AUTOINCISION AND AUTOGENIC STREAM TERRACES IN FLUVIO-DELTAIC SYSTEMS WITH THE STEADY FALL OF RELATIVE SEA LEVEL: IMPLICATIONS FROM FLUME EXPERIMENTS

A series of two- and three-dimensional flume experiments in which miniature deltas were built with relative sea-level fall were conducted. During each of the runs, any of relative sea-level fall rate (A), sediment discharge (S), and upstream water discharge (Q) were kept constant with time. The experiments have substantiated an inbuilt geomorphic process that inevitably forms discrete stream terraces on the subaerial delta slope.

In the early stage of relative sea-level fall, the delta can sustain its original geometry and grow basinward without downcutting. Within a finite time interval after the beginning of sea-level fall, however, the delta plain inevitably becomes affected by valley incision. This valley incision, occurring with the steady forcing of the basin, is here referred to as autoincision. After the autoincision event has been attained, stream terraces are formed one after another without any change in A, S, Q. Nondimensionalized height and length, occurrence frequency, and shape of the terraces are dependent on the inclination of the flume slope and the Q/S ratio.

The terrace topography is caused by the autocyclic migration of active stream channels that, because of the increasing surface area of the feeder system, inevitably fail to erode out the entire fluvial deposit of older cycles. The autogenic formation of stream terraces with steady sea-level fall arises from the principle of autoretreat combined with autocyclic lateral shifting of the feeder system, and are analogous with autostepping of delta-front lobes with steady sea-level rise. Any discrete changes of A, S, A/S or Q/S are not required to explain the origin of autoincision and autogenic stream terraces in an overall sea-level fall regime. Allocyclic changes in A and S, as commonly invoked in the sequence stratigraphy literature would only enhance or reduce the autogenic terracing processes in fluvio-deltaic systems of falling sea-level stages.