Upper Oligocene to Upper Pliocene in well 24/12-1
Modified after Eidvin & Rundberg (2007).
Based on analyses of benthic and planktonic foraminifera, Bolboforma, pyritised diatoms and Sr isotopes in well 24/12-1 (59º02'29.80''N, 01º52'57.93''E, Map 1), we recorded 110 m with Upper Oligocene sediments, a 310 m-thick column with Lower Miocene deposits, 100 m with Middle Miocene sediments, 170 m with Upper Miocene sediments, approximately 55 m with Lower Pliocene deposits and approximately 10 m with Upper Pliocene sediments. The base of the Upper Oligocene and the top of the Upper Pliocene were not investigated. The units are investigated with 49 ditch-cutting samples at mainly ten metre intervals. However, where stored samples are depleted to the archive limit the interval between the samples is much larger (Figs. 1 and 2). A detailed stratigraphy of these units (including fossil range charts) was presented in Eidvin & Rundberg (2007).
Upper Oligocene (1240-1130 m, Hordaland Group)
Pyritised diatoms of the Diatom sp. 3 assemblage and benthic foraminifera of the Turrilina alsatica assemblage (lower, main part), together with a Sr isotope age (25.1 Ma) from 1210 m, date this unit to Late Oligocene (Fig. 1). In addition to the nominate species, the benthic foraminiferal fauna include G. soldanii girardana and Rolfina arnei. The in situ planktonic foraminiferal fauna includes a few specimens of G. praebulloides, G. ciperoensis and G. angustiumbilicata. The diatom assemblage is correlated with upper part of Subzone NSP 9c, and the benthic foraminiferal assemblage 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 (1130-820 m, Hordaland Group, Skade Formation and lowermost part of the Nordland Group)
Benthic foraminifera of the Turrilina alsatica assemblage (lowermost part), Plectofrondicularia seminuda 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 (Fig. 1). The lowermost part of the Bolboforma reticulata assemblage is also a part of the unit, but the occurrence of B. reticulata may be caved (Fig. 1). In addition to the nominate species, the Globorotalia zealandica - Globigerina ciperoensis assemblage includes Globorotalia praescitula. The upper part of Turrilina alsatica assemblage is correlated with the upper part of Zone NSB 8, and the Plectofrondicularia seminuda assemblage is correlated with Zone NSB 9 of King (1989) from the North Sea. The Uvigerina tenuipustulata - Astigerina guerichi staeschei assemblage is correlated with Zone NSB 10 and Zone 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 (820-720 m, Nordland Group and lowermost part of the Utsira Formation)
Bolboforma of the Bolboforma reticulata assemblage and Bolboforma badenensis 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 include E. variabilis, B. elongata (lower part), Uvigerina semiornata saprophila (lower part), Trifarina gracilis (lower part) and M. communis (agglutinated). Planktonic foraminifera include G. bulloides, N. atlantica (sinistral) and N. atlantica (dextral).
Spiegler & Müller (1992) described a B. badenensis Zone and a B. reticulata Zone from the North Atlantic and Müller & Spiegler (1993) described a B. badenensis/B. reticulata Zone from the Vøring Plateau. These zones are recorded from deposits with an age slightly older than 14 to 11.7 Ma (Spiegler & Müller 1992). 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.
Upper Miocene (720-550 m, Utsira Formation)
Benthic foraminifera of the Uvigerina venusta saxonica assemblage (lower, main part), Bolboforma of the Bolboforma fragori assemblage and planktonic foraminifera of the Neogloboquadrina atlantica (dextral) assemblage, together with a large number of Sr isotope analyses, give a Late Miocene age to this unit (Fig. 2). In addition to the nominate species, the benthic foraminiferal assemblage includes F. boueanus, S. bulloiodes and Globulina gibba myristiformis. The Bolboforma fragori assemblage also includes B. clodiusi, B. subfragori (few) and B. laevis (few), and the Neogloboquadrina atlantica (dextral) assemblage also includes N. atlantica (sinistral) and G. bulloides.
A B. fragori/B. subfragori Zone is known from deposits with an age of approximately 11.7-10.3 Ma from the North Atlantic and the Vøring plateau (Spiegler & Müller 1992, Müller & Spiegler 1993). 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 Uvigerina venusta saxonica is correlated with the lower part of Subzone NSB 13b of King (1989) from the North Sea.
Lower Pliocene (550 to approximately 495 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 and Globigerina bulloides assemblage (lower part) give an Early Pliocene age for this unit (Fig. 2). In addition to the nominate species, the upper part of the Uvigerina venusta saxonica assemblage and Monspeliensina pseudotepida assemblage also includes F. bouanus, G. gibba myristiformis, S. bulloides, Nodosaria konincki and Bolivina imporcata. The Globorotalia puncticulata assemblage also includes N. atlantica (sinistral).
The upper part of the 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). According to King (1989) M. pseudotepida is reported from the uppermost Upper Miocene to the lower part of Upper Pliocene. This indicates that the uppermost part of this unit may be of early Late Pliocene age. However, some of the other species in the assemblage do not, or seldom, extend into the Upper Pliocene. An Early Pliocene Globorotalia puncticulata assemblage is described by Weaver & Clement (1986) from the North 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 (approximately 495 to 480 m, Nordland Group)
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. 2). 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 last 0.5 Ma in the North Atlantic (Kellogg 1977).
Sr isotope stratigraphy
Thirty-one samples with mollusc fragments from twelve depths, and three samples with calcareous foraminiferal tests from three depths were analysed for Sr isotopes in well 24/12-1. The obtained 87Sr/86Sr ratio from the sample at 1210 m (based on foraminiferal tests) gave 25.1 Ma (Upper Oligocene) and supports the biostratigraphical correlations (Table 1, Fig. 1).
Six samples (based on mollusc fragments) from 960, 930 and 920 m in the middle part of the Skade Formation gave ages from 19.2 to 17.8 Ma (Early Miocene), which also support the biostratigraphical correlations.
Two samples (based on foramiferal tests) from 840-830 m gave 15.3 and 14.6 Ma (earliest Middle Miocene). These samples are from the uppermost part of the unit given an Early Miocene age by the biostratigraphical correlations (Fig. 1). The Sr isotope ages are close to the Early/Middle Miocene boundary and the small discrepancy is within the precision of the method.
The part of the well which was given Late Miocene and Early Pliocene ages by the biostratigraphical correlation (Utsira Formation) was investigated with 22 mollusc fragments samples from nine depths. The results varied from a minimum of 2.1 Ma (uppermost sample) to a maximum of 9.0 Ma. There is a relatively distinct trend of younger ages in the upper part of the Utsira Formation (560-500 m), in which five of the samples gave ages of about 5.8-5.2 (close to the Late Miocene/Early Pliocene boundary), and older ages in the lower part (710-650 m), in which 17 analyses displayed isotopic compositions corresponding to ages between 9.0 and 5.2 Ma. This is broadly in agreement with the biostratigraphical correlations. Two samples are quite obviously caved (see Fig. 2), and some of the other samples in the lower part probably contained mollusc fragments which also are caved from a somewhat shallower level in the Utsira Formation.
|Litho. Unit||Sample (DC)||Corrected 87/86Sr||2S error||Age (Ma)||Comments||Analysed fossils|
|Utsira Fm||500 m||0.709083||0.000007||2.09||Caved||One mollusc fragments|
|Utsira Fm||520 m||0.709005||0.000009||5.76||Two mollusc fragments|
|Utsira Fm||530 m||0.709018||0.000007||5.51||One mollusc fragments|
|Utsira Fm||550 m||0.708999||0.000009||5.84||One mollusc fragments|
|Utsira Fm||560 m||0.709037||0.000009||5.00||One mollusc fragments|
|Utsira Fm||560 m||0.709053||0.000009||4.14||Two mollusc fragments|
|Utsira Fm||560 m||0.709078||0.000007||2.24||Caved||Two mollusc fragments|
|Utsira Fm||650 m||0.708914||0.000009||8.96||Two mollusc fragments|
|Utsira Fm||650 m||0.708999||0.000009||5.84||One mollusc fragments|
|Utsira Fm||650 m||0.708986||0.000008||6.00||Three mollusc fragments|
|Utsira Fm||650 m||0.709032||0.000007||5.16||One mollusc fragments|
|Utsira Fm||650 m||0.709073||0.000009||2.41||Caved||One mollusc fragments|
|Utsira Fm||670 m||0.709026||0.000009||5.30||One mollusc fragments|
|Utsira Fm||670 m||0.709018||0.000009||5.51||One mollusc fragments|
|Utsira Fm||670 m||0.708929||0.000008||7.95||Two mollusc fragments|
|Utsira Fm||670 m||0.708933||0.000009||7.69||One mollusc fragments|
|Utsira Fm||670 m||0.708970||0.000009||6.26||One mollusc fragments|
|Utsira Fm||670 m||0.708922||0.000009||8.58||Two mollusc fragments|
|Utsira Fm||700 m||0.708962||0.000008||6.54||Three mollusc fragments|
|Utsira Fm||700 m||0.709006||0.000009||5.75||Three mollusc fragments|
|Utsira Fm||700 m||0.708971||0.000008||6.23||One mollusc fragments|
|Utsira Fm||700 m||0.708919||0.000009||8.74||One mollusc fragments|
|Utsira Fm||710 m||0.708918||0.000008||8.79||Two mollusc fragments|
|Utsira Fm||710 m||0.708952||0.000009||6.87||Two mollusc fragments|
|Utsira Fm||710 m||0.709014||0.000008||5.61||Two mollusc fragments|
|Nordland Gr||830-840 m||0.708792||0.000009||14.56||21 tests of Astigerina guerichi staeschei and Siphonina reticulate|
|Nordland Gr||840 m||0.708773||0.000009||15.28||24 tests of Elphidium inflatum|
|Skade Fm||920 m||0.708523||0.000009||18.95||Two mollusc fragments|
|Skade Fm||920 m||0.708617||0.000009||17.76||One mollusc fragments|
|Skade Fm||930 m||0.708570||0.000009||18.30||One mollusc fragments|
|Skade Fm||930 m||0.708577||0.000009||18.22||One mollusc fragments|
|Skade Fm||960 m||0.708538||0.000008||18.73||Two mollusc fragments|
|Skade Fm||960 m||0.708509||0.000009||19.16||One mollusc fragments|
|Hordaland Gr||1210 m||0.708187||0.000017||25.08||14 tests of Gyroidina soldanii girardana|
Table 1: Strontium isotope data from well 24/12-1. 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. Modified after Eidvin & Rundberg (2007).
Upper Oligocene to lower part of Lower Miocene (1240 to approximately 1010 m, Hordaland Group)
Clay dominates the ditch cutting samples in this part, but some sand (quartzose (dominant), glauconitic and biotitic) and silt are recorded throughout. Limestone is also recorded in a few samples (Fig. 1). Some of the sand may be caved from the immediately overlying Skade Formation.
Upper (main part) of Lower Miocene (approximately 1010 to approximately 820 m, Skade Formation and lowermost part of the Nordland Group)
Medium to fine sand dominates most of the samples in this unit, but some parts are also rich in clay and silt. Quartz dominates the sand fraction in most samples, but glauconite is dominant in some. Minor mica is recorded throughout. In some samples mollusc fragments are quite common (Fig. 1).
Middle Miocene (lower main part, 820-720 m, Nordland Group)
Clay dominates Middle Miocene samples. Minor sand and silt and some limestone are also recorded (Fig. 2).
Middle Miocene (uppermost part) to Lower Pliocene (720 to approximately 495 m, Utsira Formation)
Sand dominates this part, but clay and silt are also common. The sand is mainly quartzose with minor glauconite and mica. The unit is very rich in molluscs and mollusc fragments especially in the upper part (Fig. 2).
Upper Pliocene (approximately 495 to 480 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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