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Abstract: 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 25.93Ma, while for the Centinela Formation the ages ranged between 21.24 and 26.38Ma. The 40Ar/39Ar analysis of a sample of the Centinela Formation yielded an age of 20.48±0.27Ma. 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. [Copyright &y& Elsevier]