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.     

Counter point bar deposits: lithofacies and reservoir significance in the meandering modern Peace River and ancient McMurray Formation, Alberta, Canada

Counter point bar deposits in the meandering Peace River, North‐central Alberta, Wood Buffalo National Park, are distinct from point bar deposits in terms of morphology, lithofacies and reservoir potential for fluids. Previously referred to as the distal‐most parts of point bars, point bar tails and concave bank‐bench deposits, counter point bar deposits have concave morphological scroll patterns rather than convex as with point bars. The Peace is a large river (bankfull discharge 11 700 m³ sec^?1, width 375 to 700 m, depth 15 m, gradient 0·00004 or 4 cm km^?1) in which counter point bar deposits are dominated by silt (80% to 90%), which contrasts with sand‐dominant (90% to 100%) point bar deposits. Beginning at the meander inflection (transition from convex to concave), counter point bar deposit stratigraphy thickens as a wedge‐like architecture in the distal direction until the deposit is nearly as thick as the point bar deposits. The low permeability silt‐dominant lithofacies in counter point bar deposits will limit reservoir extent and movement of fluids in both modern and ancient subsurface fluvial deposits. In the exploration and extraction of bitumen and heavy oil in subsurface fluvial rocks, identification and mapping of reservoir potential of point bar deposits and counter point bar deposits is now possible in the fluvial‐dominated tidal estuarine Lower Cretaceous Middle McMurray Formation, North‐east Alberta. Recent geophysical advances have facilitated imaging of some ancient buried point bar deposits and counter point bar deposits which, on the basis of morphological shape of sedimentary bodies observed from seismic amplitude, can be interpreted and mapped as depositional elements or blocks that contain associated sandstone or siltstone dominant lithofacies, respectively. As counter point bar deposits exhibit poor permeability and thus limit reservoir potential for water, natural gas, light crude, heavy oil and bitumen, counter point bar deposits should be avoided in resource developments. Geophysical imaging, interpretation and mapping of point bar deposit and counter point bar deposit elements provide new opportunities to improve recovery of bitumen and heavy oil and reduce development costs in subsurface cyclic steam stimulation and steam‐assisted gravity drainage projects by not drilling into counter point bar deposits.     

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.     

Alluvial deposits of a mud-dominated stream: the Red River, Manitoba, Canada

Abstract The Red River, Manitoba, is a mud‐dominated, meandering stream that occupies a shallow valley eroded into a clay plain. The valley‐bottom alluvium is the product of incision and lateral migration of river meanders. As revealed by a transect of five boreholes located across the floodplain at each of two successive river meanders, the alluvial deposits range from about 15 to 22 m thick and are composed primarily of silt. Sedimentary structures in the cores are weakly defined and consist mostly of beds of massive silt, thick (>0·4 m) massive silt and disturbed silt. Interlaminated sand and silt, and sand beds form relatively minor deposits, principally within the lower half of the alluvium, and thin beds of medium‐coarse sand and pea gravel can be present locally within the lower metre of the alluvium. The alluvium is interpreted to consist of overbank deposits from 0 to 2–3 m depth, oblique accretion deposits from 2–3 to 8–12 m depth and oblique accretion and/or channel deposits from 8–12 m to the base of the sequence. The massive bedding within the oblique accretion deposits is interpreted to represent the remnants of couplet deposits that were initially composed of interbedded, muddy silt and sand‐sized silt aggregates, as is consistent with the contemporary bank sedimentation. The post‐depositional disintegration and/or compaction of the aggregates has caused the loss of the sand‐sized texture. The disturbed silt beds are interpreted as slump structures caused by large‐scale rotational failures along the convex banks. Overall, the Red River represents a portion of a continuum of muddy, fine‐grained streams; where the alluvium lacks a distinct coarse unit, oblique accretion deposits form a majority of the floodplain, and large‐scale slump features are present.     

卵石沙洲发育与冲刷试验

沙洲是塑造分汊型河道最重要的形态因子,其发育与蚀退由于上游来水来沙变化呈现冲淤交替,从而影响分汊河道输水输沙平衡.通过单个卵石沙洲的淤积和冲刷试验,揭示不同加沙速率、粒径和来流量条件下,沙洲淤积和冲刷规律,并建立简化理论模型分析沙洲淤积速率.结果表明,4组加沙试验中,分流点后出现明显淤积下延至洲头,左汊和右汊成为输沙通道,洲尾中心线两侧的左右汊道有泥沙淤积,洲尾未出现淤积.7组清水冲刷试验中,洲头最先承受冲刷和蚀退,并沿洲体冲刷延伸,洲头冲刷的泥沙沿左右汊水流带到下游,洲尾未出现明显冲刷.卵石沙洲以洲头淤积为主导发育模式,泥沙粒径、洲头坡角和分流角是决定淤积速率的关键因子.     

Controls on channel deposits of highly variable rivers: Comparing hydrology and event deposits in the Burdekin River,Australia

Discharge event frequency, magnitude and duration all control river channel morphology and sedimentary architecture. Uncertainty persists as to whether alluvial deposits in the rock record are a time-averaged amalgam from all discharge events, or a biased record of larger events. This paper investigates the controls on channel deposit character and subsurface stratigraphic architecture in a river with seasonal discharge and very high inter-annual variability, the Burdekin River of north-east Australia. In such rivers, most sediment movement is restricted to a few days each year and at other times little sediment moves. However, the maximum discharge magnitude does not directly correlate with the amount of morphological change and some big events do not produce large deposits. The Burdekin channel deposits consist of five main depositional elements: (i) unit bars; (ii) vegetation-generated bars; (iii) gravel sheets and lags; (iv) antidune trains; and (v) sand sheets. The proportions of each depositional element preserved in the deposits depend on the history of successive large discharge events, their duration and the rate at which they wane. Events with similar peak magnitude but different rate of decline preserve different event deposits. The high intra-annual and inter-annual discharge variability and rapid rate of stage change make it likely that small to moderate-scale bed morphology will be in disequilibrium with flow conditions most of the time. Consequently, dune and unit bar size and cross-bed set thickness are not good indicators of event or channel size. Antidunes may be more useful as indicators of flow conditions at the time they formed. Rivers with very high coefficient of variance of maximum discharge, such as the Burdekin, form distinctive channel sediment bodies. However, the component parts are such that, if they are examined in isolation, they could lead to misleading interpretation of the nature of the depositional environment if conventional interpretations are used.     

Morpho‐sedimentary characteristics of proximal gravel braided river deposits in a Froude‐scaled physical model

A Froude‐scaled physical model of a proximal gravel‐bed braided river was used to connect the river morphological characteristics, and sedimentary processes and forms, to deposit geometry. High resolution continuous three‐dimensional topographic data were acquired from sequential photogrammetric digital elevation models paired with grain‐size surface maps derived from image analysis of textural properties of the surface. From these data, the full three‐dimensional development of the braided river deposit and grain‐size sorting patterns was compiled over an experimental time period of 41 h during which the model river reworked a large portion of the braided channel. The minimum surface of the deposit is developed progressively over time by erosion, migration and avulsion of channels, and by local scour at channel confluences. The maximum surface of the deposit is formed by amalgamation of braid bar surfaces and has less overall relief than the minimum surface. Confluence scour constitutes about 5% of the area of the minimum surface. Migration of individual confluences is limited to distances of the order of the width and length of the confluence, so that confluences do not form laterally extensive deposits and basal surfaces. Maximum and minimum surfaces have very similar grain‐size distributions, and there is no extensive basal coarse layer. Deposit maximum thickness is strongly associated with large channel confluences which occur as deeper areas along the main channel belt and make up a large proportion of the thickest portions of the deposit.     

新疆额尔齐斯河边滩砾石定向性特征及其意义*

综合运用玫瑰图以及Curray的计算方法,对额尔齐斯河某处边滩上的20个观测点的砾石定向性进行了研究。结果表明,研究区砾石长轴存在定向排列且最优方向和河流流向平行,但是砾石定向性特征还和其他因素有关。颗粒支撑方式对砾石定向性有一定影响,多级颗粒支撑的砾石定向性最好,其次是颗粒支撑的砾石。水动力强弱会影响砾石定向排列,当水动力比较弱时,砾石在流水作用下以滚动的方式被搬运,砾石长轴和河流流向垂直;水动力比较强时,砾石在流水作用下以跳跃方式被搬运并且长轴和流向平行,在这种情况下其所受阻力最小,能够保持稳定状态。砾石粒径也会影响砾石定向性,在水动力比较弱时,小砾石(粒径小于2cm)长轴和河流流向垂直,大砾石(粒径大于2cm)和河流流向平行;但是当水动力较强时,砾石粒径大小对颗粒定向性特征的影响就可以忽略不计。因此运用砾石长轴来判断古流向时,要综合考虑研究区的古气候、沉积时的水动力等其他因素,然后确定河流流向和砾石长轴的关系,最后再确定研究区的古流向。     

珠江磨刀门河口交杯四沙现代沉积特征及其形成机理

通过对交杯四沙表层现代沉积考察,分析了滩面沉积微地貌类型,并结合粒度分析和成分分析,探讨不同微地貌沉积特征,进而讨论了交杯四沙的形成过程。研究结果表明:交杯四沙表层发育有低潮线以下、低潮位波浪冲洗带、高潮位冲洗带、风暴潮冲洗带、滩顶冲越带、滨后冲越带、冲越扇中部、冲越扇前缘斜坡以及分流间湾区等9个微地貌单元,代表了9个微相分区。交杯四沙滩面主要以粉砂和细砂为主,整体上分选性中到差,矿物成分主要为石英和黏土矿物。交杯四沙东侧是磨刀门主河槽,以径流为主;西侧沿白藤海、灯笼沙至三灶岛水道以潮流为主。交杯三沙以南,四砂以北构成了分流间凹地环境。交杯四沙以南为东南向的波浪作用带。交杯四沙和其他交杯沙系列沙体具有相同的形成过程,首先由河槽底流将拦门沙沉积物向西搬运沉积形成浅滩,浅滩受潮流和波浪的改造其平面形态呈酒杯状,浅滩的东南面受东南向波浪的作用,前坡遭受侵蚀,在后坡产生堆积,由此沙滩逐渐向陆迁移,先后形成交杯一沙、二沙、三沙和四沙,交杯一沙、二沙和三沙已经合并成陆。随时间的推移,交杯四沙也将与交杯三沙相接成陆,新的交杯五沙有望形成,磨刀门西侧浅滩区将演变成由一系列反曲沙脊和潮滩相间而成的三角洲“滩-脊”平原。     

Architectural Model of a Dryland Gravel Braided River, based on 3D UAV Oblique Photogrammetric Data: A Case Study of West Dalongkou River in Eastern Xinjiang, China

Three-dimensional unmanned aerial vehicle(UAV) oblique photogrammetric data were used to infer mountainous gravel braided river lithofacies, lithofacies associations and architectural elements. Hierarchical architecture and lithofacies associations with detailed lithofacies characterizations were comprehensively described to document the architectural model, architectural element scale and gravel particle scale.(1) Nine lithofacies(i.e., Gmm, Gcm, Gcc, Gci, Gcl, Ss, Sm, Fsm and Fl) were identifi...     

Controls on the morphology of braided rivers and braid bars: An empirical characterization of numerical models

Braided rivers exhibit highly variable morphologies, morphodynamic behaviours and resulting depositional records. To evaluate relationships between characteristics of braided-river channel belts and river depth, water discharge and streambed gradient, 39 numerical modelling experiments were conducted with the software Delft3D to simulate braided-river evolution under a broad range of boundary conditions. Data from model outputs were integrated with observations from 63 natural braided rivers differing with respect to river depth and streambed gradient. The modelled rivers each underwent similar evolutions, yet each culminated in markedly different final river morphologies, dependent on discharge and riverbed gradient. The rivers underwent evolutionary stages of: (i) formation of transverse unit bars with limited relief from an initially featureless bed; (ii) channel development around bars and in some cases dissecting transverse unit bars; (iii) formation of relatively simpler compound bars; and (iv) amalgamation of these simpler compound bars into more complex compound bars. Quantitative relationships relating to braided-river channel-belt morphology and organization are established, and the following results are noted: (i) bar elongation (length-to-width ratio) is correlated positively with riverbed gradient; (ii) bar height and area are correlated positively with discharge, and negatively with riverbed gradient; (iii) the river depth is the main predictor of mean braid-bar area; and (iv) the degree of braiding is primarily associated with river width-to-depth ratio and riverbed gradient. Results arising from this research improve our understanding of controls on the morphology and architectures of braided fluvial channel belts; they provide a novel empirical characterization that can be applied for predicting channel depth, bar morphology, streambed gradient, and degree of braiding of modern fluvial systems and of the formative rivers of ancient preserved successions.     

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.     

Recognition and significance of sharp‐based mouth‐bar deposits in the Eocene Green River Formation,Uinta Basin,Utah

Sandstone bodies in the Sunnyside Delta Interval of the Eocene Green River Formation, Uinta Basin, previously considered as point bars formed in meandering rivers and other types of fluvial bars, are herein interpreted as delta mouth‐bar deposits. The sandstone bodies have been examined in a 2300 m long cliff section along the Argyle and Nine Mile Canyons at the southern margin of the Uinta lake basin. The sandstone bodies occur in three stratigraphic intervals, separated by lacustrine mudstone and limestone. Together these stratigraphic intervals form a regressive‐transgressive sequence. Individual sandstone bodies are texturally sharp‐based towards mudstone substratum. In proximal parts, the mouth‐bar deposits only contain sandstone, whereas in frontal and lateral positions mudstone drapes separate mouth‐bar clinothems. The clinothems pass gradually into greenish‐grey lacustrine mudstone at their toes. Horizontally bedded or laminated lacustrine mudstone onlaps the convex‐upward sandstone bars. The mouth‐bar deposits are connected to terminal distributary channel deposits. Together, these mouth‐bar/channel sandstone bodies accumulated from unidirectional jet flow during three stages of delta advance, separated by lacustrine flooding intervals. Key criteria to distinguish the mouth‐bar deposits from fluvial point bar deposits are: (i) geometry; (ii) bounding contacts; (iii) internal structure; (iv) palaeocurrent orientations; and (v) the genetic association of the deposits with lacustrine mudstone and limestone.     

大同盆地中侏罗世河流沉积体系及古河型演化

通过对大同云冈出露的部分中侏罗统的沉积物特征、沉积构造特征、岩相和结构单元的研究,识别出该剖面是由下伏的曲流河沉积体系和上覆的辫状河沉积体系组成的。由此认为研究区在中侏罗世早期为曲流河沉积环境,此后开始转化为辫状河流沉积环境。河型的转化受制于流域基底差异抬升的影响,而流域气候由湿润向半干旱-干旱的转化也起了促进作用。     

Alternate bar dynamics in response to increases and decreases of sediment supply

Gravel‐bed rivers can accommodate changes in sediment supply by adjusting their bed topography and grain size in both the downstream and cross‐stream directions. Under high supply aggradational conditions, this can result in spatially non‐uniform stratigraphic patterns, and the morphodynamic influence of heterogeneous stratigraphy during subsequent degradational periods is poorly understood and has not been studied through physical modelling. A flume experiment was conducted to analyse channel response where alternate bars were developed in a gravel–sand mixture under constant discharge and sediment supply before two supply increases led to the development of heterogeneous stratigraphy beneath alternate bars. The supply was then reduced back to the initial supply rate, causing degradation through that self‐formed stratigraphy. Stratigraphic samples were collected, and the bed topography and flow depth were measured frequently, which were used with a two‐dimensional hydrodynamic model to characterize flow conditions. Migrating alternate bars stabilized during the first equilibrium phase, creating bed surface sorting patterns of coarse bar tops and fine pools. During the first supply increase, the bars remained stable as the pools aggraded. During the second supply increase, the pools aggraded further, causing the boundary shear stress over the bar tops to increase until the bars gained the capacity to migrate and eventually stabilize in new locations. As aggradation occurred, the original sediment sorting patterns were preserved in the subsurface. During the degradational phase, the pools experienced incision and the bars eroded laterally, but this lateral erosion ceased when coarse sediment previously deposited during the bar‐building phase became exposed. The results suggest that if a sediment supply increase is capable of filling the pools, it can cause stable bars to migrate and the bed to be reworked. These findings also show that heterogeneous stratigraphy can play an important role in determining whether bars persist or disappear after a sediment supply reduction.     

A quantitative, three-dimensional depositional model of gravelly braided rivers

A quantitative, three‐dimensional depositional model of gravelly, braided rivers has been developed based largely on the deposits of the Sagavanirktok River in northern Alaska. These deposits were described using cores, wireline logs, trenches and ground‐penetrating radar profiles. The origin of the deposits was inferred from observations of: (1) channel and bar formation and migration and channel filling, interpreted from aerial photographs; (2) water flow during floods; and (3) the topography and texture of the river bed at low‐flow stage. This depositional model quantitatively represents the geometry of the different scales of strataset, the spatial relationships among them and their sediment texture distribution. Porosity and permeability in the model are related to sediment texture. The geometry of a particular type and scale of strataset is related to the geometry and migration of the bedform type (e.g. ripples, dunes, bedload sheets, bars) associated with deposition of the strataset. In particular, the length‐to‐thickness ratio of stratasets is similar to the wavelength‐to‐height ratio of associated bedforms. Furthermore, the wavelength and height of bedforms such as dunes and bars are related to channel depth and width. Therefore, the thickness of a particular scale of strataset (i.e. medium‐scale cross‐sets and large‐scale sets of inclined strata) will vary with river dimensions. These relationships between the dimensions of stratasets, bedforms and channels mean that this depositional model can be applied to other gravelly fluvial deposits. The depositional model can be used to interpret the origin of ancient gravelly fluvial deposits and to aid in the characterization of gravelly fluvial aquifers and hydrocarbon reservoirs.     

Chronology of the shell bar section and a discussion on the ages of the Late Pleistocene lacustrine deposits in the paleolake Qarhan, Qaidam basin

Shell bar, composed of abundant fossil shells of Corbicula fluminea müller and Corbicula largillierti philippi and located at the southeastern end (36°30′N, 96°12′E) of the paleolake Quarhan, is one of the most prominent features in the Qaidam basin. It is the highest site where such species of fossil shells have been found in the Late Pleistocene age. A 2.6-m-thick fresh profile was manually excavated to determine the formation ages and the scope of the high paleolake levels. Accelerator mass spectrometry (AMS), conventional radiocarbon dating, and sector inductively coupled plasma-mass spectroscopy ²³⁰Th methods were used to investigate the reliability and accuracy of dating results in different laboratories. Ages of various components (e.g. acid residual and acid soluble fraction of the organic matter) from the same sample were determined. Age differences of a variety of materials (e.g. organic matter, fossil shells, and salt crystals), and age differences at the same sampling position were evaluated. Dating shows that the AMS ages given by alkali residue and acid soluble fractions vary greatly, e.g., from 124.5 cm upwards. The dates given by the acid soluble fraction were normal. Down the column, the dates show a reverse pattern and those of alkali residues, especially the lower part of the section, show an unstable pattern. These imply that organic matter had been influenced by two separate processes. One possible explanation is that the alkali residual fraction most likely had been contaminated by dead carbon-bearing reworked material because the study section is located near the edge of the paleolake and could have been easily influenced by old eroded deposits. Another is that the acid soluble fraction could have been contaminated by upward-flowing groundwater containing soluble organic matter. The amount of this soluble organic material should be very small because the strongest age reversal is in the lower part, where the TOC content remains low, meaning that the change of soluble organic fraction alters neither TOC content nor the δ ¹³C dramatically. It is concluded that a uniform mega-paleolake developed in the Qaidam basin in the northeastern Tibetan plateau between 39.7 and 17.5 ¹⁴C kaB.P. During the period when the high paleolake level of Qarhan was formed, the huge paleolake covered a vast area with dramatic lake level fluctuations. It is found that the ages determined may be influenced by either radioactive ¹⁴C variations of repeated deposition or content variations in the atmosphere. The ages given by fossil shells are 15–18 ka older than those given by organic matter. The large differences between the ages of fossil shells and organic matter might have resulted from the large water areas and huge water volume and the special location of the high-elevation Qaidam basin. This study also shows the reliability and accuracy of the ²³⁰Th dating method on the salt crystals but further study is needed to determine whether this method could be applied to the study area. __________ Translated from Quaternary Sciences, 2007, 27(4): 511–521 [译自: 第四纪研究]     

Unit bar architecture in a highly-variable fluvial discharge regime: Examples from the Burdekin River,Australia

Unit bars are relatively large bedforms that develop in rivers over a wide range of climatic regimes. Unit bars formed within the highly-variable discharge Burdekin River in Queensland, Australia, were examined over three field campaigns between 2015 and 2017. These bars had complex internal structures, dominated by co-sets of cross-stratified and planar-stratified sets. The cross-stratified sets tended to down-climb. The development of complex internal structures was primarily a result of three processes: (i) superimposed bedforms reworking the unit bar avalanche face; (ii) variable discharge triggering reactivation surfaces; and (iii) changes in bar growth direction induced by stage change. Internal structures varied along the length and across the width of unit bars. For the former, down-climbing cross-stratified sets tended to pass into single planar cross-stratified deposits at the downstream end of emergent bars; such variation related to changes in fluvial conditions whilst bars were active. A hierarchy of six categories of fluvial unsteadiness is proposed, with these discussed in relation to their effects on unit bar (and dune) internal structure. Across-deposit variation was caused by changes in superimposed bedform and bar character along bar crests; such changes related to the three-dimensionality of the channel and bar geometry when bars were active. Variation in internal structure is likely to be more pronounced in unit bar deposits than in smaller bedform (for example, dune) deposits formed in the same river. This is because smaller bedforms are more easily washed out or modified by changing discharge conditions and their smaller dimensions restrict the variation in flow conditions that occur over their width. In regimes where unit bar deposits are well-preserved, their architectural variability is a potential aid to their identification. This complex architecture also allows greater resolution in interpreting the conditions before and during bar initiation and development.     

Grain‐size variability within a mega‐scale point‐bar system,False River,Louisiana

Point bars formed by meandering river systems are an important class of sedimentary deposit and are of significant economic interest as hydrocarbon reservoirs. Standard point‐bar models of how the internal sedimentology varies are based on the structure of small‐scale systems with little information about the largest complexes and how these might differ. Here a very large point bar (>25·0 m thick and 7·5 × 13·0 km across) on the Mississippi River (USA) was examined. The lithology and grain‐size characteristics at different parts of the point bar were determined by using a combination of coring and electrical conductivity logging. The data confirm that there is a general fining up‐section along most parts of the point bar, with a well‐defined transition from massive medium‐grained sands below about 9 to 11 m depth up into interbedded silts and fine–medium sand sediment (inclined heterolithic strata). There is also a poorly defined increase in sorting quality at the transition level. Massive medium sands are especially common in the region of the channel bend apex and regions upstream of that point. Downstream of the meander apex, there is much less evidence for fining up‐section. Finer sediment accumulated more readily after the establishment of a compound bar in the later stages of construction, at the terminal apex and in the bar tail. This work implies that the best reservoir sands are likely to be located in the centre of the point bar, deposited in a simple bar system. Reservoir quality decreases towards the bar edge. The early‐stage channel plug is largely composed of coarsening‐upward cycles of silt to clay and is dominated by clay and clayey silt material with poor reservoir characteristics.