The Carboniferous-Permian boundary in the central western Argentinean basins : paleontological evidences

The central western Argentinean basins of Río Blanco, Calingasta-Uspallata and western Paganzo, enclose the most complete marine successions used for examining the Carboniferous-Permian boundary in Gondwana. A detailed review of the key stratigraphical sections has allowed the identification of the latest Carboniferous assemblages; NBG, Interval megafloristic Zone and DM (Raistrickia densa-Convolutispora muriornata) Palynological Zone in the lower part, and the earliest Permian T-S (Tivertonia jachalensis-Streptorhynchus inaequiornatus) invertebrate Zone and FS (Pakhapites fusus-Vittatina subsaccata) Palynological Zone in the upper part. This diagnostic mega/microflora and marine invertebrate paleontological evidence provides a biostratigraphical framework for the definition of the CarboniferousPermian boundary inside the basins of central western Argentina.


Introduction
The Carboniferous-Permian (Gzhelian-Asselian) boundary has been biostratigraphically defined in stratotype sections in the latest Carboniferous and earliest Permian marine sequences from the Uralian Region (Russia and Kazakhstan), by key conodonts and fusulinid foraminiferid species.The beginning of the Asselian stage is indicated by the first appearances of the conodont Streptognathodus isolatus, the fusulinid Sphaeroschwagerina aktjubensis (=vulgaris) and the distinctive ammonoid species of the genera Boesites, Daixites, Glaphyrites, Artinskia, Svetlanoceras, Prostacheoceras and Prothalassoceras (Bogoslovkaya et al., 1995;Davydov et al., 1997;Mei et al., 1999;Lucas et al., 2001).However in Gondwana and the peripheral gondwanan regions, these faunal groups, considered the principal tools for correlation, are absent or more often, extremely rare.The marine faunas and terrestrial microflora data from the early Permian (Asselian-Early Artinskian) interval of these regions (Australia, Afghanistan-Pakistan, Himalaya, China, Southeast Asia, India, Africa, Arabia, Antarctica and South America), has been compiled by Archbold (2001).This author has also suggested the importance of using palynological data to link the marine successions in the definition of the Carboniferous-Permian boundary.Recently early Permian faunal and palynological records from eastern Australia and the Argentine Precordillera have been discussed by Archbold et al. (2004).The authors concluded that integrating the studies of marine faunas with the palynological record is required to both date the record and to strengthen Gondwanan wide correlations.
The central western Argentinean basins (Fig. 1A) examined here, appear to enclose one of the more complete successions for the study of the Carboniferous-Permian boundary in this part of Gondwana.Because more precise biostratigraphical elements, such as conodonts and fusulinids that would define the boundary in the type section of the Uralian region are absent, this paper attempts to match the marine faunal data (primarily brachiopods, the most biostratigraphically important group available and the conspicuous bivalve fauna), with the extensive megafloristic and palynological records in different key sections inside the Argentine Precordillera.A biostratigraphic local scheme of the Carboniferous-Permian boundary, based in the integrated study of these fossil groups, and the possible correlation with other Gondwan localies, is provided herein.

The Carboniferous-Permian boundary in the marine successions of the Argentine Precordillera
The latest Carboniferous-earliest Permian interval in the Argentine Precordillera is associated with a Paleo-Pacific transgressive event that represents the transition from a glacial to postglacial condition, with the posterior climatic amelioration in this area (López Gamundí, 1989;Limarino et al., 2002).
Previous studies of the authors (Cisterna and Sabattini, 1998;Cisterna and Simanauskas, 2000;Cisterna and Sterren, 2007;Cisterna et al., 2002Cisterna et al., , 2005Cisterna et al., , 2006bCisterna et al., , 2006c;;Coturel and Gutiérrez, 2005;Gutiérrez et al., 2005;Sterren, 2000Sterren, , 2004)), indicate that the most complete and fossiliferous gondwanan successions identified for this interval of time have been documented in the central western Argentinian basins, i.e. the Río Blanco basin (Río del Peñón and Quebrada Larga Formations) and in the west part of the Paganzo basin (Tupe Formation) (Fig. 1B).Paleontological evidence, located throughout each stratigraphical section of these units, have allowed for integrated studies of the marine faunal data with the associated palynological and megafloristic records.
Recent field work in the Agua del Jagüel Formation (Calingasta-Uspallata basin), in the southernmost part of the Argentine Precordillera, allows suggesting a probable new key section to define the Carboniferous-Permian boundary.This stratigraphical unit is characterized by a sedimentary succession grading upwards from a deglaciation facies to a fluvial and shallow marine facies (Henry et al., 2008).The lower part of this unit (deglaciation phase) includes the diagnostic late Carboniferous Rhipidomella-Micraphelia faunal assemblage, located immediately above the diamicitic horizons, in the lower part of the section (Martínez et al., 2001).Additionally, a K-Ar date in biotite of 307±5.2Ma (Lech, 2002) from dacitic pillow lavas, located about 150 m above of the Rhipidomella-Micraphelia faunal assemblage, indicates a Late Carboniferous age.The upper part of the Agua del Jagüel Formation, distinguished by shoreface sandstones and offshore shales, contains the lower Permian Costatumulus amosi fauna (Cisterna, 2010).

Key sections for the definition of the Carboniferous-Permian boundary in Argentina
Here we review the stratigraphic and paleontological evidence of key stratigraphical sections for the Carboniferous-Permian boundary in the central western Argentinean basins.The completed study places emphasis on the Río del Peñón and Quebrada Larga formations of the Río Blanco basin and on the Tupe Formation in western Paganzo Basin (Fig. 1B).

Río del Peñón Formation
The main outcrops of the Río del Peñón Formation (Borrello, 1955;González and Bossi, 1986) are exposed at the Rincón Blanco syncline, located 35 km west of the village of Jagué in the La Rioja province (Fig. 1B).This unit, about 1,200 m thick in the examined reference section (Fig. 2), encloses several fossiliferous horizons containing rich and diversified invertebrate marine fauna and megafloristic assemblages.The marine faunal assemblages in this section are more abundant and diversified than those from the other sections studied herein.Three informal members are recognized in the Río del Peñón Formation.The lower member of the section is characterized by an abundance of conglomerates probably deposited in a proximal glacimarine environment (Gulbranson et al., 2008), and by sedimentary associations representing fluvial distributary channels and interdistributary bay deposits identified by coal beds.The middle-upper member of the section is characterized by vertically-stacked, coarsening-upward successions that suggest mouth bars overlaying prodeltaic mudstones.Locally bioclastic sandstone-mudstone horizons with marine invertebrate fauna have been recognized in the middle member.An interval approximately 25 m thick, characterized by carbonate and siliciclastic facies with an excellent record of marine invertebrates has been identified in the middle member (Fig. 2).This interval is interpreted to be a marginal marine environment, such as a tidal and subtidal flat with mixed sedimentation and deltaic front with a reworking wave in the upper part of the interval (Cisterna, 1997;Sterren, 2000).In the upper part of the section, complexes of fluvial distributary channels associated with overbank deposits have been identified (Cisterna, 1997;Sterren, 2000).The Río del Peñon Formation and the underlying Punta del Agua Formation have been objects of several radiometric studies.Radiometric ages reported from andesites of the Punta del Agua Formation (295 Ma, Fauqué et al., 1999), as well as from a riodacitic sill intercalated in the lowermost part of the Río del Peñón Formation (287.8±0.7 Ma Ar/Ar method, i.e., a Sakmarian age, Coughlin, 2000), suggests a Stephanian age for the base of the Río del Peñón Formation, whose lower member would have been deposited before the Sakmarian.
Three different faunal marine assemblages, closely related to the associated paleofloristic record, have been identified in the stratotype section of the Río del Peñón Formation (Cisterna and Simanauskas, 2000).

Quebrada Larga Formation
Outcrops of the Quebrada Larga Formation (Scalabrini Ortiz, 1972) can be recognized on the west flank of the Punilla Hill in the Carrizalito area, San Juan province.In the reference section located to the left margin of the Blanco River (Fig. 6), about 60 km northern of Malimán, the succession is characterized by a predominance of sandstones, occasionally interbedded fine conglomerates and mudstones that contain remains of NBG flora (Scalabrini Ortiz and Arrondo, 1973).Besides the reference section, remains of plants (Botrychiopsis weissiana, Malanzania nana, Nothorhacopteris sp., Paracalamites sp., and Cordaites sp.), that characterize the NBG Zone have been recognized in the basal part of the Quebrada Larga Formation (Cingolani et al., 1992;Caminos et al., 1993) exposed to the northwest of the Punilla Hill, close to the Llanos de Chaparro locality in La Rioja Province.The NBG Zone has also been identified in the uppermost part of the Cortaderas Formation (Scalabrini Ortiz, 1973), in horizons that would otherwise be included in the Quebrada Larga Formation (Cardó et al., 2001;Carrizo, 1990).The records of the NBG Zone in the Quebrada Larga Formation have been documented in the lower part of the section.
In the upper part of the reference section of the Quebrada Larga Formation (Fig. 6), two marine fossil assemblages are described in a 120 m thick stratigraphical interval (Cisterna and Sterren, 2007) sp.aff.S. jaguelensis and scarce gastropods.From outcrops located on the left margin of the Blanco River, immediately to the north of the Quebrada Larga reference section, another marine faunal assemblage has been recognized (Cisterna and Sterren, 2007).This assemblage appears in calcareous horizons interbedded in mudstones and is dominated by the bivalves Schizodus sp., Pleurophorella sp., Edmondia sp., Modiolus?sp., Aviculopecten sp. and Streblochondria sp., accompanied by gastropods (Peruvispira?sp.) and the brachiopods Septosyringothyris sp. and Productida indet.Unfortunately the diagnostic elements of the T-S fauna have not been found in this isolated assemblage, and therefore it cannot be correlated with the fauna from the Quebrada Larga reference section.
The marine fauna from the Quebrada Larga Formation can be included in the Tivertonia jachalensis-Streptorhynchus inaequiornatus Biozone (Sabattini et al., 1991).However some compositional variations have been observed, including the occurrence of Coronalosia sp. and Svalbardia sp., which probably represents two new species, both with Permian gondwanan affinities.The reference section of the Quebrada Larga Formation is a potential key section to study of the Carboniferous-Permian boundary (Cisterna and Sterren, 2007), as early Permian Tivertonia jachalensis-Streptorhynchus inaequiornatus fauna occurs above beds carrying elements of the Late Carboniferous megafloristic assemblage NBG, such as Nothorhacopteris argentinica.

Western Paganzo basin
From a paleogeographic viewpoint, outcrops of the Tupe Formation in the western region of the Paganzo basin (La Herradura creek, La Delfina creek, Mina La Ciénaga and Paslean localities, on the west flank of Perico Hill, San Juan province) (Fig. 1B), belong to the 'Guandacol embayment' (López Gamundí et al., 1994).The Tupe Formation consists primarily of sandstone, conglomerate, mudstone and coal, and uncomfortably overlies the Guandacol Formation.Although the Tupe Formation typically suggests deposition in continental environments, in some localities this continental succession is punctuated by a marine interval interpreted as the Panthalassan marine ingression, indicating an extensive geographical breach of the Proto-Precordillera at that time (López Gamundí et al., 1994).

Tupe Formation at La Herradura creek
Outcrops of the Tupe Formation at La Herradura creek are located about 20 km northeast of San José de Jáchal locality in the San Juan province (Fig. 1B).This stratigraphic section is considered to be the stratotype of the Tivertonia jachalensis-Streptorhynchus inaequiornatus Biozone (Sabattini et al., 1991).The marine fossiliferous horizons that contain this fauna are located in the upper part of the Tupe Formation, in a stratigraphical interval approximately 20 m thick, composed of mudstones interbedded with carbonate beds (Fig. 7).
The fossil assemblage that characterizes the Tivertonia jachalensis-Streptorhynchus inaequiornatus Biozone in the Tupe Formation at La Herradura by Cisterna et al. (2002); and bivalves reviewed by Sterren (2004), who suggested an assemblage mostly dominated by cosmopolitan genera, although some of them show tethyan affinities.

Tupe Formation at La Delfina creek
The stratigraphical section of the Tupe Formation, located south of the Mina La Delfina locality, about 29 km northeast of San José de Jáchal (Figs. 1B,  8), encloses an important paleontological record, composed of megafloristic, palynological and marine invertebrate assemblages.The lower part of Tupe Formation at La Delfina Creek, characterized by sandstone and mudstone with coal facies, has been interpreted as a deltaic, swamp system containing a high proportion of organic matter (Ottone and Azcuy, 1986).Two megafloristic assemblages have recently been identified in this part of the section (Coturel and Gutiérrez, 2005).The lower assemblage is composed of Fedekurtzia argentina Nothorhacopteris argentinica (Fig. In the middle part of the section, in a stratigraphical interval of alternating sandstones and claystones with mudstones lenses, a marine fossil assemblage composed of brachiopods (Costatumulus sp. and Orbiculoidea sp.dominant, accompanied by Pericospira pericoensis, Kochiproductus sp., Streptorhynchus inaequiornatus, Septosyringothyris sp. and scarce Tivertonia jachalensis), bivalves (Heteropecten?sp., Myofossa sp., Sanguinolites? sp. and Anomalodesmata indet.),gastropods and scarce crinoids, have been recognized (Cisterna et al., 2006b).This association, dominated by Costatumulus and Orbiculoidea, is compositionally comparable to the fauna described in the lowest marine horizon of the Tupe Formation at La Herradura creek (Cisterna and Simanuskas, 2002) and it could represent the oldest part of the Tivertonia jachalensis-Streptorhynchus inaequiornatus Zone in the Paganzo basin (Cisterna et al., 2006b).The marine interval that contains this invertebrate fauna represents the maximum flooding surface; the trace-fossils situated in the strata located over them evidence low diversity and high abundance, suggesting a stressed environment (Desjardins et al., 2009(Desjardins et al., , 2010)).

Tupe Formation at Mina La Ciénaga
A detailed sedimentologic and high-resolution sequence stratigraphic analysis of the lower member of the Tupe Formation at Mina La Ciénaga locality (Figs.1B, 8), carried out by Desjardins et al. (2009), has provided insight into the geographically larger latest Carboniferous-early Permian transgression within the Paganzo Basin.The lower member of Tupe Formation shows the transition from a coastal-plain to a marine embayment.The detection of a transgressive surface within the coastal-plain deposits has significantly expanded the volume of deposits than can be included as part of the latest Carboniferousearly Permian transgression (Desjardins et al., 2009).
Although the transgression is well represented at the Mina La Ciénaga locality, only one fossiliferous horizon, about 110 m from the base, has been recognized.The faunal assemblage integrates a coquina horizon composed of large gastropods; these appear to be closely related to those identified in the middle part of the fossiliferous interval in La Herradura creek.However other diagnostic elements of the Tivertonia jachalensis-Streptorhynchus inaequiornatus Zone have not yet been found at Mina La Ciénaga.

Discussion
The Late Paleozoic marine sedimentary sequences in Argentina are characterized by the absence of the most important key faunal groups, such as conodonts and fusulinid foraminiferids, which when present, provide a precise correlation with the defined Carboniferous-Permian boundary in transgression and the relationship between the fossil assemblages (Fig. 8).
The marine interval of the Tupe Formation, characterized by changes in thickness between adjacent localities, records the maximum transgression of the Paganzo Basin (Dejardins et al., 2009).In several sections of the Tupe Formation in the western Paganzo basin (Fig. 8), the marine horizons containing diagnostic elements of the T-S Zone are bounded by the upper and lower part of the micro and megafloristic key assemblages.The best and most developed section of the T-S Zone within the basin is documented in the Tupe Formation at La Herradura Creek, the westernmost fossiliferous locality.A distinctive vertical distribution of the T-S fauna has been recognized along a 15 m thick fossiliferous interval, allowing the identification of the possibly oldest T-S faunal assemblages (Cisterna et al., 2002).The assemblage of brachiopod genera such as Pericospira-Kochiproductus-Costatumulus, that appear to be dominant in the oldest part of this zone, represent the main component of the T-S fauna in La Delfina Creek section, where the fauna is vertically concentrated.The bivalves that appear associated in the T-S Zone in the La Herradura Creek section have also been identified in the upper part of Río del Peñón Formation, which overlies the T-S zone.The more characteristic Permian bivalves from this assemblage are Nuculavus levatiformis and Septimyalina sp.whose tethyan affinities have been discussed previously by Sterren (2004).
In the Tupe Formation at the Mina La Ciénaga locality, the latest Carboniferous-early Permian transgression is well represented in stratigraphical terms, but the fauna is not abundant and the the diagnostic elements of the T-S Zone have not been identified.The marine horizons of this section, apart from those of La Delfina Creek, represent the eastern most location that the Panthalassan marine ingression reaches in the western Paganzo Basin; the fauna here is comparatively impoverished in relation to those registered in other localities inside of the western part of the Paganzo basin (Cisterna et al., 2006b).
A similar biostratigraphical pattern that includes the invertebrate, palynological and megafloristic zones has also been recognized in the thicker succession of the Río del Peñón Formation in its reference section (Rincón Blanco locality, Río Blanco basin).The T-S Zone appears in a 38 m thick succession, in the upper part of the marine member, showing a particular vertical distribution.Although the key species Tivertonia jachalensis and Streptorhynchus inaequiornatus are present, they are not in abundance in the lower part of the fossiliferous interval; the brachiopod faunal assemblage is dominated by Pericospira riojanensis.In the upper part of the interval, brachiopods and ostracods constitute the dominant taxa.Tivertonia jachalensis is abundant and the species identified in the lower part disappear.As it was noted in the Tupe Formation, in the youngest part of the T-S Zone, the diagnostic species Tivertonia jachalensis is Streptorhynchus inaequiornatus appear to be more abundant.Some brachiopod genera that characterize the T-S fauna (i.e., Streptorhynchus, Pericospira, Costatumulus), have also been identified in the lowest part of the Río del Peñón section, immediately above the latest Carboniferous megafloristic assemblages.However, the fossil material that integrates this assemblage (Assemblage I, Cisterna and Simanuskas, 2000), is fragmentary and has not been determined at specific level.Also, the brachiopod species Septosyngothyris jaguelensis, similar to those described from the Tupe Formation, has been recognized in the uppermost brachiopod assemblage of the Río del Peñón section, informally identified as Neochonetes-Rhynchopora (Assemblages III, Cisterna and Simanauskas, 2000).This assemblage could represent the latest Early Permian marine horizons inside the Río Blanco and Paganzo basins, but its biostratigraphical relationship with the Early Permian Costatumulus amosi fauna, from the Calingasta-Uspallata basin, has not yet been examined in detail.
Palynological records have been well documented along the fossiliferous marine interval of the Río del Peñón Formation (Gutiérrez and Limarino, 2006).The T-S Zone is associated with elements of the Early Permian FS (Pakhapites fusus-Vittatina subsaccata) Palynological Zone and below this faunal assemblage, with the Latest Carboniferous DM (Raistrickia densa-Convolutispora muriornata) Zone, which has also been identified by Gutiérrez and Limarino (2006).The new palynological evidence from the upper part of the Río del Peñón section, presented in this article, suggests the presence of the Pakhapites fusus-Vittatina subsaccata (FS) Zone (late Cisuralian-early Guadalupian).The palynological record associated with the T-S Zone in the Río del Peñón Formation would define the age of the Panthalassan marine transgression into the Río Blanco Basin, as well as provide another tool for correlation.
The oldest part of the T-S Zone appears to be not present in the Quebrada Larga Formation, but the faunal assemblage shows a compositional variation along the fossiliferous interval.Septosyringothyris sp.aff.S. jaguelensis, the dominant brachiopod species, accompanied by Tivertonia jachalensis, Orbiculoidea sp. and very scarce Streptorhynchus inaequiornatus, characterizes the lower part of the interval, and Coronalosia sp. and Svalbardia sp.(two new species with Permian gondwanan affinities that are being studied by one of the authors), the upper.Although there is no palynological evidence from the Quebrada Larga Formation, the occurrence of Tivertonia jachalensis-Streptorhynchus inaequiornatus fauna above a bed carrying elements of the Late Carboniferous megafloristic assemblage NBG, allows to consider this unit as a potential key section for studying the Carboniferous-Permian boundary.
The geographical extension of the T-S Zone is clearly restricted to the Río Blanco and western Paganzo basins; however some of their brachiopods (Tivertonia jachalensis, Pericospira rioajensis, Pericospira pericoensis), bivalves (Modiolus (Modiolus), Schizodus, Edmodia, Wilkingia, Myalinidae and Parallelodontidae), and gastropods (Knightites (Cymatospira) montfortianus and 'Peruvispira' sp.), are common and show affinities with those recognized in the lower member of the Del Salto Formation in the Calingasta-Uspallata Basin.Furthermore, the identification of the Tivertonia jachalensis in the Del Salto Formation (Taboada, 2006), appears to extend the geographical distribution of Tivertonia jachalensis-Streptorhynchus inaequiornatus fauna into the Calingasta-Uspallata Basin.The stratigraphical relationship between the T-S Zone and the Costatumulus amosi Zone (Taboada, 1998), from the early Permian successions of the Agua del Jagüel and Santa Elena formations (Mendoza province, Calingasta-Uspallata Basin) is not clear.Taboada (2006) has proposed a youngest age (latest Asselian-Sakmarian?) for the Costatumulus amosi Zone, but several common diagnostic species have been suggested for both zones.Further studies will allow the correct biostratigraphical relationship between Tivertonia jachalensis-Streptorhynchus inaequiornatus Zone and the Costatumulus amosi Zone.A better biostratigraphical and paleobiogeographic knowledge of the Precordilleran Permian marine faunas will provide a better understanding of their relationship with those fauna from the Patagonian basins (Tepuel Genoa and San Rafael basins), as well as with faunas from Perú, Bolivia and Chile, which have recently been studied (Cisterna and Niemeyer, 2010;Cisterna et al.,2009).

FIG. 1 .
FIG. 1. A. location maps with the paleogeography of the Río Blanco and Paganzo basins in western Argentina; b. generalized map showing the geographical location of the studied key sections; LH.La Herradura creek; LD.La Delfina creek; LC.Mina La Ciénaga.

FIG. 6 .
FIG. 6. Stratigraphical section of the Quebrada Larga Formation in the type locality (modified from Cisterna and Sterren, 2007) and vertical distribution of the fossiliferous horizons.

FIG. 7 .
FIG. 7. Stratigraphical section of Tupe Formation at La Herradura creek showing the vertical distribution of the invertebrate and mega/microflora assemblages (modified from Cisterna et al., 2005).
5.7-8), and accompanied by Kochiproductus riojanus (Fig.5.12,15), Costatumulus sp.B, Septosyringothyris sp., Spiriferellina sp. and Orbiculoidea sp.However, Tivertonia jachalensis and Streptorhynchus inaequiornatus, which define the T-S Zone, are scarce in this location.In the upper part of the fossiliferous interval, the faunal assemblage is associated with mudstone horizons and it is composed of brachiopods and ostracods.
The dominant brachiopod species is T. jachalensis accompanied by S. inaequiornatus, Kochiproductus?sp., Productidae indet.(a possible new species for this zone) and very scarce linguliforms.
FIG. 8. Stratigraphic correlation as the hypothetical matching of the sections studied, showing the latest Carboniferous-early Permian transgression with the record of the Tivertonia jachalensis-Streptorhynchus inaequiornatus Invertebrate Zone and its relationship with the mega and microflora assemblages.Captions as previous figures.