Exploring freshwater influence in epicontinental seas through integrated sedimentology and geochemistry approaches

ABSTRACT Fine‐grained successions are formed through various sedimentary processes, including high‐energy ones, within dynamic and complex settings. However, the monotonous appearance of fine‐grained rocks poses challenges in understanding these processes. Even when the rocks are silty‐sandy and exhibit preserved sedimentary structures, their interpretation is complicated, as these structures are not always diagnostic of specific processes. The fine‐grained Devonian rocks in the Paraná Basin used in this case study are linked to an extensive epicontinental sea influenced by frequent storm wave activity. Over recent decades, researchers have acknowledged a wide range of high‐energy processes, including storm waves, gravitational flows and hyperpycnal flows, producing deposits that require detailed analysis to distinguish. This study proposes a multi‐proxy approach, integrating sedimentological (texture, sedimentary structure and ichnofabric index), mineralogical (quantitative XRD), geochemical (oxide quantification by XRF) and organic matter characterization (TOC, isotopic organic carbon and palynofacies) data. This approach aims to identify high fluvial discharge events capable of forming long‐lived hyperpycnal flows that effectively alter the environment and leave deposits that, despite textural similarities with tempestite and turbidite facies, show unique compositional features. This study identified chemical fingerprints of fluvial input events in a succession of fine‐grained rocks, both in mudstones and silty‐sandy deposits. Associated with weakly bioturbated rocks, the organic material within hyperpycnal layers exhibits a terrestrial signature, with less negative δ 13 C org excursions around −25‰, associated with TOC values <0.5%. Palynofacies data corroborate this pattern by showing >50% continental elements (e.g. spores, cuticles, pseudoamorphous organic matter and opaques). Additionally, increased zircon percentages are observed in rocks with an SiO 2 affinity, exhibiting high SiO 2 /Al 2 O 3 and TiO 2 /Al 2 O 3 ratios. Affinity with MnO and high MnO/Al 2 O 3 ratios are also expected due to the oxygenation of distal platform areas with large volumes of freshwater influx. Such freshwater influx and oxygenation are closely associated with river floods and, in extensive platforms with gentle slopes, as interpreted for the studied unit, are favoured during periods of sea‐level rise and under storm wave conditions.