Tectonic influence on the Jurassic sedimentary architecture in the northern North Sea with focus on the Brent Group - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 409

that fault-slippage during this period was caused
by pulses of mild extension (Ravnås et al ., 2000).
For the most part, these inter-rift fault movements
occurred on Permo-Triassic faults that propagated
upward during deposition of the inter-rift sedimen-
tary sequence. Such faults include the Statfjord
East fault (Fig. 4) and the Ninian-Hutton-Dunlin
fault, both of which are reported to have been
active throughout the Jurassic period (Hampson,
et al ., 2004).
There are at least two end-member models that
can explain such fault activity during the inter-rift
period. One is that extension continued more or
less continuously at a very low rate, activating
faults at different locations in the basin at differ-
ent times. The other is that the basin experienced
a number of short-lived (some millions of years)
pulses of extension, as advocated by Ravnås et al .
(2000). At present the resolution of available data
makes it difficult to discriminate between these
two models.
The suggested time of initiation for the main
Middle-Late Jurassic rift phase varies from study
to study, ranging from Late Bajocian (Helland-
Hansen et al ., 1992; Mitchener et al ., 1992,
Johannessen et al ., 1995) to Kimmeridgian (Badley
et al ., 1988). This wide range may indicate that
rift-initiation was not synchronous throughout
the basin (Ravnås & Bondevik, 1997) because
wedge-shaped stratal packages (with internal
stratal thinning or expansion) in a depositional
strike section are taken to indicate syn-deposi-
tional fault-block rotation (i.e. syn-rift deposition;
e.g. Yielding et al ., 1992; Ravnås & Steel, 1998).
Considering the data presented in this study, we
suggest that the Jurassic rift episodes of the north-
ern North Sea rift system initiated in the Early
Bajocian within the central part of the basin,
marked by the base of the wedge-shaped Ness and
Tarbert formations in the Gullfaks-Kvitebjørn area
(Figs 4 and 6).This would correspond approxi-
mately at the base of the Early Bajocian. Work by
Graue et al . (1987) and Fält et al . (1989) suggests
that the Ness Formation shows evidence of syn-
sedimentary faulting along major structural linea-
ments that most likely represent Permo-Triassic
faults. Rifting first occurred in the proto-Viking
Graben and the adjacent areas and then expanded
to affect the lateral platform-areas at a later stage
to cause the syn-rift activity reflected in the
Tarbert Formation in the East Shetland Platform
(Davies et al ., 2000) and the Horda Platform
(Ravnås et al ., 1997).

The initial rift stage (Early Bajocian) was prob-
ably characterised by a combination of increased
movement of the Permo-Triassic faults as well as
the development of a new Jurassic fault-population
at the base of the large-scale Ness and Tarbert
stratal wedge (Fig. 4) that created scattered local
depocentres (see also Fig. 3; Gawthorpe & Leeder,
2000; Sharp et al ., 2000; Davies et al ., 2000). We
refer to the anomalously thick sandstone units,
which formed in the fault-induced local depocen-
tres, as the 'base Ness sandstone unit' (Figs 8, 9A,
9C and 10D). Davies et al . (2000) found fault-
induced local depocentres at the rift-initiation
level within the Tarbert Formation on the East
Shetland Platform which appears to be analogous
to the 'base Ness sandstone unit'. In other loca-
tions with limited sand supply either further
landward or offshore, these local depocentres may
have had a different sedimentological expression
than seen in Fig. 9A and C.
The base Ness sandstone unit in well 34/10-23
(Fig. 9A) has earlier been interpreted as the result
of rapid sea-level rise, with a step-up of the shore-
line with cross-bedded sandstones representing
shoreface (Bullimore & Helland-Hansen, 2009).
Such a rapid sea-level rise is not indicated in any
of the laterally adjacent wells (see Figs 4 and 8). In
the UK sector, a similar unit with tens of metres of
cross-bedded sandstones has been interpreted as
incised-valley fills (Morris et al. , 2003; Hampson
et al ., 2004). This latter interpretation explains the
aggradational style and isolated occurrence of the
sandstones but is at odds with the lack of a
time-equivalent seaward shoreline detached
sedimentary wedge, at least in the Norwegian sec-
tor of the 'Brent delta' (Graue et al. , 1987; Helland-
Hansen et al ., 1992; Fjellanger et al ., 1996;
Bullimore & Helland-Hansen, 2009; Mjøs, 2009).
The transition from the rift-initiation stage to a
phase of increased fault movement has been cor-
related to a sharp increase in the rate of basin-
wide subsidence early in the rift event (e.g.
Steckler et al. , 1988). Gupta et al . (1998) sug-
gested that the transition from rift-initiation to
syn-rift occurs as fault activity becomes local-
ised into linked arrays (Table 2 and Fig. 3). With
a decline in the number of active faults, the rate
of fault displacement increased; hence, the rate
of tectonic subsidence increased. Observa-
tions from the East Shetland Basin (Dawers &
Underhill, 2000; McLeod et al ., 2000; Hampson
et al ., 2004), the Tampen Spur area (Bruaset
et al ., 1999; Ravnås et al ., 2000) and the Horda