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Where are deltas found?
1. Deltas can occur in lakes and inland seas as well as in the
ocean, but they are most important in the open ocean
2. Deltas are particularly common in the modern ocean
owing to post-Pleistocene sea-level rise coupled with
high sediment loads carried by many rivers.
3. They often contain extensive wetlands, whose high
biological productivity makes them vital nursery grounds
4. Huge amounts of coal, oil, and natural gas are
found in subsurface ancient deltaic deposits.
Deltas seldom form on active, subducting
continental margins because there is no stable
shallow shelf on which sediments can accumulate.
5. Twenty-five of the world’s largest deltas are found
on passive margins of continents. They are
categorized according to features within the delta
plain and by energies acting on the delta (tide,
wave, and fluvial). The size of a delta is dependent
on the size of the drainage basin, the elevation of
the land, and climate.
Three geomorphological elements
1. Delta Plain
• Subaerial; Flat swampy plain
• Fluvial process generally predominate
• Incised by fluvial distributary channels
2. Delta front
• Shallow submarine platform
• Most sand accumulates (mouth bars)
• Most distal; Water depth > 10-15 m
• Bulk suspension settles out
Levee Crevasse Splay
Types of Deltas
Morphology reflects the type and intensity of the
1. River-dominated delta
• elongated distributary and mouth bar
2. Tide-dominated delta
• usually funnel-shaped mouth
• mouth bar developed as a series of ridges or isolated
3. Wave-dominated delta
• mouth bar reworking parallel to the coastline
Fluvially-dominated deltas are primarily controlled by the
water density difference between the inflowing river water
and the standing water on the basin.
Different flow types that determine the distribution of
sediment and sedimentary structures formed in the delta
are homopycnal flow, hyperpycnal flow, and hypopycnal
Homopycnal flow occurs when the density of the river
water is equal to the density of the standing water in the
Hyperpycnal flow is produced when the density of the
river water entering the basin is greater than the density of
the standing water in the ocean basin. This higher density
river water will flow below the standing water in the basin
because of the difference in density.
Hypopycnal flow is associated with a lower river water
density entering a higher density standing water in the
basin. Under these conditions, the river water will flow out
over the standing water, gradually depositing the
suspended clay portion of the sediment load on the
- Deltas which undergo strong tidal interaction
- As sediment travels out of the delta into the sea, high tides and flood
tides confine sediment on the delta plain and low tides carry
- Tide-dominated deltas typically occur in locations of large tidal ranges
or high tidal current speeds. In situations such as these, where the
sediment supply is over powered by strong tidal currents, the delta
tends to be very small.
Characteristic of Tidal Delta
1. Heights of up to 15-20 meters close to the shoreline.
2. Ridges are greater in relief and size when there is more
tidal discharge than fluvial discharge.
3. The delta has a gradational, upward coarsening sequence
of muds; interbedded sands, silts, and muds; and then
fine to medium sands
4. Due to the eroding effect of tides, the sands display coss-bedding,
the sand ridge field can be truncated through
erosion, and the distributary channel sediments contain
more slack-water mud drapes than usual.
- Open ocean basin accepts more water input, which means there
is potential for greater wave energy
- High wave interference causes conflicted or deflected river
- Less influence from fluvial sources & breaking waves cause
immediate mixing of fresh and salt water.
A bar may form in the immediate vicinity of the
distributary mouth, often supplemented by landward
migrating swash bars.
The wave action reworks the sediment, making it much
sandier than other types of deltas, coarsening upward.
Sediment is delivered by the river and is transported
along the coast. The sediment is then deposited as
beaches and bars and the development of distributaries is
Characteristics of deltaic deposits (conclusion)
lithologies – gravel, sandstone and mudstone
mineralogy – variable, delta-front facies may be compositionally mature
texture – moderately mature in delta-top sands and gravels, mature in
wave-reworked delta-front deposits
bed geometry – lens-shaped delta channels, mouthbar lenses variably
elongate, prodelta deposits thin bedded
sedimentary structures – cross-bedding and lamination in delta-top and
palaeocurrents – topset facies indicate direction of progradation, wave and
tidal reworking variable on delta front
fossils – association of terrestrial plants and animals of the delta top with
marine fauna of the delta front
colour – not diagnostic, delta-top deposits may be oxidised
facies associations – typically occur overlying shallow- marine facies and
overlain by fluvial facies in an overall progradational pattern.