An ichnofabric approach to Upper Devonian mudstones of North America and its implications for redox reconstruction

Ichnofabric analysis is widely used to infer paleo-oxygenation in ancient marine successions; however, most approaches primarily assess redox levels and frequency of redox shifts remains largely unexplored. Here, we integrate ichnofabric distribution with discrete ichnoguild characterisation to refine reconstructions of bottom- and pore-water oxygenation dynamics in the Upper Devonian North American Seaway. Detailed sedimentological analysis reveals common interbeds of cross-laminated sandstone and light-grey to greenish mudstone beds within thick black, organic-rich mudstone successions. Integrating these data with an ichnofabric approach allowed identification of seven ichnofabrics: Mottled (MI), mud-infilled Planolites (MPI), sand-infilled Planolites (SPI), dark-on-light Planolites–Teichichnus–Cylindrichnus (DOLI), Zoophycos–Chondrites–Planolites (ZCPI), subvertical spreiten (SSI), and unbioturbated (UI). The distribution of these ichnofabrics reflects variations in oxygenation levels and pulse duration within a generally anoxic setting, enabling discrimination among extreme (SPI), severe (MI, MPI), and moderate (DOLI, ZCPI) dysoxic conditions, fully oxic conditions (SSI), and anoxic intervals (UI). Nevertheless, ichnofabric indices alone do not capture the temporal complexity of redox fluctuations. By integrating ichnodiversity, tiering structure, burrow size, and the distinction between transitory and permanent biogenic structures within each ichnofabric (i.e. ichnoguild identification), we reconstruct both the duration and frequency of redox shifts. Shallow-tier and vagile biogenic structures (e.g. mottled and Planolites ichnoguilds), produced by opportunistic tracemakers, indicate high-frequency redox oscillations and brief colonization windows. Conversely, shallow- to deep-tier permanent structures—particularly Zoophycos and Chondrites ichnoguilds—and dark-on-light traces (Planolites, Teichichnus, Cylindrichnus) record longer-lasting dysoxic intervals and more sustained benthic activity. Ichnofabric distribution throughout the study units allows identification of the temporal structure of redox fluctuations, distinguishing between intervals characterised by low-frequency redox oscillations (commonly reflecting persistently anoxic conditions) and intervals with moderate- to high-frequency fluctuations, marked by the predominance of permanent and transitory biogenic structures, respectively. Thus, this integrated framework demonstrates that an ichnofabric approach, when incorporating ichnoguild analysis, can disentangle redox severity from temporal persistence.