On the Use of Random Material in Dam Cores: Case of the Manso Dam,Brazil

Geotechnical and Geological Engineering - The Manso Dam, in the Brazilian State of Mato Grosso, intended for flood control and electricity generation, was built by a state-owned energy company in...     

Dynamics of phytoplankton associations in three reservoirs in northeastern Brazil assessed using Reynolds’ theory

The aim of the present study was to evaluate the influence of seasonality on the behavior of phytoplankton associations in eutrophic reservoirs with different depths in northeastern Brazil. Five collections were carried out at each of the reservoirs at two depths (0.1 m and near the sediment) at three-month intervals in each season (dry and rainy). The phytoplankton samples were preserved in Lugol's solution and quantified under an inverted microscope for the determination of density values, which were subsequently converted to biomass values based on cellular biovolume and classified in phytoplankton associations. The following abiotic variables were analyzed: water temperature, dissolved oxygen, pH, turbidity, water transparency, total phosphorus, total dissolved phosphorus, orthophosphate and total nitrogen. The data were investigated using canonical correspondence analysis. The influence of seasonality on the dynamics of the phytoplankton community was lesser in the deeper reservoirs. Depth affected the behavior of the algal associations. Variation in light availability was a determinant of changes in the phytoplankton structure. Urosolenia and Anabaena associations were more abundant in shallow ecosystems with a larger eutrophic zone, whereas the Microcystis association was more related to deep ecosystems with adequate availability of nutrients. The distribution of Cyclotella, Geitlerinema, Planktothrix, Pseudanabaena and Cylindrospermopsis associations was different from that seen in subtropical regions and the substitution of these associations was related to a reduction in the eutrophic zone rather than the mixture zone.     

Geochronological evidence for pervasive Miocene weathering,Minas Gerais,Brazil

⁴⁰Ar/³⁹Ar laser incremental‐heating analyses of 22 individual grains of supergene cryptomelane from three weathering pro?les, up to 400 km apart, in the Rio Doce valley and Barbacena regions at Minas Gerais, Brazil, show that the formation of weathering pro?les in these regions is contemporaneous, suggesting a strong weathering event in the Middle to Late Miocene (10–8 Ma). The preservation of these Miocene samples at or near the present surface suggests that either erosion rates have been very low in the region since the Miocene or that a much thicker weathering mantle was present in the region originally. Assuming a constant thickness of weathering pro?les in the region throughout the Tertiary, we may calculate weathering front propagation rates of 4–8 m Myr^?1 during the past 10 Ma. Copyright © 2004 John Wiley & Sons, Ltd.     

Sm–Nd isotopic investigation of Neoproterozoic and Cretaceous igneous rocks from southern Brazil: A study of magmatic processes

Nd-evolutionary paths for diversified igneous suites from southern Brazil are here re-evaluated using published results. We interpret the ε_Nd paths considering the secondary fractionation of ¹⁴⁷Sm/¹⁴⁴Nd due to major petrogenetic processes. The inclusion of Nd isotopes and geochemical data for Precambrian and Mesozoic basic rocks allow improving the discussion on the subcontinental lithosphere beneath southern Brazil. Late Neoproterozoic rocks, mostly granitoids, are exposed in two regions of the southern Brazilian shield, an eastern collisional belt and a western foreland. The latter included two geotectonic domains amalgamated at this time, the São Gabriel Arc (900–700 Ma), and the Taquarembó cratonic block. Magma genesis mainly involved mixture of crustal and incompatible-element-enriched mantle components, both with a long residence time. Continental segments are the Neoarchaean–Paleoproterozoic lower crust (ca. 2.55 Ga) in the western foreland, and Paleoproterozoic–Neoproterozoic recycled crust (2.1–0.8 Ga) in the collisional belt. Granitoids with a single crustal derivation are limited in the southern Brazilian Shield. Mixing processes are well-registered in the western foreland, where the re-enriched old mantle was probably mixed with a 900–700 Ma-old subducted lithosphere and a 2.55 Ga-old lower crust. The contribution of the latter increased from the early 605–580 Ma to the later 575–550 Ma Neoproterozoic events, which may be due either to crustal thickening or to delamination of the lithosphere. Magma sources were diversified in the 660–630 Ma collisional belt. Initially, they involved the mixing between two components with similar Nd isotopic ratios, a 2.1–0.8 Ga-old recycled crust and a subduction-processed old mantle. Regional heating and abundant production of granitic melts, with diversified contribution of enriched mantle components, mark the end of the collisional period, at 630–580 Ma. We can also attribute this to the delamination of the lithosphere, so that the same geodynamic process may explain the magmatism in the whole shield at the end of the Dom Feliciano Orogeny. Mesozoic rocks include flood basalts from the Cretaceous Paraná Province and sub-coeval alkalic suites. Multiple processes of metasomatism affected the lithospheric mantle, resulting in some complexity but they mainly register two enriched-mantle components, both generated during Neoarchaean–Paleoproterozoic events. One end-member has a more pronounced subduction signature. The other one probably resulted from the re-enrichment of the first component at the end of the Camboriú collisional orogeny (2.0 Ga).     

Fish and ostracod remains from the Santos Basin (Cretaceous to Recent), Brazil

For the first time, ichthyoliths are described from the Santos sedimentary basin, offshore southern Brazil. Isolated teeth, dermal scales and the first documented otoliths from Cretaceous (Albian) to Recent cuttings from five wells are described. The following groups are represented: Chondrichthyans: Triakidae, Carcharhinidae; Ginglymostomatidae: ?Ginglymostoma sp., Lamnidae indet., Scyliorhinidae; Osteichthyans: Teleostei; Myctophiidae: Diaphus aff. splendidus sp. complex, Diaphus spp., Diaphus cf. garmani, Ceratoscopelus aff. warmingii; Sternoptychidae: Valenciennellus tripunctulatus, teeth of indeterminate Teleostei. The majority of these ichthyofossils represent extant forms, known to occur in the Atlantic Ocean, and are of potential value for stratigraphical correlations between oil‐yielding basins in the region. Ostracods are not well preserved but can be identified to generic level indicating marine environments. The ostracod faunas offer potential for intrabasinal correlation in the Eocene and Oligocene. Copyright © 2002 John Wiley & Sons, Ltd.     

Heterogeneity and homogeneity of Southern Brazilian beaches: A morphodynamic and statistical approach

The characterization and beach morphodynamic differentiation along 620 km of Rio Grande do Sul state (RS) coastline has been principally done using beach profiles and their resulting morphometric parameters from the early 1990s through the present. The objective of the present work is to make a regional characterization, grouping the beaches with similar morphodynamic behavior along the RS coastline using multivariate statistical tools. For such purpose, a database containing topographic profiles and environment characteristics of 32 sites along the state coastline was built. A clustering analysis followed by a multidimensional scaling (MDS) analysis and an analysis of similarities (ANOSIM) was performed. The clustering and MDS results suggest that the data could be merged into six groups, one with dissipative tendencies, another with intermediate to reflective tendencies and three groups with intermediate characteristics. Each group showed distinct morphodynamic characteristics. Such differences are shown by the ANOSIM to be statistically significant. The three intermediate groups were differentiated in terms of beach mobility and percentage of medium-size sand, with higher mobility related to higher amounts of medium-size sand. The use of multivariate statistical techniques allowed a good differentiation between the RS beaches. Based on the results, it is suggested that the RS coastline can be segmented into seven different morphodynamic sectors. This approach can be used by management agencies to reduce potential oil spill impacts as well as decrease other beach hazards as well as for differentiate morphodynamical stages.     

Large dam-reservoir systems: guidelines and tools to estimate loads resulting from natural hazards

This paper examines hydrodynamic pressure diagrams due to earthquakes acting on distinct configurations of the upstream face of a dam, considering the reservoir length and different bathymetries of the reservoir bottom. Two suitable mathematical models are used to obtain the dynamic pressure on the dam, and the reservoir free surface oscillation. Conceptual mathematical models are proposed to study the impact of a landslide triggered by an earthquake, and its progress into the reservoir. The waves formed when a landslide advances into the reservoir are tested with the help of laboratory results. These waves and their propagation in the reservoir are studied using experimental data and numerical results, including wave-type analyses based on dimensionless parameters. Two distinct 1DH and 2DV numerical models based on different mathematical formulations are tested. A discussion of physical and numerical results is detailed in a general risk context and uncertainty associated with the input data in a deterministic model. Numerical simulations are performed for the upper and lower limits of the sliding mass velocity diagram which is obtained as a result of the intrinsic uncertainty of the stochastic nature of the friction angle. Finally, the findings are discussed and some conclusions drawn.     

Geology of the Elephanta Island fault zone,western Indian rifted margin,and its significance for understanding the Panvel flexure

The Panvel flexure is a 150-km long tectonic structure, comprising prominently seaward-dipping Deccan flood basalts, on the western Indian rifted margin. Given the active tectonic faulting beneath the Panvel flexure zone inferred from microseismicity, better structural understanding of the region is needed. The geology of Elephanta Island in the Mumbai harbour, famous for the ca. mid-6th century A.D. Hindu rock-cut caves in Deccan basalt (a UNESCO World Heritage site) is poorly known. We describe a previously unreported but well-exposed fault zone on Elephanta Island, consisting of two large faults dipping steeply east–southeast and producing easterly downthrows. Well-developed slickensides and structural measurements indicate oblique slip on both faults. The Elephanta Island fault zone may be the northern extension of the Alibag–Uran fault zone previously described. This and two other known regional faults (Nhava–Sheva and Belpada faults) indicate a progressively eastward step-faulted structure of the Panvel flexure, with the important result that the individual movements were not simply downdip but also oblique-slip and locally even rotational (as at Uran). An interesting problem is the normal faulting, block tectonics and rifting of this region of the crust for which seismological data indicate a normal thickness (up to 41.3 km). A model of asymmetric rifting by simple shear may explain this observation and the consistently landward dips of the rifted margin faults.