A geochronology for the Sarmiento Formation at Gran Barranca

RÉ G.H.,; MADDEN R.H.; CARLINI AA; BELLOSI, EDUARDO; M. HEIZLER; VILAS, JF; KAY, RF; VUCETICH M.G.,

The Paleontology of Gran Barranca: Evolution and Environmental Change through the Middle Cenozoic of Patagonia”, Madden, R.H., Carlini, A.A., Vucetich, M.g. and Kay R.F. (eds).

CAMBRIDGE UNIVERSITY PRESS

Lugar: Cambridge; Año: 2010; p. 46 - 60

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 39.0Ma or late Lutetian–Bartonian (Chron C19r to C18n.1n). The Rosado Member includes both transitional Barrancan–Mustersan andMustersan 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 33.3Ma 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