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

the south side of the valley (Figure 1.3). The fold axis currently plunges 20-30° to the
northeast, matching the general dip of the surrounding Cretaceous strata (Bohannon,
1983; Flodin, 2003; Eichhubl et al., 2004). This indicates that the folding largely predates
regional tilting.
4. Structural geology of compaction bands
Extensive, anastomosing arrays of subparallel CBs pervade the upper 600 m of the
Aztec sandstone with a dominant north-northwest trend and steep (~70°) dip to the east.
There also are common, but generally isolated outcrops exhibiting more complicated
patterns of bands, including multiple sets of CBs and abundant shear bands dissecting
them (Sternlof et al., 2004). Phenomenal exposure of the Aztec throughout the main part
of the Valley of Fire State Park permits an integrated view of the structural geology of
CBs from thin-section to regional scale (Figure 1.4).
4.1. Thin-section to outcrop scale
Viewed in thin section (Figure 1.4b), the boundaries of tabular CBs are clearly
delineated as abrupt drops in porosity directly attributable to inelastic mechanical
compaction associated with grain crushing—specifically, micro-fracture accommodated
plasticity of the quartz grains (Sternlof et al., 2005) apparent under backscatter electron
imaging (Figure 1.4c and d). The total volume change realized, as measured by the
relative volume fraction occupied by detrital grains inside versus outside the band, is
about 10% and appears to be consistent along the bands from tip to middle. Judging from
the overall continuity of depositional bedding cutting across the bands (Figure 1.4b), and
the almost complete absence of granular disaggregation within them despite intense grain
fracturing, the volume change appears to have resulted from uniaxial compaction directed
normal to the plane of the band in the absence of appreciable shear accommodated across
it (Sternlof et al., 2005).
In the otherwise weakly lithified Aztec sandstone, CBs tend to weather out in positive
relief as distinctive tabular fins, rendering them readily visible in outcrop and
accentuating the absence of appreciable shear accommodated (Figure 1.4e). The relative
resistance of CBs is due primarily to the preferential precipitation of kaolinite as a pore-
filling, grain-bridging cement (Sternlof et al., 2005). This post-compaction clay
accumulation further exacerbates porosity loss and consequent permeability
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