Pennsylvanian epicontinental sea dynamics in Western Gondwana: Stratigraphic, petrographic, and C, O, and Sr isotopic insights from the Itaituba Formation, Amazonas Basin

During the Pennsylvanian period, an epicontinental sea significantly influenced the paleogeography of Western Gondwana, as evidenced by the Pennsylvanian units of the Tapajós Group (Monte Alegre, Itaituba, and Nova Olinda formations) in the Amazonas Basin, Brazil. The carbonate deposition within the Itaituba Formation represents a Pennsylvanian transgressive event linked to the Panthalassa Ocean. Comprehensive stratigraphic, petrographic, and isotopic analyses were conducted on a carbonate succession in the Itacimpasa Quarry, one of the best-exposed carbonate outcrops in the southern Amazonas Basin, notable for its rich fossil content. The petrographic analysis allowed the identification of eleven microfacies grouped into three facies associations: tidal flat (FA1), lagoon (FA2), and outer platform (FA3) representative an epicontinental carbonate platform system. The general transgressive trend is evidenced by the superimposition of marine facies (FA3) over coastal facies (FA1 and FA2). Fossil content was quantified through detailed petrography, revealing a clearly marine fossiliferous diversity, including brachiopods, echinoderms, bivalves, ostracods, foraminifers, trilobites, and bryozoans. Isotopic data, including carbon, oxygen, and strontium isotopes, refined previous chemostratigraphic framework. δ¹³C values ranged from +2.15‰ to +4.54‰, and δ¹⁸O values spanned −6.99‰–2.82‰, consistent with the expected values for the Pennsylvanian carbonates. An upward δ¹³C trend within the studied interval suggests significant organic productivity during a predominantly transgressive phase, although minor sea-level falls explain variations in the δ¹³C trend. The ⁸⁷Sr/⁸⁶Sr values indicate epicontinental marine conditions, ranging from 0.70852 to 0.70858. However, samples affected by diagenetic processes can reach up to 0.70940. A representative δ¹³C curve for the Itaituba Formation was constructed for the first time, integrating isotopic data using a sixth-degree polynomial fit. The resultant curve for the carbonate succession shows a positive upward trend, following the δ¹³C curve trends of other contemporaneous carbonates from the Panthalassa and Paleo-Tethys oceans and with the global relative sea-level curve. This correlation, in addition to reinforcing the validity of the δ¹³C trends observed in the Itaituba Formation, also suggest a response to paleoceanographic and paleoenvironmental changes during the Pennsylvanian period linked to a deglaciation-induced transgressive event.