NEW AGE CONSTRAINTS FOR THE FOSSILIFEROUS SALAMANCA FORMATION AND LOWER RIO CHICO GROUP IN THE WESTERN SAN JORGE BASIN, PATAGONIA, ARGENTINA

The Salamanca Formation of southern Argentina preserves one of the most informative Paleocene stratigraphic and paleontological records from South America, yet its precise age remains poorly resolved. Some studies suggest that its base ranges into the Cretaceous, whereas others indicate its top ranges to the Selandian. Given that surface sections of the Salamanca are often less than 50 meters in total thickness, this represents significant chronostratigraphic uncertainty. Understanding the age of the Salamanca is complicated further by the great distances over which it is exposed and its transgressive character, creating the possibility of significant diachroneity. We report a multidisciplinary geochronological study of the Salamanca Formation in the western part of the San Jorge Basin, near Sarmiento city, to further constrain its age and improve comparisons with other fossiliferous exposures farther to the east. New micropaleontological results from planktonic foraminifera, calcareous nannofossils, and dinoflagellates all indicate an early Danian age for the base of the Salamanca. This is further corroborated by a new 40Ar/39Ar age determination from a basalt flow that is reported to lie directly below the Salamanca. The Salamanca Formation is overlain by the Río Chico Group, starting with the “Banco Negro Inferior” (BNI), a mature putatively widespread paleosol unit, and the Peñas Coloradas Formation. New U/Pb ages for zircons from volcanic ash beds in the upper Peñas Coloradas provide a minimum age for the top of the Salamanca Formation, an approximate age for "Carodnian" vertebrates from eastern exposures of the Peñas Coloradas, and a minimum age for vertebrates from eastern exposures of the BNI at Punta Peligro. Paleomagnetic results indicate that the Salamanca is entirely normal polarity, with reversals occurring in the overlying Río Chico units. Given the new micropaleontological and isotopic age constraints, we correlate the Salamanca Formation in the Sarmiento area to Chron C29n and/or C28n. The Banco Negro Inferior is also normal polarity in this area but may represent a younger normal Chron (e.g. C27n), given the likelihood of a hiatus or unconformity at this level. The BNI has been reported to have reverse polarity in areas farther to the east, suggesting it is at least partly time-transgressive.