2.44 Submarine Canyons
These are valley-shaped depressions which extend
from the top to the bottom of the slopes, down to
2,000–4,000 m. In some cases they may start in shallow water near the beach, in others close to the edge of
the shelf. The height from the bottom of the canyon to
the top of the slope on each side may be up to 2,000 m.
We are dealing with enormous topographical features,
which would have been very impressive indeed if they
had been on land, towering structures on the scale of
the Grand Canyon.
Shepard et al. (1979) systematically gathered data
on currents and sediment transport in submarine
canyons and found that tidal currents are of great
importance also at great depths in submarine canyons.
Current meters have shown that currents flow both up
and down the submarine canyons, and that they switch
every 6 h like tidal currents in shallow water. Current
velocity is often only 10–20 cm/s, but in many
canyons velocities of up to 40 cm/s occur sometimes,
powerful enough to transport fine to medium-grained
sand. The flow velocity tends to be greatest in the
upper part of the canyon and diminish downvalley.
Most sediment transport takes place during these episodic and unusually high flow velocities which may be
linked with storms which create wind-induced shear
forces which sweep the water up against the coast
(storm tides) and may cause currents to develop
along the bottom and down the submarine canyons.
However, high flow velocities have also been
measured without it being possible to associate them
with storms or wind stress. The most powerful flow
velocity is most commonly directed downvalley, but
upvalley-directed streams have been observed with
velocities of up to 90 cm/s.
Detailed measurements in the submarine canyons
off the coast of California reveal that the highest flow
rates are oriented up the canyon in a way which seems
to indicate that they are generated by internal waves
from the ocean basin, and not by gravitational forces.
Currents may then develop when the waves “break”
against the coast or the continental shelf.
Powerful currents in submarine canyons are capable of transporting sand, sometimes in rare instances
even coarser material. These are frequently not
turbidity currents, but traction currents, which transport and deposit better-sorted material. Turbidity
currents have been observed in submarine canyons as
well, but definite (observed) examples were only lowvelocity, low-density turbidity currents with a maximum velocity of 70–100 cm/s. In submarine canyons
we thus have both traction currents, which are controlled partly by tidal forces, and turbidity currents.
Downward-moving currents driven by tidal forces
may, if they contain much suspended material, turn
into turbidity currents. The downward-moving
currents have both a component of traction and
gravitation.
The relief of submarine canyons is due partly to
erosion down into the underlying sediments, and
partly to lack of deposition in the canyon while the
adjacent beds were being deposited. During low sea
level stands rivers may prograde closer to the shelf
edge and hence supply more sediment to the submarine canyons, thus feeding submarine fans. At sea level
highstands, currents in the submarine canyons and the
shelf may erode shelf and slope sediments and deposit
pure sand onlapping an erosional unconformity at the
toe of the canyon.
Most of the canyon itself is an area of sediment
transport and erosion. Deposition takes place where
there is a change of slope near the basin floor. Here the
channel defined by the canyons splits up into several
channels which build depositional lobes called
suprafan lobes (Fig. 2.45).
As the lobes build up, the gradient of the slope is
reduced and a new channel will form in a part of the
fan where there is a steeper slope (Fig. 2.46). This
produces lobe-shifting similar to that observed in
fluvially dominated deltas. Each lobe will tend to
build a fining-upwards sequence, with conglomerate
and coarse sand near the base. On the sides of the
channels fine-grained material in suspension is deposited as thin-bedded turbidites. The levee builds up on
both sides of the channel, and resembles a river levee.
On the submarine Amazon delta slope there are welldeveloped meandering channels. The distal fan is also
dominated by fine-grained sediments deposited as
thin, graded fine sand, silt and clay. Progradation of
submarine fans may also produce an upwardscoarsening sequence (Fig. 2.47).
2 Introduction to Sedimentology
85
These are valley-shaped depressions which extend
from the top to the bottom of the slopes, down to
2,000–4,000 m. In some cases they may start in shallow water near the beach, in others close to the edge of
the shelf. The height from the bottom of the canyon to
the top of the slope on each side may be up to 2,000 m.
We are dealing with enormous topographical features,
which would have been very impressive indeed if they
had been on land, towering structures on the scale of
the Grand Canyon.
Shepard et al. (1979) systematically gathered data
on currents and sediment transport in submarine
canyons and found that tidal currents are of great
importance also at great depths in submarine canyons.
Current meters have shown that currents flow both up
and down the submarine canyons, and that they switch
every 6 h like tidal currents in shallow water. Current
velocity is often only 10–20 cm/s, but in many
canyons velocities of up to 40 cm/s occur sometimes,
powerful enough to transport fine to medium-grained
sand. The flow velocity tends to be greatest in the
upper part of the canyon and diminish downvalley.
Most sediment transport takes place during these episodic and unusually high flow velocities which may be
linked with storms which create wind-induced shear
forces which sweep the water up against the coast
(storm tides) and may cause currents to develop
along the bottom and down the submarine canyons.
However, high flow velocities have also been
measured without it being possible to associate them
with storms or wind stress. The most powerful flow
velocity is most commonly directed downvalley, but
upvalley-directed streams have been observed with
velocities of up to 90 cm/s.
Detailed measurements in the submarine canyons
off the coast of California reveal that the highest flow
rates are oriented up the canyon in a way which seems
to indicate that they are generated by internal waves
from the ocean basin, and not by gravitational forces.
Currents may then develop when the waves “break”
against the coast or the continental shelf.
Powerful currents in submarine canyons are capable of transporting sand, sometimes in rare instances
even coarser material. These are frequently not
turbidity currents, but traction currents, which transport and deposit better-sorted material. Turbidity
currents have been observed in submarine canyons as
well, but definite (observed) examples were only lowvelocity, low-density turbidity currents with a maximum velocity of 70–100 cm/s. In submarine canyons
we thus have both traction currents, which are controlled partly by tidal forces, and turbidity currents.
Downward-moving currents driven by tidal forces
may, if they contain much suspended material, turn
into turbidity currents. The downward-moving
currents have both a component of traction and
gravitation.
The relief of submarine canyons is due partly to
erosion down into the underlying sediments, and
partly to lack of deposition in the canyon while the
adjacent beds were being deposited. During low sea
level stands rivers may prograde closer to the shelf
edge and hence supply more sediment to the submarine canyons, thus feeding submarine fans. At sea level
highstands, currents in the submarine canyons and the
shelf may erode shelf and slope sediments and deposit
pure sand onlapping an erosional unconformity at the
toe of the canyon.
Most of the canyon itself is an area of sediment
transport and erosion. Deposition takes place where
there is a change of slope near the basin floor. Here the
channel defined by the canyons splits up into several
channels which build depositional lobes called
suprafan lobes (Fig. 2.45).
As the lobes build up, the gradient of the slope is
reduced and a new channel will form in a part of the
fan where there is a steeper slope (Fig. 2.46). This
produces lobe-shifting similar to that observed in
fluvially dominated deltas. Each lobe will tend to
build a fining-upwards sequence, with conglomerate
and coarse sand near the base. On the sides of the
channels fine-grained material in suspension is deposited as thin-bedded turbidites. The levee builds up on
both sides of the channel, and resembles a river levee.
On the submarine Amazon delta slope there are welldeveloped meandering channels. The distal fan is also
dominated by fine-grained sediments deposited as
thin, graded fine sand, silt and clay. Progradation of
submarine fans may also produce an upwardscoarsening sequence (Fig. 2.47).
2 Introduction to Sedimentology
85