Evaporites in a rift setting - the alpine halitemudrock-polyhalite tectonite (Haselgebirge Fm., Eastern Alps, Austria)

The salt of the late Permian Haselgebirge Fm. exposes large proportions of mudrock. The mud was trapped in a basin without a connection to the open sea. The mudrock comprises earthquake structures. An enrichment of magnesium minerals (mainly magnesite and chlorite) and "deep water" anhydrite point to a residual brine basin. Pre-saturation happened in the surroundings represented by the gypsum/anhydrite deposits of the Haselgebirge Fm. The basin itself was probably narrow (tens to fifties of kilometers), but elongated (>1000 km). Igneous rocks with an intermediate chemistry are in sedimentological contact with the salt. A late syn-thinning stage is assumed for the deposition of the Haselgebirge Fm., due to low diapiric activity and the paleogeographic arrangement of the salt rocks in the context of the Northern Calcareous Alps. Heating was probably enhanced by the overlapping processes of orogenic collapse of the Variscan mountain belt and the formation of a rift. The first occurrence of polyhalite [K2Ca2Mg(SO4) 4.2H2O], which represents a mineralogic peak of the diagenetic history coincides with the final opening of the Meliata Ocean.