Late Triassic to Early Jurassic climatic change, northern North Sea region: impact on alluvial architecture, palaeosols and clay mineralogy - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 65

area (Nystuen et al ., 1989), represents a marked
change in depositional environment, from lacus-
trine or terminal basins to alluvial fans and wide
alluvial plains (Nystuen et al ., 1989; Steel &
Ryseth, 1990; Steel, 1993; Nystuen & Fält, 1995).
The Lunde Formation is composed of inter-
changing fluvial channel sandstone bodies and
reddish-brown floodplain mudstones, altogether
about 800 m to 850 m thick. The Lunde Formation
is an overall fining-upwards (FU) succession and
the Statfjord Group a coarsening-upwards (CU)
succession, about 150 m to 330 m thick in the
Tampen Spur area. The lower boundary of the
Statfjord Group was defined by Deegan & Scull
(1977) as the base of the coarsening-upwards suc-
cession. Later, Vollset & Doré (1984) redefined the
lower boundary as the base of the first thick chan-
nel sandstone body occurring above the mud-
stone-dominated interval that was defined as the
upper part of the Lunde Formation. The original
definition is applied in the present study, as the
trend change from FU to CU can be identified in
most wells penetrating this interval in the north-
ern North Sea area (Nystuen et al ., 1989). The
turnaround from fining-upwards to coarsening-
upwards in grain size-trend is likely to reflect a
major regional change in the depositional system
(Nystuen et al ., 1989; Steel & Ryseth, 1990; Steel,
1993; Nystuen & Fält, 1995) and is discussed
further below.
The Raude Formation is dominated by red-
dish-brown mudstone and single-storey fluvial
channel sandstone bodies and associated thin
sheet sandstones of crevasse splay origin. The
Eiriksson Formation has a higher frequency of
fluvial channel sandstone bodies compared with
the Raude Formation. Individual channelised sand-
stone bodies commonly form composite multi-
storey sandstone bodies. Mudstones are dominantly
greenish-grey. The Nansen Formation is predomi-
nantly sandstone of shallow-marine origin, over-
lain by marine mudstone and sandstone of the
Amundsen Formation in the Dunlin Group
(Charnock et al ., 2001; Goldsmith et al ., 2003).

lake system located in the central and northern
North Sea area (Mckie & Williams, 2009). The
Lunde Formation has been interpreted as having
formed within a wide alluvial plain and the conti-
nental part of the Statfjord Group as deposited as
alluvial fans, alluvial plains or braidplains (Røe &
Steel, 1985; Nystuen et al ., 1989; Steel & Ryseth,
1990; Nystuen & Fält, 1995; Ryseth, 2001)
(Fig. 3). The ultimate base level during deposition
of the Lunde Formation is believed to have been
the Borealic Sea to the north (Müller et al ., 2005)
and the Tethys to the south-east (McKie &
Williams, 2009), both seaways several hundred
kilometres from the 'Lunde basin'. The transgres-
sion during deposition of the upper part of the
Statfjord Group took place both from the south
and the north (Nystuen & Fält, 1995; Ryseth, 2001;
Goldsmith et al ., 2003).
The total thickness of the Lunde Formation
(about 800 m to 850 m) is relatively uniform in the
Tampen Spur area. The thickness of the Statfjord
Group, about 150 m to 330 m in the Tampen Spur
area, increases further to the south; the variation
may be due to syndepositional tectonics or differ-
ential subsidence (Vollset & Doré, 1984; Fält,
1987; Steel & Ryseth, 1990; Ryseth & Ramm, 1996).
Estimated ages of the Lunde Formation and the
Statfjord Group (Fig. 2) indicate an overall
marked decline in sedimentation rates from the
Norian-Rhaetian to the late Sinemurian. By apply-
ing the time scale of Gradstein et al . (2004)
(Fig. 2), the Lunde Formation tentatively spans
about 15 million years, giving an average rate of
vertical deposition in the order of 0.055 mm/year.
The Statfjord Group may represent about 12 mil-
lion years, giving an average depositional rate in
the Tampen Spur area of about 0.012 mm/year to
0.027 mm/year. These numbers are only approxi-
mate but they do illustrate a marked decline in
average sedimentation rate and/or accommoda-
tion rate from late Triassic to earliest Jurassic.
The source areas of clastic debris supplied to the
Lunde Formation and the Statfjord Group, i.e. the
Shetland Platform to the west and north-west and
south-western Norway to the east, were composed
of Precambrian gneisses, Caledonian metamor-
phic rocks and Devonian sandstones (Mearns
et al ., 1989, Nystuen & Fält, 1995; Knudsen, 2001).
The Triassic to Middle Jurassic succession in the
Snorre Field was uplifted and partly eroded dur-
ing a major rifting episode in the North Sea region
during Late Jurassic to Early Cretaceous times.
The uplift was followed by subsidence and burial

Basin configuration and structural framework

Most of the Hegre Group and the whole of the
Statfjord Group were deposited during the post-
rift phase of the Late Permian-Early Triassic rifting
(Badley et al ., 1988; Gabrielsen et al ., 1990;
Odinsen et al ., 2000). Throughout most of the
Triassic, alluvial systems fed into a large terminal