Autostepping during the transgressive growth of deltas; results from flume experiments

A series of three-dimensional flume experiments on the transgressive growth of deltas, conducted with constant rate of relative sea-level rise (A), constant sediment discharge (S), and constant upstream water discharge (Q), reveals an inbuilt geomorphic process that inevitably forms discrete steps on the abandoned subaqueous deltaic slope. The stepped topography and its process of formation are here referred to as autostep(s) and autostepping, respectively. The stepped topography is caused by the lateral migration of delta- front lobes that inevitably shift landward but fail to cover lobes of older cycles. The autostepping arises from the principle of autoretreat combined with autocyclic lateral shifting of the feeder system. Discrete changes of A, S, A/S, or Q/S are not required to explain the origin of episodic backstepping in an overall transgressive regime. Allocyclic changes in A and S, as commonly invoked in the sequence stratigraphy literature, are not an alternative to autostepping, but would only enhance or reduce the inbuilt process of autostepping in river-dominated delta systems.