(From NPD Bulletin no. 5)
updated to follow: Stratigraphic Guide to the Cromer Knoll, Shetland and Chalk Groups of the North Sea and Norwegian Sea. Felix M. Gradstein & Colin C. Waters (editors), Mike Charnock, Dirk Munsterman, Michelle Hollerbach, Harald Brunstad, Øyvind Hammer & Luis Vergara (contributors). Newsletter on Stratigraphy, vol 49/1 pp71-280, 2016
Cromer Knoll Group
The formation is dominated by light to dark grey, olive-grey, greenish and brownish, often calcareous claystones, and passes into light grey, light greenish-grey and light olive-grey marlstones and stringers of limestone. Mica, pyrite and glauconite are common. The claystones may be silty, and siltstones or very fine-grained sandstone layers or laminae are present. Where major sandstone layers occur they are regarded as belonging to the Ran sandstone unit, defined below.
In a few Norwegian wells in the Central North Sea (eg. 1/9-3, 2/3-1, 2/7-2, 2/10-1, 2/11-1, 7/3-1, 7/8-2, 7/12-4, 7/12-5 and 8/1-1) a sequence of calcareous claystone, marlstone and limestone interbeds is recognised as the basal part of the Åsgard Formation (Figures 12 and 14 in Isaksen & Tonstad (1989)). This sequence is very difficult to correlate in the Norwegian Sector, even over small distances, and is therefore regarded as representing local variations in the lowermost part of the Åsgard Formation. It might be more useful stratigraphically as a set of local members. In the Danish Sector this sequence is defined as the Leek Member (Jensen et al., 1986).
The formation is 492 m thick in the type well 2/11-1, and 608 m thick in reference well 17/11-2. In the Central Trough area the thickness varies from a few metres to more than 500 m over short distances, showing the complex pattern of small, restricted Early Cretaceous basins.An even thicker Åsgard unit was penetrated in the Norwegian-Danish Basin, and especially in the Asta Graben, where more than 700 m were encountered in well 17/12-3. The formation is thickest in the Sogn Graben where it is probably more than 1200 m thick, as indicated by seismic data.
The Åsgard Formation is widespread in the North Sea (Figures 7-11 in Isaksen & Tonstad (1989)), as are the partial equivalents in the Danish Sector (Valhall Formation, Jensen et al., 1986; Vedsted Formation, Larsen, 1966), UK Sector (Speeton Clay, Rhys, 1974) and Dutch Sector (Vlieland Shale Member, NAM & RGD 1980).
In the Norwegian Sector, the formation is absent from the highest parts of the Mandal, Jaeren and Utsira Highs, the Lomre Terrace, the Troll area, Tampen Spur and locally over salt pillows and diapirs in the Central Trough and the Norwegian-Danish Basin.
Norwegian well 17/11-2: 2410 to 1802 m, Lat. 58°06'54.91"N, Long. 03°22'09.81"E (Figure 13 in Isaksen & Tonstad (1989)). No cores.
Danish well 1-1: 3358 to 2986 m, Lat. 56°03'10"N, Long. 04°14'60"E (Figure 14 in Isaksen & Tonstad (1989)). No cores.
The characteristics of the upper boundary vary with the overlying formations. Where the Tuxen Formation occurs, the boundary is defined by an upward decrease in the gamma-ray readings and an increase in velocity, reflecting slightly more calcareous claystones, marl-stones and limestones compared with the underlying Åsgard Formation (Figures 14 and 15 in Isaksen & Tonstad (1989)). Where the Tuxen Formation is missing, and the Sola Formation is deposited on the Åsgard Formation, the boundary is defined by an upward increase in gamma-ray readings and a decrease in velocity (Figures 17, 18 and 23 in Isaksen & Tonstad (1989)).
If both the Tuxen and Sola Formations are missing, the boundary to the overlying Rødby Formation is defined by an upward decrease in gamma-ray readings and an increase in velocity.
The lower boundary is defined by a marked upward decrease in gamma-ray response and an increase in velocity in areas. This is true where the underlying sediments are slightly calcareous or non-calcareous, organic-rich claystones and shales, usually belonging to the Mandal, Flekkefjord, Tau or Draupne Formations (Figures 15 and 22 in Isaksen & Tonstad (1989)). Where the claystones and shales are less organic rich and more calcareous, the boundary on logs may be more difficult to identify.