Carboniferous outcrops at La Herradura Creek, San Juan Province (Western Argentina), revisited: age of the transgressions

At the outcrops of the Tupe Formation in the La Herradura Creek, San Juan Province, two transgressive events are recorded, which range from few meters to tens of meters in thickness. These strata correspond to the stratotype of the faunal Tivertonia jachalensis-Streptorhynchus inaequiornatus Biozone. An evaluation of the timing of the two marine transgressions is made on the basis of palynological assemblages and available radiometric ages. A short-lived transgression is recorded in the latest Bashkirian-early Moscovian, and then a substantial sea level fall took place forming an irregular incision surface. Later, a major high eustatic sea-level occurred during the middle Moscovian resulting in a more important marine ingression. The palynological assemblages associated to the older transgressive event indicate the presence of the Raistrickia densa-Convolutispora muriornata (DM) Biozone (Subzone B). Palynofloras referred to the Subzone C of the DM Biozone characterize the strata of the younger transgression that also bear marine fauna of the Tivertonia jachalensis-Streptorhynchus inaequiornatus Biozone. Correlations with other Pennsylvanian outcrops in the region give support to the stratigraphic scheme here proposed.


Introduction
Changes of sea levels have been determined for the Carboniferous and Permian in Gondwana (Veevers and Powell, 1987;Visser, 1997;Limarino et al., 2006;López Gamundí, 2010), but an accurate history of sea levels is steel pending. Evaluation of the timing of individual sea-level events was problematic to estimate accurately in Argentinian northwestern basins.
The Paganzo Basin is one of the best known and most extensive late Paleozoic depositional areas of Argentina, characterized by continental sedimentation punctuated by short Pennsylvanian marine transgressions. The Paganzo Group includes the upper Paleozoic deposits of the Paganzo Basin and comprises, from base to top, the Guandacol, Tupe and Patquía formations. An important approach to constrain the age of the transgressive events in the Carboniferous successions of the Paganzo Basin was given by the definition of several biozones based on marine invertebrates. The marine deposits of the Tupe Formation contain a key invertebrate fauna referred to the Tivertonia jachalensis-Streptorhynchus inaequiornatus (TS) Biozone by Sabattini et al. (1990) who suggested a late Carboniferous age for the assemblage. The outcrops of the La Herradura Creek were proposed as the holostratotype of the biozone and its Pennsylvanian age was accepted by different authors (González, 1993;Sabattini et al., 1990;Taboada, 1997Taboada, , 2010. However, the age of the biozone was a matter of debate and a Cisuralian age was proposed for the fauna (Archbold et al., 2004;Cisterna et al., 2002Cisterna et al., , 2005Cisterna et al., , 2011. Recently, Gulbranson et al. (2010) obtained an U-Pb zircon age of 312.82±0.11 Ma for a tonstein from shallow marine deposits at the nearly locality of La Ciénaga. These transgressive deposits at La Ciénaga were considered coeval of the transgressive event recognized at La Herradura Creek (Desjardins et al., 2009). Another Pennsylvanian age was obtained at the Río del Peñón section (parastratotype of the TS Biozone), where an absolute U-Pb date of 310.63±0.07 Ma was provided by Gulbranson et al. (2010) from an ignimbrite closely related to the TS Biozone. Therefore, the transgresive deposits that contain the Tivertonia jachalensis-Streptorhynchus inaequiornatus Biozone were assigned to the Moscovian by Césari et al. (2011) and Gulbranson et al. (2010).
The emergence of a radiometric control on the age of many palynological biozones and sedimentary units of the Paganzo Basin Césari et al., 2011) encourages the application of palynostratigraphic control in key stratigraphic sections. Given the importance of the upper Paleozoic succession exposed at the La Herradura Creek (Fig. 1), where two transgressive events are recorded, a revision of the palynological content of the succession is presented. The aim of this study is to provide a detailed stratigraphic context and to highlight the biostratigraphic correlations of the strata with their equivalents in the Paganzo Basin in order to clarify how recent radiometric ages may be used to constrain the age of the marine transgressions. For this purpose, fifteen palynological samples corresponding to different stratigraphic horizons are studied and compared with previous information. Moreover, a sedimentary section of the stratotype is provided in which seven facies associations are described and interpreted.

Geological and paleontological setting
The transgressive deposits of the Tupe Formation at La Herradura Creek yield abundant marine fossils since the first works by Frenguelli (1946) and Leanza (1946). Initially the marine and megafloristic assemblages were considered Visean in age (Frenguelli, 1946;Leanza, 1946) but later studies by Sabattini et al. (1990) and González (1997) relocated in the late Carboniferous these associations. Indeeed, Sabattini et al. (1990) proposed the La Herradura Creek outcrops as the stratotype of the Tivertonia jachalensis-Streptorhynchus inaequiornatus (TS) Biozone. A Late Pennsylvanian age was suggested for this biozone by Sabattini et al. (1990); González (1997); Taboada (1997); Kelly et al. (2001) and Dineen et al. (2012). However, Archbold et al. (2004) and Cisterna et al. (2002Cisterna et al. ( , 2006Cisterna et al. ( , 2011 assigned the Tivertonia jachalensis-Streptorhynchus inaequiornatus Biozone to the early Permian based on brachiopod correlations with similar species of Australia. The assignment to the Permian of the TS Biozone is not consistent neither with recent chronostratigraphic information  nor regional biostratigraphic schemes based on plant remains (Césari et al., 2011).
Therefore the outcrops of the La Herradura Creek are key for determining the stratigraphy of the Pennsylvanian transgressions. In order to analyze the stratigraphic position of the fossiliferous levels and to establish the sedimentary pattern of the Late Paleozoic deposits, a detailed stratigraphic study was carried out in the La Herradura Creek. First, a regional geologic reconnaissanse was done, a geologic map of the region was surveyed, (Fig. 2) and a detailed section of the late Paleozoic sequence was logged (Fig. 3). This information allowed defining seven facies associations (5 in the Tupe Formation and two in the Patquía Formation, Table  1). Moreover all the upper Paleozoic section was sampled for palynological studies with the purpose of establishing the correlation between palynological and invertebrates zones. Frenguelli (1946)  Scalabrini Ortiz (1972) identified as formations the 'Estratos' defined by Frenguelli (1946) and measured a thickness of 204 m for the Tupe Formation in which marine deposits were described in the upper third. On the basis of marine fauna and plant remains, Leanza (1946) and Frenguelli (1946) assigned a Visean age for the deposits. The marine fauna was revised by Sabattini et al. (1990) and later by Cisterna et al. (2002) who identified at the upper section of the Tupe Formation nine fossiliferous horizons, in a stratigraphical interval of about 20 m. Unlike previously suggested by previous authors (Frenguelli, 1946 andScalabrini Ortiz, 1972), Cisterna et al. (2002) proposed a thickness of about 80 m for the Tupe Formation and Cisterna et al. (2011) approximately 120 m (including strata here considered Patquía Formation). The marine deposits are overlain by a conspicuous marine limestone  . Asterisk show the location of the studied stratigraphic section. level that were considered by Frenguelli (1946) andScalabrini Ortiz (1972) indicative of the boundary with the overlying Patquía Formation (Fig. 4). However, Cisterna et al. (2002) positioned much higher up the boundary between both formations, including in the Tupe Formation a rich palynological association (P9a in Cisterna et al., 2002) that was recovered above the marine beds and few tens of meters above the uppermost horizon of limestone (Vergel, 2008). In this paper, we follow the approach suggested by Frenguelli (1946) andScalabrini Ortiz (1972) for establishing the boundary between the Tupe and Patquía formations. For characterizing the stratotype, a stratigraphic section referred to Tupe and Patquía formations, was measured at the La Herradura Creek (Fig. 3) identifying five facies associations in the former unit and two in the latter (Table 1). Each of them was sampled for palynological studies and the results are shown in Table 2. Palynological results together with stratigraphic correlations and chronostratigraphic information allow obtaining additional evidence for the age of the TS Biozone.

Facies Association 1 (FA 1, coarse grained cross-bedded sandstones and conglomerates)
This facies association sits on a low-relief incision surface carved into the underlying Guandacol Formation. It comprises cross-bedded coarse-grained sandstones, gravely sandstones and some conglomerates stratified in stacked lenticular beds (38 m thick, Fig. 3, Table 1). These rocks form channel complexes bounded by five-order bounding surfaces (Miall, 1996), frequently showing downstream accretion surfaces (DA, surfaces by Miall, 1996). Fine grained-sandstones and mudstones are scarce; with the exception of a thin level of carbonaceous mudstones bearing fragmentary plant remains located 10 m above the base of the Carboniferous outCrops at La Herradura Creek, san Juan provinCe (Western argentina)...
This interval is interpreted as deposited in braided alluvial plains with restricted development of floodbasins under low-accomodation space conditions. FA 1 correlates with that described by Tedesco et al. (2010) as FA 1 at the base of the Tupe Formation in the Huaco area which was interpreted as formed under low-accommodation conditions.

Facies Association 2 (FA 2, coarse grained cross-bedded sandstones and laminated mudstones)
This interval (15 m thick, Fig. 3) is similar in terms of sandstone dominance to the previously described FA 1, but differs in the presence of carbonaceous mudstones and fine-grained sandstones as well as in the more tabular form of the beds and the occurrence of finning-upward cycles. Each cycle can be divided in two terms, the lower one comprises cross-bedded coarse-grained sandstones and finegrained conglomerates (forming more than 80% of the cycle), overlying a low-relief erosive surface. The upper term is composed of thin beds (up to 20 cm) of mudstones and fine-grained sandstones bearing plant remains and showing paleosoil structures (roots). Palynologically, monosaccate pollen is abundant in the FA 2 where more diverse spores also appear ( Table 2). Some species seems to be restricted to this interval, like Raistrickia sp. cf. R. paganciana Azcuy (Fig. 6J) and Florinites sp. (Fig. 6AA). Other species of broader stratigraphic record, but constrained to the Tupe Formation, are: Raistrickia densa Menéndez (Fig. 6F), Raistrickia rotunda Azcuy (Fig. 6D) and Convolutispora muriornata Menéndez (Fig. 5O).
Probably the depositional environment was very similar to that of the FA 1, but the higher amount of fine-grained deposits indicates the presence of flood plains. A network of braided (or anastomosing) channels separated by relatively broad, vegetated and likely poorly-drained flood plains is inferred for FA 2. Therefore the transition from braided to anastomosing river deposits indicates an increase in the sedimentary accommodation space during the deposition of FA 2.

Facies Association 3 (FA 3, fine-grained sandstones and carbonaceous mudstones)
Interbedded green to dark-gray fine-grained sandstones and mudstones form a monotonous

Pollen grains
Potonieisporites triangulatus (Fig. 6N) X X X sequence up to 25 m thick. Mudstones, frequently organic-rich, are composed of laminated siltstones (less frequently claystones) stratified in tabular beds, up to 10 cm thick, showing poorly preserved plan remains. Fine-grained sandstones appear in tabular beds exhibiting current and wave ripple-cross lamination, heterolithic structures and more rarely small-scale cross-bedded sets. Several pollen species appear for the first time in the FA 3: Caheniasaccites elongatus Bose and Kar (Fig. 6X), C. ovatus Bose and Kar emend. Archangelsky and Gamerro (Fig. 6V), C. densus Lele and Karim emend. Gutiérrez (Fig. 6W), Tuberisaccites varius Lele and Makada (Fig. 7R), Potonieisporites brasiliensis (Nahuys, Alpern and Ybert) Archangelsky and Gamerro (Fig. 7Q), P. magnus Lele and Karim (Fig. 7F) and P. densus Maheshwari (Fig. 7G). Palynological samples from the FA 3 are characterized by the exclusive presence (Table 2) of Ahrensisporites cristatus Playford and Powis (Fig. 5A), Cyclogranisporites rinconadensis Césari and Limarino (Fig. 5T) and Brevitriletes leptoacaina Jones and Truswell (Fig. 5J), the last record of Raistrickia rotunda, and the presence of Tetraporina punctata (Tiwari and Navale) Kar and Bose (Fig. 8K). Fossil plants were collected from the upper levels of the FA 3 and are preserved as imprints of foliage and molds of stems. Botrychiopsis weissiana Kurtz emend. Archangelsky and Arrondo is the most abundant species in the assemblage. This species is represented by bipinnate fragments characterized by pinnules obliquely inserted, decurrent, subopposite to alternate, spathulate to obovate (Fig. 9E). Distal pinnules are spathulate to oval with rounded margin and venation open and dichotomous (Fig. 9A, C). A long stem mold of lycophyte, 33 cm in length and 4 cm in diameter, exhibits rhomboidal leaf cushions in spiral arrangement (Fig. 9F). Its imprint on the host rock shows the rhomboidal leaf cushions, about 0.8 cm in width, lacking details about leaf scar or ligule (Fig. 9D). Probably these specimens correspond to the ones described by Frenguelli (1946) as Lepidodendron australe Mc Coy, but their preservation hampers to confirm this assignment. According to the observed characteristics in other Gondwanan lycophytes, outline and size of the leaf cushions vary on axes of different orders in the same species (Souza de Faria et al., 2009).
Several sphenopsid stems of small diameter (0.8-12 mm) are also present and probably represent herbaceous plants (Fig. 9B).
The presence of wave ripple cross-lamination, heterolithic structures and the dominance of finegrained rocks suggest a restricted (estuarine?) marine depositional environment. FA 3 corresponds to a short-lived early Moscovian marine transgression which is represented by shallow marine and estuarine deposits. Invertebrate remains were not recognized in these levels.

Facies Association 4 (FA 4, coarse-grained lenticular sandstones)
The base of the FA 4 is marked by an incision surface carved into FA 3 which is in turn covered by coarse-grained sandstones, conglomerates, and thin beds of fine-grained sandstones and mudstones (Table 1). FA 4, 85 m thick, is mainly composed of tabular and trough cross-bedded coarse-grained sandstones and conglomerates stratified in lenticular beds. Thin beds (up to 25 cm) of laminated finegrained sandstones and carbonaceous mudstones appear towards the top of this facies association. Only one palynological sample was recovered from this facies association; its composition is similar to the underlying assemblages ( Table 2).
The irregular topography recorded by the incision surface was initially filled by braided river deposits which initially did not preserve fine-grained sediments of the flood plain area. Nevertheless, floodplain accumulations were becoming more important towards the top of the facies, to the extent that the accommodation space increased.

Facies Association 5 (FA 5, fine-grained sandstones, shales and limestones)
This interval, from 75 to 95 m thick, is easily identified by the dominance of fine-grained rocks, including mudstones, shales, fine-grained sandstones and interlaminated successions of very fine-grained sandstones and mudstones. The lower part of the FA 5 (about 15 m thick) comprises a retrogradational sequence of medium-and fine-grained sandstones interbedded with mudstones being coarse-grained sandstones very scarce and restricted to the base of the succession. The middle part is mainly composed of dark gray shales and massive mudstones, frequently showing poorly preserved plant remains, together with centimetric lenses of massive marls. Finally, the upper part (70 m thick) is made up by an upward-coarsening and thickening succession composed of shales, mudstones and fine-to mediumgrained sandstones which finishes with a level of limestone (3 m thick) at the top of the succession. Sandstones frequently show horizontal lamination, ripple-cross lamination and ripple (both symmetrical and asymmetrical) at the top of the beds. Samples from the FA 5 contain Navifusa variabilis Gutiérrez and Limarino (Fig. 8H) as exclusive species and the first appearances of Raistrickia cephalata Bharadwaj, Kar and Navale (Fig. 6G, I), Limitisporites monosaccoides Bose and Maheswari (Fig. 7I), Cannanoropollis trilobatus (Höeg and Bose) Gutiérrez (Fig. 6Z), Potonieisporites marleniaei Playford and Dino (Fig. 7L), Protohaploxypinus sp. cf. P. haigii Foster (Fig. 8B), Protohaploxypinus limpidus (Balme and Hennelly) Balme and Playford (Fig. 8C), Striatoabieites multistriatus (Balme and Hennelly) Hart (Fig. 8A), Hamiapollenites insolitus (Bharadwaj and Saluha) Balme (Fig. 8D, E) and Protohaploxypinus amplus (Balme and Hennelly) Hart (Fig. 8N). Abundance of taeniate pollen varies from 0.3% to 0.7%. There is little doubt that FA 5 records the Pennsylvanian transgression and seems to be is a key level for regional correlation between the upper Paleozoic sequences of the western Paganzo Basin and Río Blanco Basin . The above described lower and middle parts of the facies association represent transgressive systems tract deposits while the upper part corresponds to sedimentation under highstand conditions. The TS Biozone studied by Cisterna et al. (2002Cisterna et al. ( , 2011 was recovered from these levels. The fossiliferous levels are located immediately below the thicker package of limestones and a few meters over the thinner package of limestones (see figure 2 in Cisterna et al., 2002).

Facies Association 6 (FA 6, variegated sandstones and conglomerates)
Fine-grained marine deposits are abruptly covered by fluvial coarse-grained sandstones and conglomerates reaching 35 in thickness which rest over an important incision surface carved into the marine deposits. While conglomerates are gray in color, sandstones are light gray to purple at the base of the facies association and pass upward to pink and red in color. Arkosic sandstones are stratified in cross-bedded lenticular channelized beds whereas conglomerates are mainly massive and composed of quartz, k-feldspar, granite and metamorphic clasts. Despite the fact that fine-grained rocks are missing in the first meters of the facies association, red mudstones appear from the middle part and increase progressively their participation towards the top.
The lowermost levels of the described succession are interpreted as deposited by braided rivers whilst a progressive increase in flood plain deposits occurs towards the top of the facies. It is possible that the growth in the amount of flood plain deposits reflects a change in the type of fluvial system from braided to wandering. The erosive surface that separates FA 5 and 6 in the La Herradura Creek correlates with an important incision surface of regional expression, which points out the Pennsylvanian sea-level fall in the Late Paleozoic basins of western Gondwana.

Facies Association 7 (FA 7, red mudstones and sandstones)
The top of the upper Paleozoic sequence, 45 m thick in the La Herradura Creek, is a classical redbed succession composed of channel complexes separated by thick flood plain deposits. Channel complexes reach up to 5 m in thickness and comprise coarse-grained cross-bedded sandstones and conglomerates stratified in amalgamated beds. Flood plain accumulations can be divided in two major types: 1. monotonous successions (up to 5 m thick) of horizontally laminated or ripple cross-bedded mudstones likely deposited in flood-basin areas and 2. upward-coarsening sequences of interbedded mudstones and sandstones probably corresponding to crevasse splay progradation within the floodplain.

Previous palynological studies
Vergel (2008) described three palynological associations, P(1a), P(1b) and P(9a) (see Cisterna et al., 2002, Fig. 2). The sample P(1a) was located at the base and the P(1b) in the middle part of Tupe Formation, while the sample P(9a) comes from the top levels of the Tupe Formation (Cisterna et al., 2002) or the lower part of the Patquía Formation according to stratigraphic boundaries followed here (Fig. 4).
The palynoflora P(1a) from the lower part of the Tupe Formation was characterized by Vergel (2008) Vergel (2008) referred this assemblage to the DM Biozone (Césari and Gutiérrez, 2001) and assigned a late Carboniferous age. Vergel (2008) noted that all these species are common with other associations of the Tupe Formation, and equivalent units, in other localities.
The sample from Patquía Formation, P(9a), contains taeniate pollen (5%) and was characterized by Vergel (2008) Menéndez. Vergel (2008) accurately mentioned that the palynoflora recovered from beds above the marine horizon includes species never before recorded from the Tupe Formation. The author proposed correlations with assemblages from western Argentina and other Gondwanan regions suggesting an early Permian age (not older than Asselian-Sakmarian) for the palynoflora.

Palynological results and comparisons
The palynological analysis of the Tupe and Patquía formations at the La Herradura Creek was performed on 15 samples collected from almost all the facies associations except the FA 6 that resulted palynologically unproductive. Several palynological associations comparable to those analyzed here have been described for central western Argentina successions. The palynological content of the FA 1, FA 2, FA 3, and FA 4 are equivalent to those included in the Subzones A and B of the DM Biozone defined for the Carboniferous of Argentina (Césari and Gutiérrez, 2001). Both subzones were characterized by the presence of monosaccate pollen grains and the appearance of Protohaploxypinus spp. in the Subzone B which was defined on the basis of palynofloras from the Tupe and other coeval units of the Paganzo Basin (e.g., Loma Larga, Trampeadero formations, the upper member of the Agua Colorada Formation and the lower parts of the Santa Máxima Formation) and the El Imperial Formation (San Rafael Basin, Fig. 10). The percentages of taeniate pollen in those palynofloras, is always lower than 1.5%.
Palynological assemblages from FA 5 are distinguished by the presence of Navifusa (a marine marker) and taeniate pollen, suggesting a correlation with the Subzone C of the DM Biozone. Subzone C was originally recognized in marine deposits of the El Imperial, Santa Máxima and Agua Negra Formations (Fig. 10). The palynofloras from the upper part of the 'Estratos de Mascasín' (Pérez Loinaze and Césari, 2004), the Libertad Formation (di Pasquo et al., 2010) in the Paganzo Basin and the upper part of the Río del Peñón Formation (sensu Gutiérrez and Limarino, 2006) in the Río Blanco Basin are characterized by some species of Protohaploxypinus and the first records of Vittatina but lack key species as Pakhapites fusus (Bose and Kar) Menéndez, indicative of the overlying FS Biozone. Libertad Formation at Los Sauces locality (Fig. 1) also bear plant remains of the Interval Biozone (defined by Archangelsky et al., 1987) confirming the relation between this biozone and the Subzone C.
Two samples from the Patquía Formation at La Herradura Creek provided palynological assemblages characterized by many species identified in underlying strata together with the incoming of Illinites unicus, Vittatina costabilis, Vittatina wodehousei and Vittatina subsaccata ( Table 2). Vergel (2008) also recognized specimens of Pakhapites fusus in these stratigraphic levels which were included in the Tupe Formation by Cisterna et al. (2002). Thus, palynological assemblages from the Patquía Formation can be compared with those recovered from the Bajo de Véliz and Tasa Cuna formations (Paganzo Basin) and the upper part of the El Imperial Formation. Palynological content of the Bajo de Véliz and Tasa Cuna formations was studied by Menéndez (1971), Gutiérrez and Césari (2000), Césari et al. (1999) and Balarino and Gutiérrez (2006). Palynofloras from both units share the taeniate pollen: Hamiapollenites fusiformis Marques-Toigo emend. Archangelsky and Gamerro, Protohaploxypinus spp., Pakhapites fusus and Vittatina spp. García (1996) analyzed palynological assemblages from the upper section of the El Imperial Formation and identified taeniate pollen such as Protohaploxypinus spp., Pakhapites fusus, Hamiapollenites fusiformis and Vittatina sp. All these palynofloras, were included in the FS Biozone (Césari and Gutiérrez, 2001) and are associated with plant remains of the Gangamopteris flora (Archangelsky and Cúneo, 1984;Césari et al., 2007).

Age and discussion
Palynological assemblages of the FA 1 to FA 4 are correlative of the Subzones A and B of the DM Biozone. These subzones were constrained to the Late Serpukhovian-Bashkirian according with radiometric data (Césari et al., 2011). Plant remains of the NBG Biozone (Nothorhacopteris-Botrychiopsis-Ginkgophyllum) supported the age assigned to the fossiliferous strata. The short-lived transgression, represented by FA 3, is characterized by palynological assemblages of those subzones without evidence of taeniate pollen. Invertebrate remains were not recognized in this interval, but regional correlation with fossiliferous localities in the west of the basin is possible. This short transgressive event may be constrained approximately to the latest Bashkirian-early Moscovian by 206 Pb/ 238 U age of 315.46±0.07 Ma obtained from a tonstein included in coal bed of flood plain deposits of the Tupe Formation at the near locality of Huaco . Thus, there is a strong correlation with strata bearing the Marginovatia-Maemia fauna (formerly known as Balakhonia peregrina-Geniculifera tenuiscostata) that characterizes the Río Blanco and Calingasta-Uspallata Basins (Taboada, 1999;Taboada, 2010), which was considered late Bashkirian-earliest Moscovian in age according to worldwide records of the homonyms species (Taboada, 1997(Taboada, , 2010. FIG. 10. Correlation of the main stratigraphic units bearing palynological assemblages mentioned in the text. Palynological biozones are based on Césari and Gutiérrez (2001). Radiometric ages are based on Gulbranson et al. (2010).
The main transgression, represented by FA 5, is characterized at La Herradura Creek by the oldest faunal assemblages of the TS fauna (Cisterna et al., 2011). Palynological assemblages here analyzed from the same beds bearing marine invertebrates confirm the original Pennsylvanian age assigned to the TS Biozone by Sabattini et al. (1990). Palynofloras recovered from the AF5, characterized by the presence of several species of Protohaploxypinus genus, are referred to the Subzone C of the DM Biozone. This subzone is referred to the Moscovian (Césari et al., 2011).
Although an scarce fauna was recognized in marine beds from the easternmost records of the transgression at La Ciénaga locality (17 km to southeast of the La Herradura Creek), these horizons were correlated with the marine interval identified in the middle part of the La Herradura Creek by Desjardins et al. (2009) and Cisterna et al. (2011). A sample of a tonstein from shallow marine deposits at La Ciénaga (facies association VII by Desjardins et al., 2009) yields a 206 Pb/ 238 U age of 312.82±0.11 Ma. Thus, as was suggested by Césari et al. (2011), radiometric ages discard an early Permian age for the top of the Tupe Formation. Moreover, the palynoflora (P9a) described by Vergel (2008) some tens of meters above the transgressive deposits (Fig. 4) and referred to the Asselian-Sakmarian suggests just that the underlying fauna would be not younger than this age.
The middle-upper part of the Río del Peñón Formation (Río Blanco Basin, Fig. 1) contains faunal assemblages described by (Cisterna and Sabattini, 1998;Cisterna and Simanauskas, 2000), one of them referred undoubtedly to the TS Biozone. The marine fossiliferous interval of 38 m in thickness was located approximately at 600 m above the base of the unit (Cisterna et al., 2011). Therefore, although thickness variations are considered accompanying the paleogeography, the TS Biozone is recognized above the radiometric 206 Pb/ 238 U dating of 310.63±0.07 Ma provided by Gulbranson et al. (2010) from an ignimbrite in fluvio-deltaic deposits, approximately at the level 'F' described by Gutiérrez and Limarino (2006) that contain palynological assemblages of the DM Biozone. A Moscovian age for the TS Biozone in the Río del Peñón Formation is also proposed by Dineen et al. (2012).
Radiometric K-Ar age of 307.2±5.2 Ma from the middle part of the Agua de Jagüel Formation (Mendoza Province) where Lech (2002) recognized the presence of the TS Biozone, also constrains its age range.

The faunal assemblages of the nearby La Delfina
Creek were also correlated by Cisterna et al. (2006) with those from La Herradura Creek, located 10 km to northwestern (Fig. 1). According to Cisterna (2010) and Cisterna et al. (2011) the TS Biozone in the Tupe Formation at La Delfina Creek is located above horizons that contain diagnostic species of the Carboniferous NBG and Interval megafloristic biozones and Raistrickia densa-Convolutispora muriornata palynological Biozone described by Coturel and Gutiérrez (2005) and Gutiérrez et al. (2005). Cisterna et al. (2011) noted that the faunal association is dominated by Costatumulus Waterhouse and Orbiculoidea d´Orbigny and is compositionally comparable to the fauna described in the lowest marine horizon of the Tupe Formation at La Herradura Creek.
The FA 7 considered here part of the Patquía Formation contains palynofloras assigned to the FS Biozone. The age of this biozone was originally considered early Cisuralian (Césari and Gutiérrez, 2001), but recently Césari et al. (2011) proposed its possible beginning in the latest Pennsylvanian. Some species present in this facies association, that usually were considered markers of Carboniferous-Permian in Gondwana include: Converrucosisporites confluens, Vittatina spp., Pakhapites fusus, among others. Although Vittatina species are abundant in Permian sequences, the genus is also known from the Moscovian of Arabia (Owens and Turner, 1995), the Stephanian B of Western Europe (Clayton et al., 1977) and the section slightly below the Carboniferous-Permian boundary stratotype in Kazakhstan (Dunn, 2001) and the late Bashkirian-early Moscovian of the Subangara area (Zhu et al., 2005). Moreover, di Pasquo (2009) has recorded well-calibrated Vittatina species in the Moscovian of Bolivia.
Converrucosisporites confluens is a good index of the latest Carboniferous-earliest Permian in Australia, Antarctica, Brazil, India, Oman, Saudi Arabia, South Africa and South America. Stephenson (2009), proposed the incoming of C. confluens in the Gzhelian or Kasimovian based on palynological assemblages from the Ganigobis Shale Member of Namibia, radiometrically dated as 302.0±3.0 Ma. The difficulty in detecting good specimens of C. confluens in sparse assemblages has rendered its identification uncertain in most palynofloras of the Paganzo Basin.
Previous records of taeniate pollen recovered from glossopterid sporangia have revealed that Protohaploxypinus spp. (especially P. limpidus) and Striatopodocarpidites cancellatus (Balme and Hennelly) Hart are species not useful for high resolution biostratigraphic analysis since they have been found in a single sporangium (Lindström et al., 1997 and cites therein). The same coincidence of the two pollen types was advertised in Angaraland pollen organs (Zavialova and van Konijnenburg-van Cittert, 2011) and primitive Protohaploxypinus (1%) were reported in the early Bashkirian of the Subangara area (Zhu et al., 2005).
The upper part of the Río del Peñón Formation also provided palynological assemblages (levels G-L in Gutiérrez and Limarino, 2006) with up to 2% of Vittatina (see page 703 in Gutiérrez and Limarino, 2006). Those authors suggest the presence of the Pakhapites fusus-Vittatina subsaccata (FS) Biozone by the occurrence of Vittatina sp., Barakarites rotatus and Polarisaccites bilateralis. Cisterna et al. (2011) add new palynological species, although the precise stratigraphic range of the new records (e.g., Pakahpites fusus) is not given throughout the succession. The illustrated specimens are poorly preserved casting doubt on some of the assignments, e.g., B. rotatus and P. fusus (Cisterna et al., 2011, fig. 4.1, 4.2). If the presence of the biozone can be confirmed, it would have begun in the late Pennsylvanian according to the radiometric age obtained by Gulbranson et al. (2010) for the Río del Peñón Formation.
The FS Biozone was correlated by Césari and Gutiérrez (2001), Césari (2007) and Césari et al. (2011) with the Brazilian Vittattina costabilis (VcZ) Interval Zone Souza and Marques-Toigo (2003), characterized by the abundance and diversity of taeniate pollen grains and registered in the upper Itararé Subgroup and Río Bonito Formation (Marques-Toigo, 1991). Guerra-Sommer et al. (2005) obtained a time of deposition based on ID-TIMS U-Pb zircon ages ranging from 299±2.6 to 296±1.4 Ma for tonsteins located in the middle section of the Río Bonito Formation. A similar age (298.5±2.6 Ma) was obtained by Rocha-Campos et al. (2006, 2008 for the same stratigraphic section. Thus, a latest Kasimovian or earliest Asselian age may be considered at least for the lower part of the VcZ in Brazil. In summary, radiometric data in Argentina and Brazil in conjunction with palynological correlation allow to propose a Late Pennsylvanian age for the high eustatic conditions and associated TS Biozone recognized in the Tupe Formation at the locality of La Herradura Creek. Recently, Césari et al. (2011) proposed a Moscovian age for the TS Biozone without discarding that can reach slightly younger ages. Although radiometric data from Brazil and Argentina suggest an age no younger than 299 Ma, a palynological revision and absolute ages in key sections bearing TS Biozone are necessary to precise the upper limit of the faunal biozone.

Conclusions
The Tupe Formation is constrained to the Pennsylvanian in contrast to the proposal that extends its chronostratigrafic range into the Permian on the basis of the faunal association located in its upper part. Two marine incursions are identified in the middle-upper part of the unit at La Herradura Creek. The lower is characterized by palynofloras of the Subzone B of Raistrickia densa-Convolutispora muriornata Biozone and constrained to approximately 315 Ma. The higher is more significant and the strata contain marine invertebrates of the Tivertonia jachalensis-Streptorhynchus inaequiornatus Biozone and palynofloras referred to the Subzone C of Raistrickia densa-Convolutispora muriornata Biozone. Radiometric controls in near localities corroborate the presence of the Tivertonia jachalensis-Streptorhynchus inaequiornatus Biozone at approximately 312 Ma.
Palynological samples recovered from strata above the marine deposits, in the Patquia Formation, provide palynological assemblages that allow their assignment to the Pakhapites fusus-Vittatina subsaccata Biozone.
Comparison of the palynofloras studied here with other described from the Paganzo Basin, suggests a gradual increase in abundance of taeniate pollen towards the Carboniferous-Permian boundary and encourages the revision of key stratigraphic sections for a better definition of the Subzone C of Raistrickia densa-Convolutispora muriornata Biozone and the Pakhapites fusus-Vittatina subsaccata Biozone.