The Carboniferous Itaituba Formation represents expressive retrograding carbonate sedimentation included in the sedimentary evolution of the Tapajós Group of the Amazonas
basin. These carbonate consist of fossiliferous limestones, fine grained dolostones, fine to
coarse grained sandstones and subordinate reddish siltstones, black shales and evaporites.
Facies and microfacies analysis of drill cores from the Uruará region, State of Pará , allowed
to individualize nineteen facies grouped into five facies associations : tidal plain ( FA1 ) , tidal
channel ( FA2 ), lagoon ( FA3 ), bioclastic bar ( FA4 ) and outer shelf ( FA5 ). FA1 is
composed of fine grained sandstone with rip-up clasts and mud cracks , marl with quartz and
feldspar grains, laminated dolomudstone with fine terrigenous grains and silicified dolostone
with intercalation of mudstone and disseminated quartz grains dolomitized and locally with
microcrystalline quartz. FA2 consists of medium to coarse grained sandstone with trough
cross-bedding and mud drapes on foresets, very fine to fine grained sandstone with wavy
bedding, laminated siltstone, synsedimentary faults and convolute lamination. FA3 consists of
massive red siltstone, mudstone with fossils, brachiopods and floatstone with disseminated
pyrite and massive mudstone with frequent quartz grains. AF4 and AF5 exhibit abundant
bioclasts represented by spines and fragments of echinoderms, shells, spines and fragments of
brachiopods, ostracods, foraminifera, red algae and bilvave shells. AF4 consists of
fossiliferous oolitic grainstone and grainstone with terrigenous grains, mainly monocrystalline
quartz grains and the AF5 consists of fossiliferous wackestone, wackestone with terrigenous
grains and massive mudstone with monocrystalline quartz grains. Subarkoses (AF1), arkoses
(AF2) and lithic arkoses (AF2) predominate in the Itaituba Formation and are composed by of
polycrystalline and monocrystalline quartz grains, K-feldspar, plagioclase, pyrite, detrital
muscovite, mudstone, metamorphic and chert fragments and rare bioclasts. The cement is of
nonferrous calcite, iron oxides/hydroxides, silica overgrowth and intergranular, moldic and
sometimes elongated porosities reaching up to 11 % of the rock. The diagenetic processes in
sandstone are physical compaction, quartz overgrowth, non-ferrous calcite cementation,
chemical compaction, grain replacement, pyrite autigenesis, formation of iron
oxides/hydroxides and alteration of plagioclase. The diagenetic processes in carbonates are:
micritization, neomorphism, dolomitization, fracturing, chemical compaction, calcite
cementation, secondary dissolution and mineral autigenesis. The Itaituba succession is
interpreted as a lagoon/tidal flat system linked to the marine carbonate platform. Tidal flats
developed on the margins of the lagoons were periodically supplied by fine (silt) terrigenous
influxes that inhibited the carbonate precipitation. Bioclastic bars were cut by tidal channels
(inlet) connected the lagoon with the shallow platform rich in benthic organisms.
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