COSMOGENIC 10BE EXPOSURE DATING OF COLORADO RIVER TERRACES AT LEES FERRY, ARIZONA

Precise age control of fluvial deposits using cosmogenic ¹⁰Be is problematic due to complications with inheritance, bioturbation, and exhumation. A deposit's inheritance could also be influenced by sampling technique since distinct transport histories may exist for different grain sizes. The stream terraces at Lees Ferry reside in an arid environment and are overlain by a well developed desert pavement—suggesting the effects of bioturbation and exhumation are minimal. We adopted a sampling strategy with the goals of 1) determining the inheritance and dating the abandonment of a Colorado River terrace at Lees Ferry, Arizona, and 2) investigating how sampling different grain sizes effects the measurement of inheritance. The results of this work will be combined with OSL results from the underlying fill in order to unravel the fluvial history of this key location—which lies in the transition between Glen and Grand canyons.

A 220 cm depth profile was created by dating both sand- and pebble-sized quartz grains (at intervals of 30 – 40 cm), as well as a surface sample of the desert pavement. Preliminary results from the sand-sized depth profile yield a minimum exposure age of 87.3 ± 4.2 ka (2σ systematic error) with an inherited ¹⁰Be signal of 1.09 × 10⁵ atoms/g (equivalent, locally, to ~11.5 ka). Our raw data fit the theoretical equation—an exponential decay curve defined by an average particle attenuation length of 150 g/cm²—to an R²=0.99. This result reinforces our assumption that bioturbation and exhumation have had a negligible effect on the stream terrace; furthermore, it is consistent with the stratigraphic correlations made in the field to the chronology in eastern Grand Canyon. These observations demonstrate the effectiveness of this method for dating classically difficult fluvial landforms.