The continental record of Ediacaran volcano‐sedimentary successions in southern Brazil and their global implications     

Liliane Janikian  Renato Paes De Almeida  Ricardo Ivan Ferreira Da Trindade  Antonio Romalino Santos Fragoso‐Cesar  Manoel Souza D′Agrella‐Filho  Elton Luis Dantas  Eric Tohver 《地学学报》2008,20(4):259-266

The Ediacaran is one of the most important periods on Earth evolution, including the first appearance of soft‐bodied macrofossils, major climatic changes and a supposed rise in free oxygen. In southernmost Brazil, this period is represented by Camaquã Supergroup, including the Bom Jardim Group and the Acampamento Velho Formation, both of which record continental palaeoenvironmental changes in a more than 5000 m thick stratigraphic succession. Age constraints are given by seven Ar‐Ar and U‐Pb determinations on volcanic rocks, which bracket these units between c. 605 and 574 Ma, revealing the best dated and most continuous documented Ediacaran continental succession to date. Depositional systems evolution supports a Phanerozoic‐type glacial context during the last Neoproterozoic glacial event and presents the Picada das Graças Formation (580 ± 3.6 Ma) as the first dated non‐glacial unit coeval to the Gaskiers Formation.  相似文献   

Magnetic polarity zonation within the El Teniente copper–molybdenum porphyry deposit, central Chile     

Natalia Astudillo  Pierrick Roperch  Brian Townley  Cesar Arriagada  Annick Chauvin 《Mineralium Deposita》2010,45(1):23-41

El Teniente porphyry copper deposit, the world’s greatest intrusion-related Cu–Mo ore body, is hosted within basaltic–andesitic volcanic and gabbroic rocks (mafic complex). This ore body is strongly affected by multiple events of alteration/mineralization with pervasive potassic and chloritic alteration and coetaneous with associated copper mineralization. We present paleomagnetic results obtained from oriented samples at four locations within the mine and from two drill cores, 200 and 400 m long, respectively. Samples are representative of all the main hydrothermally altered rock units, with emphasis on the mafic host rock and dacitic (Teniente dacite porphyry) and dioritic porphyry intrusions. Magnetic experiments [hysteresis loop, isothermal remanent magnetization (IRM), k–T curves, thermal, and alternating field demagnetization] show the presence of prevailing magnetite. Microscope and SEM observations show two families of magnetite, (a) large multidomain magnetite grains, associated with biotite and chlorite of various different hydrothermal alteration events, and (b) abundant small to medium grain-size magnetite (<10 μm) contained within plagioclase, either related to an early Na–Ca–Fe alteration or included within plagioclase during magmatic crystal growth. While the Teniente dacite porphyry and the quartz diorite–tonalite have low magnetic susceptibility (<0.0005 SI) and low natural remanent magnetization (NRM, 10⁻⁴–10⁻³ Am⁻¹), the mineralized mafic host rocks have usually high susceptibility (>0.01 and up to 0.2 SI) with NRM in the range 0.1–2 Am⁻¹. Most mafic complex rock samples have univectorial magnetizations during alternating field or thermal demagnetization. Within the mine, the magnetic polarity is spatially distributed. In the northern part of the deposit, the Teniente dacite porphyry, the associated hydrothermal breccias, and the hosting mafic complex record a reverse polarity magnetization, also observed in the El Teniente sub-6 mine sector immediately to the east and southeast. In the eastern part of the deposit, a normal polarity is observed for samples of the mafic complex from the two long drill cores. There is no evidence for superimposed magnetizations of opposite polarities in samples of the mafic complex. Anhysteretic remanent magnetization (ARM) in a DC field of 40 μT and NRM have similar magnitude and comparable behavior upon alternating field demagnetization. The well-defined strong remanent magnetizations associated with high unblocking temperatures (>500°C) indicate an acquisition of remanent magnetization during mineralization by circulating high temperature fluids related with ore deposition. Paleomagnetic results and the recorded polarity zonation suggest multiple mineralization events occurred at El Teniente, each one with its own evolution stages, superimposed within the district. These results indicate that a simplified broad four-stage model for El Teniente, as presented and overly employed by many authors, divided in (1) late magmatic, (2) main hydrothermal, (3) late hydrothermal, and (4) posthumous stage, does not recognize various short-lived single mineralization events, some superimposed and some distinctly separated in time and space. There is no paleomagnetic evidence for post-mineralization deformation  相似文献   

Influence of Particle Morphology on the Hydraulic Behavior of Coal Ash and Sand     

Karla Salvagni Heineck  Rosemar Gomes Lemos  Juan Antônio Altamirano Flores  Nilo Cesar Consoli 《Geotechnical and Geological Engineering》2010,28(4):325-335

Flexible-wall hydraulic conductivity tests were carried out on bottom ash, fly ash and compacted specimens of sand with additions of 0, 3, 6, 9 and 18% of bentonite. In order to study the effect of bentonite inclusion and particle morphology on the hydraulic conductivity of the admixtures, an investigation was undertaken based on thin section micrographs. It was found that, for both bottom and fly ash admixtures, bentonite addition reduced only one order of magnitude the hydraulic conductivity, from 1.78 × 10⁻⁶ m/s to 1.39 × 10⁻⁷ m/s. On the other hand, the sand hydraulic conductivity was reduced five orders of magnitude, from 3.17 × 10⁻⁵ m/s to 5.15 × 10⁻¹⁰ m/s. Among several factors that can be responsible for the difficulty in reducing hydraulic conductivity, such as ash grain size distribution and elevated cation concentration (leached from the ash) in pore water, it can also be recalled the high particle voids observed in the ash by means of microscopic analysis. The same is not true with the sand, which has solid particles, without inner voids.  相似文献   

Natural resource exploitation and regional development in Latin America     

Cesar N. Caviedes 《GeoJournal》1985,11(1):5-6

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Modeling monthly mean air temperature for Brazil   总被引：1，自引：1，他引：0  

Clayton Alcarde Alvares  José Luiz Stape  Paulo Cesar Sentelhas  José Leonardo de Moraes Gonçalves 《Theoretical and Applied Climatology》2013,113(3-4):407-427

Air temperature is one of the main weather variables influencing agriculture around the world. Its availability, however, is a concern, mainly in Brazil where the weather stations are more concentrated on the coastal regions of the country. Therefore, the present study had as an objective to develop models for estimating monthly and annual mean air temperature for the Brazilian territory using multiple regression and geographic information system techniques. Temperature data from 2,400 stations distributed across the Brazilian territory were used, 1,800 to develop the equations and 600 for validating them, as well as their geographical coordinates and altitude as independent variables for the models. A total of 39 models were developed, relating the dependent variables maximum, mean, and minimum air temperatures (monthly and annual) to the independent variables latitude, longitude, altitude, and their combinations. All regression models were statistically significant (α?≤?0.01). The monthly and annual temperature models presented determination coefficients between 0.54 and 0.96. We obtained an overall spatial correlation higher than 0.9 between the models proposed and the 16 major models already published for some Brazilian regions, considering a total of 3.67?×?10⁸?pixels evaluated. Our national temperature models are recommended to predict air temperature in all Brazilian territories.  相似文献   

GEOGRAPHY AND THE LESSONS FROM EL NIÑO*     

Cesar N. Caviedes 《The Professional geographer》1984,36(4):428-436

Recognized for years as a major environmental disruption, El Niño generated intense public interest in 1982–1983. During its last occurrence, this phenomenon, which develops in the tropical Pacific and usually affects rimlands of the Pacific basin, exceeded its boundaries and its effects were transmitted to continental North America, Europe, Africa and East Asia. Notwithstanding its vast areal extent, the interest of geographers in El Niño events, past and recent, has been comparatively less than that demonstrated by natural scientists and ecologists: little has been accomplished to place these climatic-ecological crises within global perspective. Lessons from El Niño include the acknowledgement of new concepts of climatic transitivity, ocean dynamics and energy exchanges that must find a place within the conceptual wealth of geography if this science intends to keep up with the rapid progress of other geosciences.  相似文献   

Climate Change Impacts for the Conterminous USA: An Integrated Assessment     

Allison?M.?ThomsonEmail author  Robert?A.?Brown  Norman?J.?Rosenberg  R.?Cesar?Izaurralde  Verel?Benson 《Climatic change》2005,69(1):43-65

Here we simulate dryland agriculture in the United States in order to assess potential future agricultural production under a set of general circulation model (GCM)-based climate change scenarios. The total national production of three major grain crops—corn, soybeans, and winter wheat—and two forage crops—alfalfa and clover hay—is calculated for the actual present day core production area (CPA) of each of these crops. In general, higher global mean temperature (GMT) reduces production and higher atmospheric carbon dioxide concentration ([CO₂]) increases production. Depending on the climatic change scenarios employed overall national production of the crops studied changes by up to plus or minus 25% from present-day levels. Impacts are more significant regionally, with crop production varying by greater than ±50% from baseline levels. Analysis of currently possible production areas (CPPAs) for each crop indicates that the regions most likely to be affected by climate change are those on the margins of the areas in which they are currently grown. Crop yield variability was found to be primarily influenced by local weather and geographic features rather than by large-scale changes in climate patterns and atmospheric composition. Future US agronomic potential will be significantly affected by the changes in climate projected here. The nature of the crop response will depend primarily on to what extent precipitation patterns change and also on the degree of warming experienced.  相似文献   

Coral reef ecosystems protect shore from high-energy waves under climate change scenarios     

Cesar Villanoy  Laura David  Olivia Cabrera  Michael Atrigenio  Fernando Siringan  Porfirio Aliño  Maya Villaluz 《Climatic change》2012,112(2):493-505

Coral reefs and other coastal ecosystems such as seagrasses and mangroves are widely recognized to provide protection against the devastating effects of strong waves associated with tsunamis and storms. The predicted warming climate brings to fore the role of these ecosystems in providing protection against stronger typhoons that can result in more devastating waves of greater amplitude. We performed a model simulation of storm generated waves on a Philippine reef, which is located along the path of tropical storms, i.e., at least 10 typhoons on the average pass through the study site yearly. A model to simulate wave propagation was developed using Simulating Waves Nearshore (SWAN) and DELFT3D-WAVE computer simulation software. Scenarios involving local monsoonal wind forcing and storm conditions were simulated. In addition, as climate change may also result to increased relative sea level, a 0.3 m and 1 m rise in sea level scenarios were also used in the wave model simulations. Results showed that the extensive reef system in the site helped dissipate wave energy that in turn reduced wave run-up on land. A significant reduction in wave energy was observed in both climate change, i.e., stronger wind and higher sea level, and non-climate change scenarios. This present study was conducted in a reef whose coral cover is in excellent condition (i.e., 50 to 80% coral cover). Estimates of coral reef growth are in the same order of magnitude as estimates of relative sea level rise based on tide gauge and satellite altimeter data, thus it is possible that the role of reefs in attenuating wave energy may be maintained if coral reef growth can keep up with the change in sea level. Nonetheless, to maintain reef growth, it is imperative to manage coral reef ecosystems sustainably and to eliminate the stressors that are within human control. Minimizing activities such as illegal and destructive blast and poison fishing methods, pollution and siltation, is crucial to minimize the impacts of high-energy waves that may increase with climate change.  相似文献   

Volumetric strains in neutral surface folding     

Richard J. Lisle  Jesús Aller  Fernando Bastida  Nilo C. Bobillo-Ares  Noel C. Toimil 《地学学报》2009,21(1):14-20

Neutral surface folding is a significant contributor to fold development. This mechanism produces contrasting strains in the inner and outer arcs of the folded layer that arise from principal stress orientations that are approximately parallel and perpendicular to the layer. We demonstrate that such stress patterns imply significant gradients of mean stress across the folding layer, being more tensional on the outer arc and more compressive in the inner arc. This could pump fluids towards the outer arc during folding and result in heterogeneous volume changes. We conclude that the neutral surface folding model should be adapted to accommodate volumetric strains, in order to explain dilatational structures (e.g. open fractures, veining) on the extrados and volume-loss structures (e.g. pressure solution seams, stylolitic cleavages) on the intrados. This dilatation has economic implications as it allows prediction of sites of mineralization and zones of secondary permeability in fold-related hydrocarbon traps.  相似文献   

Deposition and tectonic setting of the Palaeoproterozoic Castelo dos Sonhos metasedimentary formation,Tapajós Gold Province,Amazonian Craton,Brazil: age and isotopic constraints     

Evandro L. Klein  Joseneusa B. Rodrigues  Joana D.S. Queiroz  Roberto G. Oliveira  Stella B. Guimarães  Cesar L. Chaves 《International Geology Review》2017,59(7):864-883

The Castelo dos Sonhos Formation (CSF) represents a relic of a sedimentary basin located in the southeastern Tapajós Gold Province (TGP), at its boundary with the Iriri–Xingu Domain (IX), south of the Amazonian Craton. The formation comprises mature, coarse-grained metasandstones (quartz-arenites) intercalated with auriferous metaconglomerates with clasts of quartz and subordinately of banded iron formation, quartzite, schist, and felsic metavolcanic rocks. Lithology, planar, channelled, and large-scale cross-bedding suggest deposition in continental setting by the braided fluvial system associated with alluvial fans and subordinate aeolian dunes. The rocks underwent very low metamorphism and gently synformal folding, and were intruded by andesites and granitoids (2011–1918 Ma). U–Pb LA-ICP-MS zircon data indicate maximum depositional ages of 2050 Ma (metaconglomerate) and 2074, 2088, and 2104 Ma (metasandstones). Hence, deposition occurred at 2050–2011 Ma, slightly preceding or being coeval with the onset of the orogenic phase in TGP (2030–1956 Ma). The quartz-arenite composition, zircon U–Pb data, and negative εHf (?1.3 to ?13.0) and εNd (?2.9 to ?5.3) values indicate: (1) quartzose provenance, (2) prolonged transport and recycling of sedimentary sources, (3) multiple age peaks (2050–3700 Ma), with predominant sources of 2100 and 2750 Ma, and (4) long crustal residence time for the source rocks. Source areas were Rhyacian–Siderian orogenic belts to Mesoarchaean terranes of the Amazonian Craton located to the east, southeast, and northeast of TGP, along with Palaeoarchaean and Eoarchaean detrital zircons recycled from older sedimentary rocks. We interpret CSF as part of a larger foreland system related to the evolution of Rhyacian orogens, currently represented by the Bacajá and Santana do Araguaia domains. The present location of CSF in the easternmost TGP, close to its boundary with IX, is due to rifting (1.89–1.80 Ga) that produced the Uatumã Silicic Large Igneous Province (Uatumã SLIP), and the juxtaposition of the crustal domains is supported by gravity data.  相似文献