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 37

Sea High to the south and limited evidence of
basin margin pinch-out of shale members until
the more marginal areas, e.g. the Egersund and
Stord basins. However, deep erosion leaves limited
evidence of these trends across the major Jurassic
structural highs such as the Forties-Montrose and
Jaeren highs.

at the top of the Joanne Sandstone Member (Archer
et al ., 2010) is constrained by fewer well data, but
does appear to have involved some erosion of the
upper parts of the succession.
The Judy and Joanne Sandstone members show
moderate thickness variations, reflecting on-going
halokinesis during deposition. However, thickness
changes are gradational, the sandstones remain
sheet-like across the basin and they retain their
internal wireline log and facies character across
wide areas with little evidence of abrupt facies
changes or changes in sand : shale ratio. The
smooth and gradational changes in character sug-
gest that there was minimal surface expression of
any underlying salt movement during deposition
of these fluvial sheets and that sediment supply
was sufficient to infill any emerging floodplain
irregularities. Onlap of Middle to Late Triassic
deposits onto Zechstein salt walls does however
occur locally, indicating that the alluvial plain,
although largely unconfined, locally experienced
salt deformation at surface.

Extensional basin architecture

Seismic resolution of the Triassic is hampered by
the poor impedance contrast between the Smith
Bank Formation and the underlying Zechstein
Group halite. However, a broad eastward thicken-
ing pattern is suggestive of the former presence of a
large-scale, half graben wedge geometry across the
central North Sea, which has been subsequently
dissected by a narrower Jurassic rift (Fig. 3), with
rift flank erosion on the Norwegian margin locally
creating the impression of thinning and wedging
(Fig. 3A). However, stratigraphic thickening pat-
terns on east-west well sections (Fig. 14) locally
confirm the gross half graben geometry seen on
seismic, indicating that the thicker successions
seen on the Norwegian margin were probably the
sites of active faulting and the depositionally thin-
ner (albeit heavily eroded) UK margin was less
active. Internal unconformities can be recognised
within the Skagerrak Formation succession, but
are expressed subtly by thinning and truncation
beneath stratigraphic markers correlated using
well logs, and are rarely visible in seismic sec-
tions (cf. Tomasso et al ., 2002; Archer et al ., 2010).
The Hardegsen Unconformity can be traced as the
variable truncation of Bunter and Smith Bank for-
mation successions beneath a widespread shale
package at the base of the Judy Sandstone Member
(Fig. 14), in a remarkably similar style to the onlap
of the Solling Formation across the erosionally
truncated Bunter, Detfurth and Hardegsen forma-
tion successions in the adjacent Southern Permian
Basin (e.g. Geluk, 2005; Radies et al ., 2005). This
feature is locally visible on seismic, but resolution
of the deeper Triassic succession is insufficient to
enable regional mapping. The lower, pre-Hardegsen
Unconformity, succession is typically highly var-
iable in total thickness and relative proportion
and thickness of Smith Bank and Bunter forma-
tion sandstones, reflecting both unconformity
erosion and the strong effects of halokinesis and
minibasin formation (which is largely smoothed-
out in the widely spaced well log correlations in
Fig. 14, but cf. Fig. 3). The Cimmerian I Unconformity

Pods and interpods - deposition
versus preservation

Although salt controlled facies changes are not
common in the post-Hardegsen Unconformity
succession, salt tectonics clearly exerted a signifi-
cant control on the stratigraphic accommodation
and preservation of Early Triassic sediments.
Regional seismic data show the widespread devel-
opment of minibasins infilled by Smith Bank
Formation (Fig. 3), particularly on the basin flanks
where Zechstein halite was thinner and more
deformed. Minibasin formation was likely to have
been driven by a combination of salt dissolution,
sediment loading and regional evacuation (Clark
et al ., 1999; Stewart & Clark, 1999; Cartwright
et al ., 2001). It has been proposed that the geome-
try of these minibasins exerted a fundamental
control on the deposition of the Skagerrak
Formation fluvial systems, defining accommoda-
tion, local palaeoslope and sediment routing (e.g.
Hodgson et al ., 1992; Smith et al ., 1993). In areas
of reduced salt thickness, such as the UK Western
Platform and Norwegian Jaeren High, it has also
been suggested that the Smith Bank Formation
minibasins grounded at a relatively early stage in
the Triassic basin evolution and that subsequent
deposition of the Skagerrak Formation occurred
within interpod palaeovalleys which provided
the conduits to feed Late Triassic fluvial sediment