1、第三章 三角洲沉积体系DELTA SYSTEMS三角洲的环境特点及其沉积作用第二节 三角洲的沉积一、曲流河三角洲沉积一、曲流河三角洲沉积1.1.三角洲的亚环境和沉积特征三角洲的亚环境和沉积特征两套两套术语术语（two sets of terms for deltas）水上三角洲水上三角洲水下三角洲水下三角洲所有三角洲都可以分所有三角洲都可以分为水上三角洲（三角为水上三角洲（三角洲平原：又分上部和洲平原：又分上部和下部）和水下三角洲下部）和水下三角洲（三角洲前缘和前三（三角洲前缘和前三角洲）角洲）Physiographic and Sediment Characteristics of Allu
2、vial 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 subaqueous comp
3、onents,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,is commonl
4、y called the delta front.（1 1）三角洲平原）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 tida
5、l 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 1）三角洲平原）三角洲平原 （DELTA PLAIN）Upper Del
6、ta 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
7、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 channels(辫状河道辫状河道),meandering channels(曲流河道
8、曲流河道),lacustrine delta fill,backswamps,and floodplain environments such as swamps,marshes,and freshwater lakes(淡水淡水湖泊湖泊)(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 depos
9、its.湿地湿地（1 1）三角洲平原）三角洲平原 （DELTA PLAIN）Lower Delta Plain SedimentsThe 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
10、 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 bays(分流间湾分流间湾),bay fills(crevasse splays),marshes,and swamps(C
11、oleman 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 deposits.In very arid climates,evaporites(蒸发盐蒸发盐)also may be depos
12、ited 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 channels after they have been abandoned by the main stream owing to bloc
13、king or other processes that cause channel shifting.（1 1）三角洲平原）三角洲平原 （DELTA PLAIN）（2 2）三角洲前缘三角洲前缘(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 sediments.It may extend outward for distance
14、s 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,may be dominated by high-energy marine
15、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 finer sands and coarse silts that settle
16、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 2）三角洲前缘三角洲前缘(DELTA FRONT)（3 3）前三角洲前三角洲 (PRODELTA)The remaining seaward part of th
17、e 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.It may not exist in shallow-water areas.It comprises well laminated mud and silts deposited from suspe
18、nsion 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）（底辟）are also a common feature
19、 of many subaqueous deltas.These structures are piercement-type（刺穿）（刺穿）bodies of soft sediment that is squeezed upward and intruded into overlying sand bodies owing to sediment loading.（3 3）前三角洲前三角洲 PRODELTA三角洲前缘和前三角洲的同生变形特征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
20、和前三角洲的同生变形特征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、曲流河三角洲沉积一、曲流河三角洲沉积A delta and adjacent environmentsA delta and adjacent environments上粗下细上粗下细内陆架或浅水三角洲内陆架或浅水三角洲（陆架边缘或深水三角洲陆架边缘或深水三角洲（河控三角洲河控三角洲浪控三角洲浪控三角洲潮控三角洲潮控三角洲A variety of shoal-water delta内陆架浅水三角洲内陆架浅水三角洲River-d
21、ominated type of the shoal-water deltasWave-dominated type of the shoal-water deltasTide-dominated type of the shoal-water deltas洞穴 滑坡界面滑坡体（3 3）三角洲沉积旋回与演化）三角洲沉积旋回与演化n三角洲沉积旋回分为两个时期：即建设期和破坏期：三角洲沉积旋回分为两个时期：即建设期和破坏期：n三角洲向海推进增长发育时期称三角洲的建设期三角洲向海推进增长发育时期称三角洲的建设期n三角洲被海水淹没遭受侵蚀破坏的时期称三角洲的三角洲被海水淹没遭受侵蚀破坏的时期称三角洲的
22、破坏期破坏期n每个单一的三角洲沉积体在垂向上就代表了一个沉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，每个旋回都是以三角洲的建设相开始，以积旋回，每个旋回都是以三角洲的建设相开始，以破坏相的出现而告终破坏相的出现而告终n在垂向剖面上，建设相和破坏相周期性地交替重复在垂向剖面上，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沉积旋回三角洲体系的演化三角洲体系的演化相对海平面变化与三角洲演化相对海平面变化与三角洲演化近海的二、辫状河三角洲沉积二、辫状河三角洲沉积1.辨状河三角洲的定义辨状河三角洲的定义2.辨状河三角洲
23、的发育条件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。含沙和砾石的三角洲沉积体。定义定义需要特定的构造、地形和气候条件需要特定的构造、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，而三角洲往往发育在坳陷盆地陡坡边界地带，而曲流河曲流河三角洲往往形成在坳陷盆地缓坡边界地带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只有盆地
24、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，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，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，冲积平原必然向盆地方向扩展，冲积平原沿陡倾的盆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。率大于盆地沉降速率。辨状河三角洲的发育条件辨状河三角洲的发育条件辨状河三角洲的发育
25、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。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，可潮湿气候。在这种气候条件下，有一定的降水
26、量，可形成稳定的水系，同时物理风化亦较为强烈，可有大形成稳定的水系，同时物理风化亦较为强烈，可有大量的碎屑物供应。量的碎屑物供应。海平面的升降海平面的升降也会影响三角洲沉积类型的变化。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。辫状河三角洲沉积仅见于海平面上升减缓的时期。3.辨状河三角洲的辨状河三角洲的亚环境及沉积特征亚环境及沉积特征三角洲平原三角洲平原三角洲前缘三角洲前缘前三角洲前三角洲沉积物重力沉积物重力流主要为碎流主要为碎屑流、液化屑流、液化流和浊流沉流和浊流沉积。积
27、。三、扇三角洲沉积三、扇三角洲沉积 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 water.Fan deltas we
28、re 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).1.1.扇三角洲的概念扇三角洲的概念Fan DeltasThe alluvial fans can include any of the fan types and may form in settings ranging from glacial
29、to humid to arid.扇三角洲的水下部分分扇三角洲的水下部分分河流、波浪和潮汐为主河流、波浪和潮汐为主的三种类型的三种类型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 Copper and the Yallahs)are considered to be fan deltas.沉积物在水下部分的沉积作用包括：沉积物在水下部分的沉积作用包括：碎屑崩
30、落、浊流、惯性（重密度）流碎屑崩落、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 flood stages;the riverborne load achieves sufficient densi
31、ty 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 avalanching,inertia flow,and turbidity-current flow.These mo
32、dels 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.2.2.扇三角洲的发育的条件扇三角洲的发育的条件control over the growth control over the growth ofCOARSEofCOARSE-GRAINED DELTA-GRAINED DELTA3.3.扇三角洲环境的划分及沉积相特点扇三角
33、洲环境的划分及沉积相特点 退积型退积型进进积积型型第三章 三角洲沉积体系DELTA SYSTEMS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第三节 古代三角洲的识别标志古代三角洲的识别标志 与油气资源的特征与油气资源的特征The subenvironments of delta systems range from normal marine(beach,barrier,lagoonal)to nonmarine(fluvial marsh,eolian),and a variety of different sed
34、iment types can be deposited in these subenvironments.Deltaic sedimentary successions 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
35、series of steps,eliminating other possibilities and using distinguishing characteristics 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三角洲沉积的识别标志Some general characteristics of delta deposits that can be useful in their recogniti
36、on include:(1)几何形态几何形态 geometry,(2)侧向相变关系侧向相变关系lateral facies relationships,(3)相的垂向序列相的垂向序列vertical successions of facies,(4)沉积构造和化石沉积构造和化石sedimentary structures and fossils(5)地震剖面上的构造形态地震剖面上的构造形态 the structural attitude of deltaic deposits revealed in seismic reflection profiles.Geometry Ideally,de
37、ltas are triangular in areal shape;however,much variation from this ideal shape can occur,particularly with tide-and wave-dominated 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
38、from small bodies a few thousand square kilometers in size to huge deltas exceeding 100,000 km2.Nonetheless,delta deposits have a restricted lateral extent,with the center of deposition fixed around a river mouth.Lateral Facies Relationships Deltaic origin is suggested by a wedge-or lobeshaped depos
39、it of nonmarine to shallow marine sand,silt,mud that grades landward into totally nonmarine,largely fluvial sediments and basinward into 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 dis
40、tributary-channel deposits to finer grained marsh or interdistributary-bay or lacustrine deposits.The lateral associations of delta-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
41、 facies relation-ships.Coarser,delta-front sediments may grade seaward into prodelta silts and clays which,in turn,grade 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 contai
42、n more mud-turbidite units than shelf muds.Vertical Facies SuccessionsProgradation（进积进积）of deltaic deposits during active delta growth produces a generally coarsening-upward sedimentary succession,which passes upward from offshore(marine)facies to nonmarine fluvial facies.As discussed,growth of delt
43、as tends to be cyclic（旋回性）旋回性）.Migration of delta-front sands over prodelta silts and clays generates fairly well-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),c
44、oarsening-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 si
45、liciclastic coasts and estuaries.Sedimentary Structures and Fossils Numerous types of sedimentary structures-such as cross-bedding,ripple marks,bioturbation structures(生物扰动构造生物扰动构造),root traces(marsh deposits),slump structures(滑塌构造滑塌构造),and mud diapirs(泥岩底辟泥岩底辟)-occur in deltaic deposits.With the po
46、ssible exception of mud diapirs,all of these structures are found also in many other environments;none is uniquely diagnostic(诊断诊断)of delta environments.Suites of structures may help to define a particular type of deltaic deposit such as fluvial deposits.Paleocurrent directions in deltaic deposits c
47、an be highly variable,ranging from unidirectional patterns in fluvial-dominated portions of deltas to bidirectional patterns in tide-dominated portions.There are no specific fossil taxa that are characteristic of deltas alone.A transition from freshwater to brackish water to saltwater fauna in facie
48、s having the characteristics described under Geometry and Lateral Facies Relationships is indicative of deltaic origin;however,such transitions can occur also in estuarine sediments.Seismic Reflection ProfilesSediments deposited on the slope of a delta front commonly have a fairly steep initial dip.
49、These seaward dipping beds are referred to as clinoforms.Clinoforms may be recognizable in some well-exposed outcrops that have not been tectonically tilted;however,they are best recognized in seismic reflection profiles.泥质沉积区泥质沉积区黄海槽黄海槽二、三角洲沉积体系与油气资源的关系二、三角洲沉积体系与油气资源的关系1.生（前三角洲）储（前缘）盖（河道间）组合生（前三角洲）
50、储（前缘）盖（河道间）组合2.变形构造和泥岩底辟形成的圈闭变形构造和泥岩底辟形成的圈闭3.岩性、地层圈闭岩性、地层圈闭END OF THIS SESSIONEND OF THIS SESSION第三章 三角洲沉积体系DELTA SYSTEMS三角洲的环境特点及其沉积作用第二节 三角洲的沉积三角洲沉积体系三角洲沉积体系内容内容 1.三角洲分类、三角洲发育的影响因素及形成过程三角洲分类、三角洲发育的影响因素及形成过程 2.扇三角洲沉积（发育条件：地形条件，地质构造扇三角洲沉积（发育条件：地形条件，地质构造条件与气候条件；沉积相特征：扇三角洲平原，前缘与条件与气候条件；沉积相特征：扇三角洲平原，前缘
