Upper Oligocene to Upper Pliocene in well 30/6-3

Based on analyses of benthic and planktonic foraminifera, Bolboforma and Sr isotopes in well 30/6-3 (60°34’52.98’’N, 02°47’1.41’’E), Map 1), we recorded 100 m with Upper Oligocene deposits, 50 m with Lower Miocene sediments, a 210 m-thick column with Middle Miocene deposits, 10 m with Upper Miocene sediments, 60 m with Upper Miocene-Lower Pliocene deposits and 80 m with Upper Pliocene sediments. The base of the Upper Oligocene and the top of the Upper Pliocene are not investigated. The units are investigated with 48 ditch-cutting samples at mainly 10 metre intervals (Figs. 1 and 2).

Well summary figure for well 30/6-3, fig 1

Well summary figure for well 30/6-3, fig 2

Biostratigraphy

Upper Oligocene (1110-1010 m, Hordaland Group)

Benthic foraminifera of the Gyroidina soldanii girardana assemblage (in the upper part) indicate a Late Oligocene age (Fig. 1). The benthic foraminiferal fauna is correlated with Zone NSB 8 of King (1989). The age of the lower undefined part is based on correlation with the nearby well 30/5-2.

Lower Miocene (1010-960 m, Hordaland Group)

Benthic foraminifera of the Uvigerina tenuipustulata assemblage and Astigerina guerichi staeschei assemblage (lower part) and planktonic foraminifera of Globorotalia zealandica-Globigerina woodi assemblage (upper main part) give an Early Miocene age to this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal assemblages also include Ceratobulimina hauerii, Trifarina gracilis var. A, Cibicides dutemplei and Hoeglundina elegans. The planktonic foraminiferal assemblage also includes Globigerina praebulloides and Globigerina angustiumbilicata. The Uvigerina tenuipustulata assemblage and lower part of the Astigerina guerichi staeschei assemblage are correlated with Zone NSB 10 of King (1989, North Sea) and Zone NSR 8B and the 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 (960-750 m, Hordaland Group (uppermost part) and Nordland Group)

Bolboforma badenensis-Bolboforma reticulata assemblage and benthic foraminifera of Astigerina guerichi staeschei assemblage (upper part), Bulimina elongata assemblage and Florilus bouanus-Globocassidulina subglobosa assemblage, together with a large number of Sr isotope ages, give a Mid Miocene age to this unit (Figs. 1 and 2). In addition to the nominate species, the Astigerina guerichi staeschei assemblage (upper part) and Bulimina elongate assemblage also include Sphaeroidina bulloides, G. subglobosa, Martinottiella communis, Trifarina gracilis and Trifarina gracilis var. A. A few, probably reworked, specimens of G. soldanii girardana are also recorded in the middle of the Bulimina elongata assemblage. The planktonic foraminiferal fauna includes a few specimens of Sphaeroidinellopsis disjuncta and Globigerinoides quadrilobatus triloba in the lower part of the unit.

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 benthic foraminiferal assemblages are correlated with Zone NSB 11, Zone NSB 12 and the lower part of Subzone NSB 13a of King (1989) from the North Sea area.

Upper Miocene (750-740 m, Utsira Formation (lower part))

Bolboforma of the Bolboforma subfragori assemblage, benthic foraminifera of the Florilus bouanus assemblage and the occurrence of the planktonic foraminifera Neogloboquadrina atlantica (dextral) give a Late Miocene age to this unit (Fig. 2). The benthic foraminiferal fauna also includes Ehrenbergina variabilis and Uvigerina venusta saxonica. A B. fragori/B. subfragori Zone is known from deposits with an age of approximately 11.7-10.3 Ma from the North Atlantic and the Vøring plateau (Spiegler & Müller 1992, Müller & Spiegler 1993). The benthic foraminiferal assemblage is tentatively correlated with the upper part of Subzone NSB 13a 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 Miocene-Lower Pliocene (740-680 m, Utsira Formation (upper main part))

Benthic foraminifera of the Florilus bouanus assemblage and planktonic foraminifera of the Globigerina bulloides-Neogloboquadrina atlantica (sinistral) assemblage (lower part) together with number of Sr isotope ages give a Late Miocene to Early Pliocene age to this unit (probably Late Miocene in the lower part and Early Pliocene in the upper part). In addition to the nominate species, the benthic foraminiferal fauna also includes Globulina gibba myristiformis, Pullenia bulloides and U. venusta saxonica (lower part). The benthic foraminiferal assemblage is tentatively correlated with the upper part of Subzone NSB 13b and the lower part of Subzone 14a of King (1989, North Sea). The planktonic foraminiferal fauna in this unit is correlated with the lower part of the Neogloboquadrina atlantica (sinistral) Zone of Spiegler & Jansen (1989, Norwegian Sea).

Upper Pliocene (680-600 m, Nordland Group)

Benthic foraminifera of the Cibicides grossus–Elphidiella hannai assemblage and planktonic foraminifera of the Globigerina bulloides-Neogloboquadrina atlantica (sinistral) assemblage (upper part), Globigerina bulloides assemblage and Neogloboquadrina pachyderma (sinistral) assemblage give a Late Pliocene age (on the time scale of Berggren et al. 1995) to this unit (Fig. 2). The benthic foraminiferal fauna is correlated 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). A non-encrusted form of N. pachyderma (sinistral) is described from the upper part of the Upper Pliocene to the Pleistocene on the Vøring Plateau (Norwegian Sea, Spiegler & Jansen 1989).

Sr isotope stratigraphy

Thirty-six samples based on mollusc fragments and six samples based on foraminiferal tests from 27 depths were analysed for Sr isotopes (Figs. 1 and 2, Table 1). Three samples were taken from the upper part of the unit which the biostratigraphical correlations gave an Early Miocene age (16.5-16.3 Ma) supporting the biostratigraphical correlations. Twenty-seven samples were taken from the unit which the biostratigraphical correlations gave a Middle Miocene age. Twelve of these analyses gave Middle Miocene ages from 14.1 to 11.3 Ma and twelve gave earliest Late Miocene ages from 11.1 to 9.8 Ma (Figs. 1 and 2, Table 1). Two samples gave very young ages (1.4 and 3.9), which obviously came from caved mollusc fragments, and one sample gave a very old age (21.7 Ma) which clearly came from reworked fragments (Fig. 1, Table 1).

Eleven samples were taken from the unit which the biostratigraphical correlations gave a Late Miocene age and a Late Miocene to Early Pliocene age (Utsira Formation). Four samples in the upper half of this unit gave ages from 7.0 to 4.0 Ma and one sample in the lower part gave 8.7 Ma (Fig. 2, Table 1). These samples support the biostratigraphical correlations. Three samples gave old ages of 14.5, 12.7 and 12.3 Ma, and these probably represent reworked mollusc fragments. Three samples gave very young ages of 2.4, 2.3 and 2.3 Ma and these probably represent fragments caved from the Upper Pliocene unit (Fig. 2 and Table 1).
The Sr isotope analyses indicate that there is a hiatus between the lower part of the Nordland Group and Utsira Formation representing about three million years, and a hiatus between the Utsira Formation and the Upper Pliocene part of the Nordland Group representing about two million years.

Well 30/6-3

Litho. Unit	Sample (DC)	Corrected 87/86Sr	2S error	Age (Ma)	Comments	Analysed fossils
Utsira Fm	680 m	0.709077	0.000008	2.27	Caved	One mollusc fragment
Utsira Fm	680 m	0.709032	0.000008	5.13		One mollusc fragment
Utsira Fm	690 m	0.709018	0.000008	5.51		One mollusc fragment
Utsira Fm	690 m	0.708950	0.000008	6.95		One mollusc fragment
Utsira Fm	700 m	0.709056	0.000008	3.98	Caved	One mollusc fragment
Utsira Fm	710 m	0.708949	0.000008	6.99		One mollusc fragment
Utsira Fm	710 m	0.708823	0.000009	12.71	Reworked	One mollusc fragment
Utsira Fm	720 m	0.709148	0.000008	0.763	Caved	One mollusc fragment
Utsira Fm	720 m	0.708830	0.000009	12.31	Reworked	One mollusc fragment
Utsira Fm	730 m	0.708794	0.000009	14.46	Reworked	One mollusc fragment
Utsira Fm	740 m	0.709072	0.000008	2.44	Caved	One mollusc fragment
Utsira Fm	740 m	0.708920	0.000009	8.69		One mollusc fragment
Nordland Gr	760 m	0.708895	0.000008	9.75		One mollusc fragment
Nordland Gr	760 m	0.708829	0.000007	12.38		One mollusc fragment
Nordland Gr	770 m	0.708831	0.000007	12.24		One mollusc fragment
Nordland Gr	780 m	0.708887	0.000009	10.08		One mollusc fragment
Nordland Gr	780 m	0.708827	0.000008	12.49		One mollusc fragment
Nordland Gr	790 m	0.708360	0.000009	21.68	Reworked	One mollusc fragment
Nordland Gr	800 m	0.709057	0.000009	3.89	Caved	One mollusc fragment
Nordland Gr	800 m	0.708848	0.000008	11.41		13 large tests of F. bouanus
Nordland Gr	800 m	0.708833	0.000009	12.10		One mollusc fragment
Nordland Gr	810 m	0.709112	0.000009	1.359	Caved	One mollusc fragment
Nordland Gr	820 m	0.708817	0.000008	12.98		One mollusc fragment
Nordland Gr	820 m	0.708799	0.000008	14.12		One mollusc fragment
Nordland Gr	830 m	0.708874	0.000008	10.55		One mollusc fragment
Nordland Gr	840 m	0.708874	0.000009	10.26		One mollusc fragment
Nordland Gr	840 m	0.708863	0.000008	10.92		One mollusc fragment
Nordland Gr	850 m	0.708890	0.000008	9.96		One mollusc fragment
Nordland Gr	860 m	0.708863	0.000009	10.92		One mollusc fragment
Nordland Gr	860 m	0.708850	0.000009	11.34		One mollusc fragment
Nordland Gr	870 m	0.708798	0.000009	14.21		One mollusc fragment
Nordland Gr	880 m	0.708896	0.000008	9.72		One mollusc fragment
Nordland Gr	880 m	0.708859	0.000007	11.03		One mollusc fragment
Nordland Gr	890 m	0.708858	0.000009	11.06		One mollusc fragment
Nordland Gr	900 m	0.709157	0.0.00009	0.569	Caved	One mollusc fragment
Nordland Gr	900 m	0.708826	0.000008	12.55		One mollusc fragment
Nordland Gr	900 m	0.708826	0.000010	12.55		36 tests of T. gracilis var. A, E. serrata
Nordland Gr	910 m	0.708879	0.000008	10.37		One mollusc fragment
Nordland Gr	910 m	0.708857	0.000010	11.10		32 tests of T. gracilis var. A, E. serrata
Hordaland Gr	970 m	0.708730	0.000010	16.30		36 tests of T. gracilis, T. gracilis var. A, U. tenuipustulata, G. zealandica, G. woodi, A. guerichi staeschei
Hordaland Gr	980 m	0.708726	0.000008	16.38		42 tests of T. gracilis var. A, U. tenuipustulata, G. zealandica, A. guerichi staeschei
Hordaland Gr	990 m	0.708716	0.000007	16.54		44 tests of T. gracilis var. A, U. tenuipustulata, G. zealandica, A. guerichi staeschei

Table 1: Strontium isotope data from well 30/6-3. 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 to lower part of Middle Miocene (1110 to approximately to 920 m, Hordaland Group and lowermost part of Nordland Group)

The samples in this unit are mainly dominated by clay and silt, but sand is also quite common throughout especially in the lower part where several thin sections are dominated by sand. Quartz dominates the sand fraction, especially in the lower part, but glauconite is quite common in most parts. Mica and sponge spicules also constitute considerable parts of the fraction (Fig. 1).

Upper part of Middle Miocene to Upper Miocene-Lower Pliocene (approximately 920 to 680 m, Nordland Group and Utsira Formation)

Most of the samples in this unit are dominated by medium sand, but thin fine-grained sections are also present. Quartz dominates the sand fraction, but glauconite, mica and mollusc fragments are also quite common in parts of the unit (Figs. 1 and 2).

Upper Pliocene (680-600 m, Nordland Group)

The samples in the lower part of the Upper Pliocene unit 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.

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.