5.4 Gravity Mass Flow
often evolve into turbulent suspension currents which
generate specific beds termed turbidites. Particularly
in orogenie belts, these bed types form thick flysch
sequences.
In this chapter, only mass flow deposits of the marine realm are described. Their most characteristic
features and depositional environments are demonstrated by a few, simplified conceptual models. Most
of these can also be applied to lake sediments (Sect.
2.5), but large-scale phenomena are commonly missing in small and shallow aquatic environments.
For summaries on terrestrial mass wasting and sediment
gravity flows see, e.g., Brunsden et al. (1979), Selby
(1994), Iverson et al. (1997). Special features are described
by Major (1997), Blair and McPherson (1998).
On marine gravity mass flow deposits and turbidites numerous articles and some special books have been published in the last decades (e.g. Mutti and Ricci Lucchi
1978; Walker 1978, 1984a, c; Stanley and Kelling 1978;
Kelts and Arthur 1981; Saxov and Nieuwenhuis 1982;
Schwarz 1982; Mutti et al. 1984; Prior and Coleman 1984;
Stow and Piper 1984; Thornton 1984; Stanley 1985; Mutti
and Norrnark 1987; Mutti 1992).
The flow behavior of the different types of gravity mass
movememts is treated, e.g., by Middleton and Hampton
(1976), Blatt et al. (1980), Stow (1980), Allen (1982),
Lowe (1982), Komar (1970, 1985), Postma (1986), Einseie
(1989), Middleton (1993); Piper and Savoye 1993; Mulder
and Cochonat 1996; Mohrig et al. 1998; and summarized
by Stowet al. (1996).
In the following figures, displaying individual beds, bed
sets, and their internal sedimentary structures, a number of
symbols are introduced. These are purely descriptive and
partially supplement or rcplace symbols which have been
used earlier (e.g. those for sandy turbidites after Bouma
1962; see also Lowe 1982; Walker 1984a, c; summaries in
Ghibaudo 1992; Stowet al. 1996). For example, sandy
turbidites are discriminatcd trom mud turbidites by using
the symbols TS and TM, respectively; ig signifies inversed
grading, Im laminated umd, etc.
5.4.2 Gravity Mass Movements and Mass Flows
in the Ocean
Types of Gravity Mass Transport
The most important types of gnlvity mass movements
found in both ocean and lake basins are summarized
in Fig. 5.11. They can be subdivided into several
groups:
- Mass movements of lithified, jointed rocks: Rockfall along eoastal cliffs or steep submarine slopes and
fault scarps (Fig. 5.11 a). The transport distance of
such fallen rocks is commonly limited. However on
steep slopes, such as present around volcanic islands,
huge block-rich rock falls and slides extending SO100 km out in to the sea have been observed (e.g. on
the flanks of the island of Hawaii; Moore et al.
211
1995). Another means of producing larger transport
distances is a composite mass movement, as indicated in Fig. 5.lli.
- Creep, sliding and slumping of semi-solid to soft
sediments (Fig. 5.llc through e) on slopes ofvarious
angles (as little as a few degrees). Movement takes
place if the shear stress exceeds the shear strength of
the sediment at some depth below the sedimentary
surface, which is usually tested by stability analysis
(Fig. 5.l1b). The shear stress increases with the
slope angle and depth below the sea floor. Deep below the surface of a gentle slope, the shear stress can
be as high as on a steep slope at shallow depth. For
this reason, there is a tendency for thick mass movements to develop on gentle slopes, whereas thin ones
are characteristic for steep slopes. The shear stress
can, in addition, be significantly enhanced by earthquakes and, in shallow water, by the effect of storm
waves on the sea bed. Furthermore, the sudden loading by an approaching slide or slump often generates
seeondary failure planes and propagation of the mass
movement. Similarly, a drop in sea level or tectonic
uplift can lead to an additional loading of emerging
sediments which, when losing buoyancy, may trigger
slope failure.
The shear strength of sediments usually increases
with depth below the sea floor, but is reduced by
high porc pressure in underconsolidated sediments.
Such a situation frequently occurs in areas where
fine-grained sediments are deposited rapidly, for example in front of deltas. A further reduction in shear
strength is caused by the rather common development of bio genie gas in the uppermost tens of meters
in slope sediments rich in organic matter. Finally, the
release of methane from crystallized gas hydrates
within the sediment may locally diminish the shear
strength and thus cause large mass movements.
In creep and slides, the sediment masses do not
change their mechanical state, i.e., they move as a
kind of rigid plug without significant internal disturbance. The distance of transport is normally short.
Slumps show considerable internal disturbance, for
example slump folding, and frequently several slip
faces (Fig. 5.lle). They often evolve into debris or
mud flows.
Gas hydrates have been recently mentioned by several authors as a mechanism of triggering mass movements (e.g.
Haq 1993. 1998; Paull et al. 1996; Maslin et al. 1998).
Methane gas can be released from solid gas hydrates when
the bottom water temperature increases (resulting from
climatic changes or shifting of currcnt systems), and/or
when the water pressure decreases due to relative sea-level
fall.
Gravitational collapse structures are associated with
faults and often produce aseries of slides and slumps. They
are ascribed to earthquakes and high fluid pressures
and may affect loose sediments and consolidated rocks
often evolve into turbulent suspension currents which
generate specific beds termed turbidites. Particularly
in orogenie belts, these bed types form thick flysch
sequences.
In this chapter, only mass flow deposits of the marine realm are described. Their most characteristic
features and depositional environments are demonstrated by a few, simplified conceptual models. Most
of these can also be applied to lake sediments (Sect.
2.5), but large-scale phenomena are commonly missing in small and shallow aquatic environments.
For summaries on terrestrial mass wasting and sediment
gravity flows see, e.g., Brunsden et al. (1979), Selby
(1994), Iverson et al. (1997). Special features are described
by Major (1997), Blair and McPherson (1998).
On marine gravity mass flow deposits and turbidites numerous articles and some special books have been published in the last decades (e.g. Mutti and Ricci Lucchi
1978; Walker 1978, 1984a, c; Stanley and Kelling 1978;
Kelts and Arthur 1981; Saxov and Nieuwenhuis 1982;
Schwarz 1982; Mutti et al. 1984; Prior and Coleman 1984;
Stow and Piper 1984; Thornton 1984; Stanley 1985; Mutti
and Norrnark 1987; Mutti 1992).
The flow behavior of the different types of gravity mass
movememts is treated, e.g., by Middleton and Hampton
(1976), Blatt et al. (1980), Stow (1980), Allen (1982),
Lowe (1982), Komar (1970, 1985), Postma (1986), Einseie
(1989), Middleton (1993); Piper and Savoye 1993; Mulder
and Cochonat 1996; Mohrig et al. 1998; and summarized
by Stowet al. (1996).
In the following figures, displaying individual beds, bed
sets, and their internal sedimentary structures, a number of
symbols are introduced. These are purely descriptive and
partially supplement or rcplace symbols which have been
used earlier (e.g. those for sandy turbidites after Bouma
1962; see also Lowe 1982; Walker 1984a, c; summaries in
Ghibaudo 1992; Stowet al. 1996). For example, sandy
turbidites are discriminatcd trom mud turbidites by using
the symbols TS and TM, respectively; ig signifies inversed
grading, Im laminated umd, etc.
5.4.2 Gravity Mass Movements and Mass Flows
in the Ocean
Types of Gravity Mass Transport
The most important types of gnlvity mass movements
found in both ocean and lake basins are summarized
in Fig. 5.11. They can be subdivided into several
groups:
- Mass movements of lithified, jointed rocks: Rockfall along eoastal cliffs or steep submarine slopes and
fault scarps (Fig. 5.11 a). The transport distance of
such fallen rocks is commonly limited. However on
steep slopes, such as present around volcanic islands,
huge block-rich rock falls and slides extending SO100 km out in to the sea have been observed (e.g. on
the flanks of the island of Hawaii; Moore et al.
211
1995). Another means of producing larger transport
distances is a composite mass movement, as indicated in Fig. 5.lli.
- Creep, sliding and slumping of semi-solid to soft
sediments (Fig. 5.llc through e) on slopes ofvarious
angles (as little as a few degrees). Movement takes
place if the shear stress exceeds the shear strength of
the sediment at some depth below the sedimentary
surface, which is usually tested by stability analysis
(Fig. 5.l1b). The shear stress increases with the
slope angle and depth below the sea floor. Deep below the surface of a gentle slope, the shear stress can
be as high as on a steep slope at shallow depth. For
this reason, there is a tendency for thick mass movements to develop on gentle slopes, whereas thin ones
are characteristic for steep slopes. The shear stress
can, in addition, be significantly enhanced by earthquakes and, in shallow water, by the effect of storm
waves on the sea bed. Furthermore, the sudden loading by an approaching slide or slump often generates
seeondary failure planes and propagation of the mass
movement. Similarly, a drop in sea level or tectonic
uplift can lead to an additional loading of emerging
sediments which, when losing buoyancy, may trigger
slope failure.
The shear strength of sediments usually increases
with depth below the sea floor, but is reduced by
high porc pressure in underconsolidated sediments.
Such a situation frequently occurs in areas where
fine-grained sediments are deposited rapidly, for example in front of deltas. A further reduction in shear
strength is caused by the rather common development of bio genie gas in the uppermost tens of meters
in slope sediments rich in organic matter. Finally, the
release of methane from crystallized gas hydrates
within the sediment may locally diminish the shear
strength and thus cause large mass movements.
In creep and slides, the sediment masses do not
change their mechanical state, i.e., they move as a
kind of rigid plug without significant internal disturbance. The distance of transport is normally short.
Slumps show considerable internal disturbance, for
example slump folding, and frequently several slip
faces (Fig. 5.lle). They often evolve into debris or
mud flows.
Gas hydrates have been recently mentioned by several authors as a mechanism of triggering mass movements (e.g.
Haq 1993. 1998; Paull et al. 1996; Maslin et al. 1998).
Methane gas can be released from solid gas hydrates when
the bottom water temperature increases (resulting from
climatic changes or shifting of currcnt systems), and/or
when the water pressure decreases due to relative sea-level
fall.
Gravitational collapse structures are associated with
faults and often produce aseries of slides and slumps. They
are ascribed to earthquakes and high fluid pressures
and may affect loose sediments and consolidated rocks