New age constraints for early Paleogene strata of central Patagonia, Argentina: Implications for the timing of South American Land Mammal Ages

IBAÑEZ-MEJÍA, MAURICIO; BELLOSI, EDUARDO S.; CLYDE, WILLIAM C.; BARNUM, TIMOTHY; KRAUSE, J. MARCELO; SCHMITZ, MARK D.; WILF, PETER

GEOLOGICAL SOCIETY OF AMERICA BULLETIN

GEOLOGICAL SOC AMER, INC

Año: 2017 vol. 129 p. 886 - 903

0016-7606

The Río Chico Group in the San JorgeBasin of central Patagonia (Argentina) preservessome of South America?s most significantPaleogene records of biotic and climaticchange. Three of its constituent formations,the Peñas Coloradas, Las Flores, and Koluel-Kaike, host vertebrate faunas referred to the?Carodnia faunal zone,? the Itaboraian SouthAmerican Land Mammal Age (SALMA),and the Riochican SALMA. However, theprecise absolute ages of these units, and thustheir associatedfaunas and paleoclimate records,are poorly resolved. Herein, we reportnew paleomagnetic and geochronologic resultsfrom these formations in south-centralChubut Province, Argentina. U-Pb dating offour volcanic ashes, using both laser ablation?multicollector?inductively coupled plasma?mass spectrometry and high-resolutionchemical abrasion?isotope dilution?thermalionization mass spectrometry, indicates agesof igneous crystallization of 51.403 ± 0.037(0.045) [0.071] Ma for a level within themiddle Las Flores Formation and 46.714 ±0.012 (0.026) [0.057] Ma, 44.579 ± 0.013(0.026) [0.054] Ma, and 42.856 ± 0.017 (0.027)[0.054] Ma for levels in the lower, middle, andupper Koluel-Kaike Formation, respectively.Combining these with previous isotopic agesin our new magnetostratigraphic framework,we correlate the Peñas Coloradas Formationto chrons C27n-26r (ca. 62.5 to ca. 61.6 Ma;late Danian) and the section from the middleLas Flores to the uppermost Koluel-Kaike tochrons C23n to C19r (ca. 51.4?42.2 Ma; midYpresian?late Lutetian). We combine thesedata with other recently published chronostratigraphicresults from Paleogene unitsin Patagonia to better constrain the ages ofnoteworthy Paleogene plant and mammalfossil sites in Patagonia and to develop a revisedtemporal calibration of the Las Flores,Vacan, and ?Sapoan? faunas.