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

7. Concluding observations
The data and analyses presented above indicate that CBs formed as anticracks in the
Aztec while it comprised a shallowly buried, weakly lithified sandstone aquifer subjected
to regional compression associated with the earliest phases of Sevier tectonism in the area.
Interpreted as such, the presence of two orthogonal, coeval and coexisting sets of CBs
can be used to constrain the 3-D paleostress state in which they formed, with the
dominant set orthogonal to the maximum compressive stress (σ1) and the secondary set
orthogonal to intermediate compressive stress (σ2). Furthermore, the mutually cross-
cutting presence of both sets in the same outcrop suggests that σ1 ≈ σ2. That the inferred
paleostress orientations depart radically from classical Andersonian expectations can
reasonably be interpreted as evidence of as yet unrecognized regional structures,
specifically a relatively shallow detachment along which a coherent structural block
containing the Aztec now exposed in the Valley of Fire moved relatively westward.
Based on an average spacing between CBs of the dominant, north-south trending set
of 0.7 m, and an average band thickness of 1 cm representing 10% uniaxial compaction
(~1.1 mm), the Aztec sandstone experienced less than 0.2% total shortening as the result
of CB formation. Even assuming an average band spacing of only 10 cm hardly bumps
the shortening past 1%. If homogeneously distributed, dropping overall porosity from
25% to 24%, such a mild deformation event would be of no consequence to fluid flow.
The highly localized, yet pervasively interconnected nature of the CB fabric formed,
however, constitutes a considerable network of low-permeability baffles to fluid flow
(CB permeability ~0.1% of the inherent sandstone permeability) that has been shown
capable of exerting profound effects at scales of practical interest (Sternlof et al., 2004;
Sternlof et al., 2006).
The analysis presented here of compaction bands as indicators of paleostress in and
tectonic structure around the exhumed analog aquifer/reservoir that is the Aztec
sandstone can also be applied in reverse. That is, given independent knowledge of the
tectonic stress and material history of a sandstone similar to the Aztec, reasonable
forecasts of the possible presence and gross geometry of CBs in the subsurface can be
made. Such insights could prove highly valuable in the efficient development aquifers
and reservoirs.
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