Hordaland Group
Introduction
The Hordaland Group was originally described by Deegan & Scull (1977) to cover a
series of Eocene to Early Miocene marine claystones with minor sandstones (i.e. their
Frigg Formation) in the North Sea Tertiary Basin. Subsequently the Hordaland Group has
been extended northwards to include the contemporaneous
Brygge Formation in the
Norwegian Sea (Dalland et al. 1988). In the revised lithostratigraphy of the Norwegian
North Sea published by Isaksen & Tonstad (1989) three new sandstones formations were
formally assigned to the Hordaland Group, namely the
Grid Formation, the Skade
Formation and the Vade Formation.
Subsequently, Knox & Holloway (1992) subdivided the terrigenous mudstones of the
Hordaland Group in the UK sector into two new separate formations: a lower
Horda
Formation (Eocene in age) and an upper Lark Formation
(Oligocene to Early Miocene in
age). The further introduced two new groups to replace the Hordland Group; the lower
Stronsay Group (including the Horda Formation and Frigg
Sandstone Member) and the
upper Westeray Group (including the Larke and Skade formations).
We here suggest to include the use of Horda
Formation and the Lark Formation on the
Norwegian sector of the North Sea, and to change the status of the sandstone units to
members comparable to what has been done in the UK sector. We see, however, no
arguments for changing the status of the Hordaland Group and propose to retain this
lithostratigraphic unit as originally described by Deegan & Scull (1977).
On Haltenbanken the Hordaland Group consists of claystones and minor sandstones,
assigned to the Brygge Formation (Dalland et al., 1988).
Presently, no further subdivision
exists for the Hordaland Group in the Norwegian Sea. Lateral facies changes and breaks in
the sequence may form the basis for future subdivision. The contemporaneous deep-sea
(Eocene to mid Miocene) sediments to the west in the Vøring area comprises somewhat
different lithologies and may also require a separate lithostratigraphic nomenclature.
Name
English / Norwegian
Hordaland Group / Hordalandgruppen
Derivatio nominis
Named by Deegan & Scull (1977) after the county of Hordaland in Norway.
Original definition
Deegan, C. E. & Scull, B. J. 1977. A standard lithologic nomenclature for the Central and
Northern North Sea. Institute of Geological Sciences Report 77/25. NPD Bulletin no. 1, 33 pp.
Lithology
In the North Sea area the Hordaland Group consists of marine claystones with minor
sandstones. The claystones are normally light grey to brown, fissile and fossiliferous.
Red and green claystones sometimes occur at the base. Thin limestones and streaks of
dolomite are present. Sandstones are developed at various levels in the group. These are
generally very fine to medium grained, and are often interbed-ded with claystones.
On Haltenbanken the Hordaland Group consists of claystones and minor sandstones, herein
assigned to the Brygge Formation. The sandstone content increases
to the east.
Thickness
In the North Sea area the group has a thickness of 1060 m in well
2/2-1
and 1365 m in well
24/12-1. Its average thickness is around 1100-1200 m in the central and
southern part of the Viking Graben, but in the northern
Viking Graben the group only
reaches a thickness of a few hundred metres. Maximum thicknesses in the central and
southern part of the Viking Graben are approximately 1300 m and 1400 m, respectively.
The thickness decreases towards the basin margins.
Geographical distribution
The group is distributed over most of the North Sea Basin. It is incomplete at the basin
margins, owing to erosion or non-deposition. The Hordaland Group is also present on
the Mid Norwegian Shelf (Dalland et al. 1988), where it occurs throughout Haltenbanken.
It thins eastwards and is eroded on the Nordland Ridge.
Boundaries
Lower boundary (basal stratotype)
The lower boundary shows an increase in gamma-ray intensity and a decrease in
velocity from the laminated tuffs of the Balder Formation
into the claystones of the Hordaland Group.
Where the Frigg Member is present
at the base of
the Hordaland Group the lower boundary normally
shows a decrease in gamma-ray
response and an increase in velocity from the Balder Formation into the Frigg
Member.
Upper boundary (characteristics)
The upper boundary is placed at the contact with undifferentiated grey to grey-brown
claystones of the Nordland Group. It represents an
unconformity of Middle Miocene age, which may be difficult to identify in some wells.
In the Central Trough, a zone occurs which has high gamma-ray readings and usually a
slightly lower velocity than the underlying and overlying claystones. The upper boundary
of the Hordaland Group is placed at the base of this zone.
On seismic sections, the sediments below this horizon normally have a distorted
signature whilst those above it have a smoother one. The boundary shows a small
angular unconformity; it is not clear whether a small hiatus is present. In the Viking
Graben, the upper boundary is normally the base of the Utsira
Member. The contact is
then marked by an upward decrease in gamma-ray intensity (Fig. .
..). Where the basal
part of the Nordland Group is developed as claystone the boundary is placed at log
breaks associated with a change in claystone colour.
Age
The group is of Eocene to Early Miocene age. Biostratigraphic correlations to wells 2/2-1, 2/2-2
and 2/2-3 indicate that the uppermost part of the Group may be of Middle
Miocene age in the Central Trough.
Depositional environment
The Hordaland Group was deposited in an open marine.
Remarks
We here suggest to include the use of the Horda and
Lark formations on the Norwegian
sector of the North Sea, and to change the status of the sandstone units to members
(Grid,
Frigg,
Vade
and Skade members) comparable to what has been done in the UK sector.
We see, however, no arguments for changing the status of the Hordaland Group and
propose to retain this lithostratigraphic unit as originally described by Deegan & Scull
(1977).
