Seafloor iron-rich halos of pebbles in flat-pebble conglomerates: Decoding Middle Cambrian-Early Ordovician shallow-shelf redox fluctuations

Red halos of pebbles in flat-pebble conglomerates （FPCs） are well-developed in the Cambrian-Ordovician transitional strata in North China， yet their formation mechanism and environmental significance remain enigmatic. Here， detailed petrography， in situ analyses of elemental compositions， and stratigraphic correlations are performed. The iron-rich halos of pebbles， including hematite （reddish） or pyrite （black） halos， are manifested as either submicronic， authigenic hematite or microscopic framboidal pyrite （2-5 μm） dispersed within micropores between calcite crystals. Both iron-rich halos are cross-cut by trace fossils. These features indicate that iron-rich halos formed at a similar depth during seafloor diagenesis， controlled by redoxcline fluctuations. Fe in iron-rich halos was derived from diagenetic pore water beneath the redoxcline， likely released by the dissolution of ilmenite， volcanic glass， and other minerals. Hematite halos formed when Fe²⁺ was oxidized during redoxcline deepening driven by advective transport， whereas pyrite halos formed when the redoxcline shoaling upward. In addition， trace fossils featured by pyrite halos may indicate the expansion of anoxic conditions into the bottom seawater. These red-halo-bearing FPCs are largely restricted to the late Guzhangian to Jiangshanian stages， coinciding with the Steptoean Positive Carbonate Isotope Excursion （SPICE） event; while the pyrite-halo-bearing FPCs occur throughout the rest of the middle Cambrian-lower Ordovician strata. These observations indicate that， across the Cambrian-Ordovician transition， the shallow shelf experienced increased oxygenation during the SPICE relative to background intervals.