Sedimentology and stable isotopes from a lacustrine-to-palustrine limestone deposited in an arid setting, climatic and tectonic factors: Miocene–Pliocene Opache Formation, Atacama Desert, Chile

Field relations, petrography, GIS, and geochemistry of a Late Miocene–Pliocene limestone, the Opache Formation, Calama Basin, Atacama Desert, Chile shows that depositional facies, diagenetic histories, and isotopes differ from eastern to western parts of the basin. We recognize two facies; lacustrine, characterized by ostracod-diatom mudstone and wackestone, and palustrine, characterized by both shallow water and pedogenic features. GIS analysis of the basin topography reveals an intrabasinal low in the eastern area and steeper hydrological gradients to the west, with the “Calama constriction”, restricting flow between east and west. Petrography and geochemical results confirm that the Calama constriction influenced depositional parameters, ponding groundwater to the east, forming a shallow lake and creating palustrine conditions to the west. Our analyses indicate that lake waters were less enriched in 13C or 18O than palustrine waters even though petrologic studies show that the lacustrine carbonates have a complex diagenetic history, derived from groundwater-rich in dissolved minerals. This seemingly contrary observation can be reconciled if an important source of water for the western carbonates was sourced from the Pacific Ocean and was less evolved, whereas eastern limestones precipitated from water which had become depleted in 18O during transport from the east. Today, moisture from the Pacific very rarely, if ever, reaches inland as far as Calama, indicating that during the Late Miocene–Mid-Pliocene breakdown in the temperature inversion due to weakened or absent upwelling in the cold coastal Pacific current allowed moisture from the Pacific to reach inland. This changing atmospheric circulation pattern may have also affected upper level airflow that today blocks eastern sourced moisture from crossing the Andes, allowing it to reach farther westward into the Atacama region during the Late Miocene–Pliocene. Together, these moisture sources were sufficient to produce carbonate-rich palustrine and lacustrine environments in the Calama Basin.