Gelson Luís Fambrini

Recurring extensional and strike-slip tectonics after the Neoproterozoic collisional events in the southern Mantiqueira province

In Eastern South America, a series of fault-bounded sedimentary basins that crop out from Southern Uruguay to Southeastern Brazil were formed after the main collisional deformation of the Brasiliano Orogeny and record the tectonic events that affected the region from the Middle Ediacaran onwards. We address the problem of discerning the basin-forming tectonics from the later deformational events through paleostress analysis of more than 600 fault-slip data, mainly from the Camaqua Basin (Southern Brazil), sorted by stratigraphic level and cross-cutting relationships of superposed striations, and integrated with available stratigraphic and geochronological data. Our results show that the Camaqua Basin was formed by at least two distinct extensional events, and that rapid paleostress changes took place in the region a few tens of million years after the major collision (c.a. 630 Ma), probably due to the interplay between local active extensional tectonics and the distal effects of the continued amalgamation of plates and terranes at the margins of the still-forming Gondwana Plate. Preliminary paleostress data from the Castro Basin and published data from the Itajai Basin suggest that these events had a regional nature.

Estratigrafia, arquitetura deposicional e faciologia da formação Missão Velha (Neojurássico-Eocretáceo) na área-tipo, bacia do Araripe, nordeste do Brasil: exemplo de sedimentação de estágio de início de rifte a clímax de rifte

Este trabalho enfoca revisao da Formacao Missao Velha na area-tipo (Bacia do Araripe, Nordeste do Brasil), atraves de analises de facies, elementos arquitetonicos, sistemas deposicionais e paleocorrentes. As principais facies reconhecidas foram: 1. conglomerados de seixos e grânulos, com abundantes troncos fosseis silicificados, formando ciclos granodecrescentes, estratificacoes cruzadas acanaladas e tabulares, de pequeno e medio portes e geometria predominantemente lenticular; 2. arenitos grossos a medios, conglomeraticos, com grânulos, quartzosos, com troncos fosseis silicificados, com estratificacoes cruzadas acanaladas de medio e grande porte e geometria lenticular; 3. conglomerados e arenitos medios com seixos esparsos, mal selecionados, com estratificacao plano-paralela; 4. arenitos finos a muito finos, silticos, tabulares, laminados, intercalados com 5. camadas decimetricas de horizontes peliticos com estratificacao plano-paralela e laminacoes cruzadas cavalgantes. Dez elementos arquitetonicos foram reconhecidos: CH: canais, GB: formas de leito cascalhosas, SG: barras, SB: formas de leitos arenosos, SB(p): formas de leitos arenosos com estratificacao cruzada tabular (p), OFch: depositos de transbordamento (overbank) como diques, crevasse e channel splays, DA: macroformas de acrescimo para jusante, LS: lencois de areia laminada, LA: deposito de acrescimo lateral e FF: depositos finos de planicie de inundacao. Nem todos esses elementos foram observados em cada afloramento. Estes elementos, definidos pela geometria e superficies delimitantes, formam a base para interpretacao dos ambientes deposicionais. A Formacao Missao Velha foi interpretada como: a. sistemas fluviais entrelacados de alta energia com as seguintes caracteristicas: ciclos granodecrescentes, estratificacoes cruzadas tabulares e acanaladas, truncamentos entre estratos cruzados, geometria lenticular, feicoes canalizadas, seixos esparsos e b. sistemas fluviais meandrantes pela presenca de depositos de planicie de inundacao, crevasse splays e barras em pontal. Ocorrem tambem depositos eolicos.

Evolução Estratigráfica e paleogeográfica do grupo Santa Bárbara (Ediacarano) na sub-bacia Camaquã Central, RS

As exposicoes do Grupo Santa Barbara (Ediacarano) na Sub-bacia Camaqua Central sao representadas por uma sucessao de depositos arenosos e ritmicos marinhos/transicionais e depositos conglomeraticos de leques aluviais, que compoem ciclos progradacionais-retrogradacionais. Nesta sub-bacia, o Grupo Santa Barbara apresenta mais de 2.000 m de espessura, com excelentes exposicoes nas regioes das Minas do Camaqua e de Bom Jardim. Estudos estratigraficos de paleoambientes, paleocorrentes e de proveniencia realizados nestas regioes possibilitaram a seguinte subdivisao do Grupo Santa Barbara na Sub-bacia Camaqua Central, a partir da base: 1. Formacao Passo da Capela: unidade areno-siltitica subdividida em turbiditos distais de leque subaquoso (Outer-fan), e turbiditos proximais a distais de leque subaquoso (Inner-Fan a Outer-fan) e tempestitos de costa-afora (offshore); 2. Formacao Seival: arenitos e siltitos de depositos litorâneos e de planicie de mares; 3. Formacao Rincao do Mouras: unidade areno-conglomeratica formada por depositos de leques aluviais e de sistemas fluviais entrelacados e 4. Formacao Joao Dias: composta principalmente por arenitos interpretados como depositos litorâneos de antepraia e de face litorânea e tempestitos de face litorânea. Estes estudos permitiram ainda o reconhecimento de uma notavel correlacao entre as unidades encontradas nestas regioes legitimando, desta forma, a proposta de englobar os depositos da regiao das Minas do Camaqua, classicamente relacionadas a Formacao Arroio dos Nobres, no redefinido Grupo Santa Barbara.

ANÁLISE ESTRATIGRÁFICA DO GRUPO SANTA BÁRBARA (EDIACARANO) NA SUB-BACIA CAMAQUÃ ORIENTAL, RS

In the Eastern Camaqua Sub-basin the Santa Barbara Group (Ediacaran) is over 6,000 m thick. The group is represented by a succession of marine to coastal sandstones and rhythmites and alluvial conglomerates and sandstones, disposed in progradational-retrogrational cycles. Detailed stratigraphic studies of paleoenvironments, facies and depositional systems have lead to the subdivision of the Santa Barbara Group, in the Eastern Camaqua Sub-basin, into two formations: (i) Passo da Capela Formation: unit composed of subaqueous fan turbidites and (ii) Rincao dos Mouras Formation : alluvial fan and pebbly braided river unit. The Passo da Capela Formation is the thickest unit of the Santa Barbara Group, reaching a thickness of about 4,000 m in the eastern sub-basin. The main lithofacies is composed of conglomerates and coarse-grained sandstones deposited by subaqueous gravity mass flows, and sandstones and rhythmites deposited by turbidity currents, in subaqueous fans. These fan deposits are intercalated with marine sandstones and rhythmites (indicated for glauconita minerals) dominated by storm waves. This formation also contains two main levels of seismic deposits which indicate the action of syn-sedimentary tectonic events. The Rincao dos Mouras Formation, up to 2,000 m thick, is present in all sub-basins, and is composed of conglomerates and conglomeratic sandstones accumulated mainly in alluvial fans and braided rivers. Stratigraphic analyses show that basement highs must have been the source areas for these alluvial deposits, which suggest that these areas were up-lined during the course of sedimentation of this unit. The Rincao dos Mouras Formation therefore records the separation of the Camaqua Basin into sub-basins through uplift or internal highs. Three depositional sequences were recognized in the Eastern Camaqua Sub-basin. The Sequence 1 is composed mainly of turbidite sandstones which represent the transgressive and highstand tracts. The Sequence 2 is formed by subaqueous fan systems, representing the lowstand tract, and is overlained by marine succession with shallow-water deposits - transgressive and highstand tracts. The Sequence 3 registers the tectonic reorganization of the Camaqua Basin which becomes separated into three sub-basins by the elevation of the Serra das Encantadas basement high. This uplift lead to the installation or alluvial fans and flood plains which are typical of this sequence. The integration of the data also suggests that the Santa Barbara Group and, by extension, the entire Camaqua Supergroup, was deposited in an extensional rift, whose border faults had normal or oblique throw without large slip movements, and whose sedimentary infilling was controlled mainly by tectonic subsidence, the clastic supply and the sedimentary transport under the influence of sea level changes.

Tafonomy of macroinvertebrates and Albian marine ingression as recorded by the Romualdo Formation (Cretaceous, Araripe Basin, Brazil)

The Romualdo Formation, Araripe Basin, crops out in the municipality of Exu, Pernambuco State, exposing coquinas. Fauna survey, taphonomic analysis of the macrobenthos, and identification of sedimentary facies of the outcrop were carried out to contribute to paleoenvironmental understanding of the Romualdo Formation. The association of molluscs (Cerithium sp., Tylostoma ranchariensis, Aguileria dissita, Brachidontes araripensis and unidentified cassiopids) and echinoids of the species Bothryopneustes araripensis suggests sedimentation in the marine environment. The taphonomic similarities made possible the grouping of coquinas into two types. Coquinas type 1 exhibit characteristics of transport and reworking of bioclast generated by storms in proximal setting. Coquinas type 2 demonstrates echinoids in situ and articulated with spines connected to shells, suggesting a fast sedimentation by distal tempestite. Regarding sedimentary facies, the concentrations of invertebrates would have been formed in the transitional offshore zone of siliclastic platform. The results presented here evidence a broader distribution of echinoids and expand the geographic limits of the shallow marine environment in the Romualdo Formation.

Caracterização geoquímica de folhelhos betuminosos da formação Ipubi, bacia do Araripe, NE Brasil

The Araripe Basin is characterized predominantly by continental sedimentary systems related to the fragmentation of Gondwana and formation of the South Atlantic Ocean. This study aims to characterize the organic matter of four samples of bituminous black shale from Ipubi Formation, collected in different mines of gypsum, using total organic carbon (TOC), Rock-Eval pyrolysis,liquid chromatography (LC) and gas chromatography-mass spectrometry (GCMS) in order to make an interpretation of the environmental conditions during the deposition event. The samples show high values of TOC, and data provided by pyrolysis show a very immature rock endowed with a large hydrocarbon generating potential and classified as Type I kerogen. Gas Chromatography-Mass Spectrometry data obtained from saturated and aromatic fractions, provided parameters and specific compounds that points to an imature organic matter deposited in lacustrine, anoxic and hypersaline environment, showing contribution of algae and higher plants.