ABSTRACT

Stylolites in evaporites are very uncommon. In the Gorleben Zechstein salt dome, stylolite horizons have been found in the so-called "Banderanhydrit" (z3HA11) at the top of the Zechstein Hauptanhydrit (Main Anhydrite). These pressure-solution features seems to be related to the presence of thin magnesite-clay interlayers. Five principal stylolite morphologies can be distinguished in the core sections studied. The orientation of the sutured seams is generally bedding-parallel with the stylolite axes vertical to the bedding surface. This demonstrates that pressure-solution features clearly predate the formation of the salt dome and that there is no relation between their formation and halokinesis. Solution rates of up to about 26% have been calculated by measuring maximum stylolite amplitudes in certain core intervals. Considering the bedding-parallel occurrence of the stylolites and the fact that the stylolite seams are crosscut by anhydrite crystals that we interpret as pseudomorphs after gypsum, it must be concluded that the stylolites formed before the dehydration of gypsum to anhydrite was completed. The lower parts of the Hauptanhydrit were excluded from these processes because they had already been converted to anhydrite by the overburden of overlying Zechstein units and by ascending compaction fluids of the underlying Zechstein 2 Salt. We assume that the dehydration of gypsum to anhydrite was completed before the end of Zechstein sedimentation and that the formation of stylolites thus must be attributed to the same time interval.