A geochronology for the Sarmiento Formation at Gran Barranca

RE G., E. BELLOSI, M. HEIZLER, J. VILAS, R. MADDEN, A. CARLINI, R. KAY & M. VUCETICH

The Paleontology of Gran Barranca: Evolution and Environmental Change through the Middle Cenozoic of Patagonia, eds. R. Madden, A. Carlini, M. Vucetich & R. Kay

Cambridge University Press

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

The Sarmiento Formation (middle Eocene – earlyMiocene) is a terrestrial pyroclastic succession in centralPatagonia broadly recognized by its abundant fossilmammals which comprise the standard succession forthe South America mammal record. Age calibration of eachsubdivision of the formation and the internal discontinuitieshave been controversial and in need of better resolution.Until now, few radiometric dates were available.Age calibration of the Sarmiento Formation wasobtained at the type locality at Gran Barranca in southcentralChubut Province where the most continuous exposuresoccur. This age calibration integrates information fromAr/Ar dates on plagioclase and volcanic glass mineralseparations from ten levels in all six members and fivemagnetic polarity sequences at different stratigraphic profiles.With this information, ten bounding surfaces withdifferent origins and geometries were correlated along theexposure. The Sarmiento Formation transitionally overliesthe Koluel-Kaike Formation. At the top of Gran Barranca isa poorly preserved remnant of the marine Chenque Formation.There are six members of the Sarmiento Formation atGran Barranca containing seven distinct mammal faunas.The Gran Barranca Member yields Barrancan age fossilsand spans the temporal interval from 41.6 to 39.0Ma or lateLutetian–Bartonian (Chron C19r to C18n.1n). The RosadoMember includes both transitional Barrancan–MustersanandMustersan age faunas and spans the interval from about38.4 to 38.0 Ma or late Bartonian (Chron C18n.1n). TheLower Puesto Almendra Member contains Mustersan agefossils between 37.3 Ma and 36.58 Ma or early Priabonian(Chron C17n). The Vera Member includes Tinguiriricanage mammals and spans the interval between 35.0 Ma and33.3Ma or Priabonian to early Rupelian (Chron C15n toC13n). Tinguirirican age assemblages within this memberare between 33.3 and 33.7 Ma. The Upper Puesto AlmendraMember contains a pre-Deseadan age fauna (GVB-19“La Cantera”) and poorly sampled faunules of more typicalDeseadean aspect. The member comprises three units andspans an interval as old as 31.1 Ma to as young as 24.2 Maor late Rupelian–Chattian (Chron C12n to C7n). GVB-19 isbetween 31.1 and 29.5 Ma, and the Deseadan assemblagesare between 29.2 Ma and 26.3 or younger in age. TheColhue-Huapi Member with Colhuehuapian and Pinturanfaunal zones spans the interval between 20.4Ma and18.7Ma or late Aquitanian–Burdigalian (C6An.1r to ChronC6n). The Lower Fossil Zone of the Colhue-HuapiMembercontaining Colhuehuapian mammals is 20.0 Ma to 20.4Maand the Upper Fossil Zone containing Pinturan mammals isbetween 18.7 and 19.7Ma. Primary and secondary pyroclastic sedimentation spansnearly 23 million years in the vicinity of Gran Barranca.During this interval, 8.4 m.y. are not preserved mainlybecause of significant erosion events and subordinately bynon-deposition and soil formation. Temporal hiatuses betweenmembers increase from 0.7 m.y in the middle Eoceneto 3.48 m.y. in the early Miocene, probably in response toprogressively increasing tectonic and landscape instability.