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depositional systems during latest Triassic and
Early Jurassic in this part of the northern North
Sea region.
channel sandstones towards the top of the MLF
may indicate a change from ephemeral to more
perennial streams. Increased perennial flow would
have transported more clay material in suspension
to be deposited on levees and floodplains; together
with denser river bank vegetation, this would
have stabilised river channels and formed alluvial
ridges. A high clay production by chemical weath-
ering in the hinterland is reflected with the total of
45% to 50% floodplain mudrocks within the L09
to L05 succession (Figs 3 and 4) (see also discus-
sion below). Reddish-brown laminated mudstone,
very high hematite/goethite ratios and entisols and
inceptisols also indicate semi-arid conditions and
well-drained plains during the deposition of the
MLF (Fig. 15) (cf. Schwertmann, 1988).
There is an apparent contradiction in the mid-
dle part of the Lunde Formation between various
climatic signals: an increasing frequency of mean-
dering streams indicates rising humidity, precipi-
tation and perennial runoff, whereas palaeosol
types and clay minerals still reflect a dominantly
semi-arid climate. A shift from ephemeral and
braided to meandering streams was probably con-
trolled by an increase in precipitation in upland
catchments, whereas clay minerals and palaeosols
still reflect an overall arid to semi-arid climate in
the basin and on the floodplain. The hinterland
altitude is not inferred to have risen during the
time of lower to middle Lunde Formation (Fig. 3).
Such variations in precipitation and tempera-
ture between highland and adjacent lowland areas
are well known from the modern world; rivers in
the semi-arid Ebro Basin in northern Spain are
controlled by precipitation in the high Pyrenees
(López-Moreno & Garcia-Ruiz, 2004). A similar
relationship between precipitation and relief has
been described from semi-arid regions in Morocco
(Elmouden
et al
., 2005). During the late Triassic,
western mainland Norway was at an altitude of
between 500 m and 1000 m (Gabrielsen
et al
.,
2010) and probably received more rain than adja-
cent lowland basins. A general shift from an over-
all semi-arid to a semi-humid climate continued
through the late Rhaetian, as revealed in the upper
part of the MLF and the ULF by the increasing
frequency of major meandering stream belts, small
single storey floodplain rivers (FSA3 and 4) and
high chroma vertisols (PA3) (Figs 3 and 15). The
overall increase in the clay/sand ratio upward
through the ULF may indicate an increase in pro-
duction of clay relative to sand in the catchment
area, or a general increase in the accommodation
DISCUSSION
Climate as a controlling factor
on sedimentary trends
The high degree of stratigraphic coincidence
between the trends in alluvial style, palaeosols
and mineralogy in the Lunde-Statfjord succession
(Fig. 15) indicates a common genetic relationship,
controlled by one or several extrinsic factors. The
impact of climate as a controlling factor is dis-
cussed below and then climate
versus
tectonics
and eustasy.
The Lunde Formation
The ephemeral braided stream channel sand-
stones in the lower part of the Lunde Formation
(LLF) can be ascribed to episodic flash flooding,
separated by long time intervals with low rates
of mud deposition, oxidation and pedogenesis, as
typical for dryland regions with an arid to semi-
arid climate (Graf, 1988). An arid to semi-arid
environment is also supported by thin, high
chroma entisols/inceptisols and poorly devel-
oped composite and compound pedocomplexes.
Shallow and narrow desiccation cracks, as in the
L12 to L10 interval, are characteristic of soils
with low water content; deeper and wider desic-
cation cracks reflect more humid conditions
(Dudal & Eswaran, 1988). An arid and semi-arid
climate allows chlorite, mica and illite to survive
due to low to moderate chemical weathering. The
changes in the smectite/kaolinite ratio through
the Lunde Formation likely reflect variation in
wetness and drainage of the floodplain; an incre-
asing content of smectite may reflect wetter peri-
ods and/or higher groundwater levels compared
to periods when kaolinite dominated (Table 4,
Fig. 14). A general arid to semi-arid climate would
also explain the rather high content of feldspar in
both mudrocks and channel sandstones found
through most of the Lunde Formation (Tables 2
and 3, Fig. 15).
Mobile braided streams continued to dominate
during deposition of the LLF and MLF; some vio-
lent floods carried extrabasinal clasts far out into
the basin. The transition into meandering stream
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