Stratigraphy of Counter-Point-Bar and Eddy-Accretion Deposits in Low-Energy Meander Belts of the Peace-Athabasca Delta, Northeast Alberta, Canada
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Over the last couple of decades, fluvial geomorphology and fluvial sedimentary geology have been developing in parallel, rather than in conjunction as might be desired. This volume is the result of the editors' attempt to bridge this gap in order to understand better how sediments in modern rivers become preserved in the rock record, and to improve interpretation from that record of the history of past environmental conditions. The catalyst for the volume was a conference with the same that was hosted at the University of Aberdeen School of Geosciences, in Aberdeen, Scotland, on 12-14 January 2009. The conferences brought together a broad spectrum of geomorphology and sedimentology researchers, from academia and industry. This interdisciplinary mix of experts considered and discussed ideas and examples ranging through timescales from the annual movement of individual river bars to sequence stratigraphic analysis of major sedimentary basins spanning millions of years. The articles in this volume are a mixture of novel concepts, new evaluations of the perceived wisdom about rivers and their sediments, and improved understanding derived from recent experience in interpreting the rock record. This volume usefully illustrates the current state of knowledge and will provide a stimulus for further research, particularly work that integrates geomorphological and sedimentological approaches and emphasizes crossdisciplinary communication.Keywords:
Point bar
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Geologic record
Reservoir architecture is an important basis for studying the sedimentary evolution of underground reservoirs and the distribution of remaining oil. Taking meander-river reservoir of Guantao Formation in Chengdao Oilfield as an example, characteristics of the evolution of subsurface meander-belt deposits were studied by seismic sedimentology. Seismic lithology is performed using -90° phase wavelet and frequency fusions; seismic geomorphology is conducted on seismic stratal slices. Six meandering belts were defined in the target layer, and the sediment at the bottom is characterized by the transformation from “braided” river to “meandering” river. The width of meander-river belt has no correlation with the maximum thickness of meander-river belt. But it has obvious negative correlation differences in the minimum thickness of meander–river belt. With the increase of width of the meander-river belt, the minimum thickness of the drilled meander-river belt decreases as a whole. The forward model based on the shape of point bar reveals the seismic architecture characteristics formed by the superposition of sand bodies at different positions of point bars. According to these characteristics, the point bars in the meandering zone can be explained. In the Ng33, Ng42, and Ng45 sub-layers of the study area, the corresponding meander-belt widths where the point bar (or point bar connecting body) cannot be effectively identified by well logging and seismic data, are, respectively, 800, 800, and 500 m. At the same time, when the width of the meander belt is greater than 1000 m, three or more than three point bars (or point bar connecting body) can be basically identified in the meander belt. The width of the point bar segments in the meander belt of the three main sub-layers is mostly 300–400 m, and the maximum width of the point bar segments is about 550 m, while the scale of point bars in Ng45 sub-layer meander belt is relatively small. The quantitative characteristics of point bar debris and the determined location of SBM and SIA sand bodies provide limited and valuable data support for the formation and simulation of underground meandering rivers, which are also useful for the model of the target reservoir.
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Meandering fluvial reaches exhibit a wide range of morphology, yet published interpretations of ancient meander-belt deposits do not reflect the stratigraphic complexity known to be associated with such variability. An improved understanding of processes that generate stratigraphic heterogeneity is important to improve predictions in sedimentary facies distributions in sub-surface settings. Quantification and classification of planform geomorphology of active fluvial point bars and their recently accreted deposits enables determination of spatio-temporal relationships between scroll-bar pattern and resultant meander shape. Scroll-bar deposits describe an overall pattern of lateral accretion that records how a meander has grown incrementally over time. Analysis of 260 active meander bends, from 13 different rivers, classified by a range of parameters including climatic regime, gradient and discharge, has been undertaken. Assessment of scroll-bar morphology and growth trajectory has been undertaken using remotely sensed imagery in Google Earth Pro. Twenty-two distinct styles of meander scroll-bar pattern are recognised within active meander bends. These are grouped into 8 types that reflect growth via combinations of expansion, extension, rotation and translation. A novel technique for predicting the variable distribution of heterogeneity in fluvial point-bar elements integrates meander-shape and meander scroll-bar pattern. The basis for predicting relative lithological heterogeneity is the observation that deposited sediments fine downstream around a meander bend and outwards as a barform grows and tightens due to bend expansion. Observations of these trends are seen in experimental models, modern fluvial systems and in the ancient record at both outcrop and in the sub-surface. These trends permit planform geometries to be compared with distributions of bar-deposit lithology types. The method is applied to predict heterogeneity distribution in both sub-surface and outcrop settings. Seismic-reflection data that image point-bar and related elements of the McMurray Formation (Cretaceous, Alberta, Canada) are used to test the predictive capability of the method by comparing predicted heterogeneity to trends known from analysis of gamma-ray data available from densely distributed well-log records. Outcrop data from a point-bar deposit in the Montanyana Group (Ypresian, southern central Pyrenees, Spain) are used to test the method by comparing heterogeneity predictions with lithologies observed seen in the outcrop. This novel method constrains heterogeneity predictions in fluvial point-bar deposits for which direct lithological observations are not possible or are limited. The method therefore provides the basis of a predictive tool for improving understanding of a fragmentary geological record, including prediction of lithological heterogeneity from outcrops of limited spatial extent, or from subsurface seismic datasets.
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Many of the world's major river systems seemingly have one or a few disproportionally large meanders, with tight bends, in the fluvial-tidal transition (e.g., the Thames in the UK, and the Salmon River in Canada). However, quantitative studies on meanders have so far primarily focused on rivers without tidal influence or on small tidal meanders without river inflow, providing relations between channel geometry and meander characteristics (length, amplitude, and sinuosity). Physics-based predictions of meander size and shape for the fluvial-tidal transition zone remain untested for a lack of data. Therefore, it remains unclear whether the dimensions of meanders in the fluvial-tidal transition zone are indeed disproportionally large, and whether meander characteristics can be used as an indicator for tidal influence. Here, data from 823 meanders in 68 fluvial-tidal transition zones worldwide are presented that reveal broad-brush relations between channel geometry and meander dimensions. Our results show that fluvial-tidal meanders indeed become larger in the seaward direction, but the dimensions are proportional to local channel width, as in rivers. Sinuosity maxima are an exception, rather than the rule, in the fluvial-tidal transition zone. Surprisingly, the width of the upstream river correlates with estuarine channel width and tidal meander size even though river discharge constitutes only a fraction of the tidal prism. The new scaling relations can be used to constrain dimensions of rivers and estuaries and their meanders.
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Abstract Fluvial point‐bar evolution commonly involves multiple stages of bar development driven by changes in the style of meander transformations. Complicated planform morphologies are widely recognized in remote‐sensing imagery, but the relationships between meander‐bend evolutionary behaviour and stratigraphic architecture, facies distribution, and sand volumes remain poorly understood. This study applies a geometric forward stratigraphic model (Point‐Bar Sedimentary Architecture Numerical Deduction – ‘PB‐SAND’) to simulate the internal sedimentary architecture of 24 meander‐belt segments that evolved via a broad range of meander‐bend transformation styles. Modelling inputs are constrained by channel trajectories inferred from high‐resolution Light Detection and Ranging (LiDAR) datasets, lithological information from a sedimentological database (Fluvial Architecture Knowledge Transfer System – ‘FAKTS’) and geological knowledge of trends in point‐bar lithology (for example, decrease in sand proportion with sinuosity, downstream of bend apices, and beyond the transition from point‐bar to counter‐point‐bar deposits) and in channel bathymetry (depth variations across pools and riffles). Modelling results are used to explore how the relative distribution of sand and mud is controlled by the styles of point‐bar transformation, quantified by the relative degree of meander translation versus expansion, and by the amount of bend rotation. The 24 models are classified into three groups based on cluster analysis of their mean migration angle, mean apex rotation, mean sinuosity, standard deviation of channel circular variance and preservation ratio; these quantities are known to be controlled by meander transformation types. Quantitative comparisons across these groups and relationships between metrics of planform change and quantifications of point‐bar deposits demonstrate how meander planform evolution controls point‐bar thickness and sand volume. Locally, the thickness of sand in bar deposits is controlled by the interplay of facies trends and spatial variations in bar thickness that reflect bathymetric changes, both related to local hydrodynamics. The proposed workflow establishes linkages between planform morphologies and three‐dimensional facies distributions; it can be employed to characterize the distribution of subsurface porous volumes where the planform history of meander bends can be reconstructed.
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Geomorphologic features and sediment distribution from meander-belts informs our understanding of ancient deposits, with specific application to predicting heterogeneity in petroliferous strata. The point bar to counter-point bar transition has been of recent interest and its common occurrence in modern fluvial environments suggests that they are often over-looked and under-recognised in ancient datasets. In this study, six point bar to counter-point bar transitions are examined along meander bends from rivers with varying channel scale, discharge and tidal influence. The data indicate that observed trends of grain-size fining from point bar to counter-point bar are consistent regardless of channel scale, discharge and tidal influence. High net sand to gross thickness (> 0.7) point bars and low net-to-gross counter-point bars (< 0.3) are documented. The average decrease in net-to-gross across the transition is 57%; the transition length scales to channel size and is approximately three times channel width. Tidally-influenced counter-point bar deposits are recognised despite the absence of concave scroll patterns in tidal flat areas. The lack of scroll bar topography contributes to the challenges in identifying counter-point bar deposits in tidally-influenced settings. Recognising and predicting heterogeneity related to the point bar to counter-point bar transition in ancient fluvial and tidal-fluvial deposits is considered, with specific implications for steam chamber growth during development of the Athabasca Oil Sands, Alberta, Canada.
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Progress in Chinese fluvial sedimentology, beginning with the initial issue of ACTA SEDIMENTOLOGICA SINICA in 1983, have been reviewed respectively from the aspects of classification of stream, modern present deposits investigation, fluvial facies and depositional models, fluvial deposits sequence stratigraphy, fluvial architecture element analysis and fluvial sedimentological simulation. The great achievements, which have been made in this duration by Chinese fluvial sedimentologists, including the application of the new concepts of anastomosing stream, architecture element analysis, lithofacies and facies assemblages, sedimentary structure research, outcrop investigation, physical and digital simulation of fluvial sedimentology and fluvial sequence stratigraphy, have been concluded in this paper. However, there are no our own new theory which has been known worldwide.To make Chinese fluvial sedimentology front rank in the world fluvial sedimentology, the organization of such research should be reinforced, achievements in geography and hydrology should be absorbed, the domain of the research should be expanded with the demanding of the gradual development of the social economy , international exchange activities should be encouraged positively.
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