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. Suitable host rocks for a repository for high-level radioactive waste (HLW) inGermany include not only clay and crystalline rocks but also rock saltformations in so-called flat and steep bedding (StandAG, 2017). Favourablerepository relevant properties of rock salt are, e.g. the high heatconductivity, low porosity and permeability, and its viscoplastic deformationbehaviour. Thicker salt deposits can be attributed to the formation ofapprox. 700 salt structures that have formed under various geologicalconditions in the North German Basin (NGB) over the last 250 millionyears. According to their shape and genesis, salt structures are classified assalt pillows (considered as flat bedding) or salt diapirs (steep bedding). Outof a total of 74 sub-areas in rock salt, 60 sub-areas in salt diapirsconsisting of Permian evaporates were identified within the first phase of theGerman site selection procedure (BGE, 2020). At the current stage of the site selection process, a conservative approachhas been adopted and the internal structure of the salt structures have notyet been taken into account for further classification of the identifiedsub-areas. However, the interior of the salt structures not only consists ofrock salt but also of varying proportions of clay, carbonate and anhydriterocks, as well as potassium salts formed by progressive evaporation of marinebrines. Multi-phase salt tectonics has led to the folding of these differentlycomposed layers and to complex internal structures. Therefore, detailedknowledge of the salt structure compositions is necessary to identify suitablerock 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 rocksrepresent a well-studied host rock in Germany. The use of differentexploration methods and multidisciplinary data evaluation have led to acomprehensive understanding of the internal composition of some well-studiedsalt structures. Systematic studies have shown that several factors haveinfluenced the formation of salt structures as well as their external shapes,sizes, and spatial distribution (e.g., Pollok et al., 2020). Furthermore, theamount and distribution of suitable host rocks varies greatly in differentsalt structures and is closely related to their internal structure. Since the interior of salt structures has not been considered in the siteselection process so far, a classification of salt structures (or sub-areas)into certain types with varying internal composition and complexity ispresented. By examining their lithofacial composition, genesis, and structuralgeological position within the basin, it is possible to narrow down thesetypes to specific areas in the NGB. Without the acquisition of additionalexploration data in this phase of the site selection process, this saltstructure classification provides important data for the legally demandedassessment of the overall favourable geological situation.

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