|Premise||Definition after NAM and RGD (1980) . The status of the unit has been raised to formation level in order to avoid problems with subdivision of the Werkendam Formation if the Middle Werkendam Member cannot be recognised.|
|Derivatio nominis||Name derived from the German lithostratigraphy, where the Lower Toarcian is called ‘Posidonienschiefer’. The unit was named after the dominant fossil type, a lamellibranch genus called Posidonia. Although the genus name was subsequently amended to Posidonomya, the unit name had become so well-established that it remained in use.|
|Type section||Location map||See figure (pdf)|
|Depth||2205 to 2228 m|
|Length||23 m along hole|
|Reference||NAM and RGD (1980)|
|Definition||Dark-grey to brownish-black, bituminous, fissile claystone unit, which can be recognised on wire-line logs by its high gamma-ray and resistivity readings. The unit is a very distinctive interval throughout the Netherlands and shows up as an excellent reflection horizon on seismic sections. Boundaries:|
|Upper Boundary||The unit is overlain more or less conformably by the predominantly non-bituminous, marine silty claystones and siltstones of the Werkendam Formation. Minor bituminous intercalations locally occur in the basal parts of the Werkendam Formation.|
|Lower Boundary||The unit overlies conformably the non-bituminous, open-marine claystones of the Aalburg Formation.|
|Distribution||The unit is restricted to the basin centres of the Roer Valley Graben, the West Netherlands Basin, the Central Netherlands Basin, the Broad Fourteens Basin, the Central Graben and to some isolated locations in the Lower Saxony Basin.|
|Age||The formation yields palynofloras and very poor microfaunas of Toarcian (Lias-e to Lias-z) age.|
|Depositional Setting||The fauna, lacking benthic elements, the flora, and the bituminous character of the unit reflect pelagic deposition under anoxic conditions. The wide distribution of the unit, which occurs from the United Kingdom (Jet Rock Member) to Germany (Posidonienschiefer), suggests deposition during a period of high sea level and restricted basin-floor circulation.|
|References||See References Lower and Middle Jurassic|
Van Adrichem Boogaert, H.A. & Kouwe, W.F.P., 1993-1997. [Stratigraphic unit]. In: Stratigraphic Nomenclature of the Netherlands.
Retrieved [Datum] from [url].