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 26

(A) (B) (D)

(C)

Fig. 5. Lower Triassic facies. (A) Smith Bank Formation dry playa heterolithic deposits, UK 31/26a-8. Cores 1 m long.
(B) Structureless, well sorted siltstone within the Smith Bank Formation, UK 20/25-1 (image 10 cm wide). (C) Smith Bank
Formation wet playa deposits, UK 22/14b-4, with irregular salt crust fabric and isolated deflation granules (image 10 cm
wide). (D) mud-prone, Bunter Formation, wet playa deposits showing an irregularly laminated salt crust fabric enclosing
an isolated aeolian dune set defined by grain flow cross-stratification and pinstripe lamination, northern Dutch sector well
NL F18-11 (image 10 cm wide).

These mud-prone deposits represent low-
energy deposition in a setting characterised by
episodic sand deposition and erratic water-table
conditions that fluctuated between subaqueous
and desiccated in response to ephemeral flood
events. No significant thicknesses of fully sub-
aqueous, lacustrine mudstones or halite beds
are present and it is likely that these deposits
represent extensive dry playa rather than the
damp, fluctuating margins to perennial lakes or
salina. The origin of the structureless siltstones
is enigmatic but, given their sorting and lack
of structure, they could represent the accumu-
lation of wind-blown dust or record the rapid
deposition of fluvially reworked aeolian mate-
rial, with the locally discernible wispy fabric
the product of clay infiltration or dewatering
fabrics.
Wet playa deposits (Snyder, 1962) occur in inter-
vals tens of metres thick within the Smith Bank
and Bunter Sandstone formations and become par-
ticularly prevalent in the broadly time-equivalent

sections in the Southern Permian Basin. The dom-
inant facies within these sections consists of
irregularly laminated and deformed, bimodally-
sorted, mud-prone sandstones with common
granule lags and scattered grains (Fig. 5C and D).
The distinctive, irregular fabric is composed of
adhesion lamination, together with salt ridge and
crust structures (Fryberger et al ., 1983; Goodall
et al ., 2000). These wavy bedded deposits are
punctuated by thin (<0.5 m) intervals of laminated
mudstone. Fine-grained to medium-grained cross-
stratified sandstones (Fig. 5D) showing pinstripe
lamination (Fryberger & Schenk, 1988) and grain
flow-toe pinch-outs form isolated cross-sets (<1 m
thick). Fine-grained to medium-grained, mud
clast bearing, climbing current ripple laminated
and plane bedded sandstones up to 0.5 m thick
occur sporadically throughout these sections,
together with isolated sandstones of a similar
composition to the cross-stratified sandstones but
lacking any internal structure, or with indistinct
deformation structures.