1. Sedimentologi
Kamal Roslan Mohamed
DELTA

2. INTRODUCTION
The mouths of rivers may be places where the accumulation of detritus brought down by the flow forms a sediment body that builds out into the sea or a lake.
In marine settings the interaction of subaerial processes with wave and tide action results in complex sedimentary environments that vary in form and deposition according to the relative importance of a range of factors.
Delta form and facies are influenced by the size and discharge of the rivers, the energy associated with waves, tidal currents and longshore drift, the grain size of the sediment supplied and the depth of the water.
They are almost exclusively sites of clastic deposition ranging from fine muds to coarse gravels.
Deposits formed in deltaic environments are important in the stratigraphic record as sites for the formation and accumulation of fossil fuels.

3. RIVER MOUTHS, DELTAS AND ESTUARIES
The mouth of a river / Muara Sungai is the point where it reaches a standing body of water, which may be a lake or the sea. These are places where a delta may form (this chapter), an estuary may occur (next chapter) or where there is neither a delta nor an estuary.
A delta can be defined as a ‘discrete shoreline protuberance formed at a point where a river enters the ocean or other body of water’, and as such it is formed where sediment brought down by the river builds out as a body into the lake or sea.
A delta fed by a river prograding
into a body of water.
An estuary / estuari is a river mouth where there is a mixture of fresh water and seawater with accumulation of sediment within the confines of the estuary, but without any build-out into the sea.

4. TYPES OF DELTA
The forms of modern deltas:
(a) the Nile delta, the ‘original’ delta,
(b) the Mississippi delta, a river-dominated delta,
(c) the Rhone delta, a wave-dominated delta,
(d) the Ganges delta, a tide-dominated delta.

5. Controls on delta environments and facies.

6. DELTA ENVIRONMENTS AND SUCCESSIONS
Marine deltas form at the interface of continental and
marine environments. The processes
associated with
river channel and
overbank settings
occur alongside
wave and tidal action
of the shallow marine realm.
Deltas are fed by a river and there is
a transition between the area that is considered part of the fluvial/alluvial environment and the region that is considered to be the delta top or delta plain. Delta channels can be as variable in form as a river and may be meandering or braided, single or divided channels.
Delta deposition can be divided into two subenvironments, the delta top and the delta front.

7. DELTA ENVIRONMENTS AND SUCCESSIONS
At the mouth of the channels the flow velocity is
abruptly reduced as the water enters
the standing water of the
lake or sea. The
delta front
Immediately
forward of the channel
mouth is the site of deposition
of bedload material as a subaqueous
Mouth bar.
The delta slope, is often shown as a steep incline away from the delta
top, but the slope varies from only 1o or 2o in many fine-grained deltas to as much as 30o in some coarsegrained deltas.
Delta deposition can be divided into two subenvironments, the delta top and the delta front.

8. DELTA ENVIRONMENTS AND SUCCESSIONS
Fresh river water with a suspended load may have a lower density than saline seawater and the plume of
suspended fine particles
will be buoyant,
spreading out
away from the river
mouth.
As mixing occurs deposition
out of suspension occurs, with the
finest, more buoyant particles travelling
furthest away from the delta front
before being deposited in the
prodelta region.
Delta deposition can be divided into two subenvironments, the delta top and the delta front.

9. DELTA ENVIRONMENTS AND SUCCESSIONS
The delta slope, is often shown as a steep incline away from the delta
top, but the slope varies from only 1o or 2o in many fine-grained deltas to as much as 30o in some coarsegrained deltas.

10. VARIATIONS IN DELTA MORPHOLOGY AND FACIES
Effects of grain size:
Differences in the grain size of
the sediment supplied affect the
form of a delta:
(a) a high proportion of suspended
load results in a relatively small mouth bar deposited from bedload and extensive delta-front and prodelta deposits;
(b) a higher proportion of bedload results in a delta with a higher proportion of mouth bar gravels and sands.
A modern Gilbert-type
Coarsegrained delta.

11. VARIATIONS IN DELTA MORPHOLOGY AND FACIES
Water depth: shallow- and
deep-water deltas
(a) A delta prograding into shallow water will spread out as the sediment is redistributed by shallow-water processes
to form extensive mouth-bar and delta-front facies.
(b) In deeper water the mouth bar is restricted to an area close to the
river mouth and much of the sediment is deposited by mass-flow processes in deeper water.

12. VARIATIONS IN DELTA MORPHOLOGY AND FACIES
Water depth: shallow- and
deep-water deltas
(a) A delta prograding into shallow water will spread out as the sediment is redistributed by shallow-water processes
to form extensive mouth-bar and delta-front facies.
(b) In deeper water the mouth bar is restricted to an area close to the
river mouth and much of the sediment is deposited by mass-flow processes in deeper water.
A schematic sedimentary log of a sandy delta prograding into shallow water.
A schematic sedimentary log of a sandy delta prograding into a deep-water basin.

13. Process controls: river-dominated deltas
A delta is regarded as river-dominated where the effects
of tides and waves are
minor.
This requires a microtidal regime
and a setting where wave energy is effectively dissipated before the waves reach the coastline. Under these conditions, the form of the delta is largely controlled by fluvial processes of transport and sedimentation.
The unidirectional fluvial current at the mouth of the river continues into the sea or lake as a subaqueous flow.
A river-dominated
delta with the distributary channels building out as extensive lobes due to the absence of reworking by wave and tide processes. Low-energy, interdistributary bays are a characteristic of river-dominated deltas.

14. Process controls: river-dominated deltas
When a delta channel
avulses a new lobe
starts to build out at
the new location of the
channel mouth.
The abandoned lobe subsides by
dewatering until completely submerged.
Through time the channel will eventually switch
back to a position overlapping the former delta lobe.
This results in a series of delta-lobe successions, each coarsening-up.

15. Process controls: river-dominated deltas
A schematic graphic sedimentary log of riverdominated delta deposits.

16. Process controls: wave-dominated deltas
Waves driven by strong winds have
the capacity to rework and
redistribute any
sediment deposited in
shallow water, especially
under storm conditions.
The river mouth and mouth-bar areas of a
delta are susceptible to the action of waves,
resulting in a modification of the patterns
seen in river-dominated Deltas.
Progradation of the channel outwards is limited
because the subaqueous levees do not form
and bedload is acted upon by waves as quickly
as it is deposited.
A wave-dominated
Delta brought to the delta
front to form coastal sand bars and extensive mouth-bar deposits.

17. Process controls: wave-dominated deltas
A schematic graphic sedimentary log of
Wave dominated delta deposits.

18. Process controls: tide-dominated deltas
Coastlines with high tidal ranges
experience onshore
and offshore tidal
currents that move
both bedload and suspended
load. A delta building out into a region
with strong tides will be modified into a
pattern that is different to both river- and wave-
dominated deltas.
The delta-top channel(s) are subject
to tidal influence with reverses of flow
and/or periods of stagnation as a flood
tide balances the fluvial discharge.
A tide-dominated delta
in a macrotidal regime will show extensive reworking of the delta front by tidal currents
and the delta top will have a region of intertidal deposition.

19. Process controls: tide-dominated deltas
This may be seen in strata as
reversals of palaeocurrent
indicated by cross-
stratification, and the
formation of mud drapes.
The tidal currents rework sediments
at the river mouth into elongate bars that
are perpendicular to the shoreline.
These are modified mouth bars, which
May show bidirectional cross-stratification
and mud drapes on the cross-bed foresets
due to the reversing nature of the ebb and
flood tidal currents
A tide-dominated delta
in a macrotidal regime will show extensive reworking of the delta front by tidal currents
and the delta top will have a region of intertidal deposition.

20. Process controls: tide-dominated deltas
A schematic graphic sedimentary log of
Tidedominated delta deposits.

21. Facies succession
Delta cycles: the facies succession preserved depends on the location of the vertical profile relative to the depositional lobe of a delta.

22. Characteristics of deltaic deposits
. lithologies – conglomerate, 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.

23. SEKIAN