? Middle Eocene to Lower Oligocene in well 6610/3-1
Modified after Eidvin et al. (1998a and 2007).
Based on analyses of benthic foraminifera, dinoflagellate cysts and Sr isotopes, approximately 25.5 m of ?Middle Eocene sediments and an approximately 65 m-thick unit of Lower Oligocene deposits were investigated in well 6610/3-1 (66°55’29.70’’N, 10°54’6.28’’E, Map 1). Seven sidewall cores at 25.5-20 metre intervals were analysed from the units (Fig. 1). The top of the Lower Oligocene was not sampled; see Eidvin et al. (2007) for the section 733-611 m. Seismic interpretation indicates no clear hiatus below the Lower Oligocene unit. The unit from 525 to 460 m was first investigated by Eidvin et al. (1998a) and was given an Oligocene age. Eidvin et al. (2007) interpreted the Oligocene fossils to be reworked and suggested a post-Mid Miocene age for the unit. The current view is that the fossils are not reworked that the correct age is Early Oligocene (Eidvin & Riis 2013).
Well summary figure for well 6610/3-1
Biostratigraphy
?Middle Miocene (611-585.5 m, Brygge Formation)
This unit has been investigated only for dinocysts. According to Eidvin et al. (2007) the assemblage is a rich and relatively diverse marine microflora, which includes several typical Eocene species. The presence of Enneadocysta fenestrata at 611 m points to an age not younger than early Late Eocene (early Priabonian), while the record of Rottnestia bourssica in the same sample restricts the age to not younger than latest Middle Eocene (Powell 1992, Bujak & Mudge 1994). The appearance of Corrudinium incompositum up to the sample at 585.5 m is further evidence for a Middle Eocene age for this interval (following the range for this species given in Powell, 1992). Reworked dinocysts include Endoscrinium pharao (Late Jurassic to Early Creataceous) found at 585.5 m.
Lower Oligocene (555-460 m, Molo Formation)
Benthic calcareous foraminifera of Gyroidina soldanii girardana assemblage supported by strontium-isotope ages date this unit to Early Oligocene. In addition to the nominate species, the benthic foraminiferal fauna also includes T. alsatica, Stilostomella adolphina, C. telegdi, G. subglobosa, C. dutemplei, S. adolphina and T. gracilis (see also Eidvin et al., 1998a). The foraminiferal assemblages can probably be correlated with Zone NSB 7 of King (1989) and Zone NSR 7A and 7B of Gradstein & Bäckström (1996).
Sr isotope stratigraphy
Three Sr isotope analyses from two sidewall cores were performed. Test mainly of calcareous benthic foraminifera were used. In one sample some Echinoderma spicules were included. The obtained 87Sr/86Sr ratio from the sidewall core at 525 m gave an age of 33.6 Ma (earliest Early Oligocene). The obtained 87Sr/86Sr-ratio from the sidewall core at 555 m gave 36.9 and 37.5 Ma (latest Late Eocene, Table 1, Fig. 1). According to King (1989), the analysed foraminifera do not have a range which extends down to the Upper Eocene, indicating that these results are slightly too old probably due to minor contaminations or some minor analytical error. However, if not the foraminiferal range according to King (1989) is quite correct, the Sr ages might be correct. The fact that the foraminiferal tests in the sample at 555 m were analysed at two different laboratories (see Table 1), both giving late Eocene ages, is in favour of the latter interpretation.
Well 6610/3-1
| Litho. unit | Sample (SWC) | Corrected 87/86Sr | 2S error | Age (Ma | Laboratory | Analysed fossil species |
| Molo Fm | 525 m | 33.6 | IFE | Calcareous benthic index foraminifer | ||
| Molo Fm | 555 m | 36.9 | IFE | Calcareous benthic index foraminifera | ||
| Molo Fm | 555 m | 0.707745 | 0.000014 | 37.53 | Approx. 40 tests foraminifera and fragments of Echinoderma including Guttulina spp., Stilostomella spp., Bolivina cf. antique, Gyroidina soldanii girardana |
Table 1: Strontium isotope data from well 6610/3-1. The samples were analysed at the University of Bergen and the Institute for energy technology (IFE, at Kjeller, Norway). Sample 525 and the first analysis of 555 m are modified after Eidvin et al. (1998a). 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
?Middle Miocene (611-585.5 m, Brygge Formation)
This unit contains mostly fine-grained sediments (mainly clay with some silt and sand, Fig. 1)
Lower Oligocene (555-460 m, Molo Formation)
The Lower Oligocene section contains mostly sand and pebbles. In the upper part the sand and pebble grains are rust stained. The lower part is not oxidised and contains grey, mica-rich sand (Fig. 1).
References
Bujak, J. & Mudge, D., 1994: A high-resolution North Sea Eocene dinocyst zonation. Journal of the Geological Society of London 151, 449-462.
Eidvin, T. & Riis, 2013: The Lower Oligocene – Lower Pliocene Molo Formation on the inner Norwegian Sea continental shelf (Extent and thickness, age from fossil and Sr isotope correlations, lithology, paleobathymetry and regional correlation). NGF Abstracts and proceedings, no. 1, 2013, p. 31. Poster available from the internet: http://www.npd.no/Global/Norsk/3-Publikasjoner/Presentasjoner/NGF-Vinterkonferanse-2013/Poster-4-til-NGF-vintermotet-nett.pdf
Eidvin, T., Brekke, H., Riis, F. & Renshaw, D. K., 1998a: Cenozoic Stratigraphy of the Norwegian Sea continental shelf, 64 N - 68 N. Norsk Geologisk Tidskrift, 78, 125-151.
Eidvin, T., Bugge, T. & Smelror, M., 2007: The Molo Formation, deposited by coastal progradation on the inner Mid-Norwegian continental shelf, coeval with the Kai Formation to the west and the Utsira Formation in the North Sea. Norwegian Journal of Geology 87, 75-142. Available from the internet: : http://www.npd.no/Global/Norsk/3-Publikasjoner/Forskningsartikler/Eidvin_et_al_2007.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.
Powell, A. J., 1992: Dinoflagellate cysts of the Tertiary System. In Powell, A. J. (ed.): A Stratigraphic Index of Dinoflagellate Cysts. Chapmann and Hall, London, 155-251.