Sandstone diagenesis in a halite deposit, from surface to high-grade diagenesis (Haselgebirge Formation, Eastern Alps)

Abstract

The evaporitic Haselgebirge Formation of the Northern Calcareous Alps was deposited in an evolving rift during the Upper Permian. The Jurassic to Cretaceous Alpine orogenesis caused an overprint of ~200 °C. The aim of the study was to investigate the diagenetic evolution of these evaporite-dominated sediments from intact sandstone lenses. Field work, thin section analysis, SEM and XRD were completed by measurements of organic material and age dating. In this paper, we show that all authigenic minerals developed step by step on a prograding path, but under low temperatures of <30–50 °C. All of them developed in the presence of various, saline fluids during eo- and mesodiagenetic stages: Aluminium phosphate-sulfate minerals, magnesite, quartz and K-feldspar overgrowths, vermiculite (now clinochlore), halite, anhydrite and polyhalite. The dissolution of detrital K-feldspar grains left behind K-feldspar overgrowth rims. The large amounts of halite cement and anhydrite or polyhalite replacement cements suggest closure of the intergranular pore spaces under a thin overburden. Age dating of the polyhalite crystallization event by the 40Ar/39Ar method here and in earlier work revealed a first crystallization stage of ~15–20 My after deposition. At that time, the overburden was only ~500 ± 100 m. The high-grade diagenetic conditions of ~170 ± 20 °C measured by vitrinite reflectance relate to the late Jurassic to Cretaceous Alpine tectono-thermal overprint. Folding and thrusting formed a tectonic mélange, but left no additional mineral phases in the sandstones. Our investigation contributes to knowledge of the diagenetic evolution of evaporite-dominated sediments deposited in a lagoonal environment of a rift.