A key factor in the downstream dispersal and fractionation of sediment is the grain size distribution of sediment supplied by upstream catchments. Modeling of the grain size distribution of modern bedload in the main trunk channels of tectonically uplifting catchments, including the sediment at their outlets, and the weathering products of a range of bedrock lithologies in southern Italy and Sicily reveals fractal dimensions of 2.3-2.7, similar to the fractal dimension of many natural materials undergoing fragmentation. We examine the impact of changing statistical properties of the grain size distribution of the sediment supply in simulating grain size trends in sedimentary basins. Model simulations show a marked movement of the gravel front and patterns of progradation and retrogradation in basin stratigraphy. These grain size trends and sedimentary architectures are generated simply by variations in the grain size mix of the sediment supply, without variations in base level or sediment discharge. Variation in the grain size distribution of the sediment supply may therefore act as a first-order control on sequence stratigraphic architectures in sedimentary basins.