Constraining Ages of Desert Alluvial Surfaces According to Their Geomorphic Roughness as Measured from Space

Determining the age of depositional geomorphic surfaces presents a challenge in studying geologic processes and landscape evolution in arid environments, where ‘datable' materials are often scarce. Age constraints in such environments are typically opportunistically obtained for surface features amenable to conventional dating methods (e.g., ¹⁴C, OSL, CRN), and commonly require qualitative extrapolations between dated surfaces and the actual surfaces of scientific interest. Our goal is to establish a new method for quantitatively constraining age extrapolations between conventionally dated surfaces and near-by surfaces of interest using remotely sensed image data. This approach is a calibrated relative-dating method that builds on: 1) the observed time-dependent roughness development of alluvial surfaces in desert environments from boulder-dominated surfaces at deposition stage into smooth desert pavements at maturity, and 2) our ability to measure surface roughness at these scales quantitatively using remotely sensed data. The latter is achieved using stereoscopic image data to measure differential darkening of surfaces due to different amounts of unresolved shadows cast by sub-pixel roughness elements as a proxy for sub-pixel surface roughness itself. Stereoscopic roughness measurements for independently dated surfaces are utilized to establish age-roughness calibration curves for inferring surface age from surface roughness for surfaces that could not otherwise be dated. Additional factors affecting age-roughness evolution trends, such as lithologic composition, climate and local dust influx, may require local adjustment of the calibration curves used. This ‘age from roughness' approach has been successfully tested in Death Valley, southwestern USA, the Gobi desert, southwestern Mongolia and the Negev desert, southern Israel. It is proposed as a widely applicable complementary method for constraining the age of surfaces that could not otherwise be dated, and as a new tool for gaining a wide spatial perspective in the study of regional-scale geologic and landscape evolution processes in arid environments.