3.5 Marine Delta Complexes
Strong waves and tides hamper or prevent the out
building of fluvial sediments into the sea, whereas
low-energy conditions favor this process. Using these
criteria, one can distinguish several types of marine
deltas (Fig. 3.31b):
Fluvial-dominated birdfoot delta, elongate. Due to
high sediment input and low counter-action by the
sea forces (low-energy conditions), the single main
distributaries of the delta prograde seaward separately and create a coastline similar to the shape of a
birdfoot. A prominent example of this type is the
modem Mississippi delta.
Mixed fluvial- to wave-dominated, "classical"
delta, lobate. Higher wave energy, which may be
accompanied by moderatetides, prevents the separate
outbuilding of fluvial systems. Instead, all
distributaries advance more or less uniformly and
thus generate a characteristic protruding lobate or
triangular coastline.
The Greek compared this delta shape with their capital
letter "delta". Many modem rivers entering low-energy
adjacent seas build this type of delta, for example the Danube into the Black Sea, the Nile and Ebro into the Mediterranean (e.g., Sestini 1989). In all these cases, the influence
oftides can be neglected.
However, the deltas of the Niger and Orinoco entering
the Atlantic Ocean also belong to this type, although they
are moderately affected by tides.
Wave-dominated deltas, cuspate. Strong wave action prevents the local outbuilding of a delta front.
Temporarily deposited river load is reworked and
transported from the river mouth along the high-energy coast or into deeper water. Splitting of the main
river into several distributaries and avulsion (shifting) of channels are less frequent than in birdfoot
deltas. Overall, wave-dominated deltas advance more
slowly over a broader front.
Modem examples include the deltas of the Rhone (Mediterranean), the Brazos entering the Gulf ofMexico, and the
Sao Francisco and other South American rivers discharging
into the South Atlantic.
Tide-dominated delta, estuarine. Strong tides and
tidal currents migrate some distance up the river
(Sect. 3.2) and therefore widen the river mouth to
form an estuary. In combination with wave action,
the river load is swept out into the shallow sea or
transported longshore into areas of lower energy conditions, where it can settle in tidal flats or deeper water. The main river may split into several distributaries with islands in between, and part of the sand
load forms separate subaqueous sand ridges in front
of the river mouth. The coastline does not protrude
seaward and the total delta complex pro grades comparatively slowly.
155
Present-day examples of this delta type are the GangesBrahmaputra River entering the Bay of Bengal, the Colorado Delta at the northem end of the Gulf of Califomia,
and the Rhine Delta in the Netherlands.
3.5.3 Sedimentary Processes and Facies
of Various Delta Types
Fluvial-Dominated and Lobate Deltas
Distributary Channels and Levees
The pro ces ses operating at the mouth of an individual distributary channel of the elongate to lobate
delta type are schematically shown in Fig. 3.32. On
both sides of the channel, flat natural levees are built
up during times of peak flood. In places where the
levees are not high enough, the flood spills over into
the adjacent marshland and generates sandy crevasse
splays. The levees consist of fine sand and silt (cf.
Sect. 2.2.3) and can also be traced as subaqueous
sand bars from the river mouth some distance into
the sea. Because large delta areas usually subside but
are flooded discontinuously, their sub aerial levees
display irregular vertical aggradation. Plant cover and
soils may develop in times of non-deposition. The
walls of channels cutting such levees, marsh deposits, or bay muds can be very steep, but the channels
often migrate laterally, eroding one of their banks
and aggrading on the other. Channels mayaiso be
filled with sand bars or mud.
Interdistributary Delta Plain
The interdistributary space between the rapidly
progradicg subaerial levees of a birdfoot delta or
behind the uniformly advancing beach ridge barrier
of a lobate delta is occupied by marshland, swamps,
and fresh water lakes. In a tidal regime with outlets
to the sea, tidal flats develop behind the beach ridge
barrier. The sediments of these areas are predominantly siliciclastic and strongly influenced by the
climate ofthe region (cf. Sect. 3.2).
Under humid conditions, the interdistributary, slowly subsiding swamps offer ideal prerequisites for the accumulation and preservation of peat and allochthonous plant debris. Many coal deposits were formed in such an environment, and hydrocarbons found in deltaic sediments also
partially derive from source rocks of this type (Whateley
and Pickering 1989).
Shallow, large lakes developing in the upper part of the
delta plain may be filled by rapidly prograding lacustrine
deltas fed by fine-grained material of the main river as observed on the Mississippi delta plain (Tye and Coleman
1989). These deltaic sediments represent short basin-filling
episodes and differ from the common, Gi/bert-type lacustrine deltaic sequence (Sect. 2.5.3) in displaying extensive
Strong waves and tides hamper or prevent the out
building of fluvial sediments into the sea, whereas
low-energy conditions favor this process. Using these
criteria, one can distinguish several types of marine
deltas (Fig. 3.31b):
Fluvial-dominated birdfoot delta, elongate. Due to
high sediment input and low counter-action by the
sea forces (low-energy conditions), the single main
distributaries of the delta prograde seaward separately and create a coastline similar to the shape of a
birdfoot. A prominent example of this type is the
modem Mississippi delta.
Mixed fluvial- to wave-dominated, "classical"
delta, lobate. Higher wave energy, which may be
accompanied by moderatetides, prevents the separate
outbuilding of fluvial systems. Instead, all
distributaries advance more or less uniformly and
thus generate a characteristic protruding lobate or
triangular coastline.
The Greek compared this delta shape with their capital
letter "delta". Many modem rivers entering low-energy
adjacent seas build this type of delta, for example the Danube into the Black Sea, the Nile and Ebro into the Mediterranean (e.g., Sestini 1989). In all these cases, the influence
oftides can be neglected.
However, the deltas of the Niger and Orinoco entering
the Atlantic Ocean also belong to this type, although they
are moderately affected by tides.
Wave-dominated deltas, cuspate. Strong wave action prevents the local outbuilding of a delta front.
Temporarily deposited river load is reworked and
transported from the river mouth along the high-energy coast or into deeper water. Splitting of the main
river into several distributaries and avulsion (shifting) of channels are less frequent than in birdfoot
deltas. Overall, wave-dominated deltas advance more
slowly over a broader front.
Modem examples include the deltas of the Rhone (Mediterranean), the Brazos entering the Gulf ofMexico, and the
Sao Francisco and other South American rivers discharging
into the South Atlantic.
Tide-dominated delta, estuarine. Strong tides and
tidal currents migrate some distance up the river
(Sect. 3.2) and therefore widen the river mouth to
form an estuary. In combination with wave action,
the river load is swept out into the shallow sea or
transported longshore into areas of lower energy conditions, where it can settle in tidal flats or deeper water. The main river may split into several distributaries with islands in between, and part of the sand
load forms separate subaqueous sand ridges in front
of the river mouth. The coastline does not protrude
seaward and the total delta complex pro grades comparatively slowly.
155
Present-day examples of this delta type are the GangesBrahmaputra River entering the Bay of Bengal, the Colorado Delta at the northem end of the Gulf of Califomia,
and the Rhine Delta in the Netherlands.
3.5.3 Sedimentary Processes and Facies
of Various Delta Types
Fluvial-Dominated and Lobate Deltas
Distributary Channels and Levees
The pro ces ses operating at the mouth of an individual distributary channel of the elongate to lobate
delta type are schematically shown in Fig. 3.32. On
both sides of the channel, flat natural levees are built
up during times of peak flood. In places where the
levees are not high enough, the flood spills over into
the adjacent marshland and generates sandy crevasse
splays. The levees consist of fine sand and silt (cf.
Sect. 2.2.3) and can also be traced as subaqueous
sand bars from the river mouth some distance into
the sea. Because large delta areas usually subside but
are flooded discontinuously, their sub aerial levees
display irregular vertical aggradation. Plant cover and
soils may develop in times of non-deposition. The
walls of channels cutting such levees, marsh deposits, or bay muds can be very steep, but the channels
often migrate laterally, eroding one of their banks
and aggrading on the other. Channels mayaiso be
filled with sand bars or mud.
Interdistributary Delta Plain
The interdistributary space between the rapidly
progradicg subaerial levees of a birdfoot delta or
behind the uniformly advancing beach ridge barrier
of a lobate delta is occupied by marshland, swamps,
and fresh water lakes. In a tidal regime with outlets
to the sea, tidal flats develop behind the beach ridge
barrier. The sediments of these areas are predominantly siliciclastic and strongly influenced by the
climate ofthe region (cf. Sect. 3.2).
Under humid conditions, the interdistributary, slowly subsiding swamps offer ideal prerequisites for the accumulation and preservation of peat and allochthonous plant debris. Many coal deposits were formed in such an environment, and hydrocarbons found in deltaic sediments also
partially derive from source rocks of this type (Whateley
and Pickering 1989).
Shallow, large lakes developing in the upper part of the
delta plain may be filled by rapidly prograding lacustrine
deltas fed by fine-grained material of the main river as observed on the Mississippi delta plain (Tye and Coleman
1989). These deltaic sediments represent short basin-filling
episodes and differ from the common, Gi/bert-type lacustrine deltaic sequence (Sect. 2.5.3) in displaying extensive