Lower Oligocene to Upper Pliocene in well 16/1-4
Modified after Eidvin & Rundberg (2007).
Unusually for post-Eocene sections, well 16/1-4 (58º51'55.20''N, 02º17'56.12''E, Map 1) is also sampled with sidewall cores.
Unfortunately, very few samples, including both sidewall cores and ditch cuttings, were available from the Utsira Formation, but this unit has been investigated in the nearby well 16/1-2. Based on analyses of benthic and planktonic foraminifera, Bolboforma, pyritised diatoms and Sr isotopes, we recorded 140 m with Lower Oligocene sediments, 81.5 m with Upper Oligocene deposits, a 366 m-thick column of Lower Miocene sediments, 52.5 m with Middle Miocene deposits, a 77 m-thick undefined sediment column, 10 m with Lower Pliocene deposits and 40 m with Upper Pliocene sediments. The base of the Lower Oligocene was not investigated and the top of the Upper Pliocene is not included. The units were investigated with 34 sidewall cores and 61 ditch-cutting samples at ten metre intervals except from 860 to 770 m (Figs. 1 and 2). A detailed stratigraphy of these units (including fossil range charts) was presented in Eidvin & Rundberg (2007). In that paper we also presented the section from 720 to 357.5 m (upper part of Upper Pliocene and Pleistocene on the time scale of Berggren et al. 1995)
Lower Oligocene (1400.5-1260 m, Hordaland Group)
Benthic foraminifera of the Roaliatina buliomoides assemblage and pyritised diatoms of the Diatom sp. 3 assemblage (lower part), together with a Sr isotope age from the interval 1310-1290 m, give an Early Oligocene age to this unit (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna also contains G. soldanii mamillata and T. alsatica. In situ planktonic foraminifera are very scarce and include just a few specimens of G. praebulloides and G. ciperoensis. The benthic foraminiferal assemblages are correlated with Subzone NSB 7b of King (1989) and Zone NSR 7B of Gradstein & Bäckström (1996) from the North Sea. The diatom assemblage is correlated with the lower part of Subzone NSP 9c of King (1989, North Sea).
Upper Oligocene (1260-1178.5 m, Hordaland Group)
Pyritised diatoms of the Diatom sp. 3 assemblage (upper part) and benthic foraminifera of the Turrilina alsatica assemblage and the lowermost part of the Spirosigmoilinella compressa (agglutinated) assemblage, together with four Sr isotope ages, date this unit to Late Oligocene (Fig. 1). In addition to the nominate species, the Turrilina alsatica assemblage also includes G. soldanii girardana and A. guerichi guerichi. In situ planktonic foraminifera are scarce and include G. praebulloides and G. ciperoensis. The diatom assemblage is correlated with the upper part of Subzone NSP 9c, and the benthic foraminiferal fauna is correlated with Zone NSB 8 of King (1989) and probably Zone NSR 8A and the upper part of Zone NSR 7B of Gradstein & Bäckström (1996, all from the North Sea).
Lower Miocene (1178.5-912.5 m, Hordand Group, Skade Formation and lowermost part of the Nordland Group)
Benthic foraminifera of the Spirosigmoilinella compressa assemblage and Uvigerina tenuipustulata - Astigerina guerichi staeschei assemblage, pyritised diatoms of the Diatom sp. 4 assemblage and planktonic foraminifera of the Globorotalia zealandica - Globigerina ciperoensis assemblage, together with a number of Sr isotope analyses, give an Early Miocene age to this unit (Figs. 1 and 2). In addition to the nominate species, the Spirosigmoilinella compressa assemblage also includes T. gracilis and C. contraria. The Uvigerina tenuipustulata - Astigerina guerichi staeschei assemblage also includes B. elongata, Elphidium inflatum, U. semiornata saprophila and Uvigerina semiornata semiornata. The Globorotalia zealandica - Globigerina ciperoensis assemblage also includes G. praescitula. The Spirosigmoilinella compressa assemblage is correlated with Zone NSA 10 of King (1989, North Sea). The Uvigerina tenuipustulata - Astigerina guerichi staeschei assemblage is correlated with Zone NSB 10 and NSB 11 of King (1989, North Sea), Zone NSR 8B of Gradstein & Bäckström (1996, North Sea) and Zone FD of Doppert (1980) from the Netherlands. The Diatom sp. 4 assemblage is correlated with zone NSP 10 of King (1983) and the Globorotalia zealandica - Globigerina ciperoensis assemblage is correlated with Zone NSP 11 and probably the lower part of Zone NSP 12 of King (1983), Zone NSR 8B and the lower part of Zone NSR 9A of Gradstein & Bäckström (1996) from the North Sea.
Middle Miocene (912.5-860 m, Nordland Group)
Bolboforma attributed to the Bolboforma badenensis - Bolboforma reticulata assemblage and Benthic foraminifera of the Uvigerina pygmea langeri - Uvigerina pygmea langenfeldensis assemblage date this unit to the Middle Miocene (Fig. 2). In addition to the nominate species, the benthic foraminiferal fauna also includes E. variabilis, M. communis and B. elongata. Plantonic foraminifera include G. bulloides, N. atlantica (sinistral) and G. glutinata.
A B. badenensis Zone and a B. reticulata Zone are recorded from the North Atlantic, and a B. badenensis/B. reticulata Zone is recorded from the the Vøring Plateau from deposits with an age slightly older than 14 to 11.7 Ma (Spiegler & Müller 1992, Müller & Spiegler 1993). The Uvigerina pygmea langeri - Uvigerina pygmea langenfeldensis assemblage is correlated with Zone NSB 12 and Subzone NSB 13a of King (1989) from the North Sea.
Lower Pliocene (770-760 m, Utsira Formation and Nordland Group)
Benthic foraminifera of the Uvigerina venusta saxonica assemblage and planktonic foraminifera of the Globorotalia puncticulata assemblage give an Early Pliocene age for this unit (Fig. 2). In addition to the nominate species, the benthic foraminiferal fauna also includes M. pseudotepida, F. boueanus, S. bulloides and G. subglobosa. The planktonic foraminiferal fauna also includes G. bulloides and N. atlantica (sinistral). The benthic foraminiferal fauna is correlated with the upper part of Subzone NSB 13b of King (1989, North Sea). 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.
Upper Pliocene (760-720 m, Nordland Group)
Benthic foraminifera of the Cibicides grossus assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (sinistral) assemblage give a Late Pliocene age (on the time scale of Berggren et al. 1995) for this unit (Fig. 2). 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 at approximately 2.4 Ma.
Sr isotope stratigraphy
Twelve samples based on mollusc fragments from eleven depths, and nine samples based on calcareous foraminiferal tests from four depths were analysed in well 16/1-4. The obtained 87Sr/86Sr ratio from 1310-1290 m (calcareous foraminifera in ditch-cutting material) gave 28.5 Ma (Early Oligocene). Three analyses of foraminiferal tests from 1194.5 m (sidewall core) all gave similar ages of about 27 Ma (Late Oligocene). However, one analysis from the sidewall core at 1210.5 m, slightly down hole, gave a somewhat younger age of 24.5 Ma (Late Oligocene, Table 1, Fig. 1). Broadly speaking, all the obtained ages from the Oligocene deposits support the biostratigraphical correlations. The obtained age from 1210.5 m may be slightly too young due to either contamination or an analytical error. However, the discrepancy is close to the precision of the method.
The twelve samples based on mollusc fragments are all from the sandy interval of the Skade Formation. The samples gave results showing very consistent increasing ages with depth, from 16.9 (at 940 m) to 20.4 Ma (at 1030 m, Table 1, Fig. 2). Such a systematic trend suggests that the samples represent in situ mollusc fragments, probably giving a very reliable age for the sand which also fits well with the biostratigraphical correlations.
In the fine-grained deposits just above the Skade Formation, two samples based on foraminiferal tests from 914.5 m (sidewall core) gave 15.5 and 14.7 Ma and two samples based on the same kind of fossils from 912.5 m (sidewall core) gave 16.7 and 16.5 Ma. All these ages are close to the Lower/Middle Miocene boundary which fit very well with the biostratigraphical correlations. The small discrepancies are within the precision of the method.
|Litho. Unit||Sample||Corrected 87/86Sr||2S error||Age (Ma)||Analysed fossils|
|Nordland Gr||912.5 m (SWC)||0.708719||0.000009||16.49a||16 large tests of F. Boueanus|
|Nordland Gr||912.5 m (SWC)||0.708707||0.000009||16.68a||45 tests of A. guerichi staeschei, F. boueanus, P. bulloides, U. tenuipustulata, G. ciperoensis, G. angustiumbilicata and G. praebulloides|
|Nordland Gr||914.5 m (SWC)||0.708790||0.000010||14.66a||33 tests of A. guerichi staeschei, F. boueanus, P. bulloides, U.
tenuipustulata, G. ciperoensis, G. praebulloides and G. angustiumbilicata
|Nordland Gr||914.5 m
|0.708768||0.000009||15.45a||45 tests of A. guerichi staeschei, F. boueanus, P. bulloides, U.
tenuipustulata, B. elongata, G. ciperoensis, G. praebulloides,
G. woodi and G. angustiumbilicata
|Skade Fm||940 m (DC)||0.708623||0.000008||17.69||Six mollusc fragments|
|Skade Fm||950 m (DC)||0.708688||0.000009||16.95||Eight mollusc fragments|
|Skade Fm||950 m (DC)||0.708689||0.000008||16.94||Five mollusc fragments|
|Skade Fm||960 m (DC)||0.708671||0.000009||17.16||Ten mollusc fragments|
|Skade Fm||980 m (DC)||0.708593||0.000009||18.04||Eight mollusc fragments|
|Skade Fm||980 m (DC)||0.708637||0.000009||17.55||Six mollusc fragments|
|Skade Fm||990 m (DC)||0.708619||0.000009||17.74||Nine mollusc Fragments|
|Skade Fm||1000 m (DC)||0.708552||0.000009||18.54||Nine mollusc fragments|
|Skade Fm||1010 m (DC)||0.708458||0.000009||20.02||Eleven mollusc fragments|
|Skade Fm||1020 m (DC)||0.708455||0.000009||20.07||Six mollusc fragments|
|Skade Fm||1020 m (DC)||0.708444||0.000009||20.23||Three mollusc fragments|
|Skade Fm||1030 m (DC)||0.708430||0.000009||20.44||Seven mollusc fragments|
|Hordal. Gr||1194.5 m (SWC)||0.708087||0.000008||27.47a||35 tests of A. guerichi guerichi|
|Hordal. Gr||1194.5 m (SWC)||0.708106||0.000008||26.78a||35 tests of A. guerici guerici|
|Hordal. Gr||1194.5 m (SWC)||0.708090||0.000009||27.26a||40 tests of A. guerichi guerici|
|Hordal. Gr||1210.5 m (SWC)||0.708225||0.000009||24.51a||42 tests of T. alsatica and A. guerichi guerichi|
|Hordal. Gr||1290-1310 m (DC)||0.708042||0.000008||28.53a||18 tests of R. Bulimoides|
Table 1: Strontium isotope data from well 16/1-4. 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), a from Nødtvedt (1999).
Lower Oligocene to lowermost part of Lower Miocene (1400.5 to approximately 1170 m, Hordaland Group)
Clay dominates the samples. Minor sand and silt are also recorded throughout. Sand is most common in the upper part, but some of this may be caved from the immediately overlying Skade Formation. Limestone is also recorded in some samples, especially in the lower and upper parts (Fig. 1).
Lower Miocene (main part, approximately 1170 to approximately 916 m, Skade Formation)
Medium to fine sand dominates the samples from the Skade Formation. Quartz dominates the sand fraction in most samples, but minor glauconite and mica are recorded throughout (Figs. 1 and 2).
Middle Miocene (912.5-857.5 m, Nordland Group)
Clay dominates Middle Miocene samples. Some sand (mainly glauconitic) and silt are also recorded (Fig. 2).
Lower Pliocene (770-760 m, uppermost part of the Utsira Formation and Nordland Group)
This part contains sand rich in clay and silt (Fig. 2).
Upper Pliocene (760-720 m, Nordland Group)
The samples in the Upper Pliocene section contain a clay-rich diamicton with some sand and silt and minor pebbles.
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