In Northern Switzerland, the thickness of the Wildegg Formation decreases from west to east (Fig. 4‑47). In the JO siting region, it has a thickness of a ca. 250 to 260 m, in NL ca. 50 to 100 m, and in ZNO ca. 10 to 20 m. The Wildegg Formation consists mostly of limestone, argillaceous limestone and calcareous marl (Fig. 4‑48, Fig. 4‑49; Allenbach 2001, Deplazes et al. 2013). Individual limestone beds are mostly bundled in few metre- to decametre-scale thick successions which alternate with calcareous marl successions. The most characteristic, the Gerstenhübel Bed, and also the thinner of these limestone successions (Fig. 4‑48) can be followed over large distances of 25 km or more in the study area (Gygi & Persoz 1986, Deplazes et al. 2013).

The Wildegg Formation can be subdivided into the Birmenstorf Member (only up to few metres thick) at the base and the younger Effingen Member, which makes up the vast majority of the formation:

Birmenstorf Member: In the JO siting region, the Birmenstorf Member has a thickness of up to ca. 4 m. It consists of bioclastic and glauconitic limestone (biomicrite) and is very fossiliferous, with sponges, ammonites and belemnites (Fig. 4‑49a). At the base it may include some iron-ooids. Towards the east, the Birmenstorf Member is gradually replaced by condensed units from bottom to top: «Glaukonitsandmergel Bed», «Mumienkalk Bed» and «Mumien­mergel Bed» (Gygi 2012). The latter two units were incorporated into the Birmenstorf Member by Jordan & Deplazes (2019); together they generally reach a thickness of mostly ca. 1 m, rarely up to few metres (e.g. TRU1). They consist of fossiliferous, glauconitic, partly iron-oolitic to oncolitic calcareous to argillaceous marl and argillaceous limestone (biomicrite) (Fig. 4‑49b). Characteristic for some parts of this interval are so-called "Mumien" (mumies) consisting of partly encrusted, subangular to rounded limestone oncolites and lithoclasts.

Effingen Member: This member forms by far the largest part of the Wildegg Formation. The principal lithologies of (argillaceous) limestone (micrite) and calcareous marl are partly bioturbated, bioclastic, often pyritic and contain some quartz and feldspar (silty-sandy) (Fig. 4‑49d – g). In the lower part of the Effingen Member, sponge bioherms may also occur (Fig. 4‑49c).