Upper Oligocene to ?Upper Pliocene in well 35/3-1

Based on analyses of benthic and planktonic foraminifera, pyritised diatoms and Sr isotopes in well 35/3-1 (60º50’41.89’’N, 03º43’41.36’’E, Map 1), we recorded a 280 m-thick column of Upper Oligocene sediments, an approximately 75 m thick undefined unit of probably Early Miocene age and approximately 95 m with probably Upper Pliocene deposits. The base of the Upper Oligocene and the top of the ?Upper Pliocene were not investigated. The units were investigated with 46 ditch-cutting samples at ten metre intervals (Fig. 1).

Well summary figure for well 35/3-1

Biostratigraphy

Upper Oligocene (1100-820 m, Hordaland Group)

Benthic calcareous foraminifera of the Elphidium subnodosum assemblage and Turrilina alsatica 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. Between the Elphidium subnodosum and Turrilina alsatica assemblage there is a 110 m interval which is barren of foraminifera and other index fossils (Fig. 1). In addition to the nominate species, the Elphidium subnodosum assemblage also includes T. gracilis, G. subglobosa, R. arnei, G. soldanii mamillata (one specimen) and G. soldanii girardana. The Turrilina alsatica assemblage also includes T. gracilis (common), T. gracilis var. A, C. tenellus, B. elongata, N. granosum, E. subnodosum, R. arnei, R. bulimoides and G. soldanii mamillata. R. bulimoides and G. soldanii mamillata are probably reworked. Abundant radiolaria and sponge spicules are recorded, but no in situ planktonic foraminifera. The benthic foraminiferal assemblages are correlated with Zone NSB 8 and the diatom assemblage is correlated with Subzone NSP 9c of King (1989, North Sea).

Undefined interval (820 to approximately 745 m, Hordaland Group)

No index fossils are recorded in this interval. The unit is barren of in situ foraminifera and pyritized diatoms, but it is similar to the underlying units in that it is rich in radiolaria and sponge spicules. Lithologically it is also similar to the underlying units, and it is likely that there is no sedimentological break between this unit and the underlying unit. Sr analyses gave an age close to the Oligocene/Miocene boundary for the upper part of the underlying unit. Consequently, an Early Miocene age is likely for the undefined interval of 820-745 m.

?Upper Pliocene (approximately 745 to 650 m, ?Nordland Group)

The benthic foraminifera of the Cibicides grossus-Elphidiella hannai assemblage are all typical Late Pliocene to Pleistocene forms including Cibicides grossus and Elphidiella hannai which are index species for the Late Pliocene (on the time scale of Berggren et al. 1995) in the North Sea area (King, 1989). In addition to the nominate species, the fauna also includes Nonion affine, Cibicides lobatulus, Elphidium excavatum, Elphidium albiumbilicatum, Cassidulina teretis (common) and Islandiella norcrossi. The planktonic foraminiferal fauna of the Neogloboquadrina atlantica (sinistral)-Globigerina bulloides assemblage also includes Neogloboquadrina pachyderma (sinistral) and few specimens of Globigerinita glutinata. We correlate the benthic foraminiferal fauna with Subzone NSB 15a of King (1989, North Sea) and Zone NSR 12 of Gradstein & Bäckström (1996, North Sea and Haltenbanken area). Spiegler & Jansen (1989) described a N. atlantica (sinistral) Zone from the Vøring Plateau (Norwegian Sea) from Upper Miocene to Upper Pliocene deposits. The LAD of N. atlantica (sinistral) in that area is approximately 2.4 Ma. A G. bulloides Zone is described from the North Atlantic (DSDP Leg 94) in Pliocene sediments as young as 2.2 Ma (Weaver & Clement 1986). On the Vøring Plateau, G. bulloides is common in Pliocene deposits older than 2.4 Ma (Spiegler & Jansen 1989). G. bulloides is also common in the warmest interglacials of the last 0.5 Ma in the North Atlantic (Kellogg 1977).

Both the benthic and the planktonic foraminiferal faunas are typical of Late Pliocene deposits as recorded in other wells in the North Sea area. However, the sediments are not so typical (see below) and may point to an older age of Miocene to Early Pliocene. If that is the case, all the recorded foraminifera may be caved and the unit may be barren of in situ forms.

Sr isotope stratigraphy

Two samples based on calcareous foraminiferal tests and 22 samples based on mollusc fragments from 11 depths were analysed for Sr isotopes. The mollusc samples were taken from the sandy interval of 1100-1000 m, and all gave very similar ages within the range of 27.5-25.2 Ma (early Late Oligocene). The samples based on foraminiferal tests were taken from 830 and 820 m and gave ages of 24.1 and 23.4 Ma, respectively (close to the Oligocene/Miocene boundary at 23.8 Ma, Table 1, Fig. 1).

Well 35/3-1

Litho. Unit	Sample (DC)	Corrected 87/86Sr	2S error	Age (Ma)	Analysed fossils
Hordaland Gr	820 m	0.708292	0.000008	23.35	30 tests of T. alsatica, T. gracilis, T. gracilis var. A, R. bulimoides, C. tenellus
Hordaland Gr	830 m	0,708252	0.000009	24.08	49 tests of T. alsatica, T. gracilis, T. gracilis var. A, R. bulimoides, C. tenellus
Hordal. Gr	1000 m	0.708092	0.000008	27.20	One mollusc fragment
Hordal. Gr	1000 m	0.708155	0.000009	25.68	One mollusc fragment
Hordal. Gr	1010 m	0.708128	0.000009	26.26	One mollusc fragment
Hordal. Gr	1010 m	0.708104	0.000008	26.84	One mollusc fragment
Hordal. Gr	1020 m	0.708183	0.000008	25.15	One mollusc fragment
Hordal. Gr	1020 m	0.708083	0.000008	27.47	One mollusc fragment
Hordal. Gr	1030 m	0.708119	0.000009	26.45	One mollusc fragment
Hordal. Gr	1030 m	0.708082	0.000009	27.50	One mollusc fragment
Hordal. Gr	1040 m	0.708111	0.000009	26.65	One mollusc fragment
Hordal. Gr	1040 m	0.708085	0.000008	27.41	One mollusc fragment
Hordal. Gr	1050 m	0.708124	0.000008	26.34	One mollusc fragment
Hordal. Gr	1050 m	0.708100	0.000009	26.96	One mollusc fragment
Hordal. Gr	1060 m	0.708149	0.000009	25.81	One mollusc fragment
Hordal. Gr	1060 m	0.708110	0.000009	26.67	One mollusc fragment
Hordal. Gr	1070 m	0.708086	0.000008	27.38	One mollusc fragment
Hordal. Gr	1070 m	0.708111	0.000007	26.65	One mollusc fragment
Hordal. Gr	1080 m	0.708089	0.000007	27.29	One mollusc fragment
Hordal. Gr	1080 m	0.708114	0.000007	26.57	One mollusc fragment
Hordal. Gr	1090 m	0.708139	0.000009	26.01	One mollusc fragment
Hordal. Gr	1090 m	0.708112	0.000008	26.62	One mollusc fragment
Hordal. Gr	1100 m	0.708083	0.000008	27.47	One mollusc fragment
Hordal. Gr	1100 m	0.708091	0.000007	27.23	One mollusc fragment

Table 1: Strontium isotope data from well 35/3-1. 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

Upper Oligocene (lower part, 1100 to approximately 1000 m, Hordaland Group)

Coarse to medium sand dominates this part. Quartz dominates with minor glauconite and mica. The unit is quite rich in molluscs fragments (Fig. 1).

Upper Oligocene (middle part, approximately 1000 to approximately 850 m, Hordaland Group)

The deposits in this unit are mainly fine grained with small portions of sand (Fig. 1).

Upper Oligocene (upper part) to ?Lower Miocene (approximately 850 to approximately 745 m, Hordaland Group)

The samples in this interval are dominated by medium to fine sand with minor silt and clay. Quartz and glauconite dominate the sand fraction with minor mica (Fig. 1).

?Upper Pliocene (approximately 745 to 650 m, Nordland Group)

The Upper Pliocene unit contains mainly medium to coarse sand. Quartz is the dominant mineral, but glauconite is also very common. Some pebbles of crystalline rock are also recorded (Fig. 1). Many of the quartz grains are rust stained. We have not recorded similar sediment in Upper Pliocene sections in any other wells on the Norwegian continental shelf. However, the sediment is very similar to what we recorded in the Molo Formation in wells 6407/9-1, 6407/9-2, 6407/9-5 and 6510/2-1 on the Norwegian Sea shelf (see Map 1 and Eidvin et al., 2007). The Molo Formation in those wells is of Late Miocene to Early Pliocene and Early Miocene age. Additional seismic work should try to investigate if the Molo Formation extends so far south as the 35/3-1 area. If the section in well 35/3-1 corresponds to the Molo Formation, the recorded angular pebbles of crystalline rock (probably ice rafted) have to be caved similar to the recorded foraminifera.

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.

Eidvin, T., Bugge, T. & Smelror, M., 2007: The Molo Formation, deposited by coastal progradation on the inner Mid-Norwegian continental shelf, coeval with the Kai Formation to the west and the Utsira Formation in the North Sea. Norwegian Journal of Geology 87, 75-142. Available from the internet: http://www.npd.no/Global/Norsk/3-Publikasjoner/Forskningsartikler/Eidvin_et_al_2007.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.

Kellogg, T. B., 1977: Paleoclimatology and Paleo-oceanography of the Norwegian and Greanland Seas: The Last 450,000 years. Marine Micropalaeontology 2, 235-249.

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.

Weaver, P. P. E. & Clement, B. M., 1986: Synchronicity of Pliocene planktonic foraminiferid datums in the North Atlantic. Marine Micropalaeontology 10, 295-307.