Early Miocene Sr-geochronology of Patagoniense marine units from southern Argentina: Implications for molluscan assemblages and interbasinal correlation

Early geological investigations in southern South America addressed the Oligocene-Miocene Patagoniense deposits, a fossiliferous marine succession distributed in several basins that had been flooded by three different transgressive events. Chronological calibration and correlation of these transgressive events, lithological units and fossil assemblages through the Patagonian basins still need further definitions in some areas. To tackle this problem, new Sr-isotope ages of pectinid shells from four early Miocene key sections located at the coastal sector of Patagonia, were obtained to improve correlations among the Magallanes-Austral (MAB), San Jorge (SJB) and Mazarredo (MB) basins. A bed in the middle part of the Punta Guanacos section (MAB) was dated in a mean age of 19.8 Ma allowing its correlation with the lower section of the Monte Leon Formation. In the SJB, the lowermost bed of the Chenque Formation at Punta Borja has an age of 19.6 Ma, while lower bed of Sequence 2 of this unit has an age of 17.7 Ma at Punta Delgada. Patagoniense deposits exposed along the cliffs of the southern San Jorge Gulf (MB), and overlying the continental Sarmiento Formation, are recognized herein as the Mazarredo Formation with a mean age of 20.9 Ma at Canadón El Lobo. It is younger than the San Julián Formation and correlates with the lower part of the Monte León Formation at Playa La Mina (MAB). The radiometric age of the Mazarredo Formation is of outstanding importance because the Mazarredo area constitutes a geographic link between the MAB and SJB and precludes the presence of the “Juliense” transgression in central Patagonia as had been proposed, supporting the early Miocene age previously provided by the authors for the Jorgechlamys centralis –Reticulochlamys borjasiensis Molluscan Assemblage. This assemblage is recorded in the Mazarredo Formation and in the lower section of the Chenque Formation (SJB). Moreover, faunal similarity with the Monte León Formation allows the identification of the Reticulochlamys zinsmeisteri - Struthiolarella patagoniensis - Pleuromeris cruzensis Assemblage at Punta Guanacos in the northeastern sector of the MAB. •Four new Sr-ages for early Miocene marine beds of central Patagonia are calculated.•Marine rocks exposed at southern San Jorge Gulf are named Mazarredo Formation (MF).•The MF is dated at 20.9 Ma, precluding the expansion of the Juliense sea in central Patagonia.•The JR Molluscan Assemblage lasted from 20.9 Ma to 17.7 Ma in the S