Late Quaternary alluvial fan stratigraphy and chronology, Elqui, Turbio and Claro valleys, semiarid Andes of Chile

Abstract

New mapping and luminescence geochronology of tributary alluvial fan deposits from the semiarid Subtropical Andes of Chile are used to infer Late Pleistocene–Holocene paleoclimate conditions for the region. During this period, alluvial fans descending from tributary catchments to major river valleys encroached on fluvial deposits, creating a record of interbedded fluvial-alluvial deposition, and in places, generated extensive river damming. The oldest sequence of fan deposition (~40 ka) appears eroded and disconnected from active channel deposition. Other regionally distributed fan sequences are dated to 14–11 ka, ~8 ka, and 3–1 ka. Common depositional timing of the thick 14–11 ka fans in catchments sourced from high and intermediate elevations suggests a common factor in generating these fans, possibly linked to a regional precipitation increase derived from Pacific moisture sources in the context of a gradual temperature rise since the local last glacial maximum. A short duration fan sequence occurred centered at ~8 ka in some outlets sourced from high-elevation catchments. The occurrence of the 8 ka fans during the most arid period of the Holocene in the subtropical Andes suggests an easterly moisture source for the precipitation events that generated the fans, that could be associated with an effect of the 8.2 ka event (Bond event 5) leaking into the Southern Hemisphere. A minor Late Holocene aggradation period is linked to a regional humid period with westerlies as the principal moisture source and an increase in ENSO-warm signal frequency and intensity as a driver. Fan aggradation occurs during periods with both ascent and descent of the major regional vegetation ecotones. This observation suggests that, in this setting, hillslope hydrology and rainfall style control aggradation periods rather than vegetation change.