Geology Reference
In-Depth Information
Fig. 5.17
Cross section and sidescan sonar images (
top and
bottom
) of a dune on the bed of the Weser River, showing the
presence of fluid mud in the troughs between the dunes. The
ellipses show locations where the fluid mud becomes so soft that
no acoustic reflection is detected in the sidescan sonar record.
The firm sand on the dune crest that is not buried by fluid mud
appears dark on the sidescan sonar record (Modified after
Schrottke et al.
2006
, Fig.
5.9b
)
can be intense in this mud layer, and consists of a rela-
tively diverse assemblage (Fig.
5.3e
). At their inner
end, the high-tide beaches interfinger with mudflat and
salt-marsh deposits, and form coarse-grained cheniers
encased in muddy deposits (Fig.
5.18b
) (Lee et al.
1994
; Pye
1996
; Tessier et al.
2006
).
The mudflats that flank the channels in the inner
estuary become broader in a seaward direction, rang-
ing from only a few meters wide in the largely filled
innermost part of the estuary (Fig.
5.10c, d
), to several
10s to 100 s of meters wide near the seaward end of
active mudflat sedimentation, which typically occurs
in the middle estuary (Fig.
5.10b
). At any given loca-
tion, the width of the mudflats decreases through time
as the estuary fills. In the inner estuary where the mud-
flats lie closest to the fast currents in the channels, and
where, consequently, the delivery of sediment to the
mudflats is rapid, the sedimentation rate can reach sev-
eral meters per year, generating well-developed tidal
rhythmites (Fig.
5.19a
; Dalrymple et al.
1991
; Tessier
1993
; Choi
2010
). Further seaward where the mudflats
are, on average, a greater distance from the strong cur-
rents in the channel, the sedimentation rate is lower
(several millimeters to several decimeters per year),
allowing the development of annual cyclicity as a
result of seasonal changes in temperature and/or the
intensity of wave action (Van den Berg
1981
; Dalrymple
et al.
1991
; Allen and Duffy
1998
). These cycles typi-
cally consist of alternations of layers with physical
lamination, in which tidal rhythmites might be present,
and intensely bioturbated sediment (Fig.
5.19b
).
Although this bioturbation can be intense, the diversity
of traces is usually lower than in areas further seaward
(Fig.
5.3e
) because of the lower salinity. Overall, there
is considerable diversity in the intensity of bioturba-
tion spatially, with a much lower level of bioturbation
in areas of higher sedimentation rate near channels,
and a higher level in the more slowly aggrading tidal
flats further from the channels. Deformation structures
produced by grounding ice are present in mudflats in
temperate to polar settings (Dionne
1985
; Dalrymple
et al.
1991
). Seasonal cyclicity can also occur in the
innermost, fluvially dominated portion of the estuary,
but here the primary seasonal signal appears to be vari-
ations in river discharge. The diversity and intensity of
bioturbation in these inner-estuarine mudflats are low
because of the stress imposed by the low salinity.
A salt-marsh (see Chap. 8), or mangrove swamp in
tropical areas, lies at a greater distance from the chan-
nel, typically in the elevation range between about neap
and spring high tide. The deposits here are intensely
rhizoturbated (Fig.
5.19b
), and contain a variable
amount of organic material. The development of a levee
along the margin of the channel can lead to the develop-
ment of boggy conditions at greater distances from the
channel, commonly in the area adjacent to the valley
walls (Woodroffe et al.
1989
). Organic-rich sediments,
including potentially peat, accumulate in such areas.