Lower Oligocene to Upper Pliocene in well 6407/9-5
Modified after Eidvin et al. (2007).
Based on analyses of benthic and planktonic foraminifera, pyritised diatoms, dinoflagellate cysts and Sr isotopes in well 6407/9-5 (64º16’42.35’’N, 07º44’14.66’’E, Map 1), we recorded 50 m of Lower Oligocene sediments, approximately 23 m with Lower Miocene deposits, an approximately 37 m-thick unit with Upper Miocene sediments, an 80 m-thick unit of Lower Pliocene deposits and 40 m with Upper Pliocene sediments. The base of the Lower Oligocene and the top of the Upper Pliocene were not investigated. The units were investigated with 24 ditch-cuttings samples which were sampled at ten metre intervals (Fig. 1).
Well summary figure for well 6407/9-5
Biostratigraphy
Lower Oligocene (860-810 m, Brygge Formation)
According to Eidvin et al. (2007), benthic calcareous foraminifera of the Rotaliatina bulimoides assemblage and dinoflagellate cysts of the Svalbardella cooksoniae Zone and Areoligera semicirculata Zone date this unit to Early Oligocene (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains T. alsatica, G. soldanii girardana, G. soldanii mamillata and Frondicularia budensis. The foraminiferal assemblage is correlated with Subzone NSB 7b of King (1989) from the North Sea, Rotaliatina bulimoides zone of Stratlab (1988, Norwegian Sea continental shelf) and Zone NSR 7A or 7B of Gradstein & Bäckström (1996) from the North Sea and Haltenbanken area. Manum et al. (1989) found that the dinocyst Svalbardella cooksonia 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) corresponded to the upper boundary of their Early/Late Oligocene Areosphaeridium? actinocoronatum in ODP Hole 643 in the Norwegian Sea. The LAD of this species also defines the upper boundary of the Oli4 Zone of Poulsen et al. (1996) as defined in ODP Hole 908 on the Hovgaard Ridge between Svalbard and NE Greenland.
Lower Miocene (810-787 m (log), Brygge Formation)
According to Eidvin et al. (2007), the occurrence of Diatom sp. 4 (King 1983) and dinoflagellate cysts of Cordosphaeridium cantharellum Zone date this interval to Early Miocene (Fig. 1). The diatom assemblage is correlated with Zone NSP 10 of King (1989) from the North Sea. Powell (1992) calibrated the LAD of the dinocyst C. cantharellum to the lower NN 4 Zone in the British Tertiary, while de Verteuil & 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.
Upper Miocene (787(log)-750 m, Molo Formation)
According to Eidvin et al. (2007), Bolboforma of the Bolboforma metzmacheri assemblage date this interval to a Late Miocene age (Fig. 1). The benthic foraminiferal fauna includes G. subglobosa and S. bulloides and the planktonic foraminiferal fauna includes N. atlantica (sinistral), N. atlantica (dextral) and G. bulloides. A B. metzmacheri Zone is described from sediments with an age of 10.0-8.7 Ma from the North Atlantic and the Vøring Plateau (Spiegler & Müller 1992, Müller & Spiegler 1993).
Lower Pliocene (750-670 m, Molo Formation)
According to Eidvin et al. (2007), benthic calcareous foraminifera of the Monspeliensina pseudotepida assemblage and Eponides pygmeus assemblage give an Early Pliocene age to this unit (Fig. 1). In addition to the nominate species, the Monspeliensina pseudotepida assemblage also contains G. subglobosa, Florilus boueanus and S. bulloides. The planktonic foraminiferal fauna includes N. atlantica (sinistral) and G. bulloides. The benthic foraminiferal assemblages are correlated with Subzone NSB 14a of King (1989) and Zone NSR 12A of Gradstein & Bäckström (1996) from the North Sea.
Upper Pliocene (670-630 m, Naust Formation)
According to Eidvin et al. (2007), benthic foraminifera of the Elphidiella hannai assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) assemblage (upper part) and Globigerina bulloides assemblage give a Late Pliocene age (on the time scale of Berggren et al. 1995) to this unit (Fig. 1). In addition to the nominate species the benthic foraminiferal assemblage also includes Elphidium excavatum, Cassidulina teretis, Cibicides lobatulus, Nonion affine, Bulimina marginata and Elphidium albiumbilicatum. 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). Spiegler & Jansen (1989) described a N. atlantica (sinistral) Zone from the Vøring Plateau (Norwegian Sea) from Upper Miocene to Upper Pliocene deposits. The LAD of N. atlantica (sinistral) in that area is approximately 2.4 Ma. A G. bulloides Zone is described from the North Atlantic (DSDP Leg 94) in Pliocene sediments as young as 2.2 Ma (Weaver & Clement 1986). On the Vøring Plateau, G. bulloides is common in Late Miocene to Pliocene deposits older than 2.4 Ma (Spiegler & Jansen 1989). G. bulloides is also common in the warmest interglacials of the Pleistocene in the North Atlantic (Kellogg 1977).
Sr isotope stratigraphy
Calcareous benthic foraminiferal tests from 830-810, 780-770, 760-750, 740 and 730-710 m were analysed for Sr isotopes. The obtained 87Sr/86Sr ratios from 830-810 m gave an age of 30.3 Ma (Early Oligocene). The four samples from 780 to 710 m gave 5.8 Ma (latest Late Miocene), 5.2 Ma (earliest Early Pliocene), 5.8 Ma and 5.7 Ma (latest Late Miocene) respectively (Table 1, Fig. 1). The obtained 87Sr/86Sr ratios from 780-770 and 760-750 m gave ages somewhat younger than the age obtained from the Bolboforma correlation, and the obtained 87Sr/86Sr ratios from 740 and 730-710 m gave ages slightly older than the age obtained from the correlation of benthic foraminifera. However, these discrepancies are within the precision of the method.
Well 6407/9-5
| Litho. Unit | Sample (DC) | Corrected 87/86Sr | 2S error | Age (Ma) | Analysed fossil species |
| Molo Fm | 730-710 m | 0.709010 | 0.000010 | 5.69 | Approximately 60 tests of G. subglobosa, S. bulloides and Monspeliensina pseudotepida |
| Molo Fm | 740 m | 0.709006 | 0.000012 | 5.75 | 34 tests of G. subglobosa and S. bulloides |
| Molo Fm | 760-750 m | 0.709030 | 0.000026 | 5.18 | Approximately 60 tests of G. subglobosa and S. bulloides |
| Molo Fm | 780-770 m | 0.709003 | 0.000007 | 5.79 | 33 tests of G. subglobosa and S. bulloides |
| Hordaland Gr | 830-810-m | 0.707972 | 0.000010 | 30.30 | 25 tests of G. soldanii girardana, T. alsatica, Frondicularia budensis, Bolivina cf. antique and A. scitula |
Table 1: Strontium isotope data from well 6407/9-5. The samples were analysed at the University of Bergen. Sr ratios were corrected to NIST 987 = 0.710248. The numerical ages were derived from the SIS Look-up Table Version 3:10/99 of Howard & McArthur (1997). NIST = National Institute for Standard and Technology.
Lithology
Lower Oligocene (860-810 m, Brygge Formation)
Clay dominates the samples in this unit, but minor sand (mainly glauconitic) and silt are recorded throughout (Fig. 1).
Lower Miocene (810-787 m (log), Brygge Formation)
The samples in this unit contain some clay and silt, but are dominated by glauconitic sand (Fig. 1)
Upper Miocene (787 (log)-750 m, Molo Formation)
This unit is also dominated by glauconitic sand. Some clay and silt are also present (Fig. 1).
Lower Pliocene (750-670 m, Molo Formation)
The ditch-cutting samples in this unit contain common clay, silt and sand. In the lower part the sand is mainly glauconitic (Fig. 1). In the upper part the sand is mainly quartzose, and this part also contains pebbles of crystalline rocks. The pebbles and most of the quartzose sand are probably caved from the Upper Pliocene unit.
Upper Pliocene (670-630 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 (Fig. 1). Most of the pebbles are of crystalline rocks, but some are also of sedimentary rocks. 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.
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