Characterization and timing of diagenetically altered sedimentary gypsum in Luneburg and Othfresen (Northern Germany) by using modern OSL, petrographic and geochemical investigations

Gypsum of cap rocks from Upper Permian (Zechstein) salt structures in Luneburg and Othfresen, both Northern Germany, were investigated for geochronological, mineralogical-geochemical, stable isotopic (δ³⁴S and δ¹⁸O of the SO₄^2- ion; δ¹⁸O of the hydration water of gypsum) and petrographic characteristics. Cap rocks are formed during salt diapirism and groundwater contact (dissolution, re-precipitation, transition of evaporite minerals). For dating the precipitation of gypsum, Optical Stimulated Luminescence (OSL) was used (1).

First uncorrected age estimates range from 0.4 ka ± 0.3 ka to 7.1 ka ± 2.8 ka for the Luneburg samples and for the gypsum pit Othfresen, respectively. The age estimates represent the lower detection limit of the method for these samples. Some individual subsamples yield uncorrected age estimates from 14.4 ka ± 2.8 ka to 114 ka ± 25 ka for Luneburg samples and 9.3 ka ± 5.0 ka up to > 394 ka for Othfresen gypsum. Age estimates of these subsamples could be part of an unbleachable OSL-component or it could reflect the timing of different solution/ precipitation phases of gypsum crystals during alteration.

δ³⁴S- and δ¹⁸O- values of the sulfate ion average 11.5 ‰ (VCDT) and 12.1 ‰ (VSMOW), representing a typical Upper Permian Zechstein signature. In contrast, δ¹⁸O- values of bulk gypsum average 5.8 ‰ (VSMOW), indicating the influence of isotopically light hydration water from the transformation of Permian anhydrite to gypsum. These results are in line with the young ages derived from OSL and the petrographic observations.

1: Mahan, Kay (2012): Quaternary Geochronology 10: 345-352