Evidence of euxinia in the Norian of western Canada: Implications to halobiid and monotid paleoecology and the sedimentology of the Pardonet formation
Creators: Morton, Riley C.
Publisher: name: University of Alberta Library
Year: 2024

<p>The Norian Pardonet formation tops a well-studied Triassic stratigraphy in Western <br>
Canada which can be viewed in outcrop along the shores of Williston Lake, B. C. The Pardonet <br>
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 <br>
bivalve shell-beds. Most studies on the Pardonet have focussed on its biostratigraphy thanks to <br>
its abundance of biostratigraphically useful fossil taxa including ammonoids, conodonts, <br>
bivalves, and ichthyoliths. From a sedimentological perspective, the formation appears to have <br>
been deposited in deep water by dominantly pelagic deposition with more minor bioclastic <br>
debris-flows. High TOC and lack of bioturbation have led to the belief that the Pardonet was <br>
deposited in anoxic bottom-waters. Such a situation, however, seems inconsistent with the <br>
abundance of fossil material, especially the bivalves which do not appear to have been <br>
transported, even in shell-bed accumulations. Further, multiple genera can be observed replacing <br>
earlier forms meaning multiple species were able to exploit this deep, anoxic benthic <br>
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.<br>
Pyrite framboid size distributions and trace metal indices (Mo/U, V/Cr, V(V+Ni)) are <br>
applied as geochemical paleo-redox proxies in order to help resolve some of the questions <br>
outlined above. Firstly, to determine if the Pardonet was truly deposited in persistently anoxic <br>
water and if so, were the bivalves able to survive in such conditions. In addition, the <br>
sedimentology is reassessed in greater detail to refine the interpretations of the depositional <br>
environment. Given a reinterpreted depositional system and geochemical data, some inferences <br>
can be made about the studied intervals’ position in a sequence stratigraphic scheme.<br>
All applied proxies agree that the studied intervals were, indeed, deposited in persistent <br>
anoxic conditions with varying degrees of euxinia. There is some evidence for brief <br>
reoxygenation events but not long enough to produce observable facies changes. In many cases, <br>
bivalves are confirmed to be autochthonous and therefore survived in low-oxygen waters. The <br>
distribution of autochthonous shell material is controlled by the degree of euxinia, suggesting <br>
there was a limit to the tolerance of reducing conditions for the bivalves. Reexamination of <br>
sedimentological features, especially the observation of allochthonous shell material in euxinic <br>
facies, reveals that the Pardonet represents deposits of multiple aspects of a bioclastic turbidite <br>
system with more minor pelagic deposition. Applying models of sequence stratigraphy in <br>
turbidite systems and chemostratigraphy allows an interpretation that the upper Pardonet saw a <br>
regression followed by transgression resulting in facies changes from bioclastic turbidites to <br>
suboxic pelagic deposition followed by pelagic deposition in euxinic water.</p>