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space. The degree of soil development increased
from the lower part of the middle Lunde Formation
(MLF) towards the lower part of the Statfjord
Group (LSG) (Fig. 15). This may be explained by a
decrease in rate of accommodation and sediment
accumulation, giving more time for pedogenic
processes to operate. This is substantiated by the
decline in average rate of sedimentation calcu-
lated for the Lunde Formation and the Statfjord
Group (see above). The overall fining-upward
trend of the Lunde Formation, particularly in the
ULF and lowermost part of the Statfjord Group is
thus interpreted to have been primarily controlled
by increasing chemical weathering and clay
production (Fig. 11). Subordinate fluctuations
in accommodation space may have controlled
formation of allostratigraphic units, as expressed
in the vertical variation in the CDP value (Fig. 4).
Alternatively, these cycles may also have been
controlled by climatic variations, for example
on an astronomic scale (see below).
The thick, multi-storey, sandstone body at the
base of L02 most probably represents a time inter-
val with increased sand influx. The great vertical
thickness of the sandstone body could also imply
an increase in accommodation space giving rise
to aggradational stacking of several channel suc-
cessions. However, this unit has been correlated
over a wide area (Figs 1 and 16), showing that the
rate of creation of accommodation space was not
higher than it allowed the fluvial channel system
to develop a wide meandering stream belt rather
than isolated meander channel belts as would be
the case with a high rate of subsidence (cf. Ethridge
et al
., 1998).
Modern vertisols occur commonly in semi-arid
to semi-humid regions with a pronounced dry
season (Wilding & Tessier, 1988; Retallack, 2001)
(Table 6). However, vertisols are highly complex
and other factors than duration of dry and wet sea-
sons are important; parent material (clay content,
clay mineralogy, grain-size distribution) and time
available for pedogenic processes may impact
greatly on vertisol development (McGarry, 1996).
Thick cumulative pedocomplexes of high chroma
vertisols and with thin horizons with coalesced
carbonate nodules, as in the uppermost part of
the Lunde Formation (Figs 3 and 15), are favoured
by low rates in creation of accommodation and
sediment aggradation (see above). Vertisols of
high chroma (reddish colour) are presumed to cor-
respond to well-drained floodplains and increased
duration of the dry season, whereas deep and wide
desiccation cracks associated with intensively
slickensided high chroma vertisols, as in ULF2,
can be explained by a deep wet zone in soils
(Dudal & Eswaran, 1988). The abrupt increase in
smectite at the base of ULF (Fig. 14), followed by
a fluctuating decline in the smectite content, may
indicate an increase in water content of the soil,
followed by a time interval with drier climate or
improved floodplain drainage.
Coalesced carbonate nodules, comprising c.
50% to 60% of the total mudrock volume at cer-
tain horizons in PA3 in the ULF, represent the
most advanced calcareous palaeosols. Pedogenic
carbonate is useful for understanding palaeocli-
matic conditions (Mack, 1992, Driese & Mora,
1993; Retallack, 2001). Calcic horizons are mainly
restricted to areas with less than 1000 mm annual
precipitation and are most common where the
yearly precipitation is below 750 mm (Cerling,
1984). Royer (1999) documented that soils with
more than 95% pedogenic carbonate horizons
correlate with an annual precipitation below
760 mm. Some carbonate pedofeatures though,
like micritic root structures, can form in environ-
ments with up to 1500 mm annual precipitation
(Reeves, 1970).
The dominance of hematite relative to goethite
in mudrocks in the upper part of the Lunde
Formation (Figs 14 and 15) favours that the flood-
plain was dry and the soil aerated most of the
year. However, the presence of complex mottling
with pseudo-gleying and the occurrence of goe-
thite along desiccation cracks and peds show
that there was a high water content in the soil
and reduction of some Fe
3+
to Fe
2+
. The mottling
pattern is probably caused by fluctuating ground-
water and/or periodic stagnant surface water (cf.
Piipujol & Buurman, 1994). In modern vertisols
decreasing soil colour value and chroma gener-
ally reflect an increase in soil moisture (Dudal &
Eswaran, 1988). The presence of lower chroma
(greyish-green) vertisols in the ULF may thus
correlate to periods of more prolonged flooding
associated with wetter soil conditions. The wet
season of plant growth on the floodplain must
have been long enough to supply food for herds
of plant-eating dinosaurs on migration through
the lowland, like
Plateosarus
(Hurum
et al
. 2006).
Altogether, palaeosols and clay minerals within
the ULF indicate that as well as seasonal variation
in precipitation there was periodic, longer term,
change at the basin floor from an overall semi-arid
to semi-humid climate.
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