Pennsylvanian mixed plataform deposits of the Amazonas Basin, Uruará region, state of Pará

Abstract

Carbonate rocks are widely reported in the west and center-west regions of the state of Pará, south edge of the Amazon Basin, corresponding to the Neocarboniferous period. These deposits are included in the Pennsylvanian-Permian sequence, corresponding to a second order super sequence represented by the Tapajós Group. The Itaituba Formation corresponds to the transgressive portion preceding the expressive continentalization and formation of Pangaea. The unit consists of fossiliferous carbonates, cross-stratified sandstones and subordinate shales and evaporites, associated with coastal marine environments. Nineteen facies and microfacies were grouped into five facies associations corresponding to tidal flat deposits (FA1), tidal channel (FA2), lagoon (FA3), bioclastic bar (FA4) and outer platform (FA5). The FA1 comprises cycles of subaerial exposure with formation of mud cracks and subaqueous deposition, composed of dolomitic breccia, laminated fine dolostone and terrigenous dolomudstone, separated by a layer of pyritic massive sandstone and thin dolomite silicified. The FA2 is composed of fine sandstone with cross rolling straight base and pelitic coating on the sets with synsedimentary faulting, siltstones cross-lamination with a predominance of mud and synsedimentary faulting, marl with cross-lamination and cross-bedded coarse sandstones with politic coating on the sets. The FA3 consists of massive reddish siltstones, mudstones fossiliferous, brachiopods floatstone and black shale massive with pyrite. The FA4 is mainly represented by fossiliferous oolitic grainstone and grainstone with terrigenous with abundant fossils of brachiopods, echinoderms, bivalves, mollusks, and secondly by arthropods, bryozoans and foraminifera. The FA5 is associated with a platform, composed of fossiliferous wackestone, fossiliferous wackestone with terrigenous and massive mudstone. The fossiliferous assembly is represented by foraminifera, brachiopods, bivalves and, subordinately, echinoderms and bryozoans that attest a coastal environment of moderate salinity and shallow with intense proliferation of benthic organisms. The depositional model of Itaituba Formation, Uruará region, was assigned to a Lagunar/tidal flat system connected to a marine carbonate platform system. The lagoons were bordered by tidal flat at the continental portion and periodically supplied by fine terrigenous intake (silt) that inhibited the carbonate precipitation. Bioclastics bars cut by tidal channels (inlet) separating the lagoon from the shallow platform with greater abundance of benthic marine fossils, recording the beginning of a neocarboniferous marine transgression in the Amazonas Basin arising from NNW.