SEDIMENTOLOGY OF TWO ACID SALINE LAKES IN THE HIGH ANDES OF NORTHERN CHILE

Salars Gorbea and Ignorado are acid saline lakes at elevations of 4000 m and 4200 m, respectively, in the hyperarid Atacama Region of northern Chile. These shallow lakes are surrounded by steep alluvial fans composed of volcanic sediments. Approximately 30% of the surface of both salars contain shallow (<~2 m deep) ponds actively precipitating gypsum crystals. The remainder of the salar surfaces are dry and covered by dunes and sandflats of both reworked and effluorescent salts, including abundant gypsum and sulfur. Ponds and shallow groundwaters at Salar Gorbea have pH of 1.8 - 8.5 and salinities of ~3 - 29% TDS. At Salar Ignorado, just upstream from Salar Gorbea, ponds and shallow groundwaters have pH of 4.0 - 4.7 and salinities of 0.5 - 3% TDS. Pink crinkly organic mats exist in the most acidic areas of Salar Gorbea and black organic mats are associated with the neutral waters there. At Salar Ignorado, green algae live on some subaqueous gypsum crystals. Sedimentary processes at the salars are a combination of distal alluvial fan deposition, chemical precipitation by acid saline waters, dissolution and local karst by acid saline waters, and strong reworking of sediments by winds. The topographic relief, volcanic host rocks, and rare macroscopic life initially suggest that these acid salars may be analogous to martian strata. However, the mineralogy, mainly their paucity of iron oxides and jarosite, suggest that they are not the best terrestrial analogs for Mars. These Chilean acid salars also show differences in water geochemistry, mineralogy, sedimentology, and diagenetic features relative to other modern acid saline lakes, confirming that acid saline systems have as much diversity as alkaline saline systems.