Facies analysis of the Codó Formation (Late Aptian) in the Grajaú Area, Southern São Luís-Grajaú Basin

Abstract

Facies descriptions of the Codó Formation in the Grajaú area are provided for the first time, and its sedimentary characteristics compared to those from the Codó area to allow paleoenvironmental reconstructions. Deposits in the Grajaú area include evaporites, limestones and argillites bearing features indicative of a shallow, low energy, subaqueous, saline environment exposed to meteoric and/or capillary conditions. Floodingevaporative concentration-desiccation cycles suggest a saline pan complex surrounded by extensive evaporitic mudflats. The location of the system, whether coastal or inland, is a matter open for debate. However, the later hypothesis is favored considering: 1. Sr isotopic data, with values higher than those expected for Late Aptian marine waters; 2. calcitic composition of limestones (instead of dolomitic and/or magnesitic as expected in coastal settings); and 3. presence of continental ostracods and lack of marine fauna. This interpretation is consistent with that proposed for Upper Aptian deposits of the Codó area, but the depositional system there was one dominated by more stable, well-stratified, anoxic waters and evaporite precipitation in central lacustrine areas, while in the Grajaú area the salt pan was more oxygenated and ephemeral, with salt precipitation mainly in marginal areas or along surrounding mudflats.

Upper Aptian mixed carbonate-siliciclastic sequences from Tucano Basin, Northeastern Brazil: Implications for paleogeographic reconstructions following Gondwana break-up

“…The evaporites characterize the transitional stage between the rift and drift sequences and are present in most Brazilian marginal basins (Chang et al, 1988(Chang et al, , 1992. Gypsum layers occur even west of the Parnaíba Basin, where the evaporitic-carbonate section of the Cod o Formation overlays the fluvio-deltaic facies of the Grajaú Formation (Rossetti et al, 2004;Paz et al, 2005;Paz and Rossetti, 2006;Bahniuk et al, 2015).…”

Caracterização e significado paleoambiental da fauna de ostracodes da Formação Codó (Neoaptiano), leste da bacia de Grajaú, MA, Brasil

“…O estágio pré-rift ocorreu no Neoaptino, sendo registrado por folhelhos negros, evaporitos e calcários lacustres da Formação Codó, além de depósitos flúvio-deltaicos da Formação Grajaú. Estes estratos são recobertos por uma espessa sucessão sedimentar formada a partir do Albiano, inserida no Grupo Itapecuru (Rossetti & Truckenbrodt, 1997), o qual inclui a Unidade Indiferenciada (Eo/Mesoalbiano), a Formação Alcântara (Neoalbiano-Cenomaniano), e a Formação Nos últimos anos, diversos estudos enfocando análises faciológicas, estratigráficas e geoquímicas foram realizados nestes depósitos, visando a sua reconstituição paleoambiental Paz & Rossetti, 2001Rossetti et al, 2004e Paz, 2005. Como resultado, houve a proposição de um sistema deposicional lacustre, hipersalino, dominantemente anóxico e sujeito a flutuações periódicas da lâmina de água, característica esta revelada pela organização faciológica em ciclos de arrasamento ascendentes (Paz & Rossetti, 2005).…”

“…A caracterização sedimentológica da Formação Codó exposta na área de estudo já foi apresentada na literatura (Paz, 2005;Paz & Rossetti, 2001 (Paz, 2005;Rossetti et al, 2004). Estes estratos são organizados em ciclos de arrasamento ascendentes atribuídos à progradação de ambientes marginais sobre ambientes centrais, em sistema lacustre.…”

Linking lacustrine cycles with syn-sedimentary tectonic episodes: an example from the Codó Formation (late Aptian), northeastern Brazil

“…A detailed facies analysis of the Codó Formation was presented elsewhere (Paz & Rossetti, 2001;Rossetti, Paz & Góes, 2004). However, given its importance in providing a general overview of the depositional setting and defining the shallowing-upward cycles discussed here, the main descriptions and interpretations will be summarized in this section, together with some new information that helps to support the proposed palaeoenvironmental model.…”