Upper Oligocene to Upper Pliocene in well 35/11-14 S

Based on analyses of benthic foraminifera, pyritised diatoms and Sr isotopes in well 35/11-14 S (61º09’02.42’’N, 03º32’51.76’’E, Map 1), we recorded approximately 50 m with Upper Oligocene sediments, approximately 55 m with Upper Miocene deposits and approximately 125 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 20 ditch-cutting samples at ten metre intervals (Fig. 1).

Well summary figure for well 35/11-14 S

Biostratigraphy

Upper Oligocene (850 to approximately 800 m, Hordaland Group)

The greater proportion of the fossils recorded in this unit are sponge spicules (both rod-shaped and Geodia sp.). Calcareous benthic foraminifera are recorded in the lower part, and pyritised diatoms are recorded in the upper half of the unit. In the diatom assemblage the index fossil Diatom sp. 3 is recorded and the assemblage is correlated with Subzone NSP 9c of King (1989). T. Alsatica is the benthic nominate foraminifera. The foraminiferal assemblage also contains R. arnei, and is correlated with Zone NSB 8 of King (1989). According to King (1989), these zones are of latest Early Oligocene to earliest Early Miocene age. The obtained ⁸⁷Sr/⁸⁶Sr ratios from four Sr analyses from the lower part of the unit gave Late Oligocene ages (Fig. 1).

Upper Miocene (approximately 800 to approximately 745 m, Utsira Formation)

As in well 35/11-1, the whole of the Utsira Formation consists of quite coarse sand. Probably all in situ foraminifera were winnowed out during deposition, and all of the recorded species were probably caved from the Upper Pliocene. However, nine mollusc fragments are recorded throughout, and the obtained ⁸⁷Sr/⁸⁶Sr ratios from eight of these gave Late Miocene ages (Fig. 1, see below).

Upper Pliocene (approximately 745-660 m, Nordland Group)

Benthic foraminifera of the Cibicides grossus 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 fauna also includes Elphidium excavatum (common), Elphidium albiumbilicatum, Ephidiella hannai (few), Cibicides lobatulus and Haynesina orbiculare. The planktonic foraminiferal fauna is very sparse and just one specimen each of Neogloboquadrina pachyderma (dextral) and Turborotalia quinqueloba were recorded. The benthic foraminiferal fauna is correlated with Subzone NSB 15a of King (1989, North Sea) and NSR 12 of Gradstein & Bäckström (1996, North Sea and Haltenbanken area).

Sr isotope stratigraphy

Calcareous benthic foraminifera from 850 and 840 m (Hordaland Group) were analysed for Sr isotopes (two samples from each level). The obtained ⁸⁷Sr/⁸⁶Sr ratios gave ages from 26.5 to 25.0 Ma (Late Oligocene) for the four samples which support the biostratigraphical correlations (Table 1, Fig. 1). Mollusc fragments from 780, 770, 760 and 750 m (2-3 samples from each level, Utsira Formation) were also analysed for Sr isotopes. Except for one sample from 750 m which represents a caved mollusc fragment from the Upper Pliocene unit, the obtained ⁸⁷Sr/⁸⁶Sr ratios all gave Late Miocene ages from 9.3 to 5.9 Ma. However, some of the younger ages, in the lower part of unit, represent caved fragments from a higher level in the Utsira Formation (see Table 1 and Fig. A1).

Borehole 35/11-14 S

Litho. Unit	Sample (DC)	Corrected 87/86Sr	2S error	Comments	Age (Ma)	Analysed fossils
Utsira Fm	750 m	0.708988	0.000008		5.98	One mollusc fragment
Utsira Fm	750 m	0.709077	0.000009	Caved	2.27	One mollusc fragment
Utsira Fm	760 m	0.708976	0.000008	Caved	6.13	One mollusc fragment
Utsira Fm	760 m	0.708969	0.000009		6.29	One mollusc fragment
Utsira Fm	770 m	0.708905	0.000007		9.34	One mollusc fragment
Utsira Fm	770 m	0.708943	0.000008		7.25	One mollusc fragment
Utsira Fm	770 m	0.708975	0.000009	Caved	6.14	One mollusc fragment
Utsira Fm	780 m	0.708993	0.000008	Caved	5.92	One mollusc fragment
Utsira Fm	780 m	0.708965	0.000007	Caved	6.42	One mollusc fragment
Hordaland Gr	840 m	0.708166	0.000009		25.47	27 tests of T. Alsatica, T. Gracilis, R. arnei
Hordaland Gr	840 m	0.708118	0.000009		26.48	Approximately 33 tests of N. affine
Hordaland Gr	850 m	0.708195	0.000008		24.85	Approximately 37 tests of T. Alsatica, T. Gracilis, R. arnei, C. hiltermanni
Hordaland Gr	850 m	0.708170	0.000008		25.39	Approximately 27 tests of N. affine

Table 1: Strontium isotope data from borehole 35/11-14 S. The samples were analysed at the University in 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

Upper Oligocene (850 to approximately 800 m, Hordaland Group)

The Upper Oligocene deposits consist mainly of silty mudstones (Fig. 1).

Upper Miocene (approximately 800 m to approximately 740 m, Utsira Formstion)

This unit consists of quite coarse quartzose sand, but somewhat less coarse than the even more proximal well 35/11-1 to the northeast (Map 1). Minor glauconitic sand is also recorded in parts of section (Fig. 1).

Upper Pliocene (approximately 740 to 660 m, Nordland Group)

The Upper Pliocene unit contains a quartzose sand-rich diamicton with silt, clay and common pebbles (Fig. 1). The pebbles are mainly of crystalline rock and are interpreted to have been ice-rafted. According to Fronval & Jansen (1996), on the Vøring Plateau (Norwegian Sea) there is a marked increase in the supply of ice-rafted material after about 2.75 Ma which reflects the expansion of the northern European glaciers. The maximum age for the Upper Pliocene unit is therefore considered to be 2.75 Ma, belonging mainly to the Gelasian Stage.

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.

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.

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.