Quantitative Matching of Landform Evolution Models With High-Resolution DEMs as a New Quaternary Surface Dating Technique

A model for 3-D hillslope diffusion is used to date Quaternary alluvial surfaces in the southwestern United States. This technique complements established geochemical surface dating methods and may be extended to areas where geochemical analysis is not feasible. Alluvial fan surfaces develop drainage networks immediately following the time of their deposition. Hillslopes flanking the channels erode in a manner similar to the much studied diffusion of fault scarps. The extent of hillslope degradation on the surfaces of Quaternary deposits has long been used to correlate units of similar age, but we propose to use the slope degradation to obtain absolute surface ages. The proposed technique is based solely on surface morphology and can be rapidly and inexpensively applied to a large number of piedmonts. We focus on the arid southwestern United States, where sediment pulses have already been qualitatively correlated, and in some cases quantitatively dated. The model starts with an initial landscape with pre-defined channels then evolves towards a real-world digital elevation model (DEM) according to the diffusion equation. DEMs of up to one-meter resolution are created for areas of interest from high-resolution aerial photographs. A resultant age is obtained when the model landscape most closely represents the input DEM. To assess the accuracy of our technique, we compare our model results with ages obtained by other geochronological techniques on the same surface. We also examine the effect of adjusting channel depth, channel distribution, hillslope diffusivity, and slope of the alluvial surface on the overall landscape evolution. By comparing the resultant surface ages to paleoclimatic records from local lake sediments, this technique may elucidate the role of climatic change on alluvial fan formation.