158
Wave-Dominated Deltas
These deltas are characterized by beach
ridge/foreshore sands advancing with the active delta
lobe in a wide coastal-parallel front basinward (Fig.
3.33a). In relation to the wave energy and wave base,
these coastal barrier sands form sheets of considerable thiclmess (up to tens of meters). Due to intensive reworking, the coastal barrier sands tend to be
better sorted than the distributary channel and mouth
bar sands of the fluvial-dominated deltas. They
coarsen upward as a result of the prograding
shoreface and often contain abundant shell debris
which is produced by shallow-water organisms and
added during the processes of coastal sand transport
(Fig. 3.33a and c). Sideward shifting and return of
the active delta lobe to the subsiding initial site may
generate a second or several coastal barrier sand
sheets on top of the first one (see below). Behind the
active coastal beach ridge, the space between older
barrier sands and other parts of the interdistributary
area may be occupied by mangrove swamps (tropical
to subtropical clirnate), marshes, or lakes.
The suspended river load of wave-dominated deltas is usually removed from the river mouths and
widely distributed in the adjacent basins. As a result,
the prodelta facies of fluvial-dominated deltas is
poorly developed or more or less missing.
Tide-Dominated Deltas
In contrast to the other delta types, tide-dominated
deltas lack an advancing subaerial delta front. They
accumulate most of the river sediment in front of
their wide, estuarine mouth in shallow water orin the
adjacent deeper sea (Fig. 3.33b). Thus, an extensive
subaqueous delta platform is produced, on which at
least part of the bedload and suspended river load
comes to rest. Unlike the fluvial-dominated deltas,
the sands are not dropped as localized mouth bar
sands, but repeatedly reworked and redistributed by
tidal currents mainly directed perpendicular to the
general coastline. Hence, the river-borne sand forms
elongate subparallel ridges between the estuarine
distributaries of the river mouth, as weIl as on the
deeper delta platform. The sideward migrating
distributaries are filled with tidal current-influenced
sands, exhibiting bidirectional cross-bedding and
ripple lamination, in addition to estuarine muds with
mixed fauna (fresh water to normal saline).
Although the estuaries of a tide-dominated delta
give little evidence of progradation, the total complex
of such a delta also slowly advances seaward. The
resulting facies association, however, resembles that
of anormal tide-dominated coastline (some distance
away from an entering river, see Sect. 3.2) rather
than the associations found in the other delta types. A
Chapter 3 Coastal and Shallow Sea Sediments
typical vertical facies succession of such a constructional phase includes (from top to bottom):
- Alluvial plain sediments.
- Supratidal marsh or, depending on the climate,
evaporitic beds of a coastal sabkha, or other
supratidal deposits.
- Extensive intertidal sediments.
- Point bar and tidal-influenced sands of distributary
channels, estuarine muds with variable fauna.
- Tidal-influenced sediments of the delta platform
with subtidal sand ridges and sandwaves.
- Shelf sediments, still more or less influenced by
tidal currents.
Figures 3.33d and e display characteristic vertical
successions of an estuarine channel fill and a sand
ridge on adelta platform. Coastal barrier sands playa
subordinate role in this type of delta (for further details see Sects. 3.2 and 4.2).
3.5.4 Constructional and Destructional
Delta Phases
Facies Architecture of Lobate Deltas
The final facies architecture of a delta complex is
controlled not only by its constructional phase during
outbuilding, but also by destructional periods of reworking and redistribution of sediments delivered by
rivers. The lobate and wave-dominated delta types
are less susceptible to extensive destruction than the
birdfoot delta. In the latter case, the far seaward protruding distributaries can be reworked easily by
waves and currents, especially when they are cut off
from their sediment source by crevasse splays which
evolve into new distributaries.
(1) Constructional phase. Figure 3.34a shows the
pure constructional facies association of a lobate
delta consisting of five main facies groups (from bottom to top):
- Prodelta muds (with some intercalated silt and sand
layers) prograding seaward.
- River mouth and coastal barrier sands (delta front
sands) prograding seaward similar to the prodelta
muds.
- Point bars and infillings of distributary channels
(predominantly sand) and levee sands and silts (possibly including soils).
- Sediments of the interdistributary delta plain (bay
or lagoonal muds, possibly sediments of tidal flats,
marshes, lakes) , locally overlain or interrupted by
crevasse splay silts. Delta plain deposits are characterized by relatively slow upbuilding (aggradation)
on top of the subsiding delta complex.
Wave-Dominated Deltas
These deltas are characterized by beach
ridge/foreshore sands advancing with the active delta
lobe in a wide coastal-parallel front basinward (Fig.
3.33a). In relation to the wave energy and wave base,
these coastal barrier sands form sheets of considerable thiclmess (up to tens of meters). Due to intensive reworking, the coastal barrier sands tend to be
better sorted than the distributary channel and mouth
bar sands of the fluvial-dominated deltas. They
coarsen upward as a result of the prograding
shoreface and often contain abundant shell debris
which is produced by shallow-water organisms and
added during the processes of coastal sand transport
(Fig. 3.33a and c). Sideward shifting and return of
the active delta lobe to the subsiding initial site may
generate a second or several coastal barrier sand
sheets on top of the first one (see below). Behind the
active coastal beach ridge, the space between older
barrier sands and other parts of the interdistributary
area may be occupied by mangrove swamps (tropical
to subtropical clirnate), marshes, or lakes.
The suspended river load of wave-dominated deltas is usually removed from the river mouths and
widely distributed in the adjacent basins. As a result,
the prodelta facies of fluvial-dominated deltas is
poorly developed or more or less missing.
Tide-Dominated Deltas
In contrast to the other delta types, tide-dominated
deltas lack an advancing subaerial delta front. They
accumulate most of the river sediment in front of
their wide, estuarine mouth in shallow water orin the
adjacent deeper sea (Fig. 3.33b). Thus, an extensive
subaqueous delta platform is produced, on which at
least part of the bedload and suspended river load
comes to rest. Unlike the fluvial-dominated deltas,
the sands are not dropped as localized mouth bar
sands, but repeatedly reworked and redistributed by
tidal currents mainly directed perpendicular to the
general coastline. Hence, the river-borne sand forms
elongate subparallel ridges between the estuarine
distributaries of the river mouth, as weIl as on the
deeper delta platform. The sideward migrating
distributaries are filled with tidal current-influenced
sands, exhibiting bidirectional cross-bedding and
ripple lamination, in addition to estuarine muds with
mixed fauna (fresh water to normal saline).
Although the estuaries of a tide-dominated delta
give little evidence of progradation, the total complex
of such a delta also slowly advances seaward. The
resulting facies association, however, resembles that
of anormal tide-dominated coastline (some distance
away from an entering river, see Sect. 3.2) rather
than the associations found in the other delta types. A
Chapter 3 Coastal and Shallow Sea Sediments
typical vertical facies succession of such a constructional phase includes (from top to bottom):
- Alluvial plain sediments.
- Supratidal marsh or, depending on the climate,
evaporitic beds of a coastal sabkha, or other
supratidal deposits.
- Extensive intertidal sediments.
- Point bar and tidal-influenced sands of distributary
channels, estuarine muds with variable fauna.
- Tidal-influenced sediments of the delta platform
with subtidal sand ridges and sandwaves.
- Shelf sediments, still more or less influenced by
tidal currents.
Figures 3.33d and e display characteristic vertical
successions of an estuarine channel fill and a sand
ridge on adelta platform. Coastal barrier sands playa
subordinate role in this type of delta (for further details see Sects. 3.2 and 4.2).
3.5.4 Constructional and Destructional
Delta Phases
Facies Architecture of Lobate Deltas
The final facies architecture of a delta complex is
controlled not only by its constructional phase during
outbuilding, but also by destructional periods of reworking and redistribution of sediments delivered by
rivers. The lobate and wave-dominated delta types
are less susceptible to extensive destruction than the
birdfoot delta. In the latter case, the far seaward protruding distributaries can be reworked easily by
waves and currents, especially when they are cut off
from their sediment source by crevasse splays which
evolve into new distributaries.
(1) Constructional phase. Figure 3.34a shows the
pure constructional facies association of a lobate
delta consisting of five main facies groups (from bottom to top):
- Prodelta muds (with some intercalated silt and sand
layers) prograding seaward.
- River mouth and coastal barrier sands (delta front
sands) prograding seaward similar to the prodelta
muds.
- Point bars and infillings of distributary channels
(predominantly sand) and levee sands and silts (possibly including soils).
- Sediments of the interdistributary delta plain (bay
or lagoonal muds, possibly sediments of tidal flats,
marshes, lakes) , locally overlain or interrupted by
crevasse splay silts. Delta plain deposits are characterized by relatively slow upbuilding (aggradation)
on top of the subsiding delta complex.