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

Figure 1.4. (opposite page) Photo collage depicting compaction bands in the Aztec
sandstone. (a) Study area air photo from Figure 1.1 with arrows indicating approximate
locations where other photos were taken. (b) Photomicrograph mosaic of a compaction
band at high angle to depositional bedding, which passes through the band with no
apparent shear offset, change in direction or change in thickness (white grains are quartz,
dark grains are mostly stained orthoclase and some hematite, blue is epoxy-filled pore
space. (c) Backscatter electron image (BEI)from just outside the compaction band (light
gray is quartz, off-white is orthoclase, dark gray is kaolinite, stark white is hematite,
black is pore space). (d) BEI from inside the compaction band. Note the abrupt drop in
porosity, pore size and pore connectivity accommodated by intense damage to some
quartz grains. This damage, and attendant grain interpenetration, is predominantly due to
pervasive microfracturing. (e) Close-up view of compaction band fin capturing its
essential tabular-planar aspect, cm thickness and lack of obvious shear displacement. (f)
Typical view of subparallel, anastomosing compaction band array (resistant fins) in
outcrop. View is north-northwest along the dominant trend). (g) Cross-hatch pattern
formed by two distinct band orientations at high angle to each other. Bands belonging to
the dominant trend set (view is northward) pass through both the upper and lower crossbed
boundaries, while the secondary set of bands is constrained within the middle
bedding package.
reduction—from ~25% and ~1,500mD in the sandstone to ~10% and 1.5 mD in the bands
(Sternlof et al., 2004; Sternlof et al., 2006; Chapter 5 this thesis).
In outcrop, individual CBs appear to be grossly penny-shaped and elliptical in profile,
ranging in maximum thickness up to a few cm (averaging ~ 1 cm) and in length to more
than 100 m. They often exhibit well-defined, elliptically tapered tips, and a variety of
meter-scale patterns of near-tip interactions that are diagnostic of in-plane propagation as
anticracks (Sternlof et al., 2005; Chapter 4 this thesis). By far the dominant pattern at the
outcrop-scale, however, is an anastomosing planar fabric trending north-northwest and
dipping steeply eastward (Figure 1.4f). Spacing between adjacent bands within this fabric
can vary from 1 mm to several meters, averaging ~0.7 m (based on 286 m of orthogonal
scan lines intersecting 423 bands at 23 locations—see Section 5 below). Also distinctly
notable, although relatively rare and spatially isolated, are patterns of CBs that cross and
sometimes turn into each other at high angle (Figure 1.4g). These “cross-hatch” patterns
(Sternlof et al., 2004) are comprised of through-going bands belonging to the dominant
orientation set, and a subsidiary local set trending generally southwest, dipping steeply
northwest and spaced ~1.2 m on average (based on 73 m of scan lines intersecting 61
bands at 5 locations). Bands from these two sets generally cross-cut each other at a
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