Lower-Upper Oligocene to Pleistocene in well 35/11-1
Based on analyses of benthic foraminifera, Bolboforma and pyritised diatoms in well 35/11-1 (61º10’59.27’’N, 03º39’44.95’’E, Map 1), we recorded 20 m with Lower-Upper Oligocene deposits, 80 m with deposits which were given a general ?Mid to Late Miocene age, 50 m with Upper Miocene sediments, 30 m with deposits which were given a general Late Miocene to Early Pliocene age and 30 m with Pleistocene sediments. The base of the Oligocene and the top of the Pleistocene were not investigated. Unfortunately, no sample contains a sufficient number of calcareous index fossils to give reliable Sr isotope ages. The units were investigated with 21 ditch-cutting samples at ten metre intervals, except between 600 and 580 m (Fig. 1).
Well summary figure for well 35/11-1
Biostratigraphy
Lower-Upper Oligocene (730-710 m, Hordaland Group)
Benthic foraminifera of the Turrilina alsatica assemblage and pyritised diatoms of the Diatom sp. 3 assemblage together with seismic and log correlations indicate a latest Early Oligocene to earliest Late Oligocene age for this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains Nonion granosum. No in situ planktonic foraminifera were recorded. The foraminiferal assemblage is correlated with the upper part of Subzone NSB 7b and lower part of Subzone 8a of King (1989) and probably the upper part of Zone NSR 7B of Gradstein & Bäckström (1996).
?Middle-Upper Miocene (710-630 m, Utsira Formation)
In this well the whole of the Utsira Formation consists of coarse sand. Microfossils are usually winnowed out during deposition of such sediments. Most of the recorded species were probably caved from the Pleistocene unit. Exceptions include a few specimens of the benthic foraminifera G. subglobosa and S. bulloides, which are common in the Utsira Formation in other wells and point to a general Miocene to Lower Pliocene age. A few specimens of the planktonic foraminifer G. bulloides point to a general Mid Miocene to Pleistocene age, and one specimen of B. badenensis points to a latest Mid Miocene age. However, a Late Miocene age is most likely.
Upper Miocene (630-580 m, Utsira Formation)
Microfossils, interpreted to be in situ, are also very scarce in this unit. However, S. bulimoides and B. subfragori occur continuously throughout, and the occurrences of these species give a Late Miocene age to this unit. In addition to the nominate species, the S. bulloides assemblage includes a few specimens of G. subglobosa, E. pygmeus, E. variabilis and U. venusta saxonica which all are typical for the Utsira Formation in other wells. In addition to the nominate species, the B. subfragori assemblage also contains a few specimens of B. clodiusi and one specimen of B. metzmacheri. The last species is interpreted to be caved from a higher level in the Utsira Formation even thought it was not recorded there.
A Bolboforma fragori/B. subfragori Zone is described from deposits with an age of 11.7-10.3 Ma from the North Atlantic and the Vøring Plateau, and a Bolboforma metzmacheri Zone is described from sediments with an age of 10.0-8.7 Ma from the same areas (Spiegler & Müller 1992, Müller & Spiegler 1993).
Upper Miocene-Lower Pliocene (580-550 m, Utsira Formation)
Benthic foraminifera attributed to the uppermost part of the Sphaeroidina bulloides assemblage indicate a Late Miocene to Early Pliocene age for this interval. In addition to the nominate species the assemblage includes E. pygmeus, G. subglobosa and E. variabilis which all are typical for the Utsira Formation in other wells.
Pleistocene (550-520 m, Nordland Group)
Benthic foraminifera of the Nonion labradoricum assemblage and planktonic foraminifera of the Neogloboquadrina pachyderma (sinistral) assemblage give a Pleistocene age (on the time scale of Berggren et al. 1995) for this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal assemblage also contains Elphidium excavatum (common), Cassidulina reniforme (common), Virgulina loeblichi, Cibicides lobatulus, Islandiella norcrossi and Islandiella islandica. The planktonic foraminiferal fauna also includes a few specimens of N. pachyderma (dextral) and Globigerina bulloides. The benthic foraminiferal fauna is correlated with Subzone 16x of King (1989, North Sea) and Zone NSR 13 of Gradstein & Bäckström (1996, North Sea and Haltenbanken area). The planktonic foraminiferal fauna is correlated with Neogloboquadrina pachyderma (sinistral) Zone of Spiegler & Jansen (1989). The encrusted form of N. pachyderma (sinistral) has its first frequent occurrence in the Norwegian Sea at 1.8 Ma (Spiegler & Jansen 1989).
Lithology
Lower-Upper Oligocene (730-710 m, Hordaland Group)
The Oligocene deposits consist mainly of silty mudstones (Fig. 1).
?Middle-Upper Miocene to Upper Miocene-Lower Pliocene (710-550 m, Utsira Formation)
This unit consists primarily of coarse, quartzose sand. The bulk of the sand is coarser than 0.5 mm. Glauconitic sand is also recorded in parts of section (Fig. 1).
Pleistocene (550-520 m, Nordland Group)
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.
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.
Müller, C. & Spiegler, D., 1993: Revision of the late/middle Miocene boundary on the Voering Plateau (ODP Leg 104). Newsletter on Stratigraphy, 28 (2/3), 171-178.
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.
Spiegler, D. & Müller, C., 1992: Correlation of Bolboforma zonation and nannoplankton stratigraphy in the Neogene of the North Atlantic: DSDP sites 12-116, 49-408, 81-555 and 94-608. Marine Micropaleontology 20, 45-58.