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

Based on analyses of benthic and planktonic foraminifera, Bolboforma, pyritised diatoms and Sr isotopes in well 25/10-2 S (59º53'11.80''N, 02º30'08.33''E, Map 1). we recorded 40 m with Lower Oligocene sediments, 280 m with Upper Oligocene sediments, a 530 m-thick column with Lower Miocene deposits, 110 m with the Middle Miocene sediments, 40 m with Lower Pliocene sediments and 220 m with Upper Pliocene deposits. There is a break between the Lower and Middle Miocene. The base of the Lower Oligocene and the top of the Upper Pliocene were not investigated. The units were investigated with 92 ditch-cutting samples at 10 to 20 metre intervals (Figs. 1-3).

Well summary figure 25/2-10 S, fig. 1

Well summary figure 25/2-10 S, fig. 2

Well summary figure 25/2-10 S, fig. 3

Biostratigraphy

Lower Oligocene (1440-1440 m, Hordaland Group)

Benthic foraminifera of the Rotaliatina bulimoides assemblage and Gyroidina soldanii mamillata assemblage and pyritised diatoms of the Diatom sp. 3 assemblage (lower part) give an Early Oligocene age for this unit (Fig. 1). The planktonic foraminiferal fauna contains just a few specimens of G. ciperoensis in one sample. In addition to the nominate species, the benthic foraminiferal assemblages also include G. subglobosa and T. alsatica and the Rotaliatina bulimoides assemblage also contains C. placenta and C. rotundidorsata. The benthic foraminiferal assemblages are correlated with Subzone NSB 7b of King (1989) and Zone NSR 7B of Gradstein & Bäckström (1996). The diatom assemblage is correlated with the lower part of Subzone NSP 9c of King (1989, North Sea).

Upper Oligocene (1440-1160 m, Hordaland Group)

Pyritised diatoms of the Diatom sp. 3 assemblage (upper main part) and benthic foraminifera of the Turrilina alsatica assemblage and Gyroidina soldanii girardana assemblage (lower main part), together with one Sr isotope age, date this unit to Late Oligocene (Fig. 1). The planktonic foraminiferal fauna contains just a few specimens of G. praebulloides. In addition to the nominate species the benthic foraminiferal assemblages also include C. tenellus and G. subglobosa and the Gyroidina soldanii girardana assemblage also contains R. arnei. The diatom assemblage is correlated with upper part of Subzone NSP 9c, and the benthic foraminiferal faunas are correlated with Zone NSB 8 of King (1989) and probably the upper part of Zone NSR 7B and NSR 8A of Gradstein & Bäckström (1996, North Sea).

Lower Miocene (1160-630 m, Hordaland Group and Skade Formation)

Benthic foraminifera of the Gyroidina soldanii girardana assemblage (uppermost part), Elphidium subnodosum assemblage, Plectofrondicularia seminuda assemblage, Asterigerina guerichi staeschei assemblage and planktonic foraminifera of the Globigerina angustiumbilicata assemblage, together with a large number of Sr isotope ages, give an Early Miocene age to this unit (Figs. 1 and 2). The benthic foraminiferal assemblages are correlated with Zone NSB 9 and Zone NSB 10 of King (1989, North Sea).

Middle Miocene (630-520 m, Nordland Group)

Planktonic foraminifera of the Globigerinoides quadrilobatus triloba assemblage, Bolboforma of the Bolboforma badensis - Bolboforma reticulata assemblage and benthic foraminifera of the Bulimina elongata assemblage and Florilus boueanus - Spaeroidina bulloides (assemblage, lowermost part), together with a number of Sr isotope ages, date this unit to the Middle Miocene (Fig 3). The Globigerinoides quadrilobatus triloba assemblage is correlated with Zone NSP 12 of King (1989). 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 faunas are correlated with Zone NSB 11, NSB 12 and lowermost part of Zone NSB 13 of King (1989) from the North Sea.

Lower Pliocene (520-480 m, Utsira Formation)

Benthic foraminifera of the F. boueanus - Spaeroidina bulloides (assemblage, upper main part) and Eponides pygmeus - Cibicides telegdi assemblage and planktonic foraminifera of the Globorotalia puncticulata assemblage, together with a few Sr isotope ages, give an Early Pliocene age for this unit (Fig. 3). The benthic foraminiferal assemblage is tentatively correlated with the upper part of Subzone NSB 13b and the lower part of Subzone NSB 14a of King (1989, 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 is at approximately 2.5 Ma.

Upper Pliocene (480-260 m, Nordland Group)

Benthic foraminifera of the Cibicides grossus-Elphidiella hannai assemblage and Elphidium excavatum 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 15 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

Fifty-two samples based on mollusc fragments from 26 depths and nine samples based on foraminferal tests from eight depths were analysed for Sr isotopes in well 25/2-10S.

The obtained 87Sr/86Sr ratios from 1200-1190 m (based on foraminiferal tests) gave an age of 26.2 Ma (Late Oligocene) which support the biostratigraphical correlations (Table 1, Fig. 1).

Thirty-eight analyses based on mollusc fragments and eight 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). The results gave ages of approximately 24 to 19 Ma (Early Miocene) with slightly increasing ages with depth for most of the section (Table 1, Fig. 2).

Twelve samples based on mollusc fragments were taken from the unit which was given a Mid Miocene age by the biostratigraphical correlations (lowermost part of the Nordland Group).

Three of the four uppermost samples were obviously based on caved fragments while the fourth was obviously based on a reworked fragment. The other eight samples gave ages from approximately 13.4 to 11 Ma and consequently support the biostratigraphical correlations (Table 1, Fig. 3).

Two samples based on mollusc fragments were taken from the unit given an Early Pliocene age by the biostratigraphical correlation (Utsira Formation). The Sr values fall within the flat part of the 87Sr/86Sr curve and the corresponding ages are therefore less precise. Nevertheless, the sample which gave 4.5 Ma supports the biostratigraphical correlations (Table 1, Fig. 3).

Well 25/2-10S

Litho. Unit Sample (DC) Corrected 87/86Sr 2S error Age (Ma) Comments Analysed fossils
Utsira Fm 490 m 0.709055 0.000007 4.09   One mollusc fragment
Utsira Fm 490 m 0.709029 0.000008 5.21   One mollusc fragment
Utsira Fm 500 m 0.709048 0.000008 4.52   One mollusc fragment
Utsira Fm 500 m 0.709102 0.000007 1.504 Caved One mollusc fragment
Utsira Fm 500 m 0.709041 0.000008 4.86   One mollusc fragment
Utsira Fm 500 m 0.709040 0.000008 4.90   One mollusc fragment
Utsira Fm 510 m 0.709042 0.000009 4.82   One mollusc fragment
Utsira Fm 510 m 0.709022 0.000008 5.41   One mollusc fragment
Utsira Fm 520 m 0.709076 0.000008 2.30 Caved One mollusc fragment
Utsira Fm 520 m 0.709031 0.000008 5.16   One mollusc fragment
Nordland Gr 540 m 0.708428 0.000008 20.47 Reworked One mollusc fragment
Nordland Gr 540 m 0.709054 0.000008 4.10 Caved One mollusc fragment
Nordland Gr  560 m 0.708809 0.000008 13.34   One mollusc fragment
Nordland Gr 560 m 0.708809 0.000009 13.34   One mollusc fragment
Nordland Gr 580 m 0.708808 0.000009 13.40   One mollusc fragment
Nordland Gr 580 m 0.708864 0.000009 10.89   One mollusc fragment
Nordland Gr 600 m 0.708808 0.000008 13.40   One mollusc fragment
Nordland Gr 600 m 0.708826 0.000008 12.55   One mollusc fragment
Nordland Gr 620 m 0.708818 0.000009 12.93   One mollusc fragment
Nordland Gr 620 m 0.708855 0.000008 11.17   One mollusc fragment
Nordland Gr 620 m 0.708834 0.000008 12.05   One mollusc fragment
Skade Fm 630 m 0.708467 0.000008 19.89   29 tests of A. guerishi staeshei
Skade Fm 640 m 0.708496 0.000008 19.37   26 tests of A. guerishi staeshei
Skade Fm 640 m 0.708538 0.000008 18.73   One mollusc fragment
Skade Fm 640 m 0.708493 0.000009 19.42   One mollusc fragment
Skade Fm 650 m 0.708466 0.000008 19.91   30 tests of A. guerishi staeshei
Skade Fm 650 m 0.708351 0.000009 21.89   One mollusc fragment
Skade Fm 650 m 0.708496 0.000007 19.37   One mollusc fragment
Skade Fm 670 m 0.708433 0.000008 20.39   One mollusc fragment
Skade Fm 670 m 0.708419 0.000007 20.61   One mollusc fragment
Skade Fm 680 m 0.708464 0.000008 19.94   45 tests of A. guerishi staeshei
Skade Fm 690 m 0.708497 0.000009 19.35   One mollusc fragment
Skade Fm 690 m 0.708461 0.000008 19.98   One mollusc fragment
Skade Fm 710 m 0.708441 0.000009 20.28   One mollusc fragment
Skade Fm 710 m 0.708504 0.000008 19.23   One mollusc fragment
Skade Fm 720 m 0.708464 0.000008 19.94   One mollusc fragment
Skade Fm 720 m 0.708444 0.000007 20.23   One mollusc fragment
Skade Fm 740 m 0.708514 0.000008 19.08   One mollusc fragment
Skade Fm 740 m 0.708429 0.000008 20.45   One mollusc fragment
Skade Fm 770 m 0.708460 0.000008 19.99   One mollusc fragment
Skade Fm 770 m 0.708499 0.000009 19.19   One mollusc fragment
Skade Fm 790 m 0.708487 0.000008 19.52   One mollusc fragment
Skade Fm 790 m 0.708426 0.000008 20.50   One mollusc fragment
Skade Fm 810 m 0.708340 0.000007 22.14   One mollusc fragment
Skade Fm 810 m 0.708418 0.000009 20.62   One mollusc fragment
Skade Fm 850 m 0.708253 0.000009 24.07   19 large tests of Lenticulina sp.
Skade Fm 850 m 0.708471 0.000008 19.81   One mollusc fragment
Skade Fm 850 m 0.708296 0.000009 23.27   One mollusc fragment
Skade Fm 870 m 0.708262 0.000008 23.92   15 large tests of E. subnodosum
Skade Fm 870 m 0.708263 0.000007 23.90   19 large tests of Lenticulina sp.
Skade Fm 870 m 0.708439 0.000008 20.31   One mollusc fragment
Skade Fm 870 m 0.708221 0.000008 24.57   One mollusc fragment
Skade Fm 890 m 0.708259 0.000008 23.97   14 large tests of E. subnodosum
Skade Fm 890 m 0.708250 0.000008 24.12   One mollusc fragment
Skade Fm 890 m 0.708469 0.000008 19.85   One mollusc fragment
Skade Fm 910 m 0.708421 0.000009 20.58   One mollusc fragment
Skade Fm 910 m 0.708808 0.000008 13.40   One mollusc fragment
Skade Fm 930 m 0.708265 0.000007 23.86   One mollusc fragment
Skade Fm 930 m 0.708436 0.000007 20.35   One mollusc fragment
Skade Fm 950 m 0.708536 0.000008 18.36   One mollusc fragment
Skade Fm 950 m 0.708428 0.000009 20.47   One mollusc fragment
Skade Fm 970 m 0.708429 0.000008 20.45   One mollusc fragment
Skade Fm 970 m 0.708450 0.000008 20.15   One mollusc fragment
Skade Fm 990 m 0.708479 0.000008 19.66   One mollusc fragment
Skade Fm 990 m 0.708501 0.000009 19.28   One mollusc fragment
Skade Fm 1010 m 0.708444 0.000008 20.23   One mollusc fragment
Skade Fm 1010 m 0.708466 0.000008 19.91   One mollusc fragment
Hordaland Gr 1200-1190 m 0.708131 0.000008 26.18   40 small tests of G. soldanii giradana

Table 1: Strontium isotope data from well 25/2-10 S. 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.

Lithology

Lower Oligocene to lowermost part of Lower Miocene (1440 to approximately 1085 m, Hordaland Group)


The samples in the lower half of this unit are dominated by clay and silt. Sand (quartzose, glauconitic and biotitic) is present but not common. In the upper half of the unit sand is more common (mainly quartzose with minor glauconite and mica). The sections from 1270-1250 m and 1150-1120 m contain mainly quartzose sand (Fig. 1).

Lower Miocene (approximately 1085 to 630 m, Skade Formation)


The samples in most of the section are dominated by medium to coarse sand, but fine-grained sand is also common in parts of section. Quartz dominates the sand fraction, but glauconite is quite common in some parts. Minor mica is recorded throughout. Mollusc fragments are quite common in parts of section, and in these parts lignite coal is usually also quite common (Fig. 2).

Middle Miocene and Lower to Upper Pliocene (630-260 m, Nordland Group including Utsira Formation)


These parts are also dominated by medium to coarse sand. Quartz dominates the sand fraction, but mica and glauconite are quite common in some parts. Mollusc fragments are very common throughout, especially in the upper part of the Nordland Group and Utsira Formation (Fig. 3).

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.

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., 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.

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., 1987: Late Miocene to Recent planktonic foraminifers from the North Atlantic: Deep Sea Drilling Project Leg 94. Initial Reports of the Deep Sea Drilling Project 94, 703-727.

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

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