flow, and the sediments will settle out of suspension
more rapidly. Hyperpycnal flow, where the river water
is denser than the water in the basin, takes place only
in lakes as a rule and leads to the flow of river water
along the bottom of the delta slope, with erosion of the
delta front and formation of turbidites on the basin
floor.
In studies of modern deltas we must also take into
account the fact that most deltas are rather out of
balance as regards progradation in relation to sea
level because the Holocene transgression after the
last glaciation raised sea level by more than 100 m.
In the Niger delta we find beyond the present delta
front deposits which are 12–25,000 years old, which
corresponds to the last advance of the last glaciation.
2.30 River-Dominated Deltas
(Mississippi Type)
The Mississippi drains a huge area of the North American continent (about 3.2 Â 10
6 km
2
) with an average
precipitation of 685 mm/year. It carries vast quantities
of sediment (about 5 Â 10
8 tonnes/year) with a high
clay and silt content, and the gradient of the lower part
of the river is extremely low (about 5 cm/km).
In the 200–300 years during which bathymetric
measurements have been taken in the area, it has been
possible to record considerable progradation. The high
clay and silt content gives the sediments great cohesion,
making the fluvial channels relatively stable. Clay and
silt sediments which are deposited on the delta plain
have a high porosity and water content (60–70%), but
they loose much of their porosity by compaction at
shallow depth. Sand will be deposited chiefly in the
channels, and in mouth bars were the channels enter
the sea. Well-sorted sandy sediments, which have only
about 40–45% porosity immediately after deposition,
will then sink into the underlying clay because of their
higher density. The fluvial channels therefore sink into
the mud and this contributes to their stabilisation, with
the consequence that channels change course (avulsion)
less frequently. Because of this subsidence while sedimentation is continuing, the channel sand may be
thicker than the depth of the channel. The long strings
of sand which may then be preserved in the mud-rich
environment are called bar-finger sands.
During floods large quantities of silt and clay are
deposited in overbank areas between channels, and
these sediments are stabilised by vegetation. In the
Mississippi delta, sedimentation is very rapid so that
the channel and the levees build up above their surroundings. This means that the average gradient of the
channel decreases. Sooner or later the channel will
have to find a new, shorter route (through avulsion)
to the ocean, and which therefore has a somewhat
greater slope. At any given time most of the sediments
are deposited in one delta lobe prograding into the
ocean until the slope becomes to low. A major avulsion will then start the formation of a new delta lobe.
We can see that the Mississippi has constantly
shifted course, and has consequently been a focus of
sedimentary activity in historic as well as modern
times (Fig. 2.32). The present course has extended
the modern delta far out and should have been abandoned for a shorter course towards the southwest, to
the Atchafalaya basin. However the flow in this direction has been artificially limited because of its importance for transport to and from towns like New Orleans
which lie on the present channel.
When a delta lobe is abandoned, it slowly subsides
because of compaction and tectonic subsidence, while
sedimentation takes place elsewhere. The abandoned
lobe may sink below sea level before the fluvial supply
returns to this part of the delta and another delta lobe is
deposited in the same area, allowing intervening deposition of thin marine beds. Thin layers of carbonate or
shale represent periods of local transgression (abandonment facies) which are time equivalent with
regressions in the prograding delta lobes. Between
the delta lobes in the interdistributary bay facies
wave energy is very low and there may be little or no
beach (sand) deposits between the marine mud and
mud deposited above sea level.
In vertical profile we observe alternations of fluvial
channel sediments, levee deposits, crevasse splays and
possibly marine sediments (Figs. 2.33 and 2.34).
Even small variations in sea level have a strong
influence on delta sedimentation. In a section where
a fluvial facies gives way to a marine bed, it is often
difficult to know whether this represents a transgression due to a rise in sea level or whether this part of the
delta was abandoned by the fluvial system and is
subsiding. Only if we can correlate a transgressive
bed over the whole area can we assume that it is due
to changes in sea level. Transgressive carbonate or
thin sandstone beds are the most useful for regional
correlation.
70
K. Bjørlykke
more rapidly. Hyperpycnal flow, where the river water
is denser than the water in the basin, takes place only
in lakes as a rule and leads to the flow of river water
along the bottom of the delta slope, with erosion of the
delta front and formation of turbidites on the basin
floor.
In studies of modern deltas we must also take into
account the fact that most deltas are rather out of
balance as regards progradation in relation to sea
level because the Holocene transgression after the
last glaciation raised sea level by more than 100 m.
In the Niger delta we find beyond the present delta
front deposits which are 12–25,000 years old, which
corresponds to the last advance of the last glaciation.
2.30 River-Dominated Deltas
(Mississippi Type)
The Mississippi drains a huge area of the North American continent (about 3.2 Â 10
6 km
2
) with an average
precipitation of 685 mm/year. It carries vast quantities
of sediment (about 5 Â 10
8 tonnes/year) with a high
clay and silt content, and the gradient of the lower part
of the river is extremely low (about 5 cm/km).
In the 200–300 years during which bathymetric
measurements have been taken in the area, it has been
possible to record considerable progradation. The high
clay and silt content gives the sediments great cohesion,
making the fluvial channels relatively stable. Clay and
silt sediments which are deposited on the delta plain
have a high porosity and water content (60–70%), but
they loose much of their porosity by compaction at
shallow depth. Sand will be deposited chiefly in the
channels, and in mouth bars were the channels enter
the sea. Well-sorted sandy sediments, which have only
about 40–45% porosity immediately after deposition,
will then sink into the underlying clay because of their
higher density. The fluvial channels therefore sink into
the mud and this contributes to their stabilisation, with
the consequence that channels change course (avulsion)
less frequently. Because of this subsidence while sedimentation is continuing, the channel sand may be
thicker than the depth of the channel. The long strings
of sand which may then be preserved in the mud-rich
environment are called bar-finger sands.
During floods large quantities of silt and clay are
deposited in overbank areas between channels, and
these sediments are stabilised by vegetation. In the
Mississippi delta, sedimentation is very rapid so that
the channel and the levees build up above their surroundings. This means that the average gradient of the
channel decreases. Sooner or later the channel will
have to find a new, shorter route (through avulsion)
to the ocean, and which therefore has a somewhat
greater slope. At any given time most of the sediments
are deposited in one delta lobe prograding into the
ocean until the slope becomes to low. A major avulsion will then start the formation of a new delta lobe.
We can see that the Mississippi has constantly
shifted course, and has consequently been a focus of
sedimentary activity in historic as well as modern
times (Fig. 2.32). The present course has extended
the modern delta far out and should have been abandoned for a shorter course towards the southwest, to
the Atchafalaya basin. However the flow in this direction has been artificially limited because of its importance for transport to and from towns like New Orleans
which lie on the present channel.
When a delta lobe is abandoned, it slowly subsides
because of compaction and tectonic subsidence, while
sedimentation takes place elsewhere. The abandoned
lobe may sink below sea level before the fluvial supply
returns to this part of the delta and another delta lobe is
deposited in the same area, allowing intervening deposition of thin marine beds. Thin layers of carbonate or
shale represent periods of local transgression (abandonment facies) which are time equivalent with
regressions in the prograding delta lobes. Between
the delta lobes in the interdistributary bay facies
wave energy is very low and there may be little or no
beach (sand) deposits between the marine mud and
mud deposited above sea level.
In vertical profile we observe alternations of fluvial
channel sediments, levee deposits, crevasse splays and
possibly marine sediments (Figs. 2.33 and 2.34).
Even small variations in sea level have a strong
influence on delta sedimentation. In a section where
a fluvial facies gives way to a marine bed, it is often
difficult to know whether this represents a transgression due to a rise in sea level or whether this part of the
delta was abandoned by the fluvial system and is
subsiding. Only if we can correlate a transgressive
bed over the whole area can we assume that it is due
to changes in sea level. Transgressive carbonate or
thin sandstone beds are the most useful for regional
correlation.
70
K. Bjørlykke