Análise da evolução termomecânica da Bacia do Amazonas

Os dados geológicos e geofísicos escolhidos para o tema de estudo pertencem a Bacia do Amazonas, na região centro-norte do Brasil. A Bacia do Amazonas é uma bacia intracratônica com cerca de 500.000 km. A mesma está limitada ao norte pelo Escudo das Guianas e ao sul pelo Escudo Brasileiro. O limite oeste com a Bacia do Solimões é marcado pelo Arco de Purus, ao passo que o Arco de Gurupá constitui seu limite leste. Possui características inerentes a uma bacia intracratônica paleozóica, com uma longa história evolutiva, marcada por discordâncias expressivas e com uma cunha sedimentar relativamente rasa se comparada às bacias cretáceas brasileiras, o que levanta controvérsia a respeito da suficiência do soterramento para a eficiência de geração de hidrocarboneto. Podem ser reconhecidas nos 5000 m do preenchimento sedimentar da Bacia do Amazonas, duas seqüências de primeira ordem: uma paleozóica, intrudida por diques e soleiras de diabásio, na passagem do Triássico para o Jurássico, e uma mesozóica-cenozóica que representam um aspecto importante na evolução térmica da matéria orgânica que ocorre na primeira seqüência. Com relação à exploração de petróleo, apesar do fomento exploratório ocorrido nos últimos anos, a bacia ainda é considerada pouco explorada sendo sua maior reserva a da província de Urucu. Um dos fatores que dificultam bastante a exploração desta bacia assim como a bacia do Solimões a oeste é o acesso restrito, pois estão situadas em áreas remotas e florestadas, de difícil acesso, com muitas reservas indígenas e florestais, o que causa restrições logísticas, operacionais e legais. O efeito térmico das intrusões ígneas é considerado como o responsável pelo acréscimo de calor necessário à maturação da matéria orgânica e conseqüente geração de hidrocarbonetos. Este trabalho contribui com a reconstrução da história térmica desta bacia a partir da modelagem das variáveis termais e da história de soterramento. Para isso, foram utilizados modelos consagrados na literatura, que permitem, de forma simples, a estimativa do fluxo térmico através do embasamento e da seqüência sedimentar. Na análise da influência de intrusões ígneas na estrutura térmica da bacia, o modelo bidimensional desenvolvido pelo método de diferenças finitas se mostrou apropriado. Utilizou-se o fluxo térmico basal calculado nas condições de contorno da modelagem da influência térmica das ígneas. Como resultado obteve-se a estruturação térmica da bacia e a historia maturação de suas rochas geradoras

As relações entre tectonismo e seqüências deposiconais no rifte potiguar - porção SW do Graben de Umbuzeiro, bacia potiguar emersa

Through an integrated approach, using litho, chrono and biostratigraphic data, the relative importance of climate variations and tectonics were recognized in rift sediments of the onshore Potiguar Basin, Northeast Brazil. Concepts of sequence stratigraphy were applied as a template to integrate sedimentological and geochemical data (oxygen isotopes), as well as quantitative palynologic methods to address and recognize the main depositional patterns produced in a rift basin. The main objective was to address the relative importance of climate changes and tectonics to the resultant stratigraphic architecture. The results of computer simulations of sedimentary basin fills of rift basins were quite useful to test working hypothesis and mimic the process of filling a half graben during a rift event. The studied section includes a neovalanginian-eobarremian (Lower Cretaceous) rift interval from the Pendência Formation, located in the southwestern portion of Umbuzeiro Graben, in the offshore Potiguar Basin. The depositional setting is interpreted as progradational deltaic system entering a lake from its flexural margin. Sismoestratigraphyc and well logs analyses allowed to interpret two regressive intervals (Green and Yellow Sequences), separated by a broad transgressive interval (Orange Sequence), known as the Livramento Shale. The depositional history encompass three stages: two tectonically active phases, during the deposition of the Green and Yellow Sequences, and a tectonically quiescent phase, during the deposition of the Orange Sequence. Paleoclimatic interpretation, based on quantitative palynology and geochemical data (��18O), suggests a tendency to arid conditions during the tectonically active phases and wet conditions during the tectonically quiescent phase. Stratigraphic modeling and backstripping techniques, supported by paleoclimatic/paleoecologic interpretations provide a powerful methodology to evaluate the tectonic and climatic controls on tectonic lakes

The stratigraphic record of the quaternary sea level fluctuations and the impact of the post-glacial sea level rise (Termination I) in the Adriatic basin (Mediterranean sea)

The modern stratigraphy of clastic continental margins is the result of the interaction between several geological processes acting on different time scales, among which sea level oscillations, sediment supply fluctuations and local tectonics are the main mechanisms. During the past three years my PhD was focused on understanding the impact of each of these process in the deposition of the central and northern Adriatic sedimentary successions, with the aim of reconstructing and quantifying the Late Quaternary eustatic fluctuations. In the last few decades, several Authors tried to quantify past eustatic fluctuations through the analysis of direct sea level indicators, among which drowned barrier-island deposits or coral reefs, or indirect methods, such as Oxygen isotope ratios (δ18O) or modeling simulations. Sea level curves, obtained from direct sea level indicators, record a composite signal, formed by the contribution of the global eustatic change and regional factors, as tectonic processes or glacial-isostatic rebound effects: the eustatic signal has to be obtained by removing the contribution of these other mechanisms. To obtain the most realistic sea level reconstructions it is important to quantify the tectonic regime of the central Adriatic margin. This result has been achieved integrating a numerical approach with the analysis of high-resolution seismic profiles. In detail, the subsidence trend obtained from the geohistory analysis and the backstripping of the borehole PRAD1.2 (the borehole PRAD1.2 is a 71 m continuous borehole drilled in -185 m of water depth, south of the Mid Adriatic Deep - MAD - during the European Project PROMESS 1, Profile Across Mediterranean Sedimentary Systems, Part 1), has been confirmed by the analysis of lowstand paleoshorelines and by benthic foraminifera associations investigated through the borehole. This work showed an evolution from inner-shelf environment, during Marine Isotopic Stage (MIS) 10, to upper-slope conditions, during MIS 2. Once the tectonic regime of the central Adriatic margin has been constrained, it is possible to investigate the impact of sea level and sediment supply fluctuations on the deposition of the Late Pleistocene-Holocene transgressive deposits. The Adriatic transgressive record (TST - Transgressive Systems Tract) is formed by three correlative sedimentary bodies, deposited in less then 14 kyr since the Last Glacial Maximum (LGM); in particular: along the central Adriatic shelf and in the adjacent slope basin the TST is formed by marine units, while along the northern Adriatic shelf the TST is represented by costal deposits in a backstepping configuration. The central Adriatic margin, characterized by a thick transgressive sedimentary succession, is the ideal site to investigate the impact of late Pleistocene climatic and eustatic fluctuations, among which Meltwater Pulses 1A and 1B and the Younger Dryas cold event. The central Adriatic TST is formed by a tripartite deposit bounded by two regional unconformities. In particular, the middle TST unit includes two prograding wedges, deposited in the interval between the two Meltwater Pulse events, as highlighted by several 14C age estimates, and likely recorded the Younger Dryas cold interval. Modeling simulations, obtained with the two coupled models HydroTrend 3.0 and 2D-Sedflux 1.0C (developed by the Community Surface Dynamics Modeling System - CSDMS), integrated by the analysis of high resolution seismic profiles and core samples, indicate that: 1 - the prograding middle TST unit, deposited during the Younger Dryas, was formed as a consequence of an increase in sediment flux, likely connected to a decline in vegetation cover in the catchment area due to the establishment of sub glacial arid conditions; 2 - the two-stage prograding geometry was the consequence of a sea level still-stand (or possibly a fall) during the Younger Dryas event. The northern Adriatic margin, characterized by a broad and gentle shelf (350 km wide with a low angle plunge of 0.02° to the SE), is the ideal site to quantify the timing of each steps of the post LGM sea level rise. The modern shelf is characterized by sandy deposits of barrier-island systems in a backstepping configuration, showing younger ages at progressively shallower depths, which recorded the step-wise nature of the last sea level rise. The age-depth model, obtained by dated samples of basal peat layers, is in good agreement with previous published sea level curves, and highlights the post-glacial eustatic trend. The interval corresponding to the Younger Dyas cold reversal, instead, is more complex: two coeval coastal deposits characterize the northern Adriatic shelf at very different water depths. Several explanations and different models can be attempted to explain this conundrum, but the problem remains still unsolved.

Eocene benthic foraminiferal community DSDP of Holes 95-612, 95-613 and 11-108

Benthic foraminiferal biofacies may vary independently of water depth and water mass; however, calibration of biofacies and stratigraphic ranges with independent paleodepth estimates allows reconstruction of age-depth patterns applicable throughout the deep Atlantic (Tjalsma and Lohmann, 1983). We have attempted to test these faunal calibrations in a continental margin setting, reconstructing Eocene benthic foraminiferal distributions along a dip section afforded by the New Jersey Transect (DSDP Sites 612, 108, 613). The following independent estimates of Eocene depths for the transect were obtained by "backtracking," "backstripping," and by assuming increasing depth downdip ("paleoslope"): Site 612, near the middle/lower bathyal boundary (about 1000 m); Site 108, in the middle bathyal zone (about 1600 m); and Site 613, near the lower bathyal/upper abyssal boundary (about 2000 m). Within uncertainties of backtracking (hundreds of meters), these estimates agree with estimates of paleodepth based on comparison of the New Jersey margin biofacies with other backtracked faunas. The stratigraphic ranges of many benthic taxa correspond to those found at other Atlantic DSDP sites. The major biofacies patterns show: (1) a depth dichotomy between an early to middle Eocene Nuttallides truempyidominated biofacies (greater than 2000 m) and a Lenticulina-Osangularia-Alabamina cf. dissonata biofacies (1000- 2000 m); and (2) a difference between a middle and a late Eocene biofacies at Site 612. The faunal boundary at about 2000 m, between bathyal and abyssal zones, occurs not only on the margin, but also throughout the deep Atlantic. The faunal change between the middle and late Eocene at Site 612 was due to a decrease of Lenticulina spp., the local disappearance of N. truempyi, and establishment of a Bulimina alazanensis-Gyroidinoides spp. biofacies. Although this change could be attributed to local paleoceanographic or water-depth changes, we argue that it is the bathyal expression of a global deep-sea benthic foraminiferal change which occurred across the middle/late Eocene boundary.

Eocene benthic foraminifera of DSDP Hole 93-605

This paper describes and illustrates early to middle Eocene benthic foraminifers from northwest Atlantic Site 605, on the continental rise off New Jersey. Benthic foraminiferal faunas are dominated by Bulitnina spp., Nuttallides truempyi, Lenticulina spp., and Cibicidoides spp. Other common taxa include Oridorsalis spp., Gyroidinoides spp., uniserial taxa, arenaceous taxa, and Globocassidulina subglobosa. Together, these taxa usually make up 70% or more of the total fauna. The assemblages are interpreted as indicating a lower bathyal environment of deposition during the Eocene at Site 605. This is corroborated by an independent water depth estimate through backstripping, indicating a water depth for the beginning of the Eocene to late middle Eocene of approximately 2300 to 2000 m.

Neodymium isotopic ratios and clay mineral composition from ODP Hole 1147A and Site 184-1148

Ocean Drilling Program sampling of the distal passive margin of South China at Sites 1147 and 1148 has yielded clay-rich hemipelagic sediments dating to 32 Ma (Oligocene), just prior to the onset of seafloor spreading in the South China Sea. The location of the drill sites offshore the Pearl River suggests that this river, or its predecessor, may have been the source of the sediment in the basin, which accounts for only not, vert, similar ~1.8% of the total Neogene sediment in the Asian marginal seas. A mean erosion depth of not, vert, similar ~1 km over the current Pearl River drainage basin is sufficient to account for the sediment volume on the margin. Two-dimensional backstripping of across-margin seismic profiles shows that sedimentation rates peaked during the middle Miocene (11-16 Ma) and the Pleistocene (since 1.8 Ma). Nd isotopic analysis of clays yielded epsilonNd values of -7.7 to -11.0, consistent with the South China Block being the major source of sediment. More positive epsilonNd values during and shortly after rifting compared to later sedimentation reflect preferential erosion at that time of more juvenile continental arc rocks exposed along the margin. As the drainage basin developed and erosion shifted from within the rift to the continental interior epsilonNd values became more negative. A rapid change in the clay mineralogy from smectite-dominated to illite dominated at not, vert, similar 15.5 Ma, synchronous with middle Miocene rapid sedimentation, mostly reflects a change to a wetter, more erosive climate. Evidence that the elevation of the Tibetan Plateau and erosion in the western Himalaya both peaked close to this time supports the suggestion that the Asian monsoon became much more intense at that time, much earlier than the 8.5 Ma age commonly accepted.