Production of macropores by gypsum replacement of carbonate, and of intergranular porosity by pervasive dolomitization constitute two important mechanisms for generating high porosity in carbonate rocks in and adjacent to evaporite basins.

Exposures of Upper Permian Capitan Group carbonate beds in Carlsbad Caverns and other caves of the Guadalupe Mountains, New Mexico, indicate that one of the earliest stages of speleogenesis was the massive replacement of carbonate by sulfate. Field relations indicate that replacement took place in a mixing zone between a meteoric freshwater lens and gypsum-saturated brines. Replacement is most pronounced along joints, indicating a possible correlation with the rate of freshwater input. Replacement probably began with the development of the lens following orogenic uplift of the Guadalupe Mountains, or solutional deflation of basinal evaporites. As uplift and deflation continued, hydrologic base level fell with respect to replacement gypsum pods, exposing them first to freshwater phreat c conditions, and later to vadose conditions, causing their partial or complete recrystallization or dissolution and creating enormous voids. These underwent limited enlargement in the freshwater phreatic zone, and collapsed following draining of the caves.

Replacement of the gypsum may be recognized by primary carbonate structures preserved as remnant inclusions. Replacement gypsum crystals are generally equant with complex boundaries, but may derive their size and shape from carbonate macrostructures replaced

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(usually laminae within pisolites). Replacement crystals range in diameter from 0.1 to 10 mm. Inclusions which reproduce primary fabrics are most commonly small (to 50 µm), irregular rounded blebs of unknown composition, but are rarely dolomite rhombs.

Generally associated with sulfate replacement zones are massive sucrosic dolomites displaying abrupt lateral transitions with unaltered limestones. These dolomites may form because of increased magnesium:calcium ratios in mixing-zone pore waters, because during replacement magnesium is not accommodated in the gypsum lattice.

Work is under way on the recognition of secondary carbonate fabrics related to these mixing-zone environments which would allow identification of gypsum replacement in rocks in which gypsum has totally dissolved or recrystallized.

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