Modelling grain-size distributions. A comparison of two models and their numerical solution

Three main diffusion-based models are currently used to study grain-size di stributions. In this paper, two of these approaches - perfect sorting and i mperfect sorting - are compared in a parameter study. First, the numerical solution of the imperfect-sorting model is extensively discussed, and numer ical tests are performed. Then, the two sedimentation models are compared f or a basin under varying conditions. For some of the imposed variations, pr edictions of both models differ markedly due to the different approach. The position of the gravel front in the perfect sorting model depends on grave l input and proximal accommodation space. The position of grain-size bounda ries in the imperfect-sorting model is strongly controlled by gravel input, the position of the basin axis and the difference in diffusivities. As a r esult, those two models may predict gravel progradation for different situa tions. Both models suggest that gravel progradation should always be couple d with sedimentation rates in order to suggest an explanation of gravel pro gradation observed in the geological record. Simulations with the imperfect sorting model show that this criterion may also fail, showing that a uniqu e interpretation of gravel progradation may be impossible. (C) 2000 Elsevie r Science B.V. All rights reserved.