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 194

mudrock representing a regional transgression
(Henriksen et al ., 2011). Above this boundary, the
formation comprises a thick coarsening upwards
unit of mudrocks and sandstones, followed by
heterolithic and commonly coal-bearing strata.
The whole interval is related to large-scale delta
progradation, with the establishment of a wide-
spread delta plain during Carnian to Early Norian
times. The contact with the overlying Fruholmen
Formation is marked by another organic mudrock,
recording a major Early Norian transgression in
the area (Henriksen et al . 2011).
The Fruholmen Formation (Early Norian to
Rhaetian) shows variable thickness, but can
exceed 250 m thickness in the Hammerfest Basin
(Mørk et al ., 1999). Akin to the Snadd Formation,
it comprises a lower coarsening upwards interval
of dark mudrocks and sandstones, overlain by
coal-bearing, heterolithic deposits. The formation
has been related to widespread delta progradation,
with the coal-bearing strata representing the delta
plain (Gjelberg et al ., 1987; Mørk et al ., 1999).
The Tubåen Formation (Rhaetian to Sinemurian)
is dominated by sandstones with subordinate
mudrocks and thin coal layers and is up to
150 m thick in the western part of the Hammerfest
Basin (Mørk et al ., 1999). The lower boundary
with the Fruholmen Formation is marked by an
abrupt increase in sandstone content, whereas
the upper contact with the Nordmela Formation
(Pliensbachian) is defined by a transition into
finer grained sandstones and siltstones. Sedimen-
tological studies indicate that the Tubåen
Formation is of fluvial, deltaic, origin (Berglund
et al ., 1986; Gjelberg et al ., 1987), with the main
sandstone bodies representing a variety of fluvial
and distributary channel types. The formation
seems to mark a significant change in fluvial style
across the Barents Sea and comprises more
densely stacked channel sandstones with less
inter-bedded fine-grained deposits than the under-
lying units. Gjelberg et al . (1987) noted that coars-
ening upwards sandstone bodies in the uppermost
part of the Tubåen Formation represent deltaic
mouth bars marking initial Early Jurassic marine
incursion from the west. Sandstones in the Tubåen
Formation are used for CO 2 storage in the Snøhvit
petroleum development in the axial part of the
Hammerfest Basin (Spencer et al ., 2008).
Together, the Fruholmen and Tubåen formations
record significant Late Triassic to Early Jurassic
delta progradation and re-establishment of a wide-
spread fluvial dominated delta plain. It is assumed

that the maximum regression was reached during
the Hettangian to Sinemurian stages (Tubåen Fm.;
Henriksen et al ., 2011). Nøttvedt et al . (1993) indi-
cated that the boundary between Fruholmen and
Tubåen formations is partly erosive. Available
biostratigraphic data from well 7121/4-F-2H and
other wells in the Hammerfest Basin yield a
Rhaetian age for the topmost part of the Fruholmen
Formation in the basin position. In contrast, Late
Norian and younger strata are apparently trun-
cated or missing in wells 7124/3-1 and 7226/11-1
to the east, where Early Norian strata are preserved
in the topmost part of the Fruholmen Formation.
Furthermore, the basal strata in the Tubåen
Formation are of Rhaetian age in the Hammerfest
Basin, but yield a Hettangian age on the Bjarmeland
Platform and towards the Finnmark Platform.
These observations accord with a Rhaetian uncon-
formity towards the basin margins and across the
Bjarmeland Platform.
Late Triassic (Carnian to Rhaetian) strata in the
Viking Graben (North Sea; Fig. 1) and the Halten
Terrace (mid-Norway) are predominantly terres-
trial (Fig. 4). Alluvial red beds deposited in gener-
ally dry and hot conditions, dominate the Late
Triassic succession in the North Sea Viking Graben
and seem to reflect fluvial run-off from
Fennoscandia and other source areas, draining
into terminal basins dominated by ephemeral
playa lakes (Alke and Lunde fms.; Fig. 4; e.g.
Nystuen et al ., 1989; Frostick et al ., 1992; Morad
et al ., 1998; McKie & Williams, 2009). On the Halten
Terrace, evaporitic salt layers (halite) of inferred
Ladinian to Carnian age are interbedded with red-
dish mudrocks and are eventually succeeded by
alluvial red beds which seem to dominate the Late
Triassic succession (Müller et al ., 2005).
Early Jurassic strata from the Viking Graben
comprise the Statfjord Formation (Rhaetian to
Sinemurian) and the Dunlin Group (Pliensbachian
to Toarcian). Of these, the Statfjord Formation is
generally related to widespread alluvial deposi-
tion (Nystuen et al ., 1989; Steel & Ryseth; 1990;
Ryseth, 2001). Furthermore, it shows a gradual
transition from alluvial red beds in its lower part,
to coal-bearing grey beds in the upper part.
However, the uppermost part of the Statfjord
Formation is marginal marine and records Middle
to Late Sinemurian marine incursion in the Viking
Graben (Steel & Ryseth, 1990; Ryseth, 2001).
Overlying strata in the Dunlin Group are predomi-
nantly of offshore and shallow marine origin
(Charnock et al ., 2001).