Viking Group (new)

(From NPD Bulletin no. 3)

Name

Taken from the Viking Graben where the group attains maximum development. It replaces the Humber Group, extended into the Northern North Sea by Deegan and Scull (1977).

Type area

The type area is the Northern North Sea north of 58° N and east of the East Shetland Platform boundary faults. In this report the following wells are used to illustrate the Viking Group: UK well 211/21-1A (Shell) and Norwegian wells 33/9-1 (Mobil), 31/2-1 (Shell) and 15/3-1 (Elf).

Thickness

The thickness of the group may vary considerably since the sediments were deposited on a series of tilted fault blocks produced by predepo-sitional and syndepositional tectonic activy. The measured thicknesses vary from a few metres up to 1039 metres. The thicknesses of the above selected wells are 110.5 m (21/21-1 A), 21 m (33/9-1), 210 m (31/2-1) and 1039 m (15/3-1).

Lithology

In the type area the group consists of dark, grey to black, marine mudstones, claystones and shales. Locally these argillaceous sediments are replaced by sandstones and occasionally conglomerates.

Boundaries

In the type area, the lower boundary is marked by the distinct contrast between the fine grained sediments and the sands of the Brent and Vestland Groups. In the northernmost area (between approximately 61°30' N and 62°N) where the Brent Group is not recognized, the Viking Group is often unconformable on the Dunlin Group. The upper boundary of the group in most wells in the Northern North Sea is an unconformity, normally overlain by higher velocity and lower radioactivity Cretaceous to Paleocene sediments.

Distribution

The group is distributed as defined under the type area.

Age

The group ranges from Bathonian to Ryazanian in age. The group is approximately time equivalent to the new Boknfjord Group of the Norwegian-Danish Basin (Norwegian sector), and the new Tyne Group of the Central Graben (Norwegian sector).

Subdivision

Five formations are defined within the group in the Norwegian sector. The Heather and Draupne Formations are widely distributed. However, the Krossfjord, Fensfjord and the Sognefjord Formations, which represent marginal sandy developments, are more restricted.

Elsewhere within the group thick local sandstones and conglomerates occur (Brennand and Siri, 1975), which are not given any formal status in this report. They are described from the Magnus area by De' Ath and Schuyleman (1981) and from the Brae area by Harms et al. (1980, 1981).

Remarks

The Norwegian Lithostratigraphic Nomenclature Commitee formally proposes substitution of the names "Tyne Group" in the Northern ben area and "Viking Group" in the Northern North Sea fox intervals formerly termed the "Humber Group".

The name "Humber Group" was first proposed by Rhys (1974) for the Middle-Late Jurassic claystones in the Southern North Sea. Deegan and Scull (1977) extended the usage to the Northern North Sea where three formations were recognised, namely the Heather, Kimmeridge Clay and Piper Formations. The Piper Formation is restricted to the area around the Piper Field, while the two other formations are widely distributed.

Recognition of three Middle Jurassic to earliest Cretaceous "claystone" formations in the Central Graben area, none of which can alone be referred to the Heather Formation has led us to restrict the Heather Formation to the part of the North Sea north of 58°N (e.g. Ofstad 1983, and this report). We have further recommended restriction of the term "Kimmeridge Clay Formation" to the area south of the Mid-North Sea High and its replacement by local nomenclature in both the Central Graben and the Northern North Sea (see "Remarks" on the Draupne Formation).

The differences in mode of subdivision which exist between the Northern North Sea, the Central Graben and the type area of the Southern North Sea would, perhaps, not alone constitute grounds for replacement of the Humber Group with more localized group names. However, there is a major conceptual difference between the northern and southern Norwegian North Sea in the way the "group" is defined. In the north, the practise of Deegan and Scull of including the marginal sands (e.g. Piper Formation) in the group has been continued with units such as the Sognefjord Formation. In the south it was found to be practical and convenient to separate out the marginal sands as the Vestland Group. This suggested to the subcommittees the desirability of erecting locally defined units (Viking and Tyne Groups) to replace the Humber Group, a term which was in any case becoming unwieldy and virtually a synonym for the Upper Jurassic. We nevertheless recognise the unifying concept of an "Upper Jurassic claystone" suite throughout the North Sea, reflecting the general transgressive nature of the epoch. This overall character could perhaps be expressed as a "Humber Supergroup", although we have not for the present made this a formal proposal.

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