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Resumen: Durante el Neoproterozoico tardío y el Paleozoico, el actual segmento argentino-chileno de la Cordillera de los Andes y desde finales del Paleozoico la Península Antártica, formaron parte del margen suroccidental de Gondwana. Durante este periodo de tiempo, a dicho margen se fue acrecionando un conjunto de fragmentos continentales de tamaño y aloctonía variable, dando lugar en el Paleozoico a seis orogenias de diferente extensión temporal y espacial: Pampeana (Ediacárico-Cámbrico temprano), Famatiniana (Ordovícico Medio-Silúrico), Chánica (Devónico Medio-Carbonífero temprano), Oclóyica (Ordovícico Medio-Devónico), Gondwánica (Devónico Medio-Pérmico medio) y Tabarin (Pérmico tardío-Triásico). Todas estas orogenias son colisionales, salvo la Tabarin y la Gondwánica al sur de los 38º S.Palabras clave: Evolución geodinámica, Paleozoico, Andes argentino-chilenos, Península Antártica, Orógeno Oclóyico, Orógeno Famatiniano, Orógeno Chánico, Orógeno Gondwánico, Orógeno Tabarin.Abstract: During the late Neoproterozoic and Paleozoic times, the Argentine-Chilean Andes, -and since the late Paleozoic the Antarctic Peninsula,- formed part of the southwestern margin of Gondwana. During this period of time, several continental fragments of variable extensión and allochtonie were successively accreted to that margin, resulted in six Paleozoic orogenies of different temporal and spatial extension: Pampean (Ediacaran-early Cambrian), Famatinian (Middle Ordovician-Silurian), Chanic (Middle Devonian-early Carboniferous), Ocloyic (Middle Ordovician-Devonian), Gondwanan (Middle Devonian-middle Permian) and Tabarin (late Permian-Triassic). All these orogenies had a collisional character, with the exception of the Tabarin and the Gondwanan south of 38º S.Keywords: Geodynamic evolution, Paleozoic, Argentine-Chilean Andes, Antarctic Peninsula, Ocloyic orogen, Famatinian orogen, Chanic orogen, Gondwanan orogen, Tabarin orogen.

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New data on some neopalaeozoic lithostratigraphic units of the central sector of the Argentinean Precordillera are presented. The western Precordillera displays occasional outcrops of siliciclastic rocks of the Late Palaeozoic. These rocks are separated by two main unconformities related to the development of the Chanic (Upper Devonian-early Carboniferous) and Gondwanan (late Carboniferous-early Permian) orogens. A major unconformity marks the beginning of the Andean cycle in middle Permian times. The Del Ratón Formation (800 m) in the studied area is made up of conglomerates and can be divided into two units. The basal part (Lower Member) was deposited in a fjord environment. The Upper Member was generated by an alluvial system that was mainly dominated by upper flow-regime plane beds, whereas its uppermost part accumulated in a fan delta setting. The clast provenance and the main palaeocurrents suggest the existence of a significant topographic high to the W and NW of the present outcrops. The El Planchón Formation (1400 m) is mainly composed of shales and sandstones as a result of sedimentation in a deep-sea fan environment. These lutite-dominated materials graded laterally northwards into conglomerates in a glacial setting. The Del Ratón and El Planchón formations of the Lower Carboniferous correspond to the synorogenic Chanic sedimentation (Chanic foreland basin). Thereafter, the stratigraphic succession underwent deformation which led to its prolonged subaerial exposure and rubefaction. In Permian times, a marine transgression gave rise to the Del Salto Formation (600 m), which is constituted by beach deposits with some aeolian reworking and which lies unconformably over the Lower Carboniferous rocks. Conglomerates (Quebrada del Alumbre Formation, 100 m) produced by an alluvial system prograded episodically into coastal areas. The Escombrera Formation (350 m) is composed of beach deposits prograding seawards. This formation probably represents the end of the Palaeozoic sedimentary record. Thereafter, a playa-lake and an alluvial fan system were generated (Quebrada de la Arena Formation). This unit (750 m) was the last infill of the Gondwanan retroarc foreland, the total thickness of which was about 4000 m.Keywords: Lithostratigraphic units, Carboniferous, Permian, Triassic, Chanic orogen, Gondwanan orogen, Andes Resumen Se presentan nuevos datos sobre las unidades neopaleozoicas aflorantes en la parte occidental de la Precordillera andina. Esas unidades litoestratigráficas estan separadas por dos discordancias angulares principales asociadas al desarrollo de las orogenias Chánica (Devónico tardío-Carbonífero temprano) y Gondwánica (Carbonífero tardío-Pérmico temprano) respectivamente. Otra nueva discordancia sugiere el inicio del ciclo andino durante el Pérmico medio. La Formación Del Ratón (800 m) está constituida principalmente por conglomerados y puede dividirse en dos unidades de rango menor: la parte basal y la parte superior respectivamente. La parte basal (Miembro inferior) fue acumulada en ...

In the Argentinian Andes (Frontal Cordillera) the upper part of the late Carboniferous-early Permian San Ignacio Formation is made up of lacustrine-palustrine microbial carbonates and interbedded volcanic deposits. In this lacustrine-palustrine environment a natural monospecific forest was developed. The deposits of this sedimentary-volcanic succession were repeatedly subjected to subaerial exposure and modified by pedogenesis to varying degrees giving rise to paleosoils development. Diagenetic microfabrics were well preserved in the carbonates and volcanic rocks. The carbonate microfabrics comprise a wide spectrum of features consisting of root marks and stumps-related structures (rhizoliths, alveolar texture, tunnel-like structures and coprolites of arthropods), pisoids, coated grains and pseudomicrokarst, cracking, brecciated and nodular fabrics, and grainification also occur corresponding to different stages in the pedogenic evolution. Meteoric dissolution and cementation processes are observed; examples are well identified by scanning electron microscope showing silica-filled voids in partially dissolved carbonates and growths of inorganic carbonate microcrystals or of microbial origin in voids. Other different types of cements can be seen such as discontinuous carbonate crusts, ribbon spar, cavities with silt infillings and pendant cements. The whole set of these microfabrics are indicative of wetting, desiccation and meteoric conditions (vadose and phreatic). The abundance of plant roots and associated micro-organisms mainly of bacterial origin (micro-rods, short rod-shapes, nano-fibres, filaments and nano-spheres) played an important role in the pedogenic and subaerial diagenetic processes affecting these deposits. The immature character of the paleosoils and absence of calcretes point out to short intervals of subaerial exposure due to oscillating fluctuations in water level, intermittent volcanic supply, tectonic subsidence and oscillating climatic conditions. The whole of the macro and microfabrics reveals that the prevailing weather could correspond to an intermediate between semi-arid to sub-humid, however the alternating wetting and drying conditions in which the fossil forest developed and the abundance and diversity of micro-organisms, suggest a transition to sub-humid climate conditions.