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The sandstone beds are erosionally based and
variable in internal character, ranging from struc-
tureless to plane bedded, dominated by wave-
ripple and climbing current-ripple lamination, or
more rarely exhibiting humpback antidune lami-
nation (Fielding, 2006). Soft-sediment deforma-
tion and dewatering fabrics are common (Fig. 10B).
The interbedded mudrocks are typically black
coloured and laminated. Generally there is little
organisation to these deposits beyond a weakly
developed cleaning-upward motif on a scale of
0.5 m to 1 m, but intervals with well-developed
coarsening-upward motifs tend to be character-
ised by a predominance of climbing current and
wave-ripple lamination (Fig. 10E). However, inter-
bedding of organised and disorganised end-member
types is also common.
These heterolithic deposits record a continuum
between two end-member depositional settings.
The poorly organised intervals characterised by
interbedded laminated mudstones and dewatered,
massive and normally graded sandstones record
episodic density-current deposition into standing
water and their association with playa margin
facies suggest that they represent hyperpycnal flow
of flood events directly into floodplain lakes and
ponds (cf. Sáez
et al
., 2007). In contrast, the well-
developed coarsening-upward cycles with climb-
ing current ripple lamination probably record
lacustrine delta progradation into standing water.
Together, the interbedding of these end-members
suggest deposition within subareal terminal splay
complexes which episodically formed deltas
during lake-filling episodes, in a manner similar to
that observed on the margins of modern Lake Eyre
(Lang
et al
., 2004; North & Warwick, 2007).
Fluvial deposits are generally sand-prone and
expressed by sharp-based, fining-upward (or
locally trendless) cycles 1 m to 6 m thick com-
posed of very coarse-grained to very fine-grained
sandstone (Fig. 4). The cycles are stacked into
multi-storey intervals up to 20 m thick. The basal
surfaces of these cycles are erosional and mantled
by intraformational mud clasts, reworked calcic
palaeosol and rhizolith material (Fig. 11A). The
upper parts of the cycles commonly grade upwards
into pedogenically modified mudstones and very
fine-grained heterolithic facies, with common
adhesive meniscate bioturbation (cf. Smith
et al
.,
2008), root traces and calcic rhizoliths which
penetrate into the underlying sands. Low-angle
accretionary deposits dominate this association
and cross-stratified fills are subordinate (McKie,
2011). Less common mud-prone fills (Fig. 11B)
are characterised by up to 4 m of heterolithic,
locally rooted, burrowed and slumped siltstones
and very fine-grained sandstones which grade
upwards from sharp based lags of intraclast-bearing,
fine-grained to medium-grained sandstone. These
mud-prone deposits tend to be more common
in the south of the central North Sea area and
in stratigraphic intervals transitional into the
regional shale members. Palaeocurrent data
derived from oriented core and borehole images
show a wide scatter, but with an overall domi-
nance of flow towards the south (Fig. 2B). More
locally, discrete stratigraphic intervals show
orientations to the south-west and south-east
(Fig. 2C, D), suggesting that the overall data are a
composite of axial flow down the central North
Sea and transverse flow off the Norwegian margin
(McKie
et al
., 2010; McKie, 2011), in keeping
with the heavy mineral provenance data (Mange-
Rajetzky, 1995).
These coarse-grained, erosionally based and
upward-fining intervals show widespread evidence
of floodplain soil erosion and are interpreted as the
products of channel migration across the Skagerrak
Formation alluvial plain. Such facies are widely
distributed, with the lack of evidence for areas of
sustained floodplain deposition possibly indicat-
ing a dominance of mobile, potentially multiple
channel systems and frequent avulsion leading to
a uniformly channel-dominated architecture over
wide areas (up to 10 km). The predominance of
low-angle planar lamination and low-angle cross-
bedding suggests that bar accretion took place under
shallow, high-velocity flows transitional between
lower and upper flow regimes, possibly analogous
to the examples described by Allen (1982). The
muddy upper sections of the fining-upward cycles
represent accretion of the upper parts of bars
and these were commonly subaerially exposed,
sparsely vegetated and supported a fauna of bur-
rowing insects. This may reflect a variable discharge
regime, with dry-season colonisation or desiccation
of upper bar surfaces and wet-season, bank-full
conditions reworking desiccated mud drapes and
soil material. The more heterolithic and mud-prone
deposits are indicative of channels which experi-
enced infrequent bank-full conditions and sus-
tained periods of reduced or limited flow (albeit
with sufficient year-round water availability to
maintain biogenic activity and avoid complete
channel desiccation). The increased frequency of
such deposits in a down-palaeoflow direction
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