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 207

the Hammerfest Basin. As noted for well 7121/4-
F-2H, the terrestrial section (Fig. 10) comprises
approximately 78% channel fill deposits. On aver-
age, more than 60% of channelised deposits are
observed from the alluvial part of the Tubåen
Formation in the Hammerfest Basin.
With reference to process-based alluvial stratig-
raphy modelling (e.g. Allen, 1978; Bridge & Leeder,
1979; Bridge & Mackey, 1993), the very limited
channel deposit proportions seen in the upper
part of the Snadd Formation, and also in the delta
plain association of the Fruholmen Formation,
would be characteristic of a depositional regime
dominated by a relatively high subsidence rate
and plentiful sediment supply. Under such condi-
tions, which are also referred to as high accommo-
dation settings by other workers (e.g. Martinsen
et al ., 1999), fine-grained material deposited out-
side the main channels have a high potential to be
preserved. Furthermore, scattered thick channel
sandstones will probably also form due to persist-
ing vertical aggradation within channels and chan-
nel belts, but would be more or less encased within
the surrounding fine-grained material, particularly
if the actual proportion of channels is lower than
approximately 40% (Bridge & Mackey, 1993).
By contrast, the multilateral stacking of channel
sandstone bodies observed in the Tubåen Formation
(Fig. 12), coupled with the observed low content of
fine-grained material in the vertical sections (Figs 8
to 10), can, in particular, be related to a slower sub-
sidence rate. Under such conditions of low accom-
modation (e.g. Martinsen et al ., 1999), persistent
fluvial reworking may lead to predominant preser-
vation of the coarsest (deeper) parts of the channel
fills, as inferred for the section in well 7226/11-1
(Fig. 9). Some spatial variation in subsidence rates
can also be inferred for the Tubåen Formation. As
shown in Figs 10 and 12, fine-grained material is
better preserved between the channelised sand-
stones towards the west, where the formation is
thickest. This tendency can be related to less effec-
tive fluvial reworking in areas undergoing more
rapid subsidence (e.g. Allen, 1978).
The presented architectural data are essentially
indicative of a regional reduction in tectonic sub-
sidence rate across the Triassic to Jurassic bound-
ary in the south-west Barents Sea. Delta plain
deposits of Late Carnian and Early to Late Norian
age are characterised by abundant fine-grained
material and isolated channel sandstone deposits,
whereas the Early Jurassic fluvial system (Tubåen
Formation) is completely dominated by channel

sandstone deposits and with little preserved fine-
grained material, even though some limited lateral
variation is observed.
Considerations of architecture versus accumu-
lation rates, based upon absolute time scale, yield
similar results. As noted above, the entire Snadd
Formation can reach a thickness of more than
1400 m (e.g. in well 7222/11-1; Table 1) and is
commonly more than 500 m thick. These thick-
nesses include pro-deltaic and marginal marine
strata of Late Ladinian to Early Carnian age, in
addition to the Late Carnian to Early Norian delta
plain deposits described here. However, if a total
duration of approximately 20 Ma to 25 Ma can be
inferred for the accumulation of the Snadd
Formation (Late Ladinian to Early Norian; app.
233.3 Ma to 207.6 Ma; cf. Ogg et al ., 2008), accu-
mulation rates (ignoring effects of compaction)
should amount to several tens (20 to 70) of metres
per million years during the Late Triassic.
For the Tubåen Formation, one can firstly con-
sider the section in well 7226/11-1 (Fig. 9). Here,
the available biostratigraphy yields a general
Hettangian to Sinemurian age for the entire forma-
tion. Accordingly, the 34 m thick alluvial succes-
sion appears to be stratigraphically condensed and
spanning a possible time interval of up to 10 Myr
(189.6 Ma to 199.6 Ma, cf. Fig. 4 and Ogg et al .,
2008), yielding an accumulation rate of only 3 m to
4 m per million years. If the lateral thickness vari-
ation is included, alluvial successions of about
50 m to 100 m in thickness, deposited mainly dur-
ing the Hettangian to Sinemurian time interval,
can be considered. Given duration of approxi-
mately 3 Ma to 10 Ma, accumulation rates would
be in the order of 5 m to 30 m per million years,
which is still lower than those inferred for the Late
Triassic strata. Such considerations serve to illus-
trate that net accumulation rates were higher dur-
ing the Late Triassic than they were during the
earliest Jurassic. Thus, for fluvial and/or chan-
nelised depositional settings, one should expect to
see a denser stacking of channel fill deposits in the
Hettangian to Sinemurian succession than in the
underlying units, as is clearly pictured in Fig. 12.

Changing provenance areas and hinterland
rejuvenation

Mineralogical differences between the sandstones
of the Snadd Formation and the overlying units
have been addressed briefly above. Bergan &
Knarud (1993) provided a detailed mineralogical