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Large fan-shaped sediment distributary systems (103 – 105 km2), typically referred to as fluvial megafans, are found proximal to topographic barriers within terrestrial basins. Concepts regarding their formation have been focused on water-dominated processes associated with monsoonal climates, high sediment bedload, and flow rate. But are there other processes behind the erratic avulsive channel behavior of some of the largest fans on the planet? This study presents remotely mapped geomorphic observations from megafans including playa lakes, dunes, vegetation patterns, and basin elevation profiles of fans. Corroborating these concepts are field observations drawn from the Chaco Plain of Argentina. Active tectonics set the stage for multiple monsoon-affected eastward-flowing fluvial systems to interact with the northerly prevailing winds from the South American lower-level jet. The Chaco megafans exhibit low overall slope and limited drainage integration creating playas in both abandoned channels within the active fluvial belt and small depressions on the loess-covered floodplains. Chaco forebulge stratigraphy shows that distal from the main fluvial belt multiple loess-paleosol sequences are present up to four meters below the surface. This suggests that where subsidence outpaces fluvial avulsion rates, fine-grain wind-blown detritus aggrades in packages defined by soil profiles. Loessic paleosols are also located proximal to the orographic front, where the development of uplands relative to modern/recent fluvial incision protects the eolian sediments from erosion. Varying temporal cyclicity in aridity and seasonality of precipitation determines the climatic regime that the megafan sediments experience, resulting in megafans covered in complex fabrics of slope-oriented fluvial/pluvial features superimposed on wind-oriented eolian features and vice versa. The interaction of these depositional environments in the Chaco foreland basin highlights the smoothing effect of eolian transport and deposition, which may allow for high-avulsion rates that enhance megafan development. These geomorphic features can be observed on other globally notable megafan basin systems, highlighting the complexity of sediment transport pathways within a terrestrial realm. 

South America, from southernmost Bolivia through central Argentina, contains a useful Late Miocene to Holocene record of eolian sedimentation that can be used to advance our understanding of atmospheric circulation and dust production pathways over that interval. Our research indicates that loess provinces in the eastern Andes, Chaco Plains, and Pampean Plains had quasi-independent dust production pathways. A summary of our findings is as follows. 1) Detrital zircon crystals in the high-elevation upper Pleistocene loess deposits in the eastern Andes area of Tafí del Valle were primarily derived from the Puna Plateau to the west. At a latitude of ~27° S, this necessitates a several-degree equatorward shift in the upper- and lower-level westerlies during intervals with high dust accumulation in Tafí del Valle. 2) Upper Pleistocene to Holocene eolian sand deposits of the Pampean Sand Sea and loessic strata in the central and eastern Pampas contain detrital zircon U-Pb age spectra indicating derivation from the Río Desaguadero, Río Colorado, and Río Negro which drain the central Andes. Although the present-day Puna-Altiplano Plateau is hyperarid, the presence of major Argentine river systems in the dust production pathways of the Pampas is important for identifying the relative importance of precipitation and river courses on dust production, which parallels the relationship between the Yellow River and Chinese Loess Plateau in East Asia. 3) Upper Miocene strata of the Cerro Azul Formation, deposited between ~8.9 and ~5.5 Ma, include loess and aggradational paleosols. These eolian strata yield detrital zircon U-Pb age spectra that are consistent with the present-day Río Colorado and Río Negro, and similar to the Upper Pleistocene to Holocene deposits of the Pampas. This suggests a Late Miocene establishment of the Pampean eolian system. Interestingly, the Pampean eolian system and Chinese Loess Plateau both cover the same latitudes (~33°-39°) but in different hemispheres, and both were established at roughly the same time during the Late Miocene. These observations point to bihemispheric intensification of Hadley circulation in forcing the establishment of these two large eolian provinces.