Identification and characterisation of the Oligocene–Miocene boundary (base Neogene) in the eastern North Sea Basin — based on dinocyst stratigraphy, micropalaeontology and δ13C-isotope data

Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 11–22

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Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo

Identification and characterisation of the Oligocene–Miocene boundary (base Neogene) in the eastern North Sea Basin — based on dinocyst stratigraphy, micropalaeontology and δ 13C-isotope data Karen Dybkjær a,⁎, Chris King b, Emma Sheldon a a b

Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark 16A Park Rd., Bridport DT6 5DA, UK

a r t i c l e

i n f o

Article history: Received 22 February 2012 Received in revised form 31 July 2012 Accepted 13 August 2012 Available online 24 August 2012 Keywords: North Sea Basin Chattian Aquitanian Mi-1 glaciation event Frida-1 Palynology Micropalaeontology δ13C-isotopes

a b s t r a c t For the first time a combined palynological and δ13C-isotope study has identified the Oligocene–Miocene boundary — and thus the base of the Neogene — within the North Sea Basin. The type section, the Lemme– Carrosio section in northern Italy, is correlated with the Frida-1 well in the eastern (Danish) part of the North Sea Basin using a combination of data from a previous dinocyst stratigraphic study and new δ13C-isotope data. The results show that the Oligocene–Miocene boundary is located at a depth of 1440 m in the Frida-1 well. The Frida-1 δ13C-isotope curve further reflects the Mi-1 glaciation event also recorded in the Lemme–Carrosio section. The dinocyst events bracketing the boundary in Frida-1 are; the last occurrence (LO) of Distatodinium biffii at 1630 m, below the boundary, the LO of Chiropteridium spp. at 1370 m and first occurrence (FO) of Ectosphaeropsis burdigalensis at 1330 m, both above the boundary. An influx of Deflandrea phosphoritica is found in an interval immediately below the boundary (1532–1490 m), while the genus Homotryblium occurs abundantly in a broader interval (1650–1330 m) encompassing the boundary. Hitherto unpublished data combined with new data provide a series of stratigraphically important nannoand micropalaeontological events that frame/characterise the Oligocene–Miocene boundary within the North Sea Basin; an almost monospecific assemblage of Reticulofenestra bisecta at 1630 m, the LO of Elphidium subnodosum at 1625 m, the LO of Aulacodiscus insignis quadrata (Diatom sp. 3 of King, 1983) at 1610 m, the LO of Karreriella seigliei at 1580 m and the LO of Pararotalia canui at 1570 m, all below the boundary, and the FO of Aulacodiscus aemulans (Diatom sp. 5 of King, 1983) at 1410 m and the LO of Aulacodiscus aemulans at 1250 m, both above the boundary. The dinocyst, nanno- and micropalaeontological studies thus provide a series of bioevents and abundance variations which can be used to locate and to correlate the Oligocene–Miocene boundary within the eastern North Sea Basin more precisely than was previously possible. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The purpose of the present study is to identify the Oligocene– Miocene (O/M) boundary in the North Sea Basin and to pinpoint the biostratigraphic events (palynological, micropalaeontological and nannopalaeontological) that characterise the boundary in this (semi-) enclosed basin. The Global Stratotype Section and Point (GSSP) for the O/M boundary, and thus for the base of the Neogene, was ratified in 1996, at the Lemme–Carrosio section in Northern Italy (Steininger et al., 1997a,b) (Fig. 1). The boundary is located 35 m below the top of the section, at the base of magnetochron C6Cn.2n. Correlating the chronostratigraphic

⁎ Corresponding author. Tel.: +45 38142720; fax: +45 38142050. E-mail address: [email protected] (K. Dybkjær). 0031-0182/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2012.08.007

boundary of the Tethyan stratotype section with the North Sea Basin is problematic using biostratigraphy alone. Factors such as differing climatic and depositional environments result in different assemblages of microfossils with only a few species in common. Combining biostratigraphy with isotope stratigraphy facilitates correlation between these far removed sections. Dybkjær and Rasmussen (2007) presented the results of a palynological study of an expanded (>800 m thick) Upper Oligocene/Lower Miocene succession in the offshore well, Frida-1, located in the Danish part of the North Sea Basin (Figs. 1, 2). A series of stratigraphically important dinoflagellate cyst (dinocyst) events were identified and 6 dinocyst assemblages were proposed. Based on a combination of the dinocyst stratigraphy, well logs and seismic data, the basinal succession in Frida-1 was correlated with the onshore succession, which in turn made it possible to subdivide the Frida-1 succession into the sequences A–C, as defined onshore.

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K. Dybkjær et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 11–22

ian Sh

ield

1.1. Isotope curves and the Mi-1 glaciation event

N North Sea Basin

Fenno -Scan d

North Atlantic Ocean

Frida-1 DK UK sif

n Mas

ohemia

h-B Rhenis

LemmeCarrosio PTB

Italy

Studies by Miller et al. (1987, 1991a), Wright and Miller (1992), Oslik et al. (1994) and Zachos et al. (1997, 2001) in the southern and equatorial Atlantic Ocean have proved the existence of a distinct positive oxygen isotope (δ 18O) excursion and a corresponding δ 13C excursion, spanning the O/M boundary interval. This excursion, the Mi-1 glaciation event, has been recorded in several locations worldwide (e.g. western North Atlantic; New Jersey, the US Atlantic margin; northeastern North Atlantic; southern and equatorial Atlantic Ocean; South China Sea; equatorial Pacific; western Ross Sea, Antarctica) (Miller et al., 1985, 1991b; Pekar and Miller, 1996; Zachos et al., 2001; Zhao et al., 2001; Lear et al., 2004; Naish et al., 2008), strongly suggesting that this event can be correlated on a global scale. Similar excursions in the δ18O and δ13C isotope curves, interpreted as reflecting the Mi-1 event, also characterise the GSSP boundary section (Steininger et al., 1997a). According to Zachos et al. (1997, 2001), Paul et al. (2000), and Palike et al. (2006), the increase in δ18O during the Mi-1 glaciation event reflects a long-term, 400 kyr, eccentricity cycle, which led to a period with a colder climate. A climatic deterioration is supported by investigations of deposits around Antarctica, which indicate that a large ice sheet formed and covered large parts of Antarctica during a period of approximately 200 kyr at the O/M transition (e.g. Kennett, 1977; Fairbanks and Matthews, 1978; Wright and Miller, 1992; Flower et al., 1997; Naish et al., 2001, 2008; Barret, 2009). Furthermore, a pronounced eustatic sea-level fall occurred at this time, reflecting the large amounts of water stored as ice. The sea-level fall was in the order of 60–70 m according to Haq et al. (1987) and 50 m according to Miller et al. (2005), Pekar and DeConto (2006) and Naish et al. (2008). An overall δ 18O isotope curve for the Cenozoic in the North Sea Basin was presented by Buchardt (1978). The curve shows an increase in temperatures in the Late Oligocene and Early Miocene, culminating in the Middle Miocene, but does not show any minor excursions such as the Mi-1 excursion. Until now, no detailed δ 18O or δ 13C isotope data for the Upper Oligocene–Lower Miocene succession in the North Sea Basin have been published. 2. Geological setting

Fig. 1. Palaeogeography in the early Aquitanian showing the location of the Italian GSSP section for the Oligocene–Miocene boundary, the Lemme–Carrosio section, the enclosed North Sea Basin and the location of the Frida-1 well. Modified from Rasmussen et al. (2008), Rögl (1998) and Knox et al. (2010). UK=United Kingdom, Dk=Denmark, N= Norway.

The Mi-1 glaciation event of Miller et al. (1987, 1991a) and Wright and Miller (1992) is a stratigraphically useful isotopic excursion identified close to the O/M boundary. Based on the dinocyst stratigraphy and sequence stratigraphy, Dybkjær and Rasmussen (2007) suggested that the Mi-1 event correlates with Sequence Boundary B in Frida-1 (1478 m below KB) and that the O/M boundary is located at or very near this sequence boundary. The dinocyst assemblages in the O/M boundary succession in the Frida-1 well can be compared with the assemblages in the type section in northern Italy, while the stratigraphically significant calcareous microfossil groups in the North Sea Basin, including Frida-1, are not comparable with the Italian Tethyan taxa. The Mi-1 event records a distinct excursion in both oxygen and carbon isotope curves in the stratotype section and in other widespread locations. By combining dinocyst- and stable isotope stratigraphy this event is now for the first time recognised in the North Sea Basin, in the Frida-1 well, enabling the O/M boundary to be more precisely defined in this northern, restricted basin.

2.1. The Italian type-section The O/M GSSP boundary succession is situated in northern Italy, north of Genova, on the bank of the river Lemme and to the east of the village of Carrosio (Fig. 1; Steininger et al., 1997a). The succession was deposited within the Piedmont Tertiary Basin (PTB). The PTB formed during the Late Eocene/Early Oligocene as the result of extensional tectonism behind the arc of the Western Alps and has been incorporated into the Apennine compressional history since the Late Oligocene (Falletti et al., 1994). The Late Eocene to Late Miocene succession forms a large homocline, gently dipping to the northwest. The facies evolution of the PTB was strongly controlled by the synsedimentary tectonic activity and allows the recognition of three large-scale facies belts from west to east: the Langhe, the Visone– Lemme and the Borbera–Staffora facies belts. The GSSP succession is situated within the Visone–Lemme facies belt and comprises turbidites and hemipelagic mudstones (Vervloet, 1966; Galbiati, 1976; Andreoni et al., 1981). The lithology and benthic foraminifera assemblages indicate a middle bathyal environment with 600–1000 m water depth (Steininger et al., 1997a,b). The O/M boundary is located 35 m below the top of the section (Fig. 3), at a distinct polarity reversal corresponding to the base of magnetochron C6Cn.2n (Steininger et al., 1997a,b) — in the following this level will be referred to as “metre 35”. Shackleton et al. (1999; 2000) suggested the age of the boundary to be 22.92 +/− 0.04 Ma. The “Geological Time Scale” of Gradstein et al. (2004) and the “Concise Geological Time Scale” of Ogg et al. (2008) indicate an age of 23.03 Ma

K. Dybkjær et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 11–22

A

4°E

8°E

12°E

57°N

N Frida-1

Jylland

UK

DK D

NL 55°N

B

4°E

8°E

12°E

57°N

N Frida-1 UK

DK Ring

købin

D

g-Fyn

NL

High

55°N

C

4°E

8°E

12°E

57°N

N Frida-1 UK

DK

NL

D

Rin

gkø

bing

-Fy

nH

igh

55°N

Fig. 2. Palaeogeography in the eastern North Sea Basin, a) latest Chattian, b) at the Oligocene/Miocene boundary, c) earliest Aquitanian.

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K. Dybkjær et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 11–22

Foraminifera (Iaccarino et al. 1997)

Dinocysts (Steininger et al. 1997a,b) 0

Calcareous nannofossils (Aubry and Villa 1996)

Isotope curves modified after (Steininger et al. 1997)

δ13 C‰ PDB

δ18 O‰ PDB

S. conerae M. ?picena

Neogene Miocene

5

H. carteri

10

D. apenninicum

15

Deflandrea spp. abundant

P. kugleri G. altiaperturus U. spinicostata

S. disbelemnos

20

G. dehiscens

ent

Deflandrea spp. abundant

30

45

E.burdigalensis Chiropteridium spp. abundant

S. capricornutus S. delphix

O/M boundary

S. capricornutus

D. biffii

50

55

60

Deflandrea spp.

Palaeogene Oligocene

40

S. delphix P. kugleri U. spinicostata

Chiropteridium spp.

Chiroptendium spp.

35

Mi-1 ev

25

S. ciperoensis

-4

1 -3

-2

Fig. 3. Biostratigraphic events characterising/defining the Oligocene–Miocene boundary (located at 35 m) in the Stratotype Section, Lemme–Carrosio, and the δ18O- and δ13C-isotope curves (modified from Steininger et al., 1997a,b). In addition, the last occurrence of Distatodinium biffii at 9 m below the GSSP is included, based on Zevenboom (1996). The distinct increase in the δ18O isotope curve and the excursion in the δ13C-isotope curve have been interpreted to reflect the Mi-1 glaciation event.

based on astronomical tuning. Astronomical dating was also carried out on ODP Leg 154 sites at Ceara Rise in the equatorial Atlantic, these sites were therefore suggested as an auxiliary stratotype section for the O/M boundary and as the unit stratotype for the Aquitanian by Hilgen et al. (2006). 2.2. The North Sea Basin The formation of the North Sea Basin was initiated with active rifting in the Triassic (Jacobsen, 1982; Ziegler, 1990; Vejbæk, 1997). Renewed rifting followed during the Middle–Late Jurassic and Early Cretaceous but then ceased, leaving the area as a failed rift-system (e.g. Koch et al., 1982; Ziegler, 1988, 1990; Vejbæk, 1992, 1997; Ziegler et al., 1995; Møller and Rasmussen, 2003). During the Late Cretaceous and Palaeogene, tectonic inversions, probably partly related to the collision of North Africa with Europe and partly to the initial opening of the North Atlantic, resulted in major structural changes (e.g. Andersen et al., 1982; Ziegler, 1988, 1990). The North Sea Basin became an epicontinental basin flanked by landmasses. To the north-east it was bounded by the Fenno-Scandian Shield, to the south by the RheinishBohemian Massif, and towards the west by landmasses probably covering most or all of the present day England, Scotland, Northern Ireland and the Shetland Isles (Ziegler, 1990; Knox et al., 2010) (Fig. 1). The deepest parts of the basin were located in the Central Graben area. The sedimentation changed from the chalk deposits characterising the Late Cretaceous to siliciclastic deposits in the Palaeogene. The North Sea Basin was probably a (semi-)enclosed basin during the latest Oligocene–earliest Miocene, with only a narrow connection

to the North Atlantic Ocean between the Shetland Isles and Norway (Fig. 1; Rasmussen et al., 2008; Knox et al., 2010). There was therefore no direct connection to the PTB where the succession of the O/M boundary type section was deposited. Brackish water conditions may have prevailed in parts, or all of the North Sea Basin. Elevation of the western part of the Fenno-Scandian Shield during most of the Palaeogene resulted in a gradual southward progradation of the coastline. Sediments were transported to the North Sea Basin from the north (the present day Norway), and by the earliest Miocene large rivers and delta systems occupied the northern and central parts of present day Jylland, to the east of the Frida-1 well (e.g. Rasmussen, 2009; Rasmussen et al., 2010) (Fig. 2). The Frida-1 well is located in the more distal parts of the basin (Figs. 1, 2). The uppermost Oligocene (upper Chattian) succession was deposited at water depths of up to 500 m and comprises gravity flow deposits inter-fingering with deep marine clay. The coastline was probably located north of Jylland and south of the present Norwegian coast (Fig. 2a). In the latest Chattian, a shallowing occurred, probably reflecting the Mi-1 glaciation event, and the coastline prograded southeastwards and was located in the central and northern parts of Jylland (Fig. 2b). This was followed by a transgression in the early Aquitanian, during which the coastline retrograded to the northern parts of Jylland while parts of the Ringkøbing-Fyn High probably formed major islands (Dybkjær and Rasmussen, 2007; Rasmussen et al., 2010) (Fig. 2c). In Frida-1, the depositional setting changed in the latest Chattian towards a more shallow-marine environment and the uppermost Chattian and Aquitanian succession consists of contouritic clays (Dybkjær and Rasmussen, 2007).

K. Dybkjær et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 11–22

3. Biostratigraphy 3.1. The Italian Type-section The mid latitude, open marine setting was probably responsible for the rich dinocyst, microfossil and nannofossil assemblages at Lemme– Carrosio. The preservation of calcareous micro- and nannofossils from the GSSP section is moderate to poor (Steininger et al., 1997a). The bioevents characterising the O/M boundary succession are shown on Fig. 3. 3.1.1. Dinocysts Powell (1986), Brinkhuis et al. (1992), Zevenboom et al. (1994) and Zevenboom (1996) analysed the dinocyst asemblages of the type section. According to Steininger et al. (1997a, 1997b) the dinocyst preservation is good to excellent. The events bracketing the boundary are: the last (highest) abundant occurrence of Chiropteridium spp. at 39 m, the first occurrence (FO) of Ectosphaeropsis burdigalensis at 38 m; the last occurrence (LO) of Chiropteridium spp. at approximately 34 m; an influx of Deflandrea spp. from approximately 25 m to approximately 15 m; the FO of Distatodinium apenninicum at 10 m; the FO of Membranilarnacia ? picena at 3 m and the FO of Stoveracysta conerae at 1 m (Fig. 3). In addition to these events, the LO of Distatodinium biffii, recorded at 44 m according to Zevenboom (1996), should be emphasised, as this event is of great importance for correlation to other areas, including the North Sea Basin. Zevenboom (1996) indicated sporadic occurrences of Chiropteridium spp. throughout the Lemme–Carrosio section, but Steininger et al. (1997a, 1997b) interpreted the occurrences above 39 m as reworked. 3.1.2. Micropalaeontology Planktonic foraminifera were studied by Iaccarino in collaboration with M. Biolzi, A.M. Borsetti, F. Rögl and S. Spezzaferri (in Steininger et al., 1997a). They found the FO of Paragloborotalia kugleri at 33 m, the FO of Globoquadrina dehiscens at 23 m, the FO of Globigerinoides altiaperturus at 13 m and the LO of Paragloborotalia kugleri at 10 m. Benthic foraminifera were studied by M. Biolzi and F. Rögl (in Steininger et al., 1997a), who found the FO of Uvigerina spinicostata at 34 m and its LO at 14 m (Fig. 3). 3.1.3. Nannopalaeontology Nannofossils from the type-section were studied by Aubry and Villa (1996) (Fig. 3) who found the FO of Sphenolithus ciperoensis at 56 m, the FO of Sphenolithus capricornutus at 40 m, the FO of Sphenolithus delphix at metre 35, the LO of Sphenolithus capricornutus at 34 m and the LO of Sphenolithus delphix at 31 m. The FO of Sphenolithus disbelemnos (useful at O/M boundary ODP sites at Ceara Rise) is at 13 m and the FO Helicosphaera carteri is at 6 m. Aubry and Villa (1996) also state that reworking prevents the use of LO datums which elsewhere have proven to be useful, such as the LO of Reticulofenestra bisecta. 3.2. North Sea Basin Identification of the O/M boundary within the North Sea Basin is problematic. The stratigraphy of this time interval was originally based partly upon calcareous microfossils, in spite of their sparse occurrence in the central Basin (e.g. King, 1983, 1989; Gradstein et al., 1992; Laursen and Kristoffersen, 1999). Gallagher (1990) noted the difficulty in pinpointing the O/M boundary due to mixed nannofossil assemblages and monotonous sediments. Hydrocarbon exploration in the North Sea Basin has resulted in many “oil-company” Palaeogene biostratigraphic studies, but these remain unpublished due to their confidential nature. Lack of commercial interest in deposits of Miocene age or younger has meant that these have not been extensively studied regarding nannofossils. The restricted, probably partly brackish water, basin and its high latitudinal position are factors probably responsible

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for the lack of the calcareous microfossil taxa that define the boundary at the Italian type section. In contrast, studies by e.g. Gradstein et al. (1992), Köthe (2003, 2005a,b), Munsterman and Brinkhuis (2004), Dybkjær (2004a), Schiøler (2005), Dybkjær and Rasmussen (2007) and Dybkjær and Piasecki (2010) have shown that most of the dinocyst species characterising the boundary at the type section also occur in the North Sea Basin. 3.2.1. Dinocysts The use of dinocyst events for identifying the O/M boundary in the North Sea Basin has been rather consistent. Costa and Manum (1988) used the LO of Chiropteridium partispinatum (jun. syn. of C. galea) and C. lobospinosum and the FO of Tuberculodinium vancampoae to define the boundary between their zones D15 and D16, indicated as corresponding to the O/M boundary. In the study by Manum et al. (1989), the O/M boundary was suggested to be located between the highest appearance of Chiropteridium partispinatum and the lowest appearance of Ascostomocystis granosa (now Cyclopsiella granosa). At that time, the LO of the genus Chiropteridium was generally considered a reliable marker for the top of the Oligocene, although the authors were aware that it had been reported from the lowermost Miocene. Powell (1992) indicated that the LO of Chiropteridium spp. was located below the O/M boundary. He further indicated that the FO of Tuberculodinium vancampoae occurred just below the boundary. Gradstein et al. (1992) concluded that the LO of C. lobospinosum (and C. partispinum) is the most suitable event for indicating the upper Chattian boundary and that the range of Distatodinium biffii seems to be a Chattian marker. In later studies, the LO of D. biffii (below) and the LO of Chiropteridium spp. (above) were used to bracket the boundary in the North Sea Basin (e.g. Köthe, 2003, 2005a; Munsterman and Brinkhuis, 2004; Schiøler, 2005; Dybkjær and Rasmussen, 2007; Dybkjær and Piasecki, 2010). Ectosphaeropsis burdigalensis has only been recorded sporadically from the Aquitanian (e.g. Powell, 1992; Munsterman and Brinkhuis, 2004; Dybkjær and Rasmussen, 2007). Furthermore, most studies from the North Sea Basin have been carried out on ditch cuttings samples, which are problematic with respect to caving which often prevents a reliable use of first occurrences, see e.g. the records of E. burdigalensis by Schiøler (2005). The FO of this species has therefore generally not been used as a reliable stratigraphic event. In the Frida-1 well, however, the FO of E. burdigalensis is used with confidence, as a casing point above the O/M boundary interval effectively limited caving (Dybkjær and Rasmussen, 2007, see below). 3.2.2. Micropalaeontology King (1983, 1989) presented important, and still commonly applied, studies of the Cenozoic micropalaeontology of the North Sea Basin, incorporating data from foraminifera, radiolaria, diatoms and the probable chrysophyte Bolboforma. He erected an NSB zonation (North Sea benthic foraminifera), an NSP zonation (North Sea planktonic foraminifera, diatoms, radiolarian and Bolboforma) and an NSA zonation (North Sea non-calcareous agglutinating foraminifera). These zonations can be supplemented by biostratigraphic data based on Bolboforma from the North Sea and Atlantic (Spiegler and von Daniels, 1991). Other useful, local micropalaeontological studies include Doppert (1980): Neogene of the Netherlands, Doppert and Neele (1983): Palaeogene of the Netherlands, and Laursen and Kristoffersen (1999): onshore Miocene of Denmark. The mid and northern part of the North Sea Basin has been documented by e.g. Gradstein et al. (1992, 1994), Gradstein and Bäckström (1996), Eidvin and Rundberg (2007) and more recently by Anthonissen (2009). The zonations of King (1983, 1989) have recently been reviewed and revised, and integrated into a single unified zonal scheme. This is based on the interpretation of many biostratigraphic events as reflecting eustatic sea-level fluctuations or regional tectonic events

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K. Dybkjær et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 11–22

which influenced the whole basin simultaneously. For instance, a major sea-level rise can permit planktonic organisms to enter the basin from oceanic areas, and at the same time cause replacement of poorly oxygenated bottom waters by open marine circulation, causing an influx of benthic organisms. So benthic and planktonic faunas will change simultaneously. The new zonal scheme incorporates almost all the previous events, adds new ones, and updates the taxonomy, intercalibration and chronostratigraphic calibration of all events. It comprises 48 NS (North Sea) zones (NS1-NS48). These are interval zones, with boundaries based on the FO, LO or influx of specific taxa. Full details will be published elsewhere. The relevant part of the new zonal scheme, compared with the previous schemes, is summarised in Fig. 4. Accurate chronostratigraphic calibration of Oligocene and Miocene events based on benthic organisms relies mainly on indirect calibration via planktonic taxa, primarily dinoflagellate cysts, planktonic foraminiferids and Bolboforma.

3.2.3. Nannopalaeontology The global nannofossil zonation of Martini (1971), amended by Young (1998), can be applied to the North Sea (Fig. 4). Zone NN1 spans the O/M boundary. The base of NN1 is defined by the LO Sphenolithus ciperoensis, but Varol (1998) suggests that the LO Reticulofenestra bisecta is used to approximate the top of NP25 (and by inference the base of NN1) in the North Sea. Young (1998), however, places the LO of R. bisecta below the O/M boundary, within NN1. The base of NN2 is defined by the FO of Discoaster druggii, but this species is not reliable in the North Sea (Liam Gallagher, pers. comm., 2011). Gallagher (1990) presented a Tertiary North Sea nannofossil study and noted the absence of the established global marker Sphenolithus species from the Late Oligocene, and other traditional marker species from the Early Miocene; and subsequently the difficulty in pinpointing the O/M boundary. He also noted the LO of Zygrhablithus bijugatus and R. bisecta as important uppermost Oligocene events, and assemblages Nannofossil (King 1983, 1989) zones (Martini 1971) NSA NSP NSB

15

12

Lang.

Middle

Age (Ma)

Burdigalian

Miocene

Early

NS36 c

NN4

20

11

Composite zonation (King, in prep.)

Cibicidoides dutemplei peelensis 11

11

10

NS35 b

Aulacodiscus allorgei Plectofrondicularia seminuda

NN3

NS34

Aquitanian

10

10

9 Aulacodiscus aemulans

c

8c

Chattian

Aulacodiscus insignis quadrata Elphidium subnodosum

NS32

9

8b

NP25

a b

9c 8a

NS31a

Asterigerinoides guerichi influx

a Rotaliatina bulimoides

Rupelian

Oligocene

Late

b

Early

4. Materials and methods The samples forming the basis for the present study are from the Frida-1 well (Fig. 1), located in the Danish part of the North Sea Basin. Frida-1 is located within the deeper parts of the basin and comprises an expanded Upper Oligocene–Lower Miocene boundary succession, more than 800 m thick (Dybkjær and Rasmussen, 2007; Fig. 7). The studied succession comprises the interval from 980 to 1740 m. Borehole casing placed around 970 m prevented caving of younger sediments into the studied succession, while the samples above the casing point were severely contaminated by caved material.

4.1. Palynology The dinocyst data presented here were published by Dybkjær and Rasmussen (2007), where a description of the preparatory methods was given. A total of 37 ditch cuttings samples and 4 core samples were included in the study.

4.2. Carbon Isotope stratigraphy Bulk kerogen material from the Frida-1 well was analysed for δ 13C-isotopes by “Applied Petroleum Technolog AS”, Kjeller, Norway, and the following description of the method is taken from their “Experimental Procedures” report. The kerogen comprised the material left after palynological preparation for the dinocyst analysis of Dybkjær and Rasmussen (2007). Approximately 3 mg of each sample was weighed and transferred to tin capsules. Combustion of the samples was carried out in the presence of O2 and Cr2O3 at 1700 °C in a Carlo Erba NCS 2500 element analyser. Reduction of NOx to N2 occurred in a Cu oven at 650 °C. H2O was removed using a chemical trap of KMnO4 before separation of N2, CO2 and SO2 on a 2 m Poraplot Q GC column. CO2 is flushed on-line in a He flow to a Micromass Optima, Isotope Ratio Mass Spectrometer for determination of δ 13C. A standard (USGS-24) is analysed for each 10th sample. The δ 13C value obtained for this standard is − 16.01 ± 0.06 ‰ VPDB (one standard deviation) in this project. The given value from IAEA is − 15.99 ± 0.11 ‰ VPDB (one standard deviation).

NS33 NN1

30

Uvigerina tenuipustulata Uvigerina hemmooriensis

a

NN2

25

Globoconella praescitula Uvigerina tenuipustulata Asterigerinoides staeschei influx

comprising Helicosphaera carteri, H. ampliaperta and H. mediterranea associated with Early Miocene Zone NN2. Varol (1998) provided a refined zonation for the North Sea Oligocene, and reported the LO of Z. bijugatus as preceeding the LO of R. bisecta in the uppermost Oligocene, and poorly preserved, low diversity assemblages also including Discoaster deflandrei, Reticulofenestra spp. and Cyclicargolithus floridanus.

NP24 (pars)

8 7b

NS30

4.3. Micropalaeontology The data presented here are taken partly from the RPS-Paleo report (Mears et al., 1997) and partly from a new study (herein). Unfortunately, very small amounts of sample material were available for the new micropalaeontological study, resulting in limited additional information. Twenty nine ditch cuttings samples (washed and dried) were sieved through 1 mm, 250 μm and 100 μm sieves; the material b100 μm was scanned.

4.4. Nannofossils Fig. 4. Composite micropalaeontological zonation for the Early Oligocene to Middle Miocene of the North Sea Basin (mainly outer neritic and bathyal environments) and on the left column, correlation with mid-latitude nannofossil zonation. NSA: North Sea Agglutinated, NSP: North Sea Planktonic, NSB: North Sea Benthic, NS: North Sea. The definitions and details of this new composite zonation will be published elsewhere.

Fifteen nannofossil smear slides were prepared from 12 levels covering the O/M boundary interval, using the simple smear slide technique described in Bown and Young (1998).

K. Dybkjær et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 11–22

17

5.1. Location of the O/M boundary in Frida-1, based on palynology and the δ 13C-isotope curve The dinocyst stratigraphy from Frida-1, used herein, is based on the study by Dybkjær and Rasmussen (2007). The LO of Distatodinium biffii at 1630 m and of Chiropteridium spp. at 1370 m, respectively, are used to identify the broader O/M boundary interval (Fig. 5). The new δ13C-isotope data from Frida-1 are presented in Figs. 5 and 6. In Fig. 5 the Frida-1 δ13C curve is shown together with that of the Lemme– Carrosio type section. The Frida-1 curve clearly reflects the Mi-1 isotope event (Figs. 5, 6) and is comparable with the curve from the typesection; the overall δ13C-isotope curve pattern in Frida-1 shows an increase of approximately 1.5 ‰ δ13C in the interval from immediately

Depth (m)

C Sequence

below the LO of D. biffii to immediately above this event (Fig. 5, point a to b). This is followed by an overall high but variable level of δ13C up to immediately above the LO of Chiropteridium spp. (g), from which the curve decreases with approximately 1.5‰ again (h). Within the overall high δ13C interval two decreasing-increasing δ13C cycles are present (b to d and d to f). The negative excursion at “e” shows a characteristic “low valley”-pattern. Detailed correlation of the Lemme–Carrosio and Frida-1 δ13C curves show that the O/M boundary is located at 1440 m below KB in Frida-1. In order to support the correlation of the Lemme–Carrosio and Frida-1 δ 13C-isotope curves, curves from two additional locations, from ODP Leg 154 Site 929 at Ceara Rise and DSDP Site 522 (Shackleton et al., 2000; Zachos et al., 2001), have been added to the correlation panel (Fig. 6). The four curves clearly show similar trends, e.g. the “low valley”-pattern indicated in green. The location

5. Results

1050

Frida-1 T. pelagica

1100 D. phosphoritica

Lemme-Carrosio section Depth (m) 0

1150

Occurrence of T. pelagica 1200

S. conerae M. ?picena

5

Miocene

20

Aquitanian

15

D. apenninicum

B

1250 10

h

h

1300

Deflandrea spp. influx

E.burdigalensis 1350

g

g Chiropteridium spp.

25 1400

30 Chiropteridium spp.

35

O/M

f

E. burdigalensis

e

D.biffii

c

f

1450

40

e

1500

-2

-1

δ13 C‰ PDB

0

1650

0

3 %

0 5 10 %

10 30 %

d

c

Homotryblium tenuispinosum

-3

Homotryblium plectilum

D. biffii

a

Deflandrea spp.

50 0 30 % %

1600

Chiropteridium spp.

0

A

1550 Deflandrea spp.

60

d b

Chiroptendium spp.

55

Chattian

50

Oligocene

45

b a

10 %

-26 -25.5

-25

-24.5

δ13 C‰ PDB

Fig. 5. Comparison of the dinocyst- and δ13C-isotope data from the Lemme–Carrosio section and Frida-1 shows that the location of the Oligocene–Miocene boundary can be placed at 1440 m in Frida-1.

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K. Dybkjær et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 11–22

Lemme-Carrosio section

DSDP 522 ODP 929

Chattian

Oligocene

Miocene Aquitanian

Frida-1

-4

-3

-2

-1

δ13 C‰

0

-26

-25.5

-25

24.5

δ13 C‰ 1

1.5

2

δ13 C‰

2.5

3 0

0.5

1.5

2.0

δ C‰ 13

Fig. 6. Correlation of the Lemme–Carrosio, Frida-1, DSDP 522 (modified from Shackleton et al., 2000) and ODP 929 (modified from Zachos et al., 2001) δ13C-isotope curves. The correlations between the Lemme–Carrosio, DSDP 522 and ODP 929 curves are based on Shackleton et al. (2000).

of the O/M boundary in the ODP and DSDP curves, based on a combination of palaeomagnetostratigraphy, nannofossil stratigraphy (the FO of Sphenolithus disbelemnos and the entire range of S. delphix) and correlation of carbon isotope curves (Shackleton et al., 2000; Zachos et al., 2001; Hilgen, et al., 2006), further strengthens the suggested location of the boundary in Frida-1. Abundance variations of the most abundant dinocyst taxa from the two localities are included for comparison; Chiropteridium spp. and Deflandrea spp. from the Lemme–Carrosio section and Chiropteridium spp., Deflandrea spp., and two species of Homotryblium, H. plectilum and H. tenuispinosum from Frida-1. According to Dybkjær (2004a, 2004b) the former species may be more tolerant to low salinities than the latter. 5.2. Biostratigraphic events in Frida-1 characterising the boundary interval Fig. 7 presents a summary of all the stratigraphically useful biostratigraphic (dinocyst, microfossil and calcareous nannofossil) events and abundance variations encompassing the O/M boundary interval in Frida-1, together with the δ13C-isotope curve. The dinocyst zonation of Dybkjær and Piasecki (2010), the global nannofossil zonation of Martini (1971) and the new composite micropalaeontological zonation (see Section 3.2.2.) are shown. The diatoms cited in this zonation are illustrated by King (1983) and Bidgood et al. (1999). In this area, as for much of the central North Sea Basin, the microfauna comprises mainly non-calcareous agglutinating foraminiferids; calcareous foraminiferids and nannofossils are rare and usually poorly preserved. This is probably due to restricted circulation inducing dysoxia at the sea floor, and the resulting acidic environment attacking planktonic foraminiferids and nannofossils ending up here. The large diatoms used in the microfossil zonation are however well-represented, preserved as pyrite moulds. 5.2.1. Dinocysts In addition to the dinocyst events used for identifying the boundary interval — the LOs of Distatodinium biffii at 1630 m and Chiropteridium spp. at 1370 m — other useful events are the FO of Ectosphaeropsis burdigalensis at 1330 m (110 m above the boundary), an influx of Deflandrea phosphoritica in an interval below the boundary

(1532–1490 m) and an influx of the genus Homotryblium in a broader interval (1650–1330 m) across the boundary. 5.2.2. Microfossils The samples analysed in the present study yielded moderately common agglutinated foraminifera and diatoms. The diatoms and some foraminifera are pyritised. Calcareous benthic and planktonic foraminifera are rare but age diagnostic. Other components comprise rare radiolaria, Bolboforma, fish teeth and sponge spicules. The data interpreted here are from the present study and from Mears et al. (1997). Taxa cited here are foraminifera unless stated otherwise. The LO of Rotaliatina bulimoides (top of Zone NS30) is probably represented by a single record at 1710 m. A higher record of a single specimen at 1650 m is considered suspect, as this is above the Asterigerinoides guerichi influx. The A. guerichi influx (Subzone NS31b) is well-developed mainly in neritic environments. In the Central Graben A. guerichi is generally represented by rare and small specimens (personal observations). Here it is represented by consistent records of A. guerichi (up to 5 specimens/sample) between 1655 m- c. 1690 m. The base of this influx is near the base of the Chattian. Single specimens of Elphidium subnodosum at 1625 m and 1630 m probably indicate Subzone NS32b, characterised in mid-neritic environments by the influx of E. subnodosum. This is a ‘mid-Chattian’ event, but is not accurately calibrated chronostratigraphically.The LO of the diatom Aulacodiscus insignis quadrata [Diatom sp. 3 of King, 1989] is at 1610 m. This event defines the top of Zone NS32. However, in the overlying interval (1410 m–1610 m) no zonally diagnostic diatoms have been recorded. This probably reflects the lowstand associated with the emplacement of the Freja Member. It is likely that the LO of A. i. quadrata here is synchronous with its LO elsewhere. The FO of the diatom Aulacodiscus allorgei [Diatom sp. 4 of King, 1989] is found in a cuttings sample at 1500 m. This species is restricted to Zones NS33 and NS34, but this may be a caved specimen and is not shown in Fig. 7. The diatom Aulacodiscus aemulans [Diatom sp. 5 of King, 1989] has its FO at 1410 m. This gives more confidence in the identification of Zone NS33. The most appropriate interpretation is to regard the interval from 1410–1610 m as not allocated to a

1050

Microfossil and calcareous nannofossil events

δ13 C( ‰ PDB) *: Nannofossil event

-25

Occurrence

19

-26

60

Microfossil zonation

Sonic 100 190

First occurrence

Homotryblium tenuispinosum

Gr 0

Last occurrence

Homotryblium plectilum

Core

Dinocyst events

Deflandrea phosphoritica

Casing

Dinocyst zonation (Dybkjær and Piasecki, 2010)

Depth 1000

Sample

S. hamulatum

Sequence stratigraphy C

Lithostratigraphy Member

Formation

Age Burdigalian

Period/epoch

Chronostratigraphy

K. Dybkjær et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 11–22

NS35b

Cordosphaeridium cantharellus

Uvigerina tenuipustulata

Thalassiphora rota NS35a Aulacodiscus allorgei common agglutinating foraminifera, Plectofrondicularia seminuda

Thalassiphora pelagica T. p. Caligodinium amiculum

1100

Rolfina arnei Spirosigmoilinella compressa

Deflandrea phosphoritica NS34 C. amiculum

1200

Aquitanian

Membranophoridium aspinatum 1250

B

Aulacodiscus aemulans

1350

Homotryblium spp.

1300

Lark Formation

Miocene

1150

Ectosphaeropsis burdigalensis Homotryblium spp. influx Ectosphaeropsis burdigalensis

NS33

Chiropteridium spp. 1400

C. galea

1450

Deflandrea phosphoritica, influx

1600

D. phosphoritica

A

Freja Member

Chattian

Oligocene

1500

1550

Aulacodiscus aemulans

(No diagnostic taxa)

Deflandrea phosphoritica, influx

?

Pararotalia canui Karreriella seigliei

NS32c 1650

Distatodinium biffii Homotryblium spp. influx

NS32a-b

Aulacodiscus insignis quadrata Elphidium High abundance subnodosum Reticulofenestra bisecta bisectus* Asterigerinoides guerichi influx

NS31b 1700

Asterigerinoides guerichi influx

Wetzeliella gochtii/ W. symmetrica group A. semicirculata R. draco S. cooksoniae

NS31a Rotaliatina bulimoides NS30 10% 10%

10%

Fig. 7. Chronostratigraphy, lithostratigraphy, sequence stratigraphy, geophysical logs, biostratigraphic (palynological, micro- and nannopaleontological) events and abundance variations, and δ13C-isotope data from the Frida-1 well, offshore Denmark, modified from Dybkjær and Rasmussen (2007). T.p.: T. pelagica.

zone. The general absence of Aulacodiscus in this interval is probably due to environmental factors related to the climatic and/or sea-level fluctuations at the O/M boundary. The LO of A. aemulans defines the top of Zone NS33. One specimen is recorded at 1170 m; the next specimens are at 1250 m, and it occurs consistently downsection from this level. As broken specimens can be difficult to differentiate from A. allorgei, the highest record is here disregarded.

The LO of Spirosigmoilinella compressa [S. sp. A of King, 1989] is at 1130 m. This event was used as a datum by King (1983, 1989), corresponding to the top of Zone NS34, but can be found at a lower level in some areas. It is not used as a primary datum in the revised zonation. The LO of A. allorgei is one of the events defining the top of Zone NS34. Here it is at 1040 m. The Aquitanian/Burdigalian boundary is interpreted to lie within Zone NS34. This is consistent with the dinocyst data in this well. The LO of Plectofrondicularia seminuda is

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K. Dybkjær et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 11–22

the other event defining the top of Zone NS34, regionally coincident with the LO of A. allorgei. It is however a small species, liable to be overlooked, and occurs in relatively low numbers. Here a single specimen is recorded at 1050 m. The FO of Uvigerina tenuipustulata is at 1000 m. This event is at the base of Subzone NS35b. It is normally difficult to identify due to downhole caving, but is considered accurate in view of the casing set at c. 970 m. Later events (e.g. the Asterigerinoides staeschei influx, top of Zone NS35, in the Langhian), are above the casing point and are not included in this study. Other events shown in Fig. 7 (the presence of Karreriella seigliei, LO of Pararotalia canui, LO of Rolfina arnei and LO of common agglutinating foraminiferids) may also prove to be of biostratigraphic significance in this interval. Chronostratigraphic calibration of the zones introduced by King (1983, 1989) was based on relatively limited data, and was of variable accuracy. The current revision is based on a much more extensive database. Accurate calibration depends on high-resolution biostratigraphic analysis of sections in which other fossil groups are represented; in the Oligocene and Early Miocene it relies mainly on dinocysts. The present study has provided important new data with respect to constraining the O/M boundary. King (1989) placed the Oligocene-Miocene boundary c. 1.5 My below the LO of Aulacodiscus insignis quadrata [Diatom sp. 3] (which is placed at the top of Zone NS32 in the new zonation). The present study places the O/M boundary above the top of Zone NS32 (Figs. 4, 7). This study confirms the importance of the diatoms in providing a reasonably reliable proxy for the Oligocene/Miocene boundary in the central North Sea Basin, in an interval in which calcareous fossils are rare or absent, though not well calibrated. 5.2.3. Nannofossils Calcareous nannofossils were absent from most samples, or rare (and then badly preserved or broken). The long-ranging calcareous dinocyst Thoracosphaera spp. was occasionally present, but also rare. One sample at 1630 m contains an almost monospecific assemblage of Reticulofenestra bisecta and small Reticulofenestra spp. R. bisecta has its LO in the uppermost Oligocene.

primary production related to increased nutrient availability, e.g. upwelling areas and river mouths. Chiropteridium spp. (also extinct) are recorded in marginal marine, inner neritic settings (Brinkhuis, 1994). As the Lemme–Carrosio section and the Frida-1 well represent two different basins, the Piedmont Tertiary Basin and the North Sea Basin, respectively, it cannot be expected that the abundancevariations of these taxa within two far removed basins coincide. c) The high abundance of Homotryblium spp. encompassing the O/M boundary succession in Frida-1 is known from throughout the eastern North Sea Basin (e.g. Dybkjær, 2004a,b; Munsterman and Brinkhuis, 2004; Dybkjær and Piasecki, 2010), and was suggested to reflect low salinity conditions by Dybkjær (2004b). The abundance variations of the two species H. plectilum and H. tenuispinosum in Frida-1 possibly reflect changes in salinity, with the former species indicating lower salinities than the latter (Dybkjær, 2004a,b). A similar high abundance of Homotryblium was not recorded in the Lemme– Carrosio section, probably due to the different oceanographic setting. d) We have not found Distatodinium apenninicum, Membranilarnacia picena or Stoverocysta conerae — species present at Lemme– Carrosio — in the Danish area. As in previous studies from the North Sea Basin, none of the micro- or nannopaleontological marker taxa characterising the O/M boundary in the Italian GSSP section were found in Frida-1. However, the micropalaeontological data from Frida-1 indicate that the new composite NS zonation can be applied successfully in this area. The close calibration between micropalaeontology and dinocyst stratigraphy in this section permits more accurate calibration of the composite NS zonation to the standard stratigraphic scale than has been possible previously. It can therefore be applied elsewhere in the central North Sea Basin. This enhances the value of the micropalaeontological zonation, which can be used increasingly accurately as a chronostratigraphic proxy in sections for which dinocyst data is not available. Due to sea floor dysoxia induced by restricted circulation, nannofossil (and calcareous foraminifera) recovery is particularly poor in this part of the central North Sea, however, the occurrence at 1630 m of an almost monospecific assemblage of the nannofossil R. bisecta is a useful Oligocene species and supports the microfossil and dinocyst data.

6. Discussion The O/M boundary in Frida-1 is located in the lower part of the Chiropteridium galea dinocyst Zone (Dybkjær and Piasecki, 2010). The boundary between the D. phosphoritica Zone and the Chiropteridium galea Zone is thus located within the uppermost Oligocene rather than at the O/M boundary as suggested by Dybkjær and Piasecki (2010). However, this zonal boundary is very close to the O/M boundary and at present the most precise palynological indication of the boundary. The detailed correlation between the Italian GSSP section and the succession in Frida-1, provided by the combined dinocyst and δ13C-isotope stratigraphy, reveal some differences in abundance variations and diachronity of specific dinocyst taxa; a) The FO of E. burdigalensis is above the O/M boundary in Frida-1, while it is found immediately below the boundary in the Lemme– Carrosio section. Strong diachronism of this event is also known from the Southern Hemisphere mid-latitudes (FO 26.4 Ma) to the equatorial regions (FO 23.7 Ma), Williams et al. (2004). The later appearance of E. burdigalensis in the North Sea area could be due to environmental factors, such as lower sea surface temperatures or low salinity conditions. b) The abundance variations of Deflandrea spp. and Chiropteridium spp. probably reflect variations in the depositional facies. According to Brinkhuis (1994) the extinct genus Deflandrea may represent heterotrophic peridinioids, which are characteristic of areas with high

7. Conclusions A combined dinocyst and δ 13C-isotope study has for the first time located the Oligocene–Miocene boundary — and thus the base of the Neogene — within the North Sea Basin. Data from the Frida-1 well are correlated with data from the GSSP section at Lemme–Carrosio in northern Italy. The LOs of Distatodinium biffii and Chiropteridium spp. were used to identify the broader O/M boundary interval. New δ 13C-isotope data from Frida-1 clearly reflects the Mi-1 isotope event and is comparable with the curve from the type-section. The correlation showed that the boundary is located at 1440 m in the Frida-1 well. As in previous studies in the North Sea Basin, neither the nannonor micropaleontological marker taxa characterising the O/M boundary in the Italian GSSP section were found. Instead, a series of dinocyst-, nanno- and micropalaeontological events encompassing the boundary has been identified, and can be used in future studies, to locate and correlate the boundary interval within the North Sea Basin in higher detail than hitherto. The study presented here further shows the importance of integrating palynology, stable isotope-data and micro- and nannopalaeontology in order to strengthen the stratigraphic framework and establish a high stratigraphic resolution for Oligocene–Miocene successions in the North Sea Basin.

K. Dybkjær et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 11–22

Acknowledgements Several colleagues from GEUS are gratefully thanked for their assistance, Yvonne Desezar for laboratory assistance, Stefan Sølberg and Eva Melskens for producing the figures, and Erik Skovbjerg Rasmussen for useful discussion on an early version of the manuscript. Xx and xx are particularly thanked for reviewing and improving the quality of the manuscript.

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