Geology Reference
In-Depth Information
deposited together with mineral grains (sand, sortable
silt) having similar settling velocities. Field evidence
suggests that the largest aggregates have equivalent
'grain sizes' corresponding to sand grains about
180 Pm in diameter (fine sand) (Chang et al.
2007
). As
a consequence, the mud content of the sediment gradu-
ally increases toward finer-grained sediments in accor-
dance with the rapidly increasing number of smaller
aggregates having lower settling velocities than the
larger ones. Once deposited, the aggregates are mixed
into the ambient sediment, which will become increas-
ingly more cohesive once the clay content of the total
sediment exceeds 5-10% (van Ledden et al.
2004
).
Laboratory analyses of dispersed mud thus introduce
mechanical artefacts into grain-size distributions that
suggest poor sorting. In hydraulic terms, such sedi-
ments are actually very well sorted, the standard devia-
tion of the sand fraction being a good approximation
of the 'true' sorting of the total sediment. The deposi-
tion of mud on tidal flats is thus controlled by the set-
tling velocities of the differently sized flocs and
aggregates and not by those of the constituent particles.
At a water temperature of 18°C the critical lower size
limit for individual 'sortable' silt particles (8 Pm) cor-
responds to a settling velocity of ~0.01 cm s
−1
, smaller
particles being rapidly scavenged to be incorporated
into aggregates ranging from floccules to fecal pellets.
Conceptually this is in excellent agreement with ear-
lier findings about the settling velocity of suspended
matter in a variety of environments (Nichols and Biggs
1985
, based on data of Migniot
1968
; Haven and
Morales-Alamo
1968
; Owen
1971
, and Krone
1972
)
(Fig.
10.8
).
The diagram in Fig.
10.8
shows the settling velocity
range of dispersed clay particles at 18°C relative to
that of composite particles (flocs and aggregates) in
quiet and turbulent water, as well as that of fecal pellets.
The corresponding equivalent grain size of quartz
spheres shows that the bulk of aggregated material
generally exceeds the critical size, fecal pellets and
some of the flocs and aggregates being hydraulically
equivalent to grain sizes as large as fine sand. While
sortable silt particles, as in the case of sand, respond in
a predictable way to changing hydrodynamic condi-
tions, flocs and aggregates constantly change their size
and composition in the course of transport, deposition
and resuspension due to continually changing shear
forces in the course of a tidal cycle (Chang et al.
2006b
). Because the number of aggregates increases
Fig. 10.6
Sediment classification based on sand/mud ratios
(After Flemming
2000
)
It allows the distinction of six sediment types
(Fig.
10.6
). These are: sand (<5% mud), slightly muddy
sand (5-25% mud), muddy sand (25-50% mud), sandy
mud (50-75% mud), slightly sandy mud (75-95%
mud), and mud (>95% mud). The scheme provides a
good spatial resolution of textural sediment composi-
tion, the textural classes also forming good descriptors
of sedimentary environments or facies. For example,
an intertidal area consisting of muddy sand would be
called a 'muddy sand flat' or a 'muddy sand facies',
etc. A more detailed scheme based on sand/silt/clay
ratios, constructed by adding lines to the diagram of
Fig.
10.6
fanning out from the sand endmember
toward the silt-clay baseline, can be found in
Flemming (
2000
).
In the past, it was generally thought that mixed sed-
iments were immature, being principally more poorly
sorted than sand. In recent years, however, it has been
recognised that mud is composed of two major particle
groups, one comprising non-cohesive 'sortable' silt
(McCave et al.
1995
) consisting of particles coarser
than about 8 Pm (medium, coarse and very coarse silt),
the other comprising flocs and aggregates consisting of
particles finer than about 8 Pm (fine silt, very fine silt,
and clay) (Chang et al.
2007
). The aggregated nature
of suspended sediment is illustrated in Fig.
10.7
, in
which a laser-based
in situ
size distribution (a) is com-
pared with that of a dispersed sample (b) collected at
the same location.
As the aggregates also get size-sorted according
to the principle of hydraulic equivalence, they are
