Features of cross-stratified units due to random and other changes in bed forms |
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| Authors: | J R L ALLEN |
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| Institution: | Sedimentology Research Laboratory, University of Reading, Reading, Berks. |
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| Abstract: | Because cross-stratified units depend upon the movement of bed forms, any change in the shape, size and direction of travel of the forms is reflected in the geometry of the units, notably in their relative length, breadth and thickness, mode of termination upstream and downstream, and internal discontinuities. Most models of cross-stratification so far published are unsatisfactory because they ignore the fact that real bed forms are subject to change. The changes are thought to occur at two levels of detail independently. Those at the coarser level depend on the essential non-uniformity, unsteadiness and multi-directionality of natural flows, when assessed on a suitably large scale. At the finer level, change is related to the random behaviour of individual bed forms as they interact with the adjacent flow, and it proceeds even when the flow is an equilibrium one overall. Flume experiments on current ripples show that many features of cross-stratified units can be explained by the random behaviour of bed forms. The finite streamwise length of such units, and their upstream and downstream erosional termination, is governed by the life-span (finite) of individual ripples and by the extent of net deposition on the bed. Internal discontinuities, closely resembling features described as reactivation structures, were also found to depend on the relative motion of ripples, no change of flow discharge and stage being involved. The degree of relative motion in the ripple assemblages was substantial, as measured by the fluctuating component of the ripple celerity. |
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