Weibull vs. lognormal distributions for fluvial gravels

Krumbein and Tisdel (1940) suggested that the particle size distribution of a weathered source rock tends to follow the Rosin distribution, whereas with distance downstream, fluvially transported sediment tends to follow the lognormal distribution. This issue tan be illuminated somewhat by recognizing that the Rosin distribution is the same as the well-studied Weibull distribution. However, it can be difficult to distinguish the lognormal, Rosin (Weibull), and even gamma distributions in particle size distribution curves. Brennan and Durrett (1987) suggest that differences in splitting rates can give rise to the lognormal vs. Weibull distributions, but the geological implications of these concepts, and the integration of the effects of sorting by fluvial transport, remain to be addressed.