The Ordovician Basin of Northern Argentina

The Ordovician Basin of Northern Argentina

María Cristina MOYA1

1 CONICET-CIUNSa. Universidad Nacional de Salta, Geología, Buenos Aires 177, 4400 Salta. Argentina.E-mail: crismoya@unsa.edu.ar

Abstract: THE ORDOVICIAN BASIN OF NORTHERN ARGENTINA. The marine-clastic Ordovician strata of Northern Argentina bears an abundant benctic and planctic fossil fauna that allows to stablish acceptable correlations. The Ordovician basin in Northern Argentina was developed in the margins of the Pampean Craton. These, had a elonged N-S geometry and asymmetric disposition of the sedimentary environments. A wide stable marine shelf (Chaco Shelf) was developed during all the Famatinian Cycle (Upper Cambrian-Upper Devonian) in the areas today occupied by the Cordillera Oriental, Sierras Subandinas and Llanura Chaqueña. The Puna region was a tectonically unstable area, whose famatinian history started with clastic and volcano-sedimentary accumulation (Lower Tremadocian-Middle Arenig) over a shelf afectted by volcanism (Altiplano Shelf). At the end of the volcanism, the latter records a strong subsidence with thick accumulation of volcaniclastic turbidites (Upper Arenig-Caradoc). Silurian-Devonian strata are restricted to the western border of Puna under Upper Carboniferous red beds. The evolution of the Famatinian Cycle in the Chaco Shelf was controlled by relative sea-level fluctuations. These, allowed the development of three second order tectono-eustatic cycles separated by important unconformities: The Mesonian Cycle (Upper Cambrian) whose strata covers unconformably the slates and intrusives of the basement (Upper Proterozoic-Lower Cambrian), are placed below strata of the Victorian Cycle (late Upper Cambrian-Caradoc) with an erosive unconformity (Iruya Unconformity). Another erosive unconformity (Ocloya Unconformity) separates the Victorian Cycle to the Cordilleran Cycle (Upper Ashgill-Upper Devonian). These cycles include other third or fourth order cycles related to transgressive-regressive episodes whose sedimentary result is an alternation of shale and sandstone bodies. Inside the Victorian Cycle are recognized seven lower order cycles separated among them by sedimentary unconformities of different magnitude and regional expression. The First Cycle was preceded by a pronounced eustatic event (LREE) with areal erosion and development of fluvial system. This cycle start with the FAD of Parabolina (Neoparabolina) frequens argentina. During its evolution the basin inherit the structural elements of the Mesoninan Cycle with the sediments accumulating on tide-dominated shelfs. The Second Cycle starts with the volcanism on the Puna, and the Chaco Shelf is affected by tectonic events producing the collapse of certain areas. Within this cycle the Cambrian-Ordovician boundary is recorded with Jujuyaspis keideli and Rhabdinopora flabelliformis. The Third Cycle start a generalized flooding of Chaco Shelf pointed by Anisograptus matanensis. The Fourth Cycle was preceded by subaerial exposures with the accumulation of fluviomarine and mineralized fan deltas deposits. Paleontologically is characterized by the FAD of Kainella meridionalis. The Fifth Cycle represents the ending of the Tremadocian times with the maximum flooding of Chaco Shelf. During this cycle occurred a tectonic episode generating important paleogeographic modifications (Tumbaya Phase). The paleontological elements of the last cycle are different species of Anisograptus, Adelograptus and Bryograptus. The Sixth Cycle starts during the Tremadoc-Arenig transition. Along its evolution the different volcanic episodes are closed, while different species of didymograptids and tetragraptids characterize its strata. Finally the Seventh Cycle represent a quieter basinal interval with conodonts and trilobites (Llandeilian- Lower Caradoc). This cycle rests below ashgill strata with hirnantia fauna of the Zapla Formation and equivalents, which starts up the Cordilleran Cycle.

Resumen: LA CUENCA ORDOVÍCICA DEL NORTE ARGENTINO. Los depósitos ordovícicos en el norte argentino son marino-clásticos y contienen abundante fauna fósil bentónica y planctónica que permite establecer correlaciones de aceptable precisión. La cuenca Ordovícica del norte argentino se desarrolló en las márgenes del Cratón Pampeano. Tuvo una geometría alargada en sentido N-S y una disposición asimétrica de los ámbitos de sedimentación. Una plataforma marina amplia y relativamente estable (Plataforma Chaqueña) se desarrolló durante todo el Ciclo Famatiniano (Cámbrico Superior-Devónico Superior) en las áreas actualmente ocupadas por la Cordillera Oriental, Sierras Subandinas y Llanura Chaqueña. En contraposición, la Puna fue un área tectónicamente inestable, cuya historia famatiniana se inició con la acumulación de depósitos clásticos y volcano-sedimentarios (Tremadoc Inferior-Arenig Medio) en una plataforma marina afectada por vulcanismo (Plataforma Altiplánica). Al culminar el vulcanismo, la Plataforma Altiplánica registró una marcada subsidencia, cuyo resultado fue la acumulación de espesas sucesiones de turbiditas volcaniclásticas (Arenig Superior-Caradoc). Los estratos silúricos y devónicos se restringen puntualmente al borde occidental de la Puna, donde subyacen a red beds del Carbonífero Superior. En la Plataforma Chaqueña en cambio, la evolución del Ciclo Famatiniano estuvo controlada, principalmente, por cambios relativos del nivel del mar. Esto posibilitó el desarrollo de tres ciclos tectonoeustáticos de segundo orden, separados por importantes discordancias: El Ciclo Mesoniano (Cámbrico Superior), cuyos depósitos cubren angularmente a las metamorfitas e intrusivos del basamento (Proterozoico Superior- Cámbrico Inferior) y subyacen a través de una discordancia de erosión (Discordancia Iruya) a los depósitos del Ciclo Victoriano (Cámbrico Superior tardío-Caradoc). Otra discordancia de erosión (Discordancia Ocloya) separa al Ciclo Victoriano del Ciclo Cordillerano (Ashgill Superior-Devónico Superior). Estos ciclos a su vez, incluyen otros de tercero y cuarto orden, los que se corresponden con episodios de transgresión-regresión; el arreglo estratigráfico resultante es una sucesión alternante de cuerpos de pelita y de arenisca. Dentro del Ciclo Victoriano se reconocieron siete ciclos de orden menor, los que están separados por discontinuidades sedimentarias de distinta magnitud y expresión regional. El Primer Ciclo estuvo precedido por una descenso eustático pronunciado (LREE), durante el cual ocurrieron procesos de erosión subaérea y desarrollo de un sistema fluvial. Se inicia con la aparición de Parabolina (Neoparabolina) frequens argentina. Durante su evolución la cuenca hereda los elementos estructurales del Ciclo Mesoniano y los depósitos se acumulan en ámbitos dominados por mareas. El Segundo Ciclo, con él se inicia el vulcanismo en la Puna. La Plataforma Chaqueña es afectada por eventos tectónicos que producen el colapso de algunas áreas. En este ciclo se registraría el límite Cámbrico-Ordovícico, dado que los fósiles más conspicuos son Jujuyaspis keideli y Rhabdinopora flabelliformis. El Tercer Ciclo inicia un período de inundación generalizada señalado por la aparición de Anisograptus matanensis. El Cuarto Ciclo estuvo precedido por procesos de exposición subaérea que dieron como resultado la acumulación de depósitos fluvio-marinos y de fan-deltas mineralizados. Paleontológicamente está caracterizado por la aparición de Kainella meridionalis. El Quinto Ciclo representa la culminación del tiempo Tremadociano y en él se registra la máxima inundación de la Plataforma Chaqueña. Durante su evolución se desarrolla un episodio tectónico que produce importantes modificaciones paleogeográficas (Fase Tumbaya). El contenido paleontológico de los depósitos incluye distintas especies de Anisograptus, Adelograptus y Bryograptus. El Sexto Ciclo se inicia durante la transición Tremadoc-Arenig. Durante su evolución culminan los episodios volcánicos. Distintas especies de didymográptidos y tetragráptidos caracterizan a sus estratos. Finalmente, el Séptimo Ciclo corresponde a un estadio de mayor tranquilidad en la cuenca. Los depósitos contienen conodontes y trilobites del Llandeiliano y Caradoc Inferior. Este ciclo subyace a los depósitos ashgillianos portadores de fauna hirnantiana (Formación Zapla y equivalentes), con los que se inicia el Ciclo Cordillerano.

Key words: Ordovician. North Argentina. Stratigraphical evolution.

Palabras clave: Ordovícico. Norte Argentino. Evolución estratigráfica.

Introduction

The aim of this contribution is to discuss the main sedimentary events recognized in the marine-clastic Ordovician successions exposed in Puna, Cordillera Oriental and Sierras Subandinas of northern Argentina (Figs. 1a, 1b).

Figure 1. 1a. Map of Geologic Regions. 1b. Map of Proterozoic-Paleozoic outcrops. 1. Metamorphic basement (Proterozoic-Lower Cambrian). 2. Upper Cambrian sedimentary rocks. 3. Ordovician sedimentary rocks. 4. Silurian-Devonian sedimentary rocks (shelf deposits). 5. Devonian - Carboniferous sedimentary rocks (deep deposits). 6. Proterozoic-Middle Cambrian granites. 7. Ordovician granites. 8. Proterozoic-lower Paleozoic granites. 9. Upper Paleozoic granites. 1c. Paleogeologic sketch according to the age and nature of the stratigraphic nucleus. 1. Pan-American Domain. 2. Areas of uncertain age. 3. Pampean Domain. 4. Famatinian Domain. Famatinian sedimentary rocks and granites (Famatinian or younger). 5. Ordovician sinsedimentary volcanism. 6. Granites without differentiated age. 7. Old tectonic front (previous to the Upper Cambrian). 1d-f. Paleogeographic sketchs of the ordovician basin. 1. Shallow areas. 2. Area occupied by the old Cambrian basin (Mesón Group).

3. Emerged to submerged areas. 4. Shallow to submerged areas. 5. Deep areas. 6. Turbidites. 7. Positive areas . 8. Sinsedimentary Volcanism. 9. Intrusives with radiometric age. 10. Storm-dominated shelf. 11. Isolated areas. 12. Lipán Swell. 13. Maximum subsidence axis. 14. Old tectonic front. 15. Erosional western border of the ordovician basin.

References about Ordovician deposits in these regions are very abundant, great part of them were analysed in recent synthesis papers (Aceñolaza et al., 1999; Bahlburg and Zimmermann, 1999; Coira et al., 1999; Moya, 1999), where the interested colleague can consult the mentioned complementary bibliography.

The Ordovician System subdivision used here corresponds to the accepted British Series, because all the International Stages of the Ordovician System has not been fully defined yet. The chronology is the proposed by Cooper (1999).

Geological Framework

The cratonic areas of South America were attached to Gondwana during the Upper Proterozoic- Cambrian. The extensional collapse that happened to the orogenic processes generated, in a diachronic way, intracratonic and pericratonic basins. These are marine-clastic and evolved in the margins of the South America cratons.

The general scheme above mentioned is applicable to the Cambrian-Ordovician basins of North Argentina, which were development in the margins of Pampean Craton (Pampean Domain, Figs. 1c-1f). The annexation to Gondwana of the Pampean Domain terranes could have been previous to the one of the basement of the Cordillera Oriental (Pan-American Domain, Fig. 1c). The Pampean Domain basement is characterized by plutonic rocks and medium to high-grade metamorphic rocks. Folded sedimentary rocks, affected by low to very low-grade metamorphism and intruded by plutonic bodies, compose the Pan-American Domain basement. Two magmatic-metamorphic episodes could have been recorded in both domains; the oldest, with ages of 560-540 M.a. (Vendian) and the youngest, of 525-505 M.a. (Lower to Middle Cambrian) (Becchio et al., 1999).

In the Cordillera Oriental, the metamorphic and the intrusive rocks of the basement underlie in clear angular unconformity (Tilcara Unconformity), deposits of the Mesón Group (Middle? -Upper Cambrian). The Tilcara Unconformity points out the end of the sedimentary, metamorphic, tectonic and magmatic events occurred during the Pampean Cycle.

The Mesón Group begins the Famatinian Cycle (Upper Cambrian-Upper Devonian). The deposits of the Famatinian Cycle are clastic being accumulated in a marine shelf environment; the deep basin facies are restricted in time (Ordovician) and space (Famatinian Domain, Fig. 1c).

The Famatinian Domain understands the Argentinean-Chilean Puna, where the Pampean Cycle rocks are poorly represented. In this region, the Ordovician deposits does not expose its base, representing the stratigraphic nucleus. The Famatinian Domain was a tectonically unstable area affected by volcanism during the Lower Tremadocian-Middle Arenig (Figs. 1d, 1e); the deposits of

Figure 2. Correlation Chart of the Ordovician of northern Argentina. 1. Concordant stratigraphic relationship. 2. Tectonic relationship, not exposed or unknown. 3. Unconformity. 4. Probable younger age. 5. The identification to GROUP. 6. Idem to Formation. 7. Radiometric age. 8. Granites. 9. Volcanic rocks. 10. Piroclastics rocks. 11. Correlation Time-Lines (see above). 12. Conglomerate. 13. “Diamictite”. 14. Quartz sandstone. 15. Wacke. 16. Sandstone/Shale interbedded. 17. Silstone/Mudstone. 18. Shale. 19. Trace fossils abundant. 20. Sulfides. 21. Calcareous coquinite with Lower Tremadoc trilobites. 22. Concretions. 23. Phosphates (nodules, crusts or lingulid coquinites). 24. Iron ore deposits (hematite, chamosite). 25. Fine-grain Turbidites. 26 Coarse Turbidites. 27. Global Eustatic and/or Regresión Events (in text). Observation: The references 19 to 24 are the best characteristics to identified the differ units. Correlation Time-Lines: LT1. Coquinites with Lower Tremadocian trilobites. LT2. A. murrayi, LT3. T. approximatus. LT4. B. vacillans, A. filiformis, B. deflexus. LT5. D. bifidus/minutus/nitidus. LT6. A. eivionicus, P. minor, X. svalbardensis. LT7, G. acanthus, C. antennarius, U. austrodentatus, A. cucullus. LT8, D. cf. murchisoni, G. hinckii fimbriatus, G. cf. ciliatus. LT9, Dicellograptus sp. LT10, Hoekaspis schlagintweiti. LT11, Dalmanitina subandina, cf. Eohomalonotus, cf. Chattiaspis LT12, Talacastograptus leanzai, Normalograptus aff. normalis, N. rectangularis.

this age integrate volcano-sedimentary successions (CVP, Fig. 2), accumulated in shelf marine environments (Altiplano Shelf, Fig. 1d). A pronounced subsidence recorded in the Upper Arenig, gave place to the accumulation of thick volcaniclastic turbidite successions (CTP, Fig. 2; Upper Arenig-Caradoc). In the Famatinian Domain, the Silurian-Devonian deposits are restricted to the western border of the Puna, where they underlie discordantly to red beds of Upper Carboniferous age (Figs. 1b, 2).

The more complete Cambrian-Devonian successions crop out in the Cordillera Oriental and in the Sierras Subandinas (Figs. 1a, 1b); also, they extend toward the east (out of the limits seen in the figure), in the subsurface of the Chaco Plain. These regions were part of a wide marine shelf during all the Famatinian Cycle (the Chaco Shelf, Fig. 1d). An emerged to submerged structure (the Lipán High, Figs. 1d-f) divided the Chaco Shelf into an eastern and a western area. The western area was bounded to the west by the Cobres High, effective only during the Cambrian-Lower Tremadoc (cf. Figs. 1d and 1e-f). The Cobres High is an extension of the Pampean Craton; their western limit is an old and important tectonic front (Fig. 1d).

The evolution of the Famatinian Cycle in the Chaco Shelf was controlled, mainly, by relative sealevel changes which had, as results, the development of three second order tectonic-eustatic cycles, limited in base and top by important unconformities.

The Mesonian Cycle (Upper Cambrian). It is developed above the Tilcara Unconformity and includes two transgression-regression episodes, represented in the Mesón Group (MG). The first episode corresponds to the prograding sequence that K1 and K2 integrate; the deposits of the second episode (K3) are truncated by the Iruya Unconformity (Fig. 2). The MG deposits were accumulated in tide-dominated paleoenvironments (sub, inter and supratidal environments) characterized by a high mineralogical and textural maturity. These indicate that the MG was deposited under conditions of high stability of basement. The MG basin is restricted to the Cordillera Oriental (Figs. 1b, 1d, 2); its genesis is linked with the extensional processes to which the Pampean Cycle concluded.

The Victorian Cycle (late Upper Cambrian-Caradoc). It is represented in the Santa Victoria Group (Cordillera Oriental) and in the Tamango Group (Sierras Subandinas). This cycle is limited in base and top by two erosional unconformities (Iruya and Ocloya unconformities; Fig. 2). These unconformities were generated by relative sea level falls, periods where wide areas of the Chaco Shelf were exposed to subaerial erosion. These events belong together, respectively, with the Lange Ranch Eustatic Event (LREE, Upper Cambrian) and with the Hirnantia Regressive Even (HRE, Ashgill) recorded in other regions of the world. The Iruya Unconformity is only observed in the Cordillera Oriental, where it separates the Mesón Group of the Santa Victoria Group.

The Cordilleran Cycle. (Upper Ashgill-Upper Devonian). It was defined in Bolivia (fide Suárez Soruco, 2000). In northern Argentina, the Cordilleran Cycle is better represented in the Sierras Subandinas (Fig. 1b), where is bounded in base and top by the Ocloya and Chánica Unconformities.

The oldest deposits of the Cordilleran Cycle is represented by the Zapla Formation (Upper Ashgill), that starts the Cinco Picachos Supersecuence (Upper Ashgill-Lower Devonian). The last one represents the first of the three cycles of third order that characterize the Silurian-Devonian successions in the Sierras Subandinas (Vistalli, 1999).

Figure 3. 3a. Location map of the stratigraphic columns. 3b. Paleogeographic sketch of the Ordovician Chaco Shelf: Western area (inclined lines), profiles 1, 2, 3. 3c. 1a, Fluvial conglomerate; 1b, Fluvial-coastal marine conglomerate. 2, Sandstone. 3, Shale. 4, Shale and sandstone. 5, Calcareous mudstone. 6, Gravity flows (beds and blocks). 7, Mudflakes. 8, Fe-Mn nodules. 9, Concretions with calcareous and iron oxide nucleous. 10, Coquinite. 11, Storm succession and storm beds. 12. Bioturbation, trace fossils abundant. 13, Cross-beds. 14, Profiles: La Quesera (1), Pascha (2), Angosto del Moreno (3), Lesser Range (4), Sierra de Mojotoro (5) and Quebrada de Humahuaca area (6). 15, Unconformity.

Evolution of the Victorian Cycle

Except for the Zapla Formation, the rest of the Ordovician deposits accumulated in the Chaco Shelf belong to the Victorian Cycle, which includes cycles of third and quarter order, representing transgression-regression episodes. The resulting succession is composed by alternating sandstone and shale bodies, which were identified with a wide nomenclature (Fig. 2). The four cycles recognized by Moya (1999) are represented in the Fig. 2, where it is observed that the first cycle (Ss1, Sh1, Ss2) is the only one that doesn’t have a time-line of correlation. In a similar way, the second cycle (Sh2, Ss3, Fig. 2) only has a reference (LT1, Fig. 2).

Recently obtained information in the units allowed to define seven stratigraphic intervals characterized by distinct fossiliferous assemblages, based on which Moya et al. (this symposium) present a more detailed correlation scheme (I-VII, Figs. 3, 4). This scheme is valid for the Cambrian- Tremadocian deposits of the Santa Victoria Group allowing to adjust the regional correlation chart (Fig. 2).

Although the stratigraphic arrangement of the Chaco Shelf deposits are notably different to that of the Puna Ordovician successions (CVP, CTP, Fig. 2), the fossiliferous assemblages pointed out in the Fig. 2 and those mentioned by Moya et al. (this symposium), allow to discuss the relationship among the tectonic, eustatic and sedimentary processes happened in the Ordovician basin of northern Argentina. The base of the discussion will be the transgression-regression cycles recognized, whose limits are sedimentary discontinuities of different magnitude and regional expression. These discontinuities represent relative sea level falls and they would be equivalent with the events LREE (Lange Range Eustatic Event), ARE 1 and ARE 2 (multiple event sensu Cooper and Nowlan, 1999; Acerocare Regressive Event), BMEE (Black Mountain Regressive Event), GARE (Grés Armoricain Regressive Event), VRE (Vallhallfonna Regressive Event) and HRE (Hirnantia Regressive Event) (Figs. 2, 3, 4). The paleontological control doesn’t still allow to specify if identified events here in the Kainella Regressive Event (KRE) and the Notopeltis Regressive Event (NORE) belong together with the Peltocare Regressive Event (PRE) and the one with the Ceratopyge Regressive Event (CRE), respectively.

First Cycle It is limited by the LREE and ARE 1 events (Stratigraphic Interval I, Figs. 3, 4). This Cycle includes the deposits of Ss1, Sh1 and the lower member of Ss2. It is represented in the centralnorthern areas of the Cordillera Oriental (Sections 3, 6; Figs. 3 and 4), in the Sierras Subandinas and Puna Oriental (Fig. 2). The LREE generated processes of subaerial erosion that removed the deposits of the MG, even reaching levels of K2. The resulting discontinuity (Iruya Unconformity) is recorded in the whole basin of the MG, settling fluvial systems over it. The lowstand systems tract is represented by sandstones and conglomerates of fluvial braided bars and channels that filled paleovalleys inserted over the MG deposits and of the basement. These deposits grade vertically and laterally to coastal facies. The transgressive systems tract begins with a sandy body (Ss1). In this unit there is the FAD of Parabolina (Neoparabolina) frequens argentina (Kayser). This sandstones grade to shales (Sh1) of intermediate to distal shelf environment with conodonts from the Hirsutodondus hirsutus Subzone (Upper Cambrian). Within Sh1 unit the maximum flooding is recorded, starting the highstand systems tract. The distal to intermediate shelf facies are progressively replaced by sandy deposits of bars and subtidal sand sheets (lower member of Ss2).

The deposits of this cycle were accumulated in tide-dominated marine shelf environments and

Figure 4. Eastern area (horizontal lines), profiles 4, 5, 6.

overlap the borders of the MG basin, lying sometimes directly over the basement in the Sierras Subandinas and in the Puna Oriental (Fig. 2). However, the basin preserved the same structures of those during the MG (Fig. 1d). In addition, although there is no direct data, it is probable that the Altiplano Shelf has begun its evolution during this cycle.

Second Cycle

It is bounded by the ARE 1 and ARE 2 events (Stratigraphic Interval II, Figs. 3, 4). It includes part of Las Vicuñas Formation (CVP, Fig. 2), the middle and upper members of the Alfarcito and Cardonal Formations in the Humahuaca and Angosto del Moreno areas and the San José and Caldera Formations in the Sierra de Mojotoro and Cordón de Lesser (Figs. 3, 4). Although this cycle had a short duration, its depositional timming is one of the most critical and important in the evolution of the Ordovician basin. With this cycle it is settled the volcanic arch in the Altiplano Shelf (Fig. 1d, 2) and the Chaco Shelf becomes a storm-dominated environment. The areas located in the borders of the Lipán Swell collapse along normal faults, through which spill out mud and brines metal-bearing (Pb, Zn, Ag, Fe). The activity of the faults generates frequent mineralized gravitational flows lying between the deposits (Figs. 3, 4). Dark gray or black shales and mudstones with autigenic sulfides represent the transgressive systems tract. In this cycle would be documented the Cambrian- Ordovician boundary, given that this time interval records the appearance of Rhabdinopora flabelliformis (Eichwald) that generally accompanies Jujuyaspis keideli Kobayashi and exceptionally to P. (N.) frequens.

The highstand systems tract is represented by fine-grained quartz sandstones with hummocky cross-stratification deposited in lower to middle shoreface environments.

Third Cycle

This cycle belongs together with Stratigraphic Interval III (Figs. 3, 4), being limited by the ARE 2 and BMEE events. It includes the lower member of Sh2 in the Cordillera Oriental and Sierras Subandinas; in the Puna Occidental, includes the Tolar Chico Formation and the upper member of Las Vicuñas Formation (Fig. 2).

The transgressive systems tract rests with a clear ravinement surface on the top of the previous cycle deposits; the maximum flooding coincides with the appearance of Anisograptus matanensis Ruedemann. The high systems tract is represented by brown, pink and purple, generally bioturbated, sandstones and mudstones, which were deposited in subtidal and intertidal environments.

Fourth Cycle

This cycle belongs together with the Stratigraphic Interval IV (Figs. 3, 4), being bounded by the BMEE and KRE events. The latter includes the lower member of Sh2 in the Cordillera Oriental an Subandean Range. The relative sea level fall is assigned to the BMEE, being pronounced. In the areas next to the basin borders (Angosto La Quesera, Section 1, Fig. 3), subaerial erosion removed deposits of previous cycles and even great part of Ss2. This unit underlie with a clear erosional unconformity quick discharge fluvial-coastal deposits. represented by coarse to coarser conglomerate.

In the flanks of the Lipán High, fan deltas were developed with mineralized debris and mud flows (Fig. 4). A pelitic wedge is developed in the more distal parts of the shelf. These fine-grain deposits represent the transgression systems tract that is covered successively by sandy and sandy-calcareous shoreface tempestites. This cycle records the appearance of Kainella meridionalis Kobayashi; which is present in all the deposits of this cycle; even, in the matrix of the fluviomarine conglomerates.

Fifth Cycle

The Fifth Cycle is bounded by the KRE and NORE events, including the Stratigraphic Intervals V and VI (Figs. 3, 4). The deposits of this cycle are well represented eastwards of Lipán Swell.

Westwards this structure, the Tumbaya Unconformity truncates great part of these deposits (Figs. 2, 3). During the deposition timing of this cycle, the Tremadocian basin reached its maximum extension.

The transgression systems tract is represented by clayey and muddy shales accumulated in distal outer-shelf environments. These deposits contain abundant graptolites of Lower and Upper Tremadoc (Anisograptus spp., Adelograptus spp. Bryograptus spp.). The maximum flooding is reached within the levels with Adelograptus, starting the settling of the highstand system tract, represented by shales of middle to proximal offshore grading quickly to sandy tempestites of inner shelf (shoreface).

These deposits include abundant coquina levels with Notopeltis orthometopa (Harrington) and Ceratopyge forficuloides Harrington and Leanza; for this reason, in the Fig. 2 it is drawed the accumulation of these deposits during the CRE. Nevertheless, focussing on this data (Section 5, Fig. 4), is clear that the stratigraphic position of the CRE assigned by other authors (to the Araneograptus murrayi Zone) is younger that NORE, which is here considered the sequence boundary.

Westwards Lipán Swell, deposits of the highstand systems tract were suppressed by the Tumbaya Unconformity (Upper Tremadoc). The tectonic processes during this time caused important paleogeographics modifications (cf. Figs. 1d, 1e): The Chaco Shelf tilted to the west, the maximum subsidence axis located to eastwards the Lipán Swell (Fig. 1d), was displaced to the eastern border of the Famatinian Domain (Fig. 1e). The Cobres High collapses being affected by magmatic events (Eruptive Belt of the Puna Oriental; FEP, Fig. 2); while the Lower Tremadoc shallow environment deposits (Matancilla, Taique and Potrerillo Formations) were covered by turbidites (Coquena Formation). The collapse of Cobres High during the Upper Tremadoc had a greater magnitude that the one affecting the flanks of Lipán High at the beginning of the Tremadoc. If the emplacement model of the mineral deposits is similar in both areas, it is probable that in the old Cobres High, they were housed in the basal Lower Arenig deposits and/or underlain it.

Sixth Cycle

This cycle includes Sh3 and Ss4 in the Cordillera Oriental and Sierras Subandinas and its temporarily equivalent units in Puna (upper CVP and lower CTP) (Fig. 2). Eastwards of Lipán Swell this cycle starts from the Aorograptus victoriae Zone (Interval VII, section 5, Fig. 4). Westwards the Lipán Swell, the oldest levels over the Tumbaya Unconformity correspond to distal outer-shelf deposits with Tetragraptus phyllograptoides and T. approximatus. Towards the northwest and west, they couple with fine-grain volcaniclastic turbidites slightly older (Tolillar Formation) with A. murrayi. It is understood that these last ones correspond to the shelf-margin wedge systems tract, previous to the transgression systems tract represented by Sh3 and their equivalent units in the Puna Oriental (Chiquero Formation).

It is probable that the Altiplano Shelf had a similar evolution, although up to day, the base of the Aguada de la Perdíz Formation is not known. The high systems tract is developed starting from the late Lower Arenig. Shales and sandstones with Didymograptellus bifidus (Middle Arenig), accumulated in a storm-dominated proximal shelf (lower to middle shoreface), prograding over the old muddy shelf. D. bifidus is also present in the uppermost part of the volcano-sedimentary successions (CVP, Chiquero and Aguada de la Perdíz Formations, Fig. 2); allowing a Middle Arenig age for the culmination of the volcanic episodes of the Puna. When the volcanic activity decreased, the old Altiplano Shelf overdeepened rapidly, the maximum subsidence axis is displaced again towards the west (Fig. 1f). This situation gave place to the beggining of the deposition of thick successions of sandy and sandy-conglomeraditic volcaniclastic turbidites of the CTP (Coquena Formation, Fig. 2).

With the end of volcanism in the Puna, the Chaco Shelf turns again into a tide-dominated area; with a general shallowing trend started on this region. The coastal progradation process ends with sandstones and bioturbated mudstones accumulated in subtidal and intertidal environments, with crust and phosphatic nodules (Ss4). (Fig. 1f, 2).

Seventh Cycle

This cycle includes Sh 4 and Ss5 in Cordillera Oriental and Sierras Subandinas and the upper CTP in the Puna (Fig. 2). In the Cordillera Oriental, Sh4 contains conodonts of the lower Llandeilian- Caradoc (Albanesi and Moya, 2002). The relative emersion of the Lipán High starting from the Middle Arenig isolated the eastern area of the Chaco Shelf, where was installed a tide-dominated shelf only influenced by relative sea-level changes. The Sh4 deposits correspond to mudstones and fine dark wackes, without visible stratification, frequently bioturbated; scarce and thin banks of dark coquinoideous limestone (assigned to sporadic storms) interbedded with clastic-sedimentites. These deposits contain well preserved shelly-fauna being rich in organic matter and phosphates. The early mentioned characteristics, together with the absence of current sedimentary structures, denote restricted circulation and slow sedimentation conditions. The coastal progradation is also gradual; the deposits (Ss5) correspond to bars and subtidal sand waves complexes. Westwards Lipán High, the conditions were less restricted and the Sh4 interbedded bigger quantity of tempestites. Likewise, its relationship with the Ss5 is signed by a clear discontinuity surface.

In the Chaco Shelf, strata referred to the Victorian Cycle discordantly underlain (Ocloya Unconformity) the Zapla Formation (Upper Ashgill). In most of cases, the Zapla Formation is represented by pebbly mudstones with hirnantian fossils. These deposits are assigned to mass flows accumulated in coastal and submarines fans. In the basin borders, the Zapla Formation begins with conglomerates and sandstones accumulated in bars and channels of a braided fluvial system. The Ashgillian deposits of the last mentioned unit represents the lowstand systems tract that starts the Cinco Picachos Supersequence (Late Ashgill-Early Devonian). This sequence represents the first of the three cycles of third order that integrate the Cordilleran Cycle.

A traditionally accepted concept is that the Ocloya Unconformity point out the end of an orogeny (Oclóyic Phase) that would have folded the Cambrian and Ordovician (pre-Ashgillian) deposits. None of the authors who support this hypothesis indicates any section or area in Northern Argentina, where the ashgillian deposits overlie in angular unconformity the Caradocian deposits.

The only folded deposits that underlie the ashgillian diamictites are those of the Lower Tremadocian (Las Vicuñas Formation, Puna Occidental, Fig. 2). These means that the deformation of Las Vicuñas Formation could have been previous, specially if it is taken into account that during the Upper Tremadocian a tectonic event (Tumbaya Phase) produced important paleogeographics modifications (cf. Figs. 1d and 1f).

Acknowledgements. To the Prof. Dr. Florencio Gilberto Aceñolaza, for his gentle invitation to be part of this work.

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