require current velocities exceeding 60–70 cm/s.
Cross-stratification may be symmetrical or asymmetrical on both sides of the sandwave, depending on the
relative strength of the opposing currents.
Relatively low-energy environments (<50 cm/s)
are characterised by sand patches and mud. The sand
forming the patches probably only moves during
storms.
Shelf sediments characteristically accumulate at
relatively low overall sedimentation rates (1–10 mm/
1,000 years).
2.41 Continental Slopes
Continental slopes are the areas between the edge of
the mostly very flat continental shelf which commonly
lies at a depth of 200–500 m and the continental rise,
where the ocean deep begins at a depth of 2–4,000 m.
The continental slope gradient is typically 2–6
, and it
is 20–100 km broad. The gradient is a function of a
number of different factors, but the stability of the
shelf edge constitutes a major control factor. The
steepest slopes are therefore to be found off carbonate
banks with well-cemented coral reefs and carbonate
beds which have high shear strength. In areas with
rapid sedimentation, loose sediments have little shear
strength and submarine slides, slumping and formation
of turbidites occur, maintaining relatively gentle
slopes (1–2
). Where sedimentation is slower the
sediments have more time to consolidate, and will be
more stable. The steepest submarine slopes in clastic
sediments (greater than 10
) are therefore found in
submarine canyons, where erosion cuts into older,
well-consolidated sedimentary strata.
Along passive continental margins the continental
slope is associated with the transition from continental
crust to oceanic crust. In areas with a large supply of
sediment, the shelf may have prograded beyond this
boundary.
2.42 Organic Sedimentation on the
Slope of the Continental Shelf
The continetal slopes are enriched in organic matter
compared to the shelf and the deep ocean.
This is because the slope is where we have the
greatest upwelling of nutrients from the deep.
We also find low oxygen content in the water column
on continental slopes, allowing much of the organic
matter produced to be retained in the sediments. There
will also be high productivity on shallower slopes in
front of deltas because of the large nutrient supply
from river water, but the organic matter may be greatly
diluted by rapid clastic sedimentation. On the continental shelf the supply of nutrients is small and the
prevalence of stronger currents and turbulence means
that most of the organic production there will be
oxidised.
Out in the open ocean basin organic production is
relatively low, due to a limited supply of nutrients.
Much of the planktonic organic matter is oxidised near
the seafloor by deep currents of cold, oxygenated
water from the polar areas.
Sediments deposited on the continental slope are
therefore more promising as source rocks for oil than
shelf and deep-water facies.
2.43 Sediment Transport on Submarine
Slopes
Gravitational processes are naturally important on
submarine slopes.
Gravity forces can be represented by forces
(vectors) normal to, and parallel to, the slope. The
component parallel to the slope consists of shear
forces which may overcome the shear strength of the
sediment, causing slumping. Sliding of large volumes
of sediments downslope produces extensional faulting
in the upper part of the slope and compression in the
lower part. Gravitational instability on the slopes may
also develop into debris flows and turbidity currents.
Collapse of the sediment grain framework may cause
sudden compaction and liquefaction of the slope
sediments.
Traction currents may, however, also play a part on
the submarine slopes, particularly in canyons but also
near the toe of the slope, where we may have
contourites – deposited by currents flowing parallel
to the slope contours.
84
K. Bjørlykke
Cross-stratification may be symmetrical or asymmetrical on both sides of the sandwave, depending on the
relative strength of the opposing currents.
Relatively low-energy environments (<50 cm/s)
are characterised by sand patches and mud. The sand
forming the patches probably only moves during
storms.
Shelf sediments characteristically accumulate at
relatively low overall sedimentation rates (1–10 mm/
1,000 years).
2.41 Continental Slopes
Continental slopes are the areas between the edge of
the mostly very flat continental shelf which commonly
lies at a depth of 200–500 m and the continental rise,
where the ocean deep begins at a depth of 2–4,000 m.
The continental slope gradient is typically 2–6
, and it
is 20–100 km broad. The gradient is a function of a
number of different factors, but the stability of the
shelf edge constitutes a major control factor. The
steepest slopes are therefore to be found off carbonate
banks with well-cemented coral reefs and carbonate
beds which have high shear strength. In areas with
rapid sedimentation, loose sediments have little shear
strength and submarine slides, slumping and formation
of turbidites occur, maintaining relatively gentle
slopes (1–2
). Where sedimentation is slower the
sediments have more time to consolidate, and will be
more stable. The steepest submarine slopes in clastic
sediments (greater than 10
) are therefore found in
submarine canyons, where erosion cuts into older,
well-consolidated sedimentary strata.
Along passive continental margins the continental
slope is associated with the transition from continental
crust to oceanic crust. In areas with a large supply of
sediment, the shelf may have prograded beyond this
boundary.
2.42 Organic Sedimentation on the
Slope of the Continental Shelf
The continetal slopes are enriched in organic matter
compared to the shelf and the deep ocean.
This is because the slope is where we have the
greatest upwelling of nutrients from the deep.
We also find low oxygen content in the water column
on continental slopes, allowing much of the organic
matter produced to be retained in the sediments. There
will also be high productivity on shallower slopes in
front of deltas because of the large nutrient supply
from river water, but the organic matter may be greatly
diluted by rapid clastic sedimentation. On the continental shelf the supply of nutrients is small and the
prevalence of stronger currents and turbulence means
that most of the organic production there will be
oxidised.
Out in the open ocean basin organic production is
relatively low, due to a limited supply of nutrients.
Much of the planktonic organic matter is oxidised near
the seafloor by deep currents of cold, oxygenated
water from the polar areas.
Sediments deposited on the continental slope are
therefore more promising as source rocks for oil than
shelf and deep-water facies.
2.43 Sediment Transport on Submarine
Slopes
Gravitational processes are naturally important on
submarine slopes.
Gravity forces can be represented by forces
(vectors) normal to, and parallel to, the slope. The
component parallel to the slope consists of shear
forces which may overcome the shear strength of the
sediment, causing slumping. Sliding of large volumes
of sediments downslope produces extensional faulting
in the upper part of the slope and compression in the
lower part. Gravitational instability on the slopes may
also develop into debris flows and turbidity currents.
Collapse of the sediment grain framework may cause
sudden compaction and liquefaction of the slope
sediments.
Traction currents may, however, also play a part on
the submarine slopes, particularly in canyons but also
near the toe of the slope, where we may have
contourites – deposited by currents flowing parallel
to the slope contours.
84
K. Bjørlykke