Boknfjord Group (new)
(From NPD Bulletin no. 3)
Name
From the main fjord in Rogaland, Norway.
Type area
The Fiskebank and Egersund Sub-Basins comprise the type area. The group
is illustrated in the Norwegian well
9/4-3 (Conoco), (Fig. 36).
Thickness
In the type area wells show thicknesses of up to 500 m. In well 9/4-3 the thickness of the group is 290 m. Towards the basin margins the section thins considerably.
Lithology
The sediments of the group are dominated by shales. Varying amounts
of siltstone, sandstone, limestone stringers and differences in organic content make it possible, however, to subdivide the group into formations (Olsen and Strass, 1982).
Boundaries
The lower boundary is characterized by a distinct log break with the
underlying sandstones of the Vestland
Group. The upper boundary is usually characterized by abrupt changes in
log response to lower gamma ray and interval transit times in the overlaying
Valhall Formation. In the Egersund Sub-Basin this
boundary may be difficult to identify due to a large supply of clastic
material.
Distribution
The group is confined to the Fiskebank and Egersund Sub-Basins although
the upper two formations extend further westwards than those lying below.
Occurrences of group tops in wells
Isopach map VESTLAND-BOKNFJORD
Age
The group ranges in age from Callovian to
Ryazanian.
Subdivisions
The group can be subdivided into four formations, the
Egersund (base), Tau,
Sauda and Flekkefjord
Formation (top).
Remarks
The term "Bream Formation" was first used by Deegan and Scull (1977) to
describe a Callovian-Portlandian (Volgian) sequence, mainly claystones and
siltstones, distributed throughout the Norwegian-Danish Basin. The formation
comprised the Egersund Member, Børglum Member and Fredrikshavn Member.
The Bream Formation was adopted with some modification by Michelsen (1978)
for the Danish Sub-Basin where it comprises the Borglum and Fredrikshavn
Members. Recent correlation work in the Egersund and Fiskebank Sub-Basin
(e.g. Olsen and Strass, 1982) shows the existence of four argillaceous units,
ranging in age from Callovian-Ryazanian, which are considered
stratigraphically useful. They are widespread enough to deserve formation
status and different enough from the Danish deposits to merit separate
nomenclature. These units are the Egersund, Tau, Sauda and Flekkefjord
Formations. The formal definitions of the unit in this volume outline
their relationship to the Egersund/Barlum/ Fredrikshavn Members of
Deegan and Scull (1977). Note that the Flekkefjord Formation was formerly
part of the early Cretaceous Valhall Formation, also defined by Deegan and Scull.
The four formations fall naturally into a single "claystone" group. It is not however considered proper to elevate the former Bream Formation to a "Bream Group" which would encompass these units, since it may still desirable to retain the formation status of the Bream Formation and the member status of the Borglum/Fredrikshavn Members in the Danish sector.
It is therefore proposed that the term "Bream Formation" should be abandoned
for the Fiskebank and Egersund Sub-Basins, and replaced by the Boknfjord Group, which is defined above.
