LATE MIOCENE-PLIOCENE PALEOENVIRONMENTAL EVOLUTION OF THE LAUCA BASIN, ALTIPLANO, CHILE

The Lauca Basin comprises an intermontane basin located in the westernmost margin of the Andean Altiplano in northern Chile. It is of interest for paleoenvironmental studies due to its unique position at high elevation (averaging ~4100 m) that was likely reached prior to deposition and its continuous sedimentary record that spans from late Miocene to early Pleistocene. Deposition within the basin is concurrent with the global cooling event of the late Miocene to early Pliocene as well as the transition to the warm, temperate climate of the Pliocene. We infer that these global climate changes have resulted in detectable differences in paleoenvironments within the Lauca Basin. To test this hypothesis, we use stratigraphic and sedimentological field observations, stable isotopes from lacustrine carbonates and zircon U-Pb dating to reconstruct the paleoenvironmental history of the basin. We measured an ~150 m thick stratigraphic section that spans ~5.6 Ma to 2.5 Ma. Depositional environments consist of fluvio-lacustrine mudstone and sandstone with minor amounts of evaporite. Stable isotope analysis of lacustrine carbonates indicates that the basin experienced three distinct stages of δ¹⁸O and δ¹³C trends during this timeframe with δ¹⁸O(VPDB) ranging from ~-11‰ to ~0‰ and δ¹³C ranging from ~-10‰ to ~-2.5‰. δ¹⁸O and δ¹³C values remain generally stable with little correlation during the late Miocene to early Pliocene, indicative of stable basin hydrology with open lake and semi-humid to semi-arid conditions. This stable condition is followed by a large positive shift in δ¹⁸O and δ¹³C by as much as ~5.5‰ and ~6‰ respectively, with positive correlation. We infer this transition to reflect increased evaporation in an increasingly arid environment, perhaps the result of closed lake conditions. Finally, an abrupt decrease in δ¹⁸O and δ¹³C by as much as ~6‰ and ~4‰ respectively, marks the return to more negative and more stable δ¹⁸O and δ¹³C values with little correlation that continues until deposition of the ~2.7 Myr Lauca Ignimbrite. This shift is inferred to a indicate a transition back to semi-humid to semi-arid, open lake conditions. Further effort is underway to develop an age model to understand whether these large isotopic shifts correspond with the global climate change events of the late Miocene and Pliocene.