Sub-areas in steep rock salt formations – what do we know about the geological composition of North German salt structures?

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.