An experimental examination of the theory of shoreline autoretreat

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The theory of shoreline autoretreat predicts that even if the rate of change of relative sea level [A] is constant (>0) and if any other basin dynamic conditions including the rate of sediment supply [S] (>0) and remain constant, the shoreline inevitably begins to retreat landward at a relatively early stage of delta growth and finally reaches the break point where the clear angular configuration of the delta front is lost suddenly. This autoretreat theory was examined with flume experiments in which a miniature of delta is built under a controlled steady state (A=const>0, S=const>0). The experimental results and related notions are summarized as below.l. The autoretreat of shoreline has been substantiated. Just as predicted from the theory, the deltas experienced the landward turnaround of a shoreline and subsequently the occurrence of a geometrical break event, without any change of basin conditions.2. A semiquantitative method to detect temporal changes in ancient basin dynamic conditions has been found from the discussion based on the theory of shoreline autoretreat. The applicability of this method was illustrated with the Paleogene regressive succession in the Central Tertiary Trough, west Spitsbergen.3. A series of the three-dimensional experiments has given negative evaluation to the conventional notion that any significant change in the entire sediment-stacking pattern of a sedimentary system is principally due to a significant temporal change of an external or allocyclic factor rather than to inbuilt or autocyclic mechanisms within the depositional system itself. In the transgressive phase, the delta plain retreats in a step-like manner, and the abandoned deltaplains, being submerged with water-level rise, are preserved as multiple stepped terraces. This particular behavior of a delta is related to the autoretreat effect combined with the autocyclic lateral-shifting processes of sediment-feeding streams.