Search

Cut-fill terraces occur throughout the western Andes, where they have been associated with pluvial episodes on the Altiplano. The mechanism relating increased rainfall to sedimentation is, however, not well understood. Here, we apply a hillslope sediment model and reported cosmogenic nuclide concentrations in terraces to examine terrace formation in semi-arid escarpment environments. We focus on the Pisco river system in western Peru in order to determine probable hillslope processes and sediment transport conditions during phases of terrace formation. Specifically, we model steady-state and transient hillslope responses to increased precipitation rates. The measured terrace distribution and sediment agree with the transient predictions, suggesting strong climatic control on the cut-fill sequences in western Peru primarily through large variations in sediment load. Our model suggests that the ultimate control for these terraces is the availability of sediment on the hillslopes, with hillslope stripping supplying large sediment loads early in wet periods. At the Pisco river, this is manifest as an approximately 4-fold increase in erosion rates during pluvial periods. We suggest that this mechanism may also control terrace occurrence other semi-arid escarpment settings. [ABSTRACT FROM AUTHOR]

Cut-fill terraces occur throughout the western Andes where they have been associated with pluvial episodes on the Altiplano. The mechanism relating increased rainfall to sedimentation is however not well understood. Here, we apply a hillslope sediment model and reported cosmogenic nuclide concentrations in terraces to examine terrace formation in semi-arid escarpment environments. We focus on the Rio Pisco system in western Peru in order to determine probable hillslope processes and sediment transport conditions during phases of terrace formation. Specifically, we model steady state and transient hillslope responses to increased precipitation rates. The measured ter race distribution and reconstructed sediment loads measured for the Rio Pisco agree with the transient model predictions, suggesting strong climatic control on the cut-fill sequences in western Peru primarily through large variations in sediment load. Our model suggests that the ultimate control for these terraces is the availability of sediment on the hillslopes with hillslope stripping supplying large sediment loads early in wet periods. At the Rio Pisco, this is manifest as an approximately 4x increase in erosion rates during pluvial periods. We suggest that this mechanism may also control terrace occurrence in other semi-arid escarpment settings. [ABSTRACT FROM AUTHOR]