TRABAJOS DE INVESTIGACIÓN

Sr ages for the Chenque Formation in the Comodoro Rivadavia region (Golfo San Jorge Basin, Argentina): Stratigraphic implications

 

José I. Cuitiño ^1*; Roberto A. Scasso ^2,3; Roberto Ventura Santos ⁴; Luis H. Mancini ⁴

¹ Centro Nacional Patagónico (CENPAT), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). Boulevard A. Brown 2915, Puerto Madryn 9120, Chubut, Argentina. jcuitino@cenpat-conicet.gob.ar
² Universidad de Buenos Aires, Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales.
³ Instituto de Geociencias Básicas y Ambientales de Buenos Aires (IGeBA), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). Intendente Güiraldes 2160 (C1428EHA), Ciudad Autónoma de Buenos Aires, Argentina. rscasso@gl.fcen.uba.ar
⁴ Universidade de Brasília, Instituto de Geociências. Campus Universitário Darcy Ribeiro (CEP 70910-900), Brasília, Brazil. rventura@unb.br; geochronos.lais@gmail.com

Received March 21, 2015
Accepted August 2, 2015
Available online August 19, 2015

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ABSTRACT

The age and stratigraphic subdivisions for the late Oligocene-Miocene, marine deposits of Patagonia (Patagoniense succession) have been largely debated. Most studies for the Comodoro Rivadavia region were focused on its biostratigraphy and stratigraphic relations with the underlying and overlying mammal-bearing terrestrial strata. We report here thirteen new 86Sr/87Sr ages obtained from wellpreserved oyster valves of the Chenque Formation in the classical Patagoniense outcrops around the city of Comodoro Rivadavia (Golfo San Jorge Basin, Argentina). According to these ages, the deposition of the unit in the region occurred between 19.69 and 15.37 Ma (Burdigalian-early Langhian). A lower interval of normal sedimentation rate (Sequences 1 and 2) and an upper interval of high sedimentation rate (Sequence 3) are defined according to these new ages of the Chenque Formation. Subsequently, the depositional age of the Chenque Formation is compared with other well-dated marine successions of southern Patagonia located within the Austral Basin. This analysis suggests that the Austral and Golfo San Jorge basins were flooded by the Atlantic sea at about the same time (~21-20 Ma), inundating what was before a vast continental region of southern Patagonia. A renewed phase of continentalization occurred at about 18 Ma in the Austral Basin and at about 15 Ma in the study region of the Golfo San Jorge Basin. This 3 my difference combined with the elevated sedimentation rate estimated for the upper part of the study interval, suggest that the Golfo San Jorge Basin may have locally experienced relatively high subsidence rates during the early Miocene, allowing prolonged marine sedimentation in the Comodoro Rivadavia area. Consequently, the accumulation of the terrestrial deposits assigned to the Santa Cruz Formation started at about 15 Ma in the study region, later than in other localities of southern Patagonia. This results clearly indicate a remarkable interdigitation between the Santa Cruz and Chenque formations in a west-east transect across central Patagonia.

Keywords: Chenque Formation; Miocene; Sr isotopes; Patagoniense transgression; Golfo San Jorge Basin.

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INTRODUCTION

The shallow-marine Patagoniense deposits comprise a sedimentary succession mostly of late Oligocene- Miocene age that crops out extensively in Patagonia (Fig. 1). This succession accumulated in several basins, particularly in the extra-Andean region, as a result of a major Atlantic transgression. The depositional age, stratigraphic subdivisions, and correlations in such a large area have been a matter of debate since the first studies by Darwin, D'Orbigny, Florentino and Carlos Ameghino, Hatcher, and Ortmann (for a detailed list of contributions see Ameghino 1906; Feruglio, 1949-1950; Bertels, 1970; Camacho, 1995; del Río, 2004; Parras et al., 2012; Cuitiño et al., 2012). Recently, a number of publications devoted to the geochronology of the Miocene deposits in the Austral Basin provided a high-resolution chronostratigraphic framework for the Patagoniense succession in that region (Cuitiño et al., 2012; Parras et al., 2012; Perkins et al., 2012; Cuitiño et al., 2015). However, the precise age of the Patagoniense and related terrestrial Miocene deposits in the Golfo San Jorge Basin remained uncertain.

Figure 1. Regional location of the studied sites. The shadowed area represents the distribution of Patagoniense deposits after Malumián et al. (1999). The approximate boundaries of the Golfo San Jorge (GSJ) and Austral basins are indicated with brown, dashed lines.

The objective of this paper is to report new 87Sr/ 86Sr ages from oyster shells that allow constraining the depositional age of the marine deposits belonging to the Chenque Formation in the Comodoro Rivadavia region, a classic Patagoniense locality of the Golfo San Jorge Basin. The new data, checked by means of stable isotopes and stratigraphic coherence, is consistent with age determinations for the Austral Basin, providing a solid across-basin chronostratigraphic framework for the Patagoniense succession and overlying terrestrial units (i.e., the Santa Cruz Formation). In light of the new results, a second objective is to discuss the implications of these new ages for the stratigraphy of marine and terrestrial strata of the Golfo San Jorge Basin during the early Miocene.

STRATIGRAPHIC BACKGROUND

Stratigraphy of the Patagoniense succession
Ameghino (1906), based on mollusk biostratigraphy and stratigraphic relations with terrestrial mammal-bearing strata, subdivided his marine Formación Patagónica into an older Juliense and a younger Leonense stages. This author also recognized a younger marine stage within his overlying Formación Santacruceña, called the Superpatagoniense stage. This stratigraphic scheme was proposed for most of the Cenozoic coastal outcrops of Chubut and Santa Cruz provinces, including the study area of this contribution. It is not the objective of this work to focus on the long-lived discussion about these stages. An exhaustive analysis on the evolution of the stratigraphic proposals for this unit can be found in Feruglio (1949-1950), Camacho (1979) and Griffin and Parras (2012). Camacho (1979, 1995) analyzed the historical meaning and use of terms such as Formación Patagónica, Patagoniano and Patagoniense, whereas Legarreta and Uliana (1994) discussed the biostratigraphic criteria used to date these units. In this contribution we refer as "Patagoniense" to all the late Oligocene-early Miocene marine successions of Patagonia related to the Atlantic transgression. In the Austral Basin (Fig. 1), the Patagoniense succession comprises several lithostratigraphic units (Fig. 2). In the southeast region of Santa Cruz it is represented by strata of the San Julián and Monte León formations (Bertels, 1970)), whereas in the southwest of Santa Cruz and Sierra Baguales in Chile, it is represented by the Estancia 25 de Mayo Formation (Cuitiño and Scasso, 2010; Bostelmann et al., 2013). In the northwest of Santa Cruz and the region of Aisén in Chile, it comprises the El Chacay/ Guadal formations (Chiesa and Camacho, 1995; De la Cruz and Suárez, 2006). Finally, in the Golfo San Jorge Basin (Fig. 2), strata of the Patagoniense comprise the Chenque Formation (Bellosi, 1990). The older San Julián Formation recognized in southeastern Santa Cruz was traditionally assigned to the Late Eocene-Oligocene, but recent studies suggest its age is limited to the Oligocene (Chattian; Parras et al., 2012; Fig. 2). This unit is excluded from our analysis because time-equivalent deposits are not present in the Comodoro Rivadavia area.

Figure 2. Chronostratigraphic chart of the studied succession and correlative units in southern Patagonia (Austral Basin) based on isotopic age determinations. Modified after Cuitiño et al. (2012). Superscript numbers indicate source of information: 1) Cuitiño et al. (2012); 2) Bostelmann et al. (2013); 3) Cuitiño et al. (2015); 4) Blisniuk et al. (2005); 5) Parras et al. (2012); 6) Perkins et al. (2012); and 7) This work. S1 to S5 indicate the approximate age of depositional sequences defined by Bellosi (1987, 1990). Green color represents units composed largely of marine deposits; orange color represents units comprising mostly continental sediments.

Age estimations for the Patagoniense succession have been approached from different disciplines, which include physical correlation and sequence stratigraphy (Legarreta and Uliana 1994; Bellosi, 1995), palynology (Bellosi and Barreda, 1993), invertebrate biostratigraphy (Ameghino, 1906; Camacho and Fernández, 1956; Bertels 1970; del Río 2004), and isotopic dating (Parras et al. 2008, 2012; Cuitiño et al., 2012; Cuitiño et al., 2015). The Patagoniense deposits were considered as old as Eocene (Camacho and Fernández, 1956) because it bears the bivalve Venericardia planicosta. Based on foraminifera studies Bertels (1970) assigned the Monte León Formation to the late Oligocene (Chattian), Bertels and Ganduglia (1977) gave a similar age to the Patagoniense deposits at Astra (near the city of Comodoro Rivadavia), and Malumián et al. (1999) suggested an age ranging between 26 and 20 Ma for the "Transgresión Patagoniana", mostly equivalent to the Monte León Formation. Later on, Malumián (2002) extended the age of the transgression in the Golfo San Jorge Basin up to the middle Miocene (Neogene climax). More recently, del Río (2004) defined several molluscan assemblages within the Patagoniesne succession. Two of them, the JR (Jorgechlamys centralis-Reticulochlamys borjasensis of late Oligocene-early Miocene age) and the NVG (Nodipecten sp.-Venericor abasolensis- Glycymerita camaronesia of latest early Miocene to earliest middle Miocene), were found in the sections close to Comodoro Rivadavia and Astra (Fig. 3). There, however, the age of the JR assemblage seems to be not older than early Miocene (see figure 15 of del Río, 2004), in agreement with the palynological associations of Barreda and Palamarczuk (2000).

Figure 3. Simplified geologic map of the study area after Sciutto (2000, 2008). Sample localities are highlighted with black stars.

Palynological studies and sequence-stratigraphic correlation with the Global Cycle Chart (Haq et al., 1987, 1988) led Bellosi and Barreda (1993) to assign ages from 25.5 to about 16 Ma for the Patagoniense in the Golfo San Jorge Basin. Mammal biostratigraphy and sequence-stratigraphic correlation with the same global chart allowed Legarreta and Uliana (1994) to propose an age between 30 and 21 Ma for the lower part of the Patagoniense in the area of Comodoro Rivadavia, and an age between 21 and 10.5 Ma for the upper part of the unit. For the same strata, Bellosi (1995) proposed two separated transgressions, an older one (Leonense) with ages between 26 to 21 Ma, and a younger one (Superpatagoniense), with ages between 19 to 16 Ma. Later on, Barreda (1996) assigned the Chenque Formation to the late Oligocene? - early to middle? Miocene in the area of Comodoro Rivadavia-Cañadón Bauman, based on four spore and pollen assemblages and their comparison with New Zealand, Australia and Antarctica assemblages. Palamarczuk and Barreda (1998) arrived to similar conclusions studying dynoflagellate cysts, but suggested a slightly younger age (early Miocene) for the lower part of the Patagoniense in the Cerro Chenque area. Barreda and Palamarczuk (2000) proposed four informal Angiosperm pollen and dinocyst associations that span from the late Oligocene to the late early/middle Miocene. The three younger associations (early Miocene- middle Miocene) are present in the area of Comodoro Rivadavia.

Isotopic ages for the Patagoniense succession
Recent advances in geochronology methods allow several authors to date the Miocene successions of the Austral Basin more precisely. This provided a more accurate chronostratigraphic framework for some key regional geologic events, such as the deposition of the Patagoniense succession, which allows for comparison to other areas of Patagonia. 87Sr/86Sr studies for the Patagoniense succession of several regions of Santa Cruz Province (Parras et al., 2008) yielded ages between 23.8 and 25.9 Ma (Chattian, late Oligocene) for the San Julián Formation at its type locality; between 21.2 and 22.8 Ma (Aquitanian, early Miocene) for the lower part of the Estancia 25 de Mayo Formation in Lago Argentino; and between 25 and 26.3 Ma (Chattian) for the equivalent Patagoniense El Chacay Formation deposits in the NW of the province. Additionally, Parras et al. (2008) provided a 40Ar/39Ar age of 20.48 Ma (early Miocene) in a tuff interbedded in the lower part of the Patagoniense section in SW Santa Cruz. Later on, Parras et al. (2012) reinforced the Chattian age of the San Julián Formation with additional 87Sr/86Sr ages and provided new Sr ages for the Monte León Formation between 22.1 and 17.9 Ma (Aquitanian-Burdigalian) in SE Santa Cruz province (Fig. 2). U-Pb ages in zircons from tuffs at two separated levels combined with87Sr/86Sr ages in oysters support a Burdigalian age-range between 20 and 18.8 Ma for the entire Patagoniense column at the Lago Argentino region (Estancia 25 de Mayo Formation), in southwest Santa Cruz (Cuitiño et al., 2012) (Fig. 2). The equivalent marine Patagoniense succession in NW Santa Cruz (El Chacay Formation) was dated by 87Sr/86Sr from 20.1 Ma near the base to ~18 Ma at the top (Cuitiño et al., 2015). The Burdigalian ages of the Patagoniense from SW to NW of Santa Cruz suggest that previous Chattian 87Sr/86Sr ages provided by Parras et al. (2008) in the same regions are anomalously older in comparison with the present chronostratigraphic scenario (Fig. 2).
Radiometric ages of the Santa Cruz Formation overlying all the marine Patagoniense successions in the Austral Basin are in concordance with the ages of the marine deposits (Fig. 2). In southeastern Santa Cruz the unit is dated by numerous 40Ar/39Ar data from tuff layers between 17.7 and 16 Ma (Perkins et al., 2012). Blisniuk et al. (2005) obtained six 40Ar/39Ar ages varying from 22.36 to 14.24 Ma for the Santa Cruz Formation in the NW part of the Santa Cruz province, although Perkins et al. (2012) and Cuitiño et al. (2015) suggested that accumulation of the continental Santa Cruz Formation started at about 18 Ma. Using detrital zircon U-Pb dating, Bostelmann et al. (2013) estimated the beginning of deposition of the Santa Cruz Formation in southwestern Santa Cruz at about 18.8 Ma, in concordance with the age of the underlying Patagoniense deposits in that region (Cuitiño et al., 2012) (Fig. 2). In contrast to the Austral basin, previous isotopic ages for the Patagoniense succession of the Golfo San Jorge Basin are rare. Low-precision K-Ar radiometric ages between 25±5 and 30±3 Ma (Linares, 1979; Riggi, 1979) were obtained for the Astra section, while Bellosi (1990) mentioned a radiometric age of 26±2 Ma for the base of the Chenque Formation at Pico Salamanca.

STUDY AREA

The study area is located in the northern flank of the Golfo San Jorge Basin, in the coastal zone around the city of Comodoro Rivadavia, Chubut Province, where the Sarmiento, Chenque and Santa Cruz formations crop out (Sciutto 2000, 2008; Figs. 2 and 3). The Sarmiento Formation is composed mainly of whitish tuffs, deposited in a terrestrial environment (Bellosi, 1995, 2010; Sciutto 2000, 2008). This unit is exposed in the badlands just north of Comodoro Rivadavia, around Astra and Caleta Córdova (Fig. 3), where it is overlain by the Chenque Formation. Abundant tuff beds allowed the Sarmiento Formation to be precisely dated at Gran Barranca, to the west of the study area. U-Pb ages constrained the age of this unit from 40 to 19 Ma (late Eocene to early Miocene; Dunn et al., 2013).
In the study area the Sarmiento Formation is solely represented by the Gran Barranca Member (Bellosi, 1995), which is dated at about 40 Ma (Dunn et al., 2013). The contact between the Sarmiento and Chenque formations dips gently to the south and "sinks" below the sea-level at Punta Borja (Fig. 3), where it can be traced in the subsurface (Bellosi, 1987; Sciutto, 2000). The Chenque Formation makes up most of the hills around Comodoro Rivadavia like the Cerro Chenque, Cerro Viteau and Cerro Antena. Five stratigraphic sequences were defined for the Chenque Formation at a regional scale (Bellosi, 1987; Paredes and Colombo, 2001; Paredes, 2003). The lower three of them, and the lower part of the fourth, are nicely exposed in the study area (Fig. 4). The Santa Cruz Formation, composed of sandstone and mudstone beds of fluvial origin (Sciutto 2000; 2008), transitionally overlies the Chenque Formation. It crops out in the northwestern sector of the study area, along the elevated cliffs of the Pampa del Castillo tableland (Fig. 3).

Figure 4. Stratigraphic position of oyster samples that were used to calculate the 87Sr/86Sr ages. a) Schematic composite section for the Chenque Formation integrating different outcrops from the Comodoro Rivadavia region (after Bellosi 1987, 1990; Paredes, 2003, and our observations). Sequences are numbered following Bellosi (1987) and Paredes (2003). SB: sequence boundary. b) Detailed section of the sampled interval at Cerro Viteau. c) Detailed section of the sampled interval at Caleta Córdova. Marker horizons after Feruglio (1949-1950). Green numbers are the 87Sr/86Sr ages (see Table 1).

METHODOLOGY

We sampled unbroken, well-preserved entire oyster valves for Sr-dating in the Chenque Formation. Most of the samples come from Cerro Chenque, Cerro Viteau and Cerro Antena, but the base of the unit was sampled at Caleta Córdova (Figs. 3 and 4). Each valve was cut and polished for microstructure observation and microsampling, following the criteria established by Cuitiño et al. (2012, 2013). About 30 mg of calcite powder was obtained from each valve using a microdrill and avoiding chalky layers. From each microsample, a small aliquot of carbonate powder was used to perform δ18O and δ13C isotopic analyses, in order to check for diagenetic alterations or freshwater input into the marine environment where the oysters lived. The δ18O and δ13C isotopic composition of the carbonates were determined by a Thermo Fisher DELTA V Plus Isotope Ratio Mass Spectrometer attached to Gas Bench II analyzer. The associated errors are 0.1‰ for δ18O and 0.05‰ for δ13C. Stable isotope results are expressed in the Pee Dee Belemnite (PDB) notation (Table 1).

Table 1. Sr, C, and O isotope data. Ages are derived from the Look Up table of McArthur et al. (2001).

* Average ages for samples of the same stratigraphic horizon, used for sedimentation rate calculations.

Aliquots used for Sr ratio determination were initially attacked with an acid solution following by Sr separation using Teflon columns filled with 0.5 ml of Eichrom Sr Resin/50-100 μm. The 87Sr/86Sr isotopic ratio determinations of the samples were obtained by a Neptune ICP Mass Spectrometer and the analytical accuracy was estimated by the analysis of the NBS standard 987, which produced a mean value of 0.710243 ± 0.0000102 (N = 21). Age calculations from 87Sr/86Sr are based on the Look-Up Table Version 4B: 08/04 of McArthur et al. (2001) with an uncertainty of 0.07 my derived from the mean curve. All 87Sr/86Sr, δ13C and δ18O analyses were carried out at the Geochronos Laboratory of the University of Brasilia, Brazil.

RESULTS

Thirteen 87Sr/86Sr ages were obtained from the oysters shells (Table 1). Carbon and oxygen isotopic studies were carried out in order to discard diagenetic recrystallization of the carbonate in the shells. The older 87Sr/86Sr ages were obtained in the Caleta Córdova region, where the contact between the Sarmiento and Chenque formations is well exposed. There, the lowermost 25 m of the Chenque Formation, composed of highly bioturbated, finegrained tuffaceous deposits with a few shell beds, overlies the continental white tuffs of the Sarmiento Formation (Fig. 4c). Three beds showing wellpreserved, large oysters in life position yielded ages between 19.69 and 19.02 Ma (Fig. 4c). At Cerro Chenque in Comodoro Rivadavia city, at about 80 to 90 m from the bottom of the unit, and within the upper half of a coarsening-upward succession (Sequence 1 of Bellosi, 1987; lower sequence of Paredes, 2003), autochthonous oysters yielded 87Sr/86Sr ages varying between 17.63 and 17.03 Ma (Fig. 4a). Upward in the section, at about 100 m stratigraphically above the samples of Cerro Chenque, three samples from Cerro Antena and other three from Cerro Viteau yielded consistent results between 15.85 to 15.5 Ma (Fig. 4a,b). Five of the sampled beds occur just below the "Banco del Mangrullo" and one just above the base of this bed (Fig. 4a,b). This prominent bed is a 4- m thick, coarse-grained complex shell bed with highly reworked shells (Fig. 4b), which can be used as a stratigraphic marker in all the sections around Comodoro Rivadavia, and, according to Bellosi (1987), represents the base of his Sequence 3 (Fig. 4a). Therefore the ages of the underlying beds would represent a minimum age (15.85 to 15.5 Ma) for the top of the underlying Sequence 2 of Bellosi (1987). The youngest sample analyzed in this contribution comes from a fossiliferous bed located 70 m stratigraphically above the Banco del Mangrullo at Cerro Antena (Fig. 4a). This sample, corresponding to the top of the Sequence 3 of Bellosi (1987) yielded an age of 15.37 Ma.

DISCUSSION AND FINAL REMARKS

The Chenque Formation is dated for the first time by means of 87Sr/86Sr. Age results are stratigraphically coherent (i.e., stratigraphically lower samples are older while the stratigraphically upper samples are younger) and carbon/oxygen isotopes indicate neither diagenetic alteration nor large freshwater input into the sea during deposition. The dated interval (Fig. 4a) comprises the lower three sequences of the unit as defined by Bellosi (1987) and Paredes (2003). The uppermost beds of the Chenque Formation (sequences 4 and 5 of Bellosi, 1987) are not present in the study area. The temporal interval involved in the deposition of the study interval at the Comodoro Rivadavia region is established from 19.69 to 15.37 Ma, younger than previously reported by other authors such as Camacho and Fernández (1956), Bertels and Ganduglia (1977), Bellosi (1995), Barreda (1996) and Malumián et al. (1999). The position of the oyster beds from Caleta Córdova into the sequence stratigraphic scheme of Bellosi (1987; 1995) and Paredes (2003) is somehow uncertain. These beds lie 2 to 20 m above the contact with the Sarmiento Formation (Fig. 4c) and provided the oldest ages (mean 19.29 Ma) for the Chenque Formation in the study area (Table 1). The Cerro Chenque-Punta Borja section is located 15 km south of the Caleta Córdova section (Fig. 3). There, the lower 100 m of the Chenque Formation were assigned to its Sequence 1 by Bellosi (1987, 1995) or to the lower sequence (Paredes, 2003). As it is the oldest sequence defined for the Chenque Formation in Comodoro Rivadavia, we assume that the beds from Caleta Córdova belong to this sequence. Because the basin depocenter is located to the south of the Comodoro Rivadavia area (Fitzgerald et al., 1990; Sylwan, 2001), it is possible that even older Patagoniense deposits might be preserved in the subsurface of that area (Fig. 2).
Assuming that the age of the lowermost beds at Caleta Córdova is synchronous with the lowermost beds at Cerro Chenque-Punta Borja (Fig. 4), and using the average ages for the overlying samples of Cerro Chenque (mean age 17.34 Ma, Table 1) the lower 80 m of the Chenque Formation in the Cerro Chenque- Punta Borja region (interval 1, Table 2) accumulated during an interval of about 1.9 my. This part of the unit belongs mostly to the Sequence 1 of Bellosi (1987) and the estimated duration points to sedimentation rates of about 41 m/my, on the average of terrigenous shelves (Ricken, 1991). Samples of Cerro Chenque are separated by a stratigraphic interval of about 110 m (interval 2, Table 2) from samples of Cerro Antena and Cerro Viteau, which occur at the top of Sequence 2 of Bellosi (1987). The estimated duration for this interval (1.55 my for Cerro Antena and 1.66 my for Cerro Viteau, Table 2) would imply that sedimentation rates during the accumulation of this sequence ranged from about 71 to 66 m/my, somewhat higher than for Sequence 1. The following interval up to the sample Antena-3 (Fig. 4a) comprise a stratigraphic interval of about 60 m (interval 3, Table 2), which corresponds mostly to Sequence 3 of Bellosi (1987). Using the average ages (Table 1), the temporal interval ranges from 0.42 my (Cerro Antena) to 0.31 my (Cerro Viteau), thus providing an estimated sedimentation rate varying from 143 to 194 m/my (Table 2). These values fall at the highest end for terrigenous shelves (Ricken, 1991), probably suggesting a relatively increase of sediment supply during the upper part of the study interval.

Table 2. Summary of sedimentation rates estimations derived from the 87Sr/86Sr ages. Because the sample position in the column does not coincide with sequence boundaries, calculations are made for intervals 1, 2 and 3 which are roughly equivalent to depositional Sequences 1, 2 and 3 (after Bellosi, 1987), respectively.

The ages reported in this paper are in agreement with other ages obtained for the Patagoniense succession elsewhere in southern Patagonia (Fig. 2). At Lago Posadas the Patagoniense (El Chacay Formation) is dated from 20.3 to 18 Ma (Cuitiño et al., 2015), whereas the equivalent Estancia 25 de Mayo Formation at Lago Argentino is dated between 20 to 18.8 Ma (Cuitiño et al., 2012; Bostelmann et al., 2013). In the southeastern sector of Santa Cruz, the Monte León Formation is dated from 22.12 to 17.9 Ma (Parras et al., 2012) (Fig. 2). All these partially equivalent marine units, together with the Chenque Formation, represent the Miocene Patagoniense deposits in southern Patagonia (Austral and Golfo San Jorge basins). Considering all the available isotopic ages, we argue that the sea flooded large areas of continental Patagonia at about 21-20 Ma. After the maximum depths were achieved, regressive marine deposits accumulated, displaying different thickness and facies style, in different temporal intervals. All across the Austral Basin a renewed phase of continentalization was established at about 18 Ma (Perkins et al., 2012; Parras et al., 2012; Cuitiño et al., 2012; Cuitiño et al., 2015), whereas in the Golfo San Jorge Basin the terrestrial systems were established later than 15 Ma (Fig. 2). The difference of at least 3 my in the timing of continentalization suggests that the Golfo San Jorge Basin might have experienced a relatively high subsidence rate in its eastern depocenter, close to Comodoro Rivadavia. In this area, even the highest sedimentation rates reported for the uppermost part of the successions (Table 2) were not capable of filling all the marine accommodation space being created.
The new isotopic ages for the lower three sequences of the Chenque Formation imply that the overlying Santa Cruz Formation in this region has to be younger than 15.37 Ma, therefore younger than the classic fossiliferous sections at the southeast of Santa Cruz province (Fig. 2), dated between 18 to 16 Ma (Perkins et al., 2012), and only partially coeval with the strata of Santa Cruz Formation near Lago Posadas, dated between 18 and 14 Ma (Blisniuk et al., 2005; Cuitiño et al., 2015). Moreover, the older strata of the Santa Cruz Formation in the Andean region are mostly coeval with the Patagoniense succession of Comodoro Rivadavia and suggest interdigitation of both units in a west-east transect across Patagonia.

Acknowledgements

We thank the comments and suggestions provided by Eduardo Bellosi and Silvio Casadío. The manuscript was also improved through the revision of the editor Ernesto Schwarz. We thank the staff of the Geochronos laboratory of the University of Brasilia (Brazil) for their help in Sr isotope analyses, and Roberto Pujana (MACN-CONICET) for his help in oyster preparation. This work was funded through grants of the Consejo Nacional de Investigaciones Científicas y Técnicas and the University of Buenos Aires.

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