Lower Oligocene to Upper Pliocene in well 34/7-1
Modified after Eidvin & Rundberg (2001).
Based on analyses of benthic and planktonic foraminifera, pyritised diatoms and Sr isotopes in well 34/7-1 (61º28’21.92’’N, 02º13’24.71’’E, Snorre Field, Map 1), we recorded 30 m with uppermost Lower Oligocene sediments, 20 m with Upper Miocene-Lower Pliocene deposits and 150 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 21 ditch-cutting samples at ten metre intervals and 13 sidewall cores (Fig. 1).
Well summary for well 34/7-1
Biostratigraphy
Lower Oligocene (1200-1170 m, Hordaland Group)
Benthic foraminifera of the Roaliatina bulimoides assemblage and pyritised diatoms of the Diatom sp. 3 assemblage, together with a Sr isotope age from the interval 1200-1190 m indicate an Early Oligocene age close to the Early/Late Oligocene boundary for this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains T. alsatica and G. soldanii girardana. No in situ planktonic foraminifera were recorded. The benthic foraminiferal assemblages are correlated with Subzone NSB 7b of King (1989) and Zone NSR 7B of Gradstein & Bäckström (1996) from the North Sea. The diatom assemblage is correlated with the lower part of Subzone NSP 9c of King (1989, North Sea).
Upper Miocene-Lower Pliocene (1170-1150 m, Utsira Formation)
Benthic foraminifera of the Cibicides dutemplei assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) assemblage (lower part) indicate a Late Miocene-Early Pliocene age for this interval (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains E. pygmeus, C. telegdi and U. venusta saxonica (one specimen). The planktonic foraminiferal fauna also includes G. bulloides and one specimen of N. atlantica (dextral). The benthic foraminiferal assemblage is tentatively correlated with Subzone NSB 13b of King (1989, North Sea) and the G. subglobosa – E. variabilis zone of Stratlab (1986, Norwegian Sea continental shelf).
C. dutemplei is known from the Upper Oligocene to Lower Pliocene sediments in the Netherlands (Doppert 1980) and from Upper Oligocene to Upper Miocene sediments on the Norwegian continental shelf (Skarbø & Verdenius 1986). 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). Ehrenbergina variabilis is not recorded in this assemblage, but in wells 34/8-1, 34/8-3 A, 34/4-6, 34/4-7 and 34/2-4 this species was recorded in corresponding sediments. However, E. variabilis was recorded in the uppermost sample in the underlying assemblage, and was probably caved from the current assemblage even though we did not observe it there. 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 (1150-1000 m, Nordland Group)
Benthic foraminifera of the Cibicides grossus assemblage and Elphidiella hannai 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 assemblages also includes Elphidium excavatum (common), Cassidulina teretis (common), Nonion affine and Bulimina marginata. The benthic foraminiferal assemblages are 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 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).
Sr isotope stratigraphy
The obtained 87Sr/86Sr-ratio from the interval 1200-1180 m gave an age of 26.1 Ma (earliest Late Oligocene, Table 1, Fig. 1) which is slightly younger than the age indicated by the foraminiferal correlation. However, this discrepancy is within the precision of the method.
Well 34/7-1
| Litho. unit | Sample (DC) | Corrected 87/86Sr | 2S error | Age (Ma | Laboratory | Analysed fossils |
| Hordaland Gr | 1200-1190 m | 0.708133 | 0.000029 | 26.14 | IFE | Calcareous benthic index foraminifera |
Table 1: Strontium isotope data from well 34/7-1. IFE = Analysed at the Institute for energy technology (at Kjeller, Norway). The Sr ratio is corrected to NIST 987 = 0.710248. The numerical age was derived from the SIS Look-up Table Version 3:10/99 of Howard & McArthur (1997). NIST = National Institute for Standard and Technology. Modified after Eidvin & Rundberg (2001).
Litology
Lower Oligocene (1200-1170 m, Hordaland Group)
The Lower Oligocene contains mainly silty mudstones (Fig. 1).
Upper Miocene to Lower Pliocene (1170-1150 m, Utsira Formation)
The Upper Miocene to Lower Pliocene is dominated by glauconitic sand. Some quartzose sand, silt and clay are also recorded, but most of these may be caved (Fig. 1).
Upper Pliocene (1150-1000 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. Between approximately 1050 and 1125 m there is an interval consisting of two blocky sand bodies separated by a thinner mudstone (Fig. 1). This unit can be correlated between all the wells we have investigated from the Snorre Field and well 34/2-4 to the north (Map 1, Eidvin & Rundberg (2001)).The sand fraction is dominantly quartzose, but glauconite is present in most samples. The ice-rafted pebbles consist mainly of crystalline rocks, but in the lower part pebbles of sedimentary rocks are also common. 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.
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