Quaternary alluvial fan evolution in the western Atacama Desert, North Chile

The Atacama Desert is considered the driest and oldest non-polar desert on Earth featuring numerous indications for an Early Miocene onset of its hyperaridity. Despite vast evidence for long-term hyperaridity dominating the landscape evolution of the desert’s core, younger Quaternary fluvial modification has been deduced from various archives across the Atacama Desert. Located within the N-S-extending Coastal Cordillera, a significant portion (>10%) of the western Atacama Desert’s surface geology is recognized as Quaternary deposits predominantly related to past alluvial fan activity. However, only few and patchy stratigraphic and geochronological constraints exist on the formation of those depositional landforms. We therefore systematically studied the alluvial fans across the Coastal Cordillera at ~21°S by combining DEM-based morphometric assessment, establishing morphochronological frameworks by in situ terrestrial cosmogenic nuclide dating, and stratigraphic analyses by geophysical surveys and sedimentological-pedological analyses of a soil pit. Significant fan-catchment morphometric relationships indicate intact source-to-sink connectivity and signal propagation. The youngest and last abandoned surface generations of four multi-stage alluvial fans were dated by ¹⁰Be exposure dating, complemented by reappraisal of ¹⁰Be data from an alluvial fan system published by Baker et al. (2013). Site-specific timing of terminal fan aggradation dates to the late Middle Pleistocene and beginning of the Early–Middle Pleistocene Transition (EMPT) for the youngest and penultimate stages, respectively. Interpreted in the form of a regional geochronological compilation, fluvial-alluvial activity shows additional peaks during the Last Interglacial Complex. Late and Middle Pleistocene palaeoclimatic signals are largely in agreement with other Quaternary sediment records from the central Atacama Desert. While alluvial fan evolution within the Coastal Cordillera is rooted in the tectonic evolution of the basin settings, integrated results strongly indicate a close coupling between Quaternary fluvial activity and palaeoclimatic variability. Our study provides first evidence for a major fluvial modification during the EMPT. Moreover, we can infer a dominant effect of the maximum inland extent of advective fog on preventing long-term landscape stabilisation and conservation.

Reference
Baker, A., Allmendinger, R.W., Owen, L.A., Rech, J.A. (2013). Permanent deformation caused by subduction earthquakes in northern Chile. Nature Geoscience 6, 492–496. https://doi.org/10.1038/ngeo1789