HILLSLOPE PROCESSES AND FORM IN THE HYPERARID ATACAMA: LEGACIES, BOUNDARY CONDITIONS, CLIMATIC FLUCTUATIONS, AND SALT

The landscape of the Atacama Desert is a complex legacy resulting from pulsative erosion and deposition driven by spatially and temporarily varying tectonics and climate. Much of the topography developed pre-Pliocene during periods of erosion much faster than possible under the current hypearid climate. Within the present climate, however, there is slow but active soil formation and erosional shaping of the landscape. Hillslopes differ depending on boundary conditions, i.e. whether there is active incision at their base. Along the coastal Cordillera, active bedrock incision and sediment transport by channels draining to the sea, undercuts hillslopes, removes sediment waste and leads to hillslopes with extensive bedrock exposure. Farther inland, in the Central Depression, channel incision is subdued, and hillslopes are largely surrounded by sediment fills and pediments. These hillslopes are surprisingly smooth with strong convex up profiles and a distinct soil mantle. Salt, derived from atmospheric deposition, mantles the slopes, infiltrates with the rare rain, fractures the bedrock, and mixes with the spalled rock to produce a soil mantle of mixed salt/bedrock origins. Overland flow erosion is limited to a sheetwash that displaces stones into distinct rows, and to localized rilling. Some steep hillslopes are bounded by shallow, slowly incising bedrock valleys, and there soil formation via bedrock erosion is keeping pace with the slow stream incision. Cosmogenic radionuclide measurements reveal that soil formation and erosion rates are on the order 1 m/My or less, demonstrating that the relief and gradient of hillslopes are legacies of earlier wetter conditions that were capable of driving much higher rates of erosion. Based on examinations of erosion rates and hillsopes to the south, which experience increasing amounts of rain with latitude, we suggest that the hillslopes of the Atacama Desert were once mantled with bedrock-derived soils in a more pluvial climate, and subsequent aridification first stripped this soil mantle from the hillslopes, exposed relatively fresh bedrock to the landsurface, which ultimately was mantled with salts during the present hyperarid phases which has existed since at least the late Pliocene.