Descripción: In this paper new geologic evidences allow us to complete the stratigraphy of Ramez de Velasco ranges. Granitoids of the igneous basement are included into the Ojo de Agua-Ambargasta complex as the Sumampa Granite which is composed of three facies: facies Para Yacu, Sumampa Viejo and Los Telares. Thin aplitic dykes were included into the same basement as the last magmatic event of the same geological setting. The geochemical signature of these rocks shows meta to peraluminous compositions, which evidence magmatic arc granitoides. Basic dykes, few meters wide and more than one kilometer long, also intrude these granitoids. They are composed of basalts and andesites with calcoalkaline signatures that were correlated with Balbuena formation due to their petrological features. Finally, few and small outcrops of sandstones appears as the only expression of the Neopaleozoic sedimentation. On the basis of all the data obtained we suggest that the Ramíez de Velasco ranges preserve evidences of several geologic processes that took place since the Paleozoic.

Descripción: In the south of Santiago del Estero Province and in the north of Cordoba Province, the regional outcrops of Late Precambrian igneous-metamorphic basement show a N to NE tectonic trend which varies from the mainly N-S trend of the hills of central and southern Cordoba ranges. This ancient basement is dominated by granites and acidic porphyritic rocks which are best exposed in the Ambargasta Hills. Minor outcrops of medium-high grade metamorphic rocks (gneisses, schists, amphibolites and marbles) are better exposed in the central and southern. Cordoba ranges. In the main, this paper deals with unconformable overlying Eopalaeozoic folded quartzose sandstones, conglomerates and pelitic hornfels that compose the El Escondido and La Clemira formations, almost exclusively confined to the Ambargasta hills. The folded rocks are partially affected by contact metamorphism associated with intrusive granites dated as being of Late Cambrian (K/Ar 517± 15 Ma, intruding La Clemira Formation) and Carboniferous age (K/Ar 325±10 Ma, intruding in El Escondido Formation). The lithologic characteristics and location of these units, at the western boundary of the Chacoparana Basin, allow correlation with the subsurface marine Early Paleozoic units of that basin. However, tectonic studies by means of statistical mapping of ductile deformation structures (So; fold axis measurements), show the preservation of D2 (328) and D3 (18) deformations of Late Cambrian to Ordovician-Silurian age in El Escondido Formation, and recognized in several locations in the regional Late Precambrian metamorphic rocks of the Cordoba Pampean ranges. The stratigraphical position of the La Clemira and El Escondido Formations is restricted to the Cambrian-Early Ordovician, and could probably correlate with Las Brenas Formation or the Cambrian-Ordovician deposits of the Las Brenas Basin. A modification of the boundary of the Early Paleozoic Chacoparana and the Las Brenas basins is therefore suggested, extending them towards the southwest over the southern Santiago del Estero Hills.

Descripción: A major and trace element based characterization of the metasedimentary protoliths of three metamorphic units of Sierra de San Luis (Pringles Metamorphic Complex, San Luis Formation and Conlara Metamorphic Complex) is presented. Geochemistry indicates a dominance of shales in the protoliths of San Luis Formation, whereas greywackes and shales made up the Conlara Metamorphic complex and mainly greywackes, the Pringles Metamorphic Complex. Both major element data and trace element ratios (i.e. Th/Sc, Th/U,) indicate a source with an average upper crustal composition for the protoliths of the Pringles Metamorphic Complex, the San Luis Formation and the shales of the Conlara Metamorphic complex. A component with less evolved signature may be inferred for the metagreywackes of the Conlara metamorphic Complex. Mixed sourced detritus are indicated for the three units with clastic material resulting mainly from both andesitic and acidic/recycled detritus. The overall data consistently suggest a continental island arc and/or active margin setting as the more probable geodynamic scenario for the deposition of the sedimentary precursors of the studied units. In this context, a back-arc setting can account for the mixed nature of the inferred source areas with uplifted old basement and arc-related detritus as the end members of the mixtures. The inferred back-arc basin would have evolved through the Cambrian receiving the sediments derived from the Pampean Orogen to the east combined with probably some old crust exposures and to the west the source might have been controlled by the active continental margin. © 2003 Asociación Geológica Argentina.

Descripción: In the southern segment of the Copahue- Pino Hachado block, next to the Moquehue-Alumine lacustrine basin, an anomalous concentration of calderas, resulting from a Pliocene-Quaternary volcanic activity was recognized. The Ñireco caldera, which was proposed as part of this caldera complex, is the reason of the present work. The authors object the interpretation of such a morphostructural feature on the basis of a detailed geomorphologic, structural and stratigraphic analyses of the area. The limits of the proposed caldera coincide mostly with fluvial valleys that seem to be controlled by previous structures like faults and regional lineaments. There are no evidences that could suggest the presence of a volcanic complex that theoretically contains such a depression, or for its external and internal flanks. The central sector of the proposed caldera presents the highest altitude values of the region (± 2,900 m a.s.l.), which highly surpass those of the Principal Cordillera at these latitudes. The local stratigraphy of the area, is dominated by the oldest rocks of the region (Upper Paleozoic granitoids and Triassic volcanic and volcaniclastic sequences), and shows not temporal coincidence with the Pliocene-Quaternary volcanic activity, well represented immediately to the north. The study area falls on the contrary into a thick-skinned west-verging fold-and-thrust-belt, where tectonic inversion played a central role.

Descripción: A palaeomagnetic study was made on the Early Permian Del Salto Formation, exposed in the Western Precordillera (31.3°S, 69.2°W). Eleven sites were sampled for siltstone and fine-grained sandstone in the middle-upper part of the succession exposed in the creek at kilometre 114, San Juan. Standard demagnetization techniques permitted the identification of a characteristic remanence that showed good within-site consistency of direction at eight sites. Application of a tectonic correction showed that the characteristic remanence is pre-tectonic. A palaeomagnetic pole for the Del Salto Formation was computed, SC: 26.8°S, 357.0°E.δp = 4.4°, δm = 6.4°, N = 8. The position of SC indicates that the sampling locality underwent a counterclockwise rotation of about 40° around a vertical axis. The age of the rotation cannot be determined from the available data but it is most likely to have happened in either Permian or Late Cenozoic times.

Descripción: Group of microstructures present in the Ordovician Yerba Loca and Los Sombreros Formations (cropping out in the vicinity of Road 150, between cuesta del Viento and Los Túneles), are described and interpreted. Cleavage, joints and fractures, shear zones, planar and sigmoid (en echelon) veins and kink bands are most common among them. Folded and domino faulted quartz veins, resulting from pressure solution contraction during formation of axial plane cleavage, are an outstanding feature and allowed calculation of contraction associated to cleavage development. The assemblage of minor structures shows coherency with the major structures hosting them and evidences a systematic displacement and stacking of rock masses from west to east. They also show the important participation of pressure solution and re-crystallization processes during deformation, along with progressive deformation due to rotation of local structures in the regional stress field.

Descripción: The Sierras Bayas Group of Late Precambrian age is formed by sedimentary Villa Mónica, Cerro Largo and Loma Negra formations. It is located in the northwest area of Sierras Septentrionales of Buenos Aires Province. The outcrops of this group are located around Olavarría city, mainly in the low isolated hills of Sierras Bayas and also though in a limited way, in the Barker city area. The morphstructure of these sedimentary rocks in the Northern sector of Sierras Bayas is defined by an asymmetric basin subtly tilted to south-southwest. Two interfering fold systems, F1(NE) and F2(NW) which define a type 1 domes and basins pattern, were recognized based on systematic structural mapping of de Sierras Bayas Group formations. Both F1 and F2 foldings with interference angles between 70° to 90° have subvertical axial planes and subhorizontal B axis. In adopted mapping scales, folding systems were observed in three fold orders which exert a direct control over natural an artificial outcrops of the Sierras Bayas Group. The less important local faulting exerts a poor control over the attitude of Late Precambrian folded Sierras Bayas Group and field relationships supports that it's initial activity postdates folding deformation. Ductile deformation of Sierras Bayas Group sedimentary cover, as was locally observed, is thought to be associated with a tectonic reactivation of the basement, Buenos Aires Complex, furnished by pre-existing planar anisotropies of the mylonitized granitic rock mass. The resemblance F1 and F2 patterns of foldings, the overall basin morphostructure and absence of a clear interference sequence between the foldings could be related with a sole episode of constrictional deformation. But also, a type 1 folding interference pattern could have been formed by two regional, orthogonal contractional deformations, in rehologic conditions of low ductility contrast. Folding interference of SBG is considered a result of tectonic activity occurred during Late Precambrian to Lower Early Paleozoic, and predates Cambro - Ordovicic Balcarce Formation of the Sierras Septentrionales. The ductile deformation of Sierras Bayas Group with basement involvement by reactivation of main penetrative anisotropies is correlated with comparable structural patterns owing to Pan African Tectonics, in coetaneous Damara Sequence and Nama Group of Southwest Africa. © 2005 Asociación Geológica Argentina.

Descripción: Structures cropping out in the Jachal Valley from the Cuesta del Viento dam to the locality of Los Tuneles are described and analyzed. These structures affect the Ordovician Yerba Loca and Los Sombreros Formations and the Paleogene-Neogene Vallecito and Rodeo Formations. The oldest units form the heights, Sierra Negra de Rodeo and Sierra de los Túneles, which are limited in their eastern flanks by the La Tranca and Los Blanquitos overthrusts, and by the angular unconformity underlying Rodeo Formation in their western ones. The regional folds show asymmetrical shapes and a wavelength comparable to the width of the Sierras, but minor structures ranging from meters to tens of meters are very common in their eastern limbs. Regional folds axis trend varies from Az 315 to Az 360 and the vergence of pre-carboniferous and andean structures point to the east, what makes very difficult to use orientation as a criterion to discriminate the differentiated deformational episodes. The comparison of sedimentological and structural features in the Rodeo area with those at the San Juan River evidences strong similarities but also important differences. Among the first ones, it can be mentioned the lithological and structural resemblance of the Ordovician units, which show NS trending axis in the eastern outcrops and point N-NW in the western ones. Main differences are the absence -in the Rodeo area- of the metapelites of the Silurian Calingasta Formation. and that of the conglomerates and rhythmites of the Devonian (?) El Planchón Formation. present in the San Juan River section.

Descripción: The geological observations made by Darwin in 1835 during his crossing of the Andes from Santiago to Mendoza via the Piuquenes Pass and Cordón del Portillo are compared with the present geological knowledge of the Cordillera Principal and Cordillera Frontal at 33°-34°S. The analysis of the complex stratigraphy of the Cordillera Principal, the imbricated structure of the Aconcagua fold and thrust belt, as well as the stratigraphy and structure of the inter mountain foreland Tunuyán Basin, allows to assess the pioneer observations of Darwin. He recognized the old metamorphic basement and the granitoids and volcanic sequences of late Paleozoic to Triassic age of the Cordillera Frontal, established the Cretaceous age of the marine successions cropping out along the eastern Cordillera Principal and studied the conglomeratic deposits associated with the uplift of the Cordillera in the Alto Tunuyán Basin. Based on the study of clast provenance of the synorogenic deposits of the Alto Tunuyán Basin, Darwin recognized that the Cordillera Frontal was uplifted later than the Cordillera Principal. The present knowledge of this sector of the Andean Cordillera confirms his pioneer observations and show that Darwin was one of the first scientists ever in realizing that in an orogenic system the sequence of uplift and deformation proceeds from hinterland towards foreland, according to a process that is exceptionally well-illustrated along the Piuquenes-Cordón del Portillo transect.

Descripción: During the Early Permian an extensive deformation belt well-known as Gondwánides developed in the southwest Gondwana margin. A foreland basin has been settled during the Late Carboniferous - Early Permian in the western end of this strip, and is represented by the El Imperial Formation, which crops out in the San Rafael Block. The upper section of this unit records an inversion of the ancient slopes, which has been interpreted as the first evidence of the migration of the orogenic front towards the foreland. Therefore, a paleomagnetic study has been carried out on this unit, searching for elements that allow to constraint the deformation age to evaluate the possible diachronism of it. First paleomagnetic results obtained in EI Imperial Formation, from the lower member in the Represa Agua del Toro and from the upper member in the Puesto La Josefa, are presented and discussed. These rocks have reverse magnetizations carried by hematite, of possible Permian age. Pole positions are discordant with the apparent polar wander path from South America. These differences could be explained because: secular variation was not properly averaged; rotations about vertical axes of the sampling localities; or the combination of these factors. © 2006 Asociación Geológica Argentina.

Descripción: Ciénaga del Río Huaco Formation was deposited during the Cretaceous in the Precordillera Central (La Rioja and San Juan provinces). The unit forms part of a thick red-bed sequence including rocks of different ages (from Late Paleozoic to Tertiary) and problematic identification. In the La Troya river area, the red bed sequence is composed of the Panacán (Permian), Santo Domingo (Late Triassic- Early Jurassic), Ciénaga del Río Huaco (Cretaceous) and Puesto La Flecha Formations (Oligocene- Early Nfiocene). In the past, all these formations were included in the same stratigrapbic unit and only in recent years the different formations could be separated mainly based on their fossil contain. In the case of Ciénaga del Rio Huaco Formation, the finding of microfossils (palynomorphs and ostracods) allowed to infer a Late Cretaceous age for these rocks. However, a radiometric age of 108,1 ± 4,4 Ma presented in this paper, clearly suggest that the unit was deposited at the end of the Early Cretaceous. This age is the first Cretaceous absolute age in the Precordillera Central and enhances the time constraints of of the Cretaceous Basin.

Descripción: Quartzitic sandstones of the sierras Bayas Group in Sierras Septentrionales of Buenos Aires are studied. Outcrops of these deformed rocks are observed at sierras Bayas and Barker localities, where quartzitic sandstones from Precambrian units are present. These rocks are compared with similar fault breccias in Sierras Australes of Buenos Aires, which were develop in quartzitic rocks of several Paleozoic units of this region. At both regions, fault zones are subvertical layers with acute contacts on the wall rocks. An original sedimentary protolith is recognized in textures and structures preserved in all samples of Sierras Bayas. Sedimentary textures at Sierras Australes are obliterated by penetrative deformation in greenschists facies methamorphism and also show a metaquartzitic protolith. In both analyzed regions, similar cataclastic textures are observed. All these breccias and microbreccias would be associated with a major extensional tectonic event of Jurassic - Cretaceous age, which could be related to the origin of Colorado and Salado extensional basins during the opening of the South Atlantic Ocean.