Werkendam Formation ATWD

 

Premise Definition modified after NAM and RGD (1980) . The Posidonia Shale Member of NAM and RGD (1980) has been split off into a separate formation. The lithological affix has been dropped.
Derivatio nominis Named after the municipality of Werkendam in ‘Het Land van Altena’ (northern part of the Dutch province of Noord-Brabant).
Type section Location map See figure (pdf)
  Well Werkendam-2 (pdf)
  Location N 51°47’29.7
E 04°50’22.1
  Depth 1746 to 2205 m
  Length 459 m along hole
  Reference amended after NAM and RGD (1980)
Definition Sequence of grey, slightly marly, shaly claystones. Towards the middle of the succession a more calcareous, silty to sandy interval occurs, occasionally developed as silt- or sandstone beds. Pyrite, carbonaceous matter, macrofossils (ammonites, belemnites, gastropods, etc., frequently pyritised), iron oolites and up to dm-size ferruginous siderite nodules can be common.
Upper Boundary The formation is generally unconformably overlain by the Schieland Group or a younger unit. In more complete successions, which are only encountered in the central parts of the Roer Valley Graben, the West Netherlands Basin and the Broad Fourteens Basin, the Werkendam Formation is conformably overlain by the sandy limestone/marl alternation of the Brabant Formation. In the latter basin this succession is inferred from seismic evidence. Because of the Mid-Kimmerian unconformity in the Dutch Central Graben, the Lower Graben Formation of Middle to Late Callovian age unconformably overlies Werkendam Formation not younger than Early Bajocian in that area. In the southern half of the country, in places where the Werkendam Formation is found, this unconformity is only expressed by a mild shoaling, combined with an influx of sandy sediments.
Lower Boundary In the new definition, the base of the formation has been placed at the top of the bituminous, shaly claystones of the underlying Posidonia Shale Formation.
Distribution The unit is restricted to synclinal erosional remnants in the Roer Valley Graben, the West Netherlands Basin, the Central Netherlands Basin, the Broad Fourteens Basin, the Central Graben and some isolated occurrences in the Lower Saxony Basin and the eastern Central Netherlands Basin (Achterhoek).
Age The formation yields palynofloras and rich microfaunas of Late Toarcian (Lias-z) to latest Bajocian (Dogger-e4), locally even earliest Bathonian (Dogger-e5) age.
Depositional Setting The fossils and lithofacies associations encountered in this formation indicate deposition in a shallow, open-marine conditions.
Subdivision The redefined Werkendam Formation can be subdivided into three members:
  ATWDU Upper Werkendam Member
  ATWDM Middle Werkendam Member
  ATWDL Lower Werkendam Member
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].