CONICET | Buscador de Institutos y Recursos Humanos

Morpho-sedimentary data of one of the largest fluvial fans of South America, the Salado-Juramento megafan (ca.185,000 km2), are presented. It is one of the six megafans of the Chaco aggradational plain, representing part of the foreland basin along the eastern flank of the Central Andes. The applied methods combine assessment of landforms and processes and geomorphological mapping from remote sensing images analysis and field data. The lack of exposed stratigraphy across the flat megafan surface and the sub-environments complexity conducted to a program of research boreholes. Sedimentological analyses of cores were performed by laser diffractometry, optical microscopy and DRX; also geochemical data and OSL datings were obtained. The surface of the Andean basin of the formative transverse river of the megafan is 38,000 km2. The river undergoes large seasonal fluctuations in discharge (from ca. 30 to 2,000 m3/s) that result from monsoonal precipitation (South American Monsoon System), with high sediment yields favouring an avulsive dynamic. The longitudinal profile of the megafan is upward concave, with gentle slopes (0.18?0.01°) and maximum width and length of 150 km and 650 km respectively. Aggradation and progradation have been conditioned by flexural and tectonic controls regulating the interaction between the megafan and its axial trunk river (Paraná). The defined nine morpho-sedimentary units of the megafan and the ten sub-units of its distal zone represent the complexity of subenvironment interactions produced by different processes. Fan-shaped depositional lobes and a radiating network of palaeochannel belts are typical landforms. Flexure and dynamic topography favoured subsidence, avulsion and accumulation, with high potential of preservation of swamp, channel, pond, soil, and loessic facies in the distal zone of the megafan. A Late Quaternary wetland (ca.29,000 km2)occupies this zone preserving high-resolution palaeoenvironmental data. Shallow cores and a 30 m depth borehole (29°10´28"S and 61°11´57"W) conducted to stratigraphic correlations. The representative Pleistocene sequence (19.5 to 6.5 m) is formed by clayey silts accumulated in temporary to permanent swamps with an interbedded hydric paleosoil. A sandy palaeochannel of the megafan was drilled on top of the wetland (6.5 to 5.5 m), covered by a loessic deposit (5.5 to 1.5 m). Late Holocene swamp sediments compose the upper part of the sequence. The palaeowetland deposits are lying on Plio-Pleistocene fluvial sands deposited by the Paraná system (30 to 19.5 m). First geochronological data and regional correlation indicate that Chaco megafans would have reached their maximum development during the MIS 3 and the first part of the MIS 2, and experienced aridity in the LGM. The recent expansion of deforestation and agricultural land on the megafan increased its vulnerability to flooding events. Megafan evolution information is the key to support socio-economic decisions on aquifer exploitation, land uses and infrastructure planning.