Influence of bedform superimposition and flow unsteadiness on the formation of cross strata in dunes and unit bars — Part 2, further experiments

This experimental investigation examines the processes that control the grain-size sorting and geometry of cross strata in dunes and unit bars, in particular the effects of bedform superimposition and flow unsteadiness. Cross stratification formed by dunes and unit bars is the most common sedimentary structure in river-channel deposits, and is common in many other depositional environments. Previous work by Reesink and Bridge (2007) shows that the grain-size variation that allows cross strata to be recognized is determined by three main factors: (1) pre-sorting by superimposed bedforms and flow unsteadiness; (2) sorting during deposition on the lee slope, and; (3) reworking of the slope by currents in the lee of the bedform. Differences in the relative importance of these three factors cause cross strata to be more varied in their geometry, grain-size sorting and permeability than commonly realized, and provide information for detailed quantitative interpretation of river-channel deposits. The experiments presented in this paper cover a wide range of steady and unsteady flow conditions with different combinations of host and superimposed bedforms. Lee-side deposition was highly variable at the time required for the buildup of cross strata, but systematic variations related to bed morphology provided good indicators of flow conditions. Water currents in the lee-side flow separation zone affect initial deposition on the lee slope and can re-distribute sediment on the lee slope. Centimeter-scale turbulent eddies were observed to increase the tangential shape, decrease the (vertical) grain-size separation, and reduce the thickness of the cross strata. These effects of turbulent eddies were found to increase with increasing flow velocity, decreasing grain size, and decreasing bedform height. Flow unsteadiness was primarily expressed in a change of host and/or superimposed bedform type. The presorting pattern formed by the superimposed bedforms is lost if the pre-sorting involves less sediment than the processes that re-sort the sediment on the lee slope (e.g. grainflows). If the pre-sorting pattern of the superimposed bedforms can be identified, the cross strata they form can be used to interpret host and superimposed bedform geometries. The presence of reactivation surfaces indicates that superimposed bedform heights (Hs) exceed 25% of the host bedform height (H; Hs/H>0.25). Pre-sorting by superimposed bedforms is recognizable as cross stratification where Hs/H<0.25, and comprises: (i) drapes of fine-grained sediment that settles from suspension during the passage of the superimposed trough, and; (ii) a body of coarse-grained bedload with a cross-sectional area that equals the cross-sectional area of the formative superimposed bedform. The distinctiveness and lateral extent of fine-grained drapes represent the distinctiveness and lateral extent of their formative superimposed troughs. The lateral extent and internal grain-size sorting of the thick, coarser-grained body represents the lateral extent and internal grain-size sorting of the body of the superimposed bedform. The plan view shape of the host lee slope controls the cross-stream geometry of cross strata. Thus, cross strata can be used to interpret type, size, and geometry of the host and superimposed bedforms, hence flow and sediment transport condition on the back of the host bedform.