Journal of South American Earth Sciences 26 (2008) 204–216 Contents lists available at ScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier .com/locate / jsames Correlation of marine beds based on Sr- and Ar-date determinations and faunal affinities across the Paleogene/Neogene boundary in southern Patagonia, Argentina Ana Parras a,b,*, Miguel Griffin a,b, Rodney Feldmann c, Silvio Casadío a,b, Carrie Schweitzer d, Sergio Marenssi b,e a Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa, Uruguay 151, 6300 Santa Rosa, La Pampa, Argentina b CONICET Consejo Nacional de Investigaciones Cientı́ficas y Técnicas, Argentina c Department of Geology, Kent State University, Kent, OH 44242, USA d Department of Geology, Kent State University, Stark Campus, Canton, OH 44720, USA e Instituto Antártico Argentino, Cerrito 1248, Buenos Aires (1010), Argentina a r t i c l e i n f o Article history: Received 22 February 2005 Accepted 10 December 2007 Keywords: Isotopic ages Paleogeography Paleogene/Neogene Patagonia 0895-9811/$ - see front matter � 2008 Elsevier Ltd. A doi:10.1016/j.jsames.2008.03.006 * Corresponding author. Address: Facultad de Ci Universidad Nacional de La Pampa, Uruguay 151, Argentina. Tel.: +54 2954 453943; fax: +54 2954 432 E-mail address: aparras@exactas.unlpam.edu.ar (A a b s t r a c t The San Julián and Monte León formations (‘‘Patagonian”) are exposed along the Atlantic coast of Pata- gonia, whereas in the west equivalent rocks are known as Centinela Formation. Sixteen 87Sr/86Sr mea- surements 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 25.93 Ma, while for the Centinela Formation the ages ranged between 21.24 and 26.38 Ma. The 40Ar/39Ar analysis of a sample of the Centinela Formation yielded an age of 20.48 ± 0.27 Ma. 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 mid- dle 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. � 2008 Elsevier Ltd. All rights reserved. 1. Introduction During the Late Cretaceous-Neogene interval Patagonia experi- enced alternating periods of sedimentation associated with trans- gressions of the Atlantic Ocean and periods of non-marine sedimentation and erosion (Malumián, 1999). Two of these Atlan- tic transgressions resulted in the deposition of marine sediments grouped under the informal name of ‘‘Patagoniano” (Patagonian). Sediments deposited in the ‘‘Patagonian” sea extend from the present-day Atlantic coast to the foothills of the Andes. Unfortu- nately, exposures of these rocks are separated by broad expanses of grassland. This makes it difficult to establish correlations among the various outcrops. Therefore, correlation and age of the widely separated exposures have been controversial and poorly con- ll rights reserved. encias Exactas y Naturales, 6300 Santa Rosa, La Pampa, 535. . Parras). strained ever since d’Orbigny (1842) and Darwin (1846) first as- signed them to the Tertiary. Modern estimated ages range from middle Eocene to early Miocene (Bertels, 1970, 1975, 1977; Furque and Camacho, 1972; Furque, 1973; Camacho, 1974, 1979; Riccardi and Rolleri, 1980; Zinsmeister, 1981; Ramos, 1982; Náñez, 1988; Parma, 1989; Griffin, 1990; Barreda, 1997; Casadío et al., 2000a,b, 2001; Parras and Casadío, 2002). Such discrepant ages have made it difficult to elucidate the Cenozoic geologic history of Patagonia and thus to establish a temporal framework for paleo- environmental and paleogeographic reconstructions. Recent progress in the correct age determination of these mar- ine deposits has been based on data drawn from micropaleontolog- ical and isotopic studies (Náñez, 1988; Barreda, 1997; Casadío et al., 2000b, 2001; Parras and Casadío, 2002; Guerstein et al., 2004). The age of associated terrestrial beds has been based mainly on radiometric and magnetostratigraphic data (Marshall et al., 1986; Fleagle et al., 1995). While radiometric ages proved useful in dating terrestrial facies, these rocks have eluded correlation with the Paleogene–Neogene marine beds (Legarreta and Uliana, 1994). mailto:aparras@exactas.unlpam.edu.ar http://www.sciencedirect.com/science/journal/08959811 http://www.elsevier.com/locate/jsames A. Parras et al. / Journal of South American Earth Sciences 26 (2008) 204–216 205 Disagreement over the stratigraphic relationships between the ter- restrial and marine beds still remains one of the most controversial aspects of Patagonian geology. The dating of the marine beds are problematic and one way to address this is through the use of Sr and Ar isotope ratios. At other localities, 87Sr/86Sr ratios in rock and fossil samples have been used successfully to date and correlate marine sediments, estimate the duration of stratigraphic gaps, underpin speculations about major geochemical cycles, estimate the duration of biozones and stages, and distinguish between marine and non-marine environments (McArthur et al., 2001). Burke et al. (1982) demonstrated that Sr- isotope stratigraphy provides a precise means of correlating and dating marine carbonates for certain periods within the past 100 Ma, and they compiled the Phanerozoic 87Sr/86Sr ratios then available. From that moment onwards, 87Sr/86Sr isotopic variations in marine carbonates have received growing attention as a valu- able chronostratigraphic tool (DePaolo and Ingram, 1985; Elder- field, 1986; Howarth and McArthur, 1997; Veizer et al., 1997, 1999; McArthur et al., 2001). Most of the advances have been par- ticularly valuable for the Cenozoic, due to the more precise and definitive geochronology and biostratigraphy for all Cenozoic time intervals except for the early Eocene. The results obtained from low-Mg calcite shells, such as foraminifers, belemnites, brachio- pods and oysters, are typically more reliable than those from phos- phatic skeletal components (Veizer et al., 1997). Here, we show that measurements of 87Sr/86Sr in the oyster Crassostrea? hatcheri (Ortmann) from the ‘‘Patagonian” rocks as- signed to the San Julián and Centinela formations, and a 40Ar/39Ar-dating from a whole-rock sample from the Centinela For- mation, yield ages for sections exposed in the eastern and western parts of Patagonia, enabling correlation between the two areas. In order to establish the faunal relationships and adjust correlations, we compared the invertebrate content of the two formations at dif- ferent locations with each other and with those present in other Paleogene and Neogene units exposed in Patagonia. Such a correla- tion allows more precise adjustment of the relative sea-level changes and paleoenvironmental conditions during the Paleo- gene–Neogene interval in southern Patagonia. 2. Geological setting and previous age determinations Along the eastern seaboard of southern Patagonia (Fig. 1), in the areas known as Gran Bajo de San Julián and Cabo Curioso, the well- exposed lower part of the ‘‘Patagonian” beds is referred to as the San Julián Formation (Bertels, 1970). This is a shallow marine unit that includes both siliciclastic and calcareous sediments and repre- sents upward-shoaling cycles developed in a sand-dominated shelf sequence (Manassero et al., 1997). Bertels (1977) divided this unit into two members, the lower Gran Bajo Member and the upper Meseta Chica Member. The former is characterized by yellowish to dark-red mudstones, siltstones, and sandstones and the second by green-yellowish and brown-greenish sandstones and coquinas. The San Julián Formation unconformably overlies the Jurassic vol- canic and sedimentary rocks of the Bahı́a Laura Group, and is over- lain by the marine Monte León Formation (late Oligocene–early Miocene). In the stratigraphic sections considered in this study (Gran Bajo, Nido de Águila, La Colmena and Cabo Curioso, Fig. 2), the San Julián Formation consists of up to 34 m of yellowish-brown sandstone and mudstone (Gran Bajo Member), and up to 28.5 m of yellow- ish-brown and green sandstone and limestone (Meseta Chica Member). The massive, laminated and trough-cross bedded, bio- turbated sandstones and heterolithic carbonaceous mudstones with wood and leaf fragments and coal beds of the lower part of the San Julián Formation are associated with a sea-level rise and infilling of an incised topography. They are interpreted as the beginning of a transgressive system tract (Parras and Casadío, 2002, 2005), and were deposited in a marshy environment under warm and humid climatic conditions (Barreda, 1997). The overly- ing part of the San Julián Formation is characterized by a flooding surface, corresponding to a ravinement surface, formed by shore- face erosion within the transgressive system tract. This surface was interpreted by Parras and Casadío (2002, 2005) as a sedimen- tological concentration deposited in an upper shoreface environ- ment. Overlying the flooding surface, marine transgression and facies retrogradation resulting from continued sea-level rise are represented by another sedimentological concentration deposited in intermediate energy conditions in a lower shoreface environ- ment (Parras and Casadío, 2002). The maximum flooding surface separating the transgressive system tract from the highstand sys- tem tract reflects maximum water depths and associated sediment starvation on the shelf and is represented by a biogenic concentra- tion of articulated adult and juvenile Crassostrea? hatcheri (Ort- mann) in life position, forming bunches. Parras and Casadío (2002, 2005) interpreted this level as deposited in a lower shore- face to offshore environment, with a low sedimentation rate. Final- ly, the highstand system tract is represented by several sedimentological concentrations interpreted as storm events, which are common in this part of the sequence. This part of the section was interpreted as deposited in a shallow shelf storm-wave dominated depositional environment (Manassero et al., 1997; Par- ras and Casadío, 2002, 2005). Different opinions exist regarding the age of the San Julián For- mation (Fig. 3). Based on the invertebrate fauna it contains, Cama- cho (1974) and Zinsmeister (1981) proposed a middle-late Eocene age for the San Julián Formation. According to Camacho (1979, 1985) an Eocene age is supported by the presence of the bivalves Parinomya patagonensis (Ihering) and Neoinoceramus ameghinoi Ihering. A late Eocene–early Oligocene age is suggested by the foraminiferans (according to Bertels, 1975, 1977 and Náñez, 1988) and echinoderms (Parma, 1989). A late Oligocene (Chattian) age was suggested by Bertels (1970), also based on foraminifera, and by Casadío et al. (2001) and Parras and Casadío (2002), based on 87Sr/86Sr datings drawn from shells of the oyster Crassostrea? hatcheri (Ortmann). An Oligocene age was proposed by Barreda (1997), who studied the spore–pollen assemblages. Such an age was based on the ranges of known species and the similarity with associations from other areas, stating that the samples examined by her could be as young as late Oligocene. Overlying the San Julián Formation are yellowish-gray, fine- tuffaceous sandstones and tuffaceous siltstones, tuffs, and coquin- as of the upper part of the ‘‘Patagonian”. These are included in the Monte León Formation (Bertels, 1970), which is exposed along a large portion of the eastern seaboard. This unit records the highest relative sea-level and the influx of Antarctic waters onto the Argen- tine Continental Shelf (Malumián, 2002). Although some discrep- ancies still persist about the age of this unit, most authors consider it to be late Oligocene (Bertels, 1970) or late Oligocene– early Miocene (Malumián, 1999) (Fig. 3). Based on foraminifera, Náñez (1988) restricted its age to the Oligocene (middle section of Zone P19 and top of Zone P21), although she indicated that the top beds could extend into the early Miocene. A radiometric 40Ar/39Ar age of the Monte León Formation at the Monte León locality indicated 19.33 Ma (Burdigalian, early Miocene) for the top of this unit (Fleagle et al., 1995). Although not exposed in central Patagonia, the ‘‘Patagonian” beds have been recorded there in subsurface samples (Malumián, 2002). They outcrop again in the west along the foothills of the An- des (Fig. 1), between Lago Posadas and Río Turbio. In this area, the ‘‘Patagonian” is known under the formal name of Centinela Forma- tion (Furque and Camacho, 1972). Furque and Camacho (1972) Fig. 1. Map showing the location of the studied sections and sites. (1) Estancia 25 de Mayo, (2) Cerro Pirámides, (3) Veranada de Cárcamo, (4) Estancia La Siberia, (5) Gran Bajo, (6) Nido de Águila, (7) La Colmena, (8) Cabo Curioso. Localities: 1–4, Centinela Formation; 5–8, San Julián Formation. 206 A. Parras et al. / Journal of South American Earth Sciences 26 (2008) 204–216 proposed this name for the marine beds exposed in the Lago Argentino area and intercalated between the terrestrial Río Leona Formation below and the overlying terrestrial Santa Cruz Forma- tion. Later, Riccardi and Rolleri (1980) extended this use also to the marine deposits exposed in the Patagonian Cordillera in north- western Santa Cruz, and intercalated between the Paleocene–Eo- cene Posadas Basalt and the terrestrial middle Miocene Santa Cruz Formation. In this study, we follow the stratigraphic nomen- clature proposed by the latter authors. The preserved Oligocene–early Miocene section in the Lago Argentino area at Estancia 25 de Mayo (Fig. 2), south of El Calafate city, comprises a vertical arrangement of facies passing upwards from high-energy fluvial to low-energy fluvial-coastal plain envi- ronments included in the Río Leona Formation (Marenssi et al., 2005). These are covered by marine deposits of the Centinela For- mation. The lower Centinela Formation is composed of fine sand- stones, siltstones and claystones with calcareous concretions containing mollusks, echinoderms, and a diverse fauna of crusta- ceans, including nine families of Decapoda and one of Isopoda (Casadío et al., 2000a). This part of the section is interpreted to be a biogenic concentration, characterized by articulated Crassos- trea? hatcheri (Ortmann) forming bunches in life position. A layer of white, massive tuff overlies these rocks, followed by siltstones, tuffs, and fine-grained sandstones with decapods and the bivalve Panopea sp. in life position. These sediments were deposited in pre- dominantly subtidal environments (Casadío et al., 2000a). The upper part of the Centinela Formation comprises fine- to med- ium-grained sandstones with intercalated siltstones showing sig- moid and planar bi-directional cross stratification and also biogenic concentrations of oysters, characterized by articulated specimens forming bunches in life position, and a few convex-up and convex-down disarticulated valves. Assemblages from the lower part of the section suggest that the beds were deposited un- der marine, nearshore paleoenvironmental conditions with a strong continental influence; in the middle part, the dinoflagellate cyst ratios mark the highest relative sea-level rise, and toward the top the sporomorph assemblage reflects the development of vege- tation adapted to coastal environments (Guerstein et al., 2004). In the Lago Viedma area, at the locality known as Cerro Pirá- mides (Fig. 2), the Centinela Formation is at the top of the section and consists of only 3 m of fine sandstone and muddy sandstone, massive or with trough-cross stratification, and undulose and fla- ser lamination (Marenssi et al., 2003). At this locality, the forma- tion contains specimens of Crassostrea? hatcheri (Ortmann) and wood fragments. It was deposited in a shallow marine environ- ment and probably represents sandy and mixed subtidal flats (Marenssi et al., 2003). Southeast of Lago Cardiel, at Estancia La Siberia (Fig. 2), the exposures of the ‘‘Patagonian” comprise 100 m of medium sand- stones with trough-cross, planar cross, and hummocky cross strat- ification, with lenses and beds of medium massive sandstones that constitute biogenic concentrations dominated by Crassostrea? Fig. 2. Studied stratigraphic sections of the San Julián and Centinela formations, showing provenance of the samples and 87Sr/86Sr ages (Ma). Dashed line shows boundary between rocks deposited during the late Oligocene and the latest Oligocene–early Miocene. A .Parras et al./Journal of South A m erican Earth Sciences 26 (2008) 204– 216 207 SAN JULIÁN FORMATION MONTE LEÓN FORMATION CENTINELA FORMATIONTI M E (M y) EP O C H AG E M IO C EN E O LI G O C EN E EO C EN E M ID D LE EA R LY M ID D LE LA TE EA R LY LA TE EA R LY 55 50 45 40 35 30 25 20 15 23.8 33.7 Camacho, 1974; 1979; 1985; Zinsmeister, 1981 Bertels, 1975; 1977; Náñez, 1988; Parma, 1989 Bertels, 1970; Barreda, 1997; Casadío et al., 2001; Parras and Casadío, 2002 Bertels, 1970 Náñez, 1988; Malumián, 1999; Fleagle et al., 1995 Ameghino, 1906; Camacho, 1974; Chiesa and Camacho, 1995; Casadío et al., 2000a Ortmann, 1902; Furque and Camacho, 1972; Ricardi and Rolleri, 1980; Carrizo et al., 1990; Casadío et al., 2001; Guerstein et al., 2004 Furque, 1973; Griffin, 1990 Casadío et al., 2000b Fig. 3. Diagram showing the different ages of the stratigraphic units considered, according to various authors. 208 A. Parras et al. / Journal of South American Earth Sciences 26 (2008) 204–216 hatcheri (Ortmann) in life position. Intercalated are a few beds of massive and laminated mudstones, tuffs, and chonites. We inter- pret the section as deposited in a shallow shelf, storm-dominated depositional environment, with facies of bars and channels and fre- quent storm events. These were deposited under intertidal to sub- tidal conditions. Ostracods studied by Echeverría (1998) in this area suggest a shelf environment with moderately high-energy conditions, shallow depths (0–20 m), sandy substrate, and warm waters with normal salinity. In the Lago Posadas area, at Veranada de Cárcamo (Fig. 2), the Centinela Formation is characterized by 145 m of sandstones and mudstones overlying the volcanic rocks of the Basalto Posadas (Paleocene–Eocene). At the base of the section there are conglom- erates and conglomeratic sandstones, massive or with trough-cross or planar cross stratification, with marine invertebrates and intra- clasts of the underlying volcanic rocks. Biogenic concentrations formed mostly by Crassostrea? hatcheri (Ortmann) and a bed of tuff with specimens of Pinna sp. in life position are intercalated. Fine- grained sandstone and mudstone characterize the middle part of the section with concretions containing very well preserved mar- ine invertebrates. The upper part is characterized by medium to coarse-grained sandstone with biogenic and sedimentological con- centrations formed mostly by C.? hatcheri and C. orbignyi (Ihering). Towards the top, there are frequent plant fragments. We interpret these rocks as deposited in a nearshore, shallow marine environment. There are different opinions regarding the age of the Centinela Formation too (Fig. 3). Based on the invertebrates collected by Hatcher south of Santa Cruz, Ortmann (1902) concluded that the marine fossiliferous rocks had been deposited during the late Oli- gocene–early Miocene. Ameghino (1906) supported an Eocene age based on the studies undertaken by Ihering (1897) on the mol- luscan fauna. An Eocene age was also proposed by Camacho (1974) due to the presence of the bivalve Venericor. A similar age was in- ferred by Chiesa and Camacho (1995) based on molluscan assem- blages, and by Casadío et al. (2000a) by means of a 40Ar/39Ar determination of about 46 Ma in a tuff layer at the lower part of the unit at Estancia 25 de Mayo. Furque and Camacho (1972), Ric- cardi and Rolleri (1980), and Carrizo et al. (1990) suggested a late Oligocene–early Miocene age for this unit based on its molluscan and foraminiferan assemblages, whereas Furque (1973) and Griffin (1990) placed it in the Miocene. Palynological data (Guerstein et al., 2004) and a 87Sr/86Sr age of 23.19 Ma on a valve of Crassos- trea? hatcheri (Ortmann) collected at Estancia 25 de Mayo (Casadío et al., 2000b) yielded a late Oligocene–early Miocene age. 3. Materials and methods 3.1. Samples We measured 87Sr/86Sr ratios of biogenic carbonate from 16 samples of Crassostrea? hatcheri (Ortmann), which is one of the more characteristic fossils of the Cenozoic marine deposits in Pat- agonia (Table 1). We also measured 40Ar/39Ar ratios in a whole- rock sample from the Centinela Formation at the La Siberia locality. Each of the localities from which oysters were collected was also carefully examined for the presence of decapod crustaceans, fossil crabs and lobsters. Extensive work on this group of organ- isms throughout Patagonia has made it possible to develop models of paleobiogeographic patterns that can serve as a test for models based upon studies of mollusks (Feldmann and Schweitzer, 2006). In the present study, it was anticipated that the decapods could provide independent, useful information regarding distributional patterns and temporal relationships. The specimens collected were studied in the laboratories at Kent State University, several papers describing the identification and classification of the material have either been published (Feldmann et al., 1995, 1997; Schweitzer and Feldmann, 2000; Casadío et al., 2004, and others) or are in pro- gress. This work, coupled with the extensive published record of Aguirre-Urreta (1987, 1990, 1992) is summarized in Table 2. Likewise, the mollusk content of the localities where the oysters were collected was recorded in order to identify as closely as pos- sible the mollusk fauna associated with the dated oysters. The geo- graphic and stratigraphic distributions of the bivalves and gastropods reflect the different temporal and paleobiogeographic settings in which the rocks involved were deposited and agree well Table 1 87Sr/86Sr ratio and calculated age Locality Sample Formation Sr (ppm)a 87Sr/86Srb N 87Sr/86Src 87Sr/86Srd Limiting ages (Ma)e Age (Ma)f Cabo Curioso CC11 (*) San Julián 487.1 0.708215 (9) 17 0.708220 0.708226 24.05–24.92 24.49 0.708224 (8) (0.708206–0.708246) Cabo Curioso CC11 (*) San Julián 531.0 0.708243 (8) 23 0.708220 0.708226 24.05–24.92 24.49 0.708216 (8) (0.708206–0.708246) 0.708200 (7) Cabo Curioso CC18 San Julián 624.0 0.708240 (9) 15 0.708244 0.708250 23.63–24.56 24.12 0.708247 (6) (0.70823–0.70827) Cabo Curioso CC18 (*) San Julián 619.0 0.708266 (13) 28 0.708261 0.708267 23.30–24.30 23.83 0.708261 (7) (0.708287–0.708247) 0.708257 (8) La Colmena LC6 San Julián 288.0 0.708214 (10) 19 0.708215 0.708221 24.13–25.00 24.57 0.708216 (9) (0.708241–0.708201) Nido de Águila NA3 San Julián 582.0 0.708258 (8) 16 0.708253 0.708259 23.46–24.42 23.97 0.708248 (8) (0.708279–0.708239) Gran Bajo GB11 San Julián 678.0 0.708132 (6) 12 0.708137 0.708143 25.35–26.60 25.93 0.708147 (6) (0.708163–0.708123) Gran Bajo GB11 (*) San Julián 680.0 0.708137 (7) 7 0.708137 0.708143 25.35–26.60 25.93 (0.708163–0.708123) Gran Bajo GB39 San Julián 324.0 0.708169 (7) 18 0.708170 0.708176 24.81–25.86 25.28 0.708171 (11) (0.708196–0.708156) La Siberia LS1 Centinela 411.0 0.708117 (9) 9 0.708116 0.708122 25.76–27.18 26.38 (0.708142–0.708102) La Siberia LS1 (*) Centinela 402.0 0.708116 (10) 10 0.708116 0.708122 25.76–27.18 26.38 (0.708142–0.708102) Veranada de Cárcamo VC1 (*) Centinela 518.3 0.708187 (7) 25 0.708186 0.708192 24.58–25.53 25.00 0.708179 (7) (0.708212–0.708172) 0.708193 (11) Cerro Pirámides CP1 (*) Centinela 636.9 0.708378 (8) 15 0.708370 0.708376 20.83–22.00 21.34 0.708362 (7) (0.708396–0.708356) Cerro Pirámides CP1 (*) Centinela 665.8 0.708371 (7) 14 0.708375 0.708381 20.75–21.88 21.24 0.708379 (7) (0.708401–0.708361) Estancia 25 de Mayo E25M1 (*) Centinela 453.9 0.708316 (7) 23 0.708323 0.708329 21.72–23.16 22.45 0.708328 (8) (0.708349–0.708309) 0.708324 (8) Estancia 25 de Mayo E25M1 (*) Centinela 396.2 0.708309 (9) 25 0.708308 0.708314 22.05–23.48 22.86 0.708318 (10) (0.708334–0.708294) 0.708298 (6) Each entry with a sample number represents a separate dissolution. When two fragments from the same sample were analyzed, the one indicated by the asterisk (*) was dissolved using acetic acid while the other one was dissolved in dilute HCL. a Minimum concentrations of Sr for the sample used for analysis. b Measured values of 87Sr/86Sr normalized assuming normal Sr with 86Sr/88Sr = 0.119400 and using a reference value of the SRM987 = 0.710242. c Average 87Sr/86Sr from measured values and recommended value for that dissolution considering replication of 87Sr/86Sr determinations. d Reference value 87Sr/86Sr corrected to make the data concordant with SRM987 = 0.710248 used in the construction of used Look-Up Table (number in parentheses is 2r of the mean). e Calculated limiting ages using the SIS Look-Up Table Version 3:10/99 of McArthur et al. (2001), at 95% confidence limits. f Preferred numerical age. A. Parras et al. / Journal of South American Earth Sciences 26 (2008) 204–216 209 with the patterns shown by the decapod fauna. Table 3 summa- rizes the distribution of mollusks in the two stratigraphic units considered in this work (i.e., San Julián and Centinela formations) and in the other equivalent units exposed in the area (i.e., Monte León and Guadal formations). Uneven preservation of the material and inaccurate identifications cloud the true affinities among the marine faunas contained in these formations. Therefore, we have only included in the table those taxa that could be positively iden- tified and that occur in at least two of the lithostratigraphic units. Data on Table 3 was compiled from a number of sources (Sowerby, 1846; Philippi, 1887; Ihering, 1897, 1907, 1914; Ortmann, 1897, 1902; Feruglio, 1937, 1954; Zinsmeister, 1981; Camacho, 1985; Camacho and Zinsmeister, 1989; Erdmann and Morra, 1985; Mor- ra, 1985; Olivera et al., 1994; Chiesa et al., 1995; del Río, 1995, 2004a,b; del Río and Camacho, 1996, 1998; Frassinetti and Covace- vich, 1999) and from our own field observations. 3.2. Sample preparation and analytical techniques Oyster samples were collected from sections of the San Julián Formation measured at Cabo Curioso (49�120S; 67�390W), Gran Bajo (49�310S; 68�140W), La Colmena (49�270S; 68�50W), and Nido de Águila (49�360S; 68�140W) along the Atlantic coast, and from sections of the Centinela Formation exposed at Estancia 25 de Mayo (50�300S; 72�150W), Estancia La Siberia (49�000S; 71�050W), Veranada de Cárcamo (47�290S; 71�130W), and Cerro Pirámides (49�520S; 72�130W) along the foothills of the Andes (Figs. 1 and 2). Almost all of the samples were from biogenic or biogenic-sedi- mentological concentrations with oysters in life position (Fig. 4). For each sample, a small piece of the hinge area of a shell was broken off and manually crushed. One of the resulting fragments was selected and part of it was used for thin-section petrographic analysis under a petrographic microscope in order to determine Table 2 Stratigraphic array of decapod crustacean taxa known from the Cenozoic of Argentina TAXON Maast/Danian Oligocene Oligocene/Miocene Miocene Late Miocene Nephropidae Hoploparia cf H. arbei Aguirre-Urreta, 1989 R Parastacidae Huxley, 1879 Lammuastacus longirostris Aguirre-Urreta, 1992 FW Callianassidae Dana, 1852 Callianassa burkhardi Böhm, 1911 R Callianassa sp. Aguirre-Urreta, 1990 P Protocallianassa sp. R Callianopsis australis Casadío et al., 2004 F Ctenochelidae Manning and Felder, 1991 Ctenocheles sp. R N Galatheidae Munida casadioi (Schweitzer and Feldmann, 2000) F C Paguroidea Latreille, 1802 Paguroidea, family and species undetermined J Family Raninidae de Haan, 1833 Raninoides sp. C Majidae Samouelle, 1819 Eoinachoides cf. E. senni Van Straelen, 1933 G Leurocyclus primigenius Aguirre-Urreta, 1990 P Notomithrax sp. J C Rochinia sp. C V Calappidae Milne Edwards, 1837 ?Calappilia n.sp. N Hepatidae Stimpson, 1871 Hepatus n. sp. N Aethridae Dana, 1851 Osachila n. sp. V Retroplumidae Gill, 1894 Costacopluma australis Feldmann et al., 1995 R Costacopluma salamanca Feldmann et al., 1997 S Atelecyclidae Ortmann, 1893 Trichopeltarion levis Casadío et al., 2004 F N Cancridae Latreille, 1802 Metacarcinus sp. C Notocarcinus sulcatus Schweitzer and Feldmann, 2000 C Romaleon n. sp. V Geryonidae Colosi, 1923 Chaceon peruvianus (d’Orbigny, 1842) C V Chaceon fuegianum (Colosi, 1823) T Portunidae Rafinesque, 1815 Proterocarcinus lophos Feldmann et al., 1995 R Proterocarcinus latus (Glaessner, 1933) J C, G V Proterocarcinus corsolini Casadío et al., 2004 F Proterocarcinus n. sp. N Portunidae, genus and species undetermined J Palaeoxanthopsidae Schweitzer, 2003 Lobulata lobulata (Feldmann et al., 1995) R Rocacarcinus gerthi (Glaessner, 1930) R Hexapodidae Miers, 1886 Palaeopinnixa rocaensis (Feldmann et al., 1995) R Panopeidae Ortmann, 1893 Chirinocarcinus wichmanni (Feldmann et al., 1995) R Panopeus n. sp. V Pilumnidae Samouelle, 1819 Baricarcinus mariae Casadío et al., 2004 F Pilumnus n. sp. N Rhizopinae, genus and species undetermined N Pseudorhombilidae Alcock, 1900 Pseudorhombilia patagonica Glaessner, 1933 ? Xanthidae MacLeay, 1838, sensu stricto Atergatris sp. C Pinnotheridae de Haan, 1833 Asthenognathus microspinus Casadío et al., 2004 F Asthenognathus urretae Schweitzer and Feldmann, 2001 C Pinnixa n. sp. N Ocypodidae Rafinesque, 1815 Ocypode n. sp. V Footnotes: C, Centinela Formation; F, Río Foyel Formation; G, Gran Bajo del Gualicho Formation; N, Navidad Formation (Chile); P, Patagoniano undifferentiated; R, Roca Formation; T, Tierra del Fuego unit undifferentiated; V, Puerto Madryn Formation, Penı́nsula Valdés; J, San Julián Formation, Gran Bajo de San Julián; S, Salamanca Formation; FW, Fresh water. 210 A. Parras et al. / Journal of South American Earth Sciences 26 (2008) 204–216 the state of textural preservation; small chips of about 100 mg were taken from the remaining part of it for isotopic analysis. Pet- rographic analysis showed that shell structure is well preserved and retains the original calcitic composition and foliated micro- structure. Evidence of postdepositional diagenesis could not be de- tected. The chips were washed in water, alcohol, and acetone prior Table 3 Stratigraphic array of mollusk taxa in the San Julián, Monte León, and Centinela formations and in the Guadal Formation in southern Chile which is at least partly equivalent to the Centinela Formation TAXON Centinela Fm. (North) Centinela Fm. (South) San Julián Fm. (at Bahı́a San Julián) Monte León Fm. (at M.León) Guadal Fm. (southern Chile) Bivalvia Neilo sp. * * Glycymeris cuevensis (Ihering, 1897) * * * Cucullaea alta Sowerby, 1846 * * * * Lithophaga patagonica d’Orbigny, 1846 * * Modiolus arctus Feruglio, 1937 * * Atrina magellanica (Ihering, 1899) * * * Neopanis quadrisulcatum (Ihering, 1897) * * Reticulochlamys proxima (Sowerby, 1846) * * * Zygochlamys geminata (Sowerby, 1846) * * * * Zygochlamys jorgensis (Ihering, 1907) * * Zygochlamys sebastiani Morra, 1985 * * Crassostrea orbignyi (Ihering, 1897) * * Crassostrea? hatcheri (Ortmann, 1897) * * * * * Pteromyrtea crucialis (Ihering, 1907) * * * * Macoma santacruzensis (Ihering, 1899) * * Patagonicardium? guadalense Frassinetti and Covacevich, 1999 * * Patagonicardium iheringi Frassinetti and Covacevich, 1999 * * Patagonicardium philippii (Ihering, 1897) * * * Trachycardium puelchum (Sowerby, 1846) * * * Cardium patagonicum Ihering, 1907 * * Lahilla patagonica Ihering, 1907 * * * Fasciculicardia patagonica (Sowerby, 1846) * * * Dosinia meridionalis Ihering, 1897 * * Eumarcia arenosa (Ortmann, 1899) * * Eurhomalea? navidadiformis Frassinetti and Covacevich, 1999 * * Chione patagonica (Philippi, 1887) * * * Ameghinomya argentina (Ihering, 1897) * * * Ameghinomya meridionalis (Sowerby, 1846) * * * Ameghinomya darwini (Philippi, 1887) * * * Panopea quemadensis (Ihering, 1897) * * * ‘‘Panopea” cf. nucleus (Ihering, 1899) * * * * Pholadidea patagonica (Philippi, 1887) * * * * Gastropoda Valdesia dalli (Ihering, 1897) * * * Calyptraea pileolus d’Orbigny, 1841 * * * * Crepidula gregaria Sowerby, 1846 * * * Cirsotrema rugulosa (Sowerby, 1846) * * Turritella ambulacrum Sowerby, 1846 * * * Struthiolarella ameghinoi (Ihering, 1897) * * * * Ficus? carolina (d’Orbigny, 1847) * * * Trophon santacruzensis Ihering, 1897 * * * ‘‘Siphonalia” sp. * * Aeneator? annae (Ortmann, 1900) * * Penion subrectus Ihering, 1899 * * Neoimbricaria patagonica (Ihering, 1897) * * * Proscaphella cossmanni Ihering, 1907 * * * Proscaphella quemadensis (Ihering, 1897) * * * Proscaphella santacrucensis Chiesa, Parma and Camacho, 1995 * * Adelomelon? burmeisteri (Ihering, 1907) * * * Sigapatella americana (Ortmann, 1900) * * * Peonza torquata Olivera et al., 1994 * * * Only the species in common from one locality to another are included. Not included in the table are most of the mollusk species of the Monte León Formation, which are known to occur only within this unit in which the preservation of aragonitic shells in some beds results in a large number of ‘‘exclusive” taxa. The mollusk taxa contained in the Centinela and San Julián formations are relatively few and comprise either species with calcitic shells or molds. Of these, only the taxa positively identifiable are included in the tables. A significant number of the specimens collected are impossible to identify adequately for meaningful comparisons between these two units. A. Parras et al. / Journal of South American Earth Sciences 26 (2008) 204–216 211 to dissolution. Carbonate was dissolved with dilute acetic or hydrochloric acid. For hydrochloric acid dissolution, aliquots of 2 N HCl were incrementally added to the carbonate samples for 15 min until visible reaction ceased. For acetic acid dissolution, 4 N acetic acid was added to the carbonate for two days. After dis- solution, the solution was centrifuged to separate any residue, and the supernatant was spiked with a highly enriched 84Sr tracer, dec- anted and evaporated. Then the Sr was isolated from this fraction by ion-exchange chromatography. All chemical preparations were carried out with low blank procedures in a clean laboratory, fol- lowing the general analytical procedures for Sr isolation, isotope dilution, and mass spectrometry separation described by Foland and Allen (1991). The Radiogenic Isotopes Laboratory in the Department of Geological Sciences of the Ohio State University performed the 87Sr/86Sr analyses. 87Sr/86Sr determinations were made using dynamic multicollec- tion of all Sr-isotopes on a Finnigan MAT 261A thermal ionization mass spectrometer as outlined by Foland and Allen (1991). Mea- sured values of 87Sr/86Sr were normalized assuming normal Sr with 86Sr/88Sr = 0.119400. Data are presented in Table 1, where each en- try with a sample number represents a different sample dissolu- tion and replicate entries indicate complete replicate analyses. Multiple entries for 87Sr/86Sr values for a given dissolution repre- sent replicate mass spectrometer analyses. Each entry is for a sep- arate loaded filament and represents a run of 100 ratio measurements with an 88Sr ion signal intensity of approximately Fig. 4. Field photograph of a biogenic concentration at Cabo Curioso, consisting of a 20 cm-thick continuous bed of Crassostrea? hatcheri (Ortmann) in life position, fo- rming clumps. early Miocene late Oligocene 0,7081 0,70815 0,7082 0,70825 0,7083 0,70835 0,7084 21 22 23 24 25 26 27 28 Age (My) 87 Sr /86 Sr San julián Formation Centinela Formation (North) Centinela Formation (South) Fig. 5. 87Sr/86Sr average compositions of 16 Crassostrea? hatcheri (Ortmann) shells from the San Julián and Centinela formations plotted on the secular Sr-isotope curve for marine water (solid line) showing 95% confidence interval (shaded lines) of McArthur et al. (2001). 0.0000 0.0004 0.0008 0.0012 0.0016 0.0020 0.0024 0.0028 0.0032 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 39Ar/40Ar 36 A r/ 40 A r Age = 20.48 ± 0.27 Ma 40/36 = 317 ± 5.5 MSWD = 2.0 Steps 1 through 10 La Siberia wr tuff 10 15 20 25 30 35 40 0 20 40 60 80 100 Fraction 39Ar Released A ge (M a) Fig. 6. 40Ar/39Ar age spectrum of the tuff from the Centinela Formation, based upon total fusion and incremental heating procedures, and isotope correlation. 212 A. Parras et al. / Journal of South American Earth Sciences 26 (2008) 204–216 6 � 10�11 to 5 � 10�11 amps. Uncertainties refer to the last digit(s) and are two standard deviations of the mean within-run uncertain- ties. The reference value of 87Sr/86Sr for the SRM987 is 0.710242 ± 0.000010 (one sigma external reproducibility). The 87Sr/86Sr values of the samples were converted to numerical ages using the SIS (Strontium Isotope Stratigraphy) Version 3:10/ 99 of the Look-Up Table of McArthur et al. (2001). The reference value 87Sr/86Sr used (=0.710242) was corrected to make the data concordant with SRM987 of 0.710248 used in the construction of this Look-Up Table. The time scale used is that of Berggren et al. (1995). Results are plotted in Fig. 5. The Nevada Isotope Geochronology Laboratory of the University of Nevada, Las Vegas, made the 40Ar/39Ar analyses. A rock sample ta- ken at 35 m from the bottom of the section at Estancia La Siberia was analyzed by the furnace step heating method on crushed and sieved whole rock material. The sample was wrapped in Al foil and stacked in 6 mm inside diameter Pyrex tubes. It was in-core for 14 h in the D3 position on the core edge of the 1 MW TRIGA type reactor. Correction factors for interfering neutron reactions on K and Ca were determined by repeated analysis of K-glass and CaF2 fragments. Cor- rection factors were (40Ar/39Ar)K = 0.00 ± 0.0002, (36Ar/37Ar)Ca = 2.78 ± 2.99 � 10�4, (39Ar/37Ar)Ca = 6.81 ± 2.94 � 10�4. J factors were determined by fusion of 3–5 individual crystals of neutron fluence monitors which gave reproducibilities of 0.09–0.29% at each stan- dard position. An error in J of 0.5% was used in age calculations. Irra- diated crystals together with CaF2 and K-glass fragments were placed in a Cu sample tray in a high vacuum extraction line and were fused using a 20WCO2 laser. Measured 40Ar/36Ar ratios were 290.12 ± 0.38% during this work, thus a discrimination correction of 1.01857 (4 AMU) was applied to measured isotope ratios. Com- puter automated operation of the sample stage, laser extraction line, and mass spectrometer as well as final data reduction and age calcu- lations were done using LabSPEC software written by B. Idleman (Le- high University). An age of 27.9 Ma was used for the Fish Canyon Tuff sanidine flux monitor in calculating age for the sample. Results of the analyses are given in Fig. 6. 4. Results and discussion 4.1. Isotopic and radiometric ages Although the samples have less Sr content (<800 ppm) than modern low-Mg calcitic shells, it is still possible that they may not have suffered diagenetic loss of Sr, since concentrations <800 ppm have been reported in oyster samples from Upper Creta- ceous and Miocene deposits (McArthur et al., 2000; Scasso et al., 2001). Veizer et al. (1999) stated that the lower Sr concentration may be in part due to the inclusion of distinct small domains of secondary calcite, and not to partial re-crystallization, leaving the bulk of the shell unaltered. Nine measurements were performed using six oyster valves (three were analyzed twice) from the San Julián Formation (Table 1, Fig. 5). The ages range between 23.83 and 25.93 Ma (late Oligo- cene), and Sr contents lie between 288 and 680 ppm. Seven mea- surements were performed using four oyster valves (three were analyzed twice) from the Centinela Formation. The ages range be- tween 21.24 and 26.38 Ma (late Oligocene to early Miocene), and Sr content ranges between 396.2 and 665.8 ppm. A. Parras et al. / Journal of South American Earth Sciences 26 (2008) 204–216 213 The results provide a well-constrained depositional age for the ‘‘Patagonian” deposits examined here. The 87Sr/86Sr ages suggest that in the eastern areas of Patagonia, the rocks included in the San Julián Formation were deposited during the late Oligocene (Chattian). In the west, the rocks belonging in the Centinela Forma- tion were deposited between the late Oligocene and the early Mio- cene (Chattian–Aquitanian). At the two northernmost sections where this unit is exposed (Veranada de Cárcamo and Estancia La Siberia), 87Sr/86Sr data from oysters collected in the lowermost beds suggest a late Oligocene age. Samples from further South – also from the base of the sections – at Cerro Pirámides and Estancia 25 de Mayo, yielded younger (early Miocene) ages. The sample of the tuff from the Centinela Formation, based upon the furnace step heating method, exhibits a discordant 40Ar/39Ar age spectrum (Fig. 6) with high initial ages (�27 Ma), fall- ing to ages of �21 Ma at �50–90% gas released, and then rising again to older final step ages. This type of age spectrum (called U-shaped, or saddle-shaped) is associated with excess argon, which can cause anomalously old ages for samples. The total gas age of 22.64 ± 0.25 Ma is equivalent to conventional K/Ar age and is likely an overestimate of the true eruptive event. This sample has a valid isochron, using the first 10 out of 14 steps, which de- fines an age of 20.48 ± 0.27 and does confirm some initial excess argon present, as the y-intercept is 317. We considered that this isochron is the best estimate of the age of the Estancia La Siberia sample. A strong discrepancy exists between the ages derived from 87Sr/86Sr (this study) and 40Ar/39Ar dating methods (Casadío et al., 2000a) on the Centinela Formation at Estancia 25 de Mayo, near Calafate. The initial radiometric age published for the Centin- ela Formation was based upon an 40Ar/39Ar analysis of a thick vol- canic ash bed within the formation and bounded above and below by fossiliferous sediments (Casadío et al., 2000a). The age of approximately 46 Ma was judged to be a minimum age. Subse- quently, multiple 87Sr/86Sr ages taken on oyster shells from the same stratigraphic section, and reported herein (Table 1) docu- ment a much younger age. In an attempt to resolve this difference, another 40Ar/39Ar analysis was conducted at a different laboratory on a different split of the same sample of the volcanic ash, and the resultant age was not conclusive. The youngest age on the age spectrum, presumed to be the minimum age, was 30.2 Ma, whereas the statistically valid isochron yielded and age of 64.8 ± 1.6 Ma (T. Spell, personal communication to RMF). Although this age determination embraced the previous 40Ar/39Ar determi- nation, the uncertainty in the results, coupled with the similarity of the 87Sr/86Sr ages taken on multiple oyster shells from the same stratigraphic section leads us to the conclusion that the latter ages are a more reliable estimate of the age of the Centinela Formation. 4.2. Paleontological affinities In the Monte León Formation, most of the mollusk and decapod fauna is exquisitely preserved, showing a high diversity with a large number of mollusk taxa represented. In contrast, the San Julián Formation yields mostly taxa with calcitic shells such as oys- ters, pectinids, muricids, epitoniids, bryozoans, barnacles, and echinoids, among others. Very rarely are aragonitic shells pre- served and only as calcite replacements or molds. The Centinela Formation is somewhat intermediate, although most of the arago- nitic shells have suffered a fair degree of diagenetic alteration. Con- sequently, comparison among the faunas from different localities must by necessity take into account the preservation of the fossils considered, in order to avoid taxonomic misplacements that may artificially cloud or enhance any existing affinities. The decapod record (Table 2) suggests that different geographic regions, for example the areas of Calafate and Gran Bajo de San Julián, had some affinities. Both areas yielded Proterocarcinus latus (Glaessner); however, this species was extremely common at the former locality and rare at the latter. Furthermore, the species is widespread geographically and temporally. Notomithrax sp. was also present at both localities but the material collected from Cal- afate is fragmentary and it may not be possible to determine whether it is the same species as that from Gran Bajo de San Julián. Unfortunately, no decapods were collected at the other localities from which age determinations were made. Although decapods are not typically considered to be good stratigraphic indices, the more detailed analysis of ages of Paleo- gene rock units does permit description of one morphological trend that may have stratigraphic implications. Proterocarcinus Feldmann, Casadío, Chirino-Gálvez and Aguirre-Urreta, is a genus of primitive portunids, or swimming crabs, that first appear in the fossil record in the Paleocene Roca Formation of central Argen- tina. The geologically oldest species, P. lophos Feldmann, Casadío, Chirino-Gálvez and Aguirre-Urreta, is characterized by having four well-developed anterolateral spines. Another species within the genus, P. latus (Glaessner), is distinguished from the type species in having the central two anterolateral spines reduced to small protuberances. As can be seen in Table 2, that genus is geographi- cally widespread and geologically long-ranging with representa- tives known from the Oligocene and the Miocene. The trend in reduction of the medial spines is apparently reflected in two other species. Proterocarcinus corsolini Casadío, de Angeli, Feldmann, Gar- assino, Hetler, Parras and Schweitzer, from the Oligocene Río Foyel Formation near Bariloche exhibits only three anterolateral spines, the central one of which is strongly reduced. Finally, a newly dis- covered species, yet to be named, from the Miocene Navidad For- mation in Chile, bears all the diagnostic features of Proterocarcinus except that the medial spine(s) are absent alto- gether. The age of the part of the Navidad Formation from which crabs were collected is Tortonian (late Miocene, ca. 10–11 Ma) (Finger et al., 2003). Thus, there seems to be one lineage of Prote- rocarcinus, including P. lophos and P. latus, that retains the original condition of four anterolateral spines, although the latter exhibits reduced medial spines, whereas a second lineage, characterized by P. corsolini and the new species from the Navidad Formation, has experienced reduction and ultimate loss of the medial antero- lateral spines. Additional studies of its presence both in Argentina and Chile will permit testing this hypothesis. The occurrence of Proterocarcinus spp. on both sides of the Andes suggests that there may have been a connection through the Andes at this time such that faunal mixing of Pacific and Atlantic faunas was possible. That potential connection is further reinforced by the distribu- tional patterns of two other taxa. The Cancridae is represented by three genera, two noted from the Centinela Formation and one from the late Miocene Puerto Madryn Formation. Examination of the geographic distribution of extant members of the family indicates that none is known from the eastern coast of South Amer- ica, south of the Caribbean (Nations, 1975; Boschi, 2000). The occurrence of representatives of this family in the Oligocene and Miocene of Argentina suggests either that the family was formerly more widespread in the Atlantic Ocean or that the fossil species document a Pacific connection through the Andes during this time. These possibilities can be explored by additional studies in both Chile and Argentina to develop a more detailed history of the Can- cridae in South America. Perhaps even more striking is the occur- rence of the atelecyclid species, Trichopeltarion levis Casadío, de Angeli, Feldmann, Garassino, Hetler, Parras and Schweitzer, in the Río Foyel Formation in Argentina and in the Navidad Formation in Chile. Recent studies on the Atelecyclidae (Schweitzer and Salva, 2000; Salva and Feldmann, 2001) document a fossil record of the family that is strictly circum-Pacific so the occurrence in Argentina represents the first fossil record in rocks presumed to have been 214 A. Parras et al. / Journal of South American Earth Sciences 26 (2008) 204–216 connected to the Atlantic basin. Extant atelecyclids are known from both coasts of South America and the Caribbean (Boschi, 2000), as well as West Africa (Crosnier, 1981). Finally, another decapod taxon of interest is Chaceon peruvianus (d’Orbigny). Although it is formally recognized from only two occurrences herein (Table 2), it probably has a wider occurrence. The species occurs in museum collections around the world and is probably the best know fossil crab from South America. Unfortu- nately, the stratigraphic and geographic information accompany- ing most of the material is inadequate to locate the collecting sites and verify the occurrences. The known occurrences of the spe- cies in the Centinela and the Puerto Madryn formations, however, do document a range extending from the late Oligocene to the late Miocene, suggesting that additional studies could yield more occurrences. A survey of the mollusk taxa collected from the four strati- graphic units (i.e., San Julián, Monte León, Centinela, and Guadal formations; Table 3) reveals that forty-one taxa are common to the Monte León and Centinela formations; in contrast, only six are in common between the Centinela and San Julián formations. If the Centinela Formation is divided into North and South (accord- ing to the area where the samples were taken) then the species in common differ slightly. A cluster analysis using Euclidean Distance (Simple Linkage), and considering the Centinela Formation as a unit (Fig. 7A) or separated into North and South (Fig. 7B), reveals divergent clustering patterns that in some cases agree well – but not always – with the accepted paleobiogeographic and biostrati- graphic patterns for the Patagonian marine Paleogene–Neogene faunas proposed by del Río (2004b). These patterns are not always consistent with the 87Sr/86Sr ages distribution obtained from oyster shells from these localities. The most obvious reason for this are the strong preservational bias resulting from the absence of arago- nitic shells in the San Julián Formation and the generally poor pres- ervation in the Centinela Formation, which produce a significant distortion of the record. Therefore, as only relative proportions can be used in estimating any possible affinities, the evidence pro- vided by mollusks regarding the affinities between the different faunas must remain presently inconclusive. Nevertheless, it is interesting to note some peculiarities in the distribution of certain well preserved mollusks. Such is the case of the distribution of the large pectinids from Patagonia. Reticulochlamys proxima (Sowerby) is a species that appears in the Monte León Formation and also oc- curs in the Centinela Formation but is entirely missing from the San Julián Formation, despite the fact that pectinids are among the most conspicuous elements in the latter unit. A similar strati- graphic distribution is also shown by Zygochlamys geminata (Sow- San Julián Fm. Guadal Fm. Monte León Fm. Centinela Fm. 2.5 3.0 3.5 4.0 4.5 5.0 5.5 Li nk ag e D is ta nc e Fig. 7. Tree diagrams illustrating association of lithostratigraphic units based on cluster considering the entire Centinela Formation as a unit; (B) discriminating the Centinela F erby). Conversely, the most abundant pectinid in the San Julián Formation, i.e., Zygochlamys dominator Morra, is never present in the overlying Monte León Formation and is also missing in the Cen- tinela Formation and the equivalent Guadal Formation in southern Chile. Likewise, Crassostrea orbignyi (Ihering) is only found within the uppermost beds of the Monte León and Centinela formations, whereas it is missing in the lower part of these units and is not present at all in the San Julián Formation. Further collections and study of different sections within the area will be instrumental in elucidating whether these differences can be attributed to age or else to different paleoecological conditions. 5. Conclusions The 87Sr/86Sr data from the lower part of the Centinela Forma- tion at the northernmost localities – Estancia La Siberia and Vera- nada de Cárcamo (25.00 and 26.38 Ma) – suggest that these rocks temporally correlate in part with the San Julián Formation (23.83– 25.93 Ma; see Table 1 for localities) exposed in the eastern areas of southern Patagonia. Despite the agreement between Sr-isotope data there are insufficient species in common to establish a close paleontological correlation between the San Julián Formation and the lower part of the Centinela Formation at the northernmost localities. 87Sr/86Sr data from the lower part of the Centinela For- mation at the southern localities – Estancia 25 de Mayo and Cerro Pirámides – yield younger ages (21.24 to 22.86 Ma), suggesting a possible correlation with the Monte León Formation exposed in the eastern part of southern Patagonia and there overlying the San Julián Formation. No isotopic data are available on the lower part of the Monte León Formation, and the only radiometric 40Ar/39Ar age of 19.33 Ma is from the top of the unit (Fleagle et al., 1995). The 87Sr/86Sr age of 23.83 and 24.12 Ma (latest Oligo- cene) drawn from top of the San Julián Formation immediately be- low its contact with the overlying Monte León Formation at Cabo Curioso, suggest that the age of the latter at the localities along the coast should lie between 24.12 and 19.33 Ma, i.e., latest Oligo- cene or more probably early Miocene (see Fig. 8). The faunal affin- ities among the Centinela Formation at its southernmost localities, the upper part of the same unit at its northernmost localities, and the Monte León Formation agree with this correlation. It should be noted, however, that the present state of knowledge on the faunas from all these units is inconclusive. The isotopic data obtained and the identification and compari- son of the taxa present in the different units somewhat modifies the extent of the ‘‘Patagonian” transgressions as described by pre- vious authors (Camacho, 1974; Malumián, 1999, 2002), and refines San Julián Fm. Centinela Fm. (South) Monte León Fm. Guadal Fm. Centinela Fm. (North) 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 Li nk ag e D is ta nc e analysis (single linkage) of the mollusk fauna using Euclidean distance indexes; (A) ormation into northern and southern exposures. 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 Santa Cruz Fm. Santa Cruz Fm. TI M E (M a) EP O C H AG E SANTA CRUZ PROVINCE SOUTHWEST EAST LA N G H IA N SE RR AV AL LI AN BU R D IG AL IA N M IO C E N E O L IG O C E N E AQ U IT AN IA N C H AT TI AN R U PE LI AN Centinela Fm. Río Leona Fm. ? Monte León Fm. San Julián Fm. NORTHWEST ? Santa Cruz Fm. Centinela Fm. ? Fig. 8. Proposed lithostratigraphic correlation of units exposed in the western part of Santa Cruz province with those exposed along the Atlantic coast. A. Parras et al. / Journal of South American Earth Sciences 26 (2008) 204–216 215 the age of ‘‘Patagonian” rock units and the Oligocene–Miocene paleogeographic boundaries. Acknowledgements We thank the Ariztizabal Family for support during the field season in Calafate and Margarita Igalú for granting permission to work in the Gran Bajo de San Julián. We thank A. Dutton and an anonymous reviewer for their helpful comments. This study was funded in part by the Facultad de Ciencias Exactas y Naturales, Uni- versidad Nacional de La Pampa and CONICET, Argentina. Field and laboratory work related to the decapods was supported by NSF Grants OPP 9417697 to RMF and OPP 9909184 to RMF and Karen Bice. References Aguirre-Urreta, M.B., 1987. La familia Geryonidae (Crustacea: Brachyura) en el Terciario de Patagonia y Tierra del Fuego, Argentina. 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Veizer, J., Ala, D., Azmy, K., Bruckschen, P., Buhl, D., Bruhn, F., Carden, G.A.F., Diener, A., Ebneth, S., Godderis, Y., Jasper, T., Korte, Ch., Pawellek, F., Podlaha, O.G., Strauss, H., 1999. 87Sr/86Sr, d 13C and d 18O evolution of phanerozoic seawater. Chemical Geology 161, 59–88. Zinsmeister, W.J., 1981. Middle to late Eocene invertebrate fauna from the San Julián Formation at Punta Casamayor, Santa Cruz province, Argentina. Journal of Paleontology 55, 1083–1103. Correlation of marine beds based on Sr- and Ar-date determinations and faunal affinities across the Paleogene/Neogene boundary in southern Patagonia, Argentina Introduction Geological setting and previous age determinations Materials and methods Samples Sample preparation and analytical techniques Results and discussion Isotopic and radiometric ages Paleontological affinities Conclusions Acknowledgements References