Lower Oligocene to Upper Pliocene in well 6305/5-1
Based on analyses of planktonic and benthic foraminifera, Bolboforma, pyritised diatoms and Sr isotopes in well 6305/5-1 (63º32’27.5’’N, 05º20’14.9’’E, Map 1), we recorded an 170 m-thick unit of Lower Oligocene sediments, an 160 m-thick unit of Upper Oligocene sediments, 30 m with Upper Miocene deposits and 30 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 40 ditch-cutting samples at ten metre intervals (Fig. 1).
Lower Oligocene (2000-1830 m, Brygge Formation)
Benthic calcareous foraminifera of the Rotaliatina bulimoides – Gyroidina soldanii mamillata assemblage date this interval to the Early Oligocene (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains T. alsatica, G. soldanii girardana, G. subglobosa, Trifarina gracilis and C. tenellus. Globigerna praebulloides dominates a sparse planktonic foraminiferal fauna. Diatom sp. 3 is quite common throughout. The Rotaliatina bulimoides – Gyroidina soldanii mamillata assemblage is correlated with the Zone NSB 7 of King (1989, North Sea) and Zone NSR 7A and NSR 7B of Gradstein & Bäckström (1996, North Sea and Haltenbanken). The Diatom sp. 3 assemblage is correlated with the lower part of Subzone NSP 9c of King (1989, North Sea).
Upper Oligocene (1830-1670 m, Brygge Formation)
Benthic calcareous foraminifera of the Turrilina alsatica assemblage and pyritised diatoms of the upper part of the Diatom sp. 3 assemblage give a Late Oligocene age to this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains G. soldanii girardana, T. gracilis and C. tenellus. Planktonic foraminifera are nearly absent. The Turrilina alsatica assemblage is correlated with the Zone NSB 8 of King (1989) and probably Zone NSR 8A of Gradstein & Bäckström (1996) from the North Sea. The Diatom sp. 3 assemblage is correlated with the upper part of Subzone NSP 9c of King (1989, North Sea).
Upper Miocene (1670-1640 m, Kai Formation)
Bolboforma attributed to the Bolboforma fragori – Bolboforma subfragori assemblage date this interval to the Late Miocene (Fig. 1). In addition to the nominate species, the Bolboforma assemblage also contains B. clodiusi. The planktonic foraminiferal fauna includes G. bulloides, N. atlantica (sinistral) and N. atlantica (dextral), and the benthic foraminiferal fauna includes U. pygmea langeri, C. telegdi, Eponides pygmeus, E. variabilis and S. 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). According to Berggren et al. (1995) the Middle/Late Miocene boundary is at 11.2 Ma; consequently, the lower part of the unit can be of latest Mid Miocene age. However, the Sr isotope analyses point to a Late Miocene age (see below).
Upper Pliocene (1640-1610 m, Naust Formation)
Benthic foraminifera of the Cibicides grossus assemblage and planktonic foraminifera of the Globigerina bulloides 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 fauna also include Nonion affine (common), Angulogerina fluens (common), Elphidium excavatum (common), Cassidulina teretis (common) and Bulimina marginata. The planktonic foraminiferal fauna also includes Turborotalia quinqueloba, Globorotalia inflata, Neogloboquadrina pachyderma (dextral) and Globigerinita glutinata. 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 Sr isotope analyses on calcareous benthic foraminiferal and Bolboforma tests from ditch cuttings in three intervals. The 87Sr/86Sr ratios obtained from the benthic foraminiferal tests in the interval 1960-1930 m gave an age of 28.1 Ma (Table 1) which is close to the Lower/Upper Oligocene boundary (28.5 Ma). The foraminiferal correlation indicates that the Lower/Upper Oligocene boundary is higher up in the well (Fig. 1), but this discrepancy is within the precision of the method. The 87Sr/86Sr ratios obtained from the benthic foraminiferal tests in the sample at 1720 m gave an age of 23.1 Ma (Table 1) which is close to the Upper Oligocene/Lower Miocene boundary (23.8 Ma). However, biostratigraphical correlation indicates that Lower Miocene sediments are not present in the well. The 87Sr/86Sr ratios obtained from the Bolboforma tests in the interval 1660-1650 m gave an age of 10.6 Ma (Table 1, Fig. 1, early Late Miocene) which is within the time interval derived from the Bolboforma correlation.
|Litho. Unit||Sample (DC)||Corrected 87/86Sr||2S error||Age (Ma)||Analysed fossil species|
|Kai Fm||1660 m||0.708872||0.000008||10.62||More than 100 tests of Bolboforma fragori, Bolboforma subfragori, Bolboforma clodiusi|
|Brygge Fm||1720 m||0.708304||0.000005||23.09||20 tests of T. alsatica|
|Brygge Fm||1960-1930 m||0.708059||0.000004||28.10||Ten tests of Rotaliatina bulimoides, Gyroidina soldanii mamillata|
Table 1: Strontium isotope data from well 6305/5-1. 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.
Lower Oligocene to Upper Miocene (2000-1640 m, Brygge and Kai formations)
The samples contain mainly pelagic sediments (biogenic ooze) with minor sand and silt. Mainly siliceous ooze is recorded in the samples from the Brygge Formation. In the Kai Formation the ooze is also rich on calcareous microfossils (Fig. 1).
Upper Pliocene (1640-1610 m, Naust Formation)
This unit also contains dominantly pelagic sediments (both siliceous and calcareous ooze).
However, some silt, sand and small pebbles are present throughout (Fig. 1). 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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