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Case Study 3: Post-Permian Dissolution over the Central Basin Platform in
the Pecos Valley Area

detail

Detail of east–west stratigraphic cross section near the east edge of the Midland Basin showing salt-character changes controlled by post-Permian dissolution over the Capitan Reef tend. Cross-section location is shown on study area index map.

Part of the north-south stratigraphic cross section across the Pecos Valley and uplifted southern Central Basin Platform was selected in an area at the west end of the uplifted area where some salt is preserved and several useful caliper logs are available. Multiple changes in salt quality are noted on this structural cross section of this area. Salt has been completely dissolved on the crest of the structure at Pecos County well 10. Salt has been dissolved to depths between 700 and 800 ft beneath the Pecos valley alluvium, and here it forms a depression in the top Alibates structure on top of the more regional structural positive. Where salt occurs at greater depths away from the uplift, less salt has been dissolved.

Anhydrite beds thicken across the Central Basin Platform, probably in response to increased water depth and better circulation during deposition in this area of slightly greater subsidence. Thicker anhydrite beds begin at about the same place that dissolution cuts deeply into the section, compounding the problem of determining how much salt has been dissolved. Measured salt thickness in the interval where salt is preserved documents the relatively low percent salt, which is between 46 and 64 percent in the percent salt map. Although percent salt could potentially be in error because of the salt dissolution, inspection of the logs and cross section supports the conclusion that the percent salt decrease is because of increased anhydrite bed thickness. Potential but discounted sources of error are: (1) sampling effects because a different stratigraphic interval is included in each calculation as the top salt varies stratigraphically across the dissolution zone, and (2) some effects of dissolution, if some salt has been removed interstratally within the salt section. Predictions of residue thickness based on stratigraphy of adjacent areas where salt is preserved yielded values similar to those observed. For example, Pecos 1 contains 215 ft of residue stratigraphically equivalent to 620 ft of salt section in Crane 11; this reduction could occur in a section containing 65 percent salt.

Interpretation of this cross section is complemented by maps and cross sections from the Yates Field area (Wessel, 1988a) that show structure of the Cretaceous in outcrop. In the Yates area, at the east end of the south part of the Central Basin Platform, the Cretaceous strata have been deformed on the north side of the structure in the Pecos Valley, but have not been deformed across the top of the structure or on the south side. This supports the conclusions of Adams (1940), based on stratigraphic interpretation, that salt dissolution across much of the structure was pre-Cretaceous. Cenozoic and potentially ongoing dissolution has occurred in the Pecos Valley. This is a common model for understanding salt dissolution; active dissolution may be found on the flanks of the structure where initial dissolution removed accessible salt from the crest of the structure. The surface mapping by Wessel (1988a) also emphasizes the role of faults and fractures formed by salt dissolution in focusing further dissolution.

This relationship between the structural high, topographic low, and area of salt dissolution is similar to the relationship localizing the Canadian River on the crest of the Amarillo Uplift because of dissolution of salt in that area (Gustavson, 1986). The Rolling Plains, where Permian rocks crop out at the surface, lie at lower elevations than the adjacent Edwards Plateau and Southern High Plains, indicating that the Permian rocks have been eroded more rapidly than the Cretaceous carbonates or the Ogallala Formation that overlie preserved salt (Gustavson and Simpkins, 1989).

Case Study 4: Post-Permian Dissolution over the Capitan Reef