Sola Formation (Solaformasjonen)
(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
Informally named by Hesjedal & Hamar (1983) after a village in southwestern
Norway. Formally named by Jensen et al. (1986).
Lithology
The Sola Formation consists of shales interbedded with stringers of marlstone and limestone. The carbonate content is lower than that in the underlying Tuxen and Åsgard Formations and the overlying Rødby Formation. The colour is black or dark grey, but olive-grey, brown and red colours occur. The shales are finely laminated and often very pyritic.
Thickness
The thickness in the type well is 39 m, and in reference well 2/11-1 it is 78 m. It generally varies between 20 m and 200 m. The formation is thick in the Viking Graben and Asta Graben, and thin in the East Shetland Basin and parts of the Fiskebank Sub-Basin.
Geographical Distribution
The formation is widespread in the North Sea. It is absent or thin on structural highs, salt-induced structures and in parts of the Central Trough and Norwegian-Danish Basin.
Type Well
Well name: 1-1
Danish well 1-1 from 2898 to 2859 m. Coordinates N
56°03'10", E 04°14'60" (Fig. 14 in Isaksen & Tonstad (1989)). No cores.
Reference Wells
Norwegian well 2/11-1 from 2988 to 2910 m, coordinates N 56°14'16.98", E 03°27'07.05" (Fig. 12). No cores.
Norwegian well 24/12-2 from 4043 to 3985 m, coordinates N 59°12'00.75", E 01o52'53.34" (Fig. 18 in Isaksen & Tonstad (1989)). No cores.
Upper and lower boundaries
Upper boundary
The upper boundary is most often placed where the carbonate content starts to increase rapidly into the overlying Rødby Formation (Figures 12, 14, 18, and 22 in Isaksen & Tonstad (1989)). In some areas where, the Sola Formation is overlain by the Ran Member (Figure 23 in Isaksen & Tonstad (1989)), the boundary is defined by an upward decrease in gamma-ray response and an increase in velocity.
Lower boundary
The lower boundary is usually placed on the Tuxen or Åsgard Formations (Figures 12, 14, 17 and 18 in Isaksen & Tonstad (1989)). Generally, the gamma-ray response increases and the velocity decreases from the calcareous and sandy sediments up into the shaly and organic rich Sola Formation. In some wells in the east, on the Horda Platform, the gamma-ray response does not increase when the boundary from the Asgard Formation up into the Sola Formation is crossed. In such wells, a lower, more stable velocity identifies the Sola Formation.
Age
The Sola Formation is of Mid Aptian-Early Albian age. A possible Middle (Late) Barremian-Albian age is recorded from Danish wells (Heilmann-Clausen 1986).
Depositional environment
The Sola Formation was deposited in a marine environment with alternating anoxic and oxic bottom conditions. Hesjedal & Hamar (1983) suggested that the formation was deposited during a regressive period, while Rawson & Riley (1982) held the opposite view.
