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

Abstract

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 storm-dominated 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.