The Opalinus Clay host rock exists in all three siting regions with similar facies characteristics and thickness. It was deposited during the Aalenian at the beginning of the Middle Jurassic. During this time period, Central Europe was affected by continued N-S-extension during the opening of the Piemont – Liguria Ocean (Chapter 3). These processes led to subsidence and created accommodation space for the deposition of the thick Opalinus Clay succession. This unit was deposited in a shallow marine epicontinental sea.

Fig. 4‑27:Map of depositional environment during the Middle Jurassic (mainly Early Aalenian) preserved in the Opalinus Clay

Refer to Fig. 4‑3 for a schematic representation of depositional environments. The north arrow refers to present-day geographic north.