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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.

Optically stimulated luminescence dating of young fluvial deposits of the Middle Elbe River Flood Plains using different age models

Kunz A., Pflanz D., Weniger T., Urban B., Krüger F., Chen Y.-G.

Geochronometria

2014

Vol. 41, no. 1

36--56

In the last few decades optically stimulated luminescence (OSL) dating has become an im-portant tool in geochronological studies. The great advantage of the method, i.e. dating the deposi-tional age of sediments directly, can be impaired by incomplete bleaching of grains. This can result in a scattered distribution of equivalent doses (DE), leading to incorrect estimation of the depositional age. Thoroughly tested protocols as well as good data analysis with adequate statistical methods are important to overcome this problem. In this study, samples from young fluvial sand and flood plain deposits from the Elbe River in northern Germany were investigated to compare its depositional ages from different age models with well-known historical dates. Coarse grain quartz (100-200 μm and 150-250 μm) and polymineral fine grains (4-11 μm) were dated using the single aliquot regenerative (SAR) dose protocol. The paleodose (DP) was calculated from the DE data set using different ap-proaches. Results were compared with the development of the Elbe River, which is well-documented by historical records and maps covering the last 1,000 years. Depending on the statistical approach it can be demonstrated that depositional ages significantly differ from the most likely depositional age. For the investigated coarse grain quartz samples all ages calculated from the MAM-3UL, including their uncertainties, are within the historical documented age. Results of the polymineral fine grain samples are overestimating the historically documented depositional age, indicating undetectable in-complete bleaching. This study shows the importance of using an adequate statistical approach to cal-culate reliable OSL ages from fluvial sediments.