Lower and Upper Oligocene in well 2/2-2
Based on analyses of agglutinated and calcareous benthic and planktonic foraminifera, pyritised diatoms and Sr isotopes in well 2/2-2 (55º56’40.61’’N, 03º22’46.81’’E, Map 1). We recorded a 50 m-thick unit of Lower Oligocene deposits and 160 m with Upper Oligocene sediments. The base of the Lower Oligocene and the top of the Upper Oligocene were not investigated. The units were investigated with 22 ditch-cutting samples at ten metre intervals and two core samples (Fig. 1).
Well summary figure 2/2-2, fig. 1
Biostratigraphy
Lower Oligocene (2110-2060 m, Hordaland Group)
Benthic calcareous foraminifera of the Rotaliatina bulimoides assemblage and Gyroidina soldanii mamillata assemblage and pyritised diatoms of the Diatom sp. 3 assemblage date this interval to Early Oligocene (Fig. 1). In addition to the nominate species, the benthic calcareous fauna also contains A. guerichi guerichi, G. soldani girardana and T. alsatica. Agglutinated benthic foraminifera include B. eocenicus, Ammodiscus sp. B, S. compressa, Ammodiscus cretaceous, C. rotundidorsata and D. seigliei. Planktonic foraminifera include G. praebulloides. The benthic foraminiferal assemblage is correlated with Subzone NSB 7b of King (1989, North Sea) and Zone NSR 7B of Gradstein & Bäckström (1996, North Sea). The diatom assemblage is correlated with the lower part of Subzone NSP 9c of King (1989).
Upper Oligocene (2060-1900 m, Vade Formation and Hordaland Group)
Benthic calcareous foraminifera of the Turrilina alsatica assemblage, agglutinated foraminifera of the Ammodiscus sp. B assemblage and pyritised diatoms of the Diatom sp. 3 assemblage together with two Sr isotope ages from a core sample at 1984.5 m date this unit to Late Oligocene (Fig. 1). In addition to the nominate species, the benthic foraminifera also contain the calcareous G. soldanii girardana, R. arnei and A. guerichi guerichi, and the agglutinated B. eocenicus, C. rotundidorsata, S. compressa and A. cretaceous. Planktonic foraminifera include G. praebulloides and G. angustiumbilicata. The benthic foraminiferal assemblages are correlated with NSB 8 and NSA 9 of King (1989, North Sea) and probably Zone NSR 8A of Gradstein & Bäckström (1996, North Sea). The diatom assemblage is correlated with the upper part of Subzone NSP 9c of King (1989).
Sr isotope stratigraphy
Two pieces of a whole bivalve from a core sample at 1984.5 m were analysed for Sr isotopes. The obtained 87Sr/86Sr ratios gave 26.3 and 26.0 Ma (Late Oligocene, Table 1, Fig. 1).
Well 2/2-2
| Litho. Unit | Sample (Core) | Corrected 87/86Sr | 2S error | Age (Ma) | Analysed fossils |
| Vade Fm | 1984.5 m | 0.708127 | 0.000009 | 26.3 | Fragment of one large Bivalve |
| Vade Fm | 1984.5 m | 0.708127 | 0.000009 | 26.0 | Fragment of one large Bivalve |
Table 1: Strontium isotope data from well 2/2-2. The samples are 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 and McArthur (1997). NIST = National Institute for Standard and Technology.
Lithology
Lower Oligocene (2110-2060 m, Hordaland Group)
The ditch-cutting samples are dominated by clay, but minor sand (glauconitic, biotitic and quartzose) and silt are also recorded throughout (Fig. 1).
Upper Oligocene (2060-1900 m, Vade Formation and Hordaland Group)
The samples in the lower part are also dominated by clay, but sand (glauconitic, biotitic and quartzose) are slightly more common than in the immediately underlying unit. In the upper part of the Vade Formation, sand is common but not dominant (glauconitic, biotitic and quartzose). The Hordaland Group above is less sandy (Fig. 1).
References
Eidvin, T., Riis, F. & Rasmussen, E. S., work in progress. Oligocene to Lower Pliocene deposits of the Norwegian continental shelf, with correlation to the Norwegian Sea, Greenland, Svalbard, Denmark and their relation to the uplift of Fennoscandia.
Gradstein, F. & Bäckström, S., 1996: Cainozoic Biostratigraphy and Paleobathymetry, northern North Sea and Haltenbanken. Norsk Geologisk Tidsskrift 76, 3-32.
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.
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.