Stratigraphy and facies

The rock laboratory is situated in the Opalinus Clay rock formation, a marly argillaceous claystone with different proportions of sand and carbonates. Opalinus Clay has an age of approximately 174 million years (Aalenian-Toarcian stages of the Lower/Middle Jurassic). The name Opalinus Clay is derived from a particular fossil ammonite, Leioceras opalinum, whose shell consists of aragonite and, when backlit, shimmers with a bluish “opalescent” glow. This marine cephalopod is ubquitous in the upper part of this formation.


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.

Palaeographic distribution of land and sea
Palaeographic distribution of land and sea at the time of deposition of the Opalinus Clay, ca. 174 Ma (million years ago). Mainly carbonates precipitataed in the shallow water zones, clays formed in deeper, basinal zones. Sand occurs in areas closer to land masses.


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.


Geological map of the Mont Terri rock laboratory
Geological map of the Mont Terri rock laboratory. The three different facies types of Opalinus Clay can be clearly identified (earth colours).

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).


Sandy facies
Mostly layered argillaceous rock, pervaded by thin sand layers and pockets.

Argillaceous facies
Laminated and partly bioturbated claystone with some biodetritus and quartz.

Carbonate-rich sandy facies
Sandy limestone with bioclastic material (fragments of echinoderms and molluscs) and quartz grains (thin section).