Strategies for Exposure Dating Coarse Alluvium Strain Markers

More than 150 measurements of 10Be and 36Cl are used to test methods of dating Quaternary alluvium surfaces in the US southwest. Sample targets include large surface boulders, desert pavement clasts, subsurface clasts (cobbles, pebbles), and depth profiles (pebbles or sand). Experiments were designed considering sedimentological and soils stratigraphy, catchment erosion and lithology, sediment storage, sediment mixing, post-depositional surface processes, terrace and fan construction, and climate change: these factors are significant at the timescales of interest. The results indicate that no single approach is ubiquitously optimal however several approaches are evidently advantageous. For example, (1) sediment derived from glaciated catchments appear to have lower probabilities for inheritance and yield more reproducible ages than ages on individual boulders and subsurface clasts from non-glaciated basins. (2) Age variation among samples of amalgamated desert pavement clasts seems to vary with elevation (possibly linked through degree of bioturbation, not sediment transport distance). (3) Amalgamated samples of sand consistently provide a tighter depth profile than pebble amalgamates particularly in catchments with low erosion rates. Soils analysis provides useful criteria for sample site selection, recognition of buried surfaces, and interpretation of exposure ages.