Eastwest structural cross section near the east edge of the Midland Basin showing salt-character changes controlled by post-Permian dissolution overprinted on Permian facies changes. Cross-section location is shown on study area index map.
Regionally, the salt and salt-bearing interval thins toward the east edge of the Midland Basin. This east-west structural cross section across the Howard-Glasscock high shows salt character changes in this area. Structure on top Yates shows that the gentle west-dipping basin structure is complicated in this area by a well-defined east-west striking uplift along the Howard-Glasscock county line. South of this uplift, irregularities on the Yates surface suggest a complex structure at depth, interpreted to be a graben.
The Salado salt-bearing interval progressively thins from 580 ft offstructure at the west end of the cross section, to no salt at the east end. Structure on the Alibates shows a reversal of dip from the regional trend and from the dip in the Yates in the area of no salt. This is the typical geometry produced by salt dissolution in the burial environment. Anhydrite and polyhalite beds within the salt-bearing interval can be traced into the insoluble residue. Closer inspection shows that the burial dissolution crosscuts a Permian trend toward thinner units, most clearly seen in the lower Tansill carbonate-anhydrite unit. The lower Tansill thins from 65 ft offstructure to 15 ft on the east end of the cross section. The salt-bearing interval also thins by 100 ft between the two westernmost wells, and relationships between the top salt and correlated horizons within the salt show that this is not the result of dissolution of the uppermost salt but of incremental thinning of each unit, a pattern similar to that seen toward the north basin margin in case study 1. The siliciclastic unit in the upper Tansill shows a reverse trend, becoming thicker on the structural high. This is partly an effect of merging insoluble residue with mudstone beds, but may also include an effect of increased mudstone thickness toward the paleo-high, reflecting more exposure in an area of decreased accommodation. A calculation to approximate the amount of residue expected from dissolution of 580 ft of salt from GL4, at typical regional values of 75 percent salt and 25 percent insoluble, yields a residue thickness of 145 ft. The measured thickness of residue between markers in the easternmost well GL 12 equivalent to the 580 ft of salt section in the GL 4 well is only about 100 ft, further supporting an interpretation of a depositional thinning trend that parallels and is accentuated by burial dissolution.
Post-Permian dissolution overprints on Permian facies changes are common in the Midland Basin. Where this relationship exists, it indicates that the post-Permian uplift responsible for exposing the salt to a near-surface setting where it underwent dissolution has reactivated the structures that caused reduced subsidence during the Permian. Post-Permian dissolution overprints on Permian facies changes were seen throughout the eastern shelf beneath the Rolling Plains and on the Ozona Platform beneath west Edwards Plateau.
The area of dissolution
and subsidence south of the Howard-Glasscock high lies at depths of 1,500
ft below land surface, which makes it one of the deepest areas of salt
dissolution seen in the study area. Surface geology at a 1:250,000 scale
(Eifler and others, 1994) shows relatively flat-lying Cretaceous strata
at the surface above the salt-dissolution area, suggesting that most of
the salt dissolution in this area preceded the deposition of Cretaceous
units. This timing might also indicate that dissolution took place under
shallower burial conditions than presently exist. Complex Pleistocene
deposits in this area may be indicators of post-Cretaceous salt dissolution
in this area but further localized study is needed to confirm salt dissolution
in this area. Deformation of Cretaceous strata can be seen in exposures
at the spring in Big Spring, Howard County.