Process-landform relationships at variable spatio-temporal scales in the Southern Chaco fluvial megafans (Argentina)

KRÖHLING, DANIELA M.

Viena

Congreso; EGU General Assembly 2024 (EGU-European Geosciences Union); 2024

Copernicus GmbH

The Chaco Plain (South America) is an aggradational lowland (40-600 m asl) with divergentdrainage patterns, at present under a tropical–subtropical climate. Late Quaternary coalescentfluvial megafans constitute the most extensive depositional system of the Central Andean forelandbasin. A complex assemblage of macro- and mesoscale landforms of the Southern Chaco plain(22°–31°S) was analyzed at variable spatio-temporal scales, based on remote-sensing data coupledwith field and sedimentological datasets. The Pilcomayo, Bermejo, Salado-Juramento and DulceRiver megafans are between the largest distributive fluvial systems (DFS) in modern landscapes ofEarth. The surface covered by these megafans reaches ca. 5,5 million km2, deeply advancingbasinward and achieving megafan radii of 600/750 km, most of these to the axial fluvial trunk(Paraná). They are characterized by highly dynamic fluvial processes related to flooding andpartial avulsions. A process-based geomorphological research focusing on Late Quaternary-Present fluvial and wetland dynamics from fluvial valley scale to DFS scale is presented. The spatio-temporal pattern of fluvial aggradation and progradation on these megafans is related to climaticand neotectonic forcings. The influence of the South American Summer Monsoon System duringthe Late Quaternary produced high seasonal fluctuations in the discharge in their Andean rivers,associated with the high sediment input and short-term sedimentation of the formative rivers.Fluvial distributive channels, alluvial ridges, aggradational lobes and crevasse and terminal splaysare the typical landscape elements of the megafans. Foreland basin configuration determined thelandscape patterns along these megafans. The mosaic of the Late Pleistocene and Holocenelandforms influences the present hydro-geomorphological dynamic of each megafan. Extensivepermanent to temporary wetlands were developed in the middle/distal reaches of these DFS,which are characterized by surfaces of extremely low gradients (0.03°–0.18°). These wetlandsfunctioned as sediment sinks. Late Pleistocene (MIS 5/3) wetland sedimentary facies andhydromorphic palaeosols were analyzed from cores obtained from research boreholes in distalareas, considering the lack of exposed stratigraphy across the extensive and flat megafansurfaces. Partial avulsions are frequent in the Holocene river belts in middle/distal areas of themegafans, where large, floodplain-filling splays dominate (overbank processes). The acceleratedexpansion in deforestation and agricultural land use on the Chaco megafans enhanced their floodvulnerability under climate change. Hence, understanding the landscape dynamics of the ChacoPlain is fundamental to risk mitigation strategies.