Overall, the Opalinus Clay has similar sedimentological characteristics in all investigated boreholes of the three siting regions. The lateral and vertical lithological variability is generally small compared with other Mesozoic formations of Northern Switzerland. The succession of sub-units or subfacies trends can generally be correlated over all borehole sites (Fig. 4‑28, Fig. 4‑30). However, local facies differences occur and the possibility to correlate units improves with a detailed look at the individual siting regions. For example, the approximately uppermost part of the Opalinus Clay retrieved from the boreholes of JO is richer in fine sand and silt, and less argillaceous compared to NL and ZNO. Special attention must be paid to the occurrence of characteristic calcareous and/or iron-rich beds to further investigate correlatable strata within the Opalinus Clay. Wohlwend et al. (2024) and Zimmerli et al. (2024) propose six potential marker beds or horizons, at least at local scale (Fig. 4‑30). Most of these beds occur in the approximately uppermost quarter of the Opalinus Clay. Within the NL and ZNO siting regions, the correlation of these beds appears to be quite robust, with the exception of Bed-II which represents a correlation with high uncertainty. The correlation towards and within JO is generally less reliable.
Fig. 4‑29:Selected core photos of the Opalinus Clay (slabbed cores)
(a) Dark grey claystone (silty), sparsely pyritic, scattered fine bioclasts and pyritised burrows (SF1); (b) dark to medium grey, laminated claystone (silty) with light grey layers and lenses of siltstone with beige siderite nodules (SF1); (c) dark to medium grey, laminated silty claystone (calcareous) with light grey, more silty to sandy layers (SF1); (d) dark grey sandy claystone with lenticular-bedded light grey lenses of siltstone and light grey sandy marl, rippled, bioturbated (SF2); (e) medium grey laminated to bioturbated sandy claystone (silty), with abundant light grey lenses of silt to sand, rippled (mostly SF2, towards SF3); (f) partly iron-rich succession of several firm- to hardgrounds, lithoclasts, calcareous marls and bioclastic limestone (argillaceous), ooids (former iron-mineral composition replaced by calcite); (g) medium to dark grey silty to sandy claystone (calcareous), with light grey lenses of mostly siltstone, rippled with beige limestone lithoclasts (904.18 to 904.48 m), densely bored (SF1 to SF2 with iron-rich limestone).
Fig. 4‑30:Subfacies classification and lithologies of the Opalinus Clay and adjacent formations in boreholes of Northern Switzerland
Modified after Zimmerli et al. (2024).
The Middle Jurassic Opalinus Clay was mainly deposited during the Opalinum ammonite Zone of the Early Aalenian. To improve the chronology within the Opalinus Clay, the combination of biostratigraphy, based on ammonites and palynomorphs, and stable carbon-isotope chemostratigraphy, measured on inorganic carbonates and organic matter, was applied (Wohlwend et al. 2024). In the uppermost part of the Staffelegg Formation (Rietheim Member), the global negative carbon-isotope excursion of the Toarcian Oceanic Anoxic Event was detected in all boreholes of the three siting regions. This demonstrates the reliability of C-isotopic chemostratigraphy (Section 4.2.5). In the lower part of the Opalinus Clay, an additional, slightly less negative C‑isotope shift in the organic matter was detected in the Opalinum Subzone of the Opalinum Zone. This negative shift is as well correlatable across all boreholes of the three siting regions. Within most of the Opalinus Clay, the C-isotopes of the organic matter are very constant or only show slight changes. However, a clear increase in C-isotope values of organic matter can be traced in the uppermost part of the Opalinus Clay that can be allocated to the Bifidatum Subzone of the Opalinum Zone by ammonites and palynomorphs. The positive isotope increase coincides with a calcareous, iron-rich bed and is well expressed in the boreholes of NL (Bed-VI in Fig. 4‑30). The facies (e.g. amount of iron-oolites) and thickness of this bed change laterally, but it apparently marks a more or less synchronous event of condensation in all boreholes. Therefore, these types of beds can be used in a first step to define timelines on a local to regional scale. In the Opalinus Clay, six beds were picked in the boreholes that might represent timelines (Fig. 4‑30). Bed-I, -III to -VI have a medium to good confidence level, whereas Bed-II has a low confidence level.