Towards the Field Classification of Alluvial Architecture or Sequence
- J. D. Collinson and
- J. Lewin
Published Online: 29 APR 2009
Copyright © 1983 The International Association of Sedimentologists
Modern and Ancient Fluvial Systems
How to Cite
Friend, P. F. (1983) Towards the Field Classification of Alluvial Architecture or Sequence, in Modern and Ancient Fluvial Systems (eds J. D. Collinson and J. Lewin), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444303773.ch28
- Published Online: 29 APR 2009
- Published Print: 7 FEB 1983
Print ISBN: 9780632009978
Online ISBN: 9781444303773
- towards field classification – alluvial architecture or sequence;
- river sedimentation;
- environmental classification;
- elongate depressions;
- ancient fluvial formation
Present models of river sedimentation are based largely on a classification of the geomorphology of present-day river channel patterns. The classes are difficult to apply with confidence to ancient sediments, even where good, three-dimensional exposures are available. Although many workers have often referred to the deposits of (a) meandering or (b) braided rivers, present knowledge suggests that these terms should only be used where there is direct evidence of (a) high sinuosity and/or lateral migration or (b) emergent bars (islands).
A broad transport-mode classification can be useful, based on grain-size and distinguishing ‘mainly bed-transport’, ‘mainly suspension’, and ‘bed-transport and suspension’. A descriptive classification of two- or three-dimensional exposures is proposed, which is based on the shape and interrelations of distinctive sediment bodies, e.g. sheet or ribbon body form, or, more genetically, channel-plugs or bar forms.
Environmental classification tends to concentrate on the degree and mobility of channelization during deposition. Useful classes are: (1) sheet flood, (2) fixed channel, (3) mobile channel. Terms such as meandering or braided may then be suffixed to any of these classes, and some estimation of indices of sinuosity or braiding may locally be possible.
All too often, knowledge of ancient alluvium is limited to more-or-less one-dimensional, ‘sequence’ information (‘vertical’ exposure logs, borehole logs). Without two-dimensional data any classification must be limited to transport-mode generalizations, although detailed studies of vertical patterns of palaeocurrent direction, structure and grain size may suggest more precise environmental models.