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Facies association 10 (FA10):
Interdistributary-bay deposits
sharp-based to erosively-based sandstone-
dominated heterolithic deposits (F5.1)  showing
cross-bedding and ripple cross-lamination are
present. The heterolithic deposits are commonly
overlain by medium-bedded very-fine grained
to  silty, rooted sandstones (F6.5) containing
organic- matter lenses.
The nature of the basal muddy heterolithic depos-
its, the presence of bipolar current to ripple cross-
lamination and the trace-fossil assemblage suggest
deposition by tidally influenced conditions in a
low-energy brackish-water setting. The overlying
mixed sandstone-mudstone heterolithics containing
both small-scale wave-generated and current-gener-
ated structures indicate a relative increase of energy,
probably due to the interaction of tides and waves
reworking and redistributing the fine suspended
sediment. This, coupled with the trace-fossil assem-
blage, suggests deposition in an embayment (Elliott,
1974). The overlying rooted sandstones and silt-
stones indicate aggradation to the water level and
colonisation by plants, where the total disruption of
the original sedimentary structures and the mottled
texture of these deposits indicate extensive biogenic
activity under low energy and slow rates of sediment
accumulation (Turner et al ., 1981). Local, sharp to
erosively based sandstone-dominated heterolithics
are interpreted as tidal-fluvial crevasse channels.
The overall coarsening upward of the sandy portion
within the  heterolithic facies, together with evi-
dence of tidal and wave processes and a moderate
to diverse trace-fossil assemblage suggest that FA10
was deposited in a prograding minor mouth-bar/
interdistributary-bay or tidal plain area. The local
coarser cross-bedded deposits suggest that the
excess discharge from the distributary channels
was diverted into the bay during flood periods
(Elliott, 1974; Brown & Richards, 1989).
FA10 (Fig.  18) consists of mixed siltstone, very
fine sandstone and mudstone. In general, FA10
successions (3m to 4m thick) commonly thicken
and become sandier/siltier upward. The base of
these deposits is characterised by sharp-based
thin-bedded mud-dominated lenticular (F6.5)
and disturbed heterolithic (F6.3) deposits (1m to
2m thick) showing sandstone lenses with ripple
cross-lamination (occasionally bidirectional) and
a mixed Cruziana-Skolithos trace-fossil suite.
Locally, the mudstone-dominated heterolithics
pass upward into mixed  sandstone-mudstone
heterolithics (F5.2.1) showing current-and
wave- generated sedimentary structures. Locally,
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STRATIGRAPHIC MODEL
AND PALAEOENVIRONMENTS
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In the most general terms, the upper Åre and Tilje
interval in the study area shows an overall
shallowing upward succession. Internally, the two
sequences that are recognised in the Tilje Formation
(sequences 2 and 3; Martinius et al ., 2001; Ichaso,
2012; Fig. 3) are each composed of several sandier
upward trends (i.e. parasequences 2 m to 5 m thick)
of shoreface and deltaic origin that are punctuated
by smaller-scale (i.e. 0.5 m to 1 m thick) fining-
upward channel successions (common in sequence
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Fig. 18. Facies association FA10: Interdistributary-bay
deposits. Muddy to very fine-grained bioturbated sandstone
showing lenticularly-bedded, wave-dominated and rare
current-dominated heterolithic deposits capped by rooted
intervals. Organic debris is common locally (black arrow).
Example taken from reservoir zone Tilje 4 (T4).
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