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

Abundant deeply etched and/or kaolin-replaced orthoclase grains indicate that much of
the clay present is internally derived (Eichhubl et al., 2004; Sternlof et al., 2005). Small
amounts of inter-layered smectite and illite are also present (Eichhubl et al., 2004).
Abundant mutual indentation of quartz grains via pressure solution has been reported
(Flodin et al., 2003; Eichhubl et al., 2004), although much of this texture may in fact be
due to mechanical indentation accommodated by pervasive intra-grain microfracturing
(Sternlof et al., 2005).
3.2. Diagenesis
Syndepositional precipitation of hematite grain coats resulted in the Aztec being
stained a uniform red color during initial burial (Eichhubl et al., 2004). The color from
this earliest diagenetic alteration still dominates the lower half of the sandstone. The
upper Aztec, however, has been subjected to at least two stages of bleaching and iron
oxide redistribution apparently associated with upward and eastward expulsion of
reducing basinal brines from beneath Sevier-related thrust sheets advancing from the
west (Eichhubl et al., 2004). These fluid-flow events are responsible for the vividly
colorful patterns of iron mineralization from which the Valley of Fire derives its name
(Figure 1.1).
Despite its long alteration history, however, the presence of only limited quartz
overgrowth cementation and diagenetic illite, both restricted to the very bottom of the
pile (and within the damage zones of some major faults), indicate that the base of the
Aztec in the Valley of Fire has likely never been buried deeper than about 3 km (~ 80° C
for a normal geothermal gradient). Interestingly, this equals the sum of its own thickness
and of all overlying sedimentary deposits through (Eichhubl et al., 2004; Sternlof et al.,
2005). In any case, the Aztec is today at best moderately well cemented toward the
bottom, where it is still red, and poorly cemented toward the middle and top (Flodin et al.,
2003), where it has been extensively bleached and now exhibits an unconfined uniaxial
compressive strength of only 2-3 MPa (Haimson and Lee, 2004). Neither is there
evidence to suggest that the state of lithification anywhere in the Aztec was ever
appreciably greater than it is today. Our own limited triaxial testing, however, does
indicate that the resistance of present-day Aztec to uniform compaction with increasing
pressure approaches that of quartzite.
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