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 195

On the Halten Terrace, the Triassic red beds
pass vertically into alluvial grey beds of Rhaetian
age (Morris et al ., 2009) and eventually coal-bearing
grey beds in the Åre Formation (Rhaetian to
Pliensbachian; Dalland et al ., 1988; Morris et al .,
2009). Sedimentological studies of the Åre
Formation (Svela, 2001) indicate that the lower
part is of fluvial/alluvial origin, whereas the upper
part comprises paralic facies of interbedded wave-
dominated mouth bars, distributary channels and
wave-influenced bay fill deposits. Based on
biostratigraphic data, a general Hettangian to
Sinemurian age can be inferred for the lower, flu-
vial part of the Åre Formation (Morris et al ., 2009).
The Late Triassic red bed associations from the
Viking Graben and Halten Terrace are approxi-
mately time equivalent with the upper Snadd
Formation and the Fruholmen Formation in the
Barents Sea (Fig. 4). It also follows from the
presented age assignments that the Tubåen
Formation is more or less time-equivalent with
the Statfjord and lower Åre formations to the
south. Accordingly, terrestrial and dominantly
fluvial conditions seem to have persisted over
most of the Norwegian shelf during the Carnian to
Sinemurian stages, although with some more
marine influence to the north.
Regional climatic scenarios were indicated
above and will be addressed at the end of this
study. However, the dominance of red beds in the
Late Triassic successions to the south can clearly
be related to generally dry and hot conditions
(e.g. McKie & Williams, 2009). In contrast, con-
current coal-bearing successions in the Barents
Sea testify to a much more humid climate to the
north, e.g. during deposition of the Snadd and
Fruholmen formations (Fig. 4). Also, the common
occurrence of coal-bearing strata in the Statfjord
and lower Åre formations to the south and in the
Tubåen Formation to the north, indicate a more
or less equable and generally humid climate
across the entire area from the North Sea to the
Barents Sea during the Early Jurassic (see also
Hallam, 1985).
In the following paragraphs, the Late Triassic to
Early Jurassic depositional setting in the Barents
Sea area will be outlined, with a discussion
of possible mechanisms controlling deposition.
Subsequently, some effects of different Late Triassic
climatic settings will be addressed from a regional
perspective, with additional comments on the
inferred increase in humidity at the Triassic to
Jurassic boundary.

Late Carnian to Early Norian delta plain
(Snadd Formation)

The Late Carnian to Early Norian succession in
the Barents Sea area represents a possible regres-
sive maximum, with a widespread delta plain
environment extending across large parts of the
shelf (Riis et al ., 2008; Glørstad-Clark et al ., 2010;
Henriksen et al ., 2011). Fig. 5 shows a graphic
core log from the uppermost Snadd Formation in
well 7222/11-1 (Loppa High) together with a
Gamma Ray curve. Core photographs from the
same interval are shown in Fig. 6.
The cored section comprises a lower unit (Fig. 5;
below app. 798 m core depth) of thin coal beds and
inter-bedded mudrocks and sandstones (Fig. 6A
to C). Thin root horizons are usually seen below
the coal beds, occasionally with siderite concre-
tions. The associated fine-grained strata comprise
laminated siltstones and very fine-grained sand-
stones with various types of ripple lamination and
bioturbation (Fig. 6B). Coarser sandstone beds
range in thickness from approximately 1 m to
2.5 m. They are characterised by rather flat upper
and lower boundaries (Fig. 5) and are dominated
by planar lamination and ripple lamination.
The overlying thicker sandstone unit in Fig. 5
(app. 778 m to 798 m core depth) is sharply based
and comprises a basal intraclast conglomerate
(Fig. 6D), followed by fine-grained to medium-
grained sandstones. Primary sedimentary features
include dm-scale cross-stratification with possible
paired drapes (Fig. 6E and F) and associated mas-
sive sandstones. The log data (Gamma Ray curve;
Fig. 5) show that the sandy unit has a total thick-
ness of approximately 25 m. Furthermore, available
wire-line log data from well 7222/11-1 (not included
in Fig. 5) show that it is the only significant sand-
stone body present within a more than 500 m thick,
mudrock-dominated succession. Seismic ampli-
tudes calibrated to well data (Fig. 7) indicate that
the cored sandstone in Fig. 5 (labelled A0 in Fig. 7)
forms a low-sinuosity ribbon with a general SW to
NE orientation and a width of approximately
1000 m. The amplitude map also show that other
ribbon-like features of variable width, sinuosity
and orientation are present away from the well.
Data from the uppermost part of the Snadd
Formation in well 7124/3-1 are shown in Fig. 8.
Here, the cored section is divided into two main
facies associations. The lower association com-
prises thin-bedded sandstone layers and grey
muddy strata with a thin coal bed, abundant roots