Silvio H. PERALTA1 and Stanley C. FINNEY 2

Abstract: THE UPPER ORDOVICIAN GRAPTOLITE FAUNAS OF THE CUYANIA TERRANE: THEIR BIOSTRATIGRAPHIC AND PALEOGEOGRAPHIC SIGNIFICANCE IN THE WESTERN MARGIN OF GONDWANA. The stratigraphic distribution of the Upper Ordovician graptolite faunas in the Cuyania Terrane, Western Argentina, and its paleogeographic significance in the Gondwana – Laurentia relationship is focused in this paper. In that tectono-stratigraphic unit, Upper Ordovician marine sequences are very conspicuous owing to their stratigraphic, sedimentologic and paleobiologic features. From a stratigraphic point of view, they are characterized by siliciclastic and mixed unconformities- bounded marine sequences, showing as a rule, thinning-fining upward arrangement. From a sedimentological point of view, their petrologic features, and paleocurrent patterns of these deposits indicate varied sources of provenance, and are related to platformal environments in Eastern and Central Precordillera, exhibiting facies changes west-ward, with deep facies in the Western Precordillera, which in parts includes basaltic, mafic and ultramafic rocks related. From a biostratigraphic point of view, the upper Ordovician deposits of Precordillera Terrane, includes several graptolite assemblages, which are not recognized in the rest of South America and, consequently neither in Western Gondwana. These faunas belong to the Nemagraptus gracilis and Climacograptus bicornis Zones, in the Caradoc stage, and to the Normalograptus extraordinarius, Dicellograptus complanatus, Dicellograptus ornatus and Normalograptus persculptus Biozones, in the Asghillian stage. Graptolites assemblages of the N. gracilis and C. bicornis shows a widespread record in the Precordillera Terrane, and in part occurs associated with conodonts, trilobites, brachiopods, among others, whereas, all the later three biozones show localized occurrence. Farther to the southeast, in the La Matras Block, at La Pampa Province, crops out the last platformal remnants of the Cuyania Terrane, which lack of graptolite record. During the occurrence of the N. gracilis and C. bicornis Zones Precordillera remained at low latitude like Laurentia.

Then, during the Late Ordovician, graptolite assemblages recorded in several units of Precordillera terrane, such as D. complanatus and D. ornatus in Mendoza Precordillera, and N. persulptus in the San Juan Precordillera, are pandemic and, by then, their are useless to recognize its paleogeographic position for themselves. However, the widespread distribution of glaci-marine deposit along Western margin of Gondwana, and its stratigraphic relationships with the mono-typic Normalograptus persculptus Zone, indicates that at least in the Late Asghillian (Hirnantian) the Precordillera terrane had reached a high latitude (Atlantic Province), showing from this time the same stratigraphic framework that the rest of Gondwana. In this scene, the uppermost Ordovician Normalograptus persculptus Zone, grade upward to P. acuminatus zone, indicating the beginning of the typically silici-clastic Latest Ordovician-Silurian and Devonian marine sequence. To explain these paleoclimatic and paleogeographic shifts, and taking into account the isotopic data from individual detrital zircons that provide strong evidences of Gondwanian affinities to Lower Cambrian and Upper Ordovician clastic units at the northern part of the Precordillera terrane, we suggest the paleogeographic evolution of Precordillera could be fairly explained from a transcurrent (para-autochtonous) model, in agree with paleogeographic models proposed by other authors, rather than by mean of an allochtonous model, as a Laurentian-derived fragment.

Resumen: LAS FAUNAS DE GRAPTOLITOS DEL ORDOVÍCICO SUPERIOR DEL TERRANE DE CUYANIA: SU SIGNIFICADO EN LA EVOLUCIÓN PALEOGEOGRÁFICA Y GEODINÁMICA DEL MARGEN OCCIDENTAL DEL GONDWANA. En este trabajo se analiza la distribución estratigráfica de las faunas de graptolitos del Ordolvícico superior del Terrano de Cuyania, y su significado paleogeográfico en la relación Laurentia-Godwana. En dicha unidad tectono-estratigráfica las secuencias marinas del Ordovícico superior se caracterizan por sus atributos estratigráficos, sedimentológicos y paleobiológicos.

Desde el punto de vista sedimentológico, se relacionan con ambientes de plataforma en la Precordillera Oriental y Central, con un marcado cambio a facies más profundas, hacia el oeste, en la Precordillera Occidental, donde en partes, se observan relacionadas con rocas basálticas, básicas y ultrabásicas. Sus rasgos petrológicos y diseños de paleocorrientes indican, en general, una variada proveniencia. Desde el punto de vista bioestratigrafico, estos depósitos contienen asociaciones de graptolitos no reconocidas en el resto de Sudamérica, y en consecuencia, en Gondwana Occidental. Estas faunas corresponden a la Zona de N. gracilis y C. bicornis, en el Caradociano, y a las Zonas de Normalograptus extraordinarius, D. complanatus, D. ornatus y N. persculptus para el Ashgilliano. Entre ellas, las faunas de la Zona de N. gracilis muestran una distribución restringida al ámbito de la Provincia Geológica Precordillera de La Rioja, San Juan y Mendoza, mientras que las asociaciones de la Zona de C. bicornis, se extienden fuera del ámbito Precordillerano, hasta el Bloque de San Rafael.

La composición paleobiológica de estas dos biozonas, en especial la de N. gracilis¸ indica una franca afinidad con las faunas de graptolitos homólogas de la zona de Alabama,Ouachita, demostrando una clara vinculación paleogeográfica en baja latitud, entre Laurentia y el margen occidental de Gondwana, por lo menos durante el Caradociano. Las asociaciones de graptolitos post- Zona de C. bicornis, tales como D. complanatus y D. ornatus en la Precordillera de Mendoza y N. persculptus en la Precordillera Oriental de San Juan, han demostrado ser pandémicas y por lo tanto, no son muy útiles para el desarrollo de modelos de reconstrucción paleogeográfica. Sin embargo, la amplia distribución de depósitos glaci-marinos en Gondwana Occidental, y su franca relación estratigráfica con la fauna mono-típica de N. persculptus en la Precordillera, indica que al menos en el Ashgilliano tardío (Hirnantiano) el Terreno de Cuyania ya habría alcanzado un posición a alta latitud (Provincia Atlántica), exhibiendo desde ese tiempo un desarrollo estratigráfico similar al del resto de Gondwana Occidental. En este contexto, la asociación mono-específica de la Zona de N. persculptus, en la Precordillera Central de San Juan, muestra un pasaje gradual hacia la Zona de P. acuminatus, indicando el comienzo de la típica sucesión silici-clástica marina cuya extensión temporal se verifica en Gondwana Occidental, desde el Ashgilliano tardío al Devónico. Para explicar estos cambios paleogeográficos y paleoclimáticos, y considerando los datos isotópicos obtenidos a partir de granos detríticos de Zircón, que demuestran una franca afinidad Gondwánica para unidades clásticas del Cámbrico inferior y Ordovícico superior de la Precordillera, se sugiere que la evolución paleogeográfica del Terreno de Cuyania, podría ser explicada a partir de un modelo de transcurrencia, considerandolo como un terreno desplazado o para-autóctono, acordando con modelos similares ya propuesto por otros autores, antes que por medio de un modelo de terreno alóctono, en el cual el Terrane de Cuyania es considerado como un fragmento derivado de Laurentia.

Key words: Upper Ordovician. Precordillera. Graptolites. Biostratigraphy. Palaeogeography.

Palabras clave: Ordovícico Superior. Geodinamica. Precordillera. Paleogeografía.

The CuyaniaTerrane, Western Argentina, is a tectono-stratigrphic unit composed by the Precordillera of La Rioja, San Juan and Mendoza, the San Rafael and Las Mahuidas blocks and the Angaco Belt (Ramos et al., 1986, 1996, Astini et al., 1996) (Fig. 1). The goal of this paper is to analyze the Upper Ordovician stratigraphy of this unit, emphasizing in its graptolite faunas contents and paleogeographic significance. Herein, we will refer to Precordillera as an equivalent term of Precordillera Geological Province, as well as Cuyania Terrane is used here as an equivalent to Precordillera Terrane.

The knowledge of the Upper Ordovician graptolite faunas stratigraphy of the Cuyania Terrane (Precordillera Terrane) presently is not well constrained, and in fact a review it is necessary ot be carry out. Upper Ordovician, Caradoc to Ashgillian, graptolite faunas occurs mainly in the Precordillera Geological Province (Precordillera of La Rioja, San Juan and Mendoza Provinces) (Fig. 2) according to Furque and Cuerda (1979) definition, but outside of Precordillera, graptolite fauna of the Bicornis Zone occurs only in the San Rafael Block, at the southern part of Mendoza Province. In this scenary, the earlier reference about the Upper Ordovician graptolite fauna was given by Borrello and Garecca (1951), who recorded Nemagraptus gracilis in the northern part of San Juan Precordillera. A more complete and extensive work on the graptolite fauna of the Precordillera was provided by Turner (1959) in its monographic work on graptolites of South America. Since then, the knowledge of Upper Ordovician graptolite fauna of Precordillera and San Rafael Block increased due to the contributions of Cuerda et al. (1982, 1985, 1988, 1987a, b), Ortega and Brussa (1990), Ortega et al. (1985, 1996), Brussa (1996, 1997, 2000), Brussa et al. (1999), Peralta (1986, 1990, 1998) among others. Most of these works, reported the finding of graptolite faunas belonging to C. bicornis Zone, and only some few of them registered the occurrence of graptolites ranging into N. gracilis Zone.

Later Ross (1975) attempted to explain such widespread dispersion of ollenids by mean of larval transfer by oceanic currents, and Bond et al. (1984) provided stratigraphic evidences to support the Laurentian affinity of Precordillera. From the paper of Ramos et al. (1984), much workers have published a lot a papers in agreement with the "allochtonous character" of the Precordillera Terrane (Ramos et al., 1986; Ramos, 1992; Dalziel, 1993; Dalziel et al., 1994, 1996; Astini et al., 1995; 1996; Thomas and Astini, 1996; Huff et al., 1998; Dickerson and Keller, 1998; Astini, 1998; Keller et al., 1998; Astini and Thomas, 1999; Keller, 1999), as well as to explain the geometry and mechanisms leads to consider the Precordillera as a Laurentian fragment, rifted from Southern Appalachian (present coordinates) in the Lower Cambrian, drifting during Middle and Upper Cambrian to Arenigian, and colliding against western margin of South America (Gondwana) in the Llanvirnian (Astini et al., 1995, 1996; Thomas and Astini, 1996; Astini, 1998a, b; Astini and Thomas, 1998; Thomas and Astini, 1999; Thomas et al., 2002). This is the so-called "the funeral ship" model by Dalziel (1997), who in turn suggested that Precordillera terrane is a result of continent-continent collision between Laurentia and Gondwana; which is the so-called "calling card" model (Astini & Thomas, 1998). Another model, termed as the "Laurentian Occidentalia Terrane", is proposed by Dalla Salda et al. (1992a, b; 1998) to include all known Precambrian outcrops along the Andean margin as result of a continent-continent collision between eastern Laurentia and the southwestern Gondwana along the whole length of the Andean margin is postulated in this model. Anyway, in all three models the Precordillera is considered as allochtonous terrane, Laurentia-derived.

Earlier, Baldis et al. (1989) suggested a Gondwanian (para-autochtonhous) origin to Precordillera, to explain its emplacement to the west of Famatina System (present coordinates) by mean of a transcurrent mechanism (right-lateral fault). In addition to the Ross’s idea, this model has been successfully applied by Aceñolaza et al. (2000; 2002), to support their para-autochtonous model of Precordillera Terrane, based on transcurrent fault mechanism, similar to that proposed by Baldis et al. (1989). Recently, Finney et al. (2002, 2003) have also proposed a Gondwanian origin to Precordillera Terrane, on the basis of geological (mainly stratigraphic) and isotope evidences. Earlier, Finney and Peralta (2000) suggested to Precordillera a similar latitudinal location that Laurentia, at least up to Nemagraptus gracilis/Climacograptus bicornis Zones, based on paleobiogeographic (Pacific realm) affinities.

Geological setting of the Cuyania Terrane

The Precordillera is a typical thrust- fold belt, striking North-South with minor variations, which were outlined formerly by Keidel (1921, 1949) and Heim (1952), and subsequently detailed by Baldis and Chebli (1969). Structural style of the Precordillera Oriental is similar to that of the Sierras Pampeanas, showing vergence to the west, and thick-skinned deformation evolving basement rocks (Zapata and Allmendinger, 1996a, b; Zapata, 1998). In addition, the Precordillera Central shows thin-skinned deformation and eastward vergence. On the other side, structural deformation in

Figure 1. Geological map of Cuyania Terrane, including Precordillera of La Rioja, San Juan and Mendoza Provinces, San Rafael Block (Mendoza Province), Las Mahuidas Block (La Pampa Province), besides showing its boundaries with the neighboring units: mainly Cordillera Frontal, Sierras Pampeanas Occidentales and the  Sistema de Famatina, and the distribution of the Upper Ordovician graptolites fauna, indicating N. gracilis, C. Bicornis, Normalograputs extraordinarius, Dicellograptus complanatus and Dicellograptus ornatus, Noramalograptus persculptus occurrences.

Precordillera Occidental is more complex, showing mainly eastward vergence, but in part the lower Paleozoic complex shows west-wards vergence (Quartino et al., 1971; Ramos, 1999). The Ordovician rocks of the Precordillera Central and Oriental are included in the Eastern Tectofacies, and those of the Precordillera Occidental, in the Western Tectofacies (Astini, 1992). However, it is noteworthy that stratigraphic frame-work and facies distribution of the early Paleozoic rocks of the Precordillera Geological Province, don’t keep strict correspondence with such a morpho-structural subdivision, such as it is suggested by Keller (von Gosen, 1992; 1999).

The Upper Ordovician succession of the Cuyania Terrane, in the Precordillera and San Rafael Block, have been developed on a erosional surface, unconformity, related to Guandacol Tectonic Phase, which has been formerly defined by Furque (1972), and subsequently included into the Famatinian Geotectonic Cycle (Aceñolaza and Toselli, 1973). This unconformity can be traced in most part of the Precordillera (La Rioja, San Juan and Mendoza Provinces), as well as in the San Rafael Block, indicating the beginning of the Upper Ordovician siliciclastic succession. Latterly, Furque and Cuerda (1982, 1984), established the age of this tectonic phase as Arenigian to Caradocian, by then, recent stratigraphic and biostratigraphic studies indicate that its ranges between N. gracilis to C. bicornis Zones, and so, the age the Upper Ordovician succession extends up to N. persculptus Zone.

In the Precordillera Oriental, at Villicum range, three erosional unconformities are recognized: the first one is the so called Guandacol Phase, which occurs at the base of the La Cantera Formation, which yields graptolite faunas of the N. gracilis Zone and late Llanvirnian pebbles in its basal conglomerate (Albanesi and Benedetto, 1992). The second one is placed between the top of La

Figure 2. The Precordillera of La Rioja, San Juan and Mendoza Provinces, showing distribution of the typical Upper Ordovician localities and geological setting.

Cantera Formation and the base of the La Pola Formation (Astini, 2001), which is assigned to Middle Caradocian to late Ashgillian (Brussa, 2000; Astini, 2001) that indicates the age of this unconformity could be established as Middle Caradocian. The third one is placed at the base glacimarine diamictite of the Don Braulio Formation (Peralta and Carter, 1990, 1999), late Ashgillian in age (Baldis and Blasco, 1975; Levy and Nullo, 1974; Bendetto, 1986), whether at the top of the La Pola Formation as well at the top of the La Cantera Formation due to erosive effect Baldis and Chebli (1969), and Baldis (1975) (Fig. 3). Upwards succession, the boundary between late Ordovician and early Silurian transitionally.

Towards the south and south-west, the Guandacol Phase occurs as a para-conformity surface, which displays at the base of the black shale sequence of the Los Azules (Upper Member) Formation at the Cerro Los Azules as well of the Los Azules Formation at the Cerro La Chilca sections, which underlay Late Ashgillian deposits (Astini and Benedetto, 1992). In that way, the Guandacol Phase occurs at the base of the carbonate and mixed (carbonate-siliciclastic) deposits of the Las Aguaditas Formation, at the Los Blanquitos range and the Las Chacritas creek, unconformably overlain by Silurian deposits. Towards south, from the Gualilán-Talacasto to San Juan River areas, including La Deheza range, in general, Upper Ordovician rocks have not been recorded up to date, except in the Talacasto section, where Late Ashgillian (Hirnantian) graptolite (Cuerda et al., 1982; Cuerda et al., 1988b) and palynomorphs (Melendi and Volkheimer, 1982) have been registered at the base of the La Chilca Formation, and at the Sassito creek, in the San Juan River area, where the mixed (carbonatesiliciclastic) sequence of the Sassito Formation (Astini and Cañas, 1995) is interposed between the early Ordovician limestones of the San Juan Formation, and the Silurian deposits of the Tambolar Formation.

In the Precordillera Occidental, (Western Tectofacies of Astini, 1992) the Upper Ordovician sequences including ofiolitc rocks, is much more complex to recognize because of the tectonic signature, and there, every Upper Ordovician sequence is fault-bounded and pervasively deformed.

Towards west of the Sierra del Tontal, Upper Ordovician graptolite faunas have been recorded in the Cabeceras and Cortaderas Formation, at the Mendoza Province. All mafic and ultramafic rocks of the Cortaderas and Bonilla area, at the Precordillera Oriental (Western Belt of Mendoza Precordillera), thought as an ophiolite complex, have been recently reinterpreted as four distinct units, of Upper Precambrian to Cambrian age, combined with meta-sedimentary rocks (Davis et al., 1999). This Pre- Carboniferous complex was juxtaposed along previously unrecognized ductile, top-to-the-east ductile shear zones in the early to Middle Devonian ocean closure, between Chilenia and Precordillera Terrane, along western margin of the Precordillera Terrane.

In the Las Matras Block (Fig. 1), at La Pampa Province, the limestones of the San Jorge Formation and the quartzites of the Agua Escondida Formation crop out in close spatial association within Las Matras Block, although the stratigraphic relationship between them is not yet clear (Sato et al., 2000).

Up to date, these units lack of fossil record, however, the recently obtained deposition age provided from K-bentonite levels, interbedded in the limestones of the San Jorge Formation (Melchor et al., 1999a, b) indicates it is coeval with the Cambrian-Ordovician carbonates rocks of the Precordillera and San Rafael Block. On the other side, the quartzites of the Agua Escondida Formation, assigned to the Carboniferous by Linares et al. (1980), could also be correlated with the clastic facies of those carbonate shelf deposits extending from the Precordillera (Sato et al., 2000).

Outside of the Cuyania Terrane, Lower Paleozoic sedimentary and basic-ultrabasic rocks crop out to the west, in the Cordillera Frontal, at Los Andes Hill (Caminos, 1979, 1993; Pöthe de Baldis et al., 1987), but either its age as well as the stratigraphic relationship with the Grenvillian (1069 Ma) (Basei et al., 1999) metamorphic rock of basement are not clear, due to tectonic deformation and lacking radiometric dating and scarce fossil record. The basic-ultrabasic rocks have been thought as part of an Upper Ordovician ophiolite complex developed during the Famatinian Cycle, along hundreds of kilometers in a continental suture zone between Precordillera and the Cordillera Frontal (Chilenia) (Haller and Ramos, 1984) (Fig. 2). However, such is suggested by these authors, the age of these rocks, is not well known up to date.

Upper Ordovician units from Precordillera terrane bearing graptolite assemblages

The biostratigraphic analysis of the Upper Ordovician graptolite faunas, shows that the Lower Caradoc units are distributed in the Precordillera Terrane, ranging into the N. gracilis and C. bicornis Zones up to N. perscupltus Biozone. Among them, Caradoc graptolite assemblages are widely distributed mainly on Precordillera as well in San Rafael Block, but occurrence of the Asghillian assemblages are only restricted to Precordillera Geological Province. In order to provide an adequate description of these biozones, their biostratigraphic and paleontologic features are considered, as well as its stratigraphic correlations and boundaries, on those basis an up to date information give a more complete synthesis of these assemblages.

Graptolite of the Nemagraptus gracilis biozone stratigraphy

Graptolites assemblages of the N. gracilis Biozone are clearly recorded, up to date, only in the San Juan Precordillera, among them, four sections occur at the Central Precordillera: the Los Azules Formation at cerro La Chilca area, (Ramos and Blasco, 1975; Peralta, 1998), the Las Aguaditas Formation to the west of Jáchal city (Baldis and Caballeri, 1988; Baldis, 1995; Brussa, 1996, 1997), at the Los Blanquitos and Las Chacritas sections, and the Sierra de La Invernada Formation at the homonymous locality. In the Precordillera Oriental, graptolites of that biozone occur in the La Cantera Formation, at Don Braulio creek, on the eastern slope of the Villicum range, Precordillera Oriental (Baldis et al., 1982; Peralta, 1986, 1990). In the Precordillera Occidental, these faunas has been recorded in the Yerba Loca Formation, in the northern part, and in the Portezuelo del Tontal Formation, in the southern part, respectively. However, in all of these, the lower and upper boundaries of N. gracilis Zone are not exposed because of the erosive relationship with the over and under laying units, except Portezuelo del Tontal and Yerba Loca Formations, which are fault-bounded.

The lower stratigraphic boundary of these sequences in the Precordillera Central and Oriental is represented by and erosional surface at the base of the conglomerate of the Las Vacas and La Cantera Formations, overlaying Llanvirnian black shales of the Gualcamayo Formation (Baldis et al., 1982; Peralta, 1986, 1990, 1993). On the other hand, in the Precordillera Central, correlative stratigraphic discontinuity are represented by the para-conformity at the base of the black shales of the Los Azules Formation, at La Chilca Hill, which overlie conformably, mixed deposits the Gualcamayo Formation, and at the base of the mixed (pelitic-calcareous) sequence of the Las Aguaditas Formation which overlie mudstones deposits equivalents to Las Chacritas Formation (Peralta and Baldis, 1994; Astini, 1995; Peralta et al., 1999).

On the other side, despite all these sections include graptolite faunas of the the N. gracilis Zone, they show some taxonomic differences in its compositions and also different ages. In this way, in agreement with the review on Upper Ordovician graptolite fauna from Precordillera that are being carrying out, preliminarily data indicates clearly that the older assemblage belong to the La Cantera Formation, then, the next upward is the graptolite fauna of the Portezuelo del Tontal Formation, which in turn is older than those of the Los Azules Formation, at the cerro La Chilca section, the Yerba Loca Formation, at rio Jáchal section, the Las Aguaditas Formation at the Los Blanquitos section, and the Sierra de La Invernada Formation. All of these assemblages are markedly different among them, but the absence of Climacograptus bicornis, in all of them, indeed the occurrence of conodont faunas of the P. anserinus Zone and B. variabilis Subzone of the A. tvaerensis Zone recorded in the Las Aguaditas Formation, support this age. On the other side, lithology and paleoenvironment are clearly contrasted between these units, and also, they are unconformities-bounded stratigraphic sequences (Peralta, 1994; Benedetto et al., 1995).

The La Cantera Formation is made up of siliciclastic deposits, showing thinning-fining upward sequence, developed in shallow water environment (Peralta, 1998), which is composed by three members (Peralta, 1993): the lower member is made up of a basal conglomerate passing upward to medium to coarse grained sandstones and wackes and pelites interbedded, which yields a graptolite fauna (basal assemblage of Peralta), composed by Dicellograptus gruleyi gurley, Hustedograptus teretiusculus, Glossograptus aff. G. ciliatus, Pseudoclimacograptus modestus, Climacograptus sp., Amplexograptus sp., Nemagraptus gracilis, Cryptograptus sp., and isolated rhabdosomes of an unidentified dendroid.

In the middle member remains of D. gurleyi, Climacograptus modestus, Hustedograptus teretiusculus, and fragmented stipes of N. gracilis are common. In the upper member, the assemblage is dominated by Dicellograptus gruleyi gurley, Dicellograptus alabamensis, Reteograpgtus geintizianus, Climacograptus modestus, Climacograptus sp., Amplexograptus sp., and Glyptograptus sp. These assemblages are the older graptolite fauna recognized to the Caradocian successions on the Cuyania Terrane, and it is characterized by the absence of Leptograptus and Dicranograptus genera.

On the other side, the Los Azules Formation, is composed mainly of black shales, which are plenty of graptolites, whit scattered dark mudstone levels, which became thicker and laterally more continuous towards the top, evidencing lightly thickening upward sequence (Peralta, 1998). Lensshaped mudstones in the lower part of this units, contains abundant graptolites well preserved, while towards the upper part, graptolite faunas became scarce in the laterally continuous mudstone levels, in which brachiopods and trilobites, among other, are dominant. According with our revision, the graptolite fauna of this unit is constituted by Dicellograptus gurleyi, Leptograptus trentonensis, Leptograptus sp., Hustedograptus teretiusculus, Pseudoclimacograptus modestus, Dicellograptus sp., Dicellograptus alabamensis, Climacograptus sp., Dicranograptus sp., Glossograptus aff. G. ciliatus, Cryptograptus sp., and Pseudoclimacograptus sp.

To the north, in the Los Blanquitos ridge, crops out the Las Aguaditas Formation, which is a typically mixed-carbonate sequence, made up mainly of evenly laminated hemipelagic mudstones and blacks shales containing abundant fossils, which overlaying Llanvirnian laminated mudstonewackestone- shale association of the Las Chacritas Formation, and at the top is unconformably underlain by the basal cherty pebble conglomerate of the La Chilca Formation (Ashgillian-early Llandovery to early Wenlock, Cuerda et al, 1982; 1988). In this section, the Las Aguaditas Formation bears an abundant graptolite faunas belonging to N. gracilis Zone (Baldis and Cabaleri, 1988; Brussa, 1996, 1997), constituted by Nemagraptus gracilis, Pseudoclimacograptus scharenbergi, Pseudoclimacograptus cf. scharenbergi, Glossograptus ciliatus (s.l.), Dicranograptus nicholsoni, Cryptograptus tricornis, Didymograptus (s.l.) superstes, Corynoides curtus cf. pristinus¸Thamnograptus capillaris, Dicellograptus cf. divaricatus, Reteograptus geinitzianus, Dicellograptus salopiensis, Orthograptus? sp. This graptolite assemblage is associated with abundant shelly faunas, mainly trilobites of the Protoincai ancestor and Incaia deormaecheai Zones (Baldis and Pöthe de Baldis, 1994; Baldis, 1995), besides of rests of brachipods, crinoids, and ostracods, among others. Conodonts of the P. anserinus Zone and B. variabilis Subzone of the A. tvaerensis Zone (Lehnert, 1995), occur in several levels associated with the graptolite faunas, supporting that age. Striking to the south, in the La Trampa ridge, Las Aguaditas Formation is well exposed in the Las Chacritas creek, showing the same stratigraphic relationships that in the type

locality. The predominantly dark mudstone and wackestone sequence of the Las Aguaditas Formation starts herein, with a conspicuous rich-graptolites black shale level, passing upward to mixed (peliticcarbonate) deposits exhibiting thickening-upward sequence, and bearing abundant graptolite faunas of the N. gracilis Zone (Peralta and Baldis, 1994).

In the Precordillera Occidental, in the Cerro Alto de Mayo, at the Rio Jáchal section, a Caradoc graptolite assemblage from the Yerba Loca Formation, has been described by Ramos and Blasco (1976), that it is constituted by Nemagraptus gracilis yerbensis, Corynoides tricornis turneri, Leptogratpus minusculus, Glyptograptus sp., and Dicellograptus divaricatus salopiensis, which is assigned to N. gracilis Zone by the mentioned authors. However, Ortega et al. (1991) have proposed a younger age for that association on the basis of its biostratigraphic correlation with the graptolite faunas from the Las Plantas Member of the Las Vacas Formation, and the upper member of the Los Azules Formation in the Cerro Viejo section, Sierra de Huaco. On the other side, a recent study carried out on the same deposits sampled by Blasco and Ramos (1976), indicate strong faunal affinities with that of the Los Azules Formation, where a graptolite fauna of the N. gracilis has been recognized (Peralta, 1998) at Cerro La Chilca section, in accordance to Blasco and Ramos (1976). In the Sierra de La Invernada and Sierra del Tigre outcroppings of the Sierra de La Invernada Formation bears graptolite faunas of the N. gracilis Zone (Caballe et al., 1992). Towards south of the San Juan River, in the Sierra del Tontal, crops out a thick sequence of coarse to fine grained deposits of the Portezuelo del Tontal Formation characterized by alternated packages of sandstones and pelites which in places bears graptolite faunas, formerly described by Cuerda et al. (1986), and recently assigned to the N. gracilis Zone (Peralta et al., in review), which includes: Dicellograptus gurleyi gurley, Dicellograptus alabamensis, Dicranograptus sp., Glyptograptus teretiusculus and Retiograptus geintizianus Hall, Cryptograptus sp., Glossograptus hincksii (Hopkinson), Glossograptus sp., Amplexograptus sp. This deposits has been recently interpreted as wave modified turbidites deposits, evolved in a platform setting (Basilici et al., in review a, b).

A possible post-N. gracilis Zone assemblage

Towards south, in the Cuesta de los Castañeros section, at the Sierra del Tigre, Western Precordillera, a graptolite assemblage composed of Dicellograptus sp., Orthoretiolites sp. y Orthograptus sp., has been registered from the Yerba Loca Formation by Ortega et al. (1991), which is interpreted as post-N. gracilis Zone age, possibly Climacograptus spiniferus Zone, in agree with the mentioned authors. However, these exposures have been refered by Caballé et al. (1993) and Caballé (1997), to the Cantaro de Oro Formation.

Graptolite faunas of the Climacograptus bicornis biozone stratigraphy

Recently, Brussa (2000) and Astini (2001), have pointed out that in the Precordillera Oriental, the record of the bicornis Zone graptolite faunas, in the La Pola Formation, at Villicum range, which is composed of thick-bedded coarse-graineddebris flows, and interbedded pebbly mudstone, quartzbioclastic- rich sandstone, and few turbidites and silty shale. Its age is constrained by graptolite faunas in the undelaying La Cantera Formation, Early Caradoc, and the overlaying Don Braulio Formation, late Ashgillian (Hirantian), respectively. However, graptolites from the debris flow themselves suggests a mid-late Caradoc age, but taking into account ist stratigraphic relationships, this unit could reach the Hirnantian stage, in agree with the mentioned authors. These litho and paleobiological evidences could reflect that La Pola Formation represent a silici-clastic, conglomerate dominated erosive remnat of the late Ordovician glaciation. Further, the La Pola Formation deposits coul be correlated, in general sense, with those of the Sassito Formation, in the San Juan River area, at the Precordillera Central, and with the upper member of the Empozada Formation in the San Isidro creek, at the Eastern Belt of the Mendoza Precordillera (Astini, 2001). However, it is noteworthy there is not other bioestratigraphic evidence to support this suggestion.

At the Precordillera Central, graptolite faunas of the bicornis Zone show widespread distribution, and could be traced from the northern part, in La Rioja Province, throughout of the San Juan Province, up to the southern part, in the Mendoza Province. In the northern part of the Precordillera Central, at Guandacol area, La Rioja Province, the Upper Ordovician sequence of the Trapiche Group (Furque, 1972), is composed by the Las Vacas and Trapiche Formations, in agreement with the stratigraphic arrangement given by Astini (1998a). The Las Vacas Formation rests unconformably on the black shales of the Gualcamayo Formation, or in places, it rests directly upon the San Juan Formation limestones, because erosion prior to the deposition of the basal conglomerate, created a considerable relief (Astini, 1998a, Keller, 1999). At the top, the Trapiche Group underlay unconformably Carboniferous sedimentary rocks, which have been deposited on an impressive erosional surface evolved on the Upper Ordovician rocks, and it is related to the Chánica tectonic phase (Salfity et al., 1984), which is a major event in Western Gondwana since it indicates the beginning of the Gondwana geotectonic cycle.

The Las Vacas Formation as it was redefined by Astini (1998a, b), in its type section at Guandacol locality, is composed mainly of conglomerates and subordinated pelites and sandstones, yielding two graptolite faunas assigned to C. bicornis Zone (Astini and Brussa, 1997; Caballé et al., 2002), which indicates a Lower Caradoc age. The first one includes the zonal eponymous species associated to Cryptograptus cf. insectiformis Ruedemann, Orthograptu sp. and Amplexograptus sp. The second one is composed by Pseudoclimacograptus cf. modestus (Ruedemann), Amplexograptus sp., Orthograptus sp., Cryptograptus sp. and Reteograptus sp. At the top, the Las Vacas Formation is unconformably overlain (angular unconformity) (Astini, 1998a) by the Trapiche Formation deposits, which age has been established by the occurrence of Amorphognathus superbus which indicates the homonymous conodont zone, late Caradoc-early Ashgill (Albanesi et al., 1990) to the lower part of the succession, and brachiopods and trilobites indicating late Ordovician (Hirnantian) at the upper part (Benedetto and Herrera, 1987). Graptolite are scarce and badly preserved, and only rests of Dicellograptus have been recorded by Turner (1960) in the lower part of this unit, that it is unconformably (Chánica Tectonic Phase) overlain by Carboniferous continental deposits, which herein indicates the beginning of the Gondwanian Geotectonic Cycle.

Towards the south, graptolite assemblages of the Bicornis Zone are recorded in the Los Azules Formation by Ortega (1995) at Cerro Viejo section, and in the Las Vacas Formation, at the Cerro Potrerillos by Ortega and Albanesi (1998). At both sections, several genera and species of graptolites of that biozone have been described, including the nominal taxon. Besides, Ortega (1987) also describes, in the lower part of the Los Azules Formation, a graptolite assemblage of the Hustedograptus teretiusculus Zone, which is scarcely recorded in the Cuyania Terrane. In the cerro Potrerillo section, the fine-grained deposits of the Las Plantas Member, of the Las Vacas Formation, according to redefinition given by Astini (1998a), overlie unconformably black shales of the Gualcamayo Formation, Llanvirnian in age, involving a stratigraphic gap that span the late Llanvirnian-earliest Caradoc (Ortega, 1995). In this section, the basal part of the Las Plantas Formation bears an abundant, diverse and well preserved graptolite fauna composed of Cryptograptus tricornis, Leptograptus cf. validus, Dicellograptus divaricatus, Dicranograptus nicholsoni, D. n. longibasalis, D. spiniferus, Climacograptus brevis brevis, C. bicornis, Orthgraptus ex. gr. calcaratus, Orthograptus sp., Pseudoclimacograptus scharenbergi and Lasiograptus cf. harknessi. N. gracilis and Reteograptus geinitzianus are very scarce forms in this assemblage, and the associated conodont fauna is assigned to the Baltoniodus gerdae Subzone of the A. tvaerensis Zone, indicating Middle Caradoc (Albanesi et al., 1998). The upper part of the Las Plantas Formation, is made up of black shales and contains D. nicholsoni and Climacograptus tridentatus (Benedetto et al., 1991).

At cerro Viejo section, to the south of cerro Potrerillo, the Los Azules Formation is composed mainly of black shales and calcareous shales, which paraconformably overlies the San Juan Formation, and at the top is in turn unconformably overlain by Carboniferous deposits. Three members have been recognized in this unit: the lower member, early Llanvirnian in age (Paraglossograptus tentaculatus Zone); the middle member, Middle-Upper Llanvirnian in age (Pterograptus elegans to Hustedograptus teretiusculus Zones); and the upper member which includes a Caradoc graptolite assemblage composed mainly by N. gracilis and C. bicornis, associated with Thamnograptus capillaris, Acrograptus superstes, Reteograptgus geinitzianus, Glossograptus ciliatus, C. tricornis, Dicellograptus divaricatus, D. salopiensis, Dicranograptus nicholsoni, Pseudoclimacograptus scharenbergi, P. modestus, Glyptogratpus? sp., Orthograptus sp. and occasional dendroid remains. A conodont fauna appears associated with the graptolite fauna, belonging to uppermost span of the Baltoniodus variabilis Subzone or the subsequent B. gerdae Subzone of the A. tvaerensis Zone (Ottone et al., 1999).

Upper Ordovician sequences of the Empozada Formation (Harrington and Leanza, 1957), crops out in the San Isidro Creek, Precordillera Central, Mendoza Province, which is overlain unconformbly (erosive surface) by siliciclastic marine deposit of the Villavicencio Formation, Silurian- Lower Devonian in age, in agree with its paleofloristic and palynologic contents (Cuerda, et al., 1987a; Edwards et al., 2001; Rubinstein, 1993). At the base, the Empozada Formation overlies a sedimentary complex of varied composition composed of megabreccias and breccias, green shales, dark conglomerates, mudstones and sandstones, paraconglomerates and black shales, including olistoliths containning trilobite and conodont fauna, associated with an early Ordovician graptolite fauna (T. approximatus Zone) (Bordonaro and Peralta, 1987) in an allochtonhous sequence, deposited in the Mid-Upper Ordovician (Heredia et al., 1990). According to the mentioned authors, two members can be recognized in the Empozada Formation: The Lower Member composed mainly of black shale deposits, which in the lower and middle part bears graptolites of the C. bicornis Zone (Cuerda, 1978; Alfaro, 1988; Alfaro and Fernández, 1985; Cuerda et al., 1988a; Cuerda and Alfaro, 1992), and in the upper part yields graptolites of the Orthograptus quadrimucronatus Zone (Upper Caradocian- Lower Ashgillian) (Alfaro, 1988) and of the Dicellograptus complanatus and D. ornatus Zones (Mitchell et al., 1998) of Hirnantian age. Graptolite fauna of the C. bicornis Zone is constituted mainly of Nemagraptus gracilis, Climacograptus bicornis, C. tridentatus, C. tubuliferus, C. cf. C. caudatus, Pseudoclimacograptus scharenbergi, according to Toro and Brussa (2001). Cuerda (1978) registered the Amphigraptus genus, besides Alfaro (1988) registered C. pygmeus, Diplograptus cf. D. minutus, Climacograptus cf. C. tubuliferus and Reteograptus sp, which was assigned to the Orthograptus quadrimucronatus Zone (Upper Caradoc) by the mentioned author.

In the Precordillera Occidental, farther to the north, in the Jagüé area, Upper Ordovician graptolite faunas composed by Glossograptus ciliatus Emons var. duglasi (Lapworth), Glossograptus sp. and cf. Glyptograptus sp., all them badly preserved due to tectonic deformation, were recordedby Aceñolaza and Bernasconi (1969) and Aceñolaza (1970) in the Esquistos del Cerro Condor Member of the Río Bonete Formation, at the Bonete River section, and tentatively could be assigned to the Upper Ordovician, Caradocian, on the basis of lithostratigraphic correlation. In the Sierra de la Invernada (Precordillera Central) and Sierra del Tigre (Precordillera Occidental) Upper Ordovician siliciclastic rocks belonging to the Cantaro de Oro Formation, yields several graptolite assemblage (Caballé et al., 1993) composed by Crytograptus tricornis (Carruthers), Leptograptus sp., Dicranograptus ramosus cf. D. r. cf. spinifer (Lapworth), Dicranograptus nicholsoni nicholsoni Hopckinson, Dicranograptus nicholsoni diapason Gurley, Dicranograptus kirki Ruedemann, Climacograptus sp., Didymograptus sp., Climacograptus bicornis bicornis (Hall), Climacograptus sp., Diplograptus sp., Amplexograptus sp., Orthograptus cf. O. spinigerus Elles and Wood, Neurograptus cf. N. margaritatus (Lapworth), Orthoretiolites cf. O. hami Whittington, Orthoretiolites cf. O. tigris Brussa. In agree with Caballé et al. (1997), these graptolites indicates a Lower Caradoc age, ranging into the Bicornis Zone, however, their stratigraphic relations with the other units on the area, are not well known due to the tectonic effect.

Farther to the south of the San Juan River, in El Leoncito and Santa Clara areas, graptolite faunas of probably Upper Ordovician age, have been recorded in the Cabeceras Formation (Cuerda et al., 1982) and Cortaderas Formation (Cuerda et al., 1987a, b). In the former, rests of the genus Climacograptus are described, and in the later, an assemblage composed by Tetragraptus sp., Nemagraptus sp., Dicellograptus sp., and Dichograptus sp., is described by the mentioned authors. For all that, their biostratigraphic positions are not well constrained and they remains still dubious, owing to graptolites are not well preserved and its description has been done only up to genus level. In this way, taxonomic and biostratigraphic data are insufficient to assign them to one of the Upper Ordovician biozones. In the Uspallata area, northern Mendoza, the Bonilla and Farallones Formations, are considered as equivalent deposits of the Cortadera Formation, as stated by Caminos (1993).

Towards south, at the San Rafael Block, Mendoza Province, sedimentary rocks of the Pavón Formation bears three graptolite assemblages belonging to the C. bicornis Zone (Marquat and Menéndez, 1985; Cingolani and Cuerda, 1994; Cuerda et al., 1998). The first one is composed by: Climacograptus bicornis, C. tridentatus, Lasiograptus constatus, Orthoretiolites sp.; the second one by Dicranograptus ramosus ramosus, D. ramosus cf. longicaulis, D. nicholsoni; and the third one by Cryptograptus tricornis insectiformis, Pseudoclimacograptus scharenbergi, Climacograptus tridentatus, Orthograptus aff. apiculatus and Dicellograptus salopiensis. In the northern end of the Cuyania Terrane, in the Las Matras Block (in the sense of Sato et al., 2000) as a part of the Las Mahuidas Block (sensu Ramos, 1999), Upper Ordovician sedimentary rocks have not been recognized at the present.

Late Ordovician graptolite faunas stratigraphy

The occurrence of late Ordovician graptolites fauna in Cuyania Terrane is restricted, such as it is demonstrated by the localized record of the Dicellograptus complanatus and Dicellograptus ornatus in the upper part of the Empozada Formation, in the San Isidro creek, at Eastern Belt of the Mendoza Precordillera (Mitchell et al., 1997, 1998), and of the Normalograptus extraordinarius, in the Alcaparrosa Formation, at the Precordillera Occidental (Brussa et al., 1999). Besides, Normalograptus persculptus is registered in the Talacasto section, Central Precordillera (Cuerda et al., 1988), in the Hirnantian deposits of the Don Braulio Formation, at Sierra de Villicum, Precordillera Oriental (Peralta and Baldis, 1990), indeed, in the unnamed unit outcropping in the río Escondido section (Rickard et al., 1996). In the northern part of the Precordillera Terrane, at the Guandacol area, southernmost La Rioja Province, Hirnantian deposits bearing brachipods fauna and trilobites of the Dalmanatina Zone, were registered by Benedetto et al. (1986) in the Trapiche Formation. However, no diagnostic graptolite faunas has been recorded at that age.

Graptolite faunas of the Dicellograptus complanatus and Dicellograptus ornatus Zones stratigraphy

In the Mendoza Province, at San Isidro creek, in the upper part of the lower member of the Empozada Formation, graptolite faunas of the Dicellograptus complanatus and Dicellograptus ornatus Zones, have been recorded in the upper part of the lower member by Mitchell et al. (1998). The former is composed by Climacograptus tubuliferus, Normalograptus miserabilis, Dicellograptus minor, Dicellograptus flexuosus and Dicellograptus complanatus. The later yields Dicellograptus ornatus, Cryptograptus cf. insectiformis, Orthoretiolites cf. hami, Dicranograptus ramosus. These Asghillian faunas have been used by Mitchell et al. (1997) in the paleogeographic analysis of the Precordillera Late Ordovician path migration, and its relation with Gondwana.

Graptolite faunas of the Normalograptus extraordinarius Zone stratigraphy

Ashgillian graptolites faunas of this zone have been recorded only in the black shales deposits of the Alcaparrosa Formation (Brussa et al., 1999), to the east of the Los Patos River, in the Western Precordillera, San Juan Province, which is composed by Normalograptus sp. cf. N. normalis, Climacograptus tubuliferus, Dicellograptus sp. cf. D. flexuosus, Dicellograptus ornatus, Normalograptus miserabilis, Normalograptus extraordinarius, Dicellograptus sp. cf. D. complanatus, Amplexograptus sp. cf. A. latus. However, taking into account the structural complexity affecting the lower Paleozoic units over all Western Precordillera setting, stratigraphic relations among them remain unknown, which in fact difficults to recognize with certainty the stratigraphic frame-work, such as has been pointed out by Baldis and Peralta (1999) and Brussa et al. (1999). In this way, the stratigraphic relation of the Alcaparrosa Formation fossiliferous levels with the Caradocian deposits of the Portezuelo del Tontal Formation, as well as with the other early Paleozoic units, remains unknown up to date, like towards south with the Cabeceras and Cortaderas Formations. Recently Astini et al. (2000) suggested a stratigraphic framework to the Ordovician units of the Precordillera Occidental, but their stratigraphic boundaries as well as their relations remain still ambiguous.

The mono-type N. persculptus fauna stratigraphy

The mono-type fauna of the N. persculptus Zone is represented in the Don Braulio Formation, Villicum range at the Precordillera Oriental, by the occurrence of the nominal taxon (Peralta and Baldis, 1990; Peralta and Carter, 1999) as an exclusive component, as well as in the Precordillera Central, in the Talacasto area, at the base of the La Chilca Formation (Cuerda et al., 1988), and in the Cerro del Fuerte, at the Rio Escondido section (Benedetto et al., 1986; Benedetto, 1986; Rickards et al., 1996). The typical locality to the Hirnantian section, out crops in the Don Braulio creed, there Peralta and Baldis (1990) and Peralta (1993) recognized in the Don Braulio Formation four members: the lower member composed mainly by pebbly mudstones, with clasts scattered in a fine-grained matrix, and showing glacial features. This member unconformably overlays deposits of the La Cantera Formation, as well of the La Pola Formation, due to its erosive nature, and at the top is capped by a transgressive conglomerate level, which represents the base of the overlaying member, composed by fossiliferous mudstones, bearing brachipods of the Hirnantia Fauna (Benedetto, 1986), trilobites of the Dalmanitina Zone (Baldis and Blasco, 19759, and in the uppermost level, the mono-type N. persculptus assemblage (Peralta and Baldis, 1990). This deposits pass continuously towards the Ocher Member, made up of yellowish bioturbated mudstones, which might include the Ordovician-Silurian boundary. The upper member, is composed mainly of ironstones (ferriferous oolites) bearing palynomorphs (Volkheimer et al., 1980) and black shales interbedded which yield graptolites (Peralta, 1986). These faunas indicate an early Llandovery age; At the top, Don Braulio Formation is capped by an impressive erosional surface, owing to deposition of the Upper Silurian sedimentary mélange of the Rinconada Formation (Peralta, 1984; 1993).

The N. persculptus occurrence in Cuyania Terrane, are useful in the paleogeographic reconstruction

of the Western Gondwana, since it is closely related to glaci-marine deposits of the Hirnantian glacial event. Towards the north of the Precordillera Terrane, in the Cordillera Oriental of Salta and Jujuy provinces, similar Hirnantian faunas show the same stratigraphic relation with de glaci-marine deposits of the Zapla Formation (Peralta and Baldis, 1994), continuing in the Cordillera Oriental of Bolivia, up to Andean region of Perú (Cancañiri Formation). As it is known, this Hirnantian fauna and the glaci-marine diamictite deposits related, are widely distributed on Gondwana, and they mark in the Cuyania Terrane the beginning of the classically continuos late Ordovician to Devonian sedimentary cycle.

Other non-graptolite Upper Ordovician fossiliferous units

Although some Upper Ordovician units of the Cuyania Terrane, or parts of them, lack of graptolite fauna record, some of them contain shelly fauna, such as the correlative deposits of the Don Braulio Formation outcropping at the western flank of the Cerro La Chilca, Precordillera Central, which are composed of a basal coarse grained debris flow 3 m thick, covered by graygreenish fine-grained sequence, 12 m thick, bearing Hirnantian brachiopod faunas and trilobites of the Dalmanitina Zone (Baldis, 1995), that has been correlated with the Don Braulio Formation, in Precordillera OrientaL (Astini and Benedetto, 1992; Peralta and Baldis, 1992). At the top, this unit is capped by the basal cherty pebble conglomerate of the underlying La Chilca Formation of, probably, late Ashgillian (Hirnantian)-early Llandovery age, taking into account the age of the equivalent conglomerate level in the Talacasto river area, established by graptolite faunas (Cuerda et al., 1982; 1988) and palynomorphs (Melendi and Volkeheimer, 1982). The lower boundary is represented by an erosive surface at the base of the debris flow deposits, evolved on the black shales of the Los Azules Formation. This basal unconformity could be correlated with that recognized at the base of the Don Braulio Formation in the Sierra de Villicúm, such as has been suggested by Astini and Benedetto (1992), and Peralta and Baldis (1992), but as yet not sedimentological evidences, as glacial features, such as has been reported in the type section of the Don Braulio Formation (Peralta and Carter, 1990), support that suggestion.

On the other side, conodont faunas have been useful in the stratigraphic record of the Upper Ordovician sequences in the Precordillera terrane, such as in the Gualcamayo area, at the upper part of the Trapiche Formation, Benedetto and Herrera (1987) registered brachipods of the Hirnantia Fauna and trilobites of the Dalmanitina Fauna. However, the stratigraphic relationship whit the lower part of this unit, which yields Amorphognatus superbus remains still not well known. Besides, the Las Aguaditas Formation, at the Jáchal area, Central Precordillera, yields conodont faunas of the Pygodus anserinus and Amorphognathus tvaerensis (Keller et al., 1993; Lehnert, 1995a, b) associated with graptolite assemblages spanning Bircornis Zone. Further, the Sassito Formation (Astini and Cañas, 1995), at the San Juan River area, Precordillera Central, has provided a rich conodont fauna which indicate Middle-Upper Caradocian, and might have persisted even into the Ashgillian based on the occurrence of A. politus (Lehnert, 1995a, b). Likewise in Mendoza Precordillera, where in the upper member of the Empozada Formation, Heredia et al. (1990) have registered conodonts which age range from the upper Caradoc to, probably, lower Ashgill. This member ovelies unconformably the lower Member (considered in the C. bicornis and D. complanatus and D. ornatus Zones), and it is composed mainly of very fine grained sanstones and mudstones alternating, carbonate paraconglomerates and carbonate sandstones and mudstones alternating, showing hummocky cross-stratification, lenticular bedding slumping and load casts. This deposits are thought as platform facies (Gallardo et al., 1988). The unconformity at the base of this member has been related to the Ashgillian glacial event, "Discontinuidad S2 o Villicúmica", by Peralta (1994), and recently also by Keller (1999). Two conodont zones have been recognized from resedimented carbonate clasts in this part: Oepikodus evae and Amorphognathus superbus.

Farther to the south, in the San Rafael block, at the Ponón Trehue area, crops out the Lindero Formation, which is composed by two members: The lower, Peletay Member, yielding conodonts which age span in the Darriwillian, and the upper Los Leones Member, bearing conodonts of the Pygodus anserinus Zone, indicating a Caradoc age (Heredia, 1996; Bordonaro et al., 1996). In this unit, the Guandacol Phase shall be placed at the base of the Los Leones Member, in agree with the description given by Bordonaro et al. (1996).

Paleogeographic discussion

As well the Cambrian-early Ordovician bank of Cuyania Terrane, due to its faunal contents and lithostratigraphic features, has been used as a strong evidence for a Laurentian affinities, in the same way the Upper Ordovician sequences of this terrane could be used as evidence of unconnection among them, taking into account that graptolites of the N. gracilis and C. bicornis Zones appears exclusively in the Cuyania Terrane, but not on other place of South America (Western Gondwana).

That is so, despite some authors have included in the Upper Ordovician (except Ashgillian) some lithostratigraphic units although no paleontological nor radiometric data has been done. This is the case of the Anzaldo Formation in the Cordillera Oriental of Bolivia, and the Centinela Formation in the Sierras Subandinas of Argentina. On the other side, it is clearly demonstrated that in the Famatina System, there are not evidences of Upper Ordovician sedimentary rocks, except the extensive granitic bodies, of Upper Ordovician to, probably, Silurian ages, intruding the meta-sedimentary and meta-volcanic Upper Precambrian-Lower Precambrian complex correlative to Puncoviscana Formation, and the volcanic-sedimentary early Ordovician rocks developed in a volcanic arc setting (Aceñolaza and Toselli, 1988). In this way, we assume that the Famatina System (Orogeno Famatiniano in the sense of Aceñolaza and Toselli, 1998) could have been a positive relief during the Upper Ordovician as well as in the Silurian and Devonian. A similar interpretation is assumed to Puna Oriental and Cordillera Oriental of North West of Argentina, taking into account the similar geotectonic evolution of these areas, from the Upper Precambrian up to early Ordovician.

The occurrence of N. gracilis Zone as an exclusive paleobiologic component of Cuyania Terrane, disagree with an allochtonous origin for Precordillera, as a micro-continent rifted from Laurentia in the early Cambrian, drifting during the early Ordovician and colliding and docking on the western margin of Gondwana in the middle Ordovician (Upper Arenig-Llanvirnian) (Astini et al., 1995; Thomas and Astini, 1996; Rapallini and Astini, 1998, Rapallini et al., 1999). As well, is known that the graptolite faunas of the N. gracilis Zone show Pacific affinities, like coeval faunas of Laurentia, which indicate that Precordillera at least during the Upper Ordovician was close of Laurentia (Finney and Peralta, 1999, Finney, et al., 2002; 2003). These authors provide strong evidences that sedimentary supply to the Cambrian and early Ordovician basins in Precordillera, have been the cratonic areas of Gondwana and not the Laurentian basements, such as has been interpreted by other authors (Ramos et al., 1986; Astini et al., 1995; 1996; Thomas and Astini, 1997, Benedetto, 1998, Benedetto et al., 1999, among others). This gondwanian provenance not associated with any known part of Laurentia, has been provided by the U/Pb in Zircons detrital grains sampled in the Cambrian and Ordovician units of the Precordillera (Finney et al., 2002; 2003). Additional data have been provided from Neodymium-graptolite stratigraphy of Upper Ordovician shales from the Precordillera (Gleason et al., 2001), in the discussion of the Laurentian or Gondwanian source to that deposits. The zircon ages and the occurrence of graptolites belonging to the N. gracilis Zone, fit bet into a geodynamic model in which the transcurrence mechanism can explain satisfactorily the affinities between Precordillera Terrane and Laurentia, and the Gondwanian signature of the Precordillera basements.

In agree with the data provided by the graptolites assemblages of the N. gracilis and C. bicornis Zones, strong evidence arises concerning the diachronous character of the several units of the upper Ordovician, mainly taking into account the distribution and age of the graptolite faunas contents, and the stratigraphic analysis of their upper and lower boundaries. In fact, this point has not been considered adequately in previous contributions concerning this matter, then, the psefitic sequence of the Las Vacas Formation, which includes the Las Plantas Member, and the basal conglomerate of the La Cantera Formation, have been considered erroneously coevals. The biostratigraphic evidence indicates that the erosional surface at the base of the Las Vacas Formation is younger than that of the La Cantera Formation. On the other side, comparing the graptolite fauna of the later unit with that of the Los Azules Formation, cerro La Chilca, and Las Aguaditas Formation at Los Blanquitos and Las Chacritas section, we assume that the faunas of these units, are younger than that of the La Cantera Formation. Matching with the Precordillera Occidental units, a recent study allow to suggest that the graptolite fauna of the Portezuelo del Tontal is younger that that of the La Cantera Formation, and compare the graptolite fauna of the Yerba Loca Formation, at the rio Jáchal section, with that of the Los Azules Formation at cerro La Chilca section.

In this way, an erosive diachronous overlapping can be traced from the Las Vacas to La Cantera Formation, which indicates high complexity in the structural framework of the Eastern and Central Precordillera and, consequently, over all Precordillera terrane, taking into account that the age of the para-conformity at the base of the Los Azules Formation at its type-locality, is non-coeval to that recognized at the base of the Las Aguaditas Formation, and Los Azules Formations and Cerro La Chilca. Astini (1997) explained, in part, this complex Ordovician architecture as result of the Middle to Late Ordovician widespread extension, related to post-collisional disruption and major change in plate motions following collision. Diachronous character of both, Las Vacas and La Cantera Formation is not considered for this author in its model discussion. On the other side, since a new constrained age and paleoenvironment to Portezuelo del Tontal Formation have been done (Basilici et al., in review), then, a critical re-evaluation of widely held assumptions regarding the geotectonic and paleogeographic evolution of Cuyania Terrane is required.

The biostratigraphic data provided in this contribution agree with that interpretation of Mitchell et al. (1997) respect to the approach of the Precordillera terrane to low latitude (Atlantic Province), coming from high latitude area (Pacific Province). But, this migration of the Precordillera Terrane, only can be coherent with the isotopic studies on Zircon detrital grains from Las Vacas and Cerro Totora Formation carried out by Finney et al. (2002, 2003), which indicates Gondwana affinities, if a continent-parallel movement model is out-lined, such as has been suggested by Baldis et al. (1989), Aceñolaza and Toselli (2000) and Aceñolaza et al. (2000, 2002). In this way, the progressive influence of the typical faunas proper from the Atlantic Province or Realm, and extensional tectonic event in the Lower Caradocian, can be explained by mean of the migration of CuyaniaTerrane along of transcurrent (shear) zone, as much has been suggested by the mentioned authors. Also, it is noteworthy that the Upper Ordovician graptolite faunas of the N. gracilis from Precordillera, show strong affinities with the coeval Alabama basin, in the Appalachian margin of Laurentia (Finney et al., 1996), bearing several common elements. In fact, these evidence support a strong paleogeographic conection between Laurentia and Precordillera on Western Gondwana, at least up to C.bicornis Zone.

Conclusions

On the basis of biostratigraphic analysis, stratigraphic distribution of the Upper Ordovician units in the Precordillera Terrane, indeed, radiometric ages (U/Pb) obtained from Zircon detrital grain, the following conclusions rise:

- Graptolites faunas of the N. gracilis are only recognized in the San Juan Precordillera Geological Province; they occur in the Central Precordillera, in the Las Aguaditas Formation (Los Blanquitos and Las Chacritas sections) and in Los Azules Formation at La Chilca hill section, also in the La Cantera Formation in the Don Braulio creek, at Sierra de Villicum, Precordillera Oriental, and in the Yerba Loca and Portezuelo del Tontal Formations at the Precordillera Occidental.

- Graptolites fauna of the N. gracilis Zone indicates that those faunas of the Los Azules, Las Aguaditas and Yerba Loca Formations, are younger than those of La Cantera and Portezuelo del Tontal Formations, but also, the late is younger that that La Cantera Formation. - Graptolite faunas of the C. bicornis Zone are widespread in the Cuyania Terrane, and occurs in several unit of the Precordillera Geological Province, and also, in the San Rafael Block. However, despite of the biostratigraphical constraints, it is not possible to discern a stratigraphic framework to the graptolite-bearer units, in the same way, has been done on the basis of the N. gracilis graptolite faunas distribution.

- The westward facies change, to the Upper Ordovician sequences in Precordillera Geological Provinces, it is not possible to be correlated with compositional changes in the C. bicornis Zone, in that direction, owing to the poor record of graptolite faunas in the Precordillera Oriental, mainly in Cabeceras and Cortaderas Formations.

- Records of the late Ordovician graptolite faunas, are well constrained from the Alcaparrosa Formation, at Western Precordillera, with graptolites of the N. extraordinarius Zone, likewise in the Empozada Formation, Eastern Belt of the Mendoza Precordillera, with graptolites of the D. complanatus and D. ornatus Zones. The uppermost Ordovician graptolite fauna recognized in the Precordillera Terrane, occurs in the Don Braulio Formation, at the Precordillera Oriental, and in the Uppermost Ordovician-Lower Silurian unnamed unit outcropping in the Rio Escondido creek, at the Precordillera Central.

- Stratigraphic evidences indicates that mono-type N. persulptus fauna occurs after the Hirnantian glacial event, and, in like way, N. extraordinarius, D. complanatus and D. ornatus Zones, might be related to the Hiranantian glacial event.

- Record of the diamictite glaci-marine deposits in Precordillera Terrane, under-laying fossiliferous self deposits containing Hirnantian brachiopod faunas and trilobites of the Dalmanitina Fauna, associated with the mono-type N. persulptus fauna, are restricted to the Precordillera Oriental, and possibly, also to the Precordillera Central.

- On the basis of litho and paleobiological evidence, the basal deposits of the Don Braulio Formation and equivalent units, indicate clearly that, in the Late Ordovician (Ashgillian) Precordillera terrane had reached a high latitude emplacement, but no close of the Famatina System still.

- Based on the Upper Ordovician graptolites fauna analysis, and isotopic data, U-Pb on Zircon grains from Lower Cambrian and Upper Ordovician rocks of Precordillera, a strong paleogeographic and paleobiologic affinity at low latitude between Precordillera (Western Gondwana) and Laurentia is suggested, at least up to C. bicornis time.

- Taking into account these affinities and the Gondwanian provenance to the Cambrian and Ordovician rocks of the Precordillera, a parallel-continent movement is suggested to explain such affinities and the Gondwanian provenance. In this way, a para-autochtonous model seems to be more suitable rather than an allochtonhous model, to explain the paleogeographic evolution of Precordillera at that time.

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Recibido: 12 de diciembre de 2002

Aceptado: 2 de febrero de 2003