Lower Oligocene and Pleistocene in well 31/3-1

Modified after Eidvin & Rundberg (2001).

Based on analyses of benthic and planktonic foraminifera and Sr isotopes in well 31/3-1 (60º46’47.86’’N, 03º44’03.40’’E, Troll Field, Map 1) we recorded a 40 m-thick section of Lower Oligocene deposits and 40 m of Pleistocene sediments. The base of the Lower Oligocene and the top of the Pleistocene were not investigated. The units were investigated with nine ditch cutting samples at ten metre intervals, except between 580 and 560 m (Fig. 1).

Well summary figure for well 31/3-1


Lower Oligocene (580-540 m, Hordaland Group)

Benthic foraminifera of the Gyroidina soldanii mamillata assemblage and Turrilina alsatica assemblage, supported by a strontium-isotope age obtained from the interval 580-570 m and seismic and log correlations, give an Early Oligocene age for this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal assemblages also contain G. soldanii girardana, Stilostomella longiscata and Stilostomella hirsuta. 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.

Pleistocene (540-500 m, Nordland Group)

Benthic foraminifera of the Nonion labradoricum assemblage and planktonic foraminifera of the Neogloboquadrina pachyderma (sinistral) assemblage give a Pleistocene 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 contains Elphidium excavatum, Cibicides lobatulus, Haynesina orbiculare, Angulogerina angulosa, Bolivina skagerakensis, Islandiella norcrossi and Islandiella helena. The planktonic foraminiferal fauna also includes a few specimens of N. pachyderma (dextral), Turborotalia quinqueloba and Globigerina bulloides. The benthic foraminiferal fauna is correlated with Subzone 16x of King (1989, North Sea) and Zone NSR 13 of Gradstein & Bäckström (1996, North Sea and Haltenbanken area). The planktonic foraminiferal fauna is correlated with the Neogloboquadrina pachyderma (sinistral) Zone of Spiegler & Jansen (1989). An encrusted form of N. pachyderma (sinistral) has its first frequent occurrence, in the Norwegian Sea, at 1.8 Ma (Spiegler & Jansen 1989).

Sr isotope stratigraphy

Calcareous benthic foraminifera from the interval 570-580 m were analysed for Sr isotopes. The obtained 87Sr/86Sr ratios gave an age of 32.6 Ma (early part of Early Oligocene, Table 1, Fig. 1).

Well 31/3-1

Litho. Unit Sample (DC) Corrected 87/86Sr 2S error Age (Ma) Analysed fossil species
Hordaland Gr 570-580 m 0.707876 0.000012 32.57 50 tests of Stilostomella longiscata, Stilostomella adolphina, Stilostomella hirsute, Cibicides aknerianus, P. Bulloides

Table 1: Strontium isotope data from well 31/3-1. The sample were analysed at the University of Bergen. Sr ratio was 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.


Lower Oligocene (580-540 m, Hordaland Group)

The unit consists primarily of silty mudstones (Fig. 1).

Pleistocene (540-500 m, Nordland Group)

The samples of the Pleistocene unit contain clay-rich diamictons with sand, silt and common pebbles. The pebbles are mainly of crystalline rock (Fig. 1) and are interpreted to have been ice-rafted.


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. & Rundberg, Y., 2001: Late Cainozoic stratigraphy of the Tampen area (Snorreand Visund fields) in the northern North Sea, with emphasis on the chronology of earlyNeogene 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

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.

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.