Blodøks Formation (new) (Blodøksformasjonen)
(After NPD Bulletin no. 5)
Shetland Group
Name
Named after Eirik Haraldson Bloødks, a Norwegian king who reigned in Norway
(A.D. 930-934) and in Northumberland (A.D. -954).
Well type section
Norwegian well 25/1-1
from 3807 to 3790 m, coordinates N 59°53'17.40", E 02°04'42.70".
No cores.
Well reference sections
Norwegian well 35/3-2
from 3207 to 3190 m, coordinates N 61°51'O5.98", E 03°46'28.22". No cores.
Norwegian well 1/3-1
from 4371 to 4343 m, coordinates N 56°51'21.00", E 02°51'05.00". No cores.
Danish well BO-1 from 2220 to 2213 m, coordinates N 55°48'02.22",
E 04°34'18.66". Cored throughout.
Thickness
The formation is 17 m thick in the type well (25/1-1), 17 m in well 35/3-2,
28 m in well 1/3-1 and 7 m in well BO-1. It rarely exceeds 20 m in thickness.
Lithology
The formation consists of red, green, grey and black shales and mudstones
which are non-calcareous to moderately calcareous. In the central North Sea
the formation may show a varied influx of marls, limestones and chalky
limestones.
Basal stratotype
The lower boundary is generally characterised by a distinct log break with
an upward increase in gamma-ray intensity and a distinct decrease in velocity
from the Svarte Formation (Fig. 33) or
Hidra Formation (Fig. 24) into the Blodøks Formation.
This is due to the lower content of carbonate in the Blodøks Formation.
Characteristics of the upper boundary
The upper boundary shows a decrease in gamma-ray intensity and an increase
in velocity from the Blodøks Formation upwards into the more calcareous
Tryggvason Formation or the chalky
Narve Formation.
Geographical distribution
The formation is present throughout the North Sea, lacking only on local
highs such as the Sørvestlandet High, the Utsira, Mandal, Jæren and Sele
highs and the Grensen Ridge as well as above many salt diapirs.
Occurrences of formation tops in wells
Isochore map HIDRA-BLODØKS
Isochore map SVARTE-BLODØKS
Biostratigraphy and Stage/Age
Latest Cenomanian to early Turonian; base taken at the base of UC4 near 94.5 Ma.
Depositional environment
The formation was deposited during a period characterised by anoxic bottom
conditions (e.g. Hart & Leary 1989). Presence of carbonates may indicate
periods of more oxic conditions or supply of allochthonous limestones and
chalks (e.g. Norwegian wells 1/3-1 and 2/5-1).
Remarks
The Blodøks Formation is equivalent to the former Plenus Marl Formation
and the informal "formation B" of Deegan & Scull (1977). A black
shale of Early Turonian age is also widespread outside the North Sea, e.g.
the Yorkshire Black Band in England (Jeffries 1963) and similar facies on
Helgoland and in northwestern Germany (Schmidt and Späth 1980).
References
Isaksen, D. and Tonstad, K. (eds.) 1989: A revised Cretaceous and Tertiary lithostratigraphic
nomenclature for the Norwegian North Sea. NPD-Bulletin No. 5, 59 pp.
