Mineralogy and geochemistry of the Tonmittelsalz (z3TM) and Tonbrockensalz (z4TS) as “zuber” equivalents in the German Zechstein (Upper Permian)

Abstract

Two main lithostratigraphic units of clay-bearing rock salt, the Tonmittelsalz of the Leine formation (z3) and Tonbrockensalz of the Aller formation (z4), occur in the German Zechstein (Upper Permian) succession. These units could be equivalents of the Brown Zuber (Na3t) and Red Zuber (Na4t) in the PZ3 and PZ4 cyclothemes of the Polish Zechstein basin. Mineralogical-geochemical investigations of the Tonmittelsalz and Tonbrockensalz were carried out on samples taken from a deep borehole in the Gorleben salt dome in Northern Germany. Even though these units are characterized by a similar mineralogical composition of mainly halite with subordinate quantities of anhydrite and clay minerals, variations in mineral content and fabric were observed. The older Tonmittelsalz rocks document some primary features like chevrons in halite crystals and idiomorphic halite crystals in clay-bearing layers. A brecciated fabric and a vague layering, shown by polarizing microscopy and CT-imaging, indicate a deformation of the younger Tonbrockensalz, which is folded in the deep borehole Go1004. Nevertheless, internal fabrics of clay clasts in the z4TS show an early brecciated and folded fabric during sedimentation or diagenesis. Main component chemistry and REE are comparable in both units, but significant differences were observed for trace element and isotope data. The z3TM rocks contain higher values of trace elements like Li and higher values in REE, while the z4TS rocks are enriched in K. Isotope data of anhydrites of both units correspond to those of the Zechstein. The δ¹⁸O values of samples from the Tonbrockensalz display a relatively large range (8.5–11.9‰) and may indicate changing conditions during its formation. In contrast, only minor variations in the δ³⁴S of samples from the Tonbrockensalz and in both isotope compositions of samples from the Tonmittelsalz have been documented.