the canyon as well. These traction currents are capable
of transporting coarse sand and at times even coarser
material.
2.12 High Density Mass Flows – “Debris
Flows” and “Mud Flows”
Debris flows occur both on land and under water, and
represent a type of mass transport where the sediment/
water ratio is very much greater than in turbidity
currents, resulting in high viscosity and high internal
friction during flow. This also means that the density
of the mass is from 1.5 to 2.0 g/cm
3 , while most
turbidity currents have a density of 1.1–1.2 g/cm
3 or
less. The high density of a debris flow means that all
clasts have increased buoyancy because of the dense
matrix. The high viscosity of the matrix also means
that large stones do not sink rapidly towards the bottom of the flow. In flows with high density and viscosity, blocks may remain near the surface of the flow
until it solidifies, through loss of water or reduced
gradients, and becomes quite rigid as a result of
increased density and cohesion. The shear strength of
the matrix is often referred to as matrix strength.
Debris flows may be rich in stones and other coarse
material because of the matrix strength. Mud flows
typically have a more clay-rich matrix. But there is
no clear distinction between the two.
Because of the high matrix strength, little sorting
takes place in debris flows (Fig. 2.11). Large blocks
are often concentrated at the front or on the sides of the
flows, and there may be less coarse material near the
base of the flow because of the shear movements.
On land, the density difference (Δρ) between the
flow and its surrounding fluid (air) is much higher than
under water. Flows with a particularly low water content and high shear strength will only flow slowly
down a slope, and we get transitions into what we
call solifluction (creep). Sediment flows with a higher
water content can move faster, however. In debris
flows with their high internal friction most of the
shear forces will be released along the bottom of the
flow, so that the overlying mass moves more or less as
a coherent mass with little internal deformation. This
helps to reduce the total frictional resistance to movement. Shear strength and viscosity tend to decrease
with increasing rate of shear, and this means that when
a flow first gets going it will tend to accelerate.
Debris flows and mud flows normally have thixotropic properties: the shear forces must reach a critical
threshold before deformation (shear) takes place, and
the material loses much of its shear strength following
deformation. In clay containing smectite (montmorillonite) this property is particularly well developed.
Under shear stress, water will be released. The
house-of-cards packing of clay minerals is destroyed,
the clay particles tending to develop parallel alignment, with the release of water which will reduce the
shear strength causing shear weakening. Some of the
water in the bottom layer in which deformation is
taking place will be lost to the other sediments, however, and friction will mount again. If large clasts enter
the basal shear zone, friction will also increase and the
flow may stop.
The stability of mud on slopes and the flow
properties of mud flows depend on the clay mineral
composition of the mud and on the geochemistry of
the porewater and the ions adsorbed on the clay
minerals. The presence of potassium and sodium
tend to stabilise the mud.
Debris flows are often described in continental
deposits, but are also found on submarine slopes. In
water the density difference between sediment and
surroundings is far less than on land, so that the
angle of the slope must be greater for flows with the
same internal friction. Submarine mud flows, on the
other hand, will not dry up and they can easily take up
more water as they move.
Debris flows are particularly common in desert
deposits. This is because of the often powerful
rainstorms which mobilise sediments that in a wetter
climate would have been transported by fluvial processes. In addition, there is little vegetation in deserts
to bind the sediments, so they are more easily set in
motion. Also of great significance is the fact that the
clay mineral smectite is formed particularly through
weathering in desert environments. Clay containing
smectite will expand when it begins to rain, preventing
the water from filtering rapidly through the soil profile.
Instead, water will be bound to the sediments and the
viscosity may be reduced enough for mud flows with
thixotropic properties to form. In continental
environments with freshwater the content of
stabilising salt (K
+
, Na
+
) is low.
The term “quick clay” is used for extremely thixotropic clays. Undisturbed clays have a relatively high
shear strength, but after shaking or some other type of
2 Introduction to Sedimentology
47
of transporting coarse sand and at times even coarser
material.
2.12 High Density Mass Flows – “Debris
Flows” and “Mud Flows”
Debris flows occur both on land and under water, and
represent a type of mass transport where the sediment/
water ratio is very much greater than in turbidity
currents, resulting in high viscosity and high internal
friction during flow. This also means that the density
of the mass is from 1.5 to 2.0 g/cm
3 , while most
turbidity currents have a density of 1.1–1.2 g/cm
3 or
less. The high density of a debris flow means that all
clasts have increased buoyancy because of the dense
matrix. The high viscosity of the matrix also means
that large stones do not sink rapidly towards the bottom of the flow. In flows with high density and viscosity, blocks may remain near the surface of the flow
until it solidifies, through loss of water or reduced
gradients, and becomes quite rigid as a result of
increased density and cohesion. The shear strength of
the matrix is often referred to as matrix strength.
Debris flows may be rich in stones and other coarse
material because of the matrix strength. Mud flows
typically have a more clay-rich matrix. But there is
no clear distinction between the two.
Because of the high matrix strength, little sorting
takes place in debris flows (Fig. 2.11). Large blocks
are often concentrated at the front or on the sides of the
flows, and there may be less coarse material near the
base of the flow because of the shear movements.
On land, the density difference (Δρ) between the
flow and its surrounding fluid (air) is much higher than
under water. Flows with a particularly low water content and high shear strength will only flow slowly
down a slope, and we get transitions into what we
call solifluction (creep). Sediment flows with a higher
water content can move faster, however. In debris
flows with their high internal friction most of the
shear forces will be released along the bottom of the
flow, so that the overlying mass moves more or less as
a coherent mass with little internal deformation. This
helps to reduce the total frictional resistance to movement. Shear strength and viscosity tend to decrease
with increasing rate of shear, and this means that when
a flow first gets going it will tend to accelerate.
Debris flows and mud flows normally have thixotropic properties: the shear forces must reach a critical
threshold before deformation (shear) takes place, and
the material loses much of its shear strength following
deformation. In clay containing smectite (montmorillonite) this property is particularly well developed.
Under shear stress, water will be released. The
house-of-cards packing of clay minerals is destroyed,
the clay particles tending to develop parallel alignment, with the release of water which will reduce the
shear strength causing shear weakening. Some of the
water in the bottom layer in which deformation is
taking place will be lost to the other sediments, however, and friction will mount again. If large clasts enter
the basal shear zone, friction will also increase and the
flow may stop.
The stability of mud on slopes and the flow
properties of mud flows depend on the clay mineral
composition of the mud and on the geochemistry of
the porewater and the ions adsorbed on the clay
minerals. The presence of potassium and sodium
tend to stabilise the mud.
Debris flows are often described in continental
deposits, but are also found on submarine slopes. In
water the density difference between sediment and
surroundings is far less than on land, so that the
angle of the slope must be greater for flows with the
same internal friction. Submarine mud flows, on the
other hand, will not dry up and they can easily take up
more water as they move.
Debris flows are particularly common in desert
deposits. This is because of the often powerful
rainstorms which mobilise sediments that in a wetter
climate would have been transported by fluvial processes. In addition, there is little vegetation in deserts
to bind the sediments, so they are more easily set in
motion. Also of great significance is the fact that the
clay mineral smectite is formed particularly through
weathering in desert environments. Clay containing
smectite will expand when it begins to rain, preventing
the water from filtering rapidly through the soil profile.
Instead, water will be bound to the sediments and the
viscosity may be reduced enough for mud flows with
thixotropic properties to form. In continental
environments with freshwater the content of
stabilising salt (K
+
, Na
+
) is low.
The term “quick clay” is used for extremely thixotropic clays. Undisturbed clays have a relatively high
shear strength, but after shaking or some other type of
2 Introduction to Sedimentology
47