Geology Reference
In-Depth Information
Box 13.3
TYPES OF DELTA
Based on their overall morphology, six basic delta types
are recognized that may be classified according to the
importance of river, wave, and tidal processes (Wright
1985; Figure 13.20). The characteristics of the six types
are as follows (Trenhaile 1997, 227-8):
at right-angles to the coast, while the waves redis-
tribute the riverine sand to produce beach-dune
ridge complexes and barriers running parallel to the
coast. Type 3 deltas are common in areas of inter-
mediate wave energy, moderate to high tides, and
low littoral drift. Examples are the Irrawaddy Delta
in Burma, the Mekong Delta in Vietnam, and the
Danube Delta in Romania.
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Type 4 deltas
consist of finger-like bodies of sand
deposited as distributary mouth bars, or they may
coalesce to form a broad sheet of sand. They pro-
grade into lagoons, bays, or estuaries sheltered by
offshore or baymouth barriers. Their development
is encouraged by intermediate wave energy, low off-
shore slopes, low sediment yields, and a low tidal
range. Examples are the Brazos Delta in Texas and
the Horton Delta in Canada.
•
Type 5 deltas
have extensive beach ridges and dune
fields. These extensive sand sheets are shaped by
wave redistributing of river-borne sands. They form
where moderate to high wave energy is unremitting,
where littoral drift is low, and where offshore slopes
are moderate to steep. Examples are the deltas of the
São Francisco in Brazil and the Godavari in India.
•
Type 6 deltas
form on coasts totally dominated
by wave action. The waves straighten the coasts,
and deltas consist of numerous sandy spit barri-
ers running parallel to the coastline that alternate
with fine-grained, abandoned channel fills. They
are found in environments with strong waves, uni-
directional longshore transport, and steep offshore
slopes. Examples are the deltas of the Shoalhaven
in New South Wales, Australia, and the Tumpat in
Malaysia.
•
Type 1 deltas
are dominated by river processes.
They are elongated distributary mouth bar sands,
aligned roughly at right-angles to the overall line
of the coast. The protrusions are called bar-
finger sands. In the modern birdfoot delta of the
Mississippi River, seaward progradation of the prin-
cipal distributaries has formed thick, elongated
bodies of sand up to 24-32 km long and 6-8
km wide. Type 1 deltas form in areas with a low
tidal range, very low wave energy, low offshore
slopes, low littoral drift, and high, fine-grained
suspended load. Examples are the deltas of the
Mississippi in the USA, the Paraná in Brazil,
the Dnieper in the Ukraine, and the Orinoco in
Venezuela.
•
Type 2 deltas
are dominated by tides. They have
broad, seaward-flaring, finger-like channel sand
protuberances. Sandy tidal ridges produced by tidal
deposition and reworked river sediments at dis-
tributary mouths front them. They occur in areas
with a high tidal range and strong tidal currents,
low wave energy, and low littoral drift. Examples
are the deltas of the Ord in Western Australia, the
Indus in Pakistan, the Colorado in the USA, and
the Ganges-Brahmaputra in Bangladesh.
•
Type 3 deltas
are affected by waves and tidal cur-
rents. The tidal currents create sand-filled river
channels and tidal creeks running approximately
