(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
Named after a god from Norse mythology who was considered to be very wise.
The formation is dominated by limestones and marls. It often contains impure carbonates that are reworked and mixed with smaller quantities of sand and silt. The formation is sometimes chalky. The matrix is usually calcareous, and oolites are observed in some wells in the East Shetland Basin. The formation colour is usually white or light pink, but may vary slightly on account of the sand/silt mixture.
The formation is found only as narrow zones along structural highs. On the flanks of the Viking Graben it may be seen almost continuously from approximately 58° to 62° N. It is also found as a thin carpet over most of the East Shetland Basin and along the Fladen Ground Spur, the Utsira High-Lomre Terrace, the northwest side of the Sele High, and the Jaeren High.
The formation is not encountered in the more central parts of the basins, and it is doubtful if it is present along the boundaries of the Fennoscandian Shield.
The upper boundary is usually defined at the bottom of more or less calcareous shales in the Åsgard Formation. This boundary is reflected on the logs as an upward increase in gamma-ray readings and a reduction in velocity (Figure 17 of Isaksen & Tonstad (1989)). The upper boundary can also be defined by the overlying shales of the Sola or Rødby Formations. The boundary will normally be reflected on logs as described above (Figure 16 of Isaksen & Tonstad (1989)).