Bioturbation as a trigger for early diagenetic processes in miocene mixed carbonate-siliciclastic platforms: The Pirabas Formation, Equatorial Brazilian Margin

During the Miocene, the mixed carbonate–siliciclastic deposits of the Pirabas Formation record decapod crustacean trace fossils that provide valuable insights into paleoenvironmental conditions and early diagenetic processes linked to bioturbation. This study integrates sedimentological, ichnological, and geochemical data to explore the interplay between substrate colonization and mineral precipitation. Facies analysis identified three associations: FA1 – tidal flat, FA2 – marginal mangrove, and FA3 – restricted lagoon. Systematic ichnological analysis recognized four ichnogenera and five ichnospecies, with bioturbation indices ranging from BI1 to BI5, corresponding to the Cruziana Ichnofacies, which is characteristic of shallow-marine, softground, and well-oxygenated settings. Petrographic descriptions and geochemical analyses revealed three main types of trace fossil infill—calcite, pyrite, and siderite—corresponding to distinct early diagenetic redox zones: oxic/suboxic, oxic/anoxic. These results highlight the role of decapod activity in modifying shallow-marine substrates and mediating biogeochemical cycles, offering new perspectives on the interactions between bioturbation, paleoenvironmental dynamics, and early diagenesis in Miocene mixed carbonate platforms of the equatorial Brazilian margin. • The Miocene Pirabas Formation preserves decapod crustacean trace fossils that record shallow-marine depositional environments. • Ichnological analysis identifies the Cruziana Ichnofacies, characterized by softground substrates with dominant domichnia and fodinichnia structures. • Trace fossil fills exhibit three mineral types—calcite, pyrite, and siderite—corresponding to distinct redox zones (oxic/suboxic, oxic/anoxic, suboxic/anoxic). • Decapod bioturbation played a key role in modifying substrates and mediating early diagenetic iron and sulfur cycles. • Paleoenvironmental reconstruction highlights the influence of tidal flats, mangroves, and restricted lagoons on trace fossil distribution and diagenetic processe