Correlation of marine beds based on Sr- and Ar-date determinations and faunal affinities across the Paleogene/Neogene boundary in southern Patagonia, Argentina

The San Julián and Monte León formations ("Patagonian") are exposed along the Atlantic coast of Patagonia, whereas in the west equivalent rocks are known as Centinela Formation. Sixteen 87Sr/86Sr measurements on the oyster Crassostrea? hatcheri (Ortmann) from the San Julián and Centinela formations and an 40Ar/39Ar-date from a whole-rock sample from the Centinela Formation yielded ages that allow more precise correlation between the two areas. 87Sr/86Sr measurements from the San Julián Formation yielded ages between 23.83 and 25.93 Ma, while for the Centinela Formation the ages ranged between 21.24 and 26.38 Ma. The 40Ar/39Ar analysis of a sample of the Centinela Formation yielded an age of 20.48 ± 0.27 Ma. The age data suggest a late Oligocene (Chattian) age for the San Julián Formation and the lowermost beds of the Centinela Formation (northernmost exposures). The Monte León Formation along the East coast and the entire section of the Centinela Formation in the southern area - and the middle and upper beds of this same unit in the northernmost localities - were deposited at the end of the Oligocene and early Miocene (Chattian-Burdigalian). The invertebrate fauna present in these units shows diverse preservation patterns that makes these fossils, especially the mollusks, not useful - at this stage - for correlation purposes.Fil: Parras, Ana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Ciencias de la Tierra y Ambientales de La Pampa. Universidad Nacional de La Pampa. Facultad de Ciencias Exactas y Naturales. Instituto de Ciencias de la Tierra y Ambientales de La Pampa; ArgentinaFil: Griffin, Miguel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de La Pampa. Facultad de Ciencias Exactas y Naturales; ArgentinaFil: Feldmann, Rodney. Kent State University; Estados UnidosFil: Casadio, Silvio Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigación en Paleobiología y Geología; Argentina. Universidad Nacional de La Pampa; ArgentinaFil: Schweitzer, Carrie. Kent State University; Estados UnidosFil: Marenssi, Sergio Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Ministerio de Relaciones Exteriores, Comercio Interno y Culto. Dirección Nacional del Antártico. Instituto Antártico Argentino; Argentin