Autostepping during the transgressive growth of deltas: Results from flumeexperiments

A series of three-dimensional flume experiments on the transgressive growth of deltas, conducted with constant rate of relative sea-level rise (A), co nstant sediment discharge (S), and constant upstream water discharge (Q), r eveals an inbuilt geomorphic process that inevitably forms discrete steps o n the abandoned subaqueous deltaic slope. The stepped topography and its pr ocess of formation are here referred to as autostep(s) and autostepping, re spectively. The stepped topography is caused by the lateral migration of de lta-front lobes that inevitably shift landward but fail to cover lobes of o lder cycles. The autostepping arises from the principle of autoretreat comb ined with autocyclic lateral shifting of the feeder system. Discrete change s of A, S, AIS, or Q/S are not required to explain the origin of episodic b ackstepping 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.