Geoscience Reference
In-Depth Information
mechanisms. In aqueous environments, solute
transport can sometimes also be an important
medium for the transport of contaminants (see
Chapters 3 and 6). These transport mechanisms,
which operate in a range of fluid mediums and
in different environments, produce very different
types of sedimentary deposits. These are discussed
below in the context of (i) aqueous environ-
ments, (ii) aeolian environments and (iii) glacial
environments. Consideration is also given to the
movement of material by gravitational processes.
settling velocities and thus different transport
and settling thresholds. Differences in settling
velocities are even more complex in systems
dominated by skeletal carbonates because the
grains not only have very different internal
skeletal structures (and hence densities), but also
very different morphologies (see Chapter 9).
1.4
SEDIMENT TRANSPORT IN DIFFERENT SEDIMENTARY
ENVIRONMENTS
1.3.3 Sediment settling
1.4.1 Sediment transport in aqueous
environments
As with sediment entrainment and transport,
the major controls on sediment deposition relate
to grain size and flow velocity. The rate at
which sediment settles (the settling velocity
W
)
within a fluid of a given density is determined
by Stokes law:
Sediment transport within aqueous environ-
ments occurs primarily in association with either
traction or turbidity currents. Within traction
currents the primary mechanisms of sediment
movement are rolling and saltation (Fig. 1.7b),
whereas within density currents transport is asso-
ciated both with traction and suspension. Con-
sideration is given first to traction currents, which
are important within both fluvial and shallow
marine environments. Within fluvial systems,
current flow is nearly always unidirectional and
progressive reworking of fine sediment commonly
leads to the finest material being transported
furthest downstream. Hence, fluvial systems are
typically characterized by downstream reductions
in mean grain size (see Chapter 3). In contrast,
within nearshore settings and in the marine-
influenced lower reaches of rivers, currents tend
to be bi-directional (owing to the variable flood-
and ebb-tide influence) and hence sediments will
be reworked both on- and offshore during the
tidal cycle (see Chapters 7 and 8).
Studies of flow regimes under unidirectional
conditions have highlighted clear changes in
sediment transport mechanisms and sediment-
ary structures associated with different current
velocities. As flow increases, the critical velocities
required to entrain sediment particles are reached
and at this stage sediment starts to move by
rolling and saltation. This leads to the develop-
ment of ripples and, at slightly higher velocities,
dunes (Fig. 1.6b). Such structures are associated
with lower flow regimes (Froude numbers
]
d
2
W
=
[(
P
1
−
P
)
g
/18
μ
where (
P
1
−
P
) is the density difference between
the fluid and particle,
g
is the acceleration due
to gravity,
is the fluid viscosity and
d
is the
grain diameter. The law states that the settling
velocity of a spherical particle is related both to
its diameter and to the difference between the
density of the particle and that of the surround-
ing fluid. In simple terms this means that larger
sediment grains will settle faster than smaller
grains providing they are of equal density. Within
most sedimentary systems this process is com-
plicated, however, by three factors:
1
the fact that few grains are completely
spherical;
2
the fact that grains are often continually in
contact within one another and hence disrupt
settling;
3
the fact that different minerals have differ-
ent densities (e.g. quartz 2.65 g cm
−3
, feldspars
2.55-2.76 g cm
−3
, biotite 2.80 -3.40 g cm
−3
; Allen
1985).
Hence, in the case of terrigenous sands, which
are commonly dominated by quartz, but with
variable amounts of other detrital and heavy
minerals, the different particles have different
μ
<
1).
