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 Be-10, Al-26, and Ne-21 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 < 0.1 m/Myr. The survival of these gravels, specifically, and more generally, the stability of landform features in this geographic area is made possible by the attaimnent of hyperarid conditions in the Atacama Desert resulting from global climatic cooling about 15 Myr ago combined with the rain shadow effect caused by uplift of the Central Andes. The landform features observed presently in the Atacama Desert are remarkably stable and, despite the inevitable erosion that is detectable using cosmogenic nuclides, undoubtedly bear considerable resemblance to conditions as they existed in the Miocene. Over geologic time, the Atacama landscape is evolving in such a manner as to erode the higher bedrock ridges relative to the more stable, but stratigraphically lower depositional surfaces through which elastic detritus now travels occasionally along the floors of incised drainage systems leaving the older permeable alluvial fan surfaces largely intact as widespread remnants of a Miocene fossil landscape. (c) 2005 Elsevier B.V. All rights reserved.