Lower-Upper Oligocene and Pleistocene in well 6404/11-1
Modified after Riis et al. (2005).
This well is from the erosional domain of the Storegga Slide area. Based on analyses of benthic and planktonic foraminifera, pyritised diatoms and Sr isotopes, a 180 m-thick unit (2400-2220 m) of probably mainly autochthonous Upper Oligocene deposits was investigated in the well 6404/11-1 (64º31’11.91’’N, 04º21’36.65’’E, Map. 1). Early Oligocene index foraminifera are not recorded in the investigated interval, but Sr isotope analysis of the sample at 2300 m indicates a latest Late Oligocene age close to the Early/Late Oligocene boundary. The unit is capped with allochthonous Pleistocene sediments (2220-2200 m). The base of the Oligocene and the top of the Pleistocene were not investigated. The units were investigated with 21 ditch-cutting samples at ten metre intervals (Fig. 1.).
Well summary figure for well 6404/11-1
Biostratigraphy
Uppermost Lower to Upper Oligocene (2400-2220 m, Brygge Formation)
Agglutinated foraminifera of the Cyclammina placenta – Cyclammina rotundidorsata assemblage (2400-2320 m), benthic calcareous foraminifera of the Turrilina alsatica – Gyroidina soldanii girardana assemblage (2320-2220 m) and pyritised diatoms of the Diatom sp. 3 assemblage (2300-2220 m), together with a strontium-isotope age obtained from 2300 m, indicate a latest Early to Late Oligocene age (Fig. 1).
The Cyclammina placenta – Cyclammina rotundidorsata assemblage is correlated with the Cyclammina acustidorsata – Cyclammina rotudidorsata Zone of Verdenius & van Hinte (1983) and probably Zone NSA 8 to 9 of King (1989). The Turrilina alsatica – Gyroidina soldanii girardana assemblage is correlated with the Zone NSB 8 of King (1989) and probably Zone NSR 8A of Gradstein & Bäckström (1996) from the North Sea. The Diatom sp. 3 assemblage is correlated with the Subzone NSP 9c of King (1989, North Sea).
Pleistocene (2220-2200 m, Naust Formation)
Planktonic foraminifera of the Neogloboquadrina pachyderma (sinistral, encrusted) assemblage and benthic foraminifera of the Elphidium albiumbilicatum assemblage give a Pleistocene age (on the time scale of Berggren et al. 1995) for this unit (Fig. 1). In addition to abundant encrusted forms of N. pachyderma (sinistral), the planktonic foraminiferal assemblage also includes a few specimens of N. pachyderma (dextral), Turborotalia quinqueloba and Globigerina bulloides. The occurrence of N. pachyderma (sinistral, encrusted) determines an age younger than 1.8 Ma for this unit (Spiegler & Jansen 1989). The benthic foraminiferal assemblage also includes Elphidium excavatum (common), Bulimina marginata (common), Cassidulina teretis (common), Nonion affine, Epistominella exigua, Angulogerina angulosa and Uvigerina peregrina. A few specimens of Cibicides grossus and Elphidiella hannai from the Late Pliocene to earliest Pleistocene, a few Inoceramus prisms (mollusc fragments) from the Late Cretaceous and a few agglutinated foraminifera from the early Cenozoic are also recorded. The benthic foraminiferal assemblage shows that the unit contains mainly Pleistocene and Holocene sediments primary deposited in deep, intermediate and shallow-water environments. Also in the mix are some Cenozoic and Mesozoic sediments. This is typical for allochthonous deposits from the erosional domain of the Storegga Slide area (Jansen et al. 1987, see also borehole 49-23).
Sr isotope stratigraphy
We performed Sr isotope analysis on calcareous benthic foraminiferal tests from the ditch-cutting sample at 2300 m, and the obtained 87Sr/86Sr ratio gave an age of 27.9 Ma (earliest Late Oligocene, Table 1, Fig. 1).
Well 6404/11-1
| Litho. unit | Sample (DC) | Corrected 87/86Sr | 2S error | Age (Ma) | Analysed fossil species |
| Brygge Fm | 2300 m | 0.708066 | 0.000005 | 27.92 | 18 tests of Turrilina alsatica, G. soldanii girardana |
Table 1: Strontium isotope data from well 6404/11-1. The sample were 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
Lower-Upper Oligocene (2400-2220 m, Brygge Formaton)
The ditch-cutting samples contain mainly pelagic sediments (siliceous ooze) with minor sand and silt (Fig. 1).
Pleistocene (2220-2200 m, Naust Formation)
The Pleistocene unit contains a clay-rich diamicton with sand, silt and common pebbles. The pebbles are mainly of crystalline rock (Fig. 1) and are interpreted to have been ice-rafted.
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.
Jansen, E., Befring, S., Bugge, T., Eidvin, T., Holtedahl, H. & Sejrup, H. P., 1987: Large submarine slides on the Norwegian Continental Margin: Sediments, transport and timing. Marine Geology 18, 503-512.
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.
Riis, F., Berg, K., Cartwright, J., Eidvin, T. & Hanch, H., 2005: Formation of large, crater-like evacuation structures in ooze sediments in the Norwegian Sea. Possible implications for the development of the Storegga Slide. Marine and Petroleum Geology 22, 257-273.
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.
Verdenius, J. G. & van Hinte, J. E., 1983: Central Norwegian-Greenland Sea: Tertiary (sic) arenaceous foraminifera, biostratigraphy and environment. In Verdenius, J. G., van Hinte, J. E. & Fortuin, A. R. (eds.), Proceedings of the First Workshop on Arenaceous Foraminifera, 173-223. Continental Shelf Intitute, Trondheim, Norway.