A geochronology for the Sarmiento Formation at Gran Barranca

Abstract

The Sarmiento Formation (middle Eocene – early Miocene) is a terrestrial pyroclastic succession in central Patagonia broadly recognized by its abundant fossil mammals which comprise the standard succession for the South America mammal record. Age calibration of each subdivision of the formation and the internal discontinuities have been controversial and in need of better resolution. Until now, few radiometric dates were available. Age calibration of the Sarmiento Formation was obtained at the type locality at Gran Barranca in southcentral Chubut Province where the most continuous exposures occur. This age calibration integrates information from Ar/Ar dates on plagioclase and volcanic glass mineral separations from ten levels in all six members and five magnetic polarity sequences at different stratigraphic profiles. With this information, ten bounding surfaces with different origins and geometries were correlated along the exposure. The Sarmiento Formation transitionally overlies the Koluel-Kaike Formation. At the top of Gran Barranca is a poorly preserved remnant of the marine Chenque Formation. There are six members of the Sarmiento Formation at Gran Barranca containing seven distinct mammal faunas. The Gran Barranca Member yields Barrancan age fossils and spans the temporal interval from 41.6 to 39.0 Ma or late Lutetian–Bartonian (Chron C19r to C18n.1n). The Rosado Member includes both transitional Barrancan–Mustersan and Mustersan age faunas and spans the interval from about 38.4 to 38.0 Ma or late Bartonian (Chron C18n.1n). The Lower Puesto Almendra Member contains Mustersan age fossils between 37.3 Ma and 36.58 Ma or early Priabonian (Chron C17n). The Vera Member includes Tinguirirican age mammals and spans the interval between 35.0 Ma and 33.3 Ma or Priabonian to early Rupelian (Chron C15n to C13n). Tinguirirican age assemblages within this member are between 33.3 and 33.7 Ma. The Upper Puesto Almendra Member contains a pre-Deseadan age fauna (GVB-19 “La Cantera”) and poorly sampled faunules of more typical Deseadean aspect. The member comprises three units and spans an interval as old as 31.1 Ma to as young as 24.2 Ma or late Rupelian–Chattian (Chron C12n to C7n). GVB-19 is between 31.1 and 29.5 Ma, and the Deseadan assemblages are between 29.2 Ma and 26.3 or younger in age. The Colhue-Huapi Member with Colhuehuapian and Pinturan faunal zones spans the interval between 20.4 Ma and 18.7 Ma or late Aquitanian–Burdigalian (C6An.1r to Chron C6n). The Lower Fossil Zone of the Colhue-Huapi Member containing Colhuehuapian mammals is 20.0 Ma to 20.4 Ma and the Upper Fossil Zone containing Pinturan mammals is between 18.7 and 19.7 Ma. Primary and secondary pyroclastic sedimentation spans nearly 23 million years in the vicinity of Gran Barranca. During this interval, 8.4 m.y. are not preserved mainly because of significant erosion events and subordinately by non-deposition and soil formation. Temporal hiatuses between members increase from 0.7 m.y in the middle Eocene to 3.48 m.y. in the early Miocene, probably in response to progressively increasing tectonic and landscape instability.