Pelotas Basin Research Articles

Natural gas hydrates in the Rio Grande Cone (Brazil): A new province in the western South Atlantic

The Rio Grande Cone is a large-scale fanlike feature in the continental slope of the Pelotas Basin, Southern Brazil, where ubiquitous world-class bottom simulating reflectors (BSRs) are readily observed in seismic records. With the purpose of searching for natural gas hydrate deposits in the Cone area, four oceanographic cruises were carried out between May 2011 and July 2013, leading to the discovery of two pockmark fields, active faults and gas hydrates in shallow sediments. Multichannel seismic, multibeam echo sounder, side scan sonar and sub-bottom profiler records were used to map the shallow section and select sites for piston core sampling. Gas hydrates were recovered in several piston cores within muddy sediments collected inside pockmarks displaying high backscatter in the multibeam and side scan sonar data. We present two representative piston cores where numerous levels of gas hydrates occur, along with degassing features, authigenic carbonate and soupy sediments. Gas dissociated from gas hydrate samples is dominantly methane (>99.78%) with minor quantities of ethane. The chemical and isotopic compositions of the gas strongly suggest a biogenic origin for the analyzed samples. These new findings are regarded as strong enough evidence to consider the Rio Grande Cone as a new gas hydrate province.

The Pelotas Basin oil province revealed – new interpretation from long offset 2D seismic data

The Pelotas Basin oil province revealed – new interpretation from long offset 2D seismic data

Aptian/Albian (Early Cretaceous) paleogeography of the South Atlantic: a paleontological perspective

Paleontological data obtained in recent years reinforce the hypothesis that Aptian marine sedimentation in the sedimentary basins of the Brazilian continental margin - except the Pelotas basin, the southernmost Brazilian basin - took place under the domain of waters coming from the north through the Tethys Sea (Central Atlantic). Tethyan waters could reach the basins of the Brazilian continental margin via the seaway then existing in the present-day region of northeastern Brazil. Here there are records in several basins, notably in the Sao Luis (Codo Formation), Parnaiba (Codo Formation), Araripe (Santana Formation), Tucano (Marizal Formation), Sergipe (Riachuelo Formation) and Camamu (Algodoes Formation) basins. Despite irrefutable marine evidence - e.g., dinoflagellates, echinoids, foraminifera, molluscs and fishes, conspicuously present in the Araripe Basin - there are very few paleogeographic reconstructions that include the seaway which is totally ignored in the international literature. The skepticism is even greater in relation to the Tethyan affinity although the evidence has been well documented by molluscs and dinoflagellates, together with ammonoids in the Sergipe Basin. That skepticism may be due to the fact that, in tectonic and geodynamic terms, the opening of the South Atlantic indeed proceeded from south to north, at least in the part that extends from Argentina to the northeastern Brazilian state of Paraiba.

Strontium isotope stratigraphy of the Pelotas Basin

Dados da razao isotopica de estroncio foram obtidos a partir da analise de conchas de foraminiferos recuperadas dos depositos terciarios da Bacia de Pelotas visando o refinamento do arcabouco cronoestratigrafico dessa secao. Este artigo representa a primeira abordagem a obtencao de idades numericas para esses estratos. A estratigrafia de isotopos de estroncio permitiu a identificacao de oito hiatos deposicionais na secao Eoceno-Plioceno, aqui classificados como desconformidade, alem de uma secao condensada. O reconhecimento de hiatos deposicionais representa um importante avanco, considerando a baixa resolucao cronoestratigrafica da secao cenozoica da Bacia de Pelotas. Alem disso, foi identificado um substancial aumento na taxa de sedimentacao na secao neomiocenica. Tendencias gerais de paleotemperatura e produtividade foram identificadas com base em dados das razoes isotopicas de oxigenio e carbono da secao Oligoceno-Mioceno. Essas tendencias sao coerentes com eventos globais, evidenciando as condicoes ambientais durante a sedimentacao.

Três Forquilhas Valley in Southern Brazil - evidence for the uplift of the volcanic plateau

Os padrões de drenagem e a encosta foram estudados no vale do rio Três Forquilhas que é situado próximo da localidade de Terra de Areia, entre o mar e a borda nordeste do planalto vulcânico, no Rio Grande do Sul. Esta área foi estudada para explicar por que a escarpa do planalto vulcânico é mais próxima ao mar nesta sua por- ção nordeste e estabelecer a história erosional desta porção da escarpa. Aqui dois rios relativamente curtos, os rios Três Forquilhas e Maquiné, seguem a Sinclinal de Torres em direção ao Atlântico. Esses rios, provavelmente, iniciaram a erodir significativamente a sinclinal e a borda do planalto a partir do Mioceno Médio a Superior, em resposta à tectônica Andina de soerguimento tectônico de colisão ao longo da maior parte da América do Sul. A erosão subsequente, provavelmente, ocorreu em pulsos e não em uma fase uniforme em ambos os divisores de águas, muito como as chuvas torrenciais atualmente, que a cada dois ou três anos “impulsionaram” a erosão local da escarpa. O estudo da sequência estratigráfica no Terciário quando da sedimentação da Bacia de Pelotas adjacente à área de estudo, nos ajudou a estabelecer esta história erosiva local (sete discordâncias entre as sucessões de depósitos, variações de espessura entre estas discordâncias e a grande escala de espessuras). O estudo de uma seção transversal do continente em direção ao litoral identificou dois paleovales em rochas do Mesozoico abaixo do nível de base dos atuais rios Três Forquilhas e Maquiné. Há um controle estrutural forte da drenagem em todas as escalas. Essas considerações sobre os processos que se desenvolvem sobre a vertente contribuem, significativamente, para a compreensão sobre a evolução da drenagem e a origem da escarpa do planalto vulcânico.

Cenozoic synthem stratigraphic architecture of the SE Brazilian shelf and its global eustatic context: evidence from the Pelotas Basin (offshore Brazil)

Abstract The Pelotas Basin, located on the SE Brazilian shelf, has evolved since the Aptian. Stratigraphical data from the basin can be used for delineation of the unconformity-bounded units (synthems) on the shelf, which is a first step towards a full understanding of its stratigraphic architecture, evolution, and hydrocarbon potential. Hiatuses in the Cenozoic succession of the Pelotas Basin are established with both biostratigraphic (planktonic foraminifers and calcareous nannofossils) and isotopic (87Sr/86Sr) data. The seven recognised hiatuses are dated respectively as (1) Palaeocene (Danian- Thanetian), (2) Palaeocene/Eocene boundary (Thanetian-Ypresian), (3) Eocene (Ypresian-Lutetian), (4) Eocene-Oligocene (Lutetian-Rupelian), (5) early-late Oligocene (Rupelian-Chattian), (6) early Miocene (Aquitanian-Burdigalian), and (7) middle-late Miocene (Serravallian-Tortonian). These intervals between the hiatuses are correlated with those of the Santos and Campos Basins north from the Pelotas Basin. The breaks in sedimentation that these basins have in common occurred (1) at the Palaeocene-Eocene and (2) Eocene-Oligocene transitions, (3) in the early Miocene, and (4) in the middle-late Miocene. These main unconformities outline five synthems on the SE Brazilian shelf, viz. the SEBS-1 (Palaeocene), SEBS-2 (Eocene), SEBS-3 (Oligocene), SEBS-4 (early-middle Miocene) and SEBS-5 (late Miocene-Holocene). The above unconformities are correlated with those established in the Cenozoic sedimentary successions of different regions such as Western Siberia, Arabia, NW and NE Africa, peninsular India, S Australia, the Gulf of Mexico, NW Europe, and South Africa. The only regional unconformity, near the Oligocene/Miocene boundary, coincides with the nearly-global sedimentation break. The latter was resulted from a climatic event, i.e., the ‘Mi-1 glaciation’. Thus, a eustatic origin is supposed for this regional unconformity. The other regional unconformities also correspond to global sea-level falls (probably with an exception for the Palaeocene/Eocene surface), which suggests that global eustatic movements controlled the development of the regional synthem architecture.

The evolution of rifting on the volcanic margin of the Pelotas Basin and the contextualization of the Paraná–Etendeka LIP in the separation of Gondwana in the South Atlantic

The Pelotas Basin is the classical example of a volcanic passive margin displaying large wedges of seaward-dipping reflectors (SDR). The SDR fill entirely its rifts throughout the basin, characterizing the abundant syn-rift magmatism (133–113 Ma). The Paraná–Etendeka Large Igneous Province (LIP), adjacent to west, constituted the pre-rift magmatism (134–132 Ma). The interpretation of ultra-deep seismic lines showed a very different geology from the adjacent Santos, Campos and Espírito Santo Basins, which constitute examples of magma-poor passive margins. Besides displaying rifts totally filled by volcanic rocks, diverse continental crustal domains were defined in the Pelotas Basin, such as an outer domain, probably constituted by highly stretched and permeated continental igneous crust, and a highly reflective lower crust probably reflecting underplating.The analysis of rifting in this portion of the South Atlantic is based on seismic interpretation and on the distribution of regional linear magnetic anomalies. The lateral accretion of SDR to the east towards the future site of the breakup and the temporal relationship between their rift and sag geometries allows the reconstitution of the evolution of rifting in the basin. Breakup propagated from south to north in three stages (130–127.5; 127.5–125; 125–113 Ma) physically separated by oceanic fracture zones (FZ). The width of the stretched, thinned and heavily intruded continental crust also showed a three-stage increase in the same direction and at the same FZ. Consequently, the Continental-Oceanic Boundary (COB) shows three marked shifts, from west to east, from south to north, resulting into rift to margin segmentation. Rifting also propagated from west to east, in the direction of the final breakup, in each of the three segments defined. The importance of the Paraná–Etendeka LIP upon the overall history of rupturing and breakup of Western Gondwanaland seems to have been restricted in time and in space only to the Pelotas Basin.

Paleoecologia do sistema Pinguela-Palmital-Malvas, Holoceno da Bacia de Pelotas, RS, Brasil: uma abordagem focada na utilização de análises multivariadas para obtenção de diatomáceas paleoindicadoras

No Brasil, o potencial paleontológico das diatomáceas ainda é pouco explorado.Para a Planície Costeira do Rio Grande do Sul, o papel suplementar das diatomáceas nos estudos paleoecológicos efetivados torna-se manifesto em função do inexpressivo número de espécies citadas. Além disso, no que tange ao processamento dos dados nestes estudos, a definição de intervalos fundamentou-se em critérios subjetivos. Uma pesquisa foi desenvolvida visando reconstituir os sucessivos paleoambientes do sistema lacustre Pinguela-Palmital-Malvas na porção emersa norte da Bacia de Pelotas, durante o Holoceno, a partir de 89 amostras do testemunho de sondagem PM-RS-D01 com 4,87 m de profundidade total, com base em diatomáceas. Maior objetividade dentro deste enfoque foi obtida pela definição de espécies indicadoras de conjuntos de unidades amostrais oriundos de níveis de particionamentos significativos em análises de agrupamento. Os resultados demonstraram que, entre 4.600 ±70 anos AP e 3.950 ± 70 anos AP, ocorre alternância entre estratos compostos pela preponderância de espécies mixohalinas e intervalos caracterizados pela supremacia de espécies dulciaqüícolas em um contexto transgressivo; os registros determinados por Actinocyclus normanii refletem períodos de clima mais seco, quando o volume de água doce drenado para a bacia era menor e a evaporação mais intensa. De forma inversa, Aulacoseira cf. agassizii determinou o esclarecimento de etapas de incremento do afluxo lótico por variação positiva do regime pluviométrico. Após 3.950 ± 70 anos A.P., alterações sedimentológicas e bióticas significativas como o estabelecimento de fácies com predomínio de areia e a maior diversidade e abundância de vestígios de espécies perifíticas, caracterizam o início do processo de regressão holocena da Planície Costeira gaúcha. A manutenção da coerência das interpretações paleoambientais obtidas, quando confrontadas a estudos pregressos, e a detecção de processos originais para a região, assinalam a eficiência dos procedimentos estatísticos empregados baseados no estabelecimento de agrupamentos significativos e destaque a espécies reguladoras destes particionamentos.

Seabed acoustic classification in the Pelotas basin, Brazil

The present study sought to develop a seabed map of the region of the Pelotas Basin using acoustic methods. A total number of 1,507,823 seabed reflectivity data, collected during six oceanographic surveys, were processed to generate a seabed map. Data processing consisted of the classification of the acoustic parameter BSBS (Bottom Surface Backscattering Strength) obtained with the Scientific Echosounder EK 500 operating at a frequency of 38 kHz. BSBS is expressed in decibels (dB), and corresponds to a logarithm of the ratio between incident acoustic energy and the energy reflected by the seabed. Four BSBS value classes, associated with different sediment types, were established. High BSBS values are associated with coarse sediments, whereas low values indicate fine sediments. A seabed reflectivity map was generated using the Natural Neighbor method to interpolate the BSBS values organized according to the pre-established classes. Four features with high BSBS values were identified at 100-200 m depth. The largest one was found in the region of Santa Marta Cape and attributed mainly to consolidated seabed and/or the presence of biodetritic material, according to comparison with maps available in the literature. Above 500m depth, there was a predominance of acoustically low reflectivity sea floor, which was attributed to the presence of muddy sediment. Considering the lack of information on the seabed at great depths, the acoustic method was shown to be an alternative tool to obtain data on seabed characteristics in these regions.

Preservation potential of foredunes in the stratigraphic record

Barboza, E.G., Rosa, M.L.C.C., Dillenburg, S.R and Tomazelli, L.J., 2013. Preservation potential of foredunes in the stratigraphic record. In: Conley, D.C., Masselink, G., Russell, P.E. and O'Hare, T.J. (eds.)Foredunes occur worldwide on shorelines, especially those which are wave-dominated and with moderate to high sediment supply. Pelotas Basin, located in southern Brazil, has a significant record of foredunes which are present on modern beaches as well as in the geological record. Many locations along the Holocene coastal plain of Pelotas Basin were investigated with a Ground Penetrating Radar (GPR). Foredunes are present in almost all the investigated areas. However some peculiarities were found along these areas. At some places it is still possible to see foredune morphology in the field. At these places GPR profiles show a convex ridge morphology preserved and its separation by concave, sometimes filled swales. At other sites foredunes are covered by transgressive dunes, the morphology is not preserved and erosive surfaces are found on the top of the unit. Along the progradational sectors, dip profiles show that even where foredunes have their morphology preserved, some erosive scarps are found at the seaward face. These patterns indicate that high frequency events lead to the erosion of foredunes. The combination of sedimentary budget with climatic events sets foredune preservation and its pattern. Where sedimentary budget is high, progradation occurs faster and foredunes are preserved. However, where the sedimentary budget is not very expressive, the progradation is slower and part of the foredune is destroyed forming transgressive dunes or sand sheets.

WITHDRAWN: 3D crustal architecture of a magma-poor passive margin, Santos, Campos and Espírito Santo Basins – Comparisons with a volcanic passive margin, Pelotas Basin – Offshore Brazil

WITHDRAWN: 3D crustal architecture of a magma-poor passive margin, Santos, Campos and Espírito Santo Basins – Comparisons with a volcanic passive margin, Pelotas Basin – Offshore Brazil

Relating petroleum system and play development to basin evolution: Brazilian South Atlantic margin

We review the structural genesis and evolutionary history of basins along the Brazilian South Atlantic margin from the Sergipe-Alagoas (north) to the Pelotas basin (south), and demonstrate the links with petroleum system and play development. In our approach, we first break basins down into their tectonostratigraphic megasequences and define their characteristics, particularly focused on the development of characteristic source- and reservoir-rock intervals. We then compare these megasequences with similar types of megasequences in other basins, thereby providing a means to learn through a greater population of analogues. We demonstrate, using trajectory plots, that these basins experienced a similar tectonostratigraphic basin evolution, resulting in the deposition of many analogue potential source- and reservoir-rock intervals. These give rise to the development of similar types of potential petroleum systems and play (level)s. Although the area is currently being actively explored, large areas remain poorly understood, with unknown source-rock maturity distributions and many unknown/untested reservoirs/plays. This approach allows us to make analogue comparisons between the Brazilian marginal basins in order to evaluate and predict the presence of potential, yet undiscovered or under-explored, hydrocarbon accumulations. Supplementary material: Published source-rock and play properties used in this analysis are available at http://www.geolsoc.org.uk/SUP18536

Proveniência dos depósitos de areia do rio Jacuí (RS), utilizando a análise de minerais pesados

Este artigo discute a origem dos depósitos arenosos atuais do rio Jacuí, no sul do Brasil. O Jacuí constitui o principal rio do Estado do Rio Grande do Sul e contém os principais depósitos de areia utilizados na indústria de concreto do estado. O rio Jacuí e seus afluentes drenam as mais diversas áreas-fonte, com diferentes idades e associações litológicas, incluindo as rochas do Escudo Sul-riograndense (Pré-Cambriano), da Bacia do Paraná, onde está incluída a Formação Serra Geral (Cretáceo), e da Formação Tupanciretã (Terciário). As amostras foram coletadas de sedimentos fluviais em transporte para a análise de minerais pesados, ao longo do curso principal do rio e de seus principais afluentes. Os resultados demonstram uma grande diversidade de minerais pesados, incluindo: granada, turmalina, hornblenda, zircão, apatita, estaurolita, opacos (magnetita, ilmenita, hematita e pirita) e, minoritariamente, rutilo, piroxênio (hiperstênio principalmente), cianita, sillimanita, andaluzita, titanita, actinolita e tremolita. Esta assembleia de minerais pesados indica que a área-fonte principal dos sedimentos é o Escudo Sul-rio-grandense, proveniente dos afluentes da margem direita do Rio Jacuí. As rochas sedimentares da Bacia do Paraná também constituem uma importante área-fonte para os sedimentos. As rochas vulcânicas da Formação Serra Geral e as sedimentares fluviais da Formação Tupanciretã fornecem uma menor contribuição. Uma redução pequena, mas consistente, dos índices ATi e GZi ao longo do curso principal do rio, de montante para jusante, indica que os sedimentos sofreram decréscimo destes minerais ao longo da planície aluvial possivelmente devido ao intemperismo. Os valores do índice ZTR e ARi são relativamente constantes ao longo do rio principal. Além disso, o ARi é relativamente baixo. Os valores consistentes destes índices indicam uma forte contribuição dos afluentes do rio Jacuí. A assinatura de minerais pesados do rio Jacuí poderá ser utilizada para avaliar a contribuição dos sedimentos do rio no preenchimento da porção mais jovem da Bacia de Pelotas.

Cretaceous calcareous nannofossils from Pelotas Basin, Brazil: Biostratigraphic and paleoecological inferences

Abstract Biostratigraphic and paleoecological inferences are made on the distribution of calcareous nannofossils in three wells drilled in the Pelotas Basin, southern Brazil. The succession is composed mainly of siliciclastic rocks sampled by cutting and short core intervals. Eleven biozones were recognized, comprised the late Albian and the Maastrichtian. The frequency and relative abundance of some Cretaceous calcareous nannofossils species suggest a prevalence of warm-water conditions, with cooling events during the late Turonian, late Turonian/early Coniacian, late Coniacian and late Santonian/early Campanian.

A new protacanthopterygian fish from the Upper Cretaceous (Turonian) of the Pelotas Basin, southern Brazil

The Turonian age in the Pelotas Basin is represented by the Atlântida Formation, which consists mainly of marine dark shales and siltstones with intervening layers of carbonates. It has yielded a neritic fish assemblage that includes lamniform, pycnodontid, cladocyclid, clupeomorph, basal euteleostean, dercetid, enchodontid and holocentrid remains. In this assemblage there is a small fish, here described as Parawenzichthys minor gen. et sp. nov. It is characterized by having a low and elongate body, prominent snout, drop-shaped nasal, smooth dermal bones, toothless jaws, a mandible-suspensorium joint placed at the posterior portion of the orbit, very small pleural ribs, and at least 50 smooth vertebrae. Although superficially very similar to Santanasalmo elegans, known from the Albian of the Araripe Basin, it is separated by a combination of features of skull and, particularly, in the caudal endoskeleton. The first uroneural shows reduced anterior outgrowth and is fused to the compound centrum (first preural + ural centra), and the second uroneural is very long. There are six hypurals, the first of which is the largest. Taking into account the presence of derived conditions in the skull and caudal endoskeleton, the fish is assigned to Protacanthopterygii, sharing certain derived features with the argentinoid Wenzichthys congolensis from the non-marine Lower Cretaceous of Gabon (Africa).

The continental margin of Uruguay: Crustal architecture and segmentation

The Uruguayan continental margin comprises three sedimentary basins: the Punta del Este, Pelotas and Oriental del Plata basins, the genesis of which is related to the break-up of Gondwana and the opening of the Atlantic Ocean. Herein the continental margin of Uruguay is studied on the basis of 2D multichannel reflection seismic data, as well as gravity and magnetic surveys. As is typical of South Atlantic margins, the Uruguayan continental margin is of the volcanic rifted type. Large wedges of seaward-dipping reflectors (SDRs) are clearly recognizable in seismic sections. SDRs, flat-lying basalt flows, and a high-velocity lower crust (HVLC) form part of the transitional crust. The SDR sequence (subdivided into two wedges) has a maximum width of 85 km and is not continuous parallel to the margin, but is interrupted at the central portion of the Uruguayan margin. The oceanic crust is highly dissected by faults, which affect post-rift sediments. A depocenter over oceanic crust is reported (deepwater Pelotas Basin), and volcanic cones are observed in a few sections. The structure of continental crust-SDRs-flat flows-oceanic crust is reflected in the magnetic anomaly map. The positive free-air gravity anomaly is related to the shelf-break, while the most prominent positive magnetic anomaly is undoubtedly correlated to the landward edge of the SDR sequence. Given the attenuation, interruption and/or sinistral displacement of several features (most notably SDR sequence, magnetic anomalies and depocenters), we recognize a system of NW–SE trending transfer faults, here named Río de la Plata Transfer System (RPTS). Two tectono-structural segments separated by the RPTS can therefore be recognized in the Uruguayan continental margin: Segment I to the south and Segment II to the north.

Palinomorfos neogenos do poço 2-CA-1-RS, Bacia de Pelotas, Brasil: significado bioestratigráfico e paleoecológico

Dados de microfósseis calcários são conhecidos para a seção pré-quaternária da Bacia de Pelotas. Contudo, informações palinológicas são relativamente escassas, com estudos iniciados, de forma sistematizada, somente nos últimos anos. Este trabalho apresenta resultados palinológicos do poço 2-CA-1-RS, locado na porção onshore da bacia, no Rio Grande do Sul. Um total de 20 amostras foi selecionado no trecho entre 262 - 145 m de profundidade, das quais cinco revelaram associações de palinomorfos abundantes e diversificadas; quinze apresentaram predomínio de matéria orgânica amorfa ou fitoclastos. Espécies de esporos, grãos de pólen, cistos de dinoflagelados compreendem os palinomorfos mais comuns, incluindo ainda palinoforaminíferos, escolecodontes, acritarcos, algas Chlorococcales (Botryococcus) e esporos de fungos. A análise da distribuição quantitativa dos componentes da matéria orgânica particulada ao longo da seção permitiu a individualização de dois conjuntos palinológicos. Em termos gerais, os resultados indicam natureza marinha para intervalo estudado, com variações da base para o topo. O conjunto 1 (262 - 248 m) reflete condições mais distais, em mar aberto, enquanto o conjunto 2 (190 - 160 m) é interpretado como marinho mais proximal. Com base na amplitude estratigráfica de determinadas espécies de dinoflagelados, distintas idades entre o Mioceno e o Quaternário são indicadas.

Upper Campanian calcareous nannofossils from a core of the well 2-Rss-1, Pelotas Basin, Brazil

Upper Campanian calcareous nannofossils from a core of the well 2-Rss-1, Pelotas Basin, Brazil

Cretaceous–Lower Paleogene ostracods from the Pelotas Basin, Brazil

A total of 479 samples from five drill-holes of the Pelotas Basin were analysed. Despite the huge number of samples, only 98 specimens were recovered, belonging to nine families, 21 genera and 34 species (most in open nomenclature). Due to the paucity and poor preservation of this material, just seven taxa were identified or tentatively identified at the species level: Cytherella cf. C. araucana Bertels, Cytherelloidea spirocostata Bertels, Bairdoppilata triangulata Edwards, Actinocythereis indigena Bertels, Brachycythere gr. sapucariensis Krommelbein, Wichmannella araucana Bertels and Wichmannella meridionalis Bertels. Four taxa could not even be identified at the generic level and are referred as “gen. et sp. indet.”. Trachyleberididae and Cytherellidae were the most diversified and abundant families while Cytherella and Paracypris were the most diversified genera, with seven and four species, respectively. The K–Pg boundary in the Pelotas Basin is marked by a faunal change with the local disappearance of the genera Cytherelloidea, Argilloecia, Cythereis, Brachycythere, Pondoina and Rostrocytheridea, and the first local occurrence of Neonesidea?, Bairdoppilata, Ambocythere, Buntonia, Langiella?, Trachyleberis and Krithe. The genera Cytherella, Paracypris, Wichmannella and Actinocythereis are recorded in the Cretaceous as well as the Paleogene. This faunal study suggests a neritic marine environment for the analysed interval.

Geological and geophysical interpretation of the Rio Grande Rise, south-eastern Brazilian margin: extensional tectonics and rifting of continental and oceanic crusts

ABSTRACT This paper discusses the geological and geophysical interpretation of rift structures in the region extending from the Rio Grande Rise, in the Southeastern Brazilian margin, towards the Cabo Frio High, which separates the Campos and Santos basins. We have analysed potential field data (gravity and magnetic) from the Argentine to the Brazilian oceanic basins and extending over the Pelotas, Santos and Campos basins. The Rio Grande Rise shows a relatively negative Bouguer anomaly in an area that corresponds to a major positive bathymetric feature between the Argentine and Brazil basins. North–south propagators related to the early spreading centres of the Atlantic Ocean are observed from Argentina towards the southern Santos Basin, which is characterized by an elevated basement topography relative to the Pelotas Basin. The region adjacent to the Florianopolis Fracture Zone between the Santos and Pelotas basins is also characterized by an elevated basement region aligned in an east–west direction, and locally it is marked by rift structures aligned along a NW–SE direction, forming a lineament or shear zone (Cruzeiro do Sul lineament) that extends from the Cabo Frio High towards the Rio Grande Rise, thus involving both continental and oceanic crusts. The Rio Grande Rise is associated with the east–west-trending fracture zones, which are characterized by several aligned magnetic anomalies in the southern Santos Basin. The Rio Grande Fracture Zone continues landward as the Sao Paulo Ridge, and extends towards the platform as the Florianopolis High. Oceanic propagators are identified from Argentina towards the Pelotas and Santos basins, and locally we observe rupturing of the salt layer by igneous intrusions or possibly by mantle exhumation. The Florianopolis (or Rio Grande) Fracture Zone is marked by an abrupt topographic offset separating the Pelotas Basin from the southern Santos Basin, and the associated volcanic belts limit the southernmost occurrence of the late Aptian evaporite sequence. The evaporite sequence in this segment of the continental margin shows remarkable layering of halite, anhydrite and carnalite. Conjugate to the Rio Grande Rise, the Walvis Ridge, offshore Namibia, is similarly a topographic high, but rift structures as observed in the Brazilian side are apparently unique in the South Atlantic. Alternative interpretations for the origin of the Rio Grande Rise include: a volcanic edifice or plateau rooted in the mantle; an intraplate shear zone affecting both continental and oceanic crust; an oceanic area of igneous over-productivity caused by a hotspot or a thermal anomaly in the mantle; a palaeo-spreading centre in the Cretaceous Atlantic Ocean; an area of excessive volcanic activity resulting from mantle differentiation due to adiabatic decompression; or perhaps an isolated remnant of continental crust left outboard of the Brazilian continental margin during the drifting process.