Sedimentology and oceanography of Early Ordovician ironstone, Bell Island, Newfoundland: Ferruginous seawater and upwelling in the Rheic Ocean

Stephanie Todd, Peir Pufahl, Brendan Murphy, Kevin Taylor

    Research output: Contribution to journalArticlepeer-review


    The Bell Island and Wabana groups together comprise a ca. 150-m-thick succession of interbedded clastic and
    chemical sedimentary rocks composed of eight distinct lithofacies that accumulated along the northern margin
    of the Rheic Ocean. Lithofacies stacking patterns indicate that deposition occurred during a marine transgression
    with superimposed small-scale sea level fluctuations producing at least six parasequences. Parasequences containing
    ironstone are 10 to 20-m-thick and composed of hummocky cross-stratified sandstone interbedded
    with organic-rich mudstone and phosphatic Fe-silicate-bearing siltstone,which is overlain by hematitic granular
    ironstone capped by an erosive flooding surface.
    This lithofacies association is interpreted to record the deposition of upwelling-related ironstone on a stormdominated
    shelf. The close relationship between Fe-silicates and phosphorite typical of upwelling systems
    suggests that Fe was delivered from deep, anoxic, nutrient-rich seawater that also stimulated high surface productivities.
    The result was the precipitation of authigenic sedimentary apatite in anoxic organic-rich sediments
    that accumulated near the upwelling front. The gradual advection of Fe-rich waters away from the upwelling
    front, initiated precipitation of Fe-silicate coated grains and cements in suboxic pore-waters. Iron pumped
    into shallower environments through advection and Fe-redox cycling is interpreted to have precipitated Fe-
    (oxyhydr)oxide grains in sediment of the oxygenated middle shelf. These coated grains were subsequently
    concentrated by fairweather and storm currents on the shoreface to create granular economic Fe deposits.
    We challenge conventional models of Paleozoic ironstone deposition that rely on a continental source of Fe by
    proposing a hydrothermal source that supplied Fe2+ to the shelf through upwelling. It also highlights the potential
    connection between the delivery of anoxic, ferruginous seawater to the margins of the Rheic Ocean and the
    Early Ordovician extinctions that punctuated the beginning of the Great Ordovician Biodiversification Event.
    Original languageEnglish
    Pages (from-to)1-15
    Number of pages16
    JournalSedimentary Geology
    Early online date3 Nov 2018
    Publication statusPublished - Jan 2019


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