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the intensity of reworking and mixing of grains increases
with water depth, as the result of increasing time and
decreasing sediment supply through the post-glacial
transgression (Wilson 1988 ). In the Miocene cool-water
carbonates of SE France, recurring associations between
the fauna and tidal bedforms have been noted (Descote
2010 ). The largest and coarsest grained dunes contain
the highest content of red algae, whereas the small and
finer-grained dunes show a larger amount of benthic
forams and molluscans. This partitioning is also
reflected in the sequence-stratigraphic organization of
the deposit. The coarse bioclastic TST deposits are
dominated by a bryozoa/echinoderm (Bryonoderm)
fauna, which is succeeded by a red algae (Rhodalgal)
association, whereas the more muddy HSTs are domi-
nated by a mollusc/benthic foraminifera (Molechfor)
association.
of fine sand) and gravel size (Carling 1999 ), and by
current speeds above about 0.5 m/s. Water depth is
not a significant limiting factor on the occurrence
of dunes, provided the current speed is sufficient,
although an increase in water depth commonly leads
to a decrease in current speed and, hence, the disap-
pearance of dunes.
The size and shape of dunes vary widely. Following
Ashley ( 1990 ) and Dalrymple and Rhodes ( 1995 ) we
suggest the size distinctions given in Table 13.1 . The
maximum height of a shelf tidal dune is not well
defined, but tidal dunes up to 15 m high are reported on
modern shelves (e.g. Berné et al. 1989 ). The larger the
dunes, the lesser their relative relief: in general, the
dune wavelength-to-height ratio (= the ripple index;
RI) is less than 10 for small dunes but commonly
exceeds 30 for large dunes, and may reach 100 for very
large ones.
The size and shape of dunes are controlled by water
depth, current speed and grain size. Studying dunes in
flumes and rivers, Van Rijn ( 1982 , also Southard and
Boguchwal 1990 ) showed that, in the lower part of the
dune stability field, increasing current speed brings
about an increase of the equilibrium height of dunes.
As well, for a given depth and current speed, the dune
height increases slightly with grain size (Flemming
1980 ; Van Rijn 1982 ). Water depth, which is a proxy
for boundary-layer thickness, is generally regarded as
being the most important control on dune size, with
dune height (H) and wavelength (L) increasing as
water depth (h) increases (Ashley 1990 ). Following
Yalin ( 1964 ) and based on many examples in nature
summarized by Allen ( 1982 ), the widely accepted
relationships are:
13.4
Tidal Dunes
The sandy sediments that are present over large parts
of tidal shelves are very commonly molded into a
complex array of large bedforms, ranging from flow-
transverse dunes of various sizes to nearly flow-
parallel tidal-current ridges. Dunes are the most
ubiquitous bedforms on continental shelves, occur-
ring both on sand ridges and flat sand sheets, and are
responsible for much of the sedimentary record of
offshore tidal environments. Therefore, they are dis-
cussed at length here.
13.4.1 Morphological Response to Flow
L6h
(13.1)
Dunes is the generally accepted term that replaces the
older terms megaripple and sandwave (Ashley 1990 ).
Flume experiments and observations in nature have
defined the stability field of dunes as a function of
grain size, current speed and water depth (e.g. Rubin
and McCulloch 1980 ; Allen 1982 ; Southard and
Boguchwal 1990 ). Dunes can be formed in grain sizes
between approximately 0.15 mm (i.e. within the range
H
0.167h
(13.2)
These relationships are only applicable in cases
where the dunes are fully developed in equilibrium
with the flow, and where the sea floor is completely
covered by mobile sediment, a condition called
fullbedded (Ashley 1990 ). These relationships do
Table 13.1 Size classes for
dunes (From Dalrymple and
Rhodes 1995 )
Small
Medium
Large
Very large
Wavelength (L)
0.6-5 m
5-10 m
10-100 m
>100 m
Height (H a )
0.05-0.25 m
0.25-0.5 m
0.5-3 m
>3 m
a Calculated from the Flemming ( 1988 ) relationship: H = 0.0677 L 0.8098
 
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