Lower-Middle Eocene to Upper Pliocene in well 6607/5-1
Modified after Eidvin et al. (1998a).
Based on analyses of benthic and planktonic foraminifera, radiolaria, Bolboforma and Sr isotopes in well 6607/5-1 (66º38'09.67''N, 07º32'21.38''E, Map 1) we recorded 80 m of Lower-Middle Eocene deposits, 111 m with Upper Miocene sediments, 135 m with Upper Miocene-Lower Pliocene deposits and 198 m with Upper Pliocene sediments. The base of the Lower-Middle Eocene and the top of the Upper Pliocene were not investigated. The units were investigated with 37 ditch cuttings at 24 to 6 metre intervals (Fig. 1).
Lower-Middle Eocene (2528-2448 m, 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. the assemblage also includes some sponge spicules. The assemblage is correlated with Zone NSP 6 of King (1989, North Sea).
Upper Miocene (2448-2262 m, Kai Formation)
Bolboforma of the of the Bolboforma subfragori assemblage and Bolboforma metzmacheri assemblage, planktonic foraminifera of the Neogloboquadrina atlantica (dextral) assemblage and benthic foraminifera of the Ehrenbergina variabilis-Globocassidulina subglobosa assemblage (lower part) together with several Sr isotope ages, give a Late Miocene age for this unit (Fig. 1). In addition to the nominate species, the Bolboforma subfragori assemblage includes B. fragori (one sample) and B. clodiusi (two samples). Planktonic foraminifera include N. atlantica (dextral), N. atlantica (sinistral) and G. bulloides. In addition to the nominate species, the benthic foraminiferal fauna includes E. pygmeus, C. telegdi, P. bulloides, C. dutemplei, U. venusta saxonica (one sample) and M. communis.
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). Spiegler & Jansen (1989) reported a lower N. atlantica (dextral) Zone from Upper Miocene sediments on the Vøring Plateau, and Weaver & Clement (1987) described a N. atlantica (dextral)/N. acostaensis Zone from Upper Miocene sediments in the North Atlantic. The Ehrenbergina variabilis-Globocassidulina subglobosa assemblage is correlated with Zone NSB 13 of King (1989, North Sea) and the G. sublobosa-E. variabilis zone of Stratlab (1988) from the Norwegian Sea continental shelf.
Upper Miocene-Lower Pliocene (2262-2202 m, Kai Formation)
Benthic foraminifera of the Ehrenbergina variabilis-Globocassidulina subglobosa assemblage (upper part) and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral)-Globigerina bulloides assemblage (lower part) indicate a Late Miocene-Early Pliocene age for this interval (Fig. 1). The benthic foraminiferal assemblage is correlated with the upper part of G. sublobosa-E. variabilis zone of Stratlab (1988,) and tentatively with the Subzone 13b of King (1989).
Upper Pliocene (2202-2010 m, Naust Formation)
Benthic foraminifera of the Cibicides grossus assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral)-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 Elphidium excavatum (common), Cassidulina reniforme (common), Cibicides scaldisiensis, Nonion affine, Cassidulina teretis, Bulimina marginata, Buccella tenerrima. A few specimens of Eponides pygmeus and Cibicides telegdi are also recorded in some samples. The planktonic foraminiferal fauna also includes Turborotalia quinqueloba, Neogloboquadrina pachyderma (sinistral), Globorotalia inflata (few) and Globigerinita glutinata (few). 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 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).
Eponides pygmeus and Cibicides telegdi are recorded from deposits from Oligocene to Late Pliocene on the Norwegian continental shelf, but have been reported from Upper Miocene-Lower Pliocene deposits (Stratlab 1988, Eidvin & Rundberg 2001, 2007, Eidvin et al. 2007). Micropalaeontologists working with deposits from the Norwegian shelf have speculated if the occurrences in the Upper Pliocene represent in situ or reworked tests. The tests are very small and are probably easily reworked. Their small sizes had made it difficult to collect enough tests for Sr analyses which could have solved the problem. However, recently we were able to collect enough tests for two analyses from one sediment sample, each based on more than 100 tests of E. pygmeus, in a cored section of the Upper Pliocene in well 34/8-A-33 H (Tampen area, northern North Sea). The Sr analyses gave ages of 2.52 and 1.84 Ma and showed that at least E. pygmeus lived in the Late Pliocene (T. Eidvin, unpublished data).
Sr isotope stratigraphy
Seven samples from well 6607/5-1 were analysed for Sr isotopes. Tests of benthic and planktonic foraminifera and Bolboforma were used for the analyses. The 87Sr/86Sr ratios obtained from Bolboforma tests in the sample from 2391 m gave an age of 11.0 Ma (Table 1, Fig. 1, early Late Miocene) which is just within the time interval derived from the Bolboforma correlation. Three samples based on benthic and planktonic foraminifera from 2361 to 2337 m gave ages from 6.9 to 5.7 Ma (Table 1, Late Miocene) which is slightly younger than the age obtained by the correlation of B. metzmacheri. However, this discrepancy is close to the precision of the method. Three samples based on benthic foraminifera from 2331 to 2301 m gave ages from 6.2 to 5.1 Ma (Table 1, Late Miocene to earliest Early Pliocene). This is close to the age obtained by the foraminiferal correlation.
|Litho. unit||Sample (DC)||Corrected 87/86Sr||2S error||Age (Ma)||Analysed fossil species|
|Kai Fm||2322-2301 m||0.709021||0.000010||5.22||25 tests of G. subglobosa, Ehrenbergina variabilis|
|Kai Fm||2331-2322 m||0.709027||0.000010||5.07||Seven tests of Pullenia bulloides|
|Kai Fm||2331-2322 m||0.708967||0.000008||6.24||Six tests of Cibicides dutemplei|
|Kai Fm||2361-2337 m||0.708947||0.000008||6.89||Ten tests of C. dutemplei|
|Kai Fm||2361 m||0.708993||0.000009||5.74||18 tests of P. bulloides, E. variablis|
|Kai Fm||2361-2337 m||0.708992||0.000011||5.92||Ten tests of Neogloboquadrina atlantica (dextral)|
|Kai Fm||2391 m||0.708859||0.000009||11.01||Approximately 100 test of B. subfragori|
Table 1: Strontium isotope data from well 6607/5-1. 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-Middle Eocene (2528-2448 m, Brygge Formation)
This unit consists of claystone, limestone and tuff (Fig. 1).
Upper Miocene to Upper Miocene-Lower Pliocene (Kai Formation)
All the samples contain mostly clay with minor sand (quartzose and glauconitic) and silt (Fig. 1).
Upper Pliocene (2202-2010 m, Naust Formation)
The Upper Pliocene unit consists of a clay-rich diamicton with coarse- to medium-grained sand and silt. Subangular to angular pebbles of mostly crystalline and some sedimentary rocks occur 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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