5.4 Gravity Mass Flow
(Hesthammer and F ossen 1999). Such collapse structures
have been identified in the northern North Sea where they
range in scale from small to very large features (up to ~ 100
km).
- Sediment gravity flows. Flow may occur in several
different ways:
(1) Viscoplastic flows with internal shear planes and
virtually no movement at the base of the flow. Such
gravity movements usually need a rather high slope
angle (5 to 10°) and reach limited thicknesses.
(2) Slide-debris flows, or slideflows move as a more
or less rigid plug over a basal shear zone. In this zone
the water pressure is in excess of hydrostatic pressure and thus reduces the shear strength of this material, which may even become liquefied. It seems that
a basal lubricating layer of water underneath the debris flow enables acceleration and long run-out distances of the mass flow (Mohrig et al. 1998). Such
flows can reach great thicknesses and occur on very
gentle slopes (0.1 to 1°). in some cases, the flows can
develop erosional features in the form of broad furrows or channels.
(3) Liquefied mass flows. Sediment masses on subaqueous slopes can be frequently transformed, e.g.
by earthquake shocks, into fluids of high density and
viscosity. Aprerequisite to this behavior is a high insitu water content and meta-stable grain packing.
Partial or entire remolding of the sediment creates a
small surplus of pore water which cannot immediately escape. As a result, the shear strength of the
material drops drastically and approaches zero without the uptake of additional water. The liquefied
masses start to flow downslope, even on very gentle
slopes (::;OS), and become further remolded and disorganized. Similarly, already moving slump masses
may be converted irito slow, plastic debris flows or
mud flows. Typical examples of this type of liquefaction are non-cohesive or low-cohesive sands and
silts, but it appears that many sediments rich in diatoms, nannofossils and other micro-organisms are
also susceptible to this process.
(4) Grainflows consisting ofpure sand are characterized by their frictional strength. To overcome friction
between the grains, a kind of dispersive pressure
must develop. This can only be achieved on fairly
steep slopes (18 to 37°), as at the head of submarine
canyons and on some prodelta slopes. Grain flows
require an environment with ready supply of sand,
...
Fig. 5.11. Summary of submarine mass movements
and gravity mass flows. (Based on many sourees,
e.g., Middleton and Hampton 1976, Walker 1978,
Moore DG et al. 1982, Prior and Coleman 1984;
Stow 1986, Einseie 1989). Rockfalls (a), slides (d),
and slumps (e) occur if the shear stress exceeds the
shear strength of the rock or sediment at some depth
(b stability analysis). g and h Debris and mud flows
213
and they usually travel only short distances. The deposits of individual grain flow processes are thin
(several centimeters) and may show reverse grading.
Grain flows have been described, e.g., by Shepard and Dill
(1966), Lowe (1976), Dingler and Anima (1989).
Because many sediment gravity flows evolve from laminar to fully turbulent systems (flow transformation; see also
Lowe 1979; Postma 1986), an exact correlation of natural
tlows to idealized tlows (tluidized flow, liquefied flow,
grain flow, mud flow or cohesive debris flow) is often difficult. The preserved gravity flow deposit and its internal
fabric and sedimentary structures only record the mode of
final deposition, but cannot fully reveal the (possibly
changing) character of the tlow on its path from a failed
slope to the site of redeposition (Einseie et al. 1996 ).
In the oceans, the most important flow types are mud
flows and cohesive debris flows. They contain varying proportions of mud, which provide them with cohesive matrix strength supporting larger particles.
When. their excess pore water dissipates, the flow
masses come to rest.
By uptaking additional water from the overlying
water body, individual gravity flows or parts of them
can evolve into masses of lower density and viscosity
and, if there is a long, sufficiently steep gradient,
finally generate turbulent suspension currents of high
velocity (turbidity currents).
- Compound mass movements result from the sudden
loading of slope sediments by other masses, e.g.,
slides, slumps, or grain flow deposits originating
from higher slope areas (Fig. 5.11i). In this way, the
underlying sediment is transformed into a loaded,
undrained condition with reduced shear strength.
Compound mass movements provide a mechanism
for transporting coarse gravel over long distances and
on very gentle slopes into the deep sea.
Moore DG et al. (1982) reported an interesting case from
the southern Gulf of California, where coastal sand and
gravel were first transported as a grain tlow and debris flow
via a steep submarine canyon, down to amid-fan and lower
slope region (around 2500 m deep). There, the superposition of a great load of coarse c1astics onto siliceous silty
clays (porosity 70 to 80%) triggered a second, considerably
larger mass movement on a much gentIer slope (about
1.5°), which carried the coarse material "piggyback" down
to a 3000 m deep marginal basin plain (slope angle 0.1°).
The mass spread over an area of approximately 300 km 2,
forming a sheet several tens to about 100 m thick. Due to
the remolding and differential settling of the flow mass
usually originate from slides and slumps by liquefaction of the primary, metastable grain packing (in situ
water content, w, greater than liquid limit, w L ). Turbidity currents evolve from gravity flows by uptake
of additional water. i A composite, two-step grainmud flow mechanism can explain long-distance
transport of pebbles and gravel into the deep sea
(Hesthammer and F ossen 1999). Such collapse structures
have been identified in the northern North Sea where they
range in scale from small to very large features (up to ~ 100
km).
- Sediment gravity flows. Flow may occur in several
different ways:
(1) Viscoplastic flows with internal shear planes and
virtually no movement at the base of the flow. Such
gravity movements usually need a rather high slope
angle (5 to 10°) and reach limited thicknesses.
(2) Slide-debris flows, or slideflows move as a more
or less rigid plug over a basal shear zone. In this zone
the water pressure is in excess of hydrostatic pressure and thus reduces the shear strength of this material, which may even become liquefied. It seems that
a basal lubricating layer of water underneath the debris flow enables acceleration and long run-out distances of the mass flow (Mohrig et al. 1998). Such
flows can reach great thicknesses and occur on very
gentle slopes (0.1 to 1°). in some cases, the flows can
develop erosional features in the form of broad furrows or channels.
(3) Liquefied mass flows. Sediment masses on subaqueous slopes can be frequently transformed, e.g.
by earthquake shocks, into fluids of high density and
viscosity. Aprerequisite to this behavior is a high insitu water content and meta-stable grain packing.
Partial or entire remolding of the sediment creates a
small surplus of pore water which cannot immediately escape. As a result, the shear strength of the
material drops drastically and approaches zero without the uptake of additional water. The liquefied
masses start to flow downslope, even on very gentle
slopes (::;OS), and become further remolded and disorganized. Similarly, already moving slump masses
may be converted irito slow, plastic debris flows or
mud flows. Typical examples of this type of liquefaction are non-cohesive or low-cohesive sands and
silts, but it appears that many sediments rich in diatoms, nannofossils and other micro-organisms are
also susceptible to this process.
(4) Grainflows consisting ofpure sand are characterized by their frictional strength. To overcome friction
between the grains, a kind of dispersive pressure
must develop. This can only be achieved on fairly
steep slopes (18 to 37°), as at the head of submarine
canyons and on some prodelta slopes. Grain flows
require an environment with ready supply of sand,
...
Fig. 5.11. Summary of submarine mass movements
and gravity mass flows. (Based on many sourees,
e.g., Middleton and Hampton 1976, Walker 1978,
Moore DG et al. 1982, Prior and Coleman 1984;
Stow 1986, Einseie 1989). Rockfalls (a), slides (d),
and slumps (e) occur if the shear stress exceeds the
shear strength of the rock or sediment at some depth
(b stability analysis). g and h Debris and mud flows
213
and they usually travel only short distances. The deposits of individual grain flow processes are thin
(several centimeters) and may show reverse grading.
Grain flows have been described, e.g., by Shepard and Dill
(1966), Lowe (1976), Dingler and Anima (1989).
Because many sediment gravity flows evolve from laminar to fully turbulent systems (flow transformation; see also
Lowe 1979; Postma 1986), an exact correlation of natural
tlows to idealized tlows (tluidized flow, liquefied flow,
grain flow, mud flow or cohesive debris flow) is often difficult. The preserved gravity flow deposit and its internal
fabric and sedimentary structures only record the mode of
final deposition, but cannot fully reveal the (possibly
changing) character of the tlow on its path from a failed
slope to the site of redeposition (Einseie et al. 1996 ).
In the oceans, the most important flow types are mud
flows and cohesive debris flows. They contain varying proportions of mud, which provide them with cohesive matrix strength supporting larger particles.
When. their excess pore water dissipates, the flow
masses come to rest.
By uptaking additional water from the overlying
water body, individual gravity flows or parts of them
can evolve into masses of lower density and viscosity
and, if there is a long, sufficiently steep gradient,
finally generate turbulent suspension currents of high
velocity (turbidity currents).
- Compound mass movements result from the sudden
loading of slope sediments by other masses, e.g.,
slides, slumps, or grain flow deposits originating
from higher slope areas (Fig. 5.11i). In this way, the
underlying sediment is transformed into a loaded,
undrained condition with reduced shear strength.
Compound mass movements provide a mechanism
for transporting coarse gravel over long distances and
on very gentle slopes into the deep sea.
Moore DG et al. (1982) reported an interesting case from
the southern Gulf of California, where coastal sand and
gravel were first transported as a grain tlow and debris flow
via a steep submarine canyon, down to amid-fan and lower
slope region (around 2500 m deep). There, the superposition of a great load of coarse c1astics onto siliceous silty
clays (porosity 70 to 80%) triggered a second, considerably
larger mass movement on a much gentIer slope (about
1.5°), which carried the coarse material "piggyback" down
to a 3000 m deep marginal basin plain (slope angle 0.1°).
The mass spread over an area of approximately 300 km 2,
forming a sheet several tens to about 100 m thick. Due to
the remolding and differential settling of the flow mass
usually originate from slides and slumps by liquefaction of the primary, metastable grain packing (in situ
water content, w, greater than liquid limit, w L ). Turbidity currents evolve from gravity flows by uptake
of additional water. i A composite, two-step grainmud flow mechanism can explain long-distance
transport of pebbles and gravel into the deep sea