4-河流沉积体系

FLUVIALSYSTEM

（河流沉积体系）TERMINOLOGY(1)Fluvial:usedforanythingassociatedwithrivers;Channel:confinedtodepressionsorscoursinlandsurfacewhichcontaintheflows;Overbankorfloodplain:areaoflandbetweenorbeyondchannels,receivingwateronlywhenriverisinflood;Catchment:areaofgroundthatsupplieswatertoariversystemDischarge:volumeofwaterflowingintheriverinatimeperiod;Thalweg:thelineofthedeepestpartofthechannelTERMINOLOGY(2)Levees:ridgesbuiltoneithersideofachannelbutcommonlybetterdevelopedontheoutermarginsofbends;mainlyfine-grainedsandandsiltsdominatedbyripplecross-laminationsandsmall-scalecross-bedding.Someleveesareactuallytheproductsoflaterallycoalescingsplaylobes.Crevassesplays:discretelobesofsiltyorsandysediments,extendingdowntheleveefromshallowcrevassesonthecrestandspreadontothefloodplain.Braidplains:regionswherebraidedriversrepeatedlychangepositiononthealluvialplain,abroad,leadingtoanextensiveregionofgravellybardepositsmanytimeswiderthantheriverchannel.Chutechannel:ashort-cutoverthetopofapointbarduringperiodofhighstageflow.Mainmorphologicalfeaturesofmeanderingriver

Classicalpoint-barmodelinmeanderingriverchannel

TERMINOLOGY(3)Avulsion:aprocessbywhicholdrivercourseiscompletelyabandonedandanewchannelisscouredintothelandsurface.Oxbowlakesareanexampleofabandonmentofasectionofriverchannel.Lateralmigration:sidewaygrowthofinnerbanksofriversduetodepositionattheexpenseoferosionattheouterbanks,formingthepointbarsofmeanderingriver;Autocyclicsequence:controlledbyprocessestakingplacewithinthebasinitself,suchasmigrationandsuperpositionofchannelsystemsAllocyclicsequence:causedbyvariationsexternaltothebasin,suchasclimaticchanges,tectonicactivity,andsea-levelfluctuations;RiverTypes(1)

(intermsofchannelsinuosity)Sinuosity:calculatedbydividingthedistancebetweentwopointsmeasuredalongachannelbythestraight-linedistancebetweenthosepointsinadownstreamdirection;Meanderingriver:

havingahighsinuosity(>1.5),andcharacteristicallypossessingbendsthatchangeshapeswithtimebyerosionononesideanddepositionontheother.Braidedriver:havingalowsinuosity(1.1–1.2),andpossessingavarietyofbars;Straightriver:havingalowsinuosity,andasinglechannelwithdividingbars.

Anastomosingriver:consistinganumberofseparatechannelsthatdivideandjoinalongtheriver.Formsofriverchannel

Evolutionofdepositionalfluvialsystem

RiverTypes(2)

(intermsofmodeofsedimenttransportinthechannel)Bed-loadfluvialsystem:typicallyhavesteepgradient,highwidth/depthratio(>40),channelpatternsoflowsinuosity;Mixed-loadfluvialsystem:preserveahigherpercentageoffloodplaindepositsconsistingofsilts,mudandbackswampcarbonaceousmudandclays.Channelsaremorestable,havealowwidth/depthratiothanbed-loadsystem,filledmainlywithsandsandminorproportionofsiltandclays,andflankedbyleveessand-siltandcrevasse-splaysand;Suspended-loadfluvialsystem:characterizedbyhigh-sinuositysinglechannelsofgreatstabilityandlowwidth/depthratio(<10).Channelfillscontainhighproportionofsiltsandmuds,withwell-developedsiltyormuddyleveedepositsandoverbankfines;RiverTypes(3)

(intermsofmodeofdischarge)Ephemeralstreams:

dryforlongperiodoftimeandonlyexperienceflowwhenthereisrainincatchmentarea;Perennialstreams:

featuresbyconsistentflow,especiallyinmorehumidregionsPatternsofriversTributarydrainagesystem:

smallstreamsconvergetoformlargetrunkchannel,efficientatcarryingsediments;Distributarydrainagesystem:

splitting,bifurcating,orbranchingintosmallerriversinadownstreamdirection,lessefficientatcarryingsediments.Channelsbecomesmaller,transmitlesswater,andbedloadtendstobedeposited

Tributaryversusdistributaryriverpatternsanddownstreamchangeofriversystem

AvarietyofBARSLongitudinalbars(alsocalledasmid-channelbars):Elongatealongchannelaxis,andevolvedfromtransversebars;Transversebars(alsotermedascross-channelbars):

Wider(thantheyarelong)oracrossthechannel;Linguoidbars:

Crescentic,withtheirapexpointingdownstream;Bank-attachedbars(alsocalledalternatebars):

Occuratchannelbends;Pointbars:

Presentatinnerbankandformedbylateral-accretiondeposition;Scrollbars:

Elongatesandridgesonpointbar,broadlyparalleltothecontour,andoriginatinglowonpointbarandmigrateupwardsuntiltheyreachbank-fulllevel;MeanderingRivers

(mixed-loadriver)Mainfaciesassociationsofmeanderingriver

Idealizedmodeloflateralaccretioncrossbedding

Evolutionofchannelavulsionfromacrevasseevent

Crevasse-splaydepositsModelofMeanderingriversystemBraidedRivers

(bed-loadriver)Mainmorphologicalfeaturesofabraidedriver

CompositefaciesmodelforsandybraidedriverDepositionalarchitectureofabraidedriverlateralmigrationofchannelandabandonmentofbarsledtothebuild-upofchannel-fillsuccessionModelforapebblybraidedriver

dominatedbylongitudinalandtransversebars,withsomebarsdevelopingwithin-channellateralaccretion

Large-scalebedformsinfluvialchannelModelofbraidedriversystemAnastomosingrivers

(suspended-loadriver)AnastomosingriverchannelpatternsChannel-filldepositsofanancientanastomosingriversystem(Nadon,1994)Modelofanastomosingriversystem“ARCHITECTUREELEMENT”ANALYSIS

Anapproachtostudylarge-scalegeometryandinternalorganizationoffluvialsystemsBasicarchitecturalelementsoffluvialdepositstwomainFluvialArchitecturalElements(CH&DA)definedbyaccretionalstyle,andtype&scaleofassociatedboundingsurfaceThreeseparatechannel-fillsuccessionIfexposureislimited,itisdifficulttodistinguishthem.

低曲度河流水道的砂体几何形态和内部组成TypicalchannelconglomeratefaciesTypicalchannelsandstonedepositionalfacies控制水道砂体连接的主要因素Mainfaciesinfluvialsediments

AncientFluvialSystemAncientfluviallithofaciesCoarsersedimentsoccurringinchannel;Finermaterialsdepositedonfloodplain;ChannelLithofaciesCross-beddedconglomerate:representingbardepositsingravellybraidedriverchannels;Thinlayersofcoarsedebrislyingonerosionalscours:representinglagdepositsatthebottomoftheriverchannel;Cross-beddedsandstone:formedbysubaqueousdunemigrationonbarsinsandybraidedriveroronlowerpartsofpointbarsofmeanderingrivers;Massiveandliquefiedsandstone:resultingfromcollapseofriverbankmaterialsintochannel;bardepositsingravellybraidedriverchannelsbardepositsingravellybraidedriverchannelsBardepositsingravellybraidedriverchannelsImbricationofGravelsConglomerateingravellybraidedriverchannelsChannelfillofgravellybraidedriverBardepositsingravellybraidedriverchannelsBardepositsinsandybraidedriverchannelsChannelfillofgravellybraidedriverChannelfillofbraidedriverChannelfillofbraidedriverChannelfillofgravellybraidedriverLagdepositsandchannelfillofmeanderingriverBardepositsBankcollapse-relateddepositsBankcollapse-relateddepositsofameanderingriverLateralaccretionofapointbarLateralaccretionofapointbarAbandonedchannel(Oxbowlakedeposits)

FloodplaindepositsThinbedsnormallygradedfromsandtomud;Evidencefrominitialrapidflow(plane-parallellamination)quicklywaningandaccompaniedbyrapiddeposition(climbingripplelamination);Thinsheetsofsediment,oftenonlyafewcentimetersthickbutextendingforhundredsofmeters,evenkilometers,awayfromthechannels;Erosionatthebaseofsandstonebeds,normallylocalizedtoareasnearthechannelwheretheflowismostvigorousDesiccationcracksinmudsatthetopofthenormallygradedbeds;Evidenceofsoilformation;LeveedepositsFloodplaindepositsFloodplaindepositsFloodplaindepositsDesiccationcracksinFloodplainsettingPaleosolsinfloodplainsettingSomethingyoushouldbearinmi