Evolution and geochemical signatures in a Neogene forearc evaporitic basin: the Salar Grande (Central Andes of Chile)

Abstract

The Salar Grande is a 45-km-long (N–S axis), 4–5-km-wide, fault-bounded (pull-apart) Neogene forearc evaporitic basin, located in the Cordillera de la Costa of northern Chile whose sedimentary record is composed almost exclusively of a massive halite salt body. Underground waters coming from the east (now the Precordillera and Altiplano) were progressively enriched in solutes by interaction with volcanic rocks. These fluids drained into a lacustrine system located in the Depresión Central (in the Llamara–Quillagua area) and thick sequences of diatomites, carbonates, evaporites and clastic sediments were deposited. These lacustrine areas eventually dried out and became a salar with anhydrite and halite. During this stage (Late Miocene–Pliocene), evolved brines of the Llamara–Quillagua area migrated eastwards through structural paths, reaching the Salar Grande basin which acted as a final sink. A proof of this connection is the anhydrite level that links the western Llamara and southeastern Salar Grande areas. The accessory minerals and geochemical signatures in the rock salt are a record of the inputs arriving to the former lake and reflect variations in climate, and in tectonic and volcanic activity. The arrangement of halite textures along the sequence shows an evolution from a very shallow ephemeral saline lake (some cm–dm deep on average) to a salar where interstitial processes controlled by phreatic brines dominate. This evolution is coherent with a slight increase in the terrigenous content of the rock salt, and documents the increasing aridity of the climate as a response to the tectonic uplift of the Andean Ranges. In Pliocene times pore brines were definitively lost as a response to the downfall of the water table generated by the beginning of the exorheism due to the opening of the Loa River canyon towards the sea. The aim of this paper is to describe the Salar Grande through the study of its geological setting and its salt record. A detailed description of the salar will enable us to reach valuable conclusions about environmental conditions in the area during the Neogene, prior to the opening of the hydrologic system to the Pacific Ocean.