The rate of change of accommodation (at the shoreline) [ A ] and the rate of sediment supply [ S ] are the primary factors controlling regression and transgression in the geological record, or retreat and advance of a coastal depositional system. The generally accepted notion, here referred to as the A/S ratio concept, stating that the migration of a shoreline is controlled principally by the magnitude of the ratio of A to S is insufficient and somewhat misleading, partly owing to the problem of dimensional confusion. The interplay of A and S inevitably results in autoretreat of the shoreline, whereby the seaward advance of any shoreline is halted and is subsequently turned to landward retreat, provided there is a continuous rise of relative sea level. How effective the autoretreat process is depends, for a given period of relative sea level rising, upon the length of potential time period for the seaward advance of shoreline, and is proportional to A 2 / S and A (super 1.5) / S (super 0.5) in two- and three-dimensional sediment dispersal modes, respectively. Autoretreat becomes more effective as A increases and/or S decreases: A functions more critically to the effectiveness than S does. The A/S ratio concept is only approximately applicable when autoretreat is less effective (i.e., with very low A and/or very high S ). In this sense, the autoretreat theory and the A/S ratio concept are complementary to each other. The autoretreat mechanism is expected to be substantially useful not only in fluvial deltas but also in many other depositional systems (e.g., barrier islands) whose evolution has been considered in terms of the A/S ratio concept.

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