The Itaituba Formation (Pennsylvanian age) in the Monte Alegre region is
characterized by limestones interbedded with carbonatic shales and less frequently
sandstones. The fauna of this formation is represented mainly by brachiopods,
echinoderms, molluscs, foraminifera and sporadically by ostracods, trilobites,
bryozoans and algae. The microfacies study determinated 8 types of microfacies,
being the most predominant the mudstone with terrigenous grains and few bioclasts,
followed by grainstones, dolomites, and hybrid packstones and wackstones. Unless
the grainstones these microfacies suggest a lagoon environment influenced by
continental/transicional sedimentation. Regarding the oolithic bioclastic grainstone
and oolithic peloidal grainstone they indicate more agitated waters of tidal bar or
shoal environment of the internal platform. The diagenesis of the limestones is
characterized by bioturbation and micritization, followed by cementation, silicification,
dolomitization and pyritization. The mechanical compaction is evidenced by the
presence of fractures, while stylolithes and dissolution seams indicate chemical
compaction. The insoluble fraction is predominantly silty clay, and together with the
sand fraction (> 62μm) constitutes 2 to 30% of the rock. The most abundant clay
mineral is illite, followed by smectite and traces of chlorite and kaolinite. The climate
during the deposition of the Itaituba Formation is considered to be moderate with a
trend to aridity supported by the presence of Illite, smectite and feldspars.
Furthermore, the terrigenous silty quartz in the limestones suggests eolic transport.
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