Upper Oligocene to Upper Pliocene in well 25/1-8 S
Based on analyses of benthic and planktonic foraminifera and Sr isotopes in well 25/1-8 S (59º54'03.283''N, 02º06'09.798''E, Map 1), we recorded 160 m with Upper Oligocene sediments, a 290 m thick column with Lower Miocene sediments, 90 m with Middle Miocene deposits and 160 m with Upper Pliocene deposits. There is a break between the Lower and Middle Miocene. The Upper Miocene is capped by Upper Pliocene deposits. The base of the Upper Oligocene and the top of the Upper Pliocene were not investigated. The units were investigated with 71 ditch-cutting samples at 10 meters intervals (Figs. 1 and 2).
Well summary figure for well 25/1-8 S, fig 1
Well summary figure for well 25/1-8 S, fig 2
Biostratigraphy
Upper Oligocene (900-740 m, Hordaland Group)
Benthic foraminifera of the Almaena osnabrugensis – Gyroidina soldanii girardana assemblage and planktonic foraminifera of the Globigerina angustiumbilicata – Globigerina praebulloides assemblage (lower part), together with a number of Sr isotope ages, give a Late Oligocene age for this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal assemblage also contains R. arnei, E. subnodosum, M. communis, G. subglobosa and T. alsatica (one sample). The planktonic foraminiferal assemblage also contains Globoquadrina baroemoensis and G. woodi in some samples. 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) from the North Sea.
Lower Miocene (740-450 m, Skade Formation)
Benthic foraminifera of the Elphidium subnodosum assemblage, Bolivina cf. antique assemblage and Astigerina guerichi staeshei – Bulimina elongate – Elphidium inflatum assemblage and planktonic foraminifera of the Globigerina angustiumbilicata – Globigerina praebulloides assemblage (upper part), together with a large number of Sr isotope ages, give an Early Miocene age for 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 (450-360 m, Nordland Group)
Planktonic foraminifera of the Globigerinoides quadrilobatus triloba assemblage and benthic foraminifera of the Florilus boueanus assemblage, together with a number of Sr isotope ages date this unit to the Mid Miocene (Fig. 2). No Bolboforma of the Bolboforma badensis - Bolboforma reticulata assemblage were recorded in this unit as in the Middle Miocene unit in the close by well 25/2-10 S. This is probably due to the fact that the Middle Miocene in 25/1-8 S was deposited in a more shallow-marine environment than in 25/2-10 S, and oceanic forms as Bolboforma did not enter the 25/1-8 S site. The benthic foraminiferal faunas are correlated with Zone NSB 11, NSB 12 and the lowermost part of Zone NSB 13 of King (1989) from the North Sea. The Globigerinoides quadrilobatus triloba assemblage is correlated with Zone NSP 12 of King (1989).
Upper Pliocene (360-200 m, Nordland Group)
Benthic foraminifera of the Elphidiella hannai assemblage and Elphidiella hannai - Cibicides grossus assemblage and planktonic foraminifera of Neogloboquadrina atlantica (sinistral) - 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. 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 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
Forty-five samples based on mollusc fragments from 39 depths and 18 samples based on foraminiferal tests from 15 depths were analysed for Sr isotopes in well 25/1-8 S.
The obtained 87Sr/86Sr ratios based on foraminiferal tests from 860-850 m, 810-800 m, 780-770 m and 770 m all gave quite similar ages of 26.3 to 25.8 (Late Oligocene, Table 1, Fig. 1). These ages support the biostratigraphical correlations. One sample, based on a mollusc fragment from 780 m, gave a somewhat younger age of 23.8 Ma and probably represents a caved fragment.
Twenty-nine analyses of mollusc fragments and eight analyses of foraminferal tests have been carried out for the part of the well which was given an Early Miocene age by the biostratigraphical correlations (including all of the Skade Formation). The obtained 87Sr/86Sr ratios gave ages with consistently increasing ages with depth from approximately 17 to 24 Ma, suggesting that the samples represent mainly in situ calcareous tests (Table 1, Figs. 1 and 2).
Fourteen samples based on mollusc fragments and six samples with foraminiferal tests were taken from the unit which was given a Middle Miocene age by the biostratigraphical correlations (lowermost part of the Nordland Group). Four of the samples were obviously based on caved tests. The other 16 samples gave ages from approximately 15.5 to 12 Ma. These results support the biostratigraphical correlations and show that there is a hiatus between the Lower and Middle Miocene (Table 1, Fig. 2). The obtained 87Sr/86Sr ratios of the four obvious caved samples (three based on mollusc and one based on foraminiferal tests) gave ages from 5.8 to 4.7 Ma (Late Miocene-Early Pliocene). We suggest that these tests were either reworked during the Late Pliocene and caved into the Middle Miocene unit or caved from a very condensed unit of Late Miocene-Early Pliocene age situated between the Middle Miocene and the Upper Pliocene.
Well 25/1-8 S
| Litho. Unit | Sample (DC) | Corrected 87/86Sr | 2S error | Age (Ma) | Comments | Analysed fossils |
| Nordland Gr | 360 m | 0.708837 | 0.000008 | 11.90 | 28 tests of F. Bouanus, Virgulinella pertusa | |
| Nordland Gr | 360 m | 0.708818 | 0.000008 | 12.93 | 15 tests of F. Bouanus, V. pertusa, E. inflatum | |
| Nordland Gr | 360 m | 0.709004 | 0.000009 | 5.78 | Caved | 13 tests of Amonia batavus, G. quadrilobatus triloba |
| Nordland Gr | 360 m | 0.709037 | 0.000009 | 5.00 | Caved | One mollusc fragment |
| Nordland Gr | 370 m | 0.708814 | 0.000008 | 13.12 | 17 tests of F. bouanus, V. pertusa, E. inflatum | |
| Nordland Gr | 370 m | 0.708822 | 0.000007 | 12.76 | One mollusc fragment | |
| Nordland Gr | 380 m | 0.708783 | 0.000009 | 14.95 | One mollusc fragment | |
| Nordland Gr | 380, 390, 400 m | 0.708795 | 0.000008 | 14.41 | 13 tests of F. bouanus, V. pertusa. | |
| Nordland Gr | 390 m | 0.708813 | 0.000008 | 13.17 | One mollusc fragment | |
| Nordland Gr | 390 m | 0.7087777 | 0.000008 | 15.15 | One mollusc fragment | |
| Nordland Gr | 400 m | 0.708829 | 0.000009 | 12.38 | One mollusc fragment | |
| Nordland Gr | 400 m | 0.708818 | 0.000008 | 12.55 | One mollusc fragment | |
| Nordland Gr | 410 m | 0.709017 | 0.000008 | 5.54 | Caved | One mollusc fragment |
| Nordland Gr | 410 m | 0.708809 | 0.000009 | 13.34 | One mollusc fragment | |
| Nordland Gr | 410, 420, 430, 440 m | 0.708766 | 0.000007 | 15.48 | 22 tests of F. bouanus, V. pertusa, E. inflatum | |
| Nordland Gr | 420 m | 0.709081 | 0.000010 | 2.15 | Caved | One mollusc fragment |
| Nordland Gr | 420 m | 0.708813 | 0.000008 | 13.17 | One mollusc fragment | |
| Nordland Gr | 430 m | 0.709045 | 0.000008 | 4.70 | Caved | One mollusc fragment |
| Nordland Gr | 430 m | 0.708810 | 0.000008 | 13.30 | One mollusc fragment | |
| Nordland Gr | 440 m | 0.708816 | 0.000007 | 13.03 | One mollusc fragment | |
| Nordland Gr | 440 m | 0.708829 | 0.000009 | 12.38 | One mollusc fragment | |
| Skade Fm | 450 m | 0.708629 | 0.000009 | 17.63 | One mollusc fragment | |
| Skade Fm | 450 m | 0.708659 | 0.000009 | 17.21 | One mollusc fragment | |
| Skade Fm | 450, 460, 470 m | 0.708726 | 0.000008 | 16.38 | 16 tests of A. guerichi staeschei | |
| Skade Fm | 460 m | 0.708608 | 0.000008 | 17.86 | One mollusc fragment | |
| Skade Fm | 470 m | 0.708676 | 0.000008 | 17.11 | One mollusc fragment | |
| Skade Fm | 480 m | 0.708621 | 0.000009 | 17.71 | One mollusc fragment | |
| Skade Fm | 480, 490, 500 m | 0.708655 | 0.000008 | 17.35 | 32 tests of A. guerichi staeschei, B. elongata | |
| Skade Fm | 490 m | 0.708574 | 0.000007 | 18.26 | 75 tests of G. angustiumbilicata | |
| Skade Fm | 490 m | 0.708570 | 0.000011 | 18.30 | 50 tests of G. angustiumbilicata | |
| Skade Fm | 490 m | 0.708596 | 0.000009 | 18.00 | One mollusc fragment | |
| Skade Fm | 500 m | 0.708573 | 0.000009 | 18.27 | 40 tests of G. angustiumbilicata | |
| Skade Fm | 500 m | 0.708556 | 0.000008 | 18.49 | One mollusc fragment | |
| Skade Fm | 510 m | 0.708504 | 0.000008 | 19.23 | One mollusc fragment | |
| Skade Fm | 520 m | 0.708408 | 0.000007 | 20.79 | One mollusc fragment | |
| Skade Fm | 530 m | 0.708375 | 0.000009 | 21.36 | One mollusc fragment | |
| Skade Fm | 550 m | 0.708372 | 0.000007 | 21.42 | One mollusc fragment | |
| Skade Fm | 560 m | 0.708561 | 0.000008 | 18.42 | One mollusc fragment | |
| Skade Fm | 570 m | 0.708342 | 0.000008 | 22.09 | One mollusc fragment | |
| Skade Fm | 580 m | 0.708365 | 0.000007 | 21.57 | One mollusc fragment | |
| Skade Fm | 590 m | 0.708354 | 0.000009 | 21.82 | One mollusc fragment | |
| Skade Fm | 600 m | 0.708328 | 0.000008 | 22.48 | One mollusc fragment | |
| Skade Fm | 610 m | 0.708399 | 0.000008 | 20.93 | One mollusc fragment | |
| Skade Fm | 620 m | 0.708381 | 0.000009 | 21.24 | One mollusc fragment | |
| Skade Fm | 630 m | 0.708342 | 0.000008 | 22.09 | One mollusc fragment | |
| Skade Fm | 640 m | 0.708296 | 0.000008 | 23.27 | One mollusc fragment | |
| Skade Fm | 650 m | 0.708285 | 0.000008 | 23.50 | One mollusc fragment | |
| Skade Fm | 660 m | 0.708258 | 0.000008 | 23.99 | One mollusc fragment | |
| Skade Fm | 670 m | 0.708235 | 0.000009 | 24.36 | One mollusc fragment | |
| Skade Fm | 680 m | 0.708280 | 0.000009 | 23.59 | One mollusc fragment | |
| Skade Fm | 690 m | 0.708296 | 0.000008 | 23.27 | One mollusc fragment | |
| Skade Fm | 690 m | 0.708204 | 0.000007 | 24.81 | 21 tests of E. subnodosum | |
| Skade Fm | 700 m | 0.708304 | 0.000007 | 23.04 | One mollusc fragment | |
| Skade Fm | 700 m | 0.708195 | 0.000009 | 24.95 | 23 tests of E. subnodosum | |
| Skade Fm | 710 m | 0.708352 | 0.000008 | 21.87 | One mollusc fragment | |
| Skade Fm | 710 m | 0.708196 | 0.000008 | 24.94 | 20 tests of E. subnodosum | |
| Skade Fm | 720 m | 0.708242 | 0.000008 | 24.25 | One mollusc fragment | |
| Skade Fm | 730 m | 0.708243 | 0.000008 | 24.23 | One mollusc fragment | |
| Hordaland Gr | 740 m | 0.708254 | 0.000008 | 24.05 | One mollusc fragment | |
| Hordaland Gr | 770 m | 0.708136 | 0.000009 | 25.91 | 25 tests of P. bulloides, R. arnei, G. sodanii girardana | |
| Hordaland Gr | 770, 780 m | 0.708125 | 0.000008 | 26.32 | 23 tests of Almaena osnabrugensis, P. bulloides | |
| Hordaland Gr | 780 m | 0.708267 | 0.000008 | 23.83 | Caved | One mollusc fragment |
Table 1: Strontium isotope data from well 25/1-8 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 & McArthur (1997). NIST = National Institute for Standard and Technology.
Lithology
Upper Oligocene (900-740 m, Hordaland Group)
The samples in this unit consist mainly of coarse, medium (dominant) and fine sand. Quartz is the dominant mineral. Mica and glauconite are common to rare. Mollusc fragments are common throughout and lignite coal is common in the lower part (Fig. 1).
Lower Miocene (740-450 m, Skade Formation)
The samples in the Lower Miocene unit consist mainly of sand, but in this unit the coarse fraction is dominant, the medium fraction is much smaller than in the Upper Oligocene and the fine fraction is even less frequent. Quartz is the dominant mineral. Mica and glauconite (less frequent) are also present throughout the medium sand fraction. Mollusc fragments are abundant, and lignite coal is common throughout (Figs. 1 and 2).
Middle Miocene and Upper Plicene (450-200 m, Nordland Group)
These parts are also dominated by sand. As in the underlying unit the coarse fraction is dominant, the medium fraction is less frequent and the fine fraction is least frequent. Quartz is the dominant mineral. Mica is common to rare, and glauconite is rare. Mollusc fragments are abundant and lignite coal is rare (Fig. 1).
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.
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.