Mime Formation (new) (Mimeformasjonen)
(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
Name
Named after a god from Norse mythology who was
considered to be very wise.
Lithology
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.
Thickness
In the type well the Mime Formation is 11 m, and in the reference well it is 42 m thick. Usually, the thickness varies between 5 and 20 m.
Geographical Distribution
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.
Type Well
Well name: 34/10-18
Norwegian well 34/10-18
from 2351 to 2340 m, coordinates N 61°14'22.48", E O2°O3'18.83" (Fig. 16 of Isaksen & Tonstad (1989)). Cores (lower half of the formation).
Well reference section
Norwegian well 17/4-1 from 2122 to 2080 m, coordinates N 58°35'54.00", E 03°16'05.00" (Fig. 17 of Isaksen & Tonstad (1989)). No cores.
Upper and lower boundaries
Upper boundary
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)).
Lower boundary
In those wells where the Mime Formation is present it defines the lower boundary of the Lower Cretaceous, lying on Upper Jurassic sediments or older rocks. This boundary is always an unconformity, and can most often be seen on the logs as a decrease in gamma-ray readings and an increase in velocity upwards from underlying Jurassic sediments.
Age
The formation is time-transgressive, and is dated late Valanginian to Albian. It is oldest in the deeper parts along the basin margins and becomes younger up along the flanks. In most of the East Shetland Basin, along the Utsira, Bergen, Sele and Jaeren Highs, and along the flanks of the Viking Graben, it is usually of Barremian/Hauterivian age (Figure 4 of Isaksen & Tonstad (1989)).
Depositional environment
Paleontological investigations, together with the observation of oolites, indicate a transgressive, shallow marine, depositional environment.
Remarks
Hesjedal & Hamar (1983) described the impure, reworked limestones resting directly on the Base Cretaceous unconformity over the structural highs, as the Utvik Formation. This formation is formally defined as the Mime Formation in this paper, since the name suggested was not in accordance with existing recommendations.
