三角洲沉积体系-

第三章三角洲沉积体系DELTASYSTEMS第一节三角洲的环境特点及其沉积作用第二节三角洲的沉积特征及沉积模式第三节古代三角洲沉积鉴别标志及其与油气资源的关系第二节三角洲的沉积特征及沉积模式一、曲流河三角洲沉积体系二、辫状河三角洲沉积体系三、扇三角洲沉积体系一、曲流河三角洲沉积１．三角洲的亚环境和沉积特征２．三角洲的主要类型、沉积相组合、沉积旋回及演化1.三角洲的亚环境和沉积特征两套术语（twosetsoftermsfordeltas）））Basedongradientofdeltaprofile（吉尔伯特，美国，1885）:顶积层或三角洲台地topsetordeltaplatform前积层或三角洲斜坡foresetordeltaslope底积层或前三角洲bottomsetorprodelta推荐使用的术语Basedondelta-buildingprocesses:三角洲平原deltaplain上部高潮面以上下部低潮面以上三角洲前缘deltafront浪基面以上前三角洲prodelta浪基面以下水上三角洲水下三角洲所有三角洲都可以分为水上三角洲（三角洲平原：又分上部和下部）和水下三角洲（三角洲前缘和前三角洲）PhysiographicandSedimentCharacteristicsofAlluvialDeltaicSystemsVariationsinsedimentinput,outflowvelocity,waveandcurrentenergy,andotherfactorsdiscussedcausethedepositionalfeaturesofdeltastoexhibitahighdegreeofvariabilityfromonedeltatoanother.Nevertheless,alldeltascanbedividedintosubaerialandsubaqueouscomponents,eachofwhichcanbefurthersubdivided(Fig.10.15).Thesubaqueousdeltaplainliesseawardofthelowerdeltaicplainbelowlow-tidewaterlevelandischaracterizedbyrelativelyopenmarinefaunas.Theuppermostpartofthesubaqueousdelta,lyingatwaterdepthsdownto10morso,iscommonlycalledthedeltafront.（1）三角洲平原（DELTAPLAIN）Thesubaerialcomponentofdeltasisgenerallylargerthanthesubaqueouscomponentandisdividedintoanupperdeltaplain,whichlieslargelyabovehigh-tidelevel,andalowerdeltaplain,lyingbetweenlow-tidemarkandtheupperlimitoftidalinfluence.Theupperdeltaplainiscommonlytheoldestpartofthedeltaandisdominatedbyfluvialprocesses.Thelowerdeltaplainisexposedduringlowtidebutiscoveredbywaterduringhightide.Thus,itissubjectedtobothfluvialandmarineprocesses.（1）三角洲平原（DELTAPLAIN）UpperDeltaPlainSedimentsTheupperdeltaplainliesmainlyabovetidalinfluenceandislittleaffectedbymarineprocesses,andthussimilartofluvialenvironmentsinmanyrespects,exceptthatareasofswamp,marshandlakeareusuallywidespreadandthatchannelsmaybifurcatedownstream;Sedimentationontheupperdeltaisdominatedbydistributary-channelmigrationandassociatedfluvialsedimentationprocessessuchaschannelandpoint-bardeposition,overbankflooding,andcrevassingintolakebasins.Theprincipaldepositionalenvironmentsincludebraidedchannels(辫状河道),meanderingchannels(曲流河道),lacustrinedeltafill,backswamps,andfloodplainenvironmentssuchasswamps,marshes,andfreshwaterlakes(淡水湖泊)(ColemanandPrior,1982).Therefore,upperdelta-plainsedimentsarepredominantlyfluvialsands,gravels,andmudsthatmaybecloselyassociatedwithlacustrine,swamp,andmarshdeposits.湿地定义:指海洋和内陆常年有浅层积水或土壤过湿的地段。表达湿地的词约有30个左右,但学术上有个比较统一的名词———沼泽.在湿地研究影响最大的英语国家中比较普遍的名词有四五个,如swamp,marsh,bog,moor,fen等.某些文献中指出其间有些差别,例如swamp指木本植物多于marsh而排水情况优于bog的一种湿地;marsh指软、湿、低的草本覆被为主的有由水过渡到陆的趋势的湿地;marsh土常多碱性,而bog常多酸性;moor常指较开阔、地势较高、排水较差、有小灌木丛以致泥炭的湿地;fen则是英国常用的名词等.（1）三角洲平原（DELTAPLAIN）LowerDeltaPlainSedimentsAffectedbybothfluvialandbasinalprocesses.Thewidthofthelowerdeltaplainisgreatestondeltaswheretidalrangeislarge.Thisplainincludestheactivedistributarysystemofthedelta,aswellasabandoneddistributary-filldeposits,andmaybeflankedbymarginal-basinorbay-filldeposits.Distributarychannelsarenumerous,butenvironmentsbetweenchannelsmakeupthelargestpercentageofthelowerdeltaplain.Theseenvironmentsincludeactivelymigratingtidalchannels,naturallevees,interdistributarybays(分流间湾),bayfills(crevassesplays),marshes,andswamps(ColemanandPrior,1982).Sediments:Themajorsandbodiesgeneratedinthisenvironmentarebay-filldeposits,whichmayformthinsandwedgesstackedoneontopoftheotherandseparatedbyfiner-grained,interdistributary-bayandmarshdeposits.Inveryaridclimates,evaporites(蒸发盐)alsomaybedepositedinsomepartsofthelowerdeltaplain.Depositsofthelowerdeltaplainalsocommonlyincludeabandoneddistributarydeposits.Theseconsistoflocallyderivedsands,muds,andorganicdebristhatgraduallyfilldistributarychannelsaftertheyhavebeenabandonedbythemainstreamowingtoblockingorotherprocessesthatcausechannelshifting.（1）三角洲平原（DELTAPLAIN）两个主要单元（TwoprincipalcomponentsofDeltaplain）:分流河道沉积（Distributarychannels）为沙和粉沙沉积物。横剖面形态为近于对称的透镜体。为向上变细的沉积序列。分流河道间沉积（Inter-distributaryarea）天然堤--发育在河道两侧，由细沙、粉沙和泥质组成。沙沼泽及湖泊--排水不良，但植物繁茂，沉积物主要为富含有机质的暗色泥岩夹泥炭和煤层。植物化石丰富，根土构造发育。湖泊的规模通常较小，沉积物主要为泥岩。决口扇沉积--发育在天然堤两侧和分流间海湾，呈扇状分布，由细沙和粉沙组成。常见粒序层、沙纹层和平行层理。分流间湾沉积--分流间湾是指分流河道间与海洋相通的相对低凹的地区。为泥质，含少量粉沙和细沙，具水平层理和透镜状层理。含海相动物化石。（2）三角洲前缘(DELTAFRONT)Thesubaqueousdeltaplainconstitutesthatareaofadeltathatliesseawardoflowtidelevelandabovewavebase,andactivelyreceivesfluvialsediments.Itmayextendoutwardfordistancesofafewkilometerstotensofkilometerstowaterdepthsasmuchas300m.Depositsofthesubaqueousdeltathusformthebaseoverwhichsubaerialdeltadepositsprogradeasthedeltabuildsseaward.Theuppermostpartofthesubaqueousdelta,thedeltafront,maybedominatedbyhigh-energymarineprocesses,includingwaves,longshorecurrents,andtidesinsomecases.Thedepositstypicallyconsistinpartofsands,andpossiblygravels,depositedneartherivermouths,formingdistributary-mouth-bardeposits.Inageneralway,thesedepositsgradeseawardtofinersandsandcoarsesiltsthatsettlefromsuspensiontoformthedistalbar.Onwave-andtide-dominateddeltas,sedimentisreworkedandwinnowedbytheseprocesses,creatingwell-sorteddelta-frontsheetsandsthatarecross-beddedonavariet