Advances on Conodont-Graptolite Biostratigraphy of the Ordovician System of Argentina

Guillermo L. ALBANESI and Gladys ORTEGA1

Abstract - ADVANCES ON CONODONT-GRAPTOLITE BIOSTRATIGRAPHY OF THE ORDOVICIAN SYSTEM OF ARGENTINA- Present state of knowledge on conodont-graptolite biostratigraphy of the Ordovician System of Argentina is reviewed. A dual biostratigraphic chart is controlled by conodont-graptolite links throughout the system. Conodont faunas from the Argentine Precordillera were dominated by Midcontinent type components during the Lower Ordovician, while an increasing influx of Atlantic taxa was received through the system. Conodont faunas from northwestern Argentine basins include mixed assemblages from both realms, though characterizing transitional environments. 21 conodont and 29 graptolite biostratigraphic units are recognized in the Ordovician System of Argentina. Upper Ordovician conodont faunas are still not well registered. Best controlled graptolite records derive from the Lower Ordovician of north-west Argentina, and the Middle and Upper Ordovician of the Precordillera. The assemblages of the Eastern Cordillera are characterized by Atlantic and Baltic types across the Lower Ordovician. During the Middle Ordovician the graptolite faunas show a Pacific affinity in the Precordillera.

Resumen - AVANCES EN BIOESTRATIGRAFÍA DE CONODONTES Y GRAPTOLITOS DEL SISTEMA ORDOVÍCICO DE ARGENTINA. Se revisa el estado actual de conocimiento sobre bioestratigrafía de conodontes y graptolitos del Sistema Ordovícico de Argentina. La carta bioestratigráfica dual está controlada por registros conjuntos de conodontes y graptolitos guías a través del sistema. Las faunas de conodontes de la Precordillera Argentina están dominadas por componentes típicos del Reino Midcontinent durante el Ordovícico Inferior, con una creciente diversidad de taxones atlánticos a través del sistema. Las faunas de conodontes de las cuencas del noroeste de Argentina incluyen asociaciones mixtas de ambos reinos, caracterizando ambientes transicionales. Se reconocen 21 y 29 unidades bioestratigráficas de conodontes y graptolitos, respectivamente, en el Sistema Ordovícico de Argentina. Las faunas de conodontes del Ordovícico Superior aún carecen de un registro adecuado. Los registros de graptolitos más completos proceden del Ordovícico Inferior del noroeste argentino, y del Ordovícico Medio a Superior de la Precordillera. Las asociaciones de la Cordillera Oriental muestran afinidad atlántica y báltica en el Ordovícico Inferior. Durante el Ordovícico Medio las faunas de graptolitos presentan afinidad pacífica en la Precordillera.

Keywords: Ordovician. Argentina. Biostratigraphy. Conodonts. Graptolites.

Palabras clave: Ordovicico. Argentina. Bioestratigrafía. Conodontes. Graptolitos.

Introduction

The purpose of present contribution is to review the state of the art on conodont-graptolite biostratigraphy of the Ordovician System of Argentina. The objective considers most precise ties between key species of these important orthochronologic fossil groups (Leitfossils), which allow the establishment of a composite biozonal scheme (Figure 1).

The history of conodont knowledge from Argentine basins begins in 1951, when Youngquist and Iglesias mention the finding of conodont species proceeding from Lower Ordovician strata of Jujuy Province. First Argentine authors to publish a conodont report were Hünicken and Gallino (1970); since then, but in particular during last decade, conodont biostratigraphy received a strong impulse allowing for the proposal of a preliminary biozonation for the Argentine Precordillera (Albanesi et al., 1995d, 1998), and the organization of a series of biostratigraphic intervals for northwestern Argentina.

Conodont zones of the Argentine Precordillera are characterized by assemblages that incorporate a major proportion of species from the Midcontinent Realm during the early Lower Ordovician, though the contribution of Atlantic representatives increases through the late Lower Ordovician (Bagnoli and Stouge, 1991; Albanesi and Bergström, in press). A mixture of faunas prevails during the Middle Ordovician, while characteristic cold-water forms dominate the late part of the period (e.g., Lehnert, 1995a; Albanesi et al., 1998; Lehnert et al., 1999). Conodont faunas from northwestern Argentina are still not well understood. Apparently, a predominant Atlantic affinity occurs during the Lower and Middle Ordovician. However, typical species from the Midcontinent Realm are already present in the early Lower Ordovician, being the complex fauna assigned to a “transitional realm” (Rao, 1999; Albanesi et al., 1999b).

First graptolite record in Argentina corresponds to Brackebush (1883), who found a didymograptid specimen in the Ordovician of the Portezuelo of Salta, subsequently described by Kayser (1897). Since that time, graptolite contributions were documented in numerous papers (particularly profuse during last decade), which provided significant information for regional and intercontinental correlation, and paleobiogeographic relationships. Older graptolite faunas are best represented in north-western Argentine basins (Cordillera Oriental and Famatina System), where Lower Ordovician faunas are well documented. Arenig faunas show Atlantic affinity (Maletz and Ortega, 1995); however, particular associations include some Pacific, Baltic, and Chinese elements (Toro, 1999a) that suggest an intermediate latitudinal position for this peri-Gondwanan region.

Graptolite assemblages of the Precordillera are registered from lower Arenig to Ashgill rocks. They exhibit a remarkable Pacific affinity linked to the particular paleogeographic evolution of the Precordillera, in relation to the neighbouring Gondwanan environments (cf., Benedetto et al., 1999; Aceñolaza et al., 2002).

Current advances on the reconstruction of the global time scale for the Ordovician Period (Webby, 1998; Ordovician News, 2002), as provided by respective working groups of the International Subcommission on Ordovician Stratigraphy, are incorporated in the chrono-biostratigraphic chart (Figure 1). Recent progresses on the establishment of intra and inter-systemic boundaries include Global Stratotype Sections and Points for the Cambrian / Ordovician boundary by the FAD of Iapetognathus fluctivagus (Cooper et al., 2001), the boundary between the Lower (Tremadocian) and Upper Stages of the Lower Ordovician Series by the FAD of Tetragraptus approximatus (Maletz et al., 1996), the base of the Upper Stage (Darriwilian) of the Middle Ordovician Series by the FAD of Undulograptus austrodentatus (Mitchell et al., 1997), and the boundary between the Middle and Upper Ordovician Series by the FAD of Nemagraptus gracilis (Bergström et al., 2000). The Lower and Middle Ordovician Series boundary is currently under discussion (cf., Finney and Ethington, 2000), as well as that one between both stages of the Upper Ordovician Series (Ordovician News, 2002). The GSSP for the Ordovician / Silurian boundary, as marked by the FAD of Parakidograptus acuminatus, is open to revision (Silurian Times, 2002).

Ordovician rock sequences of Argentina were traditionally referred to by using the classic British chronostratigraphic scheme. An attempt for applying a regional nomenclature was introduced by Aceñolaza (1992), without development so far. In spite of enormous efforts carried out during last decades, the Ordovician System is still awaiting for completion of outlining and denomination of its global geochronologic units. Taking into account this situation we maintain the British Series as currently used (Fortey et al., 2000), and the Australian Stages (Vandenberg and Cooper, 1992), to refer to the conodont-graptolite biozonation of the Ordovician System of Argentina.

Figure 1. Chrono-biostratigraphic chart of the Ordovician System. Reference zones of conodonts and graptolites from different faunal regions are drawn as outlined by Cooper (1999), Fortey et al. (2000), and Chen et al. (2001).

Conodont and graptolite zones of the Precordillera and north-west Argentine basins are depicted as discussed in the text.

Biostratigraphy

Conodonts

Iapetognathus Zone (interval zone). Lower Tremadocian Famatina System, La Rioja.

The base of the Iapetognathus Zone is adopted as the Cambrian-Ordovician boundary (FAD of I. fluctivagus), according to recommendations of the International Subcommission on Ordovician Stratigraphy (ICS, IUGS). This boundary is accurately approached in the Volcancito Formation of the Famatina System, as section for South America (Albanesi et al., 1999a), and could be identified in the Cardonal Formation or equivalent strata of the Cordillera Oriental as previous studies suggest (Rao, 1999; Tortello et al, 1999; Moya and Albanesi, 2000). The Cambrian-Ordovician boundary interval spans the middle part of the La Silla Formation, Argentine Precordillera, and the lower part of the Santa Victoria Group, Cordillera Oriental, where the referred biozone might be recognized (Lehnert, 1995a).

Cordylodus angulatus Zone (interval zone). Lower Tremadocian. Famatina System, La Rioja; Cordillera Oriental, Salta and Jujuy.

Following Rao and Hünicken (1995), Rao (1999), and Tortello and Rao (2000), the C. angulatus Zone extends through the lower part of the Cardonal Formation at Cajas locality, and other classical sections of the Cordillera Oriental of Jujuy. However, in recent contributions by Rao and Tortello (1998) and Tortello et al. (1999), the base of the biozone is identified in the upper strata of that formation. The C. angulatus Zone was inferred or recognized in diverse units of NW Argentina (Suárez Riglos et al., 1982; Albanesi et al., 1999b; Moya and Albanesi, 2000; Ortega and Albanesi, 2002). The appearance of the characteristic Midcontinent species Rossodus manitouensis in northwestern Argentine basins (Alfarcito Formation), and the Famatina System (upper member of the Volcancito Formation), reveals the upper part of the C. angulatus Zone (Albanesi et al., 2001, Zeballo, 2002). In the Argentine Precordillera, this biozone should extend through the middleupper part of the La Silla Formation (Lehnert, 1995a).

Paltodus deltifer Zone (interval zone). Early Upper Tremadocian. Precordillera, San Juan and Mendoza; San Rafael Block, Mendoza; Cordillera Oriental, Salta and Jujuy.

A biostratigraphic interval assigned to the P. deltifer Zone was documented for the first time by Keller et al. (1994) in La Silla Formation at the homonymous section, Precordillera of San Juan. This zone can be divided into two subunits considering the presence of the genus Cordylodus in the lower part, and the two subspecies P. d. pristinus and P. d. deltifer, which nominate both intervals as recognized in north-west Argentina (Albanesi et al., 2001). A series of reports mention the key species P. deltifer, or particular assemblages, which are interpreted to be restricted in this interval, either in the Precordillera (Lehnert, 1995a,b; Heredia, 1995; Albanesi et al., 1998) or north-west Argentina (e.g., Rao and Flores, 1998; Ortega and Albanesi, 2002). Lowest carbonate strata of the San Rafael Block were correlated with this biozone by Lehnert et al. (1998).

Paroistodus proteus Zone (interval zone). Late Upper Tremadocian – early Lower Arenig.

Precordillera, San Juan and La Rioja; Cordillera Oriental, Salta and Jujuy.

P. proteus - Stiptognathus borealis and Oelandodus elongatus - Acodus deltatus subzones (assemblage subzones).

The absence of graptolites in calcareous facies of the San Juan Formation led Albanesi et al. (1998) to propose the base of the O. elongatus - A. deltatus Subzone as the boundary between the Tremadoc and Arenig in the Argentine Precordillera (cf. Lehnert 1995b). The presence of P. proteus, and a correlation with respective biostratigraphic interval, was documented for several sections of the Precordillera (e.g., Keller et al., 1994; Lehnert, 1995a,b), the San Rafael Block (Lehnert et al., 1998), and diverse sections of the Cordillera Oriental of Salta and Jujuy (Albanesi et al., 2001; Ortega and Albanesi, 2002).

Prioniodus elegans Zone (interval zone). Lower Arenig. Precordillera, San Juan and La Rioja; San Rafael Block, Mendoza; Famatina System, La Rioja.

P. elegans - Tropodus sweeti and P. elegans - Oepikodus communis subzones (assemblage subzones).

The first record of P. elegans from the lower part of the San Juan Formation was published by Hünicken and Sarmiento (1980) for the Guandacol River Section, Precordillera of La Rioja. After this reference, the species was reported by several authors in diverse localities of the Precordillera (e.g., Lehnert, 1993, 1995a, Albanesi et al., 1998). The presence of O. communis is frequently registered in association with P. elegans, and its first appearance verifies a subdivision of the biozone. An interval correlative with the P. elegans Zone was also recognized in the Suri Formation, by the association of typical species from that interval (Lehnert et al., 1997). This biozone has also been recognized in the San Rafael Block (Lehnert et al., 1998).

Oepikodus evae Zone (interval zone). Middle Arenig. Precordillera, Mendoza, San Juan, and La Rioja; San Rafael Block, Mendoza; Famatina System, La Rioja.

O. evae - Juanognathus variabilis and O. evae - Scolopodus oldstockensis subzones (assemblage subzones).

The Fauna B of Serpagli (1974) is recognized by the stratigraphic range of O. evae in the San Juan Formation, Precordillera of San Juan. The lower subzone of the O. evae Zone as defined by Albanesi et al. (1998) for the Yanso Section, Central Precordillera, is identified by the appearance of Juanognathus variabilis associated with the eponymous species. In the upper subzone, the species Scolopodus oldstockensis, Oepikodus intermedius and Paroistodus originalis appear for the first time. The O. evae Assemblage Zone established by Lehnert (1993, 1995a) at the Niquivil Section can be correlated with the O. evae - Juanognathus variabilis Subzone of Albanesi et al. (1998). The O. evae - Scolopodus oldstockensis Subzone corresponds to the O. evae/O. intermedius Assemblage Zone of Lehnert. The interval characterized by O. evae was recognized in numerous sections of the precordilleran region (e.g., Lemos, 1981; Bultynck and Martin, 1982; Hünicken and Sarmiento, 1985; Rao, 1988; Sarmiento, 1990). Strata of the Acoite Formation, cropping out at the Purmamarca area, and the Sepulturas Formation at the Espinazo del Diablo Range, Cordillera Oriental of Jujuy, yielded a low diversity conodont fauna typified by Baltoniodus crassulus andinus (= Gothodus crassulus andinus) and Drepanoistodus pitjanti (= D. costatus) that can be correlated with the O. evae Zone (Rao et al., 1994, Rao, 1999).

Likewise, a similar association, including O. evae elements, was reported for the Suri Formation of the Famatina System, confirming that assignment (Lehnert et al., 1997, Albanesi and Astini, 2000a).

Oepikodus intermedius Zone (interval zone). Middle Arenig. Precordillera, San Juan.

O. intermedius is diagnostic for the homonymous zone when it does not appear associated to O. evae or T. laevis sensu lato, following the original definition for the Yanso Section, Central Precordillera (Albanesi et al., 1998). The Fauna C of Serpagli (1974) was determined in accordance with the range of O. intermedius. This Fauna was homologated by Lehnert (1993, 1995a) with Assemblage Zone IV as defined in the Niquivil Section. His Assemblage Zone V, which has also been identified in the carbonate sequence of the San Rafael Block (Lehnert et al., 1998) can be correlated with part of the O. intermedius Zone. The FAD of Protoprionodus aranda (as well as Texania heligma, another short range and widely distributed species), wich is precisely registered in this zone at Niquivil Section, was proposed as reference biohorizon for the base of the global Middle Ordovician Series (Albanesi and Carrera, 2001).

Tripodus laevis Zone (interval zone). Middle Arenig. Precordillera, San Juan and La Rioja.

This zone is typified by T. laevis sensu lato (apparently, a form related to T. laevis sensu stricto from Northamerica), as defined by Albanesi et al. (1998). It apparently correlates with Fauna D of Serpagli (1974) in the Pachaco Section, San Juan River, and with Assemblage Zone VI of Lehnert (1993, 1995a) in the Niquivil Section. However, despite the presence of several common species in these sections; e.g., Juanognathus jaanussoni, Oistodus lanceolatus, Paroistodus originalis, Protopanderodus nogamii, Semiacontiodus potrerillensis, other significant forms, such as Triangulodus brevibasis (Sergeeva), documented for both sections, suggest a correlation of the upper part of these intervals with the overlying Baltoniodus navis Zone. The T. laevis Zone has also been identified in diverse sections between the Gualcamayo and Guandacol rivers, Precordillera of San Juan and La Rioja (Albanesi et al., 1999b).

Baltoniodus navis Zone (interval zone). Middle Arenig. Precordillera, San Juan; San Rafael Block, Mendoza; Famatina System, Catamarca.

The B. navis Zone was first recognized in the Precordillera of San Juan by Lemos (1981), in the Buenaventura Luna Section. Albanesi et al. (1998) established this biozone by the FAD of B. navis, and the presence of key taxa, such as Triangulodus brevibasis and Histiodella altifrons, permits a correlation of the biostratigraphic interval through different lithofacies of the basin (Serpagli, 1974, Lehnert, 1993, 1995a, Albanesi et al., 1999b) including peripheral environments of the San Rafael Block (Lehnert et al., 1998). The B. navis Zone was identified in the upper part of the Suri Formation, Narváez Range, Famatina (Albanesi and Vaccari, 1994; cf., Lehnert et al., 1997).

Microzarkodina parva Zone (interval zone). Upper Arenig. Precordillera, San Juan; San Rafael Block, Mendoza.

This zone has been defined in the Yanso Section (Albanesi et al., 1998) and has been recognized in the stratotype section of the Gualcamayo Formation, Precordillera of San Juan (Albanesi et al., 1999b). The Fauna E defined by Serpagli (1974) in the Pachaco Section probably correlates with the M. parva Zone because the last occurrence of the key species T. brevibasis occurs in its Fauna D. Instead, following Lehnert (1993) this species appears in the uppermost part of the Niquivil Section, verifying that the bearer levels precede this biozone. H. altifrons, whose range partly corresponds to that of the M. parva Zone, has been documented for the La Silla and Puesto Los Potrerillos sections, Precordillera of San Juan (Lehnert, 1995a). Other associated species allow to correlation part of the Ponón Trehué Formation, San Rafael Block (Lehnert et al., 1998).

Lenodus variabilis Zone (assemblage zone). Upper Arenig – Lower Llanvirn. Precordillera, San Juan.

Periodon gladysi Subzone (interval subzone), Upper Arenig, and Paroistodus horridus Subzone (interval subzone), Lower Llanvirn.

The L. variabilis Zone was first recognized in the Argentine Precordillera by Sarmiento (1985, 1991), whose records correspond to the outcrops of the San Juan and Gualcamayo formations in the Villicum Range. The three assemblage biozones of the lower member of the Gualcamayo Formation at Potrerillo Mountain Section, as proposed by Ortega et al. (1995), were revised by Albanesi et al. (1998) and formally redefined with two subzones. The uppermost levels of the San Juan Formation were examined for conodonts in diverse localities of the Central Precordillera (e.g., Los Azules, Las Aguaditas), where the L. variabilis Zone was determined by means of key taxa (Albanesi et al., 1998). Hünicken and Ortega (1987) refer the uppermost part of this unit in Viejo Hill at Huaco to the E. suecicus Zone; however, a careful assessment by means of new collections indicates a close correspondence with the L. variabilis Zone (Ottone et al., 1999). Sarmiento et al. (1988) recovered a typical conodont fauna of the L. variabilis Zone from the lower strata of the Rinconada Formation. At Las Chacritas, Central Precordillera, the P. horridus Subzone has been documented by Albanesi and Astini (1994, 2000b). Albanesi et al. (1995a) identified the L. variabilis Zone in the basal strata of the Yerba Loca Formation, which overlies the Los Sombreros Formation at Puerta de Ancaucha, Alto de Mayo Mountain, Precordillera of San Juan.

Eoplacognathus suecicus Zone (assemblage zone). Lower Llanvirn. Precordillera, San Juan.

Histiodella kristinae and Pygodus anitae subzones (assemblage subzones).

The presence of the E. suecicus Zone was initially recognized in the Argentine Precordillera by Hünicken and Ortega (1987), in the Los Azules Formation at Viejo Hill, Huaco. Specimens from the lower levels identified as E. suecicus are considered synonyms of Lenodus variabilis (Ottone et al., 1999). Nevertheless, some representative taxa of the E. suecicus biozone (e.g., Dzikodus, Polonodus, Pygodus) indicate its presence in the section; in particular, the upper interval of the P. anitae Zone. The E. suecicus Zone has been identified in diverse localities of the Precordillera; e.g., the lower boundary of this unit was defined in the Don Braulio section, Villicum Range (Sarmiento, 1991). According to published data from a number of sections (Hünicken and Ortega, 1987; Sarmiento et al., 1988; Lehnert, 1995a; Ortega et al., 1995; Albanesi et al., 1995a, Albanesi et al., 1998, Albanesi and Astini, 2000b) the co-existence of E. suecicus with Histiodella kristinae (cf., H. kristinae Zone sensu Lehnert, 1995a) in lower levels, and E. suecicus with Pygodus anitae in upper levels suggest the E. suecicus Zone be divided into lower and upper subzones by the FAD of key species. Stratotype sections for the definition of these subunits can be located at Las Chacritas and Las Aguaditas localities, Central Precordillera of San Juan (Albanesi et al. 1998). Apparently, part of the E. suecicus Zone is present in the Chica de Zonda Range and La Chilca Hill; likewise, strata close to the top of the San Juan Formation at Talacasto bear a conodont fauna that could be referred to this zone (Lehnert, 1995b).

The E. suecicus Zone is also represented in the Sierra de La Invernada and Las Aguaditas formations (Brussa, 1997; Ortega and Albanesi, 1998). The Puerta de Ancaucha and El Divisadero sections correspond to the easternmost outcrops of the Yerba Loca Formation, where representative conodonts of the E. suecicus Zone were found (Albanesi et al., 1995a).

Pygodus serra Zone (interval zone). Late Lower Llanvirn. Precordillera, San Juan; San Rafael Block, Mendoza.

Eoplacognathus foliaceus, E. reclinatus, E. robustus, and E. lindstroemi subzones (interval subzones).

After a first report by Hünicken and Ortega (1987), the P. serra Zone was registered with detail from the classic section of the Los Azules Formation at Viejo Hill, Central Precordillera of San Juan (Ottone et al., 1999). Eoplacognathus foliaceus and E. robustus subzones were identified in the Yerba Loca Formation, Central Precordillera, by Albanesi et al. (1995a). E. robustus was also identified by Heredia (1998) in the Ponón Trehué Formation (= Lindero Formation sensu Bordonaro et al. 1996), San Rafael Block. This unit has recently been thoroughly studied by Lehnert et al. (1999) and Heredia (2001), who documented the index species of the four subzones of the Pygodus serra Zone. The uppermost of these subunits, represented by E. lindstroemi, was firstly registered in the La Cantera Formation, Precordillera of San Juan, by Albanesi et al. (1995c).

Pygodus anserinus Zone (interval zone). Upper Llanvirn – Lower Caradoc. Precordillera, San Juan; San Rafael Block, Mendoza. First report of Pygodus anserinus from the San Rafael Block was published by Heredia (1982); however, the boundaries of the homonymous biozone were not determined until recently.

Incidentally, the lower limit of this biozone was determined in the carbonate sequence of the San Rafael Block (Lehnert et al., 1999; Heredia, 2001), while the upper boundary can be referred to the Las Aguaditas Formation, Precordillera of San Juan, by the FAD of Amorphognathus tvaerensis (Lehnert, 1995a, Albanesi and Ortega, 1998). This biozone could be divided into two subzones by means of the FAD of characteristic taxa; e.g., Baltoniodus variabilis and Cahabagnathus sweeti, as it has been shown by Lehnert et al. (1999).

No nominated interval. The Santa Gertrudis Formation exposed in the Mojotoro Range, Cordillera Oriental of Salta, bears a conodont assemblage that could be attributed either to the lower part of the Amorphognathus tvaerensis Zone (Albanesi and Rao, 1996) or underlying units, but the lack of key species preclude a reliable assignment.

Amorphognathus tvaerensis Zone (interval zone). Lower Caradoc. Precordillera, Mendoza and San Juan.

Segments of the Baltoniodus variabilis and B. gerdae subzones of the A. tvaerensis Zone are represented in the upper member of the Los Azules Formation (Ottone et al., 1999), as well as in the Las Plantas Formation at Potrerillo Mountain (Albanesi et al., 1998; Ortega and Albanesi, 1998), and the Las Aguaditas Formation of the Central Precordillera of San Juan (Lehnert, 1995a; Albanesi and Ortega, 1998). Specimens of A. tvaerensis are preserved as casts on bedding plane surfaces of black shales of the lower member of the Empozada Formation (Albanesi and Ortega, 1998). The conodont fauna of the Sassito Formation was preliminary assigned to the A. tvaerensis Zone (Keller and Lehnert, 1998); however, the absence of index species does not allow for precise recognition, and a subsequent biostratigraphic interval could be represented.

PLATE 1

Selected conodont species from the Ordovician System of Argentina (figures: 4, 6, 7, 8, 10-15, 17-20, 22, 23, 25-28, SEM photomicrographs; figures: 1-3, 5, 9, 16, 21, 24, optical pictures).

Figure 1 – Amorphognathus tvaerensis Bergström, Pa element, upper view, x 35, CORD-MP 10010. Amorphognathus tvaerensis Zone, Los Azules Formation (upper member), Arbol Seco Section, Viejo Hill, Precordillera, San Juan.

Figure 2 – Pygodus anserinus (Lamont and Lindström), Pa element, upper view, x 35, CORD-MP 10011. Pygodus anserinus Zone, Las Aguaditas Formation (upper member), Las Aguaditas Section, Precordillera, San Juan.

Figure 3 – Erismodus quadridactylus (Stauffer), Pb element, posterior view, x 35, CORM-MP 10012. No nominated interval, Santa Gertrudis Formation, Gallinato Creek Section, Cordillera Oriental, Salta.

Figure 4 – Histiodella tableheadensis Stouge, Pa element, lateral view, x 200, CORD-MP 10013. Lenodus variabilis Zone, uppermost San Juan Formation, Las Aguaditas Section, Precordillera, San Juan.

Figure 5 – Phragmodus undatus Branson and Mehl, Sc element, lateral view, x 35, CORD-MP 10014. Amorphognathus tvaerensis Zone. Los Azules Formation (upper member), Arbol Seco Section, Viejo Hill, Precordillera, San Juan.

Figure 6 – Paroistodus horridus (Barnes and Poplawski), Sc element, lateral view, x 180, CORD-MP 10015. Lenodus variabilis Zone, uppermost San Juan Fomation, Los Azules Section, Viejo Hill Precordillera, San Juan.

Figure 7 – Periodon aculeatus Hadding, Pa element, lateral view, x 150, CORD-MP 10016. Lenodus variabilis Zone, uppermost San Juan Formation, Los Azules Section, Viejo Hill, Precordillera, San Juan.

Figure 8 – Baltoniodus norrlandicus Löfgren, M element, lateral view, x 100, CORD-MP 10017. Lenodus variabilis Zone, uppermost San Juan Formation, Las Aguaditas Section, Precordillera, San Juan.

Figure 9 – Polonodus magnum Albanesi, Pb element, upper view, x 35, CORD-MP 10018. Eoplacognathus suecicus Zone, Las Chacritas Formation, Las Chacritas Section, Precordillera, San Juan.

Figure 10 – Fahraeusodus marathonensis (Bradshaw), Sc element, lateral view, x 180, CORD-MP 10019. Lenodus variabilis Zone, uppermost San Juan Formation, Las Aguaditas Section, Precordillera, San Juan.

Figure 11 – Lenodus variabilis (Sergeeva), Pa element, upper view, x 120, CORD-MP 10020. Lenodus variabilis Zone, uppermost San Juan Formation, Las Aguaditas Section, Precordillera, San Juan.

Figure 12 – Microzarkodina parva Lindström, Pa element, lateral view, x 200, CORD-MP 10021. Microzarkodina parva Zone, uppermost San Juan Formation, Puesto Los Alamos Section, Precordillera, San Juan.

Figure 13 – Protoprioniodus aranda Cooper, Sc element, lateral view, x 100, CORD-MP 10022. Tripodus laevis Zone, lowermost Gualcamayo Formation, Los Sapitos Section, Precordillera, La Rioja.

Figure 14 – Baltoniodus navis (Lindström), Pa element, upper view, x 180, CORD-MP 10023. Baltoniodus navis Zone, lower Gualcamayo Formation, Puesto Los Alamos Section, Precordillera, San Juan..

Figure 15 – Protopanderodus gradatus Serpagli, Sb element, lateral view, x 80, CORD-MP 10024. Lenodus variabilis Zone, uppermost San Juan Formation, Los Azules Section, Precordillera, San Juan.

Figure 16 – Bergstroemognathus extensus Serpagli, Sa element, posterior view, x 35, CORD-MP 10025. Oepikodus evae Zone, San Juan Formation, Niquivil Section, Precordillera, San Juan.

Figure 17 – Erraticodon balticus Dzik, Pa element, antero-lateral view, x 150, CORD-MP 10026. Lenodus variabilis Zone, uppermost San Juan Formation, Los Azules Section, Viejo Hill, Precordillera, San Juan.

Figure 18 – Drepanodus reclinatus (Hadding), Pb element, lateral view, x 90, CORD-MP 10027. Lenodus variabilis Zone, uppermost San Juan Formation, Los Azules Section, Viejo Hill, Precordillera, San Juan.

Figure 19 – Eoconodontus notchpeakensis (Miller), Sc element, lateral view, x 100, CORD-MP 10028. Cordylodus angulatus Zone, lower Volcancito Formation, Volcancito River Section, Famatina, La Rioja.

Figure 20 – Drepanoistodus forceps (Lindström), M element, lateral view, x 150, CORD-MP 10029. Tripodus laevis Zone, lower Gualcamayo Formation, Los Sapitos Section, Precordillera, La Rioja.

Figure 21 – Iapetognathus aengensis (Lindström), Sb element, posterior view, x 120, CORD-MP 10030. Iapetognathus Zone, lower Volcancito Formation, Volcancito River Section, Famatina, La Rioja.

Figure 22 – Cordylodus angulatus Pander, Pa element, lateral view, x 100, CORD-MP 10031. Cordylodus angulatus Zone, lower Volcancito Formation, Volcancito River Section, Famatina, La Rioja.

Figure 23 - Utahconus utahensis (Miller), Sb element, posterior view, x 100, CORD-MP 10032. Cordylodus angulatus Zone, Alfarcito Formation, Casa Colorada River Section, Alfarcito Area, Cordillera Oriental, Jujuy.

Figure 24 – Acodus deltatus Lindström, Pa element, lateral view, x 35, CORD-MP 10033. Paroistodus proteus Zone, Parcha Formation, Abra de Sococha Section, Eastern Cordillera, Salta.

Figure 25 – Rossodus manitouensis Repetski and Ethington, Sb element, posterior-lateral view, x 250, CORD-MP 10034. Paltodus deltifer Zone, Volcancito Formation, Bordo Atravesado Section, Famatina, La Rioja.

Figure 26 – Paltodus deltifer pristinus (Viira), M element, lateral view, x 120, CORD-MP 10035. Paltodus deltifer Zone, Rupasca Formation, Casa Colorada River Section, Alfarcito Area, Cordillera Oriental, Jujuy.

Figure 27 – Teridontus nakamurai (Nogami), Sc element, lateral view, x 150, CORD-MP 10036. Cordylodus angulatus Zone, lower Volcancito Formation, Volcancito River Section, Famatina, La Rioja.

Figure 28 – Monocostodus sevierensis (Miller), Sc element, lateral view, x 120, CORD-MP 10037. Cordylodus angulatus Zone, Cardonal Formation, Amarilla Creek Section, Cajas Range, Cordillera Oriental, Jujuy.

Amorphognathus superbus Zone (interval zone). Upper Caradoc – Lower Ashgill. Precordillera, San Juan and Mendoza.

Representative elements of the A. superbus Zone were recovered from reworked clasts of the Empozada Formation (Heredia et al., 1990), Precordillera of Mendoza, as well as casts on bedding plane surfaces of the Trapiche Formation (Albanesi et al., 1995b) in the Precordillera of San Juan. As discussed above, the Sassito Formation at San Juan River Section might be a potential stratotype for this biozone (Keller and Lehnert, 1998).

Graptolites

Rhabdinopora flabelliformis interval. Lower Tremadocian (Lancefieldian, La1a). Famatina System, La Rioja; Cordillera Oriental, Jujuy and Salta.

Numerous records of R. flabelliformis from north-western Argentina were published since Bodenbender´s (1916) first report from the Peña Negra area, Famatina System, La Rioja. This form was registered in several localities of the Eastern Cordillera, sometimes referred to as subspecies (e.g., Santa Victoria Range, Angosto del Moreno, Aguilar Range, Cajas Range, Mojotoro Range, Humahuaca Creek, Yala). Some of these findings should be revised under the new concepts regarding Rhabdinopora to determine the presence of different forms of significant biostratigraphic value (Cooper et al, 1998). R. f. parabola and anisograptids referable to Anisograptus matanensis group were found in the Saladillo Formation, Angosto del Moreno, western border of Cordillera Oriental (Moya et al., 1994). Rhabdosomes studied by Bodenbender (1916) and Turner (1959) in the Volcancito River, Famatina System, were attributed to R. f. anglica indicating a late Lower Tremadocian for the bearer rocks (Aceñolaza and Durand, 1984).

Bryograptus Zone (assemblage zone). Early Upper Tremadocian (Lancefieldian, La1b). Cordillera Oriental, Jujuy and Salta. Specimens of the Bryograptus genus from the Mojotoro Range and Angosto de Lampazar, Cordillera Oriental of Salta (González Barry and Alonso, 1984; Moya et al., 1994; Ortega et al., 1998) were referred to the Bryograptus Zone suggesting an early Upper Tremadocian age (Ortega and Albanesi, 2002). Biradiate rhabdosomes assigned to Adelograptus tenellus from the Cardonal Formation, Cajas Range (Ortega and Rao, 1995) would correspond to this interval.

Kiaerograptus Zone (interval zone). Early Upper Tremadocian (Lancefieldian, La 2). Cordillera Oriental, Salta; Puna, Jujuy. Specimens of the Kiaerograptus genus comparable with K. kiaeri, paradelograptids and clonograptids were cited for the Chiquero Formation at Las Burras creek, Puna of Jujuy (Benedetto et al., 2002), and the Saladillo Formation at Angosto de Lampazar, Cordillera Oriental of Salta (Ortega and Albanesi, 2002). Strata between the Kiaerograptus and A. murrayi zones could be attributed to the Kiaerograptus supremus Zone of Scandinavia and Bolivia, according to Ortega and Albanesi (2002).

Araneograptus murrayi Zone (interval zone). Late Upper Tremadocian (Lancefieldian, La2).

Cordillera Oriental, Salta; Puna, Catamarca.

This unit can be recognized in several localities of the Cordillera Oriental (Gutiérrez Marco and Aceñolaza, 1987) by the appearance of the index fossil. A. murrayi (J. Hall) (= Dictyonema yaconense Turner) was considered to represent early Arenig strata in Argentina (e.g., Turner, 1959) but the record of Hunnegraptus copiosus immediately above these levels, in the Abra de Sococha Section, indicates a late Tremadocian age for this biozone (Ortega and Albanesi, 2002). The record of A. murrayi in the volcaniclastic Tolillar Formation shows the presence of this biozone in southern Puna, Catamarca (Zimmermann et al., 1999).

Hunnegraptus copiosus Zone (interval zone). Late Upper Tremadocian. Cordillera Oriental, Salta; Puna, Jujuy.

This unit is recognized by the appearance of H. copiosus associated with paradelograptids, didymograptids and tetragraptids in the Chiquero Formation, Las Burras Creek, Puna of Jujuy (Benedetto et al., 2002), and the basal part of the Parcha Formation, Abra de Sococha, Cordillera Oriental of Salta (Ortega and Albanesi, 2002).

Aorograptus victoriae Zone (assemblage zone). Upper Tremadocian. Cordillera Oriental, Salta.

A graptolite assemblage including A. victoriae, Paradelograptus antiquus, and P. onubensis was localized in the lower part of the San Bernardo Formation, Mojotoro Range (Monteros and Moya, 2002).

This late Tremadocian assemblage would be equivalent to the Kiaerograptus, A. murrayi, and H. copiosus zones of the scheme provided by Ortega and Albanesi (2002).

Tetragraptus phyllograptoides Zone (assemblage zone). Lower Arenig (Lancefieldian, La 3). Famatina System, Catamarca; Cordillera Oriental, Jujuy.

The biozone was erected by Toro (1994) for the lower part of the Acoite Formation at Agua Blanca Creek, south of Los Colorados area. Subsequently, it was identified in the Cristóbal River Section, Cajas Range (Ortega et al., 1998), Angosto del Moreno (Moya et al., 1998) in the western flank of the Cordillera Oriental of Jujuy, and in the Santa Victoria Range, eastern side of Cordillera Oriental of Salta (Toro, 1998). The FAD of T. phyllograptoides is not present in the investigated section because the basal part of the Acoite Formation is truncated by fault, or a basal unconformity biases the lowest record of the species. T. phyllograptoides was also recovered from the middle part of the La Alumbrera Formation, Famatina System, Catamarca (Toro, 1997b). In this section the index fossil ranges into the next biozone (Tetragraptus akzharensis Zone).

Tetragraptus approximatus Zone (assemblage zone). Lower Arenig (Lancefieldian, La3).

Precordillera, San Juan.

Graptolites assigned to the T. approximatus Zone were found by Banchig and Moya (2002) in the autochthonous shally strata of the Los Sombreros Formation at El Tontal Range, Precordillera of San Juan. The association is composed by T. a. approximatus, T. cf. acclinans, and T. decipiens, among others. T. approximatus records from Eastern Cordillera were referred to the upper part of the T. phyllograptoides Zone (Toro, 1997a).

Tetragraptus akzharensis Zone (assemblage zone). Lower Arenig (Bendigonian, Be1). Famatina System, La Rioja and Catamarca; Cordillera Oriental, Jujuy.

This biozone was recognized by the T. akzharensis – D. (E.) constrictus assemblage in the lower part of the Acoite Formation, Los Colorados Creek, Cordillera Oriental (Toro, 1997a). It is also present in the Agua Blanca Creek, south of Los Colorados, Cajas River, and Agua Chica and Lumará creeks in Aguilar Range, where it is recognized by the associated fauna (Toro, 1997a). A long list of taxa was cited for this biozone. The forms assigned to the T. approximatus Zone in the Aguilar Range (Martín et al., 1987), and specimens of B. vacillans from Purmamarca (Ortega and Rao, 1994) could be attributed to the T. akzharensis Zone. The graptolite fauna studied by Loss (1951) (= Grapolite Association VII of Moya et al., 1994) in the San Bernardo Hill, probably represents this unit. T. akzharensis (= T. cf. approximatus in Lavandaio, 1973) was identified in the Portezuelo de Las Minitas, western flank of the Famatina System. Aceñolaza and Gutiérrez Marco (2000) referred the graptolite assemblage of this locality to the T. akzharensis Zone. The index fossil was also found in the La Alumbrera Formation, eastern flank of the Famatina System, Catamarca (Toro, 1997b).

Pendeograptus fruticosus Zone (assemblage zone). Lower Arenig (Bendigonian). Precordillera, Mendoza. P. cf. fruticosus was registered in the Los Sombreros Formation, San Juan (Cuerda et al., 1983) suggesting the presence of the Bendigonian P. fruticosus Zone in the Precordillera. Subsequently, a graptolite fauna that consists of T. approximatus and P. fruticosus was collected from basal allochthonous rocks of the Empozada Formation, San Isidro Creek, Precordillera of Mendoza (Bordonaro and Peralta, 1987).

Baltograptus deflexus Zone (interval zone). Lower Arenig (Bendigonian, Be2-Be4). Famatina System, La Rioja; Cordillera Oriental, Jujuy and Salta; Puna, Salta.

This biozone was erected by Toro (1994, 1997a) from the middle part of the Acoite Formation, in the Los Colorados and Chamarra creeks, Cordillera Oriental of Jujuy. The biozone was also

PLATE 2

Selected nominal species of graptolite zones from the Ordovician System of Argentina (all figures about 2.5 x).

Figure 1 – Normalograptus extraordinarius (Sobolevskaya), CEGH-UNC 17764 (after Brussa et al., 1999). Normalograptus extraordinarius Zone, Alcaparrosa Formation, Calingasta, Precordillera, San Juan.

Figure 2 – Normalograptus persculptus (Salter), CEGH-UNC 9512 (after Rickards et al., 1996). Normalograptus persculptus Zone, La Chilca Formation (lower part), El Fuerte Hill, Precordillera, San Juan.

Figure 3 – Climacograptus bicornis (J. Hall), CORD-PZ 13952. Climacograptus bicornis Zone, Los Azules Formation (Upper Member), Viejo Hill, Precordillera, San Juan.

Figure 4 – Dicellograptus ornatus Elles and Wood, CEGH-UNC 16348 (after Mitchell et al., 1998). Dicellograptus ornatus Zone, Empozada Formation (Middle Member), San Isidro Creek, Precordillera, San Juan.

Figure 5 – Dicellograptus complanatus Lapworth, CEGH-UNC 16355a (after Mitchell et al., 1998). Dicellograptus complanatus Zone. Empozada Formation (Middle Member), San Isidro Creek, Precordillera, San Juan.

Figure 6 – Nemagraptus gracilis (J. Hall), CORD-PZ 13893. Climacograptus bicornis Zone, Los Azules Formation (Upper Member), Viejo Hill, Precordillera, San Juan.

Figure 7 – Pterograptus elegans Holm, CORD-PZ 31195a. Pterograptus elegans Zone, Gualcamayo Formation (Middle Member), Corridita Creek, Precordillera, San Juan.

Figure 8 – Holmograptus lentus (Törnquist), CORD-PZ 31079. Holmograptus lentus Zone, Gualcamayo Formation (Middle Member), Corridita Creek, Precordillera, San Juan.

Figure 9 – Undulograptus dentatus (Hall), CEGH-UNC 7824. Undulograptus dentatus Zone Gualcamayo Formation (Middle Member), Potrerillos Creek, Precordillera, San Juan.

Figure 10 – Cardiograptus morsus Harris and Keble, CORD-PZ 21462. Cardiograptus morsus Zone, Gualcamayo Formation (Lower Member), Los Sapitos Creek, Precordillera, La Rioja.

Figure 11 – Oncograptus upsilon (T.S. Hall), CORD-PZ 13786. Oncograptus upsilon Zone, Gualcamayo Formation (Lower Member), Los Sapitos Creek, Precordillera, La Rioja.

Figure 12 – Isograptus victoriae maximus Harris, CEGH-UNC 8656b. Isograptus victoriae maximus Zone, Gualcamayo Formation (Lower Member), Los Sapitos Creek, Precordillera, La Rioja.

Figure 13 – Hustedograptus teretiusculus (Hisinger), CORD-PZ 13596. Hustedograptus teretiusculus Zone, Los Azules Formation (Middle Member), Viejo Hill, Precordillera, San Juan.

Figure 14 – Undulograptus austrodentatus (Harris and Keble), CEGH-UNC 7850. Undulograptus austrodentatus Zone, Gualcamayo Formation (Middle Member), Potrerillos Creek, Precordillera, San Juan.

Figure 15 – Baltograptus deflexus (Elles and Wood), CEGH-UNC 14117 (after Toro and Brussa, 1997). Baltograptus deflexus Zone, Suri Formation, (lower part), Saladillo River, Famatina System, La Rioja.

Figure 16 – Hunnegraptus copiosus Lindholm, CORD-PZ 19090. Hunnegraptus copiosus Zone, Parcha Formation (lower part), Abra de Sococha, Cordillera Oriental, Salta.

Figure 17 – Tetragraptus akzharensis Tzaj, CEGH-UNC 9357 (after Toro, 1997). Tetragraptus akzharensis Zone, Acoite Formation (lower part), Los Colorados Creek, Cordillera Oriental, Jujuy.

Figure 18 – Didymograptellus bifidus (J. Hall), CEGH-UNC 7535 (after Toro, 1994). Didymograptellus bifidus Zone, Acoite Formation, Los Colorados Creek, Cordillera Oriental, Jujuy.

Figure 19 – Kiaerograptus sp., CORD-PZ 19849. Kiaerograptus Zone, Saladillo Formation, Angosto de Lampazar, Cordillera Oriental, Salta.

Figure 20 – Rhabdinopora flabelliformis flabelliformis (Eichwald), CORD-PZ 30500. Rhabdinopora flabelliformis interval, Alfarcito Formation (upper part), Alfarcito area, Eastern Cordillera, Jujuy.

Figure 21 – Bryograptus sp., CORD-PZ 18939. Bryograptus Zone, Saladillo Formation, Angosto de Lampazar, Cordillera Oriental, Salta.

Figure 22 – Tetragraptus phyllograptoides Strandmark, CORD-PZ 21058. Tetragraptus phyllograptoides Zone, Acoite Formation (lower part), Cristóbal River, Cajas Range, Cordillera Oriental, Jujuy.

Figure 23 – Araneograptus murrayi (J. Hall), PIL 11290 (after Aceñolaza et al. 1996). Araneograptus murrayi Zone, Parcha Formation, Gólgota Hill, Cordillera Oriental, Salta.

found in the Cajas and Aguilar ranges, Jujuy, and in the Santa Victoria Range, Salta (Toro, 1998, 1999a,b), which is recognized by the entrance of the index fossil. A graptolite fauna containing T. a. approximatus, P. fruticosus, and B. deflexus, recovered in the upper part of the T. akzharensis Zone, at Lumará Creek (Toro,1997a) should be considered the basal part of the B. deflexus Zone. Graptolites described for the Parcha Formation, eastern Puna, Salta (Ramos, 1974) and Escaya Range, Puna of Jujuy (Bahlburg et al., 1990) apparently correspond to this biozone. The B. deflexus Zone was also recorded in the lower part of the Suri Formation at Saladillo River, Famatina System, La Rioja (Toro and Brussa, 1997).

Didymograptellus bifidus Zone (interval zone). Middle Arenig (Chewtonian, Ch1-Ch2).

Famatina System, La Rioja; Eastern Cordillera, Jujuy.

The association of D. bifidus, Baltograptus minutus, and Phyllograptus anna allows to recognize the D. bifidus Zone in the upper part of the Acoite Formation, at Chamarra Creek, Cordillera Oriental of Jujuy (Toro, 1997a). Likewise, the biozone is recorded in the Los Colorados Creek (Toro, 1997a), and Santa Victoria Range (Toro, 1998).

This biozone was also identified in the lower part of the Suri Formation, at Saladillo River, Famatina System, La Rioja, located immediately above the B. deflexus Zone (Toro and Brussa, 1997), and in the volcano-sedimentary Muñayoc succession, Quichagua Range, north-eastern Puna (Martínez et al., 1999).

No nominated interval. Middle Arenig (Castlemainian, Ca1-Ca2). Puna, Jujuy.

A Castlemainian assemblage with Azigograptus eovionicus and Dichograptus octobrachiatus was found in the Muñayoc section, Quichagua Range, north-eastern Puna (Martínez et al., 1999).

Isograptus victoriae maximus Zone (interval zone). Middle Arenig (Castlemainian, Ca3- Ca4). Precordillera, La Rioja.

A rich Pacific graptolite fauna, characterized by the entrance of I. v. maximus was assigned to this biozone by Ortega et al. (1993). It is recognized in the basal part of the Gualcamayo Formation at Potrerillos and Los Sapitos creeks, southwest Guandacol, in northern Precordillera of La Rioja (Ortega and Albanesi, 1999). The FAD of I. v. maximus can be localized in the Peña Sombría section, Guandacol River, where this fossil and I. v. victoriae were found together (Ortega et al., 1985).

Oncograptus upsilon Zone (interval zone). Upper Arenig (Yapeenian, Ya1). Precordillera, La Rioja.

This biozone is defined by the appearance of the index fossil in the lower part of the Gualcamayo Formation at Los Sapitos Creek, Precordillera, La Rioja (Ortega and Albanesi, 1999). The Oncograptus Zone, preliminary documented for the Potrerillos Creek (Ortega et al., 1993) comprises the O. upsilon and Cardiograptus morsus zones.

Cardiograptus morsus Zone (interval zone). Upper Arenig (Yapeenian, Ya2). Precordillera, La Rioja.

The base of this biozone is marked by the FAD of the nominal taxon. The biozone was erected for the lower part of the Gualcamayo Formation at Los Sapitos Creek, Precordillera, La Rioja (Ortega and Albanesi, 1999). Exigraptus clavus, Glossograptus acanthus, the first biserial graptolites (Undulograptus sinodentatus and Undulograptus sp.) and Paraglossograptus sp. appear within the biozone. The fauna from the Nazareno Creek (Brussa et al., 1998), and the assemblage with C. morsus recorded in the Corridita Creek, Gualcamayo River, Precordillera of San Juan (Brussa and Astini, 2001), also pertain to this biozone.

Undulograptus austrodentatus Zone (interval zone). Late Upper Arenig (Darriwilian, Da1).

Precordillera, La Rioja and San Juan.

The first appearance of U. austrodentatus allows the recognition of this biozone in the middle member of the Gualcamayo Formation at Los Sapitos Creek, Precordillera of La Rioja (Ortega and Albanesi, 1999). Arienigraptus zhejiangensis and Undulograptus sinicus subzones are present in the Los Sapitos and Potrerillos creeks, and in the Potrerillo Mountain. The major part of U. austrodentatus faunal records in the Precordillera, usually assigned to the P. tentaculatus Zone, pertain to the upper association (U. sinicus Subzone) (Brussa, 1997; Mitchell et al., 1998; Ortega and Albanesi, 1999).

Graptolites of the A. zhejiangensis Subzone were mentioned in the Nazareno and Gualcamayo creeks (Brussa and Astini, 1998; Brussa et al., 1998).

Undulograptus dentatus Zone (interval zone). Early Lower Llanvirn (Darriwilian, Da2).

Precordillera, San Juan; Puna, Jujuy.

The biozone was identified in the lower member of the Los Azules Formation at Viejo Hill of Huaco, Precordillera of San Juan (Mitchell et al., 1998) containing C. antennarius, A. angulatus, U. austrodentatus, U. dentatus, U. cumbrensis, U. primus, and P. tentaculatus, among others. The first appearance of the index fossil can be recognised in the Potrerillos Creek, La Rioja, where it is also associated with A. cf. reliquus and T. acanthonotus (MásperoCastro, 2002).

Graptolite remains from strata of the northern Puna, Esquina Colorada and Olaroz Grande sections, apparently correspond to the U. dentatus Zone (Bahlburg et al., 1990).

Holmograptus lentus Zone (interval zone). Late Lower Llanvirn (Darriwilian, Da2-Da3).

Precordillera, San Juan.

This biozone was recently localized in the Corridita Creek, Precordillera of San Juan (MásperoCastro, 2002), which is identified by the appearance of H. lentus. A low diversity graptolite association containing forms of the underlying U. dentatus Zone is recorded. Holomograptus spinosus was documented for the upper part of the P. tentaculatus Zone in the Sierra de la Invernada Formation, western Precordillera (Brussa, 1999). It indicates a Darriwilian (Da3) age and possibly equates the upper part of the H. lentus Zone of our biostratigraphic scheme. Apparently, correlative levels to this zone are present in uppermost layers of the lower member of the Los Azules Formation at Viejo Hill of Huaco (Ortega, unpubl. coll.).

Pterograptus elegans Zone (interval zone). Early Upper Llanvirn (Darriwilian, Da4a).

Precordillera, San Juan.

The P. elegans Zone was recognized for the basal part of the middle member of the Los Azules Formation, at Viejo Hill section, Precordillera of San Juan (Ortega, 1995). It is also present in the Gualcamayo Formation at Potrerillo Mountain (Ortega and Albanesi, 2000) and in the Corridita Creek (Ortega and Máspero-Castro, 2002). This biozone is characterized by the appearance of the index fossil, and other typical forms such as, Kalpinograptus parallelus, Wuninograptus spp., Reteograptus geinitzianus, Cryptograptus schaeferi, and abundant biserial graptolites. In the Viejo Hill and Potrerillo Mountain sections, graptolites are associated with conodonts of the Eoplacognathus suecicus Zone (Pygodus anitae Subzone).

Hustedograptus teretiusculus Zone (assemblage zone). Late Upper Llanvirn (Darriwilian, Da4b). Precordillera, San Juan.

A graptolite assemblage dominated by biserial graptolites range throughout the upper part of the Los Azules Formation middle member, at Viejo Hill, Precordillera of San Juan (Ortega, 1995).

It is located immediately above the last appearance of P. elegans, and was referred to as H. teretiusculus Zone. The entrance of Dicellograptus cf. vagus and Nemagraptus sp. was registered in the upper part of this biozone. Graptolites are associated with conodonts of the Pygodus serra Zone. A stratigraphic gap is registered at the top of the biozone (Ortega, 1995; Ottone et al., 1999).

Nemagraptus gracilis Zone (assemblage zone). Early Lower Caradoc (Gisbornian, Gi1).

Precordillera, San Juan.

Graptolites assigned to the N. gracilis Zone were described for the Las Aguaditas Creek, Central Precordillera, San Juan (Brussa, 1996; Ortega and Albanesi, 1998), and the La Chilca Hill (Blasco and Ramos, 1976; Peralta, 1998). Presumably, the FAD of N. gracilis would be located at Las Aguaditas Section, but a barren interval is recorded in the passage between the P. tentaculatus and N. gracilis zones. Graptolites recorded in the La Cantera Formation, Villicum Range, Eastern Precordillera (Peralta, 1986, 1990; Sánchez et al., 1991; Brussa, 2000) probably correspond to the N. gracilis Zone.

Climacograptus bicornis Zone (assemblage zone). Lower Caradoc (Gisbornian, Gi2).

Precordillera, La Rioja, San Juan and Mendoza; San Rafael Block, Mendoza.

The C. bicornis Subzone (N. gracilis Zone) was proposed to enclose most of the N. gracilis findings in the Precordillera (Ortega and Brussa,1990). Subsequently, it was raised to the category of zone by Ortega and Albanesi (1998). It can be recognized in the Los Azules Formation at Viejo Hill (Ortega, 1995), the Las Plantas and Las Vacas formations of northern Precordillera (Ortega and Albanesi, 1998; Astini and Brussa, 1997), the Cántaro de Oro Formation at El Tigre Range (Caballé et al., 1993), the Empozada Formation in the Precordillera of Mendoza (Alfaro, 1988; Cuerda and Alfaro, 1993; Mitchell et al., 1998), and in the Bola Hill, San Rafael Block (Cuerda and Cingolani, 1998). A stratigraphic gap is registered at the base of the biozone in most investigated sections.

Biserial graptolites and dicranograptids dominate the assemblage.

No nominated interval. Upper Caradoc (Eastonian, Ea2-Ea3). Precordillera, San Juan.

An early Eastonian graptolite assemblage probably equivalent to the Dicranograptus clingani Zone is present in the Yerba Loca Formation at El Tigre Range, western Precordillera (Ortega et al., 1991; Brussa, 1995; Brussa et al., 1999). It encloses Corynoides calicularis, Cryptograptus insectiformis, Dicellograptus flexuosus, Phormograptus sooneri, and Orthograptus quadrimucronatus, among others.

Dicellograptus complanatus Zone (assemblage zone). Lower Ashgill (Bolindian, Bo1).

Precordillera, Mendoza.

A graptolite fauna of the D. complanatus Zone was discovered in the middle member of the Empozada Formation, Precordillera of Mendoza (Mitchell et al., 1998). Dicellograpthus morrisi, D. complanatus, D. flexuosus, and Normalograptus miserabilis were identified. The low diversity of this fauna and the absence of spinous climacograptids were remarked by Mitchell et al. (1998).

Dicellograptus ornatus Zone (assemblage zone). Lower Ashgill (Bolindian, Bo2). Precordillera, Mendoza.

This biozone was identified in the middle member of the Empozada Formation, Precordillera of Mendoza (Mitchell et al., 1998). A graptolite fauna integrated by Dicellograptus ornatus, D. flexuosus, Dicranograptus ramosus, and others was mentioned. An unconformity present between the middle and upper members of the Empozada Formation, does not allow for infer the actual extent of the biozone.

Normalograptus extraordinarius Zone (assemblage zone). Upper Ashgill (Bolindian, Bo4).

Precordillera, San Juan.

A graptolite assemblage with the nominal taxon, Normalograptus normalis, N. miserabilis, Climacograptus tubuliferus, Dicellograptus ornatus, and others was recorded in the Alcaparrosa Formation, western Precordillera, San Juan (Brussa et al., 1999). It is identified by the presence of N. extraordinarius, and shares many species with the older D. complanatus and D. ornatus zones of the Empozada Formation.

Normalograptus persculptus Zone (assemblage zone). Upper Ashgill (Bolindian, Bo5).

Precordillera, San Juan.

Graptolites of the N. persculptus Zone were identified in the Baños de Talacasto locality, Talacasto Range, central Precordillera of San Juan (Cuerda et al., 1988). The assemblage is integrated by N. perscultus, C. angustus, C. cf. medius, and Metaclimacograptus robustus. This biozone was also recorded in the Don Braulio Formation at Villicum Range (Peralta and Baldis, 1990; Sánchez et al., 1991), and the La Chilca Formation at the Escondido River Section, El Fuerte Hill (Rickards et al., 1996).

Conclusions

An integrated conodont-graptolite biostratrigraphic chart for the Ordovician System of Argentina is assembled, with linkages provided by index species from both groups. 14 conodont zones are determined in the Argentine Precordillera, which include typical Midcontinent taxa in the early Lower Ordovician, and increasing amounts of Atlantic components throughout the system. 7 biostratigraphic intervals are recognized in the Ordovician of north-west Argentine basins. Early Lower Ordovician biostratigraphic intervals are well established in these basins. Conodont zones include mixed faunas from both the Midcontinent and Atlantic realms, which reveal transitional environments. 17 graptolite units are determined for the Argentine Precordillera. The graptolite fauna presents a Pacific affinity with a diversity peak that is reached by the early Middle Ordovician. A most precise graptolite biostratigraphy is developed for this span of time. 12 biostratigraphic intervals based on graptolites are reported for north-west Argentine basins. Graptolite assemblages are dominated by Atlantic and Baltic forms in these basins. A significant biostratigraphic resolution is accomplished for the Lower Ordovician Series, while the Middle and Upper Series are still awaiting detailed studies.

Acknowledgements. The authors acknowledge the continued support by CONICET for conodont and graptolite studies.

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Recibido: 18 de Septiembre de 2002

Aceptado: 17 de Diciembre de 2002