Evidence of euxinia in the Norian of western Canada: Implications to halobiid and monotid paleoecology and the sedimentology of the Pardonet formation

  • Author / Creator
    Morton, Riley C.
  • The Norian Pardonet formation tops a well-studied Triassic stratigraphy in Western
    Canada which can be viewed in outcrop along the shores of Williston Lake, B. C. The Pardonet
    itself, however, has remained rather enigmatic. The formation is composed of dominantly finegrained, organic-rich limestone and shale complicated by the occurrence of dense, monospecific
    bivalve shell-beds. Most studies on the Pardonet have focussed on its biostratigraphy thanks to
    its abundance of biostratigraphically useful fossil taxa including ammonoids, conodonts,
    bivalves, and ichthyoliths. From a sedimentological perspective, the formation appears to have
    been deposited in deep water by dominantly pelagic deposition with more minor bioclastic
    debris-flows. High TOC and lack of bioturbation have led to the belief that the Pardonet was
    deposited in anoxic bottom-waters. Such a situation, however, seems inconsistent with the
    abundance of fossil material, especially the bivalves which do not appear to have been
    transported, even in shell-bed accumulations. Further, multiple genera can be observed replacing
    earlier forms meaning multiple species were able to exploit this deep, anoxic benthic
    environment. Some researchers have proposed that the bivalve taxa present were adapted to lowoxygen conditions but this hypothesis has never been tested in the Pardonet formation.
    Pyrite framboid size distributions and trace metal indices (Mo/U, V/Cr, V(V+Ni)) are
    applied as geochemical paleo-redox proxies in order to help resolve some of the questions
    outlined above. Firstly, to determine if the Pardonet was truly deposited in persistently anoxic
    water and if so, were the bivalves able to survive in such conditions. In addition, the
    sedimentology is reassessed in greater detail to refine the interpretations of the depositional
    environment. Given a reinterpreted depositional system and geochemical data, some inferences
    can be made about the studied intervals’ position in a sequence stratigraphic scheme.
    All applied proxies agree that the studied intervals were, indeed, deposited in persistent
    anoxic conditions with varying degrees of euxinia. There is some evidence for brief
    reoxygenation events but not long enough to produce observable facies changes. In many cases,
    bivalves are confirmed to be autochthonous and therefore survived in low-oxygen waters. The
    distribution of autochthonous shell material is controlled by the degree of euxinia, suggesting
    there was a limit to the tolerance of reducing conditions for the bivalves. Reexamination of
    sedimentological features, especially the observation of allochthonous shell material in euxinic
    facies, reveals that the Pardonet represents deposits of multiple aspects of a bioclastic turbidite
    system with more minor pelagic deposition. Applying models of sequence stratigraphy in
    turbidite systems and chemostratigraphy allows an interpretation that the upper Pardonet saw a
    regression followed by transgression resulting in facies changes from bioclastic turbidites to
    suboxic pelagic deposition followed by pelagic deposition in euxinic water.

  • Subjects / Keywords
  • Graduation date
    Spring 2024
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.