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The AAPG/Datapages Combined Publications Database

CSPG Special Publications


Fluvial Sedimentology — Memoir 5, 1977
Pages 849-849
Symposium Abstracts

Keynote Address: Achievements and Prospects in Fluvial Sedimentology: Abstract

J. R. L. Allen1

Our present knowledge and understanding of the sedimentology of fluviatile deposits has been obtained through the combined efforts of geologists, geomorphologists, and hydraulic engineers. Useful future advances will depend on the continued close collaboration between these workers.

Channel pattern and process. Channel pattern and interconnectedness vary widely in contemporary streams but can be regarded as functions of stream power, bank erodibility, and time. The pattern of a river reach may change with time, and different reaches on the same river may differ strongly in pattern at a given time. If a satisfactory basis for palaeohydraulic interpretation is to be obtained, the attack on the question of the dependence of channel geometry on hydraulic and drainage-basin controls must be renewed in a wider and more vigorous manner than hitherto. There is a fair understanding of meandering stream processes, obtained through a combination of theoretical, experimental and field studies of contemporary rivers, but an inadequate knowledge of the processes operating in streams of low sinuosity, where secondary flow may not be important. Our inadequate knowledge of the time scales appropriate to river processes, particularly of channel-shifting and bar-building, is holding back understanding of the full implications of channel facies encountered in the older geological record. Knowledge of the processes affecting the overbank is particularly inadequate and retards appreciation of the corresponding facies.

Bed forms. In the analysis of fluvial channel facies, bed forms and their dependent internal structures afford potential information about transport directions, hydraulic conditions and regime, and channel pattern and connectedness. Extensive data have been obtained experimentally on the characteristics and hydraulic limits of the common one-way bed forms in sand-sized sediments. There is little corresponding data on bed forms in gravel, and our knowledge of bed forms in silts also cannot be described as satisfactory. Efforts are, however, being made to remedy these defects. These experiments, almost without exception, were carried out under conditions of steady-state equilibrium (flow quasi-uniform and quasi-steady). Flows in real rivers are non-uniform and unsteady, and field studies show that many bed forms display lag effects in relation to the patterns of hydraulic conditions. Are bed forms which differ geometrically necessarily different hydrodynamically, or can operational differences be explained by lag effects? How might lag affect our judgement of regime from the deposits which chance to be preserved? What are the bed forms of gravel-bed streams?

Facies and facies models. A combination of studies of contemporary fluvial deposits with the older geological record has led to the presentation of general facies models for the deposits of meandering and a variety of low-sinuosity streams. In refining and applying these models in the future, their limitations should not be overlooked. Firstly, the models are of local relevance, for what may be true of a stream or fluvial sand body at one restricted site may not necessarily be true either overall or even at sites close by. A lateral change of facies may be expected in point bars, for example, because of the influence of local channel curvature on local processes. Secondly, these models emphasize the facies originating in channels. Just as overbank processes are ignored in comparison with channel processes, so the relatively fine-grained sediments of the overbank are neglected at the expense of the more immediately attractive channel sands and gravels. Overbank facies are the more important volumetrically in many alluvial suites and are the “matrix” in which the channel sand-bodies lie. A better understanding of overbank facies, particularly those dependent on pedogenic and other superficial processes, would greatly assist our understanding of how alluvial suites are put together by the flows in and emanating from channels, under the influence of subsidence, base-level change, and climatic variation. A future emphasis on this aspect of fluvial deposits would go far towards bridging the present gap between the traditional stratigrapher’s treatment of alluvial suites on the macroscale and the sedimentologist’s perhaps undue concern to date with the microscale aspects of channel facies.

Acknowledgments and Associated Footnotes

1 Sedimentology Research Laboratory, University of Reading, Whiteknights, Reading, England, RG6 2AB

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