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Deposition from steady currents
We have explained earlier (Chapter 1) that unsteady (e.g. waning flow) conditions are not prerequisite for deposition as has sometimes been assumed, but that deposition (e.g. of massive ignimbrite) can occur during waning, quasi-steady and even waxing flow conditions. In order to explore the processes of deposition it is useful first to consider the simplest case, that of deposition during steady conditions, before considering possible consequences of waxing and waning flow.
Deposition during steady flow proceeds at a constant rate, so that the surface of the deposit rises steadily with time (Fig. 4.1 A). We call this type of aggradation 'gradual' to distinguish it from 'stepwise' aggradation, which proceeds in a series of abrupt jumps (Branney & Kokelaar 1992). Note that this use of gradual does not imply a slow rate. Gradual aggradation was first invoked to describe the formation of massive sediment-gravity flow deposits by Carter (1975): 'a surface separating stationary from moving particles moves rapidly up ... deposition is therefore not simultaneous throughout the bed. If the depositional surface migrates up gradually, a comparatively homogenous bed might be expected'.
Any clast depositing from a pyroclastic density current must cross the lower flow boundary of the current. All flow-boundary zones must exhibit gradients in velocity and concentration (see p. 4), so any pyroclast undergoing deposition must descend through successive levels of different shear intensity and concentration as
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