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Analyzing the dynamics of microstructural response on natural deformation in rock salt, we present microfabric, EBSD, geochemical and U–Pb data, obtained from Permian salt formations of the Kiel-Honigsee salt wall in Northern Germany. The samples were recovered from deep drillings, which penetrated through an overturned rock salt sequence of both Rotliegend and Zechstein deposits. The bromide concentration in halite indicates a continental and marine origin for the Rotliegend and Zechstein deposits, respectively. Despite intense deformation, relics of early diagenetic fabrics are still preserved. Deformation of the impure Rotliegend rock salt was accommodated by pressure solution and hydrofracturing as is indicated by the microfabrics and bromide concentration in halite. Fractures in siliciclastic domains were filled with fibrous halite and deformed by subgrain rotation recrystallization (SGR). Fluid-rich Zechstein rock salt, on the other hand, was deformed by formation of subgrains and grain boundary migration (GBM). The distribution of mineral phases and fluids had a significant impact on the fabric evolution and on strain localization. U–Pb dating of carbonate phases of the Rotliegend sequence yielded Permian depositional ages and Jurassic to Cretaceous deformation ages, the latter related to diapiric ascent. The combination of results traces a dynamic evolution of the rock fabric inside the diapir structure driven by locally active deformation processes that can be correlated with early stages of halite deposition and diagenesis and syntectonic fabric reorganization related to diapirism in an extensional setting.

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Abstract. Halite forms the main constituent of rock salt, which is regarded as a possible host rock for nuclear waste repositories and storage caverns. The deformation behavior of pure halite and rock salt has been revealed by microfabric analyses of naturally and experimentally deformed samples. Such studies, however, are rare for rock salt with significant amount of secondary phases (e.g., siliciclastics, clay, anhydrite), although these are also common in nature. In order to determine the influence of siliciclastic material on the deformation behaviour of rock salt, we performed deformation experiments using rock salt samples with variable siliciclastic content (1, 7, 38 and 53 vol. %). The experiments were conducted under bulk flattening strain, elevated temperature (345 °C), low differential stress (< 4.6 MPa), and a strain rate of 10-7 s-1. To gain inside in the 3D distribution of the siliciclastic material and in the deformation mechanisms of the constituent minerals, computer tomographic (CT), microstructural and electron backscatter diffraction (EBSD) analyses were applied to both initial and experimentally deformed samples. The EBSD and microstructural data suggest that, independent of the amount of siliciclastic content, the deformation of the halite matrix was largely accommodated by subgrain formation without subgrain rotation recrystallization. The deformation of the siliciclastic domains, on the other hand, was entirely brittle. CT images show open fractures, oriented sub-perpendicular to the least principal stress, σ3. An increase in the siliciclastic content leads to an increase in differential stress of the halite matrix. The new results suggest that the barrier properties of rock salt is significantly reduced by larger content of siliciclastic material, particularly in cases where the siliciclastic parts and their fractures are interconnected making pervasive ascendant fluid transport possible in these lithological units. Future thermomechanical experiments of impure rock salt should focus on the effect of confining pressure, which is expected to reduce the number and width of open fractures in the siliciclastic domains.

Abstract. Suitable host rocks for a repository for high-level radioactive waste (HLW) in Germany include not only clay and crystalline rocks but also rock salt formations in so-called flat and steep bedding (StandAG, 2017). Favourable repository relevant properties of rock salt are, e.g. the high heat conductivity, low porosity and permeability, and its viscoplastic deformation behaviour. Thicker salt deposits can be attributed to the formation of approx. 700 salt structures that have formed under various geological conditions in the North German Basin (NGB) over the last 250 million years. According to their shape and genesis, salt structures are classified as salt pillows (considered as flat bedding) or salt diapirs (steep bedding). Out of a total of 74 sub-areas in rock salt, 60 sub-areas in salt diapirs consisting of Permian evaporates were identified within the first phase of the German site selection procedure (BGE, 2020). At the current stage of the site selection process, a conservative approach has been adopted and the internal structure of the salt structures have not yet been taken into account for further classification of the identified sub-areas. However, the interior of the salt structures not only consists of rock salt but also of varying proportions of clay, carbonate and anhydrite rocks, as well as potassium salts formed by progressive evaporation of marine brines. Multi-phase salt tectonics has led to the folding of these differently composed layers and to complex internal structures. Therefore, detailed knowledge of the salt structure compositions is necessary to identify suitable rock salt areas for the designation of the containment providing rock zone. As a result of decades of research through extensive salt and potash mining, cavern storage and exploration for final waste disposal, Permian salt rocks represent a well-studied host rock in Germany. The use of different exploration methods and multidisciplinary data evaluation have led to a comprehensive understanding of the internal composition of some well-studied salt structures. Systematic studies have shown that several factors have influenced the formation of salt structures as well as their external shapes, sizes, and spatial distribution (e.g., Pollok et al., 2020). Furthermore, the amount and distribution of suitable host rocks varies greatly in different salt structures and is closely related to their internal structure. Since the interior of salt structures has not been considered in the site selection process so far, a classification of salt structures (or sub-areas) into certain types with varying internal composition and complexity is presented. By examining their lithofacial composition, genesis, and structural geological position within the basin, it is possible to narrow down these types to specific areas in the NGB. Without the acquisition of additional exploration data in this phase of the site selection process, this salt structure classification provides important data for the legally demanded assessment of the overall favourable geological situation.