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Conclusions


This report compiles basic descriptive information about the geometry of salt in the Midland Basin as well as guidance for site-specific evaluation of salt quality and geometry in the context of use of this salt for solution-mined caverns. Thick and laterally homogeneous bedded salt is found in the Salado Formation beneath a 31-county area including the Midland Basin, Central Basin Platform, and associated areas. Regional and local variation in salt thickness, percent salt, structure on the top and bottom of the salt, and depth to salt are mapped throughout this region.

The geometry of salt is the product of interaction between depositional trends and postdepositional dissolution; reference to these two controls are used to aide both in describing the salt geometry and as a mechanism for interpreting relationships. Depositional geometry of the Salado Formation was fairly simple, with a gentle westward thickening from areas of little or no accumulation on the east to a maximum thickness in the Delaware Basin. The salt is divided into high-frequency genetic cycles composed of a basal anhydrite, overlain by halite, muddy halite, and mudstone. Many incomplete cycles containing only the halite, muddy halite, and mudstone facies are recognized within the master cycles defined by anhydrite beds at the base. Examination of the high-frequency cycles defining the stratigraphy within the Salado Formation shows that the observed westward thickening is an effect of greater accommodation (greater relative subsidence) during salt deposition, so that each individual salt bed thickens toward the west. The cycles in the upper part of the Salado Formation show a change from this pattern in that they are thickest along the present Midland Basin axis, contain thick but laterally discontinuous beds, and pinch out into mudstone toward the edges of the Midland Basin.

Depositional geometry of the salt has been modified by several episodes of postdepositional dissolution. The first postdepositional dissolution events probably occurred in terrestrial environments that preceded and followed Alibates deposition. A significant episode of dissolution occurred after significant warping of the Permian strata but prior to Cretaceous deposition. Dissolution occurred during the Cenozoic and continues today.

Substantial thicknesses of salt have been dissolved along the east margin of the basin, along the Central Basin Platform in the Pecos Valley, and over the Capitan Reef margin in Ward and Winkler Counties. Minimum postdepositional dissolution is seen in areas where the salt lies at depth below the most active near-surface hydrologic regime, typically at depths of more than 1,000 ft in the structural basin.

Thin salt generally corresponds to positive structural elements. Inspection of facies relationships in the Midland Basin and comparison with relationships seen in detailed studies in adjacent areas indicate that the salt thinned toward the basin margins because of reduced accommodation during deposition. The present-day structure on the top of the Alibates Formation/Rustler anhydrite follows the long-lived structural pattern of the basin, so that positive areas during deposition have been uplifted more strongly than basinal areas. Postdepositional warping has therefore exposed thin marginal salt to more intense dissolution by placing it at higher elevations than basinal salts.

A change from this pattern is noted where salt has been dissolved in the Winkler-Ward County area. The general trend of thickening of the salt-bearing unit across the Central Basin Platform suggests that this area was subsiding during Salado deposition and is an area of subsidence west of the Central Basin Platform structural positive. In this area, a hydrologic model where salt dissolution is related to interstratal dissolution above the highly transmissive Capitan aquifer is accepted.

Modern landforms are overprinted on the structural elements. Areas where salt is present at shallow depths may influence landform development because salt has been dissolved, creating low areas, and overlying strata have collapsed, been brecciated, and are therefore easily eroded. The Pecos Valley generally overlies an area of salt dissolution on the south end of the Central Basin Platform. In this area, salt was probably relatively thick during deposition but has been removed over the uplift and at the hydrologically active areas along the valley.


Acknowledgments