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Autocyclic behaviour of alluvial and deltaic systems
Fan deltas are excellent recorders of fan-building processes because of their high sedimentation rate, particularly in tectonically active settings. Although previous research has focused mainly on allogenic controls, there is clear evidence for autogenically produced storage and release of sediment by flume and numerical modelling that demands further definition of characteristics and significance of autogenically forced facies and stratigraphy. Analogue experiments were performed on fan deltas with constant extrinsic variables (discharge, sediment supply, sea-level and basin relief) to demonstrate that fan-delta evolution consists of prominent cyclic alternations of channelised flow and sheet flow. The channellised flow is initiated by slope-induced scouring and subsequent headward erosion to form a channel that connected with the valley, while the removed sediment is deposited in a rapidly prograding delta lobe. The resulting decrease in channel gradient causes a reduction in flow strength, mouth-bar formation, flow bifurcation and progressive backfilling of the channel. In the final stage of channel filling, sheet flow coexists for a while with channellised flow (semi-confined flow), although in cycle 1 this phase of semi-confined flow was absent. Subsequent autocyclic incisions are very similar in morphology and gradient. However, they erode deeper into the delta plain and, as a result, take more time to backfill. The duration of the semi-confined flow increases with each subsequent cycle. During the period of sheet flow, the delta plain aggrades up to the 'critical' gradient required for the initiation of autocyclic incision. This critical gradient is dependent on the sediment transport capacity, defined by the input conditions. These autogenic cycles of erosion and aggradation confirm earlier findings that storage and release of sediment and associated slope variation play an important role in fan-delta evolution. The erosional surfaces produced by the autocyclic incisions are well-preserved by the backfilling process in the deposits of the fan deltas. These erosional surfaces can easily be misinterpreted as climatically, sea-level or tectonically induced bounding surfaces.