Lower-Middle Eocene to Upper Pliocene in well 34/10-17
Based on analyses of benthic and planktonic foraminifera, pyritised diatoms and Sr isotopes in well 34/10-17 (61º03'58.93''N, 02º0'58.78''E, Map 1), we recorded 80 m of Lower-Middle Eocene deposits, a 390 m-thick column with Upper Oligocene sediments, a 170 m thick undefined unit (also probably of Late Oligocene age), approximately 60 m with Upper Miocene sediments and 140 m of 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 84 ditch-cutting samples at ten metre intervals except between 1550 and 1580 m (Figs. 1-3).
Lower-Middle Eocene (1600-1510 m, Hordaland Group)
Benthic agglutinated foraminifera of the Reticulophragmium amplectens-Ammolagena clavata assemblage give an Early-Middle Eocene age for this unit (Fig. 1). In addition to the nominate species, the benthic agglutinated foraminiferal assemblage also includes Ammodiscus cretaceous, Bathysiphon eocenicus, Cyclammina rotudidorsata, Haplophragmoides kirki and Cyclammina rotundidorsata. The benthic foraminiferal assemblage is correlated with Zone NSA 5 and Zone NSA 6 of King (1989, North Sea).
Upper Oligocene (1510-1120 m, Hordaland Group)
Pyritised diatoms of the Diatom sp. 3 assemblage, benthic agglutinated foraminifera of the Bathysiphon eocenicus assemblage and Spirosigmoilinella compressa assemblage (lower part) together with a number of Sr isotope ages give a Late Oligocene age for this unit (Figs. 1 and 2). In addition to the nominate species, the Bathysiphon eocenicus assemblage also includes common C. rotundidorsata. The Spirosigmoilinella compressa assemblage is nearly barren of benthic foraminifera. Abundant radiolaria and sponge spicules are recorded, but no in situ planktonic foraminifera or calcareous benthic foraminifera. The diatom assemblage is correlated with the upper part of Subzone NSP 9c and the benthic foraminiferal assemblages are correlated with the upper part of Zone NSA 8 and Zone NSA 9 of King (1989, North Sea).
Undefined interval (Upper Oligocene?, 1120-950 m, Hordaland Group)
This unit is nearly barren of foraminifera, but a few specimens of S. compressa and R. placenta are recorded in the uppermost part. As in the immediately underlying unit, the unit contains pyritised diatoms, radiolara and abundant sponge spicules. Log correlation and seismic investigation indicate no depositional break between this and the underlying unit. Consequently, even if the most important index fossils are lacking, it is likely that this unit was also deposited during the Late Oligocene. The benthic foraminiferal assemblage is tentatively correlated with the Zone NSA 9 of King (1989, North Sea).
Upper Miocene (950-890 m, Utsira Formation)
Benthic calcareous foraminifera of the Uvigerina venusta saxonica - Globocassidulina subglobosa assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (dextral) assemblage together with a number of Sr isotope ages give a Late Miocene age to this unit (Fig. 3). In addition to the nominate species, the benthic foraminiferal fauna also includes Plectofrondicularia advena, E. pygmeus, C. telegdi and Florilus bouanus. 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 benthic foraminiferal fauna is correlated with lower part of Subzone NSB 13b (King 1989, North Sea).
Upper Pliocene (890-750 m, Nordland Group)
Benthic foraminifera of the Cibicides grossus – Elphidiella hannai assemblage and planktonic foraminifera of Neogloboquadrina atlantica (sinistral) assemblage (upper main part) give a Late Pliocene age (on the time scale of Berggren et al. 1995) for this unit (Fig. 3). 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. Some specimens of U. venusta saxonica, Sphaeroidina bulloides, G. subglobosa and F. bouanus are recorded in the lower part of the unit and these are probably reworked from the Utsira Formation.
Sr isotope stratigraphy
Twenty-nine samples with mollusc fragments from 17 depths and 16 samples with fish teeth from nine depths were analysed for Sr isotopes. Mollusc fragment are sparse in the Oligocene and Upper Miocene sections, but are common in the Upper Pliocene. Except for two samples from the Utsira Formation, all the obtained 87Sr/86Sr ratios from the mollusc fragments gave ages of latest Pliocene to recent. It is obvious that the analysed fragments are caved, and consequently we have not included these results in tables and figures. We recorded no in situ calcareous foraminifera in the Oligocene unit, and it is likely that corrosion, due to excess CO2 in the bottom water, has dissolved any mollusc and foraminiferal tests.
Fish teeth, which are more resistant to carbonate corrosion, were recorded mainly in two separate parts of the investigated sediment succession, namely in the in the sandy lower part of the Hordaland Group and the sandy Utsira Formation. The obtained 87Sr/86Sr ratios from the lower part of the Oligocene unit gave ages from 27.1-23.2 (Late Oligocene, Table 1, Fig. 1). One exception is the sample at 1330 m which gave an age of 15.9 Ma and which is probably an error. A Late Oligocene age is in agreement with the biostratigraphical correlations. Two samples from the upper part of the underlying Early-Middle Eocene unit gave ages of 28.7 and 27.7 Ma (close to the Early/Late Oligocene boundary), and the fish teeth in these samples are probably caved from the base of the overlying unit.
The obtained 87Sr/86Sr ratios from the Utsira Formation gave ages of 10.9-6.5 Ma (Late Miocene) which support the biostratigraphical correlations in that the Early Pliocene part of the Utsira Formation is missing in this well. The similar ages of 9.4-8.3 Ma, which are recorded in the uppermost part of the underlying Hordaland Group, were probably based on fish teeth caved from the Utsira Formation (Table 1, Fig. 3).
|Litho. Unit||Sample (DC)||Corrected 87/86Sr||2S error||Age (Ma)||Comments||Analysed fossils|
|Utsira Fm||900 m||0.708962||0.000008||6.54||One mollusc fragment|
|Utsira Fm||900 m||0.708956||0.000008||6.71||One fish tooth|
|Utsira Fm||910 m||0.708864||0.000008||10.89||Two small fish teeth|
|Utsira Fm||940 m||0.708952||0.000008||6.87||Two small fish teeth|
|Utsira Fm||950 m||0.708903||0.000009||9.42||One fish tooth|
|Hordaland Gr||960 m||0.708924||0.000012||8.48||Caved from the Utsira Fm||Two small fish teeth|
|Hordaland Gr||970 m||0.708907||0.000008||9.25||Caved from the Utsira Fm||One fish tooth|
|Hordaland Gr||970 m||0.708927||0.000012||8.33||Caved from the Utsira Fm||Four small fish teeth|
|Hordaland Gr||980 m||0.708904||0.000009||9.38||Caved from the Utsira Fm||Four small fish teeth|
|Hordaland Gr||1310 m||0.708298||0.000022||23.23||One fish tooth|
|Hordaland Gr||1320 m||0.708277||0.000011||23.65||Two fish teeth|
|Hordaland Gr||1330 m||0.708748||0.000008||15.92||Two fish teeth|
|Hordaland Gr||1350 m||0.708095||0.000014||27.11||Four small fish teeth|
|Hordaland Gr||1390 m||0.708259||0.000010||23.97||Two small fish teeth|
|Hordaland Gr||1400-1420 m||0.708237||0.000008||24.33||Five small fish teeth|
|Hordaland Gr||1450 m||0.708237||0.000008||24.33||One fish tooth|
|Hordaland Gr||1520-1540 m||0.708034||0.000009||28.73||Caved from a slightly higher level||13 small fish teeth|
|Hordaland Gr||1580-1600 m||0.708073||0.000010||27.74||Caved from a slightly higher level||Ten small fish teeth|
Table 1: Strontium isotope data from well 34/10-17. 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. A large number of mollusc fragment samples from both the Hordaland Group and Utsira Formation obtained 87Sr/86Sr-ratios which gave ages of latest Pliocene to recent. It is obvious that the analysed fragments are caved from much higher levels, and we have not included these results in the table and figures.
Upper Oligocene (including undefined interval, 1510-950 m, Hordaland Group)
Medium- to fine-grained sand dominates this unit, but clay and silt are also common especially in the upper part. Quartz dominates the sand fraction, but mica and glauconite are also recorded throughout. In the upper part glauconite is common (Figs. 1 and 2).
Upper Miocene (950-890 m, Utsira Formation)
Sand dominates this unit with minor silt and clay. The sand fraction is dominated by quartz, but glauconite, mica and mollusc fragments also occur throughout (Fig. 3).
Upper Pliocene (890-750 m, Nordland Group)
The samples in the lower and upper parts of the Upper Pliocene section contain sand-rich diamictons with some silt, clay and pebbles. Quartz dominates the sand fractions. The middle part of the Upper Pliocene contains a clay-rich diamicton with silt sand and pebbles (Fig. 3). The pebbles are interpreted as ice-rafted. According to Fronval & Jansen (1996), on the Vøring Plateau (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 the Upper Pliocene unit is therefore considered to be 2.75 Ma belonging mainly to Gelasian Stage.
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