Cromer Knoll Group (UK Sector)

(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


The Cromer Knoll Group in the North Sea is essentially a Lower Cretaceous argillaceous unit. 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. The Cromer Knoll Group is partly equivalent to the Speeton Clay Formation together with the Red Chalk Formation of the UK Sector (Rhys 1974).


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

Type area

The type area 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 thickness of the group varies considerably since the sediments were deposited in response to an active Late Jurassic tectonic phase. The group is thickest, exceeding 1500 m, in the Inner Moray Firth Basin (Figure), adjacent to the Little Halibut Fault (Andrews et al., 1990). In the Magnus Trough, Northern North Sea, well 210/15-4 proved over 1590 m of Lower Cretaceous strata (Johnson et al., 1993). In the Witch Ground Graben and Ettrick Basin the group is more than 900 m thick (Andrews et al., 1990). In the Viking Graben, the Asta Graben and locally in the Central Trough the thickness is often more than 600 m, (e.g., 653 m recorded in well 2/11-1) gradually thinning towards the basin margins (Johnson & Lott, 1993). The average thickness in the Central North Sea is generally between 91 and 244 m (Deegan & Scull, 1977).


Distribution and thickness of the Cromer Knoll Group and main component sandstones (derived from Cameron et al., 1992, fig. 79; Gatliff et al., fig. 39; Andrews et al., 1990, fig. 34; Johnson et al., 1993, fig.47).


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.

Upper and lower boundaries

Upper Boundary

South of approximately 59° N, the upper boundary is the base of the chalk facies of the Chalk 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 Chalk 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 Kimmeridge Clay Formation to the overlying Spilsby Sandstone Formation (e.g., 48/22-2) or Valhall Formation (e.g., 47/9-1).


Extensively across the UK Sector of the Southern, Central and Northern North Sea at outcrop or beneath Cenozoic deposits.


Early Cretaceous, Berriasian to Albian.

Depositional environment

Open marine, with generally low energy.


Several formations are recognized within the group in the North Sea. In ascending order the Cromer Knoll Group includes the Valhall, Carrack and Rødby formations extending across the Northern, Central and Southern North Sea. More arenaceous units include the Spilsby Sandstone Formation of the Southern North Sea, Britannia Sandstone Formation of the Central North Sea and Wick Sandstone Formation of the Moray Firth.