The geomorphology and Late Quaternary sedimentary record of the Salado/Juramento fluvial megafan, Central Andes foreland basin (Chaco Plain, Argentina)

Abstract

This research provides geomorphological and sedimentary data on the Salado-Juramento fluvial megafan, the second largest megafan (204,747 km2) of the Chaco aggradational plain, generated by the Salado-Juramento River in NE Argentina. It comprises one of the most extensive depositional systems of the Central Andean foreland basin system in South America, also being among the largest megafans on Earth. Despite this, it has not been previously studied from the perspective of its geomorphology, stratigraphy and dynamic evolution. An integrated approach focusing on the analysis of morpho-sedimentary processes, the resulting landforms and the reconstruction of the Late Quaternary evolution is presented here. The applied methods combine geomorphological mapping from remote sensing images analysis, assessment of landforms and processes, and field data. Historical documents were also analysed. Insights into the near sub-surface stratigraphy were obtained from analysis of research boreholes (up to 30 m depth), integrated with groundwater borehole information. The Fluvial Megafan of the Salado-Juramento Geomophological System comprises four geomorphological units: 1) the middle-proximal area of the megafan, 2) the distal area of the megafan, 3) the complex fluvio-aeolian belt of the Golondrinas-Calchaquí, and 4) the present fluvial belt of the Salado. Thirteen geomorphological sub-units were defined and mapped, representing the complexity of sub-environment interactions. Amalgamated braided channel deposits and aggradation lobes with limited floodplain muds are common in the proximal area, while mixed-load, low-energy, narrow, shallow, and sinuous palaeochannels and associated floodplain marsh deposits characterize the distal area, with groundwater discharges and hydromorphic soils. The megafan construction was dominated by short-term sedimentation and channel avulsions to overbank areas. Fan-shaped depositional lobes of the proximal area and sandy channel deposits and sand sheets linked to terminal splays that are covering mud-flat deposits (floodplains, including palaeosols) in the middle area suggest a high-flow regime. Neotectonics favoured subsidence, avulsion, and prolonged flooding events, with high potential of preservation of an extensive Late Quaternary wetland unit, ponds, lakes, soils, and loessic deposits in the distal megafan. Aggradation and progradation patterns have been conditioned by tectonic control regulating the interaction between the megafan and its axial trunk river (Paraná). Furthermore, the Late Quaternary megafan sequences would also have responded to climatic changes, primarily related to strong hydroclimatic seasonality linked to changes in the South American monsoon system strength. Seasonal high sediment load of the formative river linked to the characteristics of the Andean basin (38,000 km2) and its climatic patterns are considered as the main control on megafan dynamics over at least the Upper Quaternary. Morpho-stratigraphical data of the megafan presented here and the first interpretation of its Quaternary evolution is the key to support socio-economic decisions on aquifer exploitation, land uses, and infrastructure planning.