Sedimentology and fossil vertebrates of the Santa Cruz Formation (early Miocene) in Lago Posadas, southwestern Patagonia, Argentina

Lago Posadas is located at the foot of the Southern Patagonian Andes, in southwestern Argentina, where the early Miocene Santa Cruz Formation (SCF) shows thick and laterally continuous exposures. This region has been scarcely explored for fossil vertebrates since the first efforts by J.B. Hatcher in 1898-99. In this contribution, we performed sedimentologic and paleontological studies in order to reconstruct depositional environments and the associated fossil vertebrate fauna. Sedimentologic data suggest that the sedimentary record begins with restricted marine-estuarine deposits grading upward to fluvial floodplains and fluvial channels. Extensive floodplains, occasionally interrupted by low-sinuosity, sand-dominated channels, show dominant reddish coloration, moderate to low paleosol development, abundant crevasse splay sandstones and lack of vegetal remains, suggesting deposition in a low gradient, oxygenated setting under elevated sedimentation rates. Vertical stratigraphic trends are subtle, suggesting little paleoenvironmental changes during deposition of the whole SCF in this region. Paleocurrent directions, sandstone composition and paleogeographic reconstructions all indicate that deposition of the SCF was strongly associated to the contemporaneous uplift of the Andes. Fossil vertebrates analyzed are the result of our collecting effort and revision of museum collections. The faunal assemblage includes 31 taxa: 28 mammals and three birds. Mammals belong to the main groups recorded in other areas of the SCF (metatherians, xenarthrans, notoungulates, litopterns, astrapotheres and rodents). The assemblage allows a Santacrucian Age sensu lato assignment for the fauna at Lago Posadas. Taxonomic revisions of several taxa are necessary to further adjust the biostratigraphic significance of this association. 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. Frugivores, among primary consumers, and the secondary consumers guild are under-represented due to sample and fossil remain size biases. The sedimentologic and paleontological record of the SCF in Lago Posadas suggests that the uplift of the Southern Patagonian Andes acted as a primary control on basin subsidence and sediment supply, providing a special signature for sub-andean localities. However, previously registered climatic changes are poorly recorded in this study.


Introduction
The Santa Cruz Formation (SCF) is one of the most relevant stratigraphic units of southern South America in terms of Miocene terrestrial stratigraphic record and vertebrate paleontology. It is widely distributed in southern Patagonia, being part of the synorogenic deposits of the Austral (or Magallanes) Basin and representing a sedimentary archive of a period that unraveled significant environmental changes in the region, driven mostly by the uplift of the adjacent Andean orogenic belt (Blisniuk et al., 2005;Ghiglione et al., 2016a). Vertebrate fossils from this unit have been collected since 1845 especially in the southern and eastern parts of Santa Cruz Province, and thousands of specimens, mostly mammals, are preserved in museums around the world Vizcaíno et al., , 2016a. They constituted the basis for the establishment of the early Miocene Santacrucian South American Land Mammal Age (SALMA; Pascual et al., 1965).
Santacrucian sensu lato fauna is also present in the West, in the Andes or nearby, from Río Turbio and Lago Argentino in the South, to Río Pinturas (Pinturas Formation) and Lago Posadas in the North.
The lower levels at some of these localities show faunal peculiarities that have been traditionally considered as corresponding to earlier stages than the typical Santacrucian faunas from exposures along the Río Santa Cruz and the Atlantic coast (Ameghino, 1906;Simpson, 1940;Marshall et al., 1983;Kramarz and Bellosi, 2005). Recently, Cuitiño et al. (2016) proposed that the temporal scenario now available (see below) suggests that at least some of these faunal differences must reflect ecological or geographic, rather than temporal, factors.
From a geologic perspective, the terrestrial deposits of the SCF are a significant element for the reconstruction of the Miocene paleoenvironments and the tectonic history of the adjacent Southern Patagonian Andes, which had been uplifted simultaneously to the deposition of this unit (Fosdick et al., 2013;Ghiglione et al., 2016a, b). The Miocene Andean uplift represents a major control on the paleogeographic reconfiguration as well as on the climatic systems of southern Patagonia (Blisniuk et al., 2005;Palazessi et al., 2014) that supposes significant environmental changes that should be recorded to some extent in the adjacent foreland basin deposits and its associated fossil content.
In this sense, Lago Posadas, located in the northwest of the Santa Cruz Province, is a key locality for studying the depositional environments and paleontology of the SCF for several reasons: a. the exposures are abundant, thick , laterally continuous and of variable quality; b. it is one of the few relatively accessible areas today in the remote northwestern sector of the Austral Basin; c. it is located at the foot of the Andes, where few studies for the SCF are available in the literature; d. it has been the focus of historical paleontological expeditions for fossil vertebrates; e. its age is relatively well established (Blisniuk et al., 2005;Cuitiño et al., 2015).
Although some paleoenvironmental interpretations have been made for the SCF in Lago Posadas (e.g., Giacosa and Franchi, 2001;Blisniuk et al., 2005), we know little about the stratigraphy, sedimentology, composition, paleoenvironments of deposition and paleoecology of the associated fauna for this unit in the northwestern sector of the Santa Cruz Province. The SCF in Lago Posadas yielded fossil vertebrates that were first collected by the pioneer expeditions by John B. Hatcher (Princeton University; PU) in 1898 and 1899, and described in the Reports of the Princeton University Expeditions to Patagonia, 1896-1899 (Hatcher, 1903). After more than a century from those discoveries, neither thorough paleontological work have been carried out nor publications about the area are available.
The objective of this work is to provide new field sedimentologic descriptions and to update the fossil vertebrate composition, based on recent fieldwork and a revision of museum collections, from this historic locality of northwestern Santa Cruz Province. We aim to integrate geologic and paleontological information in order to reappraise this locality and to integrate it in a broader geologic, paleontological and paleoenvironmental framework. We provide novel data for the reconstruction of the Miocene paleoenvironments and vertebrate paleoecology of southern Patagonia as well as some hypotheses about the evolution of the foreland basin associated to the uplift of the Southern Patagonian Andes.

Geology of the study area
The study area is located in the northwest of the Santa Cruz Province (Patagonia, Argentina), at the foot of the Southern Patagonian Andes and the northern end of the Austral Basin. Lago Posadas is a small lake next to the larger Lago Pueyrredón, both located in a glacial trough oriented NW-SE. Hatcher (1900Hatcher ( , 1903 refers to the area of Lago Posadas, i.e., between the Río Furioso to the west, and Río Tarde to the east (Fig. 1), as "Lake Pueyrredon". Fossil vertebrates coming from the SCF in this area collected by the Princeton University expeditions are also catalogued as coming from "Lake Pueyrredon".  Giacosa and Franchi (2001) and Cuitiño et al. (2015).
The study area shows exposures of the Cretaceous-Miocene infill of the Austral (or Magallanes) Basin (Giacosa and Franchi, 2001). The section of the SCF studied herein is exposed along the north margin of the Meseta Belgrano (Fig. 1), where several older stratigraphic units are also exposed. North of this margin, a wide and deep glacial valley develops, exposing the basement volcaniclastic rocks of the basin assigned to El Quemado Complex (Mid to Late Jurassic). To the west, cretaceous sedimentary units are exposed, including the marine Springhill, Río Mayer and transitional Río Belgrano formations, all mapped as "Cretaceous units" in figure 1. Above the former and outcropping at the base of the Lago Posadas-Meseta Belgrano cliffs, is the Río Tarde Formation (Aptian; Ghiglione et al., 2015) which is covered by the Eocene Posadas Basalt, which in turn is covered by the early to middle Miocene marine and continental deposits. The early Miocene marine deposits are considered as part of the Patagonian Transgression and were formally named as the El Chacay Formation (Chiesa and Camacho, 1995;Cuitiño et al., 2015), which in turn are followed by the continental deposits of the SCF. The top of the Miocene succession in this area, roughly about 1200-1,400 m above sea level (m a.s.l.), is truncated by an erosional surface which forms a plateau slightly dipping to the east. South of the northern margin and forming the core of the Meseta Belgrano is the Belgrano Basalt (late Miocene; Gorring et al., 1997) ( Fig. 1), which is located at the highest elevation within the plateau reaching up to 2000 m a.s.l.

Previous geo-paleontological studies of the SCF in the area
The first mention regarding the Miocene continental deposits and its fossil content in this area is that of John Bell Hatcher (Hatcher, 1900(Hatcher, , 1903, who visited the area for the first time in 1898 as part of the Princeton University Expeditions to Patagonia. Hatcher climbed the cliffs of the eastern end of what he first called "Lake Princeton", but time after recognized that the name Pueyrredón had priority (Hatcher, 1903). Detailed descriptions of the geography and geology of the area by Hatcher allow confirming that the exposures he visited are the same as that we describe in this work (Fig. 2). There Hatcher made a collection of vertebrate fossils from the beds he called the "Santa Cruzian formation" (Hatcher, 1903). The discovery of several vertebrate taxa, including a phorrusrhacoid bird, the notoungulate Nesodon, litopterns and carnivorous marsupials, allowed him to consider this fossil association as having a Santacrucian Age. Hatcher went back to the area in 1899 with Barnum Brown from the American Museum of Natural History (AMNH, New York, USA). They stayed a longer period, making a larger collection of Santacrucian fossil vertebrates. Most of his collection is nowadays in the Yale Peabody Museum (YPM, New Haven, USA), and a few, mostly unprepared, specimens are in the AMNH. Since then, few paleontological expeditions visited the area, probably due to the difficulty for accessing these outcrops. There was a lack of effort in making a systematic collection of new fossils, excepting brief visits by a team from the Museo de La Plata (MLP, Argentina) in 1984, and a joint expedition of the Universidad Nacional de la Patagonia San Juan Bosco (UNPSJB, Argentina) and the State University of New York at Stony Brook (SUNY, USA) in 2007(González Ruiz, 2012. In regard to the stratigraphic and sedimentologic knowledge, the SCF in this area is relatively unknown in comparison to other better known localities of Santa Cruz Province. Most studies were focused on easily accessible, fossiliferous equivalent strata in the southeast (Tauber, 1994(Tauber, , 1997aMatheos and Raigemborn, 2012;Vizcaíno et al., 2012a;Raigemborn et al., 2015), or central-western Cuitiño et al., 2016) areas of the province, or nearby Pinturas Formation in the west (Bown and Larriestra, 1990;Fleagle et al., 1995;Kramarz and Bellosi, 2005). Probably, the abundance of fossil vertebrates in those localities prompted the continuous interest of geoscientists. Albeit widely distributed, descriptions of the SCF in the southwest of Santa Cruz Province are scarce (Furque, 1973;Giacosa and Franchi, 2001;Cuitiño and Scasso, 2010;Bostelmann et al., 2013;Cuitiño et al., 2014Cuitiño et al., , 2016. Besides Hatcher, the oldest geological description of the SCF deposits of the study area corresponds to Riggi (1957), who described the regional stratigraphy of the area south of lakes Posadas and Pueyrredón. In this contribution the author described 100 m of the marine Patagonian beds (currently the El Chacay Formation, Chiesa and Camacho, 1995) overlying directly on the Posadas Basalt. Conformably covering the marine deposits, the author describes the Santacrucian (currently the SCF), a succession of continental mudstones, sandstones and few conglomerates that reach 800 m-thick.
Other mentions to these continental strata are included in regional studies, which provide little new data. Feruglio (1949) mentioned 500 m of continental Santacrucian deposits for the Lago Posadas region without detailed descriptions. Zambrano and Urien (1970) and Russo and Flores (1972), referring to all Miocene continental deposits overlying the Patagonian marine deposits of Austral Patagonia, used the term SCF for the first time. Ramos (1989) described more than 800 m of intercalated sandstones and mudstones rich in vertebrate fossils in the southwestern margin of Meseta Belgrano, which he assigned to the SCF.
More recently, Giacosa and Franchi (2001) added some descriptions of the sediments composing the SCF and mapped its distribution around the Meseta Belgrano and adjacent areas. Blisniuk et al. (2005) provided radiometric ages for the SCF within the study area and several stable isotope analyses from paleosol carbonate nodules, which allow the authors to estimate some climatic changes, occurred during the deposition of the unit and related to the Andean uplift. Bande (2007) described the lower part of the SCF and interpreted a fluvial paleoenvironment of deposition. Cuitiño et al. (2015) described and dated the underlying marine deposits, including the transitional deposits and the lower 20 m of the SCF in the area, and suggested the later was deposited in a low energy fluvial setting. In agreement with the original proposal of Feruglio (1938), the authors highlighted the conformable and gradational passage between the underlying shallow marine deposits and the low-energy fluvial deposits of the SCF above. Ugarte (1956) defined the Río Zeballos Group to include all the continental Miocene deposits that crop out in the western and southern border of the Meseta del Lago Buenos Aires, just north of the study area. This group includes from base to top the Río Jeinimeni, the Cerro Boleadoras and the Río Correntoso formations, and compose a coarsening upward succession, from mud dominated strata at the base, to gravel dominated strata at the top (Ugarte, 1956;Dal Molin and Colombo, 2003). Other contributions correspond to De Barrio et al. (1984) who described the lithology and vertebrate paleontology of the SCF deposits to the east of the study area. Part of the beds described by De Barrio et al. (1984) where later defined as Pinturas Formation by Bown and Larriestra (1990) in the surroundings of the Río Pinturas, some 100 km northeast of the study area. The age of the SCF in Lago Posadas is based on six 40 Ar/ 39 Ar dates provided by Blisniuk et al. (2005), with an age span of 22-14 Ma. Based on regional tephrochronology and sedimentation rates estimations, Perkins et al. (2012) questioned the age for the lower strata of the SCF of Lago Posadas and concluded it should be close to 18 Ma. The latter was also supported by Cuitiño et al. (2015) based on Sr-stratigraphy for the underlying marine deposits, whose younger beds are chronologically located in 18 Ma. These age estimations provide an age span of roughly 4 my, from 18 Ma to 14 Ma, for the SFC in Lago Posadas. A note of caution has to be made related to the younger age (14.24±0.78 Ma) obtained by Blisniuk et al. (2005), since the stratigraphic position of this dated level corresponds to a tuff located 3 m beneath the top of the SCF in a place located 10 km southeast of the locality studied herein. That locality, shows thicker exposures of the SCF, which means that the dated level is probably not represented in the locality studied in this work and consequently the age of the top of our section should be somewhat older than 14.24±0.78 Ma.

Materials and Methods
Geological and paleontological fieldwork was conducted in the area in three opportunities since 2010. Geological observations in the field included measurement and facies description for a sedimentary column located in the northern border of Meseta Belgrano, just south of Lago Posadas (47°35' S; 71°51' W), in one of the most complete sections for the unit (Fig. 1). This section is close to that measured by Blisniuk et al. (2005). Observations include bed thickness and geometry, sedimentary structures, grain size, paleocurrent directions as well as sampling for sandstone composition.
Besides the stratigraphic section, for some extended lateral exposures, the 2D geometry of sandstone bodies was evaluated by means of the width to thickness (W/T) ratio (Gibling, 2006), taking care of observing outcrop faces oriented perpendicular to mean paleocurrent directions. Paleocurrent directions were measured in cross bedded sandstones when a 3D exposure of the bed allows determining the true dipping direction. These were complemented in nearby outcrops between Río Furioso and Río Tarde ( Fig. 1) that allowed 3D examination of sedimentary bodies. Lithofacies analysis was done following the criteria of Miall (1985Miall ( , 1996, from which most of the lithofacies codes were taken. Coarse to medium sandstones were sampled for petrographic studies, selected from thick and indurated beds at stratigraphic intervals of approximately 40 m, covering the complete thickness of the studied strata. These were analyzed in thin section under polarized microscope and, in order to obtain a quantitative compositional scheme, 450 grains were counted for each. Sandstones are classified following the Q.F.L. (quartz, feldspar and rock fragments) ratios following Folk et al. (1970) and the tectonic provenance diagrams of Dickinson et al. (1983).
In total, paleontological prospecting and collecting efforts sum eight days of an experimented field paleontologist. The specimens collected belong to the Museo Regional Provincial P.M.J. Molina (MPM; Río Gallegos, Santa Cruz Province, Argentina). In addition, the collections made by Hatcher for PU (currently at YPM), Brown for the AMNH, and Rosendo Pascual and others for the MLP, were studied. The material revised is listed in Appendix 1.
A preliminary paleoecological analysis of the fauna recorded in Lago Posadas was performed following a general protocol based on body mass, diet and substrate preference and use (Vizcaíno et al., 2016b), applied by Kay et al. (2012) and Toledo (2016).
Body mass values and dietary and substrate preference and use categories were estimated by direct assignation to genera present in the Atlantic coastal exposures of SCF analyzed by Kay et al. (2012: Table 17.1). Mass values and categories for the genera not present in the coastal fauna were taken from Toledo et al. (2014) and Toledo (2016) for the sloths, Cassini et al. (2012) and Muñoz (2017) for notoungulates. M.E. Pérez (personal communication, 2017) provided mass estimation for the rodent Schistomis rollinsi, not present in the coastal fauna.

Sedimentology and facies analysis
In order to provide order to the field sedimentologic data for the SCF in Lago Posadas, twelve lithofacies were discriminated (Table 1), which are grouped into three Facies Associations (FA), namely FA 1, FA 2 and FA 3. Biostromes formed by the growing of a monospecific oyster community.

Ht
Heterolithic deposits composed of a millimeter to-centimeter-thick alternation of fine sandstones and mudstones. Current ripple lamination is common in sandstone layers.
Mud-sand alternations are produced by fluctuation in the energy of the currents, which produce alternating traction and decantation processes, probably caused by tidally-modulated currents.

Fl
Laminated claystones and siltstones. Desiccation cracks are present in some occasions.
Decantation of fines in lentic environments or deposition of silt by tractive currents during floods. Desiccation cracks indicate subaerial exposure.
Deposition of silt to clay sized sediments in lentic environments or tractive silt deposits produced during floods.

Fr
Dark reddish, greenish to gray mudstones with pedogenic structures such as mottles, rhizoliths, cutans and soil aggregates.
Soil development in fine-grained sediments. Primary sedimentary structures obscured especially by root emplacement.

Sm
Moderately to well sorted, massive fine to medium sandstones. Occasionally with pedogenic features.
Tractive deposits that lost the primary sedimentary structures by pedogenic modification after deposition, or deposition by flows with elevated sediment concentration producing primary structureless deposits.

Sh
Well sorted very fine to medium sandstones with parallel lamination.
Subaqueous sedimentation during upper flow regime flows probably caused by unconfined flows in the floodplain.

Sr
Moderately well sorted very fine to fine or medium sandstones with current ripple lamination.
Subaqueous sedimentation through migration of asymmetrical ripples during unidirectional lower flow regime tractive flows.

St
Well sorted fine to very coarse sandstones with medium scale trough cross stratification. Sets are usually of 20-40 cm-thick.

Sp
Well sorted fine to very coarse sandstones with medium scale tabular cross stratification. Sets are usually of 20-30 cm-thick.
Subaqueous sedimentation produced by migration of dunes with straight crests under unidirectional currents.

Sd
Cross-bedded sandstones showing evidence of deformation, such as overturned cross-beds, convolute stratification or dish structures.
Soft sediment deformation of sand deposits due to the friction of the overlying flow or sediment overloading.

Gh
Fine conglomerates with horizontal stratificacion forming thin and lenticular bodies. Sometimes with mud intraclasts.
Tractive currents forming longitudinal gravel bars in channel thalwegs.

Facies Association 1, coastal marine-terrestrial transition deposits
The boundary between the terrestrial SCF and the underlying marine El Chacay Formation is not a sharp contact, as early noted by Feruglio (1938) and endorsed by Cuitiño et al. (2015); consequently, the location of this limit in the field is not straightforward. On the contrary, it is a stratigraphic interval having particular sedimentological features with a thickness of about 20 m, which in this work is included into FA 1. Thus, FA 1 includes both the upper part of the El Chacay Formation and the lowermost part of the SCF. From a strictly lithostratigraphic point of view, the boundary between the El Chacay Formation and the SCF in this area is placed in the uppermost level of the oyster Crassostrea orbignyi, same criteria used in former publications (e.g., Cuitiño et al., 2015).
This FA represents only the 5% of the section and is dominated by lithofacies Ht, corresponding to a centimeter-to millimeter-thick alternation of sandstone, siltstone and mudstone layers (Fig. 3), arranged in lenticular, wavy or flaser bedding patterns (Reineck and Wunderlich, 1968). Sandstone and coarse siltstone layers usually reach up to 0.2 m-thick and show current ripple (lithofacies Sr) or parallel (lithofacies Sh) lamination. Occasionally, thicker sandstone beds, of up to 0.3 m, show tabular cross bedding (lithofacies Sp), parallel lamination (lithofacies Sh) or are massive (lithofacies Sm). Bioturbation is scarce, only represented by isolated Planolites isp., Teichichnus isp. and Thalassinoides isp. Some outcrops of the area show the presence of lenticular, monospecific oyster accumulations (lithofacies B) intercalated within heterolithic deposit. These are formed by articulated or disarticulated Crassostrea orbignyi specimens (see Figure 5C of Cuitiño et al., 2015) and are interpreted as biostromes. In some layers, leaf or particulate vegetal remains where observed in muddy heterolithic sediments. In all outcrops visited, FA 1 grades transitionally upward to FA 2 (see below).
Interpretation: according to the abundance of heterolithic deposits, which suggest tidal modulated currents, the presence of oysters in life position (i.e., not transported), and the presence of some trace fossils of marine origin such as Teichichnus isp. and Thalassinoides isp., FA 1 is interpreted having marine influence, most probably a coastal marine to terrestrial transitional environment. Invertebrate-rich marine deposits, as those observed in underlying deposits  are not observed in FA 1, suggesting stressing environmental conditions for marine life. Crassostrea orbigny beds were referred as estuarine organisms in the study area  as well as in other parts of the Austral Basin , capable of thriving in low salinity estuarine water bodies . The lack of coarsening upward meter-scale tabular sandstone deposits typical of beach or shorefaces sub-environments in this FA, suggest a protected coastal environment receiving low wave energy. Thin sandstone beds with tabular cross bedding (Sp) may be explained by occasional unidirectional currents probably induced by tides within small tidal creeks. Some complementary paleoenvironmental interpretations for these deposits in association to others similar are described as the Facies D (estuarine complex) in Cuitiño et al. (2015).

Facies Association 2, fluvial floodplain deposits
Facies Association 2 represents the 65% of the stratigraphic thickness of the SCF in the studied section. This FA is dominated by structureless mudstones (lithofacies Fm) and mudstones with paleosol structures (lithofacies Fr), with minor millimeter-to centimeter-thick sandstone intercalations (Fig. 4). Those levels showing massive mudstones with no visible soil structures are referred as lithofacies Fm. Beds of FA 2 are tabular and vary in thickness from some decimeters to up 15 m. Dark red hues are dominant for the mudstones although in some vertical cliffs, where floodplain mudstones are better FIG. 4. Superb exposure (albeit inaccessible) of the lower to middle part of the SCF showing thick floodplain fluvial deposits (FA 2), composed of intercalated mudstone and thin sandstone beds. Changes in mudstone coloration from reddish to greenish suggest high frequency changes in oxidation conditions. Sandstone beds show sharp bases, sometimes erosive, and commonly wedge shape. White arrows point to evident wedging out of some of these beds. Few thicker, lenticular fluvial channels (Ch) also intercalate. In the background, the Río Tarde (whitish), the Posadas Basalt (dark grey) and the El Chacay (greenish) formations are observed. exposed, some centimeter to decimeter-thick greenish to gray layers are visible. Usually, these layers are composed of coarser (silt to fine sand) material and appear intercalated within mudstone intervals along the entire sedimentary section (Figs. 4 and 5A). Pedogenic structures such as small root traces (Fig. 5C), slickensides and blocky structures are commonly observed in the mudstones (lithofacies Fr). Excepting for the coloration changes, paleosol horizonation is hardly recognized, although caution on this observation has to be made due to the poor quality of many of the exposures of the fine-grained levels. Isolated or partially coalescent carbonate nodules are observed in several mudstone layers (Fig. 5D). In some occasions, they show tubular shape and vertical orientation and can be regarded as rhizocretions. Lamination in mudstones (lithofacies Fl) as well as ripple or horizontal bedding (lithofacies Sr and Sh, respectively) are visible (Fig. 5A, B), although rare. Desiccation cracks are present occasionally (Fig. 5E), associated to laminated mudstones (lithofacies Fl). Tabular to wedge-shaped fine sandstone beds intercalate with mudstone deposits, ranging in thickness from a few centimeters to up to 0.8 m. They show sharp bases, in some cases showing erosive features (Fig. 4) and internal parallel lamination (lithofacies Sh) or are structureless (lithofacies Sm). In addition, some lenticular sandstone beds show planar base and convex-up top ( Fig. 6) and internal structures such as planar cross-bedding (lithofacies Sp) or parallel lamination (lithofacies Sh). Few small and isolated lenticular sandstone bodies with concave up base and planar top were also recognized. Interpretation: the dominance of mudstones, either structureless or with paleosol structures, suggests deposition in a low-energy, low-gradient environment with frequent subaerial exposure where soils developed, typical of fluvial floodplains (Miall, 1996). Exposures of structureless mudstones (Fm) showing a weathered surface, pose some doubts about the recurrence of paleosol horizons. Well-exposed mudstone beds usually show some paleosol structures, suggesting these horizons are more common in the SCF than those observed. Dark reddish mudstone beds alternate with greenish to gray beds, suggesting cumulative paleosols which undergone periodic changes of oxygenation conditions (Krauss, 1999). Reddening of floodplain mudstones is usually referred as the consequence of well-drained conditions and rapid episodes of a fluctuating precipitation (Eren et al., 2015). Isolated carbonate nodules suggest weak development of calcretes, produced by precipitation of carbonate around small roots. This soil structure usually forms in semiarid to arid environments (Alonso-Zarza, 2003). Thin sandstone sheets with current ripple or parallel lamination indicate the action of tractive currents which can be interpreted as stages of flooding, producing unconfined flows on the floodplain (sheet floods). The interfingering of mudstone and sandstone beds sheets, wedges or lobeshaped fine sandstones with parallel lamination, and in some occasions with erosional bases, are interpreted as levees or crevasse splays deposits, produced after channel margin surpass during flood stages (Burns et al., 2017). These deposits imply fast sedimentation episodes, and in some occasions were associated to vertebrate fossil remains, suggesting fast burial of death organisms. The small and isolated channeled sandstones, intercalated within mudstone deposits, are interpreted as secondary channels crossing the floodplains, especially active during flood stages.

Facies Association 3, fluvial channel deposits
Facies Association 3 represents nearly 30% of the measured section and is mostly composed of lenticular or tabular bodies of trough cross-bedded sandstones FIG. 6. View of the lower 30 m of strata of the SCF showing the dominance of dark reddish mudstones. Arrows highlight thin (less than 1 m), lobate sandstone bodies interpreted as crevasse splay deposits.
(lithofacies St) with a sharp base and homogenous grain size (Fig. 7A, B), although an internal finning upward trend is evident in some cases. Grain-size is typical medium to coarse sandstone at the base and may grade to fine sandstone at the top. Some minor thin pebble sandstones or fine conglomerates (lithofacies Gh) are present at the base of the beds (Fig. 7C). In some sandstone bodies, a thin lag of mudstone intraclasts is observed covering the bottom surface. Thicknesses of the sandstone bodies vary from 1 to 14 m although rarely exceed 3 m. Internal major erosion surfaces are rarely observed in the channeled sandstones, although the thicker bodies show internal reactivation surfaces covered by thin layers of coarse-to-pebble sandstones. Sedimentary structures within these bodies are usually arranged as basal trough cross-bedded coarse sandstones (lithofacies St), followed by parallel laminated (lithofacies Sh) or massive sandstones (lithofacies Sm), in turn covered by fine, massive sandstones (lithofacies Sm) or siltstones (lithofacies Fm and Fr). In some occasions, when the lateral limit of a channel is well exposed, wings are observed wedging out into floodplain mudstones (see Figure 5E of Cuitiño et al., 2015). Measured width-to-thickness (W/T) ratios for the channeled sandstone bodies are in the range of 16 to 94, although most fit between 20 and 50. Following Gibling (2006) these channels are classified as narrow sheets. Interpretation: lenticular sandstone bodies intercalated between floodplain deposits of FA 2 are interpreted as fluvial channel deposits. The internal cross-bedded sandstones, the occasional finning upward trend and the erosive concave-up base are also indicative of channel deposition (Miall, 1985(Miall, , 1996. Given the simple infill of the channels and the general reduced thickness of individual bodies, most are interpreted as shallow, single story channels. Internal erosion surfaces observed only for some thicker bodies, suggest reactivations of multistory channels. Channel amalgamation is rarely observed, and most are encased in floodplain deposits. The lack of lateral accretion evidence suggests these were fixed channels in a low sinuosity fluvial system (Miall, 1996;Gibling, 2006). These channels remain stable by levees reaching higher elevations than the adjacent floodplains. Figure 5C of Cuitiño et al. (2015) shows a channel margin in which vertically stacked wings wedge out into floodplain mudstones, indicating the channel remain stable in its position and levees allow the channel to reach an elevated position in relation to the floodplain. Deformed cross-stratified sandstone layers, occurring exclusively within channel bodies, are interpreted as produced by liquefaction of water-saturated sand triggered by overloading after fast sedimentation episodes in channels. This phenomenon may be the consequence of elevated sedimentation rates during high-discharge episodes.

Paleocurrents and sandstone composition
Thirty confident paleocurrent measurements were recorded from cross-bedded sandstones (lithofacies St and Sp) representing channeled sandstones (FA 3). Measurements were recorded along the entire column of the SCF in Lago Posadas, taking care of measuring only on 3D exposures of the structures in order to obtain the true dune migration direction. Figure 8A shows the distribution and proportion of measurement directions grouped into clusters of 30° range. The mean paleocurrent direction is 93° (i.e., to the east) and 76% of the paleocurrent measurements fall into the NE-SE (i.e., N45° to N135°) interval, indicating a unidirectional pattern pointing roughly to the east. A minor (23%) proportion of the measurements indicate paleocurrents toward the north and south. FIG. 8. A. Plot of data for paleocurrent measurements from channel cross-bedded sandstones. Note the clustering of vectors pointing mostly towards the east, as well as northeast and southeast; B. Dickinson et al. (1983) Qm-F-L provenance triangle. The overall reduced proportion of quartz is evident, and most of the samples plot close to the "lithic" apex (lithic recycled and undissected arc).
All the sandstones analyzed in thin section are dominated by lithic particles and show low quartz proportion (all below 20%). Most of the samples classify as litharenites excepting two samples that classify as feldspathic litharenites (samples MB1 and MB14), following Folk et al. (1970). Plotting the data into the tectonic provenance triangle of Dickinson et al. (1983) most of them cluster into the "undissected arc" field whereas three of them fall into the "lithic recycled" and one (MB14) into the "transitional arc" (Fig. 8B).
Lithic particles compose 60% to 85% of the grain fraction and are fairly dominated by volcanic particles, with minor proportions of low-grade metamorphic grains and very few sedimentary and plutonic grains. Particles of primary pyroclastic origin are virtually absent. Feldspar particles usually compose less than 20% of the population, with only two samples above 30% (Fig. 8B). They are represented by plagioclase as the dominant component. Two samples (those classified as feldspathic litharenites) show higher proportion of plagioclase.
Accessory minerals include micas (biotite and muscovite), amphiboles, pyroxenes and opaques. The predominant cement is zeolitic and in some cases carbonate with a pervasive alteration of the clastic components. Occasionally, clay rim (illitesmectite series) cement is developed surrounding the grains.

Vertical stratigraphic trends
The measured section of the SCF south of Lago Posadas is 460 m-thick and shows a general homogeneous vertical facies composition (Fig. 9). The base of the unit is transitional with the underlying marine to estuarine deposits of the El Chacay Formation , whereas the top in this area is truncated by an erosional unconformity and is covered by modern gravel deposits.
Despite its homogeneity, some slight variations can be recognized along the SCF column. Sandstone/ mudstone proportion along the measured column is essentially constant, although a slightly higher proportion of mudstone deposits is recognized in the lower 30 m of the unit, immediately above the transition with estuarine deposits (Fig. 6). This lower muddy interval shows a distinctive dark reddish coloration, which is visible and correlatable along several sites in the region, especially with the Jeinimeni Formation at the Zeballos-Jeinimeni Rivers to the north. In addition, this lower portion shows a higher frequency of paleosol horizons in floodplain deposits, and also shows a higher proportion of lobe-shaped sandstone bodies (Fig. 6). Upward in the column, a change of the dominant coloration is recognized with dominance of gray colors in the upper part.
Besides the sandstone/mudstone proportion, the thickness of individual sandstone beds shows slight vertical stratigraphic variations. The lower 100 m of the unit shows sandstone beds rarely overcoming 3 m-thick (Fig. 6), whereas from 100 m upwards, thicker sandstone beds (4 to 5 m-thick) are more common (e.g., Fig. 7A), reaching to a maximum of 14 m-thick (Fig. 9). The middle and upper portions of the SCF studied herein are lithologically correlatable with equivalent strata to the north assigned to the Cerro Boleadoras Formation.
Grain-size trends are hardly recognized in the measured column. When considering the sandstone beds alone, a dominance of medium sandstone is observable throughout the column. Coarse sandstones and thin lags composed of granule or fine conglomerates are more common from 100 m upward. Only one channeled sandstone bed shows a relatively thick (up to 1 m) fine conglomerate (lithofacies Gh) interval at the base (Fig. 7C).
No vertical trends in paleocurrent direction neither in sandstone composition are recognized for the SCF in this locality. Given the lack of major compositional modal variations in the analyzed sandstones along the section, one single petrofacies is defined for the SCF in this site.

Vertebrate paleontology
The paleontological prospection was focused in the hills that are immediately southwest of the cliff that limits the plateau, southwest of Lago Posadas (Fig. 1). These hills expose the SCF, and are part of the areas prospected by Hatcher (Fig. 2) and correspond to the site where Blisniuk et al. (2005) measured their stratigraphic section. The vertebrate fossil collection is composed of 23 specimens of mammals. The material is fragmentary and was recovered loose on the surface, except for two specimens that were found in situ (the toxodonts Homalodotherium and Nesodon). Most of them come from the lower to mid-levels of the stratigraphic column (Fig. 9). The specimens recovered by Hatcher 's expeditions for the Princeton University (currently at YPM), total about 50 specimens sourced from the same general area where we worked, and include some articulated specimens in nodules (Fig. 10A). The specimens collected by Brown for the AMNH are fewer than Hatcher's. In an unpublished report to the AMNH, Brown stated that he collected remains of four typotheres. These specimens do not appear in the catalogue of the Vertebrate Paleontology Department, but there are four unprepared specimens in the collection with original handwritten cards that indicate their provenance from "Lake Pueyrredon". Furthermore, the sediment matrix is composed of medium to coarse sandstones that clearly resembles rocks of the SCF in Lago Posadas (Fig. 10B). Particularly, the sediment matrix of some specimens is similar to that of the sheet sandstone beds of the FA 2, and also similar to the sediment matrix of the few specimens found in situ by us. A collection of about 20 specimens from Lago Posadas is housed in the MLP (Argentina), of which only 12 were identified. Albeit fragmentary, the material is in an acceptable state of preservation making their identification possible. The exact stratigraphic provenance of this material is not confidently established.
The complete list of specimens (collected in the field and housed in museums) from Lago Posadas and Pueyrredón, with catalogue numbers and information about the materials, collectors and provenance is provided in Appendix 1.
The faunal assemblage of the Lago Posadas area includes 31 taxa: 28 mammals and three birds (see Table 2; Fig. 11). Due to the poor quality of some specimens and/or the lack of recent systematics revision, they could be identified only at a high taxonomic level. Mammals belong to the main groups recorded in other areas of the SCF (metatherians, xenarthrans, notoungulates, litopterns, astrapotheres and rodents).

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.

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 . 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 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, poorlydeveloped 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) widthto-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).

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 timecorrelation 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.

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;, Monte León Cuitiño et al., 2016) and Lago Posadas . 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 km 2 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 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.

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 andFernicola 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., 2015Vizcaíno et al., , 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 . 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., 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;, 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 . 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 wellpreserved 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. (2015Raigemborn et al. ( , 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.

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 ref lecting either temporal, geographic or ecological differences, as it was also proposed for other Santacrucian faunas 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.

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 welldeveloped 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., 2015Raigemborn et al., , 2018, from which most of the paleoecological data for the SCF is derived.
In these later examples, a marked seasonality was proposed Raigemborn et al., 2015Raigemborn et al., , 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  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.

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 f loodplain 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 f loodplain 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.