Large geographic and temporal extensions of the Río de la Plata Craton,South America,and its metacratonic eastern margin

Integration of existing isotopic and geological data allows a reconsideration of the distribution and age of the Río de la Plata Craton within South America. The reinterpretation increases the area of the craton to about 2,400,000 km² with implications for the tectonic map of South America and for global reconstruction of palaeocontinents. Four areas previously considered as separate cratons (Luís Alves, Curitiba, Tebicuary, and Paranapanema) are interpreted as part of the same Río de la Plata Craton. The craton is organized into six provinces and domains: Buenos Aires–Piedra Alta, Taquarembó, Tebicuary, Luís Alves, Encantadas, and Nico Pérez. The term ‘Transplatense’ is proposed to replace ‘Trans-Amazonian’ for Rhyacian events that occurred within the Río de la Plata Craton. The craton is formed not only by dominant Rhyacian rocks and local Archaean rocks, but also by Statherian and Mesoproterozoic rocks. The domains are all partially to totally covered by Phanerozoic basins (Paraná, Chacoparanense, Claromecó, Salado, Balcarce, and Colorado) which makes their investigation difficult. The Ediacaran–Cambrian collisions of the Brasilian orogen generated tectonic mixtures of orogenic and cratonic zones. This is more evident in the eastern margin of the craton, which behaved as a metacraton.     

Shear zone evolution and timing of deformation in the Neoproterozoic transpressional Dom Feliciano Belt,Uruguay

New structural, microstructural and geochronological (U-Pb LA-ICP-MS, Ar/Ar, K-Ar, Rb-Sr) data were obtained for the Dom Feliciano Belt in Uruguay. The main phase of crustal shortening, metamorphism and associated exhumation is recorded between 630 and 600 Ma. This stage is related to the collision of the Río de la Plata and Congo cratons at ca. 630 Ma, which also involved crustal reworking of minor crustal blocks such as the Nico Pérez Terrane and voluminous post-collisional magmatism. Subsequent orogen-parallel sinistral shearing gave rise to further deformation up to ca. 584 Ma and resulted from the onset of the convergence of the Kalahari Craton and the Río de la Plata-Congo cratons. Sinistral shear zones underwent progressive strain localization and retrograde conditions of deformation during crustal exhumation. Dextral ENE-striking shear zones were subsequently active at ca. 550 Ma, coeval with further sinistral shearing along N- to NNE-striking shear zones. The tectonothermal evolution of the Dom Feliciano Belt thus recorded the collision of the Río de la Plata and Congo cratons, which comprised one of the first amalgamated nuclei of Gondwana, and the subsequent incorporation of the Kalahari Craton into Western Gondwana.     

Neoproterozoic geodynamic evolution of SW-Gondwana: a southern African perspective

Our current understanding of the tectonic history of the principal Pan-African orogenic belts in southwestern Africa, reaching from the West Congo Belt in the north to the Lufilian/Zambezi, Kaoko, Damara, Gariep and finally the Saldania Belt in the south, is briefly summarized. On that basis, possible links with tectono-stratigraphic units and major structures on the eastern side of the Río de la Plata Craton are suggested, and a revised geodynamic model for the amalgamation of SW-Gondwana is proposed. The Río de la Plata and Kalahari Cratons are considered to have become juxtaposed already by the end of the Mesoproterozoic. Early Neoproterozoic rifting led to the fragmentation of the northwestern (in today??s coordinates) Kalahari Craton and the splitting off of several small cratonic blocks. The largest of these ex-Kalahari cratonic fragments is probably the Angola Block. Smaller fragments include the Luis Alves and Curitiba microplates in eastern Brazil, several basement inliers within the Damara Belt, and an elongate fragment off the western margin, named Arachania. The main suture between the Kalahari and the Congo-S?o Francisco Cratons is suspected to be hidden beneath younger cover between the West Congo Belt and the Lufilian/Zambezi Belts and probably continues westwards via the Cabo Frío Terrane into the Goiás magmatic arc along the Brasilia Belt. Many of the rift grabens that separated the various former Kalahari cratonic fragments did not evolve into oceanic basins, such as the Northern Nosib Rift in the Damara Belt and the Gariep rift basin. Following latest Cryogenian/early Ediacaran closure of the Brazilides Ocean between the Río de la Plata Craton and the westernmost fragment of the Kalahari Craton, the latter, Arachania, became the locus of a more than 1,000-km-long continental magmatic arc, the Cuchilla Dionisio-Pelotas Arc. A correspondingly long back-arc basin (Marmora Basin) on the eastern flank of that arc is recognized, remnants of which are found in the Marmora Terrane??the largest accumulation of oceanic crustal material known from any of the Pan-African orogenic belts in the region. Corresponding foredeep deposits that emerged from the late Ediacaran closure of this back-arc basin are well preserved in the southern areas, i.e. the Punta del Este Terrane, the Marmora Terrane and the Tygerberg Terrane. Further to the north, present erosion levels correspond with much deeper crustal sections and comparable deposits are not preserved anymore. Closure of the Brazilides Ocean, and in consequence of the Marmora back-arc basin, resulted from a change in the Río de la Plata plate motion when the Iapetus Ocean opened between the latter and Laurentia towards the end of the Ediacaran. Later break-up of Gondwana and opening of the modern South Atlantic would have followed largely along the axis of the Marmora back-arc basin and not along major continental sutures.     

Development and validation of a storm surge forecasting/hindcasting modelling system for the extensive Río de la Plata Estuary and its adjacent Continental Shelf

Natural Hazards - The Río de la Plata (RdP) Estuary is affected by significant surges several times per year. This phenomenon has historically caused catastrophic water-level enlargements of...     

The transpressional connection between Dom Feliciano and Kaoko Belts at 580�C550?Ma

A new U?CPb SHRIMP age of 551?±?4?Ma on a mylonitic porphyry that intruded into the Sierra Ballena Shear Zone (Southernmost Dom Feliciano Belt, Uruguay) and a review of relevant published data make possible a more refined correlation and reconstruction of Brasiliano/Pan-African transpressional events. Paleogeographic reconstruction, kinematics and timing of events indicate a connection between the shear systems of the Dom Feliciano and Kaoko Belts at 580?C550?Ma. Sinistral transpression recorded in shear zones accommodates deformation subsequent to collision between the Congo and Río de la Plata Cratons. The correlation is strengthened by the similarity of magmatic and metamorphic ages in the Coastal Terrane of the Kaoko Belt and the Punta del Este Terrane of the Dom Feliciano Belt. This post-collisional sinistral transpression brought these units near to their final position in Gondwana and explains the different evolution at 550?C530?Ma. While in the Kaoko Belt, an extensional episode resulted in exhumation as a consequence of collision in the Damara Belt, in the Dom Feliciano Belt, sinistral transpression occurred associated with the closure of the southern Adamastor Ocean due to Kalahari-Río de la Plata collision.     

Magnetotelluric characterization through the Ambargasta-Sumampa Range: The connection between the northern and southern trace of the Río de La Plata Craton – Pampean Terrane tectonic boundary

The South American Platform was part of the Western Gondwana, a collage of plates of different ages assembled in late Neoproterozoic to Cambrian times. The Transbrasiliano Lineament, a continental shear belt that transversely intersects this platform from NE to SW, has its southern expression in the tectonic boundary between the Río de La Plata Craton and the Pampean Terrane. Magnetotelluric long-period data in a W–E profile (29°30′ S) that crosses the Ambargasta-Sumampa Range and the Chaco-Pampean Plain were obtained to connect information of this mostly inferred tectonic boundary. A 2-D inversion model shows the Chacoparanense basin, Río Dulce lineament, Ambargasta-Sumampa Range and Salina de Ambargasta in the upper crust. At mid-to-lower crust and 40 km to the east of the Ambargasta-Sumampa Range, a discontinuity (500–2000 Ω m) of 20-km-wide separates two highly resistive blocks, the Río de La Plata Craton (6000–20,000 Ω m) in the east, and the Pampean Terrane (5000–20,000 Ω m) in the west. This discontinuity represents the tectonic boundary between both cratons and could be explained by the presence of graphite. The geometry of the Pampean Terrane suggests an east-dipping paleo-subduction. Our results are consistent with gravimetric and seismicity data of the study area. A more conductive feature beneath the range and the tectonic boundary was associated with the NE–SW dextral transpressive system evidenced by the mylonitic belts exposed in the Eastern Pampean Ranges. This belt represents a conjugate of the mega-shear Transbrasiliano Lineament and could be explained by fluid–rock interaction by shearing during hundreds of years. The eastern border of the Ambargasta-Sumampa Range extends the trace of the Transbrasiliano Lineament. The electrical Moho depth (40 km to the west and 35 km to the east) was identified by a high electrical contrast between the crust and upper mantle. The upper mantle shows a resistive structure beneath the Río de La Plata Craton that could have been originated by stationary delamination by the presence of hydrated lithosphere.     

The Record of the Varanger Glaciation at the Río De La Plata Craton,Vendian-Cambrian of Uruguay

A sedimentary succession included in the lower section of the Playa Hermosa Formation from the Playa Verde Basin, Uruguay, is described. Two facies associations, one mainly coarse- to medium-grained and other one fine-grained, have been defined (FA I-II). In the first one, breccias, conglomerates, sandstones and minor mudstones were deposited in a subaqueous depositional setting (proximal) suggesting slope instability and resedimentation processes. The second one contains diamictites, rhythmites, sandstones and mudstones and presents abundant evidence of soft-deformation, also interpreted to be deposited in a subaqueous environment (distal). Dropstones, clast layers, diamictites, rhythmites and varve-like deposits are interpreted as ice rafting processes generated during a glacial episode. This glacial-related succession constitutes the first record from the Varanger glaciation at the Río de la Plata Craton of the late Neoproterozoic age and also represents one of the oldest sedimentary records after the collision of the Río de la Plata and Kalahari Cratons. A combined interaction of extensional faulting and glaciation in a tectonically active basin with locally high subsidence rates, resulted in high rates of sedimentation and resedimentation processes. As a whole, the sedimentary succession sets a relevant datum to be used in future paleogeographic reconstructions of the Vendian glacial record in southern South America.     

The R��o de la Plata Craton: a review of units, boundaries, ages and isotopic signature

A review of the lithostratigraphic units in the Río de la Plata Craton and of new and previously published geochronological, isotopic and geophysical data is presented. Sm?CNd T_DM model ages between 2.6 and 2.2?Ga characterize the Piedra Alta Terrane of this craton. Crystallization ages between 2.2 and 2.1?Ga for the metamorphic protoliths and 2.1?C2.0?Ga for the post-orogenic granitoids indicate juvenile crust, followed by a short period of crustal recycling. Cratonization of this terrane occurred during the late Paleoproterozoic. Younger overprinting is not observed, suggesting it had a thick and strong lithosphere in the Neoproterozoic. A similar scenario is indicated for the Tandilia Belt of Argentina. Sm?CNd T_DM model ages for the Nico Pérez Terrane show two main events of crustal growth (3.0?C2.6?and 2.3?C1.6?Ga). The crystallization ages on zircon ranges between 3.1 and 0.57?Ga, which is evidence for long-lived crustal reworking. The age for cratonization is still uncertain. In the Taquarembó Block, which is considered the prolongation of the Nico Pérez Terrane in southern Brazil, a similar scenario can be observed. These differences together with contrasting geophysical signatures support the redefinition of the Río de la Plata Craton comprising only the Piedra Alta Terrane and the Tandilia Belt. The Sarandí del Yí Shear Zone is regarded as the eastern margin of this Craton.     

Interactions Between An Alluvial-Aquifer Wellfield And The Scioto River,Ohio, USA

Data from wellfields in the Scioto River valley aquifer near Piketon, Ohio, USA, were used to evaluate groundwater quantity and quality and to formulate wellfield-management concepts for future use of the alluvial aquifer. The percentage of induced river infiltration pumped from wells next to the river was estimated using a numerical groundwater-flow model and chemical mass-balance techniques. These analyses indicate that 50 to 88 percent of the wellfield water is derived from induced infiltration of river water. Model simulations correlate the percentage of well water from induced infiltration to wellfield pumpage, river stage, proximity of wells to the river, riverbed hydraulic conductivity, aquifer hydraulic conductivity, and river-stage level. Induced infiltration of river water has influenced the quality of groundwater, making the wellfield water more similar to the chemical character of the river than to that of the alluvial aquifer.

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Resumen: Los datos procedentes de un conjunto de pozos en el acuífero de valle del Río Scioto, cerca de Pinkerton, Ohio; se usaron para evaluar la cantidad y calidad de las aguas subterráneas, así como para planificar la gestión de los pozos cercanos al mismo, fue estimado mediante un modelo numérico de flujo y técnicas de balance químico de masa. Los análisis indican que entre el 50 y 88 porciento del agua bombeada procede de infiltración inducida. Simulaciones realizadas con el modelo permitieron correlacionar el porcentaje del agua bombeada que proviene de infiltración inducida, con distintas variables: caudal de bombeo, tramo del río, proximidad de los pozos al río, conductividad hidráulica del lecho del río, conductividad hidráulica del acuífero y nivel del tramo de río. La infiltración inducida ha afectado a la calidad del agua de los pozos, cuyo carácter químico es más parecido al del agua del río que al del acuífero aluvial.

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Résumé: Les données provenant de champs captants dans l'aquifère de la valée de la Scioto, près de Piketon (Ohio), ont été utilisées pour évaluer la quantité et la qualité de l'eau souterraine et pour définir les conditions de gestion de l'exploitation future de cet aquifère alluvial. La part de la réalimentation par la rivière, sous l'effet de pompages, dans les eaux des puits voisins du cours d'eau, a été estimée au moyen d'un modèle numérique d'écoulement souterrain et par des méthodes chimiques du bilan de matière. Ces analyses indiquent que 50 à 88 pourcent de l'eau pompée provient de la réalimentation par la rivière. Les simultations corrèlent la part pompée provenant de la réalimentation avec les pompages, le niveau de la rivière. La réalimentation provoquée à partir de la rivière a influencé la qualité de l'eau souterraine en conférant à l'eau des puits des caractères plus proches de ceux de la rivière que de ceux de l'aquifère alluvial.

     

Mesoproterozoic evolution of the R��o de la Plata Craton in Uruguay: at the heart of Rodinia?

Mesoproterozoic volcanosedimentary units and tectonic events occurring in the Río de la Plata Craton (RPC) are reviewed. A belt consisting of volcanosedimentary successions exhibiting greenschist-facies metamorphism is exposed in the eastern RPC (Nico Pérez Terrane) in Uruguay. The Parque UTE Group consists of basic volcanics and gabbros at the base (1,492?±?4?Ma, U?CPb on zircon), carbonates in its middle part and interbedded carbonates, shales and acid volcanics (1,429?±?21?Ma, U?CPb on zircon) at the top. The Mina Verdún Group is made up of rhyolites and acid pyroclastics at its base and top, and Conophyton-bearing limestones and massive dolostones in the middle. A U?CPb LA-ICP MS zircon age of 1,433?±?6?Ma is reported here for lapilli-tuffs at the base of the Mina Verdún Group (Cerro de las Víboras Formation). This age shows that the Mina Verdún Group immediately postdates the Parque UTE Group, a fact supported by carbon isotope chemostratigraphy. Both units were deformed and metamorphosed between 1.25 and 1.20?Ga, as shown by K?CAr and Ar?CAr ages. This tectonic event affected most of the RPC and led to the accretion of the Nico Pérez Terrane to the remainder of the RPC along the Sarandí del Yí megashear. We report a U?CPb LA-ICP MS zircon age (upper intercept) of 3,096?±?45?Ma for metatonalites of the La China Complex (Nico Pérez Terrane), which yield a lower intercept age of 1,252?Ma. A proto-Andean, Mesoproterozoic belt is envisaged to account for abundant Mesoproterozoic detrital zircon ages occurring in Ediacaran sandstones of the RPC. If the RPC is fringed at both sides by Mesoproterozoic, Grenville-aged belts it is likely that it occupied a rather central position in Rodinia. A possible location between Laurentia and the Kalahari Craton, and to the south of Amazonia, is suggested.     

Assembly of Pampia to the SW Gondwana margin: A case of strike-slip docking?

Different hypotheses have been proposed to account for the geologic evolution of the southwestern margin of Gondwana in the Early Paleozoic, involving accretion and displacement of different terranes in a protracted convergent margin. In order to constrain and understand the kinematic and paleogeographic evolution of the Pampia terrane a paleomagnetic study was carried out in different Cambrian to Devonian units of the Eastern Cordillera (Cordillera Oriental) and the Interandean Zone (Interandino) of NW Argentina. Paleomagnetic poles from the Campanario Formation (Middle to Upper Cambrian): 1.5°N 1.9°E A₉₅ = 9.2° K = 37.46 N = 8; and Santa Rosita Formation (Lower Ordovician): 8.6°N 355.3°E A₉₅ = 10.1° K = 26.78 n = 9, representative of the Pampia terrane, are interpreted to indicate a Late Cambrian significant displacement with respect to the Río de la Plata and other Gondwana cratons. A model, compatible with several geological evidences that explains this displacement in the framework of the final stages of Gondwana assembly is presented. We propose a simple dextral strike-slip kinematic model in which Pampia and Antofalla (? Arequipa?) blocks moved during Late Cambrian times from a position at the present southern border of the Kalahari craton into its final position next to the Rio de la Plata craton by the Early Ordovician.     

Multiple accretion at the eastern margin of the Rio de la Plata craton: the prolonged Brasiliano orogeny in southernmost Brazil

The Neoproterozoic-Eoplalaeozoic Brasiliano orogeny at the eastern margin of the Rio de la Plata craton in southernmost Brazil and Uruguay comprises a complex tectonic history over 300?million years. The southern Brazilian Shield consists of a number of tectono-stratigraphic units and terranes. The S?o Gabriel block in the west is characterized by c.760?C690?Ma supracrustal rocks and calc-alkaline orthogneisses including relics of older, c. 880?Ma old igneous rocks. Both igneous and metasedimentary rocks have positive ??Nd(t) values and Neoproterozoic TDM model ages; they formed in magmatic arc settings with only minor input of older crustal sources. A trondhjemite from the S?o Gabriel block intruding dioritc and tonalitic gneisses during the late stages of deformation (D₃) yield an U?CPb zircon age (LA-ICP-MS) of 701?±?10?Ma giving the approximate minimum age of the S?o Gabriel accretionary event. The Encantadas block further east, containing the supracrustal Porongos belt and the Pelotas batholith, is in contrast characterized by reworking of Neoarchean to Palaeoproterozoic crust. The 789?±?7?Ma zircon age of a metarhyolite intercalated with the metasedimentary succession of the Porongos belt provides a time marker for the basin formation. Zircons of a sample from tonalitic gneisses, constituting the Palaeoproterozoic basement of the Porongos belt, form a cluster at 2,234?±?28?Ma, interpreted as the tonalite crystallization age. Zircon rims show ages of 2,100?C2,000?Ma interpreted as related to a Palaeoproterozoic metamorphic event. The Porongos basin formed on thinned continental crust in an extensional or transtensional regime between c. 800?C700?Ma. The absence of input from Neoproterozoic juvenile sources into the Porongos basin strongly indicates that the Encantadas and S?o Gabriel blocks were separated terranes that became juxtaposed next to each other during the Brasiliano accretional events. The tectonic evolution comprises two episodes of magmatic arc accretion to the eastern margin of the Rio de la Plata craton, (i) accretion of an intra-oceanic arc at c. 880?Ma (Passinho event) and (ii) accretion of the 760?C700?Ma Cambaí/Vila Nova magmatic arc (S?o Gabriel event). The latter event also includes the collision of the Encantadas block with the Rio de la Plata craton to the west. Collision and crustal thickening was followed by sinistral shear along SW?CNE-trending orogen-parallel crustal-scale shear zones that can be traced from southern Brazil to Uruguay and have been active between 660 and 590?Ma. Voluminous granitic magmatism in the Pelotas batholith spatially related to shear zones is interpreted as late- to post-orogenic magmatism, possibly assisted by lithospheric delamination. It marks the transition to the post-orogenic molasse stage. Localized deformation by reactivation of preexisting shear zones continued until c. 530?Ma and can be assigned to final stages of the amalgamation of West Gondwana.     

Identification and isotopic studies of early Cambrian magmatism (El Carancho Igneous Complex) at the boundary between Pampia terrane and the Río de la Plata craton,La Pampa province,Argentina

We have identified late Early Cambrian metaigneous rocks very poorly exposed at the Estancia El Carancho, in central La Pampa province, Argentina. They comprise calc–alkaline metadiorite and metagranite, and tholeiitic metapyroxenite and metagabbro. They are jointly referred to as the El Carancho Igneous Complex, and regarded to pertain to the Pampean magmatic arc and backarc, respectively. Titanite U–Pb SHRIMP dating of the metapyroxenite yielded 528 ± 5 Ma, and zircon U–Pb SHRIMP dating of the metadiorite yielded 520 ± 1.4 Ma. Hafnium isotope determinations on the dated zircons show ¹⁷⁶Hf/¹⁷⁷Hf ratios corresponding to positive εHf values from + 7.18 to + 9.37; Hf model ages of the Cambrian zircons yielded 884 Ma. It is interpreted that the metadiorites of the Complex crystallized from an Early Neoproterozoic (Tonian) juvenile source. We argue that the inferred occurrence of juvenile Tonian magmatic rocks in the (otherwise, mostly Paleo-Mesoproterozoic) substratum of the southern Pampia terrane could indicate a zone of thinned basement possibly associated with the early stage of Rodinia's breakup. In addition, the studied segment of the Pampean magmatic arc is contaminated by also juvenile, Late Mesoproterozoic crust, as evidenced by the presence of xenocrystic cores of 1140–1194 Ma – T_DM-Hf 1720 Ma and εHf values of + 3.24 to + 4.85 – in the Cambrian zircons, hence suggesting that the studied segment of the Pampean magmatic arc was intruded into juvenile Late Mesoproterozoic magmatic arc rocks. The El Carancho Igneous Complex would be located at the tectonic boundary between the Pampia terrane and the Río de la Plata craton. This boundary stands out in the aeromagnetic data as a change in the structural orientation about a roughly N-S line located approximately at 65° W and representing the suture zone between the Pampia terrane and the Río de la Plata craton. Our geotectonic model envisages westward dipping subduction of oceanic crust beneath the Pampia terrane; the El Carancho Igneous Complex would, therefore, have been originated on the Pampia side (upper plate) of the suture. Slivers of the arc- and backarc-type rocks would have been tectonically imbricated in the suture zone during the Pampean orogeny.     

Trophic Impacts of Marine Mammals and Seabirds in the Río de la Plata Estuary and the Nearshore Oceanic Ecosystem

As top predators, marine mammals and seabirds play an important role in structuring marine ecosystems, and their trophic role have become a major issue. In this study, we quantify the trophic impact of marine mammals and seabirds in the Río de la Plata estuary based on a previous trophic mass balance model of this estuary. The model comprised 37 functional groups: 3 species of marine mammals (2 cetaceans: Tursiops truncatus, Pontoporia blainvillei and 1 pinniped: Otaria flavescens), 1 coastal bird group comprising 11 species, 17 fish, 12 invertebrates, 2 zooplanktons, 1 phytoplankton, detritus, and 5 fishing fleets. Model results showed that seabirds, O. flavescens and P. blainvillei, are apex predators, with high levels of niche overlap among them, suggesting competition for similar resources such as fish species. Marine mammals and seabirds produce negative effects on commercially important species, while, at the same time, indirect positive effects (increase of the biomass) were also detected in some groups related to trophic cascade effects.     

Hydrogeological environment and groundwater occurrences of the alluvial aquifers in parts of the Central Ganga Plain, Uttar Pradesh, India

A detailed hydrogeological investigation was carried out in parts of the Central Ganga Plain, India, with the objective of assessing the aquifer framework and its resource potential. The area was studied because of its dual hydrogeological situation, that is water logging and soil salinization in the canal command areas and depletion of aquifers in the western part of the basin. A comprehensive investigation of the aquifer system between the Ganga River and Kali River revealed its lateral and vertical dimensions and hydrogeological characteristics. Moreover, study of the groundwater occurrences, movements and behaviour, in terms of water level fluctuation with time and space, confirms the coexistence of over exploitation as well as water logging in the area.

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Electronic Supplementary Material Supplementary material is available for this article at

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Resumen Una investigación hidrogeológica detallada se llevó a cabo en partes de la Llanura de Ganga Central, India, con el objetivo de evaluar la estructura del acuífero y su potencial del recurso. El área fue estudiada por su doble situación hidrogeológica, es decir la saturación con agua y salinización de suelos en las áreas dominadas por el canal y vaciamiento de acuíferos en el la parte occidental de la cuenca. Una investigación completa del sistema acuífero entre el Río Ganga y el Río Kali, reveló sus dimensiones verticales y laterales y las características hidrogeológicas. Además, los estudios sobre la ocurrencia del agua subterránea, sus movimientos y comportamiento, en lo que se refiere al nivel de agua, y a su fluctuación en el tiempo y el espacio, confirma la co-existencia en el área de sobre- explotación así como de saturación con agua.

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Résumé L'objectif de cette étude hydrogéologique détaillée de portions de la Plaine Centrale du Gange est de déreminer la structure aquifère et la ressource potentielle. L'intérêt de la zone repose sur sa dualité du point de vue hydrogéologique, les zones influencées par le canal présentent une remontée de la nappe avec une salinisation des sols, la portion Ouest du bassin présente une baisse du niveau des aquifères. Par cette étude, le système aquifère compris entre la rivière du Gange et la rivière Kali révèle ses dimensions latérales et verticales ainsi que ses caractéristiques hydrogéologiques. De plus, l'étude des événements, des mouvements et du régime hydrogéologique affectant le niveau phréatique confirme la co-existence de surexploitation et de saturation des sols dans la région.

     

On the accuracy of atmospheric forcing for extra-tropical storm surge prediction

The ability of the SMARA storm surge numerical prediction system to reproduce local effects in estuarine and coastal winds was recently improved by considering one-way coupling of the air–sea momentum exchange through the wave stress, and best forecasting practices for downscaling. The inclusion of long period atmospheric pressure forcing in tide and tide/surge calculations corrected a systematic error in the surge, produced by the South Atlantic Ocean quasi-stationary pressure patterns. The maximum forecast range for the storm surge at Buenos Aires provided by the real-time use of water level observations is approximately 12 h. The best available water level prediction is the 6-h forecast (nowcast) based on the closest water level observations. The 24-h forecast from the numerical models slightly improves this nowcast. Although the numerical forecast accuracy degrades after the first 48 h, the improvement to the full range observation-based prediction is maintained at the inner Río de la Plata area and extends to the first 3 days at the intermediate navigation channels.     

Correlation and paleoenvironments of middle Paleogene marine beds based on dinoflagellate cysts in southwestern Patagonia,Argentina

An understanding of paleonvironmental and paleoceanographic evolution of the Southwestern Atlantic Ocean during the Palaeogene is prevented by the lack of precise tools to date and correlate the sedimentary units. Palynological samples collected in the upper portion of the Man Aike Formation, which crops out southern Lago Argentino area, in the southwest of the Austral Basin (50°21′45″S–72°14′30″W), contain well preserved marine organic dinoflagellate cysts, which are potentially important biostratigraphic and paleoenvironmental indicators. Herein we describe the composition of the Man Aike Formation's dinoflagellate cyst assemblages and compare them to similar assemblages collected in the same basin in the upper member of the Río Turbio Formation using Compositional Statistical Analysis. The dinoflagellate cyst assemblages from the upper part of the Man Aike Formation are highly correlated to the assemblages from the upper member of the Río Turbio Formation from outcropping sections (51°31′13″S–72°15′11″W) and with the lower part of sediment cores drilled by Yacimientos Carboníferos Fiscales in the Río Turbio Formation area. These dinoflagellate cyst assemblages show a very low correlation with the assemblages from the upper part of the Yacimientos Carboníferos Fiscales's cores. The comparison of our results with the high-resolution Southern Pacific Ocean dinoflagellate cyst zonation for the late Palaeocene to late Eocene allow us to date some of the dinoflagellate events recorded in formations of southwestern Patagonia. The assemblages from the Man Aike Formation and the lower part of the upper member of the Río Turbio Formation relate to the zones SPDZ11 and SPDZ12 and are assigned to the mid-middle Eocene (late Lutetian to early Bartonian). The biostratigraphy proposed herein constrains the age of the Man Aike Formation and equivalent units based on calcareous microfossil data, mollusks affinities and ⁸⁷Sr/⁸⁶Sr isotopic values to an age ranging between ∼42 and 39 Ma. The assemblages from the upper part of the upper member of the Río Turbio Formation relate to the SPDZ13 Zone, which corresponds to the late Eocene (early Priabonian). The dinoflagellate cyst assemblages indicate that they were under the influence of relatively warm, marine open waters in an inner-shelf environment for the lower part of the sections. In the upper part of the sections, the assemblages suggest shallow marine waters associated with coastal areas and high trophic levels, possibly as a result of freshwater inputs.     

Conceptual hydrodynamic model of the Pamukkale hydrothermal field, southwestern Turkey, based on hydrochemical and isotopic data

The results of study on the hydrochemical and isotope characteristics of shallow and deep waters at Pamukkale hydrothermal field Turkey are described in order to obtain a better understanding of the hydrological circulation. The field can be grouped into two groundwater sub-systems; cold water springs of Ca–HCO₃ type (10–12 °C), and CO₂-rich thermal waters of Ca–HCO₃–SO₄ type (25–58 °C). The occurrence of these water types is closely related to the morphology of the region, where intense tectonism formed horst and graben structures. Hence, two hydrogeological systems were defined: a deep geothermal system which is related to extensive and deep circulation of meteoric water in the regional flow system, and a shallow system which is related to local groundwater flow through sedimentary strata. The meteoric water falling at higher elevations percolates to the local groundwater system at a shallow level and flows to the deep geothermal system. During a deep convection cycle from a recharge to discharge area, the cold water attains heat from the asthenospheric intrusions, causing it to ascend. Variations of chemical and isotopic composition of thermal waters result from their mixing with cool groundwater in a shallow aquifer during their ascent to the surface.

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Résumé Les résultats d'une étude portant sur les caractéristiques hydrochimiques et isotopiques de puits phréatiques et profonds situés dans le champ hydrothermal de Pamukkale, sont décrits de telle manière à éclairer le fonctionnement des circulations hydrologiques. Le champ peut étre divisé en deux sous-systèmes d'eaux souterraines, l'un avec des eaux de sources froides (10–12 °C) de type Ca–HCO3, et les eaux thermales (25–58 °C) riches en CO₂ et de type Ca–HCO₃–SO4. L'occurrence de ces types d'eaux est fermement liée à la morphologie de la région, oùne tectonique intense a engendré des structures en horsts et en grabens. Dés lors deux systèmes hydrogéologiques ont été définis : un système profond, qui est lié à la circulation extensive et profonde des eaux météoritiques dans le système régional d'écoulement, et un système phréatique lié aux écoulements locaux des eaux souterraines à travers les strates sédimentaires. Les eaux météoritiques aux altitudes élevées, percolent jusqu'aux systèmes locaux phréatiques, puis coulent jusqu'aux systèmes géothermaux plus profonds. Durant le cycle de convection profond des zones de recharge jusqu'aux zones de décharge, l'eau froide atteint les zones chaudes liées aux intrusions athenosphériques, provoquant la remontée. Les variations de la composition chimique et isotopique des eaux thermales résultent dans leurs mélanges avec des eaux souterraines froides dans les aquifères phréatiques durant leur remontée jusqu'à la surface.

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Resumen Se describen los resultados del estudio de las características isotópicas e hidroquímicas de las aguas someras y profundas para obtener un mejor entendimiento de la circulación hidrológica del campo hidrotermal Pamukkale. El campo puede agruparse en dos sub-sistemas de agua subterránea: manantiales de agua fría del tipo Ca–HCO₃ (10–12°C) y aguas termales ricas en CO₂ del tipo Ca–HCO₃–SO₄(25–58°C). El ambiente de estos tipos de aguas se relaciona estrechamente con la morfología de la región donde el tectonismo intenso ha formado estructuras extensionales tipo graben y horst. Se definieron dos sistemas hidrogeológicos: un sistema geotermal profundo que se relaciona con la circulación profunda y extensa de agua meteórica en el sistema regional de flujo y un sistema somero el cual se relaciona con flujo local de agua subterránea a través de estratos sedimentarios. El agua meteórica que cae en altas elevaciones percola al sistema local de agua subterránea en un nivel somero y fluye hacia el sistema geotermal profundo. Durante un ciclo de convección del área de recarga hacia la zona de descarga, el agua fría se calienta a partir de los intrusivos astenosféricos lo que ocasiona que asciendan. Como resultado de la mezcla de las aguas recalentadas, con agua subterránea fría en un acuífero somero durante el ascenso hacia la superficie, se derivan variaciones en la composición química e isotópica de las aguas termales.

     

Zircon and Titanite U–Pb SHRIMP dating of unexposed basement units of the Buenos Aires region,southeastern Río de La Plata Craton,Argentina

ABSTRACT

We present U–Pb Sensitive High Mass Resolution Ion MicroProbe (SHRIMP) data of unexposed igneous-metamorphic basement rocks from two areas of the southeastern Río de la Plata craton (RPC; Buenos Aires city) located within the Buenos Aires–Piedra Alta (BAPA) terrane – and the Tapalqué area (Buenos Aires province) – located within the Tandilia terrane, and discuss the tectonic evolution of that portion of the craton based on both the new data and previous work. The newly obtained geochronological data of drill cores indicate that: (a) arc magmatism occurred at 2164–2186 Ma corresponding to early ‘Trans-Amazon’ (early Rhyacian) arc magmatism; (b) the age of collision between the BAPA and Tandilia terranes is inferred to have commenced at ca. 2110 Ma; (c) peak metamorphism occurred at ca. 2069 Ma; and (d) the presence of rocks related to the RPC is confirmed under Cenozoic sediments in a large area between Martin Garcia Island and Tandil. We envisage an early Rhyacian divergent double subduction scheme between the BAPA and Tandilia terranes – i.e. N-dipping towards BAPA and S-dipping towards Tandilia – prior to the late Rhyacian collision between these two terranes.     

Accretion of Grenvillian terranes to the southwestern border of the R��o de la Plata craton, western Argentina

A comprehensive review of the geological, geochronological, and isotopic features of the Mesoproterozoic Grenvillian terranes attached to the southwest of the Río de la Plata craton in Early Paleozoic times is presented in this paper. They are grouped into the northern (sierras de Umango, Maz and del Espinal and surroundings), central (Sierra de Pie de Palo, southern Precordillera and Frontal Cordillera), and southern (San Rafael and Las Matras Blocks) segments. The Mesoproterozoic basement consists mainly of arc related, intermediate to acidic and mafic?Cultramafic rocks of 1,244?C1,027?Ma, with juvenile, Laurentian affinity. Exception to it is the Maz Group, with a protracted history and reworked character. They are affected by 846?C570?Ma, extensional magmatism in the northern and central segments, which represents the Neoproterozoic breakup of the Rodinia supercontinent. Successive passive margin sedimentation is registered in Late Neoproterozoic (~640?C580?Ma) and Cambro-Ordovician (~550?C470?Ma) times. The southern segment is noted for the younger sedimentation alone, and for showing the exclusive primary unconformable relationship between the Mesoproterozoic basement and Early Ordovician cover. The effects of Early Paleozoic Famatinian orogeny, associated with the collisions of Cuyania and Chilenia terranes, are recorded as main phase (480?C450?Ma), late phase (440?C420?Ma), and Chanic phase (400?C360?Ma). Among them, the tectonothermal climax is the Ordovician main phase, to which klippe and nappe structures typical of collisional orogens are related in the northern and central segments. Preliminary data allow us to suggest a set of paired metamorphic belts, with an outboard high-P/T belt, and an inboard Barrowian P/T belt.