Climatic and tectonic controls on Triassic dryland terminal fluvial system architecture, central North Sea - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 30

conditions. These apparently alternated with desic-
cated and evaporitic playa. The palynological data
suggest a mix of drought-resistant shrubland and
wetland environments, possibly with a maritime
influence (cf. Kelber & van Konijnenburg-van
Cittert, 1998; Hubbard & Boulter, 2000) although
no definitively marine facies have been identified
in the central North Sea region.
Floodplain deposits form mud-rich, hetero-
lithic sections 1 m to 30 m thick (Fig. 4D) which
are ubiquitously pedogenically modified and
burrowed (Fig. 9D). These deposits tend to occur
as discrete packages vertically separating multi-
storey channel-belt deposits and are more com-
mon in the south of the central North Sea and in
the basal parts of the Skagerrak Formation sand-
stone members. The mudstones are disrupted
pervasively by palaeosols which are variable in
character, ranging from weakly developed enti-
sols, through aridisols with relatively immature
calcrete profiles and rooted alfisols with abun-
dant illuviated clay. Ped fabrics with crystallaria
and soil slickensides are locally common in the
aridisol intervals. Interbedded sand-prone intervals
are metre-scale in thickness, composed of poorly
sorted, very fine-grained to fine-grained sandstone
containing a high proportion of intraformational
clay and carbonate material and commonly have a
structureless to disrupted appearance with abun-
dant adhesive meniscate burrows and root traces.
The alfisol deposits record vegetated, shrubland,
floodplain conditions, with the interbedded aridi-
sol intervals showing calcrete, ped fabrics and
slickensides suggesting an alternation with dry,
well-drained, but seasonally wet conditions. The
overprinting by burrows suggests episodically
higher soil-moisture conditions favourable for insect
larvae (cf. Smith & Hasiotis, 2008; Smith et al ., 2008)
and indicates the former presence of abundant
organic detritus derived from plant material.
The interbedded sandstones represent weakly
confined to unconfined splay deposition which
was subject to pervasive, post-depositional palae-
osol modification. The sharp bases and localised
lags suggest some scouring and preservation of
small-scale bedforms, but in general these depos-
its were probably deposited as unconfined lateral
and terminal splays to the channel facies (cf. Tooth,
2005; Fisher et al ., 2007).
Terminal splay-lacustrine delta deposits com-
prise 1 m to 5 m of normally graded, very fine-
grained to fine-grained sandstones and mudrocks
interbedded on a scale of 0.01 m to 0.1 m (Fig. 10).

(A) (B) (C) (D) (E)

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182

(F) (G)

Fig. 8. Biogenic indicators appear relatively abruptly in
the Middle Triassic (possibly in the Late Anisian) section
and remain common throughout the overlying Middle to
Late Triassic interval. Diversity however is relatively low
and dominated by common calcareous rhizoliths (A and B,
UK 22/29-5RE), root traces (C, UK 22/29-5RE), glossifungites
burrows in playa heterolithic deposits (D, NO 7/8-4),
microbial laminites in playa and marsh deposits (E, UK
22/29-5RE) and pervasive adhesive meniscate burrows (F
and G, UK 29/10-3) which occur in floodplain and fluvial
upper bar deposits. Cores 10 cm wide.

of current and small-scale wave ripple lamination,
indeterminate burrow mottling, glossifungites and
adhesive meniscate burrows, microbial laminites,
root-traces and flat-pebble conglomerates (Figs 8
and 9). Deeply penetrating (up to 0.3 m) desicca-
tion cracks are common. The disrupted muddy
sandstones locally show fabrics typical of evapor-
itic salt crusts (Goodall et al ., 2000) and rare
chicken wire anhydrite. Palynological recovery is
locally common in Middle Triassic examples of
this facies association, assemblages of which show
a dominance of pteridophytes, bryophytes and
arborescent lycopods (Goldsmith et al ., 1995).
The black mudrocks record subaqueous condi-
tions in relatively persistent water bodies, either as
floodplain lakes or small ponds. In the heterolithic
sections more marginal, distal sheetflood condi-
tions predominated, with the plant rooting, desic-
cation, microbial structures and early carbonate
cementation within the mudstones indicative
of episodically stressed, palustrine and marsh