Susana B. ESTEBAN

E-mail: insugeo@unt.edu.ar

Abstract: THE EARLY ORDOVICIAN IN THE SISTEMA DE FAMATINA: STRATIGRAPHIC AND GEOTECTONIC FRAMEWORK. The Ordovician rocks of the Sistema de Famatina comprise late Cambrian to Llanvirnian? fossiliferous epiclastic and volcaniclastic deposits and several intrusive bodies related to an active margin environment. The analysis of the early Ordovician rocks provides information concerning the western margin of Gondwana previous to the development of a calc-alkaline volcanism and to the formation of a back-arc basin. The early Ordovician deposits, traditionally enclosed under the denomination of Volcancito Formation, are characterized mostly by epiclastic sediments where no features of volcanic influence is observed. Detailed stratigraphic and sedimentologic studies have documented the diversity of the early Ordovician deposits, that has lead to a divition of the same in two different lithostratigraphical units. The Volcancito Formation name is restricted, as defined originally, to the deposits outcropping in the Rio Volcancito – Peña Negra region. This unit comprises pelites, marls and carbonatic sandstones with fauna of the Parabolina (N.) frequens argentina Biozone and conodonts of the Cordylodus proavus, C. "intermedius", C. lindstromi, Iapetognathus and Cordylodus angulatus biozones (Late Cambrian – earliest Tremadocian) and black shales with graptolites of the Rhabdinopora f. anglica and Anisograptus matanensis biozones (mid to late Lower Tremadocian). In this paper the early Ordovician rocks of the Bordo Atravesado region is defined as a new lithostratigraphic unit named as Bordo Atravesado Formation (nom. nov.). This new unit comprises clayey siltstones, silty sandstones and calcareous sandstones bearers of a scantly diverse fauna of conodonts of the Paltodus deltifer Biozone (Late Tremadocian) and a trilobite association corresponding to the cyclopygid biofacies, which has been assigned to the latest Tremadocian. The paleontological information of the early Ordovician deposits allow to reaffirm the posture for a genuinely Gondwanic origin for the Famatina region during the above mentioned time lapse, in contrast with the ideas that suggest that this region may represent a terrane independent from Gondwana.

Resumen: EL ORDOVÍCICO TEMPRANO EN EL SISTEMA DE FAMATINA: MARCO ESTRATIGRÁFICO Y GEOTECTÓNICO. Las rocas ordovícicas del Sistema de Famatina comprenden depósitos epiclásticos y volcaniclásticos fosilíferos del Cámbrico tardío al Llanvirniano? y varios intrusivos relacionados a un ambiente de margen activo. El análisis de las rocas del Ordovícico basal provee información acerca del margen occidental del Gondwana previo al desarrollo de un volcanismo calcoalcalino y a la génesis de una cuenca de retroarco. Los depósitos correspondientes al Ordovícico basal, englobados tradicionalmente bajo la denominación de Formación Volcancito, se caracterizan por presentar sedimentos mayormente epiclásticos en donde no se observan rasgos de influencia volcánica. Estudios estratigráficos y sedimentológicos detallados han documentado la diversidad de los depósitos del Ordovícico basal, a los cuales se propone dividirlos en dos unidades litoestratigráficas diferentes. Se restringe el nombre de Formación Volcancito, como fuera definida originalmente, a los depósitos aflorantes en la región de Río Volcancito - Peña Negra. Dicha unidad comprende a las pelitas, margas y areniscas carbonáticas con fauna de la Biozona de Parabolina (N.) frequens argentina y conodontes de las Biozonas de Cordylodus proavus, C. "intermedius", C. lindstromi, Iapetognathus y Cordylodus angulatus (Cámbrico tardío - Tremadociano temprano) y las lutitas negras con graptolitos de las biozonas de Rhabdinopora f. anglica y Anisograptus matanensis (Tremadociano Inferior, mediotardío).

En tanto que se define a las rocas del Ordovícico basal de la región de Bordo Atravesado, como una nueva unidad litoestratigráfica a la cual se propone denominarla Formación Bordo Atravesado. Esta nueva unidad comprende limolitas arcillosas, areniscas limosas y areniscas calcáreas portadoras de una fauna poco diversa de conodontes de la Biozona de Paltodus deltifer (Tremadociano Superior) y una asociación de trilobites correspondiente a la biofacies de ciclopígidos, la cual ha sido asignada al Tremadociano Superior tardío. La información paleontológica de los depósitos del Ordovícico basal permiten reafirmar la postura de un origen genuinamente gondwánico para la región de Famatina en dicho lapso de tiempo, en contraposición de las ideas que sugieren que dicha región puede representar un terreno independiente al Gondwana.

Key words: Famatina, Early Ordovician. Stratigraphy. Geotectonic framework.

Palabras clave: Famatina. Ordovícico temprano. Estratigrafía. Marco geotectónico.

In the last decade, a series of sedimentological, stratigraphic and paleontological studies of the Ordovician sequences of Famatina have been carried out (Aceñolaza and Esteban, 1996; Aceñolaza and Rábano, 1990; Aceñolaza and Gutiérrez-Marco, 2000; Albanesi and Astini, 2000; Albanesi and Vaccari, 1994; Albanesi et al., 1999, 2000a, b; Astini,1998, 1999a, b, 2001a, b; Astini and Benedetto, 1996; Astini and Dávila, 2000, 2002; Benedetto, 1994, 1998; Clemens, 1993; Clemens and Miller, 1996; Esteban, 1992, 1993, 1994, 1996; Esteban and Gutiérrez-Marco, 1997; Esteban and Rigby, 1998; Mángano and Buatois, 1990a, b, 1992a, b, c, 1994 a, b, 1996a, b, 1997; Mángano et al., 1996; Rubinstein and Astini, 2000; Sánchez, 2001; Sánchez and Babin, 1994; Toro, 1997, 1999; Toro and Brussa, 1997; Tortello and Esteban, 1995, 1997, 1998, 1999; Vaccari et al., 1993; Vaccari and Waisfeld, 1994; Zimmermann and Esteban, 2002). Also, the nature of the Famatinian Ordovician igneous rocks and their geochemistry have been studied by many authors (Toselli et al., 1990, 1993, 1996; Cisterna, 1992, 2000, 2001; Cisterna and Toselli, 1996; Mannheim, 1993; Mannheim and Miller, 1996).

The oldest levels of the Ordovician basin of Famatina correspond to the Volcancito Formation (Late Cambrian-Tremadocian), defined by Harrington (In: Harrington and Leanza, 1957) in the region of the homonymous river, on the eastern flank of the Sierra de Famatina (La Rioja province).

This author characterized this unit as integrated by: 1- "dark green and dark greenish-blue, thinly bedded, silty shales, in places some what marly, with abundant intercalations of dark blue marls and marly limestones", these beds yield abundant remains of trilobites and brachiopods and 2- bluish-black to dark greenish-blue, thinly laminated shales with graptolites".

The lower deposits (= Filo Azul member, Esteban 1999) are composed of pelites and marls with interbedded carbonatic sandstones and laminated siltstones. The mineralogical composition of the rocks is dominated by quartz and calcite with subordinate plagioclase (albite), muscovite, chlorite, kaolinite and illite. Detritical volcanic material has not been observed in these rocks (Zimmermann and Esteban, 2002). Faunistically, the deposits of Volcancito River contain trilobites belonging to the Parabolina (Neoparabolina) frequens argentina Biozone (Harrington and Leanza, 1957).

Using new collections, Tortello and Esteban (1997) assigned the basal parts of the Río Volcancito sections to the Upper Cambrian. The discovery in the upper parts of an association of trilobites and graptolites integrated by Jujuyaspis keideli trilobite and graptolites of the Rhabdinopora flabelliformis (R. f. cf. socialis and R. f. cf. norvegica) group have permitted to assign this part to the early Tremadocian (Tortello and Esteban, 1998, 1999). The presence of trilobites diagnostic of the uppermost Cambrian in the lower part and graptolites and trilobites (upper sector) characteristic of the earliest Ordovician makes the "Filo Azul member" section a biostratigraphical referent of regional importance for the Cambro-Ordovician transition (Tortello and Esteban, 1998, 1999).

A more precise approximation of the Cambro-Ordovician boundary has been done by means of conodont fauna in the Río Volcancito section, where a succession of five biozones can be distinguished: Cordylodus proavus, C. "intermedius", C. lindstromi, Iapetognathus and Cordylodus angulatus (Albanesi et al., 1999a). The approximate determination of the boundary in this section is given by the first appearance of the conodont Iapetognathus aengensis, which occurs at 85 m from the base (Albanesi et al., 1999a).

Recently, Astini (2001a) has described calcareous algae (Nuia y Girvanella) in different levels of the Río Volcancito section, between the biozones of Cordylodus proavus and Cordylodus angulatus.

Esteban, 2002). The black shales contain an abundant fauna of graptolites, accompanied only in a few levels of the profile by rests of hexactinellid sponges limited to spicules and some fragments of net (Esteban and Rigby, 1998). Dendroid graptolites represent the dominant fauna of these levels, both juvenile and adult, with R. flabelliformis flabelliformis, R. f. socialis and R. f. anglica. This fauna, especially in the Peña Negra section, is associated in numerous levels with anisograptids, which follow Rhabdinopora in abundance. Based on the graptofauna, these rocks are assigned to the mid to late Lower Tremadocian (Cressagiense) (Esteban and Gutierrez-Marco, 1997).

The Ordovician of this region crop out in the Portezuelo de la Alumbrera Creek, 2 km north of Bordo Atravesado locality (Cuesta de Miranda). Based on the discovery of Lower Tremadocian trilobites, these rocks were assigned to the Volcancito Formation by Alderete (1968). This assignation was maintained in the different investigations realized in the region, although Esteban (1999) indicated that they show lithological and faunistical differences with the type locality and proposed its inclusion as a different member from the already considered for the Volcancito Formation (= Figure 1. Location map of the Sistema de Famatina (A) and the Rio Volcancio-Peña Negra (B) and Bordo Atravesado (C) regions (La Rioja province, western Argentina).

Bordo Atravesado member of Esteban, 1999). In this paper, the eariest Ordovician rocks of the Bordo Atravesado region are defined as a new lithostratigraphic unit which we propose to name Bordo Atravesado Formation.

- The Bordo Atravesado Formation (nov. nom.) is formed by a 160 m thick sedimentary sequence of fine grained and dark colored siliciclastic rocks, affected by very low grade metamorphism.

This unit does not constitute a continuous strip but instead is characterized by small hills with discontinuous outcrops, which are intruded by several porfiric bodies of riolitic-dacitic composition assigned to the Middle Ordovician (Toselli et al., 1996).

Portezuelo de La Alumbrera Creek, located on the eastern flank of Filo Aspero, is considered the type locality of this unit. The type section is defined in the mid section of this valley and on its right margin, where the most complete stratigraphical sucesion is exposed.

Underlying rocks are represented by Tertiary andesites tectonically related (Alderete, 1968), while its upper boundary display unconformable Neopaleozoic sedimentites of the Paganzo Group.

In the type section four sedimentary facies have been recognized: massive or laminated clayey silstones (A), very fine sandstone/siltstone couplets (B), hummocky cross-stratified calcareous sandstones and mudstones (C), and massive silty sandstones (D) (Fig. 2).

Layers of this facies are tabular and sharp-based. Beds commonly are very thin, with thickness up to 20 cm. Although they are generally massive or thinly laminated, some beds show a micro-graded stratification. In thin section the lamination shows dark organic-rich laminae alternated with lighter colored quartz-rich silty laminae. Some levels of this facies are highly silicificate and other present calcareous concretions.

Fossils, mainly cyclopygid trilobites, are found at two levels in the middle to upper part of the sequence. The trilobite association is represented by pelagic and benthic forms as Corrugatagnostus sp., Degamella famatinensis Esteban, Prospectatrix sp., Sagavia sp. nov. A, Illaenopsis sp. nov. B, Hapalopleura aff. longicornis, Proteuloma sp. nov. C, Shumardia sp. and Bienvillia sp. (Esteban, 1996; Tortello and Esteban, in press). Phyllocarid crustaceans (Caryocaris bodenbenderi Aceñolaza and Esteban) occur within this facies (Aceñolaza and Esteban, 1996). Ocassionally, within greenish levels, display scarce bioturbation with Trichichnus (Esteban, 1993), Gyrophyllites and Tomaculum (Esteban, 2001).

This facies mostly represents sedimentation from suspension fall-out in a very low energy setting below storm wave base. Presence of trilobites mainly pelagic indicate deep water shelf conditions. Rare micro-graded beds represent isolated turbiditic events. Dilute turbidity currents have been considered responsible for lamination in shales (O’ Brien et al., 1980; O’ Brien, 1996).

The preservation of thin lamination, the scarce bioturbation and the dark colour of these sediments suggests restricted conditions attributed to anoxic or dysoxic bottoms (Sageman et al., 1991; Bottjer and Savrda, 1993; Allison et al., 1995). Facies distribution studies of Trichichnus in turbidites of the Marnoso-arenacea Formation (Miocene, Northern Apennines) have demonstrated that Trichichnus-making animals had a great tolerance for low oxygen levels (McBride and Picard, 1991).

This facies is represented by green-black to dark grey-light grey very fine-grained sandstonesiltstone couplets. Bed thickness is 3 - 10 cm, with sharp upper and lower boundaries and tabular geometry. Some units exhibit normal grading towards the base of the deposit. Each lamina of the couplet is commonly from 3 to 7 mm in thickness. Amalgamation of beds is quite uncommon, each bed being separated by a facies A siltstone interval.

Facies B partially resembles facies C2.3 of Pickering et al. (1986) (Table II). According to these authors, thin-bedded sand-mud couplets like those of the Bordo Atravesado region have been

deposited rapidly from low-concentration sandy-silty turbidity currents. The presence of sharp bases and normal grading indicates rapid influx of sediment, most likely by episodic, short-lived turbidity currents capable of transports sand far away of the nearshore. Such deposits typically occur in deep water successions, but graded beds also have been reported from shelf settings, interpreted as storm-induced turbidites (Hamblin and Walker, 1979; Brenchley, 1985).

This facies is characterized by thin beds, 5 cm thick, of light brown calcareous very fine-grained sandstone to coarse-grained siltstone. Beds typically have parallel lamination at the base and microhummocky cross-stratification toward the top, representing the P (lower planar laminated) and H (hummocky cross-stratified) divisions of the "type bed" of Walker et al. (1983). The basal massive zone (B) is commonly absent. Bases are sharp. Tops commonly exhibit symetrical to quasisymetrical undulations. Fossils, mainly parautochthonous inarticulate brachiopods (disarticulated valves) are dispersed within the calcareous sandstones and siltstones at some levels. Scarce and moderately preserved conodont fauna have been found in this facies to 25 m of the sequence base.

Recent studies indicate that this material correspond to Paltodus deltifer Zone of latest Tremadocian age (Albanesi et al., 2000b).

Facies C is interpreted as storm deposit. Storm generated currents are the main agents by which sand is carried out onto shelves. In shelf areas above storm wave base it is reworked by storm waves to form hummocky cross-stratification (Harms et al., 1975; Hamblin and Walker, 1979; Dott and Bourgeois, 1982; Brenchley, 1985; 1989). The presence of P zone in the lower part of the studied beds, and ocasionally the B zone, is considered to reflect the first phase of the formation of a bed which emplaces the sand on the shelf. Hummocky cross-stratification in the top of the beds (H zone) reflect the second phase when the sand is reworked by waves. Brenchley et al. (1986) have indicated that differences in hummocky cross-stratification facies can be explained in terms of proximal- distal trends respect from shore. Thin, non-amalgamated beds of very fine sandstone and coarse siltstone such as those described herein have been regarded as distal tampestites (Brenchley et al., 1986).

This facies comprises dark grey massive silty sandstone. Bed thickness is 10 - 20 cm, with sharp upper to lower boundaries, and tabular geometry. These beds are interbedded with finer sediment (Facies A) in the lower and upper part of the succession and occasionally are associated with facies B beds. Soft-sediment deformation structures can occasionally be detected, pseudonodules of 7-10 cm large and internally laminated have been seen in some levels. However most facies D beds show no internal sedimentary structures. Bioturbation is absent.

Lack of lamination may reflect conditions of deposition or it may be the result of destruction of original lamination (Collinson and Thompson, 1989). Absence of bioturbation by organisms living within the sediment indicate that this facies may be the result of a rapid deposition. Facies D is thought to record deposition from fluidized or liquefied sediment gravity flows (Pickering et al., 1989). Lack water-escape structures and the presence of pseudonodulos internally laminated implies that the sand was not totally liquefied.

Thin-bedded very fine sandstone/siltstone couplets (Facies B), massive silty sandstones (Facies D) and massive or laminated clayey silstones (Facies A) are grouped as facies assemblage 1. It is present in great part of the Bordo Atravesado Formation (Figure 2).

Facies assemblage 1 is marked by a clear predominance of fine-grained strata. Facies A clayey siltstones carrying a trilobite-dominated faunal assemblage are predominant, with sandstone/ siltstone couplets (Facies B) and massive silty sandstones (Facies D) locally interbedded. This arrangement suggest that suspension fall-out in relatively deep shelf waters was punctuated by ocassional event sedimentation, recording storm-induced turbidity currents and gravity flows. No physical structures indicative of wave reworking were detected. Accordingly, both sedimentologic and paleoecologic data suggest that facies assemblage 1 records deposition below storm wave base in an outer shelf setting.

The dominance of dark thinly laminated sediments in this assemblage, the presence of planktonic organisms (cyclopygids and phyllocarids) and the pausicity of benthic biota suggest deposition in oxygen-depleted environments. However, green levels of facies A showing scarce bioturbation (Trichichnus, Gyrophyllites, Tomaculum) can be indicate as more oxigenated intervals (dysoxic facies) in the Bordo Atravesado succession.

Facies assemblage 2 includes massive or laminated clayey siltstones (Facies A) and hummocky cross-stratified calcareous sandstones and mudstones (Facies C). This assemblage occur at both the lower and upper part of the sequence studied here (Figure 2). The package at the lower part is approximately 10 m thick, whereas the thickness of the upper package is 20 m. The most distinctive feature of facies assemblage 2 is the presence of hummocky cross-stratified calcareous sandstone and siltstone beds, that evidence storm action. Thin, non-amalgamated very fine-grained storm beds are regarded as distal tempestites. Accordingly, we suggest that facies assemblage 2 records the alternation of suspension fall-out periods and brief episodes of storm in a middle shelf setting above storm wave base.

The older papers referring to the Ordovician sedimentites of the Portezuelo de La Alumbrera Creek mention the discovery of few fragmentary trilobites, which were referred to the early Tremadocian (De Alba 1956, 1979; Alderete 1968). During the last decade, additional paleontologic material has provided more biostratigraphical information: the record of the cyclopygid trilobite Degamella Marek (see Esteban 1996, 1999), Plesioparabolina Harrington and Leanza (Esteban 1992), ?Corrugatagnostus Kobayashi (Tortello and Esteban, 1995) and numerous phyllocarids (Aceñolaza and Esteban, 1996) have suggested a younger age than early Tremadocian (Late Tremadocian - ?earliest Arenigian).

The discovery of conodonts near the base of the section, corresponding to the Paltodus deltifer Biozone, has permitted to narrow the age restricting it to the Upper Tremadocian (Albanesi et al., 1999b). Using a more precise determination of the outcrop ages of the Portezuelo de La Alumbrera Creek, Tortello and Esteban (in press) have postulated that the trilobite association of the upper section of the secuence would correspond to the earliest cyclopygid biofacies of the record.

The Bordo Atravesado Formation can be correlated with other units of the Upper Tremadocian of Precordillera, Bloque San Rafael and Cordillera Oriental where the presence of the Paltodus deltifer Biozone has been recorded.

While studying the evolution of the Sistema de Famatina, some authors referred to the geotectonic setting of the Ordovician, for which they considered a deposition in a back-arc basin associated to a continental margin arc (Clemens, 1993; Mannheim, 1993; Astini and Benedetto, 1996). On the other hand, while analyzing the Arenigian-Llanvirnian lapse in the Famatina basin, Aceñolaza and Toselli (1984, 1986 and 1988) interpreted the volcanic activity during this period as proper of a magmatic arc and the associated plutonism as characteristic of a back-arc.

Although there exists a general consensus between the different authors that the Ordovician sediments of Famatina would have deposited in basins associated with a subduction margin, no agreement yet exists concerning the types of basins involved. Clemens (1993) and Mannheim (1993) the Famatina basin is a back-arc basin. A basin of this type was assumed in other reconstructions such as those postulated by Benedetto and Astini (1993), Astini et al. (1995) and Astini and Benedetto (1996). Mángano and Buatois (1992a, 1996b, 1997) mention the development of distentional or transtentional phenomena within the arc, which gave place to small graben-style basins that controlled the emplacement of stratovolcanoes and calderas. Although the suggestion of a magmatic arc in the Sierra de Fiambalá (Grissom et al., 1991) is difficult to adjust due to the presence of arc roots in the Sistema de Famatina (Toselli et al., 1990; 1993; 1996; Pankhurst et al., 1998), a possible alternative was postulated by Mángano y Buatois (1996c) suggesting the existence of two different arcs coinciding with Sierras Pampeanas (to the east) and the Sistema de Famatina (to the west), separated by a marine basin. A similar model was presented by Astini and Benedetto (1996), where two hypothesis concerning the possible locations of the Famatinian arc are considered.

The existence of a volcanic arc before the Arenigian in the Famatina region, is still a matter of debate (Upper Cambrian – Tremadocian).

The geotectonical significance of Volcancito Formation is a much more complex question than the significance of the younger Ordovician units which have a clear volcanic influence. Although some authors interpret the presence of a volcanic arc already during the earliest Ordovician in the Famatinan region (Clemens, 1993; Mannheim, 1993: Benedetto and Astini, 1993 and Astini et al., 1995), the evidence for this is not altogether conclusive.

Though Pankhurst et al. (1998) confirmed a Tremadocian magmatic activity (SHRIMP data 490 +/- 5 Ma) associated to a subduction zone on the protoandean Gondwanic margin, recent geochemical studies carried out on the earliest Ordovician sedimentites of Famatina (Volcancito and Bordo Atravesado formations) have permitted to determine that these rocks have a UCC (Upper Continental Crust) composition and show an active continental margin signature. Neither fragments indicating a mafic or volcanic source, nor typical geochemical trends for subduction related volcanism could be found (Zimmermann and Esteban, 2002).

From a paleogeographic point of view, the Famatina has been recently postulated as a independent terrane from Gondwana, accreted to the protoandean margin before the formation and collision of the Precordillera (Astini et al., 1995; Ramos 1999, 2000). This hypothesis is sustained by biogeographic data, specially the characterization of several fossil associations interpreted as of warm to template waters whose affinities would imply connections with the faunas of Precordillera and Laurentia (e.g. Benedetto, 1998) and the recent discovery of calcareous algae (Nuia y Girvanella) in the "lower member" of Volcancito Formation indicative of a warm water deposition (Astini and Dávila, 2000; Astini, 2001a, 2001b). This last finding has permitted to postulate for Famatina a low latitude position, at least during the late Cambrian – early Tremadocian (Astini, 2001a, b). The conodont associations of the Río Volcancito profile (= Filo Azul member of Esteban, 1999), however, do not include any of the characteristic elements of the Mid Continent province, proper of warm and shallow waters such as the epicratonic basins of Laurentia and the carbonatic facies of Precordillera (Albanesi et al., 1999a). On the contrary, they are comparable to those of other contemporaneous localities of the Cordillera Oriental (Lampazar and Cardonal Formations) assigned to the "transition" environments (Transitional Faunal Realm) between cold and warm waters along the Gondwanic margin (Albanesi et al.1999a, 2000). Also, the trilobite fauna of the above mentioned profile, assigned to the Parabolina (N.) frequens argentina Biozone shows a strong affinity with those of Lampazar Formation in the Cajas Range (Cordillera Oriental) (Tortello and Esteban, 1997, 1999) indicating fluid interchange between contemporary communities along the South American Gondwanic margin. Another faunistic evidence, concerning the position of the Famatina basin in the Gondwanic margin for the early Ordovician, is the presence of a genuine biofacies of cyclopygid trilobites in the Bordo Atravesado Formation (Tortello and Esteban, in press). During the early Ordovician, this kind of facies was confined to high latitudes in peripherical sites marginal to the Gondwana continent (Fortey and Owens, 1987).