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

Onneken J., Schramm M., Hammer J.

Geological Quarterly

2018

Vol. 62, No. 4

896--916

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 Liand 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.

Contribution to mineralogy and geochemistry of Tonmittelsalz (z3TM) and Tonbrockensalz (z4TS) in the German Zechstein

Onneken J., Schramm M., Hammer J.

Przegląd Solny

2017

T. 13

94--106

In the German Zechstein basin, two main clay-bearing salt strata occur, in fact Tonmittelsalz in Z3 (Leine formation) and Tonbrockensalz in Z4 (Aller Formation). These units may correlate with the Brown Zuber (Na3t) and Red Zuber (Na4t) in the Polish Zechstein basin. To mark out the characteristics of Tonmittelsalz and Tonbrockensalz, BGR started detailed mineralogical-geochemical investigations. Even though these units are characterized by a similar mineral composition of mainly halite with subordinate quantities of anhydrite and clay minerals, differences were observed. Based on microscopical observations, carbonate is only existing in Tonmittelsalz and polyhalite is more common in Tonbrockensalz. Characteristic for Tonmittelsalz seem to be clay bands with idiomorphic halite and a breccious fabric for Tonbrockensalz. Similarities to the equivalent zuber beds were observed. This ongoing project will be continued with more samples from other locations.

W niemieckiej części basenu cechsztyńskiego występują dwie główne warstwy soli ilastych: Tonmittelsalz w cyklotemie Z3 (formacja Leine) i Tonbrockensalz w cyklotemie Z4 (formacja Aller). Warstwy te korelują się z Brązowym Zubrem (Na3t) i Czerwonym Zubrem (Na4t) w polskiej części basenu cechsztyńskiego. BGR wykonało badania mineralogiczne i geochemiczne soli Tonmittelsalz i Tonbrockensalz, aby uzyskać ich szczegółową charakterystykę. Pomimo podobieństwa składu mineralnego, w którym głównymi minerałami są halit, anhydryt i iłowce zaobserwowano różnice. Na podstawie obserwacji mikroskopowych stwierdzono, że węglany występują tylko w Tonmittelsalz, natomiast polihalit jest częściej spotykany w Tonbrockensalz. Ponadto, w Tonmittelsalz iłowce tworzą pasma z idiomorficznymi kryształami halitu, a w Tonbrockensalz występują brekcje. Badania opisanych soli są nadal prowadzone.