Modern Depositional Dynamics of Saline Playas Associated with the Pampa Plain

Abstract

The region associated with the Pampa plain of Argentina is occupied by intermontane basins, currently containing playa systems: Salina de Ambargasta (29° S), Salinas Grandes (30° S) and Salina del Bebedero (33° S). These closed depressions are under a pervasive water deficit condition (-700 mm yr-1) allowing the development of sedimentary subenvironments. Salina de Ambargasta and Salinas Grandes show a clear lateral zonation from alluvial fans (marginal zone) to ephemeral saline lakes (central zone). Sandflats, sandy dunes, and dry-capillary-saline mudflats are developed. The southernmost Salina del Bebedero has a different sedimentary setting, with an extensive ephemeral stream floodplain grading downwards to an ephemeral saline lake, surrounded by marginal sedimentary facies (paleo-shorelines and mudflats). The dune systems are developed over the dry mudflat. Saline waters are classified as sodium-chloride type in each saline playa, due to the constant evaporation. During wet conditions, water chemistry presents total dissolved solids varying between 33 to 55 g L-1. Evaporation process results in the progressive precipitation of carbonates and sulphates. According to the annual precipitation/evaporation balance, the playas undergo seasonal evaporitic sedimentation phases: flooding (mesosaline condition and salt dissolution), evaporative concentration (hypersaline lakes and subaqueous salt precipitation), and desiccation (evolution into salt pans with halite).