Delta

1. Delta System
Lecture 6#

2. Introduction
• Why studying deltaic system?
• Their sandstones constitute the majority of the producing
reservoirs in siliciclastic basin

3. Rivision
• 1. What type of river systems exist?
• 2. What are the characteristics of flood plain can be seen from
the imagery?
• 3. Describe lithofacies at Point A

4. Answer:
1. Braided, meandering and anastomosing
2. i. Adjacent to the channels
ii. Crevasse splay deposits
3.
Electric log (E-log)

6. Typical delta look like

7. 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
(Boggs).
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
for fisheries.

8. 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.

10. Morphology
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)
3. Prodelta
• Most distal; Water depth > 10-15 m
• Bulk suspension settles out

11. Distributary
Channels
Prodelta
Distributary
Mouth Bar
Aluvial Plain
Delta Plain
Delta Front
Previous Coastline
Floodplain Bay
Levee Crevasse Splay
Low River
Flow
Flood Level

13. Types of Deltas
Morphology reflects the type and intensity of the
hydrodynamic processes
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
bar
3. Wave-dominated delta
• mouth bar reworking parallel to the coastline

17. Fluvial-Dominated Deltas
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
flow

19. Homopycnal flow occurs when the density of the river
water is equal to the density of the standing water in the
basin.
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
prodelta.

20. Fluvial Dominated

21. Tide-Dominated Deltas
- 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
sediment seaward.
- 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.

24. 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.

25. Tide
Dominated

26. Wave-Dominated Deltas
- Open ocean basin accepts more water input, which means there
is potential for greater wave energy
- High wave interference causes conflicted or deflected river
mouths.
- Less influence from fluvial sources & breaking waves cause
immediate mixing of fresh and salt water.

27. 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
limited

38. 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
mouth-bar facies
 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.