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Previous Work: Geologic Setting of
the Bedded Salt in the Permian Basin

The evolution of the Permian Basin is very well known because of the long and intense history of hydrocarbon exploration in the sub-salt section. The Permian Basin formed as an area of rapid Mississippian-Pennsylvanian subsidence in the foreland of the Ouachita Foldbelt. Complex faulting, creating platform or arch areas of slower subsidence, subdivided the Permian Basin. Subdivisions of significance to this report are, from southwest to northeast: the Delaware Basin, Central Basin Platform, Sheffield Channel, Midland Basin, Ozona Arch, and Matador Arch (map of structure on top of the Yates Formation, 66k).

The geometry, quality, and stability of salt depend on interactions among the depositional character, thickness, and composition of the salt; postdepositional uplift and subsidence; and landscape development and resulting ground-water circulation patterns. Few studies have described the salt within the Midland Basin. Extensive research on the salts in the adjacent Delaware and Palo Duro Basins, conducted during characterization of the salts in these areas as potential hosts for radioactive waste, can be readily applied to understanding the similar salt in the Midland Basin.

Permian basin filling began with Pennsylvanian marine shales, limestones, and arkoses (Cys and Gibson, 1988). By early to middle Permian (Leonardian), the north and east parts of the Permian Basin had been infilled with sediments. The Delaware Basin, at the western edge of the study area, was a structural and topographic basin that provided the inlet for marine water during most of the Permian (index map of the Permian Basin, Texas-New Mexico, 99k). Sedimentary patterns show that by the Guadalupian, sedimentation had mostly leveled topography east of the Delaware Basin, so that the major structural elements such as the Central Basin Platform, Midland Basin, Northern Shelf, Matador Arch, Eastern Shelf, and Ozona Platform (map of structure on top of the Yates Formation, 66k) were expressed only by subtle contrasts in subsidence rates. This relationship is apparent in the continuity of strata across structural positive areas with only minor changes in thickness or composition (Adams, 1968; Feldman, 1962; Matchus and Jones, 1984; Fracasso and Hovorka, 1986). Connection with marine environments to the west therefore became poorer and saline brines began to form, first in the marginal parts of the Permian Basin and then, progressively, throughout the entire basin. Evaporite sediments, initially anhydrite and then halite, began to accumulate in the Palo Duro Basin during the Leonardian (Wichita and Clear Fork Groups and lower San Andres Formation).

Salt precipitation began in the Midland Basin during the Guadalupian; salt occurs in the Grayburg, Queen, and Seven Rivers Formations (details in next section). The thickest salts are generally observed on the parts of the shelf away from the Delaware Basin toward the east and north. The classic and extensively studied Capitan Reef is a strongly aggradational Guadalupian carbonate accumulation that rims the Delaware Basin (King, 1942; Garber and others, 1989; Bebout and Kerans, 1993). Several cycles of sandstones, anhydrite, and halite of the Yates Formation were deposited across the platform during a sea-level lowstand; the corresponding deposits in the Delaware Basin are in the Bell Canyon Formation. The deposits of the following highstand, also composed of a number of cycles, are carbonate, anhydrite, halite, and sandstone of the Tansill Formation. The Lamar Limestone at the top of the Bell Canyon Formation is the basinal equivalent to the Tansill (Garber and others, 1989).

During the Ochoan, evaporites began to precipitate in the Delaware Basin. The topographic depression was filled by the Castile Formation (Snider, 1966; Adams, 1944; Anderson and others, 1972). Deposition of thick salts in the Salado Formation followed. The Salado Formation, like preceding Permian units throughout the Permian Basin (Meissner, 1972; Fracasso and Hovorka, 1986; Hovorka, 1987), is highly cyclic on a meter scale throughout the Permian Basin (Dean and Anderson, 1978; Lowenstein and Hardie, 1985; Lowenstein, 1988; Hovorka, 1990; Holt and Powers, 1990). Cycles began with a flooding event that typically precipitated anhydrite. Sediment aggradation caused restriction, limiting water movement and causing halite precipitation. In the Salado Formation, highly evaporated brines ponded on the saline flat altered previously deposited gypsum to polyhalite. Mud, silt, and sand deposited by eolian and arid-region fluvial processes are interbedded with the halite. Interbedding of anhydrite, polyhalite, halite, and fine-grained clastics on a centimeter scale reflects the variation in the depositional environment (Fracasso and Hovorka, 1986; Lowenstein, 1988; Hovorka, 1990; Hovorka, 1994). Facies within the salt-depositional environment control variations in the amount, mineralogy, and distribution of impurities; in the crystal size, shape, and interrelationships; and in the amount, distribution, and chemistry of included water. The facies are complex vertically and horizontally; however, analysis of the facies relationships can be used to map the characteristics of the salt (Kendall, 1992; Hovorka and others, 1993).

Salt deposition within most of the Permian Basin ended with a major transgression that deposited the Alibates Formation. This unit contains thin but extensive carbonate and anhydrite beds separated by a siltstone or sandstone (McGillis and Presley, 1981). Although stratigraphic nomenclature and relationships are complex in the Delaware Basin (Powers and Holt, 1990), genetic equivalence and correlation of the upper Rustler carbonate-anhydrite unit (Magenta and Forty-Niner Members) with the upper carbonate-anhydrite unit of the Alibates appears reasonable. Overlying the Alibates and the upper Rustler anhydrite are fine sandstones, siltstones, and mudstones of the Dewey Lake Formation, or equivalent upper Rustler Formation that were the final Permian deposits.

Basin evolution after evaporite deposition is significant for salt cavern siting because the salt geometry was modified by burial dissolution. Triassic deposition of lake-deposited mudstones and fluvial sandstones of the Dockum Formation occurred following subtle warping and reconfiguration of the basin to a large centripetally draining lake basin (McGowen and others, 1979). Inferred uplift along the margins may have permitted salt dissolution to begin at this time, although no dissolution features that unequivocally formed at this time have been identified. Complex sedimentation within the Dockum Group and later crosscutting episodes of salt dissolution have obscured the record of any dissolution that occurred at this time.

A long unconformity followed Dockum deposition and is represented by erosion and truncation preceding deposition of Cretaceous sandstones and carbonates over most of the area. Dissolution prior to Cretaceous deposition is reported in many parts of the Permian Basin (Adams, 1940; Gustavson and others, 1980; Wessel, 1992a). Regional uplift occurred during the Cenozoic, and gravel, sand, and finer grained clastics of the Miocene?Pliocene Ogallala Formation were deposited in fluvial and upland eolian settings (Seni, 1980; Gustavson, 1996). Other significant Cenozoic deposits include Pecos River gravel (Bachman, 1984) and surficial sand, terrace, and colluvial deposits (Barnes, 1992). The current structure of this region (generalized geologic map of the study area, 50k) is the result of post-Cretaceous uplift and tilting that reactivated structural elements with the same sense of motion as they had during the Permian (McGookey, 1984), so that beneath the Southern High Plains and in the center of the Midland Basin the top of the Alibates is at 500 ft above sea level, while over the Eastern Shelf of the Midland Basin at shallow depths beneath the Rolling Plains it has been elevated to 1,800 ft above sea level. The Permian has also been uplifted over the Central Platform where it lies beneath Triassic units in the Pecos Valley. In the Delaware Basin, Permian rocks dip gently toward the east; in the Eastern Shelf, Permian rocks dip gently toward the west. Cretaceous rocks are preserved only in the southeast part of the study area, and Permian, Triassic, and Cretaceous units have been partly covered by Cenozoic deposits. These units are now undergoing erosion to create the Caprock Escarpment that rims the Southern High Plains (generalized geologic map of the study area, 50k).

Stratigraphic Units and Type Logs