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).
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).
|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).
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).
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