High Precision U/Pb Geochronology of Eocene-Miocene South American Land Mammal Ages at Gran Barranca, Argentina

Constraining the ages and duration of Cenozoic South American Land Mammal Ages (SALMAs) has been based on 40Ar/39Ar dating and magnetic polarity stratigraphy. At Gran Barranca (68.7°W, 45.7°S) - South America’s most important site and sequence for constraining SALMAs -uncertainties of ~ 1 Myr persist. To better constrain the ages of mammalian and plant assemblages and stable isotope stratigraphies, we employed high-precision (±50 kyr) single-crystal zircon U/Pb dating. These results generally confirm previous chronologies, but change the timing or duration of some SALMAs at Gran Barranca by 0.5-1 Myr. We collected 23 tuffs from six members of the Sarmiento Formation that contain 7 successive formally recognized SALMAs spanning the middle Eocene through the early Miocene. These strata include the type faunas for the Barrancan, Mustersan and Colhuehuapian SALMAs. Zircons were separated and chemically treated using standard techniques, spiked with EARTHTIME ET535, and analyzed for U-Pb ratios at Boise State University. Simpson’s Y, a prominent marker tuff within the Barrancan SALMA, yielded a date of ∼40.0 Ma. The Rosado Tuff, in the Rosado Member, contains Mustersan SALMA age mammals and yields a date of ∼38.3 Ma. Two tuffs in the Lower Puesto Almendra Member (Bed 10 and the Kay Tuff, stratigraphically above a Mustersan SALMA mammal assemblage) yielded ages of ∼37.0 and ∼36.9 Ma respectively. The Big Mammal Tuff at the base of the Colhuehuapian SALMA is ∼20.9 Ma, and the MMZ24.5 Tuff between the Colhuehuapian and Pinturan SALMAs is ∼19.1 Ma. Together with published magnetostratigraphy, these U/Pb dates have the following implications: (a) The known duration of the Barrancan SALMA is shortened by ~ 1 Myr and spans 40.5-39.0 Ma, (b) The Mustersan SALMA at Gran Barranca is between ∼38.3 and ∼37.0 Ma, (c) The Colhuehuapian SALMA must fall between ∼20.9 and ∼19.8 Ma, and (d) the fossil levels referred to the Pinturan SALMA are bracketed between ∼19.1 and ∼18.7 Ma. Additional work is planned on tuffs in relation to Tinguirrirican and Deseadan levels at Gran Barranca, in part to better constrain correlations of faunal and floral changes with global climate records.