The sedimentology and alluvial architecture of the sandy braided South Saskatchewan River, Canada

Ground penetrating radar (GPR) surveys of unit and compound braid bars in the sandy South Saskatchewan River, Canada, are used to test the influential facies model for sandy braided alluvium presented by Cant & Walker (1978) . Four main radar facies are identified: (1) high‐angle (up to angle‐of‐repose) inclined reflections, interpreted as having formed at the margins of migrating bars; (2) discontinuous undular and/or trough‐shaped reflections, interpreted as cross‐strata associated with the migration of sinuous‐crested dunes; (3) low‐angle (< 6°) reflections, interpreted as formed by low‐amplitude dunes or unit bars as they migrate onto bar surfaces; and (4) reflections of variable dip bounded by a concave reflection, interpreted as being formed by the filling of channel scours, cross‐bar channels or depressions on the bar surface. The predominant vertical arrangement of facies is discontinuous trough‐shaped reflections at the channel base overlain by discontinuous undular reflections, overlain by low‐angle reflections that dominate the deposits near the bar surface. High‐angle inclined reflections are only found near the surface of unit bars, and are of relatively small‐scale (< 0·5 m), but can be found at a greater range of depths within compound bars. The GPR data show that a high spatial variability exists in the distribution of facies between different compound bars, with facies variability within a single bar being as pronounced as that between bars. Compound bars evolve as an amalgamation of unit bars and other compound bars, and comprise a facies distribution that is representative of the main bar types in the South Saskatchewan River. The GPR data are compared with the original model of Cant & Walker (1978) and reveal a much greater variability in the scale, proportion and distribution of facies than that presented by Cant & Walker (1978) . Most notably, high‐angle inclined strata are over‐represented in the model of Cant and Walker, with many bars being dominated by the deposits of low‐ and high‐amplitude dunes. It is suggested that further GPR studies from a range of braided river types are required to properly quantify the full range of deposits. Only by moving away from traditional, highly generalized facies models can a greater understanding of braided river deposits and their controls be established.     

Role of vegetation in shaping Early Pennsylvanian braided rivers: Architecture of the Boss Point Formation,Atlantic Canada

Vegetation is a major driver of fluvial dynamics in modern rivers, but few facies models incorporate its influence. This article partially fills that gap by documenting the stratigraphy, architecture and palaeobotany of the Lower Pennsylvanian Boss Point Formation of Atlantic Canada, which contains some of the Earth's earliest accumulations of large woody debris. Braided‐fluvial systems occupied channel belts of varied scale within valleys several tens of metres deep and more than 12 km wide, and their deposits predominantly consist of sandy and gravelly bedforms with subordinate accretionary macroforms, high flow‐strength sand sheets and rippled abandonment facies. Discrete accumulations of clastic detritus and woody debris are up to 6 m thick and constitute at least 18% of the in‐channel deposits; they represent lags at the base of large and small channels, fills of minor channels and sandy macroforms that developed in central positions in the upper parts of channel fills. Sandstones with roots and other remnants of in situ vegetation demonstrate that vegetated islands were present, and the abundance of discrete channel fills suggests that the formation represents an anabranching, island‐braided sandbed river, the earliest example documented to date. Although some sphenopsid and lycopsid remains are present, most woody fragments are derived from cordaitalean trees, and the evolution of this group late in the Mississippian is inferred to have exerted a significant influence on fluvial morphodynamic patterns. The formation records a landscape in which active channel belts alternated with well‐drained floodplains colonized by dense, mature forests and local patches of pioneering, disturbance‐tolerant vegetation. Lakes and poorly drained floodplains dominated by carbonate and organic deposition, respectively, were also present. A large supply of woody debris triggered channel blockage and avulsion, and active channel margins and islands within the channel belts were initially colonized by pioneer vegetation and subsequently stabilized by large trees. A similar alternation of stable and unstable conditions is observed in modern braided rivers actively influenced by vegetation.     

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.     

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.     

Fluvial and lacustrine palaeoenvironments of the Miocene Siwalik Group, Khaur area, northern Pakistan

The Miocene Siwalik Group (upsection, the Chinji, Nagri, and Dhok Pathan Formations) in northern Pakistan records fluvial and lacustrine environments within the Himalayan foreland basin. Thick (5 m to tens of metres) sandstones are composed of channel bar and fill deposits of low-sinuousity (1·08–1·19), single-channel meandering and braided rivers which formed large, low-gradient sediment fans (or ‘megafans’). River flow was dominantly toward the south-east and likely perennial. Palaeohydraulic reconstructions indicate that Chinji and Dhok Pathan rivers were small relative to Nagri rivers. Bankfull channel depths of Chinji and Dhok Pathan rivers were generally ≤ 15 m, and up to 33 m for Nagri rivers. Widths of channel segments (including single channels of meandering rivers and individual channels around braid bars) were 320–710 m for Chinji rivers, 320–1050 m for Nagri rivers, and 270–340 m for Dhok Pathan rivers. Mean channel bed slopes were on the order of 0·000056–0·00011. Bankfull discharges of channel segments for Chinji and Dhok Pathan rivers were generally 700–800 m³s^?1, with full river discharges possibly up to 2400 m³s^?1. Bankfull discharges of channel segments for Nagri rivers were generally 1800–3500 m³s^?1, with discharges of some larger channel segments possibly on the order of 9000–32 000 m³s^?1. Full river discharges of some of the largest Nagri braided rivers may have been twice these values. Thin (decimetres to a few metres) sandstones represent deposits of levees, crevasse channels and splays, floodplain channels, and large sheet floods. Laminated mudstones represent floodplain and lacustrine deposits. Lakes were both perennial and short-lived, and likely less than 10 m deep with maximum fetches on the order of a few tens of kilometres. Trace fossils and body fossils within all facies indicate the former existence of terrestrial vertebrates, molluscs (bivalves and gastropods), arthropods (including insects), worms, aquatic fauna (e.g. fish, turtles, crocodiles), trees, bushes, grasses, and aquatic flora. Palaeoenvironmental reconstructions are consistent with previous palaeoclimatic interpretations of monsoonal conditions.     

Evolution and deposits of a gravelly braid bar, Sagavanirktok River, Alaska

The evolution, migration and deposits of a gravelly braid bar in the Sagavanirktok River, northern Alaska, are described in unprecedented detail using annual aerial photographs, ground‐penetrating radar (GPR) profiles, trenches and cores. Compound braid bars in the Sagavanirktok River form by chute cut‐off of point bars and by growth of mid‐channel unit bars. Subsequent growth is primarily by accretion of unit bars onto their lateral and downstream margins. The upstream ends of braid bars may be sites of erosion or unit bar deposition. Compound braid bar deposits vary in thickness laterally and are thickest in medial sections and near cut banks. Compound bar deposits are typically composed of three to seven sets of simple large‐scale inclined strata, each simple set formed by a unit bar. The simple large‐scale strata contain medium‐scale cross‐strata (from dune migration) and planar strata (from migration of bedload sheets). The upstream and medial parts of compound braid bar deposits show very little vertical variation in grain size, but downstream and lateral margins tend to fine upwards. The deposits are mostly poorly sorted sands and gravels, although sands tend to be deposited at the top of the braid bar, and open‐framework gravels preferentially occur near the top and base of the braid bar. The patterns of braid bar growth and migration, and the nature of the deposits, described from the Sagavanirktok River are generally similar to other sandy and gravelly braided rivers, and consistent with the theoretical braid bar model of Bridge (1993).     

Abandoned channel fill sequences in the tidal estuary of a small mountainous,dry‐summer river

This study proposes a modification of the current model for abandoned channel fill stratigraphy produced in unidirectional flow river reaches to incorporate seasonal tidal deposition. Evidence supporting this concept came from a study of two consecutive channel abandonment sequences in Ropers Slough of the lower Eel River Estuary in northern California. Aerial photographs showed that Ropers Slough was abandoned around 1943, reoccupied after the 1964 flood, and abandoned again in 1974 with fill continuing to the present. Planform geomorphic characteristics derived from these images were used in conjunction with sub‐centimetre resolution stratigraphic analyses to describe depositional processes and their resultant sedimentary deposits. Both abandonment sequences recorded quasi‐annual scale fluvial/tidal deposition couplets. In both cases, tidal deposits contained very little sand, were higher in organic and inorganic carbon content than the sandier, fluvially dominated deposits, and possessed millimetre‐scale horizontal laminations. The two abandonment fills differed significantly in terms of the temporal progression of channel narrowing and fluvial sediment deposition characteristics. Aerial photographic analysis showed that the first abandonment sequence led to a more rapid narrowing of Ropers Slough and produced deposits with a positive relationship between grain size/deposit thickness and discharge. The second abandonment resulted in a much slower narrowing of Ropers Slough and generally thinner fluvial deposits with no clear relationship between grain size/deposit thickness and discharge. The δ¹³C values and organic nitrogen to organic carbon ratios of deposits from the first phase overlapped with Eel River suspended sediment characteristics found for low flows (one to five times mean discharge), while those of the second phase were consistent with suspended sediment from higher flows (seven to ten times mean discharge). When considered together, the results indicate that the early fill sequence recorded a reach experiencing regular fluvial deposition through flow conditions during the wet season, while the latter fill sequence records a reach more disconnected from the main stem in terms of flow and sediment. The major factor affecting the difference in sedimentation between the two fill periods appears to have been the morphology of the upstream river bend in relation to the position of the bifurcation node. During the first fill period, the upstream entrance to Ropers Slough seems to have remained open, in part due to the placement of its entrance on the outside of the mainstem river bend, and despite stronger tidal effects caused by a larger tidal prism and closer proximity to the tidal inlet. By the second fill sequence, the upstream bend morphology had altered, placing the entrance to Ropers Slough on the inner bank of the mainstem bend, which resulted in more rapid plug bar formation. The role of tidal effects in the geomorphic trajectory of the two abandonment sequences is unclear, but appears to have been less important than local bifurcation geometry.     

Quantification of the relation between surface morphodynamics and subsurface sedimentological product in sandy braided rivers

This study uses digital elevation models and ground‐penetrating radar to quantify the relation between the surface morphodynamics and subsurface sedimentology in the sandy braided South Saskatchewan River, Canada. A unique aspect of the methodology is that both digital elevation model and ground‐penetrating radar data were collected from the same locations in 2004, 2005, 2006 and 2007, thus enabling the surface morphodynamics to be tied explicitly to the associated evolving depositional product. The occurrence of a large flood in 2005 also allowed the influence of discharge to be assessed with respect to the process–product relationship. The data demonstrate that the morphology of the study reach evolved even during modest discharges, but more extensive erosion was caused by the large flood. In addition, the study reach was dominated by compound bars before the flood, but switched to being dominated by unit bars during and after the flood. The extent to which the subsurface deposits (the ‘product’) were modified by the surface morphodynamics (the ‘process’) was quantified using the changes in radar‐facies recorded in sequential ground‐penetrating radar surveys. These surveys reveal that during the large flood there was an increase in the proportion of facies associated with bar margin accretion and larger dunes. In subsequent years, these facies became truncated and replaced with facies associated with smaller dune sets. This analysis shows that unit bars generally become truncated more laterally than vertically and, thus, they lose the high‐angle bar margin deposits and smaller scale bar‐top deposits. In general, the only fragments that remain of the unit bars are dune sets, thus making identification of the original unit barform problematic. This novel data set has implications for what may ultimately become preserved in the rock record.     

Morphology and evolution of bars in a wandering gravel-bed river; lower Fraser river, British Columbia, Canada

A hierarchical typology for the channels and bars within aggradational wandering gravel-bed rivers is developed from an examination of a 50 km reach of lower Fraser River, British Columbia, Canada. Unit bars, built by stacking of gravelly bedload sheets, are the key dynamic element of the sediment transfer system, linking sediment transport during individual freshets to the creation, development and remoulding of compound bar platforms that have either a lateral or medial style. Primary and secondary unit bars are identified, respectively, as those that deliver sediment to compound bars from the principal channel and those that redistribute sediment across the compound bar via seasonal anabranches and smaller channels. The record of bar accretion evident in ground-penetrating radar sequences is consistent with the long-term development of bar complexes derived from historical aerial photographs. For two compound bars, inter-annual changes associated with individual sediment transport episodes are measured using detailed topographic surveys and longer-term changes are quantified using sediment budgets derived for individual bars from periodic channel surveys. Annual sediment turnover on the bars is comparable with the bed material transfer rate along the channel, indicating that relatively little bed material bypasses the bars. Bar construction and change are accomplished mainly by lateral accretion as the river has limited capacity to raise bed load onto higher surfaces. Styles of accretion and erosion and, therefore, the major bar form morphologies on Fraser River are familiar and consistent with those in gravelly braided channels but the wandering style does exhibit some distinctive features. For example, 65-year histories reveal the potential for long sequences of uninterrupted accretion in relatively stable wandering rivers that are unlikely in braided rivers.     

Scales and causes of heterogeneity in bars in a large multi‐channel river: Río Paraná, Argentina

To date, published studies of alluvial bar architecture in large rivers have been restricted mostly to case studies of individual bars and single locations. Relatively little is known about how the depositional processes and sedimentary architecture of kilometre‐scale bars vary within a multi‐kilometre reach or over several hundreds of kilometres downstream. This study presents Ground Penetrating Radar and core data from 11, kilometre‐scale bars from the Río Paraná, Argentina. The investigated bars are located between 30 km upstream and 540 km downstream of the Río Paraná – Río Paraguay confluence, where a significant volume of fine‐grained suspended sediment is introduced into the network. Bar‐scale cross‐stratified sets, with lengths and widths up to 600 m and thicknesses up to 12 m, enable the distinction of large river deposits from stacked deposits of smaller rivers, but are only present in half the surface area of the bars. Up to 90% of bar‐scale sets are found on top of finer‐grained ripple‐laminated bar‐trough deposits. Bar‐scale sets make up as much as 58% of the volume of the deposits in small, incipient mid‐channel bars, but this proportion decreases significantly with increasing age and size of the bars. Contrary to what might be expected, a significant proportion of the sedimentary structures found in the Río Paraná is similar in scale to those found in much smaller rivers. In other words, large river deposits are not always characterized by big structures that allow a simple interpretation of river scale. However, the large scale of the depositional units in big rivers causes small‐scale structures, such as ripple sets, to be grouped into thicker cosets, which indicate river scale even when no obvious large‐scale sets are present. The results also show that the composition of bars differs between the studied reaches upstream and downstream of the confluence with the Río Paraguay. Relative to other controls on downstream fining, the tributary input of fine‐grained suspended material from the Río Paraguay causes a marked change in the composition of the bar deposits. Compared to the upstream reaches, the sedimentary architecture of the downstream reaches in the top ca 5 m of mid‐channel bars shows: (i) an increase in the abundance and thickness (up to metre‐scale) of laterally extensive (hundreds of metres) fine‐grained layers; (ii) an increase in the percentage of deposits comprised of ripple sets (to >40% in the upper bar deposits); and (iii) an increase in bar‐trough deposits and a corresponding decrease in bar‐scale cross‐strata (<10%). The thalweg deposits of the Río Paraná are composed of dune sets, even directly downstream from the Río Paraguay where the upper channel deposits are dominantly fine‐grained. Thus, the change in sedimentary facies due to a tributary point‐source of fine‐grained sediment is primarily expressed in the composition of the upper bar deposits.     

Bar structure in an arid ephemeral stream

This study describes the structure of gravel bars in Nahal Zin, an ephemeral stream in the Negev desert. The internal structure of the bars was examined along trenches and in shallow pits. Gravel sheets and unit bars form during transporting flow events in the main channel, on intra-bar channels and near bar heads. Unit bars are dominated by the Go facies. Compound bars develop from accretion around, and modification of, unit bars. Compound bars are active under the current flow regime and the average depth of the fill layer is about 35 cm. The structure of compound bars is dominated by Gm (massive), containing large amounts of sand. The second most common facies is clast-supported, openwork, and well sorted sediments of the Go (pebbles) facies. Bar formation, and the development of the range of facies evident in the bars is controlled by sediment supply, particularly the high volumes of sand-sized sediment, the passage of gravel sheets and bedforms during floods, and the lateral and vertical instability of the channel. Repeated scour and fill events have produced a diverse arrangement of facies, with numerous erosional contacts between depositional units. Lateral and downstream shifts in the pattern of scour and fill due to flow and antecedent conditions shape the channel morphology and bar internal structure. Ephemeral river bars differ from those of humid and proglacial rivers in terms of the dominant facies present, the arrangement of the facies within the bars, and the sedimentary structures developed within the depositional units and on the bar surface.     

Large-scale structure of Calamus River deposits (Nebraska, USA) revealed using ground-penetrating radar

The large-scale (i.e. bar-scale) structure of channel deposits of the braided, low-sinuosity Calamus River, Nebraska, is described using ground-penetrating radar (GPR) profiles combined with vibracores. Basal erosion surfaces are generally overlain by medium-scale, trough-cross-stratified (sets 3–25 cm thick), very coarse to medium sands, that are associated with relatively high amplitude, discontinuous GPR reflectors. Overlying deposits are bioturbated, small-scale cross-stratified (sets <3 cm thick) and vegetation-rich, fine to very-fine sands, that are associated with low-amplitude discontinuous reflectors. Near-surface peat and turf have no associated GPR reflectors. In along-stream profiles through braid and point bars, most GPR reflectors dip downstream at up to 2° relative to the basal erosion surface, but some reflectors in the upstream parts of bars are parallel to the basal erosion surface or dip upstream. In cross-stream profiles through bars, GPR reflectors are either approximately parallel to bar surfaces or have low-angle inclinations (up to 6°) towards cut banks of adjacent curved channels. Basal erosion surfaces become deeper towards cut banks of curved channels. These structures can be explained by lateral and downstream growth of bars combined with vertical accretion. Convex upwards forms up to 0·5 m high, several metres across and tens of metres long represent episodic accretion of unit bars (scroll bars and bar heads). Stratal patterns in channel fills record a complicated history of erosion and deposition during filling, including migration of relatively small bars. A revised facies model for this type of sandy, braided river has been constructed based on this new information on large-scale bedding structure.     

Grain-size sorting within river bars in relation to downstream fining along a wandering channel

The textural variability of river bed gravels at bar scales is poorly understood, as are the relations between variability at this scale and at reach and river scales. Surface and subsurface grain‐size distributions were therefore examined at reach, bar and bedform scales along lower Fraser River, British Columbia, Canada. Grain‐size variations within compound bars are conditioned by longitudinal position, elevation and morphological setting. Surface and subsurface sediments tend to decrease in median size from bar head to bar tail by 33% and 17%, respectively. Sediment size is constrained at some upper limit that is inversely related to bar surface elevation and which is consistent with competence considerations. The surface sediments on unit bars are finer and better sorted than the bed materials in bar‐top channels and along the main bar edges. Secondary unit bars tend to have a lower sand content than other features, a consequence of sediment resorting. Individual unit bars and gravel sheets exhibit streamwise grain‐size fining and lee‐side sand deposition. Over time, significant amounts of cut and fill do not ipso facto cause changes in surface grain sizes; yet, sediment characteristics can change without any significant morphological adjustment taking place. At the reach scale there is a clear downstream fining trend, but local variability is consistently high due to within‐bar variations. The surface median grain‐size range on individual bars is, on average, 25% of that along the entire 50 km reach but is 68% on one bar. While the overall fining trend yields a downstream change in surface median size of 0·76 mm km^?1, the average value for ‘head‐to‐tail’ size reduction on individual bars is 6·3 mm km^?1, an order of magnitude difference that highlights the effectiveness of bar‐scale sorting processes in gravel‐bed rivers. Possibilities for modelling bar‐scale variability and the interaction of the different controls that are identified are discussed.     

砂质辫状河储层构型表征及其对剩余油的控制--以渤海海域P油田为例

砂质辫状河内部不同级次的构型模式,特别是心滩坝及其内部夹层的构型特征尤为重要,直接控制了油田开发中后期剩余油富集与分布。采取现代地质调查与地下地质解剖相结合的思路,以渤海海域P油田为例,分别从辫流带、心滩坝及心滩坝内部夹层三个级次,探讨了砂质辫状河构型特征及其对剩余油分布规律的影响。结果表明砂质辫状河道具有三种充填样式,且以砂质充填为主,心滩坝与辫状河道呈"宽坝窄河道"的分布样式;坝内落淤层具有穹窿式和水平式两种分布样式。此外,从5级至3级构型单元逐步细分,通过三种识别标志划分单一辫流带,平均宽厚比约178;心滩坝辫状河道宽度与心滩坝宽度比约为1:3.8,心滩坝宽度与长度比约为1:2.2;心滩坝内部单期落淤层近水平分布,多期之间互相平行,一般2~4个。不同级次储层构型要素对剩余油分布的控制存在差异性。对于辫流带级次,层间隔层直接控制剩余油分布;心滩坝级次泥质半充填河道底部砂体连通,侧翼上部水淹程度较低,剩余油富集;受落淤层和物性界面的影响,心滩坝内部剩余油主要呈"分段式"富集在落淤层下方垂积体的顶部和水驱较弱的边部。该成果在油田老区综合调整方案和新区开发方案设计上得到了应用,也在调整井部署,尤其是水平井挖潜剩余油等方面,提供了地质依据。     

砂质辫状河心滩坝的发育演化过程探讨--沉积数值模拟与现代沉积分析启示

心滩坝作为辫状河中一种重要的沉积单元,国内外学者基于地下储层、露头、现代沉积进行了大量研究,但并未建立完整的心滩坝发育演化模式,明确心滩坝两侧辫状水道对其的影响。因此,采用沉积数值模拟技术（Delft3D）再现心滩坝的生长与演化过程,并且通过现代沉积相互验证,建立了心滩坝演化模式。研究表明:1）心滩坝的演化过程可划分为三个阶段,心滩坝的形成、生长及向下游方向迁移,心滩坝的侧向迁移,"坝尾沉积"及复合心滩坝的形成;2）由于两侧辫状水道的冲刷作用,心滩坝头部两侧均遭受侵蚀,表现为坝体向下游方向迁移,随着发育演化时间的增长,向下游迁移速率逐渐减小,最终坝体稳定;3）心滩坝中下部由于两侧辫状水道的不对称性,一侧侵蚀,一侧沉积,心滩坝侧向迁移;4）沉积物在心滩坝尾部卸载,形成"坝尾沉积",其发育程度取决于两侧辫状水道水动力强度,水动力越强,"坝尾沉积"越发育,随着"坝尾沉积"的不断延长,最终充填心滩坝之间的辫状水道,形成复合心滩坝。     

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.     

Pool-fills: a window to palaeoflood history and response in bedrock-confined rivers

Channel‐scale sedimentary units associated with bedrock‐controlled riffle‐pool morphology are examined in detail along Sandy Creek gorge, an ephemeral stream in arid south‐eastern central Australia. Pool‐fills comprise cut‐and‐fill assemblages of poorly sorted sediments ranging in texture from muds to boulders. Five unit types are defined based on particle size, sedimentary structures, geometry and bounding surface character: (1) coarse‐grained bar platform; (2) fine‐grained bar supraplatform; (3) fine‐grained pool‐fill; (4) fine‐grained bench; and (5) modern pool‐fill. The last coarse‐grained unit currently lining the pools suggests an altered sedimentation style over the post‐settlement period (post‐ad 1860s). Situated at bedrock valley constrictions, pool‐fills are compared with other sedimentary units associated with recirculating currents: eddy bars and slackwater deposits. But only the fine‐grained bench units reflect eddy recirculation; the pool‐fills are principally forced‐bars associated with bedrock‐controlled or ‘forced’ riffle‐pool morphology. A late Holocene palaeoflood history is proposed based on radiocarbon ages from the pool‐fills: multiple phases of cut‐and‐fill activity were preceded by a superflood 3400–1900 years ago that eroded the pool‐fills to bedrock. The resilience of the pool‐fills was illustrated by the passage of a 1‐in‐100‐year flood in 1992, which caused only minor erosion. The presence of pool‐fills may provide a window to past phases of river activity that cannot be extracted from either historical records/observations or palaeoflood slackwater sediment analyses. The formation and sedimentary preservation potential of these landforms reflect a combination of hydraulic and structural influences, but the occurrence of high‐magnitude floods exerts the dominant control.     

鄂尔多斯盆地南部直罗—店头地区侏罗系直罗组沉积特征及铀成矿*

鄂尔多斯盆地南部中侏罗统直罗组下段砂岩中已发现店头砂岩型铀矿床及多个铀矿点,但对于盆地南部直罗组沉积相研究较为薄弱,制约了砂岩型铀矿的进一步找矿工作。通过剖面实测、岩心编录,结合石油、煤炭、核工业地质钻孔资料,对鄂尔多斯盆地南部直罗—店头地区直罗组沉积特征进行精细刻画。在此基础上,探讨了沉积相与铀成矿的关系及下一步找矿方向。结果显示,直罗组下段砂体厚度30~65 m、砂地比在0.6~0.75之间,为砂质辫状河沉积。直罗组上段早期砂体厚度10~38 m、砂地比值在0.15~0.45之间,为曲流河沉积;直罗组上段沉积晚期研究区位于湖盆中心所在位置,为滨浅湖沉积。沉积环境、沉积相、辫状河河道交汇部位、砂体厚度、沉积物粒度及泥岩夹层对铀成矿具有重要控制作用。直罗组下段辫状河河道交汇或分叉部位应作为勘查重点,心滩亚相的含炭屑、黄铁矿砂质碎屑岩可作为铀矿化的找矿标志。     

Facies model for a coarse‐grained,tide‐influenced delta: Gule Horn Formation (Early Jurassic), Jameson Land,Greenland

Tide‐dominated deltas have an inherently complex distribution of heterogeneities on several different scales and are less well‐understood than their wave‐dominated and river‐dominated counterparts. Depositional models of these environments are based on a small set of ancient examples and are, therefore, immature. The Early Jurassic Gule Horn Formation is particularly well‐exposed in extensive sea cliffs from which a 32 km long, 250 m high virtual outcrop model has been acquired using helicopter‐mounted light detection and ranging (LiDAR). This dataset, combined with a set of sedimentological logs, facilitates interpretation and measurement of depositional elements and tracing of stratigraphic surfaces over seismic‐scale distances. The aim of this article is to use this dataset to increase the understanding of depositional elements and lithologies in proximal, unconfined, tide‐dominated deltas from the delta plain to prodelta. Deposition occurred in a structurally controlled embayment, and immature sediments indicate proximity to the sediment source. The succession is tide dominated but contains evidence for strong fluvial influence and minor wave influence. Wave influence is more pronounced in transgressive intervals. Nine architectural elements have been identified, and their internal architecture and stratigraphical distribution has been investigated. The distal parts comprise prodelta, delta front and unconfined tidal bar deposits. The medial part is characterized by relatively narrow, amalgamated channel fills with fluid mud‐rich bases and sandier deposits upward, interpreted as distributary channels filled by tidal bars deposited near the turbidity maximum. The proximal parts of the studied system are dominated by sandy distributary channel and heterolithic tidal‐flat deposits. The sandbodies of the proximal tidal channels are several kilometres wide and wider than exposures in all cases. Parasequence boundaries are easily defined in the prodelta to delta‐front environments, but are difficult to trace into the more proximal deposits. This article illustrates the proximal to distal organization of facies in unconfined tide‐dominated deltas and shows how such environments react to relative sea‐level rise.