Differentiation of incised valley systems from mobile streams: some examples from the oseberg field, Norwegian North Sea

51

Differentiation of incised valley systems from mobile streams: some examples from the Oseberg Field, Norwegian North Sea AIf Ryseth and Hege Fjellbirkeland

The Statfjord Formation (Rhaetian-Sinemurian) and the Ness Formation (Bajocian) in the Oseberg Field (Viking Graben) feature interstratification of fluvial sandstone bodies, and packages of mudrock-dominated floodplain deposits. Together, these two formations contain a set of different continental depositional environments, including braided stream (Statfjord Formation) and humid-climate delta plain deposits (Ness Formation). A comparative study offers a potential of testing sequence stratigraphic concepts in a broad range of continental environments, including the problem of distinguishing incised valley fill from mobile stream deposits. Thick sandstone bodies (tens of metres) of fluvial origin may represent incised valleys cut during base-level falls and filled during subsequent periods of base-level rise. Consequently, their basal bounding surfaces attract attention as likely sequence boundaries within alluvial units. This simple allocyclic model contrasts with current autocyclic models of alluvial stratigraphy, which are based on rivers being erosive and mobile and able to sweep across entire alluvial plains. Using the autocyclic models, thick fluvial sandstone bodies are likely to form due to vertical aggradation of fixed channels during periods of rapid subsidence, or by vertical stacking of mobile channel belts during periods of slower subsidence. The latter models also relate channel deposit abundance and sandstone body geometry to such factors as avulsion period, floodplain width and deposition rate, in addition to subsidence. This study investigates fluvial sandstone bodies of variable thickness on a local scale, as a contribution to establishing sedimentological criteria for differentiating between deposits that can be related to valley incision and those that are the product of deposition within mobile streams that erosively sweep across depositional plains. Sandstone body thickness and width, character of the encasing deposits, and the relationship between erosional surfaces and scours at sandstone bases and palaeosol units in the surrounding sediments form the basis for interpretations regarding the origin of these sandstone bodies. In both formations, the existence of incised fluvial valleys can be disputed, and the observed alluvial architectures indicates that deposition was governed by generally declining long-term rates of accommodation space development.

Stratigraphy of alluvial deposits: outline Alluvial and fluvially dominated delta plain deposits are important components of the stratigraphic record, and occur in a wide range of tectonic settings, including rift basins such as the Viking Graben of the North Sea. Here, Triassic continental red beds are the prime reservoir units of the giant Snorre Field (e.g. Nystuen et al., 1989), and younger (Early and Middle Jurassic) continental deposits are also significant reservoirs of other major fields such as the Brent, Statfjord, Gullfaks and Oseberg. Facies models derived from such deposits are numerous (see Collinson, 1986, and Miall, 1992 for summaries), yet the main components of continental deposits are rather similar, irrespective of, for example, geological time and basin types, and comprise the sandy to gravelly fill of the main channel systems and the generally finer-grained deposits of the sur-

rounding alluvial to deltaic plains. Most important, in vertical sections the contact between a channel fill deposit and the subjacent fine-grained interval is almost invariably sharp and well-defined, and is also most commonly considered to be erosive. On the other hand, the upward transition from a channel fill deposit to a floodplain deposit is much more gradational due to the tendency of channel fill deposits to fine upwards. This relationship between the main subenvironments (channels and plains) is probably the main reason that depositional cycles in continental deposits are usually defined as fining upwards, and related to a set of major events, including the initial erosional channel cut to account for the sharp basal boundary, the deposition of coarse sediment by fluvial processes, the abandonment of the channel (gradual or abrupt) and the covering of the coarse material by finer deposits. Repetition of this sequence of events

Sequence Stratigraphy on the Northwest European Margin edited by R.J. Steel et al. NPF Special Publication 5, pp. 51-73, Elsevier, Amsterdam. 9 Norwegian Petroleum Society (NPF), 1995.

B2

A. Ryseth and H. FjeUbirkeland

Fig. 1. Outcrop example of continental deposits comprising coarse, conglomeratic sandstones interbedded with finer-grained units. Each of the five depositional cycles noted along the vertical profile has a fining-upward character above an erosional surface, and is related to channel deposition followed by floodplain aggradation. Escanilla Formation (Eocene, Spain). This alluvial formation has been considered a possible field analogue for the Statfjord Formation and Triassic reservoirs in the North Sea (Dreyer et al., 1993). Photograph used by permission from the SAFARI Group.

in an area undergoing sufficient subsidence would produce a stratigraphic section of the type presented in Fig. 1, with each fining upward unit representing a package of genetically related strata, bounded at the top and base by erosional surfaces. Autocyclic mechanisms (referring to processes intrinsic to the depositional environment) such as lateral channel migration and avulsion induced by the build-up of alluvial ridges above the general level of a depositional plain (a response to aggradation rates being highest along the reach of a channel) can be used to explain the pattern of stacked fining upward units (see numerous examples in Miall, 1978; Collinson and Lewin, 1983; Ethridge et al., 1987). Additionally, theoretical models based more or less entirely on autocyclic processes (in addition to general net subsidence) have been generated to predict and explain the 2-dimensional architecture of highly variable alluvial units (Allen, 1978; Bridge and Leeder, 1979; Bridge and Mackey, 1993). Whereas sophisticated facies models based upon autocyclic mechanisms and burial subsidence can adequately explain the complex nature of many ancient continental units, there is little doubt that external controls, including eustatic sea-level fluctuations exert a pronounced control on deposition (and erosion), particularly in deltaic and coastal settings.

Events of base (sea)-level fall may lead to valley incision and/or widespread subaerial erosion, thus forming sequence-bounding unconformities (Posamentier and Vail, 1988). A key problem in the analysis of continental deposits is how to distinguish between erosive surfaces that are truly autocyclically controlled, and those that should be related to external (allocyclic) controls such as base-level falls induced by eustasy or tectonism. Van Wagoner et al. (1990, p. 36) pointed at the importance of distinguishing between incised valleys and local channels, including distributary channels, in constructing chronostratigraphic frameworks. These workers also listed some criteria for differentiation between local channels and incised valleys, and pointed at differences in width (incised valleys being about one order of magnitude wider than distributary channels) and the nature of the encasing deposits. Notably, incised valleys are likely to be encased in middle to outer neritic mudstones, whereas distributary channels are surrounded by delta plain or stream mouth bar deposits (van Wagoner et al., 1990, p. 37). There is little doubt that fluvial deposits encased in shelfal deposits must record a major basinal shift of facies that can be related to valley incision during a relative low-stand. The problem of recognition is probably more complex than suggested by the above

Differentiation of incised valley systems from mobile streams

criteria. Incised valleys encased within other fluvial or coastal plain deposits have been reported (e.g. Blakey and Gubitosa, 1984; Retallack, 1986; Kraus and Middleton, 1987; Aubrey, 1989; Shanley et al., 1992). Further, mobile (or local) channels are not only of the deltaic distributary type, but embrace the whole spectrum of fluvial morphologies, from proximal braided streams, meandering and anastomosing systems, to deltaic channels. Hence, channel widths and the lateral extent of the sandstone deposits they leave behind are bound to vary significantly due to differences in channel morphology and of course due to the set of parameters that governs fluvial style (slope, climate and annual discharge, sediment supply, etc.). Thus, simplistic considerations on differences in width between incised valleys and local delta plain distributary channels are probably inadequate to make a distinction between mobile channel deposits and incised valley fills. The relationship between palaeosols and present fluvial deposits is of prime importance in interpreting alluvial stratigraphy. Blum (1990) identified four allostratigraphic units in Pleistocene to Holocene deposits of the lower Colorado River, each representing episodes of channel aggradation and floodplain construction, bounded by erosional unconformities and/ or surfaces of non-deposition and soil formation. Allen (1973) and Retallack (1986) noted that mature palaeosols would form on the dissected topography flanking incised valley axes due to prolonged sediment starvation, in Devonian and Tertiary strata, respectively. The main factor controlling soil maturity in these two cases is the deprivation of clastic input to the dissected surface flanking an incised valley system. Bown and Kraus (1987) also demonstrated that palaeosol maturity would increase away from active channel systems in areas of floodplain aggradation, but in such situations the more mature soils would develop in areas some tens of kilometres away from the active channels. Hence, a successful correlation of an erosional surface to a well-developed palaeosol in nearby deposits may indicate a valley incision. Palaeosol drainage condition.s may also provide important information on the behaviour of fluvial systems. Besly and Turner (1983) and Besly and Fielding (1989) demonstrated how red beds and coalbearing cycles with stagnant palaeosols could form simultaneously in a moist tropical climate due to differences in drainage, and Besly and Turner (1983) also pointed at channel incision as a possible cause of groundwater lowering and improved drainage. A similar result has been presented by Retallack (1986), who could correlate events of deep fluvial incision to mature, well-drained palaeosols formed during periods of lowered groundwater table.

53

The stratigraphic model presented in Fig. 2 shows an incised valley deposit and an associated mature palaeosol encased in autocyclically controlled mobile channel and floodplain deposits with less mature palaeosols. The overall architecture of this diagram can be explained in terms of a base-level drop leading to valley incision and formation of a mature palaeosol on the dissected landscape, followed by deposition of fluvial sandstones within the valley as deposition starts again due to base-level rise. At some stage, the incised topography is "overtopped", and the channel system can again start to migrate across a wide floodplain by autocyclic processes. This paper examines the continental deposits of the Statfjord Formation (Rhaetian-Sinemurian) and the Ness Formation (Bajocian) in the Oseberg Field of the Norwegian North Sea (see lithostratigraphic nomenclature in Fig. 3), with emphasis on the possibility of differentiating between incised valley fill and mobile channel deposits from subsurface core and well log data. The main part of the study is related to the Ness Formation, and an example from the Statfjord unit is included to demonstrate the lateral character of a multistorey, approximately 70 m thick fluvial sandstone that may represent a valley fill encased in other fluvial deposits. Also, a consideration and discussion of sequence stratigraphic interpretations of these two units are presented.

Local stratigraphy and depositional settings Figure 4 outlines the main structural elements of the Oseberg Field and shows the position of the field at the western flank of the Horda Platform. The Oseberg structure (Larsen et al., 1981), is a major Mesozoic rotated fault block with a structural closure. Badley et al. (1984) interpreted its evolution in terms of two major rift events involving listric faulting during Late Permian to Early Triassic and Late Jurassic to Early Cretaceous, respectively. The Early and Middle Jurassic units including the Statfjord and Ness Formations were deposited during a period of post-rift thermal subsidence (Steel, 1993). Differential subsidence was, however, accommodated across the main faults also during the thermal sag phase, as all units, including the Statfjord Formation, the Dunlin Group and the Brent Group tend to increase their thicknesses in an east-to-west direction across major faults in the area (Badley et al., 1988; Steel and Ryseth, 1990). The present study is based on data from a single major fault block (Alpha structure; Fig. 4). Here, the Ness Formation thickens gradually from about 40 m in the southern parts to approximately 80 m in

54

A. Ryseth and H. Fjellbirkeland

/\

- "

~___,~.:.:.:.-.-.-.:.:~~.....:.:..

-

,,

""

Mobile channel fill

Incised valley fill

',

. :...

-

I~

I\

~

~

I\

/-'~

/~--'-

IIIIIIIIIIII!!11111111111

Imature paleosol

Mature paleosol

Floodplain deposits Fig. 2. Schematic alluvial architecture model containing an incised valley fill associated with a mature palaeosol, encased in other fluvial deposits of mobile channels and aggrading floodplains. The incised valley/mature palaeosol system records a base-level fall, whereas the encasing deposits record periods of base-level rise.

the northern areas, whereas the Statfjord Formation thickness fluctuates between 250 and 300 m. The Ness Formation is generally taken as the delta plain environment of the Brent Group deltaic unit. Following the work of Graue et al. (1987) and Helland-Hansen et al. (1992), the main phase of delta progradation (and regression) occurred during the late Aalenian to early Bajocian stages, with a subsequent phase of transgressive back-stepping and retreat of the delta during early Bajocian to Bathonian times. It is important that the chronostratigraphic correlations presented by these workers show that the continental Ness type of facies was deposited during both the progradational and retrogradational stages of delta evolution. However, within the Oseberg area the main part of the Ness Formation falls below the early Bajocian time line of Helland-Hansen et al. (1992; cf. their fig. 8, wells 30/9-2 to 30/6-5), Thus, it can be assumed that the main portion of the Ness Formation at Oseberg was deposited during the regressive phase of the Brent Delta. The Statfjord Formation (Rhaetian-Sinemurian; ROe and Steel, 1985; Lervik et al., 1989; Steel and Ryseth, 1990) comprises a thick package of

sandstone-dominated alluvial deposits capped by a thin unit of shoreline sandstones. The continental Statfjord deposits span part of the (late) Rhaetian, the Hettangian and part of the Sinemurian stages, whereas the shallow marine deposits are of middle to late Sinemurian age, as dated from the presence of dinocysts (P. van Veen, personal communication, 1988). Palaeogeographic reconstructions indicate that the Statfjord alluvium on the Horda Platform (including the Oseberg area) was derived from the Fennoscandian hinterland to the east, with deposition having taken place within braid plains, braided streams and associated floodplains. Moreover, the Statfjord Formation represents the last major stage of continental deposition in the area, as the overlying Dunlin Group is generally dominated by marine mudrocks. The Statfjord Formation also shows a vertical transition from continental red beds in the lower part, to coal-bearing grey-beds in the upper half, that is generally related to a climatic change during the Triassic-Jurassic junction, from a semi-arid to a more humid setting (Roe and Steel, 1985). The example presented from the Oseberg Field is taken from the coal-bearing interval. Considerations regarding sediment accumulation

Differentiation of incised valley systemsfrom mobile streams

,•

~ Lithostratigraphic 2 i formations i Draupne

t'..~

Stage

9

Oxfordian !

Heather

i

Callovian

! Late Bathonian Tarbert

Bathonian

Ness t,-"

._o r o)

rn

Etive

Bajocian

L._

Rannoch

.-j

Oseberg

Aalenian

Drake

Toarcian

Cook

Pliensbachian

Amundsen

Late Sinemurian

Statfjord

Sinemurian Hettangian

Lunde

Rhaetian Norian

t-.m e..

Cl

I9 "~

|

._~

~m

-r-

Fig. 3. Lithostratigraphic nomenclature of the Late Triassic and Jurassic deposits (modified from Vollset and Dor6, 1984, and Helland-Hansen et al., 1992).

rates are of importance in stratigraphic analysis, but require that absolute datings are available for the sections involved. Steel (1993) suggested that deposition of the Brent "megasequence" required some 7-8 million years (late Aalenian to early Bathonian). The biostratigraphic data presented by Helland-Hansen et al. (1992) show that the Ness Formation at Oseberg was deposited prior to the early-late Bajocian (below their time-line 6) and a time interval of some 4-5 million years can probably be estimated for the deposition of the Ness Formation in this area (L. Lcmo, personal communication, 1993). This is less than the 7.4 Ma duration of the Bajocian stage as dated by Harland et al. (1989). Steel (1993) also suggested that deposition of his "Statfjord megasequence" took place during a time span of approximately 12 million years (late Rhaetian to mid Sinemurian). This package (sensu Steel) also includes some marine deposits above the Statfjord Formation, and a rough estimate of approximately 10 million years involved in the deposition of the continental part of the Statfjord Formation will be used here. Thus, from the thickness data presented above, rock accumulation rates in the order of 0.01 m/1000 years to 0.02 m/1000 years can be calculated for the

55 Ness Formation, whereas some 0.02 to 0.03 m/1000 years can be estimated for the Statfjord Formation. These accumulation rates will be discussed below. Facies a s s e m b l a g e s and sandstone body dimensions

Both the Ness and Statfjord Formations feature interstratification of sharply based fining upward sandstone bodies and heterolithic intervals of mudrocks, fine-grained sandstones, rootlet horizons and coal beds. For simplicity, only two main facies assemblages are defined within each formation, and a series of representative core logs are presented in Figs. 5 and 6. Facies Assemblage I comprises the main sandstone bodies of each formation, and is characterized by a general fining upwards grain size distribution (FU), or within the thicker units, a vertical stack of several fining-upward units. Sandstone bases are sharp and apparently erosive, and are usually associated with intraclasts and coarse- to medium-grained sandstones which grade upwards into commonly micaceous fineto very fine-grained sandstones that are followed by deposits of Facies Assemblage 2. It should be noted from Fig. 5 that the thickest sandstone body has a multistorey character due to the existence of two internal surfaces marked by intraclast conglomerates, and also from Fig. 6 that thick sandstone bodies in the Statfjord Formation are composed of several stacked fining upward units. Facies Assemblage 2 is markedly finer grained and generally heterolithic. It contains coal beds, rooted horizons, mudrocks and thin sandstone beds that separate the thicker sandstone packages of Assemblage 1. For both the Statfjord and Ness Formations, Assemblage 1 is considered to represent the fluvial channels and potential incised valley deposits, whereas Assemblage 2 records deposition, pedogenesis and peat accretion within interfluvial plains. The main difference between the upper Statfjord and Ness Formations in the Oseberg area relates to the character and proportion of Assemblage 1 deposits. Due to differences in depositional environment (braided streams versus deltaic channels), Statfjord sandstones are coarser grained, with more abundant high-energy features such as planar lamination and cross-stratification than the corresponding deposits of the Ness Formation. The upper half of the Statfjord Formation also contains significantly more channel deposits than the Ness Formation, on the average about 70% and 27%, respectively. Figure 7 shows the distribution of measured sandstone body thicknesses in the two formations. In the Ness unit, 91 measurements show that the bulk

A. Ryseth and H. Fjellbirkeland

56

1~

0o

//i'X

~x,.

2~ '

Ea/st']\ Basin A

,,

_r /// lxe l

./

Graben~~ t t II IJ /

a

~ i

0

Alapha % East~ ' ~ V G~aki~~n/ / / C ~ t : ' Shetland\ \~ V/~/,~" r I}, Platform'k ~ k" I'x s',u:=l~ I H~d/a /' ,~~~/~/ ~ Platform ir ,

60030 '

60030 ' I

i 0

2

4

Fig. 4. Regional structural framework and local structure of the OsebergField, with well locations. Data presented in this study are taken from the Alpha and Alpha North structures.

proportion of Assemblage 1 sandstones falls within the thickness range of 2 to 8 m, and that sandstone bodies thicker than 12 m are generally rare. The lack of data in the thickness range from 16 to 24 m and the existence of two particularly thick units (24-28 m) may indicate that two different sandstone body types (mobile channels and incised valleys) have been recorded, with the possible (?) interpretation that the two thickest bodies (at least) may represent incised valley deposits. However, the similar grain size and sedimentary structures seen in the thickest sandstone body presented in Fig. 5 as compared to the thinner ones should be noted, as these sedimentological characteristics indicate that rather similar hydraulic conditions (flow velocity) persisted during deposition, irrespective of the measured vertical thickness of individual deposits. Sandstone body thickness data are more plentiful in the Statfjord Formation due to the greater thickness of the formation itself. The observed thickness range is markedly broader than in the Ness Formation, with a maximum sandstone body thickness of 83 m, with no clear breaks (though measurements in the range from 30 to 90 m are scattered). The verti-

cal core section presented in Fig. 6 is representative for the Statfjord Formation in the area, and as in the Ness unit, there is no evidence that the thinner sandstone bodies of Assemblage 1 are different from the thicker ones in terms of sediment calibre and sedimentary structures (see also Fig. 14). However, the vertical thickness of the main sandstone body in Fig. 6 is close to 70 m. Clearly, such a sandstone body is not deposited from a single mobile channel, and a mechanism involving either valley incision or a stacking of several channel systems through time (or a combination) must be invoked.

Vertical and lateral relationships in the Ness Formation Ryseth (1989) described the sedimentology of the Ness Formation in the Oseberg area, and divided the formation into nine environmental facies, that were related to deposition within a fluvio-lacustrine upper delta plain environment. Of these, Facies 9 (thick sandstones) is identical to Assemblage 1 as described here, and comprises both single-storey and multistorey vertical sections that can be related to

Differentiation of incised valley systems from mobile streams 9

~

57 -u-

.

T=Z

2 i

,

.

~

_

r

_

_ . . . _

~"~_T 9

.

.

9

.

.

2

S

--.

" ~

_~

~

U-

-u-

V 9

.

.

9 9

, . . .

J<_

. .

9 9

, .

9 9

. .

,

Scale (m)

,

9

.

,

9

9

9

.

.

.

9

.

9 ."

._)<_

10

,

8

9

" , ' . ' , 9t t - - 7 1 ~ . " 9

~ . .

,

1

6

.

9

9

9

S

,

9 9

"

.

.

.

.

~

4

".',

2 0

u.

~

S

- - 1 - - I -

2

r-r-I--"1r

' ~

u~

!

I -u-

S

i 2

,

-.__

1

S I Clay I Si Ivfl

f l m I c Ivr162

Sandstone

IT
Mudrock

I

Coal

I CI~

~ ~ ~

I Si Ivfl

f I m I c IvcIr

I Clay I Si Ivfl

f I m I c IvcIogl

I

Ripple lamination

-U-

Bioturbation (mainly Planolites)

~

Nodules (siderite/pyrite)

Cross-stratification

ufa

Convolute lamination

,~)

Plant litter

Rootlets (paleosols)

._)L_ Water escape

Fig. 5. Cored sections from the Ness Formation, illustrating the vertical alternation of the two main facies assemblages. The logged sections contain the whole spectrum of measured sandstone-body thicknesses (Assemblage 1). The similarity in grain size (mainly medium- to coarse-grained sand) and sedimentary structures seen in the sandstone bodies, irrespective of sandbody thickness, should be noted.

channel deposition. The remaining facies of Ryseth (1989) are components of Assemblage 2 of this study, and represent the delta plain environment with peatforming swamps, soils, lake and pond systems, levees and crevasses. The water-saturated nature of Assemblage 2 deposits in the Ness Formation is interpreted from lithofacies types such as those presented in Fig. 8. These are in general thin layers of very fine- to medium-grained sandstones intercalated with laminated, deformed (convolute lamination) and weakly bioturbated (usually by Planolites) grey mudrocks. The fine-grained material is related to low-energy deposition in shallow lakes and ponds on the delta plain, whereas the sandstone layers are interpreted in terms of flood-driven sheetflows including crevasse deltas and splays. In cored sections, about 25% to 50% of the Ness Formation is made up of this material, depending on the amount of channel deposits (Assemblage 1) seen in the same sections.

Evidence for subaerial exposure comes from the coal beds and rootlet intervals (palaeosols). However, coal beds (peat) require a high groundwater level to form (McCabe, 1984), and the pedogenic units (Fig. 9) are invariably grey to greyish green (ferrous iron), with preserved carbonaceous material and pyrite nodules, which together indicate that pedogenesis took place in a water-saturated (stagnant) environment. Moreover, the common preservation of sedimentary structures and absence of clear soil horizons indicate that these pedogenic intervals are rather immature. Thus, the cored intervals show no evidence of evolved soil maturity and improved drainage that could be expected from a significant lowering of the water table (following a base-level drop) even in humid and tropical areas. Ryseth (1989) defined a depositional cycle for the Ness Formation at Oseberg, and suggested that each depositional cycle was triggered by an event of drowning, during which a vegetated surface (peat

58

A. Ryseth and H. Fjellbirkeland

Differentiation of incised valley systems from mobile streams

Fig. 7. Distribution of measured sandstone-body thicknesses (Assemblage 1), Ness Formation and Statfjord Formation.

swamp or palaeosol) became submerged to form a body of standing water (lake/pond). Subsequently, these lake and pond systems were filled in by deposition to the point of emergence, and the previously submerged areas became the site of renewed peat accretion and/or soil formation. The photograph in Fig. 10 shows a cored section through a single depositional cycle. Each cycle commences with the drowning of a swamp or soil, hence boundaries are picked at the top of coal beds and palaeosol horizons, where they pass upwards into non-rooted lithologies deposited in standing waters. Notably, mudrocks recording a rather quiet, low-energy lacustrine environment are the most typical lithology above the cycle boundaries, whereas sandstone beds of crevasse splay and levee origin dominate in the middle and central parts, where they are interbedded with mudrocks. Finally, palaeosol units and coal beds form the upper part of the depositional cycle (uppermost 2 m in Fig. 10).

59

The vertical succession of facies that defines the depositional cycles form the main basis for reconstructing channel behaviour in the Ness Formation. Crevasse and levee deposits are invariably derived from adjacent channel systems, and their occurrence in the middle part of the depositional cycle, and usually well above the basal drowning (flooding) surface is a good indication that a channel may have avulsed into the established shallow lake system some time after its formation, and then supplied the sediment required to fill it to the point of emergence. The fine-grained palaeosol unit and the coal bed developed at the final stage of the cycle may record the abandonment of the channel system. Peat-forming swamps (coals) are particularly sensitive to sediment influx, and are not likely to form in close proximity to an active sediment source like a channel system (McCabe, 1984). The correlation presented in Fig. 11 is based on the lateral continuity of depositional cycles between two closely spaced wells (ca. 250 m apart), and illustrates the lateral relationship between two major fluvial sandstone bodies (each about 15 m thick) and the adjacent fine-grained facies assemblage. The interstratification of crevasse splay sandstones and non-rooted, sub-aqueously deposited mudrocks well above the basal cycle boundary, and the correlations of the crevasse deposits (Cycles 3 and 8) with nearby fluvial channel deposits indicate that these channel systems manifested themselves at times when the area was submerged. Figure 12 gives further information about the geometries of Assemblage 1 deposits in the Ness Formation (for location of wells see Fig. 13). Curiously enough, the thickest sandstone bodies in both the coal-bearing lower part of the Ness, and in the upper, less coal-prone interval show a strong tendency to pinch out laterally over rather short distances (approx. 1500 m or less, measured approximately normal to the assumed fluvial flow direction). Moreover, a clear, off-set stacking of major sandstone units in the upper Ness interval (wells 3 and 4, Fig. 12) can be noted, along with the (possible) existence of thin, but laterally more persistent sandstone sheets at the base of the Ness Formation, and also in the middle part of the unit (at the base of the upper Ness interval in Fig. 12). Also, the large-scale architecture of the Ness Formation should be noted, as this is of importance to the sequence stratigraphic discussion below. The lower Ness interval as correlated through Fig. 12 is

Fig. 6. Stratigraphy and core sedimentology of the Statfjord Formation. The formation features interstratification of fine- and coarsegrained alluvial deposits, and an uppermost shallow marine package separated from the subjacent alluvium by a flooding surface. The fluvial sandstone units form composite bodies of stacked, fining-upward sequences, and are related to deposition in a braided stream environment.

60

A. Ryseth and H. Fjellbirkeland

Fig. 8. Characteristic lithofacies types of Assemblage 2, Ness Formation. (a) Laminated, very fine-grained sandstone (distal crevasse splay) encased in laminated/bioturbated grey mudrock (lake/pond). (b) Intercalated beds of laminated mudrock and fine-grained sandstone. The lower sandstone layer shows a weak coarsening-upward tendency, with unidirectional ripple lamination in the upper part, whereas the upper unit fines upwards, with a thin unit of softly deformed mudclasts at the base. Packages of such heterolithic intervals are related to subaqueous levee deposition. (c) Unidirectional, climbing-ripple lamination recording rapid sedimentation from a concentrated flow at the base of a coarsening-upward sandstone unit of a crevasse-related minor mouth bar. (d) Sharp-based fine-grained sandstone layer (crevasse splay) encased in grey mudrock. Escape burrows at the base of the sandstone bed and the preserved lamination indicate an event of rapid sand supply into a low-energy setting.

Differentiation of incised valley systems from mobile streams

61

Fig. 9. Representative palaeosol units from the Ness Formation. Typical sections are light grey to brownish grey and commonly darken towards overlying coal beds. Diagnostic features of a soil hydromorphism include preserved carbonaceous rootlets and the reduced colours. Sedimentary bedding features are commonlypreserved, indicating rather immature soil development.

the most coal-prone part of the formation, and is also characterized by a low proportion of Assemblage 1 deposits, on the average about 25% or less. In contrast, the upper Ness interval contains less coal, and a higher proportion of Assemblage 1 sandstones (up to 40%; see also Ryseth, 1989). Figure 13 shows an attempt to map out the planform of the two channel systems seen in Fig. 11. This reconstruction constrains the width of the lowermost sandstone body (well 10A; Fig. 11) to less than 500 m, more or less independent of the orientation of the sandstone body. The uppermost sandstone body (well 10A; Fig. 11) probably has a larger lateral extent, and is correlated into well 3 in Fig. 12. Nevertheless, it does not seem to give a realistic match with the well data for widths larger than about 1000 to 1500 m. Hence, width/thickness ratios are in the order of 30:1 to 100 : 1, or within the range of narrow sheets in the classification of Friend et al. (1979). Apparently, rather similar width/thickness ratios should be expected for the majority of Assemblage 1 deposits shown in Fig. 12. In comparison, the sandstone sheet at the base of the upper Ness interval (Fig. 12) has a minimum width of about 3000 m (in the east-west direction), with a minimum width/thickness ratio of 500: 1.

The amount of apparent erosion/incision at the base of the lowermost sandstone body in well 10A (Fig. 11) should also be noted. Assuming the entire sandstone body is represented by a single major cut and fill event, erosion in the order of 5 to 10 m into the underlying cycle can be estimated at the base of this sandstone body. Moreover, similar amounts of scouring are also possible below the deposits of the narrow channels in Fig. 12. The sandstone sheet referred to above is also characterized by a distinctively sharp base, but the actual amount of scouring is apparently smaller, judging from its laterally persistent thickness of about 5 to 6 m, and the apparently conformable boundary with the subjacent deposits. Several lines of evidence argue against the existence of incised valley deposits in the Ness Formation. The similar sediment grain size and structures seen in all sandstone bodies assigned to Assemblage 1, and the similar flow conditions that can be implied has already been noted. Though not entirely conclusive, one should perhaps expect to find evidence for higher discharge and flow velocity within an incised valley fill than in deposits of widespread (mobile) deltaic distributaries. For instance, van Wagoner et

62

A. Ryseth and H. FjeUbirkeland

Fig. 10. Cored section illustrating the architecture of a depositional cycle, Ness Formation. Flooding surfaces are recorded above the coal beds (lower left and upper right). The alternation of sandstone beds (crevasse splays and sheet floods) and non-rooted mudrocks in the central part of the unit should be noted. al. (1993) indicated that incised valley fill in the Brent Group in the Statfjord Field had the facies character of braided streams, and that these could be differentiated from other fluvial deposits. The cored sections presented in Figs. 5 and 11 suggest that no such differentiation is possible at Oseberg, as Assemblage 1 deposits of highly variable thickness are characterized by similar grain sizes and sedimentary features. In fact, the multistorey character of the thickest sandstone bodies in the Ness Formation (Fig. 5) may indicate that these have formed by amalgamation of thinner units. Additionally, the analysis of the depositional cycle (Fig. 10) and the correlation of crevasse sandstones with nearby Assemblage 1 deposits (Fig. 11) indicate that the channel systems appeared at times when the depositional area was occupied by bodies of standing water, and the most simple interpretation would be that they avulsed into a water-filled, topographic depression from a nearby area (an autocyclic mechanism). The erosive nature and incision into subjacent deposits seen below some of the Ness Formation channels can probably be related to scouring following the initial stage of avulsion. Farrell (1987) recognized several stages in the evolution of channel belts by avulsion in the Mississippi False River region, including a low-lying lake and backswamp system as

the pre-avulsion stage. She also demonstrated how an avulsing stream could scour into previous floodbasin deposits (lake, backswamp) and at the same time deposit sandy sheet-flood units along its margin. This may represent a modern analogue to the depositional pattern suggested for the Ness Formation. In addition, the nature of the cycles and the correlation presented in Fig. 11 indicate that no genetic relationship exists between the interpreted channel deposits and the adjacent surfaces of subaerial exposure (coal beds and water-saturated palaeosols), other than that the coal beds (peat) may have formed at times of channel abandonment. It might be questioned whether peat accretion and soil hydromorphism are likely responses to valley incision at all, since the lowered base level could cause the groundwater table to descend, with the formation of more well-drained and oxidized soil features. The width/thickness ratios suggested for the majority of Assemblage 1 deposits (narrow sheets) do not seem to support an incised valley origin, in particular when considering the thickest sandstone bodies, which apparently are the most laterally restricted. The indicated sandstone sheet at the base of the upper Ness interval (Fig. 12) is probably a much better candidate for an incised valley, but can be just as easily interpreted in terms of limited lateral migration

Differentiation of incised valley systems from mobile streams

63

Fig. 11. Correlation of facies assemblages and depositional cycles in the Ness Formation. Note how the two main sandstone bodies in well 10A can be traced directly into subaqueously deposited minor mouth bar and crevasse splay deposits in the adjacent well.

of a mobile stream, for example by bank erosion and point bar accretion. Finally, some comments should be made on the off-set stacking pattern of channel deposits presented in Fig. 12. Such stacking patterns are likely to form

in areas where buried channels have an influence on the topography of the floodplain above, so that new channels avulsing into the area take a position in a depression that is off-set from the buried channel (Allen, 1978). It seems rather unlikely that erosive

64

i.

f~l

~ ,...4

A. Ryseth and H. Fjellbirkeland

0

o ,,.4

~

o,,,~

0

o,.,~

,~ ~.~m

0

"~~

. ,.,.~

f21.

~o"~ ~'.~ m

~

~

m

. ,-w

Z

m

f2~

0

0 ,.~

a,

~ "~

N o ,

,.~ *'~

"~8~

.o g i -~ "~ ~

l::z,.,-,

~ ~ "~ ,~ P~.w r4

Differentiation of incised valley systems from mobile streams

65

Lower Ness (Cycle 3)

,,i,,

_q/_

iL

4 ,/

N \

,m

\5/

9

9~

Figure \

[i'"'"'"]

12 (Ioc.,\~L Channel sandstond~

~_

"-~ . . . . . .

,,v

~,

e

~. ~

,J,

" ".2"2"2.2..

~

~

'J'

.'.'.'.'.'."2">-..

--~ e-~

l

.......

~, l Well

Upper Ness (Cycle 8)

,v

-

9 Well

-~-

,

-~-

1_.~_ _.~_[Overbank

,a,

deposits

0

1

2

[

.

.

.

.

.

i sandstone

....

,4, 3

~

] Overbank

"-

deposits

4

5

Kilometers

Fig. 13. Tentative palaeogeographic reconstruction of the two main channel systems presented in Fig. 10. T he available well data indicate that both systems g e n e r a t e d sandstone bodies with a rather limited lateral extent.

events caused by external factors would respond to the local floodplain topography in such a predictable way, though local depressions may be possible precursors of incised valleys. Thus, the observed stacking pattern along with the above interpretation of channel behaviour is most compatible with a mobile stream origin for the Ness channels. Vertical and lateral relationships in the Statfjord Formation The lateral continuity of facies assemblages presented in Fig. 6 (Statfjord Formation) is reconstructed in the well correlation in Fig. 14. Both wells are deviated, and thicknesses are adjusted to vertical thickness (TVT) for comparison. The horizontal distance between the two sections is approximately 600 m at the top Statfjord level, and decreases with depth. As shown, both measured sections contain two main levels of fluvial reservoir sandstones (Assemblage 1) separated by a coal-bearing, finer-grained interval (Assemblage 2). In the uppermost part of the formation, shallow marine sandstones occur below the marine mudrocks of the Dunlin Group. Three major surfaces are identified in Fig. 14. These include an abandonment surface at the top of

the lowermost Assemblage 1 unit, at the transition from interpreted passive channel deposits to unconfined floodplain sediments. Notably, this surface aids correlation in the following coal-bearing unit, but has otherwise no relevance to the following discussion. Additionally, an erosive bounding surface is indicated below the uppermost Assemblage 1 complex, and the transition from continental to shallow marine deposits is indicated by a transgressive surface, with an apparently rather conformable relationship to the subjacent continental deposits. Over the measured horizontal distance, apparently good lateral continuity of coal-bearing floodplain deposits below the main fluvial sandstone complex constrains the interpretation of the erosional bounding surface depicted in Fig. 14. From the reconstruction it can be inferred that approximately 10 m of mudrocks and coal have been truncated along the section between wells A and B. Hence, the erosive nature of this surface is well documented, and it is clearly a candidate for a sequence boundary. In the upper part of the formation, correlation of the transgressive surface and overlying shallow marine sandstones indicates that the vertical thickness between the two main surfaces is relatively constant between the wells, hence it can be assumed that the

66

A. Ryseth and H. Fjellbirkeland

Differentiation of incised valley systems from mobile streams

deposits bounded by the erosional surface and the transgressive surface form a more or less tabular body along the investigated section. The main feature to notice, however, is the lateral splitting of the main Assemblage 1 deposits in well B into four thinner intervals separated (at two of three occasions) by rooted mudrocks. Hence, it may be argued that at least the upper half of the sandstone body in well B has formed by amalgamation of mobile channel systems that avulsed into the area during three different time intervals separated by floodplain aggradation and pedogenesis. Allen (1978) and Bridge and Leeder (1979) demonstrated that thick, multistorey and laterally persistent sandstone bodies could form by amalgamation of mobile channel belts to generate alluvial suites with more than 70% of channel deposits (as in the upper part of the Statfjord Formation). In such situations, the expected lateral continuity of floodplain deposits is rather limited, and the packages of fine-grained sediment are most commonly truncated at channel margins and bases. The similarity with the upper Statfjord unit at Oseberg is striking. Assemblage 2 deposits in the upper Statfjord Formation have a strong resemblance with the equivalent deposits in the Ness Formation. The core log presented in Fig. 15 shows that the whole package of sediment above the fluvial channel sandstones was deposited in a subaqueous/water-saturated environment. As in the Ness Formation, the only evidence for subaerial exposure comes from the present coal beds and associated rootlet horizons. Further, depositional cycles bounded by drowning surfaces (grey mudrocks above coal beds) and filled with crevasse sandstones can also be defined. Thus, many of the arguments against the presence of incised systems in the Ness Formation may also be valid in this part of the Statfjord Formation, irrespective of the higher sandstone content. If the erosional bounding surface depicted in Fig. 14 actually represents the base of an incised valley (which is possible despite the reservations presented above), the valley fill itself is probably restricted to the lowermost sandstone body in well A, and the corresponding deposits in well B. In this case one is confronted with the problem of recognizing an incised fluvial valley from mobile stream deposits with almost identical lithofacies characteristics. Objectively, this seems more or less impossible, at least from subsurface data. On a local scale, it seems

67

20-

"15"-

18-

Distal crevasse splay Floodplain lake/pond

Drowning surface Swamp

16-

§ +

Paleosol

~4 14-'u-

Crevasse splay complex) crevasse subdelta

12-

Drowning surface Swamp

E U) C

10-

0 el-

"

.,n

--U"

Watersaturated (grey) paleosol

Drowning surface 9Swamp/paleosol

_

j-

Crevasse sub-delta

Floodplain lake/pond _

Inactive channel fill/ subaqueous levee

_

Fluvial channel O

_

SClay~Si~vf' f ~m c IVCl cgl Fig. 15. Detailed core log of an Assemblage 2 deposit in the upper half of the Statfjord Formation. Note the small-scale cyclicity as defined by the drowning surfaces above each coal bed (swamp) and the water-saturated/subaqueous nature of the interbedded clastic sediments.

Fig. 14. Correlation of facies assemblages in the Statfjord Formation. The thick fluvial sandstone body seen in well B splits into four smaller fining-upwards units separated by palaeosol-bearing finer-grained deposits. Below the main erosional surface, correlation of coal-bearing intervals indicates that some ten metres of erosional relief is present along the section (about 600 m horizontal distance).

A. Ryseth and H. Fjellbirkeland

68

more fruitful to interpret the depositional processes from the core data and relate these environmental reconstructions to observations regarding the alluvial architecture, than to speculate about external controls that need to be documented across the whole basin. This point will be further discussed below.

Sequence stratigraphic considerations Before going into a stratigraphic discussion, some comments should be made on the accumulation rates presented above. Apparently, accumulation rates in the range from 0.01 to 0.03 m/1000 years are rather low, and it is tempting to argue that both the Ness and Statfjord Formations represent intervals characterized by low rates of accommodation space development. However, the data presented by Steel (1993) show that rather similar accumulation rates can be calculated for other megasequence packages of comparable duration (approx. 106-107 years) of the mid-Triassic to Late Jurassic post-rift stage of the north Viking Graben (see Table 1). Notably, the duration of these megasequences corresponds to 3rd order (and 2nd order?) cycles of Vail et al. (1977). From Steel's data, it appears that the Statfjord Formation records a period of lower accumulation rate than the subjacent Triassic megasequences (Lomvi and Lunde), and that similar accumulation rates persisted during deposition of the following three megasequences (Johansen, Cook, Drake: midSinemurian-late Aalenian, Table 1). Moreover, it is noted that the accumulation rate derived for the Staffjord Formation at Oseberg is similar to the rate presented in Table 1. The sediment accumulation rate presented for the Ness Formation at Oseberg (0.01-0.02 m/103 years) is also comparable to the calculated rates for the older Statfjord, Johansen, Cook and Drake megasequences of Steel (1993), but significantly lower than

the maximum rate indicated for the Brent megasequence (0.08 m/103 years; Table 1), of which the Ness Formation is a main component. This maximum rate is however based on a Brent thickness of 700 m, which is rather unusual. Helland-Hansen et al. (1992) presented one well of comparable thickness (30/11-4), which they correlated into the Oseberg well 30/9-2. From their data it is clear that much of the regional thickness variation seen in the Brent Group is due to a rather dramatic expansion of the Ness unit in axial parts of the basin relative to the surrounding platform areas. Hence, it is rather obvious that the accumulation rate of the Ness Formation must have varied significantly on a basinal scale. In the reported case (well 30/11-4) a total thickness of about 320 m of Ness Formation can be correlated into the Oseberg Field, giving a comparable accumulation rate of approximately 0.06 m/103 years. This change in thickness and accumulation rate occur over a distance of less than 30 km and must probably reflect a strong effect of differential subsidence during deposition, with the implication that the low accumulation rate derived for the Ness Formation at Oseberg is mainly controlled by a comparably low subsidence rate during deposition, relative to more basinal areas. In this context, it is interesting that Sadler (1981) and Sadler and Strauss (1990) documented that accumulation rates depend upon the time span over which they are measured. For time intervals of 106 t o 107 years Sadler (1981) also found that accumulation rates in a wide range of depositional environments, including fluvial settings and terrigenous shelves converge toward values below 0.1 m/103 years, and argued that this was related to the behaviour of the crust during long-term loading and subsidence. Hence, from the data presented in Table 1 it seems that the Ness and Statfjord Formations were affected by similar long-term rates of accommodation space

TABLE 1 Gross depositional setting, thickness, duration and calculated rock accumulation rates for a series of Triassic-Jurassic post-rift megasequences of the north Viking Graben (modified from Steel, 1993). Accumulation rates calculated over similar time intervals for the Ness and Statfjord Formations at Oseberg are within the same range, indicating that these depositional systems were subjected to similar long-term rates of accommodation space development as other post-rift units in the area. The data also indicate that the effects of variable depositional environment are of only limited importance to the accumulation rates calculated over time intervals of more than 1 Ma Megasequence

Gross depositional setting

Thickness (m)

Duration (Ma; approx.)

Rock accumulation rate (m/103 years)

Lomvi Lunde Statfjord Johansen Cook Drake (Oseberg) Brent Krossfjord Sognefjord

Alluvial Alluvial Alluvial/marine Shelf/shoreline Shelf Shelf/shoreline/fan delta Deltaic Shelf/shoreline Shelf/shoreline

800-1200 700-1000 200- 350 100- 200 100- 250 100- 300 Up to 700 Up to 300 150- 200

18 9 12 6 6 10 8 10 8

0.04-0.07 0.07-0.10 0.02-0.03 0.02-0.03 0.02-0.04 0.01-0.03 <0.08 <0.03 0.01-0.03

Differentiation of incised valley systems from mobile streams

development as other Triassic-Jurassic post-rift intervals in the Viking Graben, and that these rates basically were controlled by basin subsidence. This should be kept in mind when going into a discussion of sequence stratigraphic approaches and higher order of resolution. Regional surveys (e.g. Graue et al., 1987; Lervik et al., 1989; Steel and Ryseth, 1990; Helland-Hansen et al., 1992; Steel, 1993) show that both the Statfjord Formation and the Ness Formation are widespread systems that can be correlated across more or less the entire north Viking Graben. However, these studies have failed to recognize basin-wide unconformities within the Ness and Statfjord units, whereas marine incursions are at least locally common in the Ness Formation (Livera, 1989). Steel (1993) did indicate that a local unconformity exists within the Statfjord Formation on the Horda Platform, but from biostratigraphic data it seems that this feature is structurally controlled (P. van Veen, personal communication, 1993). Thus, over the time intervals discussed here (approx. 106-107 years: 3rd order events) there is no clear evidence for stratigraphic breaks of basinal significance that need to the explained by eustatic changes or other external agents. However, the question of recognizing incised valleys in the Ness and Statfjord Formations is most relevant to the definition of 4th order sequences (see Van Wagoner et al., 1993). Sequence stratigraphic approaches to the interpretation of fluvial/continental deposits are not dramatically different from studies of shallow marine deposits, but generally, the former are more complex due to the difficulties of separating local and regional erosional surfaces. Studies of high-quality outcrops (e.g. Shanley et al., 1992; Shanley and McCabe, 1993) show that regional, sequence-bounding unconformities and transgressive surfaces can be traced from the shallow marine realm and into time-equivalent alluvial deposits, and aid the chronostratigraphic correlation between marine and non-marine strata. Some of the main problems in sequence stratigraphic interpretation of continental deposits are related to the definition of systems tracts. Posamentier and Vail (1988) describe two sequences of fluvial deposition that reflect relative changes of sea level (see summary in Wright and Marriott, 1993). The first type contains incised valley fill of the late low-stand and early high-stand systems tracts, and widespread (mobile) fluvial deposits of the late high-stand systems tract. The second type lacks the incised valley fill, and is also characterized by widespread fluvial deposits of the late high-stand systems tract. The notion that widespread fluvial deposition describes a highstand situation is also shared by Shanley et al. (1992)

69 and Shanley and McCabe (1993), who interpreted fluvially dominated valley fill in terms of a transgressive systems tract, and overlying coal-bearing strata with isolated fluvial channel deposits as a high-stand systems tract. An alternative view has recently been presented by Wright and Marriott (1993), who suggested that the maximum period of fluvial accumulation occurs during the transgressive and early high-stand phases. Importantly, these workers also indicated that the transgressive systems tract, being characterized by the highest relative rate of accommodation space development, would favour the formation of thin, immature and water-saturated palaeosols, thick packages of floodplain deposits and scattered fluvial channel deposits. In comparison, the high-stand phase, recording a decrease in the rate of accommodation space development, can be recognized by a vertical increase in the density of fluvial channel deposits. Apparently, the basinal (widespread) distribution of the Ness Formation, along with the published notion that the Ness unit at Oseberg accumulated during the progradational phase of the Brent Delta, point toward deposition during a high-stand phase, at least when considering the time scale of a 3rd order cycle or event. Such an interpretation should however be reflected in the overall facies and architecture of the formation itself. The observed depositional cyclicity seen in the Ness Formation (this study) forms the basis for fieldwide reservoir correlations, and can be further discussed in sequence stratigraphic terms. Clearly, the depositional Ness Formation cycle as defined in Fig. 10 is bounded by flooding surfaces (or drowning surfaces), and the overall facies sequence has a shallowing-upwards character that may resemble a parasequence as defined by van Wagoner et al. (1988, 1990). No marine influence can be related to the flooding events, and the overall grain-size pattern is composite, coarsening upwards to fining upwards rather than simply coarsening upwards, which would be most typical for a progradational unit. Van Wagoner et al. (1990) stated that marine flooding surfaces can be correlated into the non-marine (coastal plain) environment, and showed one example of a "fining upward" parasequence. Here, they picked the boundaries above coal beds and palaeosols, at the vertical transition into non-marine mudrocks. The similarity with the depositional cycles outlined above is clear, though the cited example shows shallow marine (tidal and intertidal) deposits in the central part of the sediment package, rather than lacustrine and fluvial deposits. Stacked depositional cycles (or parasequences) in the Ness unit show no dramatic vertical changes of

70 facies and grain size, and may resemble an aggradational parasequence set (van Wagoner et al., 1990). The vertical architectural change seen in the Ness Formation, with the lower Ness interval containing the bulk amount of coal, and generally less fluvial sandstone than the upper interval (Figs. 11, 12) should be taken into consideration, as the vertical increase in sandstone content may indicate a progradational parasequence set for the Ness unit (at Oseberg). The aggradational to progradational stacking patterns reported here, coupled with the interpretation of the main fluvial sandstones in the Ness as mobile systems, the water-saturated nature of the Ness interval and the lack of evidence for improved drainage during soil formation, seem most compatible with an interpretation in terms of a declining rate of accommodation space development through the time interval of deposition. Thus, from the regional correlations of Helland-Hansen et al. (1992) we find that the Ness deposits at Oseberg are most adequately related to deposition during an early to late highstand phase. More specifically, the coal-bearing lower Ness interval in Fig. 12, with its scattered fluvial channel deposits, may record a relatively high rate of accommodation space development during an early high-stand phase, whereas the more sand-prone upper Ness interval may record lower net aggradation and more dense stacking of fluvial deposits induced by lower rates of accommodation space development during the later stages of the high-stand phase. This long-term pattern of deposition may form the basis for discussing the influence of shorter-term events. For instance, van Wagoner et al. (1993) recognized eleven high-frequency sequences in the Ness Formation in the Statfjord Field, each bounded by unconformities including incised valleys. If incised systems are actually present in the Ness Formation at Oseberg, they should perhaps be most prominent in the upper part of the formation, which accumulated during the phase of minimum (long-term) creation of accommodation space. As stated above, the apparent behaviour of the Ness channels, their width/ thickness ratios, and the observed stacking pattern argue against the existence of incised valleys at this stratigraphic level. Moreover, the water-saturated nature of floodplain deposits throughout the Ness Formation, and the dominance of immature palaeosol units are in our opinion most compatible with relatively high short-term rates of accommodation space development and a persistently high groundwater level (cf. Wright and Marriott, 1993). The notion of short-term high accumulation rates in the Ness Formation need not contradict the apparently low long-term rates presented above. Sadler

A. Ryseth and H. Fjellbirkeland

(1981) has demonstrated that calculated short-term accumulation rates generally are higher than longterm rates in a wide range of environments, and also that short-term accumulation rates are poor guides to long-term accumulation regimes, in particular when discussing fluvial systems. Periodic deposition may account for some of this discrepancy, but need not be reflected by erosive events. For instance, coal beds of about 1 m of thickness are common in the lower Ness interval, and may require a time span of about 10 4 to 105 years to form (e.g. McCabe, 1984), during which little or no clastic deposits accumulated. Much of the discussion presented for the Ness Formation may be of relevance to the understanding of the Statfjord Formation. Steel and Ryseth (1990) noted a vertical increase in the proportion of fluvial channel deposits in the Statfjord Formation that can be recognized on a regional scale, and suggested that this pattern was controlled by decreasing rates of basinal subsidence through time, or as in the Ness Formation, a decreasing rate of creation of accommodation space. This study has documented the existence of coal-bearing cycles akin to the Ness floodplain intervals in the upper part of the unit, and also the possibility that some of the erosional surfaces seen below major sandstone bodies may record valley incision, despite the splitting of overlying deposits into thinner sandstone bodies and the lack of evidence for improved drainage. Notably, the potential of recording high-frequency base-level changes should be highest in the upper half of the formation. The question of whether or not an incised fluvial valley fill exists at the base of the sandstone body in Fig. 14, needs some further discussion. Only continental deposits have been recognized below and above the possible sequence boundary/valley fill in question. Little is actually known about the position of the contemporary shoreline at this stage, though Steel and Ryseth (1990) indicated that continental (alluvial) conditions persisted in the Viking Graben throughout the Hettangian and into the middle Sinemurian. The nearest shoreline may have been more than 100 km away. In such a situation one might question whether valley incisions can be induced by sea-level changes at all. In this respect, it should be noted that Blum (1990) demonstrated that glacio-eustatic sea-level cycles had no influence on the episodic delivery of sediment and the genesis of bounding surfaces in Quaternary deposits of the Colorado River, except for influencing the sediment architecture in the far downstream reaches. Blum (1990) also noted that climate has a much stronger influence in up-stream parts of the river system, and noted that this also was the case for the upper reaches of the Mississippi River. Even though climate and sea

Differentiation of incised valley systems from mobile streams

level may not be completely independent variables, it is clear that these findings give good grounds to question the likelihood that high-frequency eustatic changes will have any major effect on continental deposits, which are far removed from any shoreline. It is clear that the location of the contemporary shoreline has to be considered before changes in alluvial architecture are related to relative sea-level changes. Relative sea-level falls can surely cause valley incision near the shoreline, but very little is known about the rate at which a valley incision propagates into the continental environment from the shoreline, and about the various factors that governs the final landward position of the incised valley. In addition, river response to base-level changes is apparently more complex than predicted by, for example, Posamentier and Vail (1988). Schumm (1993) noted that channels can adjust to base-level falls by changing their sinuosity and pattern, shape and bed roughness, and that fluvial incision occurs only if the base-level change is large enough. Hence, a base-level fall may be reflected in fluvial architecture by a series of subtle facies changes rather than simply by valley incision. The corollary of this is that any claim for a base-level change demands documentation of changes in architectural style through detailed facies description. It is also our opinion that the interpretation of incised valleys in situations similar to the Statfjord example presented here, should be qualified by the recognition of major incisive or forced regressive events in correlatable marine deposits, or by equally important events recording changes in the delivery of water and sediment from the source terrain. To summarize these considerations, it is likely that both the Ness and Statfjord Formations record a long-term decrease in the rate of accommodation space development, as is reflected by an increasing proportion of fluvial sandstones in the upper parts of the units relative to the lower intervals. This longterm variation suggests, in turn, that shorter-term cycles and events affecting the relative sea level (e.g. 4th order events) and causing possible valley incisions, should have eventually been recorded in the upper parts of these formations. However, the analysis of vertical and lateral facies relationships in the Ness Formation suggests that channels are generally mobile and free to move by autocyclic mechanisms. This is also the case in the Statfjord Formation, though the possible existence of an incised fluvial valley in the lower parts of a major sandstone complex cannot be discounted. Long-term accumulation rates in the Ness Formation are apparently variable and controlled by differential subsidence, and it may be of importance to investigate the stratigraphic and architectural effects of this differential subsidence.

71

Finally, sequence stratigraphic interpretations of the Statfjord Formation would benefit from palaeogeographic studies which document the position of the time-equivalent shoreline, and also from an evaluation of the rates at which incisions at the shoreline can propagate into the continental environment. Conclusions

This paper has investigated possible ways of differentiating the deposits of incised valley systems from those of mobile streams using subsurface data. Factors such as width/thickness ratios of the main sandstone deposits, possible differences in depositional facies (grain size and bedding features), lateral continuity of the main sandstone bodies, and the depth of erosion seen at possible sequence bounding surfaces are considered important in the analysis. However, the character of the encasing fine-grained sediments (floodplain), and in particular the maturity of palaeosols and their degree of oxidation (improved drainage) are equally important factors that may indicate whether major base-level drops have controlled the depositional history or not. In the two cases discussed here (Ness Formation, upper Statfjord Formation), floodplain deposits encasing the fluvial sandstones are primarily subaqueously deposited. Evidence for prolonged subaerial exposure and improved floodplain drainage is lacking, and, on the contrary, the floodplain deposits contain grey, hydromorphic palaeosols and coal beds that require wet and stagnant conditions to form. It is unlikely that such conditions would have prevailed during periods of major valley incision. Vertical stacking patterns of interpreted parasequences in the Ness Formation at Oseberg provide evidence for deposition during a long-term phase of decreasing rates of accommodation space development. This was penecontemporaneous with the main progradational phase of the Brent Delta, most likely during a relative high-stand of sea level. A similar long-term trend of accommodation space development is also suggested for the Statfjord Formation. Differential subsidence during deposition clearly affected the Ness Formation, and an analysis of architectural and stratigraphic effects that can be related to variable rates of subsidence may be of value to the sequence stratigraphic interpretation of continental deposits. Moreover, in any discussion regarding base-level changes and the interpretation of the Statfjord Formation it is important that the location of the contemporary shoreline is established. Also, the fluvial response to base-level changes of variable magnitude has to be considered, and rates of incision propagation into the continental environment need

72

to be established, in particular when erosive events in continental areas are claimed to be controlled by short-term changes of relative sea level.

Acknowledgements We wish to thank Norsk Hydro, Statoil, Elf, Total, Mobil and Saga for permission to publish data from the Oseberg Field. Ole Martinsen reviewed an earlier version of this manuscript, and the constructive criticism of Ron Steel, Torben Olsen and an anonymous reviewer is also acknowledged.

References Allen, J.R.L., 1973. Studies in fluviatile sedimentation: implications of pedogenic carbonate units, Lower Old Red Sandstone, Anglo-Welsh outcrop. Geol. J., 9: 181-208. Allen, J.R.L., 1978. Studies in fluviatile sedimentation: an exploratory quantitative model for the architecture of avulsioncontrolled alluvial suites. Sediment. Geol., 21: 129-147. Aubrey, W.M., 1989. Mid-Cretaceous alluvial plain incision related to eustasy, southeastern Colorado Plateau. Geol. Soc. Am., Bull., 101: 443-449. Badley, M.E., Egeberg, T. and Nipen, O., 1984. Development of rift basins illustrated by the structural evolution of the Oseberg feature, Block 30/6, offshore Norway. J. Geol. Soc. London, 141: 639-649. Badley, M.E., Price, J.D., Rambech Dahl, C. and Agdestein, T., 1988. The structural evolution of the northern Viking Graben, and its bearing upon extensional modes of basin formation. J. Geol. Soc. London, 145: 455-472. Besly, B.M. and Fielding, C.R., 1989. Palaeosols in Westphalian coal-bearing and red-bed sequences, central and northern England. Palaeogeogr., Palaeoclimatol., Palaeoecol., 70: 303330. Besly, B.M. and Turner, P., 1983. Origin of red beds in a moist tropical climate (Etruria Formation, Upper Carboniferous, U.K). In: R.C.L. Wilson (Editor), Residual Deposits. Geol. Soc., Spec. Publ., 11: 131-147. Blakey, R.C. and Gubitosa, R., 1984. Controls of sandstone body geometry and architecture in the Chinle Formation (upper "Ii'iassic), Colorado Plateau. Sediment. Geol., 38:51-86. Blum, M.D., 1990. Climatic and eustatic controls on Gulf coastal plain fluvial sedimentation: an example from the late Quaternary of the Colorado River, Texas. In: Sequence Stratigraphy as an Exploration Tool, Concepts and Practices in the Gulf Coast, l lth Annu. Res. Conf. Gulf Coast Sect., Soc. Econ. Paleontol. Mineral. Found., pp. 71-83. Bown, T.M. and Kraus, M.J., 1987. Integration of channel and floodplain suites, I. Developmental sequence and lateral relations of alluvial paleosols. J. Sediment. Petrol., 57: 587601. Bridge, J.S. and Leeder, M.R., 1979. A simulation model of alluvial stratigraphy. Sedimentology, 26: 617-644. Bridge, J.S. and Mackey, S.D., 1993. A theoretical study of fluvial sandstone body dimensions. In: S. Flint and I.D. Bryant (Editors), The Geological Modelling of Hydrocarbon Reservoirs and Outcrop Analogues. Int. Assoc. Sedimentol., Spec. Publ., 15: 213-236.

A. Ryseth and H. Fjellbirkeland Collinson, J.D., 1986. Alluvial sediments. In: H.G. Reading (Editor), Sedimentary Environments and Facies, 2nd ed. Blackwell, Oxford, pp. 20-62. Collinson, J.D. and Lewin, J. (Editors), 1983. Modern and Ancient Fluvial Systems. Int. Assoc. Sedimentol., Spec. Publ., 6, 575 pp. Dreyer, T., Falt, L.M., Hey, T., Knarud, R., Steel, R. and Cuevas, J.-L., 1993. Sedimentary architecture of field analogues for reservoir information (SAFARI): a case study of the fluvial Escanilla Formation, Spanish Pyrenees. In: S. Flint and I.D. Bryant (Editors), The Geological Modelling of Hydrocarbon Reservoirs and Outcrop Analogues. Int. Assoc. Sedimentol., Spec. Publ., 15: 57-80. Ethrigde, EG., Flores, R.M. and Harvey, M.D. (Editors), 1987. Recent Developments in Fluvial Sedimentology. Soc. Econ. Paleontol. Mineral., Spec. Publ., 39, 389 pp. Farrell, K.M., 1987. Sedimentology and facies architecture of overbank deposits of the Mississippi River, False River region, Louisiana. In: EG. Ethrigde, R.M. Flores and M.D. Harvey (Editors), Recent Developments in Fluvial Sedimentology. Soc. Econ. Paleontol. Mineral., Spec. Publ., 39: 111-120. Friend, P.E, Slater, M.J. and Williams, R.C., 1979. Vertical and lateral building of sandstone bodies, Ebro basin, Spain. J. Geol. Soc. London, 136: 39-46. Graue, E., Helland-Hansen, W., Johnsen, J.R., Lcmo, L., Ncttvedt, A., Rcnning, K., Ryseth, A. and Steel, R., 1987. Advance and retreat of the Brent Delta system, Norwegian North Sea. In: K. Brooks and K. Glennie (Editors), Petroleum Geology of North West Europe. Graham and Trotman, London, pp. 915-937. Harland, W.B., Armstrong, R.D., Cox, A.V., Craig, L.E., Smith, A.G. and Smith, D.G., 1989. A Geologic Time Scale. Cambridge University Press, Cambridge, 263 pp. Helland-Hansen, W., Ashton, M., Lcmo, L. and Steel, R., 1992. Advance and retreat of the Brent delta: recent contributions to the depositional model. In: A.C. Morton, R.S. Haszeldine, M.R. Giles and S. Broom (Editors), Geology of the Brent Group. Geol. Soc., Spec. Publ., 61: 109-127. Kraus, M.J. and Middleton, L.T., 1987. Dissected paleotopography and base level changes in a Triassic fluvial sequence. Geology, 15: 18-21. Larsen, V., Aasheim, S.M. and Masset, J.M., 1981. 30/6-Alpha structure, a field case study in the Silver Block. In: Norwegian Symposium on Exploration (NSE-81). Norwegian Petroleum Society, Oslo, 14, pp. 1-32. Lervik, K.S., Spencer, A.M. and Warrington, G., 1989. Outline of Triassic stratigraphy and structure in the central and northern North Sea. In: J.D. Collinson (Editor), Correlation in Hydrocarbon Exploration. Norwegian Petroleum Society, Graham and Trotman, London, pp. 173-189. Livera, S.E., 1989. Facies associations and sand-body geometries in the Ness Formation of the Brent Group, Brent Field. In: M.K.G. Whateley and K.T. Pickering (Editors), Deltas: Sites and Traps for Fossil Fuels. Geol. Soc., Spec. Publ., 41: 269286. McCabe, P.J., 1984. Depositional environments of coal and coalbearing strata. In: R.A. Rahmani and R.M. Flores (Editors), Sedimentology of Coal and Coal-bearing Sequences. Int. Assoc. Sedimentol., Spec. Publ., 7: 13-42. Miall, A.D. (Editor), 1978. Fluvial Sedimentology. Can. Soc. Pet. Geol., Mem., 5, 859 pp. Miall, A.D., 1992. Alluvial sediments. In: R.G. Walker and N.P. James (Editors), Facies Models: Response to Sea Level

Differentiation of incised valley systems from mobile streams Change. Geological Association of Canada, St. John's, Nfld., pp. 119-142. Nystuen, J.P., Knarud, R., Jorde, K. and Stanley, K.O., 1989. Correlation of Triassic to lower Jurassic sequences, Snorre Field and adjacent areas, northern North Sea. In: J.D. Collinson (Editor), Correlation in Hydrocarbon Exploration. Norwegian Petroleum Society, Graham and Trotman, London, pp. 273289. Posamentier, H.W. and Vail, P.R., 1988. Eustatic controls on clastic deposition, II. Sequence and systems tract models. In: C.K. Wilgus, B.S. Hastings, C.G.St.C. Kendall, H.W. Posamentier, C.A. Ross and J.C. Van Wagoner (Editors), Sea-Level Changes: An Integrated Approach. Soc. Econ. Paleontol. Mineral., Spec. Publ., 42: 125-154. Retallack, G.J., 1986. Fossil soils as grounds for interpreting long-term controls on ancient rivers. J. Sediment. Petrol., 56: 1-18. Roe, S.-L. and Steel, R., 1985. Sedimentation, sea-level rise and tectonics at the Triassic-Jurassic boundary (Statfjord Formation), Tampen Spur, northern North Sea. J. Pet. Geol., 8: 163-186. Ryseth, A., 1989. Correlation of depositional patterns in the Ness Formation, Oseberg area. In: J.D. Collinson (Editor), Correlation in Hydrocarbon Exploration. Norwegian Petroleum Society, Graham and Trotman, London, pp. 313-326. Sadler, P.M., 1981. Sediment accumulation rates and the completeness of stratigraphic sections. J. Geol., 89: 569-584. Sadler, P.M. and Strauss, D.J., 1990. Estimation of completeness of stratigraphical sections using empirical data and theoretical models. J. Geol. Soc. London, 147: 471-485. Schumm, S.A., 1993. River response to base level change: implications for sequence stratigraphy. J. Geol., 101: 279-294. Shanley, K.W. and McCabe, P.J., 1993. Alluvial architecture in a sequence stratigraphic framework: a case history from the Upper Cretaceous of southern Utah, USA. In: S.S. Flint and I.D. Bryant (Editors), The Geological Modelling of Hydrocarbon Reservoirs and Outcrop Analogues. Int. Assoc. Sedimentol., Spec. Publ., 15: 21-56. Shanley, K.W., McCabe, P.J. and Hettinger, R.T., 1992. Tidal influence in Cretaceous fluvial strata from Utah, USA: a key to sequence stratigraphic interpretation. Sedimentology, 39:

A. RYSETH H. FJELLBIRKELAND

73 9O5-930. Steel, R.J., 1993. Triassic-Jurassic megasequence stratigraphy in the northern North Sea: rift to post-rift evolution. In: J.R. Parker (Editor), Petroleum Geology of North West Europe. Proceedings of the 4th Conference, Geol. Soc., London, pp. 299-315. Steel, R. and Ryseth, A., 1990. The Triassic - - early Jurassic succession in the northern North Sea: megasequence stratigraphy and intra-Triassic tectonics. In: R.EP. Hardman and J. Brooks (Editors), Tectonic Events Responsible for Britains Oil and Gas Reserves. Geol. Soc., Spec. Publ., 55: 139-168. Vail, P.R., Mitchum, J.M. and Thompson, S., 1977. Seismic stratigraphy and global changes of sea level, Part 4. Global cycles of relative changes of sea level. In: C.E. Payton (Editor), Seismic Stratigraphy- Applications to Hydrocarbon Exploration. Am. Assoc. Pet. Geol., Mem., 26: 83-97. Van Wagoner, J.C., Posamentier, H.W., Mitchum, R.M., Vail, P.R., Sarg, J.F., Loutit, T.S. and Hardenbol, J., 1988. An overview of the fundamentals of sequence stratigraphy and key definitions. In: C.K. Wilgus, B.S. Hastings, C.G.St.C. Kendall, H.W. Posamentier, C.A. Ross and J.C. Van Wagoner (Editors), Sea-Level Changes: An Integrated Approach. Soc. Econ. Paleontol. Mineral., Spec. Publ., 42: 39-45. Van Wagoner, J.C., Mitchum, R.M., Campion, K.M. and Rahmanian, V.D., 1990. Siliciclastic Sequence stratigraphy in well logs, cores and outcrops. Am. Assoc. Pet. Geol., Methods Explor. Ser., 7, 55 pp. Van Wagoner, J.C., Jenette, D.C., Tsang, P., Hamar, G.P. and Kaas, I., 1993. Abstract. Application of high resolution sequence stratigraphy and facies architecture in mapping potential additional hydrocarbon reserves in the Brent Group, Statfjord Field. In: Sequence Stratigraphy: Advances and Applications for Exploration and Production in North West Europe. Norwegian Petroleum Society, Stavanger, p. 2. Vollset, J. and Dor6, A.G., 1984. A revised Triassic and Jurassic lithostratigraphic nomenclature for the Norwegian North Sea. Norw. Pet. Dir., Bull., 3, 53 pp. Wright, V.P. and Marriott, S.B., 1993. The sequence stratigraphy of fluvial depositional systems; the role of floodplain sediment storage. Sediment. Geol., 86: 203-210.

Norsk Hydro Research Centre, Sandsliveien 90, 5020 Bergen, Norway Norsk Hydro Production, Sandsliveien 90, 5020 Bergen, Norway