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This thesis examines a late Miocene to Pliocene sedimentary succession in the Lauca Basin, a topographically isolated, fluvio-lacustrine basin in the northern Chilean Altiplano. A sedimentary section within the Lauca Basin, composed of Lauca Formation sediments, was investigated using stratigraphic and petrographic analyses to define the Miocene to Pliocene paleoenvironmental history of the region. Geochronologic results indicate the section ranges in age from approximately 5.57 +/- 0.20 Ma to 5.44 +/- 0.16 Ma. The sedimentology of 36 samples was described, and twelve samples were analyzed petrographically. Five lithofacies were characterized: laminated mudstone (Fl), volcaniclastic mudstone (Fv), carbonate (C), evaporite (E), and muddy sandstone (SF). These lithofacies were used to characterize sedimentary structures and interpret depositional environments. These ranged from deposition from suspension in a moderate to deep lacustrine environment (Fl and Fv); deposition and precipitation of carbonates and evaporites in closed, possibly ephemeral environment (C and E); and deposition in a shallow high-energy lacustrine environment (SF). The dominant presence of evaporites and the dominantly laminated sedimentary units throughout the section suggest a closed lacustrine environment, and are indicative of seasonal or periodic changes in sediment supply. The evolution of the sedimentary section is inferred from the depositional environments combined with prior studies of the Miocene-Pliocene transition to produce a regional history at higher temporal resolution. The evolution of the Lauca Basin was influenced strongly by evaporation and precipitation, with significantly lesser influences of volcanism, tectonism, and sedimentary supply. The bulk of sediments deposited in the sedimentary section were from an arid, saline, occasionally ephemeral lacustrine environment.
Advisor: Sherilyn Fritz