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A mid-Oligocene cooling (Oi-2b) reflected in the dinoflagellate recordand in depositional sequence architecture. An integrated studyfrom the eastern North Sea Basin

K.K. �Sliwi�nska*, O.R. Clausen, C. Heilmann-ClausenDepartment of Earth Sciences, Aarhus University, Høegh-Guldbergs Gade 2, DK-8000 Aarhus C, Denmark

a r t i c l e i n f o

Article history:Received 7 October 2009Accepted 15 March 2010Available online 20 March 2010

Keywords:3D seismicsDinoflagellatesOi-2b cooling eventOligoceneRupelian/Chattian boundarySequence stratigraphy

a b s t r a c t

Integrated micropaleontological (dinoflagellate and foraminifera) and 3D seismic studies of Oligocenesurfaces were carried out in the eastern North Sea in order to investigate the influence of the climate onthe evolution of depositional geometries and surface morphologies.

Age-indicative dinoflagellates allowed a correlation of the succession with the global time scale andthereby with published oxygen isotope curves. Temperature-indicative dinoflagellate taxa furthermoreallowed to infer paleoclimatic changes during the mid-Oligocene.

One of the surfaces studied on 3D seismic datasets shows features such as low-angle landwarddirected onlaps, incised valleys and pockmarks, indicating a prominent, relative sea-level fall. The cold-water dinoflagellate Svalbardellawas recorded immediately above this surface. The coincidence of a cold-water indicator with this distinctive sequence boundary shows that deposition of this sequence wascontrolled by climatically induced sea-level changes, and that 3D seismic analysis combined with high-resolution dinoflagellate analysis substantially improves the understanding of the depositional historyand processes within the North Sea Basin. The Svalbardella event represents the Oi-2b glaciation andcoincides with the onshore NW European Rupelian/Chattian boundary.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

It is generally assumed that sequence development is closelyrelated to eustatic change and hence may reflect growth andwaning of major polar ice caps (Catuneanu et al., 2009). During theOligocene climates were highly variable, with a number of coolingintervals (Miller et al., 1991) and one major warming event (Zachoset al., 2001). The second largest d18O isotope maximum in theOligocene is the Oi-2b cooling event (Miller et al., 1998; Wade andPalike, 2004) known also as the ‘Oligocene Glacial Maximum’ (VanSimaeys et al., 2005a). This, and other major Cenozoic glaciationsshould, therefore, be expected to be reflected in the development ofsequences.

The Eocene-Oligocene high latitude dinoflagellate Svalbardella isknown to be a reliable indicator of cold waters (Head and Norris,1989; Brinkhuis et al., 2003; Van Simaeys et al., 2005a). A mid-Oligocenewidespread migration of Svalbardellawas recogniszed byVan Simaeys et al. (2005a). These authors showed that the event

was globally synchronous on both hemispheres, took place duringChron C9n and coincided with the early Chattian Oi-2b glaciationevent of Miller et al. (1991, 1998).

In this paper we wish to investigate the relationship betweensequence development and climate change. It will be done byexamining the Oi-2b Svalbardella event and a prominent seismicsurface reflecting sequence development. Furthermore, we wish toelucidate the relationship between the Oi-2b cooling event and thetraditional Rupelian/Chattian boundary in NW Europe.

For these two purposes we have chosen the eastern North Sea asstudy area (Fig. 1), and here we have studied the Danish North Seawell Nini-1 and a 3D seismic study (NODAB) of the surroundingarea (Fig. 1).

During the Cenozoic the North Sea area constituted an epicon-tinental sea surrounded by landmasses delivering sediments to thebasin (Ziegler, 1990; Huuse et al., 2001). At the Eocene-Oligoceneboundary a dramatic change in source and type of sediments tookplace in the eastern North Sea (Kristoffersen and Bang,1982; Huuseand Clausen, 2001). At this time the sedimentation of mainlyhemipelagic clays of the Eocene was succeeded by progradingmuddy and locally sandy sediments from Fennoscandia (Michelsenet al., 1998; Schiøler et al., 2007). The Oligocene clinoforms exceed

* Corresponding author. Tel.: þ45 8942 9542.E-mail address: kasia.sliwinska@geo.au.dk (K.K. �Sliwi�nska).

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1 km in thickness in the Norwegian-Danish Basin (Schiøler et al.,2007) and are subdivided into a number of sequences showingprogradation from the east e northeast (Michelsen and Danielsen,1996; Danielsen et al., 1997). The structure and development ofthese sequences have been the subject of many sequence strati-graphic studies (Michelsen and Danielsen, 1996; Danielsen et al.,1997; Clausen et al., 1999, 2000) but attempts to correlate thesequence development to glacioeustatic changes (Miller et al.,1991) have previously been hampered by the poor datings of thesequence boundaries (Clausen et al., 1999).

2. Study area

The study area is located in the western part of the Norwegian-Danish Basin close to the Central Graben area within the Danishpart of the North Sea (Fig. 1). Because of the hydrocarbon producingfields in the area (early Paleogene reservoir Hamberg et al., 2005)the area has been covered by a number of 3D seismic surveys, andnumerous wells have been drilled in the area. The 3D survey,NODAB (Table 1), used in this study is located above a mid-Oligo-cene depocenter characterized by clinoforms prograding from thenortheast (Michelsen et al., 1998). The Nini-1 well (Table 2) islocated within the NODAB survey area (Fig. 1). The well penetratesapproximately 1000 m of Oligocene sediments and cuttings fromthis interval were used for the dating of stratigraphic surfacesobserved on seismic as well as the biostratigraphic analysis pre-sented here.

3. Palynological preparation and analysis

The sediment material from Nini-1 (ditch cuttings samples,washed and dried) was provided by the Geological Survey ofDenmark and Greenland (GEUS) and processed in the palynologicallaboratory of the Department of Earth Sciences, Aarhus University.

Samples were processed using a standard palynological prepa-ration technique including treatment with HCl, HF and HNO3(details of methods are described in Wood et al., 1996). Sampleswere sieved on 20 mm sieves. Slides for light-microscopy weremade in gelatine-jelly. In each sample at least 200 specimens werecounted, when possible. Additionally, at least one more slide wasscanned for rare species. Preservation of the dinoflagellates ismoderate to very good.

4. Results

4.1. Dinoflagellate and foraminifera record

The age control was based on a study of dinoflagellates in 44ditch cuttings samples from the interval 640e1540 m in the Nini-1well. The dating relied on the NSO (North Sea Oligocene) dinofla-gellate zonation of Van Simaeys et al. (2005b) whichwas developedin the southern North Sea Basin. The zonation has a high resolutionand has proved applicable also in the present study area.

Fig. 1. Map of the study area. A magnification of the area in the red rectangle is shown on Fig. 4a.

Table 1Key data of the NODAB survey.

Survey name NODAB-97Area covered 323,2 km2

Recording Date 31-07-1997Acquired by Compagnie Generale de GeophysiqueInline distance 12.5 mCrossline distance 12.5 m

Table 2Key data for the Nini-1 well.

Welll name NINI-1Longitude 05�19015,87" ELatitude 56�38031,09" NWell block no. 01-10-5605TD drill 1813.1 m bMSLThe well is VerticalCompleted 24-08-2000Operator DANOP

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The most important dinoflagellate events for the NSO zonationin the studied interval (as shown on Fig. 2) are: LCO (last commonoccurrence) of Enneadocysta pectiniformis (marks the top of NSO-4a, Pl.1, Fig. 5), FO (first occurrence) of Distatodinium biffii (marksthe base of NSO-5 zone, Pl.1, Fig. 3). The top of the NSO-5 zone ismarked by the LO (last occurrence) of Rhombodinium draco (Pl.1,

Fig. 1). The top of the NSO-6 zone coincides with the LO of Areoli-gera semicirculata (Pl.1, Fig. 6) andWetzeliella symmetrica. The LO ofWetzeliella gochtii is usually recorded in the lowermost part of NSO-7 zone.

In the Nini-1 well the preservation of the closely related species:W. gochtii and W. symmetrica precludes their unambiguous

(1) 22,36 Ma

(2) 24,5 (Ma)

(3) 26,6 Ma

(5) 29,3 Ma

(6) 31,0 Ma

(8) 33,6-33,4 Ma(7) 33,3 Ma

(4) 28,5 Ma

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PERSISTENT OCCURENCEEROSION

>50% 20-50%

Depth-age correlation for Nini-1 showing an

accumulation rate in the Oligocene

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(1) LO Chiropteridium spp. (Williams et al. 2004)(2) LO Distatodinium biffii (Williams & Manum 1999)(3) LO Areoligera semicirculata (Williams et al. 2004)

(6) LO Spiniferites manumii (Eldrett et al. 2004)

(8) LO Areospheridium diktyoplokum (Eldrett et al. 2004)(7) LO Areospheridium diktyoplokum (Williams et al. 2004)

(4) FO Distatodinium biffii (Williams et al. 2004)(5) LO Enneadocysta pectiniformis (Williams et al. 2004)

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Fig. 2. Chronostratigraphy, gamma log, seismic surfaces, biostratigraphy and deptheage correlation plot for the Nini-1 well. Range of the stratigraphically important species ofbenthic foraminifera and dinoflagellates are shown. The presence of Svalbardella is shown as percentages of total dinoflagellates. The deptheage correlation plot is based onbiostratigraphically important dinoflagellate events recorded in the Nini-1 well with published absolute ages as specified.

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distinction and the two species were therefore combined into onegroup, W. symmetrica þ gochtii, during the analysis. The top of theNSO-8 zone coincides with the LO of D. biffii. Artemisiocysta clado-dichotoma and Triphragmadinium demaniae, the FO’s of which markthe base of the NSO-5b subzone and the top of the NSO-7 zonerespectively, were not recognized in the studied well. The NSO-7and NSO-8 zones were therefore combined into one NSO-7þ8 zone.

Most of the key events of the NSO dinoflagellate zonation of VanSimaeys et al. (2005b) were identified in the Nini-1well (Fig. 2). It istherefore believed that the succession is not only very thick but alsostratigraphically complete. The LCO of E. pectiniformis is observed at1520 m and indicates the top of the NSO-4a subzone of mid-Rupelian age (Fig. 2). In general, in situ FO’s of species in ditchcuttings samples can not be precisely indicated due to the down-hole contamination. However, our observations on the Nini-1samples show that the ditch cuttings from this well are of goodquality with only minor downhole contamination. Therefore the FOof D. biffii at 1400 m is probably in situ, marking the base of theNSO-5a subzone of mid-late Rupelian age at this level (Fig. 2). At1040e1100 m co-occurring Svalbardella cooksoniae (Pl.1, Fig. 2) andSvalbardella sp. 1 (Pl.1, Fig. 4) were observed. The frequency ofSvalbardella in the interval is up to 1,5% of total dinoflagellates. TheFO of Svalbardella sp.1 is recorded at 1100 m, while single speci-mens of S. cooksoniae occur sporadically in the lowermost part ofthe NSO-5a subzone (Fig. 2)

The LO of R. draco, marking the top of the earliest Chattian NSO-5b subzone, was recognized at 790 m (Fig. 2). The LO of A. semi-circulata and the LO of W. symmetrica þ gochtii are recorded at750 m, indicating the top of the early Chattian NSO-6 zone at thisdepth.

The dinoflagellate studies were supplemented with forami-nifera data from the completion report of the Nini-1 well (Fenton,2001). Foraminifera data are shown in Fig. 2. Based on thebenthic foraminifera data in Fenton (2001), we recognized two

zones of De Man et al. (2004) in the studied interval (Fig. 2). The LOof Rotaliatina bulimoides at 1400m indicates the top of the DeMan’szone VI. This event is calibrated to occur within the basal part of theNSO-5a dinoflagellate subzone (De Man et al., 2004; Van Simaeyset al., 2005b). The co-occurrence of D. biffii and R. bulimoides at1400 m confirms the presence of the lower part of the NSO-5asubzone at this depth (Fig. 2).

Fenton (2001) recorded the top of abundant (20e50%) Aster-igerina guerichi guerichi at 750 m, the top of superabundant A.guerichi guerichi (>50%) at 780 m, while the base of superabundantA. guerichi guerichiwas observed at 880m and the base of abundantA. guerichi guerichi was found at 980 m (Fig. 2). This 230 m longinterval is referred to zone IX of De Man et al. (2004) and can becorrelated with the ‘Asterigerina Horizon’ at the base of the Chat-tian type section in Doberg, Germany and elsewhere in the basalChattian onshore NW Europe (King, 1983; De Man and VanSimaeys, 2004; Van Simaeys, 2004).

4.2. Stratigraphic sequences and 3D seismic data

The previously used sequence stratigraphic subdivision(Michelsen et al., 1998; Michelsen and Danielsen, 1996; Danielsenet al., 1997) was primarily based on well log analysis supportedby 2D seismic sections. However, the introduction of 3D seismics insequence stratigraphic analysis calls for a revision of the subdivi-sion. A number of sequence stratigraphic surfaces sensu Catuneanuet al. (2009) in the studied interval are indicated in Figs. 2 and 3.The ages of the surfaces as indicated in Fig. 2 are based on thepresent biostratigraphy in the Nini-1 well.

The present study is focused on the surface named OSS-A(Oligocene Stratigraphic Surface A) as recognized in Nini-1 at1100 m below kb because of the biostratigraphy and the closeassociation with the cold-water indicator Svalbardella. Themorphology of the OSS-A surface is shown on Fig. 4.

Fig. 3. Seismic section and gamma log showing the position of a number of sequence stratigraphic surfaces (coloured lines), including the OSS-A surface studied here (green). Blackarrows indicate low-angle onlaps. The location of the seismic section is shown as a red line on Fig. 4a. (For interpretation of the references to colour in this figure legend, the readeris referred to the web version of this article.)

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The OSS-A is characterized by a high amplitude reflector atpresent dipping toward the SW and having a clinoform breakpointlocated east of the Nini-1 well (Fig. 3). The surface is characterizedby erosive truncations at the top of the clinoforms, and low-anglelandward directed onlaps. The erosion located at the clinoformbreakpoint has the character of aligned linear gullies which areeroded into the top of the clinoforms (Fig. 4a and b). South-west of

the clinoform breakpoint (i.e., basinward of the clinoform break-point) a number of pockmarks are present. The pockmarks areelongate parallel to the contours of the slope, indicating contourparallel erosion andmodification of the pockmarks (Andresen et al.,2008) (Fig. 4c).

The analysis of Nini-1 shows that the OSS-A coincides with thebase of the interval with Svalbardella (Fig. 2).

5. Discussion

The present results reveal the presence of the cold-waterdinoflagellate Svalbardella in a narrow interval of the expandedmid-Oligocene Nini-1 section. A similar mid-Oligocene Svalbardellaevent was previously recorded from the central North Sea wellMona-1 by Van Simaeys (2004). Using dinoflagellate biostratig-raphy Van Simaeys et al. (2005a) showed that the event in Mona-1occurs in the same biostratigraphic position as in central Italiansections with multidisciplinary age control studied by Coccioniet al. (2008). On this basis Van Simaeys et al. (2005a) furthershowed that the event in Mona-1 represents the Oi-2b coolingevent. In the present study the short interval with Svalbardella(Fig. 2) occurs in the same biostratigraphical position as in Mona-1,i.e., between the FO of D. biffii and LO of R. draco. It may therefore beconcluded, that the Svalbardella interval in Nini-1 and Mona-1 areof the same age, and in both sections represents the Oi-2b coolingevent. This event is dated as ca. 27.1 Ma in the Italian sections (VanSimaeys et al., 2005a; Coccioni et al., 2008).

So far the mid-Oligocene Svalbardella event has not beenrecognized in any onshore section in the North Sea area. In NWEurope the base of the traditional Chattian is a distinct unconfor-mity and the earliest Chattian deposits are known as the ‘Aster-igerina Horizon’ (e.g. Gramann and Spiegler, 1986; De Man and VanSimaeys, 2004) characterized by an abundance of the warm waterforaminifera A. guerichi guerichi and dated with KeAr methods at26.2 � 0.5 Ma (Gramann et al., 1980). Relying on this dating VanSimaeys (2004) concluded that the ‘Asterigerina Horizon’ isyounger than the 27.1 Ma Svalbardella event, and that the Oi-2bglaciation and the associated sea-level fall coincides with theunconformity at the base of the traditional Chattian in NW Europe.It may be assumed, following Van Simaeys (2004), that the timespan of the onshore hiatus coincides with the Svalbardella intervalobserved in the North Sea wells.

Nini-1 is the first section recording both the Svalbardella intervaland an interval with abundant A. guerichi guerichi. The Svalbardellainterval occurs below the Asterigerina interval, which clearlyrepresents the ‘Asterigerina Horizon’. The observation confirms theassumption of Van Simaeys (2004) that the Svalbardella eventpredates the Asterigerina bloom. The mid-Oligocene Svalbardellaevent in the eastern North Sea is characterized by a co-occurrenceof S. cooksoniae and Svalbardella sp.1. This contrasts to an earlyOligocene Svalbardella event characterized by presence of S. cook-soniae only (�Sliwi�nska et al., 2009). The co-occurrence of S. cook-soniae and Svalbardella sp.1 in the mid-Oligocene event can beobserved in other wells: Mona-1 (personal observations, K.�S.) andAlma-1X (Schiøler, 2005).

As mentioned above, the Svalbardella interval coincides with thebase Chattian unconformity onshore. For this reason the Svalbar-della interval was not included in the onshore NSO-zonationscheme of Van Simaeys et al. (2005b). The unzoned interval withSvalbardella falls between the NSO-5a and NSO-5b subzones(Fig. 2). In Nini-1 these two zones are expanded and the deptheagecorrelation (Fig. 2) indicates a faster accumulation rate in the mid-Oligocene than in the remaining part of the Oligocene succession.

Just below the Svalbardella interval (Fig. 2), the OSS-A seismicsurface is observed. The features observed at the OSS-A, such as

Fig. 4. a. The figure shows the area covered by the NODAB survey and the morphologyof the OSS-A surface. The position of the seismic section in Fig. 3 and the Nini-1 wellare marked. Magnifications of the green and blue areas are shown on Fig. 4b and crespectively. b. The figure shows a magnification of the green area marked in Fig. 4a.The trace of NNE-SSW oriented gullies is clearly visible. The gullies are orienteddowndip of the OSS-A surface. c. The figure shows a magnification of the blue area inFig. 4a. Yellow arrows indicate the position of pockmarks. (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version ofthis article.)

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erosion, pockmarks and gullies indicate that this surface representsa sequence boundary associated with a significant drop in baselevel. The seismic defined interval in which the Svalbardella ispresent onlaps onto the slope, and the configuration of the Sval-bardella-bearing strata may suggest that the interval with Sval-bardella is not recorded in the more proximal settings of the basin(�Sliwi�nska et al., 2009). The age of the OSS-A sequence boundarydemonstrates that it was formed during (and as a result of) the Oi-2b glaciation, which is emphasized by the indications of coolerNorth Sea waters during this episode, i.e., the OSS-A originated dueto a glacioeustatic sea-level drop.

Finally, and relying on the Van Simaeys (2004) correlation of theSvalbardella interval with the onshore area, it follows that the OSS-A surface represents the classical NW European base Chattianunconformity as typified at Doberg, Germany.

It should be noted that a new Rupelian/Chattian boundarydefinition (a GSSP) has recently been proposed, based on an Italiansection (Coccioni et al., 2008). To avoid confusion, in the presentpaper reference in made exclusively to the traditional NW Euro-pean Rupelian/Chattian boundary, not to the new boundaryproposal of Coccioni et al. (2008), which may be numerically olderthan the traditional boundary, according to data in Van Simaeys(2004) and Coccioni et al. (2008).

6. Conclusions

The integrated biostratigraphic and 3D seismic study presentedhere leads to two major conclusions

� The recognition of a section where both the ‘AsterigerinaHorizon’ and the Svalbardella interval are present, which allowto correlate the Rupelian/Chattian transition in NW Europewith the global time scale and with the offshore North Seasequence stratigraphy. The short interval with Svalbardellarecorded in the mid-Oligocene deposits in the Nini-1 well issynchronous with the Oi-2b event.

� The features observed on the OSS-A seismic surface by meansof 3D seismic analysis, such as erosion, gullies and pockmarksare characteristic for a prominent relative sea-level drop. Thepresence of the interval with the Arctic dinoflagellate Sval-bardella immediately above the OSS-A surface indicates thatthe sea-level drop in the study area was driven by glacio-eus-tacy. The dating and the indications of the glacioeustaticcontrol of the sequence boundary generation along the OSS-Asurface allows to trace the traditional NW European baseChattian unconformity into the offshore area.

The study furthermore illustrates that integrated micropaleon-tology, e.g. involving bothdinoflagellates and foraminifera, combinedwith 3D seismic analysis is important for the further development ofthe sequence stratigraphic analysis and for the understanding of thecauses of an interpreted relative sea-level change.

Acknowledgments

Søren Bo Andersen (Aarhus University, Denmark) is thanked forpreparing the plate with dinoflagellate photographs. Bartosz

Plate 1. Scale bar in Fig. 1 ¼100 mm applies also to Figs. 2 and 3. 1. Rhombodinium draco, Sample K52 (depth 750 m). 2. Svalbardella cooksoniae, Sample K158 (depth 1100 m).3. Distatodinium biffii, Sample K50 (depth 840 m). Scale bar in Fig. 4 applies also to Figs. 5 and 6. 4. Svalbardella sp.1, Sample K 158 (depth 1100 m). 5. Enneadocysta pectiniformis,Sample K125 (depth 1630 m). 6. Areoligera semicirculata, Sample K149 (depth 1420 m).

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Goledowski (Aarhus University, Denmark) is thanked for preparingthe map of the study area. Katarzyna K. �Sliwi�nska acknowledgesfinancial support for her PhD project from Aarhus University(Denmark) and Fur Museum (Denmark). Reviewers are thanked fortheir constructive suggestions and comments that had improvedthe manuscript.

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