Gypsum domes and diapirs: common features in the Zechstein (Upper Permian) of Germany

The Upper Permian Zechstein Group contains three gypsum horizons of variable thickness. All of them are affected by gypsum diapirism which is not related to salt diapirism. Best areas for observations and descriptions of this phenomenon are outcrops and mining districts around the Harz Mts. The lowermost gypsum member (A1) is overlain by Main Dolomite (Ca2) of the Stassfurt Formation - dolomite showing mudstone and wackestone textures. The gypsum forms cupolas or elongate ridges up to 50 m high, several km long, and may project through its roof. Above the ridges, the Ca2 roof is thinner or completely removed by slumping and sliding. The uplift of the gypsum started very early before the carbonates were completely lithified. All the ridges strike between 90 and 120°, most likely, drawn out by tectonic forces. The Basal Anhydrite (A2) and the Main Anhydrite (A3) are overlain by rock salt and behave in the same manner. Here, too, only the upper parts of the gypsum are involved in the movements. Reasons of the uplift are high water content of gypsum, additional release of water by the conversion of gypsum to anhydrite and, last not least, bouyancy of the light gypsum mud against compact rock salt or semi-lithified carbonates.