Lower-Middle Eocene to Upper Pliocene in well 6609/5-1
Modified after Eidvin et al. (2007).
Based on analyses of planktonic and benthic foraminifera, Bolboforma, radiolarians and Sr isotopes in well 6609/5-1 (66º37’42.73’’N, 09º24’52.17’’E, Map 1), we recorded 50 m with Lower-Middle Eocene deposits, a 150 m-thick unit of Upper Miocene sediments, 25 m with sediment which we were only able to give a general Late Miocene – Lower Pliocene age, and 65 m with Upper Pliocene deposits. The units were investigated with 28 ditch-cutting samples at ten metre intervals (Fig. 1). For the section 1860-1790 m, see Eidvin et al (2007).
Well summary figure for well 6609/5-1
Biostratigraphy
Lower-Middle Eocene (1790-1740 m (log), Brygge Formation)
Radiolarians of the Cenosphaera sp. assemblage and benthic agglutinated foraminifera of the Bathysiphon eocenicus assemblage give an Early-Middle Eocene age to this unit (Fig. 1). In addition to abundant Cenosphaera sp. and other radiolarians the planktonic assemblage also includes pyritised diatoms. The planktonic assemblage is correlated with Zone NSP 6 of King (1989, North Sea), and the benthic assemblage is correlated with Zone NSA 4 of King (1989).
Upper Miocene (1740 (log)-1590 m, Kai Formation)
Bolboforma of the Bolboforma subfragori – Bolboforma fragori assemblage, Bolboforma laevis assemblage and Bolboforma metzmacheri assemblage and benthic foraminifera of the Martinottiella communis assemblage, Uvigerina venusta saxonica assemblage and Globocassidulina subglobosa assemblage (lower part) give a Late Miocene age to this interval (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains S. bulloides and C. telegdi. The planktonic foraminiferal fauna includes N. atlantica (dextral), N. atlantica (sinistral) and G. bulloides.
AB . 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 (Spiegler & Müller 1992, Müller & Spiegler 1993). The same authors have recorded a Bolboforma laevis/B. capsula Zone from the North Atlantic and a Bolboforma laevis Zone from the Vøring Plateau from deposits with an age of approximately 10.3-10.0 Ma. They have also recorded a B. metzmacheri Zone from sediments with an age of 10.0-8.7 Ma in the same areas. The Martinottiella communis assemblage is tentatively correlated with Zone FC of Doppert (1980) from the Netherlands. The Uvigerina venusta saxonica assemblage and the Globocassidulina subglobosa assemblage are correlated with Subzone NSB 13b of King (1989) from the North Sea and probably G. subglobosa – Ehrenbergina variabilis zone of Stratlab (1988).
Upper Miocene – Lower Pliocene (1590-1565 m (log), Kai Formation)
Benthic foraminifera of the Globocassidulina subglobosa assemblage (upper part) indicate a Late Miocene – Early Pliocene age for this unit (Fig. 1). G. bulloides is the sole planktonic foraminiferal species recorded in this interval. The assemblage is tentatively correlated with the upper part of Subzone NSB 13b and 14a of King (1989) from the North Sea and probably the G. subglobosa – Ehrenbergina variabilis zone of Stratlab (1988) from the Norwegian Sea continental shelf.
Upper Pliocene (1565 (log)-1500 m, Naust Formation)
Benthic foraminifera of the Cibicides grossus assemblage give a Late Pliocene 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 includes Elphidiella hannai (few), Cassidulina teretis, Elphidium excavatum, Cibicides scaldisiensis, Bulimina marginata, Buccella tenerrima and Elphidium albiumbilicatum. The planktonic foraminiferal fauna includes just a few specimens of Neogloboquadrina pachyderma (sinistral) and G. bulloides in some samples. The benthic foraminiferal fauna is correlated with Subzone NSB 15a of King (1989, North Sea) and Zone NSR 12 of Gradstein & Bäckström (1996, North Sea and Haltenbanken area).
Sr isotope stratigraphy
We performed Sr isotope analysis on calcareous benthic foraminiferal tests from the interval 1650-1660 m. The obtained 87Sr/86Sr ratio gave an age of 9.6 Ma (Late Miocene, Table 1, Fig. 1) which supports the biostratigraphical correlation.
Well 6609/5-1
| Litho. unit | Sample (DC) | Corrected 87/86Sr | 2S error | Age (Ma) | Analysed fossil species |
| Kai Fm | 1660-1650 m | 0.708900 | 0.000016 | 9.55 | 29 small tests of U. venusta saxonica |
Table 1: Strontium isotope data from well 6609/5-1. 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.
Lithology
Lower-Middle Eocene (1790-1740 m (log), Brygge Formation)
The samples from the Lower-Middle Eocene are dominated by clay (with some silt) and limestone (Fig. 1).
Upper Miocene-Lower Pliocene (1740 (log)-1565 m (log), Kai Formation)
The samples are mostly fine grained and clay dominates, but the contents of silt, sand (mainly quartzose) and pebbles of crystalline rocks are quite high in the upper part. The pebbles and most of the sand are probably caved from the Upper Pliocene section. The lower part is rich in glauconitic sand and some glauconite is present throughout (Fig. 1).
Upper Pliocene (1565 (log)-1500 m, Naust Formation)
The ditch-cutting samples from the Upper Pliocene interval contain a clay-rich diamicton which is also rich in sand, silt and ice-rafted pebbles of mainly crystalline rocks (Fig. 1). According to Fronval & Jansen (1996), in the Norwegian Sea there is a marked increase in the supply of ice-rafted material after about 2.75 Ma which reflects the expansion of the northern European glaciers. The maximum age for this unit is therefore considered to be 2.75 Ma, belonging mainly to the Gelasian Stage.
References
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Doppert, J. W. C., 1980: Lithostratigraphy and biostratigraphy of marine Neogene deposits in the Netherlands. Mededelingen Rijks Geologische Dienst 32-16, 2, 3-79.
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
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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.
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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.
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