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

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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 trend. Cross-section location is shown on study area index map.

The salt-dissolution feature in Ward and Winkler Counties is another significant variation from those described in cases 1, 2, and 3. A depression of as much as 1,500 feet in the top Alibates structure is filled with post-Permian sediments to depths of as much as 2,000 ft below land surface. Net salt thins from 600 ft on the Central Basin Platform to a measured minimum of 128 ft in the depression. Net salt thickens again west of the depression to 1,000 ft.

Cross-section relationships show that: (1) the thin in the salt is the result of dissolution, not facies changes, and (2) salt has locally been dissolved from the bottom of the salt as well as from the top. The facies changes in this area are readily understood in the context of case 1 and case 3. Anhydrite beds start to thicken across the Central Basin Platform in western Ector County, east of the dissolution feature. Although individual bed correlations are tentative through the area of salt dissolution, caliper log character shows that salt is missing from the base of the Salado above a thick Tansill siliciclastic and anhydrite section. The salt-dissolution interval is condensed relative to adjacent areas, although some of the halite is represented by siliciclastic insoluble residue between anhydrite beds.

The Ward-Winkler salt-dissolution area lies along the trend of the Capitan Reef. Hiss (1975b, 1976, 1980) has proposed a genetic relationship based on a model where fresh ground water, moving through the highly transmissive Capitan aquifer from the Glass Mountains recharge area, has moved up through fractures into the salt. The Ward-Winkler salt dissolution is part of a larger system of depressions on the Alibates that follow the Capitan Reef trend into New Mexico toward its outcrop in the Guadalupe Mountains (Hiss, 1976). The geometry on top of the Guadalupian strata (top Yates, Capitan, and Bell Canyon) shows that beneath the dissolution area these units are dipping steeply to the west. Although the Capitan Reef or back reef may have been a relatively positive feature during deposition, Late Permian and post-Permian deformation has warped the western reef edge downward, relative to the platform. The present structural high on both the top Yates and the top Alibates maps and lies east of the main Capitan Reef (Hiss, 1975a) and east of the salt-dissolution zone. Therefore, the style of dissolution contrasts with that observed in case 3 on the southern Central Basin Platform, where dissolution was focused on the crest as well as the flanks of the structural uplift. The observations made in this study support the aquifer dissolution model of Hiss (1976).

The Ward-Winkler salt-dissolution feature is not related to a surface depression. The relationship between this feature and past drainage has been explored by Bachman (1984) and Hiss (1976). The timing of dissolution is not well constrained. Historic subsidence and recent formation of the collapse feature at the Wink Sink (Baumgardner and others, 1982; Johnson, 1987) on the east edge of the paleodissolution feature indicates that salt dissolution may be ongoing in this area.

Methods for Site Evaluation