160
- Fluvial sediments of the prograding alluvial plain
consisting of point bars, channel fills, and finer
grained deposits of the flood plain (cf. Sect. 2.2.3).
The elevation of the actual delta front deposits is always adjusted to the sea level, but older equivalents
of the deposits are affected by sediment compaction
and subsidence ofthe crust under the increasing sediment load. Therefore, the delta front sediments buried below the prograding subaerial delta plain dip
slightly landward. The different types of sediment
accretion (i.e. pro gradation of prodelta and delta
front deposits) generate an unusual pattern of the
isochrones within the total delta complex (Fig. 3.34a)
and contrast to the prevailing vertical aggradation of
delta plain sediments.
(2) Destructional phase. Destruction of delta lobes
commonly takes place where main distributaries have
prograded too far into the sea and lose their sediment
supply by channel diversion. Such a development is
mostly initiated by breaches in the levees, crevasse
splays, and crevasse channels that find a shorter and
steeper course into the sea. Another cause of coastal
retreat is subsidence and/or rising sea level (see below). Destruction is brought about by wave action
and wave-induced currents (cf. Sect. 5.2) which predominantly rework and redistribute the river mouth
and delta front sands, the coastal sand barrier elose to
the distributaries, and part of the interdistributary
sediments behind the sand barrier.
This process may produce a chain of retreating
beach ridges and barrier islands (Fig. 3.34b) which
protect flooded portions of the delta plain from further erosion. As a result, widely extended shallow
bay muds and salt marsh deposits of limited thickness can accumulate.
In front of the retreating coastal sands, aveneer of
reworked, relatively coarse foreshore and shoreface
sands rnay rest on truncated mouth-bar sands or directly on prodelta muds. The sands frequently contain shell concentrations of mixed origin, part of
which are derived from eroded bay and lagoon deposits. Fine-grained material reworked from the
interdistributary areas is swept into deeper water by
currents.
Such a destructional phase continues until a new
equilibrium is established between the forces of the
sea and the capability of the accumulated sediments
to resist erosion. A new delta lobe may prograde and
Fig. 3.34. Facies association of a classical lobate
delta, generalized, with landward dipping coastal and
fluvial deposits due to continuous subsidence (similar to the Niger delta, see, e.g., Allen 1970; Doust
and Omatsola 1990). a Constructive phase with permanent outbuilding (progradation) of prodelta muds
and aggradation of sediments of the delta plain. Note
the unusual position of isochrones. b Discontinuous
Chapter 3 Coastal and Shallow Sea Sediments
incorporate the retreated coastal sands into the
subaerial delta plain of a subsequent constructional
phase.
(3) Vertical facies successions. Vertical sections in
delta sediments can be very variable, particularly in
cases, when the delta was subjected to destructional
periods (Fig. 3.34c through g). A characteristic feature is coarsening-upward of prodelta silts and elays,
mouth bar sands, and shore facelbeach ridge sands.
The opposite tendency is observed in vertical sections through the infillings of maj or distributary
channels and minor channels associated with crevasse splays. On or near the channel floor, mud
elasts, logs, and sometimes reworked nodules of
ironstone, formed during early diagenesis, are common. An irregular coarsening-upward trend is also
common in interdistributary sequences, i.e.,
- from lagoonal muds to tidal flats,
- from salt marsh and crevasse splay silts to sands,
or
- from lake bottomsets to lake delta foresets and
topsets of the alluvial delta plain.
Such coarsening-upward sections may occur repeatedly; they are mostly interpreted as autocyelic deltaic
sequences (cf. Sect. 7.1).
The most important sedimentary structures of
these subunits are described in the previous chapters.
Normally graded sand and silt layers may occur in
the prodelta environment as a result of density currents (cf. Fig. 3.32), as weIl as in crevasse splay sheet
sands and silts due to waning flow conditions after
peak flood. Delta abandonment and coastal retreat
lead to truncated vertical sequences (Fig. 3.34g).
Usually, the upper part of a prograding constructional sequence is more or less eroded and, after deposition of a thin vene er of reworked material, overlain by sediments of a younger constructional phase.
(4) Lateral facies transitions. The facies in the
prodelta region gradually change laterally from
prodelta silts and elays to shelf sediments (Fig. 3.28d
and e; cf. Sect. 3.3). In shallower water and particularly within the delta plain, the facies strongly vary
laterally and may display the following transitions:
progradation, interrupted by periods of partial delta
destruction (erosional faces J and 2), which may remove river mouth bar and subaqueous levee sands
and silts, as weIl as part of the primarily overlying
sediments of shallow bays, lagoons, marshland, and
lakes. c-g Vertical sections (locations shown in a and
b), see text for explanation
- Fluvial sediments of the prograding alluvial plain
consisting of point bars, channel fills, and finer
grained deposits of the flood plain (cf. Sect. 2.2.3).
The elevation of the actual delta front deposits is always adjusted to the sea level, but older equivalents
of the deposits are affected by sediment compaction
and subsidence ofthe crust under the increasing sediment load. Therefore, the delta front sediments buried below the prograding subaerial delta plain dip
slightly landward. The different types of sediment
accretion (i.e. pro gradation of prodelta and delta
front deposits) generate an unusual pattern of the
isochrones within the total delta complex (Fig. 3.34a)
and contrast to the prevailing vertical aggradation of
delta plain sediments.
(2) Destructional phase. Destruction of delta lobes
commonly takes place where main distributaries have
prograded too far into the sea and lose their sediment
supply by channel diversion. Such a development is
mostly initiated by breaches in the levees, crevasse
splays, and crevasse channels that find a shorter and
steeper course into the sea. Another cause of coastal
retreat is subsidence and/or rising sea level (see below). Destruction is brought about by wave action
and wave-induced currents (cf. Sect. 5.2) which predominantly rework and redistribute the river mouth
and delta front sands, the coastal sand barrier elose to
the distributaries, and part of the interdistributary
sediments behind the sand barrier.
This process may produce a chain of retreating
beach ridges and barrier islands (Fig. 3.34b) which
protect flooded portions of the delta plain from further erosion. As a result, widely extended shallow
bay muds and salt marsh deposits of limited thickness can accumulate.
In front of the retreating coastal sands, aveneer of
reworked, relatively coarse foreshore and shoreface
sands rnay rest on truncated mouth-bar sands or directly on prodelta muds. The sands frequently contain shell concentrations of mixed origin, part of
which are derived from eroded bay and lagoon deposits. Fine-grained material reworked from the
interdistributary areas is swept into deeper water by
currents.
Such a destructional phase continues until a new
equilibrium is established between the forces of the
sea and the capability of the accumulated sediments
to resist erosion. A new delta lobe may prograde and
Fig. 3.34. Facies association of a classical lobate
delta, generalized, with landward dipping coastal and
fluvial deposits due to continuous subsidence (similar to the Niger delta, see, e.g., Allen 1970; Doust
and Omatsola 1990). a Constructive phase with permanent outbuilding (progradation) of prodelta muds
and aggradation of sediments of the delta plain. Note
the unusual position of isochrones. b Discontinuous
Chapter 3 Coastal and Shallow Sea Sediments
incorporate the retreated coastal sands into the
subaerial delta plain of a subsequent constructional
phase.
(3) Vertical facies successions. Vertical sections in
delta sediments can be very variable, particularly in
cases, when the delta was subjected to destructional
periods (Fig. 3.34c through g). A characteristic feature is coarsening-upward of prodelta silts and elays,
mouth bar sands, and shore facelbeach ridge sands.
The opposite tendency is observed in vertical sections through the infillings of maj or distributary
channels and minor channels associated with crevasse splays. On or near the channel floor, mud
elasts, logs, and sometimes reworked nodules of
ironstone, formed during early diagenesis, are common. An irregular coarsening-upward trend is also
common in interdistributary sequences, i.e.,
- from lagoonal muds to tidal flats,
- from salt marsh and crevasse splay silts to sands,
or
- from lake bottomsets to lake delta foresets and
topsets of the alluvial delta plain.
Such coarsening-upward sections may occur repeatedly; they are mostly interpreted as autocyelic deltaic
sequences (cf. Sect. 7.1).
The most important sedimentary structures of
these subunits are described in the previous chapters.
Normally graded sand and silt layers may occur in
the prodelta environment as a result of density currents (cf. Fig. 3.32), as weIl as in crevasse splay sheet
sands and silts due to waning flow conditions after
peak flood. Delta abandonment and coastal retreat
lead to truncated vertical sequences (Fig. 3.34g).
Usually, the upper part of a prograding constructional sequence is more or less eroded and, after deposition of a thin vene er of reworked material, overlain by sediments of a younger constructional phase.
(4) Lateral facies transitions. The facies in the
prodelta region gradually change laterally from
prodelta silts and elays to shelf sediments (Fig. 3.28d
and e; cf. Sect. 3.3). In shallower water and particularly within the delta plain, the facies strongly vary
laterally and may display the following transitions:
progradation, interrupted by periods of partial delta
destruction (erosional faces J and 2), which may remove river mouth bar and subaqueous levee sands
and silts, as weIl as part of the primarily overlying
sediments of shallow bays, lagoons, marshland, and
lakes. c-g Vertical sections (locations shown in a and
b), see text for explanation