The Permian Castile formation is a favorable starting point for a study of precipitated marine sediments because of its relatively simple stratigraphy. It is 1,300 to 2,000 feet thick and consists principally of “banded” or laminated anhydrite, but it contains some salt and other materials. It was deposited within the deep steep-walled Delaware Basin of western Texas and southeastern New Mexico. Throughout Castile time communication with the open sea was maintained through a restricted channel near the southwest corner of the basin. Paucity of detrital material indicates that the deposit consists almost exclusively of marine evaporites.
It is suggested that the water within the basin consisted of a body of brine lying below average wave base and a less dense surface layer lying above average wave base. Some of the dense brine continuously returned to the open sea; otherwise the volume of halite deposited would have been about 30 times as great as the volume of anhydrite instead of somewhat less than the volume of anhydrite. The barrier must therefore have been incomplete. Either the top lay partly below wave base or the barrier was permeable. This theory of reflux is presented as a modification of Ochsenius’ bar theory.
The calcium sulphate presumably was deposited as anhydrite rather than as gypsum because of the salinity of the brine. The banding is ascribed to seasonal variation in temperature. The average net rate of evaporation in Castile time is calculated as 114 inches a year.