environment influenced by freshwater fluvial and saline
marine waters, the vegetative landscape generally contains species tolerant of both brackish and saline water.
For example, the lower modern Mississippi River delta
is characterized by Phragmites australis, an invasive species that tolerates fluctuating salinities, river stage, and
storms (White, 1993). The resiliency of this plant community is largely responsible for stabilizing deltaic deposits
and greatly reducing erosion and reworking during frequent hurricane impacts. Mangroves serve a similar function in low-latitude deltas, such as Ganges-Brahmaputra.
Subaqueous delta plain This is the shallow, seaward
sloping part of the delta that is characterized by sedimentation at the ends of distributary channels and the
reworking of these deposits onshore and laterally by
waves and tides. The subaqueous delta plain forms a broad
apron of interfingering deltaic and marine sediments that
transitions seaward into continental shelf sediments.
Major components of the subaqueous delta plain consist
of distributary bar mouth sands incised by channels bordered by subaqueous levees. Seaward of this region is an
area of increasingly finer-grained sediments and less
marine reworking as the depositional environment transitions from distributary bar mouth sand to sandy silt of
the delta front and finally to the silty clay of the prodelta.
At some deltas where progradation has reached at or close
to the shelf edge, sediment from distributaries is conveyed
directly into submarine canyons (e.g., Indus Goodbred
and Kuehl, 2000; Giosan et al., 2006; GangesBrahmaputra).
Deltaic sediments
In general, deltaic sedimentary lithosomes contain a coarsening upward sequence that reflects seaward progradation
of the delta into its receiving basin. At any given time,
there is typically a well-developed fining grain size pattern
extending seaward from the subaerial to subaqueous portions of a delta. The relatively coarsest-grained material
is deposited proximal to the river mouth where transport
competence is highest, whereas the finest-grained material
is carried farther seaward and deposited in more distal
locations. Thus, as sediments are delivered to the receiving basin from the river mouth, they accumulate in a subaqueous, fine-grained depositional zone referred to as the
prodelta. The prodelta forms the platform across which
the delta progrades and subsequently aggrades. The
prodelta sedimentary package is a widespread laterally
continuous interval composed primarily of the finest sediment fraction transported by the fluvial and ocean system.
Thus, the overall finest sediment is found at the base of the
prodelta sequence and the whole sequence coarsens
upward. Bioturbation of prodelta sediments is a function
of the rate of deposition.
The delta front is located between the fine-grained
prodelta deposition and the landward, coarser-grained distributary mouth deposits of a progradational deltaic
system. The relatively coarser-grained sediment of the
delta front generally consists of interbedded clays, silts,
and sands. This zone is dominated by the interaction of
fluvial and marine processes, resulting in sand-rich accumulations landward of the advancing prodelta that are
commonly a result of high-stage fluvial discharge or storm
reworking. They are usually highly bioturbated. The delta
front also represents a zone of transition between deposits
representative of progradation and aggradation.
In a progradational deltaic sequence, distributary channel deposits overlie the relatively finer-grained delta front
and prodelta deposits. The framework of distributary
channels consists of distributary mouth bars overlain and
bordered by natural levee deposits. Distributary mouth
bar deposits are primarily subaqueous deposits that grade
laterally into relatively finer-grained deposits; locally,
mouth bar deposits may contain fine-grained beds within
the generally sandy matrix of the mouth bar. The presence
of fine-grained deposits represents deposition during
low-stage conditions when current velocities are relatively
weak. During high-stage flood conditions, natural levees
can be overtopped and breached creating crevasse splays.
These splays occur within the distributary network and
provide conduits for sediment dispersal into interdistributary bays. Crevasse splays are one of the major landform processes in riverine-dominated deltas (e.g.,
Mississippi) contrasting to the preponderance of beach
ridges at wave-dominated deltas (e.g., Ebro, Danube).
However, crevassing can produce loss of stream power
downstream of the cut due to loss of flow, leading to
decreased channel competency and sedimentation within
the channel. A case study in South Pass, one of the Mississippi River distributaries, in Louisiana, USA, reported that
lateral flow loss from crevassing, including flow into a relict distributary channel, diminished stream power significantly such that South Pass aggraded immediately
downstream (Clark et al., 2013).
Interdistributary bay and marsh sediments are generally
as volumetrically significant as prodelta sediments. The
interdistributary area is a low-energy depositional environment and less dynamic than areas of the delta plain
characterized by multiple channels, channel bifurcations,
and channel avulsions. Interdistributary deposits characteristically consist of fining upward deposits consisting
of clay-rich bay sediments overlain by organic-rich marsh
deposits. This fine-grained depositional environment is
punctuated by sandy depositional events associated with
overbank flooding and crevasse splay events. Channels
within the crevasse splay also create elongated sand
deposits.
Delta processes
Deltas develop when sediment-laden river water decelerates upon entering a receiving basin and sediment transport
competence of the river is lost. Consequently, the interplay
between fluvial and river-mouth processes constitutes an
important controlling factor in the nature of deltaic
176
DELTAS
marine waters, the vegetative landscape generally contains species tolerant of both brackish and saline water.
For example, the lower modern Mississippi River delta
is characterized by Phragmites australis, an invasive species that tolerates fluctuating salinities, river stage, and
storms (White, 1993). The resiliency of this plant community is largely responsible for stabilizing deltaic deposits
and greatly reducing erosion and reworking during frequent hurricane impacts. Mangroves serve a similar function in low-latitude deltas, such as Ganges-Brahmaputra.
Subaqueous delta plain This is the shallow, seaward
sloping part of the delta that is characterized by sedimentation at the ends of distributary channels and the
reworking of these deposits onshore and laterally by
waves and tides. The subaqueous delta plain forms a broad
apron of interfingering deltaic and marine sediments that
transitions seaward into continental shelf sediments.
Major components of the subaqueous delta plain consist
of distributary bar mouth sands incised by channels bordered by subaqueous levees. Seaward of this region is an
area of increasingly finer-grained sediments and less
marine reworking as the depositional environment transitions from distributary bar mouth sand to sandy silt of
the delta front and finally to the silty clay of the prodelta.
At some deltas where progradation has reached at or close
to the shelf edge, sediment from distributaries is conveyed
directly into submarine canyons (e.g., Indus Goodbred
and Kuehl, 2000; Giosan et al., 2006; GangesBrahmaputra).
Deltaic sediments
In general, deltaic sedimentary lithosomes contain a coarsening upward sequence that reflects seaward progradation
of the delta into its receiving basin. At any given time,
there is typically a well-developed fining grain size pattern
extending seaward from the subaerial to subaqueous portions of a delta. The relatively coarsest-grained material
is deposited proximal to the river mouth where transport
competence is highest, whereas the finest-grained material
is carried farther seaward and deposited in more distal
locations. Thus, as sediments are delivered to the receiving basin from the river mouth, they accumulate in a subaqueous, fine-grained depositional zone referred to as the
prodelta. The prodelta forms the platform across which
the delta progrades and subsequently aggrades. The
prodelta sedimentary package is a widespread laterally
continuous interval composed primarily of the finest sediment fraction transported by the fluvial and ocean system.
Thus, the overall finest sediment is found at the base of the
prodelta sequence and the whole sequence coarsens
upward. Bioturbation of prodelta sediments is a function
of the rate of deposition.
The delta front is located between the fine-grained
prodelta deposition and the landward, coarser-grained distributary mouth deposits of a progradational deltaic
system. The relatively coarser-grained sediment of the
delta front generally consists of interbedded clays, silts,
and sands. This zone is dominated by the interaction of
fluvial and marine processes, resulting in sand-rich accumulations landward of the advancing prodelta that are
commonly a result of high-stage fluvial discharge or storm
reworking. They are usually highly bioturbated. The delta
front also represents a zone of transition between deposits
representative of progradation and aggradation.
In a progradational deltaic sequence, distributary channel deposits overlie the relatively finer-grained delta front
and prodelta deposits. The framework of distributary
channels consists of distributary mouth bars overlain and
bordered by natural levee deposits. Distributary mouth
bar deposits are primarily subaqueous deposits that grade
laterally into relatively finer-grained deposits; locally,
mouth bar deposits may contain fine-grained beds within
the generally sandy matrix of the mouth bar. The presence
of fine-grained deposits represents deposition during
low-stage conditions when current velocities are relatively
weak. During high-stage flood conditions, natural levees
can be overtopped and breached creating crevasse splays.
These splays occur within the distributary network and
provide conduits for sediment dispersal into interdistributary bays. Crevasse splays are one of the major landform processes in riverine-dominated deltas (e.g.,
Mississippi) contrasting to the preponderance of beach
ridges at wave-dominated deltas (e.g., Ebro, Danube).
However, crevassing can produce loss of stream power
downstream of the cut due to loss of flow, leading to
decreased channel competency and sedimentation within
the channel. A case study in South Pass, one of the Mississippi River distributaries, in Louisiana, USA, reported that
lateral flow loss from crevassing, including flow into a relict distributary channel, diminished stream power significantly such that South Pass aggraded immediately
downstream (Clark et al., 2013).
Interdistributary bay and marsh sediments are generally
as volumetrically significant as prodelta sediments. The
interdistributary area is a low-energy depositional environment and less dynamic than areas of the delta plain
characterized by multiple channels, channel bifurcations,
and channel avulsions. Interdistributary deposits characteristically consist of fining upward deposits consisting
of clay-rich bay sediments overlain by organic-rich marsh
deposits. This fine-grained depositional environment is
punctuated by sandy depositional events associated with
overbank flooding and crevasse splay events. Channels
within the crevasse splay also create elongated sand
deposits.
Delta processes
Deltas develop when sediment-laden river water decelerates upon entering a receiving basin and sediment transport
competence of the river is lost. Consequently, the interplay
between fluvial and river-mouth processes constitutes an
important controlling factor in the nature of deltaic
176
DELTAS