A modern subtropical playa complex: Salina de Ambargasta, central Argentina

摘要

Salina de Ambargasta is a playa located at mid latitudes in central Argentina (29 °S-64 °W). Because of its hydrological behaviour, this playa complex can be subdivided into a closed system "sensu-stricto" and an "open-like system" due to the presence of a seasonal outflow. Geomorphological and sedimento-logical features enable the separation of the Ambargasta playa into well-defined western, eastern and northern zones, where aeolian processes, groundwater supply and surface inflow, respectively, define distinctive sedimentary environments and typical processes of deposition. The following depositional settings were recognized: (1) alluvial fan; (2) sandflat; (3) springs; (4) dunes and palaeo-dune field; (5) dry mudflat; (6) capillary mudflat; (7) ephemeral saline lake (includes saline mudflat and salt pan). The dry mudflat is present at the highest topographical zone, where the groundwater influence is less important and only intermittent ponds are present. In the capillary mudflat, evaporite sedimentation is limited to the development of efflorescence by evaporative pumping. The ephemeral saline lake, placed in the lowermost topographical region, alternates cycles of lake expansion and contraction responding to inter-annual hydrological variability as well as to seasonal variability. Lake expansion takes place during early austral summer (December-March), while continuous evaporation of brine leads to the growth of halite crystals during late summer. Sodium chloride-type brines result from both chemical fractionation as evaporation increases and salt dissolution. The isotopic compositions of surficial and underground waters (δ~(18)O and δ~2H) indicate that lake waters become isotopically enriched during summer when evaporation plays a significant role in the playa complex.