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mudstone member deposition. However, shale
colour in the Skagerrak Formation is unlikely
to be a reliable indicator of palaeogroundwater
conditions and invariably shows reduction within
hydrocarbon columns and reddening in dry holes
and aquifers. Archer
et al
. (2010) also propose that
fluvial expansion was more likely to have taken
place during wetter conditions and cite the absence
of lacustrine deltas and presence of evaporites
within the shale members as evidence that fluvial
runoff was reduced and the climate was drier
during shale deposition.
rooted alfisols (pointing to widespread shrubland
vegetation) and lack of evaporite and aeolian
facies after the Early Triassic. One possible expla-
nation for the year-round availability of water
within the Skagerrak Formation fluvial drainage
network could be that the headwaters extended
into higher latitude, generally wetter catchments,
allowing some runoff or groundwater flow into
the basin during the dry season to maintain ripar-
ian vegetation. This hypothesis can be tested
by estimating the possible size of the Skagerrak
Formation catchments. Davidson & North (2009)
outline a simple, albeit imprecise, method for esti-
mating ancient catchment size based on bankfull
depth and using regional relationships between
depth and catchment area based on modern
streams from a range of climate settings. For the
Skagerrak Formation streams two estimates of
bankfull depth of 3 m and 6 m were applied based
on the typical thickness of fining-upward cycles
seen in core and validated by bankfull depth
estimates calculated from the mean thickness
of stacked cross-sets using the method outlined by
Bridge & Tye (2000). Two regional curves from
modern dryland regions were used to relate bank-
full depth and catchment area (U.S. Western
Cordillera and Basin and Range Province; cf.
Davidson & North, 2009). The two depth estimates
and regional curves combine to give drainage
areas ranging from c. 14,800 to 69,500 km
2
to
c. 129,400 to 1,083,000 km
2
. As Davidson & North
(2009) note, there is a paucity of regional curves
for modern dryland climatic zones and the results
can only be used to give a general sense of the
possible scale of fluvial catchment size in these
settings. However, the latter range is excessively
large for the scale of the Skagerrak Formation
fluvial systems within the central North Sea and,
more critically, fails to fit within the available area
of uplifted Caledonian basement on the UK mar-
gin of the basin, for which there is established
provenance data (Mange-Rajetzky, 1995). The
smaller range is more realistic, with the advantage
of scaling with the UK Caledonian catchments
and is more likely to be representative of Skagerrak
Formation catchment size (Fig. 19). In neither case,
however, does the calculated area (using plausible
catchment aspect ratios) extend the drainage
sufficiently into a higher latitude region that
would have been consistently subject to more
perennial precipitation (Kutzbach & Gallimore,
1989; Sellwood & Valdes, 2006, 2007). This would
appear to suggest that the year-round maintenance
Fluvial expansion driven by catchment
wettening
Hinterland pollen and spores from the upper
part of the Julius Mudstone and lower Joanne
Sandstone members are dominated by gymno-
sperms (Goldsmith
et al
., 1995, 2003), indicating
semi-arid conditions in the UK and Fennoscandian
headwaters in the Early Ladinian. However, out-
side of these intervals there is little palynological
evidence of catchment conditions, particularly
during peak fluvial expansion when conditions
may have been wetter. The barren nature of Early
Triassic fluvial sections may have been the result
of the prevailing aridity at this time (Péron
et al
.,
2005), combined with deposition during recovery
from the end Permian extinction (Payne
et al
.,
2004). Hochuli & Vigran (2010) document diverse
spore and pollen assemblages in the Early Triassic
of the Barents Sea region, indicating that low
latitude aridity may well have been the key
controlling factor in limiting recovery from the
central North Sea. The barren Norian section in
the central North Sea may likewise be indicative
of general aridity in both the basin and catchment
regions.
General circulation models for Pangaea predict
a strong seasonality to the precipitation across the
north-west European region, with arid, evaporative
conditions across the central North Sea during the
dry season (Kutzbach & Gallimore, 1989; Sellwood
& Valdes, 2006, 2007; Goddéris
et al
., 2008). This
is consistent with the regional evidence of aeolian
activity, evaporite precipitation, paucity of per-
ennial lacustrine facies and the terminal nature
of the fluvial drainage, but does not predict the
vegetated, burrowed character of the Skagerrak
Formation (indicating year-round soil moisture),
laminated mudstone channel abandonment-fills
(indicating standing water; McKie, 2011), common
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