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Principles
of Shoreline Migration: the Interplay between
Sediment Supply and Accommodation Space
MUTO, TETSUJI, and RON J. STEEL
A sequence-stratigraphic notion, here referred to as the A/S ratio concept,
stating that shoreline migration is controlled principally by the magnitude of
the ratio of the rate of accommodation space change (at the shoreline) [A] to
the rate of sediment supply [S], is insufficient and somewhat misleading.
The interplay of sediment supply and increasing accommodation inevitably
results in autoretreat of the shoreline, whereby the seaward advance of any
shoreline is halted and turned to a landward retreat, provided there is a
continuous rise of relative sea-level. It is erroneous to discuss the migration
of shorelines simply in terms of equilibrium disequilibrium between
accommodation and sediment supply.
Effectiveness of the autoretreat process depends, for a given period of relative sea-level rising, upon the time period for the seaward advance of the shoreline, and is proportional to S/A{2} ~ S{0.5}/A{1.5}. Autoretreat becomes more effective as accommodation rate increases and/or sediment supply rate decreases. Accommodation rate functions more critically to the effectiveness than sediment supply rate does. The A/S ratio concept becomes more applicable when autoretreat is least effective (i.e., with low accommodation rate and/or very high sediment supply rate). In this sense, the autoretreat theory and the A/S ratio concept are complementary to each other.