A new stratigraphic framework for the – Lower Pliocene deposits offshore Scandinavia: A multiscale approach

Dybkjær, Karen; Rasmussen, Erik Skovbjerg; Eidvin, Tor; Grøsfjeld, Kari; Riis, Fridtjof; Piasecki, Stefan; Sliwinska, Kasia K.

Published in: Geological Journal

DOI: 10.1002/gj.3982

Publication date: 2021

Document version Publisher's PDF, also known as Version of record

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Citation for published version (APA): Dybkjær, K., Rasmussen, E. S., Eidvin, T., Grøsfjeld, K., Riis, F., Piasecki, S., & Sliwinska, K. K. (2021). A new stratigraphic framework for the Miocene – Lower Pliocene deposits offshore Scandinavia: A multiscale approach. Geological Journal, 56, 1699-1725. https://doi.org/10.1002/gj.3982

Download date: 30. Sep. 2021 Received: 4 April 2020 Revised: 17 July 2020 Accepted: 11 August 2020 DOI: 10.1002/gj.3982

RESEARCH ARTICLE

A new stratigraphic framework for the Miocene – Lower Pliocene deposits offshore Scandinavia: A multiscale approach

Karen Dybkjær1 | Erik Skovbjerg Rasmussen1 | Tor Eidvin2 | Kari Grøsfjeld3 | Fridtjof Riis2 | Stefan Piasecki4 | Kasia K. Śliwinska 1

1Stratigraphy Division/Geophysical Division, Geological Survey of Denmark and Greenland Here, we present an updated stratigraphic subdivision of the Oligocene to Pleisto- (GEUS), Copenhagen, Denmark cene succession (880–610 m) in the newly proposed type well for the Molo Forma- 2Norwegian Petroleum Directorate (NPD), Stavanger, Norway tion, the industrial 6407/9-5 well, located on the continental shelf in the eastern 3Geological Survey of Norway, The team for Norwegian Sea. Furthermore, new data from the Danish North Sea wells Nora-1, Marine Geology in the Earth Surface and Vagn-2, and Tove-1 are presented. The studied succession in the 6407/9-5 well is Seabed Division, Geological Survey of Norway, Trondheim, Norway composed of five sedimentary units separated by hiati. The dating of these five units 4GLOBAL Institute, Section for GeoBiology, is based on correlation to the stratigraphically more complete Neogene succession in University of Copenhagen, Copenhagen K, the (Danish) central North Sea area. In this study, a robust stratigraphic framework of Denmark these five units, based on a combination of dinoflagellate cysts (dinocyst) stratigraphy Correspondence and seismic data, is established. The Oligocene succession is referred to the NSO Karen Dybkjær, Stratigraphy Division, Geological Survey of Denmark and Greenland zonation of Van Simaeys et al., Review of Palaeobotany and , 2005, (GEUS), Øster Voldgade 10, DK-1350, 134, 105–128, while the Miocene–Pliocene succession is referred to the dinocyst Copenhagen, Denmark. Email: [email protected] zonation of Dybkjær and Piasecki, Review of Palaeobotany and Palynology, 2010, 161, 1–29. In well 6407/9-5 the two lowermost units, located below the Molo For- Handling Editor: I. Somerville mation, comprise a Rupelian (Lower Oligocene) succession up to 803 m, and an Aqui- tanian/Burdigalian (Lower Miocene) succession from 803 to 787 m, respectively. Both of these units are referred to the Brygge Formation. The Molo Formation is separated from the underlying Brygge Formation by an unconformity. Furthermore, and in contrast to previous studies, our study shows that the Molo Formation (787–703 m) has an unconformity within it. The lower part of the formation (787– 724 m) is dated to late (Late Miocene), and referred to the Hys- trichosphaeropsis obscura dinocyst Zone. The upper part (724–703 m) is dated to Zanclean (Early Pliocene), and referred to the Melitasphaeridium choanophorum dinocyst Zone. The uppermost unit studied (703–670 m) in the well is referred to the Gelasian (Lower Pleistocene) and is included in the Naust Formation. The regional correlation of this stratigraphy with the complete succession in the Danish North Sea reveals that the hiati found in the Miocene succession on the Norwegian Sea shelf are controlled by tectonism, while the internal depositional patterns of the Molo For- mation were controlled by eustatic sea-level changes.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2020 The Authors. Geological Journal published by John Wiley & Sons Ltd.

Geological Journal. 2021;56:1699–1725. wileyonlinelibrary.com/journal/gj 1699 1700 DYBKJÆR ET AL.

KEYWORDS Dinoflagellate cyst zonation, Molo Formation, Neogene, North Sea, Norwegian Sea shelf, sequence stratigraphy

1 | INTRODUCTION sector, the Miocene–Pliocene succession is located off the major deltas and is more expanded and complete compared to the onshore The present study presents an improved age constraint of the Molo succession. The succession is fully marine and thus provide the basis Formation (Norwegian continental shelf) by correlating to the solid for a detailed dinocyst stratigraphy for all of the Miocene–Pliocene stratigraphic framework recorded for the complete Miocene–Pliocene interval. The dinocyst stratigraphy is combined with new seismic succession in the Danish North Sea. interpretations and sequence stratigraphic analysis. This combination The Miocene–Pliocene sedimentary succession on the Norwegian provides an improved stratigraphic framework for the Miocene– continental shelf is characterized by a high number of unconformities Pliocene succession for the North Sea Basin. due to a combination of tectonic movements, shifts in ocean current The Molo Formation is a narrow depositional body located along systems, and eustacy. The variations in sediment flux from the hinter- the Norwegian continental shelf (from 67.5N to 63.3N, Eidvin land add to the incompleteness of the succession off mid Norway. et al., 2007). The formation is composed of a distinct prograding sedi- Massive reworking resulting from both submarine erosion and uplift mentary package and was originally defined from seismic data of the hinterland complicate the establishment of an unambiguously (Eldholm & Nysæther, 1968). biozonation and dating of the lithostratigraphic units (Eidvin, Bugge, & In 2007, Eidvin et al. published a proposal for the 6610/3-1 well Smelror, 2007; Eidvin, Riis, & Rasmussen, 2014; Løseth, Kyrkjebø, as the type well for the Molo Formation. They presented a detailed Hilde, Wild, & Bunkholt, 2017; Grøsfjeld et al., 2019). Furthermore, summary of the previous work done on this lithostratigrahic unit, the only sediment material (with a few exceptions) available for analy- including biostratigraphic and Sr-isotope datings. These authors pro- sis from the Miocene–Pliocene succession on the Norwegian conti- posed the type section as spanning the interval from 555 m to nental shelf is from ditch cuttings from industrial wells. This type of approximately 349 m in the 6610/3-1 well (Figures 1 and 2). Unfortu- samples always carries a risk of downhole contamination (caving). nately, the upper part of the interval was neither sampled nor logged However, the almost complete Miocene–Pliocene succession in in this well. The 6407/9-5 well was proposed as a reference well. In the Danish part of the North Sea Basin provides a key important ref- this well, the formation was defined to range from 787 to 670 m, erence to the understanding of the Miocene–Pliocene succession off- comprising a continuous succession of Late Miocene to Early Pliocene shore Norway. During the last two decades, a robust dinoflagellate age. According to Eidvin et al. (2007), the Molo Formation is uncon- cyst (dinocyst) zonation for the Miocene and Pliocene has been formably bracketed by the Lower Miocene (Brygge Formation) below, established in the eastern (Danish) North Sea Basin (Dybkjær & and an Upper Pliocene succession referred to the Naust Formation Piasecki, 2010). The zonation is mainly based on data from onshore above, respectively. The succession referred to the Naust Formation Denmark, comprising a number of outcrops, a single cored borehole was at that time dated as Late Pliocene, but as the Gelasian recently and more than 50 high-quality conventional stratigraphic boreholes. has been moved from the Late Pliocene to the Early Pleistocene, the The proposed ages of the dinocyst zones were mainly based on corre- basal part of the Naust Formation in this well probably belongs to the lation with international zonations and stratigraphic schemes, where Early Pleistocene (Cohen et al., 2013; updated 2018). lowest occurrences (LO) and highest occurrences (HO) of dinocyst For unknown reasons this proposal of the 6610/3-1 well as the type taxa were dated on the basis of Sr-isotopes and magnetostratigraphy. well for the Molo Formation was never ratified by the Norwegian Strati- A few Sr-isotope datings from Danish wells were also included. A graphic Committee and at present there is thus no official type well for more comprehensive study correlating the Danish dinocyst zones the Molo Formation. However, in a recently submitted paper Eidvin et al. directly with Sr-isotope datings was published by Eidvin, Ullmann, present a new proposal, now with the the well 6407/9-5 as the type well Dybkjær, Rasmussen, and Piasecki (2014). The results from that study for the Molo Formation and the type section as the interval from 787 to strongly supported the previously suggested age model of the Danish 703 m (Figure 2), while the reference section for this formation is defined Lower – Middle Miocene succession. Correlation in the late Middle – as the interval from 642 to 460 m in the 6610/3-1 well (Eidvin, pers. Upper Miocene part of the succession, however, indicated that Sr- comm. 2020). Based partly on the new age constraints in the present datings in that interval are problematic. study, the location of the upper boundary of the Molo Formation in the The dinocyst zonation of Dybkjær and Piasecki (2010) is mainly newly proposed type well has been moved 33 m downwards, so that the based on data from onshore Denmark but includes also data from a formation now is suggested to comprise the interval from 787 to 703 m few offshore wells, for example, from the Tove-1. This dataset is in well 6407/9-5. In line with the current study, Eidvin et al. further spec- included herein. In order to improve the stratigraphic frame for the ified the formation to span in age from the Tortonian to Zanclean, with Danish offshore area further, we included two new datasets from the an unconformity separating the Tortonian and the Zanclean sedimentary Nora-1 and the Vagn-2 wells, respectively. In the Danish offshore units (Eidvin, pers. comm. 2020). DYBKJÆR ET AL. 1701

FIGURE 1 Location of the wells included in the present study and the geographical distribution of the Molo Formation. Modified from Eidvin, Riis, and Rasmussen (2014). The location of the cross-sections shown in Figures 7–9 are indicated. The full line show the location of the seismic line in Figure 7, while the stipled line show the location of geosection of Figure 9b. The cross-sections of Figures 8a,b and 9a are so short that a line marking them would not be visible. The locations of these cross-sections are therefore just marked by indicating the locations of the two wells they run through, well 6407/9-5 and 6610/3-1, respectively [Colour figure can be viewed at wileyonlinelibrary.com]

The aim of the present study is to develop an improved stratigraphic quality and, not least, the large amounts of reworked material. The framework for the Molo Formation in the well 6407/9-5 (Figure 1). An interpretations of the age have varied from Oligocene to Pliocene updated dating of the sedimentary succession of well 6407/9-5 will con- (e.g., Eidvin et al., 2007; Eidvin, Brekke, Riis, & Renshaw, 1998; Eidvin, tribute to the understanding of the geological history of the Norwegian Rasmussen, Riis, Dybkjær, & Grøsfjeld, 2019; Eidvin, Riis, & Sea and in particular the sedimentary regimes onshore and offshore Nor- Rasmussen, 2014; Eidvin, Riis, Rasmussen, & Rundberg, 2013; Grøsfjeld wayduringtheNeogene. et al., 2019). Eidvin et al. (1998) assigned a Rupelian (Early Oligocene) age for the Molo Formation in the northern part of the Trøndelag Platform, 1.1 | Previous biostratigraphic studies of the Molo based on benthic foraminifera, dinocysts and strontium isotope analy- Formation sis of sidewall cores from the 6610/3-1 well. Based on ditch cutting samples from three industrial wells Dating the Molo Formation on the basis of calcareous microfossils, (6407/9-1, 6407/9-2, and 6407/9-5) Eidvin et al. (2007) assigned the strontium isotopes, and dinocysts has proved very difficult. This is Molo Formation a Late Miocene to Early Pliocene age. The authors partly due to the few wells penetrating the formation, the poor sample explained the presence of Rupelian (well 6610/3-1) and Early 1702 DYBKJÆR ET AL.

FIGURE 2 The previous definition and the new definition of the Molo Formation in the well 6407/9-5, of Eidvin et al. (2007) and the subdivision suggested in the present study and by Eidvin et al. (pers.comm. 2020), respectively. Note that in 2007 the Gelasian was part of the Late Pliocene, whereas it now represents the Early Pleistocene

Miocene (well 6510/2-1) microfossils in the Molo Formation as a Miocene microfossils in 6510/2-1 to be contemporaneous with the result of reworking and suggested a post-middle Miocene age for the deposition, not reworked after all. entire formation. Based on the regional geological framework, Løseth and Interpretation of new seismic data led Eidvin et al. (2013) and Henriksen (2005), Løseth, Kyrkjebø, Hilde, Wild, and Bunkholt (2016), Eidvin, Riis, and Rasmussen (2014) to propose that the northern, prox- and Løseth et al. (2017) referred the Molo Formation to the Early Plio- imal part of the Molo Formation is of Rupelian age and that the forma- cene. Løseth et al. (2017) included the interval from 810 to 787 m in tion gets gradually younger towards the west and south. They further the 6407/9-5 well in the Molo Formation and referred also this inter- suggested the Rupelian microfossils in 6610/3-1 and the Early val to the Early Pliocene. DYBKJÆR ET AL. 1703

Recently, Grøsfjeld et al. (2019) presented a palynostratigraphic 2.2 | Stratigraphic framework study of seven sidewall cores from the lower part of the Molo Forma- tion (611–460 m) in the well 6610/3-1. This well penetrated the most By combining a series of wells in the eastern part of the North Sea proximal – and northernmost – part of the Molo Formation (Figure 1). Basin, onshore and offshore Denmark, a complete Miocene–Pliocene The studied interval consists of a prograding unit forming the lower succession can be studied. A dinocyst zonation for the uppermost Oli- part of the Molo Formation. Based on a very sparse in situ dinocyst gocene (upper Chattian) – Lower Pleistocene (Gelasian) was published assemblage, diluted by high numbers of reworked dinocysts, the age by Dybkjær and Piasecki (2010) (Figure 3). The lower part of the zona- of this unit could be restricted to the Tortonian (Late Miocene). tion (upper Chattian – mid Tortonian) is based on a comprehensive dataset from onshore Denmark including one cored borehole (the Sdr. Vium borehole), more than 50 conventional boreholes and 25 outcrop 2 | DESCRIPTION sections onshore Denmark. As the uppermost Miocene–Pliocene is missing onshore, a hand- The well 6407/9-5 is located at the southern part of the Trøndelag ful of offshore wells was included in the study in order to cover this Platform (Figure 1). It penetrates the Molo Formation in the southern, stratigraphically younger interval as well. Due to several factors most distal part of its distribution area, in a setting off the main including the smaller dataset, the poorer sample quality [ditch cut- prograding packages. The microfossil assemblages indicate that depo- tings samples (DCS) only] and the gradual fall in sea-surface temper- sition took place on the middle to outer shelf (Eidvin et al., 2007). This atures – resulting in more sparse and less diverse dinocyst location is optimal in order to date the marine intervals based on assemblages, the dinocyst zonation for this interval is less robust dinocysts and to minimize the amounts of reworked material in the than for the older parts of the Miocene. Therefore, a combination of studied samples. dinocyst stratigraphy, log-correlation and seismic interpretation have The stratigraphic subdivision presented here is based on correla- here formed the basis for a stratigraphic frame for the Miocene to tion with the almost complete Miocene–Pliocene succession in the lowermost Pleistocene succession in the Danish part of the North Danish North Sea area. This long-distance correlation between the Sea Basin, based on three key wells, Tove-1, Vagn-2, and Nora-1. two areas is based on a combination of dinocyst stratigraphy and seis- Subsequently, this framework was applied for dating the proposed mic data. In addition, a similar sequence stratigraphic development of type well for the Molo Formation, well 6407/9-5, located on the the succession in the Danish North Sea and on the Norwegian Sea Norwegian Sea shelf. shelf strongly supports the biostratigraphic correlation suggested here, for example, distinct sea-level falls found at both locations. 2.3 | Geological setting

2.1 | Palynological studies of the Neogene The Miocene epoch was an important time for the evolution of the Nor- succession in the Norwegian Sea and the North Sea wegian Sea shelf and the North Sea Basin. During the Early Miocene, the Norwegian Sea and the Norwegian continental shelf were characterized Previous dinocyst studies of the Miocene–Pliocene succession in the by a sediment starved deep marine setting dominated by deposition of Norwegian Sea are rather sparse, comprising for example, Manum, siliciclastic mud and diatom ooze, referred to as the upper part of the Boulter, Gunnarsdottir, Rangnes, and Scholze (1989), Mudie (1989), Brygge Formation (Eidvin et al., 2007, 2013; Eidvin, Riis, & Eidvin et al. (1998, 2007), Piasecki, Gregersen, and Johannessen Rasmussen, 2014; Figures 4 and 5). The base of the upper Brygge For- (2002), De Schepper et al. (2015), De Schepper, Beck, and Mangerud mation, dated as Late Eocene (Eldholm, Thiede, Taylor, et al., 1989) was (2017), and Grøsfjeld et al. (2019). defined by a regional unconformity, probably formed by deep marine In contrast, dinocysts of the Neogene succession in the North bottom currents associated with initial uplift of the margin. Sea Basin have been studied in more details, including for example, A similar sediment starved basin existed during the Early Miocene Piasecki (1980), Strauss and Lund (1992), Powell (1992), Head (1993, in the northern North Sea Basin. Only a few distal density flow 1996, 1997), Louwye (1999, 2002), Dybkjær and Rasmussen (2000, deposits of the Skade Formation, sourced from the Shetland Platform, 2007), Strauss, Lund, and Lund-Christensen (2001), De Schepper, reached the central part of the North Sea Basin (Eidvin & Rundberg, Head, and Louwye (2004, 2009), Dybkjær (2004a, 2004b), Louwye, 2007; Eidvin, Ullmann, et al., 2014; Rundberg & Eidvin, 2005). There- Head, and De Schepper (2004), Munsterman and Brinkhuis (2004), fore, mud dominated deposits of the Hordaland Group characterized Schiøler (2005), Louwye, De Schepper, Laga, and Vandenberghe the sedimentation in this area. (2007), Louwye and Laga (2008), Louwye and De Schepper (2010), In the eastern North Sea Basin (onshore Denmark), a huge Dybkjær and Piasecki (2010), Dybkjær, King, and Sheldon (2012), delta system introduced a high clastic input to the basin. Conse- Eidvin, Ullmann, et al. (2014), Śliwinska et al. (2014), and De Schepper quently, the Lower Miocene succession of the central and east- and Mangerud (2017). Recently Dybkjær et al. (2019) presented a pal- ern North Sea is dominated by siliciclastic mud laid down in front ynofacies study of different depositional environments from the of this delta system (the Ribe Group; Rasmussen, Dybkjær & Lower Miocene succession onshore Denmark. Piasecki, 2010). 1704 DYBKJÆR ET AL.

FIGURE 3 Dinocyst zonation of Dybkjær and Piasecki (2010). Abbreviations of dinocyst zones used in Figure 6 and of dinocyst zones and events used in the supplementary material are indicated. Nannoplankton zonation of Martini (1971). Timescale after Cohen et al. (2013). Modified from Dybkjær and Piasecki (2010). Photoes of key dinocysts observed in this study are shown in Figures 10 and 11

In the Middle Miocene, uplift of the western margin of the Sea (Eidvin et al., 2013; Eidvin, Ullmann, et al., 2014). This updoming Fennoscandian Shield resulted in the formation of a distinct unconfor- of the margin was coincident with the formation of major anticlines in mity across the Norwegian Sea and in the northern part of the North the North Atlantic area, for example, the Faroe Islands and the Ormen DYBKJÆR ET AL. 1705

FIGURE 4 (a,b) Palaeogeography, Early and Late Miocene respectively, and the position of the studied wells. Modified from Rasmussen et al. (2008) [Colour figure can be viewed at wileyonlinelibrary.com]

Lange structure (Boldrell & Andersen, 1995; Løseth et al., 2017; strong erosion of the upper part of the Molo Formation the genuine Stoker, Holford, Hillis, Green, & Duddy, 2010). In the central and east- development of the Pliocene succession in the Norwegian Sea shelf is ern North Sea accelerated subsidence commenced and the former unclear (Ottesen, Rise, Andersen, Bugge, & Eidvin, 2010). In the north- delta system of the Ribe Group was flooded (Rasmussen, 2005; Ras- ern North Sea Basin, sands of the upper Utsira Formation character- mussen & Dybkjær, 2014). This quite different evolution of the depo- ized the depositional environment. The Central and eastern North Sea sitional setting, with formation of an unconformity on the Norwegian experienced an accelerated subsidence during the Pliocene. This pro- Sea shelf and in the northern North Sea Basin on one hand and cess was probably caused by sediments load from the so-called flooding in the central and eastern North Sea Basin on the other hand, Eridanos Delta that dominated the depositional setting in the south- is interpreted as a rejuvenation of the tectonic setting in NW Europe ern part of the North Sea Basin (Gibbard & Levin, 2016; Knox (Rasmussen, 2005; Stoker et al., 2010; Ziegler, 1990). A change from et al., 2010). Consequently, drowning of the delta system of the Måde a predominantly N-S oriented Alpine compressional regime to an E-W Group occurred in the eastern North Sea Basin and marine, mud- oriented Atlantic compressional regime took place in the Middle and dominated deposits were laid down in this area. However, sand-rich Late Miocene. It resulted in the westward progradation of a huge sediments of the Eridanos Delta began to dominate also in the central coastal plain system, the Molo Formation, on the Norwegian Sea shelf North Sea by the end of the Pliocene. (Bullimore, Henriksen, Liestøl, & Helland-Hansen, 2005; Eidvin The Miocene climate in the North Sea region was humid and et al., 2007; Henriksen et al., 2005; Løseth et al., 2017; Løseth & warm temperate (Larsson et al., 2011; Pound & Riding, 2016; Henriksen, 2005). Distally to the Molo Formation, mud and diatom Mosbrugger, Uteshcer anov, & Dilcher, 2005). Also in the Atlantic ooze referred to the Kai Formation were deposited (Eidvin realm, Iceland Sea region, a warm temperate, humid climate per- et al., 2007). sisted (De Schepper et al., 2017; Denk, Grimson, & Kvacek, 2005; During the latest Middle Miocene and Late Miocene, a sandy unit Schreck, Matthiessen, & Head, 2012). A climatic deterioration com- (the Utsira Formation) was deposited in the northern North Sea Basin mencedinthelateMiddleMiocene(Denketal.,2005;Mosbrugger, (Eidvin et al., 2007, 2013; Eidvin, Riis, & Rasmussen, 2014; Eidvin & Utescher, & Dilcher, 2005; Stein et al., 2016). Significant cooling Rundberg, 2001; 2007; Galloway, 2002; Gregersen & events have been detected by ice-rafted debris at 12.6, 11, and Johannessen, 2007; Rundberg & Eidvin, 2005). The occurrence of 6.4 Ma (Fronval & Jansen, 1996). The decrease in temperature, glaucony-rich layers within the Utsira Formation indicates episodes of especially in the high latitude areas, also resulted in intensification local sediment starvation (Eidvin et al., 2007, 2013; Eidvin & of deep-water circulation in the North Atlantic Ocean (Stoker Rundberg, 2001; Rundberg & Eidvin, 2005). In the eastern North Sea et al., 2005; Tsikalas, Faleide, & Kalac, 2019). Basin, resumed deltaic progradation of the Gram and Marbæk formations took place during the Late Miocene (Michelsen et al., 1998; Møller, Ras- mussen, & Clausen, 2009; Rasmussen, Dybkjær, & Piasecki, 2010; Ras- 3 | METHODS AND MATERIALS mussen, Vejbæk, Bidstrup, Piasecki, & Dybkjær, 2005). During the Early Pliocene, the progradation of the Molo Forma- New palynological analyses were carried on 214 ditch cutting samples: tion continued and within the basinal portion of the Norwegian Sea 27 from the Norwegian 6407/9-5 well, 60 from the Danish Vagn-2 shelf, mud and ooze of the Kai Formation settled (Eidvin, 2009; Eidvin well, and 87 from the Danish Nora-1 well. In addition, data from 40 et al., 2013; Eidvin, Riis, & Rasmussen, 2014). However, due to a DCSs from the Danish Tove-1 well are included. Due to different 1706 DYBKJÆR ET AL.

FIGURE 5 Lithostratigraphy of the Danish North Sea and the Norwegian Sea. Modified from Eidvin et al. (2019) [Colour figure can be viewed at wileyonlinelibrary.com] standards of operator companies, depths in Tove-1, Vagn-2, and by the DCS. This approach has resulted in minor changes in the depth Nora-1 wells are given in feet, while depths in the 6407/9-5 well are intervals of the dinocyst zones when compared with the depth inter- given in metres. vals of those of the zones which were defined in Tove-1 in Dybkjær Approximately, 20 g of sediment from each sample have been and Piasecki (2010). The age range of the various species, referred to processed using standard palynological preparation methods, includ- below, can be found in Dybkjær and Piasecki (2010). Note that ing treatment with HCl, HF, neutralization, brief oxidation with KOH, although the timescale used in Dybkjær and Piasecki (2010) was that heavy liquid separation, and sieving on 20 μm nylon mesh. The of Gradstein, Ogg, and Smith (2004) while we here use that of Cohen organic residues were mounted on glass slides in glycerin gelatin and et al. (2013, updated 2018), the changes of absolute ages between studied in a normal light microscope. Counting of dinocysts, freshwa- these two timescales for the chronostratigraphic units in the Miocene ter algae, and acritarchs were done until a minimum of 200 dinocysts are few and very small, and does not influence the age ranges of the had been registered (if possible) and referred to species or (if not pos- dinocyst species suggested by Dybkjær and Piasecki (2010). sible) genus. Furthermore, two full slides from each sample were scanned in order to record rare taxa. The dinocyst nomenclature used here follows Williams, Fensome, and MacRae (2017). The timescale 4 | RESULTS AND INTERPRETATION used is that of Cohen, Finney, Gibbard and Fan (2013, updated in 2018). 4.1 | Dinocyst stratigraphy The Miocene–Pliocene dinocyst zonation defined for the Danish North Sea area by Dybkjær and Piasecki (2010) has been applied to 4.1.1 | Dinocyst stratigraphy in the three Danish the three Danish key wells (Tove-1, Vagn-2, and Nora-1) and to the key-wells, Tove-1, Vagn-2, and Nora-1, Danish Norwegian wells 6407/9-5 and 6610/3-1 (Figure 6; Supporting North Sea Information 1–6). The zonal boundaries are based on a mix of LO's, HO's, and abundance variations. As the present study is based on Tove-1 ditch cutting samples it is somewhat problematic to use LO's due to Tove-1 is selected as one of the key wells from the Danish North the risk of caving. LO's have therefore mainly been used in intervals Sea, because it is penetrating a complete Upper Miocene–Pliocene closely below casings, where the interval from which caving may succession. Approximately 200 m of Upper Miocene and 520 m of occur is reduced, or where distinct changes in abundances have been Pliocene deposits are present in this well (Figure 6; Supporting recorded. Information 1). The data presented here were generated by Piasecki Here, the HO of a species in a DCS is indicated as the top depth and Rasmussen (2004). All dinocyst zones representing the Upper of the interval represented by the DCS in which the HO is recorded, Miocene–Pliocene defined in Dybkjær and Piasecki (2010) are pre- while the LO is indicated as the base depth of the interval represented sent and Tove-1 was one of the reference wells for several of these DYBKJÆR ET AL. 1707 zones. The log-correlation between Tove-1 and Vagn-2 is very well- upper boundary of the zone is defined based on the LO of defined, due to very similar log-patterns. Also, the seismic correla- Amiculosphaera umbraculum. However, in Tove-1 the LO of this spe- tion between these two wells is very good. Compared to Nora-1, cies is found in the sample at 40100–39800, below the sample com- the Upper Miocene succession in Tove-1 is condensed, while the prising the HO of P. miocaenicum. This indicates that the occurrence Pliocene succession is expanded (Figure 6). of A. umbraculum in this sample is due to caving and thus not useful The dinocyst assemblages are somewhat impoverished from the for locating the upper boundary of this zone. base of the Zanclean (Lower Pliocene; approximately from the sample The A. umbraculum Zone (early Tortonian, Late Miocene) is inter- at 3290' to 3260' and upwards). preted to be present in the interval from 39500 to ?38900. The lower Each DCS covers an interval of the studied succession. In the boundary is based on the HO of P. miocaenicum in the sample at Tove-1 well, the DCS's from the interval 48000–40400 each cover an 39800–39500. The upper boundary is defined based on the LO of Bar- interval of 200, while the DCS's from the interval 40400–3500 each ssidinium evangelineae. However, it is somewhat problematic to locate cover an interval of 300. this boundary in Tove-1 due to caving. Sporadic occurrences of B. evangelineae are found below 38900, while this species is recorded Dinocyst zonation of the Tove-1 well more consistently and in higher numbers in the samples above this In Tove-1 no samples were studied from the Lower Miocene. level. Seismic correlation to Nora-1 support the location of this The Labyrinthodinium truncatum Zone (partial; early Langhian, boundary as suggested here. Middle Miocene). The base of this zone, based on the LO of the nomi- The Hystrichosphaeropsis obscura Zone (late Tortonian, Late Mio- nate species, is not identified in this study, as L. truncatum is present cene) is interpreted to be present in the interval from ?38900 to in the lowermost sample at 42800–42600. The upper boundary of the 36200. The lower boundary is based on the possible LO of L. truncatum Zone is interpreted to be present at ?41400, based on the B. evangelineae in the sample at 38900–38600. The HO of H. obscura, HO of Cousteaudinium aubryae in the sample at 41600–41400. The HO defining the upper boundary of this zone, are in Tove-1 found in the of C. aubryae occurs within the L. truncatum Zone as described in the sample at 38000–37700. However, the HO's of L. truncatum and definition of the zone by Dybkjær and Piasecki (2010) (not at Impagidinium ‘densiverrucosum’, both secondary markers for the upper the lower boundary of the zone as shown in their figures 6 and 7). boundary, occur in the sample at 3650'–3620' and the boundary is The presence of Palaeocystodinium miocaenicum in the sample at thus placed at the top of this sample interval. The location of this 42800–42600 support the presence of the L. truncatum Zone in this boundary is further supported by good seismic- and log-correlation to interval as the LO of this species occur within the zone. Vagn-2 (Figures 6 and 7). A common occurrence of the informal spe- The Unipontidinium aquaductum Zone (late Langhian – early cies Impagidinium ‘densiverrucosum’ of Zevenboom and Santarelli in , Middle Miocene) is interpreted to be present in the inter- Zevenboom (1995) is present from 3800' to 3620' (Supporting val from ?41400–40800. The location of the lower boundary is based Information 1). on the HO of C. aubryae in the sample at 41600–41400, while the loca- The Tove-1 well is one of the reference wells for this dinocyst zone tion of the upper boundary is based on the HO of U. aquaductum in and the zone was defined as occurring from 38900 to 36500 (Dybkjær & the sample at 41000–40800. Piasecki, 2010). The difference in location of the upper boundary is sim- The Achomosphaera andalousiensis Zone (Serravallian, Middle ply due to different ways of interpreting a HO in ditch cutting samples. Miocene) is interpreted to be present in the interval from 40800 to In the study by Dybkjær and Piasecki (2010) the depth of for example, 39800. The lower boundary is based on the HO of U. aquaductum in the HO in the sample representing the interval from 36500 to 36200 was the sample at 41000–40800. The upper boundary of the zone is reported as 36500 (as the basal depth of the the sample interval), while in defined at the LO of Gramocysta verricula (Dybkjær & Piasecki, 2010). the present study the depth of a HO is reported as the top of the sample However, no G. verricula is recorded in the Tove-1 well and according interval. This difference in principle is the cause for minor adjustments of to Dybkjær and Piasecki (2010) the taxon is generally absent in off- several zonal boundaries in the following. shore settings. Therefore, we used the HO of Cerebrocysta poulsenii, The Selenopemphix armageddonensis Zone (latest Tortonian – ear- since the event may be used as an alternative marker for the top of liest Zanclean, Late Miocene – earliest Pliocene) is interpreted to be the A. andalousiensis Zone. The HO of C. poulsenii is recorded in the present in the interval from 36200 to 33500. The lower boundary of sample at 40100–39800. The HO of Cannosphaeropsis passio in the this zone is defined based on the HO's of both L. truncatum and same sample support the presence of the A. andalousiensis Zone in Impagidinium ‘densiverrucosum’ in the sample at 3650'–3620'. The this interval. The LO's of both A. andalousiensis and C. passio are upper boundary is based on the HO of B. evangelineae in the sample recorded below this interval. These occurrences are interpreted to be at 3380'–3350'. The HO of Spiniferites solidago in the sample at due to caving and thus not useful for locating the zonal boundaries. 3590'–3560' support the presence of the S. armageddonensis Zone in The G. verricula Zone (Serravallian – Tortonian, Middle – Late this interval. The location of this boundary is supported by good seis- Miocene) is interpreted to be present in the interval from 39800 to mic correlation to Vagn-2 (Figure 6). 39500. The lower boundary is based on the HO of C. poulsenii in the The Tove-1 well is one of the reference wells for this dinocyst sample at 40100–3980, while the upper boundary is based on the HO zone and the zone was defined as occurring from 36500 to 33800 of P. miocaenicum in the sample at 39800–39500, respectively. The 1708 DYBKJÆR ET AL.

FIGURE 6 Correlation panel showing the gamma-log, chronostratigraphy and lithostratigraphy, dinocyst zonation, seismic markers (green lines), sequence stratigraphy and casings (triangles). The sequence stratigraphic subdivision used here is that of Rasmussen (2004). Abbreviations for the dinocyst zones used here are explained in Figure 3. See Supporting Information 2 and 3 for the dinocyst zonation in the Lower Miocene of the Vagn-2 and Nora-1 wells [Colour figure can be viewed at wileyonlinelibrary.com] DYBKJÆR ET AL. 1709

(Dybkjær & Piasecki, 2010). The different depths of the zonal bound- M. choanophorum. However, the location of this HO in Tove-1, regis- aries are due to the different principles discussed above. tered in the sample at 23300–23000, is not well-defined and seismic The Melitasphaeridium choanophorum Zone (Zanclean, Early Pliocene) correlation to Vagn-2 and Nora-1 strongly indicate that this location is interpreted to be present in the interval from 33500 to ?17900.The is too low, and that the upper boundary of the M. choanophorum lower boundary is based on the HO of B. evangelineae in the sample Zone in Tove-1 should be found at approximately 17900 (Figure 6). at 33800–33500. The upper boundary is defined based on the HO of The missing records are probably due to the generally very sparse

FIGURE 6 (Continued) 1710 DYBKJÆR ET AL.

FIGURE 7 NNW-SSE striking seismic section from the Danish North Sea Sector. Note that the interpreted surfaces are indicated in the correlation panel (Figure 6). Location of the seismic section is shown in Figure 1 [Colour figure can be viewed at wileyonlinelibrary.com]

dinocyst assemblage in the interval from 23000 to 16400,possibly (Dybkjær & Piasecki, 2010; Harland, 1992; Head, 1998; Powell, 1992; reflecting an increased freshwater influx due to progradation of the Verhoeven, Louwye, Eiríksson, & De Schepper, 2011; Williams, coast. The HO of Reticulosphaera actinocoronata should have been Brinkhuis, Pearce, Fensome, & Weegink, 2004). De Schepper found within this zone, however, it was found just below, in the sam- et al. (2009) show a mid ‘Calabrian’ HO of A. umbraculum in DSDP ple at 33800–33500. Hole 610A (eastern North Atlantic) and further suggest an age of The Tove-1 well is one of the reference wells for this dinocyst zone 1.44 Ma for this event. and the zone was defined as occurring from 33800 to 23300 (Dybkjær & Piasecki, 2010). The different depth of the lower zonal boundary is due Vagn-2 to the different principles discussed above. The different depth of the This well is selected as key well in the present study due to a com- upper boundary is based on the seismic correlation to this zonal bound- plete Upper Miocene–Pliocene succession and the straight-forward ary in Vagn-2 and, especially, in Nora-1 (Figure 6). correlation to Tove-1. Compared to the Nora-1 well, the Upper Mio- The Barssidinium pliocenicum Zone (Piacenzian – earliest Gelasian, cene succession in the Vagn-2 well is condensed, while the Pliocene Late Pliocene – earliest Pleistocene) is interpreted to be present from succession is expanded (Figure 6). The dinocyst assemblages become ?17900 to 14900. gradually more and more impoverished, especially from the middle The location of the base of this zone is in disagreement with that Zanclean (approximately from the sample at 22900–23100) and indicated in Dybkjær and Piasecki (2010). According to these upwards. authors, the location of the base of the zone is at 23300 in Tove-1, DCS intervals: 50200–40300:100; 40300–40000:300; 40000–39900: which is the reference well for this zone. However, seismic correla- 100; 39900–3600:300. tion to the HO's of (consistent) M. choanophorum in Vagn-2 and Nora-1, indicate that this location should be revised to approxi- Dinocyst zonation of the Vagn-2 well mately 17900 in Tove-1 (Figure 6). The upper boundary of the The Deflandrea phosphoritica Zone (Chattian, Late Oligocene) is inter- B. pliocenicum Zone is based on the LO of Impagidinium multiplexum preted to be present in the interval from 43900 to 43300 based on the in the sample at 14900–14600. The depth of the upper boundary in HO of Distatodinium biffii in the sample at 44000–43900 combined Tove-1 was erroneously indicated at 16400 in Dybkjær and with the highest common occurrence of D. phosphoritica in the sample Piasecki (2010, p.25) and is revised to 14900 here. at 43400–43300. The I. multiplexum Zone (Gelasian – mid ‘Calabrian’, Early Pleisto- The Chiropteridium galea Zone and the Homotryblium spp. Zone cene) is interpreted to be present from 1490' and at least up to 1310' (early Aquitanian, Early Miocene) are both interpreted to be represented based on the LO of I. multiplexum in the sample at 1490'–1460' and in the interval from 43300 to 43100 based on the HO's of both C. galea the HO of A. umbraculum in the sample at 1340'–1310'. The upper and of abundant Homotryblium spp. in the DCS at 43200–43100. boundary is not found in the present study. Tove-1 is the reference The Caligodinium amiculum Zone (late Aquitanian, Early Miocene) wells for this dinocyst zone and the zone was defined from 1490' to is probably present in the interval from 43100 to 42900, although the 1340' (Dybkjær & Piasecki, 2010). recorded specimen found in the sample at 43000–42900 is question- The occurrence of A. umbraculum in the sample at 13400–1310 ably assigned to C. amiculum. strongly indicates an age no younger than the Early Pleistocene DYBKJÆR ET AL. 1711

The Thalassiphora pelagica Zone (late Aquitanian – early Bur- sample interval. The HO of Cleistosphaeridium placacanthum in the digalian, Early Miocene) is interpreted to be represented in the inter- same sample indicates that also the A. umbraculum Zone is present val from 42900-42700 based on the HO of T. pelagica in the sample at within this interval. The LO of A. umbraculum, defining the lower 42800–42700. boundary of the A. umbraculum Zone, is recorded in the sample at The Sumatradinium hamulatum Zone and the Cordosphaeridium 33300–33000 in the Vagn-2 well. However, the biostratigraphic and cantharellus Zone (early Burdigalian, Early Miocene) are recognised in the seismic correlation to Nora-1 clearly shows that this LO in Vagn-2 the interval from 42700 to 42300. It is not possible to subdivide the is found too high up in the section. interval into the two zones as caving hinders the identification of The H. obscura Zone (late Tortonian, Late Miocene) is interpreted the LO of Exochosphaeridium insigne, defining the boundary between to be present in the interval from 3750' to 33000. The location of the the two zones. The base of the S. hamulatum Zone is based on the base of the zone is based on the HO of C. placacanthum in the sample HO of T. pelagica in the sample at 42800–42700. The top of the at 37800–37500. As was the case for the LO of A. umbraculum men- C. cantharellus Zone is based on the HO of C. cantharellus in the sam- tioned above, also the LO of B. evangelineae, defining the base of the ple at 42400–42300. H. obscura Zone is found too high in the Vagn-2 well. The misplace- The E. insigne Zone (mid Burdigalian, Early Miocene) is interpreted ment of these LO's may be due to poor dinocyst recovery from 36600 to be present in the interval from 42300 to 41100 based on the HO of to 33300. C. cantharellus in the sample at 42400–42300 and the HO of E. insigne The top of the zone is placed at the HO of H. obscura in the sam- in the sample at 41200–41100. ple at 33300–33000. This is further supported by the HO of The C. aubryae Zone (late Burdigalian, Early Miocene) is interpreted L. truncatum in the same sample and the HO of Impagidinium to be present in the interval from 41100 to 40600 based on the HO of ‘densiverrucosum’ at 3390'–3360'. The location of this boundary is E. insigne in the sample at 41200–41100 and the LO of L. truncatum in supported by seismic- and log-correlation to Tove-1. As also seen in the sample at 40600–40500. Use of the LO of C. aubryae to identify the Tove-1, this zone is characterized by an interval with common occur- lower boundary of this zone is hindered by caving. rences of Impagidinium ‘densiverrucosum’. The L. truncatum Zone (early Langhian, Middle Miocene) is The S. armageddonensis Zone (latest Tortonian – earliest Zanclean, interpreted to be present in the interval from 40600 to 38850.The Late Miocene – earliest Pliocene) is interpreted to be present in the lower boundary is based on the LO of L. truncatum in the sample interval from 33000 to 30600. The lower boundary of this zone is at 40600–40500.Thelocationofacasingat40050 reduces the risk based on the HO's of H. obscura and L. truncatum in the sample at of caving and thus support the use of this lowest occurrence. The 33300–33000. The upper boundary is based on the HO of combined HO of C. aubryae and LO of U. aquaductum in the sam- B. evangelineae in the sample at 30900–30600. ple at 39000–38700 is used for placing the upper boundary of the The M. choanophorum Zone (Zanclean, Early Pliocene) is inter- zone. The zonal boundary is arbitrarily located in the middle of the preted to be present in the interval from 30600 to 17400. The lower interval represented by this sample, at 38850.TheLOof boundary is based on the HO of B. evangelineae in the sample at P. miocaenicum in the sample at 39600–39300 further supports this 30900–30600. The upper boundary is based on the HO of interpretation. M. choanophorum in the sample at 17700–17400. The occurrence of a The U. aquaductum Zone (late Langhian – early Serravallian, Mid- single specimen of R. actinocoronata in the sample at 19800–19500 dle Miocene) is interpreted to be present in the interval from 38850 to supports the presence of the M. choanophorum Zone. 38100 based on the LO of U. aquaductum in the sample at 3000–38700 The occurrence of the nominate taxa becomes very sparse in and the HO of the same species in the sample at 38400–38100. the upper part of the zone due to an increasingly impoverished The A. andalousiensis, the G. verricula, and the A. umbraculum dinocyst assemblage upwards. The identification of the upper Zones (Serravallian – Tortonian, Middle – Late Miocene) are recog- boundary of this zone in the Vagn-2 well is thus problematic nized in the interval from 38100 to 37500. The base of the basedontheHOofM. choanophorum alone. However, the loca- A. andalousiensis Zone is interpreted based on the HO of tion of the boundary and the correlation between Nora-1 and U. aquaductum in the sample at 38400–38100. The upper boundary of Vagn-2 based on dinocysts is strongly supported by the seismic the zone is defined based on the LO of G. verricula. However, no data (Figure 6). G. verricula is recorded in the Vagn-2 well and according to Dybkjær The B. pliocenicum Zone (Piacenzian – earliest Gelasian, Late Pli- and Piasecki (2010) G. verricula is generally absent in offshore settings. ocene – earliest Pleistocene) is interpreted to be present from Fortunately, the HO of C. poulsenii may be used as an alternative 17400 to 14700 based on the HO of M. choanophorum in the sample marker for the top of the A. andalousiensis Zone. The HO of at 17700–17400 and the LO of I. multiplexum in the sample at C. poulsenii is recorded in the sample at 37800–37500. The LO of com- 14700–14400. The HO of Invertocysta lacrymosa in the sample at mon A. andalousiensis and the presence and common occurrence of 18300–18000 (closely below the suggested zonal interval) adds sup- C. passio in the sample at 37800–37500 further support the presence port to the location of the lower boundary of this zone, although of the A. andalousiensis Zone in this interval. recorded a bit too low. The HO's of both C. poulsenii and P. miocaenicum in the sample at The I. multiplexum Zone (partially; Gelasian – mid ‘Calabrian’, Early 37800–37500 indicate that the G. verricula Zone is present within this Pleistocene) is interpreted to be present from 1470' based on the LO 1712 DYBKJÆR ET AL. of I. multiplexum in the sample at 1470'–1440'. The top of the zone is approximately 54600 based on the LO of E. insigne (see discussion not identified in the present study. above), while the upper boundary is interpreted to be located at 52100 based on the LO of C. aubryae in the sample at 52100–51800. Nora-1 As was the case for the LO of E. insigne, also the LO of C. aubryae is The Nora-1 well is in the present study selected as one of the key- somewhat speculative due to caving. Its proposed location is based on wells from the Danish North Sea due to an optimal location in the a change in abundance, interpreted as representing a change from central part of the basin and a complete Upper Miocene–Pliocene sporadic, caved, occurrences to common (in situ) occurrences in the succession. Approximately 450 m of Upper Miocene and 400 m of Pli- sample at 52100–51800 and above. The HO of E. insigne in the sample ocene are represented in this well (Figure 6). All dinocyst zones rep- at 52900–52600 indicates the presence of the E. insigne Zone in this resenting the Upper Miocene–Pliocene defined in Dybkjær and interval. Piasecki (2010) are present. The C. aubryae Zone (late Burdigalian, Early Miocene) is inter- DCS intervals: 60000–42900:300; 42900–42800:100; 42800–42600: preted to be present in the interval from 52100 to 51000 based on the 200; 42600–3300:300. LO of C. aubryae in the sample at 52100–51800 and the LO of L. truncatum in the sample at 51000–50700. The locations of these Dinocyst zonation of the Nora-1 well boundaries are somewhat speculative due to caving, but are based on The C. galea Zone (early Aquitanian, Early Miocene) is interpreted to changes in abundances. be represented in the interval from approximately 59400 to 57900. The L. truncatum Zone (early Langhian, Middle Miocene) is The lower boundary is based on a combination of seismic data and interpreted to be present in the interval from 51000 to ?48000. This log-data, indicating the location of the Oligocene-Miocene boundary is based on the LO of L. truncatum in the sample at 51000–50700 at approximately 59400. This boundary is found in the lower part of and the HO of C. aubryae in the sample at 48600–48300. The latter the C. galea Zone (Dybkjær et al., 2012). The lower boundary is not event occurs within the L. truncatum Zone and the boundary is covered by palynological data. The interpretation of the location of therefore located slightly above this event. The upper boundary of the upper boundary is based on the HO of C. galea in the sample at this zone is defined as the LO of U. aquaductum. However, caving 58200–57900. hinders the use of this LO in Nora-1. The LO of P. miocaenicum in The Homotryblium spp. Zone (early Aquitanian, Early Miocene) is the sample at 49200–48900 supports the presence of the interpreted to be represented in the interval from 57900 to 57300 L. truncatum Zone in this interval as this event occurs within the based on the HO of C. galea in the sample at 58200 to 57900 and the zone. However, caving may also have influenced the location of this HO of abundant Homotryblium spp. in the sample at 57600–57300. LO. The upper boundary of the L. truncatum Zone is thus located The C. amiculum Zone (late Aquitanian, Early Miocene) and the arbitrarily here, at?4800 ft, slightly above the HO of C. aubryae and T. pelagica Zone (late Aquitanian – early Burdigalian, Early Miocene) the LO of P. miocaenicum. cannot be differentiated. The upper boundary of the C. amiculum The U. aquaductum Zone (late Langhian – early Serravallian, Mid- Zone is based on the HO of C. amiculum, while the upper boundary of dle Miocene) is interpreted to be present in the interval from?48000 the T. pelagica Zone is based on the HO of T. pelagica. However, in to 46500. This is based on the HO of C. aubryae in the sample at this well these two HO's were recorded in the same DCS. The interval 48600–48300 (see further above) and the HO of U. aquaductum in the represented by these two zones are interpreted to be present in the sample at 46800–46500. interval from 57300 to 56100 based on the HO of abundant Homo- The A. andalousiensis Zone (Serravallian, Middle Miocene) is inter- tryblium spp. in the sample at 57600–57300 and the HO's of both preted to be present in the interval from 46500 to 44100. This is based C. amiculum and T. pelagica in the sample at 56400–56100. on the HO of U. aquaductum in the sample at 46800–46500 and the The S. hamulatum Zone (early Burdigalian, Early Miocene) is inter- HO of C. passio in the sample at 44400–44100. The LO of preted to occur in the interval from 56100 to 54600 based on the HO of A. andalousiensis in the sample at 46200–45900 strongly support the T. pelagica in the sample at 56400 to 56100 and the possible LO of presence of this zone within this interval. E. insigne in the sample at 54600–54300. The LO of E. insigne is some- The G. verricula Zone (Serravallian-earliest Tortonian, Middle – what speculative due to caving. However, the interpretation of its LO is Late Miocene) is interpreted to be present in the interval from 44100 based on a change from sporadic (possibly caved) occurrences in the to 43800. This is based on the (HO) of C. passio in the sample at samples below 56400 to more common (in situ) occurrences in the sam- 44400–44100 and the (LO) of A. umbraculum in the sample at ple at 54600–54300 and in the samples above, up to the HO of this spe- 43800–43500. cies. The location of the S. hamulatum Zone in this interval is supported The A. umbraculum Zone (early Tortonian, Late Miocene) is inter- by the HO of Thalassiphora rota in the sample at 55800–55500. preted to be present in the interval from 43800 to 43500.Thisisbased The C. cantharellus Zone (early Burdigalian, Early Miocene) and on the co-occurrence of the LO of A. umbraculum and the LO of the E. insigne Zone (mid Burdigalian, Early Miocene) cannot be differ- B. evangelineae in the DCS representing this interval. The HO of entiated as the HO of C. cantharellus, defining the boundary between C. placacanthum further support the presence of this zone at this level. the two zones, are recorded too low. The lower boundary of the inter- The H. obscura Zone (late Tortonian, Late Miocene) is interpreted val represented by these two zones is interpreted to be located at to be present in the interval from 43500 to 32100. The lower boundary DYBKJÆR ET AL. 1713 is based on the LO of B. evangelineae in the sample at 43800–43500, locations of the boundaries between the five units are based on a while the upper boundary is based partly on the HO of Impagidinium combination of dinocyst stratigraphy, log-patterns, and seismic data. ‘densiverrucosum’ in the sample at 3240'–3210' and partly on seismic The boundaries between the units (the unconformities) are located correlation to the upper boundary of the H. obscura Zone in the within the interval of 10 m' represented by a DCS. Therefore, DCS's Vagn-2 well (Figure 6). The HO of H. obscura (formally defining the in several occasions comprise palynomorphs from two dinocyst zones upper boundary of this zone) and the HO of L. truncatum (normally of different ages. found close to the upper boundary), are both found in the sample at Both caving and reworking obscure the palynological record in 3540'–3510' in Nora-1. However, the seismic correlation to Vagn-2 the well. Therefore, a combination of HO's, LO's, and abundance vari- strongly suggests that HO's of these two zonal marker species in ations have been used to subdivide the succession (see range chart Nora-1 should ideally have been located at approximately the level with selected taxa in Supporting Information 6). In addition, the corre- indicated by the HO of I. ‘densiverrucosum’ (Figure 6). The HO's of lation to the Danish wells support the proposed stratigraphic frame- H. obscura and L. truncatum in Nora-1 are thus here interpreted to work of the five units (Figure 6). occur too low. DCS intervals: 880 m–610 m: 10 m. As observed in both Tove-1 and Vagn-2, also in Nora-1a common occurrence of Impagidinium ‘densiverrucosum’ is found within the Dinocyst zonation of the 6407/9-5 well H. obscura Zone (Supporting Information 3). The NSO2-NSO4 Zone sensu Van Simaeys, Munsterman, and The S. armageddonensis Zone (latest Tortonian – earliest Brinkhuis (2005) (Rupelian, Early Oligocene) is interpreted to be pre- Zanclean, Late Miocene – earliest Pliocene) is interpreted to be sent in the interval from 880 to 803 m. The location of the upper present in the interval from 32100 to ?27500. The lower boundary boundary at 803 m is based on a combination of the biostratigraphy of this zone is defined as the HO of H. obscura,whichinNora-1 and the gamma-log pattern. At this depth a distinct shift in the occurs at 35400. However, based on seismic correlation to Vagn-2, gamma-log towards lower values can be observed (Figure 6; the boundary is moved upwards to 32100 (see further discussion Supporting Information 4 and 6). This is interpreted as representing above). The upper boundary is based on seismic correlation of the an unconformity spanning from the Rupelian to, at least, the late HO of B. evangelineae in Vagn-2 (in the sample at 30600–30900)to Aquitanian. Nora-1 (Figure 6). The HO of S. solidago inthesampleat32100– The zone is represented by eight DCSs spanning the interval from 31800 support the presence of the S. armageddonensis Zone in this 880 to 800 m. The uppermost sample (810–800 m) is interpreted as interval. transitional between the Lower Oligocene and Lower Miocene. Based The M. choanophorum Zone (Zanclean, Early Pliocene) is inter- on the log-patterns, the Rupelian succession is interpreted to reach preted to be present in the interval from? 27500–25500. The lower only up to 803 m. boundary is based on seismic correlation to the HO of B. evangelineae The co-occurrence of for example, Chiropteridium lobospinosum, in Vagn-2. The upper boundary is based on the HO of Deflandrea heterophlycta, D. biffii, Enneadocysta arcuata/pectiniformis M. choanophorum in the sample at 25800–25500. Group, Licracysta? semicirculata, Phthanoperidinium amoenum, The B. pliocenicum Zone (Piacenzian – earliest Gelasian, Late Phthanoperidinium comatum, Svalbardella cooksoniae, Spiniferites man- Pliocene – earliest Pleistocene) is interpreted to be present umii, and Wetzeliella gochtii suggest a Rupelian age (Egger et al., 2016; from 25500 to 16800 basedontheHOofM. choanophorum at Heilmann-Clausen & Van Simaeys, 2005; Lund, 2002; Manum 25500–25800 and the LO of I. multiplexum inthesampleat et al., 1989; Schiøler, 2005; Śliwinska et al., 2012; Van Simaeys, De 16800–16500. Man, Vandenberghe, Brinkhuis, & Steurbaut, 2004; Van Simaeys The I. multiplexum Zone (Gelasian – mid ‘Calabrian’, Early Pleisto- et al., 2005). The dinocyst assemblage recovered in this interval is sim- cene) is interpreted to be present from 1680' to 1170' based on the ilar to the assemblage from the middle Rupelian succession recovered LO of I. multiplexum at 1680' and the HO of A. umbraculum in the sam- from the central North Sea Basin, the 11/10-1 well (Śliwinska, 2019). ple at 1200'–1170'. The upper boundary may, however, be placed too An undefined Lower Miocene unit (?late Aquitanian-Burdigalian, low here as the samples above 1170' show very sparse dinocyst Early Miocene) is interpreted to be present in the interval from 803 to assemblages and the ‘true’ HO of A. umbraculum may therefore not 787 m. This interval is thus represented by 3 DCSs from 810 to have been recorded. 800, 800 to 790, and 790 to 780 m. The location of the lower bound- ary of the interval is based on a combination of the above-mentioned shift in gamma-log pattern at 803 m, combined with a change in the 4.1.2 | Dinocyst stratigraphy in the Norwegian dinocyst assemblage with the lowest common occurrence of well 6407/9-5 in the eastern Norwegian Sea Apteodinium tectatum in the sample at 800–790 m together with a distinct increase in abundances of C. placacanthum and The present study of the 6407/9-5 well suggests the presence of five Achomosphaera ramulifera and a decrease in abundances of typical depositional units separated by unconformities (Figure 6; Supporting Lower to mid Oligocene taxa (e.g., D. heterophlycta, E. arcuata/ Information 4 and 6). The datings of these five depositional units are pectiniformis Group and L. semicirculata). However, the ranges of these primarily based on dinocyst stratigraphy. However, the precise taxa continue into this interval, probably due to reworking. 1714 DYBKJÆR ET AL.

The top of the unit is based on a combination of a shift in High abundances of A. ramulifera are recorded in the interval from gamma-log pattern at 787 m and the HO's of both C. amiculum and 780 to 740 m. Similar high abundances of Achomosphaera are charac- C. cantharellus in the sample at 790–780 m (Figure 6; Supporting teristic for the Tortonian in the Norwegian Sea (Eidvin et al., 2007; Information 4 and 6). The HO of T. pelagica in the sample at 800– Poulsen, Manum, Williams, & Ellegaard, 1996). In these studies, it is 790 m further support a Lower Miocene age (De Verteuil & Nor- referred to Achomosphaera sp. 1 (see further the discussion). ris, 1996; Dybkjær & Piasecki, 2010). Reworking of Upper to Lower deposits is Due to the massive reworking, it is problematic to assign this indicated by the occurrences of for example, Cribroperidinium granu- interval to specific dinocyst zones. The dinocyst assemblage does not ligerum Group, Cribroperidinium hansenii, Gochteodinia villosa and yield any of the characteristics of the lower Aquitanian zones of Sirmiodinium grossii, while reworking of Palaeogene and possibly also Dybkjær and Piasecki (2010), as there is no registration of the key- Lower Miocene deposits is indicated by the presence of species C. galea, and only a single occurrence of Homotryblium spp. e.g. A. gippingensis, C. cantharellus, and Wetzeliella spp. The presence The HO's of C. amiculum, T. pelagica, and C. cantharellus, defining of I. multiplexum is interpreted as the result of caving. some of the zonal boundaries in the upper Aquitanian and Burdigalian The M. choanophorum Zone (Zanclean, Early Pliocene) is inter- in the zonation of Dybkjær and Piasecki (2010) are of low confidence preted to be present in the interval from 724 to 703 m. The location here. Presumed reworked specimens of both C. amiculum and (fre- of the lower boundary is based on a combination of a gamma-log shift quent) C. cantharellus are registered in the succession above, referred at 724 m and the HO of Impagidinium ‘densiverrucosum’ in the sample to the Upper Miocene, Pliocene, and Pleistocene, while the occur- at 740–730 m, in the zone below. The location of the upper boundary rences of T. pelagica are very rare. The 803–787 m interval may thus is based on a change in lithology, a small negative gamma ray spike represent a very condensed succession representing several of the and a small positive sonic spike (Eidvin et al. pers. comm. 2020), com- Lower Miocene dinocyst zones. bined with a well-defined highest consistent occurrence of Reworking of Cretaceous strata is indicated by the occurrence of R. actinocoronata in the sample at 710–700 m (Figure 6; Supporting Hystrichodinium voigtii. Massive reworking of Palaeogene deposits are Information 4 and 6). This HO of R. actinocoronata indicates an early indicated by the occurrences of for example, Areoligera gippingensis, Zanclean age for this interval (De Schepper et al., 2017; Kuhlmann Areosphaeridium dictyoplokum, C. lobospinosum, D. heterophlycta, et al., 2006; Louwye et al., 2004; Verhoeven et al., 2011). The HO of Eatonicysta ursulae, Enneadocysta peciniforme, Palaeocystodinium pyro- B. pliocenicum in the same sample and the absence of Tortonian key- phorum, Thalassiphora fenestrata, and Wetzeliella spp. The presence of species (e.g., H. obscura and I. ‘densiverrucosum’) further support a Filisphaera filifera, Habibacysta tectata, Impagidinium ‘densiverrucosum’ Zanclean age and the presence of the M. choanophorum Zone and I. multiplexum in the sample from 800 to 790 m is interpreted to (De Schepper et al., 2017; Dybkjær & Piasecki, 2010; Kuhlmann result from caving, while the presence of the two former taxa et al., 2006; Louwye et al., 2004; Verhoeven et al., 2011). The HO of (F. filifera and H. tectata) in the sample representing the interval from M. choanophorum, defining the upper boundary of this zone, is found 790 to 780 m probably reflects that the uppermost 7 m of that inter- in the sample at 730–720 m. This HO is interpreted to have been val actually belongs to the Upper Miocene. recorded too low, as it was found below the well-defined HO of The H. obscura Zone (late Tortonian, Late Miocene) is interpreted R. actinocoronata. This mis-location probably reflects the rather sparse to be present in the interval from 787 to 724 m and represented by dinocyst assemblage in this interval. 7DCSsfrom790–780 m to 730–720 m. The location of the lower Reworking of Upper Jurassic to Lower Cretaceous deposits is boundary of this zone – and thus the lower boundary of the Molo For- indicated by the occurrences of Cyclonephelium hystrix, mation – at 787 m is based on an upward increase in the gamma-ray Dichadogonyaulax culmula, G. villosa, and Kleithriasphaeridium spp., response and an upward decrease in velocity, in combination with the while reworking of Palaeogene deposits is indicated by the presence lowest consistent occurrence of Impagidinium ‘densiverrucosum’ in the of for example, A. gippingensis, A. dictyoplokum, Cerodinium striatum, sample at 780–770 m (the single specimen recorded in 800–790 m is E. ursulae, L. semicirculata, P. pyrophorum, and Wetzeliella spp. The interpreted as caved; Figure 6; Supporting Information 4 and 6). The presence of I. multiplexum is interpreted as the result of caving of upper boundary is based on a combination of a gamma-log shift at Lower Pleistocene deposits. 724 m and the HO of Impagidinium ‘densiverrucosum’ in the sample at The I. multiplexum Zone (Gelasian – mid ‘Calabrian’, Early Pleisto- 740–730 m. The range of common I. ‘densiverrucosum’ is restricted to cene) is interpreted to be present in the interval from 703 to 669 m. the H. obscura Zone in all the Danish key wells (Tove-1, Vagn-2, Nora- The lowest common occurrence of I. multiplexum in the sample at 1) and the HO of this species is found closely below or at the upper 700–690 m, strongly indicates an early Gelasian age (De Schepper boundary of the H. obscura Zone in these wells, see further discussion. et al., 2017; Dybkjær & Piasecki, 2010; Head, 1998; Köthe, 2012; The location of the upper boundary at 724 m is further supported Kuhlmann et al., 2006). The less common occurrences of I. multiplexum by the HO of H. obscura in the sample at 760–750 m, although this in the samples below – actually down into the Lower Miocene – are HO, which define the upper boundary of the H. obscura Zone, seems interpreted to be due to caving. The occurrence of A. umbraculum in to be located too low here. The lowest recorded occurrence of the sample at 670–660 m suggests an age no younger than the Early A. umbraculum in the sample at 770–760 m further support a Tor- Pleistocene (Dybkjær & Piasecki, 2010; Harland, 1992; Head, 1998; tonian age for this interval (Dybkjær & Piasecki, 2010). Powell, 1992; Verhoeven et al., 2011; Williams et al., 2004). As DYBKJÆR ET AL. 1715 mentioned previously, De Schepper et al. (2009) show a mid ‘Cala- The Upper Miocene succession reveals a constant thinning brian’ HO of A. umbraculum in DSDP Hole 610A (eastern North Atlan- towards the south, especially the lower part which pinches in the tic) and further suggest an age of 1.44 Ma for this event. southernmost portion of the study area. The Upper Miocene is Reworking of Upper Jurassic to Lower Cretaceous deposits is subdivided into two sequences; F1 and F2 (Rasmussen, 2017). indicated by the occurrences of for example, C. hansenii, G. villosa, ThePliocenesuccessionshowsasystematic onlap onto the Mio- Gonyaulacysta jurassica, Rhynchodiniopsis chladophora, and S. grossii, cene section and is quite thick in the southern part. Around the while reworking of Palaeogene deposits is indicated by the presence Nora-1 well, a distinct wedge that pinches out just south of the of for example, Apectodinium augustum, A. tectatum, A. gippingensis, well characterizes the uppermost Pliocene. The Pliocene are sub- A. dictyoplokum, Cerebrocysta bartonensis, C. striatum, D. heterophlycta, divided into sequences G, H, and I (Rasmussen, 2017; Rasmussen Diphyes ficusoides, E. ursulae, P. pyrophorum, P. comatum, and et al., 2005). Wetzeliella spp. An undefined, Gelasian-mid Calabrian (Early Pleistocene) unit is present in the interval from 669–610 m. 4.2.2 | Norwegian Sea shelf seismic This upper part of the studied succession showed a poor dinocyst assemblage (Supporting Information 6). However, the rare occur- In this study, the 6407/9-5 well, located on the Norwegian Sea shelf, has rences of I. multiplexum indicate an early Gelasian age also for this been dated and correlated by using the dinocyst zonation of Dybkjær and interval (Head, 1998; Köthe, 2012; Kuhlmann et al., 2006). Further- Piasecki (2010) from the Danish North Sea (Figure 6). Recently, Grøsfjeld more, an age no younger than the Early Pleistocene is indicated by et al. (2019) published a study of the Miocene Molo Formation in the north- the consistent occurrence of F. filifera (Mudie, 1989; De Vernal ern part of the Norwegian Sea shelf, in the 6610/3-1 well. In both wells an et al., 1992. De Schepper et al. (2009) recorded the HO of Filisphaera Upper Miocene section has been penetrated and dated to the Tortonian. in the mid ‘Calabrian’ in the DSDP hole 610A in the eastern North This section is in 6407/9-5 overlain by a succession also referred to the Atlantic. Molo Formation and dated as Zanclean. In the 6610/3-1 well this interval Reworking of Upper Jurassic to Lower Cretaceous deposits is was not sampled and has thus not been dated palynologically. indicated by the occurrences of C. granuligerum Group, A seismic dip section tying to the well shows the overall architecture Hystrichodinium vogtii and S. grossii, while reworking of Pal- of the Miocene–Pliocene succession offshore Norway at the 6407/9-5 aeogene deposits is indicated by the presence of for example, well location (Figure 8a). The Lower Miocene succession is characterized Apteodinium australiense, A. gippingensis, C. amiculum, C. striatum, by a very irregular package bounded by distinct unconformities. The and W. gochtii. internal reflection pattern is parallel to sub-parallel with a local high- amplitude reflector. The succeeding Upper Miocene succession is char- acterized by a clinoformal reflection pattern. The clinoformal 4.2 | Seismic interpretation section shows a tripartite development comprising a progradational pat- tern, a progradational/aggradational pattern and again a progradational 4.2.1 | North Sea seismic reflection pattern, respectively. The upper progradational unit shows a tendency to successively deeper erosion to the west. The overlying Plio- In order to support the biostratigraphic correlation of the three key cene succession is a very thin clinothem with no internal characteristics. wells, Tove-1, Vagn-2, and Nora-1, these wells have been tied seismi- Well 6610/3-1 penetrates only one of the clinoform packages, cally based on a new seismic study (Figures 6 and 7). The surfaces and thus probably represent only a small part of the Upper Miocene identified on the seismic data provide timelines that supplement the (Tortonian) successions. correlations of the dinocyst zones, and further help to identify bio- However, the penetrated Tortonian interval can be tied with the events, which are misplaced due to for example, reworking, caving, 6607/9-5 well where a similar seismic sequence stacking pattern can and/or sparse occurrences. be seen (Figure 8a,b). The similarity in stacking patterns may be asso- In general, the key seismic surfaces are easy to follow between ciated with eustatic sea-level changes and thus indicate that the the three wells and thus form a confident framework for the Miocene Upper Miocene succession on the Norwegian Sea shelf can be sub- and Pliocene successions in the area. divided into two sequences. This is similar to the development of The Lower Miocene succession shows a quite uniform thick- sequences in the North Sea where the Upper Miocene is subdivided ness from the north to the south. Marked thinning, however, into the F1 and F2 sequences (Rasmussen, 2017; Figure 7). The Upper occurs over both salt structures, where Vagn-2 and Tove-1 are Miocene sequences, F1 and F2, are referred to the Tortonian and located. From a regional study, including both onshore and off- Tortonian-Messinian, respectively. Therefore, it is appealing to corre- shore data, three sequences named B, C, and D have been identi- late the first progradational system, formed during the late Tortonian, fied in the Lower Miocene succession (Rasmussen, 2004, 2017). to sequence F1 (Figure 8a,b). The succeeding system, which shows a The Middle Miocene succession shows a similar uniform thickness progradation/aggradation to progradational pattern, may represent with distinct thinning above salt structures. The Middle Miocene the late Tortonian-Messinian F2 sequence of the North Sea. Espe- is only subdivided into one sequence, E (Rasmussen, 2004, 2017). cially, the down-stepping pattern seen in the F2 sequence west of 1716 DYBKJÆR ET AL.

FIGURE 8 (a,b) Two W-E striking seismic sections tying to the wells 6407/9-5 and 6610/3-1, respectively. The sequences F1, F2, and G, defined in the Danish North Sea area (Rasmussen, 2017), are indicated on the sections. Note the similar seismic pattern of the progradational Molo Formation in the two wells: Prograding – prograding/aggrading (yellow) – prograding pattern. Note also the marked down-stepping pattern within Sequence F2 marked with red arrows in the well 6610/3-1 section. Location of the seismic sections are shown in Figure 1 [Colour figure can be viewed at wileyonlinelibrary.com]

6610/3-1 (Figure 8b) mimic the development in the North Sea 5. The lower part of the Molo Formation in the 6407/9-5 well, (the sequence development (Figure 7), where deep incision has been rec- succession from 787 to 724 m) can be referred to the upper Tor- ognized within the Messinian interval, sequence F2 (Møller tonian (Upper Miocene) H. obscura Zone. et al., 2009; Rasmussen, 2017). The Pliocene clinothem correlates 6. An unconformity at approximately 724 m is suggested to represent with sequence G of the North Sea sequence stratigraphy. the Messinian to lowermost Zanclean. 7. The upper part of the Molo Formation (the succession from 724 to 703 m) can be referred to the Zanclean (Lower Pliocene) M. cho- 4.3 | Summary of results anophorum Zone. 8. The Zanclean succession is unconformably overlain by lower The present study of the Molo Formation in the well, 6407/9-5 Gelasian (lowermost Pleistocene) deposits from approximately supports the general interpretation of Eidvin et al. (2007) of a Lower 703 m (the lowermost DCS comprising a common occurrence of Miocene succession unconformably overlain by an Upper Miocene – lower Gelasian dinocysts represents the interval from 700 to Lower Pliocene succession (Figure 2). However, because the dinocyst 690 m). The dinocyst assemblage strongly indicates that this suc- zonation of Dybkjær and Piasecki (2010) has shown to be applicable for cession should be referred to the I. multiplexum Zone. This is in dating and subdividing the succession, a more accurate age constraint contrast to the interpretation by Eidvin et al. (2007) who referred fortheMoloFormationontheNorwegian shelf have been achieved (Fig- the interval from 690 to 670 m to the Lower Pliocene. The upper ure 6, Supporting Information 4 and 6); boundary of the Molo Formation is here suggested to be located at 703 m based on a combination of the dinocyst stratigraphy and 1. The studied succession in well 6407/9-5 comprises five deposi- log-patterns. tional intervals separated by unconformities. 2. The precise locations of these unconformities, as suggested here, are based on a combination of dinocyst data, log-patterns, and seismic data. 5 | DISCUSSION 3. The Oligocene deposits present in the lowermost part of the studied succession in 6407/9-5, up to 803 m, can be referred to the Rupelian, 5.1 | Correlation between the North Sea and the dinocyst zone NSO2-NSO4a sensu Van Simaeys et al. (2005). Norwegian Sea shelf 4. The Lower Miocene succession (803–787 m) probably consists of deposits of ?late Aquitanian – Burdigalian age. However, due to The Miocene succession is incomplete both on the Norwegian Sea reworking, it has not been possible to refer the interval to the shelf and in the northern part of the North Sea. Discussions about dinocyst zonation. The interval may represent a condensed Lower ages of different formations are numerous for example Løseth and Miocene succession comprising several dinocyst zones. Henriksen (2005), Eidvin et al. (2013); Eidvin, Riis, and DYBKJÆR ET AL. 1717

FIGURE 9 (a,b) W-E striking geosections. The vertical lines indicate the correlated wells. (a) From the Norwegian Sea. (b) From the Danish North Sea. The yellow parts of the geosections mark the Messinian succession which in both the Norwegian Sea and the North Sea were formed associated with a sea-level fall. The location of these sections are shown in Figure 1. Note the complete and expanded Neogene succession in the Danish North Sea area, forming the basis for the robust stratigraphic frame [Colour figure can be viewed at wileyonlinelibrary.com]

Rasmussen (2014), Rundberg and Eidvin (2016a, 2016b), Løseth, 5.2 | Comparison with previous palynological Raulline, and Nygård (2013), Løseth et al. (2016), Løseth and datings of the Molo Formation Øygarden (2016), Eidvin and Rundberg (2016a, 2016b), Løseth (2016); Grøsfjeld et al. (2019), and Eidvin et al. (2019). The incomplete succes- Eidvin et al. (2007) proposed the 6407/9-5 well as a reference well sions in these areas are due to a number of reasons; variable sediment for the Molo Formation and defined the interval of the formation as flux from Scandinavia, erosion by bottom currents and large- and ranging from 787 to 670 m, comprising a continuous succession of small-scale tectonic movements (e.g., inversion structures and salt Late Miocene to Early Pliocene age (Figure 2). They indicated that the diapirism). In contrast, sediment supply to the eastern North Sea (pre- Molo Formation unconformably overlies a succession of Early Mio- sent-day Denmark) was high and relatively stable during the Miocene cene age. Furthermore, they suggested that the Molo Formation is and Pliocene. Therefore, a complete archive for dating the Miocene – unconformably overlain by an Upper Pliocene succession referred to Pliocene succession exists here (Dybkjær & Piasecki, 2010). During the Naust Formation. It should be noted here that at that time the the period in question, there has been an open connection and thus Gelasian was part of the Upper Pliocene after Berggren, Kent, Swisher water mass exchange, between the North Sea and the North Atlantic III, and Aubry (1995) and Gradstein et al. (2004), while it now repre- (Eidvin et al., 2019). Consequently, most dinocyst species found in the sents the lowermost Pleistocene after Cohen et al. (2013). North Sea were also found in the successionontheNorwegian Eidvin et al. (2007) defined a number of dinocyst zones in the Sea shelf. We therefore believe that the dinocyst zonation 6407/9-5 well in addition to establishing some undefined intervals: established for the Danish North Sea Basin by Dybkjær and Two zones, the S. cooksoniae Zone (860 m) and the Areoligera semi- Piasecki (2010) applied in this study represents the most optimal circulata Zone (860–810 m), represent the Lower Oligocene, while methodology for dating Miocene – Pliocene successions on the their Cordosphaeridium cantharellum Zone (810–790 m) represents the Norwegian Sea shelf. Lower Miocene succession. They referred the lower part of the Molo We have here provided a stratigraphic framework for the succes- Formation (790–750 m) to the Achomosphaera sp. 1 Assemblage Zone sion referred to the Molo Formation in the well 6407/9-5, based on and dated it as Late Miocene – Early Pliocene. According to Eidvin this dinocyst zonation, which is further refined by seismic data. Fur- et al. (2007), the age is ‘partly based on benthic foraminiferal and thermore, we suggest a correlation to the succession referred to the planktonic fossil evidence’. Specimens referred to Achomosphaera Molo Formation in the well, 6610/3-1, see also discussion below sp. 1, an informal taxon previously described by Poulsen et al. (1996), (Figure 6). A similar sequence stratigraphic development in the Danish occurs in high abundances in the DCSs representing this interval. In North Sea and the Norwegian Sea shelf – showing coincident sea- the present study, we also found high abundances of Achomosphaera level falls and -rises – strongly support the suggested correlation in the interval referred to the late Tortonian H. obscura Zone. These between these two areas. specimens are here referred to A. ramulifera. Consistent recordings of 1718 DYBKJÆR ET AL.

FIGURE 10 (a–p) Photos of selected taxa from the Danish North Sea well, Tove-1. The scale bar is 20 μm. (a) Barssidinium evangelineae. (b) Hystrichosphaeropsis obscura. (c, d) Reticulatosphaera actinocoronata. (e) Cousteaudinium aubryae. (f, g) Labyrinthodinium truncatum. (h) Palaeocystodinium miocaenicum. (i, m) Amiculosphaera umbraculum. (j) Cannosphaeropsis passio. (k, l) Impagidinium multiplexum. (n) Impagidinium ‘densiverrucosum’. (o, p) Melitasphaeridium choanophorum [Colour figure can be viewed at wileyonlinelibrary.com]

specimens referred to Achomosphaera sp. 1 have previously been The LO of B. evangeliniae defines the lower boundary of the reported from the ODP Sites 907, 908, and 909 in the Norwegian- H. obscura Zone. However, as they record this species also in the Greenland Sea in successions referred to as Upper Miocene – Lower interval referred to the Lower Miocene, probably due to caving from Pliocene (Poulsen et al., 1996). In addition to the high abundances of the Upper Miocene, the LO of this species is not useful. The HO of Achomosphaera, Eidvin et al. (2007) reported several Upper Miocene – B. evangelineae defines the upper boundary of the S. armageddonensis Pliocene index taxa from this interval for example, A. umbraculum, Zone, located in the lowermost Zanclean. The R. actinocoronata Zone B. evangelineae, B. graminosum, and B. pliocenicum. In the present of Eidvin et al. (2007) comprises the interval from 750 to 690 m, in study, these taxa were also recorded from this interval, except for the upper part of the interval they referred to the Molo Formation. B. evangelineae. The presence of B. evangelineae strongly supports that They reported a moderately rich and diverse in situ dinocyst assem- this interval should be referred to the Tortonian (Eidvin et al., 2007). blage including A. umbraculum, Barssidinium graminosum, DYBKJÆR ET AL. 1719

FIGURE 11 (a–n) Photos of selected taxa from the proposed type well for the Molo Formation, 6407/9-5. The scale bar is 20 μm. (a) Caligodinium amiculum. (b) Melitasphaeridium choanophorum. (c, d) Achomosphaera ramulifera. (e, f) Impagidinium ‘densiverrucosum’. (g) Cleistosphaeridium placacanthum. (h) Hystrichosphaeropsis obscura. (i) Reticulatosphaera actinocoronata. (j) Impagidinium multiplexum. (k) Cordosphaeridium cantharellus. (l) Bitectatodinium tepikiense. (m) Spiniferites manumii. (n) Distatodinium biffii [Colour figure can be viewed at wileyonlinelibrary.com]

B. pliocaenicum, and R. actinocoronata. The HO of R. actinocoronata et al. (2013) show that E. pygmaeus also existed in the earliest Pleisto- defined the upper boundary of this zone and dated it as early cene. The latter interval was dated as Late Pliocene (Pleistocene Zanclean (Early Pliocene). Above these two zones they referred an according to the timescale of Cohen et al., 2013) based on the pres- ‘undefined interval’ from 690 to 670 m to the uppermost part of the ence of the Elphidiella hannai foraminiferal assemblage. They found Molo Formation while another ‘undefined interval’ (undefined with only a few in situ and no age-diagnostic dinocysts from this interval. respect to the dinocyst zonation) from 670 to 620 m was referred to Reworked Jurassic, Cretaceous, and Palaeogene dinocysts were the Naust Formation. The former of these two intervals was dated as reported as common to abundant in the R. actinocoronata Zone and in Early Pliocene, mainly based on the presence of the Eponides the two overlying undefined intervals, as also recorded in the present pygmaeus foraminiferal assemblage but later investigations of Eidvin study. In the present study, the upper boundary of the Molo 1720 DYBKJÆR ET AL.

Formation in the 6407/9-5 well is suggested to be located at 703 m above the HO's of both H. obscura and L. truncatum andusedthe and the succession above 703 m is suggested to belong to the Naust HO of I. ‘densiverrucosum’ to mark the top of the Miocene. How- Formation. Part of the R. actinocoronata Zone and the two above- ever, as seen in Nora-1, the succession of these HO's are not con- mentioned ‘undefined intervals’ of Eidvin et al. (2007) thus now repre- sistent, probably partly due to sparse occurrences of dinocysts in sent the Naust Formation of Pleistocene age. the Late Miocene. In the study of Munsterman et al. (2019) the HO The results of the study by Grøsfjeld et al. (2019) of sidewall of I. ‘densiverrucosum’ ranges up to the ‘Late Miocene Unconfor- cores from the lower part of the Molo Formation in the well mity’. It may be questioned if this unconformity could correlate 6610/3-1 compare very well with the results of the present study. In with the Messinian glacial events and be the result of the lowered spite of a very sparse in situ dinocyst assemblage, diluted by high global sea-level in the Messinian (see Jiménez-Moreno et al., 2013 numbers of reworked, mainly Palaeogene dinocysts, the age could be and references therein). It can further be questioned if this uncon- restricted to the Tortonian (Late Miocene), based on the co- formity comprise the lower part of the Messinian, so that the HO occurrence of the dinocysts B. evangelineae and Minisphaeridium of I. ‘densiverrucosum’ actually also here is recorded in the upper latirictum (senior synonym for Cordosphaeridium minimum)and Tortonian rather than in the Messinian. the presence of the acritarch Lavradosphaera lucifer. The presence I. ‘densiverrucosum’ was also recorded by De Schepper et al. (2017) of A. andalousiensis, Barssidinium graminosum,andB. pliocenicum in their study of the upper Neogene (Upper Miocene to Upper Pliocene) areingoodagreementwithaTortonianage.Theco-occurrence successionintheOceanDrillingProgram(ODP)hole642BontheVøring of B. evangelineae and M. latirictum further indicates that the Plateau in the Norwegian Sea. They defined three interval biozones, studied interval probably belongs to the basal part of the VP1-VP3,basedonthedinocyst-andacritarchassemblagesinthe H. obscura Zone of Dybkjær and Piasecki (2010) and refers it to 40 core samples studied and further provided calibrated ages for a series thelateTortonian. of bioevents, based on the palaeomagnetostratigraphy of Bleil (1989) for the studied well. Unfortunately, only the <63 μmfractionwasprocessed for the palynological study, while the 150–63 μmfractionwasusedfor 5.3 | Comments on the dinocyst stratigraphy foraminiferal stable isotopes. Larger dinocyst taxa (those larger than 63 μm) may therefore be missing from their dataset. De Schepper The dinocyst species Impagidinium ‘densiverrucosum’ seems to be very et al. (2017) show a consistent occurrence of Impagidinium important for the correlation between the Danish North Sea and the Nor- ‘densiverrucosum’ throughout their VP1 interval biozone which they wegian Sea shelf and the occurrences of this species are therefore inserted dated as late Messinian to earliest Zanclean (>5.89–5.29 Ma), based on in Supporting Information 1–4.ItwasnotfoundintheNorwegianwell correlation with the palaeomagnetostratigraphy. The authors report that 6610/3-1 (Grøsfjeld et al., 2019) and is thus not shown in Supporting the recorded specimens of I. ‘densiverrucosum’ are often broken and usu- Information 5. This species has previously been reported from the Middle- ally present in low numbers only and therefore they decided not to use Late Miocene of the Netherlands and Italy (Zevenboom, 1995), from the the HO of this species to define the upper boundary of the VP1 biozone. Upper Miocene of the Danish North Sea area (Dybkjær & Piasecki, 2010) Instead, they used the HO of the more abundant acritarch ‘Veriplicidium and from the Roer Valley Graben (Munsterman et al., 2019). franklinii’ of Anstey (1992). The H. obscura Zone of Dybkjær and Piasecki, dated as late Tor- They also recorded the dinocyst species C. poulsenii in the VP1 tonian (8.8–7.6 Ma), is in the present study characterized by a com- biozone, a species with a HO at the top of the A. andalousiensis Zone mon occurrence of I. ‘densiverrucosum’ in both Tove-1, Vagn-2 as well of Dybkjær and Piasecki (2010), dated as early to mid-Serravallian as in Nora-1. The correlation between the intervals with common I. (Middle Miocene). ‘densiverrucosum’ in these three wells is strongly supported by seismic The presence of S. armageddonensis in the VP1 biozone of De data (Figure 6). The HO of I. ‘densiverrucosum’ is found closely below, Schepper et al. (2017) indicates an age not older than the latest Tor- or at, the upper boundary of the H. obscura Zone, as also reported in tonian (7.6 Ma; De Verteuil & Norris, 1996; Dybkjær & Dybkjær and Piasecki (2010). A common occurrence and the HO of Piasecki, 2010 and references therein), which is in accordance with this informal species thus seems to be characteristic for the H. obscura the age based on the palaeomagnetostratigraphy. The absence of Zone in offshore settings in the North Sea Basin. On this basis I. Tortonian index-taxa (e.g., H. obscura and L. truncatum) support this ‘densiverrucosum’ is considered an important biostratigraphic marker. interpretation. However, rather than prolonging the ranges of the The age of the HO of I. ‘densiverrucosum’ and whether this event two taxa I. ‘densiverrucosum’ and C. poulsenii upwards to 5.29 and occurs below or above the HO's of H. obscura and L. truncatum seems 5.35 My, respectively, as suggested by De Schepper et al. (2017), it is however to be disputable. One of the problematic issues in this context is suggested here that the presence of these two taxa within VP1 that the lower and upper boundary of the Messinian stage is at present should be interpreted as due to reworking of Langhian to Tortonian not well-documented within the North Sea Basin as this stage was defined deposits. The sparse occurrences and the broken specimens of in the Mediterrenean and was deposited during quiet different climatic and I. densiverrucosum support this interpretation. Examples of reworking depositional conditions, resulting in the ‘Messinian salinity crisis’. of for example, R. actinocoronata and the Batiacasphaera micro- In a study in the Roer Valley Graben of the Netherlands, papillata complex is suggested by De Schepper et al. (2017) further Munsterman et al. (2019) recorded the HO of I. ‘densiverrucosum’ up in the studied succession, in the VP3 biozone. DYBKJÆR ET AL. 1721

The absence of I. ‘densiverrucosum’ in the H. obscura Zone in prograding unit, that is, west of well 6610/3-1, indicate deposition 6610/3-1 (Grøsfjeld et al., 2019; Supporting Information 5) is proba- during a relative sea-level fall for the later part of the Molo Formation. bly due to the proximal location of this well. According to Dybkjær This development shows a remarkable correlation with the incised and Piasecki (2010), this species is only recorded in offshore succes- valley in the Danish North Sea described by Møller et al. (2009) and sions. The less abundant occurrences in 6407/9-5 may likewise be Rasmussen (2017). Such a similar development calls for an allotonic due to a more proximal setting for this well compared to the three process, which could be the Messinian sea-level fall described by for Danish key-wells. example Ohneiser et al. (2015). The highest (consistent) occurrence of M. choanophorum also seems to be a useful stratigraphic marker in the study area, although it should be used with caution for correlation over longer distances. 6 | CONCLUSIONS M. choanophorum is a warm water demanding dinocyst species and its HO/highest common occurrence may indicate the time when more Five sedimentary units separated by unconformities represent the Oli- cold surface water comes into Norwegian Sea/the North Sea after the gocene to basal Pleistocene succession in the well 6407/9-5. These warm interval in the Early Pliocene. This interpretation is further units have been dated using dinocyst stratigraphy supported by seismic supported by a concurrent income of abundant coldwater tolerant data and sequence stratigraphy. The new data support the previous dinocysts, especially F. filifera. interpretations indicating the presence of a Rupelian (Lower Oligocene) The somewhat impoverished dinocyst assemblages seen in succession (up to 803 m) and an Aquitanian/Burdigalian (Lower Mio- the Tove-1 and Vagn-2 wells from the base of the Zanclean cene) succession (803–787 m) below the Molo Formation. The dating (Lower Pliocene) and upwards possibly reflect the changes in the of the Molo Formation is here specified to be composed of two inter- depositional regime from an offshore, fully marine setting to a vals, an upper Tortonian (Upper Miocene) interval from 787 to 724 m setting characterized by progradation of more coarse-grained referred to the H. obscura dinocyst Zone and a Zanclean (Lower Plio- units from the south-southeast (the Eridanos Delta) and cene) interval from 724 to 703 m referred to the M. choanophorum increased freshwater influx. dinocyst Zone. Both of these zones are defined in the Danish sector of the North Sea. In well 6407/9-5 these two intervals are separated by an unconformity, probably comprising the uppermost Tortonian and 5.4 | Allotonic processes the Messinian. The data presented here resulted in a revision of the upper boundary of the Molo Formation, which moved the boundary The Neogene basinal history of the Norwegian Sea shelf and the east- downwards from 670 to 703 m. The interval from 703 to 670 m, previ- ern North Sea are quite different (Figure 9a,b). However, some major ously referred to the Lower Pliocene and included in the Molo Forma- events can be recognized in both areas. The basal Miocene unconfor- tion, is here interpreted to be of Gelasian (Early Pleistocene) age. This mity is present in both areas (Eidvin, Riis, & Rasmussen, 2014; interval is thus suggested to be part of the Naust Formation. Grøsfjeld et al., 2019; Rasmussen, 2017; Rasmussen et al., 2010; Furthermore, the lower part of the Molo Formation in the well Tsikalas et al., 2019) and is interpreted to be a regional tectonic event, 6610/3-1, recently dated as Tortonian, is here interpreted to correlate the Savian Event (Knox et al., 2010; Pharaoh et al., 2010). In the east- with the H. obscura Zone, as suggested for the 6407/9-5 well. This is ern North Sea Basin, a relatively large fluvio-deltaic system developed based on a combination of the dinocyst assemblage referred from this during the Early Miocene. During the Middle Miocene this delta was interval in well 6610/3-1 and seismic data from the areas represented drowned as a result of accelerated subsidence of the North Sea Basin by these two wells. and consequently a deeper marine environment with deposition of fine-grained sediments was established. ACKNOWLEDGEMENTS On the Norwegian Sea shelf, deeper marine mud was deposited Financial support from the Norwegian Petroleum Directorate (NPD) is during the Early Miocene (Eidvin, Riis, & Rasmussen, 2014; Løseth greatly appreciated. The authors are also very grateful to Total for et al., 2017), while the Middle Miocene was characterized by erosion, permission to publish the data from the Vagn-2 well. Annette Ryge either caused by uplift and/or by deep water bottom currents. In the and Charlotte Olesen are thanked for processing the samples and Late Miocene, a marked progradation occurred in both basins Annabeth Andersen and Jette Halskov are thanked for drawing the (Grøsfjeld et al., 2019; Henriksen & Weimer, 1996; Løseth & figures. Sverre Henriksen is thanked for his valuable comments to an Henriksen, 2005; Rasmussen et al., 2005). The Tortonian sedimentary early version of the manuscript. The two reviewers, Dirk Munsterman packages are in both places characterized by a progradational unit and Lucy Edwards, are thanked for their very helpful suggestions and followed by a progradational-aggradational unit (Figure 9a,b). The corrections to the manuscript. lower boundary of the latter unit represents a sequence boundary, F2, defined in Rasmussen (2017). The development of this sequence, F2, ORCID where the progradational-aggradational unit is succeeded by Karen Dybkjær https://orcid.org/0000-0002-8420-3379 progradational unit and in turn followed by a down stepping Kasia K. Śliwinska https://orcid.org/0000-0001-5488-8832 1722 DYBKJÆR ET AL.

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