A STRATIGRAPHIC SIGN OF ALLUVIAL CYCLIC STEPS PRESERVED IN DELTA FORESET BEDS: FLUME EXPERIMENTS

Upper regime bed forms associated with spatially-periodic hydraulic jumps are called ‘cyclic steps’. They develop in various modern sedimentary environments ranging from inland bedrock rivers to deep-sea floors, but have rarely been reported from stratigraphic records. It is generally accepted that they have intrinsically low preservation potential and are hardly recognizable in cross section. However, cyclic steps might leave their indirect traces in delta foreset bedding in different appearance than that in the modern surface. This hypothesis has been tested using a series of flume experiments in which alluvial cyclic steps were produced on the topset surface of a Gilbert-type delta. During each run, sediment feed rate and upstream water discharge were kept constant, and base level remained stationary. Sediment material used in the runs was a bimodal quartz-sand mixture (D = 0.1 mm and 0.2 mm). The experimental results suggest that there exists clear synchronism between delta’s episodic/intermittent progradation and upstream migration of cyclic steps. When a new hydraulic jump occurs in the vicinity of river mouth and starts to migrate upstream, coarser-grained sand existing on the topset is remobilized and transports to the foreset slope of the delta. This coarse-grain deposit accelerates progradation of the delta tentatively. As the hydraulic jump recedes upstream from the river mouth, reworking of the topset sediment progressively declines, giving rise to quiet settling of finer-grained sand onto the foreset slope. Alternate/cyclic occurrences of these sequential events result in distinctive bedding structure of the delta foreset. Such structure has commonly been attributed to daily/seasonal fluctuations of river discharge and autocyclic shifting/avulsion of an active feeder channel.