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Sedimentary Geology 220 (2009) 60–76

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Sedimentary Geology

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Evolution and strike variability of early post-rift deep-marine depositional systems:Lower to Mid-Cretaceous, North Viking Graben, Norwegian North Sea

Anna-Jayne Zachariah a,b,⁎, Rob Gawthorpe a, Tom Dreyer b

a School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UKb StatoilHydro, Sandsliveien 90, NO-5020, Bergen, Norway

⁎ Corresponding author. StatoilHydro, Sandsliveien 90E-mail address: [email protected] (A.-J. Zachar

0037-0738/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.sedgeo.2009.06.006

a b s t r a c t
a r t i c l e i n f o
Article history:Received 10 January 2008Received in revised form 3 June 2009Accepted 9 June 2009

Keywords:Post-riftDeep-marine depositional systemsCretaceousOsebergNorth Sea

The controls and development of early-post-rift, deep-water depositional systems are poorly understooddue to their commonly deeply-buried nature. As a consequence, in the subsurface there is usually a lack ofwell penetrations and/or weak seismic imaging. At outcrop, early post-rift strata have commonly beendeformed beyond reasonable recognition by later inversion tectonics. In contrast to these systems, the NorthViking Graben shows a well-imaged Cretaceous early post-rift package with good well control and little effectfrom inversion. Therefore, this paper examines the early post-rift, deep-water depositional systems of theNorth Viking Graben to determine the controls on their stratigraphic position, geometry and evolution, andthus provide an analogue for comparable systems. Greater understanding of such systems will allow for theenhanced prediction of reservoir units in the subsurface and development of new play models since post-riftintervals are generally under-explored.

The basin configuration inherited by the Cretaceous early post-rift in the northern North Sea was set upby Permo-Triassic and Late Jurassic rifting. In the North Viking Graben this established considerable along-strike variability, resulting in a northern basin segment surrounded by steep slopes and faulted-boundedstructural highs and a southern basin segment margined by slopes with noticeably gentler gradients.Associated with the Cretaceous post-rift is an overall transgressional trend, which drowned local sourceareas, resulting in prevalent carbonate and hemipelagic mudstone deposition in the basins. In the NorthViking Graben, the uplifted Oseberg fault-block provided the sub-aerial clastic source area until it wassubmerged in the early Upper Cretaceous.

The early post-rift infill of the North Viking Graben was divided into four key seismic stratigraphic units(K1, K2, K3 and K4) using an integration of seismic and well data. Inside this stratigraphic framework, thedepositional systems within each K-unit were resolved from characteristic seismic facies, amplitudeanomalies, relationship with adjacent reflections, and geomorphologies. In the northern basin segment, theearly post-rift stratigraphy contains basin-floor fans, a channel complex and a shoreline-like geometry,whereas the southern basin segment is solely characterised by hemipelagic and carbonate deposition. Thisspatial variability indicates that one of the dominant controls on the development of the early post-riftdepositional systems in the North Viking Graben was the inherited syn-rift fault-controlled topography. Thesteep slopes bounding the northern basin segment aided the delivery of sediment from the sub-aerialOseberg source area to the grabenwhereas the submerged, gentle slopes in the southern basin segment wererelatively sediment-starved.

Long- and short-term changes in relative sea-level also heavily influenced the evolution of the early post-rift basin stratigraphy. Short-term relative sea-level fall allowed basin-floor fan emplacement whereas short-term relative sea-level stand-still favoured deposition of a channel complex. Deposition of the shoreface-likegeometry is associated with a short-term relative sea-level rise. This temporal difference in the style andscale of the depositional systems is also interpreted to reflect the gradual denudation and drowning of theOseberg source area. Regional short-term trangressive and anoxic events in the northern North Sea furtherinfluenced the early post-rift strata, resulting in the deposition of stratigraphic units that can be correlatedacross the North Sea.

© 2009 Elsevier B.V. All rights reserved.

, NO-5020, Bergen, Norway.iah).

l rights reserved.

1. Introduction

Early post-rift, deep-marine depositional systems are poorlydocumented due to the common lack of well penetrations, deeply

61A.-J. Zachariah et al. / Sedimentary Geology 220 (2009) 60–76

buried nature, and their deformation at outcrop resulting from post-depositional inversion. As a consequence, there is a significant lackof understanding with regard to the controls and development ofsuch systems. In the North Viking Graben, northern North Sea, theCretaceous post-rift basin only experienced very minor inversion inthe Tertiary (Gabrielsen et al., 2001), shows a clearly-defined post-riftpackage on 3D seismic, and has reasonable well control. It is thereforean ideal candidate to study sedimentary and tectonic evolution in apost-rift basin.

The Cretaceous early post-rift deep-marine depositional systemsof the northern North Sea inherited fault-bounded structural highsand deep basins from the Late Jurassic rifting events. In many places,the structural highs developed into local source areas and duringperiods of relative sea-level fall, clastic sediment was derived from theuplifted footwalls, such as the Oseberg footwall island and the proto-Scottish mainland in the UK sector and the Trøndelag Platform, MåløyTerrace and Halten Terrace in the Norwegian Sea (Fig. 1). Clasticsediments were delivered into the deep-marine basins by way ofslumping and mass-flow processes, as commonly recognised onseismic profiles and in core samples (e.g. Shanmugam et al., 1995;Martinsen et al., 2005). Due to the high stand of sea-level thatprevailed during the Cretaceous, limited shelfal, shallow marine andcoastal systems are preserved in the North Sea (Brekke et al., 2001;Copestake et al., 2003).

Most studies of the North Sea Cretaceous post-rift system overlookthe influence of local controls on the development of the deep-marinedepositional systems (e.g. Shanmugam et al., 1995; Skibeli et al., 1995;Brekke et al., 1999; Argent et al., 2000; Garrett et al., 2000; Law et al.,2000; Brekke et al., 2001; Bugge et al., 2001; Copestake et al., 2003).Similarly, no detailed seismic stratigraphic analysis of early post-riftbase-of-slope to basin-floor depositional systems exists for rift basins.In order tofill this knowledge gap, this paper investigates the evolution

Fig. 1. The black box in the left inset outlines the North Viking Graben study area shown instructure map (after Fraser et al., 2002). The thin dashed lines in the right inset represent thshown by the solid white V–V', W–W', X–X', Y–Y' and Z–Z' lines. ESP, East Shetland Platformexcept for the graben areas, UK and Scotland, represent areas of palaeoshelf during the Cre

of, and documents the local versus regional controls on, early post-riftclastic depositional systems of the Cretaceous within the North VikingGraben. In particular, the seismic facies and seismic geomorphology ofthe depositional systems are analysed and compared with analogoussystems to unravel the controls on their stratigraphic position,geometry and evolution. Since individual depositional systems haverarely been documented within early post-rift, deep-marine strata,this study aims to provide greater insight into a system that is poorlyunderstood, and an excellent analogue for the documentation andinterpretation of comparable systems.

2. Regional setting

The North Viking Graben is a major N–S trending, deep-waterbathymetric feature in the northernNorth Sea, located to the northeastof theUK and to thewest of theNorwegian coastline (Fig.1). It is part ofthe failed trilete rift system that accompanied rifting in the Permo-Triassic and Middle to Upper Jurassic. Permo-Triassic rifting isresponsible for the dominant N–S-trending Oseberg fault complex,Brage fault and southern basin segment faults in the study area,whereas the Jurassic events created the deep-water basins andintervening structural highs, including the North Viking Graben(Fig. 1). It has been established that rifting ceased in the northernNorth Sea during the Volgian (Johnson, 1975; Færseth et al., 1995;Færseth and Ravnås, 1998; Færseth and Lien, 2002; Zachariah et al.,2009). The basin then underwent post-rift thermal subsidence (Badleyet al., 1984; Gabrielsen et al., 1990; Prosser, 1993; Nøttvedt et al., 1995;Gabrielsen et al., 2001) and was affected by relative changes in sea-level, in association with the far-reached effects of renewed NorthAtlantic rifting and the Austrian Orogeny during the Aptian (Skibeliet al., 1995; Brekke et al., 2001; Bugge et al., 2001; Kjennerud et al.,2001; Kyrkjebø et al., 2001; Copestake et al., 2003; Oakman 2005).

the right inset. All maps represent the TWTT (ms) present-day Base Cretaceous (BCU)e major structural trends at BCU level. The seismic sections shown in Figs. 4, 5 and 7 are; MT, Måløy, Terrace: SG, Sogn Graben; VG, Viking Graben. All the features highlighted,taceous.

Fig. 2. Tectonostratigraphic chart showing the seismic stratigraphic framework thatcomprises the four main seismic units in this study (Ravnås and Bondevik, 1997; Skibeliet al., 1995; Davies et al., 2000; Bugge et al., 2001; Løseth, 2001; Copestake et al., 2003).Within the relative sea-level curve column (after Haq et al., 1988), the five seismicreflections discussed within this paper are shown. The stratigraphic position of the fiveseismic reflections in the chart represents their stratigraphic position within the basinonly. BCU = Base Cretaceous Unconformity.

62 A.-J. Zachariah et al. / Sedimentary Geology 220 (2009) 60–76

Thickness variations associated with fault-related topography,wedge-shaped sedimentary bodies and the deposition of coarseclastics in the post-rift interval have often been interpreted to beassociated with Cretaceous tectonism elsewhere in the northernNorth Sea (Badley et al., 1984; Alhilali and Damuth,1987; Skibeli et al.,1995; Nøttvedt et al., 1995; Hesthammer and Fossen, 1999; Buggeet al., 2001; Gabrielsen et al., 2001; Kyrkjebø et al., 2004). However, inand around the North Viking Graben these features do not accompanyfault movement but are rather associated with the passive infilling ofthe syn-rift topography (Færseth et al., 1995; Zachariah et al., 2009).

During the early post-rift, water depths fluctuated between 400 mto 600 m in the North Viking Graben, whist shelfal and terrace areasand structural highs remained shallow or exposed (up to 200 m)(Kjennerud et al., 2001; Kyrkjebø et al., 2001). Throughout most of theCretaceous, however the crestal area of the uplifted Oseberg fault-block (Fig. 1) remained above sea-level as a footwall island until thelate Maastrichtian (Oakman and Partington, 1998; Gabrielsen et al.,2001; Kjennerud et al., 2001; Kyrkjebø et al., 2001). This is recognisedfrom the lack of post-rift hardground development in the well data ofthe Oseberg footwall crest and the progressive onlap of the footwall byCretaceous strata.

A deep-marine environment prevailed during the Cretaceous inthe northern North Sea where hemipelagic clay was dominantlydeposited and episodic clastic material was shed into the basinsduring periods of relative sea-level fall (Fig. 2; Skibeli et al., 1995;Argent et al., 2000; Garrett et al., 2000; Law et al., 2000; Bugge et al.,2001; Copestake et al., 2003). During periods of transgression andrelative sea-level highs, carbonate deposition was favoured in theshelf areas around the structural highs (Fig. 2; Bugge et al., 2001).

The study area is located in the Norwegian sector of the North Seabetween 60° and 61° N, where the North Viking Graben is bounded tothe east by the Horda Platform and to the west by the East ShetlandPlatform (Fig. 1). In the study area, the N–S trending Oseberg faultcomplex and the southern basin segment faults of the Permo-Triassicare dominant (Fig. 1). These were reactivated along with a number oflarge NE–SW trending normal faults in the Jurassic rifting phase sothat the faults overlapped and switched polarity along the basinedges, creating transfer zones and relay ramps, and defining theViking Graben as a number of en échelon segments (Færseth et al.,1997; Færseth and Ravnås, 1998) (Fig. 1). The North Viking Graben inthe study area is semi-enclosed, approximately 25 kmwide and 80 kmlong, with a narrow (~10 kmwide) continuation over a small ridge tothe NE and a similar opening to the south (~3–10 kmwide) (Fig. 1). Inthe study area, the Base Cretaceous reflection in the basin is ~3900 msTWTT, shallowing up to ~2100 ms TWTT on the flanks, and dipstowards the south (Fig. 1). Structural features upslope include back-basins and shelfal and terrace areas, which widen southwards (~5–20 km) (Fig. 1). The early post-rift Ryazanian–Turonian package has amaximum thickness of 866 ms in the North Viking Graben and isabsent on the fault-block crests.

3. Dataset and methodology

The study area covers 5230 km2 and contains five overlapping 3Dseismic data surveys with an inline and crossline spacing of 12.5 m:SH9004 (GECO, 1990), NVG96_MERGE (PGS, 1996), NH0402 (Wester-nGeco, 2004), NH02M1 (Ensign, 2002), and NH05M01 (Norsk Hydro,2005). The frequency of the data are approximately 30 Hz and thus thevertical stratigraphic seismic resolution is estimated at around 30 m.All surveys were processed to zero phase so that a peak (blackreflection) represents a decrease in acoustic impedance whereas atrough (red reflection) represents an increase in acoustic impedance.

Fifty-three exploration and production wells were examined forgamma ray, sonic, neutron, and density logs to interpret lithology.Biostratigraphic zonations (StatoilHydro) were used for age calibra-tion of key seismic horizons (Fig. 2). The 30/4-1 and 30/5-1 basinal

wells were dominantly used to tie in the seismic horizons since themajority of wells are located around the basinmargins on comparativestructural highs and contain large hiati in their Cretaceous strati-graphy due to their dominantly sub-aerial position during this period(Figs. 1 and 3). The biostratigraphic age calibrations were based ona combination of micropalaeontological (foraminifera, radiolaria)and palynological (dinoflagellate cysts) analyses of ditch cuttings,

Fig. 3. Summary log for well 30/5-1 illustrating lithostratigraphy, biostratigraphy and seismic stratigraphy derived in this study. See Fig. 1 for location. See Fig. 2 lithostratigraphy forlithology key. DC=ditch cutting. SC=sidewall core. LO=last occurrence. LCO=last common occurrence. INC.=increase. DEC.=decrease. FO=first occurrence. FCO=firstcommon occurrence. The well was drilled in 1972 by A/S Norske Shell, who also performed the biostratigraphic interpretation.


supplemented by selective study of sidewall cores (an example of thewell data available is shown in Fig. 3). A single core, fromwell 30/5-1,exists for the Cretaceous in the study area but only biostratigraphicreports from its analysis were available for this study (Fig. 3).

Micropalaeontological data were also used for palaeoenviron-mental interpretation and approximate palaeobathymetries. Palaeo-water depth estimates taken from palaeobathymetry studies in thenorthern North Sea were used to supplement the data (Kjennerud etal., 2001; Kyrkjebø et al., 2001; Wien and Kjennerud, 2005). Thesestudies used micropalaeontological analyses from 12 wells, including

well 30/4-1, structural restorations of regional transects and seismicstratigraphic analysis to determine the palaeo-water depth develop-ment of the northern North Sea basin.

The time structure maps from the key seismic stratigraphichorizons, which were correlated to the biostratigraphy, were used tocreate volume amplitudemaps for themain seismic units. The volumeamplitude maps were used together with seismic sections todetermine the presence of discrete, comparatively thin, coarse clasticdepositional systemswithin the dominantly mud-rich basin fill. It wasassumed that coarser clastic material represented higher amplitudes


and thus by examining the areas of relative low versus high amplitudeover an interval the discrete coarse clastic systems could bedistinguished more readily (Brown, 2004). This methodology wascalibrated with well data because carbonate-rich sequences can oftenalso be distinguished by higher amplitudes. The depositional elementswithin each seismic unit were resolved from their characteristicseismic facies and associated amplitude anomalies, relationship withadjacent reflections, and cross-sectional and planform morphology.

A semblance cube was generated from the NH05M01 seismicsurvey for a small region over the Oseberg footwall crest, its adjacentslope and basin-floor. A semblance cube, also known as a varianceor coherency cube, is a measure of discontinuity in seismic data.Abrupt changes in waveform result in low coherence and can be usedto indicate, in this case, the juxtaposition of high angle, rotated andtilted, syn-rift strata against the contrasting low angle, overlying post-rift strata. The semblance cube was extracted across the BaseCretaceous surface and well data were added to make a Cretaceoussubcrop map for the area.

4. 3D seismic facies analysis of early post-rift depositional systems

Five key seismic surfaces were identified on the basis of majorchanges in seismic facies and reflection terminations within the earlypost-rift Ryazanian to Turonian interval: Base Cretaceous Unconfor-

Fig. 4. TWTT (ms) seismic sections highlighting the depositional geometries and seismic faciSee Fig. 1 for the location of the seismic sections.

mity (BCU), Intra-Aptian, Top Albian, Top Cenomanian and TopTuronian (Fig. 2). The five key seismic surfaces reported here arealso major biostratigraphic markers in the wells (Fig. 3). Theiridentification allows the Ryazanian to Turonian interval (137–89 Ma,Fig. 2) to be subdivided into four main seismic units, termed K1, K2,K3, and K4 (Fig. 2). The seismic stratigraphic framework for theRyazanian to Turonian interval is described with reference to theseismic sections shown in Figs. 4 and 5, and is summarised in Fig. 2.

4.1. K1 (Base Cretaceous Unconformity to Intra-Aptian)

The K1 package comprises the latest Ryazanian to intra-Aptiansuccession (~137–115 Ma). It is bounded at its base by the BCUreflection and at its top by the Intra-Aptian marker. The BCU reflectionis an interpreted horizon that is simply correlated between thebasinal, slope and crestal areas of the North Viking Graben. It isdefined by a high amplitude peak (soft kick) and consistent characterwithin the basin. On the structurally high areas surrounding thegraben, however, the reflection often weakens amplitude or changespolarity to a negative red trough. The BCU reflection is mapped at thetop of the Lower Cretaceous Ryazanian Draupne Formation in thebasin as a result of its preferentially strong seismic signal between itand the overlying Cromer Knoll Group. On the slopes, the BCUreflection marks a mid/late Volgian Draupne Formation unconformity

es of each K-unit within the northern basin segment, a) N–S section, and b) E–Wsection.

Fig. 5. TWTT (ms) seismic sections highlighting the depositional geometries and seismic facies of each K-unit within the southern basin segment, a) N–S section, and b) E–WsectionSee Fig. 1 for the location of the seismic sections.


and on the crestal areas it represents a far more complex compoundunconformity from the syn-rift and post-rift events combined(Zachariah et al., 2009). As this study focuses on depositional systemswithin the graben, the Lower Cretaceous Ryazanian Draupne Forma-tion is not included in the K1 unit. The Intra-Aptian marker ischaracterised by a negative red, relatively continuous and highamplitude reflection that can be tied to the biostratigraphy of basinalwell 30/5-1.

The dominant seismic facies of K1 throughout the study area istransparent, of low to moderate amplitude with relatively continuousreflections (Figs. 4 and 5). The reflections are noticeably of higheramplitude towards the edge of the northern basin segment (Fig. 4).However, within the K1 interval some subordinate features withdiffering seismic facies and geometries can be distinguished. Thelowermost 200 ms of K1 down-dip of the Oseberg fault-block withinthe northern basin segment consists of a higher amplitude, black,positive seismic facies (Fig. 4). Within this lower part of K1, moundedfeatures, up to 20 kmwide and 100 ms thick, bi-directionally downlaponto the BCU and comprise laterally continuous reflections (Fig. 4a).K1 high amplitudes expand into up to 20–40 km wide lobe-shapedgeometries extending up to 30 km into the northern basin segmentfrom the base of the Oseberg fault-block (Fig. 6a). These lobe-shapedplanform high amplitude anomalies (Fig. 6a) correlate to the lower-most, high amplitude mounded features seen in seismic section(Fig. 4a).

In the southern basin segment there is an intra-slope low and on itsnorth-western side is a scoop-shaped scour, which is defined at itsupper limit by arcuate faults and has a concave-up profile in section(Fig. 7a). Sub-parallel and linear gullies, 0.3 kmwide and 80 ms deep,

line the intra-slope low (Fig. 7a). Within the intra-slope low is amounded feature, approximately 10 kmwide and nearly 200ms thick,that downlaps on the BCUwith a chaotic and transparent seismic faciesand has reflections that appear to be segmented and slumped (Fig. 7b).There are no high amplitudes in K1 in the southern basin segment. Theamplitude anomalies are dominated by the high amplitude characterof the BCU reflection, where K1 thins which leads to anomalouslyapparent high amplitudes in the K1 interval (Fig. 6b).

There are no K1 sediments recorded on the crest of the Osebergfault-block. Elsewhere, the well data records the dominant depositionof Åsgard Formation claystones, which commonly have a calcareousnature and are interbedded with limestone stringers for the K1interval. Deposition of Mime Formation limestones are occasionallydocumented around the Oseberg footwall crest and on the shallowershelfal and terrace areas. The micropalaeontology shows thatdeposition of the Åsgard Formation was associated with a moderatelyto poorly oxygenated outer shelf to bathyal, deep-water environment(30–600 m palaeobathymetry). Alternatively, deposition of theMime Formation occurred in well oxygenated, inner to outer shelfpalaeoenvironments (0–200 m palaeobathymetry) (Kjennerud et al.,2001; Kyrkjebø et al., 2001; Wien and Kjennerud, 2005). Theprevalent mudstone-rich facies is responsible for dominant lowamplitude and heterogeneous character of K1 whereas the higheramplitude reflections close to basin edges can be explained by thepreference of carbonate deposition in shallower environments(Fig. 6). Given the low amplitude, relative lateral continuity of thereflections, dominant mud-rich lithology and palaeoenvironment,these sediments of K1 are interpreted as pelagic and hemipelagicdeposits. Pelagic and hemipelagic sediments from comparable deep-

Fig. 6. Volume-based amplitude map for K1 (BCU to Intra-Aptian reflection) draped on the BCU surface a) within the northern basin segment, and b) in the southern basin segment.


marine slope to basin-floor settings share these similar characteristics(e.g. Galloway, 1998; Beaubouef and Friedmann, 2000; Fowler et al.,2004).

The different seismic facies and geometry of the mounded featuresrelative to the dominant background seismic facies within thenorthern basin segment suggests that the mounds comprise adifferent lithology and formed by different depositional processes.The mounded features have the high amplitude, downlapping cross-sectional appearance and lobate planform geometry, which are allcharacteristics of deep-marine basin-floor fans (Fig. 4a) (Shanmugamet al., 1995; Galloway, 1998; Demyttenaere et al., 2000; Fowler et al.,2004; Martinsen et al., 2005). Their location at the base-of-slopefurther supports this interpretation (Fig. 6a).

Many K1 sediments around the Oseberg footwall crest recordslightly silty to silty and very fine sand intervals and have a subcrop thatis dominated by sand-rich lithologies (e.g. wells 30/9-7 and 30/9-10).This suggests that coarse clastic sediments were stripped off the crestbypassed the slope and deposited as basin-floor fans during K1. This issupported by the presence of siltstone traces in the K1 interval of thebasinal well 30/4-1. The coarse clastic nature of these basin-floor fans isthe reason for their differing seismic appearance.

The mounded topography of the feature seen within the intra-slope low in the southern basin segment may also be a basin-floor fan.

However, its lack of nearby sediment source, low amplitude,transparent and chaotic nature implies it is more likely to be mud-rich (Fig. 7). The mound is flanked on its western side by a scoop-shaped scour and arcuate faults, which may represent breakawayfaults and slump scars, suggesting that this feature is composed ofslump material from the slope (Martinsen, 1994; Hesthammer andFossen, 1999). The segmentation of the reflections within the moundmay correspond with small contractional faults associated withslumping (Martinsen, 1994; Hesthammer and Fossen, 1999). Thesub-parallel, linear features seen on the BCU in the intra-slope lowsuggest mass flows of an erosive nature were also active during thistime (Fig. 7). They may have be the distal product of the slump event,or may have preceded it and/or originated from the other north-westfacing slope that surrounds the low. Either way, this contributionmeans that the mounded feature within the low is most likely to be amud-rich, integrated slump and mass-flow deposit, which are well-documented amongst the Cretaceous of the North Sea (Shanmugamet al., 1995).

4.2. K2 (Intra-Aptian to Top Albian)

The K2 package is intra-Aptian to Albian (~115–99 Ma) in age andis bounded by the Intra-Aptian reflection at its base and at its upper

Fig. 7. a) Close-up of an intra-slope low in the southern basin segment on the TWTT (ms) BCU surface map (Fig. 1), showing gully features and arcuate faults associated with a slumpscour, b) TWTT (ms) seismic section of the slumped, mounded feature seenwithin the K1 unit that infills the intra-slope low seen in a. See Fig.1 for the location of the seismic sectionin context of the study area.


limit by the Top Albian reflection event, a negative event that is tied tothe basinal well 30/5-1. K2 is dominated by transparent andheterogeneous seismic facies with some parallel and continuousreflections, especially close to the basin edges where they aredistinctly higher amplitude (Figs. 4 and 5b). Seismic sections reveala mounded seismic body, 10 km wide and 200 ms thick, that iscomposed of higher amplitudes within the northern basin segment. Itis defined by steeply dipping reflections and has further internaldipping reflections, which appear to divide concave-up reflectionevents, each about 4 kmwide, which consist of high amplitude eventsat their base that grade up into lower amplitude events (Fig. 4a). Thebase of each of the high amplitude, concave-up units appears to inciseslightly deeper into the underlying K1 unit relative to the remainder ofthe K2 unit (Fig. 4a). K2 thins either side of the mound and its seismicfacies reverts to the characteristic K2 lower amplitude facies (Fig. 4a).The K2 amplitude map shows that down-dip of the Oseberg fault-block, high amplitude anomalies form an elongate, parallel to strike

fringe about 10 km wide that corresponds to the higher amplitudereflections seen close to the basin edges on seismic sections (Figs. 4aand 8a). From this, a narrow neck of high amplitudes (~2 km wide)extends about 15 km before expanding into a lobate geometry,approximately 10 km wide, along the basin-floor (Fig. 8a). The highamplitude, mounded feature seen in seismic section (Fig. 4a)corresponds to the lobate geometry seen on the amplitude map(Fig. 8a). In the southern basin segment, there are some linear, sub-parallel high amplitude anomalies against the basin edges that are upto 2 km wide and 4 km long at the edge of the 3D seismic coverage(Fig. 8a).

There are no K2 sediments preserved on the Oseberg fault-blockcrest but the well data show that on the shelfal and terrace areas andin the basin, K2 dominantly comprises the mud-dominated Sola andRødby formations, which contain rare intervals of limestone. Theenvironment of deposition for both formations has been determinedfrom the micropalaeontology as predominantly open marine, inner to

Fig. 8. Volume-based amplitude maps for K2 (Intra-Aptian to Top Albian reflection) draped on the Intra-Aptian surface a) in the northern basin segment, and b) in the southern basinsegment.


outer shelf to upper bathyal (0–400 m palaeobathymetry) with theSola Formation generally being deposited under poorly oxygenated todysoxic bottom-water conditions (Kjennerud et al., 2001; Kyrkjebøet al., 2001;Wien and Kjennerud, 2005). This mudstone-rich lithologyis consistent with the dominant heterogeneous and transparentseismic facies of K2, and similar to K1, the higher amplitude reflectionsclose to the basin edges in both basin segments, are possibly due to thepreference of carbonate-prone facies in these locations (e.g. wells 29/3-1, 30/3-1, 30/11-3, 30/11-4, 30/11-5, 30/12-1) (Figs. 5 and 8). Justlike deposits in similar basin and depositional settings, the dominanttransparent and heterogeneous seismic facies, parallel and continuousnature of the reflections, mud-rich lithology and open marine, innerto outer shelf to upper bathyal environment suggest that this seismicfacies represents pelagic and hemipelagic drape (Figs. 4 and 5)(Galloway, 1998; Beaubouef and Friedmann, 2000; Fowler et al.,2004).

The different reflection geometries and seismic facies of themounded body in the northern basin segment suggest it is notcomprised of the same dominant mud-rich lithology and wasdeposited under different conditions than the dominant hemipelagicand pelagic mudstones. The steeply dipping bounding reflections andhigh amplitude, mounded cross-sectional appearance and lobate

plan-view geometry are suggestive of a channel complex. Suchcharacteristics have been interpreted in comparable settings aschannel deposits (e.g. Posamentier, 2003; Fowler et al., 2004). Theseparate, internal concave-up reflections represent individual chan-nels within the complex, and the transition from strong, highamplitudes to weaker ones represents increasing shale content asthe channel became abandoned and mud-filled (Fig. 4a). There issome evidence of these channels eroding into the underlying K1 unit,however this incision is not great compared to more establishedchannel systems in deep-water settings such as Angola (e.g. Gee at al.,2007) and the Gulf of Mexico (e.g. Posamentier, 2003). This maysuggest that they are immature, shallow channels that had yet todevelop into more sinuous, erosional systems (Gee et al., 2007). Thelower amplitude, thinner K2 sediments to the side of the channelcomplex are penetrated by well 30/5-1 and show, as elsewhere, mud-rich sediments for K2. It is possible that these dimmer amplitudesrepresent the associated mud-rich, overbank deposits, which aremostly likely inter-mixed with the background pelagic and hemi-pelagic sediments (Figs. 4a and 8a). Poor biostratigraphic andlithological data from well 30/5-1 prevents a definite interpretation.

There are no well data for the possible channel complex in thenorthern basin segment, but as seen in K1, many of the wells that


contain K2 sediments around the crest of the Oseberg fault-blockrecord silty to very silty intervals with occasional fine sands (e.g. wells30/3-1, 30/9-8 and 30/9-10). These intervals could represent residualclastic-rich material that was eroded from the exposed Osebergfootwall crest and subsequently re-deposited in the basin afterbypassing the slope. Siltstone traces in the basinal well 30/4-1support the idea that eroded clastic material was deposited in thebasin during K2 times. The higher amplitude character of theinterpreted channel complex is a consequence of its more clastic-rich nature. Differential compaction over a more robust, coarse clasticlithology would also explain the mounded topography of the channelcomplex (Fig. 4a).

4.3. K3 (Top Albian to Top Cenomanian)

The K3 package is earliest-to-latest Cenomanian (~99–94Ma), andis bounded by the Top Albian reflection at its base and by theTop Cenomanian reflection at its top, which is a continuous, highamplitude event tied to the biostratigraphy of the slope-penetratingwells 30/3-1 and 30/5-1 and the basinal wells 30/4-1 and 30/5-1.

K3 is characterised by heterogeneous, parallel and dominantlycontinuous seismic reflections, which are higher in amplitude close to

Fig. 9. Volume-based amplitude maps for K3 (Top Albian to Top Cenomanian reflection) dsouthern basin segment.

the basin edges (Figs. 4 and 5b). The K3 amplitude map shows thatthese high amplitude reflections correspond to a 10 km wide fringeadjacent to the footwall of the Oseberg fault-block in the northernbasin segment (Fig. 9a). In the southern basin segment, these highamplitudes occur as a 5 kmwide fringe along the basin edges (Fig. 9b).Seismic sections also reveal subtle, downlapping, mounded features,up to 10 km wide and 100 ms thick in the northern basin segment(Fig. 4a). These subtle, mounded featuresmatchwith the discrete highamplitude, lobate geometries seen on the 3D amplitude map (Figs. 4aand 9a). These lobe geometries are up to 20 kmwide and extend about20 km across the basin-floor (Fig. 9a).

There are no K3 sediments on the crest of the Oseberg fault-block.In the data elsewhere, however, K3 is composed of calcareoussediments of the Svarte Formation. Commonly, these are calcareousmudstones and marlstones with minor limestone horizons but inmore shelfal and terrace areas limestone is the dominant sediment,which is interbedded with lesser mudstones and marlstones (e.g.wells 30/8-3 and 30/12-1). Micropalaeontological data for the SvarteFormation deposits show that it was deposited in a marine, inner toouter shelf to possibly upper bathyal moderate-to-well oxygenatedenvironment (0–500 m palaeobathymetry) (Kjennerud et al., 2001;Kyrkjebø et al., 2001; Wien and Kjennerud, 2005). This calcareous-

raped on the Top Albian surface, a) within the northern basin segment, and b) in the


prone lithology corresponds well with the high amplitude, paralleland continuous seismic facies for the K3 unit. Its heterogeneousappearance on seismic can be explained by the interbedding of lesserlimestones within mudstones (Figs. 4 and 5). Since well data showthat deposition of limestone occurred in shallower environments likethe shelfal and terrace areas, this may explain the high amplitudefringes around the basin edges (Fig. 9). Overall, the characteristic lowamplitude, relatively parallel and continuous reflections, mud-richlithology and palaeoenvironmental interpretation from the micro-palaeontology suggest that in the basin these sediments representpelagic and hemipelagic deposits (Figs. 4 and 5) (e.g. Galloway, 1998;Beaubouef and Friedmann, 2000; Fowler et al., 2004).

The different cross-sectional form and high amplitudes of themounded, lobate geometries that are seen on the 3D amplitudemap inthe northern basin segment suggest theycomprise a different lithologyand mode of deposition to the dominant mud-prone, pelagic andhemipelagic basinal lithology (Figs. 4a and 9a). They are interpreted asbasin-floor fans since they sharemany of the same characteristics withthe fans seen in K1 (Figs. 4a and 6a) and with documented deep-marine basin-floor fans (e.g. Shanmugam et al., 1995; Galloway, 1998;Demyttenaere et al., 2000; Fowler et al., 2004; Martinsen et al., 2005).

Fig. 10. Volume-based amplitude maps for K4 (Top Cenomanian to Top Turonian reflection) dthe southern basin segment.

Some of the wells around the crest of the Oseberg fault-blockrecord a slightly silty to silty nature for K3 (e.g. wells 30/9-7 and 30/9-8). These silty sediments could signify remnant clastic material fromthe erosion of the exposed fault-block crest. The eroded coarse clasticmaterial bypassed the slope and was subsequently deposited in thenorthern basin segment as basin-floor fans. The higher amplitude andmounded appearance of the basin-floor fans is the consequence oftheir coarse clastic composition (Figs. 4a and 9a). The delivery of suchcoarse clastic material to the basin during K3 times would alsoaccount for the traces of siltstone and generally silty nature of the K3sediments in the basinal wells 30/4-1 and 30/5-1 respectively. Thesemay be the fine-grained, distal deposits of the basin-floor fan systems.

4.4. K4 (Top Cenomanian to Top Turonian)

The K4 package is late Cenomanian to latest Turonian (~94–89 Ma), and is bounded by the Top Cenomanian reflection at its baseand by the Top Turonian reflection at its upper limit, a red, negativereflection event which is tied to the biostratigraphy of the 29/3-1, 30/3-1, 30/5-2, 30/6-27, 30/11-3 and 30/11-4 basin-flanking wells andthe basinal 30/4-1 and 30/5-1 wells.

raped on the Top Cenomanian surface, a) within the northern basin segment, and b) in


The K4 unit is dominantly comprised of high amplitude, contin-uous and parallel reflections, which are distinctly higher in amplitudewhere they onlap the basin edges (Figs. 4 and 5a). These basin-edgeadjacent higher amplitudes correspond to a 10 km wide highamplitude fringe against the footwall of the Oseberg fault-block(Fig. 10a), and a more extensive high amplitude sheet, extending forover 20 km to the end of the seismic survey, in the southern basinsegment (Fig. 10b). The high amplitude fringe in the northern basinsegment (Fig. 10a) corresponds to downlapping reflections up to100 ms thick, which extend up to 10 km into the basin (Fig. 4b).

K4 sediments are absent from the crest of the Oseberg fault-blockbut well data from the remainder of the study area record prevalentdeposition of Blodøks Formation mudstones, overlain by calcareousclaystones and interbedded limestones of the Tryggvason Formation.Themicropalaeontological data for these deposits describe an inner toouter shelf environment with an open marine influx (0–400 mpalaeobathymetry) (Kjennerud et al., 2001; Kyrkjebø et al., 2001;Wien and Kjennerud, 2005). The Blodøks Formation is also associatedwith poorly-oxygenated bottom-water conditions. This dominantmudstone-rich lithology with sporadic layers of limestone matcheswell to the high amplitude, continuous, parallel seismic facies of K4.The high amplitude sheet in the southern basin segment is probablydue to the preference of carbonate deposition in shallow waters(Fig. 10b). The seismic, micropalaeontological and lithological char-acteristics of the dominant K4 seismic facies corresponds well to thetraits of pelagic and hemipelagic sediments (e.g. Galloway, 1998;Beaubouef and Friedmann, 2000; Fowler et al., 2004).

In the older K-units, features similar to the 10 km high amplitudefringe in the northern basin segment adjacent to the Oseberg fault-block have been taken to represent preferential carbonate deposition.However, within the high amplitude fringe in K4, downlappingreflections are present, prompting a different explanation for thisamplitude anomaly (Figs. 4b and 10a). The characteristics of thisfringe are representative of a progradational, clastic-rich shoreline.This interpretation is backed up by the lithology of the K4 unit in well30/5-1, which records calcareous and mottled mudstones inter-bedded with limestones, silty shales, traces of sandstone and siltstonewith minor shell debris, and suggests that clastic material was beingdelivered to the basin during this time. No high amplitudes areassociated with the well 30/5-1 on the amplitude map, which impliesthat the presence of any clastic material at this location is masked bythe dominant argillaceous basinal lithology (Fig. 10a). Rare wells closeto the crest of the Oseberg fault-block have siltstone and calcite-cemented sandstone stringers (wells 30/6-11 and 30/9-8) withintheir K4 stratigraphy, which further supports that clastic material hasbeen eroded from the Oseberg fault-block, bypassed the slope anddelivered to the basin.

5. Discussion

Seismic data records the progressive onlap and eventual drowningof the North Viking Graben by the late Upper Cretaceous with nosediments older than Turonian age draping the Oseberg fault-blockcrest (Fig. 11). Biostratigraphic data indicate a prevailing open marine,inner to outer shelf to upper bathyal environment, whereas waterconditions varied from well-oxygenated to dysoxic and the palaeo-bathymetry fluctuated between 0–200 m and 0–500 m, throughoutthe early post-rift in the North Viking Graben. Subsequently,hemipelagic and pelagic sediments dominate the early post-riftstratigraphy with carbonate facies recognised where the strata onlapsthe basin. The preference of fine-grained, argillaceous sedimentdeposition indicates that only the Oseberg footwall island providedthe only sediment source area for the early post-rift in the study area.Seismic and well data show the crests of other rotated syn-rift fault-blocks in the area were submerged in palaeo-water depths of up to600 m (Færseth et al., 1995; Bugge et al., 2001; Kjennerud et al., 2001;

Kyrkjebø et al., 2001; Copestake et al., 2003; Wien and Kjennerud,2005). The contribution from the Norwegian hinterland or otherregional structures was also negligible, despite evidence of clinoformsindicatingwesterly sediment transport across the Horda Platform. Thebroad nature and similar submergence of the platform meant thatlittle to no sediment was delivered to the North Viking Graben(Gabrielsen et al., 1990). The clastic-rich depositional geometries,such as the basin-floor fans and channel complexes, identified in theearly post-rift stratigraphy in the North Viking Graben, are thereforebelieved to have been sourced mainly from the Oseberg footwallisland. The temporal and spatial variabilities of these depositionalgeometries are suggestive of the controls affecting the development ofthe early post-rift depositional systems within the North VikingGraben.

A sequence stratigraphic interpretation of the North Viking Grabenearly post-rift infill has not been presented due to the low resolutionof the biostratigraphic data and the bias towards wells on thestructural highs. The wells on the structural highs contain hiati thatrepresent complex compound unconformities from the syn-rift andpost-rift events combined (Fig. 11). Thus, the presence of key stratalsurfaces in the study area is limited, which restricts the confidentcorrelation of time equivalent surfaces from the structural highs intothe basin.

5.1. Controls on the temporal evolution of early post-rift deposition

In the northern basin segment, it is the basin-floor fans in theoldest K-unit, K1, that have the most unconfined and widespreaddeposition and the highest amplitudes compared to the depositionalgeometries in the other K-units (Figs. 6 and 12). This is incontradiction to what is documented in other similar depositionalsettings, where during the early stages of basin infill sediment gravityflows preferentially infill topographic lows, such as small fault-bounded basins and base-of-slope areas (Anderson et al., 2000;Demyttenaere et al., 2000; Posamentier and Kolla, 2003; Lomas andJoseph, 2004). Widespread, unconfined deposition is more commonlyassociated with the later stages of basin fill, i.e. during the K4 stage,when successive stratigraphy has onlapped further upslope, allowingthe basin to become more in-filled with a reduced gradient (Ravnåsand Steel, 1998; Posamentier and Kolla, 2003). Whereas gradual infilland onlap of the North Viking Graben did occur in the latter stages,there was also a continued significant reduction in the Osebergfootwall island sediment source area due to erosion and degradation,and gradual drowning as a consequence of the overall rise in sea-levelduring the early post-rift interval (Figs. 2 and 12). This resulted in lesscoarse clastic sediment being supplied to the basin in the successive K-units and thus less extensive sediment gravity flow deposits.

Commonly, large volumes of sediment on the basin-floor, such asthat represented by basin-floor fans, are associated with a fall ofrelative sea-level (e.g. Posamentier and Kolla, 2003; Fowler et al.,2004). The basin-floor fans in K1 can therefore be explained by thehigh frequency, relatively prolonged periods of relative sea-level fallseen during this time interval (Fig. 2). These decreases in relative sea-level were regionally significant and are commonly associated withdeposition of the Åsgard sandstones elsewhere in the northern NorthSea during K1 times. Similarly, deposition of the Mime Formationlimestones in the study area (Fig. 2) was influenced by regionaltransgressional periods during K1 because these limestones can becorrelated across large areas in the North Sea (Oakman andPartington, 1998; Bugge et al., 2001; Copestake et al., 2003).

Variations in sea-level also explain the switch from deposition ofbasin-floor fans in K1 to a more confined channel complex in K2(Fig. 12). In contrast to basin-floor fan deposition, channel complexesare interpreted to form during times of relative sea-level stand-still, orslow rise, when large amounts of fine-grained sediment were fedpreferentially into the deep-marine environments (e.g. Posamentier

Fig. 12. Depositional block diagrams for each of the K-units.


and Kolla, 2003; Fowler et al., 2004). This explains the channelcomplex in K2, which is associated with long-term rising sea-levelpunctuated by comparatively very minor, intermittent falls in sea-level (Fig. 2). Elsewhere in the northern North Sea, K2 times wereassociated with the development of the Sola and Agat sandstones,which have been ascribed to tectonism linked to North Atlantic riftingduring the Aptian and Albian (Fig. 2) (Skibeli et al., 1995; Oakman andPartington, 1998; Brekke et al., 2001; Bugge et al., 2001; Copestakeet al., 2003; Oakman, 2005). In the North Viking Graben, however,no evidence of such tectonism has been found.

The change back to basin-floor fan deposition in K3 is explained, asin K1, by the reasonably extensive periods of relative sea-level fallduring this time (Figs. 2 and 12). K3 also has a predominance ofcarbonates compared to the other K-units. This may be been linked to

Fig. 11. Upper inset is a TWTT (ms) seismic section across the Oseberg footwall crest. Lowesubcropping stratigraphy to the BCU at the well locations and highlights the potential clastic-the formations and groups see Fig. 2. Horseshoe-shaped drainage catchments in the stratigraThe subcrop data are draped on a discontinuity map; the darker the colours, the greater th

an increase inwater temperature from the Lower to Upper Cretaceousas a result of rising global temperatures (Surlyk et al., 2003). Thisincrease in carbonate content may have been further enhanced by themore arid climate and periods of relative sea-level highs in K3 thatflooded and reduced the sediment source area (Fig. 12). This wouldhave encouraged clastic starvation.

Regional influences continue to be seen in the early post-riftstratigraphy in the form of the Blodøks Formation at the base of K4,which represents a major regional anoxic event. The Blodøks For-mation is a major condensed section of organic-rich clay thatcorrelates to the Plenus Marl Formation further south in the NorthSea (Fig. 2) (Copestake et al., 2003). Also associated with K4 isa shoreline-like geometry that is not recognised in any of the otherK-units. This difference is probably a function of sediment supply and

r inset is Cretaceous subcrop map for the Oseberg footwall crest area, which shows therich subcrop units for erosion and re-deposition in the basin. For stratigraphic context ofphy are interpreted as sediment fairways to the basin during the early post-rift interval.e discontinuity.


water depth (Prior and Bornhold, 1988). In K1–K3, water depths weretoo great, due to sediment ‘under-filling’ of the graben, to allowsediment aggradation to be sufficient enough to build a significantnear-shore/sub-aerial platform. In addition, the influxof sedimentwasgenerally episodic and quite possibly could have been high-energy.This would have meant that most the sediment introduced to theOseberg footwall margin was sufficiently mobile to bypass theshoreline and accumulate in deep-water on the basin-floor (Priorand Bornhold, 1988). K4 was a more advanced, steady-state systemwith sufficient sediment supply and sufficiently shallow water depthsto allow stable sediment aggradation above/close to sea-level andprogradation (Prior and Bornhold,1988). The 100ms thick shoreline inK4 suggests water depths of approximately 100 m adjacent to theOseberg footwall. Furthermore, the remaining exposed footwall areaduring K4 would have been so slight and peneplained into a less steepterrain compared to K1–K3, that very little sediment would be shedinto the basin, additionally enhancing the development of just anarrow facies belt adjacent to the footwall slope (Figs. 10 and 12)(Ravnås and Steel, 1998).

5.2. Along-strike variability in early post-rift depositional systems

All the clastic-rich depositional geometries in the early post-riftbasin infill occur within the northern basin segment located adjacentto the footwall of the Oseberg fault-block (Figs. 4–6, 8–10). In thesouthern basin segment, aside from a mud-rich integrated slumpand mass flow deposit seen in K1 (Fig. 7), the early post-rift basinstratigraphy dominantly consists of hemipelagic, carbonate-pronesediments. This along-strike variability is attributed to the differingstructural configurations of the northern and the southern basinsegments (Fig. 1). The northern basin segment is bordered byrelatively steep and short slopes, inherited from the underlying,uplifted, tilted syn-rift fault-blocks, specifically the Oseberg faultcomplex (Fig. 1). In contrast, the southern basin segment is defined bya lower gradient and wider basinmargin, reflecting the lack of a singleJurassic border fault and significantly uplifted fault-blocks (Fig. 1).These differences meant that during the early post-rift the Osebergfault-block crest was sub-aerial and provided a source area forsediment in the northern basin segment whereas the southern basinsegment was dominantly submerged.

Whilst significant footwall erosion would have taken place duringthe syn-rift interval (Ravnås and Bondevik, 1997; Davies et al., 2000),the Oseberg footwall island would also have been subject to sub-aerialdenudation during the early post-rift, and therefore still acted as asediment source area. Severe degradation of the crest allowed thedirect juxtaposition of the tilted and rotated strata of the Dunlin andBrent groups against the Base Cretaceous (Fig. 11). In some crestalwells, most of the well-known, sand-rich subcrop formations, like theBrent Group (Morton et al., 1992) and Cook Formations, are missing(Fig. 11). For example, the crestal well 30/6-27 (Fig. 1) recordsPliensbachian sediments of the Amundsen Formation overlain byTuronian Tryggvason Formation, representing a possible hiatus of upto 100 Ma (Figs. 2 and 11). Coarse clastic material deposited in theearly post-rift stage in the wells around the Oseberg footwall crestand in the basin indicates that the coarse clastic subcrop was erodedduring the early post-rift interval. These sediments bypassed the slopeand were re-deposited in the basin.

In the northern basin segment and back-basin behind the Osebergfootwall crest, Upper Jurassic Draupne and Heather shales are thesubcrop to the Cretaceous and, even though hiatuses do exist betweenthe Jurassic and Cretaceous in some of the wells in these areas, theyare not as great as those seen on the footwall crest (Fig. 11). Thissuggests that these areas were dominantly submerged, and thus notsignificantly eroded, during the Cretaceous.

Continued erosion of the Oseberg footwall crest during the earlypost-rift is further supported by the subcrop map, which shows

horseshoe-shaped drainage catchments that can be traced from theOseberg footwall crest, downslope towards the northern basinsegment (Fig. 11). It is interpreted that these represent streamnetworks carved into the Oseberg footwall block during its sub-aerialexposure. The Jurassic and Triassic sediments exposed on the Osebergfootwall crest would have been fairly unconsolidated, given theirrelatively recent deposition and subsequent uplift, in relation to theearly post-rift Cretaceous interval. This would have greatly facilitatedtheir erosion. Turonian sediments are the oldest sediments drapingthe Oseberg footwall crest so the stream networks are interpreted tohave been active up until the footwall crest was drowned in the LateCretaceous (Fig. 11). These stream networks would have depositedtheir sediment load down-dip from the Oseberg footwall crestpossibly close to the slope break. Sediment overloading along theslope break may have encouraged the initiation of sediment gravityflows into the basin (Fig. 12).

Overall, the gradient in the southern basin segment is muchgentler than in the north and consequently no such depositionalgeometries, apart from an integrated slump and mass-flow depositseen within an intra-slope low in K1, are recognised there. Theintegrated slump and mass-flow deposit feature does share somecharacteristic features with the depositional geometries in thenorthern basin segment, but it has been interpreted as mud-richdue to the significant lack of a nearby sediment source. This deposit ismost likely to be the result of oversteepened slope collapse into theadjacent intra-slope low (Fig. 7) and therefore simply comprisesreworked hemipelagic and pelagic slope sediments.

5.3. Comparisons with analogous systems

The basin-floor fans and channel complexes in this study sharemany comparable characteristics in planform, shape and size withthose seen in other deep-marine depositional environments, such asthe Gulf of Mexico, Indonesia and other northern North Sea basins(Beaubouef and Friedmann, 2000; Demyttenaere et al., 2000;Posamentier, 2003; Posamentier and Kolla, 2003; Fowler et al.,2004; Martinsen et al., 2005). For example, the basin-floor fans seenin K1 and K3 are up to 20 km wide and 100 ms thick (Fig. 4). In theKutei Basin, offshore Indonesia, the deep-water basin-floor fans fromthe Mio-Pliocene are approximately 6 km wide and 125 ms thick,whereas the more recent examples are about 20 kmwide and 200 msthick (Fowler et al., 2004). The channel complex in K2 is approxi-mately 10 km wide and 200 ms thick (Fig. 4) and in the Kutei Basinsimilar Mio-Pliocene channels are observed at up to 5 km width and125 ms thick, and recent examples are approximately 6 km wide and150 ms thick (Fowler et al., 2004). This observation was not expectedgiven the limited (approximately 125 km2; Fig.11) sediment source areaprovided by the Oseberg footwall island compared to the much greatervolumes of sediment supplied to these other deep-marine depositionalenvironments. It was thought that this limited spatial extentmeant thatthe footwall island would have been capable of only producing limitedamounts of sediment (Ravnås and Steel,1998). An explanation for thesecomparatively-sized depositional geomorphologies could be the steep,short slopes adjacent to the northern basin segment, which facilitatedthe transfer of sediments to thedeep-water environment. Steeper slopesallow for a greater velocity of the sediment gravity flows beingdischarged from the sediment source area, and therefore sedimentdischarge is increased (Posamentier and Kolla, 2003).

Whilst detailed accounts of sedimentation patterns in early post-rift settings are not particularly abundant, some outcrop studieshave been carried out in Greenland. Larsen et al. (2001) document theLower Cretaceous sandstones of the Steensby Berg Formation, whichcomprises an early post-rift coarse-grained clastic wedge deposited inan otherwise mudstone-dominated succession during a period ofoverall transgression, similar to the that of the North Viking Graben.The clastic wedge comprises several facies associations, including a


shoreface deposit, analogous to that in K4, and channel sandstones,like those in K2. Graded sandstones, which have been interpreted topossibly represent low-density turbidites deposited in an outer shelfenvironment, are comparable to the basin-floor fans seen in K1 and K3(Larsen et al., 2001). As in the North Viking Graben, the differingdepositional geometries seen the Steensby Berg Formation arebelieved to be the consequence of changes in relative sea-level(Larsen et al., 2001). The presence of comparable facies associationsin outcrop with analogous controls on their deposition to those inthe North Viking Graben further validates the results of this study.The integration of outcrop and subsurface work with seismicstudies such as this provides higher resolution in the constraint ofsedimentary architecture in less well-documented basins in similarsettings. Knowledge that even mud-dominated successions, such asthose presented here, can yield significant sand-rich units is especiallyimportant in subsurface exploration and may establish new playmodels in both under-explored and mature basins.

6. Conclusions

This study focuses on the controls on the evolution and spatialvariability in the early post-rift deposition of the North Viking Grabenin the Norwegian North Sea. The openmarine, shelfal to upper bathyalenvironment that existed during this time explains the dominant,basinal hemipelagic and pelagic deposition, with carbonates lappingthe graben flanks. Clastic-prone depositional geometries in thenorthern basin segment illustrate that one of the major controls onthe development of the early post-rift basin stratigraphy was theinherited syn-rift fault-controlled topography. This determined thelengths, gradients and orientation of the slopes adjacent to the NorthViking Graben and established a basin physiography that varied along-strike. The short, steep slopes bounding the northern basin segmentfacilitated the delivery of sediment gravity flows from the sub-aerial,unconsolidated coarse clastic-rich subcrop of the Oseberg footwallisland to the graben. In contrast, the submerged slopes with gentlergradients in the southern basin segment were relatively sediment-starved.

Long- and short-term changes in relative sea-level also heavilyimpacted the evolution of the early post-rift basin. Long-termtransgression resulted in the drowning of the Oseberg footwall islandand an overall reduction in sediment supply to the basin. Times ofshort-term relative sea-level fall allowed for greater exposure anderosion of the Oseberg footwall crest and the emplacement of basin-floor fans during K1 and K3. In contrast, the channel complex seen inK2 occurred during a period of short-term relative sea-level stand-stillor slow rise, and the shoreface-like geometry in K4 is associatedwith aperiod of relative sea-level rise. Regional short-term trangressive andanoxic events in the northern North Sea further influenced the earlypost-rift strata, resulting in the deposition of carbonate units such asthe Mime Formation, and the condensed mudstone Blodøks Forma-tion, respectively.

Degradation of the Oseberg footwall island over time resulted in ashift in the scale of the depositional geometries in the northern basinsegment from extensive basin-floor fans and channel complex geome-tries in K1, K2 and K3 to a restricted shoreline-like geometry in K4.Shoreline geometries were not seen in K1–K3 because deep relativepalaeobathymetries and episodic, high-energy sediment dischargepromoted bypass of the shoreline and deep-water deposition. In K4,shallowpalaeobathymetryand sufficient sediment supply promoted thepreservation of a shoreline-like, progradational facies belt.

Comparisons to subsurface and outcrop analogues with similardepositional systems and settings provides further validation of thepresence of the interpreted seismic facies in the North Viking Grabenand the controls on their deposition. This study is an ideal analogue forthe constraint of sedimentary architecture in less well-documentedbasins in comparable settings. Since post-rift intervals are generally

under-explored an important implication of this is enhanced accuracyin the prediction of reservoir units during subsurface exploration andthe subsequent creation of new play models in both under-exploredand mature basins.

Acknowledgments

This work was funded by NERC (NER/S/C/2003/11694) with CASEsupport from StatoilHydro. The authors are grateful to StatoilHydro forpermission to use their seismic data and to publish this paper. Thanksare due to Schlumberger Ltd for use of their GeoFrame software. MikeCharnock and Valerie Charnock are acknowledged for their thoroughand detailed biostratigraphic work, without which much of thisstudy would not have been possible. Dave Peacock is thanked for hisexcellent proof reading in the final stage of this work.

References

Alhilali, K.A., Damuth, J.E., 1987. Slide block (?) of Jurassic sandstone and submarinechannels in the basal Upper Cretaceous of the Viking Graben: Norwegian North Sea.Marine and Petroleum Geology 4, 35–48.

Anderson, J.E., Cartwright, J., Drysdall, S.J., Vivian, N., 2000. Controls on turbidite sanddeposition during gravity-driven extension of a passive margin: examples fromMiocene sediments in Block 4, Angola. Marine and Petroleum Geology 17,1165–1203.

Argent, J.D., Stewart, S.A., Underhill, J.R., 2000. Controls on the Lower Cretaceous PuntSandstone Member, a massive deep-water clastic deposystem, Inner Moray Firth,UK North Sea. Petroleum Geoscience 6, 275–285.

Badley, M.E., Egeberg, T., Nipen, O.V., 1984. Development of rift basins illustrated by thestructural evolution of the Oseberg feature, Block 30/6, offshore Norway. Journal ofGeological Society 141, 639–649.

Beaubouef, R.T., Friedmann, S.J., 2000. High resolution seismic/sequence stratigraphicframework for the evolution of Pleistocene intra slope basins, Western Gulf ofMexico: depositional models and reservoir analogs. GCSSEPM Foundation 20thAnnual Research Conference, Deep-Water Reservoirs of the World. Houston, Texas,USA, pp. 40–60.

Brekke, H., Dahlgren, S., Nyland, B., Magnus, C., 1999. The prospectivity of the Vøring andMøre basins on the Norwegian continental margin. In: Fleet, A.J., Boldy, S.A.R.(Eds.), PetroleumGeology of Northwest Europe: Proceedings of the 5th Conference.The Geological Society of London, London, UK, pp. 261–274.

Brekke, H., Sjulstad, H.I., Magnus, C., Williams, R.W., 2001. Sedimentary environmentsoffshore Norway — an overview. In: Martinsen, O.J., Dreyer, T. (Eds.), SedimentaryEnvironments Offshore Norway — Palaeozoic to Recent, vol. 10. NPF SpecialPublication, pp. 7–37.

Brown, A.R., 2004. Interpretation of three-dimensional seismic data. AAPG Memoir 42.Bugge, T., Tveiten, B., Bäckström, S., 2001. The depositional history of the Cretaceous in

the north-eastern North Sea. In: Martinsen, O.J., Dreyer, T. (Eds.), SedimentaryEnvironments Offshore Norway — Palaeozoic to Recent, vol. 10. NPF SpecialPublication, pp. 279–291.

Copestake, P., Sims, A.P., Crittenden, S., Hamar, G.P., Ineson, J.R., Rose, P.T., Tringham,M.E.,2003. Lower Cretaceous. In: Evans, D., Graham, C., Armour, A., Bathurst, P. (Eds.), TheMillennium Atlas: Petroleum Geology of the Central and Northern North Sea. TheGeological Society of London, London, UK, pp. 191–211.

Davies, S.J., Dawers, N.H., McLeod, A.E., Underhill, J.R., 2000. The structural andsedimentological evolution of early syn-rift successions: theMiddle Jurassic TarbertFormation. Basin Research 12, 343–365.

Demyttenaere, R., Tromp, J.P., Ibrahim, A., Allman-Ward, P., Meckel, T., 2000. Bruneideep water exploration: from sea floor images and shallow seismic analogues todepositional models in a slope turbidite setting. In: Weimer, P., Slatt, R.M., Coleman,J., Rosen, N.C., Nelson, H., Bouma, A.H., Styzen, M.J., Lawrence, D.T. (Eds.), DeepWater Reservoirs of the World. Gulf Coast Section SEPM Foundation, Houston,pp. 304–317.

Fowler, J.N., Gurinto, E., Sherwood, P., Smith, M.J., Algar, S., Busono, I., Goffey, G., Strong,A., 2004. Depositional architecture of Recent deepwater deposits in the Kutei Basin,East Kalimantan. In: Davies, R.J., Cartwright, J.A., Stewart, S.A., Lappin, M., Underhill,J.R. (Eds.), 3D Seismic Technology: Application to the Exploration of SedimentaryBasins. Memoirs, vol. 29. Geological Society, London, UK, pp. 25–33.

Fraser, S.I., Robinson, A.M., Johnson, H.D., Underhill, J.R., Kadolsky, D.G.A., Connell, R.,Johannessen, P., Ravnås, R., 2002. Upper Jurassic. In: Evans, D., Graham, C., Armour,A., Bathurst, P. (Eds.), The Millennium Atlas: Petroleum Geology of the Central andNorthern North Sea. The Geological Society of London, London, UK, pp. 157–189.

Færseth, R.B., Ravnås, R., 1998. Evolution of the Oseberg Fault-block in the context of thenorthern North Sea structural framework. Marine and Petroleum Geology 15,467–490.

Færseth, R.B., Lien, T., 2002. Cretaceous evolution in the Norwegian Sea — a periodcharacterized by tectonic quiescence.Marine and PetroleumGeology 19,1005–1027.

Færseth, R.B., Sjøblom, T.S., Steel, R.J., Liljedahl, T., Sauar, B.E., Tjelland, T., 1995. Tectoniccontrols on Bathonian — Volgian syn-rift successions on the Visund fault-block,northern North Sea. In: Steel, R.J., Felt, V., Johannessen, E., Mathieu, C. (Eds.),Sequence Stratigraphy on the Northwest European margin, vol. 5. NPF SpecialPublication, pp. 325–346.


Færseth, R.B., Knudsen, B.-E., Liljedahl, T., Midbøe, P.S., Søderstrøm, B., 1997. Obliquerifting and sequential faulting in the Jurassic development of the northern NorthSea. Journal of Structural Geology 19, 1285–1302.

Gabrielsen, R.H., Færseth, R.B., Steel, R.J., Idil, S., Kløvjan, O.S., 1990. Architectural stylesof basin fill in the northern Viking Graben. In: Blundell, D.J., Gibbs, A.D. (Eds.),Tectonic Evolution of the North Sea Rifts, vol. 181. Publication, InternationalLithosphere Program, pp. 158–179.

Gabrielsen, R.H., Kyrkjebø, R., Faleide, J.I., Fjeldskaar, W., Kjennerud, T., 2001. TheCretaceous post-rift basin configuration of the northern North Sea. PetroleumGeoscience 7, 137–154.

Galloway, W.E., 1998. Siliciclastic slope and base-of-slope depositional systems:component facies, stratigraphic architecture, and classification. AAPG Bulletin 82,569–595.

Garrett, S.W., Atherton, T., Hurst, A., 2000. Lower Cretaceous deep-water sandstonereservoirs of the UK Central North Sea. Petroleum Geoscience 6, 231–240.

Gee, M.J.R., Gawthorpe, R.L., Bakke, K., Friedmann, S.J., 2007. Seismic geomorphologyand evolution of submarine channels from the Angolan continental margin. Journalof Sedimentary Research 77, 433–446.

Haq, B.U., Hardenbol, J., Vail, P.R., 1988. Mesozoic and Cenozoic chronostratigraphy andcycles of sea-level change. In: Wilgus, C.K., Hastings, B.S., Ross, C.A., Posamentier,H.W., Van Wagoner, J.C., Kendall, C.G. (Eds.), Sea-level Changes: An IntegratedApproach. SEPM, pp. 71–108. Special Publication.

Hesthammer, J., Fossen, H., 1999. Evolution and geometries of gravitational collapsestructures with examples from the Statfjord Field, northern North Sea. Marine andPetroleum Geology 16, 259–281.

Johnson, R.J., 1975. The base of the Cretaceous: a discussion. In: Woodland, A.D. (Eds.),Petroleum and the continental shelf of north-west Europe. Applied SciencePublishers Ltd., John Wiley & Sons, New York, USA, pp. 389–402.

Kjennerud, T., Faleide, J.I., Gabrielsen, R.H., Gillmore, G.K., Kyrkjebø, R., Lippard, S.J.,Løseth, H., 2001. Structural restoration of Cretaceous–Cenozoic (post-rift) palaeo-bathymetry in the northern North Sea. In: Martinsen, O.J., Dreyer, T. (Eds.),Sedimentary Environments Offshore Norway — Palaeozoic to Recent: NPF SpecialPublication, vol. 10, pp. 347–364.

Kyrkjebø, R., Kjennerud, T., Gillmore, G.K., Faleide, J.I., Gabrielsen, R.H., 2001. Cretaceous–Tertiary palaeo-bathymetry in the northern North Sea; integration of palaeo-waterdepth estimates obtained by structural restoration and micropalaeontologicalanalysis. In: Martinsen, O.J., Dreyer, T. (Eds.), Sedimentary Environments OffshoreNorway — Palaeozoic to Recent: NPF Special Publication, vol. 10, pp. 321–345.

Kyrkjebø, R., Gabrielsen, R.H., Faleide, J.I., 2004. Unconformities related to the Jurassic–Cretaceous synrift–postrift transition of the northern North Sea. Journal of theGeological Society, London, UK 161, 1–17.

Larsen, M., Nedkvitne, T., Olaussen, S., 2001. Lower Cretaceous (Barremian–Albian)deltaic and shallow marine sandstones in North-East Greenland — sedimentology,sequence stratigraphy and regional implications. In: Martinsen, O.J., Dreyer, T.(Eds.), Sedimentary Environments Offshore Norway — Palaeozoic to Recent: NPFSpecial Publication, vol. 10, pp. 259–278.

Law, A., Raymond, A., White, G., Atkinson, A., Clifton, M., Atherton, T., Dawes, I.,Robertson, E., Melvin, A., Brayley, S., 2000. The Kopervik fairway, Moray Firth, UK.Petroleum Geoscience 6, 265–274.

Lomas, S.A., Joseph, P., 2004. Confined turbidite systems. In: Lomas, S.A., Joseph, P.(Eds.), Confined Turbidite Systems, vol. 222. Geological Society, London, UK, pp.1–7.Special Publication.

Løseth, T.M., 2001. Rannoch and Etive Formations depositional model, Oseberg Field,Northern North Sea. Internal Report. Norsk Hydro Research Centre, Bergen, Norway.

Martinsen, O., 1994. Mass movements. In: Maltman, A. (Ed.), The Geological Defor-mation of Sediments. Chapman and Hall, London, UK, pp. 129–165.

Martinsen, O.J., Lien, T., Jackson, C., 2005. Cretaceous and Palaeogene turbidite systemsin the North Sea and Norwegian Sea Basins: source, staging area and basinphysiography controls on reservoir development. In: Dore, A.G., Vining, B.A. (Eds.),Petroleum Geology: North-West Europe and Global Perspectives — Proceedings ofthe 6th Petroleum Geology Conference. The Geological Society of London, London,UK, pp. 1147–1164.

Morton, A.C., Haszeldine, R.S., Giles, M.R., Brown, S., 1992. Geology of the Brent Group,vol. 61. The Geological Society, London, UK. Special Publication.

Nøttvedt, A., Gabrielsen, R.H., Steel, R.J., 1995. Tectonostratigraphy and sedimentaryarchitecture of rift basins, with reference to the northern North Sea. Marine andPetroleum Geology 12, 881–901.

Oakman, C., 2005. The Lower Cretaceous plays of the Central and Northern North Sea:Atlantean drainage models and enhanced hydrocarbon potential. In: Dore, A.G.,Vining, B.A. (Eds.), PetroleumGeology: North-West Europe and Global Perspectives—Proceedings of the 6th Petroleum Geology Conference. The Geological Society ofLondon, London, UK, pp. 187–198.

Oakman, C.D., Partington, M.A., 1998. Cretaceous, In: Glennie, K.W. (Ed.), PetroleumGeology of the North Sea — Basic Concepts and Recent Advances, 4th ed. BlackwellScience Ltd., London, UK, pp. 294–349.

Posamentier, H.W., 2003. Depositional elements associated with basin floor channel-levee system: case study from the Gulf of Mexico. Marine and Petroleum Geology20, 677–690.

Posamentier, H.W., Kolla, V., 2003. Seismic geomorphology and stratigraphy ofdepositional elements in deep-water settings. Journal of Sedimentary Research73, 367–388.

Prosser, S., 1993. Rift-related linked depositional systems and their seismic expression.In: Williams, G.D., Dobbs, A. (Eds.), Tectonic and Seismic Sequence Stratigraphy,vol. 71. Geological Society, London, UK, pp. 35–66. Special Publications.

Prior, D.B., Bornhold, B.D., 1988. Submarine morphology and processes of fjord fandeltas and related high-gradient systems; modern examples from British Columbia.In: Nemec, W., Steel, R.J. (Eds.), Fan Deltas; Sedimentology and Tectonic Settings.Blackie and Son, Glasgow, UK, pp. 125–143.

Ravnås, R., Bondevik, K., 1997. Architecture and controls on Bathonian–Kimmeridgianshallow marine syn-rift wedges of the Oseberg–Brage area, northern North Sea.Basin Research 9, 197–226.

Ravnås, R., Steel, R.J., 1998. Architecture of marine rift-basin successions. AAPG Bulletin82, 110–146.

Shanmugam, G., Bloch, R.B., Mitchell, S.M., Beamish, G.W.J., Hodgkinson, R.J., Damuth,J.E., Straume, T., Syvertsen, S.E., Shields, K.E., 1995. Basin-floor fans in the NorthSea: sequence stratigraphic models vs. sedimentary facies. AAPG Bulletin 79,477–512.

Skibeli, M., Barnes, K., Straume, T., Syvertsen, S.E., Shanmugam, G., 1995. A sequencestratigraphic study of the Lower Cretaceous deposits in the northernmost NorthSea. In: Steel, R.J., Felt, V., Johannessen, E., Mathieu, C. (Eds.), Sequence Stratigraphyon the Northwest European Margin: NPF Special Publication, vol. 5, pp. 389–400.

Surlyk, F., Dons, T., Clausen, C.K., Higham, J., 2003. Upper Cretaceous. In: Evans, D.,Graham, C., Armour, A., Bathurst, P. (Eds.), The Millennium Atlas: petroleumgeology of the central and northern North Sea. The Geological Society of London,London, UK, pp. 213–233.

Wien, S.T., Kjennerud, T., 2005. 3D Cretaceous to Cenozoic palaeobathymetry of thenorthern North Sea. In: Wandas, B.T.G., Eide, E.A., Gradstein, F., Nystuen, J.P. (Eds.),Onshore–Offshore Relationships on the North Atlantic Margin: NPF SpecialPublication, vol. 12, pp. 241–253.

Zachariah, A.-J., Gawthorpe, R., Dreyer, T., Corfield, S., 2009. Controls on early post-riftphysiography and stratigraphy, lower to mid-Cretaceous, North Viking Graben,Norwegian North Sea. Basin Research 21, 189–208.

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early postrift strata

theility of early postrift

development of early

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basin conguration

shortterm changes

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