Stratigraphy

From a stratigraphic point of view, Opalinus Clay is underlaid by the Staffelegg Formation  and overlain by the Passwang Formation, both calcareous facies. At the rock laboratory, the Opalinus Clay exhibits a thickness of around 130 m and was deposited over a period of some 400,000 years.

Palaeography

The Opalinus Clay was formed in a shallow sea with an average water depth of 20 to 50 m. During storms or due to changes in current direction fine sand layers were washed in from the mainland in some areas. In the north, the shallow sea was bordered by the Rhenish Slate Mountains and, in the east and south, by the Bohemian Massif or the Alemannic Islands. The area of deposition roughly covered a triangle between Bern, Munich, and Strasbourg. Due to this large extent, we conclude that extrapolation from conditions at Mont Terri to other areas (e.g. potential site areas) is largely valid.

Three types of facies

Mineralogically, Opalinus Clay consists of 40-80% clay minerals (including mixed layers of swellable illite and smectite), 10-40% quartz, 5-40% calcite, and smaller proportions of siderite, pyrite, and organic carbon. At Mont Terri, we discern three types (facies) of Opalinus Clay each exhibiting different percentages of clay minerals, quartz, and calcite.

The three types of facies are: shaly facies, sandy facies, and carbonate-rich sandy facies. These resulted from changing sedimentation (sea depth and current direction) in a shallow sea with a water depth of 20 to 50 m (see above).