Lower-Middle Eocene to Upper Pliocene in well 6508/5-1
Modified after Eidvin et al. (2007).
Based on analyses of Bolboforma, planktonic and benthic foraminifera, radiolarians and Sr isotopes in well 6508/5-1 (65º42’51.23’’N, 08º28’35.44’’E, Map 1), we recorded 42 m with Lower-Middle Eocene sediments, a 128 m-thick unit of Upper Miocene deposits, a 65 m-thick unit of Upper Miocene – Lower Pliocene sediments and 35 m with Upper Pliocene deposits. The base of the Lower-Middle Eocene and the top of the Upper Pliocene were not investigated. The units were investigated with 28 ditch-cutting samples at ten metre intervals (Fig. 1).
Well summary figur for well 6508/5-1
Biostratigraphy
Lower-Middle Eocene (1400-1358 m (log), Brygge Formation)
Radiolarians of the Cenosphaera sp. assemblage give an Early-Middle Eocene age for this unit (Fig. 1). In addition to abundant Cenosphaera sp. and other forms of radiolarians, the assemblage also includes common sponge spicules and a few pyritised diatoms (upper part). The assemblage is correlated with Zone NSP 6 of King (1989, North Sea).
Upper Miocene (1358 (log)-1230 m, Kai Formation)
Bolboforma of the Bolboforma subfragori – Bolboforma fragori assemblage and Bolboforma metzmacheri assemblage and benthic foraminifera of the Martinottiella communis assemblage and Uvigerina venusta saxonica assemblage give a Late Miocene age to this interval (Fig. 1). The planktonic foraminiferal fauna includes N. atlantica (dextral), N. atlantica (sinistral), N. acostaensis and G. bulloides.
A B. fragori/B. subfragori Zone is described from deposits with an age of 11.7-10.3 Ma and 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). The Martinottiella communis assemblage is tentatively correlated with Zone FC of Doppert (1980) from the Netherlands and the Uvigerina venusta saxonica assemblage is correlated with Subzone NSB 13b of King (1989) from the North Sea.
Upper Miocene – Lower Pliocene (1230-1165 m (log), Kai Formation)
Benthic foraminifera of the Eponides pygmeus – Cibicides telegdi assemblage and the Eponides pygmeus assemblage indicate a Late Miocene – Early Pliocene age for these units (Fig 1), and the assemblages are tentatively correlated with the upper part of Subzone NSB 13b and NSB 14a of King (1989) from the North Sea. The planktonic foraminiferal fauna includes N. atlantica (sinistral) and G. bulloides.
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).
Upper Pliocene (1165 (log)-1130 m, Naust Formation)
Benthic foraminifera of the Cibicides grossus assemblage and planktonic foraminifera of the Globigerina bulloides assemblage (upper 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, Nonion affine (common), Cassidulina teretis (common), Elphidium excavatum, Cibicides lobatulus, Angulogerina fluens and Bulimina marginata. The planktonic foraminiferal fauna also includes Turborotalia quinqueloba, Neogloboquadrina pachyderma (sinistral) and N. pachyderma (dextral). 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). 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 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
We performed two Sr isotope analyses on calcareous benthic foraminiferal tests from the ditch cutting sample at 1260 m, and the obtained 87Sr/86Sr ratios gave ages of 6.4 and 6.0 Ma (Late Miocene, Table 1, Fig. 1), which broadly support the biostratigraphical correlations.
Well 6508/5-1
| Litho. unit | Sample (DC) | Corrected 87/86Sr | 2S error | Age (Ma) | Analysed fossil species |
| Kai Fm | 1260 m | 0.708991 | 0.000008 | 5.95 | Approximately 90 small tests of Uvigerina venusta saxonica |
| Kai Fm | 1260 m | 0.708965 | 0.000012 | 6.42 | Approximately 75 small tests of U. venusta saxonica |
Table 1: Strontium isotope data from well 6508/5-1. 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-Middle Eocene (1400-1358 m (log), Brygge Formation)
This unit consists of claystone and limestone. Minor sand (mainly glauconitic) is also recorded (Fig. 1).
Upper Miocene (1358 (log)-1230 m, Kai Formation)
The ditch cutting samples from this interval are mostly fine grained. However, the upper and lower parts are rich in glauconitic sand (Fig. 1). Some quartzose sand and pebbles of crystalline rocks are recorded in most samples, but these are probably caved.
Upper Miocene – Lower Pliocene (1230-1165 m (log), Kai Formation)
The Upper Miocene - Lower Pliocene samples are also mostly fine grained. Clay dominates the samples, but the content of silt, sand (mainly quartzose) and pebbles of crystalline rocks is also considerable. The pebbles and most of the sand are probably caved from the Upper Pliocene glacial section.
Upper Pliocene (1165 (log)-1130 m, Naust Formation)
The samples from the Upper Pliocene contain a clay-rich diamicton with sand, silt and ice-rafted pebbles of mainly crystalline rocks. The pebbles are interpreted as ice-rafted. 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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