Lower-Upper Oligocene to Upper Pliocene in well 34/8-3 A

Modified after Eidvin & Rundberg (2001).

Based on analyses of benthic and planktonic foraminifera, pyritisied diatoms and Bolboforma in well 34/8-3A (61º24’28.04’’N, 02º32’45.06’’E, Visund Field, Map 1), we recorded 60 m with Lower-Upper Oligocene deposits, 60 m with Lower Miocene sediments, 40 m with Upper Miocene deposits, 20 m with Upper Miocene-Lower Pliocene sediments and 120 m with Upper Pliocene deposits. The Lower Miocene lies unconformably on the Lower-Upper Oligocene, and Upper Pliocene deposits lie unconformably on the Upper Miocene-Lower Pliocene. The base of the Oligocene and the top of the Upper Pliocene were not recorded. The units were investigated with 30 ditch cutting samples at ten metre interval, except between 1260 and 1240 m (Fig. 1). Unfortunately, no sample contains a sufficient number of calcareous index fossils to give reliable Sr isotope ages.

Well summary figure for well 34/8-3A

Biostratigraphy

Lower-Upper Oligocene (1300-1240 m, Hordaland Group)

Benthic foraminifera of the Turrilina alsatica assemblage together with seismic and log correlations indicate a latest Early Oligocene to earliest Late Oligocene age for this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains G. soldanii girardana. No in situ planktonic foraminifera were recorded. The foraminiferal assemblage is correlated with the upper part of Subzone NSB 7b and the lower part of Subzone 8a of King (1989) and probably the upper part of Zone NSR 7B of Gradstein & Bäckström (1996).

Lower Miocene (1240-1180 m, Hordaland Group)

Pyritised diatoms of the Diatom sp. 4 – Diatom sp. 5 assemblage and benthic foraminifera of the Eponides umbonatus – Stilostomella sp. assemblage indicate an Early Miocene age for this unit (Fig. 1). The diatom assemblage is correlated with Zone NSP 10 and the benthic foraminiferal assemblage is correlated with Zone NSA 10 of King (1989) from the North Sea.

Upper Miocene (1180-1140 m, Utsira Formation)

Benthic foraminifera of the Uvigerina pygmea langeri assemblage and Ehrenbergina variabilis assemblage (lowermost part) and Bolboforma of the Bolboforma fragori assemblage date this unit to Late Miocene (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna includes G. subglobosa, C. telegdi, M. communis (agglutinated) and S. bulloides. The Bolboforma assemblage also includes one specimen each of B. metzmacheri and B. clodiusi.

A Bolboforma fragori/B. subfragori Zone is described from deposits with an age of 11.7-10.3 Ma from the North Atlantic and the Vøring Plateau (Spiegler & Müller 1992, Müller & Spiegler 1993). A Bolboforma metzmacheri Zone is described from sediments with an age of 10.0-8.7 Ma from the same areas (Spiegler & Müller 1992, Müller & Spiegler 1993). However, we interpret the occurrence of B. metzmacheri as being due to caving, even though it is not recorded above. Another interpretation might be that the Bolboforma forms have a slightly different range in this shelfal area than in the open ocean sites. It can also be that B. metzmacheri is in situ and the other Bolboforma forms are reworked. However, this is least likely since only one specimen of B. metzmacheri is recorded.

U. pygmea langeri is known from Subzone NSB 13a (upper part of Middle Miocene to lower part of Upper Miocene) in the North Sea area (King 1989). The lower part of the Ehrenbergina variabilis assemblage is correlated with the lower part of the G. subglobosa – E. variabilis zone of Stratlab (1986) from the Norwegian Sea continental shelf and tentatively the lower part of Subzone 13b of King (1989, North Sea).

Upper Miocene-Lower Pliocene (1140-1120 m, Utsira Formation)

Benthic foraminifera of the Ehrenbergina variabilis assemblage (upper, main part) and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) assemblage indicate a latest Late Miocene to earliest Early Pliocene for this interval (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna includes C. telegdi and E. pygmeus. According to Skarbø & Verdenius (1986) E. variabilis is known from Oligocene to Lower Pliocene deposits on the Norwegian continental shelf, but is most common in the Upper Miocene to Lower Pliocene. C. telegdi and E. pygmeus are recorded in deposits from the Oligocene to the Upper Pliocene in the North Sea and on the Norwegian Sea continental shelf, but are mostly found in Upper Miocene to Lower Pliocene sediments in those areas (Stratlab 1988, Eidvin & Rundberg 2001, 2007, Eidvin et al. 2007, see also well 6607/5-1). The planktonic foraminiferal fauna also includes G. bulloides and one specimen of N. atlantica (dextral). The benthic foraminiferal assemblage is tentatively correlated with the upper part of Subzone NSB 13 b of King (1989, North Sea) and the upper part of G. subglobosa – E. variabilis zone of Stratlab (1986, Norwegian Sea shelf). N. atlantica (dextral) occurs both in the Upper Miocene and the Upper Pliocene in the Norwegian Sea according to Spiegler & Jansen (1989). Consequently, the occurrence of N. atlantica (dextral) may point to a Late Miocene age, but it may also be caved from the Upper Pliocene.

Upper Pliocene (1120-1000 m, Nordland Group)

Benthic foraminifera of the Cibicides grossus assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) assemblage (upper main part) and Globigerina bulloides assemblage give a Late Pliocene age (on the time scale of Berggren et al. 1995) for this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal assemblage also includes Elphidiella hannai (upper part), Elphidium excavatum, Cassidulina teretis (common), Cibicides lobatulus, Nonion affine (common) and Buccella tenerrima. The benthic foraminiferal fauna is correlated with Subzone NSB 15a of King (1989, North Sea) and Zone NSR 12 of Gradstein & Bäckström (1996, North Sea and Haltenbanken area). Spiegler & Jansen (1989) described a N. atlantica (sinistral) Zone from the Vøring Plateau (Norwegian Sea) from Upper Miocene to Upper Pliocene deposits. The LAD of N. atlantica (sinistral) in that area is approximately 2.4 Ma. A G. bulloides Zone is described from the North Atlantic (DSDP Leg 94) in Pliocene sediments as young as 2.2 Ma (Weaver & Clement 1986). On the Vøring Plateau, G. bulloides is common in Late Miocene to Pliocene deposits older than 2.4 Ma (Spiegler & Jansen 1989). G. bulloides is also common in the warmest interglacials of the Pleistocene in the North Atlantic (Kellogg 1977).

Lithology

Lower-Upper Oligocene and Lower Miocene (1300-1180 m, Hordaland Group)

These units contain mainly silty mudstones (Fig. 1).

Upper Miocene-Lower Pliocene (1180-1120 m, Utsira Formation)

The samples in this unit contain sand, silt and clay. The sand is mainly glauconitic (Fig. 1). Some quartzose sand is recorded in the upper part, but most of this is probably caved from the Upper Pliocene unit.

Upper Pliocene (1120-1000 m, Nordland Group)

This unit consists of poorly sorted clastics (diamicton) with clay (dominant), silt, sand and ice-rafted pebbles. Sand and pebbles are most common in the upper part of the unit (Fig. 1). The sand is dominantly quartzose, but glauconite is present throughout and quite common in the lowermost part. Studies of ice-rafted detritus (IRD) in ODP-cores from the Norwegian Sea show that the first expansion of the northern glaciers down to sea level started at about 2.75 Ma (Jansen & Sjøholm 1991, Fronval & Jansen 1996). The maximum age for this unit is therefore considered to be 2.75 Ma, belonging mainly to the Gelasian Stage.

References

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