New age constraints for the Salamanca Formation and lower Río Chico Group in the western San Jorge Basin, Patagonia, Argentina: Implications for cretaceous-paleogene extinction recovery and land mammal age correlations

The Salamanca Formation of the San Jorge Basin (Patagonia, Argentina) preserves critical records of Southern Hemisphere Paleocene biotas, but its age remains poorly resolved, with estimates ranging from Late Cretaceous to middle Paleocene. We report a multi-disciplinary geochronologic study of the Salamanca Formation and overlying Río Chico Group in the western part of the basin. New constraints include (1) an 40Ar/39Ar age determination of 67.31 ± 0.55 Ma from a basalt flow underlying the Salamanca Formation, (2) micropaleontological results indicating an early Danian age for the base of the Salamanca Formation, (3) laser ablation HR-MC-ICP-MS (high resolutionmulti collector-inductively coupled plasmamass spectrometry) U-Pb ages and a highresolution TIMS (thermal ionization mass spectrometry) age of 61.984 ± 0.041(0.074) [0.100] Ma for zircons from volcanic ash beds in the Peñas Coloradas Formation (Río Chico Group), and (4) paleomagnetic results indicating that the Salamanca Formation in this area is entirely of normal polarity, with reversals occurring in the Río Chico Group. Placing these new age constraints in the context of a sequence stratigraphic model for the basin, we correlate the Salamanca Formation in the study area to Chrons C29n and C28n, with the Banco Negro Inferior (BNI), a mature widespread fossiliferous paleosol unit at the top of the Salamanca Formation, corresponding to the top of Chron C28n. The diverse paleo botanical assemblages from this area are here assigned to C28n (64.67- 63.49 Ma), ̃2-3 million years older than previously thought, adding to growing evidence for rapid Southern Hemisphere floral recovery after the Cretaceous-Paleogene extinction. Important Peligran and "Carodnia" zone vertebrate fossil assemblages from coastal BNI and Peñas Coloradas exposures are likely older than previously thought and correlate to the early Torrejonian and early Tiffanian North American Land Mammal Ages, respectively.Fil: Clyde, William C.. University of New Hampshire; Estados UnidosFil: Wilf, Peter. State University of Pennsylvania; Estados UnidosFil: Iglesias, Ari. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; ArgentinaFil: Slingerland, Rudy L.. University of Pennsylvania; Estados UnidosFil: Barnum, Timothy. University of New Hampshire; Estados UnidosFil: Bijl, Peter K.. University of Utrecht; Países BajosFil: Bralower, Timothy J.. State University of Pennsylvania; Estados UnidosFil: Brinkhuis, Henk. University of Utrecht; Países BajosFil: Comer, Emily E.. State University of Pennsylvania; Estados UnidosFil: Huber, Brian T.. Smithsonian Institution; Estados UnidosFil: Ibañez Mejia, Mauricio. University of Arizona; Estados UnidosFil: Jicha, Brian R.. University of Wisconsin; Estados UnidosFil: Krause, Javier Marcelo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Museo Paleontológico Egidio Feruglio; ArgentinaFil: Schuet, Jonathan D.. State University of Pennsylvania; Estados UnidosFil: Singer, Bradley S.. University of Wisconsin; Estados UnidosFil: Raigemborn, María Sol. Universidad Nacional de La Plata; ArgentinaFil: Schmitz, Mark D.. University of Idaho; Estados UnidosFil: Sluijs, Appy. University of Utrecht; Países BajosFil: Zamaloa, María del Carmen. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ecología, Genética y Evolución; Argentin