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 208

(A) (B)

Quartz Chert K-feldspars Plagioclase

Fig. 13. Relative mineralogical composition of framework (detrital) grains in fluvial sandstones from the Tubåen Formation
(A) and Snadd Formation (B). The data are obtained from petrographic thin sections from a well on the Bjarmeland
Platform (Fig. 2). Modal data are courtesy of L. Sæbø (Statoil).

account of sandstones in well 7124/3-1 (Fig. 8).
They observed a significant change between the
Snadd Formation, which in particular is character-
ised by abundant plagioclase feldspars, and the
overlying Fruholmen and Tubåen formations,
which contain coarser and more mature, quartz-
rich arenites depleted in feldspars and with
K-feldspars being more common than plagioclases.
A comparable dataset is shown in Fig. 13. The
relative percentages of detrital framework grains
(quartz, chert, K-feldspars and plagioclases) in
channelised sandstones from the Tubåen and
Snadd formations are shown from a well on the
Bjarmeland Platform. Here, sandstones in the
Tubåen Formation are dominated by quartz grains
(95%), with minor amounts of K-feldspar (app.
4%) and with only traces (<1%) of chert grains
and plagioclases. In comparison, sandstones in
the Snadd Formation contain relatively abundant
chert grains (10%), K-feldspars (10%) and plagio-
clases (app. 8%), in addition to quartz. Following
Bergan & Knarud (1993) and Mørk (1999), this
mineralogical change is the most distinct and
regionally important petrographic turn-around in
the Arctic region. The change occurs at the Early
Norian marine flooding surface, defining the top
of the Snadd Formation, and can be attributed to
several factors:

• A change or redistribution in provenance areas
following the Early Norian transgression;
• Temporal change in the composition of erosion
products from the east (Uralides) to a more
quartz-rich composition in the Late Triassic to
Early Jurassic.

Given a predominantly humid climate in the
Barents Sea area from the Late Carnian (Hochuli &
Vigran, 2010), a significant climatic change seems
to pre-date the observed mineralogical turn-
around and it therefore seems unlikely that change
to a more humid type of climate alone can account
for the temporal variation in sandstone composi-
tion. Nevertheless, enhanced tropical weathering
from Late Carnian times will dissolve feldspars
more effectively and play a part in controlling
sandstone composition.
A detailed account of possible provenance areas
is beyond the scope of this paper. However, it is
noted that the Baltic shield and Caledonides of
north-west Scandinavia are the most probable
sources for quartz-rich sandstones, particularly
in the south-west Barents Sea (Mørk, 1999).
Accordingly, the denudation history of northern
Scandinavia may provide a direct relationship
with the offshore sedimentation patterns, as has
been indicated above. Hendriks & Andriessen
(2002) inferred rapid Triassic to Jurassic uplift and
denudation along the onshore Atlantic rift margin
of Troms County from fission track data. They also
related this denudation phase to rift shoulder

• Change to a more humid type of climate which
would lead to more pronounced leaching of
feldspars;