Tom Saylor IV
Simon Kattenhorn
Article Review
4/23/12
• Fossen, Haakon, Richard A. Schultz, and Anita Torabi. "Conditions and Implications for
Compaction Band Formation in the Navajo Sandstone, Utah." Journal of Structural Geology
33.10 (2011): 1477-490. Print.
Fossen, Schultz and Torabi conducted research on a Laramide aged anticline known as the
Kaibab Monocline in the Buckskin gulch area of Utah. They looked at the Navajo sandstone, which is
the counterpart of the Aztec sandstone seen in the Valley of Fire near Lake Mead, Nevada. Their
main focus was to understand the controls on how specific types of deformation bands occur in
nature. The key questions addressed were why deformation bands are so rare in the field and how
rock lithologies and stress states affect the evolution of these features. The three scientists
identified three separate style of deformation banding which they named Pure (or crooked)
compaction bands, shear-enhanced compaction bands and compactional shear banding.
Understanding how and where deformation bands occur is important because these bands (along
with faults) control the flow of fluids, such as oil and water, through the substrate. Knowing how they
developed can lead to a greater knowledge of where to look for these precious fluids.
Pure deformation bands have a typical sinusoidal pattern whose wavelength is ~ 0.5 – 2.0
cm with an average thickness of 1 mm. They are different from deformation bands found in Aztec
sandstone which are classified as Chevron-style. Chevron-style compaction bands are formed from
conjugate shear-enhanced compaction bands and have a zigzagging geometry rather than a wavy
one. Crooked deformation bands show no visible offset along laminations or sedimentary structures
and so it is believed that there was no shear component to the stress state in which they formed.
Interestingly, in thin section these structures show a noticeable amount of grain dissolution at their
boundaries.
Shear-enhanced compaction bands are much larger in scale than their pure compaction
band counterparts. Also called thick compaction bands (Mollema et al, 1996), their general width
ranges from 2 mm to multiple cm. These deformation bands have a high amount of compaction and
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a significant amount of shear shown by cross-cutting each other or other laminae. Fossen et al found
that these bands commonly formed conjugate sets between each other at 73˚. A general reverse
sense of shear is found with displacement being only a millimeter or so. These deformation bands
usually have eye-structures which occur when two separate bands mechanically feel each other and
move towards the other. These bands are of the same style as the ones found in the Valley of Fire.
The final style of deformation bands are known as compactional shear bands. These bands are
almost exactly like shear-enhanced bands except that they cut through other sandstone units and
are much larger in size with centimeter scale offsets. It is believed that these are the oldest style of
banding in the Navajo formation. An early stress state caused their formation, and after the rock
reached a certain decrease in porosity they switched over to the shear-enhanced bands.
The Kaibab monocline formed in response to tectonic stresses during the Laramide orogeny,
and is coupled with several other structures which all formed when pre-Laramide basement rock
faults were reactivated. It is believed to have formed around 80-76 ma and strikes approximately NS. The Navajo sandstone, which is a major formation in the Kaibab, has a thickness of approximately
1220 meters. Using a variety of methods, Fossen et al found several parameters for specific
deformation band formation. Permeability of the rock turned out to be a major factor in deformation
band style, and was measured in three separate ways. The first was by using a TinyPerm II portable
hand-held permeameter in situ. The second was through laboratory gas-based plug measurements
and the final was by analyzing images procured in the field. A relationship was found where the
higher the rocks porosity, then the more likely it would have shear-enhanced deformation bands, and
the lower the porosity the more likely it would have pure compaction bands. Deformation band
frequency was another thing Fossen et al looked at. It was found that the deformation bands would
form more favorably above the base of crossbeds where the permeability was the highest, and would
occur less and less towards the top of the dune. Grain coarseness was also studied in thin section.
This related a lot to rock porosity and it was found that the coarser the grains, the more permeable
the rock and therefore the more likely that deformation bands would form.
At the end of the article, Fossen et al decided that there were two types of compaction bands
which they studied. Planar compaction bands are interpreted as having a component of shear while
crooked compaction bands involve only compaction. The Valley of Fire has crooked compaction
bands but with several differences. First, their wavelength is much larger at around 1-15 cm. Another
is that their limbs are straight which implies an opposite sense of shear was present during growth.
The geometry of the different deformation bands shows a regional stress state where σ1 is aligned
perpendicular to the Kaibab monocline and trends at approximately 100° NE. The shear-enhanced
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and compactional shear deformation bands formed at an oblique angle to this trend. Fossen et al
calculated that pure compaction bands can form at a much lower stress state than the other two and
believe that they formed first during the Laramide orogeny. Later, as the local stresses increased,
compactional shear bands formed and after they reduced the porosity of the rock by approximately
3-4 orders of magnitude, shear-enhanced compaction banding dominated. Finally, it was found that
due to the orientations of the deformation bands, fluid flow would be retarded in the same direction
as σ1 (~ E-W), while in the N-S direction, normal flow would find little reduction in the rocks
permeability.
References
•
Mollema, P.N., Antonellini, M.A., 1996. Compaction bands: a structural analog for anti-mode I
cracks in Aeolian sandstone. Tectonophysics 267, 209-228.
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