Lower Oligocene to Middle Miocene in well 15/9-13
Modified after Eidvin & Rundberg (2007).
The stratigraphy of well 15/9-13 (58º22'25.96''N, 01º56'02.86''E, Map 1, Figs. 1 and 2) is based on analyses of benthic and planktonic foraminifera, Bolboforma, pyritised diatoms and Sr isotopes. In the lowermost sample we recorded Lower Oligocene deposits, further up we recorded 150 m with Upper Oligocene sediments, a 210 m-thick column with Lower Miocene sediments and 80 m with Middle Miocene deposits. The base of the Upper Oligocene and the top of the Middle Miocene were not investigated. The units are investigated with 37 ditchcutting samples at mainly ten metre intervals, but in some parts, where stored samples are depleted to the archive limit, the interval is 20 metres or more (Figs. 1 and 2). A detailed stratigraphy of these units (including fossil range charts) was presented in Eidvin & Rundberg (2007).
Well summary figure for well 15/9-13, fig 1
Well summary figure for well 15/9-13, fig 2
Biostratigraphy
Lower Oligocene (1550 m, Hordaland Group)
Benthic foraminifera of the Rotaliatina bulimoides assemblage date this sample to the Early Oligocene (Fig. 1). Planktonic foraminifera include G. angustiumbilicata, G. praebulloides and G. ciperoensis. The benthic foraminiferal fauna is correlated with Zone NSB 7 of King (1989) and probably Zone NSR 7B of Gradstein & Bäckström (1996) from the North Sea.
Upper Oligocene (1550-1400 m, Hordaland Group)
Pyritised diatoms of the Diatom sp. 3 assemblage and benthic foraminifera of the Turrilina alsatica assemblage and Plectofrondicularia seminuda assemblage (lowermost part), together with Sr isotope ages at 1520 m (24.5 Ma) and 1500 m (25 Ma) give a Late Oligocene age to this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna includes R. arnei, Elphidium hiltermanni and B. eocenicus. Planktonic foraminifera include G. angustiumbilicata, G. praebulloides and G. ciperoensis. The diatom assemblage is correlated with upper part of Subzone NSP 9c, and the benthic foraminiferal assemblage is correlated with Zone NSB 8 of King (1989) and probably Zone NSR 8A and the upper part of Zone 7B of Gradstein & Bäckström (1996) from the North Sea.
Lower Miocene (1550-1190 m, Hordaland Group and lowermost part of the Nordland Group)
Benthic foraminifera of the Plectofrondicularia seminuda assemblage (upper main part) and Uvigerina tenuipustulata assemblage, pyritised diatoms of the Diatom sp. 4 assemblage and planktonic foraminifera of the Globorotalia zealandica - Globigerina ciperoensis assemblage and Globigerina praebulloides - Globigerinoides quadrilobatus triloba assemblage (lowermost part), together with several 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 S. compressa and the Uvigerina tenuipustulata assemblage also includes A. guerichi staeschei. The Globorotalia zealandica - Globigerina ciperoensis assemblage also includes G. praescitula. The Plectofrondicularia seminuda assemblage is correlated with the uppermost part of Subzone NSB 8c and Zone NSB 9, the Uvigerina tenuipustulata assemblage is correlated with Zone NSB 10 and the Diatom sp. 4 assemblage is correlated with Zone NSP 10 of King (1989, North Sea). The Globorotalia zealandica - Globigerina ciperoensis assemblage is correlated with Zone NSP 11 (King 1983 and 1989) and Zone NSR 8B and the lower part of Zone NSR 9A of Gradstein & Bäckström (1996, North Sea). The lowermost part of the Globigerina praebulloides - Globigerinoides quadrilobatus triloba assemblage is correlated with the lowermost part of Zone NSP 12 of King (1983 and 1989, North Sea).
Middle Miocene (1190-1110 m, Nordland Group)
Benthic foraminifera of the Asterigerina guerichi staeshei assemblage and Uvigerina pygmea langeri - Uvigerina Pygmea langenfeldensis assemblage, planktonic foraminifera of the Globigerina praebulloides - Globigerinoides quadrilobatus triloba assemblage (upper main part) and Bolboforma of the Bolboforma reticulata assemblage and Bolboforma badenensis assemblage, together with a few Sr isotope ages, give a Mid Miocene age to this unit (Fig. 2). In addition to the nominate species, the benthic foraminiferal assemblages also include B. elongata. The Asterigerina guerichi staeshei assemblage is correlated with Zone NSB 11 of King (1989, North Sea), probably Zone FD of Doppert (1980, the Netherlands) and probably Zone NSR 9A of Gradstein & Bäckström (1996, North Sea). The Uvigerina pygmea langeri - Uvigerina pygmea langenfeldensis assemblage is correlated with Subzone NSB 13a of King (1989, North Sea). The Globigerina praebulloides - Globigerinoides quadrilobatus triloba assemblage is correlated with Zone NSP 12 of King (1983 and 1989) and the upper part of Zone NSR of Gradstein & Bäckström (1996) from the North Sea. 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.
Sr isotope stratigraphy
Three samples based on mollusc fragments and seven samples based on benthic and planktonic foraminifera and Bolboforma were analysed in well 15/9-13. The obtained 87Sr/86Sr ratios from 1520 and 1500 m gave ages of 24.5 and 25 Ma, respectively (Late Oligocene), and confirm the biostratigraphical correlations (Table 1, Fig. 1). Five samples from the upper half of the unit which were given an Early Miocene age by the biostratigraphical correlation, gave ages from 16.8 to 15.1 Ma. These ages broadly confirm the biostratigraphical correlations. However, one sample (based on mollusc fragments) from the uppermost part of this unit (1190 m) gave an age 12.2 Ma. This age is probably slightly too young, probably due to the fact that the mollusc fragments were caved (Table 1, Fig. 2). Two samples from the unit which were given a Mid Miocene age by the biostratigraphical correlation, gave 14.7 Ma (middle part) and 10.7 Ma (upper part, Table 1, Fig. 2). These ages also confirm the biostratigraphical correlations.
Well 15/9-13
| Litho. Unit | Sample (DC) | Corrected 87/86Sr | 2S error | Age (Ma) | Comments | Analysed fossils |
| Nordland Gr | 1110 m | 0.708871 | 0.000008 | 10.66 | Approximately 50 tests of B. badenensis and B. clodiusi | |
| Nordland Gr | 1160 m | 0.708790 | 0.000008 | 14.66 | 25 tests of A. guerichi staeshei, Bolivina cf. antiqua, Bolivina sp. | |
| Nordland Gr | 1190 m | 0.708831 | 0.000009 | 12.24 | Caved | Ten mollusc fragments |
| Hordaland Gr | 1200 m | 0.708779 | 0.000011 | 15.09 | 20 tests of G. praebulloides and Stilostomella spp. | |
| Hordaland Gr | 1210 m | 0.708776 | 0.000008 | 15.19 | Ten mollusc fragments | |
| Hordaland Gr | 1220 m | 0.708748 | 0.000009 | 15.92 | Two mollusc fragments | |
| Hordaland Gr | 1260 m | 0.708722 | 0.000015 | 16.44 | 13 tests of G. woodi | |
| Hordaland Gr | 1280 m | 0.708696 | 0.000010 | 16.84 | Ten tests of G. woodi | |
| Hordaland Gr | 1500 m | 0.708201 | 0.000019 | 25.00 | Ten tests of T. alsatica | |
| Hordaland Gr | 1520 m | 0.708237 | 0000041 | 24.45 | Ten tests of T. alsatica |
Table 1: Strontium isotope data from well 15/9-13. 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. Modified after Eidvin & Rundberg (2007).
Lithology
Lower Oligocene to Lower Miocene (1550 to approximately 1195 m, Hordaland Group)
Clay dominates the ditch cutting-samples. Minor sand and silt are also present in most samples. Sand is more common in the upper part, but some of this may be caved from the lower part of the Nordland Group. Mollusc fragments are quite common in parts of section, and limestone is recorded in some samples (Figs. 1and 2).
Uppermost Lower Miocene to Middle Miocene (approximately 1195 to 1110 m, Nordland Group)
Medium sand dominates the lower part of this unit, but clay and silt are also recorded. The sand is dominantly quartzose with minor glauconite and mica. The upper, main part of this unit is dominated by clay with minor sand and silt (Fig. 2)
References
Doppert, J. W. C., 1980: Lithostratigraphy and biostratigraphy of marine Neogene deposits in the Netherlands. Mededelingen Rijks Geologische Dienst 32-16, 2, 3-79.
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.
Eidvin, T. & Rundberg, Y., 2007: Post-Eocene strata of the southern Viking Graben, northern North Sea; intergrated biostratigraphic, strontium isotopic and lithostratigraphic study. Norwegian Journal of Geology 87, 391-450. Available from the internet: http://www.npd.no/Global/Norsk/3-Publikasjoner/Forskningsartikler/Eidvin_and_Rundberg_2007.pdf
Gradstein, F. & Bäckström, S., 1996: Cainozoic Biostratigraphy and Paleobathymetry, northern North Sea and Haltenbanken. Norsk Geologisk Tidsskrift 76, 3-32.
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.
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.