Permian brachiopods from the Tupe formation, San Juan Province, Precordillera, Argentina

The brachiopod fauna from the Tupe Formation at La Herradura Creek, located on the west flank of Perico Hill, San Juan Province, Argentina, palaeogeographically belongs to the western sector of the Paganzo basin ('Guandacol embayment'). The stratigraphical section of the Tupe Formation at La Herradura Creek is the stratotype of the Tivertonia jachalensis-Streptorhynchus inaequiornatus biozone, was previously regarded as being of Late Carboniferous age but here is assigned to the earliest Permian (Asselian). We describe and review the biozone assemblage, which consists of Streptorhynchus inaequiornatus, Tivertonia jachalensis, Kochiproductus sp., Costatumulus sp., Coronalosia argentinensis, Tupelosia paganzoensis, Trigonotreta pericoensis, Septosyringothyris sp. aff. Septosyringothyris jaguelensis and Crurithyris? sp. This brachiopod assemblage is related to Indian and Australian Early Permian faunas and its presence in the La Herradura Creek section provides new evidence in support of an Asselian (Early Permian) age for the Tivertonia jachalensis-Streptorhynchus inaequiornatus biozone. This assemblage is also important for intra- and inter-basinal correlation because several of its characteristic species have been identified from other sections of the Paganzo basin and the Riacuteo Blanco basin. The proposed age for this biozone is consistent with the age of palynological data from slightly above the marine faunas from the stratotype locality.

Gravity collapse structures in the Central Precordillera fold-andthrust belt, Argentina

Several landforms found in the fold-and-thrust belt area of Central Precordillera, Pre-Andes of Argentina, which were often associated with tectonic efforts, are in fact related to non-tectonic processes or gravitational superficial structures. These second-order structures, interpreted as gravitational collapse structures, have developed in the western flank of sierras de La Dehesa and Talacasto. These include rock-slides, rock falls, wrinkle folds, slip sheets and flaps, among others; which together constitute a monoclinal fold dipping between 30º and 60º to the west. Gravity collapse structures are parallel to the regional strike of the Sierra de la Dehesa and are placed in Ordovician limestones and dolomites. Their sloping towards the west, the presence of bed planes, fractures and joints; and the lithology (limestone interbedded with incompetent argillaceous banks) would have favored their occurrence. Movement of the detached structures has been controlled by lithology characteristics, as well as by bedding and joints. Detachment and initial transport of gravity collapse structures and rockslides in the western flank of the Sierra de la Dehesa were tightly controlled by three structural elements: 1) sliding surfaces developed on parallel bedded strata when dipping >30° in the slope direction; 2) Joint’s sets constitute lateral and transverse traction cracks which release extensional stresses and 3) Discontinuities fragmenting sliding surfaces.  Some other factors that could be characterized as local (lithology, structure and topography) and as regional (high seismic activity and possibly wetter conditions during the postglacial period) were determining in favoring the steady loss of the western mountain side in the easternmost foothills of Central Precordillera.

Famatina: dos siglos de violencia

El Cerro Nevado de Famatina también conocido como General Belgrano es una montaña de 6.250 metros de altura. Se encuentra en el centro de la provincia argentina de La Rioja. Por su altura podría pertenecer a la Cordillera de los Andes, ya que además su cima permanece nevada siempre, el cerro está dentro de la Precordillera, por eso tiene valles al Este y al Oeste. Es la cima de la denominada Sierra de Famatina, inserta del cordón montañoso de las Sierras Pampeanas. El más elevado de los picos del Cerro Famatina sirve como hito en donde coinciden los límites de los siguientes departamentos riojanos. Al sudeste del mismo se encuentra Chilecito; al noreste, Famatina; al noroeste, Vinchina y al suroeste, General Lamadrid. Famatina viene de Wamatinaj, tal como eran llamados eran llamados los pueblos originarios de la etnia Diaguita y parcialidad Capayán que habitaban en el valle ubicado al oriente de esta montaña. El cerro conoce una larga historia en materia de explotación de sus recursos.

30 million years of Permian volcanism recorded in the Choiyoi igneous province (W Argentina) and their source for younger ash fall deposits in the Parana Basin: SHRIMP U-Pb zircon geochronology evidence

We present four SHRIMP U-Pb zircon ages for the Choiyoi igneous province from the San Rafael Block, central-western Argentina. Dated samples come from the Yacimiento Los Reyunos Formation (281.4 +/- 2.5 Ma) of the Cochico Group (Lower Choiyoi section: andesitic breccias, dacitic to rhyolitic ignimbrites and continental conglomerates). Agua de los Burros Formation (264.8 +/- 2.3 Ma and 264.5 +/- 3.0 Ma) and Cerro Carrizalito Formation (251.9 +/- 2.7 Ma Upper Choiyoi section: rhyolitic ignimbrites and pyroclastic flows) spanning the entire Permian succession of the Choiyoi igneous province. A single ziron from the El Imperial Formation, that is overlain unconformably by the Choiyoi succession, yielded an early Permian age (297.2 +/- 5.3 Ma). while the main detrital zircon population indicated an Ordovician age (453.7 +/- 8.1 Ma). The new data establishes a more precise Permian age (Artinskian-Lopingian) for the section studied spanning 30 Ma of volcanic activity. Volcanological observations for the Choiyoi succession support the occurrence of explosive eruptions of plinian to ultraplinian magnitudes, capable of injecting enormous volumes of tephra in the troposphere-stratosphere. The new SHRIMP ages indicate contemporaneity between the Choyoi succession and the upper part of the Parana Basin late Paleozoic section, from the Irad up to the Rio do Rasto formations, encompassing about 24 Ma. Geochemical data show a general congruence in compositional and tectonic settings between the volcanics and Parana Basin Permian ash fall derived layers of bentonites. Thickness and granulometry of ash fall layers broadly fit into the depletion curve versus distance from the remote source vent of ultraplinian eruptions. Thus, we consider that the Choiyoi igneous province was the source of ash fall deposits in the upper Permian section of the Parana Basin. Data presented here allow a more consistent correlation between tectono-volcanic Permian events along the paleo-Pacific margin of southwestern Gondwana and the geological evolution of neighboring Paleozoic foreland basins in South America and Africa. (C) 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

The Guarguaraz Complex and the Neoproterozoic-Cambrian evolution of southwestern Gondwana: Geochemical signatures and geochronological constraints

The Guarguaraz Complex, in western Argentina, comprises a metasedimentary assemblage, associated with mafic sills and ultramafic bodies intruded by basaltic dikes, which are interpreted as Ordovician dismembered ophiolites. Two kinds of dikes are recognized, a group associated with the metasediments and the other ophiolite-related. Both have N-MORB signatures, with epsilon(Nd) between +3.5 and +8.2, indicating a depleted source, and Grenville model ages between 0.99 and 1.62 Ga. A whole-rock Sm-Nd isochron yielded an age of 655 +/- 76 Ma for these mafic rocks, which is compatible with cianobacteria and acritarchae recognized in the clastic metasedimentary platform sequences, that indicate a Neoproterozoic (Vendian)-Cambrian age of deposition. The Guarguaraz metasedimentary-ophiolitic complex represents, therefore, a remnant of an oceanic basin developed to the west of the Grenville-aged Cuyania terrane during the Neoproterozoic. The southernmost extension of these metasedimentary sequences in Cordon del Portillo might represent part of this platform and not fragments of the Chilenia terrane. An extensional event related to the fragmentation of Rodinia is represented by the mafic and ultramafic rocks. The Devonian docking of Chilenia emplaced remnants of ocean floor and slices of the Cuyania terrane (Las Yaretas Gneisses) in tectonic contact with the Neoproterozoic metasediments, marking the Devonian western border of Gondwana. (C) 2009 Elsevier Ltd. All rights reserved.

The Permian brachiopod genus Trigonotreta Koenig and its occurrence in Argentina

A review of the Permian spiriferid brachiopod genus Trigonolrela and its occurrence in the Early Permian of Argentina is provided herein. Several species are known from the Late Palaeozoic sequences of the Argentinean Precordillera. These include Trigonolrela sp. and Trigonolrela riojanensis (Lech and Acenolaza), from the Rio del Peii.on Formation (Rio Blanco Basin), Trigonolrela pericoensis (Leanza), from the Tupe Formation at the La Herradura creek locality (Paganzo Basin) and Trigonolrela sanjuanensis (Lech and Acenolaza), from Del Salto Formation (Calingasta-Uspallata Basin). These species are characterised by being small to medium sized, relatively transverse, with cardinal extremities often strongly angular. Costae are weakly bifurcated and superimposed on weakly developed lateral flank plications adjacent to the fastigium and sulcus. The Argentinean species are close to the oldest known Indian species of the genus, Trigonotreta hesdoensis (Salmi and Dutt), particularly with respect to the nature of its weakly fasciculated costae. Further study will refine the details of the relationship of the South American species with those from elsewhere in Gondwana and may permit the recognition of a distinctive lineage. The presence of the genus in Argentina in the earliest Permian is an important palaeobiogeographical observation that raises questions about the probable migration routes of the genus from the western Gondwanan South American margin to eastern Australia and India. The Precordilleran region appears to be the likely site of the first appearance of Trigonolrela. Species with relatively simple costae appeared first. These gave rise to more complex species with a more elaborate costal pattern indicating an evolutionary progression through time.

Carboniferous brachiopods from the "Levipustula Fauna" in central-western Argentina : biostratigraphic, palaeoclimatic, and palaeobiogeographic implications

The “Levipustula fauna” is a relatively diverse fossil assemblage composed of brachiopods, bivalves, bryozoans, gastropods and crinoids that appears in glaciomarine sequences related to the Carboniferous glacial event that affected the central-western Argentinean basins. Brachiopods that characterize this fauna have been studied in some of the classical localities of the Argentine Precordillera in San Juan province.

In the Hoyada Verde Formation, the “Levipustula fauna” is usually located immediately above the glacial diamictite horizons and appears to be associated with the mudstones facies of the postglacial transgression. From a palaeoecological study, three brachiopod subfaunas are distinguished in this formation: the lower “Neospirifer” (“Trigonotretidae” gen. nov.) subfauna, above which the more diversified Kitakamithyris subfauna occurs, and the upper Levipustula subfauna, with the dominant species being attributed to Levipustula levis Maxwell. The Hoyada Verde fauna, as well those identified in the La Capilla Formation (Cerro El Morado locality), have been proposed as the typical “Levipustula fauna”. However, in the Leoncito Formation, the “Levipustula fauna” occurs in sandstones horizons located below the glacial diamictite beds. This fauna is poorly diversified and the brachiopods are characterized by “Neospirifer” (“Trigonotretidae” gen. nov.)-Septosyringothyris assemblage and Levipustula is not abundant. This fauna has been considered a colder “pre-interglacial fauna”.

The significant taxonomic, palaeoecologic and taphonomic variations of the “Levipustula fauna”, as well as its position in the glacial sequences, suggest an important palaeoenvironmental control related to Carboniferous glacial dynamics. From the palaeobiogeographic viewpoint, this fauna shows the highest affinity with the Eastern Australian basins from where it was previously described.

Ordovician klippen structures of the Sierra de Umango: New insights on Tectonic evolution of the Western Sierras Pampeanas, Argentina

The basement rock of the Pampean flat-slab (Sierras Pampeanas) in the Central Andes was uplifted and rotated in the Cenozoic era. The Western Sierras Pampeanas are characterised by meta-igneous rocks of Grenvillian Mesoproterozoic age and metasedimentary units metamorphosed in the Ordovician period. These rocks, known as the northern Cuyania composite terrane, were derived from Laurentia and accreted toward Western Gondwana during the Early Paleozoic. The Sierra de Umango is the westernmost range of the Western Sierras Pampeanas.This range is bounded by the Devonian sedimentary rocks of the Precordillera on the western side and Tertiary rocks from the Sierra de Maz and Sierra del Espinal on the eastern side and contains igneous and sedimentary rocks outcroppings from the Famatina System on the far eastern side. The Sierra de Umango evolved during a period of polyphase tectonic activity, including an Ordovician collisional event, a Devonian compressional deformation, Late Paleozoic and Mesozoic extensional faulting and sedimentation (Paganzo and Ischigualasto basins) and compressional deformation of the Andean foreland during the Cenozoic. A Nappe System and an important shear zone, La Puntilla-La Falda Shear Zone (PFSZ), characterise the Ordovician collisional event, which was related to the accretion of Cuyania Terrane to the proto-Andean margin of Gondwana. Three continuous deformational phases are recognised for this event: the D1 phase is distinguished by relics of 51 preserved as internal foliation within interkinematic staurolite por-phyroblasts and likely represents the progressive metamorphic stage; the D2 phase exhibits P-T conditions close to the metamorphic peak that were recorded in an 52 transposition or a mylonitic foliation and determine the main structure of Umango; and the D3 phase is described as a set of tight to recumbent folds with S3 axial plane foliation, often related to thrust faults, indicating the retrogressive metamorphic stage. The Nappe System shows a top-to-the S/SW sense direction of movement, and the PFSZ served as a right lateral ramp in the exhumation process. This structural pattern is indicative of an oblique collision, with the Cuyania Terrane subducting under the proto-Andean margin of Gondwana in the NE direction. This continental subduction and exhumation lasted at least 30 million years, nearly the entire Ordovician period, and produced metamorphic conditions of upper amphibolite-to-granulite facies in medium- to high-pressure regimes. At least two later events deformed the earlier structures: D4 and D5 deformational phases. The D4 deformational phase corresponds to upright folding, with wavelengths of approximately 10 km and a general N-S orientation. These folds modified the S2 surface in an approximately cylindrical manner and are associated with exposed, discrete shear zones in the Silurian Guandacolinos Granite. The cylindrical pattern and subhorizontal axis of the D4 folds indicates that the S2 surface was originally flat-lying. The D4 folds are responsible for preserving the basement unit Juchi Orthogneiss synformal klippen. This deformation corresponds to the Chanica Tectonic during the interval between the Devonian and Carboniferous periods. The D5 deformational phase comprehends cuspate-lobate shaped open plunging folds with E W high-angle axes (D5 folds) and sub-vertical spaced cleavage. The D5 folds and related spaced cleavage deformed the previous structures and could be associated with uplifting during the Andean Cycle. (C) 2012 Elsevier Ltd. All rights reserved.