U-Pb zircon constraints on the age of the Cretaceous Mata Amarilla Formation, Southern Patagonia, Argentina: its relationship with the evolution of the Austral Basin

Despite the abundant fossil content of the Mata Amarilla Formation (Southern Patagonia, Santa Cruz Province, Argentina), its age has always generated a considerable number of questions and debates. The chronological data provided by invertebrates, dinosaurs, fish, turtles, plesiosaurs and fossil flora are contradictory. In this work, twenty U-Pb spot analyses by laser ablation were carried out on the outer parts of the zoned zircon crystals from a tuff layer of the middle section of the Mata Amarilla Formation, yielding a U-Pb concordia age of 96.23±0.71 Ma, which corresponds to the middle Cenomanian. The deposition of the lower section of the Mata Amarilla Formation marks the onset of the foreland stage of the Austral Basin (also known as Magallanes Basin); this transition is characterized by the west-east shift of the depositional systems, which is consistent with the progradation of the Cretaceous fold-and-thrust belt. Thus, the onset of the foreland stage could have occurred between the upper Albian and lower Cenomanian, as the underlying Piedra Clavada Formation is lower Albian in age. On comparing the data obtained with information from the Última Esperanza Province in Chile, it can be suggested that the initiation of the closure of the Rocas Verdes Marginal Basin occurred simultaneously.


Introduction
The study area is located in the southwest of the Santa Cruz Province, Argentina, near the locality of Tres Lagos (Fig. 1).
The Mata Amarilla Formation is a key unit in the development of the Austral Basin, as it marks the transition between the thermal subsidence stage and the foreland stage (Varela, 2009(Varela, , 2011)).The exact time of deposition of the Mata Amarilla Formation, however, has always been unclear, and therefore the onset of the compressional phase has also led to dating speculations on the basis of its fossil content.In this respect, the absence of chronological dating has increased the lack of certainty over the stratigraphy of the Austral Basin.In turn, the presence of both continental and littoral fossil specimens has created confusion as to the stratigraphic nomenclature of the unit, since the fossil content was used to refer to lithostratigraphically identical sediments (cf.Goin et al., 2002;Varela, 2011).
In this work, a U-Pb concordia age obtained by laser ablation of primary zircons from a tuff layer of the middle section of the Mata Amarilla Formation is presented.On the basis of such radiometric dating, certain regional considerations regarding the evolution of the Austral Basin (also known as Rocas Verdes Marginal Basin+Magallanes Basin) are introduced.

Geological background
The Austral Basin, also known towards the south as Rocas Verdes Marginal Basin -when sea-floor spreading occurs-and Magallanes Basin, is located on the southwestern edge of the South American Plate, and it is bounded to the south by the Scotia Plate (Fig. 1).It covers an area of approximately 230,000 km 2 which extends over the southernmost end of the Argentine and Chilean territories, and it is surrounded to the east by the Deseado Massif (Macizo del Deseado).With an elongated shape in a north-south direction, bounded to the east by the Río Chico High, which separates it from the Malvinas Basin.Its tectonic western edge is constituted by the Patagonian-Fuegian Andes (Andes Patagónico-Fueguinos, Fig. 1).
This basin consists of a thick sedimentary succession which has a maximum thickness of approximately 8,000 m, with the almost exclusive occurrence of siliciclastic rocks.In the study area, it overlies the Bahía de la Lancha Formation (Biddle et al., 1986;Robbiano et al., 1996;Ramos, 2002;Kraemer et al., 2002;Peroni et al., 2002;Rodríguez and Miller, 2005), and towards the south, the Tierra del Fuego igneous and metamorphic basement complex of Hervé et al. (2010).Carbonate sediments are only present in certain locations within the basin and they are of limited thickness (Peroni et al., 2002).
As shown in figure 1, outcrops of Mesozoic ophiolite complexes are well south of the study area, which indicates that in the study area the basin was not subject to sea-floor spreading.The evolution of the Austral Basin in this sector differs from the evolution of the Rocas Verdes Marginal Basin-Magallanes Basin.The Rocas Verdes Basin is a Late Jurassic-Early Cretaceous backarc basin units (modified from Hervé et al., 2007a, b;Calderón et al., 2007a, b;Fildani y Hessler, 2005).
characterised by rifting and sea-floor spreading along the western margin of the Patagonian-Fuegian Andes (Katz, 1963;Dalziel et al., 1974;Dalziel, 1981;Biddle et al., 1986;Calderón et al., 2007a).A change from regional extension to compression occurred in the mid-Cretaceous and produced a retroarc fold-thrust belt and linked foreland basin, known in Chile as Magallanes Basin (Biddle et al., 1986;Wilson, 1991;Fildani et al., 2003;Fildani and Hessler, 2005;Fosdick et al., 2011).The geological history of the Austral Basin in the study area is characterized by three main tectonic stages (Biddle et al., 1986;Robbiano et al., 1996;Ramos, 2002;Kraemer et al., 2002;Peroni et al., 2002;Rodríguez and Miller, 2005).These are the rift stage, the stage of thermal subsidence and, finally, the foreland stage (Fig. 2).The rift and thermal subsidence stages of the Austral Basin mainly coincide with the deposition of the Rocas Verdes Marginal Basin, whereas the foreland stage of the Austral Basin is simultaneous with the evolution of the Magallanes Basin.

Rift Stage
This rifting stage is connected to the breakup of Gondwana (Uliana and Biddle, 1988;Pankhurst et al., 2000), which took place during the Middle to Upper Jurassic, approximately 170 Ma ago.It affected mainly the Deseado Massif and it is represented in the Patagonian-Fuegian Andes by the El Quemado Complex, which is known as the Tobífera Formation in Chile.The El Quemado Complex consists of very thick layers of dacites and andesites controlled by direct faults and intercalated with conglomerates, sandstones and, in certain cases, even mudstones.In this initial stage, grabens and half-grabens developed, which were filled with volcaniclastic and volcanic rocks intercalated with epiclastic sediments (Biddle et al., 1986;Uliana and Biddle, 1988).The sediments become progressively younger towards the east.
To the south of the study area, this stage is characterized by rifting and sea-floor spreading along the western margin of the Patagonian-Fuegian Andes (Katz, 1963;Dalziel et al., 1974;Dalziel, 1981;Calderón et al., 2007a), which led to the development of a marginal basin (i.e., the Rocas Verdes Marginal Basin), connected to the opening of the Weddell Sea, in the southwestern section of the basin (Dalziel, 1981;Biddle et al., 1986).

Thermal Subsidence Stage
Subsequently, once the tectonic activity had ceased the thermal subsidence stage began, the typical transgressive deposits of the Springhill Formation developed, characterized by quartz arenites with bipyramidal quartz clasts.The Springhill Formation broadly overlaps the margins of the initial halfgraben, and it was overlaid by a thick deep-marine succession, characterized by the alternating black mudstones and marls of the Río Mayer Formation, which extends to the Barremian.Towards the end of this cycle (early Aptian-Albian), in the northern and eastern sectors of the basin a large passive-margin delta system developed, referred to as the Piedra Clavada Formation, and its equivalent in the Lago San Martín area, the Kachaike Formation.
During this stage, to the south of the study area, the basin acts as a back-arc basin (Calderón et al., 2007a).

Foreland Stage
A regional change from an extensional to a compressional phase took place in the mid-Cretaceous, as a result of the convergence of an arc and/or a craton, thus causing the obduction of the ophioliteswhich constituted the sea floor of the Rocas Verdes Marginal Basin-over the cratonic continental margin (Dalziel, 1981;Ramos, 1989).It has not been possible to determine the exact timing of obduction, but the compression associated with the early stages of the orogeny caused the development of a retroarc fold-and-thrust belt along the Patagonian-Fuegian Andes (Ramos et al., 1982;Biddle et al., 1986;Wilson, 1991;Fildani et al., 2003;Fildani and Hessler, 2005).This fold-and-thrust belt is associated along its eastern margin with a foreland basin (Austral Foreland Basin).This compression process extended from the Upper Cretaceous to the Neogene (Ramos et al., 1982;Biddle et al., 1986;Wilson, 1991;Spalletti and Franzese, 2007;Fosdick et al., 2011).
Between the middle Cenomanian and the lower Coniacian (96-84 Ma), a deformation event occurred which could be related to the closure of the Rocas Verdes Marginal Basin (Ramos et al., 1982;Biddle et al., 1986).The onset of the compressional phase in the middle sector of the Austral Basin is characterized by the west to east progradation of the fluvial-estuarine facies of the Mata Amarilla Formation (Varela, 2009(Varela, , 2011)).This change towards a compressional phase was also interpreted in the northern sector of the Lago Viedma (Lake Viedma), on the basis of paleocurrent data and facies changes in the Lago Viedma Formation, which is upper Albian to upper Cenomanian in age (Canessa et al., 2005).In Chile, on the other hand, in the region of the Torres del Paine National Park and in the Última Esperanza Province (Fig. 1), the compressional phase of the Magallanes Basin occurs in the transition from the Zapata Formation to the Punta Barrosa Formation, the former being a shelf marine environment, and the latter a deep-marine environment (Wilson, 1991;Fildani et al., 2003;Fildani and Hessler, 2005).The transition was dated by means of detrital zircons, and the age was estimated as being no older than 92±1 Ma (Fildani et al., 2003).Recently, Fosdick et al. (2011) obtained new zircon U-Pb ages from an interbedded volcanic ash in the Zapata-Punta Barrosa transition, suggesting an age of 101±1 Ma.They indicated that the deformation at 51º30'S began ~100 Ma and progressed during six main stages (Fosdick et al., 2011).
This formation has a maximum thickness of approximately 350 m in outcrops, and it is composed of gray and blackish siltstone and claystone, alternating with banks measuring between 1 and 10 m constituted by whitish and yellowish-grey fine-to medium-grained sandstone, deposited in littoral and continental environments (Arbe, 1989(Arbe, , 2002;;Poiré et al., 2004;Russo and Flores, 1972;Russo et al., 1980;Varela and Poiré, 2008;Varela et al., 2008;Varela, 2009Varela, , 2011)).According to the classification of Folk et al. (1970), the sandstones of the Mata Amarilla Formation are feldspathic litharenites, except for three samples which were labelled as lithic feldarenites (Varela, 2011).The petrographic study of 48 thin sections concluded that the main provenance of the sandstones of the Mata Amarilla Formation derives from a magmatic arc and, to a lesser extent, from an orogenic area (Varela, 2011).The type section is located on the southern margin of the Río Shehuen or Chalía, approximately 23 km east of the locality of Tres Lagos, in the area surrounding the Estancia Mata Amarilla, also known as Estancia La Soriana, (locality 3, Fig. 3).It overlies the Piedra Clavada Formation, which is lower Albian in age, with transitional contact and it is unconformably covered by the Campanian sediments of the La Anita Formation (Varela and Poiré, 2008;Varela, 2009Varela, , 2011) ) (Fig. 4).This formation (see Fig. 4) was deposited during the early Upper Cretaceous, extending from the Cenomanian to the Santonian (Poiré et al., 2007 1 ;  Varela and Poiré 2008; Varela, 2009).
On the basis of facies analysis, Varela (2009Varela ( , 2011) ) recently divided the Mata Amarilla Formation into three sections, according to the different conditions of accommodation space creation with respect to sediment supply (Fig. 4).
The lower section of the Mata Amarilla Formation consists of fine-grained sediments with palaeosol development interbedded with laminated shale and coquinas.In the eastern part of the study area, eight littoral facies associations or sedimentary units were recognised: sabulithic bars, large-scale bars, bioclastic lobes, sand bars with herringbone cross-stratification, sand bars with hummocky cross-stratification, smallscale gravelly channels and diamictites, fine-grained sediments with shells, and heterolithic deposits with marine fossils.These facies associations correspond to littoral marine, lagoon, estuary, and bayhead delta palaeoenvironments (Varela, 2011).In turn, in the western sector of the study area, the facies associations identified were large-scale simple ribbons and fine-grained sedimentation, which is consistent with a distal fluvial system, with palaeocurrents coming from the west (Varela, 2011).
The middle section of the Mata Amarilla Formation is characterized by conglomerates, sandstones, siltstones and mudstones.In the western part of the study area, four facies associations or sedimentary units were defined: gravelly sheets, sandy sheets, small-scale bars and fine-grained sedimentation.These correspond, from west to east, to a gravel-bed braided fluvial system and a sandy high-sinuosity meandering fluvial system (Varela, 2011).The palaeocurrents indicate flow from west to east, into the main fluvial system located to the east.In the eastern part of the study area six facies associations were recognised: complex ribbons, small-scale simple ribbons, small-scale bars, lobes, fine-grained sedimentation, and heterolithic deposits with continental fossils.These are associated with a low-sinuosity meandering fluvial system with aggradation, which is the main drainage system, and the palaeocurrents indicate flow from the northeast to the southwest (Varela, 2011).
As regards the upper section of the Mata Amarilla Formation, it is similar to the lower section, as it consists of fine-grained sediments with palaeosol development interbedded with laminated shale and coquinas (Varela, 2011).In the eastern part of the study area, facies associations or sedimentary units identified-i.e., bioclastic lobes, sand bars with herringbone cross-stratification, fine-grained sediments with shells, and heterolithic deposits with marine fossils-correspond to littoral marine, lagoon, and estuary palaeoenvironments (Varela, 2011).The western sector of the study area is characterized by two facies associations or sedimentary units: large-scale simple ribbons and fine-grained sediments, which can be assigned to a distal fluvial system, with palaeocurrents coming from the west (Varela, 2011).
The lower and upper sections show conditions with high rates of accommodation/sediment supply, whereas the middle section shows low rates of accommodation/sediment supply (Varela, 2009(Varela, , 2011)).These changes are inferred to be promoted by relative sea-level oscillations in response to the tectonic evolution of the Austral fold-and-thrust belt (Varela, 2009(Varela, , 2011)).

Problems of dating on the basis of fossil content
Despite the abundant fossil content of the Mata Amarilla Formation, a considerable number of doubts and debates have arisen concerning the age of this formation.On one hand, Exogyra guaranitica, Pterotrigonia aliformis, Corbula sehuena, and the gastropod Potamides (Pirenella) patagoniensis (Ihering, 1907;Wilckens, 1907;Feruglio, 1936Feruglio, , 1937Feruglio, , 1938;;Piatnizky, 1938;Griffin and Varela, 2012) present in these rocks are not index fossils which could be used to determine ages and/or limit options.Yet, this association suggests a Coniacian age (e.g., Riccardi and Rolleri, 1980).
The most reliable chronological information is given by the presence of ammonites at Cerro Índice (Índice Hill, locality 9, Fig. 3), which are assigned to the 'Estratos' de Mata Amarilla and were placed in a new species of the genus Peroniceras (P.santacrucense) (Leanza, 1970).Subsequently, Blasco et al. (1980) and Nullo et al. (1981) described specimens of Placenticeras sp. in the Mata Amarilla Formation at Cerro Índice, and regarded them as Santonian in age.On the basis of the few specimens found by Leanza (1970) in the locality mentioned above, Riccardi (1984a, b) described the faunule of Peroniceras santacrucense, assigning it a Coniacian age.Riccardi et al. (1987) relocated the species to the genus Gauthiericeras, adding that it occurs approximately 75 m above levels with Placenticeras and approximately 20 m below the point of contact with the Pari Aike Formation (currently regarded as the middle section of the Mata Amarilla Formation; Varela, 2011).In turn, they found ammonites in the Bajo del Cerro Índice (Cerro Índice Depression) which belong to the same species (Gauthiericeras santacrucense), and located this faunule to the upper Coniacian, according to the stratigraphic distribution of the genus Gauthiericeras (Riccardi et al., 1987).Finally, Riccardi (2002) established the Gauthiericeras santacrucense zone and dated it as late Coniacian, in agreement with the stratigraphic range of the genus Gauthiericeras suggested by Grossouvre (Riccardi, 2002).Regardless of the exact stratigraphic location of the ammonite-bearing level or levels, it is difficult to locate them in any of the three sections of the Mata Amarilla Formation, as they have not been found during the field work.
The record of the turtle family Chelidae in South America ranges from the Albian until today (Broin and de la Fuente, 1993;de la Fuente et al., 2001;Lapparent de Broin and de la Fuente, 2001).The turtles collected in the lower section of the Mata Amarilla Formation, at the Estancia Bajada de los Orientales (Bajada de los Orientales Farm, locality 8) and the Estancia Mata Amarilla (locality 3, Fig. 3) have much in common with various taxa that constitute the Campanian-Maastrichtian cheloniafauna (Goin et al., 2002).
Lungfish (Dipnoi) were described by Cione et al. (2007) based on 200 complete and partial tooth plates.These authors described two ʻCeratodus' iheringi species and a new genus, Atlantoceratodus iheringi Cione et al., 2007, which is similar to the ʻCeratodus' madagascariensis Priem 1924 of the Upper Cretaceous (Campanian) from Madagascar, to the extent that they are regarded as belonging to the same genus.
Recently found plesiosaur remains have been studied by O'Gorman and Varela (2010), who compared them with the diagnosis of Polyptychodon patagonicus described by Ameghino (1893).The remains found in the lower section of the Mata Amarilla Formation at the Estancia Mata Amarilla (locality 3) and Estancia La Blanca (locality 15, Fig. 3) include some vertebrae, a propodium and over 56 teeth which could be assigned to Plesiosauria indet.and Elasmosauridae indet., and can be compared to the plesiosaurs of the Cenomanian from Antarctica and Australia (O'Gorman and Varela, 2010).
The trunks of the María Elena Petrified Forest are almost exclusively gymnosperms of the Podocarpaceae family and, to a lesser extent, Araucariaceae (Zamuner et al., 2004(Zamuner et al., , 2006(Zamuner et al., , 2008)), they do not have biostratigraphic value.
In short, the chronological information provided by invertebrates, theropod dinosaurs and sauropods, fish, turtles, plesiosaurs, and fossil flora does not coincide.Owing to this, and to the lack of radiometric data, the need to have a reliable age which would dispel all doubts on the age of the Mata Amarilla Formation has become of the utmost importance.

Methodology
In order to obtain a reliable radiometric age which would solve the problem concerning the age of the formation, a sample was collected from a primary tuff located 29 m above the point of contact between the lower and middle sections of the Mata Amarilla Formation, at the Estancia Mata Amarilla (locality 3, Figs. 3 and 4).
Zircon crystals from the primary tuff sample were recovered from a heavy mineral concentrate by hand-picking under a binocular microscope.Once extracted, they were mounted and polished to approximately half their thickness.The crystals were observed and imaged by cathodoluminescence (CL) using a Jeol JSM-6490 scanning electron microscope (SEM) equipped with a Gatan MiniCL detector at the Goethe-Universität Frankfurt am Main (GUF), in Germany.The U-Th-Pb analyses were performed at the GUF using a sector field inductively coupled plasma mass spectrometer (SF-ICP-MS) coupled with a new ultraviolet (UV) laser ablation system, the New Wave Research UP-213, with a teardrop-shaped, low-volume (<2.3 cm 3 ) laser cell.The instrument setup and the analytical method used in this study were described in detail by Gerdes andZeh (2006, 2009), and Frei and Gerdes (2009).Analytical spots (30 µm in diameter) were chosen on the basis of the internal structure of the grains, as observed in the CL images.Raw data were corrected for background signal.Common Pb, laserinduced elemental fractionation, instrumental mass bias, and time-dependent elemental fractionation of Pb/U were calculated using an Excel © spreadsheet (Gerdes and Zeh, 2006).Drift-correction was performed by fitting a linear regression through all measured ratios, excluding the outliers (±2 standard deviation, 2σ), and using the intercept with the y-axis as the initial ratio.The laser-induced elemental fractionation and instrumental mass bias were corrected by normalization to the reference zircon GJ-1 (at GUF, ID-TIMS U-Pb age 608±1 Ma).Reported uncertainties (2σ) of the 206 Pb/ 238 U were propagated by quadratic addition of the external reproducibility (2SD, standard deviation) obtained from the GJ-1 zircon during each analytical session and the within-run precision of each analysis (2SE, standard error).In the case of the 207 Pb/ 206 Pb ratio, a 207 Pb signal-dependent uncertainty propagation was used (Gerdes and Zeh, 2009).The accuracy of the method was verified by means of the analysis of reference zircons 91.500 (1064.8±4.3Ma, MSWD of concordance and equivalence=0.86),Plešovice (337.7±1.6 Ma, MSWD C+E =0.84), and Temora (416.6±2.5 Ma, MSWD C+E =0.9).Plots and age calculations were made using the ISOPLOT software (Ludwig, 2003).

Tuff level
The tuff level is located 29 m above the contact between the lower and middle sections of the Mata Amarilla Formation, at the Estancia Mata Amarilla (locality 3, Figs. 3 and 4).This tuffaceous level is 60 cm thick, massive and white coloured, and it is interbedded with fine-grained floodplain deposits with abundant palaeosol development (Fig. 5a).The tuff has no overimposed pedogenesis, its grain size is very fine to fine (Fig. 5b), and based on its fragmental composition it could be classified as a vitric tuff with minor crystal content (Fig. 5c).Even though this level has lateral continuity in locality 3 (Estancia Mata Amarilla) and other tuff levels which display no pedogenesis were found in other localities, the nature of the fluvial palaeoenvironment prevents its direct correlation.Added to this, all palaeosols of the Mata Amarilla Formation are composed almost entirely of smectite clays, which have a genesis associated with the alteration of volcanic glass (Varela, 2010(Varela, , 2011)).

Radiometric data
Cathodoluminescence studies on the prismatic zircon crystals from the tuff revealed that their growth is characterized by oscillatory zoning, which is typical for primary volcanic crystals.It could be observed that certain crystals were fractured at right angles, which could be connected to eruptive violence, that is, clast collision during the eruptive event (Fig. 6).On occasion, rounded inherited cores could be distinguished, characterized by a darker luminescence (Fig. 6).Xenocrystic zircon typically shows signs of resorption and original oscillatory zoning is blurred or obliterated (Fig. 6).In this manner, twenty U-Pb spot analyses by laser ablation were carried out on the outer parts of the zoned zircon crystals (Table 1), which yielded a concordia age of 96.23±0.71Ma (Fig. 7A).This age is interpreted as dating the crystallisation of zircons from the tuff layer and, as it constitutes a primary deposit, it coincides with the time of deposition of the middle section of the Mata Amarilla Formation.
Fifteen ages older than 100 Ma were obtained: 6 spot analyses by laser ablation were carried out on zircon cores with younger overgrowth rims and 8 spot analyses were obtained on xenocrystic zircon (Table 1).The datings of the inherited zircon cores and xenocrystic zircon suggest crustal involvement, and three magmatic and/or metamorphic events prior to the eruption that deposited the tuff were registered.These are, in increasing order of age, a lower Permian event (280-300 Ma), a Lower Jurassic event (180-200 Ma) and finally a younger group of ages indicating an Upper Jurassic event (~157 Ma, Fig. 6B).

Discussion
The new dating of the Mata Amarilla Formation indicates the occurrence of magmatism at 96.2±0.7 Ma, which would be located to the west of the study area.This magmatic arc may have been active both during the thermal subsidence stage and the foreland stage (Fig. 2).However, on the basis of the sedimentological data -which indicate a marked west-east shift in the proximal-to-distal sedimentological facies-and the paleocurrent data, Varela (2009Varela ( , 2011) ) suggests that such a change reflects the onset of the foreland stage at the Austral Basin.The lower section of the Mata Amarilla Formation shows distal fluvial facies to the west, whereas to the east the deposits occur in a prograding sequence coarsening and thickening upwards from an estuary towards a bayhead delta.The middle section of the Mata Amarilla Formation shows, from west to east, a clear transition from gravel-bed braided systems to sandy high-sinuosity meandering fluvial systems; finally, in the eastern section the fluvial systems are of the anastomosing type (low-sinuosity meandring fluvial systems with aggradation).The upper section of the Mata Amarilla Formation shows a transition from distal fluvial deposits to the west -wherein the floodplain has been preserved to a great extent-to purely littoral facies to the east of the study area.The marked west-east change in the evolution of the fluvial systems, as well as the west-east transition from distal continental facies to estuarine facies, is coherent with the propagation direction of the Cretaceous fold-and-thrust belt of the Austral Basin.Therefore, the relative sea level changes of the Mata Amarilla Formation are related to tectonic control more than to purely eustatic fluctuations (Varela, 2009(Varela, , 2011)).
The onset of the foreland basin is regarded as coinciding clearly with the onset of deposition of the lower section of the Mata Amarilla Formation (Varela, 2009(Varela, , 2011)), as the increase in the rate of accommodation/sediment supply represents the flexural response of the lithosphere due to the increase in weight of the fold-and-thrust belt.Hence, according to this study, the age of compression is older than 96.2±0.7 Ma and is located within the range between the lower Cenomanian and the upper Albian (~100 Ma), since the Piedra Clavada Formation, underlying the Mata Amarilla Formation, is lower Albian (Fig. 7), as indicated by Mohria-like spores (Archangelsky, 2009).
The study area within this sector of the Austral Basin coincides with the edge of the Rocas Verdes Marginal Basin, which according to several contributions opened by 'unzipping' from south to north (de Wit and Stern, 1981;Biddle et al., 1986;Alabaster and Storey, 1990;Hervé et al., 2007a;Fildani and Hessler, 2005).Nevertheless, this 'unzipping' model was questioned by Calderón et al. (2007a) on the basis of new zircon ages.The Rocas Verdes Marginal Basin widens to the south and is narrower to the north, where the study area for this work is located (Fildani and Hessler, 2005;Romans et al., 2010Romans et al., , 2011)).Therefore, when the basin is under compression-i.e., during the foreland stage-it is logical for the onset of the deformation to become more evident from north to south, as if the 'zipper' were closing.The depocentre of the basin is located in the Última Esperanza Province in Chile, to the southwest of the study area.In this region, the presence of the deep-marine sediments of the Punta Barrosa Formation over the shelf-marine sediments of the Zapata Formation indicates the occurrence of a well-developed subaerial fold-and-thrust belt (Wilson, 1991;Fildani et al., 2003;Fildani and Hessler, 2005;Romans et al., 2010;2011;Fosdick et al., 2011).The Punta Barrosa Formation was dated on the basis of detrital zircon analysis, suggesting an age not older than 92±1 Ma (Fildani et al., 2003).Fildani and Hessler (2005) suggest the occurrence of thrusting during the deposition of the Zapata-Punta Barrosa transition on the basis of provenance data.Recently, this incipient thrust belt formation was dated as being in progress at 101 Ma (Fosdick et al., 2011), but it has not caused significant changes in the sedimentary sequences (Fildani and Hessler, 2005;Romans et al., 2010).Yet, on the northern edge of the basin the crust is more rigid, as it is less thinned (Romans et al., 2010;Varela, 2011).That is why small uplifts in the arc and in the fold-and-thrust belt may have caused major changes in the sedimentary environments.In this regard, these uplifts have modified the general direction of the drainage network (northto-south), which in this case has shifted towards a marked west-to-east direction of flow.
In addition, the age obtained for the middle section of the Mata Amarilla Formation coincides with the peak age of the histogram for detrital zircons of the Punta Barrosa Formation, which is between 95 and 100 Ma (see Fildani et al., 2003;Fig. 3B) and recently Fosdick et al. (2011) constrains the age of Punta Barrosa at 101±1 Ma (Fig. 7).
On the basis of the previously discussed data, it may be concluded that the compression and the ensuing development of the foreland basin occurred simultaneously throughout the basin (Fig. 8).In this sense, the onset of the compressional phase at 49.5º south latitude in the lower section of the Mata Amarilla Formation must be older than 96.2±0.7 Ma, whereas this compression becomes evident at 51.5º south latitude in the Punta Barrosa Formation ~100 Ma (Fosdick et al., 2011;Fig. 8).These ages coincide with the beginning of the Andean uplift at the 34º-35ºS latitude (98.6 Ma-88 Ma) suggested by Tunik et al. (2010).However, Suárez et al. (2009aSuárez et al. ( , 2009bSuárez et al. ( , 2010 and references therein) and references therein) argue that the uplift of the Aisén Basin (43º-48ºS latitude), equivalent to the northern part of the Austral Basin, must have taken place some time between 121 and 118 Ma.
Finally, regarding the ages of the inherited zircon cores, an older group was found, which was located in the lower Permian (280-300 Ma, Fig. 7B); these ages are coherent with the age of the metamorphic rocks of the Austral Basin (see Hervé et al., 2003Hervé et al., , 2010)).In Argentina, it is known as the Bahía de la Lancha Formation and the Río Lácteo Formation; whereas in Chile it is represented in the metamorphic complexes of Aysén and Magallanes.These two were dated at the same latitude as the study area by Hervé et al. (2003Hervé et al. ( , 2007b) ) and Augustsson et al. (2006).
In the graph of the inherited zircon cores (Fig. 7B), there is an absence of ages in the range between the lower Permian and the Jurassic, which could be related to the collision of the allochthonous Madre de Dios terrane (Thomson and Hervé, 2002).Likewise, Hervé et al. (2007a) pointed out the absence of plutonic rocks in the South Patagonian Batholith in the same time interval.
The second group of zircon cores with inherited ages lies between 180 and 200 Ma (Lower Jurassic), which probably corresponds to another metamorphic event in the Chonos Metamorphic Complex and, less probably, in the Eastern Metamorphic Complex (Hervé et al., 2007a, b), or else to the Subcordilleran Batholith (Rapela et al., 2005).The youngest inherited age event is approximately 157 Ma (Upper Jurassic), which is contemporary with the final stage of V3 volcanism (157-153 Ma) in Pankhurst et al. (2000).This extensive rhyolitic volcanism comprises the El Quemado Complex in Argentina and the Ibáñez and Tobífera Formations in Chile (Pankhurst et al., 2000;Suárez et al., 2009b).This magmatism is associated with a process of extensional continental rifting which caused the breakup of Gondwana (Pankhurst et al., 2000).On the other hand, the 157 to 145 Ma ages group could be more precisely related to a granitic magma emplacement in the South Patagonian Batholith (Hervé et al., 2007a).Therefore, Hervé et al. (2007a) assigned ages to the first constructional phase of the South Patagonian Batolith ranging from 157 to 145 Ma (Late Jurassic), which is in part coeval with the rhyolitic ignimbrites of the Tobífera Formation (Pankhurst et al., 2000;Calderón et al., 2007a;Suárez et al., 2009b).

Conclusion
The time of deposition of the middle section of the Mata Amarilla Formation was U-Pb dated by means of zircons collected from a tuff layer at 96.2±0.7 Ma, which corresponds to the middle Cenomanian (Walker and Geissman, 2009).The onset of the foreland stage is considered to coincide clearly with the deposition of the lower section of the Mata Amarilla Formation (upper Albian-lower Cenomanian, ~100 Ma).
Dating the middle section of the formation allowed us to suggest a more specific time frame for the Austral Basin (Fig. 8), as well as being of the utmost importance to understand the paleontological implications of the flora and fauna under study collected in the area.
The presence of Cenomanian deposits in continental facies is of considerable relevance as the Cenomanian is an extremely important period in the biological evolution of the Earth (Iglesias et al., 2007).
On the basis of this new data, it may be concluded that the compression and ensuing development of the Austral Foreland Basin occurred simultaneously, being the onset of compression older than 96.2±0.7 Ma (upper Albian-lower Cenomanian, ~100 Ma) at 49.5º south latitude in the lower section of the Mata Amarilla Formation, which coincide with the compression at 51.5º south latitude in the Punta Barrosa Formation ~100 Ma (Fosdick et al., 2011).

FIG. 2 .
FIG. 2. Schematic stages of geological evolution of the Austral Basin from Middle Jurassic to the Cenozoic.a. Rift Stage (Middle to Upper Jurassic), characterized by rifting in a series of narrow grabens.b.Thermal Subsidence Stage (Upper Jurassic to Lower Cretaceous), once the tectonic activity ceased thermal subsidence began with a transgression that broadly overlapped the margins of the initial half-grabens.c.Foreland Stage (mid-Cretaceous to Cenozoic), occurrence of the Andean uplift, development of a foreland thrust belt and basin.
FIG. 3. Geological map of the study area showing the Cretaceous units of the Austral Basin (after Varela, 2011).

FIG. 5
FIG. 5. a. Photograph of the tuff layer of the middle section of the Mata Amarilla Formation at Estancia Mata Amarilla (black arrow), interbedded with fine-grained floodplain deposits with palaeosol development (white arrows); b.Detail of the tuff level, where the massive structure and very fine to fine grain size can be observed (hammer for scale is 33 cm long); c.Microphotographs of the tuff, without polarised light (left), showing ash glass and crystals; and with polarised light (right), only crystals can be distinguished due to the isotropic nature of volcanic glass.

FIG. 8 .
FIG. 8. Stratigraphic summary and geological history of the Austral Basin compared to the Rocas Verdes Marginal Basin-Magallanes Basin.Stratigraphy and data on the Rocas Verdes Marginal Basin-Magallanes Basin taken from Fildani et al. (2003) and Fosdick et al. (2011).

TABLE 1 . DATA FROM ISOTOPIC ANALYSIS BY LASER ABLATION OF 35 SPOT ANALYSES FROM TUFF LAYER (MIDDLE SECTION, MATA AMARILLA FORMATION).
c: spot analysis on zircon core; x: spot analysis on xenocrystic zircon; r: spot analysis on outer overgrowth rim of zircon; gr: zircon grain number.