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Structure of coarse grained braided stream alluvium*

Published online by Cambridge University Press:  06 July 2012

B. J. Bluck
B. J. Bluck
Affiliation:
Department of Geology, University of Glasgow
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Synopsis

Grain size characteristics of the sediment and the flow stage characteristics of the river are the two most important factors influencing sedimentation in the channel zone. Climate is important in its influence on floodplain (overbank) sediment.

Supra-bar platform (upper) parts of mature bars from a wide range of climatic conditions consistently show a sequence in the development of contained sedimentary structures which is related to the flow stage. At high stage, in rivers where the bed materials are quite mobile, the bar form is not thought to be present: the form develops on the falling stage, and the bar is dissected on the lowest stage. The various bar forms have a related diagnostic structure which determined from the nature and distribution of the stratification types, and the manner of their deposition on the falling flow stage. The exposed (supra-platform) parts of lateral bars have a side and longitudinally filled inner channel which comprises fine sediment. Medial bars have converging cross-strata in sands which are overlain partly by gravels in an upward coarsening sequence.

Lateral migration of the channel zone, brought about by the preferential bar accretions to one side, results in building of a lithosome, the facies structure of which is partly determined by the nature of the bar accretions. As the channel migrates the abandoned bars may be covered with fine sediments so as to build up an upward fining sequence. The nature of the fine overbank part of the braided stream cycle is as complex as the coarse lower, so that a highly variable vertical sequence of lithological types is likely to be constructed.

Upward fining cycles are also built by some fine braided streams but they are the product of a single flood rather than lateral channel migration. Braided stream deposits are distinguishable from those of small, coarse meandering stream deposits by the presence of inner accretionary banks in the latter, and inner channels in the former.

Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 70 , Issue 10-12 , 1979 , pp. 181 - 221
Copyright
Copyright © Royal Society of Edinburgh 1979

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