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the system. Subsequently, the fluvial environment
seems to have retreated gradually eastwards dur-
ing Sinemurian to Pliensbachian times.
channel unit seems to have been penetrated in the
three wells; an approximately 30 m thick sand-
stone body in well 7124/3-1. The remaining strata
are entirely dominated by fine-grained lithologies,
including thin coal beds as inferred from density/
porosity wire-line log data, however with some
scattered thinner channel sandstones indicated.
In summary, the proportion of channel deposits in
the upper part of the Snadd Formation is appar-
ently very low. This is similar to well 7222/11-1
on the Loppa High, where only a single thick
sandstone body (Fig. 5) was encountered in more
than 500 m of delta plain deposits. Based on
regional data it seems unlikely that the proportion
of channelised sandstone in the upper Snadd
Formation is larger than approximately 10% to
20% in the south-west Barents Sea. Some large
channels are identified clearly from wells and
seismic data, but these appear to be isolated
within the surrounding fine-grained material.
In the Fruholmen Formation, pro-deltaic
mudrocks and overlying mouth bar sandstones
seem to be correlated readily between the wells
(Fig. 12), although they expand in thickness to the
west into the Hammerfest Basin, along with the
rest of the formation. The large channelised sand-
stone unit observed in well 7124/3-1 (Fig. 8) does
not correlate with similar bodies in the studied
wells, whilst the overlying lower delta plain asso-
ciation is widespread. A small distributary chan-
nel sandstone is recognised in well 7124/3-1
(Fig. 8) and some possible thin, channelised,
sandstones are also recognised in the other two
wells. The actual proportion of channel deposits
in the Fruholmen delta plain association may
exceed that of the Snadd Formation but would
nevertheless be limited. Accordingly, the delta
plain association in the Fruholmen Formation
also appears to be dominated by fine-grained
material and the few channels present are likely to
be isolated within the surrounding fines.
The architecture of the Tubåen Formation is
markedly different. Between wells 7226/11-1 and
7124/3-1 the formation seems to form a multi-
lateral, 20 m to 34 m thick, sheet of amalgamated
channel sandstones, more or less devoid of fine-
grained material. The formation thickens
westwards toward well 7121/5-1 (65 m), where
some thin, fine-grained and coal-bearing units are
preserved between the interpreted channel
sandstones. Despite a slightly more heterolithic
character to the west, the Tubåen Formation
maintains a high content of channel sandstone in
DEPOSITIONAL TRENDS ACROSS THE
TRIASSIC TO JURASSIC BOUNDARY
Changes in depositional trends across the Triassic
to Jurassic boundary can be discussed in terms of
temporal variations in sedimentary architecture
and development of accommodation space avail-
able for deposition, in terms of changing prove-
nance areas and hinterland rejuvenation, and
from both local and regional climatic variations,
as inferred from the Carnian to Sinemurian strata.
Temporal variation in accommodation
space development
In the present case, where fluvial-dominated delta
plain successions are recorded from the upper
Snadd Formation, from the middle and upper parts
of the Fruholmen Formation and from the Tubåen
Formation, an investigation of alluvial architecture
and in particular the stacking density of channel
deposits, can provide a reference for temporal
changes in basin subsidence; or accommodation
space development (e.g. Allen, 1978; Bridge &
Leeder, 1979; Bridge & Mackey, 1993; Martinsen
et al
., 1999). Thematically, abundance of channels
can be related to slow rates of accommodation
space generation, whereas fewer channels and
more abundant preservation of floodplain fines
reflect a higher rate of accommodation space gen-
eration (Bridge & Mackey, 1993). Accommodation
space measured over millions of years, as in the
present case, is probably governed by tectonic sub-
sidence rates (e.g. Sadler, 1981; Sadler & Strauss,
1990), but also includes space generation related
to, for instance, sea-level rise. These are not treated
explicitly here and it is assumed that sediment
accumulation rates are indeed controlled by tec-
tonic subsidence.
The architectural changes inferred from the
Late Triassic to Early Jurassic succession is illus-
trated in the correlation panel shown in Fig. 12. In
the Snadd Formation, the correlated strata com-
prise more than 200 m of delta plain deposits
capped by shallow marine sandstones (as demon-
strated from core data in Fig. 8). Despite the large
vertical thickness (>200 m) of the delta plain
association (Snadd Formation), only one major
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