Lower Oligocene to Upper Pliocene in well 6609/11-1
Modified after Eidvin et al. (2007).
The biostratigraphical investigation of well 6609/11-1 (66º08’13.90’’N, 09º33’47.89’’E, Map 1) is based on analyses of Bolboforma, benthic and planktonic foraminifera, pyritised diatoms, dinoflagellate cysts and Sr isotopes. In the lowermost investigated sample we recorded Middle Eocene deposits. Above, we recorded 25 m with Upper Eocene sediments, 20 m with Lower Oligocene, 10 m with Upper Oligocene, 65 m with Lower Miocene, approximately 25 m with Upper Miocene, approximately 38 m with Upper Miocene – Lower Pliocene and approximately 32 m with Upper Pliocene deposits. The base of the Middle Eocene and the top of the Upper Pliocene were not investigated. The units were investigated with 24 ditch-cutting samples at mainly ten metre intervals (Fig. 1).
Well summary figure for well 6609/11-1
Biostratigraphy
Middle Eocene (1410 m, Brygge Formation)
According to Eidvin et al. (2007) dinoflagellate cysts of the Diphyes colligerum Zone and benthic agglutinated foraminifera of the Spiroplectammina spectabilis assemblage give a Middle Eocene age to this sample (Fig. 1). The Diphyes colligerum Zone also includes Phthanoperidinium distictum and the presence of the latter species suggests a correlation to Subzone E6b of Bujak & Mudge (1994) dated as late Lutetian. According to King (1989), S. spectabilis is known from Lower to Middle Eocene in the North Sea area. According to Gradstein & Bäckström (1996) this species is described from Upper Paleocene to Middle Eocene deposits in the North Sea and Haltenbanken areas. The Spiroplectammina spectabilis assemblage is correlated to Zone NSA 5 of King (1989) and probably Zone NSR 5A or Zone NSR 5B of Gradstein & Bäckström (1996). The present dinocysts data suggest that there is a minor hiatus between the Diphyes colligerum Zone and the overlying Areosphaeridium dictyoplokkus Zone and that Bartonian strata (correlatable to the H. porosa Zone of Bujak & Mudge 1994) are missing.
Upper Eocene (1410-1385 m, Brygge Formation)
According to Eidvin et al. (2007), dinocysts of to the Areosphaeridium dictyoplokkus Zone give a Late Eocene age to this interval (Fig. 1). The last occurrence datum (LAD) of A. dictyoplokkus appears to be a well documented marker of the uppermost Eocene in the North Sea area (Powell 1992, Bujak & Mudge 1994) and the Norwegian-Greenland Sea (Firth 1996, Mangerud & Charnock 1999).
Lower Oligocene (1385-1365 m, Brygge Formation)
According to Eidvin et al. (2007), dinoflagellate cysts of to the Svalbardella cooksoniae Zone and the Areoligera semicirculata Zone and pyritised diatoms of the Diatom sp. 3 assemblage (lower part) date this unit to the Early Oligocene (Fig. 1). The diatom assemblage is correlated with the lower part of Subzone NSP 9c of King (1989) from the North Sea. Mannum et al. (1989) found that S. cooksoniae was restricted to their Early Oligocene Chiropteridium lobospinosum Zone in ODP Hole 643 in the Norwegian Sea. According to Powell (1992), the LAD of A. semicirculata lies within the lower NP25 calcareous nannoplankton biozone in Britain and in the North Sea area. Manum et al. (1989) found that the LAD of this species (named Glaphyrocysta intricata in their publication) corresponds tothe upper boundary of their Early/Late Oligocene Areospheridium? actinocoronatum Zone in ODP Hole 643 in the Norwegian Sea.
Upper Oligocene (1365-1355 m, Brygge Formation)
According to Eidvin et al. (2007), dinocysts attributed to the Distatodinium biffi Zone and the Chiropteridium spp. Zone (lowermost part) and pyritised diatoms of the Diatom sp. 3 assemblage (upper part) give a Late Oligocene age to this unit (Fig. 1). The diatom assemblage is correlated with the upper part of Subzone NSP 9c of King (1989) from the North Sea. According to Brinkhuis & Biffi (1993) and deVerteuil & Norris (1996), the LAD of D. biffi is found just below the Oligocene/Miocene boundary in the Mediterranean and on the U.S. Coast Plain. A concurrent LAD has also been recorded by M. Smelror (unpublished data) in the Norwegian-Greenland Sea.
Lower Miocene (1355-1285 m (log), Brygge Formation)
According to Eidvin et al. (2007), pyritised diatoms of the Diatom sp. 4 assemblage, benthic foraminifera of the Trifarina gracilis assemblage and dinoflagellate cysts of Chiropteridium spp. Zone (upper main part), Corodosphaeridium cantharellum Zone, Cribroperidinium tenuitabulatum Zone and Distatodium paradoxum Zone date this unit to the Early Miocene (Fig. 1). The diatom assemblage is correlated with Zone NSP 10 (King 1989), and the benthic foraminiferal assemblage can probably be correlated with Zone NSB 10 and NSB 9 of King (1989) and probably Zone NSR 8B of Gradstein & Bäckström (1996) from the North Sea area. Powell (1992) calibrated the LAD of C. cantharellum to the lower NN 4 Zone in the British Cenozoic, while deVerteuil & Norris (1996) placed the LAD in the upper NN 2 in their study on the Miocene of the U.S Atlantic Margin. Williams & Manum (1999) gave an age of 17.95 Ma for the LAD of C. cantharellum, which is in agreement with a calibration to the lower NN 4 Zone. The present Cribroperidinium tenuitabulatum Zone can be correlated to the Mio2 Zone defined by Poulsen et al. (1996) at ODP Site 909 in the Greenland-Spitsbergen Sill.
Upper Miocene (1285 (log)-1260 m, Kai Formation)
According to Eidvin et al. (2007), Bolboforma of the Bolboforma subfragori assemblage and B. laevis assemblage and benthic foraminifera of the Uvigerina venusta saxonica assemblage date this interval to Late Miocene (Fig. 1). The planktonic foraminiferal fauna includes N. atlantica (dextral), N. atlantica (sinistral) and G. bulloides.
A B. 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.
Upper Miocene – Lower Pliocene (1260-1222 m (log), Kai Formation)
According to Eidvin et al. (2007), benthic foraminifera of the Eponides pygmeus – Globocassidulina subglobosa assemblage and Cibicides telegdi assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) assemblage and Globigerina bulloides assemblage indicate a Late Miocene – Early Pliocene age for this interval (Fig. 1). The benthic foraminiferal assemblages are tentatively correlated with the upper part of Subzone NSB 13b and NSB 14a of King (1989) from the North Sea. The lowermost part of the Cibicides grossus assemblage constitutes the uppermost part of this unit, but the occurrence of C. grossus is probably caved.
E. pygmeus and C. telegdi are described from the Oligocene in Denmark and Germany (Grossheide & Trunko 1965, Hausmann 1964, Kummerle 1963, Ulleberg 1974). These species are recorded in deposits from the Oligocene to the Upper Pliocene in the North Sea and on the Norwegian Sea continental shelf, but are mostly found in Upper Miocene to Lower Pliocene sediments in those areas (Stratlab 1988, Eidvin & Rundberg 2001, 2007, Eidvin et al. 2007, see also well 6607/5-1). G. subglobosa is recorded from the Oligocene to the Lower Pliocene in the North Sea (Eidvin & Rundberg 2001, 2007).
Upper Pliocene (1222 (log)-1190 m, Naust Formation)
According to Eidvin et al. (2007), benthic foraminifera of the Cibicides grossus assemblage (upper main part) 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, Elphidium excavatum, Cibicides lobatulus, Nonion affine, Cassidulina teretis, Bulimina marginata, Elphidium albiumbilicatum and Buccella tenerrima. 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). Log correlations show the top of the Kai Formation of Late Miocene-Early Miocene age as high as 1225 m (Fig. 1). However, index fossils for Late Miocene-Early Pliocene are not recorded higher than 1240 m.
Sr isotope stratigraphy
We performed Sr isotope analysis on calcareous benthic foraminiferal tests from the ditch-cutting sample at 1260 m, and the obtained 87Sr/86Sr ratio gave an age of 6.3 Ma (Late Miocene, Table 1, Fig. 1), which broadly supports the biostratigraphical correlations.
Well 6609/11-1
| Litho. Unit | Sample (DC) | Corrected 87/86Sr | 2S error | Age (Ma) | Analysed fossil species |
| Kai Fm | 1260 m | 0.708968 | 0.000013 | 6.32 | Approximately 34 tests of U. venusta saxonica |
Table 1: Strontium isotope data from well 6609/11-1. The sample was 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
Middle to Upper Eocene (1410-1385 m, Brygge Formation)
This unit consists of claystone with some limestone in the lower part. Minor sand is also recorded (Fig. 1).
Lower Oligocene to Lower Miocene (1385-1285 m, Brygge Formation)
Clay dominates the samples, but the content of glauconitic sand is quite high in the upper part and some limestone is recorded in the lower part (Fig. 1).
Upper Miocene – Lower Pliocene (1285-1225 m (log), Kai Formation)
The ditch-cutting samples from this interval are also mostly fine grained. Clay dominates the samples, but the content of silt, sand (mainly quartzose) and pebbles of crystalline rocks are also quite considerable especially in the upper part. However, the pebbles and most of the sand are probably caved from the Upper Pliocene glacial section. Glauconitic sand is common in the lower part (Fig. 1).
Upper Pliocene (1225-1190 m, Naust Formation)
The ditch-cutting samples from the Upper Pliocene unit contain mostly fine-grained material. Clay dominates the samples, but the content of sand and silt is also considerable. Some pebbles of crystalline rocks are also recorded. The pebbles are interpreted as ice-rafted and indicate that the sediments were deposited after the marked increase in the supply of ice-rafted detritus to the Norwegian Sea, which started at about 2.75 Ma (Fronval & Jansen 1996).
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