Sedimentation at the Jurassic–Triassic boundary, south-west Barents Sea: indication of climate change - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 201

grey colour indicating a slightly depleted oxygen
level in the bottom sediment. The recorded wave-
generated structures and bioturbation in the over-
lying sandstone unit, coupled with its relatively
sharp base and the inter-bedding of sandy and
muddy layers, seem most compatible with deposi-
tion in a delta mouth bar environment. The rela-
tively sharp base of the interpreted mouth bar unit
accords with a sudden incursion of sand and may
reflect a lateral delta lobe shift driven, for exam-
ple, by river avulsion. The coarsening upwards
pattern in the lower part of the Fruholmen
Formation is widespread (Mørk et al ., 1999;
Henriksen et al ., 2011), hence the interval is
related to significant delta progradation following
the Early Norian transgression.
Strata in the middle and upper parts of the
Fruholmen Formation can be related to deposi-
tion in a variety of sub-environments of a delta
plain (as summarised in Figs 8 and 9). All the pri-
mary sedimentary features (e.g. cross-stratification
and current ripple lamination) in the 25 m-thick
sandstone body capping the mouth bar deposits
(Fig. 8) can be related to deposition from sus-
tained traction currents in a channelised setting
(e.g. Collinson, 1996), with the massive sand-
stone beds recording rapid sediment fall-out of
suspended material during waning flows (as was
also inferred for the Snadd Formation channel;
Fig. 5). Accordingly, this sandstone body is
related to deposition from a major fluvial distrib-
utary channel complex, possibly representing a
proximal feeder system for the underlying mouth
bar system. The organisation of the main sand-
stone body (Fig. 8) into two stacked fining
upwards units may also indicate that it formed in
response to lateral migration and aggradation
within a channel belt.
Common rootlet horizons and thin coal beds in
the upper part of the Fruholmen Formation record
soil formation and peat accretion in a terrestrial
setting and thus confirm the delta plain environ-
ment. In this context, the 3 m-thick sandstone unit
with observed dm-scale cross-stratification (Fig. 8;
1325 m) may represent a smaller distributary
channel. Associated muddy intervals with thin-
bedded sandstone layers are related to deposition
within lakes and small bays, with the sandy inter-
beds representing crevasse systems and possible
levees derived from nearby channels. Observations
of bioturbation in these beds probably reflect some
marine impact, thus implying a lower delta plain
setting for the investigated strata, as was also

inferred for the underlying Snadd Formation. The
grey colour of the fine-grained sediment testifies
to deposition in generally water-saturated and
stagnant conditions. However, scattered occur-
rences of red/grey coloured mottling point to
infrequent (?) events of drying and partial oxida-
tion of the delta plain. Such changes can be driven
by vertical shifts in the ground water level (e.g.
Besly & Fielding, 1989) and may reflect seasonal
variations in precipitation.

Late Rhaetian to Sinemurian fluvial system
(Tubåen Formation)

The Tubåen Formation is observed in three well
sections, which will be described and interpreted
separately, followed by a short discussion of the
depositional system.

Well 7124/3-1

The Tubåen Formation is approximately 20 m-thick
in well 7124/3-1 and is truncated beneath Late
Jurassic mudrocks (Fig. 8). The basal boundary is
apparently sharp and characterised by an abrupt
vertical transition from heterolithic deposits
(Fruholmen Formation) to thick-bedded sand-
stones in the Tubåen Formation. The cored sec-
tion comprises fine-grained to very coarse-grained
sandstones with dm-scale cross-stratification,
planar lamination and current ripple lamination.
Quartzite pebbles and other rock fragments
(<0.5 cm), angular mud-clasts (up to 10 cm), cm-
size coal fragments and fine-ground organic
material are also present throughout the sand-
stone unit. The logged section (Fig. 8) comprises
at least two stacked and sharply based fining
upwards units.
The relatively coarse nature of the sandstones
and the association of primary sedimentary struc-
tures, with abundant cross-stratification, planar
lamination and current ripple lamination, all tes-
tify to deposition from strong and sustained trac-
tion currents in a channelised setting (Collinson,
1996). Also, the abundance of eroded organic
material is indicative of a more or less fully ter-
restrial, alluvial setting. The organisation of strata
into stacked fining upwards units is related to jux-
taposition of several channel units, possibly
within mobile channel belts. As shown in Fig. 8, a
braided fluvial setting is inferred for the Tubåen
Formation in well 7124/3-1; a point that will be
further discussed below.