Middle Miocene to Lower Oligocene in well 6610/2-1S
Modified after Eidvin et al. (2007).
Based on analyses of benthic foraminifera, dinoflagellate cysts and Sr isotopes, ten metre of Middle Eocene sediments and a 70 m-thick unit (1020 to 950 m) of Lower Oligocene deposits were investigated in well 6610/2-1S (66°48’48.73’’N, 10°30’26.70’’E, Map 1). The Lower Oligocene rests unconformably on the Middle Eocene. Eleven ditch-cutting samples at ten metre intervals were analysed (Fig. 1). The base of the Lower Oligocene was not investigated and the top of the Middle Miocene was not sampled.
Well summary figure for well 6610/2-1S
Biostratigraphy
Middle Eocene (1050-1040 m, Brygge Formation)
The age of this unit is mainly based on analysis of dinocysts. According to Eidvin et al. (2007), the interval contains relatively abundant and diverse Middle Eocene marine microfloras. An age not younger then Middle Eocene at 1040 m is inferred by the last appearance datum of Cassiculosphaera magma at this level. According to Bujak & Mudge (1994), the youngest occurrence of this species is within the middle NP15 Zone. The presence of Cerebrocysta bartonensis at 1040 m and Rottnestia borussica at 1050 m are further taken as evidence for an age not younger than Middle Eocene for this un-zoned interval. The single occurrence of Diphyes colligerum at 1050 m may suggest that strata correlatable with the Diphyes colligerum Zone are present at this level. According to Bujak & Mudge (1994), the D. colligerum Zone is of late Lutetian age. The interval also contains reworked, Late Paleocene, dinoflagellate cysts.
Lower Oligocene (1040-950 m, Brygge Formation)
Benthic calcareous foraminifera attributed to the Gyroidina soldanii mamillata – Rotaliatina bulimoides assemblage and the Turrilina alsatica assemblage and dinoflagellate cysts attributed to the Svalbardella cooksoniae Zone and Areoligera semicirculata Zone, supported by a strontium-isotope age obtained from the interval 1010-990 m, date this unit to Early Oligocene. The foraminiferal assemblages are correlated with Zone NSB 7 of King (1989) and Zone NSR 7A or 7B of Gradstein & Bäckström (1996).
Sr isotope stratigraphy
The 87Sr/86Sr ratio obtained from calcareous benthic foraminiferal tests in the interval 1010-990 m gave an age of 32.2 Ma (earliest Oligocene, Table 1, Fig. 1)
Well 6610/2-1S
| Litho. unit | Sample (DC) | Corrected 87/86Sr | 2S error | Age (Ma) | Analysed fossil species |
| Brygge Fm | 1010-990 m | 0.707894 | 0.000015 | 32.22 | Calcareous benthic index foraminifera |
Table 1: Strontium isotope data from well 6610/2-1S according to Eidvin et al (2007). The sample was analysed at the University of Bergen. The Sr ratio was corrected to NIST 987 = 0.710248. The numerical age was derived from the SIS Look-up Table Version 3:10/99 of Howard & McArthur (1997). NIST = National Institute for Standard and Technology.
Lithology
The ditch-cutting samples contain mainly clay. Small portions of fine-grained sand and silt are also recorded (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., 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.