(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 by Jensen et al. (1986) from a bathymetric feature west of Blåvandshuk, the westernmost point of Jylland.
The formation is dominated by white to greyish-pink, calcareous claystones and marlstones. Along some of the structural highs the marlstones grade into purer limestones. Generally, the formation terminates vertically upwards with a chalk sequence containing subordinate marlstone layers. This chalk is white to pale orange or yellowish-grey, occasionally greenish and reddish. The marlstones are generally light grey to greenish-grey or olive-grey, but may be reddish-brown in some wells.
A 0.3-1 m thick, radioactive, marlstone bed is frequently encountered within the Tuxen Formation in the Danish Sector where it is defined as the Munk Marl Bed (Jensen et al., 1986). This characteristic unit has also been recognised in some wells in the central Norwegian Sector (e.g. 2/1-2, 2/1-3, 2/1-8, 2/6-2, 2/11-7, 6/3-1, 16/8-1 and 16/10-1), (see also Figure 15 in Isaksen & Tonstad (1989)). In the Norwegian Sector, the Tuxen Formation above the Munk Marl Bed is often more calcareous than the rest of the sequence.
The thickness of the formation varies from 1 m along structural highs to about 100 m in basinal areas. In the reference wells the thicknesses are 75 m (2/11-1) and 71 m (2/6-2). In the type well (1-1) the thickness is 88 m.
The Tuxen Formation is widely distributed in the Norwegian and Danish Sectors (Jensen et al., 1986). In the Norwegian Sector it is developed in the Central Trough, along the Jæren High and in parts of the Norwegian-Danish Basin.
In basinal areas in the Norwegian Sector it inter-fingers laterally with claystones and marlstones of the Åsgard Formation (Figures 4 and 7 in Isaksen & Tonstad (1989)).
Danish well 1-1 from 2986 to 2898 m, coordinates N 56°03'10", E 04°(14160" (Fig. 14 in Isaksen & Tonstad (1989)). No cores.
Upwards, the Tuxen Formation is generally in contact with the micaceous claystones and organic-rich shales of the Sola Formation (Figures 12 and 14 in Isaksen & Tonstad (1989)). This boundary is marked by an upward increase in gamma-ray readings and a decrease in velocity. Where the Sola Formation is missing, the Tuxen Formation is in contact with the marlstones of the overlying Rødby Formation (Figure 15 in Isaksen & Tonstad (1989)). The boundary is usually defined by an upward increase in gamma-ray readings.
The lower boundary is defined as the base of an upward decrease in gamma-ray readings and an increase in velocity, reflecting the passage from the slightly calcareous claystones of the underlying Åsgard Formation up into the more calcareous claystones and marlstones of the Tuxen Formation (Figures 12, 14 and 15 in Isaksen & Tonstad (1989)). The transition is generally gradual in basinal areas. Purer limestones were deposited along some structural highs, causing more distinct log breaks.