INFLUENCE OF SUPERIMPOSED BEDFORMS AND FLOW UNSTEADINESS ON FORMATION OF CROSS STRATA IN DUNES AND UNIT BARS

Formation of angle-of-repose cross strata on the lee-side of dunes and unit bars is dependent on three main grain-sorting mechanisms: (1) pre-sorting of sediment that arrives at the lee-side of the bedform, related to superimposed bedforms and longer-term variations in water flow and sediment transport; (2) sorting due to differential deposition of sediment on the lee-side and associated grain flows, and; (3) movement of sediment on the lee-side by the water currents in the lee-side flow separation zone. New experiments and field data reveal that the role of superimposed bedforms is of primary importance, even though most emphasis has been put on mechanism (2). It is now well established that bedload sheets or ripples are ubiquitous on the back of dunes, and that bedload sheets, ripples or dunes occur on the back of unit bars. The exact nature of bedform superposition depends on the nature of flow and sediment transport. The size of a superimposed bedform relative to the host bedform controls not only the thickness of the cross stratum, but also its angle of inclination, and the spatial distribution of texture and permeability. In particular, the 3-D character of high permeability zones and permeability barriers is intimately related to the nature of bedform superposition. Longer-term changes in the rate and grain size of sediment supply to the crests of dunes and unit bars, caused by flow unsteadiness (e.g. tides, snowmelt), act simultaneously with sorting caused by superimposition of bedforms, and with that caused by differential deposition and grain flows on bedform lee-sides. Relatively coarse-grained cross strata are commonly thought to be deposited at high flow stage, whereas fine-grained cross strata are deposited at low flow stage. New experiments reveal that relatively fine-grained cross strata can be formed by: (1) reduction in size of sediment supplied due to falling flow stage, or; (2) reworking of pre-existing fine-grained deposits at any flow stage.