GEOCHRONOLOGY OF THE SARMIENTO FORMATION AT GRAN BARRANCA AND ELSEWHERE IN PATAGONIA: CALIBRATING MIDDLE CENOZOIC MAMMAL EVOLUTION IN SOUTH AMERICA

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

La Plata

Congreso; XVI Congreso Geológico Argentino, Simposio Gran Barranca; 2005

GEOCHRONOLOGY OF THE SARMIENTO FORMATION AT GRAN BARRANCA AND ELSEWHERE IN PATAGONIA: CALIBRATING MIDDLE CENOZOIC MAMMAL EVOLUTION IN SOUTH AMERICA R.H. Madden1, Bellosi, E.2, Carlini, A.A.3, M. Heizler4, J.J. Vilas5, G.H. Re5, R.F. Kay 1 and M.G. Vucetich3 1 Dept. Biological Anthropology & Anatomy, Duke Univ. Medical Center, Durham, NC (USA) 2Museo Argentino de Ciencias Naturales “B. Rivadavia”, Angel Gallardo, Buenos Aires. 3Facultad de Ciencias Naturales y Museo, Paseo del Bosque, 1900 La Plata 4New Mexico Bureau of Mines & Mineral Resources, New Mexico Tech, Socorro, NM (USA) 5Lab. de Paleomagnetismo, Facultad de Ciencias Exactas y Naturales, Univ. de Buenos Aires, Buenos Aires (Argentina) 40Ar/39Ar dating results for twenty-one tuffs and five basaltic lavas at Gran Barranca and type faunas for Vacan and Deseadan faunas in the Sarmiento Formation in Chubut and Santa Cruz provinces Argentina are within analytical error and with few exceptions consistent with stratigraphic superposition. Volcanigenic sediments evidence two periods of intense pyroclastic activity, one between 36.5 and 39 Ma and another between 19 and 20 Ma. An important episode of basaltic activity occurred between 28.2 and 23.7 Ma. Numerous tuffs within Barrancan levels in the Gran Barranca Member suggest this interval range in age from between 41.0 to 38.2 Ma. The Mustersan interval in the lower Puesto Almendra Member ranges in age from between 38.0 and 36.4 Ma, dating a Mustersan assemblage. The Eocene-Oligocene transition occurs within the Vera Member (with the epoch boundary just below the “Astraponoteen plus Superieur” level of Ameghino. This interval represents all or part of Chrons C15n, C15r and C13n. Deseadan taxa occur in association with the basalt complex dated to between 28.1 and 27.4 Ma. The fossil zone corresponding to the Colhuehuapian Land-Mammal Age spans the complete interval of Chron C6n (20.131 to 19.048 Ma), and the uppermost fossil zone at Gran Barranca or Pinturan is in Chron C5En (18.281 to 18.781 Ma).