Geology Reference
In-Depth Information
(Van der Wal et al.
2002
). However, when the estuary
becomes nearly filled and broad tidal flats and salt
marshes occupy most of the area, the locus of maxi-
mum deposition shifts to the channel margins as has
been noted in Arcachon Bay (Allard et al.
2009
).
Overall, the width of the intertidal flats increases sea-
ward. In some cases, the mudflats slope gently into the
main channels, producing smooth point-bar surfaces.
In other situations, cliffed margins are created by epi-
sodic erosion of the outer edge of the mudflats, either
because of shifts in the location of the channels, or
because of channel enlargement during river floods.
Aggradation of the area at the foot of the cliff occurs
when the channel migrates away, or the river-flow
decreases, leading to the development of a terraced
channel-margin morphology (Fig.
5.10d
).
The tidal flats and salt marshes are dissected by net-
works of smaller channels (see Chap. 11) that are ori-
ented approximately at right angles to the larger
channels (Fig.
5.10b, c
). Some of these small channels
connect to terrestrial drainage, but many have no fresh-
water input, except for local rainfall. They have a
meandering pattern and appear to show the 'straight'—
meandering—'straight' pattern described above
(Fagherazzi et al.
2004
). The larger pattern is typically
dendritic, with the first-order tributaries consisting of
small rills only a few decimeters wide. Higher-order
channels become progressively wider. The banks of
these runoff channels are gentle in sandy sediments,
but may be steeper than 20° in muddy sediments.
and marine sources (cf. Figs.
5.2
and
5.3
), and the
sediment-sorting process that occurs within the estuary.
The sediment supplied by the river can range from
gravel-dominated, as is the case in the Cobequid
Bay—Salmon River estuary (Figs.
5.1
a and
5.12
), to
quite fine grained and predominantly mud, as a result
of differences in the nature of the river's catchment
area. Because there is deposition in the river-domi-
nated inner portion of the estuary, the river-supplied
sediment becomes finer in a downstream direction (see
the general discussion of the causes of fining in
Dalrymple
2010a
). The sediment supplied by marine
processes can also be quite variable in caliber. Most
commonly, the sediment entering the mouth of the
estuary consists of sandy material that can be quite
coarse. This occurs because transgressive erosion
(i.e. ravinement) of coastal and shallow-marine areas
commonly reworks older fluvial deposits that are char-
acteristically relatively coarse grained. This marine-
sourced sediment also becomes finer as it moves into
the estuary, again because of deposition. Consequently,
the sediment in tide-dominated estuaries is typically
coarsest at its mouth and head, and finest in the vicin-
ity of the bedload convergence (Fig.
5.12
; Lambiase
1980
; Dalrymple et al.
1990
).
Superimposed on this general trend, there can be an
abrupt decrease in grain size at the inner end of the
complex of elongate sand bars that occupies the outer
part of the estuary (Fig.
5.12
). As explained by
Dalrymple et al. (
1990
), this is attributable to the dif-
ferential transport speeds of the sediment fractions
moving as traction load (generally medium sand and
coarser) and in intermittent suspension (mainly fine and
very fine sand). Sediment entering the estuary by way
of the headward-terminating flood channels must pass
through or over an ebb-dominated region before con-
tinuing its migration into the estuary. The slow-moving
traction material cannot do this and is recycled back out
of the estuary, and remains 'trapped' in the zone of
elongate sand bars. By contrast, the fast-moving grains
that travel by intermittent suspension are capable of
reaching the inner parts of the estuary. Thus, sediment
in the outer estuary, and in the flood-dominant areas in
particular, tends to be composed of medium to coarse,
or even very coarse, sand, whereas the middle and inner
estuary are characterized by fine and very fine sand.
The ebb-dominant channels in the outer estuary that
pass through the inner estuary first also tend to be finer
grained than the adjacent flood channels. This pattern
5.4
Sediment Facies
As described above, the axial portion of tide-domi-
nated estuaries is occupied by a network of channels
that contain sandy and, locally, gravelly sediment,
whereas the fringing tidal flats and salt marshes consist
of muddy deposits. The spatial organization of sedi-
ment caliber and sedimentary facies is relatively pre-
dictable because of the process organization discussed
above.
5.4.1
Axial Grain-Size Trends
The grain size and its spatial distribution within tide-
dominated estuaries is a function of two factors: the
nature of the sediment supplied by the terrestrial
