Geological and tectonic investigations in the former Morsleben salt mine (Germany) as a basis for the safety assessment of a radioactive waste repository

摘要

The safety assessment of a repository for radioactive waste is based on the results of model simulations and scenario analysis. To obtain a well founded geological basis for this, extensive geological and structural studies have been carried out for the Morsleben repository.The Morsleben repository for radioactive waste was constructed in Zechstein strata. The location of the Werra Formation (z1) was located in boreholes and the Stassfurt (z2), Leine (z3) and Aller (z4) Formations are exposed in the repository mine. The Zechstein Salt has a total thickness in the western part of the salt body of 580 m. In the eastern part it is only 380 m thick. The Morsleben repository is in the Allertal zone salt structure, the root zone of a deeply eroded salt body. This is evidenced by the relatively small thickness of the Zechstein Salt, as well as by the many blocks of the Hauptanhydrit (z3HA) in the salt, which are present mostly in the synclinal hinge zones, i.e. in the deeper parts of the structure, as a result of tectonic separation of ductile salt from competent anhydrite.The tectonics of the structure are documented by polyphase folds, characteristic of the deformation of ductile material. The general trend of the fold axes is horizontal, NW-SE, parallel to the trend of the Allertal zone.The western part of the salt structure is characterised by high, NE-vergent, isoclinal folds. The z2 to z4 strata involved in the folding extend from the salt table (top of the salt body) down to the lowest synclinal trough.In the eastern part the folds are open and symmetric with considerably smaller amplitude. Correspondingly, z4 strata mostly occur just below the salt table, z3 at the repository level, and z2 below that.The different styles of folding in the east and west are due to the structure of the rocks beneath the Zechstein Salt. The boundary between the two types of folds lies above a major fault in the rocks below the Zechstein Salt, separating a downthrown block in the west from an upthrown block in the east. NE-vergent, isoclinal folds occur above the downthrown block, where the major ascent of the salt occurred. In contrast, the folding in the upthrown eastern block has hardly been affected by the ascending sa hence the wide, open, symmetrical folds. Thus, the structure of the rocks beneath the Zechstein determined the folding style in the overlying salt.The present-day fold structures chiefly result from Late Cretaceous compression that affected the North German basin. The salt body is, therefore, primarily a tectonic structure and is not halokinetic.