Modeling Paleo-Flow Events on Alluvial Fans in the Atacama Desert, Chile

The Atacama Desert of Northern Chile has experienced hyper-aridity since ~10-15 Ma. Abundant alluvial fans form relict surfaces in the Antofagasta region that are undated and presumed to be ancient. Morphological and sedimentological characteristics of these alluvial fans and their associated catchments can be used to understand regional paleohydrology and climate. By understanding the past precipitation conditions, we can improve our understanding of the role of large, rare storms in hyper-arid regions. This study attempts to use alluvial fan surficial deposits to evaluate paleo-flow events and to determine under what climatic conditions these deposits formed. We collected positional and slope data using a differential GPS, as well as grain size data on three steep alluvial fans (radii ~ 0.5 km) and associated catchments (basin area ~0.2 km²) during the winter of 2008. Measurements of surficial deposits were used to model paleo-flows using an integrated one-dimensional approach for sediment transport based on d_md and d₉₅ grain sizes, slope, and basin characteristics of each system. This model determines the minimum flow required to transport a given particle based on differing flow regimes –i.e., fluvial vs. debris flow modes of transport. Preliminary results from this study show that paleo-flow conditions were such that discharges approximate 1 m³/s with maximum flow depths of ~0.5 m in small alluvial fan-catchment systems. These results can be used to infer the effects of rare catastrophic flooding events that may become increasingly common during a shift in climate from hyper-arid to semi-arid. Modeling paleo-flows in other systems regionally will aid in understanding basin development and sediment transport in hyper-arid regions.