Miocene deposits from Pirabas Formation occurs discontinuously in the northeastern state of Pará, and are characterized by the predominance of fossiliferous carbonate rocks, which represent one of the best records of the Cenozoic marine sedimentation in Brazil. The Pirabas Formation has revealed the presence of an abundant invertebrate both siliciclastic and carbonatic ichnofauna, it remains poorly studied. The trace fossils analyzed in this research occur in the B-17 Mine (Capanema), Atalaia beach (Salinópolis) and Aricuru village (Maracanã).The microfacies analysis allowed the identification of seven microfacies: grainstone with terrigenous grains and algae (Gt), wackestone/packstone with planar lamination(W/P), foraminifera rich packstone and molluscs (P), rudstone with molluscs fragments (R), wackestone with terrigenous grains(W), calci-mudstone (Cm) and massive dolomudstone (Dm). Five types of trace fossils were identified represented by Gyrolithes davreuxi, Palaeophycus tubularis, Thalassinoides callianassae, Thalassinoides suevicus, Megathalassinoides isp., and megatrace fossil probably made of Turbinella gastropods. These trace fossils occur in soft ground substrates (unconsolidated substrates, rich in water), characterized by trace fossils with diameter irregularity that depending on substrate consistence and abundant horizontal burrows. This feature is typical of Cruziana ichnofacies that generally consisting of dwellings (Domichnia) and food (Fodinichnia) burrows building in regions below the wave action level. Cruziana ichnofacies suggests low energy environment, indicating that the animals responsible for these traces fossils prefer calm water environments with terrigenous sporadic influxes. This ichnofacies in Pirabas Formation are rather poor in terms of ichnodiversity suggesting brackish water conditions. The ichnofossils distribution in Pirabas Formation suggests hallow marine environment, in which there were lower rates of salinity in upward as shown by the abundance of trace fossils and their low diversity toward the profile top. The presence of Gyrolithes ichnofabric suggests the occurrence periods with salinity fluctuations. This is corroborated by generally low bioturbation rates, and the trace fossil diameters and sizes that generally do not exhibit anomalies or significant differences from their counterparts described in other regions and deposits. This suggests that there was a predominance of oxygen, brackish and shallow water. Low bioturbation levels may be related to some environmental stress during the occupation of the substrate as changes in oxidation levels and salinity. The presence of Cruziana icnofácies reflects the coastal environment with unconsolidated substrate, characterized by moderate variations in the energy level in shallow water under the action of normal waves and low energy levels in deep and calm waters. These features and the predominance of horizontal traces fossils indicate a predominantly brackish environment with oxygenation and salinity fluctuating levels. The very low bioturbation levels may also be related to some environmental stress during the occupation of the substrate. The megatrace fossils are restricted to fine microfacies and to a single location and level, suggesting
that only specialist organisms have inhabited in these substrates. The "mega" diameter trace fossils suggest that the salinity and oxygen conditions have allowed the surface colonization by big animals. The anoxic substrate below the sediment-water interface was another important factor to the lack of burrowed organisms in Pirabas Formation. Anoxia of the substrate is indicated by the abundance of organic matter preservation in these microfacies. Thus, after microfacies and trace fossils analyses, the Pirabas carbonate succession was interpreted here as shallow marine carbonate platform with bioclastic sandy bar lagoons and mangroves, where changes in salinity and oxygen near the substrate triggered periods of seafloor occupation.
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