Deciphering the role of microbial influence in trace fossil diagenesis in sandstone (Ipu Formation, Silurian, Parnaíba Basin, Brazil)

ABSTRACT Microbial communities present in biofilms and microbial mats can modify and stabilise the substrates they colonise. Consequently, the preservation of trace fossils, resulting from the interaction of organisms with such substrates, can also be influenced by microbial activity. This study focused on the trace fossils preserved in the medium‐ to coarse‐grained sandstones of the Ipu Formation (Silurian, Serra Grande Group, Parnaíba Basin, Brazil), where the occurrence of microbially induced sedimentary structures indicates microbial biostabilisation, to investigate the role of microbenthos in the taphonomy of ichnofossils in coarse sandstones. The ichnofossil assemblage includes Circulichnis , Conichnus , Palaeophycus , Planolites and Beaconites , with a predominant bioturbation index of 2. Petrographic, X‐ray fluorescence, Raman and X‐ray diffraction studies helped determine the main steps in the diagenesis of sandstones containing this ichnoassemblage in Pacujá (Ceará). In addition, U‐Pb dating of detrital zircons confirmed that the studied ichnofossils belong to the Ipu Formation, resolving previous uncertainties regarding their lithostratigraphic attribution. Fossil diagenesis began with early cementation by iron sulphide minerals (e.g. pyrite), likely influenced by microbial activity, as suggested by Fe and S correlations observed in the X‐ray fluorescence. Anatase precipitation also occurred during early diagenesis, followed by compaction and subsequent mineral alterations. The oxidation of pyrite may have occurred either during early diagenesis or, more extensively, during late diagenesis, leading to the formation of goethite and haematite, which now constitute the main cement phases. The application of multiple analytical techniques is essential for identifying key microbial signatures, even when partially overprinted by subsequent diagenetic alteration, as demonstrated in this study.