Cenozoic sequence stratigraphy of the central and northern North Sea Basin: tectonic development, sediment distribution and provenance areas

Marine and Petroleum Geology, Vol. 12, No. 8, pp. 845-879, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0264-8172/95 $10.00 + 0.00

E~UTTERWORTH I'~IE I N E M A N N

Cenozoic sequence stratigraphy of the central and northern North Sea Basin: tectonic development, sediment distribution and provenance areas Henrik Jordt*, Jan Inge Faleide, Knut Bjerlykke and Maged T. Ibrahim University of Os/o, Department of Geology, PO Box 1047, B/indem, N-0316 Os/o, Norway Received 15January 1995; revised 1May 1995; accepted 9 May 1995 The Cenozoic succession in the central and northern North Sea has been investigated to establish a regional sequence stratigraphic framework. Changes in sediment distribution indicate a complex pattern of regional vertical movements along older Palaeozoic and Mesozoic structures in Cenozoic times. These vertical movements, mainly related to tectonic processes along the continental margin to the north and north-west, were responsible for the generation and removal of provenance areas for sediments delivered to the North Sea basin through Cenozoic time. During the early through middle Palaeogene, sediments were mainly sourced from areas in the west and from the Atlantic margin in the north. Uplift in the northern North Sea in the late Eocene was, in the earliest, Oligocene followed by marked basin subsidence and tectonic uplift of southern Norway. A similar pattern of tectonic movements resulted in subaerial exposure of the northern North Sea in the early Miocene, followed by significant tectonic subsidence in the basin and along the Atlantic margin in late Miocene-Pliocene times. At the same time, southern Norway was uplifted and became a major sediment source. After the Miocene-Pliocene subsidence-uplift events, glacial processes in southern Norway and fluvial processes in a large drainage area east and south-east of the North Sea were responsible for the main sediment influx to the North Sea. The Cenozoic depositional sequences in the North Sea developed in close interaction with regional tectonic movements and changes in provenance areas. Tectonic movements in north-west Europe overprinted global sea-level changes, so that the generation of depositional sequences and sequence boundaries apparently occurred independently of the rate of eustatic sea-level change. Keywords: Cenozoic sequence stratigraphy; outbuilding directions; subsidence-uplift patterns; sea-level variations

The sequence stratigraphic concept has received considerable attention and acknowledgement in recent years. The present paper concerns the Cenozoic sequence stratigraphic development of the North Sea Basin (Figure 1) and it is based on the sequence stratigraphical concepts established by Mitchum and Vail (1977) and Mitchum et al. (1977). We provide a regional sequence stratigraphic study with conceptual and regional significance. The main objectives are to document a sequence stratigraphic framework and to interpret the geological history in terms of provenance area, relative tectonic movements and sea-level changes for post-Danian sediments in the North Sea. We have compared the ages and occurrences of our sequences with glacioeustatic sea-level changes and will demonstrate that tectonic processes played a dominant part in the development of depositional sequences and sequence * Correspondence to D r H. Jordt

boundaries, and that this development is intimately related to events along the north-west European continental margin. The North Sea Basin is an intra-cratonic basin situated to the north-west of the European Platform (Figure 1). The Norwegian North Sea comprises several major structural elements (Figure 2). These are the northern Central Graben, the Viking Graben, the Norwegian-Danish Basin and the Horda Platform. Several fault zones and alignments were important for the Cenozoic development: the Fjerritslev Fault, defining the continuation of the Tomquist Zone; the Highland Boundary Fault Alignment and its northeastward continuation defined by the Patchbank Ridge and the onshore Caledonian Deformation Front; the ~ygarden Fault Zone; the faults defining the western boundaries of the Viking and the Central graben; and the Hitra Fault Alignment defining the northern boundary of the present North Sea. Since crustal accretion terminated during Early

Marine and Petroleum Geology 1995 Volume 12 Number 8

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Cenozoic sequence stratigraphy of the North Sea Basin: H. Jordt et al.

Major river system] I:igute 1 Regional map of the North Sea region with main structural elements. The present study is outlined with a solid line and the study area of Michelsen etal. (in press) with a broken line. Abbreviations: HBFA, Highland Boundary Fault Alignment; and EVC, Erlend Volcanic Complex. After Catliff et aL (1984), Dor~ and Gage (1987), Gibbard (1988) and Herngren and Wong (1989)

Devonian time, the North Sea area has been dominated by several episodes of extensional tectonism (Devonian, Permian-Early Triassic, Late JurassicEarly Cretaceous). Most of the rift topography was filled by the end of the Cretaceous. In general, the Cenozoic evolution of the North Sea Basin was characterized by regional subsidence and infill from the uplifted areas to the west and east. Conventional multi-channel seismic reflection profiles ( - 1 5 000 km) comprise the main part of the database (Figure 2). Biostratigraphic data and well logs from more than 30 wells are integrated to correlate the seismic data with the chronostratigraphic time-scale. The study was carried out as a part of the IBS-DNM project (Integrated Basin Studies - - the Dynamics of the Norwegian Margin). This extensive study involves geological and geophysical interpretation and forward and inverse modelling of the geological succession from the Moho to sea level, carried out in close and fruitful co-operation between European and Norwegian universities and research institutions as well as participants from the Norwegian petroleum industry. Exchange of scientific results has been a main objective for the IBS-DNM project. Two common regional seismic transects were used to transfer information between the studies of deep structures, the syn-rift and post-rift sequences and the inverse and forward modelling studies (Figure 3). One of the earliest examples of regional sequence stratigraphic mapping and interpretation from the

846

Tertiary North Sea Basin was presented by Vail et al. (1977a; 1977b). Based on geophysical well logs Nielsen et al. (1986) subdivided the Cenozoic succession in the North Sea into eight stratigraphic units. Seismic and log data were used to subdivide the Palaeocene and earliest Eocene deposits in the British North Sea into 10 depositional sequences by Stewart (1987). The ages of Stewart's sequences were revised and the sequence stratigraphic framework was extended into the Middle and Upper Eocene by Jones and Milton (1994). In the northern North Sea, Rundberg (1989) used seismic data and well logs to subdivide the Cenozoic into eight map units. Galloway et al. (1993) subdivided the post-Danian to Miocene deposits in the central and northern North Sea Basin into tectono- and depositional sequences based on an open seismic grid and a comprehensive database of well logs. In a regional seismic study, Ibrahim (1993) divided the Upper Mesozoic and Cenozoic into eight sequences. Using seismic, log and new biostratigraphic data, Michelsen et al. (1995) subdivided the Tertiary and Quaternary succession in the Danish North Sea and the adjacent parts of the Dutch, German and Norwegian areas into seven major sequence stratigraphic units constituting 21 depositional sequences and correlated these with outcrops and wells in Denmark. We provide a coherent stratigraphic framework that extends from the Atlantic margin in the north to the central Dutch North Sea in the south, and we have compared and incorporated results from previous

M a r i n e a n d P e t r o l e u m G e o l o g y 1995 V o l u m e 12 N u m b e r 8

Cenozoic sequence stratigraphy of the North Sea Basin: H. Jordt et al.

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studies. The model for the generation of depositional sequences (Vail et al., 1977a; 1977b; Posamentier et al., 1988) and the Cenozoic part of the eustatic sea-level curve in Haq et al. (1987; 1988) are to a large extent based on investigations of the Tertiary in the North Sea. Their interpretations are based on the assumption that relative changes of coastal onlap are mainly caused by global sea-level changes. Because we have inte-

grated multi-disciplinary data from our own study area with data from other studies in the North Sea, we have been able to extract the influence of tectonic movements on the development of depositional sequences. Therefore, our results have important implications for the use of sequence stratigraphy concepts and for the understanding of the development of depositional sequences. Our results are also important for the

M a r i n e and Petroleum G e o l o g y 1995 V o l u m e 12 N u m b e r 8

847

Cenozoic sequence stratigraphy of the North Sea Basin: H. Jordt et al.

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Figure 4

understanding and modelling of oil generation and migration in reservoirs at deeper stratigraphic levels.

Sequence division and chronostratigraphy The post-Danian succession in the study area has been subdivided into ten main Cenozoic Seismic Sequences (CSS-1-CSS-10) by correlation and calibration to biostratigraphic studies of Norwegian, Danish and

848

British wells undertaken by Steurbaut et al. (1991), Eidvin et al. (1991), Eidvin and Riis (1992), Eidvin et al. (1993), Gradstein et al. (1994), Van Veen et al. (1994), Mudge and Bujak (1994) and Michelsen et al. (1995) (Figure 4). Direct ties between our database and the seismic database and sequence stratigraphic framework of the study area of Michelsen et al. (1995) was established for the base of CSS-1, CSS-2, CSS-3, CSS-5, CSS-6 and CSS-7 (Figure 1).

Marine and Petroleum Geology 1995 V o l u m e 12 N u m b e r 8

Cenozoic

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Figure 4 Sequence stratigraphic correlation scheme. The ages of the CSS sequences are based on different kinds of fossil zonations, which are correlated with the nannofossil zonation of Martini (1971). The discrepancy between the top CSS-1.2 and the top Balder sequence of Mudge and Bujak (1994) is caused by a slightly younger age indicated by the biostratigraphic age determination of the Balder Formation in the British North Sea

The biostratigraphic zonation is based on analyses of calcareous nannofossils, dinoflagellates and foraminifera, and the zonation scheme for the calcareous nannofossils established by Martini (1971) is used as a common reference for the stratigraphic correlation. This means that changes in the age of the nannofossil zones may affect the age of the sequences; however, such changes will probably neither affect the correlations between sequences from the different studies nor the correlations to plate tectonic events presented later in the paper. We use the time-scale of Harland et al. (1990) to indicate absolute ages for the sequences. Different definitions are used for the base Quaternary in the North Sea. The base of the Quaternary is generally placed at a time level of 2.3 Ma in the area (Zagwijn, 1989) but, in this paper, we use the younger time level at 1.6 Ma as proposed by Harland et al. (1990). This means that we use the Tiglian-Eburonian transition as the base Quaternary.

Marine

CSS-1 is of Late Palaeocene-earliest Eocene age (Figure 4). Lithostratigraphically, it correlates with the Rogaland Group. It rests on the Danian chalk deposits to the south and Maastrichtian in the north. CSS-1 has been subdivided into two sub-sequences: CSS-1.1 and CSS-1.2. No direct tie of the boundary between these sub-sequences has been established to wells with biostratigraphic data, but lithostratigraphic correlation has been carried out in several wells. It appears that the sequence boundary between CSS-1.1 and CSS-1.2 correlates with the boundary between the Lista and Sele formations. CSS-2 is the Eocene succession above the Balder Formation. The base of CSS-2 correlates lithostratigraphically with the base of the Hordaland Group. CSS-2 has been subdivided into two sub-sequences: CSS-2.1 and CSS-2.2. Based on the datings in well 3501-l and 34/4-l (Steurbaut et al., 1991), an early Middle Eocene age is suggested for the sequence boundary between CSS-2.1 and CSS-2.2.

and Petroleum

Geology

1995 Volume

12 Number

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Cenozoic sequence stratigraphy of the North Sea Basin: H. Jordt et al. CSS-3 is of Early Oligocene age. The lower sequence boundary correlates with the Eocene-Oligocene transition. The datings suggested by Steurbaut et al. (1991) in wells 35/11-1 and 34/4-1 indicate a late Rupelian age for the upper boundary of CSS-3. No direct seismic tie was established with the Danish study area in the central North Sea (Michelsen et al., 1995), but we have correlated the top of CSS-3 with the top of their sequence 4.2 based on a similar marked change from progradation to aggradation across the upper boundaries of these sequences. The base of CSS-3 probably correlates with the Pyrenean unconformity of Ziegler (1982; 1990). CSS-4 is of late Early-latest Oligocene age. In well 34/8-1 a Late Oligocene age is indicated for the upper sequence boundary (Eidvin and Riis, 1992; Gradstein and Bfickstr6m, unpublished data) and in wells 34/4-1 and 35/11-1 it correlates with a hiatus covering a major part of Early-Middle Miocene times (Steurbaut et a/.,1991). The base of CSS-4 correlates with the marked mid-Oligocene eustatic sea-level fall which is indicated by Vail et al. (1977a; 1977b) in the upper part of nannofossil zone NP23, and so it correlates with the second-order cycle boundary between supercycle TA4 and TB1 on the Cenozoic chronostratigraphic and eustatic cycle chart of Haq et al. (1988). CSS-5 is of latest Oligocene-earliest Miocene age. A 30 m thick unit of this age is found in well 34/8-1 (Eidvin and Riis, 1992; Gradstein and Bfickstr6m, unpublished data). Where the overlying sequence CSS-6 is absent, the top of CSS-5 is correlated with the top of the Hordaland Group. CSS-6 is of late Early-early Middle Miocene age. The sequence is dated in well 2/4-B-19 (Eidvin et al., 1993) and 2/2-1. A hiatus correlating with CSS-6 is found in well 34/8-1 (Eidvin and Riis, 1992; Gradstein and Bfickstr6m, unpublished data). The upper sequence boundary correlates with the top Hordaland Group. CSS-7 is of late Middle-Late Miocene age. The sequence is dated in 2/4-B-19, (Eidvin et al., 1993), and 34/8-1 and 34/4-1 (Eidvin and Riis, 1992). The upper boundary corresponds to the Miocene-Pliocene transition. The base of CSS-7 correlates with the base of the Nordland Group. CSS-8 is of Pliocene age, including Praetiglian and Tiglian time. The sequence is present in all wells used for age dating, although a discrepancy apparently exists. Eidvin and Riis (1992) have not registered Lower Pliocene sediments in 34/8-1 and 34/4-6, whereas biostratigraphic evidence for both the Lower and Upper Pliocene is found in the same area by Steurbaut et al. (1991) and Gradstein and Bfickstr6m (unpublished data). CSS-9 is of Early Quaternary age (Eburonian?Menapian) and CSS-10 is of Middle-Late Quaternary age (?Menapian-present). In well 2/4-B-19, the base Quarternary at 1.6 Ma (Eidvin et al., 1993) correlates with the base of CSS-9. The upper CSS-10 sequence boundary is represented by the seafloor. Age information has not been available for top CSS-9-base CSS-10, thus the chronostratigraphic relationship between these sequences is uncertain. The oldest crystalline rock fragments from the Scandinavian area found in the Baltic river system are from late Menapian

850

time (Bijlsma, 1981) and coevally a change from marine to subglacial facies occurred in the central North Sea (Cameron et al., 1987). A contemporaneous hiatus dated to ~0.9 Ma is present in the Netherlands (Zagwijn, 1989). CSS-9 is dominated by marine clay, whereas CSS-10 is dominated by sand; therefore, we correlate the lithological change between CSS-9 and CSS-10 with the climatic facies change reported by Cameron et al. (1987), which apparently represents a regional hiatus and suggests an age of approximately 0.9 Ma for the CSS-9-CSS-10 boundary. The dating of the main seismic sequences CSS-1 to CSS-10 is based on several types of biostratigraphic zonations and each set of fossil group zonations has its own age interpretation with possibilities for uncertainties when comparing different fossil groups. In addition to this, age determinations of sequence boundaries in each of the investigated wells are influenced by biostratigraphic resolution and by uncertainties related to the depth conversion of the seismic two-way travel times. We have observed that regional seismic mapping and high resolution biostratigraphic data in many instances complement each other. The seismic data indicate that CSS-6 pinches out northward and it is missing in the northern North Sea; this interpretation is supported by the biostratigraphy, which indicates a hiatus correlating with CSS-6. Eidvin et al. (1991) examined seven wells in the northern North Sea to date the sediments above and below the angular discordance, which defines the base of CSS-10, and they concluded that it defines the Pliocene-Pleistocene transition. In addition, they note that the lowermost Pleistocene appears to be absent, in agreement with our seismic interpretation. The oldest sediments within CSS-10 mapped at the Horda Platform are probably older than 1.1 Ma (Sejrup et al., in press), hence the boundary between CSS-9 and CSS-10 may be older than 900 ka, or more probably CSS-9 is present on the platform. We do not think that new age information will change the suggested ages for the established sequences significantly, nor do we think that it will change our interpretation of the general geological development presented here. However, new age data may provide better constraints on some of the tectonic events.

Controls on sequence development: methodology The mapped sequences are separated by seismic unconformities, defined by reflection terminations, by differences in the amplitude and frequency spectra of adjoining sequences or by regionally continuous reflections. The nature and genesis of such unconformities are under debate (Vail et al., 1977a; 1977b; Vail, 1987; Cloetingh and Kooi, 1989; Einsele, 1989; Galloway, 1989; Galloway et al., 1993; Van Wagoner et al., 1990). The key point in this debate is how to discriminate between sea-level changes controlled by eustasy and sea-level fluctuations controlled by tectonics. Relative sea-level changes in the North Sea basin have been related to eustatic sea-level changes (Vail et al., 1977a; 1977b), variations in intra-cratonic stresses (Cloetingh and Kooi, 1989), lithospheric stretching (Hall and White, 1994) and to igneous underplating (Brodie and White, 1994). The aspects

Marine and Petroleum Geology 1995 Volume 12 Number 8

Cenozoic sequence stratigraphy of the North Sea Basin: H. Jordt et al. presented here provide aims to discriminate between eustatic sea-level changes and tectonic causes of sequence development. Reflection terminations such as onlap and toplap may reflect an apparently unconformable relationship between strata generated by pinch-out of sedimentary units (Schlager, 1993; Stafleu and Schlager, 1993). It is not possible to differentiate between continental onlap, coastal onlap and marine onlap without having palaeobathymetric information directly indicated by the sediment composition (e.g. coal layers) or fossil content (Bertram and Milton, 1988; Roberts et al., 1993). Thus we have not used seismic onlap as a sea-level indicator without integrating evidence from the investigated wells. Michelsen et al. (1995) observed seismic stratification in very fine-grained marine sediments and interpreted depositional sequence boundaries situated within these. Their observations indicate that mineralogical composition and diagenesis also play an important part in seismic reflectivity. It has been shown that the acoustic impedance of pelagic carbonates is a function both of original carbonate content and of stratigraphically controlled diagenesis (Winterer, 1991). Mineralogical analyses of the Cenozoic succession in the central and northern North Sea show marked shifts in the composition of clays across sequence boundaries (Thyberg et al., unpublished data). Therefore, we think that the generation of sequence boundaries is related to changes in the sediment supply system, in contrast with Van Wagoner et al. (1990), who suggest that the sequence boundary forms independently of sediment supply.

Sediment input and progradational directions The seismic data contain quantitative information about the lateral extent, thickness, geometry and stratigraphy of the deposits. Outbuilding is indicated by progradation, onlap-downlap patterns and the overall sequence geometry. Reflection stacking and onlap and downlap patterns at the base of a sequence may reveal information about the topography and morphology of the basin floor during deposition. Progradation of clastic sediments occurs both in deep-sea settings (Reading and Richards, 1994) and on the shelf (Orton and Reading, 1993), in each case resulting in similar geometries. It may therefore be difficult or impossible to distinguish between shallow depths and water depths >200 m from seismic data alone. In both environments, however, outbuilding will generally proceed downslope towards deeper parts of the basin, generating onlap against the sequence boundary updip and basinward downlap. Aggrading reflections contain no information about outbuilding directions or the possible catchment area for the sediments. Sediment accumulation and mechanisms affecting the rate and style of basin infilling in the North Sea may have many similarities to fjord basins (Syvitski and Farrow, 1989). Outbuilding clinoforms may reach the opposite basin margin if the sediment influx is sufficiently high, resulting in distal onlap. Often, the distally onlapping reflections will comprise aggrading deposits. Outbuilding on the shelf is caused by progradation of delta systems and by progradation of the coastal

plain (Galloway, 1989). Even if indications of shelf environment exist from outcrops on land or rock samples from wells, it may be difficult or impossible to distinguish between coastal plain progradation and delta progradation (Alexander, 1989). The actual sediment transport direction may be parallel, oblique or perpendicular to the direction of progradation. In deltaic headlands it is mainly parallel to progradation and caused by sediments supplied to the delta by rivers. However, progradation between deltas is mainly caused by coast-parallel sediment transport and deposition. Because of the complexities in the sediment transport systems on the shelf, it may be difficult or impossible to interpret sedimentary environments based on reflection terminations and reflection stacking patterns. Therefore, we use the term progradation in a broad sense and do not implicitly imply (or exclude) the presence of large deltaic environments when we use this term. Internal gravitative faulting in a prograding wedge can be triggered by earthquakes. Such faulting may be seen in the landward part of a prograding complex as basinward inclined extensional faults (Galloway, 1986). In the distal part of the depocentre the faults are compressional and inclined towards the basin margin. We use internal extensional fault patterns to indicate the direction of gravitative induced sliding.

Grain size distribution and clinoform geometry The geometry of individual sequences and sequence stacking patterns may contain information about grain size and depositional environment. The size and relief of the catchment area, the shape, gradient and long-term stability of the shoreline, the width of 'shoreface' and the gradient of the subaqueous delta front are, among several other parameters, related to the grain size of the depositional system (Orton and Reading, 1993). A common tendency is that steep gradients tend to reflect coarse-grained sediments, and great lateral extent tends to reflect fine-grained deposits. However, coarse-grained sediments do not always have steep gradients and fine-grained sediments do not always have a great lateral extent. A facies change from fine-grained marine sedimentation to coastal coarse-grained progradation commonly reflects tectonic activity in the source area (Orton and Reading, 1993). We suggest that a shift from an aggrading to a prograding stacking pattern caused by an increase in the sediment input rate and grain size may reflect tectonic uplift in the provenance area.

Cenozoic geological development The sediments in the North Sea fundamentally built out from two catchment areas during the Cenozoic (Figure 1). The Palaeogene clinoforms building out from westerly and north-westerly directions were mainly sourced from provenance areas along the Atlantic continental margin, e.g. the East Shetland Platform and the British Highland (Stewart, 1987; Milton et al., 1990; Galloway, 1993; Mudge and Bujak, 1994). The sediments building out from easterly directions may have had a larger and probably at times a more distant source area. The Scandinavian region was probably an

Marine and Petroleum Geology 1995 Volume 12 Number 8

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Cenozoic sequence stratigraphy of the North Sea Basin." H. Jordt et al.

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852

Marine and Petroleum Geology 1995 Volume 12 Number 8

Cenozo~ sequence stratigraphy of the North Sea Basin: H. Jordt et al. important provenance area in the Neogene (Spjeldn~es, 1975; Bijlsma, 1981); however, sediments from other parts of north-western Europe were also delivered to the North Sea (Gibbard, 1988). Scandinavia's role as a source area in the Palaeogene is more uncertain as almost all sediments from this time were removed from the region during the Late Pliocene and Quaternary glaciations.

Upper P a l a e o c e n e - l o w e r m o s t CSS-1

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(Figure 5). The main depocentre is situated outside Sognefjorden, achieving a thickness of >800 ms TWT and inclined to the west. This deposit is characterized by eastward dipping internal faults that apparently terminate against the upper sequence boundary and pinch out against the lower boundary (Figure 6). The other large depocentre which is located in the Viking Graben area, achieving a thickness of >500 ms TWT, built out eastwards from the margin of the East Shetland Platform and through the Moray Firth Basin (Figure 5). A minor depocentre built out from the north-east in the Norwegian-Danish Basin. On sections along the Norwegian coast CSS-1 is subdivided into two sub-sequences CSS-I.1 and CSS-I.2 (Figure 6). The lower sub-sequence CSS-I.1 that built from the east during the Late Palaeocene has a progradational stacking pattern and thickens pronouncedly eastward. The overlying uppermost Palaeocene-lowermost Eocene CSS-1.2 is an aggrading sequence with an even sediment thickness of 1-200 ms TWT. Its thickness increases slightly west of the underlying CSS-I.1 depocentre. The thickness distribution of CSS-1 indicates that deep marine conditions existed along the Central and Viking graben and towards the continental margin in the north. A similar interpretation was made by Gradstein et al. (1994) based on biostratigraphic data; they suggested that the sea level exceeded 800 m in the deepest parts of the basin during Late Palaeocene times. Shallow marine conditions probably prevailed along the basin margins in the east and west. The

position of the main depocentre and the westward progradation of CSS-I.1 indicates that sediments were delivered from the Sognefjorden area from a Scandinavian provenance area. Although CSS-I.1 has been subjected to later erosion (Figure 6), the present external geometry and lithology indicate that it was deposited close to sea level. A relative sea-level rise in latest Palaeocene time is indicated by aggradation and the even sediment thickness of CSS-1.2 and by the marine Balder Formation in the upper part of CSS-1.2. If such a sea-level rise was caused by a eustatic rise, then an eastward (i.e. landward) migration of the CSS-1.2 depocentre relative to the underlying CSS-I.1 and continued progradation from the east should be expected; however, the opposite is observed. Furthermore, a eustatie rise in CSS-1.2 time cannot explain the observed extensional fault pattern within CSS-1. In the distal part of a prograding complex, compressional faults may develop due to gravitative instabilities (Galloway, 1986). As the faults in CSS-I.1 are extensional, they indicate marked tectonic subsidence to the east during earliest Eocene times. The pyroclastic deposits found within the Balder Formation were generated by explosive volcanism in the British and the Faeroe-Greenland Tertiary volcanic provinces (Spjeldn~es, 1975; Knox and Morton, 1988), coeval with uplift and eastward tilting of the East Shetland Platform and subsidence in the central and northern North Sea Basin (Milton et al., 1990). The position of the Erlend volcanic complex in the Faeroe-Shetland Basin (Gatliff et al., 1984; Kanaris-Sotiriou et al., 1993) and the extensive intrusive and extrusive Tertiary magmatic activity within this basin (Ridd, 1983; Smythe et al., 1983; Wood et al., 1987; Fitch et al., 1988; Hitchen and Ritchie, 1987) strongly suggest that a significant catchment area was exposed to erosion north-west of the North Sea in Late Palaeocene time. The sand content of the Balder Formation in the north-eastern North Sea (Rundberg, 1989) may indicate the presence of a Devonian source rock in the Scandniavian area.

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Marine and Petroleum Geology 1995 Volume 12 Number 8

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Cenozoic sequence stratigraphy of the North Sea Basin: t4. Jordt et al.

Eocene (CSS-2)

Michelsen et al., in press), and the regional thinning of CSS-2.2 on the Horda Platform and further south, may suggest that the Scandinavian topography was too limited to supply sufficient clastic material and to allow progradation from the east, so that large parts of Scandinavia, east of the study area, may have been submerged in Middle-Late Eocene times. The thickness increase of CSS-2 towards the Atlantic continental margin (Figure 7) indicates sediment supply from that direction, and it also indicates that the basin floor had a dip towards the south or south-east. This means that the depositional environment in the northern North Sea changed from pelagic deep marine deposition to sedimentation controlled by clastic input from the uplifted Atlantic continental margin in the north-west. At the V0ring margin off Mid-Norway, dynamic uplift created a main western source area for the Palaeogene V0ring basin sediments (Skogseid and Eldholm, 1989; Stuevold et al., 1992). Therefore, we suggest that this Early Eocene regional uplift along the Norwegian continental margin was caused by processes related to the opening of the North Atlantic ocean. Absence of the upper Eocene in the North Sea (e.g. Gradstein et al., 1994) indicates that the tectonic uplift along the Atlantic margin, that was initiated in the Early Eocene, continued throughout Eocene times, and that the uplifted areas extended laterally into the northern North Sea and caused shallow marine conditions or emergence in Late Eocene time. Further south is a complete deep marine Eocene succession preserved in the Norwegian Danish Basin and in the Central Graben (Heilman-Clausen et al., 1985). A possible causal link between uplift along the continental margin in the north and tectonic subsidence in the south has to be studied in further detail.

CSS-2 is mainly derived from the west and is characterized by >600 ms TWT thick depocentres along the Viking and the Central graben (Figure 7). On seismic sections CSS-2 is subdivided into two sub-sequences (Figure 8), CSS-2.1 dominating the isopachs in the south-east and CSS-2.2 dominating in the west. CSS-2.1 built out from both easterly and westerly directions in the early Eocene, whereas CSS-2.2 (Middle-Upper Eocene) mainly built out from the west and north-west. A cyclic succession of upward thickening Lower Eocene sand units interrupted by thin clay intervals north of the Horda Platform (Rundberg, 1989) indicates that CSS-2.1 prograded out as coastal plain or deltaic deposits, suggesting a Scandinavian source to the north-east. In the west, stratigraphic equivalents to CSS-2.1 (Figure 4), Frigg I and II, consist of stacked, thickly bedded immature sands with interbeds of clayand siltstone at the margin of the East Shetland Platform and in the Moray Firth Basin and generally of mudstones in the Viking Graben and Central Graben (Mudge and Bujak, 1994). The opening of the Norwegian-Greenland Sea and the termination of volcanism in the British area at the Ypressian-Lutetian transition (Knox and Morton, 1988) occurred at the CSS-2.1-CSS-2.2 transition. The dominance of fine-grained Middle-Upper Eocene deposits in eastern North Sea wells (Steurbaut et al., 1991; Rundberg, 1989; Mudge and Bujak, 1994;

The lower boundary of CSS-3 is recognized by high amplitude reflections and by a shift in seismic reflection pattern from the underlying Eocene sequence (Figure 9). It is weakly defined in the Viking Graben, but is characterized by more continuous reflections along the margins and is there easier to identify. Remobilization of the sequence in the Stord Basin has reduced the continuity of the seismic reflections and, in parts of this area, it is difficult to separate CSS-3 from the overlying CSS-4. However, with the available biostratigraphic data and additional infilling seismic lines it has been possible to map the general sediment distribution of importance for interpretation. The sediments in a >300 ms TWT thick elongated depocentre built out from the east at the Horda Platform and a >600 ms thick deposit prograded from the north into the Norwegian-Danish Basin with a steeply inclined depocentre front (Figure 10) (figure 2 in Jordt, in press). The lower Oligocene deposits in the Norwegian Danish Basin consist of sand and sandstones with scattered rock fragments (Strass, 1979). Outbuilding from the west along the margin of the East Shetland Platform continued, resulting in the development of a >600 ms thick depocentre in the northern Viking Graben. CSS-3 is absent north and north-west of the Horda Platform, probably due to

Milton et al. (1990) have shown that in the Late Palaeocene, one tectonic cycle encompassed in the central North Sea; however, uplift of the East Shetland Platform continued contemporaneously along the Atlantic margin. CSS-I.1 built from the east and west during the uplift phase, and CSS-1.2 transgressed the basin margins during subsidence (Figure 6). Thus this tectonic cycle resulted in similar evolutionary patterns in both the eastern and western North Sea. The outbuilding observed in the Norwegian-Danish Basin shows that the seafloor had a dip towards the Central Graben during the deposition of CSS-1 (Figure 5). Although marine conditions prevailed in the Scandinavian area in CSS-1.2 time, not all parts of southern Norway may have been submerged. The fossil content in the Danish Fur Formation indicates that the lowermost Eocene sediments (i.e. CSS-1.2) were deposited relatively close to a coastline in the Norwegian-Danish Basin (Spjeldn~es, 1975), so that erosional products from exposed areas on the Fennoscandian Shield may have contributed to progradation. The fossil content and the lithology indicates that the Upper Palaeocene-Lower Eocene strata in Denmark (Heilmann-Clausen et al., 1985), central and northern Sweden (Cleve-Euler, 1941; Brotzen, 1948; 1960) and Finland (Hirvas and Tynni, 1976) were deposited in a deep marine environment, hence large parts of Scandinavia were probably deeply submerged by the end of CSS-1 time. Thus the end of sediment supply from the east off the Sogne Fjorden area indicates that the area north of the Caledonian Deformation Front was subjected to a marine transgression that caused a significant reduction in the sediment supply to the North Sea.

854

Lower Oligocene (CSS-3)

Marine and Petroleum Geology 1995 Volume 12 Number 8

Cenozoic sequence stratigraphy of the North Sea Basin: H. Jordt et al.

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Marine and Petroleum Geology 1995 V o l u m e 12 N u m b e r 8

Cenozoic sequence stratigraphy of the North Sea Basin: H. Jordt et al. subaerial erosion, and it is also eroded along the western margin of the basin. West of the Oygarden Fault Zone, CSS-3 is thin along a narrow belt, probably indicating that this area was uplifted during or after deposition of the sequence. South-east of the western depocentre, CSS-3 is dominated by large, up to 300 ms TWT, local thickness variations caused by remobilization of clays in the sequence. The lower sequence boundary appears nearly unaffected by the internal movements (Figure 9). The concentration of CSS-3 sediments in the centre of the northern North Sea basin and contemporaneous erosion on the flanks indicate an increased topographic basin relief (Figure 10), which was probably caused by uplift along the eastern and the western basin margins and coeval basin subsidence in the early Oligocene. In the Norwegian-Danish Basin, the marked shift from the late Eocene deep water sedimentation to shore progradation in the earliest Oligocene indicates rapid tectonic uplift of southern Norway. Thus the marked glacio-eustatic sea-level fall that was caused by expansion of ice sheets in Antarctica in the earliest Oligocene and reported by Miller et al. (1987) and Zachos et al. (1992) was overprinted by local vertical tectonic movements in the North Sea area. The thickness of CSS-3 indicates that water depths in the central North Sea may have exceeded 600 m in the early Oligocene. Separation of north-east Greenland and Svalbard was initiated at the Eocene-Oligocene transition (Myhre and Eldholm, 1988). At the same time, a north-south oriented compressional stress regime was generated on the European platform by strong AfricaEurope convergence (Bergerat, 1987; Dewey and Windley, 1988) and by the opening of the North Atlantic (Biddle and Rudolph, 1988). West of the study area, the British Isles were uplifted and minor inversion occurred in the central North Sea (Pegrum and Ljones, 1984; Biddle and Rudolph, 1988). Koii et al. (1989) have shown that intra-plate compression may have a significant effect on basin stratigraphy. Hence the early Oligocene North Sea basin relief and stratigraphy were probably influenced by the compressional stress regime, but van der Beck (1995) has shown that this mechanism does not provide a satisfactory explanation for the late Palaeogene-Neogene uplift pattern in southern Norway.

the Norwegian-Danish Basin in CSS-4 time (Figure 11). CSS-4 thins towards the north-west along the Hitra Fault Alignment and it is eroded and overlain by CSS-10 along the Oygarden Fault Zone. CSS-4 is commonly underlain by Lower Oligocene CSS-3 sediments, with the exception of the areas along the Atlantic continental margins and over the Ling Depression. In those areas, it is probably underlain by the Eocene CSS-2. CSS-4 sediments are more uniformly distributed than underlying sequences and the sequence is dominated by even aggrading reflections, suggesting a reduced basin relief and an increased accommodation space. The presence of CSS-4 above lower Oligocene CSS-3 deposits, both in the northern North Sea and in the Norwegian-Danish Basin, reflect a regional marine transgression of the basin margins (Figures 3 and 11). The end of sediment supply from the east on the Horda Platform suggests that a marine transgression towards the east probably occurred in the same area that was also transgressed in earliest Eocene times. A reduced basin topography can also explain the observed onlap and increase in accommodation space reported by van Veen et al. (1994). The CSS-4 depocentre on the Patchbank Ridge (Figures 11 and 12) reflects outbuilding towards the north-west from the Stavanger Platform, suggesting that this area was not influenced by the regional transgression. The absence of underlying CSS-3 sediments along the Atlantic margin and over the Ling Depression reflects tectonic uplift before CSS-4 time. It is probable that such uplift was related to the mid-Tertiary transpressional phase along the extension of the Tornquist Zone suggested by Pegrum (1984) and Pegrum and Ljones (1984). The change in outbuilding direction towards the south-west and the change from progradation to aggradation in the Norwegian-Danish Basin (Figure 13) apparently occurred contemporaneously with the change to a south-west-north-east extensional stress regime at the European Platform reported by Bergerat (1987). Cloetingh and Kooi (1989) have shown that such a change may influence the basin stratigraphy and cause margin subsidence relative to the basin centre. Thus the shift in stacking pattern from CSS-3 to CSS-4 coevally with the change in stress regime indicates that variations in the intra-continental stress pattern affected the development of CSS-3 in the middle Oligocene.

Upper Lower-Uppermost Oligocene (CSS-4) Outbuilding continued from the East Shetland Platform in the northern North Sea, resulting in a regional >300 ms thick sequence characterized by lateral continuous, even reflections (Figures 9 and 10). Local thickness variations (1-200 ms TWT) in this depocentre and internal faulting suggest the remobilization of clays. The lower CSS-4 boundary appears almost unaffected by this disturbance. A sequence containing laterally continuous, even reflections built out towards the west in the northernmost part of the study area, and a >300 ms TWT thick sedimentary wedge prograded north-west along the Patchbank Ridge (Figure 11). Sediments in a >400 ms TWT thick depocentre dominated by aggrading reflections built out from the north-east in

Lower Miocene (CSS-5) The last episode of significant outbuilding from the west occurred in latest Oligocene-Early Miocene times. A >400 ms TWT thick sand-rich prograding wedge with discontinuous reflections built out into the central Viking Graben from the East Shetland Platform (Figure 14). The sequence is absent north and north-west of the Horda Platform, probably as result of erosion (Figure 15). In the east CSS-5 pinches out eastward north of the Stavanger Platform and the position of the eastern coastline is unknown as no sediments derived from Scandinavia have been observed (Figure 15). To the south this, probably shallow marine, basin was bounded by the Caledonian

Marine and Petroleum Geology 1995 Volume 12 Number 8

859

Cenozoic sequence stratigraphy of the North Sea Basin: H. Jordt et al. 2

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Marine and Petroleum Geology 1995 Volume 12 Number 8

Cenozoic sequence stratigraphy of the North Sea Basin: t4. Jordt et al. Depositional Sequences

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during the Oligocene and early Miocene, to dominantly eastern sources in Middle Miocene until the present, and it reflects a marked change in the palaeogeography around the North Sea (Figures 15 and 18). Southern Norway was uplifted and the sea trangressed into the subsiding basin, resulting in the development of the Utsira Formation. The oxygen isotope record indicates a low glacio-eustatic sea level in the Middle-Late Miocene (Miller et al., 1987). Thus the marine transgression in the northern North Sea occurred during a low glacio-eustatic sea level, indicating a significant phase of tectonic subsidence in the basin. Increased surface water circulation occurred at the mid-Norwegian Margin at 15 Ma (Poole and Vorren, 1993); this means latest in CSS-6 time, probably as a result of subsidence of the Iceland-Faeroe Ridge (Stille, 1992). However, data from ODP-leg 105 in the Norwegian-Greenland Sea indicate that overflow of the Greenland-Scotland Ridge was first reflected in deep water circulation in Late Miocene at ~7.5 Ma (Kaminski et al., 1989). Nevertheless, the subsidence and increase in surface water circulation in the North Atlantic occurred contemporaneously with the initiation of subsidence in the northern North Sea. The distribution of CSS-7 (Figure 18) and the subsidence pattern inferred for CSS-6 time suggest that a hinge line for the subsidence in the northern North Sea was located along a trend defined by the Highland Boundary Fault Alignment and the Caledonian Deformation Front.

Pliocene (CSS-8) The sediments of the CSS-8 depocentre built out westward as prograding clinoforms north of the Viking Graben with thicknesses >900 ms TWT basinward of the underlying CSS-7. A minor >400 ms TWT thick wedge built out on the Horda Platform (Figure 19). Internal reflections are truncated by CSS-10 along the upper sequence boundary (Figure 20). CSS-8 is more aggrading further west and truncation along the upper boundary is less pronounced. It is overlain by Pleistocene sediments and pinches out towards the Stavanger Platform (Figures 19 and 21). Erosion in the top of the sequence is observed on the Patchbank Ridge. The outbuilding pattern in the north indicates marked uplift in the source area and we suggest that CSS-8 mainly consists of sediments derived from southern Norway (Figure 20). The position of the main depocentre basinward of the underlying CSS-7 and the stacking pattern indicate increasingly restricted accommodation space for CSS-8 towards the east; thus basinal subsidence in the north initiated in CSS-6 time may have slowed down and/or was exceeded by the sediment accumulation rate. Distal onlap towards the East Shetland Platform reflects a pronounced basin floor topography at the onset of CSS-8 time (Figure 14) and the sediment distribution may suggest that the marine North Sea Basin evolved as a narrow elongated sea. The

Marine and Petroleum Geology 1995 Volume 12 N u m b e r 8

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Cenozoic sequence stratigraphy of the North Sea Basin: H. Jordt et al.

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Marine and Petroleum Geology 1995 V o l u m e 12 N u m b e r 8

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Cenozoic sequence stratigraphy of the North Sea Basin: H. Jordt et al. aggrading stacking pattern may suggest limited differential subsidence south of the main depocentre. The occurrence of Lower Pliocene sediments in the northern North Sea has been questioned. Steurbaut et al. (1991) found biostratigraphic indications of Lower Pliocene in wells 34/4-1, 34/10-11 and 35/11-1 in the northern North Sea. In well 34/8-1, Gradstein and B~ckstr6m (unpublished data) have found Lower Pliocene sediments above 1090 m; however, the same strata are referred to the Upper Pliocene by Eidvin and Riis (1992). The composition of biogenic components on the V0ring Plateau suggests cooling at 6 and 4.8 Ma (Locker and Martini, 1989). Maximum water depths in the North Sea were close to 300 m and depocentres were generated in the south (Cameron et al., 1993; Gradstein et al., 1994), and a moist and oceanic subtropical climate in the lowlands surrounding the southeastern North Sea prevailed in the Early Pliocene (Zagwijn, 1990). Climatic evidence from the Netherlands indicates that the west coast of Norway was also characterized by a moist climate, and that at the coast the temperature was probably not colder than at present. A prominent reorganization of spreading directions and rates at 2.5 Ma along the entire Atlantic spreading centre (Klitgord and Schouten, 1986) was followed by dramatically colder climatic conditions. Onset of extensive glacial erosion in mid-Pliocene times at 2.45 Ma is indicated by the occurrence of ice-rafted Mesozoic sediments sampled in ODP Leg 104 on the V0ring Plateau (H61emann and Henrich, 1994). In the Dutch area, the climatic change is indicated by pollen analysis, and a considerable sea level drop of 80-100 m followed the first distinctly cold conditions recorded around 2.3 Ma (Zagwijn, 1989). The tectonic subsidence of the northern North Sea basin, which was initiated in the middle Miocene, increased the preservation potential for the Lower Pliocene sediments (Figure 19) and the sea-level drop in the middle Pliocene was probably not sufficient to cause subaerial exposure and erosion in the basin; therefore, we suggest that Lower Pliocene sediments are preserved in the northern North Sea. A delta plain environment developed by the merged Baltic and Rhine river systems transported sediments northward and the marine basin may have been confined to the central and northern North Sea (Cameron et al., 1987; Gibbard, 1988; Zagwijn, 1989). The size of the delta system was similar to that of the largest deltas of the world, and it was several times larger than that of the current Mississippi (Zagwijn, 1989; Cameron et al., 1993). Strike-feeding along the coast may have caused progradation off Norway of sediments originating from the Baltic and Alpine areas in Late Pliocene times. Glacial material in the Pliocene in the northern North Sea (Eidvin and Riis, 1992) suggests that these sediments are in close proximity to the provenance area. As discussed earlier, it is unlikely that Lower Pliocene sediments are absent in the northern North Sea and the occurrence of glacial material in the CSS-8 depocentre may suggest that extensive glaciers were already present in the Scandinavian area in Early Pliocene times. However, it is also unlikely that the climate was significantly colder than at present, thus

872

the generation of glaciers must have been favoured by a significant Scandinavian topography causing precipitation of snow instead of ram and the accumulation of ice. The glacial material in the northern North Sea may have been caused by increased melting of outlets of glaciers and icebergs caused by their contact with the northward flowing, warmer freshwater originating from the river systems in the south. A similar evolutionary model for the lower Pliocene sediments on the V0ring margin is proposed by Stuevold and Eldholm (in press).

Quaternary (CSS-9 and 10) CSS-9 is found in the Norwegian Danish Basin and south of the Viking Graben (Figure 21). It is below seismic resolution in the northern North Sea, although it is probably present. The lower boundary of CSS-9 is defined by the base Quaternay in the Central Graben and the upper by truncation of CSS-10. The truncational limit of CSS-9 follows the late Weichselian ice limit proposed by Jansen (1976). A pronounced >400 ms thick depocentre is found south-west of the Ling Depression. Internally, CSS-9 is aggrading and characterized by high amplitude reflections with variable continuity that onlap towards the east and north-east and diverge in the opposite direction. A pronounced dip of the lower sequence boundary is observed in the Norwegian-Danish Basin (Figure 13). A >400 ms TWT thick CSS-10 depocentre built from the east, north of the Horda Platform (Figure 22). The position of this depocentre is controlled by faulting along the Oygarden Fault Zone (Figures 3 and 20). CSS-10 pinches out towards the Stavanger Platform and it is thin across the northern Viking Graben (Figure 22). The lower boundary of CSS-10 is a pronounced regional angular discordance, truncating all underlying CSS sequences (Figure 3). South of 60°N both CSS-10 sequence boundaries are poorly defined due to a low signal to noise ratio and noise introduced by the processing in the shallow part of the seismic data. The underlying CSS-8 appears unaffected by differential compaction (Figure 19), hence the thickness of CSS-9 indicates tectonic subsidence along the Ling Depression of the order of 500 m (~ 700 m/Ma) (Figure 21). The inclination of CSS-9 and 10 in the Norwegian-Danish Basin and the distribution of CSS-10 indicate uplift, probably along the Tornquist Zone (Figures 13 and 22). Hence an even greater subsidence rate relative to the adjacent areas in CSS-9 time may have taken place. The aggrading stacking pattern within CSS-9 indicates limited sediment input directly from the land areas in the east adjacent to the central North Sea. Thus the massive input of sediments in the south suggests that CSS-9 consists of erosional products from both the Scandinavian, the Baltic and the Alpine regions. Along the Oygarden Fault Zone the underlying Upper Palaeocene to Miocene sequences (CSS-1CSS-7) are inclined towards the west, indicating postMiocene uplift in the east (Figure 3). The Pliocene (CSS-8) and Lower Pleistocene depocentres (CSS-9) are also truncated by CSS-10, but the geometry of CSS-8 has been changed by Late Quaternary faulting

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Cenozoic sequence stratigraphy of the North Sea Basin: H. Jordt et al. along Oygarden Fault Zone. The development of the angular discordance at the base of CSS-10 is probably a combined result of uplift caused by isostatic rebound and glacial erosion. The regional extent of the basal angular discordance indicates extensive erosion before the deposition of CSS-10. In the British North Sea it is inferred that the unconformity at the top of the Aberdeen Ground Formation was generated during the low sea level in the Elsterian glacial period (Cameron et al., 1987). However, King et al. (1995) suggested that the northern North Sea was glaciated as early as 1.1 Ma, thus erosion mapped at the base of CSS-10 may have been caused by an earlier glaciation in CSS-9 time, possibly in the Eburonian.

Summary and conclusions In the Late Palaeocene, erosional products were delivered to the deep marine North Sea basin from the elevated land masses of the Shetland and Scandinavian continental platforms (Figure 23). A regional transgression of the basin margins associated with explosive vulcanism in the west at the Palaeocene-Eocene transition was followed by uplift along the Atlantic continental margin and by regional tectonic subsidence to the east and south-east (Figure 24). During the Middle and Late Eocene mainly fine-grained, smectiterich sediments generated by the erosion of basaltic lavas along the Atlantic continental margin in the north built into the basin from the East Shetland Platform. Uplift in the north or north-west resumed in the late Eocene and caused erosion or starvation in large parts of the northern North Sea. At the Eocene-Oligocene transition, southern Norway was uplifted again and coarse-grained sediments built towards the west from coastlines along the Oygarden Fault Zone and towards the south in the Norwegian-Danish Basin. Coevally with this tectonic uplift, a global sea-level fall was caused by growth of glacial ice sheets in Antarctica (Zachos et al., 1992). In mid-Oligocene time the stratal stacking pattern changed regionally from margin progradation to widespread aggradation in the basin and on the margins. Vail et al. (1977a; 1977b) interpreted this change in reflection stacking pattern as a basinward shift in coastal onlap caused by a significant eustatic sea-level fall. The oxygen isotope measurements undertaken by Miller et al. (1987) do not indicate any significant glacio-eustatic sea-level changes during the Oligocene and, in addition to this, we have not been able to document the occurrence of coastal sediments in the basin. Therefore, we suggest that the change in stratal stacking pattern was caused by a relative sea-level rise along the margins of the basin and a concomitant reduction in sediment supply to the basin centre. The coeval change from a compressional to an extensional stress regime indicated on the European Platform by Bergerat (1987) indicates that the change in the stratal stacking pattern in mid-Oligocene time was related to a shift in the intra-continental stress regime. In latest Oligocene time, uplift in the north-west caused the influx of sand-rich sediments from the East Shetland Platform to the basin. Inversion with northward increasing uplift rates affected the central

874

and northern North Sea until early Middle Miocene times and resulted in emergence and subaerial erosion. A new episode of significant basinal tectonic subsidence coevally with the uplift of southern Norway reflected by the development of a basinwide regional downlap surface was initiated in the mid-Miocene and was characterized by increasing subsidence rates towards the Atlantic continental margin. Under the moist climate that dominated the North Sea area in Pliocene time, (Zagwijn, 1989), the elevated land masses of Scandinavia favoured the precipitation of snow and the generation of glacial ice sheets, and large amounts of elastic material were glacially transported to the coastline. The other important source for the Pliocene sediments in the North Sea was the Baltic river system that entered the basin in an area from southern Denmark to the Netherlands. The drainage area for this river system probably reached from the Scandinavian and Baltic areas in the north and to the eastern Alps in the south. A new episode of glacially induced subsidence of Scandinavia was probably initiated by ice growth following the climatic deterioration at ca. 2.3 Ma, and resulted in continued marine sedimentation, probably both in the central and the northern North Sea. The Quaternary ice-caps that covered major parts of the North Sea area eroded into the underlying pre-Quaternary sediments and generated the angular discordance at base Quaternary in the northern North Sea. The last episode of relative tectonic movements in the North Sea that we have observed occurred as normal faulting along the Oygarden Fault Zone in CSS-10 time. The present geometry of the Cenozoic sequences is therefore a result of tectonic uplift through Oligocene to Pliocene times, and further uplift related to late Pliocene-Pleistocene glacial erosion and isostatic adjustments. We have observed that the generation of and the ability to recognize depositional sequences are closely related to provenance area, so that sequence boundaries separating sequences consisting of sediments originating from different source areas are easier to recognize than sequence boundaries separating sequences delivered from the same source. We have observed that sequence boundaries develop independently of grain size and that even the most marked basinward shifts in onlap (i.e. CSS-4/CSS-5 sequence boundary in Figure 13) do not necessarily reflect any sea-level changes. This means that the generation reflection terminations and internal sequence architecture may be controlled by mechanisms other than eustatic sea-level changes. Our observations suggest that the development of depositional sequence boundaries are closely related to tectonic movements and to changes in the sediment supply systems. We have observed, however, that sequence boundaries (i.e. CSS-2/CSS-3 and CSS-6/CSS-7) develop close to significant eustatic sea-level falls, but in each case regional tectonic movements, related to the development of the adjacent Atlantic continental margin, overprint the eustatic signal. This means that the development of depositional sequences is related to variations in spreading rates and directions of continental plates and to deep lithospheric processes.

Marine and Petroleum Geology 1995 Volume 12 Number 8

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Acknowledgements

References

The research was funded by the Commission of the E u r o p e a n Union and the Norwegian Research Council (NFR) in the framework of the D G XII-Joule Programme, sub-programme: Energy From Fossil Sources: Hydrocarbons, Integrated Basin Studies - - The Dynamics of the Norwegian Margin. Norsk Hydro as., Saga Petroleum as. and Statoil as. provided data and research efforts for this study. We thank PGS Nopec for permission to publish their seismic sections. We are specially indebted to those people who shared their knowledge and thoughts with us and gave valuable comments.

Alexander, J. (1989) Delta or coastal plain? With an example of the controversy from the Middle Jurassic of Yorkshire. In: Deltas: Sites and Traps for Fossil Fuels (Eds M. K. G. Whateley and K. T. Pickering), Spec. PubL GeoL Soc. London No. 41, 11-19 Bergerat, F. (1987) Stress fields in the European Platform at the time of Africa-Eurasia collision Tectonics 2, 99-132 Bertram, G. T. and Milton, N. J. (1988) Reconstructing basin evolution from sedimentary thickness; the importance of palaeobathymetric control, with reference to the North Sea Basin Res. 1,223-236 Biddle, K. T. and Rudolph, K. W. (1988) Early Tertiary structural inversion in the Stord Basin, Norwegian North Sea J. Geol. Soc. London 145, 603-611 Bijlsma, S. (1981) Fluvial sedimentation from the Fennoscandian

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bounded depositional units; II application to Northwest Gulf of Mexico Cenozoic Basin Am. Assoc. Petrol GeoL Bull. 2, 125-154 Galloway, W. E., Garber, J. L., Liu, X. and Sloan, B. J. (1993) Sequence stratigraphic and depositional framework of the Cenozoic fill, Central and Northern North Sea. In: Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference (Ed. J. R. Parker), Geological Society, London, 33-43 Gatliff, R. W., Hitchen, K., Ritchie, J. D. and Smythe, D. K. (1984) Internal structure of the Erlend Tertiary volcanic complex, north of Shetland, revealed by seismic reflection J. GeoL Soc. London 141,555-562 Gibbard, P. L. (1988) The history of the great northwest European rivers during the past three million years Phil. Trans. R. Soc. London B 317, 559-602 Gradstein, F. M., Kaminski, M. A., Berggren, W. A., Kristiansen, I. L. and D'ioro, M. A. (1994) Cenozoic biostratigraphy of the North Sea and Labrador Shelf Micropaleontology SuppL 40, 152 pp Hall, B. D. and White, N. (1994) Origin of anomalous Tertiary subsidence adjacent to North Atlantic continental margins Mar. Petrol GeoL 6, 702-713 Harland, W. B., Armstrong, R. L., Cox, A. V., Craig, L. E., Smith, A. G. and Smith, D. G. (1990) A Geologic Time Scale 1989, Cambridge University Press, Cambridge, 263 pp Haq, B. U., Hardenbol, J. and Vail, P. R. (1987) Chronology of fluctuating sea levels since the Triassic Science 235, 1156-1167 Haq, B. U., Hardenbol, J. and Vail, P. R. (1988) Mesozoic and Cenozoic chronostratigraphy and eustatic cycles. In: Sea-level Changes: an Integrated Approach (Eds C. K. Wilgus, B. S. Hastings, H. Posamentier, J. V. Wagoner, C. A. Ross and C. G. S. C. Kendall), Spec. Pub/. Soc. Econ. PaleontoL Mineral No. 42, 71-108 Heilmann-Clausen, C., Nielsen, O. B. and Gersner, F. (1985) Lithostratigraphy and depositional environment in the Upper Paleocene and Eocene of Denmark Bull GeoL Soc. Denmark 33, 287-323 Herngreen, C. F. W. and Wong, T. E. (1989) Revision of the 'Late Jurassic' stratigraphy of the Dutch Central North Sea Graben GeoL Mijn. 68, 73-105 Hirvas, H. and Tynni, R. (1976) Tertiary clay deposits at Savukoski, Finnish Lapland, and observations of Tertiary microfossils, preliminary report GeoL Geol. se/s. Finland28, 33-40 Hitchen, K. and Ritchie, J. D. (1987) Geological review of the West Shetland area. In: Petroleum Geology of North West Europe (Eds J. Brooks and K. W. Glennie), Graham & Trotman, London, 737-749 H61emann, J. A. and Henrich, R. (1994) AIIochtonous versus autochtonous organic matter in Cenozoic sediments of the Norwegian North Sea: evidence for the onset of glaciations in the northern hemisphere Mar. GeoL 121, 87-103 Ibrahim, M. T. (1993) Post-Jurassic basin fill in the northern North Sea, Cand. Scient. Thesis, Univ. Oslo, 137 pp Isaksen, D. and Tonstad, K. (1989) A revised Cretaceous and Tertiary lithostratigraphic nomenclature for the Norwegian North Sea NPD-Bull. 5, 59 pp Jansen, J. H. F. (1976) Late Pleistocene and Holocene history of the northern North Sea, based on acoustic reflection records Neth. J. Sea Res. 1, 1-43 Jones, R. W. and Milton, N. J. (1994) Sequence development during uplift: palaeogene stratigraphy and relative sea-level history of the Outer Moray Firth, UK North Sea Mar. Petrol GeoL 2, 157-165 Jordt, H. Regional Cenozoic uplift and subsidence patterns in the southeastern North Sea DGU Ser. B, in press Kaminski, M. A., Gradstein, F. M., Scott, D. B. and Mackinnon, K. D. (1989) Neogene benthic foraminifera biostratigraphy and deep-water history fo Sites 645, 646 and 647 Baffin Bay and Labrador Sea. In: Proc. ODP, Sci. Res. (Eds S. P. Srivastava et aL), 105, 731-747 Kanaris-Sotiriou, R., Morton, A. C. and Taylor, P. N. (1993) Palaeogene peralumionous magmatism, crustal melting and continental break-up: the Erland complex, Faeroe-Shetland Basin, NE Atlantic J. GeoL Soc. London 15D, 903-914 King, E. L., Sejrup, H. P., Haflidason, H., Aarseth, I. and Elverh¢i, A. (1995) The North Sea fan and outer Norwegian Channel:

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