Bromide characteristics and deformation mechanisms of naturally deformed rock salt of the German Zechstein Basin

Abstract

This dissertation includes four studies that were concerned with geochemical characteristics and deformation mechanisms of rock salt. The studies focused especially on the rock salt of the Stassfurt Formation (Zechstein 2) and samples were taken from different salt deposits of Germany and the Netherlands. The first study investigated the relationship between deformation intensity and bromide characteristics of the rock salt. For this, the bromide distribution of a defined stratigraphic section (Hauptsalz) of the Stassfurt Formation (Zechstein 2) was investigated in three salt deposits, with bedded salt (Teutschenthal) compared to domal salts (Morsleben, Gorleben). The comparison between the locations yielded the following findings: (1) The characteristic trend of the bromide profile can be observed in both the bedded and the more intensely deformed domal salts. (2) The dispersion of bromide contents along the running average curve is lower in Morsleben and Gorleben than in Teutschenthal, especially in the lower half of the Hauptsalz. (3) The Kristallbrocken are absent in the lower half of the Hauptsalz of Morsleben and Gorleben. On the one hand, the lower dispersion of the bromide contents in the domal salts and the absence of the Kristallbrocken are clear indications for the influence of salt migration-related processes on the bromide distribution characteristics. It shows that these processes are associated with a redistribution of bromide, which eventually results in a homogenisation of the originally varying bromide contents. On the other hand, preservation of the characteristic trend in all bromide profiles indicates that large-scale brecciation, folding processes, or circulating bromide-rich fluids could play only a minor role during the formation of the salt domes. The topic of the second and third study of this work was the laminated halite type Kristallbrocken that is characteristic for the Stassfurt Formation (Z2) rock salt. The investigations of both studies contributed to the understanding of the structure, the formation and the deformation mechanisms of this halite type. X-ray texture analyses on 4 Kristallbrocken samples from the salt deposit Teutschenthal demonstrated that the Kristallbrocken are single crystals and that they can be regarded as relics of formerly larger single crystal-layers . Microscopic investigations showed that the internal lamination of the Kristallbrocken consists of several types of sulfate inclusions (anhydrite, polyhalite). Especially characteristic are anhydrite aggregates with surrounding brine. The original size of the monocrystalline beds, their high amount of relatively large solid inclusions as well as the characteristic sedimentary features make it unlikely that these laminated single crystals were grown directly from a solution. A more probable scenario is that an originally fine-grained, laminated halite sediment was converted into monocrystalline Kristallbrocken in a post-sedimentary grain growth process. A plausible explanation for the formation of the characteristic anhydrite aggregates is that, initially, gypsum crystals were incorporated in the halite sediment, which converted into anhydrite within the later formed Kristallbrocken. The solid volume reduction as well as the release of water in consequence of a complete gypsum-anhydrite conversion explains the present appearance of the characteristic solid inclusions within the Kristallbrocken. The brittle deformational behavior of the monocrystalline Kristallbrocken can be explained by the originally large size and the high amount of solid inclusions forming the internal lamination, since these features make this halite type behave rheologically more competent compared to the surrounding fine- to coarse-grained polycrystalline rock salt. The aims of the fourth study were the investigation of textures in naturally deformed rock salt and their correlation with micro- and macrostructures as well as large scale salt structures. This dissertation presents the results of first neutron texture measurements on four samples from the salt deposits Teutschenthal, Morsleben, and Gorleben. The samples from Morsleben and Gorleben show a clear grain shape anisotropy, whereas the samples from Teutschenthal display only a weak grain elongation. The pole figures of all samples show no crystallographic preferred orientation (texture). The lack of texture indicates that Intracrystalline deformation mechanisms played no or only a minor role during the deformation of the salts. So, alternative deformation mechanisms have to be taken into account such as solution precipitation processes, diffusion creep, or fluid assisted grain boundary migration.