Lower and Upper Oligocene in well 11/10-1

Based on analyses of calcareous benthic and planktonic foraminifera and Sr isotopes in well 11/10-1 (57º0’46.00’’N, 06º10’04.70’’E, Map 1). We recorded a 457.2 m-thick column of Lower Oligocene deposits and 152.4 m with Upper Oligocene sediments. The base of the Lower Oligocene was not investigated. The top of the Upper Oligocene are not sampled. The units were investigated with 27 ditch-cutting samples taken at intervals varying from approximately 60 to 3.5 m (Fig. 1).

Well summary figure for well 11/10-1

Biostratigraphy

Lower Oligocene (914.4-457.2 m, Hordaland Group and the lowermost part of Vade Formation)

The dating of the lower part of the unit (914.4-762 m) is based on benthic calcareous foraminifera of the Rotaliatina bulimoides assemblage together with three Sr isotope ages (Fig. 1). In addition to the nominate species the benthic foraminiferal fauna also includes T. alsatica, G. soldanii mamillata, Ceratobulimina contraria, Cibicides mexicanus and Frondicularia budensis. Planktonic foraminifera include G. praebulloides.

The middle part of the unit (762-487.7 m) contains very few foraminiferal index fossils. The dating of this part is mainly based on Sr isotope ages (Fig. 1)

The dating of the upper part of the Lower Oligocene unit (487.7-457.2 m) is based on benthic calcareous foraminifera of the Pararotalia canui assemblage together with two Sr isotope ages (Fig. 1). The planktonic foraminiferal fauna includes G. praebulloides and G. angustiumbilicata.

The benthic foraminiferal assemblage in the whole of the Lower Oligocene unit can probably be correlated with Subzone NSB 7b of King (1989) and Zone NSR 7B of Gradstein & Bäckström (1996) from the North Sea.

Upper Oligocene (457.2-304.8 m, Vade Formation)

Benthic calcareous foraminifera of the Astigerina guerichi guerichi – Gyroidina soldanii girardana assemblage together with six Sr isotope ages give a Late Oligocene age to this unit (Fig. 1). The benthic foraminiferal fauna also includes E. subnodosum and Ehrenbergina serrata. Planktonic foraminifera include a few specimens of G. ciperoensis. The benthic foraminiferal assemblage is correlated with NSB 8 of King (1989) and probably Zone NSR 8A of Gradstein & Bäckström (1996) from the North Sea.

Sr isotope stratigraphy

Calcareous benthic foraminifera were analysed for Sr isotopes at 914.4 m, 883.9-853.5 m, 823 m, 640.1m, 472.4 m, 457.2 m and 304.8 m (two samples). Mollusc fragments were analysed for Sr isotopes at 640.1 m, 518.2 m, 502.9 m, 487.7 m, 426.7 m and 365.8 m (two samples at each depth). For the part 914.4-457.2 m, for which the foraminiferal correlation gave an Early Oligocene age, the obtained ⁸⁷Sr/⁸⁶Sr ratios also gave Early Oligocene ages except for two mollusc samples at 640.1 m which probably were caved. The other samples show a pattern of older ages in the lower part of the unit and younger ages in the upper part (Table 1, Fig. 1).

For the part 457.2-304.8 m, for which the foraminiferal correlation gave a Late Oligocene age, the obtained ⁸⁷Sr/⁸⁶Sr ratios also gave Late Oligocene ages except for one of two mollusc samples at 426.7 m. This gave a late Early Oligocene age; the analysed mollusc fragments might be reworked, but the discrepancy is within the precision of the method (Table 1, Fig. 1).

Well 11/10-1

Litho. Unit	Sample (DC)	Corrected 87/86Sr	2S error	Age (Ma)	Comments	Analysed fossils
Vade Fm	304.8 m	0.708177	0.000007	25.26		Nine large tests of Elphidium subnodosum
Vade Fm	304.8 m	0.708118	0.000008	26.48		Ten large tests of E. subnodosum, Astigerina guerichi guerichi and Guttulina sp.
Vade Fm	365.8 m	0.708091	0.000008	27.23		One mollusc fragment
Vade Fm	365.8 m	0.708101	0.000007	26.93		One mollusc fragment
Vade Fm	426.7 m	0.708008	0.000009	29.39		One mollusc fragment
Vade Fm	426.7 m	0.708227	0.000009	24.48	Caved?	One mollusc fragment
Vade Fm	457.2 m	0.707979	0.000008	30.11		Approximately 30 fragments of Bryozoa and Echinoderma
Vade Fm	472.4 m	0.708039	0.000012	28.61		50 small tests of Pararotalia canui, G. angustiumbilicata and G. praebulloides
Hordal. Gr	487.7 m	0.708015	0.000008	29.22		One mollusc fragment
Hordal. Gr	487.7 m	0.707991	0.000007	29.81		One mollusc fragment
Hordal. Gr	502.9 m	0.707991	0.000008	29.81		One mollusc fragment
Hordal. Gr	502.9 m	0.707973	0.000007	30.27		One mollusc fragment
Hordal. Gr	518.2 m	0.707951	0.000008	30.87		One mollusc fragment
Hordal. Gr	518.2 m	0.707937	0.000008	31.24		One mollusc fragment
Hordal. Gr	640.1 m	0.707938	0.000007	31.22		Nine large tests of G. soldanii girardana, Nodosaria sp. A (King, 1989), Lagena sp., Alabamina scitula
Hordal. Gr	640.1 m	0.708077	0.000008	27.69	Caved?	One mollusc fragment
Hordal. Gr	640.1 m	0.708250	0.000008	24.10	Caved?	One mollusc fragment
Hordal. Gr	823.0 m	0.707875	0.000008	32.59		19 tests of Stilostomella spp., T. alsatica, R. bulimoides, Ceratobulimina contraria, G. soldanii girardana, Cibicides mexicanus
Hordal. Gr	853.4-883.9 m	0.707871	0.000008	32.66		26 tests of Stilostomella spp., Bolivina cf. antique, T. alsatica, R. bulimoides, Hoeglundina elegans, G. soldanii girardana,
Hordal. Gr	914.4 m	0.707871	0.000008	32.66		Twelve tests of G. soldanii mamillata, H. elegans, G. soldanii girardana, Stilostomella spp., G. subglobosa, Frondicularia budensis, T. alsatica, C. contraria, G. praebulloides

Table 1: Strontium isotope data from well 11/10-1. 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

Lower Oligocene (main part, 914.4 to approximately 500 m, Hordaland Group)

The ditch-cutting samples in this part are dominated by clay, but some sand (dominantly quartzose, minor biotitic and glauconitic) and silt are also present. Sand is most common in the upper part of the unit. In parts of the section we have also recorded mollusc fragments and limestone (Fig. 1).

Uppermost part of Lower Oligocene to Upper Oligocene (approximately 500-304.8 m, Vade Formation)
Sand dominates the samples in this unit. Some clay and silt is recorded in the lower part, but the sediments are coarsening upwards and the upper part consists of coarse, mainly quartzose sand. Minor glauconite and biotite are recorded in the lower part, and mollusc fragments are recorded throughout (Fig. 1).

Uppermost part of Lower Oligocene to Upper Oligocene (approximately 500-304.8 m, Vade Formation)

Sand dominates the samples in this unit. Some clay and silt is recorded in the lower part, but the sediments are coarsening upwards and the upper part consists of coarse, mainly quartzose sand. Minor glauconite and biotite are recorded in the lower part, and mollusc fragments are recorded throughout (Fig. 1).

References

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.