Studies in fluviatile sedimentation: an exploratory quantitative model for the architecture of avulsion-controlled alluvial suites |
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| Authors: | JRL Allen |
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| Institution: | Sedimentology Research Laboratory, Department of Geology, The University, Reading Great Britain |
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| Abstract: | A river crossing a coastal plain is assigned a zone of influence within which it may abruptly move and construct new channel sand-bodies following avulsion. Assuming uniform and steady subsidence at a fixed site, the alluvium formed beneath the zone is constructed over a series of equal time-steps, at the start of each of which an avulsion occurs, and during each of which an increment of sediment is accumulated beneath the plain. Each increment comprises an overbank sequence of a thickness proportional to subsidence rate and avulsion period, laterally equivalent to a generally weakly multi-storey sand body of a thickness proportional to channel depth, avulsion period, and subsidence rate. The choice of sand-body width is determined by stream size and mode of behaviour. The new position of the river after an avulsion is chosen using random-number tables combined with rules for the avoidance of older, relief-creating sand bodies. The model otherwise ignores sequence compaction, and so is valid only when operated for comparatively short periods.In accordance with a theoretical model of sand-body connectedness, in which uniform bodies are regularly packed, experimental sand bodies are virtually unconnected in those alluvial suites containing 50% or more of overbank fines. The degree of connectedness grows very rapidly as the proportion of sand bodies increases above 50%. The use of rules for avoidance does not prevent the experimental suites from being somewhat disordered internally, as shown by the wide variation in the composition of one-dimensional profiles through individual sequences. The results obtained from the model suggest that coarsening-upward alluvial suites of coastal origin may owe their character as much to progressively decreasing subsidence as to any independent decline in channel sinuosity. |
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