Tracking internal sedimentary structure of linear dune using cellular automaton
Yukun Liu, Andreas Baas
King's College London, London, UK
Linear dunes are very common on Earth and other planetary bodies (e.g. Titan, Mars). The internal sedimentary structure however is not known so well, and linear dune deposits are difficult to recognize in the rock record. Here we use a cellular automaton (CA) approach to track the dynamic of linear dune internal structure. Algorithms are based on Werner 1995 sand dune model, but more focused on properties of each discrete sediment unit. Here we use new rules to track aeolian transport and deposition types, associated with burial age of sediments to visualise different cross-beds. Initiated from a flat bed under bi-modal winds, we use three wind regimes with varying alternations: from entirely equal alternations for two winds, to slightly, and significantly asymmetric wind regimes. The degree of asymmetry between the two winds in the regime has a strong impact on the internal stratigraphy of cross-beds and dipping directions. The variations between the winds create periodical bounding surfaces separating chronological gaps. During the pattern coarsening process, we monitor a few standard defects/junctions where superposition occurs.
We also explore three collision cases of maturely developed straight linear dunes: forward merging, backward merging and ejection. The first case occurs when dune size difference is large enough so that the small dune is absorbed by the big one. The second case occurs when a superposition relationship forms and absorption takes place during the weaker wind's duration. The superposition is characterized by escaping of the dune body in the lee of the dominant wind. This dune keeps leaving its sand in the shadow zone so the available escaping volume becomes smaller and smaller: ejection occurs if the volume is large enough to maintain a dune shape after a completed escape.