156
distributary mouth-bar sands and paralIel-laminated
prodelta muds instead of coarser grained, steeply inelined
prodelta foresets.
River Mouth and Delta Front
The bedload of the distributary channels is deposited
directly in front of the subaerial delta, commonly at
water depths in the range of 5 to 30 m. The sediment
load of large river systems draining extensive alluvial
plains (apart from highlands far from the coast) is
commonly fine grained. Consequently, the river
mouth bars are composed rnainly of fine sand displaying large- and small-scale trough cross-bedding
of rather consistent current direction (Fig. 3.32b). At
a lobate delta, river mouth bars, subaqueous levees,
and foreshore sands of the beach ridge barrier may
combine to form a more or less continuous sand
sheet (delta front sands) of locally varying thickness
(5 to 30 m).
The beach ridge barrier is fed by sands transported alongshore from the river mouths. Separately
advancing channels of the birdfoot delta type, however, generate isolated elongate sand bodies, the socalled bar finger sands (Fig. 3.32b). Typical features
of a rapidly prograding delta front are growth faults,
slope gullies, and mud diapirs originating from differential compaction and failure of under-consolidated sediments (see below).
Delta Platform, Prodelta Slope and Shelf
The river mouth and its accompanying shoreface
sands prograde as the delta front. During times of
peak flood, however, the river water with its suspended load rnay become denser than sea water.
Then it flows basinward as undercurrent and drops
its sandy and silty bedload in deeper water.
The delta platform develops seaward of the delta
front at shallow water depths (10 to 30 m). This feature results from the combined action of rapid deposition and wave action during the constructive phase,
but mayaiso be shaped during a subsequent destructive phase ofthe delta development (see below).
The prodelta slope farther offshore is generally
very gently inclined (less than one up to a few degrees) toward the floor of a shelf sea or a deeper oceanic basin. A great part of the silt- and clay-sized
material of the river load is deposited on the prodelta
slope.
The transport and settling behavior of sand and silt partieIes is little affected at the transition from fresh water to sea
water. The suspended elay minerals, however, tend to
flocculate and form aggregates as soon as they pass, under
conditions of normal river discharge, from the less dense,
overlying fresh-water wedge into the zone of mixed brackChapter 3 Coastal and Shallow Sea Sediments
ish water or normal sea water (Fig. 3.32. The clay aggregates sink much faster than isolated elay particles and thus
significantly contribute to the prodelta slope sediments.
Since comparatively large clay minerals such as
kaolinite produce denser aggregates and settle earlier than
do very small ones (e.g. montmorillonite), these minerals
can be separated from each other (lateral fractionation).
Kaolinite tends to accumulate near the river mouth, while
illite and montmorillonite are transported further into
deeper water.
The shelf sediments in front of a large delta also tend
to be dominated by fine-grained siliciclastic material
delivered by the river. With decreasing sedimentation
rates bioturbation becomes a significant process
masking or obliterating all kinds of bedding.
Sedimentary Strdctures
Characteristic sedimentary structures of all these environments are shown in Fig. 3.32b through e (see
also Sects. 2.5 and 3.1 through 3.3). Of particular
interest are the prodelta sediments, because they usually reach considerable thiclmesses and have a good
chance of being preserved, even if the upper part of
the deltaic sequence is eroded. The clayey silts or
silty clays (hernipelagic terrigenous material) of
prodelta sediments typically display thin (mm to cm)
intercalations of fine sand or coarse silt reflecting
episodes of rninor and major river floods. Plant fragments rnay be abundant, but the percentage of
autochthonous marine biogenic components is generally low. Due to a high sedimentation rate (on the
order of a few mm to tens of cm per year), burrowing
by bottom dwelling organisms is sparse. Therefore,
primary larninations within the clayey silts, although
indistinct and irregular, are frequently preserved.
Growth Faults and "Depobelts"
The prograding pro delta sediments of major rivers
form huge clastic wedges which may reach thicknesses in excess of 10 km and overlie oceanic crust
(e.g., at the deltas of the Ganges-Brahrnaputra in
Bangladesh and the Niger in West Africa). Such sediment wedges rnay be strongly affected by syn-sedimentary and postsedimentary normal faults. In these
cases, the prograding delta complex can be divided
into a number of major, growth fault-bounded sedimentary units or "depobelts". Within each depobelt,
the sediment buildup may occur stepwise or cyclic in
response to continued deformation and sea-Ievel
changes. Even the gentle foot of the prodelta slope
may undergo deformation characterized by
imbricated "toe thrusts".
distributary mouth-bar sands and paralIel-laminated
prodelta muds instead of coarser grained, steeply inelined
prodelta foresets.
River Mouth and Delta Front
The bedload of the distributary channels is deposited
directly in front of the subaerial delta, commonly at
water depths in the range of 5 to 30 m. The sediment
load of large river systems draining extensive alluvial
plains (apart from highlands far from the coast) is
commonly fine grained. Consequently, the river
mouth bars are composed rnainly of fine sand displaying large- and small-scale trough cross-bedding
of rather consistent current direction (Fig. 3.32b). At
a lobate delta, river mouth bars, subaqueous levees,
and foreshore sands of the beach ridge barrier may
combine to form a more or less continuous sand
sheet (delta front sands) of locally varying thickness
(5 to 30 m).
The beach ridge barrier is fed by sands transported alongshore from the river mouths. Separately
advancing channels of the birdfoot delta type, however, generate isolated elongate sand bodies, the socalled bar finger sands (Fig. 3.32b). Typical features
of a rapidly prograding delta front are growth faults,
slope gullies, and mud diapirs originating from differential compaction and failure of under-consolidated sediments (see below).
Delta Platform, Prodelta Slope and Shelf
The river mouth and its accompanying shoreface
sands prograde as the delta front. During times of
peak flood, however, the river water with its suspended load rnay become denser than sea water.
Then it flows basinward as undercurrent and drops
its sandy and silty bedload in deeper water.
The delta platform develops seaward of the delta
front at shallow water depths (10 to 30 m). This feature results from the combined action of rapid deposition and wave action during the constructive phase,
but mayaiso be shaped during a subsequent destructive phase ofthe delta development (see below).
The prodelta slope farther offshore is generally
very gently inclined (less than one up to a few degrees) toward the floor of a shelf sea or a deeper oceanic basin. A great part of the silt- and clay-sized
material of the river load is deposited on the prodelta
slope.
The transport and settling behavior of sand and silt partieIes is little affected at the transition from fresh water to sea
water. The suspended elay minerals, however, tend to
flocculate and form aggregates as soon as they pass, under
conditions of normal river discharge, from the less dense,
overlying fresh-water wedge into the zone of mixed brackChapter 3 Coastal and Shallow Sea Sediments
ish water or normal sea water (Fig. 3.32. The clay aggregates sink much faster than isolated elay particles and thus
significantly contribute to the prodelta slope sediments.
Since comparatively large clay minerals such as
kaolinite produce denser aggregates and settle earlier than
do very small ones (e.g. montmorillonite), these minerals
can be separated from each other (lateral fractionation).
Kaolinite tends to accumulate near the river mouth, while
illite and montmorillonite are transported further into
deeper water.
The shelf sediments in front of a large delta also tend
to be dominated by fine-grained siliciclastic material
delivered by the river. With decreasing sedimentation
rates bioturbation becomes a significant process
masking or obliterating all kinds of bedding.
Sedimentary Strdctures
Characteristic sedimentary structures of all these environments are shown in Fig. 3.32b through e (see
also Sects. 2.5 and 3.1 through 3.3). Of particular
interest are the prodelta sediments, because they usually reach considerable thiclmesses and have a good
chance of being preserved, even if the upper part of
the deltaic sequence is eroded. The clayey silts or
silty clays (hernipelagic terrigenous material) of
prodelta sediments typically display thin (mm to cm)
intercalations of fine sand or coarse silt reflecting
episodes of rninor and major river floods. Plant fragments rnay be abundant, but the percentage of
autochthonous marine biogenic components is generally low. Due to a high sedimentation rate (on the
order of a few mm to tens of cm per year), burrowing
by bottom dwelling organisms is sparse. Therefore,
primary larninations within the clayey silts, although
indistinct and irregular, are frequently preserved.
Growth Faults and "Depobelts"
The prograding pro delta sediments of major rivers
form huge clastic wedges which may reach thicknesses in excess of 10 km and overlie oceanic crust
(e.g., at the deltas of the Ganges-Brahrnaputra in
Bangladesh and the Niger in West Africa). Such sediment wedges rnay be strongly affected by syn-sedimentary and postsedimentary normal faults. In these
cases, the prograding delta complex can be divided
into a number of major, growth fault-bounded sedimentary units or "depobelts". Within each depobelt,
the sediment buildup may occur stepwise or cyclic in
response to continued deformation and sea-Ievel
changes. Even the gentle foot of the prodelta slope
may undergo deformation characterized by
imbricated "toe thrusts".