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1、第三章 三角洲沉积体系DELTA SYSTEMS,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,三角洲的亚环境和沉积特征三角洲的主要类型、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（two sets of terms for deltas）,Based on gradient of delta profile（吉尔伯特，美国，1885）:顶积层或三角洲台地 tops
2、et or delta platform前积层或三角洲斜坡 foreset or delta slope底积层或前三角洲 bottomset or prodelta推荐使用的术语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缘 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）Physiographic and Sediment Characterist
3、ics 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 into subaerial and suba
4、queous 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 depths down to 10 m or so
5、,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-tide mark and the upper limit of
6、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.,（1）三角洲平原（DELTA PLAIN）,Upper Delta
7、Plain SedimentsThe 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 bifurcate downstream;Sedimentation on the
8、 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 channels(辫状河道),meandering channels(曲流河道),lacus
9、trine 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 associated with lacustrine,swamp,and marsh deposits.,湿地,定义:指海洋和
10、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（DELTA PLAIN）,Lower Delta Plain Sediment
11、sAffected 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 distributary-fill deposits,and may be flanked by marginal-basin or bay-fill deposit
12、s.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 bays(分流间湾),bay fills(crevasse splays),marshes,and swamps(Coleman and Prior,1982
13、).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 deposits.In very arid climates,evaporites(蒸发盐)also may be deposited in some parts of th
14、e 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 channels after they have been abandoned by the main stream owing to blocking or other processes
15、that cause channel shifting.,（1）三角洲平原（DELTA PLAIN）,两个主要单元（Two principal components of Delta plain）:分流河道沉积（Distributary channels）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-distributary area）天然堤-发育在河道两侧，由细沙、粉沙和泥质组成。沙质沉积内部具沙纹交错层和流水波痕。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
16、，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(DELTA FRONT),The subaqueous delta plain constitutes that area of a delta that lies seaward of low tide level and above wave base,and actively receives fluvial sedim
17、ents.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 delta deposits prograde as the delta builds seaward.The uppermost part of the subaqueous delta,the delta front,
18、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,deposited near the river mouths,forming distributary-mouth-bar deposits.In a general way,these deposits grade seaward to fi
19、ner 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,creating well-sorted delta-front sheet sands that are cross-bedded on a variety of scales.,（2）三角洲前缘(DELTA FRONT),水下分流河道沉积-是陆上分流河道的延伸部分。
20、沉积物中常含粘土碎块，具交错层理、流水沙纹层理和侵蚀面等，可见向上变细的序列。水下天然堤沉积-由极细沙和粉沙组成。可见沙纹层理、浪成波痕、虫孔和包卷纹层等。河口沙坝沉积-由沙质沉积物组成。质地纯净，分选良好，具多方向槽状交错层和冲刷面。河口坝末端通常称为末端坝，为细沙、粉沙和泥组成的交互层。具反粒序，有水平纹层、波状及透镜状层理，河口坝单元形成向上变粗的序列。前缘席状沙-河口坝经海洋营力改造形成。成层性好，沙纹层理发育。,（3）前三角洲(PRODELTA),The remaining seaward part of the subaqueous delta is called the prod
21、elta,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.AREA that is not affected by wave and tide processes.It may not exist in shallow-water areas.It comprises well laminated mud and silts depo
22、sited from suspension in deep-water areas.Previously deposited sediments may be reentrained,transported,and redeposited farther downslope on the subaqueous delta by gravity-driven mass-movement processes that include landslides,slumps,turbidity-current flows,and mudflows.Mud diapirs（底辟）are also a co
23、mmon feature of many subaqueous deltas.These structures are piercement-type（刺穿）bodies of soft sediment that is squeezed upward and intruded into overlying sand bodies owing to sediment loading.,（3）前三角洲 PRODELTA,前三角洲位于前缘的向海一侧，水深通常在浪基面以下。沉积物主要为泥和粉沙质泥，富含有机质，可见海绿石等自生矿物。层理主要是水平纹层。在近前缘的部分可出现少数粉砂薄层，其中有小型交错
24、纹层等构造。含海相动物化石，有较多的虫孔及生物扰动构造。三角洲前缘沉积物的再搬运、沉积形成的沉积物重力流，泥岩底辟、变形改造等。,三角洲前缘和前三角洲的同生变形特征Syn-deformational features in delta front and prodelta,三角洲前缘和前三角洲的同生变形特征Syn-deformational features in delta front and prodelta,一、曲流河三角洲沉积,三角洲的亚环境和沉积特征三角洲的主要类型、沉积相组合、沉积旋回及演化,三角洲沉积相组合、沉积旋回及演化,（1）平面相组合及垂向层序（相序）,A delta an
25、d adjacent environments,三角洲沉积相组合、沉积旋回及演化,（1）平面相组合及垂向层序（相序）,A delta and adjacent environments,浪控三角洲下细上粗的反旋回具浪蚀海滩脊序列以层序顶部三角洲平原的沼泽和分支河道沉积与海岸沉积的海滩脊相区别。,（2）三角洲类型及三维空间组合,A variety of shoal-water delta,内陆架浅水三角洲,河控三角洲是在河流输入泥沙量大，波浪、潮汐作用微弱：鸟足状三角洲 朵状三角洲,浪控三角洲的平面形态呈鸟嘴状，海洋的波浪作用大于河流的作用，河流输入泥沙量少，形成平行于海岸的海滩、沙嘴、沙坝,潮
26、控三角洲-河流流入三角港或其他形状的港湾，由于潮汐作用远大于河流作用，在港湾中堆积的泥沙受潮汐作用的强烈改造，仅形成小型三角洲。外形受港湾控制,发育潮汐沙坝。,River-dominated type of the shoal-water deltas,Wave-dominated type of the shoal-water deltas,Tide-dominated type of the shoal-water deltas,洞穴,不管三角洲沉积体系的成因类型如何，它们都是在浅水背景上发育起来的，其垂向剖面序列自下而上依次为前三角洲泥三角洲前缘沙三角洲平原的沙、泥互层夹煤层其中水下沉积
27、部分是一个向上变粗的沉积序列。,三角洲前缘沉积是其最特征部分，它不仅有别于其它沉积单元，也是区分不同三角洲类型的重要依据。河控三角洲前缘以河口沙坝沉积为特征，交错层倾角较大，古流向比较集中(单峰态)；浪控三角洲前缘沙具海岸沙坝沉积序列，交错层方向多变，倾角较小；潮控三角洲前缘沙具双向交错层，再作用面和粘土纹层，其上三角洲平原有潮坪及潮道沉积等。,滑坡界面,滑坡体,（3）三角洲沉积旋回与演化,三角洲沉积旋回分为两个时期：即建设期和破坏期：三角洲向海推进增长发育时期称三角洲的建设期三角洲被海水淹没遭受侵蚀破坏的时期称三角洲的破坏期每个单一的三角洲沉积体在垂向上就代表了一个沉积旋回，每个旋回都是以三
28、角洲的建设相开始，以破坏相的出现而告终在垂向剖面上，建设相和破坏相周期性地交替重复出现，就形成了多旋回三角洲复合体系,三角洲沉积旋回,三角洲体系的规模,构造与规模 三角洲沉积的厚度和保存状况与水深和盆地的沉降速率有密切关系 大型的河控三角洲主要发育在构造稳定下沉的地区。因其沉积物供应充足，沉速较大，所以比相邻的近岸沉积往往具有较大的厚度。,浅水型三角洲主要发育在基底稳定，水深较浅(数米至十余米)的克拉通盆地，三角洲沉积薄，推进快；盆地营力较弱，河流作用显得特别突出；相的关系主要表现在侧向上。进积相易于被后继河流冲刷破坏，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，压实沉陷作用微弱；三
29、角洲前缘及前三角洲沉积规模小，发育差，三角洲平原相占居很大比例；分流河道沙体分支多，延伸远，有的甚至可以沿盆地倾向延伸数百千米。三角洲形态呈不规则的分枝状浅水三角洲分布很广，在地质记录中保存较好。,深水型三角洲在深水条件下，三角洲沉积厚，推进慢；易于形成巨厚的前三角洲泥，如果前缘沙体推进到巨厚的前三角洲泥之上，快速的压实与沉陷将有利于完整进积序列和厚的前缘沙体的保存；突出特征是具有厚而完整的进积序列，骨架沙体呈巨；现代的密西西比三角洲是其典型例子。,水深变化与三角洲演化 河控三角洲是一种高建设性三角洲。因其地层样式受水深和基底性质的控制，所以随着盆底地形及底质性质的变化，河流将建造出沉积样式不
30、同的三角洲体系。在古代大型的海盆或湖盆中；缓坡一侧往往存在有浅而平坦的广阔浅水台地，其与深盆地往往以明显的斜坡过渡。在台地上形成分布广泛的浅水型三角洲。当三角洲推进到台地边缘的深盆地区，浅水型逐渐为深水型三角洲取代。深水型三角洲由于巨厚的前三角洲沉积的不稳定性和水下斜坡的存在，易于引起三角洲沉积滑塌破坏，所以分布比较局限。而滑塌破坏的产物也可以沉积物重力流的方式进入盆地底，形成独立的浊积扇体系。,三角洲体系的演化,相对海平面变化与三角洲演化,近海的,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,二、辫状河三角洲沉积,1.辨状河三角洲
31、的定义2.辨状河三角洲的发育条件3.辨状河三角洲的亚环境及沉积特征,辨状河三角洲是由辨状河体系前积到水体中富含沙和砾石的三角洲沉积体。,定义,需要特定的构造、地形和气候条件 扇三角洲往往形成在盆地断陷边界地带，辫状河三角洲往往发育在坳陷盆地陡坡边界地带，而曲流河三角洲往往形成在坳陷盆地缓坡边界地带。只有盆地坳陷较陡边界最有利于辫状河三角洲发育。这是因为较陡的盆地坳陷边界地带地形坡降大，易于辫状河人湖后形成独具特色的沉积体；随着进入盆地的沉积物数量的增加，辫状河三角洲的向前推进，冲积平原必然向盆地方向扩展，冲积平原沿陡倾的盆地边缘向盆地内进积，盆地的依次被充填表明沉积速率大于盆地沉降速率。,辨状
32、河三角洲的发育条件,辨状河三角洲的发育条件,古气候条件决定了一定区域的降水量、水系的特征、风化的强度以及碎屑物的供应，其对湖泊辫状河三角洲的影响尤为重要。干旱气候向潮湿气候转化会引起沉积物数量的增加，从而引起扇三角洲沉积向辫状河三角洲沉积转化。因此最适合辫状河三角洲形成的古气候是半干旱半潮湿气候。在这种气候条件下，有一定的降水量，可形成稳定的水系，同时物理风化亦较为强烈，可有大量的碎屑物供应。海平面的升降也会影响三角洲沉积类型的变化。一般认为，扇三角洲沉积与海平面快速上升有关，而辫状河三角洲沉积仅见于海平面上升减缓的时期。,3.辨状河三角洲的亚环境及沉积特征,三角洲平原三角洲前缘前三角洲,(1
33、)辫状河三角洲平原 辫状河三角洲平原是辨状河三角洲的水上部分，沉积特征与辨状河沉积特征相似，主要包括：辫状河道 河道间泛滥平原及沼泽微相并以辫状河道最具代表性。,辫状河道充填物为宽厚比高的、宽平板状的砂带，多个河道砂体叠合可形成分布广泛的粗粒厚层单元；由颗粒支撑的砂、砾组成，碎屑颗粒一般为次棱角-次圆状，分选中等，填隙物以泥质为主。测井曲线表现为较高的视电阻率，呈齿状或尖峰状起伏，自然电位曲线一般呈箱形或波状起伏。,泛滥平原主要为泥夹粉-细砂，而沼泽微相主要由煤层、碳质泥夹粉砂组成。主要以水平层理和波状层理为特征。古植物化石丰富。,(2)辫状河三角洲前缘 水下分流河道 河口沙坝 水下河道间 席
34、状沙,水下分流河道-系平原辫状河道入湖后在水下的延续部分，它在辫状河三角洲中所占的厚度是最大的。岩性一般较粗，以砂和砂砾为主。从剖面上看，水下分流河道大都从冲刷面开始。向上依次为具层理构造的砂、砾，顶部为薄层状泥质粉砂。河道砂频繁交互，形成多层楼式的正韵律的砂、砾组成的叠合砂。,水下河道间-主要为灰、灰绿色粉沙质泥、泥质粉沙夹薄层状砂。块状层理、小型交错层理及波状层理发育。由于水下分流河道冲刷力强，改道频繁，一旦发生改道，这些沉积物就被冲刷减薄，甚至全被冲刷掉，因此常以细粒碎屑夹层的形式出现。,河口砂坝-岩性为分选、磨圆较好的细砂、中砂，偶含泥砾。具板状、楔状及槽状交错层理。与下伏席状砂沉积过
35、渡，并一起构成明显的反韵律，电测曲线形态呈漏斗型。,席状砂-岩性一般为分选相对较好的细砂、粉砂与泥互层。波状层理、小型交错层理及水平层理发育。一般与前辫状河三角洲沉积物呈薄互层。,(3)前辫状河三角洲 细粒泥质沉积 沉积物重力流沉积,沉积物粒度较细，多为灰、灰绿、深灰色泥、粉砂质泥夹粉-细砂组成，在剖面上为砂泥薄互层。水平层理、块状层理及沙纹交错层理发育。,沉积物重力流主要为碎屑流、液化流和浊流沉积。,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,三、扇三角洲沉积,1.扇三角洲的概念 2.扇三角洲发育的条件 3.扇三角洲环境的划分
36、4.扇三角洲的沉积特点,Hotmes(1965),Mxgrowen(1970)的定义：由相邻高地进积到安静水体中的冲积扇 扇三角洲是由冲积扇(包括旱地扇和湿地扇)提供物源，在活动的扇体与稳定水体交界地带沉积的沿岸沉积体系。它是含有大量沉积载荷的冲积扇与海或湖相互作用的产物。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 depos
37、ited,mainly or entirely subaqueously,at the interface between the active fan and a standing body of water.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
38、洲的概念Fan Deltas,The alluvial fans can include any of the fan types and may form in settings ranging from glacial to humid to arid.扇三角洲的水下部分分河流、波浪和潮汐为主的三种类型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 Cop
39、per and the Yallahs)are considered to be fan deltas.沉积物在水下部分的沉积作用包括：碎屑崩落、浊流、惯性（重密度）流Sediments 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 flo
40、od stages;the riverborne load achieves sufficient density to overcome buoyancy and 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 avala
41、nching,inertia flow,and turbidity-current flow.These models were developed by study of fan deltas in fiords(Prior and Bornhold,1990),but serve to illustrate the general principal of subaqueous transport on fan deltas.,地形条件-临近山区的盆地边缘，高差变化大，坡度陡是扇三角洲发育的最有利和最基本的条件。地质构造条件-扇三角洲多发育在活动的构造区，常与同沉积期大型断裂带相伴。从大地
42、构造背景来看，沿大陆碰撞海岸、岛弧碰撞海岸以及克拉通内部的裂谷盆地或其它断陷盆地的岸边对扇三角洲发育最为有利。气候条件-在不同的气候区有不同类型的扇三角洲形成。（a）干旱半干旱地区多发育旱地扇三角洲(如死海西岸扇三角洲)；（b）潮湿的热带和温带地区易形成湿地扇三角洲和辫状河三角洲(如牙买加的耶拉斯三角洲)；（c）在寒冷潮湿的冰水冲积平原有利于辫状河平原三角洲或辫状 河三角洲的发育(如阿拉斯加科珀河扇三角洲)。,2.扇三角洲的发育的条件control over the growth ofCOARSE-GRAINED DELTA,扇三角洲多发育在活动的构造区,3.扇三角洲环境的划分及沉积相特点,(
43、1)扇三角洲平原是扇三角洲的陆上部分，包括从扇端至岸线之间的近海平原地带。在干旱半干旱地区的扇三角洲平原具有旱地冲积扇的沉积特征；在潮湿区的扇三角洲平原则以发育砾质辫状水系沉积为特征。,(2)扇三角洲前缘亦称过渡带，位于岸线至正常天气浪基面之间的浅水区 河控型扇三角洲前缘是湖泊扇三角洲的特点，以各种粒级的砂和粉砂为主，也常有砾石沉积。粒度变化向盆地方向变细，砂层中交错层理发育，底部具大型冲刷面。,(3)前扇三角洲是指扇三角洲的浪基面以下部分，向下与陆架泥或深水盆地沉积过渡。发育在宽阔陆棚上的扇三角洲的前三角洲沉积主要为临滨-远滨的粉砂和泥质沉积，与陆棚泥呈互层产出。,前扇三角洲 扇三角洲前缘
44、扇三角洲平原,退积型扇三角洲,进积型扇三角洲,(1)单个的扇三角洲的陆上部分一般比较小，平面形态多为扇形。(2)扇三角洲沉积体向陆方向通常都以断层为界，其近源沉积物(扇根)常以角度不整合超覆在古老的基岩地层上，(3)扇三角洲的组成均为砾石、含砾砂和砂等粗碎屑沉积物，成分和结构成熟度均比较低，反映其距物源区比较近，搬运距离短，沉积迅速的特点。(4)扇三角洲沉积体几何形态和粒度变化:一般为楔形，从山前向盆地(海或湖)方向变薄变细，逐渐过渡为盆地相而消失；(5)单个扇三角洲的垂直层序一般呈向上变粗的特点；(6)扇三角洲层序的厚度和延展范围受边缘断裂差异升降幅度控 制。单个扇三角洲沉积层序厚可达几十米
45、，而发育在板块边缘经历很长时期的扇三角洲层序可厚达几千米，延长几十千米。,扇三角洲沉积特征,第三章 三角洲沉积体系DELTA SYSTEMS,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,第三节 古代三角洲的识别标志 与油气资源的特征,一、三角洲体系的主要识别标志二、三角洲体系与油气资源的关系,一、三角洲体系的主要识别标志,三角洲沉积是河流注入蓄水盆地后，在浅水区快速建造的近岸碎屑沉积体系。尽管三角洲体系的影响因素多、沉积环境复杂、沉积物类型多，但其突出特征是具有特定的剖面沉积序列和骨架砂体的几何形态。不同类型的三角
46、洲体系，可以根据骨架砂体的成因特征、几何形态、定向习性、进积砂体与分流河道的空间关系，以及河道充填物的特征等加以识别。（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
47、are characterized by assemblages of lithofacies,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 characteristi
48、cs of facies types,bed geometry,and types of cyclic successions to focus gradually on the correct delta model.,三角洲沉积的识别标志Some general characteristics of delta deposits that can be useful in their recognition include:(1)几何形态 geometry,(2)侧向相变关系lateral facies relationships,(3)相的垂向序列vertical successions
49、 of facies,(4)沉积构造和化石sedimentary structures and fossils(5)地震剖面上的构造形态 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-domin
50、ated deltas.In cross 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
