Biostratigraphical and Sr isotope correlations

T. Eidvin, F. Riis, E. S. Rasmussen & Y. Rundberg, 2013. New layout 2021

Lower Oligocene deposits (Fig. 1 and Fig. 2) are recorded at Forlandsundet (outcrop), in wells 7316/5-1, 6305/5-1, 6610/3-1, 6610/2-1 S, 6609/11-1, 6507/12-1, 6407/9-2, 6407/9-1, 6407/9-5, 36/1-2, 34/4-6, 34/7-1, 34/8-1, 31/3-1, 25/2-10 S, 25/10-2, 16/1-4, 15/9-13, 2/4-C-11, 9/12-1, 2/2-2 and 11/10-1. King (1989) described a Rotaliatina bulimoides Zone (NSB7b) from the Lower Oligocene of the North Sea. Gyroidina soldanii mamillata is also common in this zone, and both species have their last appearance datums (LADs) close to the Lower/Upper Oligocene boundary. In most of the wells/outcrops we have investigated, from the northwestern Norwegian-Danish Basin in the south to the Forlandsundet (northwestern Svalbard) in the north (Map 1), we have used R. bulimoides or G. soldanii mamillata as index fossils for the Lower Oligocene. The Early Oligocene age is also supported by Sr isotope analyses in most of the wells and outcrops. The exceptions include wells 7316/5-1 and 6609/11-1 where the identification of the Lower Oligocene is based on palynological evidence and Sr isotope analyses, and wells 6610/3-1 and 36/1-2 where the occurrence of the Lower Oligocene is based on a number of Sr analyses. It is probably due to environmental factors that neither R. bulimoides nor G. soldanii mamillata occur in these deposits. In wells 6610/2-1 S and 6407/9-1 the occurrence of R. bulimoides and G. soldanii mamillata does not reach to the top of the Lower Oligocene units (also probably due to environmental factors), and the top is based solely on palynological evidence. In both of these wells, Sr isotope analyses support the Early Oligocene age.

Upper Oligocene deposits (including Lower-Upper Oligocene deposits, Fig. 1 and Fig. 2) are recorded in wells 6404/11-1, 6305/4-1, 6305/5-1, 49-23 (core), 6609/11-1, 6507/12-1, 36/1-2,35/3-1, 34/2-4, 34/4-6, 34/4-7, 34/8-3A, 34/8-1, 34/7-2, 35/11-1, 35/11-14 S, 34/10-17, 30/5-2, 30/6-3, 25/1-8 S, 25/2-10 S, 25/10-2, 24/12-1, 16/1-4, 15/9-13, 15/12-3, 2/4-C-11, 9/12-1, 2/2-2, 11/10-1 and in the Hjøllund and Rødding boreholes (Denmark). In the North Sea area, the LAD of Diatom sp. 3 is close to the Late Oligocene/Early Miocene boundary according to King (1989, the top of Zone NSP 10). The LADs of the calcareous benthic foraminifera Turrilina alsatica, Gyroidina soldanii girardana and Almaena osnabrugensis are also close to this boundary according to King (1989). In CaCO3-poor deposits, the LAD of the agglutinated foraminifera Ammodiscus sp. B is a reliable marker for an event close to the Late Oligocene/Early Miocene boundary, according to King (1989).

In most of the wells and boreholes from the northern Central Graben in the North Sea to the Trøndelag Platform (Norwegian Sea continental shelf, Fig. 6 and Fig. 7) we have used the LAD of Diatom sp. 3 as a marker for top Upper Oligocene. In many of these wells, the LADs of T. alsatica and G. soldanii girardana occur at or close to the same level. In a few wells, where Diatom sp. 3 is scarce or missing, we have used the LADs of T. alsatica and/or G. soldanii girardana as markers for the top of the Upper Oligocene. The Late Oligocene age is also supported by Sr isotope analyses in many of these wells (usually based on analyses of tests of T. alsatica and/or G. soldanii girardana). In well 25/1-8 S, the LAD of A. osnabrugensis is used. In well 2/2-2, the Upper Oligocene is scarce in calcareous foraminifera, and in this well the top Oligocene is based on the LAD of Ammodiscus sp. B. In wells 34/7-2 and 35/11-14 S, Diatom sp. 3 does not reach to the top of the Upper Oligocene unit, but similar pyritised diatoms are common towards the top of the unit. Seismic correlation also indicates that these units are of Late Oligocene age. In the Hjøllund and Rødding boreholes from onshore southern Denmark (Map 1), the foraminiferal fauna has a more shallow-marine affinity than in most North Sea wells. In these boreholes the top of the Upper Oligocene is based on LAD of Pararotalia canui, which is also close to the Late Oligocene/Early Miocene boundary according to King (1989). Some of the index fossils used in wells from the North Sea area are absent in these boreholes. However, T. alsatica and Diatom sp. 3 are recorded somewhat lower in the unit in the Hjøllund borehole. In most of the wells the biostratigraphical correlations are supported by Sr isotope analyses. In well 7316/5-1, a section was given an unspecified Late Oligocene to Early Miocene age based mainly on palynological evidence (see Eidvin et al. 1998b). In well 29/3-1, we have not recorded any Upper Oligocene index fossils, but the general fossil assemblage and seismic correlation indicate a Late Oligocene age.

Onshore Denmark (Map 1), Upper Oligocene deposits of the Brejning Formation (Rasmussen et al. 2010) are recorded from several boreholes and outcrops. In some of these we have supported the palynological datings of Dybkjær & Piasecki (2008, 2010) with strontium isotope datings of mollusc tests (Eidvin et al. work in progress a and b, Link to Danish Sr isotope ages).

Lower Miocene sediments (Fig. 1 and Fig. 2) are recorded in wells 7316/5-1, 6510/2-1, 6609/11-1, 6507/12-1, 6407/9-5, 34/8-3A, 34/8-1, 30/5-2, 30/6-3, 25/1-8 S, 25/2-10 S, 25/10-2, 24/12-1, 16/1-4, 15/9-13, 15/12-3, 2/4-C-11 and in the Rødding borehole (Denmark). In the North Sea area, the LADs of the calcareous benthic foraminifera Uvigerina tenuipustulata and the planktonic foraminifera Globorotalia zealandica and Globorotalia praescitula are close to the Lower/Middle Miocene boundary (the top of Zone NSB 10 and Zone NSP 11, respectively, according to King 1983, 1989). In some wells, also the LADs of Globigerina angustiumbilica and/or Globigerina ciperoensis are close to the Lower/Middle Miocene boundary. As marker fossils for the lower part of the Lower Miocene, King (1983, 1989) used the LAD of the calcareous benthic foraminifera Plectofrondicularia seminuda (Zone NSB 10), the LAD of the agglutinated foraminifera Spirosigmoilinella sp. A (synonymous with Spirosigmoilinella compressa, Zone NSA 10) and the LADs of the Diatom sp. 4 and sp. 5 (Zone NSP 10).

The registration of the lower part of Lower Miocene in the wells from the Trøndelag Platform (Norwegian Sea continental shelf) and the Tampen area (northern North Sea) is mainly based on the occurrence of Diatom sp. 4 and sp. 5. The registration of the top of the Lower Miocene is based on the LAD of U. tenuipustulata in almost all of the wells from the southern Viking Graben (Fig. 6 and Fig. 7). In addition, the LADs of G. zealandica and/or G. praescitula are also used in most of the wells. The exception is well 25/1-8 S and 25/2-10 S where the LADs of G. angustiumbilica and G. ciperoensis are used. The recording of the lower part of Lower Miocene in wells 25/10-2, 24/12-1, 15/9-13 and 15/12-3 is based on the occurrence of Diatom sp. 4 and P. seminuda, only P. seminuda in well 25/2-10S and Diatom sp. 4 and S. compressa in well 16/1-4. In nearly all the wells from the southern Viking Graben (Fig. 7) the biostratigraphical correlations are supported by a large number of Sr isotope analyses. The benthic index foraminifera U. tenuipustulata is missing in the shallow-marine deposits in the Molo Formation in well 6510/2-1 and in the Rødding borehole (Denmark). However, the shallow-marine dwelling Asterigerina guerichi stashei is recorded, and this species is known from the Lower Miocene and the lowermost Middle Miocene in the North Sea (King 1989). According to King (1983) and Eidvin & Rundberg (2007), the LADs of A. guerichi stashei and U. tenuipustulata are approximately coincident in some areas of the North Sea. The obtained ⁸⁷Sr/⁸⁶Sr ratios from a number of Sr isotope analyses support an Early Miocene age.
Onshore Denmark, Lower Miocene deposits of the Ribe Group are recorded from a number of boreholes and outcrops. In many of these we have supported the palynological datings of Dybkjær & Piasecki (2008, 2010) with strontium isotope datings of mollusc tests (Map 1 and Link to Danish Sr isotope ages).

Middle Miocene sediments (Fig. 1 and Fig. 2) are recorded in boreholes 6704/12-GB1 and 6403/5-GB1, wells 6507/12-1, 30/5-2, 30/6-3, 25/1-8 S, 25/2-10 S, 25/10-2, 24/12-1, 16/1-2, 16/1-4, 15/9-13, 15/12-3, 2/4-C-11 and in the Rødding borehole (Denmark). The planktonic fossils of genus Bolboforma are very reliable index fossils for the Middle Miocene. Spiegler & Müller (1992) described a Bolboforma reticulata Zone from deposits with an age slightly older than 14 to 12.3 Ma, a very short Bolboforma danielsi Zone from around 12.3 Ma and a Bolboforma badenensis Zone from 12.3-11.7 Ma from the North Atlantic. On the Vøring Plateau, B. reticulata and B. badenensis occur together in the same samples in a B. badenensis – B. reticulata Zone (Müller & Spiegler 1993). As marker fossils for the lower part of the Middle Miocene, King (1983, 1989) use the LAD of the calcareous benthic foraminifera A. guerichi stashei (Zone NSB 11) and the LADs of the planktonic foraminifera Sphaeroidinellopsis disjuncta and Globigerinoides quadrilobatus triloba (Zone NSP 12) for the North Sea area. In some areas in the North Sea, Zone NSB 11 is not easily recognisable, and may be very condensed or absent, and in these areas the LADs of A. guerichi stashei and U. tenuipustulata are approximately coincident (King 1983, Eidvin & Rundberg 2007). In such areas the planktonic foraminiferal Zone NSP 11 and NSP 12 also seem to merge (Eidvin & Rundberg 2007).

B. badenensis was used as index fossil for the upper part of the Middle Miocene (younger than 12.3 Ma) in borehole 6704/12-GB1. A Bolboforma reticulata assemblage and a B. badenensis or a B. badenensis – B. reticulata assemblage are recorded in borehole 6403/5-GB1, wells 6507/12-1, 30/5-2, 30/6-3 (northern Viking Graben, Fig. 7) and all of the wells from the southern Viking Graben (except 25/1-8 S) and in the Rødding borehole onshore Denmark (Map 1). In well 2/4-C-11, neither B. badenensis nor B. reticulata is recorded, indicating that some of the Middle Miocene is missing. However, this is probably a local hiatus due to salt tectonics and polygonal faulting. Benthic and planktonic foraminiferal assemblages corresponding to Zone NSB 11 and Zone NSP 12 (King 1983, 1989), respectively, are present, indicating that at least the lower the part of the Middle Miocene is present. Neither B. badenensis nor B. reticulata is recorded in the shallow-marine sandy deposits in well 25/1-8 S, even though Sr analyses indicate an age of approximately 15 to 12 Ma for the sediments. However, the occurrence of G. quadrilobatus triloba indicates the presence of Zone NSP 12 (King 1989). Benthic and planktonic foraminiferal assemblages corresponding to Zone NSB 11 and Zone NSP 12 are also present in well 15/12-3, benthic foraminifera corresponding to Zone NSB 11 are present in well 25/10-2 and planktonic foraminifera corresponding to Zone NSP 12 are present in well 25/2-10 S. In boreholes 6704/12-GB1 and 6403/5-GB1 and wells 30/5-2, 30/6-3, 25/2-10 S, 15/12-3, 15/9-13 and 2/4-C-11 the Middle Miocene age is supported by Sr isotope analyses.

Upper Miocene deposits (Fig. 1 and Fig. 2) are recorded in boreholes ODP Site 987 (off Scoresby Sund, East Greenland, Map 2), 6704/12-GB1, 6403/5-GB1, wells 6305/5-1, 6609/5-1, 6508/5-1, 6609/11-1, 6507/12-1, 6407/9-5, 34/8-3A, 35/11-1, 35/11-14 S, 34/10-17, 30/5-2, 30/6-3, 25/10-2, 24/12-1, 16/1-2, 15/12-3, 2/4-C-11, and in the Rødding borehole (Denmark). First and foremost we have used several planktonic fossils as index fossils for the Upper Miocene. For the lowermost part of the Upper Miocene we have used Bolboforma fragori and Bolboforma subfragori. A B. fragori/B. subfragori Zone is described from deposits with an age of 11.7-10.3 Ma from the North Atlantic and the Vøring Plateau (Spiegler & Müller 1992, Müller & Spiegler 1993). According to Berggren et al. (1995), the Middle/Late Miocene boundary is at 11.2 Ma, consequently the oldest part of this unit is within the Middle Miocene. Bolboforma metzmacheri is also used as an index fossil for the lower part of Upper Miocene. A B. metzmacheri Zone is recorded 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). We have also used the LAD of Neogloboquadrina atlantica (dextral). Spiegler & Jansen (1989) described a lower N. atlantica (dextral) Zone from Upper Miocene sediments on the Vøring Plateau, and Weaver (1987) and Weaver & Clement (1987) recorded a Neogloboquadrina atlantica (dextral)/Neogloboquadrina acostaensis Zone from Upper Miocene sediments in the North Atlantic. However, Spiegler & Jansen (1989) also described an upper N. atlantica (dextral) Zone from the Upper Pliocene and, consequently, caved specimens of N. atlantica (dextral) can be recorded in the Lower Pliocene and Upper Miocene.

The calcareous benthic foraminifera Globocassidulina subglobosa, Ehrenberina variabilis, Sphaeroidina bulloides, Florilus boueanus, Eponides pygmeus, Cibicides telegdi and Martinottiella communis (agglutinated) are common in Upper Miocene and Lower Pliocene deposits in the North Sea, on the Norwegian Sea continental shelf and in the Netherlands. However, these species also occur, though less numerous, in the lower Neogene and upper Paleogene, and E. pygmeus and probably C. telegdi also in the Upper Pliocene (see below, Doppert 1980, King 1983 and 1989, Skarbø & Verdenius 1986, Stratlab 1988, Gradstein & Bäckström 1996, Eidvin & Rundberg 2007, Eidvin et al. 2007).

Either a B. fragori - B. subfragori assemblage, a B. fragori assemblage or a B. subfragori assemblage is recorded in the lower part of Upper Miocene in borehole 6704/12-GB1, wells 6607/5-1, 6609/5-1, 6508/5-1, 6609/11-1, 6507/12-1, 34/8-3A, 35/11-1, 30/6-3, 25/10-2, 24/12-1 and 2/4-C-11. A B. metzmacheri assemblage is recorded in wells 6607/5-1, 6609/5-1, 6508/5-1, 6407/9-5, 30/5-2, 25/10-2, 2/4-C-11 and Rødding borehole onshore Denmark. In borehole 6403/5-GB1 we have recorded a Bolboforma pseudohystrix - Bolboforma compressibadenensis assemblage and a Bolboforma clodiusi assemblage and these forms are known from, respectively, Middle to Upper Miocene sediments and Upper Miocene deposits on the Vøring Plateau and in the North Atlantic. In well 6507/12-1 and the Rødding borehole we have described a Bolboforma laevis assemblage, and this species is known from the Upper Miocene in the same areas (Qvale & Spiegler 1989, Spiegler & Müller 1992, Müller & Spiegler 1993). A N. atlantica (dextral) assemblage is recorded in wells 6507/12-1, 34/10-17, 25/10-2, 24/12-1, 16/1-2, 15/12-3, 2/4-C-11, and a N. atlantica (dextral)- N. acostaensis assemblage in well 15/12-3. A large number of Sr isotope analyses based on calcareous foraminifera, Bolboforma and mollusc fragments support the biostratigraphical correlations.

The uppermost part of the Upper Miocene in well 6508/5-1 is defined on basis of the common occurrence of the calcareous benthic foraminifera Uvigerina venusta saxonica and Sr isotope analyses of tests of this species.

In borehole ODP Site 987 off Scoresby Sund (East Greenland, Map 2), thorough magneto-stratigraphic investigations of continuous cores have resulted in a very good stratigraphic resolution (Channell et al. 1999). The magneto-stratigraphic data give an age of approximately 7.43 Ma to the base of the borehole. The Upper Miocene section is nearly barren of planktonic foraminifera, but contains a sparse but continuous calcareous benthic foraminiferal fauna which we denoted as the Globocassidulina subglobosa - Ehrenbergina variabilis - Cibicides dutemplei assemblage. The assemblage is very similar to calcareous benthic foraminiferal assemblages which we have recorded in the North Sea (e.g. in the Utsira Formation) and on the Norwegian Sea continental shelf (e.g. in the Kai Formation). Unfortunately, the samples do not contain sufficient numbers of calcareous index fossils to give reliable Sr isotope ages. The Upper Miocene unit in well 35/11-14 S (coarse sand) is barren of microfossils. However, mollusc fragments were recorded at four levels, and the nine Sr isotope analyses all gave Late Miocene ages.

Sediments of a general Late Miocene-Early Pliocene age and Early Pliocene age (Fig. 1 and Fig. 2) are recorded at ODP Site 987 (off Scoresby Sund, East Greenland) and in wells 6609/5-1, 6508/5-1, 6609/11-1, 6507/12-1, 6407/9-2, 6407/9-1, 6407/9-5, 34/2-4, 34/4-6, 34/4-7, 34/7-1, 34/8-3A, 34/8-1, 34/7-2, 35/11-1, 30/6-3, 25/2-10 S, 25/10-2, 24/12-1, 16/1-2, 16/1-4, 15/9-A-23, 15/12-3 and 2/4-C-11. An exact definition of the Lower Pliocene is usually difficult in the areas we have investigated, since there are no planktonic or benthic foraminifera which have a range limited to the Lower Pliocene. The use of Sr isotope stratigraphy is also problematic for the interval 4.5-2.5 Ma. The Sr isotope seawater curves of Hodell et al. (1991), Farrell et al. (1995) and Howarth & McArthur (1997) are relatively flat in this interval due to small variations in the Sr composition. The LAD of N. atlantica (dextral) indicates the top of the Upper Miocene, but this species is sometimes absent due to environmental factors, and in several wells there are no Lower Pliocene deposits overlying the Upper Miocene. The benthic calcareous foraminiferid Monspeliensina pseudotepida is an easily recognisable and reliable index fossil for the Early Pliocene and the early Late Pliocene (before the start of extensive glaciations), but this species is a shallow-marine form and consequently has a limited geographic range (King 1983, 1989). When M. pseudotepida occurs together with some of the calcareous benthic foraminifera G. subglobosa, E. variabilis, S. bulloides, F. boueanus, E. pygmeus and C. telegdi, which are typical for both Upper Miocene and Lower Pliocene deposits, a Late Pliocene age is unlikely. However, in the North Sea, tests of E. pygmeus and C. telegdi are also recorded in basal Upper Pliocene deposits (Stratlab 1988, Eidvin & Rundberg 2007). Micropalaeontologists working with deposits from the Norwegian shelf have discussed if the occurrences in the Upper Pliocene represent in situ or reworked tests. The tests are very small and are probably easily re-worked. Their small sizes had made it difficult to collect enough tests for Sr analyses which could have solved the problem. 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 grain sorting mechanism of turbidite currents has probably concentrated this species in this sample. 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). However, in areas on the Norwegian continental shelf where E. pygmeus occurs in both the Lower and the Upper Pliocene, there is a clear unconformity between the units visible both on seismic, wire line log data and lithology (see the chapter “The base of the Upper Pliocene and important Upper Pliocene foraminifera” below).

Neogloboquadrina atlantica (sinistral), Globigerina bulloides and Globorotalia puncticulata are the most common planktonic foraminifera in Lower Pliocene deposits. N. atlantica (sinistral) and G. bulloides occur through the Upper Miocene and Lower Pliocene and into the Upper Pliocene, and G. puncticulata occurs through the Lower Pliocene and into the Upper Pliocene (Weaver 1987, Weaver & Clement 1987, Spiegler & Jansen 1989). However, according to Eidvin & Rundberg (2007) G. puncticulata seems to disappear from the southern Viking Graben area close to the Early/Late Pliocene boundary. This observation is based on co-occurrence with the calcareous benthic foraminifera listed above and on Sr isotope analyses.

A general Late Miocene-Early Pliocene age is based on the occurrence of an E. variabilis – G. subglobosa assemblage and C. telegdi – E. pygmeus assemblage in well 6607/5-1, a G. subglobosa assemblage in well 6609/5-1, an E. pygmeus - C. telegdi assemblage and E. pygmeus assemblage in well 6508/5-1, an E. pygmeus - G. subglobosa assemblage and C. telegdi assemblage in well 6609/11-1, an E. pygmeus - C. telegdi assemblage and E. pygmeus asseblage in well 6507/12-1, a dinoflagellate Achomosphaera sp. 1 Zone in well 6407/9-1, an E. variabilis assemblage in wells 34/2-4, 34/4-7, 34/8-3A and 34/8-1, the upper part of a S. bulloides assemblage in well 35/11-1 and a F. boueanus assemblage in well 30/6-3. In wells 34/2-4, 34/4-6, 34/4-7, 34/7-2 and 34/8-1, Sr isotope analyses based on tests of some of these foraminifera gave ages of about 5 Ma and an indication of ages near the Late Miocene/Early Pliocene boundary (5.32 Ma).

An Early Pliocene age is based on the occurrence of a Monspeliensina pseudotepida assemblage capped by an E. pygmeus assemblage in wells 6407/9-2 and 6407/9-5, a dinoflagellate Reticulatosphaera actinocoronata Zone and an E. pygmeus - S. bulloides assemblage in well 6407/9-1, the upper part of a benthic U. venusta saxonica assemblage and G. subglobosa assemblage and a planktonic G. puncticulata assemblage in well 25/10-2, a F. boueanus - S. bulloides assemblage, E. pygmeus - C. telegdi assemblage and G. puncticulata assemblage in well 25/2-10 S and the upper part of an U. venusta saxonica assemblage, M. pseudotepida assemblage and G. puncticulata assemblage in wells 24/12-1, 16/1-2, 15/12-3, an U. venusta saxonica assemblage and G. puncticulata assemblage in wells 16/1-4, 15/9-A-23 and 15/12-3 and the upper part of an U. venusta saxonica assemblage and lower part of a Cibicides limbatosuturalis assemblage in well 2/4-C-11. In some of these wells an Early Pliocene age is supported by Sr isotope analyses.

In borehole ODP Site 987, an Early Pliocene age is based on magneto-stratigraphy. The lower part of the Lower Pliocene is nearly barren of benthic calcareous and planktonic foraminifera. In the upper part we defined a benthic Cassidulina teretis - Nonion affine - Elphidium excavatum assemblage and a planktonic N. atlantica (sinistral) - G. bulloides assemblage. All these taxa are common throughout most of the upper Neogene on the Norwegian continental shelf. It is noteworthy that the FAD of Cibicides grossus is very close to the Early/Late Pliocene boundary.