Characterization of the Opalinus Clay and its upper confining unit: a multi-proxy approach

The Opalinus Clay (OPA) is an argillaceous to silty mudstone formation of Jurassic age (Toarcian to Aalenian), particularly known in Switzerland as being the selected host rock for deep geological disposal of radioactive waste. It is overlain by successions of sandy bioclastic marls and limestones capped by ooidal ironstone beds (termed here as the upper confining unit; abbreviated UCU).

In collaboration with Nagra, swisstopo, the University of Bern and the Mont Terri Project, one part of the project focuses on the development of a subfacies classification scheme for characterizing the horizontal and vertical lithological variability within the OPA at dm- to cm-scale. This subfacies model is implemented with petrographic, mineralogical, geochemical and petrophysical data, and meant to homogenize routine OPA drillcore descriptions within large and interdisciplinary projects (e.g. Mont Terri Project; site selection for radioactive waste repository). An additional emphasis is devoted to the development of an automatic pattern recognition program allowing the subfacies identification on the base of drillcore images (in collaboration with geo7 AG).

A second part of the project focuses on the petrographic and geochemical characterization of the OPA/UCU lithostratigraphic transition. This transition is diachronous across northern Switzerland and evidences high vertical and lateral variability. X-ray fluorescence core scanning, in particular, is used to unravel the depositional and diagenetic processes that shaped this complex transition, as well as to identify characteristic chemofacies. A special attention is also drawn and the formation and origin of iron ooids.