The morphodynamics of fluvial sand dunes in the River Rhine, near Mainz, Germany. I. Sedimentology and morphology

The dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply‐limited reach of the River Rhine, Germany. Bed sediments, dune geometry, bedform migration rates and the internal structure of dunes are considered in this paper. Hydrodynamic and sediment transport data are considered in a companion paper. The pebbles and cobbles (D₅₀ of 10 mm) of the flat lag layer are rarely entrained. Dunes consist of well‐sorted medium to coarse sand (D₅₀ of 0·9 mm). Small pebbles move over the dunes by ‘overpassing’, but there is a degree of size and shape selectivity. Populations of ripples in sand (D₅₀ < 0·6 mm), and small and large dunes are separated by distinct breaks in the bedform length data in the regions of 0·7–1 m and 5–10 m. Ripples and small dunes may have sinuous crestlines but primarily exhibit two‐dimensional planforms. In contrast, large dunes are primarily three‐dimensional barchanoid forms. Ripples on the backs of small dunes rarely develop to maximum steepness. Small dunes may achieve an equilibrium geometry, either on the gravel bed or as secondary dunes within the boundary layer on the stoss side of large dunes. Secondary dunes frequently develop a humpback profile as they migrate across the upper stoss slope of large dunes, diminishing in height but increasing in length as they traverse the crestal region. However, secondary dunes more than 5 m in length are rare. The dearth of equilibrium ripples and long secondary dunes is probably related to the limited excursion length available for bedform development on the parent bedforms. Large dunes with lengths between 20 m and 100 m do not approach an equilibrium geometry. A depth limitation rather than a sediment supply limitation is the primary control on dune height; dunes rarely exceed 1 m high in water depths of ≈4 m. Dune celerity increases as a function of the mean flow velocity squared, but this general relationship obscures more subtle morphodynamics. During rising river stage, dunes tend to grow in height owing to crestal accumulation, which slows downstream progression and steepens the dune form. During steady or falling stage, an extended crestal platform develops in association with a rapid downstream migration of the lee side and a reduction in dune height. These diminishing dunes actually increase in unit volume by a process of increased leeside accumulation fed by secondary dunes moving past a stalled stoss toe. A six‐stage model of dune growth and diminution is proposed to explain variations in observed morphology. The model demonstrates how the development of an internal boundary layer and the interaction of the water surface with the crests of these bedload‐dominated dunes can result in dunes characterized by gentle lee sides with weak flow separation. This finding is significant, as other studies of dunes in large rivers have attributed this morphological response to a predominance of suspended load transport.     

河流相储层构型研究新理论、新方法——海上油田河流相复合砂体构型概念、内容及表征方法

基于陆上油田密井网的河流相储层构型研究,经过数十年积累,理论基础及表征方法已比较成熟,然而,将其应用于海上油田稀疏井网条件时,依然面临较大的挑战。笔者团队经过多年探索,基于海上油田开发地质研究方法,形成了井震结合、以"复合砂体"为核心的海上河流相复合砂体构型理论与表征方法,其特点在于运用三维原型建模、地震构型相预测及地震驱动确定性建模等技术,重点表征影响海上油田开发的五、六、七级构型单元(复合河道带、单一河道带、复合点坝),针对开发阶段的小层细分对比及不连续渗流屏障刻画等方面具有明显的优势。目前,应用这套理论及方法开展精细地质建模和数值模拟依然存在挑战,需要进一步发挥地震资料作用并探索人工智能的地震解释新途径。     

Reconstructing fluvial bar surfaces from compound cross‐strata and the interpretation of bar accretion direction in large river deposits

The interpretation of fluvial styles from the rock record is based for a significant part on the identification of different types of fluvial bars, characterized by the geometric relationship between structures indicative of palaeocurrent and surfaces interpreted as indicative of bar form and bar accretion direction. These surfaces of bar accretion are the boundaries of flood‐related bar increment elements, which are typically less abundant in outcrops than what would be desirable, particularly in large river deposits in which each flood mobilizes large volumes of sediment, causing flood‐increment boundary surfaces to be widely spaced. Cross‐strata set boundaries, on the other hand, are abundant and indirectly reflect the process of unit bar accretion, inclined due to the combined effect of the unit bar surface inclination and the individual bedform climbing angle, in turn controlled by changes in flow structure caused by local bar‐scale morphology. This work presents a new method to deduce the geometry of unit bar surfaces from measured pairs of cross‐strata and cross‐strata set boundaries. The method can be used in the absence of abundant flood‐increment bounding surfaces; the study of real cases shows that, for both downstream and laterally accreting bars, the reconstructed planes are very similar to measured bar increment surfaces.     

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

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

Bar and dune development during a freshet: Fraser River Estuary, British Columbia, Canada

Bi-weekly multi-track sonar surveys collected along a 2-km reach of the estuarine South Arm of the Fraser River, British Columbia, Canada, during seasonal high flows document the initiation, development and interaction of dune and bar morphologies. Bedforms of several scales developed in well-delineated fields. Bedforms that appear to fit accepted equilibrium depth-scaling developed in the main channel during rising discharge when there was little local aggradation. During the rising stage, a bar also formed along the tidal shelf of the channel, migrated downstream and expanded into the main channel. Dunes that formed along the bar crest, a region of rapid deposition, lagged flow conditions and were larger than expected based on depth-scaling relations. The larger dunes developed simultaneously, although bar growth lagged dune development and was initially partially obscured by the more rapidly developing dune field. It appears that rapid deposition enhances dune development along the channel tidal shelf to dimensions larger than would be expected by simple depth-scaling rules. Smaller dunes that fit equilibrium depth-scaling relations re-established themselves throughout the study area during falling discharge when there was again little or no aggradation. Bed-material transport rates estimated from the migration rates of the large dunes suggest that, at high flows, the dunes transport the majority of the material found within the bar.     

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

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

Supply limited sediment transport in a high-discharge event of the tropical Burdekin River, North Queensland, Australia

Interactions between catchment variables and sediment transport processes in rivers are complex, and sediment transport behaviour during high‐flow events is not well documented. This paper presents an investigation into sediment transport processes in a short‐duration, high‐discharge event in the Burdekin River, a large sand‐ and gravel‐bed river in the monsoon‐ and cyclone‐influenced, semi‐arid tropics of north Queensland. The Burdekin's discharge is highly variable and strongly seasonal, with a recorded maximum of 40 400 m³ s^?1. Sediment was sampled systematically across an 800 m wide, 12 m deep and straight reach using Helley‐Smith bedload and US P‐61 suspended sediment samplers over 16 days of a 29‐day discharge event in February and March 2000 (peak 11 155 m³ s^?1). About 3·7 × 10⁶ tonnes of suspended sediment and 3 × 10⁵ tonnes of bedload are estimated to have been transported past the sample site during the flow event. The sediment load was predominantly supply limited. Wash load included clay, silt and very fine sand. The concentration of suspended bed material (including very coarse sand) varied with bedload transport rate, discharge and height above the bed. Bedload transport rate and changes in channel shape were greatest several days after peak discharge. Comparison between these data and sparse published data from other events on this river shows that the control on sediment load varies between supply limited and hydraulically limited transport, and that antecedent weather is an important control on suspended sediment concentration. Neither the empirical relationships widely used to estimate suspended sediment concentrations and bedload (e.g. Ackers & White, 1973) nor observations of sediment transport characteristics in ephemeral streams (e.g. Reid & Frostick, 1987) are directly applicable to this river.     

Variations in the architecture of hydraulic‐jump bar complexes on non‐eroding beds

Sediment accumulation downstream of hydraulic jumps can occur in many settings but the architectures of such deposits are poorly documented. Here, three flume runs were used to examine the influence of sediment grain size and transport rate on the characteristics of hydraulic‐jump unit bars. In one of these runs six hydraulic‐jump unit bars formed a hydraulic‐jump bar complex. In another, the same sediment was supplied more quickly and only two unit bars formed. In the third run with the same sediment supply rate, but different grain size, only one large unit bar formed. All unit bars developed in a similar way but their size and internal architecture differed; they all resulted from a reduction in sediment transport capacity at the transition from supercritical flow to subcritical flow in the hydraulic jump. After initial onset of sedimentation and unit bar formation, generation of subsequent unit bars may be: (i) related to small changes in sediment flux; and (ii) independent of changes in the hydraulic jump. Continued sedimentation caused changes from oscillating to weak hydraulic jumps and hydraulic‐jump unit bars formed in both circumstances. The flow of water and suspended sediment becomes shallower over the lee of the bar complex. This leads to flow acceleration and a return to supercritical flow conditions. In turn, a chain of such features can form and generate a chute and pool bed morphology. There is an inherent upper size limit to a hydraulic‐jump bar complex due to the changing flow conditions over the growing deposit as the water above it becomes shallower. There is also an amplitude minimum for the development of foresets and subsequent unit bar growth. Hydraulic‐jump unit bars have architectures that should be recognizable in the rock record and because their size is constrained by the flow conditions, their identification should be useful for interpreting palaeoenvironment.     

The Table Rocks Sandstone: A fluvial, friction-dominated lobate mouth bar sandbody in the Westphalian B Coal Measures, NE England

Westphalian B (Duckmantian) alluvial Coal Measures along the Northumberland coast, NE England, comprise coal-capped coarsening-upward crevasse-splay sequences of shale, siltstone and sandstone, interbedded with a number of major distributary channel sandbodies, including the Table Rocks Sandstone. Lithofacies, architectural analysis and outcrop geometries divide the Table Rocks Sandstone into flaggy sandstone, massive sandstone, heterolithic, and mudstone facies associations, each comprising up to 7 lithofacies types. The three sandy facies associations are characterised by lenticular bed geometries on different scales producing a hierarchy of lensoid packages and associated bounding surfaces, all showing typical offset stacking patterns: (1) lenses, represent individual lenticular cross-bed sets, bounded by 1st order surfaces; (2) packages of lenses, called lens clusters are bounded by 2nd order surfaces, and are the basic architectural building block of the sandy facies associations; and (3) vertically stacked lens clusters called amalgamated lens clusters, bounded by 3rd order surfaces. The Table Rocks sandbody has a laterally extensive, irregular, lobate subsurface plan geometry, it displays a radial palaeocurrent pattern with 180° dispersion, and it forms part of a 14-m thick coarsening-upward regressive sequence. It is interpreted as a composite, lobate crevasse-splay delta system that prograded into a shallow interdistributary fresh to brackish water lake up to 14 m deep. The shallow lake water, fluvial input, and extensive development of traction structures such as cross-bedding and ripple cross-lamination suggests a friction-dominated delta, in which the four facies associations can be interpreted in terms of discrete elements of the mouth bar environment. The flaggy sandstone facies association represents the main, axial part of the mouth bar system, the erosively based massive sandstone facies association major subaqeous distributary channels, the lithologically more variable heterolithic facies association the medial mouth bar, and the mudstone facies association the distal mouth bar fringe and prodelta. Within this environmental setting amalgamated lens clusters are interpreted as small, discrete mouth bar sand lobes, whose offset, imbricate stacking pattern reflects channel spacing and bifurcation, the rate of channel shifting, or shallow depths and lack of accommodation space. Thus, lens clusters are interpreted as discrete growth elements of the mouth bar sand lobes, and lenses as individual bedforms making up these growth elements. Because of the high rate of channel shifting, lack of extensive erosion of the mouth bar lobes, and deposition of low discharge fines, the lobes retained much of their original depositional geometry, thereby providing advantageous gradients for offset deposition and stacking of adjacent sand lobes. Although the delta complex was maintained by frequent crevassing from the feeder channel, and by subsidence due to contemporaneous compaction and/or local tectonism, it was deeply incised on two occasions by subaqeous channels in response to high magnitude floods or falling lake level.     

Multi‐scale classification of fluvial architecture: An example from the Palaeocene–Eocene Bighorn Basin,Wyoming

Fluvial channel geometry classification schemes are commonly restricted in relation to the scale at which the study took place, often due to outcrop limitations or the need to conduct small‐scale detailed studies. A number of classification schemes are present in the literature; however, there is often limited consistency between them, making application difficult. The aim of this study is to address this key problem by describing channel body geometries across a depositional basin to ensure that a wide range of architectures are documented. This was achieved by studying 28 locations over 4000 m of vertical succession in Palaeocene‐aged and Early Eocene‐aged deposits within the Bighorn Basin, Wyoming, USA. Five different channel body geometries have been defined based on the external geometric form, and internal arrangement and nature of storey contacts. These include the massive channel body geometry, semi‐amalgamated channel body geometry, internally amalgamated channel body geometry and offset stacked channel body geometry, which are considered to be subdivisions of the sheet geometry of many other classifications. An isolated channel body geometry has also been recognized alongside splay channel and sheet sandstone geometries in the floodplain facies associations. Field evidence, including the stacking style of storey surfaces, suggests that the different geometries form a continuum. The nature and degree of amalgamation at the storey scale are important in producing the different geometries and are related to the degree of channel migration. It is speculated that this is the result of differences in sediment supply and available accommodation. In contrast to previous schemes, the classification scheme presented here recognizes the importance of transitional geometries. This geometrical range has been recognized because of the basin‐scale nature of the study.     

Temporal and environmental significance of microbial lamination: Insights from Recent fluvial stromatolites in the River Piedra,Spain

Despite extensive research, the environmental and temporal significance of microbial lamination is still ambiguous because of the complexity of the parameters that control its development. A 13 year monitored record of modern fast‐accreting calcite stromatolites (mean 14 mm year^?1) from artificial substrates installed in rapid flow in the River Piedra (north‐east Spain) allows comparison of the sedimentological attributes of successive six‐month depositional packages with the known climatic, hydrophysical and hydrochemical parameters of the depositional system. The stromatolites are formed of dense, porous and macrocrystalline composite laminae. The dense and porous composite laminae, which are composed of two to eight laminae consisting largely of calcified cyanobacteria, are characterized by: (i) dense composite laminae, up to 15 mm thick, mostly with successive dense laminae and minor alternating dense and porous laminae; and (ii) porous composite laminae, up to 12 mm thick, consisting mainly of porous laminae alternating with thinner dense laminae. Most of the dense composite laminae formed during the warm periods (April to September), whereas most of the porous composite laminae developed in the cool periods (October to March). Each dense and porous composite lamina represents up to or slightly longer than six months. The alternation of these two types of composite laminae parallels seasonal changes in temperature. The dense and porous laminae result from shorter (for example, intraseasonal) variations in temperature, insolation and hydrological conditions. The macrocrystalline laminae, with crystals >100 μm long, occur isolated and grouped into composite laminae up to 1·7 mm thick. Their occurrence suggests the absence or poor development of microbial mats over periods of weeks to several months. Thus, stromatolite lamination can record different‐order, periodic and non‐periodic changes in the magnitude of environmental parameters over a single year. These results hold important implications for the temporal and environmental interpretation of lamination in microbial structures.     

黄河中下游焦作区段现代边滩沉积中的生物遗迹*

沉积物中的生物遗迹记录是生物与环境相互作用的结果,是沉积环境、气候及生态环境的良好指示标志。文中主要研究了黄河中下游焦作区段现代边滩沉积物中的生物遗迹及其造迹者的组成与分布特点,即：（1）近岸边滩：几乎无植被,以层面上的生物觅食迹为主,其造迹生物主要有鸟类、长泥甲虫（鞘翅目长泥甲科）和隐翅虫（鞘翅目隐翅虫科）等;（2）远岸边滩：有大量植物根出现,大多数生物遗迹是层内U形、Y形和W形潜穴以及层面上的F形、Y形爬行迹和觅食迹,其主要造迹生物为蝼蛄（直翅目蝼蛄科）、蟋蟀、狼蛛和蠕虫类等;（3）泛滥平原：出现大量植物根,以层内J形、漏斗形等的生物居住迹和进食迹为主,主要造迹生物为蝼蛄、蟋蟀、田鼠、蚁狮和狼蛛等。基于沉积物粒度分析、总有机碳测定、菌落平板计数法等手段分析得知,研究区现代生物遗迹的深度随沉积物中微生物群落数量的增加及植物根长度的增长而增加,而丰度和分异度随沉积物粒度的递减及总有机碳含量的增加而增大。该成果一方面能为研究区黄河边滩上生物成因的沉积构造提供新资料,另一方面也能为地史时期河流沉积环境的研究乃至古地理、古生态环境重建提供生物遗迹学方面的科学依据。     

人类活动影响下的干旱区河流地貌演变——以塔里木河为例

全球范围内干旱区河流正日益受到高强度人类活动的扰动,但较少研究报道这种扰动对河流地貌过程的影响。采用历史文献、水文数据和遥感影像相结合的方式,详细分析了人类活动影响下中国最大内陆河塔里木河（简称塔河）的河流地貌变化过程。结果表明:塔河流域人类活动的规模和强度日趋上升,对河流水沙过程和地貌形态等影响显著。近50年来,塔河干流低流量过程发生频率呈显著上升趋势,而中、高流量过程则呈降低趋势,河道径流和输沙量减少显著。塔河干流上游现为游荡河道,冲淤变化剧烈且总体处于淤积抬升状态, 但河道平均河宽呈减小趋势,可能是因为塔河两岸冲积平原的开垦和河岸加固。塔河中游弯曲河道蜿蜒系数在近几十年呈缓慢上升趋势,但明显低于废弃古河道。     

Integration of outcrop and borehole image logs for high-resolution facies interpretation: example from a fluvial fan in the Ebro Basin, Spain

Electrical borehole image logs yield high-resolution information about variations in micro-resistivity along the borehole wall. To interpret these variations in terms of sedimentary structures and lithofacies types, calibration with real rock is needed. Normally, the only real rock available is core, and this only provides one-dimensional information. In this paper, the interpretation of fluvial facies types from borehole image logs was established by direct comparison with outcrops. Four fluvial facies associations were established in an outcrop study of a low net-to-gross fluvial succession: (i) meandering rivers, (ii) braided rivers, (iii) crevasse deltas, and (iv) crevasse splays. The lithofacies characteristics and palaeocurrent distributions of each fluvial facies association were established. Two 200 m deep wells were drilled behind the cliff face outcrops. One well was cored to a depth of 150 m and borehole image logs were recorded in both wells. The wells were correlated with the outcrop. The borehole image logs were analysed by their vertical colour succession and the dipmeter pattern. Image log facies were established, and these were interpreted in terms of the fluvial facies associations encountered in the corresponding outcrops. The study of borehole image logs yields a set of diagnostic criteria for a detailed fluvial facies interpretation and the establishment of depositional trends, and thus provides a powerful tool for the direct interpretation of fluvial facies in a reservoir setting.     

Fluvial Geochemistry through a Short-Duration, Tropical-Cyclone Induced Discharge event in the Burdekin River and Hann Creek, North Queensland, Australia

The chemical composition of river water integrates a number of factors such as weathering, land use, climate, vegetation cover and human activity that individually affect its chemistry. Short term variations may also be significant. The Burdekin River, NE Australia, is an example of a class of tropical streams which experiences two to four orders of magnitude variation in discharge in response to seasonal but erratic monsoonal and cyclonic rainfall. In these systems individual discharge events last for days to weeks. Given the inherent difficulty sampling these events published data on water chemistry (and thus calculated fluxes and global budgets) may tend to be biased to low flow conditions. One such discharge event in February 1996 has been investigated for its impact on the chemistry of the water. Major cations (Na, Mg, K, Ca) all decreased in concentration as the water level rose, as did the minor elements Sr, Ba and U. Some other trace elements, notably Rb, Cr, Pb and REE were enriched in the peak flow waters. The flux of all measured elements increased substantially during the seven days of the discharge event. Such short term but significant events will have a major impact on the annual fluxes of elements delivered to the oceans from the land and global discharge budgets may need to take them into account when refining databases in the future.     

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.     

Facies and palaeosol analysis in a progradational distributive fluvial system from the Campanian–Maastrichtian Bauru Group,Brazil

The Upper Cretaceous Bauru Group in south‐east Brazil consists of alluvial strata whose characteristics and distribution indicate a fluvial system developed in a semi‐arid to arid climate. Sections exposed within a 90 000 km² study area in Minas Gerais State (in south‐eastern Brazil) were evaluated using facies and palaeosol analysis to formulate depositional and pedogenic models that may account for geomorphic and climate features. From east to west, the study succession records a gradual decrease in grain size, an increase in the width/thickness ratio in channels, a decrease in the lateral and vertical connectivity of channel deposits, and an increase in overbank deposits. The fluvial architecture indicates a braided channel belt, ephemeral ribbon–channels, and an unconfined fluvial facies from east to west in the study area. The lateral and vertical distribution of facies, stratigraphic architecture and palaeocurrent data suggest proximal, medial and distal portions of a progradational distributive fluvial system. The sedimentary dynamics were marked by the building and abandonment of channels related to processes of aggradation, vegetation growth and palaeosol generation. Macromorphological and micromorphological analyses have identified pedological and mineralogical features that indicate an arid to semi‐arid climate with a provenance from the north‐eastern part of the basin (Alto Paranaiba Uplift). From the proximal to the distal portions of the distributive fluvial system, the palaeosol development is different. In the proximal portion, the palaeosols are absent or poorly developed, allowing a possible general comparison with the present soil order: Inceptisols and Aridisols. In the medial portion of the fluvial system, the palaeosols are well‐developed and characterized by Bt, Btk, C and Ck horizons (Alfisols, Aridisols, Inceptisols and Entisols). Poorly drained to well‐drained palaeosols from the base to the top in the distal plain (Aridisols and Inceptisols) are associated with geomorphic and hydromorphic changes in the fluvial system due to progradational evolution. The genetic relationship between the fluvial architecture and the palaeosols enhances the understanding that the sedimentation and pedogenesis that occurs in different portions of the distributive fluvial system are related to the tectonic and climatic evolution of the basin.