Origin of cross-laminas in a laboratory experiment

A quantitative explanation is cited for the origin of the cross-laminated structure of a microdelta deposited in a laboratory flume under conditions of constant flow discharge and sediment input. The structure originated when the sediment load carried over the front of the delta was differentiated into finer and coarser size fractions by hydrodynamic sorting processes. Detailed sampling of the sediment load for grain size distributions demonstrated that the most important sorting process was selective transport and deposition related to differential settling velocity. The cross-laminas composed of the coarser grain sizes were produced by the downslope transfer of material from the upper foreset slope of the delta. This material was primarily of bed-load origin. On the other hand, the cross-laminas composed of the finer grain sizes were produced by the fallout of suspended sediment onto the middle and lower parts of the foreset slope. This material was derived mostly from the sediment carried in suspension by the heavy fluid layer flowing along the stream bed. It was possible, therefore, to correlate the stratified nature of the foreset with the density-stratified nature of the heavy fluid layer. The cross-laminated structure of the deposit wras quite distinct even though the size differential between the fine and coarse layers was only 0.05mm. Based on the results of this paper, a possible procedure is outlined for predicting the size differential between the fine and coarse laminas of the foreset for other conditions of experimentation. Specifically, the procedure entails a reconstruction of the inferred path lines of the particles settling out in the flow transition and a calculation of the rate of decrease of median grain size in the downstream direction.