Thickness of interval containing Salado salt from the top of Alibates Formation and equivalent top of upper Rustler anhydrite to top of Yates Formation and top of Lamar Limestone.

Synsedimentary effects influence the Alibates-Salado-Tansill isopach, which shows a general area of thick accumulation along the present structural axis of the Midland Basin. Comparison of the map view with cross sections shows that much of this thickening results from a combination of (1) regional thickening throughout the Salado from the north and east basin margins toward the west and (2) accumulation of thick Salado units at the top of the formation above bed 60. The thickest interval in the Alibates-Salado-Tansill isopach (2,000 to 4,000 ft) is in the Delaware Basin in the southwest part of the study area (western Pecos, Ward, and Winkler Counties). This is the margin of the very thick and extensive salt of the Delaware Basin. The lower half of this interval is made up of anhydrite of the Castile Formation. A thick Ochoan interval (>1,200 ft) also fills the San Simon channel (western Gaines County). Comparing the salt thins to the top Alibates structure shows more depressions than the structural elements seen on the top Yates. One deep depression on top Alibates and thin in the isopach is found in central Winkler and Ward Counties. This corresponds to thin, absent, and dissolved salt along the Capitan Reef trend (Girard, 1952; Hiss, 1976; Baumgardner and others, 1982; Johnson, 1987; 1989a). Depressions are found along the Capitan Reef trend into New Mexico (Bachman, 1984; Hiss, 1976). Southward along a related trend is a large depression in the Alibates structure and corresponding thin in the Alibates-Salado-Tansill interval that lies above the south part of the Central Basin Platform in east Pecos and west Crockett Counties extending east to the Yates oil field area (Adams, 1940; Wessel, 1988a; 1988b; 1992a; 1992b). Other areas of thinning over short distances are noted over structural features marking the Midland Basin margins. Thinning is noted in Crockett County over the Ozona Platform. Regional cross sections (Humble Oil and Refining Company, 1960; 1964a; Vertrees, 1962; 1963) show erosional truncation of the Permian beneath the Cretaceous in this area. Thinning of the interval to 300 or 200 ft corresponds to complete dissolution of the salt in the interval toward its truncated edge, leaving only the Tansill, Alibates, and insoluble residue after salt dissolution. The trend of thinning of the salt-bearing interval continues along the eastern shelf (Reagan, Glasscock, Howard, Borden, Garza, and Crosby Counties). Depositional thinning, salt dissolution, and erosional truncation beneath the Cretaceous and toward the outcrop are all factors in this thinning. Some areas of abrupt lateral thinning and complex geometries are noted in Glasscock and Howard Counties, generally corresponding to a structurally high area (Humble Oil and Refining Company, 1960; Vertrees, 1962; 1963; Geomap, 1986). Another area of salt thinning lies south of the Howard-Glasscock high. The thin area in the isopach is on the north side of a structural depression in both the top Yates and top Alibates structure, so that both the closed depression in the top Alibates is larger than in the top Yates because the interval thins along the northeast edge of the structural depression. A general trend in salt thinning continues around the north of the Midland Basin along the Matador Arch and Northern Shelf structural and depositional positive elements. No areas of abrupt thinning were noted in this area.