Lower and Upper Oligocene in well 9/12-1

Based on analyses of calcareous benthic and planktonic foraminifera and Sr isotopes in well 9/12-1 (57º11’40.00’’N, 04º57’21.00’’E, Map 1), we recorded 146.3 m with Oligocene deposits (609.6-463.3 m). The lowermost sample is given an Early Oligocene age, and the rest of the investigated column is given a Late Oligocene age. The interval 463.3-414.5 m was also investigated, but this unit is barren of foraminifera. This is probably due to post-depositional dissolution since the biostratigraphical consultant commissioned by the oil company reported calcareous benthic foraminifera also from this unit. The assemblage they reported is similar to the one we recorded from the unit immediately below, and consequently these deposits are also probably of Late Oligocene age. The base of the Lower Oligocene was not investigated, and the top of the Upper Oligocene was not sampled. The units were investigated with 17 ditch-cutting samples at ten metre intervals (Fig. 1).

Well summary figure 9/12-1, fig. 1

Biostratigraphy

Lower Oligocene (609.6 m, Hordaland Group)

Benthic foraminifera of the Gyroidina soldanii mamillata assemblage together with a Sr isotope age give a late Early Oligocene age for this sample (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains Siphonina reticulata (common), Karriella siphonella (agglutinated) and Stilostomella spinescens. No planktonic foraminifera are recorded. The foraminiferal assemblage is correlated with Subzone NSB 7b of King (1989) and Zone NSR 7B of Gradstein & Bäckström (1996) from the North Sea.

Upper Oligocene (609.6- 463.3 m, Hordaland Group)

Benthic foraminifera of the Astigerina guerichii guerichii – Gyroidina soldanii girardana assemblage together with a number of Sr isotope ages give a Late Oligocene age for this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal assemblage also contains R. arnei, Ehrenberina serrata, Cibicides aknerianus and T. alsaticsa. The plantonic foraminiferal fauna includes G. angustiumbilicata, G. ciperoensis and G. praebulloides. 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 from 609.6 m, 573 m, 548.6 m, 536.5 m, 524.3 m, 512.1 m and 487.6 m were analysed for Sr isotopes. The obtained 87Sr/86Sr ratios all gave earliest Late Oligocene ages except for the sample at 609.6 m which gave a latest Early Oligocene age and the sample 573 m which gave an age exactly at the Early/Late Oligocene boundary. Analyses of mollusc fragments from 512.1 m (three samples), 499.9 m (three samples), 487.6 m, 475.5 m (three samples) and 463.3 m (two samples) all gave early Late Oligocene ages, except for 487.6 m which gave a late Early Oligocene age probably due to the fact that the analysed fragment was reworked (Table 1, Fig. 1).

Well 9/12-1

Litho. Unit	Sample (DC)	Corrected 87/86Sr	2S error	Age (Ma)	Comments	Analysed fossils
Hordal. Gr	463.3 m	0.708135	0.000008	26.10		One mollusc fragment
Hordal. Gr	463.3 m	0.708150	0.000008	25.79		One mollusc fragment
Hordal. Gr	475.5 m	0.708152	0.000008	25.75		One mollusc fragment
Hordal. Gr	475.5 m	0.708145	0.000008	25.89		One small mollusc
Hordal. Gr	475.5 m	0.708140	0.000007	25.99		One mollusc fragment
Hordal. Gr	487.6 m	0.708103	0.000008	26.87		One mollusc fragment
Hordal. Gr	487.6 m	0.708159	0.000008	25.61		Approximately 20 tests of A. guerichi guerichi, A. guerichi staeschei
Hordal. Gr	499.9 m	0.708080	0.000008	27.55		One mollusc fragment
Hordal. Gr	499.9 m	0.708128	0.000009	26.25		One mollusc fragment
Hordal. Gr	499.9 m	0.708111	0.000008	26.65		One mollusc fragment
Hordal. Gr	512.1 m	0.708231	0.000008	24.42		Approximately 15 tests of A. guerichi guerichi, A. guerichi staeschei
Hordal. Gr	512.1 m	0.708136	0.000008	26.08		One mollusc fragment
Hordal. Gr	512.1 m	0.708113	0.000007	26.60		One mollusc fragment
Hordal. Gr	512.1 m	0.708114	0.000008	26.57		One mollusc fragment
Hordal. Gr	524.3 m	0.708092	0.000008	27.20		Approximately 35 tests of A. guerichi guerichi, A. guerichi staeschei
Hordal. Gr	536.5 m	0.708075	0.000008	27.69		Approximately 23 tests of A. guerichi guerichi, A. guerichi staeschei
Hordal. Gr	548.6 m	0.708103	0.000007	26.87		Approximately 55 tests of A. guerichi guerichi, A. guerichi staeschei
Hordal. Gr	573.0 m	0.708042	0.000007	28.53	Reworked?	Approximately 40 tests of A. guerichi guerichi, A. guerichi staeschei
Hordal. Gr	609.6 m	0.708037	0.000007	28.66		Ten tests of Siphonina reticulata

Table 1: Strontium isotope data from well 9/12-1. 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 and Upper Oligocene (609.6-536.5 m, Hordaland Group)

The ditch-cutting samples in this part are dominated by clay, but some sand (glauconitic, biotitic and quartzose) and silt are also recorded throughout (Fig. 1).

Upper Oligocene (536.5-463.3 m, Hordaland Group)

The samples in this part are also dominated by clay, but sand (glauconitic, biotitic and quartzose) are slightly more common. This part is also rich in mollusc fragments. Lignite coal is also recorded in most samples (Fig. 1).

Undefined interval (Upper Oligocene? 463.3-414.5 m, Hordaland Group)

Clay is most common in this part, but it is also rich in glauconitic sand (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.