IRONSTONES OF THE KATHERINE GROUP, YUKON: SHALLOW MARINE IRON CYCLING DURING THE TONIAN PERIOD (Invited Presentation)

Iron-rich chemical sediments deposited in shallow marine settings, known as ironstones, are important tracers of ancient ocean chemistry. Typically characterised by the presence of ooids composed of iron oxides (goethite / hematite) or ferrous iron silicates (berthierine / chamosite), ironstones can potentially offer valuable insights into the redox conditions of shallow seawater and porewaters. However, the use of ironstone geochemistry as a paleoredox proxy is limited due to conflicting interpretations regarding the genesis and primary mineralogy of these enigmatic deposits. Here we describe ca. 850 Ma ironstones from the Katherine Group (Wernecke Mountains, northwestern Canada), the first reported example of an ironstone from the Tonian Period. These ironstones are hosted by quartz arenites and siltstones of the Katherine Group, interpreted to have been deposited in a peritidal setting characterised by migrating sandbars. The Katherine ironstones are well-preserved and feature iron ooids with concentric coatings of interlaminated hematite and iron-rich phyllosilicates. Iron isotope evidence indicates that the hematite is the product of partial oxidation of ferrous iron. We suggest that these ironstones formed by the variable precipitation of iron oxyhydroxides and authigenic iron-rich clays from ferruginous seawater near a redox boundary between suboxic and anoxic conditions, with vigorous iron cycling. This evidence for ferruginous conditions and partial iron oxidation in shallow seawater must be linked to markedly low atmospheric oxygen levels during the middle Tonian, which may have exerted an environmental control on organismal physiology and ecological complexity during this time.