More than one hundred masses of limestone are associated with gypsum of the Castile Formation in the western half of the Delaware basin. The carbonate masses, many of which form buttes 70 ft (21 m) or more above the surface of the Gypsum Plain, are the result of replacement of anhydrite by calcite. The buttes represent masses of calcite that have been exposed by erosion of Castile gypsum. The calcite is a by-product of the Cenozoic reduction of calcium sulfate by natural gas and possibly oil.

Natural gas, derived from reservoirs in underlying Permian units, was utilized by sulfate-reducing bacteria and served as the chief source for the isotopically "light" carbon (enriched in carbon-12) now present in the calcite of the carbonate masses. Isotopically "light" native sulfur is associated with some buttes and is also the result of the activity of bacteria, the only agent by which pronounced fractionation of isotopes of sulfur can be achieved during sedimentation or diagenesis.