THE PHYSICAL AND CHEMICAL SEDIMENTOLOGY OF TWO HIGH-ALTITUDE ACID SALARS IN CHILE: SEDIMENTARY PROCESSES IN AN EXTREME ENVIRONMENT

摘要

The range of environments that host acid brines has not been fully described. Salar Gorbea and Salar Ignorado are two salars in the Andes Mountains of northeastern Chile with surface and groundwater pH as low as 1.8 and salinity as high as 290 parts per thousand total dissolved solids (TDS). These salars are surrounded by active volcanoes and are hosted by volcaniclastic sediments. Volcanogenic groundwaters are low-temperature brines that range from extremely acid to neutral. These groundwaters are the source of waters fed upward into small pools of surface waters scattered throughout the salars. The extreme water chemistry in this arid setting leads to precipitation of an unusual assemblage of chemical sediments, including gypsum and other Ca and Mg sulfates, alunite, jarosite, halite, hematite, kaolinite, and native sulfur. Winds sculpt the salar surface, forming blowouts and large ripples and dunes. In particular, large gravel devils, as well as other wind phenomena, rework both volcaniclastic grains and chemical sediments, including gypsum crystals up to 27 cm long. Extremes in temperature cause freezing and thawing and result in cracks and buckled and puffed polygons. Microbial mats are associated with both the lowest- and highest-pH surface waters. Early diagenesis in the top few centimeters of sediment forms displacive crystals, cements, and concretions. The combination of volcanogenic setting with the extreme conditions of high elevation, high solar radiation, intense aridity, strong winds, high salinity, and low pH makes these among the most extreme surface environments on Earth. This paper provides a comprehensive documentation of the sedimentary processes and products of these rare acid salars for the first time. In addition, this study compares and contrasts salars Gorbea and Ignorado to acid saline lakes elsewhere in the world. These Chilean settings, with their acid-brine sedimentary and diagenetic minerals, strong winds, and volcanic terrain, may serv