Contribution of lateral processes to stone pavement formation in deserts inferred from clast orientation patterns

Abstract

Stone pavements are widespread landforms of arid environments. They typically develop by accretionary rise above a thickening aeolian mantle which usually contains a surficial vesicular unit. Hence, stone pavements are regarded as stable landforms and, thus, are often used as surface-age indicators. We studied orientation patterns of pavement clasts and depict their geometrical properties. We introduce a new statistical approach for quantitative description of circular data. From four study regions in a wide variety of environmental settings we show that preferred, non-random orientation of clast length axes is a recurrent feature of modern stone pavements. Orientation is primarily determined by no other relief parameter than slope aspect. In 40 out of 52 measured plots an angle of 72 ± 9° separates two angle maxima. The same patterns are present in stone lines buried under a cover of aeolian dust. We interpret these features as fossil stone pavements, accordingly. Furthermore, on artificially cleared plots disturbed stone pavements begin to recover within few months by a process which creates essentially the same orientation pattern. We suggest that lateral surficial processes such as unconcentrated overland flow and creep contribute to the formation and maintenance of stone pavements as well as to this preferred clast orientation. They arrange clasts symmetrical to slope aspect with an angle determined by the axes ratio of the transported object. This results in the frequent bimodal pattern of stone length axes. This special type of transport requires previous accumulation of aeolian material forming a fine grained, even vesicular layer. Thus, there is close relationship between stone pavement and vesicular layer. We conclude that lateral processes take part in stone pavement formation. The commonly accepted model of dust accretion, which is not rendered obsolete by our findings, has to be complemented by this lateral component. This has implications for the stability of stone pavement-covered surfaces and for their applicability as age indicators. Quickly resealing disturbances as well as fossilised stone pavements imply this prominent desert surface feature may become fragile and transient.