Tectonic controls on bathonian-volgian syn-rift successions on the visund fault block, northern north sea

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Tectonic controls on Bathonian-Volgian syn-rift successions on the Visund fault block, northern North Sea R.B. Fa~rseth, T.S. Sj~blom, R.J. Steel, T. Liljedahl, B.E. Sauar and T. Tjelland

The 23-26 km wide Visund fault-block (Tampen Spur area of the northern Viking Graben) had a varied structural and sediment-infill history during some 10 degrees of tilt development from earliest Bathonian to late Volgian/Ryazanian times. During an early syn-rift phase (Bathonian to mid Oxfordian) a muddy Heather Formation wedge (0-700 m thick) accumulated during an initial 2 degrees of half-graben tilt. Despite the development of a base syn-rift unconformity around the structural crest-area, there are no significant resedimented sands in the early half-graben, and there was no major footwall island during most of this interval. Increased extension and an accelerated fault-block rotation saw a subsequent 4 degrees of tilt develop during the next 6-7 million years, by mid-Kimmeridgian times. There was also the partial collapse of the then-crestal area at this time, leading to a decoupling of the present southeastern terrace from the present crestal area. More significant emergence of a footwall island saw erosion of the present crestal area and took place during a subsequent 8-9 million years with a further 4 degrees of tilting. Erosion of some 90 km3 of Triassic-Jurassic strata from the structural crest (during early Kimmeridgian to mid-Volgian times) led to sediment dispersal and resedimentation of clastic material both down the dip slope to the northwest as well as over the scarp slope to the southeast. The northwesterly sub-basin received several major clastic pulses in the form of sediment gravity flows which formed submarine fan lobes or aprons, but from mid-Volgian times was sediment-starved and shows no late syn-rift clastic wedge infill. The southeasterly sub-basin shows a thick conglomeratic infill beyond the NE-SW trending fault-scarp, derived by erosion (probably largely Volgian) from this footwall. The sub-basin infill is dominated by upward-fining motifs (tens of metres thick) of resedimented conglomerates to turbiditic sandstones and shales. Some of the motifs show a basal progradational trend, a few metres thick. In contrast to well-fed syn-rift basins, the Visund sub-basins do not show late-stage shallowing and progradation. The sediment starvation signature on Visund is one of overall upwards fining. Decreased rotation rate and generally increased subsidence led to a latest Jurassic submergence of the fault-block crest and draping by late Volgian-Ryazanian mudstones, possibly indicating a transition then to a post-rift stage of development.

Introduction D e s p i t e m u c h r e s e a r c h and publication on the structural m e c h a n i s m s and controls of extension and rifting, as well as on the stratigraphy and sedimentation in rifts and half-grabens, t h e r e is still a significant lack of precision as regards the s e d i m e n t a r y architecture and signatures expected to f o r m f r o m specific e p i s o d e s / m a g n i t u d e s / r a t e s of extension and block-rotation in a syn-rift setting. A l t h o u g h this is u n d e r s t a n d a b l e ( t h e r e are a whole host of additional variables affecting the relationship, such as source a r e a extent/characteristics, sea level changes, s e d i m e n t supply and t r a n s p o r t mechanisms, for exa m p l e ) it r e p r e s e n t s a m a j o r weakness in our ability to use the t e c t o n i c s / s e d i m e n t a t i o n relationship in a predictive m a n n e r , for example to identify and m a p p o t e n t i a l reservoir sand resulting f r o m the syn-rift process. S e q u e n c e stratigraphy which has precisely this predictive ambition, has not yet b e e n able to address

successfully this puzzle of syn-rift sedimentation. T h e restricted and variable n a t u r e of tilt-block d r a i n a g e areas, as a result of strong local tectonics, d e m a n d s a detailed u n d e r s t a n d i n g of local p a l e o g e o g r a p h y before any sand-shale distribution can be predicted. T h e r e are broadly 4 groups of r e c e n t studies which have advanced our u n d e r s t a n d i n g of the relationship b e t w e e n extensional tectonics and the r e s p o n d a n t s e d i m e n t a r y architecture: (1) Work which has e m p h a s i s e d the g e n e r a l lithological signature of subsidence rate and the a s y m m e t ric g e o m e t r y of fault controlled s e d i m e n t a r y packages. Particularly critical has b e e n the realisation that the shale-prone intervals (where time-equivalent c o n g l o m e r a t e s are confined to n a r r o w fault scarp belts) in syn-tectonic successions c o m m o n l y reflect the most rapid differential subsidence periods (Steel, 1988). In contrast, the intervals w h e r e the coarsest facies extend farthest f r o m the fault scarp are the periods of relative tectonic stability (Blair, 1987; Blair and Bilodeau, 1988; H e l l e r and Paola, 1993). T h e s e

Sequence Stratigraphy on the Northwest European Margin edited by R.J. Steel et al. NPF Special Publication 5, pp. 325-346, Elsevier, Amsterdam. 9 Norwegian Petroleum Society (NPF), 1995.

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R.B. Faerseth, T.S. SjCblom, R.J. Steel, T. Liljedahl, B.E. Sauar and T. Tjelland

themes have recently been summarised, discussed and applied to seismic data sets by Prosser (1993). (2) The description of rift-basin facies models (mainly non-marine) (Frostick and Reid, 1987; Leeder and Gawthorpe, 1987) as well as studies on Pleistocene and recent earthquake created faulting and concomitant control on slope and drainage development has explored the interplay between extensional tectonics and sedimentation. (Leeder and Alexander, 1987; Collier, 1990; Leeder and Jackson, 1993). (3) Numerical and analogue structural modelling which has highlighted and evaluated the significance of the most important structural parameters (Barr, 1987a, b; Kuszinir et al., 1991). Fault spacing is critical to tilt-block size and behaviour (Yielding, 1990) and therefore to drainage area dimensions and sediment yield potential. Variable stretching rate during rifting relative to the thermal component of basin subsidence, as well as variable half-graben load have been shown to affect onlap or offlap on the half-graben dip-slope (Hardy, 1993; Roberts et al., 1993a; Waltham et al., 1993), and should therefore have important consequences for stacking patterns and sedimentary architecture. (4) Field-based studies in marine syn-rift successions have revealed potentially significant tectonic signatures expressed in the rock strata. One of the most clear signatures of likely tectonic importance is an upward-fining (conglomerate-sandstone-shale) trend, registered at varying scales (10's to 100's of metres) in syn-rift strata of East Greenland (Surlyk, 1989, 1991) and described in data from the southern Viking Graben Brae field (Turner et al., 1987). Similar trends are inferred within component packages of the Inner Moray Firth syn-rift wedge (Underhill, 1991). In the present study, in the 4th category above, we present a case based on seismic and well data from the Visund fault block on the Tampen Spur area of the northern Viking Graben. Syn-rift wedges of contrasting types are documented within both the "Heather" and the "Draupne" intervals, through a Bathonian to Volgian time period. We attempt to show how both general and specific aspects of Jurassic tectonics have affected the distribution of conglomerates, sandstones and shales.

Structural framework The northern North Sea encompasses the northern part of the Viking Graben and the flanking East Shetland Basin/Tampen Spur and Horda Platform (Fig. 1). The study area comprises the easternmost major fault block on the Tampen Spur, adjacent to the western margin of the Viking Graben.

A considerable crustal thinning is evident in the northern North Sea, following late Permian-early Triassic and Jurassic extensional tectonics which was also the main control on basin evolution. In the deepest parts of the Viking Graben the thickness of basement (pre-Triassic) is interpreted to be ca. 15 km (Klemperer, 1988; Holiger and Klemperer, 1989, 1990; Fichler and Hospers, 1990) as compared to 32 km in western Norway and on the Shetland Platform (assumed to be the original crustal thickness). The structure of the northern North Sea is, at Jurassic levels, dominated by faults with an overall N-S orientation, whereas a NE-SW trend is evident either as individual faults or as segments on major N-S striking faults. An E - W to W N W - E S E basement grain is reflected as steep faults with the same orientation and, in places, by complex structuring representing accommodation zones where major normal faults change polarity. The major, basement-involved faults trending N S, which are typically 15-20 km apart, define the large-scale, tilted fault blocks which are characteristic structural features, at pre-Cretaceous levels, both to the east and to the west of the Viking Graben. These fault blocks contain some 4 km (compacted) of Jurassic-Triassic sediments which, at least in crestal position, may rest directly on basement. The fault blocks are buried beneath an infill of Cretaceous and Cenozoic sediments exceeding 5 km (compacted) in the graben and typically 2-3 km on the flanking areas. The tilted fault blocks, with their characteristic eroded crests, represent the main trapping style on the Horda Platform and in the East Shetland Basin/Tampen Spur areas, which have proven to be the most prolific oil and gas provinces in the North Sea. The Visund fault block in the Tampen Spur area which was first drilled in 1985/86 is now penetrated by eleven wells and established as an oil/gas discovery containing some 61 mill. tons recoverable oil equivalent.

Mesozoic stretching phases Two major tectonic phases in the Permo-Triassic and mid-late Jurassic, exerted the main structural control during the Triassic to Tertiary infiU of the northern North Sea basin. The clearest evidence for major Triassic fault activity in this area is seen across the Horda Platform. Wedge-shaped packages of Triassic rocks overlie tilted basement blocks (Badley et al., 1984, 1988; Lervik et al., 1989; Gabrielsen et al., 1990). The top of the syn-rift sequence is within the Teist Formation and is of Scythian age (Steel and Ryseth, 1990). Triassic fault activity is also documented

Tectonic controls on Bathonian-Volgian syn-rifi successions on the Visund fault block, northern North Sea

327

Fig. 1. Map showing some of the main late Jurassic faults in the northernmost North Sea and the study area on the Tampen Spur.

west of the Viking Graben, e.g. the Hutton fault, the Pobie fault (Johns and Andrews, 1985) and its possible northeasterly continuation, the Tern-Eider fault (Fig. 1) (Roberts et al., 1990a; Yielding et al., 1992). The significance of this phase of extension has been highlighted by Giltner (1987) who has suggested that underestimation of thermal subsidence associated with Triassic extension has caused overestimates of late Jurassic extension in the Viking Graben. Total extension in the axial areas of the basin (I3 = 1.8) calculated by means of subsidence modelling, has been suggested to consist of Triassic stretching by a factor of about 1.5 and late Jurassic extension of about 1.2 (Giltner, 1987). Marsden et al. (1990) and Kusznir et al. (1991) applying the flexural cantilever model on a ca. 270 km transect across the northern North Sea infer whole-basin Triassic and Jurassic extension of 38 km and 22 km respectively. On the Horda Platform the extension associated with the Triassic faulting is said to correspond to a stretching factor of 1.3-1.4, whereas the Jurassic-lower Cretaceous reactivation has a 13 < 1.1 (Yielding et al., 1992). Also in the Tampen Spur area significant early Triassic-?late Permian fault movement (/3 of ca. 1.4, Yielding et al., 1992) was followed by a smaller amount (/3 of ca. 1.15, Roberts et al., 1993b) of late Jurassic offset. Middle and upper Triassic, lower Jurassic and middle Jurassic strata (ca. 70 Ma) are termed post-rift

and interpreted to represent a response to thermal subsidence following ? late Permian-early Triassic stretching. Although there are significant thickness changes of lower Triassic (Steel and Ryseth, 1990) and early-middle Jurassic (Yielding et al., 1992) strata across some of the major faults, the most striking feature of the post-rift stratigraphy is the presence of some 9 major clastic wedges, originating from the Shetland and Norwegian hinterlands. It has been argued that these reflect the pulsed nature of thermal subsidence/compaction in this early postrift interval (Steel, 1993). As thermal re-equilibrium of the lithosphere takes place on a time-scale of the order of 100 Ma (McKenzie, 1978), it is likely that thermal effects from the early extensional phase would not have relaxed completely by the onset of renewed extension in the Jurassic. The geological effect of this background thermal anomaly during later extension would be to enhance subsidence of the basin floor and therefore to reduce Jurassic footwall uplift as discussed by Yielding et al. (1992) and Roberts et al. (1993a). In the North Sea, sedimentary units (VikingHumber Group) from the late middle Jurassic to the earliest Cretaceous were deposited during and after a major basin-deepening event. They show evidence of wedge-shaped, syn-sedimentary geometries adjacent to major faults, suggesting syn-depositional faults movement. This second rift phase affected

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R.B. Fcerseth, T.S. SjCblom, R.J. Steel, T. Liljedahl, B.E. Sauar and T. Tjelland

the northern North Sea from the Bathonian to the Ryazanian, although the onset of block tilting was not synchronous throughout the basin. Models of extensional faulting The presence of large, tilted fault blocks is a characteristic feature of extensional basins. An understanding of fault block behaviour through time, and especially of footwall uplift, is of considerable relevance to the exploration for hydrocarbons because the upper flanks of half-grabens represent the most economically attractive and thoroughly explored parts of such sedimentary basins. Their uplift histories have particularly affected the extent to which pre-rift sediments have been eroded across the crests of fault blocks and the facies of nearby syn-rift sediments. The formation of an extensional basin can be described by the uniform lithospheric stretching model of McKenzie (1978), which predicts the vertical movement of the basin floor through time. However, this model does not take into account the presence of upper crustal fault blocks. Barr (1987a, b), coupled together the McKenzie model for whole-lithosphere extension with a geometric model for upper-crustal normal faulting and used the rotated planar fault or domino mechanism to establish a predictive model, both for the magnitude of uplift, and the subsidence associated with the evolving half-graben as well as for the patterns of stratigraphic fill within the halfgraben. Kusznir and Egan (1989) and Kusznir et al. (1991) have constructed mathematical models of geometric, thermal and flexural-isostatic response of the lithosphere to extension by faulting (simple-shear) in the upper crust and plastic, distributed deformation (pure-shear) in the lower crust and mantle. The resulting model, termed the "flexural cantilever" model, has been used to study the development of basin profiles in the northern North Sea (Marsden et al., 1990; Kusznir et al., 1991; Roberts et al., 1993b). Footwall uplift may have exceeded 1000 m adjacent to major faults according to Yielding et al. (1992), whereas Marsden et al. (1990), Kusznir et al. (1991) and Roberts et al. (1993b) found that modest footwall uplift (a few hundred metres above sea-level) occurred during the late Jurassic. Sedimentation at this stage, however, was sometimes unable to keep pace with the rapid hangingwall subsidence, and therefore substantial water depths (ca. 700 m average) developed rapidly. For a set of adjacent planar faults, evenly spaced, and all of similar displacement the lateral superposition of flexural footwall uplift and hanging wall collapse generates the familiar rigid block-rotation,

i.e. the domino model of Barr (1987a, b). The domino model has been applied to fault blocks in the northern North Sea by Barr (1987b, 1991), White (1990), Yielding (1990) and Yielding and Roberts (1992). Model predictions of footwall uplift have been found to be in close agreement with observed depth of erosion and hanging wall subsidence across a whole range of structures. Hence, while the structural predictions of the domino model are now well tested, the stratigraphic predictions have been less well investigated. In a recent paper Roberts et al. (1993a) discuss deviations from the basic domino assumptions. They state that due to the geological effect of a background subsidence anomaly (following early Triassic extension) during Jurassic extension, and change to depositional environments with deeper water and reduced sediment input through time, the geological history of the northern North Sea does not fulfil the assumptions built into the basic domino model of Barr (1987a, b). They further conclude that the basic domino model, or published variants of this model cannot be used to investigate the complex syn-rift stratigraphy of the East Shetland Basin/ Tampen Spur province fault blocks. Instead they have devised further modifications to the domino model which allow the prediction of somewhat more detailed stratigraphic geometries within the evolving half-graben. In their models, Roberts et al. (1993a) apply an initial fault-plane spacing of 15 km and an initial fault-plane dip of 60~. However, predictions of footwall uplift are favoured both by a steep initial fault-plane dip and a large initial fault-plane spacing (McKenzie, 1978; Barr, 1987b). By varying the density of the half-graben load by increments of tilt during extension Roberts et al. (1993a) model the amount of uplift of fault-block crest above sea level, and the distance from the fault-block crest at which the pre-rift sequence becomes emergent. Variations in the width of this "emergent island", caused by variable stretching rates and half-graben loads, should lead to variable onlap and offlap patterns in the stratigraphy of the dip-slope strata. In this paper we have adopted the model predictions by Roberts et al. (1993a). The latter are based on values which we consider to be consistent with the Visund fault-block parameters. This allows us to test stratigraphic as well as structural predictions of the modified domino model. The Visund Fault Block The Visund fault block represents the easternmost structure in an array of N-S oriented and westerly rotated fault blocks between the Viking Graben and the

Tectonic controls on Bathonian-Volgian syn-rift successions on the Visund fault block, northern North Sea

East Shetland Platform (Fig. 1). The average spacing of N-S to NNE-SSW trending, block-bounding and basement involved master faults is ca. 20 km. Yielding et al. (1991) in a study of the geometry of major faults on the Horda Platform and in the East Shetland Basin/Tampen Spur area, claim that a common feature is that offsets at top basement are much larger than at top middle Jurassic (ca. 5 km and ca. 1 km respectively) implying significant Permo-Triassic movement followed by a smaller amount of Jurassic offset. However, the main faults bounding the tilted fault blocks located adjacent to the Viking Graben exhibit much larger Jurassic offsets. The fault representing the boundary between the Visund fault block and the Viking Graben has a throw of ca. 5 km at top middle Jurassic. The seismic data do not allow the top basement to be identified with any confidence across this fault. However, applying a basement thickness of 15 km in the Viking Graben (Klemperer, 1988; Holiger and Klemperer, 1989, 1990; Fichler and Hospers, 1990) would not allow a considerably larger offset at top basement than that recorded at top middle Jurassic. Hence, all Jurassic master faults were not necessarily preceded by Permo-Triassic movement of similar or larger magnitude. This is supported by observations on the master fault bounding the Visund fault block to the west and which has been named the Inner Snorre Fault following Gabrielsen (1986). Nelson and Lamy (1987, their fig. 5) indicate throws across this fault of ca. 3 km and ca. 4 km at top middle Jurassic and top(?) basement respectively, showing that the main offset is related to Jurassic stretching. Within the Snorre fault block (Fig. 1), Dahl and Solli (1993) argue that N-S striking, west-dipping faults had most impact on Triassic thickness distribution. Depth-migration shows that major basementinvolved faults on the Horda Platform and in the East Shetland Basin/Tampen Spur have planar geometries. However, the dip changes from typically 300-35 ~ in the basement to 400-50 ~ in the upper part where they cut through the sedimentary column (Yielding et al., 1991). The original fault dips through Jurassic sediments were probably 500-60 ~ before compaction which is the dip that the faults would have had when active, and prior to Cretaceous and Tertiary sedimentation. Nelson and Lamy (1987, their fig. 5) show a true-scale depth crosssection from the Inner Snorre Fault. Here the lower basement-involved part of the fault is planar down to a depth of 12 km with a present dip of 250-30 ~ whereas the upper part becomes steeper, i.e. a similar geometry to that observed by Yielding et al. (1991) on other basement-involved faults in the northern North Sea.

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The present day eastern boundary of the Visund fault block is defined by a fault complex where faults change orientation from N-S to N E - S W as they converge towards the Inner Snorre Fault (Fig. 2). In the northeastern part of the study area where the boundary fault possesses a N-S orientation, vertical displacement may exceed 5 km at middle Jurassic level. From the point where the present day boundary fault changes orientation and runs in a NE-SW direction, the N-S lineament continues to the south and is interpreted as the boundary fault of the incipient (Bathonian-Oxfordian) Visund fault block. In the southern part of the study area this lineament is represented by flexuring at base Cretaceous and fault displacement at deeper levels. Further south the lineament, with a change in polarity represents the eastern boundary of the Viking Graben. It bounds the Huldra- and Oseberg fault blocks (Fig. 1), and is therefore of regional significance. This master fault at ca. 2~ together with the Inner Snorre Fault, define the Visund fault block with a present width of 23-26 km. Hence, the Visund fault block is wider than average in this part of the North Sea. With a current average dip of 9 ~ at pre-rift level it is also, together with the Gullfaks fault block located immediately to the southwest, the block which possesses the steepest dip at pre-rift level. The average dip of the fault blocks in the East Shetland Basin and Tampen Spur province (Yielding, 1990) is otherwise about 6 ~. Following increased extension and rotation, the southeastern part of the mega-block collapsed during early Kimmeridgian times to form a structural terrace between the present crestal area and the Viking Graben proper to the east (Figs. 3 and 4). A characteristic feature of the faults activated at this stage is their curved nature (Fig. 7). Depth conversion reveals ramp-fiat-ramp geometries with a present maximum dip of 50 ~, whereas flat segments may possess dips below 10~ The faults tend to converge at depths to create a listric fan. The instability of the footwall of the mega-block as well as a considerable relief during the late Jurassic is also suggested by observations of Alhilali and Damuth (1987). They describe a slide block composed of Brent Group strata that apparently detached from the easternmost part of the Visund fault block in the northeast (area with N-S trending boundary fault) and slid downslope on a listric fault. Following decoupling of the SE-flank, the present NE-SW oriented crestal area became the site of late Jurassic erosion. Over a timespan of maximum 8-9 Ma, strata of early Oxfordian-Bathonian (Heather Formation) to Triassic age and representing ca. 1200 m (compacted thickness) of section were

R.B. Faerseth, T.S. SjCblom, R.J. Steel, T. Liljedahl, B.E. Sauar and T. Tjelland

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2040 .

2020 ,

i

61~

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JO t~ It_

O O") C =.,.

= u

61o15 ,

5 km i

Brent Group Dunlin Group

--.._...-----> Sediment feeder conduit 9 Well location

Statfjord Formation Triassic

Fig. 2. Pre-rift units subcropping at base Draupne Formation together with distribution of Draupne units A 2 - A 4 on southeast flank of the Visund structure.

progressively removed from the crestal area (Fig. 4). The present day crest of the Visund fault block is 3.5-5 km away from the major bounding faults (Fig. 3) with the eroded area becoming wider to the north (Fig. 2). The large volume of sediments removed was transported across the block-bounding faults onto the terrace area to the southeast, but also down the dip-slope into the evolving half graben to the northwest. The half-graben development is a part of a largerscale stratigraphic transition in depositional environment from fluvio-deltaic (Ness Formation) to shallow-marine sandstones (Tarbert Formation) and marine siltstones (Heather Formation), to marine shales (Draupne-/Kimmeridge Clay Formation), to deepwater marine limestones (Lower Cromer Knoll Group). This transition is characterized by a reduction in the input of clastic sediments, followed by a reduction in the input of suspended sediment. At the

same time an overall increase in bathymetry is indicated, from above sea level (Ness Formation) to several hundred metres (Cromer Knoll Group). The crestal erosion surface of the Visund fault block is covered by condensed late Volgian-Ryazanian shales of the Draupne Formation, implying that the crest was below sea level during the waning stretching phase.

Early Bathonian-Mid Oxfordian syn-rift (Heather Formation)

Syn.rift wedge and basal unconformity In the northern North Sea the cross-sectional wedge-shaped nature of the Heather Formation adjacent to major faults is a characteristic feature. The Heather Formation may represent a major part of the total volume of the Bathonian-Ryazanian wedge on some of the tilted fault blocks. On the Visund fault

Tectonic controls on Bathonian-Volgian syn-rift successions on the Visund fault block, northern North Sea

331

Fig. 3. Structural cross-section and stratigraphy of the Visund fault block; note the collapsed segment on the south-east flank of the structure, which was part of the fault-block crest at an early stage.

block the Heather and Draupne Formations have about equal thickness within the half-graben (Fig. 3). Roberts et al. (1993a) by applying a modified domino model to the development of the Brent/ Statfjord fault block (Fig. 1), claim that during initial (Bathonian) stage of stretching, tilting of the fault block (which was less than 1~ did not result in uplift of the fault block crest above sea-level. Their model predicts submergence of the whole block, such that the lower (Bathonian-Callovian) part of the Heather Formation caps the whole fault block. Hence, there is no unconformity at the base of the syn-rift sequence, in contradiction to the domino model of Barr (1987b; his figs. 8 and 9). On the Visund fault block, however, the lower (early-middle Bathonian) part of the Heather Formation, which exhibits a clear wedge-shaped geometry, may be interpreted partly in terms of a regional subsidence increase which led to the drowning of the Brent Group delta and partly in terms of local, gentle synrift tilt-subsidence. The tilt of the fault block during the Bathonian was probably less than 1~ and it is possible that a thin Heather Formation may have capped some of the crestal area of the Visund fault block at some points in Bathonian time. However, the presence of a partly eroded Tarbert Formation

overlain by early Callovian Heather Formation, indicates that even modest rotation was sufficient to create a base syn-rift unconformity in the crestal region. As mentioned above, there is increasing evidence and documentation that there was no synchronity for the initiation of syn-rift conditions in the different areas of the northern North Sea. This contradicts the geometric requirements built into the domino model (e.g. Barr, 1987a, b) that all fault blocks move simultaneously. This assumption has, nevertheless, been used in the modelling of syn-tectonic relationships in the North Sea, implying that fault blocks in any transect across the basin developed synchronously and that syn-rift fill of individual half-grabens is of the same age range (Callovian-Ryazanian) (e.g. Bertram and Milton, 1989; Roberts et al., 1990b; Barr, 1991; Roberts and Yielding, 1991; Roberts et al., 1993b). Graue et al. (1987) and Helland-Hansen et al. (1992) argued that the latest early Bajocian retreat of the Brent delta could have signalled the initiation of extension, and the same notion has been taken further by Johannessen et al. (1995) who document late Bajocian block tilting in the Statfjord/Gullfaks area. On the other hand, Yielding et al. (1992) calculated the syn-Brent Group extension to be negligible

R.B. Fcerseth, T.S. SjCblom, R.J. Steel, T. Liljedahl, B.E. Sauar and T. Tjelland

332

Sea

t

orahan

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,w•

~'~

level

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Bathymetry

Bathymetry

Bathymetry

Fig. 4. Schematic illustration of the main stages of syn-rift development of the Visund fault block, from 2 ~ tilt in early Oxfordian to 10~ tilt by early Ryazanian.

(fl = 1.01) along a 200 km, E-W basinal transect, and suggested that the use of a syn-rift terminology for this time interval is misleading. In the Inner

Moray Firth area the onset of significant extension is dated to early Kimmeridgian (Underhill, 1991). However, dip-lines presented show wedge-shaped ge-

Tectonic controls on Bathonian-Volgian syn-rift successions on the Visund fault block, northern North Sea

ometries for Bajocian-mid Oxfordian strata also in that area (Underhill, 1991, his figs. 6 and 7).

Faulting and tilting prior to footwall collapse The Heather Formation appears to have capped the entire Visund fault block, at least in the earlymiddle Callovian, though it is now absent in a 3.55 km wide crestal area adjacent to the major fault zone which presently bounds the structure to the east-southeast (Fig. 2). The exact timing of the erestal erosion within the Jurassic is uncertain, but it did not include the early-middle Callovian. Table 1 shows the situation in different parts of the Visund fault block. An early-middle Bathonian Heather Formation conformably overlies the Tarbert Formation both in the half-graben and towards the crest before it is finally truncated. Until the early Kimmeridgian times the area between the Inner Snorre Fault and the major N-S trending fault at ca. 2~ 40'E represented one structural unit (Fig. 4). Before the Kimmeridgian-Volgian footwall collapse, the present SE-terrace area was part of the Visund mega fault-block and represented the crestal area. In this structural position erosion penetrated down to the Tarbert Formation. Erosion ceased and the crestal area became drowned and draped by shales during early-middle Callovian. Whether the crestal area continued to accumulate Heather Formation shales during the late Callovian to early Oxfordian interval is uncertain, but the geometric evidence from the deeper half-graben areas suggest erosion or condensed deposition as discussed below. The wedge-shaped Callovian Heather Formation on the southeastern structural terrace, although partially a result of base Draupne Formation erosion, may indicate initial movement on the NE-SW striking faults which bound the terrace to the northwest. However, as Kimmeridgian Draupne Formation shales unconformably overlie the Heather Formation (Fig. 4), the terrace remained in a structurally high position at this stage with the crest close to the sea level. At the end of the Heather Formation deposition (mid Oxfordian), the tilt of the pre-rift sequence is estimated to be a minimum of 2~ assuming that the half-graben was filled to sea-level with sediments. Subsidence in the half-graben was gentle with ca. 700 m uncompacted Heather Formation deposited over a span of 18 Ma. The throw on the Inner Snorre Fault during Heather Formation interval is the net result of half-graben subsidence and the footwall uplift processes of the Snorre fault-block crest above sea-level. Roberts et al. (1993a, their fig. 8) present a model which estimates 120-150 m footwall uplift

333

with 2-3 ~ of fault-block tilt. A total fault throw of ca. 850 m during deposition of the Heather Formation indicates a minimum of 45 m of throw per million years. However, Dahl and Solli (1993) have predicted the footwall uplift of the Snorre fault-block to have been some 600-700 m by the end of the deposition of Heather Formation. The estimated tilt of the Visund fault block during deposition of Heather Formation would have generated a 3.2-3.5 km wide footwall island, following Roberts et al. (1993a, their fig. 8), which would then have become a likely site of erosion. On the SE-terrace, representing the crestal area prior to footwall collapse, a base syn-rift unconformity is represented by early Callovian Heather Formation overlying eroded Tarbert Formation approximately 2 km west of the incipient master fault. This implies that wells located close to the present day crest of the Visund fault block were more than 5 km west of the incipient (Bathonian-Oxfordian) block-bounding fault, and therefore should not exhibit a base syn-rift unconformity. This is in accordance with observations of Bathonian Heather Formation conformably overlying a Tarbert Formation. As a result of the slow Bathonian-early Oxfordian uplift the uncompacted deposits in the footwall crest would have been eroded/degraded faster than they could rise above sea-level and a footwall island with a significant topography is unlikely to have developed at this stage. Based on well-correlations ca. 30 m of Tarbert Formation was eroded at a location ca. 2 km west of the incipient master fault during an early (Bathonian) stage of footwall uplift, and it is most likely that this crestal area remained close to sea-level until mid Oxfordian time.

Syn-rift stratigraphy The Heather Formation is a calcareous, silty mudstone, ranging in age from early Bathonian to mid Oxfordian. It was deposited in a basinal marine environment, and is lithologically homogenous with very similar wireline log patterns over the eastern Tampen Spur area. Only traces of sand have been penetrated in the Heather Formation. Using decompacted thicknesses give a sedimentation rate in the basin centre of about 40 mm per 1000 year. Figure 5 shows a chronostratigraphical correlation of the Heather Formation over the Visund fault block based on palynological data. Four correlative intervals have been defined, i.e. early-mid Bathonian, late Bathonian-early Callovian, early-mid Callovian and late Callovian-mid Oxfordian. Early Bathonian Heather Formation appears to conformably overlie the Tarbert Formation in the

R.B. Faerseth, T.S. Sjcblom, R.J. Steel, T. Liljedahl, B.E. Sauar and T. Tjelland

334 TABLE 1

Summary of stratigraphy for the intra-Middle Jurassic to intra-Early Cretaceous interval for the various segments of the Visund fault block

! Age Ma

Stage

Down-flank area

Present crestal area

Collapsed footwall

Tectonic setting

Valanginian

Cromer Knoll Gp.

Cromer Knoll Gp.

Cromer Knoll Gp.

--121

--128

--131

Post-rift

Ryazanian

B2 Draupne Fm.

Draupne Fm.

- A3 ---A2

Volgian

_ ,,, | -~ ~=a. I I 8-~ 0"4~tilt

~Z,cK~ -Dr~tipn~.rr0.'l~ .

--140 --145

Debris flow

Kimmeridgian

.

.

.

.

.

LLO m ~

B1 Draupne Fi

m

c.4 ~ tilt

Syn -rift

Oxfordian - - 152

Callovian

Heather Fm.

Heather Fm.

- - 157

I c.2 ~ tilt

L, Heather Fr?

Bathonian 165

Brent Gp.

Bajocian

Pre-rift

Brent Gp.

171

Thickness (m) 0 --

100--

Hanging wall of

Dip slope

Present Crest

Inner Snorre Fault ~.

L. CallovianM. Oxfordian

Collapsed footwall area

Location

m,pJ / visur~

fault block

E.-M .

.

.

.

.

.

.

.

.

.

.

.

.

/

/

p

. "",,l,,,,,,,1%

200-

L L_.BathonianBathonian-

300

~

m

Fig. 5. Chronostratigraphic correlations are compacted, present day thickness of the Heather Formation across the Visund fault block. Note the geometry of successive stratigraphic segments.

eastern Tampen Spur area, and represents the drowning and transgression of the Brent delta. This transgression appears to have been nearly synchronous over the eastern Tampen Spur area. There are, however, uncertainties in the datings of this boundary due to a barren underlying Tarbert Formation, and differences in the zonation of the mid-late Jurassic section between the different contractor companies analyzing the biostratigraphy of the wells (B. Pilskog, pets. commun., 1993). The oldest of the four correlative intervals, the early-mid Bathonian, shows a clear wedge shape. In the basin centre, close to the Inner Snorre Fault,

more than 170 m (decompacted) of this part of the Heather Formation was deposited, whereas less than 60 m was deposited higher on the upflank area of the Visund fault block. Over the paleo-crest of the structure this correlative unit is absent, but it is not clear whether it was deposited and removed by later erosion, or whether the crest was then in a position of erosion/non-deposition. If the geometry of the wedge shape is used as an indication of fault movement and rotation, it shows the rotation of the Visund fault block within this period to have been ca. 0.5 ~. The wedge-shaped geometry of the overlying late Bathonian-early Callovian unit is even more distinct

Tectonic controls on Bathonian-Volgian syn-rifi successions on the Visund fault block, northern North Sea

(Fig. 5), giving a total rotation of the Visund fault block in the early Callovian of ca. 1~ It is likely that this rotation and footwall uplift had caused erosion in the crestal areas of the footwall by early Callovian times, and that this is the reason for the absence of the uppermost Tarbert Formation and lowermost Heather Formation (Fig. 5 and Table 1). However, as no sand is found downflank within this unit, and the fact that the Heather Formation in general is complete, the erosion of the crest is not likely to have been extensive. Based on well-correlations an estimate of some 50 m eroded (30 m and 20 m of Tarbert and Heather Formations respectively) seems plausible. The geometry of the overlying early-mid Callovian unit differs clearly from the underlying wedges (Fig. 5). It has a tabular shape with a nearly uniform thickness (ca. 60 m, decompacted) in both the basinal and crestal areas. This tabular shape is interpreted to represent a period of only mild tectonic activity, and suggests that the sediment input during the earlier intervals had filled the accommodation space created, resulting in a low relief area during early Callovian times. The youngest (late Callovian-mid Oxfordian) unit of the Heather Formation again shows a clear wedgeshaped geometry, and is not present in the crestal area (represented by the southeastern terrace), at least, within a distance of ca. 2 km west of the block-bounding fault. However, along the present N E - S W trending crest which was located west of the late Callovian-early Oxfordian footwall island, thin remnants of this unit are locally preserved in fault-controlled depressions which indicates deposition and later erosion following Kimmeridgian footwall collapse and generation of an uplifted area northwest of the N E - S W trending fault zone. Sandstones and siltstones within the Heather Formation at this level are interpreted as the erosional products of crestal areas. The Heather Formation is clearly part of a syn-rift sequence, although the larger magnitudes of fault activity and fault block rotation occurred later. The early rotation was sufficient to create a base syn-rift unconformity in the crestal areas.

Late Oxfordian-Volgian syn-rift (Draupne Formation) Rotation, refief development and partial structural collapse The Draupne Formation of the Visund area consists of late Oxfordian-Ryazanian sedimentary units deposited over a period of ca. 19 Ma. These strata

335

have a clear wedge-shaped geometry both in the main Visund half-graben as well as in the downfaulted subbasin to the southeast of the block-crest (Figs. 3 and 4). The main half-graben, with more than 1000 m of Draupne Formation in its axial area (units B1 and B2 in Figs. 4 and 6) and nearly thinning out on the crested area, appears to be dominated by shales. However, wells have been drilled only on the terraced area near the Inner Snorre Fault (Fig. 2) and upflank in the half-graben. The downfaulted basin to the southeast has up to 700 m of Draupne Formation ( A 1 - A 4 in Figs. 4 and 7) and contains thick conglomeratic intervals. The Draupne Formation was deposited under conditions of increased extension and accelerated fault-block rotation rate (compared to Heather Formation) and reflects a pulsed syn-rift sedimentation. During deposition of the Draupne Formation the tilt of the pre-rift sequence (Brent Group) in the Visund fault block increased from ca. 2 ~ in mid Oxfordian (top Heather Formation) to a present tilt of 9~ (top Draupne Formation) and by this time the area was in transition to the post-rift stage. Assuming the same topographic level for the crest of the Visund fault block as for the fiat-topped Snorre fault block to the west, taken to represent late syn-rift erosion near sea level, it appears that the Visund fault block has been rotated 1-1.5 ~ clockwise due to CretaceousTertiary thermal subsidence and increased sediment loading on the Visund fault block. A tilt of the prerift sequence of ca. 10~ is therefore likely, by late Volgian-Ryazanian times. There was increased bathymetry in the half-graben to the northwest as a consequence of the rotation. Because the rate of sediment input was not sufficient to keep pace with the subsidence created, the water depth increased. In the Visund fault block, 8~ of tilt at the top Heather Formation surface would have created a topographic difference of ca. 1400 m between the deepest part of the half-graben and the line of emergence (preservation limit of the Heather Formation) to the east. An uncompacted Draupne Formation thickness of some 1000 m was deposited in the deepest part of the half-graben, indicating that, at least during parts of the Draupne Formation interval, a considerable part of available half-graben space was water-filled. A bathymetry of ca. 600 m is estimated at the end of the Jurassic period as a result of tectonic tilt and compaction of late Jurassic sediments. The low-density of the water-rich basin fill would also have resulted in increased footwall uplift (Barr, 1987b). Assuming a footwall uplift of 400 m west of the adjoining Inner Snorre Fault during the Draupne Formation interval, which is in accord with a model of Roberts et al. (1993a, their

336

R.B. Fcerseth, T.S. SjCblom, R.J. Steel, T. Liljedahl, B.E. Sauar and T. Tjelland

Fig. 6. Seismic section NVGTI-92-105 and line drawing of the interpreted section across the western part of the Visund half-graben, adjacent to the Snorre fault block to the west.

fig. 10), and 1400 m of half-graben subsidence, the resultant estimated fault throw would have been ca. 1800 m during deposition of the Draupne formation i.e. ca. 100 m per million years. This is probably a minimum estimate, as both Yielding (1990) and Dahl and Solli (1993) have suggested total footwall uplift of the Snorre fault block in the order of 1.1-1.5 km (increasing to the north) following Bathonian-Kimmeridgian stretching, although unlike Roberts et al. (1993a) the former have not considered the effects of a background thermal anomaly on Jurassic footwall uplift. In the Visund half-graben the Heather Formation was followed in mid-late Oxfordian times by the deposition of Draupne Formation (Table 1).

The Draupne Formation thins to the east, and is present only as condensed Volgian-Ryazanian strata in the present day deeply eroded crestal area. During late Oxfordian-earliest Kimmeridgian times the presently down-faulted, SE-flank was still part of the Visund mega-fault-block (Fig. 4). This is evidenced by truncated Callovian Heather Formation overlain unconformably by a thin unit of Draupne Formation shales (unit A1 discussed below) of Kimmeridgian age (Table 1) on this downfaulted terrace (Fig. 7). An unconformity between the Draupne and the Heather Formations in crestal areas has been recorded on fault blocks in the Tampen Spur province and other parts of the North Sea basin by several workers. This unconformity is either interpreted as an intra syn-rift

Tectonic controls on Bathonian-Volgian syn-rift successions on the Visund fault block, northern Noah Sea

337

Fig. 7. 3D-seismic section NH 9001-0789 and line drawing of the interpreted section across the southeastern flank of the Visund fault block. Note the Draupne Formation intervals A2-A4.

wedge unconformity (e.g. Roberts et al., 1993a) or more often as a base syn-rift wedge unconformity. The gravity collapse of the crestal area with the decoupling of the SE-flank to become a structural terrace, possibly started in the mid-late Kimmerid-

gian although the main activity and full development of the conglomeratic fans is believed to have been in the Volgian. In the latest Jurassic the strain rate had slowed sufficiently such that background subsidence had begun to exert a significant effect so that the

338

R.B. Fterseth, T.S. SjCblom, R.Z Steel, T. Liljedahl, B.E. Sauar and T. Tjelland

crestal area of the Visund fault block became submerged and covered by late Volgian Draupne Formation muds. Assuming a constant rate of rotation and that the tilting of the Visund fault block had ended in the late Volgian, the tilt of the pre-rift sequence would have increased to ca. 6~ at the time of the main decoupling of the SE-flank. Hence, the development of an emergent footwall island and removal of sediments along the present NE-SW trending crest was associated with the subsequent 4~ of syn-rift tilt and took place over a time span of 8-9 Ma. Applying models published by Roberts et al. (1993a, figs. 12 and 13) and bearing in mind that the initial faultspacing for the Visund fault block exceeds the 15 km used in their models, a footwall island approximately 3 km wide and ca. 400 m above sea level would have become emergent to the east. This is generally consistent with observations of a 3.5 km wide zone with eroded strata along the NE-SW trending segment of the bounding fault. As thin (1-10 m) Draupne Formation shales of late Volgian-Ryazanian age cap major parts of the present day crestal area, the degradation of the uplifted footwall culminated in a mature topography close to sea level in the late stretching phase. This uppermost unit of the Draupne Formation reaches a thickness of some 100-150 m (decompacted) in the half-graben to the northwest. In contrast to the underlying syn-rift units, it thins up the Inner Snorre Fault and in total exhibits a geometry similar to the overlying post-rift Cromer Kroll Group (Fig. 6). It is likely that the uppermost late Volgian-Ryazanian shale unit of the Draupne Formation represents passive infill of earlier rift-generated topography and hence, the syn-to post-rift transition in this area is within the latest Volgian. As discussed above, a significant late Volgian-early Ryazanian bathymetry (ca. 600 m) is estimated in the half-graben to the northwest. As Cretaceous sediments were added to the half-graben fill, which resulted in burial of earlier topography in the Turonian, the underlying sediments have been continually compacted. Compaction of the Heather and Draupne Formations during this time period would have made room for some 400 m of additional sediment thickness in the deepest part of the half-graben, which again would have modified the early Cretaceous sedimentary thickness even though there was no actual fault movement. The Cromer Knoll Group which represents a time span of some 35 Ma is present as a highly condensed (1-5 m) sequence on the crest of the Visund fault block. It is too thin to be resolved on seismic data but reaches a maximum of ca. 25 m in fault-controlled topographic depressions across the crest. The Cromer Knoll Group sequence on either side of the Vi-

sund fault block crest thickens down-structure as a stratigraphic wedge, which could give the impression of accompanying fault movement. In contrast to the wedge-shaped, syn-rift units, the lower Cretaceous thins towards the faults on the limbs of compaction synclines (Fig. 3). We would therefore follow the interpretations made elsewhere in the North Sea Basin by Bertram and Milton (1989), Barr (1991), Cartwright (1991) and Roberts et al. (1993a) and attribute thickness variations within the lower Cretaceous primarily to passive infill of pre-existing bathymetry.

Development of erosion and drainage systems Tectonic activity on the Visund structure probably culminated in the Volgian, having caused repeated phases of footwall uplift along the NE-SW trending fault zone. Over a time span of maximum 8-9 Ma, sediments from Callovian-Bathonian (Heather Formation) to Triassic (Lunde and Lomvi Formations) in age and representing ca. 1200 m (compacted thickness) of section were progressively removed from the crest northwest of the fault zone and probably also from the retreating submarine fault scarp. The Visund fault block, is in general, characterized by a rounded profile, but becomes relatively flat to the south. The present structural crest is ca. 3.5 km northwest of the southeastern bounding fault zone (Fig. 2). A large volume of sedimentary strata were removed from the crest, probably by a range of sediment gravity flow and collapse processes. In an early stage (late Oxfordian-early Kimmeridgian) of footwall uplift, a northwesterly, dip-slope transport of deep water sandstones is likely to have taken place. Later Kimmeridgian rotation continued to cause partial infill of the half-graben to the northwest, as well as initial collapse of the crest and early infill to the southeast. Accelerated (Volgian) footwall uplift and associated mass flow, resulted in collapse of poorlyconsolidated sands and shales and resedimentation into the evolving half-graben to the southeast. These latter deposits are now present as a major wedgeshaped conglomeratic body at the base of the N E SW trending fault scarp. In an attempt to determine the sediment dispersal directions and sedimentary products which resulted from the deep erosion into the crest of the Visund structure, the following aspects were evaluated: (1) On the basis of the structure, what was the paleotopography and where were the likely erosional/ drainage domains? Which of these domains dispersed sediment to the northwest and which to the southeast? There would also have been some shift in these domains with time because of the tilt configuration

Tectonic controls on Bathonian-Volgian syn-rifi successions on the Visund fault block, northern North Sea

and because of the collapse of the southeast flank. (2) What proportion of mapped eroded volume is likely to have been sand and how much of this sand is likely to have been transported respectively to the NW and SE? (3) What are the volumes in the present late Oxfordian-Volgian syn-rift wedges to the NW and SE of the crestal area? The proportion of these volumes believed to have been drained from the Visund crest was checked against the erosional calculations. (4) Account has also to be taken of the likely pulsed nature of the structural rotation and footwall uplift, as well as of the erosion and the changes in relative and absolute sea level (as this affects erosion). This is important for the age of the redeposited sand, and together with the depositional processes/ environments determines the stratigraphic position, the location and the geometry of the potential reservoir sands. A volume balance was performed using decompacted thickness of lithologies both in the footwall and the hanging wall. Net/gross values from wells on the Visund Field were applied for removed lithologies. To restore the crestal area, an initial dip of 60 degrees for upper parts of the NE-SW trending faults was assumed. Calculations indicate that ca. 24 km 3 of removed sediments were transported to the southeast. This shows a strong correlation with the volume of 20 km 3, represented by the clastic wedges in the hanging wall to the southeast. The volume balance infers that more than half of the volume deposited in the southeast is represented by eroded material from the clay-dominated Dunlin Group. Trough-like depressions (maximum 200 m deep and 1.5 km wide), N-S oriented and partly fault controlled have been mapped from the crestal area and down the fault scarp to the southeast (Fig. 2) (A. Groth, pers. commun., 1993). They may represent canyons or slide areas, i.e. conduits for sediment transport which remained active up to Ryazanian time. Mass transport continued in the post-rift stage, as two broad channel systems have been identified and mapped by Alhilali and Damuth (1987), interpreted to have been formed by a combination of turbidity currents and mass-movements (slumps, slides, debris flows, etc.). These channels are located on the terrace area in the southeastern part of the study area and they head along the fault zone which is the present eastern/southeastern limit of the Visund fault block and trend south and southeast into the Viking Graben. These channel systems are locally more than 300 m deep and a few kilometres wide, and are said to be of Cretaceous age, having been filled by the end of the Turonian (Alhilali and Damuth, 1987).

339

Comments on the influence of relative sea level It is important to note that relative sea level around the crest of the Visund structure would have been critically important for optimal erosion-rates and volumes of sediment available for resedimentation. Although submarine erosion can be important (especially for mass collapse) the most intense erosion and sediment production is likely to have been during time intervals when sea level (or fair weather wave-base level) lay below the Visund structural crest and when this subaerial island was of maximum areal extent. Although the final cumulative volume of strata eroded from the Visund uptilted crest can be estimated directly from the seismic data it is important to determine or predict the specific time intervals (within the 19 million years study period) when emplacement of resedimented sandy strata could have been expected. This was done by predicting intervals of low relative sea level, when there was a favourable combination of the following factors: maximum structural uplift as determined by rate of block rotation and footwall uplift; - low eustatic sea level; minimal regional extensional subsidence. Determination of the time intervals when these factors combined most favourably can be achieved by having independent knowledge of when the rotational pulses occurred and of when eustatic sea level was low, or by being able to date the deposits or hiatuses in and around the rift basins. The next section deals with the characteristics and ages of the infilling sediments, but prior to drilling it could have been postulated, assuming the general correctness of published eustatic trends for the late Jurassic, that early-mid Volgian would have been favourable from the eustatic (low) point of view, whereas late Kimmeridgian and late Volgian times would have been less favourable because of their tendency to high relative sea level (Fig. 8). -

-

Syn.rift infill (Draupne Formation) The Draupne Formation is dominated by black shales, generally more organic rich than the Heather Formation. In addition, the depositional setting was considerably more tectonically active, resulting in high relief and deeper erosion into a more varied stratigraphy than earlier. Mapping of the uplifted crestal area of the Visund structure has shown that a total of some 90 km 3 (decompacted)were eroded and removed during the period of deposition of the Draupne Formation. Of this, up to 30-35% was sand, and this was redistributed partly down the northwest-

340

R.B. Fcerseth, T.S. SjCblom, R.Z Steel, T. Liljedahl, B.E. Sauar and T. Tjelland

Mill. yrs 125

\ 2 SHORT ~ ,oNG~i'k~ RM HiGH <, Y~ TERM ~

,/

__

__

rr"

o

0 >

"r" F-

"'

141

z_

~ z~

om

_

03 0

z

co

< a ft.

123 if3 ..I

0 x 0

1

rr Ill

~z

z

,3> 0

~ -rr

< Y~ O "1-

HAQ ETAL. 1987 0

I

~. 9

O0 m

EAST GREENLAND ( S U R L Y K 1989)

I

165

Fig. 8. Eustatic sea level curve for late Jurassic interval (from Haq et al., 1987), compared with a similar curve reconstructed from East Greenland (from Surlyk, 1989).

erly dip slope, partly over the southeasterly scarp slope. The Draupne Formation is therefore expected to have a more varied facies development than the Heather. The major challenge in such a setting is then to predict, locate and map the sandstones (Fig. 9). Our subdivision of the syn-rift stratigraphy comes from (1) sparse well data on and near the crestal area, on the southeast downfaulted flank and on the "terraced" area near the Inner Snorre Fault, and (2) a seismic data base which could be tied to the wells. The succession is subdivided as follows: (1) Biostratigraphic dating of sediments in the wells within the Visund half-graben allowed late Oxfordian, late Kimmeridgian and middle-late Volgian shale packages to be distinguished. (2) The same datings allowed inferences of hiatuses or strongly condensed levels of late Oxfordianearly Kimmeridgian and of early Volgian ages, on the flank- and terraced area of the main half-graben. In the deeper parts of the basin these intervals are less likely to be missing but may still show some condensation. This tentative and somewhat uncertain dating was then used with the seismic and well data to provide a picture of the syn-rift infill history. The late Oxfordian shale unit was identified only in the deeper basin and on the terraced areas of the Visund half-graben (Figs. 4 and 6). The unit contains very organic-rich shales but these arise, nevertheless, somewhat transitionally from the underly-

ing early Oxfordian Heather Formation shales. The organic-rich shales contain some few thin turbiditic sandstones, suggesting either a slight shallowing or simply initiation of upslope erosion. Eroded material would consist of Heather Formation mudstones, redeposited in a basinal setting. The litho- and biostratigraphy of the earliest Draupne Formation deposits are thus strongly dominated by reworked older Heather Formation facies. The unit onlaps the half-graben dip-slope and is locally present in upslope wells in fault-controlled topographic depressions. The unit is absent from the southeast flank, providing part of the evidence for this area having been attached to the tilt-block crest at that time. The late Kimmeridgian shale unit has been drilled only on the terraced area in the west of the halfgraben and in the presently downfaulted southeastern flank (which by then had partially collapsed to become a deeper basinal area) (Figs. 4 and 6). It consists of organic-rich shales with considerable numbers of interbedded deep water sands on the western terraces and of shales interbedded with thin sandstones and conglomerates on the southeast flank. In the deeper, undrilled basinal areas there is likely to be a more complete Kimmeridgian succession (unit B1 in Fig. 4). Seismic reflection patterns here are rather chaotic, with low amplitudes and low continuity, defining a marked lensoid package above the base of the unit (late Oxfordian-early Kimmeridgian age?). This unit is also absent on the crestal areas, and can be seen to onlap the basal reflector some way upflank. The NE-SW trending fault bounding the Visund fault block to the east had become the active western edge of a new easterly half-graben by this time, as shown by the conglomerates in this area. The strata of Kimmeridgian age here consist of the A1 unit (Fig. 4) which is wedge-shaped with a few low-amplitude internal reflectors. The unit as a whole as well as the internal horizons diverge markedly towards the Visund fault in a "syn-rift" manner. The A1 unit has been drilled as a 9 m thick shale-prone unit, some 2 km southeast of the main fault, i.e. as the distal tip of the wedge. The middle-late Volgian unit (B2 in Fig. 4) is present as organic-rich mudstones and the upper part covered most of the Visund sub-basin, including much of the crestal region. The upper part of this unit has high amplitude, continuous reflectors, characteristic of the shale-prone levels which flooded high onto the upflank areas. The lower part of B2 is less well defined on the seismic data, but is believed to contain low continuity lensoid packages which onlap the B1/B2 boundary upflank, thus represented only by hiatus in the crestal wells and in the westerly ter-

Tectonic controls on Bathonian-Volgian syn-rift successions on the Visund fault block, northern North Sea

341

Fig. 9. Interpretive chronostratigraphic diagram for the Bathonian-Ryazanian interval for the various segments of the Visund block. Vertical pattern indicates hiatus.

raced area. This Volgian interval is less well dated in the easterly sub-basin but probably is represented by the A2 and A 3 syn-rift wedges (Figs. 4 and 7) drilled as conglomerates and interbedded shales some 2 km from the main fault line (Fig. 2). The A2/A3 wedges have prograded considerably farther into the southeastern basin than the A1 wedge, possibly as a result of greater footwall uplift, more widespread tectonic collapse of the crestal area, and a generally falling Volgian sea level. An A 4 unit is also present in the southeastern area. This unit is restricted more closely to the fault line (Figs. 2 and 7), it varies greatly in thickness (from 0 to 250 m) along strike and the thicker parts have a characteristic cone-shape. This unit terminates updip towards the fault plane against a series of depressions or canyons which could possibly have been feeder conduits both for A4 and earlier fan wedges. The age of the A 4 wedge is uncertain, but may range from late Volgian into early Ryazanian and may even be classed as post-rift.

Sedimentary signatures in syn-rift stratigraphy Upward-fining motifs In order to make some generalisations about synrift stratigraphy, and thus to improve our ability to predict and locate potential reservoirs with a minimum of data, it is worth reviewing briefly what other researchers have reported about syn-tectonic sedimentary signatures, to compare this with the Visund data set. Only data from marine rift basins are dealt with because stratigraphic/facies models for

non-marine syn-rift infill are well established and are less relevant to the Visund fault block. Marine rift basin successions are commonly shalerich, with significant volumes of conglomerate and sandstone deposited from rock falls, debris flows and high- to low-density turbidites. The dominance of sediment gravity flows is due to the steep slopes generated by extension and block rotation, as well as to the tendency for most of the contemporaneous shoreline and nearshore deposits to be subject to collapse and resedimentation (both on dip and scarp slopes). Nearshore deposits may be more common as part of the latest syn-rift infill when tilting is mild or has ceased. Feeder fluvial systems often tend to be short-headed and only locally developed due to the restricted size of drainage areas, and are commonly cannibalised by repeated episodes of base-level fall associated with footwall uplift. A recurring sedimentary signature in marine synrift infill is the "fining-upward" motif, often on at least 3 scales; a few metres, tens of metres, and hundreds of metres. These signatures have been particularly well documented by Surlyk (1984, 1989, 1990) and Surlyk et al. (1993) within the syn-to early postrift infill of the Wollaston Forland Group (VolgianValanginian) of East Greenland. Surlyk (1990) and Surlyk et al. (1993) have interpreted the small-scale motifs in terms of local processes on the surface of the submarine depositional system, such as filling and abandonment of scours, retrogressive flow slides, or surging high-density turbidity currents. The intermediate and larger scale motifs were originally interpreted as reflecting directly major phases of ro-

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tational block faulting. Tilting caused rejuvenated source areas, rapid influx of coarse-grained sediment, followed by diminishing sediment supply as the source area retreated and was aided by a relative sealevel rise (Surlyk, 1978). More recently, Surlyk (1991) and Surlyk et al. (1993) have somewhat downplayed the direct tectonic signal and increased emphasis on the possible contribution of sea level change to the resultant upward-fining and upward-thinning cycles. Similar sedimentary signatures are also present in the Upper Jurassic, syn-rift infill of the south Viking Graben (Brae Formation), as can be seen from the descriptions of this submarine succession by Stow (1984), F~erseth and Pederstad (1987) and by Turner et al. (1987). The relationship of the upward-fining motifs (of intermediate scale) to the 2dimensional geometry of fan-channel complexes has been illustrated by Frostick and Steel (1993). The latter are multistorey and multilateral units in the fan architecture, and thus are probably not caused by single events or local processes. The upper Jurassic syn-rift infill of the Inner Moray Firth has also been interpreted to show fairly large-scale upward-fining trends in the footwallderived conglomerate-sandstone-shale deposits of the deeper reaches of the rift-wedge (Underhill, 1991). Such motifs are not yet documented, however, from the onshore-exposed submarine-fan succession of the same basin, where it is banked against the footwall of the Helmsdale Fault. A vertical profile through the submarine fan deposits derived from the crest of the KimmeridgianVolgian Visund (fault block), and dispersed into the tilt-block sub-basin to the southeast of the present crestal area of the Visund fault block, is shown in Fig. 10. About 8 cycles, each of the order of a few tens of metres, with a clear overall upward-fining trend of conglomerates-sandstones-shales can be seen. Of interest here, however, is a tendency for many of the cycles to be less asymmetric than has been described from East Greenland and from the south Viking Graben. There can be detected a slight upwardcoarsening and thickening component, usually in the form of sandy, high-density turbidites, below the main conglomeratic component of some cycles. This implies that the fan lobes and aprons were probably subject to an initial progradational phase prior to a retreat and back-filling phase. Successive cycles are believed to reflect rapid influx of sediment and the more gradual exhaustion of stored sediment from the crestal area of the fault block. These sediment influx phases need not necessarily correspond directly with pulses of footwall uplift, though fault movements are likely to have caused sediment storage or instability thresholds to have been exceeded.

Fig. 10. Conglomeratic development within the Draupne Formation on the southeast flank of the Visund fault block. The vertical distribution of conglomerates, massive/graded sandstone and black shales allows the definition of a series of well-defined upward-fining units (few tens of metres thick), and several less well-defined, larger scale upward coarsening-to-fining units.

Tectonic controls on Bathonian-Volgian syn-rift successions on the Visund fault block, northern North Sea

Overall syn.rift signature The largest scale infill signature of the south Viking Graben and the East Greenland rift basin appears to have been one of upward fining on a scale of hundreds of metres, as referred to above. The same trend is found in the main Visund half-graben on a ca. 1000 m scale, where the Draupne Formation clastics evolve upwards into a mud-rich succession. This type of trend appeared to be symptomatic of underfilled or sediment-starved rift basins. With a more abundant or continual, sediment supply, the latest phase of infill (after the climax of extension and rotation) should show a renewed progradational clastic wedge, with an upward coarsening and shallowing of facies. This has been argued by Prosser (1993), and demonstrated for segments of the Moray Firth Rift by Sinclair and Riley (1995).

Summary (1) A domino model has recently been applied in efforts to improve our understanding of late Jurassic extensional faulting and syn-rift stratigraphy in the northern North Sea. This modelling has assumed that the tilting of individual fault blocks along any transect developed synchronously and that the synrift infill of the series of half-grabens is of the same age. The modelling has also focussed largely on structural aspects. A data set from the Visund fault-block allows us to test stratigraphic as well as structural predictions of a modified version of the domino model. Our interpretation of the synrift wedges on the Visund fault-block is in good agreement with published model predictions. (2) The Visund fault-block has a present width of 23-26 km, i.e. is wider than normal in this part of the North Sea, and has tilted the pre-rift surface to ca. 10 degrees by the end of the syn-rift interval i.e. is steeper than normal in the area. (3) Visund fault-block tilting had been initiated by the early Bathonian, and both Heather and Draupne Formations have clear wedge-shaped geometries. This syn-rift succession is generally absent on the present crestal area but is up to 1600 m thick (decompacted) in the deepest part of the half-graben. (4) The Heather Formation has been eroded from the present crestal area of the fault block. However, the pre-to syn-rift transition has nevertheless been preserved in a collapsed footwall segment, now present as a structural terrace between the present crestal area and the Viking Graben proper to the east. A base syn-rift unconformity has been documented, with early Callovian Heather Formation overlying an eroded Brent Group (Tarbert Formation) succession.

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(5) A tilt of the pre-rift surface is estimated to at least 2 degrees by mid-Oxfordian (top Heather Formation) times. Four stratigraphic units have been documented within the syn-rift Heather Formation, but because of slow crestal uplift, stratigraphic condensation and some degradation, it is unlikely that any major footwall island developed at this time. (6) Draupne Formation accumulated during increased extension and accelerated fault-block rotation rates, with the pre-rift surface undergoing a pulsed syn-rift tilt from ca. 2 degrees (mid-Oxfordian) to ca. 10 degrees (late Volgian-Ryazanian). A ca. 6 degree tilt by mid-Kimmeridgian times saw partial footwall collapse and a decoupling of the present southeast flank. The development of an emergent footwall island and erosion of the present crestal area happened largely during the subsequent 4 degrees of tilting, over an interval of some 8-9 Ma. (7) The ca. 90 km 3 (decompacted) of strata removed by erosion and collapse from the crestal area was dispersed and resedimented both across the fault scarp into the terrace area to the southeast, as well as down the dip slope into the half-graben to the west. The latter area received two major influxes of sediment gravity flow sands during the KimmeridgianVolgian tilting, but was otherwise sediment-starved and shale-prone, showing only incomplete syn-rift stratigraphic signatures. (8) Accelerated (Volgian) footwall uplift and associated erosion/collapse and resedimentation to the southeast of the crest resulted in the development of a major conglomeratic wedge at the base of the NE-SW trending fault scarp. Definition of erosion and drainage domains in the crestal area, and calculation of eroded volumes dispersed to the southeast (ca. 24 km 3) allows a good comparison with calculated deposited volumes (ca. 20 km 3) in the clastic wedges of the southeast. The latter show KimmeridgianVolgian upward-fining signatures, sometimes modified by a basal prograditional (upwards coarsening) segment. (9) Decreased strain rate and increased background subsidence in the latest Jurassic caused submergence of the Visund crestal area and draping by late Volgian-Ryazanian muds. The latter unit therefore represents a passive, late stage infill of the syn-rift area, probably indicating transition to a post-rift stage already in the latest Volgian.

Acknowledgements We thank Norsk Hydro a.s., Conoco Norway Inc., Elf Petroleum Norge A/S, Saga Petroleum a.s. and Statoil for permission to publish this manuscript. Alan Roberts, Graham Yielding and Richard Hodgkin-

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s e n a r e t h a n k e d for c o n s t r u c t i v e c o m m e n t s o n t h e work.

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Norsk Hydro a.s., P.O. Box 200, 1321 Stabekk, Norway Norsk Hydro a.s., P.O. Box 200, 1321 Stabekk, Norway Norsk Hydro a.s., P.O. Box 200, 1321 Stabekk, Norway Norsk Hydro a.s., P.O. Box 200, 1321 Stabekk, Norway Norsk Hydro a.s., P.O. Box 200, 1321 Stabekk, Norway Geological Institute, University of Bergen, 5007 Bergen, Norway