Lower Oligocene to Upper Pliocene in well 25/10-2

Modified after Eidvin & Rundberg (2007).

Based on analyses of benthic and planktonic foraminifera, Bolboforma, pyritised diatoms and Sr isotopes in well 25/10-2 (59º09'38.40''N, 02º11'38.20''E, Map 1), we recorded 18 m with Lower Oligocene sediments, 91 m with Upper Oligocene sediments, a 330 m-thick column with Lower Miocene deposits, 46 m with Middle Miocene sediments, 100 m with Upper Miocene sediments, 119 m with Lower Pliocene deposits and 64 m with Upper Pliocene sediments. The base of the Lower Oligocene and top of the Upper Pliocene were not investigated. The units were investigated with 82 ditch-cutting samples at mainly ten metre intervals. However, in some parts, where stored samples are depleted to the archive limit, the intervals are 20 metres or more (Figs. 1-3). A detailed stratigraphy of these units (including fossil range charts) was presented in Eidvin & Rundberg (2007).

Well summary figure for well 25/10-2, fig 1

Well summary figure for well 25/10-2, fig 2

Well summary figure for well 25/10-2, fig 3

Biostratigraphy

Lower Oligocene (1210-1192 m, Hordaland Group)

Benthic foraminifera of the Rotaliatina bulimoides assemblage and pyritised diatoms of the Diatom sp. 3 assemblage (lower part) give an Early Oligocene age to this unit (Fig. 1). The planktonic foraminiferal fauna include a few specimens of G. angustiumbilicata, G. praebulloides and G. ciperoensis. The benthic foraminiferal assemblages are correlated with Subzone NSB 7b of King (1989) and Zone NSR 7B of Gradstein & Bäckström (1996) from the North Sea. The diatom assemblage is correlated with the lower part of Subzone NSP 9c of King (1989, North Sea).

Upper Oligocene (1192-1101 m, Hordaland Group)

Pyritised diatoms of the Diatom sp. 3 assemblage (upper main part) and benthic foraminifera of the Turrilina alsatica assemblage (lower main part), together with two Sr isotope ages, date this unit to Late Oligocene (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also includes B. eocenicus, G. soldanii girardana, C. placenta and C. rotundidorsata. The planktonic foraminiferal fauna includes a few specimens of G. angustiumbilicata, G. praebulloides and G. ciperoensis. The diatom assemblage is correlated with the upper part of Subzone NSP 9c, and the benthic foraminiferal fauna is correlated with Zone NSB 8 of King (1989) and probably the upper part of Zone NSR 7B and Zone NSR 8A of Gradstein & Bäckström (1996, all from the North Sea).

Lower Miocene (1101-771 m, Hordaland Group and Skade Formation)

Benthic foraminifera of the Turrilina alsatica assemblage (uppermost part), Plectofrondicularia seminuda assemblage and Uvigerina tenuipustulata assemblage, pyritised diatoms of the Diatom sp. 4 assemblage and planktonic foraminifera of the Globorotalia zealandica assemblage, together with a large number of Sr isotope ages, give an Early Miocene age to this unit (Figs. 1 and 2). In addition to the nominate species, the Plectofrondicularia seminuda assemblage also includes T. gracilis, and the Uvigerina tenuipustulata assemblage includes A. guerichi staeschei. The Globorotalia zealandica assemblage also includes G. praebulloides and G. angustiumbilicata. The Plectofrondicularia seminuda assemblage is correlated with Zone NSB 9 of King (1989, North Sea). The Uvigerina tenuipustulatai assemblage is 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 Diatom sp. 4 assemblage is correlated with Zone NSP 10 of King (1983, 1989). The Globorotalia zealandica 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 (771-725 m, Skade Formation (uppermost part) and Nordland Group)

Bolboforma of the Bolboforma badensis - Bolboforma reticulata assemblage and benthic foraminifera of the Astigerina guerichi staeshei assemblage and Uvigerina semiornata saprophila assemblage give a Middle Miocene age to this unit (Fig. 2). The planktonic foraminiferal fauna includes a few specimens of G. bulloides and N. atlantica (sinistral). 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 Asterigerina guerichi staeshei assemblage is correlated with Zone NSB 11 of King (1989, North Sea) and probably Zone FD of Doppert (1980, the Netherlands). The Uvigerina semiornata saprophila assemblage is correlated with Zone NSB 12 of King (1989, North Sea).

Upper Miocene (725-625 m, Utsira Formation)

Benthic foraminifera of the Uvigerina venusta saxonica assemblage (lower, main part), Bolboforma of the Bolboforma fragori assemblage and Bolboforma metzmacheri assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (dextral) assemblage, together with a large number of Sr isotope ages give a Late Miocene age to this unit (Fig. 3). The Uvigerina venusta saxonica is correlated with the lower part of Subzone NSB 13b of King (1989) from the North Sea. A Bolboforma fragori - B. subfragori Zone is described from deposits with an age of 11.7-10.3 Ma and a Bolboforma metzmacheri Zone is known 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 Neogloboquadrina atlantica (dextral) assemblage is correlated with the lower N. atlantica Zone on the Vøring Plateau (Upper Miocene, Spiegler & Jansen 1989) and the upper part of the N. atlantica (dextral)/N. acostaensis Zone of Weaver & Clement (1987) from the Late Miocene in the North Atlantic (DSDP Leg 94).

Lower Pliocene (625-506 m, Utsira Formation)

Benthic foraminifera of the Uvigerina venusta saxonica assemblage (upper part) and Globocassidulina subglobosa assemblage and planktonic foraminifera of the Globorotalia puncticulata assemblage and Neogloboquadrina atlantica (sinistral) assemblage (lower, main part), together with a number of Sr isotope ages, give an Early Pliocene age for this unit (Fig. 3). In addition to the nominate species, the benthic foraminiferal assemblages also include F. boueanus, S. bulloides (few) and M. pseudotepida (few). The Uvigerina venusta saxonica assemblage is correlated with the upper part of Subzone NSB 13b and the Globocassidulina subglobosa assemblage is correlated with the lower part of Subzone NSB 14a of King (1989) from the North Sea. An Early Pliocene Globorotalia puncticulata assemblage is described by Weaver & Clement (1986) from the North Atlantic. The first occurrence of G. puncticulata in that area is at approximately 4.6 Ma and its last occurrence at approximately 2.5 Ma.

Upper Pliocene (506-442 m, Nordland Group)

Benthic foraminifera of the Cibicides grossus assemblage and planktonic foraminifera of Neogloboquadrina atlantica (sinistral) assemblage (upper part), Globigerina bulloides assemblage and Neogloboquadrina pachyderma (dextral) assemblage give a Late Pliocene age (on the time scale of Berggren et al. 1995) for this unit (Fig. 3). 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 latest Pliocene N. pachyderma (dextral) Zone is described by King (1989) from the North Sea, by Weaver (1987) and Weaver & Clement (1986) from the North Atlantic and by Spiegler & Jansen (1989) from the Vøring Plateau. On the Vøring Plateau the zone is dated to 1.9-1.8 Ma.

Sr isotope stratigraphy

One hundred and four samples based on mollusc fragments from 44 depths, two samples based of fish teeth and six samples based on foraminifera were analysed for Sr isotopes in well 25/10-2.

The obtained ⁸⁷Sr/⁸⁶Sr ratios from 1146-1137 m and 1128-1110 m (based on foraminiferal tests) gave 23.1 and 23.8 Ma, respectively (latest Late Oligocene) and support the biostratigraphical correlations (Table 1, Fig. 1).

Sixty-two analyses based on molluscs and three analyses based on foraminiferal tests have been carried out for the part of the well which was given an Early Miocene age by the biostratigraphical correlations (Skade Formation). Three samples were analysed at most depths to discriminate between in situ and caved tests. The results showed consistently increasing ages with depth, from about 15 Ma at the top of the sands to about 20 Ma in the lower part. Approximately 30-40 % of the samples gave ages indicative of caving from the upper Neogene (Table 1, Figs. 1 and 2).

Forty-two samples based on mollusc fragments, two based on fish teeth and one based on foraminiferal tests have been carried out for the sandy Utsira Formation which was given a Late Miocene to Early Pliocene age by the biostratigraphical correlations. In this part we deliberately also analysed fossil tests which we had expected could be caved, and not surprisingly, we obtained a high proportion of samples which gave Late Pliocene ages (Table 1, Fig. 3). Taking the uncertainty of the method into account, however, many of these samples fall within the flat part of the seawater curve (see Farrell et al. 1995) and could have ages between 4.5 - 2.5 Ma (the Early/Late Pliocene boundary is at 3.5 Ma according to Berggren et al. 1995 ). Between 661 and 524 m, the bulk of the data (17 out of 31 samples) gave ages between 5.8-4.1 Ma, whereas two samples gave older ages of 10.1 and 6.4 Ma; the latter may be reworked. In the lower part of the formation, between depths of 716 and 670 m, the results showed much scatter, with many samples obviously including cave tests. However, excluding these, five other samples gave ages between 12.1 and 7.1 Ma and support the biostratigraphical correlations (Table 1, Fig. 3).

Well 25/10-2

Litho. Unit	Sample (DC)	Corrected 87/86Sr	2S error	Age (Ma)	Comments	Analysed fossils
Utsira Fm	524 m	0.709080	0.000008	2.18	Caved	One mollusc fragment
Utsira Fm	570 m	0.709021	0.000009	5.44		One mollusc fragment
Utsira Fm	570 m	0.709287	0.000008	0.00	Caved	One mollusc fragment
Utsira Fm	570 m	0.709053	0.000007	4.14		One mollusc fragment
Utsira Fm	579 m	0.709052	0.000008	4.21		One mollusc fragment
Utsira Fm	579 m	0.709068	0.000009	2.62	Caved	One mollusc fragment
Utsira Fm	579 m	0.709016	0.000009	5.56		One mollusc fragment
Utsira Fm	588 m	0.709078	0.000008	2.24	Caved	One mollusc fragment
Utsira Fm	588 m	0.708999	0.000008	5.84		One mollusc fragment
Utsira Fm	588 m	0.709071	0.000008	2.48	Caved	One mollusc fragment
Utsira Fm	598 m	0.709083	0.000009	2.09	Caved	One mollusc fragment
Utsira Fm	598 m	0.708998	0.000009	5.85		One mollusc fragment
Utsira Fm	598 m	0.709013	0.000008	5.63		One mollusc fragment
Utsira Fm	625 m	0.709060	0.000007	3.59	Caved	One mollusc fragment
Utsira Fm	625 m	0.708980	0.000008	6.07		One mollusc fragment
Utsira Fm	625 m	0.709002	0.000008	5.80		One mollusc fragment
Utsira Fm	634 m	0.709126	0.000008	1.19	Caved	One mollusc fragment
Utsira Fm	634 m	0.709018	0.000007	5.51		One mollusc fragment
Utsira Fm	634 m	0.709038	0.000007	4.97		One mollusc fragment
Utsira Fm	661-634 m	0.709071	0.000008	2.48	Caved	Approximately 20 tests of U. venusta saxonica
Utsira Fm	643 m	0.709028	0.000009	5.24		One mollusc fragment
Utsira Fm	643 m	0.709028	0.000009	5.24		One mollusc fragment
Utsira Fm	643 m	0.709133	0.000008	1.11	Caved	One mollusc fragment
Utsira Fm	652 m	0.708887	0.000008	10.08	Reworked	One fish tooth
Utsira Fm	652 m	0.708967	0.000008	6.35		One mollusc fragment
Utsira Fm	652 m	0.709015	0.000008	5.58		One mollusc fragment
Utsira Fm	652 m	0.709025	0.000008	5.30		Two mollusc fragment
Utsira Fm	661 m	0.709006	0.000008	5.75		One mollusc fragment
Utsira Fm	661 m	0.709019	0.000009	5.49		One mollusc fragment
Utsira Fm	661 m	0.709028	0.000008	1.17	Caved	One mollusc fragment
Utsira Fm	670 m	0.708946	0.000009	7.12		One mollusc fragment
Utsira Fm	670 m	0,708892	0.000007	9.88		One mollusc fragment
Utsira Fm	670 m	0.709045	0.000008	4.70		Two mollusc fragments
Utsira Fm	680 m	0.708891	0.000007	9.92		One mollusc fragment
Utsira Fm	680 m	0.708833	0.000008	12.10		One fish tooth
Utsira Fm	680 m	0,709274	0.000008	0	Caved	Two mollusc fragments
Utsira Fm	680 m	0.709055	0.000008	4.09	Caved	Two mollusc fragments
Utsira Fm	689 m	0.709078	0.000009	2.24	Caved	One mollusc fragment
Utsira Fm	689 m	0.708995	0.000010	5.90		One mollusc fragment
Utsira Fm	689 m	0.709098	0.000008	1.60	Caved	One mollusc fragment
Utsira Fm	707 m	0.709059	0.000008	3.68	Caved	Two mollusc fragments
Utsira Fm	707 m	0.709099	0.000008	1.57	Caved	Two mollusc fragments
Utsira Fm	716 m	0.709032	0.000008	5.13	Caved	One mollusc fragment
Utsira Fm	716 m	0.708852	0.000008	11.27		Three mollusc fragments
Utsira Fm	716 m	0.709028	0.000008	5.24	Caved	Two mollusc fragments
Skade Fm	771 m	0.708900	0.000008	9.55		Three mollusc fragments
Skade Fm	771 m	0.709039	0.000009	4.94	Caved	Three mollusc fragments
Skade Fm	771 m	0.708765	0.000008	15.51		Two mollusc fragments
Skade Fm	780 m	0.709090	0.000008	1.84	Caved	One mollusc fragment
Skade Fm	780 m	0.708767	0.000009	15.45		One mollusc fragment
Skade Fm	780 m	0.708748	0.000008	15.92		Two mollusc fragments
Skade Fm	790 m	0.708696	0.000008	16.84		One mollusc fragment
Skade Fm	790 m	0.708767	0.000006	15.45		One mollusc fragment
Skade Fm	790 m	0.708688	0.000008	16.94		Two mollusc fragments
Skade Fm	799 m	0.709031	0.000009	5,16	Caved	One mollusc fragment
Skade Fm	799 m	0.708707	0.000008	16.68		One mollusc fragment
Skade Fm	799 m	0.709105	0.000008	1.45	Caved	One mollusc fragment
Skade Fm	799 m	0.709068	0.000007	2.62	Caved	One mollusc fragment
Skade Fm	808 m	0.708935	0.000008	7.56	Caved	Three mollusc fragments
Skade Fm	808 m	0.708677	0.000007	17.09		Three mollusc fragments
Skade Fm	808 m	0.708754	0.000008	15.79		Four mollusc fragments
Skade Fm	817-808 m	0.708698	0.000009	16.82		Approximately 60 tests of U. tenuipustulata, G. zealandica, A. guerichi staeschei
Skade Fm	817 m	0.708673	0.000008	17.14		Two mollusc fragments
Skade Fm	817 m	0.709048	0.000008	4.52		Three mollusc fragments
Skade Fm	817 m	0.708726	0.000009	16.38		Four mollusc fragments
Skade Fm	817 m	0.708674	0.000007	17.13		Approximately 30 tests of U. tenuipustulata
Skade Fm	826 m	0.709071	0.000008	2.48	Caved	Two mollusc fragments
Skade Fm	826 m	0.709048	0.000009	4.52	Caved	One mollusc fragment
Skade Fm	826 m	0.708651	0.000008	17.40		Three mollusc fragments
Skade Fm	835-826 m	0.708720	0.000009	16.48		Approximately 40 tests of U. tenuipustulata
Skade Fm	835 m	0.708878	0.000008	10.41		Three mollusc fragments
Skade Fm	835 m	0.708798	0.000008	14.21		Four mollusc fragments
Skade Fm	835 m	0.708698	0.000008	16.82		Three mollusc fragments
Skade Fm	844 m	0.708852	0.000009	11.27		Two mollusc fragments
Skade Fm	844 m	0.708760	0.000009	15.64		Three mollusc fragments
Skade Fm	844 m	0.708648	0.000008	17.43		One mollusc fragment
Skade Fm	854 m	0.709057	0.000009	3.89		One mollusc fragment
Skade Fm	854 m	0.708721	0.000008	16.46		Two mollusc fragments
Skade Fm	854 m	0.709137	0.000009	1.05	Caved	One mollusc fragment
Skade Fm	863 m	0.708753	0.000008	15.81		Three mollusc fragments
Skade Fm	863 m	0.708777	0.000008	15.15		Three mollusc fragments
Skade Fm	863 m	0.708554	0.000007	18.51		Three mollusc fragments
Skade Fm	872 m	0.708609	0.000008	17.85		Two mollusc fragments
Skade Fm	872 m	0.709110	0.000008	1.38	Caved	Three mollusc fragments
Skade Fm	881 m	0.708651	0.000007	17.40		Three mollusc fragments
Skade Fm	881 m	0.708587	0.000007	18.10		Three mollusc fragments
Skade Fm	890 m	0.708516	0.000008	19.05		Three mollusc fragments
Skade Fm	890 m	0.709073	0.000009	2.41	Caved	Two mollusc fragments
Skade Fm	899 m	0.708428	0.000008	20.47		One mollusc fragment
Skade Fm	899 m	0.708535	0.000007	18.77		Two mollusc fragments
Skade Fm	909 m	0.708553	0.000008	18.66		Three mollusc fragments
Skade Fm	909 m	0.708575	0.000008	18.24		One mollusc fragment
Skade Fm	918 m	0.708722	0.000007	16.44		Three mollusc fragments
Skade Fm	918 m	0.708828	0.000008	12.44		Three mollusc fragments
Skade Fm	927 m	0.708502	0.000008	19.27		Three mollusc fragments
Skade Fm	927 m	0.708986	0.000007	6.00	Caved	Three mollusc fragments
Skade Fm	936 m	0.708989	0.000008	5.97	Caved	Four mollusc fragments
Skade Fm	936 m	0.708697	0.000008	16.83		Four mollusc fragments
Skade Fm	945 m	0.708493	0.000008	19.42		Four mollusc fragments
Skade Fm	954 m	0.709035	0.000008	5.06	Caved	One mollusc fragment
Skade Fm	954 m	0.708932	0.000007	7.81	Caved	Three mollusc fragments
Skade Fm	963 m	0.708938	0.000009	7.43	Caved	Four mollusc fragments
Skade Fm	973 m	0.709064	0.000007	3.31	Caved	One mollusc fragment
Skade Fm	973 m	0.708761	0.000007	15.62		Four mollusc fragments
Skade Fm	991 m	0.708888	0.000009	10.04		Four mollusc fragments
Skade Fm	1000 m	0.708925	0.000009	8.43	Caved	Five mollusc fragments
Skade Fm	1009 m	0.708715	0.000009	16.56		Four mollusc fragments
Skade Fm	1027 m	0.708945	0.000009	7.16	Caved	Three mollusc fragments
Skade Fm	1037 m	0.708664	0.000008	17.24		Six mollusc fragments
Skade Fm	1043 m	0.708895	0.000008	9.76	Caved	Four mollusc fragments
Hordaland Gr	1128-1101 m	0.708271	0.000009	23.76		40 tests of B. elongata
Hordaland Gr	1146-1137 m	0.708302	0.000008	23.14		30 tests of B. elongata and T. alsatica

Table 1: Strontium isotope data from well 25/10-2. 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 and McArthur (1997). NIST = National Institute for Standard and Technology. Modified after Eidvin & Rundberg (2007).

Lithology

Lower Oligocene to lower part of Lower Miocene (1210 to approximately 1055 m, Hordaland Group)

The ditch cutting samples in this part are dominated by clay, but sand (quartzose (dominant), glauconitic and biotitic) is also quite common throughout. Limestone is recorded in the upper part of the unit (Fig. 1). Some of the sand is probably caved from the immediately overlying Skade Formation.

Lower Miocene (upper main part) to lowermost part of Middle Miocene (approximately 1055 to approximately 758 m, Skade Formation)

The samples are dominated by medium to fine sand, but some parts are also quite rich in clay and silt (Fig. 2). Quartz dominates the sand fraction, but glauconite is quite common in some parts. Minor mica is also recorded throughout. Mollusc fragments are present in most samples.

Middle Miocene (upper main part, approximately 758 to approximately 725 m, Nordland Group)

Clay dominates this unit. Sand and silt are also recorded throughout, but most of the sand is probably caved from the immediately overlying Utsira Formation (Fig. 2).

Upper Miocene to Lower Pliocene (approximately 725 to 506 m, Utsira Formation)

Medium- to fine-grained sand dominates this part, but clay and silt are also quite common. Quartz dominates the sand fraction with minor glauconite and mica. The unit is very rich on molluscs and mollusc fragments, especially in the upper part (Fig. 3).

Upper Pliocene (506-442 m, Nordland Group)

The samples in the Upper Pliocene section contain a clay-rich diamicton with some sand, silt and pebbles (Fig. 3).

References

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Doppert, J. W. C., 1980: Lithostratigraphy and biostratigraphy of marine Neogene deposits in the Netherlands. Mededelingen Rijks Geologische Dienst 32-16, 2, 3-79.

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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.

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