Garza 13 John J. Eisner No. 1A Porter shows resistivity log response to dissolution.

Dewey Lake Formation Overlying the Alibates is the Dewey Lake Formation, a 100- to 200-ft-thick siliciclastic red-bed sequence. This interval has moderately high, fairly uniform gamma-ray-log response. In the Palo Duro Basin, where this unit was examined in core, it is composed of siltstone and very fine sandstone deposited in pedogenically modified eolian-flat and cross-bedded wadi-channel environments. Alibates Formation The uppermost evaporite units in the Midland Basin are a pair of anhydrite beds of the Alibates Formation. These 10- to 50-ft-thick anhydrite beds and the siliciclastic interval that separates them forms a stratigraphic marker across most of the study area. Where this unit has been examined in core in the Palo Duro, the anhydrite beds are similar to other anhydrite beds in the section. They contain abundant pseudomorphs after bottom-grown gypsum, indicating that the unit formed in shallow, areally extensive brine pools. The pair of anhydrite beds of the Alibates are homogeneous and widespread over most of the basin. Complexities noted in this pattern include local thinning or absence of one or both anhydrite beds and change in log character, suggesting replacement of anhydrite by less dense, more porous, and more radioactive carbonate or chert. Thinning and compositional changes are common toward the north and east Midland Basin margins. More than two thick carbonate-anhydrite beds are common in the areas over and adjacent to the Capitan Reef, but the geometry of these units was not resolved in this study.

Where they have been examined in the Palo Duro Basin, diagenetic alteration in Alibates anhydrite beds has followed a more complex path than diagenesis of other anhydrite beds (Hovorka, 1992). In the Alibates, gypsum has been pseudomorphically replaced by dolomite, so that in places, the Alibates is a carbonate unit (McGillis and Presley, 1981). Locally in the Palo Duro Basin, the Alibates has been extensively replaced by chert. Silicification is a common diagenetic alteration of anhydrite but is very minor in other Permian anhydrite beds. In core from the Oldham nose structural positive on the northwest margin of the Palo Duro Basin, I observed cross-bedded, reworked, doubly-terminated quartz crystals with anhydrite inclusions in the upper Alibates dolomite bed. I have never observed halite overlying Alibates anhydrite beds, but brecciated, corroded, diagenetically altered anhydrite-siliciclastic contacts are areas where original halite may have been dissolved. This complex diagenesis is significant because it shows that Alibates deposition was preceded by an episode of reworking and silicification of older evaporites at least locally on the basin margins. Conforming to current stratigraphic nomenclature, this break is described as a sequence boundary. Additional alteration throughout the Alibates but not penetrating far into the underlying salt suggests that periods of alteration occurred before substantial warping of the Alibates, before or during Dewey Lake or Dockum deposition. These observations provide context in which to interpret heterogeneities observed within and beneath the Alibates in the Midland Basin.

Insoluble Residue
Above the halite-bearing part of the Salado Formation is an interval of insoluble residue. Insoluble residue thickness varies depending on the amount of salt dissolved and the impurity content of the salt. In cores from the Palo Duro and Delaware Basins, examination of the insoluble residue showed that this interval is composed of impurities in the salt, including anhydrite beds, mudstone beds, and impurities disseminated within the salt. Water sampling from this interval in the Palo Duro Basin (Dutton, 1987) showed that the insoluble residue contained brines that have dissolved evaporite but are not saturated with respect to halite. Anhydrite beds within insoluble residue are partly to completely altered to gypsum. The insoluble residue interval is commonly slightly to strongly brecciated containing horizontal fractures, small faults, high-angle fractures, abundant joints, or collapse breccia. Because the insoluble residue is commonly poorly understood and because it is a potential engineering challenge for caverns sited in the underlying salt interval, insoluble residues and the salt dissolution process are described in a following separate section.

Insoluble residue is recognized on logs by high gamma-ray-log response reflecting concentration of clayey and arkosic mudstone, low resistivity because of saline pore water in residue, which is more permeable than the underlying salt, and cycle skipping in sonic logs as a result of fracturing (Crane 5, W. H. Black No. 1 Shannon Estate, shows sonic-log response to fracturing and collapse). Comparison of insoluble residue intervals with adjacent logs where salt is preserved shows condensed thickness and concentration of anhydrite beds as intervening salt has been removed. Where anhydrite has been partly hydrated to gypsum, increased water content causes higher neutron count rates. As discussed in detail in a later section, salt dissolution in most areas is coincident with depositional changes in unit thickness and facies; this is one of the challenges in understanding these variations. As well as the common occurrence of insoluble residue at the top of the Salado, salt has also locally been dissolved from the base of the formation.