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

Although systematic arrays of DBs pervade the middle and upper Aztec, DB-poor
exposures tens to hundreds of meters or more on a side are also common. The reasons for
this patchwork quality to the DB fabric at a variety of scales is as yet poorly understood,
but may be related to the æolian sedimentary architecture of the Aztec and differences in
how distinct dune packages responded to tectonic loading during DB formation.
Nonetheless, any given characteristic DB pattern recognized in outcrop can be seen to
persist at length scales ranging from meters (the thickness of small cross-bed packages)
to tens of meters (the thickness of large cross-bed packages) to hundreds of meters (for
DBs that pass through multiple dune boundaries).
4.1. Parallel
Patterns of approximately parallel DBs crop out extensively throughout the Aztec
over areas ranging from tens to hundreds of meters on a side. Individual bands within the
parallel sets commonly range up to ~1.5 cm thick and are spaced anywhere from
centimeters to meters apart (Figure 6.3). It is not uncommon, however, to see groups of
individually distinct DBs running parallel to each other only millimeters apart, sometimes
merging into what appears in outcrop to be a single band 10 cm or more thick. Also,
groups of closely spaced DBs often form distinct sets spaced a meter or more from the
next adjacent, closely spaced set. DBs cropping out in parallel patterns throughout the
Aztec cluster around three mean strike/dip orientations—350°/70°, 210°/60° and
240°/30° (using the convention of dip direction oriented 90° clockwise from the strike
azimuth). The first two of these sets cut across depositional bedding at a high angle,
while the third generally runs sub-parallel to bedding. The north-trending, steeply east-
dipping DB orientation (350°/70°) strongly dominates in terms of abundance, persistence
and distribution, and is the one shown in both Figures 6.2 and 6.3.
4.2. Cross-hatch
Cross-hatch patterns of DBs, which generally crop out over areas from meters to 10s
of meters on a side, consist of two distinct sets of parallel bands that commonly intersect
each other at a high angle (Figure 6.4). In the Aztec, individual bands within the cross-
hatch patterns exhibit the same range of widths as bands in the parallel patterns and the
same tendency to occur as closely spaced pairs. Spacing between bands ranges from 2 cm
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