Cromer Knoll Group (updated) (Cromer Knollgruppen)

(Original definition in 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

Introduction

The original definition of the Cromer Knoll Group is in Isaksen and Tonstad (1989). In the Norwegian Sea the group originally included the Lyr, Lange and Lysing Formations, and extended across the Lower/Upper Cretaceous boundary. Thus, it was of greater stratigraphic extent than the Cromer Knoll Group in the North Sea, where it essentially is a Lower Cretaceous argillaceous unit. We have redefined the stratigraphic extent of the Group such that the Norwegian and North Sea definitions agree more closely.

Name

From the Cromer Knoll buoy in the southern North Sea. Named by Deegan & Scull (1977).

Type area

The type area for the Cromer Knoll Group is in the southern North Sea. Rhys (1974) used UK well 48/22-2 to illustrate a typical section of the group, and Deegan & Scull (1977) used UK wells 29/25-1, 22/1-2A and 3/29-1, and Norwegian well 2/11-1.
The emphasis in this paper is on the Norwegian sector, and for this purpose wells 6506/12-1 and 12-4 are used to illustrate local developments of the group. Other wells useful to study the Group are 2/6-2, 2/7-15, 2/11-1, 7/3-1, 17/11-2, 34/10-18, 35/3-4, 35/3-5, 31/6-3, 24/12-2 and 17/4-1; the Danish well DK1-1 has also been used.

In well 6506/12-1 the Cromer Knoll was previously defined from 3175 m to 3836 m (NPD definition). This is corrected in this study from the clear log break at 3705 m and the top of  Apteodinium grande and Osangularia schloenbachi to the log break at 3836 m, top of Viking Group. In well 6506/12-4, the Cromer Knoll Group was previously defined from 3132.5m – 3855m (NPD definition). This broad usage is corrected in this study in well 6506/12-4 from the clear log break at 3708m, just above the LO of A.grande, to 3855m, to encompass the Lyr and redefined Lange Formations (Figure 3.4).

Thickness

The thickness of the group varies considerably. In the Viking Graben, the Asta Graben and locally in the Central Trough the thickness is often more than 600 m, gradually thinning towards the basin margins. The group is 667 m thick in Norwegian well 2/11-1 and 643 m thick in Norwegian well 17/11-1. Seismic data indicate that the group is thickest in the Sogn Graben, where it probably reaches up to 1400 m in thickness.

Figure 1 shows depth to the base of the Cromer Knoll Group (revised definition) in the Norwegian Sea based on released well data. The wells displayed are those that penetrated the base of the group.

Figure 2 shows depth to the top of the Cromer Knoll Group (revised definition) in the Norwegian Sea based on released well data. This follows the revised definition that corresponds to the top Lower Cretaceous, and coincides with the level applied in the North Sea. The wells displayed are those that penetrated the group.

The thickness of the group varies considerably since the sediments were deposited in response to an active Late Jurassic tectonic phase. In the Viking Graben, the Asta Graben and locally in the Central Trough the thickness is often more than 600 m, gradually thinning towards the basin margins. The group is 667 m thick in Norwegian well 2/11-1 and 643 m thick in Norwegian well 17/11-1. Seismic data indicate that the group is thickest in the Sogn Graben, where it probably reaches up to 1400 m.

Fig 1: Depth to the base of the Cromer Knoll Group (revised definition) in the Norwegian Sea based on released well data. The wells displayed are those that penetrated the base of the group. Click for PDF.

Fig 2: Depth to the top of the Cromer Knoll Group (revised definition) in the Norwegian Sea based on released well data. This follows the revised definition that corresponds to the top Early Cretaceous and coincides with the level applied in the North Sea. The wells displayed are those that penetrated the group. Click for PDF.

3D image of the depth to the base of the Cromer Knoll Group (revised definition) in the Norwegian Sea based on released well data. Note that the figure is viewed from a location in the SE (Møre Basin) looking towards the NW (Trøndelags Platform). The green arrow is the North pointer. The grid is based on the updated NorLex well data for this group and the revised NorLex definition of the Cromer Knoll Group. Click for large version.

3D image of the depth to the top of the Cromer Knoll Group (revised definition) in the Norwegian Sea based on released well data.

Regional isopach of the Cromer Knoll Group thickness in the Norwegian Sea based on released well data. The group thicknesses follow the revised definition that corresponds to the top Early Cretaceous and coincides with the North Sea level.

The isochore map is generated from Norlex data using thin plate splines (thickness constrained to original range). Thicknesses in metres. Circled well contain both horizons. The red wells have Norlex biostratigraphy.

Lithology

The Cromer Knoll Group consists mainly of fine-grained, argillaceous, marine sediments with a varying content of calcareous material. Calcareous claystones, siltstones and marlstones dominate, but subordinate layers of limestone and sandstone occur. The claystones are generally light to dark grey, olive-grey, greenish and brownish, often becoming light grey, light greenish-grey and light olive-grey marlstones. Mica, pyrite and glauconite are common. Generally, marlstones become the more dominant lithology in both the upper and lower parts of the group.

Logs of reference well 6506/12-1, former stratigraphic extent

Logs of reference well 6506/12-1, redefined extent

Reference well 6506/12-1 (click to enlarge)

Logs of reference well 6506/12-4, former stratigraphic extent

Logs of reference well 6506/12-4, redefined extent

Reference well 6506/12-4 (click to enlarge)

Upper and lower boundaries

Upper Boundary

South of approximately 59° N, the upper boundary is the base of the chalk facies of the Shetland Group, defined by the onset of a decrease in gamma-ray response and an increase in velocity into the overlying carbonates. The uppermost Rødby Formation of the Cromer Knoll Group often appears on logs as a transition between the overlying carbonates of the Shetland Group and the more argillaceous parts of the Cromer Knoll Group. Further north, the upper boundary is the base of the siliclastic facies of the Shetland Group. This boundary is normally also shown by a decrease in gamma-ray response and an increase in velocity when passing into the overlying, generally more calcareous, Svarte Formation of the Shetland Group.

Lower Boundary

The lower boundary is usually well defined and is recognised by a distinct decrease in gamma-ray response and an increase in velocity when passing upward from the generally more organic-rich shales of the underlying Upper Jurassic formations (see Figures 12-14 and 22 in Isaksen and Tonstad, 1989).

Distribution

The group is widely distributed in the Norwegian sector of the North Sea. It is absent from the highest parts of the Mandal High, Jæren High, Utsira High and Lomre Terrace in the Troll area and locally from the Tampen Spur.

Age

Early Cretaceous .

Depositional environment

Open marine, with generally low energy.

Subdivision

Several formations and members are recognized within the group. In the Norwegian Sea this includes the Langebarn and Lyr Formations. In the northern North Sea the Cromer Knoll Group includes the åsgard, Tuxen, Mime, Sola, Rødby and Agat Formations and Ran Member.

Remarks

The group was erected by Rhys (1974) to embrace three marine, arenaceous, argillaceous to marly formations of mainly Early Cretaceous age recognisable onshore and offshore. Deegan & Scull (1977) formally defined the group to include the sediments between the underlying Humber Group and Bream Formation and the overlying Shetland and Chalk Groups. Vollset & Dorset (1984) replaced the Humber Group of the northern North Sea by the Viking Group, and the Bream Formation in the Central Trough and the Norwegian-Danish Basin by the Tyne and Boknfjorden Groups, respectively. The tops of the Draupne Formation of the Viking Group, the Mandal Formation of the Tyne Group and the Flekkefjord Formation of the Boknfjorden Group define the base of the Cromer Knoll Group.

The Cromer Knoll Group is partly equivalent to the Rijnland Group of the Dutch sector (NAM & RGD 1980, Crittenden 1982) and the Speeton Clay Formation together with the Red Chalk Formation of the UK sector (Rhys 1974).