Lakehead University Library Logo
    • Login
    View Item 
    •   Knowledge Commons
    • Electronic Theses and Dissertations
    • Electronic Theses and Dissertations from 2009
    • View Item
    •   Knowledge Commons
    • Electronic Theses and Dissertations
    • Electronic Theses and Dissertations from 2009
    • View Item
    JavaScript is disabled for your browser. Some features of this site may not work without it.
    quick search

    Browse

    All of Knowledge CommonsCommunities & CollectionsBy Issue DateAuthorTitleSubjectDisciplineAdvisorCommittee MemberThis CollectionBy Issue DateAuthorTitleSubjectDisciplineAdvisorCommittee Member

    My Account

    Login

    Statistics

    View Usage Statistics

    Macro- and micro-structure comparison of ancient and modern iron-rich stromatolites

    Thumbnail

    View/Open

    ArtsA2015m-1b.pdf (16.33Mb)

    Date

    2015

    Author

    Arts, Adrian

    Degree

    Master of Science

    Discipline

    Geology

    Subject

    Regional geology
    Mineralogy
    Modern stromatolites (ferromanganese nodules)

    Metadata

    Show full item record

    Abstract

    The Gunflint and Biwabik Formations comprise the middle units of the Paleoproterozoic Animikie Group that crop out near the northwestern shore of Lake Superior. Stromatolitic units were investigated and sampled at 9 outcrop sites, 4 mine sites and in 12 drill-cores. Controversy exists as to whether the stromatolitic units that are now composed of chert were deposited as a silica gel or represent carbonate replaced by chert. There is also debate in the literature as to whether the iron oxides that coat many microfossils are primary or deposited during later diagenesis. Younger examples of siliceous stromatolites are very rare and iron hydroxides coating bacteria are only known from limited modern environments. Thus, understanding the primary composition of the Gunflint microbialites will provide information on ocean chemistry during this interval. Three stromatolitic members have previously been described within these formations: the Lower Stromatolitic Member containing stromatolites that grow directly on the peneplained Archean basement or on the conglomerate which forms the base of the Animikie Group, a Middle Member 45 to 50 meters above the base, and the Stromatolitic Limestone Member at the top of the formation directly beneath the Sudbury impact layer. Based on this work, a new Upper Stromatolitic Member 50 meters below the Limestone Member in the Gunflint Formation was recognized. This member contains abundant iron-silica-carbonate-manganese oncolites, and is correlative to stromatolitic units described from the Biwabik. The oncolites represent a primary-to-very-early authigenic, precipitated mineral assemblage. The accretion of lacustrine ferromanganese nodules at the sediment-water redox boundary provides a modern analogue for the development of the oncolites described in this work. The presence of exceptionally well-preserved, hematite coated microfossils, encased in chert in the relatively unmetamorphosed Middle Stromatolitic Member together with positive cerium and europium anomalies indicates that these sediments were deposited at a redox boundary in ocean waters near saturation with respect to silica and iron. Conversely, precipitates containing carbonaceous filaments in the Lower Stromatolitic Member contain a distinct negative cerium anomaly, indicating deposition in an oxidized environment. Alternating manganese- and iron-rich laminae within the Upper Stromatolitic Member indicate a fluctuating redoxcline and oxygen levels in the shallow subtidal to peritidal environments within the Animikie Basin. The fluctuation reduces the Fe:Mn ratio, allowing for the less readily oxidized manganese to precipitate. The presence of dehydration cracks which cross-cut original ooid and stromatolitic laminae indicate that deposition of the silica and iron most probably occurred as amorphous gels of opal-A and iron hydroxides. This study strongly indicates the Gunflint and Biwabik stromatolites were originally siliceous and formed by a different precipitation mechanism than that of Proterozoic carbonate stromatolites or modern agglutinated forms.

    URI

    https://knowledgecommons.lakeheadu.ca/handle/2453/4126

    Collections

    • Electronic Theses and Dissertations from 2009

    Lakehead University Library
    Contact Us | Send Feedback

     


    Lakehead University Library
    Contact Us | Send Feedback