218
marine canyons associated with major rivers. The
driving force of turbidity currents is primarily a function of the difference in the densities of the suspension and the overlying water body, the submarine
relief, i.e. the angle and length of slope, and the
thickness of the suspension current.
High-density and thick turbidity currents usually
originate from large gravity mass movements on the
slopes of deep basins well supplied with sediment.
Their densities range from 1.10 to 1.17 glcm 3 and they
reach thicknesses of several hundred meters (e.g. Piper and
Shor 1988). Due to their high current velocities (up to 1020 mlsec), they can carry relatively coarse-grained sand,
pebbles, and intraclasts.
Within the confines of submarine channels, largescale turbidity currents have the competence for
transporting gravel (up to at least 10 cm in diameter)
as bedload, and may thus generate lenses of conglomerate at the foot of prodelta slopes. On deep-sea
fan lobes and basin plains, suspension flows often
erode the uppermost mud layer in extensive areas
(Fig. 5.15b-d). The eroded material feeds the suspension with new sediment which replaces coarser material settling out of suspension in the slackening body
and tail of the current. In this way, and by the maintenance ofturbulence by gravitational forces (auto-suspension), the current is kept in motion and can travel
over long distances.
Low-density turbidity currents flow slowly and
can therefore keep only silt and elay-sized material or
larger aggregates of fine particles in suspension.
Their erosional capacity is very low or non-existent,
but weak turbulence maintains such suspension currents for relatively long periods of time. They can
attain considerable thicknesses and distribute their
suspended load as a thin bed over wide areas. It is
assumed that low-density, muddy turbidity currents
often are the final stage of sand-bearing suspension
currents, which have lost their coarser grain size fraction underway (Fig. 5.15d).
A more comprehensive treatment of this topic including a
specific nomenclature for the various processes involved in
the generation ofthese event beds is given, e.g., by Middleton (1993), Stowet al. (1996).
Types of Turbidites and Their Characteristics
The final products of turbidity currents are various
types of turbidite beds (turbidites). Texture, internal
sedimentary structures, and composition of turbidites
vary greatly. At least four types of turbidites can be
distinguished:
(1) Coarse-grained turbidites are commonly found in
proximal regions in relation to the sediment source.
They are generated by high-density turbidity currents
Chapter 5 Oceanic Sediments
carrying pebbles and elasts as bed load and finergrained material in suspension. They often show an
initial stage of traction sedimentation (coarse-grained
conglomeratic sand with plane lamination, cross-bedding, internal swur, and possibly some inverse grading (Figs. 5.14a and 5.15d). This division is followed
by sedimentation from suspension creating either a
structureless or normally graded higher division
(Bourna division Ta; Bourna 1962). Typical features
are water-escape structures (pillar and dish structures).
Erosional truncation of the upper part of the bed by a
subsequent suspension flow is eommon, leading to
"amalgamation" of several turbidite beds.
Coarse-grained turbidites seem to occur preferentially in
the wide feeder channels of deep-sea fans. The high-velocity turbidity currents have the capacity of eroding sandy
and gravelly beds of the channel floor where they leave
behind large-scale bed forms such as flutes and antidunes
(Morris et al. 1998).
The transformation of a cohesionless debris flow into
a high-density turbidity eurrent, generating a basal
traction carpet, may take plaee within a short distanee downslope (Falk and Dorsey 1998).
(2) Medium-grained sandy turbidites (silieielastics
and carbonate) re fleet deposition from suspension
currents of moderate density. The event bed is graded
from bottom to top, but grading may beeome indistinct if the souree area does not provide material of a
wide range of grain sizes. Due to the uptake of
eroded deep-sea mud, the amount of autoehthonous
fauna (nekton, plankton) often inereases toward the
top of a turbidite bed.
A "elassie", medium-sized sand turbidite, shows
the following sequenee (or divisions) of internal sedimentary struetures (from top to bottom; Figs. 5.14a
and 5.l5b and d):
- Struetureless and indistinetly graded mud interval
(Bourna Te)' originating solely from suspension
sedimentation.
- Laminated mud (Bouma T d)' explained as mixed
traetionlsuspension sedimentation.
- Ripple eross-bedded sand (Bouma division Tc) due
to fallout of sand from suspension and current
traction in the lower flow regime.
- Plane laminatedsand (Bouma division Tb) refleeting traction struetures of the upper flow regime.
- Graded division (Bourna T J, mostly struetureless,
often with water eseape struetures, oceasionally
displaying antidunes which indicate a high flow
regime.
The lowermost Bouma division may rest on a coarsegrained, inverse graded traetion carpet as mentioned
above.
marine canyons associated with major rivers. The
driving force of turbidity currents is primarily a function of the difference in the densities of the suspension and the overlying water body, the submarine
relief, i.e. the angle and length of slope, and the
thickness of the suspension current.
High-density and thick turbidity currents usually
originate from large gravity mass movements on the
slopes of deep basins well supplied with sediment.
Their densities range from 1.10 to 1.17 glcm 3 and they
reach thicknesses of several hundred meters (e.g. Piper and
Shor 1988). Due to their high current velocities (up to 1020 mlsec), they can carry relatively coarse-grained sand,
pebbles, and intraclasts.
Within the confines of submarine channels, largescale turbidity currents have the competence for
transporting gravel (up to at least 10 cm in diameter)
as bedload, and may thus generate lenses of conglomerate at the foot of prodelta slopes. On deep-sea
fan lobes and basin plains, suspension flows often
erode the uppermost mud layer in extensive areas
(Fig. 5.15b-d). The eroded material feeds the suspension with new sediment which replaces coarser material settling out of suspension in the slackening body
and tail of the current. In this way, and by the maintenance ofturbulence by gravitational forces (auto-suspension), the current is kept in motion and can travel
over long distances.
Low-density turbidity currents flow slowly and
can therefore keep only silt and elay-sized material or
larger aggregates of fine particles in suspension.
Their erosional capacity is very low or non-existent,
but weak turbulence maintains such suspension currents for relatively long periods of time. They can
attain considerable thicknesses and distribute their
suspended load as a thin bed over wide areas. It is
assumed that low-density, muddy turbidity currents
often are the final stage of sand-bearing suspension
currents, which have lost their coarser grain size fraction underway (Fig. 5.15d).
A more comprehensive treatment of this topic including a
specific nomenclature for the various processes involved in
the generation ofthese event beds is given, e.g., by Middleton (1993), Stowet al. (1996).
Types of Turbidites and Their Characteristics
The final products of turbidity currents are various
types of turbidite beds (turbidites). Texture, internal
sedimentary structures, and composition of turbidites
vary greatly. At least four types of turbidites can be
distinguished:
(1) Coarse-grained turbidites are commonly found in
proximal regions in relation to the sediment source.
They are generated by high-density turbidity currents
Chapter 5 Oceanic Sediments
carrying pebbles and elasts as bed load and finergrained material in suspension. They often show an
initial stage of traction sedimentation (coarse-grained
conglomeratic sand with plane lamination, cross-bedding, internal swur, and possibly some inverse grading (Figs. 5.14a and 5.15d). This division is followed
by sedimentation from suspension creating either a
structureless or normally graded higher division
(Bourna division Ta; Bourna 1962). Typical features
are water-escape structures (pillar and dish structures).
Erosional truncation of the upper part of the bed by a
subsequent suspension flow is eommon, leading to
"amalgamation" of several turbidite beds.
Coarse-grained turbidites seem to occur preferentially in
the wide feeder channels of deep-sea fans. The high-velocity turbidity currents have the capacity of eroding sandy
and gravelly beds of the channel floor where they leave
behind large-scale bed forms such as flutes and antidunes
(Morris et al. 1998).
The transformation of a cohesionless debris flow into
a high-density turbidity eurrent, generating a basal
traction carpet, may take plaee within a short distanee downslope (Falk and Dorsey 1998).
(2) Medium-grained sandy turbidites (silieielastics
and carbonate) re fleet deposition from suspension
currents of moderate density. The event bed is graded
from bottom to top, but grading may beeome indistinct if the souree area does not provide material of a
wide range of grain sizes. Due to the uptake of
eroded deep-sea mud, the amount of autoehthonous
fauna (nekton, plankton) often inereases toward the
top of a turbidite bed.
A "elassie", medium-sized sand turbidite, shows
the following sequenee (or divisions) of internal sedimentary struetures (from top to bottom; Figs. 5.14a
and 5.l5b and d):
- Struetureless and indistinetly graded mud interval
(Bourna Te)' originating solely from suspension
sedimentation.
- Laminated mud (Bouma T d)' explained as mixed
traetionlsuspension sedimentation.
- Ripple eross-bedded sand (Bouma division Tc) due
to fallout of sand from suspension and current
traction in the lower flow regime.
- Plane laminatedsand (Bouma division Tb) refleeting traction struetures of the upper flow regime.
- Graded division (Bourna T J, mostly struetureless,
often with water eseape struetures, oceasionally
displaying antidunes which indicate a high flow
regime.
The lowermost Bouma division may rest on a coarsegrained, inverse graded traetion carpet as mentioned
above.