Fluvial fill terraces in intermontane basins are valuable geomorphic archives that can record tectonically and/or climatically driven changes of the Earth-surface process system. However, often the preservation of fill terrace sequences is incomplete and/or they may form far away from their source areas, complicating the identification of causal links between forcing mechanisms and landscape response, especially over multi-millennial timescales. The intermontane Toro Basin in the southern Central Andes exhibits at least five generations of fluvial terraces that have been sculpted into several-hundred-meter-thick Quaternary valley-fill conglomerates. New surface-exposure dating using nine cosmogenic 10Be depth profiles reveals the successive abandonment of these terraces with a 100 kyr cyclicity between 75±7 and 487±34 ka. Depositional ages of the conglomerates, determined by four 26Al/10Be burial samples and U–Pb zircon ages of three intercalated volcanic ash beds, range from 18±141 to 936±170 ka, indicating that there were multiple cut-and-fill episodes. Although the initial onset of aggradation at ∼1 Ma and the overall net incision since ca. 500 ka can be linked to tectonic processes at the narrow basin outlet, the superimposed 100 kyr cycles of aggradation and incision are best explained by eccentricity-driven climate change. Within these cycles, the onset of river incision can be correlated with global cold periods and enhanced humid phases recorded in paleoclimate archives on the adjacent Bolivian Altiplano, whereas deposition occurred mainly during more arid phases on the Altiplano and global interglacial periods. We suggest that enhanced runoff during global cold phases – due to increased regional precipitation rates, reduced evapotranspiration, or both – resulted in an increased sediment-transport capacity in the Toro Basin, which outweighed any possible increases in upstream sediment supply and thus triggered incision. Compared with two nearby basins that record precessional (21-kyr) and long-eccentricity (400-kyr) forcing within sedimentary and geomorphic archives, the recorded cyclicity scales with the square of the drainage basin length.
Bibliographical noteFunding Information:
This study was funded by the Emmy-Noether-Programme of the Deutsche Forschungsgemeinschaft (DFG) grant number SCHI 1241/1-1 awarded to T. Schildgen. M. Strecker acknowledges DFG funding (grant number STR 373/32-1 , StRATEGy ). We thank George Hilley for providing data from the sampling campaign he performed in 2003, and Walter Düsing for field assistance. Jan Schüürmann, Lilian Pollozek, David Käter, Martin Lang, and Cathrin Schulz are thanked for their help during sample processing and Steve Binnie and Stefan Heinze for performing the AMS measurements. We thank Luca Malatesta and one anonymous reviewer for their very detailed and constructive reviews that helped to improve the manuscript.
© 2017 Elsevier B.V.
Copyright 2017 Elsevier B.V., All rights reserved.
- Be depth-profiles
- Eastern Cordillera
- aggradation–incision cycles
- glacial–interglacial cycles
- landscape response to climate change
- surface inflation