Tetsuji Muto¹, Ron J. Steel²

(1) University of Wyoming, Laramie, WY
(2) University of Wyoming

ABSTRACT: The 'Autostepping' of A Transgressive Delta: An Implication from Flume Experiments

Under conditions of constant sediment supply [S] and constant rise in relative sea level [A], the theory of shoreline autoretreat predicts that (1) at a relatively early stage of delta growth the seaward advance of deltaic shoreline is inevitably halted and replaced by a landward turnaround ('autoretreat'), and subsequently (2) the entire delta system inevitably undergoes a significant geometrical change (from the point of 'autobreak') in the retreat phase. The concept is corroborated with flume experiments in which a miniature delta is constructed under constant A and S. The experiments provide further insight as to how transgressive deltas evolve during the post-autobreak phase. The transgressive delta shifts landwards and leaves discrete steps on the delta's subaqueous slope, despite constant A and S. The stepped topography, representing abandoned delta fronts, and its process of formation are referred to as 'autostep(s)' and 'autostepping', respectively. The autostepping arises from the autoretreat principle combined with autocyclic lateral migration of feeder channels and active lobes. Some steps are generated even before the autobreak state is attained, but disappear when an active delta lobe covers them completely. During the post-autobreak phase, the topographic steps remain unburied because the increased surface area of the delta does not allow a new lobe to reach the distal end of a previous delta front. One of the main factors controlling the occurrence and dimensions of the autosteps is the inclination of basin slope; the steps develop better with the lower inclination. Possible examples of autostepping deltas will be discussed.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado