New Age Constraints for Early Paleogene Strata of Central Patagonia

New age constraints for early Paleogene strata of central Patagonia, Argentina New age constraints for early Paleogene strata of central Patagonia, Argentina: Implications for the timing of South American Land Mammal Ages J. Marcelo Krause1,2,†, William C. Clyde3, Mauricio Ibañez-Mejía4,5, Mark D. Schmitz6, Timothy Barnum3, Eduardo S. Bellosi7, and Peter Wilf8 1CONICET–Museo Paleontológico Egidio Feruglio, Avenida Fontana 140, 9100, Trelew, Argentina 2Departamento de Geología, Universidad Nacional de la Patagonia San Juan Bosco, Ruta Provincial 1 s/n, 9000, Comodoro Rivadavia, Chubut, Argentina 3Department of Earth Sciences, University of New Hampshire, 56 College Road, Durham, New Hampshire 03824, USA 4Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA 5Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA 6Department of Geosciences, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA 7CONICET–Museo Argentino de Ciencias Naturales, Ángel Gallardo 470, 1405 Buenos Aires, Argentina 8Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA ABSTRACT in our new magnetostratigraphic framework, rec ords of regional biotic responses to global we correlate the Peñas Coloradas Formation climatic events (e.g., Raigemborn et al., 2009, The Río Chico Group in the San Jorge to chrons C27n-26r (ca. 62.5 to ca. 61.6 Ma; 2014; Bellosi and González, 2010; Krause et al., Basin of central Patagonia (Argentina) pre- late Danian) and the section from the middle 2010; Dunn et al., 2015; Kohn et al., 2015; serves some of South America’s most signif- Las Flores to the uppermost Koluel-Kaike to Selkin et al., 2015). cant Paleogene records of biotic and climatic chrons C23n to C19r (ca. 51.4–42.2 Ma; mid Currently, the chronostratigraphy for central change. Three of its constituent formations, Ypresian–late Lutetian). We combine these Patagonia is well constrained for both early the Peñas Coloradas, Las Flores, and Koluel- data with other recently published chrono- Paleo cene (Clyde et al., 2014; Comer et al., Kaike, host vertebrate faunas referred to the stratigraphic results from Paleogene units 2015) and middle-late Paleogene (Ré et al., “Carodnia faunal zone,” the Itaboraian South in Patagonia to better constrain the ages of 2010a, 2010b; Dunn et al., 2013) sequences. American Land Mammal Age (SALMA), noteworthy Paleogene plant and mammal However, the middle Paleocene–middle Eocene and the Riochican SALMA. However, the fossil sites in Patagonia and to develop a re- interval, encompassing the Las Flores and precise absolute ages of these units, and thus vised temporal calibration of the Las Flores, Koluel- Kaike Formations, is still poorly re- their asso ciated faunas and paleoclimate rec- Vacan, and “Sapoan” faunas. solved (see also Woodburne et al., 2014b). Prior ords, are poorly resolved. Herein, we report age constraints on these units came mostly from new paleomagnetic and geochronologic re- INTRODUCTION the underlying and overlying formations. Re- sults from these formations in south-central cent magnetostratigraphic and geochronological Chubut Province, Argentina. U-Pb dating of Paleogene continental sequences from the results from the underlying Salamanca–Peñas four volcanic ashes, using both laser abla tion– San Jorge Basin, central Patagonia, Argentina, Coloradas sequences from the Sarmiento area multicollector–inductively coupled plasma– are globally signifcant due to their extraor- constrain the age of the Salamanca Formation to mass spectrometry and high-resolution dinary preservation of South American fossil ca. 65.7–63.5 Ma (early-middle Danian; chrons chemical abrasion–isotope dilution–thermal faunas (e.g., Simpson, 1935a, 1935b, 1948; C29n and C28n, where the time scale of Grad- ionization mass spectrometry, indicates ages Cifelli, 1985; Pascual and Ortiz-Jaureguizar, stein et al. [2012] is used throughout this paper) of igneous crystallization of 51.403 ± 0.037 1990; Bonaparte et al., 1993; Bond et al., 1995; and the Peñas Coloradas Formation to ca. 62.5– (0.045) [0.071] Ma for a level within the Flynn and Swisher, 1995; Ortiz-Jaureguizar 61.6 Ma (latest Danian; Chrons C27n-bottom middle Las Flores Formation and 46.714 ± and Cladera, 2006; Gelfo et al., 2009; Madden C26r; Clyde et al., 2014; Comer et al., 2015). 0.012 (0.026) [0.057] Ma, 44.579 ± 0.013 et al., 2010; Woodburne et al., 2014a, 2014b) The base of the Sarmiento Formation, which (0.026) [0.054] Ma, and 42.856 ± 0.017 (0.027) and foras (Berry, 1937; Romero, 1968; Arch- overlies the Koluel-Kaike Formation, is well [0.054] Ma for levels in the lower, middle, and angelsky, 1973, 1976; Petriella and Archangel- constrained at the Gran Barranca locality (Gran upper Koluel-Kaike Formation, respectively. sky, 1975; Archangelsky and Zamaloa, 1986; Barranca Member) to 42.11–41.70 Ma (latest Combining these with previous isotopic ages Zamaloa and Andreis, 1995; Brea et al., 2005, Lutetian, base of chron C19r) based on mag- 2007, 2008, 2011; Iglesias et al., 2007; Zucol netostratigraphic and radioisotopic geochrono- †mkrause@ mef .org .ar et al., 2010). These sequences also provide logic data from the area (Ré et al., 2010a, 2010b; GSA Bulletin; Month/Month 2016; v. 128; no. X/X; p. 1–18; doi: 10.1130/B31561.1; 8 fgures; 1 table; Data Repository item 2017078.; published online XX Month 2016. For permission to copy, contact [email protected] Geological Society of America Bulletin, v. 1XX, no. XX/XX 1 © 2016 Geological Society of America Krause et al. Dunn et al., 2013, and references cited therein). 1949; Legarreta and Uliana, 1994; Krause et al., Crocodilian coprolites were also described from Two radioisotopic dates were also recently re- 2010; Raigemborn et al., 2010; Comer et al., the Las Flores Formation in the coastal area ported from the Las Flores Formation, which 2015). Our study site, Las Flores, is located on of Pico Salamanca, 4–5 km to the east of Bajo underlies the Koluel-Kaike Formation. One is a the eastern margin of the San Bernardo fold belt Palangana (Krause and Piña, 2012). 40Ar/39Ar date of 49.512 ± 0.019 Ma on alkaline in the San Jorge Basin, southern Argentina (Fig. The Koluel-Kaike Formation, in the feldspars from a tuff in the lower portion of the 1A–1C), at the eastern extreme of the fossilifer- past known as the Argiles Fissilaires facies formation, 20 m above the beds bearing the Las ous Gran Barranca escarpment (Fig. 1C). An ad- (Ameghino, 1906; McCartney, 1934) or “Tobas Flores fauna (Woodburne et al., 2014b). The ditional locality, Cerro Blanco (Fig. 1C), 26 km de Koluel-Kaike” (Feruglio, 1949), is mainly other is a maximum age of sedimentation inter- SW of Las Flores, also was included in order composed of pyroclastic sediments in the form preted from laser ablation–inductively coupled to sample a volcanic tuff from the middle Las of fne tuffs and bentonites (Krause et al., 2010; plasma–mass spectrometry (LA-ICP-MS) U-Pb Flores Formation for isotopic dating. Raigemborn et al., 2010). Conspicuous lenticu- analyses of detrital zircons, which constrained Under the Las Flores Formation sits the Peñas lar bodies with manganese nodules are pres- the bed bearing the Las Flores fauna to be no Coloradas Formation, a fuvial unit up to 42 m ent at the base, recording shallow pond facies older than ca. 47 ± 1 Ma (Kohn et al., 2015). in thickness (e.g., at Estancia la Rosa; Raigem- (Krause et al., 2010). Strongly developed red- These conficting ages confound chronostrati- born et al., 2010) but typically less than 30 m dish paleosols with a moderate to high degree of graphic interpretation of the Las Flores Forma- thick (Krause, 2010). The rocks composing silicifcation are frequent throughout the section tion in this area. the Peñas Coloradas Formation are mainly (Krause et al., 2010). At Las Flores, the Koluel- In this study, we constrain the ages of the medium- dark reddish, cross-bedded sandstones, Kaike Formation exhibits gradational contacts Peñas Coloradas, Las Flores, and Koluel-Kaike related to sinuous channel flling, and subordi- with both the underlying Las Flores Formation Formations using new radioisotopic and paleo- nate mudstones and tuffs deposited in foodplain and the overlying Sarmiento Formation (Bellosi magnetic data in the Las Flores area, which lies settings (e.g., Raigemborn et al., 2009, 2010; and Madden, 2005; Krause et al., 2010, their at the eastern extreme of the Gran Barranca in Comer et al., 2015). According to Bond et al. fgs. 3A and 3B). south-central Chubut Province, Patagonia, (1995) and Woodburne et al. (2014a, 2014b), Fossils are scarce in the Koluel-Kaike For- Argen tina (Figs. 1A–C; Table 1). Our goals are to among others, the Peñas Coloradas Formation mation, but silicifed wood (Brea et al., 2009) develop a well-defned stratigraphic framework contains vertebrate remains from the “Carodnia and cicada trace fossils (Krause et al., 2008) for the San Jorge Basin, so as to better constrain faunal zone,” originally described by Simpson have been recorded. Mammalian remains have the ages of some of the most signifcant records (1935a, 1935b) from Bajo Palangana and Cerro never been found in the Koluel-Kaike Forma- of Paleogene biotas in South America, and to Redondo (Fig. 1B); however, this fauna is prob- tion itself (e.g., Simpson, 1933a). However, establish a chronostratigraphic framework for ably contained within the Las Flores Formation siliciclastic rocks exposed along the Atlantic improved interpretation of the important early (see Discussion). Fossil wood has also been de- coast, correlated with the Koluel-Kaike Forma- Paleogene global climatic events recorded in scribed from the Peñas Coloradas Formation in tion by Legarreta and Uliana (1994), contain South America. Improved chronostratigraphic the area around the town of Sarmiento and at mammal fossils from the Ernestokokenia faunal data allow more precise correlation to other Punta Peligro along the coast (Brea et al., 2004; zone (= Riochican SALMA; Bond et al., 1995).

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