Interpretation of Polymodal Sediments

Commonly used statistical treatments of grain size distributions are inappropriate if the sediment in question is polymodal. 190 sediment samples from a river-lake system were analyzed by one or more sizing methods. Cumulative probability plots revealed most samples to be polymodal with inflection points at approximately 2 ø, 5 ø, and 8.5 ø. Partitioning of representative curves into overlapping lognormal populations was accomplished by utilizing a standard graphical technique. Samples from the river consisted of two populations: medium sand (population A) with a mean of 1.25 ø and fine sand (population B) with a mean of 3.8 ø. Shear velocities calculated from velocity-depth data suggest that the coarse population would be transported as bedload and the finer one as suspension. Samples from the lake lack the medium sand population, but could be partitioned into 3 overlapping populations: fine sand (population B) with a mean of 3.8 ø, silt (population C) with a mean of 5.8 ø, and clay (population D) with a mean of 10.3 ø. Population B apparently moves as suspension in the river but represents traction transport in the lower energy lake environment. Population C and D are interpreted as suspension populations. This technique of partitioning polymodal grain size curves should prove an important tool in the understanding of sedimentary processes and interpreting environments of deposition.

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