Surface Rock Controls on the Development of Desert Varnish in the Mojave Desert

Abstract

Desert varnish is a commonly occurring feature on surface rocks of stable landforms in arid regions. The objectives of this study were to investigate how desert varnish is related to the properties of the rocks on which it forms and how varnish is related to landform surface age and stability. To accomplish these objectives, approximately 350 varnished rocks from previously dated sites in the Mojave Desert were collected, photographed, converted to 3-D models, and analyzed to determine the extent, intensity, and patterns of desert varnish and how the desert varnish was related to land surface age and stability. Our results show a link between increasingly stronger varnish expression and both landform age and stability. We found a potential interaction between vesicular (V) horizons and the formation of the rubified ventral varnish. The rocks in this study showed a maximum varnish expression at a depth below the embedding plane that corresponded to the depth of V horizons when present and the lowest portion of the rock when absent. When V horizons were present, the varnish tended to be strongest near the lower boundary of these horizons. This interaction between the V horizons and location of maximum varnish expression on the rock may be due to the effect of V horizons on infiltration and the retention of water at those depths. The relationship between ventral rubification, rock size, and age shown in this study suggest that stable land surface environments (i.e., stable landforms and large surface rocks) create conditions needed for strongly expressed varnish. In the absence of traditional dating techniques, these relationships could be used to estimate the ages of Mojave Desert landforms.