Evaluating the life expectancy of a desert pavement

This paper integrates prior scholarship on desert pavements with a case study of pavements on stream terraces in the Sonoran Desert to analyze the processes and site conditions that facilitate the survival of ancient desert pavements. This synthesis identifies vital factors, key factors, and site-specific factors promoting pavement stability. Hyperaridity is the vital factor in pavements surviving for 10⁶ years or more, aided by minimal bioturbation and clast-size reduction. Three key factors aid in pavements surviving for 10⁴ to 10⁵ years: accumulation of allochthonous dust underneath pavement cobbles; a flat topography; and a lack of headward retreating swales or gullies. A unified explanation for pavement longevity, however, did not emerge from a literature review, because a variety of site-specific factors can also promote pavement antiquity including: resistant bedrock beneath the pavement; disk-shaped cobbles to promote dust accumulation; and microclimatological and ecological reasons for minimal bioturbation. Both key and site-specific explanations for pavement longevity apply well to a case study of pavements on stream terraces in the Sonoran Desert, central Arizona. The buildup of cosmogenic¹⁰Be and in situ¹⁴C, optically stimulated luminescence and varnish microlamination ages reveal stable pavements range in age between ~ 30 and 332 ka with conditions for longevity including: flat surface topography; pavements underlain by consolidated granitic bedrock; a lack of headward-retreating gullies and swales;⁸⁷Sr/⁸⁶Sr analyses indicating the infiltration of allochthonous dust floating disk-shaped pavement cobbles; and a quartzite lithology resistant to disintegration. However,¹⁰Be ages also indicate evidence for the instability of desert pavements on stream terraces underlain by unconsolidated playa clays and unconsolidated fanglomerate; these weaker materials allowed the growth of headward-retreating swales, that in turn promoted exposure of newer gravels by surface erosion.