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stream channel sandstone bodies with sheet-like
geometry in FSA6 demonstrate a decrease in
the A/S ratio towards the upper termination of
the unit S1. The quartz-arenitic composition of
sandstones in the upper part of the Statfjord Group
(Sinemurian) is interpreted to reflect advanced
chemical weathering in a humid and warm cli-
mate in the hinterland. The palaeosol types with
overall greenish-grey colour, absence of carbonate
nodules, rhizocretions and mottling reflect a humid
climate with overall water-saturated soils on the
alluvial plains. This is also documented by coal
fragments in fluvial channel conglomerates and
mudstones with high contents of plant debris.
The clay mineralogy, being characterised by a
sudden drop in smectite followed by an abrupt
increase in kaolinite (Figs 14 and 15), can be
explained by a warm humid climate promoting
production of kaolinite through advanced chemi-
cal weathering of hinterland bedrocks and/or of
rather well-drained floodplain sediments (cf. Girty,
1991). Improved drainage in the USG, relative to
LSG, would result from an increasing influx of
sand, forming steeper-sloping and very wide allu-
vial fans or braidplains. Ryseth (2001) also empha-
sised that the highest depositional gradients in the
Statfjord Formation were in the Tampen Spur area.
The relatively high occurrence of greenish-grey
entisols/inceptisols
versus
low chroma vertisols
in the allostratigraphic units S2 and S1 (Fig. 15)
may be explained by low sediment stability on
alluvial plains/fans due to reworking from highly
mobile braided streams during annual monsoonal
seasons.
The Statfjord Group
The fluvial style of the uppermost part of the
Lunde Formation is continued into S5 and S4 of
the Statfjord Group by predominantly isolated
fluvial channel sandstones, inferred to represent
single and isolated rivers within muddy flood-
plains. The high mudrock: sandstone proportion
in S5 to S4 and clay mineral composition reflect
a continuing high rate of chemical weathering
(Table 6, Fig. 14).
The transition from the mostly reddish-brown
mudrocks in S5 to the increasingly mottled and
greenish to grey mudrocks upwards in S4 is
accompanied by dominantly mottled and low-
chroma vertisols (MF 4), replacing red-coloured
entisols/inceptisols or high chroma vertisols. All
these features indicate increasing water content
in the soil. The rare occurrence of aggregated
horizons in LSG may also imply wetter condi-
tions, as the depth and degree of self-mulching
and soil aggregation are closely related to soil
moisture (Probert
et al
., 1987; Dudal & Eswaran,
1988, McGarry, 1996).
The presence of organic material and coalified
plant debris in mudrocks and channel sandstones
in the S4 unit indicates a high groundwater table
on the floodplain and thus an increase in vegeta-
tion cover and preservation of organic material in
swamps and depressions. The rich occurrence of
Deltoidospora spp
. in the SG is particularly indic-
ative of humid conditions (Hubbard & Boulter,
2000). The influx of these palynomorphs might
also reflect changes in vegetation type. Increasing
humidity is also reflected by the decrease in carbon-
ate nodules and rhizocretions through S5 to their
absence in S4. The LSG is also typified by a high
content (up to 9%) of goethite in some mudrock
units (Table 6).
Palaeosol development up through the ULF and
LSG generally shows better developed palaeosol
types and thicker cumulative pedocomplexes.
This general trend is also reflected in the clay min-
eralogy by the increase in abundance of kaolinite
and smectite at the expense of mixed-layer clay
minerals, chlorite and illite. The marked increase
in the quartz : feldspar ratio in fluvial channel sand-
stones and mudrocks from the ULF to the LSG is
probably the result of increased chemical weath-
ering, triggered by increased humidity in a warm
climate (Tables 2 and 3, Figs 14 and 15).
The characteristic CU pattern of the upper part
of the Statfjord Group and the increase in braided
Tectonics and eustasy
versus
climate as cause
of change in depositional trends
In most alluvial basins, tectonics, eustasy and cli-
mate interact. Unless the basin is terminal and
beyond the reach of sea-level fluctuations, eustasy
would also to some degree influence the sedimen-
tation patterns in such basins. Interpreting the
relative contribution of the various factors can be
difficult. This problem is well demonstrated by
the Lunde-Statfjord succession in the northern
North Sea.
Steel & Ryseth (1990) and Steel (1993) explained
the vertical grain-size trends in the Lunde-Statfjord
succession primarily as controlled by variation
in tectonic subsidence. The increase in coarse sand
in the Statfjord Group has mostly been explained
in terms of tectonic uplift and consequent increased
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