²⁶Al/¹⁰Be dating of an aeolian dust mantle soil in western New South Wales, Australia

Aeolian dust mantle soils are an important element of many landscapes in south-eastern Australia, though the age of these aeolian deposits has not been radiometrically determined. At Fowlers Gap in western New South Wales, surface cobbles of silcrete and quartz overlie a stone-free, aeolian dust mantle soil, which has a thickness of about 1.6m. The clay-rich aeolian dust deposit in turn lies upon a buried silcrete and quartz stone layer. Modelling in-situ cosmogenic ²⁶Al and ¹⁰Be concentrations measured in both the surface quartz stones and in the buried quartz layer of rocks, reveals that each has experienced a complex exposure-burial history. Due to the absence of quartz stones or sand at intermediate depths, our cosmogenic ²⁶Al and ¹⁰Be modelling was not able to determine a definitive mechanism of stone pavement formation and stone burial. Various scenarios of stone formation, transport, burial and exhumation were tested that constrain the age of the deposit to range from 0.9±0.2Ma to 1.8±0.2Ma, based largely on different assumptions taken for the time-dependency of the net sedimentation rate. This corresponds with the initiation of the Simpson Desert dune fields and the deflation of lakes in central Australia, which probably responded to the shift to longer-wavelength, larger-amplitude Quaternary glacial cycles at around 1Ma. Sensitivity analyses were carried out to identify those parameters which better constrained model outputs. Within model errors, which largely are the result of analytical errors in measured ²⁶Al and ¹⁰Be concentrations, all three competing theories of colluvial wash, upward displacement of stones, and cumulic pedogenesis are possible mechanisms for the formation of the surface stone pavement.