Laboratory Study of the Distribution of Grain Sizes in Cross-Bedded Deposits
A preliminary study was made of the movement and dispersion of particles in the flow transition over the foreset front of a tabular unit of cross–bedding (a micro–delta). The approximate path lines of particles settling out on the bottomset were derived by making simplifying assumptions concerning the flow pattern over the foreset front. Specifically, the path line for any given particle was derived by plotting the vectorial resultant of the longitudinal velocity of stream flow in the transition region and the settling velocity of the particle.
The velocity, depth, and suspended sediment concentration were determined for the flow upstream of the foreset slope and the path lines of the various size grades of particles were plotted for several basins of differing depths. The theoretical grain size compositions for ‘synthetic’ samples at selected positions along the bottom– set were calculated by integrating the path line data for the various size grades. The assumption was made that the relative proportions of particles of differing size grades settling out on the bottomset in unit time would determine the average composition at the selected sample position. There was a reasonable measure of agreement between predicted and actual grain size compositions, suggesting that the path line hypothesis affords a rational approach to the problem of downstream sorting.
The principle involved in the path line method can be used to a limited extent in the reconstruction of paleo– flow regimes. This is based on the premise that the dispersion of grain sizes in a bottomset affords an index to the depth of the stream. The latter can be estimated from an analysis of ‘inverse’ path lines.