Upper Paleocene-Lower Eocene to Pleistocene in well 36/1-2

Based on analyses of benthic and planktonic foraminifera, pyritised diatoms and Sr isotopes in well 36/1-2 (61º63'00.25''N, 04º0'52.19''E, Map 1), we recorded 10 m with Upper Paleocene-Lower Eocene sediments, 100 m with Lower Oligocene deposits, 40 m with Lower-Upper Oligocene deposits, an 180 m-thick column with Upper Oligocene sediments and 50 m with Pleistocene deposits. The base of the Upper Paleocene-Lower Eocene and the top of the Pleistocene were not investigated. The units were investigated with 38 ditch-cutting samples at ten metre intervals except between 610 and 590 m (Fig. 1).

Well summary figure for well 36/1-2

Biostratigraphy

Upper Paleocene-Lower Eocene (950-940 m, Balder Group)

Benthic agglutinated foraminifera of the Reticulophragmium amplectens assemblage give a Late Paleocene-Early Eocene age for this unit. In addition to the nominate species, the benthic foraminiferal fauna also includes Reticulophragmium sp. A and Psammosiphonella gr. discreta, and the assemblage is correlated the zones NSA 1 to NSA 3 of King (1989, North Sea).

Lower Oligocene (940-840 m, Hordaland Group)

A large number of Sr isotope analyses, based on mollusc fragments, are the main factors in dating this unit to the Early Oligocene (Fig. 1). The recorded benthic foraminifera, including A. guerichi guerichi (most common), T. alsatica and T. gracilis, can only give a general Oligocene to Lower Miocene age. No planktonic foraminifera and diatom index forms are recorded in this unit. The foraminiferal fauna is correlated with Zone NSR 7A and Zone NSR 7B of Gradstein & Bäckström (1996) and probably Zone NSB 7 of King (1989) from the North Sea. However, according to King (1989) A. guerichi guerichi did not occur in the North Sea area before the early Late Oligocene. According to Gradstein & Bäckström (1996), however, A. guerichi guerichi occurred from the early part of Early Oligocene.

Lower - Upper Oligocene (840-800 m, Hordaland Group)

The fossil assemblage in this unit is similar to the one from 940-840 m. However, the Sr isotope analyses give Early and Late Oligocene ages close to the Early/Late Oligocene boundary (Fig. 1).

Upper Oligocene (800-620 m, Hordaland Group)

Benthic foraminifera of the Elphidium subnobosum assemblage and Gyroidina soldanii girardana assemblage and pyritised diatoms of the Diatom sp. 3 assemblage together with a number of Sr isotope ages, give a Late Oligocene age for this unit (Fig. 1). The units are nearly barren of planktonic foraminifera. In addition to the nominate species, the benthic foraminiferal fauna also includes T. alsatica, G. subglobosa, T. gracilis, T. gracilis var. A and R. arnei. The Gyroidina soldanii girardana assemblage and the uppermost part of the Elphidium subnodosum assemblage contain a few specimens of R. bulimoides, but these may be reworked. The benthic foraminiferal assemblages are correlated with Zone NSB 8 of King (1989) and probably Zone NSR 8A of Gradstein & Bäckström (1996, North Sea). The diatom assemblage at 670-660 m is correlated with Subzone NSP 9c of King (1989, North Sea).

Pleistocene (620-570 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 (common), Cassidulina reniforme, Virgulina loeblichi, Islandiella norcrossi, Bulimina marginata (common) and Islandiella islandica. A few specimens of T. alsatica are also recorded in some samples, and these are reworked from Oligocene to Early Miocene deposits. The planktonic foraminiferal fauna also includes a few specimens of N. pachyderma (dextral), Globigerina bulloides and Heterohelix sp. The latter is reworked from the Upper Cretaceous. 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). The 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

The part of the well where the biostratigraphical correlations indicated an Oligocene age, were analysed with 29 samples (27 depths) based on mollusc fragments and three samples based on calcareous foraminiferal tests. Most of the samples from 930-840 m gave Early Oligocene ages, i.e. approximately 32.5-30.5 Ma in the lower part of this interval and approximately 28.5 Ma in the upper part. Two samples from the immediately underlying Upper Paleocene-Lower Eocene unit gave similar ages, but these were probably based on caved tests. The samples from 840-800 m gave ages from 28.8-27.1 Ma (latest Early to earliest Late Oligocene). The samples from 760 m and above gave ages from 28.5 to 25.2 Ma (early Late Oligocene). The two uppermost samples gave considerably younger ages and are probably based on caved mollusc fragments (Table 1, Fig. 1).

Well 36/1-2

Litho. Unit	Sample (DC)	Corrected 87/86Sr	2S error	Age (Ma Comments	Comments	Analysed fossils
Hordal. Gr.	620 m	0.709380	0.000009	undefined	Caved	One mollusc fragment
Hordal. Gr.	630 m	0.708530	0.000007	18.85	Caved	One mollusc fragment
Hordal. Gr.	640 m	0.709134	0.000008	1.1	Caved	One mollusc fragment
Hordal. Gr.	650 m	0.708050	0.000007	28.53		One mollusc fragment
Hordal. Gr.	670 m	0.708126	0.000009	26.29		One mollusc fragment
Hordal. Gr.	670-680 m	0.708148	0.000008	25.99		27 tests of E. subnodosum
Hordal. Gr.	680 m	0.708134	0.000009	26.12		One mollusc fragment
Hordal. Gr.	690 m	0.708111	0.000009	26.65		One mollusc fragment
Hordal. Gr.	700 m	0.708133	0.000008	26.14		One mollusc fragment
Hordal. Gr.	710 m	0.708099	0.000008	26.99		One mollusc fragment
Hordal. Gr.	720 m	0.708125	0.000009	26.32		One mollusc fragment
Hordal. Gr.	730 m	0.708121	0.000008	26.41		One mollusc fragment
Hordal. Gr.	740 m	0.708091	0.000008	27.23		One mollusc fragment
Hordal. Gr.	750 m	0.708179	0.000008	25.22		One mollusc fragment
Hordal. Gr.	760 m	0.708104	0.000007	26.84		One mollusc fragment
Hordal. Gr.	800 m	0.708069	0.000008	27.84		One mollusc fragment
Hordal. Gr.	800 m	0.708048	0.000007	28.38		One mollusc fragment
Hordal. Gr.	810 m	0.708095	0.000008	27.11		One mollusc fragment
Hordal. Gr.	820 m	0.708097	0.000009	27.05		One mollusc fragment
Hordal. Gr.	830 m	0.708033	0.000007	28.76		One mollusc fragment
Hordal. Gr.	840 m	0.708027	0.000008	28.91		One mollusc fragment
Hordal. Gr.	840 m	0.708046	0.000009	28.43		One mollusc fragment
Hordal. Gr.	840-850 m	0.708092	0.000009	27.44		23 tests of E. subnodosum, R.arnei, T. alsatica, A. guerichi guerichi
Hordal. Gr.	850 m	0.708049	0.000009	28.35		One mollusc fragment
Hordal. Gr.	860 m	0.708049	0.000009	28.35		One mollusc fragment
Hordal. Gr.	870 m	0.708083	0.000008	27.47		One mollusc fragment
Hordal. Gr.	880 m	0.708090	0.000008	27.50		20 tests of E. subnodosum, T. alsatica, A. guerichi guerichi
Hordal. Gr.	880 m	0.708074	0.000008	27.72		One mollusc fragment
Hordal. Gr.	890 m	0.708040	0.000009	28.58		One mollusc fragment
Hordal. Gr.	900 m	0.708018	0.000008	29.14		One mollusc fragment
Hordal. Gr.	910 m	0.707964	0.000009	30.51		One mollusc fragment
Hordal. Gr.	920 m	0.707869	0.000008	32.69		One mollusc fragment
Hordal. Gr.	930 m	0.707927	0.000008	31.49		One mollusc fragment
Hordal. Gr.	940 m	0.707965	0.000008	30.49		One mollusc fragment
Hordal. Gr.	950 m	0.707975	0.000007	30.22		One mollusc fragment

Table 1: Strontium isotope data from well 36/1-2. 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

Upper Paleocene-Lower Eocene (950-940 m, Balder/Sele Formation)

The Upper Paleocene-Lower Eocene contains silty claystone (Fig. 1).

Lower Oligocene and Lower-Upper Oligocene (940-820 m, Hordaland Group)

Coarse to medium sand dominates this part. Quartz dominates the sand fraction. The contents of glauconite and mica vary from minor to common. In the lower part of the unit, rounded, subrounded and subangular pebbles of mainly quartzite are common. This part is also quite rich in mollusc fragments (Fig. 1).

Lower-Upper Oligocene and Upper Oligocene (lower part, 820-720 m, Hordaland Group)

Clay dominates this part, but sand (quartzose, glauconitic and biotitic) is also common in parts of section (Fig. 1).

Upper Oligocene (upper main part, 720-620 m, Hordaland Group)

The samples in this part are dominated by medium to coarse sand with minor silt and clay. The sand fraction is dominated by quartz with common glauconite and minor mica. Mollusc fragments are common in parts of section (Fig. 1).

Pleistocene (620-570 m, Nordland Group)

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

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.

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.