High-resolution bio- and chemostratigraphy of the Toarcian to Aalenian of Northern Switzerland with a focus on the Opalinus Clay

The Opalinus Clay (OPA), a claystone to silty claystone formation deposited during the latest Toarcian to mainly Early Aalenian, has been selected as the host rock for the disposal of radioactive waste in Switzerland. To establish which of the proposed repository sites is the best, Nagra (National Cooperative for the Disposal of Radioactive Waste) has drilled nine additional deep boreholes in Northern Switzerland, covering the complete Lower to Middle Jurassic successions. The present study compares these new boreholes with five existing drill cores, covering a distance of around 50 km. This extensive underground recovery allows a detailed and unique investigation of the Early to Middle Jurassic period. To establish a high-resolution chronology in the Jurassic successions, we combined carbon-isotope chemostratigraphy with ammonite and palynomorph biostratigraphy, as well as detailed lithological correlation of individual calcareous beds. The metre-step sampling for C-isotope chemostratigraphy for both carbonates and organic matter provides sufficiently distinct negative and positive trends to establish a high-resolution correlation between the drill cores. The geochemical data also support the lateral correlation of six calcareous beds or horizons within the OPA, identified as Bed-I to Bed-VI. The high-resolution C-isotope data can be correlated with existing data from Northern Switzerland, Southern Germany (e.g. Dotternhausen), France (e.g. Le Brusquet), and other European successions. The major perturbation in the carbon cycle observed in the Toarcian Rietheim Member confirms previous findings of the negative C-isotope excursion (CIE), which can be linked with the Toarcian Oceanic Anoxic Event (T-OAE). Additionally, a newly identified negative CIE (Opalinum nCIE) in the lower Opalinum Subzone of the Opalinum ammonite Zone can be correlated across all Nagra boreholes in Northern Switzerland. This nCIE follows almost the horizon with the calcareous concretions located approximately 10 m above the base of the OPA (Bed-I). In the uppermost Opalinus Clay, a positive CIE (Bifidatum pCIE) was documented within Bed-VI, which can be tied by ammonites and palynomorphs to the Bifidatum Subzone of the Opalinum Zone. Bed-VI can be interpreted as a sequence boundary possibly related to global sea-level variations. The integration of the C-isotope chemostratigraphy, biostratigraphic data, and lithological observations enables the establishment of a detailed chronostratigraphic framework.