Loess of southern South America

Abstract

Loess and loessoid (loess-like, reworked loess) sediments extend across the Chaco-Pampean plains and the NW mountain environments of Argentina, and in neighbouring countries (Paraguay, Brazil, Uruguay, Bolivia). Loessoid sediments are much more abundant than primary loess. The beginning of the loessoid sedimentation cycle was related to a phase of Late Miocene orogeny in the Andes. The Plio-Pleistocene record is mostly composed of loessoid sediments modified by pedogenesis, which produced welded palaeosols. The Late Pleistocene/Holocene loess record reveals a heterogeneous composition across the region. Coarse textures and an Andean-derived volcanic composition prevail in the southern Pampas. Finer textures and material coming from the Andes and the other two sources (Sierras Pampeanas and the Paraná basin) characterize the loess deposits of the northern Pampas and the eastern Chaco. A southern Patagonian source is also suggested for the mountain valley loess of Tucumán, although a western Andean provenance has been proposed. It is believed that the material in the western Chaco was derived from the Bolivian Andes. Loess deposition was related to a multistage transport mechanism, involving fluvial and aeolian processes. Inferred westerly and southwesterly wind directions, as dominant carriers of the aeolian deposits, are in agreement with westerly palaeowind simulations using climate models. However, the role played by westerly tropospheric winds and northerly winds remains to be established.