Integrating facies and mineralogy: new Insights from the Opalinuston Formation in southern Germany

The Opalinuston Formation (OPA) is designated as host rock for the disposal of high-level radioactive waste in Switzerland. In southern Germany, it remains one of several potential candidates under investigation. For the present study, samples from the Mont Terri rock laboratory (Switzerland) and from the BGR project “Sequence Stratigraphy of the Aalenian in southern Germany” (SEPIA) were investigated using a facies-based approach including mineralogical and geochemical analyses. Particular emphasis was placed on the degree of ordering of the illite-smectite interstratified clay minerals (I-S), which are responsible for sorption of radionuclides and swelling properties.

The results support the classification of the OPA into several (sub-)facies types. The clay fraction present in the samples varies according to these facies types, consistent with variations in cation exchange capacity. For southern Germany, crystal structure-based Rietveld refinement indicates a proportion of illitic layers in the I-S averaging between 77% and 83% across all refinements and ordering types. No significant differences in the illite:smectite ratio of the I-S was observed between the two sites studied. However, a notable difference in the degree of ordering was identified. This variation can be attributed to differences in the burial history of the OPA at the two localities.

A preliminary comparison with results from the OPA in Switzerland reveals that the average proportion of illite layers in the I-S (approximately 77% for Mont Terri) closely aligns with the results observed in southern Germany. Our ongoing effort is to compare these results with crystal-structure based Rietveld measurements from other claystone formations.