Upper Oligocene in well 6305/4-1

In well 6305/4-1 (63º34’17.76’’N, 05º17’55.93’’E, see Map 1) the interval 1778.9-1761 m, in the upper part of the Brygge Formation, was cored with 94.3 % recovery. The core was given a latest Late Oligocene age based on analyses of benthic and planktonic foraminifera, pyritised diatoms and Sr isotopes. The top and base of the Upper Oligocene was not seen. The core was investigated with six samples (Fig. 1, Diagrams 1 and 2).

Well summary figure for well 6305/4-1

Biostratigraphy

Benthic calcareous foraminifera of the Turrilina alsatica – Gyroidina soldanii girardana assemblage and pyritised diatoms of the Diatom sp. 3 assemblage, supported by strontium-isotope ages, date the unit to Late Oligocene (Fig. 1). No planktonic foraminifera were recorded. In addition to the nominate species, the benthic foraminiferal fauna also contains Cibicides tenellus, Bulimina elongata, Pullenia bulloides, G. subglobosa and Sphaeroidina bulloides and the assemblage is correlated with the Zone NSB 8 of King (1989) and probably the 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).

Sr isotope stratigraphy

Four samples from three depths in the core were analysed for Sr isotopes. The obtained ⁸⁷Sr/⁸⁶Sr ratios all gave latest Late Oligocene ages close to the Miocene/Oligocene boundary (23.8 Ma, Table 1, Fig. 1).

Well 6304/4-1

Litho. Unit	Sample (cores)	Corrected 87/86Sr	2S error	Age (Ma)	Analysed fossil species
Brygge Fm	1762.4 m	0.708230	0.000009	24.44	17 tests of Cibicides tenellus, Gyroidina soldanii girardana, Bulimina elongata
Brygge Fm	1765.8 m	0.708267	0.000005	23.83	34 tests of B. elongata, Turrilina alsatica
Brygge Fm	1771.8 m	0.708251	0.000005	23.94	28 tests of Cibicides tenellus, G. soldanii girardana
Brygge Fm	1771. 8 m	0.708269	0.000006	23.79	35 tests of Cibicides tenellus, G. soldanii girardana

Table 1: Strontium isotope data from cored section in well 6305/4-1. The samples were 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.

Lithology

The core samples contain mainly pelagic sediments (siliceous ooze) with some mineralogical sand and silt (Fig. 1).

References

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.