Fig. 11. Vertebrate specimens from Lago Posadas; some from the old collections at YPM and MLP, and new specimens collected by SFV, that belong to the MPM (Río Gallegos, Santa Cruz Province, Argentina). A. The phorusrhacid Psilopterus bachmanni (YPM-VPPU 15904)=Pelecyornis pueyrredonensis (Holotype); B. The cavioid rodent Schistomys rollinsii (YPM-VPPU 15959 (Holotype); C. The litoptern Tetramerorhinus mixtum (YPM-VPPU 15838)=Proterotherium dodgei (Holotype); D. The toxodontid Nesodon cornutus (YPM-VPPU 16012, Holotype); E., F. and G. The rodents Perimys sp. (MLP 84-III-2-40) and Scleromys sp. (MLP 84-III-2-9), scale bar=0.5 cm, and Neoreomys sp. (MPM-PV 17471), scale bar=1 cm; H. The typothere Interatherium sp. (MLP 84-III-2-32), scale bar=1 cm; I. The toxodontid Homalodotherium sp. (MPM-PV 17474), scale bar=5 cm; J. The dasypodid armadillo Proeutatus sp. (MPM-PV 17465), scale bar=1 cm; K. A Propalaehoplophorinae glyptodont (MPM-PV 17472), scale bar=1 cm; L. A megatherioid sloth cf. Hapalops sp. (MPM-PV 17460), scale bar=1 cm.

4.5. Paleoecological analysis

The paleoecological analysis was based on body mass, diet and substrate preference and use. Results are displayed in table 2, which includes categories for these biological attributes following Kay et al. (2012).

Most mammals recorded fall in body mass categories III and IV (19 taxa between one and 100 kg). Category I (10-100 grams) is not represented. The only mammal certainly smaller than one kilogram (II) is the rodent Perimys. About one third (nine) of the taxa fall within category III (1-10 kg): one marsupial, one armadillo, all typotheres, and two rodents. Another third (10 taxa) fall within category IV (10-100 kg): one armadillo, glyptodonts, most sloths and proterotheriid litopterns. The remaining third fall in categories V (100-500 kg) and VI (>500 kg). Category V includes one sloth, the homalodotheriid toxodont and the macraucheniid litoptern; category VI the toxodontid and the astrapothere. The largest taxa are the notoungulates Homalodotherium (400 kg) and Nesodon (600 kg) and one astrapothere (900 kg).

Among herbivorous mammals, grazers are represented by one rodent (Schistomys) of about one kilogram, four genera of typotheres of less than ten kilograms and one toxodontid (mixed-feeder) of about 600 kg. Browsers are represented by one genus of a mostly arboreal sloth (Hapalops) that ranged between 30 and 50 kg, and a more terrestrial megatheriid sloth of about 120-200 kg, four genera of litopterns between 10 and 120 kg, a 400 kg homalodotheriid and an almost one-ton astrapotheriid. The only proposed frugivore is the seven-kilogram rodent Neoreomys. There are not specialized seed feeders. There are no strict arboreal taxa suchs microbiotheres, porcupines, and primates.

The carnivore guild is represented only by one hathliacynid metathere, of about six kilograms and mainly arboreal. Scavengers are represented by two armadillos between one and 15 kg. There are not strict insectivorous neither specialized myrmecophagous.

Among birds there is one omnivorous rhea of about 50 kg, one small predatory phorusrhacid of about 5 kg, and one scavenging New World vulture.

5. Discussion

5.1. Depositional environments and temporal stratigraphic changes

The SCF south of Lago Posadas is composed of an alternation of mudstone and sandstone beds, rich in terrestrial vertebrate remains, being the mudstones the dominant deposits.

The base of the SCF in Lago Posadas is not a sharp surface but a transition interval of about 20 m-thick composed of estuarine fine-grained deposits (Cuitiño et al., 2015). Small biostromes formed by the oyster Crassostrea obignyi are a distinctive element within this interval. Since no distinctive boundary between the SFC and other underlying marine deposits is present, these oysters where used in other localities as marker horizons (Griffin and Parras, 2012; Cuitiño et al., 2013; Cuitiño et al., 2015). This interval represents the passage from fully marine-estuarine to terrestrial deposits without any major temporal gap. This is supported by the lack of major physical discontinuities, as well as by the existing geochronological data (Blisniuk et al., 2005; Perkins et al., 2012; Cuitiño et al., 2015) which shows no significant temporal gaps throughout the Miocene sedimentary column of Lago Posadas. The fluvial system of the SCF prograded into a shallow marine embayment that occupied a large NW-SE trough (Windhausen, 1924; Malumián and Náñez, 2011; Cuitiño et al., 2015), comprising the northern extreme of the Austral Basin and named as the Posadas Sub-Basin (Bellosi, 1995). The transitional passage from marine to terrestrial deposits allows describing this interval as a normal regression, part of a Highstand Systems Tract (Catuneanu, 2006).

Facies analysis for the overlying terrestrial portion of the SCF indicates that the whole unit was deposited by a low-gradient fluvial system that developed extensive fine-grained floodplains, crossed by sand-bed fluvial channels. Conglomerates are nearly absent in the system, only forming thin lags or lenticular bodies at the base of the larger channel deposits. The overall fine-grained nature of the fluvial system represented by the SCF in Lago Posadas, and the lack of thick conglomerate deposits typical of settings located close to mountain ranges (e.g., gravel-bed braided fluvial systems or alluvial fans, Miall, 1996), suggests the analyzed section of the SCF was located in a distal position in relation to the (Miocene) Andean orographic front. We hypothesize that because of high subsidence rates, the gravel sediments originated in proximal areas was retained in alluvial fans located at the foot of the early Miocene Andes, west of the study area, which were subsequently eroded by eastward migration of the deformational front (Fosdick et al., 2013; Ghiglione et al., 2016a, b) and finally not preserved in the study area. Following this idea, only the fine sediments (mud and sand) surpassed the alluvial fans and arrived to the adjacent eastern plains.

Fine-grained, floodplain-dominated fluvial systems are often referred as High Accommodation Systems Tract (Shanley and McCabe, 1994; Catuneanu, 2006). The SCF in Lago Posadas shows several features similar to those systems, such as channel deposits encased in floodplain mudstones, poorly-developed paleosols, large thicknesses and low sand/mud ratios (Catuneanu, 2006). Additionally, fine-grained, floodplain-dominated fluvial systems lacking evidences for lateral migration of channels, combined with low to medium (10 to 100) width-to-thickness (W/T) ratios for channel bodies, are regarded as formed by anastomosed rivers (Miall, 1996; Makaske, 2001). These systems have fixed channels that change positions through avulsions producing abundant crevasse splays deposits, as observed in the SCF in Lago Posadas. One of the main limiting agents for lateral channel migration observed in modern anastomosed rivers is floodplain stabilization by vegetation (Makaske, 2001). Although immature, the recurrent paleosol horizons may account for this vegetation growth and channel margins stabilization. This is particularly more evident for the lower third of the SCF, where paleosol horizons appear more frequently. River anastomosis can be considered as the consequence of upstream or downstream control. Modern examples suggest that sediment overloading of fluvial systems (upstream control) entering a low gradient foreland area may be the cause of anastomosis (Makaske et al., 2017). However, the long term anastomosing fluvial architecture can be achieved if the basin is constantly subsiding.

Dark reddish floodplain deposits (e.g., Fig. 6) suggest oxygenated environments with low proportion of preserved organic matter. The low organic matter content, plus the absence of any micro or macrofossil plant remains, indicates fast oxidation of organic matter after deposition. Abundant paleosol structures, including carbonate nodules, slickensides, soil peds and root traces, suggest recurrent paleosol formation (Retallack, 2001). However, paleosol horizonation is hardly recognized, suggesting poor soil development (inceptisols). Each poorly developed paleosol represents relatively short periods of landscape stability. The lack of well-developed paleosol horizons, the abundance of crevasse splay deposits and the floodplain-dominance in the whole fluvial system, suggest overall continuous aggradation under moderate to high sedimentation rates. These rates however, seem to have been relatively slower for the lower third of the unit where paleosols appear in a higher frequency. This is in agreement with the sedimentation rates calculated by means of geochronologic data by Blisniuk et al. (2005) for this locality.

From a paleoclimatic point of view, carbonate nodules (rhizocretions) indicate arid to semiarid conditions (Alonso-Zarza, 2003), which is in agreement with previous estimations for this locality (Blisniuk et al., 2005). Raigemborn et al. (2018) showed similar features for the SCF in the southeast of the Santa Cruz Province, interpreted as fluctuations of wetter and dryer paleoclimatic conditions at different temporal hierarchies.

Paleocurrent data indicate the fluvial system that the SCF represents drained towards the east (Fig. 8). Thus, the origin of the sediments most probably was the Southern Patagonian Andes. Sandstone composition shows a clear dominance of volcanic lithic particles. The lack of fresh, primary volcaniclastic particles (e.g., shards, pumice) in sandstones suggest their origin is through weathering and erosion of ancient volcanic rocks. West of the study area the Southern Patagonian Andes show abundant exposures of the Jurassic El Quemado Complex volcanic and volcaniclastic rocks (Fig. 1), which can be considered as the most probably source for the SCF sandstones, albeit other minor sources cannot be discarded.

Vertical stratigraphic changes in the SCF of Lago Posadas are subtle, and no easily recognized. Paleocurrent directions and sandstone composition show slight changes although without significant trends, suggesting the source of sediments for this sedimentary system remained the same. However, the detailed field observations of lithofacies and stacking patterns allowed pointing some slight stratigraphic trends. From base to top, the 460 m of terrestrial sediments show a slight increase in the ratio of sandstone/mudstone, as well as an increase in channel deposits grain-size and thickness (Fig. 9). The lower 100 m of the SCF show dominance of floodplain mudstones and sandstones with few channel deposits (Fig. 6). Coarse sandstone deposits are only present, although never dominant, in the upper part of the SCF, indicating a slight progradation of the sedimentary system. However, the mudstone/sandstone ratio remains essentially the same for the whole column, suggesting the accommodation space was uniform through time during the deposition of the unit. All these evidences allows defining the entire depositional system of the SCF as a High Accommodation Systems Tract (Catuneanu, 2006), although the vertical changes described points to a subtle upward lowering of accommodation.

An additional slight vertical trend is recognized within the floodplain mudstones. Paleosol horizons are relatively more abundant in the lower third of the unit. Although color variations can be observed at a high frequency (decimeters to meters, e.g., Fig. 4), a change of the dominant coloration is recognized along the column, from dark reddish-dominated at the lower third, to gray-dominated at the upper part. Primary sedimentary structures, such as lamination and ripple lamination are more common in floodplain deposits to the upper part, which is also associated to a low proportion of pedogenically modified levels. The cause of this better preservation of primary structures in the floodplain deposits of the upper part of the section can be associated to a lack or a weak paleosol development (Krauss, 1999; Retallack, 2001). This can be associated to recurrent episodes of high sedimentation rates in the floodplain for the upper two thirds of the SCF. Higher upward, sedimentation rates seem to be in contradiction to the upward reduction of accommodation suggested above. Thus, it is likely that these subtle changes observed for floodplain mudstones could be the consequence of the prevalence of sedimentary processes occurring at fast velocity, such as flash floods. Alternatively, this can be explained through a reduction of the velocity at which pedogenic processes occur, probably controlled by colder and/or dryer climatic conditions. Geochronologic data provided by Blisniuk et al. (2005) indicates low sediment accumulation rates in the lower part and relatively high rates in the upper part of the SCF. This differential accumulation rates can explain the relatively higher abundance of paleosol horizons to the lower part of the unit.

Based on stable isotopes from calcareous paleosol nodules, Blisniuk et al. (2005) detected a trend to more arid environments upward in the SFC column of Lago Posadas. They interpreted a major ecosystem change at ~16.5 Ma, which is roughly at the half of the column of the SCF. This means a climate change that could have been produced by the growth of the Andes and the subsequent instauration of the rain shadow effect in the foreland area. Similar conclusions, based mainly on sedimentological attributes, were obtained by Tauber (1999) for the exposures of the unit in the southeastern region of Santa Cruz Province (i.e., roughly 450 km southeastward), suggesting this climatic deterioration could be a regional phenomenon. This isotope-based proposed climate change is roughly recognized in the sedimentary facies of the SCF at Lago Posadas. Only some color changes in floodplain mudstones and frequency of paleosols can be observed, although these features could be explained through differential depositional rates. Whether these changes were controlled by climatic effects is somewhat uncertain. This implies that the proposed climate change had a minor effect on the fluvial depositional style, which remains essentially the same for the entire SCF in Lago Posadas.

The top of the SCF in the exposures studied south of Lago Posadas is truncated by modern glacio-fluvial deposits that form a flat terrace. Farther south, close to the central part of the Meseta Belgrano and at higher elevations, the unit is covered by the Belgrano Basalt (Fig. 1).

5.2. The SCF of Lago Posadas in a basinal correlation context

The terrestrial sediments that compose the SCF (and equivalents) are recognized in a huge area of southern Patagonia, particularly well-recorded in the Austral Basin. Several nomenclature proposals exist, and it is not the objective of this work to discuss the formal terminological issue.

North of the study area, two main regions expose early Miocene sediments equivalent to those described here. In the northern extreme the early Miocene fluvial deposits of the Pampa Guadal and Meseta Cosmelli of the Aysén Region of Chile, referred as the Galeras Formation or SCF (Niemeyer et al., 1984; Flint et al., 1994; Flynn et al., 2002; De la Cruz and Suárez, 2006; Bostelmann and Buldrini, 2012). Recently, Ugalde et al. (2015) defined these deposits as the SCF, which were interpreted as low to moderate energy fluvial deposits, suggesting temporal and paleoenvironmental equivalence to the SCF of Lago Posadas. Fossil mammals of Santacrucian times (Flynn et al., 2002) also confirms this time-correlation to that area.

About 50 to 100 km north of Lago Posadas and south of Lago Buenos Aires in the Santa Cruz province of Argentina, a thick column of terrestrial early Miocene strata is grouped as the Río Zeballos Group (Ugarte, 1956). From base to top the Río Jeinimeni, Cerro Boleadoras and Río Correntoso formations are defined (Ugarte, 1956), forming a clear coarsening up succession of approximately 1,000 m-thick (Escosteguy et al., 2003). Based on facies similarity, the mudstone-dominated lower part of the SCF at Lago Posadas can be correlated with the Río Jeinimeni Formation, whereas the sandier upper part of the SCF can be correlated to part of the Cerro Boleadoras Formation. Similar grain-size trends are also described for the SCF in Meseta Cosmelli (De la Cruz and Suárez, 2006; Ugalde et al., 2015), a large but isolated outcrop area in the Aysén region (Chile) that show remarkable stratigraphic similarities with the Argentinean exposures. The sand-dominated part of the Cerro Boleadoras Formation and the conglomeratic Río Correntoso Formation (Ugarte, 1956) both have no equivalent in our studied section of the SCF. Fossil mammal descriptions from these beds are nearly absent. Based on the record of certain anteater and rodents taxa, Carlini et al. (1993) and Vucetich (1994) indicated a Santacrucian ages for the Cerro Boleadoras Formation. The Río Zeballos Group suggests the proximal depositional systems appear to have moved eastward into the foreland basin as a response to eastward migration of the Andean deformational front. The lack of these coarse deposits in Lago Posadas suggests that the upper part of the SCF could have been removed by subsequent erosion. The lack of conglomeratic deposits in the upper reach of the SCF in Lago Posadas, such as those described for the Río Correntoso Formation (Ugarte, 1956; Escosteguy et al., 2003), can be explained by the removal of that part of the unit by subsequent erosion, evidenced by the flat modern terrace with a thin veneer of gravels that cover the unit in the area. This poses a question regarding the cause of this erosional episode, that appears to be absent in the northern region. One plausible explanation to this phenomenon could be the protection against erosion produced by modern plateau basalts. To the north, the equivalent Neogene succession shows a thick conglomerate succession which is directly covered by the Lago Buenos Aires Basalt (Escosteguy et al., 2003).

About 100 km to the northeast of Lago Posadas, some isolated outcrops of early Miocene terrestrial deposits were assigned to the Pinturas Formation (Bown and Larriestra, 1990). This unit is composed of a thin succession of pyroclastic rich deposits of aeolian origin, with mature paleosol development and several internal erosional unconformities (Bown and Larriestra, 1990). Some ages for this unit indicate partial synchronicity with the SCF in the Atlantic coast, varying roughly between 18 to 16 Ma (Fleagle et al., 1995; Perkins et al., 2012). Other dates, plus biostratigraphic and sedimentological evidence, indicate a somewhat older age for the lower section of the Pinturas Formation (Bown and Fleagle, 1993; Kramarz and Bellosi, 2005; Perkins et al., 2012). The abundant pyroclastic material, the aeolian deposits and the mature paleosols all mark a strong difference with the SCF studied herein. Although partly contemporaneous, the connection between both sedimentary systems is no clear since the sedimentologic and stratigraphic differences are notorious. The Pinturas Formation is a thin package of sediments that rest directly on the Jurassic volcanic basement rocks, suggesting that this area was part of the Austral Basin margin, receiving sediments mostly from wind-derived explosive volcanism.

South of the study area, the SCF is extensively exposed along a narrow belt associated to the foothills of the Southern Patagonian Andes and farther south these exposures extends to the east reaching the Río Chalía (=Sehuén) and Río Santa Cruz, and the coastal area from Monte León to Río Gallegos. Most of the geological knowledge for the SCF comes from this southern area (e.g., Tauber, 1997a, b; Matheos and Raigemborn, 2012; Fernicola et al., 2014; Raigemborn et al., 2015; Cuitiño et al., 2016). In contrast, few studies describe the exposures along the western Andean foothills zone. In the southwestern margin of the Meseta Belgrano, 20 to 30 km south of Lago Posadas, the SCF is 650 m-thick (Giacosa and Franchi, 2001). Farther south near Lago Argentino, Furque (1973) roughly described 500 m for the SCF. For the same area, Fernicola et al. (2014) described an isolated 100 m-thick outcrop of tuff-rich deposits of the lower part of the SCF at Río Bote. Detailed stratigraphic and paleoenvironmental reconstructions for the SCF at these western exposures are lacking.

5.3. Tectonic implications

Several lines of evidence allow inferring that the deposition of the SCF is strongly linked with the evolution of the Southern Patagonian Andes. The most obvious link between Andean evolution and sedimentation is the outcrop and thickness distribution for this unit. The SCF of Lago Posadas (460 m-thick) is part of a narrow north-south belt that lies just east of the Andean foothills. As pointed above, the unit shows the greatest thickness close to the Andes, whereas it thins or pinches out to the east and southeast, where Tauber (1999) reports 220 m for the SCF in coastal localities.

In most of the exposures of the Austral Basin, the basal strata of the SCF (and its equivalents) are described as a transitional passage from the underlying marine or estuarine sediments (Feruglio, 1938). In particular, this passage is described in detail at Lago Argentino (Cuitiño and Scasso, 2010; Cuitiño et al., 2013), Monte León (Griffin and Parras, 2012; Cuitiño et al., 2016) and Lago Posadas (Cuitiño et al., 2015). This implies that the SCF is part of a high hierarchy sedimentation period that also involves the Patagonian marine sediments occurred during the early-mid Miocene. This early Miocene sedimentary package is regionally distributed in a large area of the Austral Basin, involving at least 90.000 km² only in the Santa Cruz Province. Age constraints also suggest a close relationship between these early Miocene marine and continental deposits. The marine deposits were dated as Aquitanian-Burdigalian (23-18 Ma; Cuitiño et al., 2012; Parras et al., 2012; Cuitiño et al., 2015) whereas the terrestrial deposits were dated as Burdigalian-Langhian (18-14 Ma; Blisniuk et al., 2005; Perkins et al., 2012; Cuitiño et al., 2016). This means that no major breaks in sedimentation occurred during deposition of the early Miocene deposits in this portion of the Austral Basin.

The available uplift age constraints for the adjacent Southern Patagonian Andes indicate increased denudation rates starting at 30-23 Ma in the (western) basement domain and migrating eastward until 12 Ma (Thomson, 2002). Other geochronometers reflect synchronous regional exhumation across the orogen between 22 and 18 Ma, while lower temperature chronometers give exhumation ages between 10 to 4 Ma concentrated on the eastern fold and thrust belt (Thomson et al., 2010; Fosdick et al., 2013). The timing of uplift and erosion for the Southern Patagonian Andes coincides in part with the timing of deposition of the adjacent SCF. Considering that paleocurrent data for this unit suggest that the sediments arrived to the basin from the west (i.e., the Andes), it is reasonable to affirm that the SCF is a consequence of erosion in an actively rising orogen and subsequent deposition in the adjacent retroarc foreland basin.

Sandstone composition for the SCF in Lago Posadas indicates an immature tectonic setting for the origin of sediments (Fig. 8). Most of the particles are of volcanic composition typical of undissected volcanic arcs (Dickinson et al., 1983), suggesting the erosion of adjacent volcanic terrains. The scarcity of ash layers or juvenile volcanic particles such as glass shards or pumice particles, suggest little explosive volcanic activity during deposition of the SCF in this area. This is in marked contrast to the outcrops of the unit around Lago Argentino were primary pyroclastic material is abundant (Cuitiño and Scasso, 2013; Fernicola et al., 2014; Cuitiño et al., 2014), suggesting that for the time of deposition of the SCF, explosive volcanic activity was noticeably stronger at the southern part of the Austral Basin.

5.4. Considerations on the fossil collection sizes and preservation

The vertebrate fossil collection obtained from the SCF in the area of Lago Posadas is rather limited in relation to the prospecting effort, especially when compared with those localities of the southeast of Santa Cruz Province and Río Santa Cruz (see localities in Vizcaíno et al., 2012b and Fernicola et al., 2014). For instance, Bown and Fleagle (1993) reported that the joint expeditions by SUNY (USA) and Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” (MACN, Argentina), between 1982 and 1992, collected more than 500 specimens of the tiny Santacrucian palaeothentid marsupials (and many others belonging to other taxa), much of them from the single locality Monte Observación (now Cañadón de las Vacas, see Vizcaíno et al., 2015, 2016c). Joint expeditions of the MLP (Argentina) and Duke University (DU, USA), starting in 2003, collected more than 1,600 specimens (many well-articulated) from the coastal localities between Río Coyle and Cabo Buen Tiempo (Vizcaíno et al., 2012a). Finally, recent collections by a tripartite team (MACN-MLP-DU) have collected probably more than 2000 specimens from the outcrops of the Río Santa Cruz that are currently under study (J.C. Fernicola, personal communication, 2017). The collection made by Hatcher housed at YPM includes about 954 specimens, mostly mammals, and some birds, while Brown’s collection at AMNH includes about 94 specimens (Vizcaíno et al., 2013). Other old collections by H.T. Martin (1904) for the University of Kansas and E. Riggs for the Field Museum of Natural history of Chicago, collected between 235 and 395 specimens the former and 282 the latter, from the area between Río Gallegos and Río Coyle (Vizcaíno et al., 2013; Vizcaíno et al., 2016a).

Despite Hatcher’s impression during his first visit to Lago Posadas in 1898 (Hatcher, 1903), other expeditions, including his own second try, were not much successful in terms of number of specimens collected. In February-March 1899, Hatcher returned to these outcrops with a second collector from PU and the novel paleontologist of the AMNH, B. Brown. According to Hatcher (1903) they spent two weeks in the area, where they gathered a “…considerable collection of vertebrates and invertebrates of the Santa Cruzian, Cape Fairweather and Patagonian beds”. As already mentioned they must have collected between 50 and 60 specimens. Certainly, considering the number of personnel and days spent in the field, these amounts are much less than what can be collected from the SCF in other regions. As a guise of comparison with a contemporary collection, in 1887 Carlos Ameghino collected more than 2000 fossil pieces, comprising 120 new taxa (Rusconi, 1965; Fernicola, 2011a, b; Vizcaíno et al., 2013), from outcrops nearby the Río Santa Cruz and working alone for a month (Vizcaíno, 2011).

More recent expeditions did not gather larger collections either. In 1984, two members (Alfredo Carlini and Omar Molina) of the MLP expedition led by R. Pascual, collected about 20 specimens (Appendix 1), but observed some specimens in situ in the few hours they could spend in the outcrops (A. Carlini, personal communication, 2016); their exact location could not be established. In 2007 six members of the UNSJB-SUNY expedition spent two days in the area, and collected a few small mandibles (L. González Ruiz, personal communication, 2017) and some scutes of the armadillos Proeutatus and Stenotatus (Dasypodidae), and the glyptodonts Eucinepeltus and Propalaehoplophorinae indet. (Glyptodontidae) (González Ruiz, 2012).

As the whole sample of Lago Posadas is small (barely reaching hundred specimens), some groups are under-represented in comparison with other Santacrucian outcrops, for example those of the Atlantic coastal and nearby localities (Vizcaíno et al., 2012b). In the east, metatherians are represented by 11 species of sparassodonts (Prevosti et al., 2012), four microbiotheres and about 10 species of paucituberculatans (Abello et al., 2012). In Lago Posadas, only the hathliacynid Cladosictis patagonica was recorded, and no paucituberculatans and microbiotheres were documented so far.

Within xenarthrans, in the east region cingulates are represented by at least five genera of armadillos and four genera of glyptodonts (Vizcaíno et al., 2012c), and pilosans by one genera of anteater and 11 genera of sloths (Bargo et al., 2012). In Lago Posadas only two genera of armadillos, some osteoderms of glyptodonts indet., and four genera of sloths were recorded. Among native ungulates, notoungulates are diverse and abundant in eastern Santacrucian outcrops, including three genera of Toxodontia (two toxodontids and one homalodotheriid), and four genera of Typotheria (two interatheriids and two hegetotheriids) (Cassini et al., 2012). The record in Lago Posadas is good in terms of taxa: two genera of Toxodontia and four of Typotheria. The same is true for litopterns, represented by five genera in the coastal and nearby localities, four of them recorded at Lago Posadas. Astrapotheres include one genera and one species, and one specimen catalogued as Astrapotheriidae indet. Last, rodents are very frequent and diverse in the eastern localities, being represented by about 14 genera of the main four groups of caviomorphs (Cavioidea, Chinchilloidea, Octodontoidea and Erethizontoidea; Candela et al., 2012). At Lago Posadas only four genera of cavioids were recorded.

The preservation of the new remains collected by our team since 2010 is rather poor. We found only two specimens in situ, which consist of fragmentary skulls and mandibles of large notoungulates (Appendix 1). The fossils collected by the 1984 MLP and 2007 UNSJB-SUNY expeditions are also fragmentary, but according to A. Carlini, personal communication, 2017, the specimens in situ observed were in good conditions. This is in accordance with the fact that in the collection at YPM there are several well-preserved specimens, including an almost complete bird skeleton and skulls of different sizes, most of them extracted from concretions (Fig. 10).

The possibility exists that certain facies in restricted areas provided the concretions that contained the best-preserved specimens collected and those observed by the 1984 MLP crew, but we have been unable to find those particular spots yet. Based on the few in situ specimens collected by us, plus the observation of the sediment matrix from specimens housed in collections, it seems that many vertebrate fossils are preserved in sheet sandstone levels of FA 2 (floodplain deposits). These sandstones represent episodes of fast sedimentation rates in the floodplain produced during flood stages or during channel avulsions, which promoted quick burying of carcasses. Beyond that, it seems clear that the Lago Posadas area is not as fossiliferous as other extra-Andean outcrops of the SCF. Furthermore, the tiniest mammals, which are highly frequent in other localities, are lacking in all the collections made so far. For instance, as mentioned above, in taxonomic terms it implies the complete absence of pauciturbeculatan and microbiotherian marsupials, otherwise frequent in other places (Bown and Fleagle, 1993; Abello et al., 2012).

The cause of the poor fossil content relative to other exposures of the SCF of the Austral Basin is not clear, although different explanations can be considered. Paleoenvironments of deposition interpreted from SCF exposures from Lago Posadas and southern Santa Cruz region are similar. Matheos and Raigemborn (2012) and Raigemborn et al. (2015, 2018) highlight that the SCF in SE Santa Cruz Province was produced by a fluvial system with channels of low energy and low sinuosity, enclosed in a broad floodplain, conclusions that are analogous to those arrived in this work. Also, sedimentation rates for the SCF calculated for both regions are nearly similar (Perkins et al., 2012; Cuitiño et al., 2016) suggesting that this variable cannot be used as the cause of the difference in overall fossil abundance. In contrast, two main differences between them can be recognized. One is sediment composition, with very high proportion of pyroclastic material recognized for the SCF in the southeastern regions, whereas very few pyroclastic sediments are described for the SCF in Lago Posadas (see section 5.2). The recurrent supply of fine-grained pyroclastic material in the southeastern region that repeatedly mantled the paleosurfaces (Perkins et al., 2012; Matheos and Raigemborn, 2012; Raigemborn et al., 2015), could be the cause of the relatively higher preservation potential through fast burial of death animals, such as was previously proposed (e.g., Hunt, 1990). The second cause of differential fossil preservation is related to present-day rock stability. Rocks of the SCF in Lago Posadas are more resistant to erosion than in other outcrops further south and east, so the “release rate” of fossils could be comparatively much lower.

5.5. Biostratigraphic implications

The reduced number of specimens collected in the field effort performed in the last decades, and the absence of key taxa among them, makes only possible to assign the fossil collection to a Santacrucian Age sensu lato. Furthermore, as most specimens were found loose and, in some cases, the exact provenance of this material is not confidently established, their biostratigraphic value remains limited.

The study of the older collections at YPM allows postulating some interesting working hypothesis. In a series of exhaustive monographs in the Reports of the Princeton University Expeditions to Patagonia, edited by W. Scott between 1903 and 1932, several new fossil taxa were described based on material from Lago Pueyrredón. Up to date, these taxa have not been cited for other SCF localities. Among them, it is the phorusrhacid bird Pelecyornis pueyrredonensis Sinclair and Farr, which is now considered a synonym of Psilopterus bachmanni (Moreno and Mercerat) (Alvarenga and Höfling, 2003), a taxon present in other Santacrucian localities (Degrange et al., 2012 and references therein). According to Pérez (2010), Schistomys rollinsii Scott is a valid species of “ecoardid” rodent. However, because this species can only be distinguished from others of the same genus from cranial features, it cannot be ruled out its presence in other localities where the genus is represented only by teeth (M.E. Pérez, personal communication, 2016). Following González Ruiz (2010), Metopotoxus anceps Scott, corresponds to a different taxon from other Propalaehoplophorinae glyptodonts, although its inclusion within that genus is questionable. The sloth Hapalops vulpiceps Scott, and the toxodontid Nesodon cornutus Scott, have not been recorded in other areas besides Lago Posadas. The taxonomic validity of both taxa has not been formally questioned, but preliminary observations suggest that their morphology falls within the range of variability of other species of those genera (see Hernández del Pino, 2018 for Nesodon spp). Taxonomic revisions of all these taxa are necessary to adjust their biostratigraphic significance.

If Schistomys rollinsii, Metopotoxus anceps, Hapalops vulpiceps and Nesodon cornutus were confirmed as valid species which are absent in other localities of the SCF, this may be interpreted as reflecting either temporal, geographic or ecological differences, as it was also proposed for other Santacrucian faunas (Fernicola et al., 2014; Cuitiño et al., 2016). Certainly, if they were absent in synchronous levels of other localities, differences would imply ecological or geographical, rather than temporal, factors. A larger sample with more precise chronologic/stratigraphic control is also needed.

5.6. Paleoecology

As it was discussed above, the facies analysis for the SCF in Lago Posadas indicates that the unit was deposited in a low-gradient fluvial system with extensive floodplains traversed by sand-dominated fluvial channels. The abundance of dark reddish floodplain deposits and the lack of vegetal fossil remains, suggest oxygenated environments with low proportion of preserved organic matter. Some calcareous nodules, cutans and root traces suggest paleosol formation (Retallack, 2001), most of which show evidences of poor maturity. The lack of well-developed paleosol horizons suggests continuous, relatively high sedimentation rates. The presence of some paleosol horizons, in some occasion with carbonate nodules (calcretes), suggests the occurrence of short periods of floodplain stability and a semi-arid environment. This is especially evident for the lower third of the SCF, while for the upper two thirds the scarcity of paleosol horizons suggests environmental changes, probably linked to augmented sedimentation rates and/or climate deterioration. Similar sedimentary paleoenvironments are proposed for the SCF in the southeast of Santa Cruz (Tauber, 1994; Matheos and Raigemborn, 2012; Raigemborn et al., 2015, 2018), from which most of the paleoecological data for the SCF is derived. In these later examples, a marked seasonality was proposed (Kay et al., 2012; Raigemborn et al., 2015, 2018).

The integrated list of taxa recorded by the PU and MLP expeditions, and by us do not allow performing an exhaustive paleoecological analysis such as that of Kay et al. (2012) for the exposures of the SCF in the Atlantic coast. The sample considered is not only small, but it may also be time-averaged, allowing only broad considerations about the paleoecology of the Lago Posadas fauna.

The record of arboreal, browser and frugivore mammals indicates the presence of trees. The occurrence of grazer mammals and rheas suggests also the existence of open environments. With the available evidence, it cannot be asserted if these two environments coexisted or alternated during all the time of deposition of the formation. However, it is clear that they did coexist in the Eastern areas, under mean annual temperature of about 20 °C and about 1,500 mm of mean annual precipitation, with a marked seasonality in day length (Kay et al., 2012).

Among primary consumers, most of the categories analyzed for the East are present in the mammal assemblage of Lago Posadas. Within them, the frugivores are underrepresented and there are not specialized seed feeders, although this could be an artifact. Albeit larger, the number of frugivore taxa in the East is also low, and the majority falls within the smaller body mass categories for mammals (I and II). Kay et al. (2012) speculated about the low number of frugivores, suggesting that could be a consequence of the extreme seasonality of fruit production at these latitudes or that the fruits may have been present but the frugivores were birds, which have much lower preservation potential than mammals.

Conversely, the secondary consumers are clearly underrepresented at Lago Posadas when comparing with other Santacrucian faunas. For instance, among mammals, only one sparassodont (hathliacynid) was recorded, a semiarboreal predator of body mass category III (less than 10 kilograms). In the coastal localities, the predator niche includes three genera of hathliacynids and five genera of borhyaenoids, comprising both semiarboreal and ground dwelling forms within the body mass categories II-IV (up to 50 kilograms). As secondary consumers are always much less abundant than primary consumers, this difference could be due to the comparatively small sample size of Lago Posadas.

Only one scavenger/insectivorous armadillo is recorded in Lago posadas, while scavengers are represented by two or three genera of armadillos in the coastal localities. Specialized insectivores are not recorded in Lago Posadas. In the coastal localities they are represented by a diversity of palaeothentiid and microbiotheriid metatheres and the enigmatic Necrolestes (all of less than one kilogram), and the proposed strict anteater Protamandua (four kilograms). Among birds, only one genera of small phorusrhacid appears at Lago Posadas, while in the coastal and nearby localities four genera of phorusrhacids and a seriema occupied the predator niche along with metatheres.

6. Conclusions

Sedimentologic field survey of the SFC in Lago Posadas revealed that the unit is composed of 460 m of a mud-dominated succession with sandstone intercalations, interpreted as deposited in a low-gradient fluvial system. The lowermost interval of the unit represents the transition from restricted marine deposits, which grade upward to fluvial deposits without any recognizable break in sedimentation. Floodplain deposits dominate the succession being composed of mudstones and thin sheet to wedge sandstones, the later deposited by crevasse splay episodes. In turn, relatively thick sandstone lenses indicate deposition within fluvial channels. Conglomerates are nearly absent in the succession. The dominance of crevasse splay deposits and the lack of evidence for channel lateral migration suggest deposition in an anastomosed fluvial setting. The ratio of floodplain/channel deposits allows classifying these deposits as a high accommodation fluvial system, associated to active subsidence during the time of deposition.

Reddish floodplain deposits dominate the lower third of the SCF, including horizons of poorly developed paleosols under well-drained, oxidizing conditions. The lack of well-developed paleosol horizons suggests continuous, relatively high sedimentation rates. The presence of some paleosol horizons, in some occasion with carbonate nodules (calcretes), suggests short periods of floodplain stability and a semi-arid temperate environment. This is especially evident for the lower third of the SCF, while for the upper two thirds the scarcity of paleosol horizons suggests environmental changes, probably linked to increased sedimentation rates and/or climate deterioration. Paleocurrent data from cross-bedded channeled sandstones indicate a strong unidirectional pattern for sediment dispersal from west to east, i.e., from the Southern Patagonian Andes to the foreland region. In turn, sandstone composition indicates dominance of lithic volcanic particles, which are thought to be originated after erosion of the basin basement assigned to the Jurassic El Quemado Complex, located to the west.

A slight coarsening upward trend is recognized for the SCF, as well as a subtle upward increase in sandstone proportion, suggesting progressive reduction in accommodation.

The exposures of the SCF in Lago Posadas are significantly less fossiliferous in terms of vertebrates than other classical localities in the south, such as those from the Atlantic coast and the Río Santa Cruz. Consequently, the list of fossils may under represent the real taxonomic richness of the locality. In addition, the vertebrate fossil sample is size-biased, with a noticeable under representation of the smaller taxa well recorded in other outcrops of the SCF. The cause of the reduced fossil vertebrates abundance may be explained by different sediment composition or different “release rate” of fossils.

The fossil vertebrates collected so far allows a Santacrucian Age sensu lato assignment for the fauna of the SFC at Lago Posadas. Taxonomic revisions of several taxa are needed in order to adjust their biostratigraphic significance. The specimens recently collected, summed to those in museum collections, show some differences with the typical Santacrucian association. These faunal differences in relation to other synchronous localities may reflect ecological or geographical, rather than temporal, controlling factors. The combined record of arboreal, browser and frugivores on one side, and grazer mammals and rheas on the other, suggest the presence of both trees and open environments for the SCF in Lago Posadas. Frugivores, among primary consumers, and the secondary consumers guild are under-represented due to sample and fossil remain size biases.

For sub-Andean localities of the SCF, the uplifting orogen acted as a primary control on basin subsidence, sediment supply and climate, thus providing a special signature on the sedimentary and paleontological record.

Acknowledgments
We especially thank the owners and workers of Estancia El Chacay for their logistic support during fieldwork in the area, and people who helped during fieldwork: J.C. Fernicola (MACN-CONICET), L. Cataldi (UBA), G. Ronda (IDEAN-UBA) and P. Alonso Muruaga (IGEBA-UBA). We also thank the collection and archives managers D. Brinkman (YPM), J. Gaulkin and S. Bell (AMNH), M. Reguero y A. Scarano (MLP) for access to the specimens and information, and several colleagues for the identification of specimens: F. Degrange, J.C. Fernicola, A. Racco, N. Muñoz, M. Fernández, M. Arnal, M.E. Pérez, A. Forasieppi, F. Prevosti, S. Hernández del Pino. We are grateful to the reviewers R. Ugalde and E. Bellosi for their comments and suggestions, which allowed improving this manuscript. This work was funded by Agencia Nacional de Promoción Científica y Tecnológica through the grants PICT 2013-398 (JIC), PICT 2013-389 (SFV) and PICT 2017-1081 (MSB), a postgraduate grant from the International Association of Sedimentologists (JIC), and National Science Foundation grants EAR0851272 and EAR1348259, and CP-030R-17 (R.F. Kay and SFV).

References

Abello, M.A.; Ortiz-Jaureguizar.E.; Candela, A.M. 2012. Paleoecology of the Paucituberculata and Microbiotheria (Mammalia, Marsupialia) from the late Early Miocene of Patagonia. In Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation (Vizcaíno, S.F.; Kay, R.F.; Bargo, M.S.; editors). Cambridge University Press: 156-172. Cambridge.

Alonso-Zarza, A.M. 2003. Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record. Earth Science Reviews 60: 261-298.

Alvarenga, H.M.F.; Höfling, E. 2003. Systematic revision of the Phorusrhacidae (Aves: Ralliformes). Papéis Avulsos de Zoologia 43 (4): 55-91. São Paulo.

Ameghino, F. 1906. Les formations sédimentaires du Crétacé Superieur et du Tertaire de Patagonie avec un parallele entre leurs faunes mammalogiques et celles de l’ancien continent. Anales del Museo Nacional de Buenos Aires 15: 1-568.

Bande, A.E. 2007. Geología y sedimentología del Cenozoico de la margen sur del Lago Posadas, provincia de Santa Cruz. Trabajo Final de Licenciatura (Inédito), Universidad de Buenos Aires: 97 p.

Bargo, M.S.; Toledo, N.; Vizcaíno, S.F. 2012. Paleobiology of the Santacrucian sloths and anteaters (Xenarthra, Pilosa). In Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation (Vizcaíno, S.F.; Kay, R.F.; Bargo, M.S.; editors). Cambridge University Press: 216-242. Cambridge.

Bellosi, E.S. 1995. Paleogeografía y cambios ambientales de la Patagonia central durante el Terciario medio. Boletín de Informaciones Petroleras 44 (5): 50-83.

Blisniuk, P.M.; Stern, L.A.; Chamberlain, C.P.; Idleman, B.; Zeitler, P.K. 2005. Climatic and ecologic changes during Miocene surface uplift in the Southern Patagonian Andes. Earth and Planetary Science Letters 230: 125-142.

Bostelmann, J.E.; Buldrini, K.E. 2012. Late-early Miocene fossil mammals of the Pampa Guadal area, Meseta Cosmelli, Aysén Region, Chilean Patagonia. In Simposio de Paleontología de Chile, No. 3, Abstract Book: 38-41.

Bostelmann, J.E.; Le Roux, J.P.; Vásquez, A.; Gutiérrez, N.M.; Oyarzún, J.L.; Carreño, C.; Torres, T.; Otero, R.; Llanos, A.; Fanning, C.M.; Hervé, F. 2013. Burdi­galian deposits of the Santa Cruz Formation in the Sierra Baguales, Austral (Magallanes) Basin: age, depositional environment and vertebrate fossils. Andean Geology 40 (3): 458-489. doi: 10.5027/andgeoV40n3-a04.

Bown, T.M.; Larriestra, C.N. 1990. Sedimentary paleoenvironments of fossil platyrrhine localities, Miocene Pinturas Formation, Santa Cruz Province, Argentina. Journal of Human Evolution 19: 87-119.

Bown, T.M.; Fleagle, J.G. 1993. Systematics, biostratigraphy, and dental evolution of the Palaeothentidae, later Oligoceneto Early-Middle Miocene (Deseadan-Santacrucian) Caenolestoid Marsupials of South America. Journal of Palaeontology, Memoir 67 (29): 1-76.

Burns, C.E.; Mountney, N.P.; Hodgson, D.M.; Colombera, L. 2017. Anatomy and dimensions of fluvial crevasse-splay deposits: Examples from the Cretaceous Castlegate Sandstone and Neslen Formation, Utah, USA. Sedimentary Geology 351: 21-35.

Candela, A.M.; Rasia, L.L.; Pérez, M.E. 2012. Paleobiology of Santacrucian caviomorph rodents: a morphofunctional approach. In Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation (Vizcaíno, S.F.; Kay, R.F.; Bargo, M.S.; editors). Cambridge University Press: 287-305. Cambridge.

Carlini, A.A.; Scillato-Yané, G.J.; Vizcaíno, S.F. 1993. Un myrmecophagidae (Vermilingua) del Mioceno de Cerro Boleadoras (Santa Cruz, Argentina). Ameghininana 30 (1): 102.

Cassini, G.H.; Cerdeño, E.; Villafañe, A.L.; Muñoz, N.A. 2012. Paleobiology of native ungulates (Meridiungulata: Astrapotheria, Litopterna and Notoungulata). In Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation (Vizcaíno, S.F.; Kay, R.F.; Bargo, M.S.; editors). Cambridge University Press: 243-286. Cambridge.

Catuneanu, O. 2006. Principles of Sequence Stratigraphy. Elsevier: 375 p. Amsterdam.

Chiesa, J.O.; Camacho, H.H. 1995. Litoestratigrafía del Paleógeno marino en el noroeste de la provincia de Santa Cruz, Argentina. Monografías de la Academia Nacional de Ciencias Exactas, Físicas y Naturales de Buenos Aires 11, Parte I: 9-15.

Cuitiño, J.I.; Scasso, R.A. 2010. Sedimentología y paleoambientes del Patagoniano y su transición a la Formación Santa Cruz al sur del Lago Argentino, Patagonia Austral. Revista de la Asociación Geológica Argentina 66 (3): 406-417.

Cuitiño, J.I.; Scasso, R.A. 2013. Reworked pyroclastic beds in the early Miocene of Patagonia: Reaction in response to high sediment supply during explosive volcanic events. Sedimentary Geology 289: 194-209.

Cuitiño, J.I.; Pimentel, M.M.; Ventura Santos, R.; Scasso, R.A. 2012. High resolution isotopic ages for the “Patagoniense” transgression in southwest Patagonia: stratigraphic implications. Journal of South American Earth Sciences 38: 110-122.

Cuitiño, J.I.; Ventura Santos, R.; Scasso, R.A. 2013. Insights into the distribution of shallow marine/estuarine early Miocene oysters from Southwestern Patagonia: sedimentologic and stable isotope constraints. Palaios 28: 583-598.

Cuitiño, J.I.; Fernicola, J.C.; Vizcaíno, S.F.; Bargo, M.S.; Kay, R.F. 2014. Variaciones proximales-distales (O-E) en la Formación Santa Cruz, Mioceno de la Cuenca Austral. In Reunión Argentina de Sedimentología, No. 14, Abstracts Book: 89-90. Puerto Madryn.

Cuitiño, J.I.; Ventura Santos, R.; Alonso Muruaga, P.J.; Scasso, R.A. 2015. Sr-stratigraphy and sedimentary evolution of early Miocene marine foreland deposits in the northern Austral (Magallanes) Basin, Argentina. Andean Geology 42 (3): 364-385. doi: 10.5027/andgeoV42n3-a05.

Cuitiño, J.I.; Fernicola, J.C.; Kohn, M.; Kay, R.F.; Trayler, R.; Naipauer, M.; Bargo, M.S.; Vizcaíno, S.F. 2016. U-Pb geochronology of the Santa Cruz Formation (early Miocene) at the Río Bote and Río Santa Cruz (southernmost Patagonia, Argentina): implications for the correlation of fossil vertebrate localities. Journal of South American Earth Sciences 70: 198-210.

Dal Molin, C.N.; Colombo, F. 2003. Sedimentación neógena en la Cuenca del Río Zeballos y del Río Jenimeni (47º de Latitud Sur). Antepaís Patagónico, Argentina. Geogaceta 34: 139-142.

De Barrio, R.E.: Scillato-Yané, G.; Bond, M. 1984. La Formación Santa Cruz en el borde occidental del Macizo del Deseado (Provincia de Santa Cruz) y su contenido paleontológico. In Congreso Geológico Argentino, No. 9, Actas IV: 539-556. San Carlos de Bariloche.

Degrange, F.J.; Noriega, J.I.; Areta, J.I. 2012. Diversity and paleobiology of the Santacrucian birds. In Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation (Vizcaíno, S.F.; Kay, R.F.; Bargo, M.S.; editors). Cambridge University Press: 138-155. Cambridge.

De la Cruz, R.; Suárez, M. 2006. Geología del área Puerto Guadal-Puerto Sánchez, Región Aisén del General Carlos Ibáñez del Campo. Servicio Nacional de Geología y Minería, Carta Geológica de Chile, Serie Geológica Básica 95: 58 p., 1 mapa escala 1:100.000. Santiago.

Dickinson, W.R.; Beard, L.S.; Brakenridge, G.R.; Ejavec, J.L.; Ferguson, R.C.; Inman, K.F.; Knepp, R.A.; Lindberg, F.A.; Ryberg, P.T. 1983. Provenance of North American Phanerozoic sandstones in relation to tectonic setting. Geological Society of America Bulletin 94: 222-235.

Eren, M.; Kadir, S.; Kapur, S.; Huggett, J.; Zucca, C. 2015. Colour origin of Tortonian red mudstones within the Mersin area, southern Turkey. Sedimentary Geology 318: 10-19.

Escosteguy, L.; Dal Molin, C.; Franchi, M.; Geuna, S.; Lapido, O. 2003. Hoja Geológica 4772-II Lago Buenos Aires. Programa Nacional de Cartas Geológicas de la República Argentina 1:250.000. Servicio Geológico Minero, Boletín 339: 80 p.

Fernicola, J.C. 2011a. Implicancias del conflicto Ameghino-Moreno sobre la colección de mamíferos fósiles realizada por Carlos Ameghino en su primera exploración al río Santa Cruz, Argentina. Revista Museo Argentino de Ciencias Naturales 13 (1): 41-57.

Fernicola, J.C. 2011b. 1886-1888: Ascenso, auge y caída de la sociedad entre Florentino Ameghino y Francisco P. Moreno. In Vida y obra de Florentino Ameghino (Fernicola, J.C.; Prieto, A.; Lazo, D.; editors). Asociación Paleontológica Argentina, Publicación Especial 12: 35-49. Buenos Aires.

Fernicola, J.C.; Cuitiño, J.I.; Vizcaíno, S.F.; Bargo, M.S.; Kay, R.F. 2014. Fossil localities of the Santa Cruz formation (early Miocene, Patagonia, Argentina) prospected by Carlos Ameghino in 1887 revisited and the location of the Notohippidian. Journal of South American Earth Sciences 52: 94-107.

Feruglio, E. 1938. Relaciones estratigráficas entre el Patagoniano y el Santacruciano en la Patagonia Austral. Revista del Museo de La Plata, Geología 4: 129-159.

Feruglio, E. 1949. Descripción Geológica de la Patagonia. Dirección General de Yacimientos Petrolíferos Fiscales, 3 Tomos, T1: 1-323; T2: 1-349; T3: 1-331. Buenos Aires.

Fleagle, J.G.; Bown, T.M.; Swisher III, C.C.; Buckley, G.A. 1995. Age of the Pinturas and Santa Cruz Formations. In Congreso Argentino de Paleontología y Bioestratigrafía, No. 6, Actas: 129-135. Trelew.

Flint, S.S.; Prior, D.J.; Agar, S.M.; Turner, P. 1994. Stratigraphic and structural evolution of the Tertiary Cosmelli Basin and its relationship to the Chile triple junction. Journal of the Geological Society 151: 251-268.

Flynn, J.J.; Novacek, M.J.; Dodson, H.E.; Frassinetti, D.; McKenna, M.C.; Norell, M.A.; Sears, K.E.; Swisher, C.C.; Wyss, A.R. 2002. A new fossil mammal assemblage from the southern Chilean Andes: implications for geology, geochronology, and tectonics. Journal of South American Earth Sciences 15: 285-302.

Folk, R.L.; Andrews, P.B.; Lewis, D.W. 1970. Detrital sedimentary rock classification and nomenclature for use in New Zealand. New Zealand Journal of Geology and Geophysics 13: 937-968.

Fosdick, J.C.; Grove, M.; Hourigan, J.K.; Calderón, M. 2013. Retroarc deformation and exhumation near the end of the Andes, southern Patagonia. Earth and Planetary Science Letters 361: 504-517.

Furque, G. 1973. Descripción geológica de la Hoja 58b, Lago Argentino, provincia de Santa Cruz. Servicio Nacional Minero Geológico, Boletín 140: 1-49.

Ghiglione, M.C.; Naipauer, M.; Sue, Ch.; Barberón, V.; Valencia, V.; Aguirre-Urreta, B.; Ramos, V.A. 2015. U-Pb zircon ages from the northern Austral basin and their correlation with the Early Cretaceous exhumation and volcanism of Patagonia. Cretaceous Research 55: 116-128.

Ghiglione, M.C.; Ramos, V.A.; Cuitiño, J.; Varberón, V. 2016a. Growth of the Southern Patagonian Andes (46-53°S) and their Relation to Subduction Processes. In Growth of the Southern Andes (Folguera, A.; Naipauer, M.; Sagripanti, L.; Ghiglione, M.C.; Orts, D.L.; Giambiagi, L.; editors). Springer Earth Systems Sciences: 201-240.

Ghiglione, M.C.; Sue, Ch.; Ramos M.E.; Tobal, J.E.; Gallardo, R.E. 2016b. The relation between Neogene denudation of the Southernmost Andes and sedimentation in the offshore Argentine and Malvinas Basins during the opening of the Drake Passage. In Geodynamic Evolution of the Southernmost Andes (Ghiglione, M.C.; editor). Springer Earth Systems Sciences: 109-135.

Giacosa, R.; Franchi, M. 2001. Hojas Geológicas 4772-III y 4772-IV Lago Belgrano y Lago Posadas, Provincia de Santa Cruz. Programa Nacional de Cartas Geológicas de la República Argentina. Servicio Geológico Minero Argentino, Instituto de Geología y Recursos Minerales, Boletín 256.

Gibling, M.R. 2006. With and thickness of fluvial channel bodies and valley fills in the geological record: a literature compilation and classification. Journal of Sedimentary Research 76: 731-770.

González Ruiz, L. 2010. Los Cingulata (Mammalia, Xenarthra) del Mioceno temprano y medio de Patagonia (edades Santacrucense y “Friasense”). Revisión sistemática y consideraciones bioestratigráficas. Ph.D. Thesis (Unpublished), Universidad Nacional de La Plata: 486 p.

González Ruiz, L. 2012. Los Cingulata (Mammalia, Xenarthra) del Santacrucense sensu lato de Lago Posadas (Argentina). Ameghiniana 49 (4): R28.

Gorring, M.L.; Kay, S.M.; Zeitler, P.K.; Ramos, V.A.; Rubiolo, D.; Fernández, M.I.; Panza, J.L. 1997. Neogene Patagonian plateau lavas: continental magmas associated with ridge collision at the Chile Triple Junction. Tectonics 16: 1-17.

Griffin, M.; Parras, A. 2012. Oysters from the base of the Santa Cruz Formation (late Early Miocene) of Patagonia. In Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation (Vizcaíno, S.F.; Kay, R.F.; Bargo, M.S.; editors). Cambridge University Press: 83-90. Cambridge.

Hatcher, J.B. 1900. Sedimentary rocks of southern Patagonia. American Journal of Science (1880-1910), Serie 4 (9): 85-108.

Hatcher, J.B. 1903. Narrative of the expedition. In Reports of the Princeton University Expeditions to Patagonia, 1896-1899. Vol. 1, Narrative and geography (Scott, W.B.; editor). Princeton University Press: 296 p.

Hernández del Pino, S.E. 2018. Anatomía y Sistemática de los Toxodontidae (Notoungulata) de la Formación Santa Cruz, Mioceno Temprano, Argentina. Ph.D. Thesis (Unpublished), Universidad Nacional de La Plata: 1-415.

Hunt, R.M.Jr. 1990. Taphonomy and sedimentology of Arikaree (lower Miocene) fluvial, eolian, and lacustrine paleoenvironments, Nebraska and Wyoming; A paleobiota entombed in fine-grained volcaniclastic rocks. In Volcanism and fossil biotas: Boulder, Colorado (Lockley, M.G.; Rice, A.; editors). Geological Society of America, Special Paper 244: 69-111.

Kay, R.F.; Vizcaíno, S.F.; Bargo, M.S. 2012. A review of the paleoenvironment and paleoecology of the Miocene Santa Cruz Formation. In Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation (Vizcaíno, S.F.; Kay, R.F.; Bargo, M.S.; editors). Cambridge University Press: 331-365. Cambridge.

Kramarz, A.G.; Bellosi, E.S. 2005. Hystricognath rodents from the Pinturas Formation, Early-Middle Miocene of Patagonia, biostratigraphic and paleoenvironmental implications. Journal of South American Earth Sciences 18: 199-212.

Krauss, M.J. 1999. Paleosols in clastic sedimentary rocks: their geologic applications. Earth-Science Reviews 47: 41-70.

Makaske, B. 2001. Anastomosing rivers: a review of their classification, origin and sedimentary products. Earth-Science Reviews 53: 149-196.

Makaske, B.; Lavooi, E.; Haas, T.; Kleinhans, M.G.; Smith, D.G. 2017. Upstream control of river anastomosis by sediment overloading, upper Columbia River, British Columbia, Canada. Sedimentology 64: 1488-1510.

Malumián, N.; Náñez, C. 2011. The Late Cretaceous-Cenozoic transgressions in Patagonia and the Fuegian Andes: foraminifera, palaeoecology, and palaeogeography. Biological Journal of the Linnean Society 103: 269-288.

Marshall, L.G.; Hoffstetter, R.; Pascual, R. 1983. Mammals and stratigraphy: geochronology of the continental mammal-bearing Tertiary of South America. Paleovertebrata, Mémoire Extraordinaire: 1-93.

Martin, H.T. 1904. A collecting trip to Patagonia, South America. Transactions of the Kansas Academy of Science 19: 101-104.

Matheos, S.D.; Raigemborm, M.S. 2012. Sedimentology and paleoenvironment of the Santa Cruz Formation. In Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation (Vizcaíno, S.F.; Kay, R.F.; Bargo, M.S.; editors). Cambridge University Press: 59-82. Cambridge.

Miall, A.D. 1985. Architectural-Element Analysis: A New Method of Facies Analysis Applied to Fluvial Deposits. Earth-Science Reviews 22: 261-308.

Miall, A.D. 1996. The geology of fluvial deposits. Sedimentary facies, basin analysis, and petroleum geology. Springer-Verlag: 582 p. Berlin.

Muñoz, N.A. 2017. Esqueleto apendicular de tipoterios (Notoungulata) y caviomorfos (Rodentia) de la Formación Santa Cruz (Mioceno Inferior alto). Implicancias paleoecológicas. Ph.D. Thesis (Unpublished), Universidad Nacional de La Plata: 290 p.

Niemeyer, H.; Skármeta, J.; Fuenzalida, R.; Espinosa, W. 1984. Hojas Peninsula de Taitao y Puerto Aysén. Servicio Nacional de Geología y Minería, Carta Geológica de Chile, Serie Geología Básica 60-61: 80 p.

Palazzesi, L.; Barreda, V.D.; Cuitiño, J.I.; Guler, M.V.; Tellería, M.C.; Ventura Santos, R. 2014. Fossil pollen records indicate that Patagonian desertification was not solely a consequence of Andean uplift. Nature Communications 5 (3558). doi: 10.1038/ncomms4558.

Parras, A.; Dix, G.R.; Griffin, M. 2012. Sr-isotope chronostratigraphy of Paleogene-Neogene marine deposits: austral Basin, southern Patagonia (Argentina). Journal of South American Earth Sciences 37: 122-135.

Pascual, R.; Ortega Hinojosa, E.J.; Gondar, D.; Tonni, E.P. 1965. Las Edades del Cenozoico mamalífero de la Argentina, con especial atención a aquellas del territorio bonaerense. Anales de la Comisión de Investigaciones Científicas de la Provincia de Buenos Aires 6: 165-193.

Pérez, M.E. 2010. Sistemática, ecología y bioestratigrafía de Eocardiidae (Rodentia, Hystricognathi, Cavioidea) del Mioceno temprano y medio de Patagonia. Ph.D. Thesis (Unpublished), Universidad Nacional de La Plata: 385 p.

Perkins, M.E.; Fleagle, J.G.; Heizler, M.T.; Nash, B.; Bown, T.M.; Tauber, A.A.; Dozo, M.T. 2012. Tephrochronology of the Miocene Santa Cruz and Pinturas Formations, Argentina. In Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation (Vizcaíno, S.F.; Kay, R.F.; Bargo, M.S.; editors). Cambridge University Press: 23-40. Cambridge.

Prevosti, F.J.; Forasiepi, A.M.; Ercoli, M.D.; Turazzini, G.F. 2012. Paleoecology of the mammalian carnivores (Metatheria, Sparassodonta) of the Santa Cruz Formation (late Early Miocene). In Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation (Vizcaíno, S.F.; Kay, R.F.; Bargo, M.S.; editors). Cambridge University Press: 173-193. Cambridge.

Raigemborn, M.S.; Matheos, S.D.; Krapovickas, V.; Vizcaíno, S.F.; Bargo, M.S.; Kay, R.F.; Fernicola, J.C.; Zapata, L. 2015. Paleoenvironmental recon­struction of the coastal Monte León and Santa Cruz formations (Early Miocene) at Rincón del Buque, Southern Patagonia: A revisited locality. Journal of South American Earth Sciences 60: 31-55.

Raigemborn, M.S.; Krapovickas, V.; Beilinson, E.; Peral, L.E.G.; Zucol, A.F.; Zapata, L.; Kay, R.F.; Bargo, M.S.; Vizcaíno, S.F.; Sial, A.N. 2018. Multiproxy studies of Early Miocene pedogenic calcretes in the Santa Cruz formation of southern Patagonia, Argentina indicate the existence of a warm temperate vegetation adapted to a fluctuating water table. Palaeogeography, Palaeoclimatology, Palaeoecology 500: 1-23.

Ramos, V.A. 1989. Andean foothills structures in northern Magallanes Basin, Argentina. American Association of Petroleum Geologists 73: 887-903.

Reineck, H.; Wunderlich, F. 1968. Classification and origin of flaser and lenticular bedding. Sedimentology 11: 99-104.

Retallack, G.J. 2001. Soils of the Past. An introduction to paleopedology. Blackwell Science: 404 p.

Riggi, J.C. 1957. Resumen geológico de la zona de los Lagos Pueyrredón y Posadas, Provincia de Santa Cruz. Revista de la Asociación Geológica Argentina 12 (2): 65-96.

Rusconi, C. 1965. Carlos Ameghino. Rasgos de su vida y su obra. Revista del Museo de Historia Natural de Mendoza 17: 1-162.

Russo, A.; Flores, M. A. 1972. Patagonia austral extraandina. In Geología Regional Argentina (Leanza, A.F.; editors). Academia Nacional de Ciencias de Córdoba: 809 p.

Shanley, K.W.; McCabe, P.J. 1994. Perspectives on the Sequence Stratigraphy of Continental Strata. American Association of Petroleum Geologists, Bulletin 78: 544-568.

Simpson, G.G. 1940. Review of the mammal-bearing Tertiary of South America. Proceedings of the American Philosophical Society 83: 649-709.

Tauber, A. 1994. Estratigrafía y vertebrados fósiles de la Formación Santa Cruz (Mioceno Inferior) en la costa Atlántica de Santa Cruz, República Argentina. Ph.D. Thesis (Unpublished), Universidad Nacional de Córdoba: 422 p.

Tauber, A.A. 1997a. Bioestratigrafía de la Formación Santa Cruz (Mioceno inferior) en el extremo sudeste de la Patagonia. Ameghiniana 34: 413-426.

Tauber, A.A. 1997b. Paleoecología de la Formación Santa Cruz (Mioceno inferior) en el extremo sudeste de la Patagonia. Ameghiniana 34: 517-529.

Tauber, A.A. 1999. Los vertebrados de la Formación Santa Cruz (Mioceno Inferior-Medio) en el extremo sureste de la Patagonia y su significado paleoecológico. Revista Española de Paleontología 14: 173-182.

Thomson, S.N. 2002. Late Cenozoic geomorphic and tectonic evolution of the Patagonian Andes between latitudes 428S and 468S: An appraisal based on fission-track results from the transpressional intra-arc Liquiñe-Ofqui fault zone. Geological Society of America Bulletin 114: 1159-1173.

Thomson, S.T.; Brandon, M.T.; Tomkin, J.H.; Reiners, P.W.; Vásquez, C.; Wilson, N.J. 2010. Glaciation as a destructive and constructive control on mountain building. Nature 467: 313-317. doi:10.1038/nature09365.

Toledo, N. 2016. Paleobiological integration of Santacrucian sloths (early Miocene of Patagonia). Ameghiniana 53: 100-141.

Toledo, N.; Cassini, G.H.; Vizcaíno, S.F.; Bargo, M.S. 2014. Mass estimation in Santacrucian sloths from the Early Miocene Santa Cruz Formation of Patagonia, Argentina. Acta Paleontologica Polonica 59: 267-280.

Ugalde, R.; Bostelmann, E.; Buldrini, E.K.; Oyarzún, J.L. 2015. Lithofacies, architecture, and depositional environments of the Santa Cruz Formation in Chilean Patagonia. In Congreso Geológico Chileno, No. 14, Actas 1: 816-820. La Serena.

Ugarte, F.R.E. 1956. El grupo Río Zeballos en el flanco occidental de la Meseta Buenos Aires (Provincia de Santa Cruz). Revista de la Asociación Geológica Argentina 11 (3): 202-216.

Vizcaíno, S.F. 2011. Cartas para Florentino desde la Patagonia. Crónica de la correspondencia édita entre los hermanos Ameghino (1887-1902). Publicación Especial de la Asociación Paleontológica Argentina 12: 51-67.

Vizcaíno, S.F.; Bargo, M.S. 2013. Patagonia a Budapest: caminos europeos de fósiles santacrucenses (1845-1956). Revista de la Fundación Museo de La Plata 26: 36-42.

Vizcaíno, S.F.; Kay, R.F.; Bargo, M.S. 2012a. Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation. Cambridge University Press: 392 p. Cambridge.

Vizcaíno, S.F.; Kay, R.F.; Bargo, M.S. 2012b. Background for a paleoecological study of the Santa Cruz Formation (late Early Miocene) on the Atlantic Coast of Patagonia. In Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation (Vizcaíno, S.F.; Kay, R.F.; Bargo, M.S.; editors). Cambridge University Press: 1-22. Cambridge.

Vizcaíno, S.F.; Fernicola, J.C.; Bargo, M.S. 2012c. Paleobiology of Santacrucian glyptodonts and armadillos (Xenarthra, Cingulata). In Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation (Vizcaíno, S.F.; Kay, R.F.; Bargo, M.S.; editors). Cambridge University Press: 194-215. Cambridge.

Vizcaíno, S.F.; Bargo, M.S.; Fernicola, J.C. 2013. Expediciones paleontológicas durante los siglos XIX y XX a la Formación Santa Cruz (Mioceno Inferior, Patagonia) y destino de los fósiles. In Congreso Argentino de Historia de la Geología, No. 3, Actas: 231-246. Salta.

Vizcaíno, S.F.; Fernicola, J.C.; Bargo, M.S.; Cuitiño J.I.; Kay, R.F.; Toledo, N.; Muñoz, N.A.; Hernández del Pino, S.; Spradley, J. 2015. Afloramientos fosilíferos de Cañadón de las Vacas (Formación Santa Cruz; Mioceno Temprano). Importancia bioestratigráfica y paleoecológica. In Jornadas Argentinas de Paleontología de Vertebrados, No. 29. Diamante.

Vizcaíno, S.F.; Brinkman, P.D.; Kay, R.F. 2016a. On the objectives and results of the Handel T. Martin paleontological expedition (1903-04) to the Santa Cruz Formation in southern Patagonia. Revista del Museo de La Plata, Special Number 1: 316-333.

Vizcaíno, S.F.; Bargo, M.S.; Cassini, G.H.; Toledo, N. 2016b. Forma y función en paleobiología de vertebrados. Editorial de la Universidad Nacional de la Plata (Edulp): 268 p. http://sedici.unlp.edu.ar/handle/10915/55101.

Vizcaíno, S.F.; Bargo, M.S.; Fernicola, J.C.; Cuitiño, I.; Trayler, R.; Kohn, M.J.; Kay, R.F.; Toledo, N.; Muñoz, N.A. 2016c. The Cañadon de las Vacas-Rincón del Buque area: preliminary correlations with other Santacrucian (EarlyMiocene, Patagonia) localities. In Congreso de la Asociación Paleontológica Argentina, No. 11, Resúmenes: 102-103. Río Negro. Argentina.

Vucetich, M.G. 1994. La fauna de roedores de la Formación Cerro Boleadoras (Mioceno inferior?) en la provincia de Santa Cruz (Argentina). Acta Geologica Leopoldensia 39 (1): 365-374.

Windhausen, A. 1924. Líneas generales de la constitución geológica de la región situada al oeste del Golfo San Jorge. Boletín de la Academia Nacional de Ciencias 27: 167-320.

Zambrano, J.J.; Urien, C.M. 1970. Geological Outline of the Basins in Southern Argentina and Their Continuation off the Atlantic Shore. Journal of Geophysical Research 75: 1363-1396.

Manuscript received: December 4, 2017; revised/accepted: August 1, 2018; available online: February 4, 2019.

Andean Geology, ISSN: 0718-7106

Journal Edited by Servicio Nacional de Geología y Minería 
Chilean Government

  This work is licensed under a Creative Commons Attribution 4.0 License.