Recognition of Interstitial Anhydrite Dissolution: A Cause of Secondary Porosity, San Andres Limestone, New Mexico, and Upper Minnelusa Formation, Wyoming

Abstract

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 subsurface dissolution of interstitial anhydrite in both dolomite grainstones and quartz sandstones 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. We deduced the following sequence of anhydrite dissolution within dolomite grainstones and quartz sandstones. 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 the 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 then repeated in adjacent interstices.