STRATIGRAPHY, GEOMORPHOLOGY, AND GEOCHRONOLOGY OF LATE PLEISTOCENE TO MIDDLE HOLOCENE EOLIAN DEPOSITS IN THE CHACO PLAINS OF ARGENTINA

Upper Pleistocene to Holocene eolian sediments within the Chaco Plains in central South America present a unique opportunity to examine a monsoon-driven eolian system. The Chaco Plains cover over 800,000 km² within the greater retroarc foreland basin of the Andean Orogeny. The low relief of the Chaco plains formed by alluvial megafans of the Ríos Salado, Bermejo, Pilcomayo, and Parapetí. Present day, this region is covered by dense vegetation and contains playa lakes, which commonly form during wet intervals due to limited internal drainage of the basin. Previously generated detrital zircon data suggest that the eolian material in the Chaco Plains was derived directly from the Puna Plateau (to the west-northwest) or from the rivers that drain that region. New field data characterizing stratigraphic relationships was combined with wind-indicators from geomorphic landforms within northern and central Argentina to better define the transport and preservation mechanisms for eolian sediments in this region. Stabilized parabolic dunes and playa lakes were mapped using surficial imagery and digital elevation models. The geometries of these features were used to generate an effective paleo-surficial wind coverage map. The relic landforms and unit thicknesses support a northernly wind direction for most of the Argentine Chaco and suggests that the rivers acted as an eolian source. In addition, the average particle-size (coarse silt to fine sand) casts doubt on substantial input from the distant Puna Plateau. Rather than having a single phase of wind transport dispersing sediments, the eolian system in the Chaco appears to operate by repeated re-entrainment and remobilization of previously deposited eolian sediments. During wet intervals, runoff washes sediment into the playa lakes and the ponded water kills vegetation. Once the lakes evaporate, the desiccated sediments are well suited for eolian transport. However, foliage downwind may have captured wind-blown materials, setting the stage for the next wet/dry cycle. This monsoon-driven playa-eolian system allows for movement of eolian material across the landscape to the south. The low relief of the Chaco allows for eolian material to be spread to distal portions of the poly-megafan system over relatively long periods of time.