Middle Miocene to Upper Pliocene in well 16/1-2
Modified after Eidvin & Rundberg (2007).
Based on analyses of benthic and planktonic foraminifera, Bolboforma and Sr isotopes in well 16/1-2 (58º56'09.16''N, 02º13'20.06''E, Map 1), we recorded 10 m of Middle Miocene sediments, 100 m with Upper Miocene sediments, 40 m with Lower Pliocene deposits and 30 m with Upper Pliocene deposits. The base of the Middle Miocene and the top of the Upper Pliocene were not investigated. The units are investigated with 17 ditch-cutting samples which were sampled at ten metre intervals except between 870 and 850 m (Fig. 1). A detailed stratigraphy of these units (including fossil range charts) was presented in Eidvin & Rundberg (2007).
Middle Miocene (880-870 m, Nordland Group)
Bolboforma of the Bolboforma badenensis and benthic foraminifera of the Uvigerina pygmea langeri - Uvigerina pygmea langenfeldensis assemblage give a Middle Miocene age for this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains B. elongata and E. variabilis. The planktonic foraminiferal fauna includes N. atlantica (sinistral) and N. acostaensis.
A Bolboforma badenensis Zone of Spiegler & Müller (1992) is known from deposits with an age of 12.3-11.7 Ma from the North Atlantic. The benthic foraminiferal fauna is correlated with Subzone NSB 13a of King (1989) from the North Sea.
Upper Miocene (870-780 m, Nordland Group and Utsira Formation)
Benthic foraminifera of the Uvigerina venusta saxonica assemblage (lower main part) and planktonic foraminifera of the Neogloboquadrina atlantica (dextral) assemblage, supported by several Sr isotope ages, give a Late Miocene age to this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal assemblage also contains G. subglobosa, M. communis and S. bulloides. In addition to the nominate species, the planktonic foraminiferal fauna also contains N. acostaensis (few) and N. atlantica (sinistral).
Spiegler & Jansen (1989) described a lower N. atlantica (dextral) Zone from Upper Miocene sediments on the Vøring Plateau, and Weaver & Clement (1987) reported a N. atlantica (dextral)/N. acostaensis Zone from Upper Miocene sediments in the North Atlantic. The benthic foraminiferal fauna is correlated with lower part of Subzone NSB 13b (King 1989, North Sea).
Lower Pliocene (780-740 m, Utsira Formation)
Benthic foraminifera of the Uvigerina venusta saxonica assemblage (upper part) and Monspeliensina pseudotepida assemblage and planktonic foraminifera of the Globorotalia puncticulata assemblage, together with some Sr isotope ages, give an Early Pliocene age to this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal assemblages also contains G. subglobosa, S. bulloides and F. bouanus. The planktonic foraminiferal fauna also contains G. glutinata and Orbulina universa.
An Early Pliocene Globorotalia puncticulata assemblage is described by Weaver & Clement (1986) from the North Atlantic. The first occurrence of G. puncticulata in that area is at approximately 4.6 Ma and its last occurrence at approximately 2.5 Ma. The upper part of Uvigerina venusta saxonica assemblage is correlated with the upper part of Subzone NSB 13b and the Monspeliensina pseudotepida assemblage is correlated with the lower part of Zone NSB 14 of (King 1989, North Sea).
Upper Pliocene (740-710 m, Nordland Group)
Benthic foraminifera of the Cibicides grossus assemblage and planktonic foraminifera of Neogloboquadrina atlantica (sinistral) assemblage give a Late Pliocene age (on the time scale of Berggren et al. 1995) for this unit (Fig. 1). The benthic foraminiferal fauna is correlated with Subzone NSB 15a of King (1989, North Sea) and 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.
Sr isotope stratigraphy
Nine samples with mollusc fragments were analysed for Sr isotopes in well 16/1-2. Three samples from the upper subunit of the Utsira Formation had isotopic compositions corresponding to ages of 4.8, 4.7 and 3.4 Ma, whereas one sample gave a younger age of 1.9 Ma (Table 1, Fig. 1). The Sr values fall within the flat part of the 87Sr/86Sr curve and the corresponding ages are therefore less precise. Nevertheless, they support the Early Pliocene age given for this subunit by the biostratigraphical correlation. Below this interval, within the main Utsira sand-body, three samples gave Sr ages of about 16-15 Ma, whereas two other samples both gave ages of 11.5 Ma. The older ages most likely represent reworked material.
|Litho. Unit||Sample||Corrected 87/86Sr||2S error||Age (Ma)||Comments||Analysed fossils|
|Utsira Fm||740 m (DC)||0.709087||0.000009||1.94||Caved||One mollusc fragments|
|Utsira Fm||740 m (DC)||0.709063||0.000008||3.38||Two mollusc fragments|
|Utsira Fm||770 m (DC)||0.709043||0.000009||4.78||Four mollusc fragments|
|Utsira Fm||770 m (DC)||0.709045||0.000009||4.70||Eight mollusc fragments|
|Utsira Fm||790 m (DC)||0.708709||0.000009||16.65||Reworked||Three mollusc fragments|
|Utsira Fm||790 m (DC)||0.708846||0.000009||11.49||Five mollusc fragments|
|Utsira Fm||800 m (DC)||0.708766||0.000009||15.48||Reworked||Five mollusc fragments|
|Utsira Fm||800 m (DC)||0.708749||0.000009||15.90||Reworked||Five mollusc fragments|
|Utsira Fm||820 m (DC)||0.708846||0.000008||11.49||Eight mollusc fragments|
Table 1: Strontium isotope data from well 16/1-2. 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. Modified after Eidvin & Rundberg (2007).
Middle Miocene ( 880-870 m, Nordland Group)
Clay dominates the Middle Miocene. Some sand and silt are also recorded in the ditch-cutting samples, but most of this is probably caved from the overlying Utsira Formation (Fig. 1).
Upper Miocene to Lower Pliocene (870 to 740 m, Nordland Group and Utsira Formation)
Sand dominates this unit, but clay and silt are also common in the lower part. The sand fraction is dominated by quartz, but some glauconite and mica are also recorded. Molluscs and mollusc fragments are common (Fig. 1).
Upper Pliocene (740 to 710 m, Nordland Group)
The samples in the Upper Pliocene section contain a clay-rich diamicton with some sand and silt and minor pebbles (Fig. 1).
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Weaver, P. P. E. & Clement, B. M., 1987: Magnetobiostratigraphy of planktonic foraminiferal datums: Deep Sea Drilling Project Leg 94, North Atlantic. In Ruddiman, W. F., Kidd, R. B., Thomas, E., et al. (eds.): Initial Reports of the Deep Sea Drilling Project 94, 815-829.