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Previous issue date: 2003-02-14 / This thesis deals with the sedimentological/stratigraphic and structural evolution of the sedimentary rocks that occur in the NW continental border of the Potiguar Basin. These rocks are well exposed along coastal cliffs between the localities of Lagoa do Mato and Icapu?, Cear? State (NE Brazil).
The sedimentological/stratigraphic study involved, at the outcrop scale, detailed facies descriptions, profile mapping of the vertical succession of different beds, and columnar sections displaying inferred lateral relationships. The approach was complemented by granulometric and petrographic analyses, including the characterization of heavy mineral assemblages. The data set allowed to recognize two kinds of lithological units, a carbonate one of very restricted occurrence at the base of the cliffs, and three younger, distinct siliciclastic units, that predominate along the cliffs, in vertical and lateral extent. The carbonate rocks were correlated to the late Cretaceous Janda?ra Formation, which is covered by the siliciclastic Barreiras Formation.
The Barreiras Formation occurs in two distinct structural settings, the usual one with nondeformed, subhorizontal strata, or as tilted beds, affected by strong deformation. Two lithofacies were recognized, vertically arranged or in fault contacts. The lower facies is characterized by silty-argillaceous sandstones with low-angle cross bedding; the upper facies comprises medium to coarse grained sandstones, with conglomeratic layers. The Tibau Formation (medium to
coarse-grained sandstones with argillite intercalations) occurs at the NW side of the studied area, laterally interlayered with the Barreiras Formation. Eolic sediments correlated to the
Potengi Formation overly the former units, either displaying an angular unconformity, or simply an erosional contact (stratigraphic unconformity). Outstanding structural features, identified in the Barreiras Formation, led to characterize a
neocenozoic stress field, which generated faults and folds and/or reactivated older structures in the subjacent late cretaceous (to paleogene, in the offshore basin) section.
The structures recognized in the Barreiras Formation comprise two distinct assemblages, namely a main extensional deformation between the localities of Ponta Grossa and Redonda, and a contractional style (succeeded by oblique extensional structures) at Vila Nova. In the first
case, the structural assemblage is dominated by N-S (N?20?Az) steep to gently-dipping extensional faults, displaying a domino-style or listric geometry with associated roll-over
structures. This deformation pattern is explained by an E-W/WNW extension, contemporaneous with deposition of the upper facies of the Barreiras Formation, during the time interval Miocene to Pleistocene. Strong rotation of blocks and faults generated low-angle distensional faults and,
locally, subvertical bedding, allowing to estimate very high strain states, with extension estimates varying between 40% up to 200%. Numerous detachment zones, parallel to bedding, help to acommodate this intense deformation. The detachment surfaces and a large number of faults
display mesoscopic features analoguous to the ones of ductile shear zones, with development of S-C fabrics, shear bands, sigmoidal clasts and others, pointing to a hydroplastic deformation regime in these cases. Local occurrences of the Janda?ra limestone are controled by extensional faults that
exhume the pre-Barreiras section, including an earlier event with N-S extension. Finally, WNWtrending extensional shear zones and faults are compatible with the Holocene stress field along the present continental margin. In the Vila Nova region, close to Icapu?, gentle normal folds with fold hinges shallowly
pluging to SSW affect the lower facies of the Barreiras Formation, displaying an incipient dissolution cleavage associated with an extension lineation at high rake (a S>L fabric). Deposition of the upper facies siliciclastics is controlled by pull-apart graben structures, bordered
by N-NE-trending sinistral-normal shear zones and faults, characterizing an structural inversion. Microstructures are compatible with tectonic deformation of the sedimentary pile,
burried at shallow depths. The observed features point to high pore fluid pressures during deformation of the sediments, producing hydroplastic structures through mechanisms of
granular flow. Such structures are overprinted by microfractures and microfaults (an essentially brittle regime), tracking the change to microfracturing and frictional shear mechanisms accompanying progressive dewatering and sediment lithification. Correlation of the structures observed at the surface with those present at depth was
tested through geophysical data (Ground Penetrating Radar, seismics and a magnetic map). EW and NE-trending lineaments are observed in the magnetic map. The seismic sections display several examples of positive flower structures which affect the base of the cretaceous sediments; at higher stratigraphic levels, normal components/slips are compatible with the negative structural inversion characterized at the surface. Such correlations assisted in proposing
a structural model compatible with the regional tectonic framework. The strong neogenepleistocene deformation is necessarily propagated in the subsurface, affecting the late
cretaceous section (A?u and Janda?ra formations), wich host the hydrocarbon reservoirs in this portion of the Potiguar Basin. The proposed structural model is related to the dextral transcurrent/transform deformation along the Equatorial Margin, associated with transpressive terminations of E-W fault zones, or at their intersections with NE-trending lineaments, such as the Ponta Grossa-Fazenda Bel?m one (the LPGFB, itself controlled by a Brasiliano-age strike-slip shear zone). In a first step (and possibly during the late Cretaceous to Paleogene), this lineament was activated under a sinistral transpressional regime (antithetic to the main dextral deformation in the E-W zones), giving way to the folds in the lower facies of the Barreiras Formation, as well as the positive flower structures mapped through the seismic sections, at depth. This stage was succeeded (or was
penecontemporaneous) by the extensional structures related to a (also sinistral) transtensional movement stage, associated to volcanism (Macau, Messejana) and thermal doming processes during the Neogene-Pleistocene time interval. This structural model has direct implications to hydrocarbon exploration and exploitation activities at this sector of the Potiguar Basin and its offshore continuation. The structure of the reservoirs at depth (A?u Formation sandstones of the post-rift section) may be controlled (or at
least, strongly influenced) by the deformation geometry and kinematics characterized at the surface. In addition, the deformation event recognized in the Barreiras Formation has an age close to the one postulated for the oil maturation and migration in the basin, between the Oligocene to the Miocene. In this way, the described structural cenario represents a valid model to understand the conditions of hydrocarbon transport and acummulation through space openings, trap formation and destruction. This model is potentially applicable to the NW region of the Potiguar Basin and other sectors with a similar structural setting, along the brazilian
Equatorial Atlantic Margin / Esta tese contempla a caracteriza??o sedimentol?gica/litoestratigr?fica e estrutural das
rochas sedimentares na por??o continental NW da Bacia Potiguar, e que afloram em fal?sias no setor litor?neo compreendido entre as localidades de Lagoa do Mato e Icapu?, Cear? (NE do Brasil).
Os estudos sedimentol?gicos /estratigr?ficos das unidades mapeadas, em escala de afloramento, envolveram descri??es faciol?gicas, levantamento de perfis com empilhamento
vertical das diferentes camadas, e a constru??o de se??es colunares caracterizando as rela??es laterais observadas. Ainda foram realizados estudos granulom?tricos e petrogr?ficos, bem como a identifica??o das assembl?ias de minerais pesados. O conjunto desses dados permitiu identificar dois grupos de litologias, representados por
uma unidade carbon?tica, que ocorre de forma restrita na base das fal?sias, e tr?s unidades silicicl?sticas, que predominam lateral e verticalmente, nas fal?sias. As rochas carbon?ticas, correlacionadas ? Forma??o Janda?ra, do Cret?ceo superior, s?o capeadas pelas rochas
silicicl?sticas da Forma??o Barreiras. A Forma??o Barreiras ocorre em dois contextos estruturais distintos, sob a forma de estratos horizontalizados e n?o deformados, a situa??o mais usual, ou com camadas basculadas e afetadas por forte deforma??o. Foram individualizadas duas litof?cies, dispostas
verticalmente em contatos normais, ou por falhas. A denominada f?cies inferior ? composta por arenitos s?ltico-argilosos com estratifica??o cruzada de baixo ?ngulo; a f?cies superior ? caracterizada por arenitos m?dios a grossos, maci?os, com intercala??es conglomer?ticas. No
extremo NW da ?rea ocorre uma segunda unidade, a Forma??o Tibau (arenitos m?dios a grossos com intercala??es de argilitos), que est? lateralmente interdigitada com a Forma??o Barreiras. Finalmente, no topo das unidades anteriores ocorrem sedimentos e?licos correlacionados ? Forma??o Potengi. A discord?ncia na base desta ?ltima unidade (a Forma??o Potengi) torna-se n?tida quando os estratos sotopostos est?o basculados e falhados
(discord?ncia angular). Para os demais casos, o contato ? erosional (discord?ncia estratigr?fica). Na ?rea estudada, chamam aten??o as fei??es estruturais identificadas na Forma??o Barreiras, que permitem caracterizar o campo de tens?es neocenoz?ico, que gerou falhas e dobras e/ou reativou estruturas mais antigas na se??o neocret?cica (a pale?gena, no setor offshore da bacia) subjacente.
As fei??es estruturais identificadas nas rochas da Forma??o Barreiras permitem distinguir duas situa??es distintas, caracterizadas por uma deforma??o principal distensional, entre as localidades de Ponta Grossa e Redonda, e um estilo contracional (sucedido por estruturas distensionais obl?quas), em Vila Nova. No primeiro caso, as fei??es observadas envolvem falhas distensionais de dire??o N-S (N?20?Az), com mergulhos de alto a baixo ?ngulo. Em geral possuem arranjo em domin? ou geometria l?strica com estruturas roll-over associadas. Este padr?o deformacional pode ser explicado por uma distens?o E-W/WNW, contempor?nea ? deposi??o da f?cies superior da Forma??o Barreiras, no intervalo Mioceno-Pleistoceno. A pronunciada rota??o de blocos e falhas delimitantes, gerando falhas distensionais de baixo
?ngulo e, localmente, verticaliza??o do acamamento, traduz a forte magnitude deste evento, com estimativas que variam de 40% at? 200% de distens?o. S?o freq?entes as zonas de
descolamento paralelas ao acamamento, que permitem acomodar a intensa deforma??o observada. As superf?cies de descolamento e boa parte das falhas exibem fei??es
mesosc?picas an?logas ?s de zonas de cisalhamento d?ctil, com desenvolvimento de superf?cies S-C, shear bands, clastos sigmoidais e outras, caracterizando no caso um regime de
deforma??o hidropl?stico. As ocorr?ncias localizadas do Calc?rio Janda?ra s?o condicionadas por falhas
distensionais que exumam a se??o pr?-Barreiras, incluindo um evento precoce com distens?o N-S. Finalmente, s?o observadas zonas de cisalhamento e falhas distensionais com dire??o WNW, tardias, explicadas por um evento compat?vel com o campo de tens?es holoc?nico, neste setor da margem continental. Na regi?o de Vila Nova, pr?ximo a Icapu?, observam-se dobras suaves afetando a f?cies inferior da Forma??o Barreiras, com eixos mergulhando para SSW, planos incipientes de clivagem de dissolu??o e uma linea??o de estiramento de alto rake, compondo um fabric S>L.
A deposi??o dos silicicl?sticos da f?cies superior ? controlada por estruturas de graben pullapart, delimitados por falhas e zonas de cisalhamento sinistrais-normais, com dire??o N-NE,
caracterizando uma invers?o estrutural. Os aspectos microestuturais s?o compat?veis com deforma??o tect?nica em um contexto de soterramento raso do pacote sedimentar. As fei??es identificadas sugerem altas press?es de fluidos nos poros dos sedimentos, gerando microestruturas de car?ter hidropl?stico, indicativos da atua??o do mecanismo de fluxo granular. Tais estruturas s?o superpostas por
microfraturas e microfalhas (car?ter mais fr?gil), o que denota a passagem para mecanismos de microfraturamento e deslizamento friccional, com a progressiva desidrata??o e litifica??o dos sedimentos. A correla??o das estruturas observadas em superf?cie com aquelas presentes em
subsuperf?cie foi realizada a partir de dados geof?sicos (Ground Penetrating Radar, s?smica e mapa magn?tico). Devem ser destacados os lineamentos E-W e NE, observados no mapa
magn?tico e, nas se??es s?smicas, os v?rios exemplos de estruturas em flor positiva, que afetam a base da se??o neocret?cea; em n?veis superiores, tamb?m s?o discern?veis
rejeitos/componentes normais, o que seria compat?vel com a invers?o negativa caracterizada em superf?cie. Tais correla??es auxiliaram na proposi??o de um modelo estrutural compat?vel com o arcabou?o tect?nico regional. A deforma??o ne?gena-pleistoc?nica, de grande magnitude, est? necessariamente propagada em subsuperf?cie afetando, neste caso, a se??o neocret?cea (forma??es A?u e Janda?ra), hospedeira dos reservat?rios de hidrocarbonetos
neste setor da Bacia Potiguar. O modelo estrutural proposto foi relacionado ? deforma??o transcorrente/transformante dextral da Margem Equatorial, associada com termina??es em
transpress?o de zonas E-W, ou na interse??o destas com zonas NE, a exemplo do Lineamento Ponta Grossa-Fazenda Bel?m (o LPGFB, controlado por uma zona de cisalhamento
transcorrente, de idade brasiliana). Numa primeira etapa (e possivelmente no Cret?ceo superior e Pale?geno), este lineamento funcionou em regime transpressional sinistral (antit?tico ? deforma??o dextral nas zonas E-W), e pode ter gerado as dobras na f?cies inferior da Forma??o Barreiras, bem como as estruturas em flor positiva em subsuperf?cie. Este est?gio foi sucedido (ou foi penecontempor?neo) pelas estruturas distensionais que sugerem uma etapa
de movimenta??o transtracional (tamb?m sinistral) associada a processos de vulcanismo (Macau, Messejana) e domeamento t?rmico, no intervalo Ne?geno-Pleistoceno.
Este modelo estrutural tem implica??es diretas nas atividades de explora??o e de explota??o de hidrocarbonetos neste setor da Bacia Potiguar e em sua extens?o offshore. A
estrutura??o dos reservat?rios em subsuperf?cie (arenitos da Forma??o A?u, da sequ?ncia p?srifte) pode estar condicionada (ou pelo menos, fortemente influenciada) pela geometria e cinem?tica da deforma??o, caracterizada em superf?cie. Em adi??o, o evento deformacional registrado na Forma??o Barreiras tem idade pr?xima ? da janela de matura??o e migra??o de ?leo na bacia, estimada entre o Oligoceno e o Mioceno. Desta forma, o cen?rio estrutural
descrito representa um modelo v?lido para entender as condi??es de transporte e aprisionamento de hidrocarbonetos, promovendo abertura de espa?os, forma??o e destrui??o
de trapas. Este modelo ? potencialmente aplic?vel ? regi?o NW da Bacia Potiguar e a setores com contexto estrutural semelhante, ao longo da Margem Equatorial Atl?ntica brasileira
| Identifer | oai:union.ndltd.org:IBICT/oai:repositorio.ufrn.br:123456789/18357 |
| Date | 14 February 2003 |
| Creators | Sousa, Debora do Carmo |
| Contributors | CPF:06892922449, http://lattes.cnpq.br/4094827215552998, Matos, Renato Marcos Darros de, CPF:18404421153, http://lattes.cnpq.br/2387084052091889, Vital, Helenice, Sa, Emanuel Ferraz Jardim de |
| Publisher | Universidade Federal do Rio Grande do Norte, Programa de P?s-Gradua??o em Geodin?mica e Geof?sica, UFRN, BR, Geodin?mica; Geof?sica |
| Source Sets | IBICT Brazilian ETDs |
| Language | Portuguese |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, info:eu-repo/semantics/doctoralThesis |
| Format | application/pdf |
| Source | reponame:Repositório Institucional da UFRN, instname:Universidade Federal do Rio Grande do Norte, instacron:UFRN |
| Rights | info:eu-repo/semantics/openAccess |
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