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

pore-clogging clay—due presumably to groundwater filtering by the compacted band
coupled with mechanically enhanced feldspar degradation—at least partially explains the
greater resistance to weathering exhibited by CBs, as well as their strong impact on fluid
flow. That clay remains the primary cement in the weakly indurated Aztec today further
suggests that cementation was light to nonexistent during CB formation.
Six sets of backscatter images were collected at locations ranging from the middle to
the tip of CB-A. Each set comprises images taken from inside and immediately outside
the band within the same depositional layer(s). Porosity-reduction volume loss related to
CB formation at these locations ranged from 9.4% to 11.7% (Figure 2.8). These results
suggest that mechanical compaction throughout the band is uniform at about 10%,
regardless of the local variations in sandstone porosity through which it propagated.
Cathodoluminescence imaging indicates that redistribution of quartz via pressure solution
is volumetrically insignificant and has not appreciably contributed to the spatial
distribution of porosity now observed. This is not surprising given the shallow burial
history of the study area (< 2 km), which, for any reasonable estimate of geothermal
gradient, would have made quartz pressure solution an extremely slow to nonexistent
process.
Cracking of quartz grains at point contacts, due both to overburden and tectonic
loading, is pervasive throughout the thin sections. Wholesale micro-fracture-
accommodated plastic deformation of quartz grains, however, comprises the dominant
micro-structural characteristic of the band, and is the obvious mechanism by which
granular rearrangement and porosity loss compaction was accommodated (Figure 2.9).
Roughly half of the quartz grains within the band exhibit plasticity, creating the hint of a
mild shape fabric of deformed grains elongated parallel to the band trace (i.e. orthogonal
to the inferred direction of maximum compression). Far more striking is the relative
absence of granular disaggregation, despite often intense micro-fracturing and distortion.
Taken together, these observations corroborate the field interpretation of the band as
representing nearly pure uniaxial compaction with little or no shear, and suggest a stable,
quasi-static process of plastic collapse.
Isolated examples of grain plasticity and collapse can also be found outside the band
(Figure 2.10). We interpret these as incipient CBs, representing an early stage of
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