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 28

These interbedded facies record a variety of
depositional processes. The wavy bedded depos-
its record extensive disruption by haloturbation
under evaporitic conditions close to the water
table, with episodic adhesion as sand was blown
across damp substrates. The laminated mudstones
record brief episodes of raised water tables and
the development of localised subaqueous ponds.
The pinstripe cross-stratified sandstones repre-
sent the migration of isolated aeolian dunes across
this wet playa surface, with the current ripple
laminated sandstones representing episodic, low
energy fluvial deposition. The structureless and
dewatered sandstones are texturally identical to
the dune sands and may record fluvial reworking
of aeolian dunes or mass wasting by rainfall during
convective desert storms.
Terminal splay deposits consist of micaceous,
very fine-grained to fine-grained sandstones and
mudstones organised into coarsening-upward
cycles up to 10 m thick (and locally grouped into
composite cycles up to 40 m thick; Fig. 4). At the
bases of these cycles are thin shales and heterolithic
units (typically < 0.5 m thick), which are wave and
current ripple laminated or structureless, com-
monly desiccated and very rarely burrowed. The
contact with the overlying sandstones is either
sharp (with mud clasts or desiccation cracks) or
gradational and passes upwards into successions
dominated by planar-lamination and climbing-
current-ripple lamination, with scattered mud-clast
concentrations and locally developed dewatering
fabrics (Fig. 6A).
The basal shales to these cycles record subaque-
ous deposition and the paucity of biogenic activ-
ity and common desiccation suggest that these
were ephemeral water bodies. The overlying
plane-bedded and climbing-ripple-laminated
sandstones represent the product of deposition
from shallow, sediment-charged, unconfined flows
and their arrangement into coarsening-upward
cycles is suggestive of progradation. The variably
sharp (with evidence of desiccation) or grada-
tional contacts suggest that these progradational
sand systems alternated between entering stand-
ing water bodies or covering subareally exposed
surfaces. The overall setting appears to have
been of flood-dominated systems characterised
by rapid deposition into ephemeral water bodies
or across dry lake floors.
Fluvial deposits occur in close association
with the terminal splay deposits described above
(Fig. 4) and comprise up to 3 m thick, typically

vertically isolated, sharply or erosionally based,
fining-upward or trendless packages of fine-grained
to medium-grained sandstones with abundant
mud-flake lags (Fig. 6). Mudclast-filled gutter
casts and scours are common throughout. The
sandbodies have an internal fabric varying
from structureless to indistinctly dewatered, or
dominated by horizontal to gently inclined
planar lamination (with abundant mica concen-
trations defining laminae) and climbing current
ripple lamination. Cross-bedding is generally
uncommon but locally dominant in discrete strati-
graphic intervals, typically at the top of large-
scale, terminal splay coarsening-upward successions
(Fig. 4C).
These deposits record deposition under a range
of variably channel-confined conditions. The
abundance of mud-flake lags may either be indica-
tive of channel reworking of overbank muds, or
local reworking of desiccated, intra-channel, mud
drapes. The structureless to dewatered sandstones
suggest catastrophic deposition, either through
bank collapse or rapid deposition from waning,
sediment-laden flows, whilst the plane-bedded
and current ripple-laminated intervals, in associa-
tion with common scours and gutter casts, indi-
cate shallow, high-velocity, waning flows with
common scouring and erosion. These features
indicate a dominance of ephemeral flood pro-
cesses (cf. Fielding, 2006). The vertically isolated
nature of these sandbodies, encased within splay
and playa deposits, together with evidence of
rapid cut and fill, suggest that these fluvial sys-
tems did not form established channel belts and
instead comprised discrete ribbons prone to sedi-
ment plugging and avulsion and that floodwaters
expanded and shallowed into lateral and terminal
splays on the margins of playa.
Overall, the Early to early Middle Triassic
succession appears to have been deposited under
relatively arid conditions as terminal fluvial systems
which discharged into desiccated and evaporitic
playas (Fig. 7). Aeolian processes are recorded, but
generally high water tables and low sand supply in
distal reaches appears to have acted to suppress
dune field development. The overall setting may
have been broadly analogous to the present day
Lake Eyre basin (cf. Lang et al ., 2004; Fisher et al .,
2008), although with considerably less vegeta-
tion cover. The provenance of these Early Triassic
sediments appears to have been from Caledonian
metasediments from the UK margin (Mange-
Rajetzky, 1995) and reworked Carboniferous