César Ulisses Vieira Veríssimo

Geology and geochemistry of paleoproterozoic magnesite deposits (~1.8Ga), State of Ceará, Northeastern Brazil

Five important magnesite mines and small occurrences in the state of Ceará form a sequence of lenses that extend, discontinuously, for over 140km. The magnesite rocks are hosted by metadolomites with lutecite, sulphate nodules pseudomorphosed by fiber-radiated quartz, scapolite and dissolution breccias. This metacarbonate sequence is more voluminous and more calcic in the southwestern extremity of the belt, but less voluminous and more magnesian towards the northeastern portion of the belt, the highest Mg contents being observed in the Alencar-Orós region. Magnesite deposits pass gradually into metadolomites and then to almost pure calcitic marbles westward the belt. This group is hosted in a greenschists to amphibolite-facies metavolcanic-sedimentary sequence crosscut by basic sills and granitic intrusions of variable size, form, composition and age (middle to late Proterozoic ∼550Ma).Two types of magnesite marble may be distinguished: (1) medium-grained (1 to 9mm) and (2) sparry-magnesite marble (1 to 15cm). The latter displays wide textural variation (porphyric, rosette, layered and palisadic types) and exhibits remnants of the original sedimentary features. The sparry crystals, in spite of their deformation, are hypidiomorphic and pinolitic. The colors vary from white to light or dark gray, or even red. The dark banded term presents traces of microfossils and stromatolites structures. The medium-grained magnesite marble, on the other hand, shows more homogeneous texture and color and anhedral crystals are more abundant.The sparry-magnesite marble is less enriched in SiO2, Fe2O3, Al2O3 and CaO and richer in MgO than the medium-grained ones. The magnesitic marbles display positive Ce, and negative Eu anomalies. This suggests a shallow marine platform environment with continental, reduced influence (lagunar). These magnesitic marbles also show positive Ce and Eu anomalies in relation to the present seawater, suggesting compositional differences, materialized mainly by an enrichement of these elements in the past seawater.Fluid inclusions in the medium-grained magnesites of the Riacho Fundo deposit are aqueous inclusions, while in the sparry magnesite deposits, particularly in Cabeça de Negro, aquo-carbonic inclusions dominate. Hydrocarbons, however, are found in both cases.The calcitic marble, deposited on open-marine environment, displays positive13C (+0.5 to 2.1 ‰ PDB), while dolomitic to dolostone marbles, deposited on less open marine environment, display negative shift (0.7 to −3.2‰ PDB). This13C fluctuation is within the range for carbonates deposited around 1.8Ga.The depositional environment of these carbonates is close to a paralic system, lagoon, with strong evaporitic conditions. The depressions were of variable depth and lengths and could be isolated, in particular by stromatolitic barriers, dried and flooded by seawater with continental inflows. The largest and deepest ones correspond to the sparry magnesite deposits. These magnesites are located in a lateral way to almost pure calcitic marbles, through dolomitic marbles. This points to a process of chemical differentiation within a paralic system, from open sea to landwards, more calcic in the first to more magnesian in the confined environment, in which lagoon waters are progressively purified from Ca through preferential precipitation of calcic carbonates and secondly of gypsum/anhydrite, which increases the Mg/Ca ratio, and lead carbonate deposits towards the magnesite pole. These metacarbonates also reveal a fractionation of the light REE, starting from the calcitic, through the dolomitic to magnesitic marbles. In the calcitic marbles, REE are in association with the clay fraction, while in magnesitic marbles, they seem to be in association with organic and/or soluble complexes. The magnesitic marbles are of sedimentary origin and underwent important diagenetic evolution before being metamorphosed during the Brasiliano orogenic cycle.

ANÁLISE MULTITEMPORAL DO PROCESSO DE DEGRADAÇÃO DA VEGETAÇÃO DA BACIA HIDROGRÁFICA DO RIO JAIBARAS NO ESTADO DO CEARÁ

This study aimed to analyze a series of multitemporal LANDSAT 5, where they have gone through techniques of digital image processing, allowing to generate NDVI index (normalized difference vegetation), so it was possible to identify and quantify the levels of degradation of vegetation in the area of River Basin Jaiba / EC within 24 years, and specifically, the research conducted in the years 1985, 1992, 1996, 2007 and 2009. Through the use of classification techniques of digital image processing and field work were prepared thematic maps of the levels of degradation of the vegetation of river basin Jaiba / EC, the vegetation is classified into vegetation conserved, partially degraded, degraded, severely degraded and exposed soil, showing the wear and changes in levels of conservation of vegetation.

Carste em Rochas Não Carbonáticas: contribuição ao estudo geomorfológico em cavernas de arenito da Amazônia Paraense.

A Geomorfologia Carstica e um ramo das geociencias que se dedica a investigar a natureza, a morfogenese e a morfodinâmica das cavidades naturais subterrâneas, as quais incluem diferentes denominacoes como cavernas, grutas, abrigos, etc., alem de entender suas feicoes relacionadas. O termo carste surgiu relacionado a rochas carbonaticas (calcarios, dolomitas), porem existem tambem cavernas desenvolvidas em rochas siliciclasticas (arenitos, conglomerados, argilitos), metassedimentares (quartzito, formacao ferrifera), igneas (granito, basalto), dentre outras. Na regiao Amazonica, rica em feicoes geomorfologicas resultantes da interacao sistemica de elementos da natureza, ocorrem muitas cavernas em arenitos. Como exemplo, tem-se a Provincia Espeleologica Altamira-Itaituba (estado do Para), situada na faixa de contato dos dominios geologicos da Bacia Sedimentar do Amazonas e do Embasamento Cristalino do Complexo Xingu. Nesse sentido, o estudo proposto, com base na analise geossistemica da paisagem, apresenta uma ampliacao dos estudos sobre carste em rochas nao carbonaticas, contribuindo para os estudos espeleologicos no Brasil. A estrutura geologica das cavernas da Provincia apresenta-se notadamente em arenitos friaveis da For¬macao Maecuru, pertencente ao Grupo Urupadi. Os processos que envolvem a formacao das cavernas estao associados principalmente a acao mecânica da agua (erosao hidrica), embora a acao quimica da agua apresente pa¬pel fundamental na dissolucao intergranular.

Levantamento de feições estruturais lineares a partir de sensoriamento remoto - uma contribuição para o mapeamento geotécnico na Serra de Baturité, Ceará

The use of techniques of digital image processing remote sensing allows applications of great importance for the geosciences. One of the possible fields of application of these resources is the geotechnical mapping. Filtering techniques for radar and optical images emphasize natural linear structures that can be interpreted as fractures, drainages, ridges, valley bottoms and foliation. With the lifting of these data, combined with field work, the geotechnical mapping gains in precision and agility. The high incidence of slope instability events recently occurred in some parts of Brazil, especially in the Southeast, has attracted the interest of researchers in engineering and geoscience areas in the preparation of monographs, dissertations, theses and academic articles that address this issue. For subsequent generation of a diagram rosettes and visualization of regional trend of the structures and preparation of lineament density map, the vectors corresponding to the guidelines direction were measured in azimuth, and the length was measured in meters. The importance of applying frequency spectrum filters is therefore to quantify and qualify these geological and geomorphological structures, which are among the main components of conditions of mass movements and erosion.