2.31 Delta Front Sedimentation
At the channel mouth the fluvial water flow rapidly
loses its energy, and all the sand which has been
transported along the bottom (the bed load) is deposited in the form of a channel mouth bar (Fig. 2.34).
Most of the suspended material is also deposited
fairly rapidly, at a relatively short distance from the
mouth. While the river water, which is lightest, flows
out of the channel, salt and brackish water flow back
in along the bottom, and may encroach a long way up
the channel when the rate of fluvial flow is low.
Marine fossils can therefore be transported quite a
way into the fluvial environment. During floods the
saltwater wedge is forced back over the bank at the
mouth. The wedge reduces the cross-section of the
river, and the velocity increases somewhat in consequence (Fig. 2.35). Spring tides will force the river
water up the channel and may cause it to overflow its
banks.
The channel mouth bar itself is a very characteristic
deposit which grades upwards from delta mud to wellsorted sand on top. Because of brackish water and the
high rate of sedimentation, there are few organisms
which live right at the mouth of the channel, but
bioturbation may occur in the surrounding sediments.
The delta front is the area where fluvial and marine
forces meet, and we can virtually quantify the marine
influence on the delta. Tidal forces are a function of
the tidal range, and from wave measurements we can
F
D
A
B
Modern Mississippi River subdeltas
A Dry Cypress Bayou complex
B Grand Liard complex
C West Bay complex
D Cubits Gap complex
E Babtiste Collette complex
F Garden Island Bay complex
C
E
East Bay
West Bay
Garden
Island
Bay
S o u th
P a s s
Burrwood
Port
Lads
P a ss L o u tr e
S o u th w e s t P a s s
1838
1862
1891
29°15'
29°00'
89°30'
89°15'
89°00'
0
15 km
Carto Sect.,Csi,Lsu
N
b
Gulf of Mexico
1874
1
3
5
6
7
4
2
New
Orleans
a
Baton
Rouge
Prairie Terrace
Sale Cypremort
1
2
3
Cocodrie
Teche
St. Bernard
Lafourche
Plaquemine
Balize
4
5
6
7
M is s . R .
30°
29°
92°
91°
90°
89°
0
5 0 k m
After Kolb & Van Lopik, 1958
N
Fig. 2.32 Sedimentation in the modern Mississippi delta. When the river water breaks through the levees, crevasse channels and
splays are formed, which help to fill the areas between the channels (Coleman and Prior 1980). (a) Delta lobes which show how the
sedimentation has changed during the past 7,000 years. Each lobe of the delta appears to be active for 1,000–1,500 years (Coleman
and Prior 1980). (b) Sedimentation in the last few 100 years
Delta plain
silty sand and clay
Delta front
clay and silt,
little fauna
Prodelta clay
rich in fauna
Clay
diapir
Pure sand
Sand and
silt
Levée
Swamp
Fig. 2.33 Fluvial channels in a clay-rich delta environment. The channel sand is denser than the clay and will sink into the
underlying mud. Clay diapirs may form as a result
2 Introduction to Sedimentology
71
At the channel mouth the fluvial water flow rapidly
loses its energy, and all the sand which has been
transported along the bottom (the bed load) is deposited in the form of a channel mouth bar (Fig. 2.34).
Most of the suspended material is also deposited
fairly rapidly, at a relatively short distance from the
mouth. While the river water, which is lightest, flows
out of the channel, salt and brackish water flow back
in along the bottom, and may encroach a long way up
the channel when the rate of fluvial flow is low.
Marine fossils can therefore be transported quite a
way into the fluvial environment. During floods the
saltwater wedge is forced back over the bank at the
mouth. The wedge reduces the cross-section of the
river, and the velocity increases somewhat in consequence (Fig. 2.35). Spring tides will force the river
water up the channel and may cause it to overflow its
banks.
The channel mouth bar itself is a very characteristic
deposit which grades upwards from delta mud to wellsorted sand on top. Because of brackish water and the
high rate of sedimentation, there are few organisms
which live right at the mouth of the channel, but
bioturbation may occur in the surrounding sediments.
The delta front is the area where fluvial and marine
forces meet, and we can virtually quantify the marine
influence on the delta. Tidal forces are a function of
the tidal range, and from wave measurements we can
F
D
A
B
Modern Mississippi River subdeltas
A Dry Cypress Bayou complex
B Grand Liard complex
C West Bay complex
D Cubits Gap complex
E Babtiste Collette complex
F Garden Island Bay complex
C
E
East Bay
West Bay
Garden
Island
Bay
S o u th
P a s s
Burrwood
Port
Lads
P a ss L o u tr e
S o u th w e s t P a s s
1838
1862
1891
29°15'
29°00'
89°30'
89°15'
89°00'
0
15 km
Carto Sect.,Csi,Lsu
N
b
Gulf of Mexico
1874
1
3
5
6
7
4
2
New
Orleans
a
Baton
Rouge
Prairie Terrace
Sale Cypremort
1
2
3
Cocodrie
Teche
St. Bernard
Lafourche
Plaquemine
Balize
4
5
6
7
M is s . R .
30°
29°
92°
91°
90°
89°
0
5 0 k m
After Kolb & Van Lopik, 1958
N
Fig. 2.32 Sedimentation in the modern Mississippi delta. When the river water breaks through the levees, crevasse channels and
splays are formed, which help to fill the areas between the channels (Coleman and Prior 1980). (a) Delta lobes which show how the
sedimentation has changed during the past 7,000 years. Each lobe of the delta appears to be active for 1,000–1,500 years (Coleman
and Prior 1980). (b) Sedimentation in the last few 100 years
Delta plain
silty sand and clay
Delta front
clay and silt,
little fauna
Prodelta clay
rich in fauna
Clay
diapir
Pure sand
Sand and
silt
Levée
Swamp
Fig. 2.33 Fluvial channels in a clay-rich delta environment. The channel sand is denser than the clay and will sink into the
underlying mud. Clay diapirs may form as a result
2 Introduction to Sedimentology
71