Chalk Group

Code
CK
Status
Formal (NAM & RGD 1980).
Lithological description

Predominantly carbonate rocks. The bulk 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 in the SE Netherlands coarser bioclastic limestones and sand tongues are present. Sand is also present near inversion structures (e.g. Central Graben, Peel Block).

Depositional setting

Except in the SE Netherlands, entirely marine. 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 m. 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.

Definition of lower boundary

Base of the limestone succession on top of the (mainly red brown) 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.

Definition of upper boundary

Base of the sands, marls and clays of the Lower North Sea Group (Upper Paleocene) or, where this group is absent, of the Middle North Sea Group.

Thickness indication
Up to 2000 m.
Geographical distribution
Present in 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 Q quadrants and large parts of the provinces of Noord-Brabant, Noord-Holland and Zuid-Holland, Gelderland and Utrecht. On top of the inverted Central Graben and Broad Fourteens, West Netherlands and Central Netherlands Basins areas a patchy, thin chalk cover can be present, often of the youngest, Maastrichtian to Danian sediments. The basin-margin sediments crop out in the southern part of the province of Limburg. In the Achterhoek a single, isolated outcrop is found.
Regional correlation
UK: Chalk Group; GER: various Upper Cretaceous formations; BEL: various Upper Cretaceous formations.
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 late 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). In some cases these are even Danian in age.
Holostratotype
Depth (thickness) AH:
822 - 1720 m (898 m)
Origin of name
The name Chalk is commonly used in the UK for light-coloured, soft limestones that have an earthy texture.
Previous name(s)
Chalk is commonly used throughout northwestern Europe for Late Cretaceous limestones.
Reviewed by (date)
Mark Geluk, Geert-Jan Vis (2017).
References
NAM & RGD 1980. Stratigraphic nomenclature of The Netherlands. Verhandelingen van het Koninklijk Nederlands Geologisch Mijnbouwkundig Genootschap 32, 77 p.
Cite as
TNO-GDN ([YEAR]). Chalk Group. In: Stratigraphic Nomenclature of the Netherlands, TNO – Geological Survey of the Netherlands. Accessed on [DATE] from http://www.dinoloket.nl/en/stratigraphic-nomenclature/chalk-group.