Geology Reference
In-Depth Information
Fig. 13.10
Conceptual model of flow over an offshore tidal
ridge that is slightly oblique to the tidal currents. The bending of
the flow across the ridge is a consequence of friction at the sea-
bed, which delays the shallower edge of the flow. The crest of
the ridge is a convergence zone because of opposed net transport
directions on either side of the ridge crest. The circulation of
sand around the ends of the ridge has been documented by
McCave and Langhorne (
1982
) and Howarth and Huthance
(
1984
), but it is not a requirement for ridge formation (After
Houbolt
1968
and Caston
1981
)
must have a spacing that is of the order of 250 times
the water depth (i.e. ridge spacing is many kilometers)
in order for the residual rotary circulation to be estab-
lished (Huthnance
1982a
). This prediction is supported
by observations and may explain why linear tidal
ridges are largely restricted to open shelves and seem
to be absent in narrow seaways (Harris
1988
; Malikides
et al.
1988
).
on the side facing the dominant current (McCave
and Langhorne
1982
; Dalrymple and Rhodes
1995
;
Reynaud et al.
1999b
; Fig.
13.12
). This happens when
the ridge height is a significant fraction of the water
depth, such that bottom friction, which slows the flow
toward the ridge crest, is greater than the tendency for
acceleration as a result of the flow constriction. In this
case, the cross-ridge flow can become accelerated
through local low points along the crest, forming
oblique channels called
swatchways
. These channels
can then lead to splitting of the ridge into two,
en ech-
elon
parts as described by Caston (
1972
) (Fig.
13.13
).
By this mechanism, the ridge field can expand through-
out the area of the tidal transport pathway, provided
there is sufficient sand.
It must be stressed that the available knowledge on
the internal structures of offshore tidal ridges is based
almost entirely on seismic data and surface morphol-
ogy. Therefore, it is difficult to propose a generalized
facies model that can be used to interpret the rock
record. From the facies point of view, it is likely that
the deposits within tidal ridges are composed predomi-
nantly of crossbedded sand produced by the dunes that
mantle the ridges in most situations. However, it is also
likely that the deposits record, at least locally, the
imprint of storm waves, in the form of gravel lags with
large gravel wave ripples, coarse graded storm beds
or even HCS if the grain size is too fine to form dunes
(cf. Yoshida et al.
2007
). Because of the very large
sediment volume within a ridge, one or more storms
will not destroy the ridge but can be very prominently
recorded in its architecture and facies (Houthuys and
Gullentops
1988a, b
). The deposits are thought to
13.5.2 Ridge Architecture
As a consequence of the tidal asymmetry, the ridges
migrate in the direction of the dominant regional cur-
rent, but at a rate that is so slow that it cannot be mea-
sured with confidence over a few years (Lanckneus
et al.
1994
). Because of the slight obliqueness of the
tidal flow relative to the ridge axis, the ridges migrate
laterally, as documented first by Houbolt (
1968
)
(Fig.
13.11
), especially in areas with a strong residual
transport. In areas of weaker tidal asymmetry, accre-
tion can occur on both sides of the ridge (Davis and
Balson
1992
). Banner banks, because they are anchored
in a coastal eddy, may be aggradational rather than
migratory (Fig.
13.11
). Lateral migration and aggrada-
tion are likely to be disrupted during periods of
increased storminess, which can either accelerate
deposition or cause significant erosion of the crest,
producing flat, wave-planation surfaces, as showed for
Sark and Shamble banks in the Western Channel
(M'Hammdi et al.
1992
; Bastos et al.
2003
; Fig.
13.11
).
Although accretion on the regional down-current
flank might be most common, accretion can occur
