 |
|
|
|
Formations assigned to the group: Polarrev Formation Ulv Formation IsbjГёrn Formation
Bjarmeland was used by the Vikings to describe the area immediately south of the Barents Sea. The area was visited and described by the Norwegian Viking, Ottar, in the 9th century. The name was more recently used to name a structural element on the Barents Shelf: the Bjarmeland Platform (Gabrielsen et al. 1990). The group was introduced and briefly reviewed by Dallmann et al. (1999) in the knowledge of the ongoing more detailed work presented herein.
The Bjarmeland Platform in the southern Norwegian Barents Sea is here defined as type area for the group since the offshore successions are best displayed in wells from this area, including the eastern flanks of the Loppa High (Fig. 38). Three wells show typical developments of the group: 7124/3-1 (4271 m to 3900 m), 7226/11-1 (4334 m to 4103 m) and 7121/1-1 (3990 m to 3502 m): the base of the group is defined by the basal stratotype of the biohermal Polarrev Formation in well 7229/11-1 on the northern Finnmark Platform.
Fig. 38. Correlation of the Bjarmeland Group in selected wells from the Finnmark Platform, Nordkapp Basin, Bjarmeland Platform and the Loppa High.
Well 7228/9-1S (4361 m to 4065 m) (Fig. 38) located on the Finnmark Platform’s northern margins towards the Nordkapp Basin illustrates the group’s development in a deeper water basinal setting. The Finnmark Platform itself is an important reference area since the group has been drilled in a variety of settings from the outer platform areas to the north (7229/11-1, 4282 m to 3970 m, Fig. 38) across the central platform (7128/4-1, 1820 m to 1704 m) and 7128/6-1, 1835 m to 1745 m) to the southern updip areas represented by core 7128/12-U-01, 569.2 m to 557.5 m). The succession assigned to the group in 718/4-1 and 7128/6-1 corresponds to lithological unit L-8 of Ehrenberg et al. (1998a).
Hambergfjellet on the southern mountain massif of BjГёrnГёya is designated as an onshore reference area. Hambergfjellet is the type area of the Hambergfjellet Formation, the only onshore unit to be assigned to the Bjarmeland Group at the present time.
The group attains a maximum thickness of 488 m in well 7121/1-1 at the eastern flank of the Loppa High. It is thinner, 233-371 m, in wells 7226/11-1 and 7124/3-1 on the Bjarmeland Platform (Fig. 38). On the Finnmark Platform the group thins from 312 m in well 7229/11-1 to 116-89 m in wells 7128/4-1 and 7128/6-1 central on the platform and less than 50 m in the IKU cores further updip. The Hambergfjellet Formation on the southern cliffs of BjГёrnГёya shows a similar thickness of up to 60 m, but this unit wedges out and disappears northwards on the island.
The group is dominated by white to light grey bioclastic limestones containing a typical cool-water fauna of crinoids, bryozoans, brachiopods and siliceous sponges. Silty, dark grey to black, locally bituminous limestones characterise the deeper-water succession. Minor cherts occur, especially in the uppermost part. Siliciclastics are rare, except on the Polheim Subplatform where the group is unusually developed and dominated by finegrained siliciclastics and marls (well 7120/1-1). The Hambergfjellet Formation on BjГёrnГёya consists of basal sandstones which onlap all older units from basement to Gipsdalen Group, passing up into sandy bioclastic limestones with a fauna dominated by crinoids, bryozoans and brachiopods (Worsley et al. 2001).
The group is most thickly developed at the eastern flanks of the Loppa High and eastward across the Bjarmeland Platform. The thickest development is in outer platform settings north and south of the Nordkapp Basin where thick bryozoan-dominated buildups occur as isolated mounds or merge to form elongated complexes (Gerard & Buhrig 1990; Nilsen et al. 1993). Intermound and basinal areas are dominated by more fine-grained and thinly bedded limestones, which in well 7120/1-1 are interbedded with siliciclastic shales. The platform areas of the eastern Finnmark Platform are characterised by relatively uniform successions of bedded crinoid- and bryozoan-dominated packstones and grainstones.
The group is seen to onlap palaeohighs and the margins of the depositional basin such as the eastern flank and crestal areas of the Loppa High and the southern parts of the Finnmark Platform. It is missing in wells 7120/12-2 and 7120/12-4 from the southern Hammerfest Basin – western Finnmark Platform and onshore it is only known from the Hambergfjellet Formation on Bjørnøya, - although future work may well demonstrate that the uppermost Gipsfjorden Formation and the Vøringen Member (Kapp Starostin Formation) of the Tempelfjorden Group both age- and facies-wise represent lateral equivalents of parts of the group on Spitsbergen.
Fusulinids suggest a mid-Sakmarian to late Artinskian age in 7128/6-1 (Ehrenberg et al. 2000). The base of the group is thought to be highly diachronous, oldest in the more distal settings and youngest on the platforms (Fig. 4). The Hambergfjellet Formation on BjГёrnГёya is dated as late Artinskian based on fusulinids and conodonts (Nakrem 1991; Nakrem et al. 1992). Fusulinids indicate a similar age in cores 7128/12-U-01 and 7129/10-U-01 (Bugge et al. 1995; Ehrenberg et al. 2000).
The group is characterised by deposition of carbonates dominated by crinoids, bryozoans, brachiopods and siliceous sponges. The fauna is markedly different from the foraminiferdominated warm-water fauna of the underlying Gipsdalen Group and is believed to reflect deposition in more temperate cool-water environments (Stemmerik 1997). Deposition took place in a variety of cool-water carbonate environments and deposits range from shallow inner shelf bioclastic grainstones to outer shelf bryozoan-dominated buildups and thinly bedded bioclastic wackestones and packstones. Siliciclastic input to the basin was limited - except locally in the west where deeper water shales are interbedded with resedimented carbonates in 7120/1-1; sand input was also significant on BjГёrnГёya, immediately adjacent to the subaerially exposed parts of the Stappen High. The bryozoan-dominated carbonate buildups formed along the margins of the Nordkapp Basin on the outer part of the platforms. They are often located above older buildups. Distally to the trend of buildups more marly sediments have been recorded in well 7228/9-1S.
The Bjarmeland Group is represented by three formations in the offshore areas of the southern Norwegian Barents Sea. The formations are formally described herein and named after predators common to Arctic Norway. The Polarrev and Ulv formations show an interfingering of the carbonate buildups of the former and the inter-buildup lithofacies of the latter formation. The Ulv Formation was also developed in the outer platform and basinal areas throughout deposition of the group, while the uppermost IsbjГёrn Formation in inner shelf areas overlies earlier buildups but does not extend into deeper waters characteristic of the Ulv Formation.
The Hambergfjellet Formation of BjГёrnГёya, defined by Worsley & Edwards (1976) is included in the group, as it appears to represent a lithologically similar but highly condensed (<60 m thick) development of the IsbjГёrn Formation.
Polarrev Formation Ulv Formation IsbjГёrn Formation