Evolução tectono-sedimentar da Formação Araripina, Bacia do Araripe: registro dos esforços tectônicos atuantes durante o Albiano no Nordeste do Brasil
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Data
2019-05-03
Autores
Andrade, André Santiago Martins de
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Universidade Estadual Paulista (Unesp)
Resumo
O evento de ruptura do Gondwana no Cretáceo foi responsável pela reativação de estruturas do Pré-Cambriano e geração de diversas bacias interiores do tipo rifte no Nordeste do Brasil. Nesse contexto, a sucessão sedimentar da Bacia do Araripe registra as sequências pré-, sin- e pós-rifte formadas durante os distintos estágios tectono-sedimentares envolvidos na formação do Oceano Atlântico Sul. A Formação Araripina do Albiano Médio faz parte da sequência pós-rifte, é limitada na base por discordância litológica com rochas do embasamento ou por discordância angular com a Formação Romualdo, marinha. No topo, é limitada por discordância erosiva ou angular com os depósitos fluviais da Formação Exu. A Formação Araripina é composta principalmente de intercalações rítmicas de arenitos e lamitos, e a sucessão sedimentar é dividida em duas sequências deposicionais, SD1 e SD2, por uma superfície de truncamento regional. Sob essa superfície tectônica, em SD1, a associação de fácies mostra que a sedimentação foi em planícies intermediárias a distais de sistemas fluviais distributivos sujeitas a inundações episódicas associadas a canais efêmeros e à fluxos laminares, com evidências de complexos de splays terminais e laterais, pequenos lagos efêmeros e tectônica ativa. Acima da superfície, a SD2 apresenta menor evidência de tectonismo e de aporte sedimentar. O ambiente deposicional imediatamente após a superfície é compatível com depósitos de playa lake em diversos afloramentos estudados, provavelmente em resposta à atividade tectônica anterior. De maneira geral representam porções distais de sistemas fluviais distributivos também sujeitas a inundações episódicas, com canais efêmeros e delgadas evidências de splays terminais. No topo da sequência há evidências de exposição subaérea da planície, com pedogênese associada. O padrão de paleocorrentes sugere deposição para sudeste-leste, com áreas-fonte a noroeste-oeste. Esse dado é oposto às paleocorrentes para oeste medidas na Formação Exu e indicam uma mudança polarizada no padrão de dispersão da sedimentação. Essa mudança é provavelmente relacionada ao soerguimento epirogênico da porção leste da Bacia do Araripe que promoveu o rearranjo de toda paleodrenagem continental durante o Albiano Médio. Diversas evidências de deformações sin-deposicionais estão presentes nos ritmitos (pseudonódulos, brechas intraformacionais, injectitos, dobras convolutas, microfalhas) e são aqui interpretadas como produzidas por atividade sísmica, constituindo sismitos. As evidências de atividade sin e pós tectônica na Formação Araripina são relacionadas a reativações de estruturas do embasamento durante o Albiano, provavelmente em resposta ao começo da abertura da margem equatorial brasileira. De fato, a Formação Araripina parece ser confinada por falhas pré deposição da Formação Exu, entre a Zona de Cisalhamento Farias Brito e a Falha de Iara/ Lineamento de Patos. Além disso, a ocorrência de fraturas (falhas e juntas) que mostram padrões NE-SW, E-W e NW-SE é concordante com estruturas tectônicas regionais do embasamento cristalino. Por fim, a Formação Araripina mostra sua importância como um registro sedimentar das atividades tectônicas do Albiano no nordeste do Brasil durante os últimos estágios da ruptura dos continentes sulamericano e africano.
The Cretaceous Gondwana breakup event was responsible for the reactivation of Precambrian structures producing several interior rift basins in the Northeast Brazil. In this context, the sedimentary succession of the Araripe Basin record the pre, syn and post-rift sequences formed during the distinct tectono-sedimentary stages involved in the formation of the South Atlantic Ocean. The Middle Albian Araripina Formation is part of the post-rift sequence, bounded at base by basement rocks (nonconformity) or by the marine Romualdo Formation (angular unconformity). It is bounded at the top by a disconformity/angular unconformity with the fluvial deposits of the Exu Formation. The Araripina Formation is mainly composed of rhythmically interbedded sandstones and mudstones, and the succession is divided in two (DS1 and DS2) by a major truncating surface. Under this tectonic surface, in DS1, facies association shows that sedimentation was likely in a plain with medial to distal distributive fluvial systems. This plain was subjected to episodic inundations associated with ephemeral channels and sheetfloods, with evidences of terminal splays complexes, lateral splays, small playa lakes and active tectonics. Above the surface, the DS2 presents a more lenient tectonic setting and less sediment suppy. The depositional environment immediately after the surface is compatible with playa lake deposits in several studied places, probably in response to the previous tectonic activity. Overall, it represents distal portions of the distributive fluvial systems, with the plain also subjected to episodic inundations and presenting minor ephemeral channels and thin terminal splays. The top of the sequence shows subaerial exposition of this plain with pedogenesis associated. The paleocurrent patterns suggest deposition towards southeast and east, with source-areas at northwest and west. This data is opposite to the westward paleocurrents measured in the Exu Formation, indicating a polarized change in the dispersal pattern of the sedimentation. This change is probably related with the epirogenetic uplift of the eastern portion of the Araripe Basin that rearranged all the continental paleodrainage during the mid-Albian. Several evidences of soft-sediment deformation are present in the rhythmites (i.e., pseudonodules, ball-and-pillows, injectites and convoluted folds) and are here interpreted as produced by seismic activity, constituting true seismites. The evidences of syn and post tectonic activity in the Araripina Formation is related to reactivations of basement structures during the Albian, probably in response to the beginning of the opening of the Brazilian equatorial margin. Indeed, the Araripina Formation seems to be constrained by faults pre deposition of Exu Formation, between the Farias Brito Shear Zone and the Iara Fault/ Patos Lineament. In addition, the occurrence of fractures (faults and joints) that show NE-SW, E-W and NW-SE patterns, is concordant with tectonic regional structures of the crystalline basement. Finally, the Araripina Formation plays an important role as a sedimentary record of the Albian tectonic activity in the Northeast of Brazil during the last stages of the South America and Africa breakup.
The Cretaceous Gondwana breakup event was responsible for the reactivation of Precambrian structures producing several interior rift basins in the Northeast Brazil. In this context, the sedimentary succession of the Araripe Basin record the pre, syn and post-rift sequences formed during the distinct tectono-sedimentary stages involved in the formation of the South Atlantic Ocean. The Middle Albian Araripina Formation is part of the post-rift sequence, bounded at base by basement rocks (nonconformity) or by the marine Romualdo Formation (angular unconformity). It is bounded at the top by a disconformity/angular unconformity with the fluvial deposits of the Exu Formation. The Araripina Formation is mainly composed of rhythmically interbedded sandstones and mudstones, and the succession is divided in two (DS1 and DS2) by a major truncating surface. Under this tectonic surface, in DS1, facies association shows that sedimentation was likely in a plain with medial to distal distributive fluvial systems. This plain was subjected to episodic inundations associated with ephemeral channels and sheetfloods, with evidences of terminal splays complexes, lateral splays, small playa lakes and active tectonics. Above the surface, the DS2 presents a more lenient tectonic setting and less sediment suppy. The depositional environment immediately after the surface is compatible with playa lake deposits in several studied places, probably in response to the previous tectonic activity. Overall, it represents distal portions of the distributive fluvial systems, with the plain also subjected to episodic inundations and presenting minor ephemeral channels and thin terminal splays. The top of the sequence shows subaerial exposition of this plain with pedogenesis associated. The paleocurrent patterns suggest deposition towards southeast and east, with source-areas at northwest and west. This data is opposite to the westward paleocurrents measured in the Exu Formation, indicating a polarized change in the dispersal pattern of the sedimentation. This change is probably related with the epirogenetic uplift of the eastern portion of the Araripe Basin that rearranged all the continental paleodrainage during the mid-Albian. Several evidences of soft-sediment deformation are present in the rhythmites (i.e., pseudonodules, ball-and-pillows, injectites and convoluted folds) and are here interpreted as produced by seismic activity, constituting true seismites. The evidences of syn and post tectonic activity in the Araripina Formation is related to reactivations of basement structures during the Albian, probably in response to the beginning of the opening of the Brazilian equatorial margin. Indeed, the Araripina Formation seems to be constrained by faults pre deposition of Exu Formation, between the Farias Brito Shear Zone and the Iara Fault/ Patos Lineament. In addition, the occurrence of fractures (faults and joints) that show NE-SW, E-W and NW-SE patterns, is concordant with tectonic regional structures of the crystalline basement. Finally, the Araripina Formation plays an important role as a sedimentary record of the Albian tectonic activity in the Northeast of Brazil during the last stages of the South America and Africa breakup.
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Geologia, Sedimentologia, Fácies, Bacia do Araripe, Geologia estratigráfica, Paleogeografia do Cretáceo, Deformação sinsedimentar, Sedimentology, Soft-sediment deformation, Synsedimentary tectonics, Paleoenvironments, Araripina Formation, Araripe Basin