AGE AND PALEOGEOGRAPHY OF THE MARINE TRANSGRESSIONS AT THE PALEOGENE-NEOGENE BOUNDARY IN PATAGONIA, SOUTHERN ARGENTINA

Atlantic transgressions in southern Patagonia across the Paleogene-Neogene boundary resulted in deposition of richly fossiliferous marine sediments grouped under the informal name of “Patagoniano” (Patagonian). These occur along the present-day Atlantic coast and the foothills of the Andes but are separated by broad expanses of grassland, making correlations difficult. Modern estimates of the age of “Patagonian” rocks range from middle Eocene to early Miocene. Along the eastern seaboard, they are included in the San Julián and Monte León formations. In the west, these rocks are referred to the Centinela Formation. Sixteen measurements of ⁸⁷Sr/⁸⁶Sr in the oyster Crassostrea? hatcheri (Ortmann, 1897) from the San Julián and Centinela formations, and an ⁴⁰Ar/³⁹Ar date from a whole-rock sample from the Centinela Formation, yield ages for these units, enabling correlation between the two areas and more precise interpretation of relative sea-level changes and paleoenvironmental conditions during the Paleogene-Neogene interval. ⁸⁷Sr/⁸⁶Sr measurements from the San Julián Formation yielded ages between 23.83 Ma and 25.93 Ma. ⁸⁷Sr/⁸⁶Sr measurements from the Centinela Formation resulted in ages between 21.24 Ma and 26.38 Ma. The ⁴⁰Ar/³⁹Ar analysis of a sample of the Centinela Formation yielded an age of 20.48±0.27 Ma. Mollusks and decapod crustaceans present in the formations were identified; the number and percentage of common species was calculated and compared with one another. The age data suggest deposition of the San Julián Formation along the Atlantic coast and the lowermost beds of the Centinela Formation in its northernmost exposures during the late Oligocene (Chattian). A second transgression, at the end of the Oligocene and early Miocene (Chattian-Burdigalian), resulted in deposition of the Monte León Formation along the coast, all of the Centinela Formation in the southwestern localities, and the middle and upper part of the Centinela Formation in its northernmost localities. The faunal affinities between the Centinela Formation at the southernmost localities, the middle and upper Centinela Formation in the northernmost localities, and the Monte León Formation support this correlation. Thus, the age of “Patagonian” rock units and limits of Oligocene-Miocene paleogeography are refined.