Foraminíferos, ostracodes e microfauna associada da plataforma continental equatorial norte-rio-grandense, NE Brasil: área Porto do Mangue a Galinhos

This study was conducted in the adjacent Brazilian equatorial inner shelf to Rio Grande do Norte, between the region of Porto do Mangue and Galinhos. The main objective is the characterization of biogenic sediments, especially foraminifera and ostracod collected on the surface of the seafloor. The methodology involved standard procedures including literature, surveys, processing of samples in the laboratory and identification of foraminifera and ostracods by genera or species under stereo microscopy and scanning electron microscopy (SEM). Multivariate statistical analyzes and study of ecological indexes were also applied to the study of foraminifera. Three transects, from inner shelf to slope were sampled: profile 01 (east, near Galos), profile 02 (center, near the city of Macau) and profile 03 (west, near Ponta do Mel). Results indicated the predominance of benthic foraminifera and little plankton occurrence. Benthic foraminifera genera observed in abundance were Quinqueloculina, Textularia, Globigerina and Pyrgo, Quinqueloculina, Textularia, Pyrgo, Ammonia, Elphidium, Pseudononion, Peneroplis, Bolivina and Poroeponides, occurred more frequently. Less frequently been described Amphistegina, Archaias, Bigenerina, Cibicides, Cassidulina, Amphicorina, Cornuspira, Paterina, Hopkunsina, Oolina, Uvigerina, Fusenkoina, Nonionella, Amphisorus, Wiesrella, Reussella, Reophax, Nodosaria, Marginulina and Cyclogyra. Six genera of ostracods were also identified: Puriana variabilis / P. convoluted?, Loxoconcha sp, Bairdiidae, Xestoleberis sp, Hemicytheridae and Ruggiericythere sp. Groups of organisms found in the studied shelf presented chemical composition of Ca, C, O, Na, Cl, Al, Mg, and Si. The proportions of chemical elements may vary according to the type of biogenic sediment, with the highest values identified as Ca, C, Cl, Na and O. The absolute dating by carbon 14 method indicated sediments of different colors (light and dark), correspond to a single age from 3000 to 6000 years BP, related to the Quaternary. These data intend to complement information about biogenic sediments in the Brazilian continental shelf, especially in the Northeast, where there is a lack of such studies.

Pirometamorfismo em calcários da formação Jandaíra, bacia potiguar, nordeste do Brasil

The present study aims the characterization of thermally affected carbonate rocks from Jandaíra Formation in contact with Paleogene and Neogene basic intrusions in the region of the Pedro Avelino and Jandaíra municipalities (RN), northeastern Brazil. For this study, field, petrographic, x-ray diffraction, electron microprobe, and whole rock litogeochemistry data of carbonates were undertaken. The thermally unaffected limestones are classified like wackstones, grainstones and packstones. They may constitute carbonates grains of benthic foraminifera, echinoderm spines, ostracods, algae, corals, bivalves, gastropods, peloids and intraclasts. The porosities are classified like vug, intraparticle, interparticle, intercrystal and moldic types. The major minerals are calcite, ankerite and dolomite; the detrital are montmorillonite, pyrite, limonite, quartz and microcline. The thermally affected limestones are very coarse to very fine-grained and light to dark gray color. The fossiliferous components totally disappear, and the porosity tends to disappear. With the data obtained, it can be inferred that the carbonate protoliths would be calciferous to dolomitic limestones, both with small amount of clay minerals. Crystalline carbonates from dolomitic protolith have rhombohedral calcite and iron oxides / hydroxides, making the rocks much darker. The carbonates from calciferous protolith have a wide variation of grain size according to the recrystallization degree, increasing toward contact with the basic bodies. In this group, it was identified the minerals lizardite and spinel in weakly to moderately affected samples, and spinel and spurrite in strongly affected rocks, as well as calcite, that occur everywhere. The geological context (shallow level diabase intrusions), the crystallization of the pyrometamorphic minerals spurrite and olivine, and comparison with diagrams from the literature allow estimating temperatures and pressures around 1050-1200 °C and 0.5-1.0 kbar, respectively, for PTOTAL=PCO2. The post-intrusion cooling would have afforded the releasing of metasomatic / hydrothermal fluids, allowing the opening of the metamorphic system, with possible contribution of chemical elements from host units (sandstones, shales) and from basic intrusions. This would induce hydration of previous phases, allowing the formation of serpentine, chlorite and brucite. The results discussed here reveal the strong influence of the heat from basic intrusions within the sedimentary pile. Whereas in the offshore portion of the basin occur sills with up to 1000 m thickness, the understanding of pyrometamorphism might be useful for understanding and measuring the thermally affected rocks.

Pirometamorfismo em calcários da formação Jandaíra, bacia potiguar, nordeste do Brasil

The present study aims the characterization of thermally affected carbonate rocks from Jandaíra Formation in contact with Paleogene and Neogene basic intrusions in the region of the Pedro Avelino and Jandaíra municipalities (RN), northeastern Brazil. For this study, field, petrographic, x-ray diffraction, electron microprobe, and whole rock litogeochemistry data of carbonates were undertaken. The thermally unaffected limestones are classified like wackstones, grainstones and packstones. They may constitute carbonates grains of benthic foraminifera, echinoderm spines, ostracods, algae, corals, bivalves, gastropods, peloids and intraclasts. The porosities are classified like vug, intraparticle, interparticle, intercrystal and moldic types. The major minerals are calcite, ankerite and dolomite; the detrital are montmorillonite, pyrite, limonite, quartz and microcline. The thermally affected limestones are very coarse to very fine-grained and light to dark gray color. The fossiliferous components totally disappear, and the porosity tends to disappear. With the data obtained, it can be inferred that the carbonate protoliths would be calciferous to dolomitic limestones, both with small amount of clay minerals. Crystalline carbonates from dolomitic protolith have rhombohedral calcite and iron oxides / hydroxides, making the rocks much darker. The carbonates from calciferous protolith have a wide variation of grain size according to the recrystallization degree, increasing toward contact with the basic bodies. In this group, it was identified the minerals lizardite and spinel in weakly to moderately affected samples, and spinel and spurrite in strongly affected rocks, as well as calcite, that occur everywhere. The geological context (shallow level diabase intrusions), the crystallization of the pyrometamorphic minerals spurrite and olivine, and comparison with diagrams from the literature allow estimating temperatures and pressures around 1050-1200 °C and 0.5-1.0 kbar, respectively, for PTOTAL=PCO2. The post-intrusion cooling would have afforded the releasing of metasomatic / hydrothermal fluids, allowing the opening of the metamorphic system, with possible contribution of chemical elements from host units (sandstones, shales) and from basic intrusions. This would induce hydration of previous phases, allowing the formation of serpentine, chlorite and brucite. The results discussed here reveal the strong influence of the heat from basic intrusions within the sedimentary pile. Whereas in the offshore portion of the basin occur sills with up to 1000 m thickness, the understanding of pyrometamorphism might be useful for understanding and measuring the thermally affected rocks.

Petrologia e geocronologia U-Pb do plúton granítico Serra da Rajada, porção central do domínio Rio Piranhas - Seridó, Província Borborema, NE do Brasil

The ediacaran plutonic activity related to the Brasilian/Pan-African orogeny is one of the most important geological features in the Borborema Province, represented along its extension by numerous batholiths, stocks, and dikes.The object of this study, the Serra Rajada Granitic Pluton (SRGP), located in the central portion of the Piranhas-Seridó River Domain is an example of this activity. This pluton has been the subject of cartographic, petrographic, geochronological and lithogeochemical studies and its rocks were characterized by two facies. First, the granitic facies were described as monzogranites consisting of K-feldspar, plagioclase (oligoclase - An23-24%), quartz and biotite (main mafic) and opaque minerals such as titanite, allanite, apatite, and zircon as accessories. Alteration minerals are chlorite, white mica and carbonate. Second, the dioritic facies consist of rocks formed by quartz diorite containing plagioclase (dominant mineral phase), quartz and K-feldspar. Biotite and amphibole are the dominant mafic minerals; and titanite, opaque minerals, allanite, zircon and apatite are the accessories. However, previous geological mapping work in the region also identified the presence of other lithostratigraphic units. These were described as gneisses and migmatites with undifferentiated amphibolite lenses related to the Caicó Complex (Paleoproterozoic) and metasedimentary rocks of the Seridó Group (Neoproterozoic) composed of paragneiss with calc-silicate lenses, muscovite quartzite and biotite schist (respectively, the Jucurutu formations, Equador and Seridó), the host rocks for the SRGP rocks. Leucomicrogranite and pegmatite dikes have also been identified, both related to the end of the Ediacaran magmatism and colluvial- eluvial and alluvial deposits related to Neogene and Quaternary, respectively. Lithogeochemical data on the SRGP granite facies, highlighted quite evolved rocks (SiO2 69% to 75%), rich in alkalis (Na2O+K2O ≥ 8.0%), depleted of MgO (≤ 0.45%), CaO (≤ 1.42%) and TiO2 (≤ 0.36%) and moderate levels of Fe2O3t (2.16 to 3.53%). They display transitional nature between metaluminous and peraluminous (predominance of the latter) with sub-alkaline/monzonitic (High K calcium-alkali) affinity. Harker diagrams show negative correlations for Fe2O3t, MgO, and CaO, indicating mafic and plagioclase fractionation. REE spectrum shows enrichment of LREE relative to heavy REE (LaN/YbN = 23.70 to 10.13), with negative anomaly in the Eu (Eu/Eu* = 0.70 to 0.23), suggesting fractionation or accumulation in the feldspars source (plagioclase). Data integration allows to correlate the SRGP rocks with those described as Calcium-Alkaline Suite of equigranular High K. The crystallization conditions of the SRGP rocks were determined from the integration of petrographic and lithogeochemical data. These data indicated intermediate to high conditions of ƒO2 (mineral paragenesis titanite + magnetite + quartz), parent magma saturated in H2O (early biotite crystallization), tardi-magmatic processes of fluids rich in ƒCO2, H2O and O2 causing part of the mineral assembly to change (plagioclase carbonation and saussuritization, biotite chloritization and opaques Sphenitization). Thermobarometrical conditions were estimated based on geochemical parameters (Zr and P2O5) and CIPW normative minerals, with results showing the liquidus minimum temperature of about800°C and the solidus temperature of approximately 700°C. The final/minimum crystallization pressure are suggested to be between 3 and 5 Kbar. The presence of zoned minerals (plagioclase and allanite) associated with lithogeochemical data in bi-log diagrams for Rb vs. Ba and Rb vs. Sr suggest the role of fractional crystallization as the dominant process in the magmatic evolution of SRGP. U-Pb Geochronological and Sm-Nd isotope studies indicated, respectively, the crystallization age of biotite monzogranite as 557 ± 13 Ma, with TDM model age of 2.36 Ga, and εNd value of -20.10 to the crystallization age, allowing to infer paleoproterozoic crustal source for the magma.

Petrologia e geocronologia U-Pb do plúton granítico Serra da Rajada, porção central do domínio Rio Piranhas - Seridó, Província Borborema, NE do Brasil

The ediacaran plutonic activity related to the Brasilian/Pan-African orogeny is one of the most important geological features in the Borborema Province, represented along its extension by numerous batholiths, stocks, and dikes.The object of this study, the Serra Rajada Granitic Pluton (SRGP), located in the central portion of the Piranhas-Seridó River Domain is an example of this activity. This pluton has been the subject of cartographic, petrographic, geochronological and lithogeochemical studies and its rocks were characterized by two facies. First, the granitic facies were described as monzogranites consisting of K-feldspar, plagioclase (oligoclase - An23-24%), quartz and biotite (main mafic) and opaque minerals such as titanite, allanite, apatite, and zircon as accessories. Alteration minerals are chlorite, white mica and carbonate. Second, the dioritic facies consist of rocks formed by quartz diorite containing plagioclase (dominant mineral phase), quartz and K-feldspar. Biotite and amphibole are the dominant mafic minerals; and titanite, opaque minerals, allanite, zircon and apatite are the accessories. However, previous geological mapping work in the region also identified the presence of other lithostratigraphic units. These were described as gneisses and migmatites with undifferentiated amphibolite lenses related to the Caicó Complex (Paleoproterozoic) and metasedimentary rocks of the Seridó Group (Neoproterozoic) composed of paragneiss with calc-silicate lenses, muscovite quartzite and biotite schist (respectively, the Jucurutu formations, Equador and Seridó), the host rocks for the SRGP rocks. Leucomicrogranite and pegmatite dikes have also been identified, both related to the end of the Ediacaran magmatism and colluvial- eluvial and alluvial deposits related to Neogene and Quaternary, respectively. Lithogeochemical data on the SRGP granite facies, highlighted quite evolved rocks (SiO2 69% to 75%), rich in alkalis (Na2O+K2O ≥ 8.0%), depleted of MgO (≤ 0.45%), CaO (≤ 1.42%) and TiO2 (≤ 0.36%) and moderate levels of Fe2O3t (2.16 to 3.53%). They display transitional nature between metaluminous and peraluminous (predominance of the latter) with sub-alkaline/monzonitic (High K calcium-alkali) affinity. Harker diagrams show negative correlations for Fe2O3t, MgO, and CaO, indicating mafic and plagioclase fractionation. REE spectrum shows enrichment of LREE relative to heavy REE (LaN/YbN = 23.70 to 10.13), with negative anomaly in the Eu (Eu/Eu* = 0.70 to 0.23), suggesting fractionation or accumulation in the feldspars source (plagioclase). Data integration allows to correlate the SRGP rocks with those described as Calcium-Alkaline Suite of equigranular High K. The crystallization conditions of the SRGP rocks were determined from the integration of petrographic and lithogeochemical data. These data indicated intermediate to high conditions of ƒO2 (mineral paragenesis titanite + magnetite + quartz), parent magma saturated in H2O (early biotite crystallization), tardi-magmatic processes of fluids rich in ƒCO2, H2O and O2 causing part of the mineral assembly to change (plagioclase carbonation and saussuritization, biotite chloritization and opaques Sphenitization). Thermobarometrical conditions were estimated based on geochemical parameters (Zr and P2O5) and CIPW normative minerals, with results showing the liquidus minimum temperature of about800°C and the solidus temperature of approximately 700°C. The final/minimum crystallization pressure are suggested to be between 3 and 5 Kbar. The presence of zoned minerals (plagioclase and allanite) associated with lithogeochemical data in bi-log diagrams for Rb vs. Ba and Rb vs. Sr suggest the role of fractional crystallization as the dominant process in the magmatic evolution of SRGP. U-Pb Geochronological and Sm-Nd isotope studies indicated, respectively, the crystallization age of biotite monzogranite as 557 ± 13 Ma, with TDM model age of 2.36 Ga, and εNd value of -20.10 to the crystallization age, allowing to infer paleoproterozoic crustal source for the magma.

Análise de sedimentos associados à atividade de perfuração na plataforma continental externa, Bacia Potiguar - Brasil

This study was developed in an area located on the outer shelf in the Potiguar Basin, Brazilian equatorial margin; this tropical shelf represents a modern, highly dynamic mixed carbonate-siliciclastic system. Field sampling was carried out during 3 cruises surrounding a shallow-water exploratory well to compare sediment properties of the seafloor, including grainsize, texture, mineral composition, carbonate content, and organic matter. Cruise 1 (C1) was carried prior to drilling, while Cruise 2(C2) and 3 (C3) respectivelly 3 and 12 months after drilling. The sample grid used had 16 stations located along 4 radials from 50 m the well up to a distance of 500 m. Sediments were analyzed in the first 0-2 cm, and 0-10 cm layers. The results show that sedimentary cover around the well is poor to very poorly sorted, with the particle size predominantly in the medium to coarse sand fraction, followed by gravel and mud. The content of calcium carbonate is greater than 96%, associated to bioclastic sediments, and the content of organic matter is less than 12%. Only minor sedimentological variations occured in the area affected by drilling operations. Mainly observed during the second cruise, in terms of a change in grain size distribution associated to an increase in siliciclastic content, This impact occurred in the most surficial sediment (0-2 cm), in the radial closest to the well (50 m), and could suggest the effects of drilling. However, in the third cruise, one year after drilling, the sediments return to show the same characteristics as in the first cruise. These results show no significant sedimentological variations due to drilling activity, and indicate that ocean dynamics in this area was high enough to recover the environment original characteristics.

Análise de sedimentos associados à atividade de perfuração na plataforma continental externa, Bacia Potiguar - Brasil

This study was developed in an area located on the outer shelf in the Potiguar Basin, Brazilian equatorial margin; this tropical shelf represents a modern, highly dynamic mixed carbonate-siliciclastic system. Field sampling was carried out during 3 cruises surrounding a shallow-water exploratory well to compare sediment properties of the seafloor, including grainsize, texture, mineral composition, carbonate content, and organic matter. Cruise 1 (C1) was carried prior to drilling, while Cruise 2(C2) and 3 (C3) respectivelly 3 and 12 months after drilling. The sample grid used had 16 stations located along 4 radials from 50 m the well up to a distance of 500 m. Sediments were analyzed in the first 0-2 cm, and 0-10 cm layers. The results show that sedimentary cover around the well is poor to very poorly sorted, with the particle size predominantly in the medium to coarse sand fraction, followed by gravel and mud. The content of calcium carbonate is greater than 96%, associated to bioclastic sediments, and the content of organic matter is less than 12%. Only minor sedimentological variations occured in the area affected by drilling operations. Mainly observed during the second cruise, in terms of a change in grain size distribution associated to an increase in siliciclastic content, This impact occurred in the most surficial sediment (0-2 cm), in the radial closest to the well (50 m), and could suggest the effects of drilling. However, in the third cruise, one year after drilling, the sediments return to show the same characteristics as in the first cruise. These results show no significant sedimentological variations due to drilling activity, and indicate that ocean dynamics in this area was high enough to recover the environment original characteristics.

Evolução estratigráfica da sequência neocarbonífera-eotriássica da Bacia do Parnaíba, NE do Brasil

The Parnaíba Basin consists in an intracratonic basin whose sucession of rocks is arranged in five supersequences. The Upper Carboniferous-Lower Triassic Sequence represents the third major sedimentary cycle and corresponds to Balsas Group, which is divided into four units: Piauí Formation, Pedra de Fogo Formation, Motuca Formation and Sambaíba Formation, from base to top. Different interpretations have been made by several authors in recent decades to interpreted the depositional system and environments related to each unit that belongs to this sequence. In general way, it is described as a thick pack of siliciclastic sediments deposited under complex conditions, varying from clastic/evaporitic shallow marine to lacustrine and desert environment. Aiming to clarify the sedimentary sequence evolution, this work underwent a stratigraphic analysis of the Upper Carboniferous-Lower Triassic deposits by applying modern concepts of the sequence stratigraphy based on well and seismic database. Three main depositional sequences of higher frequency were identified in each well analyzed. The sequence 1 corresponds to rocks initially deposited by a fluvial system with braided channel characteristics which evolved to shallow marine with coastal sabkha conditions related to a transgressive stage, that later evolved to a deltaic system. The Sequence 2 corresponds to rocks deposited in a lacustrine/desert environment associated with sabkha generated during a period of increased aridity in which the area occupied by the Parnaíba Basin had been suffering. The registration of a major regressive phase is shown in Sequence 2 which evolved to a dominantly desert environment recorded in Sequence 3. Seismic stratigraphy analyses allow to define a series of stratigraphic surfaces and related genetic units, as well as to infer its lateral expression. Seismic facies associated with such sequences are dominantly parallel and sub-parallel, with good lateral continuity, suggesting the sedimentary rate was relatively constant during deposition.

Evolução estratigráfica da sequência neocarbonífera-eotriássica da Bacia do Parnaíba, NE do Brasil

The Parnaíba Basin consists in an intracratonic basin whose sucession of rocks is arranged in five supersequences. The Upper Carboniferous-Lower Triassic Sequence represents the third major sedimentary cycle and corresponds to Balsas Group, which is divided into four units: Piauí Formation, Pedra de Fogo Formation, Motuca Formation and Sambaíba Formation, from base to top. Different interpretations have been made by several authors in recent decades to interpreted the depositional system and environments related to each unit that belongs to this sequence. In general way, it is described as a thick pack of siliciclastic sediments deposited under complex conditions, varying from clastic/evaporitic shallow marine to lacustrine and desert environment. Aiming to clarify the sedimentary sequence evolution, this work underwent a stratigraphic analysis of the Upper Carboniferous-Lower Triassic deposits by applying modern concepts of the sequence stratigraphy based on well and seismic database. Three main depositional sequences of higher frequency were identified in each well analyzed. The sequence 1 corresponds to rocks initially deposited by a fluvial system with braided channel characteristics which evolved to shallow marine with coastal sabkha conditions related to a transgressive stage, that later evolved to a deltaic system. The Sequence 2 corresponds to rocks deposited in a lacustrine/desert environment associated with sabkha generated during a period of increased aridity in which the area occupied by the Parnaíba Basin had been suffering. The registration of a major regressive phase is shown in Sequence 2 which evolved to a dominantly desert environment recorded in Sequence 3. Seismic stratigraphy analyses allow to define a series of stratigraphic surfaces and related genetic units, as well as to infer its lateral expression. Seismic facies associated with such sequences are dominantly parallel and sub-parallel, with good lateral continuity, suggesting the sedimentary rate was relatively constant during deposition.

Utilização de sismos regionais para a determinação de um modelo 1D de velocidades da onda P na Província Borborema - NE do Brasil

In this work it was performed a study to obtain parameters for an 1D regional velocity model for the Borborema Province, NE Brazil. It was used earthquakes occurred between 2001 and 2013 with magnitude greater than 2.9 mb either from epicentres determined from local seismic networks or by back azimuth determination, when possible. We chose seven events which occurred in the main seismic areas in the Borborema Province. The selected events were recorded in up to 74 seismic stations from the following networks: RSISNE, INCT-ET, João Câmara – RN, São Rafael – RN, Caruaru - PE, São Caetano - PE, Castanhão - CE, Santana do Acarau - CE, Taipu – RN e Sobral – CE, and the RCBR (IRIS/USGS—GSN). For the determination of the model parameters were inverted via a travel-time table and its fit. These model parameters were compared with other known model (global and regional) and have improved the epicentral determination. This final set of parameters model, we called MBB is laterally homogeneous with an upper crust at 11,45 km depth and total crustal thickness of 33,9 km. The P-wave velocity in the upper crust was estimated at 6.0 km/s and 6.64 km/s for it lower part. The P-wave velocity in the upper mantle we estimated at 8.21 km/s with an VP/VS ratio of approximately 1.74.

Utilização de sismos regionais para a determinação de um modelo 1D de velocidades da onda P na Província Borborema - NE do Brasil

In this work it was performed a study to obtain parameters for an 1D regional velocity model for the Borborema Province, NE Brazil. It was used earthquakes occurred between 2001 and 2013 with magnitude greater than 2.9 mb either from epicentres determined from local seismic networks or by back azimuth determination, when possible. We chose seven events which occurred in the main seismic areas in the Borborema Province. The selected events were recorded in up to 74 seismic stations from the following networks: RSISNE, INCT-ET, João Câmara – RN, São Rafael – RN, Caruaru - PE, São Caetano - PE, Castanhão - CE, Santana do Acarau - CE, Taipu – RN e Sobral – CE, and the RCBR (IRIS/USGS—GSN). For the determination of the model parameters were inverted via a travel-time table and its fit. These model parameters were compared with other known model (global and regional) and have improved the epicentral determination. This final set of parameters model, we called MBB is laterally homogeneous with an upper crust at 11,45 km depth and total crustal thickness of 33,9 km. The P-wave velocity in the upper crust was estimated at 6.0 km/s and 6.64 km/s for it lower part. The P-wave velocity in the upper mantle we estimated at 8.21 km/s with an VP/VS ratio of approximately 1.74.

Análise estratigráfica da sequência mesodevoniana-eocarbonífera da Bacia do Parnaíba, nordeste do Brasil

The Middle Devonian-Early Carboniferous sequence of the Parnaíba Basin, lithostratigraphically defined as Canindé Group, has been reinterpreted using the basic model of sequence stratigraphy. Therefore, lithology and gamma ray well-logs and seismic lines of central portion of the basin were analyzed, producing up from there diagrams 1D, isochore maps and stratigraphic sections. As results of this study, were defined two depositional cycles of second order, referred as Depositional Sequence 1 (SEQ1) and the Depositional Sequence 2 (SEQ2). The SEQ1, with interval about 37 Ma, is limited below by Early Devonian Unconformity and is equivalent to the formations Itaim, Pimenteiras and Cabeças. The SEQ2, which follows, comprises a range of about 15 Ma and is equivalent to the Longá Formation The SEQ1 starts with the lowstand systems tract, consisting of progradational parasequence set in the basal part, predominantly pelitic, deposited on a prodelta under influence of storms and the upper part consists in sandstones of deltaic front, with the maximum regressive surface on the upper limit. The transgressive systems tract, deposited above, is characterized by retrogradacional parasequence set composed of shallow shelf mudstones, deposited under storm conditions. The maximum flooding surface, upper limit of this tract, is positioned in a shale level whose radioactivity in gammaray well-log is close to 150 API. The highstand systems tract presents progradational parasequence set, comprising mudstones and sandstones deposited in shelf, fluvial-estuarine or deltaic and periglacial environments, with the upper limit the Late Devonian Unconformity. The SEQ2 was deposited in shelf environment, starting with the lowstand systems tract, that is characterized by a progradational parasequence set, followed by the transgressive systems tract, with retrogradational character. The upper limit of the tract corresponding to the fusion between maximum flooding surface with the upper limit of this sequence, which is the Early Carboniferous Unconformity, where the overlapping section was eroded. This section, which corresponds the highstand systems tract is restricted to portions at which the erosion that generate the Early-Carboniferous Unconformity was less effective, preserving the records of this unit.

Análise estratigráfica da sequência mesodevoniana-eocarbonífera da Bacia do Parnaíba, nordeste do Brasil

The Middle Devonian-Early Carboniferous sequence of the Parnaíba Basin, lithostratigraphically defined as Canindé Group, has been reinterpreted using the basic model of sequence stratigraphy. Therefore, lithology and gamma ray well-logs and seismic lines of central portion of the basin were analyzed, producing up from there diagrams 1D, isochore maps and stratigraphic sections. As results of this study, were defined two depositional cycles of second order, referred as Depositional Sequence 1 (SEQ1) and the Depositional Sequence 2 (SEQ2). The SEQ1, with interval about 37 Ma, is limited below by Early Devonian Unconformity and is equivalent to the formations Itaim, Pimenteiras and Cabeças. The SEQ2, which follows, comprises a range of about 15 Ma and is equivalent to the Longá Formation The SEQ1 starts with the lowstand systems tract, consisting of progradational parasequence set in the basal part, predominantly pelitic, deposited on a prodelta under influence of storms and the upper part consists in sandstones of deltaic front, with the maximum regressive surface on the upper limit. The transgressive systems tract, deposited above, is characterized by retrogradacional parasequence set composed of shallow shelf mudstones, deposited under storm conditions. The maximum flooding surface, upper limit of this tract, is positioned in a shale level whose radioactivity in gammaray well-log is close to 150 API. The highstand systems tract presents progradational parasequence set, comprising mudstones and sandstones deposited in shelf, fluvial-estuarine or deltaic and periglacial environments, with the upper limit the Late Devonian Unconformity. The SEQ2 was deposited in shelf environment, starting with the lowstand systems tract, that is characterized by a progradational parasequence set, followed by the transgressive systems tract, with retrogradational character. The upper limit of the tract corresponding to the fusion between maximum flooding surface with the upper limit of this sequence, which is the Early Carboniferous Unconformity, where the overlapping section was eroded. This section, which corresponds the highstand systems tract is restricted to portions at which the erosion that generate the Early-Carboniferous Unconformity was less effective, preserving the records of this unit.

Medium change monitoring using ambient seismic noise and coda wave interferometry: examples from intraplate NE Brazil and the Mid-Atlantic Ridge

This thesis presents and discusses the results of ambient seismic noise correlation for two different environments: intraplate and Mid-Atlantic Ridge. The coda wave interferometry method has also been tested for the intraplate data. Ambient noise correlation is a method that allows to retrieve the structural response between two receivers from ambient noise records, as if one of the station was a virtual source. It has been largely used in seismology to image the subsurface and to monitor structural changes associated mostly with volcanic eruptions and large earthquakes. In the intraplate study, we were able to detect localized structural changes related to a small earthquake swarm, which main event is mR 3.7, North-East of Brazil. We also showed that the 1-bit normalization and spectral whitening result on the loss of waveform details and that the phase auto-correlation, which is amplitude unbiased, seems to be more sensitive and robust for our analysis of a small earthquake swarm. The analysis of 6 months of data using cross-correlations detect clear medium changes soon after the main event while the auto-correlations detect changes essentially after 1 month. It could be explained by fluid pressure redistribution which can be initiated by hydromechanical changes and opened path ways to shallower depth levels due to later occurring earthquakes. In the Mid-Atlantic Ridge study, we investigate structural changes associated with a mb 4.9 earthquake in the region of the Saint Paul transform fault. The data have been recorded by a single broadband seismic station located at less than 200 km from the Mid-Atlantic ridge. The results of the phase auto-correlation for a 5-month period, show a strong co-seismic medium change followed by a relatively fast post-seismic recovery. This medium change is likely related to the damages caused by the earthquake’s ground shaking. The healing process (filling of the new cracks) that lasted 60 days can be decomposed in two phases, a fast recovery (70% in ~30 days) in the early post-seismic stage and a relatively slow recovery later (30% in ~30 days). In the coda wave interferometry study, we monitor temporal changes of the subsurface caused by the small intraplate earthquake swarm mentioned previously. The method was first validated with synthetics data. We were able to detect a change of 2.5% in the source position and a 15% decrease of the scatterers’ amount. Then, from the real data, we observed a rapid decorrelation of the seismic coda after the mR 3.7 seismic event. This indicates a rapid change of the subsurface in the fault’s region induced by the earthquake.

Medium change monitoring using ambient seismic noise and coda wave interferometry: examples from intraplate NE Brazil and the Mid-Atlantic Ridge

This thesis presents and discusses the results of ambient seismic noise correlation for two different environments: intraplate and Mid-Atlantic Ridge. The coda wave interferometry method has also been tested for the intraplate data. Ambient noise correlation is a method that allows to retrieve the structural response between two receivers from ambient noise records, as if one of the station was a virtual source. It has been largely used in seismology to image the subsurface and to monitor structural changes associated mostly with volcanic eruptions and large earthquakes. In the intraplate study, we were able to detect localized structural changes related to a small earthquake swarm, which main event is mR 3.7, North-East of Brazil. We also showed that the 1-bit normalization and spectral whitening result on the loss of waveform details and that the phase auto-correlation, which is amplitude unbiased, seems to be more sensitive and robust for our analysis of a small earthquake swarm. The analysis of 6 months of data using cross-correlations detect clear medium changes soon after the main event while the auto-correlations detect changes essentially after 1 month. It could be explained by fluid pressure redistribution which can be initiated by hydromechanical changes and opened path ways to shallower depth levels due to later occurring earthquakes. In the Mid-Atlantic Ridge study, we investigate structural changes associated with a mb 4.9 earthquake in the region of the Saint Paul transform fault. The data have been recorded by a single broadband seismic station located at less than 200 km from the Mid-Atlantic ridge. The results of the phase auto-correlation for a 5-month period, show a strong co-seismic medium change followed by a relatively fast post-seismic recovery. This medium change is likely related to the damages caused by the earthquake’s ground shaking. The healing process (filling of the new cracks) that lasted 60 days can be decomposed in two phases, a fast recovery (70% in ~30 days) in the early post-seismic stage and a relatively slow recovery later (30% in ~30 days). In the coda wave interferometry study, we monitor temporal changes of the subsurface caused by the small intraplate earthquake swarm mentioned previously. The method was first validated with synthetics data. We were able to detect a change of 2.5% in the source position and a 15% decrease of the scatterers’ amount. Then, from the real data, we observed a rapid decorrelation of the seismic coda after the mR 3.7 seismic event. This indicates a rapid change of the subsurface in the fault’s region induced by the earthquake.