Lower Oligocene to Upper Pliocene in well 6507/12-1

After Eidvin et al. (work in progress) and modified after Eidvin et al. (2007).

Based on analyses of Bolboforma, benthic and planktonic foraminifera, pyritised diatoms and Sr isotopes in well 6507/12-1 (65°07’01.62’’N, 07°42’42.61’’E, Map 1), we recorded 30 m with Lower Oligocene sediments, 20 m with Upper Oligocene, 55 m with Lower Miocene, 15 m with Middle Miocene, 70 m with Upper Miocene, 65 m with Upper Miocene – Lower Pliocene and 40 m with Upper Pliocene deposits. The base of the Lower Oligocene and the top of the Upper Pliocene were not investigated. The units were investigated with 31 ditch cutting samples at ten metre intervals (Fig. 1).

Well summary figure for well 6507/12-1

Biostratigraphy

Lower Oligocene (1600-1570 m, Brygge Formation)

Benthic calcareous foraminifera of the Rotaliatina bulimoides assemblage and pyritised diatoms of the Diatom sp. 3 assemblage (lower part) date this unit to the Early Oligocene (Fig. 1). The unit is correlated with Subzone NSB 7b and the lower part of Subzone NSP 9c of King (1989, North Sea), Rotaliatina bulimoides zone of Stratlab (1988, Norwegian Sea continental shelf) and probably Zone NSR 7A and the lower part of Zone NSR 7B of Gradstein & Bäckström (1996) from the North Sea and Haltenbanken area.

Upper Oligocene (1570-1550 m, Brygge Formation)

Benthic foraminifera attributed to the Turrilina alsatica assemblage and pyritised diatoms of the Diatom sp. 3 assemblage (upper part) date this unit to the Late Oligocene (Fig. 1). The benthic foraminiferal assemblage is correlated with Zone NSB 8 of King (1989, North Sea) and the upper part of Zone NSR 7B of Gradstein & Bäckström (1996, North Sea and Haltenbanken area). The diatom assemblage is correlated with Subzone NSP 9c of King (1989, North Sea).

Lower Miocene (1550-1495 m (log), Brygge Formation)

Pyritised diatoms of the Diatom sp. 4-Diatom sp. 5 assemblage, and benthic foraminifera attributed to the Trifarina gracilis assemblage (lower part) date the interval to an Early Miocene age (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains Trifarina alsatica var. A (Batjes 1958) and B. elongata. The planktonic foraminiferal fauna includes Globorotalia zealandica, G. praebulloides and S. disjuncta. The diatom assemblage is correlated with Zone NSP 10 (King 1989) from the North Sea.

Middle Miocene (1495(log)-1480 m, Kai Formation)

Bolboforma of the Bolboforma badenensis – B. reticulata assemblage give a Middle Miocene age to this interval (Fig. 1). The benthic foraminiferal fauna includes T. gracilis, T. gracilis var. A, G. subglobosa, M. communis and Cibicides dutemplei. The planktonic foraminiferal fauna includes N. atlantica (dextral), N. atlantica (sinistral), Globigerinita glutinata and G. bulloides.

Spiegler & Müller (1992) described a B. badenensis Zone and a B. reticulata Zone from the North Atlantic and Müller & Spiegler (1993) described a B. badenensis/B. reticulata Zone from the Vøring Plateau. These zones are recorded from deposits with an age slightly older than 14 to 11.7 Ma (Spiegler & Müller 1992).

Upper Miocene (1480-1410 m, Kai Formation)

Bolboforma of the Bolboforma subfragori assemblage and B. laevis assemblage and benthic foraminifera of the U. venusta saxonica assemblage date this unit to Late Miocene (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains E. pygmeus, C. telegdi, G. subglobosa, S. bulloides and E. variabilis. The planktonic foraminiferal fauna includes N. atlantica (dextral), N. atlantica (sinistral), G. glutinata and G. bulloides.

A B. 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. A Bolboforma laevis/B. capsula Zone is known from the North Atlantic and a Bolboforma laevis Zone is known from the Vøring Plateau in sediments with an age of approximately 10.3-10.0 Ma (Spiegler & Müller 1992 and Müller & Spiegler 1993). The U. venusta saxonica assemblage is correlated with Subzone NSB 13b of King (1989) from the North Sea.

Upper Miocene-Lower Pliocene (1410-1340 m (log), Kai Formation)

Benthic foraminifera attributed to the Eponides Pygmeus - Cibicides telegdi assemblage and Eponides pygmeus assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) and Globigerina bulloides assemblage indicate a Late Miocene – Early Pliocene age for this interval (Fig. 1). The benthic foraminiferal assemblages are tentatively correlated with the upper part of Subzone NSB 13b and the lower part of Subzone 14a of King (1989, North Sea).

E. pygmeus and C. telegdi are described from the Oligocene in Denmark and Germany (Grossheide & Trunko 1965, Hausmann 1964, Kummerle 1963, Ulleberg 1974). These species 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).

Upper Pliocene (1340 m (log)-1300 m, Naust Formation)

Benthic foraminifera of the Elphidiella hannai assemblage and planktonic foraminifera of Globigerina bulloides assemblage (upper part) and Neogloboquadrina pachyderma (dextral) 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 Cibicides grossus, Elphidium excavatum, Cibicides lobatulus, Nonion affine, Cassidulina teretis, Bulimina marginata and Elphidium albiumbilicatum. 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). 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). A latest Pliocene N. pachyderma (dextral) Zone is described by King (1989) for the North Sea, by Weaver (1987) and Weaver & Clement (1986) from the North Atlantic and by Spiegler & Jansen (1989) from the Vøring Plateau. On the Vøring Plateau the zone is dated to 1.9-1.8 Ma.

Sr isotope stratigraphy

Calcareous benthic foraminiferal tests from the intervals 1470, 1460 and 1450-1430 m were analysed for Sr isotopes. The obtained ⁸⁷Sr/⁸⁶Sr ratios gave 10.9, 8.0 and 10.0 Ma respectively (early Late Miocene). The sample at 1460 m which gave 8.0 Ma has a comparatively high standard error (see Table 1), and the age is probably not quite correct. Otherwise the obtained Sr ages support the biostratigraphical correlation.

Well 6507/12-1

Litho. Unit	Sample (DC)	Corrected 87/86Sr	2S error	Age (Ma)	Analysed fossil species
Kai Fm	1450-1430 m	0.708897	0.000008	9.57	Approximately 30 tests of U. venusta saxonica and two test of G. subglobosa
Kai Fm	1460 m	0.708931	0.000029	7.95	Approximately 50 small tests of B. fragori, B. subfragori and B. clodiusi
Kai Fm	1470 m	0.708865	0.000009	10.86	Approximately 100 small tests of B. fragori and B. subfragori

Table 1: Strontium isotope data from well 6507/12-1. The samples were analysed at the University of Bergen. Sr ratios were corrected to NIST 987 = 0.710248. The numerical ages were derived from the SIS Look-up Table Version 3:10/99 of Howard & McArthur (1997). NIST = National Institute for Standard and Technology.

Lithology

Lower Oligocene to Lower Miocene (1600-1495 m (log), Brygge Formation)

Clay dominates the samples in this unit, but some sand (mainly glauconitic) and silt is recorded especially in the upper part (Fig. 1).

Middle Miocene to Upper Miocene – Lower Pliocene (1495 (log)-1340 m (log), Kai Formation)

The ditch cutting samples from this interval are also mostly fine grained. Clay dominates the samples, but some sand and silt are also present. In the lower part the sand is mainly glauconitic. In the upper part sand and silt are more common. This sand is mainly quartzose, and pebbles of crystalline rocks are also recorded. The pebbles and most of the quartzose sand are probably caved from the Upper Pliocene glacial unit (Fig. 1).

Upper Pliocene (1340 m (log)-1300 m, Naust Formation)

The ditch cutting samples from the Upper Pliocene unit contain a clay-rich diamicton which is also rich in sand, silt and pebbles of mainly crystalline rocks. The pebbles are interpreted as ice-rafted and indicate that the sediments were deposited after the marked increase in the supply of ice-rafted detritus to the Norwegian Sea, which started at about 2.75 Ma (Fronval & Jansen 1996).

References

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