214
particles, the coarse material now forms the base of the
mass flow deposit, 70 km away from its source area.
Loading of slope sediment by a debris avalanche probably also triggered the very large Canary debris flow which
travelled 700 km from the West African slope to the Madeira abyssal plain (Masson et al. 1993; Masson et al.
1998).
Preferential Sites of Mass Movements
Figure 5.12 summarizes the most important tectonic
and environmental settings for the occurrence of
large gravity mass movements. Most of theses settings provide both high influx of sediment and strong
relief.
- The prodelta slopes of marine deltas exhibit a variety of mass movements. Very common are slides and
mud flows which occur even on extremely gentle
slopes in shallow water. Furthermore, large-scale
creep generating growth faults and mud diapirs may
form (Fig. 5.12a).
Such features have been described, e.g., for the Mississippi
delta (Prior and Coleman 1984) and Niger delta (Cohen
and McClay 1996). Weil documented examples include the
1929 Grand Banks slump and turbidity current offthe Laurentian Channel of North America (Piper and Shor 1988;
Hughes Clarke et al. 1990) and the 1979 mass-wasting at
the prodelta slope of the Var river in the northwestem
Mediterranean (Auffret et al. 1988; Piper and Savoye 1993;
Savoye et al. 1993).
- The heads of submarine canyons, extending to the
inner shelf and foreshore zones, can collect sediment
transported parallel or obliquely to the coastline (Fig.
5.12a). This sediment moves episodically by gravity
processes down the canyon into the deep sea.
- "Shelf break erosion". Rapid subsidence and active
faulting during rifting and early drifting (right-hand
side of Fig. 5.12a) can create a submarine relief particularly favorable for extensive gravity mass movements along the shelf break of young basins
(Bourrouilh 1987; Eberli 1987). The resulting debris
flows may carry soft and lithified material of various
age.
- Subduction-related depositional envIronments provide several possibilities for different types of gravity
mass movements.
Fig. 5.12. Depositional environments (overview), in
which large gravity mass movements and turbidite
sedimentation take place. a Passive continental margin in an early rifting-drifting stage with active faults.
Chapter 5 Oceanic Sediments
Deep-sea trenches adjacent to a main continent (not shown
in Fig. 5.l2b), such as the modem Peru-Chile trench, frequently collect high quantities of river material. A major
proportion of these sediments consists of sandy and muddy
gravity flow deposits (Thomburg and Kulm 1987).
Trenches far away from significant terrestrial sediment
sources (Fig. 5.12b) are fed by slumps and debris flows
originating from an accretionary wedge or young,
autochthonous slope sediments. For that reason, they often
contain material of varying age and nature (polymict clast
composition), sometimes including ophiolites and metamorphic rocks.
- The sedimentary fills of forearc basins, and to
some extent those of backarc basins (not shown in
Fig. 5.12b), are usually characterized by high proportions of volcaniclastic material, transported by gravity mass movements into the basin.
- Redeposited gravel-sized rock fragments and skeletal material (rudites), sand-sized shell fragments
(arenites), and finer-grained lutites (including pellets)
play a great part in deep-sea carbonate depositional
envirOnments (Fig. 5.12c). The main sources ofthese
materials are carbonate shelves and isolated carbonate platforms. Due to early differential lithification
and the oversteepened slopes of many reef buildups,
rock falls and collapse events are fairly common.
These processes generate large-scale slurnps and debris flows including large blocks.
Part of the dislocated coarse and fine-grained shallow-water carbonate is transported by turbidity currents farther
basinward (see e.g. Remane 1960; Wilson 1975; Cook and
Enos 1977; Scholle et al. 1983a; McIlreath and James
1984; Eberli 1987).
Deposits of debris flows and mud flows
Flow masses come to rest if the applied shear stress
drops below the shear strength of the moving material and/or, on land, if their excess pore water dissipates. The flows "freeze", which is accomplished
either by cohesive freezing or, in the case of a
cohesionless sandy matrix, by frictional freezing, or
by both processes. In subaqueous environments,
parts of the flow masses can take up additional water
from the overlying water body and evolve into turbulent suspension currents of high velocity (turbidity
currents, see below).
The principal features of debris flow and mud
flow deposits are summarised in Figure 5.13. They
reflect the final flow processes immediately before
deposition, which can be characterised as more or
b Convergent margin with forearc basin. c Mass
flows and deep-sea fan fed by detritus from carbonate shelf or platform. (Based on different sources,
e.g., Stow 1986)
particles, the coarse material now forms the base of the
mass flow deposit, 70 km away from its source area.
Loading of slope sediment by a debris avalanche probably also triggered the very large Canary debris flow which
travelled 700 km from the West African slope to the Madeira abyssal plain (Masson et al. 1993; Masson et al.
1998).
Preferential Sites of Mass Movements
Figure 5.12 summarizes the most important tectonic
and environmental settings for the occurrence of
large gravity mass movements. Most of theses settings provide both high influx of sediment and strong
relief.
- The prodelta slopes of marine deltas exhibit a variety of mass movements. Very common are slides and
mud flows which occur even on extremely gentle
slopes in shallow water. Furthermore, large-scale
creep generating growth faults and mud diapirs may
form (Fig. 5.12a).
Such features have been described, e.g., for the Mississippi
delta (Prior and Coleman 1984) and Niger delta (Cohen
and McClay 1996). Weil documented examples include the
1929 Grand Banks slump and turbidity current offthe Laurentian Channel of North America (Piper and Shor 1988;
Hughes Clarke et al. 1990) and the 1979 mass-wasting at
the prodelta slope of the Var river in the northwestem
Mediterranean (Auffret et al. 1988; Piper and Savoye 1993;
Savoye et al. 1993).
- The heads of submarine canyons, extending to the
inner shelf and foreshore zones, can collect sediment
transported parallel or obliquely to the coastline (Fig.
5.12a). This sediment moves episodically by gravity
processes down the canyon into the deep sea.
- "Shelf break erosion". Rapid subsidence and active
faulting during rifting and early drifting (right-hand
side of Fig. 5.12a) can create a submarine relief particularly favorable for extensive gravity mass movements along the shelf break of young basins
(Bourrouilh 1987; Eberli 1987). The resulting debris
flows may carry soft and lithified material of various
age.
- Subduction-related depositional envIronments provide several possibilities for different types of gravity
mass movements.
Fig. 5.12. Depositional environments (overview), in
which large gravity mass movements and turbidite
sedimentation take place. a Passive continental margin in an early rifting-drifting stage with active faults.
Chapter 5 Oceanic Sediments
Deep-sea trenches adjacent to a main continent (not shown
in Fig. 5.l2b), such as the modem Peru-Chile trench, frequently collect high quantities of river material. A major
proportion of these sediments consists of sandy and muddy
gravity flow deposits (Thomburg and Kulm 1987).
Trenches far away from significant terrestrial sediment
sources (Fig. 5.12b) are fed by slumps and debris flows
originating from an accretionary wedge or young,
autochthonous slope sediments. For that reason, they often
contain material of varying age and nature (polymict clast
composition), sometimes including ophiolites and metamorphic rocks.
- The sedimentary fills of forearc basins, and to
some extent those of backarc basins (not shown in
Fig. 5.12b), are usually characterized by high proportions of volcaniclastic material, transported by gravity mass movements into the basin.
- Redeposited gravel-sized rock fragments and skeletal material (rudites), sand-sized shell fragments
(arenites), and finer-grained lutites (including pellets)
play a great part in deep-sea carbonate depositional
envirOnments (Fig. 5.12c). The main sources ofthese
materials are carbonate shelves and isolated carbonate platforms. Due to early differential lithification
and the oversteepened slopes of many reef buildups,
rock falls and collapse events are fairly common.
These processes generate large-scale slurnps and debris flows including large blocks.
Part of the dislocated coarse and fine-grained shallow-water carbonate is transported by turbidity currents farther
basinward (see e.g. Remane 1960; Wilson 1975; Cook and
Enos 1977; Scholle et al. 1983a; McIlreath and James
1984; Eberli 1987).
Deposits of debris flows and mud flows
Flow masses come to rest if the applied shear stress
drops below the shear strength of the moving material and/or, on land, if their excess pore water dissipates. The flows "freeze", which is accomplished
either by cohesive freezing or, in the case of a
cohesionless sandy matrix, by frictional freezing, or
by both processes. In subaqueous environments,
parts of the flow masses can take up additional water
from the overlying water body and evolve into turbulent suspension currents of high velocity (turbidity
currents, see below).
The principal features of debris flow and mud
flow deposits are summarised in Figure 5.13. They
reflect the final flow processes immediately before
deposition, which can be characterised as more or
b Convergent margin with forearc basin. c Mass
flows and deep-sea fan fed by detritus from carbonate shelf or platform. (Based on different sources,
e.g., Stow 1986)