Abstract

The aeolian stratigraphic record along the central coast of Chile (32°-35°S) consists of sequences of paleodunes separated by interstratified paleosols. Previous research has linked these sedimentary units to paleoclimatic changes associated with the oscillation and intensification of the Southern Westerly Winds. As a result, these eolian deposits are considered proxies for the paleoclimate of central Chile. However, a detailed sedimentary analysis of these eolian records is currently lacking. The spatial distribution of the dune deposits (north of the mouth of the Andean rivers) suggests that the sands' origin is connected to past Andean glacial and/or fluvial processes near the coast. Nevertheless, these processes have not been thoroughly investigated. Several unanswered questions remain: When did coastal dunes in central Chile form? Which rocks contribute the sediments that form the dunes? What role do Andean glaciations play in dune morphogenesis? And what are the paleoenvironmental implications of the paleodune-paleosol sequences? Based on our stratigraphy and U-Pb dating results, we have observed not only an Andean influence but also other major sources of sand in the hinterland that vary in their relative importance over time. Our findings indicate that the sands primarily originate from rivers located immediately south of the dune fields Our study focuses on the Aconcagua and Rapel basins, which encompass the study sites comprising paleodunes, dunes, fluvial terraces, and a moraine. We collected detailed stratigraphic information from these units to understand the sedimentary processes. We conducted Optically Stimulated Luminescence (OSL) dating on the sands to determine the age of the paleodunes and estimate the age of the paleosols. Additionally, U-Pb dating was performed on detrital zircons to identify the source of the sediments. LA-ICPMS was used to date the detrital zircons via U-Pb analysis on 100 grains per sample. OSL dating relied on a pIRIR225 (post infrared, infrared stimulated luminescence @225°C) single aliquot regenerative (SAR) dose protocol specifically for potassium-rich feldspar. This method has been successfully applied in previous studies and is reliable for establishing chronologies in the aeolian sediment record of Central Chile. Within the Aconcagua basin, we selected three sites that outcrop in sections (see FIG: MAPA): Ventanas II (Fig 2), Guardia Vieja Moraine, and the fluvial terrace of the Aconcagua River. In the Rapel basin, we selected three sites: Fundo Cardonal, Las Brisas, and the fluvial terrace of the Rapel River. Altogether, 19 samples for luminescence datingwere taken from the base and the top of the different generations of aeolian sand deposition. Soil ages were estimated to lie between the age of the older parent material and the covering sands. Ventanas II exposes a detailed stratigraphic sequence of 15 m with 13 well-developed units overlying the Horcón Formation (Miocene-Pliocene). Our OSL results show that periods of dune accumulation and pedogenesis (interpreted to reflect wetter conditions) occur during MIS 4, 3, and 2 (LGM). When comparing the U-Pb detrital zircon ages with the longitudinal geologic belts making up the Aconcagua and Rapel basins we identify different rock sources (e.g., Andean and Coastal). This is evidence of a restricted local basins provenance of sands that made up the paleodunes. The results suggest that main sources of sand change their relative importance through the last ice age (MIS 2 Andean-affinity; and MIS 4 Coastal-affinity).