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The facies change from muddy bay deposits in
Åre 6.1 to tide-dominated channelling in Åre 6.2
Sub-Zone is interpreted as a basinward shift in
deposition and the base boundary may thus be
ascribed to another candidate sequence boundary
(c.SB3; Table 1, Fig. 18). However, the low relief
characteristics of the surface add some uncertain-
ties to this interpretation. Alternatively, the field-
wide belt of tidal channel deposits may be
interpreted as a progradational phase of meander-
ing tidal channels containing inclined heteroliths
strata as a part of the late stage of estuary filling
(e.g. Dalrymple
et al
., 1992). The multiple cycles
of relative sea-level rise and fall identified within
the Åre 5 and 6 Zones are proposed to represent
compound estuarine valley fill, similar to those
described from the overlying Tilje Formation on
the Heidrun Field (Martinius
et al
., 2001).
Åre 7 Zone: Transgressive shoreface
The Åre 7 Zone (15m to 20m) comprises a hetero-
lithic, but nevertheless sandstone-dominated suc-
cession (Fig. 4). The deposits display a combination
of wave-dominated and current-dominated sedi-
mentary structures and have been divided into
upper shoreface (FA 15), lower shoreface (FA 16),
proximal transition zone (FA 17) and distal transi-
tion zone facies associations (FA 18; Fig. 19).
Bioturbation intensity varies. High bioturbation
intensity is often associated with moderate to
high ichnodiversity and trace fossil assemblages
displaying strong marine affinities:
Planolites,
Palaeophycus, Teichichnus, Asterosoma, Chond-
rites
,
Diplocraterion
, bivalve burrows, plus sub-
ordinate
Rhizocorallium
and
Skolithos
. Where
bioturbation is less abundant, current ripples,
wave ripples, combined flow ripples and both
small-scale and larger-scale hummocky cross-
stratification are observed in close juxtaposition.
Although the sedimentary structures are domi-
nantly formed by wave-generated processes, silt
drapes associated with current-ripples suggest
that tidal currents may have occurred periodi-
cally. Local poorly sorted sandy interbeds within
this package of shoreface-dominated facies asso-
ciations reflect the reworking of the underlying
Åre 6.2 Sub-zone deposits.
The basal surface of the Åre 7 Zone erodes the
underlying estuarine deposits (Åre 6.2 Sub-zone)
and is locally overlain by a lag of reworked peb-
ble-sized intraformational clasts and granular
quartz. This lag is commonly associated with a
Fig. 24.
Core expression (6507/7-A-46) of the transgressive
ravinement surface (TRS) separating the Åre 6 and 7 Zones.
The surface (blue, dashed line) is overlain by a granule lag
and calcite cemented sandstone. A
Diplocraterion
(
Dipl
.)
trace is indicated 15 cm above the TRS. (Red bars on the
ruler are 10 cm long).
bed of upper shoreface sandstone (FA 15) show-
ing calcite cementation (Fig. 24). The entire Åre 7
Zone contains extensive calcite cemented hori-
zons that results in this interval being a mapable
seismic marker across the Heidrun Field. The
change in ichnological assemblage seen across
the Åre 6 and 7 Zone boundary, from low-
diversity, brackish water ichnotaxa to a relatively
diverse assemblage indicative of typical marine
palaeosalinities, confirms that an important
change took place in the depositional environ-
ment. Significantly higher palaeosalinity than
in the underlying Åre 6.2 Sub-zone is also con-
firmed by an influx of dinoflagellate cysts and
acritarchs, such as prasinophycean algae. Based
on these characteristics, the erosive surface
marking the base of the Åre 7 Zone is identified
as a transgressive ravinement surface (TRS;
Table 1, Figs 18 and 24).
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