《角洲沉積體系》課件.ppt

1、第三章 三角洲沉積體系DELTA SYSTEMS,第一節(jié) 三角洲的環(huán)境特點(diǎn)及其沉積作用 第二節(jié) 三角洲的沉積特征及沉積模式 第三節(jié) 古代三角洲沉積鑒別標(biāo)志及其 與油氣資源的關(guān)系,第二節(jié) 三角洲的沉積特征及沉積模式,一、曲流河三角洲沉積體系 二、辮狀河三角洲沉積體系 三、扇三角洲沉積體系,一、曲流河三角洲沉積,三角洲的亞環(huán)境和沉積特征 三角洲的主要類型、沉積相組合、沉積旋回及演化,1.三角洲的亞環(huán)境和沉積特征,兩套術(shù)語(yǔ)（two sets of terms for deltas） ）,Based on gradient of delta profile（吉爾伯特，美國(guó)，1885）: 頂積層或三角洲

2、臺(tái)地 topset or delta platform 前積層或三角洲斜坡 foreset or delta slope 底積層或前三角洲 bottomset or prodelta 推薦使用的術(shù)語(yǔ) Based on delta-building processes: 三角洲平原 delta plain 上部 高潮面以上 下部 低潮面以上 三角洲前緣 delta front 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為水上三角洲（三角洲平原：又分上部和下部）和水下三角洲（三角洲前緣和前三角洲） Physiographic and Sedime

3、nt Characteristics of Alluvial Deltaic Systems Variations in sediment input, outflow velocity, wave and current energy, and other factors discussed cause the depositional features of deltas to exhibit a high degree of variability from one delta to another. Nevertheless, all deltas can be divided int

4、o subaerial and subaqueous components, each of which can be further subdivided (Fig. 10.15). The subaqueous delta plain lies seaward of the lower deltaic plain below low-tide water level and is characterized by relatively open marine faunas. The uppermost part of the subaqueous delta, lying at water

5、 depths down to 10 m or so, is commonly called the delta front.,（1）三角洲平原 （DELTA PLAIN）,The subaerial component of deltas is generally larger than the subaqueous component and is divided into an upper delta plain, which lies largely above high-tide level, and a lower delta plain, lying between low-ti

6、de mark and the upper limit of tidal influence. The upper delta plain is commonly the oldest part of the delta and is dominated by fluvial processes. The lower delta plain is exposed during low tide but is covered by water during high tide. Thus, it is subjected to both fluvial and marine processes.

7、,（1）三角洲平原 （DELTA PLAIN）,Upper Delta Plain Sediments The upper delta plain lies mainly above tidal influence and is little affected by marine processes, and thus similar to fluvial environments in many respects, except that areas of swamp, marsh and lake are usually widespread and that channels may b

8、ifurcate downstream; Sedimentation on the upper delta is dominated by distributary-channel migration and associated fluvial sedimentation processes such as channel and point-bar deposition, overbank flooding, and crevassing into lake basins. The principal depositional environments include braided ch

9、annels(辮狀河道), meandering channels(曲流河道), lacustrine delta fill, backswamps, and floodplain environments such as swamps, marshes, and freshwater lakes(淡水湖泊) (Coleman and Prior, 1982). Therefore, upper delta-plain sediments are predominantly fluvial sands, gravels, and muds that may be closely associa

10、ted with lacustrine, swamp, and marsh deposits.,濕地,定義:指海洋和內(nèi)陸常年有淺層積水或土壤過(guò)濕的地段。 表達(dá)濕地的詞約有30個(gè)左右,但學(xué)術(shù)上有個(gè)比較統(tǒng)一的名詞沼澤. 在濕地研究影響最大的英語(yǔ)國(guó)家中比較普遍的名詞有四五個(gè),如swamp, marsh, bog, moor, fen等.某些文獻(xiàn)中指出其間有些差別,例如swamp指木本植物多于marsh而排水情況優(yōu)于bog的一種濕地; marsh指軟、濕、低的草本覆被為主的有由水過(guò)渡到陸的趨勢(shì)的濕地; marsh土常多堿性,而bog常多酸性; moor常指較開(kāi)闊、地勢(shì)較高、排水較差、有小灌木叢以致泥炭

11、的濕地; fen則是英國(guó)常用的名詞等.,（1）三角洲平原 （DELTA PLAIN）,Lower Delta Plain Sediments Affected by both fluvial and basinal processes. The width of the lower delta plain is greatest on deltas where tidal range is large. This plain includes the active distributary system of the delta, as well as abandoned distributa

12、ry-fill deposits, and may be flanked by marginal-basin or bay-fill deposits. Distributary channels are numerous, but environments between channels make up the largest percentage of the lower delta plain. These environments include actively migrating tidal channels, natural levees, interdistributary

13、bays(分流間灣), bay fills (crevasse splays), marshes, and swamps (Coleman and Prior, 1982). Sediments:The major sand bodies generated in this environment are bay-fill deposits, which may form thin sand wedges stacked one on top of the other and separated by finer-grained, interdistributary-bay and marsh

14、 deposits. In very arid climates, evaporites (蒸發(fā)鹽) also may be deposited in some parts of the lower delta plain. Deposits of the lower delta plain also commonly include abandoned distributary deposits. These consist of locally derived sands, muds, and organic debris that gradually fill distributary

15、channels after they have been abandoned by the main stream owing to blocking or other processes that cause channel shifting.,（1）三角洲平原 （DELTA PLAIN）,兩個(gè)主要單元 （Two principal components of Delta plain ） : 分流河道沉積（Distributary channels） 為沙和粉沙沉積物。橫剖面形態(tài)為近于對(duì)稱的透鏡體。為向上變細(xì)的沉積序列。 分流河道間沉積（Inter-distributary area） 天

16、然堤-發(fā)育在河道兩側(cè)，由細(xì)沙、粉沙和泥質(zhì)組成。沙質(zhì)沉積內(nèi)部具沙紋交錯(cuò)層和流水波痕。 沼澤及湖泊-排水不良，但植物繁茂，沉積物主要為富含有機(jī)質(zhì)的暗色泥巖夾泥炭和煤層。植物化石豐富，根土構(gòu)造發(fā)育。湖泊的規(guī)模通常較小，沉積物主要為泥巖。 決口扇沉積-發(fā)育在天然堤兩側(cè)和分流間海灣，呈扇狀分布，由細(xì)沙和粉沙組成。常見(jiàn)粒序?qū)?、沙紋層和平行層理。 分流間灣沉積-分流間灣是指分流河道間與海洋相通的相對(duì)低凹的地區(qū)。為泥質(zhì)，含少量粉沙和細(xì)沙，具水平層理和透鏡狀層理。含海相動(dòng)物化石。,（2）三角洲前緣 (DELTA FRONT),The subaqueous delta plain constitutes that

17、 area of a delta that lies seaward of low tide level and above wave base, and actively receives fluvial sediments. It may extend outward for distances of a few kilometers to tens of kilometers to water depths as much as 300 m. Deposits of the subaqueous delta thus form the base over which subaerial

18、delta deposits prograde as the delta builds seaward. The uppermost part of the subaqueous delta, the delta front, may be dominated by high-energy marine processes, including waves, longshore currents, and tides in some cases. The deposits typically consist in part of sands, and possibly gravels, dep

19、osited near the river mouths, forming distributary-mouth-bar deposits. In a general way, these deposits grade seaward to finer sands and coarse silts that settle from suspension to form the distal bar. On wave- and tide- dominated deltas, sediment is reworked and winnowed by these processes, creatin

20、g well-sorted delta-front sheet sands that are cross-bedded on a variety of scales.,（2）三角洲前緣 (DELTA FRONT),水下分流河道沉積-是陸上分流河道的延伸部分。沉積物中常含粘土碎塊，具交錯(cuò)層理、流水沙紋層理和侵蝕面等，可見(jiàn)向上變細(xì)的序列。 水下天然堤沉積-由極細(xì)沙和粉沙組成?？梢?jiàn)沙紋層理、浪成波痕、蟲(chóng)孔和包卷紋層等。 河口沙壩沉積-由沙質(zhì)沉積物組成。質(zhì)地純凈，分選良好，具多方向槽狀交錯(cuò)層和沖刷面。河口壩末端通常稱為末端壩，為細(xì)沙、粉沙和泥組成的交互層。具反粒序，有水平紋層、波狀及透鏡狀層理，河口壩

21、單元形成向上變粗的序列。 前緣席狀沙-河口壩經(jīng)海洋營(yíng)力改造形成。成層性好，沙紋層理發(fā)育。,（3）前三角洲 (PRODELTA),The remaining seaward part of the subaqueous delta is called the prodelta, or prodelta slope. The finest silts and clays are transported still farther seaward and settle on the prodelta on the outermost part of the subaqueous delta. ARE

22、A that is not affected by wave and tide processes. It may not exist in shallow-water areas. It comprises well laminated mud and silts deposited from suspension in deep-water areas. Previously deposited sediments may be reentrained, transported, and redeposited farther downslope on the subaqueous del

23、ta by gravity-driven mass-movement processes that include landslides, slumps, turbidity-current flows, and mudflows. Mud diapirs （底辟） are also a common feature of many subaqueous deltas. These structures are piercement-type（刺穿） bodies of soft sediment that is squeezed upward and intruded into overly

24、ing sand bodies owing to sediment loading.,（3）前三角洲 PRODELTA,前三角洲位于前緣的向海一側(cè)，水深通常在浪基面以下。 沉積物主要為泥和粉沙質(zhì)泥，富含有機(jī)質(zhì)，可見(jiàn)海綠石等自生礦物。 層理主要是水平紋層。在近前緣的部分可出現(xiàn)少數(shù)粉砂薄層，其中有小型交錯(cuò)紋層等構(gòu)造。 含海相動(dòng)物化石，有較多的蟲(chóng)孔及生物擾動(dòng)構(gòu)造。 三角洲前緣沉積物的再搬運(yùn)、沉積形成的沉積物重力流，泥巖底辟、變形改造等。,三角洲前緣和前三角洲的同生變形特征Syn-deformational features in delta front and prodelta,三角洲前緣和前三角洲

25、的同生變形特征Syn-deformational features in delta front and prodelta,一、曲流河三角洲沉積,三角洲的亞環(huán)境和沉積特征 三角洲的主要類型、沉積相組合、沉積旋回及演化,三角洲沉積相組合、沉積旋回及演化,（1）平面相組合及垂向?qū)有颍ㄏ嘈颍?A delta and adjacent environments,三角洲沉積相組合、沉積旋回及演化,（1）平面相組合及垂向?qū)有颍ㄏ嘈颍?A delta and adjacent environments,浪控三角洲 下細(xì)上粗的反旋回 具浪蝕海灘脊序列 以層序頂部三角洲平原的沼澤和分支河道沉積與海岸沉積的海灘脊

26、相區(qū)別。,（2）三角洲類型及三維空間組合,A variety of shoal-water delta,內(nèi)陸架淺水三角洲,河控三角洲是在河流輸入泥沙量大，波浪、潮汐作用微弱： 鳥(niǎo)足狀三角洲 朵狀三角洲,浪控三角洲的平面形態(tài)呈鳥(niǎo)嘴狀，海洋的波浪作用大于河流的作用，河流輸入泥沙量少，形成平行于海岸的海灘、沙嘴、沙壩,潮控三角洲-河流流入三角港或其他形狀的港灣，由于潮汐作用遠(yuǎn)大于河流作用，在港灣中堆積的泥沙受潮汐作用的強(qiáng)烈改造，僅形成小型三角洲。外形受港灣控制,發(fā)育潮汐沙壩。,River-dominated type of the shoal-water deltas,Wave-dominated

27、type of the shoal-water deltas,Tide-dominated type of the shoal-water deltas,洞穴,不管三角洲沉積體系的成因類型如何，它們都是在淺水背景上發(fā)育起來(lái)的，其垂向剖面序列自下而上依次為 前三角洲泥 三角洲前緣沙 三角洲平原的沙、泥互層夾煤層 其中水下沉積部分是一個(gè)向上變粗的沉積序列。,三角洲前緣沉積是其最特征部分，它不僅有別于其它沉積單元，也是區(qū)分不同三角洲類型的重要依據(jù)。 河控三角洲前緣以河口沙壩沉積為特征，交錯(cuò)層傾角較大，古流向比較集中(單峰態(tài))；浪控三角洲前緣沙具海岸沙壩沉積序列，交錯(cuò)層方向多變，傾角較?。怀笨厝侵耷?/p>

28、緣沙具雙向交錯(cuò)層，再作用面和粘土紋層，其上三角洲平原有潮坪及潮道沉積等。,滑坡界面,滑坡體,（3）三角洲沉積旋回與演化,三角洲沉積旋回分為兩個(gè)時(shí)期：即建設(shè)期和破壞期： 三角洲向海推進(jìn)增長(zhǎng)發(fā)育時(shí)期稱三角洲的建設(shè)期 三角洲被海水淹沒(méi)遭受侵蝕破壞的時(shí)期稱三角洲的破壞期 每個(gè)單一的三角洲沉積體在垂向上就代表了一個(gè)沉積旋回，每個(gè)旋回都是以三角洲的建設(shè)相開(kāi)始，以破壞相的出現(xiàn)而告終 在垂向剖面上，建設(shè)相和破壞相周期性地交替重復(fù)出現(xiàn)，就形成了多旋回三角洲復(fù)合體系,三角洲沉積旋回,三角洲體系的規(guī)模,構(gòu)造與規(guī)模 三角洲沉積的厚度和保存狀況與水深和盆地的沉降速率有密切關(guān)系 大型的河控三角洲主要發(fā)育在構(gòu)造穩(wěn)定下沉的地

29、區(qū)。因其沉積物供應(yīng)充足，沉速較大，所以比相鄰的近岸沉積往往具有較大的厚度。,淺水型三角洲 主要發(fā)育在基底穩(wěn)定，水深較淺(數(shù)米至十余米)的克拉通盆地，三角洲沉積薄，推進(jìn)快； 盆地營(yíng)力較弱，河流作用顯得特別突出； 相的關(guān)系主要表現(xiàn)在側(cè)向上。進(jìn)積相易于被后繼河流沖刷破壞，有的河道甚至可以深切到盆地的基底，垂向序列薄而不完整，壓實(shí)沉陷作用微弱； 三角洲前緣及前三角洲沉積規(guī)模小，發(fā)育差，三角洲平原相占居很大比例；分流河道沙體分支多，延伸遠(yuǎn)，有的甚至可以沿盆地傾向延伸數(shù)百千米。 三角洲形態(tài)呈不規(guī)則的分枝狀 淺水三角洲分布很廣，在地質(zhì)記錄中保存較好。,深水型三角洲 在深水條件下，三角洲沉積厚，推進(jìn)慢； 易于

30、形成巨厚的前三角洲泥，如果前緣沙體推進(jìn)到巨厚的前三角洲泥之上，快速的壓實(shí)與沉陷將有利于完整進(jìn)積序列和厚的前緣沙體的保存； 突出特征是具有厚而完整的進(jìn)積序列，骨架沙體呈巨； 現(xiàn)代的密西西比三角洲是其典型例子。,水深變化與三角洲演化 河控三角洲是一種高建設(shè)性三角洲。因其地層樣式受水深和基底性質(zhì)的控制，所以隨著盆底地形及底質(zhì)性質(zhì)的變化，河流將建造出沉積樣式不同的三角洲體系。在古代大型的海盆或湖盆中；緩坡一側(cè)往往存在有淺而平坦的廣闊淺水臺(tái)地，其與深盆地往往以明顯的斜坡過(guò)渡。在臺(tái)地上形成分布廣泛的淺水型三角洲。 當(dāng)三角洲推進(jìn)到臺(tái)地邊緣的深盆地區(qū)，淺水型逐漸為深水型三角洲取代。深水型三角洲由于巨厚的前三角

31、洲沉積的不穩(wěn)定性和水下斜坡的存在，易于引起三角洲沉積滑塌破壞，所以分布比較局限。而滑塌破壞的產(chǎn)物也可以沉積物重力流的方式進(jìn)入盆地底，形成獨(dú)立的濁積扇體系。,三角洲體系的演化,相對(duì)海平面變化與三角洲演化,近海的,第二節(jié) 三角洲的沉積特征及沉積模式,一、曲流河三角洲沉積體系 二、辮狀河三角洲沉積體系 三、扇三角洲沉積體系,二、辮狀河三角洲沉積,1.辨狀河三角洲的定義 2.辨狀河三角洲的發(fā)育條件 3.辨狀河三角洲的亞環(huán)境及沉積特征,辨狀河三角洲是由辨狀河體系前積到水體中富含沙和礫石的三角洲沉積體。,定義,需要特定的構(gòu)造、地形和氣候條件 扇三角洲往往形成在盆地?cái)嘞葸吔绲貛?，辮狀河三角洲往往發(fā)育在坳陷盆

32、地陡坡邊界地帶，而曲流河三角洲往往形成在坳陷盆地緩坡邊界地帶。 只有盆地坳陷較陡邊界最有利于辮狀河三角洲發(fā)育。這是因?yàn)檩^陡的盆地坳陷邊界地帶地形坡降大，易于辮狀河人湖后形成獨(dú)具特色的沉積體；隨著進(jìn)入盆地的沉積物數(shù)量的增加，辮狀河三角洲的向前推進(jìn)，沖積平原必然向盆地方向擴(kuò)展，沖積平原沿陡傾的盆地邊緣向盆地內(nèi)進(jìn)積，盆地的依次被充填表明沉積速率大于盆地沉降速率。,辨狀河三角洲的發(fā)育條件,辨狀河三角洲的發(fā)育條件,古氣候條件決定了一定區(qū)域的降水量、水系的特征、風(fēng)化的強(qiáng)度以及碎屑物的供應(yīng)，其對(duì)湖泊辮狀河三角洲的影響尤為重要。 干旱氣候向潮濕氣候轉(zhuǎn)化會(huì)引起沉積物數(shù)量的增加，從而引起扇三角洲沉積向辮狀河三角洲

33、沉積轉(zhuǎn)化。因此最適合辮狀河三角洲形成的古氣候是半干旱半潮濕氣候。在這種氣候條件下，有一定的降水量，可形成穩(wěn)定的水系，同時(shí)物理風(fēng)化亦較為強(qiáng)烈，可有大量的碎屑物供應(yīng)。 海平面的升降也會(huì)影響三角洲沉積類型的變化。一般認(rèn)為，扇三角洲沉積與海平面快速上升有關(guān)，而辮狀河三角洲沉積僅見(jiàn)于海平面上升減緩的時(shí)期。,3.辨狀河三角洲的 亞環(huán)境及沉積特征,三角洲平原 三角洲前緣 前三角洲,(1)辮狀河三角洲平原 辮狀河三角洲平原是辨狀河三角洲的水上部分，沉積特征與辨狀河沉積特征相似，主要包括： 辮狀河道 河道間泛濫平原及沼澤微相 并以辮狀河道最具代表性。,辮狀河道充填物為寬厚比高的、寬平板狀的砂帶，多個(gè)河道砂體疊合

34、可形成分布廣泛的粗粒厚層單元；由顆粒支撐的砂、礫組成，碎屑顆粒一般為次棱角-次圓狀，分選中等，填隙物以泥質(zhì)為主。測(cè)井曲線表現(xiàn)為較高的視電阻率，呈齒狀或尖峰狀起伏，自然電位曲線一般呈箱形或波狀起伏。,泛濫平原主要為泥夾粉-細(xì)砂，而沼澤微相主要由煤層、碳質(zhì)泥夾粉砂組成。主要以水平層理和波狀層理為特征。古植物化石豐富。,(2)辮狀河三角洲前緣 水下分流河道 河口沙壩 水下河道間 席狀沙,水下分流河道-系平原辮狀河道入湖后在水下的延續(xù)部分，它在辮狀河三角洲中所占的厚度是最大的。巖性一般較粗，以砂和砂礫為主。從剖面上看，水下分流河道大都從沖刷面開(kāi)始。向上依次為具層理構(gòu)造的砂、礫，頂部為薄層狀泥質(zhì)粉砂。河

35、道砂頻繁交互，形成多層樓式的正韻律的砂、礫組成的疊合砂。,水下河道間-主要為灰、灰綠色粉沙質(zhì)泥、泥質(zhì)粉沙夾薄層狀砂。塊狀層理、小型交錯(cuò)層理及波狀層理發(fā)育。由于水下分流河道沖刷力強(qiáng)，改道頻繁，一旦發(fā)生改道，這些沉積物就被沖刷減薄，甚至全被沖刷掉，因此常以細(xì)粒碎屑夾層的形式出現(xiàn)。,河口砂壩-巖性為分選、磨圓較好的細(xì)砂、中砂，偶含泥礫。具板狀、楔狀及槽狀交錯(cuò)層理。與下伏席狀砂沉積過(guò)渡，并一起構(gòu)成明顯的反韻律，電測(cè)曲線形態(tài)呈漏斗型。,席狀砂-巖性一般為分選相對(duì)較好的細(xì)砂、粉砂與泥互層。波狀層理、小型交錯(cuò)層理及水平層理發(fā)育。一般與前辮狀河三角洲沉積物呈薄互層。,(3)前辮狀河三角洲 細(xì)粒泥質(zhì)沉積 沉積物

36、重力流沉積,沉積物粒度較細(xì)，多為灰、灰綠、深灰色泥、粉砂質(zhì)泥夾粉-細(xì)砂組成，在剖面上為砂泥薄互層。水平層理、塊狀層理及沙紋交錯(cuò)層理發(fā)育。,沉積物重力流主要為碎屑流、液化流和濁流沉積。,第二節(jié) 三角洲的沉積特征及沉積模式,一、曲流河三角洲沉積體系 二、辮狀河三角洲沉積體系 三、扇三角洲沉積體系,三、扇三角洲沉積,1.扇三角洲的概念 2.扇三角洲發(fā)育的條件 3.扇三角洲環(huán)境的劃分 4.扇三角洲的沉積特點(diǎn),Hotmes(1965),Mxgrowen(1970)的定義：由相鄰高地進(jìn)積到安靜水體中的沖積扇 扇三角洲是由沖積扇(包括旱地扇和濕地扇)提供物源，在活動(dòng)的扇體與穩(wěn)定水體交界地帶沉積的沿岸沉積體系

37、。它是含有大量沉積載荷的沖積扇與?；蚝嗷プ饔玫漠a(chǎn)物。 A fan delta, as defined by Holmes(1965) and modified slightly by Nemec and Steel (1988a), is a coastal prism of sediments delivered by an alluvial-fan system and deposited, mainly or entirely subaqueously, at the interface between the active fan and a standing body of wat

38、er. Fan deltas were recognized first in modern settings but fan-delta deposits have now been reported in many ancient sedimentary successions (e.g., Nemec and Steel, 1988b; Chough and Orton, 1995).,1.扇三角洲的概念Fan Deltas,The alluvial fans can include any of the fan types and may form in settings rangin

39、g from glacial to humid to arid. 扇三角洲的水下部分分河流、波浪和潮汐為主的三種類型 Like other deltas, the subaqueous portion of fan deltas may be fluvial-dominated, wave-dominated, or tide-dominated. In fact, two of the deltas (the Copper and the Yallahs) are considered to be fan deltas. 沉積物在水下部分的沉積作用包括：碎屑崩落、濁流、慣性（重密度）流 Se

40、diments are deposited downslope in the subaqueous part of fan deltas by processes such as slumping and debris avalanching, turbidity-current flow, and inertia (hyperpycnal) flow that takes place particularly during flood stages; the riverborne load achieves sufficient density to overcome buoyancy an

41、d frictional effects at the river mouth and can transport even gravel and coarse sand downslope. Figure 10.14 illustrates subaqueous transport of sediment down the front of fan deltas undergoing transport by debris avalanching, inertia flow, and turbidity-current flow. These models were developed by

42、 study of fan deltas in fiords (Prior and Bornhold, 1990), but serve to illustrate the general principal of subaqueous transport on fan deltas.,地形條件-臨近山區(qū)的盆地邊緣，高差變化大，坡度陡是扇三角洲發(fā)育的最有利和最基本的條件。 地質(zhì)構(gòu)造條件- 扇三角洲多發(fā)育在活動(dòng)的構(gòu)造區(qū)，常與同沉積期大型斷裂帶相伴。從大地構(gòu)造背景來(lái)看，沿大陸碰撞海岸、島弧碰撞海岸以及克拉通內(nèi)部的裂谷盆地或其它斷陷盆地的岸邊對(duì)扇三角洲發(fā)育最為有利。 氣候條件- 在不同的氣候區(qū)有不同

43、類型的扇三角洲形成。 （a）干旱半干旱地區(qū)多發(fā)育旱地扇三角洲(如死海西岸扇三角洲)； （b）潮濕的熱帶和溫帶地區(qū)易形成濕地扇三角洲和辮狀河三角洲 (如牙買(mǎi)加的耶拉斯三角洲)； （c）在寒冷潮濕的冰水沖積平原有利于辮狀河平原三角洲或辮狀 河三角洲的發(fā)育(如阿拉斯加科珀河扇三角洲)。,2.扇三角洲的發(fā)育的條件 control over the growth ofCOARSE-GRAINED DELTA,扇三角洲多發(fā)育在活動(dòng)的構(gòu)造區(qū),3.扇三角洲環(huán)境的劃分及沉積相特點(diǎn),(1)扇三角洲平原 是扇三角洲的陸上部分，包括從扇端至岸線之間的近海平原地帶。 在干旱半干旱地區(qū)的扇三角洲平原具有旱地沖積扇的沉積特

44、征；在潮濕區(qū)的扇三角洲平原則以發(fā)育礫質(zhì)辮狀水系沉積為特征。,(2) 扇三角洲前緣 亦稱過(guò)渡帶，位于岸線至正常天氣浪基面之間的淺水區(qū) 河控型扇三角洲前緣是湖泊扇三角洲的特點(diǎn)，以各種粒級(jí)的砂和粉砂為主，也常有礫石沉積。粒度變化向盆地方向變細(xì)，砂層中交錯(cuò)層理發(fā)育，底部具大型沖刷面。,(3) 前扇三角洲 是指扇三角洲的浪基面以下部分，向下與陸架泥或深水盆地沉積過(guò)渡。 發(fā)育在寬闊陸棚上的扇三角洲的前三角洲沉積主要為臨濱-遠(yuǎn)濱的粉砂和泥質(zhì)沉積，與陸棚泥呈互層產(chǎn)出。,前扇三角洲 扇三角洲前緣 扇三角洲平原,退積型扇三角洲,進(jìn)積型 扇三角洲,(1)單個(gè)的扇三角洲的陸上部分一般比較小，平面形態(tài)多為扇形。 (2)

45、扇三角洲沉積體向陸方向通常都以斷層為界，其近源沉積物(扇根)常以角度不整合超覆在古老的基巖地層上， (3)扇三角洲的組成均為礫石、含礫砂和砂等粗碎屑沉積物，成分和結(jié)構(gòu)成熟度均比較低，反映其距物源區(qū)比較近，搬運(yùn)距離短，沉積迅速的特點(diǎn)。 (4)扇三角洲沉積體幾何形態(tài)和粒度變化:一般為楔形，從山前向盆地(?；蚝?方向變薄變細(xì)，逐漸過(guò)渡為盆地相而消失； (5)單個(gè)扇三角洲的垂直層序一般呈向上變粗的特點(diǎn)； (6)扇三角洲層序的厚度和延展范圍受邊緣斷裂差異升降幅度控 制。單個(gè)扇三角洲沉積層序厚可達(dá)幾十米，而發(fā)育在板塊邊緣經(jīng)歷很長(zhǎng)時(shí)期的扇三角洲層序可厚達(dá)幾千米，延長(zhǎng)幾十千米。,扇三角洲沉積特征,第三章 三角

46、洲沉積體系DELTA SYSTEMS,第一節(jié) 三角洲的環(huán)境特點(diǎn)及其沉積作用 第二節(jié) 三角洲的沉積特征及沉積模式 第三節(jié) 古代三角洲沉積鑒別標(biāo)志及其 與油氣資源的關(guān)系,第三節(jié) 古代三角洲的識(shí)別標(biāo)志 與油氣資源的特征,一、三角洲體系的主要識(shí)別標(biāo)志 二、三角洲體系與油氣資源的關(guān)系,一、三角洲體系的主要識(shí)別標(biāo)志,三角洲沉積是河流注入蓄水盆地后，在淺水區(qū)快速建造的近岸碎屑沉積體系。盡管三角洲體系的影響因素多、沉積環(huán)境復(fù)雜、沉積物類型多，但其突出特征是具有特定的剖面沉積序列和骨架砂體的幾何形態(tài)。不同類型的三角洲體系，可以根據(jù)骨架砂體的成因特征、幾何形態(tài)、定向習(xí)性、進(jìn)積砂體與分流河道的空間關(guān)系，以及河道充填

47、物的特征等加以識(shí)別。（排除法） The subenvironments of delta systems range from normal marine (beach, barrier, lagoonal) to nonmarine (fluvial marsh, eolian), and a variety of different sediment types can be deposited in these subenvironments. Deltaic sedimentary successions are characterized by assemblages of lith

48、ofacies, each of which can occur in other environments, such as fluvial, lacustrine, and shallow-marine environments. Identifying ancient delta deposits is best accomplished in a series of steps, eliminating other possibilities and using distinguishing characteristics of facies types, bed geometry,

49、and types of cyclic successions to focus gradually on the correct delta model.,三角洲沉積的識(shí)別標(biāo)志 Some general characteristics of delta deposits that can be useful in their recognition include: (1)幾何形態(tài) geometry, (2) 側(cè)向相變關(guān)系lateral facies relationships, (3) 相的垂向序列vertical successions of facies, (4) 沉積構(gòu)造和化石sed

50、imentary structures and fossils (5)地震剖面上的構(gòu)造形態(tài) the structural attitude of deltaic deposits revealed in seismic reflection profiles.,Geometry Ideally, deltas are triangular in areal shape; however, much variation from this ideal shape can occur, particularly with tide- and wave-dominated deltas. In cr

51、oss section, deltas are typically wedge- or lens-shaped bodies extending laterally to several hundred kilometers. Modern deltas vary in their areal dimensions from small bodies a few thousand square kilometers in size to huge deltas exceeding 100,000 km2. Nonetheless, delta deposits have a restricte

52、d lateral extent, with the center of deposition fixed around a river mouth.,Lateral Facies Relationships Deltaic origin is suggested by a wedge- or lobeshaped deposit of nonmarine to shallow marine sand, silt, mud that grades landward into totally nonmarine, largely fluvial sediments and basinward i

53、nto finer grained, deeper water marine sediments. On a smaller scale, lateral facies relationships are likely to be complex. Delta-plain deposits may range from coarse distributary-channel deposits to finer grained marsh or interdistributary-bay or lacustrine deposits. The lateral associations of de

54、lta-front sediments can also be highly variable depending upon whether deposition was dominated by fluvial, wave, or tidal processes; it is difficult to generalize about these facies relation- ships. Coarser, delta-front sediments may grade seaward into prodelta silts and clays which, in turn, grade

55、 to open-shelf muds. Prodelta muds may be difficult to differentiate from open-shelf muds except perhaps by greater thickness and higher sedimentation rates. They also tend to contain more mud-turbidite units than shelf muds.,Vertical Facies Successions Progradation （進(jìn)積） of deltaic deposits during a

56、ctive delta growth produces a generally coarsening-upward sedimentary succession, which passes upward from offshore (marine) facies to nonmarine fluvial facies. As discussed, growth of deltas tends to be cyclic（旋回性）. Migration of delta-front sands over prodelta silts and clays generates fairly well-

57、defined, large-scale (50-150 m), coarsening-upward successions. Progradation of subaerial delta-plain sediments over subaqueous delta-plain sediments tends to produce smaller-scale (2-15 m), coarsening-downward units. These distinctive cycles and the general property of upward grading from marine to fluvial deposits are particularly characteristic of fluvial-dominated deltas. They may be much less distinctive in wave- and tide-dominated deltaic deposits, which can resemble the deposits of siliciclastic coasts and estuaries.,Sedimentary Structures and Fossils Nume