Upper Oligocene to Upper Pliocene in well 2/4-C-11
Modified after Eidvin et al. (1999)
The post-Eocene sediments in well 2/4-C-11 (56º32'52.66''N, 03º12'55.69''E, Map 1, Figs. 1-3) are exceptionally well sampled with conventional cores and sidewall cores. The cores were sampled for rock mechanic investigations of the overburden due to the subsiding of the Ekofisk platform system. In no other oil well on the Norwegian continental shelf has this part of the sediment column been similarly sampled.
Well summary figure 2/4-C-11, fig. 1
Well summary figure 2/4-C-11, fig. 2
Well summary figure 2/4-C-11, fig. 3
Based on analyses of benthic and planktonic foraminifera and Sr isotopes, we recorded 31 m with Lower Oligocene sediments, a 237.3 m thick column with Upper Oligocene deposits, a 420.6 m thick column of Lower Miocene sediments, 118.4 m with Middle Miocene sediments, a 549.1 m-thick column with Upper Miocene sediments, approximately 145.6 m with Lower Pliocene deposits and approximately 163.3 m of Upper Pliocene sediments. There is probably a small hiatus between the Middle and Upper Miocene. The base of the Lower Oligocene was not investigated. The units were investigated with 90 ditch-cutting samples, 24 sidewall core samples (swc) and 13 conventional core samples. Due to lack of space a number of samples are not listed in the well summary figures (1-3) including 2303.1 m (core), 1745.0 m (swc) 1719.9 m (core), 1718.2 m (core), 1712.5 m (core), 1672.0 m (swc), 1563.2 m (core), 1561.5 m (core), 1559.6 m (core), 1557.7 m (core), 1556.0 m (swc), 1362.0 m (swc), 1307.0 m (swc), 1163.4 m (core), 1162.6 m (core) and 1160.9 m (core). Except for the Lower Oligocene and the lower part of the Upper Oligocene, an investigation of these units was presented by Eidvin et al. (1999, including fossil range charts). In that paper we have also presented the section from 804.7 to 190 m (upper part of Upper Pliocene and Pleistocene on the time scale of Berggren et al. 1995). Here, we have re-interpreted and modified the stratigraphy somewhat.
Biostratigraphy
Lower Oligocene (2472-2441 m, Hordaland Group)
Benthic calcareous foraminifera of the Rotaliatina bulimoides assemblage and Gyroidina soldanii mamillata assemblage give an Early Oligocene age to this unit (Fig. 1). In addition to the nominate species, the benthic calcareous foraminiferal fauna also contains G. soldanii girardana. The dominant agglutinated fauna includes G. charoides, B.eocenicus , C. placenta and Dorothia seigliei. Planktonic foraminifera are not recorded. The benthic calcareous fauna is correlated with Subzone NSB 7b of King (1989) and Zone NSR 7B of Gradstein & Bäckström (1996, North Sea).
Upper Oligocene (2441-2203.7 m, Hordaland Group)
Pyritised diatoms of the Diatom sp. 3 assemblage and agglutinated foraminifera of the Ammodiscus sp. B assemblage (lower part), together with a Sr isotope age, give a Late Oligocene age to this unit (Fig. 1). In addition to the nominate species, the dominant agglutinated foraminiferal fauna also includes Ammodiscus sp. A (King 1989), B. eocenicus, C. placenta, Reticulophragmium rotundidorsatum, Bathysiphon nodosariaformis and S. compressa. The planktonic foraminiferal fauna includes a few specimens of G. praebulloides, G. angustiumbilicata and G. ciperoensis. In the lower part of the unit, a few specimens of the benthic calcareous T. alsatica and G. soldanii girardana are also recorded. The diatom assemblage is correlated with the upper part of Subzone NSP 9c and the agglutinated foraminiferal fauna is correlated with Zone NSA 9 of King (1989) from the North Sea.
Lower Miocene (2203.7-1664.7 m, Hordaland Group)
Benthic foraminifera of the Ammodiscus sp. B assemblage (upper part), S. compressa assemblage, Ammodiscus sp. A assemblage and U. tenuipustulata assemblage and diatoms of the Diatom sp. 4 assemblage, together with a Sr isotope age, give an Early Miocene age for this unit (Figs. 1 and 2). Planktonic foraminifera are only recorded in the upper part and include G. praebulloides, G. angustiumbilicata, G. ciperoensis and Globigerina woodi. The upper part of the Ammodiscus sp. B assemblage is correlated with the upper part of Zone NSA 9, the Spirosigmoilinella compressa and the Ammodiscus sp. A assemblage are correlated with Zone NSA 10 and the Uvigerina tenuipustulata assemblage is correlated with Zone NSB 10 of King (1989) from the North Sea. The diatom assemblage is correlated with Zone NSP 10 (King 1989, North Sea).
Middle Miocene (1783.1-1664.7 m, Nordland Group)
Benthic foraminifera of the Bulimina elongata assemblage and planktonic foraminifera of the Globigerina praebulloides - Globigerinoides quadrilobatus triloba - Sphaeroidinellopsis disjuncta assemblage, together with two Sr isotope ages give an early Middle Miocene age for this unit (Fig. 2). In addition to the nominate species, the benthic foraminiferal fauna also contains Valvulineria mexicana grammensis. The planktonic foraminiferal assemblage also contains G. zealandica. The benthic foraminiferal assemblage is correlated with Zone NSB 11 and Subzone NSB 12a, and the planktonic foraminiferal assemblage is correlated with Zone NSP 12 of King (1989) from the North Sea.
In the Middle Miocene in wells 16/1-4, 24/12-1, 25/10-2, 15/9-13 and15/12-3 from the southern Viking Graben, well 6507/12-1 on the Norwegian Sea continental shelf, borehole 6403/5-GB1 in the Møre Basin and borehole 6704/12-GB1 on the Gjallar Ridge on the Vøring Plateau (uppermost part of the Middle Miocene), we have recorded either a Bolboforma badenensis - Bolboforma reticulata assemblage or a Bolboforma badenensis and a Bolboforma reticulata assemblage. These assemblages correlate with the B. badenensis/B. reticulata Zone of Spiegler & Müller (1992) and Müller & Spiegler (1993) from the North Atlantic and Norwegian Sea. In these areas the zone has been calibrated using nannoplankton and paleomagnetic data, and was dated to approximately 14-11.7 Ma. Since these Bolboforma are not recorded in well 2/4-C-11, a part of the Middle Miocene is probably missing. This is also supported by the two Sr isotope analyses at 1724 m which both give early Mid Miocene ages (Fig. 2).
Upper Miocene (1664.7-1115.6 m, Hordaland Group)
Benthic foraminifera of the Glomospira charoides assemblage and Uvigerina venusta saxonica assemblage (lower, main part), Bolboforma of the Bolboforma fragori - B. subfragori assemblage and Bolboforma metzmacheri assemblage and planktonic foraminifera of the Neogloboquadrina acostaensis assemblage and Neogloboquadrina atlantica (dextral) assemblage, together with several Sr isotope ages, date this unit to Late Miocene (Figs. 2 and 3). In addition to the nominate species, the Glomospira charoides assemblage also includes Chilostomellina fimbriata and the Uvigerina venusta saxonica assemblage is rich on F. boueanus and G. subglobosa.
The Glomospira charoides assemblage is correlated with Zone NSA 12 and the Uvigerina venusta saxonica assemblage is correlated with NSB 13 of King (1989, 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 acostaensis assemblage is correlated with a N. acostaensis Zone of Spiegler & Jansen (1989), which is reported from upper Middle to Upper Miocene sediments on the Vøring Plateau. It is also correlated with the lower part of N. atlantica (dextral)/N. acostaensis Zone of Weaver & Clement (1987) which is described from the Upper Miocene in the North Atlantic. Further, the Neogloboquadrina acostaensis assemblage is correlated with Subzone NSP 15a of King (1989, North Sea). 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.
Lower Pliocene (1115.6 to approximately 970 m, Nordland Group)
Lower Pliocene (1115.6 to approximately 970 m, Nordland Group)
Benthic foraminifera of the Uvigerina venusta saxonica assemblage (uppermost part) and Cibicidoides limbatosuturalis assemblage (lower half) and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) assemblage (lower part) give an Early Pliocene age to this unit (Fig. 3). The benthic foraminiferal assemblages also include M. pseudotepida, G. subglobosa and F. bouanus. The planktonic foraminiferal assemblage also contains common G. bulloides. The Cibicidoides limbatosuturalis assemblage is correlated with the Subzone NSB 14a of King (1989). King (1989) describes Subzone NSB 14a (Cibicidoides limbatosutualis Subzone) from the Lower Pliocene to lowermost Upper Pliocene in the central and southern North Sea, and suggest that the Lower/Upper Pliocene boundary is close to the middle part of the unit. Farther north in the North Sea, Subzone NSB 14a (Cibicidoides limbatosutualis Subzone) becomes thinner and eventually merges with Subzone NSB 14b (Monspeliensina pseudotepida Subzone) according to King (1989). The N. atlantica (sinistral) assemblage is correlated with Neogloboquadrina atlantica (sinistral) Zone of Weaver & Clement (1986, North Atlantic) and Spiegler & Jansen (1989, Vøring Plateau) and Subzone NSP 15d of King (1989, North Sea).
Upper Pliocene (approximately 970 to 804.7 m, Nordland Group)
Benthic foraminifera of the Cibicidoides limbatosuturalis assemblage (upper half), Monspeliensina pseudotepida assemblage and Cibicides grossus assemblage and planktonic foraminifera of the 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. The benthic foraminiferal assemblages are correlated with Subzone NSB14a (upper part), Subzone NSB 14b and Subzone NSB 15a of King (1989, North Sea). On the Vøring plateau (Norwegian Sea) N. atlantica (sinistral) and G. bulloides were common in the Late Miocene to Late Pliocene until about 2.4 Ma. N. pachyderma (dextral) was common in the Late Pliocene until about 1.8 Ma (Spiegler & Jansen 1989). Well 2/4-C-11 is the only well we have investigated where the lower part of the Upper Pliocene (Piacenzian) is well developed (upper part of the Cibicidoides limbatosuturalis assemblage and Monspeliensina pseudotepida assemblage). This is also supported by the fact that no ice-rafted detritus is recorded in this part of the Upper Pliocene.
Sr isotope stratigraphy
We performed 16 Sr isotope analyses from 15 depths in well 2/4-C-11. Samples mainly from side-wall cores and conventional cores were used. Two analyses were from ditch-cutting samples. Tests mainly of calcareous benthic foraminifera were used, but some analyses were based on tests from planktonic foraminifera, Bolboforma and molluscs (Table 1, Figs. 1-3).
The 87Sr/86Sr ratio obtained from the samples taken from 2441, 2451, 2460 m gave an age of 27.9 Ma (close to the Early/Late Oligocene boundary), and the sample from 2357.3 m gave an age of 25.3 Ma (Late Oligocene) and both support the biostratigraphical correlations. The sample from 1953 and 1972 m gave 20.7 and 16.7 Ma, respectively, and this fits well with the fact that the biostratigraphical correlation has given an Early Miocene age for this part of the succession.
One analysis based on molluscs and one based on calcareous benthic foraminifera from the core sample at 1724 m gave 14.7 and 15.5 Ma, respectively (early Middle Miocene). This also supports the biostratigraphical correlations.
The unit which was given a Late Miocene age by biostratigraphical correlations was investigated with six analyses. The obtained 87Sr/86Sr ratios gave ages of 7.4 Ma in the lower part to 5.2 Ma in the upper part, which fit very well with the correlations.
Four analyses from the part given an Early Pliocene age by the biostratigraphical correlation do not fit accurately. Three of the obtained 87Sr/86Sr ratios gave Late Miocene ages and one gave an early Late Pliocene age. The Sr isotope curve of Howard & McArthur (1997) shows little variation through the Pliocene deposits. There is a relatively flat part between 4.5 and 2.5 Ma, and the obtained 87Sr/86Sr ratios from deposits of such ages are often inaccurate (Eidvin & Rundberg 2001 and 2007). However, the obtained 87Sr/86Sr ratio from the base of the Late Pliocene unit gives an age which fits very well with the biostratigraphical correlation (Fig. 3).
Well 2/4-C-11
| Litho. Unit | Sample | Corrected 87/86Sr | 2S error | Age (Ma) | Laboratory | Analysed fossils |
| Nordland Gr | 965 m (SWC) | 0.709059 | 0.000011 | 3.68 | IFE | Calcareous benthic index foraminifera |
| Nordland Gr | 1004 m (SWC) | 0.709064 | 0.000013 | 3.31 | IFE | Planktonic index foraminifera |
| Nordland Gr | 1004 m (SWC) | 0.708972 | 0.000010 | 6.21 | IFE | Calcareous benthic index foraminifera |
| Nordland Gr | 1004 m (SWC) | 0.709011| | 0.000008 | 5.67 | IFE | Mollusc fragments |
| Nordland Gr | 1077 m (SWC) | 0.709001 | 0.000016 | 5.82 | IFE | Calcareous benthic index foraminifera |
| Nordland Gr | 1160.7 m (Core) | 0.709015 | 0.000010 | 5.58 | IFE | Calcareous benthic index foraminifera |
| Nordland Gr | 1163.4 m (Core) | 0.709030 | 0.000012 | 5.18 | IFE | Calcareous benthic index foraminifera |
| Nordland Gr | 1293 m (SWC) | 0.709012 | 0.000019 | 5.65 | IFE | Calcareous benthic index foraminifera |
| Nordland Gr | 1494 m (SWC) | 0.708959 | 0.000010 | 6.60 | IFE | Calcareous benthic index foraminifera |
| Nordland Gr | 1600.2 m (DC) | 0.708936 | 0.000036 | 7.43 | IFE | Bolboforma |
| Hordaland Gr | 1724 m (Core) | 0.708789 | 0.000009 | 14.70 | IFE | Mollusc fragments |
| Hordaland Gr | 1724 m (Core) | 0.708764 | 0.000013 | 15.54 | IFE | Calcareous benthic index foraminifera |
| Hordaland Gr | 1792 m (SWC) | 0.708705 | 0.000018 | 16.71 | IFE | Planktonic index foraminifera |
| Hordaland Gr | 1953 m (SWC) | 0.708415 | 0.000012 | 20.67 | IFE | Calcareous benthic index foraminifera |
| Hordaland Gr | 2357.3 m (SWC) | 0.708173 | 25.34 | IFE | Calcareous benthic index foraminifera | |
| Hordaland Gr | 2441, 2451, 2460 m (DC) | 0.708068 | 0.000009 | 27.87 | UiB | 14 tests of G. soldanii mamillata, G. soldanii girardana, R. bulimoides |
Table 1: Strontium isotope data from well 2/4-C-11. IFE = Analysed at the Institute for energy technology (at Kjeller, Norway). UiB = Analysed at the University of Bergen (Norway). 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 from Eidvin et al. (1999).
Lithology
Clay dominated the whole of the investigated succession in well 2/4-C-11 with minor sand (mainly quartzose) and silt in most parts. Limestone is recorded in the lower part of the Upper Miocene.
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.
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., 2001: Late Cainozoic stratigraphy of the Tampen area (Snorre
and Visund fields) in the northern North Sea, with emphasis on the chronology of early
Neogene sands. Norsk Geologisk Tidsskrift, 81, 119–160. Available from the internet: http://www.npd.no/Global/Norsk/3%20-%20Publikasjoner/Forskningsartikler/Eidvin_and_Rundberg_2001.pdf
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
Eidvin, T., Riis, F. & Rundberg, Y., 1999: Upper Cainozoic stratigraphy in the central North Sea (Ekofisk and Sleipner fields). Norsk Geologisk Tidsskrift 79, 97-127. Available from the internet: http://www.npd.no/Global/Norsk/3-Publikasjoner/Forskningsartikler/Eidvin-et-al-1999-(2).pdf
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.
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.
Weaver, P. P. E. & Clement, B. M., 1987: Magnetobiostratigraphy of planktonic foraminiferal datums: Deep Sea Drilling Project Leg 94, North Atlantic. In Ruddiman, W. F., Kidd, R. B., Thomas, E., et al. (eds.): Initial Reports of the Deep Sea Drilling Project 94, 815-829.