Rectangular and "stair-step" pore reentrants in carbonate mudstones have been recognized previously as indirect evidence for anhydrite dissolution. In this study, direct evidence for dissolution of interstitial anhydrite in subsurface rocks includes: (1) cleavage-related dissolution fringe on anhydrite crystal surfaces and (2) isolated remnants of optically continuous (formerly poikilotopic) anhydrite. Influenced by the prominent cleavages, the dissolution fringe on the surfaces of the anhydrite crystals consists of a series of sharp, right-angled projections and reentrants. Experimentally etched anhydrite surfaces exhibit features that directly compare to the dissolution fringe, whereas experimentally grown anhydrite does not.

Anhydrite in both the dolomite grainstones of the Permian San Andres Limestone in the Vacuum field, Lea County, New Mexico, and the sandstones of the upper (Permian) part of the Minnelusa Formation in the West Mellot Ranch field, Crook County, Wyoming, exhibited these direct evidences, demonstrating the presence of secondary porosity after anhydrite.

We deduced the following sequence of anhydrite dissolution within these rocks. Slow incipient dissolution began along the boundaries between anhydrite and adjacent minerals. From these intercrystalline boundaries, solutions penetrated anhydrite cleavages, leading to more rapid preferential dissolution perpendicular to the more prominent cleavage planes. The widened cleavage planes, together with intercrystalline boundaries, acted as conduits for removal of dissolved ions. In the final stage, as dissolving anhydrite borders retreated toward pore throats, dissolution slowed and was, again, restricted to intercrystalline boundaries. This process was repeated in adjacent interstices.

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