Upper Oligocene to Upper Pliocene in well 30/5-2

Based on analyses of benthic and planktonic foraminifera, Bolboforma, pyritised diatoms and Sr isotopes in well 30/5-2 (60º30'16.30''N, 02º38'12.71''E, Map 1), we recorded 160 m with Upper Oligocene sediments, 100 m with Lower Miocene deposits, a 240 m-thick column with Middle Miocene sediments, a 60 m undefined unit, 20 m with Upper Miocene sediments and 180 m with Upper Pliocene deposits. The base of the Upper Oligocene and the top of the Upper Pliocene are not investigated. The units are investigated with 53 ditch-cutting samples at mostly ten metre intervals. However, in some parts, where stored samples are depleted to the archive limit, the intervals are 20 metres or more, and between 760 and 700 m there are no samples (Figs. 1 and 2).

Well summary figure for well 30/5-2, fig 1

Well summary figure for well 30/5-2, fig 2

Biostratigraphy

Upper Oligocene (1260-1100 m, Hordaland Group)

Benthic foraminifera of the Gyroidina soldanii girardana assemblage in the middle part of the unit together with one Sr isotope age of 23.6 Ma (close to the Late Oligocene/Early Miocene boundary) at 1100 m (uppermost sample) indicate a Late Oligocene age for this unit (Fig. 1). The benthic foraminiferal fauna is correlated with Zone NSB 8 of King (1989). A palynological investigation of the same well shows the occurrence of Late Oligocene dinoflagellates up till 1090 m (K. Dybkjær, personal communication).

Lower Miocene (1100-1000 m, Hordaland Group and Skade Formation)

Benthic foraminifera of the Uvigerina tenuipustulata assemblage, planktonic foraminifera of the Globorotalia zealandica-Globigerina woodi assemblage together with a large number of Sr isotope ages, give an Early Miocene age to this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also includes A. guerichi staeshei, T. gracilis, B. elongata and T. gracilis var. A. The Globorotalia zealandica-Globigerina woodi assemblage also includes G. praebulloides and G. angustiumbilicata. The Uvigerina tenuipustulata assemblage is correlated with Zone NSB 10 of King (1989, North Sea) and Zone NSR 8B and lower part of Zone NSR 9A of Gradstein & Bäckström (1996, North Sea). The Globorotalia zealandica-Globigerina woodi assemblage is correlated with Zone NSP 11 of King (1983, 1989) and Zone NSR 8B and the lower part of Zone NSR 9A of Gradstein & Bäckström (1996) from the North Sea.

Middle Miocene (1000-760 m, Skade Formation (uppermost part) and Nordland Group)

Bolboforma of the Bolboforma badenensis-Bolboforma reticulata assemblage and benthic foraminifera of the Bulimina elongata assemblage, together with a large number of Sr isotope ages, give a Middle Miocene age to this unit (Figs. 1 and 2). In addition to the nominate species, the benthic foraminiferal assemblage also includes Elphidium antoninum, A. guerichi staeshei, T. gracilis and T. gracilis var. A. T. alsatica, G. soldanii girardana, U. tenuipustulata and Nonion granosum are also recorded sporadically throughout. T. alsatica and G. soldanii girardana are probably reworked from the Upper Oligocene and U. tenuipustulata and N. granosum are probably reworked from the Lower Miocene. The planktonic foraminiferal fauna includes a few specimens of G. praebulloides and G. quadrilobatus triloba. Spiegler & Müller (1992) described a B. badenensis Zone and a B. reticulata Zone from the North Atlantic in deposits with an age of slightly more than 14 to 11.7 Ma. The in situ benthic foraminiferal fauna is correlated with Zone NSB 11 and Zone NSB 12 of King (1989, North Sea). A palynological investigation of the same well gives a Late Miocene age for the section from 840 to 760 m (K. Dybkjær, personal communication).

Undefined interval (760-700 m, Utsira Formation?)

No samples were available from this interval (Fig. 2).

Upper Miocene (700-680 m, Utsira Formation)

Bolboforma of the Bolboforma metzmacheri assemblage, benthic foraminifera of the Globocassidulina subglobosa assemblage and the occurrence of the planktonic foraminifera Neogloboquadrina atlantica (dextral), together with some Sr isotope ages, give a Late Miocene age to this unit (Fig. 2). The benthic foraminiferal fauna also includes S. bulloides, P. advena, C. dutemplei and E. pygmeus. A B. metzmacheri Zone is described from sediments with an age of 10.0-8.7 Ma from the North Atlantic and the Vøring Plateau (Spiegler & Müller 1992, Müller & Spiegler 1993). The benthic foraminiferal assemblage is correlated with Zone NSB 13 of King (1989, North Sea). The Neogloboquadrina atlantica (dextral) assemblage is correlated with the Late Miocene lower N. atlantica (dextral) Zone on the Vøring Plateau (Spiegler & Jansen 1989).

Upper Pliocene (680-500 m, Nordland Group)

Benthic foraminifera of the Cibicides grossus–Elphidiella hannai assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) assemblage and Globigerina bulloides assemblage give a Late Pliocene age (on the time scale of Berggren et al. 1995) for this unit (Fig. 2). 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). 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).

Sr isotope stratigraphy

Sixty samples with mollusc fragments from 36 depths were analysed for Sr isotopes (Figs. 1 and 2, Table 1). Of the 14 samples taken from the unit which the biostratigraphical correlation indicates a Late Oligocene age, only one sample (1100 m) gave a similar age (23.6 Ma, close to the Oligocene/Miocene boundary). All the other samples gave Early and Mid Miocene ages. The mollusc fragments analysed in these samples are most likely caved from the sandy Skade Formation and the sandy sections in the Middle Miocene which are both quite rich in molluscs. The fine-grained Oligocene unit is probably sparse on in situ mollusc fragments.

Fourteen samples were also taken from the unit which biostratigraphical correlation gave an Early Miocene age (Fig. 1, Table 1). Most of these gave an Early Miocene age (17.5-16.1 Ma) and support the biostratigraphical correlations. Several samples gave Middle Miocene ages (15.1-13.9 Ma) and one sample gave a Pleistocene age (1.3 Ma). All of these are probably caved.

Twenty-two samples were taken from the unit which biostratigraphical correlation gave a Mid Miocene age (Figs. 1 and 2, Table 1). Nearly all of Sr analyses also gave Mid Miocene ages (15.7-11.4 Ma) and show consistent increasing ages with depth. In the upper part one sample gave a clear Late Miocene age (7.5 Ma) and is probably caved. Some samples also show ages slightly younger than the Middle/Early Miocene boundary, but these small discrepancies are within the precision of the method. One sample from the middle part of the unit (19.7 Ma) is probably reworked from the Lower Miocene.

Only samples from three depths were available from the Utsira Formation which biostratigraphical correlation gave a Late Miocene age. No samples were available from the lower, main part of the formation. Ten samples were analysed from the three depths. The obtained 87Sr/86Sr ratios displayed much scatter. Five samples gave Late Pliocene and Pleistocene ages (3.7-0.2 Ma) and are obvious caved. Two samples gave Middle Miocene ages (12.4-11.6 Ma) and are probably reworked. Three samples gave ages close to the Miocene/Pliocene boundary (5.2-4.9 Ma) and support the biostratigraphical correlations within the precision of the method (Fig. 2, Table 1).

Well 30/5-2

Litho. Unit	Sample (DC)	Corrected 87/86Sr	2S error	Age (Ma)	Comments	Analysed fossils
Utsira Fm	680 m	0.709041	0.000008	4.86		One mollusc fragment
Utsira Fm	680 m	0.709079	0.000009	2.21	Caved	One mollusc fragment
Utsira Fm	680 m	0.709059	0.000007	3.68		One mollusc fragment
Utsira Fm	690 m	0.708939	0.000009	5.16		One mollusc fragment
Utsira Fm	690 m	0.709027	0.000008	5.27		One mollusc fragment
Utsira Fm	690 m	0.708844	0.000008	11.57	Reworked	One mollusc fragment
Utsira Fm	690 m	0.709168	0.000009	0.212	Caved	One mollusc fragment
Nordland Gr	700 m	0.709070	0.000008	2.52	Caved	One mollusc fragment
Nordland Gr	700 m	0.708825	0.000010	12.44	Reworked	One mollusc fragment
Nordland Gr	700 m	0.709082	0.000007	2.12	Caved	One mollusc fragment
Nordland Gr	760 m	0.708832	0.000008	12.16		One mollusc fragment
Nordland Gr	760 m	0.708929	0.000008	7.47	Caved	One mollusc fragment
Nordland Gr	770 m	0.708834	0.000008	12.55		One mollusc fragment
Nordland Gr	770 m	0.708877	0.000008	10.44		One mollusc fragment
Nordland Gr	800 m	0.708856	0.000009	11.41		One mollusc fragment
Nordland Gr	800 m	0.708842	0.000009	11.61		One mollusc fragment
Nordland Gr	830 m	0.708867	0.000008	11.03		One mollusc fragment
Nordland Gr	830 m	0.708839	0.000009	11.80		One mollusc fragment
Nordland Gr	840 m	0.708483	0.000009	19.74	Reworked	One mollusc fragment
Nordland Gr	880 m	0.708858	0.000008	11.06		One mollusc fragment
Nordland Gr	880 m	0.708857	0.000009	11.10		One mollusc fragment
Nordland Gr	880 m	0.708845	0.000009	11.52		One mollusc fragment
Nordland Gr	900 m	0.708811	0.000009	13.26		One mollusc fragment
Nordland Gr	900 m	0.708816	0.000008	13.03		One mollusc fragment
Nordland Gr	910 m	0.708845	0.000009	11.52	Caved	One mollusc fragment
Nordland Gr	920 m	0.708783	0.000009	14.95		One mollusc fragment
Nordland Gr	930 m	0.708864	0.000024	10.89	Caved	One mollusc fragment
Nordland Gr	940 m	0.708768	0.000009	15.43		One mollusc fragment
Nordland Gr	950 m	0.708788	0.000008	14.74		One mollusc fragment
Nordland Gr	960 m	0.708805	0.000009	13.59		One mollusc fragment
Nordland Gr	970 m	0.708757	0.000009	15.72		One mollusc fragment
Nordland Gr	980 m	0.708793	0.000009	14.51		One mollusc fragment
Skade Fm	1000 m	0.708819	0.000009	12.89	Caved	One mollusc fragment
Skade Fm	1000 m	0.708801	0.000008	13.95	Caved?	One mollusc fragment
Skade Fm	1010 m	0.708643	0.000008	17.48		One mollusc fragment
Skade Fm	1020 m	0.708723	0.000008	16.43		Three small mollusc fragments
Skade Fm	1030 m	0.708778	0.000008	15.12		One mollusc fragment
Skade Fm	1040 m	0.708739	0.000008	16.13		One mollusc fragment
Skade Fm	1040 m	0.708794	0.000009	14.40		One mollusc fragment
Skade Fm	1040 m	0.708689	0.000009	16.94		One mollusc fragment
Hordal. Gr	1050 m	0.708650	0.000009	17.41		One mollusc fragment and two Lenticulina sp.
Hordal. Gr	1060 m	0.709114	0.000008	1.34	Caved	One mollusc fragment
Hordal. Gr	1060 m	0.708671	0.000008	17.16		One mollusc fragment
Hordal. Gr	1070 m	0.708676	0.000009	17.11		One mollusc fragment
Hordal. Gr	1080 m	0.708790	0.000008	14.66	Caved	One mollusc fragment
Hordal. Gr	1090 m	0.708784	0.000009	14.91	Caved	Two mollusc fragment
Hordal. Gr	1100 m	0.708280	0.000009	23.59		One mollusc fragment
Hordal. Gr	1100 m	0.708827	0.000008	12.49	Caved	One mollusc fragment
Hordal. Gr	1100 m	0.708819	0.000009	12.89	Caved	One mollusc fragment
Hordal. Gr	1110 m	0.708793	0.000009	14.51	Caved	One mollusc fragment
Hordal. Gr	1120 m	0.708788	0.000008	14.74	Caved	One mollusc fragment
Hordal. Gr	1150 m	0.708805	0.000009	13.59	Caved	One mollusc fragment
Hordal. Gr	1150 m	0.708795	0.000009	14.41	Caved	One mollusc fragment
Hordal. Gr	1150 m	0.708783	0.000009	14.95	Caved	One mollusc fragment
Hordal. Gr	1200 m	0.708694	0.000009	16.87	Caved	One mollusc fragment
Hordal. Gr	1200 m	0.708635	0.000008	17.57	Caved	One mollusc fragment
Hordal. Gr	1220 m	0.708802	0.000009	13.88	Caved	One mollusc fragment
Hordal. Gr	1220 m	0.708657	0.000009	17.33	Caved	One mollusc fragment
Hordal. Gr	1240 m	0.708647	0.000008	17.44	Caved	One mollusc fragment
Hordal. Gr	1260 m	0.708788	0.000009	17.70	Caved	One mollusc fragment

Table 1: Strontium isotope data from well 30/5-2. 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 Oligocene and lower part of Lower Miocene (1260 to approximately 1042.5 m, Hordaland Group)

The samples in this unit are dominated by clay and silt, but sand is also quite common throughout. However, some of the sand may be caved from the immediately overlying Skade Formation. Quartz dominates the sand fraction. Glauconite is quite common in most parts. Mica and sponge spicules also constitute considerable parts of the fraction (Fig. 1).

Upper part of Lower Miocene to lowermost part of Middle Miocene (approximately 1042.5 to approximately 992.5 m, Skade Formation)

Most of the samples in this unit are dominated by medium sand, but fine-grained sediments are present in parts of section. Quartz dominates the sand fraction, but glauconite and mica are also quite common (Fig. 1).

Upper (main part) of Middle Miocene and Upper Miocene (992.5 to 680 m, Nordland Group including Utsira Formation)

Most of this unit is dominated by medium sand. In the lower part, clay and silt are also quite common. Quartz dominates the sand fraction, but glauconite and mica are also present throughout. Mollusc fragments are also quite common in parts of section (Figs. 1 and 2).

Upper Pliocene (680-500 m, Nordland Group)

The samples in the lower part of the Upper Pliocene section contain a sand-rich diamicton with minor silt and clay and common pebbles (mainly of crystalline rocks). Quartz dominates the sand fraction. The upper part of the Upper Pliocene contains a clay-rich diamicton with some silt and sand and common pebbles (mainly of crystalline rocks). The pebbles are interpreted as 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.

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., 1983: Cenozoic micropaleontological biostratigraphy of the North Sea. Report of the Institute for Geological Sciences 82, 40 pp.

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.

Müller, C. & Spiegler, D., 1993: Revision of the late/middle Miocene boundary on the Voering Plateau (ODP Leg 104). Newsletter on Stratigraphy, 28 (2/3), 171-178.

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.

Spiegler, D. & Müller, C., 1992: Correlation of Bolboforma zonation and nannoplankton stratigraphy in the Neogene of the North Atlantic: DSDP sites 12-116, 49-408, 81-555 and 94-608. Marine Micropaleontology 20, 45-58.

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