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

Eichhubl et al., 2004) did not lend significant cohesion to the detrital grains (Sternlof et
al., 2005) or, at least that it did not impede CB formation. Why the lower boundary of the
alteration front is located stratigraphically where it is, and why this horizon roughly
coincides with the lower extent of compaction band formation remains a significant open
question. Given that the bands predate alteration and pure coincidence is unlikely, it is
interesting to speculate, however, that the relatively newly formed CB arrays in the
Cretaceous Aztec sandstone played an active role in attracting and concentrating flow of
the expulsing brines.
As described above in Section 3, a preponderance of evidence indicates that the Aztec
sandstone in the immediate study area just avoided being overridden by the Willow Tank
and Muddy Mountain thrust sheets and that, based on diagenetic evidence, has never
been buried by much more than the combined thickness of overlying Cretaceous and
Neogene sediments (~1,600 m). Directly overlying the Aztec is the basal conglomerate of
the Willow Tank formation, interpreted to be a gravel pediment indicative of
erosional/depositional stasis. An upward coarsening, 1,300-m-thick sequence of
Cretaceous deposition followed (Figure 1.2), dominated by more than 1,100 m of the
Baseline sandstone representing aqueous redeposition of the Aztec from western upland
sources into a foreland basin (Longwell, 1949; Bohannon, 1983). Up to 600 m of these
lower Cretaceous sediments are overridden by the Willow Tank thrust, which is
associated with shear banding in the Aztec that post-dates CB formation.
Sternlof et al. (2005) conclude that no more than 600 m of fine-grained foreland basin
sediments buried the Aztec sandstone during CB formation, and likely less. We further
suggest that the evolution of this flexural foreland basin was in its earliest stages during
CB formation and that no more than 200 m of clay-rich swampland deposits of the
Willow Tank formation covered the Aztec. The end-member scenario is that the Aztec
sandstone was essentially unburied, covered only by the gravel pediment, which is
suggestive of mild uplift related to broad regional warping of the crust at the onset of the
Sevier compression (Fleck, 1970; Brock and Engelder, 1977). In any case, it appears
certain that CB formation in the Aztec occurred well before the thrust front reached the
area. This interpretation is further corroborated by the observation that CBs exposed at
Buffington Pockets (Figure 1.3) are substantially similar in geology and orientation to
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