ABSTRACT

Paleokarst developed in sulfate deposits is common, and it is usually formed along the contact with the overlying permeable rocks or it is due to near-surface dissolution of bedded evaporites. In the Lower Werra Anhydrite (Zechstein) of northern Poland the paleokarst cavities are usually filled by bluish semitransparent anhydrite and more rarely by celestite, polyhalite, halite, and carbonate. In small cavities (a few centimeters across), a rim of rod-like anhydrite crystals arranged in narrow bundles occurs, and the inner part of the cavity is filled with a mosaic aggregate of short prismatic crystals of anhydrite and celestite as well as coarse irregular anhydrite. Celestite crystals and fan-shaped aggregates as well as spherulites of anhydrite are rare. In bigger cavities (some ten centimeters across), multiple zones of fibrous anhydrite are arranged in different directions in the middle part of the cavity fill. The innermost parts of large karst cavities remain hollow in some cases, with the cavity walls encrusted by coarse, well-developed crystals of anhydrite and celestite.

The karst cavities in the Lower Werra Anhydrite developed in the subsurface by dissolution of CaSO₄ strata in halite-rich intervals due to gypsum dehydration water. During gypsum dehydration, dissolution of that halite would have increased the sodium chloride content of the solution and thus the solubility of calcium sulfate. Dissolved calcium sulfate was removed from a leaching zone by diffusion and/or downward flow in interstitial space, and the minerals in karst cavities precipitated from the same solutions as those solutions became oversaturated because of decreases in NaCl concentration over time. This study suggests that karst in sulfate deposits can develop in the subsurface and without uplift and/or near-surface conditions.