Planform architecture,stratigraphic signature and morphodynamics of an exhumed Jurassic meander plain (Scalby Formation,Yorkshire, UK)

Modern fluvial meander plains exhibit complex planform transformations in response to meander‐bend expansion, downstream migration and rotation. These transformations exert a fundamental control on lithology and reservoir properties, yet their stratigraphic record has been poorly evaluated in ancient examples due to the lack of extensive three‐dimensional exposures. Here, a unique exhumed meander plain exposed to the north of Scarborough (Yorkshire, UK) is analysed in terms of architecture and morphodynamics, with the aim of developing a comprehensive model of facies distribution. The studied outcrop comprises tidal platforms and adjacent cliffs, where the depositional architecture of un‐tilted deposits was assessed on planform and vertical sections, respectively. In its broader perspective, this study demonstrates the potential of architectural mapping of extensive planform exposures for the reconstruction of ancient fluvial morphodynamics. The studied exhumed meander plain is part of the Scalby Formation of the Ravenscar Group, and originally drained small coastal incised valleys within the Jurassic Cleveland Basin. The meander plain is subdivided into two storeys that contain in‐channel and overbank architectural elements. In‐channel elements comprise expansional and downstream‐migrating point bars, point‐bar tails and channel fills. Overbank elements comprise crevasse complexes, levées, floodplain fines and lake fills. The evolution of the point bars played a significant role in dictating preserved facies distributions, with high flood‐stage nucleation and accretion of meander scrolls later reworked during waning flood‐stages. At a larger scale, meander belt morphodynamics were also a function of valley confinement and contrasts in substrate erodibility. Progressive valley infilling decreased the valley confinement, promoting the upward transition from prevalently downstream migrating to expansional meander belts, a transition associated with enhanced preservation of overbank elements. Strikingly similar relations between valley confinement, meander‐bend transformations and overbank preservation are observed in small modern meandering streams such as the Beaver River of the Canadian prairies and the Powder River of Montana (USA).     

Formation of point bars through rising and falling flood stages: Evidence from bar morphology,sediment transport and bed shear stress

Flow processes and sediment transport in a channel bend and associated point bar have been studied in modern rivers, theoretical models and physical experiments: however, the relationship between flow process and point‐bar morphology has rarely been explained due to the complex nature of open channel flow. Plan‐view exposures of an ancient point‐bar complex, exposed at the top of the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation, south‐central Utah, allowed reconstruction of bar morphology, sediment transport and bed shear stress, which were used to extrapolate flow processes. Studies of these outcrops show that compound point bars and scroll bars were probably formed during falling and rising flood stages, respectively. A simulation model of plan‐view channel form shows that channel dimensions, such as radius of curvature and sinuosity of the point‐bar complex, range between 205 m and 351 m and 1·04 and 1·22, respectively, throughout the evolution of the channel bend. Variations in strength of the helical flow were interpreted as the main control on facies architecture and bar morphology. Strong helical flow was related to the deposition of the scroll bars, while strength of helical flow is decreased for compound bars. The use of cross‐beds as a common palaeocurrent indicator was found to be inconsistent with mean flow directions and channel margin orientation.     

Plan‐form evolution of ancient meandering rivers reconstructed from longitudinal outcrop sections

The mode of channel‐bend transformation (i.e. expansion, translation, rotation or a combination thereof) has a direct bearing on the dimensions, shape, bedding architecture and connectivity of point‐bar sandstone bodies within a fluvial meander belt, but is generally difficult to recognize in vertical outcrops. This study demonstrates how the bend transformation mode and relative rate of channel‐floor aggradation can be deciphered from longitudinal outcrop sections aligned parallel to the meander‐belt axis, as a crucial methodological aid to the reconstruction of ancient fluvial systems and the development of outcrop analogue models for fluvial petroleum reservoirs. The study focuses on single‐storey and multi‐storey fluvial meander‐belt sandstone bodies in the Palaeogene piggyback Boyabat Basin of north‐central Turkey. The sandstone bodies are several hundred metres wide, 5 to 40 m thick and encased in muddy floodplain deposits. The individual channel‐belt storeys are 5 to 9 m thick and their transverse sections show lateral‐accretion bed packages representing point bars. Point bars in longitudinal sections are recognizable as broad mounds whose parts with downstream‐inclined, subhorizontal and upstream‐inclined bedding represent, respectively, the bar downstream, central and upstream parts. The inter‐bar channel thalweg is recognizable as the transition zone between adjacent point‐bar bedsets with opposing dip directions into or out of the outcrop section. The diverging or converging adjacent thalweg trajectories, or a trajectory migrating in up‐valley direction, indicate point‐bar broadening and hence channel‐bend expansion. A concurrent down‐valley migration of adjacent trajectories indicates channel‐bend translation. Bend rotation is recognizable from the replacement of a depositional riffle by an erosional pool zone or vice versa along the thalweg trajectory. The steepness of the thalweg trajectory reflects the relative rate of channel‐floor aggradation. This study discusses further how the late‐stage foreland tectonics, with its alternating pulses of uplift and subsidence and a progressive narrowing of the basin, has forced aggradation of fluvial channels and caused vertical stacking of meander belts.     

Recognizing the product of concave-bank sedimentary processes in fluvial meander-belt strata

Downstream migration of point bars is an important process in meander belts. Inherent to downstream migration is sediment accumulation in concave channel banks, immediately adjacent to and downstream of convex point bars. Despite this, associated concave bank processes are often overlooked, with depositional products sparsely identified in the stratigraphic record. Counter-point-bar deposits are a type of concave-bank deposit that have been positively identified in subsurface three-dimensional seismic datasets, yet outcrop examples are not well-constrained. This study characterizes and establishes recognition criteria of counter-point-bar deposits in outcrop using extensive exposure of Late-Cretaceous meander-belt deposits in eastern Alberta, Canada. Using a combination of traditional field-based sedimentological analyses and three-dimensional outcrop mapping with an Uncrewed Aerial Vehicle and Structure-from-Motion photogrammetry, point bar, counter-point bar, and associated abandoned-channel deposits, as well as adjacent floodplain deposits are identified. Bed-scale characteristics of counter-point-bar deposits include interlaminated and interbedded siltstone and fine-grained sandstone, abundant organic detritus, and evidence of deformation and slumping. At the bend scale, accretion packages bounded by internal erosion surfaces are composed of dipping siltstone and minor sandstone beds that extend from the top to the base of the meander belt. At the belt scale, positive identification relies on concave accretion surface mapping, their orientation relative to the meander-belt edge (i.e. dipping away), and consideration of meander-bend evolution. These results have implications for recognition of counter-point-bar deposits in analogous, less-constrained data sets, which provides a foundation for more complete palaeoenvironmental interpretations.     

Sedimentology of the Upper Burdekin River of North Queensland, Australia—an example of a tropical, variable discharge river

The Burdekin River is an example of a class of tropical streams which experience two to four orders of magnitude variation in discharge, in response to seasonal but erratic monsoonal rainfall. Floods of the Burdekin rise abruptly, reaching peak discharges of up to 40,000 m³ s^-1 in less than 24 h; maintain peak flow for up to a few days, and recede exponentially. The geomorphology and deposits of these rivers reflect the extreme discharge fluctuations, and have not previously been described. A stretch of the upper Burdekin River comprising four bends and one straight reach was examined near the town of Charters Towers. The river bed is largely exposed for most of any year, with a small, misfit perennial channel carrying low stage flow. Major geomorphic elements of bends include point bars with ridge-and-swale topography, three distinct types of chute channels, avalanche slipfaces up to 5 m or more high around the downstream edges of bars, and on the outer part of one point bar an elevated, vegetated ridge. Straight reaches are flat or gently inclined, sand- and gravel-covered surfaces. Much of the river bed is covered by well sorted, in places gravelly, coarse to very coarse-grained sand with local accumulations of pebble to boulder gravel. Lower parts of the river bed are periodically draped by mud which is desiccated on exposure. Dunes and plane beds are the most commonly occurring bedforms, with local development of gravelly antidunes. Most bank tops and upper, vegetated bars are covered by silt and fine-grained sand. The river bed also hosts a low-diversity but locally high-abundance, flood-tolerant flora dominated by the paperbark tree Melaleuca argentea, which plays an important role in controlling the distribution of sediment. The gross geomorphology of the river bed and most of the sedimentary features are interpreted as having formed during major (bankfull or near bankfull) flows, which have a recurrence of about 18 years (based on 65 years hydrographic data). The initial rapid drop in discharge following flood peaks appears to preserve flood peak features on upper bars more or less intact, whereas lower areas are subjected to variable degrees of modification during falling stage and by more frequent, non-bankfull discharge events.     

基于无人机航测黄河源弯曲河道泥沙亏损量计算

弯曲河流的河道冲淤变化在长时间尺度处于动态的平衡状况,其泥沙输移部分来自于凹岸冲刷与凸岸淤积的差值。2018年采用无人机航测获取黄河源典型弯曲河流(麦曲、哈曲、格曲和兰木错曲)连续弯道的低空影像数据,通过图像技术处理后生成高精度地形,进而提取单个弯道和连续弯道的断面地形,计算弯道凸、凹两侧断面面积和相邻断面间的差值。结果表明,凹岸侵蚀崩岸产生的泥沙量与凸岸点边滩淤积的泥沙量是不平衡的,即存在一个泥沙亏损量。对于单个弯道,兰木错曲单位河长的泥沙亏损量约为0.191 m^3,麦曲、哈曲和格曲单位河长的泥沙亏损量约为0.045 m^3,且相同河段的沿程亏损量具有不均匀性,其间接反映不同弯道横向的迁移速率具有差异性。     

Tidal currents and wind waves controlling sediment distribution in a subtidal point bar of the Venice Lagoon (Italy)

The present study aims to improve current understanding of the sedimentation of subtidal point bars, analyzing interaction between tidal currents and waves in shaping a submerged meander bend of the microtidal Venice Lagoon (Italy), and it is based on coupling of sedimentological studies, geophysical analyses and numerical modelling. The Venice Lagoon is characterized by an average depth of about 1·5 m over subtidal platforms and a mean tidal range of about 1·0 m. The morphodynamic evolution of the lagoon is strongly affected by intense seasonal windstorms, which promote the formation of wind waves triggering sediment resuspension and bottom erosion. The study channel is 70 to 100 m wide, it has a radius of curvature of about 260 m and cuts through a permanently submerged subtidal platform. Water depth ranges from 1·0 to 5·0 m below mean sea level on the subtidal platform and channel thalweg, respectively. Different from classical architectural models, the study point‐bar beds do not show sigmoidal geometries, but consist of horizontally‐bedded deposits abruptly overlying clinostratified beds. Sedimentation in the study bar is hypothesized to stem from the interaction between the in‐channel secondary helical flow, as for most meander bends, and wave winnowing of the subaqueous overbank areas. Laterally accreting point‐bar deposits point out that the curvature‐induced helical flow redistributed sediment from the channel thalweg to the bar top and contributed to the development of the ‘classical’ fining‐upward grain size trend. The marked truncation surface, separating clinostratified bar deposits from overlying horizontally‐bedded platform sediments is interpreted here as due to bar top wave‐winnowing, which also possibly promoted bank collapses. In the proposed model, sediments remobilized from bar top and subaqueous overbank areas were transported into the channel, forming peculiar ‘apron‐like’ accumulations, where sand accumulated through avalanching processes and mud settled down from suspension.     

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.     

Morphodynamic evolution and stratal architecture of translating tidal point bars: Inferences from the northern Venice Lagoon (Italy)

The widespread distribution of tidal creeks and channels that undertake meandering behaviour in modern coasts contrasts with their limited documentation in the fossil record, where point‐bar elements arising from the interaction between a mix of both fluvial and tidal currents are mainly documented. The sedimentary products of tidal channel‐bend evolution are relatively poorly known, and few studies have focused previously on specific facies models for tidal point bars present in modern settings. This study improves understanding of tidal channel meander bends through a multi‐disciplinary approach that combines analyses of historical aerial photographs, measurements of in‐channel flow velocity, high‐resolution facies analyses of sedimentary cores and three‐dimensional architectural modelling. The studied channel bend (12 to 15 m wide and 2 to 3 m deep) drains a salt marsh area located in the north‐eastern sector of the microtidal Venice Lagoon, Italy. Historical photographs show that, during the past 77 years, the bend has translated seaward ca 15 m. Results show that the channel bend formed on a non‐vegetated mud flat that was progressively colonized by vegetation. Seaward translation occurred under aggradational conditions, with an overall migration rate of 0·2 to 0·3 m year⁻¹, and was promoted by the occurrence of cohesive, poorly erodible outer bank deposits. Ebb currents are dominant, and translation of the channel bend promotes erosion and deposition along the landward and seaward side of the bar, respectively. Tidal currents show a clear asymmetry in terms of velocity distribution, and their offset pattern provides a peculiar grain‐size distribution within the bar. During the flood stage, sand sedimentation occurs in the upper part of the bar, where the maximum flow velocity occurs. During the ebb stage, the bar experiences the secondary helical flow that accumulates sand at the toe of the bar. Lateral stacking of flood and ebb deposits has caused the formation of localized coarsening‐upward and fining‐upward sedimentary packages, respectively.     

The genesis of lateral accretion deposits in recent intertidal mudflat channels, Solway Firth, Scotland

ABSTRACT The pattern of lateral accretion on an intertidal point bar is a response to the suspended sediment concentrations to which it is exposed. In summer, high sediment concentrations occur only during shallow (ebb) channel flows; deposition therefore takes place mainly on the point bar toe. In winter, high concentrations obtain at all depths and deposition therefore occurs on the upper point bar slopes. Accretion of the point bar toe is limited by scouring resulting from the high rainfall runoff of winter. High suspended sediment concentrations have little effect, however, if flow velocities are too high to allow deposition. Deposition is dependent on the non-uniformity of curved channel flow and is often confined to the downstream (ebb-sense) portion of the point bar. The lateral accretion deposits form a series of wedge-shaped units. Each unit represents one year's deposition, bounded by erosion scarps produced during successive winters. The base of the point bar deposit shows a gradual aggradation, keeping pace with the build-up of the adjacent interchannel flats.     

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.     

复杂曲流带识别标志及其内部点坝叠加接触模式

以现代沉积为指导,通过岩芯、测井地质、地震沉积学分析研究复杂曲流带边界砂体类型及其内部结构特征,提出了识别地下复杂曲流带的典型标志,即曲流带边界处废弃河道的长度与曲流带边界的长度比值大于65%,尤其在曲流带凹岸一侧此比值超过85%。通过对比分析正演模型、测井地质剖面及分频后地震反射轴的波形及振幅,识别并总结出研究区曲流带内部主要发育点坝主体-初期点坝-初期河道边缘、点坝伊始-废弃河道、点坝伊始-点坝伊始、废弃河道-废弃河道、废弃河道-点坝伊始、末期河道-点坝等点坝间边界叠加类型,并在此基础上建立了研究区曲流带内部结构模式。     

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.     

Architecture of an Oligocene fluvial ribbon sandstone in the Ebro Basin,North‐eastern Spain

Fluvial ribbon sandstone bodies are ubiquitous in the Ebro Basin in North‐eastern Spain; their internal organization and the mechanics of deposition are as yet insufficiently known. A quarrying operation in an Oligocene fluvial ribbon sandstone body in the southern Ebro Basin allowed for a three‐dimensional reconstruction of the sedimentary architecture of the deposit. The sandstone is largely a medium‐grained to coarse‐grained, moderately sorted lithic arenite. In cross‐section, the sandstone body is 7 m thick, occupies a 5 m deep incision and wedges out laterally, forming a ‘wing’ that intercalates with horizontal floodplain deposits in the overbank region. Three architectural units were distinguished. The lowest and highest units (Units A and C) mostly consist of medium‐grained to coarse‐grained sandstone with medium‐scale trough cross‐bedding and large‐scale inclined stratasets. Each of Units A and C comprises a fining‐up stratal sequence reflecting deposition during one flood event. The middle unit (Unit B) consists of thinly bedded, fine‐grained sandstone/mudstone couplets and represents a time period when the channel was occupied by low‐discharge flows. The adjoining ‘wing’ consists of fine‐grained sandstone beds, with mudstone interlayers, correlative to strata in Units A and C in the main body of the ribbon sandstone. In plan view, the ribbon sandstone comprises an upstream bend and a downstream straight reach. In the upstream bend, large‐scale inclined stratasets up to 3 m in thickness represent four bank‐attached lateral channel bars, two in each of Units A and C. The lateral bars migrated downflow and did not develop into point bars. In the straight downstream reach, a tabular cross‐set in Unit A represents a mid‐channel transverse bar. In Unit C, a very coarse‐grained, unstratified interval is interpreted as deposited in a riffle zone, and gives way downstream to a large mid‐channel bar. The relatively simple architecture of these bars suggests that they developed as unit bars. Channel margin‐derived slump blocks cover the upper bar. The youngest deposit is fine‐grained sandstone and mudstone that accumulated immediately before avulsion and channel abandonment. Deposition of the studied sandstone body reflects transport‐limited sediment discharges, possibly attaining transient hyperconcentrated conditions.     

Largescale ripples of the lower Wabash River

Largescale ripples in the meandering lower Wabash River of Illinois and Indiana, U.S.A., include scroll bars and three dunelike bed forms (dunes, sand waves, and transverse bars). Scroll bars are lobate crested, asymmetrical in stream-wise vertical profile, usually solitary, and oriented approximately normal to local channel strike. They form by passive flow expansion downchannel from locally emergent topographic highs, face and lie near inner banks of meander bends, enjoy a high preservation potential as leveelike ridges of ridge-and-swale topography, and migrate only during relatively low stream discharges, when water depth over bar crests is less than 0·5 m. Dunes correspond to dunes of the flow-regime classification and rarely are solitary or superimposed. Sand waves may be symmetrical or asymmetrical, are always superimposed by dunes, occur in depths greater than 4 m and in bed material coarser than 1 mm mean size, and develop at bankfull and flood flows. Transverse bars migrate in depths less than 5 m in straight reaches and near inner banks of bends, display crestal dunes, and correspond to the bars of Costello (1974) and to the sand waves of Boothroyd (1969). Hydrodynamic regimes of scroll bars and transverse bars differ from that of dunes. The omnipresence of dunes upon stoss-sides of sand waves confirms the existence of an equilibrium superimposition of dunelike largescale ripples. Depth-velocity-size diagrams appear to be a valid representation of empirical stability fields of dunelike largescale ripples in deep unsteady nonuniform aqueous flows. Stability fields of dunes and sand waves overlap greatly. Velocity profiles demonstrate an absence of leeside flow separation over dunes and an appearance (rare) over transverse bars only when the ratio of trough depth to crest depth exceeds two. Dune stratification displays (1) largescale trough cross-strata, (2) thinning of sets as bed-material size increases, and (3) an orientation within 20° of local channel strike. Transverse bars show avalanche sets up to 2 m thick, with reactivation surfaces. Scroll bars display thick avalanche sets separated by reactivation structures consisting of erratically oriented smallscale trough cross-strata. Avalanche sets of scroll bars and of transverse bars are oriented 50–150° from and within 50° of, respectively, local channel strike.     

Computer simulation of sedimentation in meandering streams

A dynamic mathematical model for simulation of sedimentation in meandering streams is briefly described. This is composed of component mathematical models which are formulated to predict the following aspects of the system for a given physical situation and a single time increment. (1) The characteristics of the plan form of the meander; (2) the movement of the meander in plan, and definition of cross-sections across the meander in which erosion and deposition are considered in detail; (3) the hydraulic properties of the channel in the bend and the erosional and depositional activity within the channel as defined in specific cross-sections; (4) the nature and occurrence of cut-off; (5) a relative measure of the discharge during a seasonal high water period, which is used in (3) and (4); (6) aggradation. The model, in the form of a FORTRAN IV computer program, has been used to simulate various aspects of sedimentation in meandering streams by performing a set of experiments with the program under different input conditions. The geometry of simulated point bar sediments, as controlled by channel migration over floodplains with variable sediment type, agrees broadly with the natural situation, however extensive sheets of point bar sediment cannot be simulated because large scale meander-belt movements are not accounted for. In the simulated sediments, successive surfaces of the point bar before falling stage deposition (lateral and vertical) may be picked out, and these delineate the epsilon cross-stratification of Allen (1963b). The epsilon unit thickness is that measured from bankfull stage down to the lowest channel position existing prior to deposition. The model records the characteristic fining upwards of grain sizes in the point bar, and the systematic distribution of sedimentary structures. Channel migration combined with seasonal scouring and filling across the channel section produces a characteristic relief in the basal scoured surfaces and facies boundaries (as defined by variation in grain size and sedimentary structure). A related lensing and inter-fingering of the facies may also be present. The model also records large-scale lateral changes in grain size and sedimentary structure associated with changes in the shape of developing meanders. When channel migration is combined with a constant aggradation rate the model predicts a general slope (relative to the land surface) of facies boundaries and scoured basal surfaces upward in the direction of channel movement. If aggradation sufficiently increases the thickness of fine-grained overbank material, there is a channel stabilization effect. It is shown that a complete sequence of point bar sediments capped by overbank sediments would rarely be preserved in the moving-phase situation. Such preservation only becomes likely when an aggrading section lies out of range of an eroding channel for a considerably longer time span than it takes a meander to move one half-wavelength downvalley. Deep channel scours have a higher preservation potential than contemporary shallower ones. Where appropriate field data exist the model can be used in the more accurate recognition of ancient fluviatile sediments. Inferences may be made about the erosion-deposition processes operating in the ancient channel system, and the geometry and hydraulics of the system can be alluded to. A representative application of the model to the quantitative interpretation of an ancient point bar deposit is illustrated. There is reasonable agreement between the natural and the simulated deposits, and a broad quantitative picture of the palaeoenvironment of sedimentation is obtained.     

曲流串沟型江心洲形成机制与演化探讨

曲流串沟型江心洲是现代河流水利学研究关注的重要河流要素,不同于曲流河点坝和辫状河心滩,但在油气储层沉积学研究中鲜有关注。选取现代松花江典型河段,开展基于水动力学的沉积数值模拟,恢复曲流串沟型江心洲的演化过程,建立了曲流串沟型江心洲演化模式。研究结果表明:1）洪水期曲流河发生漫滩冲刷,串沟发育扩张,截弯取直,形成分汊河道及江心洲地貌;2）分汊偏移角、相对梯度优势可用于预测短期汊道存在的稳定性;3）串沟型江心洲形成于曲流点坝残余沉积之上,发育洲头侵蚀、洲尾加积、翼部双侧积,但垂向加积主要发育在顶部,不构成沉积主体结构。通过上述研究建立了曲流江心洲沉积模式,揭示了曲流河江心洲内部结构及分汊河道水动力特征,对现代河流治理和地下河流沉积储层刻画有指导意义。     

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.     

Sedimentology and morphology of a low-sinuosity river: Calamus River, Nebraska Sand Hills

A study reach of the Calamus River, Nebraska Sand Hills, has a low sinuosity (less than 1.3) and braiding parameter (less than 1). Depending on sinuosity, the channel is occupied by alternate bars and point bars, the emergent parts of which form nuclei for midstream bars (islands). Channel migration occurs by bend expansion and translation, downstream and lateral growth of islands, and by chute cutoff. Channel-bed sediment is mainly medium-grained sand, but gravel and coarser sand sizes occur in thalweg areas adjacent to cutbanks and upstream parts of bars and islands, and finer sands occur on the downstream parts of bars and filling channels. Curved-crested dunes cover most of the channel bed at most flow stages, with ripples restricted to shallow areas near banks. Bed material is mostly large-scale cross-stratified, with small-scale cross-strata interbedded with plant debris occurring in topographically high areas near banks. Vibracores through channel bars show a basal erosion surface overlain by large-scale cross-stratified sands, in turn overlain by small-scale cross-stratified sand interbedded with plant debris. The overall sequence generally fines upwards, but the large-scale cross-stratified portion either fines upwards, coarsens upwards, or shows little grain size variation. Lithofacies distributions vary spatially within and between bars depending on position in the bar and local channel curvature/width, in a similar way to unbraided rivers elsewhere. Lithofacies of bar deposits are similar to those in the active channel, and the elevations of the basal erosion surface and adjacent channel thalweg correspond closely. Channels abandoned by chute cutoff are filled progressively from the upstream end, and comprise deposits similar to the downstream parts of bars (i.e. fining upwards). The downstream extremities of channel fills may contain large proportions of peat relative to sand, but little mud due to the paucity of such fine suspended load in the Calamus.     

Quantitative interpretation of an evolving ancient river system

Multistorey sandstone bodies described from the Upper Devonian-Lower Carboniferous of Kerry Head (Ireland) are interpreted as deposits of aggrading, perennial, river channels migrating laterally across alluvial plains. Point bars displayed surface features such as scroll bars, chute channels and chute bars. Relatively uncommon channel fills are both coarse- and fine-grained. Quantitative interpretation of the sandstone bodies was accomplished by comparison with a physical model that predicts the sedimentology of single point bar deposits developed in channels of prescribed geometry and hydraulics. This analysis reveals that the separate storeys (point bars) in each sandstone body were deposited in a single channel belt in which channel geometry and hydraulics varied little with time (order of 10³ yr) and space (order of 10³ m). Two southerly flowing rivers of markedly different size were responsible for all sandstone bodies: bankfull widths, depths and mean velocities of both rivers varied little with time (order of 10⁵ yr), implying a stable climatic setting. Channel sinuosities were usually 1.15–1.2 throughout the succession. Both rivers decreased in mean channel slope as time progressed, in association with a rising base-level and a shoreline encroaching from the south. Using Bridge & Leeder's (1979) alluvial stratigraphy model, the nature and distribution of channel sandstone bodies relative to overbank deposits in the succession can be explained by an average (compacted) floodplain deposition rate of about 0.005 m yr^?1, if avulsion occurred with a frequency of about once every 10³ yr. Local variation in the relative amount of channel sandstone in the succession is probably due to local tectonic control of deposition.