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.
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).
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).
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
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.