Abstract
Abstract. 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.
Highlights
High-elevation plateaus are commonly associated with either passive margins (e.g. Africa, Sri Lanka, Australia) or large convergent mountain systems (e.g. Himalaya, Andes)
Using a climate-dependent regolith production algorithm (Norton et al, 2014) coupled with simple sediment transport laws, we investigate the effects of climate change in the form of precipitation variation on the hillslope system and propose that hillslope regolith production and stripping may control cut and fill sequences during the late Quaternary
We focus on the Pisco river drainage basin, situated on the western Andean margin at ca. 17◦ S in central Peru
Summary
High-elevation plateaus are commonly associated with either passive margins (e.g. Africa, Sri Lanka, Australia) or large convergent mountain systems (e.g. Himalaya, Andes). We suggest that weathering is a dominant control on river form in escarpment settings as it is responsible for the production of sediment through the formation of regolith. The antiquity of most of these plateaus suggests that they erode through parallel retreat (Schlunegger et al, 2006) with somewhat constant topographic profiles These large topographic gradients often result in orographic precipitation on the escarpment (see, e.g., Bookhagen and Strecker, 2008). Using a climate-dependent regolith production algorithm (Norton et al, 2014) coupled with simple sediment transport laws (see, e.g., Tucker and Slingerland, 1997), we investigate the effects of climate change in the form of precipitation variation on the hillslope system and propose that hillslope regolith production and stripping may control cut and fill sequences during the late Quaternary. Norton et al.: On the potential for regolith control of fluvial terrace formation
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