Sedimentologi
Kamal Roslan Mohamed
DELTA
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.
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.
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.
Controls on delta environments and facies.
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.
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.
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.
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.
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.
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 massflow processes in deeper water.
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 massflow 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.
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,
A river-dominated
the form of the delta is largely controlled by
delta with the distributary channels
building out as extensive lobes due to the
fluvial processes of transport and
absence of reworking by wave and tide
sedimentation.
The unidirectional fluvial current at the
mouth of the river continues into the sea or
lake as a subaqueous flow.
processes. Low-energy, interdistributary
bays are a characteristic of riverdominated deltas.
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.
Process controls: river-dominated deltas
A schematic graphic sedimentary log of
riverdominated delta deposits.
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.
Process controls: wave-dominated deltas
A schematic graphic sedimentary log of
Wave dominated delta deposits.
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 wavedominated 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.
Process controls: tide-dominated deltas
This may be seen in strata as
reversals of palaeocurrent
indicated by crossstratification, and the
formation of mud drapes.
The tidal currents rework sediments
at the river mouth into elongate bars that
are perpendicular to the shoreline.
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.
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
Process controls: tide-dominated deltas
A schematic graphic sedimentary log of
Tidedominated delta deposits.
Facies succession
Delta cycles: the facies succession preserved
depends on the location of the vertical profile
relative to the depositional lobe of a delta.
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.
SEKIAN