Oligocene to Upper Pliocene in well 34/7-2
Modified after Eidvin (2009).
Based on analyses of benthic and planktonic foraminifera, pyritised diatoms, sponge spicules and Sr isotopes in well 34/7-2 (61º17'57.16''N, 02º09'40.90''E, Map 1), we recorded approximately 35 m with unspecified Oligocene sediments, approximately 10 m with Upper Miocene-Lower Pliocene sediments and approximately 109 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 16 ditch-cutting samples at ten metre intervals and eight sidewall cores (Fig. 1). A detailed stratigraphy of these units (including fossil range charts) was presented in the report of Eidvin (2009).
Well summary figure for well 34/7-2
Biostratigraphy
Oligocene (1070 m to approximately 1035 m (log), Hordaland Group)
The greater proportion of the fossils recorded in this unit are sponge spicules (both rod-shaped and Geodia sp.). Pyritised diatoms are also recorded throughout and in the lower part also radiolaria. In the diatom flora, the index fossil Diatom sp. 3 is recorded in the ditch-cutting samples at 1070 and 1060 m. King (1989) described a Diatom sp. 3 Subzone (NSP 9c) from the uppermost Lower Oligocene to the lowermost Lower Miocene of the North Sea area.
Upper Miocene-Lower Pliocene (approximately 1035 m (log) to approximately 1025 m (log), Utsira Formation)
Benthic foraminifera of the E. variabilis assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) assemblage (lower part) recorded in the ditch-cutting sample and the sidewall core at 1030 m give a Late Miocene-Early Pliocene age for this unit. In addition to the nominate species, the benthic foraminiferal fauna also includes G. subglobosa, Cibicides telegdi and Hoeglundina elegans.
The planktonic foraminiferal fauna in this unit is correlated with the lower part of the Neogloboquadrina atlantica (sinistral) Zone of Spiegler & Jansen (1989, Norwegian Sea). The benthic foraminiferal fauna is correlated with G. subglobosa-E. variabilis zone of Stratlab (1986) from the Norwegian Sea continental shelf and tentatively Subzone 13b of King (1989, North Sea).
Upper Pliocene (approximately 1025 (log) to 919 m, Nordland Group)
Benthic foraminifera of the Cibicides grossus assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) assemblage (upper part) and Globigerina bulloides assemblage give a Late Pliocene age for this unit. In addition to the nominate species, the benthic foraminiferal fauna also includes Elphidiella hannai throughout most of the section. According to King (1989) C. grossus and E. hannai are known from the northern North Sea in the Upper Pliocene to lowermost Pleistocene deposits (on the time scale of Berggren et al. 1995). In the North Sea, first appearance datums (FADs) of these species are considerably later than the Early/Late Pliocene boundary (3.56 Ma). However, N. atlantica (sinistral) is described from the Vøring Plateau in deposits no younger than 2.4 Ma. In the Norwegian Sea there is a marked dominance of this species together with G. bulloides in Pliocene deposits older than this (Spiegler & Jansen 1989). G. bulloides is also known from the North Atlantic and Norwegian Sea in deposits from the warmest interglacials during the Pleistocene (Kellogg, 1977).
The planktonic foraminiferal fauna in this unit is correlated with the upper part of the Neogloboquadrina atlantica (sinistral) Zone of Spiegler & Jansen (1989, Norwegian Sea). The benthic foraminiferal fauna is correlated with Subzone NSB 15a of King (1989, North Sea), and both the benthic and planktonic assemblages are correlated with the Cibicides grossus-Elphidiella hannai-Globigerina bulloides-Neogloboquadrina atlantica (sinistral) assemblage of Eidvin & Rundberg (2001, Tampen area, northern North Sea).
Sr isotope stratigraphy
Two samples from the sidewall core at 1030 m were analysed for 87Sr/86Sr ratios. The samples are based on 27-25 tests of the benthic calcareous foraminifera Ehrenbergina variabilis (in one sample a few tests of Globocassidulina subglobosa are also used). The obtained 87Sr/86Sr ratios gave ages of 4.7 and 4.2 Ma (Early Pliocene, Fig. 1, Table 1).
Well 34/7-2
| Litho. Unit | Sample (SWC) | Corrected 87/86Sr | 2S error | Age (Ma) | Analysed fossils |
| Utsira Fm | 1130 m | 0.709044 | 0.000008 | 4.74 | 25 tests of E. variabilis, G subglobosa |
| Utsira Fm | 1130 m | 0.709051 | 0.000008 | 4.21 | 27 tests of E. variabilis |
Table 1: Strontium isotope data from well 34/7-2. 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
Oligocene (1070 to approximately 1035 m (Log), Hordaland Group)
This unit contains mainly silty mudstone with some sand rich in sponge spicules (Fig. 1).
Upper Miocene to Lower Pliocene (approximately 1035 m (log) to approximately 1025 m (log), Utsira Formation)
Both the ditch cuttings and the sidewall core from 1030 m in this unit are dominated by glauconitic sand. Some quartzose sand, silt and clay are also recorded (Fig. 1).
Upper Pliocene (approximately 1025 m (log) to 919 m, Nordland Group)
Most of this unit consists of poorly sorted clastics (diamicton) with clay (dominant), silt, sand and a few ice-rafted pebbles in some samples. Quartzose sand is dominant from approximately 1020 m to 1000 m, but the samples at 1021.5 m (SWC) and 1020 m (DC) also contain some glauconitic sand (Fig. 1). 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).
References
Berggren, W. A., Kent, D. V, Swisher, C. C., III & Aubry, M.- P., 1995: A Revised Cenozoic Geochronology and Chronostratigraphy. In Berggren, W. A. et al. (eds.): Geochronology Time Scale and Global Stratigraphic Correlation. Society for Sedimentary Geology Special Pulication 54, 129-212.
Eidvin, T., 2009: A biostratigraphic, strontium isotopic and lithostratigraphic study of the upper part of Hordaland Group and lower part of Nordland Group in well 34/7-2, 34/7-12 and 34/7-R-1 H from the Tordis Field in the Tampen area (northern North Sea). Available from the internet:
http://www.npd.no/Global/Norsk/3%20-%20Publikasjoner/Forskningsartikler/Tordis-biostr-rapp.pdf
Eidvin, T. & Rundberg, Y., 2001: Late Cainozoic stratigraphy of the Tampen area (Snorre and Visund fields) in the northern North Sea, with emphasis on the chronology of early Neogene sands. Norsk Geologisk Tidsskrift, 81, 119–160. Available from the internet: http://www.npd.no/Global/Norsk/3%20-%20Publikasjoner/Forskningsartikler/Eidvin_and_Rundberg_2001.pdf
Fronval, T. & Jansen, E., 1996: Late Neogene paleoclimates and paleoceanography in the Iceland-Norwegian Sea: evidence from the Iceland and Vøring Plateaus. In Thiede, J., Myhre, A. M., Firth, J. V., John, G. L. & Ruddiman, W. F. (eds.), Proceedings of the Ocean Drilling Program, Scientific Results 151: College Station, TX (Ocean Drilling Program), 455-468.
Howarth, R. J. & McArthur, J. M., 1997: Statistics for Strontium Isotope Stratigraphy: A Robust LOWESS Fit to Marine Sr-Isotope Curve for 0 to 206 Ma, with Look-up table for Derivation of Numeric Age. Journal of Geology 105, 441-456.
Jansen, E. & Sjøholm, J., 1991: Reconstruction of glaciation over the past 6 Myr from ice- borne deposits in the Norwegian Sea. Nature 349, 600-603.
Kellogg, T. B., 1977: Paleoclimatology and Paleo-oceanography of the Norwegian and Greanland Seas: The Last 450,000 years. Marine Micropalaeontology 2, 235-249.
King, C., 1989: Cenozoic of the North Sea. In Jenkins, D. G. and Murray, J. W. (eds.), Stratigraphical Atlas of Fossils Foraminifera, 418-489. Ellis Horwood Ltd., Chichester.
Spiegler, D. & Jansen, E., 1989: Planktonic Foraminifer Biostratigraphy of Norwegian Sea Sediments: ODP Leg 104. In Eldholm, O., Thiede, J., Tayler, E., et al. (eds.), Proceedings of the Ocean Drilling Program, Scientific Results 104: College Station, TX (Ocean Drilling Program), 681-696.
Stratlab, 1988: Mid - Norway offshore Biozonation, Tertiary to Triassic. Fossil-atlas, bind 1 - 4, Stratlab a.s. (non-proprietary report).