Lower-Upper Oligocene to Upper Pliocene in well 34/2-4
Modified after Eidvin & Rundberg (2001).
Based on analyses of benthic and planktonic foraminifera, pyritised diatoms and Sr isotopes in well 34/2-4 (61º45’20.11’’N, 02º39’37.64’’E, Map 1), we recorded 80 m with Lower-Upper Oligocene sediments, 50 m with Upper Miocene-Lower Pliocene deposits and 170 m with Upper Pliocene sediments. The base of the Oligocene and the top of the Upper Pliocene were not investigated. The units were investigated with 30 ditch-cutting samples at ten metre intervals except between 1390 and 1370 m (Fig. 1).
Well summary figure for well 34/2-4
Biostratigraphy
Lower-Upper Oligocene (1600-1520 m, Hordaland Group)
Benthic foraminifera of the Gyroidina soldanii girardana assemblage and pyritized diatoms of the Diatom sp. 3 assemblage together with seismic and log correlations indicate a latest Early Oligocene to earliest Late Oligocene age for this unit (Fig. 1). The benthic foraminiferal assemblage is correlated with the upper part of Subzone NSB 7b and lower part of Subzone 8a of King (1989) and probably the upper part of Zone NSR 7B of Gradstein & Bäckström (1996). The diatom assemblage is correlated with the Subzone NSP 9c of King (1989). All of these fossil zones are described from the North Sea area.
Upper Miocene-Lower Pliocene (1520-1470 m, Utsira Formation)
Benthic foraminifera of the Ehrenbergina variabilis assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) assemblage (lower part), together with a Sr isotope age obtained from the interval 1510-1500 m, indicate a Late Miocene-Early Pliocene age for this interval (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains G. subglobosa, E. pygmeus, C. telegdi and C. dutemplei. The planktonic foraminiferal fauna also includes G. bulloides and a few specimens of N. atlantica (dextral). The benthic foraminiferal assemblage is correlated with the G. subglobosa – E. variabilis zone of Stratlab (1986) from the Norwegian Sea continental shelf and tentatively Subzone 13b of King (1989) from the North Sea.
E. variabilis is recorded from the Upper Oligocene to Lower Miocene of Germany (Grossheide & Trunco 1965, Spiegler 1974) and from the Upper Oligocene to Lower Pliocene on the Norwegian continental shelf (Skarbø & Verdenius 1986). N. atlantica (sinistral) is known from the Late Miocene to Late Pliocene deposits on the Vøring Plateau (Spiegler & Jansen 1989). In the same area, N. atlantica (dextral) occurs in a part of the Upper Pliocene and in the Upper Miocene. The occurrence of N. atlantica (dextral) in this unit may point to a Late Miocene age, but it may also be caved from the Upper Pliocene.
Upper Pliocene (1470-1300 m, Nordland Group)
Benthic foraminifera of the Cibicides grossus-Elphidiella hannai assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) assemblage (upper main part) 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 Elphidium excavatum, Cassidulina teretis (common), Cibicides scaldisiensis, Nonion affine (common in the lower part), Buccella tenerrima (upper part) and Bulimina marginata. 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). The planktonic foraminiferal fauna also includes Globigerina bulloides, Neogloboquadrina pachyderma (dextral), N. atlantica (dextral, few) and Globorotalia inflata (few). 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.
Sr isotope stratigraphy
Calcareous benthic foraminifera from the interval 1510-1500 m and the interval 1490-1470 m were analysed for Sr isotopes. The obtained 87Sr/86Sr ratios from the interval 1510-1500 m gave an age of 5.1 Ma (earliest Early Pliocene) which support the biostratigraphical correlation. The analysis of interval 1490-1470 m gave an age of 2.6 Ma (Late Pliocene, Table 1, Fig. 1) which is obviously too young, probably due to some analytical error.
Well 34/2-4
| Litho. unit | Sample (DC) | Corrected 87/86Sr | 2S error | Age (Ma) | Laboratory | Analysed fossils |
| Utsira Fm | 1490-1470 m | 0.709068 | 0.000011 | 2.62 | IFE | Calcareous benthic index foraminifera |
| Utsira Fm | 1510-1500 m | 0.709032 | 0.000008 | 5.13 | 16 tests of E. variabilis, G subglobosa |
Table 1: Strontium isotope data from well 34/2-4. IFE = Analysed at the Institute for energy technology (at Kjeller, Norway, from Eidvin & Rundberg 2001). The sample 1510-1500 m is new and 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-Upper Oligocene (1600-1520 m, Hordaland Group)
The Oligocene sediments consist primarily of silty mudstones (Fig. 1).
Upper Miocene-Lower Pliocene (1520-1470 m,Utsira Formation)
The samples in this unit contain sand, silt and clay. In the lower part the sand is mainly glauconitic (Fig. 1). In the upper part the sand is mainly quartzose, and this part also contains some pebbles of mainly crystalline rocks. The pebbles and most of the quartzose sand are probably caved from the Upper Pliocene unit.
Upper Pliocene (1470-1300 m, Nordland Group)
The Upper Pliocene unit consists of poorly sorted clastics (diamicton). Clay is dominant, but silt, sand and ice-rafted pebbles are recorded throughout. The sand fraction is dominantly quartzose, but glauconite is present in most samples. The pebbles are mainly of crystalline rocks, but some of sedimentary rocks and chalk are recorded. Sand is common in several layers and between approximately 1380 and 1360 m there is an interval consisting of two blocky sand bodies separated by a thinner mudstone (Fig. 1). This unit occurs at an equivalent stratigraphic position as similar units in the wells we have investigated from the Snorre Field (towards south) and is most likely the same unit (Eidvin & Rundberg (2001). 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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