sedimentologi sains laut-9 - river _ alluvial fan

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sedimentologi sains laut-9 - river _ alluvial fan Powered By Docstoc
 Kamal Roslan Mohamed

   RIVER &

Three geomorphological zones can be
recognised within fluvial and alluvial

 In the erosional zone the
streams are actively
downcutting, removing
bedrock from the valley
floor and from the valley
sides via downslope movement
of material into the stream bed.

In the transfer zone, the gradient is lower, streams and rivers are not actively
eroding, but nor is this a site of deposition.

The lower part of the system is the depositional zone, where sediment is
deposited in the river channels and on the floodplains of a fluvial system or on
the surface of an alluvial fan.

Rivers are an important feature of most landscapes, acting as the principal
mechanism for the transport of weathered debris away from upland areas
and carrying it to lakes and seas, where much of the clastic sediment is

River systems can also be depositional, accumulating sediment within
channels and on floodplains.

The grain size and the sedimentary structures in the river channel deposits
are determined by the supply of detritus, the gradient of the river, the total
discharge and seasonal variations in flow.

Overbank deposition consists mainly of finer-grained sediment, and organic
activity on alluvial plains contributes to the formation of soils, which can be
recognised in the stratigraphic record as palaeosols.

Water flows over the land surface also occur as unconfined sheetfloods and
debris flows that form alluvial fans at the edges of alluvial plains.

Water flow in rivers and streams is
normally confined to channels /
alur, which are depressions or
scours in the land surface that
contain the flow.

The overbank /lampau tebing area
or floodplain / dataran banjir is the
area of land between or beyond the
channels that (apart from rain)
receives water only when the river
is in flood.

Together the channel and overbank
settings comprise the fluvial

Alluvial is a more general term for
land surface processes that involve
the flow of water. It includes
features such as a water-lain fan of
detritus (an alluvial fan) that are not
necessarily related to rivers.

An alluvial plain / dataran aluvial
is a general term for a low-relief
continental area where sediment is
accumulating, which may include
the floodplains of individual rivers.

The area of ground that supplies
water to a river system is the
catchment area / kawasan
tadahan (sometimes also referred
to as the drainage basin).

Rivers in the depositional tract
can have a variety of forms, with
the principal variables being:

(a) How straight or sinuous the
channel is;

(b) the presence or absence of
depositonal bars of sand or
gravel within the channel;          Several types of river can be distinguished, based
                                    on whether the river channel is straight or sinuous
(c) the number of separate          (meandering), has one or multiple channels
channels that are present in a      (anastomosing), and has in-channel bars (braided).
                                    Combinations of these forms can often occur.
stretch of the river.

A straight channel / sungai lurus
without bars is the simplest form but
is relatively uncommon.

A braided river / sungai berburai
contains mid-channel bars that are
covered at bank-full flow, in contrast
to an anastomosing river, which
consists of multiple, interconnected
channels that are separated by           Several types of river can be distinguished,
areas of floodplain.                     based on whether the river channel is straight or
                                         sinuous (meandering), has one or multiple
Both braided and anastomosing            channels (anastomosing), and has in-channel
                                         bars (braided). Combinations of these forms can
river channels can be sinuous, and       often occur.
sinuous rivers that have
depositional bars only on the
insides of bends are called
meandering / sungai berliku.

Rivers with a high proportion of
sediment carried by rolling and
saltation along the channel floor are
referred to as bedload rivers.

Where the bedload is deposited as
bars of sand or gravel in the
channel the flow is divided to give     Main morphological features of a braided river.
the river a braided form.               Deposition of sand and/or gravel occurs on mid-
                                        channel bars.

The bars in a braided river channel
are exposed at low flow stages, but
are covered when the flow is at
bank-full level.

Flow is generally strongest between
the bars and the coarsest material
will be transported and deposited on   Main morphological features of a braided river.
the channel floor to form an           Deposition of sand and/or gravel occurs on mid-
                                       channel bars.
accumulation of larger clasts, or
coarse lag.
This large braided river has
moved laterally from right to left.

At the base there will be an erosion
surface representing the base of the
channel and this will be overlain by
a basal lag of coarse clasts
deposited on the channel floor.

As the flow is stronger in the lower
part of the channel the subaqueous
dunes, and hence the cross-beds,
tend to be larger at the bottom of
the bar, decreasing in set size

Finer sands or silts on the top of a
bar deposit represent the
abandonment of the bar when it is
no longer actively moving.             A schematic graphic sedimentary log of braided
                                       river deposits.

The deposits of gravelly braided
rivers are characterised by
crossbedded conglomerate
representing deposition on channel

Sandy braided river deposits
typically consist of fining-upward
successions from a sharp scoured
base through beds of trough and
planar cross-bedded, laminated and
cross-laminated sandstone.

                                     A schematic graphic sedimentary log of braided
                                     river deposits.

In plan view the thalweg in a river is
not straight even if the channel
banks are straight and parallel: it
will follow a sinuous path, moving
from side to side along the length of
the channel.

In any part of the river the bank
closest to the thalweg has relatively
fast flowing water against it while      A sandy river channel and adjacent overbank
the opposite bank has slower             area: the river is at low-flow stage exposing
                                         areas of sand deposited in the channel.
flowing water alongside.

Meanders develop by the erosion of
the bank closest to the thalweg,
accompanied by deposition on the
opposite side of the channel where
                                         Flow in a river follows the sinuous thalweg
the flow is sluggish and the bedload     resulting in erosion of the bank in places.
can no longer be carried.

The river flow may also take a short-cut between meander loops when the
river floods: this may result in a new section of channel developing, and the
longer loop of the meander built becoming abandoned.
The abandoned meander loop
becomes isolated as an oxbow
lake and will remain as an area
of standing water until it
becomes filled up by deposition
from floods and/or choked by

The deposits of an oxbow lake
may be recognised in ancient
fluvial sediments as channel      Depositional architecture of a meandering river:
                                  sandstone bodies formed by the lateral migration of
fills made up of fine-grained
                                  the river channel remain isolated when the channel
sediment.                         avulses or is cut-off to form an oxbow lake.

Meandering rivers transport and
deposit a mixture of suspended and
bedload (mixed load).

The bedload is carried by the flow in
the channel, with the coarsest
material carried in the deepest parts
of the channel.
                                        Main morphological features of a meandering
Finer bedload is also carried in        river. Deposition occurs on the point bar on the
                                        inner side of a bend while erosion occurs on the
shallower parts of the flow and is      opposite cut bank. Levees form when flood
deposited along the inner bend of a     waters rapidly deposit sediment close to the
meander loop where friction             bank and crevasse splays are created when the
reduces the flow velocity.              levee is breached.

The deposits of a meander bend
have a characteristic profile of
coarser material at the base,
becoming progressively finer-
grained up the inner bank.

                                   A schematic graphic sedimentary log of
                                   meandering river deposits.

Alluvial fans are cones of detritus
that form at a break in slope at the
edge of an alluvial plain. They are
formed by deposition from a flow
of water and sediment coming
from an erosional realm adjacent
to the basin.

The ‘classic’ modern alluvial fans
described from places such as
Death Valley in California, USA
occur in arid and semiarid

However, alluvial fans also form
today in much wetter settings, and
alluvial fan deposits occurring in the
stratigraphic record may have been       Alluvial fans in the Death Valley, USA, a region
deposited in a wide range of             with a hot, arid climate.
climatic regimes.
Morphology of alluvial fans

Alluvial fans form where there is a
distinct break in topography
between the high ground of the
drainage basin and the flatter
sedimentary basin floor.

A feeder canyon funnels the
drainage to the basin margin: at this
point the valley opens out and there
is a change in gradient allowing
                                        A colluvial fan, a mixture of scree and debris
water and sediment to spread out.       flows in a cold, relatively dry setting in the Arctic.

The flow quickly loses energy and
deposits the sediment load.
Processes of deposition on alluvial fans

The processes of deposition on an alluvial
fan will be determined by the availability of
water, the amount and type of sediment
being carried from the feeder canyon, and
the gradient on the fan surface.

Where there is a dense mixture of water
and sediment, transport and deposition
are by debris flow, a viscous slurry of
material that spreads out on the fan
surface as a lobe.

Debris flows do not travel far and a small,
relatively steep, alluvial fan cone is built
up if this is the dominant process.
Processes of deposition on alluvial fans

With more water available, the mixture of
sediment and water is more dilute:
deposition will be either by unconfined
sheetfloods, or flow will be constrained to
channels on the surface.

Dilute, water-lain fan deposits form fans
with shallower slopes and greater radial
extent (around 10 km).
Morphology of alluvial fans

A debris flow on an alluvial fan: the      Sheetflood deposits on an alluvial fan
onglomerate is poorly sorted, with the     showing well-developed stratification.
larger clasts completely surrounded by a
matrix of finer sediment.
Processes of deposition on alluvial fans

             Schematic sedimentary logs through debris-flow,
             sheetflood and stream-channel alluvial fan deposits.
Characteristics of fluvial and alluvial fan deposits

.   lithologies – conglomerate, sandstone and mudstone
.   mineralogy – variable, often compositionally immature
.   texture – very poor in debris flows to moderate in river sands
.   bed geometry – sheets on fans, lens shaped river channel units
.   sedimentary structures – cross-bedding and lamination in channel
.   palaeocurrents – indicate direction of flow and depositional slope
.   fossils – fauna uncommon, plant fossils may be common in floodplain
.   colour – yellow, red and brown due to oxidising conditions
.   facies associations – alluvial fan deposits may be associated with
    ephemeral lake and aeolian dunes, rivers