Grain size segregation and stratigraphy in aeolian ripples modelled with a cellular automaton

Abstract

AEOLIAN ripples are distinguished by coarse-grained crests, finegrained troughs, and thin veneers of coarse grains on the upwind (stoss) slopes. Migration of these sorted bedforms during periods of net deposition results in a characteristic inversely graded stratigraphy that distinguishes aeolian sandstones from those of fluvial origin¹. Here we investigate the formation and evolution of aeolian ripples using a cellular automaton model of a sandbed which incorporates two grain sizes and is subject to the impacts of episodically hopping (saltating) grains. Using this model, we identify the physical processes responsible for both the spatial sorting and the stratigraphic signature of the ripples. High-energy saltating grains are found to eject small grains preferentially from the impact site, leading to a coarsening of the heavily bombarded stoss slopes. Coarse grains do not in general leap far enough to escape the shadow zones on the downwind (lee) slopes, and therefore tend to accumulate and cycle around the ripple crests. Small grains, on the other hand, are ejected at higher velocities, enabling them to hop further. Incorporating net deposition into the model results in a stratigraphy closely resembling thin planar laminae ('pinstriping') found in aeolian sandstones: the inverse grading of grain sizes is a direct consequence of their different hop lengths.