Lower Oligocene to Upper Pliocene in well 34/4-6
Modified after Eidvin et al. (2001).
Based on analyses of benthic and planktonic foraminifera, pyritised diatoms and Sr isotopes in well 34/4-6 (61º34’14.09’’N, 02º13’19.99’’E, Snorre Field, Map 1), we recorded 120 m with Lower Oligocene deposits, 20 m with Upper Oligocene sediments, 40 m with Upper Miocene-Lower Pliocene sediments and 210 m with Upper Pliocene deposits. The base of the Oligocene and the top of the Upper Pliocene were not investigated. The units were investigated with 26 ditch-cutting samples at 10 to 20 m intervals (Fig. 1). See Eidvin & Rundberg (2001) for the section from 1000-540 m.
Lower Oligocene (1390-1270 m, Hordaland Group)
Benthic foraminifera of the Rotaliatina bulimoides assemblage and Gyroidina soldanii mamillata assemblage and pyritised diatoms of the Diatom sp. 3 assemblage indicate a latest Early Oligocene age for this interval (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains T. alsatica and G. soldanii girardana. No 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 Oligocene (1270-1250 m, Hordaland Group)
Benthic foraminifera of the Turrilina alsatica assemblage and pyritised diatoms of the Diatom sp. 3 assemblage, together with a Sr isotope age, indicate an earliest Late Oligocene age for this section (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains G. soldanii girardana. No planktonic foraminifera were recorded. The benthic foraminiferal assemblage is correlated with Subzone NSB 8a and the diatom assemblage is correlated with the upper part of Subzone NSP 9c of King (1989, North Sea).
Upper Miocene-Lower Pliocene (1250-1210 m, Utsira Formation)
Benthic foraminifera of the Ehrenbergina variabilis assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) assemblage together with a Sr isotope age obtained from the interval 1240-1220 m indicate a latest Late Miocene to earliest 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 C. dutemplei. The planktonic foraminiferal fauna also includes G. bulloides and one specimen 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. The occurrence of N. atlantica (dextral) together with the result of the Sr isotope analysis may point to a Late Miocene age, but N. atlantica (dextral) is also recorded in the Upper Pliocene unit and may consequently be caved (Spiegler & Jansen 1989).
Upper Pliocene (1210-1000 m, Nordland Group)
Benthic foraminifera of the Cibicides grossus-Elphidiella hannai assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) assemblage (upper main 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 Elphidium excavatum (common), Cassidulina teretis (common), Cibicides lobatulus, Nonion affine (lower part), Buccella tenerrima 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). 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 Late Pliocene N. pachyderma (dextral) Zone is described by King (1989) from 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 foraminifera from 1280-1250 m and 1240-1220 m were analysed for Sr isotopes. The obtained 87Sr/86Sr ratios from 1280-1250 m gave an age of 26.3 Ma (early Late Oligocene) and the interval 1240-1220 m gave an age of 5.7 Ma (latest Late Miocene, Table 1, Fig. 1), which both support the biostratigraphical correlation.
|Litho. unit||Sample (DC)||Corrected 87/86Sr||2S error||Age (Ma)||Laboratory||Analysed fossils|
|Utsira Fm||1240-1220 m||0.709010||0.000014||5.69||IFE||Calcareous benthic index foraminifera|
|Hordaland Gr||1280-1250 m||0.708127||0.000017||26.27||IFE||Calcareous benthic index foraminifera|
Table 1: Strontium isotope data from well 34/4-6. IFE = Analysed at the Institute for energy technology (at Kjeller, Norway). 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. Modified after Eidvin & Rundberg (2001).
Lower to Upper Oligocene (1390-1250, Hordaland Group)
The lowermost part of the unit contains a coarse, white quartzose sand. The rest of the unit contains mainly silty mudstones (Fig. 1).
Upper Miocene to Lower Pliocene (1250-1210, Utsira Formation)
The samples in this unit are dominated by glauconitic sand. Some quartzose sand, silt and clay are also recorded, but most of these may be caved (Fig. 1).
Upper Pliocene (1210-1000 m, Nordland Group)
This unit consists of poorly sorted clastics (diamicton). Clay is dominant, but silt, sand and ice-rafted pebbles (mainly of crystalline rocks) are recorded throughout, and between approximately 1150 and 1135 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. 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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