Two Brazilian archaeological sites investigated by GPR: Serrano and Morro Grande     

Glria da Silva Cezar  Paula Lucia Ferrucio da Rocha  Angela Buarque  Ariovaldo da Costa 《Journal of Applied Geophysics》2001,47(3-4)

The application of the ground penetrating radar (GPR) at two archaeological sites, Serrano and Morro Grande, situated in Araruama County, Rio de Janeiro, Brazil, aids the study of a prehistoric indigenous culture, associated with the “Tupinambá” that inhabited the region during prehistoric times.The archaeological remains of the study area are mainly characterized by pottery artifacts for several uses, including funerary urns, which were buried within layers of sand and clay. Several profiles were acquired using a RAMAC system, with a 200 and 400 MHz frequency antennae. At the Serrano site, the profiles were acquired around some partially exposed pottery shards, due to sand exploitation. The resultant profiles provided a response model to guide the interpretation of new profiles acquired at other sites in the area, which present similar characteristics.The results showed the great importance of the dielectric permittivity contrast which exists between the targets and the host media, in order for possibly significant features to be identified in radar data.  相似文献   

Gold deposits and occurrences of the Crixás Goldfield, central Brazil     

Hardy Jost  Paulo de Tarso Ferro de Oliveira Fortes 《Mineralium Deposita》2001,36(3-4):358-376

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Pegmatite Emplacement in the Seridó Belt, Northeastern Brazil: Late Stage Kinematics of the Brasiliano Orogen     

M.N.C. Araújo  F.C. Alves da Silva  E.F. Jardim de S 《Gondwana Research》2001,4(1)

Geometric and kinematic analysis was performed in an area located in the central part of the Seridó Belt (NE Brazil), where supracrustal rocks affected by polyphase deformation are well exposed. The first event recognized in this area (and regionally known as the D₂ deformation) is characterized by top to the south thrust tectonics while a second one (D₃ deformation) is marked by upright folds, strike-slip or transpressive shear zones and the development of flower structures. Major pegmatite swarms were emplaced during and late as regards the second event (dated ca. 580 Ma), being part of the Brasiliano orogeny; similar dyke swarms are known from the Nigerian Shield. These pegmatite swarms provide reliable kinematic markers of the late evolutionary stage of the Neoproterozoic Trans-Sahara-Borborema collisional belt. Mineralogical, geometric and kinematic features support two stages of pegmatite emplacement during the strike-slip event: (i) older, syn-D₃ homogeneous pegmatites intruded mostly along lithological and structural discontinuities, such as foliation surfaces; (ii) late, D₃ heterogeneous pegmatites were emplaced along tension gashes and other dilation structures. The heterogeneous pegmatites are economically more important, being exploited for precious metals and stones, as well as industrial minerals.  相似文献   

Fine sediment transport and accumulations at the mouth of the Seine estuary (France)     

Pierre Le Hir  Andre Ficht  Ricardo Silva Jacinto  Patrick Lesueur  Jean-Paul Dupont  Robert Lafite  Isabelle Brenon  Benedicte Thouvenin  Philippe Cugier 《Estuaries and Coasts》2001,24(6):950-963

A comprehensive study of fine sediment transport in the macrotidal Seine estuary has been conducted, including observations of suspended particulate matter (SPM), surficial sediment, and bathymetric data, as well as use of a three dimensional mathematical model. Tide, river regime, wind, and wave forcings are accounted. The simulated turbidity maximum (TM) is described in terms of concentration and location according to tidal amplitude and the discharge of the Seine River. The TM is mainly generated by tidal pumping, but can be concentrated or stretched by the salinity front. The computed deposition patterns depend on the TM location and are seasonally dependent. The agreement with observations is reasonable, although resuspension by waves may be overestimated. Although wave resuspension is likely to increase the TM mass, it generally occurs simultaneously with westerly winds that induce a transverse circulation at the mouth of the estuary and then disperse the suspended material. The resulting effect is an output of material related to wind and wave events, more than to high river discharge. The mass of the computed TM remains stable over 6 months and independent of the river regime, depending mainly on the spring tide amplitude. Computed fluxes at different cross-sections of the lower estuary show the shift to the TM according to the river flow and point out the rapidity of the TM adjustment to any change of river discharge. The time for renewing the TM by riverine particles has been estimated to be one year.  相似文献   

Late- to post-orogenic basins of the Pan-African – Brasiliano collision orogen in southern Africa and southern Brazil     

P. G. Gresse  F. Chemale  L. C. da Silva  F. Walraven  & L. A. Hartmann 《Basin Research》1996,8(2):157-171

Late- to post-orogenic basins formed on both sides of the Pan-African – Brasiliano orogen when the Congo and Kalahari Cratons collided with the Rio de la Plata Craton during the formation of western Gondwana. Trace fossil evidence and radiometric age dating indicate that deposits on both sides are coeval and span the Cambrian–Precambrian boundary. A peripheral foreland basin, the Nama Basin, developed on the subducting southern African plate. Lower, craton-derived fluviomarine clastics are overlain by marine platform carbonates and deltaic flysch derived in part from the rising subduction complex along the northern (Damara Belt) and western (Gariep Belt) orogenic margins. Rare, thin volcanic ash layers (tuffs and cherts) are present. Upper sediments consist of unconformable red molasse related to collisional orogenesis. Orogenic loading from the north and west led to crustal flexure and the formation of a remnant ocean that drained to the south and closed progressively from north to south. During final collision SE-, E- and NE-verging nappes overrode the active basin margins. Although younger than most of the post-orogenic magmatism, its setting on the cratonic edge of the subducting plate precluded marked volcanism or granitic intrusion, the only exception being the youngest intrusions of the Kuboos-Bremen Suite dated at 521±6 Ma to 491±8 Ma. Two foreland-type basins, perhaps faulted remnants of a much larger NE–SW elongated retroarc foreland basin, are found west of the Dom Feliciano Belt on the edge of the Rio de la Plata Craton in southern Brazil. In the southern Camaqua Basin, basal fluvial deposits are followed by cyclical marine and coarsening-up deltaic deposits with a notable volcanic and volcaniclastic component. This lower deformed succession, comprising mainly red beds, contain stratabound Cu and Pb–Zn deposits and is overlain unconformably by a fluviodeltaic to aeolian succession of sandstones and conglomerates (with minor andesitic volcanics), derived primarily from an eastern orogenic source and showing southerly longitudinal transport. In the northern Itajaí Basin, sediments range from basal fluvial and platform sediments to fining-up submarine fan and turbidite deposits with intercalated acid tuffs. The Brazilian basins had faulted margins off which alluvial fans were shed. They also overlie parts of the Ribeira Belt. Thrust deformation along the orogenic margin bordering the Dom Feliciano Belt was directed westward in the Camaqua and Itajaí basins, but reactivated strike-slip and normal faults are also present. Late- to post-orogenic granitoids and volcanics of the Dom Feliciano Belt, ranging in age from 568±6 Ma to 529±4 Ma, occur in the foreland basins and are geochemically related to some of the synsedimentary volcanics.  相似文献   

Controls on Quaternary coastal evolution of the east-northeastern coast of Brazil: roles of sea-level history, trade winds and climate     

JosMaria Landim Dominguez  Abilio Carlos da Silva Pinto Bittencourt  Louis Martin 《Sedimentary Geology》1992,80(3-4):213-232

East-northeastern Brazil has a wave-dominated, micro- to meso-tidal coast, lying entirely within the southern Atlantic trade wind belt. Integration of geologic mapping, radiocarbon dating and vibracoring data shows that the Quaternary coastal evolution of this area was controlled by three major factors: (1) sea-level history; (2) trade winds; and (3) climate change.

Sea-level history. Along the east-northeastern coast of Brazil, relative sea level has fallen approximately 5 m during the last 5000 y. Correlation of this sea-level history with the evolution of beach-ridge, lagoonal and coastal plain deposits shows that: (1) sea-level rise favours the formation of barrier island—lagoonal systems and the construction of intralagoonal deltas; (2) sea-level lowering is not conductive to barrier island formation. Rather, lagoons and bays become emergent and beach-ridge plains rapidly prograde.

Trade winds. Sediment dispersal systems along the coastal zone of east-northeastern Brazil have been highly persistent since Pleistocene time, as deduced from beach-ridge orientation. This persistence results from the fact that sediment dispersal in wave-dominated settings is ultimately controlled by atmospheric circulation which, for the east-northeastern coast of Brazil is associated with the South Atlantic high-pressure cell. The remarkable stability of this cell through time, has allowed the accumulation of extensive beach-ridge plains at the longshore drift sinks located along the coast.

Climate change. Effects of Quaternary climate changes on coastal sedimentation are twofold. Climate changes may affect rainfall patterns, thus exerting an important control on coastal dune development. Along the coast of northeastern Brazil, active coastal dunes are only present in those areas in which at least four consecutive dry months occur during the year. Mapping of these areas has shown that dune development during the Holocene has been episodic, these episodes being probably controlled by variations in rainfall patterns associated with climate changes. Secondly, despite its overall stability, the position of the high-pressure cell has experienced small shifts in position during the Holocene in response to climate changes. Changes in wind direction associated with these shifts have induced modifications in the coastal dispersion system, which are recorded in the strandplains as small truncations in the beach-ridge alignments.

These results have important implications in understanding accumulation of ancient sandstone shoreline sequences.  相似文献   

A methodology to estimate renewal time scales in estuaries: the Tagus Estuary case   总被引：2，自引：0，他引：2  

Frank?BraunschweigEmail author  Flávio?Martins  Paulo?Chambel  Ramiro?Neves 《Ocean Dynamics》2003,53(3):137-145

High-resolution hydrodynamic models are a common tool to simulate water dynamics in estuaries. Results from these models are, however, difficult to interpret without the aid of additional parameters to integrate the information. In this paper a methodology to understand the transport patterns in the Tagus Estuary is proposed. It is based on the computation of two renewal time scales: residence time and integrated water fraction. This last parameter is used to build a dependency matrix that gives the integrated influence of each region of the estuary at a selected point. The parameters are computed using a Lagrangian transport model coupled to the hydrodynamic model. Results show that Tagus Estuary has two different types of regions: the central part of the estuary, with low renewal efficiency, and three regions with higher renewal efficiency. Renewal mechanisms are, however, different for each region as shown by the dependency matrix. Comparison of renewal time scales with results from a water-quality model revealed that residence time is not a limiting parameter for primary production in the Tagus Estuary.Responsible Editor: Hans Burchard  相似文献   

Characterization of beryl (aquamarine variety) from pegmatites of Minas Gerais,Brazil   总被引：2，自引：0，他引：2  

R. R. Viana  H. Jordt-Evangelista  G. Magela da Costa  W. B. Stern 《Physics and Chemistry of Minerals》2002,29(10):668-679

Eight samples of the beryl variety aquamarine were selected from four pegmatites in the Governador Valadares and Araçuaí regions in northeastern Minas Gerais State, Brazil. These samples were fully characterized by chemical analysis, infrared and UV-visible spectroscopy, thermal analyses, and high-temperature X-ray diffraction (from room temperature up to 800 °C). Several physical and chemical properties of beryl were found to depend on the amount of water and ions residing in the structural channels. The thermal expansion coefficients from room temperature to about 800 °C are temperature-independent, with αa ? ?3.2 × 10^?6 ° C^?1 and αc ? ?8.7 × 10^?6 ° C^?1. The contraction of both a and c unit-cell parameters with increasing temperature and the shift of the infrared band centered at about 1200 cm^?1 were tentatively ascribed to interactions between channel water and the silicate rings.The color of beryl seems to be dictated by the relative proportions of Fe³⁺ in the octahedralsites and of fe²⁺ in the channels. Thus, deep-blue samples have little Fe³⁺, whereas greener samples have more Fe³⁺ or less channel Fe²⁺.  相似文献   

"三联式"资源定量预测与评价--数字找矿理论与实践探讨   总被引：42，自引：4，他引：38  

赵鹏大 《地球科学》2002,27(5):482-489

随着信息技术的发展,矿产勘查已步入数字化,定量化研究的新阶段。“三联式”成矿预测及资源评价途径正是“数字找矿”的创新探索。“三联式”成矿预测以地质异常分析为基础,以成矿多样性分析与矿床谱系研究为指导,将地质异常,成矿多样性及矿床谱系三方面定量化研究紧密结合形成矿产预测及定量评价的有机切入点,是实现全面数字找矿的必由之路,也是矿产勘查评价领域应用信息技术的基础和前提。  相似文献   

Sediment contamination in floodplains and alluvial terraces as an historical record of gold exploitation in the Carmo River basin,Southeast Quadrilátero Ferrífero,Minas Gerais,Brazil     

Adivane Terezinha Costa  Hermínio Arias Nalini  Paulo de Tarso Amorim Castro  Jorge Carvalho de Lena  Peter Morgenstern  Kurt Friese 《洁净——土壤、空气、水》2006,34(3):245-256

This paper describes the geochemistry of sediment samples placed in floodplains and alluvial terraces downstream from gold mines in the Carmo River basin, Quadrilátero Ferrífero, Minas Gerais, Brazil. The geochemistry signature Na₂O, K₂O, SiO₂, CaO, MgO, Al₂O₃, Fe₂O₃, TiO₂, P₂O₅, Mn, As, Cu, Zn, Ba, Ni, Cr, S, Co were analyzed in different facies from stratigraphic profiles. As, Cu, Zn, and Mn anomalies are mainly associated with the clayed facies deposited in floodplains and oxbow lakes, and with coarse‐sediment facies deposited in the channel. The facies were accumulated by the gold exploitation activity in the region. The contamination of As, Cu, and Zn was controlled by minerals such as iron oxides and hydroxides (hematite, magnetite, and mainly goethite), manganese oxides, and sulfide‐rich minerals. The As‐bearing sediments of the region characterize one of the most As contaminated area of Brazil. Their main source is associated with gold exploration in the last three centuries.  相似文献