The geological evolution of the Silala River basin, Central Andes

Abstract Improved understanding of the geology and stratigraphic architecture of the Silala River basin and its evolution, reviewed here, has been important in providing scientific evidence to an international dispute between Chile and Bolivia on the nature and origin of the waters of the Silala River. The dispute was submitted in 2016 to International Court of Justice (ICJ), which issued its judgment in 2022. The Silala River has evolved within an active volcanic chain, in the western region of the Andean plateau. Various volcanic structures, at different stages of their evolution, have determined the basin's development. The first evidence of alluvial drainage associated with the Silala fluvial system appeared in the Lower Pleistocene (ca. 2.6–1.6 Ma), a record of alluvial deposits with paleoflow directions toward the Southwest and South‐Southwest. These deposits had an important role in forming a highly permeable horizon, confined between two pyroclastic deposits (the Cabana and Silala Ignimbrites) which comprise the main regional aquifer in the basin, although there are other minor locally important aquifers. The second stage in the evolution of the river system occurred in the late Upper Pleistocene‐Lower Holocene (ca. 11–8.5 ka BP), when an erosive period carved the current trans‐boundary ravine in the Silala Ignimbrite. Morphological evidence clearly shows that the ravine was carved by fluvial action. The only documented tectonic activity during the development of the Silala River basin is the Cabana reverse fault and associated normal faults, representing an East–West shortening, which occurred between 2.6 and 1.6 Ma. This article is categorized under: Science of Water > Water and Environmental Change Science of Water > Hydrological Processes Water and Life > Nature of Freshwater Ecosystems Human Water > Water Governance