UPPER MIOCENE PALUSTRINE CARBONATES (OPACHE FORMATION) IN THE CALAMA BASIN, CHILE, AND THEIR PALEOCLIMATIC IMPLICATIONS

The Calama basin, located at ~2400 meters in the Atacama Desert along the western margin of the Andes, contains thick deposits (up to 80m) of freshwater carbonates that date between ~6-3 Ma. These deposits can be traced along the course of the Rio Loa throughout the Calama Basin and into the Pampa del Tamarugal to the west, spanning over 1 km in elevation. In order to assess the paleoclimatic implications of these deposits, we described stratigraphic sections of the Opache Formation at two localities. We determined mineralogy, stable isotope geochemistry (13C, 18O, and 87Sr/86Sr), Ca/Mg ratios, and performed petrographic analysis on collected samples. The Opache Formation consists of bioturbated, low-Mg carbonates with abundant siliciclastics, root voids, fossils, and mud cracks. Petrographic analysis shows abundant rip-up clasts, peloids, ostracode, gastropoda, and ooids. d13C and d18O values of these carbonates range from +4 to +7 ‰ PDB and –6 to –2 ‰ PDB, respectively, exhibit strong covariation, and trend towards more positive values up section. There are no evaporites within the Opache Formation. The Opache Formation was deposited in a palustrine environment that extended from the eastern Calama Basin and continued all the way to the Pampa del Tamugugal in the Central Depression. This system was dominated by broad, shallow, flowing marshes that periodically desiccated. Although small lakes may have been present locally within this hydrologic system, the lack of evaporites and the continuous occurrence of the deposits into the Pampa del Tamuragal indicate a palustrine, rather than a lacustrine, environment. This environment is present today in the Calama Basin, however, the connection of the Rio Loa to the Pacific around 2.5 Ma deeply incised the Rio San Salvador and Rio Loa and confined present-day marshes to mostly within canyons and quebradas. Therefore, wetter conditions than today were not required to produce these extensive palustrine deposits, merely a water table that was emergent across the entire basin.