OOIDAL IRONSTONES: PRODUCTS OF EXHALATIVE PALEOENVIRONMENTS IN SHALLOW EPEIRIC SEAS

The origin of ooidal ironstones remains controversial. Conventional interpretations for their formation include diagenesis of calcitic ooids or derivation from lateritic precursor materials. Some recent studies have implicated exhalative hydrothermal sources or derivation from volcanic ash. We examined typical Minette-type ooidal ironstones of Cambrian, Early Ordovician, and Cretaceous age from numerous localities in Canada and the United States. Occurrences of iron ooid facies seem directly related to tectonic activity within underlying Precambrian basement rocks. For example, episodic reactivation of the heavily-faulted Peace River Arch (Alberta, Canada) during the Late Cretaceous renewed fluid exhalation along fault conduits with ensuing deposition of at least six separate ooidal ironstone-bearing stratigraphic intervals from the Late Albian to Late Coniacian. Bad Heart Fmn and Kaskapau Fmn phyllosilicate ooids are coeval to nearby ultramafic volcanism, also interpreted to reflect tectonic reactivation of the crystalline basement. Additionally, the Bad Heart is extraordinary for its diversity and abundance of marine invertebrates (esp. inoceramid bivalves) and stands in marked contrast to comparatively barren under- and overlying shales. Abundant fauna (inoceramids, worm tubes) also occur in the Castlegate Fmn (Cretaceous, Colorado), but are rarer (inoceramids, lucinids) in the Emery Fmn (Cretaceous, Utah) and Wabana Iron Ore (Ordovician, Newfoundland; large inarticulate brachiopods), and rare to absent in the Kicking Horse Shale Member (Burgess Shale) and Bliss Fmn (Cambrian, New Mexico). We find that the available evidence favors an exhalative or seep origin for all authigenic Phanerozoic ooidal ironstones. Volcanic ash seems a less probable source because: 1) volcaniclastic precursor materials (glass shards, lapilli, etc) are rare or absent; and 2) the sheer volume of some ooidal ironstones (e.g., 1.1 billion tonnes, Bad Heart Fmn) implies incremental sedimentation over extended time. Any enclosed faunas are likely to include seep-related components and abundant “background” components attracted by locally increased productivity.