In Defense of Shelf‐Edge Delta Development during Falling and Lowstand of Relative Sea Level

Abstract

There is a continued debate about the sequence stratigraphic interpretation of deep‐marine sand accumulations. The conventional notion states that it is preferably (but not only) falling stage and lowstand of relative sea level that favor the formation of the deep‐marine systems because, at these times, the fluvial and deltaic transport system tends to extend relatively easily across a shallow‐marine shelf surface and hence has potential for sand delivery into deeper‐marine environments. It has been argued recently, however, that sand delivery across the shelf to the shelf edge is equally likely during highstand, as gauged from the estimated time periods for modern natural rivers to build their deltas to the shelf edge, during the modest relative sea level rise of the past 7000 yr. This argument and counterargument are examined here quantitatively by using, as a basis, the concept of “shoreline autoretreat.” The autoretreat theory predicts that under conditions of constant sediment supply ($$S> 0$$) to the basin and a steady rate of rise ($$A> 0$$) in relative sea level, a prograding delta inevitably attains an “autobreak” state (time: tb) after which the delta's subaqueous slope begins to be starved of sediment. This happens because the finite sediment supply becomes inadequate to fill the greatly increased accommodation (sensu Muto and Steel 2000) on the delta plain created by the rising sea level and the extended delta front slope as the delta attempts to move into deeper water. It is impossible for a delta to reach a shelf edge if the autobreak state is attained before this position is reached. Using published data for modern river‐shelf systems, numerical simulations to estimate tb were carried out under different sets of assumed conditions. We conclude that during development of highstand systems tracts along many coastlines during the past 7000 yr, tb can take a considerably small value, which tends to prevent rivers from reaching their shelf edges, especially if the delta's subaqueous slope is of relatively high inclination (ca. ≥1°). Simulations for stable and falling stages of relative sea level, however, indicate that deltas can reach a shelf‐edge location much more easily and quickly than with even modest relative sea level rise. Thus, this study defends the notion of shelf‐edge delta development and potential sand delivery to deepwater areas preferentially during falling or lowstand stages of the relative sea level cycle.