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

made of the Aztec sandstone in the Valley of Fire. A synopsis and expansion of those
observations and interpretations is presented below. Throughout the rest of this paper,
only CBS in the Aztec are considered. All other younger structural fabrics—shear bands,
joints and joint-based faults—are ignored.
3.1. Outcrop observations
Compaction bands in the otherwise weakly lithified Aztec sandstone tend to weather
out in positive relief as sharply delineated tabular fins, rendering them readily visible in
outcrop (Figure 4.2). In areas of outcrop relief, individual CBs are seen to be grossly
penny-shaped (i.e. very thin, oblate discs). Expansive exposures over more than 10 km 2
in the Valley of Fire represent an approximately horizontal observation plane through the
3-D CB array, revealing a detailed 2-D pattern. Viewed in total, the impression is one of
a very consistent, NNW-trending, sub-parallel (anastomosing) pattern of bands exhibiting
meter to centimeter spacing and interspersed with areas up to tens of thousands of m 2 that
are relatively devoid of bands. While the map-view observation plane does not actually
cut exactly perpendicular to the dominant band orientation, which currently dips eastward
at 70-75°, for the purposes of basic pattern analysis, interpretation and the 2-D
mechanical modeling to come, we make the simplifying assumptions that the current map
view represents paleo horizontal at the time of CB formation and that the bands
themselves were vertical.
In outcrop, band patterns range from nearly straight and parallel, to strongly
anastomosing, to essentially dendritic, to markedly uniform checkerboards comprised of
two distinct band sets crosscutting each other at high angle (Figure 4.3). Truly parallel,
evenly spaced band patterns are not observed at any substantial scale, while the dendritic
and checkerboard patterns occur as relatively rare, localized pockets within the dominant,
sub-parallel pattern. In fact, the dominant, through-going band trend—sometimes steeply
W-dipping, but usually steeply E-dipping—is present in every band pattern observed.
The complex æolian sedimentary architecture of the Aztec also clearly influenced the
extent and distribution of individual bands and patterns of bands, which are often
observed to warp, terminate and/or change configuration at or near cross-bed boundaries,
particularly major interdune contacts (Figure 4.4).
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