Pelotas Basin Research Articles

Seismic stratigraphy and subsidence analysis of the southern Brazilian margin (Campos, Santos and Pelotas basins)

The Campos, Santos and Pelotas basins have been investigated in terms of 2D seismo-stratigraphy and subsidence. The processes controlling accommodation space (e.g. eustacy, subsidence, sediment input) and the evolution of the three basins are discussed. Depositional seismic sequences in the syn-rift Barremian to the drift Holocene basin fill have been identified. In addition, the subsidence/uplift history has been numerically modeled including (i) sediment flux, (ii) sedimentary basin framework, (iii) relation to plate-tectonic reconfigurations, and (iv) mechanism of crustal extension. Although the initial rift development of the three basins is very similar, basin architecture, sedimentary infill and distribution differ considerably during the syn-rift sag to the drift basin stages. After widespread late Aptian–early Albian salt and carbonate deposition, shelf retrogradation dominated in the Campos Basin, whereas shelf progradation occurred in the Santos Basin. In the Tertiary, these basin fill styles were reversed: since the Paleogene, shelf progradation in the Campos Basin contrasts with overall retrogradation in the Santos Basin. In contrast, long-term Cretaceous–Paleogene shelf retrogradation and intense Neogene progradation characterize the Pelotas Basin. Its specific basin fill and architecture mainly resulted from the absence of salt deposition and deformation. These temporally and spatially varying successions were controlled by specific long-term subsidence/uplift trends. Onshore and offshore tectonism in the Campos and Santos basins affected the sediment flux history, distribution of the main depocenters and occurrence of hydrocarbon stratigraphic–structural traps. This is highlighted by the exhumation and erosion of the Serra do Mar, Serra da Mantiqueira and Ponta Grossa Arch in the hinterland, as well as salt tectonics in the offshore domain. The Pelotas Basin was less affected by changes in structural regimes until the Eocene, when the Andean orogeny caused uplift of the source areas. Flexural loading largely controlled its development and potential hydrocarbon traps are mainly stratigraphic.

Time–frequency spectral signature of Pelotas Basin deep water gas hydrates system

Pelotas Basin has the largest gas hydrate occurrence of the Brazilian coast. The reserves are estimated in 780 trillion cubic feet, covering an area of 45,000 km2. In this work we apply spectral decomposition technique in order to better understand a gas hydrate deep water system, performing a continuous time–frequency analysis of seismic trace, where frequency spectrum is the output for each time sample of the seismic trace. This allows a continuous analysis on the effects of the geologic structures and lithology over frequency content of the seismic wave. Spectral anomalies found were interpreted as variations of hydrates concentration inside the Gas Hydrate Stability Zone (GHSZ), as well free gas accumulations beneath and Below the GHSZ and gas chimneys. We concluded that this technique has a good potential to assist seismic study of structures associated with gas hydrates accumulations.

Extension of the Paraná Basin to offshore Brazil: Implications for coalbed methane evaluation

Coalbed methane (CBM) is a worldwide exploration target of the petroleum industry. In Brazil, the most important coal-bearing succession is associated with the Permian Rio Bonito Formation of the Paraná Basin. The gas-prone areas are located at the southeastern margin of the Paraná Basin and possibly in the offshore region of the northern part of the Pelotas Basin. Coalfields end abruptly at the present day shoreline, a result of rifting of Gondwana and the evolution of the South Atlantic Ocean. All geologic indicators suggest that in pre-rift times the coal seams extended further eastwards, probably now lying deeply buried below the sedimentary succession of the Pelotas Basin. The present paper discusses structural, stratigraphic, seismic and aeromagenetic data that support the preservation of continental crust beneath ocean sediment. If the coal beds had similar lateral extent to known onshore coals, and coal beds extended across the projected extension of the Parana basin, and there was a conservative 5 m of cumulative coal thickness, then a potential methane volume can be estimated for this newly inferred resource. Average onshore coal gas content is 32 scf/ton (1.00 m 3/ton). If this is similar in the offshore coal deposits, then the hypothetical methane volume in the offshore area could be in excess of 1.9 × 10 12 scf (56 × 10 9 m 3). Metamorphism from dikes associated with rifting are potential complicating factors in these deposits, and since no borehole reaching the deep-lying strata in the offshore area are available, this is a hypothetical gas resource with a certain level of uncertainty which should be tested in the future by drilling a deep borehole.

Integração de métodos potenciais (gravimetria e magnetometria) na caracterização do embasamento da região sudoeste da bacia de pelotas, Sul do Brasil

Por meio da aplicacao integrada dos metodos potenciais de gravimetria e magnetometria, visou-se contribuir com a caracterizacao do arcabouco estrutural e composicional do embasamento da Bacia de Pelotas. A area selecionada para este estudo e a regiao sul da Provincia Costeira do Rio Grande do Sul, entre as latitudes 31o45' e 34oS e as longitudes 52o e 53o30'W. A interpretacao integrada dos mapas gerados atraves dos metodos potenciais, em um Sistema de Informacoes Geograficas (SIG), permitiu verificar uma complexa estruturacao e a heterogeneidade composicional do embasamento. Essa estruturacao esta relacionada com a evolucao do Escudo Uruguaio-Sul-Rio-Grandense e com a abertura do Oceano Atlântico Sul. Uma anomalia gravimetrica negativa, na regiao do Banhado do Taim, foi interpretada como uma calha no embasamento. Nessa regiao, trabalhos anteriores identificaram uma segmentacao nos sistemas deposicionais pleistocenicos da Planicie Costeira do Rio Grande do Sul. A presenca desta calha na mesma posicao da segmentacao identificada indica que o registro estratigrafico da regiao foi condicionado pelo embasamento, sugerindo a existencia de um controle da heranca geologica na area. Esse controle e possivelmente relacionado com a reativacao de falhas e com a subsidencia diferencial devido a heterogeneidade das litologias presentes no embasamento. Estudos futuros com a aquisicao de novos dados em regioes onde nao foramrealizadas medidas (lagoas e banhados) e a integracao com outros metodos possibilitarao melhorar e testar as interpretacoes realizadas.

Braquiópodes (Rhynchonelliformea, Bouchardioidea) neógenos da bacia de Pelotas (RS) e seu significado paleoambiental

The occurrence of brachiopods in Cenozoic rocks of the Pelotas Basin is known since 1862. In spite of that, detailed systematic and taphonomic studies are still missing. Investigations made a half century ago, have suggested that these brachiopods could belong to Bouchardia cf. zitteli, a species found in the San Julian Formation, Late Oligocene, Argentina. Our data suggest that those brachiopods may resemble Bouchardia transplatina. In the Uruguayan portion of the Pelotas Basin B. transplatina is known in rocks of the Camacho Formation, Miocene. In addition, small recrystallized shells of brachiopods were also recovered from three Petrobras boreholes (2PJ-1-RS, 2PN-1-RS, and 2GA-1-RS) from the Pelotas Basin. Brachiopods come from the interval of 130 to 150 meters within the Miocene Henryhowella evax Zone. Despite the degree of taphonomic modification of those brachiopod shells they indubitably belong to Bouchardia sp. This is noteworthy for various reasons: 1- Bouchardia is a brachiopod with warm water affinities. Presently, extant members of this genus are unknown in latitudes up to 34ºS, with the main records at 23ºS. 2- Although occurring in depths down to 200 meters, the living member (Bouchardia rosea) of this genus is most abundant in shallow platformal, nutrient-rich waters. 3- The occurrence of Bouchardia in the Miocene of the Pelotas Basin indicates that, at least to the interval of Henryhowella evax Zone, warm waters of the Brazilian currents prevail. This interpretation is in strong accordance with other paleoeoceanographic and paleoclimatic data offered by various groups of co-occurring microfossils, such as ostracodes and foraminifers

Biocronoestratigrafia da Bacia de Pelotas: estado atual e aplicação na geologia do petróleo

The biostratigraphic studies of the Brazilian sedimentary basins have providing a support for the exploratory research of energetic and mineral resources, in particular at the continental margin. From new discoveries of hydrocarbon accumulations at the Santos and Campos basins, the south portion of the Brazilian continental margin were converted in an important target of the exploration. Therefore, the biostratigraphic refinement is necessary to detect gaps and to promote an increment on the precision of the lateral correlations in the delimitation and spatial analysis of the potential reservoirs. The actual stage of the bio-chronostratigraphic framework of the Pelotas Basin is presented herein, focusing on its application to the reservoir prospection. The main questions and lacunae on the knowledge are also discussed. In terms of biostratigraphy, the Pelotas Basin remains poorly evaluated, despite publication of important works on the biostratigraphy are known since the 1960’s. A comparison of biostratigraphic schemes proposed for the Tertiary section of the basin allows the inference of diverse hiatuses, identified by the absences of biozones. Hiatuses identified in the 87Sr/86Sr curve, suggest their relationship with discontinuities in the deposition. Discrepancies between the proposed schemes can be attributed to the particular resolution of the different groups used, the variations in the biostratigraphic criteria and the sampling interval, influencing the biostratigraphic resolution and therefore its potential as a correlation tool.

Biocronoestratigrafia da Bacia de Pelotas: estado atual e aplicação na geologia do petróleo

The biostratigraphic studies of the Brazilian sedimentary basins have providing a support for the exploratory research of energetic and mineral resources, in particular at the continental margin. From new discoveries of hydrocarbon accumulations at the Santos and Campos basins, the south portion of the Brazilian continental margin were converted in an important target of the exploration. Therefore, the biostratigraphic refinement is necessary to detect gaps and to promote an increment on the precision of the lateral correlations in the delimitation and spatial analysis of the potential reservoirs. The actual stage of the bio-chronostratigraphic framework of the Pelotas Basin is presented herein, focusing on its application to the reservoir prospection. The main questions and lacunae on the knowledge are also discussed. In terms of biostratigraphy, the Pelotas Basin remains poorly evaluated, despite publication of important works on the biostratigraphy are known since the 1960’s. A comparison of biostratigraphic schemes proposed for the Tertiary section of the basin allows the inference of diverse hiatuses, identified by the absences of biozones. Hiatuses identified in the 87 Sr/ 86 Sr curve, suggest their relationship with discontinuities in the deposition. Discrepancies between the proposed schemes can be attributed to the particular resolution of the different groups used, the variations in the biostratigraphic criteria and the sampling interval, influencing the biostratigraphic resolution and therefore its potential as a correlation tool.

Biostratigraphy and paleoceanographical significance of the Neogene planktonic foraminifera from the Pelotas Basin, southernmost Brazil

Analysis of the planktonic foraminifers recovered from 1-SCS-2 drill-hole, Florianópolis platform (Pelotas Basin, southern Brazilian Atlantic Margin), allowed recognition of the Catapsydrax dissimilis, Catapsydrax stainforthi, Globorotalia fohsi robusta, Globorotalia mayeri, Globorotalia margaritae evoluta and Globigerinoides trilobus fistulosus Miocene and Pliocene zones and four important hiatuses. Correlation with well 1-SCS-3B and other zonal schemes, as well as recognition of early diagenesis instead of a reworking process, allowed confident age assignment. The Miocene foraminifers constitute a tropical/sub-tropical assemblage, whereas in the Pliocene, the presence of scarce species associated to subantarctic water masses suggests that the Malvinas Current reaches the area but it was not a controlling factor in the paleoenvironmental conditions.

A NEW DERCETID FISH (NEOTELEOSTEI: AULOPIFORMES) FROM THE TURONIAN OF THE PELOTAS BASIN, SOUTHERN BRAZIL

Abstract: Dercetid fishes are common in Tethyan marine deposits of Europe, Asia and Africa. In this paper, we describe a South American dercetid fish, Brazilodercetis longirostris gen. et sp. nov., based on juvenile and adult specimens found in core samples at c. 4000 m sediment depth in the Atlântida Formation (Pelotas Basin, southern Brazil). Brazilodercetis longirostris is a slender, elongate fish recognized on the basis of the following features: prominent snout; contact between mesethmoid and frontals very anterior to the orbit; mesethmoid separating the premaxillae for half of their length; juvenile specimens bearing wedge‐like teeth on the anterior third of maxilla; long and shallow crest crossing epioccipital, parietal and posterior border of frontal; a pipe‐like preopercle; and reduced neural spine at the middle point of neural arch. Brazilodercetis shares a number of features with the other dercetids, including a pointed snout that is longer than the lower jaw (with Rhynchodercetis and Hastichthys), a medioparietal skull roof (with Cyranichthys and Benthesikyme), an unroofed posttemporal fossa (with Dercetoides, Rhynchodercetis and Pelargorhynchus), a flange on the anguloarticular (with Dercetoides, Hastichthys, Rhynchodercetis and Cyranichthys), untoothed premaxillae (with Hastichthys and probably Rhynchodercetis), and a single row of teeth on the maxillae (with Dercetis, Benthesikyme, Rhynchodercetis, Pelargorhynchus, Dercetoides and Hastichthys).

QUANTITATIVE ASSESSMENT OF REGIONAL SILICICLASTIC TOP-SEAL POTENTIAL: A NEW APPLICATION OF PROVEN TECHNOLOGY IN THE PELOTAS BASIN, OFFSHORE BRAZIL

Using the offshore Pelotas Basin, Brazil, as an example, we present a methodology by which petrophysically-derivedVclay and capillary displacement pressure data, in conjunction with interval isochore maps, can be used quantitatively to evaluate regional, siliciclastic present-day and palaeo-top-seal effectiveness and relative risk. This method has broad application to frontier and maturing exploration areas where data limitations preclude more sophisticated seismically-derived velocity-based evaluations. As much of the Pelotas Basin is deemed to be gas prone, top-seal effectiveness for normal density (0.1 to 0.2 g/cc) dry gas was assessed quantitatively by establishing relationships between density-log -derived hydrocarbon column height and overburden thickness using a most likely Vclay content. With constant Vclay, column height increases with increasing overburden due to a compaction-driven decrease in mudrock porosity, accompanied by a decrease in permeability and pore-throat diameter. Using these relationships, interval isochore maps (overburden thickness maps) can be transformed into hydrocarbon column-height maps to define spatial variation in top-seal effectiveness, expressed in metres of contained gas column. Laboratory and previously-published model data show that clay content and porosity (ultimately pore throat diameter) are the dominant controls on siliciclastic mudrock permeability, hence top-seal potential. Mudrock porosity is driven dominantly by burial-induced compaction; clay content is dependent on both depositional and diagenetic processes. Overburden can be determined with a reasonable degree of certainty from seismic and well data, whereas regional variations in clay content can, at best, only be estimated from depositional models. Therefore, for a given overburden, it is uncertainty in clay content that comprises the greatest risk in regional siliciclastic top-seal analysis. For this reason, we relate siliciclastic top-seal risk to clay content, when overburden/ mudrock-porosity relationships, fluid densities and requisite column heights are known.

Bioestratigrafia (Foraminiferida) da sondagem 1-SCS-3B, plataforma de Florianópolis, bacia de Pelotas, Brasil

BIOSTRATIGRAPHY (FORAMINIFERIDA) OF THE 1-SCS-3B DRILL-HOLE, PLATAFORMA DE FLORIANOPOLIS, PELOTAS BASIN, BRAZIL. Planktonic foraminifers recovered from 1-SCS-3B drill hole, Florianopolis Platform, State of Santa Catarina, Brazil, represent 60 species and subspecies belonging to 13 genera, whose stratigraphic range indicates a Neogene age for the sediments herein studied. The analysis of this assemblage allowed the recognition of eight zones, in stratigraphic order: Catapsydrax dissimilis, C. stainforthi, Globorotalia fohsi fohsi, Globigerinoides ruber, Globorotalia mayeri, G. acostaensis/ G. menardii, G. margaritae evoluta and Globigerinoides trilobus fistulosus. Four important hiatuses were observed in the studied sedimentary sequence, whose are situated at Oligocene-Miocene boundary, Early Miocene, Early Miocene-Middle Miocene and Miocene-Pliocene boundary. Correlations with previous zonations for the area are discussed.

Biologia reprodutiva e ecologia de Aegla leptoaectyla Buckup & Rossi (Crustacea, Anomura, Aeglidae)

The crustaeeans of Aeglidae family oeeur in South America's fresh water enviranments, featuring as important links in the food chains. The present article has the intention to give an acount of the results from investigations made from April/2000 to July/2001 on the biological and ecological features of Aegla leptodactyla Buckup & Rossi, 1977, an endemic species from the headwaters of the Rio Pelotas Basin in Rio Grande do Sul State, Brazil. The sampling was performed at the Rio da Divisa, tributary of Rio Pelotas, municipal district of Sao Jose dos Ausentes, Rio Grande do Sul. The animais were collected with a hand net (puca) at four different sites of the river. High contents of dissolved oxygen was verified, with a medium value of 108% and alow conductivity (avarage of 13.7mS/cm). The water mean temperature was 16oC and the pH remained neutral, average of 7.5. The velocity of the water current varied from 0.54 to 0.78m/s. A total of 1,323 males and 1,112 females of A. leptodactyla was sampled. Ovigerous females were observed from April through September and the smallest egg bearing female measured 14.09 mm cephalothorax length. The recruitment took place in November and December. The sex-ratio of the population was 1.19 males for 1 female.

Remarks on the enchodontid fishes from the Atlântida Formation, Pelotas Basin

Remarks on the enchodontid fishes from the Atlântida Formation, Pelotas Basin

LATE QUATERNARY GEOLOGICAL HISTORY OF RIO GRANGE DO SUL COASTAL PLAIN, SOUTHERN BRAZIL

The sedimentary deposits and geomorphic features preserved in the Rio Grande do Sul coastal plain, southern Brazil, represent a significant record of late Quaternary climatic changes with its associated glacio-eustatic sea-level fluctuations. The sediments of the coastal plain belong to two major depositional systems - an alluvial fan system developed along the inner part of the coastal plain and a barrier-lagoon complex with four distinctive transgressive-regressive cycles seaward. The alluvial fan sediments were derived from igneous and metamorphic rocks of the Precambrian shield and from sedimentary and volcanic rocks of the Parana Basin. Subsequently, they were reworked by four barrier- lagoon systems each representing a transgressive-regressive cycle. Each barrier probably originated at the landward limit of a transgression and was preserved due to regression of the shoreline forced by a glacio-eustatic sea-level fall. The four barrier-lagoon systems are believed to have formed during the last 400 ka assuming a correlation with the highstands represented by the last major peaks of the oxygen isotopic record. INTRODUCTION The Rio Grande do Sul coastal plain is an elongate (620 km) and wide (up to 100 km) physiographic province underlain by the Pelotas Basin. This basin, the southernmost of the Brazilian continental margin, has accumulated more than 10,000 m of mainly terrigenous sediments since its formation in the Early Cretaceous, when the South Atlantic began opening. The younger section of this sedimentary record is now exposed on the Rio Grande do Sul coastal plain and contains one of the most complete records of Brazilian coastal Quaternary sedimentation (Fig. 1). This paper summarizes knowledge about Rio Grande do Sul coastal plain sediments and suggests an evolutionary model for them during the Quaternary. PHYSICAL AND GEOLOGICAL SETTING OF THE STUDY AREA The coastal plain of Rio Grande do Sul has a nearly straight shoreline between about 29° S and 34° S latitude. Covering an area of about 33,000 km 2 , this large lowland embraces a great number of coastal water bodies, some of them of large dimensions, such as the Patos Lagoon with an area of 10,000 km 2 and Mirim Lagoon with an area of 3,770 km 2 . At the northern end of the coastal plain the adjacent highlands consist of Paleozoic and Mesozoic sedimentary and volcanic rocks of the Parana Basin that locally reach 1,000 m. At the southern section, igneous and metamorphic rocks of the Precambrian shield form lower highlands. At present, all sandy sediments eroded from these highlands and transported by rivers to the coast are trapped in the coastal lagoons and other backbarrier environments and none reaches the oceanic shoreline. The climate of the region is temperate, humid, with an even distribution of rain throughout the year, averaging around 1300 mm. The coastal plain is dominated by a bimodal high-energy wind re- gime. The dominant wind comes from NE and is more active in spring and summer months. The secondary W-SW wind becomes more important in the autumn-winter months. The coast of Rio Grande do Sul is a wave-dominated microtidal coast with semidiurnal tides with a mean range of about 0.5 m. The region is affected by swell waves approaching from SE that produce a net northerly alongshore transport of sediment. Besides the swell action, sea waves from E and NE and episodic storm waves from E and SE control erosional and depositional processes along the seashore. The adjacent continental shelf has an average width of 150 km and the shelf break is situated at a depth of about 170 m. Bottom sediments on the shelf are predominantly terrigenous elastics with some biodetrital concentrations that seems to be mostly relic. COASTAL PLAIN DEPOSITIONAL SYSTEMS Alluvial fan system This system includes fades formed through sediment gravity flows and alluvial processes along the inner part of the coastal plain. The sediments consist of mass flow deposits in proximal regions (mainly massive debris flow and colluvial slide deposits) grading seaward into waterlaid fades associated to braided channels with cross-stratified sandy and gravelly deposits. The composition and texture of the alluvial fan fades mainly reflects the nature (relief and composition) of their primary source area. To the north, the fades

Taxonomy and palaeoceanographical significance of the genus <i>Krithe</i> (Ostracoda) in the Brazilian margin

Abstract. The results of taxonomic, geographical distribution and palaeoecological studies of the genus Krithe from the Brazilian continental margin are presented. The following species and subspecies are recognized: Krithe reversa Bold, 1958; K. trinidadensis Bold, 1958; K. morkhoveni morkhoveni Bold, 1960; K. coimbrai sp. nov.; and K. gnoma sp. nov. Four species are left in open nomenclature. The occurrence of Krithe within the Brazilian continental margin is restricted to areas under the influence of cold waters and, considering their stratigraphic distribution in Cenozoic strata, they are here recognized as being useful as palaeoceanographic indicators. Of the above-mentioned species, only two occur within the Brazilian continental shelf; these are K. coimbrai and K. gnoma. These species are eurybathic and their occurrences in a shelf environment are restricted to areas under the influence of the Falkland Current. Considering their stratigraphic distribution within the Pelotas Basin, southern Brazil, and their association with other cold-water taxa, including species of foraminifera, it is here suggested that the Falkland Current was already operating in the Miocene.

Integração de dados de gravimetria marinha e de altimetria por satélite (GEOSAT/ERM) no Atlântico Sul (65/25ºW e 25/40ºS)

Foram integrados dados de gravimetria marinha convencional com aqueles derivados de altimetria por satélite na região do Atlântico Sul entre as coordenadas 65/25º W e 25/40º S. Esta região inclui, entre outras feições fisiográficas e tectônicas, o sul do Platô de São Paulo, a Elevação do Rio Grande, a Bacia de Pelotas e o Cone do Rio Grande. Utilizando a colocação por mínimos quadrados, a anomalia ar-livre e a altura geoidal foram representadas na forma de modelos digitais com resolução nominal de 10'. Os dados de gravimetria convencional foram obtidos, em sua maioria, do banco de dados GEODAS atualizado até 1995 e os de altimetria por satélite são provenientes de um ano de rastreio da missão GEOSAT/Exact Repeat Mission. A representação das anomalias ar-livre feita neste trabalho foi comparada com duas outras representações recentes, que foram feitas baseadas somente em dados de altimetria por satélite de diferentes missões. Os resultados indicam que a metodologia empregada no presente estudo produz representações do campo de gravidade para a região oceânica isentas de ruídos de alta freqüência, uma característica comum em estimativas do campo que utilizam somente dados de altimetria por satélite. Além disso, a integração de dados de gravimetria convencional permite estimar melhor o campo gravitacional em regiões com lâmina de água menor do que 1.000 m, e nas margens continentais, onde os dados de altimetria por satélite não são confiáveis. Como exemplo da potencialidade das representações das anomalias ar-livre e alturas geoidais obtidas, foi efetuada uma primeira análise qualitativa das principais anomalias com uma breve discussão do significado tectônico e implicações dinâmicas na evolução deste segmento sul da margem continental brasileira e litosfera oceânica adjacente.

The geoid in southeastern Brazil and adjacent regions: new constraints on density distribution and thermal state of the lithosphere

Least-squares collocation technique was used to process regional gravity data of the SE South American lithospheric plate in order to map intermediate (10–2000 km) wavelength geoid anomalies. The area between 35–10° S and 60–25° W includes the Paraná CFB Province, the Southern São Francisco Craton and its marginal fold/thrust belts, the Brazilian continental margin and oceanic basins. The main features in the geoid anomaly map are: (a) Paraná CFB Province is characterized by a 1000 km long and 500 km wide, NE-trending, 9 m-amplitude negative anomaly which correlates with the distribution of sediments and basalts within the Paraná basin. (b) A circular (600–800 km in diameter) positive, 8 m-amplitude geoid anomaly is located in the southern S. Francisco craton and extends into the northeastern border of the Paraná CFB Province. This anomaly partially correlates with Alto Paranaı́ba Igneous Province (APIP), where alkalic volcanism and tholeiitic dikes of ages younger than 80 Ma are found and where a low-velocity zone in the mantle has been mapped using seismic tomography. This positive geoid anomaly extends towards the continental margin at latitude 21° S and joins a linear sequence of short wavelength positive geoid anomalies associated with Vitoria–Trindade seamounts. (c) A NE-trending, 1000 km long and 800 km wide, 4 m-amplitude, positive geoid anomaly, which is located along the southeastern coast of Brazil, from latitude 24 to 35° S. The northern part of this anomaly correlates with the Ponta Grossa Arch and Florianopolis dyke swarm provinces. The age of this intrusive volcanism is 130–120 Ma. (d) A circular positive anomaly with 9 m of amplitude, located over the Rio Grande and Uruguay shields and offshore Pelotas basin. Few alkaline intrusives with ages between 65 and 80 Ma are found in the region and apatite fission track ages in basement rocks indicates cooling at around 30 Ma. A semi-quantitative analysis of the observed geoid anomalies using isostatic considerations suggests that the mechanism which generated Paraná CFB Province did not change, in a significant manner, the lithospheric thermal structure, since the same geoid pattern observed within this province continues northward over the Neoproterozoic fold/thrust belts systems separating the São Francisco and Amazon cratons. Therefore, this observation favours Anderson’s idea of rapid basaltic outpouring through a pull-apart mechanism along a major suture zone. A thermal component may still be present in the Southern São Francisco Craton and in the Rio Grande Shield and contiguous continental margins, sites of Tertiary thermal and magmatic reactivations.

Late Cenozoic sea level changes evidenced by ostracodes in the Pelotas basin, southernmost Brazil

Ostracodes recovered from sediments in seven onshore boreholes within the Pelotas basin, State of Rio Grande do Sul, indicate several continuous nearshore environments and at least four brief transgressive-regressive cycles are present. This interpretation is based on the distribution throughout upper Neogene and Quaternary sequences of species that characterize one or more biofacies depending on the borehole position in the basin. Bradleya pelotensis, Krithe coimbrai, Henryhowella kempfi and H. rectangulata are considered shelf-upper bathyal species whereas the shelf species are represented by Ambostracon crucicostatum, Argenticytheretta variabilis, A. laevipunctata, Brasilicythere reticulispinosa, Caudites posdiagonalis, Costa riograndensis, Coquimba bertelsae, C. tenuireticulata, Cytheretta punctata, Quadracythere eichlerae and Protocytheretta sp. Species with lagoon-estuarine shelf affinities include Callistocythere litoralensis, C. marginalis and Perissocytheridea kroemmelbeini whereas Cyprideis maxipunctata, C. mostardensis, C. posteroinflata, C. salebrosa and C. sparsopunctata represent a lagoon or estuarine environment, and Cypris cassinensis and Limnocythere sp. are freshwater species. Correlation of the ostracode biozones places the four maximum regressive events in the Late Pliocene and Pleistocene (1250-485 ka, 185 ka and 15 ka), whereas the maximum transgressive events occur in the Late Miocene and Pleistocene (1.6 Ma, 423–400 ka and 120 ka), in a barrier-lagoonal depositional system controlled by glacio-eustatic sea level changes.

Pelotius hesselae, gen. et sp. nov. (Teleostei: Holocentridae) from the Cretaceous (Turonian) of Pelotas Basin, Brazil

ABSTRACT The first holocentroid fish from South America, Pelotius hesselae gen. et sp. nov., is described from the Turonian of the Atlântida Formation, Pelotas Basin, southern Brazil. It is a small fish preserved in a dark gray neritic shale from a drill-core sample collected at a depth of around 4,000 m. The inclusion of P. hesselae in the Holocentroidei is supported by the presence of four spines in the anal fin, the penultimate one being the longer, and an expanded neural spine of the fourth preural centrum. Although preservation is insufficient to permit detailed morphological description of the skull and caudal skeleton, the high number of dorsal spines (10), the median suture in the posterior portion of the pelvic bone, and the presence of seven rays in the pelvic fin exclude Pelotius from the Trachichthyoidei, Stichocentridae and Pycnosteroididae and suggest its inclusion in the Holocentridae. On the other hand, it shares with the clade Erygocentrus + Tenuicentrinae + Myripristinae + Holocentrinae the apomorphic presence of two predorsals. Pelotius hesselae is distinguished from all other known holocentroids by having unusual coarse ornamentation in the anterior edge of preopercular and opercular, and lateral portion of the maxilla.

Abstract: Late Quaternary Biostratigraphy and Isotopic Analysis of Nannofossils and Forminiferid from a Borehole of South Atlantic (Pelotas Basin), Brazil

Abstract: Late Quaternary Biostratigraphy and Isotopic Analysis of Nannofossils and Forminiferid from a Borehole of South Atlantic (Pelotas Basin), Brazil