with clay and sand banks cut by channels which transport tidal water. The Thames and the Rhine are
examples of modern rivers with well-developed tidal
estuaries.
The mixing of fresh and marine water also causes
flocculation of mud and deposition in the more
protected parts of the estuary between sand bars. Estuarine deposits are therefore characterised by a mixture
of sandstone and mudstone, often with marine fossils
which may be transported several kilometres upstream
by the salt wedges along the bottom. Tidal flats are
often found in the inner parts of the estuary. Tidal
channels are oriented at a high angle to the coastline,
but in the outer part waves may produce elongated
bars which are oriented parallel to the coastline. The
geometry and orientation of the sandbars are very
different from those associated with wave-dominated
deltas and coastlines, where the sand bars are parallel
to the coast.
Tide-dominated deltas are characterised by the fact
that the erosion due to tidal currents is very strong
compared to erosion by waves. The fluvial channels
have cross-sections which are too small to allow transport of water out and in, and the result is the development of a broad belt of tidal channels, which farthest
out are separated only by narrow ridges of sand and
clay. Very often separate ebb and flow channels
develop, and in each channel one transport direction
will dominate. Signs of transportation in two opposite
directions have often been used as a criterion for tidal
deposits. This will apply to the whole area, but we
cannot always expect to find cross-bedding with opposite current directions in the same channel deposits.
2.34 Wave-Dominated Deltas
Wave-dominated deltas have a vertical sequence very
like that of a beach. Little of the fluvial part of the delta
is preserved, and instead of distinct deposits at each
channel mouth (distributary mouth bars), the waves
distribute the sediments in a continuous beach ridge
along the delta. The result is a coarsening-upward
profile which is not always easy to distinguish from a
beach deposit outside a delta. In a wave-dominated
delta, however, beach ridges form right in front of the
delta top deposits, which often contain coal beds from
vegetation, whereas beach profiles formed by barrier
islands have a lagoon behind them. The first
unambiguous proof that one is dealing with a delta,
however, is if the beach ridges are found to be cut by
fluvial channels. In addition to the Rhone and the
Niger deltas, the Nile delta is also a good example of
a wave-dominated delta Fig. 2.38. Here sediment supply to the delta has been greatly reduced through the
building of the Aswan Dam, which traps sediments,
and the balance between sediment supply and wave
erosion has been disturbed so that the delta is now
retreating. This is a modern example of how
progradation or destruction (transgression) of a delta
depends on a very sensitive balance between sediment
supply and erosion.
2.35 Coastal Sedimentation Outside
Deltas
Most sediment transport to the sea takes place via
rivers which drain into deltas. In fluvially-dominated
deltas, the greater part of the sediment is deposited
there.
On destructive deltas, a great deal of sediment is
eroded and transported out into the sea or along the
coast. Coastal areas between deltas are very largely
supplied with sediment which has been eroded from
deltas and transported along the coast by waves and
coastal currents. Since the surface of the coast slopes
outwards, the direction parallel with the shoreline can
be defined as the strike of the deposit, and the direction
normal to the coast is called the dip.
Sediment supply parallel with the coast is often
called “strike feeding”. If the supply of sediment
along the coast is greater than the erosion rate, the
coast will build out into the sea and we will get
regression through coastal progradation. If sediment
supply is less than the rate of erosion, the coast will
retreat, and we will get transgression.
Because there is a finite amount of sediment being
transported along any stretch of the coast, engineering
constructions which are sometimes built to accumulate
sediment and prevent erosion at a particular spot
(groynes) will as a rule lead to increased erosion
along other parts of the coast. This has great practical
significance in areas with high coastal erosion. Beach
sedimentation must therefore be seen, not in isolation,
but in the context of the overall sediment budget along
the coast.
2 Introduction to Sedimentology
75
examples of modern rivers with well-developed tidal
estuaries.
The mixing of fresh and marine water also causes
flocculation of mud and deposition in the more
protected parts of the estuary between sand bars. Estuarine deposits are therefore characterised by a mixture
of sandstone and mudstone, often with marine fossils
which may be transported several kilometres upstream
by the salt wedges along the bottom. Tidal flats are
often found in the inner parts of the estuary. Tidal
channels are oriented at a high angle to the coastline,
but in the outer part waves may produce elongated
bars which are oriented parallel to the coastline. The
geometry and orientation of the sandbars are very
different from those associated with wave-dominated
deltas and coastlines, where the sand bars are parallel
to the coast.
Tide-dominated deltas are characterised by the fact
that the erosion due to tidal currents is very strong
compared to erosion by waves. The fluvial channels
have cross-sections which are too small to allow transport of water out and in, and the result is the development of a broad belt of tidal channels, which farthest
out are separated only by narrow ridges of sand and
clay. Very often separate ebb and flow channels
develop, and in each channel one transport direction
will dominate. Signs of transportation in two opposite
directions have often been used as a criterion for tidal
deposits. This will apply to the whole area, but we
cannot always expect to find cross-bedding with opposite current directions in the same channel deposits.
2.34 Wave-Dominated Deltas
Wave-dominated deltas have a vertical sequence very
like that of a beach. Little of the fluvial part of the delta
is preserved, and instead of distinct deposits at each
channel mouth (distributary mouth bars), the waves
distribute the sediments in a continuous beach ridge
along the delta. The result is a coarsening-upward
profile which is not always easy to distinguish from a
beach deposit outside a delta. In a wave-dominated
delta, however, beach ridges form right in front of the
delta top deposits, which often contain coal beds from
vegetation, whereas beach profiles formed by barrier
islands have a lagoon behind them. The first
unambiguous proof that one is dealing with a delta,
however, is if the beach ridges are found to be cut by
fluvial channels. In addition to the Rhone and the
Niger deltas, the Nile delta is also a good example of
a wave-dominated delta Fig. 2.38. Here sediment supply to the delta has been greatly reduced through the
building of the Aswan Dam, which traps sediments,
and the balance between sediment supply and wave
erosion has been disturbed so that the delta is now
retreating. This is a modern example of how
progradation or destruction (transgression) of a delta
depends on a very sensitive balance between sediment
supply and erosion.
2.35 Coastal Sedimentation Outside
Deltas
Most sediment transport to the sea takes place via
rivers which drain into deltas. In fluvially-dominated
deltas, the greater part of the sediment is deposited
there.
On destructive deltas, a great deal of sediment is
eroded and transported out into the sea or along the
coast. Coastal areas between deltas are very largely
supplied with sediment which has been eroded from
deltas and transported along the coast by waves and
coastal currents. Since the surface of the coast slopes
outwards, the direction parallel with the shoreline can
be defined as the strike of the deposit, and the direction
normal to the coast is called the dip.
Sediment supply parallel with the coast is often
called “strike feeding”. If the supply of sediment
along the coast is greater than the erosion rate, the
coast will build out into the sea and we will get
regression through coastal progradation. If sediment
supply is less than the rate of erosion, the coast will
retreat, and we will get transgression.
Because there is a finite amount of sediment being
transported along any stretch of the coast, engineering
constructions which are sometimes built to accumulate
sediment and prevent erosion at a particular spot
(groynes) will as a rule lead to increased erosion
along other parts of the coast. This has great practical
significance in areas with high coastal erosion. Beach
sedimentation must therefore be seen, not in isolation,
but in the context of the overall sediment budget along
the coast.
2 Introduction to Sedimentology
75