Estratigrafia do Andar Alagoas na sub-bacia do Tucano Norte, Bahia
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Data
2016-04-12
Autores
Varejão, Filipe Giovanini [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
As bacias cretáceas do Nordeste do Brasil, como as do Tucano, Araripe e Grajaú, tiveram sua gênese associada ao processo de abertura do oceano Atlântico Sul. Os depósitos aptianos das bacias do Araripe e Grajaú são comumente correlacionados aos depósitos que originaram as grandes acumulações de hidrocarbonetos do pré-sal. Na Bacia do Tucano essa sucessão também se encontra preservada e constitui importante registro desta fase da evolução da margem atlântica sul-americana. A Serra do Tonã é uma feição geomorfológica em forma de mesa que se destaca do relevo dissecado com mergulho suave característico da bacia. Esta conformação se deve à existência de duas camadas carbonáticas posicionadas na base e no topo da mesa que são responsáveis por sustentar o relevo. Os dois intervalos carbonáticos, de caráter único na bacia, são separados por sucessões siliciclásticas que compõem a Formação Marizal e apesar de constituir registro ímpar de sedimentação mista siliciclástico-carbonática, não foram devidamente estudados quanto a seus aspectos sedimentológicos e estratigráficos. A análise pormenorizada da sucessão mista da Serra do Tonã permitiu a identificação de duas sequências deposicionais com padrão de afinamento textural em direção ao topo e limitadas por discordâncias regionais. A Sequência Deposicional 1 (SD1) assenta-se em discordância sobre os sedimentos da fase rifte e é marcada por arenitos e conglomerados aluviais da porção inferior da Formação Marizal, com indicadores de paleofluxo com rumo sul/sudeste. A SD1 apresenta padrão de afinamento textural para o topo. Arenitos intercalam-se com lamitos e siltitos até o contato desses com carbonatos microbiais, bioclásticos e mudstones. Estes depósitos são interpretados como associados a ambiente lacustre raso de elevada salinidade. Sobrepostos aos carbonatos encontra-se espessa camada de folhelhos lacustres, que em sua porção superior apresenta evidências de exposição subaérea, com desenvolvimento de feições típicas de paleossolos formados em zonas vadosas. Uma importante desconformidade define o limite entre as duas sequências, sendo que a Sequência Deposicional 2 (SD2) inicia com arenitos conglomeráticos fluviais sobrepostos aos paleossolos, apresentando indicadores de paleofluxo para sul. A sucessão apresenta padrão textural de afinamento granulométrico em direção ao topo, com arenitos sobrepostos por lamitos intercalados a arenitos, em ciclos de afinamento textural marcados pelo aumento da razão areia/argila. Estes sedimentos são interpretados como oriundos de deposição em barras de desembocadura e de baías interdistributário. Carbonatos e folhelhos perfazem fácies de águas relativamente mais profundas e são sucedidos por delgadas intercalações de microbialitos e mudstones, associados a águas mais rasas. Análise taxonômica de ostracodes e estudos isotópicos dos carbonatos da SD2 têm sido usados na correlação com a Formação Crato da Bacia do Araripe. As sequências aptianas da Serra do Tonã apresentam características litológicas, paleofluxo fluvial e sucessão estratigráfica muito semelhante às sequências reconhecidas na porção inferior do registro aptiano da Bacia do Araripe. Tal correlação entre ambas as bacias permite a interpretação de uma paleodrenagem continental fluindo para sul/sudeste associada ao desenvolvimento de corpos lacustres de salinidade elevada.
The Northeast Brazilian Cretaceous Basins as Tucano, Araripe and Grajaú had their origin in the breakup events responsible for the opening of the South Atlantic Ocean. Aptian deposits of Araripe and Grajaú basins are commonly related to hydrocarbon-rich accumulation in the Santos and Campos basins. In the Tucano Basin the pre-salt succession is also present and constitutes an important register of this phase in the South-American Atlantic Margin. The Serra do Tonã Plateau is a geomorphological feature called mesa that highlights in the dissected relief of the Tucano Basin. The table morphology is preserved because of two carbonate-rich intervals, one in the base, and another one in the top of the plateau. These two carbonate intervals are unique in the basin, and are separated by thick siliciclastic successions named Marizal Formation. Despite the exceptional preservation of the carbonate-silisiclastic, little attention has been given to its sedimentology and stratigraphy. Two finning upward depositional sequences separated by regional unconformities were identified in the mixed carbonate-siliciclastic succession of the Serra do Tonã Plateau. The Depositional Sequence 1 (DS1) initiate after an erosive period that marks the end of the rift phase. The DS1 is marked by alluvial sandstones and conglomerates from the lower portion of the Marizal Formation, with paleocurrent data indicating south-southeast transport. In the top of DS1 the sandstones texturally fines upward and are interbedded with shales up tp their contact with microbial and bioclastic carbonates, and mudstones. These deposits are interpreted as associated with shallow lakes of elevated salinity. Above the carbonates there is a thick layer of lacustrine shales. In the upper portion of the shales there is evidence of subaerial exposure, with development of features typical of evaporitic paleossols. A disconformity defines the limit between the two sequences. The Depositional Sequence 2 (DS2) begins with fluvial conglomeratic sandstones above paleossols. Paleocurrent suggests flux southwards. The succession has a finning upward textural pattern, with shales interlayered with sandstones in cycles of elevated sand/clay ratio, in a context of mouth bars and interdistributary bays. Carbonates and shales of DS2 are interpreted as deposited in deeper waters, characterizing a transgressive tract. The microbialites in the upper portion of the carbonate interval are interpreted as shallow deposits, suggesting a regressive stack pattern. Ostracode taxonomy and isotopic signatured of the top interval of DS2 have been used to correlate these carbonates with those of Crato Formation in the Araripe Basin. The aptian sequences from Serra do Tonã presents lithological characteristics, fluvial paleoflux and stratigraphical succession very similar to those of the Santana Group in the Araripe Basin. The correlation between both basins allows an interpretation of a continental palaeodrainage flowing towards south-southeast, associated with the development of lacustrine bodies of elevated salinity.
The Northeast Brazilian Cretaceous Basins as Tucano, Araripe and Grajaú had their origin in the breakup events responsible for the opening of the South Atlantic Ocean. Aptian deposits of Araripe and Grajaú basins are commonly related to hydrocarbon-rich accumulation in the Santos and Campos basins. In the Tucano Basin the pre-salt succession is also present and constitutes an important register of this phase in the South-American Atlantic Margin. The Serra do Tonã Plateau is a geomorphological feature called mesa that highlights in the dissected relief of the Tucano Basin. The table morphology is preserved because of two carbonate-rich intervals, one in the base, and another one in the top of the plateau. These two carbonate intervals are unique in the basin, and are separated by thick siliciclastic successions named Marizal Formation. Despite the exceptional preservation of the carbonate-silisiclastic, little attention has been given to its sedimentology and stratigraphy. Two finning upward depositional sequences separated by regional unconformities were identified in the mixed carbonate-siliciclastic succession of the Serra do Tonã Plateau. The Depositional Sequence 1 (DS1) initiate after an erosive period that marks the end of the rift phase. The DS1 is marked by alluvial sandstones and conglomerates from the lower portion of the Marizal Formation, with paleocurrent data indicating south-southeast transport. In the top of DS1 the sandstones texturally fines upward and are interbedded with shales up tp their contact with microbial and bioclastic carbonates, and mudstones. These deposits are interpreted as associated with shallow lakes of elevated salinity. Above the carbonates there is a thick layer of lacustrine shales. In the upper portion of the shales there is evidence of subaerial exposure, with development of features typical of evaporitic paleossols. A disconformity defines the limit between the two sequences. The Depositional Sequence 2 (DS2) begins with fluvial conglomeratic sandstones above paleossols. Paleocurrent suggests flux southwards. The succession has a finning upward textural pattern, with shales interlayered with sandstones in cycles of elevated sand/clay ratio, in a context of mouth bars and interdistributary bays. Carbonates and shales of DS2 are interpreted as deposited in deeper waters, characterizing a transgressive tract. The microbialites in the upper portion of the carbonate interval are interpreted as shallow deposits, suggesting a regressive stack pattern. Ostracode taxonomy and isotopic signatured of the top interval of DS2 have been used to correlate these carbonates with those of Crato Formation in the Araripe Basin. The aptian sequences from Serra do Tonã presents lithological characteristics, fluvial paleoflux and stratigraphical succession very similar to those of the Santana Group in the Araripe Basin. The correlation between both basins allows an interpretation of a continental palaeodrainage flowing towards south-southeast, associated with the development of lacustrine bodies of elevated salinity.
Descrição
Palavras-chave
Sequências deposicionais, Sedimentação mista, Microbialitos, Aptiano, Serra do Tonã, Depositional sequences, Mixed sedimentation, Microbialites, Aptian