Chalk Group CK

 

Premise The Chalk Group was originally defined by NAM and RGD (1980) .
Derivatio nominis The name Chalk is commonly used in the UK for light-coloured, soft limestones that have an earthy texture.
Type section Location map See figure (pdf)
  Well De Paauwen-1 (pdf)
  Location N 53°16’30.4
E 06°45’15.5
  Depth 822 to 1720 m
  Length 898 m along hole
  Reference NAM and RGD (1980)
Definition Thick succession, predominantly consisting of carbonate rocks. The bulk of the group comprises white, buff, cream and light-grey, hard, fine-grained, bioclastic limestones and marly limestones. Originally, these limestones had a more chalky nature, but as a result of deep burial they were compacted strongly and became denser. Locally, marls, calcareous claystones and glauconitic sands occur. Chert concretions, isolated or in layers parallel to the bedding, are common. Along the basin margin coarser bioclastic limestones and sand tongues are present.
Upper Boundary The top of the Chalk group has been taken at the base of the sands, marls and clays of the Lower North Sea Group or, where this group is absent, of the Middle North Sea Group.
Lower Boundary The base of the Chalk Group has been taken at the base of the limestone succession on top of the (mainly redbrown) marls of the Holland Formation. On wire-line logs this boundary is usually marked by a pronounced decrease in gamma-ray radiation and an increase in resistivity and sonic velocity.
Distribution The group is present over most of the Netherlands and adjoining offshore area. Owing to erosion as a result of inversion of Late Jurassic-Early Cretaceous basins, the group is missing in a northwest-southeast oriented, bifurcating zone comprising most of the offshore P and O quadrants and large parts of the provinces of Noord Brabant, Noord- and Zuid Holland, Gelderland and Utrecht (see Upper Cretaceous distribution maps by van Wijhe (1987) and Burgers and Mulder (1991) ). In the Central Graben and Broad Fourteens Basin areas a patchy, thin chalk cover can be present. The basin-margin sediments crop out in the southern part of the province of Limburg. In the Achterhoek a single, isolated outcrop is found.
Age Cenomanian-Danian. In the centres of the West Netherlands Basin and the Broad Fourteens Basin the base of the Texel Formation may be of Albian age. Towards the south, on the northern flank of the London-Brabant Massif, the basal beds of the Chalk Group become progressively younger (gradual onlap).
Depositional Setting The group consists entirely of marine sediments. Upper Cretaceous chalks are found over large areas of the earth's surface and reflect relatively stable and uniform conditions during a long period of time. Sedimentological features associated with downslope transport and abundant planktonic fossils suggest a certain minimum water depth, whereas the presence of light-dependent benthic organisms, such as Bryozoa, is indicative of a shallower environment. Ostracods with eyes, found throughout the sequence, indicate deposition within the photic zone. Most authors agree that, generally, the depositional depth of the Chalk sediments must have been between 50 and 300 metres. The lenses of coarse, bioclastic limestones and tongues of glauconitic sandstone found along the basin edge (e.g. South Limburg) reflect deposition in a shallower marine environment. Hardgrounds, present throughout the sequence, are related to subrosion, non-deposition or abrasion caused by either sea-level fluctuations or small-scale tectonic uplift.
Subdivision  
  In most of the Netherlands onshore and offshore three formations have been distinguished within the Chalk Group. From top to bottom these are:
  CK Chalk Group
  CKEK Ekofisk Formation
  CKGR Ommelanden Formation
  CKTX Texel Formation
  For the southern part of Limburg and adjacent areas in Belgium and Germany a more detailed subdivision has been established (Felder (1975) ; Albers and Felder (1979) ). In this basin-margin development five formations are distinguished, which are from top to bottom:"
  CK Chalk Group
  CKHM Houthem Formation
  CKMA Maastricht Formation
  CKGP Gulpen Formation
  CKVA Vaals Formation
  CKAK Aken Formation
  For detailed information on these formations and their subdivision, the reader is referrred to Felder's publications
References See References Upper Cretaceous

Van Adrichem Boogaert, H.A. & Kouwe, W.F.P., 1993-1997. [Stratigraphic unit]. In: Stratigraphic Nomenclature of the Netherlands.
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