Altena Group AT

 

Premise Definition after NAM and RGD (1980)
Derivatio nominis The Altena Group is named after ‘Het Land van Altena’, the northern part of the Dutch province of Noord-Brabant, where the most complete sequence of the group has been encountered in wells.
Type section Location map See figure (pdf)
  Well Werkendam-2 (pdf)
  Location N 51°47’29.7
E 04°50’22.1
  Depth 1507 to 2838 m
  Length 1331 m along hole
  Reference NAM and RGD (1980)
Definition The Altena Group comprises mainly argillaceous deposits with some calcareous intercalations in the lower part, and alternating calcareous and clastic sediments in the upper part.
Upper Boundary The group is overlain unconformably by the, generally variegated, siliciclastic deposits of the Schieland Group or the shallow-marine sediments of the Rijnland Group. In inverted parts of the West Netherlands Basin, the Roer Valley Graben, the Broad Fourteens Basin and the Central Netherlands Basin the group is often overlain unconformably by the Chalk Group, the Lower North Sea Group or the Middle North Sea Group.
Lower Boundary The group usually conformably overlies the red-brown, variegated, anhydritic beds of the Keuper Formation. Locally, it may rest unconformably upon this unit, on the light-greenish/grey carbonates and marls of the underlying Muschelkalk Formation or on older units of the Upper Germanic Trias Group, locally even on Zechstein deposits.
Distribution The present-day distribution of the group is confined to the Late Jurassic basinal areas as depicted on maps published by Heybroek (1974) , van Wijhe (1987) and Burgers and Mulder (1991) . The consistent, fine-grained lithofacies of the Altena Group indicates that the absence of the group on the highs and terraces is not primary, but due to later erosion, mainly associated with Mid- and Late Kimmerian uplift. Nevertheless, the group shows in places considerable thinning towards the highs. The group has locally been truncated further by the Austrian, Subhercynian, Laramide or Pyrenean uplift phases, which is reflected in places where the Altena Group is overlain unconformably by the Holland Formation, the Chalk Group, the Lower North Sea Group or the Middle North Sea Group. In the Achterhoek (eastern Gelderland) a few exposures of deposits of this group can be found.
Age Rhaetian to Oxfordian in age. A selection of reliable biostratigraphic marker horizons is presented in Table F.1 (see pdf)
Depositional Setting The group was deposited in a neritic environment in a generally open-marine basin. Occasionally basin circulation stagnated, resulting in the deposition of euxinic shales under anoxic basin-floor conditions. Periods of lower sea level and tectonic activity are indicated by scarce incursions of coarser clastics and deposition of sandy carbonates on shoals.
Subdivision  
  The Posidonia Shale has been raised to formation status here, in order to avoid problems with member subdivision of the Werkendam Formation where no Middle Werkendam Member can be recognised. The Werkendam Formation has been redefined accordingly.
  AT Altena Group
  ATRT Sleen Formation
  ATAL Aalburg Formation
  ATPO Posidonia Shale Formation
  ATWD Werkendam Formation
  ATBR Brabant Formation
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].