The basic genetic cycle style recognized in the Leonardian through Guadalupian of the Palo Duro Basin (Fracasso and Hovorka, 1986; Hovorka, 1994) and the Salado Formation of the Delaware Basin (Lowenstein, 1988; Hovorka, 1990) is also well displayed in the Midland Basin and provides the facies architecture needed to describe the thickness and continuity of salt beds and the distribution of impurities within them. Anhydrite beds formed during relative water-level rise form the bases of master cycles. Bundled between them are multiple intermediate cycles composed of halite, mudstone-halite, and mudstone.
Stacking of these master cycles produces a systematic regional thickening of halite from the north and east margins of the Midland Basin across the Central Basin Platform, toward the Delaware Basin. The conspicuous dissolution-induced variations in this trend over the Capitan Reef, Pecos River, and south Central Basin Platform area are discussed in following sections. Inspection of cycle patterns shows no major systematic change in salt quality with respect to salt purity, bed thickness, or spacing of anhydrite beds across the Midland Basin and Central Basin Platform. Anhydrite beds are gradually thicker and more numerous toward the Delaware Basin, but changes in anhydrite-bed thickness are specific to each master cycle, and no evidence for a consistent break is identified within the limits of the techniques used.
The l,200-ft-thick lower part of the Tansill Formation contains three to five mapped cycles of anhydrite overlain by mudstone. Log character suggests that the mapped cycles are probably composites of more thin, anhydrite-dominated cycles. Cycles lack halite except in the north and east parts of the Midland Basin, indicating that although the shelf was frequently and extensively flooded, accommodation was limited and halite either did not accumulate or was dissolved during exposure at the end of each cycle. Anhydrite thickens and contains more dolomite toward the Delaware Basin and the Sheffield Channel. The upper Tansill contains three or four halite-siliciclastic cycles that thin toward the Delaware Basin and the Sheffield Channel.
The cycle pattern in Salado Formation in the Midland Basin is composed of six regionally traceable master cycles overlain by multiple complex cycles at the top. Master cycles are defined by a regionally traceable flooding event that deposited an anhydrite overlain by multiple halite-mudstone cycles. The lowest master cycle (50 to 150 ft thick) has a thin and discontinuous anhydrite or anhydrite-polyhalite bed (bed 15) at the base; the flooding event initiating this cycle was sufficient to end the upper Tansill cycles with abundant siliciclastic beds but only locally produced an anhydrite bed.
The next master cycle is about 175 ft thick and is defined by anhydrite bed 20 at the base. This anhydrite bed is one of the thickest (5 to 30 ft) and most distinctive beds in the Salado Formation. A persistent siliciclastic interval, interpreted as an insoluble residue at the cycle base, gives bed 20 a distinctive log character. It is commonly labeled Cowden anhydrite on published and marked logs, but the relationship of bed 20 in the Midland Basin to the named Salado anhydrite units of New Mexico has not been investigated in this study and, therefore, that nomenclature is not applied. Five or six traceable mudstone-halite cycles are present within this master cycle, and several locally traceable thin anhydrite beds are mapped within it. Polyhalite has replaced anhydrite in several of the mudstone-halite cycles in the Central Midland Basin.
Anhydrite bed 30 defines the base of the next 50- to 200-ft-thick master cycle. It shows more rapid lateral facies relationships than the underlying master cycle, including the occurrence of multiple and thicker anhydrite beds in the south part of the Midland Basin and greater changes in thickness across the Midland Basin. A maximum of nine polyhalite ± anhydrite-halite-mudstone cycles are found in the thick part of the master cycle. Polyhalite replacement increases westward across the Central Basin Platform, and this interval is correlated with an interval containing polyhalite beds in the Delaware Basin.
Anhydrite bed 40, which defines the base of the next 100- to 200-ft-thick master cycle, is discontinuous across the basin, and correlation of beds within this interval is therefore somewhat arbitrary. This interval contains abundant polyhalite beds that are correlated to an interval with abundant polyhalite beds in the Delaware Basin. Six to ten cycles are found in the master cycle. This bed is tentatively correlated with the Union anhydrite of the Delaware Basin (Snider, 1966).
Anhydrite bed 50 is continuous and well defined across the Central Basin Platform and Midland Basin and forms the base of the 75-ft-thick master cycle containing three to five halite-mudstone cycles. This master cycle remains fairly consistent in thickness over much of the area, forming a stratigraphic marker. The master cycle thins in the northernmost tier of counties of the study area and there, anhydrite bed 50 lies near the top of the Salado halite section. Polyhalite is minor in this interval.
Anhydrite bed 60 parallels bed 50 throughout its extent and pinches out toward the north edge of the Midland Basin. Above this bed, the cycle pattern breaks up, and interpretation of cycle correlation is unclear. The typical character of the anhydrite bed 60 to the base of the Alibates interval varies regionally across the study area. In the center of the Midland Basin (northwest Ector, east Andrews, east Gaines, and Midland Counties), this interval is 175 to 225 ft thick and contains two or three halite-mudstone cycles with thicker-than-average mudstone beds, overlain by several cycles with thin anhydrite beds and unusually thick (as much as 100 ft), relatively clean halite beds. In some areas halite directly underlies the lower Alibates anhydrite bed. Over the northern Central Basin Platform (west Andrews and most of Winkler County), the anhydrite bed 60 to base Alibates interval thickens, but much of it is composed of thick mudstone and mudstone-halite beds, as well as thicker anhydrite beds, than in the Midland Basin. Over the southern Central Basin Platform, this interval is thinner and dominated by mudstone and insoluble residue. In the north and east parts of the Midland Basin, the interval is thin and also composed of mudstone and insoluble residue. In the Delaware Basin, several hundred feet of fairly typical anhydrite-halite-mudstone cycles with minor polyhalite are correlated with this interval.
NorthSouth cross section
EastWest cross section
Isopach and Structure Maps