Remnants of a fossil alluvial fan landscape of Miocene age in the Atacama Desert of northern Chile using cosmogenic nuclide exposure age dating

We have used cosmogenic nuclides to estimate limits on the surface exposure durations and erosion rates of alluvial fans and bedrock surfaces in the Atacama Desert in Chile. The oldest landforms we studied are extensive alluvial fans referred to as “Atacama gravels”. With the exception of samples collected in Antarctica, the cobbles collected on these alluvial surfaces have the lowest erosion rates of any samples, as determined by cosmogenic nuclides, analyzed to date. The oldest cobble has a model surface exposure age of 9 Myr, based on combined measurements of cosmogenic 10Be, 26Al, and 21Ne concentrations. Cobbles from the alluvial fans are eroding slower than the surrounding steep mountainous bedrock surfaces. Maximum erosion rates for cobbles on alluvial surfaces are uniformly