homogeneous, isotropic permeability for both the
sandstone and the CBs.
3. Mapping
The fins of CBs cropping out in the Aztec sandstone are plainly visible in low-altitude
aerial photographs. A high-resolution (3,000 dpi) digital scan made from one such
standard (23 cm x 23 cm) aerial photo color negative reveals an extensive exposure of
nearly continuous Aztec outcrops at a scale of 1:2,400 (Figure 7.1). This scan captured a
level of detail down to the grain of the negative, producing a digital image with a pixel
dimension relative to outcrop of about 2 cm. Loading this image into Adobe Illustrator®
and enlarging it, we were able to recognize and trace individual cm-thick CBs (or in many
cases, closely space clusters of CBs) for tens to hundreds of meters (Figure 7.1, inset
photos). This photo mapping effort was guided by our familiarity with the appearance of
CBs in outcrop and was checked extensively in the field.
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