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sandstone-dominated allostratigraphic unit 8 is
composed of sandy heterolithic tidal channel
infill facies associations (FA3a, 3b, 3c and 3d)
(Figs 7 and 13; Table 2). In one section at locality
N3, the allostratigraphic unit 8 consists mainly of
the wave-dominated shoreface facies association
(FA4a) and the wave-dominated sandy hetero-
lithic strata facies associations (FA2c). Outcrop
mapping and aerial photomosaics show that the
allostratigraphic unit 8 is geometrically relatively
symmetrical in shape in the northern area (Fig. 7).
At all the localities except N1, the sandstone
succession of the allostratigraphic unit 8 is topped
by a regional lithological shift into a succession
of offshore marine mudstone facies association
(FA7b) (Fig. 7; Table 2). The boundary may locally
be draped with patches of quartz pebbles (Dam &
Surlyk, 1998) and is here defined as the flooding
surface FSg. The overlying 60 to 100 m thick
Sortehat Formation succession is composed of
shaly heterolithic black to dark grey mudstones
containing bivalves, local carbonate concretions,
plant debris and wood fragments (Surlyk, 2003;
Koppelhus & Dam, 2003; Ahokas
et al
., 2014; this
study).
paleo-estuary with large sand influx from the sea-
ward side (Fig. 13B). The southern valley system
records an approximate position landward of cen-
tral basin and with a higher sediment supply from
the landward side. The differences in probable
contemporaneous facies development between
the two incised valley infills are interpreted to have
been caused by variations in hydraulic energy (e.g.
tidal contra fluvial) and different sets of physical
parameters (e.g. width : depth) as these valley
infills were deposited in different proximal-distal
estuarine settings.
The dominantly massive southern estuary suc-
cession at locality S3 suggests a locally high degree
of reworking by bioturbation (Dam & Surlyk, 1998),
whereas siderite concretions indicate variation in
oxygen content of the bottom water. Northward
thickening along the depositional strike, associ-
ated with an increasing abundance of several
metres-thick tidal cross-stratification within the
TDEC element (Fig. 7), shows that in addition to
the fluvial source the estuary was supplied with
large amounts of sand by alongshore currents.
Also, interfingering of sandy and heterolithic tidal
deposits (sandy TDEC and muddy TDDF elements;
Fig. 7) indicates that various estuarine sub-
environments were present, including wave-
dominated shoreface (locality S3). Several
tide-influenced bay-head deltaic distributaries
and tidal bars formed by oblique accretion (locali-
ties S4 to 5 and S6) and lower-energy environments
associated with the bay-head delta or central basin
(locality S5). Therefore the southern incised
valley system of the allostratigraphic unit 8 is
interpreted as upper estuary succession exposed
slightly obliquely to the outcrop belt.
Based on the size and facies the narrower north-
ern incised valley infill is interpreted as a tide-dom-
inated and more confined estuarine depositional
system than in the southern one. Fluvial sediments
at the valley base were exposed to extensive marine
transgressive ravinement by tidal currents when
the incised valley was flooded. The outcrops
indicate a middle estuary to estuary mouth setting
where large bars migrated along the bottom of the
valley during early stages of the valley infill
(Fig. 13E). These large tidal bars were dissected by
highly active transverse tidal channels and smaller
swatchways (Fig. 13B). The tidal channels migrated
laterally, eroding into the shoal on one side and
constructing the shoal on the other side, similar
to the modern Oosterschelde estuary in the
Netherlands (Martinius & Van den Berg, 2011). The
Interpretation: Allostratigraphic unit 8
The allostratigraphic unit 8 is interpreted as a
succession of incised valley infill between the
SU3/TS6 surface and the marine flooding surface
FSg (Fig. 7). The valley infill is composed of tide-
dominated estuary-complex (TDEC) and rare tide-
dominated prodelta and delta front (TDDF)
architectural elements (Figs 7, 11B, 12B and 13A;
Table 3) and interpreted as deposited in a variety of
estuarine sub-environments. Similarly to allostrati-
graphic unit 6, two separate but contemporaneous
incised valley systems are interpreted along the N-S
outcrop belt (Fig. 7).
The southern incised valley system is the deep-
est, attaining 50 m at locality S6 and is interpreted
to have been at least 40 kilometres wide (Fig. 7).
The observed cross-section of the valley is asym-
metric. The northern incised valley system is
deepest (45 m) at locality N2 and areally is about
half the size of the southern system and more
symmetrical in shape. The field observations and
facies within suggest that the present outcrops
cut these two incised valley systems in different
proximal-distal positions. The northern system
records an approximate position basinward of
the central basin and close to the mouth of the
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