Lower Oligocene to Upper Pliocene in well 34/8-1
Modified after Eidvin & Rundberg (2001).
Based on analyses of benthic and planktonic foraminifera, pyritised diatoms and Sr isotopes in well 34/8-1 (60º21’53.50’’N, 02º25’57.57’’E, Visund Field, Map 1) we recorded 100 m with Lower Oligocene deposits, 90 m with Upper Oligocene deposits, 30 m with Lower Miocene deposits, 50 m with Upper Miocene-Upper Pliocene sediments and 290 m with Upper Pliocene deposits. The Lower Miocene lies unconformably on the Lower-Upper Oligocene. The base of the Lower Oligocene and the top of the Upper Pliocene were not investigated. The units were investigated with 35 ditch-cutting samples at 10-20 metre intervals (Fig. 1). See Eidvin & Rundberg (2001) for the section from 800-440 m.
Lower Oligocene (1360-1260 m, Hordaland Group)
Benthic foraminifera of the Rotaliatina bulimoides assemblage and pyritised diatioms of the Diatom sp. 3 assemblage together with two Sr isotope ages date this interval to latest Early Oligocene (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains common T. alsatica. 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 (1260-1170 m, Hordaland Group)
Benthic foraminifera of the Turrilina alsatica assemblage and pyritised diatoms of the Diatom sp. 3 assemblage together with seismic and log correlations indicate an 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 planktonic foraminifera were recorded. The benthic foraminiferal assemblages are correlated with Subzone NSB 8a and the diatom assemblage is correlated with the upper part of Subzone NSP 9c of King (1989, North Sea).
Lower Miocene (1170-1140 m, Hordaland Group)
Pyritised diatoms of the Diatom sp. 4 – Diatom sp. 5 assemblage date this unit to the Early Miocene (Fig. 1). The diatom assemblage is correlated with Zone NSP 10 of King (1989) from the North Sea.
Upper Miocene-Lower Pliocene (1140-1090 m, Utsira Formation)
Benthic calcareous foraminifera attributed to the Ehrenbergina variabilis assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) assemblage (lower part) together with a Sr isotope age (5.0 Ma) indicate a Late Miocene-Early Pliocene age for this interval (Fig. 1). In addition to the nominate species, the planktonic foraminiferal fauna includes G. bulloides, a few specimens of N. atlantica (dextral) and one or a few specimens of several Bolboforma species including B. subfragori, B. metzmacheri, B. clodiusi, B. compressibadenensis, B. fragori and B. pseudohystrix. The benthic fossil assemblage is correlated with the G. subglobosa – E. variabilis zone of Stratlab (1986, Norwegian Sea continental shelf) and tentatively Subzone NSB 13b of King (1989, North Sea).
The occurrence of N. atlantica (dextral) and Bolboforma may point to a Late Miocene age. N. atlantica (dextral) is also recorded in the Upper Pliocene unit and may consequently be caved (Spiegler & Jansen 1989). The composition of the Bolboforma assemblage is very unlike those found in the North Atlantic and on the Vøring Plateau (Spiegler & Müller 1992 and Müller & Spiegler 1993), and it may therefore be reasonable to assume that they are reworked. In addition, the results of the Sr isotope analysis do not point to an age as old as the latest Middle to earliest Late Miocene, which is indicated by some of the Bolboforma species. However, we are not quite sure about the exact age of this unit.
Upper Pliocene (1090-800 m, Nordland Group)
Benthic foraminifera of the Cibicides grossus 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 Elphidiella hannai (parts of section), Elphidium excavatum (common), Cassidulina teretis (common), Cibicides scaldisiensis, 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). The planktonic foraminiferal fauna also includes Globigerina bulloides, Neogloboquadrina pachyderma (dextral) and N. atlantica (dextral, 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 1300 and 1280 m and from the interval 1130-1110 m were analysed for Sr isotopes. The obtained 87Sr/86Sr ratios from the sample at 1300 m gave an age of 28.2 Ma and the sample at 1280 gave an age of 28.6 Ma. Both these ages are close to the Lower/Upper Oligocene boundary (28.5 Ma). The obtained 87Sr/86Sr ratio from the interval 1130-1110 m gave an age of 5.0 Ma (earliest Early Pliocene, Table 1, Fig. 1).
|Litho. unit||Sample (DC)||Corrected 87/86Sr||2S error||Age (Ma)||Laboratory||Analysed fossils|
|Utsira Fm||1130-1110 m||0.709036||0.000017||5.03||IFE||Calcareous benthic index foraminifera|
|Hordaland Gr||1280 m||0.708039||0.000045||28.61||IFE||Calcareous benthic index foraminifera|
|Hordaland Gr||1300 m||0.708055||0.000047||28.20||IFE||Calcareous benthic index foraminifera|
Table 1: Strontium isotope data from well 34/8-1. 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 and Upper Oligocene and Lower Miocene (1360-1140 m, Hordaland Group)
These units contain mainly silty mudstones. Some glauconitic sand is present in the Lower Miocene (Fig. 1).
Upper Miocene to Lower Pliocene (1140-1090 m, 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 (1090-800 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 also quite common throughout (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 considered to be 2.75 Ma, belonging mainly to the Gelasian Stage.
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