A New Chronology for Middle Eocene-Early Miocene South American Land Mammal Ages

Abstract

Cenozoic South American Land Mam- mal Ages (SALMAs) have historically been correlated to the geologic time scale using 40 Ar/ 39 Ar dating and magnetostratigraphy. At Gran Barranca (68.7°W, 45.7°S)—one of South America's key areas for constraining SALMAs—existing radioisotopic ages have uncertainties of up to 4 m.y. To better con- strain the ages of mammalian assemblages, we employed high-precision (±<40 k.y.) U-Pb dating using single zircon crystals. We dated nine tuffs from the Sarmiento Formation containing middle Eocene-early Miocene faunas (Barrancan, Mustersan, Tinguiriri- can, Deseadan, Colhuehuapian, and "Pin- turan"). The new dates span from 39.861 ± 0.037 Ma to 19.041 ± 0.027 Ma. The La Can- cha Tuff, occurring within the Tinguirirican faunal level yielded an age of 33.581 ± 0.015 Ma, confi rming that the Vera Member con- tains the only fossiliferous geologic section encompassing the Eocene-Oligocene transi- tion in the Southern Hemisphere. The pre- Deseadan fauna, La Cantera, is ≤30.77 Ma, the age of the Colhuehuapian is expanded to 21.1-20.1 Ma, and the Pinturan may be as old as ca. 19 Ma. The new U-Pb dates confi rm that at- mospheric temperatures and vegetation remained constant across the Eocene- Oligocene transition in Patagonia and that hypsodonty occurred in South American un- gulates much earlier than on any other conti- nent. Additionally, refi nement of the SALMA boundaries will eventually provide the con- text necessary to compare faunal transitions across continents, although currently too much data are missing to allow such compar- isons. Finally, the new ages provide a high- resolution age model from which hypotheses about rates of environmental and evolution- ary change at Gran Barranca can be tested.