Sedimentology: River & Alluvial Fan

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Sedimentologi

Kamal Roslan Mohamed

RIVER &ALLUVIAL FAN


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Three geomorphological zones can berecognised within fluvial and alluvialsystems.

In the erosional zone thestreams are activelydowncutting, removingbedrock from the valleyfloor and from the valleysides via downslope movementof material into the stream bed.

INTRODUCTION

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

The lower part of the system is the depositional zone , where sediment isdeposited in the river channels and on the floodplains of a fluvial system or on

the surface of an alluvial fan.


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Rivers are an important feature of most landscapes, acting as the principalmechanism for the transport of weathered debris away from upland areasand carrying it to lakes and seas, where much of the clastic sediment isdeposited.

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

The grain size and the sedimentary structures in the river channel depositsare determined by the supply of detritus, the gradient of the river, the totaldischarge and seasonal variations in flow.

Overbank deposition consists mainly of finer-grained sediment, and organicactivity on alluvial plains contributes to the formation of soils, which can berecognised 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.

INTRODUCTION


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Water flow in rivers and streams isnormally confined to channels /alur , which are depressions orscours in the land surface thatcontain the flow.

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

Together the channel and overbanksettings comprise the fluvialenvironment .

INTRODUCTION


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Alluvial is a more general term forland surface processes that involvethe flow of water. It includesfeatures such as a water-lain fan ofdetritus (an alluvial fan) that are notnecessarily related to rivers.

An alluvial plain / dataran aluvial is a general term for a low-reliefcontinental area where sediment isaccumulating, which may includethe floodplains of individual rivers.

The area of ground that supplieswater to a river system is thecatchment area / kawasantadahan (sometimes also referred

to as the drainage basin ).

INTRODUCTION


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Rivers in the depositional tractcan have a variety of forms, withthe principal variables being:

(a) How straight or sinuous thechannel is;

(b) the presence or absence ofdepositonal bars of sand orgravel within the channel;

(c) the number of separate

channels that are present in astretch of the river.

RIVER FORMS

Several types of river can be distinguished, basedon whether the river channel is straight or sinuous(meandering), has one or multiple channels

(anastomosing), and has in-channel bars (braided).Combinations of these forms can often occur.


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Rivers with a high proportion ofsediment carried by rolling andsaltation along the channel floor arereferred to as bedload rivers.

Where the bedload is deposited asbars of sand or gravel in thechannel the flow is divided to givethe river a braided form.

BREADED RIVER / SUNGAI BERBURAI

Main morphological features of a braided river.Deposition of sand and/or gravel occurs on mid-channel bars.


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The bars in a braided river channelare exposed at low flow stages, butare covered when the flow is atbank-full level.

Flow is generally strongest betweenthe bars and the coarsest materialwill be transported and deposited onthe channel floor to form anaccumulation of larger clasts, orcoarse lag.

Main morphological features of a braided river.Deposition of sand and/or gravel occurs on mid-channel bars.



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This large braided river hasmoved laterally from right to left.


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At the base there will be an erosionsurface representing the base of thechannel and this will be overlain bya basal lag of coarse clastsdeposited on the channel floor.

As the flow is stronger in the lowerpart of the channel the subaqueousdunes, and hence the cross-beds,tend to be larger at the bottom ofthe bar, decreasing in set sizeupwards.

Finer sands or silts on the top of abar deposit represent theabandonment of the bar when it isno longer actively moving. A schematic graphic sedimentary log of braided

river deposits.



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The deposits of gravelly braidedrivers are characterised bycrossbedded conglomeraterepresenting deposition on channelbars.

Sandy braided river depositstypically consist of fining-upwardsuccessions from a sharp scouredbase through beds of trough andplanar cross-bedded, laminated andcross-laminated sandstone.

A schematic graphic sedimentary log of braided

river deposits.



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In plan view the thalweg in a river isnot straight even if the channelbanks are straight and parallel: itwill follow a sinuous path, movingfrom side to side along the length ofthe channel.

In any part of the river the bankclosest to the thalweg has relativelyfast flowing water against it whilethe opposite bank has slowerflowing water alongside.

Meanders develop by the erosion ofthe bank closest to the thalweg,accompanied by deposition on theopposite side of the channel where

the flow is sluggish and the bedloadcan no longer be carried.

MEANDERING RIVER / SUNGAI BERLIKU

A sandy river channel and adjacent overbankarea: the river is at low-flow stage exposingareas of sand deposited in the channel.

Flow in a river follows the sinuous thalwegresulting in erosion of the bank in places.


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The river flow may also take a short-cut between meander loops when theriver floods: this may result in a new section of channel developing, and thelonger loop of the meander built becoming abandoned.

The abandoned meander loopbecomes isolated as an oxbowlake and will remain as an area

of standing water until itbecomes filled up by depositionfrom floods and/or choked byvegetation.

The deposits of an oxbow lakemay be recognised in ancientfluvial sediments as channelfills made up of fine-grainedsediment.

Depositional architecture of a meandering river:sandstone bodies formed by the lateral migration ofthe river channel remain isolated when the channelavulses or is cut-off to form an oxbow lake.



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Meandering rivers transport anddeposit a mixture of suspended andbedload (mixed load).

The bedload is carried by the flow inthe channel, with the coarsestmaterial carried in the deepest partsof the channel.

Finer bedload is also carried inshallower parts of the flow and isdeposited along the inner bend of a

meander loop where frictionreduces the flow velocity.

Main morphological features of a meanderingriver. Deposition occurs on the point bar on theinner side of a bend while erosion occurs on theopposite cut bank. Levees form when floodwaters rapidly deposit sediment close to the

bank and crevasse splays are created when thelevee is breached.



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

A schematic graphic sedimentary log ofmeandering river deposits.



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Alluvial fans are cones of detritusthat form at a break in slope at theedge of an alluvial plain. They areformed by deposition from a flowof water and sediment comingfrom an erosional realm adjacentto the basin.

ALLUVIAL FANS


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The classic modern alluvial fans

described from places such asDeath Valley in California, USAoccur in arid and semiaridenvironments.

However, alluvial fans also formtoday in much wetter settings, andalluvial fan deposits occurring in thestratigraphic record may have beendeposited in a wide range ofclimatic regimes.

ALLUVIAL FANS

Alluvial fans in the Death Valley, USA, a regionwith a hot, arid climate.


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Alluvial fans form where there is adistinct break in topographybetween the high ground of thedrainage basin and the flattersedimentary basin floor.

A feeder canyon funnels thedrainage to the basin margin: at thispoint the valley opens out and thereis a change in gradient allowingwater and sediment to spread out.

The flow quickly loses energy anddeposits the sediment load.

Morphology of alluvial fans

A colluvial fan, a mixture of scree and debrisflows in a cold, relatively dry setting in the Arctic.


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The processes of deposition on an alluvialfan will be determined by the availability ofwater, the amount and type of sedimentbeing carried from the feeder canyon, andthe gradient on the fan surface.

Where there is a dense mixture of waterand sediment, transport and depositionare by debris flow, a viscous slurry ofmaterial that spreads out on the fansurface as a lobe.

Debris flows do not travel far and a small,relatively steep, alluvial fan cone is builtup if this is the dominant process.

Processes of deposition on alluvial fans


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With more water available, the mixture ofsediment and water is more dilute:deposition will be either by unconfinedsheetfloods, or flow will be constrained tochannels on the surface.

Dilute, water-lain fan deposits form fanswith shallower slopes and greater radialextent (around 10 km).



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Morphology of alluvial fans

A debris flow on an alluvial fan: theonglomerate is poorly sorted, with the

larger clasts completely surrounded by amatrix of finer sediment.

Sheetflood deposits on an alluvial fanshowing well-developed stratification.


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



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. 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

deposits. palaeocurrents indicate direction of flow and depositional slope

. fossils fauna uncommon, plant fossils may be common in floodplainfacies

. colour

yellow, red and brown due to oxidising conditions. facies associations alluvial fan deposits may be associated with

ephemeral lake and aeolian dunes, rivers

Characteristics of fluvial and alluvial fan deposits


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Sedimentology: River & Alluvial Fan

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