PLEISTOCENE SALT DEPOSITION IN THE SALAR DE UYUNI (BOLIVIA)

Salar de Uyuni (Bolivian Altiplano) is the largest salt flat in the world. Lacustrine carbonate rocks representing higher lake-water levels in the basin crop out on the margins of the Salar and on inselbergs within the salt flat. However, these exposures are limited and do not permit a thorough study of the sedimentary infill. Drill cores remain the best method to directly access the sedimentary record that underlies the salt pan. Petrological and mineralogical analysis of a new 460 m drill core has allowed us to identify the following lithofacies: massive halite, banded halite, interstitial halite, gypsum, marls and carbonate, volcanic ash, clays, and red silts. These lithofacies represent a succession of sedimentary environments varying from mud flat, salt pan, perennial shallow saline lake, and perennial freshwater lake environments. The chronology of the core was established by ⁴⁰Ar/³⁹Ar dating of sanidine phenocrysts in seven tuffs intercalated in the section, combined with magnetostratigraphy. The results demonstrate a long and discontinuous sedimentary history from the middle Pliocene to the late Pleistocene. The correlation of this core with two other published cores (121 m and 220 m deep) indicates lateral facies variations in the thickness of the different salt units. The Salar de Uyuni is a complex sedimentary system strongly affected by climatic fluctuation and river discharge, as well by exposure of nearby Mesozoic salt diapirs primarily responsible for the salt input into the basin. This study highlights the discontinuity of the Salar record, the spectacular variations in sedimentation rates during halite deposition, and the sensitivity of this system to paleoclimate.