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along the open-coast shoreface. It continues, however,
in the offshore area as a result of tidal scour in wide-
spread, bedload-parting and bypass zones, forming a
surface that we term an
offshore tidal ravinement sur-
face
(Reynaud et al.
2003
) to distinguish it from ravine-
ment surfaces formed in the coastal zone. The offshore
tidal ravinement surface can be continuous with the
coastal ravinement surface, or can be stratigraphically
distinct, as is the case where the bedload parting area
migrates over areas that were formerly depositional
(Fig.
13.19a
).
If the sand liberated from the seafloor by offshore
tidal ravinement is carried away by the residual trans-
port, erosional offshore tidal ridges can form, as is the
case in the Yellow Sea and the Korea Strait (Jung et al.
1998
; Jin and Chough
2002
, Park et al.
2006
). The
outer shape of the Celtic Banks has also been inter-
preted by some authors as purely erosional (Berné
et al.
1998
). The same circulation cells as those shown
in Figure
13.8
exist, but in an erosional, rather than
depositional, mode. Some of these ridges have been
sculpted into muddy estuarine deposits that predate the
last glacial-maximum lowstand (Fig.
13.22
), and are
mantled by a veneer of modern tidal sand. As sea level
rises, the erosional area can migrate landward and the
ridges can enter a progressively more depositional
regime while current speeds decrease. A constructional
tidal ridge may then develop above the erosional form
that acts as the nucleus for sediment deposition.
Numerous examples of offshore tidal ridges that show
this erosional-depositional history exist, including the
ridges of the Boulonnais along the northern French
coast (Lapierre
1975
; De Batist et al.
1996
) and the
Flemish Banks in the southern North Sea (D'Olier
1981
; Laban and Schuttenhelm
1981
; Berné et al.
1994
; Trentesaux et al.
1999
; Fig.
13.23
).
Thus, extending the evolutionary model proposed
by Snedden and Dalrymple (
1999
), there might be a
continuum between erosional (juvenile) and construc-
tional (fully evolved) ridges as conditions change over
a transgression (Fig.
13.24
). Juvenile ridges contain the
initial relief or bump from which they grew. In some
instances, this precursor might be composed of sand.
This is, for example, the case of one of the Flemish
Banks (Trentesaux et al.
1999
; Fig.
13.23
). There, the
'precursor' consists of a finer-grained, more biotur-
bated and less well sorted sandbody (U4 in Fig.
13.23
)
that may correspond to a shoreface-attached ridge,
Fig. 13.22
Interpreted seismic sections showing the complex
internal architecture of erosional transgressive tidal ridges in the
eastern part of the Yellow Sea. The internal bedding is overall
aggradational, comprising two seismic facies interpreted as low-
stand, probably fluvial, deposits (chaotic reflections) that rest on
sequence boundaries (circled numbers without prime), and early
transgressive tidal coastal muds (stratified deposits) that rest on
tidal ravinement surfaces (circled numbers with prime). The
ridge shape is entirely erosional, with the external surface trun-
cating strata within the ridge. Most of the sediment within the
ridge is muddy heterolithic deposits that predate the last glacial
lowstand (From Jin and Chough
2002
)
resting on a pebble lag (the wave ravinement surface).
This is overlain, above a prominent offshore tidal
ravinement surface, by the modern shelf ridge that con-
sists of well-sorted, coarser, bioclastic-rich crossbed-
ded sand (U6-7 in Fig.
13.23
). The longer the ridges are
active, the more fully developed they are likely to
become. As a consequence, the ridges that originate
early in the transgression and are, thus, located on the
outer shelf today, are more likely to be fully evolved
than those that originated close to the highstand coast,
as examplified by the ridges in the English Channel and
its Western Approaches (Fig.
13.25
).
As the water depth increases during the transgres-
sion, the tidal-current speeds can decrease (Fig.
13.26
),
causing the tidal ridges to become moribund (i.e. they
are no longer active). The Celtic Ridges are the best-
described examples of moribund ridges (Bouysse et al.
1976
; Pantin and Evans
1984
; Reynaud et al.
1999a, b
).
They have a rounded shape, and there is evidence of
storm erosion of their crest and the development of
