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 410

Platform (Fjellanger et al ., 1996; Færseth et al. ,
1997; Ravnås et al ., 1997) are in general agree-
ment with the predictions of the model described
by Gupta et al . (1998). For the Gullfaks-
Kvitebjørn mega-block, we have demonstrated
an earlier rift-initiation than has been reported
from the basin flanks. At least two possible
explanations can account for this. Either the
position of the Kvitebjørn area, as part of a major
step-over of the Viking Graben (ramp and horst,
according to Fossen et al ., 2010), had reduced
subsidence rates in that location, or increased
subsidence rates occurred more or less synchro-
nously across the Gullfaks-Kvitebjørn Permo-
Triassic mega-block.
The asymmetrical Ness and Tarbert wedge, rec-
ognised here from the Kvitebjørn area, places the
onset of initial rifting within the Ness Formation
(Fig. 6). The incipient faulting that influenced the
depositional patterns of the Ness Formation indi-
cates that the footwall part of the Gullfaks-
Kvitebjørn mega-block (Fig. 4) underwent
extension at this time. Ravnås and Steel (1998)
described a similar setting with segmentation of
the footwall collapse and Sharp et al . (2000)
showed footwall flexing with development of
half-grabens during block-rotation. These two
examples illustrate similarities to the develop-
ment of the faulted Kvitebjørn Field from the Ness
Formation (Early Bajocian) and onwards in terms
of stratigraphy.
As the population of small faults within the
Gullfaks-Kvitebjørn mega-block accumulated
strain, they propagated and linked, with some
developing into major faults through fault linkage
processes (e.g. see Cowie, 1998; Dawers &
Underhill, 2000). At this stage, other Permo-
Triassic faults were active, including the major
Snorre and Gullfaks faults (Fig. 1). The well cor-
relations of the Brent Group in this study show
that this occurred at the last stages of Brent Group
deposition or during submergence of the Brent
Group. The large fault-blocks outlined by a few
Permo-Triassic faults were active in the Early-
Middle Jurassic (Fig. 3), a collection of minor
faults during deposition of the Ness-Tarbert for-
mations (Fig. 3) that gradually developed into the
well-known Late Jurassic fault population. The
exact reason for the selective reactivation of
Permo-Triassic faults is unclear but it is likely to
relate to variations in frictional strength, geomet-
ric restrictions, fault interaction and/or orienta-
tion relative to the regional stress field.

Early syn-rift

The Ness Formation shows a general aggradational
stacking of facies, with a balance between sedi-
ment supply and accommodation space creation
(Fig. 4). The transition from the progradational
style of the advancing 'Brent delta' (Rannoch, Etive
and lower Ness formations) to an aggradational
style was explained by Ravnås et al . (1997) to be
the result of increased rate of fault-related subsid-
ence and thus added rate of accommodation space
creation - a model that is supported by the find-
ings of the present study of the Gullfaks-Kvitebjørn
mega-block. A 'Mid-Ness-Shale' unit within the
wedge-shaped Ness-Tarbert is described by
Fjellanger et al . (1996) in the western part of block
34/10 (north of Fig. 4) as an organic-rich lagoonal
mudstone. This observation adds to the picture of
a westward rotation of the Kvitebjørn-Gullfaks
Permo-Triassic mega-block, where the formation
of this thick lagoonal mudstone unit is indicative
of rapid subsidence. Hampson et al . (2004) also
reports the occurrence of a wedge-shaped 'Mid-
Ness-Shale' unit (lagoonal mudstone) on the UK
side, which thickens toward the Ninian-Hutton
fault. These 'Mid-Ness-Shale' units are associated
with the hanging wall depocentres of Permo-
Triassic faults and, together with the lithologic dis-
tribution within the Ness Formation (Fig. 4), show
a net to gross reduction westward, with preserva-
tion of thicker mudstone intervals between the
sandstone units as the river-supplied muds became
trapped and preserved in the high-accommodation
setting of the hangingwall area. The hangingwall
area probably experienced transgressive events
(see timelines in Fig. 4) that reached farther inland
than the footwall area and with more immature
and laterally restricted coal-layer accumulations
in the hangingwall than the footwall area. The
occurrence of isolated fluvial channels in the
hangingwall area is the result of axial drainage
being attracted into the subsiding areas.
The Brent Group forms a continuum from the
rapidly advancing deltaic Rannoch-Etive forma-
tions, without recordable tectonic influence, to
the delta pinch-out in Early Bajocian time where
the initiation of fault movements created an irreg-
ular delta front (Fig. 11). This can be seen as the
precursor to the retreating Tarbert Formation
(Rønning & Steel, 1987). The timelines in Fig. 4
demonstrate that the hangingwall area was trans-
gressed earlier than the footwall area of the
Gullfaks-Kvitebjørn Permo-Triassic mega-block,