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

5 mm
Figure 2.5. Composite photomicrograph of compaction band sampled 6.0 meters from
the tip, where it is about 9 mm thick (dotted black lines). Blue indicates epoxy-filled pore
space (~ 25% outside the band, ~ 10% inside). The plane of the section is vertical and
orthogonal to the trace of the band in outcrop. This is a mosaic of images taken with
plane-polarized transmitted light at 25X magnification using a digital camera mounted to
a binocular microscope. Mineral constituents identifiable in this image are: detrital quartz
(white to gray), detrital orthoclase (stained dark brown), and sparse hematite (black). As
is typical in foreset dune deposits, bedding generally consists of alternating layers of
coarser and finer grains, here accentuated by the relative concentration of stained
orthoclase in the finer-grained beds. Note that bedding is preserved, perhaps even
visually enhanced within the band. The three black arrows indicate a coarse-fine-coarse
bedding package as it passes through the band at high angle with no apparent change in
thickness. The preservation of bed thickness is observed consistently in all thin sections,
including those taken across the band and orthogonal to the orientation shown above at
four locations. This indicates that compaction strain within the band is predominantly
uniaxial and oriented parallel to the band normal direction. Natural, mm-scale variations
in bed thickness render more exact interpretations of bed distortion within the band
inconclusive.
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