Lowermost Upper Oligocene in gravity core 49-23
The short gravity core 49-23 (64º42’N, 01º38’E) was taken from the lowermost part of the erosional domain of the Storegga Slide area (see Map 1). The lower part of the core (about 30 cm) contains debris-flow deposits, and in the upper part there is about 30 cm with turbidite deposits. This is capped with about 10 cm of autochthonous Holocene pelagic sediments deposited after the slide occurred at approximately 8100±250 cal. Yrs BP (Haflidason et al. 2005, Fig. 1). The allochthonous sediments are mainly derived from late Pleistocene glacial and hemipelagic deposits containing mixed foraminiferal faunas from several habitats (Fig. 1). However, the lowermost part of the core consists of a large piece of hard pelagic ooze with an undisturbed Oligocene foraminiferal fauna (Eidvin 1984, Jansen et al. 1987). The core depth is in cm below sea floor (cmbsf).
Biostratigraphy
The benthic calcareous foraminiferal assemblage recorded from this piece (49-47 cm) contains common T. alsatica, G. subglobosa and Epistominella oveyi. Other taxa include C. tenellus, Lagena spp., Lenticulina sp. and Glandulina sp. It is noteworthy that all the tests are very small and that no planktonic foraminifera are recorded. The assemblage can probably be correlated with the lowest part of Zone NSB 8 of King (1989) and the lowest part of Zone NSR 8A of Gradstein & Bäckström (1996).
Sr isotope stratigraphy
We performed Sr isotope analysis on the tests of the calcareous benthic foraminifera described above, and the obtained 87Sr/86Sr ratio gave an age of 28.2 Ma (earliest Late Oligocene, Table 1).
Gravity core 49-23
| Litho. Unit | Sample (core) | Corrected 87/86Sr | 2S error | Age (Ma) | Analysed fossil species |
| Brygge Fm | 0.49-0.47 m | 0.708056 | 0.000013 | 28.17 | Approximately 50 small tests of T. alsatica, G. subglobosa, Lagena sp. Lenticulina sp., Glandulina sp., Cibicides tenellus, Epistominella oveyi |
Table 1: Strontium isotope data from gravity core 49-23. The sample was analysed at the University of Bergen. 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.
Origin of the piece of Late Oligocene pelagic ooze
It is only in the lower part of the erosional domain of the Storegga Slide area that the slide cut into the pre-Pliocene pelagic ooze deposits of the Kai and Brygge formations (Jansen et al. 1987, Bugge et al. 1987, Riis et al. 2005), and the piece found in core 49-23 (siliceous ooze of the Brygge Formation) has probably not been transported very far down the continental slope. Consequently, the recorded benthic foraminifera are representative for the foraminiferal fauna which dwelled on the lower continental slope close to the Early/Late Oligocene boundary.
References
Bugge, T., Befring, S., Belderson, H. R., Eidvin, T., Jansen, E., Kenyon, N. H., Holtedahl, H. & Sejrup, H. P., 1987: A Giant Three-Stage Submarine Slide Off Norway. Geo-Marine Letters 7, 191-198.
Eidvin, T., 1984: Stratigrafiske undersøkelser av kjerneprøver fra rasområdet utenfor Storegga. Cand. Sci. Thesis, Univ. Bergen, 122 pp, (unpublished).
Gradstein, F. & Bäckström, S., 1996: Cainozoic Biostratigraphy and Paleobathymetry, northern North Sea and Haltenbanken. Norsk Geologisk Tidsskrift 76, 3-32.
Haflidason, H., Lien, R., Sejrup, H. P., Forsberg, C. F. & Bryn, P., 2005: The dating and morphometry of the Storegga Slide. Marine and Petroleum Geology 22, 123-136.
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.