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The predominantly anhydritic Castile Formation (Lower Ochoan, Permian) of the Delaware Basin (Fig. 1) is commonly used to illustrate the necessity for surface-reflux in marine evaporite basins and as a textbook example of deep-water evaporite sedimentation in a barred basin. The occurrence of basin-wide, mm-calcite/anhydrite laminae and of salinity cycles (laminar anhydrite at the base, passing upward gradationally into 'nodular' anhydrite and laminar halite) have generated depositional models involving a brine-filled basin (up to 600 m deep) that exhibited periodic fluctuations in salinity. Such models have been widely applied to other evaporite-filled basins.

Pseudomorphs of bottom-grown gypsum crystals have been found in 'nodular' anhydrite intervals within the lower parts of the Castile Formation in the Union University "37" No. 4 well (Anhydrite I Member) and the much visited New Mexico roadside outcrop (the "State Line Outcrop") along U.S. 62, 1.2 km north of the Texas-New Mexico boundary (which Anderson and Kirkland, 1987, attribute to the uppermost Anhydrite II Member) (Figs. 2 and 3). Complete gradation between well-preserved pseudomorphs and nodular anhydrite layers in the Union core suggest that nodular anhydrite intervals throughout the Castile Formation were also originally composed of bottom-grown gypsum crystals. This would better explain their lateral persistence and their stratigraphic restriction to certain horizons than their being a recrystallization of anhydrite laminae, as has previously been suggested.

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