Depósitos costeiros e microbialitos da Formação Itaituba, o Pensilvaniano da borda sul da Bacia do Amazonas, região de Placas, Pará.

Abstract

The Itaituba Formation in the Amazon Basin represents a Bhaskiriana-Moscowian carbonate succession, composed of fossiliferous limestone, fine dolomite, fine to coarsegrained sandstone, gray shales and evaporites. Outcrop-based facies and stratigraphic analysis combined with microfacies study allowed the paleoenvironmental reconstitution of the Itaituba Formation, on the southern border of the Amazon Basin, Southwestern of Pará State. The 14mthick carbonate succession with occurrence of microbialites, consists of 9 facies and microfacies, such as: a) laminated shale (F1); b) laminated sandstone (Al); c) laminated siltstone (Sl); d) massive sandstone (Mm); e) mudstone with desiccation cracks (Mg); f) wackestone bioclastic (Wb); g) packstone bioclastic (Pb), h) oncolitic floatstone (Fo) and i) boundstone. These facies are grouped into two facies association (AF): AF1, lagoon / tidal flat deposits, consisting of facies Fl, Al, Sl, Mm, Mg, Wb, related to the flat and laterally continuous buildings with predominance of facies F1, interlayed by centimeter layers of facies Wb and Pb, organized in a fining upward cycle and; and AF2, shoal / barrier island, constituted by Pb facies and Fo and Bd facies, with Pb displayed in continuous flat and lateral bodies and Fo and Bd in lensshaped bioherms. Fo facies are the oncolytic microbilites and associated with microstromatolites, described as M1 and M2a morphotypes, and Bd facies, include columnar stromatolitic forms, described as M2b and M2c morphotypes. The depositional model of the Itaituba Formation in the Placas-PA region consists of a coastal system with lagoon, shoals / barrier island and tidal flat. Microbialites developed in the coastal region in the Amazon Basin during the Bashkirian-Moscovian transition, representing a transition to more restrictive environmental conditions in relation to fauna and flora, and more conducive to the proliferation of cyanobacteria, with probably increased alkalinity in water. These cyanobacteria evidences are record, for the first time, in Carboniferous rocks of the Amazon Basin improved the knowledge related to the carbonate fabric dispersion during the implantation of the Itaituba epicontinental sea in the Western Gondwana.