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.     

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.     

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).     

Palaeochannel reconstructions from point bar deposits: a three-dimensional perspective

Quantitative determination of palaeochannel geometry and hydraulics from point bar deposits requires an understanding of the interaction between channel-bend migration, temporal and spatial variation of point bar geometry and facies, and outcrop orientation. This interaction is modelled with the aid of a computer program which predicts three-dimensional (3-D) geometry and grain size variation of point bars. Synthetic deposits are produced for the cases of down-valley bend migration and/or increase in channel-bend sinuosity. Two-dimensional (2-D) cross-sections in varying orientations across these simulated deposits display lateral-accretion bedset surface geometry, and variation in mean bedset grain size and local palaeocurrent orientation. Most cross-sections show point bar deposits thickening away from the meander-belt axis due to a lateral progression from thinner bend-exit deposits to thicker bend-apex deposits (caused by down-valley channel translation), and/or due to a progression from thinner low sinuosity deposits to thicker high sinuosity deposits caused by channel bend expansion. In association with this lateral thickening, bedset surfaces become steeper and more convex upwards while the variation in mean grain size up bedsets commonly increases. Down-valley point bar translation allows preservation only of deposits formed downstream of the band apex, and produces characteristic fining upwards sequences. Marked lateral and vertical variations in palaeocurrent directions due to varying channel orientation relative to a given cross-section are also predicted. These results indicate a need in palaeochannel reconstructions, for a more detailed examination of 3-D variations in bedset surface geometry, palaeocurrent orientation and grain size distribution within and between bedsets of laterally accreted sediment.     

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。     

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).     

A meander-belt sandstone of the Lower Cretaceous of Southern England

The meander-belt deposit comprises a sandstone resting on an erosion surface and bounded above and below by massive varicoloured mudstones with rootlet traces. The sandstone unit is composed of six bodies separated from one another, horizontally, by erosion surfaces; together the bodies form a single multilateral sand body. Internally each body is composed of lateral accretion units inclined at up to 6° from the horizontal. Vertical sequences of facies show significant variations but the grain size generally fines upwards. The principal lithofacies within the sandstones are, in common ascending order, intraformational conglomerate, large-scale cross-bedded, horizontal bedded and small-scale cross-laminated sandstone, and alternate sandstones and mudstones. Current directions are normal to the true slope of accretion surfaces and show insignificant scatter within individual bodies but are very diverse overall. Five of the sand bodies are believed to represent individual point bars, and one body an abandoned channel. Together they comprise the meander belt. The river was subject to very variable discharges and carried high suspended loads. Analysis of vertical profiles indicates that grain size segregation along the length of the point bars caused differentiation of the bars into coarse-grained heads and sandy tails.     

Point bar and floodplain formation of the meandering Beatton River, northeastern British Columbia, Canada

This study examines the morphology, sedimentology and genesis of the point bars and floodplain of the Beatton River. The formation of point bars occurs in distinct stages. An initial point bar platform composed mainly of coarse sediment is formed adjacent to the convex bank of a migrating meander bend, and is the base on which develops a single scroll bar of fine traction and suspended load. With continued sedimentation, the scroll bar grows, eventually supporting vegetation and becoming a floodplain ridge. Scroll bars form with greatest size and frequency in rapidly migrating bends, and the shape of the meander bend appears to determine both the location of the initial bar deposit, and its direction of growth up or downstream. Approximately one-half of the floodplain sediment is derived from suspended load, and the initiation of a scroll bar appears to be due to excessive deposition of suspended load in a zone of flow separation over a point bar platform. The critical flow condition for the initiation of a scroll bar does not occur with the same recurrence interval on different shaped meander bends, however, the average recurrence interval within the study reach is approximately every 30 years. Sedimentation rates on point bars and on the floodplain indicate two relatively distinct stages of floodplain alluviation. The most rapid is for surfaces less than 50 years old, although sediment accumulation still persists on surfaces up to 250 years in age. Although frequently flooded, surfaces older than this accumulate very little sediment. Despite 2–3 m of overbank deposition, the amplitude of floodplain ridges is maintained by secondary currents which sweep sediment from the swales towards the ridge crests.     

Deep‐water foreland basin deposits of the Cerro Toro Formation,Magallanes basin,Chile: architectural elements of a sinuous basin axial channel belt

Coarse‐grained deep‐water strata of the Cerro Toro Formation in the Cordillera Manuel Señoret, southern Chile, represent the deposits of a major channel belt (4 to 8 km wide by >100 km long) that occupied the foredeep of the Magallanes basin during the Late Cretaceous. Channel belt deposits comprise a ca 400 m thick conglomeratic interval (informally named the ‘Lago Sofia Member’) encased in bathyal fine‐grained units. Facies of the Lago Sofia Member include sandy matrix conglomerate (that show evidence of traction‐dominated deposition and sedimentation from turbulent gravity flows), muddy matrix conglomerate (graded units interpreted as coarse‐grained slurry‐flow deposits) and massive sandstone beds (high‐density turbidity current deposits). Interbedded sandstone and mudstone intervals are present locally, interpreted as inner levée deposits. The channel belt was characterized by a low sinuousity planform architecture, as inferred from outcrop mapping and extensive palaeocurrent measurements. Laterally adjacent to the Lago Sofia Member are interbedded mudstone and sandstone facies derived from gravity flows that spilled over the channel belt margin. A levée interpretation for these fine‐grained units is based on several observations, which include: (i) palaeocurrent measurements that indicate flows diverged (50° to 100°) once they spilled over the confining channel margin; (ii) sandstone beds progressively thin, away from the channel belt margin; (iii) evidence that the eroded channel base was not very well indurated, including a stepped margin and injection of coarse‐grained channel material into surrounding fine‐grained units; and (iv) the presence of sedimentary features common to levées, including slumped units inferring depositional slopes dipping away from the channel margin, lenticular sandstone beds thinning distally from the channel margin, soft sediment deformation and climbing ripples. The tectonic setting and foredeep architecture influenced deposition in the axial channel belt. A significant downstream constriction of the channel belt is reflected by a transition from more tabular units to an internal architecture dominated by lenticular beds associated with a substantially increased degree of scour. Differential propagation of the fold‐thrust belt from the west is speculated to have had a major control on basin, and subsequently channel, width. The confining influence of the basin slopes that paralleled the channel belt, as well as the likelihood that numerous conduits fed into the basin along the length of the active fold‐thrust belt to the west, suggest that proximal–distal relationships observed from large channels in passive margin settings are not necessarily applicable to axial channels in elongate basins.     

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.     

古河道储层构型层次分析法——以弧岛油田馆陶组曲流河为例

以胜利油区孤岛油田11J11密井网区曲流河储层为例,在曲流河野外露头和现代沉积原型模型研究成果的基础上,应用岩心、测井及动态等资料,探索了一套曲流河地下储层构型分析方法。在基于构型分析的精细等时地层对比及单井构型要素解释的基础上,采用“层次分析”和“模式拟合”的研究思路,按复合曲流带、单一曲流带、单一点坝以及点坝内部侧积体的4个层次进行模式拟合,对曲流河各级次构型单元内部结构进行系统分析。拟合结果既符合曲流河沉积成因理论,又与研究区动态资料吻合,说明研究成果是可靠的,由此建立了真正意义上的三维储层构型模型,再现了曲流河点坝内部泥质侧积层空间分布特征,为分析油田开发后期储层构型控制的剩余油分布规律提供了科学依据,该方法对类似油田精细储层研究有较好的借鉴作用,并能推广应用。
     

准噶尔盆地阜东斜坡头屯河组二段沉积微相及主控因素

准噶尔盆地阜东斜坡头屯河组是重要的油气勘探目的层。基于岩心、测井及地震等资料,探究研究区的沉积特征及其展布规律,对该区今后油气滚动勘探有重要意义。研究表明:研究区目的层岩性以泥岩、粉砂岩及细砂岩组合为主,总体表现出“泥包砂”的特点;其中砂岩成分成熟度低,具有近物源特征;粒度概率累计曲线主要为跳跃悬浮式,水动力类型为牵引流;测井曲线可分为钟形、箱形和线形3类。综合岩性、粒度、测井及地震反射特征等明确了研究区主要发育曲流河沉积,可识别出曲流河道、点坝、泛滥平原和废弃河道4种沉积微相。研究区沉积物源来自东北方向,曲流河展布特征主要受地形坡度、物源远近及粒度特征影响。曲流河由北东向南西延伸,且西南部的河道弯曲度有增大的特征,河道逐渐呈现网状分布特征,点坝砂体也更加发育;同时,西北部也出现了连片分布的曲流河沉积。因此,后续油气勘探优质储层的找寻应重点考虑西北部与西南部的点坝砂体。     

Facies architecture of asymmetrical branching distributary channels: Cretaceous Ferron Sandstone,Utah, USA

Distributary channel systems are an important component of deltaic systems, but details of their branching pattern, stream‐order, internal variability and relation with adjacent levée, bay and bayhead delta are rather poorly documented in ancient examples. Photomosaic and measured sections collected along a gooseneck‐shaped canyon in southern Utah allow direct mapping of the branching pattern of an ancient distributary system. The main channel belt is ca 250 m wide and narrows to ca 200 m downstream of the branching point. A subordinate channel belt, ca 80 m wide, branches off of the main channel, forming a distinctly asymmetrical branching pattern. Water discharge in the main channel is estimated to be 85 to 170 m³ sec^?1. Comparison with palaeodischarge estimates of trunk rivers mapped in previous studies suggests that the branching documented in this study probably is a fourth‐order split. The distributary channels are characterized by a U‐shaped geometry filled with medium‐grained, cross‐bedded sandstone, and are dominated by lateral accretion, suggesting limited lateral migration and moderate sinuosity. Tidally influenced facies and limited trace fossils indicate direct marine influence. The distributary channels erode into adjacent levée and underlying heterolithic bay‐fill deposits, and the marine influence suggests that they were deposited on a lower delta plain, rather than on a non‐marine floodplain. The subordinate channel fed a bayhead delta, suggesting that it was formed by a partial avulsion, rather than bifurcation around a mouth bar, as is more characteristic of terminal distributary channels. Channel‐floor drapes, bar‐accretion drapes and abandoned channel fills within the sandstone channel belts represent the most important heterogeneity from the perspective of reservoir characterization.     

湖泊细粒沉积特征精细研究：以鄂尔多斯盆地延河剖面长7油层组为例

野外地质露头为精细刻画沉积体内部结构、建立准确地下地质模型发挥着重要作用。以鄂尔多斯盆地延河剖面长7段为例,采用岩石学、野外露头沉积学方法,详细剖析了湖泊细粒沉积的岩相类型、特征、垂向组合及沉积环境。研究结果表明,延河剖面长7段发育平行层理细砂岩相、流水交错层理细砂岩相、浪成交错层理粉砂岩相、沙纹层理粉砂岩相、变形层理粉砂岩相、水平层理(泥质)粉砂岩相、块状泥岩相、水平层理(砂质)泥岩相、水平纹层页岩相9种岩相类型。在结合区域地质特征基础上,研究认为长7段为远源的曲流河三角洲前缘和浅湖-半深湖沉积,进一步细分出7类沉积单元,其中水下分支河道、支流间湾较为发育,水下天然堤、远砂坝、席状砂发育规模较小,浅湖-半深湖沉积只在长72段下部发育,河口坝基本不发育,仅局部可见。对各沉积单元的垂向分布特征进行深入研究,识别出Ⅰ、Ⅱ、Ⅲ、Ⅳ 4类垂向分布形式,其中Ⅰ、Ⅱ组合主要分布在研究区长71、长73亚段,Ⅲ、Ⅳ组合主要分布在研究区长72亚段。剖面相分析表明,长7沉积期整体为一套先变细、再变粗的细粒沉积序列,为曲流河三角洲前缘沉积—浅湖-半深湖沉积—曲流河三角洲前缘沉积。     

曲流河古河道储层构型精细解剖——以孤东油田七区西馆陶组为例

以胜利油区孤东油田七区西为例,在沉积微相研究的基础上,主要应用研究区岩心、测井,动态等资料,对曲流河古河道砂体进行了系统的储层层次构型精细解剖。在点坝定量分布模式的指导下,根据点坝砂体垂向上典型的正韵律、砂体厚度大以及紧邻废弃河道分布等特征,在研究区Ng522单层复合河道砂体内部识别出2个单一点坝。综合露头和现代沉积的研究成果、经验公式预测以及对子井分析,建立了研究区点坝内部构型定量模式,以单井上识别的泥质侧积层作为依据,点坝内部定量模式及动态监测结果作为指导,进行模式拟合,达到系统解剖点坝内部构型的目的。建立了点坝内部泥质侧积层控制的剩余油分布模式,提出了采用在点坝砂体中上部钻水平井的方式开采侧积层控制的剩余油。点坝构型解剖成果在研究区得到了较好的应用效果,而且对类似油田储层精细研究及剩余分布预测都有较好的借鉴作用。     

Preservation of meandering river channels in uniformly aggrading channel belts

Channel belt deposits from meandering river systems commonly display an internal architecture of stacked depositional features with scoured basal contacts due to channel and bedform migration across a range of scales. Recognition and correct interpretation of these bounding surfaces is essential to reconstruction of palaeochannel dimensions and to flow modelling for hydrocarbon exploration. It is therefore crucial to understand the suite of processes that form and transfer these surfaces into the fluvial sedimentary record. Here, the numerical model ‘NAYS2D’ is used to simulate a highly sinuous meandering river with synthetic stratigraphic architectures that can be compared directly to the sedimentary record. Model results highlight the importance of spatial and temporal variations in channel depth and migration rate to the generation of channel and bar deposits. Addition of net uniform bed aggradation (due to excess sediment input) allows quantification of the preservation of meander morphology for a wide range of depositional conditions. The authors find that the effect of vertical variation in scouring due to channel migration is generally orders of magnitude larger than the effect of bed aggradation, which explains the limited impact bed aggradation has on preservation of meander morphology. Moreover, lateral differences in stratigraphy within the meander belt are much larger than the stratigraphic imprint of bed aggradation. Repeatedly produced alternations of point bar growth followed by cut‐off result in a vertical trend in channel and scour feature stacking. Importantly, this vertical stacking trend differs laterally within the meander belt. In the centre of the meander belt, the high reworking intensity results in many bounding surfaces and disturbed deposits. Closer to the margins, reworking is infrequent and thick deposits with a limited number of bounding surfaces are preserved. These marginal areas therefore have the highest preservation potential for complete channel deposits and are thus best suited for palaeochannel reconstruction.     

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.     

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.     

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.     

Shallow marine sand bar sequences: an example from the late Precambrian of North Norway

Five coarsening upward shallow marine sandstone sequences (2–10 m thick), are described from the late Precambrian of North Norway, where they occur in a laterally continuous and tectonically undeformed outcrop. The sequences consist of five facies with distinct assemblages of sedimentary structures and palaeocurrent patterns. Each facies is the product of alternate phases of sedimentation during relatively high- and low-energy periods. Facies 1 to 4 are interpreted as representing prograding, subtidal sand bars. Sand bar progradation occurred during the highest energy periods when unidirectional currents flowed to the northwest, depositing trough cross-bedded sandstones (facies 3 and 4) on the bar crests and flanks, and sheet sandstone beds (facies 1 and 2) in the offshore environments. Weaker northwesterly flowing currents continued during moderate energy fair weather periods. Low energy fair weather periods were dominated by wave processes, which formed largescale, low-angle, westerly inclined surfaces on the bar flanks (facies 4) and wave rippled sandstone beds (facies 2) and flat laminated siltstone layers (facies 1) in the offshore environments. One sand bar was dissected by channels and infilled by tabular cross-bedded sandstones (facies 5). Bipolar palaeocurrent evidence, with two modes separated into two laterally equivalent channel systems, suggests deposition by tidal currents in mutually evasive ebb and flood channels. The inferred processes of these sand bars are compared with those associated with modern storm-generated and tidal current generated linear sand ridges. Both are influenced by the interaction of relatively low and high energy conditions. The presence of the tidal channel facies, however, combined with the inferred strong bottom current regime, is more analogous to a tidal current hydraulic regime.