Aspects of the Ordovician System in Argentina

Florencio G. ACEÑOLAZA

INSUGEO (CONICET- UNT), Miguel Lillo 205 - 4000 San Miguel de Tucumán, Argentina.

E-mail: ;


Abstract: ASPECTS OF THE ORDOVICIAN SYSTEM IN ARGENTINA. A synthesis on the different aspects characterizing the Ordovician rocks of Argentina is herein presented. Several localities defining the lower and upper boundaries of the System are presented, summarizing its most relevant geological aspects (stratigraphy and paleontology a.o.).

This integrated analysis is preformed focusing on the different regions of Argentina where Ordovician rocks are widely distributed as it happens in the Puna, Cordillera Oriental, Famatina System, Sierras Subandinas, Cuyania, Tandilia, Ventania and the Antarctic sector. The Ordovician sequences have been dated by means of the fossil fauna, and isotopic processing. Finally, the sedimentary, metamorphic and plutonic events are approached within the framework of the western margin of Gondwana.

Resumen: ASPECTOS SOBRE EL SISTEMA ORDOVÍCICO DE ARGENTINA. En esta obra se hace una sintesis sobre distintos aspectos que caracterizan las rocas ordovícicas que afloran en Argentina. En primer lugar se refiere a localidades donde pueden definirse los límites inferior y superior del Sistema para luego exponer sobre las características y distribución regional de las rocas sedimentarias, igneas y metamórficas que se originaron durante el tiempo ordovícico. El análisis se realiza por regiones destacándose no solo las propiedades de las rocas sino también la abundante fauna fósil que existe en los afloramientos de Puna, Cordillera Oriental, Famatina y Sierras Subandinas (Salta, Jujuy, Catamarca y La Rioja), como asimismo en Cuyania (San Juan, Mendoza y La Pampa) y las que constituyen las sierras de Buenos Aires (Tandilia-Ventania). Los aspectos cronológicos tienen una mayor certeza no solo por la información producida por la fauna fósil sino también los que se obtienen mediante métodos de geología isotópica. También se aborda la problemática de los acontecimientos sedimentarios, metamórficos y plutónicos con la evolución del borde occidental del Gondwana y especialmente por la interacción del mismo con la placa oceánica del Paleo-Pacífico.

Key words: Ordovician. Argentina. Stratigraphy. Regional geology. Paleogeography.

Palabras clave: Ordovícico. Argentina. Estratigrafía, Geología regional. Paleogeografía.

Early studies

Rocks assigned to the Ordovician System in Argentina are distinguished by their wide regional distribution and a fully developed stratigraphical column, from the Lower Tremadocian (Iapetognathus preaengensis/Rhabdinopora flabelliformis/Jujuyaspis keideli), to the Hirnantian (Hirnantia saggitifera).

Likewise, abundant radimetric values done over different crystalline rocks display ages varying from the 500 to the 440 Ma. Between both extremes the fitting of the adopted British scale for stages that integrate the System, allowed an usage of local stages that facilitates the interpretation of the regional stratigraphical sequence (Aceñolaza, 1992).

The knowledge of the Ordovician in Argentina is relevant after the early studies of D’Orbigny in the 19th century. This French naturalist, when studying the ancient rocks of Bolivia, describes one of the characteristic fossils of the period: Cruziana, honoring the president Santa Cruz. Years later Martín de Moussy, Alfred Stelzner, Hermann Burmeister, Emanuel Kayser and Luis Brackebusch provide new data on the stratigraphy, palaeontology and distribution of the so called "Primordial and Silurian rocks".

In the first decades of the 20th century, after the creation of the Servicio Geológico Argentino (Argentine Geological Service) an increase on the information of the Ordovician was rapidly noticed as a consequence of systematized studies. Keidel, Bodenbender, Penck, Harrington, Kobayashi, Loss, Harrington and Leanza, Cecioni, Turner, Borrello, Cuerda and Hünicken are mentioned among the pioneers. Those days were followed by a generation of geologists and palaeontologists in the ‘70s, continuing today with a fairly good number of young and promising geologists, whose ideas had strong incidence in newly born ideas about the stratigraphy, fossil faunas and paleogeography of the Ordovician. With the apparition of isotopic methods, new ideas and concepts were added to the metamorphic, plutonic, mineralogenetic and tectonic processes occurred in Argentina during the Ordovician.

Taking all that in account and in views of the 9th ISOS, we have decided to take ahead this synthesis work involving distinguish Argentinean and foreign specialists, who have done a substantial contribution to the knowledge of the geology and palaeontology of the Ordovician System of Argentina. Although we know that some of the topics that also deal with the matter were left out, we highlight that the present ones offer an adequate vision of the plutonic, metamorphic, sedimentary events and fossil faunas, in the Ordovician rocks of Argentina.

Definition of Sistem boundaries (lower and upper)

The Cambrian-Odovician boundary: Three localities up to the present have conceited the greater attention as regional stratotypes for the Cambrian - Ordovician boundary. Those localities display a regular stratigraphic succession with well constrained palaeontological data and isotopic studies.

Different papers have shown sequences outcropping at the Sierra de Cajas (Jujuy), Sierra de Famatina-Volcancito (La Rioja) and Cerro La Silla (San Juan) as the most suitable to define the lower boundary of the System.

In the Sierra de Cajas (Cordillera Oriental, Jujuy province) a succession of shales (Lampazar Formation) and quartzites (Cardonal Formation) bears Jujuyaspis keideli (Trilobita), Iapetognathus preaengensis (Conodonta) and Dictyonema parabola (Graptolithina) (Rao and Hünicken 1995; Ortega et al., 1999). The record of the Cambro-Ordovician boundary is marked by the FAD of these fossils included in a normal 300 m thick sedimentary sequence.

In the Rio Volcancito (Sierra de Famatina, La Rioja province), the stratigraphical column is represented by a normal sequence of 170 m thick of shales, black limestones and quartzites (Filo Azul Member). The transitional levels between the Systems are located in the upper sector of the lower member, with the occurrence of Jujuyaspis keideli, Rhabdinopora flabelliformis socialis, Rhabdinopora f. norveguica and Iapetognathus sp. (Tortello and Esteban, 1999; Albanesi et al., 1999; Esteban, 2002).

In the Cerro La Silla (Precordillera of San Juan province), the boundary is represented in a normally stratified sequence of carbonates with algal levels (microbial laminites, stromatolites), bearing Iapetognathus (Conodonta), Plethopeltis (Trilobita), some mollusks and trace fossils (Vaccari, 1994; Lehnert, 1995; Keller, 1999; Cañas 1999).

The Ordovician-Silurian boundary: The determination of the boundary between the Ordovician and the Silurian display a lesser precision. Though its existence has been recorded in some sectors of the Sierras Subandinas of Jujuy province (Zapla/Lipeón formations), in the Sierra de Villicúm, of San Juan Province (Don Braulio Formation) and the San Isidro locality in Mendoza province (Empozada Formation).

In the Sierra de Zapla/Puesto Viejo of Jujuy province, the upper boundary of the Ordovician System is represented in the Zapla Formation, with a succession of diamictites classically interpreted as of a glacial origin. Several fossiliferous levels have been described, with trilobites Dalmanitina (Dalmanitina) subandina, brachiopod and mollusks (Boso, 1999). The early mentioned strata is followed by yellowish sandstones with ferriferous levels containing Llandovery graptolites (Rickards et al., 2002). According to Monteros et al. (1993), the basal levels could be regarded as Asghilian with the presence of Climacograptus aff. normalis (Metaclimacograptus? sensu, Rickards et al., 2002).

In the Sierra de Villicum, San Juan province, the classic sequence of the Don Braulio Formation, integrated by a succession of sandstones, brecchias, conglomerates and shales, is regarded as of a glacial origin. This contains Dalmanitina sudamericana, brachiopods of the "Hirnantia saggitifera fauna" ( Peralta and Carter, 1990; Astini and Benedetto, 1992) and graptolites of the Normalograptus persculptus biozone (Peralta and Baldis, 1990; Albanesi and Ortega, 2002).

In San Isidro region, the upper Member of the Empozada Formation is integrated by a heterolithic succession, of about 120 meters, characterized by three lithofacies: shales, sandstones and brecchial conglomerates. Glacial related slumping is still an issue of discussion. In these levels, limestone clasts with Amorphognathus superbus were described. This would imply that the sedimentary process in the area would possibly reach up to the Asghill (Heredia 1995; Albanesi and Ortega, 2002).

Development of the sedimentary, magmatic and metamorphic events

Sedimentary events: Ordovician sedimentary strata are widely distributed in the entire Argentine territory, but it is mainly in the northwestern sector (Salta-Jujuy-Catamarca and La Rioja) where they display a wide stratigraphical development, reaching thickness up to 7000 meters. In the Precordillera the stratigraphic column display near 3000 meters, while in the southern Buenos Aires ranges, reach only few hundred meters.

In northwestern Argentina, the stratigraphic sequence includes great part of the stages referred to the British column (Tremadocian to Asghill), with a greater development of the Lower Ordovician.

These levels record a siliclastic interstratified eruptive event related to a fairly good paleontological record (e.g. Coira and Koukharsky, 2002; Moya, 2002).

On the other hand, in the Precordillera the Tremadocian-Arenig sequence is reprsented by a limestone platform, that towards the Middle and Upper Ordovician turns into siliclastic. Here as well as in northwestern Argentina, the sequence is highly fossiliferous defining the chronological succession.

In the southern Buenos Aires ranges of Tandilia and Ventania, highly ichnofossiliferous siliciclastic strata are displayed (Poiré et al., 2000 with references; Aceñolaza and Aceñolaza, 2002).

Different synthesis are included in this special volume, poriferans (Beresi and Esteban, 2002), graptolites and conodonts (Albanesi and Ortega, 2002; Heredia, 2002) trilobites (Tortello et al., 2002), mollusk (Sanchez, 2002; Aceñolaza and Beresi, 2002), brachiopods (Benedetto, 2002), echinoderms (Aceñolaza and Gutiérrez-Marco, 2002) and palinomorphs (Vergel et al., 2002).

Magmatic and metamorphic events: During the Ordovician the magmatic and metamorphic events are directly binded with the collision of the Proto-Pacific oceanic plate with the western edge of Gondwana. This process gave place, first, to the collapse of the "Dorsal Calchaquí" buildt during the Tilcarica orogeny (Middle Cambrian), and to the development of an important perigondwanan basin that surpasses in time the Ordovician System (Famatinian Orogen).

The plutonic, metamorphic and deformative phenomena affected mainly rocks that conformed the base of the Ordovician sedimentary succession (Puncoviscana Formation: Neoproterozoic-Cambrian).

Plutonism: Isotopic studies with their stratigraphic relation referred emplacements between 500 and 465 (Tremadocian-Arenig) My. Others include smaller bodies with radimetric ages around 450 My. (Caradoc), reaching up to the Silurian-Devonian (Pankhurst and Rapela,1998; Toselli et al., 2002;

Sato et al., 2002; Bonalumi and Baldo, 2002 and Tickyj et al., 2002). The first group are represented by the Tremadocian-Arenig emplacements of the Sierras Pampeanas (Ancasti, Velasco, Capillitas, Famatina, Sierras de San Luis and Córdoba) and Puna. These are vinculated to the initial stages of the Famatinian cycle. This plutons, since the ‘70s were subject of numerous radimetric dating applying the different methods of study (K/Ar, Rb/Sr, U/Pb, SHRIMP). While dispersion of data is observed in several cases, there was a notable coincidence in other, allowing their temporal identification. The most ancient ones have values that go from 507/470 My. in the Sierra de Quilmes, 479/469 My. in Velasco, 475 My. in Mazán, 470 My. in Capillitas, 472/73 My. in Tacuil; 482 My. in Chepes; 486/467 My. in Sierra La Huerta; 467/484 My. in Ñuñorco (Famatina); 485 My. in Las Angosturas; 490 My. in Pocitos, 485 My. in Archibarca, 469 My. in Taca-Taca and 482 My. in Petaquitas.

It must be highligthed that the pluton of Las Angosturas-Narvaez (Famatina) intrude fossiliferous levels with Arenig graptolites (Rubiolo et al., 2002). In the Sierras de Cordoba, Lower Ordovician plutonism is dated between 499 My. (Trondhemite Güiraldes), 494 My. (Rhyolites Oncan), 474 My. (Tonalites of La Fronda and Paso del Carmen) and 478 My. (Granodiorite Charquina) (Bonalumi and Baldo, 2002, Quenardelle and Ramos,1999).

The post-Llanvirn plutonism have radimetric ages below 460 My., while an ample geographic dispersion can be quoted in the plutons of Chasquivil (456 My.); Cuchiyaco (446 My.), Chango Real (448 My.) and El Peñon (452 My.).

Volcanism: The volcanogenic events occurred during the Ordovician are varied. Some of them are related to the mentioned plutonic phenomena, while others are binded with events occurred in the active continental margin.

The first ones, of acid to mesosiliceous composition, configured  activities proper to arc and backarc settings, with development of aerial volcanic buildings; while the second ones, of basic and ultrabasic composition are greatly restricted to the Precordillera and Sierras Pampeanas. It must be pointed out that this last type of rocks have caught the attention of numerous authors not only by its petrologic importance, but also by its tectonic significance (Cingolani, 1970; Cingolani et al., 2002; Villar, 1975; Cortés and Kay, 1994; Davis et al., 1999; Ramos et al., 1984, 1986, 1998). Their origin has been generally attributed to the oceanic crust incorporating into the gondwanic continental margin, being fairly well constrained by the so called "ophilite complex" (26° LS to 33° LS, from La Rioja-San Juan-Mendoza provinces). Haller and Ramos (1984) call the same event as "Famatinian Ophiolites", corresponding to the discontinuous intercalation of ultramafic and mafic bodies placed in the occidental border of the carbonate platform of the Precordillera. Sectors with noticeable development of pillow lavas occurr, while in others these are arranged as mafic sills interlayered with sandstones, siltstones and shales. In San Juan province, these levels are located within fossiliferous strata of Caradoc age (Aparicio and Cuerda, 1976).

In the Sierra de Cortaderas (Mendoza province), the "ophiolites" are highly deformed and include serpentinized peridotites, grabbros and diabases. Chemical studies suggest that they are oceanic basalts of E-MORB type, developed by anomalous oceanic crust formed in a plume or plateau setting (Haller and Ramos, 1984).

Volcanic flows were also produced in the Precordillera of Jagüé, in La Rioja province (Cerro Chuscho Formation). An andesitic-basandesitic nature with development of pillow lavas interbedded to a Lower Ordovician sedimentary sequence of shales and sandstones (Aceñolaza et al., 1969; Toselli and Aceñolaza, 1971, Zimmermann and Van Staden, 2002).

Volcaniclastic deposits and lava flows are displayed in different Arenig-Llanvirn levels of the Famatina System. They vary from hornblende basalts to basaltic andesites, quartziferous andesites and rhyolites (Mannheim and Miller, 1996). This set could have been deposited in a volcanic framework, giving place to the development of platform, channels and slope deposits with participation of gravitational phenomenons (Mangano and Buatois, 1996). The early referred strata is represented by the Cachiyuyo Group, having well developed outcrops from the Sierra de Las Planchadas to the Sierra de Famatina s. str., bearing fossiliferous levels that helped on the dating of the beginning of the volcanic process (T. akzharensis) (Aceñolaza and Gutierrez Marco, 2000).

Northwards, in the Puna (Catamarca, Salta and Jujuy) two meridionally aligned volcanogenic belts can be analized, representing two different events binded with the geological conformation of the region. In the western one, shared with the Chilean border, the volcanism starts up with Tremadocian basaltic-andesitic effusions. Subaereal dacitic-rhyolitic piroclastites are superposed to the latter. During the Arenig volcanic buildings of basaltic-andesitic composition are also recorded in the region, ending up with a rhyolitic eruptivity related to turbiditic sequences. This set is interpreted by Coira and Koukharsky (2002) as representative of an arc-backarc magmatism. In the eastern region of Puna there is a bimodal subvolcanic lavic episode, predominantly dacitic, with backarc setting character. Its siliceous nature of peralluminous character, display a sedimentary cortical origin (Coira and Koukharsky, 2002).

Metamorphism: The interaction of the oceanic plate with the Gondwanan margin was not only favorable for the igneous processes but also for deformation and metamorphism, which developed in the entire region, with major magnitude during the Famatinian Cycle (Upper Cambrian- Devonian).

The deformation D1 and D2 could have occurred in the Upper Cambrian times, being the D3 characteristic of the Lower Ordovician, followed in time by the D4 (Silurian) and D4 (Devonian).

Among them the most notable metamorphic process (M3) is contemporaneous with D3 and succeeds to develop metamorphic levels that go from the biotite zone to the cordierite-sillimanite zone, occurred between 435 and 470 My. (Miller, 1983; Bachmann et al., 1986; Willner, 1990). In such chronological framework, it is highlighted that the rank of M3 values in different sectors of the Sierras Pampeanas (Rossi et al., 2002; Sato et al., 2002; Tickyj et al., 2002). The early mentioned values represent the penetrative deformation affecting supracrustal turbidites (Neoproterozoic/Cambrian) with development of compact folds (chevron), axial plane clevaje and synkinematic very low grade metamorphism. They may have developed since the pampean orogeny (540-507 My), corresponding the first homogenization to the 470 My., with emplacement of plutons of Sierras Pampeanas, with related phenomena of contact metamorphism, giving place to medium to high temperature-low pressure metamorphism. The early referred characteristics extend to the central region and towards Cordoba (Bonalumi and Baldo, 2002), San Luis (Sato et al., 2002) and La Pampa (Tickyj et al., 2002).

Palaeogeographic and tectonic framework

Processes occurring in the the western edge of Gondwana were conditioning the depositional phenomena and of the metamorphic, deformative and magmatic events during the Ordovician.

Several hypothesis have been stated about the structural framework in which the mentioned events originated. In this regard we must highlight that: a) the contour of the Paleo-Pacific margin display differences in its tectonic setting and b) that Cuyania as well as Patagonia were not in the present geographic position.

Different interpretations were presented, and have led to postulate that the northeast region, including the Famatina and the ranges of Buenos Aires province conformed an autochthonous region of the western gondwanic edge; while Cuyania (Precordillera + Bloque San Rafael) and Patagonia constitute an allochthonous fragment of Laurentian origin (Astini et al., 1995; Ramos 1984; Keller, 1999; Thomas and Astini, 1996; 1999; Thomas et al., 2002), with an intermediate paraautochthonous origin, presenting an initial gondwanic origen with displacement by "strike slip" processes from the intermediate zone between South America, Africa and Antartic (Aceñolaza and Toselli, 2000, Aceñolaza et al., 2002).

The impossibility to give an acceptable mechanic explanation for the Laurentian origin has led to deepen the hypothesis that binds the evolution of the region to Gondwana with an interaction of the palaeopacific oceanic plate with the western Gondwana (Aceñolaza et al 2002, Finney et al., 2002).

The development of a cambro-ordovician orogenic belt (Famatinian) over an older one (Pampean) in the gondwanic edge give the tectonic and structural frame under which the sedimentary and magmatic-metamorphic events took place within the autochthonous Gondwana. During the Tremadocian, the northeast region as well as the Famatina were covered by a platform expanded over the "Dorsal Pampeana" and the northern cratons of Guaporé/Rio de la Plata. The regional tectonic instability increased in the Arenig turning to an active margin of great magmatic and deformative activity, specially in the Famatina and in the Puna. It is possible that these events had their origin in the interaction of the oceanic plate over the continental margin, starting "strike-slip" structures that not only displaced fragments along the gondwanan edge, but also generated the first collitional and effusive magamtic events in the entire region, specially in Sierras Pampeanas.

From the Middle and Upper Ordovician, the basins of Northwestern Argentina continued with siliclastic type of sediments, culminating with sediments of a possible glacial origin. The second magmatic event, of a lower magnitude than the previous one, is also related to the regional unstable framework by the end of the Ordovician, manifested as the «Guandacol Orogenetic Phase».


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