嫩江现代河流沉积体岩相及内部构形要素分析

嫩江是松辽盆地北部一条多河型河流。本文以黑龙江省富裕县塔哈乡大马岗嫩江现代河流沉积露头为例,运用Miall结构要素分析法对嫩江现代河流沉积体岩相类型、层次界面及内部构形要素进行了系统研究,表明大马岗沉积体主要由块状层理细砾相、大型及小型低角度槽状交错层理细砂相、同沉积变形层理细砂相、波状交错层理细砂相、薄层状粉砂质泥与细砂互层相、微波状层理粉砂相、块状层理泥质粉砂相、水平层理泥相、块状层理粉砂质泥相等16种岩相构成,不同岩相空间分布变化差异较大。在大马岗沉积体内部识别出1～5级层次界面,划分出具有成因意义的7种构形要素:河道、砾质坝、侧向加积沉积体、单一侧积砂层、纹层砂席、砂底形及越岸细粒沉积,这种构形要素的划分丰富了Miall的分类方案。     

Floodplain sedimentology of the Squamish River, British Columbia: relevance of element analysis

Element analysis of modern-day floodplains provides a framework for characterizing associations amongst depositional forms, the processes responsible for them and their local depositional environment. From interpretation of the spatial association of elements, mechanisms of floodplain evolution can be analysed. The Squamish River, in southwestern British Columbia, is a high-energy, gravel-based river, which exhibits a distinct downstream gradation in channel planform type. The floodplain sedimentology of this river is evaluated using an element approach. Five elements, defined on the basis of their morphological outline, position within sediment sequences and sedimentological character, describe the floodplain sedimentology: (i) top-stratum, (ii) chute channel; (iii) ridge; (iv) bar platform; (v) basal channel gravels. The sedimentological composition of each element is described. Each of these units relates directly to morphostratigraphic units which make up contemporary bars of the Squamish River. Associations among facies defined at the bedform scale, morphostratigraphic units on bar surfaces and elemental floodplain features are described and explained. The vertical stacking arrangement of elements is analysed in trenches (dug perpendicular to the main channel) and in bank exposures. Two elemental sedimentology models are proposed. In the first model, bar platform sands are discontinuous above basal channel gravels. Chute channel, ridge and proximal topstratum elements form thick sequences above. The second model is characterized by sequences in which distal top-stratum deposits are observed. In these instances, bar platform sands are better preserved beneath the distal top-stratum element, with proximal top-stratum elements above. The applicability of these models is determined primarily by position on the floodplain. Chute channel reworking of floodplain sediments and replacement by top-stratum elements is the dominant process marginal to contemporary bars. Sites in which channel avulsion has resulted in preservation of distal top-stratum deposits in the midsequence of the present-day channel banks determine the occurrence of the second model. Although channel planform style changes down-valley in the study reach from braided to meandering, these two models apply in each reach. It is concluded that processes operative at the element scale, rather than the channel planform scale, determine floodplain sedimentology.     

The Aghajari (Upper Fars) Formation in the Folded Zagros Zone, Iran:Insights to Identify Facies, Architectural Elements, Fluvial Systems,Petrography and Provenance

The Aghajari Formation, called also the Upper Fars, develops throughout the Folded Zagros Zone and its thickness in the type section in southwest of Iran is 2966 meters. To analyze the sedimentary basin of this Formation, lithofacies, architectural elements and petrofacies of the related deposits in a section in southeast of Sarvestan in Fars Province of Iran with a thickness of 2221.45 meters were examined. Microscopically studying thin sections, the petrography and the occurrence of the deposits were determined. In this section, 16 lithofacies, 10 architectural elements and 2 sandstone petrofacies were identified. The lithofacies were divided into two major and minor groups, in which the major lithofacies consist of coarse-grained (Gh, Gp, Gt and Gm), medium-grained (Sh, Sp, St, Sl, Sm, Sr and Ss and fine-grained ones (Fm, Fl and Fsm), and the minor lithofacies were evaporative and mixed silisiclastic-carbonate. The identified architectural elements are CH, SB, GB, LA, DA, CR, CS, LV, LS, CH (FF) and FF. By combining evidences from facies analysis and architectural elements together, the Aghajari Formation was divided into three parts in which the related sedimentation environments, from top to bottom, are gravel, gravel-sand and fine-grained meandering river respectively. There have also been playas and shoreface in the lower part. Based on petrography, the sandstones of this formation were classified into two groups: litharenite and sublitharenite. The origin of these deposits (sandstones) was appointed to the recycled orogeny and the source of quartz is low and medium to high metamorphic rank. Using the field evidences, the paleocurrent direction was achieved indicating the direction of the paleocurrent from northwest to southeast at the time of deposition. It is hoped that these data can be used in the interpretation of the basin and reconstruction of the paleogeography in the local and regional scales.     

砂砾质辫状河储层构型解剖及层次建模

针对准噶尔盆地风城油田上侏罗统齐古组砂砾质辫状河储层,利用岩芯、露头及密井网资料,结合砂砾质辫状河储层构型层次结构、模式拟合和嵌入式层次建模方法,确定了不同级次的构型界面,对砂砾质辫状河5~3级构型单元进行了识别,建立了不同级别构型分布样式、规模及三维构型模型。研究表明:1）研究区砂砾质辫状河4级构型单元主要包括心滩坝和辫状水道两类,辫状水道包括泥质半充填水道和泥质充填水道等构型单元,砂砾质辫状河3级构型单元包括心滩坝内部沟道及落淤层等;2）根据辫状河不同级次构型单元规模公式,对辫状河不同级次构型单元进行定量表征,结果表明,研究区单一砂砾质河道宽度50~180 m;单一充填河道宽度30~70 m,长度100~350 m;单一心滩坝宽度120~400 m,长度300~800 m;心滩坝内部落淤层长度和宽度分别为110~330 m、60~200 m,沟道长度和宽度分别为100~200 m、15~25 m;3）在精细构型解剖研究的基础上,提出利用层次建模与嵌入式建模结合的新方法建立构型三维模型。分层次建立了4级构型模型,而在心滩坝内部则采用规模约束建立落淤层、沟道以及填充河道3级构型单元模型,最终建立了与定性模式和定量规模相适应的多级次耦合三维构型模型。     

现代渭河西安段沉积体沉积相与岩相特征

以渭河西安段草滩沉积体为例, 通过剖面和探槽的详细研究, 结合野外密集采样及样品的分析结果, 对现代渭河草滩沉积体的粒度特征、沉积相、岩相、地层层序及沉积模式进行了综合研究。结果表明, 现代渭河草滩河流沉积体主要由砂质、泥质和少量砾质沉积物组成, 是一个由三期沉积旋回组成的曲流河沉积体。在现代渭河草滩沉积体内部, 共识别出6种岩相, 即平行层理含细砾粗砂岩相、槽状交错层理中细砂岩相、平行层理中细砂岩相、块状层理含砾中砂岩相、交错层理含砾中砂岩和块状层理泥岩相;3种沉积微相, 即河床滞留沉积、边滩和泛滥平原, 这些沉积微相构成了草滩沉积体内部垂向上三套不同的沉积层序:下部为含砾粗砂和中砂组成的河床亚相层序, 砂质沉积厚度较大;中部为一个完整的曲流河沉积层序, 由滞留沉积、边滩和泛滥平原沉积构成;上部则主要为边滩和泛滥平原沉积层序。粒度分析结果表明河床滞留沉积物中的砂体粒径多分布在0.2~1.2 mm, 边滩沉积物中砂体粒径多分布在0.1~1.0 mm, 两者均是良好的砂矿体, 是建筑用砂的良好来源。     

Late Pleistocene-Holocene morphosedimentary architecture, Spiti River, arid higher Himalaya

The Spiti River drains the rain shadow zone of western Himalaya. In the present study, the fluvial sedimentary record of Spiti valley was studied to understand its responses to tectonics and climate. Geomorphic changes along the river enable to divide the river into two segments: (i) upper valley with a broad, braided channel where relict sedimentary sequences rise 15–50 m high from the riverbed and (ii) lower valley with a narrow, meandering channel that incises into bedrock, and here, the fluvio-lacustrine sediments reside on a bedrock bench located above the riverbed. The transition between these geomorphic segments lies along the river between Seko-Nasung and Lingti villages (within Tethyan Himalaya). Lithofacies analyses of the sedimentary sequences show six different lithofacies. These can be grouped into three facies associations, viz. (A) a glacial outwash; (B) sedimentation in a channel and in an accreting bar under braided conditions; and (C) formation of lake due to channel blockage by landslide activities. Seventeen optically stimulated luminescence ages derived from ten sections bracketed the phases of river valley aggradation between 14–8 and 50–30 ka. These aggradation phases witnessed mass wasting, channel damming and lake formation events. Our record, when compared with SW monsoon archives, suggests that the aggradation occurred during intensified monsoon phase of MIS 3/4 and that proceeded the Last Glacial Maxima. Thus, the study reports monsoon modulated valley aggradation in the NW arid Himalaya.     

现代荆江江心洲沉积

江心洲与边滩和心滩的主要差别在于其表面植被发育和长期稳定。河道规模的水力强度决定了由上游江心洲向下游江心洲沉积物粒度的不断细化,砂坝规模的局部水流条件决定了江心洲表面上由粒度和岩石相类型所指示的微相具有分带性。江心洲上部层序主要是溢岸水流沉积的结果,但其厚度和分布范围却受江心洲大小的限制,与正常溢岸沉积具有不同的指相意义。江心洲的沉积过程形成(1)正常向下游加积(NDA),(2)汇流区向下游加积(DAC),(3)反向双侧积(CABB)和(4)向上加积(UA)等四个三维建筑结构主要素,由此构成了江心洲砂体的三维建筑结构。     

准噶尔盆地玛湖凹陷百口泉组典型微相砾石定向性定量研究

砾石定向性可用于辅助判断粗粒沉积物沉积环境的介质性质与强度。在岩芯图像分析与测量的基础上,运用砾石长轴相对视倾角玫瑰花图中任意相邻三个小扇形的半径之和的最大值（参数a）和小扇形半径大小偏离程度（参数σ）,对玛湖地区百口泉组152种典型砾岩岩石相以及四种典型微相中的砾石定向性进行定量研究,结果表明:玛湖地区百口泉组不同岩石相中砾石定向性存在一定的差异,但是总的来说岩石相中包含交错层理属性、平行层理属性、小中砾岩属性的,砾石定向性比较好,定向性参数比较大;四种典型微相中,辫状河道沉积、水下分流河道沉积、河口沙坝沉积中砾石定向性变化在垂向上都有一定的旋回性,而泥石流沉积中砾石定向性变化在垂向上的旋回性不是很明显;四种典型沉积微相中砾石定向性参数大小也存在一定的差别。定向性由差到好依次是泥石流微相（σ最大值为6.61、最小值为3.00、平均值为4.03,a最大值为52.94%、最小值为22.95%、平均值为32.87%）、辫状河道微相（σ最大值为8.91、最小值为3.11、平均值为5.31,a最大值为65.79%、最小值为25.93%、平均值为341.41%）、水下分流河道（σ最大值为7.22、最小值为2.96、平均值为5.11;a最大值为55.71%、最小值为25.93%、平均值为38.76%）、河口沙坝微相（σ最大值为10.34、最小值为3.65、平均值为5.54;a最大值为83.02%、最小值为32.35%、平均值为44.42%）。     

The origin and evolution of the Klondike goldfields, Yukon, Canada

The world famous Klondike goldfields are located in the unglaciated part of west-central Yukon, Canada. Since their discovery over 100 years ago, they have produced an estimated 311 tonnes of gold, primarily from bench and creek placers that are fluvial in origin and range from Pliocene to Holocene in age. Historically, the placers are classified into three levels of gravel with four main units. These include the high-level White Channel Gravel (Pliocene), presently the most important gold-bearing unit, which sits nonconformably on an erosional bedrock surface (i.e., the ‘White Channel strath’) and is overlain and interbedded with the glaciofluvial Klondike Gravel (Pliocene); the intermediate-level gravel (Pleistocene), the least important economically; and the low-level gravel (Pleistocene–Holocene), historically the most important gold-bearing unit, but it has been mined three or four times now. The goldfields originated from the weathering and erosion of early Cretaceous, discordant mesothermal quartz veins, and the light grey color of the matrix of the White Channel Gravel is due mainly to weathering and diagenetic alteration by groundwater flow. The concentration of placer gold is related to a hierarchy of physical scales: at the lithofacies scale (metres), bed roughness determined sites of gold deposition; at the element scale (tens of metres), gravel bars were preferentially enriched in gold; at the reach scale (hundreds of metres), stream gradient was an important factor; at the system scale (hundreds of km), braided river environments transported large amounts of gold; and at the sequence scale (thousands of km), economic placers formed initially in the high-level White Channel Gravel and later in the intermediate-level and low-level gravel. The White Channel strath is interpreted as an erosional ‘tectonic’ terrace that formed during isostatic uplift and under conditions of dynamic equilibrium. The high-level White Channel Gravel and Klondike Gravel are interpreted as a depositional ‘climatic’ terrace that formed during a reversal in the tectonically induced downcutting, which is attributed to the initial and most extensive of the pre-Reid glaciations (3 Ma) in the Yukon. The intermediate-level gravel is interpreted as minor erosional ‘complex response’ terraces that formed during static equilibrium when there were pauses in valley-floor degradation, which are attributed to the subsequent and less extensive pre-Reid glaciations. The low-level gravel formed also during valley-floor degradation and may represent a return to dynamic equilibrium conditions. Hence, the dominant forcing mechanisms controlling the evolution of the goldfields were isostatically compensated exhumation and climatic change related to the repeated glaciation of the Yukon. In addition, the lowering of baselevel from high-level, to intermediate-level and finally to low-level gravel was accompanied by a decrease in accommodation space (as indicated by a decrease in gravel thickness), which resulted in an increase in the concentration of the placer gold.     

Sedimentary architecture and 3D ground-penetrating radar analysis of gravelly meandering river deposits (Neckar Valley, SW Germany)

Sedimentological outcrop analysis and sub‐surface ground‐penetrating radar (GPR) surveys are combined to characterize the three‐dimensional sedimentary architecture of Quaternary coarse‐grained fluvial deposits in the Neckar Valley (SW Germany). Two units characterized by different architectural styles are distinguished within the upper part of the gravel body, separated by an erosional unconformity: (i) a lower unit dominated by trough‐shaped depositional elements with erosional, concave‐up bounding surfaces that are filled by cross‐bedded sets of mainly openwork and filled framework gravel; and (ii) an upper unit characterized by gently inclined sheets of massive and openwork gravels with thin, sandy interlayers that show lateral accretion on a lower erosional unconformity. The former is interpreted as confluence scour pool elements formed in a multi‐channel, possibly braided river system, the latter as extensive point bar deposits formed by the lateral migration of a meandering river channel. The lateral accretion elements are locally cut by chute channels mainly filled by gravels rich in fines, and by fine‐grained abandoned channel fills. The lateral accretion elements are associated with gravel dune deposits characterized by steeply inclined cross‐beds of alternating open and filled framework gravel. Floodplain fines with a cutbank and point bar morphology cover the gravel deposits. The GPR images, revealing the three‐dimensional geometries of the depositional elements and their stacking patterns, confirm a change in sedimentary style between the two stratigraphic units. The change occurred at the onset of the Holocene, as indicated by ¹⁴C‐dating of wood fragments, and is related to a re‐organization of the fluvial system that probably was driven by climatic changes. The integration of sedimentological and GPR results highlights the heterogeneity of the fluvial deposits, a factor that is important for modelling groundwater flow in valley‐fill aquifers.     

Tectonic setting and sedimentology of Ganga River sediments, India

The Ganga basin provides a present-day example of a peripheral foreland basin. The course of the river is controlled by Himalayan tectonics. Three main types of architectural elements, such as channels (CH), sandy bedforms (SB) and overbank fines (OF) have been developed in Ganga River sediments. The channels (CH) include gravelly (Gs) and sandy channel (Ss) lithofacies. The sandy bedforms (SB) include trough cross-stratified (St), planar cross-stratified (Sp), horizontal stratified (Sh), sandy massive (Sm) and climbing ripple cross-laminated (Sr) lithofacies, all of which are active channel deposits. The overbank fines (OF) include massive silt and clay (Fm), parallel laminated silt and clay (FI) and climbing ripple cross-laminated (Sr) lithofacies. Mega units have been developed in the lower part of the active channel deposits, while small units have been developed in the upper part of active channel deposits, in inactive channel deposits and overbank fines. This study illustrates the seasonal and tectonic control on sedimentation. Petrofacies studies of the sediments indicate a recycled orogen provenance. The sediments are derived from rapidly uplifted fault blocks comprising granite, gneiss and basic and ultrabasic rocks. Lack of textural and compositional maturity suggests a local source of derivation. The principal control on sand composition is source lithology. The hot and humid climate may slightly increase the content of quartz in sand derived from reworked foreland basin sediments. but the effect is neither sufficient to shift the sand compositions out of the recycled orogen field nor does it obscure composition mixing patterns.     

青海油砂山油田第68层分流河道砂体解剖学

青海油砂山剖面第６８层三角洲分流河道砂体内部可识别出来Ｇｐｒ、Ｇｌｔ、Ｇｅ、Ｓｌｔ、Ｓｍｔ、Ｓｐｒ、Ｓｌａ、Ｓｂｍ、ＳＩｃｒ、ＳＩｈ和Ｍｒｉ等１２种岩石相，它们构成了６种岩石相组合，即:Ｇｅ－Ｇｐｒ－Ｇｌｔ组合，Ｇｌｔ－Ｓｌｔ组合，Ｓｌｔ－Ｓｍｔ组合，Ｇｌｔ－Ｓｂｍ组合，Ｓｓｔ－Ｍｒｌ组合，Ｇｐｒ（Ｓｐｒ）－Ｍｒｌ－Ｓｓｔ－Ｍｒｌ－ＳＩｃｒ－Ｍｒｌ组合。按照沉积界面之间的切割关系，建立一个包括７级界面的界面层次，第一级为砂体的上下边界，第七级为纹层边界，沉积界面将砂体分隔为三个沉积幕，它们由六个沉积亚幕组成。砂体建筑结构要素包括侧积体（ＬＡ），纵向砂坝（ＬＢ），横向砂坝（ＴＢ），简单砾质充填（ＧＦ），简单砂质充填（ＳＦ）和堤岸砂坪（ＬＦ）等六种类型，它们具有不同的宽度和厚度比。通过露头密网格采样和沉积学分析建立了砂体渗透率原型模型，均质模型，幕式模型以及建筑结构要素模型，其中建筑结构模型与原型模型相似性最好，证明了利用建筑结构分析可以建立符合砂体客观实际的储层地质模型。     

辫状河致密砂岩储层构型单元定量表征方法

针对现有辫状河储层构型单元定量表征方法关键参数获取困难且可靠性存疑的问题,以储层构型理论为指导,基于现代辫状河沉积、钻井岩心、录井和测井等资料,提出了基于单一辫状河道宽度的构型单元定量表征方法。首先,根据单井构型单元解释结果,确定同期辫状河道平面分布;其次,根据连井对比剖面,确定连井剖面内单一辫状河道间的界线位置;再次,根据现代辫状河单一辫状河道宽度与构型单元关系定量模型,限定辫状水道宽度、复合心滩长度及宽度等4级构型单元规模;最后,在单一辫状河道内确定4级构型单元平面分布,进而在密集井网区对复合心滩进行解剖,确定单个心滩位置和几何形态。采用该方法对苏××提高采收率试验区盒₈^下_1-3单层辫状河致密砂岩气藏进行了分析,结果表明：该单层砂岩气藏发育5条单一辫状河道,其宽度为1 050~1 890 m;辫状水道宽度为234~435 m;复合心滩长度和宽度分别为1 931~3 569 m、685~1 242 m;单个心滩长度和宽度分别为660~1 880 m、310~1 030 m。     

成都平原东郊台地中更新统合江组沉积特征及工程地质意义

通过地层沉积序列、粒度特征、砾石特征综合分析认为,成都平原东郊台地中更新统合江组为河流相沉积,垂向上具有明显的"河流二元"结构特征,主要以细粒沉积物为主,底部砾石层多为中-粗砾,磨圆较好,具有定向性。该组可细分为河道、边滩、堤岸-河漫3种沉积微相类型,对应发育3种岩相类型。河道砾石岩相沿十陵镇-红河镇-大面镇一带呈北西-南东向展布,边滩砂坝岩相集中发育于大面镇-西河镇东侧古河道凹岸处,堤岸-漫滩粘土岩相广泛分布于研究区内。这3种岩相特性及分布对工程建设的影响存在差异:河道砾石层作为地下水储集层的同时,水溶蚀使其地层结构稳定性被破坏,影响工程稳固技术的选择;边滩砂坝分布较为集中,对区内工程建设影响面最小;堤岸-河漫粘土广泛分布,是一种分布稳定的软弱层,对地下水有一定的封闭效果,同时软弱层的特性使其降低了边坡、坝体的稳定性。     

Facies relationships in Pleistocene outwash gravels, southern Ontario: a model for bar growth in braided rivers

Facies relationships in Pleistocene braided outwash deposits in southern Ontario demonstrate the presence of a large braid bar with adjacent side channel. The core of the bar is up to 6 m high, and consists of crudely horizontally stratified gravels. Downstream from the core is the bar front facies, consisting of large gravelly foresets up to 4 m high, rounded off in many places by reactivation surfaces. Upstream from the core is the bar stoss side facies consisting of several sets (individually up to 35 cm thick) of tabular cross-bedding, arranged in coarsening-upward sequences. The stoss side—core—bar front relationships are continuously exposed in one 400 m long quarry face which is cut almost parallel to the palaeoflow direction. A transverse quarry face shows the side channel facies, which consists of trough cross-bedded sands. Gravel layers can be seen to finger from the main gravelly bar into the sandy side channel, but they do not reach the base of the channel. This surprising relationship indicates that gravel moved only in the topographically higher parts of the system. After deposition in the side channel, and growth upstream and downstream from the bar core, the entire system aggraded. Crudely horizontally stratified, and imbricated gravel sheets were laid down as a bar top facies. Grain size analyses indicate strongly bimodal distributions, implying that much of the sand in the spaces between pebbles and boulders filtered in after the gravel had been deposited. This interpretation is strengthened by velocity calculations—mean velocities in excess of 300 cm/s would be needed to roll the gravel as bed load, but at such a velocity, a large amount of sand would be transported entirely in suspension. In a final section of the paper, our results are combined with other work on braided systems in an attempt to formulate a more general facies model.     

新疆克拉玛依油田三叠系克下组冲积扇内部构型*

应用研究区丰富的密井网和邻近露头资料,探讨了新疆克拉玛依油田三叠系克下组冲积扇内部构型单元的层次划分系统,并建立相应的沉积构型模式。在冲积扇的扇根、扇中和扇缘各亚相带,进一步划分了3个级次的构型要素以表达其内的沉积复杂性。扇根亚相主要由主槽、片流带和漫洪带组成,其中,在主槽内主要发育槽流砾石（岩）体（可细分为砾石坝和流沟）与泥石流沉积,在片流带内主要发育片流砂砾体（可细分为砾石坝和流沟）,在漫洪带主要发育漫洪砂体和漫洪细粒沉积,总体构型样式以广泛分布的砾石坝夹含流沟和漫洪沉积的“泛连通体”为特征;扇中亚相主要由辫流带和漫流带组成,其中,辫流带的主体构型要素为辫流水道（可细分为砂坝和沟道）,总体构型样式为复合宽带状辫流水道砂体与漫流细粒沉积侧向相间、垂向互层为特征;扇缘亚相主要由径流带和漫流带组成,其中,径流带的主体构型要素为径流水道,总体构型样式以窄带状径流水道和漫流砂体镶嵌于漫流—湿地泥岩之中为特征。     

Evolution and sedimentology of a channel fill in the sandy braided South Saskatchewan River and its comparison to the deposits of an adjacent compound bar

The depositional stratigraphy of within‐channel deposits in sandy braided rivers is dominated by a variety of barforms (both singular ‘unit’ bars and complex ‘compound’ bars), as well as the infill of individual channels (herein termed ‘channel fills’). The deposits of bars and channel fills define the key components of facies models for braided rivers and their within‐channel heterogeneity, knowledge of which is important for reservoir characterization. However, few studies have sought to address the question of whether the deposits of bars and channel fills can be readily differentiated from each other. This paper presents the first quantitative study to achieve this aim, using aerial images of an evolving modern sandy braided river and geophysical imaging of its subsurface deposits. Aerial photographs taken between 2000 and 2004 document the abandonment and fill of a 1·3 km long, 80 m wide anabranch channel in the sandy braided South Saskatchewan River, Canada. Upstream river regulation traps the majority of very fine sediment and there is little clay (< 1%) in the bed sediments. Channel abandonment was initiated by a series of unit bars that stalled and progressively blocked the anabranch entrance, together with dune deposition and stacking at the anabranch entrance and exit. Complete channel abandonment and subsequent fill of up to 3 m of sediment took approximately two years. Thirteen kilometres of ground‐penetrating radar surveys, coupled with 18 cores, were obtained over the channel fill and an adjacent 750 m long, 400 m wide, compound bar, enabling a quantitative analysis of the channel and bar deposits. Results show that, in terms of grain‐size trends, facies proportions and scale of deposits, there are only subtle differences between the channel fill and bar deposits which, therefore, renders them indistinguishable. Thus, it may be inappropriate to assign different geometric and sedimentological attributes to channel fill and bar facies in object‐based models of sandy braided river alluvial architecture.     

嫩江现代河流沉积层序及沉积模式

以黑龙江省富裕县大马岗现代嫩江沉积体为例,通过深挖探槽、探坑及野外密集采样的精细解剖,详细研究了现代嫩江大马岗沉积体内部岩相、沉积微相、沉积层序及沉积模式。研究表明,大马岗河流沉积体是一个由辫状河与曲流河沉积组成的复合沉积体,其下部以辫状河砾质坝沉积为主,上部则以低能量的曲流河沉积为主。在现代嫩江大马岗沉积体内部,共识别出16种岩相,即Gm,St1,St2,Ft,Ssd,Sh,Stcb,Shcs,Sw,Sm,Fw,Fm,Fsc1,Fsc2,Mm及Mpr,发育五种亚相、六种沉积微相,即曲流河的河漫滩沉积微相、决口扇微相、天然堤微相、点坝微相、辫状河的溢岸微相及心滩微相。这些沉积微相构成了大马岗沉积体内部垂向上三套不同的沉积层序:下部层序以砾石质沉积为主,为辫状河沉积形成;中部层序为一个完整的曲流河沉积层序,以曲流河点坝沙及溢岸漫滩泥沉积构成;上部沉积层序则为曲流河泛滥及决口沉积形成。通过沉积微相及沉积层序分析,本文建立了一种新型的河流复合沉积模式,进一步丰富了河流沉积学理论。     

Sedimentation in a meandering gravel-bed river: The River Tywi,South Wales

Previous studies of meandering gravel-bed rivers have illustrated a wide range of bar types. The River Tywi of South Wales shows that significant variations of accretionary style can also occur within a single river. There is a downstream decrease in the proportion of lateral bars to point bars and changes in the morphological characteristics of these point bars. Three types are recognized: simple, linguoid and multi-unit point bars. Sedimentation on the concave sides of meander bends is locally important. The changes of bar type are accompanied by different styles of channel behaviour. The River Tywi is interpreted to have deposited multilateral gravel sheets, composed of partially reworked and abandoned bars and dissected by palaeochannels and sloughs. Bar deposits consist of parallel-bedded gravel, inclined laterally-accreted gravel, local angle-of-repose foresets and inclined lenses of heterolithic beds. The proportion of the various sedimentary structures and the geometry of the abandoned bars varies along the Tywi valley because of the patterns of bar distribution and channel behaviour. The deposits of this river have strong affinities with Tertiary sequences in the Italian Apennines, previously interpreted as the deposits of meandering gravel-bed rivers. This type of river is not readily distinguished from ‘Scott type’ braided streams in the geological record, unless exposures are particularly good. In this respect, the presence of abundant, inclined heterolithic wedges and lenses may be a useful diagnostic criterion.