Cyclic steps: Review and aggradation-based classification

Cyclic steps are a type of upper flow-regime bedform consisting of trains of upstream- and upslope-migrating bed undulations. The overriding flow is characterised by a series of hydraulic jumps occurring in the troughs of the undulations. Cyclic steps form in open-channel flows such as rivers and comprise a common bedform in subaqueous density flows in oceans, lakes and reservoirs. Cyclic steps are associated with alternating Froude-sub-critical and Froude-supercritical flow on respectively the stoss side and lee side of individual bedforms. The transition between these flow states is embodied by the hydraulic jump in the trough of the bedform, leading to the permanent or quasi-permanent morphology of cyclic steps. Over the past decade, numerous studies affirmed the dominant role of cyclic steps in generating bed undulations in modern and ancient glacial outwash, fluvial, delta and turbidite environments as reviewed here. Cyclic steps were previously discriminated as net-depositional (climbing), transportational and net-erosional (falling) in different parts of sedimentary systems. The following cyclic step descriptors, with distinct depositional signatures, are proposed: fully depositional, partially depositional, transportational, partially erosional and fully erosional. Partially depositional cyclic steps are most common. They are associated with backset-bedded sets (high aggradation rate) and nested scours filled with massive-to-backset-bedded deposits (low aggradation rate). The new classification can be used as a predictive tool in the reconstruction of modern and ancient sedimentary successions using repeat bathymetry, seismic reflection or outcrop data. It is applied in three turbidite case studies: classical deep-sea system, small-scale delta slope and line-sourced carbonate slope.