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

Abstract

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.